炎热地区"晒不热"材料的探索研究

对具有低太阳吸收率、高发射率的碱土金属碳酸盐作为炎热地区"晒不热"涂料的热效果作了初步的探索.该材料在炎热地区白天晒不热效果明显.曝晒最大升温比普通热反射涂料低2～3℃;夜间辐射制冷最大降温比普通热反射涂料多降1～2℃;24h的循环实验仍然符合以上效果;价格低廉、来源广泛,适合于炎热地区使用.     

“晒不热”涂料的应用研究

测试了具有低太阳吸收率、高发射率的碱土金属碳酸盐的炎热地区“晒不热”涂料在不同模拟屋面上的降温效果.结果显示,在不同的屋面材料上涂刷“晒不热”涂料的降温效果不同,其中在油毛毡上涂刷“晒不热”涂料降温效果最好,可降温10.8℃;在砂浆板上涂刷“晒不热”涂料降温效果最差,降温2.5℃.在铁皮箱上测试24h的循环实验,白天与日间曝晒试验相符合,而夜间温度则要比环境温度低1～2℃,在不同季节的同一屋面的测试效果具有重复性.     

太空辐射致冷空调装置的实验研究

针对辐射致冷进行了可行性研究.首先介绍了辐射致冷的原理和实现方法,然后利用红外发射率测定仪测定了碳黑、TiO2、NaCl晶体、聚酯(PET)薄膜与聚四氟乙烯(PTFE)薄膜的红外发射率,并间接测定了低密度聚乙烯薄膜(LDPE)对于不同材料的透过率.最后研制了利用空气为冷媒介质的辐射致冷实验装置,进行了夜间静态和连续抽气实验.实验结果表明:分别用PET薄膜和PTFE薄膜作为辐射体,静态实验时,可获得与环境的最大温差分别为11℃和9℃;连续抽气实验时,装置出口处冷空气与环境的最大温差分别为4.7℃和5.4℃.通过计算,装置的有效致冷功率为74.5W/m2,故该装置可实现节能型建筑夏季夜间的连续降温.     

一种新型太空辐射致冷装置的实验研究

分析了辐射致冷的可行性,然后利用红外发射率测定仪测定了几种材料的红外发射率。研制了利用空气为冷媒介质的辐射致冷实验装置,并依据发汗致冷的原理设计了一种新型装置,在同一条件下进行了对比实验。实验结果表明:分别用PET薄膜和PTFE薄膜作为辐射体,当室外环境温度为29℃时,静态实验可获得与环境的最大温差分别为11℃和9℃;连续空气流动实验时,装置出口处冷空气与环境的最大温差分别为4.7℃和5.4℃,利用发汗致冷原理制成的装置,稳定后装置出口处冷空气与环境的最大温差为7.8℃,通过计算该工况下的致冷功率达到116W/m2,故该装置可实现建筑夏季夜间的连续降温。     

日间辐射制冷材料研究进展

辐射制冷是一种备受关注的新型降温方式,该方式利用大气透明窗口(ATW)向外太空辐射传热,实现被动降温.一般日间辐射制冷材料应在8～13μm波段内具有高发射率,在太阳光谱波段吸收率低于5％.辐射制冷研究可以绿色低耗的方式为建筑节能、服饰降温、冷藏冷凝、电池降温等提供方案,有着广阔的应用前景.如何使辐射材料较好地匹配理想辐射光谱是目前最主要的问题.近年来研究者多从辐射材料结构入手,在提高8～13μm窗口波段发射率的同时,通过构建具备光子带隙或发生Mie散射的结构等方式,减少太阳辐射吸收,并取得了显著成果.通过高聚物掺杂纳米粒子、不同折射率材料堆叠等结构设计,使辐射制冷材料的红外选择性得到了显著提升.一些高聚物在8～13μm波段内具有高发射率,同时可有效隔绝体系外的热量输入.将高聚物与发射光谱互补的掺杂纳米粒子相结合,可覆盖整个目标波段,提高其制冷性能.层堆叠模式参考了光子晶体阻断特定波长电磁波传播的特性,设计了不同折射率层交替排列,在不影响高发射层向外辐射红外能量的同时降低了材料对太阳光的吸收.本文主要综述了近年日间辐射制冷材料的研究进展,按其结构形态,将前沿辐射制冷材料分为薄膜类、涂层类、织物类和块材类,并阐述了辐射制冷器在建筑、电池降温等方面的实际应用.     

特种光谱选择性涂层的遗传算法优化设计

智能型自控涂层的辐射性质有很强的光谱选择性,特别值得关注的是这种涂层辐射性质的变化是自身根据环境的变化来调节的.其作用与目前广泛使用的被动热控涂层相比较,具有重量小、结构简单、可控温度范围宽等优点.为了满足具体应用的要求,需要在智能型热控涂层SRD的表面沉积一种光谱选择性涂层.本文采用遗传算法(GA)来最优化设计这种光谱选择性涂层.在保证SRD的半球总发射率的变化量△εH满足温度调节要求的条件下,尽可能地降低太阳吸收率αs.     

红外高发射率涂层的研究进展

红外高发射率材料具有良好的辐射与吸收性能,可有效改善物体表面的辐射系数,增强辐射传热从而达到散热的目的。随着国防科技与工业技术的发展,高发射率材料已被广泛应用于航空材料、工业窑炉材料、建筑材料、电子、冶金等领域,其中涂层材料的运用占较大比重。目前,美国、英国等发达国家每年投入数亿资金用于高发射率涂层的研究,超薄、复合、多功能的涂层材料成为近年来主要的发展趋势。概述了高发射率材料的种类,比较了高发射率涂层几种制备方法的优缺点,并总结了影响涂层发射率的因素与提高发射率的途径。     

辐射冷却材料的制备方法综述

随着能源和环境问题的日益严重,不需要额外消耗能量的辐射冷却技术引起了广泛关注.基于地球和宇宙的巨大温差,辐射冷却技术能够直接将热量以辐射的形式传递到外太空,不仅能够产生净冷却效果,而且避免了废热排放带来的诸多环保问题.早期研究利用涂敷法将具有辐射冷却能力的涂料刷涂在物体表面,该方法仅能实现材料的夜间降温.为了实现材料的日间辐射冷却则需要在其中引入特定结构,如多层膜结构、二维光子晶体结构、超结构等以提高它的可见近红外波段的反射率和大气窗口的发射率.然而,相关制备技术存在工艺复杂、制备成本高、难以宏量制备等局限性,严重阻碍了辐射冷却材料工程应用的推进.随着辐射冷却理论研究的深入以及更多先进制备技术的发展,制备方法和手段也越来越多样化,如真空蒸镀、微纳米加工技术、直接涂敷法、挤出成型技术、静电纺丝法等相继被报道,通过这些方法能够低成本地制备出较大面积的辐射冷却材料,进一步推动了辐射冷却材料及技术的应用.本文归纳了辐射冷却材料制备方法的研究现状,对不同制备技术的优缺点进行了对比分析.首先概述了辐射冷却常用材料及其成型方式;然后介绍了辐射冷却材料常用制备方法及各自优缺点,简述了适用于特殊材料或应用场景的制备方法;最后总结了目前辐射冷却材料制备存在的问题并展望了未来的发展方向.     

辐射致冷系统装置的实验研究

设计了几种辐射致冷装置,以空气作为制冷剂,利用聚酯(PET)薄膜作为辐射体材料,低密度聚乙烯膜(LDPE)作为盖板材料,进行夜间实验.在静态对比实验中,装置水平放置的实验结果表明,PET与环境的最大温差为9.9℃,装置内空气与环境的最大温差为7.2℃;装置倾斜放置时,在相同的夜晚条件下,倾斜45°角得到的致冷效果最佳,内部空气与环境的最大温差为7.6℃.在不同风速的连续抽气实验中,风速为0.6m/s～0.8m/s时,装置的出口空气与环境的温差最大,最大为5.2℃,且整夜温差相对较平稳.通过计算,装置的有效致冷功率为73.6W/m2.     

电流密度对MgO-ZnO陶瓷薄膜结构和热控性能的影响

在Zn(H₂PO₄)₂电解液中, 利用微弧氧化技术在AZ31镁合金表面原位生长MgO-ZnO热控陶瓷薄膜, 研究了电流密度对薄膜结构组成、结合强度和热控性能的影响, 以及紫外辐照作用下薄膜太阳吸收率的变化规律。结果表明：薄膜主要由MgO、ZnO和非晶态物质组成, 随着电流密度增大, 微孔数量逐渐减少而粗糙度逐步增大, 其厚度、结合强度和发射率先增大后减小, 而太阳吸收率则先减小后增大。电流密度9 A/dm²时所得薄膜的结合强度达到最大12.6 MPa, 且热控性能最佳, 其发射率为0.872, 太阳吸收率为0.363; 且随着紫外辐照时间延长, 此薄膜太阳吸收率先升高而后趋于平缓。研究结果为制备良好结合强度和抗紫外辐照能力的低吸收高发射热控薄膜提供技术 支持。     

Flexible and Dual-Sided Available Colored Films for Efficient Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) is a promising strategy to realize surface cooling of objects without any external energy consumption. While such materials typically exhibit dazzling white appearances, developing pleasingly looking colored radiative cooling materials is greatly significant but remains an arduous challenge. Herein, self-standing, flexible colored films available on both sides are prepared by using a facile and scalable strategy. The films are asymmetrically designed. One side is SiO₂-filled porous structure with highly reflective pigment distributed that can selectively reflect solar light to generate specific color, and the other side is hierarchically porous three-phase composite with less SiO₂, which maximizes the solar reflection. The breathable film achieves a relatively high near infrared reflectance on the colored side (up to 89%), and a broadband solar reflectance on the reverse side. Moreover, both sides exhibit extremely high mid-infrared emissivity (98%) allowing significant radiative heat loss. With the diverse but efficient reflectance and emittance, different sides of three colored films yield temperature drops ranging from 2.0 to 11.1 °C during daytime. Building energy simulation indicates that 655 MJ m⁻² energy can be saved over the whole summer if the dual-sided available colored film is deployed in China.     

Scalable Bacterial Cellulose-Based Radiative Cooling Materials with Switchable Transparency for Thermal Management and Enhanced Solar Energy Harvesting

Radiative cooling materials that can dynamically control solar transmittance and emit thermal radiation into cold outer space are critical for smart thermal management and sustainable energy-efficient buildings. This work reports the judicious design and scalable fabrication of biosynthetic bacterial cellulose (BC)-based radiative cooling (Bio-RC) materials with switchable solar transmittance, which are developed by entangling silica microspheres with continuously secreted cellulose nanofibers during in situ cultivation. Theresulting film shows a high solar reflection (95.3%) that can be facilely switched between an opaque state and a transparent state upon wetting. Interestingly, the Bio-RC film exhibits a high mid-infrared emissivity (93.4%) and an average sub-ambient temperature drop of ≈3.7 °C at noon. When integrating with a commercially available semi-transparent solar cell, the switchable solar transmittance of Bio-RC film enables an enhancement of solar power conversion efficiency (opaque state: 0.92%, transparent state: 0.57%, bare solar cell: 0.33%). As a proof-of-concept illustration, an energy-efficient model house with its roof built with Bio-RC-integrated semi-transparent solar cell is demonstrated. This research can shine new light on the design and emerging applications of advanced radiative cooling materials.     

一种搜寻辐射制冷材料的红外光谱分析方法

根据辐射制冷材料的光谱选择性吸收的特性,探索性地提出了采用红外光谱分析方法筛选辐射制冷材料,并对其中某些材料(CaSO4、BaSO4、PTFE,有机硅)的吸收比/法向发射比以及辐射制冷效果进行测定,证实了采用红外光谱分析方法搜寻辐射制冷材料具有一定的可靠性.     

Creating an Eco-Friendly Building Coating with Smart Subambient Radiative Cooling

Subambient daytime radiative cooling (SDRC) provides a promising electricity- and cryogen-free pathway for global energy-efficiency. However, current SDRC systems require stringent surface designs, which are neither cost-effective nor eco-friendly, to selectively emit thermal radiation to outer space and simultaneously maximize solar reflectance. Here, a generic method is developed to upgrade the conventional building-coating materials with a peculiar self-adaptive SDRC effect through combining particle scattering, sunlight-excited fluorescence, and mid-infrared broadband radiation. It is also theoretically proved that heat exchange with the sky can eliminate the use of resonant microstructures and noble metal mirrors in conventional SDRC, and also leads to enhanced daytime cooling yet suppressed nighttime overcooling. When exposed to direct sunlight, the upgraded coating over an aluminum plate can achieve 6 °C (7 °C on a scale-model building) below the ambient temperature under a solar intensity of 744 W m⁻² (850 W m⁻²), yielding a cooling power of 84.2 W m⁻². The results pave the way for practical large-scale applications of high-performance SDRC for human thermal comfort in buildings.     

Radiative Properties of Uranium Dioxide Near Its Melting Point

A new technique for measuring radiative properties of nuclear fuel materials at high temperatures has been developed. The technique is based on pulse diffuse optical probing of the sample surface and on pyroreflectometry used in measuring radiative properties of refractory materials during laser heating or cooling. Pulse diffuse optical probing of the sample has been realized for the first time in subsecond pyrometry of the open surface heated by laser radiation. Such a procedure of sample irradiation during sample laser heating or cooling enables reflectivity and emissivity measurements near high temperature phase transitions to be performed in spite of possible sharp changes of the reflection indicatrix at phase transitions in the investigated material. With the method developed in this study, the spectral emissivity and reflectivity of uranium dioxide near its premelting and melting points have been measured. It has been found that condensation of the vapor plume formed above the sample during laser heating influenced the melting and boiling temperatures of uranium dioxide. The first-order phase transitions in uranium dioxide, such as solid–vapor–solid and liquid–vapor–liquid, have been observed in uranium dioxide for the first time during laser heating. Also, new data on the spectral emissivity of uranium dioxide at a wavelength of 0.644 m and in the temperature range of 2000 to 4200 K are presented.     

Simultaneous Measurement Method of Normal Spectral Emissivity and Optical Constants of Solids at High Temperature in Vacuum

For the case of the thermal design of devices at high temperatures, radiative properties are important. The emissivity of materials depends on their surface conditions and temperatures. The goal of this study is to measure the emissivity of various materials with clear surface conditions. A system for simultaneous measurements of the normal spectral emissivity, optical constants, and thickness of materials at high temperatures has been developed. To determine an accurate surface temperature of specimens, a surface temperature measurement method is applied using the Christiansen effect. This article focuses on evaluations of the Christiansen effect under various conditions. Measurement results of the normal spectral emissivity of ZrO₂ from 773 K to 973 K are also presented.     

24-hour cooling of a building by a PCM-integrated adsorption system

This study presents a theoretical investigation into integration of phase change materials (PCMs) with an adsorption cooling system in order to provide 24-hour air conditioning. A latent heat storage unit containing PCM is used to store solar energy during the daytime, and at nighttime the conserved thermal energy and an auxiliary heater drive the adsorption chiller. The system adopts a cooling channel to reduce the air temperature. The air flow to the channel is provided by use of fans and at different fresh air ratios (FR). Room temperature and the room's maximum cooling demand for which thermal comfort can be achieved are estimated. In addition, the effects of different parameters on room temperature and solar fraction are studied. It is indicated that an optimum ACH value exists for which the room temperature is the lowest. Also, rise of ACH and FR decrease solar fraction and increase auxiliary energy consumption. It is found that when ACH = 4 and FR = 20%, daily solar fraction is 0.76 and 217 MJ of auxiliary energy is required during the 24 hours. Under this condition, thermal comfort is achieved for a maximum cooling demand of 4000 W during the 24 hours.     

Thermal camouflaging metamaterials

Thermal camouflage technologies, which aim at blending the infrared (IR) signature of targets into the background to counter the IR detection, have witnessed increasing development. To achieve thermal camouflage, the rule of thumb is to balance the thermal radiation between the target and the background, and the corresponding conductive strategy is to tune the local temperature field while the radiative strategy is to tune the local emissivity. Following these two basic strategies, the thermal metamaterials and wavelength-selective emissivity engineering to achieve thermal camouflage are first introduced. Then the more advanced dynamic strategies are reviewed that can adapt to the varying environment under the external stimuli, like electricity, light, strain, chemical, wetting, temperature, etc. Particularly the phase-changing and bioinspired materials are presented and reviewed. Finally, critical considerations on the challenges and opportunities of next-generation thermal camouflage technologies are elaborated and four future directions are cast, including temperature-responsive emissivity engineering, soft materials, multispectral camouflage, and detection-feedback system. Overall, a detailed introduction to the working principle, the state-of-the-art progress, and the critical thinking on the future development on thermal camouflage technologies are presented.     

The origin of highly efficient selective emission in rare-earth oxides for thermophotovoltaic applications

Rare-earth oxide materials emit thermal radiation in a narrow spectral region, and can be used for a variety of different high-temperature applications, such as the generation of electricity by thermophotovoltaic conversion of thermal radiation. However, because a detailed understanding of the mechanism of selective emission from rare-earth atoms has so far been missing, attempts to engineer selective emitters have relied mainly on empirical approaches. In this work, we present a new quantum thermodynamic model to describe the mechanisms of thermal pumping and radiative de-excitation in rare-earth oxide materials. By evaluating the effects of the local crystal-field symmetry around a rare-earth ion, this model clearly explains how and why only some of the room-temperature absorption peaks give rise to highly efficient emission bands at high temperature (1,000-1,500 degrees C). High-temperature emissivity measurements along with photoluminescence and cathodoluminescence results confirm the predictions of the theory.     

Spectral and Total Emissivity of High-Temperature Materials

A number of various high emissivity coatings has been investigated in detail. Oxide ceramic coatings for rotating x-ray anodes must have a total emissivity greater than 0.8 in order to ensure efficient cooling in vacuum. Only one of the investigated coatings showed sufficient long-term stability. For thermal protection of reusable space transportation systems during atmospheric reentry, in addition to high emissivity, oxidation resistance is required. SiC coatings and special polysilazane-based coatings have been tested. Results of emissivity measurements before and after flight experiments on the Russian FOTON capsule are also available. In order to improve the reliability of (high-temperature) emissivity measurements Pt–Rh alloys, SiC, Al₂O₃ doped with Cr₂O₃, and graphite have been tested to assess applicability as reference materials for comparative emissivity measurements by various facilities.