Essay Example on Shows the temperature dependence of real part of dielectric Permittivity








Fig 2 shows the temperature dependence of real part of dielectric permittivity and loss tan for x 0 00 0 01 0 05 and 0 07 at different frequencies Although we did measurements at twenty different frequencies but data of four frequencies are presented for the sake of clarity An anomalous increase in the dielectric permittivity was observed at high temperature 400oC and low frequency region 10kHz probably due to high dc conductivity of the sample 27 28 Therefore the data for lower frequency range are not presented Pure NBT shows a well defined shoulder in the dielectric curve followed by a diffuse peak near 360 C Tm where dielectric constant attains a maximum value In fact exhibits frequency dispersion at the shoulder which diminishes near 290 C considered as onset of anti ferroelectric to paraelectric phase transition The tan curve shows well defined anomalies with incisive decrease of loss near 190 C The temperature Td at which tan curve falls rapidly is frequency dependent as observed in relaxor ferroelectrics These results agree well with the previous results and the temperatures are identical to the structural phase transition temperatures of NBT 26 With increase in the BZ content the dielectric curve shows broad maxima indicating that the transition become diffuse Hence the characteristic temperatures Td TRT and Tm are not very clearly distinguishable

The broadening of phase transition is common in solid solutions and other disordered systems The broadening in NBT based systems is attributed to the presence of structural disorder and composition fluctuations To observe the changes in the dielectric properties the real part of dielectric permittivity at room temperature and at Tm are plotted against x in Fig 3 It is evident from the figure that the dielectric constant decreases gradually for x 0 03 and then it increases abruptly for x 0 05 and 0 07 Dielectric loss tan of system found to increase slightly with increase in BZ content when compared to pure NBT x 0 00 Beyond 200oC a reasonable increase in the dielectric loss is observed in terms of enhanced conductivity A rationalization of decline in the dielectric constant value with BZ for x 0 30 can be conferred by considering polarizability of oxygen octahedral into account For a given concentration x in NBT 1 x BZ x the Ti4 ions are replaced by Zr4 ions Using first principle calculation Zhou et al 29 have shown that replacing the B site Ti4 ions in NBT with Zr4 ions strongly reduces the polarizability by increasing B O bond strength This in turn results in overall reduction in the dielectric constant value It has been shown by Koo Cheong 30 that for polycrystalline materials the dielectric permittivity is higher for tetragonal structure than the monoclinic The enhancement in the dielectric constant was attributed to 90 domain rotation in the tetragonal phase Recently Singh et al 26 have observed tetragonal distortion in NBT 1 x BZ x solid solution for x 0 05 and 0 07 Therefore the enhanced dielectric constant for x 0 05 and 0 07 is due to the emergence of tetragonal distortion in these compositions The phase transition temperatures Td TR T and Tm for NBT 1 x BZ x system are summarized in Fig 4 Due to close proximity of TR T and Tm values it is difficult to distinguish the individual peaks Therefore we used peak deconvolution method to obtain the experimental values of TR T and Tm It is evident from Fig 4 that Td TR T and Tm shifts to lower temperature with increasing BZ content 

The decrease in the transition temperature implies that BaZrO3 weakens the translational symmetry of dipoles of NBT 1 x BZ x and reduces the polarizability of BO6 octahedral A reduction in the polarizability of the BO6 octahedra is known to decrease the driving force for inducing the distortion in the unit cell and the coulombic interaction between the A site ion and BO6 octahedra Further the decrease in the TR T leads to the augmentation of tetragonal distortion at lower temperature for high BZ content as observed earlier in the structural analysis 24 Using the analogy given by Himaru et al 28 we drew the phase diagram of NBT 1 x BZ x system The notations are T Td Td T TR T TR T T Tm and T Tm as phases I II III and IV respectively as shown in Fig 4 From the present structural investigation and the analogy with available literature 4 14 it can be concluded that region I and II possess rhombohedral structure with space group R3c The region III is a mixture of rhombohedral R3c and tetragonal structure P4bm It is evident from Fig 4 that the temperature range of coexistence of the rhombohedral and the tetragonal phases is independent on the BZ content In most of the studies region IV is described as pseudo cubic

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