Freeze-Drying Aerosols: A Facile Route to Metal Particles with Nanometer-Scale Pores

Michael Bagge-Hansen (15-LW-074)

Project Description

We plan to develop a new approach to producing metal foams with nanometer-scale pores by freeze-drying aerosols. Nanoporous metals are extremely compelling materials because they maintain good electrical and thermal conductivity while offering enhanced activity, tunable density, and novel electronic and mechanical behavior. Nanoporous metal particles are important to applications ranging from basic energy science to advanced manufacturing relevant to catalysis, battery and capacitor electrodes, heat sinks, hydrogen storage, filtration, and antimicrobial scaffolds. At LLNL, nanoporous metals are needed for independent control of material and density in high-energy physics experiments, as an essential ingredient for ink used in additive manufacturing of printed batteries and sensors, and for basic science experiments investigating the fundamental mechanical and chemical properties of these materials. However, they are currently very difficult to produce. We seek to understand and exploit the fundamental material science that governs the formation of nanoporous metal particles via freeze-drying through advanced characterization and process diagnostics (see figure) to produce uniform nanoporous copper and silver microscopic particles with extremely high yield and purity.

We expect to establish a very efficient method to produce uniform copper and silver nanoporous microscopic particles with very high yield and high purity. We will embed in this process a number of sophisticated diagnostics as well as material characterization to reveal the fundamental physics of assembly and facilitate rapid optimization. We will establish protocols to develop optimum starting solute concentration, generation and freezing of an aerosol, vacuum desiccation, and post-processing to achieve a desirable stability, density, and porosity of a nanoporous metal foam. We expect these data to establish a knowledge base sufficient to enable predictive capabilities, with applications in catalysis and advanced manufacturing. We further believe that this research may show that the process is a much more general route to nanoporous materials than existing protocols, providing a paradigm for the production of new porous materials.

Mission Relevance

Our proposed work is positioned to expand LLNL's capability to synthesize nanoporous materials. In particular, we have focused the scope of our proposal on copper and silver nanoporous foams because of specific demands for these materials within the Laboratory. As such, this basic research project is well-aligned with LLNL's core competency in advanced materials and manufacturing.

FY16 Accomplishments and Results

In FY16 we (1) succeeded in producing both nanoporous copper and silver foams; (2) further deployed in situ diagnostics during freezing, including temperature and pressure control; (3) conducted advanced characterization of these materials using ultra-small-angle x-ray scattering at the Advanced Photon Source at Argonne National Laboratory, which was further enhanced by laboratory-based in situ x-ray diffraction heating studies; (4) tracked, using these techniques, structural and morphological changes; and (5) explored applications in catalysis and strategies for assembling particles into complex geometries.

We are developing nanometer-scale porous metals derived from low-density salts, which requires chemical and thermal treatment while retaining the desirable low-density structure. here, thermal conversion of silver acetate (agoac) to silver metal is studied and optimized using scanning electron microscopy, which provides an overview of the dramatic changes (a). a heated sample stage was employed (b) to obtain in situ synchrotron-based ultra-small angle x-ray scattering (c), where the evolution of nanometer s — We are developing nanometer-scale porous metals derived from low-density salts, which requires chemical and thermal treatment while retaining the desirable low-density structure. Here, thermal conversion of silver acetate (AgOAc) to silver metal is studied and optimized using scanning electron microscopy, which provides an overview of the dramatic changes (A). A heated sample stage was employed (B) to obtain in situ synchrotron-based ultra-small angle x-ray scattering (C), where the evolution of nanometer silver crystallites was observed. Transmission electron microscopy was used to confirm these findings (C, inset). These studies provided new understanding of nanoporous metal foams and offer opportunities to optimize their production via spray freeze-drying.