Preparation,structure And Heat Shrinkage Mechanism Of Negative Thermal Expansion Materials ?-Cu₂V₂O₇ And Cu_1.5Mg_0.5V₂O₇

The volume of most materials in nature increases with increasing temperature and decreases with decreasing temperature.Thermal expansion and contraction is a double-edged sword.On the one hand,it can be well used by people in the practice,but on the other hand,it also brings many harms to life.For example,it is often necessary to use a plurality of materials in combination to get a component with a specific use performance.But when materials of different thermal expansion coefficients are used in combination,a difference in expansion coefficient may cause a coefficient of expansion mismatch.Mismatch in expansion coefficients can result in uneven distribution of thermal stress in components or composites,causing microcracking,reduced performance,and even fracture and device failure.The magical negative thermal expansion phenomenon of ZrW₂O₈ was discovered in 1996,which provides an effective solution to the above problem of expansion coefficient mismatch.Preparing a zero or near zero expansion composite using the materials with NTE and PTE can not only solve the expansion problem caused by thermal stress,but also combine different excellent properties to achieve Multi-functional application of materials.Therefore,the search and development of negative thermal expansion materials has great significance.And thus has become one of the hotspots of material science research.So far,although many negative thermal expansion materials have been discovered and prepared,there also some defects in these materials,which affects the expansion of their applications.Therefore,it is still negative to improve its performance and meet the application requirements.It is imperative for researchers of thermal expansion materials.The main focus of this work is to prepare a new negative thermal expansion materials?-Cu₂V₂O₇and Cu_1.5Mg_0.5V₂O₇,and carry out detailed research on its structure,properties and heat shrinkage mechanism.The main results and innovations are as follows:1.A new negative thermal expansion material?-Cu₂V₂O₇ was prepared by solid state method.?-Cu₂V₂O₇ exhibits negative thermal expansion characteristics in the temperature range of 153 K to 673 K.What's more,at the temperature range of RT to673 K,it exhibits a negative linear expansion,and the average expansion coefficient is?_l=-20.2×10^-6 K^-12.The microstructure of?-Cu₂V₂O₇ consists of a single smooth surface particle with a particle size of 10 to 20?m,arranged closely,but with tiny pores.?-Cu2V2O7 belongs to the monoclinic phase,C2/C space group.The unit cell parameters are a=7.69813?,b=8.03109?,c=10.11329?,?=110.28774°,V=586.459?³.3.?-Cu₂V₂O₇ heat-shrinking mechanism:one V⁵⁺and four O^2-form a[VO₄]tetrahedron,and two[VO₄]tetrahedra share a top-angle oxygen atom to form a[V₂O₇]structural unit.Each Cu²⁺and five O^2-are connected to form an elongated tetragonal pyramid polyhedral unit.And the quadrangular pyramids share edges,so that the sample has a layered staggered structure.When the temperature rises,the lateral thermal vibration of the bridge oxygen atom is intensified,which reduces the distance between the two tetrahedrons.And with the expansion and contraction of the polyhedron,the coupling effect of the polyhedron is strengthened.Due to the existence of the Jahn-Teller effect distortion,the polyhedron is deformed.In addition,during the temperature rise and fall process,the loss of oxygen atoms causes the chain structure to be destroyed and the oxygen atoms to be rearranged in the crystal lattice,which causing local collapse and aggravating the deformation of lattice in some cases.The above reasons affect each other,so that the sample exhibits a negative thermal expansion phenomenon at a macroscopic level.4.A new negative thermal expansion material Cu_1.5Mg_0.5V₂O₇ was prepared by solid state method.Cu_1.5Mg_0.5V₂O₇ exhibits linear negative thermal expansion characteristics in the temperature range of 153 to 673 K,and the average linear expansion coefficient?_l=-8.72×10^-6 K^-15.The microstructure of Cu_1.5Mg_0.5V₂O₇ consists of a single smooth surface particle with a particle size of 2-10?m.The particles are closely arranged,but there are tiny pores.Cu_1.5Mg_0.5V₂O₇ belongs to the monoclinic phase,C2/C space group.The unit cell parameters are a=7.64668?;b=8.10104?;c=10.11441?;?=110.611°;V=586.44364?³.6.Cu_1.5Mg_0.5V₂O₇ heat-shrinking mechanism:one V⁵⁺and four O^2-form a[VO₄]tetrahedron,and two[VO₄]tetrahedrons share a vertices oxygen atom to form a[V₂O₇]structural unit.Each Cu²⁺and five O^2-are connected to form an elongated tetragonal pyramid polyhedral unit,and the quadrangular pyramid shares the edge,so that the sample has a layered staggered structure.The Mg²⁺moiety occupies the position of Cu²⁺.When the temperature rises,the lateral thermal vibration of the bridge oxygen atom is intensified,which reduces the distance between the two tetrahedrons,and with the expansion and contraction of the polyhedron,the coupling effect of the polyhedron is strengthened.Due to the existence of the Jahn-Teller effect distortion,the polyhedron is deformed.In addition,during the temperature rise and fall process,the loss of oxygen atoms causes the chain structure to be destroyed and the oxygen atoms to be rearranged in the crystal lattice,which causing local collapse and aggravating the deformation of lattice in some cases.The above reasons affect each other,so that the sample exhibits a negative thermal expansion phenomenon at a macroscopic level.7.Different proportions of Mg-doped copper-magnesium compounds were prepared by solid state method.As the magnesium content increases,the expansion coefficient achieves a transition from negative to near zero to positive,and the expansion performance exhibits a near linear characteristic in the temperature range of 304-673K.The expansion mechanism is explained by the substitution of copper as the magnesium content increases,reducing the vibration mode of the Mg-O bond.The John-Teller effect of the cuprous oxide pentahedron is somewhat inhibited,thereby limiting the lattice expansion.