DE102013009001A1 - Crystallizing, cristobalite-free and electrically well-insulating glass solders with high thermal expansion coefficients for joining metals and / or ceramics - Google Patents

Abstract

The object was to provide glass solders with which metals and / or ceramics, in particular those used in high-temperature reactors, such as the high-temperature fuel cell, in the temperature range between 700 and 1000 ° C, can be added. The composites should be produced at temperatures below 1000 ° C and have high strength and chemical resistance. According to the invention, glass solders for joining metals and / or ceramics at temperatures <1000 ° C. are proposed with a basic composition of
50-65 mol% SiO ₂
20-34 mol% RO (R = Mg, Ca, Sr or Ba or mixtures thereof)
15-25 mol% MO (M = Zn, Co, Ni or Cu or mixtures thereof),
wherein the amount of CoO and / or NiO and / or CuO is at least 5 mol% and at least one of these three oxides is incorporated in a crystal or mixed crystal phase having a high coefficient of thermal expansion.

Description

The invention relates to crystallizing, cristobalite-free and electrically well-insulating glass solders with which metals, metal-ceramic composites (so-called cermets) or ceramics, such as those used in high-temperature reactors, for example fuel cells, can be firmly and permanently connected. The composites are manufactured at temperatures <1000 ° C and permanently withstand temperatures between 700 and 1000 ° C.

Glass fillers are often used to join such materials. These glass solders are usually brought as a powder, optionally with the aid of suitable auxiliaries, such as oils or polymer solutions, between the materials to be joined and heated to a temperature which allows sintering of the glass particles. In this case, then enters the compaction of the powder. The coefficient of expansion of the glass must be within the range of the materials to be joined and may not deviate more than ± 1 · 10 ^-6 K ^-1 from this. However, if the glass solder is also provided for high thermal loads, this often results in considerable problems, since metals usually have a high coefficient of thermal expansion and glasses with high softening temperatures have a rather low expansion coefficient. This means, for example, that glasses with softening temperatures> 800 ° C and thermal expansion coefficients> 12 · 10 ^-6 K ^{-1 are} not available.

Nevertheless, in order to be able to solve corresponding joining problems, crystallizing glass solders are used. Here, the solders are also placed between the materials to be joined and then heated to a sufficiently high temperature so that compression can occur by sintering. After extensive densification of the glass crystallizes, with specific crystal phases are excreted, which bring about the desired thermal expansion coefficient in the crystallizing glass solder. From the chemical composition of the glass and the thermal treatment in this case result in the type of precipitated crystal phase (s) and the thermal expansion coefficient.

Since the crystallized glass solder in the vast majority of cases should have a time-constant coefficient of expansion, the crystallization process is usually so lead that no further crystallization occurs during use.

The thermal expansion coefficients of high temperature steels or high nickel alloys are often in the range of (12-16) x 10 ^-6 K ^-1 . Since these alloys are usually very high temperature resistant, they are often used at temperatures in the range of 800 to 1000 ° C. Stable, ie non-crystallizing glass solders are not available for these temperature ranges, as described above. But even with crystallizing glass solders there are considerable problems in such joining applications, since only a few crystal phases come into question for this purpose, ie have a correspondingly high expansion coefficient. In the literature, especially alkaline earth silicates, in particular barium silicates, are mentioned here.

Another significant problem with the use of glass solders, particularly crystallizing glass solders, is the adhesion to metals. Most investigations have been done on emails. The adhesion of glass to metal can be described by means of various mechanisms and reactions at the interface. A distinction is often made between mechanical entanglement and chemical bonding.

The mechanical entanglement or gearing is based on the fact that the liquid glass adapts to the metal surface and can penetrate into cavities or undercuts. After solidification of the glass metal and glass are firmly connected. An increased surface roughness of the metal can have a positive effect on the adhesion.

The mechanisms of adhesion of the glass solder to the metal are very versatile and complex and will not be discussed in detail here. One of the most widely used models for describing the adhesion of glasses or enamels to metals is that of electrochemical corrosion. If glasses containing adhesive oxides (eg CuO or NiO) are used, this model is suitable for describing the processes at the interface. This is essentially known from the enamel technique and will be explained below using the example of a cobalt oxide-containing glass and steel.

First, it is assumed that at the beginning of the process, the iron oxides, which are located on the metal surface, dissolve in the glass. Then the cobalt oxide in the glass can be reduced to metallic cobalt according to the following equation ( AH Dietzel: Enamelling - Scientific fundamentals and fundamentals of technology. Springer-Verlag, Berlin, Heidelberg, New York, 1981 ): 2Fe ²⁺ + Co ²⁺ → 2Fe ³⁺ + Co.

After complete dissolution of the iron oxide, the glass strikes metal iron directly. This is then reduced by the cobalt oxide contained in the glass ( DG Moore, JW Pitts, JC Richmond, WN Harrison, J. Am. Ceram. Soc. 37, 1954, 1-6 ): Fe + Co ²⁺ → Fe ²⁺ + Co.

Local elements, ie short-circuited galvanic elements, form with liquid glass as the electrolyte ( AH Dietzel: Enamelling - Scientific fundamentals and fundamentals of technology. Springer-Verlag, Berlin, Heidelberg, New York, 1981 ). The iron, which is less noble than cobalt, dissolves. As a result, the surface of the iron is roughened and increases the liability due to mechanical entanglement. Schematically, these processes are in the drawing shown.

However, this mechanism can only take place as long as the temperature is high enough and thus the glass is electrically conductive (cation-conducting). Among other things, the adhesion depends on the amount of adhesive oxide in the glass. Both too low and too high amounts of cobalt oxide can cause a non-optimal adhesion according to the literature ( JC Richmond, DG Moore, HB Kirkpatrick, WN Harrison, J. Am. Ceram. Soc. 36, 1953, 410-416 ). The theory of galvanic corrosion is based on the electrochemical series of the metals. Accordingly, in the glass, adhesive oxides must be used whose metal components are nobler than the metal to be coated. The latter is thus dissolved in the glass and it creates a roughened interface, which increases the adhesion.

However, stainless steels and nickel alloys used in high temperature applications have higher electrochemical potentials. This complicates the reduction of cobalt oxide. In the adhesion of glasses to stainless steels and nickel alloys, therefore, as described below, other conditions are given at the interface.

On the surface of stainless steels form oxide layers only to a small extent. These often consist of chromium oxides, which dissolve only slowly in the glass ( AH Dietzel: Enamelling - Scientific fundamentals and fundamentals of technology. Springer-Verlag, Berlin, Heidelberg, New York, 1981 ). These oxides form an oxide interlayer that improves adhesion ( A. Petzold, H. Pöschmann: Enamel and Enamel Technology. Springer-Verlag, Berlin, Heidelberg, New York, 1987 ). Upon complete dissolution of the oxide layer, however, the adhesion decreases significantly. This has been reported in the literature, for example, for a glass with 38.8% SiO ₂ .44% BaO .6.5% B ₂ O _3. 4.0% CaO. 5.0% ZnO and 1.7% ZrO ₂ in% by weight) ( T. Shimohira, J. Ceram. Soc. Jpn. 67, 1959, 95-102 ).

Classic adhesive oxides, such as CoO or NiO, can not roughen the surface when coating stainless steels. According to the literature, this is due to the higher standard potential of stainless steels compared to conventional enamelling steels ( L. Hiller: Adhesion of enamels on special metals and alloys. Dissertation, Martin-Luther-University Halle-Wittenberg, 1995 ). Therefore, adhesive oxides whose metal components have a higher electrochemical potential than the material to be oxidized (eg, Ag ₂ O) were used here, whereby the adhesion to Cr / Ni steel could be improved.

By adding CuO to the glass, the adhesion to stainless steel can be improved. However, according to Moore and Eubanks, the adhesion improvement is not caused by a surface roughening but by the formation of an oxide layer on the metal surface ( DG Moore, AG Eubanks, J. Am. Ceram. Soc. 39, 1956, 357-361 ). In general, the mechanisms and the formation of adhesion in the glass-stainless steel system are not fully understood ( B. Heid: Enamels for stainless steels. Dissertation, Clausthal University of Technology, 2007 ).

There is a similar problem with nickel alloys as with stainless steels. The electrochemical potential is even higher than that of stainless steels ( L. Hiller: Adhesion of enamels on special metals and alloys. Dissertation, Martin-Luther-University Halle-Wittenberg, 1995 ). In addition, oxide layers on the surface are formed only to a very small extent ( ThyssenKrupp VDM, Material Data Sheet No. 4137, July 2007 issue ). These are often Ni, Cr, Mo or Al-containing oxides ( ThyssenKrupp VDM, Material Data Sheet No. 4137, July 2007 issue ; M. Renner, M. Kohler, E. Ernst, M. Spang, Materials and Corrosion 47, 1996, 125-132 ).

Again, by the addition of nobler Haftoxiden such. B. Ag ₂ O, the adhesion can be improved ( L. Hiller: Adhesion of enamels on special metals and alloys. Dissertation, Martin-Luther-University Halle-Wittenberg, 1995 ).

However, little is documented in the literature about high temperature resistant, crystallizing glasses for joining nickel alloys. This is because many nickel alloys, even without a coating, have a very high resistance to corrosion and oxidation, even at elevated temperatures ( ThyssenKrupp VDM, Material Data Sheet No. 4137, July 2007 issue ), ie crystallizing glasses for corrosion protection are hardly in demand. The thermal expansion coefficients of nickel-based alloys significantly exceed those of most glass-ceramic materials. Unadjusted thermal expansion coefficients would thus cause high stresses at the interface between substrate and coating material.

Crystallizing glass solders are used for various types of high temperature reactors, such as. B. the high-temperature fuel cell (HT fuel cell) is needed. In HT fuel cells, electrical energy is generated by redox reactions. In this case, air or fuel gas is passed to the cathode or anode via so-called interconnectors. The oxygen from the air is thus reduced at the cathode: O ₂ + 4e ^- → 4O ^2-.

The resulting oxygen ions migrate through the electrolyte to the anode ( MK Mahapatra, K. Lu, Mater. Sci. Eng., R 67, 2010, 65-85 ). Then they react with the fuel gas. These are usually H ₂ , CO or CH ₄ ( N. Laosiripojana, S. Assabumrungrat, J. Power Sources 163, 2007, 943-951 ). In the case of pure hydrogen, the following reaction takes place at the anode ( P. Geasee: Development of crystallizing glass solders for planar high-temperature fuel cells. Dissertation, RWTH Aachen, 2003 ): 2H ₂ + 2O ^2- → 2H ₂ O + 4e ^- .

Anodic and cathodic reactions lead to the flow of electric current. The high operating temperatures of about 700 to 1000 ° C are mainly due to the solid electrolyte. This has at lower temperatures insufficient oxygen ion conductivity ( NQ Minh, J. Am. Ceram. Soc. 76, 1993, 563-588 ; T. Schwickert, R. Sievering, P. Geasee, R. Conradt, Materials Science and Engineering 33, 2002, 363-366 ).

By contacting the anode and cathode to the electrically conductive interconnectors several cells can be connected in series. In order to prevent a short circuit, an electrically insulating layer must be introduced between the interconnectors. This should also prevent mixing of the reaction gases used ( T. Schwickert: Joining of high-temperature fuel cells. Dissertation, RWTH Aachen, 2002 ). In order for the solders to be gas-tight, they must sinter approximately close to the onset of crystallization (porosity <5%).

To keep mechanical stress in the contact zone between solder and metal low, the difference in the expansion coefficients, as already mentioned, should not exceed 1 · 10 ^-6 K ^-1 . In fuel cells, however, a wide variety of materials with different coefficients of expansion are used.

Ceramics made of yttrium-stabilized zirconium oxide (YSZ) serve as oxygen ion conductors. This has in the temperature range of 30-800 ° C an expansion coefficient of about 10.5 · 10 ^-6 K ^-1 ( F. Tietz, Ionics 5, 1999, 129-139 ). The cathode of HT fuel cells is often made from electron- and ion-conducting perovskites ( C. Sun, R. Hui, J. Roller, J. Solid State Electrochem. 14, 2010, 1125-1144 ). Thus, for example, La _1-x Sr _x MnO _{3 has} a coefficient of expansion (30-800 ° C.) of 12 × 10 ^-6 K ^-1 for x = 0.3 (F. Tietz, Ionics 5, 1999, 129-139). Cermets consisting of nickel and stabilized zirconium oxide are frequently used as the anode material for HT fuel cells ( AS Nesaraj, J. Sci. Ind. Res. 69, 2010, 169-176 ). For a nickel content of 30-45% by volume, the coefficient of thermal expansion of the cermets (25-1000 ° C.) is in the range of (12.7-13.3) × 10 ^-6 K ^-1 ( D. Skarmoutsos, A. Tsoga, A. Naoumidis, P. Nikolopoulos, Solid State Ionics 135, 2000, 439-444 ).

Interconnector materials for HT fuel cells include nickel-base alloys and high-chromium stainless steels ( JW Fergus, Mater. Sci. Eng., A 397, 2005, 271-283 or Z. Yang, KS Weil, DM Paxton, JW Stevenson, J. Electrochem. Soc. 150, 2003, A1188-A1201 ). For example, Crofer 22 APU, one of the most commonly used steels for interconnectors, has an expansion coefficient of 11.9 · 10 ^-6 K ^-1 in the temperature range of 20-800 ° C ( MJ Pascual, A. Guillet, A. Duran, J. Power Sources 169 (2007) 40-46 ; Thyssenkrupp VDM, Material Data Sheet No. 4146, January 2010 edition ). The coefficients of expansion of nickel-based alloys are sometimes significantly higher with values of (14-19) · 10 ^-6 K ^-1 ( Z. Yang, KS Weil, DM Paxton, JW Stevenson, J. Electrochem. Soc. 150, 2003, A1188-A1201 ).

Thus, the expansion coefficients of the solders used must be adapted to those of the other components. Depending on the materials used, this value can vary within certain limits. Generally, however, the expansion coefficient of the solder should be (in some cases significantly) greater than 10 · 10 ^-6 K ^-1 .

Various glass-ceramic materials are available for joining metallic and ceramic materials for HT applications. In this case, preference is given to solders which form a firm connection with the materials to be joined ( MK Mahapatra, K. Lu, Mater. Sci. Eng., R 67, 2010, 65-85 ). Glasses from the basic system SiO ₂ -BaO ( MK Mahapatra, K. Lu, Mater. Sci. Eng., R 67, 2010, 65-85 ; MJ Pascual, A. Guillet, A. Duran, J. Power Sources 169, 2007, 40-46 ; C. Lara, MJ Pascual, MO Prado, A. Duran, Solid State Ionics 170, 2004, 201-208 ; C. Lara, MJ Pascual, A. Duran, J. Non-Cryst. Solids 348, 2004, 149-155 ) used. The corresponding glass ceramics have as crystalline phases predominantly barium silicates with high coefficients of thermal expansion on. Below are some examples of such glasses or glass ceramics listed.

The WO 2011/081736 A2 describes a glass-ceramic brazing material for HT fuel cells (SOFCs) from the system BaO-Al ₂ O ₃ -SiO ₂ for joining various oxide ceramics, such. B. YSZ. The molar ratio of the solders of SiO ₂ : BaO lies between 1: 1 and 4: 1. The Al ₂ O ₃ content is 3.5-12 mol%. The glass ceramic has the crystal phases sanbornite (BaO · 2SiO ₂ ) and hexacelsian (BaO · Al ₂ O ₃ · 2SiO ₂ ) as well as a residual glass phase. The crystallized glasses can be completely dense sintered. The thermal expansion coefficient of these solders is adapted to that of typical ceramics, as used in HT fuel cells. For joining different HT alloys, such as As nickel alloys, but this is usually too low. Also, the resulting BaO · Al ₂ O ₃ · 2SiO _{2 can} undergo phase transformations at temperatures of about 300 ° C, which lead to stresses in the solder itself.

This also applies to bonus fees according to US 6,532,769 B1 in which Al ₂ O ₃ -containing silicate or borosilicate glasses are used. The Al ₂ O ₃ contents are between 2 and 15 mol%. Crystallization is also different barium silicates and BaO · Al ₂ O ₃ · 2 SiO ₂ form. Instead of BaO according to the invention also SrO, CaO and MgO can be used. The coefficients of expansion (25-1000 ° C) are between 7 · 10 ^-6 K ^-1 and 15 · 10 ^-6 K ^-1 . The sometimes relatively high Boroxidgehalte can lead at high temperatures to evaporate boron oxide from the solder. In general, the chemical stability of B ₂ O ₃ -containing glasses is greatly reduced at elevated temperatures.

A similar Al ₂ O ₃ -containing glass from the system BaO-SiO ₂ is in US Pat. No. 6,430,966 B1 described. This should be used to join ceramic materials. The expansion coefficient is also between 7 · 10 ^-6 K ^-1 and 15 · 10 ^-6 K ^-1 . In one embodiment, joining temperatures of 1150 ° C were used, which can lead to damage when joining other materials than ceramics, for example in the form of excessive oxidation.

A B ₂ O ₃ -free, glass-ceramic solder material is in US 7,378,361 B2 described. Besides SiO ₂ and BaO, CaO and / or SrO are also used here. In addition, up to 16% by weight of MgO and 10% by weight of ZnO may be contained. In the exemplary embodiments, the coefficient of thermal expansion (25-700 ° C.) is a maximum of 11.1 × 10 ^-6 K ^-1 .

In DE 10 2010 035 251 A9 glass solders or glass-ceramic solders for HT applications are described, wherein the proportion of crystalline phases after the soldering process does not exceed 50 wt .-%. The glasses consist mainly of BaO, SrO and SiO ₂ as well as various other oxides. In order to improve properties, for example wetting, the solders may each additionally contain up to 2% by weight of Cr ₂ O ₃ , PbO, V ₂ O ₅ , WO, SnO, CuO, MnO, CoO or Sb ₂ O ₃ . The expansion coefficient of the solders is given as (8-11) · 10 ^-6 K ^-1 .

In DE 10 2010 050 867 A1 describes crystallizable glasses for high temperature applications from the system SrO-SiO ₂ . The coefficient of expansion (20-300 ° C) is greater than 8 · 10 ^-6 K ^-1 . The crystalline phases are predominantly different strontium silicates. The glasses only start to sinter above 750 ° C.

Subject of the DE 198 57 057 C1 are glass-ceramics for HT fuel cells based on MgO-containing silicate glasses. In the embodiments, a maximum thermal expansion coefficient of 12.4 · 10 ^-6 K ^{-1 is} given.

WO 2010/099939 A4 describes a solder from the system BaO-B ₂ O ₃ -SiO ₂ . The boron oxide content is 5-15 wt .-%. The crystallized glass solder has a maximum thermal expansion coefficient of 13 × 10 ^-6 K ^-1 . In this case, the solder 0.5 wt .-% of V ₂ O ₅ and / or Sb ₂ O ₃ and / or CoO to improve the adhesion to metals. A similar material, but with lower levels of BaO, is found in EP 2 053 026 A1 described. However, the thermal expansion coefficient is only about 10 · 10 ^-6 K ^-1 .

The DE 10 2005 002 435 A1 describes a glass solder, which usually contains the constituents SiO ₂ , CaO, MgO, BaO, Al ₂ O ₃ and B ₂ O ₃ . This is used as a composite material with additional ceramic components and has expansion coefficients between 9 · 10 ^-6 K ^-1 and 13 · 10 ^-6 K ^-1 . To improve wetting and flowability, portions of V ₂ O ₅ , Li ₂ O and Co ₂ O _{3 are} added. The problem of solders containing Li ₂ O is often that Li ₂ O significantly increases the electrical conductivity of the solder. In the exemplary embodiments, 3% by weight of Co ₂ O _{3 were} added to the glasses.

Out DE 101 22 327 A1 Al ₂ O ₃ -containing glass solders of the system BaO-CaO-SiO ₂ with expansion coefficients of more than 11 · 10 ^-6 K ^{-1 are} known. The effects of various oxidic additives were investigated. For example, CoO is responsible for lowering the surface tension in a concentration range of 1-3% by weight. Effects of additives on adhesion to typical HT materials are not indicated.

In the US 4,385,127 A Glass-ceramic coating materials for metal substrates from the system BaO-MgO-B ₂ O ₃ -SiO ₂ with expansion coefficients between 7.5 · 10 ^-6 K ^-1 and 15 · 10 ^-6 K ^-1 (25-600 ° C) are described. Additions of Al ₂ O ₃ , CaO and ZnO of 0-15 or 0-16% by weight can be added according to the invention. B ₂ O ₃ is contained in amounts of 5-30 and 6-25 wt .-%. Barium zinc magnesium silicates are known for their high expansion coefficients. However, these show, depending on the composition, phase transformations with high volume increase at temperatures from about 300 ° C ( M. Kerstan, M. Muller, C. Russel, J. Solid State Chem. 188, 2012, 84-91 ).

Also in the WO 98/46540 A1 are MgO-containing barium silicate glass-ceramics with various additives, such. B. ZnO described.

US 3,467,534 A describes barium silicates with additions of various oxides, including 0-5 wt .-% ZnO and 0-3 wt .-% CoO or NiO. In addition, 0-10 wt .-% of alkali oxides may be included. However, these significantly reduce the electrical insulation properties.

From the foregoing, it will be seen that a variety of glass-ceramic brazing materials, in particular BaO-SiO ₂ -based, exists. However, many of the solders described for the joining of high temperature resistant alloys to low expansion coefficients or are due to other disadvantages, especially with regard to the desired joining and use temperatures, not or only slightly suitable. Especially for nickel alloys, the coefficient of expansion should also be adjustable in a range above 13 × 10 ^-6 K ^-1 by means of composition variations. Also, these expansion coefficients are often given for a temperature range of, for example, 20-300 ° C, which plays a minor role for HT applications.

Since the expansion behavior is not always linear, z. B. between 20 ° C and 850 ° C yield completely different coefficients of expansion. Also, the thermal expansion behavior is only one of many properties that determine the suitability as a solder material.

Furthermore, some of the glasses described have high concentrations of B ₂ O ₃ , which significantly reduces the chemical resistance. Also, no statement is made on the sintering behavior and on the residual porosity obtained after the joining process. The highest possible, most pressureless, compaction is essential for the application as solder. This must eventually prevent mixing of the reaction gases involved and an associated reduction in the efficiency.

Good adhesion as well as complete wetting of the various materials to be joined are of utmost importance.

Furthermore, it should be mentioned that no cristobalite may crystallize out of the glass. Cristobalite undergoes a phase transformation at about 270 ° C, which is associated with a significant volume change. As a result, considerable stresses can occur both in the glass-ceramic solder material itself and at the interface with the joining partners, which can lead to destruction of the joint connection when the temperature changes.

The invention is therefore based on the object, crystallizing, cristobalite and electrically good insulating, d. H. alkali-free, to provide glass solders with high thermal expansion coefficients, with which metals and / or ceramics, especially as they are used in high-temperature reactors, such as the fuel cell, in the temperature range between 700 ° C and 1000 ° C, can be added.

The composites should be produced at temperatures of less than 1000 ° C and have a high mechanical strength and chemical resistance.

This object was achieved by crystallizing glass solders with high coefficients of thermal expansion in the range (11-16) · 10 ^-6 K ^-1 for joining metals and / or ceramics at temperatures <1000 ° C with a basic composition of
50-65 mol% SiO ₂
20-34 mol% RO (R = Mg, Ca, Sr or Ba or mixtures thereof)
15-25 mol% MO (M = Zn, Co, Ni or Cu or mixtures thereof),
dissolved, wherein the amount of CoO and / or NiO and / or CuO is at least 5 mol% and at least one of these three oxides is incorporated in a crystal or mixed crystal phase having a high coefficient of thermal expansion.

Crystalline or mixed crystal phases with high coefficients of thermal expansion can be crystallized out of the glass compositions described so that the resulting glass ceramic has an expansion coefficient in the range of (11-16) · 10 ^-6 K ^-1 . Further embodiments of these said phases of the proposed glass solders are listed in the subclaims.

The coefficient of expansion is therefore very good on the typical HT materials, as z. B. in the HT fuel cell in the temperature range between 700 and 1000 ° C application adapted. This results in a very high thermal shock resistance of the joint. In addition, the glass solders may contain additional components, for example, for the purpose of crystallization-inhibiting effect in each case a maximum of 2 mol% ZrO ₂ , HfO ₂ , Y ₂ O ₃ , La ₂ O ₃ , B ₂ O ₃ and / or Al ₂ O ₃ , limited to Total concentration of 5 mol% of these components.

The invention will be explained below with reference to exemplary embodiments. The drawing shows:

: The electrochemical processes at the interface between glass and metal can be seen using the example of a Co ²⁺ -containing glass. In doing so, the processes already mentioned in the description of the state of the art expire. The surface of the metal is roughened and due to mechanical entanglement, the adhesion between glass and metal is considerably increased.

: The thermal expansion behavior of glasses A and B crystallized at 840 ° C for 1 h, the BaCo ₂ Si ₂ O ₇ high-expansion phase and the HT materials Crofer 22 APU (ThyssenKrupp VDM) and Nicrofer 6025 HT (Thyssenkrupp VDM) can be seen. The thermal expansion behavior was determined by dilatometry.

The basic compositions of the proposed glass solders consist of 50-65 mol% SiO ₂ , 20-34 mol% RO (R = Mg, Ca, Sr or Ba or mixtures thereof), 15-25 mol% MO (M = Zn, Co, Ni or Cu or mixtures thereof). The sum of the components CoO, NiO and CuO must not be less than 5 mol%. The high expansion coefficient is determined by the phases crystallizing out of the glasses. Various barium silicates are formed, but primarily sanbornite (BaSi ₂ O ₅ ). In addition to barium silicates, mixed crystals of the form BaZn _2-x Co _x Si ₂ O ₇ (0 ≤ x ≤ 2) are additionally responsible for the high thermal expansion. Here, it is of great importance that the adhesive oxides CoO, NiO or CuO be incorporated in crystal phases with high thermal expansion coefficients, because only then high concentrations of these adhesive oxides can be used without the coefficient of thermal expansion decreases.

The results of dilatometry are in to see. In this case, two different glasses each crystallized at 840 ° C. for 1 h are listed with the following compositions (data in mol%): Glass A: 8.5CoO · 8.5ZnO · 26BaO · 57SiO ₂ Glass B: 17CoO · 26BaO · 57SiO ₂

Also, in the curves of pure BaCo ₂ Si ₂ O ₇ as well as those of two typical HT-resistant metal alloys (Nicrofer 6025 HT and Crofer 22 APU, each from ThyssenKrupp VDM) are shown. A very good agreement of the expansion behavior of the materials over almost the entire temperature range shown can be seen. For the given glass compositions, the coefficient of thermal expansion in the temperature range of 100 to 800 ° C is 13.6 · 10 ^-6 K ^-1 (glass A) and 12.8 · 10 ^-6 K ^-1 (glass B). For a glass of similar composition as glass B (17CoO 26BaO · · 54SiO ₂ · ₂ · 1ZrO 1B ₂ O ₃ · 1La ₂ O ₃₎ was determined for the same temperature range even an expansion coefficient of 14.6 × 10 ^-6 K ^-1 , The coefficients of expansion are therefore so high that they are even suitable for the joining of high-expansion nickel-based alloys. By composition variation, the expansion coefficient can also be adjusted selectively, for example by the proportion of CoO or ZnO in the glass. Thus, glasses with lower CoO and ZnO contents crystallize less highly expanding phases of the form BaZn _2-x Co _x Si ₂ O ₇ , which lowers the expansion coefficient of the glass-ceramic.

By adding small amounts (1-2 mol%) of B ₂ O ₃ , ZrO ₂ , HfO ₂ , Al ₂ O ₃ , Y ₂ O ₃ or La ₂ O ₃ , the crystallization temperature can be shifted to significantly higher values. It is also possible to use several of these additives at the same time. Thus crystallizes glass B without these additives at 800 ° C, while by addition of 1 mol% B ₂ O ₃ , ZrO ₂ and La ₂ O _3, the crystallization temperature rises to 860 ° C.

For all glasses crystallization starts only at temperatures above the sintering temperature. As a result, the glasses can be compacted, caught and crystallized in one process step. Despite pressure-free compression, the residual porosity is less than 5%.

The very good adhesion of the glasses on metallic substrates is based on the already mentioned and known from the enamel technology effect of CoO reduction. In the process, metallic Co. is formed at the interface. These metallic particles could be detected at the interfaces between glass-ceramic and metal with the aid of EDX analyzes. They were found both at the interface to a typical SOFC stainless steel ("Crofer 22 APU") and an HT-resistant Ni-Cr-Fe alloy ("Nicrofer 6025 HT"). Furthermore, by the addition of small amounts of z. B. Ag ₂ O and / or CuO adhesion can be improved.

In order to produce a firmly adhering joint, the joining process of the BaO-containing glasses can take place under reduced oxygen partial pressure. For this purpose, the samples can be rinsed during the heat treatment, for example with argon. After completion of the joining process, the joint connection can also be exposed to oxygen-containing atmospheres. Furthermore, the metals to be joined before the actual joining process of a thermal treatment in an oxygen-containing atmosphere, for. B. in air, which leads to an oxidation on the surface (pre-oxidation). As with all glasses from the BaO-SiO _{2 system} , BaCrO ₄ may also be formed at elevated temperatures. However, a decrease in the strength of the joint due to the formation of BaCrO ₄ could not be determined.

The glasses can be added in a temperature range of 800 to 1000 ° C and crystallized. Thus, the joining temperatures are below the operating temperature of HT reactors, such. B. the HT fuel cell. Also, the joining temperatures are low enough that there is no damage to the metallic substrate material due to excessive oxidation.

Surprisingly, the glass ceramics produced according to the invention not only show a strong bond to the metal, but also to ceramics. In particular cubic or tetragonal stabilized ZrO ₂ as well as electron-conducting ceramics with a perovskite-like structure, for example the compositions (La, Sr) (Co, Mn, Cr) O ₃ , are to be emphasized here.

Embodiment 1

A glass having the composition 17CoO.26BaO.55SiO ₂ is comminuted and sieved to a particle size of 25-100 μm. Subsequently, a corundum mold with the dimensions 7 × 8 × 24 mm ^{3 is} completely filled with glass. The samples are heated at 5 K / min to 840 ° C for 1 h and then cooled at 5 K / min. The glass sinters together without external pressure to a closed residual porosity of 4.6%. The crystal phases are BaSi ₂ O ₅ and BaCo ₂ Si ₂ O ₇ . The linear thermal expansion coefficient (100-800 ° C) is 12.8 × 10 ^-6 K ^-1 .

Embodiment 2:

A glass of the composition 8,5ZnO · 8,5CoO · 26BaO · 57SiO ₂ is comminuted and sieved to a particle size of 25-100 μm. The glass powder is applied to a sheet consisting of Nicrofer 6025 HT, and then heated at 5 K / min to 950 ° C for 1 h. During the temperature treatment, the sample is in a quartz tube, which is purged with argon (10 l / h). The oxygen partial pressure inside the tube is about 10 ^-3 bar. The resulting glass ceramic has a closed residual porosity of less than 5%. The glass is crystallized to over 85%. The crystal phases are BaSi ₂ O ₅ and a mixed crystal of the form BaZn _2-x Co _x Si ₂ O ₇ (0 ≦ _x ≦ 2).

Embodiment 3

A glass composition 17COO 26BaO · · 54SiO ₂ · 1B ₂ O ₃ · ₂ · 1ZrO 1La ₂ O ₃ is crushed and sieved to a particle size of <71 microns. A 1 mm thick layer of the glass is applied to a sheet consisting of Nicrofer 6025 HT. A second Nicrofer sheet is placed on the glass layer (sandwich construction). The structure is weighted with a 100 g weight and heated at 5 K / min to 865 ° C. The temperature is maintained for 40 minutes. During the temperature treatment, the sample is in a quartz tube, which is purged with argon (10 l / h). The oxygen partial pressure inside the tube is about 10 ^-3 bar. After the temperature treatment, the layer thickness of the glass layer is 0.35 mm. The solder has a residual porosity of 4.0%. The crystal phases are BaSi ₂ O ₅ and BaCo ₂ Si ₂ O ₇ . The shear tensile test results in a shear strength of 2.65 MPa. The interface between metal and solder remains intact.

Such a sandwich construction with Crofer 22 APU achieves a shear strength of 2.04 MPa for the same joining parameters. If the samples are removed after joining at elevated temperatures under air, the strength is increased. A 50-hour aging at 800 ° C leads to an increase in strength to 2.91 MPa.

Embodiment 4

A glass of the composition 6CoO · 6NiO · 6ZnO · 22BaO · 60SiO ₂ is pulverized and a grain fraction 25-100 microns separated. The powdered glass is mixed with oil and applied to a plate made of stainless steel Crofer 22 APU.

An area of about 1 × 1 cm ² is coated with the oil-glass suspension. Then this composite is heated slowly at a rate of 2 K / min to a temperature of 880 ° C. This temperature is maintained for one hour. During the temperature treatment, the sample is in a quartz tube, which is purged with argon (10 l / h). This temperature / time program leads to a compression of 97%, ie to a residual porosity of only 3%. The glass is crystallized to about 86%. Sanbornite and mixed crystals of the form BaZn _2-xz Co _y Ni _z Si ₂ O ₇ crystallize (0 ≤ (x + z) ≤ 2).

Embodiment 5:

A glass composition 8,5ZnO 8,5CoO · · · 54SiO ₂ · 26BaO 1La ₂ O ₃ · ₂ · 1ZrO 1B ₂ O ₃ is sieved to a particle size <71 microns. A 1 mm thick layer of the glass is applied to a sheet consisting of Nicrofer 6025 HT. On the glass powder, a ceramic plate made of perovskite is applied. On this sandwich construction, a 100 g weight is applied. The mixture is then heated at 2 K / min to 865 ° C. The temperature is maintained for 40 minutes. A particularly strong bond between glass and metal is achieved when the joining process takes place under argon. The solder adheres very well to both materials.

Such a sandwich construction consisting of metal, crystallizing glass solder and ceramic, with the same glass composition shows similarly good adhesion properties using the materials Crofer 22 APU and ZrO ₂ .

Embodiment 6:

A 2 mm deep conical recess is placed in a 5 mm thick Nicrofer 6025 HT plate. The diameter at the sample surface is 10 mm. In this recess a glass (particle size fraction 25-100 .mu.m) is introduced to the composition 14NiO · 2ZnO · 28BaO · 54SiO ₂ · 1La ₂ O ₃ · 1ZrO. ₂ For this purpose, a pourable slurry with a composition customary for the production of ceramic films is first produced. After heating at 2 K / min to 900 ° C for 1 h, the solder has a residual porosity of less than%. In addition to barium silicates, solid solutions of the form BaZn _2-x Ni _x Si ₂ O ₇ (0 ≦ _x ≦ 2) are formed.

Embodiment 7:

A glass of the composition 6CuO.10NiO.2ZnO.24BaO.58SiO ₂ is sieved to a particle size of 25-100 μm. The glass, which has been stirred with 1% strength aqueous polyvinyl alcohol solution, is applied to a sheet of the nickel alloy Nicrofer 6025 HT. The mixture is then heated at 2 K / min to 880 ° C. This temperature is maintained for 40 minutes. The cooling also takes place at 2 K / min. During the joining process, the sample chamber is rinsed with 10 l / h argon. The solder shows very good adhesion to the metal substrate. The resulting main crystal phases are Sanbornite and solid solutions of the form BaZn _2-xz Cu _x Ni _z Si ₂ O ₇ (0 ≤ (x + z) ≤ 2).

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2011/081736 A2 [0029]
• US 6532769 B1 [0030]
• US 6430966 B1 [0031]
• US 7378361 B2 [0032]
• DE 102010035251 A9 [0033]
• DE 102010050867 A1 [0034]
• DE 19857057 C1 [0035]
• WO 2010/099939 [0036]
• EP 2053026 A1 [0036]
• DE 102005002435 A1 [0037]
• DE 10122327 A1 [0038]
• US 4385127 A [0039]
• WO 98/46540 A1 [0040]
• US 3467534 A [0041]

Cited non-patent literature

• AH Dietzel: Enamelling - Scientific fundamentals and fundamentals of technology. Springer-Verlag, Berlin, Heidelberg, New York, 1981 [0009]
• DG Moore, JW Pitts, JC Richmond, WN Harrison, J. Am. Ceram. Soc. 37, 1954, 1-6 [0010]
• AH Dietzel: Enamelling - Scientific fundamentals and fundamentals of technology. Springer-Verlag, Berlin, Heidelberg, New York, 1981. [0011]
• JC Richmond, DG Moore, HB Kirkpatrick, WN Harrison, J. Am. Ceram. Soc. 36, 1953, 410-416 [0012]
• AH Dietzel: Enamelling - Scientific fundamentals and fundamentals of technology. Springer-Verlag, Berlin, Heidelberg, New York, 1981 [0014]
• A. Petzold, H. Pöschmann: Enamel and Enamel Technology. Springer-Verlag, Berlin, Heidelberg, New York, 1987 [0014]
• T. Shimohira, J. Ceram. Soc. Jpn. 67, 1959, 95-102 [0014]
• L. Hiller: Adhesion of enamels on special metals and alloys. Dissertation, Martin-Luther-University Halle-Wittenberg, 1995 [0015]
• DG Moore, AG Eubanks, J. Am. Ceram. Soc. 39, 1956, 357-361 [0016]
• B. Heid: Enamels for stainless steels. Dissertation, Clausthal University of Technology, 2007 [0016]
• L. Hiller: Adhesion of enamels on special metals and alloys. Dissertation, Martin-Luther-University Halle-Wittenberg, 1995 [0017]
• ThyssenKrupp VDM, Material Data Sheet No. 4137, July 2007 issue [0017]
• ThyssenKrupp VDM, Material Data Sheet No. 4137, July 2007 issue [0017]
• M. Renner, M. Kohler, E. Ernst, M. Spang, Materials and Corrosion 47, 1996, 125-132 [0017]
• L. Hiller: Adhesion of enamels on special metals and alloys. Dissertation, Martin-Luther-University Halle-Wittenberg, 1995 [0018]
• ThyssenKrupp VDM, Material Data Sheet No. 4137, July 2007 issue [0019]
• MK Mahapatra, K. Lu, Mater. Sci. Eng., R 67, 2010, 65-85 [0021]
• N. Laosiripojana, S. Assabumrungrat, J. Power Sources 163, 2007, 943-951 [0021]
• P. Geasee: Development of crystallizing glass solders for planar high-temperature fuel cells. Dissertation, RWTH Aachen University, 2003 [0021]
• NQ Minh, J. Am. Ceram. Soc. 76, 1993, 563-588 [0022]
• T. Schwickert, R. Sievering, P. Geasee, R. Conradt, Materials Science and Engineering 33, 2002, 363-366 [0022]
• T. Schwickert: Joining of high-temperature fuel cells. Dissertation, RWTH Aachen University, 2002 [0023]
• F. Tietz, Ionics 5, 1999, 129-139 [0025]
• C. Sun, R. Hui, J. Roller, J. Solid State Electrochem. 14, 2010, 1125-1144 [0025]
• AS Nesaraj, J. Sci. Ind. Res. 69, 2010, 169-176 [0025]
• D. Skarmoutsos, A. Tsoga, A. Naoumidis, P. Nikolopoulos, Solid State Ionics 135, 2000, 439-444 [0025]
• JW Fergus, Mater. Sci. Eng., A 397, 2005, 271-283. [0026]
• Z. Yang, KS Weil, DM Paxton, JW Stevenson, J. Electrochem. Soc. 150, 2003, A1188-A1201 [0026]
• MJ Pascual, A. Guillet, A. Duran, J. Power Sources 169 (2007) 40-46. [0026]
• Thyssenkrupp VDM, Material Data Sheet No. 4146, January 2010 edition [0026]
• Z. Yang, KS Weil, DM Paxton, JW Stevenson, J. Electrochem. Soc. 150, 2003, A1188-A1201 [0026]
• MK Mahapatra, K. Lu, Mater. Sci. Eng., R 67, 2010, 65-85. [0028]
• MK Mahapatra, K. Lu, Mater. Sci. Eng., R 67, 2010, 65-85. [0028]
• MJ Pascual, A. Guillet, A. Duran, J. Power Sources 169, 2007, 40-46. [0028]
• C. Lara, MJ Pascual, MO Prado, A. Duran, Solid State Ionics 170, 2004, 201-208 [0028]
• C. Lara, MJ Pascual, A. Duran, J. Non-Cryst. Solids 348, 2004, 149-155 [0028]
• M. Kerstan, M. Muller, C. Russel, J. Solid State Chem. 188, 2012, 84-91 [0039]

Claims (8)

Alkali-free, crystallizing and cristobalite-free glass solders with high coefficients of thermal expansion in the range (11-16) × 10 ^-6 K ^-1 for joining metals and / or ceramics at temperatures <1000 ° C. with a basic composition of 50-65 mol% SiO ₂ 20 -34 mol% RO (R = Mg, Ca, Sr or Ba or mixtures thereof) 15-25 mol% MO (M = Zn, Co, Ni or Cu or mixtures thereof), the amount of CoO and / or NiO and / or CuO is at least 5 mol% and at least one of these three oxides is incorporated into a crystal or mixed crystal phase having a high coefficient of thermal expansion. Alkali-free, crystallizing and cristobalite-free glass solders according to Claim 1, characterized in that, for the purpose of a crystallization-inhibiting effect, in each case a maximum of 2 mol% ZrO ₂ and / or HfO ₂ and / or Y ₂ O ₃ and / or La ₂ O ₃ and / or B ₂ O ₃ and / or Al ₂ O ₃ , limited to the total concentration of 5 mol% of these components are included. Alkali-free, crystallizing and cristobalite-free glass solders according to claim 1, characterized in that upon thermal treatment of the glass solder at least one crystal phase having a thermal expansion coefficient ≥ 12 · 10 ^-6 K ^{-1 is} crystallized out. Alkali-free, crystallizing and cristobalite-free glass solders according to claim 1 or 3, characterized in that the crystal phase is an alkaline earth silicate (RO with R = calcium, strontium or barium). Alkali-free, crystallizing and cristobalite-free glass solders according to claim 1 or 3, characterized in that the crystal phase is an alkaline earth zinc silicate. Alkali-free, crystallizing and cristobalite-free glass solders according to claim 1 or 3, characterized in that the crystal phase is an alkaline-earth cobalt silicate. Alkali-free, crystallizing and cristobalite-free glass solders according to claim 1 or 3, characterized in that the crystal phase is an alkaline earth nickel silicate. Alkali-free, crystallizing and cristobalite-free glass solders according to claim 1 or 3, characterized in that the crystal phase contained is a mixed crystal of the form BaZn _2-xyz Co _x Ni _y Cu _z Si ₂ O ₇ (0 ≤ (x + y + z) ≤ 2) , in particular BaZn _2-x Co _x Si ₂ O ₇ (0 ≦ _x ≦ 2).

Images