Multiwalled carbon nanotubes (CNTs) as produced are usually entangled and not ready to be dispersed into fluids. We treated CNTs by using a concentrated nitric acid to disentangle CNT aggregates for producing CNT nanofluids. Oxygen-containing functional groups have been introduced on the CNT surfaces and more hydrophilic surfaces have been formed during this treatment, which enabled to make stable and homogeneous CNT nanofluids. Treated CNTs were successfully dispersed into polar liquids like distilled water, ethylene glycol without the need of surfactant and into nonpolar fluid like decene with oleylamine as surfactant. We measured the thermal conductivities of these nanotube suspensions using a transient hot wire apparatus. Nanotube suspensions, containing a small amount of CNTs, have substantially higher thermal conductivities than the base fluids, with the enhancement increasing with the volume fraction of CNTs. For the suspensions with the same loading, the enhanced thermal conductivity ratios are reduced with the increasing thermal conductivity of the base fluid. Comparison between the experimental data and the theoretical model indicates that the thermal conductivities of nanotube suspensions seem to be very dependent on the interfacial layer that exists between the nanotube and the liquid.

A bulk multi-walled carbon nanotube pellet has been produced by self-arrangement after drying the aqueous suspension of the carbon nanotubes (CNTs) oxidized by boiling CNTs in concentrated acid. The solid can be re-dispersed in water and self-assembled again. The centimeter order bulk CNT pellet unexpectedly showed compact domains of highly aligned parts of long CNTs which size is much larger than the domain size. Viscosity measurements confirmed that the system in question is completely different from a rigid-rod nematic. We reveal the mechanism of the entropy-driven self-alignment of treated worm-like CNTs in analogy to semiflexible liquid crystalline polymer. The polymer-like behavior of treated CNTs brings the CNTs into a genus of robust polymers.