Abstract:
The propensity of carbon nanotubes (CNTs) to self-organize into continuous networks of bundles has
direct implications for thermal transport properties of CNT network materials and defines the
importance of clear understanding of the mechanisms and scaling laws governing the heat transfer
within the primary building blocks of the network structures—close-packed bundles of CNTs. A
comprehensive study of the thermal conductivity of CNT bundles is performed with a combination of
non-equilibrium molecular dynamics (MD) simulations of heat transfer between adjacent CNTs and
the intrinsic conductivity of CNTs in a bundle with a theoretical analysis that reveals the connections
between the structure and thermal transport properties of CNT bundles. The results of MD
simulations of heat transfer in CNT bundles consisting of up to 7 CNTs suggest that, contrary to the
widespread notion of strongly reduced conductivity of CNTs in bundles, van der Waals interactions
between defect-free well-aligned CNTs in a bundle have negligible effect on the intrinsic
conductivity of the CNTs. The simulations of inter-tube heat conduction performed for partially
overlapping parallel CNTs indicate that the conductance through the overlap region is proportional to
the length of the overlap for CNTs and CNT-CNT overlaps longer than several tens of nm. Based on
the predictions of the MD simulations, a mesoscopic-level model is developed and applied for
theoretical analysis and numerical modeling of heat transfer in bundles consisting of CNTs with
infinitely large and finite intrinsic thermal conductivities. The general scaling laws predicting the
quadratic dependence of the bundle conductivity on the length of individual CNTs in the case when
the thermal transport is controlled by the inter-tube conductance and the independence of the CNT
length in another limiting case when the intrinsic conductivity of CNTs plays the dominant role are
derived. An application of the scaling laws to bundles of single-walled (10,10) CNTs reveals that the
transition from inter-tube-conductance-dominated to intrinsic-conductivity-dominated thermal
transport in CNT bundles occurs in a practically important range of CNT length from ~20 nm to
~4 μm.
