Carbon nanotube actuators
Encyclopedia
The exceptional electrical and mechanical properties of carbon nanotube
s have made them alternatives to the traditional electrical actuators for both microscopic and macroscopic applications. Carbon nanotubes are very good conductors of both electricity and heat, and they are also very strong and elastic molecules in certain directions. These properties are difficult to find in the same material and very needed for high performance actuators. For current carbon nanotube actuators, multi-walled carbon nanotubes (MWNTs) and bundles of MWNTs have been widely used mostly due to the easiness of handling and robustness. Solution dispersed thick films and highly ordered transparent films of carbon nanotubes have been used for the macroscopic applications.
The maximum actuator strain for electrically driven actuators of carbon nanotube sheets can be improved up to 0.7% in a 1 M electrolyte once the sheets are annealed in an inert atmosphere at very high temperatures (1100 oC) in contrast to once reported 0.1% or less for low electrochemical potentials (~1 V or less). The maximum strain for the carbon nanotube sheet actuators at low voltages is greater than that of the high modulus ferroelectric ceramic actuators (~0.1%), but it is lower than that of the low voltage (~0.4 V) conducting polymer actuators (~3% film direction, 20% thickness direction). Strokes were reported as high as 215% for strain biased low modulus electrostrictive rubbers under biases greater than 1kV (corresponding to an electric field 239 MV/m for the geometry mentioned in the reference paper). Spinks et al. realized pneumatic actuation from the carbon nanotube sheets in electrolyte solutions with high electrochemical potential (1.5 V), which cause gas generation in the electrolyte. The released gas dramatically increases the actuator stroke from the carbon nanotube sheet. Thickness of the carbon nanotube sheet expands by ~300% and the sheet plane contracts by 3%.
sheets of MWNTs can be realized by simply drawing the sheet from the sidewalls of CVD grown MWNT forests. UT researchers came up with the conventional method where they attach an adhesive tape to the sidewalls of MWNT forests and they pull the tape at a constant rate as fast as 7 meters per minute to get 3–5 cm wide aerogel sheets of aligned MWNTs which have exceptional mechanical and optical properties. The aerogel sheets have a density of ~1.5 mg/cm3, an areal density of 1-3 µg/cm2 and a thickness of ~20 µm. The thickness is decreased to ~50 nm by liquid-based densification to decrease the volume. The aerogel sheets can be stretched as much as three times along the width while low-modulus rubber like behavior is remained.
Having aerogel sheets of MWNTs, UT researchers fabricated actuators with giant strokes (~180% actuation along the width) with 5 ms delay time between applying the potential and observing the maximum stroke. Therefore the actuation rate is slightly better than that of the human muscle. This is a very important achievement considering the actuation rate for artificial muscles used in robots is typically much slower. Furthermore the use of carbon nanotubes as the building blocks as an artificial muscle also helps in terms of strength and robustness by making the artificial muscle stronger than steel in one direction and more flexible than rubber in the other two directions. The lack of electrolyte solution and temperature robustness of the aerogel sheet in inert ambient makes high temperature operation possible. The actuation stroke decreases by only 50% from its room temperature value to 1344 oC. Thus, this design of artificial muscles can be quite useful for many industrial applications with the drawback of high voltage operation for giant strokes.
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s have made them alternatives to the traditional electrical actuators for both microscopic and macroscopic applications. Carbon nanotubes are very good conductors of both electricity and heat, and they are also very strong and elastic molecules in certain directions. These properties are difficult to find in the same material and very needed for high performance actuators. For current carbon nanotube actuators, multi-walled carbon nanotubes (MWNTs) and bundles of MWNTs have been widely used mostly due to the easiness of handling and robustness. Solution dispersed thick films and highly ordered transparent films of carbon nanotubes have been used for the macroscopic applications.
Carbon nano-tweezers
Carbon nanotube tweezers have been fabricated by deposition of MWNT bundles on isolated electrodes deposited on tempered glass micropipettes Those nanotube budles can be mechanically manipulated by electricity and can be used to manipulate and transfer micro- and nano-structures. The nanotube bundles used for tweezers are about 50 nm in diameter and 2 µm in lengths. Under electric bias, two close sets of bundles are attracted and can be used as nanoscale tweezers.Nanotube on/off switches and random access memory
Harvard researchers have used the electrostatic attraction principle to design on/off switches for their proposed nanotube Random Access Memory devices. They used carbon nanotube bundles of ~50 nm in diameter to fabricate their proof-of-concept prototypes. One set of MWNT bundles are laid on the substrate and another set of bundles is trenched on top of the underlying nanotube bundles with air gap in between. Once electrical bias is applied the sets of nanotube bundles are attracted, thus changing the electrical resistance. These two states of resistance are on and off states. They have managed to get more than 10 times difference between off and on state resistances. This idea can be used as very highly packed arrays of nanoswitches and random access memory deivces if they can be applied to arrays of single-walled carbon nanotubes, which are about 1 nm in diameter and hundreds of micrometres in length. The current technical challenge with this design is the lack of control to place arrays of carbon nanotubes on substrate.Nanotube sheet electrodes as actuators
Researchers of AlliedSignal initially demonstrated the possibility of electrically powered actuators fabricated by carbon nanotube sheets. They taped carbon nanotube sheets on two sides of a double sided scotch tape and applied potential on the nanotube sheets in a NaCl electrolyte solution. Nanotube sheets are used as electrolyte-filled electrodes of a super capacitor. Nanotube sheets are electrically charged by the double layer formation at the nanotube-electrolyte interface without any need of ion intercalation. Therefore electrically driven actuators of nanotube sheets are superior to the conjugated polymer actuators which involve solid-state dopant diffusion and structural changes limiting rate, cycle life and energy conversion efficiencies. On the other hand, ferroelectric and electrostrictive materials are also very useful for direct energy conversion, but they require high operation voltages and ambient temperature of a limited range. Nanotube sheet actuators were shown to operate at low voltages (~1 Volts or less) and provide higher work densities per cycle than other alternative technologies. Later Baughman et al. showed that actuator response can be observed up to switching rates of 1 kHz and cycling the nanotube actuator at constant rate of 1 Hz for 140000 cycles decreases the stroke by ~33%. 0.75 MPa of stress were measured on the nanotube sheet actuators, which is greater than the maximum stress (0.3 MPa) that can be loaded on a human muscle.The maximum actuator strain for electrically driven actuators of carbon nanotube sheets can be improved up to 0.7% in a 1 M electrolyte once the sheets are annealed in an inert atmosphere at very high temperatures (1100 oC) in contrast to once reported 0.1% or less for low electrochemical potentials (~1 V or less). The maximum strain for the carbon nanotube sheet actuators at low voltages is greater than that of the high modulus ferroelectric ceramic actuators (~0.1%), but it is lower than that of the low voltage (~0.4 V) conducting polymer actuators (~3% film direction, 20% thickness direction). Strokes were reported as high as 215% for strain biased low modulus electrostrictive rubbers under biases greater than 1kV (corresponding to an electric field 239 MV/m for the geometry mentioned in the reference paper). Spinks et al. realized pneumatic actuation from the carbon nanotube sheets in electrolyte solutions with high electrochemical potential (1.5 V), which cause gas generation in the electrolyte. The released gas dramatically increases the actuator stroke from the carbon nanotube sheet. Thickness of the carbon nanotube sheet expands by ~300% and the sheet plane contracts by 3%.
Artificial muscles and giant strokes by MWNT aerogel sheets
Highly ordered free standing aerogelAerogel
Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...
sheets of MWNTs can be realized by simply drawing the sheet from the sidewalls of CVD grown MWNT forests. UT researchers came up with the conventional method where they attach an adhesive tape to the sidewalls of MWNT forests and they pull the tape at a constant rate as fast as 7 meters per minute to get 3–5 cm wide aerogel sheets of aligned MWNTs which have exceptional mechanical and optical properties. The aerogel sheets have a density of ~1.5 mg/cm3, an areal density of 1-3 µg/cm2 and a thickness of ~20 µm. The thickness is decreased to ~50 nm by liquid-based densification to decrease the volume. The aerogel sheets can be stretched as much as three times along the width while low-modulus rubber like behavior is remained.
Having aerogel sheets of MWNTs, UT researchers fabricated actuators with giant strokes (~180% actuation along the width) with 5 ms delay time between applying the potential and observing the maximum stroke. Therefore the actuation rate is slightly better than that of the human muscle. This is a very important achievement considering the actuation rate for artificial muscles used in robots is typically much slower. Furthermore the use of carbon nanotubes as the building blocks as an artificial muscle also helps in terms of strength and robustness by making the artificial muscle stronger than steel in one direction and more flexible than rubber in the other two directions. The lack of electrolyte solution and temperature robustness of the aerogel sheet in inert ambient makes high temperature operation possible. The actuation stroke decreases by only 50% from its room temperature value to 1344 oC. Thus, this design of artificial muscles can be quite useful for many industrial applications with the drawback of high voltage operation for giant strokes.