Mar 19, 2009
Nanocarbon suspension heats up
Increased thermal conductivity is essential to realizing more-efficient cooling systems in cars, trucks and power plants. Thermal conductivity describes the efficiency of heat transfer of a fluid. Nanofluids are of great interest because anomalous increases in thermal conductivity have been observed. These fluids are made by suspending nanoparticles as an additive within aqueous or hydrocarbon liquids. Multi-walled (carbon) nanotubes (MWNTs) are ideal as they are inert, support varied surface chemistries and are lightweight. Figure 1 shows a scanning and transmission electron microscopy image of MWNTs as used in this work.
In the study, researchers from Penn State University, US, compared carbon morphology, surface chemistry and graphitic structure to determine the factors that contribute to increased thermal conductivity. Nanoscale carbons were shown as being effective in boosting heat transfer. Moreover they can be rendered hydrophilic to form a water-based suspension. To measure the thermal conductivity, a new parallel plate cell was also designed and fabricated to circumvent problems associated with traditional hot wire techniques.
Through comparisons between nascent and functionalized forms of these MWNTs, interfacial (molecular) ordering was shown to be the major factor responsible for thermal conductivity enhancement. Surface oxygen groups facilitate hydrogen bonding with water molecules in close proximity. This bonding accelerates heat transfer between the nanocarbon and surrounding fluid, accounting for the observed increase in thermal conductivity. The next steps include testing other fluid combinations and tailoring the nanocarbon morphologies and surface chemistries to achieve enhanced interaction with the carrier fluid.
About the author
Randy L Vander Wal is an associate professor in the Department of Energy and Mineral Engineering, The EMS Energy Institute and the Penn States Institutes of Energy and the Environment at the Penn State University. He teaches courses on energy and his research encompasses energy generation, conversion, storage and efficiency.