Subsolidus phase equilibria in the PbO-poor part of RuO₂–PbO–SiO₂ system

Abstract

After long-term high-temperature firing, the conductive phase in thick film resistors based on RuO₂ remains unchanged. In contrast, ruthenates decompose, presumably due to interactions with the silica-rich glass phase. Subsolidus equilibria in the PbO-poor part of the RuO₂–PbO–SiO₂ diagram were studied with the aim of investigating possible interactions between the conductive phase (either ruthenium oxide or lead ruthenate) and silica-rich glasses in thick-film resistors. The tie lines are between Pb₂Ru₂O_6.5 and PbSiO₃, and between RuO₂ and PbSiO₃. This indicates that the lead ruthenate is not stable in the presence of the silica-rich glass phase.

Introduction

Thick film resistors materials are used for making resistors on alumina ceramics in thick-film hybrid circuits. They span very wide resistivity values from a few Ω to few MΩ, are very stable and can be laser trimmed to narrow tolerances.

In most modern thick film resistor compositions, the conductive phase is either RuO₂ or ruthenates; mainly lead or bismuth ruthenates. During the firing cycle, all the constituents of the resistor paste react with each other and the melted glass also interacts with the substrate. The resistors are fired only a short time at the highest temperature, typically 10 min at 850 °C. Because of this, the reactions between the main constituents (glass and conductive phase) of the resistor material do not reach equilibrium [1], [2], [3], [4], [5].

If thick film resistors are fired at temperatures higher than the required 850 °C, and for a relatively long time, the interactions between the conductive phase and the lead–borosilicate-based glass phase approach equilibrium. This normally tends to decrease the sheet resistivities and increase the temperature coefficients of resistivity (TCRs) of these “overfired” resistors [6], [7], [8]. Adachi and Kuno [9], [10] studied high-temperature interactions between PbO–B₂O₃–SiO₂ glasses and Pb₂Ru₂O_6.5 or RuO₂. They showed that in glasses poor in PbO, the Pb₂Ru₂O_6.5 disappears and the RuO₂ is formed, while for PbO-rich glasses, the RuO₂ reacts with the PbO from the glass and forms Pb₂Ru₂O_6.5. X-ray diffraction study of ruthenate-based “equilibrated” resistors showed that at higher firing temperatures the ruthenate decomposes forming RuO₂, while the conductive phase in RuO₂-based resistors stays unchanged. Presumably because of the interaction with the molten glass the lead ruthenate decompose into PbO, which is dissolved in the glass, and into RuO₂ [11].

The aim of this work was to investigate subsolidus phase equilibria (in air) in the PbO-poor part of the RuO₂–PbO–SiO₂ system. The results would indicate possible interactions between silica-rich glass and conductive phase (either ruthenium oxide or lead ruthenate) in thick-film resistors.

Four binary compounds, PbSiO₃, Pb₂SiO₄, Pb₃SiO₅ and Pb₄SiO₆, exist in the PbO–SiO₂ system [12]. The eutectic composition on the SiO₂-rich side of the system is at 62% PbO and the eutectic temperature is 739 °C. Two more eutectics are formed between PbSiO₃ and Pb₂SiO₄, and between Pb₂SiO₄ and Pb₃SiO₅, with melting temperatures of 714 °C and 711 °C, respectively. The Pb₄SiO₆ compound melts incongruently at 725 °C.

Phase equilibria in the RuO₂–PbO system were studied by Shaplygin et al. [13] and Hrovat et al. [14]. The pyrochlore Pb₂Ru₂O_6.5 exists in the system. Shaplygin et al. proposed the eutectic composition around 72% PbO and the eutectic temperature 760 °C while Hrovat et al. reported the eutectic composition around 95% PbO and the eutectic temperature 875 °C. The pyrochlore series, which can be described by the general formula Pb₂(Ru_2−xPb_x)O_6.5 [15], is not a part of RuO₂–PbO system because the lead ions which exchange ruthenium ions on “B” sites are Pb⁴⁺. No data relating to the RuO₂–SiO₂ system were found in the open literature.