Reduction process of RuO2 powders 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 ∼100 deg. C, at a rate essentially independent of temperature (50-150 deg. C) 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 deg. 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 ethyl cellulose were studied. Possible implications of the results on the properties of RuO2-based TFRs are discussed