The thermal rectification of a conductive thermal diode made up of two phase-change materials, whose thermal conductivities significantly change within a narrow interval of temperatures, is theoretically studied and optimized. This is done by deriving analytical expressions for the temperature profiles, heat fluxes and rectification factor. An optimal rectification factor of 60% is obtained for a thermal diode operating with terminals of VO 2 and Polyethylene with a temperature difference of 250 K spanning the metal-insulator transition of both materials. It is shown that this high rectification of a conductive thermal diode could be maximized even more by increasing the thermal conductivity variations of both diode terminals. The obtained results can thus be useful to guide the development of phase-change materials capable of optimizing the rectification of conductive heat fluxes.

We have theoretically studied and optimized the thermal rectification of plane, cylindrical and spherical radiative thermal diodes operating with terminals of two phase-change materials, whose emissivities significantly change within a narrow interval of temperatures. Analytical expressions for the optimal rectification factors of these three diodes are derived and analyzed comparatively. Optimal rectification factors of 82%, 86% and 90.5% are obtained for the plane, cylindrical and spherical diodes made up with terminals of Ge 2 Sb 2 Te 5 and VO 2 operating with a temperature difference of 450 − 300 = 150 K, respectively. The spherical geometry thus represents a suitable shape to optimize the rectification of radiative heat currents. Furthermore, it is shown that higher rectification factors can potentially be achieved by using phase-change materials with emissivities contrasts higher than those of Ge 2 Sb 2 Te 5 and VO 2. We demonstrate that higher rectification factors could be achieved based on the combination of two phase-change materials compared to single phase-change material. The obtained results thus shed light on the phase-change materials required for optimizing the rectification factor of radiative thermal diodes with different geometries.