Abnormal Thermal Expansion In Ta₂M₂O₁₁ And Co₂P₂O₇ System

Thermal expansion is an inherent physical property of materials.Negative thermal expansion(NTE),namely the lattice shrinks when heated,offers the possibility of control the coefficients of thermal expansion in high-precision instruments and equipment field.In this work,based on the characteristics of the oxide framework,a new abnormal thermal expansion material is prepared.Through crystal structure,Raman spectral and molecular dynamics investigations,the thermal expansion behavior and related mechanism of Ta₂M₂O₁₁(M=Mo,W)and Co₂P₂O₇systems,abnormal thermal expansion materials,have been in-depth revealed.The main results and innovations are as follows:Zero thermal expansion materials have broad application prospects in the field of high-precision engineering,but they are very rare.A new ZTE material Ta₂Mo₂O₁₁(?_l=0.37×10^-6K^-1,200 K?600 K)was synthesized by high temperature solid state reaction method.High resolution X-ray diffraction results show that Ta₂Mo₂O₁₁has a layered structure,stacked in the[001]direction.The TaO₆octahedrons are close to each other due to the strong thermal vibrations of O3 atoms,leading to intra-layer contraction.The reduction of Van der Waals force causes inter-layer expansion,and the bulk phase exhibits ZTE characteristics.Temperature-and pressure-dependent Raman spectroscopy studies reveal that the phonon modes above 50 cm^-1have less contribution to thermal expansion,which is consistent with the calculation results of lattice dynamics.Below 50 cm^-1,the asymmetric torsional motion of the TaO₆and MoO₄polyhedrons corresponding to the phonon modes with strongly negative Grüneisen parameters is balanced by the interlayer shear and breathing motions corresponding to the phonon modes with strongly positive Grüneisen parameters.These compensation effects lead to the ZTE behavior of Ta₂Mo₂O₁₁.This research provides ZTE materials with great application prospects and new insights into the mechanism of zero thermal expansion.A new,low-cost,wide-temperature negative thermal expansion material—Ta₂W₂O₁₁was synthesized by high temperature solid state sintering.The unit cell parameters are:a=12.27(?),b=12.27(?),c=3.88(?),?=?=?=90°,V=583.43(?)³.Experimental anylyses demonstrate that the macroscopic linear coefficient of thermal expansion of Ta₂W₂O₁₁is-1.89×10^-6K^-1,while the intrinsic linear coefficient of thermal expansion is?_l=4.4×10^-6K^-1.By using the solid state sintering method,under Ar₂atmosphere,a new low-cost giant negative thermal expansion material Co₂P₂O₇was synthesized,which shows a huge negative coefficient of thermal expansion in the temperature range of538 K-580 K,which is?_l=-82.1×10^-6K^-1.The substitution of Mn reduces the coefficient of negative thermal expansion of Co₂P₂O₇,but broadens the NTE temperature range.The coefficient of linear expansion of Co_1.4Mn_0.6P₂O₇is:?_l=-0.94×10^-6K^-1,173 K-555 K,which is a near-zero thermal expansion material covering the room temperature area.