|Aliev et. al. describe an investigation of carbon nanotube aerogels exhibiting giant-stroke, superelastic properties. Their aerogel sheets exhibit behavior similar to low-modulus rubbers when stretched in sheet width direction by factors up to 300% thereby making this material an ideal candidate for large stroke actuation.
Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network
- Aliev AE, Oh J, Kozlov ME, Kuznetsov AA, Fang S, Fonesca AF, Ovalle R, Lima MD, Haque MH, Gartstein YN, Zhang M, Zakhidov AA, Baughman RH. 2009. Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles. Science 323: 1575-1578. DOI: 10.1126/science.1168312.
Approaches to mechanical actuation have continuously been investigated for a range of existing and future applications, most notably as artificial muscle. Previous efforts in exploiting the electrostatic forces between individual carbon nanotubes have produced actuator structures such as nanoscale tweezers, mechanically-based switches, and logic elements. At a larger scale, polymer-carbon nanotube composite materials have exhibited actuation via optical, shape memory, or electromechanical means; electrically powered carbon nanotube actuators have demonstrated actuator stroke up to a few percent along with two orders of magnitude higher stress generation as compared to natural muscle.
Recently, Aliev et. al. from Ray Baughman’s group at the University of Texas-Dallas reported on their investigation of carbon nanotube aerogels exhibiting giant-stroke, superelastic properties. In their approach, the authors form carbon nanotube actuators from sheets drawn from forests of multi-walled nanotubes (MWNTs) having volumetric density on the order of 1.5 mg/cc for sheets having a thickness of 20 µm. Large area nanotube aerogel sheets drawn from MWNT forests have previously been demonstrated at production scale rates (>2 m/sec). The aerogel sheets exhibit behavior similar to low-modulus rubbers when stretched in sheet width direction by factors up to 300% thereby making this material an ideal candidate for large stroke actuation.
The authors report the voltage dependence properties of actuation in the sheet width and thickness directions. Applying a positive voltage to an electrode formed on the nanotube sheet with respect to a counter electrode ground plane, the authors observed width-direction actuation stroke of approximately 220%. Furthermore, similar actuation was observed for temperatures ranging from ambient to 1500°K. The authors were able to confirm a charge injection model that predicts the voltage dependence sheet actuation based on sheet length, width, and thickness. The voltage dependency actuation of sheet width exhibits a crossover as forces overcome internal directional stresses due to the ballooning of the sheet in the center while the ends of the sheet remain contracted where it is clamped; the crossover voltage increases with the length to width ratio of the nanotube aerogel sheet.
Actuation of nanotube sheets in the length direction generate isometric specific stresses approaching 4 MPa cm3/g, therefore densified nanotube sheets having density on the order of 0.8 g/cc would have an isometric stress-generation capability of 3.2 MPa. The authors report a maximum work per actuation cycle of 30 J/Kg, approaching that of natural muscle. Poisson ratios of 30 were observed, a factor of 30 higher than for conventional rubber sheets. The nanotube aerogel sheets also demonstrated negative linear compressibility and densification during stretching. While further exploring these unique carbon nanotube sheets for voltage actuated giant stroke and strain rates for artificial muscle, the authors are additionally investigating application whereby the properties of the carbon nanotube sheets can be tuned by actuation and subsequently frozen. Such techniques enable the sheet material to be optimized for specific applications such as transparent electrodes for displays or solar cells, or optical switching.
Image from Aliev AE, Oh J, Kozlov ME, Kuznetsov AA, Fang S, Fonesca AF, Ovalle R, Lima MD, Haque MH, Gartstein YN, Zhang M, Zakhidov AA, Baughman RH. 2009. Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles. Science 323: 1575-1578. DOI: 10.1126/science.1168312. Reprinted with permission from AAAS.