Ken Mukai

Expansion and contraction of polymer electrodes under applied voltage

The authors developed a scheme for characterizing the expansion and contraction of polymer electrodes when voltage is applied by coupling a symmetry analysis, the elasticity theory, and the experimental measurements. This scheme was applied to the bucky-gel electrodes and the expansion and contraction rates for the cathode and the anode were determined separately. For the case of the bucky-gel electrodes, it was found that the cathode expands and the anode contracts as voltage is applied. The stress exerted inside the electrode layers was also determined and the mechanical efficiency of the actuator composed of the bucky-gel electrodes is discussed.The authors developed a scheme for characterizing the expansion and contraction of polymer electrodes when voltage is applied by coupling a symmetry analysis, the elasticity theory, and the experimental measurements. This scheme was applied to the bucky-gel electrodes and the expansion and contraction rates for the cathode and the anode were determined separately. For the case of the bucky-gel electrodes, it was found that the cathode expands and the anode contracts as voltage is applied. The stress exerted inside the electrode layers was also determined and the mechanical efficiency of the actuator composed of the bucky-gel electrodes is discussed.

Mechanical behaviour of bending bucky-gel actuators and its representation

Bucky-gel actuators are ionic electromechanically active materials that bend in response to a low-voltage excitation. While bending actuators may offer new approaches in engineering solutions, the characterization of bending poses many challenges in comparison to conventional rotary motion. It is often desired to reduce the bending behaviour to a single parameter, which may lead to the loss of accuracy in modelling. A high-speed laser profilometer is utilized to characterize the bending response of different bucky-gel actuators at their full length and to critically compare the applicability of existing representation tools for bending. The best analytical representation of the bending of a bucky-gel actuator is found to be in the form of a power function. It is also observed that, along the length of the actuator, sections closer to the electrical input clamp exhibit back-relaxation (a common drawback for bending ionic actuators) already when the far end of the bending strip is still in forward motion.

Superior performance of a vapor grown carbon fiber polymer actuator containing ruthenium oxide over a single-walled carbon nanotube

The electrochemical and electromechanical properties of poly(vinylidene fluoride-co-hexafluoropropylene) actuators developed using a vapor grown carbon fiber (VGCF)–ionic liquid (IL) gel electrode containing ruthenium oxide (RuO2), formed without using ultrasonication, were compared with only-VGCF and only-single-walled carbon nanotube (SWCNT) based actuators. The double-layer capacitance of the VGCF electrode containing RuO2 was larger than that of the only-VGCF electrode. The VGCF polymer actuator containing RuO2 formed without using ultrasonication surpassed the performance of the only-VGCF and only-SWCNT actuators in terms of the strain. Both VGCFs and RuO2 were required to produce a large strain actuator that surpassed the performance of the only-SWCNT polymer actuator.

Impact of viscoelastic properties on bucky-gel actuator performance

Due to their soft polymeric backbone and metal-free compositions, bending electromechanically active bucky-gel laminates are attractive candidates for actuators in applications in a variety of fields such as medicine and space technology. However, the soft structure of these materials also proposes challenges to engineering new devices as the unorthodox composition requires that the developer really understands the mechanical nature of these actuators. As the composition of bucky-gel laminates includes porous polymer filled with ionic liquid but also carbon nanotubes, the viscoelastic nature and how it possibly affects actuator performance is of great interest. Several customised mechanical tests are implemented on both electrically active and inactive bending bucky-gel actuators to investigate their mechanical properties. It is reported that these materials are highly viscoelastic and their mechanical relaxation can be described by generalised Maxwell model for solids. Moreover, these viscoelastic effects are shown to depend on the level of input voltage, thus, altering the overall performance of the actuator. Temperature measurements reveal remarkable thermal dissipation during the actuation cycles, which, as the dynamic measurements demonstrate, alters the mechanical properties of bucky-gel actuators.

Wet spinning of continuous polymer-free carbon-nanotube fibers with high electrical conductivity and strength

We report on the fabrication of polymer-free carbon nanotube (CNT) fibers by a novel wet spinning method combined with a very easy and straightforward fabrication process. These fibers exhibited high electrical conductivity (14,284 ± 169 Scm−1) and tensile strength (887 ± 37 MPa). Such high performance was made possible by the preparation of free-standing CNT fibers from a surfactant solution containing uniformly dispersed CNTs, despite the use of an organic coagulating solvent and subsequent stretching to align the CNTs in the fiber.

Electroactive Shape-Fixing of Bucky-Gel Actuators

Electromechanically active polymers (EAP) behave as actuators, i.e., temporarily change their shape when subjected to electric stimulus. However, in applications where a steady state of an actuator is required for an extended period of time (e.g., displaying characters of Braille text), EAPs need an active driving signal. Shape memory polymers, on the other hand, can be fixed to a preferred shape using appropriate stimuli such as temperature change and mechanical stress. Combining the electromechanical properties with the shape memory functionality could increase the efficiency and utility of EAP materials. As bucky-gel actuators--a subtype of EAPs--have been reported to possess a quite characteristic viscoelastic response, this paper proposes to implement them as shape-fixable EAPs. We employ four different shape-fixing techniques, out of which two allow programming the bucky-gel actuator in a controlled direction without any external stress or temperature change. These shape-fixing methods utilize composite driving signal consisting of high-frequency section to generate Joule heating within the material while low-frequency rectangular component deforms the bucky-gel laminate. It is concluded that bucky-gel actuators can achieve steady deformation by using solely the electrical input. By using the proposed methods for adding the shape-fixing functionality to an existing EAP device, no constructional changes of the system are required.

Electrochemical impedance spectroscopy of the bucky-gel actuators and their electromechanical modeling

In this paper, we carried out the impedance measurements of the bucky-gel actuators and analyzed the results by means of the porous electrode model. We also measured the displacement of the same actuators by applying sinusoidal voltages of various frequencies. The frequency dependence of the displacement responses is discussed in relation with the impedance properties of the bucky-gel electrodes. The electrochemical equivalent circuit of the bucky-gel actuator is discussed on the basis of the impedance analysis. Accordingly, we are able to develop an electrochemical model allowing to analyze the behavior of these actuators.

The viscoelastic effect in bending bucky-gel actuators

Electromechanically active polymers (EAP) are considered a good actuator candidate for a variety of reasons, e.g. they are soft, easy to miniaturize and operate without audible noise. The main structural component in EAPs is, as the name states, a type of deformable polymer. As polymers are known to exhibit a distinct mechanical response, the nature of polymer materials should never be neglected when characterizing and modeling the performance of EAP actuators. Bucky-gel actuators are a subtype of EAPs where ion-containing polymer membrane acts as an electronically insulating separator between two electrodes of carbon nanotubes and ionic liquid. In many occasions, the electrodes also contain polymer for the purpose of binding it together. Therefore, mechanically speaking, bucky-gel actuators are composite structures with layers of different mechanical nature. The viscoelastic response and the shape change property are perhaps the most characteristic effects in polymers. These effects are known to have high dependence on factors such as the type of polymer, the concentration of additives and the structural ratio of different layers. At the same time, most reports about optimization of EAP actuators describe the alteration of electromechanical performance dependent on the same factors. In this paper, the performance of bucky-gel actuators is measured as a function between the output force and bending deflection. It is observed that effective stiffness of these actuators depends on the input voltage. This finding is also supported by dynamic mechanical analysis which demonstrates that the viscoelastic response of bucky-gel laminate depends on both frequency and temperature. Moreover, the dynamic mechanical analysis reveals that in the range of standard operation temperatures, tested samples were in their glass transition region, which made it possible to alter their shape by using mechanical fixing. The mechanical fixity above 90% was obtained when high-frequency input signal was used to heat the bucky-gel sample.

Electrochemistry of electromechanical actuators based on carbon nanotubes and ionic liquids

In this paper, we have developed electrochemical and electromechanical kinetic model of a bucky-gel actuator which is composed of an ionic liquid (IL) gel electrolyte layer sandwiched by electrode layers based on single-walled carbon nanotubes (SWNTs) and ILs. The electrochemical model can be applied to the electromechanical effect only due to the electric double-layer (DL) charging, or due to both the DL charging and redox reaction of SWNTs. The model was compared with the experimental results of the bucky-gel actuators.

Fast-moving bimorph actuator based on electrochemically treated millimeter-long carbon nanotube electrodes and ionic liquid gel

The development is described of an electromechanical bimorph actuator composed of ionic liquid gel sandwiched by electrochemically treated millimeter-long single-walled carbon nanotubes (SG-SWNT). Electrochemical doping of the SG-SWNT and electrochemical polymerization of polypyrrole on the surface of the SG-SWNT improved the performance of a previously reported actuator using non-treated SG-SWNTs. The conductivity of the SG-SWNT sheets treated at the anodic potential (doped) was found to be three times larger than that of the original film. The generated strain of the actuator prepared from the doped SG-SWNT sheets was increased compared to that prepared from non-doped sample. Moreover, the generated strain of the actuator from the doped SG-SWNT sheets swelled with ionic liquid (IL) was increased to twice that without ILs. The electropolymerization of pyrrole on the surface of the SG-SWNT sheet was carried out. The conductivity of the SG-SWNT was seven times larger after the electropolymerization. The generated strain of the SG-SWNT actuator prepared from the SG-SWNT sheets with electropolymerization was twice as large as that without the electropolymerization at low frequency. At higher frequency, both actuators provide almost the same performance. Both actuators exhibit mechanical resonance at about 100 Hz.