DEVELOPMENT OF PROCESSING AND JOINING TECHNIQUES FOR THE  FABRICATION OF A SILICON CARBIDE HEAT EXCHANGER

  

The development of a high-temperature heat exchanger made of silicon carbide (SiC) required the development of processing and joining technologies for the fabrication and integration of a prototype. Traditional ceramic forming techniques such as dry powder compaction, tape casting, or injection molding cannot effectively process complex and micron-size parts such as those required by heat exchangers to generate high surface area for improved thermal efficiency. Ceramic co-extrusion has been a successful fabrication technique to produce small structures, ceramic piezoelectric, and fibrous monolithic.

The co-extrusion process is unique in its ability to create micron-size features in two dimensions through multiple reduction steps. Using this process, the heat exchanger channels are developed to create a section with a high surface area to enhance the heat transfer between fluids.

Ceramic co-extrusion requires the development of ceramic/polymer binder systems based on SiC powder, fugitive thermoplastic binders, and low molecular weight polymeric species as processing aids. The thermoplastic binders mixed with SiC powder provided molding and extrusion capabilities to build the heat exchanger prototype. Afterward, a binder removal process and sintering were performed to densify the final component. The presence of cracks is common when working with ceramic/polymer binder systems. Ten different SiC ceramic/polymer binder systems were developed and evaluated to understand the mechanisms that generate cracks and lower the mechanical strengths of components.

A SiC heat exchanger is comprised of a main core where the fluids exchange energy and the manifolds that direct both cold and hot fluids to the respective set of channels. The integration of these components is challenging because of the high degree of covalent bonding and low self-diffusivity of SiC. Welding and other integration methods common in metals are not feasible due to the high melting point of SiC (2730 °C). Reaction bonding is a technique that has displayed the potential to integrate SiC parts by recreating the reaction of silicon (Si) and carbon (C) on an interlayer between SiC components. This work presents the development of a pressureless joining technique for SiC by reaction bonding using SiC/C loaded ceramic suspensions and the methodology to create a successful bonding region between SiC components. The approaches studied varied the thickness in the joint region to study its mechanical strength, and crystalline structure.