Bryce C. Tappan

Nanoporous Metal Foams

Nanoporous metal foams (NMFs) have been a long sought-after class of materials in the quest for high-surface-area conductive and catalytic materials. Herein we present an overview of newly developed synthetic strategies for producing NMFs along with an in-depth discussion of combustion synthesis as a versatile and scalable approach for the preparation of nanoporous, nanostructured metal foams. Current applications of NMFs prepared using combustion synthesis are also presented including hydrogen storage and catalysis.

Reaction Propagation of Four Nanoscale Energetic Composites (Al/MoO3, Al/WO3, Al/CuO, and B12O3)

Nanoscale composite energetics (also known as metastable intermolecular composites) represent an exciting new class of energetic materials. Nanoscale thermites are examples of these materials. The nanoscale thermites studied consist of a metal and metal oxide with particle sizes in the 30-200 nm range. They have potential for use in a wide range of applications. The modes of combustion and reaction behavior of these materials are not yet well understood. This investigation considers four different nanoaluminum/metal-oxide composites. The same nanoscale aluminum was used for each composite. The metal oxides used were molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ), copper oxide (CuO), and bismuth oxide (Bi 2 O 3 ). The reaction performance was quantified by the pressure output and propagation velocity using unconfined (or open burn) and confined (burn tube) experiments. We examine the optimization of each composite in terms of pressure output and propagaton speed (or burn rate) for the open burn experiment. We find that there is a correlation between the maximum pressure output and optimum propagation speed (or burn rate). Equilibrium calculations are used to interpret these results. We find that the propagation speed depends on the gas production and also on the thermodynamic state of the products. This suggests that condensing gases or solidifying liquids could greatly enhance heat transfer. We also vary the density of these composites and examine the change in performance. Although the propagation wave is likely supersonic with respect to the mixture sound speed, the propagation speed decreases with density. This behavior is opposite of classical detonation in which propagation (detonation) speed increases with density. This result indicates that the propagation mechanism may differ fundamentally from classical detonations.

Combustion and Conversion Efficiency of Nanoaluminum-Water Mixtures

An experimental investigation on the combustion behavior and conversion efficiency of nanoaluminum and liquid water mixtures was conducted. Burning rates and chemical efficiency of aluminum-water and aluminum-water-poly(acrylamide-co-acrylic acid) mixtures were quantified as a function of pressure (from 0.12 to 15 MPa), nominal aluminum particle size (for diameters of 38, 50, 80, and 130 nm), and overall equivalence ratios (0.67 < φ < 1.0) under well-controlled conditions. Chemical efficiencies were found to range from 27 to 99% depending upon particle size and sample preparation. Burning rates increased significantly with decreased particle size attaining rates as high as 8 cm/s for the 38 nm diameter particles above approximately 4 MPa. Burning rate pressure exponents of 0.47, 0.27, and 0.31 were determined for the 38, 80, and 130 nm diameter particle mixtures, respectively. Also, mixture packing density varied with particle size due to interstitial spacing, and was determined to affect the burning rates at high pressure due to inert gas dilution. The presence of approximately 3% (by mass) poly(acrylamide-co-acrylic acid) gelling agent to the nAl/H2O mixtures had a small, and for many conditions, negligible effect on the combustion behavior.

Synthesis and Characterization of Furazan Energetics ADAAF and DOATF

The synthesis and structural characterization of bis[4-aminofurazanyl-3-azoxy]azofurazan (ADAAF) and 3,4:7,8:11,12:15,16-tetrafurazano-1,2,5,6,9,10,13,14-octaazacyclohexadeca-1,3,5,7,9,11,13,15-octaene-1,10-dioxide (DOATF) are described. Explosive sensitivity properties of both materials were determined. The heat of formation of ADAAF was measured to be 300 kcal/mol and the detonation velocity and pressure of ADAAF were measured to be 7.88 km/s and 299 kbar, respectively, at 94% theoretical maximum density. We also investigated the burning rate characteristics of ADAAF.

Combustion of Aluminum Particles with Steam and Liquid Water

Experiments were conducted to study the effect of liquid and gas phases of water as the oxidizer combusting with both micron- and nano-sized aluminum particles. Combustion of aluminum with steam was achieved using a Bunsen-type dust cloud apparatus. During the experiment, the aluminum particles, ~5-8 microns in diameter, were entrained in a high- velocity steam flow while the aerosol velocity is regulated by an ejector system to maintain a stable flame at the end of the contoured nozzle. Al/steam/N2 flames were obtained for various equivalence ratios at atmospheric pressure. The flame luminosity was less than that of aluminum/air mixtures due to the reduction in flame temperature. Linear and mass- burning rates of mixtures of nanoaluminum (38 nm) and liquid water as a function of pressure and mixture composition at room temperature were measured using a constant volume optical pressure vessel. At the highest pressure studied (4.3 MPa), the linear burning rate was found to be 8.6±0.4 cm/s corresponding to a mass-burning rate of 6.1 g/cm 2 -s. The pressure exponent at room temperature was 0.47, which was independent of the overall mixture equivalence ratio for the cases considered.

Energy release characteristics of the nanoscale aluminum-tungsten oxide hydrate metastable intermolecular composite

Tungsten oxides are of interest as an oxidant for metals in metastable intermolecular composites (MICs), a reactive nanoscale powder useful for such applications as electric matches and gun primers. Smaller particles typically lead to fast reaction rates in this class of energetic material, and we have synthesized nanoscale WO3∙H2O using wet chemistry. Analysis by electron microscopy and small angle x-ray scattering revealed an approximately 100-nm-wide by7-nm-thick platelet morphology. X-ray diffraction verified the orthorhombic structure and composition of the hydrate. A MIC material was formulated using 44nm Al as the fuel. Performance was measured using a pressure cell where total enthalpy change and energy release rate was measured. This report includes the thermodynamic analysis of the pressure cell (calorimetry) that allows the determination of these metrics. Accuracy of the technique is discussed. Performance of the hydrate was found to significantly exceed that of MIC formulated with dehydrated t...

Evidence of a Kinetic Isotope Effect in Nanoaluminum and Water Combustion

The normally innocuous combination of aluminum and water becomes violently reactive on the nanoscale. Research in the field of the combustion of nanoparticulate aluminum has important implications in the design of molecular aluminum clusters, hydrogen storage systems, as well as energetic formulations which could use extraterrestrial water for space propulsion. However, the mechanism that controls the reaction speed is poorly understood. While current models for micron-sized aluminum water combustion reactions place heavy emphasis on diffusional limitations, as reaction scales become commensurate with diffusion lengths (approaching the nanoscale) reaction rates have long been suspected to depend on chemical kinetics, but have never been definitely measured. The combustion analysis of nanoparticulate aluminum with H2O or D2O is presented. Different reaction rates resulting from the kinetic isotope effect are observed. The current study presents the first-ever observed kinetic isotope effect in a metal combustion reaction and verifies that chemical reaction kinetics play a major role in determining the global burning rate.

Regression Rates of Solid Fuels Containing High Nitrogen (HiN) Materials

The combustion behavior of novel energetic formulations in solid fuels to replace conventional, low-energy, binders such as hydroxyl-terminated polybutadiene (HTPB) were examined. The proposed solid fuels will provide higher enthalpy, higher density, and improved rocket engine performance. The regression rates of these High Nitrogen (HiN) ingredients indicate significant improvement compared to the classical HTPB solid fuels. Upon decomposition, the formation of N2 yields a high heat of formation, which in turn suggests higher combustion enthalpies and improved specific impulse. Friction sensitivity, and opposed-flow burning rate experiments of candidate materials were performed. TAGzT formulations show great promise with a 25% increase in regression rate for a 25 wt% addition of Triaminoguanidium azotetrazolate (TAGzT). At the same time, although TAGzT is classified as a 1.1 high explosive, formulations containing 50% TAGzT by mass passed classical DoD / DoE safety tests including friction, impact, and static discharge test.

Steady-state shock-driven reactions in mixtures of nano-sized aluminum and dilute hydrogen peroxide

ABSTRACT Mixtures of nanoaluminum (nAl) and dilute hydrogen peroxide (HP) were studied to determine their potential to detonate when subjected to explosive shock. Results of explosively driven rate stick experiments revealed steady shock propagation for stoichiometric mixtures of nAl and 10 wt% HP. The critical diameter of this composition is estimated to be between 27.7 and 34.5 mm. Detonation velocities between 3.034 and 3.187 mm/μs were obtained, varying with charge diameter and density. This represents the first measured shock-driven, self-sustained reaction in nAl and dilute HP mixtures.

Analysis of potential hazards associated with 241Am loaded resins from nitrate media

LANL has been contacted to provide possible assistance in safe disposition of a number of 241Am-bearing materials associated with local industrial operations. Among the materials are ion exchange resins which have been in contact with 241Am and nitric acid, and which might have potential for exothermic reaction. The purpose of this paper is to analyze and define the resin forms and quantities to the extent possible from available data to allow better bounding of the potential reactivity hazard of the resin materials. An additional purpose is to recommend handling procedures to minimize the probability of an uncontrolled exothermic reaction.