High Temperature Interaction of Si-B Alloys with Graphite Crucible in Thermal Energy Storage Systems

Si-B alloys are proposed as a potential phase change material (PCM) in the novel high temperature thermal energy storage systems. For successfully introducing the new PCM, the selection of proper refractory material in the PCM container is vital. At present, graphite is chosen as a potential refractory material for the PCM container, due to its high temperature stability, low thermal expansion, and high thermal conductivity. The Si-B alloys and the high-temperature interaction with graphite are hence studied. The phase formation in the Si-B alloys and the interaction with graphite at B content of 2-11 mass % and temperatures of 1450-1750 °C were investigated. Carbides were observed at the interface between the solidified alloys and the graphite. A single SiC layer was produced at B content of 2 and 3.25 mass %. Otherwise, SiC and B₄C layers were generated at B content higher than 5 mass %. In the Si-B-C system, the phase formation is dependent on the B content. Moreover, the equilibrium B content is calculated to be 3.66 mass % in the molten Si-B alloys at 1450 °C in equilibrium with SiC and B₄C, based on the experimental results. In this regard, the eutectic alloy (3.25 mass % B) is recommended to be used as the new PCM in the graphite container, due to that it produces simple phases and also because it is expected not to deplete any B to the B₄C layer.