Spectral selective absorbing coating for solar energy receivers acting in air

Abstract

The invention relates to a spectrally selective absorber coating (2) for a receiver acting in the air of a thermal and thermodynamic solar system, comprising a substrate (1) and at least one protective structure (21) capable of protecting the metal components of the protective structure (21) and of the absorber coating (2) from atmospheric oxidants, the protective structure in turn comprises a metal layer (21a) on which a ceramic layer (21b) is arranged.

Description

Technical Field

[0001] The present invention relates to spectrally selective absorbing coatings for receivers acting in air, in particular for receiver tubes acting in air of solar thermal and thermodynamic systems for producing heat and / or electrical energy, and in particular solar thermal and thermodynamic systems with parabolic or Fresnel linear collector concentration systems, the receivers of which have a tubular geometry and which must absorb the greatest possible amount of solar radiation concentrated on them and at the same time must dissipate the least possible amount of heat to the external environment. Background Art

[0002] This function is performed by a specific surface coating applied to the tube, usually made of steel, in which the heat transfer fluid flows; in view of the fact that the coating is applied to the tube, the tube will hereinafter even be referred to as "substrate".

[0003] In order to effectively perform this function, the surface coating of the receiver must exhibit reflective optical properties that vary according to wavelength. Specifically, in order to obtain a solar receiver with high photothermal efficiency, the coating must exhibit performance as close as possible to the ideal performance, which is zero reflectivity (consistent absorptivity) in the spectral region of solar radiation (0.3μm to 2.0μm) and consistent reflectivity (zero absorptivity and emissivity) in the spectral region of the thermal infrared, with a step-like transition (cutoff) between the two regions.

[0004] Therefore, when referring to such coatings, the use of the term "spectrally selective absorption coating" is established for solar receivers. In order to obtain such properties, spectrally selective absorption coatings are usually formed by a complex multilayer structure, which, starting from the substrate, has a reflector in the infrared region, an absorber of solar radiation and finally an antireflection coating, which is used to minimize the losses caused by reflecting the solar radiation. Each of the above-mentioned elements of the absorption coating (infrared reflection coating, solar absorption element, antireflection coating) includes one or more layers of appropriately selected materials, so that the optical properties of these elements simultaneously reach a high solar absorption rate (α s ) and low thermal emissivity (ε th ), i.e., overall high efficiency (η) in the light-to-heat conversion of the absorber coating pt ).

[0005] In particular, solar radiation is usually absorbed by means of composite materials of the cermet type, comprising a ceramic phase and a metallic phase. Such materials allow high absorptivity in the spectral region of solar radiation and good transparency in the spectral region of the thermal infrared, by giving the coating properties close to the ideal properties.

[0006] A receiver tube operating in air may consist of a steel tube coated with an absorbing coating alone (non-encapsulated receiver tube) or of a steel tube coated with an encapsulated absorbing coating by means of an external tube made of glass (encapsulated receiver tube). In both cases, the coating is in contact with air and is therefore subject to degradation mechanisms accelerated by oxidation-type phenomena involving the metal parts of the coating, resulting in a degradation of the photothermal performance.

[0007] These phenomena have a negative impact on the durability of the absorber coating and become more important the higher the operating temperature of the receiver tube.

[0008] The spread of the technology for receiver tubes in air - both non-encapsulated and encapsulated - has been greatly limited because of the difficulty in achieving receiver tubes operating at average temperatures (400°C) and high temperatures (550°C) with photothermal performance and durability comparable to that of more expensive encapsulated and vacuum receiver tubes.

[0009] An example of a solution to the above-mentioned problem is provided in International Application No. WO 2022 / 024064 A1, which discloses a solar absorber coating that is capable of increasing the structural stability of medium and high reflectivity metals, thereby obtaining a solar absorber coating with improved photothermal performance at high temperatures.

[0010] Furthermore, in the art, several publications can be found describing both solar absorbing coatings using metal components capable of resisting oxidation, such as self-passivating alloys, and absorbing coatings encapsulated by a dense layer having the dual function of antireflection filter and oxidation barrier.

[0011] In the latter case, the solutions adopted to realize these coatings are very complex and expensive, and the required deposition processes are costly and thus incompatible with industrial production. Summary of the invention

[0012] The technical problem underlying the present invention is to provide a spectrally selective absorption coating, a receiving element using the spectrally selective absorption coating, and a method for realizing the spectrally selective absorption coating that avoid the disadvantages mentioned with reference to the prior art.

[0013] This problem is solved by an absorbing coating, a receiving element and a method of realization as described above as defined in the appended respective independent claims.

[0014] The main advantage of the spectrally selective absorption coating according to the present invention is that it comprises one or more structures: the one or more structures are capable of protecting the metal components of the one or more structures and the metal components of the absorption coating from the influence of atmospheric oxidants, thereby making the spectrally selective absorption coating suitable for operating directly in air at an operating temperature of up to 1200°C.

[0015] The protective structure is also suitable for being realized with the same deposition techniques as those used to realize the different parts of the absorbing coating, these being physical vapor deposition (PVD) techniques, and in particular sputtering type techniques with high repeatability and high productivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will be described hereinafter according to some preferred embodiments provided by way of example and not for limiting purposes with reference to the accompanying drawings, in which:

[0017] · Figure 1 A schematic cross-sectional view showing a protective structure of an absorbent receiving member according to the present invention;

[0018] · Figure 2 shows a schematic cross-sectional view of a first embodiment of an absorbent coating according to the present invention;

[0019] · Figure 3 shows a schematic cross-sectional view of a second embodiment of an absorbent coating according to the present invention;

[0020] · Figure 4 a schematic cross-sectional view showing a third embodiment of an absorbent coating according to the invention; and

[0021] · Figure 5 A diagram illustrating the reflection process of a protective structure deposited on a steel substrate in the spectral range of 250 nm to 2500 nm is shown.

[0022] In the drawings, for simplicity and clarity of illustration, shown elements are not necessarily drawn to scale. DETAILED DESCRIPTION

[0023] With reference to the accompanying drawings, the present invention relates to a spectrally selective absorbing coating for a receiver acting in air for a solar thermal and thermodynamic system, the absorbing coating being generally indicated at 2.

[0024] The absorbing coating 2 is arranged in the substrate 1, and the absorbing coating 2 generally comprises at least a protective structure 21 ( Figure 1), the protective structure 21 is capable of protecting both the metal components of the protective structure 21 and the metal components of the protective coating 2 from the influence of atmospheric oxidants; each protective structure 21 of the protective coating 2 includes a metal layer 21a, and a ceramic layer 21b is arranged on the metal layer 21a.

[0025] The metal layer 21a comprises an alloy containing at least two metals, in particular:

[0026] a first metal, which, in the metal layer 21a, is present in the alloy in an atomic percentage comprised between 50% and 90%, the first metal being selected from: W, Mo, Ta, Nb, Ni, Co, Cu, Hf, Fe, Ag, Au, Pt, Pd, Cr, Rh; and

[0027] A second metal, present in the alloy in an atomic percentage comprised between 10% and 50%, in the metal layer 21a, the second metal being chosen from: Cr, Al, Si, Ti, Zr.

[0028] The Gibbs free energy of the second metal for forming an oxide of the second metal must be less than the Gibbs free energy of the first metal for forming an oxide of the first metal.

[0029] The ceramic layer 21b includes a substoichiometric oxide of a metal material selected from Al, Si, and an alloy of Al and Si, wherein the Gibbs free energy of forming the oxide of the metal material must be less than or equal to the Gibbs free energy of forming the oxide of the second metal.

[0030] The metal alloy may include all possible combinations between the first metal and the second metal, provided that the metal alloy is not NiCr, HfZr.

[0031] The above-described protective structure 21 can be operated at an operating temperature of up to 1200° C., preferably up to 800° C., which constitutes data provided during planning.

[0032] The first component metal defined as "primary" must have medium to high reflective optical characteristics in the spectral range of 2μm to 40μm, and the first component metal defined as "primary" must have structural and chemical-physical stability at the operating temperature set for the absorbing coating 2.

[0033] The second constituent metal defined as "secondary" must have the ability to form, in contact with oxygen, a homogeneous and compact oxide that is particularly stable and resistant to the operating temperatures set for the absorber coating.

[0034] The metal layer 21a must be coated with said ceramic layer 21b, which must have the characteristics described below.

[0035] The ceramic layer 21b has structural and chemical-physical stability in air at the operating temperature set for the absorber coating 2, and the oxide of the ceramic layer 21b must be deposited in a substoichiometric form and with the following oxygen atomic percentage: the oxygen atomic percentage will decrease as the operating temperature set for the absorber coating 2 increases.

[0036] Furthermore, as the operating temperature set for the absorber coating 2 increases, the atomic percentage of the first metal must decrease, while the atomic percentage of the second metal must increase.

[0037] In addition to including the "primary" and "secondary" elements, the alloy of the metal layer 21a may also include a third metal, defined as "tertiary", which has the function of improving the structural stability and adhesion characteristics of the protective structure 21 at the operating temperature of the absorber coating 2. The "tertiary" metal used for this purpose can be selected from Y, Ti, Zr, and the atomic percentage of the "tertiary" metal in the alloy varies between 1% and 10%.

[0038] When the alloy is a ternary alloy or higher, the alloy includes all possible combinations between the first metal, the second metal and the third metal, provided that the metal alloy is not NiCrAlY.

[0039] The required condition for forming a barrier oxide layer at the metal layer 21a - ceramic layer 21b junction when the protection structure 21 is exposed to air at a fixed operating temperature is that selective oxidation of only the "secondary" metal elements of the alloy occurs at the metal layer 21a - ceramic layer 21b junction.

[0040] In this way, an oxide is obtained that acts as an effective barrier both to the diffusion of oxidants present in the atmosphere in the metal layer 21 a and to the diffusion of components of the metal layer 21 a in the overlying ceramic layer 21 b.

[0041] In order to promote the selective oxidation of only the "secondary" metal elements of the alloy at the junction of the metal layer 21a-ceramic layer 21b at the operating temperature, the "secondary" metal elements of the alloy must be selected to have a Gibbs free energy for forming their oxides that is smaller than the Gibbs free energy of the "primary" metal element for forming its oxide, and the amount of oxygen (the main oxidant present in the air) present at the junction of the metal layer 21a-ceramic layer 21b and the amount of the "secondary" metal elements of the alloy must be carefully managed.

[0042] In fact, in order to have a good selective oxidation, the amount of oxygen in contact with the surface of the metal layer 21 a at each moment must be equal to or less than the amount of “secondary” metal present on this surface.

[0043] If this condition is not met, excess oxygen may oxidize other elements in the metal layer 21a, thereby forming a mixed oxide or an oxide rich in the "primary" metal element of the alloy, and resulting in the loss of the uniformity, density, stability and resistance characteristics of the oxide rich in the "secondary" metal element of the alloy.

[0044] Management of the amount of oxygen and "secondary" metallic elements of the alloy present on the surface of the metal layer 21a is achieved by acting directly on the mechanism and kinetics of the oxidation reaction occurring on this surface.

[0045] Specifically, the formation of the barrier oxide layer mainly includes two stages: a first stage called a transition stage, followed by a second stage called a stable stage.

[0046] During the transition phase, nucleation and subsequent growth of the barrier layer occurs at the junction of the metal layer (21a)-ceramic layer (21b). In this case, the oxidation reaction rate during the transition phase depends mainly on the oxygen concentration present at the junction of the metal layer 21a-ceramic layer 21b and the oxygen concentration reached by diffusion through the ceramic layer 21b.

[0047] The transition phase is followed by a growth stabilization phase for the barrier oxide layer, during which the junction primarily involved in the oxidation reaction is the metal layer 21a-barrier oxide layer junction. In this case, the oxide growth rate always depends on the oxygen concentration that must diffuse through the ceramic layer 21b at this time and then diffuse through the barrier oxide layer formed during the transition phase. Therefore, the barrier oxide growth rate in the stabilization phase is much lower than the barrier oxide growth rate observed during the transition phase, because the amount of oxygen reaching the "secondary" metal at the metal layer 21a-barrier oxide layer junction is much lower.

[0048] The amount of oxygen involved in the various growth stages of the barrier oxide layer depends on the operating temperature of the protection structure 21, the stoichiometry and thickness of the ceramic layer 21b deposited on the metal layer 21a.

[0049] In the following, the reflector 23 of the absorbing coating 2 for the infrared region is referred to as comprising one or more layers; similarly, the solar radiation absorber 22 and the antireflection coating 24 again comprise one or more layers.

[0050] Reference Figure 2, a first example of an absorbing coating 2 is provided with said protective structure 21, said protective structure 21 being positioned between an absorber 22 and an anti-reflection coating 24 of the absorbing coating 2, the absorbing coating 2 being designed to work in a vacuum state.

[0051] In this example, the absorbing coating 2 of the receiving element for acting in air comprises in sequence:

[0052] - an infrared reflective coating 23, which is arranged in contact with the substrate 1;

[0053] - an absorbent element 22;

[0054] - protective structure 21;

[0055] -Anti-reflective coating 24.

[0056] The protection structure 21 protects the metal components of the spectrally selective absorption coating 2 including 23, 22 and 24 designed to work in a vacuum state, thereby making the spectrally selective absorption coating 2 suitable for working in air.

[0057] In the proposed order, the metal layer 21a of the protective structure 21 participates in the formation of the absorption element and has a thickness comprised between 1 nm and 80 nm, preferably comprised between 10 nm and 30 nm, so that the metal layer 21a is thick enough to ensure the formation of a barrier oxide at the operating temperature of the absorption coating 2 and thin enough to allow the different underlying layers to perform the functions for which these underlying layers are designed. The ceramic layer 21b of the protective structure 21 participates in the formation of the antireflection coating of the absorption coating 2 and has a thickness comprised between 1 nm and 250 nm, preferably comprised between 5 nm and 60 nm.

[0058] In another example, the absorbing coating 2 of a receiving element for acting in air comprises in sequence:

[0059] - an infrared reflective coating 23, which is arranged in contact with the substrate 1;

[0060] - an absorbent element 22;

[0061] -Protective structure 21.

[0062] In the proposed order, the metal layer 21a of the protective structure 21 participates in the formation of the absorption element and has a thickness comprised between 1 nm and 80 nm, preferably comprised between 10 nm and 30 nm, to ensure the formation of the barrier oxide and the transparency required for the underlying layers to achieve their designed functions. The ceramic layer 21b of the protective structure 21 performs the antireflection function of the absorption coating 2 and has a thickness comprised between 1 nm and 250 nm, preferably comprised between 20 nm and 120 nm.

[0063] Reference Figure 3 , additional examples of absorbent coatings 2 for receivers acting in air include, in order:

[0064] - an infrared reflective coating 23, which is arranged in contact with the substrate 1;

[0065] - protective structure 21;

[0066] an absorber, the absorber comprising a metal component including one of the alloys of the metal layer 21 a of the protective structure 21 and a ceramic component including one of the oxides of the ceramic layer 21 b of the protective structure 21;

[0067] -Anti-reflective coating 24.

[0068] The infrared reflective coating 23 includes a transition metal selected from the following or a nitride of one of the transition metals: Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W. The thickness of the infrared reflective coating 23 is included between 1 nm and 250 nm, preferably included between 90 nm and 150 nm, for effective infrared reflection.

[0069] In the case where the operating temperature of the receiver is 600° C. to 800° C., the transition metal may be selected from the group consisting of Ti, Zr, Mo, W, TiN and ZrN.

[0070] The connection between the protective structure 21 and the infrared reflective coating 23 is preferably achieved, if possible, by choosing the “primary” metal of the alloy constituting the metal layer 21 a of the protective structure 21 to be identical to the metal of the infrared reflective coating 23 .

[0071] When the operating temperature of the receiving element reaches 600°C, the infrared reflective coating 23 includes the following two layers arranged in sequence:

[0072] - a barrier layer, which is arranged in contact with the substrate 1 and has the functions of a barrier layer and an adhesion layer, the barrier layer comprising a transition metal or a nitride of one of the transition metals selected from the following: Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W;

[0073] - A layer comprising a highly reflective metal selected from the group consisting of: Au, Ag and Cu.

[0074] In this way, the advantageous optical properties of Au, Ag and Cu can be used, and the poor structural and chemical-physical stability of Au, Ag and Cu at high temperatures, their poor adhesion to the substrate 1, and their ineffective barrier to diffusion phenomena from the substrate 1 can be compensated by inserting Au, Ag and Cu into the absorbing coating 2 and between the barrier layer and the protective structure 21.

[0075] When using this combination of a barrier layer and an infrared reflecting layer, the barrier layer can have a thickness comprised between 1 nm and 250 nm, preferably comprised between 90 nm and 150 nm, while the infrared reflecting layer has a thickness comprised between 1 nm and 250 nm, preferably comprised between 90 nm and 150 nm, for effective infrared reflection.

[0076] The metal layer 21a of the protective structure 21 can have a thickness comprised between 1nm and 80nm, preferably comprised between 10nm and 30nm, so that the metal layer 21a is thick enough to ensure the formation of a barrier oxide at the operating temperature of the absorption coating 2, and at the same time thin enough to allow the infrared reflective coating 23 to perform the function of the infrared reflective coating 23.

[0077] The ceramic layer 21 b of the protection structure 21 may have a thickness comprised between 1 nm and 250 nm, preferably comprised between 5 nm and 60 nm.

[0078] Figure 3 The absorber 22 of the embodiment can be formed by a single protective structure 21 or by alternating multiple protective structures 21. As the first layer of the sequence, the absorber 22 has a metal layer 21a arranged in contact with the protective structure 21 not belonging to the absorber 22, and as the last layer of the sequence, the absorber 22 has a ceramic layer 21b arranged in contact with the anti-reflection coating 24.

[0079] The absorbent member 22 may include a minimum number of two layers to a maximum number of forty layers.

[0080] The metal layer 21a of each protective structure 21 may have a thickness comprised between 1 nm and 50 nm, preferably comprised between 5 nm and 20 nm, so that the metal layer 21a is thick enough to ensure both the formation of a barrier oxide and the absorption of solar radiation at the operating temperature of the absorbing coating 2. Finally, the ceramic layer 21b of the protective structure 21 may have a thickness comprised between 1 nm and 120 nm, preferably comprised between 10 nm and 40 nm.

[0081] As an alternative to the ceramic-metal multilayer structure, another example of the absorber 22 is configured to use one or more metal ceramic material layers in sequence, the metal ceramic material layers having a metal component and a ceramic component, the metal component including one of the alloys of the protective structure 21, and the ceramic component including one of the oxides of the protective structure 21.

[0082] Each single metal-ceramic material layer may have a constant metal component volume fraction, and the metal component volume fraction is included between 0.01 and 0.49, preferably included between 0.10 and 0.35, so that the clusters of the metal alloy are incorporated into the ceramic matrix of the oxide type. In addition, the size of the metal clusters in each metal-ceramic layer will be designed to ensure the formation of a barrier oxide layer at the junction of the metal cluster-ceramic matrix and to make the metal clusters in the metal-ceramic layer have the absorption function of the metal-ceramic material.

[0083] The barrier oxide formed at the metal cluster-ceramic matrix junction has the function of locking the diffusion of oxidants present in the atmosphere to the metal components of the absorber 22 and at the same time locking the diffusion of these metal components in the ceramic matrix by retaining the absorption function of the metal ceramic layer.

[0084] Therefore, the thickness of each cermet layer must be comprised between 1 nm and 200 nm, preferably between 20 nm and 120 nm.

[0085] The absorbent 22 may be formed from a single cermet layer, or from a plurality of layers up to twenty. Figure 3 , the anti-reflection coating 24 includes a ceramic layer composed of a substoichiometric oxide of Al, or a substoichiometric oxide of Si, or a substoichiometric oxide of an alloy of Al and Si. The oxide of the ceramic layer of the anti-reflection coating 24 is deposited in a substoichiometric form, so that when the absorbing coating 2 is at an operating temperature in air, the microstructure of the oxide of the ceramic layer of the anti-reflection coating 24 is densified. The resulting anti-reflection coating 24 allows minimizing the reflection of solar radiation incident on the absorbing coating 2 due to its dense and compact structure, and the resulting anti-reflection coating 24 allows a substantial reduction in the amount of oxygen that can reach the absorber 22 and then reach the infrared reflective coating 23.

[0086] The thickness of the substoichiometric oxide layer in the antireflective coating 24 is comprised between 1 nm and 200 nm, preferably between 20 nm and 120 nm.

[0087] Under these conditions, the formation of barrier oxides is favoured and these barrier oxides are able to ensure excellent structural and chemical-physical stability of the absorber coating 2 in air at operating temperature.

[0088] Reference Figure 4 The last example of the absorption coating 2 uses the protective structure 21 directly as the infrared reflection coating 23. Specifically, the absorption coating 2 includes:

[0089] - an infrared reflective coating 23, which is arranged in contact with the substrate 1, and the infrared reflective coating 23 comprises the metal layer 21a of the protective structure 21, and the ceramic layer 21b of the protective structure 21 is placed on the metal layer 21a;

[0090] an absorber 22 comprising a metal component including one of the alloys of the metal layer 21 a of the protective structure 21 and a ceramic component including one of the oxides of the ceramic layer 21 b of the protective structure 21 ;

[0091] -Anti-reflective coating 24.

[0092] The metal layer 21a constituting the infrared reflective coating 23 has a thickness comprised between 50 nm and 250 nm, preferably between 80 nm and 200 nm, while the ceramic layer 21b disposed thereover has a thickness comprised between 1 nm and 120 nm, preferably between 5 nm and 50 nm.

[0093] The absorber 22 and the anti-reflection coating 24 completing the absorber coating 2 do not need additional description, because the absorber 22 and the anti-reflection coating 24 completing the absorber coating 2 are functionally and structurally similar to the absorber 22 and the anti-reflection coating 24. Figure 3 The absorber 22 and the antireflection coating 24 of the absorber coating 2 described in .

[0094] According to the present invention, a receiver for a thermal and thermodynamic system for solar energy acting in air comprises one of the aforementioned solar absorbing coatings 2, and in particular, a receiver tube comprises one of the aforementioned absorbing coatings 2 arranged on the tubular surface of the receiver tube, which is typically made of steel and serves as a substrate 1.

[0095] The receiver tube can be encapsulated by means of an additional glass tube comprising an anti-reflection treatment, two glass-metal joints, and an element (bellows) compensating for the difference in thermal expansion between the steel tube and the glass tube. Atmospheric air remains in the air gap between the receiver tube and the glass tube.

[0096] According to the invention, a method is provided for realizing the above-mentioned absorber coating on a substrate 1 by means of PVD deposition technology and in particular by means of high deposition rate sputtering technology.

[0097] In this case, the following can be provided: i) technology for sputtering and magnetron co-sputtering from a metal target in argon; ii) technology for sputtering from a metal target in argon + reactive gas (nitrogen, oxygen); iii) technology for reactive magnetron co-sputtering from a metal target in argon + oxygen.

[0098] In the co-sputtering state, it refers to the simultaneous sputtering starting from different material targets in argon gas, while in the reactive co-sputtering state, it refers to the simultaneous sputtering starting from different material targets in argon gas + oxygen gas.

[0099] The present invention can provide that sputter deposition is performed at a depth of 5x10 -4 mbar and 5x10 -2 The sputtering deposition is carried out under a pressure between 1×10 -3 mbar and 2x10 -2 It is carried out under pressure conditions between mbar.

[0100] In less than 5x10 -4 mbar, preferably below 1x10 -4 The substrate 1 is subjected to a possible thermal pretreatment under a pressure of 100 mbar and under vacuum, so that the substrate 1 is at a temperature comprised between 100° C. and 300° C. On the contrary, the substrate 1 is usually pretreated in Ar gas and at a temperature below 5×10 -2 Possible plasma pretreatment is achieved by polarizing the substrate 1 at a pressure of less than 5x10 -3 A possible plasma pretreatment is carried out at a pressure of mbar with the aid of an ion source which is able to bombard the substrate 1 with high-energy ions.

[0101] The protective structure 21 comprising a metal layer consisting of an alloy can be applied by magnetron sputtering technology in Ar gas from a target material formed by a specific target alloy, or can be applied by magnetron co-sputtering technology in Ar gas from targets of individual metals constituting the alloy; in this second case, the desired composition of the alloy will be obtained by appropriately changing the input power of the target material of the individual metal.

[0102] The ceramic layer 21 b of the protective structure 21 including an oxide of Al or an oxide of Si may be applied by a reactive magnetron sputtering technique in Ar gas+O 2 using a target of Al or a target of Si.

[0103] The ceramic layer 21b of the protective structure 21 comprising an oxide of a binary alloy of Al and Si can be applied by reactive magnetron sputtering technology in Ar gas + O2 from a target material formed by a specific target alloy, or the ceramic layer 21b of the protective structure 21 comprising an oxide of a binary alloy of Al and Si can be applied by reactive magnetron co-sputtering technology in Ar gas + O2 from a target material of a single metal constituting the alloy supplied with appropriate power.

[0104] The metal layer of the infrared reflective coating 23 can be applied from a target of the above-mentioned metal by magnetron sputtering technology in Ar gas.

[0105] The layer of the infrared reflective coating 23 comprising transition metal nitride and having metallic properties can be applied by reactive magnetron sputtering technology in Ar gas + N2 from a target of the above-mentioned transition metal.

[0106] The metal ceramic layer of the absorber 22 can be applied by means of one of the following techniques:

[0107] Reactive co-sputtering, i.e., simultaneous sputtering in Ar gas + O2 from targets made of different materials, the targets having at least one target material capable of producing a metal component of a cermet and at least one target material capable of producing a ceramic component of a cermet;

[0108] Sputtering in Ar gas from at least one target material capable of producing a metallic component of a cermet, and subsequent reactive sputtering in Ar gas + O2 from at least one target material capable of producing a ceramic component of a cermet.

[0109] In this case, a cermet layer having a metallic component comprising an alloy and having a ceramic component comprising an oxide of Al or an oxide of Si or an oxide of a binary alloy of Al and Si can be applied starting from the following target:

[0110] For alloy deposition:

[0111] · Targets of the above alloys, or

[0112] A target of the individual metals that make up the target alloy, supplied with appropriate power;

[0113] For the deposition of Al oxide or Si oxide, or oxide of a binary alloy of Al and Si:

[0114] · Al target or Si target, or

[0115] A target of the above binary alloy of Al and Si, or a target of Al and Si, to which appropriate power is supplied.

[0116] The ceramic layer comprising an oxide of Al or an oxide of Si of the antireflection coating 24 of the absorber coating 2 can be applied by a reactive magnetron sputtering technique in Ar gas+O2 from a target of Al or a target of Si.

[0117] The ceramic layer of the oxide of the binary alloy of Al and Si of the anti-reflective coating 24 can be applied by reactive magnetron sputtering technology in Ar gas + O2 from a target formed of a specific target alloy, or the ceramic layer of the oxide of the binary alloy of Al and Si of the anti-reflective coating 24 can be applied by reactive magnetron co-sputtering technology in Ar gas + O2 from a target of a single metal constituting the alloy supplied with appropriate power.

[0118] In order to ensure that the oxide is present in the absorber coating 2 in a substoichiometric form, these deposition processes are carried out in a "transition state" with the aid of a reaction gas flow control system, in particular a PEM (plasma emission monitoring) or CVM (cathode voltage monitoring) type control system.

[0119] Preferably, the process includes an additional heat treatment of the absorbing coating 2 applied to the substrate 1. If the use temperature of the solar receiving element including the spectrally selective absorbing coating 2 is lower than or equal to 450°C, the additional heat treatment is carried out in air or by using a gas mixture with a controlled oxidation potential and at a temperature higher than or equal to 500°C. When the use temperature of the receiving element is higher than 450°C, the additional heat treatment is carried out in air or by using a gas mixture with a controlled oxidation potential by increasing the treatment temperature by at least 50°C relative to the use temperature of the receiving element.

[0120] Reference Figure 1 , Figure 1 Schematically shown is a cross-sectional view of a protective structure 21 comprising a metal layer 21 a made of a metal alloy, the composition of which must be at least two elements, on which a ceramic layer 21 b formed of an oxide in substoichiometric form is placed.

[0121] The primary metal of the alloy can be selected from: W, Mo, Ta, Nb, Ni, Co, Cu, Hf, Fe, Ag, Au, Pt, Pd, Cr, Rh, and the secondary metal can be selected from: Cr, Al, Si, Ti, Zr.

[0122] A possible tertiary metal may be provided, which may be selected from: Y, Ti, Zr.

[0123] According to a specific example of the protection structure 21 , the alloy of the metal layer 21 a includes W, Cr and Ti, while the ceramic layer 21 b includes an oxide of Al.

[0124] Table I below shows the composition of the protective structure 21 in a preferred embodiment of the protective structure 21 .

[0125] Table I

[0126]

[0127] A protective structure 21 having the composition shown in Table I is deposited on a substrate 1 made of steel. A metal layer of WCrTi is deposited in contact with the substrate 1 with a thickness of 140 nm and with the following atomic percentages: 71.8% W, 27.4% Cr and 0.8% Ti. Then, Al x O y The ceramic layer 21b is deposited on the metal layer 21a with a thickness of 100 nm, wherein x=1 and y=1.3. The protective structure thus constructed is subjected to two treatments in air at 500°C, the first treatment lasting 1776 hours and the second treatment lasting 2376 hours. Before and after each heat treatment, the reflectivity of the protective structure 21 in the spectral range of 250 nm to 2500 nm is measured, and Figure 5 The measurement process is shown.

[0128] From the comparison of the reflectivity, it can be seen that only after the first heat treatment, there is a change in the reflectivity, which is mainly due to the formation of barrier oxide at the junction of the metal layer 21a-ceramic layer 21b and the change in the composition of aluminum oxide converted to a stoichiometrically stable phase.

[0129] It is noted that during the subsequent heat treatment, the reflectivity does not change significantly, thus demonstrating the structural and chemical-physical stability of the protective structure 21 .

[0130] The oxide formed at the metal layer 21a -ceramic layer 21b junction thus effectively performs a barrier function to the diffusion of atmospheric oxidants towards the metals of layer 21a, while preventing these metals from diffusing towards the ceramic layer 21b.

[0131] Furthermore, the invariance of the optical properties of the structure also indicates that the growth rate of the barrier oxide at the junction of the metal layer 21a-ceramic layer 21b is very low. In conclusion, the results of the thermal treatment indicate that the protective structure 21 realized for 500°C may be suitable for realizing a barrier function against diffusion within a period of time compatible with the use cycle of the solar receiver.

[0132] For the above-mentioned spectrally selective absorption coating of receivers acting in air for thermal and thermodynamic systems for solar energy and the related receivers, a variety of additional modifications and variations may be introduced by those skilled in the art for the purpose of meeting additional and temporary requirements, but all these modifications and variations are within the scope of protection of the present invention as defined by the appended claims.

Claims

1. A spectrally selective absorption coating (2), the absorption coating (2) being used for a receiver acting in air in a thermal and thermodynamic solar energy system, the absorption coating (2) being arranged on a substrate (1), the absorption coating (2) comprising at least one protective structure (21), the protective structure (21) being capable of protecting a metal component of the protective structure (21) and a metal component of the absorption coating (2) from atmospheric oxidants, the protective structure (21) comprising a metal layer (21a), a ceramic layer (21b) being arranged on the metal layer (21a), wherein: the metal layer (21a) comprises an alloy of at least a first metal and a second metal, the first metal being present in the alloy in an atomic percentage comprised between 50% and 90%, the first metal being selected from W, Mo, Ta, Nb, Ni, Co, Cu, Hf, Fe, Ag, Au, Pt, Pd, Cr, Rh, and the second metal being present in the alloy in an atomic percentage comprised between 10% and 50%, the second metal being selected from Cr, Al, Si, Ti, Zr, the second metal having a Gibbs free energy of forming an oxide of the second metal being less than the first metal having a Gibbs free energy of forming an oxide of the first metal; as well as The ceramic layer (21b) comprises a substoichiometric oxide of a metal material, the metal material being selected from Al, Si, and an alloy of Al and Si, wherein the Gibbs free energy of forming the oxide of the metal material is less than or equal to the Gibbs free energy of forming the oxide of the second metal.

2. The absorbent coating (2) according to claim 1, wherein: The alloy is a ternary alloy comprising the first metal, the second metal and a third metal selected from Y, Ti, Zr, the third metal being present in the alloy in an atomic percentage comprised between 1% and 10%.

3. The absorbing coating (2) according to claim 1 or 2, wherein the absorbing coating (2) comprises in sequence: an infrared reflective coating (23), the infrared reflective coating (23) comprising one or more layers and arranged in contact with the substrate (1); An absorber (22), the absorber (22) comprising one or more layers; the protective structure (21); and an anti-reflection coating (24), the anti-radiation coating (24) comprising one or more layers.

4. The absorbent coating (2) according to claim 3, wherein: The metal layer (21a) of the protective structure (21) has a thickness included between 1nm and 80nm, preferably, the metal layer (21a) of the protective structure (21) has a thickness included between 10nm and 30nm, and the ceramic layer (21b) of the protective structure (21) has a thickness included between 1nm and 250nm, preferably, the ceramic layer (21b) of the protective structure (21) has a thickness included between 5nm and 60nm.

5. The absorbing coating (2) according to claim 1 or 2, wherein the absorbing coating (2) comprises in sequence: an infrared reflective coating (23), the infrared reflective coating (23) comprising one or more layers and arranged in contact with the substrate (1); an absorbent member (22), the absorbent member (22) comprising one or more layers; And the protective structure (21).

6. The absorbent coating (2) according to claim 5, wherein: The protective structure (21) is positioned to form the outermost portion of the absorbent coating (2) and to be in contact with air.

7. The absorbent coating (2) according to claim 5 and 6, wherein: The metal layer (21a) of the protective structure (21) has a thickness included between 1nm and 80nm, preferably, the metal layer (21a) of the protective structure (21) has a thickness included between 10nm and 30nm, and the ceramic layer (21b) of the protective structure (21) has a thickness included between 1nm and 250nm, preferably, the ceramic layer (21b) of the protective structure (21) has a thickness included between 20nm and 120nm.

8. The absorbing coating (2) according to claim 1 or 2, wherein the absorbing coating (2) comprises in sequence: an infrared reflective coating (23), the infrared reflective coating (23) comprising one or more layers and arranged in contact with the substrate (1); A protective structure (21); an absorber (22), the absorber (22) comprising one or more layers; and an anti-reflective coating (24), the anti-reflective coating (24) comprising one or more layers.

9. The absorbent coating (2) according to claim 8, wherein The metal layer (21a) of the protective structure (21) has a thickness included between 1nm and 80nm, preferably, the metal layer (21a) of the protective structure (21) has a thickness included between 10nm and 30nm, and the ceramic layer (21b) of the protective structure (21) has a thickness included between 1nm and 250nm, preferably, the ceramic layer (21b) of the protective structure (21) has a thickness included between 5nm and 60nm.

10. The absorbent coating (2) according to claim 8, wherein The infrared reflective coating (23) comprises a layer made of a transition metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W or a layer made of a nitride of one of the transition metals, and the infrared reflective coating (23) has a thickness between 1 nm and 250 nm. Preferably, the infrared reflective coating (23) has a thickness between 90 nm and 150 nm.

11. The absorbent coating (2) according to claim 8, wherein: The infrared reflective coating (23) for operating temperatures up to 600°C preferably comprises: a first barrier layer, which is realized by a transition metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, or a nitride of one of the transition metals, and has a thickness between 1 nm and 250 nm, preferably, the first barrier layer has a thickness between 90 nm and 150 nm; and a second layer for infrared reflection, which is realized by a metal selected from Au, Ag and Cu, and has a thickness between 1 nm and 250 nm, preferably, the second layer has a thickness between 90 nm and 150 nm.

12. The absorbing coating (2) according to claim 1 or 2, wherein the absorbing coating (2) comprises in sequence: an infrared reflective coating (23), the infrared reflective coating (23) comprising the protective structure (21) and arranged to be in contact with the substrate (1); an absorbent member (22), the absorbent member (22) comprising one or more layers; and an antireflective coating (24), the antireflective coating (24) comprising one or more layers.

13. The absorbent coating (2) according to claim 12, wherein: The metal layer (21a) of the protective structure (21) has a thickness included between 50nm and 250nm, preferably, the metal layer (21a) of the protective structure (21) has a thickness included between 80nm and 120nm, and the ceramic layer (21b) of the protective structure (21) has a thickness included between 1nm and 120nm, preferably, the ceramic layer (21b) of the protective structure (21) has a thickness included between 5nm and 50nm.

14. The absorbent coating (2) according to any one of claims 8 to 13, wherein The absorber (22) comprises one or more pairs formed by the metal layer (21a) and the ceramic layer (21b) in a sequence, wherein the first layer of the sequence is arranged in contact with the protective structure (21) and the last layer of the sequence is arranged in contact with the anti-reflective coating (24).

15. The absorbent coating (2) according to claim 14, wherein The metal layer (21a) of the absorber (22) has a thickness included between 1nm and 50nm, preferably, the metal layer (21a) of the absorber (22) has a thickness included between 5nm and 20nm, and the ceramic layer (21b) of the absorber (22) has a thickness included between 1nm and 120nm, and preferably, the ceramic layer (21b) of the absorber (22) has a thickness included between 10nm and 40nm.

16. The absorbent coating (2) according to claim 14 or 15, wherein: The absorbent (20) comprises a number of layers comprised between 2 and 40.

17. The absorbent coating (2) according to any one of claims 8 to 13, wherein The absorber (22) comprises one or more metal ceramic material layers in sequence, wherein the one or more metal ceramic material layers comprise a metal component and a ceramic component, wherein the metal component comprises the alloy of the metal layer (21a), and the ceramic component comprises one of the oxides of the ceramic layer (21b), wherein the metal component is dispersed in the ceramic component serving as a matrix.

18. The absorbent coating (2) according to claim 17, wherein The one or more metal ceramic material layers of the absorber (22) have a volume fraction of the metal component that is constant in a single metal ceramic material layer and is included between 0.01 and 0.

49. Preferably, the one or more metal ceramic material layers of the absorber (22) have a volume fraction of the metal component that is constant in a single metal ceramic material layer and is included between 0.10 and 0.

35.

19. The absorbent coating (2) according to claim 17, wherein: The one or more metal-ceramic layers of the absorber (22) have a thickness comprised between 1 nm and 200 nm, preferably, the one or more metal-ceramic layers of the absorber (22) have a thickness comprised between 20 nm and 120 nm.

20. The absorbent coating (2) according to any one of claims 17 to 19, wherein The absorbent (22) comprises a number of layers comprised between 1 and 20.

21. The absorbent coating (2) according to any one of claims 8 to 20, wherein The antireflective coating (24) comprises a layer comprising a substoichiometric oxide of Al, or a substoichiometric oxide of Si, or a substoichiometric oxide of an alloy of Al and Si, and having a thickness comprised between 1 nm and 200 nm, preferably between 20 nm and 120 nm.

22. A receiver acting in air for solar thermal and thermodynamic systems, comprising a substrate (1) and a spectrally selective absorbing coating (2) according to one of the preceding claims.

23. A method for achieving a spectrally selective absorbing coating (2) for a receiver of a solar thermal and thermodynamic system acting in air, comprising the following steps: Providing a substrate (1) of the receiving element; applying the spectrally selective absorbing coating (2) on the entire surface of the substrate or on a part of the surface of the substrate by means of a PVD deposition technique, in particular applying the spectrally selective absorbing coating (2) on the entire surface of the substrate or on a part of the surface of the substrate by means of a sputtering technique, in, The metal layer (21a) of the protective structure (21) includes an alloy of at least a first metal and a second metal, the first metal being present in the alloy in an atomic percentage comprised between 50% and 90%, the first metal being selected from W, Mo, Ta, Nb, Ni, Co, Cu, Hf, Fe, Ag, Au, Pt, Pd, Cr, Rh, and the second metal being present in the alloy in an atomic percentage comprised between 10% and 50%, the second metal being selected from Cr, Al, Si, Ti, Zr, the Gibbs free energy of the second metal forming an oxide of the second metal being less than the Gibbs free energy of the first metal forming an oxide of the first metal, and wherein the ceramic layer (21b) of the protective structure (21) deposited on the metal layer (21a) includes a substoichiometric oxide of a metal material selected from Al, Si, an alloy of Al and Si, the Gibbs free energy of the metal material forming an oxide of the metal material being less than or equal to the Gibbs free energy of the second metal forming an oxide of the second metal.

24. The method according to claim 23, wherein: The method comprises: subjecting the substrate (1) to at least one pretreatment, the at least one pretreatment being of a thermal type, or a pretreatment with the aid of plasma, or a pretreatment combining a thermal type pretreatment and a pretreatment with the aid of plasma, the at least one pretreatment being carried out before applying the absorbent coating (2) by means of the PVD deposition technique.

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