Mechanisms of hydride formation in hydrogen-absorbing alloys - with special attention to introduction of lattice defects -

由于吸氢合金被用作负极材料，并有望作为清洁能源的存储介质，因此引起了人们的极大兴趣。一般来说，吸氢压力被认为是由相应合金中氢原子的位置能决定的。然而，在目前的研究中，我们调查了是否有任何其他重要因素影响吸收压力的大小，以及它们在决定吸收压力中所起的作用有多重要。本研究使用LaNi5、TiMn2和FeTi3种添加剂。对于二元LaNi5和Co合金化LaNi5，只有在第一个循环中才有较高的吸收压力，在随后的循环中几乎保持不变。这些合金的脱附压力与循环次数无关。裂解(粉化)从第一次循环开始明显发生，一次循环后的有效颗粒尺寸与十次循环后观察到的相当，表明…更重要的是，第一个循环在这些合金的粉化中起着决定性的作用。在第一个吸收循环中引入了相当高的10^&lt；12&gt；cm^&lt；-2&gt；数量级的位错密度。这些位错大多是平行于c轴的a型刃位错，它们被认为是形成在基体和氢化物界面上的失配位错。仔细观察表明，氢化物呈板状形成，c轴和a轴之一位于板面，只在氢化物内部引入了高密度的位错。这表明，失配的基质/氢化物界面通过形成失配位错而推进，一旦形成失配位错，则在氢化物生长过程中将失配位错结合到氢化物中。因此，第一个周期的高吸收压力被认为是由于形成位错所需的超压和形成裂纹所需的超压所致。在铝合金LaNi5和TiMn2中，虽然在第一个吸收循环中出现了明显的裂纹(粉化)，但在任何一个循环中都几乎没有位错。因此，第一个循环的吸收压力仅略高于其他任何循环。认为这是由于在氢浓度(晶格参数)方面，这些合金中的母体/氢化物界面比二元和共合金LaNi5中的更宽。这意味着中等氢浓度的氢化物相对于母体和氢化物是相对稳定的。在FeTi的P-C等温线上观察到两个平台，表明存在两种不同的氢化物。第一个平台的吸收压力随循环次数的增加而减小。与所研究的其他合金相比，FeTi合金在循环过程中的裂纹程度显著降低。这表明在循环过程中位错的引入是持续存在的，吸收压力随循环次数的减少是由于引入位错密度随循环次数的减少而引起的。较少

Hydrogen-absorbing alloys have attracted considerable interest since they have been used as negative electrode materials and are expected to be used as storage media of clean energy. In general, the hydrogen-absorption pressure is believed to be determined by the site energy for hydrogen atoms in the corresponding alloys. In the present study, however, we investigated whether or not there are any other important factors that influence the magnitude of absorption pressures and how importantly they plays a role in determining the absorption pressure. LaNi_5, TiMn_2 and FeTi were used in the present study. For binary and Co-alloyed LaNi_5, the absorption pressure is high only for the first cycle and it stays almost constant for the subsequent cycles. The desorption pressure these alloys does not depends on cycle number. Cracking (powdering) occurs significantly from the first cycle and the effective particle size after one cycle is comparable to that observed after ten cycles, indicating … More that the first cycle plays a decisive role in powdering of these alloys. A considerably high density of dislocations of the order of 10^<12> cm^<-2> are introduced during the first absorption cycle. Most of these dislocations are a-type edge dislocations aligned parallel to the c-axis and they are considered to be misfit dislocations formed at the interface between the matrix and hydride. Careful inspection indicates that the hydride is formed in a plate shape with the c-and one of the a-axes being contained in the plate face and a high density of dislocations are introduced only inside the hydride. This indicates that misfit the matrix/hydride interface advances by forming misfit dislocations and that misfit dislocations once formed are incorporated in the hydride during the course of hydride growth. The high absorption pressure for the first cycle is thus considered due to the over-pressure required to form dislocations and the over-pressure required to form cracks. In Al-alloyed LaNi_5 and TiMn_2, while significant cracking (powdering) occurs during the first absorption cycle, almost no dislocations are introduced in any of cycle number. As a result, the absorption pressure for the first cycle is only moderately higher than that for any other cycles. This is considered to be due to the fact that the matrix/hydride interface in these alloys is broader than that in binary and Co-alloyed LaNi_5,in terms of hydrogen concentration (lattice parameter). This implies that hydrides with intermediate hydrogen concentrations are relatively stable with respect to the matrix and hydride. Two plateaus are observed in P-C isotherms for FeTi, indicating the existence of two different hydrides. The absorption pressure for the first plateau ever decreases with cycle number. The extent of cracking during cycling for FeTi is significantly reduced when compared to the other investigated alloys. These indicates that the introduction of dislocations occurs persistently during cycling and the decrease in absorption pressure with cycle number is due to the decrease in the density of introduced dislocations with cycle number. Less