Extrusion-based Additive Manufacturing of Magnetic Heat Exchange Structures for Caloric Applications

Currently, the commercial building sector accounts for 18% of total U.S. end-use energy consumption, of which almost a third was from on-site combustion of fossil fuels for space and water heating. Magnetic heat pumping (MHP) technology is an energy-efficient, sustainable, environmentally-friendly alternative to conventional vapor-compression cooling technology. Several MHP designs today are predicted to be highly energy efficient, on condition that suitable working materials can be developed. This materials challenge has proven to be daunting due to issues associated with intricate synthesis/post-processing protocols and complications related to shaping the mostly brittle magnetocaloric alloys into thin-walled channeled regenerator structures to facilitate efficient heat transfer between the solid refrigerant and the heat exchange fluid in an active magnetic regenerator (AMR) cooling device.

To address this challenge, this study is focused on a novel low-temperature extrusion-based additive manufacturing (AM) method to 3D print microchanneled magnetocaloric structures. The printing ink consists of magnetocaloric powders, a polymer binder, and multiple solvents to achieve desirable shear-thinning property, which is critical for a robust printing process. Acting as a sacrificial binding agent for the magnetic powders, the polymer binder holds the 3D printed structures in place and is subsequently removed using a two-step heat-treatment process. Both fundamental and applied aspects of the manufacturing process are being addressed, including precursor powder development, 3D printing process development and post heat treatment analysis of the additively manufactured part.

Overall, research efforts pertaining to AM process development using three precursor magnetocaloric powders will be discussed – (1) La0.6Ca0.4MnO3 powders prepared by a patented modified Pechini sol-gel wet chemical method, (2) AlFe2B2 powders manufactured by a unique molten salt sintering method, and (3) La(Fe,Si)13 powders produced via gas-atomization. The effect of various process parameters, such as powder-binder ratio, the viscosity of printing ink, powder size, shape and internal porosity etc., on the geometrical and mechanical characteristics of green parts will be described. Furthermore, the effects of precursor powder characteristics (morphology, particle size distribution, porosity, etc.), as well as sintering and printing parameters on the quality of the densified magnetocaloric structures, will be deliberated upon. Finally, the feasibility of 3D printing magnetocaloric structures with spatially designed microchannels of minimum dimensions of 150 µm will be demonstrated.

Overall, this study provides strategies for realizing low-cost functionally graded magnetic regenerators, thus potentially eliminating one of the main barriers to the commercialization of magnetic cooling technology. Despite the success achieved in this Dissertation, many open questions regarding the additive manufacturing of 3D-printed magnetocaloric structures persist. The concluding chapter of this Dissertation provides recommendations for future experiments that may be conducted to develop advanced magnetocaloric structures with compositionally graded anisotropic properties and enhanced mechanical and chemical stability.