辐射制冷膜的性能分析与优化设计

根据基尔霍夫定律和光学薄膜理论,利用光学设计软件Macleod研究辐射制冷膜的材料与结构对制冷性能的影响。辐射制冷膜要求在0.3～3.0μm的太阳波段具有较高反射率,阻止物体得热;在8～13μm的红外波段具有较高发射率,降低自身温度。研究发现:由于SiO_2在8～13μm波段的反射率低,在其他波段的反射率高,是制造辐射膜的理想材料。高分子聚合物PS与SiO_2交替叠加形成的复合膜在8～13μm波段会产生声子-极化子激发的共振现象,可进一步降低反射率,因此PS是提高辐射制冷薄膜性能的有效辅助材料。经过Simplex法优化,得到辐射制冷膜以SiO_2-PS-SiO_2结构组合,各层厚度依次为0.2、0.8、70μm时,其辐射力高达136.2 W/m~2,平均发射率达到92.12%。     

干涉型太阳选择性吸收涂层的光学性能设计

利用等效媒质理论对铝复合单层膜的光学性能进行了计算并以此为基础优化设计了具有干涉效应的铝复合选择性吸收涂层，该吸收层为两层时膜系谱曲线具有明显的干涉效应，与多层吸收层膜系相比其发射率低，吸收率二者相同，随温度的升高，发射率的变化前者较后者缓慢，实际制备了金属填充因子ｆａｌ为０．３８，０．２４的铝复合膜为吸收层，ＡｌＮ，Ａｌ２Ｏ３为减反射膜构成的膜系，其光谱线曲线与优化的理论曲线吻合较好。     

具有半透明镜反射表面和不透明漫射基底面的半透明介质层的表观光谱发射特性

对具有镜反射半透明表面和漫射不透明基底面的平板状半透明介质层,从介质的内部辐射出发,采用伪光源迭加法进行表观光谱发射特性的研究。得到半透明介质层外表面的表观方向光谱发射率及表观半球光谱发射率解析表达式,探讨了表观光谱辐射特性与介质层的光学厚度、介质折射率及基底面的发射率等各影响因素间的相互关系。其中,介质折射率对表观方向辐射特性的影响较大,它和光学厚度等因素一起作用,使表观方向发射呈现出复杂的各向     

阳极氧化电解着色铝选择性吸收涂层的光学性能研究

本文报道了阳极氧化电解着色铝选择性吸收涂层的光学性能,其太阳吸收率α_s和热发射率ε_n分别为0.94(AM2)和0.10。给出了Ni粒子在多孔性Al_2O_2膜中浓度成梯度变化的结构模型。应用Bruggeman有效介质理论和多层膜系矩阵方法对涂层的光谱反射率进行了计算,讨论了表面形貌对涂层光学性能的影响。     

太阳能光伏发电-辐射制冷建筑一体化复合装置的性能分析

一种光谱选择性复合板芯同时具有太阳能光伏发电和辐射制冷2种功能,在光电转换波段(0.2～1.1μm)和辐射制冷波段(8～13μm)具有高发射比,而在其余辐射波段具有低发射比。基于该复合板芯设计一种太阳能光伏发电-辐射制冷建筑一体化复合装置,该装置具有白天光伏发电和夜间辐射制冷2种功能。建立复合装置的数学模型,对该复合装置在夏季炎热潮湿地区(中国合肥)和炎热干燥地区(埃及Kharga)的光电性能和辐射制冷性能进行数值模拟。计算结果表明:在中国合肥和埃及Kharga,晴天条件下复合装置夏季白天平均光电功率可达74.60和91.00 W/m~2,夜间平均制冷功率分别为42.46和57.24 W/m~2,全天能量收益达3.37和4.27 MJ/m~2,分别比传统光伏装置的全天能量收益高53.78%和56.70%。     

基底及HMVF层厚度变化对NbSiN/NbSiNO/Si3N4 涂层光学性能的影响

采用多靶磁控溅射镀膜机,以在Si靶上加Nb片的方法分别在Cu和单面抛光不锈钢(SS)基底上制备NbSiN/NbSiNO/Si3N4和Mo/NbSiN/NbSiNO/Si3N4多层膜。用α-step台阶仪(DEKTAK IIA)测量膜层厚度,UVPC3100分光光度计和傅里叶红外光谱仪(EXCALIBUR FTS 3000)对样品的光学性质进行表征。实验表明:在Cu基底上制备的涂层吸收率α=0.90,发射率ε=0.12;以SS为基底,红外反射层Mo的厚度约为230nm时,在λ2.5μm范围内反射率最高,达到90%以上。高金属体积分数吸收层(HMVF)厚度约为80nm时吸收率α=0.95,发射率ε=0.33;真空下500℃和600℃各20h退火后吸收率、发射率变化不大。     

Al-C-F/渐变SS-C/Al选择性吸收表面

建立了铝与不锈钢两个平面磁控靶溅射系统。反应溅射沉积了一组SS-C复合材料、α-C:H与Al-C-F均匀薄膜。运用椭偏仪-分光光度计法确定了上述薄膜在太阳光谱范围(0.35—2.5μm)内的光学常数谱值(?)(λ)=n(λ)-ik(λ)。 对多层膜系的模型用计算机进行优化设计与计算。实验表明,溅射沉积Al-C-F/变渐SS-C/Al选择性吸收表面,其反射率谱值与计算出的最佳谱值基本一致,太阳吸收率α≈0.96,法向发射率ε_n≈0.06(80℃)。经真空中450℃烘烤1小时,该表面的光学性能基本没有变化。这种优质表面用于玻璃真空集热管有良好的前景。     

Al-AlN选择性吸收涂层性能分析及光学模拟

研究了双层Al-AlN吸收层加减反射层结构膜系,并对这种结构膜系涂层性能进行分析和模拟,在模拟得到单层Al-AlN层的膜厚和填充因子基础上,工艺优化制备得到的Al-AlN选择性吸收涂层吸收率达到0.942,100℃发射率为0.044,聚光比为1条件下,光热转换效率为0.89.     

Al-AlN选择性吸收涂层性能分析及光学模拟

研究了双层Al-AlN吸收层加减反射层结构膜系,并对这种结构膜系涂层性能进行分析和模拟,在模拟得到单层Al-AlN层的膜厚和填充因子基础上,工艺优化制备得到的Al-AlN选择性吸收涂层吸收率达到0.942,100℃发射率为0.044,聚光比为1条件下,光热转换效率为0.89。     

太阳能集热-辐射制冷复合表面及其试制与性能分析

提出一种可以兼顾太阳能集热和辐射制冷的光谱选择性复合表面,其在太阳辐射波段(0.2~3.0μm)和辐射制冷"大气窗口"波段(8.0~13.0μm)具有高吸收(发射)率,而在除此之外的其余波段(3.0~8.0μm、13.0~25.0μm)具有低吸收(发射)率。笔者初步实现该复合表面的制备,对样品进行光谱测试表明其在太阳辐射波段和"大气窗口"波段的平均吸收(发射)率分别为0.92和0.80,其余波段平均吸收(发射)率为0.55,具有一定的光谱选择性。通过实例计算,对该复合表面与单独太阳能集热表面、单独辐射制冷表面和理想复合表面之间在太阳能集热效率、辐射制冷平衡温度和辐射制冷功率等关键性能参数进行对比分析。     

Microstructure and optical properties of silicon nitride thin films as radiative cooling materials

SiN_X thin films were prepared by the RF plasma-enhanced chemical vapor deposition method. Composition, structure, surface morphology and optical properties of the thin films were analyzed by X-ray fluorescence, IR transmittance, IR reflectance and SEM. The results show that the composition of the films is SiN_0.35. Nitrogen atoms take part in the reaction with silicon atoms and Si–N bonds are formed. There are also some Si–H and N–H bonds in the films. The films have very low hemispherical IR reflectance across the full 8–13 μm band and high hemispherical reflectance elsewhere, which indicates that silicon nitride films can be used as good radiative cooling materials. The surface morphology and growth mechanisms of the films were also explained.     

A robust convection cover material for selective radiative cooling applications

The enhanced cooling of exposed surfaces by radiative heat loss to the cold sky was investigated using a variety of commonly available materials. Zinc sulphide was identified as a durable substance suitable for the construction of convection covers for radiative cooling radiators. With respect to polyethylene, the most commonly used convection cover to date, the new material is mechanically stronger, impervious to damage by solar ultraviolet and in practical thicknesses is more transparent in the 8-14 μm waveband. Use of this window material with a previously proposed selective radiator material, a form of anodised aluminium that reflects radiation at wavelengths shorter than 8 μm allows for the economical production of an effective selective radiator system. Measurements were made on simple radiator plates and convection covers.     

Materials for radiative cooling to low temperature

The clear sky can act as a heat sink. Cooling to low temperatures is possible with materials which are strongly emitting in the 8–13 μm band and non-absorbing elsewhere. In this paper we discuss the resource for radiative cooling and its implementation with thin solid films (SiO_0.6N_0.2 coatings on Al) and with slabs of certain gases (C₂H₄, C₂H₄O and NH₃ backed by Al). Results are given on spectrophotometric infrared reflectance and transmittance, computed parameters which govern the predicted cooling performance, and some preliminary field tests.     

Morphology-Driven Emissivity of Microscale Tree-like Structures for Radiative Thermal Management

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.     

Solution of radiative transfer in a one‐dimensional anisotropic scattering media with different boundary conditions using the DRESOR method

The DRESOR method was applied to analyze the radiative transfer process in anisotropic scattering media with different boundary conditions in this paper. The method was validated by the integral formulation of the radiative transfer equation at first. Some variation regulations about the emissivity were obtained by extensive numerical simulations. When the optical thickness of the media became very large, the emissivity converged to a constant value. The converged emissivity in the forward scattering medium was the largest and that for the backward scattering medium was the smallest. Also the converged emissivity was associated with the scattering albedo of the media. The greater the scattering albedo was, the smaller the converged emissivity was. When the scattering albedo decreased to zero the converged emissivity reached the blackbody emissivity at the same temperature. Furthermore, different boundary conditions were considered. The results showed that if the temperature of the medium and the boundary was equal, the intensity at boundary was the same as that for the blackbody emission at the same temperature, whether the boundary reflectivity was 1.0 or not. When the temperature of the boundary was lower than that of the medium, the boundary emissivity can reach 1.0 only if ρ=1.0. Finally, the radiation flux was studied with different phase functions and different boundary conditions. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(3): 138–152, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20198     

电共沉积InGaAs薄膜材料

阐述了电共沉积方法制备发射光波长近于1.3-1.5um的InGaAs薄膜材料,用能谱分析仪进行薄膜成分分析,用分光光度计和单色仪测量薄膜的透射率,同时测量了薄膜I－V特性,导电类型,厚度及其表面形貌,分析结果表明该方法是半导体薄膜材料制备的一种新途径。     

Radiative transfer in an absorbing-scattering slab bounded by emitting and reflecting surfaces

An exact integral theory of the planar radiative transfer with isotropic scattering and general boundary conditions is presented in the paper. The analytical solution to the problem is numerically processed for two different specializations of the emissivity and reflectivity properties of the bounding surfaces. Results are given for the total and angular radiation intensities as well as for the net radiative flux.     

Thermal analysis of a prototype to determine radiative cooling thermal balance

The paper describes the operation and thermal analysis of a new prototype to determine thermal balance of radiative cooling. The aim of the study is to establish a simple but accurate procedure to calculate the radiative heat exchange between two bodies to be used in the determination of sky temperature, clear sky index or plate emissivity. The radiative transfer calculation has been based on the power required to maintain a constant temperature at the radiative plate of the prototype, provided that the convection and conduction terms are well known. The methodology is applicable to a further advanced system, which avoids convection and reduces conduction to maximise radiative effects, minimising errors and providing more accurate results. Tests have been carried out at a plate temperature of 40 °C, which was optimum for the prototype size and operating conditions. Higher or lower temperatures have lead to a reduction of the fraction of radiative transfer or to the use of very low external power, which complicates the prototype design and makes the temperature control system very costly. Tests carried out in the prototype have shown a perfect matching between total heat transfer and supplied power with an error of less than 5%. This result allows high precision determination of sky temperature, clear sky index or plate emissivity from the use of the proposed methodology.     

Emissivity characteristics of polished aluminum alloy surfaces and assessment of multispectral radiation thermometry (MRT) emissivity models

Emissivity characteristics were measured for several polished aluminum alloy samples over the spectral range of 2.05-4.72 μm and temperatures of 600-800 K. Overall, aluminum alloys buck the general trend of increasing emissivity with increasing temperature for metallic surfaces in the infrared range. Only AL 7150 follows the expected trend, while the emissivity of the other alloys decreases between 600 and 700 K and increases between 700 and 800 K, and the emissivity of commercially pure aluminum (AL 1100) decreases monotonically with increasing temperature. The experimental results are used to assess the accuracy of popular multispectral radiation thermometry (MRT) emissivity models for temperature measurement. It is shown that drastic changes in the shape of emissivity distribution preclude the use of a single function to accurately represent every band of the measured spectrum. Better predictions are achieved using the simplest form of MRT emissivity models and minimum number of wavelengths required by the model. Two relatively simple models are identified for best overall predictions for different alloys and temperatures. Despite the relative success of these two models, this study clearly demonstrates that improvements are required in both instrumentation and emissivity models to achieve acceptable accuracy in the implementation of radiation thermometry in the aluminum industry.     

Inverse boundary design of a radiant furnace with diffuse-spectral design surface

An optimization technique is applied to inverse design of radiative furnaces with diffuse-spectral surfaces. The variation of emissivity with respect to the wavelength is approximated by considering a set of spectral bands with constant emissivities and then the radiative transfer equation is solved by the net radiation method for each band. The conjugate gradient method is used for estimation of temperatures over reflector and heater surfaces. The sensitivity problem is approximated by differentiation of the radiative transfer equation with respect to the unknown variables. The performance of the present method is evaluated by comparing the results with the results obtained by considering a diffuse-gray design surface.