Microstructure of Hydrogen-Implanted Polycrystalline α-SiC after Annealing

Microstructural evolution in H-implanted polycrystalline α-SiC upon thermal annealing at temperature 1100℃ is studied. After annealing, the samples are examined via cross-sectional transmission electron microscopy(XTEM)analysis. H_2 gas bubbles are formed during H implantation and some H2 molecules are released from the bubble to form cavities during thermal annealing. The distribution and size of the observed cavities are related to the implantation fluence. The results are compared to H implanted single crystal SiC and He implanted polycrystallineα-SiC. The possible reasons are discussed.     

Electron-stimulated desorption of positive ions from hexagonal α-SiC

Electron-stimulated desorption (ESD) of positive ions (H⁺, O⁺, ⁺OH, F⁺ and Cl⁺) from the Si- and C-terminated surfaces of hexagonal α-SiC has been observed for electron energies in the 5–105 eV range. Comparison of these results with those for the ESD of the same ions from the surfaces of Si and condensed hydrocarbons leads to a model for the H⁺, ⁺OH, F⁺ and Cl⁺ threshold desorption process based on transitions from deep valence Si and C “s-like” levels to states in the conduction band, followed by Auger decay to form a localized multiple valence-hole configuration. O⁺ desorption, on the other hand, is initiated by O 2s ionization. Evidence is found for a strong dependence of the F⁺ threshold on the local chemical bonding. The results indicate that the thresholds for ESD of these ions from SiC are determined more by the electronic excitation of the substrate than by direct excitation of the adsorbate bond.     

Luminescent Properties of the Surface of ZnS–Mn Nanocrystals Obtained by Self-Propagating High-Temperature Synthesis

Journal of Applied Spectroscopy - Luminescence of nanocrystalline ZnS–Mn powder obtained by self-propagating high-temperature synthesis is studied. The luminescence was excited by chemical...     

White Emitting ZnS Nanocrystals： Synthesis and Spectrum Characterization

Spherical organic-bonded ZnS nanocrystals with 4.0 4-0.2 nm in diameter are synthesized by a liquid-solid-solution method. The photoluminescence spectrum of sample （[S^2-]/[Zn^2＋] = 1.0） shows a strong white emission with a peak at 490nm and - 170 nm full widths at half maximum. By Gauss fitting, the white emission is attributed to the overlap of a blue emission and a green-yellow emission, originating from electronic transitions from internal S^2- vacancies level to valence band and to the internal Zn^2＋ vacancy level, respectively. After sealingZnS nanocrystals onto InGaN chips, the device shows CIE coordinates of （0.29,0.30）, which indicates their potential applications for white light emitting diodes.     

Synthesis and characterization of α-cobalt hydroxide nanobelts

α-Cobalt hydroxide was synthesized by a facile hydrothermal process from Co(Ac)₂ and NH₃·H₂O in the presence of 1,3-propanediol. The large-scale-prepared cobalt hydroxide has a uniform nanobelt morphology with a considerably high aspect-ratio more than 20 which may be advantageous for exploration of their physicochemical properties. This synthetic method is convenient, economical, and controllable. The samples were characterized by powder X-ray diffraction, energy dispersive spectrum, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, CHN element analysis, thermogravimetric and differential-thermogravimetric analysis, which revealed the compound is lamellar structural cobalt organic–inorganic hybrid with the chemical formula of Co(OH)_1.49(NH₃)_0.01(CO₃ ²⁻)_0.22(Ac⁻)_0.07(H₂O)_0.11 and single-crystalline.     

Synthesis of diverse N-acyl-pyrazoles via cyclocondensation of hydrazides with α-oxeketene dithioacetal

Molecular Diversity - An elegant, efficient, and highly regioselective approach for the synthesis of novel methyl 5-amino-3-(methylthio)-1-differently substituted-1H-pyrazole-4-carboxylates is...     

Luminescence of irradiated β-SiC

The influence of electron irradiation with energy of 3.5 MeV on the luminescence of cubic Silicon Carbide β-Sic) was investigated. It was shown that irradiation results in the diminution of edge emission and appearance of the red band (2.0–1.3 eV). Obtained results give an opportunity to consider the red band as transitions from excited states. The data on temperature stability of irradiated damage is given. The probable nature of irradiated defects are discussed.     

Comparison of Cavities Formed in Single Crystalline and Polycrystalline α-SiC after H Implantation

Cavities and extended defects formed in single crystalline and polycrystalline α-SiC implanted with H+ions are compared.The samples are investigated by cross-sectional transmission electron microscopy.H₂ bubbles are formed during H implantation and H₂ molecules escape the sample to form cavities during thermal annealing at 1100℃.Microcracks and the extended defects prefer to nucleate in single crystalline α-SiC,but not polycrystalline α-Si C.Grain boundaries can account for...     

Synthesis and characterization of magnetite particles covered with α-trietoxysilil-polydimethylsiloxane

New silicon magnetite ferrofluids were prepared by dispersing siloxane-coated magnetite particles in polydimethylsiloxane with low or high molecular weights. Ferrofluids are stable colloidal dispersions of ultra fine covered magnetite particles, which may be selected for a specific application. We demonstrated new methods of stabilizing the magnetic particles by reacting the hydroxyl groups on the surface of magnetite particles with terminal ethoxy groups of polydimethylsiloxane, followed by their dispersion in silicon fluids. The new silicon ferrofluids were tested from the morphology, magnetic properties/losses, and rheological properties point of view.     

Beneficial effects of an ultra-fine α-SiC incorporation on the sinterability and mechanical properties of ZrB2

A fully dense ZrB₂ ceramic containing 10 vol. % ultra-fine α-SiC particulate was successfully hot pressed at 1900 °C for 20 min and 40–50 MPa of applied pressure. Faceted ZrB₂ grains (average size ≈3 μm) and SiC particles dispersed regularly characterized the base material. No extra secondary phases were found. The introduction of the ultra-fine α-SiC particulate was recognized as the key factor that enabled both the control of the diboride grain growth and the achievement of full density. The mechanical properties offered an interesting combination of data: 4.8±0.2 MPafracture toughness, 507±4 GPa Young’s modulus, 0.12 Poisson’sratio, and 835±35 MPa flexural strength at room temperature. The flexural strength measured at 1500 °C (in air) provided values of 300±35 MPa. The incorporated ultra-fine α-SiC particulate was fundamental, sinterability apart, to enhancing the strength and oxidation resistance of ZrB₂. The latter property was tested at 1450 °C for 20 h in flowing dry air. In such oxidizing conditions, the formation of a thin external borosilicate glassy coating supplied partial protection for the faces of the material exposed to the hot environment. The oxidation attack penetrated into the material’s bulk and created a 200-μm-thick zirconia scale. The SiC particulate included in the oxide scale, lost by active oxidation, left carbon-based inclusions in the formerly occupied sites. PACS 81.05.Je; 81.20.Ev; 81.70.Bt     

Structural Characteristics and Photoluminescence of Zinc Oxide Nanocrystals

We have prepared zinc oxide nanocrystals using a new type of solid-state reaction.The nanosized grains were obtained by changing the heat treatment temperature and the sizes were determined to be from 10 to 42 nm.The lattice vibrational Raman spectra were measured and assigned at different grain sizes.We observed a strong visible emission centred at 580 nm in the nanosized ZnO at room temperature.We have investigated the photoluminescence properties under different grain sizes after annealing.The weakening of the luminescence is attributed to the decrease of intrinsic structural defects,mainly,oxygen vacancies and the increase of the grain size in the nanocrystals.     

Anti‐Stokes Luminescence of Cadmium Telluride Nanocrystals

For the first time, the antiStokes luminescence in colloidal solutions of CdTe nanocrystals on excitation below the absorption edge has been discovered. The maximum spectral shift to the shortwave region relative to the excitation energy E _con ^max = 319 meV is obtained for meansized nanocrystals (2.5 nm). The conversion efficiency of the absorbed radiant energy is 1.3·10^–2%. The rise in the antiStokes photoluminescence intensity with increasing temperature and the linear dependence on the exciting radiation intensity have been established. It is shown that the effect observed cannot be caused by twophoton excitation or by Auger recombination. It is assumed that the basic mechanism of the luminescence observed is the radiative recombination through the energy levels of the states attributable to the disturbance of the crystal structure in the nanocrystals.     

Large-scale synthesis of crystalline β-SiC nanowires

Large quantities of high-purity crystalline β-SiC nanowires have been synthesized at relatively low temperature via a new simple method, the chemical-vapor-reaction approach, in a home-made graphite reaction cell. A mixture of milled Si and SiC powders and C₃H₆ were employed as the starting materials. The results show that the nanowires with diameters of about 10–35 nm are single crystalline β-SiCwithout any wrapping of amorphous material, and the nanowire axes lie along the 〈111〉 direction. Some unique properties are found in the Raman scattering from the β-SiC nanowires, which are different from previous observations of β-SiC materials. A possible growth mechanism for the β-SiC nanowires is proposed. Received: 27 August 2002 / Accepted: 28 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-29/8491-000, E-mail: zjli-sohu@sohu.com     

Synthesis of α- and β-FeOOH iron oxide nanoparticles in non-ionic surfactant medium

Forced hydrolysis of Fe(III) ions in acidic media was performed under controlled synthetic conditions to produce α- and β-FeOOH iron oxides. The forced hydrolysis synthesis was carried out, separately, in an aqueous medium and the lamellar lyotropic liquid crystalline phase of a commercial non-ionic surfactant/water system. The FT-IR analyses confirmed formation of α- and β-FeOOH iron oxides in the aqueous and the surfactant media with slight formation of ferrihydrite and haematite. TEM micrographs have shown that particles formed in the lamellar lyotropic phase are smaller than those produced in the aqueous medium with their smallest size dimension being constrained in the nanometre scale with a size ranging between 5 and 100 nm. Particles produced in the nanoscale size appeared to have different optical properties compared to their counterparts produced in the microscale size.     

Simulation of Uniaxial Deformation of Magnesium Nanocrystals of “Rigid” and “Soft” Orientations

Physics of the Solid State - Atomistic simulation of high-rate deformation ($${v}$$ = 3 × 108 s–1) by compressing perfect and defect (5% of vacancies and 5% of hydrogen impurity atoms)...     

Low-Temperature Specific Heat of the Underdoped La2-xSrxCuO4 Single Crystals

We report the low-temperature specific heat of La2-xSrxCuO4(x=0.063) single crystals before and after annealing in the temperature region of 2-10K and magnetic fields up to 12T.The data confirm the √H dependence of the linear term coefficient γ predicted for d-wave paring in the Volovik theory at T=0 K.However,the data do not satisfy the proposed Simon-Lee scaling law at finite temperatures,i.e.CDOS/(T^2)=fT√H),where f is an unknown scaling function and CDOS is the extra specific heat contributed by the nodal quasiparticles due to the Doppler effect.Possible reasons for this failure are presented.