The electronic, mechanical properties and theoretical hardness of chromium carbides by first-principles calculations

In the present study, the ground state properties of chromium carbides (h-CrC, c-CrC, Cr₃C, Cr₃C₂, Cr₇C₃, and Cr₂₃C₆) are calculated by means of the first-principles pseudopotential method using the CASTEP code. The equilibrium crystal structures and thermodynamical stability of the six chromium carbide phases are discussed. Moreover, the chemical bonding in these carbides are interpreted by calculating the density of states, electron density distribution and Mulliken analysis; all the six chromium carbides have a combination of metallic, ionic and covalent bonding characteristic, while Cr₇C₃ exhibits the strongest metallic character. The elastic constants, elastic anisotropies and theoretical hardness of the carbides are also presented, which are important parameters for the structural materials and surface coatings.     

O2 在不同终端Fe31MnC(001)表面吸附与解离的理论研究

采用密度泛函理论中的广义梯度近似方法（GGA）研究了氧分子在 Fe31MnC(001) 表面的三种不同吸附位（顶位,桥位和面心间位）的吸附行为和C对Fe31Mn磁性的影响。通过吸附能结果发现,氧分子倾向于垂直吸附在四配位饱和的空穴位,而氧原子强烈吸附在相邻的四配位空穴中。以Mn为表层原子的表面均发生了氧分子的解离吸附,桥位吸附对应的吸附结构最稳定。C原子增加了基体的磁化强度杂化,抑制近邻Fe和Mn原子之间的反铁磁相互作用。     

Temperature-dependent mechanical properties of alpha-/beta-Nb5Si3 phases from first-principles calculations

The temperature-dependent structural properties and anisotropic thermal expansion coefficients of α-/β-Nb₅Si₃ phases have been determined by minimizing the non-equilibrium Gibbs free energy as functions of crystallographic deformations. The results indicate that the crystal anisotropy of α-Nb₅Si₃ phase is much more temperature dependence than that of β-Nb₅Si₃ phase. The total/partial density of states of α-/β-Nb₅Si₃ phases are discussed in detail to analyze their electronic hybridizations. It is demonstrated that the bonding of the two phases is mainly contributed from the hybridization between Nb-4d and Si-3p electronic states. The temperature-dependent mechanical properties of α-/β-Nb₅Si₃ phases are further investigated via the quasi-harmonic approximation method in coupling with continuum elasticity theory. The calculated single-crystalline and polycrystalline elasticity shows that both phases are mechanically stable and exhibit the intrinsic brittleness. The results also suggest that α-Nb₅Si₃ phase possesses a superior ability of compression resistance but an inferior ability of high-temperature resistance of mechanical properties than those of β-Nb₅Si₃ phase. The bonding features of α-/β-Nb₅Si₃ phases are discussed by means of charge density difference analysis in order to explain the difference of the temperature-dependent mechanical properties between the two phases.     

含有闭孔的镍基合金涂层压缩过程中的力学性能

利用等离子喷涂和重熔技术制备多孔型镍基自熔性合金涂层,采用扫描电镜、显微硬度计、材料试验机、热膨胀仪等设备对涂层的微观组织、热膨胀系数和压缩过程中的力学性能进行分析。结果表明,涂层中存在封闭孔洞,形态近似球形,直径在30-100 μm；孔隙率由0.53%上升至30.6%时,涂层材料的弹性模量下降69.5%,弹性变形范围上升68.7%；在40-220℃的范围内,多孔涂层的平均线膨胀系数随着孔隙率的增加而下降。球形闭孔的存在提升了原有材料的弹性变形范围,降低了热膨胀系数,使其可以实现低膨胀、高回弹的功能。     

第一性原理研究压力下Ni-Mo二元化合物的力学性能和电子结构

采用基于密度泛函理论的第一性原理方法研究压力对Ni-Mo二元化合物Ni4Mo、Ni3Mo(DOa)、Ni3Mo(DO22)、Ni2Mo力学性能和电子结构的影响。研究表明：0~40GPa压力范围内,随着压力的增大,相对体积V/V0不断减小且趋势减缓;形成热均为负值,且随着压力的增大形成热减小,说明增大压力可提高化合物的合金化能力;体积模量B、剪切模量G、杨氏模量E、拉梅常数λ、硬度H的计算结果表明压力可提高四种化合物的抗变形、抗压缩能力及硬度,另外,B/G和泊松比ν表明所有化合物均为延性和塑性的;进行态密度的分析,阐明增大压力可提高四种化合物的稳定性及硬度。     

Elastic Properties and Stacking Fault Energies of Borides,Carbides and Nitrides from First-Principles Calculations

Owing to the excellent elastic properties and chemical stability, binary metal or light element borides, carbides and nitrides have been extensively applied as hard and low-compressible materials. Researchers are searching for harder materials all the time. Recently, the successful fabrication of nano-twinned cubic BN(Tian et al. Nature 493:385–388, 2013) and diamond(Huang et al. Nature 510:250–253, 2014) exhibiting superior properties than their twin-free counterparts allows an efficient way to be harder. From this point of view, the borides, carbides and nitrides may be stronger by introducing twins, whose formation tendency can be measured using stacking fault energies(SFEs). The lower the SFEs, the easier the formation of twins. In the present study, by means of first-principles calculations, we first calculated the fundamental elastic constants of forty-two borides, seventeen carbides and thirty-one nitrides, and their moduli, elastic anisotropy factors and bonding characters were accordingly derived. Then, the SFEs of the {111} 112 glide system of twenty-seven compounds with the space group F43 m or Fm3m were calculated. Based on the obtained elastic properties and SFEs, we find that(1) light element compounds usually exhibit superior elastic properties over the metal borides, carbides or nitrides;(2) the 5 d transitionmetal compounds(ReB_2, WB, OsC, RuC, WC, OsN_2, TaN and WN) possess comparable bulk modulus( B) with that of cBN( B = 363 GPa);(3) twins may form in ZrB, HfN, PtN, VN and ZrN, since their SFEs are lower or slightly higher than that of diamond(SFE = 277 mJ/m~2). Our work can be used as a valuable database to compare these compounds.     

Anisotropy in elasticity and thermal conductivity of monazite-type REPO4 (RE = La,Ce, Nd,Sm, Eu and Gd) from first-principles calculations

Starting from theoretical calculations based on LSDA, the authors compute the lattice parameters, cohesive energies and formation enthalpies of monazite-type REPO₄ compounds. The calculated values are satisfactory compared with the experimental results from the elastic constants obtained, the mechanical moduli are evaluated using the strain–stress method. The predicted bulk, Young’s and shear moduli are in good agreement with the experiments. It is shown that the mechanical moduli are low (<200 GPa) and also increase from LaPO₄ to GdPO₄. The three-dimensional contours and their planar projections of Young’s modulus are plotted to illustrate the anisotropy in elasticity. It is found that Young’s moduli of all monazite-type REPO₄ show strong dependence on direction. The linear thermal expansion coefficients are calculated using the empirical method, and the values are in the range 9 × 10^?6–12 × 10^?6 K^?1. Using Clarke’s and Slack’s models, the thermal conductivities of REPO₄ compounds obtained are close to the experimental profiles. The observed anomalies of experimental thermal properties of monazite-type GdPO₄ are also explained based on the observed monazite to zircon-type transformation in experiment. Solving the Christoffel equation for monoclinic symmetry, the anisotropy in thermal conductivity is investigated. The results indicate that the total lattice thermal conductivities of monazite-type REPO₄ show weak dependence on direction. Meanwhile, their sound velocities exhibit strong anisotropic properties.     

Energetics of charged point defects in rutile TiO2 by density functional theory

The defect formation energies of all possible charge states of point defects in TiO₂, including titanium interstitials, titanium vacancies and oxygen vacancies, are calculated in the phase space of temperature, oxygen partial pressure and Fermi level by combining density functional theory (DFT) and thermodynamic calculations. The point defect phase diagram illustrates that fully charged defects dominate in most regimes. The calculations not only give reasonable defect formation energies compared with prior experimental measurements, but also predict n-type TiO₂ at high T and low P_O2, and p-type TiO₂ at low T and high P_O2, which agrees well with experimental data. In addition, we evaluate methods for correcting the effects of artificial electrostatic interactions caused by periodic boundary conditions in the DFT calculations, including the electrostatic potential alignment correction (ΔV correction) and the Makov–Payne correction.     

Lanthanide(III) halides: Thermodynamic properties and their correlation with crystal structure

Temperatures and enthalpies of phase transitions of 17 lanthanide(III) halides determined experimentally are reported. Correlations were made between temperature of fusion of lanthanide(III) halides, on the one hand, and enthalpy of fusion, on the other, versus atomic number of lanthanide. According to this classification, the lanthanide(III) halides split into groups, as also do the corresponding crystal structures. A correlation between the crystal structure of lanthanide(III) halides and their respective entropy of fusion (or entropy of fusion + entropy of solid–solid phase transition) was inferred from the aforementioned features. Fusion in those halides with hexagonal, UCl₃-type and orthorhombic, PuBr₃-type, structures entails an entropy of fusion change (or entropy of fusion + entropy of solid–solid phase transition change) by 50 ± 4 J mol⁻¹ K⁻¹. The homologous entropy change within the group of halides having the rhomboedric, FeCl₃-type, structure, is smaller and equals 40 ± 4 J mol⁻¹ K⁻¹. Halides with monoclinic, AlCl₃-type, crystal structure constitute a third group associated to an even smaller entropy change upon fusion, only 31 ± 4 J mol⁻¹ K⁻¹. The halides with lower entropies of fusion also have a lower S_1300 K − S_298 K indicating either a higher degree of order in the liquid or a higher entropy in the solid at room temperatures.     

Thermodynamic properties of stable and metastable phases of Pt metal

Isometric heat capacity cv and isobaric heat capacity cp of Pt with stable and metastable phases were calculated by using pure element systematic theory. These results are in excellent agreement with of SGTE (Scientific Group Thermodata Europe) database and JANAF (Joint Army-Navy-Air Force) experimental values. The calculation results of cv and cp of Pt metal in natural state are in good agreement with those calculated by FP(first-principles) method. It is found that the electron devotion to heat capacity is important to adjust in OA(one-atom) method while calculating heat capacity. The full information about thermodynamic properties of Pt metal with stable and metastable phases, such as entropy(S), enthalpy(H) and Gibbs energy(G) were calculated from 0 K to random temperature. The results are in good agreement with JANAF experimental value. In contrast to SGTE database, the thermodynamic properties from 0 K to 298.15 K are implemented.     

Lattice dynamics,thermodynamics and elastic properties of monoclinic Li2CO3 from density functional theory

Monoclinic Li₂CO₃ has been identified as a critical component of the solid electrolyte interphase (SEI), a passivating film that forms on Li-ion battery anode surfaces. Here, lattice dynamics, finite temperature thermodynamics and the elastic properties of monoclinic Li₂CO₃ are examined with density functional theory (DFT) and various exchange–correlation functionals. To account for LO-TO splittings in phonon dispersion relations of Li₂CO₃, which is a polar compound, a mixed-space phonon approach is employed. Bond strengths between atoms are quantitatively explored with phonon force constants. Temperature variations of the entropy, enthalpy, isobaric heat capacity and linear (average) thermal expansion are computed using the quasiharmonic approach. The single-crystal elasticity tensor components along with polycrystalline bulk, shear and Young’s moduli are computed with a least-squares approach based upon the stress tensor computed from DFT. Computed thermodynamic properties as well as structural and elastic properties of the monoclinic Li₂CO₃ are in close accord with available theoretical and experimental data. In contrast to a recent DFT study, however, computed vibrational spectra suggest that neither the monoclinic Li₂CO₃ nor its high-temperature hexagonal phase exhibits either elastic or vibrational instabilities.     

First-principles study on effect of pressure and temperature on structural, elastic, thermodynamic, and electronic properties of Ni3Al alloy

The influence of temperature and pressure on the electronic, elastic, structural, and thermodynamic properties of Ni3Al alloy was investigated by performing a first-principles study. The calculated elastic constants, equilibrium lattice constants, and elastic modulus agree well with the recorded theoretical and experimental data. The calculated elastic constants indicate that C11 is more sensitive than C12 and C44 to pressure. The Young’s modulus, bulk modulus, and shear modulus increase with an increase in pressure. The ratio of bulk to shear modulus (B/G) and anisotropy factor A were also analyzed. The Debye temperature was obtained by calculating the elastic constants, and it changed with the change in the pressure. The thermal expansion coefficient, normalized volume, heat capacity, bulk modulus, and Debye temperature Θ were determined and analyzed by using the quasi-harmonic Debye model at pressures of 0–60 GPa and temperatures of 0–1600 K. Finally, the density of states and Mulliken population were investigated and the effect of pressure on these was analyzed.     

Internal friction and elastic properties of the high pressure-induced phases of solid C60

Solid C₆₀ is a new promising material made from giant high-symmetric carbon clusters. Though C₆₀ molecules are extremely stable at ambient conditions, heat treatment under high pressures induces transformations to other phases, in which some weak van der Waal-type bonding between C₆₀ molecules is replaced with covalent sp³ bonds. Many new C₆₀ high pressure phases (HP) demonstrate a set of unique physical properties including mechanical and elastic ones. This paper is an analytic review of the experimental measurements of the elastic and dissipative properties of solid C₆₀ (from C₆₀ single crystal to C₆₀ ultrahard phase).     

First-principles investigation of magnetic properties and metamagnetic transition of NiCoMnZ(Z = In,Sn, Sb) Heusler alloys

We employed the density functional theory to investigate the structural, magnetic properties and metamagnetic transition on Mn and Co doped-Ni₂MnZ (Z = In, Sn, Sb) Heusler alloys. The calculated formation energy indicates that excess Mn and Co prefer to occupy Z and Ni sites, respectively. The energy difference between austenite and martensite phases exhibits a monotonic increase with Mn doping, and a decrease with Co doping, which are consistent with the trend of experimental martensitic transformation temperature. The evaluated magnetic exchange parameters show a strong dependence on Z element, which can be explained by the super-exchange interaction mediated by Z sp states near the Fermi level. Bond analysis of martensite phase reveals that the strength of MnSb bond is stronger than that of MnIn and MnSn bond and it explains the larger driving magnetic field in NiMnSb than NiMnZ(Z = In, Sn) is need for metamagnetic phase transformation. In addition, we predict NiCoMnZ (Z = Sn, Sb) alloys require a smaller compressive epitaxial strain for metamagnetic transition than NiCoMnIn alloys.     

High-temperature stability, structure and thermoelectric properties of phases

Polycrystalline perovskite-type CaMn_1-xNb_xO₃ phases (with x=0.02,0.05,0.08 and 0.10) were investigated with regard to their structure, microstructure and thermal stability as a function of temperature. The studied phases revealed a complex microstructure at room temperature with 90° twinned domains. At high temperatures, the manganate phases underwent a structural transition from orthorhombic to cubic symmetry, as confirmed by in situ high-temperature X-ray powder diffraction and electron diffraction data. Thermogravimetric heating/cooling studies showed a reversible thermal reduction/reoxidation process that occurred above a defined transition temperature. A possible mechanism relating the high-temperature structural transition and the thermal reduction process of slightly substituted CaMnO₃ phases was proposed. The thermal reduction process resulted in a change in the Mn³⁺/Mn⁴⁺ concentrations in the Mn sublattice, and therefore in a modification of the transport properties. A comprehensive study examined the impact of both phenomena on the electrical and thermal transport properties.     

Density functional study of the mechanical and phonon properties of Al12X (X = Mo,Tc, Ru,W, Re,and Os) compounds

By means of first principles calculations, we have studied the structural, elastic, and phonon properties of the Al₁₂X (X = Mo, Tc, Ru, W, Re, and Os) compounds in cubic structure. The elastic constants of these compounds are calculated, then bulk modulus, shear modulus, Young's modulus, Possion's ratio, Debye temperature, hardness, and anisotropy value of polycrystalline aggregates are derived and relevant mechanical properties are compared with the available theoretical ones. Furthermore, the phonon dispersion curves, mode Grüneisen parameters, and thermo-dynamical properties such as free energy, entropy and heat capacity are computed and the obtained results are discussed in detail.     

A first principles study of structural and mechanical properties of Ti2AlZr intermetallic

The present work describes the structural and mechanical behaviour of three phases namely B2, D0₁₉ and O phases of Ti₂AlZr intermetallic using first principles density functional theory (DFT) within generalized gradient approximation (GGA). The equilibrium lattice constant values of B2, D0₁₉ and O phases are in good agreement with the experimental and theoretical data, respectively. Formation energy of O phase is minimum followed by D0₁₉ and B2. Bonding characteristics of these phases have been explained based on electronic density of states and charge density. All the three phases satisfy the Born stability criteria in terms of elastic constants and are associated with ductile behaviour based on G/B ratios. The B2 phase exhibits very high anisotropy in comparison to those of the D0₁₉ and O.     

热压烧结制备Cu/FeCoCr复合材料的显微组织及热物理性能

利用相图计算的CALPHAD方法和超音雾化制粉技术,在CuFeCoCr体系中设计并制备了一系列微米级复合粉体。通过热压烧结方法在烧结温度为950℃,烧结压力为45 MPa的工艺条件下成功获得块体复合材料。研究了块体复合材料中Cu含量对显微组织,热导率,热膨胀系数以及显微硬度的影响。结果表明:CuFeCoCr块体复合材料均由fcc富铜相和fcc富铁钴铬相组成。该系列复合材料经600℃时效处理8 h后,其热膨胀系数变化范围为5.83×10-6~10.61×10-6 K-1,热导率变化范围为42.17~107.53 W·m-1·K-1。其中Cu55(Fe0.37Cr0.09Co0.54)45复合材料表现出良好的综合性能,即其热膨胀系数和热导率分别为9.08×10-6K-1和91.09 W·m-1·K-1,与电子封装半导体材料的热膨胀系数相匹配。     

Electric field gradients in Zr8Ni21 and Hf8Ni21 intermetallic compounds; results from perturbed angular correlation measurements and first-principles density functional theory

Numerous technological applications of Ni-based Zr and Hf intermetallic alloys promoted comprehensive studies in Zr₈Ni₂₁ and Hf₈Ni₂₁ by perturbed angular correlation (PAC) spectroscopy, which were not studied earlier until this report. The different phases produced in the samples have been identified by PAC and X-ray diffraction (XRD) measurements. Using ¹⁸¹Hf probe, two non-equivalent Zr/Hf sites have been observed in both Zr₈Ni₂₁ and Hf₈Ni₂₁ compounds. From present PAC measurements in Zr₈Ni₂₁, a component due to the production of Zr₇Ni₁₀ by eutectic reaction from the liquid metals is also observed. The phase Zr₇Ni₁₀, however, is not found from the XRD measurement. In Zr₈Ni₂₁, while the results do not change appreciably up to 973 K exhibit drastic changes at 1073 K. In Hf₈Ni₂₁, similar results for the two non-equivalent sites have been found but site fractions are in reverse order. In this alloy, a different contaminating phase, possibly due to HfNi₃, has been found from PAC measurements but is not found from XRD measurement. Density functional theory (DFT) based calculations of electric field gradient (EFG) and asymmetry parameter (η) at the sites of ¹⁸¹Ta probe nucleus allowed us to assign the observed EFG fractions to the various lattice sites in (Zr/Hf)₈Ni₂₁ compounds.