Subsolidus phase equilibria in the RuO2–ZnO–SiO2 system

Thick-film resistors are made by screen-printing thick-film paste on insulating, mainly alumina, substrates. After printing and drying, the thick-film pastes are fired in a belt furnace. Thick-film resistor pastes consist basically of a conducting phase, a lead borosilicate-based glass phase and an organic vehicle. The organic material is burned out between 300 and 400 C during the high-temperature processing. The ratio between the conductive and glass phases roughly determines the specific resistivity of the resistor. In most modern resistor compositions the conductive phase is either RuO2 or ruthenates. The glass phase is based on the B2O3–PbO–SiO2 glasses with the molar ratio of SiO2/PbO approximately equal to 2. Some other oxides are also added as modifiers of the temperature coefficient of expansion and/or the glass-transition temperature [1–5]. The well-known European environmental legislation, i.e., the RoHS Directive (restriction of the use of certain hazardous substances), requires the elimination of the lead, or at least a minimising of the lead content in electrical and electronic equipment to below 0.1 wt.%—starting from July 2006. Thick-film materials are currently an exemption from the directive, but the producers of thick-film materials are already developing new material systems in accordance with the directive [6]. While there are many lead-free conductor and dielectric compositions available, the development of thick-film resistors with lead-free glasses is still mainly in the experimental stage. At least to the authors’ knowledge, no commercially available thick-film lead-free resistor series with characteristics comparable to ‘‘conventional’’ resistors is on the market. However, many ‘‘experimental’’ papers have reported on the preparation and investigations of the characteristics of lead-free thick-film resistors where the conductive phase is based, like with ‘‘conventional’’ resistors, on ruthenium oxide or ruthenates [7–11]. In the glass phase the lead oxide is often replaced with the zinc oxide [12–15]. Regardless of the very different melting points (Tm is 886 and 1,975 C for PbO and ZnO, respectively) both oxides share similar characteristics. Both PbO and ZnO are classified as intermediate oxides, i.e., they can be either glass formers or glass modifiers. Some of the characteristics, i.e., the dissociation energy per MOx, the coordination number and the single bond strength, are summarised in Table 1 [16]. The aim of this work was to investigate subsolidus phase equilibria (in air) in the RuO2–ZnO–SiO2 system. The results would indicate possible interactions between silicarich glass with the addition of ZnO and the conductive phase (ruthenium oxide) in lead-free thick-film resistors. The binary Zn2SiO3 compound exists in the SiO2–ZnO system. The melting point of the eutectic on the ZnO-rich side is 1,507 C, and on the SiO2-rich side it is 1,432 C [17, 18]. Morimoto et al. reported a synthesis at a high temperature of 1,400 C and at a high pressure, and a characterisation of the binary compound ZnSiOO3 [19]. Phase equilibria in the RuO2–SiO2 [5] and RuO2–ZnO systems were studied by Hrovat et al. [20]. In both systems there is no binary compound and no liquid phase (eutectic) at temperatures up to 1,405 C, the temperature at which RuO2 decomposes (in air) to metallic ruthenium and oxygen. For the experimental work, RuO2 (Ventron, 99.9%), ZnO (Johnson Matthey, ultrapure), and SiO2 (Riedel de M. Hrovat (&) J. Holc S. Glinsek Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia e-mail: marko.hrovat@ijs.si