In order to understand the intrinsic degradation mechanism by the hydrogen absorption-desorption cyclings, the degradation behavior of $LaN_5$ and $ErFe_2$ intermetallic compounds were investigated through the changes of P-C-Isotherm curves, Electron michroscopy, X-ray diffraction, DSC (Differential Scanning Calorimetry), Thermal hydrogen desorption measurement. It is generally believed that the hydrogenation or dehydrogenation are concomitant with the following changes in the matrix. i) lattice strain which was related with volume change ii) heat of hydrogenation iii) thermal energy due to temperature change. To investigate the effect of above factors on the degradation, the change of hydrogenation storage capacity will be studied under the condition mostly applied one of these factors. And also the origin of the degradation was studied by means of the investigation on the products formed during hydrogen absorption-desorption cyclings or hydrogen charging at high temperature.
The $ErFe_2$ compounds formed crystalline $ErFe_2$ hydride below 165℃ and decomposed into $ErH_2$ and Fe above 300℃. The amophous hydride could be formed between 165℃ and 300℃. The hydrogen induced amorphization and phase separation of $ErFe_2$ could only occur during hydrogen charging at high temperature(200℃) and high pressure(11atm). The hydrogen storage capacity of $ErFe_2$ decreased due to hydrogen absorption-desorption cyclings. The degradation caused by the formation of the $ErH_2$ or a-$ErFe_2Hx$ which could not be dehydrided during cylcings. Although hydrogen absorption-desorption cyclings were performed under the condition which did not occur the phase separation and amorphization the amorphous hydride or $ErH_2$ were formed.
The hydrogen storage capacity of $LaNi_5$ compound decreased by hydrogenation at high temperature(140℃) when applied hydrogen pressure were higher than the plateau pressure of the hydrogen charging temperature. The hydrogen storage capacity promoted the thermal energy. From the thermal hydrogen desorption, DSC and X-ray results, the degration could be recovered by heat treatment at high hydrogen pressure(20 atm $H_2$). The degradation might be caused by the formation of stable hydride, which could not detect by means of X-ray diffraction technique. As the heat effect of hydrogenation reaction during pressure cyclings increases, the degree of degradation was more severe. And also the pressure induced cyclings with low heat effect at room temperture reduced the hydrogen storage capacity. The repeated lattice strain mostly affected this degradation. The degradation was also caused by the formation of stable hydride which was similar to that formed during hydrogen charging. The degree of degradation of $LaNi_5$ during thermal cyclings was more severe with increasing the applied hydrogen pressure. The $LaNi_5$ intermetallic compound is fully degraded after 3150 thermal cycles. The plateau region completely disappeared and hydrogen storage capacity is considerably recovered. From the X-ray diffraction results, it is found that most of the $LaNi_5$ alloy is decomposied into the Ni cluster and La rich hydride phase, that is, $LaNiH_{3.6}$ and $LaH_2$. As a result of annealing, the $LaNiH_{3.6}$ and Ni cluster recombined to form $LaNi_5$. Thus the degradation of $LaNi_5$ by thermal cylings were caused by the formation of $LaH_2$ and $LaNiH_{3.6}$.
Thus, the hydrogen storage capacity of $ErFe_2$ and $LaNi_5$ decreased by means of hydrogenation at high temperature(thermal energy dominant) without cyclings or hydrogen absorption-desorption cyclings under the condition of low thermal energy(strain energy dominant). The intrinsic degradation could be promoted by the thermal energy and strain energy, respectively. The strain energy played more dominant role under the practical cycling conditions. The hydrogen absorption-desorption cyclings and hydrogen charging at high temperature of $ErFe_2$ compounds induced the formation of amorphous $ErFe_2Hx$ or $ErH_2$. In the case of $LaNi_5$ systems, $LaH_2$ and $LaNiH_{3.6}$ were formed during temperature induced hydrogen absorption-desorption cyclings, and amorphous like stable hydrides were formed due to hydrogen charging at 140℃ or pressure induced cyclings at room temperature. Thus the intrinsic degradation was caused by the stable hydrides which were related to the hydrogen induced phase separation. The species of stable hydrides were dependent upon the cycling condition.
1. $LaNi_5$ 합금은 140℃, 65 기압에서 수소 주입 결과 수소 주입시간이 증가할수록 수소저장용량이 감소하였으며 1024시간 후에는 26%가 감소하였다. 이는 400℃ 20기압의 수소분위기에서 30분 유지함에 의하여 원상태의 90% 정도 회복되었다. $LaNi_5$합금은 $LaNi_5H_6$를 형성하는 조건에서는 열에너지(고온유지)에 의하여서도 degradation 이 나타나며 $LaNi_5$의 plateau pressure 보다 낮은 압력에서는 회복현상이 나타났다. 이는 $LaNi_5H_6$ 보다 안정한 수소화합물의 형성에 기인한다.
2. $LaNi_5$ 를 상온에서 pressure cycling 한 결과 수소화반응열의 효과가 클수록 degradation rate 는 증가하였다. 그런데 반응열의 효과를 크게 줄인 경우에는 수소저장용량 감소가 나타났으며 이는 strain energy 의 작용에 기인한 것이라고 생각된다. 또한 strain energy에 의한 degradation의 원인도 열에너지에 의하여 생성된 안정한 수소화합물과 유사한 안정성을 가지는 수소화합물에 기인한다고 생각된다.
3. $LaNi_5$ 를 thermal cycling 한 결과 가해주는 수소압력이 증가함에 따라 degradation 되는 정도가 변하였다. 이는 수소흡수-방출이 일어나는 온도의 증가에 기인한다고 생각된다. 또한 thermal cycling시는 열에너지 및 strain 에너지가 모두 작용한다고 생각된다. thermal cycling 에 의한 degradation 의 원인은 $LaNiH_{3.6}$ 와 $LaH_2$로 상분해 현상에 기인한 것이었다. 이 중 $LaNiH_{3.6}$ 는 열처리에 의하여 분해가 가능하여 수소저장용량이 일부 회복되었다.
4. $LaNi_5$ 의 intrinsic degradation 은 열에너지 및 strain 에너지 모두에 영향을 받으며 가해주는 에너지의 크기에 따라 생성되는 안정한 수소화합물의 종류가 변하였으며 일반적인 cycling 조건에서는 열에너지 보다는 strain energy 가 우세하게 작용한다고 생각된다.
