热致变色智能窗户材料研究进展

Advances in thermochromic smart window materials

窗户是建筑中的重要组成部分,也是室内外热交换的重要组件.频繁的热交换会增加室内能源消耗.节能型智能窗户可以调节室内外热交换,减少建筑能源消耗,符合低碳发展理念.热致变色智能窗户根据环境温度变化被动地调节太阳光辐射,无需额外输入能量,可以有效调节室内温度,在建筑节能领域具有潜在的应用价值.热致变色材料是在临界转变温度附近发生可逆相变的温度响应材料,相变前后伴随着显著的透射率/反射率变化,具有优异的太阳光调节能力,是构造热致变色智能窗的关键.本文重点介绍了VO_(2)、水凝胶、钙钛矿、离子液体和液晶5种代表性材料的热响应机理,系统地阐述了它们在智能窗户中的工作原理、最新研究进展以及仍需解决的科学问题,展望了热致变色智能窗户的发展方向.

Thermochromic smart windows can intelligently regulate indoor solar radiation in response to surrounding temperature changes.This effectively reduces energy consumption in buildings and is a promising technology for building energy saving.Thermoresponsive materials,the integral components in thermochromic smart windows,undergo reversible phase changes near the critical transition temperature.In this review,we summarize thermoresponsive materials,including VO_(2),hydrogels,perovskites,ionic liquids,and liquid crystals.A critical review of the thermochromic mechanisms and recent progress made in these materials is presented.Furthermore,current challenges and developing-trends in thermoresponsive materials are also discussed.(1)VO_(2)is the most widely studied material for thermochromic smart windows due to its unique properties,as it undergoes a metal-to-insulator transition at a critical temperature of 68℃.Various methods have been proposed to enhance specific properties of VO_(2),such as optical properties and transition temperatures,including element doping,multilayer film structures,nanocomposite films,and nano-/microstructures.However,simultaneously enhancingτ_(c),ΔT_(sol)and T_(lum)remains a challenge for VO_(2).(2)Hydrogels are regarded as highly promising materials for thermochromic smart windows due to their reversible phase transition between hydrophilic and hydrophobic phases.Well-known thermochromic hydrogels include poly(Nisopropylacrylamide)(PNIPAm),hydroxypropyl cellulose(HPC),and poly(N-vinylcaprolactam)(PNVCL).However,thermochromic hydrogels often exhibit weak mechanical properties and undesirable freezing at cold temperatures,which need to be addressed in future research to facilitate their applications in smart windows.(3)Thermochromic perovskites have apparent solubility variations at different temperatures,accompanied by the shift of absorbance band gaps,leading to optical change from transparency to opaqueness.However,there are some problems that hinder the development of perovskites in the field of thermochromic smart windows,including high phase transition temperature,poor stability,and limited ability to modulate solar radiation.(4)Ionic liquids have also been extensively studied in the field of thermochromic smart windows due to their unique conformational transformation in response to temperature changes.As the temperature increases,the configuration of ionic liquids changes from octahedra to tetrahedra,accompanied by transparency variations.Additionally,ionic liquids exhibit good ionic conductivity,making them suitable for constructing electrochromic smart windows.Consequently,thermochromic ionic liquids can be used to fabricate dual thermal/electrical-response smart windows for both passive and active temperature control.(5)Liquid crystals are new materials in thermochromic smart windows,which exhibit significant optical changes characterized by the molecular orientation.When the temperature exceeds the critical temperature,the aligned structure of liquid crystals transforms into focal conic orientation,offering high solar modulation capability.However,the response time of thermochromic liquid crystals in smart windows is currently long,so future research efforts should focus on enhancing the sensitivity of liquid crystals to improve their response time to temperature changes.