Regulation of phase transition temperature and preparation for doping-VO2 smart thermal control films

Abstract

Vanadium dioxide (VO2) is considered one of the most promising smart thermal control materials due to its insulator-metal temperature (IMT) reversible phase transition, accompanied by large changes in its optical properties. However, as the crystal defects on IMT change and the optical property of VO2 is still unclear, the preparation of doped VO2 films by magnetron sputtering is still a great challenge. In this work, the IMT of 41 kinds of doping-VO2 systems were studied by high throughput calculation based on density functional theory (DFT). It was found that the IMT increased with the decrease of the β angle in M phase and expansion of cell volume difference of M-phase and R-phase for IIA elements, VIIA elements, transition elements, and rare earth element doped VO2, and increased with the increase of the β angle in M phase and a decrease of cell volume difference of M-phase and R-phase for IA, IVA, VA, and VIA element doped VO2. According to the rule, the IMT, electronic structures, and optical properties of W doped VO2 were studied based on DFT. The results show that IMT and bandgap decrease with the increase of W6+ ion concentration, which is due to the increased cell volume difference of M-phase and R-phase in W doped VO2; each doped atom can reduce the IMT of 20.2 °C, and the IMT of V0.98W0.02O2 is close to room temperature (Tc ≈ 27 °C). The rate of infrared emissivity (∆ɛ) of V0.98W0.02O2 is about 0.2 at 8–14 μm (0.088–0.155 eV) and the average solar absorption (αs) of M phase and R phase is about 0.53 and 0.59 at 0.3–1.5 μm (0.496–4.13 eV), respectively. Finally, radio frequency magnetron sputtering was used to achieve precise doping, which solved the problem of oxygen partial pressure in reactive magnetron sputtering, and V1-xWxO2 films with IMT close to room temperature and narrow hysteresis width were prepared. This is due to the fact that higher W doping content will greatly increase the density of defect-induced nucleation sites and promote nucleation. At the same time, the experimental results of IMT were consistent with the calculated results, which proved the reliability of the calculation. This will provide a theoretical basis for the development of new thermal control materials and a new method for the preparation of doping-VO2 films in the future.