The rate of thermal decomposition of calcium hydride was studied under argon atmosphese using a thermogravimetric technique. The weight loss curves were obtained for the various conditions in the temperature range of 770 ℃ to 830 ℃. The effects of hydrogen partial pressure, total flow rate and the shape of the sample holder on the decomposition rate were discussed.
The reaction mechanism was estimated comparing the experimental results with the various reaction models. When calcium hydride is decomposed in dynamic argon atmosphere, the decomposition rate will be controlled by one of the three major mechanisms - chemical reaction, gas film diffusion or product layer diffusion. And the dominant mechanism would be determined by the various reaction conditions.
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, $X= k_2t$. The apparent activation energy is about 47.6 Kcal/mol, that is equal to the heat of dissociation.
2. When the short sample holder, 3mm long, was used, the reaction rate was controlled by the chemical reaction at the interface and the rate was expressed as $1-(1-X)^{1/3}= K1 (1-P_{H2}/P_{H2^o})$.t. An apparent activation energy of the chemical reaction is about 60.6 Kcal/mol in which the heat of dissociation of calcium hydride and the activation energy of backward reaction were included.
3. Comparing above two results, the activation energy of backward reaction, $Ca + H_2 → CaH_2$, is about 13Kcal/ mol.
calcium hydride의 열분해 속도에 관한 연구를 T.G.A 를 사용하여 argon gas 분위기 중에서 행하였다. 770 ℃ ~ 830 ℃의 범위에서 여러가지 반응조건에 따른 열분해 곡선을 구하고 또 이에 미치는 분위기중의 수소분압, total flow rate,sample holder의 영향등을 조사하였다.
또 얻은 실험결과를 여러가지 가상한 reaction model과 비교하여 실제의 reaction mechanism을 추정하였는데 dyanamic argon 중에서 calcium hydride가 분해될때 반응속도를 지배할수 있는 step은 chemical reaction,gas film diffusion,product layer diffusion 등이 있지만 우선적인 mechanism은 여러가지 반응조건에 의해서 결정된다.
실험에서 얻은 결과는 다음과 같다.
1) 길이가 20 mm의 긴 sample holder를 사용했을 경우 분해 속도는 surface gas film을 통한 수소 gas의 확산에 의해서 지배되며 이때의 속도식을 fraction decomposed X와 t로 표시하면 $X=k_2ㆍt$가 되며 이때의 apparent activation energy는 약 47.6kcal/mole 로서 calcium hydride의 분해열과 같은 값이다.
2) 3mm의 짧은 sample holder를 사용했을 경우 반응속도는 계면의 화학반응에 의해 지배되며 속도식은 $1-(1-X)\frac{1}{3} = k_1 (1-P_{H_2}/P_{H_2} ˚)ㆍt$로 표시된다. 이때의 apparent activation energy는 약 60.6kcal/mole로서 분해열과 역반응 즉 $Ca + H_2 → CaH_2$ 반응의 activation energy를 포함한 값이다.
3) 위의 두 결과를 비교하면 역반응 즉 $Ca + H_2 → CaH_2$ 반응의 activation energy는 약 13kcal/mole이다.
