Synthesis and Thermoelectric Studies in Cr Doped Cobalt Ferrites Nano Particles

M. Anis-ur-Rehman1,a, Mariam Ansari1, Zeb-un-Nisa Mughal1, M.S. Awan1 and A. Maqsood2,b

1Applied Thermal Physics Laboratory, Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan

2Thermal Transport Laboratory, SC & ME, National University of Sciences and Technology, Islamabad, Pakistan

amarehman@comsats.edu.pk, btpl.qau@usa.net

Keywords: Ferrites, crystallite size, AC electrical conductivity, drift mobility, seebeck coefficient, thermo-power

Abstract: Samples of Cr doped cobalt ferrite were prepared by co-precipitation route. These particles were characterized by X-ray diffraction (XRD) at room temperature. The structural properties were observed before and after sintering. The FCC spinel structure was confirmed by XRD patterns of the samples. The crystallite sizes lie in the range of 37-60 nm. DC electrical properties as a function composition were measured. Scanning electron microscopy was used in order to investigate the surface morphology of the prepared samples. The system for thermoelectric power measurement was designed, developed and calibrated in the laboratory. The room temperature thermoelectric power was measured for the prepared samples. The magnitude of Seebeck coefficient depends on the composition and resistivity of the samples. The obtained values of Seebeck coefficient for CoFe2O4 are in good agreement to the reported values. Determined values of Seebeck coefficient for other studied compositions are an addition to the literature.

Introduction

Ferrites having high resistivity and low eddy current losses have been found useful for technological applications as in the case of stress sensors and recording media. Cobalt ferrite possesses an inverse spinel structure [1]. The physical properties of the spinel ferrites are developed due to distribution of cations among the tetrahedral A site and octahedral B sites. The electrical and thermal properties of the ferrites depend on preparation method, chemical composition and grain size, frequency and temperature [2]. DC resistivity of ferrites as a function of temperature and drift mobility and activation energy was reported by different authors [3]. Several researchers reported the magnetic properties of spinel ferrites as well [4-7]. Thermoelectric measurements are done to know about the conduction mechanism in ferrites. The thermo emf and its sign gives appropriate information about the type of conduction in semiconductors, i.e., they are p-type or n-type. There has been sizeable interest during the past years in searching new materials and structures for use in clear, highly efficient cooling and energy conversion systems [8-10].

Experimental Methods

Samples of CoFe2-xCrxO4 spinel ferrites having fine particles with x varying from 0.0 to 0.5, was prepared by co-precipitation method. The analytical grade chemical reagents used in this work were nitrates of ferric, cobalt and chromium. The aqueous solutions of the chemicals were mixed, in alkaline medium. NaOH was used as co-precipitation agent. By fixing the co-precipitation step, the size of the precipitated particles was controlled. The pH of the reactions was kept about 13. The precipitates were thoroughly washed a number of times with distilled water. The dried form of the samples was homogenously grinded and was converted into pellets for further characterization. The pellets were sintered at 750 0C for 2 hours.

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Results and Discussion

Structural and morphological properties: The XRD patterns in Fig. 1(as prepared) and Fig. 2 (after sintering) show that the structure is cubic spinel structure with no additional phase (ICSD 01-076-2496).

Fig. 1. XRD patterns of CoFe2-xCrxO4 where X=0, 0.1, 0.2, 0.3, 0.4 and 0.5 (as prepared)

Fig. 2. XRD patterns for CoFe2-xCrxO4 where x= 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 (sintered)

The decrease in ‘a’ was due to Cr concentration. Ionic radius of Cr+3 is 0.63 Ȧ which was less than the ionic radius of Fe+3 which is 0.645 Ȧ [11]. The size of octahedral B site was greater than tetrahedral A site. The addition of smaller ionic radii at B site results in decrease in lattice constant. For structural morphology scanning electron microscopy was used. The SEM micrographs are given in Fig. 3.

30 35 40 45 50 55 60 65

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

x=0.5

x=0.4

x=0.3

x=0.2

x=0.0

x=0.1

(511)

(440)

(422)

(400)(3

11)

(220)

Int. (arb. units)

2θ θ θ θ (deg.)

Table 2. Variation in crystallite sizes and lattice constants with Cr concentration ‘x’ in

CoFe2-xCrxO4 sintered samples

Composition x

Crystallite size nm

Lattice constant ±0.01 Ȧ

0.5 60±3 8.30

0.4 51±3 8.32

0.3 56±1 8.35

0.2 40±3 8.36

0.1 37±2 8.36

0 52±3 8.38

Table 1. Variation in crystallite sizes and lattice constants with Cr concentration ‘x’

in CoFe2-xCrxO4 (as prepared samples)

Composition x

Crystallite size nm

Lattice constant ±0.01 Ȧ

0.5 39±3 8.41

0.4 38±2 8.38

0.3 29±2 8.38

0.2 34±3 8.41

0.1 31±2 8.24

0 18±2 8.25

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Fig. 3. SEM micrographs of samples of CoFe2-xCrxO4 (a)x=0.1, (b) x=0.2, (c) x=0.3

DC electrical properties: DC electrical resistivity for all the prepared samples was measured by two probe method. The observed decrease in the value of dc-electrical resistivity with increase in temperature shows the similar trend as that of semiconductor materials. Also with increase in Cr concentration an increasing trend in the resistivity is observed and is shown in Fig. 4. This is due to higher resistance of Cr as compared to the Fe.

0.0 0.1 0.2 0.3 0.4 0.5-1x10

4

0

1x104

2x104

3x104

4x104

ρ

ρ

ρ

ρ

(( (( ΩΩ ΩΩ-c

m)

Composition (x)

Fig. 4. Graph between resistivity and composition for CoFe2-xCrxO4

where x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5

Exchange of electrons between Fe+2 and Fe+3 result in conduction mechanism in ferrites. So as a result there is local displacement of charges causing polarization. The magnitude of this exchange depends on Fe+3and Fe+2 ion pairs on octahedral sites [11]. The decrease in activation energy is due to reduction of ion pairs. This is due to enhanced mobility of the charge carriers due to thermal activation and it is not due to generation of charge carriers by increase of temperature. Also charge carrier concentration is reported to be constant throughout the temperature range [11].

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Thermoelectric properties Calibration: Thermoelectric power or Seebeck coefficient measurement system was developed in the laboratory. The main components of the thermoelectric power system are heater, power supply, thermocouple, digital multimeter and interfacing to computer.

Interfacing

Hot end

Keithley meter

30 V

V

Cold end

T3

T2T1

Fig. 5. Block diagram of thermoelectric power apparatus

The block diagram of the system is shown in Fig. 5. The sample was clamped in sample holder as shown in Fig. 5. Temperature gradient was created by heating at one end called hot end using dc supply, while the other end was at room temperature as mentioned in Fig. 5. Due to temperature gradient emf was produced according to Seebeck effect. Hot end temperature T1, cold end temperature T2, room temperature T3 and voltage V produced were measured by digital multimeter, and data was collected on computer by interfacing of multimeter. Calibration of the developed setup was checked by using a copper and an aluminum sample. The Average Seebeck Coefficient calculated for copper is 1.67µV/K. The standard deviation of the Seebeck Coefficient calculated is 9.98x10-7 V/K. Already Reported value of the Seebeck Coefficient [12] at the Average Temperature of 27 ºC is 1.8 µV/K. The Average Value of the Seebeck Coefficient that was calculated in our case is 1.67µV/K at the corresponding room temperature of 20ºC. The experimentally measured (-1.23µV/K) and reported values (-1.8µV/K) of the aluminum and copper are almost similar. The trend and behavior of the measured values and the sign of Seebeck coefficient is also same as reported in the literature. The Error or difference in the measured and already reported values may be due to the material purity, surrounding atmosphere and sample temperature. The Aluminum sample used in the experiment has some impurities. If the purest form of samples are used the error value can be minimized. Measurements: Thermoelectric effects give conversion of energy between heat and electricity directly. Thermoelectric power of Cr doped Co ferrite nanoparticles as a function of composition was measured at room temperature. The conduction mechanism in Cr doped Co ferrite is due to electrons because sign of Seebeck is negative for all the prepared samples. Seebeck coefficient as a function of Cr concentration for all the prepared samples is given in Fig. 6. All the samples were sintered at 750 0C. The decrease in Seebeck coefficient ‘S’ is due to fact that if resistivity of the material increases, motion of charges becomes slow and as a result of which emf developed across the sample was low. Due to decrease in emf the Seebeck coefficient also decreases, as observed in samples with x=0.0-0.2. Further increase in x resulted in decrease in resistivity and that affects the ‘S’ also by showing a dip in Fig. 6 at x=0.2.

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0.0 0.1 0.2 0.3 0.4 0.5

-700

-600

-500

-400

-300

-200

S (

µµ µµV/K

)

Composition x

Fig. 6. Variation of Seebeck coefficient ‘S’ with composition CoFe2-xCrxO4 where x=0-0.5

Conclusions

Nanoparticles of Cr doped Co ferrite CoFe2-xCrxO4 where x=0 to 0.5 were prepared. These nanoparticles were prepared by co-precipitation method and were homogenous. The addition of Cr in Co ferrite resulted in structure, electrical, and thermoelectric properties. The patterns of XRD show that the structures of all the samples were cubic spinel. The lattice constant ‘a’ decreased with the increase in the concentration of Cr. DC electrical properties of the samples were done by two probe method. The resistivity for the prepared samples was increased with increase in Cr concentration. Seebeck coefficients show that charge carriers were electrons in our prepared samples. The decrease in Seebeck coefficient was due to increase in resistivity for that particular composition. The reported value of the Seebeck coefficient for the cobalt ferrite was in good agreement with our measured value. The Seebeck coefficients for the other compositions are an addition to the existing literature.

Acknowledgement

Higher Education Commission is highly acknowledged for the financial support through the project funded under National Research Program for Universities (NRPU#893).

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