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Glassy Zr 70 Pd 30 as well as Zr 70 Au 30 prepared by melt-spinning exhibit a “catastrophic” oxidation behavior in wet as well as synthetic air. For example, at a temperature of 270 °C, ribbons (about 30 μm in thickness) are fully oxidized within a few hours. Thermogravimetric analysis reveals an oxidation kinetics by orders of magnitude faster than those in comparable Zr–Pt or Zr–Ni, Zr–Cu as well as related ternary or quaternary glasses. Detailed microstructural investigations, in particular cross-sectional microscopy, revealed details of this reaction and indicate for example for Zr–Au glasses at 25 °C an oxygen diffusivity in the developing zirconia scale of about 2×10 −17 m 2 s −1 ; such a diffusivity is known to occur for stabilized ZrO 2 only at about 300 °C. At the as-cast free surface of the ribbon nucleation of the oxidation process starts at defects like scratches or along the grooves at the contact side. The developing “vulcano” like morphology (with diameters in the range of 10 μm or more) can be explained assuming a diffusion controlled thickening of the scale, but linear lateral growth. It is of interest that the occurrence of extreme fast oxidation is correlated with significant changes in the zirconia microstructure as indicated by a decrease in the intensity of the low-indexed diffraction lines; TEM reveals a nanocrystalline structure with ZrO 2 crystal diameters even below 5 nm. The results will be discussed in detail taking into account for example the different redox potentials of the noble metals, their catalytic activity, the Zr-noble metal bond strength as well as the density of states at the Fermi level of the glasses. |
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