Non-isothermal but temperature-driven static PCI measurement technique: a novel approach for hydrogen sorption characterization at vacuum conditions

Kurzfassung

By utilizing the gas pressure dependent thermal conductivity of porous structures, insulation materials can be transformed into thermal barriers with controllable heat flux. Hereby, controllable insulation layers can support, for instance, the thermal management of BEVs or adaptive façades for active conditioning of buildings’ energy management to variable internal and external constraints. Such a controllable (vacuum) insulation layer requires the specific adjustment of the gas pressure in the insulation panel. Reversible gas-solid reactions such as metal hydrides with hydrogen are predestined to fulfill this task. In the technically relevant temperature range (approx. 0…150 °C), metal hydrides are promising reaction systems to precisely control the gas pressure along the equilibrium line, especially below ambient pressure. For the accurate adjustment of the hydrogen pressure in this kind of application, detailed sorption characterization data of the metal hydride materials are required. Especially under vacuum conditions, measuring the equilibrium data via conventional isothermal static volumetric technique (Sieverts’ technique) turns out to be a challenge. For a reliable determination of the hydrogen charge, huge volumes have to be provided in the desorption process, which is technically hardly feasible. Therefore, in this work, a temperature-driven approach of the volumetric technique using a Sieverts’ apparatus is presented. Here, the driving parameter for the changing hydrogen charging states is the temperature of the metal hydride bulk, while the corresponding gas pressure is the dependent variable. The hydrogen concentration is fixed during programmed temperature ramps and is then varied stepwise for different initial charging states. This allows the sorption properties to be measured in the vacuum pressure ranges without having to provide large capturing volumes. In the present contribution, this novel pressure-composition-isotherm (PCI) measurement method itself as well as the accordingly built test bench and the measurement procedure is explained in detail. The method is applied to two different metal hydride materials, each at the end of range of the technically reasonable pressure range for controllable vacuum insulation panels (ca. 0.01-1000 hPa) in the given temperature interval of 0…150 °C. We carried out both absorption and desorption equilibria measurements for LaNi4.1Al0.52Mn0.38 and ZrNi. The resulting PCIs are presented, including repeating and reproduction measurement series to verify the data yielded by the novel approach. To validate the measurement method and system, the gained PCI data is additionally compared to pressure-concentration isotherms provided in the literature.