Rapid Prototyping of Ceramic Based Photonic Bandgap Structures
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Rapid Prototyping of Ceramic Based Photonic Bandgap Structures Jennifer Synowczynski, Samuel Hirsch, and Bonnie Gersten Weapons and Materials Research Directorate, Army Research Laboratory Aberdeen Proving Grounds, MD 21005-5069 ABSTRACT A three-dimensional photonic bandgap (PBG) structure was fabricated from CAD models using a method based on lost wax rapid prototyping and ceramic gelcasting. The inverse PBG mold was constructed from a low melting point thermoplastic using a high precision Sanders Rapid Toolmaker. An aqueous stable slurry (200-300 cp) containing 50-75wt% of the ceramic powders (BaSrTiO3, MgO), a 15wt% solution of monomer (Methacylamide) and crosslinker (Poly(ethylene glycol) dimethacrylate), and a free radical initiator (2,2′Azobis(2-amidinopropane) dihydrochloride) was cast into the mold. A polymerization reaction was thermally activated at 50oC to immobilize the ceramic powders. The wax mold was then removed by drying the green body in a high humidity oven at 120oC. Scanning Electron Microscopy (SEM) of the unfired part showed that atmosphere hampered the polymerization reaction at the surface of the part. The green density, sintered density, and permittivity all increased as the solids loading increased. In order to optimize the dielectric properties and minimize cracking and warping in the sintered part, the solids loading had to be greater than 80 wt%. This study investigated several steps in the lost mold / gelcasting procedure including stabilizing the ceramic suspension, the correlation between the solids loading and the green and sintered densities, binder removal, and the effect of shrinkage during sintering on the net shape. INTRODUCTION Photonic bandgap materials (PBG) are composites that are artificially engineered to have a periodic variation in the dielectric constant with a period that is on the order of the electromagnetic wavelength. If the phases have a strong refractive contrast, a bandgap is created in the frequency spectrum due to the Bragg-like reflection at the interface between the two phases. Electromagnetic waves with frequencies within the bandgap cannot exist within the composite. Theorically, PBGs are perfect reflectors. They have found many applications in optical and microwave devices including: inhibiting spontaneous emission in lasers, optical waveguiding, high efficiency antenna substrates[1], and reflectors[2]. However, PBGs have limited bandwidth and are sensitive to the angle of incidence of the incoming radiation. Recently, a three-dimensional PBG structure with a 21% omnidirectional bandgap was proposed[3]. It contained alternating layers of periodically spaced silicon rods in air and periodically spaced holes in a silicon substrate. Since the width of the bandgap increases as the dielectric contrast increases, it is possible to further widen the bandgap by replacing the silicon with 40% Ba0.45Sr0.55TiO3 / 60% MgO composite (εR = 80, tanδ = 0.0041 at 10GHz)[4]. Ba0.45Sr0.55TiO3 is also a ferroelectric material whose permittivity changes under an applied elect
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