Engineering topics

Davidson, M. R.

Publications and source records attributed to Davidson, M. R..

5 records

NASA NTRS1992

Study of the oxygen transport through Ag (110), Ag (poly), and Ag 2.0 Zr

The transport of oxygen through high-purity membranes of Ag (110), Ag (poly), Ag (nano), and Ag 2.0 Zr has been studied by an ultrahigh vacuum permeation method over the temperature range of 400-800 C. The data show that there are substantial deviations from ordinary diffusion-controlled transport. A surface limitation has been confirmed by glow-discharge studies where the upstream O2 supply has been partially converted to atoms, which, for the same temperature and pressure, gave rise to over an order of magnitude increase in transport flux. Further, the addition of 2.0 wt percent Zr to the Ag has provided increased dissociative adsorption rates, which, in turn, increased the transport flux by a factor of 2. It was also observed that below a temperature of 630 C, the diffusivity exhibits an increase in activation energy of over 4 kcal/mol, which has been attributed to trapping of the atomic oxygen and/or kinetic barriers at the surface and subsurface of the vacuum interface. Above 630 C, the activation barrier decreases to the accepted value of about 11 kcal/mol for Ag (poly), consistent with zero concentration at the vacuum interface.

Outlaw, R. A.↗

NASA NTRS1991

Auger electron intensity variations in oxygen-charged silver

Auger intensity variations over an oxygen-charged polycrystalline silver surface have been observed by studies of Auger images and line scans of selected adjacent grains which were determined to be the (421) and (221) orientations. The observed contrast (M4.5VV transition) between the grains is produced by the variation in the detected Auger electrons caused by the different directions (interatomic direction) of forward focusing in each grain. The contrast produced by the Ag Auger electrons was found to be strongly dependent on the surface order of the grains, but that of the O Auger electrons was not, presumably because the atoms were randomly distributed throughout the Ag surface or subsurface. The contrast observed between the grains at the lower Auger energies (N1VV and N1N2.3V transitions) appeared to be produced by constructive interference from multiple scattering. The N1N2.3V electrons at 29 eV, for example, gave higher contrast than that of the N1VV transition at 78 eV.

Lee, W. S.↗

NASA NTRS1989

Continuous electron stimulated desorption using a ZrO2/Ag permeation membrane

During the development of an atomic oxygen beam generator for laboratory simulation of the atmospheric conditions in low earth orbit, a new technique for performing electron stimulated desorption (ESD) in a continuous manner has been developed. In this technique, oxygen permeates through an Ag membrane at elevated temperature thereby providing a continuous supply of oxygen atoms to the 1000-A ZrO2 coating at the vacuum interface. ESD then results in a large peak of neutral O2 molecules which ultimately decay into steady-state desorption. The ESD signal is linear with respect to primary beam flux (0.035 O2 molecules per electron at a primary beam energy of 1 keV) but nonlinear with respect to primary beam energy.

Outlaw, R. A.↗

NASA NTRS1989

Auger electron intensity variations in oxygen-exposed large grain polycrystalline silver

Auger electron spectroscopic studies of the grains in oxygen-charged polycrystal-line silver show significant intensity variations as a function of crystallographic orientation. These intensity variations were observed by studies of the Auger images and line scans of the different grains (randomly selected) for each silver transition energy. The results can be attributed to the diffraction of the ejected Auger electrons and interpreted by corresponding changes in the electron mean-free path for inelastic scattering and by oxygen atom accumulation in the subsurface. The subsurface (second layer) octahedral sites increased in size because of surface relaxation and serve as a stable reservoir for the dissolved oxygen.

Lee, W. S.↗

NASA NTRS1988

Oxygen transport through high-purity, large-grain Ag

The permeability of oxygen through pure, large-grain Ag membrames has been found to be linear and repeatable over the 400-800 C range, but only at a magnitude that is a factor of 3.2 smaller than ascribed by prior research. AES data indicate the pertinence of grain-boundary considerations, due to the virtual undetectability of intragranular oxygen. Vacuum-desorption of oxygen-saturated Ag is found to occur at the critical temperature of 630 C; this is consistent with the increased mobility of oxygen atoms in the higher temperature regime.

Outlaw, R. A.↗