Spectrally selective solar absorbing coating and a method for making it

Abstract

A spectrally selective solar absorbing coating includes a multilayer stack including, from the substrate to the air interface: substrate (1), infrared reflective layer (2), barrier layer (3), composite absorbing layer (4) consisting of metal absorbing sublayer (4.1), metal nitride absorbing sublayer (4.2), and metal oxynitride absorbing sublayer (4.3), and antireflective layer (5). Therefore, the solar absorbing coating has good high and low temperature cycle stability and superior spectrum selectivity, with a steep transition zone between solar absorption and infrared reflection zones. It has a relatively high absorptance >95%, and a low thermal emissivity 4%, PC (performance criterion) =0.3. The solar absorbing multilayer stack can be obtained by reactively magnetron sputtering the metal target in argon or other inert gas with some amounts of gas containing oxygen or nitrogen or their combination.

Claims

1. A spectrally selective solar absorbing coating comprising: a substrate; an infrared reflective layer on the substrate; a barrier layer on the infrared reflective layer; an absorbing layer on the barrier layer, which is a composite absorbing layer; an antireflective layer on the absorbing layer, wherein the absorbing layer comprises three absorbing sublayers, which are metal Cr, metal nitride CrN.sub.x, and metal oxynitride CrN.sub.yO.sub.z, the value of x is between 0.9-1.5, a value of y is between 0-0.1, a value of z is about between 1.4-1.5, the thermal expansion coefficients decrease in the order of metal, metal nitride, and metal oxynitride.

2. The spectrally selective solar absorbing coating of claim 1, wherein the refractive index and extinction coefficient decreases in the order of metal, metal nitride, and metal oxynitride.

3. The spectrally selective solar absorbing coating of claim 1, wherein a thickness of the composite absorbing layer is 80-140 nm, including metal absorbing sublayer with thickness of 10-30 nm, metal nitride sublayer with thickness of 30-50 nm, and metal oxynitride with thickness of 40-60 nm.

4. The spectrally selective solar absorbing coating of claim 1, wherein the barrier layer comprises metal nitride CrN.sub.x, wherein a value of x is between 0.9-1.5.

5. The spectrally selective solar absorbing coating of claim 1, wherein the thickness of the barrier layer is about 14-20 nm.

6. The spectrally selective solar absorbing coating of claim 1, wherein the infrared reflective layer is made of a metal selected from a group consisting of Al, Cu, Au, Ag, Ni, and Cr.

7. The spectrally selective solar absorbing coating of claim 6, wherein the thickness of the infrared reflective layer is 50-200 nm.

8. The spectrally selective solar absorbing coating of claim 1, wherein the antireflective layer is a transparent dielectric material with the reflective index between 1.4 and 2.0.

9. The spectrally selective solar absorbing coating of claim 8, wherein the antireflective layer is selected from a group consisting of SiO.sub.2, Al.sub.2O.sub.3, ThO.sub.2, Dy.sub.2O.sub.3, Eu.sub.2O.sub.3, Gd.sub.2O.sub.3, Y.sub.2O.sub.3, La.sub.2O.sub.3, MgO and Sm.sub.2O.sub.3.

10. The spectrally selective solar absorbing coating of claim 8, wherein the thickness of antireflective layer is between 50 nm and 150 nm.

11. The spectrally selective solar absorbing coating of claim 1, wherein the substrate is made of glass, aluminum, copper or stainless steel.

12. A method for forming the spectrally selective absorbing coating of claim 1, the method comprising: cleaning a substrate for coating; depositing an infrared reflective layer on the substrate; depositing a barrier layer on the infrared reflective layer; depositing an absorbing layer on the barrier layer, which is a composite absorbing layer; depositing an antireflective layer on the absorbing layer; wherein the depositing methods include chemical vapor deposition and physical vapor deposition including evaporation, magnetron sputtering, and wherein the absorbing layer comprises three absorbing sublayers, which are metal Cr, metal nitride CrN.sub.x, and metal oxynitride CrN.sub.yO.sub.z, the value of x is between 0.9-1.5, a value of y is between 0-0.1, a value of z is about between 1.4-1.5, and the thermal expansion coefficients decrease in the order of metal, metal nitride, and metal oxynitride.

13. The spectrally selective solar absorbing coating of claim 2, wherein within the 500 nm-2500 nm wavelength range, the refractive index of Cr is 3.19-6.13, the refractive index of CrN.sub.x is 3.00-4.40, the refractive index of CrN.sub.yO.sub.z is 2.38-2.20; and within the 380-2500 nm wavelength range, the extinction coefficient of Cr is 3.59-6.84, the extinction coefficient of CrN.sub.x is 1.79-0.76, the extinction coefficient of CrN.sub.yO.sub.z is 0.47-0.05.

14. The spectrally selective solar absorbing coating of claim 6, wherein the infrared reflective layer is made of Al.

15. The spectrally selective solar absorbing coating of claim 7, wherein the thickness of the infrared reflective layer is 80-120 nm.

16. The spectrally selective solar absorbing coating of claim 9, wherein the antireflective layer is SiO.sub.2.

17. The spectrally selective solar absorbing coating of claim 10, wherein the thickness of antireflective layer is between 80 nm and 120 nm.

18. The method for forming the spectrally selective absorbing coating of claim 12, wherein reactively magnetron sputtering the metal target in argon or other inert gas with some amounts of gas containing oxygen or nitrogen or their combinations is selected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the spectral performance of an ideal selective solar absorbing coating and the irradiance of solar and blackbody of 100 C.-400 C.

(2) FIG. 2 illustrates a sectional view of a spectrally selective solar absorbing coating in accordance with the present invention.

(3) FIG. 3 illustrates an exemplary flowchart of a preparation method for the solar absorbing coating according to an embodiment of the invention.

(4) FIG. 4 shows the measured absorbing spectra of the solar absorbing coating prepared in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) To illustrate the technical schemes and effect of the present invention, the preferred embodiments of the spectrally selective solar absorbing coating and its manufacturing method, as well as testing results are described in detail below.

(6) As shown in FIG. 2, the absorbing stack consists of, sequentially, a substrate 1, an infrared reflective layer 2, a barrier layer 3, an absorbing layer 4, and an antireflective layer 5. These layers are coated in sequence on a substrate 1 which ordinarily acts as a heat exchange medium.

(7) The substrate 1 may be a piece of glass with a thickness of 0.5 mm-10 mm; it can also be some kind of metal such as copper, aluminum or stainless steel with a thickness of 0.2 mm-2 mm. Before coating, the substrate is cleaned by mechanical cleaning followed by RF (radio frequency) plasma cleaning, to remove various contaminants on the substrate surface.

(8) The infrared reflective layer 2 is placed on the substrate. The function of the infrared reflective layer 2 is to reflect the incident light from ultraviolet to far infrared wavelength range, and more particularly infrared light above 2.5 m. The infrared reflective layer 2 consists of aluminum and has a preferred thickness of 80-130 nm.

(9) The barrier layer 3 is placed on the infrared reflective layer, and consists of CrN.sub.x(x=0.9-1.5) with preferred thickness of 14-20 nm. The function of the barrier layer is to blocking the diffusion between infrared reflective layer and absorbing layer at higher operating temperature than 250 C.

(10) The absorbing layer 4 is placed on the barrier layer. And it has a high absorptance in the solar spectrum range and nearly transparent in the infrared range. The absorbing layer in the present invention is a composite absorbing layer including three absorbing sublayers 4.1, 4.2, and 4.3 with successively decreased optical constants. The absorbing sublayer 4.1 consists of metal Cr; its refractive index is 3.19-6.13 (500 nm-2500 nm); its extinction coefficient is 3.59-6.84 (380 nm-2500 nm); the preferred thickness is 10 nm-30 nm. The absorbing sublayer 4.2 consists of mental nitride CrN.sub.x (x=0.9-1.5); its refractive index is 3.00-4.40 (500 nm-2500 nm); its extinction coefficient is 1.79-0.76 (380 nm-2500 nm); the preferred thickness is 30 nm-50 nm. The absorbing sublayer 4.3 consists of mental oxynitride CrN.sub.yO.sub.z (y0.05, O1.45); its refractive index is 2.38-2.20 (500 nm-2500 nm); its extinction coefficient is 0.47-0.05 (380 nm-2500 nm); the preferred thickness is 40 nm-60 nm.

(11) The AR layer 5 is placed on the composite absorbing layer, and consists of SiO.sub.2. The function is to reduce the sunlight reflection at the interface between air and the absorbing coating, allowing more solar energy to enter the absorbing coating and then increasing the solar absorptance. The preferred thickness of the SiO.sub.2 AR layer is 80 nm-120 nm, and within the 380 nm-2500 nm wavelength range, its refractive index is 1.47-1.43 and its extinction coefficient is below 0.03.

(12) Preparation Method

(13) Embodiments of the present invention provide a preparation method for the above spectrally selective absorbing coating, which includes the following steps (as shown in FIG. 3).

(14) Preparation of the substrate: Obtaining a polished metal plate or glass plate acting as substrate; applying mechanical cleaning (for example ultrasonic cleaning, brush cleaning, et al) followed by RF Ar plasma cleaning to remove contaminants on the substrate surface and ensure the sufficiently high cleanness level that are pre-requisite to deposit a uniform, reproducible coating.

(15) Formation of the infrared reflective layer: Using (pulse) DC magnetron sputtering to deposit a metal infrared reflective layer on the surface of the above mentioned substrate. The sputtering target can be metal Al.

(16) Formation of the barrier layer: Using (plus) DC magnetron sputtering to deposit a barrier layer on the surface of the above mentioned infrared reflective layer. The sputtering target can be metal Cr. The reactive gas is argon and nitrogen.

(17) Formation of the composite absorbing layer: Using (pulse) DC magnetron sputtering to successively deposit metal, metal nitride, and metal oxynitride three absorbing sublayers on the surface of the above mentioned barrier layer. The sputtering target can be metal Cr. The reactive gas is argon for metal Cr, argon and nitrogen for metal nitride, argon, nitrogen and oxygen for metal oxynitride.

(18) Formation of the antireflective layer: Using (pulse) DC reactive magnetron sputtering to deposit an antireflective layer on the surface of the above mentioned composite absorbing layer. The sputtering target can be semiconductor Si (doped Al:0-30 wt %). The reactive gas is argon and oxygen.

EXAMPLES

(19) Specific examples illustrate certain embodiments of the invention are set out below.

(20) Table 1 lists the thickness of various single layers of different selective absorbing coating samples formed by reactive magnetron sputtering based on the present invention.

(21) TABLE-US-00001 TABLE 1 The thickness of all the single layers in different solar absorbing multilayer stacks sample Al/nm CrN.sub.x/nm Cr/nm CrN.sub.x/nm CrN.sub.yO.sub.z/nm SiO.sub.2/nm Example1 120 0 30 40 50 90 Example2 120 15 30 40 50 90 Example3 120 15 10 40 50 90 Example4 120 15 20 40 50 90 Example5 120 15 30 30 50 90 Example6 120 15 30 35 50 90 Example7 120 15 30 40 50 90 Example8 120 15 30 45 50 90 Example9 120 15 30 48 50 90 Example10 120 15 30 50 50 90 Example11 120 15 30 40 40 90 Example12 120 15 30 40 45 90 Example13 120 15 30 40 50 90 Example14 120 15 30 40 55 90 Example15 120 15 30 40 60 90

Example 1

(22) The specific steps of the preparation process of the above example 1 on glass substrates are as following:

(23) 1) Cleaning of the glass pane: First, A glass pane was ultrasonically cleaned in a neutral detergent solution and thereafter rinsed in deionized water. Then, the glass pane was placed in the entrance chamber of the magnetron sputtering equipment and performed second cleaning step, using an RF plasma source to bombard the glass pane surface. The process parameters were as following: RF source sputtering power was 200 w, flow rate of working gas Ar (purity 99.99%), was 45 sccm, the working pressure was 9.810.sup.2 mTorr, and sputtering time was 360 s.

(24) 2) The cleaned glass substrate was passed from the entrance chamber to the sputtering chamber of the deposition equipment. The base pressure of the sputtering chamber was lower than 610.sup.6 Torr.

(25) 3) Forming the infrared reflective layer Al on the glass substrate: A metal Al target (purity 99.7%) was magnetron sputtered using plus DC power. The processing parameters were as follows: the sputtering power was 1200 w, the working pressure was 5 mTorr, the working gas Ar (purity 99.99%) flow rate was 5 sccm, the substrate moving speed was 0.8 m/min and the substrate was moved back and forth 5 times under the Al target, and the substrate temperature was room temperature.

(26) 4) Forming the composite absorbing layer on the Al/glass: A metal Cr target (purity 99.7%) was magnetron sputtered using pulse DC power to successively deposit Cr, CrN.sub.x, and CrN.sub.yO.sub.z on the surface of Al/glass.

(27) a. The processing parameters of metal Cr absorbing sublayer were as follows: the sputtering power was 1500 w, the working pressure was 3 mTorr, the working gas Ar (purity 99.99%) flow rate was 50 sccm, the substrate moving speed was 2.3 m/min and the substrate was moved back and forth 3 times under the Cr target, and the substrate temperature was room temperature.

(28) b. The processing parameters of metal nitride CrN.sub.x absorbing sublayer were as follows: the sputtering power was 1500 w, the working pressure was 3 mTorr, the working gas Ar (purity 99.99%) flow rate was 50 sccm, the working gas N.sub.2 (purity 99.99%) flow rate was 50 sccm, the substrate moving speed was 1 m/min and the substrate was moved back and forth 3 times under the Cr target, and the substrate temperature was room temperature.

(29) c. The processing parameters of metal nitride CrN.sub.yO.sub.z absorbing sublayer were as follows: the sputtering power was 1500 w, the working pressure was 3 mTorr, the working gas Ar (purity 99.99%) flow rate was 50 sccm, the working gas N.sub.2 (purity 99.99%) flow rate was 50 sccm, the working gas O.sub.2 (purity 99.99%) flow rate was 10 sccm, the substrate moving speed was 0.45 m/min and the substrate was moved back and forth 5 times under the Cr target, and the substrate temperature was room temperature.

(30) 5) Forming the AR layer on the CrN.sub.yO.sub.z/CrN.sub.x/Cr/Al/glass: A Si target (Al content 30 wt %, purity 99.7%) was magnetron sputtered using pulse DC power to deposit SiO.sub.2 on the surface of CrN.sub.yO.sub.z/CrN.sub.x/Cr/Al/glass. The processing parameters were as follows: the sputtering power was 2000 w, the working pressure was 5 mTorr, the gas flow rates were 30 sccm for Ar (purity 99.99%), 14 sccm for O.sub.2 (purity 99.99%), the substrate moving speed was 1 m/min and the substrate was moved back and forth 9 times under the Si target, and the substrate temperature was room temperature.

(31) 6) After the above processing steps were completed, the coated sample were cooled for 20 min in the vacuum, and then removed out of the coating equipment.

(32) 7) The multilayered stack, example 1 was then heated in air at 250 C. for 200 hr.

Example 2

(33) Example 2: The coating processing was same as that of the example 1, except of adding the barrier layer CrN.sub.x between IR reflective Al and absorbing sublayer metal Cr. The processing parameters of the barrier layer CrN.sub.x were as follows: the sputtering power was 1500 w, the working pressure was 3 mTorr, the working gas Ar (purity 99.99%) flow rate was 50 sccm, the working gas N.sub.2 (purity 99.99%) flow rate was 50 sccm, the substrate moving speed was 2 m/min and the substrate was moved back and forth 3 times under the Cr target, and the substrate temperature was room temperature.

(34) The multilayer stack, example 2 was then heated in air at 250 C. for 370 hr.

Example 3-15

(35) The coating processing parameters were same as the above detailed description for example land example 2, except of the coating processing of the composite absorbing layer.

(36) 1) The substrate moving speed and the number of the passes were determined according to the thickness of the film being sputtered.

(37) 2) When coating CrN.sub.x layer, the flow rate of working gas N.sub.2 was chosen randomly between 50 sccm and 100 sccm.

(38) 3) When coating CrN.sub.yO.sub.z layer, the flow rate of working gas O.sub.2 was chosen randomly between 10 sccm and 50 sccm.

(39) For the examples in the present embodiment of the invention, the 0.3-2.5 m reflective spectra were measured using a spectrophotometer at room temperature, which can be used to calculated the absorbing spectra in accordance to spectral absorptance being expressed in terms of total reflectance for opaque materials, =100(%). The solar absorptance was obtained by numerical integration of the above absorbing curves over the solar distribution curve referenced as 1.5 AM in FIG. 1.

(40) Thermal emissivity was firstly measured at 100 C., by means of an emissometer, which performs a complete measurement in the range of 3-30 m, the uncertainty of which is 0.01. Secondly, the optical reflectance was characterized in the infrared range from 2 m to 48 m, which makes it possible to estimate the thermal emissivity at any temperature based on said reflectance and the emission curve of an ideal black body emitter at said working temperature. The service lifetime within at 10-20 years of low-temperature absorber can be described by a PC value (PC=+0.25<5%), which was obtained by exposing the absorbing coating for 200 hr at 250 C.

(41) FIG. 4 shows the absorbing spectra of the example 1 and example 2. Their solar absorptance, thermal emissivity (100 C., 250 C.) before and after the heating treatment and corresponding PC values are given in Table 2. It is shown that example 1, showing in FIG. 4 as a solar absorbing coating according without a barrier layer, provides a thermal and environmental stability for multilayer coatings with a good optical performance required for low-temperature application under 200 C. The optical properties can reach a value of >95%, and a value of <4% (100 C.), PC value of 1.0% when heated in air at 250 C. for 200 hr. Example 2, showing in FIG. 4 as absorbing coating with barrier layer, provides a thermal and environmental stability for multilayer coatings with an excellent optical performance required below 250 C. The optical properties can reach a value of >95%, and a value of 4%(100 C.), 6-7%(250 C.), PC value of 0.3% when heated in air at 250 C. for 370 hr.

(42) TABLE-US-00002 TABLE 2 The solar absorptance, thermal emissivity of example 1 and 2 before and after the heating treatmen Solar absorptance Thermal emissivity (%) example Heat treatment (%) (100 C.) (250 C.) 1 \ 95.9 3.8 6.2 250 C. (120 hr) 96.3 6.6 10.6 PC value (120 hr) 0.3 0.7 PC value (200 hr) 1.0 2 \ 95.4 4.1 6.6 250 C. (370 hr) 95.7 4.1 6.4 PC value (370 hr) 0.3 0.35

(43) For the example 3-15, the solar absorptance was found to be more than 95%, thermal emissivity (100 C.) was lower than 5%. Combining with their coating parameters, it is clear that their optical selectivity was not influenced by the fluctuation of the working gas flow rate, especially in the coating process of metal oxynitride and metal nitride absorbing sublayers.

(44) Moreover, the thickness range for the optimized optical properties was quite wide, for example, the thickness of Cr absorbing sublayer was 10-30 nm, the thickness of CrN.sub.x absorbing sublayer was 30-50 nm, the thickness of CrN.sub.yO.sub.z was 40-60 nm.