27-08-2014, 04:11 PM
Electro Active Polymers Seminar Report
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Introduction
Electro active polymers (EAP) are actuation materials that are used to drive mechanisms and are fastly replacing conventional methods. Several investigations are in its way to utilize the excellent properties of the polymer. These materials are now applied in various fields including robotics, medicine, defense etc:-and are effective alternatives for conventional sensors and actuators such as motors, gears, piezoelectric crystals, bearings, screws etc: - These are unique materials capable of soft actuation under low applied voltages. They have been called by some researchers ‘artificial muscles’ due to their large strain characteristics and electro-mechanical-chemical muscle- like behavior. They
have been shown to be quite capable of low temperature actuation as well as being quite durable when compared to other actuators in their class. This leads to the belief that there is great potential for use in space applications. EAP’s can change all the paradigm of design and they show great potential as soft robotic actuators, artificial muscles and dynamic sensors in macro-to-micro size range.
MICROSTRUCTURE AND COMPOSITION
The bending actuator is composed mainly of perflourinated ion exchange membrane metallic composite backbone called ionic polymer metallic composite or IPMC (0.18µm). IPMC have commercial name Nafion®. The ionomer background or matrix is coated on both sides with metallic electrodes made of noble metals such as Pt, Au or Pt/Au (5-10µm). It is then neutralized with a certain amount of counter-ions such as monovalent cations of alkali metals such as Li+, Na+, K+ and Rb+. A finishing layer of gold is provided to increase surface conductivity. It is then fully solvated. The most common solvent used is water but we can also use organic solvents like Ethylene glycol or Glycerol. An IPMC has to be kept moist continuously for long working (4 months) and it is done by providing a polysilicon coating.
The preparation of ionic polymer metallic composites (IPMC’s) requires involved laboratory work. The current state of the art of IPMC manufacturing technique incorporates two distinct preparation processes:
1. Initial compositing process
2. Surface electroding process
Rectangular silicone wells with size 1cm by 3cm by 2-5mm is used as containers for
IPMC fabrication. It is deposited onto glass microscopic slides. We use a multiple material freeform fabrication system with Cartesian gantry positioning system and multiple changeable deposition tools. A low vapour pressure solvent (N-N- dimethyl formamide,DMF) is added to the ionomer dispersion to help control cracking on drying.
Initial Compositing process
It is to metallize the inner surface of polymer by a chemical reduction process. Ionic polymer is soaked in salt solution of a complex salt Pt (NH3)4HCl to allow platinum containing cations to diffuse through via the ion exchange process. A proper reducing agent such as LiBH4 or NaBH4 is introduced to metallize the polymer by a chemical reduction means. The metallic platinum particles are not homogeneously formed across the membrane but concentrate predominantly near the interface boundaries. It has been experimentally observed that the platinum particulate layer is buried microns deep (typically 1–20 µm) within the IPMC surface and is highly dispersed.
The primary reaction for platinum composites is:
LiBH4 + 4[Pt (NH3)4]2+ + 8OH- ==> 4Pt + 16NH3 + LiBO2 + 6H2O
Surface Electroding process
In the subsequent surface electroding process, multiple reducing agents are introduced (under optimized concentrations) to carry out the reducing reaction similar to previous equation in addition to the initial platinum layer formed by the initial compositing process. The roughened surface disappears. Platinum will deposit predominately on top of initial Pt layer. Other metals which are also successfully used include palladium, silver, gold, carbon, graphite etc: - After the upper electrode material is deposited and allowed to air dry, the glass slide is placed in an oven and annealed at 7000C for 45minutes. The silicone well is filled with deionized water for 30 minutes to saturate the IPMC. The device is lifted from the well with tweezers and tested. To maintain the actuation capability of IPMC, the material needs to be kept moist continuously providing the media that is necessary for ion mobility that is responsible for actuation. Without coating material, the IPMC can work in air for less than 5 minutes. So a low modulus poly silicon coating is applied to the surface to trap the solvent inside IPMC.
MECHANISM FOR ELECTRICAL ACTUATION
The electrical-chemical-mechanical response of the IPMCs depends on the neutralizing cation, the nature and the degree of solvent saturation, the electrode morphology, and the chemical structure of the ionomer backbone.When a strip of solvated Nafion based IPMC sample is subjected to an electric potential of several volts (1-3 V) across its faces, it bends towards the anode. The speed and magnitude of this actuation towards the anode depends on the type of solvent. The actuation towards the anode is relatively slow with ethylene glycol comparing to water, and it is comparatively much slower with glycerol than with water or ethylene glycol as solvents. For Nafion-based IPMCs with alkali metals as counter ions, the actuation towards the anode is followed by a slow back relaxation towards the cathode .The back relaxation speed also depends on the type of solvent. The duration of the back relaxation phase can vary, from less than about 60 seconds (e.g., with most alkali metals and with water), to about 300 seconds (e.g., in K+-form with ethylene glycol), and to about 2000 seconds (e.g., in Na+-form with glycerol).
The sample eventually reaches an equilibrium state (while the electric potential is still on), which is generally different from its initial equilibrium position
When an external voltage is applied on an IPMC film, it causes bending towards the anode. The IPMC strip bends due to these ion migration induced hydraulic actuation and redistribution. Nafion IPMC has the ability to absorb considerable amount of water, which increases the cations mobility and conductivity. The cations will get hydrated while the anions sulfonate (SO3-) group remains fixed to the polymer matrix. When a voltage (1-3V) is applied the hydrated cations will move towards the cathode side. The swelling or expansion at the cathode side results due to the increase in volume at the cathode side of IPMC, as a result of the transfer of hydrated cations.This swelling is followed by a slow back relaxation towards cathode. This is because that the weak bonds associated with the hydrated cations break after prolonged exposure to the applied electric field causing the inherent ‘relaxation.’ This will cause the re-orientation of the cations in the boundary layer. Finally the EAP will come to an equilibrium position
FACTORS AFFECTING ACTUATION
Counter ion species
For neutralizing counter ions we have used Li+, Na+, K+ and Rb+. The properties of the bare ionomer as well as that of IPMC change with the cation type for the same membrane. It has been shown that using Li+ as cationic base we can get greater displacement and force density per volt.
Hydration
The speed and magnitude of the actuation towards the anode depends on the type of solvent. The actuation towards the anode is relatively slow with ethylene glycol comparing to water, and it is comparatively much slower with glycerol than with water or ethylene glycol as solvents
Potential
The deflection increases as voltage is increased and reaches saturation as the voltage rises. Under an AC voltage, the film undergoes swinging movement and the displacement level depends not only on the voltage magnitude but also on the frequency. Generally, activation at lower frequencies (down to 0.1 or 0.01 Hz) induces higher displacement and it reaches saturation as the voltage increases
Platinum penetration and dispersion
Deeper the platinum penetration lower is the surface resistance and greater is the force density. The incipient particles coagulate during the chemical reduction process .One can then realize that there is a significant potential for controlling this process in terms of platinum particle penetration, size and distribution. This could be achieved by introducing effective dispersing agents (additives) during the chemical reduction process. One can anticipate that the additives should enhance dispersal of platinum particles within the ionic polymer and thus reduce coagulation. As a result, a better platinum particle penetration in the polymer with a smaller average particle size and more uniform distribution could be obtained. This uniform distribution makes it more difficult for water to pass through. Thus; the water leakage out of the surface electrode could be significantly reduced.
THERMODYNAMIC EFFICIENCY OF IPMC
The bending force of the IPMC is generated by the effective redistribution of hydrated ions and water. The IPMC strip bends due to these ions migration-induced hydraulic actuation and redistribution. The total bending force, Ft, can be approximated as:
where f is the force density per unit length. Assuming a uniformly distributed load over the length of the IPMC, the mechanical power produced by the IPMC can be estimated from:
Pout = Ft v
Notation v is the average tip velocity of the IPMC in action. Finally, the thermodynamic efficiency, Eff,em, can be obtained as:
where Pin is the electrical power input to the IPMC.
APPLICATIONS OF EAP
For many years, electroactive polymers (EAP) received relatively little attention due to the small number of available materials and their limited actuation capability. The recent emergence of EAP materials with large displacement response changed the paradigm of these materials and their potential capability. Their main attractive characteristic is their operational similarity to biological muscles, particularly their resilience and ability to induce large actuation strains. Unique robotic components and miniature devices are being explored, where EAPs serve as actuators to enable new capabilities. Now EAP’s are revolutionizing the industrial and bio-medical field
Miniature Robotic arm
Another application of EAP actuators is the development of a miniature robotic arm with closed-loop control. A longitudinal EAP is used to lift and drop the arm, whereas a 4-finger gripper is used to grab rocks and other objects. To date, multi-finger grippers that consist of two, four, and eight fingers have been produced. This gripper was driven by a 5 V square wave signal at a frequency of 0.1 Hz and could lift a mass of 10.3 gm. The demonstration of this gripper capability to lift a rock paved the way for a future potential application of the gripper to planetary sample collection tasks.
Diaphragm pumps
Bellows pumps can be made by attaching two planar sections of slightly different sizes of IPMC sections and properly placing electrodes on the resulting cavity. This permits modulation of the volume trapped between the IPMC’s. The applied voltage amplitude and frequency can be adjusted to control the flow and volume of fluid being pumped. Single or multiple IPMC’s can function as the diaphragms that creates positive volume displacement. Such a pump produces no noise and has a controllable flow rate in the range of a few micro liters per minute
CONCLUSION
Smart materials such as EAP’s are the foundation of current state-of-the-art devices to convert energy from chemical or electrical into mechanical energy to perform useful work. In the field of sensing, these devices can provide an efficient way of converting mechanical energy into electrical or chemical forms. This seminar had summarized efforts on a number of potential applications of ionic polymer– metal composite that have proven to be a viable alternative to conventional means.
Electroactive polymers have emerged with great potential enabling the development of unique biomimetic devices. As artificial muscles, EAP actuators are offering capabilities that are currently considered science fiction. Developing such actuators is requiring development on all fronts of the field infrastructure. Enhancement of the performance of EAP will require advancement in related computational chemistry models, comprehensive material science, electro-mechanics analytical tools, and improved material processing techniques.
Making robots that are actuated by EAP, as artificial muscles that are controlled by artificial intelligence would create a new science and technology realities. While such capabilities are expected to significantly change future robots, significant research and development effort is needed to develop robust and effective EAP-based actuators