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The influence of the cylindrical exposed electrode diameter on distribution and attenuation of the surface charge in surface dielectric barrier discharges is experimentally investigated using the Pockels effect of the Bi12SiO20 crystal. For the actuator with the smaller exposed electrode diameter, more positive charges accumulate closer to the exposed electrode after positive half periodic discharge, while the quantity of positive charges deposited on the surface in a discharge period at the same distance away from the exposed electrode is smaller. At the same time, the smaller the exposed electrode diameter is, the shorter decay time the positive charge filament needs. Combined with thrust generation of actuators, it can be indicated that a positive residual surface charge closer to the exposed electrode may play a more crucial role in the discharge and consequent thrust than one far away from the exposed electrode does. Also, the effect on the discharge of the positive residual surface charge may be greater than that of the negative residual surface charge. More explorations of the residual surface charge have been done by selectively enhancing the discharge of the actuator with a 10 μm exposed electrode using the pulse-AC dual power supply, drawing the same conclusions.

Flake-shaped carbonyl iron (FCI) powder was prepared by ball milling spherical carbonyl iron (SCI) powder for 20 h. The thicknesses of the FCI particles were ∼0.1–0.3 μm with a diameter of ∼5–10 μm. The milling process increased the aspect ratio from ∼1 for spherical powder to an aspect ratio of ∼20–100 for flake one. The complex permittivity (ɛ′ − jɛ″) and permeability (μ′ − jμ″) were measured by the transmission/reflection method in the frequency range of 2–18 GHz for composites with various weight ratios of SCI and FCI. The results show that all ɛ′, ɛ″, μ′ and μ″ substantially increased after the milling treatment. Compared to SCI particles, the FCI particles exhibit higher permeability, higher permittivity, and better absorbing properties within the frequency test range. The magnetic loss tangent of FCI composites shows that FCI, with their large shape anisotropy, may have a higher resonance frequency and exceed the Snoek’s limit in the gigahertz frequency range. For the 50 wt. % FCI composite with a thickness of 3 mm, the reflection loss at 5.5 GHz reaches −23.0 dB.

The spontaneous strain associated with the structural change in the metal-insulator transition in VO2 is orders of magnitude higher than thermal expansion mismatch used in bimetallic strips. Here we show that this strain can be leveraged to thermally activate bending of crystalline VO2-based bilayer microcantilevers at extremely large curvatures, making them suitable for thermal sensors, energy transducers and actuators with unprecedented sensitivities. The single-crystallinity, deposition conditions, and postdeposition treatments were utilized to control the metal-insulator domain structure along the cantilever, by which we achieved bending curvatures a few hundred times higher than conventional bilayer cantilevers with the same geometry.