Method for sealing an oxygen transport membrane assembly

Abstract

An improved method of sealing a ceramic part to a solid part made of ceramic, metal, cermet or a ceramic coated metal is provided. The improved method includes placing a bond agent comprising an Al.sub.2O.sub.3 and SiO.sub.2 based glass-ceramic material and organic binder material on adjoining surfaces of the ceramic part and the solid part. The assembly is heated to a first target temperature that removes or dissolves the organic binder material from the bond agent and the assembly is subjected to a second induction heating step at a temperature ramp rate of between about 100? C. and 200? C. per minute to temperatures where the glass-ceramic material flows and wets the interface between adjoining surfaces. The assembly is rapidly cooled at a cooling rate of about 140? C. per minute or more to induce nucleation and re-crystallization of the glass-ceramic material to form a dense, durable and gas-tight seal.

Claims

1. A method of sealing a first ceramic part to a second ceramic part configured to receive the first ceramic part, the method comprising the steps of: placing a glass-ceramic bond agent comprising a glass-ceramic material and organic binder material in a volumetric ratio of from about 4:1 to about 1.5:1 on one or more adjoining surfaces of the first ceramic part and the second ceramic part, wherein said glass-ceramic bond agent comprises a blend of from about 2 mol % to about 15 mol % of Al.sub.2O.sub.3, from about 40 mol % to about 70 mol % SiO.sub.2 and from about 20 mol % to 55 mol % BaO, SrO, CaO, MgO or combinations thereof; heating the first ceramic part, the second ceramic part and the bond agent to remove or dissolve the organic binder material; further heating the first ceramic part and the second ceramic part at a rate of greater than about 100? C. per minute to temperatures where the glass-ceramic material reacts with carbon from the organic binder material and the glass-ceramic material flows into or wets the interface between adjoining surfaces of the first ceramic part and the second ceramic part; and cooling the first ceramic part and the second ceramic part at a rate of greater than about 100? C. per minute to nucleate and re-crystallize the glass-ceramic material disposed between adjoining surfaces of the first ceramic part and the second ceramic part and form a hermetic or gas-tight seal between the first ceramic part and the second ceramic part, said seal able to withstand temperatures of >800? C. and pressures of >100 psi, wherein the steps of heating and cooling the first ceramic part and the second ceramic part further comprise using an induction heating element to effect the temperature changes.

2. The method of claim 1 wherein the organic binder material is a combination of phosphate ester, polyvinyl butyrai and butyl benzyl phthalate, and said glass-ceramic bond agent is a paste, slurry, film, ribbon, or tape disposed on one or more adjoining surfaces of the first ceramic part and the second ceramic part.

3. The method of claim 1 wherein the step of cooling the first ceramic part and the second ceramic part further comprises cooling the first ceramic part and the second ceramic part at a rate of between about 120? C. per minute to about 160? C. per minute to nucleate and re-crystallize the glass-ceramic material disposed between adjoining surfaces of the first ceramic part and the second ceramic solid part.

4. The method of claim 1 wherein the step of heating the first ceramic part and the second ceramic part to temperatures effective to flow and wet the glass-ceramic material further comprises heating the first ceramic part, the second ceramic part and the glass-ceramic material at a rate of between about 200? C. per minute to about 250? C. per minute.

5. The method of claim 1 further comprising the step of maintaining the temperature of the first ceramic part and the second ceramic part for a prescribed duration of between about 30 seconds to 360 seconds, the step of maintaining the temperature to be effected between or during the heating and cooling steps.

6. The method of claim 1, wherein the first ceramic part is a tubular oxygen transport membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) While the specification concludes with claims distinctly pointing out the subject matter that applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:

(2) FIG. 1 is a schematic illustration of an oxygen transport membrane assembly with an close-up cut away view of a pair of ceramic oxygen transport membrane tubes sealably connected to an adaptor and connector;

(3) FIG. 2 is a schematic illustration of a ceramic oxygen transport membrane assembly sealably connected to a metal connector in accordance with another embodiment of the present invention;

(4) FIGS. 3A and 3B are 50? images of a glass-ceramic seal made using conventional joining methods;

(5) FIGS. 4A and 4B are 100? images of a glass-ceramic seal made using the joining method in accordance with the present invention;

(6) FIG. 5 is a graph depicting the time versus temperature profile suitable for use in the present method of making a glass-ceramic seal; and

(7) FIG. 6 is another graph depicting an alternate time versus temperature profile suitable for use in the present method of making a glass-ceramic seal.

DETAILED DESCRIPTION

(8) Turning to FIG. 1, there is shown an oxygen transport membrane assembly 10 including an arrangement of a pair of tubular oxygen transport membranes 20 connected at one end 22 by a U shaped pipe-like adaptor 24 or bend and connected at the other end 26 to a manifold (not shown) via a thru-block connector 28. An important characteristic of the tubular oxygen transport membranes 20 is that they are comprised of a multilayered structure comprising a porous surface exchange layer 32; a mixed phase oxygen ion conducting dense ceramic separation layer 34; an intermediate porous layer 36; and a porous support 38.

(9) The dense ceramic separation layer 34 is capable of conducting oxygen ions and electrons to separate oxygen from an oxygen containing feed and preferably comprises a mixture of a fluorite structured ionic conductive material and electrically conductive perovskite materials to conduct the oxygen ions and electrons, respectively. The porous surface exchange layer 32 or air activation layer is disposed on the outer surface of the oxygen transport membrane tube adjacent to the dense ceramic separation layer. The porous surface exchange layer 32 preferably has a porosity of between about 30 and 60 percent functions to ionize some of the oxygen in the feed. The oxygen that is not ionized at and within the porous surface exchange layer 32 will typically ionize at the adjacent surface of the dense ceramic separation layer 32.

(10) The porous support layer 38 is disposed on the inner surface of the oxygen transport membrane tube and is comprised of a fluorite structured ionic conducting material having a porosity of greater than about 20 percent and a microstructure exhibiting substantially uniform pore size distribution. The intermediate porous layer 36 is often referred to as a fuel oxidation layer and is disposed between the dense ceramic separation layer 34 and the porous support 38. Like the dense separation layer 32, the intermediate porous layer 36 is also capable of conducting oxygen ions and electrons to separate the oxygen from an oxygen containing feed.

(11) In the illustrated embodiment, the U shaped pipe-like adaptor 24 and thru-block connector 28 are fabricated from dense ceramic materials (e.g. 3 mol % yttria stabilized zirconia or MgOMgAl.sub.2O.sub.4) and have similar thermal expansion characteristics to the oxygen transport membrane tubes 20, in particular to the thermal expansion characteristics of the porous support 38.

(12) When a partial pressure difference of oxygen is applied on opposite sides of a membrane, oxygen ions will ionize on one surface of the membrane and emerge on the opposite side of the membrane and recombine into elemental oxygen. The free electrons resulting from the combination will be transported back through the membrane to ionize the oxygen. The partial pressure difference can be produced by providing the oxygen containing feed to the membrane at a positive pressure or by supplying a combustible substance to the side of the membrane opposing the oxygen containing feed or a combination of the two methods.

(13) In the illustrated embodiment, the oxygen containing feed is contacted on the retentate side or outer surface 33 of the tubular composite oxygen transport membrane 20 where it contacts the porous surface exchange layer 32 which ionizes some of the oxygen. As indicated above, Oxygen is also ionized at the adjacent surface of the dense ceramic separation layer 34. The oxygen ions are transported through the dense ceramic separation layer 34 to intermediate porous layer 36 to be distributed to the pores of the porous support 38. Some of the oxygen ions, upon passage through the dense ceramic separation layer 34 will recombine into elemental oxygen. The recombination of the oxygen ions into elemental oxygen is accompanied by the loss of electrons that flow back through the dense ceramic separation layer to ionize the oxygen at the opposite surface.

(14) At the same time, a combustible substance, for example a hydrogen and carbon monoxide containing synthesis gas, is contacted on the permeate side or inner surface 37 of the tubular oxygen transport membrane 20. The combustible substance enters the pores of the porous support 38, contacts the transported oxygen and burns through combustion supported by the transported oxygen. Optionally, the combustion may be further promoted by a catalyst that may be present in the form of catalyst particles incorporated into the porous support. The presence of combustible fuel on the permeate side 37 of the oxygen transport membrane 20, provides a lower partial pressure of oxygen. This lower partial pressure drives the oxygen ion transport as discussed above and also generates heat to heat the dense ceramic separation layer 34, the intermediate porous layer 36 and the porous surface exchange layer 32 up to an operational temperature at which the oxygen ions will be conducted.

(15) In order to operate effectively and at commercial scale, such oxygen transport membrane based systems typically require a plurality of connected oxygen transport membrane tubes. In addition, the gas flows on the permeate side 37 and retentate side 33 of the oxygen transport membrane tubes must be effectively sealed from one another with a gas-tight or hermetic seal. To achieve such gas-tight seals, the ends of the tubular oxygen transport membranes 20 are connected to a U shaped pipe-like adaptor 24 by using a glass-ceramic based seal formed using a bond agent preferably comprising a glass ceramic material and organic material. The other end of the tubular oxygen transport membranes 20 are connected to a manifold (not shown) via a thru-block connector 28. Specifically, the bond agent includes a baria-alumina-silcate glass-ceramic powder and an organic binder mixed together using various solvents and cast into a tape, ring or ribbon. In the illustrated embodiment, a glass-ceramic based bond agent is formed into a ring shape element 40 and interposed between adjoining surfaces of the oxygen transport membrane 20 and the U-shaped pipe-like adaptor 24 or thru-block connector 28.

(16) In the preferred embodiment, the high temperature glass-ceramic based bond agent is fabricated using about 1500 grams of the glass-ceramic powder having a density of about 0.98 grams/cc and 202.9 grams of organic binder materials having a overall density of about 3.89 grams/cc resulting in a bond agent having roughly 65% by volume glass-ceramic material and 35% by volume of organic material. The preferred organic material used in fabricating the present bond agent is a combination of Phosphate Ester (Emphos PS236); Polyvinyl Butyrai (Butvar B-79); and Butyl Benzyl Phthalate.

(17) The preferred glass-ceramic powder is preferably a blend of Al.sub.2O.sub.3, and SiO.sub.2 together with BaO, SrO, CaO, MgO or combinations thereof, commercially available from Ceradyne-Viox as G9 Glass Powder V2767. The G9 Glass Powder includes roughly between about 2 mol % to about 15 mol % of Al.sub.2O.sub.3; between about 40 mol % to about 70 mol % of SiO.sub.2; and between about 20 mol % to 55 mol % of BaO, SrO, CaO, MgO, or combinations thereof. Optionally, the glass-ceramic material may further contain other additives or modifiers such as metal oxides including, but not limited to, ZrO.sub.2, TiO.sub.2, Cr.sub.2O.sub.3, and combinations thereof to modify the properties of the glass phase or the final crystallized seal. Such modified properties may include wetting characteristics, glass transition temperature, glass softening temperature, and thermal expansion coefficient.

(18) The resulting glass-ceramic material preferably has a coefficient of thermal expansion that closely approximates the coefficient of thermal expansion of the tubular oxygen transport membrane 20 and the adaptor 24 or connector 28 to which it is joined. For example, if the coefficient of thermal expansion for the resulting glass-ceramic material is within about 30% or less of the coefficient of thermal expansion of the parts to be joined, one could consider that a close approximation. As indicated above, the U-shaped pipe-like adaptor 24 and the thru-block connector 28 are preferably ceramic components, although metal components, ceramic coated metal components or cermet components can be used.

(19) While the above-identified glass-ceramic sealing materials are generally known to those skilled in the art and are generally disclosed in U.S. Pat. Nos. 6,430,966 and 6,532,769; such glass-ceramic seals have heretofore been unable to produce high performance gas-tight or hermetic seals needed in many high temperature oxygen transport membrane applications. As clearly stated in these related art references, the formed seals had an oxygen leak rate of about 3.7?10.sup.?5 standard cubic centimeters per sec which is not considered a gas-tight or hermetic seal.

(20) To overcome these deficiencies in the glass-ceramic sealing materials, an enhanced or improved method of sealing has been developed using these same glass-ceramic materials. This enhanced or improved method of sealing ceramic parts includes preparing a bond agent comprising the glass-ceramic materials and organic material, the glass-ceramic material preferably comprising the known blend of Al.sub.2O.sub.3 and SiO.sub.2 together with BaO, SrO, CaO, MgO or combinations thereof. Preferably, the volumetric ratio of the glass-ceramic material to the organic material is between about 4:1 and about 1.5:1, and more preferably as indicated above, the bond agent is about 65% by volume glass-ceramic material and 35% by volume of organic material.

(21) The glass-ceramic based bond agent is then placed or disposed on one or more adjoining surfaces of the first ceramic part (i.e. oxygen transport membrane tube) and the second part (i.e. adaptor or connector) to form an assembly. The assembly, including the first ceramic part, the second solid part, and glass-ceramic based bond agent, is then heated to a temperature that removes or dissolves the organic binder material from the bond agent. The initial heating step is preferably performed using an induction heating furnace or element at a temperature ramp rate of greater than about 100? C. per minute to raise the temperature of the assembly from about room temperature to a first target temperature of between about 400? C. and 500? C. Optionally, the assembly may be held at this first target temperature for a predetermined temperature soak time, generally for 60 seconds or less.

(22) The assembly is then subjected to a second or further induction heating step at a high temperature ramp rate of greater than about 100? C. per minute, and more preferably between about 100? C. per minute and 200? C. per minute to temperatures where the glass-ceramic material starts to soften and undergo phase reactions, including reactions of the glass-ceramic material with carbon from the organic binder material to form silicon carbides. In particular, the second or further heating step continues until the assembly reaches a second target temperature where the glass-ceramic material flows and wets the interface between adjoining surfaces of the first ceramic part and the second part. Optionally, the assembly may be held at this second target temperature for a predetermined temperature soak time, generally between about 180 and 360 seconds.

(23) Finally, the assembly is rapidly cooled or quenched at a cooling rate of greater than about 100? C. per minute, and more preferably at a rate of about 140? C. per minute or more to induce nucleation and re-crystallization of the glass-ceramic material to form a dense, durable and gas-tight seal.

(24) These rapid secondary heating and rapid cooling steps (i.e. at high temperature ramp rates ?100? C. per minute) changes or alters the macrostructure of the glass seals being formed. Using the conventional slower temperature ramp rates for the secondary heating and cooling steps as disclosed in the prior art, the macroscopic nature of the glass-ceramic reveals comparatively larger pores distributed throughout the entire bulk material of the seal, as generally shown in FIGS. 3A and 3B. Conversely, as shown in FIGS. 4A and 4B, when using the rapid secondary heating and rapid cooling temperature profiles, the pores in the resulting seal are generally fewer and smaller than the pores associated with the conventional slower temperature ramp rates.

(25) Advantageously, the present glass-ceramic seal with the fewer pores and smaller pores shown in FIGS. 4A and 4B forms a stronger and tougher seals which translates to a significantly higher acceptance rate or yield for the manufactured oxygen transport membrane assemblies or modules (i.e. 50% to 90%) shown in FIG. 1 compared to the acceptance rate or yield for manufactured oxygen transport membrane assemblies or modules using the prior art sealing techniques (i.e. 10% to 20% yield). More importantly, from a comparative standpoint, the present glass-ceramic seal with the fewer pores and smaller pores is also a more hermetic seal having lower measured helium leak rates with estimated values around 1?10.sup.?9 cc per second. The porous seals shown in FIGS. 3A and 3B tended to have higher measured helium leak rates with estimated values around 1?10.sup.?5 cc per second.

(26) Use of an induction heating technique with a high temperature metal susceptor in lieu of traditional radiant heating techniques has lead to further reductions in cycle time for the present seal forming method as well as increased space for fixturing and alignment. Advantageously, the induction heating process using a metal susceptor allows precise control of the complex heating and cooling temperature profiles and also provides reliable, repeatable, non-contact heating of the assembly in a minimal amount of time.

(27) As described above, it is possible to make a gas-tight connection or seal directly between a ceramic oxygen transport membrane tube and a metal part, such as the illustrated metal connector, using the improved glass-ceramic sealing method described herein. However, in view of the greater difference in thermal expansion between the ceramic and metal parts, an area of high stress is likely to be created near the formed glass seal or proximate the sealing area. This added stress proximate the seal formed between a ceramic oxygen transport membrane tube and a metal connector can increase the number of premature failures of the ceramic tube assemblies which typically arise during the installation process or during thermal cycling of the oxygen transport membrane systems.

(28) To mitigate the problems associated the high stress areas in ceramic-metal joints or seals, the use of a dense ceramic adaptor disposed between the active ceramic oxygen transport membrane tube and the metal connector is suggested. The dense ceramic adaptor preferably has a coefficient of thermal expansion that is matched or closely matched to the coefficient of thermal expansion of the ceramic tubular oxygen transport membrane. More importantly, the dense ceramic adaptor is designed so as to absorb much of the stress that is caused by the differential thermal expansion characteristics between the ceramic part and metal connector.

(29) A further method to mitigate the problems associated with the high stress areas proximate the ceramic seals is to form a plurality of radial slots, grooves, or apertures in or through the walls of the metal connector or other metal part proximate the area of the glass-ceramic seal. By forming a plurality of slots, grooves, or apertures in the metal part radially disposed around the glass-ceramic sealing area, the glass-ceramic sealing material will tend to flow into the gaps or spaces created by the formed slots, grooves or apertures when the assembly is rapidly heated to the high temperatures required for glass-ceramic sealing and the metal part is expanded.

(30) Upon the rapid cooling of the assembly, the sealing material re-crystallizes in such gaps or spaces which would typically interfere with the contraction of the metal during the cooling process, which in turn transfers a portion of the stress from the sealing surface between the dense ceramic adapter and the metal connector to the glass-ceramic seal material in the gaps and spaces of the metal connector. As the metal connector absorbs a portion of the stress, the metal connector undergoes a slight deformation at or near the glass filled gaps thereby preventing the undesired brittle failures of the ceramic part. The stress induced deformation of the metal part does not generally affect the hermeticity of the formed seal.

(31) Turning now to FIG. 2, there is shown a cross section illustration of a seal area between a ceramic membrane adaptor 50 and a metal connector 60 that employs the present method of sealing. Although not shown, of the illustrated ceramic membrane adaptor 50 is preferably sealed to a ceramic tubular oxygen transport membrane using the improved glass-ceramic sealing method described herein. However, as discussed above, creating a strong, durable and hermetic seal between a ceramic part such as the ceramic membrane adaptor 50 and a metal part such as the metal connector 60 is more challenging due to the differences in thermal expansion characteristics between the parts. In the illustrated arrangement, the gas-tight seal is formed between the ceramic membrane adaptor 50 and a metal connector 60.

(32) The ceramic membrane adaptor 50 is a tubular shaped adaptor that includes a first female end 52 configured to receive a ceramic oxygen transport membrane tube (not shown) and a second male end 54 is configured to engage a corresponding female end 62 of the metal connector 60. The ceramic membrane adaptor 50 also defines a central bore 55 running between the first female end 52 and second male end 54 and that communicates with the interior of the oxygen transport membrane tube. In the illustrated embodiment, the central bore 55 further defines an interior shoulder 57 configured to receive and hold a corresponding structure protruding from the metal connector 60.

(33) The metal connector 60 is also a tubular shaped structure that has an annular coupling portion 64 at a female end 62 that operatively engages with the male end 54 of the ceramic adaptor 50. As seen in FIG. 2, the annular coupling portion 64 includes a base 61, an annular sheath structure 70 and a centrally disposed raised protrusion 77. The metal connector also includes a neck portion 66 extending from the annular coupling portion 64 to the other end 68 with a bore 75 that runs through a central axis of the tubular shaped metal connector 60.

(34) The centrally disposed raised protrusion 77 is configured to align and abut with the interior shoulder 57 of the ceramic membrane adaptor 50. The annular sheath structure 70 is configured to slide over and cover the male end 54 of the ceramic membrane adaptor 50 when the two parts are engaged. The annular sheath structure 70 has an interior surface 72 adjacent to the sealing area and an exterior surface 74. As described in more detail below, the annular sheath structure 70 includes a plurality of slots 75 extending from the interior surface 72 to the exterior surface 74 that are radially spaced around the circumference of the annular sheath structure 70.

(35) A glass ceramic bond agent 56, as described above, is placed at or near the interface 58 of the ceramic adaptor 50 and the metal connector 60, and more particularly proximate the annular sheath structure. When heated according to the process described herein, the glass-ceramic material flows into or wets the interface between adjoining surfaces of the ceramic membrane adaptor 50 and the metal connector 60 as well as into the slots 75 in the annular sheath structure.

EXAMPLES

(36) The following examples, disclose the making and use of a glass-ceramic based bond agent used to join the oxygen transport membrane tubes to the U shaped pipe-like adaptor at one end of the oxygen transport membrane tubes and to a gas manifold via a thru-block connector at the other end of the oxygen transport membrane tubes. The preferred glass-ceramic bond agent and resulting seal also requires a coefficient of thermal expansion that is close to or approximates the coefficient of thermal expansion of the components to be joined.

(37) For all examples, the bond agent was prepared by dissolving 12.8 grams of a phosphate ester (Emphos PS236) in a solvent comprising 337.7 grams of methyl ethyl ketone and 82.4 grams of ethyl alcohol (95%) and added to a jar containing 1000 grams of 0.5 inch diameter Burundum (96% Al.sub.2O.sub.3) milling media. About 1500 grams of Viox G9 Glass Powder (V2767) was then added and the solution was dispersion milled for about 24 hours. A plasticizer and binder were then added to the dispersion and subsequently mixed on rollers for another 24 hours. Specifically, the plasticizer and binder in the preferred embodiment included 101.9 grams of Polyvinyl Butyrai (Butvar B-79) and 88 grams of Butyl Benzyl Phthalate. The resulting slurry was placed in containers and de-aired for about 8 minutes in a vacuum at 25 Hg and then filtered through nylon mesh cloth with 150 micron openings. The final bond agent was then cast into a tape having a nominal thickness of about 0.020 and 8 inch width.

(38) The bond agent tape was disposed on adjoining surfaces of a tubular oxygen transport membrane having a dense 3YSZ dense layer and a dense 3YSZ ceramic adaptor configured to receive an end of the tubular oxygen transport membrane. The assembly of tubular oxygen transport membrane, ceramic adaptor and glass-ceramic bond agent was subjected to various temperature cycling profiles including one or more heating steps to remove or dissolve the organic binder from the tape and soften or melt the bond agent so as to react the silicon in the glass-ceramic material with residual carbon from the organic materials in the bond agent. The temperature cycling profiles also included one or more cooling steps to induce nucleation and re-crystallization of the glass-ceramic to form a seal adjoining surfaces of the tubular oxygen transport membrane and ceramic adaptor.

(39) For purposes of comparison, the original recommended temperature cycling profiles for making seals using the G9 Glass Powders involved heating the assembly in a furnace at 0.5? C./min from room temperature to about 350? C. and upon reaching the 350? C. target temperature, soaking the assembly at 350? C. for about 60 minutes. This was followed further heating of the assembly at a temperature ramp rate of about 5? C./min to 1200? C. and temperature soaking the assembly at 1200? C. for another 30 minutes. Finally, the assembly was cooled from 1200? C. to room temperature at a cooling rate of 5? C./min to form the glass-ceramic seal. The entire cycle time lasted approximately 20 hours.

Example 1

(40) Example 1 provides a baseline temperature cycling profiles for the joining or sealing of an oxygen transport membrane tube to an adaptor or connector. The baseline temperature cycling profiles of Example 1 are shown graphically in FIG. 5 and FIG. 6. As seen therein, the initial heating of the tube/adaptor/bond agent assembly was performed using an induction heating furnace at a temperature ramp rate of about 170? C./min to raise the assembly temperature from about 25? C. to 450? C. and without temperature soaking. The initial heating of the assembly continued at 170? C./min until reaching a first target temperature of about 835? C. This was followed further secondary heating of the assembly in three successive increments of 120 seconds each at different temperature ramp rates, including a ramp rate of 83? C./min to a temperature of 990? C.; a ramp rate of 46? C./min to a temperature of 1082? C.; and a ramp rate of 25? C./min to a max temperature of 1131? C. Finally, the assembly was cooled from 1131? C. to 984? C. at a cooling rate of about 49? C./min to re-crystallize the glass-ceramic seal. The resulting glass-ceramic seal was porous and had physical characteristics similar to those shown in FIGS. 3A and 3B. The entire cycle time lasted about 16 minutes.

Example 2

(41) Example 2 was performed to show the effectiveness of higher temperature ramp rates during both heating sequences and cooling sequences and with temperature soaking between steps. The temperature cycling profiles of Example 2 are shown in FIG. 5. As seen therein, the initial heating of the assembly using an induction heating furnace at a temperature ramp rate of about 200? C./min from 25? C. to 500? C. followed by a short temperature soak of 30 seconds and resuming the initial heating at about 200? C./min temperature ramp rate to about 950? C. followed by a second temperature soak lasting about 75 seconds. This was followed further heating of the assembly at a ramp rate of about 100? C./min to a temperature of 1150? C. followed by another temperature soak lasting about 330 seconds. Finally, the assembly was cooled from 1150? C. to 450? C. at a cooling rate of 140? C./min to nucleate and re-crystallize the glass-ceramic seal. The resulting seal was porous and had physical characteristics similar to those shown in FIGS. 4A and 4B. The entire cycle time lasted about 19 minutes.

Example 3

(42) Example 3 was also performed to show the effectiveness of higher temperature ramp rates during both heating sequences and cooling sequences but with different temperature soaking durations between steps. The temperature cycling profiles of Example 3 are shown in FIG. 6. As seen therein, the initial heating of the assembly using an induction heating furnace at a temperature ramp rate of about 200? C./min from 25? C. to 500? C. followed by a short temperature soak of 30 seconds and resuming the initial heating at about 200? C./min temperature ramp rate to about 950? C. followed by a second temperature soak lasting about 120 seconds. This was followed further heating of the assembly at a ramp rate of about 100? C./min to a temperature of 1150? C. followed by another temperature soak lasting only about 220 seconds. Finally, the assembly was cooled from 1150? C. to 450? C. at a cooling rate of 140? C./min to nucleate and re-crystallize the glass-ceramic seal. The resulting seal was porous and had physical characteristics similar to those shown in FIGS. 4A and 4B. The entire cycle time lasted about 18 minutes.

(43) While the present invention has been characterized in various ways and described in relation to preferred embodiments, as will occur to those skilled in the art, numerous, additions, changes and modifications thereto can be made to the present method without departing from the spirit and scope of the present invention as set forth in the appended claims.