Jingwu Zhang

First-principles study of low compressibility osmium borides

Using first-principles total energy calculations we investigate the structural, elastic, and electronic properties of OsB2 and OsB, respectively. The calculated equilibrium structural parameters of OsB2 are in agreement with the available experimental results. The calculations indicate that OsB in tungsten carbide is more energetically stable under the ambient condition than the metastable cesium chloride phase of OsB. Results of bulk modulus show that they are potential low compressible materials. The hardness of OsB2 is estimated by employing a semiempirical theory. The results indicate that OsB2 is an ultraincompressible material, but not a superhard material. The method designing superhard materials is different from one creating ultraincompressible materials.Using first-principles total energy calculations we investigate the structural, elastic, and electronic properties of OsB2 and OsB, respectively. The calculated equilibrium structural parameters of OsB2 are in agreement with the available experimental results. The calculations indicate that OsB in tungsten carbide is more energetically stable under the ambient condition than the metastable cesium chloride phase of OsB. Results of bulk modulus show that they are potential low compressible materials. The hardness of OsB2 is estimated by employing a semiempirical theory. The results indicate that OsB2 is an ultraincompressible material, but not a superhard material. The method designing superhard materials is different from one creating ultraincompressible materials.

Pressure-induced incompressibility of ReC and effect of metallic bonding on its hardness

The compressible behaviors of the selected 5d transition metal carbides MC (M=W,Re,Os,Ir) with hexagonal tungsten carbide-type structure were studied by first-principles calculations. Results indicate that the incompressibility of ReC exceeds that of diamond under higher pressure. The calculated method for hardness of crystals with partial metallic bonding is suggested and the calculated results indicate that hexagonal ReC crystal possesses excellent mechanical properties.

Influence of pressures on the crystallization process of an amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

Amorphous Fe73.5Cu1Nb3Si13.5B9 alloys, prepared by a melt-spinning technique, were annealed at a temperature of 823 K under pressures in the range of 1–5 GPa and ambient pressure. The high pressure experiments were carried out in a belt-type pressure apparatus. The microstructure of the annealed alloys has been investigated by x-ray diffraction, electron diffraction, and transmission electron microscopy. Experimental results show that the initial crystalline phase in these annealed alloys is α-Fe solid solution (named as α-Fe phase below), and high pressure has a great influence on the crystallization process of the α-Fe phase. The grain size of the α-Fe phase decreases with the increase of pressure (P). The volume fraction of the α-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (P>2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy and latent applications of high-pressure anne...

Theoretical hardness of PtN2 with pyrite structure

Using first-principles technique, the authors have investigated the structural, mechanical, and electronic properties of the PtN2 with cubic pyrite and orthorhombic FeS2 structure. The calculated results of the pyrite-type PtN2 are in agreement with the available theoretical and experimental values. The pyrite-type PtN2 is more energetically stable under the ambient condition. Results indicate that the two PtN2 phases are semiconducting materials. Based on Mulliken overlap population analysis in first-principles technique, the hardness of both the cubic and orthorhombic PtN2 is predicted. Results show that the PtN2 with pyrite structure possesses excellent mechanical properties.

Unusual rigidity and ideal strength of CrB4 and MnB4

By means of first-principles calculations, we report superior rigidity, ideal tensile, and shear strength for orthorhombic CrB4 and MnB4. The analysis of microscopic deformation mechanism reveals that the unique rectangular boron units in CrB4 and MnB4 are responsible for the consolidation of the directionality of boron-boron covalent bonds and the exceptional rigidity and ideal strength. The unusual mechanical properties of the orthorhombic tetraborides highlight their potential applications as intrinsically superhard materials. The unique rectangular boron unit also implies a criterion for designing and synthesizing transition metal boride based-materials with ultimate hardness and strength.

Study of interface structure of α-Fe/Nd2Fe14B nanocomposite magnets

In this study, the interface structure of α-Fe/Nd2Fe14B nanocomposite magnets has been investigated by employing both the positron lifetime spectroscopy and the two-detector Doppler broadening measurements of the positron–electron annihilation γ quanta. Positron lifetime studies show that there are two kinds of interface structures in the magnets. One characterized by a positron lifetime of 155 ps is determined to be the interfacial amorphous layer. The other has a slack atomic structure in which structural free volumes, which were detected to be predominantly surrounded by nonmagnetic atoms Nd and B by the Doppler broadening measurements, have a larger size than that of one to two lattice vacancies of Fe. This is believed to weaken the magnetic exchange coupling between α-Fe and Nd2Fe14B grains in the nanocomposites.

Structural stability and elastic and electronic properties of rhenium borides: first principle investigations.

The structural stability and elastic and electronic properties of rhenium borides with different boron concentration are calculated systemically by means of first principle total energy calculations. The total energy calculations reveal that the WC-type structure is more energetically favorable for ReB and that the Re(2)P-type structure is more preferred for Re(2)B. The formation enthalpy of these borides have been studied by the solid synthesis routes. The calculated elastic properties indicate that Re(2)B(3), ReB, and Re(2)B phases are also potential hard materials. Although valence-electron density was often employed to evaluate elastic properties of materials, our calculations indicate that the bulk elastic properties of these borides are not direct correlation with their valence-electron density. The analysis of electronic structure, charge density distribution, and Mulliken overlap population provides further understanding of the elastic and superconductivity properties of these borides.

Energetic stability, structural transition, and thermodynamic properties of ZnSnO3

First principles calculations were performed on ZnSnO3 polymorphs to understand their energetic stability and structural transition under high pressure environments. The experimentally-identified ilmenite (IL)-type and LiNbO3 (LN)-type ZnSnO3 may coexist at zero pressure considering the effect of zero point energy. IL-type ZnSnO3 becomes unstable under high pressure due to the appearance of imaginary frequency in phonon spectra. Enthalpy differences suggest that the phase stability follows the sequence: ZnO+SnO2 below 5.9 GPa, Zn2 SnO4 +SnO2 up to 7.1 GPa, and LN-type phase above 7.1 GPa. Pressurization at 34.5 GPa causes a phase transformation from the LN-type to the orthorhombic CdSnO3 -type. Thermodynamic properties including Helmholtz free energy, specific heat at constant volume and Debye temperature were also calculated.

Cubic γ-Be3N2: A superhard semiconductor predicted from first principles

The authors predict a superhard semiconductor phase of Be3N2 with cubic structure using first-principles calculations. The structural, mechanical, electronic, and optical properties of the Be3N2 have been investigated. Results indicate that the predicted Be3N2 phase is a wide gap semiconductor with a direct band gap of about 2.51eV. The calculated hardness of cubic γ-Be3N2 based on Mulliken overlap population analysis in first-principles technique approaches those of B4C and B6O. The higher mechanical property can be attributed to the existence of strong Be–N–Be covalent bond chains in the cubic structure. The obtained static dielectric constant of Be3N2 (4.6eV) is close to the spinel structure of Si3N4 (4.7eV).

Ordering of the crystalline phase α-Fe(Si) in annealed Fe73.5Cu1Nb3Si13.5B9 alloy

The ordering of the crystalline phase alpha-Fe(Si) in annealed Fe73.5Cu1Nb3Si13.5B9 alloy has been investigated in detail in this paper using XRD technology. The alpha-Fe(Si) phase in the Fe73.5Cu1Nb3Si13.5B9 alloy annealed at 490 degrees C for I h has been found to consist of a DO3 order region with the shape of a sphere and a disordered region. The size of the DO3 order region and the degree of order of the alpha-Fe(Si) phase increase with increasing annealing temperature. All regions in the alpha-Fe(Si) rain almost have the DO3 structure as the annealing temperature increases to 590 degrees C, The shape of the DO3 order region of the alpha-Fe(Si) phase in the Fe73.5Cu1Nb3Si13.5B9 alloy annealed at 550 degrees C changes from spheroid for 20 min into spherical for 60 min