**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).

DOI: 10.1063/5.0105161

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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