Wetting behavior of Si-13.5B alloy on polycrystalline h-BN-based substrates

Silicon and silicon-boron alloys have been very recently recognized as excellent phase change materials (PCMs) for applications in ultra-high temperature latent heat thermal energy storage (UHT LHTES) systems. Extremely high latent heat values and high melting points of both boron and silicon has been assumed as the main principles of the AMADEUS Project that allow overcoming actually existed limitations of molten salt-based LHTES systems. An implementation of these materials should guarantee obtaining very high energy densities (over 1kWh/l) that are two times higher than that of Li-ion batteries and 10-times higher than of molten salts-based counterparts. Nevertheless, a successful accomplishment of the Project goals requires a proper selection of refractories to build a container for storing molten Si and Si-B alloys at temperatures higher than 1400°C. However, due to a very high reactivity of molten silicon and actually unrecognized behavior of Si-B alloys, this task appears to be very challenging.

In this work, we show for the first time the results of experimental evaluation of the high temperature behavior of molten Si-B alloy in contact with a refractory material at temperature up to 1750°C, under static argon atmosphere (p=850‒900 mbar). The material investigated having a nominal chemical composition of Si-13.5B (at. %) was fabricated in Leibniz Institute for Solid State and Materials Research in Dresden (Germany) by using crucible-less electric arc-melting process assisted by a levitation drop method.
The wettability of the molten alloy in contact with commercial hexagonal boron nitride substrates was evaluated by means of especially developed sessile drop technique combined with a contact heating procedure. The reactivity in examined systems was investigated by using solidified sessile drop couples; by means of field emission gun scanning electron microscopy (FEG SEM) and energy dispersive X-ray spectroscopy microanalysis (EDS) techniques.