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.     

Manufacturing of cellular β-SiAlON/β-SiC composite ceramics from cardboard

Light-weight, cellular β-SiAlON/SiC ceramics were produced via dip-coating of an Al/Si-powder containing preceramic polymer slurry into corrugated cardboard. The coated cardboard preforms were pyrolyzed in Ar-atmosphere at 1200°C, where the cellulose fibres decomposed into carbon. Simultaneously the Al/Si melt infiltrated into the porous carbon and formed β-SiC. Subsequent nitridation at temperatures between 1200–1530°C resulted in the formation of a β-SiC-containing composite. Different pre-oxidation treatment resulted in a variation of the oxygen content in the solid solution phase (z = 0.6–1.2).     

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.     

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.     

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.     

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.     

Preparation and characterization of β-elemene-loaded microemulsion

Intravenously injectable emulsion of β-elemene was studied in detail. Both blank and β-elemene-loaded microemulsions were prepared using a simple water titration method. The pseudoternary phase diagram was constructed for the optimization of microemulsion. The loading capacity test, dilutability test, and especially the influence of antioxidants were conducted for further optimization of β-elemene-loaded microemulsion. Transmission electron microscope showed intact and spherical microemulsion droplets. Conductivity and viscosity measurements were used to study the phase behaviors of β-elemene-loaded microemulsions, providing convincing explanation. In vitro release study showed that β-elemene was steadily released until 12?h, which most fitted the first order.     

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.     

Synthesis and characterization of gold–polypyrrole film composite

Polypyrrole (PPy) coatings have potential applications in batteries, fuel cells, sensors, anti-corrosion coatings, and drug delivery systems. In this article, PPy film coating on the electrode of quartz crystal microbalance (QCM) was exposed to acidic aqueous HAuCl₄ solution. The reduction for gold ions took place and gold particles were produced at the film surface. The gold content at the PPy film was monitored by using QCM. The concentration of gold uptake increases as the original concentration of HAuCl₄ solution increases. The morphology of the film before and after the deposition of the gold particles was studied by the scanning electron microscopy coupled with energy dispersive X-ray spectrometry. The gold particles are of undefined shape and have diameters around 200–600 nm. However, the image of the composite powder shows that gold particles of sizes 100–120 nm are distributed over the surface of the polymer particles with some aggregation. Infrared spectroscopy and X-ray diffraction were used to characterize the composite.     

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.     

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.     

Spontaneous Magnetization of a Metal–Insulator Interface

Spontaneous magnetization of an eutectic In_0.15Ga_0.85-sapphire interface is discovered. Magnetic moment is directed from an insulator to a metal, its density corresponds to 1.8×10¹⁸ m⁻² localized spin-1/2 states.     

EPMA Study of the Interface of β-Tricalcium Phosphate Ceramies in Vivo

Electron probe and X-ray energy spectrum were used to investigate the chemical composition of the interface between material and new bone after porous tricalcium phosphate ceramie implanted in tibia of rabbits. The element changes of the interface, the materials transformation and the situation of new bone formation at different implantation period were observed. The results showed that the carbon element content decreased gradually in new bone tissue, and the content of calcium and phosphor element increased by degrees with the implantation time. At the same time, calcium-phosphor ratio in the new bone kept a higher Ievel. New bone grew into the materials interior, material dispersed and degraded simultaneously. Both composition of materials and new bone tended to be consentient. Finally, the materials were substituted by new bone. After implantation, not only the materials itself dissolved and degraded partially, but also new bone formed on the outer and pore surface of β-TCP porous bioceramics, which     

EPMA Study of the Interface of β-Tricalcium Phosphate Ceramics in Vivo

Electron probe and X-ray energy spectrum were used to investigate the chemical composition of the interface between material and new bone after porous tricalcium phosphate ceramic implanted in tibia of rabbits. The element changes of the interface, the materials transformation and the situation of new bone formation at different implantation period were observed. The results showed that the carbon element content decreased gradually in new bone tissue, and the content of calcium and phosphor element increased by degrees with the implantation time. At the same time, calcium-phosphor ratio in the new bone kept a higher level. New bone grew into the materials interior, material dispersed and degraded simultaneously. Both composition of materials and new bone tended to be consentient. Finally, the materials were substituted by new bone. After implantation, not only the materials itself dissolved and degraded partially, but also new bone formed on the outer and pore surface of β-TCP porous bioceramics, which showed that the degradation products of lifeless calcium phosphate inorganic materials took part in constituting of new bone tissue.     

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.     

Fabrication of biodegradable nano-sized β-TCP/Mg composite by a novel melt shearing technology

Biodegradable magnesium-matrix composites have attracted increasing interest for application in implant material fields. In this study, a new type of nano-sized β-tricalcium phosphate (β-TCP)/Mg–3Zn–Ca composite was proposed and produced using a novel melt shearing technology combined with high-pressure die casting (HPDC) process. The effect of the mixing methods on the distribution of β-TCP particles was investigated. Microstructure evolution during solidification process was analysed and the mechanical properties of the composite were also evaluated. Compared with the conventional mechanical stirring, the agglomerate phenomenon of the β-TCP particles in the matrix can be decreased by using the high shear unit and further decreased by melt shearing in the MCAST unit. The results also showed that the main constitutes in the matrix of the β-TCP/Mg–3Zn–1Ca composite are α-Mg and Ca₂Mg₆Zn₃ phase and most of the β-TCP particles are adjacent to the eutectic Ca₂Mg₆Zn₃ phase around the grain boundary. The average Vickers hardness, yield strength (0.2% YS), ultimate tensile strength (UTS), elastic modulus and elongation of as-cast of this composite are 79.0, 125.4 MPa, 150.0 MPa, 45.3 GPa and 2.85%, respectively.     

Effect of Solidification Interface Morphologies on Microstructures of a Ni-base Superalloy

A cast Ni-base superalloy K5 wasdirectionally solidified and various solidification in-terfaces including plane front,cellular,cellular-dendritic and dendritic were obtained in awider range of G/R ratio by using improved highwithdrawal device and liquid metal cooling experi-mental sets.The precipitation pattern of some prin-cipal phases of the alloy and correlation of the vari-ous interfaces with microstructure were studied sys-tematically.It was indicated that the morphology ofsolidification interface of superalloy K5 varied withG/R ratio and that the solidification interfacemorphologies have a considerable effect on the fea-tures of phases both precipitated duringsolidification and post-solidification.Plane frontand cellular directional solidification of superalloyK5 lead to a substantial decrease of MC carbideand elimination of γ-γ'eutectic,but makeneedle-shape M_6C carbide precipitate easily duringageing treatment.The finer dendritic structuressolidified under the condition of higher cooling ratehave less dendritic segregation and idealmicrostructure.     

A Density Functional Theory study of the chemical surface modification of β-SiC nanopores

The dependence of the electronic band structure and density of states on the chemical surface passivation of cubic porous silicon carbide (PSiC) is investigated by means of the ab-initio Density Functional Theory and the supercell method in which pores with different sizes and morphologies were created. The porous structures were modeled by removing atoms in the [0 0 1] direction producing two different surface chemistries; one with both Silicon (Si) and Carbon (C) atoms and the other with only Si or C atoms. The changes in the electronic band gap due to a Si-rich and C-rich phase in the porous surfaces are studied with two kind of surface passivation, one with hydrogen atoms and other with a combination between hydrogen and oxygen atoms. The calculations show that for the hydrogenated case, the band gap is larger for the C-rich than for the Si-rich case. For the partial oxygenation the tendency is contrary, by decreasing and increasing the band gap for the C-rich and Si-rich configuration, respectively, according to the percentage of oxygen in the pore surface.