Thermal and electrical conductivities of water-based nanofluids prepared with long multiwalled carbon nanotubes

Thermal and electrical conductivities of suspensions of multiwalled carbon nanotubes (MWCNT) in water were measured as a function of temperature, nanotube weight content, and nanotube length. Nanotubes were dispersed in water by using gum Arabic as surfactant. The thermal conductivity was measured by the steady-state method by using a coaxial-cylinder cell that allows the sample temperature to be varied from 15 to 75 degrees C. Our measurements show that the thermal conductivity enhancement as compared to water linearly increases when the MWCNT weight content increases from 0.01 to 3 wt %, reaching 64% for the MWCNT weight content of 3 wt %. The thermal conductivity enhancement is found to be temperature independent up to MWCNT weight content of 2 wt %. The average length of the nanotubes appears to be a very sensitive parameter. The thermal conductivity enhancement as compared to water increases by a factor of 3 when the nanotube average length increases in the 0.5-5 mu m range. Electrical conductivity measurements show that the electrical properties do not follow the same trend as a function of MWCNT weight content, as compared to thermal properties. The electrical conductivity is mainly constant in the studied range, but undergoes a drop when the weight content decreases to about 0.1 wt %, which suggests that the MWCNT network in the base fluid might be percolating at this very low value. By comparison, the thermal conductivity does not show any percolation threshold. (C) 2008 American Institute of Physics.