Abstract
High temperature stable selective emitters can significantly increase efficiency and radiative power in thermophotovoltaic (TPV) systems. However, optical properties of structured emitters reported so far degrade at temperatures approaching 1200 °C due to various degradation mechanisms. We have realized a 1D structured emitter based on a sputtered W-HfO2 layered metamaterial and demonstrated desired band edge spectral properties at 1400 °C. To the best of our knowledge the temperature of 1400 °C is the highest reported for a structured emitter, so far. The spatial confinement and absence of edges stabilizes the W-HfO2 multilayer system to temperatures unprecedented for other nanoscaled W-structures. Only when this confinement is broken W starts to show the well-known self-diffusion behavior transforming to spherical shaped W-islands. We further show that the oxidation of W by atmospheric oxygen could be prevented by reducing the vacuum pressure below 10−5 mbar. When oxidation is mitigated we observe that the 20 nm spatially confined W films survive temperatures up to 1400 °C. The demonstrated thermal stability is limited by grain growth in HfO2, which leads to a rupture of the W-layers, thus, to a degradation of the multilayer system at 1450 °C.
Highlights
High-temperature emitters play a crucial role in thermophotovoltaic (TPV) energy conversion[1,2,3,4,5,6,7,8,9]
The as-fabricated emitter structure shows a step function-like steep spectral cutoff around 1.7 μm and low absorptivities/emissivities above the wavelength corresponding to the bandgap of the PV cell, i.e. low emission of such photons
The metamaterial emitter structure after annealing at 1400 °C for 6 h, measured at room temperature, shows similar band-edge characteristics with even a slight improvement of the spectral characteristics, e.g. a reduction of the absorptivity/emissivity at long wavelengths (Fig. 2, red trace), which is attributed to a reduced www.nature.com/scientificreports electron collision frequency due to grain growth in the tungsten layer leading to an improved metallic reflection
Summary
High-temperature emitters play a crucial role in thermophotovoltaic (TPV) energy conversion[1,2,3,4,5,6,7,8,9]. The focus of the present work is to investigate the thermal stability limit of such thin flat refractory W and HfO2 based layered metamaterial structures which avoid surface diffusion These thin films were investigated under medium and high vacuum conditions[37], in our case 10−2 to 10−3 and 10−5 to 10−6 mbar vacuum pressures, respectively; the latter is needed in order to suppress the oxidation of W metal. This metamaterial structure is designed to operate as the thermal emitter in a TPV system using a GaSb PV cell with a bandgap of Eg = 0.72 eV (≙ 1.72 μm)[38]. These thin film structures demonstrate higher thermal stability as much coarser 2D and 3D PhCs4,23,31
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