Design,Fabrication And Thermochromic Properties Of Vanadium Dioxide-based Functional Films

Vanadium dioxide?VO₂?,as a typical phase change material,undergoes a metal-insulator transition?MIT?at the phase transition temperature of?68?,which is accompanied by a large change in its physical properties.During the MIT from the low-temperature insulating state to the high-temperature metallic state,VO₂ shows a resistivity drop of over?5 orders of magnitude and transmittance change from transpartent to opaque in the near-infared region.Because of these features,VO₂ shows great potentials in many fields,such as thermochromic smart windows,phase change electronic devices,and optical devices.VO₂ films are the main forms in real applications.At present,chemical methods?includes hydrothermal method,sol-gel method,tec.?and physical methods?includes magnetron sputtering,physical vapor deposition,etc.?are commonly used to prepare VO₂ thin films with high quality and excellent phase change properties.Among them,magnetron sputtering,due to the mature equipments,flexible and controllable deposition parameters,is the most utilized approach to fabricate VO₂ films and has been widely utilized in scientific experiments and commercial applications.However,to date,there still exist several problems in farbricating VO₂ thin films by magnetron sputtering:?1?In order to ensure the crystallinity of the final products,the preparation temperature of VO₂ thin films by magnetron sputtering often requires more than 400?.Such high preparation temperature is not suitable for industrial production,as well as fabrication of VO₂ thin films on flexible polymer substrates.?2?For VO₂ thermochromic smart films obtained by traditional magnetron sputtering,it is difficult to achieve simultaneous improvement of their luminous transmittance(T_lum)and solar modulation ability(?T_sol).?3?The intermediate V⁴⁺in VO₂ is not very stable in the environment and relevant service life cannot meet the requirements of practical applications.To solve this drawbacks,we have designed various structure of VO₂ functional films through optical simulation.Meanwhile,we have successfully obtained optimized structures at low-temperature and on flexible substrates by magnetron sputtering.Focused on the purposes for commercial applications and current research trendency,we mainly investigates the staibilty of prepared VO₂ thin films,and draws the following conclusions:?1?Cr₂O₃ could serve as template layers to obtain crystalline VO₂films in a relatively low temperature range?250-350??.The Cr₂O₃ template layers can transform the final VO₂ film from the metastable B phase to the monoclinic M phase.Meanwhile,Cr₂O₃ plays the role as the antireflection layer to improve the optical performance of VO₂ films.Low-temperature fabricated Cr₂O₃/VO₂ bilayer structure with optimal thickness shows improved thermochromic performance with T_lum,lt=46.0% and ?T_sol=12.2%,which is much better than the VO₂ single-layer films prepared at high temperature?450??.?2?By utilizing polymer polyimide?PI?as substrates,PI/Cr₂O₃/VO₂ flexible films were successfully prepared.The flexible film shows a thermally induced phase change performance,with a resistivity change of more than 2 orders of magnitude,a transmittance change of about 60%?at 2500 nm?,and solar modulation ability(?T_sol)of?6.7%.Meanwhile,the VO₂ flexible film is almost in a stress-free state,and the width of relevant hysteresis loop is less than 1?.?3?Cr₂O₃/VO₂/Si O₂ sandwich structure was designed and prepared to realize excellent thermochromic properties,while the T_lum,lt is 54.0%and the?T_sol is 16.1%.Furthermore,the Cr₂O₃/VO₂/Si O₂ structure can improve the stability of the middle VO₂ thin film.After 1000 hours of accelerated degradation test?60? and 90% relative humidity?or 4000 fatigue cycle tests,there is no obvious deterioration of the sandwich structure.Compared with the VO₂ single-layer film,the Cr₂O₃/VO₂/Si O₂ structure shows largely enhanced stability.?4?Four kinds of VO₂ film samples with different protective structures were designed and prepared,and the stability of the samples was investigated in different test environments.The results revealed that the cross section of VO₂ films can accelerate the degradation of the whole film.Encapsulating the surface and cross section of the VO₂ film with the Hf O₂ protective layer can effectively improve the stability of the VO₂ film.