Solar selective coating having high thermal stability and a process for the preparation thereof

Abstract

The present invention describes an improved multilayer solar selective coating useful for solar thermal power generation. Solar selective coating of present invention essentially consists of Ti/Chrome interlayer, two absorber layers (AlTiN and AlTiON) an anti-reflection layer (AlTiO). Coating deposition process uses Ti and Al as the source materials, which are abundantly available and easy to manufacture as sputtering targets for industrial applications. The present invention allows deposition of all the layers in a single sputtering chamber on flat and tubular substrates with high absorptance and low emittance, thus making the process simpler and cost effective. The process of the present invention can be up-scaled easily for deposition on longer tubes with good uniformity and reproducibility. The coating of the present invention also displays improved adhesion, UV stability, corrosion resistance and stability under extreme environments.

Claims

1. An improved solar selective coating having high thermal, stability comprising tandem stack of layers consisting of an interlayer of titanium (Ti)/Chrome followed by a first absorber layer comprising aluminum-titanium nitride (AlTiN); a second absorber layer comprising aluminum-titanium oxy-nitride (AlTiON); and a third antireflection layer comprising aluminum-titanium oxide (AlTiO); wherein said second absorber layer being deposited on the first absorber layer and said third antireflection layer being deposited on the second absorber layer at substrate temperature in the range 100-350° C. using a four-cathode reactive pulsed direct current unbalanced magnetron sputtering technique; and wherein the first absorber layer contains Aluminium in the range of 25-55%, Titanium in the range of 10-25% and Nitrogen in the range of 30-50%, the second absorber layer contains Aluminium in the range of 15-30%, Titanium in the range of 10-15%, Nitrogen in the range of 10-20% and Oxygen in the range of 50-60%, and the third anti-reflection layer contains Aluminium in the range of 15-30%, Titanium in the range of 5-15% and O in the range of 40-80%.

2. An improved solar selective coating as claimed in claim 1, wherein the thickness of the Titanium interlayer is in the range of 10-80 nm, thickness of the first absorber layer is in the range of 30-70 nm, thickness of the second absorber layer is in the range of 20-40 nm and thickness of the third antireflection layer is in the range of 30-55 nm.

3. An improved solar selective coating as claimed in claim 1, wherein thickness of the chrome interlayer is in the range of 5-10 μm, deposited by conventional electroplating.

4. An improved solar selective coating as claimed in claim 1, wherein the solar selective coating has absorptance greater than 0.92 and emittance less than 0.17 on stainless steel 304 substrate.

5. An improved solar selective coating as claimed in claim 1, wherein the solar selective coating has absorptance greater than 0.92 and emittance less than 0.07 on copper substrates.

6. An improved solar selective coating as claimed in claim 1, wherein the coating is thermally stable in air up to 350° C. for a duration of 1000 hrs on stainless steel substrates under cyclic heating conditions.

7. An improved solar selective coating as claimed in claim 1, wherein the coating is thermally stable in vacuum (2.0-8.0×10.sup.−4 Pa) up to 450° C. for a duration of 1000 hrs on stainless steel substrates under cyclic heating conditions.

8. An improved solar selective coating as claimed in claim 1, wherein the coating is stable under exposure to ultraviolet (UV) irradiation.

9. An improved solar selective coating as claimed in claim 1, wherein the coating is stable at temperature under −2° C. for more than 9600 hrs.

10. An improved solar selective coating as claimed in claim 1, wherein the coating is stable when exposed to sun in ambient conditions including dust, rain and mist for more than 10000 hrs.

11. An improved solar selective coating as claimed in claim 1, wherein the coating is stable when exposed to steam for up to 85 hrs.

12. An improved solar selective coating as claimed in claim 1, wherein the coating deposited on stainless steel substrates qualifies salt spray test as per ASTM B117 standard and shows improvement in the corrosion resistance by a factor of 100 in 3.5% NaCl solution.

13. An improved solar selective coating as claimed in claim 1, wherein the coating deposited on stainless steel substrates qualifies tape adhesion test and demonstrates high adhesion strength while scratching using a 5 rim diamond tip.

14. A process for the deposition of improved solar selective coating of claim 1 on a substrate, comprising the following steps: [a] metallographic or buff cleaning of substrate; [b] chemical cleaning of the substrate as obtained in step [a]; [c] degassing of the substrate as obtained in step [b] in vacuum using a substrate heater; [d] etching of the substrate as obtained in step [c] in Argon plasma to remove the impurities; [e] depositing a Titanium/Chrome interlayer on the substrate as obtained in step [d] in argon plasma by maintaining the substrate temperature in the range 100-350° C. and bias voltage in the range −50 to −200 V; [f] depositing a first absorber layer comprising aluminum titanium nitride (AlTiN) on the substrate as obtained in step [e] by sputtering two Titanium and two Aluminium targets in argon-nitrogen plasma by maintaining the substrate temperature in the range 100-350° C. and bias voltage in the range −50 to −200 V; [g] depositing a second absorber layer comprising aluminum-titanium oxy-nitride (AlTiON) on the substrate as obtained in step [f] by sputtering two Titanium and two Aluminium targets in argon-nitrogen-oxygen plasma by maintaining the substrate temperature in the range 100-350° C. and bias voltage in the range −50 to −200 V; [h] depositing a third antireflection layer comprising aluminum-titanium oxide (AlTiO) on the substrate as obtained in step [g] by sputtering two Titanium and two Aluminium targets in argon-oxygen plasma by maintaining the substrate temperature in the range of 100-350° C.; and [i] etching of the antireflection layer as obtained in step [h] in argon-oxygen plasma for a duration of 20-60 min by maintaining substrate temperature in the range 100-350° C. and bias voltage in the range −500 to −1200 V to obtain the substrate deposited with desired solar selective coating.

15. A process as claimed in claim 14, wherein the substrate used is selected from the group consisting of copper, nickel, stainless steel 304, glass nimonic, nickel coated stainless steel (SS), mild steel (MS) and aluminum.

16. A process as claimed in claim 14, wherein deposition of all the layers is done in a single sputtering chamber on flat and tubular metal and non-metal substrates.

17. A process as claimed in claim 14, wherein the solar selective coating is deposited at a sputtering power density of 2.75-3.5 watts/cm.sup.2 for Aluminium and Titanium targets.

18. A process as claimed in claim 14, wherein compositions of the first, second and third layers are independently controlled by controlling the sputtering power to the Aluminium and Titanium targets and the flow rates of N.sub.2 and O.sub.2.

19. A process as claimed in claim 14, wherein vacuum chamber is maintained at a base pressure of 3.0-6.0×10.sup.−4 Pa before deposition of the coating.

20. A process as claimed in claim 14, wherein the solar selective coating is deposited in the pressure range of 0.1-0.5 Pa.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention provides a multilayer solar selective coating having higher thermal stability and long life in the order of 1000 hrs under cyclic heating conditions in air at 350° C. It also provides a multilayer solar selective coating having higher thermal stability in vacuum at 450° C. and stability up to 1000 hrs under cyclic heating conditions. Solar selective coating of the present invention exhibits higher solar selectivity ratio in the order of 5-10 on stainless steel 304 substrates and 13 on copper substrates. The first absorber layer. AlTiN exhibits high temperature stability and better oxidation resistance (up to 750-800° C.). The second absorber layer AlTiON also exhibits higher thermal stability. Similarly, the third antireflection layer AlTiO exhibits very high oxidation resistance. The third layer (AlTiO) has been further etched in Ar+O.sub.2 plasma at a substrate temperature in the range of 100-350° C. for 10-60 min to generate micro-texturing as well as to stabilize the structure. The substrates have been sputter etched in Ar plasma (−500 to −1200 V) to remove contaminants before coating deposition and a thin Ti/Chrome interlayer between the substrate and absorber coating has been deposited. The sputter etching and the Ti/Chrome interlayer enhance the adhesion of the absorber layer significantly. Thus, the combination of layers selected in the present invention provides high thermally stable, high oxidation resistance, chemically inert, stable microstructure, highly adherent and graded composition solar selective absorber coating useful for high temperature applications.

(2) The objects of the invention have, been achieved by adopting the following steps: 1. Deposition of Ti interlayer/chrome in order to have good adhesion of coating to the substrate and then depositing a tandem stack of multilayer coating comprising two absorber layers in which the first absorber layer (AlTiN) is tailor made to have high metal volume fraction and the second absorber layer (AlTiON) having low metal volume fraction for enhancing the absorption of the coating. The first and second layers comprise of more Al content than Ti for enhancing the thermal stability of the absorber coating. 2. Providing third antireflection layer (AlTiO) for reducing the infrared emittance so as to increase the absorption further. The third layer also comprises of more Al content than Ti. 3. Carefully selecting the candidate materials and their composition for depositing AlTiN, AlTiON and AlTiO layers so that the inter-diffusion between the layers of the tandem stack is minimal and the microstructure is stable even at higher working temperatures.

(3) The present invention provides an improved multilayer solar selective coating useful for solar thermal power generation. Solar selective coating of present invention essentially consists of a Ti/Chrome interlayer, two absorber layers (AlTiN and AlTiON) and an anti-reflection layer (AlTiO). Coating deposition process uses two titanium and two aluminum targets to deposit both absorber layers and the anti-reflection layer so that the contents of Ti and Al in the absorber layers and the anti-reflection layer can be controlled independently by controlling the sputtering power to the Ti and Al targets using bipolar pulsed DC power supplies and N.sub.2 and O.sub.2 flow rates. The content of Al is higher than Ti in all the layers, which enhances the thermal stability of the absorber coating as Al very easily forms a passive amorphous Al.sub.2O.sub.3 layer when exposed to air. The present invention uses Ti and Al as the source materials, which are abundantly available and easy to manufacture as sputtering targets for industrial applications. The anti-reflection layer of the present invention has been etched with Ar+O.sub.2 plasma at a temperature in the range 100-350° C. to enhance the oxidation resistance of the absorber coating and stabilize the microstructure. The present invention also allows deposition of all the layers in a single sputtering chamber on flat and tubular substrates (metallic and non-metallic) with high absorptance (>0.93) and low-emittance (<0.16 on SS 304 and <0.07 on copper substrates), thus making the process simpler and cost effective. The process of the present invention can be up-scaled easily for deposition on longer tubes with good uniformity and reproducibility. The absorber coating of the present invention has been shown to display thermal stability in air (up to 350° C.) and vacuum (up to 450° C.) for longer durations (>1000 hrs) under cyclic heating conditions. The coating of the present invention also displays improved adhesion, UV stability, corrosion resistance and stability under extreme environments (freezing condition, exposure to steam and to atmosphere).

(4) The solar selective multilayer coating of the present invention was deposited using a four-cathode reactive pulsed direct current magnetron sputtering process. The sputtering system consists of: vacuum chamber, turbo molecular pump, rotary pump, four direct cooled unbalanced magnetron cathodes mounted horizontally in opposed-cathode configuration, four 5 kW asymmetric-bipolar pulsed plasma generators, 1.5 kW DC power supply for substrate bias and ion bombardment, substrate holder plate for mounting three-dimensional objects with planetary rotation and heating facility, vacuum gauges and control consoles.

(5) In order to deposit absorber coating, two Ti (purity=99.95%) and two Al (purity=99.99%) targets (diameter=150 mm and thickness of 12 mm) were sputtered in high purity Ar (99.999%) plasma containing N.sub.2 (99.999%) and O.sub.2 gases (99.999%). The coatings were deposited under a base pressure of 3.0-6.0×10.sup.−4 Pa and Ar+N.sub.2, Ar+N.sub.2+O.sub.2 and Ar+O.sub.2 gas pressures were in the range of 1.0-5.0×10.sup.−1 Pa. The flow rates of Ar, N.sub.2 and O.sub.2 were controlled separately by mass flow controllers. A DC substrate bias in the range of −50 to −200 V was applied to improve the mechanical properties of the coating and also to improve the adhesion of the coating. The pulsed generators were operated at the following conditions: frequency=50-150 kHz, pulse width=2000-3000 ns, duty cycle=10-40% and reverse bias voltage=+37 V. The coatings were deposited at a substrate temperature in the range of 100-350° C. The power density for Al and Ti targets was in the range of 2.75-3.5 watts/cm.sup.2. A 10-80 nm thick Ti interlayer was deposited to improve the adhesion of the coating on the substrates.

(6) The coatings were deposited on various substrates. Before putting the substrates into the vacuum chamber, they were metallographically polished or buffed to remove the surface oxides and to make the surface homogeneous. The polished/buffed substrates were then chemically cleaned using an ultrasonic agitator to remove the grease and other impurities such as dust and debris. Chemically cleaned substrates were positioned in the sputtering system. The vacuum chamber was pumped down to a base pressure in the order of 3.0-6.0×10.sup.4 Pa to remove any gaseous impurities. The substrates were degassed in vacuum using a substrate heater. In order to remove native oxides on the substrate surface further cleaning was carried out using argon ion bombardment (bias voltage: −500 to −1200 V). After cleaning the substrates a Ti interlayer of 10-80 nm was deposited on the substrates for improving the adhesion. For chrome plated samples, titanium interlayer was not used. The first absorber layer was deposited by sputtering of two Ti and two Al targets in the argon-nitrogen plasma at a pressure in the range of 0.1-0.5 Pa. The content of Al was higher than Ti in the first absorber layer. The second absorber layer was deposited by sputtering of two Ti and two Al targets in the argon-nitrogen-oxygen plasma at a pressure in the order of 0.1-0.5 Pa. The content of Al was higher than Ti in the second absorber layer. Subsequently, the third anti-reflection layer was deposited by sputtering of two Ti and two Al targets in the argon-oxygen plasma at a pressure of the order of 0.1-0.5 Pa. Again, the content of Al was higher than Ti in the third anti-reflection layer. Finally, the third layer was etched in Ar+O.sub.2 plasma at a substrate temperature in the range of 100-350° C. for a duration of 20-60 min to stabilize the microstructure of the AlTiO layer. The tandem stack of three layers effectively increased the absorptance (α) and reduced the emittance (∈) of the solar selective coating.

(7) The optical properties (α and ∈) of the samples were measured at four different positions and an average of them is reported herein. The solar selective coating was annealed in air and vacuum under cyclic heating conditions at different temperatures for testing the thermal stability. The thicknesses of the deposited layers were measured using transmission electron microscopy.

EXAMPLES

(8) The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

Example 1

(9) Before putting the substrates into the vacuum chamber the substrates (copper, nickel, stainless steel 304, Ni coated SS, mild steel, glass and nimonic, having dimensions 35 mm×35 mm×2 mm) were metallographically polished or buffed and chemically cleaned in an ultrasonic agitator in acetone, absolute alcohol and trichloroethylene. The tubular substrates (140 mm length and 30 mm diameter) were buffed and chemically cleaned as mentioned above. In order to reduce the emittance of the selective coating on stainless steel substrate, chrome plating of thickness of 5 μm was deposited using conventional electroplating process. The vacuum chamber was pumped down to a base pressure of 1.0×10.sup.−4 Pa using a turbo-molecular pump backed by a rotary pump. The substrates were cleaned in situ to remove the impurities by etching with argon ion bombardment for 60 min, wherein a DC bias of −500 V was applied to the substrate at an argon pressure of 8.0×10.sup.−1 Pa.

(10) After cleaning the substrates a Ti interlayer of 10-80 nm was deposited on the substrates for improving the adhesion. For chrome plated samples, titanium interlayer was not used. The solar absorber film was deposited on the substrates using four-cathode reactive pulsed direct current unbalanced magnetron sputtering system. Two Ti and two Al targets were used for the sputtering of AlTiN (first absorber) layer, AlTiON (second absorber) layer and AlTiO (third anti-reflection) layer. The power densities for Ti and Al targets were 2.75 and 3.0 watts/cm.sup.2, respectively. The substrates were heated at a temperature of 200° C. For the AlTiN (first absorber) layer the nitrogen flow rate was 15 standard cubic centimeter per minute (sccm). For the AlTiON (second absorber) layer the nitrogen flow was 10 sccm and the oxygen flow rate was 20 sccm. Whereas, the oxygen flow rate for the AlTiO layer was in 30 sccm. The said third anti-reflection layer was etched in Ar+O.sub.2 plasma with an oxygen flow rate 40 sccm and a substrate temperature of 200° C. A planetary rotation system was employed for achieving uniform absorber coating on flat as well as on tubular substrates.

(11) The optical properties (absorptance and emittance) of the samples were measured using standard instruments procured from M/s. Devices and Services, USA. Emittance was measured at 82° C. The accuracies for the measurements of the emittance and the absorptance were ±0.01 and ±0.002, respectively. The absorptance and the emittance values for stainless steel (SS) substrate, SS/Ti/AlTiN, SS/Ti/AlTiN/AlTiON, SS/Ti/AlTiN/AlTiON/AlTiO and SS/Ti/AlTiN/AlTiON/AlTiO/etching are given in Table 1. The absorber coating on the SS substrate with titanium interlayer exhibited absorptance of 0.927-0.930 and emittance of 0.16-0.1, whereas, for chrome plated samples absorptance was 0.930-0.935 and emittance was 0.09-0.10.

(12) TABLE-US-00001 TABLE 1 Absorptance and emittance of different layers of the solar selective coating system of the present invention. Also shown are the values of SS 304. Material α ε SS substrate (304) 0.361-0.363 0.10-0.11 SS/Ti 0.477-0.478 0.13 SS/Ti/AlTiN 0.802-0.803 0.15-0.16 SS/Ti/AlTiN/AlTiON 0.900 0.16-0.17 SS/Ti/AlTiN/AlTiON/AlTiO 0.927 0.16-0.17 SS/Ti/AlTiN/AlTiON/AlTiO/etching 0.927-0.930 0.16-0.17 SS/chrome/AlTiN/AlTiON/AlTiO/etching 0.930-0.935 0.09-0.10

Example 2

(13) The solar selective coating of the present invention as stated in Example 1 was mainly deposited on SS 304 and exhibited an average emittance of 0.16-0.17. The high emittance on SS substrate is attributed to the intrinsic property of the SS substrate (∈=0.10-0.11). The SS substrate chosen for the present invention is mainly due to the fact that for steam generation, the substrate may reach a temperature greater than 400° C. and at these temperatures copper and other commonly used substrates start diffusing to the absorber coating, thus affecting its optical properties. Additionally, copper and other metallic substrates get corroded very easily with supersaturated steam.

(14) In order to confirm that the emittance of the absorber coating of the present invention was substrate dependent, the solar selective coating was also deposited on other commercial available flat substrates such as: nickel, mild steel, aluminum, glass and nickel based superalloy (nimonic). The absorptance and the emittance values of the solar selective coating on these substrates are given in Table 2 along with the intrinsic absorptance and emittance values of the substrates. The absorber coating prepared on Ni, Cu and Al substrates exhibited emittance values of 0.06-0.07.

(15) TABLE-US-00002 TABLE 2 Absorptance and emittance of solar selective coating deposited on various substrates. Values in the bracket represent the intrinsic absorptance and emittance of the substrate. Substrate Material α ε Copper 0.932-0.933 (0.234-0.238) 0.07 (0.02-0.03) Glass* 0.927-0.928 — Aluminum 0.927-0.928 (0.169-0.173) 0.07 (0.02-0.03) Nickel    .sup. 0.27 (0.333-0.336) 0.07-0.08 (0.03-0.04)  .sup.  Stainless steel 0.932-0.934 (0.361-0.363) 0.16-0.17 (0.10-0.11)  .sup.  Ni coated 0.932-0.933 (0.334-0.337) 0.07-0.08 (0.03-0.04)  .sup.  stainless steel Mild steel    .sup.  0.930 (0.400-0.401) 0.11 (0.04-0.05) Nimonic    .sup.  0.924 (0.344-0.345) 0.19 (0.13-0.14) *Semi-transparent. The measurements may not be accurate.

(16) Additionally, the SS substrates were also coated with approximately 5.0 μm thick Ni layer and subsequently the absorber coating was deposited on this substrate. Interestingly, the emittance of this coating was as low as 0.07-0.08 and the absorptance was 0.932-0.933.

Example 3

(17) The solar selective coating of the present invention deposited on SS substrates, as deposited in Example 1, was heated in air in a resistive furnace at a temperature in the range of 300-600° C. for different durations under cyclic heating conditions to test the thermal stability. Annealing involved increasing the temperature of the sample from room temperature to the set temperature at a slow heating rate of 3° C./min and maintaining the desired temperature for 8 hrs. Subsequently, the sample was cooled down at a rate of 3° C./min. The accuracy of the temperature controller was ±1° C. at the set temperature. The absorptance and the emittance values of the absorber coating after heat-treatment indicated that the absorber coating was stable for temperature less than 400° C. for shorter durations and for temperature greater than 450° C. the absorptance decreased significantly.

(18) The thermal stability of the absorber coating deposited on Ni coated SS yielded low thermal stability. This coating when heated to higher temperature (>400° C.) in air peeled off mainly because of different thermal expansion coefficients of SS, Ni and the absorber coating.

(19) In order to test the long term thermal stability of the absorber coating of the present invention the heat treatment studies were carried out at 350° C. under cyclic heating conditions for 1000 hrs. The absorptance and emittance values were measured at different intervals and are summarized in Table 3. As seen from Table 3 the absorber coating of the present invention is highly stable in air at 350° C. for longer durations. No structural changes were observed as a result of prolonged heating. This demonstrates that the coating of the present invention can be used for applications in air wherein the temperature is less than 350° C. (for example, linear Fresnel technology).

(20) TABLE-US-00003 TABLE 3 Effect of annealing (in air at 350° C.) on optical properties of the Ti/AlTiN/AlTiON/AlTiO solar selective coating deposited on SS substrate under cyclic heating conditions. Total time of Days exposure (hrs) α ε 0 0 0.932-0.933 0.16 6 41 0.927-0.928 0.16 18 120 0.924-0.925 0.13-0.15 22 152 0.923 0.14 28 190 0.923 0.15 37 249 0.922 0.15 43 293 0.923-0.924 0.17 53 354 0.923-0.924 0.17 73 491 0.919-0.921 0.16 87 588 0.921-0.922 0.16 110 740 0.920-0.921 0.16 121 815 0.920 0.16 126 849 0.920-0.921 0.16 140 940 0.919-0.920 0.15 150 1000 0.919-0.920 0.16

Example 4

(21) The solar selective coating of the present invention, deposited on SS substrates following the procedure given in Example 1, was also subjected to heat-treatment in vacuum (2.0-8.0×10.sup.−4 Pa) for different temperatures and durations at cyclic heating conditions. Annealing involved increasing the temperature of the sample from room temperature to the desired temperature at a slow heating rate of 5° C./min and maintaining the desired temperature for 6 hrs. Subsequently, the samples were cooled down at a rate of 5° C./min. The accuracy of the temperature controller was ±1° C. at the set temperature. The absorptance and emittance values of the absorber coating are summarized in Table 4. As can be seen from Table 4, the coating retains its optical properties for temperature less than 500° C. Therefore, thermal stability tests were conducted for longer durations under cyclic heating conditions. The optical properties of the absorber coating were measured at regular intervals and are listed in Table 5. As can be seen from Table 5 the absorber coating deposited on SS substrates of the present invention retains its optical properties after subjecting to thermal annealing for long durations. No structural changes were observed as a result of prolonged heating. This demonstrates that the coating of the present invention can be used for applications in vacuum wherein the temperature is less than 450° C. (for example, receiver tubes).

(22) TABLE-US-00004 TABLE 4 Effect of annealing (in vacuum) on optical properties of the Ti/AlTiN/AlTiON/AlTiO solar selective coating deposited on SS substrates. Temper- α ε ature Duration As- An- As- An- (° C.) (Hrs) deposited nealed deposited nealed 475 125 0.923-0.924 0.922-0.923 0.16 0.17 500 37 0.932-0.933 0.927-0.926 0.16 0.16 550 35 0.928-0.929 0.917-0.918 0.17 0.16 650 10 0.930-0.932 0.925-0.926 0.17 0.16 750 15 0.931-0.932 0.924-0.925 0.17 0.16 850 20 0.930-0.931 0.899-0.904 0.17 0.17

(23) TABLE-US-00005 TABLE 5 Effect of annealing (in vacuum at 450° C.) on optical properties of the Ti/AlTiN/AlTiON/AlTiO solar selective coating deposited on SS substrates under cyclic heating conditions. Total time of Days exposure (hrs) α ε 0 0.930-0.931 0.16 1 2 0.930-929.sup.  0.15 2 11 0.929 0.16 5 35 0.928-0.929 0.15 10 82 0.927-0.930 0.16 16 128 0.928 0.14 21 171 0.927 0.14 26 213 0.927 0.14 31 254 0.926-0.927 0.15-0.16 36 297 0.927 0.17 41 340 0.927 0.15-0.16 51 420 0.927 0.16 56 463 0.928-0.929 0.16 102 839 0.926-0.927 0.14 123 1004 0.925-0.926 0.15-0.16

Example 5

(24) The solar selective coating of this invention deposited on SS substrates following the procedure given in Example 1 is also subjected to UV irradiation. The UV irradiation tests have been carried out using a 200 W Hg lamp at an intensity of 50 mW/cm.sup.2. The exposure has been done for 10 hrs under ambient conditions in successive steps. No degradation in the absorptance and emittance was observed after UV exposure. The absorptance and emittance values after UV exposure are listed in Table 6.

(25) TABLE-US-00006 TABLE 6 Absorptance and emittance data of solar selective coating deposited on stainless steel substrate after UV exposure. α ε Exposure As- After UV As- After UV duration (Hrs) deposited exposure deposited exposure 10 0.928-0.929 0.928-0.929 0.16-0.17 0.15-0.16

Example 6

(26) The solar selective coating of the present invention, deposited on SS substrate following the procedure given in Example 1, is subjected to salt spray test in 3.5% NaCl solution as per ASTM B117 standard. The tests were carried out for 168 hrs. No significant changes in the absorptance and emittance were observed as a result of the salt spray test and the data is presented in Table 7. The same sample when tested repetitively for 3 times showed a marginal increase in the emittance as shown in Table 7.

(27) TABLE-US-00007 TABLE 7 Absorptance and emittance data of solar selective coating deposited on stainless steel substrate after salt spray tests as per ASTM B117 standard. α ε Exposure As- After salt- As- After salt- duration (Hrs) deposited spray test deposited spray test 168 0.931-0.932 0.926-0.927 0.17 0.20 432 0.930-0.931 0.939-0.942 0.17 0.25

(28) The solar selective coating of the present invention, deposited on SS substrates following the procedure given in Example 1, was also subjected to corrosion testing in 3.5% NaCl solution (pH=5.8) in free air condition at room temperature as per the procedure described in Thin Solid Films 514 (2006) 204. For SS/Ti/AlTiN/AlTiON/AlTiO/etching solar selective coating deposited on SS substrate the obtained values of E.sub.corr, i.sub.corr and polarization resistance (R.sub.p) are displayed in Table 8. The corrosion current density of the coated substrate decreased by a factor of 100, showing improved corrosion resistance of the coating, which is due to the chemical inertness of the constituent layers of the solar selective coating.

(29) TABLE-US-00008 TABLE 8 Potentiodynamic polarization data of Ti/AlTiN/AlTiON/AlTiO solar selective coating deposited on SS substrate in 3.5% NaCl solution. Also, shown are the values for SS substrate. i.sub.corr E.sub.corr R.sub.p Material (μA/cm.sup.2) (V) (kΩ cm.sup.2) SS −0.241 4.80 × 10.sup.−8  2.13 × 10.sup.5 SS/Ti/TiAlN/TiAlON/TiAlO −0.282 4.04 × 10.sup.−10 3.72 × 10.sup.7

Example 7

(30) Robustness of the absorber coating is very important as the absorber coating should have a service life of more than 25 years. In order to test the robustness of the coating, adhesion tests were carried out on the absorber coating deposited on SS substrates. The adhesion tests were carried out using a conventional tape test and using a nanoscratch tester. The absorber coating of the present invention passed the tape test, wherein 18 mm wide scotch tape was fixed on the coating and one of the ends of the tape was pulled up. Similarly, the coating was also scratched using a 5 □m diameter spherical diamond indenter at a load of 200 mN as per the procedure described in Surface and Coatings Technology 205 (2010) 1937. Approximately 120 nm thick coating deposited on SS substrate demonstrated Lc.sub.1=40 mN and Lc.sub.2=75 mN, where Lc.sub.1 is initial point of coating detachment or cracking on the scratch track and Lc.sub.2 refers to the point at which complete delamination occurs and subsequent exposure of the substrate takes place. These measurements indicated very good adhesion of absorber coating on the substrate.

Example 8

(31) External environment exposure tests have been conducted by putting absorber coating prepared on SS substrates in a petri-dish without a lid. The sample was kept in open conditions for more than 10000 hrs. During the test the sample got exposed to dust, sunlight, rain, frost and mist. The absorptance and emittance values of the sample after exposing to external environment are presented in Table 9. Similarly, the sample was put in freezing conditions in a freezer for long durations (9600 hrs) to elucidate its stability at low temperatures (<−2° C.). The optical properties of the sample before and after exposure to ice are listed in Table 10. Finally, the absorber coating of the present invention was exposed to steam, wherein, the absorber coating prepared on SS substrate was kept over a beaker with boiling water. This exposure was done for 84 hrs. The absorptance and emittance values of the absorber coating after steam exposure are listed in Table 11. As can be seen from Tables 9-11, no changes in the optical properties of the absorber coating of the present invention were observed as a result of different aging tests, indicating its stability under harsh environments.

(32) The efficiency of photothermal conversion at high temperatures strongly depends on the optical properties and thermal stability of the component materials used in the solar absorbers. For concentrating solar power applications, the spectrally selective coatings should have high absorptance (>0.92), low emittance (<0.14) and thermal stability above 400° C. in air and vacuum. In addition, long term thermal stability of the coatings in air and vacuum is also an important requirement for high temperature solar selective coatings. Furthermore, the coatings should have high oxidation resistance and chemical inertness.

(33) For high temperature applications, low emittance at higher operating temperatures is an important parameter, because the thermal radiative losses of the absorbers increase proportionally by T.sup.4. As discussed in the prior-art, several transition metal based cermet coatings have been developed for high temperature solar thermal applications, because of their refractory nature. The choice of the dielectric material is also very important for the performance of the selective absorber. In general, dielectric materials with low refractive index are preferred in order to reduce the front surface reflections of the cermet coating. Al.sub.2O.sub.3 is widely used as a dielectric material in cermet coatings due to its low refractive index (n=1.65) and high thermal stability. A large number of cermet coatings have been developed using Pt, Ni, Mo, W as metals and Al.sub.2O.sub.3 as the dielectric material. Similarly, transition metal nitrides and oxinitrides have also been developed for high temperature solar selective applications.

(34) For manufacturing high temperature absorber coatings for receiver tube applications, the manufacturing process should include less processing steps and also should use raw materials, which are easily available. In the present invention, the absorber coating has been manufactured using a single sputtering process. The source materials (i.e., Ti and Al) are easily available abundantly. The coating design consists of two absorber layer and an anti-reflection layer. The content of Al in all the layers was higher than Ti. This helps in improving the thermal stability of the absorber coating as Gibbs energy of Al.sub.2O.sub.3 is low as compared to TiO.sub.2. The absorber coating has been tested under vacuum and air for long durations under cyclic heating conditions. The coating has also passed several other aging tests, confirming its use for high temperature solar selective applications.

(35) TABLE-US-00009 TABLE 9 Absorptance and emittance data of solar selective coating deposited on stainless steel substrate after ageing tests in open atmosphere. α ε Exposure As- After As- After duration (Hrs) deposited ageing test deposited ageing test 10000 0.932-0.933 0.951-0.952 0.17 0.18

(36) TABLE-US-00010 TABLE 10 Absorptance and emittance data of solar selective coating deposited on stainless steel substrate after freezing tests. α ε Exposure As- After As- After duration (Hrs) deposited ageing test deposited ageing test 9600 0.933-0.934 0.937-0.938 0.17 0.17

(37) TABLE-US-00011 TABLE 11 Absorptance and emittance data of solar selective coating deposited on stainless steel substrate after exposure to steam. α ε Exposure As- After As- After duration (Hrs) deposited ageing test deposited ageing test 84 0.933-0.934 0.937-0.938 0.17 0.17

ADVANTAGES OF THE INVENTION

(38) The present invention uses only Ti and Al sputtering targets for the manufacture of high temperature solar selective coating. Both Ti and Al are abundantly available. The process developed in the present invention is cost effective. The compositions of the first, second and third layers of the present invention are independently controlled by controlling the power to the Al and Ti targets and the flow rates of N.sub.2 and O.sub.2. The present invention uses only a single deposition chamber to manufacture high temperature solar selective coating. The present invention uses only pulsed DC power supplies to sputter transition metal nitride, oxynitride and oxide layers, which are easy to scale up as compared to RF power supplies. The present invention provides substantially improved solar selective coating in respect of thermal stability. The absorber coating of the present invention showed stability up to 450° C. in vacuum for 1000 hrs and up to 350° C. in air for 1000 hrs under cyclic heating conditions. The process of the present invention can be used to deposit high temperature solar selective coating on tubes, thus, up-scaling of the process can lead to deposition of absorber coating on long SS tubes to be used for solar thermal power generation. The solar selective coating of the present invention demonstrates UV stability, corrosion resistance, superior mechanical properties and improved adhesion to the substrate. The solar selective coating of the present invention showed solar selectivity ratio in the order of 9-10 on stainless steel substrate, which is commonly used for solar thermal power generation. The solar selective coating of the present invention qualified a large number of aging tests when exposed to extreme environments such as: steam, ice, open atmosphere, etc.