Preparation Of Vanadium Dioxide Nanopowders And Regulation Of Phase Transition Characteristics

Vanadium dioxide,as a typical thermotropic phase change oxide,undergoes a reversible phase transition from low temperature insulator to high temperature metal state at about 68?,which makes its excellent thermochromic properties in infrared band.Therefore,it has become a research hotspot in this field to further regulate the thermotropic infrared emissivity by controlling the phase transition characteristics of vanadium dioxide.However,the high phase transition temperature and the difficulty of large-scale preparation of thin film materials limit the wide application of vanadium dioxide.In this article,based on the preparation of vanadium dioxide nanopowders and the regulation of phase transition characteristics,a new method for preparing pure M-phase vanadium dioxide nanopowders was explored by means of DSC,XRD,XPS and SEM,and the effect and mechanism of Nb-doping on the phase transition characteristics of vanadium dioxide nanopowders were studied.The main results are as follows:?1?Controllable preparation of vanadium dioxide nanopowders by composite annealing process:based on vacuum annealing,a new preparation method of pure M-phase vanadium dioxide nanopowders is proposed in this paper.Pure M-phase vanadium dioxide nanopowders are prepared by optimizing oxygen entrance conditions during annealing.The phase-transition temperature of the powders is 66.06?,and the microstructure of the powders is long nanorod and irregular crystals.The nanorod structure is VO₂?M?and the irregular structure is VO₂?M?crystal.Both of them belong to M-phase VO₂ and have typical phase transition characteristics.Therefore,there are two exothermic peaks in the phase change process because of the difference of particle characteristic scale.The phase change exothermic at high temperature is caused by relatively longer VO₂?M?nanorods,while the phase change exothermic at low temperature is caused by small VO₂?M?crystals.?2?The effect of Nb-doping on the phase transition characteristics of vanadium dioxide nanopowders was studied.The results show that doping low concentration Nb element will cause the long nanorod structure of vanadium dioxide to broken and fused then formed a thick and short rod-like structure,and the phase transition temperature gradually decreased.When the doped atom percentage of Nb is 1.5%,the lowest phase transition temperature of vanadium dioxide nanopowders is 47.53?.With the increasing of doping concentration,the phase transition temperature of VO₂ begins to increase related to the doping concentration of 1.5%,and the rod-like structure appears again.Compared with the undoped VO₂ powders,the rod-like structure is much shorter.The rod-like structure appeared and the phase transition temperature increased again when the doped concentration continued to increase.Compared with the undoped vanadium dioxide nanopowders,the rod-like structure is much shorter..It was also found that the doped low concentration Nb element could promote the transformation of VO₂?B?to monoclinic VO₂ during annealing,while doped high concentration Nb element could lead to a transformation of monoclinic VO₂ to tetragonal VO₂.