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As a new type of cooling technology, radiative cooling achieves temperature reduction through spectral regulation. Compared with the traditional active cooling technologies such as absorption-based cooling and compression-based cooling, the radiative cooling offers unique advantages, which are of great significance in environmental protection and energy utilization. First of all, the basic principle of radiative cooling and the radiative cooling within the natural biological systems are introduced in this review. The biological systems achieve their regulations of radiative cooling through controlling the materials, microstructures and behaviors in the systems, which also provide inspiration for us to explore new radiative cooling materials and devices. We also summarize the various mechanisms of radiative cooling in the biological systems and the optimization of such cooling structures. The recent research progress of bio-inspired radiative cooling is also presented. At the end, the outlook of the research directions, potential applications, and the material fabrication approaches for bio-inspired radiative cooling are discussed. The radiative cooling materials and devices with high power output and intelligent control should be an important development direction of bio-inspired radiative cooling in the future. With the integration of advanced micro/nano fabrication technology, bio-inspired radiative cooling will have a broader market and much room of applications in the future.
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Keywords:
- radiative cooling /
- bio-inspired /
- micro/nano fabrication
1. 引 言
随着温室效应和全球变暖问题逐渐加剧, 制冷技术在各个领域的应用愈发重要. 传统制冷技术伴随着大量的能源消耗和温室气体(CO2, CH4, O3等)的排放, 而辐射制冷作为新型冷却技术, 因具有环境友好、无外界能量消耗、静谧性良好以及降温舒适等优点逐渐受到人们的关注. 近年来, 微纳尺度材料的研究与制备实现了对可见光和红外光的同时调控, 日间辐射制冷技术得以实现, 这引发了辐射制冷的研究热潮, 各种用于辐射制冷的材料和体系, 如光子晶体[1-3]、超材料[4]、高发射聚合物薄膜[5-9]或纤维材料[10]、金属电解质系统[11,12]等, 层出不穷, 在某些特殊情况下, 如高温, 基于有效介质理论设计的多层膜结构可以有效实现辐射制冷的目的[13]. 另一方面, 自然界中许多生物经过数亿年的生存斗争和进化, 为适应严苛的环境, 演化出特殊的散热结构、组织及器官. 人类可以从生物的散热结构和机制中获得灵感, 制备出具有高效辐射制冷性能的材料和体系以缓解能源和环境问题.
2. 辐射制冷的基本原理
辐射制冷的基本原理是将地球的热量以热辐射的形式释放到宇宙, 形成地球的能量损耗[14] (如图1(a)所示). 然而, 地面发射的电磁波在穿越大气层时, 会被大气中的混合气体吸收和散射, 使得一部分电磁波能量被吸收或重新返回到地面上. 当天气晴朗时, 存在一个大气透明窗口[15], 如图1(b)所示, 8—13 μm的电磁波在穿透大气时只有少量被吸收, 大部分以红外光的形式到达宇宙, 实现了地球能量的耗散, 达到降温的目的[22,23]. 由于太阳辐射中95%以上的能量集中在可见光及近红外波段, 故通过降低物体在可见光及近红外波段的吸收率(使用高反射材料), 使太阳光热增益最小化, 强化日间辐射制冷效应, 同时提高物体在8—13 μm波段的发射率(采用本征红外发射率高的材料), 使自身辐射热损耗最大化, 可得到最大的净辐射热损失, 有效地实现辐射制冷.
![图 1 辐射制冷原理及自然界中对仿生辐射制冷具有启发性的生物和材料 (a) 发生在地球表面的辐射热流示意图[14]. Psolar, 地球表面对太阳辐射的吸收; Prad, 地球表面发射的热辐射; Patm, 地球表面对大气热辐射的吸收. (b) AM 1.5太阳光谱和典型的大气透明窗口[15]. (c)−(h) 典型的具有辐射制冷特性的生物体和生物材料, 依次为具有不同近红外发射特性的鸟类[16] (c), 彗星蛾蚕茧[17] (d), 滴燕灰蝶及其红外图像[18] (e), 撒哈拉沙漠银蚁[19] (f), 人类皮肤褶皱及其结构示意图[20] (g), 木材纤维[21] (h)\r\nFig. 1. Principles of radiative cooling and the nature organism that are inspirational for bio-inspired radiative cooling: (a) Schematic of radiative energy flows occurring at the terrestrial surfaces[14]. Psolar, solar radiation absorbed by the terrestrial surfaces; Prad, thermal radiation emitted by the terrestrial surfaces; Patm, atmospheric thermal radiation absorbed by the terrestrial surfaces. (b) AM 1.5 solar spectrum and typical atmospheric window[15]. (c)−(h) Typical organisms and biomaterials with radiative cooling properties: (c) Birds with different near-infrared (NIR) emissivity[16]; (d) comet moth cocoon[17]; (e) butterfly Rapala dioetas and its infrared image[18]; (f) Saharan silver ant[19]; (g) human skin and the structural schematic[20]; (h) wood fiber[21].](/fileWLXB/journal/article/wlxb/2022/2/PIC/2-20211854-1.png)
图 1 辐射制冷原理及自然界中对仿生辐射制冷具有启发性的生物和材料 (a) 发生在地球表面的辐射热流示意图[14]. Psolar, 地球表面对太阳辐射的吸收; Prad, 地球表面发射的热辐射; Patm, 地球表面对大气热辐射的吸收. (b) AM 1.5太阳光谱和典型的大气透明窗口[15]. (c)−(h) 典型的具有辐射制冷特性的生物体和生物材料, 依次为具有不同近红外发射特性的鸟类[16] (c), 彗星蛾蚕茧[17] (d), 滴燕灰蝶及其红外图像[18] (e), 撒哈拉沙漠银蚁[19] (f), 人类皮肤褶皱及其结构示意图[20] (g), 木材纤维[21] (h)Fig. 1. Principles of radiative cooling and the nature organism that are inspirational for bio-inspired radiative cooling: (a) Schematic of radiative energy flows occurring at the terrestrial surfaces[14]. Psolar, solar radiation absorbed by the terrestrial surfaces; Prad, thermal radiation emitted by the terrestrial surfaces; Patm, atmospheric thermal radiation absorbed by the terrestrial surfaces. (b) AM 1.5 solar spectrum and typical atmospheric window[15]. (c)−(h) Typical organisms and biomaterials with radiative cooling properties: (c) Birds with different near-infrared (NIR) emissivity[16]; (d) comet moth cocoon[17]; (e) butterfly Rapala dioetas and its infrared image[18]; (f) Saharan silver ant[19]; (g) human skin and the structural schematic[20]; (h) wood fiber[21].在自然界中, 许多生物具有精密的光热调节能力[16-21], 如图1(c)—(h)所示. 具有可调反射、可调吸收和动态红外辐射等特性, 不仅对生物的生存和进化具有重要意义, 也为人类探究仿生辐射制冷带来了灵感. 仿生辐射制冷的研究主要分为两个方面: 一方面对生物体本身的辐射制冷性能和机制进行研究; 另一方面以生物体的辐射制冷机制为启发, 人工设计制备具有优良辐射制冷性能的新材料及新体系.
3. 自然界中的辐射制冷
在自然界中, 天然高红外发射材料很难实现日间辐射制冷, 因为它们在可见光波段也具备高的吸收率, 所以生物往往通过进化出特殊微结构, 与材料共同作用来实现自身温度的调控. 昆虫是一种变温生物, 在热带地区, 热调节机制对昆虫的生存尤为重要. 2019年, Xie等[24]研究了热带雨林的白甲虫(图2(a)) Goliathus goliatus, 其鳞片截面是一种壳/空心圆筒结构, 该结构具备两种热调节效应: 在可见光波段, 通过薄膜干涉、Mie共振的协同效应, 能够显著地增强可见光的全反射, 而在中红外波段, 这种结构又起到抗反射作用, 从而提高中红外波段的吸收率和发射率, 使甲虫能将热量以红外光的形式重新辐射到环境中. 在两种调节效应的协同作用下, 甲虫体温可降低7.8 ℃. 类似的效果在其他热带生物体内也有体现. 著名的撒哈拉沙漠银蚁(图2(b)) Catagliphis bombycina[25]的头部密排三角形毛发具有独特的辐射制冷特性, 该结构通过Mie散射和全反射增强了可见光和近红外波段的反射, 而在中红外波段毛发作为抗反射涂层又提高了发射率, 使银蚁可通过黑体辐射有效地将热量散发到环境中. 2020年, Zhang等[26]探究发现天牛(图2(c)) Neocerambyx gigas的绒毛也具有良好的温度调节作用, 其结构从根部到尖端形成向上的三角形, 可有效地反射可见光并在中红外波段辐射自身能量.
![图 2 不同种类昆虫的热调节生物结构 (a) 白甲虫Goliathus goliatus的体温调节机制[24] (i)白甲虫的光学照片, (ii)鳞片的壳/空心圆筒结构的扫描电子显微镜图及横截面的透射电子显微镜图, (iii)壳/空心圆筒结构内部全反射示意图, 插图为不同入射角(θ1和θ2)下的薄膜干涉, (iv)中红外波段壳/空心圆筒结构、空心圆筒和实心圆筒的模拟反射光谱; (b) 撒哈拉沙漠银蚁Catagliphis bombycine[25] (i)银蚁的光学照片和其头部的扫描电子显微镜图, (ii)密排三角形毛发的扫描电子显微镜图; (c) 天牛neocerambyx gigas[26] (i) 天牛的光学照片, (ii)天牛绒毛的扫描电子显微镜图; (d) (i)四种不同蝶翅鳞片的“井字形”周期微结构模型及其(ii)在中红外波段的发射率[27]; (e) 尖翅银灰蝶[28] (i)光学照片, (ii)亮银色鳞片的扫描电子显微镜图, (iii)亮银色鳞片横截面的透射电子显微镜图\r\nFig. 2. Thermoregulatory biological structure of different insect species. (a) Thermoregulatory mechanism of Goliathus goliatus[24]: (i) Photograph of a male Goliathus goliatus; (ii) scanning electron microscopy (SEM) image of the shell / hollow cylinder structure and transmission electron microscopy (TEM) image of cross-section; (iii) schematic of total reflection in the shell / hollow cylinder structure, inset is the thin-film interference at different incident angles (θ1 and θ2); (iv) simulated reflectance spectra of shell/hollow cylinder structure, hollow cylinders and solid cylinders in the mid-infrared (MIR) band. (b) Catagliphis bombycine[25]: (i) Optical photograph and SEM frontal view of the head; (ii) SEM image of the dense triangular hair. (c) Neocerambyx gigas[26]: (i) Optical photograph; (ii) SEM image of the fluffs covering on the forewing. (d) (i) Periodic microstructure models of four different butterfly wing scales and (ii) their emission spectrum in the mid-infrared band[27]. (e) Butterfly Curetis acuta Moore[28]: (i) Optical photograph; (ii) SEM image of the central part of a silver scale, (iii) cross section TEM image of the silver scales.](/fileWLXB/journal/article/wlxb/2022/2/PIC/2-20211854-2.png)
图 2 不同种类昆虫的热调节生物结构 (a) 白甲虫Goliathus goliatus的体温调节机制[24] (i)白甲虫的光学照片, (ii)鳞片的壳/空心圆筒结构的扫描电子显微镜图及横截面的透射电子显微镜图, (iii)壳/空心圆筒结构内部全反射示意图, 插图为不同入射角(θ1和θ2)下的薄膜干涉, (iv)中红外波段壳/空心圆筒结构、空心圆筒和实心圆筒的模拟反射光谱; (b) 撒哈拉沙漠银蚁Catagliphis bombycine[25] (i)银蚁的光学照片和其头部的扫描电子显微镜图, (ii)密排三角形毛发的扫描电子显微镜图; (c) 天牛neocerambyx gigas[26] (i) 天牛的光学照片, (ii)天牛绒毛的扫描电子显微镜图; (d) (i)四种不同蝶翅鳞片的“井字形”周期微结构模型及其(ii)在中红外波段的发射率[27]; (e) 尖翅银灰蝶[28] (i)光学照片, (ii)亮银色鳞片的扫描电子显微镜图, (iii)亮银色鳞片横截面的透射电子显微镜图Fig. 2. Thermoregulatory biological structure of different insect species. (a) Thermoregulatory mechanism of Goliathus goliatus[24]: (i) Photograph of a male Goliathus goliatus; (ii) scanning electron microscopy (SEM) image of the shell / hollow cylinder structure and transmission electron microscopy (TEM) image of cross-section; (iii) schematic of total reflection in the shell / hollow cylinder structure, inset is the thin-film interference at different incident angles (θ1 and θ2); (iv) simulated reflectance spectra of shell/hollow cylinder structure, hollow cylinders and solid cylinders in the mid-infrared (MIR) band. (b) Catagliphis bombycine[25]: (i) Optical photograph and SEM frontal view of the head; (ii) SEM image of the dense triangular hair. (c) Neocerambyx gigas[26]: (i) Optical photograph; (ii) SEM image of the fluffs covering on the forewing. (d) (i) Periodic microstructure models of four different butterfly wing scales and (ii) their emission spectrum in the mid-infrared band[27]. (e) Butterfly Curetis acuta Moore[28]: (i) Optical photograph; (ii) SEM image of the central part of a silver scale, (iii) cross section TEM image of the silver scales.蝴蝶作为一种典型多样化的昆虫一直备受人们关注, 人们对于蝴蝶的辐射制冷也有相关的研究. 如图2(d)所示, Anirudh等[27]对比了4种热带和寒带蝴蝶翅膀鳞片上的“井字形”周期微结构, 阐述了该结构的变化对蝶翅红外特性的影响以及对温度调节的重大作用. Tsai等[29]研究了热带地区蝴蝶的热调节机制, 蝶翅比热容小, 容易过热, 而蝶翅中含有活细胞的区域能维持较低的温度, 表明蝴蝶可以选择性地降低一些结构的温度, 如翼脉和雄性器官. 此外, 蝴蝶可利用翅膀感知可见光和红外辐射, 通过行为来防止翅膀过热. Lou等[18]研究了滴燕灰蝶Rapala dioetas的性斑鳞片的层次结构, 这种结构增大了内表面积, 强化了多重散射效应, 导致鳞片具有高发射率, 使蝴蝶利用有限体积的几丁质就能将热量有效地辐射到环境中. 除了红外波段, 蝴蝶的可见光特性也为人类在辐射制冷领域带来新的启发. Liu等[28]研究了蝴蝶的结构白度, 发现尖翅银灰蝶 (图2(e))Curetis acuta Moore腹侧的亮银鳞片具有增强宽带反射的作用, 这是鳞片上不规则的层间距和厚度引起的混色效应的结果, 有助于降低蝴蝶的温度, 具有良好的光热控制效果, 为设计低能量、宽波段高反射的仿生辐射制冷材料提供了新的思路.
总之, 生物的体温调节方式是复杂多样的, 这取决于环境条件、生物的物理特性、行为和生理[30]. 自然界是一个巨大的资源宝库, 可源源不断地为仿生辐射制冷技术的研究和发展提供灵感和启发.
4. 仿生辐射制冷材料和器件的研究进展
对生物所具有的辐射制冷性能的研究启发了人们设计和制备多种仿生辐射制冷材料和器件. Zhang等[26]研究了天牛Neocerambyx gigas的辐射制冷机制, 通过光热模拟对其绒毛结构进行了简化和优化, 如图3(a)所示, 制备出与生物结构相似的微金字塔阵列聚合物基体, 并在其中随机嵌入陶瓷颗粒, 形成高通量光子仿生制冷薄膜. 该薄膜可以反射95%的太阳辐射, 在中红外波段的发射率大于0.96, 有效辐射制冷功率密度达到90.8 W/m2, 在阳光直射下, 薄膜温度可下降5.1 ℃. 此外, 该薄膜还具有良好的疏水性、柔韧性和较高的机械强度, 在电子可穿戴器件的热管理领域具有广阔的应用前景. 2018年, Krishna和Lee [31]以闪蝶鳞片的微尺度树状结构为模型(图3(b)), 设计了一种周期性结构材料, 可以选择性地控制发射率, 通过改变材料表面周期性结构的种类和排布, 可以将陶瓷材料(具有高发射率)转变为反射材料, 将金属材料(具备高反射率)转变为发射材料. 其耦合波分析计算表明: 优化设计的树状结构可以成为近乎完美的反射体或黑体, 提供136 W/m2的净冷却功率密度或12 W/m2的净加热功率密度. 在不同的应用场合下, 这种结构可用于不同的热管理优化设计, 其结构灵活性和光谱发射率可调节性为热管理技术提供了新的途径. 蚕丝作为服装材料已有上千年的历史, 其具备良好的辐射制冷性能. Choi等[32]研究了家蚕Bombyx mori蚕丝的二维蛋白质结构, 通过局部化的强反射与生物分子在红外波段的高发射率相结合, 使蚕丝辐射的热量大于吸收的热量, 具有良好的降温效果. Shi等[17]研究了彗星蛾防止茧内温度过高的机制, 其蚕茧纤维呈现出明亮的银色金属光泽, 纤维内部随机分布着孔隙大小接近可见光和近红外波长的孔洞, 如图3(c)所示, 可作为太阳光的散射中心, 增强对太阳光的反射. 此外, 组成蚕茧的蛋白质具有高红外发射率(0.88), 使得彗星蛾蚕丝相对于家蚕蚕丝具有更好的辐射制冷效果. 受此启发, Shi等制备出基于再生丝素蛋白和聚偏二氟乙烯的仿生纳米结构纤维. 前者的太阳反射率为0.73, 热发射率为0.90, 后者为0.93和0.91. 两种纤维均具备较好的辐射制冷性能, 可作为人体热管理生物相容性材料. 此外, 丝状空隙导致光的定向散射, 可使光线沿纤维传播, 为生物相容性材料进行光信号和图像传输开辟了道路. 除了动物体系, 植物体系也在仿生辐射制冷领域得到应用. 2019年, Li等[21]对木材进行完全脱木素和致密化处理, 得到一种特殊的结构材料(图3(d)). 材料中的纳米纤维对太阳辐射进行散射, 在中红外波段有较强的发射, 实现了白天和夜间持续辐射制冷, 在建筑节能材料方面具有广阔的应用前景. 此外, 这种结构材料的机械强度高达404.3 MPa, 是天然木材的8倍以上, 可广泛应用于工程实际中. 2021年, Yang等[33]受天然植物纤维结构启发(图3(e)), 通过相分离法在聚合物基体中构建了随机的三维孔隙. 利用苯乙烯-丁二烯-苯乙烯(styrene-butadiene-styrene, SBS)和苯乙烯-乙烯-丙烯-苯乙烯(styrene-ethylene-propylene-styrene, SEPS)在聚丙烯(polypropylene, PP)中相容性的差别, 制备了两种内部具有不同孔隙的聚丙烯片材. 两种材料均能有效散射太阳光(97%), 同时保持在中红外波段的高发射率(0.81—0.67), 具有良好的辐射制冷效果, 在建筑节能领域具有潜在的应用价值.
![图 3 仿生辐射制冷材料及其制备 (a) 天牛绒毛的仿生模型[26] (i)从绒毛到仿生辐射制冷薄膜材料的生物启发示意图; (ii)仿生辐射制冷薄膜的结构及其对可见光的反射和红外发射特性. (b) 用于辐射热管理的(i)微尺度树状结构及(ii)发射光谱[31]. (c) 彗星蛾蚕丝[17] (i)蚕丝的光学显微镜图; (ii)蚕丝横截面的扫描电子显微镜图; (iii)含有高密度孔隙的再生丝素蛋白纤维(上)和聚偏二氟乙烯纤维(下)的横截面及纵截面的扫描电子显微镜图. (d) 对木材进行脱木素和致密化处理得到的辐射制冷结构材料[21] (i)光学照片; (ii)对太阳光的散射和纤维素官能团分子振动导致的红外发射示意图. (e) 受天然纤维启发的辐射制冷材料[33] (i) 棉花的光学照片及微观结构示意图; (ii) 两种微孔聚丙烯片材的顶部和横截面视图的显微照片; (iii) 微孔聚丙烯片材的反射和发射特性\r\nFig. 3. Bio-inspired radiative cooling materials and the fabrication of such materials. (a) The bio-inspired model from neocerambyx gigas[26]: (i) Bio-inspired schematic diagram from fluff to radiative cooling film; (ii) structure of the radiative cooling film and its reflection characteristics for visible light and infrared emission. (b) (i) Micro-tree structures and (ii) their emissivity spectrum for radiation thermal management[31]. (c) Comet moth cocoon fibers[17]: (i) Optical microscopy image; (ii) SEM image of the transverse cross-section of a comet moth silk fiber; (iii) SEM images of the cross-section and longitudinal-section of regenerated silk fiber (upper) and polyvinylidene difluoride fiber (lower) containing high-density pores. (d) Wood for radiative cooling[21]: (i) Optical photograph; (ii) schematic of the wood structure in scattering solar irradiance and infrared emission caused by molecular vibration of cellulose functional groups. (e) Radiative cooling materials inspired by the natural fibers[33]: (i) Photograph of raw cotton and the schematic of its microstructure; (ii) micrographs showing top and cross-section views of the macroporous PP/SBS sheet and PP/SEPS sheet; (iii) reflection and emission characteristics of the macroporous PP sheets.](/fileWLXB/journal/article/wlxb/2022/2/PIC/2-20211854-3.png)
图 3 仿生辐射制冷材料及其制备 (a) 天牛绒毛的仿生模型[26] (i)从绒毛到仿生辐射制冷薄膜材料的生物启发示意图; (ii)仿生辐射制冷薄膜的结构及其对可见光的反射和红外发射特性. (b) 用于辐射热管理的(i)微尺度树状结构及(ii)发射光谱[31]. (c) 彗星蛾蚕丝[17] (i)蚕丝的光学显微镜图; (ii)蚕丝横截面的扫描电子显微镜图; (iii)含有高密度孔隙的再生丝素蛋白纤维(上)和聚偏二氟乙烯纤维(下)的横截面及纵截面的扫描电子显微镜图. (d) 对木材进行脱木素和致密化处理得到的辐射制冷结构材料[21] (i)光学照片; (ii)对太阳光的散射和纤维素官能团分子振动导致的红外发射示意图. (e) 受天然纤维启发的辐射制冷材料[33] (i) 棉花的光学照片及微观结构示意图; (ii) 两种微孔聚丙烯片材的顶部和横截面视图的显微照片; (iii) 微孔聚丙烯片材的反射和发射特性Fig. 3. Bio-inspired radiative cooling materials and the fabrication of such materials. (a) The bio-inspired model from neocerambyx gigas[26]: (i) Bio-inspired schematic diagram from fluff to radiative cooling film; (ii) structure of the radiative cooling film and its reflection characteristics for visible light and infrared emission. (b) (i) Micro-tree structures and (ii) their emissivity spectrum for radiation thermal management[31]. (c) Comet moth cocoon fibers[17]: (i) Optical microscopy image; (ii) SEM image of the transverse cross-section of a comet moth silk fiber; (iii) SEM images of the cross-section and longitudinal-section of regenerated silk fiber (upper) and polyvinylidene difluoride fiber (lower) containing high-density pores. (d) Wood for radiative cooling[21]: (i) Optical photograph; (ii) schematic of the wood structure in scattering solar irradiance and infrared emission caused by molecular vibration of cellulose functional groups. (e) Radiative cooling materials inspired by the natural fibers[33]: (i) Photograph of raw cotton and the schematic of its microstructure; (ii) micrographs showing top and cross-section views of the macroporous PP/SBS sheet and PP/SEPS sheet; (iii) reflection and emission characteristics of the macroporous PP sheets.当前, 仿生辐射制冷技术发展迅速且多元化, 通过研究生物材料、结构和行为等多种机制的协同关系, 各种新型仿生辐射制冷材料和器件层见叠出, 辐射制冷功率逐渐提高, 应用场景也不断拓展.
5. 仿生辐射制冷的研究展望
仿生辐射制冷未来应朝向简单化、高功率、可调控、低成本、可大规模制造的方向发展. 当前限制辐射制冷材料和器件大规模应用的重要原因在于辐射制冷功率较低, 降温速度慢, 制冷效果不够理想等. 从生物的角度去思考, 自然界中, 许多生物进化出了非常完善的体温调节机制, 人们从这些结构和机制中获得启发, 模仿生物结构, 通过理论模拟对生物结构简化和优化, 制备出性能优异的辐射制冷材料. 同时, 将更多精巧的生物光热微结构集成到同一热控功能系统的设计中, 互相弥补, 扩展仿生辐射制冷材料和器件的波长作用范围, 达到更好的辐射制冷效果. 结合应用场景, 如作为服装材料, 还需要良好的透气性、亲肤性和色彩可调节性, 为此, 需要将辐射制冷性能和其他性能融合, 设计多重复合结构, 根据不同的应用场景智能化调控材料的辐射制冷特性. 此外, 随着各种先进材料制造工艺的出现, 将先进微纳加工技术融入到仿生辐射制冷材料的制备上, 能够实现仿生辐射制冷材料高效和低成本的大规模制造. 仿生辐射制冷是一个多学科创新交叉的研究领域, 需要各个领域的研究团队合力推进其发展和应用, 未来可期.
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图 1 辐射制冷原理及自然界中对仿生辐射制冷具有启发性的生物和材料 (a) 发生在地球表面的辐射热流示意图[14]. Psolar, 地球表面对太阳辐射的吸收; Prad, 地球表面发射的热辐射; Patm, 地球表面对大气热辐射的吸收. (b) AM 1.5太阳光谱和典型的大气透明窗口[15]. (c)−(h) 典型的具有辐射制冷特性的生物体和生物材料, 依次为具有不同近红外发射特性的鸟类[16] (c), 彗星蛾蚕茧[17] (d), 滴燕灰蝶及其红外图像[18] (e), 撒哈拉沙漠银蚁[19] (f), 人类皮肤褶皱及其结构示意图[20] (g), 木材纤维[21] (h)
Figure 1. Principles of radiative cooling and the nature organism that are inspirational for bio-inspired radiative cooling: (a) Schematic of radiative energy flows occurring at the terrestrial surfaces[14]. Psolar, solar radiation absorbed by the terrestrial surfaces; Prad, thermal radiation emitted by the terrestrial surfaces; Patm, atmospheric thermal radiation absorbed by the terrestrial surfaces. (b) AM 1.5 solar spectrum and typical atmospheric window[15]. (c)−(h) Typical organisms and biomaterials with radiative cooling properties: (c) Birds with different near-infrared (NIR) emissivity[16]; (d) comet moth cocoon[17]; (e) butterfly Rapala dioetas and its infrared image[18]; (f) Saharan silver ant[19]; (g) human skin and the structural schematic[20]; (h) wood fiber[21].
图 2 不同种类昆虫的热调节生物结构 (a) 白甲虫Goliathus goliatus的体温调节机制[24] (i)白甲虫的光学照片, (ii)鳞片的壳/空心圆筒结构的扫描电子显微镜图及横截面的透射电子显微镜图, (iii)壳/空心圆筒结构内部全反射示意图, 插图为不同入射角(θ1和θ2)下的薄膜干涉, (iv)中红外波段壳/空心圆筒结构、空心圆筒和实心圆筒的模拟反射光谱; (b) 撒哈拉沙漠银蚁Catagliphis bombycine[25] (i)银蚁的光学照片和其头部的扫描电子显微镜图, (ii)密排三角形毛发的扫描电子显微镜图; (c) 天牛neocerambyx gigas[26] (i) 天牛的光学照片, (ii)天牛绒毛的扫描电子显微镜图; (d) (i)四种不同蝶翅鳞片的“井字形”周期微结构模型及其(ii)在中红外波段的发射率[27]; (e) 尖翅银灰蝶[28] (i)光学照片, (ii)亮银色鳞片的扫描电子显微镜图, (iii)亮银色鳞片横截面的透射电子显微镜图
Figure 2. Thermoregulatory biological structure of different insect species. (a) Thermoregulatory mechanism of Goliathus goliatus[24]: (i) Photograph of a male Goliathus goliatus; (ii) scanning electron microscopy (SEM) image of the shell / hollow cylinder structure and transmission electron microscopy (TEM) image of cross-section; (iii) schematic of total reflection in the shell / hollow cylinder structure, inset is the thin-film interference at different incident angles (θ1 and θ2); (iv) simulated reflectance spectra of shell/hollow cylinder structure, hollow cylinders and solid cylinders in the mid-infrared (MIR) band. (b) Catagliphis bombycine[25]: (i) Optical photograph and SEM frontal view of the head; (ii) SEM image of the dense triangular hair. (c) Neocerambyx gigas[26]: (i) Optical photograph; (ii) SEM image of the fluffs covering on the forewing. (d) (i) Periodic microstructure models of four different butterfly wing scales and (ii) their emission spectrum in the mid-infrared band[27]. (e) Butterfly Curetis acuta Moore[28]: (i) Optical photograph; (ii) SEM image of the central part of a silver scale, (iii) cross section TEM image of the silver scales.
图 3 仿生辐射制冷材料及其制备 (a) 天牛绒毛的仿生模型[26] (i)从绒毛到仿生辐射制冷薄膜材料的生物启发示意图; (ii)仿生辐射制冷薄膜的结构及其对可见光的反射和红外发射特性. (b) 用于辐射热管理的(i)微尺度树状结构及(ii)发射光谱[31]. (c) 彗星蛾蚕丝[17] (i)蚕丝的光学显微镜图; (ii)蚕丝横截面的扫描电子显微镜图; (iii)含有高密度孔隙的再生丝素蛋白纤维(上)和聚偏二氟乙烯纤维(下)的横截面及纵截面的扫描电子显微镜图. (d) 对木材进行脱木素和致密化处理得到的辐射制冷结构材料[21] (i)光学照片; (ii)对太阳光的散射和纤维素官能团分子振动导致的红外发射示意图. (e) 受天然纤维启发的辐射制冷材料[33] (i) 棉花的光学照片及微观结构示意图; (ii) 两种微孔聚丙烯片材的顶部和横截面视图的显微照片; (iii) 微孔聚丙烯片材的反射和发射特性
Figure 3. Bio-inspired radiative cooling materials and the fabrication of such materials. (a) The bio-inspired model from neocerambyx gigas[26]: (i) Bio-inspired schematic diagram from fluff to radiative cooling film; (ii) structure of the radiative cooling film and its reflection characteristics for visible light and infrared emission. (b) (i) Micro-tree structures and (ii) their emissivity spectrum for radiation thermal management[31]. (c) Comet moth cocoon fibers[17]: (i) Optical microscopy image; (ii) SEM image of the transverse cross-section of a comet moth silk fiber; (iii) SEM images of the cross-section and longitudinal-section of regenerated silk fiber (upper) and polyvinylidene difluoride fiber (lower) containing high-density pores. (d) Wood for radiative cooling[21]: (i) Optical photograph; (ii) schematic of the wood structure in scattering solar irradiance and infrared emission caused by molecular vibration of cellulose functional groups. (e) Radiative cooling materials inspired by the natural fibers[33]: (i) Photograph of raw cotton and the schematic of its microstructure; (ii) micrographs showing top and cross-section views of the macroporous PP/SBS sheet and PP/SEPS sheet; (iii) reflection and emission characteristics of the macroporous PP sheets.
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