NMP 2008
 |
NICK DENNY |
ADVISOR:
Dan Frisbie |
How Heat Effects the Structure of Macroporous Tungsten Photonic Crystals for Thermal Efmssion
Nicholas
R. Denny,1 Sang Eon Han,2 David J. Norris,2 Andreas Stein1
1Department of Chemistry, University of Minnesota
2Department of Chemical Engineering and Materials Science, University of Minnesota
Millimeter-sized, monolithic, 3-dimensionally ordered macroporous (3DOM) tungsten pieces were synthesized by infiltrating PMMA colloidal crystals with acetylated peroxotungstic acid. The skeletal structures of the resulting photonic crystals were much more open than those of W inverse opals prepared by CVD or aqueous WCl6 precursors and should therefore reduce optical absorptions by the W framework that prevented the observation of strong photonic crystal effects in previous studies of W inverse opals. The order in these materials extends over limited domains, but these are large enough to observe stop bands in UV-visible spectra. As free-standing monolithic structures, these 3DOM W samples can be resistively heated to evaluate thermal emission without the need of an underlying support. Because of their small structural dimensions and relatively large interfacial areas, the heated materials readily react with trace amounts of water, leading to W needle formation by chemical vapor transport involving the volatile WO2(OH)2 phase. In addition, grain growth occurring during Joule heating or radiative heating at temperatures as low as 800 °C can destroy the periodic structure of the photonic crystal, which would lead to deterioration of its photonic properties. Alloy formation of 3DOM W with ca. 3 wt% Mo effectively suppressed needle formation during heat treatment, even when trace water was present in the surrounding atmosphere. Furthermore, it significantly decreased grain growth at 800 °C, so that the 3DOM structure was maintained at this temperature for at least 4 h. The combined advances described in this study, formation of a monolithic refractory metal with a periodic macroporous structure on an optical length scale, production of a relatively open structure, and thermal stabilization by alloy formation, provide important new opportunities for the design of metallic photonic crystals for thermal emission control