Studies of the initial oxidation of Fe-Si alloys by AES, XPS, EELS, and LEED

摘要

Thin oxide layers on polycrystalline Fe-8.75at% Si, and single crystalline Fe-6.85at% Si (110) and (111) alloys were investigated by AES (including Ar[superscript]+ and Xe[superscript]+ depth profiling), EELS, XPS and LEED during the initial oxidation stage at room temperature under very low oxygen pressure. It was observed that a very thin \u22SiO[subscript]2\u22-rich external layer is formed, as predicted by others, and established preferentially at the first stage of oxidation. A Si-depletion zone ~5 A deep was found in the polycrystalline alloy with about 25% Si-depletion at the alloy/oxide interface. The formation of an Fe silicate-like structure just beneath the Si oxide-rich top layer down to the alloy/oxide interface was also observed. Although this Fe-silicate layer was determined to be predominantly in the form of Fe[subscript]2SiO[subscript]4, gradual changes of the oxidation state of Fe from its highest oxidation state (\u22Fe[subscript]2O[subscript]3\u22-like) at the top to the lowest (\u22FeO\u22-like) at the bottom were also observed. The rates of oxidation of polycrystalline alloys were determined to be retarded to approximately 40% of the rate of pure Fe after exposure to 200 L of O[subscript]2. The best annealing conditions for each single-crystalline alloy were determined to provide the cleanest surface for the following initial oxidation experiments. Annealing at ~550°C for 20 min after Ar[superscript]+ sputter-cleaning was found to be best for Fe-Si (110). The initial oxidation results for the single-crystalline alloys were compared with those of the polycrystal alloys to show a general resemblance in the various initial oxidation features. The retardation in Fe oxidation rate with respect to that of the corresponding pure Fe surface was, however, enhanced by ~33% on (110) compared to the polycrystal case. Comparison between the (110) and (111) planes reveals only a minor crystallographic dependence of the initial oxidation of single-crystalline Fe-6.85at% Si. A general increase in the degree of the retardation of the oxidation of Fe in Fe-Si (110) was observed as the initial surface Si concentration goes up by segregation to a certain critical value, ~13at% Si. Various comparative experiments were performed to support the interpretations. ftn[superscript]1DOE Report IS-T-1343. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.