X-ray Absorption Fine Structure Studies Of Doped Vanadium Dioxide Smart Materials

Along with the rapid development of science and technology in 21 century,nanotechnology,biotechnology,intelligent-technology and other high-tech technologies shows strong and constant influence on providing advance materials.To compete with world's major advanced manufacturing countries,China proposed and implemented the plan of 'China Manufacturing 2025'.Intelligent materials,among several other emerging frontiers,obtained vigorously promotion towards innovation and industrialization.The so-called "intelligent materials" are certain materials that can perceive the environment and thereafter analyze and react based on the stimulation.Smart materials are crucial components in intelligent materials,undertaking the function of sensitizing and operating.Thus they can be considered as low-level intelligent materials.The most common smart materials include memory materials,piezoelectric materials,fiber materials,magnetostrictive materials,electroluminescent/thermochromic material,current/magnetorheological.VO₂,as the primary research object in this thesis,attracted constant attentions for a long time due to its potential to be utilized as smart windows.Specifically,VO₂ endows the merit of reversible thermochromic phase transition near room temperature?T_c=68??and can realize "through-the barrier" regulation of infrared wavelengths of light.Due to the excellent characteristics of high brightness,wide-band,polarization and energy accurately controllable,synchrotron radiation light source has become one of the irreplaceable advanced research tools for many basic and applied scientific disciplines in recent years.For instance,the work of macromolecular structural analysis based on synchrotron radiation technology so far has been awarded Nobel chemistry Prize Laurel for five times.Synchrotron radiation XAFS technology is sensitive to the local atomic and electronic structure of substance and suitable for most of the condensed matter systems researches,especially amorphous and disordered systems that traditional characterization methods are powerless.Meanwhile,thanks to the energy accuracy of synchrotron radiation,it also possesses the advantage for the study of complex,multi-element systems.In addition,combine with in situ and time-resolved techniques,XAFS can be further utilized for the precise detection of microstructure and transient changes.Therefore,in-situ and time-resolved XAFS techniques are extremely suitable for the mechanism study of phase transition smart materials,catalytic reactions,and crystal growth process.The key points of this thesis are listed below:1.The structure and phase transition studies of W-doped V_O2An appropriate proportion of VO?acac?2 and Na₂WO₄ are mixed together as raw materials and thereafter formed a series of W-VO₂ samples with different doping concentrations by solvothermal method.Through the basic characterizations include ICP-AES,XRD and DSC,the stoichiometry,long-range structure and thermochromic phase transition performance of doping samples were measured.The doping concentrations of the four W-VO₂ samples are 0.54%,0.95%,1.45% and 2.38% respectively.Based on the room temperature XAFS detection and further fitting from a proper structure model,the precise local structure of V basement and W dopants are revealed.The samples with doping concentration of 0.54%,0.95% are M-phase structure of at room temperature,while samples with doping concentration of 1.45% and 2.38% are already converted to R-phase structure at room temperature.The neighboring structure model of W dopant atoms from further data fitting exhibit that the local structure of W atoms are highly symmetric and very similar with tetragonal R phase,acting as the driving force in the structure evolution in the W-VO₂ System.2.The in-situ variable temperature XAFS studies of W-doped VO₂High temperature,low temperature in situ variable temperature XAFS instrument and testing the W-VO₂ samples.The four v k-edge in situ variable temperature XAFS spectra are analyzed and fitted following the same model as the structural analysis of W-VO₂ samples at room temperature.Combining with the DFT calculation,a mechanism is proposed for the thermochromic phase transition of W-VO₂.The local structure of W doping atoms maintain a highly symmetric structure and very similar with tetragonal R phase and this local structure influence the VO₂ overall transformation behavior in a "nucleation-diffusion" mechanism.The nearest 1?2 coordinate spheres of V present similar structure characteristics due to the impact from highly symmetric W local structure.Moreover,the "nucleation"W atoms are local metallic and thus have an impact on the electronic structure of overall W-VO₂ system during the thermochromic phase transition process from M to R phase.3.The in-situ variable temperature XAFS studies of Cr-doped VO₂An appropriate proportion of VO?acac?2 and COCl₃ are mixed together as raw materials and thereafter formed two Cr-VO₂ samples with doping concentrations of 3.5% and 5% by solvothermal method.XRD,DSC and in-situ variable temperature XAFS methods are employed for the sample characterization.Cr-VO₂ possesses orthorhombic O phase symmetry and its phase transition temperature is increasing with higher doping concentration.Structure information of Cr-VO₂ samples are obtained from the analyzing and fitting of XAFS data and provide some insights of the thermochromic phase transition process of Cr-VO₂.For Cr-VO₂,orthorhombic O phase is more stable than M phase under high temperature and therefore enhance the phase transition temperature.During the phase transition process the Cr-doped VO₂ system exhibited a SPT process involving the lattice expansion,the destroying of former V-V coupling structure and eventually forming of more stable tetragonal R phase structure.