In the study of materials at extreme pressures and temperatures, there is an enduring need to extend the range of experiments to previously inaccessible regimes. To accomplish this, improvements in diagnostics for in situ material characterization at extremes must proceed in parallel with techniques used to generate extreme states. Simultaneously, there is a need to study material phenomena – e.g. phase transformations and chemical reactions triggered by the application of extreme conditions – on their natural timescales. Here we report on recent developments in the application of ultrafast laser spectroscopic techniques to high-pressure hightemperature experiments on materials confined in a diamond-anvil cell. Using a bright broadband source coupled to ultrafast detection to discriminate signal from high thermal and fluorescent backgrounds, we conducted broadband optical spectroscopy up to 60 GPa and 1560 K. By coupling the broadband source to a monochromatic pulse, nonlinear Coherent Anti- Stokes Raman Spectroscopy (CARS) with high signal brightness was achieved. Optical absorption data in hot compressed O2 and CARS data in N2 at extreme pressures are reported.

Following the generic strategy of creating core-shell structured nanoparticles reported by our group previously [1] and exploring its applications, an aerosol route combined with iron carbonyl decomposition was developed to encapsulate strong oxidizer within mild oxidizer particles. This modified method enables the application of hygroscopic nano-energetic materials by stabilizing them within a water-insoluble shell. Fe2O3/I2O5 composite oxidizers have been created. Some of the results obtained from combustion tests show that the composite system significantly outperforms the single metal oxide (Fe2O3) system in both pressurization rate and peak pressure. The time-resolved mass spectrometry shows that a significant amount of O2 and I2 are released from the composite oxidizers. These preliminary results suggest a supplement to the previous strategy of obtaining the core-shell structured composite oxidizers and the method still needs to be further optimized.

We use microwave (MW) absorption to induce rapid, uniform heating of the secondary high explosive octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) for decomposition analysis in a fast kinetic regime (10-2 s). Because HMX alone is MW inactive, composites comprised of an HMX-polymeric binder system doped with a small mass fraction (<1 wt%) multiwalled carbon nanotubes or graphite, were fabricated. The dielectric properties were measured (real and imaginary permittivity between 1 – 18 GHz), which is necessary for describing the expected MW-materials interaction and predicted heating response. The real component of the relative dielectric permittivity ranged from 3 – 4 while the imaginary component was 0.01 – 0.07, depending on the amount of carbon dopant added. Dynamic temperature measurements of a similar explosive composite under MW exposure were accomplished using fiber-optic temperature measurement techniques, and can be reproducibly correlated to the amount of MW energy absorbed in the sample.

Multimillion-atom reactive molecular dynamics simulations were used to investigate the mechanisms which control heat-initiated oxidation in aluminum nanoparticles. The simulation results reveal three stages: (1) confined burning, (2) onset of deformation, and (3) onset of small cluster ejections. The first stage of the reaction is localized primarily at the core-shell boundary, where oxidation reactions result in strong local heating and the increased migration of oxygen from the shell into the core. When the local temperature rises above the melting point of alumina (T=2330K), the melting of the shell allows deformation of the overall particle and an increase in heat production. Finally, once the particle temperature exceeds 2800-3000 K, small aluminum-rich clusters are ejected from the outside of the shell. The underlying mechanisms were explored using global and radial statistical analysis, as well as developed visualization techniques and localized fragment analysis.

The three-stage reaction mechanism found here provides insight into the controlling factors of aluminum nanoparticle oxidation, a topic of considerable importance in the energetic materials community.

Rotary atomization was used to synthesize spheres of CaSi2-based compositions in order to understand issues relative to primer performance for military applications. Elemental silicon and calcium were used to synthesize the line compound CaSi2 or the eutectic composition between CaSi2 and Si. Fe was added to form FeSi2 as a secondary phase in selected compositions. Rietveld analysis showed that CaSi2 polytypes in the synthesized materials consisted primarily of 6R, with less 3R and some hexagonal material. Synthesized materials had low surface areas (≈0.1 m2/g), but short milling times increased the surface area by an order of magnitude. Peak pressures, pressure rise time, and ignition voltage showed no significant differences between experimentally prepared samples and existing commercial samples. Stoichiometric CaSi2 performed as well as CaSi2-Si or CaSi2-FeSi2-Si mixtures. The military specification for calcium disilicide should be changed to reflect the broad chemistry which can be used for primer performance.

The objectives of this work are to characterize thermite initiation processes and thresholds, and to develop thermite reactive trains, where a sensitive nanothermite ignites an insensitive micron thermite, which produces little gas. Nanothermites, including Al/AgIO3, Al/Bi2O3, Al/MoO3, Al/Fe3O4, and Ti/AgIO3, were characterized for their ignition behavior by spark and resistive heating. Energies for spark and thermal initiation were as low as 9 and 140 μJ, respectively. Thermal initiation results were consistent with local temperature as the main controlling factor. The propagation rate of the Al/Fe3O4 nanothermite was about 100X slower than that of the other nanothermites. This low reactivity is attributed to the high volatilization temperature and high melting point of the oxidizer. Mixing of 90% Al/Fe3O4 nanothermite with 10% of a more sensitive, high-gas-producing nanothermite gave materials with the same sensitivity as the sensitive nanothermite. Thus, the mixture provides a safer sensitive nanothermite. Thermites with micron-scale ingredients were pressed into pellets and ignited with small amounts of nanothermite. Gas production of micron thermite compositions was reduced by adding the intermetallic composite, Ti/2B, or excess iron. In both cases, a single hot mass was produced, while the pure micron Al/Fe2O3 produced a dispersion of particles.

In this study, a novel flow-based method is presented to place catalytic nanoparticles into a reactor by sol-gelation of a porous ceramic consisting of copper-based nanoparticles, silica sand, ceramic binder, and a gelation agent. This method allows for the placement of a liquid precursor containing the catalyst into the final reactor geometry without the need of impregnating or coating of a substrate with the catalytic material. The so generated foam-like porous ceramic shows properties highly appropriate for use as catalytic reactor material, e.g., reasonable pressure drop due to its porosity, high thermal and catalytic stability, and excellent catalytic behavior.

The catalytic activity of micro-reactors containing this foam-like ceramic is tested in terms of their ability to convert alcoholic biofuel (e.g. methanol) to a hydrogen-rich gas mixture with low concentrations of carbon monoxide (up to 75% hydrogen content and less than 0.2% CO, for the case of methanol). This gas mixture is subsequently used in a low-temperature fuel cell, converting the hydrogen directly to electricity. A low concentration of CO is crucial to avoid poisoning of the fuel cell catalyst. Since conventional Polymer Electrolyte Membrane (PEM) fuel cells require CO concentrations far below 100 ppm and since most methods to reduce the mole fraction of CO (such as Preferential Oxidation or PROX) have CO conversions of up to 99%, the alcohol fuel reformer has to achieve initial CO mole fractions significantly below 1%. The catalyst and the porous ceramic reactor of the present study can successfully fulfill this requirement.

The results of the present study confirm that product gas mixtures with up to 75% hydrogen content and less than 0.2% CO content can be achieved, which is an excellent result. The reactor temperature can be kept as low as 220°C while obtaining a methanol conversion of up to 70%. The used PROX catalyst showed selective CO conversion rates above 99.5% for temperatures between 80 and 100°C in presence of large molar fractions of H2O and CO2.

In development of new explosives, it is often necessary to balance a number of attributes in performance while certain formulation constraints exist. Statistical design of experiments (DOE) is a valuable tool for rapid formulation optimization and minimization of costly and hazardous testing. During the development of metal-loaded explosives designed for enhanced blast, it was discovered that upon proper formulation, aluminum additives obtained full reaction by 7 volume expansions, which resulted in extremely high Gurney energies equivalent to LX-14 and PBXN-5 but with lower loading of nitramines. The early aluminum oxidation can be described by Eigenvalue type detonations, where the fully reacted Hugoniot of the condensed phase aluminum oxide and explosive products lies below the unreacted aluminum Hugoniot. Such an analysis describes fully the agreement of aluminum consumption by 7 volume expansions from 1-inch copper cylinder expansion tests and an analytic cylinder model, as well as detonation calorimetry. With the early reaction of aluminum also comes a shift in the gaseous reaction products to higher enthalpy species such as CO and H2, leading to further augmentation of blast. Thus, both the mechanical energy (for fragmentation or “metal pushing”) and blast (for structural targets) are available in a single explosive fill. This provides capability for combined metal pushing and blast in a single explosive that was not previously possible. Development of such explosives and the importance of modern statistical design of experiments will be shared.

Two fundamental processes associated with shock compression of energetic materials (EM) are initiation and ignition. Initiation occurs just behind a shock front and ignition occurs anywhere from a few nanoseconds to hundreds of nanoseconds later. Experiments are described that probe the fundamental mechanisms of these processes on relevant length and time scales: picosecond vibrational spectroscopy of nanometer thick layers of energetic materials (EM) with laser-driven shock waves, and nanosecond emission spectroscopy of micrometer thick layers of EM using laser-driven flyer plates.

Common igniters such as black powder, benite, and boron potassium nitrate (BKNO3) are routinely employed in all calibers of gun systems. Armament Research Development Engineering Center (ARDEC) has pursued efforts to improve the ignition of gun propellants which has been demonstrated to be the root cause of many tribulations for gun systems. We have developed several extrudable nitrocellulose-BKNO3 based igniter materials that are more energetic, and exhibit smaller ignition delay times than most traditional igniters. We have demonstrated this via static firing. High speed video during static testing has demonstrated significantly more consistent, intense, and rapid flame generation in comparison to Benite leading to improved ignition effectiveness of the propellant bed.

The nickel-aluminum (Ni/Al) intermetallic system is useful for a variety of reactive material applications, and reaction characteristics are well studied at the normal self-heating rates of 103–106 K/s. Recent experiments at 1011–1012 K/s have measured the kinetic energy of material ejected from the reaction zone, indicating additional kinetic energy from the reactive system despite high heating rates. In order to better probe reaction phenomena at these time scales, and determine the presence of expected elements and their temperatures, we report on emission spectroscopy of electrically heated, patterned Ni/Al bridge wires, time resolved over 350 ns through the use of a streak camera. Unlike previous studies where emission was dominated by Ar and N from residual gasses in the vacuum test chamber, here we report on experiments with encapsulated laminates allowing better quantification of Al and Ni emission. We were able to identify all major spectral lines from the dominant elements present in the films, and found the multilayered Ni/Al laminates to exhibit a brighter and longer duration emission than either Al or Ni control samples. We also found the measured electrical energy absorption of the Ni/Al laminates to follow that of the Al samples up to 150 ns following the onset of emission, indicating that the exothermic mixing of vapor phase Ni and Al was the most likely source for the higher emission intensity. These results will be important for new, energetically enhanced, high efficiency bridge wire applications, where shock initiation of subsequent energetic reactions may be accomplished with less electrical energy than is currently required.

Metal borides (AlB2, MgB2, Mg0.5Al0.5B2, AlB12, SiB6 and MgAlB14) and boron carbide (B4C) reacted with Al were compared to B, Mg, Al, Mg-Al and Si as potential energetic fuel additives. Stoichiometric physical mixtures of powders corresponding to unreacted boride compounds (Al+2B, Mg+2B, Mg-Al+2B, Al+12B, Si+6B, Mg-Al+14B, and B4C+2Al) were also investigated in comparison to the compounds. Submicron boron was used, which resulted in very fine particle sizes for all materials studied. It was demonstrated that boride compounds were less sensitive to low-temperature oxidation in flowing air than physical mixtures or metallic fuels. Compounds with high mole fractions of boron were generally less sensitive, but their high temperature oxidation behavior showed no improvement over boron. Cylinder expansion testing of MgAlB14 exposed its poor performance in an energetic mixture. However, aluminum and magnesium diborides (AlB2, MgB2 and Mg0.5Al0.5B2) also had relatively low sensitivity and exhibited mechanisms to increase the rate of boron oxidation at high temperatures, showing promise as insensitive high-energy-density fuel additives. Detonation calorimetry of mixtures with AlB2 or Al+2B suggested that the AlB2 mixture released approximately 50% more heat per gram than Al +2B and underwent complete reaction. These results warrant further testing of the diboride compounds in energetic formulations. Due to the high cost of boron and acceptable performance of B4C-Al mixtures, B4C should also be investigated as a lower-cost alternative to boron.

A T-Jump/Time-of-Flight Mass Spectrometer (T-Jump/TOFMS) is used to probe the decomposition of several aminotetrazole containing energetic materials under very high heating rates of 105-106 K/s. Subtle differences between materials in functional group placement and anion composition allow for further understanding of the decomposition pathway of the tetrazole structure and various anions. Two decomposition pathways for the tetrazole ring are observed, which result in the primary formation of HN3 or N2. Further analysis is performed using a rapid-heating μ-DSC device, which revealed lower activation energies than previously reported.

The low vapor pressure of many energetic materials presents a challenge for detection by non-contact methods. We address this limitation by illuminating energetic materials including TNT and RDX with infrared lasers tuned to strong molecular absorption bands to efficiently heat trace amounts present on substrates. This substantially increases their vapor signatures for direct detection, obviating the need to swab surfaces for solid particles or to collect headspace vapors for extended time periods. The instantaneously generated vapor produced by Laser Trace Vaporization (LTV) can be detected by any number of techniques which can accommodate vapor sampling or spectroscopic analysis. Currently the testbed for LTV incorporates a tunable quantum cascade laser (QCL) to illuminate the sample and an ion mobility spectrometer (IMS) to validate the signal enhancement. The LTV technique works well with all tested substrates, though the thermal and spectroscopic properties of the substrate can influence the efficiency of the vaporization. Computational results from laser heating along with experimental thermal kinetic measurements were used to optimize LTV laser irradiation parameters. In addition to a range of LTV results for different explosives and substrates, we explore the effects of wavelength-dependent heating on the sample and substrate.

We show a low temperature gas-phase synthesis route to produce faceted aluminum crystals in the aerosol phase. Use of triisobutylaluminum whose decomposition temperature is below the melting point of elemental aluminum enabled us to grow nanocrystals from its vapor. Combustion tests show an increase in energy release compared to commercial nanoaluminum. Production of aluminum in an oxygen free environment resulted in a bare aluminum surface that was passivated in separate experiments with nickel and iron by decomposition of their carbonyl precursors.

Hydrogen fluoride (HF) is a known product from the combustion or detonation of explosives formulated with fluoropolymer binder systems. This presents the user with elevated risk levels, particularly during unintended combustion events or detonations in close combat situations. In an effort to remediate the production of HF, calcium disilicide was added to explosive formulations in an effort to decrease the amount of HF formed. First, VitonA/calcium disilicide mixtures were made and the thermal decomposition characteristics studied using thermal gravimetric/differential thermal analysis. The activation energy ranged from approximately 145-190 kJ/mol, indicative of C-F scission in the Viton binder prior to calcium fluoride formation. An energetic formulation was made which consisted of approximately a 5/3 mass ratio of Viton/CaSi2. Combustion calorimetry in oxygen and air and subsequent analysis of the residues using x-ray diffraction (XRD) and energy dispersive x-ray analysis (EDS) revealed that calcium fluoride formed. The amount of HF reduced was determined by trapping off gases in a cooling loop, rinsing into water, and analysis in anion exchange chromatography. Upon introduction of calcium disilicide into the formulation, a ~30% decrease in HF formation was observed along with appearance of CaF2 and free Si in the residue. Such products are consistent with the mechanism following a general decomposition path of 2F + CaSi2→CaF2+2Si. The same formulation was detonated, and upon product analysis it was determined the decomposition path followed nearly the same route with a net decrease in HF formation, but with a portion of the silicon oxidizing slightly further to SiO2.

Irradiating a planar silicon surface with femtosecond laser pulses under a sulfuric atmosphere creates first a structured surface featuring cones of up to 20 microns in height, and second a 0.1 – 1 μ m thick layer of multi-crystalline silicon on theses cones containing up to 1 at.% sulfur acting as n-type dopant. Further, the sulfur establishes energy states within the band gap of silicon allowing for the absorption of infrared (IR) light with energies below the band gap energy of silicon. This black silicon process is distinguished by the fact that only one single laser process is required to tailor three material characteristics in on step: the surface structure, the doping and the light absorption. In this work we study structural and optical material characteristics of black silicon. For the first time this work presents properties of black silicon processed with shaped femtosecond laser pulses. Finally, black silicon substrate is used as substrate for manufacturing a black silicon solar cell with a femtosecond laser pulse formed sulfur emitter. For such a black silicon solar cell we achieved a record efficiency of η =4.5%

The effects of surrounding gaseous environment on the reaction behaviors and product formation for sputter-deposited Ti/2B reactive multilayers are reported. With the surrounding environment set to different air pressures, from atmospheric conditions to 10-4 Torr, Ti/2B samples were reacted in a self-propagating mode, and the average reaction wave velocities were determined through high-speed imaging. Propagation speeds for 3.0 μm-thick multilayers were in the range of 10.89 to 0.05 m/s depending on bilayer thickness (i.e., reactant layer periodicity) and ambient pressure. X-ray diffraction analysis showed that single-phase TiB2 forms within multilayers that have small bilayer thickness. Multilayers that have a large bilayer thickness developed a mixture of TiB2, TiB and TiO2.

We studied the early stages of polymerization of CO under pressure. We performed DFT simulations of 128 and 432 atom models. Structures of random networks found at zero temperature were used for equilibration at 100 K by employing first principles MD. We found that the polymerization begins at 7 - 8 GPa and slightly depends on the size of the model. It turned out that there are several metastable phases of the extended CO solid, corresponding to different compression pressures from 7 - 8 GPa to 15-18 GPa with different numbers of CO fragments, not connected to the random network. We also found that the transition to the phases is irreversible which results in hysteresis loops. Random network structures obtained, say, under 18 GPa could exist at 3 GPa, whereas compression to 3 GPa results in the delta phase of CO crystal, with intact CO fragments and minor distortion of the cubic phase. To analyze the random structure fragments we calculated normal modes and IR intensities using the dipole approximation. Contributions from the main motifs of the random network are identified and compared with experimental IR measurements.

In the wake of the recent terrorist attacks, such as the 2008 Mumbai hotel explosion or the December 25th 2009 “underwear bomber”, our group has developed a technique (US patent #7368292) to apply differential reflective spectroscopy to the problem of detecting explosives in order to detect terrorist threats. Briefly, light (200-500 nm) is shone on a surface such as a piece of luggage at an airport or a parcel at a courier distribution center. Upon reflection, the light is collected with a spectrometer combined with a camera. A computer processes the data and produces in turn a differential reflection spectrum taken between two adjacent areas of the surface. This differential technique is highly sensitive and provides spectroscopic data of explosives. As an example, 2,4,6, trinitrotoluene (TNT) displays strong and distinct features in differential reflectograms near 420 nm. Similar, but distinctly different features are observed for other explosives such as RDX, PETN or ANFO. Our detection system uses a two dimension detector (CCD camera) which provide spatial and spectroscopic information in each of the two dimensions. By scanning (involving fixed optical equipment and scanning moving bags or parcels on a conveyor belt), the surface to be surveyed the system provide the spatial location of the potential threat. We present in this paper how our detector works and how it is applied to the problem of explosive screening for explosives at airports and mail sorting centers. Additionally, we will present the effect of the explosives morphology on the detection response. In particular we will evaluate the implication on the limit of detection of the instrument as well as discuss the sample morphology with respect to a realistic threat scenario.