Fault‐Tolerant Dielectric Elastomer Actuators using Single‐Walled Carbon Nanotube Electrodes

Abstract

Certain dielectric elastomers such as the 3M VHB 4910 acrylic adhesive films have exhibited electrically induced strains as high as 380 % in area expansion. The calculated maximum specific energy density of 3.4 J g and maximum stress of 8 MPa are attractive for a wide range of applications including robotics, prosthetic devices, medical implants, pumps, and valves. However, the performance of actuators based on the VHB films is substantially lower than the calculated values which reflect the maximum intrinsic material properties. Defects in the soft dielectric films, such as gel particles, uneven thickness, non-uniform crosslinking, and stress concentration, are possible causes for the reduced actuator performance. They also reduce the actuator reliability, hindering technology commercialization. We introduce self-clearable compliant electrode materials that could enhance the fault-tolerance of the dielectric elastomers actuators. Thin metallic layers (10–100 nm) are being used for polypropylene thin-film capacitors for fault-tolerance. The electrode materials locally evaporate, or self-clear, around defects during highvoltage breakdown. Metallic films are not sufficiently compliant for the dielectric elastomers. To enhance the compliancy of metallic electrodes, zig-zag shaped metallic lines were used to obtain 80 % area strains. Uniform metallic coatings on corrugated silicone elastomer surfaces supported linear strains up to 33 %. The commonly used compliant electrode materials for the VHB elastomers are powdered carbon graphite, carbon black, or carbon fibrils dispersed in grease or silicon oil, and electrolyte solutions. No self-clearing has been reported for these electrode materials. Our experiments showed that single-walled carbon nanotubes (SWNTs) can overcome this morass. The SWNT electrodes have exhibited flexibility as transparent electrodes in lightemitting diodes, solar cells, and thin-film transistors. With thin SWNT electrodes, the dielectric elastomer can not only be strained larger than 200 % in area expansion, but can achieve fault-tolerance through localized degradation of SWNTs. Figure 1 shows a 300 % biaxially prestrained VHB 4910 film (62 lm thick after prestrain) with the SWNT electrodes at rest (Fig. 1a) and during actuation at 5 kV (Fig. 1b). The calculated strain is 200 %. This value is comparable with the same film using conventional carbon grease electrodes. The observed strain for 300 % biaxially prestrained VHB 4905 films (31 lm thick after prestrain) was 190 % at 3.5 kV.