Metamaterial emitter for thermophotovoltaics stable up to 1400 °C

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-HfO 2 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-HfO 2 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 HfO 2 , which leads to a rupture of the W-layers, thus, to a degradation of the multilayer system at 1450 °C.

Thermal stability of tungsten based metamaterial emitter under medium vacuum and inert gas conditions

“…The amount of transported oxygen is dependent on the residual O 2 partial pressure and temperature. From our study on the stability of W-HfO 2 multilayered metamaterial at different vacuum pressures 1 , we learned that for the same annealing time a 10-fold reduction in the residual O 2 partial pressure can help to improve the thermal stability of the metamaterial structure by approximately 150 °C higher. Thermal stability of the emitters can be further improved by minimizing the residual O 2 in the annealing chamber.…”

“…A 30 nm thin film based metamaterial (Fig. 1c) shows around 2× less mid-infrared absorptivity/emissivity compared to a 20 nm thin film metamaterial emitter 1,36 . Thermal photons coming from a radiative blackbody span over a wide range of wavelengths, where most of the energy is residing in the long-wavelength regime.…”

“…Thermal stability of the selective emitters is one of the key issues in thermophotovoltaics (TPV) [1][2][3][4][5][6][7][8] . TPV provides a viable solution for efficient heat to power conversion, offering theoretical conversion efficiencies up to 85% 2,[9][10][11][12][13][14][15][16] .…”

“…The highest temperature stability of structured thermal emitters of 1400 °C was achieved so far at high vacuum conditions 1,2 , of 10 −5 -10 −6 mbar vacuum pressure, with an application of turbomolecular pumps. However, these working conditions are economically not viable in the commercialization of TPV technology.…”

“…At medium vacuum condition of 10 −2 mbar, which can be achieved without turbomolecular pump, the thermal stability of metal-based emitters drastically decreases at high temperatures 1,7,26,36 . For example, we have shown that 1D multilayered metamaterial emitters from W-HfO 2 nanolayers can be thermally stable only up to 1000 °C under 10 −2 mbar medium vacuum pressure 1,36 . The oxidation of refractory metals is limited only by diffusion of oxygen through the protection layers, such as HfO 2 4,37,38 .…”