A family of cyclo-pentazolate anion-based energetic salts was designed and synthesized by a rapid method, and these salts were evaluated as potential next generation energetic materials.

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.

One of the central problems in the study of rarefied gas dynamics is to find the steady-state solution of the Boltzmann equation quickly. When the Knudsen number is large, i.e. the system is highly rarefied, the conventional iteration scheme can lead to convergence within a few iterations. However, when the Knudsen number is small, i.e. the flow falls in the nearcontinuum regime, hundreds of thousands iterations are needed, and yet the "converged" solutions are prone to be contaminated by accumulated error and large numerical dissipation. Recently, based on the gas kinetic models, the implicit unified gas kinetic scheme (UGKS) and its variants have significantly reduced the iterations in the near-continuum flow regime, but still much higher than that of the highly rarefied gas flows. In this paper, we put forward a general synthetic iteration scheme (GSIS) to find the steady-state solutions of general rarefied gas flows within dozens of iterations at any Knudsen number. The key ingredient of our scheme is that the macroscopic equations, which are solved together with the Boltzmann equation and help to adjust the velocity distribution function, not only asymptotically preserves the Navier-Stokes limit in the framework of Chapman-Enskog expansion, but also contain Newton's law for stress and Fourier's law for heat conduction explicitly. For this reason, like implicit UGKS, the constraint that the numerical cell size should be smaller than the mean free path of gas molecules is removed, but we do not need the complex evaluation of numerical flux at the cell interface. What's more, as the GSIS does not rely on the specific kinetic model/collision operator, it can be naturally extended to quickly find converged solutions for mixture flows and even flows involving chemical reactions. These two superior advantages are also expected to accelerate the slow convergence in simulation of near-continuum flows via the direct simulation Monte Carlo method and its low-variance version. * Wei Su and Lianhua Zhu contribute equally.

A carbon-free inorganic-metal complex [Zn(HO)(N)]·4HO was synthesized by the ion metathesis of [Na(HO)(N)]·2HO solution with Zn(NO)·6HO. The complex was well characterized by IR and Raman spectroscopy, elemental analysis (EA), powder X-ray diffraction (PXRD), and differential scanning calorimetry (DSC). The structure of the complex was confirmed by single-crystal X-ray crystallography and a Zn(ii) ion is coordinated in a quadrilateral bipyramid environment in which the axial position is formed by two nitrogen atoms (N1) from two pentazolate rings (cyclo-N) and the equatorial plane is formed by four oxygen atoms (O1) from four coordinated water molecules. The thermal analysis of [Zn(HO)(N)]·4HO reveals that although water plays an important role in stabilizing cyclo-N, dehydration does not cause immediate decomposition of the anion. However, cyclo-N decomposed into N and N gas at 107.9 °C (onset). Based on its chemical compatibility and stability, the complex exhibits promising potential as a modern environmentally-friendly energetic material.