Theory of Hydrogen Absorption in Metal Hydrides
The mechanism of hydrogen absorption by transition metals and alloys has been a subject of intensive investigations in recent years because of its prospective energy storage applicationsl. The amount of hydrogen absorbed by all metals, at low pressures, is proportional to the square root of the pressure of the external hydrogen gas. This is a clear indication that the hydrogen molecule of the gas is dissociated into atoms in the metal phase. Many transition metals and alloys, such as Pd, Ti, La, Th, and FeTi, absorb a large quantity of hydrogen at relatively low temperatures and pressures. The concentration of saturation absorption of hydrogen in these metals and alloys goes as high as 100 to 200 % of metalic atoms, and the metals are regarded as forming hydrides represented conveniently by chemical formulas such as PdH and TiH2. Mott and Jones2 argued that the hydrogen atoms inside metals are ionized into electrons and protons. These protons go into some interstitial positions in the matrix of the host metal while electrons join the main body of the d-band electrons. This is most remarkably demonstrated by the paramagnetic susceptibility of Pd. Pd is a strong paramagnetic metal due to electron holes in the 4d band. When Pd metal is loaded with hydrogen, the paramagnetism decreases linearly with the concentration of hydrogen and this metal hydride becomes diamagnetic at and above 55 at% of hydrogen concentration.
KeywordsHydrogen Absorption Metal Phase Metal Hydride Interstitial Position Pair Approximation
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