Destabilized Metal Hydrides for Thermal Management

Many high temperature applications require thermal protection systems (TPS) to operate over long durations or increasing thermal loads. Metal hydrides provide an opportunity to serve as a TPS with improved energy density over current systems. Furthermore, metal hydride TPS are potentially reusable and produce hot hydrogen gas which can be used in other subsystems. This paper investigates using destabilized calcium hydride (CaH₂) for thermal management.

Destabilization of a metal hydride involves combining a metal hydride with an additive that reacts during desorption to form an intermetallic compound. This reaction has a lower enthalpy than the desorption of hydrogen from the neat metal hydride. The lower enthalpy allows the hydrogen to desorb out of the metal hydride at a lower temperature for a given equilibrium pressure.

We destabilized the metal hydrides in order to test at lower temperatures than neat CaH₂, reducing costs and the lead time for testing. To investigate metal hydrides for thermal management, we designed a test stand to measure the volume of hydrogen generation and temperature inside the metal hydride powder bed during desorption for up to 0.38 g of CaH₂. To test larger quantities of metal hydrides we used a test stand providing heat fluxes from 200-400 W/cm² for up to 30 g of CaH₂. We designed new sample containers with reduced material and machining costs than the original design, increased strength, and improved heat transfer. The reduction in material allows for lower cost iteration on the incorporated fin design to improve heat transfer. We destabilized CaH₂ using alumina, aluminum, silicon, and zinc.

The data suggests water contamination of alumina is a challenge when using it to destabilize CaH₂, requiring the use of alumina that has been stored in an inert environment. Additionally, large scale testing of CaH₂-α-alumina results in ~18% hydrogen desorption, indicating challenges regarding thermal conductivity. Furthermore, the data suggests CaH₂‑α‑alumina, CaH₂‑aluminum, CaH₂-silicon, and CaH₂-zinc do not produce significant endotherms and are subsequently not effective as TPS due to the poor effective thermal conductivity of the metal hydride powder mixture. Small scale testing showed poor hydrogen desorption for CaH₂ destabilized with α‑alumina, aluminum, and silicon with ~50% of the theoretical hydrogen desorption. Zinc showed a higher yield at ~70% desorption. However, further testing is required to confirm these results.