Joshua D. Kuntz

Ultralight, ultrastiff mechanical metamaterials

Microlattices make marvelous materials Framework or lattice structures can be remarkably strong despite their very low density. Using a very precise technique known as projection microstereolithography, Zheng et al. fabricated octet microlattices from polymers, metals, and ceramics. The design of the lattices meant that the individual struts making up the materials did not bend under pressure. The materials were therefore exceptionally stiff, strong, and lightweight. Science, this issue p. 1373 Ultralow-density materials that deform through tension or compression rather than bending show much higher stiffness. The mechanical properties of ordinary materials degrade substantially with reduced density because their structural elements bend under applied load. We report a class of microarchitected materials that maintain a nearly constant stiffness per unit mass density, even at ultralow density. This performance derives from a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. Production of these microlattices, with polymers, metals, or ceramics as constituent materials, is made possible by projection microstereolithography (an additive micromanufacturing technique) combined with nanoscale coating and postprocessing. We found that these materials exhibit ultrastiff properties across more than three orders of magnitude in density, regardless of the constituent material.

Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores

Graphene is an atomically thin, two-dimensional (2D) carbon material that offers a unique combination of low density, exceptional mechanical properties, thermal stability, large surface area, and excellent electrical conductivity. Recent progress has resulted in macro-assemblies of graphene, such as bulk graphene aerogels for a variety of applications. However, these three-dimensional (3D) graphenes exhibit physicochemical property attenuation compared to their 2D building blocks because of one-fold composition and tortuous, stochastic porous networks. These limitations can be offset by developing a graphene composite material with an engineered porous architecture. Here, we report the fabrication of 3D periodic graphene composite aerogel microlattices for supercapacitor applications, via a 3D printing technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g(-1)) and power densities (>4 kW·kg(-1)) that equal or exceed those of reported devices made with electrodes 10-100 times thinner. This work provides an example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrable energy storage devices optimized for a broad range of applications.

Transparent ceramic scintillators for gamma spectroscopy and radiography

Transparent ceramics combine the scintillation performance of single crystals with the ruggedness and processability of glass. We have developed a versatile, scaleable fabrication method, wherein nanoparticle feedstock is consolidated at temperatures well below melting to form inch-scale phase-pure transparent ceramics with optical scatter of α <0.1 cm-1. We have fabricated Cerium-doped Gadolinium Garnets with light yields of ~50,000 Ph/MeV and energy resolution of <5% at 662 keV. We have also developed methods to form sheets of the high-Z ceramic scintillator, Europium-doped Lutetium Oxide Bixbyite, producing ~75,000 Ph/MeV for radiographic imaging applications.

Electrophoretic deposition and mechanistic studies of nano-Al/CuO thermites

Electrophoretic deposition was used to deposit thin films (∼10–200 μm) of nano-aluminum/copper oxide thermites, with a density of 29% the theoretical maximum. The reaction propagation velocity was examined using fine-patterned electrodes (0.25 × 20 mm), and the optimum velocity was found to correspond to a fuel-rich equivalence ratio of 1.7. This value did not correlate with the calculated maximum in gas production or temperature, and it is suggested that it is a result of enhanced condensed-phase transport, which is speculated to increase for fuel-rich conditions. A ∼25% drop in propagation velocity occurred above an equivalence ratio of 2.0, where Al2O3 is predicted to undergo a phase change from liquid to solid. This is expected to hinder the kinetics by decreasing the mobility of condensed-phase reacting species. The effect of film thickness on propagation velocity was investigated, using the optimum equivalence ratio. The velocity was seen to exhibit a two-plateau behavior, with one plateau between 1...

Stiff and electrically conductive composites of carbon nanotube aerogels and polymers

Many challenges remain in the effort to realize the exceptional mechanical and electrical properties of carbon nanotubes in composite materials. Here, we report on highly electrically conductive and mechanically stiff composites of polymers and single-walled carbon nanotubes (SWNT). Conductive SWNT-based nanofoams (aerogels) are used as scaffolds to create polymer [poly(dimethylsiloxane)] composites. The resulting composites possess electrical conductivities over 1 S cm−1 and exhibit an ∼300% increase in the elastic modulus with as little as 1 vol% nanotube content.

Controlling Material Reactivity Using Architecture.

3D-printing methods are used to generate reactive material architectures. Several geometric parameters are observed to influence the resultant flame propagation velocity, indicating that the architecture can be utilized to control reactivity. Two different architectures, channels and hurdles, are generated, and thin films of thermite are deposited onto the surface. The architecture offers an additional route to control, at will, the energy release rate in reactive composite materials.

Light‐Directed Electrophoretic Deposition: A New Additive Manufacturing Technique for Arbitrarily Patterned 3D Composites

Dr. A. J. Pascall, Dr. F. Qian, Dr. M. A. Worsley, Dr. J. D. Kuntz Lawrence Livermore National Laboratory Livermore , CA 94550 , USA E-mail: pascall1@llnl.gov G. Wang, Prof. Y. Li Department of Chemistry and Biochemistry University of California Santa Cruz , CA 95064 , USA

Synthesis of bi-modal nanoporous Cu, CuO and Cu2O monoliths with tailored porosity

Nanoporous structures are exceptionally useful in catalytic, sensing and mechanical applications. However, precise control over the structure and composition of the nanoporous material is critical for the material to behave as desired. We report here a new bottom-up synthesis technique termed filter-casting for the creation of large scale (>1 cm) nanoporous structures which provide this precise control. Cu, CuO and Cu2O and bi-modal macro/nanoporous Cu structures were created with this technique to demonstrate the range of materials and structures which can be formed into nanoporous monoliths. Homogeneous nanoporous monoliths are synthesized using nanoparticles, and bi-modal or higher-order porosities are achieved using a sacrificial polystyrene template. The higher-order pore size is determined by the polystyrene particle diameter, and the nanopore size is set by the diameter of the nanoparticles. Surface areas as high as 34 m2 g−1, and relative densities between 12 and 58%, have been achieved. Filter-casting is a powerful new method for directly synthesizing large nanoporous monoliths with predetermined composition, pore size and pore structure.

Comparative gamma spectroscopy with SrI 2 (Eu), GYGAG(Ce) and Bi-loaded plastic scintillators

We are developing new scintillator materials that offer potential for high resolution gamma ray spectroscopy at low cost. Single crystal SrI<inf>2</inf>(Eu) offers ∼3% resolution at 662 keV, in sizes of ∼1 in³. We have developed ceramics processing technology allowing us to achieve cubic inch scale transparent ceramic scintillators offering gamma spectroscopy performance superior to NaI(Tl). Our bismuth-loaded plastic scintillator demonstrates energy resolution of ∼8% at 662 keV, for samples of ∼0.5 cm³.

Synthesis and characterization of monolithic, high surface area SiO2/C and SiC/C composites

In this study, the synthesis and characterization of high surface area carbon-supported silica and silicon carbide aerogels are presented. An activated carbon aerogel with surface area greater than 3000 m2 g−1 was used as a support for the sol–gel deposition of silica. The resulting silica-coated carbon aerogel retained a surface area greater than 2000 m2 g−1 and showed improved thermal stability in air. The carbon-supported silicon carbide aerogel was made by the carbothermal reduction of the silica-coated carbon aerogel under flowing Ar at 1500 °C. The resulting monolith maintained a surface area greater than 2000 m2 g−1 and was stable to temperatures approaching 600 °C, 100 °C higher than that of the pristine carbon aerogel.