Sintering of 6H(α)-SiC and 3C(β)-SiC powders with B4C and C additives

The sintering behavior of three fine industrial SiC powders (two 6H()-type and one 3C()-type) has been comparatively investigated. The powders were pressureless sintered with B₄C and C additives between 1950°C and 2250°C in a high temperature dilatometer with flowing Ar atmosphere. The densification and shrinkage rate curves, polytype content, and grain growth were correlated with physical and chemical characteristics of starting powders. One of 6H()-type powders presented good sinterability only after extensive milling, even though it presented small average particle size, narrow particle size distribution and high specific surface area. The main difference in densification behavior among powders was the narrower shrinkage rate curve of -SiC powder, with its maximum shifted to higher temperature. Grain growth and phase transformation simultaneously occurred. In -SiC, 6H polytype partially transformed to 4H. This transformation was favored by aluminum impurity and resulted in a microstructure with more elongated grains. In -SiC, 3C transformed mainly to 6H, 15R and 4H, introducing many stacking faults which resulted in elongated SiC grains.     

Self-diffusion of14C in polycrystalline β-SiC

The¹⁴C self-diffusion coefficients for both lattice (D _lc ^* ) and grain boundary (D _bc ^* ) transport in high purity CVDβ-SiC are reported for the range 2128 to 2374 K. The Suzuoka analysis technique revealed thatD _bc ^* is 10⁵ to 10⁶ faster thanD _bc ^* ; the respective equations are given by $$\begin{gathered} D_{I c}^* = (2.62 \pm 1.83) \times 10^8 exp\left\{ { - \frac{{(8.72 \pm 0.14)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ D_{b c}^* = (4.44 \pm 2.03) \times 10^7 exp\left\{ { - \frac{{(5.84 \pm 0.09)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ \end{gathered} $$ A vacancy mechanism is assumed to be operative for lattice transport. From the standpoint of crystallography and energetics, reasons are given in support of a path of transport which involves an initial jump to a vacant tetrahedral site succeeded by a jump to a normally occupied C vacancy.     

On the possibility of β-C3N4 carbon nitride synthesis via C and N implantation into copper

The effects of high dose carbon and nitrogen implantations into copper on the type of chemical bonds and stoichiometry of the formed C–N phases are described. The results are compared with those obtained after nitrogen implantation into diamond like carbon (DLC) layers. The striking difference between the two experiments is the stoichiometry of covalently bonded C–N phase which corresponds to C₂N or to C₃N_3.7 for N implantation into DLC and C and N implantation into copper, respectively.     

Microstructure and fracture toughness of liquid-phase-sintered β-SiC containing β-SiC whiskers as seeds

The effect of seeding on microstructural development and fracture toughness of -SiC with an oxynitride glass was investigated by the use of morphologically rodlike -SiC whiskers. A self reinforced microstructure consisting of rodlike -SiC grains and equiaxed -SiC matrix grains was obtained by seeding 1–10 wt% SiC whiskers, owing to the epitaxial growth of -SiC from the seed whiskers. Further addition of seeds (20 wt%) or further annealing at higher temperatures led to a unimodal microstructure, owing to the impingement of growing seed grains. By seeding -SiC whiskers, fracture toughness of fine-grained materials was improved from 2.8 to 3.9–6.7 MPa · m^1/2, depending on the seed content.     

Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

Aligned ultra-long single-crystalline α-Si(3)N(4) nanowires

We report the synthesis of aligned ultra-long single-crystalline α-Si(3)N(4) nanowires by pyrolysis of a polymeric precursor without any template. The length of the wires is up to several centimeters, which is significantly longer than that of any Si(3)N(4) wires reported previously. Microscopy characterization reveals that the wires are single crystals, with a uniform diameter of ～200?nm. Intense visible photoluminescence was observed between 1.3 and 3.7?eV. The wires could be useful in the fabrication of optoelectronic nanodevices and nanocomposites.     

Chemical synthesis of faceted α-Fe2O3 single-crystalline nanoparticles and their photocatalytic activity

Faceted hematite nanocrystals have been synthesized via a hydrothermal route and their different morphologies can be tuned by appropriate stabilizer molecules. Detailed observation by high-resolution transmission electron microscopy and atomic force microscopy has revealed many terraces, steps, and kinks on the faceted surface of hematite nanoparticles, and thus, one growth mechanism of the terrace-step-kink model has been suggested to play a major role in determining the equilibrium morphology, together with effect of surface chemistry via the interaction between outer surfaces of iron and oxygen ions and functional groups. The photocatalytic activities were evaluated by decomposing rhodamine B dye. It has been shown that polyhedron hematite particles enclosed by high-index surface planes exhibited higher photoactivity. Density functional theory calculations revealed that the higher photoactivity originates from the more flat band edge in directions normal to the surface planes.     

Preparation,characterization and microwave absorption properties of bamboo-like β-SiC nanowhiskers by molten-salt synthesis

Bamboo-like and cubic single-crystalline silicon carbide nanowhiskers (SiC_NWs) were synthesized using multiwalled carbon nanotube via a process of calcination in the molten-salt circumstance. The system was heated to 1,250 °C and maintained for 6 h in argon atmosphere, and obtained the sample. The as-prepared sample was characterized by a series of techniques. Especially, the microwave absorption properties of SiC_NWs/paraffin composites (30 wt%) were investigated over 2–14 GHz. The result shows the optimal reflection loss can reach ?48.1 dB at 13.52 GHz when the thickness of the match is only 1.9 mm. The excellent microwave absorption properties of the SiC_NWs/paraffin composites due to the dielectric loss would make it as a promising candidate for the application of absorbing materials. In addition, a possible growth mechanism of SiC_NWs was also discussed.     

Growth,structural and optical characterizations of LiLa(1−x)Eux(WO4)2 single-crystalline fibers by the micro-pulling-down method

Transparent and uniform LiLa_(1?x)Eu_x(WO₄)₂ single-crystalline fibers where x = 0.005, 0.01, 0.03, 0.05, 0.07, 0.1, 0.15, 0.2, 0.25 and 1.0, with an average of 20–40 mm length were obtained by the micro-pulling-down method aiming structural and optical characterization. The optimum pulling rate was found to depend on the difference between alkali and rare earth ionic radii. Rietveld analysis from X-ray diffraction data and excitation spectroscopy at room temperature were applied to investigate the lattice changes due to the Eu³⁺ incorporation in the host. LiLa_(1?x)Eu_x(WO₄)₂ crystal fibers present red emission due to the electric dipole ⁵D₀ → ⁷F₂ transition under 395 nm excitation showing a concentration quenching around 20 mol% of doping. The excitation spectra of the ⁷F₀ → ⁵D₀ transition show small changes in the Eu³⁺ surroundings as function of dopant concentration.     

Formation of elongated particles in β-SiC compacts

The mechanism of bonding of β-SiC particles in a compact in a sintering process at relatively low temperatures has been investigated by TEM. The initial sign of bonding is observed at around 1800°C. At these temperatures, SiC particles are found to bond together only when their mutual orientations are the same. Similarly oriented grains at contact reorient themselves by rotation to the same orientation and then bond, thus forming elongated particles. As the temperature increases, the tendency to form elongated particles increases. The formation of boundaries between differently oriented particles is observed only at around 2100°C and above in similar compacts. The mechanism of bonding of particles in the same orientation at low temperatures is ascribed to high self-energy of dislocations common to covalent-bonded materials and, hence, the high energy of formation of grain boundaries.     

Self-diffusion of 30Si in polycrystalline β-SiC

The ³⁰Si lattice self-diffusion coefficients in high-purity β-SiC are reported for the temperature range 2283 to 2547 K and may be represented by the expression $$D_{Si}^* = (8.36 \pm 1.99) \times 10^7 \exp \left[ {\frac{{(9.45 \pm 0.05 eV atom^{ - 1} }}{{kT}}} \right]cm^2 \sec ^{ - 1} $$ . Decomposition of the sample at the grain boundaries prevented detection of diffusion along these paths of fast transport. Lattice diffusion of Si was concluded to occur by a mechanism involving a direct jump to the nearest Si vacancy without the previous occupation of a normally unfilled position. A comparison of C and Si diffusion in this material is also given.     

Synthesis of β-Mo(2)C thin films

Thin films of stoichiometric β-Mo(2)C were fabricated using a two-step synthesis process. Dense molybdenum oxide films were first deposited by plasma-enhanced chemical vapor deposition using mixtures of MoF(6), H(2), and O(2). The dependence of operating parameters with respect to deposition rate and quality is reviewed. Oxide films 100-500 nm in thickness were then converted into molybdenum carbide using temperature-programmed reaction using mixtures of H(2) and CH(4). X-ray diffraction confirmed that molybdenum oxide is completely transformed into the β-Mo(2)C phase when heated to 700 °C in mixtures of 20% CH(4) in H(2). The films remained well-adhered to the underlying silicon substrate after carburization. X-ray photoelectron spectroscopy detected no impurities in the films, and Mo was found to exist in a single oxidation state. Microscopy revealed that the as-deposited oxide films were featureless, whereas the carbide films display a complex nanostructure.     

Importance of viscosity parameters in electrospinning: Of monolithic and core–shell fibers

Electrospun polymeric fibers are attractive candidates in the development of scaffolds for the tissue engineering and for providing new systems for delivery of bioactive molecules. Co-axial fibers have emerged as an efficient tool to protect the core material from the adverse conditions of electrospinning process, to spin difficult-to-process fluids and to generate fibers with much more control of the delivery of encapsulated bioactive molecules. Currently, there is very little reported work on the optimization of the processing parameters of electrospinning, especially core–shell electrospinning. This study extends the understanding of the role of solution viscosity as a vital material parameter for electrospinning of fibers. The spinning solutions were characterized for viscosity and optical imaging of the compound Taylor cone for spinnability, and the fibers were imaged by Scanning Electron Microscopy (SEM). Our experimental results, using PLGA as the model polymer, confirm that the solution concentration be above the entanglement concentration (C_e) to obtain uniform beadless monolithic fibers; for core–shell fibers, the shell solution must fulfill the above criterion for spinnability and, further, the ratio of the viscosities of core and shell solutions (η_core/η_shell) has to be greater than a threshold value to get a stable compound Taylor cone and therefore to obtain uniform beadless core–shell fibers. Addition of surfactant led to reduction of the threshold η_core/η_shell (from 0.55 to 0.18) for the PVA–PLGA system.     

Dielectric behavior of β-SiC nanopowders in air between 30 and 400 °C

Silicon carbide (SiC) is regarded as a semiconductor and thus characterized mainly for its electrical conductivity. However, SiC does exhibit significant electrical resistance at low ambient temperatures and represents a possible dielectric insulator. In this paper, the dielectric properties of the β-SiC nanopowders were examined by X-ray diffraction and dielectric spectroscopy within the humid Malaysian environment. Research emphasis is placed on the stable dielectric behavior of the nanopowder itself as the nanopowder phase is susceptible to hydroxyl oxidization as mentioned by the nanopowder manufacturer. The XRD results identified the presence of β-SiC peaks whereas EDX detected minor oxygen presence in the nanopowder. Dielectric permittivity response of the nanopowder pellet indicated stable Quasi-DC dielectric behavior from 30 to 400 °C with minor increments of the initial relative dielectric permittivity at the lower temperatures. The relative dielectric permittivity of the SiC nanoparticles was determined to be 44 (30 °C) to 31 (400 °C) at 1 MHz. Arrhenius plot of the dielectric data resulted in a two linear energy activation plots due to possible hopping mechanisms within the SiC nanoparticles covalent structure. Overall, the β-SiC nanopowder exhibited a stable Quasi-DC behavior at the measured temperatures.     

Growth characteristics of β-SiC by chemical vapour deposition

A twin-plane re-entrant corner effect (TPRE) in growth of chemical vapour deposited (CVD) -SiC is described by the film and particles of gas-phase homogeneous nucleation. The structural morphology has been characterized by scanning electron microscopy and transmission electron microscopy. Morphological characteristics of the deposited crystals, such as triangularity, hexagons or facets have been explained in terms of the re-entrant corner effect at twin junctions, which were proposed as preferential growth sites for perfect crystals. For real deposits, screw dislocations and/or the re-entrant corner effect are not expected to be compatible. The majority of chemical vapour deposited SiC crystals have a high defect density comprised of {111} twins and dislocations associated with the process variables. Infrared transmission spectra and electron spectroscopy of chemical analysis indicated that the major chemical bonds of CVD -SiC were Si-C and C-H bonds. The positions of the 1s or 2p corelevel peaks for deposits are described.     

Processing and Properties of Al2O3-ZrO2(3Y)-SiC Nanocomposites

本文研究了非均相沉淀法制备 Al     

Strength variations of reaction-sintered SiC heterogeneously containing fine-grained β-SiC

Strength variations of reaction-sintered SiC were examined to determine the effect of the volume fraction of fine-grained -SiC domain. The fracture strength significantly decreased with an increase in the volume fraction of the -SiC domain, and eventually fell to the strength of the -SiC domain alone. Furthermore, a substantial difference in the crack deflection was found between the indentation microfracture formed in a structure containing fine-grained -SiC and that in the typical structure of reaction-sintered SiC. This showed that the fracture toughness decreased on account of the -SiC layer.     

Microstructural characterization of amorphous SiO2 wrapped β-SiC nanowhiskers

Wrapped composite nanowhiskers were prepared during the carbothermal reduction of silica. SEM, TEM and HREM analyses have showed that each wrapped composite nanowhisker consists of an internal β-SiC nanowhisker with a uniform amorphous SiO₂ wrapper on the outside surface.     

Synthesis of β-SiC nanowhiskers by high temperature evaporation of solid reactants

β-SiC nanowhiskers were synthesized in large scale by evaporating the solid mixtures of silicon and silicon dioxide in a graphite crucible heated by a high-frequency induction system. Carbon source used for formation of the nanowhiskers came from the cheap common high-purity graphite at 1600 °C. XRD and TEM show that the nanowhiskers are crystalline β-SiC, and have diameters ranging from 15 to 50 nm and length up to several micrometers. Most of the nanowhiskers were wirelike and some nanowhiskers have high density stacking faults in the structure. The normal direction of the stacking layers ([111]) tilts by 12° with respect to the growth orientation ([223]). The growth mechanism of nanowhiskers is based on the reaction between silicon monoxide and carbon monoxide.