Silicon carbide (SiC) is a ceramic with strength retention at elevated temperatures, oxidation resistance, and high thermal conductivity, making it useful for high-temperature (> 1000 °C) heat exchangers (HXs). However, fabricating a SiC HX is challenging because of the low self-diffusion of SiC. This presentation aims to demonstrate the viability of using co-extrusion and slip casting as low-cost, scalable fabrication techniques for the creation of a SiC HX.

 

The HX body is fabricated using co-extrusion. Co-extrusion involves a series of extrusion and lamination steps of a feedrod containing multiple ceramic/polymer blends and is useful in fabricating components with micron-sized features in two dimensions. The HX header components, used for fluid direction, are fabricated using slip casting and involves pouring an aqueous ceramic suspension into a porous mold to create complex components.

 

First, the development of a co-extrusion process to create a HX unit cell without geometric distortions is discussed. SiC/polymer blends with varying sintering aid compositions and high solids loadings were developed to maximize the sintered density. Extrusion parameters, such as temperature and blend composition, on unit cell distortion were investigated. Successful unit cells were achieved using a 54 vol% SiC/polymer blend and a 45 vol% carbon black/polymer blend, extruded at 80 °C.

 

Upon removal of the polymer from the unit cell, defects termed microdelaminations (defects occurring within a single extrudate) and macrodelaminations (defects occurring between two extrudates) are revealed. The effect of polymer additives, lamination procedure, and polymer removal process on delaminations is presented. Results showed that a lack of shear during lamination caused macrodelaminations, while an oxygen-rich polymer removal atmosphere caused microdelaminations.

 

Aqueous suspensions with low viscosities and high zeta potentials are desirable for creating dense, crack-free slip cast components. The dispersion amount, solids loading, and suspension pH were varied, and their effects on viscosity were reported. The lowest viscosity suspension employed the dispersant polyethyleneimine (PEI) at a concentration of 1.2 wt%, and the highest zeta potential was achieved at a pH of 7.5. SiC header components fabricated with this suspension were crack-free and reached 96% relative densities after pressureless sintering.