An NiO single crystal was equilibrated with an atmosphere having an oxygen partial pressure (Po2) and the electric conductance (Q) was measured. Then Pat was changed and a was recorded until a new equilibrium was attained. As the variation rate of a reflects that of positive hole concentration and hence cation vacancy diffusivity, the plot of a vs time allows to estimate the diffusion coefficient of cation vacancy. The observed diffusion coefficients (D,) when Po2 was changed from 10-4 to 1 atm were somewhat smaller than those by a Po2 change from 1 to 10-4 atm, while a simple vacancy model requires the same diffusion coefficient for both cases. The smaller Dv values observed when Po2 was elevated can be practically attributed to singly ionized vacancy. The larger values observed when Po2 was dropped were due to the contribution of doubly ionized vacancy which is more mobile than the singly ionized one.

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structure, while the amount of Cr in the film was twice as much as that of Ni. By annealing of the stripped oxide film the electron diffraction pattern was changed to sharp strong patterns of Cr2O3 containing NiCr2O4 and NiO. in the oxide layer increased with oxidation time, whereas the Ni content remained constant. From the present results the following growth process can be constructed. At the beginning of oxidation, this alloy is oxidized at the alloy composition of 80Ni-2OCr. The oxide crystal structure is of NiO. The Cr content in the oxide film increases by preferential oxidation of Cr at the inner oxide layer during the oxidation. Consequently, the concentration gradient of Cr is built up in the oxide layer, from the composition of NiO at the outer layer to the composition close to Cr2O3 at the inner layer. Due to the epitaxial relationship with the outer NiO structure, the oxide continues to be of NiO structure despite an increase in the Cr content at the inner layer. As the oxidation continues, a continuous layer of stable corundum type Cr2O3 is formed at the inner layer. Then, the Cr2O3 layer acts as protective for oxidation and oxidation rate is controlled by diffusion of Cr through the Cr2O3 layer.

The mechanismby which ODS alloys are provided with exoellent oxidation resistance

The surface compositions of two types of stainless steels in vacuum were observed with AES and XPS at 1000–1100 K; a 304 stainless steel doped with nitrogen and boron (304-NB) and a 304 stainless steel doped with nitrogen, boron, and cerium (304-NBCe). The changes of surface compositions of these steels were compared with that of commercial 304 stainless steel. A thin layer of boron nitride came out from grain boundaries and spread over the surface of the 304-NB, but did not uniformly cover the surface. The spreading mechanism of the boron nitride layer at the initial stage of heating can be explained by Avrami’s model. In the case of 304-NBCe, a boron nitride layer almost uniformly covered the surface at the early stage of heating, and rendered the surface inert to the adsorption of carbon and oxygen. Therefore, the 304-NBCe stainless steel is a candidate material for vacuum vessels.