Advances in hydride phase growth: Automatic high precision calorimeter-volumetric devices, for thermodynamic and kinetics analyses

A new automatic apparatus has been specifically developed for investigating phase transformations in hydrogen absorbing intermetallic compounds, provi ding accurate characterizations of the thermodynamic properties as well as of the dynamic aspects of the hydride phase growth over a wide range of pre ssures 0-4 MPa and temperatures 250-800 K. It consists essentially of a hea t flow calorimeter coupled with high precision volumetric devices. The ense mble constitutes of a closed system in which high purity hydrogen gas withi n the system is transferred between hydrides reservoirs and reactors with h igh thermal transfer capacity. The excellent stability of the signal of the calorimeter, +/- 4 nV over a long period of time (> 10 days), allows one d irect measurement of the heat of H-2 absorption or desorption during a scan of an hysteresis loop with an average accuracy of 1%. To maintain quasi-is othermal conditions during the transformation, a reliable control of the te mperature inside the sample is insured by optimizing the hydrogen gas flow rate. Simultaneously the heat flux, pressure, temperature, composition data collected have been used to obtain kinetic parameters through two differen t and complementary techniques. The first one is based on an analysis of th e measured heat flux evolved during the reaction which gives the true rate law at the sample level by deconvolution of the measured signal. It is show n that only overall information can be expected by this method. The results of the numerical treatment raise the problem of the location of the heater used for calibration of the calorimeter. The second technique takes into a ccount the component volumes of the system. Here, the time variation of hyd rogen mass flow (excitation of the system) and hydrogen pressure in the rea ctor (response of the system) is analyzed in the frequency domain which req uires knowledge of the experimental transfer function of the volumetric equ ipment. In the complex plane, the shape of the transfer function appears as a signature of a proposed mechanism. ZrNi-H-2 and LaNi5-H-2 systems have b een used to demonstrate the detailed analysis. (C) 2000 American Institute of Physics. [S0034-6748(00)05401-0].