https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=1548959674798218270
Photonic crystals with periodic variations in dielectric constants can exhibit forbidden gaps in transmission spectrum to totally reflect electromagnetic waves through Bragg diffraction (Yablonovitch, 1987). Through introductions of artificial defects of air cavities into the periodic arrangement, localized modes are formed in the photonic bandgaps. These structural defects can localize and amplify electromagnetic wave energies at the specific frequencies and wavelengths corresponding to the sizes and dielectric constants of the resonation domains. For the last few decades, the photonic crystal has been expected to be applied for various microwave devices of resonators, filters, and directional antennas (Noda, 2000). Especially, micrometer order ceramic lattices with diamond structures were fabricated successfully by using nanoparticle stereolithography to create the perfect photonic bandgaps and reflect the electromagnetic waves totally in all directions (Kirihara and Miyamoto, 2009). The lattice constant of the diamond structure and the aspect ratio of the dielectric lattice were designed as 500 μm and 1.5, respectively. The whole size of the crystal component was 5 mm × 5 mm × 1 mm consisting of 10 × 10 × 2 unit cells. Fig. 43.1 shows a schematic illustration of the stereolithography system. Photosensitive acrylic resins including alumina particles of 170 nm in average diameter at 40% in volume content were supplied on a glass substrate from a dispenser nozzle by the air pressure. This paste was spread uniformly by using a mechanically controlled knife edge. The thickness of each layer was set at 15 μm. Two-dimensional solid patterns are obtained by a light-induced photopolymerization. High-resolution image has been achieved by using a digital micromirror device. In this optical device, microaluminum mirrors of 14 μm in edge length were assembled with 1024 × 768 in numbers. Each mirror can be tilted independently by piezoelectric actuating. Through the layer stacking, the acrylic resin component with the alumina particle dispersion was obtained. The composite precursor was dewaxed at 600°C for 2 h and sintered at 1500°C for 2 h in the air atmosphere. Fig. 43.2 shows the alumina lattice with the diamond structure fabricated by ceramic powder sintering. No cracks or deformations were observed in the obtained components. The average linear shrinkage was 25%. The lattice constant of the sintered sample was 375 μm. The relative density of the sample reached 97.5%.
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