SOLAR SELECTIVE COATING HAVING HIGHER THERMAL STABILITY USEFUL FOR HARNESSING SOLAR ENERGY AND A PROCESS FOR THE PREPARATION THEREOF

Abstract

The present invention provides an improved solar selective multilayer coating having higher thermal stability and a process for the preparation thereof. Solar selective coatings having higher thermal stability are useful in solar steam generation, solar steam turbines to produce electricity and also on automobile engine components. In the present invention, a tandem stack of three layers of TiAlN, TiAlON and Si3N4 is deposited on metal and non-metal substrates at room temperature using a planar reactive direct current magnetron sputtering process. The first two layers function as the absorber and the third antireflection layer further enhances the coating's absorptance. The solar selective coatings were annealed in air and vacuum to test the thermal stability at different temperatures and durations. The coatings of the present invention deposited on copper substrates are stable in air up to a temperature of 625° C. for a duration of 2 hours and exhibit higher solar selectivity in the order of 9-10 and these coating also show no change in the absorptance and the emittance values even after vacuum annealing at 600° C. for 3 hours. Coatings deposited on copper substrates showed no significant degradation in the optical properties even after continuous heating in air at 525° C. for 50 hours. The solar selective coatings of the present invention exhibit high hardness, high oxidation resistance, chemical inertness and stable microstructure.

Description

Claims

1. An improved solar selective coating having higher thermal stability useful for harnessing solar energy comprising first solar absorber layer of TiAlN being deposited on a substrate selected from glass, silicon and metal, the said first absorber layer being deposited by another second solar absorber layer and a third antireflection layer of TiAlON and Si3N4, respectively.

2. An improved solar selective coating having higher thermal stability useful for harnessing solar energy comprising first absorber layer of TiAlN being deposited on a substrate selected from glass, silicon and metal, the said first absorber layer being deposited by another second absorber layer and a third antireflection layer of TiAlON and Si3N4, respectively, has the following characteristics: i) having a first absorber layer thickness of 300-500 Å, a second absorber layer thickness of 400-800 Å and a third antireflection layer thickness of 300-500 Å,ii) high absorptance of 0.92-0.95 and low emittance 0.06-0.08 on copper substrates,iii) thermally stable in air up to 600° C. for a duration of 2 hours with a solar selectivity of 9-10 on copper substrate,iv) thermally stable in vacuum (5.0×104 Pa) for a duration of 3 hours at 600° C.,v) thermally stable in air at 525° C. for a duration of 50 hours with a solar selectivity of 10 on copper substrate,vi) approximately 4500 Å thick coating on copper substrate exhibits a nanoindentation hardness of 1170 kg/mm2 at a load of 3 mN,vii) coatings deposited on silicon substrate show oxidation resistance up to 850° C.

3. An improved solar selective coating according to claim 1, wherein elements used in depositing the first absorber layer are Ti, Al and N.

4. An improved solar selective coating according to claim 1, wherein the elements used in depositing the second absorber layer are Ti, Al, O and N.

5. An improved solar selective coating according to claim 1, wherein elements used in depositing the antireflection layer are Si and N.

6. An improved solar selective coating according to claim 1, wherein titanium-aluminum composite target is sputtered in the presence of nitrogen-argon plasma for the deposition of first absorber layer of TiAlN.

7. An improved solar selective coating according to claim 1, wherein titanium-aluminum composite target is sputtered in the presence of nitrogen-argon-oxygen plasma for the deposition of second absorber layer of TiAlON.

8. An improved solar selective coating according to claim 1, wherein silicon target is sputtered in the presence of nitrogen-argon plasma for the deposition of third antireflection layer of Si3N4.

9. A process for the preparation of an improved solar selective coating having higher thermal stability useful for harnessing solar energy, the said process comprising the steps of: i) polishing and chemically cleaning the substrate by known methods, followed by subsequent cleaning by argon ion bombardment under vacuum at a pressure of 1.0-6.0×10−1 Pa,ii) depositing a solar absorber layer of TiAlN on the above said cleaned substrate by DC magnetron sputtering of titanium-aluminium (Ti—Al) composite target in the presence of argon-nitrogen plasma, at a pressure of 0.1-0.5 Pa, at a flow rate of nitrogen of 2-3 standard cubic centimeter per minute (sccm), at a power density of 2.25-6.25 watts/cm2 and at a substrate temperature of 25-50° C.,iii) depositing another solar absorber layer of TiAlON on the above said layer of TiAlN deposited on the substrate by DC magnetron sputtering of titanium-aluminium (Ti—Al) composite target, in the presence of argon-oxygen-nitrogen plasma, at a pressure of 0.1-0.5 Pa, at a flow rate of nitrogen of 2-3 standard cubic centimeter per minute (sccm), at a flow rate of oxygen of 1-2 sccm, at power density of 2-6 watts/cm2 and at a substrate temperature of 25-50° C.,iv) further deposing a third antireflection layer of Si3N4 on the above said second solar absorber layer of TiAlON by using DC magnetron sputtering of silicon target, in the presence of argon-nitrogen plasma, at a pressure of 0.1-0.5 pa, at a flow rate of nitrogen of 2-3 sccm, at a power density of 2-6 watts/cm2 and at a substrate temperature of 25-50° C. to obtain the desired solar selective coating of substrate/TiAlN/TiAlON/Si3N4.

10. A process according to claim 9, wherein the substrate used is selected from metal and non metal substrates.

11. A process according to claim 9, wherein the substrate used is selected from the group consisting of copper, nickel, stainless steel, glass and nimonic.

12. A process according to claim 9, wherein the vacuum chamber is maintained at a base pressure of 3.0-6.0×10−4 Pa before deposition of the coatings.

13. A process according to claim 9, wherein the coatings are deposited at an operating pressure in the range of 0.1-0.3 Pa.

14. A process according to claim 9, wherein the solar selective multilayer coatings are deposited at a substrate to target distance of 4-6 cm.

15. A process according to claim 9, wherein the thickness of the first absorber layer coatings is in the range of 300-500 Å.

16. A process according to claim 9, wherein the thickness of the second absorber layer coating is in the range of 400-800 Å.

17. A process according to claim 9 wherein the thickness of the third antireflection layer is in the range of 300-500 Å.

18. A process according to claim 9, wherein the solar selective multilayer coatings obtained have high absorptance of 0.92-0.95 and low emittance 0.06-0.08 on copper substrates.

19. A process according to claim 9, wherein the solar selective multilayer coatings on copper substrate are thermally stable in air up to 600° C. for a duration of 2 hours with a solar selectivity of 9-10.

20. A process according to claim 9, wherein the solar selective multilayer coatings are thermally stable in vacuum (5.0×104 Pa) for a duration of 3 hours at 600° C.

21. A process according to claim 9, wherein the solar selective coatings on copper substrate are thermally stable in air at 525° C. for a duration of 50 hours with a solar selectivity of 10.

22. A process according to claim 9, wherein the solar selective coatings of approximately 4500 Å thick on copper substrate exhibits a nanoindentation hardness of 1170 kg/mm2 at a load of 3 mN.

23. A process according to claim 9, wherein the coatings deposited on silicon substrates show oxidation resistance up to 850° C.

24. A process according to claim 9, wherein the coatings deposited on copper substrate show an order of magnitude improvement in the corrosion resistance.