Overall thermal transmittance in carbon nanotube networks for thermal storage systems and composite materials

The study of thermal properties of large networks of carbon nanoparticles may have an important impact in loss-free, compact sorption-based thermal storage systems, as well as thermally conducting polymeric materials for innovative low-cost heat exchangers. Here, we both review and computationally investigate on the role that nanotechnology (and in particular carbon-based nanostructures) may have in the near future in thermal sciences. In particular, we focus on the role played by some geometrical and chemical parameters on the overall thermal transmittance of large complex networks made up of carbon nanotubes (CNTs), that can be potentially utilized as fillers for enhancing the transport properties of energy in (thermally) low-conductive materials. Several configurations of single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs), characterized by different dimensions and number of C-O-C interlinks, are considered. Based on the results reported in the published literature and using focused simulations by standard approaches in Non-Equilibrium Molecular Dynamics (NEMD), we aim at highlighting the dependence on the particle geometry, overlap and functionalizations of the boundary resistance across CNTs, which is known to be the relevant quantity affecting thermal properties of composite materials. We find that CNTs with short overlap length and only a few C-O-C interlinks already show a significant enhancement in the overall transmittance, whereas further increase in the number of connection generates marginal benefits. We believe that much understanding has been gained so far in this field thanks to the work of chemists and material scientists. Hence, it is time to draw the attention of engineers active in the energy sector and thermal scientists on such findings. Our effort, therefore, is to collect in this study a few guidelines that can be useful for the design of innovative thermal systems to be manufactured and employed in the near future for addressing some of the challenges in thermal energy storage (e.g. enhancing the heat rate during charging/discharging processes).