WO2023282336A1 - Improvements to light absorbing surfaces - Google Patents

Abstract

Disclosed is a top layer for a light absorption surface, which comprises a matrix and nanoparticles embedded therein. The top layer may constitute the outermost layer of a solar absorber coating or absorber material surface, for use in solar heat power generation. The top layer may be able to enhance light absorption and accommodate the thermal expansion/contraction of the absorption coating or that of its underlying substrate in use. The nanoparticles embedded in, or adhered to the matrix, may contribute to the light absorption of the underlying absorption surface.

Description

IMPROVEMENTS TO LIGHT ABSORBING SURFACES Cross Reference

This application claims priority based on Provisional Patent Application No. 2021902084 filed in Australia on July 8, 2021, the entire contents of which are incorporated herein by reference in their entirety.

This disclosure relates to a top layer for a light absorption surface, a method for producing such top layers and a process for refining top layers for a light absorption surface.

It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country.

High-temperature light-absorbing coatings (also referred to as ‘absorber coatings’ or ‘solar absorber coatings’) are a key element in concentrating solar power (CSP) or concentrated solar thermal (CST) plants, where they are employed to convert concentrated solar radiation into heat. Such coatings require high light absorptance (typically above 95%) and good durability, in order to maintain their light absorptance properties in extreme conditions over the service lifetime of the CSP plant, in which the operating temperatures for the coatings can exceed 700°C.

Many known light absorber coatings however suffer from relatively poor light absorptance and can be susceptible to severe degradation after only one year of operation, due to high-temperature annealing with repeated thermal cycling at various frequencies. Degradation of the coating initially manifests as reduced light absorptance (reduced efficiency in conversion of solar irradiation into heat), which is often followed by coating failure in the form of cracking, peeling off etc. Because of such coating breakdown, the CSP industry currently relies on expensive and time-consuming re-coating (or re-painting) processes to ensure that the light absorption of the solar thermal receivers is maintained above 95%, during the many years of operation of a typical CSP plant.

Spinel-based coatings have been shown to exhibit moderate absorptance values after long-term isothermal exposure at high temperature, but efforts to improve light absorptance and durability of such absorber coatings has been limited. Patent literature 1 for example, for which one of present inventors is also a named inventor, discloses spinel-based absorber coatings where the application of porous silica top layers has been made to improve durability of the underlying absorber coating. Such top layers, while contributing to the durability of the coatings, do not serve to improve the absorptance values of the underlying spinel-based coating.

Advanced light absorber coatings have also been considered, such as carbon nanotube (CNTs)-based absorbers that can absorb at least 99.995% of incoming light. However, such coatings are highly susceptible to burning in high temperature environments under normal atmospheric conditions, and thus are not practical for CSP applications.

US11002466

There is a need for improved top layers for light absorption surfaces (including existing absorption coatings), which may enhance absorptance and/or durability of the light absorption surfaces to which they are applied.

In a first aspect, there is disclosed a top layer for a light absorption surface, the top layer comprising a matrix and nanoparticles embedded therein. The top layer may constitute the outermost layer of a solar absorber coating or absorber material surface, for use in solar heat power generation for example. In this regard, the top layer may be able to enhance light absorption and accommodate the thermal expansion/contraction of the absorption coating or that of its underlying substrate in use. The nanoparticles embedded in, or adhered to the matrix, may contribute to the light absorption of the underlying absorption surface. While not wishing to be bound to a particular theory, it is likely that the nanoparticles within the top layer may contribute to forward scattering of light incident on the absorber surface, contributing to the focusing of incident light toward the underlying light absorbing surface. The matrix of the top layer may also contribute to the light absorption and/or durability of the underlying light absorber coating (e.g. a high-temperature light-absorber coating or absorber material surface).

In some embodiments, the nanoparticles may be in a size range of approximately 10-200 nm. In this regard, the size and distribution (density and layout) of the nanoparticles within/on the top layer matrix may serve to enhance the light absorptance of the underlying solar absorption coating, for example by the formation of a three-dimensional (3D) network structure within the top layer. Nanoparticles may scatter incoming light, creating regions of light resonance (high intensity) within the underlying absorption surface and improving net light absorption.

It is to be understood that the nanoparticles may be of a number of different morphologies, including spheres, ovoids, rods, etc. The top layer material (matrix and nanoparticles) should be selected based upon the optical properties and morphology of the underlying substrate or absorption layer being considered. In this regard, the nanoparticle material, size, size distribution and/or density may be selected depending upon the optical properties and morphology of the underlying substrate chosen.

In some embodiments, the nanoparticles may comprise oxide nanoparticles.

In some embodiments, the nanoparticles may comprise one or more metal oxides selected from silica, titania, zirconia, alumina, magnesia or tin oxide, for example. The top layer material (nanoparticles and matrix) may be comprised of oxides which are stable in air at high temperatures (i.e. as required in CSP applications), but other types of material such as metals (e.g. gold) could also be used.

In some embodiments, the top layer may be formed from a mixture comprising the nanoparticles, wherein the mixture comprises approximately 0.1075wt% to 0.43wt% nanoparticles. An optimum number or density/distribution of nanoparticles may be selected for the top layer, in order to maximise light absorptance of the underlying surface for example. This optimum particle density may depend upon the particle and matrix materials of the top layer and also the characteristics of the absorption surface to which the top layer is to be applied.

Too high a density of nanoparticles may lead to poor adherence of the top layer to the underlying light absorption surface, while too few nanoparticles may result in low improvements to absorptance. An optimum distribution of nanoparticles is therefore preferred, depending upon the particular application.

In some embodiments, the top layer mixture may comprise an organosilane. The organosilane may be hydrolyzed by water and polycondensed in the presence of a catalyst. Such a material may result in a top layer matrix comprised of silica, within which the nanoparticles may be incorporated.

In some embodiments, the top layer mixture may be formed from a tetraethyl orthosilicate precursor, a tetramethyl orthosilicate precursor or an oligomer thereof.

In some embodiments, the matrix of the top layer may have a thickness of approximately 5-100 nm. In this regard, the matrix thickness (and matrix material) may be tailored to the particular light absorption surface over which the top layer is to be applied. For example, differing matrix materials (i.e. silica, alumina, zirconia, titania, tin oxide etc.) may necessitate different matrix thicknesses for a given application. A particular matrix thickness may also be selected to support a required nanoparticle loading or density in the top layer.

In some embodiments, the matrix of the top layer may comprise nanopores. In this regard, the top layer should be resistant to thermal expansion when the underlying absorber surface (e.g. for solar heat power generation) and the substrate on which the absorber surface may be formed, thermally expand in use. The presence of nanopores in the top layer matrix may aid in protecting the underlying absorber coating, by accommodating a degree of thermal expansion and contraction.

In a further aspect there is disclosed a method of forming a top layer on a light absorption surface, the method comprising:
Preparing a first mixture, the first mixture comprising a precursor of a matrix for the top layer;
Preparing a second mixture, the second mixture comprising nanoparticles;
Mixing the first and second mixtures to produce a top layer formulation comprising nanoparticles;
Spraying the top layer formulation onto the light absorption surface; and
Subjecting the light absorption surface having the top layer to a curing process.

In this regard, provision of a first matrix mixture and a second mixture comprising nanoparticles may allow the nanoparticles to bind with the resulting matrix more effectively, thereby minimising unwanted agglomeration of the nanoparticles.

In some embodiments, the steps of spraying the top layer formulation onto the light-absorbing surface and subjecting the light-absorbing surface to the curing process may be repeated, so as to form a top layer having a matrix thickness of approximately 5-100 nm.

In some embodiments, the top layer matrix may be comprised of two or more layers of different materials.

In some embodiments, the nanoparticles may comprise oxide nanoparticles. In this regard, the nanoparticles may comprise particles of different oxide materials and different particle sizes.

In some embodiments, the first and the second mixtures may comprise organosilane-based mixtures, such as a tetraethyl orthosilicate precursor, a tetramethyl orthosilicate precursor or an oligomer thereof.

In some embodiments, the tetraethyl orthosilicate precursor, the tetramethyl orthosilicate precursor or an oligomer thereof may be hydrolyzed by water and polycondensed in the presence of a catalyst.

In some embodiments, the catalyst may be hydroxyacetone or an acid, such as acetic acid, nitric acid, sulfuric acid, hydrochloric acid, oxalic acid, or formic acid.

In case where the precursor of a matrix comprises an oligomer of tetraethyl orthosilicate or an oligomer of tetramethyl orthosilicate, the step of spraying the top layer formulation onto the light absorption surface is carried out at a temperature of 300°C or more. In this regard, by using the oligomer of tetraethyl orthosilicate or the oligomer of tetramethyl orthosilicate, the matrix is capable of crystallizing on the preheated substrate or target, whereas the matrix containing the tetraethyl orthosilicate precursor, the tetramethyl orthosilicate precursor may crystalize in air due to radiant heat before application.

The top layer as set forth above may be applied to a number of light absorption surfaces to improve their light absorption properties and contribute to their durability. Such surfaces include for example metals (e.g. steel and titanium alloys), ceramics and conventional absorber coatings.

The top layer may be applied, for example, to absorber coatings disclosed by one of the present inventors in Patent literature 1, which is incorporated herein by reference.

As a further example, Pyromark 2500 is a widely used high-temperature light absorber coating to which the present top layer may be applied, to improve absorptance and durability. Since the underlying material can be comprised of different morphologies, the effect (and optimum properties) of the top layer may vary, but in each case an improvement to light absorptance properties is expected.

In a yet further aspect, there is disclosed a process for refining a top layer for a light absorption surface by numerical simulation, the process comprising:
generating a simplified morphology of the light absorption surface;
generating a set of parameters defining the light absorption surface;
generating a model system comprising the light absorption surface and a top layer applied thereto; and
modelling the application of light to the model system, so as to obtain the spectral absorptance and/or the solar-weighted absorptance of the system;
wherein the top layer of the model system comprises a set of matrix parameters and a set of particle parameters.

In some embodiments, the step of generating the simplified morphology of the light absorption surface may comprise an optical-based measurement of the light-absorbing surface.

In some embodiments, the set of matrix parameters and/or the set of particle parameters may be modified and the process iterated, so as to alter the spectral absorptance and/or the solar-weighted absorptance of the system.

In some embodiments, the set of particle parameters may relate to particles including one or more metal oxide particles.

In some embodiments, the one or more metal oxide particles may comprise silica, titania, alumina and/or zirconia.

In some embodiments, the numerical simulation may comprise a computational electromagnetics simulation step. In some embodiments, the computational electromagnetic simulation step may be based on a Finite-Difference Time-Domain approach.

In another aspect, there is disclosed a mixture for use in preparing the top layer described above, the mixture comprising a precursor of a matrix, which comprises a tetraethyl orthosilicate precursor, a tetramethyl orthosilicate precursor or an oligomer thereof, nanoparticles, which comprises colloidal silica, ethanol, water, and a catalyst.

Throughout the disclosure, the terms ‘concentrating solar power (CSP)’ and ‘concentrated solar thermal (CST)’ may be used interchangeably.

Fig. 1 shows an embodiment of a modelling process for assessing a top layer of the present disclosure, where:a. Physical geometry of a nanoparticle sphere embedded in a top layer matrix, placed over an absorption surface bulk material;b. Simplified geometry for modelling;c. Embodiment of the modelled system comprising a top layer over an absorption surface bulk material;d. Top view for an array of randomly distributed nanospheres with periodic boundaries;e. Complex refractive index for the modelled bulk material. Fig. 2 shows the Finite-Difference Time-Domain simulation results for the system of Figure 1, where:a. Magnitude of the light intensity (Poynting vector) normalised against the case without top layer, from a cross-section side view at x = 350 nm and wavelength of 550 nm;b. Magnitude of the light intensity (Poynting vector) normalised against the case without top layer just below the spheres, whose projection is indicated as reference;c. Spectral absorptance of a flat bulk material with and without top layer (left axis); spectral solar irradiance (right axis) is included as reference. Fig. 3 shows a schematic of a generalised embodiment of the modelling process for optimising nanoparticle and matrix parameters; Fig. 4 shows a schematic detailing Process 1 of Figure 3; Fig. 5 shows a schematic example of Process 1; Fig. 6 shows a schematic detailing Process 2 of Figure 3; Fig. 7 shows the modelled absorptance values obtained by applying the generalised embodiment of Figures 3-5 with a square packing distribution of nanoparticles, to optimise a top layer, where:a. Modelled spectral absorptance values obtained by applying the foregoing process to optimise a top layer for a flat titanium (Ti) surface, varying nanoparticle density;b. Modelled spectral absorptance values obtained by varying thickness of matrix on the bare stainless steel surface;c. Modelled absorptance values obtained by varying top layer matrix material;d. Modelled absorptance values obtained by varying nanoparticle material; Fig. 8 shows an embodiment of a top layer for a light absorption surface, where:a. Scanning Electron Microscope (SEM) image of the top layer applied to the absorption surface and histogram showing the particle size distribution of the nanoparticles;b. An embodiment of a nanoparticle incorporated in the top layer matrix. Fig. 9 shows a schematic process of an embodiment of a precursor, to form a top layer of the present disclosure. Fig. 10 shows a schematic process of an embodiment of a precursor, to form a top layer of the present disclosure. Fig. 11 shows an embodiment of a light absorption surface, onto which a top layer according to the present disclosure is applied, where:a. An embodiment of the morphology of base, absorption and top layers;b. SEM images of the base layer (Figure 11 b, b.1) and absorption layer (Figure 11 b, b.2);c. Cross-sectional computed tomography (CT) image of the absorber coating; Fig. 12 shows:a. SEM image of the colloidal silica employed for the top layer formulations of the present disclosure, prior to incorporation into the top layer, and histogram showing the particle size distribution of the nanoparticles;b. SEM image of the top layer applied to an absorber surface, showing the colloidal silica incorporated into the top layer, and histogram showing the particle size distribution of the nanoparticles;c. Measured (solid line) and modelled (dashed line) absorptance performance of the multi-layer embodiment of Figure 11, having base, absorption and top layers; Fig. 13 shows measured spectral absorptance performance of top layers applied to various absorber surfaces, where:a. Top layers applied over 316L stainless steel alloy surface;b. Top layers applied over Inconel 625 alloy surface;c. Top layers applied over absorber coating of Figure 11, with 316L stainless steel substrate; results for three nanoparticle sizes and bare absorber coating (without top layer);d. Top layers applied over absorber coating of Figure 11, with Inconel 625 alloy substrate; nanoparticle concentration in top layer formulation varied; Fig. 14 shows SEM images of embodiments of top layers having differing nanoparticle number density, where:a. An absorber coating prior to application of the top layer;b. An applied top layer onto the absorber coating of Figure 14 a, having a top layer formulation containing 0.1075 wt% nanospheres;c. An applied top layer onto the absorber coating of Figure 14 a, having a top layer formulation containing 0. 215 wt% nanospheres;d. An applied top layer onto absorber coating of Figure 14 a, having a top layer formulation containing 0. 43 wt% nanospheres;e. Solar-weighted absorptance (SWA) of absorber surfaces with applied top layers having varying nanosphere density due to different concentration of nanospheres in top layer formulation, after thermal ageing at 900°C; Fig. 15-1 shows absorptance performance of the present absorber coatings (of Figure 11) with top layer applied. Figures a. to d. show performance of the top layer compared with Pyromark 2500 without top layer, where:a. Spectral absorptance (left axis; reflectance on right axis) on Inconel 625 substrate. Insets shows the morphology of the coral-structured absorber coating (top) and Pyromark 2500 (bottom), measured with a confocal microscope using a common colour bar scale; b. Solar-weighted absorptance (SWA; left axis) and reflection loss (right axis) as a function of isothermal annealing time at 800°C for the present coating and three known absorber coatings. Inset shows the results for a thermal cycle-and-hold test as reported by Torres, J. F., Ellis, I. & Coventry, J. Degradation mechanisms and non-linear thermal cycling effects in a high-temperature light-absorber coating. Sol. Energy Mater. Sol. Cells 218, 110719 (2020);c. Spectral absorptance (or emittance), including the infrared spectrum range;d. Solar-weighted absorptance (and reflection loss) as a function of isothermal annealing time at ≧850°C; inset shows the effect of nanosphere number density on the solar-weighted absorptance after ageing at 900°C for 500 hours; Fig. 15-2 shows absorptance performance of the present absorber coatings (of Figure 11) with top layer applied. Figures e. to i. show performance of the top under varying conditions, where:e. Solar-weighted absorptance (SWA) as a function of ageing time with top layers having differing nanoparticle loadings or matrix only (without nanoparticles) for ageing at 900°C.f. Spectral absorptance of an applied absorber coating for pristine, after ageing 400 h and after ageing 1000 h, having a top layer formulation containing 0. 215 wt% nanospheres compared with no top layer applied absorber coating in the same ageing time;g. Spectral absorptance of an applied absorber coating for pristine, after ageing 400 h and after ageing 1000 h, having a top layer formulation containing 0. 1075 wt% nanospheres compared with no top layer applied absorber coating in the same ageing time;h. Spectral absorptance of an applied absorber coating for pristine, after ageing 400 h and after ageing 1000 h, having a top layer formulation containing 0. 43 wt% nanospheres compared with no top layer applied absorber coating in the same ageing time;i. Spectral absorptance of an applied absorber coating for pristine, after ageing 400 h and having a top layer matrix only compared with no top layer applied absorber coating in the same ageing time; Fig. 16 shows the characterisation results indicating coating stability of the present absorber coatings (of Figure 11) with top layer applied, under isothermal annealing conditions, where:a. SEM image after annealing at 800°C for 3000 hours for the optimised coral-structured morphology;b. SEM images after annealing at 900°C for 850 hours; the coral morphology was kept (b.1) with occasional peeling off in discrete locations (b.2);c. Cross-section backscattered electron (BSE) images and energy-dispersive spectroscopy (EDS) images of the coral-structured coating after 1000 and 3000 h of isothermal ageing at 800°C; the coral-structured morphology was shown to be largely unchanged.d. X-ray diffractometry (XRD) patterns indicated minor changes in crystal phase structure after annealing. The pigment (black line) is Cu_0.64Cr_1.51Mn_0.84O₄, which corresponds to the pattern found in the pristine coating. Rutile TiO₂ crystalises after heat treatment at 850°C for 2 hours, as indicated with red arrows; Fig. 17 shows high and low magnification SEM images of the present absorber coating with top layer applied, after thermal ageing. Sintering of the silica nanospheres was found to be minimal; Fig. 18 shows the results of absorptance testing for Pyromark 2500 samples with (dashed line) and without (solid line) the top layer for pristine condition and after 500 hours thermal ageing at 800°C of the present disclosure applied; Fig. 19 shows embodiments of the present top layer demonstrated in large-scale absorption applications, where:a. Photograph of a commercial solar thermal receiver, prior to application of the top layer for testing; inset shows the coral-structured coatings and top layer placed under the receiver for a six-month on-sun test;b. Example of deposition of the absorption and top layers; the absorption solution has a large volume fraction of solvent, and thus the substrate needs to be kept at a suitable temperature for pyrolysis of Ti or Al to occur. The doughnut-shaped exhaust helps remove the excess of evaporated solvent, which is necessary to create multi-scale porosity, while circulation of heated oil through the receiver tubes helps ensure that the surface temperature is kept nearly constant;c. Coated tubes (absorption and top layers) overlaid with a lab-coated curved coupon;d. SEM image of the coral-structured absorber coating and the top layer on the coated tubes;e. Results of further high-temperature durability testing of the absorber coating with top layer, applied Inconel 625 (e.1) and 316L stainless steel (e.2) substrates. Samples were aged in the field (i.e. ‘on-sun’ testing for six months) and aged isothermally at 850°C for 200 hours (inside a furnace). Fig.20 shows absorptance performance (reflection) of Pyromark 2500 with or without top layers of oligomers containing nanoparticles as a function of wavelength. Fig.21 shows solar-weighted absorptance of Pyromark 2500 with top layers as a function of spray times. Fig.22 shows a comparison of absorptance performance between the US11002466 absorber coating with the top layer of oligomers containing nanoparticles coated and without the top layer as a function of aging time. Fig.23 shows simulation results for the effect on light-absorbing properties when varying the nanosphere diameter. Fig. 23a shows the simulated absorptance (left axis) and solar irradiance (right axis) as a function of wavelength. Fig. 23b shows the simulated effectiveness of the top layer as a function of nanosphere diameter when applied on three types of underlying solar absorbers. Fig.24 shows the simulated effectiveness of a top layer having a uniform nanosphere configuration on a dummy material as a function of the coverage ratio for different nanosphere diameters without matrix (Fig.24a) and as a function of both the nanosphere diameter and matrix thickness for a fixed coverage ratio of 46% (Fig.24b). Fig.25 shows the simulated effectiveness of the top layer as a function of the immersion degree of the nanospheres for different matrix thicknesses when the matrix thickness and nanosphere radii are equal.

In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

Optimisation of Top Layers for Light Absorption Surfaces
Top layers according to the present disclosure comprise a matrix and nanoparticles embedded in, adhered to, or distributed throughout the matrix. The matrix consists of a porous silica coating, which may be applied to the outermost surface of a light absorption surface, such as a light-absorbing coating for solar heat power generation. By forming the porous silica top layer (comprising the nanoparticles) on the outermost surface of a light absorption material, the matrix of the top layer can protect the underlying absorption surface in use. The matrix can also contribute to improved absorptance, while the nanoparticles can significantly enhance the absorptance of the absorption material, by interacting with incident light.

In order to form the porous silica coating, a solution of organosilanes, such as dimethyldichlorosilane, trimethoxyethylethoxysilane or tetra-ethoxysilane is prepared. The silica coating containing nanoparticles is sprayed onto the outermost surface of a light absorbing surface (for example, an absorber coating) and cured by heating, forming a porous silica coating. As an example of one method for forming a porous silica coating, an alkoxysilane such as tetraethyl orthosilicate mixed with ethanol, and a catalyst such as hydroxyacetone and water mixed with ethanol, are mixed. The resulting solution is sprayed onto the absorption surface and heated at a suitable temperature in the range of 300-500° C.

In another embodiment, the solution of organosilanes may comprise an oligomer of a hydrolyzable silane compound. The oligomer of the hydrolyzable silane compound may be produced by polycondensation reaction of the hydrolyzable silane compound. Hydrolyzable silane compounds are silane compounds with hydrolyzable groups such as alkoxy groups, alkoxyalkoxy groups, acyloxy groups, aryloxy groups, aminoxy groups, amide groups, ketoxime groups, isocyanate groups, and halogen atoms.

In the present invention, alkoxysilane compounds are suitably used. Lower alkyl groups such as methyl, ethyl, propyl, and butyl groups are examples of alkyl groups of alkoxy groups (-OR). Depending on the number of hydrolyzable groups, one to four functionalities are known. Representative examples of alkoxysilane compounds are dimethyldimethoxysilane (DMDMS), methyltrimethoxysilane (MTMS), tetramethoxysilane (TMOS), dimethyldiethoxysilane (DMDES), methyltriethoxysilane (MTES), tetraethoxysilane (TEOS), and others.

The oligomers of hydrolyzable silane compounds are the result of polymerization (oligomerization) of the above monomers by condensation. In this reaction, silanol groups (-Si-OH) are first formed by hydrolysis of alkoxy groups. At the same time, an alcohol (R-OH) is formed. Next, siloxane bond (-Si-O-Si-O-) is formed by (dehydrative) condensation of silanol groups. This condensation is repeated to form siloxane oligomers. As an oligomer of alkoxysilane compound, a tetramethoxysilane oligomer in which R is a methyl group or a tetraethoxysilane oligomer in which R is an ethyl group is preferred in terms of hydrolysis and condensation of alkoxy groups.

The structure of an oligomer can be linear, branched, cyclic, or reticular. A tetraalkoxysilane oligomer with a linear structure is represented by the following general formula.
RO(Si(OR)₂O)nR ……... (I)
In the general formula, n represents the degree of oligomerization of the oligomer. Usually available oligomers are compositions of oligomers with different n, and thus have a molecular weight distribution. The degree of oligomerization is expressed by the averaged n.

In the present invention, the degree of oligomerization n is usually from 2 to 100, preferably from 2 to 70, and Even more preferably, oligomers with a degree of multimerization of 2 to 50 are used. Such oligomers are already commercially available, so it is easy to use them.

Commercially available oligomers of hydrolyzable silane compounds include MKC Silicate MS51, MKC Silicate MS56, and MKC Silicate MS57 and MKC Silicate MS56S manufactured by Mitsubishi Chemical Corporation; Methyl Silicate 51, Methyl Silicate 53A, Ethyl Silicate 40, and ethyl silicate 48 manufactured by Colcoat Co., Ltd.; silicate 40 and silicate 45 manufactured by Tama Chemicals Co., Ltd., etc. All are oligomers of tetramethoxysilane or tetraethoxysilane.

Numerical modelling was conducted to determine optimum or near-optimum parameters for the top layer such as nanoparticle type, size and ‘loading’ or number density/particle distribution within the matrix (which can depend upon the substrate to which the top layer is to be applied), as well as matrix properties including material type and thickness for example. The model also included a bulk material (i.e. a model absorption surface), above which the top layer was situated.

The magnitude and direction of the Poynting vector for various top layer/bulk material arrangements was analysed by the Finite-Difference Time-Domain (FDTD) method using the software ANSYS Lumerical.

Referring to Figure 1, the physical geometry of a single silica (SiO₂) nanoparticle sphere embedded in a silica matrix, placed over an absorber coating bulk material (Figure 1 a.) was approximated for modelling purposes using the simplified geometry shown in Figure 1 b. In a physical top layer comprising nanoparticles (in the form of nanospheres embedded in or adhered to a matrix), a bottom section of the nanospheres overlaps with the matrix (i.e. the particles are embedded or incorporated with the matrix - Figure 1 a.), which acts to increase the effective particle diameter by approximately twice the matrix thickness. For this reason, a modelled sphere size was taken as the physical sphere diameter plus twice the matrix thickness. Thus, to model the properties of a physical arrangement having nanospheres of 100 nm diameter and a matrix of 8 nm thickness, an effective sphere diameter of approximately 116 nm-120 nm was taken.

As shown in Figure 1 c., the numerical model comprised a plane wave launched from a normal direction to interact with the modelled nano-scale structures of the top layer. The periodic boundary conditions were set along the z-axis, on the lateral planes x-z and y-z, with a ‘perfect matched layer’ (PML) boundary condition set on both bottom and top of x-y plane boundary. The thickness of the bulk material was set at to be larger than 3 μm to ensure all energy of the plane wave was absorbed before reaching the bottom boundary.

A sphere diameter of 120 nm and density of 42 spheres per μm² was initially employed, with the SiO₂ nanospheres placed randomly with an 8 nm thick SiO₂ matrix on top of the bulk absorber material . Figure 1 d. illustrates a top view for an array of 15 randomly distributed nanospheres over a 0.36 μm² area. A mesh size of 2 nm in each coordinate axis was employed for modelling.

The refractive indices for SiO₂ used in the modelling were obtained from the literature [Gao, L., Lemarchand, F. & Lequime, M. Exploitation of multiple incidences spectrometric measurements for thin film reverse engineering. Opt. Express 20, 15734 (2012)]. In order to simulate the effect of the top layer on top of a single absorption coating bulk material, with similar initial wavelength-dependent absorptance as absorber coatings having micro- and macro- scale features (such as the coatings described in US 11002466), a dummy bulk material was designed with good light-absorbing properties (Figure 1 e. illustrates the complex refractive indices for the modelled bulk material). A plane monitor collecting frequency-domain field profile was set, returning the Poynting vector and power integrated by Poynting vector in the simulation area, which were then normalised against the case without any material or structure applied in the simulation domain.

Figure 2 illustrates the Finite-Difference Time-Domain simulation results for the system of Figure 1. Figure 2 a. shows the magnitude of the light intensity normalised against the case without any material from a cross-section side view at x = 350 nm and wavelength of 550 nm. Figure 2 b. gives the magnitude of the light intensity and the profile of Poynting vector normalised against the case without any material, just below the spheres. Figure 2 c. shows the spectral absorptance of a flat bulk material, with and without top layer, whose parameters are illustrated in Figure 1.

It was found that the results are dependent on the nanosphere size, density, underlying coating complex index of refraction and underlying coating morphology. Thus, particular combinations of nanoparticles and matrices (i.e. of differing material, size and morphology), may be tailored to enhance the solar absorption of particular light-absorbing coatings.

The generalised process for optimising or improving the absorptance by changing nanoparticle and matrix parameters is outlined in Figure 3 (process steps 10’ to 19’). This process includes assessing the morphology of a candidate bulk material to which the top layer is to be applied and approximating that morphology in a simplified manner, in order to more easily conduct numerical modelling (Process 1, 14’, as further detailed in Figure 4 process steps 20’ to 24’). An example of Process 1 is illustrated in Figure 5, where a candidate absorber coating is assessed for application of the present top layer (like process steps of Figure 4 indicated).

The optimisation process then comprises a modelling step (Process 2, 19’ of Figure 3) which is illustrated in further detail by Figure 6. This modelling process is used to assess the parameters found in the optimisation process. An example of this modelling process is outlined above, in reference to the example illustrated in Figure 2.

Numerical simulations showed that the top layer material (i.e. nanoparticles and matrix) ought to be chosen based on the optical properties of the underlying absorber coating. Thus, the generalised process for optimising or improving the absorptance and finding the optimum top layer properties as outlined above, allows for the material properties of the solar absorber coating to be assessed and used to choose accordingly the optimum top layer parameters for a suitable top layer design, tailored for a given absorber coating.

Figure 7a. illustrates the modelled spectral absorptance values (i.e. as a function of the wavelength of incident light) obtained by applying the foregoing process to determine the optimum top layer parameters for a flat titanium (Ti) surface. To determine the optimal density of nanoparticles in the top layer for the titanium surface (particles per μm²), the following initial conditions were used to model the system:
- Titanium surface of flat morphology, having a known complex index of refraction for Ti;
- A surface length scale larger than the incident wavelength range (i.e. flat surface morphology);
- Shape of nanoparticles: Spherical;
- Diameter of nanoparticle spheres: 120 nm;
- Material of nanoparticles: SiO₂;
- Number of stacks (layers) of nanoparticles: 1;
- Arrangement of nanoparticles: square packing.

Using the above simulation conditions, the optimal density of nanoparticles to maximise spectral absorptance can be determined, depending upon the spectral irradiance of the incident light (e.g. solar-weighted absorptance when the incident light is sunlight). The results shown in Figure 7 a. indicate that for wavelengths longer than 500 nm, higher nanoparticle density in the top layer is preferred, while for wavelengths shorter than 500 nm, lower particle density is optimal.

The foregoing modelling process was also employed to assess the effect of various matrix thicknesses for top layers formed on 316 L stainless steel substrates (0 - 40 nm; Figure 7 b.), different matrix materials (alumina, silica, titania) with 100 nm Alumina nanospheres (Figure 7 c.) and different nanoparticle materials (alumina, silica, titania and zirconia) in a silica matrix (Figure 7 d.).

In another embodiment, the effectiveness of nanosphere diameter is simulated. Figure 23a shows the spectral absorptance of a dummy material with a top layer having monodispersed nanospheres of five different diameters (indicated in the legend). The coverage ratio in all cases is constant and equal to 46%. All solid lines are for random configurations, the dashed line is the uniform configuration for 400 nm diameter of nanospheres with the same coverage ratio, and the dotted line is bulk dummy material without top layer. The random and uniform configuration for nanosphere diameter of 100 nm was almost the same (not shown).

Figure 23b shows the effectiveness of the top layer as a function of nanosphere diameter (monodispersed top layer) for a random configuration with 46% coverage ratio on three different underlying materials. The optimum nanosphere diameter (or nano particle effective size) depends on the underlying absorber material, but it is generally within the range of 80 to 140 nm.

Figure 24a shows the simulated effectiveness of a top layer having a uniform nanosphere configuration (i.e. not random) on dummy material as a function of the coverage ratio for different nanosphere diameters (indicated in the legend). For nanospheres of 100 nm or less (dashed lines), the effectiveness increases monotonically with coverage ratio (until the analyzed value of 80%). For nanosphere diameters of 120 nm or larger (solid lines), there is an optimum coverage ratio. These results do not include the matrix.

Figure 24b shows the effectiveness of the top layer as the function of the nanospheres diameter and matrix thickness with random configuration on a dummy material, when the coverage ratio is 46%. The nanospheres are placed on the top matrix without immersion (i.e. the nanospheres are tangent to the original matrix-air boundary). The contour plot shows that the effectiveness is influenced by the combination of matrix and nanospheres. The line plots adjacent to the contour plot show the effects only by nanospheres (bottom) or matrix (left).

Figure 25 shows the effectiveness of the top layer as a function of the immersion degree of the nanospheres, when the top layer is on the dummy material with the nanospheres having a uniform configuration. In these simulations, the nanosphere diameter (D) is set to twice the matrix thickness (H), i.e. D = 2H. The definition of immersion degree is φ = L/D × 100%, where L is the penetration depth. The immersion degree φ determines how much the nanosphere is immersed into the matrix. Therefore, in the simulations of Figure 25, the maximum immersion degree is 50% (i.e. when L = H and D = 2H, φ = 50%). This result indicates that increasing the immersion degree generally decreases the maximum effectiveness (e.g. see the results for H = 60 nm). However, in some cases (H = 80 nm or 100 nm), the small immersion degree can increase the effectiveness. Experimentally, having a larger immersion ratio could improve the adherence of the nanospheres onto the matrix.

In this way, the foregoing modelling process can simulate and assess various combinations of nanoparticle material, size, morphology, and number density or loading, along with various types of matrix and absorber surfaces, in order to optimize or improve top layer properties for a given absorber surface application.

Preparation of the Top Layer
Figure 8 a. shows an embodiment of the top layer 10 for a light absorption surface 11, the top layer comprising a silica matrix 12 and silica nanoparticles 14 embedded therein or adhered thereto. The top layer is formed on the outermost surface of a light absorption surface 11. As illustrated in Figure 8 b., the nanoparticles 14 are typically embedded in and/or coated by the matrix material 12, however the nanoparticles can be incorporated to various depths within the matrix or may be adhered to the surface of the matrix.

To prepare sprayable compositions for forming the top layer, a mixture containing a tetraethyl orthosilicate with an organic solvent (Material A) was added to a solution containing nanoparticles (Material B). In the illustrative examples that follow, silica nanospheres were employed, though other nanoparticles having differing optical properties may be employed.

Preparation of Material A
Figure 9 outlines a schematic process for preparing Material A. 160 mL of 99.5% ethanol (weighing 126.7 g) was mixed with 41.7 g of 0.2 mol tetraethyl orthosilicate (TEOS; manufactured by Tokyo Chemical Industry Co., Ltd.) 22 at 25°C for 1hr at a stirring speed of 550 rpm (SIBATA SCIENTIFIC TECHNOLOGY LTD. Model: CPG-2120) 23, producing solution A1. In a separate process, 160 mL of ethanol of 99.5% purity (126.7 g ethanol) was mixed and stirred with 14.8 g of 0.2 mol hydroxyacetone (manufactured by Tokyo Chemical Industry Co., Ltd.) 24,25 and 18 g (1.0mol) of ion-exchanged water was gradually added, with continuous stirring 26. This solution was initially mixed ultrasonically for 10 minutes, 27, before further stirring (also at 25°C) for 1hr at a stirring speed of 550rpm, producing solution A2, 28.

Precursor solutions A1 and A2 were then stirred together at 40°C for 2 days at a stirring speed of 550rpm, 30. The mixture was allowed to stand at 40°C for 3 days, 32 producing approximately 330 g of Material A, 34.

Preparation of Material B
Figure 10 outlines a schematic process for preparing Material B. 63.4 g of ethanol (99.5 % purity) was mixed with 20.0 g of a commercially available colloidal silica (ORGANOSILICASOL^TM IPA-ST-ZL, manufactured by Nissan Chemical), 20.8 g (0.1 mol) of tetraethyl orthosilicate (TEOS) and 7.4 g (0.1 mol) of hydroxyacetone, 36, 37, with 9.0 g (0.5 mol) of ion-exchanged water slowly added with continual stirring of the mixture, 38. The colloidal silica comprised silica nanospheres having a mean diameter of approximately 70-140 nm and a loading of 30 wt% spheres. This mixture was first ultrasonically stirred for 10 mins, 39 followed by stirring at 40°C for 2 days, 40 at a mixing speed of 550 rpm. The mixture was allowed to stand for 3 days at 40°C, 42 producing approximately 120g of Material B, 44.

By adding the colloidal silica to TEOS in this way, unwanted agglomeration of the silica particles may be avoided and the silica nanoparticles may bind with the resulting silica matrix more effectively. In this regard, TEOS is hydrolyzed with water and the catalytic hydroxyacetone triggers a dehydration polycondensation process of TEOS. The silica spheres are coated by dehydration polycondensation silica, which assists in preventing the silica particles from aggregating in the solution. By adding colloidal silica to a mixed solution of ethanol, TEOS, and hydroxyacetone, then hydrolysing, dehydrating and polymerizing TEOS with water, unwanted agglomeration of the silica particles can be prevented.

Top Layer Formulation
To prepare the top layer formulation for application to a surface, both Material A and Material B were first diluted. Material A was diluted with sufficient ethanol with 99.5 % purity to reduce the silica concentration from approximately 4 wt% to 0.6 wt%. Material B was diluted with sufficient ethanol with 99.5 % purity to reduce the silica concentration from approximately 5 wt% to 0.86 wt%. Note that for Material B, the silica content measured before and after dilution refers to the silica content derived from the tetraethyl orthosilicate (TEOS) and does not reflect the contribution of the silica nanospheres.

Diluted Materials A and B were then mixed in a 3:1 (i.e. 3 parts A, 1 part B) to form the final composition for the top layer. Example values for preparing a suitable top layer formulation are given below:

Table1

For the above example, the TEOS-derived silica concentration of the final top layer formulation was found to be 0.665 wt% (i.e. (0.72 g+0.344 g)/160 g). The concentration of silica nanoparticles was found to be 0.215wt% (i.e. 0.344 g/160 g).

Application of Top Layer
Top layer formulations may be applied to suitable substrates (e.g. absorber surfaces such as absorber coatings) by spraying and curing of the formulation. In order to produce laboratory-scale samples for testing (3x3 cm coupons), the top layer formulations were applied using a Colani airbrush (manufactured by Harder and Steenbeck) using a nozzle size of 0.4 mm. The top layer formulations were applied to the substrates using a spray pressure of 0.3 MPa for 3 seconds at room temperature, with a distance between the nozzle and the substrate to be coated of 30 cm.

After spray application, the coupon substrate was heated to 400 ℃ by means of a hot plate and held for 30 minutes in order to cure the coating (i.e. allow formation of the silicon-based matrix, comprising the silica nanoparticles). The coupon was then removed from the hot plate and allowed to cool to room temperature. Upon reaching room temperature, additional layers or coats of the top layer formulation may be spray applied and cured as set out above, in order to achieve the required top layer thickness and nanoparticle density. Suitable top layers may exhibit an approximately 8-40 nm thick silica matrix for example, which may be achieved by 2-8 spray applications of the top layer formulation, depending upon the spray parameters. Note that several spray applications, using different spray parameters (i.e. distance from nozzle to substrate, spray pressure, nozzle diameter etc.) may be employed to achieve different top layer thicknesses and nanoparticle densities, depending on the required application.

The top layer formulations disclosed may also be applied using industrial scale on-line coating processes, as otherwise known in the art.

Depending upon the absorption surface or substrate to which the top layer is to be applied, a primer layer may also be employed to improve coating durability. For a substrate of stainless steel for example (e.g. 316L stainless steel), whose oxide layer peels off more easily than for nickel-based alloys, an additional primer layer (between the substrate and base layer) may be employed. Suitable primer layer solutions may consist of an aluminum complex diluted with 2-propanol for example.

The light absorption surface 11 to which the top layer is applied may be any number of surfaces, such as metallic alloys including steels, ceramic surfaces and indeed a number of known absorber coatings such as solar absorber coatings, including Pyromark 2500 for example and the absorber coatings of US 11002466.

Application of the Top Layer to Absorber Coatings
The top layer formulations may be applied to a range of light absorption surfaces to improve their absorption properties and contribute to their durability. Such surfaces include for example metals (steel and titanium alloys) and conventional absorber coatings such as Pyromark 2500.

Referring to Figure 11 a. to c., light absorption surfaces were prepared in the form of absorber coatings 46, onto which the top layer formulations were applied. The absorber coatings were produced on a metal substrate 48 (such as 316L stainless steel), with the absorption coating 46 comprising base layer 50 and absorption layer 52, followed by application of the top layer 54 comprising nanoparticles, as described above. The absorption coatings of these embodiments are further described in US 11002466.

The underlying substrate to which the present top layer is applied needs only to be chemically cleaned, not sandblasted as required for many conventional absorber coatings. The base layer comprised copper chromite black spinel pigment particles 56 (Cu_0.64Cr_1.51Mn_0.85O₄; Black 3250 supplied by Asahi Kasei Kogyo Co.,LTD.) bonded by Al₂O₃. The base layer is approximately ~20 μm thick and serves as the deposition surface for the absorption layer 52. The base layer 50 improves adhesion and can contribute to the light-trapping ability of the absorber coating 46, due in part to the presence of ~3 μm diameter micro-holes or ‘micropores’ 58 (Figure 11 b.1).

The absorption layer 52 comprises Cu_0.64Cr_1.51Mn_0.85O₄ spinel pigment particles 56 bonded by pyrolysed TiO₂. The absorption layer of this example has a ‘lumpy’ morphology in the macro scale, having a series of protrusions with a characteristic dimension (i.e. height and width) of ~100 μm, resembling a ‘coral structure’. Herein, the ‘characteristic dimension’ refers to the approximate diameter and height of the protrusions. The absorption layer also comprises ~3 μm diameter micro-holes 58 (Figure 11 b.2).

To prepare the solution for forming the base layer 50, an aluminium complex (aluminium ethylaceto acetate di iso-propirate) and isopropyl di glycol were mixed by screw stirring for 3 hours. Black spinel pigments were then added and mixed by screw stirring for 12 hours, at a weight ratio of ca. 1.15 : 1 of liquid to pigment ratio. To improve the adhesion with the metal substrate, a catalyst (N-2- (aminoethyl) -3-aminopropyltrimethoxysilane) was added and mixed with a screw stirrer for 3 hours.

To prepare the solution for forming the absorption layer 52, a titanium precursor (titanium(IV) isopropoxide; TTIP) was first reacted with acetylacetone at room temperature, and then heated at 80°C for 6 hours and then diluted with 2-propanol (isopropyl alcohol, IPA); the black pigments and N-methyl-2-pirrolidone were added and dispersed by ultrasonication for 30 minutes. A large liquid solution to pigment ratio of approximately 40 : 1 is needed to produce the coral-structured morphology.

Upon spray application of the base and absorption layers, desorption of the solvent (ligand) occurs, coordinated to aluminium and titanium respectively. This is followed by quick evaporation of the desorbed solvent, resulting in the formation of micro-holes (i.e. open elongated micropores). Finally, thermal decomposition (pyrolysis) of the aluminium and titanium (from the desorbed solvent) results in a matrix composed of alumina (base layer) or titania (absorption layer) strongly binding the black pigments.

The TiO₂ binder does not exhibit the nano-scale porosity found in many CSP coatings, promoting both strong bonding to the spinel pigments, while potentially hindering oxide layer growth during operation of the absorber coating, due to an expected reduction in cation diffusion. The lack of nano-scale porosity within the TiO₂ binder, however, is expected to reduce light trapping (or the solar-weighted absorptance). Hence, the addition of the top layer with nano-scale features supplements the lack of nano-scale porosity inside the binder, while potentially hindering the contact between oxygen in air and cations being diffused from the substrate.

Figure 11 b shows scanning electron microscope (SEM) images of the base layer (Figure 11 b, b.1) and absorption layer (Figure 11 b, b.2), illustrating the protrusions 53 formed in the absorption layer 52. The absorption layer protrusions 53 are further illustrated in Figure 11 c, a cross-sectional computed tomography (CT) scan showing the coral-like morphology. A key feature of the absorption layer is its self-assembled micro-scale (holes) and macro-scale (projections) features, with the latter introducing the intrinsic optical stability observed in many natural stony corals.

The top layer 54 applied to the absorption layer 52, comprises a thin (~8 nm) silica matrix and silica nanoparticles (of spherical morphology in the present example, though other morphologies are possible) of ~100 nm in diameter.

SEM analysis of the colloidal silica employed for the formulations of Table 1 (ORGANOSILICASOL^TM IPA-ST-ZL, manufactured by Nissan Chemical) was conducted prior to mixing to form the top layer (Figure 12 a.) and after application of the top layer to an absorber surface (Figure 12 b.). In the as-received state, the mean particle diameter was found to be 116.2 nm with standard deviation of 8.1 nm (histogram plot, Figure 12 a.). Once incorporated into the top layer and applied to an absorber surface, the mean ‘effective’ diameter of the silica nanospheres was found to be123.9 nm with standard deviation of 10.8 nm (histogram plot, Figure 12 b.). As described above, the effective diameter of the silica nanoparticles incorporated with the top layer was expected to approximate the diameter of the particles before incorporation, plus twice the thickness of the top layer matrix (i.e. 100 nm + 2 x 8 nm = ~116 nm).

For laboratory-scale deposition of the absorber coatings, the coatings were sprayed under normal atmospheric conditions onto 3-mm thick metallic coupons (either Inconel 625 or stainless steel SS316L/SS253MA) of 3 cm by 3 cm surface area. To deposit the base layer, the base solution was sprayed with an airbrush for 4 seconds while heating the substrate at 270°C; the rapid evaporation of the solvent (2-propernol and isopropyl di-glycol) produced the micro-hole morphology observed for the base layer. To deposit the coral-structured absorption layer, the absorption solution was sprayed multiple times onto the base layer (held at 320°C) with a large spraying nozzle. The thermal decomposition of titanium acetylacetonate complex by pyrolysis, which only occurs when the substrate is held at ~320 ±20°C, produced the macro-scale coral-like morphology while the rapid evaporation of the solvent (acetylacetone) produced the micro-scale features (holes). To deposit the top layer containing the silica nanoparticles, the coupon was first removed from the hot plate and allowed to reach room temperature, followed by spraying the top layer solution on the absorption layer with an airbrush at room temperature (as outlined above), followed by curing for 30 min at 400°C. Two applications of the top layer were made, to produce a top layer of approximately 8 nm matrix thickness.

Figure 12 c. gives measured (solid line) and modelled (dashed line) absorptance performance of a multi-layer system having base, absorption and top layers (described in greater detail, with reference to Figure 11 above). Figure 12c. shows modelled reflectance (left axis) and reference solar irradiance (right axis) as a function of wavelength, indicating the improvement in light trapping, i.e. reduction in solar-weighted reflectance, as various length scales are incorporated into the absorption coating. For light absorption surfaces used in CSP applications, photons from the sun are generally concentrated more than 1000 times onto the surface of a receiver coated with a light-absorbing coating. These photons first interact with the material at lower length scales (i.e. nano-length scales), which re-emit photons in a wide wavelength range. Importantly, some of these photons can be captured by light traps having larger length scales, in a process that can be described as “hierarchical” or “cascaded” light trapping”. The nano-scale structures of the present top layer are provided by the nanoparticles incorporated with the matrix.

The results of Figure 12 c. illustrate the relative contributions to light absorptance, as a function of wavelength, for each length scale of structures in the absorber coating. The contribution of the base layer (micro-scale) is indicated by the dotted line, the contribution of the absorption layer comprising macro-scale projections by the dashed-dotted line, and that of the top layer comprising the silica matrix and silicon nanoparticles (nano-scale) is indicated by the solid line. The ray-tracing simulation results for the macro-scale absorption layer contribution are also indicted, in dashed line.

Reflectance and spectral solar irradiance of the coatings are shown to decrease with the combined contributions of each of the nano-, micro-, and macro-structures, indicating improvement in light trapping (absorptance), i.e. reduction in solar-weighted reflectance, as each length scale is added in the deposition process.

Computational electromagnetics modelling has shown that both forward scattering and backscattering of sunlight occurs for wavelengths in the visible range, which carry the largest portion of the solar irradiation. Although backscattering reduces light absorption, the forward scattering creates regions of high light intensity underneath the SiO₂ nanospheres, which increases light absorption by a larger amount than that lost by backscattering. This light diffusion behaviour results in an improvement of light absorption, which is dependent on the nanosphere size, density and underlying coating complex index of refraction and morphology. Some forward scattered photons can also be re-absorbed by the micro- and macro- scale features of the absorption layer, further enhancing the light trapping ability of these absorber coatings.

Experimentally, the spectral reflectance was found to be reduced throughout the entire spectrum when the top layer comprising silicon nanospheres was deposited last, over the absorption coating (solid line in Figure 12 c.), while the solar-weighted reflectance (or reflection loss) was reduced by 15.6% (relative value), from an absolute value of 2.26% (without the top layer) to 1.91% (with the top layer). The use of nanospheres in the top layer is therefore an effective way of introducing a nano-texture on the absorber coating surface, to improve absorption performance.

In addition to the multiple reflections between the macro-scale protrusions, light is also trapped by multiple reflections within the micro-holes present in the absorber coating. Note, however, that the micro-holes of the present coatings are not excessively numerous, as this could worsen the structural integrity of the coating. A portion of the light that is reflected from the nano- and micro- scale morphologies is re-absorbed by the same length-scales within the macro-scale protrusions.

Nanoparticle Size and Loading.
The solar-weighted absorptance (SWA) of top layer formulations applied to various absorber surfaces, having different nanoparticle sizes and loadings (i.e. wt% of silica nanoparticles in the formulation) was assessed to determine optimum top layer properties for improved light-trapping performance. To calculate the SWA of the light absorption surfaces, measurements of spectral absorptance α(λ) were carried out in the wavelength range of λ = [280, 2500] nm. The SWA is defined by the following equation.

Math.1

where G(λ) is the standard G173-03 of the American Society for Testing and Materials (ASTM) for the spectral solar irradiance, commencing from λ = 280 nm; the upper limit of 2500 nm was deemed sufficient to capture most of the solar radiation. The spectral reflectance of the samples was measured at room temperature by a spectrophotometer (Perkin Elmer UV/VIS/NIR Lambda 1050) with an incident angle of 8 °. The spectrophotometer was set to use an integrating sphere that measured the spectral hemispheric directional reflectance ρ from the surface of the sample. As the samples are opaque, there is no transmittance and hence ρ(λ) + α(λ) = 1, where ρ is the measured hemispherical reflectance, α is the absorptance and λ is the wavelength. The spectral values were measured with intervals of Δλ =10 nm. A linear interpolation scheme was conducted to approximate the values of absorptance α at the wavelengths λ that were available for the solar irradiance G, but not for α, which occurred in the lower wavelength range (G is available for Δλ = 0.5 nm). The approximated integrals in Eq. (1) were evaluated at the same discrete values of λ. It was found that using the interpolation scheme with the data obtained in intervals of Δλ =10 nm yields comparable results to the those obtained with the data obtained in intervals of Δλ = 5 nm.

Top layer formulations according to Table 1 were initially applied to bare metallic substrates, i.e. without an absorber coating first applied to the metal substrate. Figures 13 a. and b. show respectively the measured absorptance values for stainless steel 316L and Inconel 625 alloy without top layer (solid line). In each case, results are given for the substrate having no top layer applied (bare metallic substrate), top layer matrix only applied (no silica nanospheres) and the top layer having 100 nm silica nanospheres. A significant improvement in absorptance of the stainless steel and Inconel surfaces is noted, when top layers comprising the nanospheres are applied. In particular, nanospheres were shown to improve absorption in infrared range for both substrates.

Top layer formulations according to Table 1 were also applied over coral-structured absorber coatings, which were placed on substrates of Inconel 625 alloy and 316L stainless steel, in the form of 3 cm x 3 cm coupons (as described above, with reference to Figure 11). The base or ‘normal’ number density corresponds to silica nanoparticles in the applied top layer formulations with silica nanoparticle concentration of 0.215 wt%. Top layer formulations having nanoparticle loadings of half (0.1075 wt%) and double (0.43 wt%) the base amount were also assessed, as were silicon nanoparticles having mean diameters ranging from 10 nm to 100 nm (Tables 2, 3 and Figure 13 c. and d.). Figures 13 c. and d. show the measured SWA (% absorption) as function of wavelength for top layers and absorber coatings (as per Figure 11) applied to Inconel 625 substrates (Figure 13 c.) and stainless steel 316L substrates (Figure 13 d.). The results shown in Figure 13 c. were produced by top layer formulations having silicon nanoparticle concentration of 0.43 wt% (i.e. double concentration to the formulation of Table 1) with varying particle diameters. The results of Figure 13 d. were produced by top layer formulations having silica nanoparticles of approximately 100 nm in diameter, with varying silicon nanoparticle concentration from 0.1075 wt% (half concentration), 0.215 wt% (normal concentration), and 0.43 wt% (double concentration).

Table2

Table3

It was found that for both Inconel 625 and stainless steel substrates, the highest values of SWA were achieved with top layer formulations having particles of a mean diameter (prior to incorporation within the matrix) of approximately 100 nm. In terms of silicon nanoparticle concentration, for 100 nm silica nanoparticles a concentration of 0.215 wt% was found to produce similar absorptance values as for 0.43 wt% (double concentration). These SWA values were found to agree well with the results of numerical modelling.

Figures 14 a.-d. show high magnification SEM images of the top layer deposited onto a coral-structured coating on Inconel 625; the top layer had different silicon nanoparticle concentration (with constant silica sphere size of ~100 nm) applied to the absorber coating 46 of Figure 11 b.2, with the respective solar weighted absorptance values obtained for each case. For absorber coating 46 without any application of the top layer formulation (i.e. pristine coral-structured absorber layer), a base SWA of 97.7% was measured. Application of a top layer formulation having 0.1075 wt% spheres (Figure 14 b.) produced an SWA of 97.9%, while a top layer formulation having 0.215 wt% spheres (Figure 14 c.) was found to produce an SWA of 98.1%, an identical absorptance value to that obtained with a top layer formulation having 0.43 wt% (i.e. double concentration of spheres; Figure 14 d.).

The SWA after thermal ageing of the absorber surfaces was also assessed at the various number density in the top layer (zero, half loading, normal loading and double loading), with ageing carried out at 900°C for 500 hours and 1000 hours (Figure 14 e.). A ‘normal’ number density of nanospheres in the top layer (i.e. a top layer formulation having 0.215 wt% spheres) was shown to minimise the optical degradation of the absorption surface with thermal ageing.

The top layer formulations were thus found to improve not only the light absorption properties of pristine light absorption surfaces, but also the optical durability of the surfaces when subjected to isothermal ageing.

Comparative Thermal Stability Testing
The absorptance performance of the present absorber coatings with top layer applied was compared with Pyromark 2500 (hereafter ‘Pyromark’), before and after ageing at 800°C for up to 3000 hours (Figures 15 a. and b.), and thermal cycling tests up to 3000 cycles (inset of Figure 15 b.). In pristine condition, the coral-structured coating with top layer performs better spectrally than Pyromark (with no top layer) for most wavelengths, except when smaller than 350 nm. After ageing, the present coating has a significantly higher spectral absorptance than Pyromark. It should be noted that the Pyromark deposition method was tuned to improve absorptance values for comparison to the present absorber coating with top layer.

Long-term testing (Figure 15 b.) showed the coral-structured coating with top layer had superior optical stability in comparison to Pyromark and two of the best previously reported long-term stable coatings (as reported by Rubin and Noc). The thermal cycling (Figure 15 b. inset) followed a cycle-and-hold pattern, which has been previously found to be a more stringent test compared with rapid cycling tests (as reported by Torres, J. F., Ellis, I. & Coventry, J. Degradation mechanisms and non-linear thermal cycling effects in a high-temperature light-absorber coating. Sol. Energy Mater. Sol. Cells 218, 110719 (2020)).

Cross-section EDS (energy-dispersive spectroscopy) results showed that the coating morphology was largely unchanged after thermal ageing (Figure 16) despite an oxide layer growing underneath it. Furthermore, normal spectral directional emittance measurements (Figure 15 c.) revealed that the coating with top layer is also optically stable for wavelengths in the infra-red spectrum.

An absorber coating with a preliminary macro-scale morphology was aged at a constant 850°C, and shown to be optically durable (Figure 15 d., green data points), keeping its solar-weighted absorptance >96.0% even after 4000 hours exposure on both nickel-based alloys (Inconel 625 and Haynes 230) and stainless steel 316L. The morphology was further improved by increasing the number and size of the macro-scale protrusions, which produced a more optically resilient coating (Figure 15 d., blue data points). Importantly, the top layer improved the absorptance more than 1% after ageing at 900°C (Figure 15 d. inset), whereas an improvement up to 0.4% was observed in the pristine condition (Figure 14 e.).

Figure 15 e. shows SWA as a function of ageing time with top layers having differing nanoparticle loadings (including matrix only, no nanoparticles) for ageing at 900°C.

Figures 15 f. to i. show the effect on SWA for different ageing conditions, when the number density of nanoparticles in the top layer is varied.

These results demonstrate that different length scales can be tuned to optimise light absorption, with the nano-scale top layer structure (i.e. comprised of matrix and nanoparticles) significantly enhancing absorptance of the underlying absorber surface.

Under isothermal annealing at 900°C, the coating follows a quasi-linear decrease in solar-weighted absorptance (Figure 15 d.), which was associated with the widening of cracks and peeled off regions at discrete locations (Figure 16 4b.2).

Under isothermal annealing at 900°C, the nano-scale morphology of the top layer was largely retained (Figures 16 b.3 and 17), despite the spinel pigments in the absorption layer undergoing crystal growth. In particular, SEM images confirmed that despite crystal growth in the absorption layer, the top layer exhibited only mild sintering of the silica nanospheres (Figure 17).

Application of the Top Layer to Pyromark 2500 Absorber Coating
Using the foregoing top layer application method, top layer formulations were applied to conventional Pyromark 2500 absorber coatings, to assess any improvement to absorptance. Top layer formulations having 100 nm silicon nanoparticle concentration of 0.215 wt% were applied to Pyromark 2500 samples, with a six-layer spray-curing application. Figure 18 shows the results of absorptance testing for Pyromark 2500 samples with (dashed line) and without (solid line) the top layer applied, indicating a significant improvement in absorptance where the top layer was applied, for both pristine and thermally aged samples.

Scalability and Improvement Over Conventional Coatings
The scalability of the top layer (as applied to the absorber coating described in reference to Figure 11) was demonstrated by applying it onto a commercial receiver (Figures 19 a. and b.). The receiver coating process was successful and the solar-weighted absorptance (evidenced by the dark appearance in Figure 19 c.) and coating morphology (Figure 19 d.) were consistent with samples prepares in the laboratory.

Figure 19 e. illustrates the results of further high-temperature durability testing of the absorber coating with top layer, applied Inconel 625 (e.1) and stainless steel 316L (e.2) substrates. The results of Figure 19 e. were obtained from samples aged in the field (i.e. ‘on-sun’ testing for six months), and samples aged isothermally at 850°C for 200 hours (inside a furnace).

These results suggest that the coral-structured coating with top layer not only significantly reduces reflection losses in a real solar thermal receiver, but also makes the light-absorbing coatings more reliable under thermal ageing conditions. The light-trapping could also be further increased by using recently developed fractal-shaped receivers, in combination with the present top layer.

Application of the Top Layer to Pyromark 2500 Absorber Coating in a pre-heat condition
Preparation of Material containing methyl silicate oligomer
700 g of ethanol with 99.5% purity was mixed with 100 g of ion exchanged water and homogenized by ultrasonication for 10 min. In this solution, 10 g of a methyl silicate oligomer (MKC^TM Silicate, grade name MS56 ; manufactured by Mitsubishi Chemical Corporation, which is an oligomer formed by the partial hydrolysis of tetramethoxysilane), and 6 g of a commercially available colloidal silica (ORGANOSILICASOL^TM IPA-ST-ZL, manufactured by Nissan Chemical) were added with continual stirring at 550 rpm for 1 hour at room temperature. The mixture was then added with 1.632 g of acetic acid, followed by stirring at 40°C for 2 days, at a mixing speed of 550 rpm. The mixture may be stored at 4°C in a refrigerator.

Application of Top Layer
The mixture was applied to conventional Pyromark 2500 absorber coatings by spraying and curing of the mixture. The Pyromark 2500 substrate was preheated at 300°C. The mixture was applied on the preheated substrate using a Colani airbrush (manufactured by Harder and Steenbeck) using a nozzle size of 0.4 mm. using a spray pressure of 0.25 Mpa for 3 seconds, with a distance between the nozzle and the substrate to be coated of 20 cm. When the substrate temperature returns to 300°C, the mixture was applied again under the same conditions.

In addition to the mixture as is (referred to as NFS 1 Normal), the mixture was diluted with ethanol at a ratio of mixture:ethanol = 1:1 (referred to as NFS:Etoh = 1:1) or mixture:ethanol = 2:1 (referred to as NFS:Etoh = 2:1). The number of applications was varied from 1 to 5 times. The preferable number of application seems to be 2 times for NFS 1 Normal, 5 times for NFS:Etoh = 1:1, and 4 times for NFS:Etoh = 2:1. Figure 20 shows a spectral reflectance of the Pyromark 2500 with or without top layer (NFS:Etoh = 1:1, 5 times application), as a function of wavelength, indicating the improvement in light trapping, i.e. reduction in solar-weighted reflectance of the top coated Pyromark 2500 between 500 and 1750 nm wavelength, compared to the Pyromark 2500 without top layer.

We then calculate Solar-weighted absorptance (SWA) including sunlight intensity of the above results. Figure 21 shows calculated SWA of the samples with respective top layers for pristine condition and after 100 hours thermal ageing at 800°C. As expected, the highest values were obtained upon twice applications for the NFS 1 Normal, 5 time applications for the NFS:Etoh = 1:1, and 4 time applications for the NFS:Etoh = 2:1 for both for pristine condition and after 100 hours thermal ageing at 800°C. AS shown in Figure 21, the top layer of 5 time applications for the NFS:Etoh = 1:1 has a 0.5% increase in SWA, which is nothing in the world that improves light absorption by 0.5%.

Figure 22 shows a comparison of absorptance performance between the US11002466 absorber coating with the top layer of oligomers containing nanoparticles coated and without the top layer as a function of aging time. Solar-weighted absorption (SWA) was shown as a function of aging time with and without the top layer at 900 C. Both were coated on the same underlying absorbent coating prepared by the method described in US110024666. Even after aging at 900 C for 1000 hours, the SWA of the top layer of oligomer containing nanoparticles was applied hardly decreased.

In the claims which follow and in the preceding description of the invention, except where the configuretext requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims (25)

1. A top layer for a light absorption surface, the top layer comprising a matrix and nanoparticles embedded therein.
2. A top layer according to claim 1, wherein the nanoparticles are in a size range of approximately 10-200 nm.
3. A top layer according to claim 1 or claim 2, wherein the nanoparticles comprise oxide nanoparticles.
4. A top layer according to any preceding claim, wherein the nanoparticles comprise one or more metal oxides selected from silica, titania, zirconia, alumina, magnesia or tin oxide.
5. A top layer according to any preceding claim, the top layer formed from a mixture comprising the nanoparticles, wherein the mixture comprises approximately 0.1075wt% to 0.43wt% nanoparticles.
6. A top layer according to claim 5, wherein the top layer mixture comprises an organosilane.
7. A top layer according to claim 6, wherein the top layer mixture is formed from a tetraethyl orthosilicate precursor, a tetramethyl orthosilicate precursor or an oligomer thereof.
8. A top layer according to claim 7, wherein the tetraethyl orthosilicate precursor, the tetramethyl orthosilicate precursor or an oligomer thereof is hydrolyzed by water and polycondensed in the presence of a catalyst.
9. A top layer according to any preceding claim, wherein the matrix of the top layer has a thickness of approximately 5-100 nm.
10. A top layer according to any preceding claim, wherein the matrix of the top layer comprises nanopores.
11. A method of forming a top layer on a light absorption surface, the method comprising:
    Preparing a first mixture, the first mixture comprising a precursor of a matrix for the top layer;
    Preparing a second mixture, the second mixture comprising nanoparticles;
    Mixing the first and second mixtures to produce a top layer formulation comprising nanoparticles;
    Spraying the top layer formulation onto the light absorption surface; and
    Subjecting the light absorption surface having the top layer to a curing process.
12. A method according to claim 11, wherein the steps of spraying the top layer formulation onto the light absorption surface and subjecting the light absorption surface to the curing process are repeated, so as to form a top layer having a matrix thickness of approximately 5-100 nm.
13. A method according to claim 11 or claim 12, wherein the nanoparticles comprise oxide nanoparticles.
14. A method according to any one of claims 11-13 wherein the first and the second mixtures comprise a tetraethyl orthosilicate precursor, a tetramethyl orthosilicate precursor or an oligomer thereof.
15. A method according to claim 14, wherein the tetraethyl orthosilicate precursor, the tetramethyl orthosilicate precursor or an oligomer thereof is hydrolyzed by water and polycondensed in the presence of a catalyst.
16. A method according to claim 15, wherein the catalyst is hydroxyacetone or an acid.
17. A method according to any one of claims 11-16, wherein the precursor of a matrix comprises an oligomer of the tetraethyl orthosilicate or an oligomer of the tetramethyl orthosilicate, and wherein the step of spraying the top layer formulation onto the light absorption surface is carried out at a temperature of 300°C or more.
    
18. A process for refining a top layer for a light absorption surface by numerical simulation, the process comprising:
    generating a simplified morphology of the light absorption surface;
    generating a set of parameters defining the light absorption surface;
    generating a model system comprising the light absorption surface and a top layer applied thereto; and
    modelling the application of light to the model system, so as to obtain the spectral absorptance and/or the solar-weighted absorptance of the system;
    wherein the top layer of the model system comprises a set of matrix parameters and a set of particle parameters.
19. A process according to claim 18, wherein the step of generating the simplified morphology of the light absorption surface comprises an optical based measurement of the light absorption surface.
20. A process according to claim 18 or claim 19, wherein the set of matrix parameters and/or the set of particle parameters are modified and the process iterated, so as to alter the spectral absorptance and/or the solar-weighted absorptance of the system.
21. A process according to any one of claims 18 to 20, wherein the set of particle parameters relate to particles including one or more metal oxide particles.
22. A process according to claim 21 wherein the one or more metal oxide particles comprise silica, titania, alumina and/or zirconia.
23. A process according to any one of claims 18 to 22, wherein the numerical simulation comprises a computational electromagnetics simulation step.
24. A process according to claim 23, wherein the computational electromagnetics simulation step is based on a Finite-Difference Time-Domain approach.
    
25. A mixture for use in preparing the top layer according to any one of claims 1 to 10, the mixture comprising
    a precursor of a matrix, which comprises a tetraethyl orthosilicate precursor, a tetramethyl orthosilicate precursor or an oligomer thereof,
    nanoparticles, which comprises colloidal silica,
    ethanol,
    water, and
    a catalyst.
    
    

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