DE19546483A1 - Sol-gel method for ceramic oxide coating of substrate - Google Patents

Abstract

A method of coating substrates with a ceramic high temperature superconductor layer, by means of the sol-gel process of US 3330697 (Pechini process), involves: (a) producing a solution; (b) forming a sol; (c) forming a gel; (d) coating the substrate; (e) firing the gel in a suitable (preferably oxygen-containing) atmosphere; and (f) synthesising the superconductor layer in a suitable atmosphere.

Description

The invention relates to a method for Coating at least one flat side of a substrate or other substrate shapes with a ceramic Oxide layer, especially with a metal-ceramic High temperature superconductor material.

Ceramic layers are as hard layers and as High-temperature superconductor layers interesting. Hardness Layers e.g. B. Al₂O₃ or BeO₂ are used as tools used.

High-temperature superconductors with transition temperatures (T _c ) of over 77 K are well known. Most of these compounds are metal oxide compounds, in particular cuprates, such as the Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Pb-Sr-Ca-Cu-O, Hg- (Ba, Sr) -Ca-Cu -O or Tl- (Ba, Sr) -Ca-Cu-O systems. In these systems, further metal atoms such as Pb, Y, Sr, Ca, Bi, Ag, Gd, Ho or V can be added as an at least partial replacement (substitution) of at least one atom or additionally (doping). Unless explicitly stated otherwise, these options should always be included in the following. In particular, the transition temperatures of these compounds above 77 K allow the use of liquid nitrogen as a coolant, in contrast to the more complex cooling with liquid helium. The application potential of these superconducting layers lies in electrical engineering (high-frequency technology as resonators or filters), medical technology (SQUIDS in magnetic resonance imaging) and energy technology (current limiters or energy storage devices).

Special processes such as laser ablation, sputtering or molecular beam epitaxy (MBE) have proven successful for the production of thin layers (layer thickness typically 10 ^-9 m to 10 ^-6 m). However, these processes require considerable equipment and are limited in terms of the layer size and shape of the carrier material that can be produced.

Furthermore, some chemical processes are far reduced equipment expenditure for production tried superconducting layers, such as Solution Sol-Gel Process (patents inter alia: EP 0 360 550 A2).

The Solution Sol-Gel process for the production of Oxide layers, in particular superconductor layers, are  the elements forming the ceramic layer z. B. as Metal alkoxides dissolved in alcohol. After adding H₂O forms after the 2 process steps Solution and Sol the gel in which the atoms are distributed store. However, the various affect Solubility of the metal atoms in this solution or in Gel the applicability of this procedure for the Manufacture of high temperature superconductors.

A similar process that avoids these problems was developed by M.P. Pechini (U.S. Pat. No. 3,330,697 dated 11 July 1967) for the production of compounds from Zr, Ti, or Nb and with Pb or an alkaline earth metal and for Coating electrodes with these compounds developed.

In this process, the metals are considered to contain water Oxides, as alkoxides or as alpha-hydroxycarboxylates (for Zr, Ti or Nb) or as oxides, as hydroxides, as Carbonates, or as alkoxides (for Pb and the Alkaline earth metals) in alpha-hydroxycarboxylic acid (e.g. Citric, lactic or glycolic acid) and an excess dissolved by polyhydroxy alcohol (e.g. ethylene glycol).  

This leads to the formation of chelate complexes between the metal ions and the alpha-hydroxycarboxylic acid. By the condensation reaction of the hydroxyl groups of the Polyhydroxy alcohol with the hydroxyl group of the alpha Hydroxycarboxylic acid forms a spatial network (gel matrix) in which the (metal) atoms are at atomic (or molecular level) are stored homogeneously distributed. After burning the gel and sintering in Oxides form in an oxygen atmosphere. A modification of this pechini method was developed by R.S. Liu and others made by the replacement of Ethylene glycol by ethylenediamine (Published in Japanese Journal of Applied Physics, Vol. 28 (1989) page L2155-L2157). This creates a better bond, especially of the Alkaline earth ions (e.g. Ca, Ba, Sr) and thus a more homogeneous one Distribution in the gel.

Using various application methods such as immersion (dip-in) or spraying the gel (possibly also the Solution or the sol) in a controlled amount on the Substrate applied. Then the gel is through thermal activation removed. After removing the The layer often has to support the material for formation a crystal structure, especially for training  (Synthesis) of the superconducting structure of another Temperature treatment (sintering see below). The solution sol-gel process is often only used for this used, oxide powder of the desired stoichiometry to produce, which then with an organic binder and dispersing agent mixed, for coating is used.

These methods can or have been expanded to that effect that compounds with more than 2 metal components can be produced. Furthermore, the Metals each a. as oxides, as hydroxides, as Carbonates, as water-containing oxides, as alkoxides, as Alpha-hydroxycarboxylates or used as nitrates will. Furthermore, instead of alpha-hydroxycarboxylic acid hydroxy polycarboxylic acid can also be used in general. The metal atoms can, if not already in solution are present before adding the hydroxy polycarboxylic acid and before Addition of the polyhydroxy alcohol can be dissolved; e.g. B. in H₂O or in dilute acid (e.g. HNO₃).

In the following, metals, semimetals, Lanthanoids and actinoids are understood. Are next to it also some non-metals or semi-metals such as B. B  (Boron), Si (silicon), As (arsenic), Te (tellurium), Be (Beryllium), Se (Selenium) and At (Astat).

When connecting some high temperature superconductors, especially technically at the moment promising, Pb-Sr-Ca-Cu-O, Hg- (Ba, Sr) -Ca-Cu-O or Tl- (Ba, Sr) -Ca-Cu-O systems results because of the Volatility of at least one component (e.g. Pb, Tl or Hg) a lack of concentration of this component in the sintered layer. To make this This is why high-temperature superconductors often become one Mixture that does not contain the volatile component or only for Part contains a so-called "precursor" powder mixture dissolved in the gel as described above. During the Synthesis of this precursor layer or in another Synthesis step after that, at least the volatile Component (e.g. Tl, Pb or Hg) present in the gas phase is formed by reaction of this gas phase with the Precursor layer from the superconducting layer. This additional source that is used for application and Reaction of the volatile component is allowed it mainly from technical difficulties, such as Difficulties in creating a homogeneous Vapor pressure over large volumes, only conditionally, larger ones  To coat substrate surfaces. Furthermore, it follows the problem that the sources mostly contain containers a considerable amount (up to a few hundred grams) on these volatile components, and these volatile components often toxic heavy metals, such as e.g. B. Hg, Pb or Tl.

The aim of the invention is therefore to develop a Process for coating substrates with Avoiding high temperature superconductor layers difficulties mentioned above, in particular the Formation of the superconducting layer without additional Evaporator source for the volatile components be feasible.

The task of layering ceramic Oxide layers, especially of high temperature Superconductors are invented with a modified one Solution Sol-Gel procedure according to M.P. Pechini (or R.S. Liu) and the combination of at least part of the solved the following steps as follows:

• - The total number of metal atoms in the gel is so high dimensioned that on average per 1-50 free Gel matrix places, preferably per 2-10 free ones  Gel matrix locations (or a gel matrix location corresponds e.g. 2 citric acid molecules) a metal atom is in the gel.
• - In the formation of the gel all become superconducting Layer-forming metal atoms added.
• - The more volatile components (of the superconductor) like e.g. B. Tl, Hg, Pb or Ca can in the gel (or in the Solution or in the sol) in increased concentration be (1-50 times, advantageously 1-5 times Concentration) compared to their concentration in the superconducting structure.
• - For the increased and improved training of superconducting layers can be in the gel (or in the Solution or in sol) further elements (preferably 1-3 Elements, e.g. B. Bi, Pb, Ag, V) be present as partial replacement (substitution) of one or more Elements of the superconductor (preferably the volatile Elements) or as an additive (doping). These additional Atoms improve and lighten during sintering u. a. probably due to the formation of a liquid phase the synthesis of the superconducting compound.
• - The burning of the gel happens (favored by the high concentration of metal atoms in the gel) low temperatures in the range of 200-800 ° C, preferably 350 ° C-500 ° C in a suitable atmosphere,  preferably in an atmosphere containing oxide (air or Oxygen). This minimizes the loss of it easily volatile components of the superconductor during the Combustion.
• - The coating can be made directly with the so produced Gel done, preferably by deposition of the Substrates in the gel. The Solution Sol-Gel process serves not just as a method of making Oxide powders, which are then brought back into solution (e.g. using organic binders) and as a layer be applied.
• - The removal of the carrier material (gel), i.e. H. the Burning of the gel happens while the substrates are in Gel are immersed. This type of deposition Substrates prevents the combustion of the Carrier material the layer to be deposited later Peels off substrate.
• - For the formation of the superconducting phase in Sintering process in a suitable atmosphere, e.g. B. noble gas, inert atmosphere or oxidizing atmosphere (air or Oxygen) the layers are placed in a container (e.g. Gold foil) included, which losses the volatile components minimized, but at the same time the Sinter atmosphere can penetrate.  
• - The synthesis happens without an additional source, the preferably supplies the more volatile components, but only with metal atoms after combustion of the gel are present.
• - The layer formation can be improved by Multiple coating, d. H. multiple solution sol-gel Coating and multiple synthesis of this coating. You can do this after each coating (Gel application and gel burning) or after several Coatings a synthesis of the superconducting layer be performed. This entire process (coating and synthesis) can be repeated further.

The atoms forming the superconductor are dissolved (0.1 up to 10 molar solution, preferably 1 molar solution) brought (or already used dissolved), preferably as nitrates in H₂O. The total amount of metal ions is 1 mol. The solution can be warmed for better distribution (e.g. up to approx. 100 ° C) and (with a magnetic stirrer) will. The metal ions lie among each other in the desired stoichiometry, preferably the easily volatile elements such. B. Tl, Hg, Pb or Ca. several times (1-20 times, preferably 1-5 times) in comparison with their concentration in the superconducting layer  represented. Furthermore, other elements, preferably u. a. Pb, Bi, V, Ag as doping or Substitution of at least one of the volatile elements (including Tl, Hg, Ca) are added in the ratio of 0-10, preferably 0.1 to 1 in proportion (in Superconductor).

This metal ion solution is 1-50 mol, preferably 5-10 mol hydroxy polycarboxylic acid (e.g. citric acid, possibly dissolved in H₂O) and a larger number of moles (preferably 1-10 times the number of moles) of polyhydroxy Alcohol (e.g. ethylene glycol) added.

After heating (preferably up to 150 ° C, escape of the nitrous gases) and formation of the gel, the substrates are coated with the gel. Preferably, the liquid gel is placed in a heat-resistant form (z. B. from Al₂O₃ ceramic), in which the substrates are placed (see. Fig. 1). The substrates, as shown in FIG. 1, are located in the middle and surrounded by gel. The gel is burned while the substrates are still in the gel.

The gel is burned in a suitable oven in an oxidizing atmosphere (preferably air or Oxygen) at temperatures of 200-800 ° C,  preferably 350-500 ° C instead (duration 30 min-50 h, preferably 2-10 h).

Fig. 1 shows the corundum crucible ( 1 ) with the substrates ( 3 ) located in the gel ( 2 ).

This coating process (solution sol-gel process) can be repeated several times (preferably 1-10 times) to improve the coating quality.

The coated substrates are placed in a container which is permeable to the (preferably oxidizing or inert) atmosphere during sintering, but is impermeable to the volatile components such as Tl, Hg, Ca or Pb (oxide). Gold foil in which the layer is sealed is preferably used for this purpose ( FIG. 2).

Fig. 2 shows a substrate ( 3 ) with the adhering film ( 2 ) wrapped in gold foil ( 1 ).

The sintering takes place in a suitable oven suitable atmosphere, preferably under the flow of oxygen, with such high temperatures and such long periods of time that allow the formation of the superconducting layer.

Typical sintering temperatures are 800-1000 ° C, preferably 840-970 ° C for a period of 2 minutes to 20 hours, preferably 5-60 minutes. Different temperature levels are preferably approached. In addition, a further lower temperature value (200 to 600 ° C.) in a suitable atmosphere (oxidizing or inert atmosphere) can preferably be approached and held (5 minutes to 200 hours) by changing (increasing or decreasing) the oxygen content of the layer . The heating and cooling process takes place with preferably high cooling or heating rates (e.g. 400 ° C. per hour). After the layer has cooled, it is removed from the container. Depending on the number of coatings and syntheses, the layer thickness can be controlled over a range of typically 1 _* 10 ^-6 to over 100 _* 10 ^-6 m. In the case of thicker layers, the sintering times and sintering temperatures should preferably be increased accordingly.

The synthesis of (Tl, Bi) - (Ba, Sr) -Ca-Cu-O; d. H. the Tl- (Ba, Sr) -Ca-Cu-O Connection with (partial) substitution (and or Doping) of Tl selected by Bi.

example

Production of a (Tl, Bi) (Ba _0.4 , Sr _1.6 ) Ca₂Cu₃O₉ layer on a LaAlO₃ substrate (crystal axis [100]) shown. The total amount of oxide powder used is 2 g.

The elements are used as oxides in the following stoichiometry: (Tl₂, Bi _0.25 ) (Ba _0.41 Sr _1.6 ) Ca₃Cu₃ ie 0.795 g Tl₂O₃ (1.74 _* 10 ^-3 mol), 0.101 g BiO₂ (2, 2 _* 10 ^-4 mol), 0.098 g CaO (1.74 _* 10 ^-3 mol), 0.777 g Sr₂Ca₂Cu₃O₇ (1.39 _* 10 ^-3 mol) and 0.229 g Ba₂Ca₂Cu₃O₇ (3.5 _* 10 ^-4 mol). The oxides are placed in a beaker with 20 ml bidistilled H₂O, then 10 ml concentrated HNO₃ acid is added and the solution is heated in a paraffin bath until the oxides have dissolved and until the evaporation of H₂O (approx. 100 ° C). Then 40 ml of a mixture of citric acid (0.064 mol) and ethylene glycol (0.256 mol) are added and the entire solution is heated to 140 ° C. until the (blue-green colored) gel matrix is formed. After volatilization of the nitrous gases (from 110 ° C), just before the gel solidifies (140 ° C), the liquid gel is placed in a heat-resistant form made of Al₂O₃ ceramic. The substrates are then placed in this gel (see FIG. 1). Fig. 1 shows the corundum crucible ( 1 ) with the substrates ( 3 ) located in the gel ( 2 ).

The heat-resistant corundum shape is made with the substrates and the gel in a chamber oven under oxidizing Given atmosphere (oxygen).

Burning the carbon contained in the gel happens at 450 ° C over a period of approx. 10 h.  

This coating process (solution sol-gel process) is repeated to improve the coating quality improve. The twice coated substrates are in Gold foil sealed.

Sintering takes place in a tube furnace under the flow of oxygen. The sintering temperature is 960 ° C, this peak value is only held for 1-2 minutes, then the layer is held at approx. 950 ° C for 6-12 minutes. The heating and cooling process takes place at 400 ° C per hour. After the layer has cooled, it is removed from the gold foil. The layer thickness is approx. 5 _* 10 ^-6 m.

Example 2

The procedure corresponds exactly to the procedure in previous example with the following change.

After the first synthesis of the superconducting layer this again a two-time solution sol-gel Coated and then the superconducting layer synthesized again.

This modification allows thicker layers (10 _* 10 ^-6 m) to be produced with improved homogeneity.

Substrates

For the substrate are almost any shapes such. B. Small plates, tubes, cylinders possible. Preferably be flat, rectangular plates used in which at least one flat side is coated. Furthermore are preferably such crystals as substrates choose whose crystal structures match the crystal structure the layer coincides so far that it is a oriented growth of the superconducting layer allow. Such substrates are e.g. B. LaAlO₃, SrTiO₃, MgO, Al₂O₃, yttrium-stabilized ZrO₂, NdGaO₃ or LaGaO₃. But less suitable substrates such as Si can also be used at least one suitable intermediate layer (buffer layer) which has a diffusion-inhibiting effect. In addition to these single crystals, they are also polycrystalline Materials and metallic substrates (e.g. stainless steel, NiCr steel, u. a. in plate, tube or wire form) with a sufficiently high thermal resistance. The poorer grid adjustment usually only allows one less oriented growing up, but what about others Advantages (mechanical flexibility, low costs) is balanced.

The advantages of this coating method are: Possibility of high quality with little effort  High temperature superconductor layers, in particular Superconductor with at least one volatile component, to manufacture without the usual post-annealing process in which at least one of the volatile components subsequently supplied and installed.

A high concentration of the superconductors Components related to the carrier substance (gel) causes that when removing the carrier material (gel) far more material forming superconductivity (metal atoms) is available. This lesser amount of gel based on The metal atoms allow the gel to burn lower temperatures. Because of the high concentration of the metal atoms forming the superconductor, the Burn at low temperatures and by location of the substrates in the middle of the gel (during combustion of the gel) the layer thickness increases per Coating process and the adhesion of the layer on the Substrate. In other coating processes such. B. "dip in" (i.e. immersing the substrate in the gel and Remove the substrate and only after that Removal of the gel) often results in holey, not continuous layers. Similar effects as with the "dip in "Coating also occurs with gels in which the  Concentration of superconducting components decreased has been.

By overdoping the volatile components in the Gel, by doping or substitution, which is used for Formation of the liquid phase leads, and through the Inclusion of the layer in a volatile for this Components not permeable, however, for those in the Gases used for sintering (e.g. oxygen, air or Argon) permeable container can be used for an additional Source for the volatile components of the superconductor to be dispensed with during the synthesis.

This removes the restriction to small, flat (e.g. 10 _* 10 _* 1 mm) substrates.

Furthermore, it can be added later Synthesis step in which this volatile component additionally offered and built into the superconductor will be waived.

For making thicker layers and for controlling the thickness of the coating and over the Coating process can be repeated several times with the same substrate and the same layer repeated be, the existing layer u. a. as Start and stick point for the growth of further material works.  

In summary, the advantages are especially for industrial application of this process in the Possibility of large-scale or with little effort ceramic oxide layers of any shape, in particular To produce high-temperature superconductor layers.

Claims (24)

1. Process for coating substrates with a ceramic high-temperature superconductor layer by means of a solution sol-gel process according to MP Pechini (US Pat. No. 3,330,697), in which the following is carried out:

• - Solution manufacturing
• - Sol training
• - Forming the gel
• - coating of substrates
• Burning the gel in a suitable (preferably oxygen-containing) atmosphere
• - Synthesis of the superconducting layer in a suitable atmosphere.

2. The method according to claim 1, characterized in that the metal atoms u. a. also as oxides, as hydroxides, as Carbonates, as water-containing oxides, as alkoxides, as Alpha hydroxy carboxylates or used as nitrates or dissolved in water or dilute acid (e.g. HNO₃) will.   3. The method according to at least one of the above claims characterized in that instead of alpha-hydroxycarboxylic acids generally hydroxy polycarboxylic acids are used. 4. The method according to at least one of the above claims, characterized in that the method is a modified Pechini process is d. H. Ethylene glycol is through Ethylene diamine replaced. 5. The method according to at least one of the above claims characterized in that the superconducting layer at least from one of the systems Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Hg-Ba-Ca-Cu-O, Bi-Sr-Ca-Cu-O or Tl- (Ba, Sr) -Ca-Cu-O comes from. 6. The method according to at least one of the above claims, characterized in that the superconductor has at least one additional element as substitution (partial replacement at least one element) or as doping (additional added), which is the development in the synthesis the superconducting phase. 7. The method according to at least one of the above claims characterized in that all those already in the gel Superconducting metal atoms are present.   8. The method according to at least one of the above claims, characterized in that no additional Synthesis step in which the partial or complete Absence of one or more volatile components of the Superconductor is balanced, is necessary. 9. The method according to at least one of the above claims characterized in that no additional source is required which is the volatile component of the superconductor during the synthesis of the superconducting layer for Provides. 10. The method according to at least one of the above claims, characterized in that an overstoichiometric Number of moles (compared to the stoichiometry of the Superconductor) at least one of the most volatile Components can be present in the gel. 11. The method according to at least one of the above claims characterized in that the superconducting compound comes from the Tl- (Ba, Sr) -Ca-Cu-O system. 12. The method according to at least one of the above claims characterized in that the superconducting compound  from the Tl- (Ba, Sr) -Ca-Cu-O system at least one of the Elements Bi, Pb, Ag, or V as doping or substitution is added. 13. Method according to at least one of the above Claims characterized in that the coating in that the combustion of the gel happens while the substrates are in the gel. 14. The method according to at least one of the above Claims, characterized in that the number of Metal atoms, or the atoms (except oxygen) 1 atom per 1-20 gel matrix spaces (1 gel matrix space corresponds to 2 Citric acid molecules). 15. The method according to at least one of the above Claims, characterized in that the number of Polyhydroxy alcohol molecules over the, the hydroxy polycarboxylic acid molecules (or alpha Hydroxycarboxylic acid molecules). 16. The method according to at least one of the above Claims characterized in that the escape of the volatile components during synthesis by a for these elements impermeable, whereas for the while  the gas atmosphere used for sintering is permeable Container, is prevented. 17. The method according to at least one of the above Claims characterized in that the container is a Gold or silver foil is in which the layer is sealed. 18. The method according to at least one of the above Claims characterized in that the coating and Sintering the layer several times on the same substrate can be repeated. 19. Method according to at least one of the above Claims characterized in that the used Substrates an oriented growth of the superconducting Allow shift. 20. The method according to at least one of the above Claims characterized in that the crystals u. a., MgO, LaAlO₃, SrTiO₃₁ NdGaO₃, Al₂O₃, yttrium stabilized ZrO₂, NdGaO₃, Si or LaGaO₃ are. 21. Method according to at least one of the above Claims characterized in that the substrates with  at least one intermediate layer, such as u. a. Yttrium- Stabilized ZrO₂, ZeO₂, MgAl₂O₄, Y₂O₃, RuO₂, SrTiO₃, RuO₂ BaTiO₃ or MgO are provided. 22. The method according to any one of the above claims characterized in that the substrates are polycrystalline Materials and metallic substrates (e.g. stainless steel, NiCr steel, u. a. in plate, tube or wire form). 23. The method according to at least one of the above claims, characterized in that instead of the high-temperature superconductor layer, a hard layer, ie a metal oxide layer is produced with the following composition A _x B _y O _z , in particular A _x O _z where A and B are a metal atom or a non-metallic atom is the 3rd, 4th, 5th, 6th or 7th main group. 24. The method according to at least one of the above claims, characterized in that the hard layer a Alumina, a zirconia or a Berrylium oxide layer is. (Composition e.g. Al₂O₃, ZrO₂ or BeO).