| Most materials in nature are characterized by thermal expansion and cold contraction,which may seriously affect the accuracy,precision and stability of instruments and equipment for applications in optoelectronics,communications,precision measurement,aerospace and other fields,or even fail to meet the design requirements.Therefore,zero or near-zero expansion materials become the "ideal" materials for scientific research and engineering technicians.The emergence of negative thermal expansion materials provides an effective way for the preparation of zero?near-zero?expansion materials:using negative thermal expansion materials to regulate the expansion coefficient of positive thermal expansion materials,the preparation of zero or near-zero expansion materials can not only solve the problem of thermal stress mismatch,may also be incorporated into various excellent properties together to achieve multifunctionality.In the past 20 years,great progress has been made in the preparation,thermal shrinkage mechanism,and application of negative thermal expansion materials.However,various negative expansion materials still have one or more defects that make them still not widely applicable.The thermal shrinkage mechanism and improved performance of negative expansion materials are still the focus of research in negative thermal expansion materials.This paper consists of two parts:???Study on the abnormal thermal expansion properties of Er1-xCaxMn O3;???Preparation and dielectric properties of controllable expansion Zn Nb2O6/Zr Mg Mo3O12 dielectric ceramics.The main results and innovations are as follows:???Study on the abnormal thermal expansion properties of Er1-xCaxMn O3.?1?Er1-xCaxMn O3 was synthesized using a solid-state method.When x<0.25,the expansion performance of the sample Er1-xCaxMn O3 in the temperature range of room temperature?RT?-750 K shows a monotonous positive expansion,but the expansion coefficient gradually decreasing;When 0.25?x?0.4,it exhibits abnormal thermal expansion characteristics:When x=0.25,the sample exhibits near zero expansion;When x=0.3,that is,the sample Er0.7Ca0.3Mn O3 shows negative thermal expansion performance,the average linear expansion coefficient is-3.1×10-6K-1;When x=0.4,the sample first exhibits negative thermal expansion and then changes to positive thermal expansion;When x=0.5,the sample exhibits a monotonous positive expansion.?2?The porosity of Er1-xCaxMn O3 samples is consistent with the change trend of expansion performance:As the Ca content increases,the expansion performance changes from positive expansion to negative expansion and then to positive expansion.The porosity first increases and then decreases.The sample with the replacement amount of 0.3 has the best negative expansion performance and the largest porosity.?3?The thermal shrinkage mechanism of Er0.7Ca0.3Mn O3:Er0.7Ca0.3Mn O3 is an orthogonal structure,the space group is Pbnm,the Jahn-Teller distortion of Mn3+,distorted.During the heating process,the increase of oxygen vacancies affects the bond length and bond angle in the unit cell of the material,and the thermal movement of Ca is intensified,and the diffusion ability to the surrounding is enhanced,causing the crystal lattice to exhibit significant anisotropy,which makes it different in different Thermal stress difference occurs in the axial direction,elastic deformation occurs,pores are consumed,and negative thermal expansion performance is exhibited.???Preparation and dielectric properties of controllable expansion Zn Nb2O6/Zr Mg Mo3O12 dielectric ceramics.?1?The Zn Nb2O6/Zr Mg Mo3O12 composite prepared by the solid-state method has no chemical reaction between the two materials.While maintaining the respective diffraction peaks,the particles of the two substances are mixed with each other to form an indistinguishable whole.?2?When the mass ratio of Zn Nb2O6 to Zr Mg Mo3O12 is 6:4,the dielectric constant is 9.9,which satisfies the application range of dielectric ceramics,while theexpansion performance has been greatly improved. |
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