Synthesis,characterization And First Principles Calculation Of Nb₂SB Ceramics

In this paper,the morphology,physical and mechanical properties of Nb₂SB ceramics were successfully synthesized and characterized by experiments.High purity B-containing MAX phase Nb₂SB was successfully prepared by spark plasma sintering.The phase composition of different samples was analyzed by X-ray diffraction,and the reaction path of Nb₂SB ceramics was systematically studied.The morphology of Nb₂SB was analyzed by field emission scanning electron microscope.The lattice structure of Nb₂SB was determined by Rietveld structure refinement fitting,and a complete set of X-ray diffraction data was given.In the first principles calculation based on density functional theory,the theoretical crystal structure,mechanical properties,phase stability,electronic and optical properties of Nb₂SB ceramics were studied.The experimental results show that B-containing MAX phase of Nb₂SB ceramics with high purity of 96 wt%was successfully synthesized using the molar ratio of Nb:S:B=2:1.3:1 by spark plasma sintering at 1350 ^oC at 30 MPa.The reaction path,microstructure and properties were systematically studied.It was found that the formation of Nb₂SB was from the combination of Nb,Nb S₂,and Nb₅B₆,or Nb B and Nb S.The full dense sample（RD%）possessed the fine grains about 6?m in length and3.6?m in width.The grains of Nb₂SB showed a layered structure,which was same to other MAX phases.The thermal expansion coefficient in the range of 24-1100°C was7.1×10^-6K^-1.The thermal diffusivity of Nb₂SB ceramics rose from 5.58 mm²/s at 25^oC to 7.07 mm²/s at 800°C with the increasing temperature.The heat capacity and thermal conductivity at 25 ^oC were 0.36 J·g^-1·K^-1 and 13.79 W·m^-1·K^-1,respectively.The electrical conductivity of Nb₂SB ceramics was 1.170×10⁶Ω^-1·m^-1.Additionally,theVickers hardness was tested as 11.89±0.37 GPa（10 N）,and the flexural strength,fracture toughness,and compressive strength were 249±17 MPa,4.76±0.36 MPa·m^1/2,and 1157±73 MPa,respectively.The calculation results show that theoretical lattice constants of Nb₂SB were a=b=0.3360 nm,c=1.1673 nm,c/a=3.474,α=β=90°,γ=120°,V=114.1?³.The atomic coordinates were Nb（1/3,2/3,0.6007）,S（2/3,1/3,1/4）and B（0,0,0）.Nb₂SB ceramic was a brittle material with weak plastic deformation resistance and strong stiffness.At the pressure of 0-40 GPa,the c/c₀ and V/V₀ of Nb₂SB ceramics decreased by about 11%,and the bulk modulus B and shear modulus G increased linearly with the increase of pressure.At 40 GPa,the B/G was greater than 1.75,which indicates that the plasticity of Nb₂SB ceramics is improved under the effect of pressure.Nb₂SB had negative enthalpy of formation,met the condition of mechanical stability,and the phonon frequency was positive without virtual frequency,which proves the phase stability of Nb₂SB.According to the band structure,density of states and electronic structure of Nb₂SB,it can be inferred that Nb₂SB ceramics have the characteristic of metal materials-conductivity,and the conductivity had directivity.The Nb 4d and B 2p orbitals of Nb₂SB were strongly hybridized to form a strong covalent bond.The peak value of imaginary part of dielectric function of Nb₂SB ceramics was close to 65,and the plasma frequency was 15.0 eV.Nb₂SB was more conductive in the incident photon energy range of 4.8 eV to 5.2 eV.Nb₂SB had high reflectivity in the energy range of～15 eV.30-36.0 eV was the strongest absorption region of Nb₂SB.In the photon energy range of 5 eV to 15 eV,the light cannot propagate in this region.