Nanotubes Improve Thermal Conductivity in Adhesively Bonded Joints Published March 13, 2008 By Heyward Burnette AFRL/RX WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- AFRL research scientists are working with the University of Dayton Research Institute (UDRI) to explore innovative uses of nanotechnology for reducing aircraft life-cycle costs and improving aircraft systems reliability. The team recently demonstrated the conceptual use of multiwalled carbon nanotubes aligned to enhance thermal conductivity in adhesively bonded joints, an important step towards developing electric aircraft and reducing--or even eliminating--rotary power generation devices. The experiments used conductive graphite sheets and nanotubes aligned vertically along bonded joint thickness. The findings revealed that using carbon nanotubes in this manner resulted in thermal conductivity exceeding that of conventional counterparts by several orders of magnitude. By demonstrating a significant increase in the through-thickness thermal conductivity of adhesively bonded joints, this research has created new opportunities for advancing military and commercial aviation. In nearly all cases, heat-generating devices aboard aircraft are attached to structural elements via adhesively bonded joints. Under current system design methods, these joints provide relatively poor thermal conductivity. Addressing this shortcoming will enable the optimal potential of expended heat to be leveraged rather than wasted. The approach of mixing carbon nanotubes or nanofibers in the adhesive material provides only a slight improvement. The solution, therefore, must include a substantial increase in the through-thickness thermal conductivity of the joints. AFRL and UDRI researchers carefully examined the material configuration, opting to align the multiwalled carbon nanotubes vertically (along the joint thickness) in order to enhance through-thickness thermal conductivity. Initial numerical data indicated that thermal contact of the conductive phase with the adherent surfaces is essential to achieving the desirable level of through-thickness conductivity. The team determined the thermal conductivity of the adhesive joint system by measuring thermal diffusivity via a laser flash (i.e., heat pulse) technique, which is capable of measuring the thermal diffusivity of solid materials over a temperature range of -180°C (centigrade) to 2000°C. The technique consists of first applying a brief pulse of heat (via the laser) to one face of the parallel-sided material sample. The next step is to monitor the rise in temperature that occurs on the opposite face as a function of time, obtaining measurements with an infrared detector.