For high-voltage direct current (HVDC) cable development, (23) space-charge suppression is an important issue. On the other hand, the addition of insulating particles increases the insulating properties of the composite, allowing higher voltages to be applied. Furthermore, surface modification of the high-dielectric constant particles reduces the relative dielectric constant of the composite. (22) However, since the surface of BaTiO 3 has no hydroxyl group, it is not easy to modify the surface by conventional coupling agents, such as silica (SiO 2) and titanium oxide (TiO 2). To avoid this problem, surface modification of BaTiO 3 particles has been reported. Composites containing conductive particles have the same problem. Therefore, even with a high dielectric constant, they are prone to dielectric breakdown in low electric fields before high stresses can be generated. These factors increase the leakage current and decrease the dielectric breakdown strength of the composites. Extrinsic factors include the partial agglomeration of particles, presence of air voids, and incomplete interfaces between the polymer matrix and fillers during processing. The intrinsic factor is an increase in the electrical conductivity because of a decrease in the volume resistivity caused by an increase in the dielectric constant. There is an intrinsic and an extrinsic origin for the dielectric breakdown. (18−21) In composite elastomers with high dielectric constant particles, the approach of adding barium titanate (BaTiO 3) is often used however, dielectric breakdown tends to occur in low electric fields. The general approaches include the addition of ceramic particles with high dielectric constants, (6−10) addition of conductive nanofillers, (11,12) post-modification of polar groups in polymers, (13−17) and polymerization of highly polar monomers. Various attempts to increase the relative dielectric constant have been made. To increase the generated stress, it is necessary to increase the relative dielectric constant (ε r) or apply a high electric field ( V/ d). Thus, we demonstrated that an elastomer containing a high dipole group on an insulating particle surface is capable of improving the power performance of soft actuators. The card-house structure of TiO 2 particles in the composite elastomer is assumed to suppress the dielectric breakdown in a low electric field. The generated stress of the composite elastomer increased in proportion to the relative dielectric constant, showing a maximum of 1.9 MPa. As the dielectric constant increased, the volumetric resistivity decreased however, the dielectric breakdown strength was maintained at 95 V/mm. The relative dielectric constant increased proportionally with the amount of CN-modified TiO 2 particles, showing a value of 22 at 100 Hz. The HXNBR/CN-modified and non-modified TiO 2 particle composite elastomer showed a high relative dielectric constant and generated stress in a low electric field. The CN group-containing silane coupling agent was synthesized via a thiol–ene reaction between acrylonitrile and 3-mercaptpropyltrimethoxysilane and immobilized onto the TiO 2 particle surface. We prepared a dielectric elastomer actuator composed of hydrogenated carboxylated acrylonitrile-butadiene rubber (HXNBR)/nitrile group (CN)-modified and non-modified titanium oxide (TiO 2) particles with insulation properties.
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