Structual Design,Controllable Preparation And Evaluation Of Integrated Properties On Vanadium Dioxide-based Multilayer Films

As a typical transition metal oxide,vanadium dioxide?VO₂?has the thermochromic properties,which undergoes metal to insulator transition?MIT?in response to the variation of surrounding temperature.In addition,the MIT accompanies with saltation in physical properties such as optical and electrical performance.For optical performance,VO₂ presents a high transmittance in near infrared range at the low temperature state while exhibits a strong reflectance at the high temperature state.Thus,it is a potential candidate for smart/intelligent window application.Recently,VO₂ based smart windows have attracted much concerns in researches due to their energy-related unique phase change around the room temperature.However,up to now,there are still many problems that need to face in actual application of VO₂ based films.First,the higher intrinsic transition temperature?T_c?.Second,the lower luminous transmittance(T_lum).Third,the unsatisfied solar modulation ability(?T_sol).Fourth,the metastability and poor durability.Fifth,the uncomfortable color for human eyes?brown-yellow?.In this paper,with the purpose of solving the mentioned drawbacks,we design a series of VO₂ based multilayer and composite structures,emphatically investigate the optical performance and stability of the films.Concretely,the research contents and main results are surmmerized as follows:1.By introducing WO₃ antireflection layer,we designed the WO₃/VO₂/WO₃?labeled as WVW?multilayer structure,and calculated the optimal thickness of different layers?30 nmWO₃/50 nmVO₂/30 nmWO₃?.Then,we employed magnetron sputtering to deposit WVW structure.From this work,we obtained films with well quality and found the located lattice matching at the interface between WO₃ and VO₂ layers.Meanwhile,the WVW structure exhibited a well enhanced luminous transmittance(T_lum-L=55.4%),with the increase rate of 49%.Furthermore,the transition temperature had been reduced due to the elements interdiffusion of tungsten and vanadium at the interface.At last,WO₃also served as a protective layer and improve the durability of the films.2.Glass/V₂O₃/VO₂ double layers structure had been designed,prepared and their thermochromic performance had also been evaluated.The crystallinity of top VO₂ film had been promoted through embedding V₂O₃ buffer layer,leading to the enhancement of solar modulation ability(?T_sol=13.2%,with the increase rate of 76%).In addition,the widths of optical hysteresis loops would be broaden due to the embedding V₂O₃ buffer layer.Finally,the durability of VO₂ was prolonged when embedding the V₂O₃ buffer layer.3.SiN_x was selected as the antireflection layer and passivation layer for VO₂ film,and SiN_x-VO₂ composite structures were deposited by magnetron sputtering.Then,the optical performance of films was simulated while the fatigue property was also studied.As the result,both of the solar modulation ability (?T_sol=14.2%,with the increase rate of 32%)and luminous transmittance(T_lum-L=39.1%,with the increase rate of 8.6%)had been promoted through SiN_x/VO₂/SiN_x?SVS?sandwich structure,agreed with the simulation based on transfer-matrix.Besides,fatigue cycling tests were executed on VO₂ single layer and SVS structure for more than 2 years.The results indicated that SVS exhibits an excellent fatigue stability,with the life-time over 27 years,which was 6 times compared with VO₂ single layer.The failure mechanism of VO₂ film during cycling test had been demonstrated.Summarily,the main factor for the failure were the nonlinear gradual oxidation on VO₂ surface and the generation of cracks due to the variation in coefficient of thermal expansion during the MIT cycling.4.Nano-porous VO₂ films were designed.We fabricated Nano-porous VO₂ films by combining magnetron sputtering and post annealing process.The PTFE was regarded as pore forming template for continuous VO₂ film during the special annealing route.The result indicated that ultrahigh luminous transmittance(T_lum-L=78.0%)and solar modulation efficiency(?T_sol=14.1%) had been realized through Nano-porous structure due to the Localized Surface Plasmon Resonance?LSPR?effect.Moreover,we proposed some ideal models based on Maxwell-Garnett theory to elaborate the excellent optical performance of Nano-porous VO₂ films.In the end,we proposed a new application field of VO₂ based smart window,which could serve as the front windshield of automobile(T_lum>70%,?T_sol>10%).5.Nitric acid solution was selected to modify the surface morphology of post annealed continuous VO₂ films,and isolated VO₂ nanoparticles were obtained during the normalized etching route.The results manifested that improved luminous transmittance(T_lum=59.8%)and enhanced solar modulation efficiency(?T_sol=11.9%)had been realized through isolated nanoparticle structure due to the Localized Surface Plasmon Resonance?LSPR?effect.By learning from the formation of karst landform,the etching mechanism could be speculated as follows.Amorphous and small grains would be removed by acid etching at first,and then the etching process would carried out along grain boundaries,resulting in discarding the dross and selecting the essential parts.Finally,ideal models based on Maxwell-Garnett theory were proposed to analyze the optimization of mechanism on optical performance for isolated VO₂ nanoparticle films.