Influence of the material and the shape of the container on LaNi5 hydride formation

Abstract

Intermetallic compounds, and particularly LaNi5, absorb large amounts of hydrogen under moderate conditions of temperature and pressure. The LaNis formation is accompanied by an important energy production. As shown previously [1], during the reaction the effective sample temperature is different from the imposed temperature of the furnace. This difference is about 20 K when 300 mg LaNi 5 is hydrided and is always < 2 K for a sample mass smaller than 30 mg. In this letter experimental results are reported with the aim of clarifying the role of sample temperature changes on hydriding kinetics. First, the studies were carried out using sample holders characterized by very different thermal conductivities. Secondly, with the aim of modifying the quantity of heat produced, some experiments were done by increasing the sample mass from 70 to 300 mg. In conclusion, the influence of these heat-transfer modifications on the shape of the experimental curves is analysed. Mass measurements are carried out using a high-pressure microbatance with a limiting accuracy of 1 ~g. In order to determine the sample temperature changes throughout the experiment, two chromel-alumel thermocouples were used, one in the LaNi 5 bed and the other in an inert powder. The temperature gradient was evaluated as the temperature differed between the two samples. The sample holder was a cylindrical flat-bottomed cup 0.5 mm thick and 5 mm deep with a diameter of 10 mm. Other holders had the special design shown in Fig. lb, which mainly consisted of a large lateral surface which ensured the maximum heat exchange with the surrounding gas phase. These holders were machined from materials such as stainless steel, copper or plexiglass. The LaNi5 compounds, prepared by induction melting of the pure components in vacuum, were activated by performing about 20 absorption