Thermochemical reactions between PZT and Ag/Pd powders: Relevance to cofiring of multilayer actuators

Mixtures of Ag_1-xPd_x (x=0.2, 0.3) and doped PZT ceramic powders have been heat treated in air and the resulting phase content has been analyzed by X-ray diffraction and transmission electron microscopy. Beginning around 400°C, a phase similar to PbPdO₂ is formed on the surface of the Ag_1-xPd_x particles and subsequently decomposes at temperatures <700°C. Consequently, the remaining Ag _1-xPd_x powder becomes significantly silver-rich while the reaction progresses. After decomposition the Pd appears to realloy and the initial Ag_1-xPd_x composition is recovered. We show that the reaction is all but eliminated in a nitrogen atmosphere. The occurrence of this reaction was also investigated in PZT multilayer actuators cofired with Ag_0.7Pd_0.3 electrodes. Transmission electron microscope analysis revealed the presence of distinct crystallites at the electrode-ceramic interface, most likely nucleated from a PbO liquid phase arising from the decomposition of PbPdO₂.