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Ceramic matrix composites have been identified as a potential material of core structure for the fourth generation of fission nuclear reactors. Regarding their excellent mechanical behavior in very harsh conditions (high temperature and high irradiation flux), the CVI-SiC/SiC composites with pyrocarbon interlayer are of prime interest for the fuel cladding in the gas-cooled fast reactor. Although the working atmosphere is helium in these advanced reactors, the presence of oxidizing impurities could have a significant role on the mechanical behavior of materials subjected to long-term exposures. Within this framework, this study was intended to investigate the influence of oxidation on the SiC/SiC composites mechanical properties. Different pre-damage states were intentionally introduced by mechanical tensile tests on plate specimens before performing an oxidation treatment of 1,000 h at 1,000 °C under helium with 10 ppm of O. The degradation of the composite was determined from the mechanical behavior of post-exposure specimens. Results were correlated both with microstructural observations of the damage and with characterizations of the generated oxides at the surface of the composites. The most severe decline of mechanical properties occurs for the higher predamaged loadings. Indeed in this case, the silica formed during the oxidation of SiC is not in sufficient quantities to fill the cracks. [ABSTRACT FROM AUTHOR]

Abstract: Silicon carbide based materials are foreseen candidates for next generation nuclear applications due to a combination of the following properties: high temperature strength, high thermal conductivity and low nuclear activation. Their main drawback lies in their too low toughness. A promising route to enhance such mechanical properties is to reduce the grain size down to the nanosize range. Enabling a quite low sintering time, the spark plasma sintering technique has been used to process nano size monolithic Silicon carbides with several grain sizes and with or without boron additives. The mechanical properties, including Young modulus, flexural strength and toughness, of these materials have been measured from room temperature to 1300°C and compared to those of a commercially sintered α-SiC. The results are carefully discussed in correlation with the microstructure. Despite a lower density, the obtained flexural strength and toughness properties of the nano grain silicon carbide are very promising when processed without boron additives. Thus, efforts should be focused on the processing of large size nanograins SiC components by SPS without boron and with high density. [Copyright &y& Elsevier]

Abstract: Boron nitride thin films were deposited on carbon fibres by chemical vapor deposition from the single source precursor tris(dimethylamino)borane (TDMAB). Hydrogen was used as carrier gas and additional nitrogen was supplied in the form of ammonia. The heating of the substrate was performed by Joule effect. Detailed TEM structural and chemical analyses reveal a meso-graphitic structure with a stoichiometric B/N ratio as confirmed by Auger Electron Spectroscopy (AES). A penetration of carbon element from the fibre was observed in the BN layer close to the interface. [Copyright &y& Elsevier]

An inter-laboratory comparison has been performed between seven participants on thermal diffusivity measurements by the laser flash method in the temperature range from 23 °C to 3000 °C. The main objective was to assess the variability and coherency of thermal diffusivity measurements performed at ultra-high temperatures at the European level. Three refractory materials (molybdenum, tungsten and isotropic graphite IG210) were selected for this inter-laboratory comparison, due to their high melting point. The disk-shaped specimens needed were machined from the same blocks of materials in order to reduce any potential scattering of results between participants due to inhomogeneity effects. The homogeneity of the sets of specimens was studied by the pilot laboratory (LNE) before launching the comparison process. Thermal diffusivity measurements were then carried out by the seven participants on the three materials during two successive thermal cycles up to the maximum temperatures that can be reached by the devices used. The analysis of results showed a good agreement between the participants for temperatures above 400 °C, with relative deviations within the uncertainties of measurement and lower than ± 4 % for molybdenum, ± 5 % for isotropic graphite and ± 9 % for tungsten. [ABSTRACT FROM AUTHOR]

Modern industry has become increasingly reliant on composite materials for a variety of applications. and the nuclear industry is no exception to this.Among the materials being researched as Enhanced Accident Tolerant Fuels, ceramic matrix composites such as SiC-fiber-reinforced SiC (Sicf/SiC) figure as some prime candidates due to their excellent high temperature performances. Sicf/SiC so far shows adequate nuclear, mechanical and chemical propertiex: still, the thermal properties need further investigation.The thermal behavior of a material ix an important factor for its performance as a nuclear fuel cladding, i.e. the first barrier encapsulating the fuel pellets. Many features determine the resulting properties of composite materials, such as matrix and fiber reinforcement properties and orientation, void fraction, and pore morphology. This study establishes a methodology to study the physical properties of composite materials and applies it to Sicf/SiC . A FEM model is uxed to characterize the thermal properties of a fundamental Sicf/SiC element. referred to as a "unit cell", with the objective of accurately predicting the thermal properties of this complex class of materials where experimental data is often difficult to obtain. The unit cell is built based on data acquired with high-resolution tomographic microscopy performed at the TOMCAT beamline of the Swiss Light Source. By using phase-retrieval prior to tomographic reconstruction, the pores, fibers and matrix that compose the material can be distinguished in the data analysis. The separated information is processed to obtain geometrical information about the individual pores and fibers, which is then used to parametrize them as cylindrical objects. This allows constructing a FEM model of a cubic unit cell that is used to extract the effective then mal properties of Sicf/SiC. The analysis scheme includes steady-state and dynamic thermal transport simulations, which yield directional effective thermal conductivity and diffusivity values, respectively. Both modes of analysis show isotropic thermal conductivity values in the range of 71 W/m/K at room temperature, more than three times that of currently employed nuclear cladding materials. Combining these results with the data on the larger structural features of the material will lead to realistic results on the macroscopic thermal properties. [ABSTRACT FROM AUTHOR]

Axial and hoop tensile tests are performed at room temperature on SiC fiber CVI matrix tubular samples. A layer-to-layer interlock 3D braided preform is used. An elastic damageable mechanical behavior resulting from the development of matrix and delamination cracks is evidenced. A weaker behavior is observed in the hoop direction as a consequence of the geometry of the fibrous preform. It is shown that a numerical approach can be used to establish a link between the mechanical behavior and the microstructure of the composite. [ABSTRACT FROM AUTHOR]

Abstract: Carbide-derived carbon (CDC) coatings were produced by reaction with pure chlorine gas on the surface of β-SiC nanoparticles. Various CDC thicknesses were obtained using moderate temperatures (565–635°C) associated with a short time (30min) of chlorination under atmospheric pressure. Such conditions enable controlled layer-by-layer silicon extraction from SiC material. Kinetics of CDC formation were assessed using three SiC laser pyrolysis-produced nanopowders of different average size. Under the same conditions, the smallest particle size material is more prone to chlorination and exhibits a thicker carbon coating. Effect of particle size distribution on reactivity with chlorine is also discussed. After achieving carbide to carbon partial conversion, tem observations show good covering and adherent carbon coatings on remaining SiC material, N2 adsorption analysis show that CDC coating is microporous and has a specific surface area exceeding 1000m2 g−1. Thermogravimetric analysis coupled with mass spectroscopy under He gas flow, is used to determine the thermal stability and the nature of volatile species trapped in the microporosity. Under an O2 gas flow, the amount of CDC formed is measured by burning it off at temperatures of 400–750°C, before the onset of oxidation of the remaining SiC. [Copyright &y& Elsevier]