Morphology Changes During Sonication of Carbon Nanotube Suspensions and Impact on Heat Transport

Sonication of carbon nanotube (CNT) dispersions is commonly employed in practice to disentangle agglomerates in order to promote homogeneity and finer dispersion in suspensions and, consequently, to effect more predictable end-properties of nanofluids and nanocomposites. Sonication also influences CNT morphology in terms of shape and length, affecting, in turn, the thermal performance of the resulting medium. Although sonication is generally known to improve heat transfer, increased sonication times may have adverse results due to CNT buckling and scission and, thus, shortening of the CNTs. The present work aims to quantify the distinct effects of sonication-induced structural changes on the thermal conductivity and the evolution of this relationship in time through a dual-scale simulation approach. The simulator reconstructs three-dimensional (3D) assemblages of CNTs with prescribed agglomeration density, CNT length and curvature, evaluates the thermal conductivity locally and temporally within a representative volume element (RVE), and integrates the results into a macroscopic model to extract the effective conductivity as a function of sonication time. The numerical simulations are compared with experiments that follow the thermal conductivity as a function of sonication time. The results elucidate the separate but also the synergistic influence of the agglomerate microstructure on the effective thermal conductivity, paying particular attention to CNT curvature, length and agglomeration features. It is found that the internal density of the agglomerates is of major significance for the heat conduction process, followed by CNT length and curvature.