Structure Stability, Mechanical Property, And Phase Transition Of Nb-H System

This paper illustrates the importance of hydrogen energy and the development of hydrogen storage materials through analysising the energy of our society. Firstly, it is briefly introduced that with a very high melting point, good mechanical properties, heat resistance, good thermal conductivity, and small capture cross section of neutron absorption, niobium and its alloys are widely used in many aspect of the industry. Nb is commonly believed as one of the most favorable hydrogen permeable candidates due to its highest hydrogen permeability among metals as well as its much lower price than palladium alloys. The metal niobium and its hydrides thus have attracted great research interests during the past decades. At the same time, the metal niobium has good hydrogen absorption capacity, and it has the potential to be the metal hydrogen storage material. Nevertheless, the studies on the influence of H on mechanical properties of niobium still have some contradictions and problems in the literature, and these studies are far from systematic and comprehensive. Therefore, we have systematically and comprehensively investigated the structural stabilities, mechanical properties, phase transition, as well as the intrinsic mechanism of Nb-H systems by means of first principles calculation.Results show that the NbHx BCC(0≤x≤0.5) structure with H at tetrahedral(T) site is the most thermodynamically stable one among all the BCC, FCC, and HCP phases, and its negative heat of formation decreases linearly with the increase of H composition. Calculation also reveals that the elastic moduli of BCC(T) NbHx phases all increase with the increase of H composition, and the BCC(T) NbHx phases remain ductile within the studied composition range(0≤x≤0.5). Moreover, it is found that the percentage anisotropy in shear(AG) and the universal anisotropic parameter(AU) are all appropriate to describe the elastic anisotropy of NbH phases, and that H location should play an important role in elastic anisotropy.NbHx phases(1≤x≤2) with face-centered cubic(FCC), orthorhombic(FCO), and tetragonal(FCT) structures are all energetically favorable with negative heats of formation, while FCC NbH1 and NbH1.25 could not be formed due to their mechanical unstableness. It is also revealed that FCT and FCO could coexist in NbH1 and NbH1.25, FCC and FCT coexist in NbH1.5 and NbH1.75, while only FCC in NbH2. Calculations also indicate that the magnitude of the elastic moduli of NbHx phases at each H concentration is as follows: E > B > G, and the G, E, and G/B values of NbHx phases reach a minimum when x = 1.5.The calculations of phase transitions of the Nb-H system indicate that the face-centered cubic(FCC) structure of NbH1 and NbH1.25 is energetically unstable and will automatically transform to the face-centered tetragonal(FCT) structure, and that the intrinsic composition range of the FCTâ†'FCC transition of NbHx phases is 1.5≤x<2.0. Calculations also show that FCC is the most stable structure of NbHx under high pressure, and that the face-centered orthorhombic(FCO) and FCT structures of each NbH1 and NbH1.25 as well as FCT and FCC of each NbH1.5 and NbH1.75 are very close to each other in terms of the behaviors of phase transition and electronic structure. In addition, Poisson ratio and density of states are discussed, respectively, to provide a fundamental understanding of various phase transitions in NbHx(1≤x≤2) phases.