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, providing accurate characterizations of the thermodynamic properties as well as of the dynamic aspects of the hydride phase growth over a wide range of pressures 0-4 MPa and temperatures 250-800 K. It consists essentially of a heat flow calorimeter coupled with high precision volumetric devices. The ensemble constitutes of a closed system in which high purity hydrogen gas within the system is transferred between hydrides reservoirs and reactors with high thermal transfer capacity. The excellent stability of the signal of the calorimeter, +/- 4 nV over a long period of time (> 10 days), allows one direct 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-isothermal conditions during the transformation, a reliable control of the temperature 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 different and complementary techniques. The first one is based on an analysis of the 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 shown 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 account the component volumes of the system. Here, the time variation of hydrogen mass flow (excitation of the system) and hydrogen pressure in the reactor (response of the system) is analyzed in the frequency domain which requires knowledge of the experimental transfer function of the volumetric equipment. 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 been used to demonstrate the detailed analysis. (C) 2000 American Institute of Physics. [S0034-6748(00)05401-0].