Development and Characterization of Hierarchical Cellular Structure

Lightweight hierarchical materials, which have tunable structural features on multiple length scales, have been a topic of increased research interest due to advances in techniques for fabricating these cellular solids with nano and microscale features. The study of these materials extends to a variety of applications including catalysis, optics, and batteries. However, processing and mechanical stability remain an integral part of expanding and scaling up the overall size of hierarchical structures with nano and microscale features at the base level of structural hierarchy. In this study, fabrication methods, characterization, and mechanical testing of hierarchical materials is explored on different length scales. The initial focus of this work explores a process called dealloying to develop hierarchical nanoporous (np) metal structures with nanopores on the smallest feature length scale and a complex shape on the macroscale. Using a step-by-step fabrication method, a binary alloy tube was developed and subsequently dealloyed to produce a np metal tube. These tubes were generated as a proof of concept exercise so that similar step-by-step procedures could be applied to more complex shapes. In addition to templating and dealloying as a method to produce hierarchical cellular materials, microlattice metal-polymer composites were generated using a combination of 3D direct laser writing (3D-DLW) and metal coating via magnetron sputtering. Using magnetron sputtering, metal coatings of Aluminum, Inconel 600, and Ti 6Al-4V were deposited onto microlattice structures. The cross section of the composite microlattice materials were evaluated using microtome, a microscopy preparation method where the structure is embedded in epoxy and thinly sliced, which allows for the assessment of cross sections of individual lattice struts. The inclusion of alloy coatings is significant because traditional deposition methods for metals are limited to a few select elements. Therefore, using sputtering as a deposition method has the potential to expand the achievable properties of composite microstruss structures. Uniaxial compression of the samples revealed that the compressive strength can be influenced by the metal that is deposited. Furthermore, by applying metal coatings, the deformation mode of the composite structures is shifted from node-dominated failure towards an increase in ligament failure. The final aspect of this dissertation discusses the scaling up of microcellular polymer materials produced with 3D-DLW, which has a limited print volume. In order to increase the size of the printed structures, multiple printed areas are "stitched" together to form a larger structure. The influence of the stitching along this interface on the mechanical behavior of cellular structures was explored. The tensile behavior of the samples was compared to continuously printed samples with no stitch. A custom micromechanical setup was used to complete tensile testing of the structures, and no pronounced tensile strength difference between stitched and continuously printed samples was found, although stitched samples regularly failed along the stitch interface. Combined, these results have addressed three critical aspects of the emerging field of hierarchical materials, including: 1) developing novel methods to incorporate nanoscale features in hierarchical materials; 2) combining multiple techniques towards functionalizing polymer architected materials in a way that affords a high degree of material flexibility; and 3) exploring issues with scaling up architected materials with microscale features so that 3D printing protocols can be improved to mitigate these effects. Overall the main goal of this work is to reform and contribute novel processing methods for designing and fabricating materials on multiple length scales.