Urmas Johanson

Nanoporous carbon-based electrodes for high strain ionomeric bending actuators

Ionic polymer metal composites (IPMCs) are electroactive material devices that bend at low applied voltage (1–4 V). Inversely, a voltage is generated when the materials are deformed, which makes them useful both as sensors and actuators. In this paper, we propose two new highly porous carbon materials as electrodes for IPMC actuators, generating a high specific area, and compare their electromechanical performance with recently reported RuO2 electrodes and conventional IPMCs. Using a direct assembly process (DAP), we synthesize ionic liquid (Emi-Tf) actuators with either carbide-derived carbon (CDC) or coconut-shell-based activated carbon-based electrodes. The carbon electrodes were applied onto ionic liquid-swollen Nafion membranes using a direct assembly process. The study demonstrates that actuators based on carbon electrodes derived from TiC have the greatest peak-to-peak strain output, reaching up to 20.4 me (equivalent to>2%) at a 2 V actuation signal, exceeding that of the RuO2 electrodes by more than 100%. The electrodes synthesized from TiC-derived carbon also exhibit significantly higher maximum strain rate. The differences between the materials are discussed in terms of molecular interactions and mechanisms upon actuation in the different electrodes.

A Distributed Model of Ionomeric Polymer Metal Composite

This article presents a novel model of an ionomeric polymer metal composite (IPMC) material. An IPMC is modeled as a lossy RC distributed line. Unlike other electro-mechanical models of an IPMC, the distributed nature of our model permits modeling the non-uniform bending of the material. Instead of modeling the tip deflection or uniform deformation of the material, we model the changing curvature. The transient behavior of the electrical signal as well as the transient bending of the IPMC are described by partial differential equations. By implementing the proper initial and boundary conditions we develop the analytical description of the possibly non-uniform transient behavior of an IPMC consistent with the experimental results.

Ionic electroactive polymer artificial muscles in space applications

A large-scale effort was carried out to test the performance of seven types of ionic electroactive polymer (IEAP) actuators in space-hazardous environmental factors in laboratory conditions. The results substantiate that the IEAP materials are tolerant to long-term freezing and vacuum environments as well as ionizing Gamma-, X-ray, and UV radiation at the levels corresponding to low Earth orbit (LEO) conditions. The main aim of this material behaviour investigation is to understand and predict device service time for prolonged exposure to space environment.

Nanoporous Carbide-Derived Carbon Material-Based Linear Actuators

Devices using electroactive polymer-supported carbon material can be exploited as alternatives to conventional electromechanical actuators in applications where electromechanical actuators have some serious deficiencies. One of the numerous examples is precise microactuators. In this paper, we show for first time the dilatometric effect in nanocomposite material actuators containing carbide-derived carbon (CDC) and polytetrafluoroetylene polymer (PTFE). Transducers based on high surface area carbide-derived carbon electrode materials are suitable for short range displacement applications, because of the proportional actuation response to the charge inserted, and high Coulombic efficiency due to the EDL capacitance. The material is capable of developing stresses in the range of tens of N cm-2. The area of an actuator can be dozens of cm2, which means that forces above 100 N are achievable. The actuation mechanism is based on the interactions between the high-surface carbon and the ions of the electrolyte. Electrochemical evaluations of the four different actuators with linear (longitudinal) action response are described. The actuator electrodes were made from two types of nanoporous TiC-derived carbons with surface area (SA) of 1150 m2 g-1 and 1470 m2 g-1, respectively. Two kinds of electrolytes were used in actuators: 1.0 M tetraethylammonium tetrafluoroborate (TEABF4) solution in propylene carbonate and pure ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITf). It was found that CDC based actuators exhibit a linear movement of about 1% in the voltage range of 0.8 V to 3.0 V at DC. The actuators with EMITf electrolyte had about 70% larger movement compared to the specimen with TEABF4 electrolyte.

In situ scanning electron microscopy study of strains of ionic electroactive polymer actuators

We have developed a technique to determine the bending strain of ionic electroactive polymer actuators without the use of the macroscopic bending geometry. In situ comparisons of the scanning electron microscope micrographs from a bending ionic electroactive polymer actuator, using a digital image correction methodology, identify its bi-directional deformation field. The developed technique allows verification of the factual axial and thickness strains of any notional layer of the actuator, including the outer surfaces of the electrodes. Thus, calculation of the bending and thickness strains of the ionic electroactive polymer laminate becomes possible. Moreover, the technique allows the determination of the position of the neutral layer of bending that is an important requirement for the calculation of the second area and bending moments of the beam. The four examples presented demonstrate the potential variations of the bending schemes, in cases where the neutral layer is at the centroid and shifted away from the centroid.

Lifetime measurements of ionic electroactive polymer actuators

This article is focused on proposing a unified methodology for automating the measurement procedures of ionic electroactive polymer actuators. The proposed methodology and large-scale automation would make testing ionic electroactive polymer actuators less labor-intensive and allow analyzing many ionic electroactive polymer actuators simultaneously. Defining a clear framework for testing ionic electroactive polymer actuators performance and reliability would make the testing process reproducible and provide better comparison between ionic electroactive polymer actuators of either different or similar classes. Our methodology separates two types of degradation: degradation during operation and spontaneous self-degradation.

A distributed model of IPMC

This paper presents a distributed model of an IPMC (Ionomeric Polymer-Metal Composite). Unlike other electromechanical models of an IPMC, the distributed nature of our model permits modelling the non-uniform bending of the material. Instead of modeling solely the tip deflection of the material, we model the changing curvature. Our model of the IPMC describes the actuator or sensor as a distributed one-dimensional RC transmission line. The behavior of the IPMC at its each particular position in time-domain is described by a system of Partial Differential Equations. (PDE). The parameters of the PDE-s have a clear physical interpretation: the conductivity of the electrodes, the pseudocapacitance of the arising double-layer at the boundary of the electrodes, the electric current caused by electrode reactions etc. The electromechanical coupling between the electrical parameters and the bending motion is implemented by means of distribution of electric current along the material in a time domain. The distributed nature of the model permits predicting the non-uniform bending of the IPMC actuators in time domain or to reconcile the output of an IPMC-based position sensor with its shape. Taking into account several nonlinear parameters, the model is consistent with the experimental results even when the inflexion of the actuator or sensor is large or the water electrolysis appears.

Effect of porosity and tortuosity of electrodes on carbon polymer soft actuators

This work presents an electro-mechanical model and simulation of ionic electroactive polymer soft actuators with a porous carbon electrode, polymer membrane, and ionic liquid electrolyte. An attempt is made to understand the effects of specific properties of the porous electrodes such as porosity and tortuosity on the charge dynamics and mechanical performance of the actuator. The model uses porous electrode theory to study the electrochemical response of the system. The mechanical response of the whole laminate is attributed to the evolution of local stresses caused by diffusion of ions (diffusion-induced stresses or chemical stresses). The model indicates that in actuators with porous electrode, the diffusion coefficient of ions, conductivity of the electrodes, and ionic conductivity in both electrodes and separator are altered significantly. In addition, the model leads to an obvious deduction that the ions that are highly active in terms of mobility will dominate the whole system in terms of resulting...

Effect of ambient humidity on ionic electroactive polymer actuators

Comparable electromechanical measurements were carried out with carbon-based ionic electroactive polymer actuators in vacuum, dry inert, and in ambient air environment. The results bring forward the effect of ambient humidity on the electrical and mechanical parameters of the laminates of this type. Presence of water decreases the Youngs modulus of the polymer and lowers the viscosity of the ionic liquid, which, in turn, is accompanied by the increase of ionic conductivity of the electrolyte. The factual bending behavior of the actuator is a result of the combined effect of these factors. A four-parameter model was developed for the quantitative estimation of the rates of forward-actuation and back-relaxation as well as the electrical parameters. An important outcome of the experiments is the observation that there is nearly no back-relaxation in vacuum and in dry inert environment.

Thermal behavior of ionic electroactive polymer actuators

The high spatial, temporal, and thermal resolution of the thermal imaging system Optotherm EL InfraSight 320 is used for investigation of the thermal behavior of the ionic electroactive polymer (IEAP) actuators. The resolution of 10-20 pixels in the direction of their thickness is close to the theoretical limit restrained by the infrared light wavelength registered by the imaging system. The videos, recorded with the frame rate of 30 fps, demonstrate showy the propagation of heat along the membrane. The analysis of the thermal images provides the foundation for precise modeling of the IEAP actuators, taking into account the thermally induced mechanical and electrochemical effects. Experiments conducted with the IEAP actuators of different types (ionic polymer-metal composite, carbon-polymer composite, conducting polymer actuators) allow comparing their efficiencies. The experiments show demonstrable, that the IEAPs, used improperly, overheat to the inadmissible temperatures within seconds only. This, in turn, evaporizes the volatile electrolyte, and shortens the life expectancy of the IEAP devices.