Passive and Active Control of Radiative Heat Flow

Abstract

Materials that control the absorption and emission of thermal radiation have attracted renewed interest for energy applications. Materials of interest include those with static optical properties that vary with photon wavelength in a desired manner as well as those with dynamic properties that can be actively tuned by external stimuli. The research in this thesis focuses on creating materials in both categories. First, we examine selective absorbers for solar thermal energy conversion with high absorptivity in solar wavelengths and low emissivity in infrared wavelengths. Achieving stagnation temperatures exceeding 200 °C with unconcentrated sunlight, pertinent to technologies like industrial process heat, air conditioning, and electricity generation, requires better spectrally selective absorbers with ultra-low thermal emittance. Current state-of-art surfaces are based on ceramic-metal mixtures and patterned metal or metal-dielectric structures. Semiconductor based selective surfaces with near zero absorption below the bandgap offer the potential for lower thermal emittance than that achieved with such surfaces that employ metals in the primary absorbing medium. In this thesis, we report a semiconductor-based multilayer selective absorber that exploits the sharp drop in optical absorption at the band gap energy to achieve a measured absorptance of 76% at solar wavelengths and a low emittance of approximately 5% at thermal wavelengths. In field tests, we obtain a peak temperature of 225 °C, comparable to that achieved with state-of-the-art selective surfaces. With straightforward optimization to improve solar absorption, our work shows the potential for unconcentrated solar thermal systems to reach stag- nation temperatures exceeding 300 °C, higher than any available selective surface. Our surface would eliminate the need for solar concentrators for mid-temperature solar applications such as supplying process heat. Second, we theoretically propose and experimentally implement a thermal switch for near-field radiative transfer. In the field of active thermal materials for manipulating heat flow in a controllable and reversible manner, numerous approaches to perform thermal switching have been reported. However, they typically suffer from various limitations, including small switching ratio or requiring large temperature differentials. We report the experimental implementation of a scheme to electrostatically control near-field radiative transfer in a graphene field effect heterostructure. We measure a maximum heat flux modulation of 4 ± 3% and an absolute heat flux modulation rate of 24 ± 7 mWm−2 per V bias. Employing gate dielectrics with lower surface warp and higher dielectric breakdown strength as well as reducing conductive losses would enable modulations up to 100%, substantially exceeding the switching ratios achievable by other methods. Our work paves the way for electrostatic control of near-field radiative transfer using two-dimensional materials.