Structure Prediction And Physical Properties Of New 5d Transition Metal Carbides Under High Pressure

High-hardness and high-melting point materials are widely used in high compressive strength and the hottest end parts in industrial production,aerospace,and other fields.The characteristics of high-pressure resistance and high-temperature resistance enable these compounds to work stably in extremely harsh environments,such as aircraft engines,fighter engines,missile turbojet/turbofan engines,etc.As we all know,diamond is the hardest material known at present.However,it is prone to cleavage and show brittleness under high-temperature conditions.Therefore,diamond cannot meet the application requirements in extreme high-pressure and high-temperature environment.In addition to the method of forming strong covalent bonds between light element atoms(carbon)to obtain superhard materials,the method of combining transition metals with high electron density(such as tantalum,tungsten,or rhenium,etc.)with light element atoms(such as boron,carbon,or nitrogen)can produce hard materials with good mechanical properties and high temperature resistance.Searching for the possible high-pressure structure of transition metal carbides plays an important role in the design of high-hardness and high-melting point materials.In this thesis,high-pressure stable structures of 5d transition metal carbides(Re/W/Ta-C system)are predicted,and the component-pressure phase diagrams of Re/W/Ta-C system are determined.In addition,the laws of mechanical,electrical,and thermal properties of compounds in Re/W/Ta-C system are also studied,and the thermal and mechanical mechanisms of Re/W-C system compounds are deeply analyzed.Using the software based on genetic evolutionary algorithm(USPEX)and the first-principles calculation method,the component-pressure phase diagrams of Re/W/Ta-C system in the pressure range of 0-300 GPa are determined,and the thermodynamic and kinetic stability of the new structures are judged.By analyzing the structural characteristics of Re/W/Ta-C system compounds,it is found that the stacking modes of polyhedrons in the crystal structures of Re/W-C compounds are related to the content of metal atoms(Re or W atoms).There is a phenomenon of thermal energy competition between the carbon vacancy ordered phase and carbon fault forming phase among Ta-C compounds with different carbon content.The physical properties of Re/W/Ta-C system compounds(including mechanical,electrical,and thermal properties)are studied by the first-principles calculation.For Re-C system,Re-C compounds with different carbon contents have good mechanical properties.In particular,these compounds have high bulk modulus.Then by analyzing the electronic structure and bonding of these compounds,the reason why these compounds have high hardness is explained.For W-C system,the elastic constants and shear modulus of structures show an increasing trend with the increase of carbon content.In addition,the thermal properties of WC and W₂C compounds reported as high-temperature resistant materials are analyzed.For Ta-C system,the mechanical properties of the structures are related to the carbon content.At the same time,the mechanical properties of the structures are also affected by the atomic ordering of carbon vacancies or the carbon faults,i.e.,we can adjust the hardness of the compounds by changing the carbon content and the arrangement of carbon atoms.Finally,the melting point information of cubic Ta C is calculated by using the empirical formula combined with the functional relationship between the mechanical parameter C₁₁ and the melting point.The error between the calculated result and the experimental value is about11.8%.The compounds with good thermal or mechanical properties in Re/W-C system are selected for in-depth thermal and mechanical mechanism analysis.First,the stability of WC-type Re C structure with strong incompressibility resistance is explained by the Quasi Harmonic Approximation(QHA)method and Crystal Orbital Hamiltonian Population(COHP)analysis method.Second,the origin of the high hardness of metal material Re₃C is analyzed by charge density difference and Bader charge analysis methods.Third,the effect of stress on the mechanical properties of Re₅C₃ with strong incompressibility is discussed by using electronic localization function(ELF)analysis and the method of finding the ideal tensile and compressive strength.Final,the thermal properties of WC compounds such as coefficient of thermal expansion at high temperature are discussed by using the Quasi Harmonic Approximation(QHA)method.