Philippe Hebert

Amorphization of faujasite at high pressure: an X-ray diffraction and Raman spectroscopy study

In situ X-ray diffraction and Raman spectroscopy measurements of synthetic powdered samples of faujasite 13X were carried out at high pressure using diamond anvil cells. Structural changes are detected, linked to a progressive reduction in crystallinity, before complete amorphization of the material. Three distinct compressibility regions are clearly observed, delimited by two discontinuities in the pressure dependence of the faujasite volume around 2 and 3.5 GPa. The transition from the crystal to the amorphous state is incomplete and partially reversible below pressures of between 8 and 12 GPa, depending on the pressure-transmitting medium used. This partial recovery of the initial structure, at least on a local level, could be related to the presence of hydrated sodium ions in the faujasite framework. In addition, the position of the first sharp diffraction peak in the X-ray diffraction pattern of a fully amorphous sample recovered from 24 GPa is consistent with the presence of 4-membered rings of tetrahedra and the persistence of a significant number of larger rings as compared to a dried amorphous faujasite precursor.

Raman spectroscopy study of laser-shocked tatb-based explosives

We have developed a nanosecond single-shot Raman spectroscopy experiment to study the initiation and decomposition mechanisms of heterogeneous solid explosives under shock compression. The laser-driven flyer plate technique was used to generate shock waves on TATB-based explosives samples. Photon Doppler Velocimetry (PDV) measurements were performed to estimate the generated shock pressures. The first Raman spectra obtained under shock compression are presented here. The shifts observed for the different Raman peaks are consistent with those observed under static pressure.

In-situ Raman spectroscopy and high-speed photography of a shocked triaminotrinitrobenzene based explosive

We have developed a single-shot Raman spectroscopy experiment to study at the molecular level the initiation mechanisms that can lead to sustained detonation of a triaminotrinitrobenzene-based explosive. Shocks up to 30 GPa were generated using a two-stage laser-driven flyer plate generator. The samples were confined by an optical window and shock pressure was maintained for at least 30 ns. Photon Doppler Velocimetry measurements were performed at the explosive/window interface to determine the shock pressure profile. Raman spectra were recorded as a function of shock pressure and the shifts of the principal modes were compared to static high-pressure measurements performed in a diamond anvil cell. Our shock data indicate the role of temperature effects. Our Raman spectra also show a progressive extinction of the signal which disappears around 9 GPa. High-speed photography images reveal a simultaneous progressive darkening of the sample surface up to total opacity at 9 GPa. Reflectivity measurements under...

Dynamics of the detonation products of a TATB based high explosive: Photon doppler velocimetry and high-speed digital shadowgraphy of expanding species

We have performed time-resolved free surface velocity measurements of the expanding products from the detonation of a TATB based high explosive composition, using both Heterodyne Velocimetry (i.e. Photon Doppler Velocimetry) and digital high speed shadowgraphy. The free surface velocity waveforms exhibit features which can be directly related to charateristics of the reaction zone. This allows to estimate the von Neumann spike pressure to PVN ≃41 GPa and the duration of the reaction zone to 400-500 ns, in good agreement with previous studies. 2D Eulerian direct numerical simulations performed with the wide-ranging equation of state and reaction rate model developed by Wescott, Scott Stewart and Davis are found to be in very good agreement with our experimental results.

Raman spectroscopy study of nitromethane in a shear diamond anvil cell

In addition to pressure and temperature effects, shear strain has been proposed to play an important role in the initiation and decomposition mechanisms of energetic materials. To study the effect of shear strain in more detail, a shear diamond anvil cell has been developed. In this paper, we present a Raman spectroscopy study of the effects of shear strain on the high-pressure behaviour of nitromethane (NM). Two major effects are observed in our experiments. The first one is a lowering of the pressures at which the different structural modifications of NM occur. The second one is observed at 28 GPa, where the sample decomposes suddenly just after shear application. Examination of the black residue showed that it is composed of carbon, as indicated by the Raman spectrum.

A novel method for the measurement of the von Neumann spike in detonating high explosives

We present detonation wave profiles measured in T2 (97 wt. % TATB) and TX1 (52 wt. % TATB and 45 wt. % HMX) high explosives. The experiments consisted in initiating a detonation wave in a 15 mm diameter cylinder of explosive using an explosive wire detonator and an explosive booster. Free surface velocity wave profiles were measured at the explosive/air interface using a Photon Doppler Velocimetry system. We demonstrate that a comparison of these free surface wave profiles with those measured at explosive/window interfaces in similar conditions allows to bracket the von Neumann spike in a narrow range. For T2, our measurements show that the spike pressure lies between 35.9 and 40.1 GPa, whereas for TX1, it lies between 42.3 and 47.0 GPa. The numerical simulations performed in support to these measurements show that they can be used to calibrate reactive burn models and also to check the accuracy of the detonation products equation of state at low pressure.

Expansion of the Detonation Products of a TATB Based High Explosive: Experimental Characterization by Photon Doppler Velocimetry and High-Speed Digital Shadowgraphy

A good description of the expansion of the detonation products from a chemical explosion is of fundamental importance because it constrains the ballistic performance of the explosive.We describe in this paper an experimental study of the free expansion of detonation products of an insensitive TATB-based explosive by measuring the free surface velocity of the detonating explosive with a PDV velocimeter system. Indeed, recent experiments [1] have demonstrated the ability of such system to record the free surface velocity of a detonating explosive, and also potentially the detonation velocity inside the explosive just before the shock breakout, if the explosive is not entirely opaque in the near IR walelengths, which is the case of most TATB based explosives. PDV appears to be a very promising recording technique for such measurements, because it offers a good time resolution (close to 1 ns) and ability to record over very long durations which is required to measure the reaction product expansion over a large volume range. We also used digital high-speed shadowgraphy to characterize the shape and speed of the products as they release from the bare charge free surface. The results allow to give new insight into the reactions zone of TATB based high explosive. They are compared with numerical simulations performed with different reactive flow models.

Experimental measurements of the chemical reaction zone of TATB and HMX based explosives

In order to have an insight into the chemical reaction zone of high explosives (HE), experimental measurements of the detonation wave profile of solid explosives using laser velocimetry techniques have been performed. The experiments consisted in initiating a detonation wave in a cylinder of explosive using an explosive wire detonator associated with an explosive booster and in measuring the particle velocity of an explosive-window interface. A TATB based HE and a TATBHMX based HE have been studied through several configurations where the cylinder geometry and the window material could vary. Particle velocity profiles were measured by VISAR and Heterodyne Velocimetry (HV). The results on the behavior of the explosives have been analyzed and compared with those of Photon Doppler Velocimetry of expanding species experiments. These experiments also enabled to carry on the comparison of the efficiency of VISAR and HV in such applications.

EFFECT OF SHEAR STRAIN ON THE HIGH‐PRESSURE BEHAVIOR OF NITROMETHANE: RAMAN SPECTROSCOPY IN A SHEAR DIAMOND ANVIL CELL

The effect of shear strain on the high‐pressure behavior of nitromethane has been studied by Raman spectroscopy in a shear diamond anvil cell (SDAC). Two major effects of shear strain have been observed. The first one is a lowering of the pressures at which the different spectral transformations occur. The second effect is observed at 28 GPa. At this pressure, a sudden decomposition of the sample occurs due to the shear deformation. Observation of the sample after decomposition shows the presence of a black residue, which is mainly composed of carbon soot.

Study of the laser-induced decomposition of HNO3/2-nitropropane mixture at static high pressure

The objective of the work presented here is to study the laser‐induced decomposition of a condensed HNO3/2‐nitropropane mixture containing 58% nitric acid. On the macroscopic scale, this energetic material detonates. Under static high pressure, the formation of an H‐bonded complex with that particular composition was demonstrated in a previous study. The high pressure behavior of the complex showed the presence of a solid‐solid phase transition around 18 GPa. The combustion front propagation velocity was recorded between 6 and 31 GPa. The analysis of the optical properties of the reaction products as well as the recording of their Raman spectra showed two different combustion regimes. Below 18 GPa, total combustion takes place in the sample and a black residue only composed of soot remains in the cell. Above 18 GPa, the combustion leads to a clear residue with little carbon present. However, the Raman spectra of the remaining sample show new features indicating the presence of species which are not yet cl...