The Preparation Of Low Valance Vanadium Oxides And Their Incorporations Into LAS Glass Ceramics

Low valence vanadium oxides (LVVO) undergo phase transformations at some critical temperatures. Although their electrical, magnetic and optical properties change abruptly during the phase transformations, their specific volumes also change so largely that the materials become disintegrated. How to use these properties to create thermistors, switches and other functional devices have been attracting research interests for a long time.Except for V₂O₅, other vanadium oxides are not commercially available in China. Reducing V₂O₅ to produce LVVO is the first target of this thesis. On the basis of summarizing previous work, we chose H₂ and carbon as the reductants to prepare VO₂/V₂O₃ from V₂O₅, and then prepare LVVO-LAS glass ceramics composites. Our final purpose is to obtain the right thermistor compositions which can be used in making heating plate for magnetic heating oven.In order to investigate the reduction process, X-ray diffraction (XRD) was employed to analyze the phases of the intermediate and final products. Differential scanning calorimeter and thermal gravity analyzer (DSC/TGA) were used to further study the steps of the reduction procedure. From DSC/TGA results, the reaction route was determined as V₂O₅→V₆O₁₃—VO₂→V₃O₅→V₂O₃. Stoichiometric V₂O₃ can be obtained by reducing V₂O₅ using dry H₂ first at 600 ℃ for 3hrs, and then 900¹000 ℃ for 5hrs;VO₂ can be produced by carbon reducing V₂O₅ at C∶V₂O₅=1∶2 in inert atmosphere. The techniques to prepare VO₂/V₂O₃ powder by reducing V₂O₅ were established.LVVO-LAS glass ceramics composites were prepared by conventional solid state sintering. There are mainly two crystal line phases in the LVVO-LAS glass ceramics composites: LVVO (V₂O₃ or VO2) and LiAlSi₃O₈. The electrical volume threshold quantity of the V₂O₃-LAS glass ceramics composites falls between 5%¹0%. Both of the V₂O₃-LAS and VO2-LAS composites exhibited negative temperature coefficient(NTC) characteristic. V2O3-LAS glass ceramics composite showed a sharp resistivity decrease at the temperature about 100K, which is much lower than expected (about 160K). At the same time, no sharp resistivity change was observed for both V2O3-LAS and VO2-LAS composites at other temperatures, which is out of expectation.