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Room temperature electric properties of cadmium-substituted nickel ferrites
Materials Letters 57 (2003) 4040 – 4042 www.elsevier.com/locate/matlet
Room temperature electric properties of cadmium-substituted nickel ferrites D. Ravinder *, S. Srinivasa Rao, P. Shalini Department of Physics, Osmania University, Hyderabad-500 007, A.P., India Received 15 October 2002; accepted 22 January 2003
Abstract Electric properties such as electrical conductivity and thermoelectric power studies of Ni – Cd ferrites of various compositions were investigated at room temperature. The Seebeck coefficient is negative for all the ferrites, showing that these ferrites behave as n-type semiconductors. On the basis of these results, an explanation for the conduction mechanism in Ni – Cd mixed ferrites is suggested. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Electrical conductivity; Thermoelectric power; Ni – Cd ferrites
1. Introduction
2. Experimental details
Substituted nickel ferrites have been the subject of extensive investigation because of their microwave applications. The elastic and magnetic properties of mixed Ni– Cd ferrites have been studied in detail [1,2]. As per authors knowledge, no information is available on electric properties of cadmium substituted nickel ferrites in the literature. Moreover, there is a need for a thorough study of electric properties of cadmium-substituted nickel ferrites as a function of composition at room temperature. The results of such study are presented in this communication.
2.1. Mixed Ni– Cd ferrites having the compositional formula
* Corresponding author. Department of Physics, Post-Graduate College of Science, Osmania University, Hyderabad-500 007, A.P., India. Tel.: +91-40-7175257; fax: +91-40-7017944. E-mail address: [email protected] (D. Ravinder).
Ni1 xCdxFe2O4 (where x = 0.2, 0.4, 0.6, and 0.8) were prepared by double sintering method. The details of the method of the preparation have been given in an earlier publication [3]. The Seebeck coefficients were measured by a differential method [4,5] at room temperature. The temperature gradient across the sample was measured using two pairs of copper constant thermocouples. The sample was mounted on top of two well-separated copper blocks with silver paint. The temperature difference between the two ends of the sample was kept at 10 K throughout the measured temperature range. To eliminate the effects of the reference leads, the absolute thermoelectric power of Cu was subtracted from the measured thermoelectric voltage. A Keithly 181
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00089-2
D. Ravinder et al. / Materials Letters 57 (2003) 4040–4042 Table 1 Experimental data on mixed Ni – Cd ferrites at room temperature Sl. no.
Ferrite
(1) (2) (3) (4)
Ni0.8Cd0.2Fe2O4 Ni0.6Cd0.4Fe2O4 Ni0.4Cd0.6Fe2O4 Ni0.2Cd0.8Fe2O4
Electrical conductivity (r) (V 1 cm 1)
Seebeck coefficient (S) Av/K
1.00 10 4 7.20 10 6 2.13 10 7 3.96 10 8
1093 980 748 704
nanovoltmeter was used for the voltage measurements. The thermoelectric power or Seebeck coefficient S was calculated using the relation S¼
DE DT
where DE is the thermo-e.m.f. produced across the sample due to the temperature different DT. The electrical conductivity of the samples has been measured at room temperature by the two-probe method [6], using a Keithley electrometer model 6517A. The electrical conductivity (r) of the Ni – Cd ferrites
4041
under investigation has been computed using the formula, r¼
It VA
where I, is the current passing through the specimen in amperes, V is the voltage applied to the specimen in volts, t is the thickness of the sample in centimeters and A denotes the area of the sample in square centimeters.
3. Results and discussion Experimental data on mixed Ni – Cd ferrites are given Table 1, which includes the compositional formulae of all the ferrites under investigation and the values of electrical conductivity at room temperature. It can be seen from the table that the value of electrical conductivity varies from 3.96 10 8 to 1.00 10 4 V 1 cm 1 and decreases continuously with increase in the cadmium content. This observation is in agreement with the result reported by Rezlescu et al. [7] who found that the resistivity of Li – Zn ferrites increased with the increase of zinc content. Among all the Ni – Cd
Fig. 1. Plot of electrical conductivity (r) versus composition for Ni – Cd ferrites.
4042
D. Ravinder et al. / Materials Letters 57 (2003) 4040–4042
Fig. 2. Plot of Seebeck coefficient (S) versus composition for Ni – Cd ferrites.
ferrites, the specimen with the composition Ni0.2Cd0.8 Fe2O4 exhibits the highest value of electrical resistivity (q = 2.53 107 V cm). Plot of electrical conductivity Vs cadmium composition is shown in Fig. 1. It can be seen from the figure that the value of electrical conductivity decreases with the increase of cadmium composition. The values of Seebeck coefficient (S) at room temperature, computed from the measured values of thermo-e.m.f., are given in Table 1. It can be seen from the table that the sign of the Seebeck coefficient is negative for all the Ni – Cd ferrites. On the basis of its negative sign, the cadmium-substituted nickel ferrites have been classified as n-type semiconductors. Thus, the conduction mechanism in these ferrites is predominantly due to hopping of electrons [8] from Fe2 + to Fe3 + ions. Fe3þ þ eZFe2þ It can also be seen from the table that the value of Seebeck coefficient varies from 1093 to 704 Av/ K as the cadmium content increases from 0.2 to 0.8 mol. The variation of Seebeck coefficient with composition is shown in Fig. 2. It can be seen from the
figure that the value of S is found to decrease with the increase of cadmium content. Acknowledgements The authors are grateful to Prof. M. Narsimha Chary, Head, Department of Physics and Prof. M. Prabhakar, Principal, Dr. G. Prasad, Post-Graduate College of Science, Saifabad, Osmania University for their encouragement. References [1] A. Globus, H. Pascard, V. Cagan, J. Physique, Colloq. 38 (1977) C1. [2] D. Ravinder, A. Manga, Mater. Lett. 41 (1999) 254. [3] D. Ravinder, T. Seshagiri Rao, Cryst. Res. Technol. 25 (1990) 963. [4] Z. Simsa, Czech. J. Phys. B16 (1996) 919. [5] V.D. Reddy, M.A. Malik, P.V. Reddy, Mater. Sci. Eng., B 8 (1991) 295. [6] V.R.K. Murthy, J. Sobhanadri, Phys. Status Solidi, A 38 (1976) 647. [7] N. Rezlescu, D. Condurach, P. Petairu, E. Luca, J. Am. Ceram. Soc. 57 (1974) 40. [8] L.G. Van Uitert, J. Chem. Phys. 23 (1955) 1883.
Room temperature electric properties of cadmium-substituted nickel ferrites D. Ravinder *, S. Srinivasa Rao, P. Shalini Department of Physics, Osmania University, Hyderabad-500 007, A.P., India Received 15 October 2002; accepted 22 January 2003
Abstract Electric properties such as electrical conductivity and thermoelectric power studies of Ni – Cd ferrites of various compositions were investigated at room temperature. The Seebeck coefficient is negative for all the ferrites, showing that these ferrites behave as n-type semiconductors. On the basis of these results, an explanation for the conduction mechanism in Ni – Cd mixed ferrites is suggested. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Electrical conductivity; Thermoelectric power; Ni – Cd ferrites
1. Introduction
2. Experimental details
Substituted nickel ferrites have been the subject of extensive investigation because of their microwave applications. The elastic and magnetic properties of mixed Ni– Cd ferrites have been studied in detail [1,2]. As per authors knowledge, no information is available on electric properties of cadmium substituted nickel ferrites in the literature. Moreover, there is a need for a thorough study of electric properties of cadmium-substituted nickel ferrites as a function of composition at room temperature. The results of such study are presented in this communication.
2.1. Mixed Ni– Cd ferrites having the compositional formula
* Corresponding author. Department of Physics, Post-Graduate College of Science, Osmania University, Hyderabad-500 007, A.P., India. Tel.: +91-40-7175257; fax: +91-40-7017944. E-mail address: [email protected] (D. Ravinder).
Ni1 xCdxFe2O4 (where x = 0.2, 0.4, 0.6, and 0.8) were prepared by double sintering method. The details of the method of the preparation have been given in an earlier publication [3]. The Seebeck coefficients were measured by a differential method [4,5] at room temperature. The temperature gradient across the sample was measured using two pairs of copper constant thermocouples. The sample was mounted on top of two well-separated copper blocks with silver paint. The temperature difference between the two ends of the sample was kept at 10 K throughout the measured temperature range. To eliminate the effects of the reference leads, the absolute thermoelectric power of Cu was subtracted from the measured thermoelectric voltage. A Keithly 181
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00089-2
D. Ravinder et al. / Materials Letters 57 (2003) 4040–4042 Table 1 Experimental data on mixed Ni – Cd ferrites at room temperature Sl. no.
Ferrite
(1) (2) (3) (4)
Ni0.8Cd0.2Fe2O4 Ni0.6Cd0.4Fe2O4 Ni0.4Cd0.6Fe2O4 Ni0.2Cd0.8Fe2O4
Electrical conductivity (r) (V 1 cm 1)
Seebeck coefficient (S) Av/K
1.00 10 4 7.20 10 6 2.13 10 7 3.96 10 8
1093 980 748 704
nanovoltmeter was used for the voltage measurements. The thermoelectric power or Seebeck coefficient S was calculated using the relation S¼
DE DT
where DE is the thermo-e.m.f. produced across the sample due to the temperature different DT. The electrical conductivity of the samples has been measured at room temperature by the two-probe method [6], using a Keithley electrometer model 6517A. The electrical conductivity (r) of the Ni – Cd ferrites
4041
under investigation has been computed using the formula, r¼
It VA
where I, is the current passing through the specimen in amperes, V is the voltage applied to the specimen in volts, t is the thickness of the sample in centimeters and A denotes the area of the sample in square centimeters.
3. Results and discussion Experimental data on mixed Ni – Cd ferrites are given Table 1, which includes the compositional formulae of all the ferrites under investigation and the values of electrical conductivity at room temperature. It can be seen from the table that the value of electrical conductivity varies from 3.96 10 8 to 1.00 10 4 V 1 cm 1 and decreases continuously with increase in the cadmium content. This observation is in agreement with the result reported by Rezlescu et al. [7] who found that the resistivity of Li – Zn ferrites increased with the increase of zinc content. Among all the Ni – Cd
Fig. 1. Plot of electrical conductivity (r) versus composition for Ni – Cd ferrites.
4042
D. Ravinder et al. / Materials Letters 57 (2003) 4040–4042
Fig. 2. Plot of Seebeck coefficient (S) versus composition for Ni – Cd ferrites.
ferrites, the specimen with the composition Ni0.2Cd0.8 Fe2O4 exhibits the highest value of electrical resistivity (q = 2.53 107 V cm). Plot of electrical conductivity Vs cadmium composition is shown in Fig. 1. It can be seen from the figure that the value of electrical conductivity decreases with the increase of cadmium composition. The values of Seebeck coefficient (S) at room temperature, computed from the measured values of thermo-e.m.f., are given in Table 1. It can be seen from the table that the sign of the Seebeck coefficient is negative for all the Ni – Cd ferrites. On the basis of its negative sign, the cadmium-substituted nickel ferrites have been classified as n-type semiconductors. Thus, the conduction mechanism in these ferrites is predominantly due to hopping of electrons [8] from Fe2 + to Fe3 + ions. Fe3þ þ eZFe2þ It can also be seen from the table that the value of Seebeck coefficient varies from 1093 to 704 Av/ K as the cadmium content increases from 0.2 to 0.8 mol. The variation of Seebeck coefficient with composition is shown in Fig. 2. It can be seen from the
figure that the value of S is found to decrease with the increase of cadmium content. Acknowledgements The authors are grateful to Prof. M. Narsimha Chary, Head, Department of Physics and Prof. M. Prabhakar, Principal, Dr. G. Prasad, Post-Graduate College of Science, Saifabad, Osmania University for their encouragement. References [1] A. Globus, H. Pascard, V. Cagan, J. Physique, Colloq. 38 (1977) C1. [2] D. Ravinder, A. Manga, Mater. Lett. 41 (1999) 254. [3] D. Ravinder, T. Seshagiri Rao, Cryst. Res. Technol. 25 (1990) 963. [4] Z. Simsa, Czech. J. Phys. B16 (1996) 919. [5] V.D. Reddy, M.A. Malik, P.V. Reddy, Mater. Sci. Eng., B 8 (1991) 295. [6] V.R.K. Murthy, J. Sobhanadri, Phys. Status Solidi, A 38 (1976) 647. [7] N. Rezlescu, D. Condurach, P. Petairu, E. Luca, J. Am. Ceram. Soc. 57 (1974) 40. [8] L.G. Van Uitert, J. Chem. Phys. 23 (1955) 1883.