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Abstract: To improve the oxidation resistance of SiC composites at high temperature, the feasibility of using Ti3SiC2 coated via electrophoretic deposition (EPD) as a SiC fiber reinforced SiC composite interphase material was studied. Through fiber pullout, Ti3SiC2, due to its lamellar structure, has the possibility of improving the fracture toughness of SiCf/SiC composites. In this study, Ti3SiC2 coating was produced by EPD on SiC fiber; using Ti3SiC2‐coated SiC fabric, SiCf/SiC composite was fabricated by hot pressing. Platelet Ti3SiC2 powder pulverized into nanoparticles through high‐energy wet ball milling was uniformly coated on the SiC fiber in a direction in which the basal plane of the particles was parallel to the fiber. In a 3‐point bending test of the SiCf/SiC composite using Ti3SiC2‐coated SiC fabric, the SiCf/SiC composite exhibited brittle fracture behavior, but an abrupt slope change in the strength‐displacement curve was observed during loading due to the Ti3SiC2 interphase. On the fracture surface, delamination between each layer of SiC fabric was observed. [ABSTRACT FROM AUTHOR]

Multi-layered SiC composites have been considered as a nuclear fuel cladding material of light water reactors, LWRs, because of their excellent high temperature strength and corrosion resistance under accident conditions. During a design basis accident of a LWR such as a loss-of-coolant accident, the peak temperature of the fuel clad rapidly increases as the production of decay heat continues. The emergency core cooling systems then automatically supply the reactor core with emergency cooling water. The fuel clad consequently suffers from thermal shock. In this study, the structural integrity of multi-layered SiC composite tubes after thermal shock was investigated. Several kinds of multi-layered SiC composite tubes consisting of CVD SiC and CVI SiCf/SiC were water-quenched from 1200°C to room temperature. The triplex SiC composite tube retained its tubular geometry during quenching. The strength degradation after thermal shock was <13% for the specimens with a PyC interphase. The residual stress distribution within the tubes during thermal shock was evaluated by a finite element method. [ABSTRACT FROM AUTHOR]

We performed microcompression tests on silicon carbide ( SiC) pillars with diameters ranging from 4.7 down to 0.65 μm at room temperature. The SiC micropillars were fabricated from a polycrystalline 3C-SiC plate using lithography and plasma etching. The pillars exhibited elastic loading behavior until a catastrophic brittle fracture occurred. The compressive fracture strength was found to increase with a decrease in pillar diameter. This scale dependency can be attributed to the flaw sensitivity of brittle materials, and cracking seems to start at the grain boundaries. Ductile plasticity occurred in micropillars with a diameter of 0.65 μm. Cross-sectional transmission electron microscopy observation of the plastically deformed pillars showed that a slip occurred on the {111} planes. The critical resolved shear stress for a {111}<110> slip system was evaluated to be 4.9-7.3 GPa, which is consistent with the values obtained from numerical and theoretical studies in the literature. [ABSTRACT FROM AUTHOR]

The whisker growing and matrix filling process was applied to reduce the canning effect during the CVI process. Grown whiskers may serve to divide the large natural pores between fibers or bundles and then matrix filling may be efficiently carried out through the modified pore structure. In this study, the multi-step of the in situ whisker growing and matrix filling process were performed to investigate the densification behaviors of SiCf/SiC composite. MTS (CH3SiCl3) and H2 were used as source and diluent gases, respectively. The process cycles were repeated three times. The whisker growth and densification behaviors with the number of the process cycles were evaluated. SiC whiskers were grown in the SiC fiber preforms at the input gas ratio of 60 and the total reaction pressure of 5 Torr. As the number of process cycles increased, the maximum size of the large voids decreased and homogeneity of the voids distributions increased. [Copyright &y& Elsevier]

Silicon carbide films have been grown onto graphite substrates by low pressure chemical vapor deposition using MTS (CH3SiCl3) as a source precursor and H2 or N2 as a diluent gas. The experiments were performed at fixed conditions of a flow rate of 100 sccm for each MTS and carrier gas, a flow rate of 300 sccm for diluent gas addition, and a total pressure of 5 torr. The effect of temperature from 900°C to 1350°C and the alteration of diluent gas species on the growth rate and structure of deposits have been studied. The experimental results showed that the deposition rate increased with increasing deposition temperature irrespective of diluent gases and reactant depletion effect increased especially at H2 diluent gas ambient. At MTS-H2 system, the deposition mechanism changed from chemical reaction to mass transfer controlled reaction with temperature. Otherwise, For MTS-H2-N2 system, surface chemical reaction controlled the growth process at whole deposition temperature ranges. For N2 addition, surface morphology of leaf-like structure appeared, and for H2, faceted structure at 1350°C. The observed features were involved by crystalline phase of β-SiC and surface composition with different gas ambient. [ABSTRACT FROM AUTHOR]

A hybrid process of SiC slurry impregnation (SI) and hot pressing (HP) has an advantage for the fabrication of dense SiCf/SiC (silicon carbide fiber reinforced/silicon carbide matrix) composite. An effective impregnation of the SiC slurry into the voids of a fiber preform is important to fill the dense matrix phase in the SI and HP processes. An electrophoretic deposition (EPD) technique was applied to effectively increase the densification of the matrix phases by a fast, simple, and efficient impregnation of the SiC slurry. An EPD was performed using a dual electrode system under an applied voltage of 10V for 30min with/without ultrasonification. The maximum density of SiCf/SiC composite fabricated by an EPD with ultrasonification was 3.14±0.04g/cm3. The flexural strength was 531±26MPa for a composite with a single interlayer of (200nm PyC (pyrolytic carbon)+600nm SiC). [ABSTRACT FROM AUTHOR]

The corrosion behavior of chemically vapor deposited SiC was investigated in relation to its application as fuel cladding in a pressurized water reactor (PWR). Corrosion tests were conducted in deoxygenated, pressurized water at 360 °C with control of dissolved hydrogen. The dissolved hydrogen dramatically reduced the corrosion rate of SiC compared with that in water without the dissolved hydrogen. It was revealed that the dissolution of the surface oxide of SiC primarily contributed to the weight loss at the initial stage of corrosion. Further weight loss was minimal for 210 d because dissolved hydrogen effectively retarded the formation of SiO 2 . [ABSTRACT FROM AUTHOR]

The SiC ceramics are under investigation for the fuel cladding in the light water nuclear reactors because of its excellent high temperature strength and corrosion resistance against hot steam under the severe accident conditions. In this study, the SiC triplex tubes consisting of a SiC inner layer, a SiC/PyC/SiC intermediate layer, and a SiC outer layer were fabricated by the chemical vapor processes. The hoop strength and fracture behaviors of the SiC triplex tube were investigated. The SiC triplex tubes fabricated at the high ratio of H 2 /MTS had a quite high average strength with a relatively small standard deviation. The hoop strength of the composite tubes tends to increase with the volume fraction of the reinforced fibers. The highest fiber volume fraction was obtained using Tyranno SA3-0.8k with the dense winding patterns such as bamboo-like mosaic pattern, which resulted in the high hoop strength compared to other fibers of Tyranno SA3-1.6k and Hi-Nicalon Type S. Hoop strength also increased slightly as the winding angle increased from 45° to 65°. Fracture behaviors of the SiC triplex tube were investigated via the observation of microstructure of the failed samples. [ABSTRACT FROM AUTHOR]

Alternating current electrophoretic deposition (AC–EPD) was used to infiltrate a SiC-based matrix phase containing Al 2 O 3 –Sc 2 O 3 sintering aids into the fine voids of a two-dimensionally woven Tyranno™-SA3 SiC fabric preform for the fabrication of a dense SiC fiber-reinforced SiC composite (SiC f /SiC). The zeta potential of the aqueous suspension was set to ≥−55 mV to achieve uniform and simultaneous deposition of all constituent particles. EPD was performed at 20 V AC for 30 min under the application of ultrasonic pulses for the initial 20 min to minimize the surface sealing effect. A high SiC f /SiC density of 96.5% along with a flexural strength of 413.9 ± 18.5 MPa was achieved after hot pressing at 1750 °C for 1 h due to the high degree of matrix phase infiltration. These results showed that an eco-friend AC–EPD process can be applied to the fabrication of high density SiC f /SiC. [ABSTRACT FROM AUTHOR]

SiC and ZrC are critical and essential materials in TRISO coated fuel particles since they act as protective layers against the diffusion of metallic and gaseous fission products and provide mechanical strength for the fuel particles. However, SiC and ZrC have critical disadvantages in that SiC loses its chemical integrity by thermal dissociation at high temperature and the mechanical properties of ZrC are weaker than those of SiC. In order to address these problems, we made new combinations of ZrC coating layers and deposited them on SiC using a chemical vapor deposition process. We varied the growth temperature to optimize the growth conditions and analyzed the changes of the microstructure, chemical composition, and mechanical properties of the ZrC films. The deposition temperature was varied from 1200 to 1500 °C and ZrC changed from a whisker structure to a film at a temperature of 1400 °C. The preferred orientation was changed from the (111) plane to (220) and the chemical composition became stoichiometric. Also, the hardness and elastic modulus of ZrC increased when it was deposited at a higher temperature. [ABSTRACT FROM AUTHOR]

In order to reduce the amount of excess silicon and carbon, reaction-bonded silicon carbide was fabricated by liquid silicon infiltration using a porous carbon preform prepared by chemical vapor deposition. A carbon preform was fabricated at 1100−1400 ℃ at 6.6 kPa for 1 h using acetylene. The deposited carbon preform has a porous structure with nano-sized carbon particles and uniform pore size distribution. Depending on the deposition temperature of the carbon preform, the porosity, the pore size, and the size of the carbon particles were controlled. As the deposition temperature increased, the porosity and average pore size increased, but the average carbon particle size decreased. The residual silicon of the prepared reaction-bonded silicon carbide was 2.7 % when the porosity of the carbon preform deposited at 1100 °C was 51.1 %. The hardness increased up to 21.7 GPa as the amount of the residual silicon reduced. [ABSTRACT FROM AUTHOR]

The hydrothermal corrosion behavior of chemical-vapor-deposited SiC was investigated for application as nuclear fuel cladding in a pressurized water reactor (PWR). This paper presented the effect of grain size, stacking fault, electrical conductivity, and electrochemical properties on the hydrothermal corrosion behavior of the CVD SiC in a simulated PWR water loop at 360 °C and 18.5 MPa. The hydrothermal corrosion of SiC was strongly influenced by electrical conductivity. It was revealed that high electrical conductivity leads to high exchange current density, which allows SiO 2 /Si(OH) 4 to form easily on the SiC surface, allowing it to dissolve rapidly in the hydrothermal environment. [ABSTRACT FROM AUTHOR]

Multi-layered SiC composites consisting of monolithic SiC and a SiC f /SiC composite are one of the accident tolerant fuel cladding concepts in pressurized light water reactors. To evaluate the integrity of the SiC fuel cladding under normal operating conditions of a pressurized light water reactor, the hydrothermal corrosion behavior of multi-layered SiC composite tubes was investigated in the simulated primary water environment of a pressurized water reactor without neutron fluence. The results showed that SiC phases with good crystallinity such as Tyranno SA3 SiC fiber and monolithic SiC deposited at 1200 °C had good corrosion resistance. However, the SiC phase deposited at 1000 °C had less crystallinity and severely dissolved in water, particularly the amorphous SiC phase formed along grain boundaries. Dissolved hydrogen did not play a significant role in improving the hydrothermal corrosion resistance of the CVI-processed SiC phases containing amorphous SiC, resulting in a significant weight loss and reduction of hoop strength of the multi-layered SiC composite tubes after corrosion. [ABSTRACT FROM AUTHOR]

This paper reports the processing feasibility of electrophoretic deposition combined with hot pressing in the fabrication of dense tubular SiC f /SiC composites using a cylindrical mold. A simulation of pressure distribution using ANSYS software was performed by varying the angular inclinations in a cylindrical mold with an ‘out → in’ configuration so as to ensure a maximum and uniform conversion of vertical hot press force to the lateral side of a centrally-located preform through graphite powder. The simulation revealed an inhomogeneous pressure distribution along the height of the preform, which could be minimized by mold optimization to achieve a more uniform tube density. To verify this, two different preform architectures such as 0/90° woven 2-D fabric rolled in a jelly state and filament winding with two plies having an inter-ply angle of 55° were hot-pressed using a mold fabricated based on the simulation after infiltrating the matrix phase by electrophoretic deposition. The density of the tube could be increased with more uniform microstructures. Although the tube using a filament winding preform exhibited a lower flexural strength (105 MPa) and relative density (90%) than those with the preform rolled in a jelly state (221 MPa, 95%), the results revealed a high degree of fiber pull-out due to the PyC coating on the SiC fiber. [ABSTRACT FROM AUTHOR]

The thermal conductivity of SiC ceramics and FCM fuel composites, consisting of a SiC matrix and TRISO coated particles, was measured and analyzed. SiC ceramics and FCM pellets were fabricated by hot press sintering with Al 2 O 3 and Y 2 O 3 sintering additives. Several factors that influence thermal conductivity, specifically the content of sintering additives for SiC ceramics and the volume fraction of TRISO particles and the matrix thermal conductivity of FCM pellets, were investigated. The thermal conductivity values of samples were analyzed on the basis of their microstructure and the arrangement of TRISO particles. The thermal conductivity of the FCM pellets was compared to that predicted by the Maxwell-Eucken equation and the thermal conductivity of TRISO coated particles was calculated. The thermal conductivity of FCM pellets in various sintering conditions was in close agreement to that predicted by the Maxwell-Eucken equation with the fitted thermal conductivity value of TRISO particles. [ABSTRACT FROM AUTHOR]

A manufacturing technique for fabricating a dense tubular SiC long fiber-reinforced SiC composite (SiC f /SiC) by hot pressing was developed. After infiltrating a SiC-based matrix phase, containing a 12 wt% of Al 2 O 3 –Y 2 O 3 sintering additive, into the fine voids of a Tyranno TM -SA3 SiC fabric preform by electrophoretic deposition combined with the application of ultrasonic pulses, hot pressing was performed using 2 types of specially designed molds filled with graphite powder to transfer the vertical hot press force efficiently to the sidewalls of the tubular SiC f /SiC. Compared to the low density (~60%) of SiC f /SiC hot-pressed using a conventional mold, a density >95% could be acquired using a special mold filled with graphite powder as a pressure delivering medium. This method is suitable for fabricating a dense tubular SiC f /SiC, which cannot be obtained using a conventional extrusion method. [ABSTRACT FROM AUTHOR]

Abstract: The incorporation of SiC nanowires in the SiC fiber preform was clearly shown to be effective in increasing the efficiency of the matrix infiltration for the fabrication of SiCf/SiC composites, and thus resulted in a higher density in a shorter CVI (chemical vapor infiltration) time. Generally, tailoring the interface between fibers and the matrix is known to be a way to control the mechanical behaviors of SiCf/SiC composites. In this study, the PyC interface was coated on SiC nanowires as well as SiC fibers to supply the additional interfaces and enhance the mechanical properties of SiCf/SiC composite prepared by the nanowire-growth assisted CVI process (NW-CVI). NW-CVI composites with or without PyC coating on the SiC nanowires had the average flexural strength of 450 and 510MPa and the average tensile strength of 226 and 253MPa. [Copyright &y& Elsevier]

Abstract: SiC fiber-reinforced SiC–matrix ceramic composites (SiCf/SiC) were fabricated by vacuum infiltration of a SiC slurry into Tyranno™-SA grade-3 fabrics coated with a 200nm-thick pyrolytic carbon (PyC) layer followed by hot pressing using a transient eutectic-phase. The density of the composite was improved using a special infiltration apparatus with a pressure gradient and alternating tape insertion between fabrics. Their overall properties were compared with those of monolithic SiC and composite containing chopped fibers. Although the density of the composites decreased with increasing fiber fraction, SiCf/SiC containing 50vol.% fibers had a density of 3.13g/cm3, which is the highest reported thus far. The composites containing continuous fibers had a maximum flexural strength of 607MPa and a step increase in the stress–displacement behavior during the three-point bending test due to fiber reinforcement, which was not observed in the monolith. [Copyright &y& Elsevier]

Abstract: The effect of coating temperature on properties of the SiC layer in TRISO-coated particles was investigated. An increase in coating temperature resulted in a significant coarsening of surface microstructures and an increase of the pore size and porosity of SiC layers. The SiC layers formed at 1400–1550°C had nearly stoichiometric compositions whereas the SiC layer formed at 1600°C contained a small amount of free carbon. The degradation of hardness and elastic modulus of the SiC layers coated at 1550 and 1600°C was attributed to the increased porosity of the specimens and partly to the existence of free carbon. The fracture stress of the SiC layer measured by the crush test of hemispherical shell specimen did not correlate with the hardness and elastic modulus, and there was no clear dependence of the fracture stress on the coating temperature; this lack of correlation can be explained by the large roughness of the inner surface of the SiC layer. [Copyright &y& Elsevier]

Abstract: β-SiC whiskers were grown via a vapor-solid (VS) reaction on two types of substrates; bare Si (111) and SiO2-coated Si (111). Different growth behaviors of β-SiC whiskers were observed for each substrate. β-SiC whiskers grew only on the bare Si substrate, not on the SiO2-coated Si surface. Therefore, β-SiC whiskers could be selectively grown along the patterns on the SiO2-coated Si substrate. The turn-on field and the maximum current density of the specimens using selectively grown whiskers along the patterns on the SiO2-coated Si substrate were 2.0 V/μm and 1.01 mA/cm2, respectively. The converted curve of I–V characteristics of β-SiC whiskers by the Fowler–Nordheim equation maintained a linear slope. [Copyright &y& Elsevier]