Microstructural improvement of solar salt based MgO composites through surface tension/wettability modification with SiO₂ nanoparticles

Composite phase change materials (CPMCs) are of particular interest in high-temperature thermal energy storage and waste heat utilization applications. To further enhance their thermal conductivity, CPCMs are often doped with nanosized particles (NPs). However, NP's influence the PCM's viscosity and wettability, significantly contributing to the CPCM's microstructure. Their effects on the PCM's viscosity have been extensively documented, but little has been done on its effect on wettability. To this end, we investigated, using a custom maximum bubble pressure tensiometer, the surface tension of the solar salt, as well as its contact angle on an MgO {1 0 0} surface with various concentrations of α-SiO₂ NPs. At 0.5 wt%, an increase of 2.45% on the surface tension is observed, scaling up to 11.8% at 2.5 wt%. Contrary to the pure salt, in the presence of SiO₂ the surface tension increases up to 400 °C followed by rapid decay, which is found to be related to NP agglomeration and subsequent sedimentation. In terms of contact angle NP addition leads to an increase at the melting point, but fully wetting conditions are attained at 390 °C independent of α-SiO₂ content. To capitalize on this effect, MgO-Solar salt CPCMs without and with 1.0 wt % SiO₂ are kept at 390 °C for 5 h. Silica addition leads to smaller leakage (mass loss), improved density (by 8.9%), improved porosity (by 18.9%), mean pore size reduction (by 100%) and enhanced thermal diffusivity (by 6.5%). This is suggested to be indirectly related to the surface tension/wettability modification through doping with polar NPs.