Source: https://www.osapublishing.org/oe/abstract.cfm?URI=oe-21-9-11482
Timestamp: 2019-04-23 16:06:16+00:00

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We present the results of extensive characterization of selective emitters at high temperatures, including thermal emission measurements and thermal stability testing at 1000°C for 1h and 900°C for up to 144h. The selective emitters were fabricated as 2D photonic crystals (PhCs) on polycrystalline tantalum (Ta), targeting large-area applications in solid-state heat-to-electricity conversion. We characterized spectral emission as a function of temperature, observing very good selectivity of the emission as compared to flat Ta, with the emission of the PhC approaching the blackbody limit below the target cut-off wavelength of 2 μm, and a steep cut-off to low emission at longer wavelengths. In addition, we study the use of a thin, conformal layer (20 nm) of HfO2 deposited by atomic layer deposition (ALD) as a surface protective coating, and confirm experimentally that it acts as a diffusion inhibitor and thermal barrier coating, and prevents the formation of Ta carbide on the surface. Furthermore, we tested the thermal stability of the nanostructured emitters and their optical properties before and after annealing, observing no degradation even after 144h (6 days) at 900°C, which demonstrates the suitability of these selective emitters for high-temperature applications.
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Fig. 1 Measured normal spectral emission of a Ta PhC at different temperatures (solid lines), compared to the simulated emission of a Ta PhC (dashed line), calculated emission of flat Ta (dashed-dotted line), and calculated blackbody emission (dashed black line) at 982°C. Inset: Scanning electron micrograph of the fabricated Ta PhC.
Fig. 2 Emissivity of Ta PhC as a function of temperature: solid lines are measured curves of a fabricated Ta PhC and flat Ta at RT, and the emissivity derived from measured emission at 982°C. Dashed lines are simulated emissivity of a Ta PhC with period a = 1.35 µm, radius r = 0.51 µm and depth d = 6.26 µm using Drude-Lorentz material parameters for Ta fitted to the reflectivity of flat Ta measured at RT, and to emissivity of Ta at 1205°C and 2527°C as taken from literature . Inset: schematic view of the Ta PhC.
Fig. 3 (a) Emissivity of a Ta PhC without any surface coating measured at RT before and after heating to 900°C for 3h, 24h and 72h, respectively, (b) and (c) surface of a Ta PhC after heating for 3h and 24h, respectively, showing TaC formation.
Fig. 4 (a) Emissivity of a Ta PhC with HfO2 coating measured at RT before and after heating to 1000°C for 1h and subsequently to 900°C for 144h (in 6 runs of 24h each). Inset: SEM micrograph of the surface of a Ta PhC after heating to 900°C for 144h. (b) SEM micrograph of Ta PhC with HfO2 coating before heating and (c) after heating to 900°C for 144h, showing no signs of structural degradation.
(2) ΔT T = Δε ln( 2h c 2 ε λ 5 L S +1 )( ε+ λ 5 L S 2h c 2 ) .

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