Preparation And Property Investigation Of Thermally Induced Phase Transition Material Of Vanadium Dioxide

Vanadium dioxide（VO₂） is a kind of transition metallic oxides and can undergo a phase transition from metal phase to semiconductor phase at phase transition temperature of 68℃while resistivity, magnetic susceptibility and optical transmittance are mutated. Based on the phase transition properties,vanadium dioxide can be used as an excellent thermochromic material. As the growing emphasis on building energy saving by countries around the word, application of vanadium dioxide material in the field of intelligent temperature-controlled film has become a focus recently.In this article, Vanadium dioxide（M） powders were prepared in the atmosphere of nitrogen by pyrolyzing the precursor powders（VO（OH）2） which were steadily synthesized by using liquid precipitation method,with vanadium pentoxide as raw material and hydrazine dihydrochloride as reductant.Then different doping amount of vanadium dioxide（M） powders were prepared by using the preparation process of the pure vanadium dioxide（M） powders,with the dopant of sodium tungstate dehydrate and ammonium molybdate tetrahydrate.Using the method of organic sol-gel, thermochromic film of vanadium dioxide could be prepared by adding a water-soluble polymer-polyvinylpyrrolidone（PVP） in the solution of Vanadyl dichloride,which had a good performance of phase-change optical switching. Products were analyzed and characterized by FT-IR, TG, DSC, XRD, SEM, EDX and UV-VIS, the influence on the crystal structure and morphology were discussed by the pyrolysis process of vanadium dioxide powder, the changes of doping on the phase transition temperature were investigated,and the optical properties of vanadium dioxide film were studied preliminarily, the conclusions are as follows:（1） The change of annealing temperature and holding time has great influence on the crystal structure and component of the powder when the heating rate is constant. The pure vanadium dioxide（M） powder is obtained after the precursor pyrolyzed for 60 min at 600℃; the particle is square, about 150 nm and a width of 100nm; phase transition temperature of the powder is 67.8℃.（2） Using the best preparation process of the pure vanadium dioxide（M） powders, Wdoped and Mo-doped powder are prepared with good crystal structure, and are still vanadium dioxide（M）.However, the integrity of crystal lattice is destroyed in a certain extent by doping the atoms of W and Mo, crystal lattice distortion makes X-ray diffraction peak and stretching vibration band of V=O offseted. Meanwhile, doping has a certain effect on the surface morphology and particle size. With the increase of doping amoun, crystal morphology becomes irregularity and the shape of particle is unconspicuous.（3）The phase transition temperature of vanadium dioxide（M） can be effectively reduced by doping W and Mo, the doping efficiency of W is higher than Mo.They are linear negative correlation.1at%W-doped can reduce the phase transition temperature of 22℃ and 1at%Mo-doped can reduce 5.1℃.（4）Using PVP as a dispersion stabilizer and film-forming promoter, thermochromic film of vanadium dioxide could be prepared on quartz glass substrate by spin-coating.And this preparation process is low-cost, simple, repeatable and easily controlled by using the method of organic sol-gel. The formation of vanadium dioxide（M） and destruction of the crystal structure are not affected by mixing PVP. Thermochromic film of vanadium dioxide could be obtained by pyrolyzing for 60 min at 600 ℃, and it has good transformation properties that optical transmittance in infrared band changes significantly when the phase transition temperature has changed.By mixing 10wt%PVP and spin-coating twenty layers, vanadium dioxide film is formed by an average particle size about 200 nm, the particle is square and evenly distributed; the average thickness of the film is about 0.64±0.01μm with good characteristics of uniform thickness and surface flatness.