(323e) Heat Transfer in Nanocomposites at High Volume Fraction

Enhancing the thermal conductivity of composites by incorporating carbon nanotubes (CNTs) and/or nanographene sheets (GSs) has been investigated with both experiments and simulations recently [1-4]. It has been found that the transport of heat in such cases is dominated by the resistance to the transport of heat at the inclusion-matrix interface (this resistance is known as Kapitza resistance), which is inversely proportional to the phonon transmission probability from the matrix phase to the inclusions and vice versa. A very small amount of nano inclusions (1.0%wt to 2.0%wt) can lead to an increase of the effective thermal conductivity (keff) of the composites up to 50% for the case of CNT-PS [3] or by a factor of 2.6 for the case of GS-PS [4], while this increase would have been of an order of magnitude if there had been no Kapitza resistance. Furthermore, Peters et al. [5] have studied keff of CNT-PS composites at high CNT weight fraction and different temperatures, and found that the enhancement ratio (keff/kpolymer per mass of CNT added) at high concentration of nano inclusions is not as good as the enhancement at low CNT concentration. The reason appears to be that the CNTs are not well dispersed at high volume fractions, as they form bundles. By means of Molecular Dynamics simulations, it has been found that the Kapitza resistance between CNTs in contact (as they likely are within bundles) is higher than that at the CNT-polymer interface [6]. Furthermore, like individual tubes, the size and orientation of bundles are believed to also have a strong effect on the keff of composites.

In this work, we apply Monte Carlo simulations to investigate the keff at different nano-inclusion weight fractions and at different temperatures. The simulations take into account the bundling effect (i.e., the size of the bundles and their orientation) as well as the Kapitza resistance. By validating the simulation results with the experiment data of Peters et al. [5], we found that the phonon transmission probability at the interface decreases by temperature. In addition, the poor enhancement of keff at high CNT concentration is because of not only the CNT-CNT contact resistance, but also the bundle geometry itself which is dominated by the length/diameter aspect ratio of the bundle.

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