Combustion synthesis of SiC/Al2O3 composite powders with SiC nanowires and their growth mechanism

SiC/Al₂O₃ composite powders with SiC nanowires were synthesized using a one-step combustion synthesis method taking silica fume (SiO₂), aluminum powder (Al) and carbon black (CB) as raw materials, while ferrocene (C₁₀H₁₀Fe) was used as the catalyst. The calculated results for the relationship between the equilibrium phase and temperature of the Al–SiO₂–C system show that SiC and Al₂O₃ are the only equilibrium phases in the system. In addition, the effects of C₁₀H₁₀Fe on the combustion synthesis process and products were studied. It was found that with increasing catalyst content, the amount of residual Si in the products first decreases and then increases, the combustion temperature first increases and then decreases, and the nanowire content continues to increase. For an optimal amount of C₁₀H₁₀Fe of 0.75 wt%, almost no residual Si is observed in the product, while the combustion temperature (T_c) is high (2104 K), the SiC nanowire content is relatively high, and the nanowire aspect ratio is large. In addition, two growth mechanism models for SiC nanowires: VS and VLS were validated.     

Fabrication and mechanical properties of carbon fibers/lithium aluminosilicate ceramic matrix composites reinforced by in-situ growth SiC nanowires

To improve the mechanical properties of carbon fibers/lithium aluminosilicate (C_f/LAS) composites, C_f/LAS with in-situ grown SiC nanowires (SiC_nw-C_f/LAS) were prepared by chemical vapor phase reaction, precursor impregnation, and hot press sintering, consecutively. The effect of multi-scaled reinforcements (micro-scaled C_f and nano-scaled SiC_nw) on the mechanical properties was investigated. The phase composition, microstructure and fracture surface of the composites were characterized by XRD, Raman Spectrum, SEM, and TEM. The morphology of SiC_nw has a close relation with the content of Si. Microstructure analysis suggests that the growth of SiC nanowires depends on the VLS mechanism. The multi-scale reinforcement formed by C_f and SiC_nw can significantly improve the mechanical properties of C_f/LAS. The bending strength of SiC_nw-C_f/LAS reaches to 597 MPa, achieving an increase of 19% to C_f/LAS. Moreover, the samples show a maximum fracture toughness of 11.01 MPa m^1/2, achieving an increase of 46.4% to C_f/LAS. Through analysis of the fracture surface, the improved mechanical properties could be attributed to the multi-scaled reinforcements by the pull-out and debonding of C_f and SiC_nw from the composites.     

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.     

Theoretical design and preparation of SiC whiskers catalyzed by Fe-oxides on carbon fibers

This work reported an in-situ vapor-liquid-solid (VLS) preparation method of SiC whiskers (SiC_w) catalyzed by Fe-oxides on carbon fibers, which could provide a method for preparing SiC_w/carbon fiber composites. The mechanism of the SiC_w was theoretically designed and then experimentally validated using XRD, SEM, and TEM. Fe₂O₃ was chosen as a Fe-oxide catalyst and directly loaded on carbon fibers by the impregnation process. The results showed that SiC_w were successfully prepared on carbon fibers at 1600 °C under the protection of flowing nitrogen, utilizing quartz and graphite as gas-phase generation sources. The prepared SiC_w were β-SiC and grew along the (111) crystal plane, with spherical droplets on top formed by Fe₂O₃ catalysts. SiC_w were microstructurally observed to have widths of 500–1000 nm and lengths of more than 15 μm, respectively.     

Combustion synthesis of hollow carbon fibers

Mixtures of calcium carbide with different oxidizers, such as C₂Cl₆, C₆Cl₆, (C₂F₄) _n , and (CF) _n , were investigated. Reactions between these substrates in the presence of sodium azide are exothermic enough to proceed in a high-temperature self-sustaining regime. Combustion of tested mixtures was performed in the presence/absence of ferrocene (Fc) as an agent catalyzing the growth of nanostructures. Heat effects accompanying the reactions were measured and solid reaction products were analyzed. SEM and TEM observations revealed the presence of multi-walled hollow carbon fibers in combustion products formed in the CaC₂/C₂Cl₆/NaN₃/Fc mixture. Exfoliated graphite was observed in solid combustion products when (CF) _n was used as an oxidizer. In others systems, a soot-like morphology was found to be predominant.      

Multifunction properties of SiOC reinforced with carbon fiber and in-situ SiC nanowires

Herein, the SiC nanowires were successfully fabricated via chemical vapor infiltration (CVI) into carbon fiber felts (CFs) and then the SiOC/SiCnws/CFs composites were synthesized by precursor infiltration and pyrolysis (PIP) processes. Results indicated that the lightweight composites possessed enhanced mechanical performance, low thermal conductivity, and excellent electromagnetic wave absorption properties. Detailedly, the compressive strength reached to 22.0 MPa and 9.6 MPa after two PIP processes cycles in z and x/y directions, respectively. Meanwhile, the composites exhibited tailored electromagnetic wave absorption performance with the effective absorption bandwidth of 3.06 GHz, and the minimum reflection loss (RL_min) was -48.2 dB with a thickness of 3.6 mm. The present work has a guidance to prepare and design multifunction properties for application in harsh environment.     

Enhanced microwave-absorption properties of polymer-derived SiC/SiOC composite ceramics modified by carbon nanowires

Novel microwave-absorbing SiOC composite ceramics with dual nanowires (carbon nanowires (CNWs) and SiC nanowires) with high performances were fabricated by using the polymer-derivation method and heat treatment in Ar atmosphere. The introduction of CNWs in the amorphous SiOC ceramics promotes the ceramic crystallization into SiC nanoparticles and SiC nanowires at lower annealing temperatures, which leads to multi-phases and multiple nano heterogeneous interfaces. The distinctive architectures largely increase the interfacial and dipole polarizations of the composite ceramics. The CNWs/SiC/SiOC composite ceramics exhibit excellent microwave-absorption properties in the Ku band (12.4–18 GHz). The minimum reflection coefficient (RC) is -24.5 dB at a thickness of 1.8 mm, while the maximum effective absorption bandwidth (EAB, the corresponding frequency band in which RC is smaller than -10 dB) is 4.8 GHz at a thickness of 1.9 mm, which make the CNWs/SiC/SiOC composite ceramics promising electromagnetic-wave-absorbing materials.     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

Effect of co-deposited SiC nanowires and carbon nanotubes on oxidation resistance for SiC-coated C/C composites

To protect carbon/carbon composites (C/Cs) against oxidation, SiC coating toughened by SiC nanowires (SiCNWs) and carbon nanotubes (CNTs) hybrid nano-reinforcements was prepared on C/Cs by a two-step technique involving electrophoretic co-deposition and reactive melt infiltration. Co-deposited SiCNWs and CNTs with different shapes including straight-line, fusiform, curved and bamboo dispersed uniformly on the surface of C/Cs forming three-dimensional networks, which efficiently refined the SiC grains and meanwhile suppressed the cracking deflection of the coating during the fabrication process. The presence of SiCNWs and CNTs contributed to the formation of continuous glass layer during oxidation, while toughed the coating by introducing toughing methods such as bridging effect, crack deflection and nanowire pull out. Results showed that after oxidation for 45 h at 1773 K, the weight loss percentage of SiC coated specimen was 1.35%, while the weight gain percentage of the SiCNWs/CNTs reinforced SiC coating was 0.03052% due to the formation of continuous glass layer. After being exposed for 100 h, the weight loss percentage of the SiCNWs/CNTs reinforced SiC coating was 1.08%, which is relatively low.     

Boosting biotribological and biological properties of carbon fiber composites by grafting SiC nanowires and hydroxyapatite for hip joint application

Carbon fiber composites have great potential application in hip joints, where excellent biotribological and biological properties are required. In this work, SiC nanowires (SiCnws) and hydroxyapatite (HA) were grafted into carbon fiber composites by carbothermal reduction and electrochemical deposition method to obtain carbon fiber-SiCnws-hydroxyapatite-carbon composites (CHS). Results show that SiCnws are grown on the surface of carbon fibers and fill the gaps between carbon fibers. Particle-shaped HA covers both carbon fibers and SiCnws completely. The introduction of SiCnws improves the biotribological properties of CHS. Compared with carbon fiber-hydroxyapatite-carbon composites without SiCnws, the friction coefficient of CHS increased from 0.196 to 0.269, while the wear rate decreased from 0.032 × 10^?14 m³/(N·m) to 0.016 × 10^?14 m³/(N·m). Biological tests show that CHS could promote cell proliferation and differentiation effectively. With these improved biotribological and biological properties, synthesized CHS composites exhibits a potential to be used as hip joints.     

In situ modulus and strength of carbon fibers in C/SiC composites

The in situ elastic modulus and strength distribution of carbon fibers in C/SiC composites were studied. To obtain the in situ property data, fibers were heat treated according to the fabrication process of C/SiC composites. Tensile tests were performed on the single fibers and fiber bundles. The equivalent in situ modulus and strength were proposed considering the loose and unparallel fibers in the composites. The experimental and numerical results showed that the equivalent elastic modulus and average strength of in situ fibers are much lower than that of the original fibers. In addition, the equivalent strength distribution of in situ fibers is more dispersive.     

Mechanical,thermal, and dielectric properties of SiCf/SiC composites reinforced with electrospun SiC fibers by PIP

Electrospun unidirectional SiC fibers reinforced SiC_f/SiC composites (e-SiC_f/SiC) were prepared with ∼10% volume fraction by polymer infiltration and pyrolysis (PIP) process. Pyrolysis temperature was varied to investigate the changes in microstructures, mechanical, thermal, and dielectric properties of e-SiC_f/SiC composites. The composites prepared at 1100 °C exhibit the highest flexural strength of 286.0 ± 33.9 MPa, then reduced at 1300 °C, mainly due to the degradation of electrospun SiC fibers, increased porosity, and reaction-controlled interfacial bonding. The thermal conductivity of e-SiC_f/SiC prepared at 1300 °C reached 2.663 W/(m∙K). The dielectric properties of e-SiC_f/SiC composites were also investigated and the complex permittivities increase with raising pyrolysis temperature. The e-SiC_f/SiC composites prepared at 1300 °C exhibited EMI shielding effectiveness exceeding 24 dB over the whole X band. The electrospun SiC fibers reinforced SiC_f/SiC composites can serve as a potential material for structural components and EMI shielding applications in the future.     

Fabrication of modified ultra high-temperature ceramic hybrid powders using in situ grown SiC nanowires

Ultra-high temperature ceramic (UHTC) hybrid powders modified using in situ grown SiC nanowires (SiCNWs) were successfully prepared via a simple catalytic method. The self-produce carbon and silicon source promoted the growth of SiCNWs during the pyrolysis process of ZrB₂ polymer precursors coated ZrB₂-SiC powders. The results showed that the growth of SiCNWs could be explained by a tip-growth model and vapor–liquid–solid (VLS) growth mechanism. The SiCNWs with diameter of 200 nm were single crystalline, and the content could be controlled by changing the catalyst content.     

In situ preparation of SiC/Si3N4-NW composite powders by combustion synthesis

Large-scale composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized in situ by combustion synthesis (CS). In this process, a mixture of silicon, carbon black, polytetrafluoroethylene (PTFE) and a small amount of iron powders was used as the precursor. The products were characterized by XRD, SEM, EDS and TEM. The particles are equiaxed with diameters in the micron range, and the in situ formed nanowires are straight with uniform diameters of 20-350 nm and lengths of tens of microns. The Si₃N₄-NWs are characterized to be α-phase single crystals grown along the [1 0 1] or [1 0 0] direction. VLS and SLGS processes are proposed as the growth mechanisms of the nanowires. The as-synthesized powders have great potential for use in the preparation of high-performance SiC/Si₃N₄-NW composites.     

Preparation and characterization of a graphite electrode containing carbon nanotubes grown in situ by flame synthesis

A crystalline flake graphite electrode (GE) was impregnated with nickel particles using direct current electrochemical deposition. The particles were used for in situ growth of carbon nanotubes (CNTs) by flame synthesis with a liquid ethanol flame. The obtained electrode was characterized by X-ray diffraction, and scanning and transmission electron microscopy. The results showed that the deposited Ni catalyst crystal face was mainly (1 1 1). CNTs with a diameter of about 40 nm were uniformly grown on the GE surface. The electrochemical performance of the CNT–GE was characterized by cyclic voltammetry using a [Fe(CN)6]³⁻/[Fe(CN)6]⁴⁻ solution, and showed a much greater electrochemical response than that obtained using a material in which CNTs were grown by catalytic chemical vapor deposition.     

Direct ink writing of chopped carbon fibers reinforced polymer-derived SiC composites with low shrinkage and high strength

The chopped carbon fiber reinforced SiC (C_f/SiC) composite has been regarded as one of the excellent high-temperature structural materials for applications in aerospace and military fields. This paper presented a novel printing strategy using direct ink writing (DIW) of chopped fibers reinforced polymer-derived ceramics (PDCs) with polymer infiltration and pyrolysis (PIP) process for the fabrication of C_f/SiC composites with high strength and low shrinkage. Five types of PDCs printing inks with different C_f contents were prepared, their rheological properties and alignment of carbon fiber in the printing filament were studied. The 3D scaffold structures and bending test samples of C_f/SiC composites were fabricated with different C_f contents. The results found that the C_f/SiC composite with 30 wt% C_f content has high bending strength (～ 7.09 MPa) and negligible linear shrinkage (～ 0.48%). After the PIP process, the defects on the C_f/SiC composite structures were sufficiently filled, and the bending strength of C_f/SiC composite can reach up to about 100 MPa, which was about 30 times greater than that of the pure SiC matrix without C_f. This work demonstrated that the printed C_f/SiC composites by using this method is beneficial to the development of the precision and complex high-temperature structural members.     

Integration of thermal protection and structural health monitoring for carbon fiber reinforced SiBCN composites with SiC coating enhanced interfacial performance

Carbon fiber (CF) reinforced SiBCN composites were generally considered as key candidates for thermal protection under severe aerodynamic heating in aerospace due to their excellent high-temperature properties, but they were also faced with challenges in stably monitoring the structural integrity of themselves under extreme conditions. Herein, with the introduction of SiC coating, multifunctional CF-SiC/SiBCN composites were fabricated which integrated functions of the thermal protection and the structural health monitoring. Compared to the untreated CF/SiBCN composites, the stability of sensing of CF-SiC/SiBCN composites was greatly improved and the sensitivity of CF-SiC/SiBCN composites maintained at a high level with a gauge factor of 652.65. Furthermore, additional researches revealed that the CF-SiC/SiBCN composites enjoyed a high compressive strength (155.33 MPa), a light weight (1.07–1.61 g/cm³), and a relatively low thermal conductivity (4.02 W/(m·K)), which showed a potentiality of CF-SiC/SiBCN composites to be applied as multifunctional structural components in thermal protection systems.     

Achieving high permeability of in-situ formed lightweight Cf/SiC(rGO)px/SiC porous ceramics with improved interfacial compatibility for transpiration cooling

Making lightweight porous ceramics with excellent permeability applied for transpiration cooling is still challenging. Herein, an ingenious fabrication method is proposed to successfully prepare C_f/SiC(rGO)_px/SiC porous ceramics possessing low density, high permeability and satisfactory mechanical properties. The introduction of carbon fibers for constructing channels and SiC(rGO)_p with three-dimensional (3D) honeycomb cellular net-like structure, could effectively decrease density and improve porosity. Meanwhile, self-supporting porous skeleton, high open porosity and uniform pores distribution contribute to brilliant permeability of the products. Good interfacial compatibility among SiC(rGO)_p, carbon fibers and β-SiC/SiOxCy/C_free matrix, as well as toughening effects of carbon fibers are beneficial for enhancing fracture toughness and compressive strength. Particularly, C_f/SiC(rGO)_p0.6/SiC porous ceramics exhibit low density (1.12 g·cm^?3), low linear shrinkage (3.22%), especially high permeability (1.36 ×10^?7 mm²), satisfactory fracture toughness (1.77 MPa·m^1/2), excellent hardness (3.88 GPa) and compressive strength (6.41 MPa), focusing on potential applications as coolant medium in transpiration cooling.     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection