A microstructural study of carbon–carbon composites impregnated with SiC filaments

Porous multidirectional carbon/carbon composite obtained by pulse chemical vapour infiltration (PCVI) was impregnated with silicon carbide (SiC) derived from pyrolysis of polymethylsiloxane resin (PMS). The impregnation process was made to improve oxidation resistance and mechanical properties of MD C/C composite. The resin was used as a source of silicon carbide component of the composite forming after heat treatment above 1000 °C. During this process SiC thin filaments were formed inside the porous carbon phase. The aim of this work was to investigate the structure and microstructure of the constituents of carbon composite obtained after pyrolysis of SiC PMS precursor. Microscopic observations revealed that during careful heat treatment of crosslinked polymethylsiloxane resin up to 1700 °C, the filaments (diameter 200–400 nm) crystallized within porous carbon phase. The filaments were randomly oriented on the composite surface and inside the pores. FTIR spectra and XRD analysis of the modified C/C composite showed that filaments had silicon carbide structure with the crystallite size of silicon carbide phase of about 45 nm. The Raman spectra revealed that the composite contains two carbon components distinctly differing in their structural order, and SiC filaments present nanocrystalline structure.     

Mechanical,structural and oxidation resistance enhancement of carbon foam by in situ grown SiC nanowires

A method for processing carbon foams containing both silicon carbide (SiC) nanowires and bulk SiC and silicon nitride (Si3N4) phases has been developed by reaction of powder mixtures containing precursors for carbon, sacrificial template, silicon (Si), short carbon fibers (SCF) and activated carbon (AC). In situ growth of Si nanowires during pyrolysis of the foam at 1000 °C under N2 changed the foam׳s microstructure by covering the porous skeleton inside and out. In situ-grown SiC nanowires were found smoothly curved with diameters ranging around two main modes at 30 and 500 nm while their lengths were up to several tens of micrometers. SCF were found effectively mixed and well-bonded to pore walls. Following density, porosity and pore size distribution analyses, the heat-treated (HT) foam was densified using a chemical vapor infiltration (CVI) process. Thereafter, density increased from 0.62 to 1.30 g/cm³ while flexural strength increased from 29.3 to 49.1 MPa. The latter increase was attributed to the densification process as well as to low surface defects, presence of SCF and coating, by SiC nanowires, of the entire SiC matrix porous structure. The foam׳s oxidation resistance improved significantly from 58 to 84 wt% residual mass of the heat treated and densified sample. The growth mechanism of Si nanowires was supported by the vapor–liquid–solid mechanism developed under pyrolysis conditions of novolac and reducing environment of coal cover.     

Porous SiC ceramics with dendritic pore structures by freeze casting from chemical cross-linked polycarbosilane

In this study, a commercial polycarbosilane (PCS) and divinylbenzene (DVB) were used as the preceramic polymer precursor and crosslinking agent, respectively to form porous silicon carbide (SiC) ceramics by freeze casting DVB/camphene/PCS solutions. Porous silicon carbide (SiC) with a dendritic pore structure and connecting bridges was obtained after pyrolysis at 1200 °C. The effects of DVB and PCS content on the rheological properties of the solution and the morphological characteristics and the compressive strengths of SiC ceramics were investigated. The use of DVB and the resulting chemical cross-linking yielded modified pore characteristics and much lower oxygen content in pyrolyzed SiC compared to the conventional thermal curing method. A compressive strength of 18.7 MPa was obtained for pyrolyzed SiC prepared with 20 wt% PCS and a 0.2 DVB/PCS mass ratio.     

Silicon carbide ceramics: Mechanical activation,combustion and spark plasma sintering

Single-stage fabrication of SiC ceramics by a combination of self-propagating high temperature synthesis (SHS) and spark plasma sintering (SPS) is reported. SHS+SPS is demonstrated to be an efficient method for production of SiC ceramics with density 3.1 g/cm³, hardness of 24 GPa and toughness of 5 MPa m^1/2. The starting material for the process is fine (50–300 nm in size) highly reactive powder, which involves composite particles of elemental carbon and silicon. This powder was prepared using a high-energy ball milling (HEBM). To optimize precursor preparation conditions, the structure transformation in nano-composite Si/C particles at different HEBM stages is also investigated.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.     

Thermal plasma synthesis of Si/SiC nanoparticles from silicon and activated carbon powders

In this study, Si/SiC nanocomposites were synthesized by non-transferred arc thermal plasma processing of micron-sized SiC powder. First, micron-sized SiC was synthesized by solid-state method where waste silicon (Si) and activated carbon (C) powder were used as precursor materials. The effect of Si/C mole ratio and solid-state synthesis temperature on structural and phase formation of SiC was investigated. Diffraction pattern confirmed the formation of SiC at 1300 °C. High C content was required for the synthesis of pure SiC as Si remained unreacted when Si/C mole ratio was below 1/1.5. Highly agglomerated micron-size (0.6–10 µm) SiC particles were formed after solid-state synthesis. Thermal plasma processing of solid-state synthesized micron-sized SiC resulted into the formation of uniformly dispersed (20–60 nm) Si/SiC nanoparticles. It was proposed that Si/SiC nanocomposites were formed due to partial decomposition of SiC during high temperature plasma processing. The formation of Si/SiC nanoparticles from micron-sized SiC was resulted from dissociation of grains from their grain boundary during plasma processing.     

Combustion synthesis of high porosity SiC foam with nanosized grains

High porosity silicon carbide (SiC) foam with nanosized grains was synthesized by a newly developed process involving two steps: (i) preparation of Si/C foam by gel-casting technique and (ii) fabrication of SiC foam by combustion Si/C foam in nitrogen atmosphere. The as-synthesized SiC foam with a high porosity in the range 70–90% exhibited an attractive strength up to 1.6 MPa. SEM analysis showed that the foam struts consisted of tightly bonded SiC particles with a grain size of 80–300 nm.     

New manufacturing process for nanometric SiC

Nanometric β-SiC powder was prepared by carbothermal reduction of freeze-dried gel. Initially, the gel was obtained by polycondensation of sol consisting of resorcinol and formaldehyde as a source of C and tetraethoxysilane as a source of silicon. The effect of temperature and time of heat treatment (carbothermal reduction) as well as the effect of C/Si ratio on SiC powder properties was studied. It was possible to obtain nanosized (～20 nm) β-SiC powder after one-hour heat treatment at relatively low temperature of 1200 °C. The powder was successfully synthesised without the need for excess carbon which is typical for conventional carbothermal reduction using some other sources of graphite. The increase in temperature of heat treatment to 1400 °C caused considerable growth of SiC particles up to 400 nm. It was found that prolonged heat treatment at 1200 °C is an effective way to obtain well crystallized SiC and keep the size of SiC particles below 50 nm at the same time.     

Physical,thermal and ablative performance of CVI densified urethane-mimetic SiC preforms containing in situ grown Si3N4 whiskers

A two-step process has been developed for silicon carbide (SiC) coated polyurethane mimetic SiC preform containing silicon nitride (Si₃N₄) whiskers. SiC/Si₃N₄ preforms were prepared by pyrolysis/siliconization treatment at 1600 °C, of powder compacts containing rigid polyurethane, novolac and Si, forming a porous body with in situ grown Si₃N₄ whiskers. The properties were controlled by varying Si/C mole ratios such as 1–2.5. After densification using a chemical vapour infiltration, the resulting SiC/Si₃N₄/SiC composites showed excellent oxidation resistance, thermal conductivity of 4.32–6.62 Wm⁻¹ K⁻¹, ablation rate of 2.38 × 10⁻³ − 3.24 × 10⁻³ g cm⁻² s and a flexural strength 43.12–55.33 MPa for a final density of 1.39–1.62 gcm⁻³. The presence of a Si₃N₄ phase reduced the thermal expansion mismatch resulting in relatively small cracks and well-bonded layers even after ablation testing. This innovative two-step processing can provide opportunities for expanded design for using SiC/Si₃N₄/SiC composites being lightweight, inexpensive, homogeneous and isotropic for various high temperature applications.     

C/C–SiC composite prepared by Si–10Zr alloyed melt infiltration

A high performance and low cost C/C–SiC composite was prepared by Si–10Zr alloyed melt infiltration. Carbon fiber felt was firstly densified by pyrolytic carbon using chemical vapor infiltration to obtain a porous C/C preform. The eutectic Si–Zr alloyed melt (Zr: 10 at.%, Si: 90 at.%) was then infiltrated into the porous preform at 1450 °C to prepare the C/C–SiC composite. Due to the in situ reaction between the pyrolytic carbon and the Si–Zr alloy, SiC, ZrSi₂ and ZrC phases were formed, the formation and distribution of which were investigated by thermodynamics. The as-received C/C–SiC composite, with the flexural strength of 353.6 MPa, displayed a pseudo-ductile fracture behavior. Compared with the C/C preform and C/C composite of high density, the C/C–SiC composite presented improved oxidation resistance, which lost 36.5% of its weight whereas the C/C preform lost all its weight and the high density C/C composite lost 84% of its weight after 20 min oxidation in air at 1400 °C. ZrO₂, ZrSiO₄ and SiO₂ were formed on the surface of the C/C–SiC composite, which effectively protected the composite from oxidation.     

UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

Microstructural evolution during the infiltration of boron carbide with molten silicon

The previously reported model that accounts for the formation of the core-rim structure in reaction-bonded boron carbide composites (RBBC) is expanded and validated by additional experimental observations and by a thermodynamic analysis of the ternary B–C–Si system. The microstructure of the RBBC composites consists of boron carbide particles with a core-rim structure, β-SiC and some residual silicon. The SiC carbide particles have a polygonal shape in composites fabricated in the presence of free carbon, in contrast to the plate-like morphology when the initial boron carbide is the sole source of carbon. In the course of the infiltration process, the original B₄C particles dissolve partly or fully in molten silicon, and a local equilibrium is established between boron carbide, molten silicon and SiC. Overall equilibrium in the system is achieved as a result of the precipitation of the ternary boron carbide phase B₁₂(B,C,Si)₃ at the surface of the original boron carbide particles and leads to the formation of the rim regions. This feature is well accounted for by the “stoichiometric saturation” approach, which takes into account the congruent dissolution of B₄C particles. The SiC phase, which precipitates form the silicon melt adopts the β-allotropic structure and grows preferably as single plate-like particles with an {1 1 1}_β habit plane. The morphology of the SiC particles is determined by the amount of carbon available for their formation.     

MCMB–SiC composites; new class high-temperature structural materials for aerospace applications

Graphite–silicon carbide (G–SiC), carbon/carbon–silicon carbide (C/C–SiC) and mesocarbon microbeads–silicon carbide (MCMB–SiC) composites were produced using liquid silicon infiltration (LSI) method and their physical and mechanical properties, including density, porosity, flexural strength and ablation resistance were investigated. In comparison with G–SiC and C/C–SiC composites, MCMB–SiC composites have the highest bending strength (210 MPa) and ablation resistance (9.1%). Moreover, scanning electron microscopy (SEM) and optical microscopy (OM) are used to analyze the reacted microstructure, pore morphology and pore distribution of carbon-based matrices. As a result, SiC network reinforcement was formed in situ via a reaction between liquid silicon and carbon. The unreacted carbon and solidified silicon are two phases present in the final microstructure and are characterized by X-ray diffraction (XRD). Based on the results obtained and the low-cost processing of pitch-based materials, the MCMB–SiC composite is a promising candidate for aerospace applications.     

Influence of silicon carbide particle size on the microstructure and mechanical properties of zirconium diboride–silicon carbide ceramics

The influence of silicon carbide (SiC) particle size on the microstructure and mechanical properties of zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics was investigated. ZrB₂-based ceramics containing 30 vol.% SiC particles were prepared from four different α-SiC precursor powders with average particle sizes ranging from 0.45 to 10 μm. Examination of the dense ceramics showed that smaller starting SiC particle sizes led to improved densification, finer grain sizes, and higher strength. For example, ceramics prepared from SiC with the particle size of 10 μm had a strength of 389 MPa, but the strength increased to 909 MPa for ceramics prepared from SiC with a starting particle size of 0.45 μm. Analysis indicates that SiC particle size controls the strength of ZrB₂–SiC.     

Application of the chemical vapor infiltration and reaction (CVI-R) technique for the preparation of highly porous biomorphic SiC ceramics derived from paper

Chemical vapor infiltration and reaction (CVI-R) is used to produce biomorphic high porous SiC ceramics based on biological structures such as paper. The paper fibers are first converted into a biocarbon (C_b) template by a carbonization step. In a second step methyltrichlorosilane (MTS) in excess of hydrogen is infiltrated into the C_b-template by CVI technique, depositing a Si/SiC layer around each fiber. The reaction (R) between biocarbon and excess silicon to form additional silicon carbide occurs during a subsequent thermal treatment as a third step of the ceramization process. Due to the mild infiltration conditions (850–900 °C) the initial micro- and macrostructure of the carbon preform is retained in the final ceramics. The applied characterization methods after every step of the ceramization process are X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, Thermal Gravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM). The bending strength of the resulting porous ceramics is measured by the double ring bending test. It is found that a slight excess of free Si in relation to the amount of carbon from the C_b-template must be deposited in the Si/SiC layer to achieve a nearly complete conversion of the C_b-templates into SiC ceramic. The weight gain after infiltration has to be at least 400 wt.%. Varying the infiltration conditions such as temperature, MTS concentration and infiltration time, ceramics in a wide range of porosity (55–80%) and mechanical properties (5–40 MPa) can be produced. A thermal treatment temperature of 1400 °C is found to be optimal for the reaction between the deposited Si and the biocarbon.     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

Oxidation protection of C/C composites with a multilayer coating of SiC and Si + SiC + SiC nanowires

An oxidation protective Si–SiC coating with randomly oriented SiC nanowires was prepared on the SiC-coated carbon/carbon (C/C) composites by a two-step technique. First, a porous network of SiC nanowires was produced using chemical vapor deposition. This material was subjected to pack cementation to infiltrate the porous layer with a mixture of Si and SiC. The nanowires in the coating could efficiently suppress the cracking of the coating by various toughening mechanisms including nanowire pullout, nanowire bridging, microcrack deflection and good interaction between nanowire/matrix interface. The results of thermogravimetric analysis and thermal shock showed that the coating had excellent oxidation protection for C/C composites between room temperature and 1500 °C. These results were confirmed by two additional oxidation experiments conducted at temperature of 900 and 1400 °C, which demonstrated that the coating could efficiently protect C/C composites from oxidation at 900 °C for more than 313 h or at 1400 °C for more than 112 h.     

Low-cost preparation and frictional behaviour of a three-dimensional needled carbon/silicon carbide composite

A low-cost carbon/silicon carbide (C/SiC) composite was manufactured by phenolic resin impregnation–pyrolysis combined with liquid silicon infiltration. The carbon fiber preform was prepared by three-dimensional needling. A carbon/carbon composite with a density of 1.22 g/cm³ after only one impregnation–pyrolysis cycle was achieved by using hot-pressing curing. The density of the final C/SiC was 2.10 g/cm³ with a porosity of 4.50% and SiC-content of 45.73%. The C/SiC composite had a high thermal conductivity of 48.72 W/(m K) perpendicular to the friction surface and demonstrated good friction and wear properties. The static and average dynamic friction coefficients were 0.68 and 0.32 (at a braking velocity of 28 m/s). The weight wear rates of the rotating disk and stationary disk were respectively 7.71 and 5.60 mg/cycle with linear wear rates, 1.67 and 1.22 μm/cycle, at a braking velocity of 28 m/s.     

Rapid carbothermic synthesis of silicon carbide nano powders by using microwave heating

This paper reports an improved procedure for synthesis of silicon carbide nanopowders from silica by carbothermic reduction under fast microwave-induced heating. The powders have been prepared by direct solid-state reaction in a 2.45 GHz microwave field in nitrogen atmosphere after 40 h milling. For the first time, the formation of silicon carbide (β-SiC) as a major phase can be achieved at 1200 °C in 5 min of microwave exposure, resulting in nano sized particles ranging from 10 to 40 nm under optimized synthesis condition. The Rietveld quantitative phase-composition analysis confirmed that the major SiC polytype is cubic SiC (β-SiC) with 98.5(4) weight fraction and the remained is minor hexagonal SiC polytypic (α-SiC) phases. Therefore this method is the most efficient one for SiC powder synthesis in terms of energy and time saving as well as preparation of SiC nano powders.     

Mass production of three-dimensional hierarchical microfibers constructed from silicon–carbon core–shell architectures with high-performance lithium storage

In this work, a facile approach is reported to mass produce highly porous fibers constructed from silicon–carbon core–shell structures. The C–Si microfibers are prepared using a modified electrospinning deposition method (ESD), and subsequent calcination of the carbon shells. Benefited from the step of vacuum drying, the unnecessary solvent left in the precursor will volatilize, resulting in the uniform three-dimensional hierarchical microfibers constructed from silicon–carbon core–shell architectures. The uniform covering layers of carbon formed by decomposition of polymer contribute to the improvement of conductivity and alleviation of volume change. The pores in the microfibers are helpful for the diffusion of electrolyte. When evaluated as an anode material for lithium-ion batteries, the C–Si microfibers exhibit improved reversibility and cycling performance compared with the commercial Si nanoparticles. A high capacity of 860 mAh g⁻¹ can be retained after 200 cycles at a current rate of 0.3 C. The rate capability of the C–Si microfibers is also improved. The special structure is believed to offer better structural stability upon prolonged cycling and to improve the conductivity of the material. This simple strategy using the modified ESD method could also be applied to prepare other porous energy materials.