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Flexible, all-inorganic actuators based on vanadium dioxide and carbon nanotube bimorphs He Ma, Jiwei Hou, Xuewen Wang, Jin Zhang, Zhiquan Yuan, Lin Xiao, Yang Wei, Shoushan Fan, Kaili Jiang, and Kai Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04393 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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Flexible, all-inorganic actuators based on vanadium dioxide and carbon nanotube bimorphs He Ma1, Jiwei Hou2, Xuewen Wang2, Jin Zhang1, Zhiquan Yuan2, Lin Xiao3, Yang Wei1*, Shoushan Fan1, Kaili Jiang1, Kai Liu2*

1.

State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics and

Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China

2.

State Key Laboratory of New Ceramics and Fine Processing, School of Material Science and

Engineering, Tsinghua University, Beijing 100084, China

3.

Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology,

Beijing 100094, China

Keywords Vanadium dioxide; doping; carbon nanotube; phase transition; actuator.

ACS Paragon Plus Environment

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Abstract

Flexible actuators responsive to multiple stimuli are much desired in wearable electronics. However, general designs containing organic materials are usually subject to slow response and limited lifetime, or high triggering threshold. In this study, we develop flexible, all-inorganic actuators based on bimorph structures comprised of vanadium dioxide (VO2) and carbon nanotube (CNT) thin films. The drastic, reversible phase transition of VO2 drives the actuators to deliver giant amplitude, fast response up to ~100 Hz, and long lifetime more than 1,000,000 actuation cycles. The excellent electrical conductivity and light absorption of CNT thin films enable the actuators to be highly responsive to multiple stimuli including light, electric, and heat. The power consumption of the actuators can be much reduced by doping VO2 to lower its phase transition temperature. These flexible bimorph actuators find applications in biomimetic inspect wings, millimeter-scale fingers, and physiological-temperature driven switches.


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Actuators, which convert external stimuli to mechanical motions, are essential components in modern electronics for a wide range of applications including biomimetic robots, artificial muscles, and micro-electro-mechanical systems.1-5 Fast response, long lifetime, good flexibility, and low triggering threshold are much desired in actuators especially in recent years owing to the emerging demands of wearable devices. The flexible actuators are expected to be used as electrical switches for flexible supercapacitor, lithium ion battery and other electrochromatic devices.6,

7

However, general designs of flexible actuators are based on all-organic or

organic/inorganic composite materials, usually subject to slow response, limited lifetime, or high triggering threshold. Ionic electroactive polymer (ionic EAP), electronic EAP, and thermally active polymer are major actuation materials for flexible actuators based on various driving mechanisms.2, 8 Most of ionic EAP have low driving voltages (~several volts), yet their responses are typically slow, in the range of 0.1 to 10 s.9-13 The response time of thermally active polymers is limited by their low thermal conductivity, as a result usually larger than 0.1 s.14-17 Electronic EAP could respond as fast as down to ~1 ms, whereas their high driving voltages usually over 1 kV have greatly limited the range of their applications.18-22 Therefore, it is still a great challenge to build flexible actuators, which could rapidly respond to multiple stimuli, with low requirements for driving conditions.

Here we develop flexible, all-inorganic actuators based on bimorph structures comprised of vanadium dioxide (VO2) and carbon nanotube (CNT) thin films. The drastic, reversible phase transition of VO2 drives the actuators to deliver giant amplitude, fast response up to ~ 100 Hz,


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and long lifetime more than 1,000,000 actuation cycles. The excellent electrical conductivity and light absorption of CNT thin films enable the actuators to be highly responsive to multiple stimuli including light, electric, and heat. The power consumption of the actuators can be much reduced by doping VO2 to lower its phase transition temperature. These flexible bimorph actuators find applications in biomimetic inspect wings, millimeter-scale fingers, and physiological-temperature driven switches.

VO2 attracts growing attention in recent years as an actuation material because of its structural change during its metal-insulator transition (MIT) at 68 °C. Transiting from a low-temperature, insulating phase to a high-temperature, metallic phase, VO2 shrinks along the cR axis of its metallic phase, resulting in a spontaneous strain ε as high as 1-2 %.23 It thus allows a volumetric work density up to 7-28 J/cm3 (elastic modulus ~140 GPa), which is 2-4 orders of magnitude higher than piezoceramics and human muscles.24 The energy efficiency is 7.7 %, almost 64% the value in ideal Carnot cycles.23 Combining other merits such as extremely fast transition process, facile micro-fabrication, and retaining of MIT even in nanolayers, VO2 is considered as an ideal actuation material for flexible inorganic actuators.23-30

CNT films used in this work are drawn and cross-stacked from a super-aligned carbon nanotube array by a facile dry-spinning method.31 The as-prepared CNT films are free standing, highly flexible, well conductive, and strong,32 being a promising frame for flexible actuators (Fig. S1). The VO2/CNT actuators are fabricated by depositing VOx onto CNT films and then annealing them in low-pressure oxygen (see details in Materials and Methods section). Raman


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spectra of the CNT films before and after the annealing show that the CNT films are very stable and no additional defects are introduced in the annealing process (Fig. S2). Before the deposition of VOx, CNT films are patterned as desired by a rapid laser trimming process (Fig. 1A). First, a 50-layer cross-stacked CNT film was attached firmly on a patterned Si3N4 substrate. Second, the CNT film was patterned by a computer-programmed laser scanning system (LP-YLM-12F, Leijieming Laser corp.), which had been used in our previous work.33, 34 The redundant CNT films were removed manually. Finally, VO2 film was grown on the patterned CNT films by the deposition and annealing processes (See Materials and Methods).

Based on this laser patterning technique, CNT films can be rapidly cut into various geometries for different actuation purposes. Fig. 1B shows CNT films shaped into butterflies, triangular cantilevers, and rectangular cantilevers. After the deposition of VO2, the VO2/CNT actuators inherit the excellent flexibility from the freestanding CNT films. The versatility in designing arbitrary patterns facilitates the VO2/CNT actuators for a wide range of requirements.

To study the properties of VO2/CNT actuators, triangular cantilevers (Fig. 1C) are used because of their structural and mechanical stability. Scanning electron microscopy (SEM) image in Fig. 1D shows the morphology of cross-stacked network of the VO2/CNT film. Vanadium element distributes uniformly on the CNT film, as revealed by the energy dispersive X-ray spectroscopy (EDX, Fig. 1F). Raman spectra of VO2/CNT films (Fig. 1E) peak at 141(Ag), 194(Ag), 223(Ag), 261(Bg), 309(Ag), 337(Bg), 390(Ag), 440(Bg), 500(Ag), and 617(Ag) cm-1, confirming that M1 insulating phase of VO2 form on CNT films.35, 36


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The driving force of VO2/CNT actuators originates from the MIT of VO2 triggered thermally by external stimuli. We found that VO2 is preferentially orientated along the (011) plane on CNT films (Fig. S3) and thus the cR axis of VO2 generally sits in the plane of VO2/CNT films, which causes VO2 to shrink along the in-plane direction and VO2/CNT films to bend towards the VO2 side when heated beyond the MIT temperature (Figs. 2A and 2B). As shown in Fig. 2C, a hysteresis exists during the heating and cooling processes, and the resistance of the VO2/CNT actuator decreases by ~ 11% across the MIT (60 to 80 oC) of VO2, which is much lower than the value of pure VO2 (usually 3-4 orders of magnitude). We attribute this small value of the resistance change mainly to the parallel electric connection of VO2 and CNT films, because the CNT film is much thicker than the VO2 film (1.5 µm vs. 0.18 µm), and the conductivity of the CNT film is comparable to the value of the metallic VO2 (0.001-0.01 Ω cm) but is much larger than insulating VO2 (1-10 Ω cm) (see detailed calculations in Supporting Information, Fig. S4). The bending amplitude of the VO2/CNT actuator changes drastically when the MIT of VO2 occurs (Fig. 2D, ~670 µm in bending amplitude vs. ~1700 µm in cantilever length). This bending behavior and its hysteresis feature follow well with the temperature dependence of electrical resistance (Figs. 2C and 2D), validating that the driving mechanism is the MIT of VO2 rather than the thermal expansion mismatch between VO2 and CNT films.

The strong light absorption, good electrical conductivity, and extremely small heat capacity per unit area of CNT films enable the actuators to sensitively respond to multiple stimuli such as electric and light, besides only heat.37, 38 The resulted photo-actuators and electro-actuators are


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more desirable in many applications due to their capabilities of wireless or fast control. The power to trigger the VO2/CNT actuators with both laser irradiation and Joule heating ranges from 800-1600 mW/cm2 (Fig. 2E). This value has been much lower than the reported CNT-coated, VO2-based photo-actuators (~2.9×104 mW/cm2)39, although it is almost one order of magnitude higher than the solar constant (136 mW/cm2), which results from the relatively high MIT temperature that must be reached by an increase of ~ 40 °C from room temperature.

Lowering the MIT temperature of VO2 brings many benefits including a tunable range of working temperature and a lower power consumption of VO2/CNT actuators. Therefore, we developed W-doped VO2/CNT (W-VO2/CNT) actuators based on the knowledge that the MIT temperature of VO2 decreases with W doping.40-43 With a 1.5 at % W doping in the VO2 film, the driving temperature of W-VO2/CNT actuators is decreased to approximately 34 oC (Fig. 2C and Fig. S7), slightly above the room temperature and very close to physiological temperatures. It suggests that these W-VO2/CNT actuators can be also triggered by physiological ways, for examples, touching and expiration. Although the change amplitude of electrical resistance for W-VO2/CNT is smaller than that for VO2/CNT (6.8% vs. 11%), the bending amplitudes for both types of actuators are comparable (Fig. 2D), leading a ratio of tip displacement to cantilever length in the range of 0.39-0.45 when the length is 1700 µm. Owing to the decreased MIT temperature, the power required to trigger W-VO2/CNT actuators is also much reduced to 250-800 mW/cm2 (Fig. 2E), which could be simply acquired by a focused sunlight.


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We also studied the frequency response of VO2/CNT and W-VO2/CNT actuators by applying a square-wave modulated voltage or a square-wave modulated laser. As shown in Fig. 2F, the 3 dB attenuation frequency of VO2/CNT actuators (half of the maximum amplitude) is approximately 80 Hz, corresponding to a response time of ~12.5 ms, whereas the attenuation frequency of W-VO2/CNT actuators is 35 Hz, corresponding to a response time of ~28.5 ms. This deviation in response time is caused by the much smaller temperature difference between the MIT and the ambient for W-VO2/CNT (~9 oC, versus ~43 oC for VO2/CNT), leading to a slower heat dissipation for W-VO2/CNT back to the insulating phase during the cooling process. We compare the response time of VO2/CNT and W-VO2/CNT actuators with other types of flexible actuators listed in Table 1. The responses of VO2/CNT or W-VO2/CNT actuators are superior to thermally driven organic actuators and almost all of electrochemically driven organic actuators. More importantly, our actuators could be triggered by multiple, rather than solo, external stimuli. Besides the MIT driven mechanism, the fast response of VO2/CNT actuator is also attributed to the good thermal conductivity of CNT film (~140 W/mK)44 and the lightweight, thus the low thermal capacitance, of VO2/CNT actuators. In Table 1, the thickness of each type of actuators may also influence their response time. For our VO2/CNT actuators, reducing the thickness would increase the response speed because thinner VO2/CNT actuators have lower thermal capacitances, which implies that a fast actuator (>100 Hz) could be achieved by employing fewer number of CNT layers. The reliability of VO2/CNT actuators is tested. It shows no degradation in amplitude after being worked for up to 1 million cycles (inset of Fig. 2F), suggesting a long lifetime and a high reliability of VO2/CNT actuators.


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We simulate the temperature distribution of a triangular actuator under a uniform input heating source (either laser irradiation or Joule heating). Briefly, a 3D steady-state thermal transfer equation ∇ ∙ ሺ−k∇Tሻ = Q is solved by a finite element method using COMSOL Multiphysics software, where k is the thermal conductivity of VO2/CNT film, T is the temperature and Q is the thermal source (see Supporting Information for detailed simulation model and parameters). In this model, the actuator is considered to have uniform temperature along the out-of-plane direction because of the small thickness of the actuator (~1.5 µm). Thermal conductivity of VO2/CNT film is considered to be dominated by the thermal conductivity of CNT film, because all the thermal conductivity, the electrical conductivity, and the thickness of CNT films are much larger than VO2 (~140 W/mK44 vs. ~5 W/mK45; ~105 S/m46 vs. 101~102 S/m47; 1.5 µm vs. 0.18 µm) (See more detailed discussion in Supporting Information). Fig. 3A shows a simulated result under an input power of 2065 mW/cm2. The area with the highest temperatures is around the tip of the actuator and the temperature decreases along its arm direction. In Fig. 3B, we compare the simulated temperatures at monitored points of the triangular actuator with the experimental values taken from infrared images (Fig. 3C). The simulated temperatures are well consistent with the experimental data, validating our simulation model. Based on this model, we further simulate the dependence of the temperature on the thickness of the actuator under the solar constant (Inset of Fig. 3B). When the thickness of the actuator is ~ 300 nm, the temperature of the actuator can reach to ~ 38 oC, which has exceeded


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the MIT temperature of W-VO2. It suggests that sunlight would be strong enough to trigger a thin W-VO2/CNT actuator. The temperature of the CNT film increased as the thickness of the CNT film decreased. The reason of that is the thermal dissipation of the CNT film to the substrate is much reduced as the cross-section area of the CNT film decreases when the CNT film becomes thinner, whereas the optical absorbance of the CNT film is almost maintained unchanged (Fig. S5). As the thickness of the actuator can be adjusted easily by controlling the number of layers of CNT films, this thickness-controlled method provides another way to tune the triggering power of the actuator.

The flexibility, photo-responsive, and simple machinability of VO2/CNT actuators enable their biomimetic and micro-robotic applications. Here we demonstrate two examples triggered by remotely controlled photo-actuation: one is a winged insect (Movie S1 and Fig. S9) and the other a biomimetic hand with tiny fingers (Movie S2). Fig. 4A and 4B show the winged insect flapping its wings up and down quickly, by responding to a square-wave modulated laser irradiation (10 Hz). The responding frequency, however, can reach up to 80 Hz as aforementioned (Fig. 2F), and the size of the insect wings is on the order of merely several millimeters, mimicking small insects such as mosquitoes and moths and providing good tools for spying purposes in military applications. Fig. 4C and 4D demonstrate a biomimetic VO2/CNT hand with four fingers, which are controlled to be open or close with the OFF or ON state of the laser irradiation. This biomimetic hand with fingers can be utilized to move a paper slip, which is estimated to be 30 times heavier than all of the fingers, from one position to another. As shown


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in Fig. 4E-H, the paper slip seated in the hole A is first positioned by the fingers (Fig. 4E), and then grasped closely and lifted entirely out of the hole A, which are controlled by the opening of a laser (Fig. 4F). After that, the paper slip is moved from the hole A to the hole B (Fig. 4G). When arriving at the hole B, the paper slip is released with the turn-off of the laser (Fig. 4H).

As W-doping technique enables actuators working at physiological temperatures, we also design a body-temperature driven electrical switch using a W-VO2/CNT actuator, which may work in wearable devices to protect the human body against overheated devices. The working principle of the switch is schematically shown in Fig. 5A. A normally closed switch is constructed by a W-VO2/CNT actuator and a tungsten probe, with the W-VO2/CNT actuator pre-pressed against the tungsten probe (inset of Fig. 5B). When the temperature is higher than 37 o

C, the W-VO2/CNT actuator bends down and separates from the tungsten probe, and

consequently the circuit is cut off (Fig. 5B). During the cooling process, the circuit would not be reclosed until the device was cooled down to 28 oC due to the hysteresis of the VO2 MIT. This switch functions as an active, multi-time safety device that protects a human body from injury by cutting off the circuit at an overheat temperature (i.e, >37 °C in this case) and recovers the circuit unless a very safe temperature (i.e, 28 °C in this case). The cutting-off and recovering temperatures of the switch are controlled by both the intrinsic MIT temperature of the W-VO2 and the depth to which the tungsten probe pre-presses the W-VO2/CNT actuator. The noise of the measured current results from the imperfect contact between the W-VO2/CNT actuator and the


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tungsten probe at the present simple designs with two electrodes, which could be solved by the optimization of the design and the improvement of the contact in future.

In summary, we developed VO2/CNT bimorph actuators with flexibility, versatility and fast response. The shape of VO2/CNT bimorph actuators can be arbitrarily fabricated in batch with a laser trimming technique. Through W doping, the MIT temperature of VO2 decreases from 68 oC to 34 oC. The low driving temperature of W-VO2 enables W-VO2/CNT actuators to be driven by biological energies, which is demonstrated by a body-temperature driven switch. Biomimetic applications including a winged insect and a biomimetic hand with millimeter-scale fingers are also designed and demonstrated in this work. More applications can be expected where flexile, high-speed and low-power consumption actuators are desired, for instance, optical switch, drug delivery, artificial muscles, etc.

Materials and Methods

Synthesis of VO2 or W-VO2 on CNT films. To fabricate a VO2/CNT film, VOx was first deposited onto a 50-layer cross-stacked CNT film by a DC magnetron sputtering system with high-purity vanadium metal target. The sputtering was carried out with flowing gas mixtures (49.7 sccm Ar and 0.3 sccm O2 under 0.55 Pa for 30 min (DC power of 60 W) at room temperature. After the VOx deposition, the VOx/CNT film was annealed in low-pressure O2 atmosphere (3×10-2 mbar) at 450 oC for 10 min to crystallize into a VO2/CNT film. To prepare a W-VOx/CNT film, V and W mixed target (W 1.5 at.%) was used in the DC magnetron


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sputtering. The gas flow and pressure were slightly different from those in the sputtering of VO2. The gas mixtures were 49.5 sccm Ar and 0.5 sccm O2. The pressure was maintained at 0.6 Pa during the sputtering. After the W-VOx deposition, the W-VOx/CNT film was annealed in low pressure O2 atmosphere (4.5×10-2 mbar) at 450 oC for 10 min. The thicknesses of both as-prepared VO2 and W-VO2 layers were about 180 nm, as estimated from a VO2 or W-VO2 film deposited on a quartz substrate with the same synthesizing conditions.

Characterizations. SEM imaging was conducted on a scanning electron microscope (FEI Nova NanoSEM 450). Measurements of Raman spectra were carried out by a Raman spectroscope (Jobin Yvon LabRAM HR800). Measurements of XRD were carried out by Rigaku X-Ray Diffractometer. Optical absorbance was obtained by 1-T%-R%, where optical transmission (T%) and reflection (R%) of CNT films were measured by a Perkin Elmer-Lambda 950 (ultraviolet– visible–near infrared) spectrometer.

Measurements of VO2/CNT or W-VO2/CNT actuators. Temperature of actuator samples was controlled by a home-made hot plate with a water cooling system. The temperature could be tuned precisely from 15 to 100 oC by a temperature controller (Model 332, Lake Shore Cryotronics Inc.). The resistance of VO2/CNT and W-VO2/CNT films was measured by an Agilent 2902B source meter with a four-probe method. Photo-driven or electric-driven VO2/CNT and W-VO2/CNT actuators were performed with a 980 nm infrared laser (BWT, Beijing Inc.) or a pulse generator (Agilent 8110A), respectively. In the lifetime measurement of the VO2/CNT actuator, the VO2/CNT actuator was driven by a 10-Hz square-wave voltage (0 and 2.7 V) in


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ambient environment. The movement of the actuators at various temperatures was recorded by an optical microscope equipped with a CCD camera. The thermal images of triangular CNT films were recorded by an infrared thermal camera (PI160, Optris Inc.).

Associated Content

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Figure S1. Flexibility and mechanical strength of VO2/CNT film

Figure S2. Raman spectra of CNT films before and after annealing in low-pressure O2 atmosphere

Figure S3. Preferred orientation of VO2 on CNT film after annealing.

Figure S4. Resistivity-temperature curves of VO2 films and a 50-layer CNT film with temperature.

Figure S5. Thermal transfer model for VO2/CNT actuators

Figure S6. Transmission electron microscope image of VO2/CNT

Figure S7. The derivative logarithmic plot of resistances of W-VO2/CNT and VO2/CNT versus temperature


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Figure S8. Physical characterizations of W-VO2/CNT actuators

Figure S9. Optical image of a winged insect in top view

Movie S1. A winged insect flapping its wings up and down, by responding to a square-wave modulated laser irradiation (10 Hz).

Movie S2. A paper slip was moved by the biomimetic VO2/CNT hand from hole A to hole B.

Author Information Corresponding Author：： *

Email: [email protected] (K. L.); [email protected] (Y. W.)

Author contributions: K. L., Y. W., and K. J. conceived the project. K. L. initiated the fabrication of tungsten-doped VO2 films, and H. M. prepared actuator samples and measured their performance. H. M. and Z. Y. conducted the electrical measurements. H. M. and J. H. performed the SEM characterization and the optical absorption measurements. H. M. and J. Z. performed the thermal transfer simulation. H. M. and X. W. prepared the switch device and measured its performance. K. L. and H. M. analyzed the simulation and experimental data. H. M., K. L, and Y. W. wrote the manuscript. All authors participated in discussing the results.


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Acknowledgements The work was financially supported by National Basic Research Program of China (2012CB932301), the Thousand Youth Talent Program of China, National Natural Science Foundation of China (51472142, 51102147, 51202012), and China Chinese Postdoctoral Science Foundation (2014M550701 and 2015T80070). References 1.

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Figure 1. Preparation of VO2/CNT actuators and their physical characterizations. (A) Schematic of fabrication processes of VO2/CNT actuators. (B) Various shapes of CNT films patterned by laser. (C) SEM image of a triangular VO2/CNT actuator. (D) Enlarged SEM image of the triangular actuator. (E) Raman spectrum of as-prepared VO2/CNT actuator. (F) EDX mapping of the vanadium element on the VO2/CNT actuator.


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Figure 2. Properties of VO2/CNT and W-VO2/CNT actuators response to various external stimuli. (A) Schematic of a VO2/CNT actuator responding to external stimuli. (B) Optical image of a VO2/CNT actuator showing the movement from at 60 oC to at 78 oC. (C) Resistance of VO2/CNT and W-VO2/CNT actuators as a function of temperature during heating and cooling processes. (D) Tip displacement of VO2/CNT and W-VO2/CNT actuators under heating and cooling processes. (E) Normalized tip displacement of VO2/CNT and W-VO2/CNT actuators versus power consumption under stimuli of Joule heating and laser irradiation. (F) Normalized tip displacement as a function of driving frequency for the VO2/CNT actuator triggered by laser (1900 mW/cm2) and joule heating (2380 mW/cm2), as well as the W-VO2/CNT actuator triggered by laser (624 mW/cm2). The inset shows the reliability test of the VO2/CNT actuator up to 106 cycles in air.


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Figure 3. Temperature distribution of a triangular VO2/CNT actuator. (A) Simulated temperature distribution of a triangular VO2/CNT actuator under an input power (2065 mW/cm2). (B) Simulated and experimental dependence of the temperature at the monitor point on the input power density. (C) Thermal images of a CNT film heated by laser irradiation under a variety of power densities captured by an infrared camera. Scale bar is 2 mm.


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Figure 4. Biomimetical applications of VO2/CNT bimorph: a winged insect and a biomimetical hand. (A) A winged insect made by VO2/CNT bimorph, in side view. Top-view image is shown in Figure S9. (B) The wings flap when the laser is opened. (C) A biomimetical hand made by VO2/CNT bimorph with four fingers. (D) The fingers close when the laser is ON. (E-H) Laser-controlled movements of a paper slip from the hole A to the hole B by the biomimetical hand.


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Figure 5. A human-body temperature driving W-VO2/CNT switch. (A) Schematic of a W-VO2/CNT electrical switch triggered by the human body temperature. (B) The electrical circuit in Figure 5A cuts off at 37 oC due to the actuation of W-VO2/CNT switch and recovers after cooling down. The inset is an optical image of the W-VO2/CNT connected with a tungsten probe.


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Table 1. Comparison of response time for different types of flexible actuators Driving source Mechanism

Electrothermal

Photothermal

System

Trans-cis transition

Ref

−

10

14

0.1

15

5

16

Light (mW/cm2)

CNT/BOPP

1-5

Graphene-epoxy (Su-8)

1-4

Graphene oxide–CNT/ PDMS

−

− 250

Graphene-elastin composite

−

0.2-5.7

5

17

Single wall carbon nanotube/polycarbonate membrane

−

30-100

0.3

48

SWNT/poly(N-isopropylacrylamide)

−

9.5×106

2.7

49

CNT/paraffin wax/polyimide

−

25-100

2

50

0.5-8

0.019

51

Graphitic carbon nitride nanosheet electrode and PEO-NBR supporting EMIBF4

3

0.3

9

Activated carbon/gold foil electrode and BMIMBF4 based ioniogels

1-4

10

10

CNT/Azobenzene

50-100

1

7

VO2/CNT actuator

1.3-2.7

800-1600

0.013

This work

W-VO2/CNT actuator

0.75-2

250-800

0.029

This work

Carbon nanotube electrodes and EMIBF4 Electrochemical

Response time [s]

Electric (V)

MIT

− : Unmentioned; : Unsupported


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TOC Graphic:


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