MechE PhD Dissertation Defense: Stephanie Zopf

  • Starts: 2:00 pm on Tuesday, October 22, 2024
  • Ends: 4:00 pm on Tuesday, October 22, 2024
TITLE: CHEMICALLY COALESCING LIQUID METAL EMULSIONS FOR 3D PRINTED SOFT CONDUCTORS

ABSTRACT: Gallium-based liquid metal alloys (GaLMAs) have widespread applications ranging from soft electronics, energy devices, and catalysis. GaLMAs can be transformed into liquid metal emulsions (LMEs), a composite form with mod-ified rheology, for simpler patterning, processing, and material integration in GaLMA-based device fabrication. One major drawback of using LMEs is re-duced electrical conductivity, owing to the oxides that form on the surface of dispersed liquid metal droplets. LMEs thus need to be activated by coalescing liquid metal droplets into an electrically conductive network, which usually in-volve techniques that subject the LME to harsh conditions. In this thesis, we present a way to coalesce these droplets through a chemical reaction at mild temperatures (T ∼ 80◦C). This chemical activation is enabled by inclusion of halide compounds that chemically etch the oxide on dispersed microdroplets of eutectic gallium indium (eGaIn). We investigate the use of a covalent halide compound as an activator and elucidate its activation mecha-nism. Through nuclear magnetic resonance spectroscopy, we discover the ability of eGaIn to catalyze the dehalogenation of our covalent halide activator and con-firm through X-ray photoelectron spectroscopy that chemical oxide etching is occurring. Consequently, we establish the mechanism for self-catalyzing chem-ically coalescing LMEs. We then optimize this emulsion as a functional ink for 3D printing by exploring activator concentrations that maximize post-heat electrical conductivity, compatibility with direct ink writing, and post-activation shape retention. As a result, we select a 3D printable formulation with an elec-trical conductivity of 1.5× 103 S cm−1 for further characterization and 3D print parameter optimization. We also explore LME formulations containing halide salt activators, and find that chemically coalescing LMEs can reach a high elec-trical conductivity (2.4 × 104 S cm−1) close to that of bulk eGaIn, but at the expense of shorter shelf-life and poorer shape retention. Rheology of the selected covalent halide-based emulsion reveals that the LME is shear thinning and shear yielding. Additionally, it exhibits a high plateau modulus (1.0 × 105 Pa) and high yield stress (∼ 2 kPa), thus requiring high pressure and high print velocities, which is desirable for rapid fabrication of GaLMA-based devices. To provide a parameter processing guide for our ink, we construct a print phase diagram describing extrusion pattern types across a normalized print velocity range from 0.45 to 1.35. We also show that our ink can span distances up to 3 mm in length, following a mathematical model for viscoelastic catenaries that predicts an elastic modulus in agreement with experiment. Finally, to demonstrate the utility of our shelf-stable chemically co-alescing LME, we incorporate it as a conductive ink in the hybrid 3D printing of custom-designed battery-integrated light emitting diode arrays, demonstrating simpler fabrication of GaLMA-based applications. This technology pioneers a new class of LMEs, providing the material basis for designing future chemically coalescing LMEs and patterning soft metal catalyzed multifunctional materials.

COMMITTEE: ADVISOR Professor J. William Boley, ME/MSE; CHAIR Professor Uday Pal, ME/MSE; Professor Keith Brown, ME/MSE/Physics; Professor Joerg Werner, ME/MSE; Professor Xi Ling, MSE/Chemistry

Location:
ENG 245, 110 Cummington Mall
Hosting Professor
Boley