| Most materials in nature have the property of"thermal expansion",which means,the size increases with temperature,which often adversely affects the application of materials.Generally,the equipment is made of different materials.Due to the mismatch in the expansion coefficient,thermal stress may be generated inside the mixed material when the temperature changes.Over time,the material may be damaged and affect the performance of the equipment.Therefore,people have been seeking materials with good thermal shock resistance.The research and development of negative thermal expansion?NTE?materials provides a new way to prepare materials with controllable expansion coefficient.The material with small expansion coefficient is prepared by the combination of the negative thermal expansion material and the positive thermal expansion material,and the research on the original low thermal expansion material is also carried out simultaneously.Zn2GeO4 has been reported to have negative thermal expansion properties in the temperature range below room temperature,and the positive and negative expansion with low coefficients(-3.43.9×10-6K-1).However,the literature does not make a complete study on the expansion properties of the material,even the test method of the expansion coefficient is not explained.The work of this paper is based on this idea that Zn2GeO4 is a widely used material,the study of its swelling properties is of great significance for the application of this material.In this work,the negative thermal expansion materials Zn2GeO4 and Zn2-xCoxGeO4series materials were prepared by solid state method.The crystal structure,thermal expansion properties and negative expansion mechanism of the series were discussed.The main work and achievements are shown below:The expansion properties of Zn2GeO4 were systematically studied and it was found that Zn2GeO4 exhibited negative thermal expansion before room temperature,and showed positive thermal expansion after room temperature with tiny expansion coefficient.The average linear expansion coefficient was-2.34×10-6K-1,?100-200 K?,2.56×10-6K-1,?460-673 K?,the axial CTEs are-0.17×10-6 K-1 for the a-and b-axes and 0.39×10-6 K-1 for the c-axis below 200 K,and change to 0.5×10-6 and1.31×10-6 K-1 between 200 and 460 K.The intrinsic linear expansion coefficient is also0.77×10-6K-1,?200-460 K?and 1.25×10-6K-1,?460-573 K?.The structure of Zn2-x-x CoxGeO4 series materials was analyzed by Rietveld refinement and the result shows that the series of materials adopt a rhombohedral structure,the space group was3??No.148?.The lattice parameters are a=b=14.2315?,c=9.5244?for Zn2GeO4 with acceptable values of Rexp=5.74%,Rwp=9.4%,and Rp=9.8%,respectively,and all diffraction peaks of the samples are in good agreement with the standard data of Zn2GeO4?JCPDF standard card No.11-0687?.The lattice parameter and volume increase and the expansion coefficient decreases with the doping ratio of Co2+in Zn2GeO4.The intrinsic linear expansion coefficient of Zn1.6Co0.4GeO4 is-0.35×10-6K-1,?80-160 K?,0.52×10-6K-1,?160-380 K?and0.89×10-6K-1,?380-673 K?,Zn1.6Co0.4GeO4 exhibits a near-zero thermal expansion near room temperature.The negative thermal expansion mechanism of Zn2-xCoxGeO4 series materials was studied by variable temperature and pressure experiments.Grüneisen parameters were calculated by variable pressure experiment.It was found that Grüneisen parameters of the six low energy Raman modes were negative and combined with the abnormal blue shift phenomenon of low energy state Raman mode with increasing temperature in the variable temperature Raman test and it is considered that the low-energy phonon vibration mode contributes mainly to the negative thermal expansion of the series. |
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