Outgassing time-dependence

R. M. Barrer Outgassing times depend upon the openness of the crystals, the cations present, and the size of the crystals. As a practical working rule for water removal, a final temperature of not less than 360°C maintained under the best vacuum for not less than 24 hours is desirable. 

At constant temperature, the time-dependence of outgassing follows the relationship ... 

Although an old process, compression molding is not well described technically or analytically in the literature. Basically, what is needed is an analysis that combines rheology, heat transfer, chemical reaction (the curing process), and even mass transfer (outgassing) in a dynamic (time-dependent) system. The situation is also complicated by changing temperatures and pressures. 

Fig. 11. Pressure dependence of the IR spectra of H2 adsorbed at 20 K on high-surface-area (230m g ), sintered (dOm g ). and smoke (10m g ) MgO samples, parts (a), (b), and (c), respectively. The upper curve of each series of spectra has been collected after an elapsed time allowing the surface species to reach the equilibrium conditions and corresponds to the maximum H2 coverage (Fjj, = 1010 kPa), while the bottom spectrum has been recorded after prolonged outgassing at 20 K (Ph, <10 Pa). All spectra have been vertically shifted for the sake of clarity. Note that the ordinate scale is progressively expanding when passing from part (a) to part (c), to account for the loss of band intensity with the decrease of the MgO surface area. (Adapted with permission from Gribov et al. (,124).)...

The observed noble-gas abundances and isotopic ratios on Venus are summarized in Tables 3 and 4. The helium mixing ratio is a model-dependent extrapolation of the value measured in Venus upper atmosphere, where diffusive separation of gases occurs. The main differences between Venus and Earth are that Venus is apparently richer in He, Ar, and Kr than the Earth, and the low " Ar/ Ar ratio of — 1.1 on Venus, which is —270 times smaller than on Earth. The low " Ar/ Ar ratio may reflect more efficient solar-wind implantation of Ar in solid grains accreted by Venus and/or efficient early outgassing that then stopped due to the lack of plate tectonics. Wieler (2002) discusses the noble-gas data. Volkov and Frenkel (1993) and Kaula (1999) describe implications of the " Ar/ Ar ratio for outgassing of Venus. 

At some level of calcite supersaturation, depending on the concentration and reactivity of inhibitors and other kinetic factors, calcite tends to nucleate and precipitate. Herman and Lorah (1987) noted previous field studies in which calcite precipitation did not occur until CO2 outgassing had raised calcite supersaturation levels to 5 or 10 times above equilibrium. In their own work on Falling... 

In isocyanates, the hazard potential depends to a cmisiderable extent on air moisture. Because of the high level of reactivity, with water amcmg other substances, the primary products show rapid chemical changes in moist air and lose their toxicity. It is therefore always better to work under cmiditions of high humidity and to wipe off the adhesive joints with a wet rag after fixation if possible. Outgassing in 1C and 2C PUR adhesives are reduced to practically zero within 10% of the total curing time. 

The kinetics of the copolymerization of norbornenes and maleic anhydride has been analyzed in situ by IR spectroscopy [254]. Gas phase FT-IR has been combined with TGA to investigate outgassing from the ketal-based KRS-XE e-beam resist [222]. A flow cell which allows exposure and heating of a resist-coated wafer was attached to the IR spectrometer to study exposure- and temperature-dependent outgassing. Real time deprotection of the KRS resist has been also monitored by infrared reflectance-absorbance spectroscopy [222,494]. 

The length of time for single or multiple bumping depends on the material used and the size and configuration of the part. This dwell time with release of gases can actually reduce the total cure cycle time. Outgassing also can occur after parts are molded. With certain materials this condition can last for months or years. If parts are to be metallized or plated, outgassing tendencies can actually rupture the plated film. 

Outdiffusion is when the mobile species that reaches the surface does not volatilize but remains on the surface as a contaminant. Plasticizers from molded polymers constitute an example of a material that outdiffuses from the bulk of the material. Often there is both outgassing and outdiffusion at the same time. The outgassing and outdiffusion properties of a material often depend on the fabrication and history of the material. 

As we indicated already, the pumpdown time and resulting vacuum life depend on more than just the pumping speed. The outgassing of molecules often dominates pumpdown time and vacuum integrity, so it is desirable to minimize the contaminants in the dewar. 

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