Encapsulated Thermionic Energy Converter With Stiffened Suspension

Lee, J.H.; Bargatin, Igor; Iwami, Kentaro; Littau, Karl A.; Vincent, Maxime; Maboudian, Roya; Shen, Zhi‐Xun; Melosh, Nicholas A.; Howe, Roger T.

doi:10.31438/trf.hh2012.130

Cited by 3 publications

(1 citation statement)

References 7 publications

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“…Our previous investigations demonstrated that Ba or BaO coating reduced the work function of the poly-SiC emitter to ∼2.1eV and increases the thermionic current by 5-6 orders of magnitude [25]. Our measurements for both the uncoated and the barium-coated poly-SiC emitter fit the Richardson-Dushman equation, when the uncoated poly-SiC emitter temperatures ranged from 2000-3000K and the barium-coated poly-SiC emitter temperatures ranged from 900-1400K:…”

Section: B Thermionic Emission Current Measurementssupporting

confidence: 67%

Microfabricated Thermally Isolated Low Work-Function Emitter

Lee

Bargatin

Vancil

et al. 2014

J. Microelectromech. Syst.

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In this paper, we report low work function mechanically and thermally robust microfabricated thermionic emitters. Conformal deposition of polycrystalline-silicon carbide (poly-SiC) was used to form stiff suspension legs with U-shaped cross sections, which increased the out-of-plane rigidity and helped to maintain a micrometer-scale gap between emitter and collector. The structurally robust poly-SiC suspended structure was coated with a thin tungsten layer to improve adhesion of a work function lowering coating (BaO/SrO/CaO). The measured emitter work function was 1.2 eV and emission current density was >0.1 A/cm 2 , which are promising for applications such as thermionic energy converters.[ 2013-0238]Index Terms-Thermionic emitter, thermionic cathode, energy conversion efficiency, barium coating, barium oxide coating, work function lowering, silicon carbide.

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Section: B Thermionic Emission Current Measurementssupporting

confidence: 67%

Microfabricated Thermally Isolated Low Work-Function Emitter

Lee

Bargatin

Vancil

et al. 2014

J. Microelectromech. Syst.

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Micron-gap spacers with ultrahigh thermal resistance and mechanical robustness for direct energy conversion

et al. 2019

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In thermionic energy converters, the absolute efficiency can be increased up to 40% if space-charge losses are eliminated by using a sub-10-µm gap between the electrodes. One practical way to achieve such small gaps over large device areas is to use a stiff and thermally insulating spacer between the two electrodes. We report on the design, fabrication and characterization of thin-film alumina-based spacers that provided robust 3–8 μm gaps between planar substrates and had effective thermal conductivities less than those of aerogels. The spacers were fabricated on silicon molds and, after release, could be manually transferred onto any substrate. In large-scale compression testing, they sustained compressive stresses of 0.4–4 MPa without fracture. Experimentally, the thermal conductance was 10–30 mWcm−2K−1 and, surprisingly, independent of film thickness (100–800 nm) and spacer height. To explain this independence, we developed a model that includes the pressure-dependent conductance of locally distributed asperities and sparse contact points throughout the spacer structure, indicating that only 0.1–0.5% of the spacer-electrode interface was conducting heat. Our spacers show remarkable functionality over multiple length scales, providing insulating micrometer gaps over centimeter areas using nanoscale films. These innovations can be applied to other technologies requiring high thermal resistance in small spaces, such as thermophotovoltaic converters, insulation for spacecraft and cryogenic devices.

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