Preparation And Thermochromic Properties Of Heterogeneous Vanadium Oxide Thin Films

Vanadium dioxide（VO₂）,as a functional material with semiconductor-metal phase transition characteristics,can undergo a reversible phase transition from a low-temperature monoclinic semiconductor phase to a high-temperature tetragonal metal phase at a temperature of⁶8℃,which is accompanied by remarkable changes in optics property and electricity property.Therefore,it has great application value in many fields such as smart windows,infrared stealth,and new-type optoelectronic devices.The transmittance of VO₂ film undergoes abrupt changes accompaning by the phase transition especially in the near-infrared wavelength,and is almost unchanged in the visible wavelength.It can achieve both sufficient lighting and smart temperature adjustment at the same time,and become an excellent smart window material.High-purity monoclinic VO₂（VO₂（M））was typically considered as a prerequisite for obtaining VO₂-based thermochromic coatings with high performance.The co-existence of other vanadium oxides in VO₂ coatings was regarded as an unfavorable issue.In this paper,we compared the thermochromic properties of the heterogeneous vanadium oxide thin films and pure monoclinic VO₂ films,and studied the effects of substrate types on the microstructure and thermochromic properties of the heterogeneous vanadium oxide films.The main contents are summarized as follows:1.The amorphous vanadium oxide thin film was first deposited on a quartz substrate by reactive sputtering routine at room temperature,carried out the rapid annealing process in air,and maked the heterogeneous vanadium oxide films.Structural analysis revealed that VO₂,V₃O₇ and V₂O₅ coexist in the heterogeneous vanadium oxide thin film.Different from previous reports,our results demonstrate that the proper co-existence of high valent vanadium oxides（V₃O₇,V₂O₅）in VO₂-based films can remarkably enhance the thermochromic performance of films.And the luminous transmittance(T_lum)and solar modulation ability(ΔT_sol)of the heterogeneous vanadium oxide thin film obtained by an optimal preparation process are 1.93 and 1.34 times those of the pure polycrystalline VO₂ thin film,respectively.Moreover,the heterogeneous vanadium oxide thin film exhibits lower semiconductor-to-metal transition temperature（60.8°C）than the pure VO₂（M）thin film（67.9°C）.2.Based on the research on the phase transition characteristics of heterogeneous vanadium oxide thin films,the influence of different substrates on the microstructure and properties of heterogeneous vanadium oxide thin films was further studied.In the heterogeneous vanadium oxide thin films deposited on different substrates（quartz,soda-lime glass,A-and C-plane sapphire）,their thermochromic properties are enhanceed first and then weakened with the annealing time increases.The results show that the substrates affected surface morphology of the thin films,and the crystalline A-and C-plane sapphire substrates promoted the growth and fusion of thin film particles.Compared with the heterogeneous vanadium oxide thin film deposited on the quartz substrate,the films deposited on A-and C-plane sapphire substrates still were composed of VO₂,V₃O₇ and V₂O₅ after the same annealing time,and improved the luminous transmittance(T_lum)of the film.The sodium ions in the soda lime glass substrate penetrated into the film to generate monoclinic NaV₆O₁₅,which reduced the luminous transmittance(T_lum:17.017%³0.364%),light modulation efficiency（ΔT_r:18.251%²2.974%）and transition temperature（T_c:58℃⁵8.86℃）of the films.3.The potential application of the heterogeneous vanadium oxide thermochromic thin films in smart windows was studied.We tested and analyzed the energy efficiency of the heterogeneous vanadium oxide thin film by preparing large-area heterogeneous vanadium oxide thin film combined with smart window model house.The results show that the internal temperature increase of the model house with the thermochromic window of the heterogeneous vanadium oxide film less than the pure monoclinic vanadium dioxide film is 19.5%under the similar visible light transmittance(T_lum)and the same irradiation time,that is the heterogeneous vanadium oxide film has better energy saving performance.In addition,the properties of the heterogeneous vanadium oxide thin film remained nearly original state after long-term storage（13-16 months）at room temperature,and the V⁴⁺/V⁵⁺decreased ratio before and after storage of the heterogeneous vanadium oxide thin film was about 50%of that of pure monoclinic vanadium dioxide film.Finally,studying of the ultraviolet radiation characteristics,it was found that the transmittance of UV radiation(T_UV)of the heterogeneous vanadium oxide thin film was less than 0.5%before and after the phase transition,and it can largely achieve the UV-shielding functions.