Simple Preparation And Properties Of Vanadium Dioxide

Vanadium dioxide (VO2) presents a first-order phase transition at 341 K (68℃), from a semi-conducting phase to a metallic phase. During this process abrupt changes of optical and electrical properties can be detected, included that VO2 can be used in the energy-saving techniques as a smart material. A great deal of attention has been paid to the preparation of pure phase VO2, most work on the film preparations. The studies on VO2 powder preparations were minority. In fact, the powder VO2 not only can be prepared to the thermo-sensitive switch directly by mechanical pressing, but also can be further used for film preparation by using coating and spraying methods.In this work, two methods were developed for the preparations of pure phase VO2 powders: oxalic acid reduction method and NH4VO3 pyrolysis method. The obtained VO2 samples were characterized by XRD, XPS, ESR as well as electric and magnetic measurements. The major results are listed below:(1) In oxalic acid reduction method, the mixture of V2O5 and suitable amounts of oxalic acid (H2C2O4) were sealed in a quarts tube filled with high-purity N2. The sealed tube was put into a 200℃-300℃preheated tube furnace, and then heated at 600℃for 2 h. For obtaining the pure phase VO2, the optimized molar ratio of V2O5 to H2C2O4 was 1:1.(2) In ammonium metavanadate pyrolysis method, NH4VO3 sealed in a quarts tube with high-purity N2 was heated at 600℃for 2 h. In the heating process,3 times of CaCl2 in weight was also sealed in the quarts tube.(3) The characteristics of VO2 powder made by the two methods showed that the samples have abrupt changes in thermal, electrical and magnetic properties near the phase transition temperature, in reasonable agreements with reported data.(4) Characterization by using AC impedance spectroscopy tried to analysis the electrical properties of VO2. Equivalent circuit fitting was used for the data analysis and the resistance, capacitance and inductance of VO2 were obtained. The band gap of VO2 was calculated from Arrhenius plots (1gσ～1/T) of the low temperature semiconductor phase, the value of which is about 0.6 eV, agreeing with the theoretical data previously reported.