Center for Intelligent Material Systems and Structures (CIMSS)
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Browsing Center for Intelligent Material Systems and Structures (CIMSS) by Subject "Carrier density"
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- High surface area electrodes in ionic polymer transducers: Numerical and experimental investigations of the electro-chemical behaviorAkle, Barbar J.; Habchi, Wassim; Wallmersperger, Thomas; Akle, Etienne J.; Leo, Donald J. (American Institute of Physics, 2011-04-01)Ionomeric polymer transducer (IPT) is an electroactive polymer that has received considerable attention due to its ability to generate large bending strain (> 5%) and moderate stress at low applied voltages (+/-2 V). Ionic polymer transducers consist of an ionomer, usually Nafion, sandwiched between two electrically conductive electrodes. A novel fabrication technique denoted as the direct assembly process (DAP) enabled controlled electrode architecture in ionic polymer transducers. A DAP built transducer consists of two high surface area electrodes made of electrically conducting particles uniformly distributed in an ionomer matrix sandwiching an ionomer membrane. The purpose of this paper is to investigate and simulate the effect of these high surface area particles on the electro-chemical response of an IPT. Theoretical investigations as well as experimental verifications are performed. The model used consists of a convection-diffusion equation describing the chemical field as well as a Poisson equation describing the electrical field. The two-dimensional model incorporates highly conductive particles randomly distributed in the electrode area. Traditionally, these kinds of electrodes were simulated with boundary conditions representing flat electrodes with a large dielectric permittivity at the polymer boundary. This model enables the design of electrodes with complicated geometrical patterns. In the experimental section, several transducers are fabricated using the DAP process on Nafion 117 membranes. The architecture of the high surface area electrodes in these samples is varied. The concentration of the high surface area RuO2 particles is varied from 30 vol% up to 60 vol% at a fixed thickness of 30 mu m, while the overall thickness of the electrode is varied from 10 mu m up to 40 mu m at a fixed concentration of 45%. The flux and charge accumulation in the materials are measured experimentally and compared to the results of the numerical simulations. Trends of the experimental and numerical investigations are in agreement, while the computational capacity is limiting the ability to add sufficient amount of metal particle to the electrode in order to match the magnitudes. (C) 2011 American Institute of Physics. [ doi:10.1063/1.3556751]
- Modeling the electrical impedance response of ionic polymer transducersFarinholt, Kevin M.; Leo, Donald J. (American Institute of Physics, 2008-07-01)An analytical study is presented that investigates the electrical impedance response of the ionic polymer transducer. Experimental studies have shown that the electromechanical response of these active materials is highly dependent upon internal parameters such as neutralizing counterion, diluent, electrode treatment, as well as environmental factors such as ambient temperature. Further examination has shown that these variations are introduced predominantly through the polymer's ability to convert voltage into charge migration. This relationship can easily be represented by the polymer's electrical impedance as measured across the outer electrodes of the transducer. In the first half of this study an analytical model is developed which predicts the time and frequency domain characteristics of the electrical response of the ionic polymer transducer. Transport equations serve as the basis for this model, from which a series of relationships are developed to describe internal potential, internal charge density, as well as surface current. In the second half of this study several analytical studies are presented to understand the impact that internal parameters have on the polymer's electrical response, while providing a conceptual validation of the model. In addition to the analytical studies several experimental comparisons are made to further validate the model by examining how well the model predicts changes in temperature, viscosity and pretention within the ionic polymer transducer. (c) 2008 American Institute of Physics.
- Transport modeling in ionomeric polymer transducers and its relationship to electromechanical couplingWallmersperger, Thomas; Leo, Donald J.; Kothera, Curt S. (American Institute of Physics, 2007-01-15)Ionomeric polymer transducers consist of an ion-conducting membrane sandwiched between two metal electrodes. Application of a low voltage (< 5 V) to the polymer produces relatively large bending deformation (> 2% strain) due to the transport of ionic species within the polymer matrix. A computational model of transport and electromechanical transduction is developed for ionomeric polymer transducers. The transport model is based upon a coupled chemoelectrical multifield formulation and computes the spatiotemporal volumetric charge density profile to an applied potential at the boundaries. The current induced in the polymer is computed using the isothermal transient ionic current associated with surface charge accumulation at the electrodes induced by nonzero volumetric charge density within the polymer. The stress induced in the polymer is assumed to be a summation of linear and quadratic functions of the volumetric charge density. Euler-Bernoulli beam mechanics are used to compute the bending deflection of the transducer to an applied potential. The diffusion coefficient and permittivity of the polymer is identified from the measured current density to a step change in the applied potential. A comparison between the measured data and the predicted response demonstrates that this model accurately predicts the current discharge due to the applied potential at voltages over the range of 50-500 mV. Furthermore, the measured strain response is accurately predicted by determining the two parameters of the mechanics model that relates volumetric charge density to induced stress. The coupled model with parameters identified from the step response analysis is used to predict the harmonic response of the current and the bending strain. Comparisons between measured data and simulations illustrate that the coupled transport-mechanics model accurately predicts the magnitude and trends associated with the current response and strain output. Excellent agreement is obtained at excitation periods above approximately 1 s while good agreement is obtained at longer excitation periods. The transport model highlights the importance of the asymmetry that develops at large applied potentials and long excitation periods in the volumetric charge density due to the fixed anionic species in the polymer. (c) 2007 American Institute of Physics.