Resumen de Development of doped isotropic graphites and C/SiC-B4C composites for fusion first wall and aerospace applications
Carbon-based materials are attractive materials for high-temperature applications due to their high thermal conductivity, low electrical resistivity, high strength, high modulus, excellent thermal shock resistance and lightweight. However, they present several drawbacks that substantially limit their application. The purpose of the present work has been the development of two kinds of materials for diverse applications, starting from different carbon-raw materials: Ti- and Zr- doped graphite on the one hand, and C/SiC-B4C composite on the other hand.
Ti- and Zr-doped graphites are addressed to fusion first wall application, especially for the strike point of the ITER divertor, which is the most critical area receiving the highest power and particle loads. The main drawbacks of the current candidate material for those critical areas, Carbon Fibre-reinforced Carbon, are its high reactivity with hydrogen, the tritium retention and its high manufacturing cost. Doped graphites show good thermo-mechanical properties, low chemical erosion by hydrogen bombardment, and high thermal shock resistance. A high thermal conductivity is taken as one of the main properties determining the most promising materials for the divertor armour tiles. Finally, samples with larger dimension have been manufactured to produce actively cooled mock-ups to be tested under ITER relevant conditions. The goal is to obtain a material which may be competitive with Carbon Fibre-reinforced Carbon, the current candidate material for the strike point of the ITER divertor.
C/SiC-B4C composites have been developed for application at the thruster chambers of hypersonic aircraft and for thermal protection systems. Carbon-based materials are attractive materials for aerospace applications due to their lightweight, thermal stability and excellent thermo mechanical properties. However, their application is limited by oxidation above 500 ºC. These C/SiC-B4C composites supply an effective self-passivating barrier against carbon oxidation at high temperatures and with reasonable mechanical properties.
To obtain both kinds of materials, the powder metallurgy route was used, consisting mainly of the following stages: mixing/milling, moulding, and the heat treatments, carbonization and graphitization.
For the development of the first kind of materials, two carbon precursors based on self-sintering mesophase were used: a mesophase derived from coal tar pitch, Osaka, and a synthetic mesophase pitch, AR, obtained from the polymerization of naphthalene. For the development of C/SiC-B4C composites, a carbon precursor from SGL consisting of a mixture of coke, graphite and pitch was considered. While the Osaka and SGL raw materials can be used directly, the AR mesophase has to be subjected to a stage of oxidative stabilization at low temperatures to reduce the excessive thermal plasticity, previously to its use. The main particles added were: carbides of Ti, Zr, Si and B. Almost all carbides showed a catalytic effect on the graphitization, which allowed obtaining materials with high thermal conductivities. The reinforcement effect of these carbides resulted in improved mechanical properties.
For each kind of C-raw material, and as depending on its final application, the material processing route was optimized. The general properties measured were: density, level and type of porosity, the development of highly crystalline graphite, thermal conductivity, mechanical properties, and distribution of dopants. Additionally as a function of the final application, other properties were studied such as the chemical erosion under hydrogen bombardment, the thermal shock resistance under intense thermal loads, and the oxidation resistance at high temperatures.
