First-principles Study On Structures And Performances Of Nb Based MAX Phase/MXene

Ternary transition metal carbide（or nitride）,also known as MAX phase,has broad application prospects in the corresponding fields because of their combined properties of metal materials（machinable,electrical,and thermal conductivity,high wear resistance,etc.）and ceramic materials（corrosion resistance,high elastic modulus,Low density,and other properties）.Among them,the Nb-based MAX phase has attracted widespread attention due to its unique electron transfer properties（such as superconductivity）in addition to its general properties as the MAX phase.However,the difficulty of preparation,high cost,and long research period limit the speed of applied research on the Nb-based MAX phase and its derivative Nb-based MXene.In this paper,based on previous experimental and computational explorations,taking Nb-based MAX phase and MXene as objects,there are few and in-depth studies on the electronic structure and performance characteristics of different types of atoms and vacancies in Nb-based MAX phase after the substitution,and the structural stability of Nb-based MXene after forming heterojunctions devices（Nb₂CS₂/Mo S₂,Nb₂CS₂/WS₂,and Nb₂CS₂/Ta S₂）.The relevant theoretical research was carried out by using the first-principle calculations and the details are as follows:Through the study of the crystal structure,mechanical properties,thermophysical properties,and electronic structure of the Nb₄AC₃ MAX phase（A=Al,Si,Ge,Ga;X=C,B）under the influence of several A elements and X element substitutions research,it is found that theαstructure has higher thermodynamic stability than theβstructure,andα-Nb₄Si C₃ has higher thermodynamic stability and Young’s modulus than the other threeαstructure compounds.When the A-site elements are Si and Al,they have higher lattice thermal conductivity than the other two structures.Substituting B atoms for C atoms in Nb₄AC₃ causes little change in thermal conductivity and hardness but increases its wear resistance.The effect of the M element on the structure and properties of the MAX phase was explored.By studying the microstructure,phase stability,mechanical properties,thermophysical properties,lattice dynamics properties,and electronic structure of Zr-doped Nb₄Al C₃ solid solution,the results show that all(Nb_1-xZr_x)₄Al C₃ phases have mechanical stability,thermodynamic stability,kinetic stability at 0K and high temperature,and are brittle.Among them(Nb_0.875Zr_0.125)₄Al C₃presents the best structural stability at high temperatures,compared to all Zr-containing and Zr-free structures.Electronic property calculations reveal that the(Nb_1-xZr_x)₄Al C₃ phase has metallic properties and the strength of the Nb/Zr-C bond is much higher than that of the Nb/Zr-Al bond.The occupation and behavior of impurity atoms in the material（including hydrogen storage（impurity H）and oxidation（impurity O））were studied.First,through the study of the incorporation and diffusion of H in the Nb₄Al C₃ MAX phase,it was found that the addition of H has little effect on the structural characteristics of Nb₄Al C₃.The Nb-Al layer can provide favorable sites for H storage,and the Nb-C layer can store H only when C vacancies are introduced.H can exist stably and migrate in the Nb-Al layer and C vacancies.According to the calculation results,the Nb₄Al C₃ MAX phase can be used as a potential solid hydrogen storage material.Through the study of the oxidation behavior of Nb₄AC₃（A=Al,Si,Ge,Ga）MAX phase under the influence of several A elements,it is found that the oxidation resistance of the system is the strongest when A=Al.Through the study of the oxidation mechanism of Nb₄Al C₃,it is found that O atoms preferentially exist in the Nb-Al layer of Nb₄Al C₃.When O atoms occupy Site-Ⅱ,which is composed of two Nb atoms and three Al atoms,the corresponding O doping energy is the lowest,and the lattice distortion caused by doping is the smallest.The stability of O atoms in the gap of Nb-Al layer mainly comes from the formation of O-Al bonds and O-Nb bonds,which are mixtures of ionic bonds and covalent bonds.The study of enhancing the oxidation resistance of Nb₄Al C₃ by doping M elements was carried out,and it was found that Zr and Mo could be used as representatives for significantly enhancing and weakening the oxidation resistance of Nb₄Al C₃,respectively.The possibility of heterogeneous combination of Nb₂CS₂（Nb-based MXene with S functional group）and TMDs with unique properties is explored.Three Nb-based MXene/transition metal sulfide heterojunctions under different stacking modes（S-M,C-S and S-S）were designed.The stability and electronic properties were studied,and it was found that Nb₂CS₂/Mo S₂ and Nb₂CS₂/WS₂ in the S-M stacking mode had dynamic stability,mechanical stability,and thermodynamic stability at 0K and 600K.Also,their thermophysical properties are intermediate between those of the two monolayer structures.The electronic property calculation results show that the two stable heterojunctions have metallic properties,and Nb element is the main contributing element near the Fermi level,which provides the possibility for the application in the field of ion batteries.