SiC Matrices Research Articles

Influence of polycarbosilane composition on the properties of SiC/SiC composite fabricated by precursor infiltration and pyrolysis process

AbstractA series of SiC/SiC composites were fabricated through the precursor infiltration and pyrolysis (PIP) process, using solid polycarbosilane (PCS) and liquid vinyl perhydrogen PCS (VHPCS) solutions as impregnating agents. The physicochemical characteristics of the SiC matrices derived from PCS and VHPCS were investigated and compared. The impact of the PCS composition on the microstructure, density, flexural strength, and modulus of the resulting SiC/SiC composites was also examined. As the VHPCS content increased to 60 wt%, the flexural strength of the composites gradually rose to a peak of 490.9 MPa before decreasing as the VHPCS proportion continued to increase. Meanwhile, the flexural modulus increased continually with the introduction of VHPCS. A possible mechanism is proposed to explain the different variations in flexural strength and modulus, taking into account the ceramic toughening principle and the distinct properties of PCS and VHPCS.

Micro-zone and macro mechanical behavior of SiCf/Si3N4 composite modified by intra-bundle matrix

In this work, the microstructure of the interfacial zone was optimized to improve the mechanical properties of SiCf/Si3N4 composite, and the relationship between micro-zone and macro mechanical behavior was revealed. For SiCf/Si3N4, the interphase destruction occurred during the Si3N4 deposition process, leading to the existence of discontinuous interphase in the outside of intra-bundle area. After incorporating SiBCN and SiC matrices into the fiber bundle, the BN interphase can be well protected, and the uniformity of the matrix microstructure within the fiber bundle was further enhanced, which increased the Weibull modulus of the interfacial shear strength (IFSS), resulting in the higher load sharing efficiency in fiber bundle and the enhanced macro-mechanical properties. Especially after incorporating SiC matrix into intra-bundle area, the higher IFSS also can be obtained, corresponding to the higher load transferring efficiency, and the strength and toughness of the composites were improved significantly.

Effects of starting powder on microstructure and thermal conductivity of pressureless-sintered fully ceramic microencapsulated fuels

The effects of the starting SiC powder (α or β) with the addition of 5.67 wt% AlN–Y2O3–CeO2–MgO additives on the residual porosity and thermal conductivity of fully ceramic microencapsulated (FCM) fuels were investigated. FCM fuels containing ∼41 vol% and ∼37 vol% tristructural isotropic (TRISO) particles could be sintered at 1870 °C using α-SiC and β-SiC powders, respectively, via a pressureless sintering route. The residual porosities of the SiC matrices in the FCM fuels prepared using the α-SiC and β-SiC powders were 1.1% and 2.3%, respectively. The thermal conductivities of FCM pellets with ∼41 vol% and ∼37 vol% TRISO particles (prepared using the α-SiC and β-SiC powders, respectively) were 59 and 41 Wm−1K−1, respectively. The lower porosity and higher thermal conductivity of FCM fuels prepared using the α-SiC powder were attributed to the higher sinterability of the α-SiC powder than that of the β-SiC powder.

Vapor‐mediated melt infiltration for synthesizing SiC composite matrices

AbstractA two‐step synthesis of SiC involving initial exposure of carbon surfaces to Si vapor, followed by Si melt infiltration, is investigated in this article with emphases on the mechanisms, kinetics, and microstructure evolution. Interrupted differential thermal analysis of amorphous C and Si powder mixtures and microstructure characterization are used to generate insight into the stages of the reaction. Exposure to Si vapor yields a SiC layer with nano‐scale porosity driven by the volume change associated with the reaction. This forms a continuous pore network that promotes subsequent melt access to the reaction front with the C. While the pores remain open, the vapor phase reaction proceeds at a nearly constant rate and exhibits a strong temperature sensitivity, the latter due in part to the temperature sensitivity of the Si vapor pressure. The implications for enhancing the reactive melt infiltration process and fabrication of SiC matrices for ceramic composites are discussed.

Processing of fully ceramic microencapsulated fuels with a small amount of additives by hot-pressing

The effects of processing parameters on the properties of SiC matrices and fully ceramic microencapsulated (FCM) fuel pellets were investigated with a small amount of additives by hot pressing. The increases in the applied pressure as well as the sintering time were beneficial in improving the thermal conductivity of the SiC matrix and FCM pellets. Tristructural isotropic (TRISO) particles without an outer pyrolytic carbon layer are not suitable for processing FCM pellets because of the corrosion of the chemical-vapor-deposited SiC layer by the additive-derived liquid phase during sintering. The FCM pellet sintered with 2.71 wt% AlN–Y2O3–Sc2O3–MgO additives at 1870 °C for 4 h under 30 MPa in argon exhibited an excellent thermal conductivity (61.7 Wm−1 K−1 at a TRISO particle content of 43.3 %), owing to the (i) reduced amount of additives, (ii) high sintered density (residual porosity ∼ 0.4 %), and (iii) highly thermally conductive SiC matrix (105.0 Wm−1 K−1).

Hydrothermal Corrosion Behaviors of Constituent Materials of SiC/SiC Composites for LWR Applications

The corrosion behaviors of SiC/SiC composite constituent materials in pure water at operating conditions, such as 300 °C and 8.5 MPa, were studied for potential application in accident-tolerant light water reactor (LWR) fuel cladding and core structures. Five kinds of SiC fibers, four kinds of SiC matrices, and three kinds of fiber/matrix interphase materials were examined in autoclaves. The potential constituent materials for future use in SiC/SiC composites were selected by considering corrosion rates and residual strength characteristics. The mass changes and the residual strength of each specimen were measured. SEM images of the surface layers were also inspected. The SiC fibers, regardless of their purity, crystallinity or stoichiometric ratio, decreased in strength due to the hydrothermal corrosion. For its part, the hydrothermal corrosion resistance of CVD-SiC, as a SiC matrix, was found to be affected by manufacturing conditions such as raw material gas type and synthesis temperature, as well as post-machining morphology. The CVD-carbon (CVD-C), as a fiber/matrix interphase material, showed good hydrothermal corrosion resistance. In order to protect the SiC fibers and the SiC matrices from hydrothermal corrosion, it would appear to be necessary to apply a dense CVD-C coating to both every fiber and the entire surface of the SiC matrices.

Preparation of non-oxidized Ge quantum dot lattices in amorphous Al2O3, Si3N4 and SiC matrices

The preparation of non-oxidized Ge quantum dot (QD) lattices embedded in Al2O3, Si3N4, SiC matrices by self-assembled growth was studied. The materials were produced by magnetron sputtering deposition, using different substrate temperatures. The deposition regimes leading to the self-assembled growth type and the formation of three-dimensionally ordered Ge QD lattices in different matrices were investigated and determined. The oxidation of the Ge QDs in different matrices was monitored and the best conditions for the production of non-oxidized Ge QDs were found. The optical properties of the Ge QD lattices in different matrices show a strong dependence on the Ge oxidation and the matrix type.

Predictions of thermal conductivity and degradation of irradiated SiC/SiC composites by materials-genome-based multiscale modeling

A new materials-genome-based multiscale modeling method is proposed to predict the thermal conductivity of SiC/SiC composites subjected to neutron irradiation. The modeling method bridges different scales from the atomic scale to the macro scale of a 2D SiC/SiC composite. The irradiation-induced point defects in the interphase of SiC/SiC composites are first studied using molecular dynamics simulations to compute the degradation of thermal conductivity as a function of irradiation dose and temperature. The thermal conductivities of SiC fibers, matrices and interphase are then used as input for the new materials genome model to compute the thermal conductivities of 2D SiC/SiC composites subject to neutron irradiation. The predicted thermal conductivities for both unirradiated and irradiated SiC/SiC composites are found to decrease with an increase of temperature while the irradiation effect on interphase needs to be considered for irradiated SiC/SiC composites.

Development of low-cost C/C, related technology using a film-boiling process

Carbon-carbon (C/C) composites are a material that, due to its resistance to high temperatures, has been used for the nozzle throat section of rockets, but is highly expensive when fabricated using conventional processes such as hot isostatic pressing (HIP). In this paper, we report a newly-developed low-cost manufacturing process that uses film-boiling (FB) technology, the basic process for which was originally developed by CEA Le Ripault of France. We have confirmed that, by using this technology, we can increase the density of the carbon matrix around the carbon fibers to the same degree as those prepared using chemical vapor infiltration (CVI) but in a much shorter time. Having successfully used FB technology in the fabrication of SiC matrices using liquid polysilane (LPS), we hope that this technology will be extensively used in the near future.

Grain size variation in nanocrystalline silicon carbide irradiated at elevated temperatures

AbstractThis study reports on ion irradiation‐induced grain size variations in nanocrystalline SiC films on Si substrates. The SiC grains with average size ranging from ~2 to 20 nm were embedded in amorphous SiC matrices. Irradiation was performed using 5 MeV Xe23+ ions to 1.15 × 1016 ions/cm2 at 700 K. The irradiated films were characterized using X‐ray diffraction, transmission electron microscopy, and Raman spectroscopy. Significant grain growth is observed for smaller grains that tend to saturate at ~8 nm. In contrast, irradiation of larger grains (~20 nm in size) leads to a decrease in the grain size, which could be associated with the production of lattice disorder within the grains. Homonuclear C‐C bonds in the irradiated amorphous SiC matrix are found to be graphitized. This bonding transformation could limit or inhibit grain growth and contribute to the size saturation. The results from this study may suggest nanocrystalline SiC as a promising candidate structural or cladding material for applications in advanced nuclear reactors.

Degradation mechanisms of a self-healing SiC(f)/BN(i)/[SiC-B4C](m) composite at high temperature under different oxidizing atmospheres

Chemical vapor-infiltrated self-healing SiC(f)/BN(i)/[SiC-B4C](m) composite specimens were exposed at 1300 °C for 300 h at atmospheric pressure under two different oxidizing atmospheres (i.e., wet (12%H2O:8%O2:80%Ar) and dry (0.01%O2:99.99%Ar)) representative of rich and poor oxidizing conditions, respectively. Mechanical testing, microstructural observations, and element analyses were performed on the treated specimens. The flexural strength retentions of the specimens were 47.9 and 39.4% under wet and dry oxygen conditions, respectively. The SiC and B4C matrices were severely oxidized under wet oxygen conditions, whereas the BN interphase remained intact. The BN interphase and the B4C layered phase were both partially oxidized under dry oxygen conditions. Thus, the SiC(f)/BN(i)/[SiC-B4C](m) composites exhibited improved oxidation resistance under wet oxygen atmospheres as compared to dry oxygen conditions as a result of the formation of borosilicate glasses. In addition, two different degradation mechanisms for the composites during the oxidation process were discussed.

Physical-Mechanical Properties of γ-Irradiated SiC Ceramics for Radioactive Wastes Immobilization

The interest in silicon carbide (SiC-based) ceramics and composites as matrix material for nuclear waste immobilization is grown up. Long-term chemical durability and radiation resistance of SiC are important factors for radionuclides immobilization. Advantages of SiC-based ceramics as structural materials in nuclear applications are the high-temperature properties, high density and reduced neutron activation. The use of radiation resistant materials is a strong requirement for safe and environmentally beneficial energy system. The SiC ceramics stability under irradiation for temperatures up to 1273 K is also very important for nuclear power applications. The SiC matrices doped by additives of Cr, Si were fabricated using High Speed Hot Pressing Method. Additives content was in the range from 0.5 to 3 wt %. Microstructural characteristics of silicon carbide ceramics were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and infra-red spectroscopy (IR) methods. The results of microcracking under indentation conditions were revealed the lack of cracks in the SiC ceramics with Cr additives before and after irradiation process. In addition, it was demonstrated that samples of SiC with alloying additives Cr and Si possess high mechanical parameters under γ-irradiation process. The strength of ceramics increases with the uniform and fine-grained structure formation. The modification of phase composition and mechanical properties of the SiC ceramics with Cr and Si additives under γ-irradiation were analyzed for further development of radiation resistant and matrix materials for radioactive wastes immobilization.

Internal friction behaviour of carbon fibre reinforced multilayered (PyC–SiC)n matrix composites

Abstract Carbon fibre reinforced multilayered (PyC–SiC) n matrix (C/(PyC–SiC) n ) composites were prepared by alternate deposition of pyrocarbon (PyC) and SiC inside preforms via chemical vapour infiltration. The matrix microstructures and internal friction behaviours of C/(PyC–SiC) n composites (n = 1, 2 and 4) under different testing conditions of frequency, strain amplitude and temperature were studied. The results show that with increasing the number of sequences (n), the internal friction increases due to the enhancement of interfacial internal friction. The internal friction of C/(PyC–SiC) n composites increases with the increase of frequency related to thermoelastic mechanism, but exhibits anomalous amplitude effect in the testing amplitude range. Effect of temperature on internal friction behaviours is attributed to combined effects of carbon fibres, PyC–SiC matrices and interfaces between fibres and PyC, adjoining PyC and SiC layers. It is also found that internal friction is sensitive to microstructural defects induced by damage. These results indicate that internal friction is an effective and efficient method to characterize the structural evolution and internal damage of C/(PyC–SiC) n composites non-destructively.

Concentration quenching of the green photoluminescence from terbium ions embedded in AlN and SiC matrices

The green photoluminescence emission from terbium rare earth ions embedded in the wide band gap matrices wurtzite AlN and amorphous SiC shows the same functional rapid decay with increasing concentration (concentration quenching). Thus this decay is insensitive against both the structural and the chemical nature of the matrix. A rate equation model can describe this quenching process. Special attention is paid to the adequate averaging procedures.

In-situ Observation of Fracture Behavior on Nano Structure in NITE SiC/SiC Composite by HVEM

We have been successfully done in situ observation on the sequence of fracture event at the interface of NITE SiC/SiC composite examined by using miniaturized double notched shear specimen for TEM prepared by Focused Ion Beam method. In this study, we used nano-mechanics TEM experimental apparatus to investigate not only microstructure evolution and but also load and displacement curve at once in High Voltage Electron Microscope. Our results summarize as follows. Cracks were initiated at the interface between carbon coating layer on the SiC fiber and SiC matrices, and propagated along the interface. Load drop in the load and displacement curve during in-situ TEM was clearly observed at the crack initiation. The shear strength by using the miniaturized specimen is about ten times higher than that obtained by the standard testing.

Tensile creep and rupture of 2D-woven SiC/SiC composites for high temperature applications

The tensile creep and rupture behavior of 2D-woven SiC fiber-reinforced SiC matrix composites with potential for advanced high temperature structural applications was determined in air at 1315 °C. The results are compared to similar SiC/SiC data in the literature in order to understand the underlying creep and rupture mechanisms. Focus was placed on three different near-stoichiometric SiC fiber-types and three SiC-based matrix systems produced by different process routes. In general, the creep and rupture properties of the tested composites were primarily dictated by the creep resistance of the fiber-type, with the Sylramic-iBN fiber typically showing the best behavior. However, the type of matrix did have an effect on the composite creep and rupture lives due to load-sharing differences for the different matrix types and due to stoichiometry in the case of chemical vapor infiltration SiC matrices.

Integral Textile Ceramic Structures

A new paradigm for ceramic composite structural components enables functionality in heat exchange, transpiration, detailed shape, and thermal strain management that significantly exceeds the prior art. The paradigm is based on the use of three-dimensional fiber reinforcement that is tailored to the specific shape, stress, and thermal requirements of a structural application and therefore generally requires innovative textile methods for each realization. Key features include the attainment of thin skins (less than 1 mm) that are nevertheless structurally robust, transpiration holes formed without cutting fibers, double curvature, compliant integral attachment to other structures that avoids thermal stress buildup, and microcomposite ceramic matrices that minimize spalling and allow the formation of smooth surfaces. All these features can be combined into structures of very varied gross shape and function, using a wide range of materials such as all-oxide systems and SiC and carbon fibers in SiC matrices. Illustrations are drawn from rocket nozzles, thermal protection systems, and gas turbine engines. The new design challenges that arise for such material/structure systems are being met by specialized computational modeling that departs significantly in the representation of materials behavior from that used in conventional finite element methods.

Synthesis and photoluminescence studies of silicon nanoparticles embedded in silicon compound films

High-density silicon nanoparticles with well-controlled sizes were grown onto cold substrates in amorphous SiNx and SiC matrices by plasma-enhanced chemical vapor deposition. Strong, tunable photoluminescence across the whole visible light range has been measured at room temperature from such samples without invoking any post-treatment, and the spectral features can find a qualitative explanation in the framework of quantum confinement effect. Moreover, the decay time was for the first time brought down to within one nanosecond. These excellent features make the silicon nanostructures discussed here very promising candidates for light-emitting units in photonic and optoelectronic applications.

Fracture mechanisms for SiC fibers and SiC/SiC composites under stress-rupture conditions at high temperatures

The successful application of SiC/SiC ceramic matrix composites as high-temperature structural materials depends strongly on maximizing the fracture or rupture life of the load-bearing fiber and matrix constituents. Using high-temperature data measured under stress-rupture test conditions, this study examines in a mechanistic manner the effects of various intrinsic and extrinsic factors on the creep and fracture behavior of a variety of SiC fiber types. It is shown that although some fiber types fracture during a large primary creep stage, the fiber creep rate just prior to fracture plays a key role in determining fiber rupture time (Monkman–Grant theory). If it is assumed that SiC matrices rupture in a similar manner as fibers with the same microstructures, one can develop simple mechanistic models to analyze and optimize the stress-rupture behavior of SiC/SiC composites for applied stresses that are initially below matrix cracking.

Quantum structures in SiC

Two types of nanostructures in SiC matrices are studied. Heterocrystalline structures are based on different polytypes, e.g. 3C-SiC and 4H-SiC. Multi-quantum well structures with 3C wells embedded in 4H barriers show photoluminescence (PL) that can be interpreted in terms of a type-II heterostructure character and a built-in electric field due to the pyroelectricity of 4H. Germanium is incorporated in 4H-SiC matrices in form of nanometer-sized crystals. Their size, crystal structure and orientation depend on the preparation conditions. A strong size-dependent luminescence from quantum states is predicted.