TED-AJ03-145 NUMERICAL ANALYSIS OF ABSORBING AND DESORBING MECHANISM FOR THE METAL HYDRIDE BY HOMOGENIZATION METHOD

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Storage capacity and fast reaction rate are the most important properties for hydrogen storage alloys. To understand the mechanism of the rate-controlling step, we have analyzed the hydrogen diffusion coefficients of the hydride by the homogenization method, which considers the microstructure of the alloy at the micro scale (0.1-10μm) precisely. By applying nucleation and growth model, we have discussed the absorbing and desorbing mechanism of the metal hydride. Our results and analysis show that it is necessary for the hydriding or dehydriding reaction to consider the number of nuclei and the transfer of hydrogen atom through the α-β interface. Fig. A-1 shows the effect of H/M (atomic Hydrogen/Metal ratio) on the homogenized diffusion coefficients, D^H in the case of Sh=0.01. The homogenized diffusion coefficients decrease more quickly against the hydride ratio, H/M with the increase in the number of nuclei. The thickness of a completely closed hydride layer on the hydride particle is very important because hydrogen transport is faster in the α-phase than in the β-phase and hydrogen atoms hardly pass through the α-β interface in the case of Sh=0.01. Once the β-phase hydride layer is formed, the [figure] hydriding reaction is limited to the interface. This means that the homogenized diffusion coefficients become small value as shown in Fig. A-1 because of the closed layer. Therefore, the hydriding reaction nearly stops in the case of small Sh number and large nuclei number because the closed layer of the β-phase is formed faster. With regard to the deterioration by oxidation reaction, the effect of the thickness of oxide layer, w/l on the homogenized diffusion coefficients, D^H were investigated on condition that various hydrogen diffusion coefficients, D_o. Fig. A-2 shows that the homogenized diffusion coefficients are constant regardless of the thickness if the oxide layer exists on the surface. However, the diffusion coefficients are dependent on the diffusion coefficients of the oxide layer. Therefore, adsorption of oxygen on the hydride surface affects hydrogen diffusion into the particle. Estimation of the microstructure of the metal hydride by using the homogenization method is useful for the absorbing, desorbing and deteriorating mechanism. Actually, the compositions such as the α, β phase and the oxide layer in micro region of the metal hydride should be considered for the decision of the rate-controlling step.[figure]
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TED-AJ03-145 NUMERICAL ANALYSIS OF ABSORBING AND DESORBING MECHANISM FOR THE METAL HYDRIDE BY HOMOGENIZATION METHOD

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