Huiyang Gou

Ultrahigh volumetric capacitance and cyclic stability of fluorine and nitrogen co-doped carbon microspheres

Highly porous nanostructures with large surface areas are typically employed for electrical double-layer capacitors to improve gravimetric energy storage capacity; however, high surface area carbon-based electrodes result in poor volumetric capacitance because of the low packing density of porous materials. Here, we demonstrate ultrahigh volumetric capacitance of 521 F cm−3 in aqueous electrolytes for non-porous carbon microsphere electrodes co-doped with fluorine and nitrogen synthesized by low-temperature solvothermal route, rivaling expensive RuO2 or MnO2 pseudo-capacitors. The new electrodes also exhibit excellent cyclic stability without capacitance loss after 10,000 cycles in both acidic and basic electrolytes at a high charge current of 5 A g−1. This work provides a new approach for designing high-performance electrodes with exceptional volumetric capacitance with high mass loadings and charge rates for long-lived electrochemical energy storage systems.

Discovery of a Superhard Iron Tetraboride Superconductor

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

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.

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.

Investigation of exotic stable calcium carbides using theory and experiment

It is well known that pressure causes profound changes in the properties of atoms and chemical bonding, leading to the formation of many unusual materials. Here we systematically explore all stable calcium carbides at pressures from ambient to 100 GPa using variable-composition evolutionary structure predictions. We find that Ca5C2, Ca2C, Ca3C2, CaC, Ca2C3, and CaC2 have stability fields on the phase diagram. Among these, Ca2C and Ca2C3 are successfully synthesized for the first time via high-pressure experiments with excellent structural correspondence to theoretical predictions. Of particular significance are the base-centered monoclinic phase (space group C2/m) of Ca2C, a quasi-two-dimensional metal with layers of negatively charged calcium atoms, and the primitive monoclinic phase (space group P21/c) of CaC with zigzag C4 groups. Interestingly, strong interstitial charge localization is found in the structure of R-3m-Ca5C2 with semimetallic behaviour.It is well known that pressure causes profound changes in the properties of atoms and chemical bonding, leading to the formation of many unusual materials. Here we systematically explore all stable calcium carbides at pressures from ambient to 100 GPa using variable-composition evolutionary structure predictions using the USPEX code. We find that Ca5C2, Ca2C, Ca3C2, CaC, Ca2C3 and CaC2 have stability fields on the phase diagram. Among these, Ca2C and Ca2C3 are successfully synthesized for the first time via high-pressure experiments with excellent structural correspondence to theoretical predictions. Of particular significance is the base-centred monoclinic phase (space group C2/m) of Ca2C, a quasi-two-dimensional metal with layers of negatively charged calcium atoms, and the primitive monoclinic phase (space group P21/c) of CaC with zigzag C4 groups. Interestingly, strong interstitial charge localization is found in the structure of R-3m-Ca5C2 with semi-metallic behaviour.

Peierls distortion, magnetism, and high hardness of manganese tetraboride

We report crystal structure, electronic structure, and magnetism of manganese tetraboride, MnB4, synthesized under high-pressure, high-temperature conditions. In contrast to superconducting FeB4 and metallic CrB4, which are both orthorhombic, MnB4 features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of MnB4 to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 μB /f.u., ferromagnetic spin correlations, and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders MnB4 a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

Peculiar structure and tensile strength of WB4: nonstoichiometric origin

Tungsten tetraboride (WB4) is experimentally considered as potentially superhard material and is therefore expected to have highly structural stability and enhanced resistance against plastic deformation and failure. The examinations of bond-deformation mechanism suggest a significantly soft bond-deformation pattern induced by ionic W-B bonding for nominal WB4 in experiments, largely responsible for the limitation of its strength and structural integrity. Computations on the structures and mechanical properties for WB4 show a novel thermodynamically favored MoB4-type phase with excellent mechanical properties and remarkable incompressibility along c direction. The illustrations of nonstoichiometry and x-ray diffraction spectra rationalize the experimental observation of nominal composition WB4 as defective tungstenborides (W1-x B3 (x 0.25)). The results provide new insight into the real structural and mechanical properties of tungstenborides.

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.