The purpose of this feasibility study was to combine aerogel chemistry and microfluidic techniques to produce aerogel spheres for the fabrication of foam shells for use in laser target capsules. We demonstrated a one-step drop-generation method for the fabrication of foams that can be manufactured at lower costs and faster production rates. There have been significant advancements in both aerogel chemistries (e.g., graphene, carbon nanotube, and ring-opening metathesis polymerization) and microfluidic techniques (e.g., multi-emulsion, drop centering, and cross-flow). We combined these mature technologies to achieve unprecedented control of the size, complexity, and uniformity of multilayered materials. The proposed technique could eliminate the need for foam machining, expand the types of materials that can be fielded, and enable faster production times. We leveraged Lawrence Livermore National Laboratory's previous experience with microfluidics by integrating new techniques and knowledge to create low-density foam shells at millimeter size scales. We modified a traditional microfluidics-based device using customized large capillaries (with diameters of 2.4–5.0 mm) for emulsion drop generation. The capillaries were treated with silane to ensure stable emulsion generation. During drop generation, an aerogel precursor solution flowed inside the capillary and was emulsified via an immiscible fluid that was used as both the carrier fluid and the core fluid. The aerogel precursor solutions used in this study were also modified to meet predetermined production requirements. By changing the reactant percentage and gelling condition, the gelling time of the aerogel solutions was shortened to ensure better drop stability and particle sphericity. Foams with densities as low as 40 mg/cc and sphere diameters of 0.59–2.96 mm were produced; as a result, capsule shell thicknesses of 7–254 µm were achieved.
Decades ago, the Laboratory investigated foam shell production using microfluidic-based methods. The results were promising, but the techniques developed were constrained by materials availability and technology limitations at that time. More recent work has focused on aerogel chemistry and microfluidic processing to meet more stringent requirements for the manufacture of materials used for laser targets at the Laboratory's National Ignition Facility (NIF). Aerogel, due to its low density and highly porous microstructure, has generated considerable interest as a potential target material, but machining the material into the required spherical shape is difficult (Pekala et al. 1992, Gash et al. 2001, Worsley et al. 2015). Microfluidic drop generation (see figure) has lately become a powerful tool for producing spherical particles (Utada et al. 2005, Ye et al. 2010).
We proposed to integrate these existing capabilities and materials requirements to produce aerogel foam shells with lower costs and faster production times. Promising preliminary results have already been achieved.
Our initial objectives were to produce low-density (<100 mg/cc) aerogels with shell diameters in the range of 0.5–3.0 mm and foam-layer thicknesses in the range of 10–300 µm. After discussions with the NIF target fabrication team, the density target was set to less than 50 mg/cc. All these objectives were met at the conclusion of the project. With the improved material formulation, microfluidic device, and handling procedures developed by our team, spherical particles were successfully produced. Densities between 40–60 mg/cc were achieved, which was within the required range. These particles also displayed the same microstructure as their bulk counterparts.
Additional options for future research, such as improving sphericity, bettering sphere-core centering, and producing multilayer capsule shells, were proposed by the NIF team. Although they were not pursued during this study, these options should be included in future projects.
Our research supports the NNSA goal of advancing the science, technology, and engineering competencies that are the foundation of the NNSA mission. Our research also addresses the Laboratory's research and development challenge in materials science, as well as its core competencies in advanced materials and manufacturing and high-energy-density science.
During this feasibility study, we focused on demonstrating the suite of available aerogel materials that can be processed using microfluidic techniques. We developed single-component foam microspheres and microcapsules of several relevant compositions. Emphasis was placed on making the different sol–gel chemistries (i.e., a method for producing solid materials from small molecules) and microfluidic processes compatible, selecting the alternative composition and structure options during the project, and getting the technique ready for more complex target fabrication for future research. The traditional aerogel production procedure was modified so that it can be used in microfluidic production, and the results were promising.
The success of this project put us in a good position for future research to combine techniques of drop centering and multilayer capsules to achieve one-step production of foam targets that are currently expensive and time consuming to produce for target needs at NIF and other facilities. The project also increased our understanding of large capsule production and aerogel precursor modification and has attracted new interest for aerogel feedstock production for use in areas such as high-temperature coatings.
We were able to produce aerogel particles using a microfluidic technique, and their densities were as low as 40 mg/cc. Traditional microfluidic-production devices were modified to fit the production requirement of 10–300-µm shell thicknesses. A tailored material-handling protocol was developed to be compatible with the microfluidic production technique. Capsule production using this method has a much shorter processing time than that of conventional techniques due to its fast gelation chemistry.
In the next phase of this research, formulation adjustment (especially improvement of drop stability) should be implemented. More sample characterizations should be done when material is available to produce more samples. A photo-curable polymeric shell for one-step complex structure production should also be tested. Our proposed microfluidics-based technique is a promising method for producing suitable targets for NIF experiments. It would also open doors to other application studies such as the production of feedstock for high-temperature coatings.
Gash, A. E., et al. 2001. "Use of Epoxides in the Sol–Gel Synthesis of Porous Iron(III) Oxide Monoliths from Fe(III) Salts." Chemistry of Materials 13(3): 999–1007. doi: 10.1021/cm0007611.
Pekala, R. W., et al. 1992. "Aerogels Derived from Multifunctional Organic Monomers." Journal of Non-Crystalline Solids 145: 90–98. doi: 10.1016/S0022-3093(05)80436-3.
Utada, A. S., et al. 2005. "Monodisperse Double Emulsions Generated from a Microcapillary Device." Science 308(5721): 537–41. doi: 10.1126/science.1109164.
Worsley, M. A., et al. 2015. "Ultralow Density, Monolithic WS2, MoS2, and MoS2/Graphene Aerogels." ACS Nano 9(5): 4698–705. doi: 10.1021/acsnano.5b00087.
Ye, C., et al. 2010. "Ceramic Microparticles and Capsules via Microfluidic Processing of a Preceramic Polymer." Journal of the Royal Society Interface 7(Supplement 4): S461–73. doi: 10.1098/rsif.2010.0133.focus.
Ye, C. 2018. "Microencapsulation of Chemicals Through Microfluidic-Based Design and Fabrication." Presentation at Special Seminar Series at University of California Davis, Davis, CA, September 7, 2018. LLNL-PRES-757598.
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