WEB High temperature stable Metamaterial Emitters for Thermophotovoltaic energy HarvestingFriday (25.09.2020) 09:45 - 10:00 S: Structural Materials 1 Part of:
Thermal stability of the metamaterials at high temperatures plays a critical role in obtaining high conversion efficiencies and high radiative powers in thermophotovoltaic (TPV) energy systems. TPV systems offer theoretical heat to power conversion efficiencies up to 85%, with a wide variety of sources (solar heat, waste heat and chemical and nuclear processes).[1-3] To achieve that spectrally selective thermal emitters are required. As absoption and thus emission of conventional materials can not be significantly adjusted, new structured materials are proposed, so called optical metamaterials. The TPV conversion efficiency currently reaches 29% by operating at 1000°C. At temperatures higher than 1000 °C, many structured emitters suffer from structural degradation due to various mechanisms such as oxidization, grain growth, diffusion and thermal expansion coefficient, which deteriorate the spectral selectivity. The ideal characteristics of a spectral selective emitter are, emissivity e=1 for E > Eg and e=0 for E < Eg, where E and Eg are the thermal photon energy and bandgap energy of the PV cell. According to the Stefan-Boltzmann law, the maximum of radiative power density of a blackbody is moving to higher photon energies, thus provides more radition that can be converted by PV cell. Thus, operating the nanostructures at higher temperatures and retaining the spectral selectivity is vital to achieving high conversion efficiency.
Herein, we present thermal stability of 1D metamaterial emitter structure under different vacuum conditions (ranging from 10-2 to 10-6 mbar) and temperatures up to 1450 °C. A hyperbolic metamaterial based on multilayers of W-HfO2 is fabricated using the magnetron sputtering technique. 1D metamaterial emitter exhibits a step function-like steep spectral cutoff around 1.7um and low absorptivities/emissivities above the wavelength corresponding to the bandgap of the GaSb PV cell.[6,7] We clarify the potential degradation mechanisms initiating the structural instability at high temperatures. We discuss in detail how the residual O2 partial pressure can affect the structural stability of the metamaterial emitter at high temperatures. By reducing the partial O2 pressure, our metamaterial structure exhibits unprecedented thermal stability up to 1400°C. Further, we also show how to achieve structural and spectral stability up to 1400°C under technical vacuum conditions of 10-2 mbar, with the help of inert gas encapsulation.
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