Three-dimensional fractals called the Menger sponge, with a fractal dimension D=log(20/log(3, were fabricated from epoxy resin by stereolithography. Clear attenuation of both reflection and transmission intensity was observed at 12.8 GHz for a cubic specimen with an edge size of 27 mm that was constructed up to the third stage of the self-similar patterns. The electromagnetic field was found to be confined in the central part of the specimen at this frequency. The localization is not caused by the presence of a photonic band gap as in photonic crystals but should be attributed to a singular photon density of states realized in the fractal structure. This is the first report on such localization of electromagnetic waves in three-dimensional fractal cavities.
The thermal conductivity of polymer composites was improved by loading three-dimensional (3D) brushlike AlN nanowhiskers fillers synthesized by simple combustion method. Through filling 47 vol % of the synthesized AlN nanowhiskers, the thermal conductivity of the composite was significantly increased to 4.2 W m−1 K−1, which was 2.3 times higher than that of the composite filled with the same content of commercial AlN equiaxed particles. According to Agari model analysis and microstructure observation, the thermal conductivity enhancement can be ascribed to the 3D brushlike AlN nanowhiskers promoted the formation of a more effect percolating network in the matrix with lower thermal resistance.
Three‐dimensional (3D) photonic crystals with a diamond structure made of a dense SiO2 ceramic were successfully fabricated using a CAD/CAM micro‐stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO2 paste (before sintering) was around 15 μm. After the SiO2‐resin photonic crystals were formed via micro‐stereolithography, they were converted to pure SiO2 ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO2 photonic crystals was measured by terahertz‐time‐domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ–X〈100〉 direction, between 490 and 630 GHz in the Γ–K〈110〉 direction, and between 400 and 510 GHz in the Γ–L〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.
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