The rates of decomposition of Magnesium hydride formed in Magnesium and Mg-25(w/o) Al alloy were studied under argon atmosphere using a thermogravimetic method.
Magnesium and Mg-25(w/o) Al alloy were directly hydrided in autoclave under high pressure of hydrogen.
Hydrided sample was analyzed by X-ray diffractometer.
The weight loss curves were obtained in the temperature range, 287℃-397℃, and the effect of gas flow rate, was investigated.
The reaction mechanism of experimental results was estimated by comparing with unreacted core model.
From the experiments, the result were follows;
1) The hydriding is occurred easier in the small size of magnesium that the large size.
2) Product hydrides are $MgH_2$ in Magnesium and Mg-25 (w/o) alloy.
3) The yield of Magnesium hydride formed in Mg-25(w/o) alloy was more than Magnesium.
4) When $MgH_2$ formed in Magnesium was decomposed, the initial reaction was controlled by the chemical reaction at the interface and the reaction rate was expressed as $1-(1-X)^{\frac{1}{3}}=k_1t$,X is fraction decomposed. The apparent rate constant ($k_1$), is expressed as $k_1=(7661\pm4.7)\exp (-\frac{19446\pm1893}{RT})$
5) When $MgH_2$ formed in Mg-25(w/o) Al alloy was decomposed, the initial reaction rate was also controlled by the chemical reaction at interface and apparent rate constant ($k_1$), is expressed as, $k_1=(145.6\pm2.5)\exp(-\frac{15927\pm1104}{RT})$ Apparent activation energies of the chemical reaction are about 19.5 kcal/mole, 15.9/mole, respectively which is nearly equal to the heat of dissociation of Magnesium hydride. The activation energy of backward reaction, $Mg+H_2\rightleftharpoons MgH_2$ is nearly equal to zero.
When $MgH_2$ formed in Magnesium and Mg-25(w/o) Al alloy is decomposed, decomposed $H_2$ gas is diffused through the gas pore of Magnesium layer.
The reaction rate was decreased as the thickness of product Magnesium-layer increased.
6) The reaction rate decreased as the content of Aluminum increased.
Magnesium 과 Mg-25(w/o)Al 합금에서 생성된 Magnesium hydride 의 분해반응 속도에 관한 연구를 T.G.A 방법을 사용하여 argon gas 분위기 중에서 행하였다. 분해에 사용된 hydride 는 autoclave 에서 직접 합성하였다.
생성된 hydride 를 X-ray diffraction analyis 로 확인하였다.
287℃∼397℃의 온도범위에서 분해곡선을 구하고, 반응속도에 gas flow rate 가 미치는 영향을 조사하였다. 또 얻은 실험결과를 unreacted core model 과 비교하여 실제의 reaction mechanism 을 추정하였다. 실험에서 얻는 결과는 다음과 같다.
1) Magnesium,Mg-25(w/o)Al alloy의 분말의 입자가 작을수록 합성된 hydride양이 많았다.
2) Mg-25(w/o)Al alloy에서 합성한 hydride 는 $MgH_2$ 임을 알았다.
3) Magnesium보다 Mg-25(w/o)Al alloy에서 합성된 hydride 양이 많았다.
4) Pure Magnesium hydride 의 분해반응시, 초기반응은 계면의 화학반응에 의해서 지배되며, 속도식은 $1-(1-X)^{\frac{1}{3}}=k_1\cdot t$로 표시되며, X = fraction decomposed, apparent rate constant, $k_1=(7661\pm4.7)\exp (-\frac{19446\pm1893}{RT})$ 이었다.
5) Mg-25(w/o)Al alloy hydride 의 초기반응도 계면의 화학반응에 의해서 지배되며, apparent rate contant, $k_1=(145.6±2.5)\exp(-\frac{15927±1104}{RT})$ 이었다. Mg-25(w/o)Al 합금에서 생성된 $MgH_2$ 의 분해반응시 activation energy는 각각 19.5Kcal/mole, 15.9Kcal/mole 이며 이 값은 Magnesium hydride 분해열과 거의 같은 값이다.
분해반응의 역반응, $Mg + H_2 \to MgH_2$ 의 activation energy는 거의 0이다. Magnesium hydride 의 분해반응시 분해된 수소는 porous product-layer 인 Magnesium layer 의 gas pore 를 통해 diffusion 하며, Magnesium-layer 의 두께가 커짐에 따라 반응속도가 감소한다.
6) Aluminum 의 양이 많아질수록 분해 반응속도는 감소한다.
