| The rate of thermal decomposition of calcium hydride was studied under argon atmospheare using a thermogravimetric technique in the temperature range of 770℃ to 835℃. The effects of hydrogen partial pressure, total flow rate and the shape of the sample holder on the decomposition rate were discussed. And the reaction mechanism was estimated compassing the experimental results with the vasious reaction models. From the experiments, the results were as follows; 1) When the long sample holder, 20mm long, was used, the thermal decomposition rate was controlled by hydrogen gas diffusion through the surface gas film, and its rate can be expressed as the fraction decomposed, X, in terms of the reaction time t, X=k2(1-PH2/PH02)·t. The apparent activation energy is about 47.6 kcal/mole, that is equal to the heat of dissociation. 2) When the short sample holder, 3 mm long, was used, the reaction rate was controlled by the chemical reaction at the interface and the rate was expressed as 1-(1-X)1/3=k1(1-PH2/PH02)·t. An apparent activation energy of the chemical reaction is about 60.6 kcal/mole in which the heat of dissociation of calcium hydride and the activation energy of Ca+H2→CaH2 were included. 3) Comparing above two results, the activation energy of Ca+H2→CaH2 is about 13 kcal/mole. |
|
