The Study of Oxygen Transport Through Polycrystalline, Single Crystal and Alloyed Silver

Abstract

The permeation of oxygen through large grain polycrystalline silver, through the (110), (111) and (100) single crystals of silver and through Ag0.05Zr alloy have been studied over the temperature range of 400 - 800 °C. In addition, studies were also conducted using glow discharge dissociation of the supply side ( upstream ) molecular oxygen in order to examine whether normal dissociative adsorption is a limiting step in the overall transport process. The permeability of oxygen through polycrystalline silver was found to be quite linear and quite repeatable. The diffusivity measurements were found to exhibit two distinct linear regions, one above and one below a critical temperature of 630 °C. The high-temperature data have an activation energy (11.1 kcal/mole ) similar to that reported by others, but the low-temperature data have a comparatively larger activation energy ( 15.3 kcal/mole ) which is probably due to the higher efficiency of traps. The permeability of oxygen through the (110), (111) and (100) single ·crystals of silver· all fell along a linear Arrhenius plot with the activation energy and the preexponential very similar to that found for the polycrystalline silver. This indicates that the transport limitation is approximately the same, even though there is some substantial difference in the microstructure. The magnitude of the diffusivity was found to be different for the three different crystal orientations which is inconsistent with the relation of K = DS since the solubility must be the same. The variation in the diffusivity can be explained by the relative displacement along a given crystallographic direction for a given number of jumps. The diffusivity ratios were in reasonable agreement with the ratio of the displacements squared. The permeability for Ag0.05Zr alloy was found to be a fact of 1 .86 higher than polycrystalline silver. The 0.05 percent Zr gives a much higher grain boundary and defect density to provide more reservoirs for oxygen which will increase the solubility and give the higher permeability. The diffusivity data have a higher activation energy but are very close in magnitude to the polycrystalline silver. The effect of the surface limitation on the transport was further examined by using a glow discharge on the upstream side to enhance the formation of oxygen atoms. It was found that the flux of oxygen substantially increased from the steady state dissociative adsorption level by a factor of 6. This indicates that there is a definite surface limitation for normal dissociative adsorption. The increased signal during the glow discharge suggests that the adsorption and dissolution rate for oxygen atoms is substantially greater than for the previously conducted permeation experiments where 02 molecules are dissociatively adsorbed