Photonic structures in radiative cooling

Abstract Radiative cooling is a passive cooling technology without any energy consumption, compared to conventional cooling technologies that require power sources and dump waste heat into the surroundings. For decades, many radiative cooling studies have been introduced but its applications are mostly restricted to nighttime use only. Recently, the emergence of photonic technologies to achieves daytime radiative cooling overcome the performance limitations. For example, broadband and selective emissions in mid-IR and high reflectance in the solar spectral range have already been demonstrated. This review article discusses the fundamentals of thermodynamic heat transfer that motivates radiative cooling. Several photonic structures such as multilayer, periodical, random; derived from nature, and associated design procedures were thoroughly discussed. Photonic integration with new functionality significantly enhances the efficiency of radiative cooling technologies such as colored, transparent, and switchable radiative cooling applications has been developed. The commercial applications such as reducing cooling loads in vehicles, increasing the power generation of solar cells, generating electricity, saving water, and personal thermal regulation are also summarized. Lastly, perspectives on radiative cooling and emerging issues with potential solution strategies are discussed

Advances in protective layer-coating on metal nanowires with enhanced stability and their applications

© 2020 Elsevier LtdThe demand for flexibility in transparent conductive electrodes(TCEs) is increasing due to the functionality demand and convenience of deformable electrodes. The high-cost and brittleness of the conventional indium tim oxide (ITO) films are obstacles to their utilization in TCEs. Metal nanowires are highly promising as substitutes for ITO. Metal nanowire electrodes achieved high-conductivity, high-transparency, and high-stretchability through the development of fabrication technology. Nevertheless, it is challenging to prevent their degradation by the external environment, intensive investigations that can improve the stability are required. Among measures to improve stability, a protective-layer coating on metal nanowires is shown to be a very effective way. Therefore, it is essential to organize the development of protective layer-coating. This review compiles coating methods of stable materials (overall coverage coating, core-shell coating, junction enhancement coating), and the obtained TCE performances. Electrical properties such as sheet resistance, transmittance conjunction with the enhanced stability are discussed. Finally, the recent progress in TCE applications based on improved stability metal nanowires are discussed concisely. Various applications of improved stability nanowires such as solar-cell, supercapacitor, and transparent heaters are considered and their performances as well as fabrication protocols are disclosed

Enhanced Thermoelectric Conversion Efficiency of CVD Graphene with Reduced Grain Sizes

The grain size of CVD (Chemical Vapor Deposition) graphene was controlled by changing the precursor gas flow rates, operation temperature, and chamber pressure. Graphene of average grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm was synthesized in high quality and full coverage. The possibility to tailor the thermoelectric conversion characteristics of graphene has been exhibited by examining the grain size effect on the three elementary thermal and electrical properties of σ, S, and k. Electrical conductivity (σ) and Seebeck coefficients (S) were measured in a vacuum for supported graphene on SiO2/Si FET (Field Effect Transistor) substrates so that the charge carrier density could be changed by applying a gate voltage (VG). Mobility (µ) values of 529, 459, and 314 cm2/V·s for holes and 1042, 745, and 490 cm2/V·s for electrons for the three grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm, respectively, were obtained from the slopes of the measured σ vs. VG graphs. The power factor (PF), the electrical portion of the thermoelectric figure of merit (ZT), decreased by about one half as the grain size was decreased, while the thermal conductivity (k) decreased by one quarter for the same grain decrease. Finally, the resulting ZT increased more than two times when the grain size was reduced from 4.1 µm to 0.5 µm