Cost-Efficient Fabrication of 3-D Photonic Crystals via Microtransfer Molding. TRAVIS MONK (Truman State University Kirrksville, MS 63501) KRISTEN CONSTANT (Ames Laboratory, Ames, IA, 50011)

Three-dimensional photonic crystals impede a certain frequency of light independent of the light's incident angle. Methods based on semiconductor processing cost around $250,000 to create a four-layer crystal. Using microtransfer molding, a method of soft lithography, it is possible to construct photonic crystals cost-efficiently. By pouring poly-dimethyl siloxane elastomer (PDMS) over parallel bars etched in a silicon wafer and curing the PDMS, one can repeatedly create parallel channels in the PDMS. The channels' spacing and size depend on the frequency of light to be impeded. Prying the PDMS from the silicon wafer exposes these imbedded channels. They are then filled with polyurethane (PU) and exposed to high-intensity ultraviolet light, hardening the PU. The PU-filled channels are then coated with polyacrylate (PA). After pressing a glass substrate atop the PU-filled, PA-coated channels, the PU and PA are again exposed to ultraviolet light for solidification. The PDMS is peeled from the glass substrate, leaving bars of solid PU glued to the substrate by the PA. Stacking these layers of PU bars in the 'Iowa State Woodpile Structure', where layers are rotated 90 degrees relative to each other, a mold is created from which one can make a photonic crystal. This crystal has a yield around 70%, meaning microtransfer molding is not yet an acceptable method for fabricating photonic crystals. Methods based on semiconductor processing produce very high quality samples. However, the process is so expensive that samples can only be used for proof of theory. Microtransfer molding, on the other hand, is quite cost efficient, allowing our group to focus on applications of photonic crystals.

Microwave Characterization of Composites with Tungsten Coated Filler Particles. THOMAS MALONEY (University of Cincinnati Cincinnati, OH 45221) NICOLA BOWLER (Ames Laboratory, Ames, IA, 50011)

Using filler particles in composites allows for the control of the electromagnetic properties of the material. This is useful in the absorption of electromagnetic waves in the microwave spectrum, which has applications in telecommunications, radar systems and microwave heating. In this study, the relative permittivity of composites made with four related types of filler particles was measured at microwave frequencies. The particles used were 3M™ Glass Bubbles A20/1000 sputter coated with tungsten. Two of the particle samples had an aluminum oxide (AlOx) outer coating. For each of these two different types of particle samples, one batch was dried at 150 C and the other at 350 C prior to sputter coating with tungsten. The particles were infused into a matrix material of paraffin wax. The electromagnetic properties were measured in the frequency range of ~2 to 18 GHz by a 7mm coaxial reflection/transmission line method. The data was analyzed and showed a dielectric loss in the observed frequency range for all four particle samples. This research shows that the composites filled with the AlOx coated particles have a higher real and imaginary relative permittivity than those with the non-AlOx coated particles. The drying temperature of 350 C was shown to shift the dielectric relaxation to a higher frequency while reducing its extent. This indicates that the frequency and amount of absorption can be controlled by the type of particle used, and the filler volume fraction of the sample, allowing composite materials to be tailored for a large number of applications.