The present invention relates to a method of producing metal to glass, metal to metal and metal to ceramic connections. Said connections may, for example, be used in solid oxide fuel cell (SOFC) applications.

Typically, SOFCs of the flat plate design comprise a stack of multiple cells, each of them comprising an electrolyte sandwiched by two electrodes. The electrodes of each cell are in contact with interconnect layers which make the series connection between the individual cells. Gas tight sealings are virtually important for the performance, durability and safely operation of a fuel cell.

Glasses are suitable as sealing materials for SOFCs since the physical and chemical properties of glasses can be tailored within a wide range. Different glass and glass-ceramic compositions have been examined within the group of alkali silicate, alkali aluminosilicates, alkaline earth silicates, alkaline earth aluminoborosilicates, phosphate and borosilicate glasses. However, even though promising results have been reported, none of them have been able to fulfill all the requirements of mechanical performance, e.g. viscosity and match of thermal expansion and chemical compatibility, e.g. wetting and bonding.

Composite seals with crystalline filler materials dispersed into base glass, e.g. alkali borosilicate glass, or in sodium aluminosilicate glass, have shown promising results in relation to exact matching the coefficient of thermal expansion (CTE) between the sealant and the seal surfaces. At the same time, suitable viscosities at the sealing temperature have been obtained.

However, an optimization of the bonding to metal surfaces is still required, and particularly a bonding to the corrosion scales formed on the metal surfaces as these depend on operation temperature, composition and microstructure of the pristine metals.

Bonding between the metal and a glass based seal has been demonstrated, cf. Yang, Z; Coyle, C.A.; Baskaran, S.; Chick, LA. "Making metal-to-metal and metal-to-ceramic interconnections for use in high temperature electrochemical devices by forming bond coat on first metal part, applying sealing material to bond coat, and adhering second part to sealing material" US 2004060967-A1 , by forming a single element metallic bond coat (M = Fe, Ni or Co) or by the preferred solution where a composite bond coat phase (M-CrAIY) is applied to the metallic surface prior to adhering the seal and further elements. The alumina content of the bond coat, either stemming from the steel or from the bond coat, is claimed to be essential for the bond coat performance.

Oxides of the metallic elements themselves (V, Fe, Ni, Cu, Co and Mo) are well known constituents in the so-called ground coats when bonding vitreous enamels to ferrous alloys and are characterized by their ability to partially oxidize metallic Fe and form vitreous or mixed oxide phases, often characterized by dendrite formation at various oxidation states, cf. Donald, I. W., "Preparation, properties and chemistry of glass and glass-ceramic-to-metal seals and coatings, J. Mat, Sci. 28 (1993), p. 2841-86 and Eppler, R. A "Glazes and glass coatings", The American Ceramic Society, Westen/ille, Ohio (2000).

In view of the disadvantages of the prior art, it is the object of the present invention to provide metal to glass, metal to metal and metal to ceramic connections to be used in, for instance, SOFC applications, with which the boundary phases can be engineered so as to obtain a strong bond.

Said object is achieved by a method of manufacturing metal to glass, metal to metal, and metal to ceramic connections and which is able to engineer the boundary phases, so as to obtain a strong bond independent of the metal alloy composition and a protective oxidation scale developed during use characterized by said connections being produced to a desired layer thickness as a mixture of a base glass powder and a metal oxide powder, preferably of predetermined particle sizes and in a binder system, referred to as a glassy

connection layer, the bonding being provided at elevated temperatures under controlled conditions, wherein the metal oxide powder is selected from the group consisting of:

- 0 to10 wt% boron oxide with a grain size d ₅₀ < 2 μm; - 0 to 10 wt% magnesium oxide with a grain size d ₅₀ < 2 μm;

- 0 to 8 wt% sodium and potassium oxides with a grain size d ₅ o < 2 μm; - 1 to 10 wt% manganese oxide with a grain size d ₅₀ < 1.5 μm;

- 1 to 10 wt% nickel oxide with a grain size d ₅ o < 1 -5 μm;

- 0 to 10 wt% vanadium oxide with a grain size d ₅₀ < 1.5 μm; - 0 to 5 wt% cobalt oxide with a grain size d ₅₀ < 1.5 μm; and

- 0 to 5 wt% molybdenum oxide with a grain size d ₅₀ < 1.5 μm.

- 0 to 5 wt% copper oxide with a grain size d ₅₀ < 1.5 μm.

Said object is moreover achieved by a method of manufacturing metal to glass, metal to metal, and metal to ceramic connections, characterized by said connections being produced with a mixture comprising a base glass powder and a metal oxide powder having a grain size of 5 μm or less.

Preferred embodiments are set forth in the subclaims.

The invention will be explained in the following with reference to the drawings wherein:

Fig. 1 illustrates the general concept and method.

Fig. 2A-2E illustrate the method of manufacturing the connections and specific embodiments of the method. Fig. 3A-3B illustrate a special embodiment of the method.

In the following, the invention will be described in more detail.

According to the invention the base glass powder is a glass matrix with a high coefficient of thermal expansion. The base glass powder is preferably selected

from the group consisting of alkali aluminosilicates, (NAS), alkaline earth aluminoborosilicates, (CAS), and phosphate glasses, (MAP), to which metal oxides are added, e.g. magnesium oxide, in order to control the local wetting properties, henceforth referred to as a glassy connection layer.

Particularly high chemical stability and slow crystallization behavior have been reported in connection with the sodium aluminosilicates (NAS) glass, cf. Hόland. W. and Beall, G., "Glass-Ceramic Technology", The American Ceramic Society, Westerville, Ohio (2002), and the residual vitreous glass has proved particularly beneficial in seal applications which may experience large thermal variations, e.g, rapid cycling between room temperature and operational temperatures close to the glass softening temperature.

Alkali alumosilicates, (NAS), suitable for the present invention comprise 10-25 mole % sodium oxide, 40-80 mole % silicon dioxide, 5-20 mole % aluminium oxide, and may optionally comprise 0-10 mole % boric oxide, 0-10 mole % magnesium oxide, 0-10 mole % calcium oxide, 0-10 mole % barium oxide, 0-2 mole % fluorine, and 0-5 mole % phosphor oxide.

Alkaline earth alumoborosilicates (CAS), suitable for the present invention comprise 40-80 mole % silicon dioxide, 5-20 mole % aluminium oxide, 20-45 mole % calcium oxide, and may optionally comprise 0-5 mole % sodium oxide, 0-10 mole % boric oxide, 0-10 mole % magnesium oxide, 0-2 mole % barium oxide, 0-2 mole % fluorine, and 0-5 mole % phosphor oxide.

Phosphate glasses (MAP) suitable for the present invention comprise 5-25 mole % aluminium oxide, 10-30 mole % magnesium oxide, 5-20 mole % calcium oxide, 40-60 mole % phosphor oxide and may optionally comprise 0-5 mole % sodium oxide, 0-12 mole % silicon dioxide, 0-10 mole % boric oxide, 0-5 mole % barium oxide, and 0-2 mole % fluorine.

Table 1 illustrates the compositional range for the base glasses used as defined above.

Table !

Interconnects develop a protective chromium-containing scale having a microstructure and composition which depend on the intrinsic alloy. In order to control the wetting and adhesion between the scale and the composite seal part, surface coatings are applied onto the interconnect which form strong and chemically compatible interfaces between the two components. Further, the inherent properties of the glass used in the composite seal parts have been altered locally at the metal-coat interface by addition of e.g. MgO, in order to control the viscosity and wetting, and at the same time maintain the bulk properties, e.g. high coefficient of thermal expansion of the base glass towards the seal components. The base glass composition has been selected so that a considerable amount of residual glass remains after bonding and partially crystallization of the glass composite seal and the glassy connection layer.

The grain size of the base glass powder to be used in accordance with the present invention is preferably from 0.05 μm to 100 μm, more preferably from 0.05 μm to 50 μm, and most preferred from 0.05 μm to 30 μm.

The grain size of the metal oxide powder to be used with the base glass powder in accordance with the present invention is 5 μm or less, preferably from 0.05 μm to 5 μm, more preferably from 0.05 μm to 3 μm, and most preferred from 0.05 μm to 1.5 μm.

The bonding in accordance with the present invention is provided under elevated

temperatures, depending on the specific components used, particularly the base glass powder. Preferred temperatures for the bonding are from 400 ⁰ C to 1100 ⁰ C. It is also preferred to select the temperature to be from 200 to 400 ⁰ C above the glass transition temperature in question and even more preferred from 250 to 35O ⁰ C above the glass transition temperature in question.

The conditions for the bonding are controlled to achieve optimal bonding strength of the connections by firstly supplying air to the joint for oxidizing the organic binder during heat up through a temperature range from room temperature to 55O ⁰ C. Secondly, at higher temperatures an orthogonal sealing pressure, typically between 0.1 kPa and 1000 kPa and preferably between 10 kPa and 400 kPa is applied and held constant during the bonding process, which typically lasts from 0.1 to 10 hours.

Preferably, prior to applying the base glass powder together with the metal oxide on the metal surface, the metal surface is coated with a metallic coating. Said coating may be alloyed into the surface by heating to high temperature, preferably between 750 to 95O ⁰ C for relatively short time, preferably 0.5 to 5 hours in a controlled atmosphere, where the oxygen and nitrogen partial pressures are held lower than ~10 ^"20 bar.

Alternatively, a transition-metal oxide coating may be applied to the metal surface by spraying or dip-coating prior to the application of the base glass powder and the metal oxide.

The combined action of transition metal oxides, in particular cobalt-, nickel-, and manganese oxide, which are preferably applied as a surface coating on the metal, together with the glassy connection layer, encompasses a tunable base coat, which is used for instance in a SOFC environment for providing a strong bonding component between the metal parts and the neighbor components in form of glassy composite seal parts, other metal parts or ceramic parts. The transition metal oxides may also be added to the glass in smaller amounts.

Specific connections are produced by coating a composite seal part with a dispersed metal oxide suspension. This may be carried out by, for example, dip coating or spraying.

Examples of the metal oxides to be added to the base glass powder in order to control the glass wetting properties are boron oxide, magnesium oxide, sodium oxide, potassium oxide and vanadium oxide, with boron oxide, magnesium oxide and sodium oxide being particularly preferred. Examples of the metal oxides to be added to the base glass powder or preferably as a surface coat on the metallic part in order to control the glass bonding and adhesion properties are manganese oxide, nickel oxide, cobalt oxide, molybdenum oxide, copper oxide or mixtures thereof. Preferred oxides are manganese oxide, nickel oxide, and cobalt oxide, with nickel oxide and cobalt oxide being particularly preferred.

Furthermore, in a preferred embodiment the connections are produced as a mixture of a base glass powder and a metal oxide powder in a binder system.

The invention will in the following be illustrated with reference to the detailed examples which are however not limiting the invention.

Examples:

Metallic materials have been divided into three groups; one which develops a duplex seal of chromia and Cr-Mn-spinel (exemplified by Krupp-Thyssen, Crofer 22APU), a second which develops a single layer scale of chromia (exemplified by Sandvik 0YC44), and a third which develops a protective alumina scale (exemplified by Fe Cr alloy). Interconnect materials of ferritic chromium steel were cut to shape before treated with surface coatings. In case of metallic coatings, except for nickel, the surface oxides on the interconnect metal were removed prior to coating by rinsing specimens in a ultrasonic aqueous HF/HNO ₃ -bath, followed by a rinse in ethanol and finally dried at 60 ⁰ C in air. Other coatings were applied after degreasing the metal surface in acetone and drying at 60 ⁰ C in air.

Glass sample mixtures:

Silicate glass samples were produced by melting analytical grade chemicals Na ₂ CO ₃ , SiO ₂ , AI ₂ O ₃ , B ₂ O ₃ , MgO, NaF, and CaCO ₂ in a platinum crucible at 1500 ⁰ C for 4 hr. The glass was quenched, crushed, re-melted and finall milled into powder (d50 < 5 μm, as measured by a Beckman coulter I/S particle size analyser). Phosphate glasses were prepared by mixing analytical grade MgHPO ₄ (3H ₂ O), AI(PO ₃ ) ₃ , H ₃ BO ₃ and SiO ₂ by ball milling, melting in alumina crucibles at 1500 ⁰ C for 30 minutes while stirring before quenching, crushing and final milling into powder (d50 < 5 μm). Thin films having a dried thickness of 5-45 μm were made by screen-printing of glass powder or mixtures of glass powder and metal oxide powders. Glass foils, typically in the range of 200-400 μm were made by tape casting of glass powder or mixtures of glass powder and metal oxide powder. Large bodies of glass composite sealing, for which the CTE was matched to the CTE of the steels, were prepared from MgO powder (d = 90-200 μm) or metal powder (Fe Cr alloy, d = 10-200 μm) mixed in a ball mill with glass powder for 18 h before filled into graphite moulds and sintered at 750 ⁰ C for 2 h at a pO ₂ < 3 10 ^"9 atm. The sintered seals were finally machined to obtain parallel, smooth surfaces. Three prepared base glass samples are illustrated in table 2.

Table 2 illustrates the three prepared base glass samples.

Table 2.

Example 1 :

A Crofer 22APU (c) was coated with a 15 μm thick layer of glass No

3/MgO/CO ₃ O ₄ (4 wt% MgO, 0.5 wt% Co ₃ O ₄ ) glassy connection layer by screen- printing, (b) in Fig. 2A. Glass No 3 was prepared and combined with 72 wt% metal powder (Fe Cr alloy, 90 μm < d < 120 μm) for the composite seal part (a). Metal and composite seal were held together by 400 kPa during heating to 950° for 4 h before cooling to 750 ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 2:

A Sandvik 0YC44 (c) was coated with nominally 0.1 μm metallic nickel (d) and by screen-printing coated with a 45 μm thick layer of glass No 1/MgO glassy connection layer (2 wt% MgO, d < 1.1 μm (b), cf. Fig. 2 B). Glass No 1 was prepared and combined with 55 wt% coarse MgO for the composite seal part (a). Metal and composite seal were held together by 400 kPa during heating to 950° for 4 h before cooling to 750 ⁰ C in an argon/hydrogen mixture, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 3:

A Sandvik 0YC44 (c) was coated with nommlaly 0.1 μm Mn-metal (d) and preheated in argon (pO ₂ < 10 ^'20 atm.) to alloy the coating into the surface and after cooling to room temperature coated with a 15 μm thick layer of glass No 3/MgO/NaO glassy connection layer (10 wt% MgO, d < 1.3 μm; 1 wt% Na ₂ O, (d < 2 μm) by screen printing (b), cf. Fig. 2B. Glass No 3 was prepared and combined with 79 wt% metal powder (SS316) for the composite seal part (a). Metal and composite seal were held together by 400 kPa during heating to 950° for 4 h before cooling to 75O ⁰ C in an argon/hydrogen mixture, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 4:

A Crofer 22 APU (c) was coated with metallic cobalt (d), cf. Fig. 2B, preheated at

900 ⁰ C for 2h in argon (pθ ₂ < 10 ^"20 atm.) to alloy the coating into the surface and after cooling to room temperature coated with a thin thick layer of glass No

1/MgO glassy connection layer (20 wt% MgO) by screen printing (b). Glass No 1 was prepared and combined with 55 wt% coarse MgO for the composite seal part (a). Metal and composite seal were- heated to 400 ⁰ C in air and then held together by 400 kPa during heating to 950° for 4 h and cooling to 750° in an argon/hydrogen mixture, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 5:

A Sandvik 0YC44 (c) was coated with Mn-metal and preheated (d) at 85O ⁰ C for 0.5 h in argon (pO ₂ < 10 ^"20 atm.) to alloy the coating into the surface and after cooling to room temperature coated with a 45 μm thick layer of glass No 1/MgO glassy connection layer (2 wt% MgO) by screen printing, (b) in Fig. 2 B. Glass No 1 was prepared and combined with 55 wt% coarse MgO for the composite seal part (a). Metal and composite seal were heated to 400°C in air and then held together by 400 kPa during heating to 750° for 4 h before cooling to 550°C, where the connection was aged for 500 h before being thermall cycled twice to room temperature.

Example 6: A Crofer 22 APU (c) was coated with a 5 μm thick layer of cobalt oxide (e) and then coated with a 15 μm thick layer of glass No 1/MgO glassy connection layer (5 wt% MgO) by screen-printing (b), cf. Fig. 2C. Glass No 1 was prepared and combined with 55 wt% coarse MgO for the composite seal part (a). Metal and composite seal were heated to 400 ⁰ C in air and then held together by 400 kPa during heating Io 950° for 4 h before cooling to 75O ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 7

A Sandvik 0YC44 (c) was coated with Mn-metal, (d) in Fig. 2D and preheated at 850 ⁰ C for 0.5 h in argon (pθ ₂ < 10 ^"20 atm.) to ally the coating into the surface. The metal was then slurry coated with a 10 μm thick cobalt oxide (e). preheated in air to 500 ⁰ C for 0.5 h to alloy and oxidize the coaling and after cooling to room temperature coated with 45 μm thick layer of glass No 1/MgO/ B ₂ O ₃ glassy connection layer (2 wt% MgO, d < 1.1 μm, 4 wt% B ₂ O ₃ , d < 0.8 μm) by screen printing (b). Glass No 1 was prepared and combined with 55 wt% coarse MgO for the composite seal part (a). Metal and composite seal were held together by 400 kPa during heating to 880 ⁰ C for 4 h before cooling to 750 ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 8 A Sandvik 0YC44 (c) was coated with nominally 0.1 μm metallic nickel, (d) In Fig. 2B and by screen-printing coated with a 45 m thick layer of glass No 3/MgO/Na ₂ O (10 wt% MgO, 2 wt% Na ₂ O) glassy connection layer (b). Glass No

3 was prepared and combined with 70 vol% metal powder (SS316, 50 μm < d < 140 μm) for the composite seal part (a). Metal and composite seal were held together by 400 kPa during heating to 95O ⁰ C for 4 h in air before cooling to 750 ⁰ C in an argon/hydrogen mixture, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 9 A Sandvik 0YC44 (c) was spray-coated with nominally 10 μm layer of nickel oxide (d 25 < 1.5 μm) (e) dispersed in a binder system, dried and coated with a 45 μm thick layer of glass No 3/MgO/NaO glassy connection layer (3 wt% MgO;

4 wt% NaO) by screen printing, (b), cf. Fig- 3 A. Magnesium aluminate spinel ceramic was cut to rectangular shape and ground to obtain plane parallel surfaces (h). Metal and ceramic parts were held together by 400 kPa during heating to 950° for 4 h before cooling to 750° in an argon/hydrogen mixture, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

ll

Example 10

A glass No 1 , cf. table 2, was prepared and combined with 55 wt% coarse MgO (a) in 5 Fig. 2E-1. The powders were mixed with an organic vehicle (paraffin dissolved in EtOH) and composite seal parts made by uniaxial pressing. Magnesium manganese spinel (MgMn ₂ O ₄ ) powder (d ₅ o = 2 μm) was dispersed and mixed with an organic vehicle. The composite seal parts were subsequently coated with the spinel-slurry by dip-coating (e). Crofer 22APU (c) and the coated seal pan were held together by approximately 10 kPa and heated to 900 ⁰ C where the load was increased to 40 kPa while keeping the temperature. After 2 h the temperature was reduced to 750 ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 11 A glass No 1 , cf. table 2, was prepared and combined with 55 wt% coarse MgO (a) in Fig. 2E-2, The powders were mixed with an organic vehicle (paraffin dissolved in EtOH) and composite seal parts made by uniaxial pressing. Magnesium manganese spinel (MgMn ₂ O ₄ ) powder (d ₅ o = 2 μm) was dispersed and mixed with an organic vehicle. The composite seal parts were subsequently coated with the spinel-slurry by dip-coating (e). A Crofer 22 APU (c) was coated with metallic cobalt (25 μm/cm ² ) (d) and preheated at 900 ⁰ C for 2h in argon (pO ₂ < 10 ^"20 atm.) to alloy the coating into the surface. After cooling to room temperature, the metal and the coated seal part were held together by approximately 10 kPa and heated to 900 ⁰ C, where the load was increased to 40 kPa while keeping the temperature. After 2 h the temperature was reduced to 750 ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 12 A Sandvik 0YC44 (e) was coated with nominally 0.1 μm metallic nickel, (d) in fig. 3B and by screen-printing coated with a 5 μm thick layer of cobalt oxide (e). Crofer 22APU (g) was coated with a 15 my thick layer of glass No 3/MgO/Co ₃ O ₄ glassy connection layer by screen-printing (b). A 200 μm, thick tape cast foil (i)

of glass No 5 3/MgO/Na ₂ O/CaO glassy connection layer was sandwiched in between these two metal parts. The compound was held together by 400 kPa during heating to 75O ⁰ C for 4 h in air before cooling to 55O ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 13

A Crofer 22APU (c) in Fig. 2A was coated with a 15 μm thick layer of glass No

2/CO ₃ O ₄ glassy connection layer (5wt% Co ₃ O ₄ , d < 1.5 μm) by screen-printing (b). Glass No 2 was prepared and combined with 79 wt% metal powder (0YC44) for the compos-ite seal part (a). Metal and composite seal were held together by 200 kPa during heating to 700°C for 4 h before cooling to 55O ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

Example 14

A Sandvik 0YC44 (c) was coated with Mn-metal, (d) m Fig. 2D and preheated at 85O ⁰ C for 0.5 h in argon/hydrogen (pO ₂ < 10 ^'20 atm.) to alloy the coating into the surface. The metal was then slurry coated with a 10 μm thick cobalt oxide layer (e), preheated in air to 800 ⁰ C for 0.5 h to alloy and oxidize the coating, and after cooling to room temperature coated with 45 m thick layer of glass No 2/MgO (2 wt% MgO) glassy connection layer by screen printing (b). Glass No 2 was prepared and combined with 55 wt% coarse MgO for the composite seal part (a). Metal and composite seal were held together by 100 kPa during heating to 700 ⁰ C for 4 h before cooling to 550 ⁰ C, where the connection was aged for 500 h before being thermally cycled twice to room temperature.

It should be apparent to those skilled in the art that various changes in form and detail of the invention as shown and described above may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto.