Design, Fabrication, and Testing of Silicon Infiltrated Ceramic Plate-Type Heat Exchanger

Kurzfassung

A novel concept for hydrogen production has been reported by the US Department of Energy, which combines the use of heat from a nuclear power plant (a Generation IV reactor) or a solar power tower for the production of hydrogen in a thermo-chemical reaction. Ceramic heat exchangers (HX) provide a promising technology for this concept. Novel plate-type HXs with high power densities are proposed, which are based on novel integrated flow-channel designs. The main purpose of this study is the investigation of net-shape fabrication to prototypical HX components based on these designs. To achieve net-shape plates, dry powder mixtures were molded by axial pressing. The joining to the prototypical 3D HX stack was accomplished by lamination followed by pyrolysis at temperatures of up to 1650 °C. Due to the use of carbon fibers the shrinkage could be controlled and reduced to about 5 %. Finally, accurate silicon melt infiltration by using the wick method into the porous C/C preforms led to dense C/SiSiC ceramics. Microstructural investigations and flexural strength measurements were performed to demonstrate the homogeneity of the ceramic and the quality of the joinings. The gas-tightness of the ceramic composites to helium has been qualified by gas-leakage tests. Corrosion tests with C/SiSiC coupons, both with and without a CVD pyrocarbon-SiC protective coating (bilayer) were performed using a ternary eutectic fluoride salt of LiF, NaF, and KF (FLiNaK) as the intermediate heat transfer fluid. While SiC is vulnerable to corrosion by the salt, such a coating offers a high degree of protection to the ceramic substrate.