An integrated microcombustor and photonic crystal emitter for thermophotovoltaics

Thermophotovoltaic (TPV) energy conversion is appealing for portable millimeterscale generators because of its simplicity, but it relies on a high temperatures. The performance and reliability of the high-temperature components, a microcombustor and a photonic crystal emitter, has proven challenging because they are subjected to 1000–1200◦C and stresses arising from thermal expansion mismatches. In this paper, we adopt the industrial process of diffusion brazing to fabricate an integrated microcombustor and photonic crystal by bonding stacked metal layers. Diffusion brazing is simpler and faster than previous approaches of silicon MEMS and welded metal, and the end result is more robust. Power demand for mobile electronics and robotics continues to rise but current Li-ion battery chemistry is approaching its thermodynamic limit [1]. Thermophotovoltaic (TPV) energy conversion is promising as a high specific energy, millimeter-scale portable power source capable of harnessing the energy content of hydrocarbon fuels (12.6 kWh/kg compared to 180 Wh/kg for batteries) without moving parts or working fluids. In TPV, a microcombustor heats a photonic crystal emitter to incandescence and the resulting spectrally confined thermal radiation drives low-bandgap photovoltaic (PV) cells to generate electricity, as shown in Fig. 1(a). The microcombustor reacts propane and oxygen in a catalyst-loaded serpentine channel to heat the photonic crystal to 1000–1200◦C. The microstructured tantalum 2D photonic crystal is engineered to only emit thermal radiation that is convertible by the PV cell, leading to high efficiency [2–4]. In this paper, we report a simple and stable method for fabricating an integrated microcombustor and photonic crystal (hot side assembly) by diffusion brazing. Designing a stable TPV hot side has been challenging because of the high temperatures and the thermo-mechanical stresses arising from thermal expansion mismatch between the microcombustor and photonic crystal. Microcombustors (for TPV and other applications) have been fabricated from silicon by MEMS techniques [5–7], from laminated metal layers by diffusion bonding [8], and from welded metal components [9, 10]. These methods are difficult PowerMEMS 2016 IOP Publishing Journal of Physics: Conference Series 773 (2016) 012108 doi:10.1088/1742-6596/773/1/012108 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 Microcombustor Photonic crystal PV cell Tantalum photonic crystal