Doping effect of magnesium oxide (MgO) on the enhancement of the thermal storage properties of sodium nitrate (NaNO₃)

In this work, sodium nitrate (NaNO₃) is used as the basic continuous PCM, and magnesium oxide (MgO) is dispersed inside it to enhance its global thermal storage properties, especially its thermal conductivity. The composite (NaNO₃/MgO) was prepared by mixing sodium nitrate with the addition of 1 wt.%, 2 wt.%, and 3 wt.% of MgO. Furthermore, differential scanning calorimetry (DSC) is used to evaluate the main thermal properties of the obtained composite materials, namely: their latent heat, specific heat and sub-cooling temperature. Their thermal conductivity is estimated based on a validated theoretical model from the literature. Finally, the chemical structures of the pure PCM and the three composites are investigated using Fourier transform infrared spectroscopy (FT-IR). Overall, the experimental and numerical results have indicated a clear enhancement of the thermal storage properties (especially the thermal conductivity and the sub-cooling temperature) of NaNO3 when it is doped with MgO: The thermal conductivity of the pure PCM was enhanced by 5 % and 19%, when it is doped by 3 wt.% and 10 wt.% respectively. While, the sub-cooling degree was decreased up to 46% when it is doped by 3wt.%, which is very good for the thermal cycling of this PCM inside the LTES system. As far as it concerns the other properties, they remained almost stable: the measured value of melting temperature has an average of 306,85 °C, with a heat of fusion of 172,40 J/g.