SURFACE CHARACTERIZATION OF PIEZOELECTRIC SENSOR MATERIALS ...
Transcript of SURFACE CHARACTERIZATION OF PIEZOELECTRIC SENSOR MATERIALS ...
The Pennsylvania State University
The Graduate School
SURFACE CHARACTERIZATION OF PIEZOELECTRIC SENSOR MATERIALS
FOR POTENTIAL USE IN REACTOR VESSEL SENSORS
A Thesis in
Nuclear Engineering
by
Dazhong Ding
© 2019 Dazhong Ding
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Master of Science
December 2019
The thesis of Dazhong Ding was reviewed and approved* by the following:
Leigh Winfrey
Associate Professor of Nuclear Engineering
Thesis Advisor
Arthur Thompson Motta
Professor of Nuclear Engineering
Jean Paul Allain
Department Head of Nuclear Engineering
*Signatures are on file in the Graduate School
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ABSTRACT
Piezoelectric materials can be used as in mechanical sensors in nuclear reactors.
However, the severe environment will accelerate the failure of these materials and decrease the
lifetime of the sensors. The working mechanism and two potential failure mechanisms are
discussed in the thesis. Five piezoelectric crystals are characterized for composition, surface
topography, crystal phase and mechanical parameters. These data will be used in future radiation
studies.
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TABLE OF CONTENTS
LIST OF FIGURES ................................................................................................................. v
LIST OF TABLES ................................................................................................................... ix
ACKNOWLEDGEMENTS ..................................................................................................... x
Chapter 1 Introduction to Piezoelectricity .............................................................................. 1
History of Piezoelectricity................................................................................................ 1 Mechanism of Piezoelectricity ......................................................................................... 2 Piezoelectric Constants .................................................................................................... 3 Piezoelectric Ultrasonic Transducer ................................................................................ 4 Potential Failure Mode of Piezoelectricity in Reactor ..................................................... 5
Thermal Depolarization ............................................................................................ 5 Gamma Radiation ..................................................................................................... 6 Neutron Radiation .................................................................................................... 7
Chapter 2 Material Selection .................................................................................................. 8
Aluminum Nitride ............................................................................................................ 9 Yttrium Calcium Oxoborate ............................................................................................. 9 Lanthanum Gallium Silicate ............................................................................................ 9 Lanthanum Gallium Tantalate.......................................................................................... 10 Goal of the Study ............................................................................................................. 10
Chapter 3 Sample Characterization and Surface Topography ................................................ 11
X-Ray Diffraction ............................................................................................................ 11 Optical Profilometry ........................................................................................................ 17 Nanoindentation ............................................................................................................... 20 Scanning Electron Microscope and Focused Ion Beam ................................................... 22
Chapter 4 Conclusion .............................................................................................................. 34
References ................................................................................................................................ 35
Appendix A Optical Profilometry Diagrams of Piezoelectric Crystals .................................. 38
Appendix B Scanning Electron Microscope Images of Piezoelectric Crystals ...................... 48
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LIST OF FIGURES
Figure 1-1: Schematic representation of the longitudinal (a) direct piezoelectric effect, in
which charges are accumulated with different signs when a negative or positive
strain applies, (b) converse piezoelectric effect, in which an external electrical
potential results in a negative or positive strain of the piezoelectric crystal and (c)
shear piezoelectric effect, in which the deformation is perpendicular to the electric
dipoles’ direction resulting in charges accumulating on the electrodes ........................... 3
Figure 1-2: Diagram of a Classical Single-Element Transducer ............................................. 5
Figure 3-1: Representative X-Ray Diffraction Pattern of the Aluminum Nitride Crystal ....... 12
Figure 3-2: Representative X-Ray Diffraction Pattern of the First Yttrium Calcium
Oxoborate Crystal ............................................................................................................ 13
Figure 3-3: Representative X-Ray Diffraction Pattern of the Second Yttrium Calcium
Oxoborate Crystal ............................................................................................................ 14
Figure 3-4: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium Silicate
Crystal .............................................................................................................................. 15
Figure 3-5: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium
Tantalate Crystal .............................................................................................................. 16
Figure 3-6: Representative Surface Roughness of the Aluminum Nitride Crystal .................. 17
Figure 3-7: Representative Surface Roughness of the First Yttrium Calcium Oxoborate
Crystal .............................................................................................................................. 18
Figure 3-8: Representative Surface Roughness of the Second Yttrium Calcium Oxoborate
Crystal. ............................................................................................................................. 18
Figure 3-9: Representative Surface Roughness of the Lanthanum Gallium Silicate Crystal .. 19
Figure 3-10: Representative Surface Roughness of the Lanthanum Gallium Tantalate
Crystal .............................................................................................................................. 19
Figure 3-11: Force-Displacement Curve Obtained by Nanoindentation. ................................ 20
Figure 3-12: Contact Depth, Modulus and Hardness from Nanoindentation .......................... 21
Figure 3-13: Representative Image of the Aluminum Nitride Crystal ..................................... 23
Figure 3-14: Representative Energy Spectrum of the Aluminum Nitride Crystal. .................. 24
Figure 3-15: Representative Image of the First Yttrium Calcium Oxoborate Crystal. ............ 25
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Figure 3-16: Representative Energy Spectrum of the First Yttrium Calcium Oxoborate
Crystal. ............................................................................................................................. 26
Figure 3-17: Representative Image of the Second Yttrium Calcium Oxoborate Crystal ......... 27
Figure 3-18: Representative Energy Spectrum of the Second Yttrium Calcium Oxoborate
Crystal .............................................................................................................................. 28
Figure 3-19: Representative Image of the Lanthanum Gallium Silicate Crystal ..................... 29
Figure 3-20: Representative Energy Spectrum of the Lanthanum Gallium Silicate Crystal ... 30
Figure 3-21: Representative Image of the Lanthanum Gallium Tantalate Crystal .................. 31
Figure 3-22: Representative Energy Spectrum of the Lanthanum Gallium Tantalate
Crystal .............................................................................................................................. 32
Figure 3-23: Cross-section of Yttrium Calcium Oxoborate ..................................................... 33
Figure A-1: Surface Roughness of the Aluminum Nitride Crystal 1 ....................................... 38
Figure A-2: Surface Roughness of the Aluminum Nitride Crystal 2 ....................................... 38
Figure A-3: Surface Roughness of the Aluminum Nitride Crystal 3 ....................................... 39
Figure A-4: Surface Roughness of the Aluminum Nitride Crystal 4 ....................................... 39
Figure A-5: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 1 ............... 40
Figure A-6: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 2 ............... 40
Figure A-7: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 3 ............... 41
Figure A-8: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 4 ............... 41
Figure A-9: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 1 ........... 42
Figure A-10: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 2 ......... 42
Figure A-11: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 3 ......... 43
Figure A-12: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 4 ......... 43
Figure A-13: Surface Roughness of the Lanthanum Gallium Silicate Crystal 1 ..................... 44
Figure A-14: Surface Roughness of the Lanthanum Gallium Silicate Crystal 2 ..................... 44
Figure A-15: Surface Roughness of the Lanthanum Gallium Silicate Crystal 3 ..................... 45
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Figure A-16: Surface Roughness of the Lanthanum Gallium Silicate Crystal 4 ..................... 45
Figure A-17: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 1 .................. 46
Figure A-18: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 2 .................. 46
Figure A-19: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 3 .................. 47
Figure A-20: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 4 .................. 47
Figure B-1: Image of the Aluminum Nitride Crystal 1 ............................................................ 48
Figure B-2: Image of the Aluminum Nitride Crystal 2 ............................................................ 49
Figure B-3: Image of the Aluminum Nitride Crystal 3 ............................................................ 50
Figure B-4: Image of the First Yttrium Calcium Oxoborate Crystal 1 .................................... 51
Figure B-5: Image of the First Yttrium Calcium Oxoborate Crystal 2 .................................... 52
Figure B-6: Image of the First Yttrium Calcium Oxoborate Crystal 3 .................................... 53
Figure B-7: Image of the Second Yttrium Calcium Oxoborate Crystal 1 ................................ 54
Figure B-8: Image of the Second Yttrium Calcium Oxoborate Crystal 2 ................................ 55
Figure B-9: Image of the Second Yttrium Calcium Oxoborate Crystal 3 ................................ 56
Figure B-10: Image of the Second Yttrium Calcium Oxoborate Crystal 4 .............................. 57
Figure B-11: Image of the Second Yttrium Calcium Oxoborate Crystal 5 .............................. 58
Figure B-12: Image of the Second Yttrium Calcium Oxoborate Crystal 6 .............................. 59
Figure B-13: Image of the Second Yttrium Calcium Oxoborate Crystal 7 .............................. 60
Figure B-14: Image of the Second Yttrium Calcium Oxoborate Crystal 8 .............................. 61
Figure B-15: Image of the Second Yttrium Calcium Oxoborate Crystal 9 .............................. 62
Figure B-16: Image of the Lanthanum Gallium Silicate Crystal 1 .......................................... 63
Figure B-17: Image of the Lanthanum Gallium Silicate Crystal 2 .......................................... 64
Figure B-18: Image of the Lanthanum Gallium Silicate Crystal 3 .......................................... 65
Figure B-19: Image of the Lanthanum Gallium Silicate Crystal 4 .......................................... 66
Figure B-20: Image of the Lanthanum Gallium Silicate Crystal 5 .......................................... 67
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Figure B-21: Image of the Lanthanum Gallium Silicate Crystal 6 .......................................... 68
Figure B-22: Image of the Lanthanum Gallium Tantalate Crystal 1 ....................................... 69
Figure B-23: Image of the Lanthanum Gallium Tantalate Crystal 2 ....................................... 70
Figure B-24: Image of the Lanthanum Gallium Tantalate Crystal 3 ....................................... 71
Figure B-25: Image of the Lanthanum Gallium Tantalate Crystal 4 ....................................... 72
Figure B-26: Image of the Lanthanum Gallium Tantalate Crystal 5 ....................................... 73
Figure B-27: Image of the Lanthanum Gallium Tantalate Crystal 6 ....................................... 74
Figure B-28: Image of the Lanthanum Gallium Tantalate Crystal 7 ....................................... 75
Figure B-29: Image of the Lanthanum Gallium Tantalate Crystal 8 ....................................... 76
Figure B-30: Image of the Lanthanum Gallium Tantalate Crystal 9 ....................................... 77
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LIST OF TABLES
Table 3-1: Interplanar Distance and Strain of the Aluminum Nitride Crystal ......................... 12
Table 3-2: Interplanar Distance and Strain of the First Yttrium Calcium Oxoborate
Crystal .............................................................................................................................. 13
Table 3-3: Interplanar Distance and Strain of the Second Yttrium Calcium Oxoborate
Crystal .............................................................................................................................. 14
Table 3-4: Interplanar Distance and Strain of the Lanthanum Gallium Silicate Crystal ......... 15
Table 3-5: Interplanar Distance and Strain of the Lanthanum Gallium Tantalate Crystal ...... 16
Table 3-6: Strain and Stress of the Two Yttrium Calcium Oxoborate Crystals ....................... 21
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ACKNOWLEDGEMENTS
I would like to thank my advisor Dr. Winfrey who gave me the opportunity working on
this project, as well as the help when I was trying to finish this thesis.
I would appreciate the reader of this thesis, Dr. Motta, who gave me a lot of suggestions
to complete the thesis with more details.
I would also like to thank my parents and friends who supported me during my hardest
time in the past years, I could not be able to finish this thesis without them.
Chapter 1
Introduction to Piezoelectricity
Piezoelectricity is the phenomenon whereby electric dipoles are generated in certain
anisotropic crystals when subjected to mechanical strain [1]. Dimensional change under the
influence of an electric field, which is the converse effect, happens in the same materials.
History of Piezoelectricity
In 1880, Pierre Curie and Jacques Curie summarized their understanding of
pyroelectricity and crystal structure and performed several experiments. The direct piezoelectric
effect, which converts stress to charges, was discovered during their experiments. In 1881,
Gabriel Lippmann mathematically deduced the converse piezoelectric effect, then the converse
effect was experimentally confirmed by Curies.
One of the first applications of the piezoelectric effect was sonar, an ultrasonic submarine
detector based on quartz crystals [2]. By applying a voltage on quartz crystals which were glued
between two steel plates, a 50MHz ultrasonic wave could be generated in the transducer that
allowed to measure the depth of the submarine by timing the return echo. Similar applications
such as microphones or signal filters were also developed and are widely used now.
Only two types of piezoelectric materials were known prior to about 1940, which were
Rochelle salt and quartz [3]. However, in 1941, barium titanate (BaTiO3) was discovered to have
good piezoelectric properties. This was the beginning of the development of the piezoelectric
ceramic materials, which has continued to the present day.
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Mechanism of Piezoelectricity
Piezoelectric materials belong to a larger class of materials called ferroelectrics. The
molecular structure of a ferroelectric material is oriented, which results in electric separation and
the separated charges forms electric dipoles. There is a critical temperature, called the Curie
temperature, above which the piezoelectric materials dipoles will be free to change. To make the
material piezoelectric, the material should be heated over its Curie temperature under a strong
external electric field, the randomly orientated electric dipole will reorient, such a process called
poling [4]. The dipoles will maintain their orientation after being cooled and the piezoelectric
effect will be apparent, when one either applies a force or an electric field on the material.
When a poled piezoelectric crystal is mechanically strained, the dipole will be changed,
which results in electrical polarization and produces electric charges on the surface of the
material. If electrodes are attached to the surfaces, the generated electric charge can be collected
and used. A schematic diagram is shown in Figure 1-1.
Piezoelectric materials are assumed to be elastic and have linear behavior [6]. It turns out
that at low mechanical stress and low electric fields, piezoelectric materials have a linear response
profile. However, nonlinear effects may happen if high mechanical stresses or high electric fields
are applied to the material. If the nonlinear working region is reached, a new model should be
used to describe the material behavior.
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Figure 1-1: Schematic representation of the longitudinal (a) direct piezoelectric effect, in which
charges are accumulated with different signs when a negative or positive strain applies, (b)
converse piezoelectric effect, in which an external electrical potential results in a negative or
positive strain of the piezoelectric crystal and (c) shear piezoelectric effect, in which the
deformation is perpendicular to the electric dipoles’ direction resulting in charges accumulating
on the electrodes [5]
Piezoelectric Constants
Several constants are used to describe the behavior of piezoelectric materials [7]:
(1) piezoelectric charge constant, 𝑑, is the polarization generated per unit of mechanical
stress or strain? applied to a piezoelectric material;
(2) piezoelectric voltage constant, 𝑔, is the electric field generated per unit of mechanical
stress applied;
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(3) dielectric constant, 𝜖, is the dielectric displacement per unit electric field;
(4) elastic compliance, 𝑠, is the strain produced in a piezoelectric material per unit stress
applied;
(5) Young’s modulus, 𝑌, is an indicator of the stiffness of a material.
(6) electromechanical coupling coefficient, 𝑘𝑖𝑗, is an indicator of the effectiveness with
which a piezoelectric material converts electrical energy into mechanical energy or
converts mechanical energy to electrical energy. The index 𝑖𝑗 represents that the
electrodes are applied in the 𝑖 direction and that the strain is in the 𝑗 direction. The
coefficient 𝑘𝑖𝑗 and other piezoelectric constants are related by Equation 1 [6]:
𝑘2 = 𝑑𝑖𝑗𝑔𝑖𝑗𝑌 (1)
Several other constants such as the frequency constant are also used, however, they are
not as significant to piezoelectric behavior as the constants mentioned above.
Piezoelectric Ultrasonic Transducer
Because the piezoelectric materials have such good response to the mechanical force and
electric field, they are good candidates for the detecting technique, and their non-ionizing
character, low cost and high efficiency of ultrasound sensors are among the features that attract
the interest of researchers [5].
A sample design of the transducer is shown in Figure 1-2, where the piezoelectric
element is located between electrodes. A thick layer is attached which is referred to as the
backing. It provides mechanical support and induces damping of the transducer resonance by
allowing acoustic energy to flow by the rear face. On the other side, matching layers are used
between the piezoelectric element and propagation medium which can increase the transfer of
energy from the active layer to the propagation medium.
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Figure 1-2: Diagram of a Classical Single-Element Transducer [5]
Potential Failure Mode of Piezoelectricity in Reactor
The usage of piezoelectric sensing technique is expanding rapidly. One of the interests is
the nuclear reactor monitoring system. However, there are many challenges to using piezoelectric
materials in the severe environment in the reactor. Two major problems are thermal
depolarization and radiation damage.
Thermal Depolarization
When heating the piezoelectric material to its Curie temperature, the electric dipoles tend
to reorient, and poling will happen under external electric fields. However, if the poled
piezoelectric material is heated above its Curie temperature without an external electric field, the
dipoles will orient randomly to its lowest electric potential which will make the material lose its
piezoelectricity. The thermal stability of these polarized regions strongly influences the
properties. Furthermore, the thermally activated aging may happen on the piezoelectric materials,
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which the dipoles will tend to revert to a random orientation and degrade the piezoelectricity
before reaching its Curie temperature for a long time [8].
Additional minor issues that piezoelectric materials face at high temperatures include, but
are not limited to:
Thermal instability of the dielectric, piezoelectric and electromechanical properties.
Three effects contribute to the temperature dependence: the decrease in the number of polarizable
particles per unit volume as the temperature increases, which is a direct result of the volume
expansion, the increase of the macroscopic polarizability due to the volume expansion, and the
temperature dependence of the macroscopic polarizability at constant volume [9].
Increased attenuation of acoustic waves with temperature. The attenuation can be
successfully accounted for by a theory proposed by Mason, who has obtained an expression for
the attenuation by considering the effect of strain associated with the wave on the phonons
propagated in different directions in the crystal [10].
Chemical instability: higher temperature increases thermal oscillation which results in the
increase of the free energy of the molecule or atom. For a molecule with free energy higher than
its bonding energy, the molecule will be decomposed. Atoms with high free energy cannot
maintain their position in a crystal, hence defects will be generated by holes and ions in the
crystal. Also, thermal expansion must be considered for component integration in the final device.
Gamma Radiation
When radiation interacts with matter, energy will be deposited into that matter [11].
Gamma radiation and neutron radiation typically can influence piezoelectricity even in high
radiation fields because they are electrically neutral. Other particles, such as alpha particles,
which is the helium-4 nuclei, or beta particles, the electron or positron, have electric charges that
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will be quickly attenuated when transmitted and lose most of their energy before arriving at the
piezoelectric detector. Ionizing radiation can also cause gradual crystal dielectric loss; charges,
which are induced by radiation, can be trapped near the electrodes and influence the polarization
of the material [12].
When a gamma ray hits an atom, there are three kinds of interaction that can deposit
photon’s energy, which are the photoelectric effect, Compton scattering and pair production [13].
All these three interaction mechanisms will generate free electrons and ions that change the
electric dipole, which directly influence the piezoelectricity. Furthermore, atoms may be activated
by radiation and change the microstructure.
Neutron Radiation
Unlike photons that interact with the atoms’ shell electrons and cause ionization, neutrons
directly interact with nucleus. Neutron interaction includes elastic or inelastic scattering and
change of element by nuclear absorption, reaction or decay, nuclear fission is another mechanism
for heavy nuclei. The changing of isotopes will dope the material or change the local structure
and results in a change or failure of piezoelectric behavior.
Neutron damage to material highly depends on the neutron cross section, in other word,
reaction probability. Very generally, a low energy neutron has much higher cross section
compared to fast neutron, which means lower energy may result in more serious damage.
Chapter 2
Material Selection
The most commonly used piezoelectric transducer is lead zirconium titanate (PZT) due to
its high performance. However, the significant toxicity of lead compounds requires alternative
materials and a lead-free process, which has been researched in recent years [14]. The Curie
temperature of PZT is below 400°𝐶 [15], which means it cannot be used in a high temperature
environment such as that of a nuclear reactor, which is the eventual goal of these sensors. Thus,
new types of materials should be studied and developed to meet the temperature requirement.
The application of piezoelectric detectors in a reactor requires research in the following
areas:
1. Vibration sensing, response of the signal under vibration of the system.
2. Temperature sensing and pressure sensing are performed by measuring the sound
speed of waves in a liquid or in a vessel.
3. Water level detection and structural integrity tests can be analyzed by echo
amplitudes and time-of-flight signals.
4. Radiation intensity sensing can be detected through measuring the leak current across
the high temperature radiation-hard piezoelectric.
5. Also, signal generation from laser and acoustic pulses to the piezoelectric transducer
will be considered.
To achieve the requirement of the usage in high temperature environment, aluminum
nitride, yttrium calcium oxoborate, lanthanum gallium silicate, and lanthanum gallium tantalate
are appeared to be good candidates in vessel sensors and are evaluated prior long-term radiation
studies in this work.
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Aluminum Nitride
Aluminum nitride (AlN), a hexagonal crystal, is good for surface acoustic wave
applications. It has high piezoelectric constants, which results in good response. The Curie
temperature of AlN is not been reached until 1150°𝐶 in the experiment [16]. A good quality and
large size of bulk material is difficult to grow as single crystal [17].
Yttrium Calcium Oxoborate
Yttrium calcium oxoborate (YCOB) forms as a monoclinic crystal. The Curie
temperature of the YCOB is higher than its melting temperature, which is around 1500°𝐶 [18],
This feature makes the material an excellent candidate for high temperature usage. The high
stability and reliability at about 1000°𝐶 were also confirmed experimentally for a dwell time of 9
hours [17]. A YCOB based sensor has also been developed that can tolerate 1000°𝐶 for vibration
sensing and acoustic emission sensing [19].
Lanthanum Gallium Silicate
Lanthanum gallium silicate crystal CTGS (Ca3TaGa3Si2O14) has been selected. CTGS is
an ordered langasite hexagonal crystal. An experiment shows that the CTGS has no phase
transition until 700°𝐶 [20], which means its Curie temperature is much higher. The stable
piezoelectric properties, electrical resistivity at high temperature [8], makes CTGS a potential
candidate for the high temperature sensing.
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Lanthanum Gallium Tantalate
Lanthanum gallium tantalate (Langatate, LGT, La3Ga5.5Ta0.5O14) crystal, which is a
hexagonal crystal, also has no phase transition until its melting temperature around 1450°𝐶 [21].
The main application of LGT includes high temperature sensors of surface acoustic waves and
bulk waves.
Goal of the Study
The aim of this study is to fully characterize the candidate materials prior to long term
radiation (on the order of years) to obtain:
1. Surface properties: surface topography;
2. Mechanical properties: modulus and hardness;
3. Microstructure: crystal orientation;
The electrical properties and piezoelectric response will be characterized by another
group.
Chapter 3
Sample Characterization and Surface Topography
Several techniques have been used to characterize the material of intent: An aluminum
nitride crystal, two yttrium calcium oxoborate crystals, a lanthanum gallium silicate crystal and
an lanthanum gallium tantalate crystal.
X-Ray Diffraction
The X-ray diffraction method was used to confirm the structure of the crystal and
determine the crystal orientation, data is shown below. The theory of the technique is given by
Bragg’s law in Equation 2:
2𝑑 sin 𝜃 = 𝑛𝜆 (2)
where 𝑑, 𝜃, 𝑛, 𝜆 represent the interplanar distance, the scattering angle, a positive integer and the
wavelength of the X-ray, respectively. By measuring the angle and intensity of the diffracted
beam, the interplanar spacing between different lattice plane can be calculated, and the crystal
structure can be obtained.
The residual stress has significant influence on the property of the material. Stress is
proportional to strain, which can be calculated from the X-ray diffraction. The peak shift from the
expected plane can be measured and the strain can be calculated by Equation 3:
𝜖 =𝑙 − 𝐿
𝐿 (3)
where 𝜖, 𝑙 and 𝐿 represent the strain, the measured interplanar distance and the original
interplanar distance.
12
The X-ray diffraction measurement (PANalytical 4-Circle XPert3 MRD) was performed
and analyzed (JADE8) in Penn State Material Research Institute. Copper was used to generate X-
rays whose 𝐾𝛼 wavelength is 𝜆 = 1.5406Å.
Figure 3-1: Representative X-Ray Diffraction Pattern of the Aluminum Nitride Crystal
The only aluminum nitride peak can be measured in Figure 3-1 is (0 0 2), which confirms
the crystal is highly oriented. A gold peak can be found in the diffraction pattern, which comes
from the coating on the surface. The lattice parameters of AlN are 𝑎 = 3.1114Å, 𝑐 = 4.9792Å.
The interplanar distance and the calculated strain of AlN are shown in Table 3-1:
Table 3-1: Interplanar Distance and Strain of the Aluminum Nitride Crystal
Direction Measured Distance (Å) Expected Distance (Å) Strain
(0 0 2) 2.4968 2.4896 2.8920E-03
AlN (0 0 2)
Au
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Figure 3-2: Representative X-Ray Diffraction Pattern of the First Yttrium Calcium Oxoborate
Crystal
The diffraction pattern of YCOB was obtained, as shown in Figure 3-2. The monoclinic
crystal has two peaks: (2 0 0) and (4 0 0), which indicates the orientation of the crystal is in (2 0
0) direction. Noise can be found at the low angle region due to the lack of intensity of the
measured peak. However, the YCOB peaks can be identified with acceptable statistical
significance. The lattice parameters of YCOB are 𝑎 = 8.0778Å, 𝑏 = 16.0220Å, 𝑐 = 3.5343Å.
The interplanar distance and the calculated strain of the YCOB are shown in Table 3-2:
Table 3-2: Interplanar Distance and Strain of the First Yttrium Calcium Oxoborate Crystal
Direction Measured Distance (Å) Expected Distance (Å) Strain
(2 0 0) 3.9686 4.0389 -1.7406E-02
(4 0 0) 1.9820 2.0195 -1.8545E-02
The measured strain from different group of planes have similar value with average 𝜖 =
−1.7975 × 10−2.
YCOB (2 0 0)
YCOB (4 0 0)
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Figure 3-3: Representative X-Ray Diffraction Pattern of the Second Yttrium Calcium Oxoborate
Crystal
A diffraction pattern of the second YCOB crystal was obtained, as shown in Figure 3-3.
The monoclinic crystal shows four main peaks: (0 2 0), (0 6 0), (0 8 0) and (0 10 0). The (0 4 0)
peak cannot be found, which is expected to appear at about 28°. However, the diffraction pattern
provides enough evidence that the YCOB crystal is oriented to its (0 2 0) direction. Some sub
peaks can also be found close to each of the main peak, which may come from the surface coating
layer. The lattice parameters of YCOB are 𝑎 = 8.0778Å, 𝑏 = 16.0220Å, 𝑐 = 3.5343Å. The
interplanar distance and the calculated strain of the second YCOB are shown in Table 3-3:
Table 3-3: Interplanar Distance and Strain of the Second Yttrium Calcium Oxoborate Crystal
Direction Measured Distance (Å) Expected Distance (Å) Strain
(0 2 0) 7.9948 8.0110 -2.0222E-03
(0 6 0) 2.6762 2.6703 2.1970E-03
(0 8 0) 2.0020 2.0028 -3.7449E-04
(0 10 0) 1.6020 1.6022 -1.2483E-04
The measured strain from different group of planes have been observed with big
variation, the reason will be figured out in the future research.
YCOB (0 2 0)
YCOB (0 6 0)
YCOB (0 8 0)
YCOB (0 10 0)
15
Figure 3-4: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium Silicate Crystal
A diffraction pattern of the CTGS crystal was obtained, as shown in Figure 3-4. The
hexagonal crystal shows two main peaks: (1 1 0) and (2 2 0), which indicates the crystal is
oriented in the (1 1 0) direction. The lattice parameters of CTGS are 𝑎 = 8.1056Å, 𝑐 = 4.9800Å.
The interplanar distance and the calculated strain of the CTGS are shown in Table 3-4:
Table 3-4: Interplanar Distance and Strain of the Lanthanum Gallium Silicate Crystal
Direction Measured Distance (Å) Expected Distance (Å) Strain
(1 1 0) 4.05908 4.05280 1.5495E-03
(2 2 0) 2.02905 2.02640 1.3077E-03
The measured strain from different group of planes have similar value with average
𝜖 = 1.4286 × 10−3.
CTGS (1 1 0)
CTGS (2 2 0)
16
Figure 3-5: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium Tantalate
Crystal
The hexagonal crystal has three main peaks: (1 1 0), (2 2 0) and (3 3 0) which indicates
the crystal is oriented in (1 1 0) direction. The lattice parameter of LGT is 𝑎 = 8.2410Å, 𝑐 =
5.1300Å. The interplanar distance and the calculated strain of the LGT are shown in Table 3-4:
Table 3-5: Interplanar Distance and Strain of the Lanthanum Gallium Tantalate Crystal
Direction Measured Distance (Å) Expected Distance (Å) Strain
(1 1 0) 4.11998 4.12050 -1.2620E-04
(2 2 0) 2.06001 2.06025 -1.1649E-04
(3 3 0) 1.37302 1.37350 -3.4947E-04
The calculated strain from (1 1 0) and (2 2 0) has similar value. However, the strain in (3
3 0) direction is about three times bigger than the other two direction. Further research will be
performed to figure out the reason.
The diffraction patterns obtained show that all these five materials are well oriented
single crystals. The strain of these crystals are also calculated which will be researched deeper in
the future.
LGT (1 1 0)
LGT (2 2 0) LGT (3 3 0)
17
Optical Profilometry
Optical profilometry is a technique that can measure the surface topography by
measuring the movement of interference drift. Because the surface is not flat, the optical path
length between the detection surface and reference plane will be different [22]. By moving the
reference plane’s position, the shifting of the interference fringes can be measured, and the
surface topography can be obtained.
Surface topography images of a certain region of the five crystals are given below. The
size of the diagram shown is 87.7𝜇𝑚 × 87.7𝜇𝑚. The Sa, Sq and Sz in the figures represent the
average roughness, the root mean square (RMS) roughness and the maximum height of the
surface (peak to valley height).
Figure 3-6: Representative Surface Roughness of the Aluminum Nitride Crystal
Occasional shifts can be found in Figure 3-6. Continuous peak region can be found at the
center, with sharp change to the valley region. The average roughness is close to the RMS
roughness, and the roughness is acceptable to do surface coating for further experiment.
18
Figure 3-7: Representative Surface Roughness of the First Yttrium Calcium Oxoborate Crystal
According to Figure 3-7, the upper right area of the region has more shifts than the other
area. Average roughness is close to the RMS roughness, but the peak to valley height is relatively
large compared to the average roughness. It seems the surface provides many supporting points
for surface treatment.
Figure 3-8: Representative Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal
19
According to Figure 3-8, the peak area and the valley area are relative equal with less
shifts, compare to the first YCOB crystal. The roughness value is also acceptable for further
surface treatment.
Figure 3-9: Representative Surface Roughness of the Lanthanum Gallium Silicate Crystal
Shifts can be found in Figure 3-9. Average roughness is close to the RMS roughness, but
the peak to valley height is much greater than the average roughness. There are many dark spots,
which indicate sharp height change happens in those regions.
Figure 3-10: Representative Surface Roughness of the Lanthanum Gallium Tantalate Crystal
20
According to Figure 3-10, many shifts and continuous dark regions can be found all over
the surface, which makes the surface very rough. The average roughness value is small but the
peak to valley value is ten times greater than the average. The crystal has good topography
feature for further experiment.
The surface roughness value and topography images obtained from optical profilometry
for these materials show that the surface of these crystals is good for surface treatment, and the
surface change can be figured out when further experiments are performed. More optical
profilometry diagrams can be found in Appendix A.
Nanoindentation
Nanoindentation is a technique that can measure the mechanical properties of surfaces on
a submicroscopic scale. A small diamond is used as an indenter to apply force to the surface. By
measuring the force-displacement curve, the reduced modules and hardness can be obtained [23].
However, since the surface topography is changed by the indenter, nanoindentation is a
destructive technique.
Figure 3-11: Force-Displacement Curve Obtained by Nanoindentation
21
Figure 3-12: Contact Depth, Modulus and Hardness from Nanoindentation
Sixteen indentations were performed in a small area at the surface of the second YCOB
crystal. The sixteen force-displacement curves are shown in Figure 3-11. By fitting the curves,
the mean contact depth, mean reduced modulus and mean hardness were obtained, as well as their
uncertainty. The reduced modulus 𝐸𝑟 can be converted to the Young’s modulus 𝐸𝑠 with the
diamond indenter tip’s modulus 𝐸𝑖 by Equation 4 [24]:
1
𝐸𝑟=
1 − 𝜈𝑖2
𝐸𝑖+
1 − 𝜈𝑠2
𝐸𝑠 (4)
where ν𝑖 and 𝜈𝑠 represent the Poisson’s ratio of the tip and the crystal.
The material used in the as indenter (Hysitron TI-900) was diamond (𝐸𝑖 = 1140𝐺𝑃𝑎 and
𝜈𝑖 = 0.07). The Poisson’s ratio of YCOB is 0.29 [25], then the Young’s modulus of YCOB is
𝐸𝑠 = 142.5𝐺𝑃𝑎.
The residual stress can be calculated by Equation 5:
𝜎 = 𝐸𝑠𝜖 (5)
where 𝜎, 𝐸𝑠 𝑎𝑛𝑑 𝜖 represent the stress, the Young’s modulus and the strain. Thus, the residual
stress values of the two YCOB are given in Table 3-6:
Table 3-6: Strain and Stress of the Two Yttrium Calcium Oxoborate Crystals
Direction Strain Stress (MPa) Direction Strain Stress (MPa)
(2 0 0) -1.7406E-02 -2480 (0 2 0) -2.0222E-03 -288
(4 0 0) -1.8545E-02 -2643 (0 6 0) 2.1970E-03 313
(0 8 0) -3.7449E-04 -53
(0 10 0) -1.2483E-04 -18
The variation of the values will be researched in the future. However, the Table 3-6
shows that the crystals’ residual stress cannot be ignored.
22
Scanning Electron Microscope and Focused Ion Beam
Scanning electron microscope (SEM) can provide high resolution surface information.
Secondary electrons will be emitted from the surface after the interaction between electrons from
the electron gun and surface atoms. Backscattered electrons may also be detected in the SEM.
Furthermore, the surface atoms may generate characteristic X-rays that can be used for elemental
analysis, which is the energy dispersive spectroscopy (EDS). In addition to X-ray diffraction,
EDS helps to identify the material’s composition, as well as the contamination. However, for the
nonconductive piezoelectric crystal, surface coating, which may change the surface topography,
is required to perform high accuracy EDS spectrum. For all the four crystals except the second
YCOB crystal, their surfaces were coated by platinum, palladium or gold with unknown
thickness. Quantitative EDS analysis cannot be made in this situation; however, qualitative
analysis provides confirmation of the expected elements.
23
Figure 3-13: Representative Image of the Aluminum Nitride Crystal
Figure 3-13 shows that the surface has many scratches, which is the dark region. The
dark hold at the center is a dent that the gold coating layer has been removed. Bubble like
structure on the surface can be found on the non-scratched region. The image gives similar
topography information compare to the optical profilometry diagram on Figure 3-6.
24
Figure 3-14: Representative Energy Spectrum of the Aluminum Nitride Crystal
The EDS spectrum of the AlN confirms the existence of aluminum with gold coating.
The nitrogen peak cannot be found because the expected intensity of the peak is small that is
shaded by the strong peak. The chromium peak measured shows the AlN crystal was prepared in
chromium rich environment. However, the carbon and oxygen peak shows that the surface may
be contaminated by organics.
25
Figure 3-15: Representative Image of the First Yttrium Calcium Oxoborate Crystal
Figure 3-15 shows that the surface has bubbled structures grow on the big grain. The
center region has many layers accumulated, which provides the same topography information as
the optical profilometry.
26
Figure 3-16: Representative Energy Spectrum of the First Yttrium Calcium Oxoborate Crystal
The EDS spectrum of the first YCOB confirms the existence of yttrium, calcium and
oxygen. A small boron peak can be found close to the left edge of the spectrum. Platinum and
palladium, which is commonly used as coating, can be found in the spectrum at the same time.
27
Figure 3-17: Representative Image of the Second Yttrium Calcium Oxoborate Crystal
Figure 3-17 shows that the surface has many continuous grains which makes it smooth.
However, the layered region on the top left and bubbled structure give the supporting points for
further surface treatment. Some sharp changes in height can be found at the center area.
28
Figure 3-18: Representative Energy Spectrum of the Second Yttrium Calcium Oxoborate Crystal
The EDS spectrum of the second YCOB also confirms the existence of yttrium, calcium,
boron and oxygen. The iridium is a layer of conductive coating with thickness 𝑑 = 5.53𝑛𝑚. The
second YCOB crystal is the only crystal that coated in Penn State whose coating thickness can be
given during the characterization. The weight percent shown in the spectrum is the same as the
atomic weight percent in pure YCOB, when the carbon contamination is not considered.
29
Figure 3-19: Representative Image of the Lanthanum Gallium Silicate Crystal
Figure 3-19 shows that most regions of the surface are smooth other than the cliff area at
the center of the image. However, the surface has less bubbled structure compare to the previous
samples. The cliff area shown in the image may be one of the dark spots which obtained in Figure
3-9.
30
Figure 3-20: Representative Energy Spectrum of the Lanthanum Gallium Silicate Crystal
The EDS spectrum of the CTGS confirms the existence of calcium, tantalum, gallium,
silicon and oxygen. The surface was coated by platinum and palladium, which also appear in the
EDS spectrum. Carbon contamination can be found.
31
Figure 3-21: Representative Image of the Lanthanum Gallium Tantalate Crystal
Figure 3-21 shows the local region of the surface is separated into two parts. The left part
of the image is relative smooth with small bubbled structures, while the right part is rough with
many layered structures.
32
Figure 3-22: Representative Energy Spectrum of the Lanthanum Gallium Tantalate Crystal
The EDS spectrum of the LGT confirms the existence of lanthanum, gallium, tantalum,
and oxygen. The surface was coated by platinum and palladium, whose peak can be found in the
spectrum. The gold peak is almost invisible, but the weight percent given in the spectrum shows
the crystal may be coated by a thin layer of gold before.
These SEM images provide the intuitive view of the surface, while the EDS spectrums
provide the element of the crystals. The structures found on the surface confirm the roughness
obtained from the optical profilometry, and the capability for further surface treatment. More
SEM images are given in Appendix B.
Focused ion beam (FIB) is a milling technique which can be installed in the SEM system.
When ion beam hits the surface, local sputtering will be made which removes material. Then,
cross-section can be imaged by SEM.
33
Figure 3-23: Cross-section of Yttrium Calcium Oxoborate
Figure 3-23 is the SEM image which is tilted 52° to the surface normal direction. The
continuous flat region is the cross-section polished by the ion beam, which confirms the expected
single-phase structure of the crystal. The multilayer structure is the bottom of the milling region
which is perpendicular to the cross section and cannot be polished properly. Some contamination
can be found above the surface which is expected have no influence on the material property.
The five crystals were characterized by the X-ray diffraction for structure, the optical
profilometry for topography, the nanoindentation for mechanical parameters and imaged by SEM.
These data will be used after the crystals are irradiated to identify any changes.
Chapter 4
Conclusion
Five crystals were fully characterized to obtain surface topography and crystal
orientations, elemental analysis and the mechanical parameters from nanoindentation:
1. Single crystal orientation was found from the X-ray diffraction for all the five
crystals, as well as the deformation of the crystal lattice. Furthermore, the FIB shows
that the subsurface of the second YCOB crystal is continuous with no visible
boundary. These characterizations data show that the crystals have big grain size,
which is the expected structure that can provide good piezoelectric response.
2. Optical profilometry and SEM images provided surface information, especially the
surface topography.
3. Elemental analysis from EDS qualitatively confirmed the composition of these
materials.
4. Mechanical properties measurement by nanoindentation was only performed on one
of the YCOB crystal because the technique is destructive. Hardness and modulus of
the crystal were obtained and used to calculate the residual stress.
Further work will be performed in post radiation characterization on these materials.
These works will be done in collaboration with other institutions and researchers.
The ultimate goal of the work is to develop the in-vessel reactor sensors, for vibration
sensing, temperature and pressure sensing, structural tests and radiation intensity sensing. The
current work on surface characterizing and bulk crystal characterizing are useful to identify the
changes after the radiation. After these experiments, the development of protective coating layer
on the piezoelectric crystals for reactor monitoring will start.
References
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38
Appendix A
Optical Profilometry Diagrams of Piezoelectric Crystals
Figure A-1: Surface Roughness of the Aluminum Nitride Crystal 1
Figure A-2: Surface Roughness of the Aluminum Nitride Crystal 2
39
Figure A-3: Surface Roughness of the Aluminum Nitride Crystal 3
Figure A-4: Surface Roughness of the Aluminum Nitride Crystal 4
40
Figure A-5: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 1
Figure A-6: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 2
41
Figure A-7: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 3
Figure A-8: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 4
42
Figure A-9: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 1
Figure A-10: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 2
43
Figure A-11: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 3
Figure A-12: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 4
44
Figure A-13: Surface Roughness of the Lanthanum Gallium Silicate Crystal 1
Figure A-14: Surface Roughness of the Lanthanum Gallium Silicate Crystal 2
45
Figure A-15: Surface Roughness of the Lanthanum Gallium Silicate Crystal 3
Figure A-16: Surface Roughness of the Lanthanum Gallium Silicate Crystal 4
46
Figure A-17: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 1
Figure A-18: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 2
47
Figure A-19: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 3
Figure A-20: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 4
48
Appendix B
Scanning Electron Microscope Images of Piezoelectric Crystals
Figure B-1: Image of the Aluminum Nitride Crystal 1
49
Figure B-2: Image of the Aluminum Nitride Crystal 2
50
Figure B-3: Image of the Aluminum Nitride Crystal 3
51
Figure B-4: Image of the First Yttrium Calcium Oxoborate Crystal 1
52
Figure B-5: Image of the First Yttrium Calcium Oxoborate Crystal 2
53
Figure B-6: Image of the First Yttrium Calcium Oxoborate Crystal 3
54
Figure B-7: Image of the Second Yttrium Calcium Oxoborate Crystal 1
55
Figure B-8: Image of the Second Yttrium Calcium Oxoborate Crystal 2
56
Figure B-9: Image of the Second Yttrium Calcium Oxoborate Crystal 3
57
Figure B-10: Image of the Second Yttrium Calcium Oxoborate Crystal 4
58
Figure B-11: Image of the Second Yttrium Calcium Oxoborate Crystal 5
59
Figure B-12: Image of the Second Yttrium Calcium Oxoborate Crystal 6
60
Figure B-13: Image of the Second Yttrium Calcium Oxoborate Crystal 7
61
Figure B-14: Image of the Second Yttrium Calcium Oxoborate Crystal 8
62
Figure B-15: Image of the Second Yttrium Calcium Oxoborate Crystal 9
63
Figure B-16: Image of the Lanthanum Gallium Silicate Crystal 1
64
Figure B-17: Image of the Lanthanum Gallium Silicate Crystal 2
65
Figure B-18: Image of the Lanthanum Gallium Silicate Crystal 3
66
Figure B-19: Image of the Lanthanum Gallium Silicate Crystal 4
67
Figure B-20: Image of the Lanthanum Gallium Silicate Crystal 5
68
Figure B-21: Image of the Lanthanum Gallium Silicate Crystal 6
69
Figure B-22: Image of the Lanthanum Gallium Tantalate Crystal 1
70
Figure B-23: Image of the Lanthanum Gallium Tantalate Crystal 2
71
Figure B-24: Image of the Lanthanum Gallium Tantalate Crystal 3
72
Figure B-25: Image of the Lanthanum Gallium Tantalate Crystal 4
73
Figure B-26: Image of the Lanthanum Gallium Tantalate Crystal 5
74
Figure B-27: Image of the Lanthanum Gallium Tantalate Crystal 6
75
Figure B-28: Image of the Lanthanum Gallium Tantalate Crystal 7
76
Figure B-29: Image of the Lanthanum Gallium Tantalate Crystal 8
77
Figure B-30: Image of the Lanthanum Gallium Tantalate Crystal 9