Helium glass permeability

Experimental data from the literature [15] concerning freon 113 permeability on a vycor glass membrane were simulated by the 3D network model. An average effective length of each pore was selected in a way that the (non-condensing) helium permeability predicted by the network matches the experimental values, and at the same time gives a porosity and surface area close to the experimental ones. Subsequently, the pore size distribution obtained from porosimetry and the effective pore length were used for the simulation of the condensable vapor permeability. 

It is possible to measure the partial pressure of any gas in a mixture directly if there is a membrane that is permeable to that gas alone for example, palladium is permeable to hydrogen and certain types of glass are permeable to helium. The fact that at present only a few such membranes are known does not destroy the physical reality of the concept of partial pressure. Later it will be shown that in chemical equilibria involving gases and in physical equilibria such as solubility of gases in liquids and solids, it is the partial pressures of the gases in the mixture that are significant (further confirmation of the physical content of the concept). 

Alkaline earth ions are also very effective in blocking the diffusion of helium atoms, with a similar trend toward decreasing helium permeability in the order Ba > Sr > Ca > Mg for glasses having otherwise identical compositions. The least permeable traditional commercial glasses are based on alkaline earth aluminosilicate compositions. 

Phase separation has a major effect on helium permeation in oxide glasses. The permeabilities of the two phases often differ by several... 

Helium permeability at 200 °C in potassium silicate and potassium germanate glasses... 

Table 35. The helium permeability of glasses at room temperature in thickjcm. Hg pressure...

Fig. 4.8. Helium permeability of Zerodur in comparison with two low-expansion glasses [4.22]...

A very large area of application for the glass ceramic Zerodur is the technology for modern navigation of space rockets, aircraft, and ships or the positioning of telescopes, antennas, or satellites. The negligibly small coefficient of thermal expansion and the extremely small helium permeability of the material are of particular importance for this application. 

Alternatively, helium may be separated from natural gas by diffusion through permeable barriers, such as high silica glass or semipermeable membranes. The gas is supplied commercially in steel cylinders or tanks. The United States is the largest producer of helium in the world. 

Figure 4.48 Permeability of helium through various glasses. From W. D. Kingery, H. K. Bowen, and D. R. Uhhnann, Introduction to Ceramics. Copyright 1976 by John Whey Sons, Inc. This material is used by permission of John Whey Sons, Inc.

On the other hand, actual binary mixture tests using porous alumina and glass membranes show separation factor values for helium recovery from oxygen that are lower than what Knudsen diffusion provides, as indicated in Table 7.15. Only Koresh and Soffer [1983a 1983b] show an ideal separation factor of 20 to 40 with a low permeability of 1.2x10 barrers when molecular sieve membranes with a reported pore diameter of 0.3 to 0.5 nm are used. 

It can be seen from the data presented in Table 9.2 that the parallelism of the decrease in permeability with the increase in glass transition temperature of the polymer is somewhat disrupted for He and CO2. Helium has the lowest solubility among the penetrants under study. However, the permeability of organosilicon polymers for helium is higher than could be expected from... 

The dependence of permeability, diffusion, and solubility coefficients on penetrant gas pressure (or concentration in polymers) is very different at temperatures above and below the glass transition temperature, Tg, of the polymers, i.e., for mbbery and glassy polymers, respectively. Thus, when the polymers are in the rubbery state the pressure dependence of these coefficients depends, in turn, on the gas solubility in polymers. For example, as mentioned in Section 61.2.4, if the penetrant gases are very sparsely soluble and do not significantly plasticize the polymers, the permeability coefficients as well as the diffusion and solubility coefficients are independent of penetrant pressure. This is the case for supercritical gases with very low critical temperatures (compared to ambient temperature), such as the helium-group gases, Ha, Oa, Na, CH4, etc., whose concentration in rubbery polymers is within the Heruy s law limit even at elevated pressures. 

Further studies on irreversible phenomena associated with helium diffusion through pyrex glass were made by Taylor and Rast(4i). The effects noted by them were not confined to the surface of the glass, as were those observed by Barrer (28). They found that the permeability constant rose by 10 % at about 550 C., after annealing at that temperature, and that thereafter the new permeabihty-temperature curve lay above the... 

It must, however, be remembered that permeabilities towards certain of the gases vary from specimen to specimen of glass thus much smaller permeabilities both to argon and to helium have been reported (28). In yet another series may be given the permeability of a number of glasses to helium ... 

A plot of log K versus 1000/r for helium permeation through various glasses is shown in Fig. 5.38. Note that the permeation also follows Arrhenius behavior quite well, and that the more open-struetured glasses are the more permeable. Permeability decreases rapidly with increasing size of the permeating atom. 

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