J. Feng

Mechanical properties and chemical bonding characteristics of Cr7C3 type multicomponent carbides

The first principles calculations based on density functional theory are performed to investigate the mechanical properties and chemical bonding features of several Cr7C3 type multicomponent carbides (Fe16Cr12C12, Fe12Cr12W4C12, Fe12Cr12Mo4C12, Fe12Cr12W4C8B4, and Fe12Cr12Mo4C8B4). The full set elastic constants are calculated using stress-strain method. The Voigt–Reuss–Hill approximation is used to evaluate the mechanical moduli. The mechanical anisotropy is characterized by calculating several different anisotropic indexes and factors, such as universal anisotropic index (AU), shear anisotropic factors (A1, A2, and A3), and percent anisotropy (AB and AG). The surface constructions of bulk and Young’s moduli are illustrated to indicate the mechanical anisotropy. The obtained results indicate that the presence of B could deteriorate the mechanical moduli of (Fe,Cr)7C3 phase in Cr cast iron slightly. On the other hand, the chemical stability of (Fe,Cr)7C3 carbides is improved due to chemical bonding modifi...

A comparative study of Cr7C3, Fe3C and Fe2B in cast iron both from ab initio calculations and experiments

The ground state properties of three compounds, Cr7C3 ,F e 3C and Fe2B, are investigated using ab initio calculations based on density functional theory. Formation enthalpy values indicate that Cr7C3 is the most stable crystal among the three compounds. Fe3C is metastable which has a positive heat of formation value. The calculated bulk modulus, shear modulus and Young’s modulus value of Cr7C3 are 311 GPa, 143.8 GPa and 374 GPa, respectively. The bulk modulus values of Fe2B and Fe3C are 194 GPa and 258 GPa. We also find that both the hardness and the stiffness of the Cr7C3 type carbides can be improved by doping with B, W, Mo, etc. The bulk modulus of transition metal doped Fe2B is considerably higher than pure Fe2B. The electronic structures of Fe2B and Fe3C are ferromagnetic and the evaluated average magnetic moment of Fe is 2.09µB/atom for Fe3C and 2.02µB/atom for Fe2B, respectively. Micro-indentation test results indicate that Cr7C3 is the hardest phase among the three phases and shows excellent wear resistance performance under three-body abrasive experiments. The experimental results are in agreement with the theoretical prediction that Cr7C3 is the best both in stability and mechanical performance. (Some figures in this article are in colour only in the electronic version)

Mechanical properties of rare earth stannate pyrochlores

The RE2Sn2O7 series compounds (REu2009=u2009La, Nb, Sm, Gd, Er, Yb) with a pyrochlore structure are prepared by co-precipitation method. The bulk, shear, Young’s moduli, B/G, and Poisson’s ratios are calculated using density functional theory and also measured by ultrasonic resonance method. The theoretical values of lattice constants and mechanical moduli are smaller than experimental results. The electronic structures of RE2Sn2O7 are analogous to RE2Zr2O7. La2Sn2O7 exhibits stronger ionic bonds than others. The covalent interactions are slightly enhanced in the heavy rare earth stannate pyrochlores. The Vickers harnesses of RE2Sn2O7 are measured experimentally, which are smaller than theoretical predictions.

Thermal expansions of Ln2Zr2O7 (Ln = La, Nd, Sm, and Gd) pyrochlore

The thermal expansions of the rare earth zirconate (Ln2Zr2O7, Lnu2009=u2009La, Nd, Sm, and Gd) pyrochlore were studied by the Debye and quasi harmonic approximation combined with the first principles calculations. The difference between CV and CP was obtained. The temperature dependence of thermal expansions is mainly caused by the restoration of thermal energy due to phonon excitations at relatively a low temperature. When the temperature is much higher than Debye temperature, i.e., above 600u2009K for Ln2Zr2O7 compounds, the volumetric coefficient is increased linearly by increasing the temperature. The calculations are in good agreement with the experiments.

Electronic and magnetic properties of double perovskite slab-rocksalt layer rare earth strontium aluminates natural superlattice structure

By using the LSDA and LSDAu2009+u2009U method, the electronic and magnetic properties of Ln2SrAl2O7 (Lnu2009=u2009La, Nd, Sm, Eu, Gd, and Dy) compounds have been investigated in this paper. While comparing the density of states calculated by two methods, the main difference between LSDA and LSDAu2009+u2009U calculations is only obvious for 4f states. The Ln2SrAl2O7 compounds are more like the charge transfer type insulators. The ferromagnetic and possible anti-ferromagnetic states of these compounds are calculated and discussed by Rudeman-Kittel-Kasuya-Yosida (RKKY) model.

Computer Simulation of Mechanical Properties of Nano-scale Cu/FeS Composite

The mechanical properties of nano-scale Cu/FeS composite were simulated by molecular dynamics (MD) simulation in this paper. Through the analysis on the stress-strain curves, the results of MD simulation were in good agreement with mechanisms of macroscopic deformation. When the size of particles was smaller than a certain value, the relationship between yield strength and size, which was different from the large size crystals abided by the contrary Hall-Petch relationship. Based on the discussion of nano-scale Cu/FeS composite, some interesting conclusions were obtained. For example, the “S” type curves were discovered in stress-strain curves and the anisotropy of FeS was very evident when the exposures of reinforcing phase (FeS) were different and so on. The basic theories and calculations of the composite that contains nano-scale particles were discussed. At the same time, a new modeling building method of composites, which was close to actual experiences, were considered in this paper.

Molecular dynamical simulation of the behavior of early precipitated stage in aging process in dilute Cu–Cr alloy

The aging behaviors of Cu–Cr alloys in the early stage at different temperatures are investigated by molecular dynamics simulations. First principles potentials are used for the interactions between Cu and Cr atoms. The initial behavior of precipitation is characterized by transmission electron microscope and electron energy disperse spectroscopy. The results showed that Cu–Cr supersaturated solid solution is thermodynamically unstable. The mean-square displacements of the atoms are used to describe the diffusivity. At room temperature, the atoms only show harmonic vibrations near the equilibrium positions. The mutual diffusion at 873 K is different from the unidirectional diffusion in low temperatures. The calculation shows that aging process is accelerated with increasing temperature, which is not only due to the lower diffusion activation energy of Cr at higher temperature, but also because Cu atoms are also participated in the aging process. When “aging” at 1073 K, the precipitation of Cr element is dissolved again into Cu matrix, which is an “over-aging” state of Cu–Cr alloy at high temperature.

LiRE2Si3 (RE = Nd, Sm, and Eu) as potential photovoltaic materials

The equilibrium lattice properties, electronic and optical properties of LiRE2Si3 (RE = Nd, Sm, and Eu) compounds have been investigated. LiRE2Si3 compounds show strong absorption in the entire range of solar spectrum. The conversion efficiencies are 52.4, 70.2, and 63.9% for LiNd2Si3, LiSm2Si3, and LiEu2Si3, respectively. The efficiencies of LiSm2Si3 and LiEu2Si3 are about twice of GaAs (34%), and they are three times higher than Si (20%). The presence of 4f shell and the unoccupied π* states enhance the electron transportation invoked by photon adsorption. The LiRE2Si3 compounds are the excellent candidates of photovoltaic materials so far.

Stability, Electronic and Magnetic Properties of Ca3Ru2O7

Ca3Ru2O7 is new tpye of thermoelectric materials.A theoretical study is presented for the stability, electronic and magnetic properties of three phases of this new thermoelectric materials in the framework of density functional theory (DFT). The calculated cohesive energy is -7.94eV/unit. AFM2 are less stable than other pahses. Electronic calculations indicate that Ca3Ru2O7 is metallic in nature. The covalent bonds in these structures are due to orbital overlap between p bands of O and d bands of Ru, and DOS at Fermi level are dominated by d bands of Ru. FM phase have obvious magnetic moments.