Low voltage varistors based on $SrTiO_3$ ceramics

Kutty, TRN and Philip, Sam (1995) Low voltage varistors based on $SrTiO_3$ ceramics. In: Materials Science and Engineering B, 33 (2-3). pp. 58-66.

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Abstract

Varistors exhibiting nonlinear resistance at low voltages of 4.5 to $30 V mm^{-1}$ have been realised from $SrTiO_3$ ceramics. These are processed from powders obtained from the gel-crystallite conversion technique. Doping with $\leq 1\%Y^{3+}$ enhances the average grain size to over $50\hspace{2mm} \mu m$, when sintered around 1650 K in static air. On annealing the ceramics, in $N_2 +H_2$ atmospheres, they acquire a low resistivity of 0.4 to 3.7 \Omega cm with positive temperature coefficient of resistance. They are painted with low melting point oxide mixtures of $PbO + Bi_20_3 + B_20_3$ and re-annealed. The energy dispersive X-ray results indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting point oxide constituents. The abnormally high dielectric constants, $\epsilon_ \gamma = 10^4 to 10^5$, point to the prevalence of GBL capacitance in these ceramics. Depending upon the conditions of the second annealing, the breakdown voltage could be adjusted from 0.2 to 1.5 V per grain boundary, without any change in the grain size. The nonlinearity coefficient a ranges from 6 to 15 and the barrier height from 0.15 to 0.3 eV. This can be explained on the basis of variable pinning of traps at the interface and also the extent of trap filling.

Item Type:	Journal Article
Publication:	Materials Science and Engineering B
Publisher:	Elsevier
Additional Information:	Copyright of this article belongs to Elsevier.
Keywords:	Grain boundaries;Schottky barriers;Ceramics;Electrical measurements
Department/Centre:	Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited:	27 Jun 2007
Last Modified:	19 Sep 2010 04:35
URI:	http://eprints.iisc.ac.in/id/eprint/10009

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