Properties of Metals
Hydrogen dissolves as atoms in metals. These atoms occupy octahedral and tetrahedral locations
within the lattice. The hydrogen apparently exists within nonhydriding metal lattices as proton,
deuteron, or triton, with the electron in a metal conduction band. Some metals are endothermic
(chemical change due to absorption of heat) hydrogen absorbers and others are exothermic
(chemical change that releases heat), and solubilities vary considerably (approximately 10 to 15
orders of magnitude) at room temperature.
The solubility of hydrogen in endothermic absorbers increases as the temperature increases. The
reverse is true for exothermic absorbers and the solubility decreases as the temperature increases.
For various hydride phases, plots of decomposition overpressure as a function of inverse
temperature yield negative enthalpies or heats of formation.
Permeability (F) of gas (including H2 or T2) through materials is a measure of how much gas will
migrate across a material wall of given thickness and area over a given time. It is a direct
function of the ability to diffuse and solubility. Dimensionally,
cm3(H2, STP) cm(thickness)
F = permeability
D = diffusivity
S = solubility
The following materials are listed in order of increasing permeability: ceramics and graphite,
silicas, nonhydriding metals, hydriding metals, and polymers. The permeability of many other
hydrogen-bearing molecules through polymers has been studied.
For such molecules,
permeability can be well in excess of that for hydrogen through a polymer. This must be
considered when handling tritiated water or organic solvents.
Two factors that influence the permeability of a metal are oxides on surface and surface area.
Because the permeability of hydrogen through a metal oxide at a given temperature is usually
orders of magnitude lower than it is through the metal, a thin surface oxide can markedly reduce
the permeability of hydrogen through the material.