阅读说明：本技术 高性能热喷涂的吸收涂层 (High performance thermally sprayed absorptive coating ) 是由 里德哈·哈尔扎拉赫 斯特凡·维南德 弗洛伦特·坎帕纳 简·克拉哈 迈埃文·拉尼科尔 德尔菲 于 2019-01-23 设计创作，主要内容包括：一种用于利用耐温且高吸收性陶瓷微结构涂层(1)通过热喷涂来涂覆用于太阳能应用的基材(3)的方法,其包括以下步骤：-制备包含陶瓷微粒粉末和聚酯微球粉末的粉末混合物(2),所述粉末混合物(2)中聚酯微球的百分比在10至30％w/w之间；-通过热喷涂工艺将粉末混合物(2)喷涂至基材(3)上,以将涂层(1)施加在基材(3)上；-将具有涂层(1)的基材(3)加热到至少400℃的温度,以使聚酯微球从涂层(1)蒸发,从而在聚酯微球的位置留下孔隙(4),其中选择喷涂步骤参数和粒径,从而将涂层(1)以50至150微米之间的厚度施加。(A method for coating a substrate (3) for solar applications by thermal spraying with a temperature resistant and highly absorbing ceramic microstructure coating (1), comprising the steps of: -preparing a powder mixture (2) comprising ceramic microparticle powder and polyester microsphere powder, the percentage of polyester microspheres in said powder mixture (2) being between 10 and 30% w/w; -spraying the powder mixture (2) onto the substrate (3) by a thermal spraying process to apply the coating (1) on the substrate (3); -heating the substrate (3) with the coating (1) to a temperature of at least 400 ℃ to evaporate the polyester microspheres from the coating (1) leaving pores (4) at the location of the polyester microspheres, wherein the spraying step parameters and particle size are selected such that the coating (1) is applied at a thickness of between 50 and 150 microns.)

1. A method for coating a substrate (3) for solar applications by thermal spraying with a temperature resistant and highly absorbing ceramic micro-structured coating (1), comprising the steps of:

-preparing a powder mixture (2) comprising ceramic microparticle powder and polyester microsphere powder, the percentage of polyester microspheres in the powder mixture (2) being between 10 and 30% w/w;

-spraying the powder mixture (2) onto the substrate (3) by a thermal spraying process to apply a coating (1) on the substrate (3);

-heating the substrate (3) with the coating (1) to a temperature of at least 400 ℃ to evaporate the polyester microspheres from the coating (1) leaving pores (4) at the location of the polyester microspheres;

wherein the spray step parameters and particle size are selected such that the coating (1) is applied at a thickness between 50 and 150 microns.

2. The method of claim 1, wherein the thermal spray process is a plasma spray process.

3. The method of claim 1, wherein the ceramic particulate is selected from the group of spinel-structured particles and perovskite particles.

4. The method of claim 3, wherein the spinel structure particles are manganese-cobalt oxide (MCO) particles.

5. A process according to claim 3, wherein the perovskite particles are lanthanum-manganese or lanthanum-cobalt/chromium oxide particles.

6. The method of claim 5, wherein the perovskite particles are lanthanum-strontium-cobalt-ferrite (LSCF) particles or lanthanum strontium manganite particles (LSM).

7. The method of claim 1, wherein the ceramic particles are between 5 and 50 microns in size.

8. The method of claim 1, wherein the polyester microspheres are between 40 and 150 microns in size.

9. The method according to claim 1, wherein the substrate (3) is maintained below 100 ℃ before and during spraying of the powder mixture.

10. The method of claim 1, wherein the substrate is a solar receiver comprised of heat exchange tubes, the solar receiver being made of steel or a Ni-based alloy.

11. The method according to claim 1, wherein the coating (1) is applied as a single layer or as a layer on a sublayer.

12. A coated substrate for solar applications having a temperature resistant and highly absorbing ceramic microstructured coating (1), obtained by a method according to any of the preceding claims.

13. A coated substrate according to claim 12, wherein the pores of the coating have an average diameter of 20 to 50 microns.

14. A solar receiver comprising a heat exchange tube made from the coated substrate of any one of claims 12 and 13.

Technical Field

The present invention relates to an absorbent coating exhibiting high performance, in particular high temperature resistance.

The invention also relates to a method for producing such a high-performance absorbing coating, and in particular a method using the "plasma spraying" technique.

The invention is applicable in the technical field where high thermal energy (heat exchangers, boilers, etc.) must be absorbed.

Background

In CSP molten salt solar tower technology, the heat transfer fluid is molten salt, which typically enters the solar receiving tubes at 290 ℃ and exits at 565 ℃. Average radiant heat flux (heat flux) of about 1000kW/m²And the surface temperature of the solar receiver panel is above 700 ℃.

Such very high operating temperatures require the use of spectrally selective coatings that exhibit high photo-thermal properties and excellent stability at high temperatures to ensure nominal solar receiver performance.

Solar coatings are characterized by their absorptivity in the visible range being as high as possible and by their emissivity in the infrared range having to be as low as possible. In fact, the reduction of emissivity from 0.88 to 0.4 at 650 ℃ increases the solar receiver efficiency by about 4%, and by 7% at 800 ℃. The radiation loss of the solar receiver increases with increasing temperature.

In the specific field of solar tower technology, absorptive coatings are a very big problem. In fact, the commercial market reference coating currently used is

2500, which is a silicon-based high temperature coating, has excellent optical properties (95% absorptivity at 400-. However, after 1 year at operating temperatures below 600 ℃, the performance decreases while the life expectancy is between 1 and 3 years. In this case, annual maintenance is required to maintain good solar receiver efficiency. Another problem is the increase in operating temperature required by current molten salt solar receiver plans (>700 c). At these temperatures, the currently used coatings show poor performance (reduced absorption and mechanical properties).

However, to improve the efficiency of the solar receiver, the operating temperature must be increased from 500 ℃ to 700 ℃. Currently used absorptive coatings show low performance at these temperatures. For this reason, development of a new absorption coating having a required high performance at high temperature has been developed.

Patent analysis showed that research/innovation of solar energy absorbing coatings began before 1995 and was accelerated between 2008 and 2013. These studies have focused on the united states, europe (specifically france and germany) and china. Several developments have been made in this regard by chemical and energy companies and research laboratories in these countries, in particular for photovoltaic cells, fresnel-type and parabolic trough collector technologies, which are limited to operating temperatures of up to 500 ℃. Coatings with high optical properties, i.e. high absorption and low emissivity, and high thermal stability at high temperatures, will be of particular interest.

A variety of solar selective coating designs (simple layers, multiple layers, textures), compositions (dielectrics, cermets, metals, etc.) and application methods (chemical methods such as electrochemical deposition, spray pyrolysis, dip coating, sputtering, PVD, etc.) have been developed and studied.

There are several ways to achieve a solar selective absorption surface. The simplest type of design would be to use a material with inherent solar selective properties. However, there are no natural materials that have these desirable solar selective properties. Some widely used designs are discussed below.

Multi-layer coating

Document US2014/0261390a1 discloses multilayer selective coatings intended for CSP towers. This coating has a high absorptivity (at 600 ℃, 0.95) and a low emissivity (at 700 ℃, 0.07) and consists of:

by SiOx, SiN, TiO₂A first diffusion barrier made of TiOx, metal/AlOx cermet or metal/SiOx cermet;

by SiOx, SiN, TiO₂A second diffusion barrier made of TiOx;

-a metallic infrared reflecting layer;

-a solar absorbing layer made of cermet SiO, AlO + Pt, Ni, Pd, W, Cr or Mo;

by SiOx, SiN, TiO₂A third diffusion barrier made of TiOx;

-an anti-reflection layer; and

a hard protective layer on top of the coating.

Document WO2014/045241a2 discloses coatings having an absorptivity of 0.9 and an emissivity of 0.1 at 400 ℃. The coating was applied by "dip coating" and was made of alternating 100nm thick layers of Cu-Co-Mn-O/Cu-Co-Mn-Si-O and SiO. The SiO layer is applied to protect the coating and to act as an anti-reflective layer.

Document WO2013/088451a1 relates to the formation of a barrier/absorber layer and an anti-reflection layer by alternating: a multilayer coating made of Ti/Cr/AlTiN/AlTiON. The coating was applied on a stainless steel substrate by "sputtering". It shows an absorptivity of 0.92, an emissivity of 0.17 and a thermal stability in air up to 350 ℃ and in vacuum up to 450 ℃.

In document WO2014/122667A1, a multilayer is disclosed which consists of a Cr/Ti-AlTiN-AlTiON-AlTiO layer and an organically modified silicon layer (ormosil). The coating is more thermally stable (500 ℃ in air, 600 ℃ in vacuum) than the coating disclosed in WO2013/088451a 1.

Document WO2009/051595A1 relates to a coating consisting of 9 layers of TiO deposited by "sputtering₂、SiO₂And TiSi (or Pt). These coatings have an absorptivity of 0.96 and an emissivity of 0.082 at room temperature and 0.104 at 500 ℃.

Document WO2005/121389a1 discloses coatings deposited by "DC sputtering", which are made of:

-a WN or ZrN reflector layer;

-an absorbing cermet layer (wherein the metal component is TiNx, ZrNx or HfNx and the ceramic component is AlN);

and on top of AlN or Al₂O₃And (3) preparing the anti-reflection layer.

The coating disclosed in document EP2757176a1 is a selective multilayer coating with high absorptivity and low emissivity, made up of a Mo layer, a cermet TiO layer₂a/Nb layer and SiO₂The layers are made.

Surface texture (Surface texture)

Surface texturing is a second method suitable for increasing solar absorptance by producing multiple internal reflections.

The ideal rough surface exhibits both high absorptivity at short wavelengths and low emissivity at long wavelengths. Dendritic or porous microstructures with characteristic dimensions comparable to the wavelength of the incident solar radiation can be used to tune the optical properties of the solar absorber. Short wavelength photons tend to be trapped inside the surface. Photons with wavelengths greater than the dendrite spacing, on the other hand, observe a "flat" surface.

Document US6783653B2 discloses an absorbent coating and a method of applying the same. The absorption coating is a sol-gel texture coating with a peak shape.

The coating disclosed in document US2011/0185728a1 consists of a nanotextured encapsulated, vertically oriented component to capture energy. However, as the temperature increases, the adhesion of such coatings changes.

Chemical coating composition

The chemical composition is one of the parameters that define the optical properties of the solar coating. Several formulations have been studied: cr black, Ni, Cu, Mo, Al, Ni-Sn, Ni-Cd, Co-Sn, Co-Cd, Mo-Cu, Fe-P, Cu-Ni, cermet (ceramic-metal), spinel, metal oxide, etc. The most promising formulations are based on Ni, Ce, Co and W oxides.

Kennedy, Review of Medium-to-High Temperature Solar Selective Absorber Materials (Review of Mid-to-High-Temperature Solar Selective Absorber Materials), NREL/TP-520-:

W-WOx, Mo-MoO due to oxide formation on the surface₂Cr-SiO, Ti-AlN, lithium zinc ferrite (LiFeZnO), ZrO₂、TiO₂And CeO₂Is an excellent candidate with high optical performance at high temperature;

SnO due to its high antireflection power₂Coatings that are also of interest;

materials like Mo, Pt, W, HfC and Au have high thermal stability at high temperatures (>600 ℃), but the metal oxides NiO, CoO still show higher thermal stability (>800 ℃). There are inherent solar selective properties in transition metals and semiconductors, but natural materials do not have very desirable selectivity. Generally, for more complex selective absorber designs, such as multilayer stacks or cermets, they are more suitable as base layers;

depending on the operating conditions, a variety of semiconductors may be suitable for selective solar absorbers, including silicon, germanium and lead sulphide. Due to the high refractive index present near the band edge of most semiconductors, which produces unwanted reflections for frequencies above the band gap, anti-reflective coatings are often added to reduce reflection and thus improve performance.

Kennedy et al, Progress in the development of high-temperature Solar selective coatings (Progress index of high-temperature Solar selective coating), ASME 2005 International Solar Energy Conference (International Solar Energy Conference), p 749-755, shows: the textured Ni and Cr coating oxidizes at temperatures above 350 ℃.

Coating deposition method/process

Various solar coating deposition methods (processes) have been investigated: painting, physical deposition, oxidation, and thermal spraying.

Physical Vapor Deposition (PVD) process

Selvakamar and H.C. Barshilia, in Review of Physical Vapor Deposited (PVD) selective coatings for mid-and high-temperature Solar thermal coatings, Solar Energy Materials and Solar Cells, volume Elsevier (2012)98, pages 1-23, have performed a comprehensive analysis of the most interesting developed and commercialized Solar Energy absorbing coatings coated by PVD on stainless steel substrates. These coatings were developed specifically for low to medium operating temperatures (200 to 500 ℃) for applications such as parabolic technology. These coatings are not suitable for solar receivers operating at higher temperatures (>650 ℃).

DC sputtering technique

This technique is widely used for the deposition of multilayer coatings. However, it is not suitable for solar receiver tubes due to their large size and high operating temperatures that exceed the technical limits.

Coating method

A variety of coatings deposited by painting have been developed:

document WO2012/127468a2 discloses several coating formulations. And2500 c, some of these formulations showed high absorption and low emissivity;

the document US2014/0326236a1 relates to formulations coated by painting. The coating showed high absorption (95%) and high thermal stability (at 750 ℃, minimum 1000 h). The coating formulation comprises an inorganic oxide based pigment, an organic binder, at least one organic solvent, and an inorganic filler, wherein the organic binder irreversibly converts to the inorganic binder upon curing the coating formulation at a temperature greater than 200 ℃.

Thermal spraying method

The method is widely used for corrosion and wear resistant coating applications. However, this technology has very limited development for solar coating applications.

Thermal spraying (different types of coatings and substrates) has been investigated as a very flexible coating application method. Different types of thermal spraying are used, with different energy sources (arc, flame, plasma, etc.) and filler metals (wire or powder). Thermal spraying may be applied in the shop or in situ, depending on the type of process.

Plasma thermal spraying was investigated by Sandia national laboratory due to the high performance of the applied coating in relation to the high melting point of the applied material. The results of the carried out developments are provided in the following reports:

ambronsi, High-Temperature Solar Selective Coating Development for Power Receivers, CSP Programm Summit 2016, energy. gov/Sunshot and A. Ambronsi, Improved High-Temperature Solar Absorbers for use in centralized Coating Solar Power Central Receivers Applications, ASME 2011, 5 th International conference on energy persistence (5. sup. th International conference on energy persistence)^thInt. conf. on Energy conservation failure), page 587-. In these reports, several commercial powders for thermal spraying were compared and different tests were performed. Hall et al, "Solar Selective Coatings for Concentrating Solar Power Receivers", ADVANCED MATERIALS&Proceses, report 1 month 2012, showed that Cr is due to its high thermal and chemical stability₂O₃Thermal spraying of coatings is a very interesting solution. Laser texturing of the coating surface increases its absorption rate. However, aging tests show that the efficiency of the coating decreases rapidly with increasing temperature. After aging at 700 ℃ for 2 weeks, CeO₂Are also good candidates for high efficiency.

In the above report, a.hall et al provides a comprehensive development of thermal spray for application on solar receivers, achieved by Sandia laboratories. Ni-5Al and WC-20Co coatings are shown to be good candidates, and the surface roughness after thermal spraying tends to be better than the performance of polished surfaces. He mentions that when selecting the coating material, special attention should be paid to the thermal expansion. In fact, WC — Co is a good candidate due to its high optical properties, but it shows high delamination due to differences in its thermal expansion coefficient and the substrate.

Document JP 2013-181192A aims at providing a method for producing a thermal barrier coating material with a top coating having both a porous structure and a vertical cracking structure. A process sequence for producing a thermal barrier coating comprising a base coat and a top coat on a heat resistant substrate comprises: a top coat layer forming step of thermally spraying a ceramic powder and a predetermined amount of a resinous powder on the undercoat layer under predetermined thermal spraying conditions to form a top coat layer; a crack forming step of forming a crack on the top coat layer, which expands in a thickness direction; and a hole forming step of heating the heat-resistant base material to form holes in the topcoat layer after the crack forming step.

Document US 2010/0223925 a1 discloses a solar thermal receiver capable of improving power generation efficiency in solar thermal power generation, reducing production costs, and improving thermal shock resistance of a solar thermal power plant using the solar thermal receiver. A solar thermal receiver that receives solar radiation to a hot fluid includes a heat receiving section made of metal and constituting a flow path in which at least the fluid flows; and a coating layer which absorbs solar energy and has heat resistance, disposed on at least a surface area of the heat-receiving section irradiated with sunlight.

All these data interestingly provide an overview of existing solutions and it is believed that there is no solution that simultaneously meets the current requirements in terms of high performance (> 95% absorption) and long lifetime (>5 years) at high temperatures (>700 ℃).

At present, it is very difficult to improve the performance of the absorption coating at high temperatures. In fact, in order to improve the efficiency of solar receivers, the operating temperatures are increasing (in the range of 700 ℃ to 850 ℃) and new absorbing coatings with high performance at high temperatures are urgently needed.

Objects of the invention

The present invention aims to provide a method for providing an absorbing coating with high performance at high temperatures, in particular for providing an absorbing coating with higher performance intended for solar receivers operating at temperatures above 850 ℃.

The present invention also aims to provide a coating having an extended lifetime and having a lifetime of at least 5 years without any reduction in optical and mechanical properties, resulting in reduced field maintenance.

Finally, the present invention aims to determine a coating thickness that is minimal, but at the same time provides the best compromise between performance (such as adhesion, thermal properties, electrical conductivity) and cost.

Disclosure of Invention

The invention relates to a method for coating a substrate for solar applications by thermal spraying with a temperature-resistant and highly absorbent ceramic microstructured coating, comprising the following steps:

-preparing a powder mixture comprising a powder of ceramic micro-particles and a powder of polyester microspheres, the percentage of polyester microspheres in the powder mixture being between 10 and 30% w/w;

-spraying the powder mixture onto the substrate by a thermal spraying process to apply the coating on the substrate;

-heating the substrate with the coating to a temperature of at least 400 ℃ to evaporate the polyester microspheres from the coating leaving voids (pores) at the locations of the polyester microspheres;

wherein the spray step parameters and particle size are selected such that the coating (1) is applied in a thickness between 50 and 150 micrometers.

According to a preferred embodiment of the invention, the method is further limited by one or a suitable combination of the following features:

-the thermal spray process is a plasma spray process;

-the ceramic particulate is selected from the group of spinel-structured particles and perovskite particles;

-the spinel-structured particles are manganese-cobalt oxide (MCO) particles;

-the perovskite particles are lanthanum-manganese or lanthanum-cobalt/chromium oxide particles;

-the perovskite particles are lanthanum-strontium-cobalt-ferrite (LSCF) particles or lanthanum strontium manganite (lanthanum strontium manganese oxide, lanthanum strontium manganite) particles (LSM);

-the size of the ceramic particles is between 5 and 50 microns;

-the size of the polyester microspheres is between 40 and 150 microns;

-maintaining the substrate at less than 100 ℃ before and during spraying of the powder mixture.

-the substrate is a solar receiver consisting of heat exchange tubes made of steel or Ni-based alloys;

-applying the coating as (recording) one monolayer or as one layer on a sub-layer.

The invention also relates to a coating produced by the method as described above and to a coated substrate suitable for solar applications having a temperature resistant and highly absorbing ceramic microstructured coating as described above.

Preferably, the average diameter of the pores of the coating is from 20 to 50 microns.

Another aspect of the invention relates to a solar receiver comprising a heat exchange tube made from a coated substrate as described above.

Drawings

Figure 1 schematically represents the important parameters of a coating development strategy according to the invention.

Fig. 2 schematically shows a method for implementing a solar coating according to the invention.

Fig. 3A shows an example of an electron micrograph of a coated sample after plasma spraying of a powder mixture comprising ceramic particulate powder and polyester microsphere powder according to the present invention.

Fig. 3B shows an electron micrograph of the coated sample of fig. 3A after further heat treatment according to the present invention.

Reference symbols

1 coating of

2 mixture of ceramic powder and polyester microspheres (plasma spray)

3 base material

4 pores

5 plasma torch

6 light trap

Detailed Description

The present invention relates to a new thermal spraying process for applying a simple (single) layer solar selective coating 1 on a substrate 3, typically metal, such as steel. This type of coating can be applied on the substrate 3 by different thermal spray applications, such as power flame spraying or high velocity oxygen fuel spraying (HVOF), but the method of choice is preferably a plasma spray process. In plasma spraying, a high frequency arc is ignited between an anode and a tungsten cathode. The gas flow between the electrodes is ionized, so that a plasma plume having a length of a few centimeters appears. The temperature in the plume may be as high as 16000K. The particle velocity was 100-300 m/s. The spray material is injected as a powder outside the gun nozzle into the plasma plume, where it melts and projects onto the substrate surface.

According to the invention, a mixture 2 of ceramic powder and polyester microspheres is deposited on a substrate 3 by a thermal spray process, and preferably by an air-plasma spray (APS) process using a plasma torch 5. In the plasma method, the mixture 2 is melted and projected onto the substrate 3, adhered and solidified on the surface thereof to form the coating layer 1 (see fig. 3A). Thereafter, the projected polyester microspheres present in the powder mixture will be separated in the coating 1. Furthermore, the substrate comprising the coating 1 is heated to a high temperature (>400 ℃), which causes the polyester microspheres to evaporate, leaving instead local voids 4 (see fig. 3B).

Furthermore, these pores 4 will function as light traps 6 and thus enable an increased absorption of the coating 1. Once applied to the surface of the solar receiver, the coating 1 will thus enable absorption of the maximum solar energy (94.5-95.5% absorption at 400 nm in the solar spectrum) and re-emission of the minimum solar energy (75-80% emission at 1-20 μm in the infrared spectrum), thereby raising the efficiency of the solar receiver panel from 90.5% to 91.35% (relative to prior art coatings such as Pyromark coatings, + 0.85% efficiency). The lifetime is estimated to increase from 1 year to 5 years, as 1000 additional cycles at 750 ℃ may be performed, as inferred from the bending test (not shown).

The process parameters affect the microstructure and properties of the coating. The appropriate choice of the material to be applied is fundamental (type, characteristics, geometry, dimensions). Fine particles tend to evaporate and coarse particles lead to lack of fusion, which is not suitable for forming dense coatings with good adhesion to the substrate. The inventors have found that the coating thickness is influenced by the mixture projection parameters and the projected particle size. Both may be selected to achieve a layer thickness between 50 and 150 microns. A thin coating is obtained by the smallest particle size.

According to one embodiment, the ceramic powder is preferably spinel-structured particles (having a chemical structure (AB)₂O₃Wherein A and B are metal cations). More preferably, the spinel structure material is in Mn_1.5Co_1.5O₄Manganese-cobalt oxide (MCO) in its form.

According to another embodiment, the ceramic powder may also be perovskite particles (having a chemical structure (AB)₃O₄Where a and B are metal cations), such as lanthanum-manganese and lanthanum-cobalt/chromium oxides, and preferably lanthanum-strontium-cobalt-ferrite (Sr-doped LaCo)_1-xFe_xO3 or LSCF) or Lanthanum Strontium Manganite (LSM).

The size of the ceramic powder particles is preferably between 5 and 50 microns.

The polyester spheres preferably range in size from 40 to 150 microns, and preferably have an average size of about 60 microns.

Particle size analysis or determination is obtained by methods known to those skilled in the art, such as laser diffraction methods, sieving analysis (e.g., according to ASTM B214), and the like.

According to one embodiment, the percentage of polyester spheres in mixture 2 is between 10 and 30% (w/w), and preferably 20% (w/w).

The proposed solution is to apply a mixture of specific high temperature stable powders by plasma spraying to form a coating. This technique ensures very good adhesion of the coating on the substrate by mechanical cohesion, even at very high temperatures (see electron micrographs, fig. 3A and 3B).

According to one embodiment, texturing the surface by coating aging is a proposed method to increase the solar absorptance by producing multiple internal reflections.

One advantage of the present invention is that the coating achieved by plasma spraying in the present invention shows better optical properties, which enables improved efficiency of the solar receiver, and a longer lifetime at high temperatures, which reduces field maintenance operations.

Another advantage resides in the formation of relatively thin coatings due to the use of smaller polyester and ceramic particles. This reduces the cost of the coating, since as the particle size increases, the particle cost can differ by a factor of 10.

In summary, the thermally sprayed absorption coatings obtained by the plasma spray process of the present invention show improved surface degradation performance, improved lifetime and reduced maintenance costs, while improving absorption properties. This new solution would provide the CSP customer the opportunity to save expenses by reducing the number of on-site maintenance operations and downtime of the power plant, which is a commercial advantage for the solar receiver suppliers.