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L1.08 - Refractory Metals for High Temperature Metamaterial Thermal Emitters 
December 1, 2014   11:15am - 11:30am

Thermophotovoltaic (TPV) devices can enhance the efficiency of existing energy generation infrastructure by reclaiming heat lost during production processes. In order to maximize the efficiency of these devices, the conversion efficiency of the TPV system needs to be optimized. Most TPV systems consist of three discrete stages: a thermal emitter, a filter, and a TPV diode. One avenue to increase TPV efficiency is to replace the wide-band emitters presently in use with a selective thermal emitter. The selective emitter would absorb wide-band radiation from a heat source and convert it into narrow bandwidth peaks of radiation tailored to the rest of the system. This would dramatically reduce energy loss due to reflection, diode heating, and carrier relaxation. In recent years, metamaterials (MM) have emerged as the ideal medium for the development of engineerable, narrowband selective emitters for such applications.

Most research into metamaterials has utilized gold as the conducting metal. With a bulk melting point of 1064 °C, nano-sized gold patterns will completely degrade at, if not before, the operating temperature of most TPV cells. The most common TPV cells in use today are bulk GaSb diodes. The band gap of this material is 0.7eV, which corresponds to a light wavelength of 1.7μm. According to Wien's Law, to achieve a blackbody radiation curve with this wavelength at the peak intensity the radiating body would have to be at a temperature of 1400 °C, well above the melting point of gold. Exploring alternate materials and their viability for use in emitters will lead to great advances in current achievable efficiencies.

To operate at the high temperatures required to optimize the efficiency of such photodiodes, traditional MM conductors, such as gold and copper, need to be replaced with more thermally robust metals. Our research consists of a thorough study of potential materials and looks at the design and response of metamaterial selective emitters with emission peaks at 1, 2, 3, 4, 6, 8, and 10 microns, respectively. Designs were made using gold, to compare with current literature, as well as platinum, tungsten, and irridium. Testing will consist of absorption measurements at room temperature and emission testing at increasingly higher temperatures until breakdown is achieved.

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