Additive manufacturing of cellular materials with tailored properties

The ability to pattern complex materials with high-speed and low-cost three-dimensional (3D) printing techniques is highly desirable. Here, we present progress on developing siloxane-based feedstock formulations, known as “inks,” for a unique 3D printing approach called Direct Ink Writing (DIW). DIW is a low-cost, mask-less printing route that enables rapid design and patterning of planar and three-dimensional (3D) microstructures. In this filamentary printing approach, a concentrated ink with tailored viscoelastic properties is deposited through a micro-nozzle that is translated using a multi-axis positioning stage. The ink rapidly solidifies as it is extruded so that 3D structures with fine features may be built up in a layer-by-layer fashion. We introduce the concept of tailoring the macro-scale mechanical properties by designing the 3D micro-architecture of the printed cellular silicone materials. We show the ability to obtain highly uniform or graded properties by simply adjusting the pattern design. Moreover, by understanding the materials-structure-processing property relationships, we have created a modeling-design-fabrication approach to achieve tailored mechanical properties. For example, we have created porous architectures that, in one case, are well suited for pure compression and, in a separate case, are better suited for shear environments. We expect that the ability to deterministically program mechanical performance from part-to-part and within a part will prove useful for many applications

Additively manufactured bio-based composites

The development of new materials solutions for advanced manufacturing and fabrication technologies is an increasing focus of many research and development efforts in applied materials science today. Advances in these areas are resulting in the development of novel, geometrically complex parts and functional devices in a multitude of arenas, such as the biomedical and aerospace industries. Recent progress in materials research includes; the development of polymer systems that are less reliant on petroleum-based products, and are instead based on renewable, bio-derived sources. Concurrently, new additive manufacturing (AM) technologies are allowing the production of complex parts with structures and physical response not typically achievable through conventional manufacturing means. AM has become a leader in manufacturing complex and previously difficult to fabricate structures with fine features, by employing three-dimensional printing methods such as direct ink write (DIW) and stereolithography (SL). Our materials based approach has been to develop tailored and functional polymer based feedstocks for such AM processes to expand the range of these versatile fabrication technologies and explore new design space for AM. Here we present stimuli responsive, tailorable and robust class of, printable bio-based polymer composite that has been three dimensionally printed via additive manufacturing methods to have micro and macro scale complexity in features and exhibit a strong, tunable shape memory response. The development, characterization and potential applications of these novel shape memory polymer composite AM structures will be discussed

Field responsive mechanical metamaterials.

Typically, mechanical metamaterial properties are programmed and set when the architecture is designed and constructed, and do not change in response to shifting environmental conditions or application requirements. We present a new class of architected materials called field responsive mechanical metamaterials (FRMMs) that exhibit dynamic control and on-the-fly tunability enabled by careful design and selection of both material composition and architecture. To demonstrate the FRMM concept, we print complex structures composed of polymeric tubes infilled with magnetorheological fluid suspensions. Modulating remotely applied magnetic fields results in rapid, reversible, and sizable changes of the effective stiffness of our metamaterial motifs

Planar and Three-Dimensional Printing of Conductive Inks

Printed electronics rely on low-cost, large-area fabrication routes to create flexible or multidimensional electronic, optoelectronic, and biomedical devices1-3. In this paper, we focus on one- (1D), two- (2D), and three-dimensional (3D) printing of conductive metallic inks in the form of flexible, stretchable, and spanning microelectrodes

Nanoparticle and Sol -Gel Inks for Direct -Write Assembly of Functional Metallic and Metal Oxide Materials

124 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009.The ability to pattern 1D arrays of TiO2 microwires offers precise control of filament diameter and spatial location, enabling a systematic study of microwire TiO2 gas sensors. A model gas sensor consisting of a single layer of parallel microwires is printed with the TiO2-based sol-gel ink in a well-defined, programmable pattern. The as-printed structure is heat treated in air to 600°C to form anatase TiO2. After heat treatment, the TiO2 wire diameter is measured as (628 +/- 13 nm). Gas sensing measurements on the TiO2 microwire array performed at elevated temperatures (200--300°C) indicate high sensitivity towards NO2 and CO gases, with estimated sensitivity limits in the sub-ppm range for NO2 and single ppm range for CO. Under ambient conditions, the TiO2 microwire array responds quite significantly and reversibly to low NO2 concentrations (down to 0.5 ppm). This is a highly promising result for the creation of low-power, gas sensor devices based upon direct-write assembled TiO2 microwire arrays.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Elucidating Mass Transport Regimes in Gas Diffusion Electrodes for CO2 Electroreduction

Gas diffusion electrodes (GDEs) have shown promising performance for the electrochemical reduction of CO2 (CO2R). In this study, a resolved, pore scale model of electrochemical reduction of CO2 within a liquid-filled catalyst layer is developed. Three CO2 mass transport regimes are identified in which the CO2 penetration depth is controlled by CO2 consumption in the electrolyte, CO2 conversion along the solid-electrolyte double-phase boundaries (DPBs), and CO2 conversion concentrated around the gas–solid–electrolyte triple-phase boundaries (TPBs). While it is possible for CO2R to be localized around the TPBs, in systems with submicron pore radii operating at –2 CO2R will be distributed across the DPBs within the catalyst layer. This validates the assumption of pore-scale uniformity implicit in popular, volume-averaged GDE models. The CO2 conversion efficiency depends strongly on the governing mass transport regime, and operational-phase diagrams are constructed to guide the catalyst layer design.</p

3D Flow Cell Simulation Via Accelerated Buffer Chemistry

To date the literature on flow cell reactors for CO electrochemical reduction buffered by bicarbonate solutions hasbeen dominated by 1D film models. These models assume a static boundary layer adjacent to the electrode of finitewidth, through which the CO2, HCO3-, CO32-, OH- and H+ species diffuse and react according to their own diffusion-reaction equations. These models are very popular, but are difficult to solve in higher dimensions, as the ~10 orders ofmagnitude variation among the reaction rate constants leads to a stiff system of differential equations which scaleswith computational mesh size. In this work, we reduce these equations to an alternative set of non-stiff PDE’s via aquasi-steady approximation as derived from a two-variable singular perturbation expansion. This reduced set of PDE’saccelerates the computation by over 100x in 1D while typically introducing errors <1% (Figure 1a-b.). The acceleration of the bulk carbonate chemistry allows the rigorous simulation of a 3D flow reactor (Figure 1c-d) inwhich a CO -saturated bicarbonate solution flows past a silver cathode at which COER and HER occur. We implementthe accelerated reactive model in the commercial tool StarCCM+ and simulate the coupled fluid flow, electrostatics,and reactive mass transport in the system, including the effects of electromigration and electroneutrality. Surfacekinetics are modeled via Tafel expression for COER, HER, and OER. Steady-state, 3D simulations on computationalmeshes with 2.65 M cells converge in ~ 5 core-hours. This simplified approach to modeling the reactor chemistry mayallow the rigorous simulation of more complex 3D geometries (such as minimal surfaces or structures for convectivetransport enhancement) which have been shown to increase Sherwood and Nusselt numbers significantly in the heat-exchanger and membrane-reactor literature.</p

Simplified Models of the Bicarbonate Buffer for Scaled Simulations of CO2 Electrolyzers

Bicarbonate electrolytes are used in a range of chemical processes; however, resolved simulation of these electrolytes is difficult, as disparate reaction time scales lead to numerical stiffness and the formation of fine boundary layers. Based on several physically motivated approximations, we reduce the full set of chemical reactions within a bicarbonate electrolyte to a simpler subset, eliminating the numerical stiffness. We supported this simplification via a two-variable singular perturbation expansion and demonstrated that under neutral conditions (6 2 electrolyzer, the simplifications lead to negligible error. We also discuss two alternative simplifications, one valid at high pH and another valid at arbitrary pH. These simplifications reduce the condition number of the matrices resulting from spatiotemporal discretization by up to 10 orders of magnitude and enable three-dimensional (3D) simulation of CO2 electrolyzers containing carbonate solutions.</p