Negative Thermal Expansion Properties In Cu_2-xCo_xV₂O₇ And Cu_1.8Mn_0.2V₂O₇

Most of the substances in nature have the property of thermal expansion and contraction,but this property also creates problems in some life practices,such as the loss of accuracy of precision measuring instruments caused by thermal expansion and the thermal mismatch of electronic packages,which are problems that must be solved in practical applications.The discovery of negative thermal expansion materials provides a very effective way to solve the above problems.Negative thermal expansion materials have negative expansion coefficients,which can not only regulate the expansion properties of materials and prepare zero or near-zero expansion materials,as well as improve other properties,and have broad application prospects.For example,the negative thermal expansion material Y₂W₃O₁₂ can be used in solid oxide fuel cells to greatly improve the battery cathode life and redox reaction activity.Another example is that ultra-low thermal expansion films can be used to address thermal fatigue and failure in semiconductors,biomedical sensors,and solar devices.However,the many negative thermal expansion materials that have been found often have disadvantages such as water absorption,easy decomposition by heat,large negative thermal expansion changes due to phase changes,and negative thermal expansion temperature intervals that are not within the application range,which are the problems that need to be solved for negative thermal expansion materials to move toward large-scale applications.In this work,the new negative thermal expansion materials Cu_2-xCo_xV₂O₇（x=0.1,0.2,0.3,0.4,0.5）and Cu_1.8Mn_0.2V₂O₇ are prepared based onβ-Cu₂V₂O₇ negative thermal expansion material with partial replacement of Cu²⁺by Co²⁺and Mn²⁺,which have stable performance and low preparation cost,the two negative thermal expansion materials have stable performance and low preparation cost,and achieve negative thermal expansion in a wide temperature range covering room temperature,with high application potential.The microstructure,specific structure and thermal shrinkage mechanism of Cu_2-xCo_xV₂O₇ and Cu_1.8Mn_0.2V₂O₇ were investigated in detail with the following main contents and results:（1）Research on the negative thermal expansion properties of Cu_2-xCo_xV₂O₇A series of Cu_2-xCo_xV₂O₇（x=0.1,0.2,0.3,0.4,0.5）samples were prepared by solid-phase sintering method,and thermal expansion and X-ray diffraction tests were performed for different ratios of the samples.The results showed that the negative thermal expansion coefficient of Cu_2-xCo_xV₂O₇ increased incrementally with the increase of Co²⁺substitution ratio.However,the partial substitution of Cu²⁺by Co²⁺doesn’t change the crystal structure of the system.To investigate the effect of partial replacement of Cu²⁺by Co²⁺on the intrinsic expansion ofβ-Cu₂V₂O₇,Cu_1.8Co_0.2V₂O₇with good linearity of negative thermal expansion was selected for the study of variable temperature X-ray diffraction and variable temperature Raman analysis,and the results showed that Cu_1.8Co_0.2V₂O₇ is a monoclinic structure with space group C12/c1 and negative thermal expansion coefficient-13.2×10^-6 K^-1（153-773 K）.In the crystal structure of Cu_1.8Co_0.2V₂O₇,two[VO₄]tetrahedra form the[V₂O₇]structure by sharing the top-angle oxygen atoms,and[Cu O₅]and[VO₄]are connected into chains by sharing O₂ and O₃ atoms,thus having a layered interlocking structure.The analysis suggests that the thermal shrinkage mechanism:in the Cu_1.8Co_0.2V₂O₇ crystal structure,the temperature increase leads to the intensification of the lateral vibration of the bridge oxygen atoms O₂ and O₃,while the coupling effect of[CuO₅]and[VO₄]polyhedra is enhanced,causing the angle between Cu-O₂-V and Cu-O₃-V to gradually decrease and the cell parameters a and c to gradually decrease,resulting in the gradual reduction of the inner layer spacing of the sample;In addition,the warming process causes the redistribution of oxygen atoms in the crystal lattice,which disrupts the original arrangement and leads to the local collapse within the crystal.The combination of the above factors causes the reduction of the cell volume of the sample,thus making Cu_1.8Co_0.2V₂O₇ a new negative thermal expansion material（2）Research on the negative thermal expansion properties of Cu_1.8Mn_0.2V₂O₇The anisotropic microstructural effects in Cu_1.8Mn_0.2V₂O₇ are similar to those found in Ca₂Ru O₄,but Cu_1.8Mn_0.2V₂O₇ has more potential applications compared to Ca₂Ru O₄.The solid-phase preparation of Cu_1.8Mn_0.2V₂O₇ with a negative thermal expansion coefficient of-15.5×10^-6 K^-1（173-673 K）and variable-temperature X-ray diffraction analysis showed that the partial replacement of Cu²⁺by Mn²⁺didn’t change the original crystal structure;the cell parameters a and c decreased and b increased with increasing temperature.Combined with the analysis of variable-temperature X-ray diffraction and variable-temperature Raman data,the thermal shrinkage mechanism:with increasing temperature,the transverse vibration of the bridge oxygen atoms O₂ and O₄ shared by the[VO₄]tetrahedra and[Cu O₅]（in which Cu is partially replaced by Mn）pentahedra in the crystal structure of Cu_1.8Mn_0.2V₂O₇ is enhanced,while the[Cu O₅]and[VO₄]polyhedra are constantly stretching and rotating,resulting in the expansion and rotation of the structure.so that the angle of Cu-O₂-V and Cu-O₄-V gradually decreases.In addition,the increase in temperature also leads to the strengthening of the coupling between the polyhedra,and the polyhedra also undergo flattening and levelling,which in combination leads to the reduction of the sample cell volume and gives the material Cu_1.8Mn_0.2V₂O₇negative thermal expansion properties.