Microfabricated Thermally Isolated Low Work-Function Emitter

Lee, Jae Hyung; Bargatin, Igor; Vancil, Bernard; Gwinn, Thomas O.; Maboudian, Roya; Melosh, Nicholas A.; Howe, Roger T.

doi:10.1109/jmems.2014.2307882

Cited by 37 publications

(24 citation statements)

References 20 publications

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“…Modern microfabrication techniques already enabled the development of thermally and electrically insulated spacers that withstand large temperature gradients. These techniques eventually enabled the experimental demonstration of micron-gap TIC 22,23 . Sub-micron separation distances were also experimentally realized using nano-spacers in the frame of near-field thermal radiation experiments [24][25][26][27] .…”

Section: Device Conceptmentioning

confidence: 99%

Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Datas

Vaillon

2019

Applied Physics Letters

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Conversion of medium-grade heat (temperature from 500 to 1000 K) into electricity is important in applications such as waste heat recovery, or power generation in solar thermal and co-generation systems. At such temperatures, current solid-state devices lack of either high conversion efficiency (thermoelectrics) or high-power density capacity (thermophotovoltaics and thermionics). Near-field thermophotovoltaics (nTPV) theoretically enables high power

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Section: Device Conceptmentioning

confidence: 99%

Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Datas

Vaillon

2019

Applied Physics Letters

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“…The optimal gap depends on the emitter temperature and, in the present case with T E < 1100 °C, should be <5 µm . Microtechnology applied to dielectric materials (electrically and thermally insulating) can open the route to these reduced values of interelectrode gap …”

Section: Physical Properties Of Active Materialsmentioning

confidence: 99%

Solar Thermionic‐Thermoelectric Generator (ST²G): Concept, Materials Engineering, and Prototype Demonstration

Trucchi

Bellucci

Calvani

et al. 2018

Advanced Energy Materials

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Concentrating solar technologies represent economically viable alternatives to distributed PV since the optics production cost per unit surface is far lower than that of PV cells. The reduction of the active conversion area allows for expensive multijunction cells with efficiency as high as 45% to be used in concentrating PV (CPV). [1] However, CPV is struggling commercially because high-density arrangements are prevented by issues in excess heat removal. On the other hand, concentrating solar power (CSP) technologies, [2,3] based on transfer of high-temperature thermal energy through a fluid toward thermomechanical engines, are promising for large plants characterized by thermal-to-electric efficiency around 35%, with the important capabilities of energy cogeneration and storage.The solar thermionic-thermoelectric generator (ST 2 G) explored here takes the advantages of CSP and CPV to provide an effective alternative concept for future solar technologies by combining the hightemperature operations, typical of CSP, to the compactness, scalability, reliability, and long operating lifetime of the solid-state converters, typically used in CPV systems. By combining a thermionic energy converter (TEC), operating at high but practical temperatures (<1100 °C) that can be achieved with point-focus solar concentrators and the development of low-work-function materials, with a thermoelectric energy generator (TEG) thermally connected in series, the ST 2 G technology can meet requirements of economic cost-effectiveness guaranteed by high-temperature solid-state converters. [4] The thermionic-thermoelectric solid-state technology, characterized by solar-to-electric conversion efficiency feasibly >40%, is comprehensively proposed and discussed for conversion of concentrating solar power. For the first time, the related solar generator prototype is designed and fabricated by developing advanced materials functionalized for the specific application, such as thermally resistant hafnium carbide-based radiation absorbers, surface-textured at the nanoscale to obtain a solar absorptance >90%, and chemical vapor deposition diamond films, acting as lowwork-function (2.06 eV) thermionic emitters. Commercial thermoelectric generators and encapsulation vacuum components complete the prototype. The conversion efficiency is here evaluated under outdoor concentrated sunlight, demonstrating thermionic stage output power of 130 mW at 756 °C, combined to the maximum thermoelectric output power of 290 mW. The related solar-to-electric conversion efficiency is found to be 0.4%, but, once the net thermal flux fed to the conversion stages is considered, a thermal-toelectric efficiency of 6% is revealed. Factors affecting the performance of the present prototype are analyzed and discussed, as well as a strategy to rapidly overcome limitations, in order to prepare an efficient and highly competitive solid-state conversion alternative for future concentrating solar plants.

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“…Therefore, to sustain inter-electrode insulation and micron-scale spacing, researchers previously used sparse and small-area microstructures in thermionic and thermophotovoltaic devices. Some small-scale area devices (< 1 mm 2 ) were only designed for radiative heating and therefore could not sustain significant compressive force 3,28,29 . For some centimeter-scale devices, spacers were designed around the perimeter of the electrodes 30–32 , but such device architectures were prone to shorting due to bowing or surface roughness at the unsupported center of the electrode.…”

Section: Introductionmentioning

confidence: 99%

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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Microfabricated Thermally Isolated Low Work-Function Emitter

Cited by 37 publications

References 20 publications

Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Solar Thermionic‐Thermoelectric Generator (ST²G): Concept, Materials Engineering, and Prototype Demonstration

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

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Microfabricated Thermally Isolated Low Work-Function Emitter

Cited by 37 publications

References 20 publications

Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Solar Thermionic‐Thermoelectric Generator (ST2G): Concept, Materials Engineering, and Prototype Demonstration

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

Contact Info

Product

Resources

About

Solar Thermionic‐Thermoelectric Generator (ST²G): Concept, Materials Engineering, and Prototype Demonstration