A Multi-Wall Carbon Nanotube Tower Electrochemical Actuator YeoHeung Yun, ² Vesselin Shanov, ‡ Yi Tu, § Mark J. Schulz,* ,² Sergey Yarmolenko, | Sudhir Neralla, | Jag Sankar, | and Srinivas Subramaniam ‡ Department of Mechanical Engineering, Smart Materials Nanotechnology Laboratory, and Department of Chemical and Materials Engineering, UniVersity of Cincinnati, 45211, First Nano, a DiVision of CVD Equipment Corporation, 1860 Smithtown AVenue, Ronkonkoma, New York 11779, and Department of Chemical and Mechanical Engineering, North Carolina A&T State UniVersity, Greensboro, North Carolina 27411 Received December 8, 2005; Revised Manuscript Received February 5, 2006 ABSTRACT Patterned multiwall carbon nanotube arrays up to four millimeters long were synthesized using chemical vapor deposition. Electrochemical actuation of a nanotube array tower was demonstrated in a 2 M NaCl solution at frequencies up to 10 Hz with 0.15% strain using a 2 V square wave excitation. The synthesis and electrochemical modeling approach outlined in the paper provide a foundation for the design of nanotube smart materials that actuate and are load bearing. Introduction Smart materials are called solid-state transducers because they have sensing and actuating properties that are intrinsic to the material. The transduction properties are based on piezoelectric, pyroelectric, electrostrictive, magnetostrictive, piezoresistive, electroactive, and other effects. Piezoelectric ceramic materials produce a charge when strained and conversely expand when a voltage is applied and are the most important smart material today. However, high modulus piezoceramic materials are heavy and brittle, and need high voltage to operate, and low modulus ferroelectric ceramics have reduced force. The strains of ferroelectric ceramics are 0.15% for high modulus materials and several percent for low modulus materials. Shape memory alloy materials have up to 8% strain, but they require constant power to heat and a cooling part of the cycle is needed for operation. Although shape memory alloys are generally slower in cooling than heating, the rate response can be above 10 Hz by using thin sections and active cooling of the material. Electroactive polymers have large strain, but hysteresis makes precision motion difficult. Demonstrated actuation stresses of conduct- ing polymers are as high as tens of megapascals. Other smart materials such as electrostrictive and magnetostrictive ac- tuators, thermal bimorphs, and other actuators require magnetic fields, large voltage, or have large sizes or weights. Therefore, no existing smart materials can meet the needs for many current advanced and future applications. Recently, smart materials based on nanotechnology have shown the potential to improve the way we generate and measure motion in devices from the nano- to the macroscale in size. The mechanical and electrochemical properties are coupled in carbon nanotubes (CNT), which is a characteristic of smart materials. In our experimentation, for example, operating with no electrolyte, no significant piezoelectric ef- fect was observed for multiwall carbon nanotubes (MWCNT). MWCNTs are probably too electrically conductive for piezoelectric actuation to be observable. The piezoelectric effect in boron nitride nanotube bundles is predicted to be about 20 times smaller than that for piezoelectric ceramic materials, and this material is not readily available for making a new smart material at this time. The first CNT actuator developed was a macroscale sheet of nanotubes termed “buckypaper”. 1-3 This actuator produced strain because of the change in dimension of the nanotube in the covalently bonded direction caused by an applied electric potential. Our group 4-6 developed CNF (carbon nanofiber) nanocomposite actuators for both liquid electrolyte and solid polymer electrolyte applications. Despite these advances, there are still many challenges to developing tailored practical devices mainly because of the uncontrollable properties of nanotube buckypaper and polymer nanocomposite actuators. The short length and difficulty in processing bundles of nonoriented nanotubes has restricted their applications. These actuators * Corresponding author. E-mail: Mark.J.Schulz@uc.edu. ² Department of Mechanical Engineering, Smart Materials Nanotech- nology Laboratory, University of Cincinnati. ‡ Department of Chemical and Materials Engineering, University of Cincinnati. § First Nano, a Division of CVD Equipment Corporation. | North Carolina A&T State University. NANO LETTERS 2006 Vol. 6, No. 4 689-693 10.1021/nl052435w CCC: $33.50 © 2006 American Chemical Society Published on Web 03/14/2006