HMX and LLM-105 are well known energetic materials with, respectively, high performance and low sensitivity. In this work, we present a Spray Flash Evaporation (SFE) based process to form a HMXSFE material as well as a novel energetic material containing HMX and LLM-105 (“HMX/LLM-105SFE”), with a peculiar crystalline composition. These materials contain, respectively, notable amounts of α-HMX and an intimate mix between HMX and LLM-105 with distinct crystalline structures retained. Their sensitivity was evaluated, allowing exploration of the complexity of mixed materials' mechanical sensitivity and the potentialities of the SFE process for both industrial applications and fundamental research purposes.

Spray atomization, particularly spray flash evaporation (SFE), exhibited interesting properties such as a small droplet size and rapid droplet evaporation, which facilitate the recrystallization of energetic materials (EMs) with enhanced physicochemical properties. This study, for the first time, explores the effectiveness of the drying stage after atomization in the SFE technique in removing the solvent and altering the physicochemical properties (crystallinity, size, morphology, and performance) of cyclotrimethylenetrinitramine (RDX), a widely used explosive, when obtained at 160 °C and 40 bar. Two different and successive drying steps were applied to particles. They were first dried directly in the SFE process for a short period, about 1 min following the end of atomization, at a very low pressure of 11 mbar and under dynamic vacuum. Then, particles were exposed to a second ex situ drying under 24 h at 800 mbar and 40 °C. It was found that SFE reduced the particle size as well changing the particle morphology and conserving the crystalline phase. Particles exhibited less regular, nonspherical morphologies, bridge-like structures, and some displayed cavities, likely resulting from buckling associated with fast evaporation and crystallization processes. Particles also showed the presence of a residual solvent (acetone). Unexpectedly, even the second drying process failed to fully eliminate residual acetone, which probably remained trapped within the particles. Results uncover an Ostwald ripening where prolonged drying led to particle growth and cavity disappearance, likely accentuated by the presence of residual solvent, promoting RDX diffusion on the particle surface. Importantly, the morphology and particle size also dictate the sensitivity. Smaller RDX particles with significant surface defects, produced by SFE and drying during a short time, exhibit a lower impact and friction threshold. The second drying step, restoring near-spherical morphology and removing morphology defects while maintaining a lower particle size compared to that of the raw material, raised the impact threshold to 8.5 J while leaving friction and electrostatic discharge sensitivities mainly unchanged. These results pave the way to advance the spray atomization process in EM recrystallization, with a particular emphasis on the final drying stage. Notably, the presence of even trace amounts of residual solvent within the RDX particles can highly influence their physicochemical properties, underscoring the need for precise control of the drying after atomization to optimize particle quality and performance.

Nitroglycerin (NG) is a high explosive that is difficult to handle in its liquid form, where the entrapment of gaseous NG bubbles results in its high sensitivity to impact (<0.25 J). NG-based materials are traditionally prepared by complex mixing processes where desensitisation is of major concern. Instead of using a mixing procedure here we demonstrate the successful synthesis of dynamite-like materials by using a single step bottom-up approach to form a polymeric host matrix (PM) for NG from a solution which contains the explosive and the matrix precursors. For this purpose, an epoxidized vegetable oil mixed with nitroglycerin (oil to NG mass ratio of 60/40) is polymerized at 100 & DEG;C for 30 h by using glutaric acid as crosslinking agent. No degradation of NG under these conditions is observed. The activation energy of NG desorption from the PM (51.9 kJ/mol) is of the same order of magnitude as the one reported in literature for a double-base powder (81.9 kJ/mol). An accelerated NG desorption is observed as temperature increases. This can be attributed to a slow diffusion mechanism of the explosive from the volume to the surface of the material, where it evaporates. The ignition of NG/PM in air by an open flame leads to a self-sustained combustion, in which a part of nitroglycerin decomposes in the polymer matrix. Conversely, NG/PM monolithic and granular loads are not ignited by the explosion of the primer in a 9 mm calibre casing. The shockwave released by a detonator on a small NG/PM cylindrical charge (& AP;1.5 g) does not detonate it, but only makes it deflagrate. Our results show that this novel single-step synthesis of NG trapped in a polymeric matrix is a very effective approach for desensitizing it to any form of stress.

Submicron-sized powders of silver azide (AgN3) were prepared by Curtius' reaction between silver nitrate (AgNO3) and sodium azide (NaN3) in aqueous solutions by a new process: the Spray Flash Synthesis (SFS). The SFS process consists in spraying the two precursor solutions in a heated atomization chamber (130-190 degrees C), maintained under low vacuum (15-30 kPa). The reaction occurs in the droplets which have collided; the final size of particles is limited by the fast evaporation of water and the small amount of matter available in each droplet which can be considered as an individual micro-reactor. The mean particle sizes of silver azide synthesized by the SFS process range from 220 nm to 390 nm, which means that these particles are three times smaller than those obtained by the conventional precipitation method. Submicron-sized AgN3 powders can be initiated by a photographic flash.

The cover image is based on the Research Article Synthesis of submicron-sized silver azide by multi-nozzle spray flash synthesis by Guillaume Galland et al., https://doi.org/10.1002/prep.202300018.

Melamine (Mel) was used as host matrix for liquid nitroglycerin (NG), to prepare Mel/NG solid powdered compounds containing up to 45 wt% of this explosive. The two preparation processes used for this purpose consisted in evaporating a solution of both components, either in ambient conditions or under reduced pressure by the Spray Flash-Evaporation (SFE) process. In Mel/NG materials, amorphous nitroglycerin is distributed in the crystallized melamine matrix as inclusions, which were found to be smaller in size in the material prepared by the SFE process. Mel/NG materials are not stable over time: they gradually lose the nitroglycerin they contain by evaporation.