In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.     

Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma

Ultrafine Si₃N₄ and Si₃N₄+ SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio-frequency plasma and an arc jet. The reactant, SiCl₄, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH₄ and/or NH₃, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si₃N₄ powder synthesis, reaction effieciency increased significantly by multistage injection compared to single-stage injection. The most striking result is that amorphous Si₃N₄ with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH₃/SiCl₄ molar ratio of ∼ 1.33, although the crystallinity depended on the NH₃/SiCl₄ molar ratio and the injection method. For the preparation of Si₃N₄+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH₃ and CH₄ reactant gases and the H₂ quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.     

Combustion Synthesis of Silicon Nitride-Silicon Carbide Composites

The feasibility of synthesizing silicon nitride-silicon carbide composites by self-propagating high-temperature reactions is demonstrated. Various mixtures of silicon, silicon nitride, and carbon powders were ignited under a nitrogen pressure of 30 atm (∼ 3 MPa), to produce a wide composition range of Si₃N₄-SiC powder products. Products containing up to 17 vol% of SiC, after being attrition milled, could be hot-pressed to full density under 1700°C, 3000 psi (∼ 21 MPa) with 4 wt% of Y₂O₃. The microhardness and fracture toughness of these composites were superior to those of the pure β-Si₃N₄ matrix material and compared very well with the properties of "traditionally" prepared composites.     

Joining of Silicon Carbide/Silicon Carbide Composites and Dense Silicon Carbide Using Combustion Reactions in the Titanium–Carbon–Nickel System

A new ceramic joining technique has been developed that utilizes an exothermic combustion reaction to simultaneously synthesize the joint interlayer material and to bond together the ceramic workpieces. The method has been used to join SiC/SiC composites and dense SiC ceramics using TiC-Ni powder mixtures that ignite below 1200°C to form a TiC-Ni joining material. Thin layers of the powder reactants were prepared by tape casting, and joining was accomplished by heating in a hot-press to ignite the combustion reaction. During this process, localized exothermic heating of the joint region resulted in chemical interaction at the interface between the TiC-Ni and the SiC ceramic that contributed to bonding. Room-temperature four-point bending strengths of joints produced by this method have exceeded 100 MPa.     

Synthesis of Aluminum Nitride–Silicon Carbide Solid Solutions by Combustion Nitridation

AlN–SiC solid solutions were synthesized via a combustion nitridation process. Reactions between powder mixtures of aluminum, silicon, and carbon or aluminum with β-SiC and gaseous nitrogen under pressures of 0.1–8.0 MPa are self-sustaining once they have been initiated. Investigations were made with reactant ratios of Al:Si:C = 7:3:3, 6:4:4, and 5:5:5 and Al:SiC = 7:3, 6:4, and 5:5. For the Al-Si-C system (molar ratio of 6:4:4), the maximum combustion temperature was dependent on the nitrogen pressure, increasing from 2300°C to 2480°C with an increase in pressure, from 0.1 MPa to 6.0 MPa. In all cases, the product contained the solid solution as the primary phase, with minor amounts of silicon. The amount of unreacted silicon decreased as the nitrogen pressure increased; the presence and dependence of unreacted silicon on pressure has been explained in terms of the volatilization of aluminum. The full width at half maximum for the (110) peak of the AlN–SiC solid solution decreased as the nitrogen pressure increased, which indicated the formation of a more homogeneous product.     

Production of Fine, High-Purity Beta Silicon Carbide Powders

Submicrometer SiC ( β -form) powders were synthesized by reacting silica and carbon black at temperatures between 1450° and 1800°C. Simultaneous application of vacuum and mixing provides the condition for full conversion of silica to SiC. It was shown that two different reaction mechanisms are possible, depending on the reaction temperature and the partial pressure of CO. At lower temperatures (below approximately 1400°C), the dominant mechanism for silicon carbide formation involves the solid-state reaction of silica and carbon. At higher temperatures (above approximately 1400°C), the dominant mechanism is the reaction between gaseous SiO and C. Above 1400°C, the rate of SiC formation is controlled by the rate of SiO formation. In as-synthesized form, the SiC powders typically contain < 0.2 wt% of unreacted silica and free carbon in the range between 6 and 15 wt%. Precise control of partial pressure of CO in the reaction chamber and continuous mixing of the reactants provide the conditions under which the rate of silicon carbide formation may be increased by one order of magnitude. The process is suitable for large-scale commercial production of SiC, requiring no postfabrication acid leaching or major milling.     

Laser Ablation Synthesis and Optical Characterization of Silicon Carbide Nanowires

Silicon carbide (SiC) nanowires were synthesized at 900°C by the laser ablation technique. The growth morphology, microstructure, and defects in SiC nanowires were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The Raman scattering study indicated that the Raman peaks corresponding to the TO and LO phonon modes of the SiC nanowires had larger red shifts compared to those of bulk SiC material. The red shift, broadening peak, and the asymmetry of the Raman peak could be explained by the size confinement effect in the radial and growth directions. The growth mechanism of SiC nano-wires was discussed based on the vapor–liquid–solid reaction.     

Combustion Synthesis of Silicon Carbide in Nitrogen Atmosphere

Fine SiC powders were synthesized by burning the mixed reactionts Si and C in a nitrogen atmosphere of 3 to 10 MPa. The exothermic synthesis reaction propagated spontaneously after igniting the reactants at room temperature. The SiC powders obtained had a uniform size distribution of about 0.2 μm. The combustion velocity was 0.8 to 1.5 mm / s. The maximum temperature measured at the reaction was 2500 K, which was higher than the adiabatic combustion temperature of SiC, but slightly lower than the decomposition temperature of Si₃N₄ under nitrogen pressure.     

Characterization of Silicon Carbide Whiskers

SiC whiskers from six manufacturers were characterized by bulk chemical techniques, X-ray photoelectron spectroscopy, X-ray diffraction, and scanning transmission electron microscopy or scanning electron microscopy. Major component (C, Si, and O) surface chemistries of the whiskers fell into four general categories: high oxygen content with oxide resembling a SiO₂, high oxygen content with oxide resembling a Si-O-C glass, and hydrocarbon. Several whiskers exhibited significant surface impurities—in particular, Fe. From a morphological viewpoint, significant differences in diameter, debris level, straightness, and types and quantities of defects were observed from one manufacturer to another.     

Synthesis and Characterization of Crystalline Silicon Carbide Nanoribbons

In this paper, a simple method to synthesize silicon carbide (SiC) nanoribbons is presented. Silicon powder and carbon black powder placed in a horizontal tube furnace were exposed to temperatures ranging from 1,250 to 1,500°C for 5–12 h in an argon atmosphere at atmospheric pressure. The resulting SiC nanoribbons were tens to hundreds of microns in length, a few microns in width and tens of nanometers in thickness. The nanoribbons were characterized with electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy, and were found to be hexagonal wurtzite–type SiC (2H-SiC) with a growth direction of [10[`1]0] [10bar{1}0] . The influence of the synthesis conditions such as the reaction temperature, reaction duration and chamber pressure on the growth of the SiC nanomaterial was investigated. A vapor–solid reaction dominated nanoribbon growth mechanism was discussed.     

氨基硅烷偶联剂表面改性SiC微粉的研究

采用3种氨基硅烷偶联剂WD-50、WD-52和WD-57对SiC微粉进行表面改性,制备出高固相含量、低粘度的SiC料浆,并通过红外光谱分析了SiC微粉改性前后的表面特性.结果表明,采用偶联剂WD-52改性的SiC微粉固相体积含量达到54.5%、料浆稳定性显著提高且粘度降低,与偶联剂WD-50和WD-57相比,WD-52的改性效果最好.     

Dispersion of Silicon Carbide Powders in Nonaqueous Solvents

Thirty-two pure solvents were used to disperse laser-synthesized SiC powder, oxidized laser-synthesized SiC powder, and commercially available SiC powder. Five-day sedimentation tests were used to screen the solvents. Relative turbidity of the supernatant after 1 month was used as a quantitative measure of the degree of dispersion. Coagulation kinetics were measured by photon correlation spectroscopy to determine the coagulation rate. Stabilized powders were centrifugally cast into ceramic green bodies and their green densities measured. Experimental dispersion results were correlated with various solvent properties including dielectric constant, hydrogen-bond index, acid dissociation constant (p K _a), and Lewis acid/base interaction energy. Microcalorimetry was used to measure the heat of wetting of the powders in various acidic and basic solvents. The heat of wetting was used to determine the Lewis interaction energy parameters for the powder surfaces. Oxidized SiC powder, either laser or commercial, was shown to have an acidic surface and was stabilized by basic solvents. Pure laser-synthesized SiC powder was shown to have a basic surface and was stabilized by acidic solvents. Solvents with high hydrogen-bond indices gave high packing densities. Other solvent properties had a much smaller influence on powder dispersibility. Good dispersibility gave ceramic green bodies with high green densities.     

Low‐Density Open Cellular Silicon Carbide Foams from Sucrose and Silicon Powder

Open cellular SiC foams with low densities were prepared by thermo‐foaming and setting (130°C–150°C) of silicon powder dispersions in molten sucrose followed by pyrolysis and reaction sintering at 1500°C. The bubbles generated in the dispersion by water vapor produced by the –OH condensation was stabilized by the adsorption of silicon particles on the air‐molten sucrose interface. The composition of a sucrose‐silicon powder mixture for producing SiC foam without considerable unreacted carbon was optimized. The sucrose in the thermo‐foamed silicon powder dispersion leaves 24 wt% carbon during the pyrolysis. The sintering additives such as alumina and yttria promoted the silicon‐carbon reaction. SiC nanowires with diameters in the range of 35–55 nm and length >10 μm observed on the cell walls as well as in the fractured strut region were grown by both vapor–liquid–solid and vapor–solid mechanisms. Large SiC foam bodies without crack could be prepared as the total shrinkage during pyrolysis and reaction sintering was only ~30 vol%. The relatively low compressive strength (0.06–0.41 MPa) and Young's modulus (14.9–24.2 MPa) observed was due to the large cell size (1.1–1.6 mm) and high porosity (93%–96%).     

Near-Stoichiometric Silicon Carbide from an Economical Polysilane Precursor

Na condensation of CH₃SiHCl₂ gives polysilanes of the type ((CH₃SiH) _x (CH₃Si) _y ) _n (where x + y = 1) whose pyrolysis in a stream of Ar gives SiC and a substantial amount of elemental Si. Treatment of such polysilanes with catalytic quantities of group-IV-metal–organometallic complexes, ((η-C₅H₅)₂ZrH₂) _n , (η-C₅H₅)₂Zr(CH₃)₂, (η-C₅H₅)₂ZrHCl, and (η-C₅H₅)₂-Ti(CH₃)₂, results in cross-linking processes such that pyrolysis of these polysilanes gives close to stoichiometric SiC (≥95 wt %) and only very little elemental Si.     

Characterization of Gold—Aluminum Nitride and Gold—Silicon Carbide Interfaces

Aluminum nitride and silicon carbide substrates were screen-printed with fritless gold and fired at 850°C in air. Interfacial diffusion zones up to 7 αm thick were observed, in which the concentrations of Au, Na impurities, and combined O varied together. Secondary ion mass and photoelectron spectroscopy revealed oxidized Al in the gold conductor supported by AIN. It is suggested that enhanced oxidation accompanies the diffusion of Au into the ceramics.     

Synthesis of Nanostructured Silicon Carbide through an Integrated Mechanical and Thermal Activation Process

Changes of crystal structures and microstructures of SiO₂ and graphite powder mixtures induced by high-energy milling, the effect of these changes on the reactivity of reactants, and the mechanism of enhanced SiC formation have been studied using a variety of analytical instruments, including X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, solid-state ²⁹Si nuclear magnetic resonance, and nitrogen adsorption (i.e., the BET method). High-energy milling before carbothermic reduction leads to substantial changes in the structural and energy states of the reactants, which in turn increases the reactivity of the reactants and enhances the formation of nanostructured SiC particles. Furthermore, the structural and energy-state changes contribute to the enhanced SiC formation through the increased reaction kinetics as well as the increased reaction driving force.     

Silicon Nitride–Silicon Carbide Nancocomposites Fabricated by Electric-Field-Assisted Sintering

Starting with Si-C-N(-O) amorphous powders, and using the electric field assisted sintering (EFAS) technique, silicon nitride/silicon carbide nanocomposites were fabricated with yttria as an additive. It was found that the material could be sintered in a relatively short time (10 min at 1600°C) to satisfactory densities (2.96–3.09 g/cm³) using 1–8 wt% yttria. With decreasing yttria content, the ratio of SiC to Si₃N₄ increased, whereas the grain size decreased from ∼150 nm to as small as 38 nm. This offers an attractive way to make nano-nanocomposites of silicon nitride and silicon carbide.     

High-Frequency Ultrasonic Characterization of Sintered Silicon Carbide

High-frequency 60- to 160-MHz ultrasonic nondestructive evaluation was used to characterize variations in density and microstructural constituents of sintered SiC bars. Ultrasonic characterization methods included longitudinal velocity, reflection coefficient, and precise attenuation measurements. The SiC bars were tailored to provide bulk densities ranging from 90% to 98% of theoretical, average grain sizes ranging from 3.0 to 12.0 μm, and average pore sizes ranging from 1.5 to 4.0 μm. Velocity correlated with specimen bulk density irrespective of specimen average grain size, average pore size, and average pore orientation. The attenuation coefficient was found to be sensitive to both density and average pore size variations, but was not affected by large differences in average grain size.     

Dense Layered Molybdenum Disilicide–Silicon Carbide Functionally Graded Composites Formed by Field-Activated Synthesis

Dense, layered, single- and graded-composition composites of MoSi₂ and SiC were formed from elemental powders in one step, using the field-activated pressure-assisted combustion method. Compositions ranging from 100% MoSi₂ to 100% SiC were synthesized, with relative densities ranging from 99% to 76%, respectively. X-ray diffractometry results indicated the formation of pure phases when the concentration of MoSi₂ was high and the appearance of a ternary phase, Mo_4.8Si₃C_0.6, when the concentration of SiC was high. Electron microprobe analysis results showed the formation of stoichiometric and uniformly distributed phases. A layer-to-layer variation in composition of 10 mol% was sufficient to prevent thermal cracking during formation of the layered functionally graded materials.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.