Optical Tunneling Mediated Sub-Skin-Depth High Emissivity Tungsten Radiators

Abstract

Tailoring the spectrum of thermal radiation at high temperatures is a central issue in the study of thermal radiation harnessed energy resources. Although bulk metals with periodic cavities incorporated into their surfaces provide high emissivity, they require a complicated micron metal etch, thereby precluding reliable, continuous operation. Here, we report thermally stable, highly emissive, ultrathin (<20 nm) tungsten (W) radiators that were prepared in a scalable and cost-effective route. Alumina/W/alumina multiwalled, submicron cavity arrays were fabricated sequentially using nanoimprinting lithography, thin film deposition, and calcination processes. To highlight the practical importance of high-temperature radiators, we developed a thermophotovoltaic (TPV) system equipped with fabricated W radiators and low-bandgap GaSb photovoltaic cells. The TPV system produced electric power reliably during repeated temperature cycling between 500 and 1200 K; the power density at 1200 K was fixed to be approximately 1.0 W/cm 2 . The temperature-dependent electric power was quantitatively reproduced using a one-dimensional energy conversion model. The symmetric configuration of alumina/W/alumina multiwall together with the presence of a void inside each cavity alleviated thermal stress, which was responsible for the stable TPV performance. The short-current-density (J SC ) of developed TPV system was augmented significantly by decreasing the W thickness below its skin depth. A 17 nm thick W radiator yielded a 32% enhancement in J SC compared to a 123 nm thick W radiator. Electromagnetic analysis indicated that subskin-depth W cavity arrays led to suppressed surface reflection due to the mitigated screening effect of free electrons, thereby enhancing the absorption of light within each W wall. Such optical tunneling-mediated absorption or radiation was valid for any metal material and morphology (e.g., planar or patterned).

Thermal stability of tungsten based metamaterial emitter under medium vacuum and inert gas conditions

“…Refractory oxides, such as Al 2 O 3 , HfO 2 , and ZrO 2 show dielectric properties and are stable in oxidizing atmospheres. Thermal stability of the Al 2 O 3 based emitters demonstrated so far lies below 1300 °C 28,[39][40][41] . ZrO 2 suffers from phase change transition (from monoclinic to tetragonal at around 1000 °C) at high temperatures 42,43 .…”

High-Temperature Carbonized Ceria Thermophotovoltaic Emitter beyond Tungsten

“…Engineering absorptivity/emissivity spectra at multiscale wavelengths enable thermal emission-harnessed energy devices including solar steamers, , radiative coolers, − high-efficiency incandescent lamps, and thermophotovoltaics (TPVs). − Each device is tailored to a different spectral range that relies on its working temperature and function. For example, radiative coolers working at room temperature, which present the possibility for the development of passive, heat dissipation technologies, are emissive at mid-infrared wavelengths (i.e., 5–30 μm, corresponding to the blackbody spectrum at room temperature).…”

“…In addition, if radiative coolers are designed to cool outdoor objects such as buildings, automobiles, and antennas, they must be reflective in the solar spectrum (0.3–2.5 μm). − In comparison, solar steamers working at approximately 100 °C maintain unity absorptivity in the solar spectrum but must suppress their emissivity within the blackbody spectrum at such temperatures to minimize the radiative cooling effect. , TPVs are a class of electricity-generating systems without moving parts, thus enabling off-grid, portable, and ultralight power generators for drones, recreational vehicles, and spacecraft. Because most TPVs operate at high temperatures (>800 °C), spectral management of the emissions is necessary within the visible and near-infrared regions (typically 0.5–1.7 μm); the upper and lower cutoff wavelengths are determined by the band gap of the photovoltaic cells and the temperature of the emitters, respectively. − An ideal TPV emitter exhibits a stepwise blackbody spectrum that is matched with the effective wavelength range. Therefore, the development of spectrally engineered, thermally stable emitters furthers the economic viability of TPVs.…”

“…Figure a shows a schematic of a TPV system that converts photon energy released from a high-temperature emitter into electric energy through an array of low-band-gap photovoltaic cells. A variety of heat sources, including concentrated solar energy, combustion engines, and radioisotope reactors, have been attempted to attain emitter temperatures of >800 °C. − Tungsten is the most common refractory metal with a melting temperature exceeding 1200 °C, which makes it suitable for use in a TPV emitter. − ,,, In comparison to other metals, tungsten has relatively small extinction coefficients within the visible and near-infrared regions, which leads to augmented in-band emission with reduced out-of-band emissions (Figure b). However, tungsten is susceptible to oxidation at temperatures >1000 °C, which results in temporal changes in the electric power of TPVs, precluding a long-term operation. , Carbon-based materials such as graphite and SiC exhibit near-unity in-band emissions, but their poor wavelength selectivity degrades the performance of integrated photovoltaic cells and decreases the temperature of the emitters .…”

“…(b) Schematics illustrating the spectral characteristics and thermal stability of tungsten, graphite, and carbonized ceria emitters. (Upper) EQE spectrum of the GaSb cell, adapted from ref and the 1000 °C blackbody spectrum. The cutoff wavelength of 1.7 μm divides the in-band and out-of-band emission regions.…”