Thermal conductivity tensor of gamma and epsilon- hexanitrohexaazaisowurtzitane as a function of pressure and temperature
R Perriot and MJ Cawkwell, AIP ADVANCES, 12, 085203 (2022).
Using reverse non-equilibrium molecular dynamics simulations, we have determined the dependences on temperature and pressure of the thermal conductivity tensors for the monoclinic gamma and epsilon polymorphs of hexanitrohexaazaisowurtzitane (HNIW or CL20). A recently developed non- reactive force field X. Bidault and S. Chaudhuri, RSC Adv. 9, 39649-39661 (2019), designed to study polymorphism and phase transitions in CL20, is employed. The effects of temperature and pressure are investigated between 200 and 500 K and up to 0.5 GPa for gamma-CL20 and 2 GPa for epsilon-CL20. In order to obtain the full thermal conductivity tensor, kappa(ij), for the monoclinic crystals, four distinct heat propagation directions are used. We find that kappa(ij) for both polymorphs is more isotropic than for other energetic molecular crystals, including alpha- and gamma-RDX, beta-HMX, and PETN, with a maximum difference of 9.8% between orientations observed at 300 K and 0 GPa for gamma-CL20 and a maximum difference of 4.8% for epsilon- CL20. The average thermal conductivity, kappa, of epsilon-CL20 is 6.4% larger than that of gamma-CL20 at 300 K and 0 GPa. Analytic linear functions of the inverse temperature and the pressure are provided, which fit the data well and can be used to predict the thermal conductivity of both polymorphs for any orientation, pressure, and temperature in and around the fitting range. Our predictions agree reasonably well with the limited available experimental data, for which the polymorph type is unknown.
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