Effects of thermal processes on the structure of monolithic tungsten and tungsten alloy photonic crystals

Monolithic pieces of tungsten inverse opals with millimeter dimensions were synthesized by infiltration of poly(methyl methacrylate) colloidal crystals with the aqueous tungsten precursors, acetylated peroxotungstic acid or ammonium metatungstate hydrate. Reduction in hydrogen removed the polymer spheres and formed a very open tungsten skeleton with periodic structure (325-363 nm macropores) and very different morphologies from the surface-templated structures usually obtained by tungsten CVD methods. When these tungsten photonic crystals were resistively or radiatively heated in a nitrogen atmosphere that was not completely dry, tungsten needles formed by chemical vapor transport. More rigorous exclusion of water eliminated the growth of needles, but significant grain growth diminished the structural integrity of the photonic crystal. By doping tungsten with ca. 3 wt % molybdenum, grain growth could be significantly reduced and needle formation eliminated, even in atmospheres that were not completely dry.