EP2122270A1

EP2122270A1 - Method for the production of thin layers of metal-ceramic composite materials

Info

Publication number: EP2122270A1
Application number: EP07846323A
Authority: EP
Inventors: Rolf Clasen; Mohammadreza Nejati
Original assignee: Dritte Patentportfolio Beteiligungs GmbH and Co KG
Current assignee: Dritte Patentportfolio Beteiligungs GmbH and Co KG
Priority date: 2006-11-10
Filing date: 2007-11-12
Publication date: 2009-11-25
Also published as: WO2008055496A1; CN101600915A; AU2007317053B2; US20100035081A1; KR20090080093A; CA2668736A1; AU2007317053A1; MX2009005005A; IL198351A0; JP2010509498A; BRPI0718831A2

Abstract

The invention relates to a method for producing thin layers of metal-ceramic composite materials containing metallic nanoparticles. Said method can be used for coating a solar absorber with a cermet-based selectively absorbing surface layer that is very easy and inexpensive to produce in a reliable manner and has good spectral selectivity. In said method, one or more thin cermet layers are deposited on a substrate by immersing metallic substrates into a stabilized aqueous or organic suspension. The suspension is composed of alcoholic or aqueous solution in which ceramic nanopowders are dispersed and which contains the metallic portion of the cermet in the form of metal ions.

Description

DESCRIPTION

Method for producing thin layers of metal-ceramic composite materials

The invention relates to a method for producing thin layers of metal-ceramic composites containing metallic nanoparticles and the use of the method.

In solar thermal collectors with high efficiency almost the entire radiation spectrum of sunlight is converted from the surface of the solar absorber into thermal energy. The absorber surface and connected tubes give off the heat to a heat transfer fluid flowing through the tubes, for example water.

The absorber surface is the most important component of thermal solar collectors. A high photothermal conversion yield for such collectors can be achieved through the use of spectrally selective absorbers. These are surfaces that well absorb radiation in the terrestrial solar spectrum but strongly reflect thermal wavelengths, i. they radiate little heat from the heat they absorb. Since there is no natural material with these surface properties, the selectivity must be generated by special coatings.

The effect of a spectral selectivity of an absorber can usually be achieved by an absorber-reflector tandem. An absorber layer that has low reflection (high solar absorption) for wavelengths <2.5 μm, but at the same time is transparent for thermal wavelengths in the IR range, is coated on a metal surface, the high reflection (low thermal emissivity) for wavelengths> 2 , 5 μm, deposited (Fig. 1). Thus, the absorber layer ensures the highest solar absorption with at the same time least influence on the thermal emissivity, which is dominated by the reflector layer or the metallic substrate. This optical property has numerous metal-ceramic composite layers on a metallic substrate.

Simple commercial selective solar absorbers are produced by electroplating, anodizing and chemical oxidation techniques. Black chrome, black zinc, Copper oxide, black cobalt, black nickel, iron oxide and pigmented alumina are the most commonly used, electrochemically produced, selective photothermal absorber layers.

Such absorbers have solar absorptions of 0.9 and thermal emissivities of 0.1 to 0.3 and are usually temperature stable up to temperatures of 425 to 500 degrees Kelvin. These manufacturing processes require toxic acid baths as well as complicated combinations of metal salts. In addition, the wastes occurring in this manufacturing process are toxic, not environmentally friendly and difficult to handle (disposal).

In addition, in these methods, a targeted adjustment of the optical properties of the absorber to the desired characteristics is very difficult or even impossible.

For about two decades, thin layers of metal-ceramic composites have been extensively studied for their suitability as selective solar absorbers because of their suitable and adaptable optical properties.

Metal-ceramic composites, also called cermets, consist of a ceramic matrix in which metallic nano-particles are dispersed. The rather high IR transparency and simultaneous high solar absorption of many cermet layers predestine them for use as selective absorbers. The use of such cermet layers as absorbers is therefore widespread. Furthermore, such coatings are long-term stable even under different thermal conditions.

Optical properties of a nanocomposite coating can be easily influenced by the thickness of the layer, the volume fraction of the metallic phase, the geometry and the particle size. In addition, the distribution characteristic of the conductive particles can decisively influence the normalized refractive index of cermet layers. For example, a gradual increase in the concentration of metallic particles from the air cermet to the substrate causes Cermet interface a higher degree of absorption by reducing the surface reflections.

Deposition via sputtering techniques provides a very clean process without the need for chemical baths and hazardous acids. This deposition technique enables high-quality optical coatings with controlled layer thickness to be obtained from high purity target materials. Using cylindrical or Roll-sputtering techniques, various selective metal-dielectric-B have been oatings as SS-C, SS-AlN (SS = Stainless Steel), AlN and TiNO _x produced commercially.

It proves to be problematic that the sputtering is relatively complex and expensive, since high-tech voltage sources and large vacuum chambers or clean room conditions are required, as well as a precise control and regulation system, the composition of the gas, the layer thickness and the pressure conditions to be able to adjust. Overall, the technology is also relatively energy consuming.

Considering the current conditions in the solar market in the initial phase of its development, where the absorber layers are still the most expensive component of a collector, sputtering is currently no way to economically produce cheaper solar thermal collectors.

In contrast to sputtered layers, solar absorber coatings are a cheaper option, but they have a very high thermal emissivity of 80-90%, which is caused by vibration modes of the built-in organic polymer binder, and suffer from poor long-term stability. The use of organically modified silicone resins has improved the stability of such paints. However, because of their poor optical properties, paint-based absorbers are generally classified in the group of non-selective or moderately selective absorbers.

By combining a graphite layer with a mechanically polished substrate, a cost-effective mechanically generated solar absorber was created. Such coatings are very sensitive to the polishing parameters and have a solar absorption of 0.9 and a thermal emission of up to 0.22.

DE 196 20 645 C2 describes a sol-gel technique in which conductive particles are introduced into the starting oil or into the not-too-viscous resulting gel. In this method, the conductive particles must be sputtered in an inert gas atmosphere under high pressure (10 Pa to 1000 Pa) to below 70 nm. Larger particles are then separated by sieving. In this process, the metallic particles must be coated with a dielectric layer for protection against chemical attack and diffusion. The very large reactive surface area of the produced metallic nanoparticles leads to problems due to the chemical oxidation of particles, which must be prevented. In addition, surface treatment, particle sputtering and sieving are expensive extra steps, which make this production process more expensive.

Previous research has focused mainly on the microstructure and the improvement of mechanical properties of metal-ceramic nanocomposites through changes in the distribution of nanoparticles and their plasticity. Especially Sekino et al. studied the mechanical properties of various metal-ceramic nanocomposites using conventional powder metallurgy techniques, the reduction and subsequent sintering of ceramic and metal oxide powders such as W-Al ₂ O ₃ (T Sekino, A. Nakahira and K. Nuhara, Relationship between microstructure and high temperature mechanical properties for A12O3 / W nanocomposites.Transactions of the materials research society ofJapon 16B (1994) 1513-1516 and T Sekino, A.Nakahira, M.Nawa and K.Niihara, Fabrication of A12O3 / W nanocomposites J.Japan Soc. of Powd and Powd., Metal 38_ (1991) 326-330) or by chemical processes, such as sol-gel, for the production of metal-ceramic composite powders, such as Ni-Al ₂ O ₃ (T Sekino, T. Nakajima, S. Ueda and K. Niihara, Reduction and Sintering of a Nickel-Dispersed Alumina Composite and Us Properties J. Am. Ceram Soc 50 (1997) 1139-1148 and T. Sekino, T. Nakajima and K. Nuhara, Mechanical and Magnetic properties of nickel dispersed alumina-based nanocomposites. Mater. Lett. 29_ (1996) 165-169). These methods have been used to prepare bulk samples having metal contents of 5-30% in the composite at metal particle sizes of about 40-150 nm. In recent decades, various spectrally selective Ni-Al ₂ O ₃ composite films have been produced by different methods. These were prepared on a laboratory scale by planar RF magnetron sputtering using hot pressed Ni-Al ₂ O ₃ targets. In this method, changing the nickel content in the composite layer is not easy to achieve and additional Ni pellets had to be placed in a special geometry on the composite target to obtain higher metal volume fraction. Thus, using a 78 μm SiO ₂ antireflection coating for solar absorption, values of about 0.94 and for thermal emission of 0.07 were achieved.

It is also known to anodize aluminum substrates using phosphoric acid and then to dye the anodized aluminum by AC electrolysis in a NiSO ₄ impregnation bath. Here a solar absorption of 0.93-0.96 is achieved and a thermal emissivity of 0.1-0.2. Using the same production method and examining the effect of different impregnation parameters on the optical properties of the layers, solar absorptions greater than 0.9 and a thermal emissivity of 0.14 were achieved.

Although pigmented alumina coatings are used commercially in solar collectors, they are generally not considered to be particularly selective.

Building on their previous work on sol-gel-based antireflection layers and C-SiO ₂ composite layers, Ni-aluminum layers were fabricated from a Ni-Al ₂ O ₃ -SoI having a solar absorption of 0.83 and a thermal emission of 0.03 with a cermet layer having a nickel content of 65%.

The invention has for its object to provide a method for producing thin layers of metal-ceramic composites, which is very simple, reliable and inexpensive, allows layers of good spectral selectivity, which are resistant to humidity and high temperatures and which are different Apply materials. According to the invention this object is achieved by a method having the features of claim 1. The dependent claims indicate advantageous embodiments. A preferred use of the method is the coating of cermet-based selective solar absorber.

In this process, according to the invention, one or more thin cermet layers with a thickness of 50 to 2000 nm are deposited on the substrate by immersing metallic substrates in a stabilized aqueous or organic suspension. The suspension consists of alcoholic or aqueous solution in which ceramic nanoparticles whose primary particle size is smaller than 30 nm are dispersed. In the solution, the metallic portion of the cermet is as metal ions.

The suspension is electrostatically or sterically stabilized depending on the type of solvent (water or alcohol). In order to eliminate agglomerates or aggregates, the suspension is well dispersed by means of mechanical and ultrasonic dispersion techniques.

The materials required for this are relatively inexpensive and easy to obtain. It proves to be advantageous that no toxic acid baths, which must be disposed of appropriately, are necessary. In addition, with this method, the metallic and the ceramic filling degree in the thin film or the composite material can be easily adjusted by adjusting the concentration of the dissolved metal ions in the solution.

The prepared suspension can be used up by spraying or dipping onto a reflector substrate. In addition, this procedure is also suitable for mass production for coating large surfaces. In addition to the low demands on the plant and process control, another advantage of this process is the coating of almost any surface, not just flat surfaces.

As substrates for the coating method of the present invention, any substrate suitable for solar absorber can be used. Preferably, the substrate is made of a low emissivity metal or metal alloy, for example, copper or aluminum. If glass tubes or glass substrates are used, the glass can first coated with silver using Tollens reagent to achieve a similar effect. After drying, the cermet layer can be applied.

According to the embodiment of claim 8, the metallic portion of the cermet from the group Cu, Ni, Fe, Cr, Zn, Ti, Ag, Co, Al, Pd and Zr can be formed in the form of corresponding metal salts.

In the embodiment according to claim 9 of the ceramic portion of nano-powders of the group Al ₂ O _3, AlN can, SiO _2, TiO _2, ZrO _2, Y ₂ O _3, WO _3, Ta ₂ O _5, V ₂ O ₅ , Nb ₂ O ₅ , CeO ₂ or a mixture of 2 or 3 different nanopowders.

In the embodiment of claim 10, multiple layers of different metal content (from low to high) are sequentially applied to reduce reflection loss at the surface. Advantageously, the optical properties of the coating overall can thus be adjusted particularly well. The individual layers can be applied one after the other, whenever the previously applied layer has dried.

Compared to the likewise known sol-gel systems, it proves to be advantageous that in the present invention, no metallic alkoxides must be used, which are correspondingly expensive. In addition, no complicated chemical reactions take place, which would also have to be controlled precisely for a precise adjustment of the layers and their properties. This is especially true for the hydrolysis processes that take place in the sol-gel systems. Another problem with the sol-gel systems is the short shelf life and too early onset of network formation that grows over time and makes processing accordingly difficult. It also avoids the problems that occur in the sol-gel systems due to cracking in thin layers during drying. In addition, it proves to be advantageous for the liquid powder suspensions over the sol-gel systems that the liquid powder suspensions are more stable and better storable. This is especially true when these liquid powder suspensions are stirred. Even after adjusting the stirring, the stabilized suspension can be processed for a few hours. Even aged suspensions are redispersible. An embodiment of the invention will be described in more detail below. The invention will be explained in more detail with reference to an exemplary embodiment.

Exemplary embodiment: Ni-AbC ^ Cermet solar absorber

Starting material are substrates of copper and aluminum. In order to eliminate negative influences of the surface condition on the solar absorption, the surfaces are subjected to a fine polishing before their coating. In addition, the removal of the surface roughness allows a uniform application of the layers without unwanted deposits on disturbing bumps.

Thereafter, the substrates are cleaned by means of ethanol and distilled water.

For the preparation of suspensions with solids contents of 2 to 20 wt.% In a beaker first a metal salt, eg here nickel salt (the amount depends on the desired metallic content in the layer), dissolved in 200 ml of distilled water. Thereafter, nano-Al ₂ O ₃ powder having an average particle size of 5-30 nm is added. The mixture is allowed to mechanically disperse for 30 minutes under controlled temperature (cooling) and at high speed. In order to avoid sedimentation and agglomeration, the entire suspension is stabilized electrostatically or electrosterically (depends on the solvent). Ultrasonic dispersion can additionally be used to achieve a finer particle size distribution.

Preferably, wetting and adhesion agents are added to the suspension to improve substrate wetting and film adhesion.

After 30 minutes of dispersion, the solution is filtered with sub-micron filters.

The substrates are immersed in the vorbreitete suspension. The part to be coated should remain submerged for a few seconds to reach a state of equilibrium between the substrate and the solution. Subsequently, the substrate is withdrawn from the bath under controlled conditions and at a constant rate. After the part to be coated has been removed from the bath, it is dried in a drying cabinet. Subsequently, the dried samples of a thermal treatment subjected to a corresponding hardness of the coating. This heat treatment can be done in an oven at about 500 K up to 1000 K. The sintering is carried out under a pure hydrogen or forming gas atmosphere to reduce oxide phases of nickel and to avoid any oxidation of the substrate.

The exemplary embodiment of the invention is explained with reference to FIGS. 1 to 3, FIG. 1 shows an absorber-reflector tandem for a Ni-Al ₂ O ₃ absorber with (FIG. ₁ b) and without antireflection coating (FIG. described. FIG. 2 shows microscopic 2 images of the surface (FIG. 2 a) and of the cross section (FIG. ₂ b) of a deposited Ni-Al ₂ O ₃ layer which has been deposited by the method according to claim 1.

Setting the Ni content in the cermet layer to 20% by weight and depositing individual cermet layers with different layer thicknesses (obtainable by changing the pulling rate) on a polished Al substrate gives the selectivity shown in FIG. The sample, which contains adhesives, proves the absorbance of 0.87 and a thermal emissivity of 0.08. A final antireflection coating can further enhance the optical properties.

FIG. ₃ shows the reflectance of Ni-Al ₂ O ₃ absorbers without antireflection layer, which contain 20% by weight of Ni and were deposited on an aluminum substrate by means of dipping processes with different drawing speeds (different layer thicknesses). The influence of adhesive on the reflection curve is additionally shown.

Claims

1. A method for producing thin layers of metal-ceramic composites containing metallic nanoparticles, characterized in that

a. starting from an aqueous or alcoholic solution in which the metallic portion of the cermet is dissolved metal ions,

b. above solution serves as the basis of a powder-based process, whereby a stable aqueous or alcoholic suspension is prepared in which ceramic nanoparticles are dispersed,

c. ceramic nanoparticles are sterically or electrostatically stabilized,

d. agglomerated solid particles are removed in the suspension by means of mechanical or ultrasonic dispersion technology with cooling,

e. inorganic wetting agents and adhesives are used to improve substrate wetting and layer substrate adhesion,

f. the prepared suspension is used up by spraying or dipping onto a reflector substrate,

G. After drying, the samples are subjected to sintering in an H ₂ or Formiergas- atmosphere.

2. The method according to claim 1, characterized in that the size of the metal particles is less than 40 ran.

3. The method according to claim 1, characterized in that individual layers of about 50 nm to 2 microns layer thickness are generated.

4. The method according to claim 1, characterized in that the sintering takes place at temperatures up to 1000 ° Kelvin.

5. The method according to claim 1, characterized in that the substrate consists of a reflector metal or a metal alloy with a low emissivity, for example copper or aluminum.

6. The method according to claim 1, characterized in that a glass substrate is first coated by Tollens reagent with silver, and after drying according to claim 1 is coated with the cermet.

7. The method according to claim 1, characterized in that the metallic and the ceramic degree of filling in the thin layer or the composite material is easily adjustable by changing the concentration of the metal ions in the solution.

8. The method according to claim 1, characterized in that the metallic portion of the cermet from the group Cu, Ni, Fe, Cr, Zn, Ti, Ag, Co, Al, Pd and Zr is formed in the form of corresponding metal salts.

9. The method according to claim 1, characterized in that the ceramic portion of the cermet of nanopowders of the group Al ₂ O ₃ , AlN, SiO ₂ , TiO ₂ , ZrO ₂ , Y ₂ O ₃ ,, WO ₃ , Ta ₂ O ₅ , , V ₂ O ₅ , Nb ₂ O ₅ , CeO _{2 is} formed.

10. The method according to any one of the preceding claims, characterized in that a plurality of cermet layers are applied successively, wherein the individual layers differ in metal content and in which also for final optical properties and thermal stability, a final anti-reflection layer is deposited on the cermet layers ,

11. The method according to claim 1 and 10, characterized in that an antireflection layer is applied by means of dilute stabilized ceramic suspension without metallic portion.

12. Use of the method according to claims 1 to 11 for coating cermet-based selective solar absorber.