Coefficient of gas permeability

To increase thermal conductivity of powder layer metal powders of copper, aluminium are added. Composites are compacted in pellets, which can be sintered in addition. Their main characteristics are coefficient of effective thermal conductivity and coefficient of gas-permeability. The weight fraction of powder in such compacts serves as the controlled parameter, and it has the optimum, when gas-permeability does not worsen sharply at acceptable thermal conductivity. Encapsulation of hydride powder by material with high thermal conductivity followed by compaction of pellets and their sintering is also used. 

Measurement of the coefficient of gas permeability trough foam films The coefficient of gas permeability K is defined by [132]... 

In all methods for determination of the coefficient of gas permeability through foam films the driving force of gas transfer is the gas pressures difference Ap, when the gas phase is the same on either side of the film. If the gases are different, then it is possible to have a gas transfer through a flat film when the pressure on either film side is the same. Such a method has been realised in [131] by forming a foam film in a cylindrical tube between two vessels... 

Figure 8, The coefficient of gas permeability of concrete contained the Pure-slag powder (Concrete I series) or the Compound-slag powder (Concrete 2 series) under nitrogen diffused compressive pressure of 0,05, 0,10 or 0.20 MPa versus slag content...

The sizes and concentration of the free-volume cells in a polyimide film can be measured by PALS. The positrons injected into polymeric material combine with electrons to form positroniums. The lifetime (nanoseconds) of the trapped positronium in the film is related to the free-volume radius (few angstroms) and the free-volume fraction in the polyimide can be calculated.136 This technique allows a calculation of the dielectric constant in good agreement with the experimental value.137 An interesting correlation was found between the lifetime of the positronium and the diffusion coefficient of gas in polyimide.138,139 High permeabilities are associated with high intensities and long lifetime for positron annihilation. 

Table 9 Oxygen permeability coefficients (P02) and P02/PN2 of gas permeable substituted polyacetylenes...

One method which is known under the name of permeametry [131] or Poiseuille-Knudsen method [124] is based on the law of gas permeability in a porous media in the two flow regimes molecular flow (Knudsen) and laminar or viscous flow (Poiseuille). According to Darcy s law, the gas flux through a membrane with a thickness / can be written as / = KAP/l, where K is the permeability coefficient and AP (AP = Pi - P2) the pressure difference across the membrane. If the membrane pore diameter is comparable to the mean free path of the permeating gas, K can be expressed as a stun of a viscous and a non-vis-cous term... 

Gas-Permeability Coefficient A measure of gas permeability of a barrier wall such as plastic film. Gas permeability coefficient, P, is a coelficient in Pick s first law that states that the volume (V) of a substance that penetrates a barrier wall is directly proportional to the area (A) of the wall, partial pressure differential (p) of the penetrant, and time (t) and inversely proportional to the wall thickness (s), if the wall is homogeneous in the direction of penetration. Gas permeability coefficient depends on the test temperature. 

A very large body of data on the gas permeability of many rubbery and glassy polymers has been published in the literature. These data were obtained with homopolymers as well as with copolymers and polymer blends in the form of nonporous dense (homogeneous) membranes and, to a much lesser extent, with asymmetric or composite membranes. The results of gas permeability measurements are commonly reported for dense membranes as permeability coefficients, and for asymmetric or composite membranes as permeances (permeability coefficients not normalized for the effective membrane thickness). Most permeability data have been obtained with pure gases, but information on the permeability of polymer membranes to a variety of gas mixtures has also become available in recent years. Many of the earlier gas permeability measurements were made at ambient temperature and at atmospheric pressure. In recent years, however, permeability coefficients as well as solubility and diffusion coefficients for many gas/polymer systems have been determined also at different temperatures and at elevated pressures. Values of permeability coefficients for selected gases and polymers, usually at a single temperature and pressure, have been published in a number of compilations and review articles [27—35]. 

Pc = Ppi[(l-cf)siO2)/(l+0.5cf)sio2))], where Pc is the permeability coefficient of a gas in the matrix mixed membrane, Ppi is the permeability coefficient of gas in the pure polpmide matrix and ( )si02 is the silica volume fraction [21]. Due to the fact that the decrease was deeper for... 

The investigation of gas permeability of the obtained polymers gave an opportunity to find out some correlations between polymer structure and its properties (Table 2). Introduction of SiMes pendant groups in ROMP polynorbomenes substantially increases permeability coefficients for light gases. At the same time the permselectivity for some gas pairs also increases, while in the majority of cases an increase in permeability is accompanied by reduced permselectivity and vice versa. It should be noted that cis or trans content in the obtained polycyclopentylenevinylenes did not exert some influence on the gas transport parameters [17]. 

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