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

吸氢合金作为负极材料，有望成为清洁能源的存储介质，引起了人们的广泛关注。一般认为，吸氢压力由相应合金中氢原子的位能决定。然而，在本研究中，我们研究了是否还有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的作用有多重要。本研究采用LaNi_5、TiMn_2和FeTi。对于二元和共合金LaNi_5，吸收压力仅在第一个循环中较高，在随后的循环中几乎保持恒定。这些合金的解吸压力与循环次数无关。裂纹（粉化）在第一次循环中显著发生，一次循环后的有效粒径与十次循环后的有效粒径相当，说明第一次循环对合金粉化起决定性作用。在第一个吸收周期中引入了相当高的位错密度，其量级为10^<12> cm^<-2>。这些位错大多为平行于c轴的a型边位错，被认为是在基体与氢化物界面处形成的错配位错。仔细检查表明，氢化物形成板形，c轴和其中一个a轴包含在板面中，高密度的位错只在氢化物内部出现。这表明基体/氢化物界面的失配是通过形成失配位错来推进的，而失配位错一旦形成，就会在氢化物生长过程中融入到氢化物中。因此，由于形成位错所需的超压力和形成裂纹所需的超压力，可以考虑第一个循环的高吸收压力。在铝合金LaNi_5和TiMn_2中，虽然在第一次吸收循环中会产生明显的裂纹（粉化），但在任何循环次数中几乎都不会产生位错。因此，第一个循环的吸收压力仅比其他任何循环的吸收压力略高。这被认为是由于在氢浓度（晶格参数）方面，这些合金中的基体/氢化物界面比二元和钴合金LaNi_5中的基体/氢化物界面更宽。这意味着具有中等氢浓度的氢化物相对于基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高原，表明存在两种不同的氢化物。第一平台的吸收压力随循环次数的增加而减小。与其他合金相比，FeTi在循环过程中的开裂程度显著降低。这表明，在循环过程中，位错的引入是持续发生的，吸收压力随循环次数的增加而减小是由于引入位错的密度随循环次数的增加而减小。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