Papers by Keyword: Oxygen-Ion Diffusion

Abstract: Oxygen ion diffusion coefficient was measured, for the first time, in oxide films grown by the high temperature oxidation of the AISI 304 austenitic stainless steel. The steel samples were polished and then oxidized in synthetic air in order to grow the chromia oxide (Cr₂O₃) film. The oxygen diffusion experiments in the oxide films were performed by using the¹⁸O isotope as oxygen tracer. The isotope diffusion studies were performed from 750 to 900^o C, in Ar +21%¹⁸O₂ atmosphere and the oxygen ion diffusion profiles were established by secondary ion mass spectrometry (SIMS) analysis. From the¹⁸O diffusion profiles, the bulk, effective and grain boundary diffusivities were determined. Using Wagner´s theory, it is shown that, under the used experimental conditions, the oxygen ion diffusion is large enough to assure the growth rate of the chrome oxide films formed by the oxidation of the AISI 304 stainless steel.

Abstract: In order to investigate the role of oxygen diffusion in the oxidation process of the AISI 439 ferritic stainless steel, oxygen ion diffusion coefficients were determined, for the first time, in oxide films formed by the oxidation of this steel. Steel samples were firstly oxidized from 750^o C to 900^o C, in synthetic air, in order to grow oxide films mainly made up of chrome oxide; the oxygen diffusion experiments were then performed using the stable isotope ¹⁸O as oxygen tracer. The introduction of the ¹⁸O in the oxide film was performed by means of the gas-solid isotopic exchange method, in the temperature range of 750-900^o C, in Ar+21%¹⁸O₂ atmosphere. The ¹⁸O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Parabolic oxidation constants calculated by means of Wagner´s theory, using the oxygen ion diffusion coefficient determined by our experimental process, are greater than oxidation constants previously determined in oxidation experiments from 850 to 950º C, in air, which indicates that the oxygen ion diffusion is large enough to assure the growth rate of the oxide film formed by the oxidation of the AISI 439 steel in these temperatures.

Abstract: Based on the oxygen ion conductor La1.95K0.05Mo2O8.95, a series of Fe-doped samples La1.95K0.05Mo2-xFexO9- (x=0, 0.025, 0.05, 0.1) were prepared with conventional solid reaction method. The effects of Fe doping on the structure, oxygen ion diffusion and electrical conductivity were studied. Based on the results of conductivity measurements, it can be concluded that Fe doping can improve the conductivity obviously, and doping of both Fe and K samples have better conductivity, which successfully suppresses the phase transition.

Abstract: The mechanism of the recovery of the nonlinearity voltage-current (V-I) characteristics after electrical degradation was investigated from various viewpoints. The recovery processes of ZnO varistors in various gas atmospheres, and at various pressures, temperatures, and absolute humidity levels were evaluated. The evaluation involved the analysis of V-I characteristics, capacitance-voltage (C-V) characteristics, and X-ray diffraction (XRD). The height of the Schottky barrier and the thickness of the depletion layer were almost constant for sintering times between 3 and 5 h. On the other hand, the rate of recovery exhibited a change with sintering time. The recovery speed of the nonlinearity was also temperature dependent. It is suggested that the recovery process is caused by the transport of oxide ions through the grain boundaries between ZnO grains. It was found that the recovery process is affected by the crystal structure of Bi2O3 particles, which exist at the boundaries between ZnO grains.