Despite its strong potentials in emerging energy applications, near-field thermal radiation between large planar structures has not been fully explored in experiments. Particularly, it is extremely challenging to control a subwavelength gap distance with good parallelism under large thermal gradients. This article reports the precision measurement of near-field radiative energy transfer between two macroscale single-crystalline quartz plates that support surface phonon polaritons. Our measurement scheme allows the precise control of a gap distance down to 200 nm in a highly reproducible manner for a surface area of 5×5  mm^{2}. We have measured near-field thermal radiation as a function of the gap distance for a broad range of thermal gradients up to ∼156  K, observing more than 40 times enhancement of thermal radiation compared to the blackbody limit. By comparing with theoretical prediction based on fluctuational electrodynamics, we demonstrate that such remarkable enhancement is owing to phonon-polaritonic energy transfer across a nanoscale vacuum gap.

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This article proposes a new energy harvesting concept that greatly enhances thermionic power generation with high efficiency by exploiting the nearfield enhancement of thermal radiation. The proposed near-field enhanced thermionic energy conversion (NETEC) system is uniquely configured with a low-bandgap semiconductor cathode separated from a thermal emitter with a subwavelength gap distance, such that a significant amount of electrons can be photoexcited by near-field thermal radiation to contribute to the enhancement of thermionic current density. We theoretically demonstrate that the NETEC system can generate electric power at a significantly lower temperature than the standard thermionic generator, and the energy conversion efficiency can exceed 40%. The obtained results reveal that near-field photoexcitation can enhance the thermionic power output by more than 10 times, making this hybrid system attractive for renewable energy recycling.

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In this article, we present a cost-effective approach to the precision measurement of heat flux using commercial thermoelectric modules (TEMs). Two different methods of measuring heat flux with TEMs are investigated, namely, passive mode based on the Seebeck effect and active mode based on the Peltier effect. For both modes, a TEM as a heat flux meter is calibrated to show a linear relation between the voltage across the TEM and the heat flux from 0 to ∼450 W m−2. While both modes exhibit sufficiently high sensitivities suitable for low heat flux measurement, active mode is shown to be ∼7 times more sensitive than passive mode. From the speculation on the origin of the measurement uncertainty, we propose a dual TEM scheme by operating the top TEM in passive mode while its bottom temperature maintains constant by the feedback-controlled bottom TEM. The dual TEM scheme can suppress the sensitivity uncertainty up to 3 times when compared to the single-TEM passive mode by stabilizing the bottom temperature. The response time of a 15 × 15 mm2 TEM is measured to be 8.9 ± 1.0 s for heating and 10.8 ± 0.7 s for cooling, which is slower than commercial heat flux meters but still fast enough to measure heat flux with a time resolution on the order of 10 s. We believe that the obtained results can facilitate the use of a commercial TEM for heat flux measurement in various thermal experiments.