Experimental Optimization of the Thermal Rectification of a Far-Field Diode Based on VO2

The rectification of radiative currents has been theoretically discussed in the literature by exploiting the metal-insulator transition of VO 2 , a phase-change material exhibiting a reversible and hysteretic transition at temperatures near room temperature [1] , [2] . In this work, the thermal rectification of far-field heat currents is experimentally observed by measuring the radiative heat flux exchanged between a heat flux sensor and a VO 2 nanofilm deposited on either r-sapphire or c-sapphire or silicon by means of pulsed laser ablation. The highest rectification factor of 61±3 % is obtained with r-sapphire, which is among the highest values reported in the literature with the lowest temperature difference (40°C) between the diode terminals [3] , [4] . This experimental value ( Fig. 1 ) is consistent with the theoretical one predicted by an analytical expression derived for the optimal rectification factor, as a function of the VO 2 emissivity in the metallic and insulating phases, sensor emissivity, and geometrical parameters. The obtained results thus show that the rectification factor of these diodes can be optimized, while reducing the temperature difference of their terminals, by means of VO 2 films deposited on a substrate of r-sapphire [5] .