Influence of longitudinal isotope substitution on the thermal conductivity of carbon nanotubes: Results of nonequilibrium molecular dynamics and local density functional calculations

Abstract

We report reverse nonequilibrium molecular dynamics calculations of the thermal conductivity of isotope substituted (10,10) carbon nanotubes (CNTs) at 300 K. (12)C and (14)C isotopes both at 50% content were arranged either randomly, in bands running parallel to the main axis of the CNTs or in bands perpendicular to this axis. It is found that the systems with randomly distributed isotopes yield significantly reduced thermal conductivity. In contrast, the systems where the isotopes are organized in patterns parallel to the CNTs axis feature no reduction in thermal conductivity when compared with the pure (14)C system. Moreover, a reduction of approximately 30% is observed in the system with the bands of isotopes running perpendicular to the CNT axis. The computation of phonon dispersion curves in the local density approximation and classical densities of vibrational states reveal that the phonon structure of carbon nanotubes is conserved in the isotope substituted systems with the ordered patterns, yielding high thermal conductivities in spite of the mass heterogeneity. In order to complement our conclusions on the (12)C-(14)C mixtures, we computed the thermal conductivity of systems where the (14)C isotope was turned into pseudo-atoms of 20 and 40 atomic mass units.

Reduced Thermal Transport in the Graphene/MoS2/Graphene Heterostructure: A Comparison with Freestanding Monolayers

“…The population of the out-of-plane modes in the heterostructure is enhanced because of the coupling between the layers, and the intensity of the in-plane peaks are simultaneously reduced. A similar trend is observed in isotope-substituted graphene and carbon nanotubes where the k reduction is associated with the perturbations in the phonon spectra. − On the other hand, the overlapping frequency range of the phonon modes of MoS 2 and the out-of-plane modes of graphene allows them to be coupled. The out-of-plane modes of MoS 2 are enhanced because of the coupling between the flexural phonon modes of the adjacent layers.…”

Thermal conductivity reduction in analogous 2D nanomaterials with isotope substitution: Graphene and silicene

“…Presence of isotopes in a material structure induces mass disorder, reducing phonon thermal conductivity (k) due to enhanced energy scattering for even the smallest defect concentration. The reductions in thermal conductivity for isotope substituted carbon and silicon nanomaterials have been discussed in the literature [33][34][35][36][37] . Here, we employ molecular dynamics (MD) simulations to understand the physics of heat transfer in silicene, relative to that in graphene, by examining the role of different vibrational modes in both the pure and isotope substituted nanomaterials.…”

Optimizing isotope substitution in graphene for thermal conductivity minimization by genetic algorithm driven molecular simulations

“…Likewise, the effect of defects in these materials is of notable interest and importance, primarily because the presence of mass disorder and perturbations to electronic structure can impede transport mechanisms. [4][5][6][7][8][9] Several reports have conclusively shown that the remarkably high thermal conductivity of graphene composed purely of 12 C is reduced by a factor of two for a nanostructure containing 50% 13 C isotopes. [9][10][11] Hence, the occurrences of defects in graphene, and in any nanomaterial for that matter, is of concern due to their variety (point defects and impurities could range from vacancies, isotopes, dopants, and functional groups) and distribution (random, clustered, superlattice type).…”