Measurement of thermo-optic coefficient of vanadium dioxide nanofluids using an interferometer

Abstract

The change in refractive index with respect to temperature (dn/dT) is defined as the thermo-optic coefficient. It varies according to the light wavelength and temperature, so it is a very important physical quantity that influences light propagation. In this study, we observed the metal-insulator transition (MIT) effect of VO2 on the optical properties of VO2 nanofluid and the thermo-optic coefficient of pure ethylene glycol (EG). We prepared two concentrations (1 x 10(-3) mol% and 1 x 10(-2) mol%) of EG + VO2 nanofluids by dispersing 50-nm VO2 nano-spheres and injecting them into a cubic glass cell (1 x 1 x 4 cm(3)). The thermo-optic coefficient was measured by counting the number of interference fringe movements with a Michelson interferometer. The thermo-optic coefficient obtained for pure EG agreed with the literature value within 13.3%. Below 60 degrees C, the thermo-optic coefficients of EG + VO2 were smaller than that of pure EG, but they increased rapidly as the temperature increased. Above 60 degrees C, the thermo-optic coefficients of both EG + VO2 nanofluids rapidly changed and showed a peak and valley near 62 degrees C and 67 degrees C, respectively. We used the maximum value of the derivative of the thermo-optic coefficient (d(2)n/dT(2)) to obtain the phase transition temperatures of the VO2 nanofluids, which were 64.6 degrees C and 65.0 degrees C in the samples with 1 x 10(-3) mol% and 1 x 10(-2) mol%, respectively.