Heiki Kasemägi

A self-oscillating ionic polymer-metal composite bending actuator

This paper presents an electromechanical model of an ionic polymer-metal composite (IPMC) material. The modeling technique is a finite element method (FEM). An applied electric field causes the drift of counterions (e.g., Na+), which, in turn, drags water molecules. The mass and charge imbalance inside the polymer is the main cause of the bending motion of the IPMC. The studied physical effects have been considered as time dependent and modeled with FEM. The model takes into account the mechanical properties of the Nafion polymer as well as the thin coating of the platinum electrodes and the platinum diffusion layer. The modeling of the electrochemical reactions, in connection with the self-oscillating behavior of an IPMC, is also considered. Reactions occurring on the surface of the platinum electrode, which is immersed into formaldehyde (HCHO) solution during the testing, are described using partial differential equations and also modeled using FEM. By coupling the equations with the rest of the model, ...

Molecular dynamics simulation of the LiBF4-PEO system containing Al2O3 nanoparticles

Abstract The amorphous LiBF4(PEO)20 system has been simulated alone and containing a ca. 14-A diameter Al2O3 nanoparticle and in juxtaposition with a ca. 65-A thick α-Al2O3 slab at a nominal temperature of 293 K by Molecular Dynamics (MD) methods. Li-ion mobility in the poly(ethylene oxide) (PEO) host is found to increase on the addition of the nanoparticle; the effect is also noticeable for the alumina slab. This can be seen as theoretical confirmation of the positive influence of nanoparticles on ion mobility in a PEO–salt system, as observed earlier experimentally. Other effects observed are related to this Li-ion mobility enhancement: PEO forms an immobilised coordination sphere around the particle and an immobilised layer at the surface of the α-alumina slab. No Li ions are found near the particle or at the slab surface. Instead, two to three unpaired BF4− anions are found attached to the particle within the region of immobilised PEO and at least one is found immobilised on the slab surface, leaving free Li ions in the regions away from the particle and slab surfaces. No more than 60% of the Li ions form ion pairs and ion clusters in the regions away from the particle surface and up to 87% of the Li ions form ion pairs and ion clusters in the regions away from the slab surface.

Molecular dynamics simulation of the LiPF6·PEO6 structure

Molecular dynamics (MD) simulations have been performed for the crystalline LiPF6·PEO6 system at ambient temperature in an effort to model the detail of its atomic-level structure and dynamics. Start coordinates were taken from the neutron powder diffraction analysis of Gadjourova et al., Chem. Mater., 2001, 13, 1282 (ref. ). Polymer-chain conformation, Li+-ion coordination and thermal displacement parameters are compared with experimentally determined values; the differences found are rationalised in terms of differences between the infinite-chain models investigated (both experimental and theoretical) and the finite chain-length material studied.

Molecular dynamics simulation of temperature and concentration dependence of the ‘filler’ effect for the LiCl/PEO/Al2O3-nanoparticle system

Abstract A system involving amorphous LiCl(PEO) x for x =20, 35 and 50, and a 14 A diameter Al 2 O 3 ‘filler’ particle has been simulated at nominal temperatures 290 and 330 K by the molecular dynamics method. The mobility of Li-ions is found to increase on the addition of the nanoparticle at 330 K and Li:EO ratio 1:50, but decreases or remains unchanged at other temperatures and concentrations. Lower temperature and concentration are generally associated with a lower Li–Cl coordination number and a correspondingly higher number of unpaired/unclustered ions in the system. A number of free Li + ions and some Li–Cl pairs/clusters are found in an immobilised poly(ethylene oxide) (PEO) ‘coordination sphere’ around the nanoparticle. This reduces the number of Li + ions in regions away from the particle surface. The number of ‘free’ Li + ions away from the particle surface is largest for the intermediate composition x =35 and at 290 K (∼23% of the total number of lithium ions in the system); smaller for x =50 (∼11% at both temperatures), and even smaller at x =20 (∼5% at 290 K and ∼9% at 330 K).

Finite element simulations of the bending of the IPMC sheet

This paper presents a electro-mechanical model of an IPMC sheet. The model is developed using Finite Element method. The physical bending of an IPMC sheet due to the drift of counter-ions (e.g Na+) and water in applied electric field are simulated. Our model establishes a cause-effect relationship between the charge imbalance of the counter-ions and the mechanical bending of the IPMC sheet. The model takes into account the mechanical properties of the Nafion polymer as well as the platinum coating. As the simulations are time dependent, a transient model is used and some additional parameters, such as damping coefficients, are included. In addition to electro-mechanical model, electrochemical reactions are introduced. Equations describing periodic adsorption and desorption of CO and OH on a platinum electrode of an IPMC muscle immersed into formaldehyde solution are coupled to mechanical properties of the proposed model. This permits us to simulate self-oscillatory behavious of an IPMC sheet. The simulation results are compared to experimental data.

An advanced finite element model of IPMC

This paper presents an electro-mechanical Finite Element Model of an ionic polymer-metal composite (IPMC) material. Mobile counter ions inside the polymer are drifted by an applied electric field, causing mass imbalance inside the material. This is the main cause of the bending motion of this kind of materials. All foregoing physical effects have been considered as time dependent and modeled with FEM. Time dependent mechanics is modeled with continuum mechanics equations. The model also considers the fact that there is a surface of platinum on both sides of the polymer backbone. The described basic model has been under developement for a while and has been improved over the time. Simulation comparisons with experimental data have shown good harmony. Our previous paper described most of the basic model. Additionally, the model was coupled with equations, which described self-oscillatory behavior of the IPMC material. It included describing electrochemical processes with additional four differential equations. The Finite Element Method turned out to be very reasonable for coupling together and solving all equations as a single package. We were able to achieve reasonably precise model to describe this complicated phenomenon. Our most recent goal has been improving the basic model. Studies have shown that some electrical parameters of an IPMC, such as surface resistance and voltage drop are dependent on the curvature of the IPMC. Therefore the new model takes surface resistance into account to some extent. It has added an extra level of complexity to the model, because now all simulations are done in three dimensional domain. However, the result is advanced visual and numerical behavior of an IPMC with different surface characteristics.

Molecular dynamics simulations of EMI-BF4 in nanoporous carbon actuators

AbstractAn artificial muscle composite material consisting of carbide derived carbon (CDC) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) ionic liquid was modeled using molecular dynamics (MD) simulations, in order to determine the molecular structural rearrangements causing actuation. CDC was represented as separate curved graphene-like flakes with charges of +2, 0 or −2 on each flake, with 24–27 aromatic rings each. The charge distribution in the flakes was determined by PM6 semi-empirical optimization. The pore size distribution of CDC and the density of the material were comparable to experimental data. Molecular structure analysis revealed a preferential parallel orientation for the cations over the negatively charged CDC surfaces, while cationic rotations and reorientations could be observed for positively charged CDC. Changes in the pore occupancy for each ionic type were observed for pore sizes between 4 and 7 Å, which, together with the replacement of large cations with smaller anions, could explain the volume decrease in the anodes (and, vice versa, the volume increase in the cathodes) in this type of actuator. FigureCarbide derived carbon flakes and EMI-BF4 in molecular dynamics simulation box

Application of multiphysics and multiscale simulations to optimize industrial wood drying kilns

Timber industry and export are an important part of Estonian economy, making affordable industrial scale equipment an important investment for small or starting companies. These companies often develop on-site equipment for wood processing and drying, utilizing pre-existing infrastructure to minimize cost and risk. However, under these conditions custom design of the wood drying kilns is often required.In the present study, a finite element simulation based approach is used to simulate and optimize the industrial wood drying process and the design of the custom-made kilns in a multiscale-multiphysics modeling framework. Air flow is calculated by the Navier-Stokes equations or ?-e turbulence model followed by heat transport in the solid and gas phase and moisture dynamics in wood and air. The dense packing of the processed materials is handled by utilizing a porous media approach and homogenization procedure, leading to effective simulations of the moisture and heat balance.Multiphysics-multiscale simulations are successfully adapted to optimize the industrial design of wood drying kilns. The optimization of the kiln design is achieved by estimating the necessary ventilating power and ensuring homogeneous drying of the processed material.

A new force field for Molecular Dynamics studies of Li+- and Na+-Nafion

Nafion is widely known as one of the most popular membrane materials for low temperature fuel cell applications. However, the particular exchange membrane material properties make it also valuable for other applications. One of the electroactive polymer (EAP) subclasses, ionic polymer metal composites (IPMC) commonly exploits Nafion as the ion exchange polymer membrane. The ion conducting properties of Nafion are extremely important for IPMCs. Although, ion conductivity depends strongly on the structural properties of the polymer matrix, there has been very little insight at the atomistic level. Molecular dynamics simulations are one of the possibilities to study the ion conduction mechanism at atomistic level. So far, the simulation results have been rather contradictory and very much dependent from the force fields and polymer matrix setup used. In the present work, new force field parameters for Li+ and Na+ - nafion based on DFT calculations are presented. The developed potentials and the force field were tested by molecular dynamics simulations. It can be concluded that Li+ and Na+ ions are coordinated to different Nafion side-chain terminal group (SO3-) oxygens and to very few water molecules. One cation is coordinated to three different side-chains. Oxygens of SO3 groups and cations form complicated multi-header systems. In the equilibrium state, no cations dissociated from side chains were found.

Molecular dynamics studies of interpenetrating polymer networks for actuator devices

Molecular Dynamics (MD) techniques have been used to study the structure and dynamics of a model system of an interpenetrating polymer (IPN) network for actuator devices. The systems simulated were generated using a Monte Carlo-approach, and consisted of poly(ethylene oxide) (PEO) and poly(butadiene) (PB) in a 80-20 percent weight ratio immersed into propylene carbonate (PC) solutions of LiClO4. The total polymer content was 32%, in order to model experimental conditions. The dependence of LiClO4 concentration in PC has been studied by studying five different concentrations: 0.25, 0.5, 0.75, 1.0 and 1.25 M. After equilibration, local structural properties and dynamical features such as phase separation, coordination, cluster stability and ion conductivity were studied. In an effort to study the conduction processes more carefully, external electric fields of 1×106 V/m and 5×106 V/m has been applied to the simulation boxes. A clear relationship between the degree of local phase separation and ion mobility is established. It is also shown that although the ion pairing increases with concentration, there are still significantly more potential charge carriers in the higher concentrated systems, while concentrations around 0.5-0.75 M of LiClO4 in PC seem to be favorable in terms of ion mobility. Furthermore, the anions exhibit higher conductivity than the cations, and there are tendencies to solvent drag from the PC molecules.