Prediction and Construction of Energetic Materials Based on Machine Learning Methods.

Energetic materials (EMs) are the core materials of weapons and equipment. Achieving precise molecular design and efficient green synthesis of EMs has long been one of the primary concerns of researchers around the world. Traditionally, advanced materials were discovered through a trial-and-error processes, which required long research and development (R&D) cycles and high costs. In recent years, the machine learning (ML) method has matured into a tool that compliments and aids experimental studies for predicting and designing advanced EMs. This paper reviews the critical process of ML methods to discover and predict EMs, including data preparation, feature extraction, model construction, and model performance evaluation. The main ideas and basic steps of applying ML methods are analyzed and outlined. The state-of-the-art research about ML applications in property prediction and inverse material design of EMs is further summarized. Finally, the existing challenges and the strategies for coping with challenges in the further applications of the ML methods are proposed.

Experimental Investigation of the Self-Ignition and Jet Flame of Hydrogen Jets Released under Different Conditions.

Hydrogen is a promising clean energy source and an important chemical raw material. To use hydrogen energy more safely, a high-pressure hydrogen-release platform for hydrogen self-ignition and for generating hydrogen jet flames under different experimental conditions was investigated in this study. The associated experimental analysis was based on the theory of high-pressure hydrogen tube diffusion. We found that the higher the initial release pressure, the greater the intensity of the leading shock. When the initial release pressure was high and the leading shock intensity was strong, hydrogen was more likely to ignite spontaneously inside the tube. The higher the initial release pressure, the faster the average propagation speed of the shock in the same pipe length. The time during which a stable leading shock was formed inside the tube may be related to the initial release pressure. It was found that flame combustion intensified after the passage of air through a Mach disk, and a stable flame was formed more easily at the jet boundary layer away from the orifice axis. The maximum speed of the flame tip and the flame decay speed were very high. Moreover, the flame length and the diameter of the ball flame first increased and then decreased.

The reaction pathway analysis of phosphoric acid with the active radicals: a new insight of the fire-extinguishing mechanism of ABC dry powder.

Dry powder fire-extinguishing agent is one of Halon substitutes due to its superior fire-extinguishing performance, non-toxicity, and environmental friendliness. As one of the most widely used dry powders, ABC dry powder has attracted wide attention. Understanding its reaction mechanism is important to the design of more efficient compound dry powder based on it. When ABC dry powder was applied to the flame, ammonium dihydrogen phosphate (the main fire-extinguishing component of ABC dry powder) would rapidly decompose into phosphoric acid (H3PO4) and ammonia. Therefore, in order to figure out the chemical reaction mechanism of ABC dry powder and active radicals, the main focus of this paper is on the H3PO4. Analysis of the electrostatic potential on van der Waals surface of H3PO4 was carried out. Besides, detailed theoretical investigation has been performed on the mechanism, kinetics, and thermochemistry of the reactions of H3PO4 with H, OH, and CH3 radicals and further decomposition of H3PO4 using M06-2X/6-311G(d,p)//CCSD(T)/cc-pVTZ level of theory. Mayer bond order for all intrinsic reaction coordinate points was also calculated. Finally, it is theoretically proved that ABC dry powder extinguishes the fire mainly by chemical inhibition on H and OH radicals. Grapical Abstract .

Pressure-decay and thermodynamic characteristics of subcooled liquid in the tank and their interaction with flashing jets.

The accidental releases of superheated liquid from tanks or pipes involve violent phase transformation, which would form two-phase releases and generate catastrophic flashing jets. In this work, the pressure and temperature of superheated liquid inside a 20 L tank with different storage pressures, superheats, and nozzle diameters were measured. The characteristics of flashing jets were characterized by measuring the evolution of droplet diameter and three-dimensional (3D) velocities using the Phase Doppler Anemometry (PDA). Results show that the pressure undergoes three phases during release: first rapid decrease phase, slow decrease phase and second rapid decrease phase. The pressure decreases to a critical pressure at the termination of the first phase, and the liquid reaches a critical saturation state and boils. Abundant vapor generated in the tank due to the boiling compensates for the pressure loss, causing the slow decrease of the pressure at the second phase. The critical saturation state is mainly dependent on superheat. 3D velocities and diameter of droplet in flashing jets are mainly controlled by the pressure. Boiling of the superheated liquid under critical saturation state has a vital influence on the dynamic process of two-phase releases, which accelerates the development of flashing jet.