Reduction process of RuO2 powder and kinetics of their re-oxidation

With the aim of improving knowledge of the phenomena occurring during the development of thick-film resistors (TFRs), redox reactions were studied in fine powders of Ru and RuO2 by means of thermogravimetric analyses and x-ray diffraction as well as observations in scanning electron microscope and x-ray fluorescence. Reduction of RuO2 powders by forming gas (10% H2 in N2) occurs in a few minutes, after an induction period, at temperatures as low as approximately 100°C, at a rate essentially independent of temperature (50-150°) and grain size. The oxidation process is well described by the Avrami model, according to a diffusion limited reaction with an apparent activation energy of 1.5 +- 0.1 eV and it requires long times for completion even at relative high temperatures (700-850°C). The order of magnitude for the diffusivity of the mobile species was derived. In addition, the reduction and further re-oxidation of RuO2 powders blended with Et cellulose were studied. Possible implications of the results on the properties of RuO2-based TFRs are discussed.