TWI411699B - Solar thermal selective absorbers and fabrication methods thereof - Google Patents

Abstract

Solar thermal selective absorbers and fabrication methods thereof are presented. The fabrication method of the solar thermal selective absorber includes providing a substrate in a vacuum deposition chamber. A metal is sputtered simultaneous injection a reactive gas to deposit a single-layered cermet layer on the substrate, wherein the single-layered cermet layer selectively absorbs visible light and reflects infrared light.

Description

Solar selective absorbing film and method of manufacturing same

The present invention relates to a solar selective absorbing film and a method of manufacturing the same, and more particularly to a single-layer solar selective absorbing film having a high selectivity and a method of manufacturing the same.

The research and development of the dual ceramic alloy layer structure applied to the solar selective absorbing film began in the 1990s. Dual ceramic alloy layer structures, such as metal-aluminum nitride (M-AlN) and metal-alumina (M-Al ₂ O ₃ ), traditionally use a dual target DC electromagnetic tube plasma sputtering technique (two -target DC magnetron plasma sputtering technology), which is applied to solar selective absorption film, can obtain very high photothermal conversion efficiency.

Most of the conventional selective absorption film processes are composite films produced by double target sputtering, such as a stainless steel and aluminum double target process, and a ceramic metal film (Cermet) is used as a selective absorption film. A suitable cermet composite film thickness and metal fraction can exhibit high absorption in the solar radiation region and high reflectivity in the heat radiation (infrared region). The composite is deposited on a metal substrate that is reflective to infrared light to form a solar selective absorbing film.

Figure 1 is a schematic cross-sectional view showing a conventional multilayer selective absorbing film structure fabricated on a metal substrate. In Fig. 1, a conventional multilayer selective absorbing film structure 110-130 is formed on a metal substrate 100 by double or multiple target sputtering. The absorbing film structures 130-110 have progressive optical properties, that is, the refractive indices (n), extinction coefficients (k), and film thicknesses (d) of the respective layers are different. In order to achieve high absorbivity to solar radiation areas and highly reflective selectivity to thermal radiation (infrared regions), conventional techniques rely on a multilayer ceramic metal film (Cermet) structure to achieve desired characteristics. Furthermore, the multi-layer ceramic metal film (Cermet) relies on a multi-target multi-cavity process, which consumes manufacturing resources and costs.

In recent years, the industry has begun to research and develop a single target sputtering selective absorption film. Although the single target process is superior to the double target process, such as saving the space and cost of the machine, in the single target process, the single target must simultaneously share the two layers of the traditional double target metal layer and the ceramic layer. The control of the power and the requirements of the dual target process are more precise, and thus the stability control of the coating is increased.

Embodiments of the present invention provide a method of fabricating a solar selective absorbing film, comprising: providing a substrate in a vacuum sputtering chamber; and sputtering a metal while introducing a gas to form a single layer of metal-ceramic A composite film is on the substrate; wherein the single-layer metal-ceramic composite film has a property of absorbing visible light and reflecting infrared light.

An embodiment of the present invention further provides a solar selective absorbing film comprising: a substrate; and a single-layer metal-ceramic composite film on the substrate; wherein the single-layer metal-ceramic composite film has absorption of visible light and reflects infrared The nature of light.

In order to make the invention more apparent, the following detailed description of the embodiments and the accompanying drawings are as follows:

The following is a detailed description of the embodiments and examples accompanying the drawings, which are the basis of the present invention. In the drawings or the description of the specification, the same drawing numbers are used for similar or identical parts. In the drawings, the shape or thickness of the embodiment may be expanded and simplified or conveniently indicated. In addition, the components of the drawings will be described separately, and it is noted that the components not shown or described in the drawings are known to those of ordinary skill in the art, and in particular, The examples are merely illustrative of specific ways of using the invention and are not intended to limit the invention.

Embodiments of the present invention provide a single-layer ceramic-metal structure and method for use as a solar selective absorbing film, and optically simulate the most appropriate optical constant of the single-layer absorbing film, including refractive index (n) and extinction coefficient ( k), film thickness (d), that is, the metal content and coating thickness required for the calculation of the single-layer ceramic gold absorbing film.

2A-2C is a schematic cross-sectional view showing a method of fabricating a solar selective absorbing film according to an embodiment of the present invention. Referring to FIG. 2A, a substrate 200 is first provided in a vacuum single target sputtering chamber, and a metal is sputtered while a gas is introduced to form a single-layer metal-ceramic composite film 210 on the substrate 200. on. The single-layer metal-ceramic composite film has a property of absorbing visible light and reflecting infrared light.

Referring to FIG. 2B, after exposure to L under natural atmosphere, a thin layer 215 of oxide is formed by metal atoms on the surface of the absorption film having a high metal fraction and oxygen atoms in the atmosphere. It should be noted that the oxide thin layer 215 is also an optical film, and the matched design can effectively reduce the reflectance of sunlight, achieve anti-reflection effect, and improve the absorption efficiency of the selective absorption film. The material properties of the oxide have anti-corrosion properties, that is, prevent oxidation diffusion of oxygen atoms, and can achieve the barrier effect of the absorption film and the atmosphere, and have stable performance and anti-aging properties for the product to be exposed to sunlight.

Another embodiment of the present invention provides a single metal target for producing a solar selective absorbing film by vacuum sputtering. In the vacuum sputtering process, taking an aluminum metal target as an example, the specific atmosphere and power can be adjusted to cause the plasma to bombard the aluminum metal atoms, and the aluminum atoms that are struck are reacted with the reaction gas (nitrogen) that is introduced. And deposited on the substrate to form a metal-ceramic composite film, that is, a cermet film. This pottery film has a good absorption effect on sunlight. According to an embodiment of the present invention, a natural metal surface oxidation is performed using a high metal fraction ceramic gold film to naturally form an oxide film. The oxide film can be used as an antireflection layer while increasing the flux of incident light and increasing the absorption rate of the absorption film.

It should be noted that the solar selective absorbing film manufacturing method of the embodiment of the present invention only needs a single metal material target and a single atmosphere, and through the optical film design, only a single layer of absorbing film is sputtered and formed by natural exposure. The anti-reflection effect eliminates the need for high-temperature heat treatment, which reduces the operating cost of the vacuum sputtering process and increases the mass production capability of the continuous sputtering process.

Embodiments of the present invention reduce dual targets to single targets as compared to conventional techniques. More specifically, a single metal target such as an aluminum (Al) target, a titanium (Ti) target or other metal is used as a sputtering target, and a solar selective absorbing film is formed by vacuum sputtering in a nitrogen reaction atmosphere. According to another embodiment of the present invention, a solar selective absorbing film is prepared by reactive sputtering.

Furthermore, the embodiment of the present invention controls a specific reaction sputtering parameter, such as sputtering power, sputtering reaction atmosphere concentration, and sputtering time, to prepare a film layer with a specific optical constant to obtain a solar selective absorption film by optical simulation calculation. It is only necessary to sputter a single layer of absorbing film to achieve the desired optical performance. The absorbing film layer is a ceramic-metal film structure, which has high absorption in the visible light and near-infrared light bands and high reflection in the far infrared band. After natural exposure, the oxygen atoms in the atmosphere are used to oxidize the surface of the absorption film, and an oxide film is formed on the surface. The thin oxide layer is also an optical film, which can effectively reduce the reflectance of sunlight and enhance the absorption efficiency of the selective absorption film. The material properties of the oxide have anti-corrosion properties, that is, prevent oxidation diffusion of oxygen atoms, and can achieve the barrier effect of the absorption film and the atmosphere, and have stable performance and anti-aging properties for the product to be exposed to sunlight.

Optical simulation

Figure 3 is a flow chart showing the optical simulation steps of the embodiment of the present invention and corresponding to the implementation of the second embodiment A-2C. First, the optical constants of the single layer film (the gold layer absorbing film) at different nitriding degrees are used to simulate the optimum optical characteristics, that is, the lower reflectance (step 330). Next, an oxide layer is added to simulate the oxide layer produced when the absorbing film is oxidized by atmospheric exposure, that is, double layer film matching is performed (step 320). Finally, the selective absorption film efficacy (absorption rate) of the single-layered absorber film and the additional oxide layer is simulated, as shown in step 330.

In view of this, the embodiments of the present invention only need to sputter a single layer of the absorption film, so that a single layer film having different degrees of nitriding and different film thicknesses is exhibited in the optical simulation. Table 1 shows that the simulated background is based on an aluminum target (Al) and reactive sputtering is performed under different N ₂ concentration atmospheres (3-12%). Thereby, a ceramic-metal monolayer film of a series of different degrees of nitridation can be formed, and its optical characteristics, including reflectance (%), can be analyzed. In addition to simulating single-layer films with different metal fractions, the results at different film thicknesses (30-120 nm) were also simulated. Because different film thicknesses produce different optical interference effects, which produce specific optical characteristics such as reflectance and cutoff wavelength position. In the simulation results of Table 1, when the nitrogen concentration was 7.5% and the film thickness was 90 nm, there was a low reflectance. That is, in the characteristics of the single layer of the absorption film, there is a high absorption rate.

Step 1: Change the film thickness over a wide range (30-120 nm) to find a better film thickness and find the metal fraction of the appropriate film. The Al/AlN-Al ₂ O ₃ with the best performance was simulated as a selective absorption film, and the absorption rate thereof is shown in Table 1, wherein the oxide layer was made of alumina as an oxide layer.

Step 2: Compare the reflectance at different film thicknesses and find a suitable film thickness. According to the simulation results in Table 1, the film has the lowest reflectance at a nitrogen atmosphere rate of 7.5% and a film thickness of 90 nm, so the optimum film thickness is found in a small range near the film thickness of 90 nm, as shown in Table 2, according to the simulation results. Displayed as 95nm.

Step 3: Comparing the absorption rates of the single layer and the double layer of the film with different metal fractions at a film thickness of 95 nm. Because of the limitation of the thickness of the second layer and its effect as an "anti-reflection" effect, the second film contributes only slightly to the absorption rate. If it can break through the film thickness limit of the oxide layer, it will help to achieve better anti-reflection effect. Table 3 shows the difference in the absorption rates of the single and double membranes. The single-layer aluminum ceramic gold film has a thickness of 95 nm, and the second aluminum oxide film has a thickness of 20 nm. The results show that among the simulation groups of the examples, the C group has better results showing that the absorption rate can reach 0.8.

4 and 5 respectively show schematic diagrams of reflection spectra of single and double-layer films under different nitridation degrees according to an embodiment of the present invention, wherein the single-layer film is an aluminum-ceramic single-layer film with different metal fractions, and the double-layer film is respectively It is a first layer of aluminum ceramic film and a second layer of aluminum oxide film. Referring to Figures 4 and 5, comparing sample numbers 40a-40f and 50a-50f, respectively, it can be found that the reflectance of Group C (40c and 50c) is low and the cutoff wavelength occurs at a position of 1400 nm at a longer wavelength, so The absorption rate is also preferred. In subsequent studies, different nitriding atmosphere concentrations and different film thicknesses can be used to match, and a single-layer absorbing film with a cutoff wavelength of 1600 nm is found.

Compare the best C group in Figures 4 and 5 and redraw it as shown in Figure 6. Among them, in the single and double-layer film reflectivity, the second film has a tendency to move in the long wavelength direction, and the overall reflectance tends to decrease, so the absorption rate of the double film is 0.8% than that of the single layer film. high. Therefore, in the embodiment of the present invention, a high absorption rate of 0.75 can be achieved by selecting a suitable single-layer absorption film, and the single-layer absorption film has selective characteristics, and has high absorption in the visible light and near-infrared light regions, and The far infrared light zone has a reflective property. When it is naturally heated and oxidized by being placed in the atmosphere, a thin oxide film is naturally formed, which can achieve an anti-reflection effect and enhance the absorption efficiency of the selective absorption film. In the current embodiment simulation, in addition to the selective characteristics, the absorption rate can reach 0.8.

Sputter preparation

Sputtering experiments were carried out to prepare a series of ceramic gold films with different nitriding degrees. At the same time, the properties of single-layer selective absorbing films were observed, and the actual exposure was carried out to analyze the oxidation (aging). First, 12 pieces of metal aluminum film were plated as an Al substrate, and two Al substrates and one glass substrate were placed under each different atmosphere sputtering conditions. After the sputtering is completed, the film on the Al substrate is measured for reflectance, emissivity, and absorptivity. The film on the glass substrate was analyzed for optical constants and film thickness by an ellipsometer. Table 4 shows the parameters of the coating experiment. The sputtering pressure is about 2 mtorr, the Ar flow rate is fixed at 40 sccm, the nitrogen flow rate is 0-4 sccm, the sputtering power is 150 W, and plating is performed for 10 minutes.

According to the color appearance of the ceramic gold film, the plating conditions of the two films are completely the same, only the substrate is different, respectively, the ceramic gold film is plated on the metal aluminum film, and the ceramic gold film is plated on the transparent glass substrate. The film does not appear to be brown in color as the pure aluminum film is silver after the introduction of nitrogen.

Figure 7 shows the reflection spectrum (solid) of the gold-plated aluminum film on the metal aluminum film, and the reflection spectrum (hollow) of the film after 15 days of standing, with the better absorption rate of sample numbers A3 and A4, and because of A3 The cutoff wavelength is longer, so the absorption rate is higher at 0.771. Figure 8 shows the penetration spectrum (solid) of a gold-plated aluminum film on a transparent glass substrate, and the breakthrough spectrum (hollow) of the film after 15 days of placement. From the results in Figures 7 and 8, it is known that the transmittance and reflectance of the film change after 15 days of standing, indicating that the surface of the film or the structure of the film has changed, so the absorption rate and emissivity also change. As shown in Table 5. The absorption rate has a small increase, and the absorption rate of the A3 test piece is as high as 0.785. The absorption rate of the A4 test piece decreases only. The reflection spectrum of Fig. 7 can be found because the reflection spectrum of the A3 test piece has risen after 15 days. Caused by trends. There was little change in the emissivity, and all of them were 1% amplitude change, and both were below 0.1.

The above examples of simulation and actual coating have verified that the absorption efficiency of the absorption film can be improved after the single-layer absorption film is exposed to sunlight. In the future, if the actual coating is combined with an optical simulation design, a more suitable single membrane layer can be prepared to achieve an absorption rate (α) of 0.8 or more.

The present invention has been disclosed in the above preferred embodiments, and is not intended to limit the scope of the present invention. Any one of ordinary skill in the art can make a few changes without departing from the spirit and scope of the invention. The scope of protection of the present invention is therefore defined by the scope of the appended claims.

Conventional part (Figure 1)

100. . . Metal substrate

110-130. . . Multilayer absorbent film structure

Part of this case ( Figures 2A - 8)

200. . . Substrate

210. . . Single layer metal-ceramic composite film

215. . . Thin oxide layer

310-330. . . Optical simulation step

L. . . Exposure to the natural atmosphere

40a-40f and 50a-50f. . . Sample serial number

Figure 1 is a schematic cross-sectional view showing a conventional multilayer selective absorbing film structure fabricated on a metal substrate.

2A-2C is a schematic cross-sectional view showing a method of fabricating a solar selective absorbing film according to an embodiment of the present invention.

Figure 3 is a flow chart showing the optical simulation steps of the embodiment of the present invention and corresponding to the implementation of the second embodiment A-2C.

4 and 5 respectively show schematic diagrams of reflection spectra of single and double-layer films under different nitridation degrees according to an embodiment of the present invention, wherein the single-layer film is an aluminum-ceramic single-layer film with different metal fractions, and the double-layer film is respectively It is a first layer of aluminum ceramic film and a second layer of aluminum oxide film.

Figure 6 is a schematic diagram showing the reflection spectra of the best-performing Group C in Figures 4 and 5 and redrawing.

Fig. 7 shows the reflection spectrum (solid) of the gold-plated aluminum film on the metal aluminum film, and the reflection spectrum (hollow) of the film after 15 days of standing.

Figure 8 shows the penetration spectrum (solid) of a gold-plated aluminum film on a transparent glass substrate, and the breakthrough spectrum (hollow) of the film after 15 days of placement.

200. . . Substrate

210. . . Single layer metal-ceramic composite film

L. . . Exposure to the natural atmosphere

Claims (10)

A method for manufacturing a solar selective absorbing film, comprising: providing a substrate in a vacuum sputtering chamber; and sputtering a metal while introducing a gas to form a single-layer metal-ceramic composite film on the substrate Laying the substrate with the single-layer metal-ceramic composite film in the atmosphere to form an oxide layer on the surface of the single-layer metal-ceramic composite film, wherein the single-layer metal-ceramic composite film has absorption of visible light and reflection The nature of infrared light. The method for producing a solar selective absorbing film according to claim 1, wherein the substrate comprises a copper or aluminum metal substrate, or a glass substrate. The method for producing a solar selective absorbing film according to claim 1, wherein the gas is introduced into the gas comprising nitrogen gas and argon gas, and the nitrogen/argon ratio is substantially between 0% and 10%. The method for producing a solar selective absorbing film according to claim 1, wherein the single-layer metal-ceramic composite film comprises an Al/AlN-Al ₂ O ₃ selective absorbing film. The method for producing a solar selective absorbing film according to claim 1, wherein the single-layer metal-ceramic composite film has a refractive index (n) in a range of substantially 2-3.5, and an extinction coefficient (k) is substantially in the range of The range of 0.5-3, and film thickness (d) is generally 60-120 nm. A solar selective absorbing film comprising: a substrate; a single-layer metal-ceramic composite film on the substrate; A primary oxide layer is on the surface of the single-layer metal-ceramic composite film, wherein the single-layer metal-ceramic composite film has a property of absorbing visible light and reflecting infrared light. The solar selective absorbing film of claim 6, wherein the substrate comprises a copper or aluminum metal substrate, or a glass substrate. The solar selective absorbing film according to claim 6, wherein the single-layer metal-ceramic composite film has a single homogeneous structure. The solar selective absorbing film according to claim 6, wherein the single-layer metal-ceramic composite film comprises an Al/AlN-Al ₂ O ₃ selective absorbing film. The solar selective absorbing film according to claim 6, wherein the single-layer metal-ceramic composite film has a refractive index (n) ranging from 2 to 3.5 and an extinction coefficient (k) of substantially 0.5-3. And the film thickness (d) is generally in the range of 60-120 nm.