| AVS 58th Annual International Symposium and Exhibition | |
| Vacuum Technology Division | Tuesday Sessions |
| Session VT-TuM |
| Session: | Accelerator and Large Vacuum System Design, Outgassing and Pumping |
| Presenter: | Robert Berg, National Institute of Standards and Technology |
| Correspondent: | Click to Email |
Reports of hydrogen outgassing usually involve large vacuum chambers made of stainless steel. Typically, the chambers were baked at temperatures up to 400 °C, but the outgassing was measured only at room temperature. In contrast, the chamber used for the present measurements had a volume of only 29 cm3, with a correspondingly large surface-to-volume ratio, and the outgassing rate was measured at temperatures as high as 250 ° C.
The present outgassing measurements were compared with a numerical model that included (1) diffusion of hydrogen atoms in the steel, (2) recombination at the surface into hydrogen molecules (plus the reverse process), and (3) release of hydrogen from traps. A trap is a site, such as a dislocation or a grain boundary, where the hydrogen is bound more strongly than in the surrounding metal. Traps allow stainless steel to hold much more hydrogen than implied by the small solubility of hydrogen in pure iron. The larger binding energy means that increasing the temperature does more than speed up diffusion; it also increases the amount of mobile hydrogen in the steel.
The model used the values obtained by Grant, Cummings, and Blackburn for the diffusivity, recombination, and permeation of hydrogen in stainless steel. If one assumes a plausible value for the initial hydrogen concentration, it gives outgassing rates in rough agreement with the measurements.