It has been recognized that heat-transfer fluids used to convey thermal energy produced by one device to another can exhibit significant increases in thermal conductivity with the addition of highly conductive particles. Suspensions of nano- and micro-particles have attracted the most recent interest because of their enhanced stability against sedimentation, reduction in potential for clogging a flow system, as well as the tantalizing possibility of unexpected enhancements in thermal conductivity that have been reported in some experiments. Among various suspensions, considerable attention has focused on those containing large-aspect-ratio particles, such as carbon nanotubes. Although recent experiments have demonstrated enormous heat-transfer enhancements in these fluids, such increases were reportedly not in agreement with existing macroscale theories [1–3]. In this research we report on an experimental study of the effects of particle aspect ratio on the effective thermal conductivity of micro- and nano-particle suspensions. The influence of particle aspect ratio on the thermal properties of suspensions was first studied in dispersions of micron-sized, silicon-carbide particles with varying aspect ratio. To carry out a detailed comparison with theoretical predictions, particle aspect ratio and size distributions were measured. It is shown that the conductivity of the silicon-carbide suspensions can be quantitatively predicted by an effective-medium theory (EMT), provided the volume-weighted aspect ratio of the particles is used. The particle-aspect-ratio effect was further studied in the suspensions of multi-walled carbon nanotubes. Experimental data on the thermal conductivity of nanotube suspensions could also be interpreted in terms of the aspect-ratio dependence predicted by EMT if the additional nanoscale effect of interfacial resistance was considered.
Influence of particle aspect ratio and ability to aggregate on electrical conductivity of fiber-forming polymer composites