Thermally induced polymorphic transformation of Hexanitrohexaazaisowurtzitane (CL-20)

To investigate the polymorphic transformation and crystal morphology changes of ε-CL-20 under different thermal stimuli, isothermal and non-isothermal experiments were conducted using a hot stage microscope and in-situ X-ray powder diffraction. It was found that the ε- and γ- phases coexisted at 120 °C (9.727 wt% γ-CL-20), and the ε→γ phase transformation occurred between 115 and 120 °C and reached completion at 130 °C. The ε-CL-20 expanded isotropically in the temperature range of 25 to 115 °C. About a quarter of the ε-CL-20 transformed after being heated at 120 °C for 12 h, whereas almost all of ε-CL-20 transformed after being heated at 150 °C for 12 h. Under the microscope, the expansion of crystal volume, changes of crystal morphology, and phenomena of decomposition were observed during heating. Molecular dynamics simulation and Rietveld refinement were conducted to verify the experimental results. A solid-solid phase transition mechanism is presented.