A study of the physical adsorption of nitrogen gas on polished 347 stainless steel at very low pressures

This investigation involves the study of the physical adsorption of nitrogen gas on a polished 347 stainless steel surface in the 1 x 10⁻⁹ to 1 x 10⁻⁷ torr pressure region. Characterization studies on the polished 347 stainless steel surface were also conducted utilizing a scanning electron microscope to determine topology and an ion microprobe mass spectrometer to determine surface composition. The stainless steel surface was cleaned in vacuo by thermal bakeout to 400° C for 48 hours. Adsorption measurements were made by employing a dynamic technique. Adsorption isotherms were constructed for temperatures of 87.7° K and 77.4° K. The data were tested for fit to the Dubinin-Radushkevich equation and found to fit reasonably well. Isoteric heats of adsorption were calculated from the analog of the Clausius-Clapeyron equation and plotted against relative coverage. These data indicated that the polished steel was a very heterogeneous surface. A roughness factor was determined to be about 70.

An adsorption isotherm was also constructed for nitrogen physically adsorbed on 7740 Pyrex glass at 77.4° K. This well studied experiment was repeated in order to evaluate the vacuum system and dynamic technique used before studying the polished stainless steel.

An error analysis of the dynamic technique indicated a maximum uncertainty of about ±40 percent.