Silicone-based thickness insensitive spectrally selective (TISS) paints as selective paint coatings for coloured solar absorbers (Part I)

The objective of this study was to make paints having a variety of colours and whose spectral selectivity would be independent of the thickness of the deposited layer of paint (thickness insensitive spectrally selective (TISS) paint coatings). TISS paint coatings combine the advantages of paints (longevity and chemical resistance achieved by a high thickness of the applied layer, variety of colours and simple application) with spectral selectivity. Low emittance is attained by the addition of bare aluminium, coloured aluminium flake pigments or copper flake pigments, while other inorganic pigments impart various colours to the paints. Pigments were dispersed in silicone resin binder imparting the TISS paint coatings high-temperature tolerance, excellent adhesion, uv resistance, flexibility and weather-durability, which make them suitable coatings for coloured glazed or unglazed solar absorbers.     

Selective paint coatings for coloured solar absorbers: Polyurethane thickness insensitive spectrally selective (TISS) paints (Part II)

Red, green and blue paints were prepared for use as thickness insensitive spectrally selective (TISS) paint coatings for solar façade absorbers. The paints were composed of a polyurethane resin binder in which various pigments were incorporated in such a way that they formed stable paint dispersions, satisfying stability criteria for façade coatings. A low emittance of the paints was achieved by using low-emittance aluminium flake pigments combined with iron oxide (red coloured paints). Black pigment was added to adjust solar absorptance. Blue and green paints were made by the addition of coloured aluminium flake pigment and the solar absorptance was also adjusted by the addition of black pigment. Efficiency for photo-thermal conversion of solar radiation was assessed by evaluation of the corresponding performance criteria, which enabled the selection of paints whose performance criteria values were higher than 0 (spectrally non-selective black coating). The results confirmed that blue and green paints and to minor extent red ones, combined selectivity with colour. The morphology of the paints was assessed, revealing that the colours originated from the deposition of finely dispersed colour and/or black pigment on the surface of the aluminium flakes during paint preparation.     

Spectrally selective paint coatings: Preparation and characterization

Preparation and characterization of spectrally selective paint coating for photothermal solar energy conversion are discussed. The applied methods for preparation of paints with described measurements and calculations of black-pigmented coatings were reviewed. The article represents not only possible future applications but also past and current applications of spectrally selective paint coating which are used all over the world since the 1980s. Spectrally selective paint coatings based on combinations of two types of resins, various types of pigments and three types of silica, were prepared. The influence of pigment type and pigment volume concentration (PVC) was studied by applying the Kubelka–Munk (K–M) theory. The relation between the degrees of dispersion and distribution of pigment particles across the paint layer is discussed in terms of K-M coefficients.     

Thermal stability and cross-linking studies of diisocyanate cured solar absorptance low-emittance paint coatings prepared via coil-coating process

Thickness-sensitive spectrally-selective (TSSS) paint coatings were prepared by using FeMnCuO_x pigment in combinations with phenoxy and silicone resins. The spectral selectivity expressed as a ratio between solar absorptance (a_s) and thermal emittance (e_T) was 0.92/0.14 for the first and 0.87/0.18 for the second type of the paint. The surface of phenoxy resin based coatings is characterized by pigment particles protruding from the surface, while silicone based paints exhibit a much smoother surface. Abrasion resistance and adhesion of both types of coatings were enhanced by cross linking the resin binder with diisocyanate hardener. Cured coatings withstand temperatures up to 135°C. Cross-linking and degradation mechanisms of both types of coatings were studied by using FT-IR spectroscopy and thermogravimetric measurements, cycling and ageing tests.     

Adhesion and thermal stability of thickness insensitive spectrally selective (TISS) polyurethane-based paint coatings on copper substrates

Thickness insensitive spectrally selective (TISS) paint coatings based on a polyurethane polymeric binder deposited on copper substrates were investigated to obtain information about their service lifetime. The degradation of TISS paint coatings was performed according to the methodology worked out within Task 10 of the IEA's Solar heating and the cooling programme. The activation energy (E_a) for the degradation process was derived from vibrational band changes of the polyurethane binder recorded in the infrared hemispherical reflectance spectra of TISS paint coatings exposed to different thermal loads. The results of the vibrational band analysis were correlated with cross-cut tests, showing that the coatings started to lose integrity at 190 °C but protected the copper substrate against oxidation perfectly even at 200 °C (15 days). An accelerated test procedure confirmed that TISS coatings could be safely used in solar collectors for at least 45 years.     

Polyhedral oligomeric silsesquioxane trisilanols as pigment surface modifiers for fluoropolymer based Thickness Sensitive Spectrally Selective (TSSS) paint coatings

Thickness Insensitive Spectrally Selective (TISS) paint coatings based on black pigment (PK 3060, Ferro Company) dispersed in a fluoropolymeric resin binder (Lumiflon, Asahi Company, Japan) have recently been made without added aluminium flakes and their properties have been reported for the first time. In this study we investigated in more detail the effect of trisilanol isobutyl (IB₇ T₇(OH)₃) polyhedral oligomeric silsesquioxane (trisilanol POSS) on the surface modification of PK 3060 pigment. Infrared spectral analysis of the surface modified pigment particles provided firm evidence for the formation of a POSS layer on the surface of the pigment particles, substantiated by the corresponding TEM and Energy Dispersive X-ray Spectroscopy (EDXS) measurements of functionalized and as-received pigments. SEM micrographs of the diluted dispersions in fluoropolymeric resin binder revealed uniform distribution of pigment particles with an average size of ∼300 nm and the beneficial effect of the pigment functionalization was assessed from the measured spectral selectivity of coatings of various thicknesses.     

Sol–gel derived CuCoMnOx spinel coatings for solar absorbers: Structural and optical properties

A novel black coloured coating with the composition CuCoMnO_x was prepared using sol–gel synthesis. The coatings were deposited using the dip-coating technique from alcoholic sols based on Mn-acetate and Co- and Cu-chloride precursors. Thermogravimetric analysis showed that xerogels become crystalline at 316°C while X-ray diffraction analysis revealed that the coatings and powders correspond predominantly to CuCoMnO_x spinels. Rutherford back scattering (RBS) and transmission electron microscopic (TEM) studies combined with energy dispersive X-ray spectroscopy (EDXS) measurements confirmed that Cu, Mn and Co are present in the films in stoichiometric ratios close to that in the initial sols. IR spectroscopy has been employed to study the formation of sols by following the changes in the vibrational bands of the acetate groups during both thermal hydrolysis and the ageing of sols to xerogels. It was found that ageing of xerogels was accompanied by the formation of −COO⁻ bridging units, which at 250°C are no longer visible in the IR spectra but substituted by the vibrational modes characteristic for CuCoMnO_x. The solar absorptance (a_s) and thermal emittance (e_T) of the coatings when deposited on an Al-substrate are a_s=0.9 and e_T=0.05, which rank deposited black sol–gel CuCoMnO_x spinels among the promising candidates for spectrally selective absorber coatings for solar collectors and solar facades.     

Characterisation of high-temperature-resistant spectrally selective paints for solar absorbers

The spectrally selective paint coatings were prepared from organically modified siloxane resin and inorganic pigment (FeMnCuO_x-P320). To optimise the low-emittance properties, different thicknesses of paints were applied on high-reflecting Al foil by a draw bar coater. For all paints, optical and thermal properties were determined as well as their adhesion resistance. Pigment to volume concentration ratio was 20% and for thicknesses of about 1.7–2.0 g/m², the solar absorptance for these samples were a_s=0.90–0.92 with corresponding thermal emittance of e_T=0.20–0.25. Temperature stability of these samples was followed by FT-IR spectroscopy at 300°C. The obtained results indicated good temperature stability of prepared paint coatings.     

Coloured paints based on iron oxide and silicon oxide coated flakes of aluminium as the pigment, for energy efficient paint: optical and thermal experiments

Energy efficient coloured paint coatings utilising flaky aluminium pigment with either single layer (Fe₂O₃) or double layer (Fe₂O₃ on SiO₂) interference coatings are optically and thermally characterised. Similar pigments with coatings on flaky dielectric particles and standard paints of similar colour are compared. Data presented includes hemispherical and specular reflectance spectra across visible and infra red wavelengths, thermal emittance from an emissometer and light spreading data from a photogoniometer. Solar absorptance, and colour show that, as theoretically predicted, and provided flaky metal pigments are used, a wide range of colours combined with a much lower solar absorptance than traditional paints of similar colour is achieved. Superior thermal performance to ordinary paints with similar colour is thus possible and is demonstrated for two such coloured layers via controlled heating-cooling studies under both an indoor lamp and outdoor clear sky solar illumination. It is seen that heat gains from the sun and associated cooling loads can be reduced by up to 50% for most colours, which is of some benefit for cars and metal roofs in hot climates.     

Electrochemical deposition of black nickel solar absorber coatings on stainless steel AISI316L for thermal solar cells

We report the electrochemical deposition of nanostructured nickel-based solar absorber coatings on stainless steel AISI type 316L. A sol–gel silica-based antireflection coating, from TEOS, was also applied to the solar surface by the dip-coating method. We report our initial results and analyze the influence of the stainless steel substrate on the final total reflectance properties of the solar absorber. The relation between surface morphology, observed by SEM and AFM, the composition of the electrodeposited surfaces analyzed by X-ray powder diffraction and the study of different electrodeposition conditions and silica sol–gel coatings is described. The best solar absorptance and thermal emittance values obtained on stainless steel substrates were 0.91and 0.1, respectively.     

Spectral selectivity of black and green painted surfaces

Black, green and mixed paints were prepared from organically modified siloxane resin. The solar absorptance (a_s) of prepared black paint/metal coatings was 0.90 with corresponding thermal emittance (e_T) 0.20, whereas green paint coatings did not reach satisfactory solar absorptance (a_s<0.8O). To improve the absorptance of the green coating, the black paint was admixed into the green paint. Optical properties of the prepared coatings were determined by the help of Kubelka–Munk formalism.     

Development and investigation of sol–gel solutions for the formation of TiO2 coatings

We show a technological approach for the sol–gel processing of stabilized xerogel colloidal titanium oxide films. Glycerol was used as a drying control additive agent. Glycerol helped in stabilizing the solution. The thermal transformation of a xerogel film was studied by differential thermal analysis (DTA) and powder X-ray diffraction. The optical index of the annealed coatings was evaluated using UV–VIS–NIR spectrophotometry. The results showed that a nanocrystalline titania anatase film of high optical quality (n=2.34 at 600 nm) can be obtained by the sol–gel process.     

Life expectancy prediction and application properties of novel polyurethane based thickness sensitive and thickness insensitive spectrally selective paint coatings for solar absorbers

A novel Thickness Sensitive Spectrally Selective (TSSS PU B: a_s=0.90, e_T=0.20) paint coating on aluminium substrate was prepared from commercially available polyurethane binder (Binder B) (HELIOS TBLUS, SI) and black pigment (spinel (Mn–Fe)), in combination with trisilanol polyhedral oligomeric silsesquioxane (POSS), which served as pigment dispersant. Polyurethane resin binder B was selected because of its higher thermal stability (determined from thermogravimetric measurements (TG)) than polyurethane resin binder A, which has previously been used for making Thickness Insensitive Spectrally Selective (TISS PU A) coatings (a_s=0.90, e_T=0.38) deposited on copper absorbers (Kuni?, 2009 [36]).Thermal degradation of the TSSS PU B and TISS PU A coatings, both deposited on aluminium substrates, was studied by following, as close as possible, the methodology worked out within TASK 10 of the IEA's Solar and Heating Programme. Thermal load tests were performed in the temperature range from 170 to 200 °C at various time intervals (1, 6, 10, 15, 21 days). Degradation of the coatings was assessed using a variety of degradation indicators: changes of solar absorptance and thermal emittance determined from the hemispherical IR and VIS/NIR spectra, intensity changes of selected vibrational modes attributed to the polymeric backbone and ester and urethane linkages and combined with peel-off tests used as adhesion and cohesion indicators. The results revealed that degradation of the polyurethane resin binder was attributable to the breaking of the urethane linkages, also shown from the AFM and XPS spectra measurements. For the TISS PU A coating, the life expectancy was estimated to be 22.77 years (activation energy (E_a)=163.2 kJ/mol, T_eff=113.4 °C) while for the TSSS PU B coatings, it was at least 25.96 years (activation energy (E_a)=96 kJ/mol, T_eff=102 °C).     

Optical, thermal and structural characteristics of carbon nanoparticles embedded in ZnO and NiO as selective solar absorbers

Selective solar absorber coatings of carbon embedded in ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings. The surface morphology of the samples was studied by scanning electron microscopy (SEM). Cross-sectional high-resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Chemical composition analysis was done by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The crystal structure of ZnO and NiO samples was also investigated with an X-ray diffraction (XRD) technique. Samples were subjected to an accelerated ageing test for 95 h, with condensation at relative humidity of 95% and at a climate chamber temperature of 45 °C. The thermal emittances of the samples were 6% for the ZnO and 4% for the NiO matrix materials. The solar absorptances were 71% and 84% for ZnO and NiO samples, respectively. The SEM revealed a smooth featureless surface for both C–ZnO and C–NiO samples. Some C–NiO samples showed dentritic features. X-HRTEM, EDS and EELS studies revealed a nanometric grain size for both types of samples. The C–ZnO and C–NiO coatings contained amorphous carbon embedded in nanocrystalline ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The accelerated ageing tests produced performance criterion (PC) values of 0.15 and 0.054 at 95 h for the C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have better solar selectivity behaviour than the C–ZnO counterparts.     

Anti-reflection (AR) coatings made by sol–gel processes: A review

Traditionally, various vacuum-based processes, depending on material systems and properties, and chemical etching process have been used for producing different types of anti-reflection (AR) coating on different substrate materials. In this paper, the development of sol–gel derived AR coating on different substrates for various applications in the past 40 years are reviewed. These coatings possess good uniformity in thickness and properties which have met requirements for various applications. The major approaches to fabricate AR coating and their characteristics have been discussed. This paper outlines the major solution coating processes and design principles of AR coatings. Major fabrication processes used in AR coating technologies have been compared. Different solution chemistries developed for producing different materials for AR coating preparation have been extensively reviewed. The optical performance of different types of sol–gel-derived AR coatings have been summarized and comparison to the commercial AR coating produced by traditional technologies have been discussed. The sol–gel AR coating has been shown to possess comparable or superior performance to those produced by vacuum-based processes.     

Antireflective transparent covers for solar devices

Promising solutions for low-cost antireflective (AR) covers for solar receivers are based on the principle of mixing bulk material with air on a subwavelength scale in order to obtain very low effective refractive indices. Possibilities to achieve this are given by porous media and by periodic or stochastic subwavelength surface-relief structures. In this work, the mentioned approaches were investigated experimentally. Subwavelength surface-relief structures were embossed in organically modified sol–gel materials or in acrylic materials, and porous sol–gel coatings were produced by dip-coating. It is shown that the solar transmittance of a transparent cover can be improved by up to 6% with a porous sol–gel coating at normal incidence. This improvement of the solar transmittance increases with larger angles of incidence.     

Coloured paints based on coated flakes of metal as the pigment, for enhanced solar reflectance and cooler interiors: description and theory

Flakes of metal coated with thin dielectric films, when used as various coloured paint pigments, can reflect much more of the near infra red component of solar radiation than standard paints of equivalent colour, which makes them attractive for reducing cooling loads in cars and under metal roofs, while maintaining a wide range of decorative options. Spectral responses, solar absorptance and CIE colour co-ordinates are explored theoretically for a single layer, and two layers of different refractive index, on smooth aluminium flakes. Results in air, and in a clear matrix of refractive index close to that of the paint binder, are presented to highlight the colour shifts that are observed in a matrix when colour is due to interference. Thin film models show that the single layers have weak angle of incidence dependence while double layers have attractive variations in reflected colour while retaining their high NIR reflectance. This adds to decorative appeal. The origins and impact of diffuseness are briefly discussed.     

Sol–gel niobium pentoxide: A promising material for electrochromic coatings, batteries, nanocrystalline solar cells and catalysis

In the last decade the sol–gel process became a promising method to synthesize materials in form of coatings, nanoscale powders and porous systems. The technique has been mainly used at laboratory scale and has brought interesting contributions for the development of new nanomaterials. Nevertheless, several products or devices made with such a process already exist and new ones should be available in the market in the near future. This paper briefly reviews the state of the art in the development of electrochromic (EC) coatings and devices, batteries, nanocrystalline solar cells and in the field of catalysis achieved during the last decade using sol–gel derived pure and doped niobium pentoxide.     

Comparative studies of “all sol–gel” electrochromic windows employing various counter-electrodes

Electrochromic (EC) “smart” windows for buildings represent an effective way to modulate the intensity of incoming solar radiation. While it is accepted that WO₃ films represent the best option for the working electrode, the choice of the best counter-electrode is still debatable. Optical properties of counter-electrodes such as Ce, Fe, V and Sn oxides are presented. Electrochromic windows were made with a sol–gel WO₃ active colouring film (150°C), Ce, Fe, V oxide counter-electrodes and a sol–gel organic–inorganic hybrid (Li⁺ormolyte) ion conductor. The electrochromic responses of these devices predicted from the charge capacities, photopic transmittances and coloration efficiencies of individual films are compared with measured values.     

Sol–gel deposited nickel oxide films for electrochromic applications

The electrochromic (EC) behavior, the microstructure, and the morphology of sol–gel deposited nickel oxide (NiO_x) coatings were investigated. The films were produced by spin and dip-coating techniques on indium tin oxide (ITO)/glass and Corning glass (2947) substrates.The coating solutions were prepared by reacting nickel(II) 2-ethylhexanoate as the precursor, and isopropanol as the solvent. NiO_x was heat treated at 350 °C for 1 h. The surface morphology, crystal structure, and EC characteristics of the coatings were investigated by scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), atomic force spectroscopy (AFM), X-ray diffractometry (XRD), and cyclic voltammetry (CV).SEM and AFM images revealed that the surface morphology and surface characteristics of the spin- and dip-coated films on both types of substrate were different. XRD spectra revealed that both films were amorphous, either on ITO or Corning glass substrates. CV showed a reversible electrochemical insertion or extraction of the K⁺ ions, cycled in 1 M KOH electrolyte, in both type of film. The crystal structure of the cycled films was found to be XRD amorphous. Spectroelectrochemistry demonstrated that dip-coated films were more stable up to 1000 coloration–bleaching cycles, whereas spin-coated films gradually degraded after 500 cycles.