Carbon Nanotube Actuators Research Articles

Bond order effects in electromechanical actuation of armchair single-walled carbon nanotubes

In this paper we first use ab initio simulations to study the strains induced by charging an armchair (5,5) carbon nanotube (CNT) segment. The observed behavior is far from a monotonic expansion that one might have expected from a classical point of view. Subsequently a new method is proposed to predict the nonelectrostatic part of the electromechanical actuation response of the nanotube based on the spatial distribution of its molecular orbitals. Locally bonding and locally antibonding molecular orbitals are defined for the CNT segment structure based on analogy with bonding and antibonding orbitals in diatomic molecules. The nonmonotonic overall actuation is explained based on the above proposition and the general alignment of the expanding and contracting bonds with respect to the axis or circumference of the CNT segment. Using the well-known concept of bond order, the actuation of this complex system of many atoms is predicted with close quantitative agreement with the ab initio simulations.

Nonlinear Dynamics of Electrically Actuated Carbon Nanotube Resonators

Abstract This work presents an investigation of the nonlinear dynamics of carbon nanotubes (CNTs) when actuated by a dc load superimposed to an ac harmonic load. Cantilevered and clamped-clamped CNTs are studied. The carbon nanotube is described by an Euler–Bernoulli beam model that accounts for the geometric nonlinearity and the nonlinear electrostatic force. A reduced-order model based on the Galerkin method is developed and utilized to simulate the static and dynamic responses of the carbon nanotube. The free-vibration problem is solved using both the reduced-order model and by solving directly the coupled in-plane and out-of-plane boundary-value problems governing the motion of the nanotube. Comparison of the results generated by these two methods to published data of a more complicated molecular dynamics model shows good agreement. Dynamic analysis is conducted to explore the nonlinear oscillation of the carbon nanotube near its fundamental natural frequency (primary-resonance) and near one-half, twice, and three times its natural frequency (secondary-resonances). The nonlinear analysis is carried out using a shooting technique to capture periodic orbits combined with the Floquet theory to analyze their stability. The nonlinear resonance frequency of the CNTs is calculated as a function of the ac load. Subharmonic-resonances are found to be activated over a wide range of frequencies, which is a unique property of CNTs. The results show that these resonances can lead to complex nonlinear dynamics phenomena, such as hysteresis, dynamic pull-in, hardening and softening behaviors, and frequency bands with an inevitable escape from a potential well.

Thermal gradient induced actuation in double-walled carbon nanotubes

Molecular dynamics simulations are applied to investigate the thermal gradient inducedactuation in double-walled carbon nanotubes, where a temperature difference can actuatethe relative motion of double-walled carbon nanotubes. The thermal driving forcecalculated through a stationary scheme is on the order of pico Newtons for a 1 K nm−1 temperature gradient. The driving force is approximately proportional to the temperaturegradient, but not sensitive to the system temperature. For the outer tube longer than 5 nm,the thermal driving force is nearly constant. For the outer tube shorter than 5 nm, however,the driving force decreases with decreasing tube length. The motion trace is found todepend on both the chirality pair and system temperature. A critical temperature can bedefined by the potential barrier perpendicular to the minimum energy track of potentialpatterns. When the system temperature is higher than the critical temperature, the motionshows random behavior. When the system temperature is lower than the criticaltemperature, the motion, translational and/or rotational, is confined within theminimum energy track, which is indicative of the feasibility of directional control.

Computational design and multiscale modeling of a nanoactuator using DNAactuation

Developments in the field of nano-biodevices coupling nanostructures and biologicalcomponents are of great interest in medical nanorobotics. As the fundamentals ofbio/non-bio interaction processes are still poorly understood in the design of these devices,design tools and multiscale dynamics modeling approaches are necessary at the fabricationpre-project stage. This paper proposes a new concept of optimized carbon nanotube basedservomotor design for drug delivery and biomolecular transport applications. The designof an encapsulated DNA-multi-walled carbon nanotube actuator is prototypedusing multiscale modeling. The system is parametrized by using a quantum levelapproach and characterized by using a molecular dynamics simulation. Based onthe analysis of the simulation results, a servo nanoactuator using ionic currentfeedback is simulated and analyzed for application as a drug delivery carrier.

Understanding the microscopic processes that govern the charge-induced deformation of carbon nanotubes

While carbon nanotubes have technological potential as actuators, the underlying actuation mechanisms remain poorly understood. We calculate charge-induced stresses and strains for electrochemical actuation of carbon nanotubes with different chiralities and defects, using density-functional theory and various tight-binding models. For a given deformation mode the concept of bonding and antibonding orbitals can be redefined depending on the sign of a differential band-structure stress. We use this theoretical framework to analyze orbital contributions to the actuation. These show charge asymmetric behavior which is due to next-nearest-neighbor hopping while Coulombic contributions account for approximately charge-symmetric isotropic deformations. In the typical case of a (10,10) tube strains around 0.1% with 1 nN force along the tube axis are obtained. Defects and functional groups have negligible influence on the actuation. In multiwall tubes we find charge inversion on the inner tubes due to Friedel-type oscillations which could lead to a slight magnification of charge-induced strains. Finally, we consider photoactuation of nanotubes and predict that transitions between van-Hove singularities can be expected to expand the tubes.

Actuator properties of the complexes composed by carbon nanotube and ionic liquid: The effects of additives

Two different types of additives were mixed into the electrode of carbon nanotube (CNT) actuators low-voltage-driven in air to investigate the influence of additives on the actuator performance. The electrode was composed by single-walled carbon nanotubes (SWNTs), an ionic liquid (IL), a base polymer, and an additive. The generated strain was estimated by measuring displacement changes of the actuator element in the frequency range between 5mHz and 200Hz. In case of a non-conductive additive, that is, mesoporous silica (MCM-41), the maximum strain of the actuator increased about 1.6 times as large as that of the non-additive actuator. On the contrary, a conductive additive, polyaniline (poly-An), caused a remarkable improvement of the strain (ca. 3 times) for the actuator. It is found that this improvement was achieved by the effective charge storage in the actuator element. The generated stress was also determined by Hooke's law with the parameter of the maximum strain and the Young's modulus of actuator composite.

Photo-mechanical actuation of carbon nanotubes: mechanisms and applications in micro and nano-devices

We review the current state of the art in photomechanical actuation of carbon nanotubes and their composites. The article presents key points in the photomechanical responses of carbon nanotubes and show how orientation, alignment and anisotropic optical properties of carbon nanotubes influence photomechanical actuation. The current state of the field of carbon-nanotube polymer photomechanical actuators is presented. The photomechanical responses of nanotube-polymer systems depend on the structure of the actuator, nanotube alignment, entanglement and the presence of pre-strains in the sample. While the mechanism of photomechanical actuation is not fully understood, we discuss some possible mechanisms that contribute to the overall photomechanical responses of carbon nanotubes and their polymer composites. It is expected that the interplay between elastic, electrostatic, polaronic and thermal interactions give rise to the overall photomechanical responses of carbon nanotubes. We also discuss some insights into how the photo-mechanical responses of the carbon nanotubes and their polymers may be coupled to the band structure of carbon nanotubes. Finally, we review the applications of photomechanical actuation of carbon nanotube in Micro-Electro-Mechanical Systems (MEMS) and nanotechnology.

Electromechanical actuation of macroscopic carbon nanotube structures: mats and aligned ribbons

The electromechanical actuation of macroscopic carbon nanotube (CNT) structures, including single and multi walled CNT mats and aligned ribbons, is analyzed and compared. From experimental evidence, actuation due to quantum chemical and electrostatic effects can be distinguished. Their respective contribution to the total actuation depends on two key parameters, namely Young's modulus and charge density. While mechanical energy densities of actuated structures are appreciable, electromechanical conversion efficiencies are found to be impractically low.

Biomimetic Soft Multifunctional Miniature Aquabots

Engineered locomotive systems are widespread on the macro scale (e.g., planes, trains, automobiles), yet miniaturized mobile systems are largely limited to the research lab. In particular, integration of specific functions such as sensing or drug delivery into a small robot is a great challenge. The ability to miniaturize parts affecting motion, such as MEMS-based components and electroactive polymers, while widely demonstrated has not led to practical advances in miniature multifunctional mobile robots. Rather complex construction, limits in the use of diverse materials, and cost (in case of the MEMS-based bot) all hinder the production of these robots. The small (micro-/millimeter scale) system within aqueous environments is particularly challenging, yet there are important potential applications for robots in such environments. To date, engineers have explored soft materials for creating components at the micro-/millimeter scales; for example, actuators such as ionic polymer metal composites, conducting polymers, and carbon nanotube actuators, and engineered devices housing hydrogel valves, filters, and lenses, as well as other soft material actuators that mimic natural movement (earthworm, myriapod, etc.). Still, the creation of miniature multifunctional robots that operate in aqueous environments has been elusive. Nature provides numerous examples of small, multifunctional and autonomous ‘‘robots’’ in the insect and marine worlds after which we aimed to model miniature polymeric aquabots. Here we present mini (microto millimeter) soft aquabots that combine multiple functionalities to perform multifunctional operations in aqueous environments, effectively simulating their natural counterparts. Despite extensive research on macroscale robotics and miniature micro-electromechanical systems, relatively little attention has been paid to the creation of miniature soft robots with diverse shapes, actuation mechanisms, and integrated functionalities. As a fabrication method, we describe a technique that takes advantage of the small volume required by amicrofluidic chamber; this will be used for photopolymerizing multifunctional minibots that possess several ‘‘organs,’’ allowing for movement, sensing/signaling, and capture/transport/release. A diverse group of aquabots mimicking three kinds of living organisms – octopus, sperm, and myriapod, each representing a different mode of locomotion – has been developed. These aquabots have the following key features: 1) diverse shape and small size (sub-millimeter feature sizes); 2) a variety of functions (e.g., actuation, sensing) integrated in a single robot; and 3) rapid, scalable fabrication both in dimension and quantity. The aquabots were fabricated out of soft materials, due to the ease of polymerization, as well as the ability to tailor the material composition towards achieving certain functionality (e.g., responsiveness to pH, temperature, or electric/ magnetic field). Sequential in situ photopolymerization within a microfluidic device allowed for precise fabrication of different components (arms, legs, body; see detailed procedure for photopolymerization in Experimental section). The bodies were constructed using a relatively inert polyethylene glycol (PEG) material, whereas ionic electroactive polymers (electroactive hydrogels) were chosen from a large number of available soft materials to serve as the actuators, thus facilitating controlled movement. Depending on the application, some aquabots contained temperature-, pH-, or chemical-sensitive polymers in addition to their electroactive hydrogel and PEG components. The gel network of polymerized actuator material was anionic, causing positively charged surfactant molecules to bind to its surface; this surfactant binding leads to an osmotic pressure difference between the gel interior and the external solution, thus inducing the contraction and curvature of the gel strip. Four parameters control electroactive hydrogelbased actuators: 1)magnitude of the applied potential; 2) angle of the applied potential; and both 3) width and 4) length of the actuator. Figure 1A illustrates the behavior of a rectangular gel actuator under changing parameters (width of post and applied voltage; see Supporting Information Fig. S1 for more complete parameter characterization). In situ photopolymerization facilitates the control of the geometric parameters and is amenable to the use of laser or multiphoton polymerization methods for improved resolution (e.g., further reduce actuator width to achieve faster and larger motion). To date, diverse electroactive hydrogels have been broadly applied to creating polymeric microactuators (see the detailed behavior of electric-sensitive hydrogel in Supporting Information). Here, we demonstrate the use of electroactive hydrogel for fabricating octopus-, sperm-, and myriapod-like aquabots that retain respective characteristic movement (Fig. 2; movies are available for octopus-like aquabot and sperm-like aquabots in Supporting Information – Videos 1 and 2, respectively). The repeated bending test under aquatic conditions was carried out to investigate the durability of the electroactive hydrogel strips (see Supporting Information for a detailed durability test conditions). The operation of the communications

Modeling of Carbon Nanotube Actuators: Part II — Mechanical Properties, Electro Mechanical Coupling and Validation of the Model

Carbon nanotubes have the potential to become one of the actuating materials of the future. Since the discovery of the actuating effect a lot of experimental data was collected to describe the electro-mechanical coupling. With this paper a model for the system behavior is available to enhance the understanding of the effect. An analytical model is introduced in the first part of the article. The electrical parameters of the model are determined by impedance spectroscopy. Furthermore, the mechanical parameters and the electro-mechanical coupling have to be examined. The main subject of the second part of the article is to validate the model and to compare measured and simulated responses to several excitations.

Electromechanical actuation of single-walled carbon nanotubes: an ab initio study

The mechanical actuation of a (5, 5) single-walled carbon nanotube as a result of addedcharge is simulated using first-principles calculations. It is observed that while both positiveand negative charging tend to expand the nanotube in the axial direction for most levels ofcharge, radial actuation is less even and symmetric with respect to charge. The spindistribution of the additional charges is investigated, and it is predicted that in some casesunpaired spin configurations are energetically favourable, significantly affecting actuationstrains.

Modeling of Carbon Nanotube Actuators: Part I — Modeling and Electrical Properties

The outstanding electrical and mechanical properties of single carbon nanotubes (CNT) are the motivation for an intensive research in various fields of application. The actuation effect constitutes the foundation for any application as a multifunctional material and within the field of adaptronics. The effect is in the majority of cases investigated by a CNT configuration of stochastically aligned CNT, so-called bucky-paper, in an electrolytic environment. The article presents an analytical model for a detailed understanding and investigation of the actuation process. The complete description and parameterization of the model is documented in two parts. In the first part the model is developed, the test setup for the model validation and parameter identification is elucidated, and the electrical parameters are determined. In the second part the mechanical system and the actuating effect will be examined. Finally the applicability of the model will be examined.

Single-Walled Carbon Nanotubes — Ionic Polymer Electroactive Hybrid Transducers

Ionic electroactive polymers, sometimes referred to as artificial muscles, have the ability to generate large bending strain and moderate stress at low applied voltages. Typical types of ionic electroactive polymer transducers include ionic polymers, conducting polymers, and carbon nanotubes. Preliminary research combining multiple types of materials proved to enhance certain transduction properties. Bennett and Leo (Materials Research Society Symposium Proceedings, vol. 785, 2003b) showed that the speed of response, maximum strain, and quasi-static actuation are improved by adding a layer of conducting polymer on an ionic polymer transducer. In this work, a recently developed fabrication method called the direct assembly process (DAP) plating is used to build SWNT/RuO2 hybrid transducers. The DAP consists of mixing a conducting powder with an ionomer solution. This technique has demonstrated improved response time and strain output as compared to previous methods. Electrodes applied using this new technique of mixing RuO2 (surface area 45—65 m2/g) particles and NafionTMdispersion provided 5× the displacement and 10× the force compared to a transducer made with conventional methods. Furthermore, previous studies demonstrated that the response speed of the transducer is optimized by varying the composition of metal in the electrode (Akle, B.J., Bennett, M.D., Leo, D.J., Wiles, K.B. and McGrath, J.E. 2007. "Direct Assembly Process: A Novel Fabrication Technique for Large Strain Ionic Polymer Transducers,'' Journal of Mat. Sci., 42:7031—7041). For RuO2, the optimal loading was approximately 45vol%, while carbon nanotubes electrodes have an optimal performance at 30vol%. Due to low percolation threshold, carbon nanotubes actuators perform better at a lower loading compared to other conducting powders. The addition of single-walled carbon nanotubes (SWNT) to the electrode increases both the strain rate and the maximum strain of the hybrid actuator. The strain rate of the transducer increased proportional to the ratio of SWNT to RuO2 in the electrode. A maximum peak-to-peak strain of 10.6% (±2V) is attained in a 15vol% SWNT/30 vol% RuO2 hybrid transducer. The maximum strain rate of 2.7%/s is generated by a 20vol% SWNT/25 vol% RuO2 hybrid transducer.

Electroactive carbon nanotube actuators: Soft-lithographic fabrication and electro-chemical modelling

Carbon nanotube (NT) actuators were fabricated using the micromolding method. In order to prevent the nanotubes from aggregating, they were enclosed in a partially cross-linked polyvinylalcohol–polyallylamine matrix. A unimorph matrix bender in the form of a small multi-layered strip, 5 mm × 15 mm was fabricated. The resulting composite system: NT/PVA, solid polymer electrolyte and metal contact, was characterised using impedance analysis and cyclic voltammetry. Initial measurements on electro-mechanical transduction show a conversion efficiency of 0.2 μm/mV. An electrochemical model of the impedance of the system was then developed, with which the experimental results agree fairly well.

Electrochemical actuation of carbon nanotube yarns

We report on actuation in high tensile strength yarns of twist-spun multi-wall carbonnanotubes. Actuation in response to voltage ramps and potentiostatic pulses isstudied to quantify the dependence of the actuation strain on the applied voltage.Strains of up to 0.5% are obtained in response to applied potentials of 2.5 V. Thedependence of strain on applied voltage and charge is found to be quadratic, inagreement with previous results on the actuation of single-wall carbon nanotubes, withthe magnitude of strain also being very similar. The specific capacitance reaches26 F g−1. The modulus of the yarns was found to be independent of applied load and voltage withinexperimental uncertainty.

Nanotube micro-opto-mechanical systems

While there have been numerous reports on the electromechanical actuation of carbonnanotubes (CNTs), devices based on the photomechanical actuation of CNTs have beenelusive until now. In this paper, we report the integration of single-wall carbonnanotube ensembles into micro-mechanical systems to realize a new carbon nanotubemicro-opto-mechanical system (CNT-MOMS). CNT-MOM grippers were fabricated usingCMOS compatible techniques involving nanotube film forming, wafer bonding,photolithography, plasma etching and dry release. A MOM-gripper displacement of∼24 µm was obtained froma gripper of 430 µm in length under infrared laser stimulus and continuous operation of more than 140 000 cycles wasacquired. The optical power consumption of the gripper operation was estimated to be as small as∼240 µW. Manipulation of polystyrene microspheres in air was demonstrated using CNT-MOMSgrippers. This study is one good example of how nanomaterials could be integrated intoCMOS compatible techniques for applications in high performance MEMS and nanoscaleactuation technologies.

Carbon Nanotubes as Optical Materials

AbstractThis paper discusses some of the highly interesting effects that occur when photons interact with carbon nanotubes. From position dependent photoconductivity of nanotube thin films to photon induced elastic actuation of carbon nanotubes is presented. A new field of micro-opto-mechanical systems (MOMS) is envisioned through the miniaturization of nanotube actuators using MEMS and CMOS processes. Number of remotely controlled MOMS devices including MOMS grippers, MOMS cantilevers and MOMS mirrors are presented. The performance of these devices rivals their MEMS electrostatic counterparts while consuming only fraction of energy and enabling remote controllability. Finally, the interaction of light with nanotubes for biomedical nanotechnology and photodynamic cancer therapy is presented.

Fast Carbon Nanotube Charging and Actuation

A number of new and established technologies employ electrical stimuli to induce mechanical deformation in materials. Carbon nanotube (CNT) actuators have recently attracted attention due to their low operation voltage (∼ 1 V), high operating load (26 MPa to date), and high tensile strengths (1.8 GPa, and possibly reaching 37 GPa in individual tubes). However, CNT actuators tend to exhibit a full response on a timescale of seconds rather than the milliseconds typical of electromagnetic actuators, piezoceramics, and muscle. A key objective of this work is to push the limits of CNT actuation rates. A second objective is to observe charging rates. CNT papers have been shown to exhibit specific capacitances on the order of 10 F kg and higher. If they can also be shown to charge and discharge rapidly, they will demonstrate particular promise for use as high-power energystorage devices. Previous work presented the use of resistance compensation to increase the rate of actuation in CNT devices. The rate of actuation was found to be limited to 3 % s. At high acceleration rates the inertia of the apparatus appeared to be the limiting rate. In the present work, the rate of change in force is measured while CNT films are held at a fixed length (isometric conditions). The advantage of measuring force changes under isometric conditions is that there are no moving parts, and hence inertia does not limit the peak rates that can be observed. CNTs are formed into porous sheets or fibers which are immersed in electrolyte to form electrodes in an electrochemical cell. The application of potential causes the formation of an electrical double layer at the nanotube/electrolyte interface in which the electronic charge stored in the carbon atoms is balanced by the ionic charge in the electrolyte. The accumulated nanotube charge and the resulting change in band structure is believed to cause a change in the C–C bond length during electron injection due to a number of factors, including Coulomb repulsion. Assuming that these mechanisms are correct, it is the rate at which charge can be injected that determines the strain rate. The key to increasing rate is to increase the current injected per unit volume. Achieving rapid charging rates is a challenge in fibers and sheets composed of CNTs because of the enormous capacitance of CNT electrodes, which is a consequence of the enormous internal surface area. This capacitance is more than six orders-of-magnitude higher than is observed in polyester capacitors, and is similar to that of commercial superand ultracapacitors. Internal resistances and ion-transport rates prevent rapid charging and discharging, limiting the effectiveness of CNTs both as actuators and supercapacitors. Work by Niu and colleagues has shown that it is possible to obtain fast response times by using a combination of relatively thin porous CNT electrodes and high-conductivity electrolytes. Charge and discharge times of 7 ms and specific powers of 8 kW kg were reported. In this paper, it is shown that fast response times can be obtained both for actuation and charging of CNT papers. In order to increase the rates of charging and actuation in thin, porous CNT films, millisecond-long voltage pulses of up to ± 35 V in amplitude are applied. Potentials are much higher than the ∼ 1 V used in charging of CNT paper and fiber actuators and supercapacitors, where such large potential magnitudes would typically lead to parasitic reactions and degradation. However, if the solution resistance and any contact and lead resistance are significantly greater than the CNT paper impedance, most of the voltage drop will initially be across these rather than the double layer within the nanotube films. The use of short pulses ensures that no overcharging of the nanotube double layer will occur, allowing rapid charging without unwanted reactions. The maximum allowable pulse duration is estimated by multiplying the desired potential change at the electrode (for example, 1 V) by the elecC O M M U N IC A TI O N S

Comparison of Piezoelectric and Carbon Nanotube-Based Actuators

AbstractThe goal of this study is to compare operation and performance of the piezoelectric and carbon nanotube actuators, by focusing on the conditions that such piezoelectric device can be replaced by carbon nanotube actuators in a near future. For many years piezoelectric (PZT) and electrostrictors devices have been the key elements for the construction of actuator systems such as positioners, miniature ultrasonic motors, and adaptive mechanical motors. However, in recent years major advances and accomplishments have been reported in the field of nanotube materials, which promise a breakthrough in the field of nanosensors and nanoactuators. The aim here is to make a comparison between the PZT- and CNT-based actuator/sensor devices in terms of the actuator materials, designs, and device control techniques. First, a model is established for the operation of a typical PZT actuator and the related electromechanical coupling relations and equations governing the actuation process are described. In the second study actuation behavior of carbon nanotube materials is modeled and the results concerning a similar parameter to that of PZT are reported.

탄소나노튜브 액츄에이터의 이론적 모델링

Carbon nanotubes have outstanding properties which make them useful for a number of high-technology applications. Especially, single-walled carbon nanotube (SWNT), working under physical conditions (in aqueous solution) and converting electrical energy into mechanical energy directly, can be a good substitute for artificial muscle. The carbon nanotube structure simulated in this paper is an isotropic cantilever type with an adhesive tape which is sandwiched between two SWNTs. For predicting the geometrical and physical parameters such as deflection, slope, bending moment and induced force with various applied voltages, the analytical model for a 3 layer bimorph nanotube actuator is developed by applying Euler-Bernoulli beam theory. The governing equation and boundary conditions are derived from energy Principles. Also, the brief history of carbon nonotube is overviewed and its properties are compared with other functional materials. Moreover, an electro-mechanical coupling coefficient of the carbon nanotube actuator is discussed to identify the electro-mechanical energy efficiency.