Containment of Lithium Hydride in Alkali Metal Heat Pipes

Lithium Hydride is an attractive energy storage material. Its high heat of fusion (2582 kJ/kg) and its melting point (956 K) make it suitable for many applications. The best location for LiH storage capsules is inside a liquid metal heat pipe. The heat pipe facilitates heat transfer into and out of the capsules. The major drawback to using LiH is its high equilibrium hydrogen pressure. Since hydrogen diffuses readily through iron and nickel-based alloys, the expected life time of a heat pipe with a LiH capsule is very low. However, Dynatherm has discovered that the actual hydrogen build-up in a potassium heat pipe with a LiH capsule is much less than predicted. It is postulated that the presence of the alkali metal suppresses hydrogen permeation, probably through the formation of a layer of alkali metal hydride. The objective of Phase I is to verify the initial observation and to quantify the phenomenon. The ultimate objective is to develop alkali metal heat pipes with internal LiH storage which are not sensitive to hydrogen poisoning. Potential commercial applications are solar powered Stirling engines and cooling systems for the leading edges of hypersonic aircraft.