Synthesis of resorcinol-formaldehyde/silica composite aerogels and their low-temperature conversion to mesoporous silicon carbide

Resorcinol-formaldehyde/silica composite (RF/SiO₂) gels were acid-catalyzed formed in one pot at 27 °C within 60 min, and then dried to aerogels with supercritical fluid CO₂. After carbonization in nitrogen atmosphere and a magnesiothermic reaction at 700 °C, RF/SiO₂ aerogels were successfully converted to monolithic mesoporous silicon carbide (SiC aerogels). The starting RF/SiO₂ aerogels had an interpenetrating organic/inorganic network and the resulting SiC products preserved monolithic mesoporous morphology similar to the original templates. The as-synthesized SiC aerogels consisted of nanocrystalline β-SiC, possessed a BET surface area of 232 m²/g and showed sufficient mesoporosity. They had a direct band gap of about 3.2 eV (less than that of bulk β-SiC) and showed photoluminescence at room temperature. The mechanisms about the formation of RF/SiO₂ gels and the conversion to SiC aerogels were discussed. Potentially, the reported method can be used to convert many metal or semimetal oxide/carbon composite aerogels to carbide aerogels at relatively low temperature for many catalytic, electronic, photonic and thermal applications.     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

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.     

Novel high-temperature-resistant Y2SiO5 aerogel with ultralow thermal conductivity

A novel Y₂SiO₅ ternary aerogel was prepared from tetraethoxysilane and yttrium chloride hexahydrate via the sol-gel method followed by high-temperature calcination. The effects of different calcination temperatures on the microstructure, mechanical and thermal stability of the Y₂SiO₅ aerogels were investigated. The aerogels exhibited low densities of 0.33-0.62 g/cm³, low thermal conductivities of 0.029-0.05 W/(m·K), and a relatively high strength of 0.16-56.47 MPa. Moreover, compared with the Al₂O₃–SiO₂ aerogel, the Y₂SiO₅ aerogel has higher thermal stability and more excellent high-temperature insulation, which has potential applications as a thermal protection material in hypersonic vehicles.     

Novel self-reinforcing ZrO2–SiO2 aerogels with high mechanical strength and ultralow thermal conductivity

Novel self-reinforcing ZrO₂–SiO₂ aerogels with high mechanical strength and ultralow thermal conductivity are fabricated by impregnating hydrolyzed ZrO₂–SiO₂ sol into wet gel matrix and drying. The ZrO₂–SiO₂ sol fills the macropores and defects of ZrO₂–SiO₂ aerogel matrix generating during the gelation process, which contributes to the improvement of the mechanical properties of the ZrO₂–SiO₂ aerogel matrix. The mechanical and thermal properties of the as-prepared ZrO₂–SiO₂ aerogel are investigated and discussed. The results show that the mechanical strength of the self-reinforcing aerogels obviously increases from 0.51 to 3.11?MPa with the increase of impregnation times, while the thermal conductivity of the aerogels slightly increases from 0.0235 to 0.0306?W?m^?1 K^?1. The novel self-reinforcing ZrO₂–SiO₂ aerogel could have interesting applications in aerospace and energy because of its outstanding mechanical and thermal properties.     

Facile preparation for gelatin/hydroxyethyl cellulose-SiO2 composite aerogel with good mechanical strength,heat insulation,and water resistance

The advanced thermal insulation materials with low cost and high mechanical properties play an important role in transport packaging and thermal protection fields. An inorganic/organic composite aerogel was prepared through hydrogen bonds and chemical crosslinking among silica aerogel particles, gelatin (GA), and hydroxyethyl cellulose (HEC). The as-prepared GA/HEC-SiO₂ composite aerogels were characterized by compression tests, scanning electron microscopy, Fourier transform infrared, thermogravimetric analyzer, and contact angle tests to investigate the chemical composition and physical structure. The GA/HEC-SiO₂ composite aerogels exhibited a strong mechanical strength (0.53–4.01 MPa), a high compression modulus (1.33–11.52 MPa), a lower volume density (0.035–0.081 g/cm³), thermal conductivity as low as 0.035 W/[m K]), a porosity of more than 93%, and hydrophobic angle as high as 150.01° after hydrophobic modification. These results indicate that biopolymer composite aerogels embedded with SiO₂ aerogel particles display a bright future in thermal insulation.     

Porous SiCnw/SiC ceramics with unidirectionally aligned channels produced by freeze-drying and chemical vapor infiltration

The current study introduces a methodology for the fabrication of porous silicon carbide nanowire/silicon carbide (SiC_nw/SiC) ceramics with macroscopic unidirectionally aligned channels and reports on their microstructural and mechanical properties. The material was produced by freezing of a water-based slurry of β-SiC nanowires (SiC_nw) with control of the ice growth direction. Pores were subsequently generated by sublimation of the columnar ice during freeze-drying. Chemical vapor infiltration (CVI) of SiC into the open pore network of the SiC_nw aerogel with unidirectionally aligned channels, resulted in the formation of highly porous SiC_nw/SiC ceramics which exhibited a unique microstructure as identified by scanning electron microscopy. The pore size distribution and the mechanical properties of the as-fabricated porous ceramics were examined by mercury intrusion porosimetry and three-point bending and compression tests, respectively, while phase composition was investigated through X-ray diffraction.     

Nanofibers reinforced silica aerogel composites having flexibility and ultra-low thermal conductivity

For the sake of enhancing the performance of flexible silica aerogel in practical applications, flexible SiO₂/SnO₂ nanofibers (SSNF) reinforced flexible silica aerogel composites (abbreviated as SiO₂-SSNF) were successfully prepared. Firstly, the SiO₂/SnO₂ nanofibers with fine diameter (~320 nm) and excellent flexibility were prepared by electrospinning technology. Then the aerogel composites were synthesized by adding the flexible SSNF to the silica solution and through the sol-gel method and ethanol supercritical drying technology. The effects of different content of the nanofibers on thermal conductivity and Yong's modulus of SiO₂-SSNF aerogel composites were investigated. The SiO₂/SnO₂ nanofibers were randomly dispersed in the flexible silica aerogel and the great integrity of the material result in smaller linear shrinkage, better thermal protection, and mechanical properties compared with those pure SiO₂ aerogels. The final SiO₂-SSNF aerogel composites possess excellent thermal conductivity (0.025-0.029 W/(m∙K)) and higher Yong's modulus (70 kPa), which was twice than that of the pure silica aerogel. This prepared SiO₂-SSNF aerogel composites can be better used in thermal insulation due to its excellent flexible and thermal insulation property.     

Characterization of nanocrystalline porous SiC powder by electrospinning and carbothermal reduction

Nanocrystalline porous silicon carbide (SiC) powder was prepared from various ratios with SiO₂ as silicon source and carbon black as carbon source by electrospinning method and subsequent carbothermal reduction. Their microstructure, thermal stability and porosity of prepared porous silicon carbide (SiC) powder were evaluated as function of Si to C ratios in precursor. 3C/Si could not often maintain the spherical form against oxygen–carbon reaction during calcination owing to relatively low carbon content. On the other hand, the uniformed spherical forms were presented with deep wrinkles in the other prepared samples. They were believed that the wrinkled surfaces were formed by vaporization of carbonaceous gas made from carbon and oxygen. When the carbon ratio to silicon (C/Si) was 5, the β-SiC was observed without remained silica and silicon metal. Further increase in the carbon content in the precursor led to sharp and high intensity of SiC peak. Initial decomposition temperature of prepared sample was increased with decreasing C/Si ratio. We confirmed that whereas 3C/Si and 4C/Si had higher big macropore, 5C/Si and 6C/Si had much large pore volume in the range of mesopore diameter (2–50 nm) and small macropore (50–100 nm).     

Improved antioxidative and mechanical properties of SiC coated C/C composites via a SiO2-SiC reticulated layer

To improve the oxidation resistances of SiC coated C/C composites by a pack cementation (PC) method at high temperature and alleviate the siliconization erosion of molten silicon on C/C substrate during the preparation of SiC coating, a SiO₂-SiC reticulated layer with SiC nanowires was pre-prepared on C/C composites through combined slurry painting and thermal treatment before the fabrication of SiC coating. The presence of porous SiO₂-SiC layer with SiC nanowires was beneficial to fabricate a compact and homogeneous SiC coating resulting from synergistic effect of further reaction between SiO₂ and pack powders and the reinforcement of SiC nanowires. Therefore, the results of thermal shock and isothermal oxidation tests showed that the mass loss of modified SiC coating was only 0.02 % after suffering 50-time thermal cycles between room temperature and 1773 K and decreased from 5.95 % to 1.08 % after static oxidation for 49.5 h in air at 1773 K. Moreover, due to the blocking effect of SiO₂-SiC reticulated layer on siliconization erosion during PC, the flexural strength of SiC coated C/C composites with SiO₂-SiC reticulated layer increased by 64.8 % compared with the untreated specimen.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

Opacified graphene-doped silica aerogels with controllable thermal conductivity

In this work, we developed a new type of thermal insulation materials by combining the silica aerogel (SiO₂) and graphene (G) followed by aging and supercritical drying. The effects of different G/SiO₂ mass ratios on the microstructures and properties of opacified G/SiO₂-x composite aerogels were investigated. The results showed that the graphene was well-distributed in the SiO₂ matrix. Meanwhile, the opacified composite aerogels showed high-specific surface area (~?1000 m²/g). Due to the unique bandgap feature and conjugated large π bond of graphene, the thermal insulation property of G/SiO₂-x composite aerogels was enhanced in contrast with the pure SiO₂ aerogel. Moreover, a possible mechanism of heat transfer was discussed to interpret the result.     

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.     

Effects of SDBS and ZrO2 additives on the microstructure and properties of silicon-bonded SiC porous ceramics

Porous silicon-bonded silicon carbide (SBSC) ceramics were prepared under argon atmosphere, with silicon as pore former and bonding material, simultaneously, sodium dodecyl benzene sulfonate (SDBS) and ZrO₂ as sintering additives, the effects of SDBS and ZrO₂ on the porosity, pore size, mechanical, physical and thermal properties and microstructures were investigated. The results suggested that suitable content of SDBS and ZrO₂ could not only effectively lower the sintering temperature to 1450 °C due to the sticky flow of molten silicon, but also increase the pore structure and improve the bending strength. The reason for this is that SDBS decomposed into Na₂O which reacted with ZrO₂ and impurity SiO₂, which was the native oxide film on the surface of SiC particles, to form a bonding phase between SiC particles to improve the bending strength; meanwhile, the disappearances of impurity SiO₂ would benefit the bond of molten silicon and silicon carbide particles, and silicon melt leaving pores in its original position to increase the pore structure. The optimal apparent porosity, bending strength, average pore size, gas permeance and residual bending strength after thermal shock cycles of SBSC porous ceramic sintered at 1450 °C with 5 wt% SDBS and 6 wt% ZrO₂ were 38.33%, 55.4 MPa, 11.3 μm, 106.4 m³/m²·h·kPa and 28.2 MPa, respectively.     

Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

The high temperature compressive strength behavior of zirconium diboride (ZrB₂)-silicon carbide (SiC) particulate composites containing either carbon powder or SCS-9a silicon carbide fibers was evaluated in air. Constant strain rate compression tests have been performed on these materials at room temperature, 1400, and 1550 °C. The degradation of the mechanical properties as a result of atmospheric air exposure at high temperatures has also been studied as a function of exposure time. The ZrB₂-SiC material shows excellent strength of 3.1 ± 0.2 GPa at room temperature and 0.9 ± 0.1 GPa at 1400 °C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB₂-SiC-C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB₂-SiC composites contain significant matrix microcracking due to residual thermal stresses, and show poor mechanical properties and oxidation resistance. After exposure to air at high temperatures an external SiO₂ layer is formed, beneath which ZrB₂ oxidizes to ZrO₂. A significant reduction in room temperature strength occurs after 16-24 h of exposure to air at 1400 °C for the ZrB₂-SiC material, while for the ZrB₂-SiC-C composition this reduction is observed after less than 16 h. The thickness of the oxide layer was measured as a function of exposure time and temperatures and the details of oxidation process has been discussed.     

Effect of silicon carbide on the mechanical and thermal properties of ethylene propylene diene monomer-based carbon fiber composite material for heat shield application

In this article, the preparation of silicon carbide (SiC), carbon fiber (CF), and ethylene–propylene–diene monomer composite with different parts per hundred of SiC (phr) ratio (CF/SiC; 10/0, 0/10, 10/10, 10/15, 10/20) and its effect on mechanical and thermal properties have been studied. After the incorporation of SiC up to 20 phr with a stable CF (10 phr), composites demonstrated higher tensile properties up to 12.09 MPa, elongation at break (725%), modulus (3.5 MPa), and hardness (79 Shore-A) at composition (CF10SiC20 phr). Furthermore, the density of the composites has been achieved to a maximum value of 1.081 g/cm³ at composition (CF10SiC20 phr). The synthesized composite's functional group has been analyzed using Fourier-transform infrared spectroscopy. The thermal stability of the composites increased with the inclusion of SiC up to 20 wt% while achieving the 520°C described by thermogravimetric analysis. After gamma irradiation analysis, the mechanical properties of the composites were slightly enhanced. Thermal conductivity of the composites has been maintained up to 0.21 W/mK at CF10SiC20 phr of the composites. The morphology of silicon carbide and carbon fiber was examined using scanning electron microscopy.     

High interfacial thermal resistance induced low thermal conductivity in porous SiC-SiO2 composites with hierarchical porosity

The incorporation of a thermally insulating secondary phase can significantly increase the interfacial thermal resistance attributed to its low intrinsic thermal conductivity and the creation of multiple phonon scattering interfaces between adjacent SiC particles. The newly developed porous SiC-33 wt% SiO₂ composites with SiO₂ as a thermally insulating secondary phase exhibited a very low thermal conductivity (0.047 Wm⁻¹ K⁻¹, 72.4 % porous), which is an order of magnitude lower than the previously reported lowest thermal conductivity (0.14 Wm⁻¹ K⁻¹, 76.3 % porous) for powder processed porous SiC ceramics and is even lower than the thermal conductivity (0.060 Wm⁻¹ K⁻¹, 87.9% porous) of SiO₂ aerogel. The porous SiC-(16–73 wt%) SiO₂ composites processed from nano β-SiC and a 40 wt% carbon template exhibited a hierarchical (meso-/macro-porous) pore structure that transformed to a trimodal (micro-/meso-/macro-porous) porous structure when polysiloxane was added and sintering was performed at 600–1000 °C in air.     

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.     

SiC/MoSi2 based coatings for Cf/C composites by two step pack cementation

In order to improve the oxidation resistance of C_f/Cs produced by chemical vapour infiltration, a multilayer coating based on silicon carbide and molybdenum disilicide was produced by two-step pack cementation technique. The inner SiC layer with a thickness up to 25 μm was obtained without promoted reaction additives by varying the composition, and thermal treatment conditions. The SiC/SiC-MoSi₂ coating was produced with a thickness up to 80 μm by two step pack cementation, considering the effect of the inner layer characteristic. The enhancement of the oxidation resistance, observed in SiC/SiC-MoSi₂ coated C_f/Cs by means of thermal analysis in flowing air up to 1500 °C, was due to the formation of SiO₂ promoted by the passive oxidation of silicon carbide and molybdenum disilicide.     

Facile synthesis of SiOx spheres or dumbbell-shaped β-SiC whiskers on expanded graphite by silicon vapor deposition

A facile method was developed to synthesize SiO_x spheres or dumbbell-shaped β-SiC whiskers on expanded graphite (SiO_x/EG or β-SiC/EG) by silicon vapor deposition without catalyst. With the carbon black atmosphere, the above hybrids were synthesized above 1100 °C in a graphite crucible where silicon powder was placed under the expanded graphite (EG). The growth of SiO_x spheres is controlled by vapor-solid mechanism at 1100 °C and 1200 °C. Namely, the active carbon atoms absorbed SiO (g) and Si (g) to form SiC nuclei. Then, the SiO₂, residual SiO (g) and Si (g) deposited on SiC nuclei to form SiO_x spheres. At 1300 °C and 1400 °C, the same SiO_x spheres formed on EG as well as many dumbbell-shaped β-SiC whiskers. The growth of dumbbell-shaped β-SiC whiskers is controlled by vapor-vapor and vapor-solid mechanism successively. In a word, firstly, the β-SiC whiskers with defects formed via the reaction between Si (g) and CO (g). After that, the SiO₂, residual SiO (g) and residual Si (g) preferentially deposited on defects, then deposited on other parts of β-SiC whiskers to form dumbbell-shaped SiC whiskers.