Halogenated PETN derivatives: interplay between physical and chemical factors in explosive sensitivity.

Determining the factors that influence and can help predict energetic material sensitivity has long been a challenge in the explosives community. Decades of literature reports identify a multitude of factors both chemical and physical that influence explosive sensitivity; however no unifying theory has been observed. Recent work by our team has demonstrated that the kinetics of "trigger linkages" (i.e., the weakest bonds in the energetic material) showed strong correlations with experimental drop hammer impact sensitivity. These correlations suggest that the simple kinetics of the first bonds to break are good indicators for the reactivity observed in simple handling sensitivity tests. Herein we report the synthesis of derivatives of the explosive pentaerythritol tetranitrate (PETN) in which one, two or three of the nitrate ester functional groups are substituted with an inert group. Experimental and computational studies show that explosive sensitivity correlates well with Q (heat of explosion), due to the change in the number of trigger linkages removed from the starting material. In addition, this correlation appears more significant than other observed chemical or physical effects imparted on the material by different inert functional groups, such as heat of formation, heat of explosion, heat capacity, oxygen balance, and the crystal structure of the material.

Microstructural and Sensitivity Changes of Neat, Spray-Dried RDX.

Spray drying has recently gained interest in the high explosives (HE) community for the production of novel nanocomposites and well-controlled particle size distributions. However, there is a dearth of information on spray-dried, neat energetic materials. In this work, we correlate the spray drying production parameters to the resulting microstructure and handling sensitivity properties of neat RDX. We demonstrate the capability to fine-tune the particle size distributions for "nanopowder" spray-dried RDX, as well as larger particle size distributions by simply changing the spray dryer setup. We also investigate other physical and chemical changes that RDX undergoes after being processed with spray drying. We characterize these changes with scanning electron microscopy, X-ray diffraction, ultrahigh-performance liquid chromatography, and small-scale sensitivity tests. Interestingly, although the phase and chemical properties are similar before and after spray drying, small-scale sensitivity testing reveals that size reduction of RDX does not follow the typical HE desensitization trends, generally observed for other energetic materials.

Optomechanical Effects Occurring in a Hybrid Metal-Halide Perovskite Single Crystal Based on Photoinduced Resonant Ultrasound Spectroscopy.

This letter reports optomechanical effects occurring in a hybrid metal-halide perovskite single crystal (MAPbBr3) based on resonant ultrasound spectroscopy (RUS) measurements under continuous wave (CW) laser illumination. The optomechanical effects are a new phenomenon in hybrid perovskite single crystals where the elastic constant of a single crystal is measured by RUS probed under varying excitation conditions. Our studies show that applying a CW laser (405 nm) to the single-crystal face shifts the RUS peaks to higher frequencies by about 1-4% in the perovskite single crystal at room temperature. The light-induced shift of the RUS peaks can be observed only when photoexcitation is occurring, rather than during heating, by positioning the laser wavelength within the optical absorption spectrum. In contrast, positioning the laser wavelength outside of the optical absorption spectrum leads to an absence of RUS peak shifting. Clearly, the laser-light-induced RUS peak shifts shows that the crystal elastic moduli can be changed by photoexcitation, leading to an optomechanical phenomenon via excited states. Essentially, the observed optomechanical phenomenon reflects the fact that the mechanical properties can be optically changed through internal repulsive and attractive force constants by external photoexcitation in a hybrid perovskite single crystal.

Environmental Gating and Galvanic Effects in Single Crystals of Organic-Inorganic Halide Perovskites.

Understanding the impact of environmental gaseous on the surface of organometal halide perovskites (OMHPs) couples to the electronic and ionic transport is critically important. Here, we explore the transport behavior and origins of the gas sensitivity in MAPbBr3 single crystals (SCs) devices using impedance spectroscopy and current relaxation measurements. Strong resistive response occurs when crystals are exposed to different environments. It was shown that SC response to the environment is extremely different at the surface as compared to the bulk due to the disorder surface chemistry. The nonlinear transport properties studied using ultrafast Kelvin probe force microscopy (G-KPFM) to unravel spatio-temporal charge dynamics at SC/electrode interface. The relaxation processes observed in pulse relaxation and G-KPFM measurements along with gas sensitivity of crystals suggest the presence of a triple-phase boundary between environment, electrode, and crystal. Results indicate that the environment is a nontrivial component in the operation of OMHP devices which is reminiscent of fuel cell systems. Furthermore, the triple-phase boundary can play a significant role in the transport properties of OMHPs due to the possibility of the redox processes coupled to the concentration of bulk ionic species. Although instrumental for understanding the device characteristics of perovskites, our studies suggest a new opportunity of coupling the redox chemistry of the Br2-Br- pair that defines the bulk ionic conductivity of MAPbBr3 with the redox chemistry of gaseous (or liquid) environment via a suitable electrocatalytic system to enable new class of energy storage devices and gas sensors.