Ceramic Routes for Fabrication of SiC f /SiC Composites

1. “Jožef Stefan” Institute Ceramic Routes for Fabrication of SiCf/SiC Composites S. Novak1, G. Dražić1. K. Mejak1, T. Toplišek1, T. Žagar2, M. Ravnik2 1 Department for Nanostructured Materials, Jožef Stefan Institute, Slovenia 2 Department for Reactor Physics, Jožef Stefan Institute, Slovenia • Densification of the SiC-based matrix • LIMITATIONS: • < 1500°C • Use of nano-powder • Low-activation additives • Limited number of elements allowed INTRODUCTION Due to the extremely low neutron-activation of SiC, SiC-based composites are considered as an attractive alternative to steel based materials (e.g. Eurofer) for the first-wall blanket of the future fusion reactors. However, the current SiCf/SiC composites, produced by the CVI process does not meet all the requirements (certain amount of internal porosity and high processing cost). Consequently, alternative techniques for the material production are under development. AIM The main problem that needs to be solved in the case of the ceramic routes is high sintering temperature of SiC and high neutron activation of commonly used sintering additives. The main stress in the present investigation was therefore to find a sintering additive from a limited number of low-activation elements, that would enable sintering of SiC to high density at temperature below 1500°C. Candidate sintering aids, alternative to conventionally used, was selected on the base of activation cross-section data and on the survey of relevant phase diagrams. CERAMIC ROUTE TO CMC - Wetting - Coating Low-activation additives Fracture surface of the sample with 0.5 mm SiC (SEM) Polished sample with 0.5 mm SiC (SEM) Continuous SiC fibres / woven SiC powder Al-phosphateMg-phosphate - 0.5 mm- 50 nm Liquid + SAA SiC fibre SiC-based matrix Infiltration Densification Suspension 50 nm 15 mm Vacuum Slip Infiltration (VSI) (!!) < 1500°C Polished cross-section of the sample with 50 nm SiC (SEM) Interface between SiC-fibre and matrix with 50 nm SiC (TEM) Electrophoretic Infiltration (EPI) SiC-fibres SiC-fibre SiC deposit Counterelectrode SiCf/SiC Good adhesion of the matrix material to the fibers (SEM) Crack deflection at the fibre (SEM) Deposition cell SiC bundle with SiC deposit SiC particles at SiC fiber Massive SiC deposit on SiCf/SiC after 10 min of deposition at 60 V Saturated gamma activities of impurities found in used SiC powders irradiated in the central channel of TRIGA Mark II reactoras a function of cooling time. SiC fibres are conductive and hence applicable as an electrode in electrophoretic deposition or infiltration. SiC particles and Al-phosphate are deposited from ethanol suspension and after firing produce a dense SiC-ceramics. CONCLUSIONS • Using nano-sized SiC powder and Al-phosphate as transient sintering additive (TSA), SiC-based ceramics can be densified at 1450°C. • Electrophoretic deposition was confirmed to be a promising technique for fast infiltration of ceramic suspension into SiC-fibre substrate as well as to produce gas-impermeable coating at the SiCf/SiC composite. • Effective filling the SiC-fibre woven can be achieved by modification of the fibres surface using appropriate surface active agents.The critical parameters are particles surface charge and conductivity of the SiC-ethanol suspension. • Oxide layer at the SiC particles hinders the deposition and have also a detrimental effect on densification and properties of the sintered material. • Impurities in the raw materials used in this phase of the investigation, cause high activation. • Further work: • - Replacment of Al with Mg • Use of high-purity raw materials • ... Acknowledgement: The work was performed in the frame of the project “Novel processing of SiC/SiC by slip infiltration of SIC-preforms with SiC”, Euratom-FU06-CT-2003-000323 (2004) and Fusion association contract MHST: FU06-CT-2004-00083-UT2 (2005)