Thermodynamics And Kinetics Of B-R And M-R Phase Transformation For VO₂

Vanadium dioxide is one kind of thermochromism material.It alters infrared transmittance by transforming crystal structure reversibly when it occurs phase transition at the temperature of 68?.By means of doping,the phase transition temperature be decreased to room temperature,which makes vanadium dioxide be the great potential material for smart window.Most of the work carried out on this thermochromic phase transition has been done on thin films of VO₂.However,it is extremely difficult to control the stoichiometry of this material in the thin film form for large area applications.Polymer/powder composites are in retrospect inexpensive and very simple for coating applications.Direct preparation of VO₂?M?is quite harsh,so the normal way to prepare VO₂?M?is the preparation of pure VO₂?B?and after annealing are pure VO₂?M?,but after annealing of doped VO₂?B?is not pure VO₂?M?,this phenomenon haven't been explained very well.The application prospect of vanadium oxide is widespread,but due to the diversity of vanadium oxide system,vanadium oxides composition is complicated,and the theory of the phase change process can't be explain in a simple way,because of the phase transformation mechanism is not yet clear,the current research on the theory,preparation and application of VO₂ is still in a infancy stage.Further,the solid-statephase transformation is important to the microstructure ofmetal materials,and thus deside the final properties and the performance of materials.For this,the phase transformation kinetics of vanadium dioxide is an important basic research which have potential for industrial applications.In this thesis,we usd hydrothermal process to obtain VO₂ nano sample.Differential thermal analysis method and JMAK model ard used to study the phase transition kinetics.The primary contents and results are as follows:?a?Phase transformation thermodynamics anddynamics of VO₂ nanoparticles of B-R phase:the vanadium dioxide powders were synthesized by hydrothermal method using V₂O₅ as the resource of vanadium and oxalic acid as the reducing agent.The samples are characterized by XRD,SEM,DSC.Using DSC under different heating rate?10 k/min-40 k/min?to test the phase transition of thermodynamics,combined with the analysis and calculation of the phase transformation kinetics model,get a B-R phase transformation is heavy configuration phase transformation,phase change activation energy of 213.17 kJ/mol,belongs to the one dimensional growth phase change,organizational change and grow up in the initial phase,belong to the nucleation process,from the parent phase,and the moment to form new nuclear collision model belongs to the anisotropic growth form collision.?b?Phase transformation thermodynamics anddynamics of VO₂ nanoparticles of M-R phase:after annealing the new madeVO₂?B?nano powderunder argon atmosphere,we obtain the powder ofVO₂?M?powders.The samples were characterized by XRD,SEM,DSC.Using DSC under different heating rate?10 k/min-40 k/min?to test the phase transition of thermodynamics,combined with the analysis and calculation of the phase transformation kinetics model,get a M-R phase transformation phase change activation energy of 206.85 kJ/mol,organization change is one dimensional phase change of the initial growth,belong to the nucleation process,from the parent phase,and the moment to form new nuclear grow out process to form new nuclear,belongs to the nucleation process,the crystal that continue to grow up in the original nuclear collision model belongs to the anisotropic growth form collision.?c?Influence of hydroxyl content on transformation thermodynamics of vanadium dioxide:after aging experiments of the new made of VO₂?B?,the sample were characterized by FTIR and DSC,combined with the phase diagram and the crystal structure of vanadium dioxide,we explain the impact and mechanism of hydroxyl content onthe B-R phase transition temperature:as the hydroxyl content increases,the phase transition temperature decreases,which match the layer law:TPHASE???=3.06+441.82 x???;Hydroxyl content of VO₂?B?will influence the layered structure of VO2?B?,reduces the phase transition energy barrierof B-R,so the phase change process would occur at lower temperature.