High-temperature tantalum tungsten alloy photonic crystals: Stability, optical properties, and fabrication

Stelmakh, Veronika; Rinnerbauer, Veronika; Geil, Robert D.; Aimone, Paul; Senkevich, Jay J.; Joannopoulos, John D.; Soljačić, Marin; Čelanović, Ivan

doi:10.1063/1.4821586

Cited by 71 publications

(48 citation statements)

References 17 publications

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“…In addition, at high temperatures, the mechanical stability of the selective emitters made from the Ta-W alloy is greatly enhanced in comparison to pure Ta which is beneficial for overall system stability where degradation, such as creep and deflection, can play a critical role in system failure. 19 There are several main driving factors for thermal degradation of microstructures. First, oxidation, and evaporation and redeposition are prevented by using refractory metals in a vacuum environment.…”

Section: Introductionmentioning

confidence: 99%

Performance of tantalum-tungsten alloy selective emitters in thermophotovoltaic systems

et al. 2014

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A tantalum tungsten solid solution alloy, Ta 3% W, based 2D photonic crystal (PhC) was designed and fabricated for high-temperature energy conversion applications. Ta 3% W presents advantages compared to the non-alloys as it combines the better high-temperature thermomechanical properties of W with the more compliant material properties of Ta, allowing for a direct system integration path of the PhC as selective emitter/absorber into a spectrum of energy conversion systems. Indeed metallic PhCs are promising as high performance selective thermal emitters for thermophotovoltaics (TPV), solar thermal, and solar TPV applications due to the ability to tune their spectral properties and achieve highly selective emission. A 2D PhC was designed to have high spectral selectivity matched to the bandgap of a TPV cell using numerical simulations and fabricated using standard semiconductor processes. The emittance of the Ta 3% W PhC was obtained from near-normal reflectance measurements at room temperature before and after annealing at 1200• C for 24h in vacuum with a protective coating of 40 nm HfO 2 , showing high selectivity in agreement with simulations. SEM images of the cross section of the PhC prepared by FIB confirm the structural stability of the PhC after anneal, i.e. the coating effectively prevented structural degradation due to surface diffusion. The mechanical and thermal stability of the substrate was characterized as well as the optical properties of the fabricated PhC. To evaluate the performance of the selective emitters, the spectral selectivity and useful emitted power density are calculated as a function of operating temperature. At 1200• C, the useful emitted irradiance is selectively increased by a factor of 3 using the selective emitter as compared to the non-structured surface. All in all, this paper demonstrates the suitability of 2D PhCs fabricated on polycrystalline Ta-W substrates with an HfO 2 coating for TPV applications.

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Section: Introductionmentioning

confidence: 99%

Performance of tantalum-tungsten alloy selective emitters in thermophotovoltaic systems

et al. 2014

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“…In our previous study a grass formation was present inside the cavity. 20 This effect was since reduced by optimizing the etch parameters, as shown in Fig. 3 where a PhC with r = 0.48 µm and a=1.0 µm was fabricated with the same method as described in Section 2.2.…”

Section: Photonic Crystal Characterizationmentioning

confidence: 99%

Tantalum-tungsten alloy photonic crystals for high-temperature energy conversion systems

et al. 2014

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A tantalum tungsten (Ta-W) solid solution alloy, Ta 3% W, based 2D photonic crystal (PhC) was designed and fabricated for high-temperature energy conversion applications. Metallic PhCs are promising as high performance selective thermal emitters for solid-state thermal-to-electricity energy conversion concepts including thermophotovoltaic (TPV) energy conversion, as well as highly selective solar absorbers/emitters for solar thermal and solar TPV applications due to the ability to tune their spectral properties and achieve highly selective emission. The mechanical and thermal stability of the substrate was characterized as well as the optical properties of the fabricated PhC. The Ta 3% W alloy presents advantages compared to the non-alloys as it combines the better high-temperature thermo-mechanical properties of W with the more compliant material properties of Ta, allowing for a direct system integration path of the PhC as selective emitter/absorber into a spectrum of energy conversion systems. Furthermore, the thermo-mechanical properties can be fine-tuned by the W content. A 2D PhC was designed to have high spectral selectivity matched to the bandgap of a TPV cell using numerical simulations and fabricated using standard semiconductor processes. The emittance of the Ta 3% W PhC was obtained from near-normal reflectance measurements at room temperature before and after annealing at 1200• C for 24h in vacuum with a protective coating of 40nm HfO 2 , showing high selectivity in agreement with simulations. SEM images of the cross section of the PhC prepared by FIB confirm the structural stability of the PhC after anneal, i.e. the coating effectively prevented structural degradation due to surface diffusion.

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“…Moreover, they typically offer near blackbody emission resulting from resonant phenomenon at the desired wavelengths and near zero emission elsewhere resulting from the low loss materials from which they are fabricated. Photonic crystals have been fabricated in different refractory metals: singlecrystal tungsten (W) bulk substrates [4][5][6][7][8], polycrystalline tantalum (Ta) and Ta-W alloy bulk substrates [9,10], and even sputtered Ta coatings [11]. Refractory metals are preferred in hightemperature (∼900-1200 • C) TPV applications due to their high melting point, low vapor pressure, advantageous low emissivity in the infrared, and ability to be etched.…”

Section: Introductionmentioning

confidence: 99%

Improved Thermal Emitters for Thermophotovoltaic Energy Conversion

Stelmakh

Chan

Joannopoulos

et al. 2016

Volume 1: Micro/Nanofluidics and Lab-on-a-Chip; Nanofluids; Micro/Nanoscale Interfacial Transport Phenomena; Micro/Nanoscale Bo

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Thermophotovoltaic (TPV) energy conversion enables millimeter scale power generation required for portable microelectronics, robotics, etc. In a TPV system, a heat source heats a selective emitter to incandescence, the radiation from which is incident on a low bandgap TPV cell. The selective emitter tailors the photonic density of states to produce spectrally confined selective emission of light matching the bandgap of the photovoltaic cell, enabling high heat-to-electricity conversion efficiency. The selective emitter requires: thermal stability at high-temperatures for long operational lifetimes, simple and relatively low-cost fabrication, as well as spectrally selective emission over a large uniform area. Generally, the selective emission can either originate from the natural material properties, such as in ytterbia or erbia emitters, or can be engineered through microstructuring. Our approach, the 2D photonic crystal fabricated in refractory metals, offers high spectral selectivity and high-temperature stability while being fabricated by standard semiconductor processes. In this work, we present a brief comparison of TPV system efficiencies using these different emitter technologies. We then focus on the design, fabrication, and characterization of our current 2D photonic crystal, which is a square lattice of cylindrical holes fabricated in a refractory metal substrate. The spectral performance and thermal stability of the fabricated photonic crystal thermal emitters are demonstrated and the efficiency gain of our model TPV system is characterized.

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High-temperature tantalum tungsten alloy photonic crystals: Stability, optical properties, and fabrication

Cited by 71 publications

References 17 publications

Performance of tantalum-tungsten alloy selective emitters in thermophotovoltaic systems

Performance of tantalum-tungsten alloy selective emitters in thermophotovoltaic systems

Tantalum-tungsten alloy photonic crystals for high-temperature energy conversion systems

Improved Thermal Emitters for Thermophotovoltaic Energy Conversion

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