Reaction-sintered SiC Research Articles

Preparation of high-strength silicon carbide by SLS-RS

One major limitation of selective laser sintering (SLS) plus reaction sintering (RS) SiC ceramics was the poor mechanical properties related to those of the traditional reaction sintered SiC due to the excess residual Si and C. TiC as a new carbon source in SLS was explored in this work to improve the performance of sintered SiC. It's found that TiC could react with liquid Si and form liquid TiSi2 during the processing, and then reduce the residual Si from 26.41 vol% to 0.53 vol% in the final products. As the content of TiC increased, the flexural strength of the sintered sample increased from 274.3 MPa to 434.6 MPa. The maximum value was reached when the TiC content was 25 wt%. Continuing to add TiC would generate more TiSi2, and the flexural strength of the sintered sample would decrease.

Modeling passive/active transition in high-temperature dry oxidation of reaction-sintered SiC for innovative fuel matrix in high-temperature gas-cooled reactors

Modeling passive/active transition in high-temperature dry oxidation of reaction-sintered SiC for innovative fuel matrix in high-temperature gas-cooled reactors

Integrated experimental assessment and validation of oxidation with real-scale SiC fuel compact in HTGR

Air ingress is a critical accident in HTGR design, caused by a rupture in the coolant pipe. It leads to the oxidation of hot graphite at high temperatures, resulting in mechanical degradation of graphite structures. To address this issue, the authors propose a novel approach of substituting graphite fuel compacts with polycrystalline reaction-sintered SiC (RS-SiC) fuel compacts due to their superior oxidation resistance. The adoption of SiC based fuel compact will eliminate the graphite sleeves currently used in the pin-in-block type HTGR designs. It will also make it possible to achieve the high-power density due to the absence of graphite sleeves. There is limited understanding of the oxidation kinetics and transition behavior in RS-SiC. To address this knowledge gap, integrated experiments are conducted to investigate the thermal and oxidation behavior of SiC under varying oxygen concentrations. These experiments aim to provide insights into SiC's performance in HTGR environments using similar temperature and flow velocity conditions. Experimental data obtained will be utilized to develop a numerical model for enhanced understanding. In this study, both experimental and computational fluid dynamics (CFD) analyses were performed to compare the temperatures obtained from simulations with experimental measurements. The results demonstrated good agreement between experimental and numerical data. Passive oxidation was observed in real scale SiC fuel compacts at both 900 °C and 1100 °C, with slightly lower weight gain at the higher temperature. This suggests the possibility of damage to the SiO2 oxide layer at elevated temperatures. These results are important as the experiments are done in an integrated test facility using the air ingress accident boundary conditions of HTGR with the SiC fuel compact of real scale. These experiments validate the earlier findings from thermogravimetry differential thermal analysis (TG/DTA) conducted in a smaller-scale facility. They play a role in extending our understanding of SiC oxidation behavior, thereby assisting in the advancement of HTGR designs for enhanced safety and performance.

Investigation of the structure and physico-mechanical characteristics of reaction-sintered materials B4C‒SiC

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Microstructure and Thermal Conductivity of Reaction-Sintered SiC

Microstructure and Thermal Conductivity of Reaction-Sintered SiC

Oxidation behavior of Al2O3 added reaction-sintered SiC ceramics in wet oxygen environment at 1300°C

ABSTRACTThe oxidation behavior of reaction-bonded SiC ceramics with Al2O3 was investigated at 1300°C for exposure duration up to 100 h under water vapor environment. Results show that Al2O3 is enriched on the oxidation surface, reducing the volatilization of the oxide layer and preventing the reduction of macroscopic size. Raman spectra results further testified that Al2O3 obviously improves the stability of glass structure and reduces the generation of non-bridged oxygen atoms (Si–O–H), which results in a lower reaction activity with steam of the aluminosilicate melt, therefore, significantly improving the oxidation resistance of SiC in O2/H2O atmosphere.

Figuring and Finishing of Reaction-Sintered SiC by Anodic Oxidation Assisted Process

Reaction-sintered silicon carbide (RS-SiC) is a promising material for optical components used in space, or molds for precision glass lens because of its excellent properties. For processing of RS-SiC, diamond tools are utilized because RS-SiC is difficult-to-machine material due to its high hardness. In that case, subsurface damage (SSD) and scratches are inevitably introduced on the processed surface, and they deteriorate the qualities of products. To resolve these issues, we proposed a complex machining technique named anodic oxidation assisted process (AOAP), in which localized anodic oxidation and removal of the oxidation layer by grinding or polishing were combined, for figuring or polishing of RS-SiC without introducing any scratches and SSD. The grinding or polishing tool used in AOAP has a lower hardness than that of RS-SiC, but higher than that of the oxidation products. It is possible to figure the objective shape and polish the surface by changing the conditions including the oxidation time, the composition of electrolyte, the configuration of the cathode electrode, applied voltage, and so on. In our previous study, we found that RS-SiC was oxidized efficiently by anodic oxidation with various electrolytes such as phosphoric acid, ultrapure water, and a mixture of hydrochloric acid and hydrogen peroxide. In this research, we investigated the preliminary processing characteristics of AOAP for RS-SiC. We ascertained that irradiating UV light with photon energy higher than the band gap of processed materials is very effective for increasing the oxidation rate of anodic oxidation. And we proposed a novel polishing process of RS-SiC, which combining oxidation only SiC area in RS-SiC by anodic oxidation with the electrolyte of ceria slurry, with polishing by ceria slurry which removes both oxidized layer and unoxidized layer in RS-SiC. The results of investigation for the oxidation rate and the polishing rate of SiC, Si and SiO2 with ceria slurry implies that we can remove SiC grain and Si grain in RS-SiC at the same MRR by combing the anodic oxidation and polishing with ceria slurry at the same time, and obtain the smooth surface.

Subsurface Atomic Structure of 4H-SiC (0001) Finished by Plasma-Assisted Polishing

Plasma-assisted polishing (PAP) was proposed for finishing difficult-to-machine materials, such as single-crystal SiC, reaction-sintered SiC, diamond, and sapphire. In the case of PAP application to the finishing of the 4H-SiC surface, an atomically smooth surface without any scratches was obtained. In this study, we observed 4H-SiC (0001) surfaces processed by water vapor plasma oxidation and PAP using ceria abrasives through cross-sectional transmission electron microscopy (XTEM). Water vapor plasma oxidation was conducted for 1 min, 5 min and 60 min. An intermediate layer located between SiO2 and SiC, which corresponds to silicon oxycarbide, was clearly observed in the case of a short oxidation. As oxidation time increased from 1 min to 60 min, average oxidation rate decreased from 2.7 nm/min to 0.6 nm/min. An atomically smooth 4H-SiC (0001) surface was obtained after PAP for 60 min using ceria abrasives.

A high-strength SiCw/SiC–Si composite derived from pyrolyzed rice husks by liquid silicon infiltration

A high-strength SiC composite with SiC whiskers (SiCw) as reinforcement has been fabricated by liquid silicon infiltration (LSI) using pyrolyzed rice husks (RHs) as raw material. RHs were coked and pyrolyzed subsequently at high temperature to obtain a mixture containing SiC whiskers, particles, and amorphous carbon. The pyrolyzed RHs were then milled and modeled to preforms, which were then used to fabricate biomorphic SiCw/SiC–Si composites by liquid silicon infiltration at 1,450, 1,550, and 1,600 °C, respectively. Dense composite with a density of 3.0 g cm−3 was obtained at the infiltration temperature of 1,550 °C, which possesses superior mechanical properties compared with commercial reaction-sintered SiC (RS-SiC). The Vickers hardness, flexure strength, elastic modulus, and fracture toughness of the biomorphic SiCw/SiC–Si composite were 18.8 ± 0.6 GPa, 354 ± 2 GPa, 450 ± 40 MPa, and 3.5 ± 0.3 MPa m1/2, respectively. Whereas the composites obtained at the other two infiltration temperatures contain unreacted carbon and show lower mechanical properties. The high flexure strength of the biomorphic composite infiltrated at 1,550 °C is attributed to the dense structure and the reinforcement of the SiC whiskers. In addition, the fracture mechanism of the composite is also discussed.

Optimizing Electrical and Mechanical Properties of Reaction-Sintered SiC by using Different-Sized SiC Particles in Preform

A series of reaction-sintered SiC was fabricated from preforms with varying volume fractions of two resin-coated SiC particles of different sizes (63 and 18 ㎛). The electrical resistivity and mechanical strength were eventually optimized at the small particle volume fraction of 0.3~0.4, at which point the porosity of the preform was minimized. This study experimentally proves that additional processes after the formation of the preform, such as silicon infiltration and reaction sintering, do not apparently alter the optimum volume fraction of the preform packing, predicted by an existing analytical model based on solid packing. Thus, the volume fraction of particles of different sizes can be determined practically through the solid packing model to fabricate RSSCs with optimal properties.

Reduced electrical resistivity of reaction-sintered SiC by nitrogen doping

A post heat treatment of reaction-sintered SiC at 1700 °C in nitrogen atmosphere significantly reduced electrical resistivity. A trace of insulating Si3N4 phase was detected via nitrogen heat treatment in high-resolution transmission electron microscopy observation; however, based on x-ray photoelectron spectroscopy, the evidence of nitrogen doping into SiC lattice has been claimed as the mechanism to the decreased resistivity. The increase of the total volume of SiC was apparent in x-ray diffraction during the nitrogen heat treatment, which was interpreted to stem from the growth of the nitrogen-doped intergranular SiC particles and surface doping of the primary SiC to reduce the contact resistance between the primary SiC particles.

ELID Grinding Properties of High-Strength Reaction-Sintered SiC

The high-strength reaction-sintered silicon carbide (RS-SiC) developed by TOSHIBA is one of the most excellent materials for large-scale space-borne optics. The bending strength of the high-strength RS-SiC is two times higher than other SiC ceramics. The purpose of this study is to investigate the ELID grinding properties of the high strength RS-SiC. Two types of metal bond diamond wheels (cup type and straight type) were used to grinding tests. The ground surface properties, such as roughness, subsurface damage and micro-step were made clear by measurement or observation. It was confirmed that, both the surface roughness and the depth of micro-step produced by cup-wheel were lower than those produced by straight-wheel. When a #20000 grit sized cup-wheel was used, a considerably high quality mirror surface (Ra<0.8nm) can be achived.

Application of reaction sintering to the manufacturing of a spacecraft combustion chamber of SiC ceramics

The method of reaction sintering was applied to the manufacturing of a spacecraft combustion chamber of SiC ceramics. Physical and mechanical experiments such as bending strength, coefficient of thermal expansion, coefficient of thermal conductivity, gas-tight property, heat-resisting property, etc., have been carried out. The results show that a spacecraft combustion chamber made from the reaction sintering of SiC ceramics is feasible, and that the results of experiment are satisfactory. The strength of high-temperature structural parts made by reaction sintered SiC ceramics varied with the silicon content; the optimum silicon content being 10.5% for the part investigated. The method of the reaction sintering SiC ceramics is suitable for the manufacturing of complicated spacecraft parts with a working temperature of 1500 °C.

反応焼結ＳｉＣ基長繊維複合材料の強化・高じん性化設計

It is well-known that dense SiC matrix composites reinforced by SiC fibers with BN coating can be fabricated by subsequent reaction sintering with molten Si. In this experiments, it was confirmed that the mechanical properties of continuous Si-C fibers (Hi-Nicalon) were not affected by the thickness of the BN coating using CVD process and the bending strength of reaction-sintered SiC increased from 500 to 1000MPa with decreasing the size of residual silicon. The effect of SiC matrix strength on the matrix cracking stress in unidirectional fiber-reinforced SiCf/SiC composites was investigated by use of these data. As a result, it made clear that the matrix cracking strength in SiCf/SiC composites increased with increasing the SiC matrix strength. There are the upper limits of the SiC matrix strength deduced from the SiC fiber strength and the fiber volume fraction to achieve the apparent ductile fracture of the SiCf/SiC composites.

Growth of SiC particles in reaction sintered SiC

Abstract For reaction sintered SiC (RSSC) prepared at 1600°C by conventional melt infiltration technique, experimentation with two different particle sizes of initial SiC, viz., 0.2 and 23.65 μm, showed that the large SiC particles remained unaltered and the sizes of the fine-grained SiC increased several times yielding well-developed faceted crystals in the final material. To study the process further, compacts of SiC powder of particle sizes varying between 0.20 and 8.99 μm were reacted with pure Si at 1600°C and the resulting SiC–Si boundaries were studied by optical microscopy. A distinct boundary layer with no penetration of Si in the compact of SiC of 0.2 μm was observed and the width of the SiC–Si boundary was found to be increasing linearly with time. Detailed SEM examination establishes the growth of the SiC upto around 4 μm from 0.2 μm starting powder. No such growth was observed in the case of starting SiC powder coarser than 0.2 μm. The growth of SiC is explained in terms of solution-reprecipitation mechanism.

Material Design for High Strength of Continuous Fiber-Reinforced and Reaction-Sintered SiC Matrix Composites by Paying Attention to Fiber-Pullout Behavior.

Interfacial debonding behaviors between fiber and matrix in continuous fiber-reinforced reaction-sintered SiC matrix composites have been analyzed in terms of strain energy release rate using a finite element method. In particular, the effects of Young's modulus ratio, Em/Ef, and fiber volume fraction, Vf, on the strain energy release rate have been investigated. As a result, it was confirmed that the steady-state strain release rate increased with increasing the Young's modulus ratio and decreasing the fiber volume fraction. Also, it was clarified that the quasi-yield state like carbon steels observed in stress-strain curve was explainable by the strain energy release rate of interfacial debonding between fiber and matrix.

On the oxidation kinetics and mechanisms of various SiC-coated carbon-carbon composites

SiC coatings on the surface of C-C were produced by either silicone resin impregnation/ pyrolysis or reaction sintering. Cycles of resin impregnation/pyrolysis produced an SiC coating, on the walls of fine open pores, which was effective in reducing the oxidation rate of C-C and in shifting the transition temperature to a higher value. Unless it is pre-coated with a pyrocarbon layer before sintering, plain reaction sintered SiC has oxidation behaviors similar to those of the above-mentioned SiC. The dense pyrocarbon film deposition on the surface of C-C could form a better SiC film than others. The carbon film homogenized the surface of C-C and a dense SiC film was established. The oxidation of the coated C-C can be modelled by a set of “oxidation resistors” in series and/or in parallel, with each resistor corresponding to an oxidation element. The controlling mechanism can be resolved from the activation energy. A combined resistant layer, consisting of resin impregnation, pyrocarbon film and reaction sintering SiC, showed the best oxidation resistance of any single-layer coated C-C composite.

Gas Permeability and Bending Strength of Porous SiC Ceramics

The reaction-sintered porous SiC ceramics with different particle sizes and porosities are prepared, and the effects of particle size and porosity on their gas permeability and bending strength are investigated. For porosities above 0.25, the Ergun's equation well explains the small pressure drop at large particle sizes and high porosities which were achieved by the decreased compaction pressure of raw materials. The reduction in porosity down to 0.26 by the addition of free Si under the constant compaction pressure increased the bending strength remarkably, while no change in the pressure drop was accompanied. At a porosity smaller than 0.26, an abrupt increase of the pressure drop was observed without any further increase in bending strength. These observations were examined through the analysis of morphology and pore size distribution. The abrupt change was found to occur at the point where all the smaller pores are filled with Si completely. The preferential filling of Si in smaller pores can be explained by the good wetting of SiC with Si.

Effect of a silicon sintering additive on solid state bonding of SiC to Nb

Pressureless-sintered (PLS) SiC was joined to Nb by solid state bonding in a vacuum. The joining strength of the PLS SiC/Nb joint increases to the saturated value of 108 MPa with increasing joining pressure at a joining condition of 1673 K and 7.2 ks. This saturated value of PLS SiC/Nb joint is higher than that of the reaction sintered (RS) SiC/Nb joint. The strength of SiC itself affects the strength of the SiC/Nb joint. The high stability of the intermediate phase Nb5Si3 in the interface at elevated temperature leads to the high heat resistance of the joint. The thickness of the intermediate phase Nb5Si3 in the PLS SiC/Nb system is lower than that of the RS SiC/Nb system at a constant joining time, although the activation energy, 452 kJ mol−1, of growth for the phase in the PLS SiC/Nb system is almost the same as the RS SiC/Nb system, 456 kJ mol−1. The rate constant of the growth for the phase in the PLS SiC/Nb system is lower than that in the RS SiC/Nb system. The excess silicon in RS SiC promotes the formation of the Nb5Si3 phase at the interface between SiC and Nb.

Strength-porosity-microstructure relations in porous reaction-sintered SiC with free silicon removed

The effects of controlled porosity on the fracture strength of reaction-sintered SiC (RS-SiC) with free silicon removed and of RS-SiC further heat-treated at 1780 °C in a vacuum were investigated. Although further heat-treatment did not cause significant change in porosity of the porous RS-SiC, it was found that the heat-treatment caused strengthening of the porous RS-SiC with below 38% porosity and an intergranular-to-transgranular fracture transition. The strengthening effect may be attributed to increasing bonding area between grains and a relaxation of stress concentration by changing pore shape, resulting from evaporation-condensation and/or surface diffusion by the heat treatment.