Monolithic Silicon Carbide Research Articles

Synthesis of monolithic mesoporous silicon carbide from resorcinol–formaldehyde/silica composites

Abstract Resorcinol–formaldehyde/silica composite (RF/SiO2) gels were synthesized in one pot using a facile process. RF/SiO2 aerogels were obtained after supercritical carbon dioxide fluid drying. Monolithic mesoporous silicon carbide (SiC aerogels) was prepared from RF/SiO2 aerogels after carbothermal reduction and calcination. The as-prepared SiC products exhibited monolithic mesoporous morphology and possessed a BET specific surface area of 251 m2/g and a pore volume of 0.965 cm3/g. X-ray diffraction (XRD) and transmission electron microscopy (TEM) demonstrated that the resulting SiC aerogels were composed of α-SiC nanocrystals. The bulk density and skeleton density of SiC products is 0.288 g/cm3 and 3.12 g/cm3, respectively. The porosity of SiC products is 90.8%. The SiC aerogels were stable up to temperatures near 650 °C.

Tritium trapping in silicon carbide in contact with solid breeder under high flux isotope reactor irradiation

The trapping of tritium in silicon carbide (SiC) injected from ceramic breeding materials was examined via tritium measurements using imaging plate (IP) techniques. Monolithic SiC in contact with ternary lithium oxide (lithium titanate and lithium aluminate) as a ceramic breeder was irradiated in the High Flux Isotope Reactor (HFIR) in Oak Ridge, Tennessee, USA. The distribution of photo-stimulated luminescence (PSL) of tritium in SiC was successfully obtained, which separated the contribution of 14C β-rays to the PSL. The tritium incident from ceramic breeders was retained in the vicinity of the SiC surface even after irradiation at 1073K over the duration of ∼3000h, while trapping of tritium was not observed in the bulk region. The PSL intensity near the SiC surface in contact with lithium titanate was higher than that obtained with lithium aluminate. The amount of the incident tritium and/or the formation of a Li2SiO3 phase on SiC due to the reaction with lithium aluminate under irradiation likely were responsible for this observation.

Suitability of ultra-refractory diboride ceramics as absorbers for solar energy applications

Ultra-refractory diborides are currently studied mainly as thermal protection materials for aerospace and military applications. However, their favorable properties (very high melting points and good thermo-mechanical properties at high temperatures) can be advantageously exploited to increase the operating temperature of thermodynamic solar plants in concentrating solar power systems. This paper reports on the spectral reflectance characterization of hafnium and zirconium diborides containing MoSi2 as secondary phase to evaluate their potential as novel solar absorbers. To assess the spectral selectivity properties, room-temperature hemispherical reflectance spectra were measured from the UV wavelength region to the mid-infrared, considering different levels of porosity for each system. Moreover, for zirconium diboride and hafnium diboride composites containing 10vol% of MoSi2 sintering aid, the thermal emittance was measured in the 1100–1400K temperature range. Room temperature spectral characteristics and high temperature emittance were compared to that of a monolithic silicon carbide.

Preparation of cubic ordered mesoporous silicon carbide monoliths by pressure assisted preceramic polymer nanocasting

Ordered mesoporous silicon carbide monoliths (OMSCMs) with three-dimensional (3D) bi-continuous cubic structure (Ia3d) have been successfully prepared using KIT-6 silica as the hard template and the commercial polycarbosilane (PCS-800) as the preceramic precursor. Tablet-like SiC/KIT-6 composite monoliths were formed via nanocasting of PCS-800 into the mesopores of KIT-6 silica by the wet impregnation, followed by pressing the PCS-800/KIT-6 composite powder with the addition of triblock copolymer P123 as a binder, and subsequent pyrolysis at 1073, 1273, or 1473K in argon. The KIT-6 silica template was then dissolved in hydrogen fluoride (HF) solution to generate the silicon carbide (SiC) replicated monoliths with cubic ordered mesoporous structure. The OMSCMs demonstrated good macroscopic tablet-like appearances and no any cracks could be found in spite of the evident shrinkage. They were characterized by small-angle and wide-angle X-ray diffraction (XRD), nitrogen adsorption, Fourier-transform infrared (FT-IR), elemental analysis, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Nitrogen adsorption and small-angle XRD measurements showed that the OMSCMs had very high stability even after re-treatment at 1673K under argon. And the transformation of amorphous into nano-crystalline state for SiC framework in the OMSCMs proceeded with the retention of the tablet-like morphology.

Evaluation of Irradiation Effects on Fracture Strength of Silicon Carbide using Micropillar Compression Tests

Micro-scale material testing techniques (e.g., nanoindentation, micropillar compression, etc.) have been developed and applied for the evaluation of radiation effects on the mechanical properties of nuclear reactor materials. These techniques can measure the mechanical properties from small volumes, which can be beneficial in the investigation of both neutron and ion-irradiated materials. In this study, we evaluated the compressive fracture strength of ion-irradiated silicon carbide (SiC) using micropillar compression tests. SiC and SiC-based composites are considered as candidate materials for micro electronics, sensors, and various components in next-generation reactors. Micropillars with a nominal diameter of 0.65 μm were fabricated on a monolithic polycrystalline cubic SiC plate using a lithography and plasma etching technique. Half of the total micropillars were irradiated with Si2+ ions. The SiC micropillars were uniaxially compressed using a flat diamond punch. A finite element model was used to compute the elastic stress state of the pillar. The results showed irradiation-induced strengthening, which is consistent with other investigations. In addition, plastic deformation of SiC at room temperature was observed. A slip was found to occur on {111} planes from a cross-sectional transmission electron microscopy observation. The micropillar tests are expected to significantly contribute to the study of irradiation and size effect on the mechanical properties of ceramic materials to be used in a radiation environment.

Laser-Assisted Surface Modification of Alumina and Its Tribological Behavior

High performance friction systems, e.g., dry clutches and brakes, require a good wear resistance and a friction coefficient that is nearly independent from sliding velocity and environmental conditions. Organic-based friction materials have reached their limitations regarding higher power densities. Engineering ceramics such as alumina (Al2O3) or silicon carbide (SiC) offer a great potential since remarkably higher thermal and mechanical loading is possible. However, the tribological performance of these monolithic ceramics is still insufficient. The aim of the present study was to assess the potential of a laser-assisted surface modification process in order to improve the tribological performance with regard to the application in dry friction systems. Therefore, commercially available alumina was modified using a newly developed laser-assisted preheating process and subsequent melting of the ceramic’s surface using a CO2-laser and modification by additives such as TiC, TiN, B4C, WC, ZrB2, Cr, Ni, Cu, and Ti. A systematic variation of additives and process parameters led to different multiphase microstructures. Subsequently, these were characterized using scanning electron microscopy and surface analysis methods (wavelength dispersive X-ray spectroscopy, energy dispersive X-ray spectroscopy). Finally, the tribological properties were investigated using a laboratory tribometer. The surface-modified ceramics were tested in unidirectional sliding motion against steel disks. The tribological results of the surface-modified ceramics were compared to those of monolithic Al2O3 and SiC ceramics and showed a reduced dependence of friction coefficient on sliding velocity. Moreover, the multi-phase ceramics possessed a higher wear resistance than the monolithic ones.

Sealants Aim to Reduce Noise Levels in Automobiles

Silicon carbide (SiC) ceramics have superior properties in terms of wear, corrosion, oxidation, thermal shock resistance and high temperature mechanical behavior, as well. However, they can be sintered with difficulties and have poor fracture toughness, which hinder their widespread industrial applications. In this work, SiC-based ceramics mixed with 1 wt% and 3 wt% multilayer graphene (MLG), respectively, were fabricated by solid-state spark plasma sintering (SPS) at different temperatures. We report the processing of MLG/SiC composites, study their microstructure and mechanical properties and demonstrate the influence of MLG loading on the microstructure of sintered bodies. It was found that MLG improved the mechanical properties of SiC-based composites due to formation of special microstructure. Some toughening mechanism due to MLG pull-out and crack bridging of particles was also observed. Addition of 3 wt% MLG to SiC matrix increased the Vickers hardness and Young's modulus of composite, even at a sintering temperature of 1700 °C. Furthermore, the fracture toughness increased by 20% for the 1 wt% MLG-containing composite as compared to the monolithic SiC selected for reference material. We demonstrated that the evolved 4H-SiC grains, as well as the strong interactions among the grains in the porous free matrices played an important role in the mechanical properties of sintered composite ceramics.

Synthesis, characterization, and hydrogen storage capacities of hierarchical porous carbide derived carbon monolith

Hierarchical porous carbide-derived carbon monoliths (HPCDCM) were prepared by selective extraction of silicon from ordered mesoporous silicon carbide monoliths (OMSCM) through chlorination at high temperature. The OMSCM was firstly synthesized by pressure-assisted nanocasting procedure using KIT-6 silica as the hard template and polycarbosilane (PCS-800) as the preceramic precursor. The OMSCM showed cubic ordered mesoporous structure with specific surface area of over 600 m2 g−1. After the chlorination, the resulting HPCDCM demonstrated very high specific surface area (2933 m2 g−1), large pore volume (2.101 cm3 g−1) with large volume of micropores (0.981 cm3 g−1), and narrow dual pore size distributions (micropore: 0.9 nm, and mesopore: 3.1 nm). Macropores in the micron range were observed in the HPCDCM. The mesostructural ordering was not maintained in the HPCDCM and the volume of the HPCDCM had greatly shrunk, by 21.2% compared to that of the OMSCM, but the tablet-like appearance was well retained in the HPCDCM. At −196 °C, the HPCDCM shows good hydrogen uptakes of 2.4 wt% and 4.4 wt% at 1 bar and 36 bar, respectively. The calculated volumetric hydrogen storage capacity is 11.6 g L−1 at 36 bar. The gravimetric hydrogen uptake capacity of the HPCDCM is comparable to, or higher than, those of previously reported ordered mesoporous carbide-derived carbon (CDC) powder and microporous CDC powder.

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

Resorcinol–formaldehyde/silica composite (RF/SiO 2) gels were acid-catalyzed formed in one pot at 27 °C within 60 min, and then dried to aerogels with supercritical fluid CO 2. After carbonization in nitrogen atmosphere and a magnesiothermic reaction at 700 °C, RF/SiO 2 aerogels were successfully converted to monolithic mesoporous silicon carbide (SiC aerogels). The starting RF/SiO 2 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 m 2/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/SiO 2 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.

Study on Compatibility Between Silicon Carbide and Solid Breeding Materials Under Neutron Irradiation

Compatibility of monolithic silicon carbide (SiC) with ternary lithium ceramics (Li1-xAlO2-y, Li2-xTiO3-y, Li2-xZrO3-y and Li4-xSiO4-y) under irradiation of neutrons at high temperatures was studied. Disk samples of SiC in contact with sintered ternary lithium ceramics were irradiated in High Flux Isotope Reactor (HFIR) at 800 °C to 5.9 displacements per atom (dpa). Chemical reactions of SiC as determined by appearance of the surface were relatively less significant for the systems of SiC/Li1-xAlO2-y and SiC/Li2-xTiO3-y, whereas some bonding likely due to chemical reaction between SiC and the lithium ceramics and broken samples were observed in the systems of SiC/Li2-xZrO3-y and SiC/Li4-xSiO4-y. The effect of lithium burnup due to the (n, α) nuclear reaction was also examined by using samples of lithium ceramics whose lithium ratio was hypo-stoichiometric in the fabrication process. More reaction products were observed on the surface of β-SiC in contact with Li1-xAlO2-y having the lower lithium ratio (Li/Al). It was considered that the formation of LiAl5O8 phase due to lithium loss could deteriorate the compatibility of the SiC - Li1-xAlO2-y system.

Study on stress relaxation behavior of silicon carbide by BSR method

Bend stress relaxation (BSR) experiment at temperatures of 600–1400°C for 1–100h was performed on the two types of highly crystallized monolithic silicon carbide (SiC) produced by chemically vapor deposition (CVD) and liquid phase sintering (LPS) methods. Both materials exhibited similar time-dependent trend of stress relaxation. The BSR ratio dropped rapidly during the first hour of the tests and then decreased gradually in the higher temperature tests. The CVD-SiC and the LPS-SiC showed good thermal creep resistance at the expected operating temperature (about 600–1000°C) of fusion blanket using SiC fiber-reinforced SiC matrix (SiC/SiC) composite. On the other hand, the BSR ratio of those two materials, especially of the LPS-SiC, dropped steeply at the temperatures of 1200–1400°C. The activation energy of the stress relaxation calculated by a cross-cut method increased with the temperature.

Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks

SiC retains high mechanical strength and oxidation stability at temperatures above 1500 °C, representing a viable alternative to silica, alumina, and carbon, which have been in use as catalyst supports for more than 60 years. Preparation of monolithic porous SiC is usually elaborate and porosities around 30% v/v are typically considered high. This report describes the synthesis of monolithic highly porous (70% v/v) SiC by carbothermal reduction (1200−1600 °C) of 3D sol−gel silica nanostructures (aerogels) conformally coated and cross-linked with polyacrylonitrile (PAN). Synthesis of PAN-cross-linked silica aerogels is carried out in one pot by simple mixing of the monomers, whereas conversion to SiC is carried out in a tube reactor by programmed heating. Intermediates after aromatization (225 °C in air) and carbonization (800 °C under Ar) were isolated and characterized for their chemical composition and materials properties. Data are interpreted mechanistically and were used iteratively for process optim...

Mechanical behavior of laser micro-machined bulk 6H–SiC diaphragms

Mechanical behavior of laser micro-machined monolithic hexagonal silicon carbide (6H–SiC) diaphragms was investigated to determine the effects of laser processing. Square diaphragms with a nominal size of 1.5 mm × 1.5 mm were fabricated from bulk 6H–SiC wafers using a Q-switched Nd:YAG laser operating at a wavelength of 1064 nm, an average power of 0.35W, a pulse repetition rate of 3 kHz, and a pulse width of 100 ns. These parameters were chosen, based on previous experiments, to minimize surface roughness. Analysis of laser-machined diaphragms revealed that the average thickness of a diaphragm was 151 μm which is composed of two layers. One is a soft, black layer with a thickness of about 83 μm consisting of silicon, oxygen, and carbon. The other layer was a hard, virgin SiC layer with a thickness of 68 μm. The diaphragms were subjected to micro-hardness indentation tests to obtain load versus deflection curves. The data was validated using Timoshenko’s analytical model for maximum deflection of a thin plate under concentrated loading with hinged and clamped boundary conditions. Experimental measurements of the deflection were found to be slightly higher than those predicted by the analytical model. The variations in the thickness of the diaphragms, homogeneity of the elastic properties of the laser micro-machined SiC, and possibly inappropriate boundary conditions during testing of the diaphragms chiefly account for the deviations between the experimental results and the analytical model.

Processing and characterization of fine-grained monolithic SiC ceramic synthesized by spark plasma sintering

Monolithic silicon carbide (SiC) ceramic with good mechanical properties was synthesized by spark plasma sintering (SPS) technique, using granulated SiC powders. The microstructures and the mechanical properties of the ceramics sintered at different temperature were investigated. The results reveal that the SiC ceramic sintered at 1860 °C has relative density of 98.5%, micro-Vickers hardness of 28.5 GPa, bending strength of 395 MPa, and fracture toughness of 4.5 MPa m 1/2. By comparison, due to the present SiC ceramic exhibits fine-grained microstructures and the granulated SiC powders was used during the sintering process, the mechanical properties of the SiC ceramic synthesized by SPS are much better than that prepared by common sintering methods.

Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB2+SiC and Oxygen Transport Mechanisms

Refractory diboride with silicon carbide additive has a unique oxide scale microstructure with two condensed oxide phases (solid+liquid), and demonstrates oxidation resistance superior to either monolithic diboride or silicon carbide. We rationalize that this is because the silica‐rich liquid phase can retreat outward to remove the high SiO gas volatility region, while still holding onto the zirconia skeleton mechanically by capillary forces, to form a “solid pillars, liquid roof ” scale architecture and maintain barrier function. Basic assessment of the oxygen carriers in the borosilicate liquid in oxygen‐rich condition is performed using first‐principles calculations. It is estimated from entropy and mobility arguments that above a critical temperature TC∼1500°C, the dominant oxygen carriers should be network defects, such as peroxyl linkage or oxygen‐deficient centers, instead of molecular O2* as in the Deal–Grove model. These network defects will lead to sublinear dependence of the oxidation rate with external oxygen partial pressure. The present work suggests that there could be significant room in improving the high‐temperature oxidation resistance by refining the oxide scale microstructure as well as controlling the glass chemistry.

Formation of dense silicon carbide by liquid silicon infiltration of carbon with engineered structure

Fully dense and net-shaped silicon carbide monoliths were produced by liquid silicon infiltration of carbon preforms with engineered bulk density, median pore diameter, and chemical reactivity derived from carbonization of crystalline cellulose and phenolic resin blends. The ideal carbon bulk density and minimum median pore diameter for successful formation of fully dense silicon carbide by liquid silicon infiltration are 0.964 g cm−3and approximately 1 μm. By blending crystalline cellulose and phenolic resin in various mass ratios as carbon precursors, we were able to adjust the bulk density, median pore diameter, and overall chemical reactivity of the carbon preforms produced. The liquid silicon infiltration reactions were performed in a graphite element furnace at temperatures between 1414 and 1900 °C and under argon pressures of 1550, 760, and 0.5 Torr for periods of 10, 15, 30, 60, 120, and 300 min. Examination of the results indicated that the ideal carbon preform was produced from the crystalline cellulose and phenolic resin blend of 6:4 mass ratio. This carbon preform has a bulk density of 0.7910 g cm−3, an actual density of 2.1911 g cm−3, median pore diameter of 1.45 μm, and specific surface area of 644.75 m2g−1. The ideal liquid silicon infiltration reaction conditions were identified as 1800 °C, 0.5 Torr, and 120 min. The optimum reaction product has a bulk density of 2.9566 g cm−3, greater than 91% of that of pure β–SiC, with a β–SiC volume fraction of approximately 82.5%.

Creep of monolithic and fibre-reinforced silicon carbide

The mechanisms governing tensile creep and creep fracture are identified for siliconized silicon carbide (Si–SiC) and sintered silicon carbide (SSC), as well as for various SiC-matrix composites reinforced with interwoven bundles of different fibres. With Si–SiC and SSC, creep is controlled by the rate at which intergranular damage development allows grain boundary sliding. Fracture then results from the formation and link up of grain boundary cavities and cracks, unless premature failure occurs by rapid crack propagation from a pre-existing flaw. In contrast, with the woven SiC-matrix composites, immediate failure was never encountered on loading at stresses less than the UTS, despite the presence of macroscopic pores. Instead, the longitudinal fibres control the rates of creep strain accumulation and crack growth. However, the fracture properties are determined by oxidation-assisted fibre failure, because matrix cracking permits oxygen ingress during tests in air. By clarifying the processes limiting the creep capabilities of current product ranges, possible development avenues are suggested for fibre-reinforced composites displaying improved long-term service performance in oxidizing atmospheres.

Contact Damage of Silicon Carbide/Boron Nitride Nanocomposites

To investigate the deformation mechanism of silicon carbide (SiC)/boron nitride (BN) nanocomposites, Hertzian contact tests were performed on monolithic SiC, and nanocomposite and microcomposite SiC/BN. Monolithic SiC had the typical microstructure of hot‐pressed SiC with Y2O3 and Al2O3 additives, composed of slightly large grains in small matrix grains. The microcomposite comprised large BN grains dispersed along the grain boundaries of elongated SiC grains, while the nanocomposite showed a finer microstructure with fine BN particles and small matrix grains. These microstructural differences led to differences in the mechanism of contact damage. The damage of the monolithic SiC and the SiC/BN microcomposite exhibited classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi‐plastic deformation.

Sliding Wear Characteristics and Fabrication of LPS-SIC Ceramic by Sintering Additives

Silicon carbide (SiC) materials have been extensively studied for high-temperature components in fusion blanket system and gas turbines, because they have excellent a hightemperature mechanical properties, high thermal conductivity and wear resistance. However, the brittle characteristics of SiC such as low strain-to fracture still impose a severe limitation on the practical application of SiC materials. Therefore, a study of the sliding wear characteristics and fabrication of SiC ceramic by sintering temperature and additives are need. As the result of abrasion, the friction coefficient of the monolithic SiC sintered at 1800°C was the lowest, and the friction coefficient of that sintered at 1760°C was the highest. The monolithic SiC manufactured at 1800°C showed the highest hardness and the lowest friction coefficient. The friction coefficient of the monolithic SiC sintered by the SiO2 contents of 2wt% was the lowest, and the friction coefficient that sintered by the SiO2 contents of 5wt% was the highest. 1800°C of sintering temperature and 2wt% of SiO2 contents ware shown high hardness, low friction coefficient and wear loss compare with other temperatures and SiO2 contents.

Study of machinable silicon carbide–boron nitride ceramic composites

Silicon carbide (SiC) particles coated with nano-boron nitride (BN) were synthesized. The SiC/BN ceramic nano-composites and micro-composites were sintered by plasma-activated sintering. For the nano-composites, due to the homogeneous dispersion of the nano-BN crystals around the SiC grains of the matrix, the grain size of nano-composites was smaller than the monolithic SiC and micro-composites. As a result, the bending strength of the nano-composites decreased slowly (for SiC/20% h-BN nano-composite, about 18% decrease), while their hardness decreased sharply and the machinability properties were improved noticeably.