Control of crystal structure and its defect of ε-HNIW prepared by evaporation crystallization

In the evaporation crystallization, the structural defect and density of ε-hexanitrohexaazaisowurtzitane (HNIW) crystals were controlled by operating parameters of evaporation rate, initial concentration of HNIW and temperature. The density change of the crystal was counted as the variation of the crystal structural defect because the crystal defect in the structure reduced the apparent crystal density. Due to formation of crystal defects during the crystal growth, the operating condition promoting the growth rate brought about increasing the crystal defects and decreasing crystal density. Thereby, the crystal density was significantly reduced with increasing the evaporation rate and initial concentration of HNIW. The influence of the evaporation rate on the crystal density change was much higher than that of the initial concentration. However, the crystal density was enhanced as increasing the temperature because the crystal growth process was retarded at high temperature. It was interesting to find that the crystal structure of ε-form was changed to β-form one by high evaporation rate above 0.125 ml/hr at 70°C.