Down the Dimensionality Lane: Thermal Conductivity Enhancement in Carbon-Based Liquid Dispersions.

Carbon allotropes of different dimensionality, i.e., 1D-carbon nanotubes, 2D-graphene nanoplatelets, and 3D-graphite, possess high thermal conductivity (TC > 2000 W/m K). They are, therefore, excellent candidates for filler material aiming at increasing the TC of composites used for thermal management. However, preparing aqueous dispersions of these materials is challenging due to their strong van der Waals attraction, leading to aggregation and subsequent precipitation. Reported dispersion methodologies have failed to disperse large microscale fillers, which are essential for efficient thermal management. In this work, we suggest to "kinetically arrest" the dispersion by using sepiolite, a fiberlike clay, that effectively disperses all three carbon dimensionalities. We explore the effect of filler dimensionality and properties (lateral size, thickness, defect density) on the dispersion TC enhancement. Modeling the TC by the effective medium approach allows lumping all the intrinsic properties of the filler into a single parameter termed "effective TC", providing an accurate prediction of the experimentally measured TC. We show that, by judicious choice of filler, the TC of both water and a water-ethylene glycol mixture can be enhanced by 31% using graphene nanoplatelets of 15 µm in lateral size. We believe that the guidelines obtained in this work provide a useful tool for designing future liquid composites with enhanced thermal properties.