Hydrogen, accommodation coefficient

Tables 2,3, and 4 outline many of the physical and thermodynamic properties ofpara- and normal hydrogen in the sohd, hquid, and gaseous states, respectively. Extensive tabulations of all the thermodynamic and transport properties hsted in these tables from the triple point to 3000 K and at 0.01—100 MPa (1—14,500 psi) are available (5,39). Additional properties, including accommodation coefficients, thermal diffusivity, virial coefficients, index of refraction, Joule-Thorns on coefficients, Prandti numbers, vapor pressures, infrared absorption, and heat transfer and thermal transpiration parameters are also available (5,40). Thermodynamic properties for hydrogen at 300—20,000 K and 10 Pa to 10.4 MPa (lO " -103 atm) (41) and transport properties at 1,000—30,000 K and 0.1—3.0 MPa (1—30 atm) (42) have been compiled. Enthalpy—entropy tabulations for hydrogen over the range 3—100,000 K and 0.001—101.3 MPa (0.01—1000 atm) have been made (43). Many physical properties for the other isotopes of hydrogen (deuterium and tritium) have also been compiled (44).

Fig. 5. The dependence of the recombination coefficient, the energy accommodation coefficient and the stationary concentration of adatoms on temperature for the hydrogen—tungsten system when P2 — 1 torr and the other parameters have the values specified in Fig. 4.

Fia. 2. EfiFeot of adsorbed hydrogen on the accommodation coefficient. The arrow marks the time at which the flashing current was cut off. 

The accommodation coefficient is a measure for the efficiency of the heat exchange between the neon molecules and the wire and depends on the nature of the wire surface. It is low for the bare wire, increases with the extent of the hydrogen layer and reaches a maximum value on complete coverage. 

Different structural materials have different thermal contraction coefficients, meaning that accommodations should be made for their different dimensions at cryogenic temperatures. If not, problems associated with safety (e.g., leaks) may arise. Generally, the contraction of most metals from room temperature (300 K) to a temperature close to the liquefaction temperature of hydrogen (20 K) is <1%, whereas the contraction for most common structural plastics is from 1% to 2.5% [23]. 

Here, Vi is the molar volume of the solute (as a measure of the size of the cavity to accommodate the solute i in the solvent), d is an empirical parameter which takes also account for polarizability n, a and / characterize respectively the acidity or basicity which in general represents the ability to form hydrogen bonds, and the C s are solvent characteristics independent of the solute. Meyer and Maurer [39] used this equation for 30 systems (371 substances, 947 experimental distribution coefficients) to evaluate generalized solvent Cj parameters. 

However, packing coefficients of up to 0.70 can be accommodated if the resulting complex is stabilised by strong intermolecular forces such as hydrogen bonds. Interestingly, the above rule also applies when more than one guest molecule is encapsulated in the receptor s cavity. 

Already by 1963, for a patent granted in 1966, Straschil and Lopez realized that the match of coefficient of thermal expansion between palladium membranes and (porous) substrates was critical, and stated that it would be virtually impossible to compensate for differences in dilation due to absorption of hydrogen [38]. They patented the use of dimpled or corrugated foils to accommodate differential thermal and chemical expansion [38]. Buxbaum and Hsu, in a 1992 patent, maintained that a rough substrate surface produced by abrasion with steel wool was critical for adherence of palladium on surfaces of Nb, Ta, V and Zr [39]. Other patents recommend corrugated or undulating configurations to allow for both thermal and chemical expansion [24, 26, 27, 29]. 

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