Abstract
Multi-material 3D-printing allows objects to be comprised of heterogeneous and complex combination of geometries. Since materials have a wide variety of properties, objects designed and printed with multi-materials have a wide range of applications. While multi-material 3D printing provides benefits of increasing the structural functionalities of a printed object, a desired technological need is the embedding of electrical functionalities in free-form 3D objects/products. Despite the rapid progress in recent years on printable electronics, there is a lack of study on printing of embedded electronics in free-form objects. Direct printing of electronics embedded in free-form geometry can potentially open up new design freedom not achievable with traditional or current fabrications and integration of electronics in products. Today, there is a lack of electrically conductive materials that not only fulfil the electrical properties requirement but also possess structural integrity. Furthermore, printing process parameters greatly govern both the electrical functionality and structural properties of the printed objects/products, and have not been well studied for the printing of electrically conductive materials. Highly conductive materials compatible additive manufacturing process must be developed with electrical conductivity higher than 5 x 103 S/m, as it would allow for a wider range of applications such as electrical contacts, conduits and others. This work involves the formulation and study of electrically conductive polymer composites that have been filled with a two component metal system: 1) Sn95Ag4Cu1 low melting point metal (LMPM) alloy (Tm = 216 °C). 2) Micron-sized nickel (Tm = 1455 °C)/copper powder (Tm = 1085 °C). By replacing a portion of the metal filler with LMPM, the viscosity of the polymer composite melt is reduced, which counter-balances the viscosity increase normally associated with increasing metal-filler content. Additionally, the solid metal particles in the melt aid in preventing complete coalescence of the liquid metal to result in a dispersed mixture of LMPM and metal particulates in a polymer matrix. Together, the combination of metal components allows the composite to have viscosity properties that would otherwise be unachievable with conventional metal composite systems, while reaching conductivities up to 3.3 x 104 S/m. A second goal of this thesis was to investigate the porosity and thermal oxidation of fused filament printed (FFP) conductive structures, and to optimize the FFP process for printing conductive structures. Through the combined effects of a highly conductive printable materials and optimized printing parameters (local gas environment, printing speed, layer height), suspended bridge structures of highly conductive composites has been achieved. Building on the knowledge acquired in design and printing of suspended conductive structures, compression force sensors incorporating 5 free-standing electrodes and unique spacer designs have been demonstrated. This work demonstrates the viability of using FF 3D printing to fabricate electronic components individually, as well as printing conductive structures comprised of multiple components in one step. In addition, the materials and printing strategies developed in this study are tailored towards fused filament printing of highly conductive, metal-filled polymer composites which enables a wider range of electrical applications. These results are a promising direction towards 3D electronics.