Solar selective coating for mid-high temperature solar thermal applications

Abstract

The present invention relates to a solar selective coating for a metal substrate comprising at least one absorber layer and at least one semi-absorber layer selected from the structures of AlTiN and AlTiSiN. In preferred embodiments, the solar selective coating according to the present invention is a double layer coating with AlTiN—AlTiN or AlTiSiN—AlTiSiN formation. The process for producing the coating includes a step of treatment of the metal substrate with a reactive magnetron sputtering system.

Claims

1. A solar selective coating for a metal substrate comprising at least one absorber layer and at least one semi-absorber layer, wherein the composition of the absorber layer consists essentially of the following components; Al; 25-40 at. %, Ti; 15-40 at. %, N; 20-45 at. %, and Si; 0.5-5 at. % and the composition of the semi-absorber layer consists essentially of the following components; Al; 20-35 at. %, Ti; 1-15 at. %, N; 40-80 at. %, Si; 0.5-5 at. %.

2. The solar selective coating according to claim 1 wherein the absorber layer and semi-absorber layer comprises AlTiSiN selected from the following compositions: TABLE-US-00021 AlTiSiN Main Absorber Layer (at. %) Al 33-37 Ti 22-26 Si 0.5-4.sup.  N 35-39 TABLE-US-00022 AlTiSiN Semi-Absorber Layer (at. %) Al 26-30 Ti 4-8 Si 0.5-4.sup.  N  61-65.

3. The solar selective coating according to claim 1 wherein the coating is a double layer coating with AlTiSiN—AlTiSiN formation comprising: TABLE-US-00023 AlTiSiN - AlTiSiN double layer coating AlTiSiN Absorber Layer AlTiSiN Semi-Absorber Layer (at. %) (at. %) Al 35.51 29.84 Ti 24.78 7.72 Si 2.01 1.84 N 37.70 63.09.

4. A metal structure comprising a coating according to claim 1.

5. The metal structure according to claim 4 wherein the metal is selected from stainless steel and copper.

6. The metal structure according to claim 4 which is a solar collector.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention provides a novel coating comprising basically a main absorber layer, and a semi-absorber layer which in turn behaves also as an anti-reflection layer. Both of these layers fundamentally comprise individually designed coating layers in the form of AlTiN or AlTiSiN which can be selected depending on the desired characteristics of the coating.

(2) Hence, the embodiments of the current invention provide coatings on metal substrates comprising a main absorber layer and a semi-absorber layer in the form of one of the configurations hereinbelow: AlTiN—AlTiN AlTiN—AlTiSiN AlTiSiN—AlTiN AlTiSiN—AlTiSiN

(3) Hence the present invention provides selective solar coatings selected from AlTiN and AlTiSiN with different elemental ratios. In preferred embodiments, however, main absorber layer and the semi-absorber layer selected with a configuration such that AlTiN—AlTiN and AlTiSiN—AlTiSiN. In most preferred embodiment, the desired configuration involves AlTiSiN—AlTiSiN.

(4) Accordingly, the main absorber layer of the present invention can be selected from AlTiN and AlTiSiN, wherein the composition of the coating is essentially limited with the following components; Al; 25-40 at. %, Ti; 15-40 at. %, N; 20-45 at. %, Si; 0.5-5 at. % (if AlTiSiN is used in the main absorber layer).

(5) Likewise, the semi-absorber layer of the current invention can be selected from AlTiN and AlTiSiN, wherein the composition of the coating is essentially limited with the following components; Al; 20-35 at. %, Ti; 1-15 at. %, N; 40-80 at. %, Si; 0.5-5 at. % (if AlTiSiN is used in the semi-absorber layer).

(6) The coating of the present invention is preferably a double layer coating having the main absorber layer and semi-absorber layer as identified above without a further absorber or semi-absorber layer.

(7) In more preferred embodiments, the main absorber layer and the semi-absorber layer may comprise AlTiN or AlTiSiN with the following compositions:

(8) TABLE-US-00003 AlTiN Main Absorber Layer AlTiSiN Main Absorber (at. %) Layer (at. %) Al 34-38 33-37 Ti 23-27 22-26 Si — 0.5-4.sup.  N 36-40 35-39

(9) TABLE-US-00004 AlTiN Semi-Absorber Layer AlTiSiN Semi-Absorber Layer (at. %) (at. %) Al 27-31 26-30 Ti 5-9 4-8 Si — 0.5-4.sup.  N 60-64 61-65

(10) The metal substrate on which the coating layers are applied, in the context of the present invention, can be selected from copper and stainless steel. A metal substrate made of stainless steel is particularly preferred.

(11) In another aspect, the present invention provides a novel process for producing a solar selective coating as defined above, comprising deposition of the layers onto a metal substrate via reactive magnetron sputtering. Accordingly, the process of the present invention for producing a solar selective coating on a metal substrate comprises the steps of: placing a metal substrate into a reactive magnetron sputtering system and creating vacuum, depositing a main absorber layer comprising AlTiN or AlTiSiN, wherein the composition of this layer is as identified above, depositing a semi-absorber layer comprising AlTiN or AlTiSiN, wherein the composition of this layer is as identified above, and obtaining a coating with the said main absorber layer and semi-absorber layer.

(12) The reactive magnetron sputtering system may comprise 99.95-99.99% pure Al, Ti, Al—Si(95-5), Ti—Si(95-5) targets and Ar—N.sub.2 gas mixture. Accordingly, the sputtering system may include two magnetron cathodes and power supplies, diffusion pumps, vacuum gauges and a control panel. The dimensions for each cathode may for instance be 1000×12 mm. All layers can be deposited using DC power supplies with power densities in the range of 4-7 W/cm.sup.2. During the deposition, substrate temperature can be kept in a range of 50-150° C.

(13) Before application of the magnetron sputtering, the metal substrates can be polished to remove impurities and reduce surface roughness. The polished samples can be cleaned using ethyl alcohol and acetone. Then, cleaned samples are placed in magnetron sputtering system and chamber was pumped down to a base pressure such as 1×10.sup.−3 Pa. Absorber and semi absorber layers are deposited on the substrates in Ar+N.sub.2 gas mixture.

(14) In another aspect, the present invention pertains to a structure comprising a metal substrate having the coating as identified herein. Said article is preferably a solar collector.

(15) The examples hereinbelow are given for better understanding of the current invention which are however not limiting the scope of the claims in anyway.

EXAMPLES

(16) In the following Examples, stainless steel and copper substrates were coated with double layer coatings of AlTiN—AlTiN and AlTiSiN—AlTiSiN. The atomic compositions of the coatings in Examples 4-19 were arranged as shown below:

(17) TABLE-US-00005 AlTiN - AlTiN double layer coating AlTiN Absorber Layer AlTiN Semi-Absorber Layer (at. %) (at. %) Al 36.22 29.84 Ti 25.64 7.72 N 38.14 63.09

(18) TABLE-US-00006 AlTiSiN - AlTiSiN double layer coating AlTiSiN Absorber Layer AlTiSiN Semi-Absorber Layer (at. %) (at. %) Al 35.51 29.84 Ti 24.78 7.72 Si 2.01 1.84 N 37.70 63.09

Example 1

Deposition of AlTiN—AlTiN Solar Selective Coating on Stainless Steel

(19) In order to investigate the effect of nitrogen content in first AlTiN layer on optical properties of the absorber coating, metal sample was coated with metallic Al—Ti coating. Absorber layer was deposited using Al and Ti cathodes. The flow rates of Ar and N.sub.2 were 100 sccm and 0 sccm. AlTiN semi-absorber layer was deposited using Al and Ti cathodes. Flow rates of Ar and N.sub.2 were 100 sccm and 75 sccm, respectively. For absorber layer, power density of Al was 4.15 W/cm.sup.2 and Ti cathode was 6.65 W/cm.sup.2. For semi-absorber layer, power density of Al cathode was 7 W/cm.sup.2 and Ti cathode was 4.15 W/cm.sup.2. Deposition time for absorber and semi-absorber layers was 48 seconds and 72 seconds, respectively. The absorptance value of the coating was 0.88167 and emittance value was 0.13.

Example 2

Deposition of AlTiN Solar Selective Coating on Stainless Steel (Without Second Layer)

(20) In order to investigate importance of second semi-absorber layer, the first layer was coated on stainless steel substrates and optical properties have been characterized. Stainless steel substrates of the dimension 50×50 mm were polished and cleaned using acetone and ethyl alcohol. The substrates were polished to remove impurities and reduce surface roughness. The polished samples were cleaned using ethyl alcohol and acetone. Then, cleaned samples were placed in magnetron sputtering system and chamber was pumped down to a base pressure of 1×10.sup.−3 Pa. Absorber layer was deposited using Al and Ti cathodes. The flow rates of Ar and N.sub.2 were 100 sccm and 50 sccm, respectively. For absorber layer, power density of Al was 4.15 W/cm.sup.2 and Ti cathode was 6.65 W/cm.sup.2. Deposition time for absorber layers was 48 seconds. The absorptance value of the coating was 0.87541 and emittance value was 0.12.

Example 3

Thickness Dependence of AlTiN—AlTiN Solar Selective Coating on Stainless Steel

(21) In order to investigate thickness dependence of optical properties of semi-absorber layer, the second layer was coated with different thicknesses on the first absorber layer. Stainless steel substrates of the dimension 50×50 mm were polished and cleaned using acetone and ethyl alcohol. The substrates were polished to remove impurities and reduce surface roughness. The polished samples were cleaned using ethyl alcohol and acetone. Then, cleaned samples were placed in magnetron sputtering system and chamber was pumped down to base pressure of 1×10.sup.−3 Pa. Absorber and semi-absorber layers were deposited using Al and Ti cathodes. During the deposition of absorber layer, the flow rates of Ar and N.sub.2 were 100 sccm and 50 sccm, respectively. For absorber layer, power density of Al was 4.15 W/cm.sup.2 and Ti cathode was 6.65 W/cm.sup.2. For semi-absorber layer, flow rates of Ar and N.sub.2 were 100 sccm and 75 sccm, respectively. Power density of Al cathode was 7 W/cm.sup.2 and Ti cathode was 4.15 W/cm.sup.2. Deposition times for semi-absorber layer were 63, 72, and 81 seconds. The absorptance values of the coatings were 0.94254, 0.97553 and 0.95722, respectively. The emittance values of the coatings were 0.16.

Example 4

Deposition of AlTiN—AlTiN Solar Selective Coating on Stainless Steel

(22) Stainless steel substrates of the dimension 50×50 mm were polished and cleaned using acetone and ethyl alcohol. The substrates were polished to remove impurities and reduce surface roughness. The polished samples were cleaned using ethyl alcohol and acetone. Then, cleaned samples were placed in magnetron sputtering system and chamber was pumped down to base pressure of 1×10.sup.−3 Pa. Absorber and semi absorber layers were deposited on substrates in Ar+N.sub.2 gas mixture. The absorber coating was deposited using Al and Ti cathodes. The flow rates of Ar and N.sub.2 were 100 sccm and 50 sccm, respectively, for AlTiN absorber coatings. AlTiN semi-absorber layer was deposited using Al and Ti cathodes. Flow rates of Ar and N.sub.2 were 100 sccm and 75 sccm, respectively. For absorber layer, power density of Al was 4.15 W/cm.sup.2 and Ti cathode was 6.65 W/cm.sup.2. For semi-absorber layer, power density of Al cathode was 7 W/cm.sup.2 and Ti cathode was 4.15 W/cm.sup.2. Deposition times for absorber and semi-absorber layers were 48 and 72 seconds, respectively. Layer thicknesses were 60 nm and 70 nm, respectively.

Example 5

Deposition of AlTiSiN—AlTiSiN Solar Selective Coating on Stainless Steel and Copper

(23) Stainless steel and copper substrates of the dimension 50×50 mm were polished and cleaned using acetone and ethyl alcohol. The substrates were polished to remove impurities and reduce surface roughness. The polished samples were cleaned using ethyl alcohol and acetone. Then, cleaned samples were placed in magnetron sputtering system and chamber was pumped down to a base pressure of 1×10.sup.−3 Pa. Absorber and semi absorber layers were deposited on substrates in Ar+N.sub.2 gas mixture. The absorber coating was deposited using Al and Ti cathodes. The flow rates of Ar and N.sub.2 were 100 sccm and 50 sccm, respectively, for AlTiSiN absorber coatings. AlTiSiN semi-absorber layer was deposited using Al—Si(95-5) and Ti—Si(95-5) cathodes. Flow rates of Ar and N.sub.2 were 100 sccm and 75 sccm, respectively. For absorber layer, power density of Al—Si(95-5) was 4.15 W/cm.sup.2 and Ti—Si(95-5) cathode was 6.65 W/cm.sup.2. For semi-absorber layer, power density of Al—Si(95-5) cathode was 7 W/cm.sup.2 and Ti-(95-5) cathode was 4.15 W/cm.sup.2. Deposition times for absorber and semi-absorber layers were 48 and 72 seconds, respectively. Layer thicknesses were 60 nm and 70 nm, respectively.

Example 6

Thermal Stability of AlTiN—AlTiN Solar Selective Coating (in Air) on Stainless Steel Substrate

(24) TABLE-US-00007 TABLE 1 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 200 2 0.97557 0.97910 0.16 0.16 300 2 0.97027 0.16 400 2 0.97602 0.15 450 2 0.98089 0.15 500 2 0.96625 0.14 550 2 0.96626 0.14 600 2 0.93101 0.13 650 2 0.93301 0.12 700 2 0.92242 0.12 750 2 0.91733 0.12

(25) The coating in Example 4 was heated in air for 2 hours at different temperatures. The heating and cooling rates were 10° C./min and 1° C./min, respectively. Absorptance and emittance values of before and after heat treatment are presented in Table 1 for AlTiN—AlTiN double layer coating.

Example 7

Thermal Stability of AlTiSiN—AlTiSiN Solar Selective Coating (in Air) on Stainless Steel Substrate

(26) TABLE-US-00008 TABLE 2 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 200 2 0.97663 0.97822 0.16 0.16 300 2 0.97312 0.16 400 2 0.97209 0.15 450 2 0.97441 0.15 500 2 0.97546 0.14 550 2 0.96407 0.14 600 2 0.94093 0.13 650 2 0.93129 0.12 700 2 0.93401 0.12 750 2 0.92781 0.12

(27) The coating in Example 5 was heated in air for 2 hours at different temperatures. The heating and cooling rates were 10° C./min and 1° C./min, respectively. Absorptance and emittance values of before and after heat treatment are presented in Table 2 for AlTiSiN—AlTiSiN double layer coating.

Example 8

Thermal Stability of AlTiN—AlTiN Solar Selective Coating (in Vacuum) on Stainless Steel Substrate

(28) TABLE-US-00009 TABLE 3 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 600 3 0.97557 0.98183 0.16 0.16 700 3 0.97378 0.16 800 3 0.97562 0.16 900 3 0.96611 0.16

(29) The coating in Example 4 was heated in vacuum for 3 hours at different temperatures. The heating and cooling rates were 15° C./min and 1° C./min. Absorptance and emittance values of before and after heat treatment are presented in Table 3 for AlTiN—AlTiN double layer coating. In vacuum environment, coatings remained stable up to 900° C. for 3 hours and showed increased absorptance performance due to the grain growth.

Example 9

Thermal Stability of AlTiSiN—AlTiSiN Solar Selective Coating (in Vacuum) on Stainless Steel Substrate

(30) TABLE-US-00010 TABLE 4 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 600 3 0.97663 0.97922 0.16 0.16 700 3 0.97472 0.16 800 3 0.97388 0.16 900 3 0.97245 0.16

(31) The coating in Example 5 was heated in vacuum for 3 hours duration at different temperatures. The heating and cooling rates were 15° C./min and 1° C./min. Absorptance and emittance values of before and after heat treatment are presented in Table 4 for the AlTiSiN—AlTiSiN double layer coating. In vacuum environment, coatings remained stable up to 900° C. for 3 hours and showed increased absorptance performance due to the grain growth.

Example 10

High Temperature Long-Term Thermal Stability of AlTiN—AlTiN Solar Selective Coating (in Air) on Stainless Steel Substrate

(32) TABLE-US-00011 TABLE 5 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 500 200 0.97557 0.92807 0.16 0.14 400 0.93260 0.13

(33) The coating in Example 4 was heated in air for 200 and 400 hours durations at different temperatures. The heating and cooling rates were 15° C./min and 1° C./min. Absorptance and emittance values of before and after heat treatment are presented in Table 5 for the AlTiN—AlTiN double layer coating.

Example 11

High Temperature Long-Term Thermal Stability of AlTiSiN—AlTiSiN Solar Selective Coating (in Air) on Stainless Steel Substrate

(34) TABLE-US-00012 TABLE 6 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 500 200 0.97663 0.93543 0.16 0.14 400 0.93988 0.13

(35) The coating in Example 5 was heated in air for 200 and 400 hours durations at different temperatures. The heating and cooling rates were 15° C./min and 1° C./min, respectively. Absorptance and emittance values of before and after heat treatment are presented in Table 6 for the AlTiSiN—AlTiSiN double layer coating.

Example 12

Low Temperature Thermal Stability of AlTiN—AlTiN Solar Selective Coating (in Air) on Stainless Steel Substrate

(36) TABLE-US-00013 TABLE 7 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 350 72 0.97557 0.98402 0.16 0.15 144 0.97885 0.14 216 0.97711 0.14 288 0.97463 0.14

Example 13

Low Temperature Thermal Stability of AlTiSiN—AlTiSiN Solar Selective Coating (in Air) on Stainless Steel Substrate

(37) TABLE-US-00014 TABLE 8 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 350 72 0.97663 0.98537 0.16 0.15 144 0.98172 0.14 216 0.97880 0.14 288 0.97644 0.14

Example 14

Corrosion Resistance of AlTiN—AlTiN Solar Selective Coating on Stainless Steel Substrate

(38) TABLE-US-00015 TABLE 9 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed ASTM B117 96 0.97557 0.97878 0.16 0.16 192

Example 15

Environmental Test of AlTiN—AlTiN Solar Selective Coating on Stainless Steel Substrate

(39) TABLE-US-00016 TABLE 10 Absorptance Emittance Temperature (α) (ε) (° C.), Duration As As Humidity (h) deposited Annealed deposited Annealed 40, 95% 80 0.97557 0.98090 0.16 0.16 150 0.97857 0.16 300 0.98219 0.16 600 0.97549 0.16

Example 16

Corrosion Resistance of AlTiSiN—AlTiSiN Solar Selective Coating on Stainless Steel Substrate

(40) TABLE-US-00017 TABLE 11 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed ASTM B117 96 0.97663 0.98153 0.16 0.16 192

Example 17

Environmental Test of AlTiSiN—AlTiSiN Solar Selective Coating on Stainless Steel Substrate

(41) TABLE-US-00018 TABLE 12 Absorptance Emittance Temperature (α) (ε) (° C.), Duration As As Humidity (h) deposited Annealed deposited Annealed 40, 95% 80 0.97663 0.98144 0.16 0.16 150 0.97912 0.16 300 0.97893 0.16 600 0.97884 0.16

Example 18

Thermal Stability of AlTiN—AlTiN Solar Selective Coating on Copper Substrate

(42) TABLE-US-00019 TABLE 13 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 250 100 0.97744 94.655 0.04 0.06 150 94.217 0.07 200 93.428 0.09 250 92.978 0.10

Example 19

Thermal Stability of AlTiSiN—AlTiSiN Solar Selective Coating on Copper Substrate

(43) TABLE-US-00020 TABLE 14 Absorptance Emittance (α) (ε) Temperature Duration As As (° C.) (h) deposited Annealed deposited Annealed 250 100 0.97698 93.587 0.04 0.06 150 94.132 0.08 200 92.842 0.08 250 92.502 0.11