Optimization of the rectification factor of radiative thermal diodes based on two phase-change materials

Abstract

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 Ge2Sb2Te5 and VO2 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 Ge2Sb2Te5 and VO2. We demonstrate that radiative thermal diodes based on two phase-change materials are able to operate with rectification factors higher than the corresponding ones of diodes involving a 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.