Nanotubes improve thermal conductivity in adhesively bonded joints Published Nov. 19, 2007 By Pete Meltzer, Jr. Materials & Manufacturing Directorate WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- Research scientists at the Air Force Research Laboratory Materials and Manufacturing Directorate, partnering with the University of Dayton Research Institute, are exploring innovative ways of using nanotechnology to reduce aircraft life cycle costs and improve aircraft systems reliability. The team recently demonstrated the concept of using aligned, multi-walled carbon nanotubes to enhance thermal conductivity in adhesively bonded joints, an important step towards the development of electric aircraft and reducing or even eliminating rotary power generation devices. "The challenge of developing electric-powered aircraft assumes conventional power generation devices will be replaced by stationary power sources such as rechargeable batteries, capacitors, and heat exchangers distributed throughout the aircraft," Dr. Ajit K. Roy, the project's lead scientist at the Materials and Manufacturing Directorate, explained. "These devices invariably will generate 'hot spots,' since they are attached to various structures on the aircraft," he said. "Aircraft structural systems, in turn, will require adequate thermal efficiency to effectively manage the heat generated by the stationary devices. The heat produced by stationary devices, a rechargeable battery for example, will need to be transferred to another location on the aircraft for productive usage," Dr. Roy explained. "In nearly all cases, heat generating devices aboard aircraft are attached to structural members by adhesively bonded joints, which under current system design, provide relatively poor thermal conductivity. This will have to change to get the optimal potential from expended heat that might otherwise be 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," Dr. Roy explained. The Materials and Manufacturing Directorate and UDRI researchers carefully examined the material configuration and aligned multi-walled carbon nanotubes in the thickness direction to enhance the through-thickness thermal conductivity. Initial numerical results indicated that thermal contact of the conductive phase with the adherent surfaces is essential in order to achieve the desirable through-thickness conductivity. To demonstrate this concept, the team developed a process to establish the thermal contact of the aligned carbon nanotube tips with the adherent surface. They then tested the adhesive joint device using the conductive graphite face sheets as the adherent with nanotubes aligned along the joint thickness. The measured thermal conductivity of adhesively bonded joints incorporating aligned carbon nanotubes exceeded the thermal conductivity of their conventional counterparts by several order of magnitudes. This showed that aligned, multi-walled carbon nanotubes are notably effective and has opened promising opportunities for much-needed thermal property tailoring in structural joints, Dr. Roy said. "The thermal conductivity of the adhesive joint system was determined by measuring its thermal diffusivity using a laser flash (or heat pulse) technique. This technique is capable of measuring the thermal diffusivity of solid materials over a temperature range of -180 degrees C (centigrade) to 2000 degrees C.," Dr. Roy explained. "The laser flash technique consists of applying a short duration of heat pulse to one face of the parallel-sided sample, then monitoring the temperature rise on the opposite face as a function of time. The temperature rise is measured with an infrared detector," he added.