Field-Assisted Direct Writing for Multifunctional Nanocomposites

The inclusion of reinforcing particles in materials used for AM allows for the fabrication of tailored multifunctional parts with improved mechanical properties, thermal conductivity, reduced coefficient of thermal expansion, improved dielectric permittivity and controllable resonance frequency. These techniques allow for the production of customized products with tunable functionalities for specific applications, for example, materials with conductive metal particles such as copper or silver can be used to print conductive traces for use in circuits. With the advancement of multi-material printing and the addition of reinforcing structures to existing polymers (e.g. 0D-3D particles) the functionality, strength, and design possibilities are endless. Fiber orientation control in printed composites can be complex due to the numerous forces acting on the composite material, forcing the fibers to align with the direction of material flow, particularly for extrusion-based additive manufacturing processes. Since extrusion-based AM relies on shear forces in a convergent nozzle to achieve fiber orientation, they can only be oriented in the direction of material flow or deposition, thus making it anisotropic. However, in order to manipulate the fiber orientation in flow, and provide the printed part with controllable fiber directionality, the shear forces acting on the material need to be overcome by the utilization of external forces. 1D Fibers (i.e. Nickel nanorods, 10- 50 um length) have been successfully synthesized and utilized in reinforcing polymer-inks to process as a Direct Printing application (via high-precision nScrypt machine). Previous attempts to randomly introduce reinforcing fibers via additive manufacturing methods have raised many limitations and issues with regards to fiber size, non-uniformity, fiber directionality, and the ability of the matrix material to hold its shape after deposition. This dissertation investigates the effects of using an externally applied magnetic field (40 mT) and novel nozzle designs with a composite containing ferromagnetic nickel nanowires during extrusion for real-time fiber orientation control. A testbed apparatus was first implemented to simulate the flow in an extrusion nozzle and to visually study the fundamental mechanism of 1D nanoparticles in flow. The effect of geometrical channel constraints (e.g. convergent vs. divergent nozzles) was explored based on Stokes’ theorem, Hagen-Poiseuille equations and experimental observations on the testbed. Divergent nozzles with inflection angles of 45 degrees for use on extrusion-based printers were designed based on the study and used to fabricate composites with improved mechanical properties, thus proving the advantage of in-situ fiber orientation control. Fiber orientations of 90 degrees to the direction of flow was achieved using a magnetic field, while a range of orientations between 0 degrees and 90 degrees can be obtained with the use of a divergent channel with the appropriate design.