Catalytic synthesis of SiC nanowires in an open system

SiC nanowires (NWs) are usually synthesized in a closed vacuum reaction system which limits the yield of SiC NWs. In this work, SiC NWs and carbon nanotubes were synthesized in an open tube furnace at 1550°C with Si powder as silicon sources, ethanol as carbon sources and ferrocene as catalyst. The as-synthesized products were ultralong β-SiC NWs with the diameter about 80-100 nm and the length up to several tens micrometers. The diameter of the carbon nanotubes was about 20-30 nm. The carbon nanotube yarns about 20 cm in length were obtained at the end of the tube furnace. The growth mechanism of SiC NWs and carbon nanotubes were proposed. Compared with the traditional synthetic techniques in the high vacuum closed system, the novel synthesis method in the open system provided a new approach to the synthesis of SiC NWs.     

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.     

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.     

Joining of Carbon—Carbon Composites Using Boron and Titanium Disilicide Interlayers

The feasibility of joining of 3-D carbon—carbon (C–C) composites by using B and TiSi₂ interlayers has been investigated. The optimum temperature for joining with a B interlayer was determined to be about 1995°C and that for joining with a TiSi₂ interlayer was about 1490°C. The shear strengths of the joints made at these optimum temperatures were found to increase with the shear testing temperature up to a point, followed by a decrease at higher temperatures. For C–C specimens bonded at 1995°C with a B interlayer, the maximum joint shear strength (average value 18.35 MPa) was observed at the test temperature of 1660°C. The shear strength of joints produced with a TiSi₂ interlayer showed a maximum at the test temperature of 1164°C, with an average value of 34.41 MPa. The B interlayers reacted with C–C composite pieces during joining, and the product of reaction was identified as B₄C. In specimens joined with TiSi₂ interlayers, the reaction between TiSi2 and C did not go to completion, and the bond interlayer contained TiC, SiC, and TiSi₂.     

Fabrication and Properties of Porous SiC Sheets from Clutch Lining Wastes

During the production of friction sheets with their pack assemblies used in clutch for automatic transmission of motorcars, useless sheet fragments are formed and have to be disposed of, amounting to several tens of tons per a month. These wastes are composed of cellulose, diatomaceous earth, synthetic fibers, and carbon powder/fibers, impregnated with phenol resin as the bonding medium. When two types of these sheets are heated in an inert atmosphere at 600°C, they become porous and contain excess carbon formed by decomposition of the resin. The reaction of these porous sheets with a source of silicon such as tetra-ethoxy-silane or SiO at 1500°–1600°C results in the formation of porous silicon carbide (SiC) (∼80% porosity). If the porous sheets are impregnated with phenol resin and again reacted with SiO gas at 1500°–1600°C in Ar, SiC sheets with reduced porosity (<70%) are obtained. The surfaces and cross sections of the SiC sheets were observed by scanning electron microscopy, and their pore size distribution in the samples heated at 1600° and 1900°C in Ar was determined by mercury porosimetry. The oxidation and corrosion properties of the SiC sheets with 60% and 80% porosities were determined at 1200°C in Ar/O₂ in the absence and presence of NaCl vapor.     

Oxidation of Chemically-Vapor-Deposited Silicon Carbide in Carbon Dioxide

Chemically-vapor-deposited silicon carbide (CVD SiC) was oxidized in carbon dioxide (CO₂) at temperatures of 1200–1400°C for times between 96 and 500 h at several gas flow rates. Oxidation weight gains were monitored by thermogravimetric analysis (TGA) and were found to be very small and independent of temperature. Possible rate-limiting kinetic mechanisms are discussed. Passive oxidation of SiC by CO₂ is negligible compared to the rates measured for other oxidants that are also found in combustion environments, oxygen and water vapor.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Synthesis of Silicon Nitride/Silicon Carbide Nanocomposite Powders through Partial Reduction of Silicon Nitride by Pyrolyzed Carbon

An alternative method to incorporate nanometer-sized silicon carbide (SiC) particles into silicon nitride (Si₃N₄) powder was proposed and investigated experimentally. Novolac-type phenolic resin was dissolved in ethanol and mixed with Si₃N₄ powder. After drying and curing, the resin was converted to reactive carbon via pyrolysis. Si₃N₄ powder was partially reduced carbothermally using the pyrolyzed carbon, and nanometer-sized SiC particles were produced in situ at 1530°-1610°C in atmospheric nitrogen. At temperatures <1550°C, the reduction rate was low and the SiC particles were very small; no SiC whiskers or barlike SiC was observed. At 1600°C, the reduction rate was high and the reaction was close to completion after only 10 min, with the appearance of SiC whiskers as well as curved, barlike, and equiaxial SiC, all of which were dozens of nanometers in diameter; this size is greater than that at observed temperatures <1550°C. A longer soaking time at 1600°C led to agglomerates. SiC particles were close to the surface of the Si₃N₄ particles. The SiC content could be adjusted by changing the carbon content before reduction and the reduction temperature. A reaction mechanism that involved the decomposition of Si₃N₄ has been proposed.     

Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Study of Nonstoichiometric Silicon-Carbon-Oxygen Glasses

Crystallization behavior of Si-C-O glasses in the temperature range of 1000°–1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, T^H, and methyldiethoxysilane, D^H. Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of T^H/D^H= 1, the formation of a carbon-rich glass was observed, whereas a ratio of T^H/D^H= 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although T^H/D^H= 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, T^H/D^H= 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20–50 nm), even at 1200°C. Both materials underwent a phase separation process, SiC _x O_2(1-x)→ x SiC + (1 - x )SiO₂, when annealed at temperatures exceeding 1200°C.     

Effect of Environment on the Stress-Rupture Behavior of a Carbon-Fiber-Reinforced Silicon Carbide Ceramic Matrix Composite

Stress-rupture tests were conducted in air, under vacuum, and in steam-containing environments to identify the failure modes and degradation mechanisms of a carbon-fiber-reinforced silicon carbide (C/SiC) composite at two temperatures, 600° and 1200°C. Stress-rupture lives in air and steam-containing environments (50–80% steam with argon) are similar for a stress of 69 MPa at 1200°C. Lives of specimens tested in a 20% steam/argon environment were about twice as long. For tests conducted at 600°C, composite life in 20% steam/argon was 30 times longer than life in air. Thermogravimetric analysis of the carbon fibers was conducted under conditions similar to the stress-rupture tests. The oxidation rate of the fibers in the various environments correlated with the composite stress-rupture lives. Examination of the failed specimens indicated that oxidation of the carbon fibers was the primary damage mode for specimens tested in air and steam environments at both temperatures.     

Formation of Carbide-Derived Carbon on β-Silicon Carbide Whiskers

Carbon was synthesized on β-SiC whiskers by extraction of Si atoms from SiC. In this study, three different elevated temperature extraction methods were used to remove Si atoms from SiC: treatments in either Cl₂ or HCl and vacuum decomposition. In all chlorination experiments and vacuum treatment at 1700°C, carbon preserved the original shape of SiC whiskers. At higher temperatures (2000°C), vacuum decomposition led to a distortion in the shape of the whiskers. High-resolution transmission electron microscopy and Raman spectroscopy showed that the structure of carbide-derived carbon depends on the Si extraction method and the process parameters. Chlorination of SiC resulted in the formation of mostly amorphous nanoporous carbon. High-temperature treatment of SiC in HCl environment produced fullerene-like structures, while high-temperature vacuum decomposition resulted in the formation of graphite. Transmission electron microscopy studies of the carbon coating thickness produced in Cl₂ at various chlorination times revealed linear reaction kinetics at 700°C. Raman studies showed that the carbon structure became more ordered with increasing chlorination temperature. The results obtained demonstrate that by using the silicon extraction technique, one can precisely control the thickness and morphology of the carbon coating.     

Synthesis and characterisation of medium surface area silicon carbide nanotubes

Silicon carbide nanotubes with medium surface area (30-60 m²/g) were successfully prepared by reaction between carbon nanotubes and SiO vapor according to the shape memory synthesis (SMS). The gross morphology of the carbon nanotubes was maintained during the carburization process. A calcination in air at 600 °C was performed to remove unreacted carbon domains in order to obtain pure carbon-free SiC nanotubes. The synthesized SiC nanotubes had a mean outer diameter of 100 nm and lengths up to several tens of micrometres.     

Phase Evolution in Silicon Carbide–Whisker-Reinforced Mullite/Zirconia Composite during Long-Term Oxidation at 1000° to 1350°C

A composite consisting of 30 wt% SiC whiskers and a mullite-based matrix (mullite–32.4 wt% ZrO₂–2.2 wt% MgO) was isothermally exposed in air at 1000°–1350°C, for up to 1000 h. Microstructural evolution in the oxidized samples was investigated using X-ray diffractometry and analytical transmission electron microscopy. Amorphous SiO₂, formed through the oxidation of SiC whiskers, was devitrified into cristobalite at T ≥ 1200°C and into quartz at 1000°C. At T ≥ 1200°C, the reaction between ZrO₂ and SiO₂ resulted in zircon, and prismatic secondary mullite grains were formed via a solution–reprecipitation mechanism in severely oxidized regions. Ternary compounds, such as sapphirine and cordierite, also were found after long-term exposure at T ≥ 1200°C.     

Synthesis of Silicon Carbide Thin Films with Polycarbosilane (PCS)

Polycarbosilane (PCS) thin films were deposited on silicon (and other) substrates and heat treated under vacuum (∼10_--6>torr)at temperatures in the range of 200°–1200°C. At temperatures in the range of 1000°–1200°C, the initially amorphous PCS films transformed to polycrystalline ß-silicon carbide (ß-SiC). Although PCS films could be deposited at thickness up to 2 μm, the films with thicknesses >1 μm could not be transformed to SiC without extensive cracking. The resulting SiC coatings were characterized using Fourier transform infrared spectroscopy, glancing-angle X-ray diffractometry, secondary-ion mass spectroscopy, Raman spectoscopy, transmission electron microscopy, and scanning electron microscopy. The temperature and time dependence of the amorphous-to-crystalline transition could be associated with the evolution of free carbon, oxygen, and hydrogen in the films.     

Tensile Creep and Creep Rupture Behavior of Monolithic and SiC-Whisker-Reinforced Silicon Nitride Ceramics

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200° to 1350°C using hotpressed materials with and without SiC whiskers. Stable steady-state creep was observed under low applied stresses at 1200°C. Accelerated creep regimes, which were absent below 1300°C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker-reinforced material exhibited better creep resistance than the monolith at 1200°C; however, the superiority disappeared above 1300°C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200°C. The primary creep mechanism was considered cavitationenhanced creep. Specimen lifetimes followed the Monkman–Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.     

Ultra-High-Temperature Ceramic Coatings for Oxidation Protection of Carbon–Carbon Composites

Carbon–carbon (C–C) composites are attractive materials for hypersonic flight vehicles but they oxidize in air at temperatures >500°C and need thermal protection systems to survive aerothermal heating. We investigated using multilayers of high-temperature ceramics such as ZrB₂ and SiC to protect C–C against oxidation. Our approach combines pretreatment and processing steps to create continuous and adherent high-temperature ceramic coatings from infiltrated preceramic polymers. We tested our protective coatings at temperatures above 2600°C at the National Solar Thermal Testing Facility using controlled cold-wall heat flux profiles reaching a maximum of 680 W/cm².     

Creep Behavior of a SiC-Whisker-Reinforced Alumina

Creep tests in four-point flexure loading configuration in air employing applied stresses of 37 to 300 MPa at temperatures of 1200°, 1300°, and 1400°C were performed on 20-vol%-SiC-whisker-reinforced alumina and unreinforced single-phase polycrystalline alumina. The creep rate of polycrystalline alumina was significantly reduced through the addition of SiC whiskers, although strain to failure was lower. Transmission and scanning electron microscopy results suggest that substantial increase in the creep resistance in flexure of alumina composites originates from the retardation of grain-boundary sliding by the SiC whiskers.     

Thermal Degradation of Fiber Coatings in Mullite-Fiber-Reinforced Mullite Composites

The thermal degradation behavior of single-layer BN and of double-layer BN/SiC chemically vapor-deposited fiber coatings in mullite-fiber-reinforced mullite composites was investigated by means of transmission electron microscopy after processing and heat treatment of the composites at 1000°, 1200°, and 1300°C for 6 h in air. The single-layer BN coatings were ˜0.7 mu m thick and consisted of turbostratic BN with (0001) basal planes lying parallel to the surfaces of the fibers plus nanosized areas that had no preferential orientation. This microstructure remained unchanged up to 1000°C; however, distinct coarsening of the randomly oriented BN crystallites occurred in the temperature range of 1000°-1200°C. The single-layer BN coatings were stable against oxidation, up to 1200°C. At higher temperatures, degradation of the coatings via oxidation occurred. Double-layer BN/SiC coating systems consisted of BN that was 0.08 mu m thick and SiC layers that were 0.16 mu m thick and deposited onto the mullite fibers. The turbostratic BN was highly anisotropic and did not undergo any microstructural change, up to 1300°C. The outer SiC layer of the double-layer coating system improved the oxidation resistance of BN in the 1200°-1300°C temperature range, despite a partial oxidation of SiC to SiO₂.     

Carbon Interfacial Layers Formed by Oxidation of SiC in SiC/Ba-Stuffed Cordierite Glass-Ceramic Reaction Couples

Reaction couples between α-SiC and cordierite (2MgO·2Al₂O₃·5SiO₂═ Mg₂Al₄Si₅O₁₈) were prepared by sandwiching (and enclosing) SiC single crystals between plates of Ba-stuffed magnesium aluminosilicate (Ba-MAS) glass and hot-pressing; the Ba-MAS was subsequently crystallized at 1000° to 1200°C in argon or air. No reaction occurred at the SiC/Ba-MAS interfaces during hot-pressing, but crystallization heat treatments caused formation of amorphous carbon reaction layers at the SiC/cordierite interfaces, due to concurrent oxidation via the reaction SiC + O₂→ SiO₂+ C. The thickness of the carbon of the carbon layer was variable. These results suggest that formation of C layers at SiC/silicate interfaces in other composites (containing Nicalon fibers, for example) depends more on thermochemistry and less on the details of SiC nonstoichiometry than has heretofore been supposed.     

Oxidation Behavior of Chemically-Vapor-Deposited Silicon Carbide and Silicon Nitride from 1200° to 1600°C

The isothermal oxidation of pure CVD SiC and Si₃N₄ has been studied for 100 h in dry, flowing oxygen from 1200° to 1600°C in an alumina tube furnace. Adherent oxide formed at temperatures to 1550°C. The major crystalline phase in the resulting silica scales was alpha-cristobalite. Parabolic rate constants for SiC were within an order of magnitude of literature values. The oxidation kinetics of Si₃N₄ in this study were not statistically different from that of SiC. Measured activation energies were 190 kJ/mol for SiC and 186 kJ/mol for Si₃N₄. Silicon oxynitride did not appear to play a role in the oxidation of Si₃N₄ under the conditions herein. This is thought to be derived from the presence of ppm levels of sodium impurities in the alumina furnace tube. It is proposed that sodium modifies the silicon oxynitride, rendering it ineffective as a diffusion barrier. Material recession as a function of oxide thickness was calculated and found to be low. Oxidation behavior at 1600°C differed from the lower temperatures in that silica spallation occurred after exposure.