Joseph H. Satcher

Nanostructured energetic materials using sol-gel methodologies

Abstract We have utilized a sol–gel synthetic approach in preparing nano-sized transition metal oxide components for new energetic nanocomposites. Nanocomposites of Fe 2 O 3 /Al(s), are readily produced from a solution of Fe(III) salt by adding an organic epoxide and a powder of the fuel metal. These materials can be processed to aerogel or xerogel monolithic composite solids. High resolution transmission electron microscopy (HRTEM) of the dried energetic nanocomposites reveal that the metal oxide component consists of small (3–10 nm) clusters of Fe 2 O 3 that are in intimate contact with ultra fine grain (UFG) ∼25 nm diameter Al metal particles. HRTEM results also indicate that the Al particles have an oxide coating ∼5 nm thick. This value agrees well with analysis of pristine UFG Al powder and indicates that the sol–gel synthetic method and processing does not significantly perturb the fuel metal. Both qualitative and quantitative characterization has shown that these materials are indeed energetic. The materials described here are relatively insensitive to standard impact, spark, and friction tests, results of which will be presented. Qualitatively, it does appear that these energetic nanocomposites burn faster and are more sensitive to thermal ignition than their conventional counterparts and that aerogel materials are more sensitive to ignition than xerogels. We believe that the sol–gel method will at the very least provide processing advantages over conventional methods in the areas of cost, purity, homogeneity, and safety and potentially yield energetic materials with interesting and special properties.

Solvent removal from water with hydrophobic aerogels

Hydrophobic aerogels are shown to be efficient absorbers of solvents from water. Solvents miscible with water are separated from it because the solvents are more volatile than water and they enter the porous aerogel as a vapor across the liquid water/solid interface. Solvents that are immiscible with water are separated from it by selectively wetting the aerogel. Adsorption isotherms are presented for hydrophobic silica aerogels, for several solvents (e.g., toluene, ethyl alcohol, trichloroethylene, chlorobenzene) in water mixtures. The adsorption capacities are compared with the standard activated carbon. Our measurements show that the adsorption capacity of the hydrophobic silica aerogels exceed the capacity of comparable granular activated carbon (GAC), on a gram-per-gram basis, for all of the solvents tested. The improved performance of adsorption capacity by the aerogel over the GAC ranged from factors of ∼30 times for low molecular weight, highly soluble solvents, to factors of 130 times for immiscible solvents.

Synthesis of high porosity, monolithic alumina aerogels

Abstract Many non-silica aerogels are notably weak and fragile in monolithic form. In particular, few monolithic aerogels with densities less than 50 kg/m 3 have any significant strength. It is especially difficult to prepare uncracked monoliths of pure alumina aerogels that are robust and moisture stable. In this paper, we discuss the synthesis of unusually strong, stable, monolithic, high porosity (>98% porous) alumina aerogels, using a two-step sol–gel process. The alumina aerogels have a polycrystalline morphology that results in enhanced physical properties. For an alumina aerogel with a density of 37 kg/m 3 , the specific surface area is 376 m2/g, the elastic modulus is 550 kPa, and the thermal conductivities at 30°C, 400°C and 800°C, are 29, 98 and 298 mW/mK, respectively. All of the measured physical properties of the alumina aerogels except the specific surface area are superior to those for silica aerogels for equivalent densities.

Super-Compressibility of Ultralow-Density Nanoporous Silica

Porosity generally embrittles ceramics, and low-density nanoporous oxides typically exhibit very brittle behavior. In contrast to such expectations, we find that an effective fracture strain of nanoporous silica increases with increasing porosity. At ultralow relative densities of <0.5%,[1] nanoporous monoliths start exhibiting super-compressible deformation with large effective fracture strains of >50%. We attribute such a super-compressible behavior to consequences of an increase in the average aspect ratio of ligaments with decreasing monolith density. These results have important implications for designing novel supercompressive materials and for understanding observations of super-compressibility for other low-density nanoporous systems such as carbon-nanotube-based nanofoams. Understanding effects of porosity on mechanical properties of solids has been a subject of numerous previous investigations, driven by their important technological implications. Indeed, most brittle structural materials, such as masonry materials, ceramics, and bones, are to some extent porous, with the size of pores and/or ligaments often being at the nanoscale. Porosity of different materials covers a very wide range, from zero (i.e., full density solids) to >99% for aerogels (AGs). The AGs are representative materials for the limiting case of low-density/high-porosity systems with submicron uniformity. They are sol-gel-derived solids made from nanoscale ligaments randomly interconnected into a macroscopic three-dimensional structure with open-cell porosity tunable up to ∼99.95%.[2] Numerous previous studies[2] have focused on conventional silica AGs with densities above ∼50 mg cm−3, first made by Kistler a number of decades ago.[3] Ligaments in these AGs are made of amorphous SiO2 with variable surface hydroxylation. Successful synthesis of ultralow-density[4] silica AGs has also been reported.[5–7] Ultralow-density nanofoams are currently of interest for thermonuclear fusion energy applications as scaffolds for condensed hydrogen fuel layers in fusion targets.[8] They are also attractive materials for solid-state targets for ultrabright x-ray lasers,[9] energy absorbing structures,[10] compliant electrical contacts,[11] and electromechanical devices.[12] Poor mechanical properties of nanofoams limit their use in these applications.

Comparison and analysis of zinc and cobalt-based systems as catalytic entities for the hydration of carbon dioxide.

In nature, the zinc metalloenzyme carbonic anhydrase II (CAII) efficiently catalyzes the conversion of carbon dioxide (CO2) to bicarbonate under physiological conditions. Many research efforts have been directed towards the development of small molecule mimetics that can facilitate this process and thus have a beneficial environmental impact, but these efforts have met very limited success. Herein, we undertook quantum mechanical calculations of four mimetics, 1,5,9-triazacyclododedacane, 1,4,7,10-tetraazacyclododedacane, tris(4,5-dimethyl-2-imidazolyl)phosphine, and tris(2-benzimidazolylmethyl)amine, in their complexed form either with the Zn2+ or the Co2+ ion and studied their reaction coordinate for CO2 hydration. These calculations demonstrated that the ability of the complex to maintain a tetrahedral geometry and bind bicarbonate in a unidentate manner were vital for the hydration reaction to proceed favorably. Furthermore, these calculations show that the catalytic activity of the examined zinc complexes was insensitive to coordination states for zinc, while coordination states above four were found to have an unfavorable effect on product release for the cobalt counterparts.

Final report SI 08-SI-004: Fusion application targets

Complex target structures are necessary to take full advantage of the unique laboratory environment created by inertial confinement fusion experiments. For example, uses-of-ignition targets that contain a thin layer of a low density nanoporous material inside a spherical ablator shell allow placing dopants in direct contact with the DT fuel. The ideal foam for this application is a low-density hydrocarbon foam that is strong enough to survive wetting with cryogenic hydrogen, and low enough in density (density less than {approx}30 mg/cc) to not reduce the yield of the target. Here, we discuss the fabrication foam-lined uses-of-ignition targets, and the development of low-density foams that can be used for this application. Much effort has been directed over the last 20 years toward the development of spherical foam targets for direct-drive and fast-ignition experiments. In these targets, the spherical foam shell is used to define the shape of the cryogenic DT fuel layer, or acts as a surrogate to simulate the cryogenic fuel layer. These targets are fabricated from relatively high-density aerogels (>100 mg/cc) and coated with a few micron thick permeation barrier. With exception of the above mentioned fast ignition targets, the wall of these targets is typically larger than 100morexa0» microns. In contrast, the fusion application targets for indirect-drive experiments on NIF will require a much thinner foam shell surrounded by a much thicker ablator shell. The design requirements for both types of targets are compared in Table 1. The foam shell targets for direct-drive experiments can be made in large quantities and with reasonably high yields using an encapsulation technique pioneered by Takagi et al. in the early 90s. In this approach, targets are made by first generating unsupported foam shells using a triple-orifice droplet generator, followed by coating the dried foam shells with a thin permeation barrier. However, this approach is difficult, if not impossible, to transfer to the lower density and thinner wall foam shells required for indirect-drive uses-of-ignition targets for NIF that then would have to be coated with an at least hundred-micron-thick ablator film. So far, the thinnest shells that have been fabricated using the triple-orifice-droplet generator technique had a wall thickness of {approx}20 microns, but despite of being made from a higher-density foam formulation, the shells were mechanically very sensitive, difficult to dry, and showed large deviations from roundness. We thus decided to explore a different approach based on using prefabricated thick-walled spherical ablator shells as templates for the thin-walled foam shell. As in the case of the above mentioned encapsulation technique, the foam is made by sol-gel chemistry. However, our approach removes much the requirements on the mechanical stability of the foam shell as the foam shell is never handled in its free-standing form, and promises superior ablator uniformity and surface roughness. As discussed below, the success of this approach depends strongly on the availability of suitable aerogel chemistries (ideally pure hydrocarbon (CH)-based systems) with suitable rheological properties (high viscosity and high modulus near the gel point) that produce low-density and mechanically strong foams.«xa0less