Francois-Xavier Jette

In-situ measurements of the onset of bulk exothermicity in shock initiation of reactive powder mixtures

The shock initiation process was directly observed in different powder mixtures that produce little or no gas upon reaction. The samples of reactive powder were contained in recovery capsules that permitted the samples to be analyzed after being shocked and that allowed the initiation of reaction to be monitored using three different methods. The microsecond time-scale processes were observed via a fast two-color pyrometer. Light intensity detected from the bottom of reactive samples was slightly greater compared to inert simulants in the first 10 μs after shock arrival. However, this light was much less intense than that which would correspond to the bulk of the material reacting. Thus it seemed that only small, localized zones, or hot spots, had begun to react on a time scale of less than 30 μs. Light emissions were then recorded over longer time scales, and intense light appeared at the bottom of samples a few milliseconds to a few hundreds of milliseconds after shock arrival at the bottom of the test ...

SHOCK REACTIVITY OF NON‐POROUS MIXTURES OF MANGANESE AND SULFUR

Equimolar mixtures of manganese powder and sulfur were melt‐cast into solid pellets in order to study the mechanism of shock‐enhanced reactivity in non‐porous heterogeneous mixtures. This mixture was selected due to the large exothermic heat release of the manganese‐sulfur reaction (214 kJ/mol), which causes the reaction to be self‐sustaining once initiated. The test samples were placed in planar recovery ampoules and a strong shock was delivered via the detonation of a charge of amine‐sensitized nitromethane. Various shock strengths were achieved by placing different thicknesses of PMMA attenuator discs between the explosive charge and the ampoule. The results confirmed that shock‐induced reactions can be produced in highly non‐porous mixtures. Indeed, the critical shock pressure that caused ignition of the mixture in the ampoule was found to be in the range 2.2–3.0 GPa (pressures were estimated using LS‐DYNA simulations of samples with 100% TMD).

Shock initiation of powder mixtures of aluminum with dense metal oxides

Strong and dense structural reactive materials may be produced by mixing aluminum powders with heavy metal-oxide powders (such as Bi2O3, PbO, Pb3O4, I2O5, etc.). The addition of certain additives to such mixtures, such as V2O5 and B2O3, can lower the softening point of the oxide mixture below the melting point of aluminum. This could lead to the fabrication of dense and nonporous aluminum-metal oxide structural materials. The shock sensitivity of aluminum-metal oxide mixtures was investigated in this work. The minimum shock initiating pressure was obtained for various porous and non-porous aluminum-metal oxide mixtures using the shock recovery technique. Since most reactions of Al in metal oxide mixtures produce little pressure and material velocity changes but large increases in temperatures, thermocouples were used to observe the bulk reaction onset, which relates to the overall reaction rate, in those mixtures. The mixtures tested were found to be very sensitive to shock initiation and their reaction r...

TIME‐RESOLVED TEMPERATURE MEASUREMENTS OF SHOCK INITIATION IN HETEROGENEOUS EXOTHERMIC MIXTURES

It is known that reactive exothermic powder mixtures contained in planar recovery capsules can be initiated by strong shock waves (incident pressures 2–20 GPa). The onset of initiation is difficult to observe and so very few studies report data or evidence regarding the initiation delay or the reaction mechanism. A method to experimentally measure the delay between the time of shock arrival and the time when most of the reactions have taken place using thermocouples was used to investigate various mixtures of interest to the shock initiation research community: Mn‐S, Ti‐Si, Ti‐B. The thermocouple technique provides a reliable measurement of the time at which bulk mixture temperature begins to rise sharply, which is related to the time required for most of the mixture to react. This provides an upper bound on the initiation delay time as well as information regarding the reaction mechanism. The test samples were placed in planar recovery capsules containing thermocouples and a strong shock was delivered vi...

EXPERIMENTAL INVESTIGATION OF SHOCK INITIATION IN MIXTURES OF MANGANESE AND SULFUR

Equimolar mixtures of manganese powder and sulfur at different starting densities were tested in two different types of steel recovery capsules in order to study the shock initiation phenomenon in Self‐Propagating High‐Temperature Synthesis (SHS) mixtures. Two different sizes of Mn particles were used for these experiments, <10 μm and −325 mesh (<44 μm). This mixture was selected due to the large exothermic heat release of the manganese‐sulfur reaction (214 kJ/mol), which causes the reaction to be self‐sustaining once initiated. The test samples were placed in planar recovery capsules and a strong shock was delivered via the detonation of a charge of amine‐sensitized nitromethane. Various shock strengths were achieved by placing different thicknesses of PMMA attenuator discs between the explosive charge and the capsule. The results confirmed that shock‐induced reactions can be produced in highly non‐porous mixtures. It was also found that shock interactions with the side walls of the recovery capsule can ...

CRITICAL DIAMETER PREDICTION FOR STEADY DETONATION IN GASLESS METAL‐SULFUR COMPOSITIONS

Since many heterogeneous mixtures whose reaction products contain no gas are highly exothermic, a possibility exists for steady gasless detonation. Theoretical investigations have focused on approximating the product Hugoniot, which depends to a large extent on the amount of heat released and the volumetric expansion resulting from the reaction. If the product Hugoniot curve lies above the unshocked state on the pressure‐volume plane, the Chapman‐Jouguet tangency criterion gives the detonation velocity. Such Hugoniot analysis assumes that the rate of energy loss is negligible compared to the reaction rate, a condition approached only when the charge diameter is much greater than the critical detonation diameter. For charges of practical dimension, the lateral losses are not negligible. The current study accounts for the competition between lateral losses and reaction rate in order to estimate the critical diameter of a mixture of manganese and sulfur. The reaction rate is to be based on experimental data ...

Experimental technique for direct observation of onset of reaction in shocked powder mixtures

A new experimental technique was developed to directly observe the onset of shock initiation in powder mixtures contained in recovery capsules over time scales ranging from hundreds of nanoseconds to at least hundreds of milliseconds. Simultaneously with a thermocouple embedded in the test mixture to monitor bulk temperature changes, a photomultiplier tube detected light emissions produced by the reacting sample. A window/optic fiber system was developed that remained intact and did not move during the experiment. A polycarbonate window was placed into a steel recovery capsule, which was held solidly in place in a steel anvil designed to protect the fiber optic and to prevent motion of the window. Samples of as-blended 5Ti+3Si powders and ball-milled mixtures 5Ti+3Si were tested in this setup. In all experiments, a weak light emission peak was observed upon shock passage followed by much more intense light beginning a few milliseconds to a few hundreds of milliseconds later. The intense light emissions occurred at approximately the same time as a bulk temperature increase measured with the thermocouple. These results suggest that only a very small fraction of the sample was initiated by the shock.

EFFECT OF PARTICLE MORPHOLOGY ON CRITICAL CONDITIONS FOR SHOCK‐INITIATED REACTIONS IN TITANIUM‐SILICON POWDER MIXTURES

The effect of titanium particle morphology on the shock sensitivity of titanium‐silicon powder mixtures has been investigated experimentally. The powder mixtures were tested in a planar recovery capsule, with the shock loading produced by a high explosive booster charge placed on top of the capsule and a PMMA attenuator. Reactions were not observed for equimolar mixtures of large (95 μm) spherical Ti particles with fine (15 μm) Si particles for incident peak shock pressures of up to 23 GPa, estimated with LS‐DYNA. In contrast, mixtures with fine (40 μm) spherical Ti particles or irregularly‐shaped fine (31 μm) Ti particles exhibited a threshold attenuator thickness, and hence shock pressure, for reaction initiation. Microscopy and microprobe backscatter analysis were used to identify the degree of intermixing between the particles for shock loading just below the reaction threshold. For the largest spherical Ti particles, little particle intermixing was evident. For the finer particles, penetration of Si ...

TIME‐RESOLVED TEMPERATURE MEASUREMENTS OF SHOCK INITIATION IN A MANGANESE‐SULFUR MIXTURE

Previous tests carried out in planar recovery capsules showed that strong shock waves (incident pressures 3–7 GPa), generated by a charge of amine‐sensitized nitromethane and attenuated by a PMMA layer, can initiate reactions in non‐porous equimolar mixtures of manganese and sulfur. The current study focused on the onset of these reactions using time‐resolved temperature measurements. A photomultiplier‐based two‐color pyrometer was used to record sample temperatures shortly after the passage of the shock. An experimental complication encountered when studying shock‐induced reactions in porous energetic materials using pyrometry, i.e. intense light due to large localized heating, can be mostly eliminated if a non‐porous sample is studied. Further, in order to increase the reliability of the pyrometry results, the reactive test mixture (Mn‐S) was chosen to have a high heat of reaction (214 kJ/mol) and thus a high reaction temperature, and baseline tests were performed with inert melt‐cast mixtures of WS2 an...