A new experimental method of investigating phase transitions of solids, photothermal radiometry, is described. The effects of modulation frequency, laser intensity, and heating-cooling rates on the first-order semiconductor-to-metal phase transition of a microcystalline vanadium dioxide sample have been studied. The PTR signal increases with temperature in both phases, and a negative peak occurs at the phase transition. The intensity of signal is inversely proportional to the square root of frequency at laser-beam chopping frequencies lower than 100 Hz, indicating that the sample is thermally thick, and the signal is controlled by the thermal conductivity and the heat capacity and is independent of the optical properties of the material. The relative peak intensity decreases slightly in the low-frequency range with increasing frequency and more sharply at higher frequencies. The measured temperature of the phase transition decreases linearly with the intensity of illuminating light, from which the static component of the temperature rise at the surface can be determined. From the increase of linewidth with the laser intensity one may estimate the amplitude of the temperature modulation at the surface. The measured temperature of the phase transition increases with increasing heating rate and decreases with increasing cooling rates. The effects of variable light intensity and heating-cooling rates can be eliminated by extrapolation, and the exact temperature of the phase transition is obtained.
Sandor Pekker and Edward M. Eyring, "Measurement of Phase Transitions by Photothermal Radiometry: The Semiconductor-to-Metal Transition of Vanadium(IV) Oxide, VO2," Appl. Spectrosc. 40, 397-401 (1986)