Design And Characterization Of Thermochromic Metal Complexes-VO₂ Composites

Rutile phase VO₂ is an ideal thermochromic material which can be used to regulate indoor temperature.Due to its unique phase transformation mechanism,it has attracted extensive attention from researchers.However,VO₂ also has the following problems,which limit its practical application:(1)the phase transition temperature is high,which is 68?;(2)The hydrothermal preparation of VO₂ nano-powders requires higher equipment requirements;(3)The color is not easy to be accepted by people,and there is no color change before and after the phase change,which is not conducive to the promotion of smart window and effect display;(4)The low solar modulating power of pure VO₂ film limits its wide application.Therefore,it is urgent to overcome these problems to realize the practical application of VO₂in the field of smart windows.In this study,we chose to use solid phase thermochromic complex and VO₂ to prepare a two-layer composite structure to improve the performance of vanadium dioxide.First,tungsten doped VO₂ nano-powders were prepared by ball milling.V₂O₅ was selected as vanadium source and activated carbon was used as reducing agent.W doped VO₂ powders were prepared by ball milling to reduce the phase transition temperature.Secondly,two thermochromic metal complexes[(CH₃CH₂)₂NH₂]₂Cu Cl₄ and[Cu(dieten)₂](Cl O₄)₂ were prepared and characterized to prepare the thermochromic complex films.The thermochromic complex thin film and VO₂ thin film were composite by double layer structure,and the properties of the composite film were tested and analyzed.The specific research contents are as follows:(1)W-doped VO₂ powders were prepared by ball milling and characterized.By XRD characterization of VO₂ powder,it was confirmed that it was monoclinic phase(M)VO₂.By DSC characterization of VO₂ powders,it is proved that the prepared VO₂ powders have thermally induced phase transition behavior,the critical temperature of phase transition T_t=39.8?and the lag width(?)T_t=13.7?.It is proved that the phase transition temperature of VO₂nano powders is effectively reduced due to the doping of W.The morphology of VO₂powders is studied by SEM.The powder is 50-100 nm particle size,uniform size,in line with the requirements of use.(2)VO₂ thin film was prepared from VO₂ nano powder and polyvinyl butyral,and its optical properties were tested.The optical transmission properties of the films were measured by UV-Vis-NIR spectrophotometer in the wavelength range of 250?2600 nm at 20?and80?.The results show that with the increase of temperature,the transmittance of VO₂ films in the near infrared band decreases,indicating that the film prepared based on VO₂ nano powder has the modulation power in the near infrared band.When the visible light transmittance(T_lum)is about 81.34%,the solar modulation ability((?)T_sol)is 3.37%.(3)The recombination of solid phase thermochromic metal complex[(CH₃CH₂)₂NH₂]₂Cu Cl₄ and VO₂ was investigated.The copper complex[(CH₃CH₂)₂NH₂]₂Cu Cl₄ has been prepared.The phase transition properties and thermodynamic stability of the complex have been characterized by DSC and TGA.The phase transition temperature of the complex is close to that of vanadium dioxide.Using polyvinyl butyral as film-forming polymer material,the thermochromic film was prepared with the complex,and the double layer structure film was prepared with VO₂ composite film.The optical properties of VO₂ film,complex film and composite film were tested respectively.In terms of appearance,the composite film retains the thermochromic effect of the simple complex film,which changes from emerald green to yellow when the temperature rises.In terms of optical properties,the transmittance of VO₂ films in the near-infrared region will decrease with the increase of temperature,so it has a modulation effect on the solar radiation.However,the transmittance of complex films in the visible region will increase with the increase of temperature,which has a negative impact on the solar regulation effect of VO₂in the composite films.Therefore,the optical properties of[(CH₃CH₂)₂NH₂]₂Cu Cl₄-VO₂composite film after the complex and VO₂ composite film are reduced,but this composite method retains the thermochromic property of the complex and the regulating effect of VO₂ in the near infrared,indicating that the double-layer composite structure is a feasible composite way.The optical properties of vanadium dioxide films can not be improved by changing the spectrum of the complex with the temperature,so the two-layer composite structure can be kept unchanged,and the selection of thermochromic complexes can be changed.(4)The recombination of solid phase thermochromic metal complex[Cu(dieten)₂](Cl O₄)₂ and VO₂ was investigated.The[Cu(dieten)₂](Cl O₄)₂-VO₂ composite film was obtained by covering the surface of VO₂ film with the thermochromic film of[Cu(dieten)₂](Cl O₄)₂ complex.The composite film retains the modulation power and thermochromic effect of the complex film in visible light,and has the regulating ability of VO₂ film in near infrared band.Compared with the simple VO₂ film,the solar light modulation ability of the composite film(?)T_sol is increased from 3.37%to 5.72%.Compared with the previous composite method,this method has a simple process,and the color changing temperature of the material is consistent with the phase transformation temperature of the VO₂ powder.This composite method will be beneficial to the promotion of the VO₂smart window.Moreover,this composite film has the advantages of stable property,small temperature range and simple preparation process,it can promote the practical application of thermochromic VO₂ composite film.