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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. [ABSTRACT FROM AUTHOR]

We investigated the shock initiation of energetic materials with a tabletop apparatus that uses km s-11 laser-driven flyer plates to initiate tiny explosive charges and obtains complete temperature histories with a high dynamic range. By comparing various microstructured formulations, including a pentaerythritol tetranitrate (PETN) based plastic explosive (PBX) denoted XTX-8003, we determined that micron-scale pores were needed to create high hot spot temperatures. In charges where micropores (i.e., micron-sized pores) were present, a hot spot temperature of 6000K was observed; when the micropores were pre-compressed to nm scale, however, the hot spot temperature dropped to ~4000 K. By comparing XTX-8003 with an analog that replaced PETN by nonvolatile silica, we showed that the high temperatures require gas in the pores, that the high temperatures were created by adiabatic gas compression, and that the temperatures observed can be controlled by the choice of ambient gases. The hot spots persist in shock-compressed PBXs even in vacuum because the initially empty pores became filled with gas created in-situ by shock-induced chemical decomposition. [ABSTRACT FROM AUTHOR]

Hot spots produced by long-wavelength infrared (LWIR) radiation in an energetic material, crystalline RDX (1,3,5-trinitroperhydro-1,3,5-triazine), were studied by thermal-imaging microscopy. The LWIR source was a CO2 laser operating in the 28-30THz range. Hot spot generation was studied using relatively low intensity (~100W cm-2), long-duration (450 ms) LWIR pulses. The hot spots could be produced repeatedly in individual RDX crystals, to investigate the fundamental mechanisms of hot spot generation by LWIR, since the peak hot-spot temperatures were kept to ~30K above ambient. Hot spots were generated preferentially beneath RDX crystal planes making oblique angles with the LWIR beam. Surprisingly, hot spots were more prominent when the LWIR wavelength was tuned to be weakly absorbed (absorption depth ~30 μm) than when the LWIR wavelength was strongly absorbed (absorption depth ~5 μm). This unexpected effect was explained using a model that accounts for LWIR refraction and RDX thermal conduction. The weakly absorbed LWIR is slightly focused underneath the oblique crystal planes, and it penetrates the RDX crystals more deeply, increasing the likelihood of irradiating RDX defect inclusions that are able to strongly absorb or internally focus the LWIR beam. [ABSTRACT FROM AUTHOR]

We have observed and characterized hot spot formation and hot-spot ignition of energetic materials (EM), where hot spots were created by ultrasonic or long-wavelength infrared (LWIR) exposure, and were detected by high-speed thermal microscopy. The microscope had 15-20 μm spatial resolution and 8.3 ms temporal resolution. LWIR was generated by a CO2 laser (tunable near 10.6 μm or 28.3 THz) and ultrasound by a 20 kHz acoustic horn. Both methods of energy input created spatially homogeneous energy fields, allowing hot spots to develop spontaneously due to the microstructure of the sample materials. We observed formation of hot spots which grew and caused the EM to ignite. The EM studied here consisted of composite solids with 1,3,5-trinitroperhydro-1,3,5-triazine crystals and polymer binders. EM simulants based on sucrose crystals in binders were also examined. The mechanisms of hot spot generation were different with LWIR and ultrasound. With LWIR, hot spots were most efficiently generated within the EM crystals at LWIR wavelengths having longer absorption depths of ~25 μm, suggesting that hot spot generation mechanisms involved localized absorbing defects within the crystals, LWIR focusing in the crystals or LWIR interference in the crystals. With ultrasound, hot spots were primarily generated in regions of the polymer binder immediately adjacent to crystal surfaces, rather than inside the EM crystals. [ABSTRACT FROM AUTHOR]

Slurries of powdered Bi2Sr2CaCu2O8+x (BSCCO-2212) superconductor in high-boiling alkanes were irradiated with intense ultrasound. Significant enhancements of magnetic irreversibility as well as transport critical current are reported. The effects are dependent on the concentration of the slurry and are optimal for 1.5 wt % slurry loading. Electron microscopy shows that ultrasonic treatment leads to a change in grain morphology and intergrain welding. The observed enhancement of superconducting properties is consistent with the limitations in critical currents in BSCCO superconductor being due to intergrain coupling rather than intragrain pinning strength. [ABSTRACT FROM AUTHOR]

Ultrasonic irradiation of magnesium diboride slurries in decalin produces material with significant intergrain fusion. Sonication in the presence of Fe(CO)[sub 5] produces magnetic Fe[sub 2]O[sub 3] nanoparticles embedded in the MgB[sub 2] bulk. The resulting superconductor–ferromagnet composite exhibits considerable enhancement of its magnetic hysteresis, which implies an increase of vortex pinning strength due to embedded magnetic nanoparticles. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The origin of 'sonochemistry' is acoustic cavitation: the formation, expansion, and implosive collapse of bubbles in liquids irradiated with ultrasound. The compression of such bubbles generates intense local heating, which has been quantified recently from both chemical kinetic thermometry and from high-resolution sonoluminescence spectra. The temperatures reached during cavitation are ≈5000 K, but have an effective lifetime of only a few microseconds. Consistent with this, the sonoluminescence that accompanies sonochemistry closely resembles flame emission! The chemistry generated by these hot spots is different than either ordinary thermal or photochemical processes and sonochemistry represents a fundamentally unique interaction of energy and matter. [For recent reviews see K. S. Suslick, Sci. Am. 260, 80 (Feb. 1989) and Science 247, 1439 (1990).] Recently, the use of ultrasound in liquid-powder slurries to enhance dramatically their chemical reactivity has been explored. For example, heterogeneous catalysis can be induced in normally nonreactive metals and the catalytic activity of Ni has been enhanced by 105. Using a variety of surface science techniques, it was shown that ultrasound removed the passivating oxide coating normally found on Ni and other metal surfaces, thus increasing their activity. The origin of these effects comes from extremely high-speed interparticle collisions which occur during ultrasonic irradiation of liquid-solid slurries. Turbulent flow and shockwaves produced by acoustic cavitation can drive metal particles together at sufficiently high velocities to induce melting upon collision. A series of transition metal powders have been used to probe the maximum temperatures and speeds reached during such interparticle collisions. Metal particles that are irradiated in hydrocarbon liquids with ultrasound undergo collisions at roughly half the speed of sound and generate localized effective temperatures between 2600°C and 3400°C at the point of impact. [Work supported by NSF and the UIUC Materials Res. Lab.] [ABSTRACT FROM AUTHOR]

The collapse of cavitation bubbles generates intense local energy release, resulting in extreme local heating, high local pressures, and high energy chemistry. Determining the local conditions formed during cavitation, however, has proved to be a difficult problem. In our work, sonochemical reactions and the sonoluminescence that results have been used as quantitative probes of both temperature and pressure during cavitation. The effective temperatures reached during cavitation can be probed by measurement of the relative rates of a series of chemical reactions whose temperature dependence is already known. Comparative rate thermometry of multibubble cavitation (immersion horn at 20 kHz and ≊30 W/cm2) yields effective temperatures of ≊5200 K in hydrocarbons [K. S. Suslick, Science 247, 1439 (1990)]. Consistent with this, sonoluminescence in these liquids closely resembles flame emission from excited states of C2; rotational and vibrational fine structure permits a spectroscopic determination of the emission temperature of C2 excited states at 5080±160 K [E. B. Flint and K. S. Suslick, Science 253, 1397 (1991)]. Sonoluminescence from excited state metal atoms is produced from sonolysis of organometallic compounds, for which the linewidth determines the collisional lifetimes of emitting atoms and the effective local pressures. For excited state Cr atoms produced from Cr(CO)6, emission lifetime is only 0.20 ps, corresponding to local densities of ≊0.15 g/cm3 or pressures of ≊1.5 kBar at 5000 K. [Supported by NSF.] [ABSTRACT FROM AUTHOR]

Recent advances in the use of single bubble cavitation [D. F. Gaitan et al., J. Acoust. Soc. Am. 91, 3166 (1992)] have made it possible to examine both the temporal and spectral characteristics of sonoluminescence [B. P. Barber et al., J. Acoust. Soc. Am. 91, 3061 (1992)]. Direct comparison of the spectra of single-bubble sonoluminescence to multibubble sonoluminescence has proved difficult, however, due to differences in experimental conditions. Examination has begun of both single-bubble and multibubble sonoluminescence spectra of various aqueous solutions under closely similar conditions, where, in a systematic way, a variety of dissolved gases, volatile organic liquids, and involatile inorganic salts have been introduced. Qualitative comparisons of these spectra will be discussed. [Work supported by ONR and NSF.] [ABSTRACT FROM AUTHOR]

We have observed and characterized hot spot formation and hot-spot ignition of energetic materials (EM), where hot spots were created by ultrasonic or long-wavelength infrared (LWIR) exposure, and were detected by high-speed thermal microscopy. The microscope had 15-20 μm spatial resolution and 8.3 ms temporal resolution. LWIR was generated by a CO2 laser (tunable near 10.6 μm or 28.3 THz) and ultrasound by a 20 kHz acoustic horn. Both methods of energy input created spatially homogeneous energy fields, allowing hot spots to develop spontaneously due to the microstructure of the sample materials. We observed formation of hot spots which grew and caused the EM to ignite. The EM studied here consisted of composite solids with 1,3,5-trinitroperhydro-1,3,5-triazine crystals and polymer binders. EM simulants based on sucrose crystals in binders were also examined. The mechanisms of hot spot generation were different with LWIR and ultrasound. With LWIR, hot spots were most efficiently generated within the EM crystals at LWIR wavelengths having longer absorption depths of ∼25 μm, suggesting that hot spot generation mechanisms involved localized absorbing defects within the crystals, LWIR focusing in the crystals or LWIR interference in the crystals. With ultrasound, hot spots were primarily generated in regions of the polymer binder immediately adjacent to crystal surfaces, rather than inside the EM crystals.