Carbon nanotube actuators

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The exceptional electrical and mechanical properties of carbon nanotubes 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 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.

Contents

Microscopic 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 bundles can be mechanically manipulated by electricity and can be used to manipulate and transfer micro- and nano-structures. [1] 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. [2] 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 an air gap in between them. 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. Using this approach, more than 10 times the difference between off and on state resistances has been achieved. Furthermore, this idea can be used to create very highly packed arrays of nanoswitches and random access memory devices, 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. This method is followed by some researches at Shahid Chamran University of Ahvaz as well. [3]

Carbon nano-heat engine

A research group at Shanghai University led by Tienchong Chang have found a domino-like motion in carbon nanotubes, which can be reversed by translating direction when different temperatures are applied. [4] This phenomenon makes it possible to use carbon nanotubes as a heat engine working between two heat sources.

Macroscopic applications

Nanotube sheet electrodes as actuators

Researchers of AlliedSignal initially demonstrated the possibility of electrically powered actuators fabricated by carbon nanotube sheets. [5] 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 supercapacitor. 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) [6] 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 140,000 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. [7]

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 (1,100 °C, 2,000 °F) in contrast to once-reported 0.1% or less for low electrochemical potentials (≈1 V or less). [7] 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). [8] Strokes were reported as high as 215% for strain-biased low-modulus electrostrictive rubbers under biases greater than 1 kV (corresponding to an electric field 239 MV/m for the geometry mentioned in the reference paper). [9] 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 aerogel sheets of MWNTs can be realized by simply drawing the sheet from the sidewalls of CVD-grown MWNT forests. UT Dallas researchers came up with the conventional method where they attach an adhesive tape to the sidewalls of MWNT forests and pull the tape at a constant rate as fast as 7 meters per minute (0.26 mph) to get 3–5 cm wide aerogel sheets of aligned MWNTs which have exceptional mechanical and optical properties. [10] 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. [11] 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. [12] 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 1,344 °C (2,451 °F). Thus, this design of artificial muscles can be quite useful for many industrial applications with the drawback of high-voltage operation for giant strokes.

Challenges and future applications

As a result, carbon nanotubes have been shown to be great materials for actuation-related applications. The subfield of carbon nanotube actuators have been quite successful and ready for scalable applications, considering there are quite a few conventional and scalable methods for the synthesis of large-scale carbon nanotubes. Carbon nanotube sheets used as electrodes in electrolyte solutions enable low voltage operations at room-temperature with actuation strokes and rates comparable to the conducting polymer actuators, but with higher work densities per cycle and lifetimes. However, the actuation strokes are much smaller than those of the electrostrictive rubbers which operate at voltages three orders of magnitude higher. On the other hand, realization of carbon nanotube aerogels made possible giant strokes compararable to electrostrictive rubbers at room temperature, but carbon nanotube aerogels can perform at a very wide range of temperatures and with very high actuation rates, which are even better than the actuation rate of human muscles.

See also

Related Research Articles

<span class="mw-page-title-main">Boron nitride</span> Refractory compound of boron and nitrogen with formula BN

Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic variety analogous to diamond is called c-BN; it is softer than diamond, but its thermal and chemical stability is superior. The rare wurtzite BN modification is similar to lonsdaleite but slightly softer than the cubic form.

<span class="mw-page-title-main">Carbon nanotube</span> Allotropes of carbon with a cylindrical nanostructure

A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized:

<span class="mw-page-title-main">Nanotube</span> Index of chemical compounds with the same name

A nanotube is a nanoscale cylindrical structure with a hollow core, typically composed of carbon atoms, though other materials can also form nanotubes. Carbon nanotubes (CNTs) are the most well-known and widely studied type, consisting of rolled-up sheets of graphene with diameters ranging from about 1 to tens of nanometers and lengths up to millimeters. These structures exhibit remarkable physical, chemical, and electrical properties, including high tensile strength, excellent thermal and electrical conductivity, and unique quantum effects due to their one-dimensional nature. Nanotubes can be classified into two main categories: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs), each with distinct characteristics and potential applications. Since their discovery in 1991, nanotubes have been the subject of intense research and development, with promising applications in fields such as electronics, materials science, energy storage, and medicine.

<span class="mw-page-title-main">Allotropes of carbon</span> Materials made only out of carbon

Carbon is capable of forming many allotropes due to its valency (tetravalent). Well-known forms of carbon include diamond and graphite. In recent decades, many more allotropes have been discovered and researched, including ball shapes such as buckminsterfullerene and sheets such as graphene. Larger-scale structures of carbon include nanotubes, nanobuds and nanoribbons. Other unusual forms of carbon exist at very high temperatures or extreme pressures. Around 500 hypothetical 3‑periodic allotropes of carbon are known at the present time, according to the Samara Carbon Allotrope Database (SACADA).

<span class="mw-page-title-main">Nanoelectromechanical systems</span> Class of devices for nanoscale functionality

Nanoelectromechanical systems (NEMS) are a class of devices integrating electrical and mechanical functionality on the nanoscale. NEMS form the next logical miniaturization step from so-called microelectromechanical systems, or MEMS devices. NEMS typically integrate transistor-like nanoelectronics with mechanical actuators, pumps, or motors, and may thereby form physical, biological, and chemical sensors. The name derives from typical device dimensions in the nanometer range, leading to low mass, high mechanical resonance frequencies, potentially large quantum mechanical effects such as zero point motion, and a high surface-to-volume ratio useful for surface-based sensing mechanisms. Applications include accelerometers and sensors to detect chemical substances in the air.

<span class="mw-page-title-main">Electroactive polymer</span>

An electroactive polymer (EAP) is a polymer that exhibits a change in size or shape when stimulated by an electric field. The most common applications of this type of material are in actuators and sensors. A typical characteristic property of an EAP is that they will undergo a large amount of deformation while sustaining large forces.

<span class="mw-page-title-main">Potential applications of carbon nanotubes</span>

Carbon nanotubes (CNTs) are cylinders of one or more layers of graphene (lattice). Diameters of single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) are typically 0.8 to 2 nm and 5 to 20 nm, respectively, although MWNT diameters can exceed 100 nm. CNT lengths range from less than 100 nm to 0.5 m.

<span class="mw-page-title-main">Nanobatteries</span> Type of battery

Nanobatteries are fabricated batteries employing technology at the nanoscale, particles that measure less than 100 nanometers or 10−7 meters. These batteries may be nano in size or may use nanotechnology in a macro scale battery. Nanoscale batteries can be combined to function as a macrobattery such as within a nanopore battery.

<span class="mw-page-title-main">Dielectric elastomers</span>

Dielectric elastomers (DEs) are smart material systems that produce large strains and are promising for Soft robotics, Artificial muscle, etc. They belong to the group of electroactive polymers (EAP). DE actuators (DEA) transform electric energy into mechanical work and vice versa. Thus, they can be used as both actuators, sensors, and energy-harvesting devices. They have high elastic energy density and fast response due to being lightweight, highly stretchable, and operating under the electrostatic principle. They have been investigated since the late 1990s. Many prototype applications exist. Every year, conferences are held in the US and Europe.

A paper battery is engineered to use a spacer formed largely of cellulose. It incorporates nanoscopic scale structures to act as high surface-area electrodes to improve conductivity.

<span class="mw-page-title-main">Mechanical properties of carbon nanotubes</span>

The mechanical properties of carbon nanotubes reveal them as one of the strongest materials in nature. Carbon nanotubes (CNTs) are long hollow cylinders of graphene. Although graphene sheets have 2D symmetry, carbon nanotubes by geometry have different properties in axial and radial directions. It has been shown that CNTs are very strong in the axial direction. Young's modulus on the order of 270–950 GPa and tensile strength of 11–63 GPa were obtained.

<span class="mw-page-title-main">Optical properties of carbon nanotubes</span> Optical properties of the material

The optical properties of carbon nanotubes are highly relevant for materials science. The way those materials interact with electromagnetic radiation is unique in many respects, as evidenced by their peculiar absorption, photoluminescence (fluorescence), and Raman spectra.

A device generating linear or rotational motion using carbon nanotube(s) as the primary component, is termed a nanotube nanomotor. Nature already has some of the most efficient and powerful kinds of nanomotors. Some of these natural biological nanomotors have been re-engineered to serve desired purposes. However, such biological nanomotors are designed to work in specific environmental conditions. Laboratory-made nanotube nanomotors on the other hand are significantly more robust and can operate in diverse environments including varied frequency, temperature, mediums and chemical environments. The vast differences in the dominant forces and criteria between macroscale and micro/nanoscale offer new avenues to construct tailor-made nanomotors. The various beneficial properties of carbon nanotubes makes them the most attractive material to base such nanomotors on.

A carbon nanotube field-effect transistor (CNTFET) is a field-effect transistor that utilizes a single carbon nanotube (CNT) or an array of carbon nanotubes as the channel material, instead of bulk silicon, as in the traditional MOSFET structure. There have been major developments since CNTFETs were first demonstrated in 1998.

<span class="mw-page-title-main">Supercapacitor</span> High-capacity electrochemical capacitor

A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than solid-state capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable batteries. It typically stores 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerates many more charge and discharge cycles than rechargeable batteries.

Artificial muscles, also known as muscle-like actuators, are materials or devices that mimic natural muscle and can change their stiffness, reversibly contract, expand, or rotate within one component due to an external stimulus. The three basic actuation responses—contraction, expansion, and rotation—can be combined within a single component to produce other types of motions. Conventional motors and pneumatic linear or rotary actuators do not qualify as artificial muscles, because there is more than one component involved in the actuation.

<span class="mw-page-title-main">Chemiresistor</span> Material with changing electrical resistance according to its surroundings

A chemiresistor is a material that changes its electrical resistance in response to changes in the nearby chemical environment. Chemiresistors are a class of chemical sensors that rely on the direct chemical interaction between the sensing material and the analyte. The sensing material and the analyte can interact by covalent bonding, hydrogen bonding, or molecular recognition. Several different materials have chemiresistor properties: semiconducting metal oxides, some conductive polymers, and nanomaterials like graphene, carbon nanotubes and nanoparticles. Typically these materials are used as partially selective sensors in devices like electronic tongues or electronic noses.

<span class="mw-page-title-main">Synthesis of carbon nanotubes</span> Class of manufacturing

Techniques have been developed to produce carbon nanotubes (CNTs) in sizable quantities, including arc discharge, laser ablation, high-pressure carbon monoxide disproportionation, and chemical vapor deposition (CVD). Most of these processes take place in a vacuum or with process gases. CVD growth of CNTs can occur in a vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth are making CNTs more commercially viable.

In materials science, vertically aligned carbon nanotube arrays (VANTAs) are a unique microstructure consisting of carbon nanotubes oriented with their longitudinal axis perpendicular to a substrate surface. These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled. VANTAs are consequently widely useful in a range of current and potential device applications.

<span class="mw-page-title-main">Graphenated carbon nanotube</span>

Graphenated carbon nanotubes (G-CNTs) are a relatively new hybrid that combines graphitic foliates grown along the sidewalls of multiwalled or bamboo style carbon nanotubes (CNTs). Yu et al. reported on "chemically bonded graphene leaves" growing along the sidewalls of CNTs. Stoner et al. described these structures as "graphenated CNTs" and reported in their use for enhanced supercapacitor performance. Hsu et al. further reported on similar structures formed on carbon fiber paper, also for use in supercapacitor applications. Pham et al. also reported a similar structure, namely "graphene-carbon nanotube hybrids", grown directly onto carbon fiber paper to form an integrated, binder free, high surface area conductive catalyst support for Proton Exchange Membrane Fuel Cells electrode applications with enhanced performance and durability. The foliate density can vary as a function of deposition conditions with their structure ranging from few layers of graphene to thicker, more graphite-like.

References

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