Controlled Synthesis Of Single-phase Vanadium Dioxide And Its Properties

VO₂ is a type of transition metal oxide with various crystal forms.VO₂（M）can undergo a fully reversible phase transition from insulating state to metallic state at about 68 ℃ while its electrical resistivity,infrared transmission and other physical properties will dramatically change.Therefore,it can be applied in integrated switches and smart windows.However,various valence states from +2 to +5 and mixed valence state of vanadium element cause the difficulty in its preparation of single-phase VO₂,and has limited the application and development of VO₂ materials.Sol-Gel method relies on hydrolysis and condensation reaction to persue selfsustaining by high chemical reactivity constituents.This synthesis approach possesses lots of appealing advantages including simplicity of equipment and high purity of products.However,there are still some significant problems in the synthesis of VO₂ materials.The poor chemical stability of +4 valence state of vanadium alkoxide makes it easily oxided to +4/+5 mixed-valence products in the synthesis of VO₂ sol which cause a difficult preparation for VO₂ sol.There is still a challenge of controllable valence state of vanadium element and the preparation of VO₂ materials by inorganic method.In this thesis,a preciptization-peptization method was proposed to successfully prepare VO₂ sol by using inorganic VOSO₄ as a vanadium source,NH₃·H₂O as a precipitant,and H₂O₂ as an initiating agent.The thesis systematically studied valence regulation technology of vanadium ions,microstructure,the formation process of VO₂ sol and their relationship with the preparation process parameters.The relationship between composition of VO₂ sol and crystal forms of film was studied.It indicated that the influence of film crystal control on its electrical resistance performance.And the relationship between hydrothermal crystallization process of VO₂ sol and phase composition of powders was studied.The regulation mechanism of phase transition temperature for VO₂ powders were discussed.And above work offered new ideas and methods for preparing highquality VO₂ films and powders.The related important conclusions are as follows:There is a challenge on oxidization and valence change of V⁴⁺ ions,and colloidal aggregation in the preparation process of VO₂ sol by sol-gel method.In this thesis,the valence state regulation of V⁴⁺ and controllable synthesis of VO₂ sol was realized at 25 ℃ by preciptization-peptization method using 0.2 mol/L VOSO₄ as a vanadium source,NH₃·H₂O as a precipitating agent,H₂O₂ with nH₂O₂/nv:120 as an initiating agent.The effects of pH value and H₂O₂ concentration on the valence state of V⁴⁺ ions in the prepared VO₂ sol were studied.The coordination mechanism between H₂O₂ and V⁴⁺ ions and the condensation mechanism among the coordination complexes,and their effect on the stability of V⁴⁺ ions were discussed.The effect of V⁴⁺ ions valence change on colloidal particles aggregation during the aging process of VO₂ sol was also studied.The influences of pH value and stabilizer on the state of VO₂ colloidal particles were discussed.The synergy of Methyl vinyl ether/maleic acid copolymer and hydrazine hydrate was used to make 0.2 mol/L VO₂ sol can be stable for at least 168 h.Above results provide a new technical method for the preparation of VO₂ sol.The crystal structure of VO₂ film decides its functional application,so the controlled preparation of VO₂ film crystal form is still focused.Based on the preparation of VO₂ sol,spin-coating method was proposed to prepare VO₂ films.The effects of coalescents and other factors on the controllable crystal forms of VO₂ films were studied.The results show that polyvinyl pyrrolidone or Methyl vinyl ether/maleic acid copolymer as the film forming assistant,which lead to the transformation of VO₂ films to B-phase and lose the thermal induced phase change properties by annealing at 550 ℃ for 1 h.However,VO₂ grown on the quartz substrate was prepared by using pure inorganic VO₂ sol without organic assistant.and the resistance of film was decreased from 0.78 MQ,at 25 ℃ to 11.4 kQ at 85 ℃.The phase transition temperature was 63.7 ℃ and the hysteresis loop width was6 ℃.The above results show that the inorganic VO₂ sol is beneficial to the preparation of M-phase VO₂ films,which provide a method for controllable crystal forms of VO₂ films.In order to meet the demand of VO₂ temperature response in different application areas,the Regulation of VO₂ phase transition temperature is very important.M-phase of tungsten（W）doping vanadium dioxide（W-VO₂）was prepareby one-step hydrothermal crystallization of prepared VO₂ sol with the presence of ammonium metatungstate during hydrothennal treatment for 48 h at280 ℃.The "nucleation-growth-transformation-ripening" mechanism is proposed as the formation mechanism for the hydrothermal crystallization of VO₂ sol and morphological evolution process of W-VO₂（M）powders.The results show that the phase transition temperature otf VO₂ decreases with the increase in W doping content.And the phase transition temperature of \VVO₂（M）reduces from 50.1 ℃ to 31.2 ℃ when the nominal dopant concentration is from 1.0 at.% to 3.0 at.%（atom fraction）.For further raised W doping concentration to 6.0 at.%,the phase transition temperature of W-VO₂（M）is 28.0 ℃.Above results show that the W-VO₂（M）powders prepared by one-step hydrothermal crystallization process using inorganic VO₂ sol.The prepared WVO₂（M）powders have a thermally induced phase transition property in a range of28 ℃ to 50 ℃,and provides a phase transition temperature of controlled preparation method for VO₂ powders.