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Posted: Jun 16th, 2008
NASA nanotechnology research into shape-shifting airplanes
(Nanowerk Spotlight) You might have seen our news item from a few days ago about BMW's shape shifting concept car. NASA has worked on something much more revolutionary, called the 'Morphing' program, for a few years already. The idea is that aircraft of the future will not be built of traditional, multiple, mechanically connected parts and systems. Instead, aircraft wing construction will employ fully-integrated, nanotechnology enabled embedded 'smart' materials and actuators that will enable aircraft wings with unprecedented levels of aerodynamic efficiencies and aircraft control.
Able to respond to the constantly varying conditions of flight, sensors will act like the nerves in a bird's wing and will measure the pressure over the entire surface of the wing. The response to these measurements will direct actuators, which will function like the bird's wing muscles. Just as a bird instinctively uses different feathers on its wings to control its flight, the actuators will change the shape of the aircraft's wings to continually optimize flying conditions. Active flow control effectors will help mitigate adverse aircraft motions when turbulent air conditions are encountered.
NASA computer animation intended to illustrate the concept of a morphing, or shape-changing, aircraft
NASA's Morphing program makes extensive use of nanotechnology, for instance by developing electroactive polymers to improve sensing and actuation. Researchers working in this area now have created a novel intrinsic unimorph carbon nanotube (CNT) polymer composite actuator.
"Our work demonstrates, for the first time, the actuation with CNT polymer composites in a dry state without any electrolytes" Dr. Cheol Park tells Nanowerk. "It is an electrostrictive actuation of the composite caused by increased interfacial polarization at the humongous interfaces between the nanotubes and the matrix."
Park, a Research Fellow at the National Institute of Aerospace in Hampton, VA, together with his colleague Dr. Jin Ho Kang and collaborators from the Advanced Materials and Processing Branch at the NASA Langley Research Center (Sharon E. Lowther, Dr. Robert C. Costen, Dr. Joycelyn S. Harrison (AMPB Head)), developed a novel electroactive single-walled carbon nanotube (SWCNT)–polymer composite, an intrinsic unimorph, which can actuate to a large strain (2.6%) at relatively low driving voltages (<1MV per meter) while maintaining its high performance in mechanical durability, thermal stability, and chemical resistances.
"The fact that this actuator requires very low input energy compared to reported state-of-the-art actuators available and generate much larger strains than PZT and PVDF is very important for NASA's long-term space exploration missions such as MARS trip or lunar habitats" says Park.
He also points out that this intrinsic unimorph actuator does not require adhesive or extraneous inactive layers to generate the large bending actuation. The actuating capabilities of the electroactive polymers are gained from incorporation of SWCNTs into these polymers.
Top: Bending actuation of a cantilevered intrinsic unimorph (0.05% SWNT/LaRC-EAP (Langley Research Center-ElectroActive Polyimide)) (side view): A) without, B) with an electric field (0.8MV per meter), C) Schematic structure of a cross-section of the intrinsic unimorph of SWNT/LaRC-EAP composite film without and with an electric field. (Reprinted with permission from Wiley-VCH Verlag)
"Our CNT composite forms an intrinsic unimorph actuator during the film processing, which is unprecedented" Park explains. "This is very significant because we do not need any inactive dummy layer and adhesive layer to convert longitudinal to bending strain, which is a prerequisite for most of conventional actuators. Therefore, our intrinsic unimorph is easy to make and has no interfacial aging (delamination) problems."
Several CNT-based composites have been reported so far based on ionic type actuation with electrolytes. These composites can actuate only at a wet state or in a liquid solution to provide sufficient mobility of the electrolytes and ions. In contrast, the CNT-polymer composite developed by the NASA/NIA scientists actuates mainly by electrostrictive effect, which is novel and has never been reported so far.
Applications of this novel nanocomposite material include lightweight and low power consuming actuators for aerospace vehicles as well as terrestrial vehicles. Park points out that their high temperature, flexible actuators are suitable for any complex system in harsh environments. He lists specific application areas from the Morphing program that could benefit from such actuating CNT-polymer composites:
Sonic fatigue abatement (by active control)
Noise transmission attenuation (by active control)
Wing and panel flutter control
Tail buffet alleviation control
Airframe surface shape control
The researchers caution that the long-term stability of highly concentrated CNT composite actuators during the application of high electric fields needs to be further studied. Furthermore, Park notes that the response time is not as fast as some leading rapid electrostrictive or piezoelectric actuators since their intrinsic unimorph actuates based on interfacial polarization. He is confident that this can be overcome with specially designed electrodes or layered structures.
Interestingly, Park mentions that a reliable supply of high quality carbon nanotubes is their biggest challenge and that the price of high purity, high quality CNTs is still very high.