Toshiaki Osaka1 and Tadayuki Nakayama2
1Graduate School, Waseda University, Tokyo
Niobium samples were heated in an 18-8 stainless steel autoclave filled with deionized water and water vapour. After heating, the oxide films were identified by means of reflection and transmission electron diffraction, and then component elements in the thin films were detected by using an electron probe micro-analyzer.
The main results obtained are as follows:
(1) The film formed on Nb in deaerated high-temperature water by oxidation at 300°C for 1 hr was composed of the rutile-type crystal alone. In addition, the presence of Fe and Nb were detected by means of the X-ray micro-analysis of the film, indicating that the film was composed of (Fe,Nb)O2. However, the oxide film produced by oxidation for 24 hr, gave a diffraction pattern corresponding to the pseudo-hexagonal δ -phase.
(2) The oxide films on Nb in deaerated high-temperature water vapour and in a pyrex glass tube together with deionized water at 300°C for 1 hr also consisted of a normal rutile-type oxide, and were thought to be NbO2. The unit cell dimensions for the above oxide were a0=4.75 Å, c0=3.09 Å, c0/a0=0.65. Further, after heating in deaerated water vapour for 24 hr, the electron diffractogram of the film indicated the presence of δ -phase.
(3) The film formed on Nb in aerated hight-temperature water by oxidation at 300°C for 1 hr or 24 hr was a hexagonal-type oxide; a0=5.84 Å, c0=4.69 Å, c0/a0=0.80. Characteristic spectra CrK α 13 and NbL α 11 were detected from the film, and Cr6+ ions were detected from water after oxidation. Therefore the Cr ions may contribute to the formation of the hexagonal-type oxide.
(4) After heating in aerated high-temperature water vapour at 300°C for 1 hr or 24 hr, the δ -phase was found on the Nb surface.
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