PhD defense Daniel Bruns

Atomistic modeling of phonon-mediated heat transport in single-walled carbon nanotubes (CNTs) dates to the year 2000, when Berber, Kwon and Tománek, by means of molecular dynamics (MD) simulations, predicted a thermal conductivity of up to 6600 W/mK, suggesting extremely efficient heat transfer in these one-dimensional carbon materials. Since then, many modeling efforts have been undertaken, but some aspects of CNT phonon transport, in particular its domain-size dependence, remain controversial.

In this thesis, we first revisit the two main-stream approaches of modeling phonon-mediated heat transport in CNTs: quantum mechanical calculations in the framework of the Peierls-Boltzmann transport theory

(PBTT) and classical MD simulations. Looking at domain size and temperature dependencies of CNT heat transport, we evaluate the strength and limitations of both modeling approaches. In regard to domain size effects, our numerical results offer new insights into the problem of weakly damped acoustic phonons with ever-increasing mean free path.

In the framework of the PBTT, we then focus specifically on the spectrum of three-phonon scattering channels in CNTs. From lowest-order anharmonic perturbation theory, we derive exact asymptotic scaling relations of phonon-phonon scattering rates in the limit of low phonon energies. By adopting a relaxation time approximation of phonon transport, we are then able to unambiguously clarify tube-length effects of CNT heat transport, which we further demonstrate to depend very sensitively on tensile lattice strain. With respect to earlier numerical PBTT calculations on CNTs, we show that a clear line can be drawn between physical and unphysical results.