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

Abstract

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.