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Abstract

Nuclear fusion offers a prospect of a safe, economical and environmentally friendly source of energy. For the advanced concepts of nuclear fusion, SiC-based fibre reinforced composites were proposed as a structural material for the first wall of the reactor vessel, enabling higher operating temperatures and efficiencies. Already developed techniques for fabrication of SiCf/SiC composites are currently unable to achieve all of the design assumed values for these materials, therefore further development is needed. The main drawback in fabrication of SiCf/SiC composites is densification of SiC matrix in 3D woven fabric preform using low activation elements and at temperatures at which the properties of the fibre reinforcement are preserved. Therefore, in this work, a novel combined process of electrophoretic (infiltration) deposition (EP(I)D) and polymer infiltration and pyrolysis (PIP) was introduced as a possible way to fabricate the SiCf/SiC composites with reduced porosity and increased properties. The work was performed within the European fusion research programme (FP7, EURATOM – Fusion) and was coordinated and verified by European Fusion Development Agreement (EFDA).
Feasibility of densification by the combined process of EPD and PIP was initially evaluated on bulk SiC samples. By tailoring the electrokinetic properties of the suspensions and employing optimal electrical conditions, deposits with high packing density (~60 %TD) were obtained. Polymer infiltration and polymer-to-ceramic conversion was studied in order to optimise the infiltration and pyrolysis procedure. To ensure the formation of crystalline material, needed to achieve the desired properties, additional heat treatment at 1600 °C and 1700 °C was employed. Densification of EPD deposit by six polymer infiltration and pyrolysis cycles resulted in a matrix material with ~87 %TD, with uniform submicron porosity. The material was characterised by flexural strength of 337±75 MPa, hardness of 1740 HV and elastic modulus of 260 GPa. Thermal conductivity of the matrix material achieved nearly 60 Wm-1K-1.
Based on the results of comprehensive analysis of the EPD of bulk SiC, electrophoretic (infiltration) deposition was successfully utilised to infiltrate thick conductive SiC fibre preform. By separation of fibres from the deposition electrode, electric field penetration through the thickness of the preform was ensured in order to provide a driving force for migrating particles. Due to the same polarity of zeta potential of migrating SiC particles as for the fibres, the particles were able to penetrate throughout the 5.4 thick fabric preform and thus filled the voids within it. Best results were obtained from suspension with 50 wt. % of solids loading at current density of 2.5 mAcm-2. After densification of the material by six polymer infiltration and pyrolysis cycles, a SiCf/SiC composite was formed, composed of pure, crystalline SiC with a more favourable microstructure in terms of pore size and porosity distribution in comparison to other fusion-grade composites prepared by state-of-art techniques (CVI, PIP).
The resulting SiCf/SiC composite was characterised by high thermal conductivity in achieving 60 Wm-1K-1 at room temperature and 30 Wm-1K-1 at 1000 °C, which is one of the highest thermal conductivities reported for such material. Initial mechanical properties characterisation of the composite material revealed a need for a more stable interphase layer between the matrix and the fibres.