Spectral Selective Solar Absorber Coating

Abstract

A spectrally selective solar absorber coating (2) for a receiver of a solar thermal or thermodynamic system, comprising in sequence: ·- an infrared reflecting multi-layer structure (21, 22); ·- an absorptive multi-layer metal-ceramic structure (23); and ·- an anti-reflection multi-layer structure (24). The infrared reflecting multi-layer structure (21, 22) comprises: ·(i) an infrared reflecting base layer (21) having the functions of a barrier layer and an adhesion layer, which is arranged to contact a substrate (1) forming part of the receiver and is composed of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Hf, Ta, Mo, W or of nitrides of one or more of said transition metals; and ·(ii) a multi-functional structure (22), which is placed on the infrared reflecting base layer (21) and at least comprises: ·- a plurality of additional infrared reflecting layers (22a) having the function of increasing the infrared reflectivity of the absorber coating (2), each additional infrared reflecting layer (22a) being composed of a metal selected from Au, Ag, Cu, Al, Mo and W, and ·- a plurality of stabilizing layers (22b) of the additional infrared reflecting layers (22a), each stabilizing layer being composed of: ·a ceramic material; or ·a metal-ceramic material; or ·a material composed of one or more transition metals or nitrides of one or more transition metals. The plurality of additional infrared reflecting layers (22a) and the plurality of stabilizing layers (22b) are arranged such that one reflecting layer alternates with one or more stabilizing layers. One of the stabilizing layers (22b) is placed in contact with the absorptive multi-layer metal-ceramic structure (23).

Description

[0001] Cross-reference to related applications

[0002] This patent application claims priority to Italian Patent Application No. 102020000018676, filed on July 30, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] This invention relates to a spectrally selective solar absorber coating for a receiver used in a solar thermal or thermodynamic system.

[0004] In particular, the present invention has discovered advantageous but non-exclusive applications for tubular receivers, which are explicitly mentioned in the following description without loss of generality.

[0005] In the following text of this document, “metal” means metal or metal alloy; and “material having metallic properties” means material other than metal or metal alloy, but in any case has the characteristics of good electrical conductivity and high light absorption coefficient; the transition metal nitrides and transition metal alloy nitrides provided in some embodiments of the present invention are metallic materials in the specified sense. Background Technology

[0006] In a solar thermal or thermodynamic system, solar radiation is collected by a solar collector and converted into heat in a receiver element in which a heat transfer fluid flows. The heat stored and transferred by the heat transfer fluid can be used as is (solar thermal power plant), can be transferred to another process fluid, or can be converted into electrical energy through a thermodynamic cycle “supplied” by solar energy (solar thermal power plant).

[0007] Especially for solar thermal or thermodynamic systems with parabolic or Fresnel linear manifolds, the receiver, with its tubular shape, is one of the most technologically advanced components in the entire solar system. The receiver tube must absorb as much solar radiation as possible concentrated on it, while dissipating as little heat as possible to the external environment.

[0008] This functionality is achieved through a specific surface coating applied to a metal tube (typically made of steel) through which the heat transfer fluid flows; this metal tube is referred to hereinafter as the "substrate." To effectively perform this function, the surface coating of the receiver must possess optical properties of reflectivity, absorptivity, and emissivity that vary with wavelength. In particular, to obtain a solar receiver with high photothermal efficiency, the coating must have performance as close as possible to ideal, namely zero reflectivity (unit absorbance) in the spectral region of solar radiation (0.3–2.0 μm) and unit reflectivity (zero absorptivity and emissivity) in the thermal infrared spectral region (2.0–40 μm), with a stepped cutoff between the two regions. As previously mentioned, the term "spectrally selective absorber coating" is established when referring to the coatings discussed in this context.

[0009] To achieve this performance, spectrally selective absorber coatings are typically formed from complex multilayer structures of thin layers of different materials, each layer possessing different optical properties, which contributes to the coating's performance at the receiver's operating temperature (ε). th High solar energy absorption rate (α) s and low thermal emissivity or overall high photothermal conversion efficiency (η) pt ) ) expected performance.

[0010] Typically, spectrally selective multilayer structures employ "cermet technology." The acronym "cermet" identifies a class of composite materials primarily composed of mutually nano-dispersed ceramic and metallic phases. The metallic phase can be made of metal or other metallic materials. The unique structure of cermet materials endows them with optical properties such as high absorption in the solar radiation spectral region (0.3–2.0 μm) and good transparency in the thermal infrared spectral region (4.0–40 μm). Therefore, these optical properties impart performance close to the aforementioned ideal characteristics.

[0011] Typically, a cermet solar absorber coating comprises a first layer of a material with high reflectivity in the infrared spectral region, and one or more layers of cermet material that absorbs solar radiation impacting the receiver, while simultaneously being transparent to infrared radiation and not interfering with the infrared reflection function of the first layer.

[0012] In the context of this invention, metals are classified according to their infrared reflectivity as follows:

[0013] - High reflectivity metals: Au, Ag, Cu, and Al;

[0014] -Metals with medium to high reflectivity: Mo, α-W;

[0015] -Medium reflectivity metals: Ti, V, Cr, Zr, Nb, Hf, Ta, β-W; binary or ternary alloys of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; nitrides of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; nitrides of binary and ternary alloys of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W.

[0016] The first layer has the function of ensuring a low emissivity value for the solar coating to minimize heat loss due to radiation from the receiver. A suitable material for this purpose is a metal that has high reflectivity under all conditions, even in the solar spectral region. For this purpose, a layer made of a cermet material is applied to the first metal layer, which, as described above, absorbs solar radiation.

[0017] The properties of layers made of cermet materials vary with the metal content in the cermet. Indeed, as the metal content increases, the absorption capacity of the cermet increases, but its transparency in the infrared region decreases. Therefore, in the production process of the absorption layer of a solar coating, the thickness and metal content of the cermet must be carefully selected to fully utilize the reflected power of the infrared reflector while ensuring a steep transition from the solar spectrum to the infrared region. To best perform this function, the cermet absorption layer typically has a metal content that decreases away from the infrared reflector.

[0018] Finally, the solar coating ends with one or more layers that function as anti-reflective filters. These layers are transparent in the infrared region and their purpose is to minimize the solar radiation reflected by the receiver.

[0019] In short, the absorber coating of the receiver tube used in solar thermal or thermal systems typically consists of a multi-layered structure, including:

[0020] - A metal layer with infrared reflective properties in the absorber coating;

[0021] - One or more metal-ceramic layers applied to a metal layer, which function as a solar energy absorber;

[0022] - One or more typical ceramic layers are applied to a metal-ceramic layer, which function as an anti-reflective filter.

[0023] In addition to infrared reflection, a metal layer with infrared reflective properties must also have good structural and chemical-physical stability, good adhesion to the substrate to which it is applied, a low element diffusion coefficient within the layer, and finally, a good ability to prevent migrating elements from diffusing from the substrate to which it is applied.

[0024] Materials that meet the above requirements include those already defined as having medium reflectivity, such as Ti, V, Cr, Zr, Nb, Hf, Ta, W; binary or ternary alloys of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; nitrides of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; and nitrides of binary and ternary alloys of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W.

[0025] In contrast, metals defined as highly reflective, such as Au, Ag, Cu, and Al, while offering the best performance in terms of hyperspectral reflectivity, are difficult to apply in any situation due to their poor structural and chemical-physical stability, as well as their poor adhesion to substrates, element diffusion, and barrier properties. Their poor ability to act as adhesion and barrier layers makes it problematic to reliably use Au, Ag, Cu, and Al as infrared-reflective metal layers for solar absorber coatings in medium- and high-temperature applications, including considering that this performance deteriorates significantly with increasing temperature.

[0026] In this regard, it is believed that a solar absorber coating is needed that allows the use of the aforementioned high or medium-high reflectivity metals at higher operating temperatures in order to improve the photothermal properties of the coating at these temperatures.

[0027] In addition, it is necessary to increase the stability of the solar absorber coating made of the aforementioned high or medium-high reflectivity metals to ensure less performance degradation or a longer component lifespan throughout the receiver's entire lifespan.

[0028] The inventors of this invention provide a solar absorber coating that can simultaneously meet the above requirements. A key technical feature of this invention's coating is that its structural stability is improved once the aforementioned high or medium-high reflectivity metal is inserted into the multilayer coating.

[0029] Compared to existing technologies, this improved structural stability allows for higher operating temperatures of the aforementioned high or medium-high reflectivity metals, thereby obtaining solar absorber coatings with improved photothermal performance at these temperatures.

[0030] Furthermore, this improved structural stability makes it possible to obtain absorber coatings made of the aforementioned high or medium-high reflectivity metals, which exhibit less performance degradation or longer component lifespan throughout the receiver's entire lifespan.

[0031] Finally, the solar absorber coating, which is the subject of this invention, also has the important advantage of versatility, and can be effectively used in both "vacuum receiver tubes" operating at temperatures up to at least 550°C and "non-vacuum receiver tubes" or "pneumatic receiver tubes" operating at temperatures up to at least 300°C. Summary of the Invention

[0032] The subject of this invention is a solar absorber coating for a receiver element of a solar thermal or thermodynamic system, the basic features of which are set forth in claim 1, and the preferred and / or auxiliary features of which are set forth in claims 2 to 11.

[0033] Another subject of the invention is a receiver for a solar thermal or thermodynamic system comprising a coating according to the invention. More particularly, the receiver is a vacuum receiver tube or a non-vacuum or pneumatic receiver tube.

[0034] The term "vacuum receiver tube" refers to a receiver consisting of: an inner tube (usually made of steel) with an absorber coating applied to its outer surface, an outer glass tube treated with anti-reflective coating, two vacuum-sealed glass-metal joints, and a compensating element (bellows) for the difference in thermal expansion between the inner tube and the glass tube. A vacuum is formed in the space created between the inner tube and the outer glass tube, preferably less than 1 x 10⁻⁶. -4 millibar.

[0035] The term "non-vacuum receiver tube" refers to a receiver configured like a "vacuum receiver tube," except that the space created between the inner tube and the outer glass tube is not a vacuum.

[0036] The term "pneumatic receiver tube" refers to a receiver consisting solely of a steel tube with an absorber coating applied to its outer surface.

[0037] In solar power plants, especially in solar thermal power plants that typically operate at temperatures up to 300°C and are used to generate process heat, non-vacuum tube or pneumatic tube solutions can be very attractive. Indeed, given the greater heat loss at the receiver leading to reduced solar field efficiency, non-vacuum tube or, particularly, pneumatic tube solutions are advantageous due to their decisive reduction in plant costs and simple, robust component structure. Attached Figure Description

[0038] The invention will now be described with reference to the accompanying drawings, according to embodiments for illustrative and non-limiting purposes, wherein:

[0039] · Figure 1 A cross-sectional view of a spectrally selective absorber coating according to a general embodiment of the invention, applied to the surface of a substrate, is schematically shown; and

[0040] · Figure 2 A cross-sectional view of a multifunctional structure of a spectrally selective absorber coating according to a general embodiment of the present invention is shown schematically. Detailed Implementation

[0041] The following is a list of the components shown in the attached diagram:

[0042] 1. Base;

[0043] 2. Spectral selective solar absorber coating;

[0044] 21. Infrared reflective substrate;

[0045] 22. Multifunctional structure;

[0046] 22a Additional infrared reflective layer;

[0047] 22b Stabilizing layer;

[0048] 23 Absorbent multilayer metal-ceramic structure;

[0049] 24 Anti-reflective multilayer structure

[0050] 3. Heat transfer fluid

[0051] For simplicity and clarity, the components shown may not be drawn to scale in the accompanying drawings. For example, for clarity, the dimensions of some components may be enlarged or reduced compared to other components.

[0052] In particular, the thicknesses of the single-layer and multi-layer structures of the spectrally selective solar absorber coating, which is the subject of this invention, are magnified and not necessarily to scale; furthermore, in the drawings, the thickness of the substrate 1 is not shown to scale compared to the elements (layers, multi-layer structures) of the spectrally selective solar absorber coating 2.

[0053] Finally, where convenient, reference numerals may be repeated between figures and within the same figure to indicate corresponding or similar elements. For example, in Figure 2 In the accompanying drawings, reference numerals 22a and 22b are repeated several times and represent elements of the multifunctional structure 22 that are similar (in terms of function) but not necessarily identical (in terms of constituent materials and thickness). Layers 22a are all additional infrared reflective layers, but... Figure 2 Each layer 22a can vary depending on the type of metal constituting it and the thickness of the layer. The stabilizing layer 22b can be ceramic or metallic or a metallic material; Figure 2 Each individual stabilizing layer 22b, independent of the other stabilizing layers 22b, can be ceramic or metallic ceramic or metallic or metallic material, and can be made of different materials within the same class of materials (e.g. ceramic materials); the thickness of each stabilizing layer 22b can also be different from each other.

[0054] Regarding the attached image... Figure 1 A cross-sectional view of a spectrally selective solar absorber coating 2 according to an embodiment of the present invention is schematically shown; in Figure 1In this invention, coating 2 is applied to the surface of tubular substrate 1 and includes, in sequence: an infrared reflective multilayer structure (including an infrared reflective base layer 21 and a multifunctional structure 22), an absorptive multilayer metal-ceramic structure 23, and an antireflective multilayer structure 24. Heat transfer fluid 3 flows within the receiver tube.

[0055] Figure 2 A cross-sectional view of the multifunctional structure 22 is shown schematically.

[0056] exist Figure 2 In the middle, the multifunctional structure 22 consists of some additional infrared reflective layers 22a and some stabilizing layers 22b. The stabilizing layer 22b can be a ceramic and / or a metal ceramic and / or a metal and / or a metallic material.

[0057] exist Figure 2 In accordance with the aforementioned Figure 1 As defined in this invention, the multifunctional structure is included between the infrared reflective substrate 21 and the absorptive multilayer metal-ceramic structure 23.

[0058] Infrared reflective base layer 21

[0059] As previously noted, in this invention, the infrared reflective substrate 21 has infrared reflective functions as the absorber coating 2, the blocking layer for the aforementioned multifunctional structure 22, and the adhesive layer. In some embodiments of this invention, one function may be preferred over others.

[0060] The basic properties of infrared reflective materials suitable for use in absorber coatings that must operate in vacuum at high temperatures (T≥550℃) and in air at moderate temperatures (T≥300℃) are summarized below:

[0061] a. High spectral reflectance in the range of 2.0–40 μm;

[0062] b. At the operating temperature of at least the absorber coating of the structure, in a vacuum (typically at p≤1x10). -4 The structure and chemical and physical stability of (millibars);

[0063] c. Structural and chemical-physical stability in air at an operating temperature of at least 300°C for the absorber coating;

[0064] d. Adhesion to the substrate;

[0065] e. Low elemental diffusion coefficient of the material constituting the infrared reflector within the above layer at the operating temperature of the absorber coating;

[0066] f. The ability of the absorber coating as a barrier layer to prevent the diffusion of elements migrating from the substrate to which the absorber coating is applied, at the operating temperature of the absorber coating.

[0067] Among metallic and metallic materials, many can satisfy most of the characteristics listed above; however, no single material can satisfy all of them simultaneously. Therefore, as an infrared reflective substrate, the primary selection is to determine a metallic or metallic material that can ensure the structural and chemophysical stability of the entire absorber coating under the operating conditions required for the target application. In other words, these metallic or metallic materials must possess good structural and chemophysical stability under the operating conditions of the absorber coating, good adhesion to the substrate to which they are applied, a low elemental diffusion coefficient within the aforementioned layer, and finally, a good ability to block the diffusion of migrating elements from the substrate to which they are applied. Many transition metals and their binary and ternary alloys possess the above characteristics; in particular, as an infrared reflective substrate, one of the following transition metals Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, or binary or ternary alloys of the aforementioned transition metals, can be selected. Nitrides of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, as well as nitrides of their binary and ternary alloys, are materials that possess characteristics that mean they can be used as infrared reflective substrates.

[0068] W has two crystal phases: the α phase W can be inserted between medium and high reflectivity materials, while the β phase W can be inserted between medium reflectivity materials.

[0069] Ti, Zr, Mo, W, TiN, and ZrN are preferred candidates for constituting the infrared reflective substrate 21 of the present invention. In fact, these materials can effectively function as a first infrared reflector, a barrier layer, and an adhesive layer. Furthermore, they exhibit excellent structural and chemical-physical stability at high operating temperatures and are characterized by low elemental diffusion coefficients.

[0070] In summary, the present invention requires that the infrared reflective substrate 21 be composed of a transition metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, or a binary or ternary alloy of the aforementioned transition metals, or a nitride of one of the aforementioned transition metals, or a nitride of a binary or ternary alloy of the aforementioned transition metals.

[0071] The thickness of the infrared reflective substrate 21 ranges from 1 to 250 nm, preferably from 90 to 150 nm. This ensures that the layer can effectively function as the first infrared reflector, the blocking layer, and the adhesive layer.

[0072] According to some preferred embodiments of the present invention, the infrared reflective substrate 21 is composed of Ti, Zr, Mo, W, TiN and ZrN.

[0073] Multifunctional structure 22

[0074] As described above, in this invention, the multifunctional structure 22 is applied to the infrared reflective substrate 21.

[0075] The multifunctional structure 22 consists of one or more additional infrared reflective layers 22a and at least one or more stabilizing layers 22b, wherein the stabilizing layer 22b is composed of ceramic or cermet or metal or metallic material or a combination of the same layers.

[0076] The additional infrared reflective layer 22a has the function of increasing the infrared reflectivity of the infrared reflective base layer 21 and thus increasing the infrared reflectivity of the entire absorber coating 2 or improving its thermal emissivity. In view of what has been discussed, the metals that can be used to perform the function of increasing the infrared reflectivity of the absorber coating 2 are the following:

[0077] a. A metal with high reflectivity (Au, Ag, Cu, Al), wherein the infrared reflective substrate 21 is made of a medium or medium-high reflectivity material;

[0078] b. A metal with medium to high reflectivity (Mo, α-W), wherein the infrared reflective substrate 21 is made of a medium reflectivity material.

[0079] Regarding highly reflective metals, Au, Ag, Cu, and Al exhibit very high reflectivity values ​​starting from the near-infrared (1.0–2.0 μm), achieving an optical “cutoff” with a very steep increase in emissivity transition from the solar absorption region to thermal emissivity. This property is advantageous from the perspective of the spectral selectivity of absorber coatings, allowing for low emissivity values, including in the presence of high operating temperatures. Furthermore, the refractive index (n) and extinction coefficient (k) of these metals change only slightly with increasing temperature, thus the use of Au, Ag, Cu, and Al remains advantageous. Consequently, even at high operating temperatures, the decrease in spectral infrared reflectivity is reduced, as is the deterioration of thermal emissivity.

[0080] As mentioned earlier, from an optical properties perspective, Au, Ag, Cu, and Al are the preferred metals for increasing the infrared reflectivity of absorber coatings or improving their thermal emissivity.

[0081] Given the favorable optical properties of Au, Ag, Cu, and Al, these metals possess:

[0082] - Adhesion performance is very poor on the surfaces of materials typically used as substrates for absorber coatings in solar receivers, especially on steel.

[0083] - It is less effective as a barrier layer for the diffusion of elements migrating from the substrate (on which an absorber coating is applied).

[0084] The poor ability of Au, Ag, Cu, and Al as adhesive and barrier layers hinders their use as the first layer in solar absorber coatings for medium- and high-temperature applications, including considering that this performance deteriorates significantly with increasing temperature.

[0085] Based on the above, the use of Ag, Al, Cu, and Au in the multifunctional structure 22 to increase the low heat emission performance of the spectrally selective solar absorber coating 2 always requires the presence of the following layer, which acts as both an adhesion layer and a barrier layer: This invention explicitly defines that this function is performed by the infrared reflective substrate 21, which is made of a material appropriately selected for this purpose. In other words, inserting an infrared reflective substrate 21 of appropriately selected material between the substrate 1 and the multifunctional structure 22 improves adhesion to steel (or other materials constituting the substrate) and provides effective barrier against the diffusion of elements from the substrate material.

[0086] In addition to the issues already mentioned, Au, Ag, Cu, and Al are also affected by a series of inherent phenomena that are strongly temperature-dependent, such as crystallization reorientation and changes in grain size:

[0087] Au, Ag, Cu and Al deposited in thin film form have a “primary (as-grown)” preferred crystal orientation and a series of secondary orientations; at high temperatures, these metals may undergo crystallization reorientation, i.e. recrystallization, and the structure of the deposited film will change.

[0088] Au, Ag, Cu and Al deposited in thin film form have “native” grains of a certain size; at high temperatures, aggregation (i.e., coalescence) may occur, accompanied by an increase in grain size and a subsequent change in the structure of the deposited film.

[0089] The inherent reorientation and coalescence phenomena mean that the highly reflective metal layer composed of one of Au, Ag, Cu and Al is structurally unstable and may compromise the structural stability of the entire coating to which it is inserted.

[0090] Another issue associated with the use of Au, Ag, Cu, and Al in solar absorber coatings is the high diffusion coefficient of these metals in the various materials commonly used in solar receiver absorber coatings. This high diffusion performance of Au, Ag, Cu, and Al deteriorates significantly with increasing temperature. In this case, diffusion at the interface between the metal (Au, Ag, Cu, and Al) and the cermet absorber involves changes in the composition of adjacent layers and potential chemical reactions between different materials in each layer, leading to a degradation in the performance of the infrared reflector and the absorption performance of the cermet. The poor chemical-physical stability of the absorber coating is exacerbated at high temperatures and during long-term operation due to variations in the stoichiometry or composition of the cermet layer and the infrared reflector.

[0091] In summary, as the operating temperature of solar absorber coatings increases, the use of high-reflectivity metals (such as Au, Ag, Cu, and Al) to potentially improve the infrared reflectivity of the coatings leads to structural and chemical-physical instability of the coatings, resulting in a decline in photothermal performance over time.

[0092] Therefore, the use of Au, Ag, Cu, and Al in coatings for solar absorbers that are not only highly photothermal efficient but also very durable means that these highly reflective metals need to be "stabilized" or that problems associated with crystal reorientation, coalescence, and high diffusion coefficients need to be mitigated.

[0093] To avoid problems associated with using Au, Ag, Cu, and Al as the additional infrared reflective layer 22a, the present invention provides a series of innovations, including a multifunctional structure 22 applied to the infrared reflective substrate 21. This multifunctional structure 22 consists of one or more reflective layers defined as the additional infrared reflective layer 22a and made of metals selected from Au, Ag, Cu, and Al, and one or more stabilizing layers 22b. The material of the stabilizing layer 22b is appropriately selected to perform one or more functions, such as improving the inherent structural stability of the additional infrared reflective layer 22a, acting as a barrier to hinder the diffusion of the metal constituting the additional infrared reflective layer 22a, thereby improving the overall chemical and physical stability of the absorber coating 2, and acting as an adhesion layer for the aforementioned absorbent multilayer metal-ceramic structure 23. Therefore, the multilayer structure 22 of the present invention is defined as a “multifunctional structure” 22.

[0094] In this context, the above content is achieved through the innovation described below.

[0095] The present invention relates to employing an additional infrared reflective layer 22a, which is thick enough to ensure optical performance close to that of a "bulk" material, but small enough to mitigate inherent crystal reorientation and coalescence.

[0096] Only when the additional infrared reflective layer 22a is made of Ag, this invention relates to improving the inherent stability of materials by manufacturing them using high-power sputtering in an argon + nitrogen atmosphere, rather than the typical argon-only atmosphere used in metal deposition. The high power enables the participation of very high-energy (high-mobility) Ag atoms in the growth of the Ag layer, but simultaneously, the additional nitrogen input only allows the highest-energy atoms to participate in film growth. In practice, the growth process will occur at high energy and low growth rates, which are preferred conditions for the formation of Ag film crystal structures, approaching the ideal conditions for single-crystal formation (with almost no inherent recrystallization and agglomeration).

[0097] As already described, particularly with respect to embodiments in which the additional infrared reflective layer 22a is composed of Au, Ag, Cu, and Al, the present invention defines that one or more stabilizing layers 22b of the multifunctional structure 22 can perform one or more functions, such as:

[0098] - Improve the inherent structural stability of one or more highly reflective metal layers 22a;

[0099] - As a barrier to the diffusion of highly reflective metals;

[0100] - As an adhesion layer for the above-mentioned absorbent multilayer metal-ceramic structure 23.

[0101] In order to perform these functions, the materials of one or more stabilizing layers 22b constituting the multifunctional multilayer structure 22 must be:

[0102] -At high temperatures, it is stable and chemically inert relative to the metals constituting one or more additional infrared reflective layers 22a;

[0103] - Apply the film in the infrared spectral region in a transparent or sufficiently thin form so as not to impair the high reflectivity optical properties of the entire multifunctional structure 22;

[0104] - Compact enough, and once applied to the additional infrared reflective layer 22a, ensures conformal coverage;

[0105] - A strong bond can be formed at the interface with the metal of the additional infrared reflective layer 22a, thereby achieving strong adhesion between adjacent layers 22a and 22b.

[0106] Furthermore, for a strong adhesion between the multifunctional structure 22 and the absorbent multilayer metal-ceramic structure 23, the only material constituting the last layer of the multifunctional structure 22 must be able to form a strong bond at the interface with the metal-ceramic material of the absorbent multilayer metal-ceramic structure 23.

[0107] It should be noted that materials with this property may be able to improve the inherent structural stability of Au, Ag, Cu and Al, and effectively block the diffusion process of such highly reflective metals.

[0108] The present invention identifies materials that can be used to perform one or more functions, such as improving the inherent structural stability of the metal of the additional infrared reflective layer 22a, acting as a barrier to the diffusion of the metal of the additional infrared reflective layer 22a, improving the chemical and physical stability of the entire absorber coating 2, and acting as an adhesion layer for the aforementioned absorbent multilayer metal-ceramic structure 23.

[0109] The present invention classifies the above materials into the following categories: ceramic oxides, ceramic nitrides, oxide-based cermets, nitride-based cermets, transition metals and their alloys, and metallic nitrides.

[0110] Regarding the material of the metal layer constituting the additional infrared reflective layer 22a, medium-to-high reflectivity metals (Mo and α-W) within the scope of the present invention can be used, wherein the underlying infrared reflective substrate 21 is composed of a medium reflectivity material.

[0111] The excellent structural and chemical-physical stability of Mo and W, as well as their low diffusion coefficient, can be further improved by alternating these materials, in the form of an additional thin infrared reflective layer 22a, with a ceramic, cermet, metal, or metal performance-stabilizing layer 22b within the multifunctional structure 22.

[0112] The use of these medium-to-high reflectivity metals is preferred if the aim is to provide a spectrally selective absorber coating 2 for solar receivers, especially in a vacuum (p≤1x10⁻⁶). -4 For high-temperature applications (up to at least 550°C) in millibars, the coating has at least the same photothermal properties (α) as prior art coatings. s ε th And improved stability (durability, lifespan). Furthermore, these medium-to-high reflectivity metals are the best candidates for producing spectrally selective solar absorber coatings 2, which are highly versatile and capable of delivering excellent photothermal performance (high α) and improved stability (durability, lifespan). s Low ε th It features high stability (durability, service life) for applications in air at temperatures up to at least 300°C.

[0113] In summary, the present invention specifies that the additional infrared reflective layer 22a of the multifunctional multilayer structure 22 of the spectrally selective solar absorber coating 2 is composed of one or more metals selected from Al, Cu, Ag, Au, Mo, and W.

[0114] For in a vacuum (p≤1x10) -4 For high-temperature applications (up to at least 550°C) in millibars, there is a desire to obtain photothermal performance (α) that is at least as good as existing technologies. s and ε th The absorber coating 2, which provides stability (durability, service life), is limited to the use of Ag and Au as an additional infrared reflective layer 22a, given the low melting temperature of Al relative to Au and Ag and the high diffusion coefficient of Cu.

[0115] In particular, considering that the cost of Ag is much lower than that of Au, the preferred embodiment of the present invention specifies that the additional infrared reflective layer 22a is made of Ag.

[0116] For in a vacuum (p≤1x10) -4For high-temperature applications (up to at least 550°C) in millibars, it is desirable to obtain photothermal performance (α) that is at least consistent with existing technologies. s and ε th The preferred embodiment of the invention defines the use of Mo and α-W as an additional infrared reflective layer 22a, which includes an absorber coating 2 with improved stability (durability, service life).

[0117] Finally, for applications where temperatures in the air can reach at least 300°C, there is a desire for products with excellent photothermal properties (high α). s Low ε th The preferred embodiment of the invention defines the use of Mo and α-W as an additional infrared reflective layer 22a, which is a particularly versatile absorber coating with high stability (durability, service life).

[0118] As previously stated, the present invention classifies the materials that can form the stabilizing layer 22b into the following categories: ceramic oxides, ceramic nitrides, oxide-based cermets, nitride-based cermets, transition metals and their alloys, and metallic nitrides.

[0119] In this context, some embodiments of the invention specify that the stabilizing layer 22b is ceramic, and in particular, is composed of oxides of transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, or binary or ternary alloys of these transition metals; or oxides of Al or Si or binary alloys of Al and Si; or nitrides of binary alloys of Al or Si or Al and Si. The thickness of each stabilizing layer 22b made of ceramic material is preferably in the range of 5 to 120 nm. According to some preferred embodiments of the invention, the stabilizing layer 22b made of ceramic material is composed of oxides and nitrides of Al₂O₃, SiO₂, Si₃N₄, AlN, and binary alloys of Al and Si; in such preferred embodiments of the invention, Al₂O₃, SiO₂, Si₃N₄, and AlN can be in both substoichiometric and stoichiometric forms.

[0120] Other embodiments of the invention specify that the stabilizing layer 22b is made of cermet, and in particular, is a cermet in which the metallic component consists of a transition metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, or a binary or ternary alloy of these transition metals, and the ceramic component consists of an oxide of Al or Si or a binary alloy of Al and Si, or a nitride of a binary alloy of Al or Si or Al and Si. The thickness of each stabilizing layer 22b made of cermet is preferably in the range of 5 to 50 nm. According to some preferred embodiments of the invention, in the stabilizing layer 22b, the cermet is composed of Zr-SiO2, Ti-AlN, Ti-Si3N4, Ta-Si3N4, Zr-Si3N4, Mo-Si3N4, W-Si3N4, or Cr-Si3N4.

[0121] In other embodiments of the invention, the stabilizing layer 22b is defined as metallic, and particularly, as composed of a transition metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, or a binary or ternary alloy of the aforementioned transition metals. The thickness of each metallic stabilizing layer 22b is preferably in the range of 5 to 20 nm. According to some preferred embodiments of the invention, the stabilizing layer 22b is composed of a binary alloy of Ti, Ti and W, or a binary alloy of Ta and W.

[0122] Finally, as a final embodiment, the present invention specifies that the stabilizing layer 22b is made of a metallic material, and in particular, is composed of transition metal nitrides selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, or nitrides of binary or ternary alloys of these transition metals. The thickness of each stabilizing layer 22b made of the metallic material is preferably in the range of 5 to 20 nm. According to some preferred embodiments of the invention, the stabilizing layer 22b is composed of nitrides of binary alloys of TiN, ZrN, NbN, MoN, TaN, WN, Ti, and Zr.

[0123] Taking all of the above into account, the stabilization of the additional infrared reflective layer 22a within the multifunctional multilayer structure 22 is achieved by using a stabilizing layer 22b made of a suitably selected ceramic or metal ceramic or metal material.

[0124] In principle, multiple stabilizing layers 22b can be used to stabilize the thin, highly reflective metal layer 22a, which is typically made of different materials, and each can improve the inherent structural stability of the additional infrared reflective layer 22a and / or serve as a barrier layer against metal diffusion of the additional infrared reflective layer 22a and / or as an adhesive layer.

[0125] For the sake of simplicity, economy and speed in the manufacturing process of the "solar receiver" component, it is advantageous to stabilize the additional infrared reflective layer 22a by using a single stabilizing layer 22b made of materials that perform all the above functions.

[0126] The present invention defines a minimum two-layer structure for the multifunctional structure 22, which is applied to the infrared reflective substrate 21: a first additional infrared reflective layer 22a and a second stabilizing layer 22b.

[0127] A preferred embodiment of the present invention limits the use of a multifunctional structure 22 consisting of: a layer pair formed by an additional infrared reflective layer 22a and a stabilizing layer 22b made of ceramic material, or a layer pair formed by an additional infrared reflective layer 22a and a stabilizing layer 22b made of cermet material.

[0128] It should be understood that, as shown above, the additional infrared reflective layer 22a must be thick enough to ensure optical performance close to that of a “bulk” material, but small enough to mitigate inherent crystal reorientation and coalescence.

[0129] The optical performance of the absorber coating 2's infrared reflectivity is achieved by using an additional thin infrared reflective layer 22a, which is certainly an improvement over the performance obtainable with only one infrared reflective substrate 21. However, it can be further improved by repeating the number of "additional infrared reflective layer 22a / stabilizing layer 22b made of ceramic material" pairs or "additional infrared reflective layer 22a / stabilizing layer 22b made of cermet" pairs.

[0130] In other words, as the number of "high reflectivity metal layer 22a / ceramic layer 22b" pairs increases, that is, as the number of "high reflectivity metal layer 22a / metal-ceramic layer 22b" pairs constituting the multifunctional structure 22 increases, the infrared reflectivity of the spectrally selective solar energy absorption layer 2 will increase.

[0131] However, generally, the present invention defines the multifunctional structure 22 as consisting of a maximum of 30 layers, arranged to improve the low heat emission performance and stability of the spectrally selective solar absorber coating 2. In a preferred embodiment of the invention, the multifunctional structure 22 consists of a pair of "high-reflectivity metal layer 22a / ceramic layer 22b" or a pair of layers formed of "high-reflectivity metal layer 22a / metal-ceramic layer 22b", with each pair repeated up to a maximum of 15 times. In such a preferred embodiment, the high-reflectivity metal layer 22a and the ceramic or metal-ceramic stabilizing layer 22b constituting the pair have a thickness in the range of 5 to 40 nm.

[0132] Absorbent multilayer metal-ceramic structure 23

[0133] As described above, in this invention, the absorptive multilayer metal-ceramic structure 23 functions as the absorber of the spectrally selective solar absorber coating 2.

[0134] "Cereceramics" are composite materials composed of ceramic and metallic phases that are nano-dispersed together. This special structure endows cereceramics with high absorption in the solar radiation spectral region and good transparency in the thermal infrared spectral region, thus not interfering with the infrared reflection function of the underlying multifunctional structure 22 and infrared reflective substrate 21.

[0135] For effective use in the absorber coating 2 of this invention, the cermet material must possess high inherent stability; therefore, the materials constituting its phase must exhibit high structural and chemophysical stability at high temperatures. Furthermore, these materials must possess a very low diffusion coefficient at high temperatures.

[0136] This invention identifies a series of materials for the metallic phase constituting the cermet absorber of the absorbent multilayer cermet structure 23 having the aforementioned properties. In some embodiments of the invention, the metallic phase of the cermet is composed of transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W. In other embodiments, the metallic phase of the cermet is composed of binary or ternary alloys of transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.

[0137] The present invention also identifies a series of materials for the ceramic phase constituting the cermet absorber of the absorbent multilayer cermet structure 23 having the above-described properties. In some embodiments of the invention, the ceramic phase of the cermet is an oxide, and in particular, it consists of Al oxide or Si oxide or oxides of a binary alloy of Al and Si. In other embodiments, the ceramic phase of the cermet is a nitride, and in particular, it consists of Al nitride or Si nitride or nitrides of a binary alloy of Al and Si.

[0138] In a preferred embodiment of the invention, where the materials used to construct the metal and ceramic phases of the metal-ceramic absorber are determined to be suitable and effective, the absorbent multilayer metal-ceramic structure 23 is composed of a transition metal selected from Ti, Zr, Mo, and W, and a ceramic material composed of Al oxides. In another preferred embodiment of the invention, the absorbent multilayer metal-ceramic structure 23 is composed of a transition metal selected from Ti, Zr, Mo, and W, and a ceramic material composed of Al nitrides.

[0139] Compared to cermets with nitride ceramic components, absorbent multilayer cermet structures 23 with oxide ceramic components are more difficult to manufacture.

[0140] To produce cermet layers, the most commonly used deposition technique is reactive co-sputtering, i.e., simultaneous sputtering, starting from targets of different materials in argon and reactive gases (O2, N2), where at least one target can produce the metallic component of the cermet and at least one target can produce the ceramic component of the cermet.

[0141] To obtain oxide-based cermets with desired optical properties, the amount of O2 used during deposition must be high enough to oxidize the metal components of the cermet, but low enough to minimize the oxidation of the metal components. To achieve stability and repeatability of process conditions, reactive co-sputtering must be performed in a "transition state," requiring reactive gases to flush the control system, which is difficult to regulate due to the high reactivity of O2.

[0142] Given the challenging setup of oxide-based cermet manufacturing processes, using these materials offers a significant advantage in terms of achievable maximum deposition rates compared to nitride-based cermets. In fact, the higher reactivity of O2 compared to N2 allows for the oxidation of a greater amount of metal per unit time, thus increasing the deposition rate of oxide-based cermets compared to nitride-based cermets.

[0143] As described above, a preferred embodiment of the present invention provides the use of a metal-ceramic absorber having a ceramic phase composed of Al oxides. This ceramic material exhibits high structural and chemical-physical stability even in a vacuum at very high temperatures (well above 550°C), hence this choice. Furthermore, this ceramic material is very stable even in air at high temperatures (well above 300°C), making it a suitable material for the ceramic phase of the metal-ceramic absorber coating 23 of the present invention, for applications in non-vacuum or pneumatic receivers.

[0144] Compared to cermets with oxide ceramic components, absorbent multilayer cermet structures 23 using materials with nitride ceramic components are easier to manufacture.

[0145] To obtain nitride-based cermets with desired optical properties, the amount of N2 used during deposition must be high enough to nitride the metallic components of the cermet, but low enough to minimize the nitriding of the metallic components. Due to the low reactivity of N2, reactive co-sputtering processes with high stability and reproducibility can be implemented in both the "transition state" and the "saturation state".

[0146] In the case of transition processes, while a system for controlling reactive gas flushing is still necessary, its management is more easily adjustable, allowing for higher deposition rates than in saturated process cases. It is understandable that saturated reactive co-sputtering processes for fabricating nitride-based cermets are significantly advantageous due to their simplicity of setup and high repeatability.

[0147] As described above, the preferred embodiment of the present invention limits the use of a cermet absorber having a ceramic phase composed of Al nitrides; this ceramic material was chosen because it exhibits high structural and chemical-physical stability even at very high temperatures (well above 550°C) in a vacuum.

[0148] The absorbent multilayer metal-ceramic structure 23 of the present invention can be formed from a single metal-ceramic layer or multiple metal-ceramic layers (up to 40).

[0149] The use of multiple metal-ceramic layers minimizes the optical mismatch encountered by solar radiation as it propagates within the absorber coating 2, and thus maximizes the absorption of solar radiation by the absorptive multilayer metal-ceramic structure 23. To achieve this optical performance, the present invention employs a suitable volume fraction distribution V of the metal composition in the metal-ceramic layers. 金属 / (V 金属 +V 陶瓷 In some preferred embodiments of the invention, the volume fraction of the metal component in the cermet layer decreases along the direction of the absorber coating 2 from the lower multifunctional structure 22 to the upper antireflective multilayer structure 24.

[0150] Finally, the present invention specifies that the absorbent multilayer metal-ceramic structure 23 preferably has a thickness of 5 to 120 nm, and in any case not greater than 200 nm.

[0151] Anti-reflective multilayer structure 24

[0152] In this invention, the antireflective multilayer structure 24, composed of one or more ceramic or cermet layers or combinations of the same layers, functions as an antireflective filter, i.e., minimizing the reflection of solar radiation striking the receiver. This function is achieved by reducing the optical mismatch between the last layer of the absorptive multilayer cermet structure 23 and the solar radiation propagation medium, or by applying the principle of interferometry.

[0153] Preferably, the principle of interferometry is applied to effectively perform the antireflective filter function. Utilizing available transparent materials with very different refractive indices allows for the creation of optically more effective "interference" antireflective filters, minimizing solar radiation reflected by the coating 2 while maximizing its absorption. In this field, using a cermet layer in the antireflective multilayer structure 24 provides the advantage of a layer with a higher refractive index compared to using a single ceramic layer. The significant increase in the refractive index of the cermet layer is achieved through the low volume fraction of the metal component in the cermet, which has the advantage of low absorption in the layers constituting the filter itself. As shown, the present invention defines each cermet layer of the antireflective multilayer structure 24 as characterized by a constant volume fraction of the metal component and ranging between 0.01 and 0.3.

[0154] In the case of interference, the present invention fixes the maximum number of layers constituting the anti-reflective multilayer structure 24 to 20, because this number is sufficient to design and produce a filter with the desired characteristics.

[0155] For the antireflective multilayer structure 24 used in this invention, the material must have high inherent stability; in other words, it must have high structural and chemical-physical stability at high temperatures, and in addition, it must have a very low diffusion coefficient in adjacent layers at high temperatures.

[0156] Due to their optical properties (refractive index), stability, and limited diffusion tendency, this invention identifies a series of ceramic and cermet materials suitable for constituting antireflective layers.

[0157] In this context, some embodiments of the present invention define the antireflective multilayer structure 24 of the absorber coating 2 as comprising a ceramic layer composed of an oxide of a transition metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, or W, or a binary or ternary alloy of these transition metals; or an Al or Si oxide or a binary alloy of Al and Si; or an Al or Si nitride or a binary alloy of Al and Si. The present invention defines the thickness of each ceramic layer as ranging from 1 to 200 nm, preferably from 5 to 120 nm. According to some preferred embodiments of the present invention, the ceramic layer of the antireflective structure 24 is composed of Al₂O₃, SiO₂, Si₃N₄, and AlN.

[0158] Other embodiments of the present invention specify that the antireflective multilayer structure 24 of the absorber coating 2 includes a cermet layer, wherein the metallic component is composed of a transition metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, or W, or a binary or ternary alloy of these transition metals, and wherein the ceramic component is composed of Al or Si oxides or a binary alloy of Al and Si, or Al or Si nitrides or a binary alloy of Al and Si. The present invention specifies that the thickness of each cermet layer ranges from 1 to 120 nm, preferably from 1 to 50 nm. According to some preferred embodiments of the present invention, the cermet layer of the antireflective structure 24 is composed of a transition metal selected from Ti, Zr, Mo, or W and a ceramic material composed of Al oxides or nitrides.

[0159] In some embodiments, the spectrally selective solar absorber coating of the present invention can be used in a vacuum at a maximum temperature of at least 550°C (p≤1x10⁻¹). -4 Operating at millibars. In such embodiments, the spectrally selective solar absorber coating of the present invention is characterized by excellent performance in terms of high solar absorptivity and low thermal emissivity, as well as higher high-temperature performance (up to at least 550°C) than those of the prior art. Furthermore, in such embodiments, the spectrally selective solar absorber coating of the present invention has a service life of up to 25 years and exhibits excellent stability in a vacuum at temperatures up to at least 550°C.

[0160] In another embodiment, the spectrally selective solar absorber coating of the present invention can operate in air at temperatures up to at least 300°C. In such an embodiment, the spectrally selective solar absorber coating of the present invention is characterized by excellent performance in terms of high solar absorptivity and low thermal emissivity. Furthermore, in such an embodiment, the spectrally selective solar absorber coating of the present invention has a service life of up to 25 years and exhibits high stability in air at temperatures up to at least 300°C.

[0161] This provides a versatile, spectrally selective solar absorber coating that is effective for both vacuum applications up to at least 550°C and air applications up to at least 300°C, which is superior to existing technologies for absorber coatings used in solar receivers.

[0162] Method for preparing absorber coatings

[0163] The present invention also relates to a method for producing a spectrally selective solar absorber coating 2 by means of physical vapor deposition (PVD) technology, particularly by means of sputtering technology with a high deposition rate, which is the subject of the present invention.

[0164] In this context, these techniques are i) magnetron sputtering and co-sputtering with a metal target in argon gas, and ii) magnetron reactive sputtering and co-sputtering with a metal target in argon gas + reactive gases (O2, N2).

[0165] This invention limits sputtering deposition to 5x10 -4 millibars to 5x10 -2 It is performed within a pressure range of millibars, specifically at 1x10⁻⁶. -3 millibars to 2x10 -2 It is performed within the pressure range of millibars.

[0166] It should be noted that in this invention, "co-sputtering" refers to simultaneous sputtering from targets of different materials in argon gas, while "reactive co-sputtering" refers to simultaneous sputtering from targets of different materials in argon gas and reactive gases (O2, N2).

[0167] Regarding the structure of the spectrally selective solar absorber coating 2 of the present invention, the manufacturing process includes the following steps:

[0168] - Provide base 1;

[0169] - Apply an infrared reflective substrate 21 to all or part of the surface of the substrate 1 by sputtering technology;

[0170] - Apply a multifunctional structure 22 to the infrared reflective substrate 21 using sputtering technology;

[0171] - Apply an absorptive multilayer metal-ceramic structure 23 to the multifunctional structure 22 using sputtering technology;

[0172] - Apply an anti-reflective multilayer structure 24 to the absorbent multilayer metal-ceramic structure 23 using sputtering technology.

[0173] In this invention, the substrate 1 is part of a solar receiver.

[0174] In many embodiments of the invention, the substrate 1 is composed of metal or alloy, and in particular of steel.

[0175] In a preferred embodiment of the invention, the substrate 1 consists of a steel tube for a receiver of a solar thermal or thermodynamic system having a parabolic or Fresnel linear manifold.

[0176] In the case of a tubular substrate 1, the substrate 1 is typically rotated about its own axis during the sputtering deposition process in order to apply the layer of the spectrally selective solar absorber coating 2 to the entire outer surface of the tube; in this case, the tube-substrate 1 is rotated relative to the sputtering source, i.e., in front of the electrode composed of the magnetron cathode.

[0177] The present invention specifies that, prior to the sputtering deposition process of the absorber coating 2, the substrate 1 may be subjected to thermal pretreatment or plasma pretreatment, typically Ar, or a combination of one or more thermal pretreatments or plasma pretreatments.

[0178] Possible thermal pretreatment of the substrate in a vacuum at less than 5 x 10⁻⁶ m² / h. -4 millibars, especially preferred below 1x10 -4 The process is carried out under millibar pressure so that substrate 1 reaches a temperature of 100°C to 300°C.

[0179] Possible plasma pretreatment typically occurs in Ar gas at concentrations below 5 x 10⁻⁶. -2 Under millibar pressure, polarization is carried out via substrate 1 or below 5 x 10 -3 The process is carried out under millibar pressure by an ion source capable of bombarding substrate 1 with high-energy ions.

[0180] In the case of a tubular substrate 1, this typically involves rotating it during possible pretreatment to uniformly and effectively adjust the entire surface on which sputtering deposition will be performed.

[0181] The sputtering process described below is specific to the application of various materials to form the layer of the spectrally selective solar absorber coating 2 (which is the main subject of this invention). In other words, the sputtering method defined by the manufacturing method of this invention will be "classified" according to the material to be deposited, because the choice and definition of the specific method essentially depends on the type of material to be manufactured.

[0182] This invention relates to a method for producing a spectrally selective solar absorber coating 2, wherein a metal layer composed of a transition metal or a highly reflective metal is applied in Ar gas by magnetron sputtering using a target of the aforementioned transition metal or highly reflective metal. In the case where an additional infrared reflective layer is composed of Ag, this layer can be produced by magnetron sputtering in Ar+N2 gas using an Ag target, for reasons explained above.

[0183] This invention specifies that a metal layer composed of a binary or ternary alloy of transition metals is applied in Ar gas via magnetron sputtering through a target formed of a specific alloy of interest, or via magnetron co-sputtering through a target of each metal forming the alloy in Ar gas. In this case, the desired alloy composition can be obtained by appropriately changing the feed power of the target for each metal.

[0184] This invention specifies that a metallic performance layer composed of transition metal nitrides is applied in Ar+N2 gas through a target of the aforementioned transition metal using magnetron reactive sputtering technology.

[0185] This invention defines a metallic performance layer composed of nitrides of binary or ternary alloys of transition metals, applied in Ar+N2 gas by a target formed of a specific alloy of interest via magnetron reactive sputtering, or applied in Ar+N2 gas by a target of individual metals supplied with appropriate power via magnetron reactive co-sputtering.

[0186] The present invention specifies that a ceramic layer composed of oxides of transition metals or oxides of Al or Si is applied by reactive sputtering in Ar+O2 gas via a magnetron through a target of the aforementioned transition metal or through a target of Al or Si.

[0187] The present invention defines a ceramic layer consisting of oxides of binary or ternary alloys of transition metals or oxides of binary alloys of Al and Si as being applied by magnetron reactive sputtering in Ar+O2 gas via a target formed of a specific alloy of interest, or by magnetron reactive co-sputtering in Ar+O2 gas via a target supplied with a suitable power to form a single metal of the alloy.

[0188] This invention specifies that a ceramic layer composed of Al or Si nitrides is applied to an Al or Si target by reactive sputtering in an Ar+N2 gas via a magnetron.

[0189] The present invention specifies that a ceramic layer composed of a binary alloy of Al and Si nitrides is applied in Ar+N2 gas by magnetron reactive sputtering via a target formed of a specific alloy of interest, or by magnetron reactive co-sputtering in Ar+N2 gas via a target of Al and Si supplied with appropriate power.

[0190] In order to produce a cermet layer, the present invention defines a deposition technique for reactive co-sputtering (i.e. simultaneous sputtering) starting from targets of different materials in Ar gas + reactive gas (O2, N2), wherein at least one target is capable of producing the metallic component of the cermet and at least one target is capable of producing the ceramic component of the cermet.

[0191] In this context, the present invention defines a cermet layer having a metallic composition consisting of a transition metal or a binary or ternary alloy of transition metals and a ceramic composition consisting of an Al or Si oxide or an oxide of a binary alloy of Al and Si, applied in Ar+O2 gas by magnetron reactive co-sputtering technique starting from the following target:

[0192] - Used for depositing transition metals or binary or ternary alloys of transition metals respectively:

[0193] i. Targets for transition metals;

[0194] ii. The target of the binary or ternary alloy of the aforementioned transition metal, or the target of each metal of the alloy of interest supplied with appropriate power;

[0195] -Oxides used for depositing Al or Si oxides, or for depositing binary alloys of Al and Si:

[0196] iii. Al target or Si target;

[0197] iv. The target of the aforementioned binary alloy of Al and Si, or the target of Al and Si supplied with appropriate power.

[0198] This invention defines a metal-ceramic layer having a metallic composition consisting of a transition metal or a binary or ternary alloy of transition metals and a ceramic composition consisting of an Al or Si nitride or a nitride of a binary alloy of Al and Si, applied in Ar+N2 gas by magnetron reactive co-sputtering from the following target:

[0199] - Used for depositing transition metals or binary or ternary alloys of transition metals respectively:

[0200] i. Targets for transition metals;

[0201] ii. The target of the binary or ternary alloy of the aforementioned transition metal, or the target of each metal of the alloy of interest supplied with appropriate power;

[0202] - Nitrides used for depositing Al or Si nitrides, or for depositing binary alloys of Al and Si:

[0203] iii. Al target or Si target;

[0204] iv. The target of the aforementioned binary alloy of Al and Si, or the target of Al and Si supplied with appropriate power.

[0205] To maintain the structural and optical properties of the oxide / nitride-based cermet layer, a reactive co-sputtering deposition process must be performed in such a manner that the amount of O2 / N2 is sufficiently high to oxidize / nitride the metal components of the cermet, but sufficiently low to minimize the oxidation / nitridation of the metal components. Otherwise, the presence of oxidized metal components within the cermet, due to the potential formation of volatile oxides at high temperatures, could lead to structural instability, resulting in cracking of the spectrally selective solar absorber coating 2. On the other hand, in the case of nitride-based cermets, the formation of nitrated metal components MyNz, due to the potential for high-temperature disproportionation or readjustment of the crystal structure of the metallic nitride MyNz, could lead to structural instability of the spectrally selective solar absorber coating 2. Furthermore, particularly regarding the layers of the absorptive multilayer cermet structure 23, the presence of oxidized / nitrated metal components within the cermet will result in a significant reduction in the cermet's absorption capacity and a decrease in the photothermal performance of the spectrally selective solar absorber coating 2.

[0206] As a direct result of the problem shown, it is appropriate to introduce O2 / N2 onto the target of the ceramic composition used for depositing the cermet in the reactive co-sputtering process used to produce cermet layers.

[0207] Regarding the magnetron reactive sputtering and co-sputtering techniques used to apply the spectrally selective solar absorber coating 2 to the ceramic and cermet layers described above, this invention specifies that these are performed in a "transition state" using a system for controlling the flow rate of the reactive gas, particularly a PEM (plasma emission monitoring) or CVM (cathode voltage monitoring) control system. Using magnetron reactive sputtering and co-sputtering techniques performed in a "transition state" allows for maximization of the deposition rate of the ceramic and cermet layers.

[0208] This invention specifies that the aforementioned magnetron reactive sputtering and co-sputtering techniques for applying ceramic layers and for applying spectrally selective solar absorber coating 2, which have a ceramic composition consisting of nitrides, can also be performed in a "saturated state." While the magnetron reactive sputtering and co-sputtering techniques in the "saturated state" are not as fast as the "transition state" techniques, they allow for deposition processes with simpler setups and higher repeatability.

[0209] The high deposition rates achievable using the sputtering technology defined in this invention have a positive impact on productivity and ultimately promote cost-effectiveness when mass-producing components.

[0210] Detailed description of the two preferred implementation schemes

[0211] Two preferred embodiments of the present invention will now be described.

[0212] In two preferred embodiments of the invention, the substrate 1 is made of steel, specifically, it is made of steel tubes of a receiver for a solar thermal or thermodynamic system having a parabolic or Fresnel linear manifold.

[0213] In two preferred embodiments, it is suitable for use in a vacuum (p≤1x10⁻¹⁰). -4 For applications at high temperatures (up to at least 550°C) in millibars, the infrared reflective substrate 21 is composed of W (α-W) of crystalline phase α, because it has good infrared reflectivity, high structural and chemical physical stability, excellent adhesion properties (especially on steel), excellent performance as a diffusion barrier layer for steel alloying elements, and low element diffusion coefficient in the aforementioned layers.

[0214] The thickness of the metallic infrared reflective substrate 21 composed of α-W is 120±20nm.

[0215] In both preferred embodiments, both additional infrared reflective layers 22a are composed of Ag, which, due to its excellent infrared reflectivity, allows for photothermal performance (α) that is superior to that of the prior art. s and ε th ) Spectral selective solar absorber coating 2.

[0216] The thickness of the additional infrared reflective layer 22a is equal to 20 ± 10 nm, and ensures that the optical performance is close to that of "bulk" Ag, and is small enough to mitigate the inherent instability once they are inserted into the four-layer multifunctional structure 22.

[0217] In the first of the two preferred embodiments, the two stabilizing layers 22b are composed of AlN, while in the second of the two preferred embodiments, the two stabilizing layers 22b are composed of Al2O3.

[0218] The thickness of the stabilizing layer 22b in the first and second embodiments is 15 ± 10 nm.

[0219] In a first preferred embodiment, the layers of the absorbing multilayer metal-ceramic structure 23 are composed of W and AlN. In addition to possessing optical properties perfectly suited for absorber functions, the W-AlN metal-ceramic layer exhibits high structural and chemical-physical stability in a vacuum at high temperatures, as well as extremely low diffusion coefficients for both the metal and ceramic components. Furthermore, the absorbing multilayer metal-ceramic structure with nitride ceramic components is easier to manufacture than that with oxide ceramic components.

[0220] In a second preferred embodiment, the layers of the absorptive multilayer cermet structure 23 are composed of W and Al. In addition to possessing optical properties perfectly suited for absorber function, the W-Al₂O₃ cermet layers exhibit high structural and chemical-physical stability in a vacuum at high temperatures, as well as extremely low diffusion coefficients for both the metal and ceramic components. Furthermore, W-Al₂O₃ cermets exhibit high stability even in air at high temperatures (well above 300°C), making them a preferred material for spectrally selective solar absorber coatings 2 used in non-vacuum or pneumatic receiver applications.

[0221] Absorbent multilayer metal-ceramic structure 23 Volume fraction distribution of metal composition V with metal-ceramic layer 金属 / (V 金属 +V 陶瓷 The volume fraction of the metal component in the cermet layer decreases in the direction from the lower multifunctional structure 22 to the upper antireflective multilayer structure 24. The volume fraction of the metal component in the cermet layer starts from a value preferably in the range of 0.3 to 0.7 (and particularly preferably in the range of 0.45 ± 0.15) and decreases to a value preferably in the range of 0.1 to 0.4 (and particularly preferably in the range of 0.25 ± 0.1).

[0222] The absorbent multilayer metal-ceramic structure 23 is formed of up to 40 layers with a thickness of 60±30 nm.

[0223] The anti-reflective multilayer structure 24 of the two embodiments of the present invention is composed of a ceramic layer and a cermet layer.

[0224] Due to its optical properties (refractive index) and stability, the ceramic layer is composed of Al2O3, SiO2 and AlN, with a thickness ranging from 5 to 120 nm.

[0225] The antireflective multilayer structure (24) comprises a cermet layer consisting of W and AlN or W and Al2O3. The thickness of each cermet layer ranges from 1 to 50 nm. Each cermet layer is characterized by a constant volume fraction of metal composition not exceeding 0.3%.

[0226] Specifically, the anti-reflective multilayer structure in both embodiments consists of two ceramic layers and one cermet layer. In these embodiments, the principles of interferometry are applied to obtain the anti-reflective multilayer structure 24 that effectively performs the function of an anti-reflective filter.

[0227] Table I shows the composition of the anti-reflective multilayer structure for the two implementation schemes.

[0228] Table I

[0229] Preferred Implementation Scheme No. 1 Preferred Implementation Scheme No. 2 I Anti-reflective layer AlN <![CDATA[Al2O3]]> II Anti-reflective layer W-AlN <![CDATA[W-Al2O3]]> III Anti-reflective layer <![CDATA[SiO2]]> <![CDATA[SiO2]]>

[0230] The first embodiment is preferred, wherein the absorbent multilayer metal-ceramic structure 23 is of the W-AlN type, and the second embodiment is of the W-Al2O3 type.

[0231] To gain a more complete understanding of the two implementation schemes described above, Table II shows the complete structure of each spectrally selective solar absorber coating 2.

[0232] Table II

[0233]

[0234] It should be noted that the embodiments shown in Table II are basically characterized by a four-layer multifunctional structure 22 (composed of two pairs of "additional infrared reflective layers 22a / stabilizing layers 22b"), which falls within the wider range of the preferred embodiments of the present invention. The multifunctional structure 22 is defined as being composed of a single pair of "additional infrared reflective layers 22a / stabilizing layers 22b" or the same pair of "additional infrared reflective layers 22a / stabilizing layers 22b" repeated up to 15 times.

[0235] In the particularly preferred embodiment of the invention just described, the spectrally selective absorber coating 2 of the invention is capable of operating in a vacuum at a maximum temperature of at least 550°C (p≤1x10⁻¹). -4 Operating at millibars. In such embodiments, the absorber coating of the present invention is characterized by excellent performance in terms of high solar energy absorptivity and low thermal emissivity, and its performance at high temperatures (up to at least 550°C) surpasses that of the prior art. Furthermore, in such embodiments, the absorber coating of the present invention has a durable service life of up to 25 years and exhibits excellent vacuum stability at temperatures up to at least 550°C.

Claims

1. A spectrally selective solar absorber coating (2) for a receiver in a solar thermal or thermodynamic system, comprising, in sequence: - Infrared reflective multilayer structure; - Absorbent multilayer metal-ceramic structure (23); and - Anti-reflective multilayer structure (24); The absorber coating (2) is characterized in that the infrared reflective multilayer structure comprises: (i) An infrared reflective substrate (21) having the functions of a barrier layer and an adhesive layer, designed to contact a substrate (1) as part of the receiver during use; the infrared reflective substrate (21) is composed of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Hf, Ta, Mo, W, or nitrides of one or more of the transition metals; and (ii) A multifunctional structure (22) placed on the infrared reflective substrate (21) and comprising at least: - Multiple additional infrared reflective layers (22a) that function to increase the infrared reflectivity of the absorber coating (2), each of the additional infrared reflective layers (22a) being composed of a metal selected from Au, Ag, Cu, Al, Mo, and W, and - A plurality of stabilizing layers (22b) for the additional infrared reflective layer (22a), each of the stabilizing layers comprising the following ▪ Ceramic materials; or ▪ Metal-ceramic materials; or ▪ A material composed of one or more transition metals or nitrides of one or more said transition metals; The additional infrared reflective layer (22a) and the plurality of stabilizing layers (22b) are arranged such that one additional infrared reflective layer (22a) alternates with one or more stabilizing layers (22b); One of the stabilizing layers (22b) is placed in contact with the absorbent multilayer metal-ceramic structure (23).

2. The solar absorber coating (2) according to claim 1, characterized in that The additional infrared reflective layer (22a) and the stabilizing layer (22b) are arranged in alternating positions relative to each other.

3. The solar absorber coating according to claim 1 or 2, characterized in that The stabilizing layer (22b) is composed of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, or of one or more nitrides of said transition metals.

4. The solar absorber coating according to claim 1 or 2, wherein The ceramic material of the stabilizing layer (22b) is selected from the group consisting of: - Oxides of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W; or - Al and / or Si oxides; or - Al and / or Si nitrides.

5. The solar absorber coating according to claim 1 or 2, wherein The cermet material of the stabilizing layer (22b) is made of: a metallic material composed of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W; and a ceramic material selected from Al and / or Si oxides or nitrides.

6. The solar absorber coating of claim 1, wherein, The thickness of the infrared reflective substrate (21) ranges from 1 to 250 nm; the thickness of the additional infrared reflective layer (22a) ranges from 1 to 250 nm; the thickness of the stabilizing layer (22b) composed of the ceramic material ranges from 1 to 200 nm; the thickness of the stabilizing layer (22b) composed of the cermet material ranges from 1 to 120 nm; and the thickness of the stabilizing layer (22b) made of the material composed of one or more transition metals or nitrides of one or more transition metals ranges from 1 to 50 nm.

7. The solar absorber coating of claim 1, wherein, The thickness of the infrared reflective substrate (21) ranges from 90 to 150 nm; the thickness of the additional infrared reflective layer (22a) ranges from 5 to 120 nm; the thickness of the stabilizing layer (22b) composed of the ceramic material ranges from 5 to 120 nm; the thickness of the stabilizing layer (22b) composed of the cermet material ranges from 5 to 50 nm; and the thickness of the stabilizing layer (22b) made of the material composed of one or more transition metals or nitrides of one or more transition metals ranges from 5 to 20 nm.

8. The solar absorber coating (2) according to claim 1, characterized in that The absorbent multilayer metal-ceramic structure (23) includes at least one metal-ceramic layer made of: a metallic material composed of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, and a ceramic material selected from Al and / or Si oxides or nitrides.

9. The solar absorber coating (2) according to claim 1, characterized in that, The anti-reflective multilayer structure (24) includes at least one ceramic layer and at least one metal-ceramic layer.

10. The solar absorber coating (2) according to claim 9, characterized in that, The ceramic layer of the antireflective multilayer structure (24) consists of the following: oxides of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; or Al and / or Si oxides or nitrides.

11. The solar absorber coating (2) according to claim 9, characterized in that, The metal-ceramic layer of the antireflective multilayer structure (24) is made of a metallic material composed of one or more transition metals selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, and a ceramic material selected from Al and / or Si oxides or nitrides.

12. The solar absorber coating (2) according to claim 11, characterized in that, The metal-ceramic layer of the antireflective multilayer structure (24) has a volume fraction of the metal material of 0.01 to 0.

3.

13. A receiver for a solar thermal or thermodynamic system, comprising a substrate (1) and a spectrally selective solar absorber coating (2) according to claim 1.

14. The receiver according to claim 13, characterized in that, It is a receiver tube.

15. The receiver according to claim 14, characterized in that, It is a vacuum receiver tube.

16. The receiver according to claim 14, characterized in that, It is a non-vacuum or pneumatic receiver tube.

17. A method for manufacturing a spectrally selective solar absorber coating (2) according to claim 1, comprising the following steps: - The infrared reflective substrate (21) is applied to all or part of the surface of the substrate (1) by physical vapor deposition. - A multifunctional structure (22) is applied to the infrared reflective substrate (21) using physical vapor deposition; the multifunctional structure (22) includes at least: - An additional infrared reflective layer (22a), which has the function of increasing the infrared reflectivity of the absorber coating (2), is composed of a metal selected from Au, Ag, Cu, Al, Mo and W, and - A stabilizing layer (22b) for the additional infrared reflective layer (22a) comprises the following: ▪ Ceramic materials; or ▪ Metal-ceramic materials; or ▪ A material composed of one or more transition metals or nitrides of one or more said transition metals; - An absorptive multilayer metal-ceramic structure (23) is applied to the stable layer (22b) by physical vapor deposition. - An anti-reflective multilayer structure (24) is applied to the absorbent multilayer metal-ceramic structure (23) by physical vapor deposition.

18. The method of claim 17, comprising pre-treating the substrate (1) at least before applying the infrared reflective substrate (21), said pre-treatment being a heat treatment or a plasma treatment or a combination thereof.

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