| Vanadium dioxide with multi configurations, all of them, the metal-insulator transition temperature of the rutile VO2 is 341 K. When the temperature is high than the phase-transition temperature, the structure of vanadium dioxide (VO2) transition from low-temperature monoclinic V02(M) to high-temperature tetragonal V02(R), and while the electrical property of VO2 becomes from low-temperature insulator phase to high-temperature metal formation. Because the thermal hysteresis exists during the phase transition occurred, so the phase transition is first-order. When the phase transition of VO2 is occurred the, the light reflectivity, conductivity, light transimission and susceptibility are all sudden changed. The optical transformation characteristics of VO2 materials can be applied to the various optical transformation switch, energy conversion and storage and so on.In this paper, the high quality of the VO2 (M) and doped VO2 (M) samples are hydrothermal prepared by using V2O5 as raw materials, boric acid as dopant, and oxalic acid as reducing agent. While the reaction temperature is 265℃, the pressure is 5 Mpa, and the reaction time is changing period. The structures of all prepared nano-powders were characterized, and their properties are measured. The phase transition, electrical and optical properties of VO2 are carefully investigated. The main content of the research as follows:① In third chapter, the phase transition, electrical and optical conversion properties are carefully study. Structures and morphologies of the VO2(A) nanorods were examined by X-ray diffraction (XRD), scan electron microscopy (SEM), and transmission electron microscopy (TEM). DSC curves show the phase transition of V02(A) nanorods is 167.8℃ upon heating cycle. The resistance of the VO2(A) was measured by four probe method, the resistance hysteresis indicate that the electron phase transition of VO2(A) is strongly first order in nature. The hopping activation energy was calculated based on Arrhenius plot, which is 0.39eV for the low-temperature of VO2(AL) phase, and 0.37eV for the high-temperature of VO2(AH), respectively. Variable-temperature infrared spectra reveal that the VO2(A) nanorods has good optical switching character in the infrared light region.② In fourth chapter, the preparation and characterization of stellate VO2(M) sample are carefully study. We used vanadium pentoxide as raw material, the oxalic acid as reducing agent. By changing the mole ratio of V2O5 and oxalic acid, reaction temperature and time, it is found that the high-quality of VO2(M) are obtained under a 1:1.8 mole ratio of V2O5/oxalic acid, the reaction temperature is 265℃ and reaction time is holding for 48h. Based on X-ray diffraction(XRD) spectrum, when the mole ratio of V2O5 and oxalic acid is 1:1.8, the prepared VO2 sample is pure phase of V02(M). Through transmission electron microscopy (TEM) and scanning electron microscope (SEM), it was seen that the V02(M) have a star shape, and the interplanar spacing is 0.4 nm. The differential scanning calorimetry(DSC) curve show an endothermic peak of VO2(M) sample at 65.6℃, indicating that the phase transition temperature is 65.6℃. The resistance of pure phase VO2 (M) was measured by four point probe method, the resistance appears sudden change at temperature of 67.2℃.③ In fifth chapter, the influence of the boron doped to the phase transition temperature of V02(M) was study. The differential scanning calorimetry (DSC) curve clearly displays that boron doped induced the transition temperature decrease of V02(M) phase. When the boron content was increased to 20%, the phase transition temperature of V02(M) drop to 53.9℃. Based on the infrared light spectroscopy (FT-IR) of the B doped sample, it can be seen that the clear absorption peak lie to the range of 650-900 nm, during the measuring temperature at 55℃. The X-ray photoelectron spectroscopy (XPS) can confirm that the vanadium ion lattice site was partial replaced by boron atoms, the B-O-V band peak was also observed from XPS curves. The resistance of the B doped VO2(M) was measured by four-point probe method, when the B doping content is 5%,10% and 20%, the resistance of the VO2(M) sample occurs a sharp dropping at 65.3℃,60.7℃ and 60.7℃, respectively. |
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