Theoretical Study On The Mechanical Property And Phase Transition Mechanism Of Layered Metal Carbides Materials

Two-dimensional layered transition metal carbides?MXenes?are a new branch of the two-dimensional material family.They are of long-term stability,large specific surface areas,high ion-exchange capacity and acid resistance.These unique properties enable MXenes various applications such as supercapacitors,lithium ion batteries,catalysis,spent fuel treatment,and so on.Generally,the structural stabilities as well as the mechanical properties of materials are fundamental characteristics to be considered during their potential application.Considering that chemical etching is the main preparation method of MXenes at present,MXenes usually be covered by various chemical groups and contain defects such as vacancy,which will inevitably affect their mechanical properties and further limit their practical applications.In addition,the variation in environmental temperature and pressure would also affect the microstructures of materials,leading to the changes in average bond lengths or even phase transformation,which would further affect their chemical bondings as well as the macroscopic properties of materials.In view of the high temperature and high pressure environment where the new layered transition metal carbides may be applied in,such as the irradiation-resistant material or spent fuel separation medium in the harsh environment of the advanced nuclear energy system,it is necessary to study their structural stabilities in extreme environments such as high temperature or high pressure.Therefore,we conceive to systematically study the mechanical properties of M₂CT₂?M=Ti,Zr,HF,T=O,F,OH?MXenes,and the structural transition mechanism of layered metal carbide material Ti₃C₂ under high pressure.The main conclusions are as follows:?1?The SS curves of M₂CT₂?M=Ti,Zr,Hf;T=O,F,OH?MXenes?with different metal composition and surface functional group covering?under homogenous tension in their biaxial and uniaxial directions as well as the elastic parameters such as Young's modulus,in-plane stiffness,Poisson's ratios are systematically calculated and analyzed. Furthermore,the effect of defect on the mechanical properties of MXenes is discussed.The results show that surface functionalization groups can increase the critical strain of the pristine M₂C MXenes material and enhance its toughness.The degree of influence of different functional groups on the mechanical properties of MXenes material is in a sequence of O>F>OH.The similar tensile properties of MXenes among the different transition metal compositions may arise from the similarity of the chemical properties of Ti,Zr,Hf elements.The vacancy could significantly weaken the tensile properties of MXene systems with their ideal strength,critical strain,in-plane stiffness constant and Young's modulus in ranges of 40-50%,33-50%,17-21%and 20-27%,respectively.Moreover,the defect also affects the bonding,resulting in a certain anisotropy of stress along armchair and zigzag directions even under the biaxial tension condition.Such anisotropy can mainly be attributed to the slightly difference in the linear concentration of defects along the two orthogonal directions.?2?The phase transition mechanisms of metal carbide Ti₃C₂ material under high pressure was also predicted.By using the first-principles method,the possible Ti₃C₂structures under the pressure of 0GPa,50GPa,100GPa and 150GPa are comprehensively being searched.Nearly 30 possible stable structures are predicted for the given range of pressures.And seven stable phases that may occur structural phase transition were determined after careful examination on their total energy as well as the thermodynamic stabilities.Furthermore,the phase transition relationship of Ti₃C₂material from the ground state phase?C2/c phase?to the high-pressure phase?P4/mbm?was determined by analyzing the equation of state?energy-volume relationship?of their stable phases.Finally,the enthalpy-pressure relationship of the two phases shows that the phase transition occurs at pressure of?27GPa.In this paper,the fundamental mechanical properties of two-dimensional layered carbide M₂CT₂ are systematically analyzed,and the structural phase transition of layered metal carbide Ti₃C₂ is also theoretical predicted.These results would help to understand the structural and mechanical stabilities of such layered metal carbide materials,and may provide theoretical basis for the future theoretical and experimental researches.