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

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 × 10–3 mol% and 1 × 10–2 mol%) of EG + VO2 nanofluids by dispersing 50-nm VO2 nano-spheres and injecting them into a cubic glass cell (1 × 1 × 4 cm3). 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˚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˚C, the thermo-optic coefficients of both EG + VO2 nanofluids rapidly changed and showed a peak and valley near 62˚C and 67˚C, respectively. We used the maximum value of the derivative of the thermo-optic coefficient (d2n/dT2) to obtain the phase transition temperatures of the VO2 nanofluids, which were 64.6˚C and 65.0˚C in the samples with 1 × 10–3 mol% and 1 × 10–2 mol%, respectively.