Rapid Chip Calorimetry of CL-20: Melting and the Kinetics of Decomposition

The melting point and heat of fusion of CL-20 (hexanitrohexaazaisowurtzitane) have not previously been reported because this high-performance explosive decomposes prior to melting at the heating rates used in conventional differential scanning calorimetry (DSC). Here we report results from an investigation using rapid chip calorimetry to separate the thermodynamic melting from the kinetic decomposition events by applying heating rates from 30 to 10,000 K·s⁻ 1 . Three endothermic transitions were identified prior to decomposition with onset temperatures at approximately 194, 270, and 286°C. We suggest the first two are solid-solid transitions and the third corresponds to melting. The heat associated with this third transition is 26 ± 4 J·g⁻ 1 (12 ± 2 kJ·mol -1 ) for seven samples ran at 3,000 K·s -1 where decomposition does not significantly interfere with the process. The kinetics of thermal decomposition were also investigated within the framework of the Kissinger-Akahira-Sunose (KAS) iso-conversion method, yielding an average activation energy of 214 ± 11 kJ·mol -1 , which is consistent with the literature for conventional heating rates. The data are well described by a first-order reaction with a rate constant of 1.813 s -1 at 573.15 K and an activation energy of 210 kJ·mol -1 , but an even better fit can be obtained, especially at the four lowest rates where melting occurred during or after the onset of decomposition, by assuming that the rate constant changes an order of magnitude through the melting transition, with fitting parameters k o = 3.059 s -1 at 573.15 K in the liquid state and E a = 183 kJ·mol -1 . Our findings not only provide the first experimental evidence of the melting behavior of CL-20, but also offer valuable insights into its thermal stability, solid-solid phase transition behavior, and decomposition kinetics which are critical for its application in advanced energetic formulations.