太阳能光谱选择性吸收涂层研究进展

全面阐述了5种太阳能光谱选择性吸收涂层的作用机理和制备方法,综述了国内外该领域的研究工作和最新研究成果,并对选择性吸收涂层的发展方向进行了探讨。     

Solar selective black molybdenum coatings by chemical conversion

Solar selectivity, with emphasis on a high solar absorptance and a low thermal emittance, was measured for black molybdenum films prepared by a chemical conversion process. The best values of the solar absorptance (0.88) and the thermal emittance (0.20) were achieved by optimizing the preparation parameters, e.g. the bath composition, the deposition temperature and the immersion time. The films deposited onto galvanized steel sheet and zinc-plated mild steel sheet show high adherence and good thermal stability at elevated temperatures (250 °C).     

Cobalt oxide-iron oxide selective coatings for high temperature applications

Cobalt oxide-iron oxide coatings on stainless steel have been prepared by spray pyrolysis technique. These coatings have absorptance (α) = 0·94 and emittance (ε ₁₀₀) = 0·20 which are much better values than for cobalt oxide or iron oxide alone on stainless steel substrates. The coatings have been found to be stable for temperatures upto 400°C.     

Optically selective coatings of Ti-N compounds obtained by ion-stimulated sorption

Results of investigations of the reflectance of titanium nitride films, produced at a temperature of 50°C by the ion-stimulated sorption method, are presented in this paper. An estimation of the spectral selectivity of the films was made and the coefficient of relative reflection in the wavelength region 0.2–2.5 μm was determined. A comparison of the reflectivity of these films with those of films produced by chemical and physical vapour deposition methods at high temperatures (200–1000°C) indicates the possibility of obtaining efficient selective-reflection coatings at near-normal temperatures, which is of interest for some technological applications.     

选择表面工艺改性的CIPs涂层及其氧化物的吸波性能

针对吸波涂层氧化腐蚀现象,提出一种基于选择表面的周期结构涂层维修工艺。利用腐蚀法,在以羰基铁粉(CIPs)为吸收剂的吸波涂层表面制备CIPs及其氧化物涂层。用扫描电子显微镜(SEM)对腐蚀后颗粒的形貌进行了分析。测试了8~18GHz下混合颗粒的复介电常数和复磁导率,并用等效介质理论计算氧化涂层的参数。分析腐蚀和维修工艺对反射损耗(RL)的影响。结果表明:当CIPs涂层表面被氧化时,随着氧化涂层厚度的增加,涂层的吸波性能减弱,反射损耗增量值约为2dB。当涂层厚度为0.8mm时,维修效果不理想,若涂层厚度增加至1mm,且氧化涂层厚度为0.1mm时,维修效果较好,体现为10~18GHz频率范围内的吸波带宽增加,反射损耗相应减小约为2dB。     

Influence of solvents on properties of solar selective coatings obtained by spray pyrolysis

Solar selective coatings for solar thermal flat-plate collectors consisting of crystalline copper oxides and amorphous nickel oxide composites were obtained by robotic spray pyrolyzed deposition. The parameters were optimized for increased spectral selectivity (S): high solar absorptance and low thermal emittance. The coatings were deposited using nickel and copper acetate, dissolved in mixed solvents with various water: ethanol ratios. The coatings’ properties were characterized in terms of crystalline composition (XRD), surface morphology (AFM, contact angle) and optical properties (solar absorptance, thermal emittance and spectral selectivity). Considering the precursor solutions composition (solvent, wetting behaviour), the growth processes were modelled for two different systems: predominant hydrophilic and predominant hydrophobic. The high selectivity values (S > 30) of the optimized composite coatings were explained based on two parallel mechanisms: intrinsic absorption and multiple reflections generated when absorbers with controlled roughness are deposited.     

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.     

Preparation of solar selective absorbing coatings by magnetron sputtering from a single stainless steel target

This paper presents a method to prepare solar selective absorbing coatings by radio frequency magnetron reactive sputtering using a single stainless steel target. Stainless steel/stainless steel nitride (SS/SS-N) ceramic-metal composite (cermet) thin films were produced under varied nitrogen gas flow ratios. The solar selective absorbing films have good solar absorptance of 0.91 and thermal emittance of 0.06 at 82 °C. The refractive index (n) and extinction coefficient (k) of the cermet composite layers prepared in nitrogen and argon atmospheres were determined by spectroscopic ellipsometry. The films were also analyzed by different oscillator models. The results showed a significant transformation from metal to cermet as the nitrogen gas flow ratio was increased to 10%. As the nitrogen gas flow ratio was increased to 17.5%, the film became a dielectric layer that could be used as an anti-reflection layer, suitable as the outermost layer of the solar selective absorbing coatings. A theoretical solar absorptance of 0.92 was achieved by selecting an appropriate combination of three solar absorbing layers. The experimental results agreed well with the theoretical calculations. This study proved the possibility of preparing solar selective absorbing coatings with high solar absorptance by using a single stainless steel target.     

Investigation of solar selective and microstructural properties of molybdenum black immersion coatings on cobalt substrates

Molybdenum black solar selective coatings have been produced on cobalt by immersion in a solution of ammonium paramolybdate and nickel sulphate. The cobalt was electroplated on nickel-plated copper prior to immersion in the paramolybdate solution. The maximum solar absorptance of the resulting molybdenum black coating was about 0.91. The minimum emittance was about 0.1 for coatings on cobalt deposited with an addition agent and about 0.23 for coatings on cobalt deposited without addition agent in the 60 °C plating solution. These differences have been related to the coating morphology determined by scanning electron microscopy. The emittance of coatings on cobalt deposited without addition agent decreases (or remains unchanged) during short-term heat treatment while that of coatings deposited on cobalt plated with addition agent increases somewhat. Reduction of the cobalt plating bath temperature to 45 °C can also lead to good initial coating properties but without the requirement for an addition agent in the cobalt plating bath. XPS studies show that the oxidation state of molybdenum in the coatings is approximately + 5 corresponding to Mo₄O₁₁. This reduces to + 4 after argon ion bombardment. Some cobalt may be present in the coatings in the form of CoO.     

Structured fluid theory

Summary The simplest case of structured fluid theory is shown to correspond to a type of Maxwell fluid which exhibits normal stress differences. A nonlinear problem related to the deformation of suspended liquid drops as induced by inertial effects is solved and the influence of structured fluid parameters investigated.

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Theorie strukturierter Flüssigkeiten

Zusammenfassung Es wird gezeigt, daß der einfachste Fall einer strukturierten Flüssigkeit einer Art Maxwellscher Flüssigkeit entspricht, welche Differenzen im Betrag der Normalspannungen aufweist. Ein nichtlineares Problem, welches mit der durch Trägheit verursachten Deformation eines Tropfens zusammenhängt, wird gelöst. Der Einfluß der Parameter der strukturierten Flüssigkeit wird untersucht.

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With 2 Figures     

Electrodeposition of black chromium spectrally selective coatings from a Cr(III)-ionic liquid solution

In the present study, black chromium coatings were electrodeposited from a 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]) solution containing Cr(III) under potentiostatic control on copper substrates. The electrochemical behavior of the electrolyte was studied by cyclic voltammetry. The reduction of Cr(III) species in the solution is a two-step process, from Cr(III) to Cr(II) and from Cr(II) to Cr(0). The potential of each reaction shifts positively with the increase of the electrolyte temperature. The parameters for the electrodeposition of black chromium films were optimized. Homogeneous black chromium films with spectrally selective optical properties were produced by applying a potential of −1.5 V for 1800 s, at an electrolyte temperature of 85 °C. The coatings consist of a mixture of chromium oxide/hydroxide and metallic chromium. They are amorphous and present a sub-micrometric granular structure.     

Design and manufacture of spectrally selective reflecting coatings for the use with laser display projection screens

To provide screens for laser projection that improve contrast, a spectrally selective reflecting filter was designed by using genetic algorithms to overcome the problem of unknown starting values. Colormetrics rather than fixed targets were used for evaluation. Various selective filters were deposited upon glass as well as upon solid and flexible plastic substrates by reactive mid-frequency magnetron sputtering. For process control, in situ spectroscopic ellipsometry was applied.