This article presents the evidence of possible route to the formation of ε-and α-polymorphic phases of 2, 4,6,8,10,4,6,8,10, studied through CL-20 solution using reverse and normal precipitation method. Reverse precipitation with instant addition facilitated with the opportunity to track the crystal phases from their immediate formation to end of phase stabilization. Precipitation under apparent conditions to achieve α-or ε-phases, showed initial occurrence of metastable β-phase and subsequent transformation to the intended stable phases. The β-phase showed sufficiently longer stability while under specified conditions for ε-than in a hydrated medium set to obtain the α-phase.Transformation of fine needle shaped β-CL-20 crystals to uniform diamond shaped α-or bipyramidal ε-habit had been observed to pass through an equilibrium state of dissolution and reprecipitation. This work also elaborates the effect of crystallization methodology on conversion time. Vibrational spectroscopy and microscopic techniques were employed to track the time dependent polymorphic conversions. Drastic reduction in β → ε conversion time, from 160 minutes to 10 minutes could have been affected by using ultra dispersed seed crystals. We thus also demonstrated a hazard free non-grinding method to prepare ε-CL-20 with particle size <10 µm through precipitation and their effect on thermal stability & mechanical sensitivity.

, et al. studied the crystal quality of ε-CL-20 obtained from different precursors and solvent systems.In the present study, it is focused to prepare and characterisation by both solvent evaporation and precipitation methods. The present evaporation method also relates for obtaining agglomeration free fine ε-CL-20 crystals by in-situ use of ultrasound to crystallisation solution. The obtained crystals were characterised using HPLC, FTIR, Raman, Powder XRD, SEM and DSC techniques. Particle size measurement of obtained ε-CL-20 was carried out using particle size analyser and true density by Helium gas pycnometer. EXPERIMENTAL WORKRaw CL-20 sample was obtained from Premier Explosive's Laboratories, India. All the solvents and allied chemicals used for the processes are of analytical grade with > 99 % purity. ε-CL-20 is prepared using both solvent evaporation and precipitation techniques. Raw CL-20 is dissolved in ethyl acetate solvent and then antisolvent, n-heptane is added into the solution to induce the crystallisation. For the effective recovery in the drowning-out crystallisation, the fraction of raw CL-20, ethyl acetate and n-heptane was applied as about from 1 : 15 : 35, AbSTRACT ε-CL-20 is prepared from raw CL-20 by solvent evaporation and precipitation methods. Experiments were also done using solvent evaporation coupled with in-situ ultrasonication method. Using precipitation method, ε-CL-20 is scaled up to 500 g batch. Raw CL-20 was assigned to α-CL-20. The chemical and polymorphic purity of prepared ε-CL-20 was found to be about 98 per cent and > 95 per cent, respectively. ε-CL-20 was obtained agglomeration free with well defined geometry in comparison with raw CL-20 and its crystal morphology is dominantly bi-pyramidal or lozenge crystal shapes. The obtained mean particle size of prepared ε-CL-20 by solvent evaporation method with and without in-situ ultrasonication and also by precipitation methods is about 30 µm -40 µm, 150 µm -200 µm and 150 µm -300 µm, respectively. The measured true density of prepared ε-CL-20 by precipitation method with 100 g and 500 g batch scale using Helium gas pycnometer was 2.038 g/cm 3 and 2.043 g/cm 3 , respectively. The lower value of calculated void percentage of ε-CL-20 (0.05-0.29%) indicate better crystal quality. Conclusively, prepared ε-CL20 has high true density with less percentage of voids, less total moisture content and free from agglomeration as compared with the starting raw CL-20 material.

Under ambient conditions, energetic materials may exist in one or more than one metastable crystal structure. Under compression or when heated, the material may transform into a different structure or may decompose. Mapping the phase diagram of explosive materials at high pressures and temperatures is an important component to evaluate their performance and safety aspects. In particular, a detailed knowledge of polymorphism and the structural and chemical stabilities of the various phases is necessary to understand the reactive behavior of explosive materials in the high-pressure and high-temperature range that is relevant to shock-wave initiation. Phase transformations could be rate-dependent; that is, fast compression or rapid heating could result in different transformation pressures, temperatures, or even structures compared with static compression and slow heating because shock compression could be accompanied by sudden and extreme heating effects. Nevertheless, static methods are expected to give a fair idea of the structure of the materials under different P–T conditions and, from the structure, their performance characteristics. Also, the shock-wave physics and chemistry of explosives are so complex that in shock experiments it has not been possible to identify the intermediate phases of molecules during decomposition. Hence experiments with static high pressure and high temperature are necessary to gain insight into these processes. Additionally, computational modeling and simulations have been extensively used to understand the effects of pressure on explosives. There is considerable literature on these aspects of energetic materials accumulated over the years. We will review the current status of experimental results, primarily using X-ray diffraction, Raman, and infrared spectroscopies, as probes exploring the P–T phase diagram of important secondary explosives ammonium nitrate, TNT, TATB, PETN, RDX, HMX, CL-20, TEX, FOX-7, and TKX-50.

Samples of energetic material TEX (CHNO) are studied using Raman spectroscopy and X-ray diffraction (XRD) up to 27 GPa pressure. There are clear changes in the Raman spectra and XRD patterns around 2 GPa related to a conformational change in the TEX molecule, and a phase transformation above 11 GPa. The molecular structures and vibrational frequencies of TEX are calculated by density functional theory based Gaussian 09W and CASTEP programs. The computed frequencies compare well with Raman spectroscopic results. Mode assignments are carried out using the vibrational energy distribution analysis program and are also visualized in the Materials Studio package. Raman spectra of the high pressure phases indicate that the sensitivity of these phases is more than that of the ambient phase.