Additive manufacturing of structural electronics: from process innovation to nanoscale sintering mechanisms
Creators: name: Jahangir, Md Naim
Publisher: name: No Publisher Supplied
Year: 2022

description: Additive integration of 3D electrically conductive circuits with off-the-shelf functional electronic devices inside 3D printed polymer parts can enable new paradigms in miniaturized and multifunctional smart structures. This thesis develops a hybrid printing process, called Flash light Assisted Manufacturing of structural Electronics (FLAME), to achieve this goal. FLAME integrates Fused Filament Fabrication of polymers with printing and Intense Pulsed Light sintering (IPL) of silver nanowires (NWs). Using NWs and IPL increases the conductivity by 300% for planar circuits and by 170% for vertical circuits as compared to state-of-the-art hybridized nanoparticle printing processes. This is achieved even for thermally sensitive 3D printed polymers like PLA and ABS that cannot be handled by existing approaches, with an ultra-low sintering time of 1.6 milliseconds and 6.9 milliseconds for planar and vertical circuits, respectively, over a 1-foot x 1-inch area for each intermediate IPL step, and without the changes in part design and flow limitations endemic to the liquid metal injection. FFF of the polymer on the post-IPL planar circuits further alters their conductivity in a complex non-monotonic manner, with an eventual increase in conductivity. This advance breaks the performance-material-throughput tradeoff that limits state-of-the-art hybrid nanoparticle-based printing methods for structural electronics. Our computational models reveal why vertical circuits have lesser conductivity than planar ones and uncover the crucial role of a multi-layer IPL strategy in achieving reasonable conductivity for vertical circuits.Our experiments reveal the advantages of using NWs for FLAME and show that mixing NPs of different shapes impacts the electrical performance and processing temperature of the printed circuits. To understand why, we first use Molecular Dynamics simulations to reveal the sintering mechanisms between stacked silver NW pairs with non-parallel axes, a more realistic emulation of geometric configurations in experimentally relevant NW networks than that in literature. Relative NW orientation is found to have a hitherto-unknown but significant influence on inter-NW neck growth. This is due to a dynamic interaction between surprisingly high rigid-body rotation of the NWs, atomic diffusion, and dislocation generation that demarcates diffusion-dominated and dislocation-dominated regimes of neck growth. Thus, the current assumption that the relative orientation of NWs has a purely geometric and quasi-static effect on NW fusion is incomplete. We further show the importance of considering such NW rotation and the consequently large spatial gradients in neck growth on fusion-driven electrical properties of NW networks. In addition to providing a mechanistic understanding of our experimental observations, the above knowledge will play a key role in rational design and processing of NW networks across other processes as well.Next, we use Molecular Dynamics simulations to examine the mechanisms of thermally driven sintering between nanowire-nanosphere, nanoflake-nanosphere, and nanosphere-nanosphere pairs. We show that a mismatch in nanoparticle shape enhances the rate of sintering, at both room temperature and above, in a manner that does not conform to conventional arguments based on the surface area to volume ratio of the nanoparticle pair. The driving mechanisms are revealed to be a shape, size, and temperature dependent combination of grain boundary diffusion, surface diffusion, and dislocation generation. These findings provide a mechanistic explanation for why using NWs, as compared to the typical use of nanospheres, enables greater conductivity in FLAME. Further, it also creates the foundation for rational design of nanoparticle shape mixtures to enable rapid low-temperature sintering in planar and conformal device fabrication using processes other than FLAME.