Phase transitions in Ru based thick-film (cermet) resistors

Thick-film resistive inks have been prepared starting from powders obtained by precipitation of RuCl₃ on two different lead-containing glasses. The structural and electrical properties of the films have been investigated after heat treatment at various temperatures. The evolution of the microstructure involves phase transformations from Pb-rich (Ru-deficient) pyrochlores to Ru pyrochlores and finally to RuO₂. Together with these crystallographic features, the atomic absorption spectroscopy shows a gradual decrease of the Ru dissolved in the glass when the firing temperature increases. The conductivity of the samples decreases according to the transition from pyrochlore to dioxide. These results emphasize the role played by exchange reactions in the control of transport mechanisms in thick-film cermet resistors.