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Rare Earth Nano Compounds: Preparation and
Thermophysical Characterization
By
Ali Abdullah
CIIT/FA07-PPH-002/ISB
PhD Thesis
In
Physics
COMSATS Institute of Information Technology
Islamabad-Pakistan
Spring, 2013
ii
COMSATS Institute of Information Technology
Rare Earth Nano Compounds: Preparation and
Thermophysical Characterization
A Thesis Presented to
COMSATS Institute of Information Technology, Islamabad
In partial fulfillment
of the requirement for the degree of
PhD Physics
By
Ali Abdullah
CIIT/FA07-PPH-002/ISB
Spring, 2013
iii
Rare Earth Nano Compounds: Preparation and
Thermophysical Characterization
A Post Graduate Thesis submitted to the Department of Physics as partial
fulfillment of the requirement for the award of Degree of PhD (Physics).
Name Registration No.
Ali Abdullah
CIIT/FA07-PPH-002/ISB
Supervisor
Dr. Muhammad Anis-ur-Rehman
Associate Professor, Department of Physics,
Islamabad Campus.
COMSATS Institute of Information Technology (CIIT),
Islamabad.
June 2013
iv
Final Approval
This thesis titled
Rare Earth Nano Compounds: Preparation and Thermophysical Characterization
By
Ali Abdullah
CIIT/FA07-PPH-002/ISB
has been approved
For the COMSATS Institute of Information Technology, Islamabad
External Examiner: ___________________________________________
Prof. Dr. Asghari Maqsood Professor, Department of Physics, CESET, # 61, Sector I10/3, Islamabad
External Examiner: ___________________________________________
Dr. Misbah-ul-Islam,
Associate Professor, Department of Physics, Bahauddin Zakariya University, Multan
Supervisor: ____________________________________________
Dr. M. Anis-ur-Rehman
Associate Professor, Department of Physics/Islamabad
Head of the Department: __________________________________________
Prof. Dr. Arshad Saleem Bhatti
Department of Physics/Islamabad
Chairman of the Department: ________________________________________
Prof. Dr. Sajid Qamar
Department of Physics/Islamabad
Dean, Faculty of Sciences: ____________________________________________
Prof. Dr. Arshad Saleem Bhatti
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Declaration
I Mr. Ali Abdullah, Reg. # CIIT/FA07-PPH-002/ISB, hereby declare that I have
produced the work presented in this thesis, during the scheduled period of study. I also
declare that I have not taken any material from any source except referred to wherever
due that amount of plagiarism is within acceptable range. If a violation of HEC rules on
research has occurred in this thesis, I shall be liable to punishable action under the
plagiarism rules of the HEC.
Date: _________________
___________________________
Ali Abdullah
CIIT/FA07-PPH-002/ISB
vi
Certificate
It is certified that Mr. Ali Abdullah, CIIT/FA07-PPH-002/ISB has carried out all the
work related to this thesis under my supervision at the Department of Physics,
COMSATS Institute of Information Technology, Islamabad and the work fulfills the
requirement for award of PhD degree.
Date: _________________
Supervisor:
_________________________
Dr. Muhammad Anis-ur-Rehman,
Associate Professor
Head of the Department:
_____________________________
Prof. Dr. Arshad Saleem Bhatti
Department of Physics
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To
rational, logical and
questing minds of all
times
…………a people who think deeply. Al-Quran, Chapter Al-Jasia 45, part of Verse 13
...……………men of understanding. Al-Quran, Chapter A’l-e-Imran 3, part of Verse 190
.………….a people who understand. Al-Quran, Chapter Al-Nehl 16, part of Verse 12
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Acknowledgements
In the name of Allah, the most Gracious, the ever Merciful.
All the praises be to Allah Almighty, ‗He who taught the man which he did not know’ and
Countless prays for his Holy Prophet Muhammad (P.B.U.H).
It is a pleasure to express my truthful gratitude to my supervisor, Dr. Muhammad Anis-
ur-Rehman, who provided me the opportunity to complete my PhD in Applied Thermal Physics
Laboratory. I appreciate his scientific guidance, skilled suggestions, patience and
encouragement to me during these years.
I pay my gratefulness to members of my supervisory committee; Dr. Ishaq Ahmad, Dr.
Ahmar Naweed and former members Dr. Zuhair S. Khan, Dr. M. Kamran, for their useful
suggestions. Dr. Timothy Tan, Dr. Zhang Yan (SCBE, Singapore) and Dr. Aqif Anwar, Dr.
Abdus-Samad (IRCBM, CIIT, Lahore) are thanked for their scientific support.
I am grateful to Dean, FoS and Head of the Department Prof. Dr. Arshad S. Bhatti,
Chairman Prof. Dr. Sajid Qamar and former Head of the Department Prof. Dr. Mahnaz Q.
Haseeb for allowing me to work in my desired field and using CIIT facilities.
The Higher Education Commission, Pakistan is highly acknowledged for its financial
support through 5000 Indigenous Fellowship Program for funding my PhD, International
Research Support Initiative Program to visit Nanyang Technological University, Singapore and
Research grant project NRPU # 893. The School of Chemical and Biomedical Engineering,
NTU, Singapore is acknowledged for facilities, there.
I am thankful to my colleagues, Dr. Anwar ul Haq, Dr. G. Asghar, Dr. M. Yasin, Dr.
Nasir Khisro, Mr. G. Hasnain Tariq, Mr. M. Akram, M. Ali and others at ATPL for their
helpful discussions and maintaining a scientific environment. Special thanks to my fellows Mr.
Awais Siddique Saleemi, Mr. M. Mubeen, Mr. M. Saqib and Mr. S. Muzammil H. Shah for
their assistance, support and friendship.
I will cherish memory of my stay at ―Scholars‘ Island‖, Islamabad, with, Awais,
Hafeez, Saleemi, Arslan, Shahid, Farooq, Mohsin, Aftab, Shakoor, Irfan, Ramzan, Dr. Azeem
and at ―Pak House‖, Singapore, with Arshad, Basit, Jamil, Asif, Aftab, Vinod, Shahzad,
Zeeshan, Husnain and Adnan sb.
The Acknowledgement remains incomplete without mentioning my family (abbu,
ammi, begum, bhai, baji, bhatijian (Hamnah Fatimah, Rohah Ayshah and Umaimah Zaineb)
and son (M‘aaz Abdullah)), without whose support, affection and love, I was not able to reach
at such a point.
I pray for all a happier future and express my best wishes.
Ali Abdullah
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Abstract
Rare Earth Nano Compounds: Preparation and Thermophysical
Characterization
Rare earth compounds are a big group of functional materials which have varied
applications in many fields ranging from Solid Oxide Fuel Cells (SOFCs) to biological
labeling/imaging. The newly developed materials and techniques are nontoxic,
ultrasensitive, and chemically and physically stable. The main focus of this research
work was to attempt to enhance the ionic conductivity of ceria based compounds.
Factors like decrease in grain size, doping of trivalent cations and multiple doping are
mainly focused to increase the conductivity. Also, Rare earth doped inorganic matrix is
synthesized and fluorescence is observed in stabilized fluorophore as bimodal probe for
bioimaging.
A comparative study for synthesis and characterization of nanocrystalline ceria
was done with a range of wet chemical methods including composite mediated
hydrothermal method (CMH), co-precipitation method and sol-gel method. The
calcination and sintering temperatures were 500 0C and 750
0C respectively for all the
samples. X-ray diffraction (XRD) confirmed the cubic fluorite structure. Raman
spectroscopy seconded the XRD results and characteristic feature of ceria was observed
ca. 465 cm-1
. The dc conductivities of the samples were determined in temperature
range 200-700 0C. The highest value obtained was for the sample prepared with CMH
method having value 0.345 S-cm-1
at 7000C. So, CMH was selected as the synthesis
method for the later samples.
Further, the synthesis conditions of CMH method were optimized for
nanocrystalline samples. The practical parameters were heat treatment time and
temperature. The heat treatment temperature during synthesis was held at 180 0C and
220 0C whereas treatment time was 45, 70 and 90 minutes. Better values of
conductivities were observed for sample with heat treatment time of 45 minutes and
heat treatment temperature of 180 0C. The maximum electrical dc conductivity of the
sample was 0.3386 S-cm-1
at 700 0C in this case.
To further enhance the conductivity, the doping of Gd was done in ceria and
composition made was Ce1-xGdxOδ; x = 0.1, 0.15, 0.2, 0.25. The fluorite F2g band
around 465 cm-1
reconfirmed the Gd doped ceria. No peak of Gd2O3 (480 cm-1
) was
observed. DC conductivity was measured in temperature range 300-700 0C and ac
x
conductivity was determined in frequency range 1 kHz to 3MHz at temperatures 300,
400, 500, 600 and 700 0C. The larger values of conductivities were obtained for
Ce0.75Gd0.25Oδ. The jump relaxation model can be used to explain the dc conductivity
behavior. By jump of ions to available sites, a hopping motion started thus contributing
to dc conductivity. The ‗step‘ ac conductivity in dispersion curves is confirmation of
the grain interior and grain boundary conductivities as ionic conduction is dependent on
the defect formation due to thermal energies which create vacancies to aid in hopping
motion of ions. The maximum conductivity, achieved for Ce0.75Gd0.25Oδ, was 7.4x10-3
S-cm-1
at 700 0C. The thermal conductivity values obtained using Advantageous
Transient Plane Source (ATPS) method was in low thermal conductivity region. The
thermal conduction is dependent on the scattering and mean free path, so the less mean
free path and more scattering gave rise to low conductivity values.
The effect of multiple doping on conductivity was also studied. La and Nd were
co-doped in Gd doped ceria for two samples which showed maximum conductivities in
the earlier studies i.e. Ce0.9Gd0.1Oδ and Ce0.75Gd0.25Oδ. Samples with nominal
compositions Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.1, 0.25) were prepared. The
Ce-O fluorite breathing mode was observed in Raman spectroscopy to confirm the ceria
and doping in ceria. The strong ceria band appeared at ca. 465 cm-1
and weak oxygen
vacancy bands appeared ca. 570 and 600 cm-1
. The formation of oxygen vacancies and
defects was confirmed through Raman spectroscopy. The jump relaxation model is
applicable for dc conductivity and Jonscher power law described the ac conductivity
behavior. The maximum dc conductivity achieved was 1.78 S-cm-1
for Ce0.5Gd0.25
Nd0.25Oδ. The relaxation reorientation peaks can be realized in dielectric constant and
dielectric loss plots which shifted toward higher frequencies with increase in
temperature.
Rare earth hydroxides (R(OH)3) were synthesized by hydrothermal method and
stoichiometric change in composition and morphology was observed. Ce(OH)3,
La(OH)3 and Nd(OH)3 samples were synthesized. XRD confirmed the hexagonal
structures of the prepared samples. The crystallite size corresponding to the most
intense peaks were 18, 33 and 41 nm for Nd-, La- and Ce- hydroxides. SEM revealed
very interesting and fascinating morphologies. Ce(OH)3 has belts like structures,
Nd(OH)3 has needles like structures and La(OH)3 has wires like structures. The growth
of structures can be ascribed to chemical potential, maintained through precipitating
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agent, the pressure inside the vessel, the temperature provided for the hydrothermal
treatment and time for hydrothermal treatment. The shape evolution can be explained
by Gibbs-Curie-Wulff model which relate the shape evolution with the face energies.
When the equilibrium energy is obtained for respective faces the Ostwald ripening is
stopped. On heat treatment, the La(OH)3 first converted into LaOOH at ca. 400 0C and
finally into La2O3 at ca. 600 0C as observed in DSC plot. The increase of conductivity
with temperature is evident from the plots. Nd(OH)3 achieved maximum conductivity
and Ce(OH)3 acquired minimum among the three possibly due to smaller crystallite
sizes in the former case. The smaller grains increase the grain boundaries and charges
can pile up on boundaries which increase the conductivity. The corresponding dc
conductivity values of Ce(OH)3, La(OH)3 and Nd(OH)3 were 0.372, 6.648 and 20.369
S-cm-1
, respectively.
The fluorescence characteristics of rare earths with intense emissions and
stabilized structures were observed with Yb, Er, and Tm doping in F based inorganic
matrix NaMnF3. Yb has served as sensitizer and Tm and Er were utilized as activators.
The synthesis of NaMnF3 co-doped with Yb;Er/Tm was successfully achieved through
solvothermal method. The ethylene glycol (EG) was used as stabilizing agent. Another
important feature of this synthesis method was surface functionalization of particles
with the synthesis process in a single step. Also, the choice of precursors of Na & F
and choice of stabilizing agent (EG) rendered the nanostructures to be rods like. The
PEI polymer was used for surface modification. An intense green emission is observed
for NaMnF3: Yb, Er, with increase in Yb concentration and for fixed Er at 2 mol%.
The observed emission was around 550 nm between levels 4S3/2 and
4I15/2. Yb20 Mn78
Er2 revealed red emission at 660 nm between levels 4F9/2 and
4I15/2 which became
intense with increase of Er concentration. With Tm as dopant, NEAR IR emission was
observed at 800 nm between levels 3H4 and
3H6 although blue emission was also
observed at 480 nm between energy levels 1G4 and
3H6.
The highest value of conductivity achieved for Ce0.75Gd0.25Oδ made this material
a potential candidate as an electrolyte for SOFCs. The low thermal conductivities of
R(OH)3 can be utilized in thermal barrier coatings. The pure red emission from Yb20
Mn78 Er2 and presence of Mn made this material prospective applicant in bimodal
bioprobe.
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Table of Contents
Chapter 1 Introduction.................................................................................................. 1
1.1 The Nano ................................................................................................................ 2
1.2 Rare Earths ............................................................................................................. 3
1.2.1 Importance and Applications ........................................................................... 4
1.3 Aims and Objectives .............................................................................................. 5
1.4 Thesis Synopsis ...................................................................................................... 7
1.5 Literature Review/Background .............................................................................. 9
Chapter 2 Synthesis Methods ..................................................................................... 15
2.1 Physical and Chemical Methods .......................................................................... 16
2.2 Wet-Chemical Methods........................................................................................ 16
2.2.1 Composite Mediated Hydrothermal Method ................................................. 16
2.2.2 Co-precipitation Method ................................................................................ 18
2.2.3 Sol-gel Method .............................................................................................. 19
2.2.4 Solvothermal Method/Hydrothermal Method ............................................... 20
Chapter 3 Characterization Techniques .................................................................... 21
3.1 Structural and Morphological Analysis................................................................ 22
3.1.1 X-Ray Diffraction .......................................................................................... 22
3.1.2 Scanning Electron Microscopy and Transmission Electron Microscopy ...... 23
3.1.3 Differntial Scanning Calorimetry .................................................................. 23
3.1.4 Raman Spectroscopy ..................................................................................... 24
3.1.5 Thermal Conduction Measurements .............................................................. 24
3.2 Conductivity Measurements ................................................................................. 25
3.2.1 AC Conductivity Measurements ................................................................... 25
3.2.2 DC Conductivity Measurements ................................................................... 28
3.3 Fluorescence Measurements ................................................................................ 29
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Chapter 4 Synthesis of Ceria and Choice of Synthesis Method............................... 30
4.1 Structural Analysis ............................................................................................... 32
4.1.1 X-Ray Diffraction .......................................................................................... 32
4.1.2 Differential Scanning Calorimetry (DSC) ..................................................... 33
4.1.3 Raman Spectroscopy ..................................................................................... 34
4.2 Electrical Measurements ...................................................................................... 34
4.2.1 DC Conductivity ............................................................................................ 34
4.2.2 AC Conductivity ............................................................................................ 36
4.2.3 Dielectric Constant ........................................................................................ 38
4.3 Conclusions .......................................................................................................... 40
Chapter 5 Effect of Synthesis Parameters on Ceria Synthesized by Composite
Mediated Hydrothermal Method ............................................................................... 41
5.1 Structural Analysis ............................................................................................... 42
5.1.1 X-Ray Diffraction .......................................................................................... 42
5.2 Electrical Properties ............................................................................................. 44
5.2.1 DC Conductivity ............................................................................................ 44
5.2.2 AC Conductivity ............................................................................................ 45
5.2.3 Dielectric Constant ........................................................................................ 48
5.2.4 Dielectric Loss ............................................................................................... 49
5.3 Raman Spectroscopy ............................................................................................ 51
5.4 Conclusions .......................................................................................................... 52
Chapter 6 Effect of Gd Doping on Conductivity of Ceria ........................................ 53
6.1 Structural and morphological studies ................................................................... 54
6.1.1 X-Ray Diffraction .......................................................................................... 54
6.1.2 Scanning Electron Microscopy ...................................................................... 55
6.1.3 Raman Spectroscopy ..................................................................................... 56
6.2 Electrical Measurements ...................................................................................... 57
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6.2.1 DC Conductivity ............................................................................................ 57
6.2.2 AC Conductivity ............................................................................................ 58
6.2.3 Dielectric Constant ........................................................................................ 61
6.2.4 Dielectric Loss (tanδ) .................................................................................... 64
6.3 Thermal Conduction ............................................................................................. 66
6.4 Conclusions .......................................................................................................... 67
Chapter 7 Conductivity Enhancement in Co-Doped Rare-Earth Oxides .............. 68
7.1 Structural and morphological studies ................................................................... 69
7.1.1 X-Ray Diffraction .......................................................................................... 69
7.1.2 Raman Spectroscopy ..................................................................................... 70
7.2 Electrical Measurements ...................................................................................... 71
7.2.1 DC Conductivity ............................................................................................ 71
7.2.2 AC Conductivity ............................................................................................ 72
7.2.3 Dielectric Constant ........................................................................................ 74
7.2.4 Dielectric Loss ............................................................................................... 77
7.3 Conclusions .......................................................................................................... 80
7.4 Comparison Table ................................................................................................ 80
Chapter 8 Synthesis and Thermophysical Characterization of Rare-Earth
Hydroxides .................................................................................................................... 82
8.1 Structural and morphological studies ................................................................... 83
8.1.1 Structural Analysis ........................................................................................ 83
8.1.2 Surface Morphology ..................................................................................... 84
8.1.3 Differential Scanning Calorimetry ................................................................ 85
8.2 Electrical measurements ....................................................................................... 85
8.2.1 DC Conductivity ............................................................................................ 85
8.2.2 AC Conductivity ............................................................................................ 87
8.3 Thermal Conduction ............................................................................................. 89
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8.4 Conclusions .......................................................................................................... 90
Chapter 9 Synthesis and Fluorescence in NaMnF3: Yb;Er/Tm .............................. 91
9.1 Structural and Morphological Analysis................................................................ 92
9.1.1 X-Ray Diffraction .......................................................................................... 92
9.1.2 Transmission Electron Microscope Analysis ................................................ 92
9.2 Fluorescence Measurements ................................................................................ 93
9.2.1 NaMnF3:Yb;Er............................................................................................... 93
9.2.2 NaMnF3:Yb;Tm ............................................................................................. 95
9.3 Conclusions .......................................................................................................... 97
Chapter 10 Summary and Conclusions ..................................................................... 98
10.1 Summary and Conclusions ................................................................................ 99
10.2 Future Recommendations ................................................................................. 103
Chapter 11 References ............................................................................................... 104
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List of Figures
Figure 1.1 Abundance of elements [9] ............................................................................ 3
Figure 1.2 Representative applications of rare earths (a) in SOFCs (as electrolyte
material) (b) in automobile industry [13]......................................................................... 5
Figure 1.3 Plan of work performed in different parts ...................................................... 6
Figure 2.1 Eutectic point of NaOH-KOH composite ..................................................... 17
Figure 2.2 Schematic of composite mediated hydrothermal synthesis .......................... 18
Figure 2.3 Schematics for synthesis process of co-precipitation method ...................... 19
Figure 3.1(a) Unit cell of CeO2, light atoms are O2-
and dark atoms are Ce4+
(b) Crystal
structure of ceria ............................................................................................................ 23
Figure 3.2 Block diagram of Advantageous Transient Plane Source (ATPS) method..25
Figure 3.3 Polarization mechanisms for dielectric mediums [102] ............................... 26
Figure 3.4 Types of defects helpful in ionic transport a) oxide vacancy in perovskite
structure, b) edge dislocation c) defective grain boundaries where space charges pile up
[104] ............................................................................................................................... 27
Figure 3.5 Conductivity as a function of frequency ...................................................... 27
Figure 3.6 Basis of the jump relaxation model, (a) ions (O) on a sublattice, (b) the
effective single particle potential, (c) development of potential after a hop [107]. ....... 29
Figure 4.1 XRD pattern of ceria prepared by different wet-chemical methods ............. 32
Figure 4.2 DSC plot of ceria synthesized with CMH method ....................................... 33
Figure 4.3 Raman spectrum of ceria synthesized by different wet-chemical methods . 34
Figure 4.4(a) DC conductivity of CeO2 prepared by CMH and sol-gel method ........... 35
Figure 4.4(b) DCconductivity of CeO2 prepared by co-precipitation method …….….35
Figure 4.5(a) AC conductivity of ceria synthesized by CMH method .......................... 37
Figure 4.5(b) AC conductivity of ceria synthesized by co-precipitation method……..37
Figure 4.5(c) AC conductivity of ceria synthesized by sol-gel method……………….38
Figure 4.6(a) Dielectric constant of ceria synthesized with CMH method.................... 39
Figure 4.6(b) Dielectric constant of ceria synthesized by co-precipitation method …39
Figure 4.6(c) Dielectric constant of ceria synthesized by sol-gel method…………….40
Figure 5.1 XRD patterns of CeO2 samples synthesized by different synthesis conditions
........................................................................................................................................ 43
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Figure 5.2 Temperature dependent dc conductivity of the prepared samples ............... 44
Figure 5.3 AC conductivity (ζac S-cm-1
) as a function of frequency at different
temperatures for all samples (H14, H17, H19, H24, H27 and H29). ............................. 46
Figure 5.4 Comparison of ac conductivity (ζac S-cm-1
) at 3 MHz for all samples at
500 0C, 600
0C and 700
0C. ............................................................................................ 47
Figure 5.5 Dielectric constant (ε΄) as a function of frequency at different temperatures
for all the samples (H14, H17, H19, H24, H27 and H29). ............................................ 48
Figure 5.6 Dielectric constant at 3 MHz for all the samples at different temperatures
(500 0C, 600
0C and 700
0C). ......................................................................................... 49
Figure 5.7 Dielectric loss (tanδ) as a function of frequency at different temperatures for
all samples. ..................................................................................................................... 50
Figure 5.8 Raman spectra of the prepared ceria samples at 514 nm excitation laser line
........................................................................................................................................ 51
Figure 6.1 X-ray diffraction patterns of Ce1-xGdxOδ (x= 0.10- 0.25) ............................ 55
Figure 6.2 Scanning electron micrographs of Ce1-xGdxOδ (x= 0.10 - 0.25) .................. 56
Figure 6.3 Raman spectroscopy of Ce1-xGdxOδ (x= 0.10 - 0.25) ................................... 57
Figure 6.4 DC conductivity of Ce1-xGdxOδ (x= 0.10-0.25) as a function of temperature.
........................................................................................................................................ 58
Figure 6.5(a) AC conductivity of CG10 at different temperatures ................................ 59
Figure 6.5(b) AC conductivity of CG15 at different temperatures……………………60
Figure 6.5(c) AC conductivity of CG20 at different temperatures……………………60
Figure 6.5(d) AC conductivity of CG25 at different temperatures……………………61
Figure 6.6(a) Dielectric constant of CG10 at different temperatures ............................ 62
Figure 6.6(b) Dielectric constant of CG15 at different temperatures …………………62
Figure 6.6(c) Dielectric constant of CG20 at different temperatures …………………63
Figure 6.6(d) Dielectric constant of CG25 at different temperatures ………………...63
Figure 6.7(a) Dielectric loss (tanδ) of CG10 at different temperatures……………….64
Figure 6.7(b) Dielectric loss (tanδ) of CG15 at different temperatures……………….65
Figure 6.7(c) Dielectric loss (tanδ) of CG20 at different temperatures……………….65
Figure 6.7(d) Dielectric loss (tanδ) of CG25 at different temperatures……………….66
Figure 7.1 X-ray diffraction patterns of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10,
0.25), the starred peaks are of Nd2O3 (*) and La2O3 (#). ............................................... 69
xviii
Figure 7.2 Raman spectroscopy of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x=0.10,0.25)
........................................................................................................................................ 71
Figure 7.3 DC conductivity of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25) as
function of temperature. ................................................................................................. 71
Figure 7.4(a) AC conductivity of Ce0.8Gd0.1 La0.1Oδ(CGL10) at different temperatures
…………………………………………………………………………………………73
Figure 7.4(b) AC conductivity of Ce0.5Gd0.25La0.25Oδ (CGL25) at different temperatures
……….………………………………………………………………………………...73
Figure 7.4(c) AC conductivity of Ce0.8Gd0.1Nd0.1Oδ (CGN10) at different temperatures
……….………………………………………………………………………………...73
Figure 7.4(d) AC conductivity of Ce0.5Gd0.25Nd0.25Oδ(CGN25) at different temperatures
……….………………………………………………………………………………...73
Figure 7.5(a) Dielectric constant of Ce0.8Gd0.1La0.1Oδ(CGL10) at different temperatures
........................................................................................................................................ 75
Figure 7.5(b) Dielectric constant of Ce0.5Gd0.25La0.25Oδ(CGL25) at different
temperatures ...…………………………………………………………………………75
Figure 7.5(c) Dielectric constant of Ce0.8Gd0.1Nd0.1Oδ(CGN10) at different
temperatures……………………………………………………………………………75
Figure 7.5(d) Dielectric constant of Ce0.5Gd0.25Nd0.25Oδ (CGN25) at different
temperatures……………………………………………………………………………75
Figure 7.6(a) Dielectric loss of Ce0.8Gd0.1La0.1Oδ (CGL10) at different temperatures..78
Figure 7.6(b) Dielectric loss of Ce0.5Gd0.25La0.25Oδ(CGL25) at different temperatures.78
Figure 7.6(c) Dielectric loss of Ce0.8Gd0.1Nd0.1Oδ(CGN10) at different temperatures..78
Figure 7.6(d) Dielectric loss of Ce0.5Gd0.25Nd0.25Oδ(CGN25) at different temperatures
…………………………………………………………………………………………79
Figure 8.1 XRD pattern of Ce(OH)3, Nd(OH)3 and La(OH)3 samples .......................... 83
Figure 8.2 SEM micrographs of Ce(OH)3, Nd(OH)3 and La(OH)3 samples ................. 84
Figure 8.3 DSC plot of La(OH)3 .................................................................................... 85
Figure 8.4(a) DC conductivity as a function of temperature for Ce(OH)3 sample ........ 86
Figure 8.4(b) DC conductivity as a function of temperature for Nd(OH)3 sample…....86
Figure 8.4(c) DC conductivity as a function of temperature for La(OH)3 sample……87
Figure 8.5(a) AC conductivity as a function of frequency of Ce(OH)3 sample ............ 88
Figure 8.5(b) AC conductivity as a function of frequency of Nd(OH)3 sample……....88
Figure 8.5(c) AC conductivity as a function of frequency of La(OH)3 sample……….89
Figure 9.1 XRD pattern of PEI-capped NaMnF3:Yb,Er ............................................... 92
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Figure 9.2 TEM images (a, b) for PEI-NaMnF3:Yb, Er ; Yb:Er 20/2 Mn 78 mol %
sample and (c, d) for PEI-NaMnF3:Yb, Er ; Yb:Er 60/2 Mn 38 mol % sample ........... 93
Figure 9.3 Upconversion spectra of NaMnY3:Yb,Er capped with PEI for different
molar percentage. ........................................................................................................... 94
Figure 9.4 Upconversion spectra of NaMnY3:Yb,Er capped with PEI for different
molar percentage. ........................................................................................................... 95
Figure 9.5 Upconversion of Tm doped PEI capped NaMnF3: Yb,Tm. ......................... 96
Figure 9.6 Dispersivity of NaMnF3:Yb;Er/Tm in water ................................................ 96
xx
List of Tables
Table 4.1 Crystallite size and lattice constant crossponding to the most intense peak .. 33
Table 4.2 Comparison of ac conductivity, dc conductivity and dielectric constant for the
ceria samples synthesized by CMH, co-precipitation and sol-gel method. ................... 36
Table 5.1The nomenclature designed for the samples at different optimized parameters.
........................................................................................................................................ 42
Table 5.2 The average crystallite size and that of the most intense peaks (D (1 1 1)) along
with lattice constants (a) for the samples. ...................................................................... 43
Table 5.3 DC conductivity (ζdc S-cm-1
) at different temperatures of the samples ........ 45
Table 5.4 Frequency dependent ac conductivity (ζac S-cm-1
) at different temperatures 47
Table 5.5 Frequency dependent dielectric constant (ε΄) and dielectric loss tangent (tanδ)
values at different temperatures ..................................................................................... 51
Table 6.1 The crystallite sizes (Ds (111) = Crystallite size corresponding to the most
intense peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding
to the most intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average
crystallite size estimated by Scherrer formula, Dw = Average crystallite size estimated
by Stokes and Wilson‘s formula), lattice constant and porosity of Ce1-xGdxOδ (x= 0.10-
0.25) ............................................................................................................................... 55
Table 6.2 Activation energies, ‗s‘ and conductivities of Ce1-xGdxOδ (x= 0.10 - 0.25) at
different temperatures .................................................................................................... 58
Table 6.3 AC conductivity at 3 MHz frequency for different temperatures of
Ce1-xGdxOδ (x= 0.10 - 0.25) samples ............................................................................. 59
Table 6.4 Values of dielectric constant at 1 kHz and 3 MHz for different temperatures
of Ce1-xGdxOδ (x= 0.10 - 0.25) samples. ........................................................................ 61
Table 6.5 Variation in tanδ as a function of frequency at different temperatures for
Ce1-xGdxOδ (x= 0.10 - 0.25) samples ............................................................................. 64
Table 6.6 Values of thermal conductivity and thermal diffusivity for Ce1-xGdxOδ
(x= 0.10 - 0.25) at room temperature ............................................................................. 66
Table 7.1 The crystallite sizes (Ds (111) = Crystallite size corresponding most intense
peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding most
intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average crystallite size
estimated by Scherrer formula, Dw = Average crystallite size estimated by Stokes and
xxi
Wilson‘s formula), lattice constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10,
0.25). .............................................................................................................................. 70
Table 7.2 DC conductivities of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25) at
different temperatures .................................................................................................... 72
Table 7.3 Values of dielectric constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ
(x = 0.10, 0.25) at 500, 600 and 700 0C for 1 kHz and 3 MHz ...................................... 75
Table 7.4 Comparison of conductivity values with literature. ....................................... 81
Table 8.1 Crystallite size corresponding to the most intense peak and lattice constants
for Ce(OH)3, Nd(OH)3 and La(OH)3. ............................................................................ 84
Table 8.2 DC conductivity as a function of temperature for hydroxide samples .......... 87
Table 8.3 AC conductivity as a function of temperature for hydroxide sample ............ 89
Table 8.4 Thermal conductivity and thermal diffusivity of R (OH)3 ............................ 89
xxii
List of Abbreviations
ATPS Advantageous Transient Plane Source
β Full Width at Half Maximum
CMH Composite Mediated Hydrothermal Method
CG10 Ce0.9Gd0.1Oδ
CG15 Ce0.85Gd0.15Oδ
CG20 Ce0.8Gd0.2Oδ
CG25 Ce0.75Gd0.25Oδ
CGL10 Ce0.8Gd0.1La0.1Oδ
CGN10 Ce0.8Gd0.1Nd0.1Oδ
CGL25 Ce0.5Gd0.25La0.25Oδ
CGN25 Ce0.5Gd0.25Nd0.25Oδ
Cop. Co-precipitation
Ds Crystallite size calculated by Scherrer formula
Dw Crystallite size calculated by Stokes and Wilson formula
D s (1 1 1) Crystallite size calculated by Scherrer formula corresponding to
most intense peak
D w (1 1 1) Crystallite size calculated by Stokes and Wilson formula
corresponding to most intense peak
DSC Differntial Scanning Calorimetry
Strains
εo Permittivity of free space
ε’ Dielectric constant
EG Ethylene Glycol
Thermal Diffusivity
Thermal Conductivity
λ Wavelength
NIR Near Infrared
PEI Polyethylenemine
xxiii
SEM Scanning Electron Microscopy
SOFCs Solid Oxide Fuel Cells
Sol. Sol-gel
TEM Transmission Electron Microscopy
tanδ Dielectric loss
UC Upconversion
UV Ultraviolet
XRD X-ray Diffraction
xxiv
List of publications
Papers part of the thesis
1. Comparative study of nano crystalline ceria synthesized by different wet chemical
methods
A. Abdullah, A. S. Saleemi and M. Anis-ur-Rehman, J. Supercond. Nov. Magn.
(2013) DOI 10.1007/s10948-013-2261-x
2. Conductivity dependence on synthesis parameters in hydrothermally synthesized
ceria nanoparticles
M. Anis-ur-Rehman, A. S. Saleemi and A. Abdullah, J. Alloy. Comp. 579 (2013)
450-456.
3. Synthesis and conductivity of nanocrystalline Ceria
A. S. Saleemi, A. Abdullah and M. Anis-ur-Rehman, J. Supercond. Nov. Magn. 26
(2013) 1065-1069
4. Synthesis and structural properties of Ce(OH)3 for useful application
M. Anis-ur-Rehman and A. Abdullah, J. Supercond. Nov. Magn. 24 (2011) 1095-
1098
Other publications
1. Electrospun proficient polymer based nanofibers with ceramic particles,
A. S. Saleemi, A. Abdullah and M. Anis-ur-Rehman, J. Supercond. Nov. Magn. 26
(2013) 1027-1030
2. Effects of Sintering Temperature on Structural and Electrical Transport Properties
of Zinc Ferrites Prepared by Sol-Gel Route,
M. Anis-ur-Rehman, M.A. Malik, I. Ahmad, S. Nasir, M. Mubeen, A. Abdullah,
Key Engineering Materials 510–511 (2012) 585-590
3. Preparation and thermoelectric studies of spinel ferrites in ―Thermoelectric Power‖
M. Anis-ur-Rehman and A. Abdullah, Ed. W. P. Dempsey, Nova Science
Publishers, NY, USA, (2011) 363-380
1
Chapter 1 Introduction
2
Introduction
1.1 The Nano
Nanoscience and nanotechnology mainly deals with synthesis, characterization,
probe, and utilization of nanostructured materials. Significant changes happen in the
physical and chemical properties of nanomaterials in comparison with those of the bulk
materials. The influence of nanoscale can be observed on the structural properties,
kinetics, reaction, governing forces and chemical makeup of nanostructures. Novel
devices and technologies can emerge by apt switching of the properties and response of
nanostructures. The significant foci in nanoscience are related with effects of size,
evolution of shape and quantum confinement [1].
The synthesis leitmotifs relating to nanoscience and nanotechnology are twin:
one is the bottom-up approach which is the reduction of the mechanisms as voiced by
Feynman, in 1959 lecture that ―there is plenty of room at the bottom‖ and the other is
the methodology of the self-assembly of molecular components which is affiliated with
Jean- Marie Lehn [2].
Taniguchi used the word Nanotechnology, first time, in 1974 in a paper
entitled ―On the basic concept of Nanotechnology‖ although the nanomaterials are
being used specifically in field of chemistry as old as chemistry itself e.g. the medieval
stain glass which utilized the precious metal colloids and use of cement by Romans [2-
3]. The Nanoscience can be defined like ―Nanoscience is the study of phenomena and
manipulation of materials at atomic, molecular and macromolecular scales, where
properties differ significantly from those at a larger scale‖ whereas ―Nanotechnologies
are the design, characterization, production and application of structures, devices and
systems by controlling shape and size at nano meter scale‖ [4].
From point of view of the length scale, the NASA defined it as ―The creation of
functional materials, devices and systems through control of matter on the nanometer
scale (1-100 nm) and exploitation of novel phenomena and properties (physical,
chemical, and biological) at that length scale‖ [5]. Normally the 1-100 nm length scale
is considered to be the nanoscale although it may vary depending on the property of
interest under observation [6]. Rogers et al have defined the Nanotechnology as;
―Nanotechnology means putting to use the unique physical properties of atoms,
molecules and other things measuring roughly 0.1 to 1000 nanometers‖ [2]. Size effects
3
are a vital feature of nanomaterials. The effects dependent on size trace the evolution of
structure, thermodynamics, electronics, spectroscopic and chemical characteristics of
these restricted systems.
There is unlimited vigor in the area of nanoscience & nanotechnology and vast
prospects as the nanoscience is an interdisciplinary field covering physics, chemistry,
biology, materials and engineering. New materials, new scientific observations and
innovative technological potentials are emerging from the collaboration among
scientists with different qualifications. Along with the academia, industry is an equally
important beneficiary of nanoscience and nanotechnology. The global market of
nanotechnology is expected to be $70-160 billion by 2015 [7]. The society is also
getting benefits as the nanotechnology has applications in fields like health care,
medicine, environment and energy [1,8].
1.2 Rare Earths
Rare earths are 4f-block elements which are also called Lanthanides and/or
Lanthanones. They get their name ―Rare earth‖ due to their extraction from oxides
which were known as earths and these were considered to be rare although now many
of these are plentiful than other elements in the periodic table.
Figure 1.1 Abundance of elements [9]
4
Owing to their close resemblance of chemical and physical properties all the
fifteen elements from La to Lu (51-71) are taken as the members of lanthanide series.
Rare earths form one of the major industries in the world and market of $1.4 billion in
2010 is expected to reach $4.1 billion by 2017 [10].
These elements form the tripositive lanthanide cations. These show oxidation
states of +2, +3 and +4. The +3 is considered as more stable than di- and tetra-positive
cations. Although the change in oxidation state can change the properties very
remarkably but in general, these elements show metals like behavior. Rare earths have
electrode potential values comparable to those of alkaline earths and the rare earth
oxides resemble those of alkaline earth oxides. These oxides are insoluble in water but
absorb CO2 and H2O. The rare earth hydroxides are not amphoteric but basic like
oxides and have hexagonal structure and are definitely not hydrous oxides. These also
absorb CO2 and on heating decompose to form oxides.
A characteristic feature of rare earths is what is known as lanthanides
contraction and is the decrease in atomic and ionic radii. This decrease in size is more
evident in ions but not so regular in case of atoms. An interesting effect in rare earths is
that, the additional electron enters 4f- subshell but not in the valence shell. The mutual
shielding effect of 4f electrons is very diminutive due to the shape of f sub shell which
is very much diffused although d electrons are larger than f electrons. However, with
the increase in each step the atomic number increases, whereas for the 4f electrons,
there is no similar increase in the mutual shielding effect. That‘s why; the atomic and
ionic radii go on decreasing from La to Lu due to the electrons in the outer most shell
practice increasing nuclear attraction. The chemical properties of an element or an ion
depend on the size of the atom or ion. The greater the atomic or ionic radius, greater is
the ease with which the ions or atoms will lose electrons [11].
1.2.1 Importance and Applications
Rare Earth compounds are a large group of functional materials with varied
applications in electric, magnetic, optical and catalytic fields mainly due to their unique
4f electrons. Owing to the amazing reducing property of rare earths, they are used in
metallothermic reactions. Different brands of steel are made through the alloys of these
materials. The Rare earth oxides are widely used for decolorizing glass. Also, these can
absorb ultra violet rays. Owing to their higher melting points the rare earth oxides are
5
used in refractories. Some other applications include; abrasives, paints, textile and
leather industries, lamps, oxidizing agents and ferromagnetic garnets among others.
Rare earth nanocrystals have major interest of scientists and engineers with their special
properties such as consistent optical characteristics and boosted catalytic acts allowing
them to aid in preparation of functional assemblies. The fruitful uses of rare earth bulk
materials as phosphors, magnets, catalysis, superconductors, electrolytes and electrode
materials in SOFCs, hard alloys, etc. have stimulated great research interest in their
nanoscale complements [12].
(a)
(b)
Figure 1.2 Representative applications of rare earths (a) in SOFCs (as electrolyte
material) (b) in automobile industry [13]
1.3 Aims and Objectives
This system (Rare earth compounds) is chosen for its numerous applications in
various technological, industrial and research purposes. The anticipated findings of this
6
proposed work are many fold; new synthesis routes, study of the varying behavior of
various properties at nano scale and functionalization of nano structures. The rare earth
compounds have various applications which are very attention-grabbing and beneficial
from research point of view; the R-oxides have gas sensing and electrolyte material for
SOFCs applications and R-hydroxides are potential candidates for biolabeling
applications.
Rare earth compounds including R-oxides and R-Hydroxides are synthesized.
Structural, electrical and thermal properties of synthesized compounds are investigated.
The transport phenomenon is examined in these compounds. An attempt is made to
explain the obtained results with some appropriate theories. The plan of work is
detailed in figure 1.3.
Figure 1.3 Plan of work performed in different parts
The major focus in these compounds was to seek their application to be
functional in areas of energy and nanobiotechnology, namely, electrolyte material for
SOFCs and bimodal bio probe for small animals imaging. A safe long term energy
resource is one of the major challenges in the 21st century. Energy is vital to universal
human development including the ecology, economic growth, occupation, success and
parity. The disappearing fossil based reserves and other related issues (environment,
social) demand for dedicated and vigorous efforts to change the present energy system
to sustainable one. The SOFCs, which convert chemical energy into electrical are such
CeO2
I. Rare earth Oxides
Ce1-x
GdxO
X=0.10, 0.15, 0.20, 0.25
Ce1-2x
GdxLa
xO
X=0.10, 0.25
Ce1-2x
GdxNd
xO
X=0.10, 0.25
II. Rare Earth doped & co-doped Ceria
III. Rare Earth Hydroxides
Ce(OH)3
Nd(OH)3
La(OH)3
NaMnF3:Yb,Er/Tm
IV. Rare Earth doped F based inorganic Matrix
7
an area of interest of researchers as alternate to fossil fuels. Rare earth doped ceria
based compounds are widely studied as electrolyte material for SOFCs. The researchers
are trying to enhance the ionic conductivity and lower the operational temperature of
these cells. Various strategies are utilized to increase the conductivity like material,
doping, multiple doping, composites, decrease in grain size and synthesis methods. In
this research work synthesis methods are compared for the achievement of enhanced
conductivity in ceria. The doping and multiple doping of rare earths in ceria are also
studied for the same.
The next generation personalized and targeted drugs are blessings of the
marriage of nano and bio technologies. The advent of functionalized, nontoxic and bio
compatible materials with sophisticated and precise, smart, par excellence tools and
devices have made researchers believe in a disease free world. The nanomaterials are
being used for imaging, diagnosis and treatment of various diseases these days. Rare
earth doped therapeutics and imaging probes are one of the widely synthesized and
fascinated materials. The physical properties significant to materials often made these
materials utilized for more than one modalities. Another focus in this research work
was to synthesize materials having potential for bimodalities. The tuning of emission
bands in rare earth doped inorganic matrix is also studied in this work.
1.4 Thesis Synopsis
Thesis is constructed on introduction, synthesis & characterizing techniques and
on different parts of research work. Introduction to chapter contents is presented here.
Chapter 1: The introduction to nano, rare earths, importance of rare earths and
objectives of the research work are discussed in this chapter. The vital application of
rare earths in the fields of energy and biotechnology grew the interest to work on
these materials.
Chapter 2: The requirements for the synthesis of functional nanomaterials are phase
purity, ease to prepare complex compositions and desired structures, The synthesis
techniques are categorized as physical methods and chemical methods or can be
termed as bottom-up and top-down approaches. The wet chemical methods have
provided easy, simple, often low temperature and precise control of synthesis of
nanomaterials. These different synthesis methods are discussed in this chapter and
8
brief description of wet chemical methods is given. Representative synthesis
schemes utilized in this research work are also presented.
Chapter 3: The progress in nanotechnology was highly based on new characterization
tools as the study of physical properties at nanoscale was not possible using
conventional tools. Physical, chemical, optical, thermal and electrical properties are
being studied with the aid of new state of the art characterization tools with
approaching- to- ideal precision in measurements. The importance of tools is
manifested by giving brief introduction to those employed in this research work.
Corresponding theories / models and formulae are also discussed.
Chapter 4: Ceria is one of the most studied and industrially produced rare earth
compounds. The significant applications in various fields make this compound one
of the most interesting materials for researchers. The synthesis of ceria is done using
range of wet chemical methods. The main focus of interest (conductivity) is
compared in prepared samples as a function of synthesis method. Conclusion is
drawn for the choice of better method for synthesis of ceria to achieve enhancement
in conductivity.
Chapter 5: The optimization of some synthesis process is very much important as the
properties of materials also depend on the synthesis process. A facile wet chemical
route, composite mediated hydrothermal method, was utilized to synthesize ceria,
focused for its application as electrolyte material. Different synthesis parameters
were tuned and results of the conductivity were compared to have idea of better set
of parameters.
Chapter 6: The ceria based compounds are widely used as electrolyte material in
SOFCs in quest by researchers to seek alternate energy sources instead of dying
conventional sources. The doping of cations in ceria influences the conductivity and
normally aid in enhancement in conductivity. The doping of Gd in ceria was done
and its effect on different properties is discussed in this chapter.
Chapter 7: The samples which showed maximum conductivity were further co-doped
with other rare earths (La, Nd) and different properties were observed. Multiple
doping is another noted process to help increase the conductivity. The results are
discussed in this chapter.
9
Chapter 8: The stoichiometry of oxygen in rare earth compounds is one of the very
important features as the properties like reduction, catalysis and conductivity are
highly dependent on it. The stoichiometry change with synthesis method also
influences the crystal structure, for example, R-oxide has cubic structure and R-
hydroxide has hexagonal structure at nano regime whereas the starting material has
cubic structure in both the cases. The morphology becomes attention grabbing as
many 1-D structures can be observed like rods, wires and belts. 1-D structures are
swiftly growing attention due to their fascinating properties and exclusive
applications. Nanowires are developing as important building blocks helping as
interconnects and active components in nanoscale electronic, magnetic and photonic
devices. The synthesized 1-D structures of rare earth hydroxides (La, Ce, Nd) and
their properties are discussed in this chapter.
Chapter 9: The blissful merger of nano and bio technologies promises the diagnosis
and treatment of various diseases. The area of multimodal imaging utilizes physical
materials at nanoscale to seek information from living organisms. Rare earth doped
inorganic matrices are being prepared in this regard. One such material with
potential of optical and magnetic imaging NaMnF3: Yb,Er/Tm, was synthesized and
its different properties are discussed in this chapter.
Chapter 10: Conclusions are drawn in this chapter and the future recommendations are
discussed.
Chapter 11: References and citations are provided in this chapter.
1.5 Literature Review/Background
Rare earth compounds are a large group of materials having multi applications
in fields like electric, magnetic, optical and catalysis. They have got immense interest
with special properties like optical and catalytic, reduction. Their usage in different
fields is increasing day by day due to synthesis techniques and functionalization. The
solution based or wet chemical synthesis routes have made easier, their synthesis and
functionalization and in many cases both in single step although high temperature solid
state reaction methods (bulk synthesis) are also being used. With advent of
nanotechnology the rare earth applications like phosphors, magnets, catalysis,
superconductors, electrolytes and electrode materials in solid oxide fuel cells, hard
10
alloys etc are being made with nanoscale rare earth compounds. Although the intra-4f
transitions are independent of size or shape of the materials but with reduction of size
nanocrystals may display electron-photon interaction. Also, the increase in the surface
areas improve the catalytic activity. The size and shape at nanoscale also influence the
application oriented studies, e.g. for small animals imaging , the probes need to be of
nanometer scale and for a smart, compact design of fuel cell nanometer scale is desired
[14-19].
Ceria based rare earth compounds have far-reaching applications as UV
absorbers, 3-way catalysis, and electrolyte in solid oxide fuel cells. Ceria is one of the
most studied rare earth and also one of the major industrially produced compounds with
Carbon, Titanium oxide, Silicon and Zirconia [2]. A large number of workers have
synthesized ceria utilizing different methods and by varying crystal growth techniques
[20-26].
One dimensional nanostructures include rods, wires and belts. The interesting
features mainly conduction and optical make these structures property of interest for
researchers. The template, supersaturation and capping are deciding parameters for the
directional growth. The rare earth hydroxides, orthophosphate and orthovanadates are
1-D rare earth compounds among others. Rare earth hydroxides have hexagonal
structure with P63/m which also becomes evident in morphology of the structures.
Hydrothermal method is widely utilized to get 1-D structures. Rare earth hydroxides
were prepared with hydrothermal method reported by Wang and Li [27]. KOH was
used to make precipitating gels at room temperature and then hydrothermal treatment
was given at 180 0C. As a representative reaction, 5 mol KOH was used to precipitate
the solution. When pH was between 6 and 7 nanosheets were observed for Sm(OH)3
and nanorods for pH 9-10. The crystal structure (hexagonal) of R(OH)3 manifest itself
through anisotropic crystal growth. The 1-d structures formed due to the chain of the
OH- and R 3+ cations connected to each other.
The films and quantum wells like structures fall in category of 2-d structures.
Mainly, these are made using molecular beam epitaxy, physical vapor deposition or
surface ligands. Hydrothermal synthesis provides facilities 2-d nonstructural growth
without surfactants. The control of pH with other parameters decides the visible faces
[28-30].
11
Re2O3 or RO2 have three phases; the hexagonal phase, monoclinic phase and
cubic phase. These rare earth oxides were mainly prepared using solid state reaction
methods as the hydrothermal methods mostly went through hydration. The synthesis of
Gd2O3 nanoplates in organic solvents is starting from gadolinium acetate to the thermal
decomposition of rare earth benzoylacetonate, acetylacetonate (acac) and acetate
precursors [25, 28-30]. Rare earth fluoride nanocrystals can be obtained by the
thermolysis. Different sizes and shapes can be obtained through reaction temperature,
and reaction time. The phase, cationic radii and synthesis parameters can be varied to
have shape control of RF3. Light rare earths fluorides are formed in trigonal, truncated
trigonal and hexagonal shapes. Large cations like La favored trigonal shapes whereas
small cations form hexagonal phases. In LaF3 short reaction times favored trigonal
phases whereas longer times favor hexagonal shapes. The shape evolution can be
explained through Ostwald ripening [25, 31-32].
R(OH)3 nanotubes can be observed through corrosion in RNi5 with help of
KOH. Eu2O3 assemblies can also be obtained with carbon nanotubes as templates in a
temperature range of 80 0C -180
0C. In this way rare earth oxides, oxysulfidde and
oxyfluoride nanotubes can be synthesized [33-36]. Yada at al has reported synthesis of
rare earth oxides using sodium dodecylsufate as surfactant [37]. The nitrates and
chlorides of rare earths can be mixed with urea, surfactant and water at 40 0C, and then
heating at 80 0C, the hydrolysis of urea raises pH values and precipitate rare earth
hydroxide. Rare earth nanotubes are thus obtained with smaller inner diameters of 3nm
after calcination.
Surfactant free synthesis of nanotubes of rare earth hydroxides is also reported.
Xu et al have reported hydrothermal synthesis of dysprosium and terbium hydroxides
from colloidal hydroxide precipitation at 120-160 0C. With similar procedures CeO2
and Y2O3: Er has also been reported. The nanotubes formation can be achieved with
combination of low temperature and high basicity and high temperature and low
basicity [38].
Colloidal up conversion nanoparticles of rare earth phosphates, oxides and
oxysulfides have been reported. NaRF4 are widely studied material. It has two
polymorphs, cubic α-NaRF4 and hexagonal β-NaRF4 and β phase is found to be
excellent up-conversion hosts. OA/OM/ODE solvents are used to get NaRF4 with
12
controlled size composition and surface state. The phase transitions are dependent on
Na/R ratios, solvent compositions, reaction temperatures and time. For β phase high
temperatures, long time and large Na/R ratios are required [39-45].
Adding aliovalent cations produces oxygen vacancies which increase the
conductivity. In case of ceria, Gd or Sm doping is widely studied. In addition to single
doping multiple doping is more effective in increase of conductivity [46-47]. Lee et al
have studied the CeO2 and Sc2O3 co-doped with ZrO2 for conductivity improvement
and phase stability. They have reported that the system showed much higher
conductivities than YSZ at same temperature range [48].
Ralph and coworkers reported the effect of oxide addition for Gd doped ceria on
different concentrations. With addition of Ca, Fe and Pr. The conductivity increased
than the simple Gd doped ceria [49-50]. Maricle et al have studied the Pr and Sm
doping in Ce0.8Gd0.19M0.01O2 and reported that by the two orders of magnitude, oxides
addition improved the lower oxygen partial pressure limit [51].
Mori and coworkers introduced the concept of effective index in ceria based
compounds. The effective index is multiple of ratios of average ionic radius to oxygen
ion radius and dopant radius to host radius. They observed increase in conductivity in
reducing atmospheres [52].
The decrease in defect formation energy improved the conductivity in
nanocrystalline ceria as described by Chiang et al [53].
Thermal decomposition of ceria complexes is utilized by Veranitisagul et al to
prepare Gd doped ceria, sintered at 1500 0C. The maximum conductivity achieved, for
Ce0.85Gd0.15O1.925, was 0.025 S/cm at 600 0C [54].
The effect of microwave heating and conventional heating on conductivity is
reported by Acharya [55]. Nanosized Dy doped ceria was prepared by combustion
method and sintering was done with microwave heating and resistive heating. The MH
heating improved the conductivity of the material.
Dielectric and electrical properties are observed in Gd doped ceria synthesized
by sol-gel method reported by Liu. The composition studied was Ce0.8Gd0.2O2-δ [56].
Anderson et al have theoretically optimized the ionic conductivity in ceria with
different dopants. They have predicted the relation between defect association and ionic
13
conductivity. The balance between elastic and electronic defect interactions is required
for low association and high conductivity [57].
Balaguer et al have synthesized different rare earth doped ceria with co-
precipitation method. Gd, La, Tb, Pr, Eu, Er, Yb and Nd were used as dopants. The
addition of Co is also studied. The samples with Pr and Eu doping showed higher
values of conductivity. The same compounds with Co addition showed higher values of
conductivity [58].
Hu et al have studied the effect of dopants on energy of oxygen vacancy
formation theoretically. They observed that the low valence dopants lower the energy
of oxygen vacancy formation. This effect occurred due to the creation of hole at the top
of valence band [59].
Li and coworkers studied the defect formation using Raman in doped ceria. The
dopants were Gd, Zr, La, Sm, Y, Lu and Pr. The samples were synthesized by citrate
sol-gel method. They observed two defect sites, the difference of ionic valence state
and ionic radii gave rise to creation of defect sites [60].
Synthesis of ceria with co-precipitation method is described by Shih et al. The
influence of calcination temperature is observed on crystallite size. With the increase in
calcination temperature the crystallite size is found to be increased [61].
The effect of grain size on conductivity is observed by Lenka and coworkers.
They studied that at low temperatures smaller grain size show high conductivities
whereas at higher temperatures larger grain size show higher conductivities [62].
Ren at al have reported the synthesis of hexagonal NaYF4:Ce, Tb, Gd
nanocrystals. The solvothermal process is used for the synthesis with PEI as
functionalization agent. The nanocrystals showed efficient fluorescence and T1 contrast
MRI [63].
The synthesis and biological studies of amine-functionalized Er doped La(OH)3
nanoparticles is reported by Sun et al [64]. The in vitro experiments with HeLa cells
and cytotoxicity have been studied. Mu et al [65] described the synthesis and
photoluminescence of La(OH)3. The electrical and photoluminescence behaviors of
La(OH)3 nanobelts are discussed by Zuo et al with Ce and Er used as dopants [66].
The cytotoxicity and luminescence of Gd(OH)3 nanostructures are discussed by
Hemmer et al [67]. The in vitro toxicity analysis was done with human lung A549 and
14
Caco2 cells. The photoluminescence of Gd2O3 nanocrystals doped with Eu and Tb is
reported by Seo et al [68]. The review by Mader et al included La2O3 as upconversion
material with Yb-Eras dopant ions [69].
Amongst others, an important and well-studied rare earth system is Y2O3. Das
and Tan have reported the rare earth doped and codoped Y2O3 as potential bioimaging
probes. The photoluminescence, upconversion and cytotoxicity of amine functionalized
nanoparticles are reported. Tb, Eu and Er are used as dopants. The cell viability data of
nanoparticles attached with Hep-G2 cells has been given [70]. The photoluminescence
in Y2O2S nanotubes is reported by Wang and Li [35]. The upconversion and down
conversion is obtained by doping with Yb-Er. Tb- Y2O3 nanocrystals synthesis and
luminescence properties are reported by Zhang et.al. The influence of different alkyl
amines is discussed [71]. The cell viability data and luminescence and magnetization
are also discussed [72]. Yang et al [73] used Ce-Tb and Yb-Er ion pairs for down- and
upconversion in LaF3 and NaLaF4. The other systems mentioned by Mader et al are
NaYF4, NaYbF4, CaF2, GdOF, BaTiO3, Lu2O3 and Y2O3 whereas Yb, Er, Tm and Ho
were used as dopants [69].
The review by Yuan et al included Eu, Sm, Tb and Dy complexes with β-
diketone and aromatic-derivative ligands as luminescence probes. The nanoparticles of
Ho- Y2O3, Er/Yb-ZrO2, Er/Yb-La2(MoO4)3, Tm/Yb-LuPO4 and Er/Yb-NaYF4 are also
mentioned [74]. The review by Chao et al [75] included NaYF4, CeF3, PrF3 NdF3 and
LaF3 systems and Er,Yb as dopants are used. The synthesis and emission spectrum of
YPO4-Eu nanobundles is reported by Wang et al [76]. The NaYF4 system doped with
Yb,Er is reported by Xiong et al [77]. The luminescence, toxicology studies and in vivo
& ex vivo examination with HeLa and MCF-7 cells is discussed. The in vivo toxicity of
Eu(OH)3 in mice is detailed by Patra et al [78]
Tian and coworkers have observed the effect of Mn doping in NaYF4: Yb/Er.
Solvothermal technique was utilized to obtain nanocrystals. The hexagonal to cubic
phase change and green to red UC emissions are related with the Mn contents [79].
15
Chapter 2 Synthesis Methods
16
Synthesis Methods
The control of size, shape and structure of nanomaterials are addressed in
synthesis process. Also, the knowledge of application plays its role in synthesis as the
nanomaterials are made functional and operational. Nano scale materials are generally
synthesized by following two approaches
Physical Methods
Wet-chemical Methods
The physical and wet-chemical methods are utilized for the controlled synthesis of
functional nanomaterials having pure phase, anticipated composition, even morphology
and tunable properties.
2.1 Physical and Chemical Methods
The physical methods include 1) physical vapor deposition, 2) pulsed laser
depositions and 3) sputtering methods. The chemical synthesis techniques can be
categorized as; 1) vapor-phase, 2) solution precipitation and 3) solid-state processes, 4)
water oil microemulsions 5) polyol method [1].
2.2 Wet-Chemical Methods
The wet-chemical methods consist of chemical reaction. The precipitating
colloids of nanomaterials from a solution of chemical compound have been a striking
scheme because of the simplicity. A key advantage of solution synthesis is the capacity
to form functional nanoparticles improving their stability and broader applications.
There are many wet chemical methods; some of them which are utilized in this research
work are given below
Composite Mediated Hydrothermal Method
Sol-gel Method
Co-precipitation Method
Solvothermal Method/Hydrothermal Method
2.2.1 Composite Mediated Hydrothermal Method
The composite mediated hydrothermal (CMH) method has simple procedure
and is a subsidiary of hydrothermal method. It gives less impurities (practically zero) in
final product and is relatively environmental affable. The method made use of the
eutectic point of NaOH-KOH composite. The melting temperatures of KOH and NaOH
17
are 406 0C and 323
0C respectively but at the mole ratio 0.515/0.485 of NaOH-KOH
system the eutectic point is 170 0C i.e. these melt at this temperature for this ratio. [80-
83].
Figure 2.1 Eutectic point of NaOH-KOH composite
By this technique the NaOH-KOH not only served as reactants but also as
precipitating agent. Very complex chemical compositions can be obtained with this
method. The synthesis parameters like heat treatment time and temperature can be
varied to obtain different sizes of nanomaterials.
Synthesis of Rare-earth oxides
For a typical reaction, Ce(NO3)3.6H2O, 4 gm, NaOH, 10.3gm and KOH, 9.7gm
were used. All chemicals in powder form were mixed in Teflon chamber, sealed and
were heat treated at 180 °C for 45 minutes in a pre-heated resistive heating oven. Once
the heat treatment was done and room temperature was achieved, the sample was
shifted to oven for drying after washing several times with de-ionized water. The
calcination was done at 500 0C for 2hrs. The pellets, made with 1gm powder of 13mm
diameter, were then sintered at 750 0C for 5hrs. The proposed chemical reaction is.
Ce(NO3)2+ KOH + NaOH CeO2+ by products
Synthesis of rare earth oxide NP‘s consists of following experimental steps given in
flow chart.
18
Figure 2.2 Schematic of composite mediated hydrothermal synthesis
2.2.2 Co-precipitation Method
Nucleation, growth and agglomeration processes sequentially happen in co-
precipitation. This method is considered to be appropriate and cost-effective for
synthesis of nanomaterials with narrow particle size. The chemical reactivity is
achieved through the pH maintained by some precipitating agent. The particles‘
formation, nucleation and growth are dependent on the rate with which the precipitating
agent is mixed. The molarities of precursor solution and precipitating agent solution
along with the pH are decisive parameters in co-precipitation method [84-88].
Synthesis of Ceria
Ce(NO3)3. 6H2O (cerium nitrate) was used to get cerium oxide nanoparticles by
co-precipitation method. The precipitating agent used was sodium hydroxide (NaOH)
provided the chemical reactivity through maintaining the pH. The molarity of cerium
nitrate solution was 0.2M and the molarity of sodium hydroxide solution was kept at
1M. De-ionized water was used as solvent for both the solutions.
19
Figure 2.3 Schematics for synthesis process of co-precipitation method
To achieve the required value of pH, the precipitating agent (sodium hydroxide
solution) was added to the cerium nitrate solution. After the continuous stirring for 30
minutes, washing of the sample was done several times with de-ionized water. Then the
sample was shifted to the conventional oven for overnight at 105 0C for drying. The
calcination temperature and time for the prepared powder was same as used in
composite mediated hydrothermal method (at 500 0C for 2-hours). The calcined powder
was then used to made the pellets which were further sintered for 5-hours at 750 0C.
2.2.3 Sol-gel Method
In sol-gel process, hydrolization of metal precursors is done with water which
yields suspension of colloid (the sol) following the condensation to produce a gel. In
condensation water or alcohol molecules are released. The rate of hydrolization and
condensation play an important role for the formation of final product [1].
Synthesis of Ceria
Ce(NO3)3. 6H2O was used to obtain cerium oxide nanoparticles by sol-gel
method. 0.4M solution of cerium nitrate was prepared using water. Ethylene glycol
20
(EG) was used as chelating agent. Initially at low temperatures the solution was
gradually changed into gel. With increase of temperature the gel was dried. After
rigorous stirring it was burned and nanoparticles of cerium oxide nanoparticles in
powder form were formed. The calcination of the prepared powder was done and
pellets made were sintered as previously described [89-90].
2.2.4 Solvothermal Method/Hydrothermal Method
In solvothermal process soluble metal species undergo precipitation followed by
a pressurized heat treatment in an autoclave. The temperatures more than 100 0C and
pressures more than the atmospheric pressure are utilized to form nanostructures. The
products thus achieved are phase pure and show homogeneity. If some solvent is used
other than water, the method is known as solvothermal [91].
Synthesis of rare earth hydroxides with hydrothermal method
For the synthesis of rare earth hydroxides hydrothermal method was adopted. A
nominal amount of cerium oxide was dissolved in nitric acid. Sodium hydroxide was
utilized for precipitation. These precipitates were then heat treated in a Teflon chamber
at 180 0C for overnight period of time. After the product was allowed to cool down to
room temperature the nanostructures were achieved after the washing and filtering with
de-ionized water [92].
Synthesis of NaMnF3 Yb; Er/Tm with solvothermal method
The solvothermal approach [93-94] was utilized to synthesize elongated
structures of rare earth doped NaMnF3. NH4F, NaCl and MnCl2 (Aldrich made,
analytical grade) were used to get F, Na and Mn ions respectively. Ethylene Glycol was
used to stabilize the ions and PEI was used as polymer to render nanorods hydrophilic.
Reaction:
Ethylene Glycol + NaCl (sonication)
+
MnCl2 + Yb(NO3)3 + ErCl3+PEI + NH4F + Ethylene Glycol (sonication)
Hydrothermal treatment @200 0C for 2hr
Collection of particles, washing, drying @50 0C
21
Chapter 3 Characterization
Techniques
22
Characterization Techniques
3.1 Structural and Morphological Analysis
3.1.1 X-Ray Diffraction
X-ray diffraction (XRD) is a multipurpose, nondestructive and investigative
method to study the crystallographic features. The phase purity, unwanted phases,
amorphous or crystalline structure, crystallite size, crystal structure and stresses in the
specimens can be determined using XRD data. The machine works on Bragg‘s law
[95]. As the lattice planes in materials are comparable with the x-rays wavelength so a
diffraction pattern is formed when x-rays interact with the crystalline substance which
is characteristic signature of the crystal structure. PANalytical X‘Pert pro XRD
diffractometer with Cu-Kα x-ray source was used to obtain diffraction patterns.
1.5406Å wavelength (λ) x-rays were utilized and 40kV, 30mA were the operational
parameters.
The Scherrer formula was used to estimate the crystallite sizes,
(3.1)
where, D is the crystallite size, λ; the wave length of the incident x-rays, θ ; the
diffraction angle and β; full width at half maximum (FWHM) expressed in radians.
The Stokes and Wilson‘s formula which incorporated the effect of strains in crystals is
[96]
. (3.2)
The strains were calculated using the relation
. (3.3)
The lattice constant ‗a‘ of the cubic system was calculated using the relation
√ . (3.4)
The formula to calculate the porosity of the samples was,
, (3.5)
where,
, (3.6)
and m = mass, r = radius, h = thickness
23
, (3.7)
where, 4 = formula units for fluorite cubic structure, M is Molecular weight, N is
Avogadro‘s number and V is volume of the unit cell.
Crystal structure of ceria
Cerium oxide (CeO2) is an oxide of cerium also known as ceric oxide, ceria or
cerium dioxide. The cerium oxide (CeO2) has cubic fluorite-type with FCC
arrangement. In such arrangement each cerium site is surrounded by eight oxygen
atoms [97-98].
Figure 3.1 (a) Unit cell of CeO2, light atoms are O2-
and dark atoms are Ce4+
(b)
Crystal structure of ceria
3.1.2 Scanning Electron Microscopy and Transmission Electron Microscopy
For its nondestructive nature SEM is utilized to have high resolution images of
samples to study the surface morphology. Electrons beam and sample interaction
expose information about surface morphology. The samples were studied using SEM
(HITACHI SU-1500). To study the morphology and particle size measurements of the
samples, TEM is one of the most powerful microscopy techniques. It can achieve
atomic resolution of crystals under study. Both spectroscpies rely on the interaction of
electron beam with material and the resulting elastic and inelastic scattering phenomena
provide with the information of the sample under study. The TEM analyze the
transmitted or forward scattered beam. TEM utilized was JEOL JEM 2100F operating
at 200 kV. The samples were dispersed in water and were supported on a 200 mesh,
3mm copper grid.
3.1.3 Differntial Scanning Calorimetry
Different phase transition can be observed using DSC as these transitions occur
due to change in temperatures. The machine works on the observation of differential
24
heat flow to sample and an inert reference material. The rate of increase in temperature
for both the sample and reference is kept constant. The thermal analysis was done on
DSC (TA Q200) in Ar environment at a rate of 10 0C/min. from room temperature to
1000 0C.
3.1.4 Raman Spectroscopy
Raman spectroscopy is an authoritative device to study the phase and structural
properties of materials as it is dependent on the local phonon characteristics. The
interaction of applied frequency with the material excites certain phonon modes which
are signatures of the materials. The Raman studies are done on DongWoo Optron
Raman & PL device with excitation laser of 514 nm [99].
3.1.5 Thermal Conduction Measurements
The thermal conductivity and diffusivity values were obtained by Advantageous
Transient Plane Source (ATPS) by employing modified bridge circuit as shown in
figure 3.2. Three dimensional heat flows inside the sample is studied. Resistive element
is used for this technique (TPS, a hot disk made out of a bifilar spiral) both as the
temperature sensor and the heat source. The TPS sensor was sandwiched between two
similar pellets. The change in the electrical resistance (t) increases with the increase
in time dependent temperature to be expressed by
( ) ( ( )) (3.8)
The thermal diffusivity, thermal conductivity and specific heat are related
through
(3.9)
where = thermal diffusivity, = thermal conductivity and the product in denominator
( ) is the volumetric heat capacity. The power of heat pulse and change in
temperature are related by
( )̅̅ ̅̅ ̅̅ ̅̅
⁄ ( ) (3.10)
Where = power, = thermal conductivity, r and ( ) are design parameters of the
sensor. This equation is an exact solution of hot disk using ring-source solution [100-
101].
25
Figure 3.2 Block diagram of Advantageous Transient Plane Source (ATPS) method
3.2 Conductivity Measurements
3.2.1 AC Conductivity Measurements
Wayne Kerr LCR meter 6440B was used to measure the capacitance and
dissipation factor at different temperatures (300 0C to 700
0C ±5
0C) for the frequency
ranging from I kHz to 3 MHz. The formula used to calculate the dielectric constant (ε′)
was,
. (3.11)
Where, ‗C‘ represent the capacitance, ‗d‘ symbolize the thickness of pellet sample, ‗A‘
denote the area, and ‗εo‘ ; the permittivity of free space (= 8.854 × 10-12
F/m).
The ac conductivity was calculated using the formula
, (3.12)
The error in measurement in capacitance was 0.2% and 0.002 % in dissipation
factor. The ε′, dielectric constant, is the ability of charge storing capacity of the
dielectric material. The dielectric constant at lower frequencies is due to polarization
processes. At higher frequencies the charges in polarization mechanisms become
unable to follow the applied frequency. The high values of dielectric constant at low
frequencies are due to electrode-electrolyte interface. With the increase in temperature
the relaxation peaks shifted toward the higher frequencies. These relaxation and
reorientation peaks are due to the response of ions to applied frequency The dielectric
loss has higher values at lower frequencies due to loss of energy in relaxation and
reorientation of ions whereas at higher frequencies dielectric loss has lower values
because of the higher frequencies, the ions were not able to respond to frequencies. The
mechanisms are shown in figure 3.3.
26
Figure 3.3 Polarization mechanisms for dielectric mediums [102]
Ceria exist in Ce3+
and Ce4+
and redox reaction is the reason of its use as
electrolyte and in catalysis.
( ) (3.13)
The Kroger-Vink notations describe the defects formation equations. In fluorite
structures, anti Frenkel type defects exist. The defect formation is governed by
(3.14)
which mean that an O atom existing on its original site with neutral charge can
be shifted to an interstitial site with two negative charges and two positive charges with
one vacancy will be generated at original site of oxygen. The reaction for ceria is
(3.15)
Similarly, when doping is done defects and vacancies are generated as shown in
figure 3.4. The Kroger-Vink relation for Gd doping in ceria is
(3.16)
The vacancy, Vo.. , can occupy any of the eight equivalent sites (cerium is surrounded
by eight oxygen atoms) around A3+
trivalent cation [62,103].
27
Figure 3.4 Types of defects helpful in ionic transport a) oxide vacancy in perovskite
structure, b) edge dislocation c) defective grain boundaries where space charges pile up
[104]
The ionic conduction is dependent on the defect formation due to thermal
excitations which create vacancies and these help in ionic motion to proceed. Also the
most common explanation of the increase in conductivity as a function of frequency is
the presence of the inhomogeneities which may be micro and/or macro in nature. The
ac conductivity follows a power law at higher frequencies and on lower frequencies
there is steady transition to a frequency independent region (figure 3.5). A dielectric
loss peak is also observed. The numeric value of power is less than 1 (0.5< s <0.8)
[105]. The power law is known as Jonscher power law or universal dielectric response,
( ) ( ) (3.17)
Figure 3.5 Conductivity as a function of frequency
28
The Wheatstone bridge circuit is utilized in Wayne Kerr LCR meter 6440B as
working principal.
3.2.2 DC Conductivity Measurements
The resistivity can be measured with two methods (two and four probe)
depending on the range of resistivity. If the resistivity of the sample is much higher
than the contact resistance, two probe method is engaged and if the resistivity of the
sample is comparable to the resistivity contacts then four probe method is used. The use
of common and independent set of probes for current and voltage is the difference
between these two methods. Wayne Kerr LCR meter 6440B was used to measure
resistance in temperature range 300 0C to 700
0C (±5
0C) with 1 volt applied. The
conductivity was calculated by using
(3.18)
where,
The error in measurement of resistance is 0.1 %. The physical and chemical
processes which depend on the temperature can follow what is known as Arrhenius
relation which describes the rate of change, is proportional to ( ⁄ ) where Ea
is a distinctive energy parameter specific to the process. The ionic hopping is
considered to be such a property. As the ceria based compounds are insulators at room
temperature and become conductive at higher temperatures, ( ⁄ ). The
hopping phenomenon is considered to be happening in rare earth compounds. The jump
relaxation model can be used to explain the ionic conductivity at higher temperature
more easily than only with Arrhenius relation [106]. The model can be understood with
figure 3.6. The following are the suppositions [107];
1) The hopping ions are of same kind.
2) The mobile ions are less than the sites available and sites are also of the same kind.
3) The mobile ions have repulsive interaction which results in a cage effect and ions
remain at a distance with each other.
29
Figure 3.6 Basis of the jump relaxation model, (a) ions (O) on a sublattice, (b) the
effective single particle potential, (c) development of potential after a hop [107].
An ion can hop to neighboring site, at t = 0, due to available thermal agitation.
The hops create a mismatch in terms of cage effect potential. Once this mismatch is
created, this can be reversed in two competing ways; to relax the ion may jump back to
original site or neighboring ions may arrange a hopping motion which produces another
cage effect potential for the ion at new site. In this later way, hop is successful and
contributes to dc conductivity.
3.3 Fluorescence Measurements
Fluorescence is the emission of light within nanoseconds after the striking of
light on the material which is of shorter wavelength than the emitted. The process is
explained with the upconversion phenomenon. The emissions are characteristic of the
transitions occurring in the material under observation.
UC is the process to generate visible light from NIR. It is a nonlinear process.
The process needs absorption of two or more photons so that sufficient energy for the
process is available. The processes which lead to UC include; excited state absorption,
energy transfer and photo avalanche. The excited state absorption process is considered
to be happening in materials with low dopant concentrations. The energy transfer
process comprises of absorption of photons that transfer energy from excited ion
(sensitizer) to another ion (activator) [108].
Fluoromax-4, Horiba Jobin Yvon Spectrofluometer with photon counting
detector was used in this work. The samples, dispersed in water were excited with 980
nm laser at 1W power working at 2.1 A.
30
Chapter 4 Synthesis of Ceria and
Choice of Synthesis Method
31
Synthesis of Ceria and Choice of Synthesis Method
There are many synthesis techniques employed for the enhancement of the
conductivity. Synthesis methods are optimized depending upon their synthesis
conditions to reduce the grain size. Grain size is an important factor for which the
increase of conductivity is observed as the grain size is reduced.
Following are some factors which influence the conductivity [109]
Processing conditions can change the conductivity as the sample density; level
of impurities, size of grains etc can influence the motion of ions which
contributes to conductivity.
The microdomains influence the conductivity of the materials.
Doping of aliovalent cation significantly increase the conductivity.
Multiple doping is another way to enhance the conductivity.
Space charge polarization and segregation of space charges also enhance the
conductivity which can be achieved by reducing the grain size.
In general, the conductivity of material is dependent on the composition,
microstructure and processing of the material. The phase pure composition and
microstructure, themselves are dependent on the synthesis process. Moreover, the
reduction in grain size makes charges to pile up on grain boundaries which in result
increase the conductivity [110]. The grain size and microstructure are dependent on the
synthesis routes therefore the choice of synthesis method plays an important role for the
application of enhancement of conductivity.
It has been reported that the reduction in grain size has increased the conductivity. It
might be due to the enhanced electronic conductivity and decrease in defect formation
energy at grain boundaries. The ceria samples were prepared with range of wet
chemical methods (Composite mediated hydrothermal method, Co-precipitation
method and Sol-gel method). One can hardly find direct comparison in the already
reported literature. In the present study, it is clear from the results that the composite
mediated hydrothermal method is better among the three for the enhancement in
conductivity.
The samples are named as; CMH for Composite mediated hydrothermal
method, Cop. for Co-precipitation method and Sol. for sol-gel method.
32
4.1 Structural Analysis
4.1.1 X-Ray Diffraction
The XRD patterns of CeO2 prepared by three wet chemical routes are shown in
figure 4.1. All the prepared samples are of single phase with cubic crystal structure
(card # 00-043-1002). The lattice constants were calculated by equation 3.4 which
varied between 5.38 Å to 5.41 Å. The crystallite size of samples were calculated using
Scherer formula, as given in equation 3.1.
20 30 40 50 60 70 80
2Theta (degrees)
Inte
ns
ity
(a.u
.)
CMH
Cop.
(331)(400)(222)
(311)(220)
(200)
(111)
SOl.
Figure 4.1 XRD pattern of ceria prepared by different wet-chemical methods
The pattern shown in figure 4.1 confirmed the cubic fluorite structure of ceria.
Table 4.1 is showing the values of crystallite size and lattice constants corresponding to
the most intense peak.
33
Table 4.1 Crystallite size and lattice constant crossponding to the most intense peak
4.1.2 Differential Scanning Calorimetry (DSC)
The DSC plot given in figure 4.2 showed a hump ca. 750 0C and no appreciable
change afterward. The sintering temperature can be 750 0C and above. The sintering at
higher temperature increases the density and the grain size. For uniformity in all
samples, the sintering temperature was chosen to be 750 0C and pellets were made
under similar conditions of pressure and time. The effect of density was same on all
samples.
0 200 400 600 800 1000 1200
-140
-120
-100
-80
-60
-40
-20
0 CeO2
He
at
Flo
w (
mW
)
Temperature (0C)
Crystallization
Figure 4.2 DSC plot of ceria synthesized with CMH method
CeO2 D s (1 1 1)
(nm)
Lattice Constant
Å
Sol. (Sol-gel) 10 5.38(1)
Cop. (co-precipitation) 47 5.41(1)
CMH (Composite mediated
hydrothermal method) 60 5.40(2)
34
4.1.3 Raman Spectroscopy
The oxygen-cerium F2g mode vibration for cubic fluorite showed the single
intense peak at ca. 465 cm-1
. The bands around ca. 570 and 600 cm-1
correspond to
vacancy sites and intrinsic oxygen [58].
400 500 600 700 800
520 540 560 580 600 620
2835
2870
2905
2940
Raman shift (cm-1
)
Raman shift (cm-1
)
Ram
an
in
ten
sit
y (
a.u
)
465
CMH
Cop.
Sol.
Hydro
Sol.
Cop.
600
570C546
Figure 4.3 Raman spectrum of ceria synthesized by different wet-chemical methods
4.2 Electrical Measurements
4.2.1 DC Conductivity
The dc conductivities of ceria samples synthesized by different methods
are shown in figure 4.4 (a-c). All samples show Arrhenius type dependence of
conductivity on temperature. A significant feature of CeO2 to show conductivity at high
temperature is observed whereas it is insulator at room temperature. The values of dc
conductivities at different temperatures are mentioned in table 4.2. It is clear from the
values that the CMH method has maximum value of dc conductivity.
35
200 300 400 500 600 700
0.02
0.04
0.06
0.08
0.10
0.12
0.14
d
c (
Scm
-1)
Temperature (oC)
d
c (
Scm
-1)
Sol
0.00
0.07
0.14
0.21
0.28
0.35
CMH
Figure 4.4 (a) DC conductivity of CeO2 prepared by CMH and sol-gel method
400 450 500 550 600 650 7002.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0 Cop.
dcx10
8(S
-cm
-1)
Temperature (oC)
Fig. 4.4 (b) DC conductivity of CeO2 prepared by co-precipitation method
36
Table 4.2 Comparison of ac conductivity, dc conductivity and dielectric constant for
the ceria samples synthesized by CMH, co-precipitation and sol-gel method.
CMH 400oC 500
oC 700
oC
DC Conductivity σdc
(S-cm-1
)
0.004 0.085 0.338
AC Conductivity σac
(S-cm-1
) at 3MHz
0.063 0.226 2.661
Co-precipitation 400oC 500
oC 700
oC
DC Conductivity σdc
(S-cm-1
)
2.351x10-8
2.672x10-8
3.963x10-8
AC Conductivity σac
(S-cm-1
) at 3MHz
0.247 0.472 2.344
Sol-gel 400oC 500
oC 700
oC
DC Conductivity σdc
(S-cm-1
)
0.010 0.011 0.095
AC Conductivity σac
(S- cm-1
) at 3MHz
0.019 0.316 2.511
4.2.2 AC Conductivity
The ac conductivities of ceria samples are shown in figure 4.5 (a-c). The values
of ac conductivities at 3MHz are mentioned in table 4.2 also.
The jump relaxation model can be applied to explain the behavior. At lower
frequencies the ac conductivity is due to the hopping of ion from one available site to
other. At higher frequencies dispersion occur due to hopping and relaxation of ions.
Dispersion region shifted towards the higher frequency region with the increase in
temperature. The step in dispersion is the confirmation of grain boundaries and grains
interior conduction. The confirmation of ionic hopping in addition to Arrhenius relation
is given by the Jonscher‘s law [107, 111].
37
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
700oC
600oC
500oC
400oC
300oC
log
ac (
Scm
-1)
log f (Hz)
Figure 4.5 (a) AC conductivity of ceria synthesized by CMH method
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-2.0
-1.5
-1.0
-0.5
0.0
0.5
log f (Hz)
log
ac (
S-c
m-1
)
700 0C
600 0C
500 0C
400 0C
300 0C
Figure 4.5 (b) AC conductivity of ceria synthesized by co-precipitation method
38
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
700oC
600oC
500oC
400oC
300oC
log f (Hz)
log
ac (
Scm
-1)
Figure 4.5 (c) AC conductivity of ceria synthesized by sol-gel method
4.2.3 Dielectric Constant
The dielectric constant values of all three samples are given in figure
4.6. The sample prepared by composite mediated hydrothermal method showed higher
values of the dielectric constant. At lower frequencies the dielectric constant values are
much higher but those decreased with increase in frequency. Because, at higher
frequencies, the dipoles were not able to follow the applied field. The ‗universal‘
dielectric response is evident from the plots. The Jonscher‘s power law is applicable to
such materials which do not show loss peaks [111]. The high values of dielectric
constant at lower frequencies are the manifestation of the electrode-electrolyte interface
[112]. The shift of peaks toward higher frequencies is evident from plots.
39
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
log
Die
lec
tric
co
ns
tan
t ('
) 700
0C
600 0C
500 0C
400 0C
300 0C
log f (Hz)
Figure 4.6 (a) Dielectric constant of ceria synthesized with CMH method
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
1
2
3
4
5
700 0C
600 0C
500 0C
400 0C
300 0C
log
Die
lectr
ic c
on
sta
nt ('
)
log f (Hz)
Figure 4.6 (b) Dielectric constant of ceria synthesized with co-precipitation method
40
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
1.2
1.4
1.6
1.8
2.0
2.2
2.4
log
Die
lec
tric
co
ns
tan
t ('
) 700
oC
600oC
500oC
400oC
300oC
log f (Hz)
Figure 4.6 (c) Dielectric constant of ceria synthesized with sol-gel method
4.3 Conclusions
Successful synthesis of phase pure ceria was done by composite mediated
hydrothermal method, co-precipitation method and sol-gel method. The dc
conductivities of the prepared samples were found to be increasing with the increase in
temperature and the values as high as 0.345 S-cm-1
and 0.095 S-cm-1
(at 700 0C) were
achieved for CMH and sol-gel samples, respectively. The samples prepared by CMH
method showed better values of ac and dc conductivities. Also, the higher values of
dielectric loss tangent and dielectric constant were observed for same samples. These
properties made this method suitable to prepare materials which can be utilized as the
electrolyte material in SOFCs. The presence of oxygen vacancies and cerium oxide was
also confirmed through Raman spectrum. The intensity in Raman spectrum for CMH
sample was observed very high as compared with samples prepared by co-precipitation
and sol-gel methods. Hence, to obtain higher values of conductivity in ceria, CMH
method is better synthesis approach.
41
Chapter 5 Effect of Synthesis
Parameters on Ceria
Synthesized by Composite
Mediated Hydrothermal
Method
42
Effect of Synthesis Parameters on Ceria Synthesized by
Composite Mediated Hydrothermal Method
Composite mediated hydrothermal (CMH) method was used to synthesize
cerium oxide nanoparticles. The enhancement in conduction was the focus in
optimization of synthesis conditions in addition to smaller crystallites. Hydrothermal
treatment temperature (180 oC and 220
oC) and hydrothermal treatment time (45 min,
70 min and 90 min) were optimized. The calcination and sintering temperature were
the same (500 0C for calcination and 750
0C for sintering). Structural properties of ceria
were observed through X-Ray diffraction (XRD) data. DC conductivity was observed
to be increasing with the increase in temperature and was done in temperature range
200 0C to 700
0C. Frequency dependent dielectric properties and ac conductivity were
observed at different temperatures in frequency range from 20Hz to 3MHz. 514 nm
excitation laser was used to observe the Raman spectrum of samples. The designed
nomenclature for the samples is given in table 5.1.
Table 5.1The nomenclature designed for the samples at different optimized parameters.
5.1 Structural Analysis
5.1.1 X-Ray Diffraction
Figure 5.1 showed the indexed XRD patterns of cerium oxide samples. All the
prepared samples were single phase with cubic structure. The value of lattice constant
varies between 5.40 Å to 5.43 Å. The Scherrer formula given in equation 3.1 was
utilized to calculate crystallite sizes.
The lattice constants, average crystallite sizes and crystallite size corresponding
to the most intense peak are mentioned in table 5.2.
43
Figure 5.1 XRD patterns of CeO2 samples synthesized by different synthesis conditions
Table 5.2 The average crystallite size and that of the most intense peaks (D (1 1 1)) along
with lattice constants (a) for the samples.
44
5.2 Electrical Properties
5.2.1 DC Conductivity
Temperature dependent resistance was measured from 200 0C to 700
0C for the
prepared samples. Equation 3.18 was used to calculate the resistivity. Figure 5.2 is
showing the dc conductivity (ζdc S-cm-1
) behavior for the samples as a function of
temperature from 200 0C to 700
0C. The conductivity increased with the increase in
temperature but the increase is much sharp at higher temperatures (5000C – 700
0C) than
lower temperatures.
Figure 5.2 Temperature dependent dc conductivity of the prepared samples
The comparison of values of dc conductivity (at 300 0C, 500
0C and 700
0C)
given in table 5.3. Maximum dc conductivity is achieved for the sample synthesized at
180 0C (H14) and 220
0C (H24) for 45 minutes using Arrhenius relation [106]. Jump
relaxation model can be used to explain the behavior [107, 111].
45
Table 5.3 DC conductivity (ζdc S-cm-1
) at different temperatures of the samples
5.2.2 AC Conductivity
By using relation 3.12, the frequency dependent ac conductivity at different
temperatures (from 200 0C to 700
0C) was measured and the plots are given in figure
5.3.
Hopping and relaxation of ions led to dispersion for higher frequencies. The ion
can relax in two ways after jumping from one site to other. The neighboring ions may
rearrange in the sequence of their hopping motion or it may jump back to its original
site. Frequency dispersion region in the conductivity spectra can be observed due to the
relaxation of ions.
The dispersion region is found to shift to the higher frequency region with
increase in temperature. The presence of both the grain boundary and the grain interior
conductivities is confirmed with the ‗step‘ in dispersion region [21]. Table 5.4 shows
the values of ac conductivities at frequency of 3 MHz for different temperatures.
46
Figure 5.3 AC conductivity (ζac S-cm-1
) as a function of frequency at different
temperatures for all samples (H14, H17, H19, H24, H27 and H29).
47
Table 5.4 Frequency dependent ac conductivity (ζac S-cm-1
) at different temperatures
Figure 5.4 is showing the comparison of values of ac conductivities at different
temperatures on 3 MHz frequency. The behavior is dependent on temperature as well as
on the synthesis method. The plot shown here is actual data beyond error bars. The
possible sources of error in ac measurements are mentioned in section 3.2.1. The value
of ac conductivity increased with increase in temperature. The samples H14 and H24
have maximum conductivity. AC conductivity is found to be higher for the samples
with narrow range of crystallite size at all the temperatures [113].
.
Figure 5.4 Comparison of ac conductivity (ζac S-cm-1
) at 3 MHz for all samples at
500 0C, 600
0C and 700
0C.
48
5.2.3 Dielectric Constant
Figure 5.5 is showing the trend of dielectric constant at different temperatures
as a function of frequency from 20 Hz to 3 MHz.
Figure 5.5 Dielectric constant (ε΄) as a function of frequency at different temperatures
for all the samples (H14, H17, H19, H24, H27 and H29).
49
The plots are showing that the values of dielectric constant increased with the
increase in temperature. Dielectric constant decreased sharply with the increase in
frequency which is due to the frequency increase and the dipoles did not follow the
applied field. Tables 5.5 show the comparison of values for the dielectric constant at
different temperatures at frequency of 3 MHz.
Figure 5.6 is showing the comparison of the values for the dielectric constant
for the samples synthesized with different synthesis conditions, which show that the
value of dielectric constant increased as temperature was achieved.
Figure 5.6 Dielectric constant at 3 MHz for all the samples at different temperatures
(500 0C, 600
0C and 700
0C).
5.2.4 Dielectric Loss
Dielectric loss plots are shown in figure 5.7 on different temperatures. When a
material acts like a dielectric medium between two conductor plates these losses occur
due to the polarization effects.
In lower frequency region, space charge polarization effects occur and in higher
frequency region ionic polarization effects appear [102]. The polarization peak shifted
to higher frequency with the increase in temperature which is very clear for higher
temperatures.
50
Figure 5.7 Dielectric loss (tanδ) as a function of frequency at different temperatures for
all samples.
At the higher temperatures the losses due to polarization mechanisms are very
much sharp. Table 5.5 is showing the comparison values of dielectric loss values at
different temperatures (500 0C, 600
0C and 700
0C) for the frequency 1 kHz and
dielectric constant at 3 MHz frequency.
51
Table 5.5 Frequency dependent dielectric constant (ε΄) and dielectric loss tangent (tanδ)
values at different temperatures
5.3 Raman Spectroscopy
Raman spectrum is observed at 514nm excitation laser and is given in figure
5.8. High intensity 465 cm-1
band of ceria is observed for all the samples. The
magnified view is shown in inset plot. 546 cm-1
band could be assigned to the oxygen
vacancies introduced into the ceria. The intrinsic oxygen vacancies are related to
570cm-1
and 600 cm
-1 bands [58].
400 500 600 700 800 900 1000
540 560 580 600 620
1500
1600
1700
1800
Ra
ma
n in
ten
sit
y (
a.u
)
Raman shift (cm-1
)
H29
H27
H24
H19
H17
H14
465
570
600
546
Figure 5.8 Raman spectra of prepared ceria samples at 514 nm excitation laser line
52
5.4 Conclusions
Optimization of different synthesis parameters of composite mediated
hydrothermal method was done to obtain phase pure nanocrystalline ceria. The crystal
structure, crystallite size, phase purity, and lattice constants were determined using
XRD data. 0.3386 S-cm-1
was the maximum value of electrical dc conductivity of the
samples at 700 0C. For sample H14 the maximum ac conductivity was found with a
value of 2.661 S-cm-1
at 700 0C. Strong ceria band was observed at 465 cm
-1 through
Raman spectrum which confirmed the presence of ceria. At 546 cm-1
, 570 cm-1
and 600
cm-1
week bands of oxygen vacancies are present. The material can be a strong
candidate for intermediate temperature range SOFCs.
53
Chapter 6 Effect of Gd Doping
on Conductivity of Ceria
54
Effect of Gd Doping on Conductivity of Ceria
The effect of Gd doping in ceria prepared by composite mediated hydrothermal
method is studied here. In general, the conductivity of material is dependent on the
composition, microstructure and processing of the material. The phase pure
composition and microstructure, themselves are dependent on the synthesis process. As
discussed in chapter 4, the composite mediated hydrothermal method is found to be
better in achieving the enhancement of the conductivity; hence this method is adopted
to prepare rare earth oxides (Gd doped ceria). In addition to synthesis process the
addition of doping produces oxygen vacancies which also help to enhance the
conductivity. Four nominal compositions were made with varying contents of Gd in
CeO2 i.e. Ce1-xGdxOδ (x= 0.10, 0.15, 0.20, 0.25) and corresponding samples were
named as CG10 (x= 0.10, Ce0.9Gd0.1Oδ), CG15 (x= 0.15, Ce0.85Gd0.15Oδ), CG20 (x=
0.20, Ce0.8Gd0.2Oδ) and CG25 (x= 0.25, Ce0.75Gd0.25Oδ).
6.1 Structural and morphological studies
6.1.1 X-Ray Diffraction
The XRD patterns shown in figure 6.1 of prepared samples, showed the
formation of cubic structure. All peaks were indexed with cubic fluorite structure
(space group 225 and card # 01-075-0161). All the compositions are phase pure. The
crystallite sizes were in range of 14 nm to 88 nm by using Scherrer formula given in
equation 3.1 and in range of 19 to 159 nm using Stokes and Wilson‘s formula given in
equation 3.2. The crystallite size obtained using both formulas; lattice constant and
porosity are given in table 6.1.
55
20 30 40 50 60 70 80
CG25
CG20
(420)(331)(400)(222)
(311)(220)
(200)
(111)
Inte
ns
ity
(a
.u.)
2degrees
CG10
CG15
Figure 6.1 X-ray diffraction patterns of Ce1-xGdxOδ (x= 0.10- 0.25)
Table 6.1 The crystallite sizes (Ds (111) = Crystallite size corresponding to the most
intense peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding
to the most intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average
crystallite size estimated by Scherrer formula, Dw = Average crystallite size estimated
by Stokes and Wilson‘s formula), lattice constant and porosity of Ce1-xGdxOδ (x= 0.10-
0.25)
6.1.2 Scanning Electron Microscopy
The SEM micrographs are shown in figure 6.2. The sample CG10 has
needles-like structures and the other three samples have agglomerated particles. No
conclusive information can be obtained for sizes of particles, due to magnification
limitation.
Sample D s (1 1 1)
(nm)
D w (1 1 1)
(nm)
D s
Average
(nm)
D w
Average
(nm)
Lattice
Constant
Å
Porosity
CG10 66 88 34 76 5.40(2) 0.62
CG15 30 40 24 44 5.38(3) 0.64
CG20 83 111 32 55 5.40(2) 0.62
CG25 83 111 41 76 5.40(3) 0.62
56
Figure 6.2 Scanning electron micrographs of Ce1-xGdxOδ (x= 0.10 - 0.25)
6.1.3 Raman Spectroscopy
The Raman spectra of Ce1-xGdxOδ are given in figure 6.3. The spectra show the
single intense peak corresponding to the oxygen-cerium F2g mode vibration for cubic
fluorite structure at ca. 465 cm-1
. The doping of Gd in ceria is confirmed from the
absence of Raman bands at ca. 480 cm-1
which is vibrational mode of cubic Gd2O3.
Also, the increase in dopant concentration of the Gd increased the peak bandwidth
which is due to the intermixing of bands. The increase in conductivity can be attributed
to more vacancy sites generation with increase in Gd concentration in ceria [114-115].
CG10 CG15
CG20 CG25
57
400 500 600 700
CG25CG20CG10
Sample Peak Width
(cm-1
)
CG10 16.27
CG15 14.35
CG20 18.26
CG25 19.68
CG10
CG15
CG20
CG25
Raman Shift (cm-1)
Ra
ma
n I
nte
ns
ity
(a
.u.)
465
CG15
Figure 6.3 Raman spectroscopy of Ce1-xGdxOδ (x= 0.10 - 0.25)
6.2 Electrical Measurements
6.2.1 DC Conductivity
The dc conductivity of prepared samples is shown in figure 6.4. The
activation energies (section 3.2.2) calculated are given in table 6.2. The Ceria and Gd
doped Ceria are very high resistive at room temperature. At higher temperature these
become conductive as the oxygen ion conductivity is dependent on temperature. The
values of conductivities of prepared samples at different temperatures are mentioned in
table 6.2. The sample CG25 has the highest value of conductivity. The increase in
conductivity might be credited to the increase in vacancy sites due to the increase in Gd
contents and the microstructure distribution due to the synthesis process [104, 107, 114,
116].
58
300 400 500 600 700
0.000
0.002
0.004
0.006
0.008
d
c (S
-cm
-1)
CG10
CG15
CG20
CG25
Temperature (oC)
Figure 6.4 DC conductivity of Ce1-xGdxOδ (x= 0.10-0.25) as a function of temperature.
Table 6.2 Activation energies ‗s‘ and conductivities of Ce1-xGdxOδ (x= 0.10 - 0.25) at
different temperatures
Sample
Activation
Energy
(eV)
‘s’ DC Conductivity (S-cm-1
) at
300-700 °C 300
oC 500
°C 550
°C 600
°C 650
°C 700
°C
CG10 0.89(1) 0.60 1.14×10-4
4.02×10-4
6.74×10-4
13.3×10-4
24 ×10-4
CG15 0.87(1) 0.49
4.10×10-5
1.42×10-4
2.10×10-4
2.76×10-4
4.5×10-4
CG20 1.19(2) 0.60
2.89×10-5
1.20×10-4
1.57×10-4
4.25×10-4
6.52×10
-
4
CG25 1.13(2) 0.59 1.95×10-4
4.78×10-4
9.30×10-4
25.8 ×10-4
74 ×10-4
6.2.2 AC Conductivity
The frequency spectra of Ce1-xGdxOδ (x= 0.10, 0.15, 0.20, 0.25) at
different temperatures are given in figure 6.5 (a-d). The jump relaxation model
explained the behavior of conduction mechanism [107]. At lower frequencies dc
conductivity occurs due to the jumping of ion from one vacant site to the other. At
higher frequencies dispersion occur due to hopping and relaxation of ions. The
59
dispersion region shifted toward the high frequency region with the increase in
temperature. The step in dispersion is the confirmation of grains boundaries and interior
conduction. The confirmation of ionic hopping in addition to Arrhenius relation is
given by the Jonscher‘s law [105-106, 111]. The value of exponent ‗s‘ in equation 3.17
is determined for ceria and expressed in table 6.2. The ac conductivities as a function of
frequency at different temperatures are summarized in table 6.3.
Table 6.3 AC conductivity at 3 MHz frequency for different temperatures of
Ce1-xGdxOδ (x= 0.10 - 0.25) samples
Sample name
AC Conductivity (S-cm-1
)
at 3 MHz
500 °C 600
°C 700
°C
CG10 0.0025 0.0023 0.0081
CG15 0.0014 0.0017 0.0023
CG20 0.0012 0.0023 0.0037
CG25 0.0025 0.015 0.189
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
700 0C
600 0C
500 0C
400 0C
300 0C
log
ac (
S-c
m-1
)
log f (Hz)
Figure 6.5 (a) AC conductivity of CG10 at different temperatures
60
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-14
-13
-12
-11
-10
-9
-8
-7
-6
700 0C
600 0C
500 0C
400 0C
300 0C
log
ac (
S-c
m-1
)
log f (Hz)
Figure 6.5 (b) AC conductivity of CG15 at different temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-14
-13
-12
-11
-10
-9
-8
-7
-6
700 0C
600 0C
500 0C
400 0C
300 0C
log
ac (
S-c
m-1
)
log f (Hz)
Figure 6.5 (c) AC conductivity of CG20 at different temperatures
61
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-12
-10
-8
-6
-4
-2
700 0C
600 0C
500 0C
400 0C
300 0C
log
ac (
S-c
m-1
)
log f (Hz)
Figure 6.5 (d) AC conductivity of CG25 at different temperatures
6.2.3 Dielectric Constant
The dielectric constants as a function of frequency are shown in figure
6.6 (a-d). The ‗universal‘ dielectric response is evident from the plots. The Jonscher‘s
power law is applicable to such materials which do not show loss peaks. The high
values of dielectric constant at lower frequencies are the manifestation of the electrode
-electrolyte interface [117-118]. The shift of peaks toward higher frequencies is
evident from plots. Values of dielectric constant at different temperatures and lowest
and highest frequency applied are given in table 6.4.
Table 6.4 Values of dielectric constant at 1 kHz and 3 MHz for different temperatures
of Ce1-xGdxOδ (x= 0.10 - 0.25) samples.
Sample
name
Dielectric constant (′)
at 3 MHz at 1 kHz
500 °C 600
°C 700
°C 500
°C 600
°C 700
°C
CG10 25.83 25.53 34.45 315.21 857.62 4077.75
CG15 20.22 24.05 26.06 202.21 375.91 794.14
CG20 26.62 32.45 38.85 200.46 398.12 1229.72
CG25 27.33 39.98 24.70 758.53 7033.89 108676.66
62
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.52
3
4
5
6
7
8
9
700 0C
600 0C
500 0C
400 0C
300 0C
log
Die
lec
tric
co
ns
tan
t ('
)
log f (Hz)
Figure 6.6 (a) Dielectric constant of CG10 at different temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5 700 0C
600 0C
500 0C
400 0C
300 0C
lo
g D
iele
ctr
ic c
on
sta
nt ('
)
log f (Hz)
Figure 6.6 (b) Dielectric constant of CG15 at different temperatures
63
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.52
3
4
5
6
7
8 700
0C
600 0C
500 0C
400 0C
300 0C
lo
g D
iele
ctr
ic c
on
sta
nt ('
)
log f (Hz)
Figure 6.6 (c) Dielectric constant of CG20 at different temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
3
4
5
6
7 700 0C
600 0C
500 0C
400 0C
300 0C
log
Die
lec
tric
co
ns
tan
t (')
log f (Hz)
Figure 6.6 (d) Dielectric constant of CG25 at different temperatures
64
6.2.4 Dielectric Loss (tanδ)
The frequency dependent tanδ plots are given in figure 6.7 (a-d) for
Ce1-xGdxOδ (x= 0.10, 0.15, 0.20, 0.25). At lower frequencies tanδ has larger values and
at higher frequencies the value decrease and became independent of frequency due to
the fact that the dipoles were not able to respond and reorient themselves with applied
frequency. Also, the relaxation and reorientation peaks are shifting towards higher
frequencies with increase in temperature. Values of tanδ for different temperatures and
applied frequency are shown in table 6.5.
Table 6.5 Variation in tanδ as a function of frequency at different temperatures for
Ce1-xGdxOδ (x= 0.10 - 0.25) samples
Sample
name
Dielectric loss tangent (tanδ)
at 3 MHz at 1 kHz
500 °C 600
°C 700
°C 500
°C 600
°C 700
°C
CG10 0.57 0.54 1.41 16.94 30.59 22.29
CG15 0.40 0.42 0.52 9.65 12.41 15.82
CG20 0.26 0.41 0.57 11.68 13.73 20.63
CG25 0.54 2.26 3.45 22.14 28.72 38.72
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
4
8
12
16
20
24
28
32 700 0C
600 0C
500 0C
400 0C
300 0C
tan
log f (Hz)
Figure 6.7 (a) Dielectric loss (tanδ) of CG10 at different temperatures
65
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
2
4
6
8
10
12
14
16
18
700 0C
600 0C
500 0C
400 0C
300 0C
ta
n
log f (Hz)
Figure 6.7(b) Dielectric loss (tanδ) of CG15 at different temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
4
8
12
16
20 700
0C
600 0C
500 0C
400 0C
300 0C
ta
n
log f (Hz)
Figure 6.7(c) Dielectric loss (tanδ) of CG20 at different temperatures
66
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
5
10
15
20
25
30
35
40
45
700 0C
600 0C
500 0C
400 0C
300 0C
ta
n
log f (Hz)
Figure 6.7(d) Dielectric loss (tanδ) of CG25 at different temperatures
6.3 Thermal Conduction
The room temperature thermal conductivity and thermal diffusivity values are
measured using ATPS method. This method provides the simultaneous measurements
of the conductivity and diffusivity [100, 101]. The values obtained are given in table
6.6. The samples showed the values in lower conductivity range. These low thermal
conductivity samples can be utilized for the thermal barrier coatings. The conduction of
heat inside solids is aided through the scattering of phonons. The decrease in
conductivity with increase of Gd contents might be due to the increase of vacancies
which slowed down the phonons and the mean free path reduced hence the conductivity
is reduced [119-120].
Table 6.6 Values of thermal conductivity and thermal diffusivity for Ce1-xGdxOδ
(x= 0.10 - 0.25) at room temperature
Sample Thermal Conductivity
(Wm-1
K-1
)
Thermal Diffusivity
(mm2s
-1)
CG10 0.42 0.64
CG15 0.15 0.91
CG20 0.39 0.71
CG25 0.40 0.79
67
6.4 Conclusions
Nanocrystalline Gadolinium doped Ceria, Ce1-xGdxOδ (x = 0.10, 0.15, 0.20,
0.25), were successfully synthesized with facile composite mediated hydrothermal
method. The X-ray diffraction confirmed the phase and composition of the synthesized
material. The crystallite sizes were estimated using Scherrer and Stokes and Wilson‘s
formulae. The range of crystallite size was 30-83 nm corresponding to the most intense
peak using Scherrer‘s formula. The x-ray diffraction data was used to calculate lattice
constant and porosity. The ranges obtained for lattice constant and porosity were
5.38Å-5.40Å and 0.62-0.64 respectively. The scanning electron microscopy was used
to get morphology of the materials. Ce0.9Gd0.1Oδ has needles like structures whereas
other three samples have agglomerated particles. DC conductivity was measured in
temperature range 300-700 °C and ac conductivity was determined in frequency range 1
kHz to 3MHz at temperatures 300, 400, 500, 600 and 700 °C. The larger values of
conductivities were obtained for Ce0.9Gd0.1Oδ and Ce0.75Gd0.25Oδ as well as for increase
in temperature. Arrhenius plots were used to calculate activation energies, obtained in
the range 0.87-1.19 eV for 300-700 °C. The ‗universal‘ dielectric response with
Jonscher power law and jump relaxation model explained the conduction phenomena in
the synthesized material. The maximum conductivity achieved for Ce0.75Gd0.25Oδ to be
9.30x10-4
S-cm-1
at 600 °C. The thermal conductivity values for these samples lie in low
thermal conductivity region and can be utilized for thermal barrier coatings. The
Raman spectroscopy seconded the structural, electrical and thermal results.
68
Chapter 7 Conductivity Enhancement
in co-Doped Rare-Earth Oxides
69
Conductivity enhancement in co-doped rare-earth oxides
The maximum conductivity achieved in samples with Gd doping, were further
co-doped with La and Nd. With the doping of La, Gd and Gd, Nd in ceria the nominal
compositions were Ce1-2xGdxLaxOδ (x= 0.10, 0.25) and Ce1-2xGdx NdxOδ (x= 0.10,
0.25). The samples were named as CGL10 (Ce0.8Gd0.1La0.1Oδ), CGN10
(Ce0.8Gd0.1Nd0.1Oδ) and CGL25 (Ce0.5Gd0.25La0.25Oδ), CGN25 (Ce0.5Gd0.25Nd0.25Oδ).
7.1 Structural and morphological studies
7.1.1 X-Ray Diffraction
The XRD patterns given in figure 7.1 of prepared samples showed the
formation of cubic fluorite structure (space group 225). The starred peaks are of Nd2O3
(*) and La2O3 (#).
20 30 40 50 60 70 80
2Degrees)
CGL10
Inte
ns
ity
(a
.u.)
CGL25
#
CGN10
CGN25
(111)
(2 0 0)(2 2 0)
(3 11)(3 31)
(4 2 0)
Figure 7.1 X-ray diffraction patterns of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x =
0.10, 0.25), the starred peaks are of Nd2O3 (*) and La2O3 (#).
70
The crystallite sizes were in the range of 12 to 70 nm using Scherrer formula
and in range of 22 to 92 nm using Stokes and Wilson‘s formula. The crystallite size
using both formulas and lattice constant are given in table 7.1.
Table 7.1 The crystallite sizes (Ds (111) = Crystallite size corresponding most intense
peak estimated using Scherrer formula, Dw (111) = Crystallite size, corresponding most
intense peak, estimated using Stokes & Wilson‘s formula, Ds = Average crystallite size
estimated by Scherrer formula, Dw = Average crystallite size estimated by Stokes and
Wilson‘s formula), lattice constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10,
0.25).
7.1.2 Raman Spectroscopy
The Raman spectra of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ are given in figure
7.2. The oxygen-cerium F2g mode vibration for cubic fluorite showed the single intense
peak at ca. 465 cm-1
. The bands ca. 570 and 600 cm-1
correspond to vacancy sites and
intrinsic oxygen in samples, respectively. Also, the increase in dopant concentration
increased the peak bandwidth which is due to the intermixing of bands. The increase in
conductivity can be attributed to more vacancy sites generation with increase in dopants
concentration in Ceria [58, 114-115, 121].
Sample D s (1 1 1)
(nm)
D w (1 1 1)
(nm)
D s
Average
(nm)
D w
Average
(nm)
Lattice
Constant
Å
CGL10 57 73 36 55 5.74(2)
CGL25 42 54 39 64 5.91(1)
CGN10 43 56 31 53 5.45(2)
CGN25 49 64 38 58 5.46(2)
71
400 500 600 700
CGN10
CGL15
CGN15
Sample Peak width
CGL10 15.83
CGL25 14.19
CGN10 26.91
CGN25 31.35
CGL10
CGL25
CGN10
CGN25
600570
Raman Shift (cm-1)
465
Ra
ma
n I
nte
ns
ity
(a
.u.)
CGL10
Figure 7.2 Raman spectroscopy of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x=0.10,0.25)
7.2 Electrical Measurements
7.2.1 DC Conductivity
The dc conductivity of prepared samples is shown in figure 7.3. At higher
temperatures ceria and doped ceria become conductive and the oxygen ion conductivity
is dependent on temperature although these are very high resistive at room temperature.
The values of conductivities of prepared samples at different temperatures are given in
table 7.2.
Figure 7.3 DC conductivity of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25) as
function of temperature.
300 400 500 600 700
0.00
0.02
0.04
0.06
0.08
0.10
0.12
d
c (S
-cm
-1)
Temperature (oC)
CGL10
CGL25
300 400 500 600 700
0
1
2
3
4
dc (S
-cm
-1)
Temperature (oC)
CGN10
CGN25
0.00
0.02
0.04
0.06
0.08
0.10
0.12
72
Table 7.2 DC conductivities of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.10, 0.25)
at different temperatures
Sample DC Conductivity dc (S-cm
-1) at
500 0C 550
0C 600
0C 650
0C 700
0C
CGL10 0.0082 0.0177 0.0318 0.0611 0.1120
CGL25 0.0034 0.0074 0.0159 0.0317 0.0610
CGN10 0.0066 0.0141 0.0298 0.0602 0.1051
CGN25 0.3490 0.8830 1.7830 2.9990 4.4591
7.2.2 AC Conductivity
The frequency spectra for ac conductivity of Ce1-2xGdx LaxOδ and Ce1-2xGdx
NdxOδ (x = 0.10, 0.25) at different temperatures (3000C – 700
0C) are given in figure 7.4
(a-d). The conductivity increased with the increase in frequency at all temperatures.
This behavior is manifestation of phenomenon explained by jump relaxation model.
Hoping of ions from one vacant state to other is responsible for the successful dc
contribution towards total conductivity at lower frequencies. Jumping and relaxation of
ions led to dispersion at higher frequencies. There are two ways in which these ions can
relax, either to go back to its original site or may reorder their jumping motion
[105,107]. There appeared a frequency dispersion region in the conductivity spectra
due to the relaxation of ions associated with the conduction. The dispersion region
present in conductivity spectrum, associated with relaxation of ions shifted to the
higher frequency region on increase of temperature. The contribution of grain interior
and grain boundary conduction can be confirmed from the presence of step in the
dispersion region [111-112, 117-118, 122-123].
73
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
700oC
600oC
500oC
400oC
300oC
log
a
c (
S-c
m-1)
log f (Hz)
Figure 7.4(a) AC conductivity of Ce0.8Gd0.1 La0.1Oδ (CGL10) at different temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-3.6
-3.2
-2.8
-2.4
-2.0
-1.6
-1.2
-0.8
-0.4
700oC
600oC
500oC
400oC
300oC
log
a
c (
S-c
m-1)
log f (Hz)
Figure 7.4(b) AC conductivity of Ce0.5Gd0.25 La0.25Oδ (CGL25) at different
temperatures
74
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
-3.2
-2.8
-2.4
-2.0
-1.6
-1.2
-0.8
700oC
600oC
500oC
400oC
300oC
log
ac (
S-c
m-1)
log f (Hz)
Figure 7.4(c) AC conductivity of Ce0.8Gd0.1 Nd0.1Oδ (CGN10) at different temperatures
700oC
600oC
500oC
400oC
300oC
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
log
a
c (
S-c
m-1)
log f (Hz)
Figure 7.4(d) AC conductivity of Ce0.5Gd0.25 Nd0.25Oδ (CGN25) at different
temperatures
7.2.3 Dielectric Constant
The dielectric constants as a function of frequency are shown in figure 7.5 (a -
d). The behavior of plots is explained by the ‗universal‘ dielectric response. The
absence of loss peaks in such materials can be explained through Jonscher‘s power law
[105, 124]. Higher values of dielectric constant at lower frequencies are due to the
polarization of charge carriers at the electrode-electrolyte interface. With the increase in
75
temperature, the polarization increases due to the enhancement of mobility of charge
carriers [112, 117-118]. Values of dielectric constant at different temperatures and
lowest and highest frequency applied are given in table 7.3.
Table 7.3 Values of dielectric constant of Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ
(x = 0.10, 0.25) at 500, 600 and 700 0C for 1 kHz and 3 MHz
Sample
name
Dielectric constant (′)
at 3 MHz at 1 kHz
500 °C 600
°C 700
°C 500
°C 600
°C 700
°C
CGL10 10.95 13.26 16.61 71.60 117.31 346.71
CGL25 12.99 15.39 18.34 89.46 368.29 1082.92
CGN10 15.86 17.84 21.26 88.23 190.41 398.81
CGN25 30.81 35.63 21.49 10969.08 58910.74 198212.12
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4 700oC
600oC
500oC
400oC
300oC
log
Die
lec
tric
co
ns
tan
t (')
log f (Hz)
Figure 7.5(a) Dielectric constant of Ce0.8Gd0.1 La0.1Oδ (CGL10) at different
temperatures
76
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51
2
3
4
5
6
700oC
600oC
500oC
400oC
300oC
log
Die
lec
tric
co
ns
tan
t (')
log f (Hz)
Figure 7.5(b) Dielectric constant of Ce0.5Gd0.25 La0.25Oδ (CGL25) at different
temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6 700
oC
600oC
500oC
400oC
300oC
log
Die
lec
tric
co
ns
tan
t (')
log f (Hz)
Figure 7.5(c) Dielectric constant of Ce0.8Gd0.1 Nd0.1Oδ (CGN10) at different
temperatures
77
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.51
2
3
4
5
6
700oC
600oC
500oC
400oC
300oC
log
Die
lec
tric
co
ns
tan
t (')
log f (Hz)
Figure 7.5(d) Dielectric constant of Ce0.5Gd0.25 Nd0.25Oδ (CGN25) at different
temperatures
7.2.4 Dielectric Loss
The frequency dependent dielectric loss (tanδ) plots are given in figure
7.6 (a-d) for Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ (x = 0.1, 0.25). At lower frequencies
tanδ has larger values and at higher frequencies the value decreased and became
independent of frequency. For Ce1-2xGdx NdxOδ the peak shift is clearly showing the
polarization phenomenon even at higher frequencies. Also, Vo.. can occupy any of the
eight equivalent sites around A3+
can jump from one site to the other site giving rise to
reorientation and relaxation process. With increase in temperature, the peaks shift
towards the higher frequencies. The effect is widely observed in ceria based
compounds by various researchers [56, 112, 117-118].
78
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
20
40
60
80
100
120 700oC
600oC
500oC
400oC
300oC
tan
log f (Hz)
Figure 7.6(a) Dielectric loss of Ce0.8Gd0.1 La0.1Oδ (CGL10) at different temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
5
10
15
20
25
30
700oC
600oC
500oC
400oC
300oC
tan
log f (Hz)
Figure 7.6(b) Dielectric loss of Ce0.5Gd0.25 La0.25Oδ (CGL25) at different temperatures
79
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
10
20
30
40
50
60 700
oC
600oC
500oC
400oC
300oC
tan
log f (Hz)
Figure 7.6(c) Dielectric loss of Ce0.8Gd0.1 Nd0.1Oδ (CGN10) at different temperatures
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0
20
40
60
80
100 700
oC
600oC
500oC
400oC
300oC
tan
log f (Hz)
Figure 7.6(d) Dielectric loss of Ce0.5Gd0.25 Nd0.25Oδ (CGN25) at different temperatures
80
7.3 Conclusions
Nanocrystalline Gadolinium doped Ceria, Ce1-2xGdx LaxOδ and Ce1-2xGdx NdxOδ
(x = 0.10, 0.25), were successfully synthesized with facile composite mediated
hydrothermal method. The X-ray diffraction confirmed the phase and composition of
the synthesized material. The crystallite sizes were estimated using Scherrer and Stokes
and Wilson‘s formulae. The range of crystallite size was 30-83 nm corresponding to
most intense peak using Scherrer‘s formula. The x-ray diffraction data was used to
calculate lattice constants. DC conductivity was measured in temperature range 300-
700 0C and ac conductivity was calculated in frequency range 1kHz to 3MHz at
temperatures 300, 400, 500, 600 and 700 0C. The ‗universal‘ dielectric response with
Jonscher power law and jump relaxation model explained the conduction phenomena in
the synthesized material. The Raman spectra confirmed the doping and increase of
vacancy sites. The maximum conductivity achieved is 1.78 S-cm-1
for Ce0.5Gd0.25
Nd0.25Oδ.
7.4 Comparison Table
A comparison of values of conductivities with literature is given in table 7.4 .
Ce0.5Gd0.25 Nd0.25Oδ prepared with CMH method got maximum conductivity overall.
The Gd and Dy doped materials which have larger values than this work (Gd doped
ceria) are synthesized with high temperature preparation methods.
81
Table 7.4 Comparison of conductivity values with literature.
# Composition Synthesis method Temp
.
oC
Conductivity
S-cm-1
Ref.
1 Ce0.85Gd0.15O1.9
25
Thermal
decomposition
600 0.025 Veranitisagul et al,
Ceram Int 2012 [54]
2 Ce0.85Dy0.15O2-δ Combustion
MH(microwave
heating)
CH(convention
al heating)
550
550
7.42x10-2
9.79x10-3
Acharya,
J Power Sourc 2012 [55]
3 Ce0.8Gd0.2O2-δ Citrate auto
ignition
550 1.8x10-4
Baral et al, Nanoscal Res
Lett 2010 [118]
4 Ce0.8Eu0.2O2-δ Citrate auto
ignition
550 1.39x10-4
Baral et al, Nanoscal Res
Lett 2010 [118]
5 Ce0.8Dy0.2O2-δ Citrate auto
ignition
550 1.36x10-4
Baral et al, Nanoscal Res
Lett 2010 [118]
6 Ce0.8Ho0.2O2-δ Citrate auto
ignition
550 1.4x10-4
Baral et al, Nanoscal Res
Lett 2010 [118]
7 Ce0.85Gd0.15O2-δ Citrate auto
ignition
550
600
1.2x10-4
2.6x10-4
Baral et al, Appl Phys A
2010 [117]
8
Ce0.9Gd0.1Oδ
CMH
(wire like
morphology)
550
600
4.02×10-4
6.74×10-4
This work
9
Ce0.75Gd0.25Oδ CMH 550
600
4.78×10-4
9.30×10-4
This work
10 Ce0.5Gd0.25
Nd0.25Oδ CMH
550
600
0.88
1.78
This work
82
Chapter 8 Synthesis and
Thermophysical
Characterization of
Rare-Earth Hydroxides
83
Synthesis and Thermophysical Characterization of Rare-
Earth Hydroxides
Introduction
The preparation and depiction of Ce(OH)3, La(OH)3 and Nd(OH)3 is given
here. The precipitated hydrothermal method was used to grow the nanostructures. The
prepared samples got very interesting morphologies which make these fascinating for
further functionalization in various applications. The synthesis method and
stoichiometric change in composition affect the crystal structure and morphology.
8.1 Structural and morphological studies
8.1.1 Structural Analysis
The XRD patterns of rare earth hydroxides are given in figure 8.1. The
corresponding pattern confirmed the phase and hexagonal structure of Ce(OH)3 (card #
01-074-0665), Nd(OH)3 (card # 00-006-0601) and La(OH)3 (card # 00-006-0585). The
crystallite sizes corresponding to most intense peaks and lattice constants are
mentioned in table 8.1.
20 30 40 50 60 70 80
(110)
(103) (302)(311)
2degrees
Inte
nsity
(a.u
.)
(002)(202)
(112)
Ce(OH)3
(110)
(310) (410)(311)
(220)
(300)
(120)
(201)(200)
(311)
(310)
(112)(211)
(300)
(210)
(201)
(200)
(101)
(110)
La(OH)3
(302)
Nd(OH)
3
Figure 8.1 XRD pattern of Ce(OH)3, Nd(OH)3 and La(OH)3 samples
84
Table 8.1 Crystallite size corresponding to the most intense peak and lattice constants
for Ce(OH)3, Nd(OH)3 and La(OH)3.
8.1.2 Surface Morphology
The SEM micrographs are shown in figure 8.2. The Ce(OH)3 has belts like
structures, Nd(OH)3 has needles like structure and La(OH)3 has wires like structures
with lengths in microns.
Figure 8.2 SEM micrographs of Ce(OH)3, Nd(OH)3 and La(OH)3 samples
Sample D s (1 1 0)
(nm)
Lattice Constant Å
a c
Ce(OH)3 41 6.52(2) 3.81(1)
La(OH)3 33 6.53(2) 3.86(1)
Nd(OH)3 18 6.40(2) 3.74(1)
Ce(OH)3
Nd(OH)3
La(OH)3
85
The growth of samples can be attributed to the chemical potential maintained
through the precipitating agent, the pressure inside the vessel, the temperature provided
for the hydrothermal treatment and time for heat treatment. Moreover, the crystal
structures of precursors also play role in growth formation. The resultant shape is
dependent on the equilibrium energy of the respective faces according to Curie-Wulff-
Gibbs model [27, 35, 125].
8.1.3 Differential Scanning Calorimetry
The DSC plot showed the two step conversion of La(OH)3 to LaOOH and
finally La2O3. The conversions occur at ca. 400 0C and ca. 600
0C [35].
0 200 400 600 800 1000
-20
-15
-10
-5
0
5
10
La2O
3
LaOOH
He
at
Flo
w (
mW
)
Temperature (0C)
La(OH)3
Figure 8.3 DSC plot of La(OH)3
8.2 Electrical measurements
8.2.1 DC Conductivity
The dc conductivities of R(OH)3 are shown in figure 8.4 (a-c). The
conductivities are measured in temperature range 300 0C to 500
0C. Nd(OH)3 achieved
maximum conductivity whereas Ce(OH)3 got minimum among the three. The
conductivity increase with temperature shows the Arrhenius type dependence. The high
conductivity of Nd(OH)3 might be due to smaller crystallite size as the smaller grains
86
increase the grain boundaries and charges pile up on boundaries which help in
enhancement of conductivity [110, 113, 127-129].
300 350 400 450 500-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
dc (S
-cm
-1)
Temperature (oC)
Figure 8.4 (a) DC conductivity as a function of temperature for Ce(OH)3 sample
300 350 400 450 500
0
5
10
15
20
Temperature (oC)
d
c (S
-cm
-1)
Figure 8.4 (b) DC conductivity as a function of temperature for Nd(OH)3
87
300 350 400 450 5000
1
2
3
4
5
6
7
d
c (S
-cm
-1)
Temperature (oC)
Figure 8.4 (c) DC conductivity as a function of temperature for La(OH)3 sample
Table 8.2 DC conductivity as a function of temperature for hydroxide samples
8.2.2 AC Conductivity
The AC conductivities of R(OH)3 are given in figure 8.5 (a-c). These
conductivity plots also followed jump relaxation model. At lower frequencies the dc
conductivity is due to the jumping of ion from one vacant state to other. At higher
frequencies dispersion occur due to hopping and relaxation of ions. The increase in
temperature moved the dispersion region towards the higher frequency region. The step
in dispersion is the confirmation of grain boundaries and interior conduction. The
confirmation of ionic hopping in addition to Arrhenius relation is given by the
Jonscher‘s law.
σdc (S-cm-1
) 300oC 400
oC 500
oC
Ce(OH)3 0.001 0.018 0.372
La(OH)3 0.751 1.361 6.648
Nd(OH)3 1.284 4.582 20.369
88
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
500oC
400oC
300oC
log f (Hz)
log
a
c (
S-c
m-1)
Figure 8.5(a) AC conductivity as a function of frequency of Ce(OH)3 sample
3.0 3.5 4.0 4.5 5.0 5.5 6.0
1.6
1.7
1.8
1.9
2.0
2.1
2.2
500oC
400oC
300oC
log
a
c (
S-c
m-1)
log f (Hz)
Figure 8.5(b) AC conductivity as a function of frequency of Nd(OH)3 sample
89
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0.9
1.0
1.1
1.2
1.3
1.4
1.5 500
oC
400oC
300oC
log f (Hz)
log
a
c (
S-c
m-1)
Figure 8.5(c) AC conductivity as a function of frequency of La(OH)3 sample.
Table 8.3 AC conductivity as a function of temperature for hydroxide sample
8.3 Thermal Conduction
The thermal conduction and thermal diffusivity values were measured with
ATPS method. The values of conductivities and diffusivities are given in table 8.4.
Table 8.4Thermal conductivity and thermal diffusivity of R (OH)3
σac (S-cm-1
) 300oC 400
oC 500
oC
Ce(OH)3 0.806 0.056 0.964
La(OH)3 1.006 1.317 1.522
Nd(OH)3 1.721 2.001 2.207
Samples Thermal Conductivity
(Wm-1
K-1
)
Thermal Diffusivity
(mm2/s)
Ce(OH)3 0.75(2) 0.78(2)
La(OH)3 0.59(2) 0.96(2)
Nd(OH)3 1.05(2) 0.57(2)
90
The thermal conductivity is dependent on the scattering of phonons during their
motion due to thermal agitation. The mean free path is the measure of the thermal
conduction. The larger the mean free path, lager the thermal conduction and smaller
mean free path, lower the thermal conduction [119-120, 130].
8.4 Conclusions
The synthesis of Ce(OH)3, La(OH)3 and Nd(OH)3 was done by using the
hydrothermal method. Hexagonal structures were confirmed with XRD. The crystallite
size corresponding to most intense peaks were 18, 33 and 41 nm for Nd, La and Ce
hydroxides. SEM revealed very interesting and fascinating morphologies. Ce(OH)3 has
belts like structures, Nd(OH)3 has needles like structures and La(OH)3 has wires like
structures. The growth of structures can be ascribed to chemical potential maintained
through precipitating agent, the pressure inside the vessel, the temperature provided for
the hydrothermal treatment and time for hydrothermal treatment time. Two step
transformations from hydroxide to oxide was observed in DSC plot. Nd(OH)3 achieved
maximum conductivity and Ce(OH)3 acquired minimum among the three. The larger
values of conductivities for Nd(OH)3 and smaller in other two samples might be due to
smaller crystallite size. The smaller grains increase the grain boundaries and charges
can pile up on boundaries which increase the conductivity. The thermal conductivity
values were determined using ATPS method and were found in low thermal
conductivity region. The thermal conduction is dependent on the scattering and mean
free path. The less mean free path and more scattering gave rise to low conductivity
values. Nd(OH)3 have larger value of thermal conductivity which might be due to its
smaller crystallite size.
91
Chapter 9 Synthesis and Fluorescence
in NaMnF3: Yb;Er/Tm
92
Synthesis and Fluorescence in NaMnF3: Yb;Er/Tm
9.1 Structural and Morphological Analysis
9.1.1 X-Ray Diffraction
The X-ray diffraction was done on as prepared samples of NaMnF3:Yb/Er and
the obtained pattern can be indexed with reference pattern PDF-number 00-018-1224
having orthorhombic crystal system and space group number 62. The represented
pattern is shown in figure 9.1.
20 25 30 35 40 45 50 55 60 65
(3 2 1)(2 0 2)
(1 2 1)
(2 0 0)
Inte
ns
ity
(a
.u.)
2Theta (degrees)
PEI@NaMnF3:Yb,Er (Yb18Er2Mn80)
(1 0 2) (1 3 1)(2 1 2)
(1 0 3)
(1 1 3)
(2 4 0)
Figure 9.1 XRD pattern of PEI-capped NaMnF3:Yb,Er
9.1.2 Transmission Electron Microscope Analysis
TEM revealed the nanorod structures with diameter range 50-60 nm and length
of few hundreds nm. The representative images are given in figure 9.2. The uniform
distribution, crystallinity and narrow range of particles‘ size can be observed from the
figure. The elongated structures have more intense emissions than the normal
nanocrystals [41]. Although the lack of in situ studies of solvothermal process [131]
makes it difficult to exactly assign a parameter to a specific job but the elongated
structured growth can be attributed to crystal structures of the reactants with ethylene
93
glycol as solvent [132] along with temperature and pressure [35, 133]. The fluoride
source itself is a deciding element [134-135]. Moreover, the shape evolution of crystals
is dependent on the compromise of the free energies of the faces of the crystal [125].
Figure 9.2 TEM images (a, b) for PEI-NaMnF3:Yb, Er ; Yb:Er 20/2 Mn 78 mol %
sample and (c, d) for PEI-NaMnF3:Yb, Er ; Yb:Er 60/2 Mn 38 mol % sample
9.2 Fluorescence Measurements
9.2.1 NaMnF3:Yb;Er
It is difficult to obtain up conversion (UC) system capable of pure single UC
emission because there are more than one metastable excited states in lanthanide ions,
generally [79]. For Yb/Er co-doped systems, two emissions are possible; bright green
emission ca. 550 nm and weak dark red emission ca. 660 nm. With the low penetration
depth of green and low intense red emission, this becomes a limit for tissue imaging
applications. The pure red and intense emission can penetrate into tissues (in vivio
imaging) [136].
94
The figure 9.3 show the fluorescence spectra for NaMnF3:Yb, Er. The red color
(4F9/2 to
4I15/2) emission is observed for Yb/Er 20/2. The increase in Yb concentration
gave rise to dual band red and green (4S3/2 to
4I15/2) emissions with green band
dominating [137]. To further tune the red emission obtained at Yb 20, the
concentration of Er was varied. The 10 and 20 mol % increase in Er gave rise to high
intensity red emissions as shown in figure 9.4. Both the green and red emissions in
respective samples are seen with naked eyes when excited by 980 nm laser. With such
intense emissions it can be anticipated that the deep tissue penetration is achievable.
Figure 9.3 Upconversion spectra of NaMnY3:Yb,Er capped with PEI for different
molar percentage.
The red UC emission obtained put forward that energy transfer mechanism
exchanged between Er3+
and Mn2+
ions is exceptionally proficient, which can be
mainly credited to the resonances between the Mn2+
absorption bands and available
many metastable Er3+
levels. The refinement of emission color, red, could be ascribed
to non-radiative energy transfer from the 2H9/2 and
4S3/2 levels of Er
3+ to the
4T1 level of
Mn2+
, followed by back-energy transfer to the 4F9/2 level of Er
3+ which enhance the
450 500 550 600 650 700
0
1x104
2x104
3x104
0.05.0x10
3
1.0x104
1.5x104
2.0x104
2.5x104
0.0
5.0x104
1.0x105
1.5x105
0.0
2.0x105
4.0x105
6.0x105
8.0x105
Wavelength (nm)
Yb18
Yb20
Yb60
4S
3/2
4F
9/2
4I15/2
Yb80
Inte
ns
ity
(c
ps
)
95
likelihood of the red emission [93, 138-139]. The fluorescence increase and decrease
was found to be non-linear. Also with increase of Mn, the fluorescence intensity
decreased due to quenching phenomenon which happens due to efficient energy
transfer among dopant ions [136].
Figure 9.4 Upconversion spectra of NaMnY3:Yb,Er capped with PEI for different
molar percentage.
9.2.2 NaMnF3:Yb;Tm
For NaMnF3:Yb,Tm blue color emission is observed for fixed Tm 0.02 and
changing Yb, Mn concentration as shown in figure 9.5. The observed strong emission
is NIR 3H4 to
3H6 [140] which might be due to energy transfer from
1D2 and
1G4
followed by back-energy transfer to the 3F4 level [141]. The dispersivity of sample in
water is clear as shown in figure 9.6 which was obtained due to successful
functionalization of surface of prepared samples with PEI.
450 500 550 600 650 700
0.0
5.0x103
1.0x104
1.5x104
2.0x104
2.5x104
0.0
5.0x105
1.0x106
1.5x106
2.0x106
2.5x1060
1x105
2x105
3x105
4x105
5x105
Wavelength (nm)
Er02
Inte
ns
ity (
cp
s)
Er10
4F
9/2
4I15/2
Er20
96
Figure 9.5 Upconversion of Tm doped PEI capped NaMnF3: Yb,Tm.
Figure 9.6 Dispersivity of NaMnF3:Yb;Er/Tm in water
400 500 600 700 800 900
0.0
2.0x106
4.0x106
6.0x1060.0
2.0x106
4.0x106
6.0x106
8.0x106
1.0x107
0.0
5.0x106
1.0x107
1.5x107
0.0
5.0x106
1.0x107
1.5x107
Wavelength (nm)
Yb20Tm0.02
Yb40Tm0.02
Yb60Tm0.02
Inte
nsit
y (
cp
s)
1G
4
3H
6
3H
4
3H
6Yb80Tm0.02
97
9.3 Conclusions
The synthesis and surface modification of NaMnF3 co-doped with Yb;Er/Tm
was successfully achieved in single step through solvothermal method. The PEI
polymer was used for surface modification. An intense green emission is observed for
NaMnF3:Yb, Er, with increase in Yb concentration and with fixed Er at 2 mol%. The
observed emission was around 550 nm and that was between levels 4S3/2 and
4I15/2.
Yb20 Mn78 Er2 revealed red emission at 660 nm and that was between levels 4F9/2 and
4I15/2 which became intense with increase of Er concentration. With Tm as dopant,
NEAR IR emission was observed at 800 nm between levels 3H4 and
3H6 although blue
emission was also observed at 480 nm between energy levels 1G4 and
3H6. The X-ray
diffraction confirmed the structure to be orthorhombic and TEM showed the
morphology to be nanorods with diameters 50-60 nm.
98
Chapter 10 Summary and
Conclusions
99
10.1 Summary and Conclusions
Comparative study for preparation and properties of phase pure nanocrystalline
ceria and doped ceria was done to obtain higher values of ionic conductivity. Wet
chemical methods like composite mediated hydrothermal method, co-precipitation
method and sol-gel method were adopted to synthesize the samples. X-ray diffraction
(XRD) and Raman spectroscopy were used for structural characterization on prepared
samples. Ceria, Gd doped ceria and co-doped ceria (CeO2, Ce
1-xGd
xO, Ce
1-2xGd
xLa
xO
and Ce1-2x
GdxNd
xO) showed the cubic fluorite structure. Ceria crystallite size
corresponding to most intense peaks for CMH, sol-gel and co-precipitated samples
were 60, 10 and 47 nm respectively. For the optimized results of CMH method, the
average crystallite sizes were in range 31 to 64 nm estimated with Scherrer formula. In
Ce1-x
GdxO , the range of crystallite size was 30-83 nm corresponding to most intense
peak using Scherrer‘s formula and 40-111 nm using Stokes and Wilson‘s formula. The
X-ray diffraction confirmed the phase and crystal structure of Ce1-2x
GdxLax/Nd
xO,
cubic structure except one peak each in Ce0.5Gd0.25 Nd0.25Oδ and Ce0.5Gd0.25 La0.25Oδ
corresponding to Nd2O3 and La2O3 respectively. The range of average crystallite size
was 31-39 nm corresponding to most intense peak using Scherrer‘s formula and 53-64
nm using Stokes and Wilson‘s formulae. The x-ray diffraction data was used to
calculate lattice constants which were in the range 5.45 Å to 5.91 Å. XRD confirmed
the hexagonal structures of Ce(OH)3, La(OH)3 and Nd(OH)3 and were found phase
pure. The crystallite size corresponding to the most intense peaks were 18, 33 and 41
nm for Nd-, La- and Ce- hydroxides.
Raman spectroscopy seconded the XRD results and intense peak ca.
465 cm-1
characteristic of Ce-O F2g band was observed for all samples. In addition to
this band, weak bands for intrinsic oxygen (appeared for charge neutrality due to
conversion of Ce4+
to Ce3+
) and oxygen vacancies were also observed, former ca. 546
cm-1
and later ca. 570 cm-1
& 600 cm-1
. For Raman excitation 514 nm laser was used.
The doped ceria and co-doped ceria also showed the increase in bandwidth of the peak
with doping contents which also increased the oxygen vacancies.
The dc conductivities of the samples were determined in temperature range 200-
700 0C. AC conductivities were determined in frequency range 20Hz-3MHz at
100
temperatures 300, 400, 500, 600 and 700 0C. For doped and co-doped ceria the
temperature range was 300-700 0C and frequency range was 1 kHz to 3 MHz. The
values obtained were in decreasing order with the synthesis method and were 0.345 S-
cm-1
(CMH), 0.095 S-cm-1
(Sol-gel) and 3.96x10-8
S-cm-1
(Co-precipitation) at 700 0C.
The maximum conductivity, achieved for Ce0.75Gd0.25Oδ, was 7.4x10-3
S-cm-1
at 700 0C.
The maximum conductivity achieved was 1.78 S-cm-1
for Ce0.5Gd0.25 Nd0.25Oδ. The
jump relaxation model can be used to explain the dc conductivity behavior. With the
thermal energies, the ion overcame the potential and moved to another available site
which caused a mismatch in the lattice. The relaxation could be achieved in two ways;
either the ion can move back or neighboring ion can move. The observation have put
weight in later scenario and by jump of neighboring ion, a hopping motion started thus
contributing to dc conductivity. The ‗universal‘ dielectric response or Jonscher power
law elaborates the ac conductivity phenomenon. The log of ac conductivity as a
function of log of frequency plot has a power dependence in the dispersion curves. The
dispersion region moved toward high frequency region with increase of temperature as
mobility increased. This phenomenon occurred due to the relaxation and hopping of
ions. The ‗step‘ in dispersion curves is confirmation of the grain interior and grain
boundary conductivities as ionic conduction is dependent on the defect formation due
to thermal energies which create vacancies to aid in hopping motion of ions. The high
values at low frequencies are due to electrode-electrolyte interface. The shift of
relaxation peaks toward higher frequencies with increase in temperature is also clear
from plots. These relaxation and reorientation of peaks which are due to the response of
ions to applied frequency is also evident in dielectric loss plots. The dielectric loss has
higher values at lower frequencies due to loss of energy in relaxation and reorientation
of ions whereas at higher frequencies dielectric loss has lower values because of the
higher frequencies; the ions were not able to response to frequencies. The enhancement
in conductivity was successfully achieved and this property made these materials as
potential candidates for SOFCs as electrolyte material.
Nd(OH)3 got maximum conductivity and Ce(OH)3 got minimum among the
three. The larger values of conductivities for Nd(OH)3 and smaller in other two samples
might be due to smaller crystallite size. The smaller grains increase the grain
boundaries and charges can pile up on boundaries which increase the conductivity. The
101
corresponding dc conductivity values of Ce(OH)3, La(OH)3 and Nd(OH)3 were 0.372,
6.648 and 20.369 S-cm-1
, respectively.
Very interesting and fascinating morphologies were revealed with SEM.
Ce(OH)3 has belts like structures, Nd(OH)3 has needles like structures and La(OH)3 has
wires like structures. The growth of structures can be attributed to chemical potential
maintained through pH adjusted by the precipitating agent, the pressure inside the
vessel, the temperature provided for the hydrothermal treatment and time for
hydrothermal treatment. The crystal structures of precursors also have a decisive role in
growth specially in obtaining different morphologies with different compounds. The
shape evolution can be explained by Gibbs-Curie-Wulff model which relate the shape
evolution with the face energies. When the equilibrium energy is obtained for
respective faces the Ostwald ripening is stopped. The monomer concentration is also
important which was obtained with rapid adjustment of pH of the solution. The DSC
plot showed a two-step transformation of R(OH)3 to R2O3. The La(OH)3 first
converted into LaOOH at ca. 400 0C and finally into La2O3 at ca. 600
0C. The dc
conductivities of these three samples were found in temperature range 300-500 0C. The
increase of conductivity with temperature is evident from the plots. These materials can
be utilized as thermal barrier coatings due to their low thermal conductivies.
The thermal conductivity values determined simultaneously with thermal
diffusivity using ATPS method for samples Ce1-x
GdxO, and R(OH)3 were in low
thermal conductivity region. The thermal conduction is dependent on the scattering and
mean free path. The less mean free path and more scattering gave rise to low
conductivity values which is the case here. The Gd contents increased the scattering
sites and conductivity decreased which can also be confirmed by Raman spectrum as
the bandwidth increased with Gd contents. The mean free path is the measure of the
thermal conduction. The larger the mean free path, larger the thermal conduction and
smaller mean free path, lower the thermal conduction.
To explore the fluorescence properties of rare earths intense emissions and
stabilized structures (crystal as well as morphologies), rare earths (Yb, Er, and Tm)
were exploited as Yb has maximum absorption cross section and served as sensitizer
and ladder like metastable states of Tm and Er were utilized as activators. The F based
inorganic matrix NaMnF3 was synthesized with dopants Yb, Er, Tm as F based
102
matrices are most stabilized when used with NIR lasers for biological imaging. The Mn
is anticipated to be more bio compatible as compared with other elements used in
synthesis of matrices. Moreover, the magnetic effect of Mn can be utilized for MR
imaging. The synthesis of NaMnF3 co-doped with Yb;Er/Tm was successfully
achieved through solvothermal method. The ethylene glycol was used as stabilizing
agent. Another important feature of this synthesis method was surface functionalization
of particles with the synthesis process in single step. Also, the choice of precursors of
Na & F and choice of stabilizing agent (EG) rendered the nanostructures to be rods like
which are not obtained normally in bio labels synthesis. The PEI polymer was used for
surface modification. An intense green emission is observed for NaMnF3: Yb, Er, with
increase in Yb concentration and for fixed Er at 2 mol%. The observed emission was
around 550 nm between levels 4S3/2 and
4I15/2. Yb20 Mn78 Er2 revealed red emission at
660 nm between levels 4F9/2 and
4I15/2 which became intense with increase of Er
concentration. With Tm as dopant, NIR emission was observed at 800 nm between
levels 3H4 and
3H6 although blue emission was also observed at 480 nm between energy
levels 1G4 and
3H6. For the excitation 980 nm laser was used. The red, green and blue
emission can be seen and photographed digitally when prepared samples were excited.
The X-ray diffraction confirmed the crystal structure to be orthorhombic and TEM
showed the morphology to be nanorods. The diameters of the rods were 50-60 nm
approximately. The PEI polymer made samples hydrophilic and were properly
dispersed in water. The intense emissions specially pure red and prescence of Mn made
these materials excellent applicant as bimodal imaging bioprobe in optical and MR
imaging.
103
10.2 Future Recommendations
Following are a few experiments and theoretical analysis that are suggested for future
studies.
The conductivity of ceria based compounds could be studied in oxygen
atmosphere.
The gas sensing properties of ceria and doped ceria could be studied.
These synthesized materials might be combined with cathode and anode
materials for the study of SOFCs.
The temperature dependent thermal conduction properties of R(OH)3 samples
could be studied
The NaMnF3:Yb;Er/Tm samples might be studied for the MRI and cytotoxicity
analysis
104
Chapter 11 References
105
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