Date Awarded

1994

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Physics

Advisor

H Frederick Dylla

Abstract

In this study, the outgassing rate from an electropolished stainless steel surface following exposures to H{dollar}\sb2{dollar}O vapor under various conditions was measured. The results of the experiments showed that the outgassing rate is proportional to {dollar}p\sb0\sp{lcub}n{rcub}{dollar}, where {dollar}p\sb0{dollar} is the H{dollar}\sb2{dollar}O exposure pressure and n is about 0.25. The outgassing rate is not as strongly dependent on the system temperature as one would expect if the temperature is kept the same during exposure and pump-down. The outgassing rate is also a function of the exposure time for the first several hours of exposure, indicating that the adsorption saturation time is on the order of hours. The pump-down time of a vacuum system can be reduced significantly by reducing the moisture content of venting gases. The time taken to achieve an outgassing rate of 10{dollar}\sp{lcub}-10{rcub}{dollar} Torr {dollar}\ell{dollar}/cm{dollar}\sp2{dollar} s following dry N{dollar}\sb2{dollar} venting is about one hour in comparison to one day if the system was vented to ambient air. In the second part of this study, the effect on the water outgassing of dc glow discharge cleaning fueled by common gases was studied. It is shown that the water outgassing rate following a typical air vent can be reduced by a factor of 13 when the surface is exposed to a He glow discharge at a dose of 0.8 coulomb/cm{dollar}\sp2{dollar} for two hours.;To explain the 1/t power law obeyed by the outgassing rate and other experimental results, diffusion-type outgassing models were proposed. Initially, non-uniform H{dollar}\sb2{dollar}O concentrations throughout the oxide layer were assumed and the outgassing rate expressions were derived without taking into account the oxide layer microstructure. A more comprehensive model starts with a simplified microstructure of a porous oxide layer, which consists of cylindrical pores with a pore length distribution inversely proportional to the square of pore length. This model predicts the 1/t time dependence for the outgassing rate under saturation conditions where the coverage on the inner surfaces of the pores is uniform. It also explains the observation that the quantity of adsorbed H{dollar}\sb2{dollar}O is a linear function of the logarithm of the exposure time.

DOI

https://dx.doi.org/doi:10.21220/s2-ypex-n908

Rights

© The Author

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