Learning to fly: thermochemistry of energetic materials by modified thermogravimetric analysis and highly accurate quantum chemical calculations

Too fast and too furious: thermoanalytical and quantum chemical study of the thermal stability of 4,4'-dinitro-3,3'-diazenofuroxan

Novel energetic materials (EM) often combine two intrinsically counter trends, viz., a high energy density and mediocre safety parameters, like thermal stability and sensitivity toward mechanical stimuli. A rational design of promising EMs requires a proper understanding of their thermal stability at both macroscopic and molecular levels. In the present contribution, we studied in detail the thermal stability of 4,4'-dinitro-3,3'-diazenofuroxan (DDF), an ultrahigh-performance energetic material with a reliable experimental detonation velocity being very close to 10 km s-1. To this end, we employed a set of complementary thermoanalytical (DSC and TGA in the solid state along with advanced thermokinetic models, optical microscopy, and gas products detection) and theoretical techniques (DLPNO-CCSD(T) quantum chemical calculations). According to the DSC measurements, the solid-state thermolysis of DDF turned out to be a complex three-step process. The decomposition commences at ∼85 °C and the most intense heat release occurs at ∼130 °C depending on the heating rate. In order to properly describe the kinetics of DDF thermolysis beyond the simple Kissinger and Friedman methods, we applied a "top-down" kinetic approach resulting in the formal model comprised of three independent stages. A flexible Kolmogorov-Johnson-Mehl-Avrami-Erofeev equation was applied for the first decomposition stage along with the extended Prout-Tompkins equation for the second and third processes, respectively. The formal exponent in the former equation turned out to be close to a second order, thus suggesting a two-dimensional nuclei-growth model for the first stage. We rationalized this fact with the aid of optical microscopy experiments tracking the changes in the morphology of a solid DDF sample. Then, we complemented the formal macroscopic kinetics with some mechanistic patterns of the primary decomposition channels from quantum chemical calculations. The three reactions involving all important moieties of the DDF molecule turned out to compete very closely: viz., the nitro-nitrite isomerization, radical C(heterocycle)-N(bridge) bond scission and molecular decomposition comprised of the consequent N-O and C-C bond scissions in a furoxane ring. The DLPNO-CCSD(T) activation barriers of all these reactions were close to ∼230 kJ mol-1. Most importantly, the calculations provide some mechanistic details missing in thermoanalytical experiment and formal kinetic models. Apart from this, we also determined a mutually consistent set of thermochemical and phase change data for DDF.

First Alliance of Pyrazole and Furoxan Leading to High-Performance Energetic Materials

Nitrogen heterocyclic scaffolds retain their leading position as valuable building blocks in material science, particularly for the design of small-molecule energetic materials. However, the search for more balanced combinations of directly linked heterocyclic cores is far from being exhausted and aims to reach ideally balanced high-energy substances. Herein, we present the synthetic route to novel pyrazole-furoxan framework enriched with nitro groups and demonstrate a promising set of properties, viz., good thermal stability, acceptable mechanical sensitivity, and high detonation performance. In-depth crystal analysis showed that the isomers having lower-impact sensitivity values in both types of regioisomeric pairs are those with the exocyclic furoxan oxygen atom being closer to the pyrazole ring. Owing to the favorable combination of high crystal densities (1.83-1.93 g cm-3), positive oxygen balance to CO (up to +13.9%), and high enthalpies of formation (322-435 kJ mol-1), the synthesized compounds show high calculated detonation velocities (8.4-9.1 km s-1) and excellent metal accelerating abilities. The incorporation of the 3-nitrofuroxan moiety increases the thermal stability (by ca. 20 °C) and decreases the mechanical sensitivity of target hybrid materials in both types of regioisomeric pairs. Simultaneously, the detonation performance of 3-nitrofuroxans is almost identical to that of 4-nitrofuroxans, highlighting the potential of the regioisomeric tunability in the future design of energetic materials.

2,2'-Bisdinitromethyl-5,5'-bistetrazole: A High-Performance, Multi-Nitro Energetic Material with Excellent Oxygen Balance

Bistetrazoles are highly sought after for developing innovative high-energy density materials. The 1,1'-substituted bistetrazoles, exemplified by TKX-50, have outstanding performance. However, the research of high-perfomance 2,2'-substituted bistetrazoles remains limited. In this work, dinitromethyl groups were introduced into bistetrazole structures as 2,2'-substituted bistetrazoles (BDBTZ), which was extensively characterized through NMR, thermal analysis, and single crystal X-ray diffraction, exhibiting excellent oxygen balance, moderate sensitivity, acceptable thermal stability, high crystal density, and excellent detonation performance.

Thermal Hazard Analysis of Two Non-Ideal Explosives Based on Ammonium Perchlorate/Ammonium Nitrate and Aluminium Powder

In recent years, various kinds of civil explosive detonation accidents have occurred frequently around the world, resulting in substantial human casualties and significant property losses. It is generally believed that thermal stimulation plays a critical role in triggering the detonation of explosives; consequently, the study of the thermal hazards of explosives is of great significance to many aspects of safety emergency management practices in the production, transportation, storage, and use of explosives. It is known that the thermal stability of the ammonium perchlorate-aluminium system and the ammonium nitrate-aluminium system has been extensively investigated previously in the literature. However, there is a paucity of research on the thermal hazard characteristics of non-ideal explosives under varying oxygen balance conditions within the academic sphere. Therefore, this research focused on the study of the thermal hazards of non-ideal explosives based on thermokinetic analysis. The thermal hazards of non-ideal explosive mixtures of ammonium perchlorate and aluminium and of ammonium nitrate and aluminium were studied by thermal analysis kinetics. The thermokinetic parameters were meticulously studied through differential scanning calorimetry (DSC) analysis. The results showed that the peak reaction temperature and activation energy of the ammonium perchlorate-aluminium system were significantly higher than those of the ammonium nitrate-aluminium system. Under the condition of zero oxygen balance, the peak reaction temperature of the ammonium nitrate-aluminium system was 259 °C (heating rate 5 °C/min), and the activation energy was 84.7 kJ/mol. Under the same conditions, the peak reaction temperature and activation energy of the ammonium perchlorate-aluminium system were 292 °C (heating rate 5 °C/min) and 94.9 kJ/mol, respectively. These results indicate that the ammonium perchlorate-aluminium system has higher safety under the same thermal stimulation conditions. Furthermore, research on both non-ideal explosive systems reveals that the activation energy is at its peak under negative oxygen balance conditions, recorded at 104.2 kJ/mol (ammonium perchlorate-aluminium) and 86.2 kJ/mol (ammonium nitrate-aluminium), which indicates a higher degree of safety. Therefore, the investigation into the thermal hazards of non-ideal explosive systems under different oxygen balance conditions is of utmost importance for the enhancement and improvement of safety emergency management practices.

Manipulating nitration and stabilization to achieve high energy

Nitro groups have played a central and decisive role in the development of the most powerful known energetic materials. Highly nitrated compounds are potential oxidizing agents, which could replace the environmentally hazardous used materials such as ammonium perchlorate. The scarcity of azole compounds with a large number of nitro groups is likely due to their inherent thermal instability and the limited number of ring sites available for bond formation. Now, the formation of the first azole molecule bonded to seven nitro groups, 4-nitro-3,5-bis(trinitromethyl)-1H-pyrazole (4), by the stepwise nitration of 3,5-dimethyl-1H-pyrazole is reported. Compound 4 exhibits exceptional physicochemical properties with a positive oxygen balance (OBCO2 = 13.62%) and an extremely high calculated density (2.04 g cm-3 at 100 K). This is impressively high for a C, H, N, O compound. This work is a giant step forward to highly nitrated and dense azoles and will accelerate further exploration in this challenging field.

Enthalpy of formation of 6-phenyl-1,5-diazabicyclo[3.1.0]hexane by combustion calorimetry and theoretical approach for efficient prediction of thermochemistry of diaziridines

The combustion energy and standard molar enthalpy of formation of crystalline 6-phenyl-1,5-diazabicyclo[3.1.0]hexane (PDABH) were determined using an isoperibolic calorimeter with a static bomb. PDABH is the first diaziridine for which the experimental value of the enthalpy of formation was obtained. This value was validated by the theoretical values of gas phase enthalpy of formation and enthalpy of sublimation. The gas phase enthalpy of formation was calculated using the DLPNO-CCSD(T1)/CBS method in conjunction with isodesmic-type reactions. This method was chosen in comparison to another high quality evaluative method (G4), which has been shown to provide unreliable results for cyclic nitrogen containing compounds. The descriptors of the molecular electrostatic potential (MEP) were used to estimate the enthalpy of sublimation of PDABH. The proposed MEP model is based on experimental enthalpies of sublimation for 75 compounds structurally similar to PDABH. The high-level ab initio calculations of gas phase enthalpies of formation combined with enthalpies of sublimations estimated using descriptors of MEP allow predicting the enthalpies of formation of diaziridines in the solid phase.

Isothermal structural evolution of CL-20/HMX cocrystals under slow roasting at 190 °C

As a new type of energetic material, cocrystal explosives demonstrate many excellent properties, such as high energy density and low sensitivity, due to the interaction between the molecules of the two components. The known decomposition temperature is 235 °C for CL-20/HMX cocrystals at a faster heating rate. CL-20 molecules could separate from the cocrystal matrix and decompose at a higher temperature, much lower than the decomposition temperature. The current work provided deep insight into the isothermal structural evolution of CL-20/HMX cocrystals with slow roasting at 190 °C. We found that the initial decomposition originates from separating CL-20 molecules from the surface along the (010) plane of the cocrystals. The gas products, such as NO₂ and NO, escape from the largest exposed surface of the (010) plane and generates microbubbles and microholes. At the same time, the residual HMX molecules form δ-phase HMX crystals and shrink the volume by 72%. By increasing the time held at 190 °C, the decomposition of CL-20 molecules and recrystallization of the residual HMX molecules form a gully-like structure on the (010) plane of the CL-20/HMX cocrystal. After a long time at 190 °C, the CL-20 component completely decomposes, and all HMX molecules recrystallize in the δ-HMX form. The interaction between HMX and CL-20 molecules makes the decomposition rate of the CL-20/HMX cocrystal much slower than that of the CL-20 pure crystal with a similar decomposition activation energy during isothermal heating. This work can help to deeply understand the safety of CL-20/HMX cocrystal explosives at a temperature lower than the recognized decomposition temperature.

Synthesis, Characterization, and Properties of High-Energy Fillers Derived from Nitroisobutylglycerol

Herein we report a comprehensive laboratory synthesis of a series of energetic azidonitrate derivatives (ANDP, SMX, AMDNNM, NIBTN, NPN, 2-nitro-1,3-dinitro-oxypropane) starting from the readily available nitroisobutylglycerol. This simple protocol allows obtaining the high-energy additives from the available precursor in yields higher than those reported using safe and simple operations not presented in previous works. A detailed characterization of the physical, chemical, and energetic properties including impact sensitivity and thermal behavior of these species was performed for the systematic evaluation and comparison of the corresponding class of energetic compounds.

Thermochemistry, Tautomerism, and Thermal Stability of 5,7-Dinitrobenzotriazoles

Nitro derivatives of benzotriazoles are safe energetic materials with remarkable thermal stability. In the present study, we report on the kinetics and mechanism of thermal decomposition for 5,7-dinitrobenzotriazole (DBT) and 4-amino-5,7-dinitrobenzotriazole (ADBT). The pressure differential scanning calorimetry was employed to study the decomposition kinetics of DBT experimentally because the measurements under atmospheric pressure are disturbed by competing evaporation. The thermolysis of DBT in the melt is described by a kinetic scheme with two global reactions. The first stage is a strong autocatalytic process that includes the first-order reaction (Ea1I = 173.9 ± 0.9 kJ mol−1, log(A1I/s−1) = 12.82 ± 0.09) and the catalytic reaction of the second order with Ea2I = 136.5 ± 0.8 kJ mol−1, log(A2I/s−1) = 11.04 ± 0.07. The experimental study was complemented by predictive quantum chemical calculations (DLPNO-CCSD(T)). The calculations reveal that the 1H tautomer is the most energetically preferable form for both DBT and ADBT. Theory suggests the same decomposition mechanisms for DBT and ADBT, with the most favorable channels being nitro-nitrite isomerization and C–NO2 bond cleavage. The former channel has lower activation barriers (267 and 276 kJ mol−1 for DBT and ADBT, respectively) and dominates at lower temperatures. At the same time, due to the higher preexponential factor, the radical bond cleavage, with reaction enthalpies of 298 and 320 kJ mol−1, dominates in the experimental temperature range for both DBT and ADBT. In line with the theoretical predictions of C–NO2 bond energies, ADBT is more thermally stable than DBT. We also determined a reliable and mutually consistent set of thermochemical values for DBT and ADBT by combining the theoretically calculated (W1-F12 multilevel procedure) gas-phase enthalpies of formation and experimentally measured sublimation enthalpies.

Thermal stability of emerging N6-type energetic materials: kinetic modeling of simultaneous thermal analysis data to explain sensitivity trends

A number of new high-performing energetic materials possess explosophoric functionalities, high nitrogen content, and fused heterocyclic blocks. Two representatives of these materials have been synthesized recently, namely, 1,2,9,10-tetranitrodipyrazolo[1,5-d:5',1'-f][1,2,3,4]-tetrazine (1) and 2,9-dinitrobis([1,2,4]triazolo)[1,5-d:5',1'-f][1,2,3,4]tetrazine (2). The thermal stability of these energetic materials bearing the N-N-N = N-N-N fragment and three closely related compounds has been investigated for the first time. The thermal decomposition process of analyzed compounds was complicated by the appearance of the liquid phase, sublimation of the material, and autocatalysis by reaction products. In contrast to the traditional approach to the kinetic modeling based on data from either TGA or DSC, we use both signals' data measured at the same time and perform the joint kinetic analysis using the model-fitting technique to obtain the pertinent kinetic description of the process. Of the analyzed materials, 1 and 2 show the lowest thermal stability in melt with a characteristic rate constant of 2.6 × 10^-3 s^-1 at 250 °C. The kinetic parameters and calculated detonation performance data were used in the model to describe the mechanical sensitivity. The model output and the experimental friction sensitivity data show a respectable agreement, but more data are required to draw firm conclusions. In general, the provided thermal stability and kinetic data can be used for thermal response and storage modeling of these new N6-type energetic materials. The developed thermokinetic approach, joint model-fitting of several thermal analysis signals, can be applied to other complex thermally induced processes to increase the value and credibility of the kinetic findings.

Energetic [1,2,5]oxadiazolo [2,3-a]pyrimidin-8-ium Perchlorates: Synthesis and Characterization

A convenient method to access the above perchlorates has been developed, based on the cyclocondensation of 3-aminofurazans with 1,3-diketones in the presence of HClO₄. All compounds were fully characterized by multinuclear NMR spectroscopy and X-ray crystal structure determinations. Initial safety testing (impact and friction sensitivity) and thermal stability measurements (DSC/DTA) were also carried out. Energetic performance was calculated by using the PILEM code based on calculated enthalpies of formation and experimental densities at r.t. These salts exhibit excellent burn rates and combustion behavior and are promising ingredients for energetic materials.

Decoding hexanitrobenzene (HNB) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as two distinctive energetic nitrobenzene compounds by machine learning

Energetic materials (EMs) are a group of special energy materials, and it is generally full of safety risks and generally costs much to create new EMs. Thus, machine learning (ML)-aided discovery becomes highly desired for EMs, as ML is good at risk and cost reduction. This work decodes hexanitrobenzene (HNB) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as two distinctive energetic nitrobenzene compounds by ML, in combination with theoretical calculations. Based on a series of highly accurate models of density, heat of formation, bond dissociation energy and molecular flatness, the ML predictions show that HNB is the most energetic among ∼370 000 000 single benzene ring-containing compounds, while TATB possesses a moderate energy content and very high safety, as determined experimentally. This work exhibits the significant power of ML and presents an instructive procedure for using it in the field of EMs. The ML-aided design and highly efficient synthesis and fabrication combined strategy is expected to accelerate the discovery of new EMs.

Chemical Evaluation and Performance Characterization of Pentaerythritol Tetranitrate (PETN) under Melt Conditions

Pentaerythritol tetranitrate (PETN) is a nitrate ester explosive commonly used in commercial detonators. Although its degradation properties have been studied extensively, very little information has been collected on its thermal stability in the molten state due to the fact that its melting point is only ∼20 °C below its onset of decomposition. Furthermore, studies that have been performed on PETN thermal degradation often do not fully characterize or quantify the decomposition products. In this study, we heat PETN to melt temperatures and identify thermal decomposition products, morphology changes, and mass loss by ultrahigh-pressure liquid chromatography coupled to quadrupole time of flight mass spectrometry, scanning electron microscopy, nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. For the first time, we quantify several decomposition products using independently prepared standards and establish the resulting melting point depression after the first melt. We also estimate the amount of decomposition relative to sublimation that we measure through gas evolution and evaluate the performance behavior of the molten material in commercial detonator configurations.

Mechanical stimulation of energetic materials at the nanoscale

The initiation of energetic materials by mechanical stimuli is a critical stage of their functioning, but remains poorly understood. Using atomic force microscopy (AFM) we explore the microscopic initiation behavior of four prototypical energetic materials: 3,4-dinitropyrazole, ε-CL-20, α-PETN and picric acid. Along with the various chemical structures, these energetic compounds cover a range of application types: a promising melt-cast explosive, the most powerful energetic compound in use, a widespread primary explosive, and a well-established nitroaromatic explosive from the early development of energetics. For the softest materials (picric acid and 3,4-dinitropyrazole), the surfaces were found to behave dynamically, quickly rearranging in response to mechanical deformation. The pit created by nanoscale friction stimulation on the surface of 3,4-dinitropyrazole doubled in volume upon aging for half an hour. Over the same time frame, a similar pit on the picric acid surface increased in volume by more than seven-fold. Remarkably, increased humidity was found to reduce the rate of surface rearrangement, potentially offering an origin for the desensitization of energetic materials when wetted. Finally, we identify an inverse correlation between the surface dynamics and mechanical sensitivity of our test energetic compounds. This strongly suggests that surface dynamics influence a material's ability to dissipate excess energy, acting as a buffer towards mechanical initiation.

Quest: structure and properties of BTF–nitrobenzene cocrystals with different ratios of components

Using the methods of quantum chemistry and AAP, the structure of BTF cocrystals with nitrobenzene, 1,2-, 1,3-, 1,4-dinitrobenzene, 1,3,5-trinitrobenzene, and hexanitrobenzene with different ratios of components (1 : 1, 1 : 2, 1 : 3, 2 : 1, 3 : 1) is modeled.