Ab initio study of Os0.5W0.5B2, Re0.5W0.5B2, and Os0.5Re0.5B2 with high shear modulus

By using first‐principles calculations, the authors investigate the structural, mechanical, and electronic properties of experimentally synthesized Os_0.5W_0.5B₂. The calculated structural parameters and elastic properties are in good agreement with the experimental results. In addition, two new 5d transition‐metal diborides (Re_0.5W_0.5B₂ and Os_0.5Re_0.5B₂) are predicted to have promising large shear moduli. The latter mainly come from the non‐uniform distribution of valence charge density, which raises the value of the shear moduli. We discuss potentially high hardness in these materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diamond Structure BeO, Designable Super-Hard Materials and Semiconductor Be-Diamond

It is possible for Beryllium oxide （BeO） to have a cubic diamond structure although it normally has a hexagonal structure under ambient conditions. As the solution of cubic BN and diamond, the solid solution of cubic BeO- diamond or BeO-cBN-diamond can potentially be a kind of super-hard materials with designable hardness; and this solution has also been confirmed based on our preliminary first principles calculations. In addition, the nonstoichiometry of BeO could create a mobile carrier in the cubic BeO-C or BeO BN-C system and it might lead to a new type of semiconductor Be-diamond.     

Ultra‐incompressible and hard technetium carbide and rhenium carbide: First‐principles prediction

Using density functional theory, the author predicts that the compounds ReC and TcC with the hexagonal WC‐like structure are ultra‐incompressible and hard materials. This is concluded from the very large bulk and shear moduli. The phonon dispersion reveals no soft modes indicating the stability of the two materials. The calculated density of states shows that ReC and TcC are metallic. The structural and elastic properties of OsC, IrC, and PtC with hexagonal and cubic structure are also investigated for comparison. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of GaAs “nano ice cream cones” by dual wavelength pulsed laser ablation

Harmonic generation crystals inherently offer the possibility of using multiple wavelengths of light in a single laser pulse. In the present experiment, the fundamental (1064 nm) and second harmonic (532 nm) wavelengths from an Nd:YAG laser are focused together on GaAs and GaSb targets for ablation. Incident energy densities up to about 45 J/cm² at 10 Hz with substrate temperatures between 25 and 600 °C for durations of about 60 s have been used in an ambient gas pressure of about 10⁻⁶ Torr. The ablated material was collected on electron-transparent amorphous carbon films for TEM analysis. Apart from a high density of isolated nanocrystals, the most common morphology observed consists of a crystalline GaAs cone-like structure in contact with a sphere of liquid Ga, resembling an “ice cream cone”, typically 50-100 nm in length. For all of the heterostuctures of this type, the liquid/solid/vacuum triple junction is found to correspond to the widest point on the cone. These heterostructures likely form by preferential evaporation of As from molten GaAs drops ablated from the target. The resulting morphology minimizes the interfacial and surface energies of the liquid Ga and solid GaAs.     

Molecular dynamics of binary metal nitrides and ternary oxynitrides

Ternary systems incorporating metals with oxygen and nitrogen are examined using Tersoff potentials. The apparent success of treating some binary nitride systems using the Tersoff potential is used as a way forward to obtain a new parameter set incorporates atomic features into a series of Tersoff potential for binary nitrides and ternary oxynitrides.     

First-Principles Calculations of Phase Transition and Stability of Si2CN4 under High Pressure

A pressure-induced phase transition and stability in Si2 CN4 polymorphs under high pressure are studied by firstprinciples calculations. The result shows that the phase transition pressure of α- and β-Si2 CN4 to the cubic spinal phase is 29.9 GPa and 27.5 GPa predicted by thermodynamic method respectively. Under ambient condition, all of the three Si2CN4 polymorphs are metastable with positive formation enthalpy. Unlike the stability of Si3N4 polymorphs, α-Si2 CN4 is more stable than the β phase.     

Growth of a Novel Periodic Structure of SiC/AIN Multilayers by Low Pressure Chemical Vapour Deposition

A novel lO-period SiC/A1N multilayered structure with a SiC cap layer is prepared by low pressure chemical vapour deposition （LPCVD）. The structure with total film thickness of about 1.45~m is deposited on a Si （111） substrate and shows good surface morphology with a smaller rms surface roughness of f.3 nm. According to the secondary ion mass spectroscopy results, good interface of the 10 period SiC/A1N structure and periodic changes of depth profiles of C, Si, A1, N components are obtained by controlling the growth procedure. The structure exhibits the peak reflectivity close to 30% near the wavelength of 322 nm. To the best of our knowledge, this is the first report of growth of the SiC/AIN periodic structure using the home-made LPCVD system.     

Y-branching of single walled carbon nanotubes

Y-branching was observed by scanning tunnelling microscopy (STM) in single wall carbon nanotubes grown by thermal decomposition of C₆₀ fullerene in the presence of transition metals. These novel carbon nanostructures may play an important role in carbon-based nanoelectronics. Received: 18 November 1999 / Accepted: 20 January 2000 / Published online: 8 March 2000     

Electromagnetic wave absorption properties of carbonyl iron-ferrite/PMMA composites fabricated by hybridization method

The surface of carbonyl iron powder or a mixture of carbonyl iron and ferrite was coated with polymethylmethacrylate (PMMA) microspheres by a hybridization method to make hybrid powders, and then electromagnetic wave absorption properties of the hybrid composites prepared with these hybrid powders have been investigated. As for the carbonyl iron/PMMA hybrid composite, the reflection loss less than −20 dB could be achieved in a frequency range of 1.7–5.0 GHz when the composite thickness was below 5.00 mm. In the case of the carbonyl iron-ferrite/PMMA hybrid composite, a similar reflection loss was observed in a frequency range of 4.3–13.0 GHz. Thus, the addition of ferrite was found to be useful for achieving a large absorption in a wide frequency range, especially for higher frequency values. Simulated values for the minimum reflection loss are well agreed with actually measured ones, because of homogeneous distribution of carbonyl iron and/or ferrite in these hybrid composites.     

The effect of compositional variations on the glass-transition and crystallisation temperatures in Ge-Se-In glasses

The glass-transition (T_g) and crystallisation (T_x) temperatures of glassy Ge_xSe_yIn₁₂ (7≤x≤28) have been determined from differential scanning calorimetry measurements. The variations of T_g and T_x with composition have been specified. It has been found that T_g reaches a maximum at 614 K for the composition Ge_23.33 Se_64.67 In₁₂ while T_x passes through a minimum at 740 K for the same composition. The values of the cohesive energies of the studied compositions have also been estimated using the chemical bond approach method. It is found that the composition Ge_23.33Se_64.67In₁₂ possesses the maximum cohesive energy. These results are explained in terms of the structure of Ge-Se-In glasses. Received: 12 March 2001 / Accepted: 29 March 2001 / Published online: 23 May 2001