Hafnia-plugged planar tungsten photonic crystals for thermophotovoltaic energy conversion
H.J. Lee, K.M. Smyth, S. Bathurst, N. Saka and S.G. Ki
Two-dimensional (2D) tungsten (W) photonic crystals (PhCs) effectively control photon motion and selectively emit radiation tailored to the bandgap of photovoltaic (PV) cells, to enhance the efficiency of thermophotovoltaic (TPV) energy conversion. At the high operating temperatures required for TPV, the micro- and nano-patterned structures of the PhCs quickly lose their integrity and precise spectral control, which introduces a fundamental structural stability problem. In this paper, the thermal failure modes of W TPV selective emitters are shown to be oxidation, recrystallization and grain growth, surface diffusion, and evaporation and re-condensation where the latter two curvature-dependent modes prove particularly detrimental . Accordingly, the key concept of a planar photonic crystal (PPhC), a micro-, nano-structured but geometrically planar surface, is designed and fabricated. Based on scale-accelerated failure tests of a Si PPhC, the lifetime of a W PPhC operating at 1,100 K is estimated to be at least about 30 years.