Structure,Hygroscopicity And Their Influence On Thermal Expansion Performance In HfMgW₃O₁₂ And Fe_2-2x?HfMg?_xW₃O₁₂

Negative Thermal Expansion is an anomalous physical phenomenon that shrinks in volume with increasing temperature,and has wide application prospects in the fields of functional materials and device design.The research on negative thermal expansion has been greatly developed in the past two decades,but some negative thermal expansion materials have gradually exposed problems such as phase transition,hygroscopicity and narrow negative thermal expansion temperature range.How to overcome these problems is crucial for the application of negative thermal expansion materials.Both phase transition and hygroscopicity will cause the material to expand rapidly,which is common in the frame compound A2M3O12 series and the ABM3O12 series materials derived from it,which seriously limits the application of these materialsThe main work of this article includes two parts.The main innovations and conclusions of the work are as follows:1.The hygroscopicity of HfMgW3O12 and its effect on expansion properties after water absorption.(1)It was found that HfMgW3O12 has hygroscopicity.The material can exhibit hygroscopicity in normal laboratory environment and humid environment,and the amount of water absorption is closely related to the expansion coefficient of the material.As the water absorption increases,the expansion curve changes from negative to positive expansion.In the process of temperature loss of the sample,the larger the amount of water absorption,the greater the volume expansion of the sample during water release.(2)Through the Raman scattering spectroscopy analysis of the samples before and after water absorption,it is found that water molecules have inhibited most vibration modes in HfMgW3O12 materials,especially some vibration modes that contribute to negative expansion(3)The structure of HfMgW3O12 was determined by high-resolution XRD of synchrotron radiation,and detailed lattice information such as atomic position,bond length,bond angle,and cell parameters were given.The material behaves as an orthogonal phase at room temperature and belongs to the Pna21(33)space group Anhydrous HfMgW3O12 variable temperature XRD test shows that the material has no structural phase change in the temperature range of 80-573 K(4)The results of variable pressure Raman experiments show that HfMgW3O12 has a phase transition from orthogonal to monoclinic phase at 0.19 Gpa,and amorphization will occur above 3.93 Gpa.2.Fe2-2x(HfMg)xW3O12 series phase change control and hygroscopicity research.(1)Fe2W3O12 belongs to the metastable phase structure,which is difficult to prepare by the traditional solid phase sintering method.Through the method of(HfMg)6+doped in(Fe)26+,the solid phase sintering method successfully prepared Fe2-2x(HfMg)xW3O12 solid solution.(2)Doping(HfMg)6+gradually to the position of(Fe)26+to realize the phase transition temperature gradually adjusted to below room temperature,obtain a near-zero expansion material in a wide temperature range,and expand the application of the material.(3)It is found that after Fe2W3O12 is incorporated into(HfMg)6+,the material has hygroscopicity.The difference between the temperature-variable XRD and temperature-variant Raman phase transformation process and the process of releasing crystal water is compared,and the hygroscopicity after doping is further explained.