Pneumatic Actuator Response from Carbon Nanotube Sheets
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Pneumatic Actuator Response from Carbon Nanotube Sheets Geoffrey M. Spinks1, Gordon G. Wallace1, Leonard S. Fifield2, Larry R. Dalton2, Alberto Mazzoldi3, Danilo De Rossi3, Ilyas I. Khayrullin4, Ray H. Baughman4 1 Intelligent Polymer Research Institute, University of Wollongong, NSW 2522, Australia 2 Chemistry Department, University of Washington, Seattle, WA 98195-1700, USA 3 School of Engineering, Centro “E. Piaggio”, University of Pisa , Via Diotisalvi, 2 – 56100 Pisa, Italy 4 Chemistry Department, University of Texas, Dallas, Richardson, TX, 75083-0688, USA
ABSTRACT Reversible actuation strains in excess of 2% in the sheet direction and over 300% in the thickness direction have been produced by single wall carbon nanotube mats when electrochemically charged to +1.5V (vs. SCE) in aqueous sodium chloride solution. The observed strains represent a ten-fold increase over that previously reported for carbon nanotube actuators, and is considerably larger than that achievable with polymer ferroelectric actuators. The enhanced actuator strains result from a new mechanism of electrochemically induced “pnuematic actuation” where high pressure gas forms within the porous structure of the nanotube mat causing partial delamination and swelling. An erasable “memory” effect was also observed for pneumatic actuation driven by hydrogen gas evolution/storage in the nanotube electrodes. INTRODUCTION Recently we reported a new type of electromechanical actuation arising from electrochemical double layer charging of single wall carbon nanotubes (SWNTs)[1]. Actuation strains of up to 0.2% were reported with an applied potential within the stability limit of the aqueous electrolyte (approximately +/- 1 V versus saturated calomel electrode, SCE). Our calculations indicate that the achievable strain for SWNTs is considerably higher and experimental evidence from graphite intercalation compounds [2-4] demonstrate strains in excess of 1% produced by electrochemical charging. We now present our findings of enhanced actuator strains in SWNT mats and a new mechanism of actuation for these materials based on Faradaic reactions occurring in the electrolyte. The development of improved mechanical actuators is being driven by the needs of many diverse applications[5-7]. It is envisaged that actuators having reversible stroke of 2% or higher, stress generating ability of >10MPa, fast response time (106 cycles) will have major impact in applications including robotics, prosthetics, optical fiber switches, steerable catheters, anti-vibration systems and many others. SWNTs appear attractive new actuator materials because of the projected high stroke when combined with a modulus of 640 GPa [8] gives a stress generating capability of 640 MPa per volt of applied electrochemical potential. Even more impressive is the work density per cycle which would be 24000 J/kg for
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SWNT actuators and is ~150 times higher than has been tabulated for the best known ferroelectric, electrostrictive and magnetostrictive materials [9]. EXPERIMENTAL Our carbon
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