Preparation And Properties Of Negative Thermal Expansion Materials ?-CuZnV₂O₇ And Zr₂Mo_0.5W_0.5PVO₁₂

Negative thermal expansion(NTE)of materials is an abnormal phenomenon which challenges conventional lattice thermal dynamics.It is well known that the unit cell volume of most materials expands during heating,but the unit cell volume of negative thermal expansion materials shrinks at different rates during heating.The mismatching of coefficient thermal expansion will lead to serious problems such as performance deterioration,fatigue,temporary and permanent damage,cracking and so on.Thermal expansion performance is a key factor affecting the application environment and working life of equipment.For example,there are some unfavorable factors,such as water absorption,thermal decomposition,huge negative expansion caused by phase transformation,negative expansion temperature range is not in the scope of application.On the basis of the above,in order to meet the application of NTE materials with negative thermal expansion,it is necessary to find and design new negative thermal expansion materials for frame structure.The main work of this paper is to prepare new negative thermal expansion materials ?-CuZnV2O7 and Zr2Mo0.5W0.5PVO12 by solid state method,and their properties are studied in detail.1.Preparation and properties of negative thermal expansion material ?-CuZnV2O7.Cu2V2O7 has good NTE property before 470 K,and Zn2V2O7 has good NTE property above 533 K.In order to obtain negative expansion materials in a larger temperature range,the combination of the two materials was proposed.Based on this idea,a new negative thermal expansion material with molecular formula of ?CuZnV2O7 was designed and prepared by solid-state sintering reaction.?-CuZnV2O7 exhibits negative thermal expansion in the temperature range of 130-673 K and its intrinsic expansion coefficient is-0.78×10-6 K-1.It belongs to monoclinic structure(C2/c space group),and has no phase transition.The VO4 tetrahedron of ?-CuZnV2O7 shares one oxygen atom to form V2O7 tetrahedral structure unit.(CuZn)0.5O5 pyramid shares one edge to form a distorted pyramid chain.The distorted pyramid chain and V2O7 tetrahedron are connected by oxygen atoms to form a chain,and have a layered staggered structure.The transverse vibration of bridging oxygen atoms enhances the polyhedral coupling effect and changes the cell volume.?-CuZnV2O7 sample particles are relatively smooth,the particles are about 1-2 ?m,the particle arrangement is relatively dense,but there are still some voids,holes and microcracks.The high-pressure Raman experiment demonstrates that two structural phase transitions exist at 0.94 GPa and 6.53 GPa,respectively.The mechanism of negative thermal expansion in ?-CuZnV2O7 is interpreted by the variations of the angles between atoms intuitively and the phonon anharmonicity intrinsically resorting to the negative Grüneisen parameter.2.Preparation and properties of negative thermal expansion material Zr2Mo0.5 W 0.5P VO 12.Based on Zr2WP2O12 material,Zr2Mo0.5W0.5PVO12 material was designed by replacing W and P ions with Mo and V ions,and Zr2Mo0.5W0.5PVO12 was prepared by solid state reaction.The crystal structure refinement of variable temperature XRD data shows that Zr2Mo0.5W0.5PVO12 has orthogonal structure and no structural phase transition in the temperature range of 80-673 K,space group 60(Pnca),intrinsic expansion coefficient-1.41 ×10-6 K-1.In Zr2Mo0.5W0.5PVO12 cell,ZrO6 octahedron and WO4/MoO4/PO4/VO4 tetrahedron share one vertex.In this open frame structure,with the increase of temperature,the bridge oxygen atoms vibrate laterally,the Zr-O-W/Mo/P/V chain becomes more curved,the Zr-O-W/Mo/P/V chain is easier to make the polyhedron translational and celestial translational,and finally make Zr2Mo0.5W0.5PVO12 have negative thermal expansion properties.From the scanning electron microscope and thermal analysis,it can be seen that Zr2Mo0.5W0.5PVO12 material has certain voids,good thermal stability and no water absorption.