<p dir="ltr">Silicon carbide (SiC) is a ceramic with strength retention at elevated temperatures, oxidation resistance and a high thermal conductivity. These properties make SiC a desirable ceramic for compact, high temperature (> 1000 °C), heat exchangers with improved thermal performance. However, fabricating a SiC heat exchanger is difficult due to the low self-diffusion and high melting temperature of SiC. The aim of this dissertation is to show the viability of using co-extrusion and slip casting as low-cost, scalable processes for creating a compact, high temperature, SiC heat exchanger.</p><p dir="ltr">Co-extrusion is an advantageous fabrication technique as it is capable of producing samples with micron-sized features in two dimensions. To fabricate the heat exchanger body via co-extrusion, a SiC-filled polymer blend and a carbon black (CB)-filled polymer blend (sacrificial) were developed. A 54 vol% SiC-filled polymer blend with the addition of 12 wt% alumina and yttria, sintering aids, in a 2:1 ratio, respectively produced samples with the highest relative densities of 94% while maintaining an extrudable rheology. The SiC-filled polymer blend was co-extruded at 80 °C with a 45 vol% CB-filled polymer blend to produce unit cells that were open and continuous after binder burnout and sintering. The unit cells had an average relative density of 90% with an average strength of 165 MPa.</p><p dir="ltr">The unit cell strengths were lower than expected due to the formation of defects that occurred after removal of the polymers. These defects were categorized into macrodelaminations, defects that occur between two laminated unit cells, and microdelaminations, defects that occur within a single unit cell. The mechanisms causing these defects was studied by investigating the lamination and polymer removal processes. Results showed that poor lamination between extrudates mitigated the macrodelaminations and an oxygen-rich debinding atmosphere caused the formation of microdelaminations. Defect-free unit cells were produced though a partial extrusion step and binder removal in a nitrogen atmosphere.</p><p dir="ltr">An aqueous SiC suspension for slip casting was optimized by investigating the rheological properties, zeta potential, and slip casting behavior. It was determined that a suspension with 40 vol% solids, 1.2 wt% dispersant (polyethyleneimine), and a pH of 7.5 resulted in uniform slip cast parts. This optimized suspension was used to fabricate dense, crack-free SiC headers with an average relative density of 96% and an average strength of 266 MPa.</p><p dir="ltr">This dissertation gives insight into important fabrication parameters that must be considered when fabricating high temperature, SiC heat exchanger components. Additionally, this dissertation showcases the capability of using co-extrusion and slip casting as potential pathways for fabricating a high temperature, SiC heat exchanger.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/24649938 |
| Date | 29 November 2023 |
| Creators | Olivia N Brandt (16913286) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Fabrication_Methods_of_Silicon_Carbide_for_High_Temperature_Heat_Exchanger_Applications/24649938 |
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