WO1996037739A1

WO1996037739A1 - Process for producing selective absorbers

Info

Publication number: WO1996037739A1
Application number: PCT/DE1996/000934
Authority: WO
Inventors: Thomas Eisenhammer; Miladin Lazarov; Helmut Schellinger
Original assignee: Thomas Eisenhammer; Miladin Lazarov; Helmut Schellinger
Priority date: 1995-05-22
Filing date: 1996-05-22
Publication date: 1996-11-28
Also published as: DE19620645A1; AU5810796A; DE19620645C2

Abstract

This invention relates to a process for producing a selective absorber containing one or more layers of an inhomogeneous material (cermet), in which the inhomogeneous material is produced by the sol-gel process.

Description

Process for the production of selective absorbers

The invention relates to a method for producing selective absorbers. The selective absorber contains an inhomogeneous material (cermet) made of a non-conductive or dielectric matrix with conductive particles embedded in the matrix.

Cermets consist of a non-conductive or dielectric matrix, in which conductive or metallic particles with diameters of typically 5-30 nm are embedded. Cermets have long been used as selective absorbers for solar thermal applications (G.A. Niklasson and C.G. Granqvist, J. Appl. Phys. 55, p. 3382 (1984)). The cermets are an inhomogeneous material that has a high degree of absorption in the short-wave, solar spectral range (approx. 350-1 500 nm), while the degree of absorption is low in the long-wave infrared spectral range. The industrial is known

Production of these cermets for solar thermal applications using galvanic processes (eg nickel-pigmented Al ₂ O ₃ ) or by PVD processes. Examples include molybdenum in SiO ₂ or Al ₂ O ₃ (M. Gorlin et al., In Modeling of Optical Thin Films II, MR Jacobsen (ed.), Proc. SPIE 1324, p. 214 (1990)) and Stahl in amorphous carbon aC: H (B. Window and GL Harding, Solar

Energy 32, p. 609 (1 984)).

A disadvantage of the electroplating process is that waste from the electroplating baths used is problematic for the environment. Because of the vacuum systems required, the PVD process is technically complex and therefore expensive. While a wide variety of substrate geometries can easily be coated using the galvanic processes, this is only possible with great effort using the PVD process. In addition, using PVD processes on uneven substrates is rather difficult to achieve the low layer thickness tolerance required for optical layers. The size and shape of the conductive particles and their volume fraction in the matrix can also be controlled only with difficulty and only to a limited extent with the methods mentioned. But these factors have one significant influence on the optical properties of a cermet. Nor is it possible to make chemically complex compounds. Embed conductive particles in the matrix. The choice of conductive particles is severely restricted in the above-mentioned processes, for example to particles of elementally pure metals (for example gold, copper, nickel, chromium, molybdenum, iron) or steel particles. Another problem is the oxidation and diffusion stability of the selective absorbers, which are often used at high temperatures and also in air. The selective absorbers degenerate easily by oxidation of the metal particles within the matrix.

The object of the present invention is therefore to provide a method for producing a selective absorber which is easy and environmentally safe to carry out and which avoids the disadvantages listed above. This object is achieved by a method according to claim 1. Advantageous refinements result from the subclaims.

It has been found by the inventors that it is advantageous to use the sol-gel process for the preparation of selective absorbers based on cermets for the provision of the cermets.

The sol-gel process has been a process known since the 1940s, in particular for the production of dielectric ceramics and coatings with electrochromic properties (for example Pach et al, J. of European Ceramic Society, 12 (1 993), pp. 249-255 ; Avellanieda C. et al, SPIE Vol. 2255, pp. 38-51 (1,994);

Roy, R., Science Vol. 238, pp. 1,664-1,669 (1,987)). The usual steps for a sol-gel process are the preparation of a starting solution, application of the solution to a substrate, gelation of this solution or application of the gel to a substrate and transition of the gel into a solid, for example by a drying or Sintering process. For example, it is possible to produce a moisture sensor using such a method. To do this, soot particles are dissolved in a solution by hydrolyzing silicon alkoxide in a particular Amount of water was obtained, dispersed, the resulting sol (starting solution) being gelled on an insulating substrate and the resulting gel drying and sintering (Patent Abstracts of Japan C-646, Vol. 1 3, No. 466 (October 20, 1 989)).

By means of the method according to the invention, it is possible to provide a selective absorber which is based on an inhomogeneous material (cermet) which contains any desired, but defined, conductive and dielectric components. The size, the shape and the volume fraction of the conductive particles can be varied in a wide range in a defined manner.

The coating of non-planar substrates is also possible without any problems.

According to the invention, a non-conductive or dielectric matrix, in which conductive particles are embedded, is produced by the sol-gel process. To carry out the sol-gel process, for example, niobium chloride (NbCI ₅ ) is dissolved in butanol and mixed with sodium butoxide (Na (OBu) _n ) under reflux. This leads to the formation of Nb (OBu _n ) ₅ and NaCl. After the NaCl has been separated off, a precursor sol is obtained, which is converted into a sol by mixing with glacial acetic acid. Another possibility to produce a sol (starting solution) is, for example, to produce an approx. 20% water-aluminum hydroxide (boehmite) mixture and to mix this mixture with HNO ₃ (pH = approx. 2) at 55 ° C. α-Al ₂ O ₃ seed crystals are mixed into HNO ₃ and added to the boehmite hydrogel. The two solutions are mixed thoroughly. A wide variety of dielectrics can be known in this way or to those skilled in the art

Be converted into a starting solution, including Al ₂ O ₃ , TiO ₂ , ZrO ₂ , Y ₂ O ₃ , SiO ₂ , Ta ₂ O ₅ , WO ₃ , V ₂ O ₅ , Nb ₂ O ₅ or CeO ₂ , namely in pure form or as a mixture. According to the invention, conductive components (particles) are introduced into the starting solution used in the sol-gel process, a dispersion being formed. However, it is also possible to introduce the conductive particles into the gel that is not yet too viscous. According to the invention, conductive components are understood to mean materials that operate at room temperature (20 ° C) have a specific electrical direct current resistance of less than 1 00000 μΩcm. For example, materials such as quasi-crystalline alloys (D. Shechtmann et al., Phys. Rev. Lett. 58 (1 984), 1 951; "Quasicristals", C. Janot, Oxford University Press, Oxford, (1 992)) may be mentioned. which have a specific resistance of approx. 5000 μΩcm or suitably doped superconducting perovskites which have a specific resistance of approx. 10000 μΩcm. Some alloys, such as AI ₂ Ru, also have high resistivities of 50,000 μΩcm. In contrast, dielectric materials are insulating with a specific resistance (in the pure state) over 10 ¹⁰ Ωm. The optimal properties in the infrared to visible spectral range are correspondingly different: in large parts of the wavelength range, dielectrics have a small imaginary part k of the complex refractive index n = n + ik, typically below 0.01. Conductive materials have a high k> 0.1 for wavelengths above the plasma wavelength. The materials suitable for selective absorbers also have such a high k in the solar spectral range.

According to the invention, the conductive particles preferably have dimensions in the range from (0.5 nm) ³ [= 0.15 nm ³ ] to (1 μm) ³ [= 1 μm ³ ], particularly preferably in the range from (2 nm) ³ to (100 nm) ³ .

Particles of largely pure elements can be used as conductive particles, but a low contamination of up to 5 atom percent, e.g. with oxygen or carbon. The conductive particles can be made of tungsten, chrome, platinum, gold, silver, nickel, cobalt, iron,

Titanium, zirconium, molybdenum, hafnium, aluminum, palladium, vanadium or tantalum. The particles can also consist of a metal alloy or a conductive oxide ceramic. Metal alloys are understood to mean materials which, with the exception of small impurities (below 5 atomic percent), consist of the above-mentioned metals or semiconductors (C, Ge, Si). Both

The class of quasi-crystalline materials is particularly suitable for metal alloys, since the quasi-crystalline materials have high chemical stability and have unusual optical properties. Quasicrystalline materials which fulfill the following sum form are particularly preferred:

Al _a Cu _b Fe _c X _d with 8≤ b≤ 30.8≤ c≤ 20, d≤ 12 and _a + b + c + d = 100

Al _a Cu _b Co _c X _d with 8≤ b≤ 25, 10≤ c≤ 20.d≤ 12 and a + b + c + d = 100

Al _a Pd _b Mri _c X _d with 15≤ b≤ 30, 7≤ c≤ 17, d≤ 5 and a + b + c + d = 100

Ga _a Mg _b Zn _c X _d with 30≤b≤ 35.50≤ c≤ 55, d≤ 5 and a + b + c + d = 100

Al _a Cu _b Li _c X _d with 10≤ b≤ 15.25≤ c≤ 35, d≤ 5 and a + b + c + d = 100

Al _a Cu _b Ru _c X _d with 8≤ b≤ 25, 10≤ c≤20, d≤ 12 and a + b + c + d = 100

In the above formulas, X means an impurity such as Na, O or N or one or more of the metals listed above. Quasi-crystalline materials of the following empirical formulas are very preferred: Al _{6 5} Cu ₂₀ Ru ₁₅ , Al ₆₂ Cu ₂₀ Co _{1 5} Si ₃ , Al _{63, 5} Cu _{24, 5} Fe ₁₂ , Al ₆₄ Cu ₂₄ Fe ₁₂ , Al ₆₄ Cu ₂₂ Fe ₁₄ , AI ₆₀ Cu ₁₀ Li ₃₀ , AI ₆₅ Cu ₁₀ Li ₃₀ , AI ₆₅ Cu ₂₀ Co ₁₅ , Ga ₁₆ Mg ₃₂ Zn ₅₂ or AI ₇₀ Mn ₉ Pd ₂₁ . Another group of conductive materials are conductive metal oxides, metal nitrides or metal carbides and mixtures thereof. ZrN, TiN, HfN, CrN or Ti _x Al _{1- x} N (with 0.2 <x <0.8), WC, ZrC, TiC or HfC or an oxynitride MeN _x O _y (with Me = Titanium, zirconium or hafnium and 0.2 <x 1, 5; 0.2 <y <2.2; 0.4 <(x + y) <2.2), as well as oxidic metals such as RuO _x and lrO _x ( AK Goel et al., Phys. Rev. B 24, p. 7342, (1 981)) and the perovskites that are conductive with appropriate doping. This also includes the often superconducting oxide ceramics, preferably with the following compositions:

(Me) ₂ CuO ₄ with Me: Ca, Sr, Ba, Na, K, lanthanides

MeBa ₂ Cu ₃ O ₇ with Me: Y, lanthanide

Bi ₂ Sr ₂ Ca _n Cu _{n + 1} O _{2n + 6} with n = 0, 1, 2

Ta ₂ Ba ₂ Ca _n Cu _{n + 1} O _{2n + 6} with n = 0, 1, 2

The perovskites are highly absorbent in the short-wave spectral range and often appear black. They are characterized by high oxidation stability and are obtained in an oxidation process in air at temperatures around 800 ° C. Mixtures of the different conductive particles expand the possibilities to vary the optical properties.

The conductive particles can be, for example, atomized or evaporated in an inert gas atmosphere (helium, neon, argon, krypton, xenon) or a reactive atmosphere made of oxygen or nitrogen or a mixture of the gases at a pressure in the range from 1 Pa to 10000 Pa, preferably 10 Pa to 1000 Pa. This creates small particles with diameters in the range of a few nanometers to approximately 70 nanometers. Larger particles can e.g. be prepared by grinding, the particles with

Sieving processes, air classifying or electrostatic separation processes can be separated into suitable size classes. These processes have the advantage that particles with almost any composition can be produced and the size can be set in a defined manner. Conductive materials with a perovskite structure are produced in a solid-state reaction at high temperatures. The resulting sintered body consists of individual, nanocrystalline grains, the shape and size of the resulting particles being able to be varied within a wide range by selecting the process parameters in the solid-state reaction. The particles are separated by mortars.

The conductive particles are preferably coated with a dielectric (for example oxidic or nitridic) layer in one process step before being added to the starting liquid or the gel. This can be done in a reactive atmosphere of oxygen and / or an inert atmosphere of nitrogen or noble gases or a mixture of these gases by oxidation processes. However, the coating can also be carried out in a further sol-gel process. This coating has several advantages. The coating can facilitate a homogeneous dispersion of the particles in solutions without agglomeration of the particles. Furthermore, a dense, chemically stable oxide skin can affect the chemical stability of the conductive. Increase particles and prevent diffusion of the conductive particles into the dielectric component. Stable Al ₂ O ₃ layers are particularly preferred for the purpose of oxidation stabilization. In this way conductive particles can be stabilized against chemical influences and against diffusion with dense dielectric layers. This is particularly advantageous if the dielectric or non-conductive component produced in the actual sol-gel process itself is not dense enough to protect the embedded conductive particles from oxygen supply, which would cause oxidation of the conductive particles. The properties of the inhomogeneous materials (cermets) can also be varied in a still further range if the layer surrounding the conductive particles consists of a different material than the dielectric component. With the help of the additional refractive index jump that can be set in this way, the properties of the inhomogeneous materials can be further optimized for use as a selective absorber.

The volume fraction of the conductive particles in the inhomogeneous material can be adjusted in a wide range of 0.1-160%, preferably 1-40%, with the method according to the invention, whereby the properties of the cermet are changed accordingly. Materials manufactured in the sol-gel process usually have voids in the range of 0-20%. The inhomogeneous materials (cermets) can be applied to a reflector substrate by brushing, spraying, dipping or spinning. However, it may be necessary to use conventional additives to adjust the viscosity and surface tension of the solution or gel. This makes it possible to coat complex substrate geometries, especially pipes that e.g. be used as selective absorbers in solar thermal power generation with parabolic trough power plants. For selective absorbers, at least one layer of the inhomogeneous material produced by means of the sol-gel process is applied to a reflector substrate, the substrate containing or consisting of the metals copper, aluminum, moybdenum, silver, gold or their alloys. So it's also an application with highly reflective

Layers of different types of substrates are possible. The selective properties are achieved with layer thicknesses of the cermet in the range from 1 nm to 10 μm, preferably 10 nm to 1 μm, reached. The result is a selective absorber with high absorption α _s in the solar spectral range, while the absorption in the long-wave infrared spectral range is low for wavelengths above approx. 2 μm, ie the reflection is high. The high degree of reflection in the infra-red spectral range serves to suppress the radiation losses through

Heat radiation, i.e. the selective absorber has a small emissivity ε. The selective properties can be further improved if several layers with different proportions of conductive particles are used. According to the invention, it is possible to produce corresponding layer systems by repeatedly applying layers with different volume fractions of the conductive particles. Furthermore, purely dielectric layers can also be used to improve the selectivity, i.e. can be used to increase the solar absorption level. The invention will now be further explained with reference to the figures, which show:

Fig. 1 reflectance of a first absorber (Al ₆₅ Cu ₂₀ Ru ₁₅ particles in an Al ₂ O ₃ matrix on a copper substrate with an Al ₂ O ₃ anti-reflection layer) as a function of the wavelength

2 reflectance of a second absorber (TiN particles in an Al ₂ O ₃ -

Matrix on a copper substrate with an Al ₂ O ₃ anti-reflection layer) as a function of the wavelength

The invention will now be further described with reference to the following examples:

EXAMPLE 1

Quasi-crystalline, conductive particles of a material of the composition

AI ₆₅ Cu ₂₀ Ru ₁₅ are produced by ultramilling. The resulting particles are oxidized at 400 ° C under oxygen at a pressure of 100 Pa and it an approximately 8-10 nm thick oxide skin (Al ₂ O ₃ ) is formed on the surface of the particles, with no further oxidation of the particles taking place in a further treatment at 500 ° C. in air under normal pressure. The particles are dispersed in a solution suitable for the production of a sol of Al ₂ O ₃ (described, for example, in R. Roy, Science 238, p. 1,664 (1,987)). Since the quasi-crystalline particles on the

Surface are coated with Al ₂ O ₃ , a homogeneous dispersion of the particles in the solution is not a problem. The liquid is applied to a copper substrate by spraying and a cermet layer of quasi-crystalline particles in an Al ₂ O ₃ matrix is formed by an annealing treatment at 600 ° C. This layer has a thickness of 110 nm and a volume fraction of quasi-crystalline material of 30%. In a further process step, a pure Al ₂ O ₃ layer is applied with a layer thickness of 60 nm, which serves as an anti-reflection layer. Fig. 1 shows the reflectance of this layer system as a function of the wavelength.

EXAMPLE 2

Conductive TiN particles are produced by grinding with an average grain diameter of 20 nm. The conductive particles are dispersed in a solution suitable for the production of Al ₂ O ₃ (described, for example, in R. Roy, Science 238, p. 1,664 (1,987)). A copper substrate is coated with the liquid by spinning, and an annealing treatment at 600 ° C. creates a cermet layer with a volume fraction of the conductive particles of 20% and a layer thickness of 1 30 nm. In a further process step, a pure Al ₂ O ₃ -Layer with a layer thickness of 60 nm applied as an anti-reflection layer. Fig. 2 shows the reflectance of this layer system as

Function of the wavelength.

Claims

PATENT CLAIMS

1 . Method for producing a selective absorber comprising one or more thin layers on a reflector substrate, at least one of the layers consisting of an inhomogeneous material consisting of a non-conductive or dielectric matrix containing conductive particles,

d a d u r c h g e k e n n z e i c h n e t, d a ß

(a) conductive particles whose specific electrical resistance at 20 ° C. is less than 100000 μΩcm are dispersed in the starting solution of the sol-gel process, or

(b) conductive particles whose specific electrical resistance at 20 ° C. is less than 100000 μΩcm are mixed into the gel formed during the sol-gel process,

and

(c) the dispersion (step) or the gel formed in step (b) is applied to a reflector substrate.

2. The method according to claim 1, characterized in that the starting solution is selected from a non-conductive or dielectric material from the group AI ₂ O ₃ , TiO ₂ , ZrO ₂ , Y ₂ O ₃ , SiO ₂ , Ta ₂ O ₅ , WO ₃ , V ₂ O ₅ , Nb ₂ O ₅ or CeO ₂ or a mixture thereof.

3. The method according to claim 1 or 2, characterized in that those with an imaginary part k of the complex refractive index of the conductive particles in the solar wavelength range from 350 to 1,500 nm greater than 0.1 are used as conductive particles.

4. The method according to any one of claims 1 to 3, characterized in that the conductive particles are regularly or irregularly shaped and have volumes in the range from (0.5 nm) ³ to (1 μm) ³ .

5. The method according to one or more of the preceding claims, characterized in that the conductive particles to at least 95 atomic percentages from only one metallic element selected from the group tungsten, chromium, platinum, gold, silver, nickel, cobalt, iron, titanium, zirconium , Molybdenum, hafnium, aluminum, palladium, vanadium and

Tantalum exist.

6. The method according to any one of claims 1 -4, characterized in that the conductive particles made of a metal alloy, a conductive metal oxide,

nitride or carbide or mixtures thereof or consist of an oxide ceramic.

7. The method according to claim 6, characterized in that the metal alloy is stainless steel, brass or constantan.

8. The method according to claim 6, characterized in that the metal alloy is at least partially in a quasi-crystalline phase, the volume fraction of the quasi-crystalline phase in the conductive particles exceeding 40%.

9. The method according to claim 6, characterized in that the conductive nitride is ZrN, TiN, HfN, CrN or Ti _x AI _1-x N (with 0.2 <x <0.8), the conductive carbide WC, ZrC, Is TiC or HfC or an oxynitride MeN _x O _y (with Me = titanium, zirconium or hafnium and 0.2 <x 1, 5; 0.2 <y <2.2; 0.4

<(x + y) <2.2)

10. The method according to claim 6, that the conductive oxide is RuO _x , lrO _x , a highly doped semiconductor or an oxidic ceramic with a perovskite structure.

1 1. Method according to one of the preceding claims, characterized records that a mixture of conductive particles of different compositions is used.

12. The method according to any one of the preceding claims, characterized in that the conductive particles are produced by evaporation or atomization in an inert gas atmosphere or a reactive atmosphere of oxygen or nitrogen or a mixture of the gases at a pressure in the range from 1 Pa to 10000 Pa.

1 3. The method according to any one of the preceding claims, characterized in that the conductive particles have been coated with a dielectric layer before the dispersion in the starting liquid or the gel.

14. The method according to any one of the preceding claims, characterized in that the dispersion or the gel is applied to the reflector substrate by brushing, spraying, dipping or spinning.

1 5. The method according to any one of the preceding claims, characterized in that the at least one layer of the inhomogeneous material on the reflector substrate has a thickness of 10 nm to 10 microns.

16. The method according to any one of the preceding claims, characterized in that the reflector substrate to which the at least one layer of inhomogeneous material is applied as an absorber layer, the

Contains metals copper, aluminum, moybdenum, silver, gold or their alloys.

17. The method according to any one of the preceding claims, characterized in that in addition to the at least one layer of inhomogeneous material, further absorber and / or anti-reflection layers are applied to the reflector substrate.

1 8. Selective absorber producible by the method according to any one of claims 1-17.