Lead-free ruthenium-based thick-film resistors: a study of model systems

A model system of thick-film resistor (TFR) was prepared starting from RuO₂ powders and a lead-free glass. The microstructural development was investigated by scanning and transmission electron microscopies, energy dispersive X-ray fluorescence, X-ray diffraction, thermogravimetry and other complementary techniques. The electrical properties of the resistors were analysed with particular attention to sheet resistance, temperature coefficient of resistance, size effects and piezoresistive properties. It was found that these simple systems are interesting with respect to their stability in ageing tests at relatively high temperatures but they are not promising for high-temperature piezoresistive gauges because of low strain sensitivity. Some samples were also prepared with a Bi₂Ru₂O₇; a notable exchange reaction occurs between the conductive grains and the glass matrix which prevents the formation of pyrochlore-type based resistors with this glassy matrix.