Thermal Decomposition Mechanism and Energy Release Law of Novel Cyclo-N5--Based Nitrogen-Rich Energetic Salt

High energy barrier hydroxyl radical dissociation mechanism of a low shock sensitivity dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50) explosive

As a representative of the new generation of high-energy explosives, TKX-50 has attracted widespread attention due to its remarkably low sensitivity toward shock. However, the reported decomposition barriers of TKX-50 (∼37 kcal mol-1) are comparable to those of commonly used explosives. The mechanism of its low shock sensitivity remains unclear. In this study, using an ab initio molecular dynamics method combined with a multiscale shock simulation technique and transition state calculations (at the B2PLYP-D3/Def2TZVP level), we discovered an unconventional reaction pathway of TKX-50 under shock, and its rate-controlling step is the dissociation of the hydroxyl radical (OH) from the anion ring after proton transfer, followed by ring rupture and the production of H2O and N2. The barrier for this OH dissociation reaction is as high as 51.9 kcal mol-1. In contrast, under thermal stimuli, TKX-50 prefers to open rings directly after proton transfer without losing the OH. The corresponding barrier is 35.4 kcal mol-1, which is in good agreement with previous studies. The reason for the unconventional reaction pathway of TKX-50 under shock may be the suppression of anion ring opening in thermal decomposition by steric hindrance upon shock compression. In addition, the dominant N2 generation pathway under shock releases less energy than pyrolysis which further explains the low shock sensitivity of TKX-50. This study comprehensively elucidates the different reaction mechanisms of TKX-50 under thermal and shock conditions and proposes a crucial reaction pathway leading to its low shock sensitivity. These findings will contribute to the understanding and application of tetrazole anionic energetic salts.

Initial Decomposition Mechanism of NH3OH+N5- Crystal under Thermal and Shock Loading: A First-Principles Study

NH3OH+N5- is a novel energetic material (EM) which has attracted much interest for its promising performances, including high energy density, high density, low sensitivity, and low toxicity. In this study, the initial decomposition mechanism of NH3OH+N5- crystal was investigated under thermal and shock loading by molecular dynamics simulation. First, programmed heating and constant temperature simulations were carried out by molecular dynamics simulation on the basis of density functional theory (DFT-MD). Results indicated that the initial decomposition reactions of NH3OH+N5- could be described by three reactions: proton transfer, ring-opening reaction, and cation decomposition and recombination, and three pathways of ring-opening reaction were found, including the ring-opening of N5-, HN5, and H2N6. The first two reactions are the main pathways that produce N2 molecules. Furthermore, we carried out DFT-MD simulations to study the shock decomposition behaviors of NH3OH+N5-, and three initial steps were proposed: N5-, HN5, and N6 ring-opening. The fewer N5- and HN5 ring-opening reactions were found during the shock simulation, accompanied by a significant change in the N5- bond angle. What's more, the transition states of decomposition reactions were investigated through quantum chemical calculations. The results revealed that the proton transfer reaction exhibits lower activation barriers compared to ring-opening reactions, and proton transfer would accelerate ring-opening reactions. In addition, the ring-opening reaction is the main energy-releasing reaction in the early stages of the decomposition. This work could promote the comprehension of the decomposition mechanism and energy release regularity of N5- ions.

Insight into the high-temperature oxidation kinetics of acetylene: A first-principles molecular dynamics study

The study on high-temperature oxidation kinetics and kinetic modeling of acetylene (C2H2) has significant importance for its engineering applications. In this paper, the first-principles molecular dynamics method is used to simulate the C2H2 oxidation under high temperatures for the first time. Our results show that there are 38 intermediates and 225 elementary reactions in the process of C2H2 oxidation. The formation mechanisms of "prompt" CO2, as well as gas pollutants CHOCHO and HCOOH are revealed in depth. Four intermediates, CHCHO, CHOCO, CHOCHO and HCOOH, which have significant controversy in current kinetic models, are verified. And a new intermediate, CHOCO2, is discovered. Meanwhile, our simulation also shows that radicals, such as HO2, OH, O, etc. play a key role in promoting the oxidation of intermediates in the early stage of C2H2 oxidation. Combined with quantum chemical calculations, a detailed chemical kinetic model of C2H2/air (FP-C2H2) is built and verified by simulating ignition delay time, species concentration in the flow reactor and premixed laminar flame speed. Our studies provide novel insight for understanding the complex chemical reaction kinetics, and environmental and human health threats from air pollutant formation during C2H2 combustion.

Oxidation kinetic mechanism of n-decane under high temperature and pressure: a first-principles molecular dynamics study

The n-decane/air (C10H22/air) combustion reaction kinetics has attracted much research attention because of its potential application in the aerospace field. In this work, C10H22 oxidation in O2 under high temperature and pressure is simulated based on the first-principles molecular dynamics method for the first time. Our results show that C-C bond breaking and H-abstraction are the two main initial reactions in the oxidation process of C10H22. However, there exists an obvious difference under high and atmospheric pressures. Under high pressure, C-C bond dissociation reactions of hydrocarbon molecules are the main reaction types, while H-abstraction reactions are the main reaction types under atmospheric pressure. The radicals (HO2, OH, O, etc.) play key roles in promoting the oxidation of hydrocarbon molecules. A detailed chemical kinetic model (76 species and 435 elementary reactions), the FP-C10H22 model, of C10H22/air mixture combustion is constructed and verified. The predicted values of FP-C10H22 model on the ignition delay time, laminar flame speed and species concentration of jet stirred reactor (JSR) species concentration are in good agreement with the experimental data.

Pyrolysis Kinetics and Combustion Behaviors of a High-Nitrogen Compound, 4,4′-Azobis(1,2,4-triazole)

To study the thermal decomposition behavior of 4,4′-azobis(1,2,4-triazole) (ATRZ), the non-isothermal thermal decomposition kinetics of ATRZ were studied using the thermogravimetric–differential scanning calorimetry (TG–DSC) method. The TG–DSC of ATRZ was analyzed at heating rates of 5, 10, 15, and 20 K·min⁻¹ in an argon atmosphere. The thermal decomposition kinetic parameters at peak temperature (T_p), such as apparent activation energy (E_a) and pre-exponential factor (lgA) of ATRZ, were calculated using the Kissinger, Ozawa, and Satava–Sestak methods. E_a and lgA calculated using the Kissinger, Ozawa, and Satava–Sestak methods are very close, at 780.2 kJ·mol⁻¹/70.5 s⁻¹, 751.1 kJ·mol⁻¹/71.8 s⁻¹, and 762.1 kJ·mol⁻¹/71.8 s⁻¹, respectively. Using a combination of three methods, the reaction mechanism function g(α) of ATRZ was obtained. The results show that the decomposition temperature of ATRZ is about 310 °C, and the decomposition is rapidly exothermic. The pyrolysis path of ATRZ was investigated through a pyrolysis-gas chromatography mass spectrometry (PY-GC/MS) experiment. ATRZ has three different decomposition paths and finally generates N₂, HC-N-CH, N≡C-N, and HC=N-C≡N. The laser ignition combustion duration of ATRZ was 0.5033 s and the peak temperature was 1913 °C. The laser ignition combustion duration of ATRZ+CL-20 was 1.0277 s and the peak temperature was 2105 °C. The rapid energy release rate of ATRZ promotes the combustion energy release of CL-20.