Search

Al2O3, an insulating ceramic with excellent mechanical and thermal properties, has been a kind of ideal fillers for composites. However, due to its poor charge transport capacity, composites doped with Al2O3 have a bad surface insulation property. In this study, nano-Al2O3 particles were modified by plasma fluorination using dielectric barrier discharge, and then doped into epoxy resin to prepare composites. In the preparation stage, the plasma treatment with and without fluorine were differentiated to determine whether fluorine or plasma itself had an influence on the microstructure of Al2O3. The changes of dielectric and surface insulation properties of composite materials before and after filler fluorination were studied. The variation of dielectric constant as a function of filler content has presented a Z-shape curve. Both larger particle diameter and fluorination treatment can reduce the value of dielectric constant, and lead the curve to a monotonous increasing trend. A model of dielectric distribution by the finite element method was constructed to explain the variation. Before fluorination treatment, the flashover voltage as a function of filler content presents a multi-inflection variation. After fluorination, the surface charge dissipation capacity together with flashover voltage were enhanced, and the fluorination treatment could increase the flashover voltage by up to 34.7% of the pure epoxy. The modification process and test results are of guiding significance for improving the insulation properties of epoxy composites. [ABSTRACT FROM AUTHOR]

A numerical algorithm, modelling the transformation from a Gaussian beam to a Bessel beam, is presented for the purpose to study the optimal condition for employing an axicon-generated Bessel beam to fabricate cylindrical microlens arrays (CMLAs). By applying the numerical algorithm to simulate the spatial intensity distribution behind the axicon under different defects of a rotund-apex and different diameter ratios of an incident beam to the axicon, we find that the diffraction effects formed by the axicon edge can be almost eliminated when the diameter ratio is less than 1:2, but the spatial intensity distribution is disturbed dramatically even a few tens of microns deviation of the apex, especially for the front part of the axicon-generated Bessel beam. Fortunately, the lateral intensity profile in the rear part still maintains a desirable Bessel curve. Therefore, the rear part of the Bessel area and the less than 1:2 diameter ratio are the optimal choice for employing an axicon-generated Bessel beam to implement surface microstructures fabrication. Furthermore, by applying the optimal conditions to direct writing microstructures on fused silica with a femtosecond (fs) laser, a large area close-packed CMLA is fabricated. The CMLA presents high quality and uniformity and its optical performance is also demonstrated. [ABSTRACT FROM AUTHOR]

High-aspect-ratio microchannels were fabricated by femtosecond-double-pulse-laser-assisted polarization-selective etching. The etching rate and uniformity of the microchannels were mainly determined by the double-pulse polarization and time delay. We found that when the two sub-pulses had a different polarization (one linear, the other circular), the microchannel etching rate increased by a factor of 10 compared to when both sub-pulses were linearly polarized. The maximum etching rate was obtained when the polarization combination was circular for the first sub-pulse and vertical for the second one. In this case, the etching rate was independent from the time delay. Laser confocal microscopy images showed that when the polarization was circular, the area modified by the laser was larger than when the polarization was linear, explaining the higher etching rate value obtained after irradiation with circularly polarized laser light. [ABSTRACT FROM AUTHOR]