The Preparation Of Porous VO₂ Film With Thermochromic Properties

Vanadium dioxide?VO₂?,as a smart thermochromic material,which shows metal-semiconductor transition?MST,critical temperature Tc?68^oC?from tetragonal?R phase,P42/mnm?at high temperature to monoclinic?M phase,P21/c?at low temperature.This structure transition is accompanied by significant changes in electrical conductivity and optical properties in near infrared region.Those properties make VO₂ to be provided with a wide application prospects.One of the most promising was used in smart window coatings,according to the change of the external environment temperature,which can intelligently adjust the transmittance and reduce additional energy consumption due to the use of air conditioning and other equipment.Despite with a series of excellent performance making it the material of choice for smart window coatings,we still face the three major problems for VO₂-based coatings for practical applications,which is low visible light transmittance,high radiance and the phase transition temperature higher than room temperature.In this paper,we research mainly around improving the visible light transmittance of VO₂ film and lowering its phase transition temperature.This paper is focusing on improving the visible light transmittance of VO₂ film,lowering phase transition temperature.Porous thermochromic pure and tungsten?W?-doped VO₂ films have been prepared on silica substrates by spin-coating via a sol-gel process and annealing in ammonia?NH₃?atmosphere.Then,we conducted XRD,FESEM,three-dimensional profile,UV-vis-NIR spectrophotometer,XPS,Raman and other test methods.We discussed the process parameters of the preparing film with excellent thermochromic properties.At the same time,we analyzed the reasons for forming absorption peak on the high-temperature transmittance spectra of the film.The main results are as follows:?1?We have successfully prepared porous VO₂ films via V₂O₅-H₂O₂ solution by spin-coating on a silica substrate and annealing in ammonia atmosphere,The optimum film is prepared at 550^oC for 30min with the optical results for the T_lum?20/90^oC?=54.0%/53.9%,?T_sol=7.0%,and visible light transmittance is about 70%.The morphological of VO₂ film is porous and continuous particles.?2?We added oxalic acid?H₂C₂O₄?to V₂O₅-H₂O₂ solution and successfully prepared porous VO₂ films with higher visible light transmittance and good thermochromic properties.When V₂O₅:H₂C₂O₄=1:2,the optimum annealing parameter is 500^oC for 60min.The visible transmittance is about 80%.The luminous transmittances(T_lum)are respectively 69.8%,70.2%at high and low temperature.The solar modulating ability(?T_sol)is 9.5%;When V₂O₅:H₂C₂O₄=1:3,the optimum annealing parameter of preparing VO₂ film is 500^oC for 30min,The luminous transmittances(T_lum)at high and low temperature are respectively 63%,62%.The solar modulating ability(?T_sol)is 8.0%.After adding oxalic acid to the solution,the morphology of VO₂ films present island grain structure.The high transmittance spectra of the film will appear an absorption peak at 1000nm-1500nm.When changing the volume of H₂O₂,it has a greater effect on the morphology and properties of the films.As the volume of H₂O₂ increases,the porosity of particle is greater,T_lum and?T_sol are increasing.?3?Pure and W-doped VO₂ thin films were successfully synthesized by the spin-coating of VOCl₂ solution.When the annealing pressure is 2.0×10³Pa,the pure porous VO₂?M?film with the high solar modulation efficiency(?T_sol=9.4%,?T_2000nm=52.9%)and good visible light transmittance?60%?.As the doping amount increases,the phase transition temperature of the film reduce from 55^oC?0at.%W?to28^oC?2at.%W?,and maintain good infrared modulation in the near infrared region.