Control of Crystal Density of ε-Hexanitrohexaazaisowurzitane in Evaporation Crystallization

In the case of evaporation crystallization, the density of ε-hexanitrohexaazaisowurtzitane (ε-HNIW, C 6 H 6 N 12 O 12 ) crystals was controlled using the evaporation rate, initial HNIW concentration, and temperature as the operating parameters. When crystal growth was promoted, the crystal density was found to deviate from the theoretical density, probably due to the inclusion of voids in the crystals that amplified the decrease in the crystal density. Thus, the crystal density was significantly reduced when the evaporation rate and initial HNIW concentration were increased. The influence of the evaporation rate on the crystal density was also found to be much stronger than that of the initial HNIW concentration. However, the crystal density was enhanced when the temperature was increased, due to a retardation of the crystal growth process. It was also interesting to find that the β-form of HNIW crystals was obtained with a high evaporation rate above 0.125 mL/h at 70 °C.