ABSTRACT Spectral emissivity of surface materials has a strong impact on thermal properties of systems that are exposed in the ambient environment. While the solar spectrum heating up the surface ranges from 200 to 2,500 nm, the atmospheric transmission spectrum allowed for infrared cooling ranges from 8 to 14 µm. However, conventional surface materials have emissivity values that are either high or low throughout the spectrum. For example, ceramic materials are typically emissive and metallic materials are typically reflective and not emissive. Here, we show that surface materials with artificial periodicities can have a selectively controlled emissivity and that the surface morphology can transform ceramic materials to be reflective or metallic materials to be emissive. As a model system, we use microscale tree-like structures, or briefly micro-trees, to demonstrate wide variations of morphology-driven emissivity spectra. Our computation based on the rigorous coupled-wave analysis shows that optimal designs of micro-trees can act as a nearly perfect reflector or a black body depending on the spectral range and offer radiative cooling or heating capabilities beyond the limits of conventional materials. For cooling, metallic micro-trees provide a surface temperature 10 K lower than that of bare metallic surfaces in a normal ambient condition, and for heating, ceramic micro-trees provide a surface temperature 8 K higher than that of bare ceramic materials. The morphology-driven emissivity of micro-trees can offer a net cooling power of 136 W/m2 or a net heating power of 12 W/m2 depending on the application without requiring any active devices, and these results guide optimal designs of artificial materials for thermal management.

中文翻译：

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用于辐射热管理的微尺度树状结构的形态驱动发射率

摘要 表面材料的光谱发射率对暴露在周围环境中的系统的热性能有很大影响。虽然加热表面的太阳光谱范围为 200 至 2,500 nm，但允许红外线冷却的大气透射光谱范围为 8 至 14 µm。然而，传统的表面材料在整个光谱中具有或高或低的发射率值。例如，陶瓷材料通常是发射性的，而金属材料通常是反射性的而不是发射性的。在这里，我们表明具有人工周期性的表面材料可以具有选择性控制的发射率，并且表面形态可以将陶瓷材料转变为反射性或将金属材料转变为发射性。作为模型系统，我们使用微型树状结构，或简单的微树，以展示形态驱动的发射率光谱的广泛变化。我们基于严格耦合波分析的计算表明，微树的优化设计可以根据光谱范围充当近乎完美的反射器或黑体，并提供超出传统材料限制的辐射冷却或加热能力。对于冷却，金属微树提供的表面温度比正常环境条件下裸金属表面低 10 K，而对于加热，陶瓷微树提供的表面温度比裸陶瓷材料高 8 K。微树的形态驱动发射率可以提供 136 W/m2 的净冷却功​​率或 12 W/m2 的净加热功率，具体取决于应用程序，而无需任何有源设备，