| NbSi2 is regarded as a potential high-temperature structural material due to its high melting point,high strength,moderate density and excellent thermal stability,etc.Therefore,NbSi2 with those excellent overall performances can used in various industrial fields such as advanced aerospace turbine blades,thermal barrier coating and industrial gas burners,etc.However,the recent numerous studies have shown that the poor high-temperature oxidation resistance is a big obstacle for its industrial applications.In particular,the nature of oxidation mechanism of NbSi2 is not well clear.The previous experimental research have shown that alloying elements Ti,Zr and Hf is easy to form the stable oxide under high temperature environment,which can markedly improve the oxidation resistance of Nb-Si based transition metal silicides.It is obvious that alloying is an important method to improve the oxidation resistance of NbSi2.In this paper,we apply the first principles method with density functional theory(DFT)to systematically investigate the crystal structure,mechanical properties,thermodynamic properties and electronic structure of NbSi2 with four crystal structures.The four structures are tetragonal C40 structure,tetragonal C11b structure,orthorhombic C54 structure and orthorhombic C49 structure,respectively.We further investigate the difference of structural stability and various properties of four NbSi2 phases,and the essential reasons of these differences are clarified based on the analysis of electronic structure.Following,we focus on the oxidation mechanism of NbSi2 with C40 structure.In this paper,we mainly consider two difference oxidation mechanism:oxygen atom on the NbSi2(001)surface and the diffusion behavior and occupy mechanism of oxygen atom in the bulk NbSi2,respectively.At the same time,we analyze the charge interaction between oxygen atom and its surrounding atoms.On the basis,the influence of Ti,Zr and Hf on the oxidation resistance of NbSi2 was studied.The main research results are as follows:(1)According to the calculated formation enthalpy and phonon spectra,it is found that NbSi2 with those structures are both thermodynamically and dynamically stable at the ground state.In particular,the calculated formation enthalpy of NbSi2 with C54 structure is smaller than that of other three structures,indicating that NbSi2 with C54 structure is the most stable phase among the four NbSi2 phases.The calculated elastic constants further reveal that NbSi2 with four structures are mechanically stable at the ground state.In particular,the calculated mechanical properties of NbSi2 with C54 structure are superior to the other three NbSi2 structures.In addition,the elastic anisotropy of NbSi2 with four structures is calculated by three calculated models.The calculated results show that NbSi2 with C54 and C40 structures have good elastic isotropic in comparison to other two structures.The elastic isotropic is confirmed by the given three-dimensional plots of elastic moduli.By analysis of electronic structure,including band structure,density of states,Mulliken overlap population and difference charge density,it can be seen that four NbSi2 phase exhibit metallic property at the ground state.The chemical bonding in NbSi2 are mainly composed of Nb-Si bond and Si-Si covalent bond,which play a crucial role in the structural stability and mechanical properties of NbSi2 phase.(2)The calculated melting point of C54-NbSi2 using semi-empirical formula is 1986 K,which is slightly lower than that of the NbSi2 with C40 structure.On the contrary,the calculated melting point of NbSi2 with C54 structure is higher than that of NbSi2 with C11b and C49 structures.The calculated Debye temperature and heat capacity of C54-NbSi2 are 547.8 K and 142.7 J/(mol·K),respectively,which are larger than that of the other three phases.Those results show that the thermal stability of C54 structure is better than that of the other three structures.According to the phonon density of state,we find that the difference in thermodynamic properties of NbSi2 with four structures mainly derive from the difference in the contribution of Nb and Si atoms to the vibration of system at different temperatures.(3)For(2×2)NbSi2(001)surface model,we consider the surface energy of NbSi2 with different atomic layers.The calculated result shows that the calculated surface energy of NbSi2 with different atomic layers is very close.On the other hand,the calculated adsorption energy shows that the oxygen atom is most stable when it is adsorbed at the bridge position between Nb atom and Si atom.The order of stability of oxygen atom in five different adsorption positions can be expressed as follows:Bri-1>Bri-2>Hol>Top-2>Top-1.The calculated electronic structure shows that the discrepancy of strength is attributed to the formation of Nb-O bond and Si-O bond.It is obvious that the formation of those chemical bonding derives from the localized hybridization between the oxygen atom and the near atoms.(4)According to the calculated oxygen impurity formation energy,it is found that oxygen atom prefers to occupy the tetrahedral interstitial(TI-2)site in bulk NbSi2.Here,we further find that the stability of oxygen atom in the four interstitial positions is in order:TI-2>TI-1>OI-2>OI-1.Compared to the diffusion route 1,oxygen atom tends to diffuse along route 2 between the AB atomic layer.According to the electronic structure,we find the formation of Si-O bond and Nb-O bond when the introduction of oxygen atom into the various interstitial sites.In particular,the Si-O bonds play an important role in structural stability.On the basis of the calculated bond parameters,the bond strength of Si-O bond and Nb-O bond show a trend that one of them strengthening while the other are lessening.(5)Finally,we further investigate the influence of alloying elements(Ti,Zr and Hf)on the oxidation resistance of NbSi2.The calculated results show that these alloying elements prefer to occupy the Nb position of NbSi2,which are thermodynamically and dynamically stable at the ground state.Based on the calculated oxygen impurity formation of oxygen,it can be seen that alloying element of Ti has better oxidation resistance in NbSi2 in comparison to Zr and Hf.The oxidation resistance of alloying element follows the order of Ti>Zr>Hf.The improvement of NbSi2 oxidation resistance by alloying elements is mainly attributed to the formation of TM-0 bond and the variation of chemical bonding around oxygen atom. |
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