Ultrafine SiC Powders Research Articles

Iron removal from ultra-fine silicon carbide powders with ultrasound-assisted and its kinetics

In the present work, the occurrence form of iron impurities of SiC crystals were analyzed using the field emission scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), and the leaching kinetics of iron from the ultra-fine SiC powders in the acid system with ultrasound-assisted was also examined. The effects of the different parameters such as the types of acid, acid concentration, leaching temperature, reaction time, stirring speed, ultrasound frequency and power affecting on the iron removal efficiency were comprehensively studied to obtain an optimum condition. The results indicated that iron impurities are determined to exist in the form of FexSiy and Fex-Siy-Tiz phases in SiC crystals. Under the optimized conditions with ultrasound, the removal rate of iron can reach more than 92%. The leaching kinetics fit a homogeneous control model with the second order model as the rate-controlling step, and the activation energy of the ultrasound-assisted leaching process is determined to be 53.6 kJ/mol. The presence of the ultrasound can accelerate the leaching process and improve the iron removal efficiency, compared to conventional stirring.

Low temperature synthesis of 2H-SiC powders via molten-salt-mediated magnesiothermic reduction

A novel molten-salt-mediated magnesiothermic reduction method was used to prepare 2H-SiC ultrafine powders by using silicon dioxide, active carbon and magnesium powders as raw materials. Phase compositions in the SiO2-2Mg-C system were thermodynamically evaluated by using the FactSage 6.2 package, and the effects of firing temperature, molar ratio of n(C)/n(Si) and amount of Mg on the synthesis of 2H-SiC powders were discussed. Phase compositions and microstructures of as-prepared ultrafine SiC powder were characterized by using X-ray diffraction (XRD), field emission gun scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). SiO2 could be completely converted into SiC after 3h at 1073K, this temperature was about 600K lower than required by the conventional methods, and the relatively content of 2H-SiC in the final SiC sample reached about 45wt%. Furthermore, the content of 2H-SiC could be increased to 72wt% upon firing at 1373K for 3h and using about 60wt% excessive Mg. In contrast to this, only 20wt% SiC was formed in the sample fired under the identical conditions but without using a molten salt medium. The average crystalline sizes of 2H-SiC in the product powders resultant from 3h firing at 1073 and 1373K were about 24 and 39nm, respectively. A formation mechanism of SiC powder in the case of using molten-salt-mediated magnesiothermic reduction was proposed based on the experimental results.

A Newly Developed Self-Bonded SiC Refractory

Based on SiC aggregate, SiC powder and ultra-fine SiC powder as the main starting materials and B4C as additive, the newly-developed self-bonded SiC material was pressed and fired at 2200°C for 10 hours in Ar environment in the high-temperature furnace. The phase composition and microstructure were investigated. In the paper, traditional self-bonded SiC material and Si3N4-bonded SiC material were taken for comparison with the newly-developed self-bonded SiC material in terms of strength, thermal conductivity, cryolite resistance, molten alkali resistance and oxidation resistance etc. The results show that the newly-developed self-bonded SiC material contains 98.42% α-SiC and presents obvious higher thermal conductivity than traditional self-bonded SiC material and much better cryolite resistance, molten alkali resistance and oxidation resistance than traditional self-bonded SiC and Si3N4-bonded SiC materials.

Combustion synthesis of ultra-fine SiC powders in low pressure N2-atmosphere

Ultra fine SiC nano-powders (100–300nm) of high purity were successfully produced by combustion of a powder mixture of Si and C, with the addition of poly-tetra-fluoro-ethylene (PTFE) as a chemical stimulator, in a moderately pressurized nitrogen atmosphere (1–10MPa). The experimental results showed that with the aid of mechanical activation of the starting powders, a small amount of PTFE (1.5wt%) can effectively stimulate the reaction between Si and C. Both the experimental results and thermodynamic calculations indicate that the formation of Si3N4 plays a key role in the process. The optimum conditions for producing the aforementioned SiC fine powders were 1.5wt% PTFE, 1MPa N2 pressure and no addition of diluents of SiC powder.

Fabrication of silicon carbide composites with carbon nanofiber addition and their fracture toughness

The authors have examined the fabrication conditions of SiC composites containing carbon nanofiber, i.e., vapor-grown carbon nanofiber (VGCF), to enhance the fracture toughness. Commercially available ultrafine SiC powder (specific surface area: 47.5 m2 g−1) was mixed with VGCF and sintering aid in the Al4C3–B4C system. Approximately 1.5 g of the mixture was uniaxially pressed at 50 MPa to obtain a compact with a diameter of 20 mm and a thickness of approximately 1.5 mm. The resulting compact was hot-pressed at 1800 °C for 1 h in Ar atmosphere under a pressure of 62 MPa. The relative density of hot-pressed SiC composite decreased from 98.0 to 96.3%, whereas the fracture toughness was enhanced from 3.8 to 5.2 MPa m1/2, as the amount of VGCF increased from 0 to 6 mass%. Furthermore, an acid treatment of VGCF was conducted to enhance its dispersibility within the SiC matrix, owing to the formation of COO− groups on the VGCF surface. As a result of this treatment, the relative density and fracture toughness of hot-pressed SiC composite with 6 mass% acid-treated VGCF addition increased to 99.0% and 5.7 MPa m1/2, respectively.

Preparation of Ultrafine SiC Powders by Carbothermal Reduction

SiC powder was synthesized by carbothermal reduction method using silica sol and amylum as reactants in closed and open graphite crucibles, respectively. The phase composition and morphology of the powders were characterized. Formation of phase-pure silicon carbide can be achieved at 1700°C for 1h. Increasing reaction temperature and extending holding time can greatly promote the production of SiC. The products synthesized in open graphite crucibles are mainly composed of equiaxial β-SiC particles while those synthesized in closed graphite crucibles are β-SiC particles and nanorods.

Densification and grain coarsening during pulse electric current sintering of an ultrafine SiC powder

Densification and grain coarsening during pulse electric current sintering of an ultrafine SiC powder

Simulation of CO2 laser pyrolysis during preparation of SiC nanopowders

Ultrafine SiC powders of highly controlled properties are produced in a CO2 laser flame with acetylene/ethylene and silane as gas phase precursors. In the present work, the basic theoretical concepts of the vibrational excitation of the reactant polyatomic molecules by IR-radiation of CO2 laser-induced pyrolysis are presented. A key parameter for multi-photon excitation and dissociation processes is the mean number of photons absorbed per molecule. A detailed analysis of the average number of absorbed photons during infrared multi-photon absorption process is given. In polyatomic molecules the density of vibrational states grows rapidly with the energy due to the permutations and combinations of vibrational modes increases rapidly with the available energy. An important parameter for a quantitative description of the vibrational quasi-continuum that is the density of vibrational states is calculated. The transitions between the different vibrational states are described in detail in terms of simple kinetic or rate equations.

Influence of Additives on Compaction Behaviour of SiC Powders

Using ultrafine SiC powder as the model system in the present work, an attempt has been made to analyze the influence of additives (Al, C, Si3N4) on green density. The pressure-density plots were generated for the uniaxial mode of compaction in the range of 30-300 MPa, for both pure powders and those containing 20 wt% additives. A linear dependence was observed for the pure system which is characteristic for a soft agglomerate powder mass. In contrast, for the additive-containing powder, the density profile deviated from linearity above a critical pressure. From these plots, the effective agglomerate strength was evaluated as 196 MPa for the powder mix employed. Change in density on subsequent isostatic pressing was also studied for both types of powders. The dual compaction treatment was observed to be more effective for the multicomponent system in terms of percentage enhancement in green density.

Thermoelectric properties of SiC thick films deposited by thermal plasma physical vapor deposition

SiC thick films of about 300 µm could be prepared with a deposition rate above 300 nm/s by thermal plasma physical vapor deposition (TPPVD) using ultrafine SiC powder as a starting material. The thermoelectric properties were investigated as a function of composition and doping content. The nondoped films showed n-type conduction. Although the Seebeck coefficient reached as high as -480 µV/K, the power factor was only around 1.6 × 10-4 Wm-1 K-2 at 973 K due to the relatively high electrical resistivity. In order to reduce the electrical resistivity and to deposit layers with n-type and p-type conduction, N2, B and B4C were selected as the dopants. Nitrogen-doped samples exhibit n-type characterization, B and B4C-doped samples exhibit p-type characterization, and the electrical resistivity decreased from 10-2–10-3 to 10-4–10-5 Ωm after doping. The maximum power factor of the nitrogen-doped SiC and the thick films deposited with B4C powder reached 1.0 × 10-3 and 6.4 × 10-4 Wm-1 K-2 at 973 K, respectively.© 2003 Elsevier Science Ltd. All rights reserved.

Ultrafast thermal plasma physical vapor deposition of thick SiC films

Thick silicon carbide films have been successfully deposited at a deposition rate of 125 nm/s on stationary graphite substrates by the thermal plasma physical vapor deposition technique, with ultrafine SiC powder fed into a hybrid plasma jet and completely evaporated. The relationship between the processing parameters and the morphology, deposition rate, composition and crystal structure has been investigated under the typical conditions of substrate temperature in the range of 1400–1700 °C and chamber pressure of 250 Torr, and compared with the results of rotating substrate deposition at the substrate temperature of around 750 °C. It was found that the deposition rate and composition showed different processing parameter dependences for rotating substrate deposition and stationary substrate deposition. The films showed dense cross-sections or cauliflower-like structures depending on the deposition conditions.©2003 Elsevier Science Ltd. All rights reserved.

Thermal Plasma Physical Vapor Deposition of Nanostructured SiC Coatings

ABSTRACTNanostructured and thick SiC coatings have been successfully deposited on Si and graphite substrates by thermal plasma physical vapor deposition (TPPVD) using ultrafine SiC powder as a starting material. The control of processing parameters such as substrate temperature, composition of plasma gases, permits to the deposition of SiC coatings with a variety of microstructures and with various morphologies from dense to columnar. The maximum deposition rate reached 200 nm/s. Seebeck coefficient up to −480 μV/K was obtained for the non-doped coatings with stoichiometric composition. Nitrogen doping to the coatings made it possible to decrease the electrical resistivity from 10-2∼10-3 to 10-4∼10-5 Ωm and showing the maximum power factor of 1.0×10-3 Wm-1K-2 at 973 K.

High-rate deposition of nanostructured SiC films by thermal plasma PVD

With ultrafine SiC powder as starting material, thermal plasma physical vapor deposition has been applied successfully to the deposition of SiC films on Si substrates. The control of processing parameters such as substrate temperature, powder feeding rate and composition of plasma gases, permits the deposition of SiCfilms on a wide area of around 400 cm2 with a variety of microstructures fromamorphous to nanostructured and with various morphologies from dense to columnar. For the nanostructured case, the crystallite size was between 3 and 15 nm and the maximum deposition rate calculated based on the actual deposition duty time reached 200 nm/s. The deposition mechanism is discussed briefly.

Densification and microstructural developments during the sintering of aluminium silicon carbide

Densification and microstructural developments during the sintering of aluminum silicon carbide (Al4SiC4) were examined. Two types of Al4SiC4 powders were prepared by the solid-state reactions between: (i) Al, Si, and C at 1600°C for 10 h (designated as Al4SiC4(SSR)), and (ii) chemically-vapour deposited ultrafine Al4C3 and SiC powders at 1500°C for 4 h (Al4SiC4(CVD/SSR)). The specific surface areas of the Al4SiC4(SSR) and Al4SiC4(CVD/SSR) powders were 2.7 and 15.5 m2 · g−1, respectively. Relative densities of the pressureless-sintered Al4SiC4(SSR) and Al4SiC4(CVD/SSR) compacts were as low as 60–70% for firing temperatures between 1700°C and 2000°C. The relative densities of Al4SiC4(SSR) and Al4SiC4(CVD/SSR) compacts could be enhanced using the hot-pressing technique; the relative density of the Al4SiC4(SSR) compact hot-pressed at 1900°C for 3 h was 97.0% whereas that of the Al4SiC4(CVD/SSR) compact hot-pressed at 1900°C for 1 h attained 99.0%. The former microstructure was composed of plate-like grains of width 10–30 μm and thickness ∼10 μm whilst the latter microstructure was comprised of equiaxed grains with a typical diameter of ∼10 μm. Densification of the Al4SiC4(CVD/SSR) compacts appeared to be promoted compared to the Al4SiC4(SSR) compact and this was attributed to the higher surface area, reduced agglomeration of the starting primary particles, and more homogeneous chemical composition.

Pretreatment process of SiC particles and fabrication technology of SiC particulate reinforced Zn–Al alloy matrix composite

In the present paper, a novel pretreatment process for SiC particulate and a new mechanical–electromagnetic combination stirring process for fabricating Zn–Al(ZA27)/SiCp composites are described. The optimal pretreatment route and the most appropriate SiC particle parameters were experimentally determined. The pretreated SiC particles were easily incorporated and dispersed in the ZA27 alloy melt and were not agglomerated before addition to the melt. The surface status of the SiC particles before and after pretreatment was observed and analysed by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. It was found that gas existing on the SiC particle surfaces by physical and chemical absorption was a significant hindrance to the incorporation and dispersion of SiC particles in the alloy melt. The gas absorption was induced by ultrafine SiC powders, fracture steps, and ions existing on the SiC particle surfaces. The carbon, silicon, and oxygen contents on the SiC surface were varied with different pretreatment techniques. Moreover, a dense layer of amorphous SiO2, which improves wetting of SiC particles in the ZA27 melt, was formed owing to calcination of SiC particles in air. The new combined stirring process exploits the advantages of both mechanical and electromagnetic stirring of the melt at the different processing stages during fabrication. The microstructural characteristics of the resulting composites are: homogeneously distributed SiC particles, fewer macro gas blows and inclusions, and little shrinkage porosity in comparison to composites fabricated by a mechanical stirring process. Finally, the mechanisms of degassing and reducing the porosity and the number of oxide inclusions are discussed.

Effects of excess carbon and vibrational properties in ultrafine SiC powders

Structural and vibrational properties are investigated in SiC nanopowders synthesized by a CO 2 laser pyrolysis of (SiH 4 , C 2 H 2 ) gaseous mixture and thermally treated up to 1800 °C. The structural modifications of the SiC crystallites and the arrangement of the carbon in excess are monitored at different annealing stages. A critical behaviour is revealed in the powder annealed at T a = 1500 °C through the features of the Raman spectra and the insulating-conductor transition. The significant electric conductivity ( σ ≈ 0.05 S cm −1 ) which appears above T a is discussed with respect to the powder composition and the interface effects where the carbon in excess seems to play a key role.

Effect of Supercritical Drying Method on Microstructure and Properties of Al2O3 Composites Reinforced with Ultrafine SiC Particulate.

Mixtures of 95vol% ultrafine γ-Al2O3 powder and 5vol% ultrafine SiC powder, with average particle size of 40nm, were mixed wet-ball milling and prepared by drying their slurry with a supercritical drying method and a microwave drying method. Al2O3 specimens dispersed with ultrafine-SiC particles as the secondary phase were hot-pressed, their powders being prepared by both the above drying methods. Difference between the superitical drying and the microwave drying method on the microstructure of Al2O3/5vol% SiC composites was investigated. Dense Al2O3/SiC composites were obtained at lower hot-pressing temperatures when the mixture was prepared by supercritical drying method. Influence of the drying method on the γ- to α-Al2O3 transition behavior was also examined. Al2O3/SiC powder dried by the supercritical drying method showed narrow particle size distribution and the formation of soft agglomerates. Al2O3/SiC composite sintered by hot-pressing the powder dried with the supercritical drying method indicated relatively higher strength, whereas the strength of Al2O3/SiC composite through microwave drying method was low and had the large scatter.

Effect of gas flow rate on ultrafine SiC powders synthesized through chemical vapor deposition in the SiH 4-C 2H 4-H 2 system

The renewed interest in the study of nanometer-scale materials has triggered intensive research on the synthesis of powders with improved properties. The characteristics of the starting powder are of paramount importance in the control of sintered microstructure and the resultant physical properties of the bulk body. Powders suitable for subsequent forming and consolidation should possess the following features: small size with a narrow size distribution, equiaxed morphology, highly pure chemical composition, single phase and nonagglomerated state. SiC ceramic is an important structural as well as functional material because of its strength, hardness, resistance to chemical corrosion, high thermal conductivity and good electrical insulation. SiC powders with desired particle size and composition can be synthesized by a direct gas phase reaction process which offers many technological advantages over other formation methods in terms of small particle size, narrow size distribution, relatively pure chemical composition as well as good reproducibility. Though the roles of other parameters have been well documented and widely accepted by many investigators, the effect of gas flow rate still remains to be clarified. In his work, Yang concluded that the size of powders changes as a parabolic function of gas flow rate, while the morphology and composition ofmore » the powders are almost unaffected. However, present results indicate that both the size and morphology of the SiC powders obtained can be altered by modifying the gas flow rate, for which the underlying mechanisms have been explored.« less

Oxidation behaviour of alumina–silicon carbide nanocomposites

Oxidation studies were conducted on Al2O2–SiC nanocomposites at 1400 °C. The composites were prepared by hot-pressing mixtures of commercial alumina and ultrafine SiC powders, in amounts of 5, 15 and 30 vol %. Linear kinetics were detected for the oxidation of composites containing 5 vol % SiC. Two stages were observed in composites containing 15 vol % SiC: the first linear and the second presumably parabolic. A parabolic behaviour was observed in the sample containing 30 vol % SiC. The oxidation rates were several orders of magnitude higher than those of monolithic SiC and the observed data were not consistent with the expected increase in weight associated with the oxidation reaction of SiC to SiO2; in fact the most surprising feature is that the sample containing 30 vol % SiC showed a better oxidation resistance than samples containing 5 and 15 vol % SiC. The reaction products were alumina and mullite in samples with 5 and 15 vol % SiC, while mullite and silica were found on the oxidized surface of samples containing 30 vol % SiC. Explanations are given of the influence of the oxidizable phase amount, the presence of impurities, reaction product structure and composition.

Synthesis of Ultrafine SiC Powders from Carbon-Silica Hybridized Precursors with Carbothermic Reduction

Inorganic-organic hybrid gels were prepared by simultaneous condensation of the liquid mixture of ethyl silicate, ethyl borate and water soluble phenol resin with low molecular weight. The liquid mixtures were changed into lightly colored transparent gels after a catalyst addition and stirring. The obtained gels were crushed, dried and fired in controlled conditions to yield the inorganic precursors. Since the tailored inorganic precursors mainly consisted of SiO2 and carbon, silicon carbide was formed with the heat treatments in an Ar atmosphere beyond 1773 K with carbothermic reduction. The properties and morphology of the formed silicon carbide powders in terms of the starting precursor compositions and the conditions of the carbothermic reduction were investigated with SEM, XRD and TG-DTA analysis.