Modeling and process innovation in nanowire fusion
Creators: name: Devaraj, Harish
Publisher: name: No Publisher Supplied
Year: 2021

description: Printed electronics are predominantly created by first fabricating circuits and then integrating them with functional devices using pick and place methods. A low-energy and inexpensive approach to creating the printed circuits is to deposit electrically conductive metallic nanoparticles in film or pattern form. Since the electrical resistivity after deposition is usually high, there is a need for post deposition sintering. During such sintering, the diffusion of atoms across the boundaries of the nanoparticles cause interparticle neck growth and leads to formation of electron flow paths. Previous work has shown that metallic nanowires (NWs) can achieve higher post-sintering conductivity at lower temperature than other nanoparticle shapes. The first major contribution of this dissertation is the development of the first atomistics motivated analytical model to predict thermally driven neck growth between NWs. Mechanistic observations from Molecular Dynamics simulations are combined with the fundamental equations of mass, energy and momentum balance to derive and validate analytical equations for neck growth between two NWs. This model can predict neck growth as a function of the NW radius, but importantly and uniquely it also incorporates the newly uncovered role of relative NW orientation on the sintering kinetics.The second major contribution of this thesis is to extend the above model, which is limited to NW pairs, to multi-NW ensembles. This realizes a key advance by achieving a several orders of magnitude reduction in the computational time and effort, as compared to direct use of the NW pair model or of Molecular Dynamics simulations which are computationally intractable for experimentally relevant time scales. More importantly from a design and manufacturing point-of-view, this approach not only predicts the structure of the ensemble (i.e., neck growth) but also successfully predicts a key property (i.e., electrical resistivity). The predictions of both sintering temperature and resistivity agree well with experiments.The third major contribution of this work is the creation of a new scalable process for fabrication of 3D circuits that conform to the surface of a targeted rigid 3D object. Existing processes suffer from one or more issues such as high electrical resistivity, low process throughput, poor repeatability, and poor conformance to complex surface geometries. The Form-Fuse process developed here is based on sequential vacuum-forming and Flash Light Sintering. It enables high throughput, robust integration with the target object, low electrical resistivity, and modular circuit fabrication. We experimentally examine the effect of the change in nanomaterial morphology during both the forming and the sintering stages. A combination of electron microscopy, temperature measurements, spectrophotometry, Molecular Dynamics simulations and Electromagnetic simulations is used to understand the effect of the part shape and the nanomaterial morphology on process performance. Further, the aforementioned NW sintering models are extended to the Form-Fuse to examine its applicability to designing the crucial Flash Light Sintering step.