Silicon carbide transport during carbothermic reduction of SiO<Subscript>2</Subscript>: Thermodynamic evaluation and experimental study

Mass-transfer processes during the high-temperature carbothermic reduction of silicon dioxide have been studied using thermodynamic modeling. The chemical vapor transport of silicon carbide has been investigated using SiO₂ + xSiC mixtures—major reaction products in the SiO₂-C system—as examples. Thermodynamic modeling results indicate that the vapor transport of silicon carbide is possible at temperatures from 1300 to 1500°C, and that the major gaseous species involved are Si and CO. Vapor transport processes have been studied experimentally. It is shown that the thermal reaction between carbon monoxide and silicon leads not only to direct conversion of silicon particles to silicon carbide but also to the growth of silicon oxycarbide fibers. The synthesized material has been characterized by x-ray diffraction and high-resolution optical microscopy.     

Brazing of reaction-bonded silicon carbide and Inconel 600 with an iron-based alloy

The objective of the present work was to join reaction-bonded silicon carbide to Inconel 600 (IN600, a nickel-based superalloy) for use in high temperature applications by brazing with an Fe-20wt% alloy. This joining method resulted in the molten filler metal reacting with the IN600 to form a Ni-Fe-Si solution, which in turn formed a liquid with the free silicon phase of the RBSC. This liquid reacted vigorously with the SiC component of the RBSC to form low melting point phases in both starting materials and chromium carbides at the metal-ceramic interface. By using solution thermodynamics, it was shown that a Ni-Fe-Si liquid with equimolar nickel and iron contents and silicon content of less than 30 at% Si will decompose -SiC at the experimental brazing temperatures; it was also shown that these predictions agree with the experimentally observed microstructures and line composition profiles.     

以淀粉为填充剂的碳坯渗硅制备反应烧结碳化硅陶瓷

探索了一条高性能RBSC低成本制造的新途径,本研究以石油焦粉为碳质原料制坯,玉米淀粉为填充剂调整碳坯的密度,纯碳素坯经高温渗硅得到密度为3.12g/cm³,强度为580MPa的反应烧结碳化硅陶瓷.研究结果表明掺加淀粉后素坯中含有更多的微孔,烧结体晶粒平均尺寸为2-4μm,晶粒细化是材料性能比传统RBSC材料高的原因.     

Synthesis of nanostructured SiC using the pulsed laser deposition technique

We report the new results on the direct synthesis of nanostructured silicon carbide (SiC) materials using the pulsed laser deposition technique. Scanning electron microscopy images revealed that SiC nanoholes, nanosprouts, nanowires, and nanoneedles were obtained. The crystallographic structure, chemical composition, and bond structure of the nanoscale SiC materials were investigated using X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering spectroscopy. The transverse optical mode and longitudinal optical mode in Raman spectra were found to become sharper as the substrate temperature was increased, while the material structure evolved from amorphous to crystalline.     

Interfacial reactions between SiC and aluminium during joining

Reactions between SiC and liquid aluminium were studied. Transmission electron microscopy (TEM) showed that aluminium carbide (Al₄C₃) phase was formed at the interface between pressureless sintered SiC and aluminium. In contrast, the Al₄C₃ phase was not detected at the reaction sintered SiC-Al interface. This difference in microstructures results in the change in bending strength of the joints. Mixtures of SiC and aluminium powders were heated to react in vacuum in the temperature range 973 to 1473 K and the reaction products were examined using X-ray powder diffraction. It was confirmed that Al₄C₃ and silicon were formed, and that the extent of reaction between SiC and aluminium was decreased by the addition of silicon into aluminium.     

Matrix characterization of fibre-reinforced SiC matrix composites fabricated by chemical vapour infiltration

Ceramic matrix composites (CMCs), that consist of silicon carbide (SiC) reinforced with continuous Nicalon^? or T-300^® fibres, are being developed for many high-temperature structural applications. The large potential use of CMCs has prompted an in-depth investigation and characterization of these materials. Electron microscopy and micro-Raman spectroscopy were used to characterize and compare the SiC matrix crystal structure and morphology of composite materials fabricated by two different chemical vapour infiltration (CVI) processes.     

Surface nano-structured silicon carbide thin film produced using hot filament decomposition of ethylene at low temperature on silicon wafer

The structure and spectroscopic properties of nano-structured silicon carbide (SiC) thin films were studied for films obtained through deposition of decomposed ethylene (C₂H₄) on silicon wafers via hot filament chemical vapor deposition method at low temperature followed by annealing at various temperatures in the range 300-700 °C. The prepared films were analyzed with focus on the early deposition stage and the initial growth layers. The analysis of the film's physics and structural characteristics was performed with Fourier transform infrared spectroscopy and Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction. The conditions for forming thin layer of cubic SiC phase (3C-SiC) are found. X-ray diffraction and Raman spectroscopy confirmed the presence of 3C-SiC phase in the sample. The formation conditions and structure of intermediate SiC layer, which reduces the crystal lattice mismatch between Si and diamond, are essential for the alignment of diamond growth. This finding provides an easy way of forming SiC intermediate layer using the Si from the substrate.     

Fluorination of silicon carbide thin films using pure F2 gas or XeF2

Two fluorination methods: direct fluorination using F₂ gas and fluorination by the decomposition of fluorinating agent XeF₂ have been applied to silicon carbide SiC thin films in order to form a composite of carbide derived carbon film together with residual silicon carbide. Before and after fluorination, the thin films have been characterized by Scanning Electron Microscopy, Rutherford Backscattering spectroscopy, Fourier Transformed InfraRed and Raman spectroscopies. Whereas direct fluorination leads to irreversible damages into the thin films, XeF₂ method allows a progressive etching of the silicon atoms and the formation of non-fluorinated carbon.     

The role of silicon in wetting and pressureless infiltration of SiCp preforms by aluminum alloys

Silicon plays an important role in the production of Al/SiC metal matrix composites. As an alloying element in aluminum, silicon retards the kinetics of the chemical reactions that result in the formation of the unwanted intermetallics Al₄C₃ and Al₄SiC₄. As a thin coating on silicon carbide, silicon becomes an active participant in a thermally activated chemical reaction that enhances wetting of silicon carbide by aluminum alloys. Consequently, Al/SiC composites made with siliconized silicon carbide and silicon rich aluminum alloys show mechanical properties that are significantly different from those of similar composites produced with unsiliconized silicon carbide or with aluminum alloys that do not contain silicon. It is shown that a silicon coating on SiC significantly enhances wetting of SiC particles by aluminum alloys, reduces porosity, does not affect the modulus of elasticity, but decreases the modulus of rupture of Al/SiC metal matrix composites.     

SiC Particles Coated by Chromium Silicides

The interaction of chromium vapors with powderlike silicon carbide (SiC) was investigated by X-ray phase analysis, X-ray microanalysis, the EPR method, electron microscopy, and the BET method. It has been established that in the temperature range of 147 K to 2023 K under a pressure of 1.3 Pa, the main interaction product is chromium silicide (Cr₅Si₃), which forms a surface layer on SiC particles in zones with a mean temperature (T _mean) of 1773 K to 1473 K. Moreover, Cr₅Si₃ vapors passing through the disperse SiC system condense in cold-temperature zones on SiC particles and aggregates.     

Polymer-Derived High-Temperature Nonoxide Materials: A Review

This review is focused on an attractive class of polymer-derived high-temperature ceramics, namely, polymer-derived nonoxide materials. With a brief introduction of high-temperature nonoxides, the origin of using polycarbosilane (PCS) polymer melt spinning to synthesize silicon carbide (SiC) fibers is traced back. For SiC formation, the four stages for the conversion from polymer precursors to microcrystalline ceramics are examined first: crosslinking, polymer decomposition, ceramic formation, and crystallization. Also, the important parameters related to PCS pyrolysis are explained, and polymer-derived SiC microstructures and compositions are evaluated. Solid-solution carbides and transition metal carbides are further reviewed. For boride materials, the discussion is focused on transition metal borides and boride composites. Similar to PCS conversion to SiC, nitride materials mostly start with polycarbosilazane (PSZ) precursors and form into the final materials through pyrolysis. With different carbide and nitride precursors mixed and pyrolyzed together, high-temperature nonoxide composites are formed. Such molecular-level intermixing and versatile capability of forming different shapes enable many exciting properties. Among these are mechanical and thermal properties, along with electrical conductivity, electromagnetic shielding, and charge storage capability. An overview of applications of polymer-derived nonoxides is provided, followed by a summary and outlook.     

Mechanical and structural characterisation of Nicalon SiC fibres up to 1300°C

The resurgence of interest in metal matrix composites has been fuelled by the development of new fibres with high temperature characteristics. The new family of continuous fine ceramic fibres based on SiC or Al₂O₃ offers the possibility of producing high temperature composites with metal or ceramic matrices. The toughening of ceramics by these fibres is a particularly interesting prospect.Two types of continuous silicon carbide Nicalon monofilaments (NLP 101 and NLM 102) have been tested in air and argon up to 1300°C. Tensile and creep tests have shown that the tensile strength falls and the fibres creep above 1000°C. Different behaviour was found for the two types of fibres. The NLM 102 fibre was stronger and crept less at high temperature under small strains. However its creep lifetime was less than that of the NLP 101 fibres.These differences have been interpreted with the aid of a microstructural study. The fibres were found to contain silicon, carbon and oxygen (electron microphobe and Auger spectrometer) and SiC was also detected (X-ray diffraction and transmission electron microscopy). The modification of the amorphous and microcrystalline structures during creep have been investigated. A fine segregation of free carbon particles was detected (X-ray diffraction and ESR) and was seen to disappear during heat treatment in both types of environment studied.     

Fabrication of pure SiC ceramic foams using SiO2 as a foaming agent via high-temperature recrystallization

A novel method was developed to produce the pure silicon carbide foams via the high-temperature recrystallization with the presence of a novel foaming agent-SiO₂. In this method, SiO₂ reacts with SiC to produce the gases in the silica liquid at high temperature, which leads to the formation of foams. The foams consist of the directional and interconnecting SiC crystals, and numerous intercommunicating pores that are located between them. The phase of foams was identified as 6H-SiC without the presence of SiO₂ since SiO₂ particles could react completely with SiC particles and vaporize from the sample at high temperature. The total porosity, weight loss and volume expansion rate can be increased with increasing SiO₂ contents while the three-point bending strength decreasing. The porosity of SiC foam with 25 wt.% SiO₂ as a foaming agent exhibits the maximum value while the three-point bending strength shows minimum value correspondingly. The sintered samples presented the porosities of 61-81%, the bending strength from 1.5 MPa to 4.8 MPa, and the volume expansion rate from 17.4% to 65%. This research can develop the theory for the preparation of SiC ceramics foams with controlled structure.     

Reactions at the Al/SiO2/SiC layered interface

Electron spectroscopy and thermodynamic modelling have been used to examine reactions at the Al/SiO₂/SiC layered interfaces at 800 °C. The reactions have been examined as a function of oxide thickness. Three regimes have been isolated: (i) where there is no oxide present aluminium and SiC react to produce Al₄C₃ and free silicon; (ii) where there is a thin oxide present the initial products are aluminosilicates and amorphous alumina; however, once the SiO₂ is consumed, Al₄C₃ emerges as a product; (iii) where a thick oxide is present only aluminosilicate and alumina are formed.     

A facile synthesis of silicon carbide nanoparticles with high specific surface area by using corn cob

Corn cob, which possesses low ash and high carbon contents, is a common waste material that accounts for a large amount of agricultural waste. This paper reports about a facile method to synthesize silicon carbide (SiC) nanoparticles with high specific surface area by using corn cob as a carbon source. The method is accomplished by carbothermal reduction at 1350?°C using corn cob as carbon source and silicon monoxide as silicon source. Fourier transform infrared (FT-IR) and Raman spectra results confirmed the formation of synthesized SiC particles. X-ray diffraction (XRD) results indicated the major phases of 3C-SiC. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the SiC particle size is in the range of 40–100?nm and mainly composed of sphere-shaped nanoparticles. The Brunauer–Emmett–Teller (BET) specific surface area of samples is 80.25?m²/g. In addition, we proposed the formation mechanism of SiC nanoparticles with high specific surface area by adsorption and vapor–solid mechanism. This facile method for synthesizing SiC nanoparticles provides a new idea for high-value application of corn cobs and new raw material for the preparation of silicon carbide.     

Carburization of Si microwires by chemical vapour deposition

We report the elaboration of silicon carbide (SiC) nanostructures thanks to the carburization of silicon microwires (MWs) under methane at high temperature. The produced SiC nanostructures display a tubular shape and are polycrystalline. The as-prepared silicon carbide microtubes (MTs) were characterized and studied by scanning electron microscopy (SEM), dual focused ion beam-scanning electron microscope (FIB-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The formation of microtubes can be explained by the out-diffusion of Si through the SiC during the carburization process.     

Nano-precipitation in hot-pressed silicon carbide

Heat treatments at 1300°C, 1400°C, 1500°C, and 1600°C in Ar were found to produce nanoscale precipitates in hot-pressed silicon carbide containing aluminum, boron, and carbon sintering additives (ABC-SiC). The precipitates were studied by transmission electron microscopy (TEM) and nano-probe energy-dispersive X-ray spectroscopy (nEDS). The precipitates were plate-like in shape, with a thickness, length and separation of only a few nanometers, and their size coarsened with increasing annealing temperature, accompanied by reduced number density. The distribution of the precipitates was uniform inside the SiC grains, but depleted zones were observed in the vicinity of the SiC grain boundaries. A coherent orientation relationship between the precipitates and the SiC matrix was found. Combined high-resolution electron microscopy, computer simulation, and nEDS identified an Al₄C₃-based structure and composition for the nano-precipitates. Most Al ions in SiC lattice exsolved as precipitates during the annealing at 1400 to 1500°C. Formation mechanism and possible influences of the nanoscale precipitates on mechanical properties are discussed.     

Formation of SiC whiskers from silicon nitride

Attempts have been made to produce SiC whiskers through vacuum pyrolysis of Si₃N₄ without any addition of extraneous carbon. Vacuum pyrolysis of Si₃N₄ granules and powder compacts, has been carried out at 1550 and 1700°C using a graphite resistance furnace. The products of pyrolysis have been identified through XRD and SEM as SiC whiskers and particles. Small amounts of elemental silicon at 1550°C and free carbon at 1700°C have been detected through X-ray diffraction. Detection of elemental silicon through X-ray diffraction and solidified silicon droplets at the whisker tips in the SEM provide important clues regarding the mechanism of SiC_w formation, as the one involving the reaction 2Si(l) + CO(g) SiC(s) + SiO(g) Silicon carbide whiskers, 3–4 mm long, have been grown from Si₃N₄ compacts at 1550°C over a short period of 0.5 h. It has been shown in the present study that Si₃N₄ can be completely converted to SiC_w, when a loose bed of Si₃N₄ in the form of granules is pyrolysed in the presence of CO at about 1550°C.     

Production and properties of hot-pressed materials based on silicon carbide with additions of boron and titanium carbides

Special features of pressure sintering materials based on silicon carbide with additives of boron and titanium carbides have been investigated. The kinetic parameters of the densification have been established. Special features of the structure and physico-mechanical properties of hot-pressed materials of the SiC(8–20 wt %)–(B₄C–TiC) system have been studied. Dense materials have been produced based on si-licon carbide with increased (3.8 MPa?m^1/2) fracture toughness and low (0.07 Ω?m) electrical resistance.