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

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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