Electrical Properties of Cadmium Substitution in Nickel Ferrites ()
1. Introduction
The conditions of the preparation play an important role in site preference of the cations and all other properties of the spinal ferrites. The characteristic physical properties from the ability of the cation were distributed among the available tetrahedral A-site and octahedral B-site. The NiFe2O4 is completely inverse spine, because Ni2+ has a strong preference for the electrical conductivity octahedral site (B-site) [1]. Cadmium is the non magnetic divalent ions which occupy essentially tetrahedral A-site [2] when substituted in ferrites.
As far as the authors are aware of the study, the ac electrical conductivity and thermo electric power of the mixed Ni-Cd ferrites play as a function of composition and temperature. Therefore a systematic study of the electrical conductivity and thermo electric power of the mixed Ni-Cd ferrite system from room temperature to well beyond Curie temperature was undertaken. The results of such a study presented in this communication are explained on the basis of the hopping model.
2. Experimental
2.1. Materials Preparation
Specimens of the Ni-Cd ferrites having the compositional formula Ni1–xCdx Fe2O4 where x = 0.0, 0.2, 0.4 were prepared by solgel method. The samples were presintered for 24 hr in air at 600˚C.
2.2. Methods
Electrical conductivity measurements were made by the two probe method [3] from room temperature to well beyond the Curie temperature using a Keithley electrometer model 6517A. The electrical conductivity (σ) of the Ni-Cd ferrites under investigation has been computed using the formula,
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 cm and A denotes the area of the sample in cm2.
Thermo electric power studies were carried out as a function of composition and temperature by the differential method [4]. The sample holder for measuring the thermo emf consists of 2 pairs of non magnetic copper electrodes between which the sample is firmly fixed to the upper electrode for additional heating to maintain a temperature difference about 10 K between the 2 faces of the sample. A temperature of both surfaces of the sample was measured by 2 Chromel-Alumel thermo couples.
Thermo electric power (S) was calculated using the relation
where ΔE is the thermo emf produced across the sample due to the temperature difference ΔT.
3. Results and Discussions
3.1. Composition Dependence of Lattice Parameter (a)
X-Ray Diffraction of all the ferrites under investigation has been obtained using Ni Kα radiation. The XRD for Ni-Cd ferrites are given in Figures 1-3. The lattice parameters of the mixed ferrites were calculated from d-spacings and are given in Table 1. It can be seen from the table that the value of lattice parameter increases with increase of cadmium content. The variation in the lattice parameter with Cd content is presented in Figure 4 (i.e., lattice parameter versus Cd content). It can be seen from the figure that the lattice parameter increases with increase in a Cd content.
It may be observed from the figure that the lattice parameter varies linearly with the Cd ion content. A similar linear dependence has also been observed in Ni-Zn [5], Mn-Zn [6], Li-Cd [7], Cd-Co [8] ferrites.
3.2. Composition Dependence of Seebeck Coefficient (S)
The values of the Seebeck coefficient of all mixed Ni-Cd ferrites were calculated from the observed values of the thermo emf and are given in Table 2. It can be seen that the sign of the Seebeck coefficient is negative for x = 0 and 0.2 compositions of the ferrites under investigation, indicating that the predominant conduction mechanism in these ferrites is of n-type semiconductors. From the observation, it follows that the predominant conduction mechanism in all the Ni-Cd ferrites is hopping of electrons [9] from Fe2+ to Fe3+ ions.
Fe3+ + e → Fe2+
Figure 1. X-ray diffraction pattern for NiFe2O4 ferrite.
Figure 2. X-ray diffraction pattern for Ni0.8Cd0.2Fe2O4 ferrite.
Figure 3. X-ray diffraction pattern for Ni0.6Cd0.4Fe2O4 ferrite.
Figure 4. Variation of lattice parameter with cadmium content.
Table 1. Lattice parameter, density, crystalline size and volume data for Ni-Cd ferrites.
And the composition 0.4 for the Seebeck coefficient is positive and its conduction mechanism in these ferrites is of p-type semiconductors and it follows the hopping of holes.
The values of S are found to increase continuously from 0.0 to 0.2 μV/K, with addition of cadmium from 0.0 to 0.4 mol.
The variation of the Seebeck coefficient (S) with temperature is shown in Figure 5. It can be seen that the value of Seebeck coefficient increases with increase in temperature up to a certain temperature, which is designnated as the Seebeck coefficient transition temperature Ts(K). However, beyond this temperature, the value of the Seebeck coefficient was found to decrease with increasing temperature. The negative value of the Seebeck coefficient was found over the entire temperature range studied and shows the majority charge carriers are electrons. In the case of positive value of the Seebeck coefficient were explained the majority charge carriers are holes and these results designated in Figure 6.
Table 2. Thermoelectric power data for mixed Ni-Cd ferrites.
Figure 5. Plot of Seebeck coefficient (S) verses temperature for Ni1−xCdxFe2O4 Ferrites (Where x = 0.0 and 0.2).
Figure 6. Plot of Seebeck coefficient (S) verses temperature for Ni0.6Cd0.2Fe2O4.
The values of Ts(K) for each composition are given in Table 2. The curie is for the purpose of comparison. It can be seen that the value of Ts(K) and Tc(K) are in good agreement. The transition temperatures obtained in the present investigation in all the mixed Ni-Cd ferrites may be attributed to ferrimagnetic to paramagnetic transitions. A similar result has been reported by Whall et al. [10,11] in the case of Manganese and Lithium ferrites.
3.3. Electrical Conductivity (σ)
Table 3 gives the experimental values of electrical conductivity and Seebeck coefficient values at room temperature. The carrier concentration for these ferrites was calculated using the formula given by morin and gebella [12].
where S is seebeck coefficient; e is charge of electron; k is Boltzmann constant; N is density of states or concentration of electronic levels involved in the conduction process.
An examination of Table 3 reveals that the electrical conductivity of the mixed Ni-Cd ferrites is found to decrease with an increase in cadmium content to 0.4 mol.
This shows that Ni0.6Cd0.4Fe2O4 poses the highest resistivity among all the compositions investigated from the Table 3 that the Ni0.6Cd0.4Fe2O4 composition has the high electrical conductivity correspondingly, the high electrical conductivity for this specimen has a value of 2.6469 × 10–5 ohm–1cm–1. This high value can be explained on the basis of the fact that it has a maximal no. of ferrous ions whose exchange Fe2+ ó Fe3+ gives rise to a maximal electrical conductivity. Table 3 reveals that the variation of the electrical conductivity of Cu-Cd ferrites runs parallel to the variation of available ferrous ions on octahedral sites. It is significant to note that Ni1–x CdxFe2O4, has the lowest ferrous ion concentration and possesses the lowest electrical conductivity. It is also pertinent to mention that the variation of electrical conductivity with the composition Table 3 runs parallel to the variation of the ferrous ions concentration [13]. Thus it is the number of ferrous ions on octahedral sites that place a dominant role in the process of conduction. This result is in agreement with the assumption of Rabankin and Novikova [13]. The temperature dependence of electrical conductivity of mixed Ni-Cd ferrites of different compositions has been investigated from room temperature to well beyond the Curie temperature. Figures 7-9 gives
Table 3. Electrical conductivity for Ni-Cd ferrites at room temperature.
Figure 7. Plot of Log(σT) versus 103/T for NiFe2O4.
Figure 8. Plot of Log(σT) versus 103/T for Ni0.8Cd0.2Fe2O4.
Figure 9. Plot of Log(σT) versus 103/T for Ni0.6Cd0.4Fe2O4.
gives a plot of log(σT) versus 103/T. It can be seen that the value of log(σT) increases linearly with increaseing temperature up to a certain temperature Ts(K) at which a change of slope has occurred.
The activation energies in the ferromagnetic and paramagnetic regions are calculated from the slopes of plots of log(σT) versus 103/T and presented in Table 4. The activation energy in the paramagnetic region is higher than that in the ferromagnetic region.
A high activation energy goes hand in hand with a low conductivity of the ferrites. Similar result was observed by samokhralov and rustmov [14]. The activation energy decreases with increase in cadmium content. The magnitude of the kink as determined by the difference between the activation energies in the paramagnetic and ferromagnetic regions (ΔE) which is given in column (5) of Table 4. Column 5 of the Table 3 gives the values of resistivity for the specimens under investigation. An inspection of these results reveals that (ΔE) decreases as resistivity decreases.
Generally, the change of slope is attributed to change in conductivity mechanism. The conduction at a lower temperature ( below curie temperature) is due to hopping of electrons [15] between Fe2+ and Fe3+ ions, where as at a higher temperature (above curie temperature ) due to hopping of polarons [16-18]. The calculated values of activation energy in a paramagnetic region (E2) are greater than 0.40 eV which clearly suggest that the conduction is due to hopping of polarons.
3.4. charge Carrier Mobility (μ)
The charge carrier mobility (μ) of mixed Ni-Cd ferrites was calculated from the experimental values of electrical conductivity (σ) and charge carrier concentration (n) using the relation.
where e is the exchange of electron.
The calculated values of the mobility are given in Table 4. It can be seen that the value of the mobility varies from 4.95 × 10–6 to 1.65 × 10–9 cm2/VS. Such low values of mobility have been reported by several researches [19-22]. The variation of charge carrier mobility values increases continuously with increasing temperature. The increase in mobility with an increase in temperature suggests that the conduction in these ferrites is due to a hopping mechanism of electrons from Fe2+ to Fe3+.
3.5. Temperature Variation of Charge Carrier Concentration (n)
The temperature variation of charge carrier concentration is shown in Figures 10-12. It can be seen from the figures that the charge carrier concentration increases
Table 4. Activation energies, Charge carrier concentration and mobility for Ni-Cd ferrites.
Figure 10. Variation of charge carrier concentration with temperature for Ni0.8Cd0.2Fe2O4.
Figure 11. Variation of charge carrier concentration with temperature for NiFe2O4.
Figure 12. Variation of charge carrier concentration with temperature for Ni0.6Cd0.4Fe2O4.
with increase in temperature continuously up to Tn(K). However above this temperature the carrier concentration decreases continuously for all the ferrites. The values of Tn(K) for all the ferrites under investigation are listed and compared with their respective curie temperatures in Table 2. It can be seen that values of Tn(K) and Tc(K) are in good agreement. Similar variations of n and T were also observed by Reddy and Mulay et al. in the case of Li-Ti [23] and Mg-Al [24] ferrites.
4. Acknowledgements
The authors are grateful to Prof. Naidu Ashoak, Principal Nizam college, Osmania University and Prof. P. Kistaiah, Head, Department of Physics, Osmania University for their encouragement to carry out research work.