Thesis - Oztek_Muzaffer_T_200508_MA

Any of the surface processes or bulk diffusion in the hydride phase can be the rate
determining step in hydriding. Since the hydrogen uptake experiments will depend on too
many non-intrinsic parameters, such as the morphological characterization, heat transfer
or system configuration; meaningful basic information on the microscopic mechanisms
controlling the reaction can not be deductible in this study [14].
1.2.2. Characterization of Metal Hydrides
Virgin metal surfaces are almost never free of a thin oxide layer. That layer limits
the interaction of H 2 with the metal under that layer. However, in the initial stages of
interaction with H 2 , the parent metal or alloy surface cracks from the internal stress
caused by the expansion of the crystal lattice from hydrogen insertion. This exposes clean
metal surfaces to hydrogen [15]. This makes the initial formation of a metal hydride a
kinetically auto-catalytic reaction [16]. After decomposition and release of H 2 , the metal
is left with a higher surface area relative to its original state. Consequently, with at least
one hydriding/dehydriding cycle, the metal is now considered activated and can absorb
hydrogen at lower temperatures and pressures and at a faster rate.
The composition of a hydride, c can be expressed as the ratio of the number of
hydrogen atoms to the number of atoms of hydride forming metal or alloy atoms (H/M).
The relationship of x with pressure, p, and temperature, T, is obtained from pressurecomposition
isotherms. These isotherms give information about the equilibrium pressure
of hydrogen over a metal sample at a given temperature. At a given temperature, a
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