Ji Su

High-dielectric-constant all-polymer percolative composites

We report here an all-polymer high-dielectric (dielectric constant K>1000 at 1 kHz) percolative composite material, fabricated by a combination of conductive polyaniline particles (K>105) within a poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer matrix (K>50). These high-K polymer hybrid materials also exhibit high electromechanical responses. For example, 1.5% strain, which is proportional to the square of the field applied, can be induced by a field of 9.5 MV/m, an eightfold reduction in field applied compared with that in a fluoroterpolymer matrix.

Electrostrictive Grafr Elastomers and Applications

Efficient actuators that are lightweight, high performance and compact are needed to support telerobotic requirements for future NASA missions. In this work, we present a new class of electromechanically active polymers that can potentially be used as actuators to meet many NASA needs. The materials are graft elastomers that offer high strain under an applied electric field. Due to its higher mechanical modulus, this elastomer also has a higher strain energy density as compared to previously reported electrostrictive polyurethane elastomers. The dielectric, mechanical and electromechanical properties of this new electrostrictive elastomer have been studied as a function of temperature and frequency. Combined with structural analysis using x-ray diffraction and differential scanning calorimetry on the new elastomer, structure-property interrelationship and mechanisms of the electric field induced strain in the graft elastomer have also been investigated. This electroactive polymer (EAP) has demonstrated high actuation strain and high mechanical energy density. The combination of these properties with its tailorable molecular composition and excellent processability makes it attractive for a variety of actuation tasks. The experimental results and applications will be presented.

A single crystal lead magnesium niobate-lead titanate multilayer-stacked cryogenic flextensional actuator

A “33” mode single crystal lead magnesium niobate-lead titanate flextensional actuator with large displacement, high load capability, and broad bandwidth was designed, prototyped, and evaluated at temperatures ranging from room temperature to cryogenic temperatures. Measuring 27.4 × 10 × 13.6 mm (height) overall and weighing 9.2 g, the actuator generates a 96.5 μm displacement in the Z-direction at 170 Vrms. The level of displacement remained constant under compressive loads up to 5 kg force. The actuator maintains 66% of its room temperature displacement at −196 °C. The measured displacements matched well with those modeled using ANSYS finite element analysis.

An electroactive polymer-ceramic hybrid actuation system for enhanced electromechanical performance

A hybrid actuation system (HYBAS) utilizing advantages of a combination of electromechanical responses of an electrostrictive copolymer and an electroactive single crystal has been developed. The system employs the contributions of the actuation elements cooperatively. The theoretical modeling of the performances of the HYBAS is in good agreement with experimental observation. The consistency between the theoretical modeling and the experimental tests makes the design concept an effective route for tailoring and maximizing electrically driven displacement of the hybrid actuation system.

Novel polymeric elastomers for actuation

Efficient actuators that are lightweight, high performance and compact are needed to support telerobotic requirements for future NASA missions. A new class of electrostrictive polymeric materials has been developed in our recent work. The materials are graft elastomers that exhibit not only large electric field-induced strain (4%) but also a relatively high mechanical modulus (560 MPa). Consequently, the materials offer high actuation output power for enhanced actuation performance. In addition to the promising electromechanical properties, the materials are readily processable by conventional methods. By combining this graft elastomer with a piezoelectric polymer, polyvinylidene fluoride (PVDF), an electrostrictive-piezoelectric multifunctional polymer blend system has also been developed. This multifunctional system exhibits a significantly high piezoelectric strain coefficient, d/sub 31/, of 12 pC/N while the electric field-induced strain is enhanced due to the combination of the mechanisms of electrostriction and piezoelectricity. Experimental results of the electric field-induced strain in the polymeric elastomers and the electrostrictive-piezoelectric polymer blend system are discussed. A prototype bending actuator using the materials is also presented.

Challenges to the Transition to the Practical Application of IPMC as Artificial-Muscle Actuators

In recent years, electroactive polymers (EAP) materials have gained recognition as potential actuators with unique capabilities having the closest performance resemblance to biological muscles. Ion-exchange membrane metallic composites (IPMC) are one of the EAP materials with such a potential. The strong bending that is induced by IPMC offers attractive actuation for the construction of various mechanisms. Examples of applications that were conceived and investigated for planetary tasks include a gripper and wiper. The development of the wiper for dust removal from the window of a miniature rover, planned for launch to an asteroid, is the subject of this reported study. The application of EAP in space conditions is posing great challenge due to the harsh operating conditions that are involved and the critical need for robustness and durability. The various issues that can affect the application of IPMC were examined including operation in vacuum, low temperatures, and the effect of the electromechanical and ionic characteristics of IPMC on its actuation capability. The authors introduced highly efficient IPMC materials, mechanical modeling, unique elements and protective coatings in an effort to enhance the applicability of IPMC as an actuator of a planetary dust-wiper. Results showed that the IPMC technology is not ready yet for practical implementation due to residual deformation that is introduced under DC activation and the difficulty to protect the material ionic content over the needed 3-years durability. Further studies are under way to overcome these obstacles and other EAP materials are also being considered as alternative bending actuators.

Electroactive-polymer-based MEMS for aerospace and medical applications

Electroactive polymers (EAP) demonstrate advantages over some traditional electroactive materials such as electro-ceramics and magneostrictive materials for electromechanical device applications due to their high strain, light weight, flexibility, and low cost. Electroactive polymer-based microelectromechanical systems (EAP-MEMS) are increasingly demanded in many aerospace and medical applications. This paper will briefly review recent progress in the developments and applications of EAP- MEMS. In the past few years, several new configurations of micromachined actuators/transducers have been developed using electroactive polymers. The performance of these micromachined EAP-based devices has been evaluated for both fluid and air conditions. The performance of EAP-MEMS has also been theoretically modeled based on material properties and device configurations. In general, the results obtained from modeling agree with the experimental measurements. Critical process issues, including patterned micro-scale electrodes, molded micro/nano electroactive polymer structures, polymer to electrode adhesion and the development of conductive polymers for electrodes will be discussed. The challenges to develop complete polymer MEMS will also be addressed.

Effect of bending stiffness of the electroactive polymer element on the performance of a Hybrid Actuator System (HYBAS)

An electroactive polymer (EAP)-ceramic hybrid actuation system (HYBAS) was developed recently at NASA Langley Research Center. This paper focuses on the effect of the bending stiffness of the EAP component on the performance of a HYBAS, in which the actuation of the EAP element can match the theoretical prediction at various length/thickness ratios for a constant elastic modulus of the EAP component. The effects on the bending stiffness of the elastic modulus and length/thickness ratio of the EAP component were studied. A critical bending stiffness to keep the actuation of the EAP element suitable for a rigid beam theory-based modeling was found for electron irradiated P(VDF-TrFE) copolymer. For example, the agreement of experimental data and theoretical modeling for a HYBAS with the length/thickness ratio of EAP element at 375 times is demonstrated. However, the beam based theoretical modeling becomes invalid (i.e., the profile of the HYBAS movement does not follow the prediction of theoretical modeling) when the bending stiffness is lower than a critical value.

The Load Capability of Piezoelectric Single Crystal Actuators

Piezoelectric lead magnesium niobate-lead titanate (PMN-PT) single crystal is one of the most promising materials for electromechanical device applications due to its high electrical field induced strain and high electromechanical coupling factor. PMN-PT single crystal-based multilayer stack actuators and multilayer stack-based flextensional actuators have exhibited high stroke and high displacement-voltage ratios. The actuation capabilities of these two actuators were evaluated using a newly developed method based upon a laser vibrometer system under various loading conditions. The measured displacements as a function of mechanical loads at different driving voltages indicate that the displacement response of the actuators is approximately constant under broad ranges of mechanical load. The load capabilities of these PMN-PT single crystal-based actuators and the advantages of the capability for applications will be discussed.

Theoretical evaluation of electroactive polymer based micropump diaphragm for air flow control

An electroactive polymer (EAP), high energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer, based actuation micropump diaphragm (PAMPD) has been developed for air flow control. The displacement strokes and profiles as a function of amplifier and frequency of electric field have been characterized. The volume stroke rates (volume rate) as function of electric field, driving frequency have been theoretically evaluated, too. The PAMPD exhibits high volume rate. It is easily tuned with varying of either amplitude or frequency of the applied electric field. In addition, the performance of the diaphragms were modeled and the agreement between the modeling results and experimental data confirms that the response of the diaphragms follow the design parameters. The results demonstrated that the diaphragm can fit some future aerospace applications to replace the traditional complex mechanical systems, increase the control capability and reduce the weight of the future air dynamic control systems.