Chad A. Stoltz

Electrochemical Oxidation of H2, CO, and CO ∕ H2 Mixtures on Patterned Ni Anodes on YSZ Electrolytes

Single-cell solid oxide fuel cell experiments using thin-film, sputter-deposited Ni pattern anodes microfabricated on single-crystal yttria-stabilized zirconia (YSZ) electrolyte disks have been performed to examine the electrochemical oxidation of H 2 , CO, and CO/H 2 mixtures. Porous lanthanum strontium manganate (LSM)/YSZ cathodes have been used and characterized in separate symmetric cell experiments such that Ni anode overpotentials and impedances can be isolated. Post-test scanning electron microscopy imaging revealed that at the high temperatures (735-850°C), the 100-nm thin Ni patterns broke up into interconnected regions resulting in three-phase boundary lengths that roughly correlated with the original coverage area of the pretested dense Ni patterns. Electrochemical characterization for H 2 , CO, and CO/H 2 oxidation under dry and wet (∼4% H 2 O) feeds showed that the interconnected anode overpotentials and polarization resistances correlated with the original Ni pattern area for the various pattern geometries. Higher activation overpotentials and polarization resistances observed for CO in comparison to H 2 were not observed for CO/H 2 mixtures down to 25% H 2 . Results indicated detrimental effects of H 2 O on CO oxidation power densities due to drops in open-circuit voltages without reduction in polarization resistance, and enhancement due to water-gas shift reactions was not seen. Our results provide the basis for insights into H 2 and CO electro-oxidation on Ni/YSZ anodes.

Combustion Behavior of Solid Fuels Based on PTFE/Boron Mixtures

An experimental study was conducted to understand the combustion behavior of polytetrafluoroethylene (PTFE)/boron–based solid fuels for future hybrid rocket motor applications. Fuels were loaded with 10–40% boron powder (w/w). Two different types of PTFE were examined in this study, while a single type of boron powder was considered. No significant differences in the decomposition mechanisms for PTFE and a candidate solid fuel mixture were observed by differential scanning calorimetry (DSC) and temperature-jump (T-jump)/Fourier transform infrared (FTIR) experiments. Diffusion flame studies between solid fuels and gaseous oxygen were carried out to measure regression rates and to develop a fundamental understanding of the combustion behavior. The fuels with the lowest boron content readily extinguished upon removal of the supplemental oxygen flow. The fuels with the highest loadings of boron self-propagated after ignition. X-ray diffraction on postcombustion residue of the self-propagating material revealed graphite and boron carbide as the remaining products, while particles captured leaving the surface of the fuel under normal burning conditions were found to be mostly boric acid. Boron oxidation and magnesium fluorination were observed in the flame zone of the diffusion flame by UV-Vis emission spectroscopy (magnesium is the major impurity in the elemental boron powder used). The results of this study suggest that solid fuels comprising PTFE and boron show promise for improving the energy density of hybrid rockets.

Neutron scattering study of internal void structure in RDX

We present the first small and ultrasmall angle neutron scattering (SANS/USANS) measurements of the internal void morphology of the high explosive RDX on length scales from 10 A to 20 μm. Measurements were taken on a range of RDX samples with similar densities and particle size distributions but which have significantly different sensitivities to shock initiation as measured by large-scale gap tests of the samples when formulated in standard polymer blends. Scattering measurements were performed using a contrast match technique to eliminate all features apart from internal void structures. The dominant feature in all samples is a surface fractal scattering that extends from ∼50 nm to above 20 μm, with no observable upper bound for the fractal correlation length. These features are interpreted in terms of scattering from rough surfaces of interior air-filled voids with fractal dimensionality between 2.4 and 2.9. The fractal pattern is proposed to arise from complex growth patterns on void surfaces as inter...

Equations of state of 2,6-diamino-3,5-dinitropyrazine-1-oxide

2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is an energetic ingredient that has an impact sensitivity close to that of TATB, yet a calculated energy content close to HMX. Reported tests of formulated LLM-105 reveal that it is a good candidate for a new insensitive high-performance explosive. As use of LLM-105 increases, thermodynamic parameters and phase stability will need to be determined for accurate modeling. In order to accomplish this goal, isothermal equations of state of LLM-105 at static high-pressure and temperature were investigated using synchrotron angle-dispersive x-ray diffraction and diamond anvil cells. Data at ambient temperature, 100 °C (373 K), and 180 °C (453 K) were used to obtain isothermal equations of state, and data at ambient pressure were used to obtain the volume thermal expansion coefficient. At ambient temperature, 100 °C (373 K), and 180 °C (453 K) no phase change was evident up to the highest measured pressure; and at ambient pressure, LLM-105 was stable up to 240 °C (513 K) and thermally decomposed by 260 °C (533 K).

Controlling RDX explosive crystallite morphology and inclusion content via simple ultrasonic agitation and solvent evaporation

Uniform crystallite morphology, narrow particle size distribution, and tailored inclusion content have been achieved for cyclotrimethylene trinitramine (RDX) explosive recrystallization by a combination of simple ultrasonic agitation and solvent evaporation, as characterized by optical imaging and confocal microscopy.

Phase stability of {epsilon} and {gamma} HNIW (CL-20) at high-pressure and temperature

Hexanitrohexaazaisowurtzitane (CL‐20) is one of the few ingredients developed since World War II to be considered for transition to military use. Five polymorphs have been identified for CL‐20 by FTIR measurements (α, β, γ, e, ζ). As CL‐20 is transitioned into munitions it will become necessary to predict its response under conditions of detonation, for performance evaluation. Such predictive modeling requires a phase diagram and basic thermodynamic properties of the various phases at high pressure and temperature. Therefore, the epsilon and gamma phases of CL‐20 at static high‐pressure and temperature were investigated using synchrotron angle‐dispersive x‐ray diffraction experiments. The samples were compressed and heated using diamond anvil cells (DAC). Pressures and temperatures achieved were around 5 GPa and 240 °C, respectively. The epsilon phase was stable to 6.3 GPa at ambient temperature. When heated at ambient pressure the epsilon phase was sustained to a temperature of 120 °C then underwent a tr...

Equations of state of hexanitrostilbene (HNS)

Hexanitrostilbene (HNS) is an energetic ingredient that is widely used in commercial and military explosives for its thermal stability. However, characterization of its thermodynamic parameters and phase stability is lacking. Crystalline properties, such as bulk modulus and thermal expansion, are necessary to accurately predict the behavior of shocked solids using hydrodynamic codes. In order to obtain these values, equations of state of fine-particle (type IV) HNS were investigated using synchrotron angle-dispersive x-ray diffraction experiments at static high-pressure and temperature. The samples were compressed and heated using diamond anvil cells. Pressure - volume data for HNS at ambient temperature were fit to the Birch-Murnaghan and Vinet formalisms to obtain bulk modulus and its first pressure derivative. Temperature - volume data at ambient pressure were fit to obtain the volume thermal expansion coefficient.

ISOTHERMAL EQUATIONS OF STATE OF LLM‐105

2,6‐diamino‐3,5‐dinitropyrazine‐1‐oxide (LLM‐105) is an energetic ingredient that has an impact sensitivity close to that of TATB, yet a calculated energy content close to HMX. Reported tests of formulated LLM‐105 reveal that it is a good candidate for a new insensitive high‐performance explosive. As use of LLM‐105 increases, thermodynamic parameters and phase stability will need to be determined for accurate modeling. In order to accomplish this goal, isothermal equations of state of LLM‐105 at static high‐pressure and temperature were investigated using synchrotron angle‐dispersive x‐ray diffraction experiments. The samples were compressed and heated using diamond anvil cells. Pressure—volume data for LLM‐105 at ambient temperature and 100° C were fit to the Birch‐Murnaghan formalism to obtain isothermal equations of state. Temperature—volume data at ambient pressure were fit to obtain the volume thermal expansion coefficient.

Sonocrystallization as a tool for controlling crystalline explosivemorphology and inclusion content

Several research groups have reported preparations of the explosive cyclotrimethylene trinitramine (RDX) that, when formulated into plastic-bonded explosives (PBXs), result in reduced shock sensitivities when compared to the standard formulations. We recently showed a correlation between shock sensitivity of formulated RDX and the void contents of the powders using Small Angle Neutron Scattering (SANS). With this correlation in mind, we present a method for generating RDX crystals with controlled particle size and morphology using ultrasonic agitation and slow evaporation rates.

T‐JUMP/FTIR STUDIES OF POLY‐GLYCIDYL NITRATE (PGN) PYROLYSIS

In an effort to understand the effects of hydroxyl end‐modification and isocyanate curing, decomposition of PGN prepolymer has been investigated using T‐Jump/FTIR (Fourier transform infrared) spectroscopy of PGN allowing real‐time analysis of decomposition gas products under simulated deflagration conditions. Our results identify decomposition products including: CH2O, H2O, CO2, CO, N2O, NO, NO2, HCN and HONO. Kinetic rates relative to CO2 formation lead to calculated activation energies of 22 kcal/mol and 18 kcal/mol. Much higher activation energies (32 kcal/mol) were calculated relative to CH2O formation rates, in agreement with DSC data, indicating that CH2O formation is likely an initial decomposition step while CO2 formation is due to side gas phase reactions. Additional FTIR and optical microscopy studies indicate that condensed phase, backbone scission reactions also occur, causing time delays prior to major gas production.