Anomalous Thermal Expansion Mechanism In Sm_1-xZn_xMnO₃,Controllable Expansion And Dielectric Properties Of Mg₂SiO₄/ZrMgMo₃O₁₂

With the rapid development of modern technology,in order to improve the accuracy,stability and service life of various components in high-tech fields such as precision measurement,optical devices,and digital inspection,it is necessary to overcome the effects of the common thermal expansion and cold contraction of materials.The discovery of negative thermal expansion materials provides an effective way to solve the above problems—compositing negative thermal expansion materials with common materials to prepare zero or near-zero expansion materials,which can eliminate the impact of temperature changes on instruments and equipment.In the past 20 years,negative thermal expansion materials have received more and more attention from scientific researchers,and research on negative thermal expansion materials has also been greatly developed.Some new negative thermal expansion materials have been discovered,and their corresponding thermal shrinkage mechanisms have been revealed,which has laid a solid foundation for the establishment of the theory of negative thermal expansion materials.In addition,application research on the composite of negative thermal expansion materials and functional materials to prepare controllable or near-zero thermal expansion functional materials shows that negative thermal expansion materials have broad application prospects.However,there are still some problems that need to be solved in the research of negative thermal expansion materials,whether in theoretical research or application practice.The work in this paper is divided into two parts:In the first part,a new type of perovskite anomalous thermal expansion material Sm_1-xZn_xMnO₃was prepared by solid-phase sintering.Through the investigation of the crystal structure,microstructure,and electrical properties of Sm_0.85Zn_0.15MnO₃,the abnormal thermal expansion mechanism was revealed.The main conclusions and innovations are as follows:?1?The once sintering sample?S1?of Sm_0.85Zn_0.15MnO₃exhibits an anomalous thermal expansion phenomenon:It shows the low positive thermal expansion characteristic with the thermal expansion coefficient of 1.60×10^-6K^-1in the temperature range of room temperature?RT?–519 K but the negative thermal expansion characteristic with the thermal expansion coefficient of-8.40×10^-6K^-1in the temperature range of 519–1023 K.?2?The microstructure of the samples?S1,S2,S3,S4,and S5?for Sm_0.85Zn_0.15MnO₃consists of a large number of ellipsoidal particles?3-8?m?and pores.The porosity of different samples can significantly affect its thermal expansion characteristics.Among them,the sample S3 achieves near-zero thermal expansion,and the corresponding average linear expansion coefficient is 0.25×10^-6K^-1.?3?The thermal-shrink mechanism of Sm_0.85Zn_0.15MnO₃?S1?:The lattice of Sm_0.85Zn_0.15MnO₃displays larger distortion due to the partial replacement of Sm³⁺by Zn²⁺,while the degree of lattice distortion decrease with the increasing temperature,which is accompanied by a transition from positive thermal expansion to negative thermal expansion in the a-axis direction,and the anisotropy of thermal expansion is gradually enhanced which plays a crucial role in the microstructural effect.Moreover,the high porosity of the samples is a sufficient condition for negative thermal expansion:When the porosity is greater than 9.5%,the samples exhibit negative thermal expansion.Therefore,the mechanism underlying the negative thermal expansion of Sm_0.85Zn_0.15MnO₃?S1?involves the coupling effect of the high porosity and the microstructural negative thermal expansion effect caused by crystal grains with anisotropic thermal expansion and intercrystalline pores.In the second part,the negative thermal expansion material ZrMgMo₃O₁₂is composited with the microwave dielectric ceramic material Mg₂SiO₄.The composite,thermal expansion and dielectric properties of Mg₂SiO₄/ZrMgMo₃O₁₂were studied.The main results are as follows:?1?Mg₂SiO₄/ZrMgMo₃O₁₂composite materials with different mass ratios?9:1,8:2,7:3,6:4,5:5,and 4:6?were prepared by solid-state method.The thermal expansion coefficient of the materials can be adjusted by changing the content of ZrMgMo₃O₁₂,when the mass ratio of ZrMgMo₃O₁₂is 60%,the thermal expansion coefficient of the composite sample is 4.47×10^-6K^-1,and the relative decrease is?2?The addition of ZrMgMo₃O₁₂can significantly improve the compactness of Mg₂SiO₄.The solid-state sintering sample of Mg₂SiO₄has poor density,and its density is only 2.37 g/cm³.When the mass ratio of Mg₂SiO₄and ZrMgMo₃O₁₂composite is 4:6,the density of the composite sample reaches 2.62g/cm³,which is an increase of 10.5%.The micro-morphology of Mg₂SiO₄/ZrMgMo₃O₁₂composite samples?8:2,6:4 and 4:6?consist of spherical particles,so the two substances have good compatibility.?3?Both of ZrMgMo₃O₁₂and Mg₂SiO₄are low dielectric constant ceramic materials.The addition of ZrMgMo₃O₁₂only causes small changes in the dielectric properties of Mg₂SiO₄,it is an ideal negative thermal expansion material combined with Mg₂SiO₄.Mg₂SiO₄/ZrMgMo₃O₁₂composite materials have good application potential as substrate materials in the field of integrated circuits.