Studies of the initial oxidation of iron-silicon 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{dollar}sp+{dollar} and Xe{dollar}sp+{dollar} 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 "SiO{dollar}sb2{dollar}"-rich external layer is formed, as predicted by others, and established preferentially at the first stage of oxidation. A Si-depletion zone {dollar}sim{dollar}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{dollar}sb2{dollar}SiO{dollar}sb4{dollar}, gradual changes of the oxidation state of Fe from its highest oxidation state ("Fe{dollar}sb2{dollar}O{dollar}sb3{dollar}"-like) at the top to the lowest ("FeO"-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{dollar}sb2{dollar}. The best annealing conditions for each single-crystalline alloy were determined to provide the cleanest surface for the following initial oxidation experiments. Annealing at {dollar}sim{dollar}550{dollar}spcirc{dollar}C for 20 min after Ar{dollar}sp+{dollar} 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 {dollar}sim{dollar}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, {dollar}sim{dollar}13at% Si. Various comparative experiments were performed to support the interpretations. ftn{dollar}sp1{dollar}DOE Report IS-T-1343. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.