Thesis - Oztek_Muzaffer_T_200508_MA

hydrides by dissolving large amounts of hydrogen. All phases of metal-hydrogen systems
other than random interstitial solid solutions are designated as hydrides [12]. Depending
on the nature of the metal-hydrogen bond, hydrides are classified into three types:
covalent, ionic or metallic hydrides. Covalent and ionic hydrides have non-metallic
character whereas metallic hydrides preserve the metallic properties of the original metal
or alloy.
The solution reaction predecessor to hydride formation is described by five partial
steps:
1. H 2 (g, bulk) H 2 (g, gas-solid interface)
2. H 2 (g, interface) H 2 (physisorbed on metal M)
3. H 2 (phys) 2 H (chemisorbed on M)
4. H (chem.) H (dissolved in M, solid solution α-phase)
5. diffusion of H in M
In the case of adherent hydride layer formation at the metal-gas interface, additional
reaction steps are involved [13];
6. diffusion of H through the MH x layer
7. M + x H (in MH x ) MH x (β-phase)
When the concentration of hydrogen atoms in the α-phase (solid solution phase)
exceeds the solubility limit at the given temperature, β-phase (hydride) is formed.
Hydrogen dissolution and hydride formation are physical and chemical processes,
respectively.
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