Self-clearable charcoal electrodes for dielectric elastomer actuator
Shiau, Li Lynn
Date of Issue2014
School of Mechanical and Aerospace Engineering
Dielectric Elastomer Actuators (DEAs) can produce large strains with high energy density like natural muscles do. In addition, they response swiftly while they are resilient against shock. However, they require high driving voltage and are subjected to pre-mature failure. This reliability issue hindered their widespread adoption and needs to be resolved. A basic unit of DEA consists of a layer of dielectric elastomer membrane sandwiched by two layers of compliant electrodes. Under compression by fieldinduced Maxwell stress, the dielectric elastomer can undergo a large deformation. However, the DEA performance is limited by electrical breakdown.The maximum Maxwell stress is dictated by the electrical breakdown field. The breakdown of dielectric elastomer may be attributed to partial discharge in air void, electromechanical instability of soft dielectrics, and electronic avalanche or thermal runway. Pre-stretching helps stiffen DE membrane and compress air voids in it so that it can sustain higher electric field and consequently produces a larger actuation.On the other hand, use of self-clearable electrodes can further help dielectric elastomer to stop pre-mature localized breakdowns at higher field. In this dissertation, we propose using thin charcoal powder as the self-clearable compliant electrodes for DEA. We demonstrated that charcoal powders are compliant enough for large deformation on dielectric elastomer and they can self clear localized electrical breakdown. During electrical breakdown, electrical arching heats up a localized defective spot in dielectric elastomer and result in a surge of leakage current through the spot. In tum, Joule heating further increases the temperature and result in reduced resistivity that leads to more leakage current. Such positive feedback causes thermal runway that may bum off the dielectric elastomer and electrodes at the defective spot. Burning of the electrode limit the leakage current and reduce the possibility of electrical breakdown occurred to DEA. Self-clearing happens to DEA if only the leakage current is limited during the electrical breakdown. At room temperature, DE membrane is an insulator and there is no current leakage. However, upon electrical breakdown, the defective spot is shorted and subjected to thermal runaway. As self-clearing stops pre-mature breakdowns, DEAs using charcoal electrodes can survive multiple breakdowns at different location of the defective spots. Electrode thickness has a profound effect on self-clearing of localized electrical breakdown.To realize self-clearing, a thin coating of charcoal electrode acts like a fuse and can be burnt off and stops the localized electrical breakdown.However, thicker coating of charcoal electrode acts like fuel and causes a spread of burning and melting the DE membrane. With this understanding on the self-clearing mechanism, we showed that DEA using very thin coating of charcoal powder electrode can sustain higher electrical field than that using thicker coating.