Thermodynamic characterisation and application of the ZrNi-H metal hydride system in the low-pressure regime

The metal hydride system ZrNi-H is examined in this work, focusing on its thermodynamic properties at very low pressures and its viability for vacuum applications, such as hydrogen sorption pump for thermal switch components. A substantial literature study on existing equilibrium data below ambient pressure reveals a highly scattered picture of the equilibrium data. Particularly under vacuum conditions, conventional volumetric characterisation methods reach their limits in determining pressure-composition-temperature curves. To overcome these limitations, a generic temperature-driven & isochoric p-c-T characterisation method has been developed and is successfully employed. Thus, high-precision equilibrium data of ZrNi-H below 150 degrees C respectively 10 mbar are obtained. Thermodynamic properties for desorption and absorption derived from the experimental results as well as the corresponding Van't Hoff diagram within the investigated pressure range are provided. The scattering width of the absorption/desorption equilibrium lines as well as the hysteresis can be considerably reduced, and the existing database supplemented with reliable and accurate thermodynamic data of the ZrNi-H system. However, challenges emerge when evaluating the material's technical applicability in a thermally operated reactor. Observations from the measurements indicate slowing reaction kinetics at low capacity and degradation due to cyclic thermal loads. A temperature-driven Sieverts' technique allows high-accuracy thermodynamic characterisation of the AB-type metal hydride ZrNi under vacuum. The applicability of the ZrNi-H system as a thermally operated hydrogen sorption pump is investigated.