Preparation And Properties Of Vanadium Oxide-Based Flexible Thermochromic Materials

As a typical phase change material,VO₂ presents a reversible semiconductor-metal phase transition near 68℃,accompanied by drastic changes in its photoelectric properties,and has a wide range of application prospects in the fields of thermochromic smart windows,threshold switches,laser protection,and radiation cooling.Based on the phase transformation characteristics of VO₂,the thermochromic film has a high transmittance at low temperatures,allowing sunlight to be incident,and has low transmittance at high temperatures,blocking the incidence of sunlight,and can intelligently adjust the indoor temperature,which is an ideal energy-saving material.However,the crystallization of VO₂ requires a high annealing temperature,which leads to the incompatibility between the preparation technology of VO₂ film and commercial flexible substrate,and greatly limits the application space of VO₂ thermochromic film.To solve the compatibility of VO₂ film preparation process with flexible substrate and expand the application of VO₂film,this thesis prepares a composite vanadium oxide-based flexible thermochromic material by sputtering pre-deposition and rapid thermal annealing process,systematically studies its microstructure and thermochromic properties and explores its application in the field of the thermochromic smart window.The main research contents are as follows:（1）Explore the compatibility between the preparation process of the vanadium oxide film and flexible substrate,compare the temperature resistance of four commercial PI substrates,select PI with appropriate performance as the flexible substrate of vanadium dioxide films,and study the preparation of vanadium dioxide flexible films.A 0.025 mm transparent PI substrate with 84.87%transmittance was screened out by a temperature resistance test at 400℃for 10 min.On this basis,the flexible vanadium oxide film with the thermochromic function was successfully prepared on the above substrates by magnetron sputtering and rapid thermal annealing in air.After process optimization,it is confirmed that when the thickness of the pre-deposition film is 120 nm and the annealing temperature is 400℃,the comprehensive properties of the flexible vanadium oxide film obtained by annealing are the best.（2）The effect of annealing time on the microstructure and thermochromic properties of flexible vanadium oxide films was studied.The structural analysis of the sample shows that the annealed flexible vanadium oxide film contains three components:VO₂（M）,V₃O₇,and V₂O₅.With the prolongation of annealing time,the vanadium valence state,average particle size,and surface roughness of flexible vanadium oxide films increase.The visible transmittance(T_lum)of the thin films increases with the increase of annealing time,while the solar energy modulation amplitude(ΔT_sol)increases first and then decreases.The T_lumandΔT_sol of the thin films anneal for 4 min are 50.55%and 8.64%respectively.（3）Use the spin coating process to cover the flexible vanadium oxide film with PDMS and study the thermochromic properties and environmental adaptability of flexible VO_x/PDMS bilayer films.After the addition of PDMS to the flexible vanadium oxide film,its T_lum was 49.09%,and theΔT_sol was increased from 8.64%to 13.65%.In addition,flexible VO_x/PDMS bilayer films exhibit excellent hydrophobicity,flexural resistance,and environmental stability.The addition of PDMS also improves the mid-infrared emissivity of the film and makes it have a certain radiative cooling capability.Compared with blank quartz glass,the flexible VO_x/PDMS bilayer film reduces the indoor temperature by about 10℃,which has an excellent energy-saving effect.