Thermal hysteresis measurement of the VO₂ emissivity and its application in thermal rectification

Hysteresis loops in the emissivity of VO₂ thin films grown on sapphire and silicon substrates by a pulsed laser deposition process are experimentally measured through the thermal-wave resonant cavity technique. Remarkable variations of about 43% are observed in the emissivity of both VO₂ films, within their insulator-to-metal and metal-to-insulator transitions. It is shown that: i) The principal hysteresis width (maximum slope) in the VO₂ emissivity of the VO₂ + silicon sample is around 3 times higher (lower) than the corresponding one of the VO₂ + sapphire sample. VO₂ synthesized on silicon thus exhibits a wider principal hysteresis loop with slower MIT than VO₂ on sapphire, as a result of the significant differences on the VO₂ film microstructures induced by the silicon or sapphire substrates. ii) The hysteresis width along with the rate of change of the VO₂ emissivity in a VO₂ + substrate sample can be tuned with its secondary hysteresis loop. iii) VO₂ samples can be used to build a radiative thermal diode able to operate with a rectification factor as high as 87%, when the temperature difference of its two terminals is around 17 °C. This record-breaking rectification constitutes the highest one reported in literature, for a relatively small temperature change of diode terminals.