In-situ synthesis and textural evolution of the novel carbonaceous SiC/mullite aerogel via polymer-derived ceramics route

A novel carbonaceous SiC/mullite composite aerogel is derived from catechol-formaldehyde/silica/alumina hybrid aerogel (CF/SiO₂/AlOOH) via polymer-derived ceramics route (PDCR). The effects of the reactants concentrations on the physicochemical properties of the carbonaceous SiO₂/Al₂O₃ aerogel and SiC/mullite aerogel are investigated. The mechanism of the textural and structural evolution for the novel carbonaceous SiC/mullite is further discussed based on the experimental results. Smaller reactants concentration is favorable to formation of mullite. Reactants concentration of 25% is selected as the optimal condition in considering of the mullite formation and bulk densities of the preceramic aerogels. Spherical large silica particles are also produced during heat treatment, and amorphous silica is remained after this reaction. With further heat treatment at 1400 °C, silicon carbide and mullite coexist in the aerogel matrix. The mullite addition decreases the temperature of SiC formation, when compared with the conventional methods. However, after heat treatment at 1450 °C, the amount of mullite begins to decrease due to the further reaction between carbon and mullite, forming more silicon carbide and alumina. The carbonaceous SiC/mullite can be transferred to SiC/mullite binary aerogel after carbon combustion under air atmosphere. The carbonaceous SiC/mullite has a composition of SiC (31%), mullite (19.1%), SiO₂ (14.4%), and carbon (35%). It also possesses a 6.531 nm average pore diameter, high surface area (69.61 m²/g), and BJH desorption pore volume (0.1744 cm³/g). The oxidation resistance of the carbonaceous SiC/mullite is improved for 85 °C when compared with the carbon based aerogel.     

Effect of hydrothermal corrosion on the fracture strength of SiC layer in tristructural-isotropic fuel particles

The hydrothermal corrosion behavior of SiC layer in tristructural-isotropic (TRISO) fuel particles and its effect on the fracture strength were investigated. The corrosion test was performed using the static autoclave at 400°C/10.3 MPa. The SiC layer exhibited a thickness loss and the corrosion rate followed a linear law. During corrosion, carbon was formed on the SiC surface due to the loss of Si. The corrosion was found preferentially occurred at the grain boundary of SiC, leading to the grain detachment and pit formation. The rate determining step of the corrosion was SiO₂ formation rather than SiO₂ dissolution in the hydrothermal environment. The fracture strength of SiC shell after corrosion was evaluated using the crush test. It showed a slight decrease with an increase in corrosion time, due to the thickness reduction in SiC layer. The results of this study demonstrated that the SiC in TRISO particles has good corrosion resistance in the hydrothermal environment.     

Ablation behavior of (TiZrHfNbTa)C high-entropy ceramics with the addition of SiC secondary under an oxyacetylene flame

The ablation behavior of high-entropy ceramics (HECs) was investigated in this study using an oxyacetylene flame at 2000 °C. Spark plasma sintering was used to construct a dense HEC (TiZrHfNbTa)C with a 20 vol% of SiC addition (HEC-20SiC). The densification of HEC-20SiC can be improved to a certain extent by adding SiC particles, increasing the hardness of HEC-20SiC to up to 24.6 GPa, and the crack deflection observed through the addition of SiC particles were considered to be the strengthening and toughening mechanisms. After ablation, Hf₆Ta₂O₁₇, Ti_5.1Ta_4.9O₂₀, Nb₂Zr₆O₁₇, TaZr_2.75O₈, and SiO₂ can be detected on an ablated surface and HEC-20SiC possesses the minimum mass ablation rate (?1.9 mg s^?1) and line ablation rate (2.1 μm s^?1) among the comparative ceramics. On the one hand, the SiC phase forms gaseous CO, CO₂, and SiO as well as viscous SiO₂ during ablation and some part of the heat can be dissipated by the evaporation of gaseous CO, CO₂, and SiO; further, pore defects can be healed by viscous SiO₂, thus inhibiting the diffusion of reactive oxygen species. On the other hand, the HEC phase with a lattice-distortion caused by single-phase solid-solution can effectively inhibit the invasion of reactive oxygen species and the outward migration of metal atoms. The invasion rate of reactive oxygen is considered to be the main step during HEC-20SiC ablation, and it is believed that higher principal component HECs can improve ablation performance even further.     

Carbon Interlayer Between CVD SiC and SiO2 in High‐Temperature Passive Oxidation

The oxidation behavior of high‐purity silicon carbide (SiC) prepared by chemical vapor deposition was investigated by thermogravimetry, transmission electron microscopy, and Raman spectroscopy in the temperature range 1534–1902 K in pure O₂. The carbon layer formed at the SiC/SiO₂ interface upon oxidation above 1784 K. Raman peaks corresponding to D‐ and G‐bands could be identified from the carbon layer. Bubbles were observed in the SiO₂ scale after the oxidation at 1873 K. This could be attributed to the accumulation of CO gas at the SiC/SiO₂ interface, resulting in the formation of the carbon layer and bubbles. These suggest that the oxidation rate of SiC is limited by the outward diffusion of CO in the SiO₂ scale in this temperature range.     

Synthesis of Silicon Carbide Nanocrystals Using Waste Poly(vinyl butyral) Sheet

SiC nanocrystals were prepared using waste poly(vinyl butyral) sheet as a carbon source. SiO₂/poly(vinyl butyral) mixtures are converted to SiO₂/pyrolytic carbon composites via pyrolysis at low temperatures (500°C) in an Ar atmosphere. Subsequently, low‐temperature magnesiothermic reduction and purification processes result in the formation of tiny SiC nanocrystals. The size of the synthesized SiC nanocrystals ranged from 3 to 12 nm, i.e., they are smaller than the SiO₂ precursor offering large specific surface area of 175.76 m²/g and are single phase as 3C–SiC. Hence, 3C–SiC nanocrystals were successfully synthesized using waste poly(vinyl butyral) through this simple, inexpensive, and scalable process, which will be a new application in the recycling industry.     

Fabrication of mullite-bonded porous silicon carbide ceramics by in situ reaction bonding

An in situ reaction bonding technique was developed to fabricate mullite-bonded porous silicon carbide (SiC) ceramics in air from SiC and α-Al₂O₃, using graphite as the pore-former. Graphite is burned out to produce pores and the surface of SiC is oxidized to SiO₂ at high temperature. With further increasing the temperature, the amorphous SiO₂ converts into cristobalite and reacts with α-Al₂O₃ to form mullite (3Al₂O₃·2SiO₂). SiC particles are bonded by the mullite and oxidation-derived SiO₂ to obtain porous SiC ceramics. The reaction bonding behavior, open porosity, pore size distribution and mechanical strength of porous SiC ceramics were investigated as a function of the sintering temperature, forming pressure and graphite content. In addition, the phase composition and microstructure were also studied.     

Laser ablation behavior of flake graphite and SiO2 particle-filled hyperbranched polycarbosilane matrix composite coatings

Composite coatings consisting of flake graphite and SiO₂ fillers in a hyperbranched polycarbosilane (HBPCS) matrix were designed and prepared to meet the requirements of laser protection. The laser ablation behavior of the composite coatings were investigated. Control experiments were designed to study the performance of SiO₂ during laser irradiation. The results show that the introduction of SiO₂ changes the anti-laser protective mechanism and can improve the anti-laser property of the coating. High power laser irradiation results in pyrolysis of HBPCS and the formation of SiC particles. Chemical reactions between SiO₂, graphite, and SiC play an important role in consuming energy, and provide an excellent cooling effect to the substrate, leading to decreased temperature. SiC particles formed on the surface of the laser ablation area act as a shield to prevent the laser from irradiating deeper layers of the coating. Due to the cooling effect and thermal stability of SiC, the proposed coating shows a good anti-laser property.     

Diameter controlled growth of single-walled carbon nanotubes from SiO2 nanoparticles

A rational approach is reported for the growth of single-walled carbon nanotubes (SWCNTs) with controlled diameters using SiO₂ nanoparticles in a chemical vapor deposition system. The SiO₂ nanoparticles with different sizes were prepared by thermal oxidation of 3-aminopropyltriethoxysilane (APTES) with different number of layers which were assembled on Si substrates. It was found that the size of SiO₂ nanoparticles increased with the number of assembled APTES layers. Using these SiO₂ nanoparticles as nucleation centers, the diameter distribution of as-grown SWCNTs were correlated with the size of SiO₂ particles. In addition, both the classical longitudinal optical or transverse optical bands of SiC in in situ Raman spectra during the whole growth process and the Si 2p peak of SiC in the X-ray photoelectron spectra were not observed, suggesting that the carbon sources did not react with the SiO₂ nanoparticles during the growth. Comparing to vapor–liquid–solid mechanism for metallic catalysts, vapor–solid mechanism is proposed which results in a lower growth rate when using SiO₂ nanoparticles as nucleation centers.     

Fabrication of Si3N4–SiC/SiO2 composites using 3D printing and infiltration processing

Si₃N₄–SiC/SiO₂ composites were prepared by employing three-dimensional (3D) printing using selective laser sintering (SLS) and infiltration processing. The process was based on the infiltration of silica sol into porous SLS parts, and silicon carbide and silicon nitride particles were bonded by melted nano-sized silica particles. To optimize the manufacturing process, the phase compositions, microstructures, porosities, and flexural strengths of the Si₃N₄–SiC/SiO₂ composites prepared at different heat-treatment temperatures and infiltration times were compared. Furthermore, the effects of the SiC mass fraction and the addition of Al₂O₃ and mullite fibers on the properties of the Si₃N₄–SiC/SiO₂ composites were investigated. After repeated infiltration and heat treatment, the flexural strength of the 3D-printed Si₃N₄–SiC/SiO₂ composite increased significantly to 76.48 MPa. Thus, a Si₃N₄–SiC/SiO₂ composite part with a complex structure was successfully manufactured by SLS and infiltration processes.     

In-situ synthesis of ultra-fine ZrB2–ZrC–SiC nanopowders by sol-gel method

ZrB₂–ZrC–SiC nanopowders with uniform phase distribution were prepared from cost-effective ZrOCl₂·8H₂O by a simple sol-gel method. The synthesis route, ceramization mechanism and morphology evolution of the nanopowders were investigated. ZrB₂–ZrC–SiC ceramic precursor can be successfully obtained through hydrolysis and condensation reactions between the raw materials. Pyrolysis of the precursor was completed at 650 °C, and it produced ZrO₂, SiO₂, B₂O₃ and amorphous carbon with a yield of 39% at 1300 °C. By heat-treated at 1500 °C for 2 h, highly crystallized ZrB₂–ZrC–SiC ceramics with narrow size distribution were obtained. With the holding time of 2 h, both the crystal size and the particle size can be refined. Further prolonging the holding time can lead to serious particles coarsening. Studies on the microstructure evolution of the generated carbon during the ceramic conversion demonstrates the negative effect of the ceramic formation on the structure order improvement of the carbon, due to the large amount of defects generated in it by the boro/carbothermal reduction reactions.     

Electrochemical investigation of the codeposition of SiC and SiO2 particles with nickel

The use of electrochemical impedance spectroscopy (EIS) for the in situ control of the electrolytic codeposition of Ni/SiO₂ and Ni/SiC was investigated. An attempt was made to clarify why silica particles hardly codeposit in comparison to silicon carbide particles. It was found that the presence of SiO₂ and SiC particles influences the metal deposition process in different ways. SiC particles that are being embedded in the growing metal layer cause an apparent decrease in the electrode surface area, probably due to blocking off a part of the surface by partly engulfed particles. In the case of SiO₂ particles, which embed in the metal matrix to a very limited extent, no blocking was observed. It was found that the presence of particles in the solution causes an apparent increase in the electrode surface area, probably due to increased surface roughness.     

Synthesis of SiC/SiO2 core–shell powder by rotary chemical vapor deposition and its consolidation by spark plasma sintering

SiC (core) and SiO₂ (shell) powders were synthesized via rotary chemical vapor deposition (RCVD). The SiC particles (3C, <1 μm in diameter) were coated with a layer of SiO₂ (10–15 nm in thickness). Using spark plasma sintering, the SiC/SiO₂ nanopowders were then synthesized into SiC/SiO₂ composite bodies. Although a phase transformation from 3C to 6H was observed at above 2123 K in the sintered monolithic SiC bodies, sintered SiC/SiO₂ bodies did not display such phase transformation. In addition, SiC/SiO₂ bodies did not exhibited grain growth until the sintering temperature reached 2223 K. The density and Vickers hardness of the sintered SiC/SiO₂ bodies increased with increasing sintering temperature. The highest density and hardness of SiC/SiO₂ composite bodies were 98.1% and 24.4 GPa at 2223 K, respectively, which were higher than the corresponding values of 90% and 14 GPa for monolithic SiC bodies.     

Microstructure and mechanical properties of SiC fibres after exposure at elevated temperature

To understand the service behaviour of SiC fibres, the effects of ambient environment and temperature on the microstructure, mechanical property and oxidation behaviour of these fibres were investigated. The result shows that, the surface of SiC fibres becomes rough after exposure in air from 973 to 1573?K due to the formation of small SiO₂ particles, and a smooth SiO₂ film will be formed on the SiC fibre at 1773?K. In Ar atmosphere, SiC fibres will change into clusters of large SiC crystals after heat treatment for 2?h at 2373?K. The tensile strength of SiC fibres decreased by 66 and 95% when the fibres were exposed at 1773?K for 5?min in air and 2373?K for 2?h in Ar, respectively. This degradation is associated with the evaporation of CO and SiO from the fibres as well as with SiC grain growth in the fibres.     

SiC microsphere derived from phenolic resin—Ethyl silicate heterogeneous precursors with carbothermic reduction

Heterogeneous precursors for SiC particles were synthesized from viscous phenolic resin and tetraethyl orthosilicate with hastening ethanol removal process in a concentrated state. In the pyrolysis products, spherical carbon domains with diameter of several micrometers were embedded in the SiO₂‐C matrix. After the carbothermic reduction process at 2073 K, SiC‐C spheres were obtained with an SiC yield of ～80%, while the carbon domains acted as templates of the SiC‐C spheres with complete disappearance of the SiO₂‐C matrix area. After the carbon burned out, some of the spheres possessed a hollow structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1612–1618, 2004     

SiC-assisted growth of tubular graphenic cones with carbon nanotube tips

SiC-assisted growth of tubular graphenic cones (TGCs) with carbon nanotube tip is achieved with high yield in the microwave plasma chemical vapor deposition process. No pre-existing metal or semiconductor catalyst particles are required on the substrate prior to the deposition process. Instead, the in situ grown SiC crystallites serve as the catalyst for the growth of TGCs. Thanks to the easy formation of SiC on various substrates, the process is compatible with a wide range of substrates, i.e. Si, diamond, 2H–SiC, GaN, and SiO₂, but not only limited to them. In addition to developing the approach, the mechanism for the formation of TGCs is also proposed. When Si is used as the substrate, the 3C–SiC crystallites grow epitaxially on the surface, which further initiate the epitaxial growth of graphene with its basal plane parallel to the {111} planes of 3C–SiC. The further expansion of graphene is constrained by the 3C–SiC crystallites, leading to the formation of curved graphene layers, i.e. onion-like or bowl-shaped graphene-based carbon. These curved graphene layers are believed to be the possible nucleation sites for the growth of TGCs. Inclusion of boron in the gas phase promotes the growth rates and the final yields of TGCs.     

Design and preparation of composite coatings with increased reflectivity under high-energy continuous wave laser ablation

High-energy continuous wave (CW) laser ablation can cause severe damage to structural materials in an extremely short time, which generates considerable concern in terms of material safety. For the purpose of reducing or even eliminating such laser-induced damage, a novel composite coating consisting of a boron-modified phenolic formaldehyde resin incorporating ZrC and SiC has been designed and prepared. The experimental results reveal that ZrC and SiC are rapidly oxidized to ZrO₂ and SiO₂ respectively, leading to the formation of a white ceramic layer consisting of ZrO₂ particles and melted SiO₂. After ablation at 1000 W/cm² for 50 s, elemental analysis indicates that no Si can be found in the central ablation zone because of gasification. A relatively compact ZrO₂ layer is formed through the sintering of adjacent ZrO₂ particles, which effectively improves the reflectivity of the coating from 7.3% (before ablation) to 63.5% (after ablation). The high reflectivity greatly reduces the absorption of laser energy. In addition, no obvious ablation defects are observed in the composite coating. The excellent anti-laser ablation performance of the coating makes it a promising system for protecting a material against the effects of long-term CW laser ablation.     

Influence of Y2O3 addition on the mechanical and oxidation behaviour of carbon fibre reinforced ZrB2/SiC composites

The influence of Y₂O₃ addition on the microstructure, thermo-mechanical properties and oxidation resistance of carbon fibre reinforced ZrB₂/SiC composites was investigated. Y₂O₃ reacted with oxide impurities present on the surface of ZrB₂ and SiC grains and formed a liquid phase, effectively lowering the sintering temperature and allowing to reach full density at 1900 °C. The presence of a carbon source (fibres) led to additional reactions which resulted in the formation of new secondary phases such as yttrium boro-carbides. Mechanical properties were significantly enhanced compared to the un-doped composite. Further tests at high temperatures resulted in strength increase up to 700 MPa at 1500 °C which was attributed to stress relaxation. Oxidation tests carried out at 1500 °C and 1650 °C in air showed that the presence of the Y-based secondary phases enhanced the growth of ZrO₂ grains, but offered limited protection to oxygen due to the lower availability of surficial SiO₂ formed from SiC.     

Carbothermal reduction reaction enhanced wettability and brazing strength of AgCuTi-SiO2f/SiO2 system

A carbothermal reduction reaction (CRR) approach was developed in this research to tailor the surface phase structure of the SiO_2f/SiO₂ composites with high chemical reactivity to replace the original inert surface. Results show that SiC can form after CRR treatments. For AgCuTi-SiO_2f/SiO₂ wetting interfaces, TiC and residual pyrolytic carbon layer can be found inside the reaction layer, which was the key, promoting the wettability of the AgCuTi-SiO_2f/SiO₂ system. The contact angle of the AgCuTi-SiO_2f/SiO₂ system dropped from 127° to 43° after the CRR treatments. The reliability of the bonded AgCuTi-SiO_2f/SiO₂ interface was also characterized by putting 3 different systems into comparison, i.e., the original AgCuTi-SiO_2f/SiO₂ system, the AgCuTi-SiO_2f/SiO₂ system with CRR treatments (SiC formation) and the AgCuTi-SiO_2f/SiO₂ system coated with powdered carbon (no SiC formation). The shear strength of the SiO_2f/SiO₂-AgCuTi-SiO_2f/SiO₂ system with CRR treatments was the highest, which was 3 times that of the other 2 brazing systems.     

Selective laser reaction synthesis of SiC,Si3N4 and HfC/SiC composites for additive manufacturing

Selective laser reaction sintering techniques (SLRS) techniques were investigated for the production of near net-shape non-oxide ceramics including SiC, Si₃N₄, and HfC/SiC composites that might be compatible with prevailing powder bed fusion additive manufacturing processes. Reaction bonded layers of covalent ceramics were produced using in-situ reactions that occur during selective laser processing and layer formation. During SLRS, precursor materials composed of metal and/or metal oxide powders were fashioned into powder beds for conversion to non-oxide ceramic layers. Laser-processing was used to initiate simultaneous chemical conversion and local interparticle bonding of precursor particles in 100 vol% CH₄ or NH₃ gases. Several factors related to the reaction synthesis process—precursor chemistry, gas-solid and gas-liquid synthesis mechanisms, precursor vapor pressures—were investigated in relation to resulting microstructures and non-oxide yields. Results indicated that the volumetric changes which occurred during in-situ conversion of single component precursors negatively impacted the surface layer microstructure. To circumvent the internal stresses and cracking that accompanied the conversion of Si or Hf (that expands upon conversion) or SiO_x (that contracts during conversion), optimized ratios of the precursor constituents were used to produce near isovolumetric conversion to the product phase. Phase characterization indicated that precipitation of SiC from the Si/SiO₂ melt formed continuous, crack-free, and dense layers of 93.7 wt% SiC that were approximately 35 µm thick_, while sintered HfC/SiC composites (84.2 wt% yield) were produced from the laser-processing of Hf/SiO₂ in CH₄. By contrast, the SLRS of Si/SiO_x precursor materials used to produce Si₃N₄ resulted in whisker formation and materials vaporization due to the high temperatures required for conversion. The results demonstrate that under appropriate processing conditions and precursor selection, the formation of near net-shape SiC and SiC composites might be achieved through single-step AM-compatible techniques.     

Preparation of SiO2–SiC composites with a precursor method

SiO₂–SiC composite particles were prepared through a hybrid sol–gel precursor process. Compacts were prepared by using a conventional sintering process. The techniques of DSC–TG, SEM and XRD were use to characterize the composite particles and the sintered compacts. It was found that a core–shell structure was constructed in the composite particles with cores of SiC and shells of amorphous SiO₂. Nucleation of SiO₂ occurred at about 1200 °C. The optimized sintering temperature for 30SiO₂–70SiC (vol.%) composites was about 1400 °C with a relatively homogeneous microstructure. The maximum density was about 2.03 g cm^?3.