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Optics Express, Vol. 21, Issue 9, pp. 11482-11491 (2013).
Veronika Rinnerbauer, Yi Xiang Yeng, Walker R. Chan, Jay J. Senkevich, John D. Joannopoulos, Marin Soljačić, and Ivan Celanovic.
Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, MA, Cambridge 02139, USA and
Institute of Soldier Nanotechnologies, Massachusetts Institute of Technology, 77 Massachusetts Avenue, MA, Cambridge 02139, USA.
Abstract
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 um, 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.
© 2013 OSA
Additional information:
An increased interest in thermophotovoltaic (TPV) energy conversion systems has led to new investigations of photonic crystals (PhCs) made from refractory metals as highly efficient selective emitters at high temperature. In this class of solid state thermal-to-electrical energy conversion, a suitable high temperature radiation source is driving a small bandgap PV cell, whereby the radiation of the emitter is ideally matched to the bandgap of the PV cell. Metallic photonic crystals offer the ability to tailor the photonic density of states, which allows for the efficient design of such highly efficient emitters.
These metallic 2D PhCs can be designed to exhibit precisely tailored optical properties. In addition to high spectral selectivity, the thermal stability of the nanophotonic structures and their optical properties is critical at the target high operating temperatures (> 900°C) and long expected operational lifetimes (years).
Substrates from refractory metals are advantageous as a starting point due to their high melting point, low vapor pressure, and high IR reflectivity (reducing losses due to waste heat). In this work, we discuss both the optical properties of a selective emitter based on a 2D PhC fabricated from a polycrystalline Ta substrate, as well as their stability at temperatures of up to 1000°C and the structural stability of the nanophotonic device.
At these operating temperatures, there is a high risk of structural degradation of microstructured surfaces due to surface diffusion, surface reactions in particular with oxygen and carbon, and material stress, as well as grain growth and recrystallization in polycrystalline materials. We propose a thin (20nm) conformal coating of HfO2 as a surface protective coating and demonstrate the stability of the PhC and its optical properties with this coating.
New studies and scanning electron microscope images of the cross-section of the fabricated PhCs prepared by focused ion beam confirm that the coating effectively prevents structural degradation of the PhC at temperatures up to 1200°C. In addition we investigate the use of tantalum tungsten solid solution alloys as substrates (V. Stelmakh et al, Appl. Phys. Lett. 103, 123903, 2013). The Ta-W alloy presents critical advantages compared to non-alloys as the thermo-mechanical properties such as yield strength and Young’s modulus can be tuned to achieve the desired high temperature stability. At the same time, the combination of better thermo-mechanical properties of W with the more compliant material properties of Ta facilitate system integration (e.g. by welding) and can prevent system failure from creep and deflection, which can be detrimental at the high operating temperatures over the expected long operational lifetime.
