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Structural and dielectric properties of ZrO2–TiO2–V2O5 nanocomposite prepared by CO-precipitation calcination method
Materials Science in Semiconductor Processing 41 (2016) 246–251
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Structural and dielectric properties of ZrO2–TiO2–V2O5 nanocomposite prepared by CO-precipitation calcination method N. Padmamalini a,n, K. Ambujam b a b
Department of Physics, St. Joseph's Institute of Technology, Chennai 600119, India Dr. Ambedkar Government Arts College, Chennai 600039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 13 June 2015 Received in revised form 29 August 2015 Accepted 5 September 2015
Nanocrystalline ZrO2–V2O5–TiO2 composite was synthesized by co-precipitation method and calcined at 500 and 700 °C. The formation of the composite material has been confirmed by X-ray diffraction analysis. The surface morphology was determined by SEM and HRTEM and it was seen that increase in calcination temperature increases the grain size. EDX analysis confirms the presence of zirconium, titanium and vanadium in the lattice. Optical absorption studies reveal a very low absorption in the visible region for both the samples. The dielectric constant, loss and ac conductivity of the pelletized samples have been examined at different temperatures as functions of frequency and the activation energies were calculated. The results indicated that the dielectric constant increases with calcination temperature. It was seen that the dielectric constant increases on the addition of Vanadia to zirconia–titania composite making it ideal for use as a gate dielectric material. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanostructures Dielectric properties Activation energy Optical properties
1. Introduction
2. Experimental procedure
Mixed metal oxides have been extensively studied as they exhibit high surface area, good thermal stability, and mechanical strength. Moreover, mixed oxides of high-k materials have become more attractive and are expected to be utilized as a replacement for silica in the future ultra large scale integrated circuits [1]. In this context, the dielectric properties of Zirconium oxide and their application as a gate dielectric have been thoroughly explored [2– 6] Similarly titanium oxide has also been explored for the same purpose [7–10]. Dielectric behavior of binary oxides of zirconium and titanium individually and along with lead and barium was also studied [11–13]. The addition of V2O5 increases the dielectric constant [14, 15] and enhances the ferroelectric properties of ceramics [16, 17]. In the present work, Vanadia was added to ZrO2– TiO2 composites in equal molar ratio, was synthesized by co-precipitation and calcined at 500 and 700 °C. The synthesized composites were subjected to different characterizations such as X-ray diffraction, optical, SEM, EDX, TEM, dielectric and Raman analyses and the results are discussed in detail.
A simple coprecipitation-calcination method was used to prepare the nanocrystalline ZrO2– V2O5–TiO2 composites [18]. The starting materials zirconium oxychloride, titanium tetrachloride and vanadium oxide (AR grade) were taken in the molar ratio 1:1:1 and dissolved in double distilled water. The water solution of ZrOCl2, TiCl4 and V2O5 was treated with aqueous ammonia at a pH of 9. The resultant hydrogel was washed with distilled water to remove NH4Cl and dried at 110 °C for 5 h to a constant mass. Further, the samples were calcined at 500 and 700 °C in a single zone furnace for 10 h. The synthesized samples were collected and powdered for further analyses.
n
Corresponding author. E-mail address: [email protected] (N. Padmamalini).
http://dx.doi.org/10.1016/j.mssp.2015.09.009 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
3. Results and discussion 3.1. XRD studies The XRD pattern of ZrO2–V2O5–TiO2 composites was recorded using a Bruker D8 advance powder XRD instrument. The XRD pattern (Fig. 1) indicates that the composites were crystallites in orthorhombic phase of V2O5 (JCPDS file 89-2482), orthorhombic phase of TiO2 (JCPDS file 76-1934) and monoclinic phase of ZrO2 (JCPDS file 89-9066) after calcination at both the calcining temperatures 500 and 700 °C respectively. The peaks indicate the presence of the three component oxides. The average particle size calculated using Debye–Scherrer formula was found to be around
N. Padmamalini, K. Ambujam / Materials Science in Semiconductor Processing 41 (2016) 246–251
45 nm for the sample calcined at 500 °C and around 550 nm for that calcined at 700 °C. 3.2. SEM, TEM with EDX studies The size and morphology of the synthesized samples were examined by SEM and HRTEM. Fig. 2(a) and (b) shows the SEM images of the synthesized samples at 500 °C and 700 °C. The grain size from the SEM micrographs were analyzed using ImageJ software. Fig. 3(a) and (b) shows the grain size distribution with data obtained from ImageJ software. The grain sizes are between 20 and 65 nm for the first sample and between 400 and 800 nm for the second. These images were taken by a FEI Quanta FEG 200 high resolution SEM. The HRTEM pictures, taken by a JEOL 3010 high
247
resolution transmission electron microscope, shown in Fig. 4 (a) and (b) corroborate the structures seen in the SEM pictures. It can be seen that the grain size of the composite increased with calcination temperature. This is in good agreement with the previously reported work [19,20]. The EDX (Energy Dispersive X-ray analysis) spectrum of the two samples, shown in Fig. 4(a) and (b), confirm the presence of vanadium, zirconium and titanium oxides. From the elemental count, 45.28% of V, 0.71% of Ti and 31.81% of Zr have been observed for 500 °C calcined sample whereas for 700 °C calcined sample, 44.67% of V, 0.72% of Ti and 27.95% of Zr is observed (Table 1). The remaining percentage of the samples is that of oxygen (Fig. 5).
3.3. UV–visible studies UV–vis absorption studies were carried out using a CAREY 5E UV–vis-NIR spectrometer. The UV–vis absorption spectra of the synthesized nanocomposites at 500 and 700 °C are shown in Fig. 6 (a) and (b). Both the spectra indicate very low absorption in the entire visible region which suggests its suitability for the device fabrication. The UV absorbance cut off is about the same wavelength for both the samples, but the energy absorbed in the lower wavelengths is lesser for the sample calcined at 700 °C. An absorption peak was found in the ultraviolet region (214 nm), which can be attributed to the interband transitions in m- zirconia [21,22].
3.4. Raman spectra studies
Fig. 1. XRD pattern of ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Raman spectroscopy is very sensitive to the polarizability of oxygen ions and hence can be used to determine the symmetry of a crystal system. The Raman spectra of the prepared nanocomposites was taken using a Bruker RFS 27 stand-alone FT Raman Spectrometer and is shown in Fig. 7(a) and (b). The peak at 773 cm 1 is assigned to the asymmetric stretching, 993 cm 1 to the symmetric stretching of VO4 tetrahedron. The peak at 382 cm 1 occurs due to monoclinic Zirconia, 269 cm 1 to the ZrO6 octahedron, 187 cm 1 from the lattice vibrations of zirconium vanadate, 141 cm 1 and 514 cm 1 due to zirconium titanate [23].
Fig.2. SEM images of ZrO2– V2O5–TiO2 composites calcined at 500 °C and 700 °C.
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Fig. 3. Grain size distribution for ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Fig. 4. HRTEM images of ZrO2– V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Table 1 EDX elemental Count for ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C. Element
Wt%
At%
OK TiK VK ZrK 700o C OK TiK VK ZrK
22.20 00.71 45.28 31.81
52.57 00.56 33.67 13.21
26.65 00.72 44.67 27.95
58.16 00.52 30.61 10.70
3.5. Dielectric studies The dielectric analysis was performed by using HIOKI 3532 LCR HITESTER impedance analyzer. Fig. 8(a) and (b) shows the frequency dependence of the dielectric constant (ε‘) at different temperatures. The dielectric constant or relative permittivity is calculated from [24] εr = Cd where d is the sample thickness and A is the surface area ε0A
of the sample. ε0 is the free space permittivity (8.854 10 12 F/m). It is observed that εr gradually decreases with increasing frequency in a given temperature range. On increasing temperature, εr increases apparently, which becomes even more significant at low frequency. The conductivity of grain boundaries contributes more to the dielectric value at lower frequencies because the grain boundaries are more effective at lower frequencies [25,26]. Here, the dielectric constant is greatly increased when the sample is calcined at a higher temperature. This may be caused by the increase in grain size of the sample for an increase in the calcining temperature, which in turn increases the dielectric constant [27,28]. The dielectric constant of the ZrO2–V2O5–TiO2 nanocomposite was found to be greater than that of zirconium–titanium oxide which is around 44 [29]. The dielectric constant of a material depends on the different types of polarization, namely, electronic, ionic, orientation and space charge polarization. Of these polarization mechanisms space charge polarization depends on purity and perfection of ceramics. The addition of vanadia to the composite, generates defects (dangling bonds and non-bridging oxygens) due to the action of vanadium ions present in vanadia, which contributes to space charge polarization and in turn increases the dielectric constant [30].
N. Padmamalini, K. Ambujam / Materials Science in Semiconductor Processing 41 (2016) 246–251
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Fig. 5. EDX spectrum of ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Fig. 7. Raman studies of the nanocomposites calcined at (a) 500 °C (b) 700 °C. Fig. 6. Optical absorption studies of the nanocomposites calcined at (a) 500 °C (b) 700 °C.
The dielectric loss, as shown in Fig. 9(a) and (b), is found to be greater for the sample calcined at higher temperature and is nearly a constant at higher temperatures for a longer range of frequencies. The AC conductivity (sac) is calculated using the formula [24]
σac = εoεrωtan δ where ω ¼2πf is the angular frequency, f is calculated in hertz and is given in Fig. 10 (a) and (b). It was observed that the ac electrical conductivity increases with increase of frequency. The increase in calcination temperature causes the ac conductivity to decrease and it can be seen that at higher temperatures it is nearly a constant with respect to frequency, indicating that the conduction may be due to small polarons [31]. As the frequency of the applied field increases, the conductive grains become more active, thereby promoting electron hopping. Therefore a gradual increase in conductivity was observed with frequency and temperature.
Activation energies (Fig. 11(a) and (b)) are estimated using the Arrhenius equation [32],
σ = Ae
E ( a ) kBT
Where A is the pre exponential factor, Ea is the activation energy, T, the temperature in Kelvin and kB, the Boltzmann constant. Plots between ln (sac) and 1000/T were drawn at frequencies 1 kHz and 10 kHz and were found to be very nearly linear (Fig. 11(a) and (b)). Activation energies calculated from the slopes of these plots were found to be 0.4 eV and 0.3 eV for the samples calcined at 500 °C and 700 °C respectively.
4. Conclusion Nanocrystalline ZrO2–V2O5–TiO2 composites were synthesized by co-precipitation method and calcined at 500 and 700 °C. The formation of the composite material has been confirmed by X-ray diffraction analysis. SEM and HRTEM analysis shows the increase
250
N. Padmamalini, K. Ambujam / Materials Science in Semiconductor Processing 41 (2016) 246–251
Fig. 8. Dielectric constant vs log f for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
Fig. 9. Dielectric loss vs log f for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
Fig. 10. log sac vs log f for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
in grain size with increase in calcination temperature. UV–vis studies revealed the samples to have very low absorption in the visible region, with a peak in the UV region at 215 nm.The Raman spectra corroborated the results of the XRD analysis. The dielectric
studies indicated that higher calcination temperature facilitated increased values of dielectric constant. The dielectric loss and ac conductivities were studied and the activation energy was found for both the samples. It can be seen that the dielectric constant
N. Padmamalini, K. Ambujam / Materials Science in Semiconductor Processing 41 (2016) 246–251
251
Fig. 11. ln sac vs 1000/T for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
increases on the addition of Vanadia to zirconia–titania composite. This high dielectric constant of the composite makes it suitable to be used as gate dielectric.
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Elect. 21 (2010) 168–172. [15] Z. Yan, J.L. Huang, Y.J. Gu, B. Jin, R.Y. Wang, Effects of V2O5 addition on the microstructure and microwave dielectric properties of ZnNb2O6 ceramics, Adv. Mater. Res. 476–478 (2012) 940–943. [16] Y. Wu, G. Cao, Enhanced ferroelectric properties and lowered processing temperatures of strontium bismuth niobates with vanadium doping, Appl. Phys. Lett. 75 (1999) 2650–2652. [17] H. Irie, M. Miyayama, T. Kudo, Enhanced ferroelectric properties of V- doped BaBi4Ti4O15 single crystal, Jpn. J. Appl. Phys. 40 (2001) 239–243. [18] W. Pyda, N. Moskała, A zirconia composite with in-situ synthesized titanium diboride inclusions, in: Proceedings of the 16th International Conference on composite materials, Kyoto, Japan, 2007, pp. 1–7. [19] N. Padmamalini, K. Ambuja, Optical and dielectric properties of ZrO2–V2O5 nanocomposites by co-precipitation calcination method, Superlattices Microstruct. 76 (2014) 376–384. [20] Y. Zhu, S. Wu, X. Wang, Nano CaO grain characteristics and growth model under calcination, Chem. Eng. J. 175 (2011) 512–518. [21] Raghavendra, P. Singh, R.K. Singh, Strutural characterization, electrical and dielectric relaxations in Dy- doped Zirconia, J. Alloy. Compd. 546 (2013) 238–244. [22] G. Ranga Rao, H. Ranjan Sahu, XRD and UV–vis diffuse reflectance analysis of CeO2–ZrO2 solid solutions synthesized by combustion method, Proc. Indi. Acad. Sci. (Chem. Sci.) 113 (2001) 651–658. [23] A. Verma, R. Dwivedi, R. Prasad, K.S. Bartwal, Microwave assisted synthesis of mixed metal-oxide nanoparticles, J. Nanoparticles (2013) 11. [24] P. Indradevi, K. Ramachandran, Hydrothermal synthesis and dielectric properties of star-like and dendrite-shaped PbS nanostructures, Adv. Sci. Lett. 4 (2011) 1–7. [25] C.G. Koops, On the dispersion of resistivity and dielectric constant of some semiconductors at audio frequencies, Phys. Rev. 83 (1951) 121–124. [26] P. Indradevi, H. Kamalaveni, K. Ramachandran, Dielectric and magnetic behaviour of undoped and Mn- doped ZnSe nanoparticles, Adv. Sci. Eng. Med. 4 (2012) 316–323. [27] M. Chaari, A. Matoussi, Z. Fakhfakh, Structural and dielectric properties of sintering zinc oxide bulk ceramic, Mater. Sci. Appl. 2 (2011) 765–770. [28] H.T. Martirenat, J.C. Burfoot, Grain-size effects on properties of some ferroelectric ceramics, J. Phys. C: Solid State Phys. 7 (1974) 3182–3192. [29] C.L. Wang, H.Y. Lee, F. Azough, R. Freer, The microstructure and microwave dielectric properties of zirconium titanate ceramics in the solid solution system ZrTiO4–Zr5Ti7O24, J. Mater. Sci. 32 (1999) 1693–1701. [30] A. Padmanabham, Y. Gandhi, M.V.N. Padma Rao, M.V. Ramachandra Rao, D.V. M. Appaji, The influence of V2O5 on dielectric properties of PbO–B2O3–Bi2O3 glasses, IOP Conf. Ser.: Mater. Sci. Eng. 2 (2009) 012055. [31] R.S. Devan, B.K. Chougule, Effect of composition on coupled electric, magnetic, and dielectric properties of two phase particulate magnetoelectric composite, J. Appl. Phys. 101 (2007) 014109. [32] L.K. Sudha, Sukumar Roy, K. Uma Rao, Evaluation of activation energy (Ea) profiles of nanostructured alumina polycarbonate composite Insulation materials, Int. J. Mater. Mech. Manuf. 2 (2014) 96–100.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Structural and dielectric properties of ZrO2–TiO2–V2O5 nanocomposite prepared by CO-precipitation calcination method N. Padmamalini a,n, K. Ambujam b a b
Department of Physics, St. Joseph's Institute of Technology, Chennai 600119, India Dr. Ambedkar Government Arts College, Chennai 600039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 13 June 2015 Received in revised form 29 August 2015 Accepted 5 September 2015
Nanocrystalline ZrO2–V2O5–TiO2 composite was synthesized by co-precipitation method and calcined at 500 and 700 °C. The formation of the composite material has been confirmed by X-ray diffraction analysis. The surface morphology was determined by SEM and HRTEM and it was seen that increase in calcination temperature increases the grain size. EDX analysis confirms the presence of zirconium, titanium and vanadium in the lattice. Optical absorption studies reveal a very low absorption in the visible region for both the samples. The dielectric constant, loss and ac conductivity of the pelletized samples have been examined at different temperatures as functions of frequency and the activation energies were calculated. The results indicated that the dielectric constant increases with calcination temperature. It was seen that the dielectric constant increases on the addition of Vanadia to zirconia–titania composite making it ideal for use as a gate dielectric material. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanostructures Dielectric properties Activation energy Optical properties
1. Introduction
2. Experimental procedure
Mixed metal oxides have been extensively studied as they exhibit high surface area, good thermal stability, and mechanical strength. Moreover, mixed oxides of high-k materials have become more attractive and are expected to be utilized as a replacement for silica in the future ultra large scale integrated circuits [1]. In this context, the dielectric properties of Zirconium oxide and their application as a gate dielectric have been thoroughly explored [2– 6] Similarly titanium oxide has also been explored for the same purpose [7–10]. Dielectric behavior of binary oxides of zirconium and titanium individually and along with lead and barium was also studied [11–13]. The addition of V2O5 increases the dielectric constant [14, 15] and enhances the ferroelectric properties of ceramics [16, 17]. In the present work, Vanadia was added to ZrO2– TiO2 composites in equal molar ratio, was synthesized by co-precipitation and calcined at 500 and 700 °C. The synthesized composites were subjected to different characterizations such as X-ray diffraction, optical, SEM, EDX, TEM, dielectric and Raman analyses and the results are discussed in detail.
A simple coprecipitation-calcination method was used to prepare the nanocrystalline ZrO2– V2O5–TiO2 composites [18]. The starting materials zirconium oxychloride, titanium tetrachloride and vanadium oxide (AR grade) were taken in the molar ratio 1:1:1 and dissolved in double distilled water. The water solution of ZrOCl2, TiCl4 and V2O5 was treated with aqueous ammonia at a pH of 9. The resultant hydrogel was washed with distilled water to remove NH4Cl and dried at 110 °C for 5 h to a constant mass. Further, the samples were calcined at 500 and 700 °C in a single zone furnace for 10 h. The synthesized samples were collected and powdered for further analyses.
n
Corresponding author. E-mail address: [email protected] (N. Padmamalini).
http://dx.doi.org/10.1016/j.mssp.2015.09.009 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
3. Results and discussion 3.1. XRD studies The XRD pattern of ZrO2–V2O5–TiO2 composites was recorded using a Bruker D8 advance powder XRD instrument. The XRD pattern (Fig. 1) indicates that the composites were crystallites in orthorhombic phase of V2O5 (JCPDS file 89-2482), orthorhombic phase of TiO2 (JCPDS file 76-1934) and monoclinic phase of ZrO2 (JCPDS file 89-9066) after calcination at both the calcining temperatures 500 and 700 °C respectively. The peaks indicate the presence of the three component oxides. The average particle size calculated using Debye–Scherrer formula was found to be around
N. Padmamalini, K. Ambujam / Materials Science in Semiconductor Processing 41 (2016) 246–251
45 nm for the sample calcined at 500 °C and around 550 nm for that calcined at 700 °C. 3.2. SEM, TEM with EDX studies The size and morphology of the synthesized samples were examined by SEM and HRTEM. Fig. 2(a) and (b) shows the SEM images of the synthesized samples at 500 °C and 700 °C. The grain size from the SEM micrographs were analyzed using ImageJ software. Fig. 3(a) and (b) shows the grain size distribution with data obtained from ImageJ software. The grain sizes are between 20 and 65 nm for the first sample and between 400 and 800 nm for the second. These images were taken by a FEI Quanta FEG 200 high resolution SEM. The HRTEM pictures, taken by a JEOL 3010 high
247
resolution transmission electron microscope, shown in Fig. 4 (a) and (b) corroborate the structures seen in the SEM pictures. It can be seen that the grain size of the composite increased with calcination temperature. This is in good agreement with the previously reported work [19,20]. The EDX (Energy Dispersive X-ray analysis) spectrum of the two samples, shown in Fig. 4(a) and (b), confirm the presence of vanadium, zirconium and titanium oxides. From the elemental count, 45.28% of V, 0.71% of Ti and 31.81% of Zr have been observed for 500 °C calcined sample whereas for 700 °C calcined sample, 44.67% of V, 0.72% of Ti and 27.95% of Zr is observed (Table 1). The remaining percentage of the samples is that of oxygen (Fig. 5).
3.3. UV–visible studies UV–vis absorption studies were carried out using a CAREY 5E UV–vis-NIR spectrometer. The UV–vis absorption spectra of the synthesized nanocomposites at 500 and 700 °C are shown in Fig. 6 (a) and (b). Both the spectra indicate very low absorption in the entire visible region which suggests its suitability for the device fabrication. The UV absorbance cut off is about the same wavelength for both the samples, but the energy absorbed in the lower wavelengths is lesser for the sample calcined at 700 °C. An absorption peak was found in the ultraviolet region (214 nm), which can be attributed to the interband transitions in m- zirconia [21,22].
3.4. Raman spectra studies
Fig. 1. XRD pattern of ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Raman spectroscopy is very sensitive to the polarizability of oxygen ions and hence can be used to determine the symmetry of a crystal system. The Raman spectra of the prepared nanocomposites was taken using a Bruker RFS 27 stand-alone FT Raman Spectrometer and is shown in Fig. 7(a) and (b). The peak at 773 cm 1 is assigned to the asymmetric stretching, 993 cm 1 to the symmetric stretching of VO4 tetrahedron. The peak at 382 cm 1 occurs due to monoclinic Zirconia, 269 cm 1 to the ZrO6 octahedron, 187 cm 1 from the lattice vibrations of zirconium vanadate, 141 cm 1 and 514 cm 1 due to zirconium titanate [23].
Fig.2. SEM images of ZrO2– V2O5–TiO2 composites calcined at 500 °C and 700 °C.
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Fig. 3. Grain size distribution for ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Fig. 4. HRTEM images of ZrO2– V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Table 1 EDX elemental Count for ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C. Element
Wt%
At%
OK TiK VK ZrK 700o C OK TiK VK ZrK
22.20 00.71 45.28 31.81
52.57 00.56 33.67 13.21
26.65 00.72 44.67 27.95
58.16 00.52 30.61 10.70
3.5. Dielectric studies The dielectric analysis was performed by using HIOKI 3532 LCR HITESTER impedance analyzer. Fig. 8(a) and (b) shows the frequency dependence of the dielectric constant (ε‘) at different temperatures. The dielectric constant or relative permittivity is calculated from [24] εr = Cd where d is the sample thickness and A is the surface area ε0A
of the sample. ε0 is the free space permittivity (8.854 10 12 F/m). It is observed that εr gradually decreases with increasing frequency in a given temperature range. On increasing temperature, εr increases apparently, which becomes even more significant at low frequency. The conductivity of grain boundaries contributes more to the dielectric value at lower frequencies because the grain boundaries are more effective at lower frequencies [25,26]. Here, the dielectric constant is greatly increased when the sample is calcined at a higher temperature. This may be caused by the increase in grain size of the sample for an increase in the calcining temperature, which in turn increases the dielectric constant [27,28]. The dielectric constant of the ZrO2–V2O5–TiO2 nanocomposite was found to be greater than that of zirconium–titanium oxide which is around 44 [29]. The dielectric constant of a material depends on the different types of polarization, namely, electronic, ionic, orientation and space charge polarization. Of these polarization mechanisms space charge polarization depends on purity and perfection of ceramics. The addition of vanadia to the composite, generates defects (dangling bonds and non-bridging oxygens) due to the action of vanadium ions present in vanadia, which contributes to space charge polarization and in turn increases the dielectric constant [30].
N. Padmamalini, K. Ambujam / Materials Science in Semiconductor Processing 41 (2016) 246–251
249
Fig. 5. EDX spectrum of ZrO2–V2O5–TiO2 composites calcined at 500 °C and 700 °C.
Fig. 7. Raman studies of the nanocomposites calcined at (a) 500 °C (b) 700 °C. Fig. 6. Optical absorption studies of the nanocomposites calcined at (a) 500 °C (b) 700 °C.
The dielectric loss, as shown in Fig. 9(a) and (b), is found to be greater for the sample calcined at higher temperature and is nearly a constant at higher temperatures for a longer range of frequencies. The AC conductivity (sac) is calculated using the formula [24]
σac = εoεrωtan δ where ω ¼2πf is the angular frequency, f is calculated in hertz and is given in Fig. 10 (a) and (b). It was observed that the ac electrical conductivity increases with increase of frequency. The increase in calcination temperature causes the ac conductivity to decrease and it can be seen that at higher temperatures it is nearly a constant with respect to frequency, indicating that the conduction may be due to small polarons [31]. As the frequency of the applied field increases, the conductive grains become more active, thereby promoting electron hopping. Therefore a gradual increase in conductivity was observed with frequency and temperature.
Activation energies (Fig. 11(a) and (b)) are estimated using the Arrhenius equation [32],
σ = Ae
E ( a ) kBT
Where A is the pre exponential factor, Ea is the activation energy, T, the temperature in Kelvin and kB, the Boltzmann constant. Plots between ln (sac) and 1000/T were drawn at frequencies 1 kHz and 10 kHz and were found to be very nearly linear (Fig. 11(a) and (b)). Activation energies calculated from the slopes of these plots were found to be 0.4 eV and 0.3 eV for the samples calcined at 500 °C and 700 °C respectively.
4. Conclusion Nanocrystalline ZrO2–V2O5–TiO2 composites were synthesized by co-precipitation method and calcined at 500 and 700 °C. The formation of the composite material has been confirmed by X-ray diffraction analysis. SEM and HRTEM analysis shows the increase
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Fig. 8. Dielectric constant vs log f for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
Fig. 9. Dielectric loss vs log f for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
Fig. 10. log sac vs log f for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
in grain size with increase in calcination temperature. UV–vis studies revealed the samples to have very low absorption in the visible region, with a peak in the UV region at 215 nm.The Raman spectra corroborated the results of the XRD analysis. The dielectric
studies indicated that higher calcination temperature facilitated increased values of dielectric constant. The dielectric loss and ac conductivities were studied and the activation energy was found for both the samples. It can be seen that the dielectric constant
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Fig. 11. ln sac vs 1000/T for the nanocomposites calcined at (a) 500 °C and (b) 700 °C.
increases on the addition of Vanadia to zirconia–titania composite. This high dielectric constant of the composite makes it suitable to be used as gate dielectric.
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