Phonon Lifetimes and Thermal Conductivity of the Molecular Crystal α-RDX

Abstract

The heat transfer properties of the organic molecular crystal -RDX were studied using three phonon-based thermal conductivity models. It was found that the widely used Peierls-Boltzmann model for thermal transport in crystalline materials breaks down for -RDX. We show this breakdown is due to a large degree of anharmonicity that leads to a dominance of diffusive-like carriers. Despite being developed for disordered systems, the Allen-Feldman theory for thermal conductivity actually gives the best description of thermal transport. This is likely because diffusive carriers contribute to over 95% of the thermal conductivity in -RDX. The dominance of diffusive carriers is larger than previously observed in other fully ordered crystalline systems. These results indicate than van-der Waals bonded organic crystalline solids conduct heat in a manner more akin to amorphous materials than simple atomic crystals.

Bond Strain and Rotation Behaviors of Anharmonic Thermal Carriers in ‐RDX

“…which was found to be 0.66 % indicating a consistently good Lorentzian fit of the SED for all modes. We observed that the phonon lifetimes decrease with increasing frequency up to ∼3 THz, followed by flattening and subsequent increase for some higher frequency modes (above ∼9 THz) . This is in contrast to the trend observed in simple atomic crystals like silicon , and carbon nanotubes in which phonon lifetimes monotonically decrease with frequency.…”

“…We also find that ∼70 % of contribution can be attributed to the optical mode phonons, primarily low frequency optical phonons (<4 THz) which are neglected in the CWP model. Higher frequency modes have negligible contribution to primarily due to their relatively low diffusivity (phonons with frequency ∼80 THz have diffusivity about five orders of magnitude lower than the phonons with frequency <4 THz) .…”

“…A first order central differencing scheme is used on frequencies to calculate group velocities. We use a frequency domain normal mode decomposition (NMD) technique to calculate phonon lifetimes for all branches . This technique has also been successfully applied to simple crystalline materials such as silicon and carbon nanotube .…”

“…The details of the NMD technique to calculate phonon lifetimes for a-RDX can be found in Ref. [15]. _ q � t ð Þ is the time derivative of the phonon normal mode coordinate (q � t ð Þ) and is determined using a molecular dynamics (MD) simulation of supercell size N ¼ 2 � 2 � 2 ¼ 8 at T ¼ 300 K and atmospheric pressure using the open source package LAMMPS [25] and the same molecular potential [19].…”

“…Interest has also been growing recently in phonon behaviors in complex crystalline materials where phonons can yield very low glass‐like thermal conductivities . In RDX, the strain and rotation of bonds, strong anharmonic coupling between the phonon modes and presence of both lattice and internal phonons has been shown previously to result in both propagating as well as diffusive thermal carriers, which leads to a glass‐like thermal conductivity . In order to understand the unique deformations associated with the different thermal carriers and improve the understanding of phonons in RDX, three models are employed that estimate thermal conductivity for propagating and diffusive phonons differently.…”