Ravi, V and Kutty, TRN (1991) Influence of cubic perovskite phase content on the positive temperature coefficient of resistance of n-type $BaTiO_3$ ceramics. In: Materials Science and Engineering B, 10 (1). pp. 41-52.
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The resistivity-temperature as well as the current-voltage (1-V) relations in donor-doped polycrystalline $BaTiO_3$ (tetragonal) ceramics are tremendously modified by the presence of cubic perovskites such as $BaSnO_3$, $BaZrO_3$, $SrTiO_3$ and $BaTiO_3$ in minor quantities. The minor cubic phase contents arise out of the compositional inhomogeneity in large industrial batches of titanate solid solutions or in chemically prepared $BaTiO_3$ powders or because of the grinding of raw materials for extended periods of time. Maintaining the same processing parameters, chemical compositional factors and microstructural contributions, the shape of the positive temperature coefficient of resistance (PTCR) curves are found to be completely altered by the addition of less than 10% cubic phase. The resistivity continues to increase not only across the Curie point $T_c$ but also at higher temperatures, without any apparent maximum even above 500K. Thus the PTCR region is broadened and the ratio of resistivities across $T_c$ has decreased. This is in sharp contrast with the characteristics of homogeneous solid solutions having the same chemical compositions wherein $\rho_m_a_x/\rho_m_i_n$ is over six orders of magnitude, with PTCR occurring in a narrow temperature range. The I-V curves of the mixed-phase ceramics show stable currentlimiting characteristics at higher field strength without any current maximum. The electron paramagnetic resonance spectra indicate the contribution of the disorder component induced by the minor phase. The vibronic activation of acceptor states arising mostly from barium vacancies located at the grain boundary layers takes place over a broader temperature range, as a result of the disorder contribution and the consequent spread of energy values.
|Item Type:||Journal Article|
|Additional Information:||Copyright of this article belongs to Elsevier Science.|
|Department/Centre:||Division of Chemical Sciences > Materials Research Centre|
|Date Deposited:||27 Jun 2007|
|Last Modified:||17 Jan 2012 05:49|
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