Phenomenological model of thermal transport in carbon nanotube and hetero-nanotube films.

The thermal properties of individual single-walled carbon nanotubes (SWCNTs) have been well documented in the literature following decades of intensive study. However, when SWCNTs form a macroscale assembly, the thermal transport in these complex structures usually not only depends on the properties of the individual tubes, but also is affected and sometimes dominated by inner structural details, e.g. bundles and junctions. In this work, we first performed an experimental measurement of the thermal conductivities of individual SWCNT bundles of different sizes using a suspended micro-thermometer. The results, together with the data that we obtained from a previous work, give a complete experimental understanding of the effect of bundling on the thermal conductivity of SWCNTs. With these quantitative understandings, we propose a phenomenological model to describe the thermal transport in two-dimensional (2D) SWCNT films. The term 'line density' is defined to describe the effective thermal transport channels in this complex 2D network. Along with experimentally obtained geometric statistics and film transparency, the thermal conductance of SWCNTs is estimated, and the effects of bundle length, diameter, and contact conductance are systematically discussed. Finally, we extend this model to explain thermal transport in 2D networks of one-dimensional van der Waals heterostructures, which are coaxial hetero-nanotubes we recently synthesized using SWCNTs as the template. This extended model suggests that the contribution of boron nitride nanotubes (BNNTs) to the overall performance of a SWCNT-BNNT heterostructured film depends on the transparency of the original SWCNT film. The increase in the thermal conductance of a highly transparent film is estimated to be larger than that of a less transparent film, which shows a good agreement with our experimental observations and proves the validity of the proposed phenomenological model.

Enhanced In-Plane Thermal Conductance of Thin Films Composed of Coaxially Combined Single-Walled Carbon Nanotubes and Boron Nitride Nanotubes.

Carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) are one-dimensional materials with high thermal conductivity and similar crystal structures. Additionally, BNNTs feature higher thermal stability in air than CNTs. In this work, a single-walled carbon nanotube (SWCNT) film was used as a template to synthesize a BNNT coating by the chemical vapor deposition (CVD) method to form a coaxial heterostructure. Then, a contact-free steady-state infrared (IR) method was adopted to measure the in-plane sheet thermal conductance of the as-synthesized film. The heterostructured SWCNT-BNNT film demonstrates an enhanced sheet thermal conductance compared with the bare SWCNT film. The increase in sheet thermal conductance shows a reverse relationship with SWCNT film transparency. An enhancement of over 80% (from ∼3.6 to ∼6.4 µW·K-1·sq-1) is attained when the BNNT coating is applied to an SWCNT film with a transparency of 87%. This increase is achieved by BNNTs serving as an additional thermal conducting path. The relationship between the thermal conductance increase and transparency of the SWCNT film is studied by a structured modeling of the SWCNT film. We also discuss the effect of annealing on the thermal conductance of SWCNTs before BNNT growth. Along with the preservation of high electrical conductance, the enhanced thermal conductance of the heterostructured SWCNT-BNNT films makes them a promising building block for thermal and optoelectronic applications.