Theoretical Study On Thermal And Mechanical Properties Of Novel Materials Of Mo₂Ga₂C,MgCaSi And(MgCoNiCuZn)O

Novel materials refer to the new or developing materials with excellent properties and special functions that traditional materials do not have,are the key points of science research of materials and also the hotspots of strategic competition of every country in the world,so have very broad application prospects and extremely important strategic significance.With rapid development of social economy,the demands for novel materials in various industries is increasing,and the requirements for its functions are also higher and higher.Because the embodiment of material function is closely related to its property,it is of great significance to study the properties of novel materials for accelerating its development and expanding the scope of application.The most important properties for materials are thermodynamic and elastic properties.The study of thermodynamic and elastic properties can provide important theoretical basis and guidance for the structure optimization design and application of a material.In this paper,the thermodynamic and elastic properties of three new materials with different crystal forms,namely,hexagonal Max phase ceramic Mo₂Ga₂C,orthorhombic magnesium alloy MgCaSi and cubic high entropy oxide(MgCoNiCuZn)O,were studied by using the quasi harmonic approximation Debye-Grüneisen model and first principles calculation combined with special quasi random structure(SQS)simulation.The main research contents are as follows:(1)The study of thermodynamic properties of Mo₂Ga₂C and Mo₂Ga C indicate that both of them are thermodynamically stable,and Mo₂Ga₂C show more stable due to the lower formation enthalpy.The entropy of two systems mainly comes from the contribution of vibration entropy,the entropy value increases with the increasing temperature,the Mo₂Ga₂C has higher thermodynamically stability under high temperature.The bulk modulus of Mo₂Ga₂C is relatively small,indicating that its strength is smaller.The thermal expansion coefficients,the isochoric and isobaric heat capacity of two coumpounds increases rapidly at low temperature and slowly at high temperature with the increasing temperature.The value of Mo₂Ga₂C are higher and uptrend are faster due to the formation of stronger Ga-Ga covalent bond in Mo₂Ga₂C.The Debye temperature of two coumpounds decreases slowly with increasing temperature,while the value of Mo₂Ga₂C is smaller and the downtrend is slightly faster.The Grüneisen parameter of two coumpounds increase slightly with temperature,implying that the influence of temperature is relatively small.The studies on the elastic properties of two compounds show that both of them have good mechanical stability under ground state.The weaker Mo-C bond in Mo₂Ga₂C making its mechanical strength reduced.Two compounds exhibit ductility,although Mo₂Ga₂C show slightly weaker than Mo₂Ga C.The shear and compression elasticity anisotropy of Mo₂Ga₂C is lower than that of Mo₂Ga C.Under high pressure,the shear modulus and Young's modulus of Mo₂Ga₂C decrease when the pressure exceeds 22 GPa.The ductility and anisotropy of two coumpounds are enhanced with the increasing pressure but vickers hardness goes down.Electronic structure show that two compounds have metallic characteristics,and the stronger C-Mo,Mo-Ga and Ga-Ga bond are formed in Mo₂Ga₂C.(2)The comparative investigation of thermodynamic properties of MgCaSi and Ca₂Si indicate that both of them are thermodynamically stable,and MgCaSi show more stable due to the lower formation enthalpy.The entropy of two systems mainly comes from the contribution of vibration entropy increases with temperature increaseing.The entropy of MgCaSi is smaller,indicate that it has smaller thermodynamically stability under high temperature.The anti-softening ability for MgCaSi is slightly smaller although the bulk modulus is higher.The thermal expansion coefficients,isochoric heat capacity and isobaric heat capacity of two compounds rise rapidly at low temperature and slowly at high temperature with increasing of temperature.The MgCaSi possess slightly higher Debye temperatures,implying the stronger chemical bonds and higher thermal conductivity than Ca₂Si,but the downtrend is slightly faster.The effect of temperature on Debye temperature and Grüneisen parameter of two compounds are relatively small.The investigation of the elastic properties under ground state of MgCaSi and Ca₂Si shows that the elastic moduli of MgCaSi are higher than those of the Ca₂Si,demonstrating that the hardness of MgCaSi has been favorably improved.Two compounds exhibit less brittleness,the MgCaSi brittleness is larger.MgCaSi possesses lower anisotropy than Ca₂Si based on several criteria.Under high pressure,the mechanical properties are improved with the increase of pressure,but MgCaSi exhibits less sensitive to pressure.The investigation of electronic structures show that MgCaSi have metallic characteristics,the stronger Mg-Si,Mg-Ca and Si-Si covalent bonds are formed in unit cell.(3)The comparative investigation of structural stability and thermal properties of three high entropy oxide(MgCoNiCuZn)O and its Li⁺-doped compounds(Mg Ni Co Cu Zn)_0.90Li_0.10O and(Mg Ni Co Cu Zn)_0.80Li_0.20O show that the calculated lattice parameters of considered oxides decrease with Li⁺-doping,due to the occurrence of charge compensation mechanisms.Three compounds are thermodynamically stable,and(MgCoNiCuZn)O is more stable than dopant due to lower formation enthalpy.The entropy of three compounds obviously increases with increasing temperature,the entropy increment of three systems mainly comes from the contribution of vibration entropy.The entropy of(MgCoNiCuZn)O is higher than its Li⁺-doped compound,so it has higher thermodynamically stability under high temperature.The bulk modulus of three coumpounds show softening behavior with the increase of temperature.However,with increasing of lithium content,the strength of anti-softening ability for compound is slightly smaller due to the slightly faster decrease trends of bulk modulus.The thermal expansion coefficients,the isobaric heat capacity and isometric heat capacity for three compounds rise rapidly at low temperature and slowly at high temperature with the increasing temperature.While Li⁺-doped compound possesses smaller thermal storage with lithium content increasing.The Debye temperature for three compounds are decreased with temperature increasing,whereas the Grüneisen parameter shows reversible variation behavior.The Debye temperature and Grüneisen parameter for Li⁺-doped compounds are larger than no-doped one,and the changes trend is slightly rapid with lithium content increasing.The investigation of the elastic properties under ground of(MgCoNiCuZn)O indicate that the(MgCoNiCuZn)O is mechanical stability.Compared to dioxides,the calculated elastic mechanics for(MgCoNiCuZn)O obeys the rule of mixture.The compound exhibits less ductility based on Pugh's empirical formula,Poisson's ratio and the Cauchy pressure.The study of electronic structure and performance shows that there are strong ionic characteristics of covalent bonds between metal atoms and oxygen(O)atoms.