The precise SEM investigations in different areas of the composite micro-volumes let elemental analysis of the eutectics crystallization into salt/graphite composites. The EDS imaging, made at magnification X30, proved preferable elemental areas of Na; K; N2; O2; C distribution during eutectics salt cooling into the graphite matrix.(Table 4).

The EDS element mapping confirmed both, potassium-rich and sodium-rich areas exist. The sodium-rich phase solidifies in rounded areas of up to 3 pm across, but the potassium-rich phase solidifies on the plane graphite surface. The composition ranging from high in Na to high in K nitrates areas, both with smaller concentrations of the other constituent The elemental separation of eutectics salt of the salt/graphite composite on solidification in two phases leads to the considerable segregation of the two chemical components and impact the salt crystallization and thermal capacity of the composite.

Table 4. Elemental images made by EDS of the NaNO3/KNO3eutectics/graphite (20%) composite. Brighter areas indicate distribution of the chemical elements: Na, K, N2, O2 and C, respectively.