Daniel M. Dabbs

Functionalized graphene sheet colloids for enhanced fuel/propellant combustion.

We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35-14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.

Enhanced Thermal Decomposition of Nitromethane on Functionalized Graphene Sheets: Ab Initio Molecular Dynamics Simulations

The burning rate of the monopropellant nitromethane (NM) has been observed to increase by adding and dispersing small amounts of functionalized graphene sheets (FGSs) in liquid NM. Until now, no plausible mechanisms for FGSs acting as combustion catalysts have been presented. Here, we report ab initio molecular dynamics simulations showing that carbon vacancy defects within the plane of the FGSs, functionalized with oxygen-containing groups, greatly accelerate the thermal decomposition of NM and its derivatives. This occurs through reaction pathways involving the exchange of protons or oxygens between the oxygen-containing functional groups and NM and its derivatives. FGS initiates and promotes the decomposition of the monopropellant and its derivatives, ultimately forming H(2)O, CO(2), and N(2). Concomitantly, oxygen-containing functional groups on the FGSs are consumed and regenerated without significantly changing the FGSs in accordance with experiments indicating that the FGSs are not consumed during combustion.

Sol-gel coated glass cells for spin-exchange polarized 3He

We have developed a high-purity sol-gel coating for the interior surface of glass cells used for polarizing 3He by spin-exchange optical pumping. The coating is designed to minimize spin relaxation due to wall collisions. A longitudinal spin-relaxation time T1 in a sol-gel coated Pyrex cell of 344±8 h was achieved, the longest T1 we have ever recorded for a gaseous sample. Repeated trials indicated that the coating was quite robust. Results using an uncoated Pyrex cell were also quite good, although inferior to the performance of the coated cell.We have developed a high-purity sol-gel coating for the interior surface of glass cells used for polarizing 3He by spin-exchange optical pumping. The coating is designed to minimize spin relaxation due to wall collisions. A longitudinal spin-relaxation time T1 in a sol-gel coated Pyrex cell of 344±8 h was achieved, the longest T1 we have ever recorded for a gaseous sample. Repeated trials indicated that the coating was quite robust. Results using an uncoated Pyrex cell were also quite good, although inferior to the performance of the coated cell.

Nanocomposite Mullite/Mullite Powders by Spray Pyrolysis

A mullite/mullite nanocomposite powder has been synthesized, composed of nanometer-size 3Al2O3·2SiO2 (‘3/2’) mullite precipitates within a matrix of the high alumina 2Al2O3·SiO2 (‘2:1’) mullite. Historically, the transition from the metastable high-alumina phase to the thermodynamically stable ‘3:2’ phase of mullite has been thought to be a continuous process, involving a continuous solid solution between the two forms of mullite. In contradiction to this widely held view, our high resolution transmission electron microscopic characterization confirms that a first order phase transition between two distinct mullites occurs. The high degree of interface coherence between the precipitates and the matrix allows us to speculate that the mechanical properties of the matrix could be enhanced by a process similar to the precipitation hardening of metals.

BIOMIMETIC FABRICATION OF MATERIALS-THE MINIMALIST APPROACH

The interfacial chemistry between inorganic ceramics and defined organic surfaces is the focus of intense investigation. Partially compressed Langmuir-Blodgett monolayers of anionic porphyrins have been used as modified nucleation sites for calcium carbonate. The porphyrin monolayer has an ordered array of carboxylates, and hence the system serves as a minimalist template for the modeling of complex biogenic acidic glycoproteins for biomineralization. The initial results suggest the formation of calcite with morphologically distinct calcitic rhombs with truncated, 3-edged corners and intricately articulated facial cavities. Stearic acid monolayers yield distinctly different calcite crystals, indicative that the geometrically defined carboxylate array is probably important. Phosphatidylcholine vesicles have been used as a tool for the formation of membrane encapsulated iron-oxides. Gramicindin A ion channels have been embedded in vesicles to kinetically alter the formation and growth of iron oxides, starting with intravesicular ferrous chloride. The results indicate that the presence of ion channels lead to the formation of magnetite vis-a-vis maghemite formation in vesicles lacking the ion channels. The use of ion channels has important implications in probable signal transduction processes during biomineralization pathways.

Biomimetic Processing of Ceramic Composites

Abstract : Three task areas composed the research effort: (1) optimized depth of photocuring in a resin system, (2) improved resin/ceramic particle compatibilities consistent for use in stereolithography, and (3) developing the technique of ceramic stereolithography (CSL) for the fabrication of a ceramic matrix composite test structure. The depth of photocuring in a model resin system was investigated as a function of photoinitiator concentration. Polymer solutions were photocured using varying levels of energy and photoinitiator concentration. An optimal photoinitiator concentration maximizing the cure depth was observed. Two regimes were shown to exist in which shrinkage was minimized or maximized. A quantitative model was developed to describe the systems behavior. Good agreement with experiment was obtained and the model predicted both the existence and location of the optimal photoinitiator concentration and corresponding cure depth. After optimizing the resin/particle system, complex shaped structures were fabricated from ceramic powder compacts constructed using CSL. The main processing parameters in CSL such as layer thickness, resolution, hatch spacing, and overcure were found to depend on the light propagation in a concentrated dispersion, and a model was developed and used to optimize the fabrication of the ceramic structures.

Disordered mesoporous silicates formed by templation of a liquid crystal (L3)

For a wide range of technological applications the need for optically transparent, monolithic, mesoporous silicates is readily apparent. Potential areas of utility include filtration, catalysis, and optoelectronics among many others. This laboratory has previously reported on the synthesis of such materials thatare formed through the addition of tetramethoxysilane to a liquid crystal solution of hexanol, cetylpyridinium chloride, and 0.2 M hydrochloric acid, and our investigation into the properties of these materials is a continuing process. We have achieved defect and fracture free material of suitable size (0.5 cm × 3 cm diameter disks) via supercritical drying of the silicate under ethanol or CO 2 . The dried materials are remarkably similar to ordinary glass in strength, texture, and clarity. They possess pore volumes of ca . 1.0 cm 3 /g, with BET surface areas >1000 m 2 /g. We can re-infiltrate thedried monolith with hydroxyethylacrylate, a photo-polymerizable monomer, to create an inorganic/organic nanocomposite. There is fracturing upon re-infiltration, but preliminary tests show that the polymerization proceeds despite the mechanical failure. These findings suggest many possible applications for these unique nanocomposites.