High Temperature Seal for Joining Ceramics and Metal Alloys

This invention relates to an improved seal for joining ceramics and metal alloys. More particularly, this invention relates to a seal useful for joining ceramics and metal alloys in a apparatus and system for converting methane gas to syngas with oxygen. The oxygen for the oxidation step is obtained from air by an oxygen-selective ceramic membrane extruded from SrFeCo.sub.0.50 O.sub.x (known as SFC-2). Preferably, the ceramic membrane is in the form of a tube which in the past has exhibited significant mechanical stability and good oxygen permeability. Currently, methane conversion reactors that use such SFC-2 membranes are being considered for engineering-scale and, eventually, commercial plants. The large and complex ceramic structures needed for commercial applications will require ceramic-ceramic and ceramic-metal joining of small components. Seals adequate to hold external pressures of more than 100 psig at the reactor operating temperature of about 800.degree.-900.degree. C. are required. Inconel metal alloys are leading candidates because of the oxidation-resistant nature of these nickel-based alloys. A major problem with ceramic-metal seals is the difference in the coefficient of thermal expansion between the two materials when the components are repeatedly cycled between room temperature and the reactor operating temperature. Traditional silicate bonding sealants such as those used to bond high-strength glass to Inconel 718 are not suitable because of the high temperatures and because of the reactivity with the ceramic membrane materials.

It is an object of the invention to provide a sealant and a method of making the sealant, wherein the sealant is based on a fritted compound of strontium oxide and boric oxide, together with SFC-2.

Yet another object of the invention is to provide a sealant and a method for its use in joining oxygen-selective membranes and oxidation-resistant metals.

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.

FIG. 1 is a cross-sectional view of a ceramic membrane tube, Inconel alloy, and sealant;

FIG. 2 is a cross-sectional view of a ceramic membrane tube, Inconel or other suitable alloy, and ceramic or sealant material, showing the formation of a seal at each end of the tube; and

FIG. 3 is a cross-sectional view of a ceramic membrane tube, Inconel or other suitable alloy and ceramic or sealant material.

The ceramic of choice in the conversion of methane gas to synthesis gas: CO+H.sub.2 (with oxygen from air as the oxidant) is obtained by the use of a ceramic membrane extruded from SFC-2 (SrFeCo.sub.0.50 O.sub.x, wherein x is less than 3). The membrane may be tubular or other shapes. For convenience, the tubular geometry was used. In tests, the membrane tube has lasted more than 1000 hours at 850.degree. C. during reactor operations and thus exhibited significant mechanical stability. Metal alloys considered for joining ceramic and metal are basically the Inconel alloys (nickel, chromium, iron alloys) because of superior oxidation resistance. Previous glass ceramic seals consisting of a mixture of silica, boric acid, aluminum, phosphorous pentoxide, and alkali metal oxides are inapplicable to ceramic membrane tubes of the type set forth because the seals degrade at temperatures in excess of about 370.degree. C. and tend to react with the SFC-2 material.

The invention relates to a sealant and method of making it and its applications. Several materials which have been tested appear in Table 1, but the only satisfactory materials are the combinations of strontium oxide and B.sub.2 O.sub.3.

The combination including BaO, CuO, and MgO were found to be too reactive at operating temperatures to form adequate seals.

The metal to which the SFC-2 membrane will be sealed is preferably an Inconel alloy and may be any one of a number of Inconel alloys, the most preferred being Inconel 600 and 601 and 625. The sealant material is made by starting with powders. The appropriate amount of powders were dry mixed and sifted through a 60 mesh screen. Powders were then calcined twice at temperatures between 775.degree. and 825.degree. C. for about 16 hours. The melting points of the different compounds were determined by differential thermal analysis and are set forth in Table 1. The wetting and bonding behavior was observed by placing small pellets pressed from dry powder on an Inconel alloy and sintered SFC-2 plate and heating at temperatures greater than the melting point in an air furnace for about 1/2 hour. The screening studies showed that the preferred material has an approximate melting point of 1115.degree. C., it being understood that these glass-like materials do not have a definite melting point. In 2SrO.circle-solid.B.sub.2 O.sub.3, 2 moles of SrO are present for each mole of B.sub.2 O.sub.3. The sealant is prepared by melting strontium carbonate and boric acid, wherein SrO and B.sub.2 O.sub.3 are present in the molar ratio of 2 to 1. This produces a fritted compound of 2SrO.circle-solid.B.sub.2 O.sub.3. This in turn is mixed with SFC-2 powder and melted to produce a fritted sealant compound. In this fritted compound, SFC-2 can be present in the range of 30-70 percent by weight of the total sealant material.

The preferred embodiment comprises two fritted compounds. The fritted compound of SFC-2 and the fritted compound of 2SrO.circle-solid.B.sub.2 O.sub.3 are present in the ratio of 5 to 3, respectively.

Referring to FIG. 1, an Inconel 600/601 alloy end cap 10 is used with a cavity measuring 0.265 inch diameter by 0.375 inch deep machined in the alloy end cap 10, the cap measuring 0.375 inch in diameter and 0.5 inch in length so that when the SFC-2 tube 12 is inserted into the cavity, the spacing between the tube and the Inconel alloy cap is between 0.0005 and 0.0018 inch. The spacing was filled with a powder made from the fritted combination of SFC-2 powder and 2SrO.circle-solid.B.sub.2 O.sub.3. The fritted compound sealant powder (particle size: -325 mesh) weighing 0.1-0.2 gram was placed into the cavity and compacted to form a washer or disc-like seal with or without a small 0.125 inch diameter centered hole to permit gas passage. The compaction pressure was in the range of 7 to 9 ksi depending on whether the pressed seal is a disc or washer-like and thereafter heated with a SFC-2 tube sitting on top of the pressed sealant powder and heated to a temperature of about 1155.degree. C. at a heating rate of 60.degree. C./hour. The sealant was held at this temperature up to 0.5 hour and thereafter cooled at a rate of 120.degree. C./hour to form seal 14. The seals 14 (FIGS. 1 and 2) made by this method were evaluated and subjected to internal and external pressure. Reliability of the seals 14 improves when the powdered frit from which the sealant was made was passed through -325 mesh screens. In fact, it is preferred that the powders from which the fritted sealant material is passed through screens from -200 mesh to -400 mesh. Seals were made with the fritted sealant (SFC-2+2SrO.circle-solid.B.sub.2 O.sub.3) both with and without 2-4 wt. % of silver powder with a particle size of 1-3 microns. Addition of silver to ceramics is known to enhance the strength of ceramics. Several seals 14 (FIGS. 1 and 2) have been capable of holding internal pressures of 15 to 50 psig and external pressures of 80-200 psig at room temperature. A number of these seals 14 (FIGS. 1 and 2), when heated and reheated to 900.degree. C. for 50 hours, have resulted in improved sealing rather than degradation of the seals. The sealed SFC-2 tubes also were tested at elevated temperature. In particular, seals were capable of holding 5 psig when the sealed tubes were held at 900.degree.-910.degree. C. for 48 hours. Preliminary examination of cross-sectional specimens showed that the interface between the SFC-2 and Inconel alloy is free of defects and well bonded. Scanning electron microscopy, in conjunction with energy-dispersive X-ray analysis of the interface, suggest that bonding occurred by dissolution/oxidation-reduction reactions.

FIG. 3, discloses a possible alternative end cap construction believed to be an improved design in which an annular groove 16 is provided in the Inconel metal end cap or other suitable oxidation-resistant alloy base end cap 10. (FIG. 3). The ceramic SFC-2 membrane tube 12 is inserted into the annular groove 16 with sufficient spacing on both sides of the tubular membrane 12 and the bottom of the groove 16 in order for sealant 14 to be placed on all three sides, that is, the bottom and both sides of the tubular membrane 12. No tests have been made on this conceptual alternative design.

While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.