RESUMEN
The drop hammer test is the easiest way to assess the sensitivity of explosive materials, but drop hammer results for low-velocity impacts have not been able to explain how explosives will react to other kinds of initiating stimuli. In order to do that, we have to understand the fundamental mechanisms of drop hammer initiation and how they differ from other initiation methods. For this reason, there is interest in instrumented drop hammers that help reveal what the drop hammer does at a fundamental level. We have developed a drop hammer that combines two types of mid-wavelength infrared (MWIR) imagers that, when operated simultaneously, can detect both the rapid explosion and slower combustion from impact-initiated polymer-bonded explosives with high time (1 µs) and space (15 µm) resolution. Results are presented that show how to vibration isolate the drop hammer to minimize MWIR image shaking during impact and to quantify the noise floor for MWIR temperature determinations via optical pyrometry. Experiments were performed on polymer-encased crystals of RDX ([CH2-NNO2]3) and HMX ([CH2-NNO2]4). Our experiments showed that drop-hammer initiated explosions occur in two phases with roughly 100 µs between explosions. Drop-hammer initiation is compared to an ultrasonic hammer, which initiates explosions by rapid frictional rubbing of the explosive surfaces against the surrounding polymer. The explosion rise time is faster with the drop hammer because the drop hammer inputs energy throughout the explosive volume, whereas the ultrasonic hammer produces localized heating and much more heat at the explosive surface.
RESUMEN
An emission spectrometer (450-850 nm) using a high-throughput, high numerical aperture (N.A. = 0.3) prism spectrograph with stepped fiberoptic coupling, 32 fast photomultipliers and thirty-two 1.25 GHz digitizers is described. The spectrometer can capture single-shot events with a high dynamic range in amplitude and time (nanoseconds to milliseconds or longer). Methods to calibrate the spectrometer and verify its performance and accuracy are described. When a reference thermal source is used for calibration, the spectrometer can function as a fast optical pyrometer. Applications of the spectrometer are illustrated by using it to capture single-shot emission transients from energetic materials or reactive materials initiated by kmâ s-1 impacts with laser-driven flyer plates. A log (time) data analysis method is used to visualize multiple kinetic processes resulting from impact initiation of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) or a Zr/CuO nanolaminate thermite. Using a gray body algorithm to interpret the spectral radiance from shocked HMX, a time history of temperature and emissivity was obtained, which could be used to investigate HMX hot spot dynamics. Finally, two examples are presented showing how the spectrometer can avoid temperature determination errors in systems where thermal emission is accompanied by atomic or molecular emission lines.
RESUMEN
The emission lifetimes of rhodamine 6G (R6G) were measured under shock compression to 9.1 GPa, with the dual intents of better understanding molecular photophysics in extreme environments and assessing the usefulness of fluorescence lifetime microscopy to measure spatially dependent pressure distributions in shocked microstructured media. R6G was studied as free dye dissolved in poly(methyl methacrylate) (PMMA), or dye encapsulated in silica microparticles suspended in PMMA. Thin layers of these materials in impedance-matched geometries were subjected to planar single-stage shocks created by laser-driven flyer plates. A synchronized femtosecond laser excited the dye at selected times relative to flyer plate arrival and the emission lifetimes were measured with a streak camera. Lifetimes decreased when shocks arrived. The lifetime decrease was attributed to a shock-induced enhancement of R6G nonradiative relaxation. At least part of the relaxation involved shock-enhanced intersystem crossing. For free dye in PMMA, the lifetime decrease during the shock was shown to be a linear function of shock pressure from 0 to 9 GPa, with a slope of -0.22 ns·GPa(-1). The linear relationship makes it simple to convert lifetimes into pressures. Lifetime measurements in shocked microenvironments may be better than emission intensity measurements, because lifetimes are sensitive to the surrounding environment, but insensitive to intensity variations associated with the motion and optical properties of a dynamically changing structure.
RESUMEN
Forensic identification of batches of homemade explosives (HME) poses a difficult analytical challenge. Differentiation among peroxide explosives is reported herein using a colorimetric sensor array and handheld scanner with a field-appropriate sampling protocol. Clear discrimination was demonstrated among twelve peroxide samples prepared from different reagents, with a classification accuracy >98%.
RESUMEN
The ability to direct the morphology of cobalt sol-gel materials by using the simple synthetic parameters in epoxide-driven polycondensations has been dramatically demonstrated, and the influence of such morphological differences upon the supercapacity of the materials has been explored. Precursor salt, epoxide, and solvent all influence the speed of the sol-gel transition and the size and shape of the features observed in the as-prepared materials, thereby leading to highly varied microstructures including spheres, sponge-like networks, and plate assemblies of varied size. These morphological features of the as-prepared cobalt aerogels were observed for the first time by high resolution scanning electron microscopy (HRSEM). The as-prepared aerogel materials were identified by powder X-ray diffraction and thermogravimetry as weakly crystalline or amorphous cobalt basic salts with the general formula Co(OH)(2-n)X(n) where X = Cl or NO3 according to the precursor salt used in the synthesis. For all samples, the morphology was preserved through mild calcining to afford spinel phase Co3O4 in a variety of microstructures. Wide-ranging specific surface areas were determined for the as-prepared and calcined phases by physisorption analysis in agreement with the morphologies observed by HRSEM. The Co3O4 aerogels were evaluated for their supercapacitive performance by cyclic voltammetry. The various specimens exhibit capacitances ranging from 110 to 550 F g(-1) depending upon the attributes of the particular aerogel material, and the best specimen was found to have good cycle stability. These results highlight the epoxide-driven sol-gel condensation as a versatile preparative route that provides wide scope in materials' properties and enables the analysis of structure-performance relationships in metal oxide materials.