Phase equilibria in RuO2-TiO2-Al2O3 and RuO2-TiO2-Bi2O3 systems

Abstract

Thick-film-resistor materials consist, basically, of a conductive phase, a glass phase and an organic vehicle which evaporates and burns out during the firing process. In contemporary thick-film resistors the conductive phase is usually either RuO2 or a ruthenate, usually Bi2Ru207 [1-3]. Thick-film resistors, prepared only from a mixture of the conductive and glass phases, possess a relatively high positive temperature coefficient of resistance (TCR). To decrease the TCR, small amounts of so-called TCR modifiers (that is, semiconducting oxides with a negative TCR) are added. TiO2 is a TCR modifier which shifts a TCR to the lower values and, when added in small quantities, does not influence the resistivity of thick-film-resistor material significantly [4, 5]. Thick-film circuits are made by printing and firing thick-film pastes on ceramic substrates. The ceramic used is mostly A1203. During firing, the components of thick-film materials react with each other and also with the alumina substrate. The characteristics of thick-film resistors, which are determined by the conductive phase, also depend to a certain degree on possible interactions with the alumina substrate [6, 7]. The aim of this work was to investigate phase equilibria in RuO2-TiO2-A1203 and RuO2TiO2-Bi203 systems. Both ternary systems imply possible interactions, which can occur between the conductive phase in the resistors and the alumina substrates or TiO2. In the TiO2-A1203 system studied by Lejus et al. [8], a binary compound, A12TiOs, with a melting point at 1850 °C is present. The melting point of the eutectic (80% TiO2) is at 1700 °C. A12TiO 3 is stable at temperatures above 1200°C, whereas under 1200 °C it decomposes into A1203 and rutile [9]. In RuO2-A1203 systems there is no binary compound and no liquid phase (eutectic) up to 1400 °C, the temperature at which RuO2 decomposes to metallic ruthenium and oxygen [10, 11]. A RuO2-Bi203 system was investigated by Hrovat et al. [12]. The binary compound Bi2Ru207 decomposes at temperatures over 1200 °C into RuO2 and Bi203. The melting point of the eutectic (80% Bi203) is at 745 °C [12]. In the Bi203-TiO2 systems studied by Speranskaya et al. [13] and by Levin and Roth [14], there are three binary compounds: Bi12TiO20 (TiO2-stabilized y-Bi203) , Bi4Ti3012 and BizTi4Oll. The lowest melting point (eutectic) between BiazTiO20 and Bi20 3 is at 835 °C. For experimental work, TiO2 (Fluka, pure), RuO2 (Fluka, extra pure), A1203 (Alcoa, A-16) and Bi20 3 (Merck, extra pure) were used. The anatase form of TiO2 was transformed into rutile by firing at 1200 °C. The samples were mixed in ethyl alcohol, pressed into pellets and fired with intermediate grinding. During firing, the pellets were placed on platinum foils. (The compositions of the relevant samples are shown in Figs 3 and 4.) Firing temperatures for compositions with Bi203 were up to 950 °C and for compositions without Bi203 (that is, in the RuO2-TiO2-A1203 system) up to 1300 °C. The compound A12TiO5 was synthesized by firing at 1400 °C. The results were evaluated by X-ray powder analysis, differential thermal analysis, energy-dispersive X-ray microanalysis (EDS) and wavelength-dispersive X-ray microanalysis (WDS). In the RuO2-TiO2 system, there is no binary compound and no liquid phase (eutectic) at temperatures below 1400 °C. The microstructure of a polished RuO2/TiO2 1/1 sample, fired at 1300 °C, is presented in Fig. 1. The material is a mixture of darker TiO2 and lighter RuO2 grains. Preliminary results obtained by WDS indicate the existence of solid solubilities (at 1300 °C) of approximately 8% RuO2 in TiO2 and a little over 10% TiO2 in RuO> Further experimental work, to determine the exact limits of the solid solubilities, and their temperature dependence, is currently in progress and will be reported elsewhere. An RuOz-TiO2 phase diagram is schematically presented in Fig. 2. The dashed line at 1400 °C indicates the decomposition of RuO2 to metallic ruthenium and oxygen.