Pressure and purging effects on material outgassing and evaporation

Details Pressure and purging effects on material outgassing and evaporation Pressure and purging effects on material outgassing and evaporation

Sep 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003esasp.540..203s&link_type=abstract

In: Proceedings of the 9th International Symposium on Materials in a Space Environment, 16-20 June 2003, Noordwijk, The Netherla

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Material Vacuum Outgassing, Degassing, Evaporation, Pressure Effects, Purging

Scientific paper

Full material outgassing and evaporation rates do not occur unless carried out under high vacuum. The evaporation and the outgassing rates are reduced by orders of magnitude when carried at pressures higher than about 10E-2 torr. This paper reports some experimental data on the outgassing, evaporation, and distillations rates carried at various vacuum pressures and reported in the literature. The data in conjunction with the equation for mass transport have been employed to evaluate the outgassing rates and the corresponding flow conductances for materials exposed to different residual air pressures. The conductances are controlled by the outgassing gases diffusing through the external pressure and through a film thickness on the outgassing surface. The evaporation of water, M=18 g/mole at 293 K with a saturated vapor pressure of 17 torr, has been considered. Also, the outgassing of materials with an average molecular mass of 100 g/mole and equivalent vapor pressures of 1E-8 torr have been considered. The 1E-8 vapor pressure corresponds to a combined outgassing rate of 1E-9 g/cm2/s obtained at the end of a full vacuum outgassing test of a group of materials. The results are employed to estimate the rates of evaporation and outgassing of materials under various conditions of external pressures knowing the rates obtainable in full vacuum. It has been calculated among others that the evaporation of water at a normal temperature and under normal air pressure is 9.6E-6 g/cm2/s. The outgassing for the M=100 g/mole materials with an outgassing rate of 1E-9 g/cm2/s in full vacuum is 5E-13 g/cm2/s under normal external pressure. These are about 5 orders and 4 orders of magnitude respectively lower than the rates in full vacuum. Their conductance's at normal pressure have been calculated to be 1.06 cm3/cm2/s for water and 8.51 for the M=100 g/mole. The corresponding film thicknesses on the outgassing surfaces are about 0.22 cm and 0.012 cm respectively. The effect of clean gaseous purging on the outgassing has also been considered.

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