Preparation of silicon carbide powders by chemical vapour deposition of the SiH4-CH4-H2 system

Chemical vapour deposition (CVD) of the SiH₄ + CH₄ + H₂ system was applied to synthesize-silicon carbide powders in the temperature range 1523 to 1673 K. The powders obtained at 1673 K were single-phase-SiC containing neither free silicon nor free carbon. The powders obtained below 1623 K were composite powders containing free silicon. The carburization ratio (SiC/(SiC + Si)) increased with increasing reaction temperature and total gas flow rate, and with decreasing reactant concentration. The average particle sizes measured by TEM ranged from 46 to 114nm, The particle size increased with the reaction temperature and gas concentration but decreased with gas flow rate. The-SiC particles obtained below 1623 K consisted of a silicon core and a-SiC shell, as opposed to the-SiC particles obtained at 1673 K which were hollow. Infrared absorption peaks were observed at 940 and 810 cm^–1 for particles containing a silicon core; whereas a single peak at about 830 cm^–1 with a shoulder at about 930 cm^–1 was observed for the-SiC hollow particles. The lattice parameter of-SiC having a carburization ratio lower than 70 wt%, was larger than that of bulk-SiC and decreased with the increasing carburization ratio. However, when the carburization ratio exceeded 70 wt%, the lattice parameter became approximately equal to that of bulk-SiC.     

Synthesis of beta silicon carbide powders using carbon coated fumed silica

The synthesis of beta silicon carbide (-SiC) powders by carbothermic reduction of carbon coated silica and silica mixed with carbon black was investigated. The production of -SiC powders by using carbon coated silica consists of two steps. The first step is to prepare the carbon coated silica precursor by coating fumed silica particles with carbon by pyrolytic cracking of a hydrocarbon gas (C3H6). This provides intimate contact between the reactants and yields a better distribution of carbon within the fumed silica. Fumed silica was also mechanically mixed with carbon black for comparison. Both starting mixtures were reacted in a tube furnace for 2 h at temperatures of 1300°C to 1600°C in 1 l min-1 flowing argon. The reaction products were characterized using weight loss data, X-ray diffraction (XRD), a BET surface area analyser, oxygen and free carbon analysis and transmission electron microscopy (TEM). The carbon coating process resulted in a more complete reaction, purer product and high yield SiC powders with very little agglomeration at temperatures of 1500°C and 1600°C. The -SiC powders produced at 1600°C for 2 h in argon gas flow have oxygen content of 0.3 wt%, a very fine particle size 0.1–0.3 m and uniform shape. © 1998 Chapman & Hall     

Mechanical properties and microstructure of β-SiAION-β-SiC composites by pressureless sintering

-SiAION--SiC composites containing up to 12 wt% -SiC were prepared by pressureless sintering. The strength of composites at room temperature remained relatively unchanged, whereas strength at 1200 °C increased for composites. The fracture toughness (K _IC) for composites was higher than that for -SiAION ceramics. The maximum value was 5.4 MPa m^1/2 for 6 wt% -SiC, and this was an improvement of 15% in comparison with -SiAION ceramics. From SEM observations, an improvement inK _IC values was attributed to crack deflections and branching-off of cracks. Intra-granular fractures were frequently observed in -SiAION. From TEM observations, -SiAION crystals were nanocomposites, within which existed the fine crystals in -SiAION crystal. For composite, -SiAION and -SiC crystals were directly in contact. The mismatching zone was observed in -SiC.     

Transmission electron microscopy study of the formation of a contamination layer on the surface of porous silicon

During observation of porous silicon by transmission electron microscopy a thin contamination layer was formed. This process was ten times faster for porous silicon than for the crystalline material. The aim of this work was to identify the material of the contamination layer and to explain mechanism(s) of rapid changes in spot patterns during electron beam exposure. The material of the layer was identified -SiC. The mechanism of formation of the -SiC layer is discussed in detail.     

Infrared absorption ofβ-SiC particles prepared by chemical vapour deposition

Infrared absorption characteristics of-SiC particles prepared by chemical vapour deposition were studied. These particles were either solid or had a silicon core and/or were hollow. The solid particles exhibited a single absorption peak between the transverse optical frequency (_TO = 794 cm^–1) and the longitudinal optical frequency (_LO = 976 cm^–1) of-SiC. This absorption peak shifted to a lower frequency with increasing lattice parameter of-SiC and increasing free silicon content. The particles containing a silicon core and/or were hollow exhibited double absorption peaks close to _TO and _LO. The peak at the LO side shifted to a lower frequency and that at the TO side to a higher frequency with decreasing silicon core size and increasing hollow size. Using the calculations based on the effective medium theory assuming surface phonon mode, the relationship between the infrared absorption characteristics and microstructures of the-SiC particles are explained.     

The formation ofβ-SiC fibres with SiO2-C-NaF(AlF3) components

The formation of-SiC fibres with SiO₂-C-NaF(AIF₃) components was investigated. It was found that the formation of a longer-SiC fibre was governed by the mole ratio of C/SiO₂ or C/NaF. Using a mole ratio for C/SiO₂ or C/NaF of 3 or more,-SiC fibres of length 3 mm were prepared in a closed system. On the other hand, short-SiC fibres were obtained in an open system.-SiC fibres prepared under the various experimental conditions were stable when heated in a high-concentration acidic solution such as HCl or H₂SO₄, and in an alkaline solution such as NaOH.     

Conversion of polycarbosilane (PCS) to SiC-based ceramic Part II Pyrolysis and characterisation

The conversion to ceramic of a commercial polycarbosilane (PCS) under various pyrolysis conditions has been investigated. The products of pyrolysis have been characterised by solid state ²⁹Si and ¹³C NMR spectroscopy and X-ray diffraction (XRD). Some of the phases identified in the present study were found to differ from those reported previously, particularly in the earlier literature. Oxidation-cured PCS, when pyrolyzed up to 1400 °C in argon, generally produced silicon oxycarbide (SiO _x C _y ) as the second major phase with -SiC as the major phase, and smaller amounts of free carbon. With increasing temperature above 1200 °C, the silicon oxycarbide phase decomposed to give -SiC. Silica (SiO₂) was also found to evolve from this silicon oxycarbide phase. Loss of some of the silica, probably by reaction with carbon, was found at 1400 °C, possibly yielding SiO, CO and SiC. At 1500 °C, crystalline -cristobalite was found as a minor phase with -SiC as the major phase and a lower amount of free carbon. Pyrolysis in vacuum leads to production and crystallization of -SiC at a lower temperature than required if pyrolyzed in argon flow. After pyrolysis at 1600 ° in vacuum, the cured PCS converted to almost stoichiometric -SiC.     

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from to -SiC reveals a dependency on the initial -content and the sintering atmosphere. The transformation rate decreases with an increasing -content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure -SiC starting powders when the additive system consists of 10.34 wt% Y2O3 and 2.95 wt% AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial /-ratio (>1:9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.     

Preparation of silicon carbide powders by chemical vapour deposition of the (CH3)2SiCl2-H2 system

Silicon carbide (SiC) powders were prepared by chemical vapour deposition (CVD) using (CH₃)₂SiCl₂ and H₂ as source gases at temperatures of 1273 to 1673 K. Various kinds of SiC powders such as amorphous powder, -type single-phase powder and composite powder were obtained. The composite powders contained free silicon and/or free carbon phases of about a few nanometres in diameter. All the particles observed were spherical in shape and uniform in size. The particle size increased from 45 to 130 nm with decreasing reaction temperature and gas flow rate, as well as with increasing reactant concentration. The lattice parameter of the -SiC particles decreased with increasing reaction temperature. All the lattice parameters were larger than those of bulk -SiC.     

Preparation of nanocrystal SiC powder by chemical vapour deposition

Nanosized silicon carbide powders of high purity and low oxygen content have been prepared by thermal chemical vapour deposition (CVD) of dimethyldichlorosilane at pyrolytic temperatures, 1100–1400 °C. The nanosized silicon carbide particles prepared at 1400 °C consist of small crystallites of -SiC arranged randomly in the particles. At pyrolytic temperature below 1300 °C, the particles consist of amorphous phase and -type SiC crystallites. The average particle size changed from 70 nm to 40 nm and the average size of the -SiC crystallite changed from 7.3 nm to 1.8 nm depending on the pyrolysis conditions. The C/Si molar ratios of the product powders changed from 0.5 to 1.07 with the CVD conditions. The near theoretical values of C/Si molar ratio of the product powders within 0.95–1.05 can be controlled by CVD conditions such as pyrolytic temperature and reactant concentration. Finally, the product powders were characterized by chemical analysis, X-ray diffraction, electron microscopy, and infrared spectroscopy.     

Kinetics of α- and β-Si3N4 formation from oxide-free high-purity Si powder

Available kinetic data for the nitridation of high-purity oxide-free Si powder are analysed. The analysis suggests that the - and -phases of Si₃N₄ are formed by separate and parallel reaction paths, and kinetic expressions for their formation are reported. The formation of the -phase follows first-order kinetics, while the -phase is formed by a phase-boundary-controlled rate law. These conclusions are consistent with other kinetic and micrographic analyses reported in the literature.     

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.     

Thermal conductivity of unidirectionally oriented Si3N4w/Si3N4 composites

-silicon nitride whiskers were aligned unidirectionally in silicon nitride sintered with 2 wt% Al₂O₃ and 6 wt% Y₂O₃. It was be densified by the Gas Pressure Sintering (GPS) method. Thermal conductivity of the sintered body with different amount of - silicon nitride whiskers was measured by the direct contact method from 298 K to 373 K. This unidirectionally oriented -silicon nitride whiskers grew into the large elongated grains, and improved also the thermal conductivity. The amount of -silicon nitride whiskers changed the microstrcuture, which changed the thermal conductivity.     

Observation of secondary particles grown on crystal surfaces of silicon carbide

Pressureless sintering of ultrafine SiC powder containing a large amount of free Si was carried out with the aid of boron. Epitaxial growth of secondary particles was observed by scanning electron microscopy (SEM) on some hexagonal-shaped surfaces of -SiC cube-octahedra. The other hexagonal-shaped surfaces were seen to be entirely smooth. The two-fold symmetry of the [100] axes for the appearance of secondary particles certainly reflects the polarity of -SiC. When the sintering was repeated again, coalescence growth of secondary particles was remarkable on the former surfaces and layered growth was predominant on the latter surfaces.     

Kinetic studies on β-SiC formation from homogeneous precursors

The kinetics of the carbothermic reduction of SiO₂ by carbon to produce -SiC from a homogeneous organic precursor has been investigated over the temperature range 1500 to 1800 °C in nitrogen by the use of a high-temperature thermobalance. The kinetic behaviour differed significantly from that of the heterogeneous reaction of SiO₂ and carbon particles. The weight-loss curves could be fitted well by the Avrami-Erofe'ev equation with an exponent of 1.5. The result was interpreted as showing instantaneous nucleation in a homogeneous matrix followed by the diffusion-controlled growth of -SiC. The obtained activation energy of 391 kJ mol^–1 was consistent with the assumption that the reaction is controlled by the diffusion of carbon through the amorphous matrix to the growing surface of -SiC.     

Synthesis of thermosetting copolymer of polycarbosilane and perhydropolysilazane

A copolymer of polycarbosilane and perhydropolysilazane was obtained by reacting polycarbosilane with titanium n-butoxide and perhydropolysilazane. Titanium n-butoxide and perhydropolysilazane were essential for the polymer to show a thermosetting property. The thermosetting copolymers were converted into silicon carbide-based ceramics by pyrolysis in a stream of nitrogen to 1000 °C with about 80 wt% ceramic yield. The main phase of the pyrolysis product at 1500 °C in nitrogen was small crystallite -SiC. Elemental carbon, based on rule-of-mixtures composition, in the final ceramics could be reduced by varying the ratio of polycarbosilane/perhydropolysilazane. The copolymer was dry spun and pyrolysed to produce ceramic fibre. Pyrolysis in nitrogen to 1500 °C yielded a silicon carbide-based fibre with low oxygen and low elemental carbon content. A tensile strength of 1.8 GPa and an elastic modulus of 220 GPa were obtained for the fibre which ranged from 10–12 m in diameter. Crystallization to -Si₃N₄, -SiC, and -Si₃N₄ proceeded on annealing in nitrogen at 1700 °C for 1 h.     

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.     

Preparation of pressureless-sintered SiC-Y2O3-Al2O3

Sintering additives were prepared from aluminium hydroxide and yttrium hydroxide. These additives were soluble in water and resulted in a binder. A -SiC powder was mixed with the additive solution and sintered at 2150° C without pressure. The oxides formed from the additive promoted sintering. The sintered body contained no pores. Aluminium, silicon, and yttrium oxide were precipitated in the sintered body.     

Silicon Carbide Polycrystalline Fibers and Single-Crystal Whiskers

The origin of the high inhomogeneity of powders of nonoxide refractory compounds, related to the chemical nature of the compounds and the preparation procedure (usually, solid-state reactions) is considered. Conclusive evidence is presented that vapor transport reactions have considerable potential for the reproducible synthesis of such compounds. It is shown that carbothermic reduction of silicon dioxide can be run as a vapor transport reaction using activated carbon fibers as a reductant. The forming polycrystalline SiC fibers are similar in particle morphology to the parent carbon fibers, suggesting that the reaction proceeds through the incorporation of silicon into the lattice of amorphous carbon. Experimental data are presented for the first time which demonstrate that, under similar process conditions, silicon carbide can be obtained in the form of polycrystalline fibers, single-crystal whiskers, or fine powder. The reaction products differ not only in morphology but also in the crystal structure of SiC ( or form). The mechanisms of - and -SiC nucleation and growth are discussed.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.