Sputtered Tantalum Photonic Crystal Coatings for High-Temperature Energy Conversion Applications

Recent advances in photonic crystals have enabled high-efficiency thermophotovoltaic (TPV) energy conversion by allowing near-perfect spectral control. In this paper, photonic crystals fabricated in thick tantalum (Ta) coatings, sputtered onto a metallic substrate, were investigated as a potential replacement for photonic crystals fabricated in bulk refractory metal substrates in TPV applications. A 2-D photonic crystal was fabricated in a 30-μm thick Ta sputtered coating using standard semiconductor processes as a proof-of-concept for a selective emitter/solar absorber. The reflectance of the fabricated photonic crystal coating was characterized before and after 1 h anneals at 700, 900, and 1100 °C, and after a 24 h anneal at 900 °C. To protect its surface during the anneals, a thin conformal hafnia (HfO 2 ) layer was deposited by atomic layer deposition on the photonic crystal. The photonic crystal showed high spectral selectivity in good agreement with numerical simulations and sustained the anneals with minimal structural degradation or change in its optical properties. This study presents a promising alternative to bulk substrates as a relatively low-cost and easily integrated platform for nanostructured devices in high-temperature energy conversion systems.