Contemporary thick film resistor materials are based on RuO 2 or ruthenates, mostly BieRuzO 7 [1, 2]. However, to obtain high sheet resistivity values, for example over 100 kf2 E3 -z, lead ruthenate is sometimes used because of its higher specific resistivity [3-5]. The specific resistivities of RuO2, BizRueO7 and Pb2Ru206.5 are around 40 X 10 -6 ~'~ cm, 150 X 10 -6 f2 cm and 270 x 10 -6 Q cm, respectively (the specific resistivity of metallic ruthenium is 4 x 10 -6 ~ cm) [3, 6, 7]. Around 65% of the approximately 5500 kg of ruthenium produced annually is used for the preparation of thick film resistor pastes [81. 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, TCR modifiers, i.e. semiconducting oxides with negative TCR, are added. TiO2 and NiO are two of the numerous TCR modifiers [3]. While most TCR modifiers decrease TCR and at the same time increase the specific resistivity [6], TiO2, when added in small quantities, does not influence the resistivity of thick film resistor materials significantly [9]. The aim of this work was to investigate the phase equilibria in the binary RuOz-NiO and PbO-NiO systems and in the ternary R u O z P b O T i O 2 and RuOz-PbO-NiO2 systems. Both ternary systems imply possible interactions between the conductive phase (lead ruthenate) in the high sheet resistivity thick film resistors and TiO2 or NiO, when used as the TCR modifiers. It should be mentioned, however, that during firing the thick film resistors are exposed for only a relatively short time (5 to 10 rain) to the highest temperature (850 °C), and therefore the reactions probably do not reach equilibria. Also, the resistor material is a very complex system consisting of many components which interact during firing. Still, the phase equilibria indicate the direction of the reactions. Phase equilibria in the RuOz-PbO and RuO2TiO2 systems were studied by Hrovat et al. [10, 11]. The eutectic composition in the RuOz-PbO system is around 95% PbO and the eutectic temperature is 875 °C. In the RuOz-TiO2 system there is no binary compound and no liquid phase (eutectic) at temperatures below 1400 °C, i.e. the temperature at which R u O 2 decomposes to metallic ruthenium and oxygen. The solid solubility is around 16% TiO 2 in RuO2 and 13% R H O 2 in TiOz at 1350 °C and decreases with decreasing firing temperature to less than 2% below 1100 °C [12]. In the TiOz-PbO system, studied by Eisa et al. [13], the binary compound PbTiO3 melts congruently at 1250 °C. The melting point of the eutectic with a lower melting temperature on the PbO rich side (85% PbO) is at 840 °C [13, 14]. For experimental work, TiO2 (Fluka, Anatas, +99%), RuO2 (Ventron, 99.9%), PbO (Merck, 99.9%) and NiO (Riedel de Haen, +99.9%) were used. The anatase form of T i O 2 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 pellets were placed on platinum foils. The compositions of the relevant samples are shown in Figs 3 and 4. The results were evaluated by X-ray powder analysis, differential thermal analysis (DTA), scanning microscopy and energy dispersive X-ray microanalysis. The proposed nickel oxide/lead oxide phase diagram is shown in Fig. 1. There is no binary compound. The eutectic temperature and composition were determined using a standard DTA procedure. The area of the peaks on the DTA curves was measured for samples with different compositions and the eutectic composition was extrapolated from these data. The eutectic composition is around 93% PbO and the eutectic temperature is 875 °C. The ruthenium oxide/nickel oxide "phase diagram" is shown in Fig. 2. The RuO2-NiO system is "empty"; there is no binary compound and no liquid phase (eutectic) at temperatures below 1400 °C, at which R u O 2 decomposes to Ru and O2. The ternary phase diagram (subsolidus) of the RuO2-PbO-NiO system is presented in Fig. 3. A tie line exists between PbzRu206. 5 and NiO. The ternary phase diagram (subsolidus) of the RuO2-PbO-TiO2 system is shown in Fig. 4. The tie
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Feb 12, 2022
Jiri Hlina + 5
Open Access
