Fabrication and optimisation of highly efficient cermet-based spectrally selective coatings for high operating temperature

In terms of both high photo-thermal efficiency and high stability, multi-layer structures based on metal-dielectric composites (cermet) can be considered the most attractive selective solar absorbers for receiver tubes operating at medium-high temperatures in the field of solar thermodynamic plants. The double cermet layer approach represents a very simple fabrication method and can give high performances in terms of high solar absorptance and low hemispherical emittance. Optimised solar coatings based on cermet layers were fabricated in our laboratories by sputtering technique following the double layer approach. The joined employment of ellipsometric measurements and optical simulation is proposed as an effective method to optimise and fabricate coatings showing the best performances at a fixed operating temperature of the receiver tube. Interesting results concerning an optimised spectrally selective coating are shown. Solar absorptance higher than 0.94 and hemispherical emittance at 580 °C lower than 0.13 were obtained.     

A new dynamic technique for the measurement of hemispherical total emittance

A new dynamic technique for the measurement of thermal conductivity under development at the IMGC requires accurate values of heat capacity and of hemispherical total emittance at high temperature. Until recently, these data were provided by subsecond pulse heating experiments performed on the same specimens in the same apparatus. The pulse heating technique is the most accurate method for the determination of heat capacity at high temperatures, but because of various experimental problems, the accuracy of hemispherical total emittance determinations is limited to 5%. A new method for a more accurate determination of hemispherical total emittance is proposed, which uses the same experimental data available from thermal conductivity experiments. An analysis of the temperature profiles measured during the free cooling indicates that regions with high-temperature gradients (toward the ends of the specimen) are the best regions for thermal conductivity measurements, while regions with low-temperature gradients (at the center of the specimen) are the best regions for hemispherical total emittance determinations. The new measurement method and some preliminary results are presented and discussed.Paper presented at the Second Workshop on Subsecond Thermophysics, September 20–21, 1990, Torino, Italy.     

Measurement of thermophysical properties by a pulse-heating method: Niobium in the range 1000–2500 K

Data for the heat capacity, electrical resistivity, hemispherical total emittance, and normal spectral emittance (at 898 nm) of niobium are reported for the temperature range 1000–2500 K. Measurements were based on a subsecond pulseheating technique. The results are discussed and compared with the literature values. Reported uncertainties for the properties are 3% for heat capacity, 1% for electrical resistivity, 5% for hemispherical total emittance, and 4% for normal spectral emittance.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

用稳态卡计法对热控材料半球发射率的测试研究

利用稳态卡计法设计了一种测试材料半球发射率的试验装置，并对其进行了不确定度分析。使用该装置对二次表面镜（OSR）的室温半球发射率进行了测试，测试结果不确定度小于2．00％。同时，使用该装置测试了OSR与一种新型相变材料在190～350K间的半球发射率。     

热反射隔热涂料的制备与隔热性能

系统研究了成膜基料、隔热颜料、空心玻璃微珠含量、涂层厚度以及耐玷污透明涂层各种因素对热反射隔热涂料隔热效果和隔热持久性的影响及作用规律。结果表明,以丙烯酸酯乳液为成膜基料,二氧化钛质量分数为20%,空心玻璃微珠含量为11%,涂层厚度为1.2mm时并辅助含氟硅的耐玷污透明涂层,制备出的热反射隔热涂料的太阳光反射率为91%,半球发射率为86%,均超过了JG/T235-2008《建筑反射隔热涂料》行业标准中的技术指标,且具有良好的自清洁性能,项目成果实现了产业化。     

水性建筑反射隔热涂料的研制

利用纯丙和苯丙乳液为基料,制备了具有热反射和低导热协同作用的隔热涂料。通过对颜填料太阳光反射率和隔热性能的研究,选择以空心玻璃微珠和金红石型二氧化钛为颜填料,空心微珠的最优含量为4%～10%(wt)。研制建筑反射隔热涂料基础性能优异,可见光反射比达到90%以上,半球发射率大于85%,隔热指数大于60%。     

Computational design of solar reflection and far-infrared transmission films for a variable emittance device

A smart radiation device (SRD) that is a variable emittance radiator has been studied as a method of thermal control for spacecraft. The SRD consists of manganese oxide with a perovskite-type structure, and the total hemispherical emittance of the SRD changes considerably depending on temperature. Here we propose an optimal method of designing multilayer films for the SRD by using agenetic algorithm. The multilayer films reflect solar radiation and transmit far-infrared radiation to maintain variation of the infrared optical properties of the SRD.     

Specific heat and electrical resistivity of niobium measured by subsecond calorimetric technique

This paper presents results of measurements of specific heat and electrical resistivity of niobium from ambient temperature to the experimental limit of the equipment which is close to 2500 K. The study used a contact thermometry variant of the millisecond resolution pulse calorimetry developed at the Institute of Nuclear Sciences VINA. In the experiments exceeding 1000 K, thermocouple thermometry was supplemented with parallel pyrometric temperature measurements. This, together with application of tungsten; rhenium thermocouple thermometry, increased the temperature range of measurements to 2500 K. In the range where two thermometries overlap, data on the specimen emittance were also generated. Novelties in the method, the results on electrical resistivity. specific heat, hemispherical total emittance and normal spectral emittance of niobium, and accuracies attained in different property measurements are discussed.     

Efficiency and behavior of textured high emissivity metallic coatings at high temperature

Three metallic coatings with textured surfaces, made of rhenium, tungsten and molybdenum, were studied in the frame of the Solar Probe Plus mission (NASA) as candidate materials. The role of these coatings is to dissipate a maximum of energy from a hot instrument facing the Sun, by the mean of their high total hemispherical emissivity. The total hemispherical emissivity of the three coatings was measured in the temperature range 1100–1900 K, as well as over time in order to study their high temperature stability. Various emissivity levels were obtained depending on the surface texture. The highest total hemispherical emissivity was obtained on a rhenium coating, with an emissivity of 0.8 in the temperature range 1300–1700 K. However, this rhenium coating with a fine, sharp surface texture, presented an instability at high temperature, which might limit its optimal operating temperature to about 1500 K. As for the tungsten coating, the total hemispherical emissivity was increased by a factor 2 due to the enhanced surface texturation and its great stability over the whole temperature range was shown.     

玻璃型镀铝二次表面镜的模拟空间环境辐照试验研究

主要通过对镀铝二次表面镜先后进行紫外辐照以及电子、质子的综合辐照,并结合太阳吸收率和半球发射率测试,研究了低能电子、质子以及紫外辐照对镀铝二次表面镜热控性能的影响。结果表明,镀铝OSR具有良好的抗紫外辐照以及抗低能电子、质子辐照能力。     

The influence of the oxidation time on the optical properties of the ceramic thermal control coating prepared by micro-plasma oxidation

The ceramic thermal control coating was produced in situ growth on the aluminum alloy by micro-plasma oxidation method. The coating possessed stable optical properties, high microhardness, and excellent adhesive power, low cost and so on, which can be widely used in the thermal control system of the spacecraft. In this article, the influence of oxidation time on the solar absorptance and emittance of the coating had been studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the phase composition and microstructure of the coating. The results show that the coating that is mainly composed of the α-Al₂O₃ and γ-Al₂O₃ phase has porous surface configurations. The coating can reach to a solar absorptance value higher than 0.91 and an infrared emittance value higher than 0.76, which can be used as a kind of important thermal control coating for space applications.     

A New Spectrophotometer System for Measuring Hemispherical Reflectance and Normal Emittance of Real Surfaces Simultaneously

A new spectrophotometer system is developed for the study of thermal radiation characteristics of real surfaces in thermal engineering environments. The system measures spectra of normal incidence hemispherical reflectance R _NH and normal emittance ${\varepsilon _{\rm N}}$ in the near-ultraviolet through infrared region of wavelength of 0.30 μm to 11 μm simultaneously and repeatedly with a cycle time of 4 s. The system enables evaluation of the normal incidence absorptance A _N in this wide spectral region. Transitions of spectra of specular-finished and rough-finished nickel surfaces in a high-temperature air-oxidation process are measured to demonstrate the performance of the system. Clear interference behaviors are found even in the spectra of hemispherical reflectance R _NH and emittance ${\varepsilon _{\rm N}}$ of a rough-finished surface.     

涂料型La1-xSrxMnO3智能热控涂层的研究

以固相反应法制备了高纯度La0.8Sr0.2MnO3粉体,并以其为基料,磷酸二氢铝为粘结剂,采用涂覆工艺在铝基片上制备了涂料型La0.8Sr0.2MnO3热控涂层.采用XRD、EDS对La0.8Sr0.2MnO3粉体的成分进行了表征,用稳态卡计法测量了涂层在100~100℃温度区间内热辐射率随温度的变化,并测量了涂层的太阳吸收比.研究结果表明:粉体合成过程中,经过1200℃三次热处理制备的La0.8Sr0.2MnO3粉体纯度高,合成的粉体具有均匀的微米级粒径尺寸.通过适当调整浆料中La0.8Sr0.2MnO3粉体所占质量百分比,获得辐射率变化范围大于0.3的热控涂层,该性能与采用烧结工艺制备的La0.8Sr0.2MnO3陶瓷片材料在变温条件下的辐射率变化范围接近.该涂层在航天器热控技术中具有潜在的应用前景.     

Thermophysical measurements on low carbon 304 stainless steel above 1400 K by a transient (subsecond) technique

Simultaneous measurements, by a subsecond duration transient technique, to determine the specific heat capacity, c _p , the electrical resistivity, ρ, and the hemispherical total emittance in the temperature range 1400–1700 K, and the melting point and the radiance temperature at the melting point, of AISI type 304L stainless steel are described. The results are expressed by the relations: $$c_p = 1127{\text{ }} - {\text{ }}7.265{\text{ }} \times {\text{ }}10^{ - 1} {\text{ }}T{\text{ }} + {\text{ }}2.884{\text{ }} \times {\text{ }}10^{ - 4} {\text{ }}T^2$$ $$\rho = 75.59{\text{ }} + {\text{ }}4.695{\text{ }} \times {\text{ }}10^{ - 2} {\text{ }}T{\text{ }} - {\text{ }}9.592{\text{ }} \times {\text{ }}10^{ - 6} {\text{ }}T^2$$ where c _p is in J · kg^?1 · K^?1, ρ is in ΜΩ · cm, and T is in K. The value of the hemispherical total emittance is 0.37 in the range 1700–1900 K. The melting point and the radiance temperature (at 653 nm) at the melting point are 1707 and 1590 K, respectively, yielding a value of 0.385 for the normal spectral emittance at the melting point. Estimated inaccuracies of the measured properties are: 3% for the specific heat capacity, 2% for electrical resistivity, 5% for hemispherical total emittance, and 8 K for melting point and radiance temperature at the melting point.     

Infrared emittance of medium containing nonsphere-shaped particles

The scattering and absorption sections of cylinder- and sheet-shaped particles in a binder have been investigated. The IR emittance of a coating layer composed of randomly arrayed particles embedded in the binder has been calculated in terms of the size of the particles, the volume fraction occupied by the particles, and the complex refractive indices of the particle and the binder as well as the emittance of the substrate. The calculation shows that the inhomogeneous medium including sheet-shaped particles can have lower emittance than that including cylinder- or sphere-shaped particles.     

Thermal Emittance of $$\hbox {La}_{0.7}\hbox {Ca}_{0.3-x}\hbox {K}_x\hbox {MnO}_3$$ Coatings on Aluminum Substrate

A novel thermal control coating was presented based on the thermochromism of manganite. The pigment of K-doped manganite nanoparticles was dispersed into polymer matrix to prepare the coating with curing below 200 \(^{\circ }\)C. The nanoparticles size mainly distributes around 100–200 nm, and it shows a comparable stoichiometric ratio. The phase transition of the nanoparticles was observed from ferromagnetic metallic to paramagnetic insulator state. With increasing K doping level, the phase transition temperature increases, achieving controllable adjustment. Coating surface with and without pore defect was obtained by different polymer matrix. A sharp emittance variation was observed with increasing temperature in K-doped coating. The variation magnitude of emittance is up to 0.46, which is attractive to space thermal control. It is suggested that the pigment content of 50 wt% is sufficient to realize a large emittance variation.     

Development and Characterization of Low-Emitting Ceramics

The infrared-optical properties of ceramics are correlated with the complex index of refraction of the material and the structure of the ceramic. By changing these parameters, the infrared-optical properties can be changed over a relatively wide range. The correlation of the structural properties (like the porosity or the pore sizes) and the material properties (such as the complex index of refraction on the one hand and the infrared-optical properties such as emittance on the other) are described by a solution of the equation of radiative transfer and the Mie-theory. Within this work, low-emitting ceramics, which have significantly lower emittances than conventional ceramics, were prepared by optimizing their composition and structure. The spectral emittance of these ceramics was measured, and a total emittance dependent on temperature was calculated from the spectral emittance. As a result, one obtains ceramics which have a total emittance of 0.2 at a temperature of 1,100 K. In comparison to conventional ceramics with a typical total emittance of 0.8 at 1,100 K, the use of such low-e ceramics leads to a reduction in heat transfer of about 70% via thermal radiation. The results of our calculations were compared with experimental data to validate the theory. Paper presented at the Seventh European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Spray-pyrolysed cobalt black as a high temperature selective absorber

Cobalt oxide coatings with an integrated solar absorptance of 0.93 and a hemispherical emittance at 100°C of 0.14 were prepared by spray pyrolysis on stainless steel substrates kept at 300°C. The coatings are strongly adherent and stable up to temperatures of about 600°C. Auger electron spectroscopy, X-ray photoelectron spectroscopy and X-ray and electron diffraction investigations revealed the coatings to be composed of an upper layer of Co₃O₄ and subsequent layers of CoO down to the substrate. The absorption mechanism is primarily intrinsic. Lower substrate temperatures (about 150°C) can be used for preparing coatings from equimolar aqueous solutions of cobaltous acetate and thiourea. Such coatings, containing both cobalt oxide and cobalt sulphide, have a comparable absorptance but a higher emittance and are stable only up to about 300°C.     

Absorbeurs selectifs de l'energie solaire elabores a partir d'acier inoxydable : Influence de la composition chimique et de l'etat metallurgique de l'alliage sur les proprietes optiques

Efficient photothermal converters exhibit high solar absorptance and low thermal emittance. An original method has allowed spectrally selective coatings to be obtained by chemical conversion of stainless steel. These coatings exhibit high selectivity (absorptance higher than 95 % and emittance lower than 20 %) and excellent durability.The dependance of the optical properties of these coatings on the chemical composition and metallurgical state of the substrate is studied. Low emittance appeared to be closely related to the corrosion resistance of the metallic surface in the treatment bath. Owing to this fact, convenient conditions can be found to produce selective coating for each alloy.     

Proton Irradiation Effects on Thermophysical Properties of High-Thermal-Conductivity Graphite Sheet for Spacecraft Application

This paper describes an evaluation of the proton irradiation effects on the thermal characteristics of a pyrolytic graphite sheet (PGS), which has characteristics of high thermal conductivity, light weight, and flexibility, in order to apply this material to an advanced spacecraft thermal control device, the reversible thermal panel (RTP). The results show slight changes in the in-plane thermal diffusivity and total hemispherical emittance of the PGS for 2.0 MeV proton irradiation. An RTP prototype model based on the PGS was designed and fabricated, and its thermal performance was evaluated. The effects of changes in thermal characteristics of the PGS on the thermal performance of the RTP were also discussed.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–26, 2004, Hefei and Huangshan, Anhui, P. R. China.