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Effect of calcining temperature on electrical and dielectric properties of cadmium stannate
Applied Surface Science 255 (2009) 6675–6678
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of calcining temperature on electrical and dielectric properties of cadmium stannate V.S. Sawant, S.S. Shinde, R.J. Deokate, C.H. Bhosale, B.K. Chougule, K.Y. Rajpure * Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 October 2008 Received in revised form 10 February 2009 Accepted 20 February 2009 Available online 4 March 2009
The cadmium stannate samples were prepared by chemical co-precipitation method using stannic chloride pentahydrate (SnCl45H2O) and cadmium chloride (CdCl2) as precursors by carefully controlling the preparative parameters. The effect of calcining temperature on the phase, microstructure, morphological and electrical properties of cadmium stannate has been investigated. The Xray diffraction patterns indicate the conversion of rhombohedral to spinel cubic crystal structure and polycrystallinity of the samples. SEM study of Cd2SnO4 sample shows randomly distributed cubic crystals of varying sizes. The dc resistivity was measured as a function of temperature. The dielectric constant and dielectric loss were studied as a function of frequency. To understand the conduction mechanism in the samples AC conductivity was measured. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Cadmium stannate X-ray diffraction Microstructure dc resistivity Dielectric constant
1. Introduction
2. Experimental
Recently there has been great attention for investigating new gas-sensing materials and developing the properties of known gas-sensing materials. Most of these materials are metal oxides. Cadmium stannate shows quite attractive electrical properties [1,2]. Advantages of oxides for gas detections are high sensitivity, simple design, and low weight and cost. Recently synthesis and gas-sensing properties of perovskite CdSnO3 nanoparticles have been reported by Jia et al. [3]. CdSnO3 nanoparticles were prepared by a chemical co-precipitation method and they studied the selectivity to vapors of C2H5OH. Cd2SnO4, being one of the alternatives to the present generations transparent conducting oxides (TCOs); FTO and ITO, is synthesized in thin film form by various techniques [4,5]. Nozik [6] prepared the Cd2SnO4 amorphous thin films and studied the optical and electrical properties. Hashemi [7] has employed the investigation of chemical nature of samples of Cd2SnO4 in as-fired and electrochemically reduced and reoxidised states. Valincius et al. [8] reported the electrochemical properties of sol–gel prepared nanocrystalline electrodes of cadmium tin oxide films. We present here the formation of different powder samples in the Cd–Sn–O system by co-precipitation method having enhanced dielectric properties.
2.1. Preparation of samples The cadmium stannate samples were prepared by a chemical co-precipitation method using AR grade equimolar (0.1 M) stannic chloride pentahydrate (SnCl45H2O) and cadmium chloride (CdCl2) as precursors in 1:2 composition by volume in solution. The preparation conditions were carefully controlled. Double distilled and deionized water was used for solution preparation. Solution pH, considered to be using the relation between pH and concentrations of both the solutions, was adjusted to neutral by adding aqueous ammonia to preserve the hydroxide phases of Cd and Sn. The homogeneous solution was prepared by thoroughly mixing both the solutions. White gelatinous precipitate formed and was filtered using Whatmann filter paper No.17. The precipitate was washed thoroughly until traces of Cl were removed. It was further dried at ambient temperature and sintered at different temperatures within 400– 1200 8C for 6 h in air atmosphere. These compositions were further mixed with polyvinyl alcohol as a binder and pressed into pellets of 15 mm diameter and 2–3 mm thickness using a hydraulic press. 2.2. X-ray measurements
* Corresponding author. Tel.: +91 231 2609435; fax: +91 231 2691533. E-mail address: [email protected] (K.Y. Rajpure). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.02.070
The samples were characterized by X-ray diffractometer (Philips, Model PW-1710) using Cu-Ka radiation (l = 1.54056 A˚) for crystal structure analysis.
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2.3. SEM studies Surface morphology was studied using JEOL JSM-6360 scanning electron microscope (SEM). 2.4. Electrical resistivity measurements Silver paste was applied on the flat surfaces of the pellet for good ohmic contact. Resistivity measurements were carried out by two probe method as a function of temperature within room temperature to 500 8C. 2.5. Dielectric measurements The AC parameters such as capacitance (C) and dissipation factor (tan d) of the samples were measured in the frequency range 20 Hz to 1 MHz using LCR meter (HP 4284 A). The dielectric constant (e0 ) was calculated using the relation
e0 ¼
Cpt
e0 A
(1)
where Cp is the capacitance of the pellet, t the thickness of the pellet, A the area of cross section of the pellet and e0 is the permittivity of free space (8.854 1012 Fm1). The AC conductivity of the samples was estimated from the dielectric parameters. As long as the pure charge transport mechanism is the major contributor to the loss mechanisms, the AC conductivity (sAC) may be calculated using the relation
s AC ¼ ve0 e0 tan d
(2)
where v the angular frequency and tan d is the dissipation factor. 3. Results and discussion
80-1469 (spinel cubic Cd2SnO4) and 73-0973 (spinel cubic Cd1.33Sn1.33O4). At 400 8C sample shows an amorphous as well as the nanocrystalline CdSnO3 phase. At 600 8C the XRD pattern shows rhombohedral crystal structure with only CdSnO3 phase and all the peaks could be easily identified. Sample sintered at 800 8C shows strong presence of CdSnO3 phase. But the presence of (2 2 0), (2 2 2) and (5 1 1) peaks from Cd2SnO4 phase confirms that it is changing to Cd2SnO4 phase. At higher temperatures viz. 1000 and 1200 8C the samples show mixed phases of CdSnO3, Cd2SnO4 and Cd1.33Sn1.33O4. The (2 2 2) and (5 1 1) planes of Cd2SnO4 spinel cubic structure are observed to be strongest in the samples sintered at 1000 (d222 = 2.6359 A˚) and 1200 8C (d511 = 1.7672 A˚) respectively. The shifting of strongest plane in the sample sintered at 1200 8C sample with respect to that of the one sintered at 1000 8C is due to change in single crystal d value. The other planes such as (3 1 1), (3 3 1), (4 0 0), (4 4 4), (6 2 2), (6 4 2), (7 3 1), and (8 0 0) are observed for Cd2SnO4 phase. Another new phase Cd1.33Sn1.33O4 having (4 4 2), (6 2 0) and (7 3 3) planes is observed in spinel cubic crystal structure. As the sintering temperature increases up to 800 8C the peak intensity of CdSnO3 phase increases. In case of Cd2SnO4 phase it increases as the temperature is raised up to 1000 8C because of enhancement in crystallinity. It then decreases due to the crystal reorientation effect. 3.2. Scanning electron microscopy (SEM) Fig. 2 shows SEM micrograph of Cd–Sn–O sample sintered at 1000 8C. The SEM studies reveal a randomly arranged growth of trapezoidal grains with compact nature. It can be seen that larger grains are overgrown on the buried smaller ones. Elongated grains of observed phases of Cd–Sn–O system can be seen clearly. Thus the surface seems to be the complex of various Cd–Sn–O systems. Observed morphology might have resulted due to uneven iondiffusion rates that cause agglomeration at some places and deagglomeration of grains at other.
3.1. X-ray diffraction 3.3. Electrical resistivity X-ray diffraction patterns of cadmium stannate samples sintered at different temperatures are shown in Fig. 1. The samples are polycrystalline and fit well with the rhombohedral and spinel cubic crystal structures with preferred characteristic temperatures. The intensity and number of diffraction peaks depend on the amount of corresponding phases. The data was analyzed by making use of JCPDS card nos. 34-0758 (rhombohedral CdSnO3),
Fig. 1. X-ray diffraction patterns of cadmium stannate samples annealed at different temperatures; (a) 400 8C, (b) 600 8C, (c) 800 8C, (d) 1000 8C and (e) 1200 8C.
Fig. 3 shows the variation of dc resistivity with temperature. The plots show two regions of conductivity. The resistivity decreases with increase in temperature suggesting semiconductor behavior of the samples. Slight increase in resistivity with increase in temperature from 400 to 500 8C for the samples sintered at 600, 800 and 1200 8C, might be due to phase transformation during heating. The first region observed at lower temperatures is due to impurities and the second region that occurs at higher tempera-
Fig. 2. Scanning electron micrograph of Cd2SnO4 sample sintered at 1000 8C magnification: 20,000.
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Fig. 4. Variation of room temperature dielectric constant with frequency. Fig. 3. Variation of dc resistivity with temperature.
tures is due to polaron hopping. The second region in the resistivity plots is attributed to the phase transition due to change in activation energy. In the hopping mechanism electrons can ‘hop’ from one site to another by acquiring the necessary activation energy. Such a thermally activated hopping process is possible at sufficiently high temperatures [9]. The dc resistivity decreases with increasing temperature because with the addition of thermal energy, electron could be set free from O2 ions. When an electron is introduced in the sample it might be associated with cations, which results in an unstable valence state [10]. As the sintering temperature increases, the resistivity goes on increasing up to 1000 8C and then decreases. This may be attributed to the difference in the resistivity of two phases. The resistivity of composite is an effective value of resistivity of its constituent phases. The maximum resistivity observed at 1000 8C is perhaps due to the addition of resistivities of constituent phases. The increase in resistivity of the composites as compared to constituent phases may be ascribed to the serial arrangement of grains. The activation energy was calculated by using the relation
r ¼ r0 exp
DE
kT
(3)
constant values, decrease in dielectric constant with frequency is observed. The large value of dielectric constant is associated with space charge polarization and inhomogeneous dielectric structure. These inhomogeneites are impurities, grain structure and pores. The variation of dielectric loss with frequency is shown in Fig. 5. At lower frequencies tan d is large and it decreases with increasing frequency. The tan d is the energy dissipation in the dielectric system, which is proportional to the imaginary part of dielectric constant. At higher frequencies the losses are reduced and the dipoles contribute to the polarization [14]. The loss factor curve is attributed to domain wall resonance. At higher frequencies, losses are found to be low if domain wall motion is inhibited and magnetization is forced to change by rotation. To confirm the conduction mechanism in these composites, the variation of log sac with log v was studied (Fig. 6). The plots are observed to be almost linear indicating that the conduction increases with increase in frequency. The linearity of the plots confirms small polaron mechanism of conduction. The slight decrease in conductivity is attributed to conduction by mixed polarons. In ionic solids the electrical conductivity is due to migration of ions and the ionic transport depends on angular
where DE is the activation energy, r the resistivity at room temperature, k the Boltzmann constant and r0 is temperature independent constant. Activation energy in low temperature region is of the order of 0.1 eV and that in the high temperature region varies from 0.26 to 1.13 eV. The respective values of activation energies for the samples sintered at 400, 600, 800, 1000, and 1200 8C are 0.266, 0.919, 0.524, 0.933 and 1.132 eV. 3.4. Dielectric properties The variation of dielectric constant with frequency at room temperature for the samples is shown in Fig. 4. From the figure it is clear that dielectric constant (e0 ) decreases abruptly at lower frequencies and remains constant at higher frequencies showing dispersion of dielectric constant at lower frequencies. The dielectric constant varies with applied frequency due to charge transport relaxation time. This dielectric dispersion is attributed to Maxwell [11] and Wagner [12] type of interfacial polarization in agreement with Koop’s phenomenological theory [13]. Since polarization decreases with increasing frequency and reaches
Fig. 5. Variation of dielectric loss with frequency.
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temperatures (<600 8C) and a mixed phase containing rhombohedral CdSnO3, spinel cubic Cd2SnO4, cubic Cd1.33Sn1.33O4 is formed at higher temperatures (>600 8C). SEM image of Cd2SnO4 sample shows randomly distributed trapezoidal grains. The dielectric behavior shows the electronic polarizability at higher frequencies due to space charge polarization. AC conductivity increases with increase in the frequency due to hopping mechanism of conduction.
Acknowledgements The authors are thankful to University Grants Commission for their support through UGC-DRS IInd phase programme (2004– 2009). The help rendered by R.C. Kamble during electrical measurements is highly acknowledged.
References Fig. 6. Variation of AC conductivity with frequency.
frequency. Thus, the ac conductivity (sac) is proportional to the angular frequency and it is confirmed here by linear plots of conductivity with angular frequency [15]. Relatively higher value of dielectric constant, loss tangent and ac conductivity in case of the sample sintered at 1000 8C might be due to dominance of Cd2SnO4 phase in the formed composite.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusions
[11]
The cadmium stannate samples were prepared by chemical co-precipitation method. The X-ray diffraction analysis indicates that the single phase CdSnO3 is observed at lower calcining
[12] [13] [14] [15]
R.D. Shannon, J.L. Gillson, R.J. Bouchard, J. Phys. Chem. Solids 38 (1977) 877. C.M. Cardile, R.H. Meinhold, K.J.D. MacKenzie, J. Phys. Chem. Solids 48 (1987) 881. X. Jia, H. Fan, X. Lou, J. Xu, Appl. Phys. A 94 (2009) 837. W.L. Wang, K.J. Liao, C.Z. Cai, G.B. Liu, Y. Ma, Surf. Coat. Technol. 167 (2003) 284. R. Mamazza Jr., D.L. Morel, C.S. Ferekides, Thin Solid Films 484 (2005) 26. A.J. Nozik, Phys. Rev. B 6 (1972) 453. T. Hashemi, J. Illingsworth, F. Golestanifard, J. Am. Ceram. Soc. 74 (1991) 662. G. Valincius, V. Reipa, V. Vilker, J.T. Woodward, M. Vaudin, J. Electrochem. Soc. 148 (2001) E341. H.V. Keer, Principles of the Solid State, New Age Int. Pub. Ltd., Mumbai, 2000, p. 178. R. Rai, N.C. Soni, S. Sharma, R.N.P. Choudhary, in: A.P. Tandon (Ed.), Ferroelectrics and Dielectrics, Allied Publishers Pvt. Ltd., New Delhi, 2004, p. 177. J.C. Maxwell, Electricity and Magnetism, Oxford University Press, London, 1993, p. 828. K.W. Wagner, Ann. Phys. 40 (1993) 818. C.G. Koop, Phys. Rev. B 83 (1951) 121. D.C. Agrawal, Asian J. Phys. 6 (1997) 108. K.K. Patankar, S.S. Joshi, B.K. Chougule, Phys. Lett. A 346 (2005) 337.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of calcining temperature on electrical and dielectric properties of cadmium stannate V.S. Sawant, S.S. Shinde, R.J. Deokate, C.H. Bhosale, B.K. Chougule, K.Y. Rajpure * Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 October 2008 Received in revised form 10 February 2009 Accepted 20 February 2009 Available online 4 March 2009
The cadmium stannate samples were prepared by chemical co-precipitation method using stannic chloride pentahydrate (SnCl45H2O) and cadmium chloride (CdCl2) as precursors by carefully controlling the preparative parameters. The effect of calcining temperature on the phase, microstructure, morphological and electrical properties of cadmium stannate has been investigated. The Xray diffraction patterns indicate the conversion of rhombohedral to spinel cubic crystal structure and polycrystallinity of the samples. SEM study of Cd2SnO4 sample shows randomly distributed cubic crystals of varying sizes. The dc resistivity was measured as a function of temperature. The dielectric constant and dielectric loss were studied as a function of frequency. To understand the conduction mechanism in the samples AC conductivity was measured. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Cadmium stannate X-ray diffraction Microstructure dc resistivity Dielectric constant
1. Introduction
2. Experimental
Recently there has been great attention for investigating new gas-sensing materials and developing the properties of known gas-sensing materials. Most of these materials are metal oxides. Cadmium stannate shows quite attractive electrical properties [1,2]. Advantages of oxides for gas detections are high sensitivity, simple design, and low weight and cost. Recently synthesis and gas-sensing properties of perovskite CdSnO3 nanoparticles have been reported by Jia et al. [3]. CdSnO3 nanoparticles were prepared by a chemical co-precipitation method and they studied the selectivity to vapors of C2H5OH. Cd2SnO4, being one of the alternatives to the present generations transparent conducting oxides (TCOs); FTO and ITO, is synthesized in thin film form by various techniques [4,5]. Nozik [6] prepared the Cd2SnO4 amorphous thin films and studied the optical and electrical properties. Hashemi [7] has employed the investigation of chemical nature of samples of Cd2SnO4 in as-fired and electrochemically reduced and reoxidised states. Valincius et al. [8] reported the electrochemical properties of sol–gel prepared nanocrystalline electrodes of cadmium tin oxide films. We present here the formation of different powder samples in the Cd–Sn–O system by co-precipitation method having enhanced dielectric properties.
2.1. Preparation of samples The cadmium stannate samples were prepared by a chemical co-precipitation method using AR grade equimolar (0.1 M) stannic chloride pentahydrate (SnCl45H2O) and cadmium chloride (CdCl2) as precursors in 1:2 composition by volume in solution. The preparation conditions were carefully controlled. Double distilled and deionized water was used for solution preparation. Solution pH, considered to be using the relation between pH and concentrations of both the solutions, was adjusted to neutral by adding aqueous ammonia to preserve the hydroxide phases of Cd and Sn. The homogeneous solution was prepared by thoroughly mixing both the solutions. White gelatinous precipitate formed and was filtered using Whatmann filter paper No.17. The precipitate was washed thoroughly until traces of Cl were removed. It was further dried at ambient temperature and sintered at different temperatures within 400– 1200 8C for 6 h in air atmosphere. These compositions were further mixed with polyvinyl alcohol as a binder and pressed into pellets of 15 mm diameter and 2–3 mm thickness using a hydraulic press. 2.2. X-ray measurements
* Corresponding author. Tel.: +91 231 2609435; fax: +91 231 2691533. E-mail address: [email protected] (K.Y. Rajpure). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.02.070
The samples were characterized by X-ray diffractometer (Philips, Model PW-1710) using Cu-Ka radiation (l = 1.54056 A˚) for crystal structure analysis.
V.S. Sawant et al. / Applied Surface Science 255 (2009) 6675–6678
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2.3. SEM studies Surface morphology was studied using JEOL JSM-6360 scanning electron microscope (SEM). 2.4. Electrical resistivity measurements Silver paste was applied on the flat surfaces of the pellet for good ohmic contact. Resistivity measurements were carried out by two probe method as a function of temperature within room temperature to 500 8C. 2.5. Dielectric measurements The AC parameters such as capacitance (C) and dissipation factor (tan d) of the samples were measured in the frequency range 20 Hz to 1 MHz using LCR meter (HP 4284 A). The dielectric constant (e0 ) was calculated using the relation
e0 ¼
Cpt
e0 A
(1)
where Cp is the capacitance of the pellet, t the thickness of the pellet, A the area of cross section of the pellet and e0 is the permittivity of free space (8.854 1012 Fm1). The AC conductivity of the samples was estimated from the dielectric parameters. As long as the pure charge transport mechanism is the major contributor to the loss mechanisms, the AC conductivity (sAC) may be calculated using the relation
s AC ¼ ve0 e0 tan d
(2)
where v the angular frequency and tan d is the dissipation factor. 3. Results and discussion
80-1469 (spinel cubic Cd2SnO4) and 73-0973 (spinel cubic Cd1.33Sn1.33O4). At 400 8C sample shows an amorphous as well as the nanocrystalline CdSnO3 phase. At 600 8C the XRD pattern shows rhombohedral crystal structure with only CdSnO3 phase and all the peaks could be easily identified. Sample sintered at 800 8C shows strong presence of CdSnO3 phase. But the presence of (2 2 0), (2 2 2) and (5 1 1) peaks from Cd2SnO4 phase confirms that it is changing to Cd2SnO4 phase. At higher temperatures viz. 1000 and 1200 8C the samples show mixed phases of CdSnO3, Cd2SnO4 and Cd1.33Sn1.33O4. The (2 2 2) and (5 1 1) planes of Cd2SnO4 spinel cubic structure are observed to be strongest in the samples sintered at 1000 (d222 = 2.6359 A˚) and 1200 8C (d511 = 1.7672 A˚) respectively. The shifting of strongest plane in the sample sintered at 1200 8C sample with respect to that of the one sintered at 1000 8C is due to change in single crystal d value. The other planes such as (3 1 1), (3 3 1), (4 0 0), (4 4 4), (6 2 2), (6 4 2), (7 3 1), and (8 0 0) are observed for Cd2SnO4 phase. Another new phase Cd1.33Sn1.33O4 having (4 4 2), (6 2 0) and (7 3 3) planes is observed in spinel cubic crystal structure. As the sintering temperature increases up to 800 8C the peak intensity of CdSnO3 phase increases. In case of Cd2SnO4 phase it increases as the temperature is raised up to 1000 8C because of enhancement in crystallinity. It then decreases due to the crystal reorientation effect. 3.2. Scanning electron microscopy (SEM) Fig. 2 shows SEM micrograph of Cd–Sn–O sample sintered at 1000 8C. The SEM studies reveal a randomly arranged growth of trapezoidal grains with compact nature. It can be seen that larger grains are overgrown on the buried smaller ones. Elongated grains of observed phases of Cd–Sn–O system can be seen clearly. Thus the surface seems to be the complex of various Cd–Sn–O systems. Observed morphology might have resulted due to uneven iondiffusion rates that cause agglomeration at some places and deagglomeration of grains at other.
3.1. X-ray diffraction 3.3. Electrical resistivity X-ray diffraction patterns of cadmium stannate samples sintered at different temperatures are shown in Fig. 1. The samples are polycrystalline and fit well with the rhombohedral and spinel cubic crystal structures with preferred characteristic temperatures. The intensity and number of diffraction peaks depend on the amount of corresponding phases. The data was analyzed by making use of JCPDS card nos. 34-0758 (rhombohedral CdSnO3),
Fig. 1. X-ray diffraction patterns of cadmium stannate samples annealed at different temperatures; (a) 400 8C, (b) 600 8C, (c) 800 8C, (d) 1000 8C and (e) 1200 8C.
Fig. 3 shows the variation of dc resistivity with temperature. The plots show two regions of conductivity. The resistivity decreases with increase in temperature suggesting semiconductor behavior of the samples. Slight increase in resistivity with increase in temperature from 400 to 500 8C for the samples sintered at 600, 800 and 1200 8C, might be due to phase transformation during heating. The first region observed at lower temperatures is due to impurities and the second region that occurs at higher tempera-
Fig. 2. Scanning electron micrograph of Cd2SnO4 sample sintered at 1000 8C magnification: 20,000.
V.S. Sawant et al. / Applied Surface Science 255 (2009) 6675–6678
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Fig. 4. Variation of room temperature dielectric constant with frequency. Fig. 3. Variation of dc resistivity with temperature.
tures is due to polaron hopping. The second region in the resistivity plots is attributed to the phase transition due to change in activation energy. In the hopping mechanism electrons can ‘hop’ from one site to another by acquiring the necessary activation energy. Such a thermally activated hopping process is possible at sufficiently high temperatures [9]. The dc resistivity decreases with increasing temperature because with the addition of thermal energy, electron could be set free from O2 ions. When an electron is introduced in the sample it might be associated with cations, which results in an unstable valence state [10]. As the sintering temperature increases, the resistivity goes on increasing up to 1000 8C and then decreases. This may be attributed to the difference in the resistivity of two phases. The resistivity of composite is an effective value of resistivity of its constituent phases. The maximum resistivity observed at 1000 8C is perhaps due to the addition of resistivities of constituent phases. The increase in resistivity of the composites as compared to constituent phases may be ascribed to the serial arrangement of grains. The activation energy was calculated by using the relation
r ¼ r0 exp
DE
kT
(3)
constant values, decrease in dielectric constant with frequency is observed. The large value of dielectric constant is associated with space charge polarization and inhomogeneous dielectric structure. These inhomogeneites are impurities, grain structure and pores. The variation of dielectric loss with frequency is shown in Fig. 5. At lower frequencies tan d is large and it decreases with increasing frequency. The tan d is the energy dissipation in the dielectric system, which is proportional to the imaginary part of dielectric constant. At higher frequencies the losses are reduced and the dipoles contribute to the polarization [14]. The loss factor curve is attributed to domain wall resonance. At higher frequencies, losses are found to be low if domain wall motion is inhibited and magnetization is forced to change by rotation. To confirm the conduction mechanism in these composites, the variation of log sac with log v was studied (Fig. 6). The plots are observed to be almost linear indicating that the conduction increases with increase in frequency. The linearity of the plots confirms small polaron mechanism of conduction. The slight decrease in conductivity is attributed to conduction by mixed polarons. In ionic solids the electrical conductivity is due to migration of ions and the ionic transport depends on angular
where DE is the activation energy, r the resistivity at room temperature, k the Boltzmann constant and r0 is temperature independent constant. Activation energy in low temperature region is of the order of 0.1 eV and that in the high temperature region varies from 0.26 to 1.13 eV. The respective values of activation energies for the samples sintered at 400, 600, 800, 1000, and 1200 8C are 0.266, 0.919, 0.524, 0.933 and 1.132 eV. 3.4. Dielectric properties The variation of dielectric constant with frequency at room temperature for the samples is shown in Fig. 4. From the figure it is clear that dielectric constant (e0 ) decreases abruptly at lower frequencies and remains constant at higher frequencies showing dispersion of dielectric constant at lower frequencies. The dielectric constant varies with applied frequency due to charge transport relaxation time. This dielectric dispersion is attributed to Maxwell [11] and Wagner [12] type of interfacial polarization in agreement with Koop’s phenomenological theory [13]. Since polarization decreases with increasing frequency and reaches
Fig. 5. Variation of dielectric loss with frequency.
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temperatures (<600 8C) and a mixed phase containing rhombohedral CdSnO3, spinel cubic Cd2SnO4, cubic Cd1.33Sn1.33O4 is formed at higher temperatures (>600 8C). SEM image of Cd2SnO4 sample shows randomly distributed trapezoidal grains. The dielectric behavior shows the electronic polarizability at higher frequencies due to space charge polarization. AC conductivity increases with increase in the frequency due to hopping mechanism of conduction.
Acknowledgements The authors are thankful to University Grants Commission for their support through UGC-DRS IInd phase programme (2004– 2009). The help rendered by R.C. Kamble during electrical measurements is highly acknowledged.
References Fig. 6. Variation of AC conductivity with frequency.
frequency. Thus, the ac conductivity (sac) is proportional to the angular frequency and it is confirmed here by linear plots of conductivity with angular frequency [15]. Relatively higher value of dielectric constant, loss tangent and ac conductivity in case of the sample sintered at 1000 8C might be due to dominance of Cd2SnO4 phase in the formed composite.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusions
[11]
The cadmium stannate samples were prepared by chemical co-precipitation method. The X-ray diffraction analysis indicates that the single phase CdSnO3 is observed at lower calcining
[12] [13] [14] [15]
R.D. Shannon, J.L. Gillson, R.J. Bouchard, J. Phys. Chem. Solids 38 (1977) 877. C.M. Cardile, R.H. Meinhold, K.J.D. MacKenzie, J. Phys. Chem. Solids 48 (1987) 881. X. Jia, H. Fan, X. Lou, J. Xu, Appl. Phys. A 94 (2009) 837. W.L. Wang, K.J. Liao, C.Z. Cai, G.B. Liu, Y. Ma, Surf. Coat. Technol. 167 (2003) 284. R. Mamazza Jr., D.L. Morel, C.S. Ferekides, Thin Solid Films 484 (2005) 26. A.J. Nozik, Phys. Rev. B 6 (1972) 453. T. Hashemi, J. Illingsworth, F. Golestanifard, J. Am. Ceram. Soc. 74 (1991) 662. G. Valincius, V. Reipa, V. Vilker, J.T. Woodward, M. Vaudin, J. Electrochem. Soc. 148 (2001) E341. H.V. Keer, Principles of the Solid State, New Age Int. Pub. Ltd., Mumbai, 2000, p. 178. R. Rai, N.C. Soni, S. Sharma, R.N.P. Choudhary, in: A.P. Tandon (Ed.), Ferroelectrics and Dielectrics, Allied Publishers Pvt. Ltd., New Delhi, 2004, p. 177. J.C. Maxwell, Electricity and Magnetism, Oxford University Press, London, 1993, p. 828. K.W. Wagner, Ann. Phys. 40 (1993) 818. C.G. Koop, Phys. Rev. B 83 (1951) 121. D.C. Agrawal, Asian J. Phys. 6 (1997) 108. K.K. Patankar, S.S. Joshi, B.K. Chougule, Phys. Lett. A 346 (2005) 337.