Synthesis, Characterization, and Properties of Heat-Resistant Energetic Materials Based on C-C Bridged Dinitropyrazole Energetic Materials.

Polycyclic energetic materials make up a distinctive class of conjugated structures that consist of two or more rings. In this work, 1,3-bis(3,5-dinitro-1H-pyrazol-4-yl)-4,6-dinitrobenzene (BDPD) was synthesized and investigated in detail as a polycyclic heat-resistant energetic molecule that can be deprotonated by bases to obtain its anionic (3-5) salts. All compounds were thoroughly characterized by 1H and 13C NMR, infrared spectroscopy, high-resolution mass spectrometry, and elemental analysis. The structural features of BDPD and its salts were investigated by single-crystal X-ray diffraction and analyzed by different kinds of computing software, like Multiwfn, Gaussian 09W, and so on. In addition, their thermal decomposition temperatures were evaluated by differential scanning calorimetry to be 319.8-329.0 °C, revealing that they possessed high thermal stabilities. The results of impact sensitivity and friction sensitivity analysis confirm that these energetic compounds were insensitive. The detonation properties of neutral compound BDPD and all its nonmetallic salts were calculated by the EXPLO5 v6.05.04 program. The results revealed that their detonation performances were higher than those of the widely used heat-resistant explosive 2,2',4,4',6,6'-hexanitrostilbene (HNS). Combining the above results, it is reasonable to suggest that these compounds have the potential to be heat-resistant energetic materials.

Progress of charge carrier dynamics and regulation strategies in 2D CxNy-based heterojunctions.

Two-dimensional carbon nitrides (CxNy) have gained significant attention in various fields including hydrogen energy development, environmental remediation, optoelectronic devices, and energy storage owing to their extensive surface area, abundant raw materials, high chemical stability, and distinctive physical and chemical characteristics. One effective approach to address the challenges of limited visible light utilization and elevated carrier recombination rates is to establish heterojunctions for CxNy-based single materials (e.g. C2N3, g-C3N4, C3N4, C4N3, C2N, and C3N). The carrier generation, migration, and recombination of heterojunctions with different band alignments have been analyzed starting from the application of CxNy with metal oxides, transition metal sulfides (selenides), conductive carbon, and Cx'Ny' heterojunctions. Additionally, we have explored diverse strategies to enhance heterojunction performance from the perspective of carrier dynamics. In conclusion, we present some overarching observations and insights into the challenges and opportunities associated with the development of advanced CxNy-based heterojunctions.

Enhancing Conjugation Effect to Develop Nitrogen-Rich Energetic Materials with Higher Energy and Stability.

This work reports a strategy by enhancing conjugation effect and synthesizes a symmetrical and planar compound, 1,2-bis (4,5-di(1H-tetrazol-5-yl)-2H-1,2,3-triazol-2-yl)diazene (NL24). The incorporation of azo and 1,2,3-triazole moieties manifests a synergistic effect, amplifying the conjugation effect of the azo bridge and thereby elevating the stability of NL24 (Td: 263 °C, IS: 7 J). Notably, NL24, possessing a structural configuration comprising four tetrazoles harboring a total of 24 nitrogen atoms, exhibits excellent detonation performances (ΔHf: 6.06 kJ g-1, VD: 9002 m s-1). This strategy achieves the balance of energy and stability of polycyclic tetrazoles and provides a direction for high-performance energetic materials.

Thermodynamic Analysis and Pyrolysis Mechanism of 4,4'-Azobis-1,2,4-triazole.

The nonisothermal thermal decomposition kinetics of 4,4'-azobis-1,2,4-triazole (ATRZ) at different heating rates (5, 10, 15, and 20 °C·min-1) were investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC) studies. The thermal decomposition kinetic parameters such as apparent activation energy (E) and pre-exponential factor (A) were calculated by the Kissinger, Ozawa, and Satava-Sestak methods. The E and A values calculated by the above three methods are very close, which are 391.1 kJ·mol-1/1034.92 s-1, 381.1 kJ·mol-1/1034.30 s-1, and 393.4 kJ·mol-1/1035.76 s-1, respectively. Then, the decomposition mechanism function of ATRZ is analyzed by the calculated results. The results show that the decomposition temperature of ATRZ is about 300 °C and the exothermic decomposition speed is fast. The decomposition pathway of ATRZ was analyzed by pyrolysis-gas chromatography-mass spectrometry (PY-GC-MS). The thermal decomposition kinetic equation of the ATRZ was deduced.

Polyvalent Ionic Energetic Salts Based on 4-Amino-3-hydrazino-5-methyl-1,2,4-triazole.

The synthesis of the new energetic material 4-amino-3-hydrazino-5-methyl-1,2,4-triazole, which shows excellent performance and reliable safety, has drawn attention recently. To fully characterize this material, a comprehensive analysis was performed using various techniques, including differential scanning calorimetry (DSC), infrared spectroscopy (IR), elemental analysis, and 1H and 13C NMR spectroscopy. Additionally, three compounds, 3, 5 and 9, were further characterized using single X-ray diffraction. The X-ray data suggested that extensive hydrogen bonds affect molecular structure by means of intermolecular interactions. In order to evaluate the explosive properties of these synthesized compounds, detonation pressures and velocities were calculated using EXPLO5 (V6.01). These calculations were carried out utilizing experimental data, including density and heat of formation. Among the explosives tested, compounds 7 and 8 exhibited zero oxygen balance and demonstrated exceptional detonation properties. Compound 7 achieved the highest recorded detonation pressure, at 34.2 GPa, while compound 8 displayed the highest detonation velocity, at 8887 m s-1.

Lithium-Promoted Formation of M-2AZTO-Li (M = N2H5+ or NH3OH+ and AZTO = Anion of 1-Hydroxytetrazole-5-hydrazide)-Type "Quaternary" Complexes with Nitrogen-Rich Characteristics: Construction of Novel Insensitive Energetic Materials.

Lithium-based nitrogen-rich complexes are important research objects in the field of high-energy materials. However, the weak coordination abilities of lithium ions relative to those of other metal ions with greater atomic numbers have hindered their applications in the field of nitrogen-rich complexes. Herein, we successfully prepared novel lithium-based nitrogen-rich complexes (N2H5-2AZTO-Li and NH3OH-2AZTO-Li) by exploiting the structural properties of 1-hydroxytetrazolium-5-hydrazine (HAZTO). Both N2H5-2AZTO-Li and NH3OH-2AZTO-Li were found to exhibit physicochemical parameters (including the density, stability, and energetic properties) that were intermediate between those of the simple ionic compounds (3 and 4) and the complexes (5) that formed them, enabling a favorable balance between high energy, high stability, and environmental friendliness (for N2H5-2AZTO-Li: detonation velocity (D) = 9005 m s-1, detonation pressure (P) = 35.5 GPa, decomposition temperature (Tdec) = 238.1 °C, impact sensitivity (IS) = 24 J, friction sensitivity (FS) = 210 N, and detonation product (DP) (CO) < 2%; for NH3OH-2AZTO-Li: D = 9028 m s-1, P = 35.7 GPa, Tdec = 211.2 °C, IS = 20 J, FS = 180 N, and DP (CO) < 2%). This study transcends the conventional structural forms of nitrogen-rich complexes, opening new horizons for the design of novel insensitive energetic materials.

A series of N-trinitromethyl-substituted polynitro-pyrazoles: high-energy-density materials with positive oxygen balances.

An N-trinitromethyl strategy was employed for the synthesis of polynitro-pyrazole based high-energy-density compounds with great potential as energetic materials. The new compounds were characterized by 1H and 13C NMR, IR spectroscopy, elemental analysis, differential scanning calorimetry, and single-crystal X-ray diffraction. Compound 10 exhibits high energetic properties, has a positive oxygen balance (OB) of +2.1%, and an excellent specific impulse (272.4 s), making it a potential high-energy dense oxidizer to replace AP in solid rocket propellants. The nitration of 7 with HNO3/H2SO4 yielded the green primary explosive 12, which showed higher density, higher performance, better oxygen balance and lower sensitivities to those of currently used diazodinitrophenol. Compound 13 is a nitrogen and oxygen rich secondary explosive with a high OB (+5.0%), comparable energy (D = 9030 m s-1; P = 35.6 GPa; η = 1.03) to HMX, and much lower mechanical sensitivity (IS = 12 J, FS = 240 N).

Density functional theory studies on N4 and N8 species: Focusing on various structures and excellent energetic properties.

All-nitrogen materials, as a unique branch of energetic materials, have gained huge attentions, of which cyclo-N 5 - derivatives are the representative synthetically reported materials. However, the energetic performance of cyclo-N 5 - compounds has certain limitations and cannot go beyond that of CL-20. In order to reach the higher energy, in this work, we presented two kinds of polynitrogen species, N4 and N8. Two isomers of N4 and four isomers of N8 were fully calculated by using density functional theory (DFT). Theoretical results show that all these polynitrogen materials exhibit excellent heats of formation (7.92-16.60 kJ g-1), desirable detonation performance (D: 9766-11620 m s-1; p: 36.8-61.1 GPa), as well as the remarkable specific impulses (330.1-436.2 s), which are much superior to CL-20. Among them, N 4 -2 (tetraazahedrane) (D: 10037 m s-1; p: 40.1 GPa; Isp: 409.7 s) and cube N 8 -4 (D: 11620 m s-1; p: 61.1 GPa; Isp: 436.2 s) have the highest energetic properties, which are expected to become promising high-energy-density-materials. Moreover, electrostatic surface potentials, Frontier molecular orbitals, infrared spectra, natural bond orbital charges, and weak interactions were also investigated to further understand their relationship between structure and performance.

Nitrogen-rich ion salts of 1-hydroxytetrazole-5-hydrazide: a new series of energetic compounds that combine good stability and high energy performance.

High-efficiency explosives that combine high stability and excellent energy performance are one of the key directions of energetic materials research. In this study, a novel monocyclic hydroxytetrazole derivative (3) with high stability was prepared, and a series of insensitive energetic ionic salts were derived from it. Benefiting from their outstanding performance in terms of density, 3D hydrogen bonding and π-electron interactions, these salts are excellent in both detonation performance (D = 8709 to 9314 m s-1 and P = 29.9 to 35.6 GPa) and thermal stability (Td = 193.0-232.2 °C). The hydrazine salt (2) exhibits high detonation properties (D = 9314 m s-1 and P = 35.6 GPa), due to its high density (ρ = 1.71 g cm-3) and high heat of formation (ΔfH = 563.2 kJ mol-1 = 3.19 kJ g-1). In addition, the high thermal stability (Td = 232.0 °C) and low mechanical sensitivity (IS = 30 J and FS = 360 N) of 2 are also unmatched by HMX and TKX-50. These improved properties demonstrate the great promise of 2 as an insensitive high-energy explosive.

Pentazolate coordination polymers self-assembled by in situ generated [Pb4(OH)4]4+ cubic cations trapping cyclo-N5.

A series of cyclo-N5--based lead-containing energetic coordination polymers [Pb(OH)]4(N5)4 (1), [Pb3(N5)3(H2O)9(NO3)]4(N5)8(H2O)5 (2), [Pb(OH)]4(N5)3(NO3)(H2O)3 (3), and [Pb(OH)]4(N5)3(ClO4)(H2O) (4) were synthesized by self-assembly and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, infrared and Raman spectroscopy, high resolution mass spectrometry, elemental analysis, scanning electron microscopy, and differential scanning calorimetry. In addition, their thermal decomposition kinetics have been studied theoretically and experimentally. The results revealed that the synthesized CPs possess regular structures, very high densities (2.852-4.537 g cm-3), good oxygen balances (CO2) (-2.72-+2.61%), good thermal stabilities (80-110 °C), and acceptable sensitivities (9.5-20 J; 120-240 N). This work will provide new inspiration for the development of cyclo-N5--based coordination polymers and energetic materials.

Anxiety and self-efficacy in Chinese international students' L3 French learning with L2 English and L3 French.

The present study explored the relationship between international students' Third Language Anxiety (TLA) and self-efficacy. The research data were collected through questionnaires involving 243 Chinese International students' L3 French Learning with L2 English and L3 French at one university in the U.K. Three of them were interviewed about their experience of anxiety and self-efficacy. Major findings include four underlying factors correlated with TLA and two underlying factors correlated with self-efficacy. Also, levels of these students' TLA were negatively correlated with the level of their self-efficacy, as shown in the correlational analysis. Then, two linear regression models were built to contribute to the prediction of their self-efficacy levels. Lastly, participants reported that grammatical and pronunciation similarities between English (L2) and French (L3) positively decreased their anxiety levels. All of these interviewees encountered communication apprehension. These findings can provide educational implications for L3 teaching and learning, inspiring teachers to consider international students' TLA and self-efficacy and thus propose some coping strategies.

Novel metal-organic frameworks assembled from the combination of polynitro-pyrazole and 5-nitroamine-1,2,4-oxadiazole: synthesis, structure and thermal properties.

Energetic metal organic frameworks (EMOFs) is a hot topic in the field of energetic materials research. This paper reports two kinds of EMOFs based on methylene-linked polynitropyrazole and nitroamine 1,2,4-oxadiazole. Their structures were fully characterized by crystallography and their detonation performance and stability performance were explored. The results showed that the crystals of compounds 4 and 5 exhibited a 3D stacking phenomenon due to the action of a large number of hydrogen bonds and coordination bonds inside the crystal. In terms of stability, both 4 and 5 showed good thermal stability (TSADT (4) = 204.4 °C and TSADT (5) = 216.2 °C), but due to the difference in the number of energetic groups (-NO2), the sensitivity of 4 (IS = 6.0 J and FS = 100 N) to mechanical stimuli is significantly lower than that of compound 5 (IS = 1.2 J and FS = 40 N). In terms of energy performance, it is this great advantage in the number of energetic groups that makes compound 5's (Dv = 8.059 km s-1 and P = 30.9 GPa) detonation performance superior to that of 4 (Dv = 7.704 km s-1 and P = 26.9 GPa). This research broadens the horizon for the development of EMOFs based on polynitropyrazole derivatives.

Oxygen-Enriched Metal-Organic Frameworks Based on 1-(Trinitromethyl)-1H-1,2,4-Triazole-3-Carboxylic Acid and Their Thermal Decomposition and Effects on the Decomposition of Ammonium Perchlorate.

Energetic metal-organic frameworks (EMOFs) with a high oxygen content are currently a hot spot in the field of energetic materials research. In this article, two series of EMOFs with different ligands were obtained by reacting 1-(trinitromethyl)-1H-1,2,4-triazole-3-carboxylic acid (tntrza) with metal iodide and metal nitrate, respectively. Furthermore, their structure, thermal stability, thermal decomposition kinetics, and energy performance are fully characterized. The research results revealed that the synthesized EMOFs possess a wide range of density (ρ = 1.88∼2.595 g cm-3), oxygen balance (OB(CO2) = -21.1∼ -4.3%), and acceptable energy performance (D = 7.73∼8.74 km s-1 and P = 28.1∼41.1 GPa). The difference in OB(CO2) caused by the ligand structure and metal properties has a great impact on the distribution of gas-phase products after the decomposition of these EMOFs. Noteworthy, [Ag(tntrza)]n is particularly prominent among these EMOFs, not only because of its excellent detonation performance (D = 8.74 km s-1 and P = 41.1 GPa) endowed by its extremely high density (ρ = 2.595 g cm-3) and oxygen balance (OB(CO2) = -4.3%) but also because of its effective catalytic effect on the decomposition of ammonium perchlorate (AP). This article broadens the horizon for the study of oxygen-enriched EMOFs with catalytic effects and helps understand the mechanism of thermal decomposition of EMOFs with nitroform and dinitro groups.

Variations in concentration and solubility of iron in atmospheric fine particles during the COVID-19 pandemic: An example from China.

Iron (Fe) in the atmosphere can affect atmospheric chemical processes and human health. When deposited into oceans, it can further influence phytoplankton growth. These roles of Fe fundamentally depend on its concentration and solubility. However, the sources of aerosol Fe and controlling factors of Fe solubility in megacities remain poorly understood. The outbreak of the COVID-19 pandemic causes large changes in human activities, which provides a unique opportunity to answer these key issues. Field observations were conducted before, during, and after the COVID-19 lockdown in Hangzhou, China. Our results show that in the COVID-19 lockdown stage, the concentrations of total Fe (FeT, 75.0 ng m-3) and soluble Fe (FeS, 5.1 ng m-3) in PM2.5 decreased by 78% and 62%, respectively, compared with those (FeT 344.7 ng m-3, FeS 13.5 ng m-3) in the pre-lockdown stage. The sharp reduction (81%) in on-road vehicles was most responsible for the aerosol Fe decrease. Surprisingly, the Fe solubility increased by a factor of 1.9, from 4.2% in the pre-lockdown stage to 7.8% in the COVID-19 lockdown stage. We found that the atmospheric oxidizing capacity was enhanced after lockdown restrictions were implemented, which promoted the formation of more acidic species and further enhanced the dissolution of aerosol Fe.

Atmospheric fine particles in a typical coastal port of Yangtze River Delta.

In recent decades, coastal ports have experienced rapid development and become an important economic and ecological hub in China. Atmospheric particle is a research hotspot in atmospheric environmental sciences in inland regions. However, few studies on the atmospheric particle were conducted in coastal port areas in China, which indeed suffers atmospheric particle pollution. Lack of the physicochemical characteristics of fine particles serves as an obstacle toward the accurate control for air pollution in the coastal port area in China. Here, a field observation was conducted in an important coastal port city in Yangtze River Delta from March 6 to March 19, 2019. The average PM2.5 concentration was 63.7 ± 27.8 µg/m3 and NO3-, SO42-, NH4+, and organic matter accounted for ~60% of PM2.5. Fe was the most abundant trace metal element and V as the ship emission indicator was detected. Transmission electron microscopy images showed that SK-rich, soot, Fe, SK-soot and SK-Fe were the major individual particles in the coastal port. V and soluble Fe were detected in sulfate coating of SK-Fe particles. We found that anthropogenic emissions, marine sea salt, and secondary atmosphere process were the major sources of fine particles. Backward trajectory analysis indicated that the dominant air masses were marine air mass, inland air mass from northern Zhejiang and inland-marine mixed air mass from Shandong and Shanghai during the sampling period. The findings can help us better understand the physicochemical properties of atmospheric fine particles in the coastal port of Eastern China.

Iron solubility in fine particles associated with secondary acidic aerosols in east China.

Soluble iron (FeS) in aerosols contributes to free oxygen radical generation with implications for human health, and potentially catalyzes sulfur dioxide oxidation. It is also an important external source of micronutrients for ocean ecosystems. However, factors controlling FeS concentration and its contribution to total iron (FeT) in aerosols remain poorly understand. Here, FeS and FeT in PM2.5 was studied at four urban sites in eastern China from 21 to 31 December, 2017. Average FeT (869-1490 ng m-3) and FeS (24-68 ng m-3) concentrations were higher in northern than southern China cities, but Fe solubility (%FeS, 2.7-5.0%) showed no spatial pattern. Correlation analyses suggested %FeS was strongly correlated with FeS and PM2.5 instead of FeT concentrations. Individual particle observations confirmed that more than 65% of nano-sized Fe-containing particles were internally mixed with sulfates and nitrates. Furthermore, there was a high correlation between sulfates or nitrates/FeT molar ratio and %FeS. We also found that the sulfates/nitrates had weaker effects on %FeS at RH < 50% than at RH > 50%, suggesting RH as indirect factor can influence %FeS in PM2.5. These results suggest an important role of chemical processing in enhancing %FeS in the polluted atmosphere.

Syntheses of Energetic cyclo-Pentazolate Salts.

Three energetic salts of cyclo-N5 - were synthesized via a metathesis reaction of barium pentazolate and sulfates which was driven by the precipitation of BaSO4 . All the energetic cyclo-N5 - salts were characterized by single-crystal X-ray diffraction, infrared (IR), 1 H and 13 C multinuclear NMR spectroscopies, thermal analysis (TGA and DSC), and elemental analysis. The salts exhibit relatively good detonation performance with low sensitivities and good thermal stabilities. This new method opens the door to exploring more pentazolate anion-containing high-performance energetic materials.

Tetracyclic pyrazine-fused furazans as insensitive energetic materials: syntheses, structures, and properties.

Energetic compounds with fused tetracyclic backbones were synthesized and fully characterized, and their structures were confirmed by single crystal X-ray diffraction. Changes from nonplanar to planar structures led their densities to increase from 1.800 to 1.856 g cm-3 (298 K). Hydrogen bonds and π-π interactions were analyzed to understand this phenomenon. Energetic evaluations showed that compounds 2 and 3 detonated with performance comparable to TATB. Excellent mechanical sensitivities (IS: >40 J; FS: >360 N) indicate the promise of these furazans as insensitive energetic materials.

Recent advances in the syntheses and properties of polynitrogen pentazolate anion cyclo-N5- and its derivatives.

The pentazolate anion, or cyclo-N5-, which is a five-membered ring composed solely of nitrogen atoms, has a unique structure among polynitrogen compounds. Cyclo-N5- is receiving ever-increasing levels of attention because of its potential ability to store large amounts of energy compared to the azide ion, its environmentally friendly decomposition products, and its carbon- and hydrogen-free composition, which are promising characteristics for advancing the field of high-energy-density materials (HEDMs), that include explosives, oxidisers, and propellants in closed environments. In this review, we provide a detailed introduction to cyclo-N5- and cover the following topics: (1) substituted pentazoles as precursors of cyclo-N5-, with a focus on the syntheses and stabilities of substituted pentazole derivatives; (2) routes to cyclo-N5- through cleavage of C-N bonds in substituted pentazoles, during which competitive reactions between pentazole decomposition and C-N bond cleavage need to be considered to ensure a successful outcome; (3) complexes of cyclo-N5-, summarising recent progress toward producing cyclo-N5--based complexes through the assembly of isolated cyclo-N5- with both metallic and nonmetallic components; and (4) interactions between cyclo-N5- and metal cations and non-metal species, as well as factors that influence the stability of these complexes; in particular, the thermal stabilities of prepared cyclo-N5- salts are discussed. This review summarises recent studies and is intended to improve the understanding of polynitrogen chemistry while supporting further research into its potential application as an efficient, safe, and environmentally friendly HEDM.

Alkali Metals-Based Energetic Coordination Polymers as Promising Primary Explosives: Crystal Structures, Energetic Properties, and Environmental Impact.

Coordination polymers (CPs) consisting of alkali metals (Na, K, Rb, and Cs) and a powerful nitrogen- and oxygen-rich energetic ligand (4,4'-bis(dinitromethyl)-3,3'-bisnitramide-methylene-furazanate, DBMF2- ) were developed. Molecular structures of these CPs, confirmed by single-crystal X-ray diffraction analysis, indicated that the same ligand takes on a U-shaped state for Na and an N-shaped state for K, Rb, and Cs. Explosion tests demonstrated that both Na2 DBMF and K2 DBMF efficiently detonated the secondary explosive RDX. This indicates that they are both effective primary explosives. K2 DBMF exhibits better calculated detonation performance (D: 8227 m s-1 ; P: 32.5 GPa) than the primary explosive Pb(N3 )2 . In addition, toxicity tests and evaluation of their decomposition products reveal their low impact on the environment. Both experimental results and theoretical analyses indicate that the combination of alkali metals and a powerful energetic ligand can stimulate the development of primary explosives.