Microstructure and electrical properties of ‘pre-sintered bismuth ruthenate-glass composite’ based thick film paste for resistor application

Abstract

Thick film resistors based on a bismuth ruthenium oxide (BRO) and a leadborosilicate glass have been used to study the effect of a ‘pre-sintered glass–ceramic composite’ powder in the paste preparation on the resistor characteristics. Pre-sintered glass–ceramic composite was prepared by heating a physical admixture of leadborosilicate glass frit and BRO as functional material at various temperatures above the softening point of the glass. The microstructural changes associated with ‘pre-sintering’ step were studied using scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction, and, were correlated to the electrical characteristics of these resistors. It was observed that pre-sintering at a lower temperature of 600°C helps in restricting the agglomeration of conducting BRO particles and in generating more non-sintered contacts between them, which improves consistency in sheet resistance and hot temperature coefficient of resistance (TCR). At the same time, the pre-sintering step lowers the absolute TCR value, mostly by way of compensation of the high negative TCR of glass by the positive TCR of Pb containing ruthenate phases and/or RuO2, which have positive TCR. It is explained that the Pb containing BRO phases are formed in situ through the Bi ↔ Pb exchange reaction. The pre-sintering temperature of 600°C was, thus, found to be optimum for the present processing conditions.