Application of hybrid sphere/carbon nanotube particles in nanofluids

Previous studies on nanofluids have focused on spherical or long-fibre particles. In this work, a new type of complex nanoparticle-a hybrid sphere/carbon nanotube(CNT) particle, consisting of numerous CNTs attached to an alumina/iron oxide sphere-is proposed for applications in nanofluids. In such hybrid nanoparticles, heat is expected to transport rapidly from one CNT to another through the centre sphere and thus leading to less thermal contact resistance between CNTs when compared to simple CNTs dispersed in fluids. CNTs have an extremely high thermal conductivity, but thermal resistance between the CNTs and the fluid has limited their performance in nanofluids. The proposed hybrid sphere/CNT particles are synthesized by spray pyrolysis followed by catalytic growth of CNTs. The spheres are about 70 nm in diameter on average, and the attached CNTs have a length up to 2 μm. These hybrid nanoparticles are dispersed to poly-alpha-olefin with sonication and a small amount of surfactants to form stable nanofluids. The thermal conductivity of the fluids has been measured by a 3ω-wire method over a temperature range 10-90 deg. C. The results indicate that the effective thermal conductivity of the fluids is increased by about 21% at room temperature for particle volume fractions of 0.2%