材料期刊网

在初始压力15—30atm、温度20—85℃的范围内研究了储氢材料LaNi_5在恒温时吸氢的动力学过程.实验结果表明:吸氢量与时间的对数呈线性关系;吸氢速率随初始压力的提高或吸氢温度的升高而改变.提出了表示吸氢速率对时间、初始压力、吸氢温度依赖性的关系式,实验结果与之一致.按这一关系式可以得到吸氢速率的限制性环节的激活能U.在吸氢量(以饱和度计)约70％以前,U约为2.0 kcal/mol H_2,吸氢量更高时约3.2 kcal/mol H_2.这些数据为进一步确定吸氢速率的限制性环节提供了重要参数.

The kinetics of hydrogen absorption in LaNi_5 has been investigated at initialpressures of 15--30 atm and temperatures in the range of 20--85℃. LaNi_5 was pre-pared in a ZrO_2 crucible in a graphite-tube furnace under vacuum. It is shownthat a linear relationship exists between the amount of hydrogen absorbed, W,and the time of reaction on a logarithmic scale and the rate of hydrogen absorp-tion depends on the initial pressure and the test temperature. The P-X-T curves of the LaNi_5 system have been determined. For each testtemperature there is a corresponding pressure plateau P_0 and it is therefore as-sumed that for every initial pressure P_0 there is a corresponding balance temperatureT_0, and T_0-T may be taken as the driving force of hydrogen absorption processin LaNi_5 at initial pressure P_0 and test temperature T. The absorption of hydro-gen takes place in three steps: (1) surface reaction, which seems to be rather com-plicated; (2) diffusion of hydrogen atoms and (3) phase transformation f LaNi_5 toa hydride phase. All these steps are thermal activation processes, and thereforethe e~(-U/RT) can be introduce in the hydrogen absorption process. The relatioshipbetween the absorption rate, the initial pressure, the test temperature and time isgiven as follow: dW/dt=A_0(T_0-T)e~(-U/RT)·1/tThe activation energy of the rate limiting step U is about 2 or 3.2 kcal/molaccording to whether the amount of hydrogen absorbed is less or more than 70%respectively. The parameter U of the rate-limiting step during hydrogen absorptionin LaNi_5 can be obtained as shown by our experiments.