# Patent

## Title
Process for producing tubular ceramic structures of non-circular cross section

## Abstract

Tubular ceramic structures of non-circular cross section, e.g., anode components of tubular fuel cells of non-circular cross section, are manufactured by applying ceramic-forming composition to the external non-circular surface of the heat shrinkable polymeric tubular mandrel component of a rotating mandrel-spindle assembly, removing the spindle from said assembly after a predetermined thickness of tubular ceramic structure of non-circular cross section has been built up on the mandrel and thereafter heat shrinking the mandrel to cause the mandrel to separate from the tubular ceramic structure of non-circular cross section.

## Background
BACKGROUND OF THE INVENTION
This invention relates to a process for producing tubular ceramic structures of non-circular cross section, to patterned arrays of such structures, in particular, of non-circular tubular anode-supported solid oxide fuel cell (SOFC) units, and to SOFC devices incorporating patterned arrays of non-circular tubular SOFC units.
Tubular ceramic structures are known for use as heat exchangers where corrosive liquids or gases are encountered, recuperators, catalyst bodies, as components of fuel cells devices, particularly SOFC devices, and in a variety of other applications.
Tubular ceramic structures can be produced in a broad range of lengths, wall thicknesses and cross-sectional areas and geometries employing any of several known and conventional techniques such as extrusion and clip coating. Each of these techniques for producing tubular ceramic structures generally, and tubular components of SOFCs in particular, is subject to certain inherent drawbacks and/or limitations.
In the case of extrusion, due to the need for the tubular extrudate to remain intact as it emerges from the extruder orifice, the ratio of the diameter of the tube to its wall thickness is typically low, e.g., under 15 and commonly under 10. This practical requirement tends to limit the usefulness of extrusion methods to the production of relatively thick-walled tubular ceramic structures. While relatively thick-walled tubular anodes can be advantageous for the construction of some types of SOFC devices, in particular, those intended for high power output (e.g., 20 KW and above), relatively thin-walled tubular anodes are generally preferred for the construction of SOFC devices of lower power output where their low thermal mass favors quicker start-ups and/or frequent on-off cycling.
The requirement for a relatively thick-walled extrudate, which can only be achieved with an extrudable material of fairly high viscosity, e.g., one of paste- or putty-like consistency, imposes yet another limitation on the usefulness of extrusion methods for the manufacture of tubular ceramic structures, namely, the need to carefully and completely dry the extrudate before subjecting it to such high temperature downstream processes as the burning out of organics (i.e., residual solvent(s), dispersant(s), binder(s), etc.) and sintering. The crying of the extrudate requires suitable control over such operational parameters as temperature, humidity and time. Too rapid drying and/or insufficient drying can result in the production of mechanical defects in the extrudate before and/or after carrying out either or both of the aforementioned high temperature post-extrusion processes.
Still another limitation of the extrusion technique is its inability to readily vary the composition of the extruded tube, e.g., to alter the composition of the tube in one preselected location but not in another.
In the case of dip coating, the requirement that the ceramic-forming composition be applied to a tubular substrate generally limits this technique to the production of structures in which the substrate becomes an integral, functional component of the final article. This requirement for a tubular substrate necessarily restricts the type as well as the design of those devices that can utilize a tubular ceramic article produced by the dip coating technique. Moreover, it is difficult in practice to provide tubular ceramic structures with relatively thin walls and/or with walls of uniform thickness employing dip coating.
There exists a need for a process for producing tubular ceramic structures that is not subject to any of the aforedescribed drawbacks and limitations of known and conventional extrusion and dip coating techniques. More particularly, there is a need for a process which with equal facility is capable of producing tubular ceramic structures over a broad range of wall thicknesses, i.e., from the very thin to the very thick, does not require close attention to and control of the conditions of drying, is readily capable of altering or modifying the composition of the tubular product for a defined portion thereof and does not require the use of a tubular substrate which is destined to become a permanent component of the product.
SUMMARY OF THE INVENTION
In accordance with the present invention, a process for producing tubular ceramic structures of non-circular cross section is provided which comprises:
a) rotating a mandrel-spindle assembly having a non-circular external cross section corresponding to the non-circular internal cross section of the tubular ceramic structure to be produced, the mandrel-spindle assembly comprising a mandrel component and a spindle component, the mandrel component being a heat shrinkable polymeric tube of non-circular cross section the external surface of which corresponds to the internal surface of the tubular ceramic structure of non-circular cross section to be produced and the internal surface of which defines a bore, the spindle component having a non-circular cross section corresponding to that of the bore of the mandrel and being in close fitting but slidably removable contact therewith;
b) applying a ceramic-forming composition to the external surface of the mandrel component of the rotating mandrel-spindle assembly to produce a tubular ceramic structure of non-circular cross section the internal surface of which is in contact with the external surface of the mandrel;
c) removing the spindle from the bore of the mandrel to provide a mandrel-tubular ceramic structure assembly in which the interior surface of the tubular ceramic structure of non-circular cross section remains in contact with the external surface of the mandrel; and,
d) heat shrinking the mandrel component of the mandrel-tubular ceramic structure assembly to cause the mandrel to undergo shrinkage to a reduced size in which the external surface of the mandrel separates from the interior surface of the tubular ceramic structure of non-circular cross section facilitating removal of the mandrel therefrom.
A major advantage and benefit of the foregoing process for producing a tubular ceramic structure of non-circular cross section lies in its ability to provide ceramic or cermet bodies over a wide range of ratios of length to external diameter and ratios of external diameter to wall thickness while meeting very precise predetermined dimensional tolerances.
Another advantage of the process herein for producing a tubular ceramic structure of non-circular cross section is its capability for readily and conveniently varying, or modifying, the ceramic-forming composition along the length of the structure. Ceramic-forming formulations of differing composition can be readily applied in a controlled manner to the external surface of the rotating mandrel at different rates and/or at different times during the production process. The degree of separation or blending of different ceramic-forming formulations during the production process can also be carefully controlled employing calibrated dispensing equipment known in the art to provide tubular ceramic products of non-circular cross section with enhanced performance capabilities compared with tubular products made by other fabrication techniques such as extrusion and dip coating.
The process of the invention can also utilize quick-drying ceramic-forming compositions thus dispensing with the need for a carefully conducted and monitored drying operation.
And, since the heat-shrinkable tubular mandrel upon which the tubular ceramic structure is first formed when carrying out the process of this invention is eventually separated from the tubular product, there is no requirement that the latter be permanently united to a tubular substrate as is the case with dip coating.



BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings in which like reference numerals refer to like elements:
FIG. 1A is an isometric view of a tubular SOFC unit of generally triangular cross section with portions partially cut away to better illustrate its anode, electrolyte and cathode components, the triangularly shaped anode component being advantageously produced in accordance with the process of the invention;
FIG. 1B is a cross sectional view of the tubular SOFC unit of FIG. 1A taken through a line perpendicular to the longitudinal axis of the latter;
FIGS. 2A and 2B illustrate patterned arrays of individual tubular SOFC units of FIG. 1 for incorporation in an SOFC device;
FIGS. 3A-3C illustrate other non-circular cross-sectional geometries of tubular anodes that can be produced in accordance with the process of the invention;
FIGS. 4A-4C illustrate the formation of a mandrel-spindle assembly of generally triangular cross section for use in the process of the invention;
FIGS. 5A and 5B illustrate the application of an anode-forming composition to the rotating triangularly shaped mandrel-spindle assembly of FIG. 4C employing an ultrasonic spraying operation to produce the triangularly shaped tubular anode component of the SOFC unit of FIG. 1;
FIG. 6 is a logic flow diagram for one embodiment of computerized control of the ultrasonic spraying operation shown in FIGS. 5A and 5B;
FIGS. 7A and 7B illustrate, respectively, the heating of the mandrel-tubular anode assembly to shrink the mandrel to its second further reduced size whereby the external surface of the mandrel separates from the interior surface of the triangularly shaped anode; and,
FIGS. 8A and 8B illustrate, respectively, perspective and plan views of an SOFC device incorporating the patterned array of tubular SOFC units of FIG. 2B.



DETAILED DESCRIPTION OF THE INVENTION
It is to be understood that the invention herein is not limited to the particular procedures, materials and modifications described and as such may vary. It is also to be understood that the terminology used is for purposes of describing particular embodiments only and is not intended to limit the scope of the present invention which will be limited only by the appended claims.
In the specification and claims herein, the following terms and expressions are to be understood as indicated.
The singular forms “a,” “an,” and “the” include the plural.
All methods described herein may be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language provided herein. e.g., “such as”, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
As used herein, “comprising,” including,” “containing,” “characterized by”, and grammatical equivalents thereof are inclusive or open-ended terms that do not exclude additional, unrecited elements or method steps, but will also be understood to include the more restrictive terms “consisting of” and “consisting essentially of.”
Other than in the working examples or where otherwise indicated, all numbers expressing amounts of materials, reaction conditions, time durations, quantified properties of materials, and so forth, stated in the specification and claims are to be understood as being modified in all instances by the term “about.”
It will be understood that any numerical range recited herein includes all sub-ranges within that range and any combination of the various endpoints of such ranges or sub-ranges.
It will be further understood that any compound, material or substance which is expressly or implicitly disclosed in the specification and/or recited in a claim as belonging to a group of structurally, compositionally and/or functionally related compounds, materials or substances includes individual representatives of the group and all combinations thereof.
The expressions “heat shrinkable polymer” and “shape-memory polymer” as used herein shall be understood as mutually inclusive.
The expression “ceramic-forming composition” shall be understood to include “cermet-forming composition.”
The expression “external surface of the mandrel component” shall be understood to include the initially bare, or uncoated, external surface of the mandrel, i.e., the external surface of the mandrel prior to the application of some other material thereto, and the external surface of any material that has accumulated upon the external surface of the mandrel during its deposition thereon.
The expression “tubular ceramic structure of non-circular cross section” shall be understood to include all shape-sustaining tubular ceramic structures anon-circular cross section whether in an intermediate or final stage of production. e.g., as including tubular ceramic structures in the green state, i.e., those containing organic matter such as dispersant, binder, etc., where present in the ceramic-forming composition from which the structures are formed, and the organic matter-free tubular ceramic structures resulting from the burning out of such matter or from a sintering operation.
All referenced publications are incorporated by reference herein in their entirety for the purpose of describing and disclosing, for example, materials, constructs and methodologies that can be used in connection with the presently described invention.
Tubular ceramic structures of non-circular cross section in general, and tubular anode-supported SOFCs of non-circular cross section in particular, may hold one or more advantages over tubular structures of circular cross section and comparable cross sectional area for certain kinds of applications. In the particular case of microtubular-type SOFC devices, the use of microtubular SOFC units of non-circular cross sectional geometry. e.g., that of a polygon and especially a regular polygon such as an equilateral triangle, square, equilateral pentagon, equilateral hexagon, etc., can permit a higher packing density, i.e., can allow for a patterned array containing a greater number of tubular SOFC units than that possible for an array of tubular SOFCs of circular cross section but comparable unit cross sectional area. This capability for higher packing density offers the potential for greater power output per unit volume of an SOFC device. In addition, the higher packing density that is achievable with tubular SOFC units of non-circular cross section compared with those of circular cross section may result in improved thermal stability and control of the SOFC device in which they are incorporated.
In addition to the foregoing advantages, such patterned arrays of tubular SOFC units of non-circular cross section may make it possible to maintain a smoother, less turbulent flow of oxidizing gas. e.g., air, to the cathodic surfaces and, as a result, reduced back pressure with accompanying lower energy demand on the gas flow management unit, e.g., a blower system.
Referring now to the figures, FIG. 1A is a perspective partially cut away view of tubular anode-supported SOFC unit 10 of generally triangular cross section (shown in cross section through line A-A in FIG. 1B) possessing an interior cermet-based, pore-containing anode component (i.e., fuel electrode) 11, the interior surface of which defines a bore, or passageway, 14, intermediate electrolyte component 12 and cathode component 13.
FIGS. 2A and 2B illustrate two of the many different patterns in which bundles, or arrays, of individual triangularly shaped anode units 10 of FIG. 1A (viewed in cross section as shown in FIG. 1B) may be arranged in an SOFC device. In FIG. 2A, the generally hexagonal array 20A of triangularly shaped tubular SOFC units 10 of FIG. 1A possesses a longitudinal passageway 21 for guiding the movement of air, initially directed toward the array at one end thereof, in a direction which is substantially parallel to its longitudinal axis, through the array and radially outwardly therefrom. In FIG. 2B, the generally rectangular array 20B of triangularly shaped tubular SOFC units 10 of FIG. 1 possesses a central lateral passageway 22 for guiding the movement of air, initially directed toward the length of the array in a direction which is substantially perpendicular to its longitudinal axis, through the array and radially outwardly therefrom. Such an arrangement is illustrated in the SOFC device of FIGS. 8A and 8B.
While the process of the invention is generally applicable to the production of all tubular ceramic structures of non-circular cross section. e.g., convex and non-convex polygonal cross sections, it will now be specifically illustrated for the production of triangularly shaped tubular anode component 11 of tubular SOFC unit 10 shown in FIG. 1.
Triangularly shaped tubular anode component 11 is produced from an anode-forming composition containing metal and ceramic compounds in particulate form. A slurry-forming amount of solvent or mixture of solvents such as water, organic solvent such as methanol, ethanol, propanol, 2-butoxyethanol, acetone, dimethylketone, methylethylketone, etc., or aqueous solution of one or more organic solvents such as any of the foregoing is used to provide a slurry of the particulates. Water is generally preferred for this purpose due to its negligible cost and its avoidance of environmental concerns such as flammability and toxicity which are generally associated with the use of volatile organic solvents.
The particulates may be maintained in suspension within the slurry with the aid of a dispersant, or suspending agent, of which many kinds are known in the art, e.g., polymer electrolytes such as polyacrylic acid and ammonium polyacrylate; organic acids such a citric acid and tartaric acid; copolymers of isobutylene and/or styrene with maleic anhydride, and ammonium salts and amine salts thereof; copolymers of butadiene with maleic anhydride, and ammonium salts thereof; and, phthalate esters such as dibutyl phthalate and dioctyl phthalate and mixtures of these and other dispersants.
An organic binder is incorporated in the anode-forming composition in order to solidify, or set-up, the anode-forming composition into a shape-sustaining mass as it is applied to the external surface of the rotating mandrel during the spraying operation shown in FIGS. 5A and 5B. The organic binder may be one that undergoes gelling by a physical mechanism, e.g., swelling in the presence of water and/or organic liquid, or by a chemical mechanism, e.g., crosslinking of polymer chains, or a combination of binders that individually undergo gelling, one by a physical mechanism, another by a chemical mechanism. Useful gelling binders include such water-soluble and/or water-dispersible substances as methylcellulose, hydroxymethylcellulose, polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyhydroxyethylmethacrylate, polyvinylpyrrolidone (also capable of functioning as a dispersant), polysaccharides such as starch, modified starch, alginate, gum arabic, agar-agar, and the like. Useful binders of the cross-linkable polymer variety include polyacrylamides, polyacrylates, polymethylmethacrylates, and the like, crosslinked situ employing known and conventional initiators such as peroxides, persulfates, etc.
One or more known or conventional additives such as plasticizers, e.g., polyethylene glycol, surfactants, foaming agents, defoaming agents, wetting agents, and the like, in art-recognized amounts can also be used to ensure a well-dispersed, homogeneous and eventually self-supporting composition (see R. J. Pugh et al., “Surface and Colloid Chemistry in Advanced Ceramics Processing”, Marcel Dekker. October 1993). The physical characteristics of these anode-forming compositions such as their viscosity and the time required for their transition from a fluid state to a shape-sustaining state can be controlled through selection of the components of the compositions and/or their amounts.
The ceramic material incorporated in anode component 11 (and in electrolyte component 13) can be stabilized-zirconia, preferably utilized for high operating temperature SOFCs (700° C. to 1000° C.). This includes preferably 8 mol % yttria-stabilized zirconia (“Y8SZ”), (ZrO2)0.92(Y2O3)0.08. Another useful material is doped-ceria, preferably used for intermediate operating temperature SOFCs (500° C. to 700° C.). This includes preferably gadolinium-doped ceria (“CGO”), (Ce0.90Gd0.10)O1.95. However, each of these materials may be employed over a wide range of temperatures. Of course, it is contemplated that other materials suitable for SOFC applications known in the art may be used.
The metal phase used in the anode and electrolyte components belongs, preferably, to the transition group of metals of the periodic table of elements, their alloys or physical mixtures. Nickel (Ni) is preferred, because of its high electrical conductivity under reducing atmosphere and its cost effectiveness. Metal may be introduced in the supported fuel electrode and cermet electrolyte via different precursors, known to those skilled in the art such as metal powders, metal oxide powders, and metal salts (aqueous or non-aqueous). Metal oxide powders, such as green NiO, are often preferred because of their cost effectiveness and their adaptability to ceramic processing. The use of fine metal oxide powders is particularly recommended for the cermet electrolyte processing since the metal will remain oxidized under SOFC operating conditions.
The metal phase range may vary from 30 vol to 80 vol % in the cermet anode. The thickness in the sintered state of the cermet anode will depend on the overall design of the fuel cell. For example, anode thickness in small diameter tubular fuel cells can range from 0.2 mm to 1.0 mm.
The metal phase range can vary from 0.1 vol % to 15 vol % in the cermet electrolyte. The thickness of the cermet electrolyte in the sintered state is preferably below 500 microns and most preferably is between 5-30 microns. The specific thickness chosen will often be determined by the size and design of the fuel cell as well as other factors apparent to those skilled in the art.
The viscosity of ceramic-forming composition can vary within fairly wide limits, e.g., from 1 to 500,000 cP at 20° C. For the ultrasonic spraying operation for making a tubular anode structure described, infra, in connection with FIGS. 5A and 5B, the viscosity of the anode-forming composition can range, e.g., from 1 to 100 cP at 20° C., and preferably from 5 to 20 cP at 20° C.′.
The use of a relatively thick-walled anode support, e.g., one having a wall thickness of from 0.9 to 5.0 mm with a diameter up to 500 mm, can allow the use of relatively thin subsequently formed electrolyte and/or cathode components, e.g., an electrolyte layer having a thickness of from 0.005 to 0.500 mm and/or a cathode layer having a thickness of from 0.010 to 1 mm. A reduced thickness for the electrolyte and/or cathode components can provide improved thermal shock resistance and electrochemical performance. Such improved mechanical stability and fuel cell performance may also enable the fuel cell to operate at a lower temperature. This in turn can allow the use of more cost-effective materials (e.g., stainless steel) within the fuel cell stack (e.g., for cell manifolding).
The use of relatively thin-walled anode support, e.g., one having a wall thickness of from 0.020 to 2 mm with a diameter up to 30 mm, can be advantageous for use, an noted above, in the construction of lower power output SOFC devices (e.g., below 20 KW and more commonly below 5 KW) where their lower thermal mass tends to better accommodate quicker start-ups anchor frequent on-off cycling.
The process of this invention also allows for the optional deposition of a thin interlayer between the anode and/or cathode component(s) of the SOFC and its electrolyte component. It can be advantageous to provide an optional interlayer thin film between anode 11 and electrolyte 12, between electrolyte 12 and cathode 13, or between electrolyte 12 and both anode 11 and cathode 13 as interlayer thin films can be made to increase fuel cell performance, e.g., through the use of catalytic materials, and/or prevent or inhibit adverse chemical reactions during sintering. An interlayer thin film can include one or more catalytically active materials such as doped cerium and gadolinium oxide (CGO), as previously disclosed, in a range of from 40 to 60 vol %, with the balance being Ni and Ru. Other catalytically active materials include scandium-stabilized zirconia (SSZ), again with the balance being Ni and Ru. An interlayer thin film can contain still other catalytically active components such as Pt, Pd and Rh to name but a few.
Referring to the drawings illustrating the production, in accordance with the process of the invention, of a tubular ceramic structure of non-circular cross section as exemplified by tubular anode component 11 of triangular tubular SOFC unit 10 of FIG. 1, the forming of mandrel-spindle assembly 45 of FIG. 4C is illustrated in FIGS. 4A and 4B.
As indicated above, the mandrel-spindle assembly of non-circular cross section that is employed in the process of the invention includes a mandrel component and a spindle component, the mandrel component being fabricated from a heat-shrinkable polymeric tube and having an external surface corresponding to the internal surface of the tubular ceramic structure to be produced and an internal surface defining a bore which is in close fitting but slidably removable contact with the external surface of the spindle component. Since the diameter of stock sections of heat shrinkable polymeric tubing seldom provide the requisite close fit, slidably removable contact, with the external surface of the spindle (whose external cross section and surface define the internal cross section and surface of the tubular ceramic structure, e.g., tubular anode, to be produced), it is often necessary to heat-shrink oversized stock tubing upon the spindle to provide the mandrel-spindle assembly for use in the production of a particular tubular ceramic structure in accordance with the process of the invention. One suitable procedure for providing the mandrel-spindle assembly employed in the process of the invention is illustrated to FIGS. 4A-4C.
As shown in FIG. 4A, mandrel-spindle subassembly 40 includes oversized cylindrically shaped mandrel 41 possessing a bore 42 of sufficient diameter as to loosely accommodate triangularly shaped spindle 43 and its optional closely fitting, removable or non-removable, friction-reducing polymer cladding, or covering 44. Mandrel 41 will generally possess a length corresponding to the length of tubular anode 41 but somewhat less than the full length of spindle 43.
Oversized mandrel 41 is fabricated from a heat shrinkable, or shape-memory, polymer numerous kinds of which are known in the art, e.g., those described in Lendlein et al., “Shape-Memory Polymers”, Angew. Chem. Int. Ed. 2002, 41. 2034-2057 (WILEY-VCH Verlag GmbH). Specific useful heat shrinkable polymers include, e.g., polyethylene terephthalate (PET), block copolymers of PET and polyethylene oxide (PET-PEO) and block copolymers of polystyrene and poly(1,4-butadiene) to name but a few.
Triangularly shaped spindle 43 can be formed from any suitably rigid material, i.e., one that resists flexing or other deformation when undergoing rotation, such as metal, e.g., aluminum, steel, bronze, etc., glass or other ceramic, non-reinforced or reinforced polymer, etc. Spindle 43 can be a solid structure as shown, a hollow structure such as a tube, a composite of different materials, e.g., a solid or hollow metal core whose exterior surface may optionally be clad with a friction-reducing polymer the function of which is to facilitate the removal of heat-shrunk mandrel 47 (shown in FIG. 4C) at a later point in the process of the invention. In the embodiment of triangular spindle 43 shown in FIG. 4A, the spindle is of solid metal construction, e.g., steel, clad with friction-reducing polymer layer 44. Optional cladding 44 can be fabricated from a friction-reducing polymer such as polyfluorotetraethylene (PTFE). In place of polymeric cladding 44, spindle 43 can be coated with a lubricious material. Suitable lubricious materials include organic lubricants such as liquid petroleum-based lubricants, natural and synthetic waxes, polyalphaolefins, and the like, and inorganic lubricants in particulate form such as boron nitride, graphite, molybdenum sulfide, and the like.
FIG. 4B illustrates the first heat shrinking treatment whereby expanded mandrel 41 of mandrel-spindle subassembly 40 is made to undergo shrinkage to a first reduced size providing close fitting, slidably removable mandrel 47 of mandrel-spindle assembly 45 illustrated in FIG. 4C. As shown in FIG. 4B, an array of mandrel-spindle subassemblies 40 are disposed between a pair of end plates 46, each end plate possessing an array of apertures 48 for receiving the cylindrical end portions of spindle 43 of each mandrel-spindle subassembly thereby supporting the subassemblies, in this particular case, in a substantially horizontal orientation. The supported array of mandrel-spindle subassemblies is then subjected to heat shrinking treatment carried out under conditions of temperature and time sufficient to cause each expanded mandrel 41 to undergo shrinkage to a first reduced size in which it assumes a close but slidably removable fit with its triangularly shaped spindle 43 thereby providing shrunken mandrel 47 of mandrel-spindle assembly 45 of FIG. 4C. For mandrel 41 fabricated from polyethylene terephthalate polymer, this first heat shrinking treatment can advantageously be carried out by subjecting the supported array of mandrel-spindle subassemblies 40 to a temperature within the range of from 105 to 180° C.′ for an exposure time of from 5 to 45 minutes.
As one alternative to the aforedescribed operation of heat shrinking oversized mandrel 41 directly onto triangularly shaped spindle 43, a length of oversized heat shrinkable polymeric tubing of a length equal to several lengths of oversized mandrel 41 can be heat shrunk upon a rigid support, e.g., a triangularly shaped stainless steel rod, optionally possessing a friction-reducing cladding or lubricant such as any of those mentioned, and having an external cross sectional geometry identical to that of spindle 43. Following the heat shrinking of the tubing to where it closely fits the exterior surface of the rigid support, the latter is removed, the heat-shrunk tubing is cut to individual lengths providing several heat-shrunk mandrels 47 and triangularly shaped spindle 43 is inserted in the bore of an individual mandrel 47 to provide the mandrel-spindle assembly of triangular cross section of FIG. 4C.
The selected anode-forming composition can be applied to the external surface of rotating mandrel-spindle assembly 45 employing any suitable means, e.g., spraying which is generally preferred, roller or brush-coating employing a doctor blade for removal of excess slurry, and similar procedures.
FIGS. 5A and 5B illustrate a preferred spraying procedure for applying an anode-forming composition such as those described above to the external surface of mandrel component 47 of triangularly shaped mandrel-spindle assembly 45 of FIG. 4C, namely, ultrasonic spraying, to provide anode component 11 of SOFC unit 10 of FIG. 1. Mandrel-spindle assembly 45 is mounted within traveling support frame 51 of ultrasonic spray apparatus 50 and securing held in place therein by spring-loaded spindle-locking member 52. Motor 53 rotates mandrel-spindle assembly 45 through flexible drive shaft 57 at adjustable rates, e.g., from 5 to 150 r.p.m., during operation of overhead stationary ultrasonic spray nozzle 54 which receives anode-forming composition from a remote source (not shown) and an atomizing gas, advantageously air. Feed pressures for both the anode-forming composition and the atomizing gas supplied to ultrasonic spay nozzle 54 and the distance between the tip of the spray nozzle and the external surface of mandrel 47 as with other spraying operational parameters can be adjusted to provide optimal spraying conditions for a particular anode-forming operation. In general, anode-forming composition and atomizing gas can be fed to ultrasonic spray nozzle 54 at pressures sufficient to deposit from 0.3 to 30,000 mg/sec of the composition upon the external surface of mandrel 57 with the distance between the tip of the nozzle and the external surface of the mandrel being maintained at from 0.5 to 10.0 cm.
Traveling frame 51 is repeatedly driven in back-and-forth cycles upon horizontal support track 55 by drive belt 56 at adjustable rates, e.g., from 0.1 to 100.0 cm/sec, during the spraying operation for a number of cycles sufficient to provide an anode of predetermined wall thickness, e.g., from 0.25 to 5.0 mm. The number of cycles required for a particular tubular anode structure will depend largely upon the wall thickness desired, the length of the anode, the quantity of anode-forming composition deposited upon the external surface of the rotating mandrel per unit of time and similar factors.
As shown in FIG. 5B, cam unit 59 at one end of drive shall 57 and companion cam unit 59′ at the spindle locking end of travelling support frame 51 are provided to maintain a substantially constant distance between the tip of spray nozzle 54 and the surface of mandrel 47 as triangularly shaped mandrel-spindle assembly 45 rotates. Each cam unit includes a retainer 60 for engagement with a cylindrical end portion of spindle 43 and tensioning springs 61 and 61′ attached at one end to the housing of the cam unit and at the other to retainer 60. Tensioning springs 61 and 61′ maintain rotating mandrel-spindle assembly 45 under tension as mandrel surface 74 follows spring-tensioned cam roller 62 which is held fixed in place within mount 63. As rotating mandrel-spindle assembly 45 moves up and down by the camming action through the vertical distance indicated by the arrows, flexible drive shaft 57 moves up and down in coordinate fashion by the same distance. By this simple camming action, the surface of rotating mandrel 47 will maintain approximately the same distance from the tip of spray nozzle 54.
It is of course, within the scope of this invention to change or modify One or more aspects of the spraying apparatus of FIGS. 5A and 5B, e.g., to provide a traveling (reciprocating) spray nozzle and a fixed support frame, to provide two or more spray nozzles capable of independent operation so as to alter the composition of the anode as it is being formed, to utilize a spray nozzle oriented in other than the overhead position shown, to provide 3-axis movement of the spray nozzle, etc.
FIG. 6 represents a logic flow diagram for one embodiment of a computerized system of control of the spraying operation illustrated in FIGS. 5A and 5B.
In the following operations for producing anode component 11 and as illustrated in FIGS. 7A and 7B, alter removing spindle 43 from the anode-coated mandrel-spindle assembly 45 resulting from the aforedescribed spraying operation, the spindle-free assembly, now designated mandrel-anode assembly 70 and possessing bore 71, is mounted upon vertical ceramic pin 72, there being sufficient clearance between interior surface 73 of mandrel 47 and external surface 74 of pin 72 to allow the mandrel when subjected to the second heat shrinking treatment to undergo further shrinkage thereby pulling away from interior surface 75 of tubular anode unit 11. For production efficiency, an array of vertically mounted mandrel-anode assemblies 70 as shown in FIG. 7A is subject to the second heat treating operation. As in the case of the first heat shrinking treatment shown in FIG. 4B, the temperature and time conditions for effecting this further shrinkage of the mandrel will depend to a large extent on the heat shrinking characteristics of the polymer from which the mandrel is made.
In the particular case where mandrel 47 is formed from polyethylene terephthalate, suitable conditions for the second heat shrinking treatment include a temperature of from 120 to 350 and an exposure time of from 1 to 100 minutes.
As a result of this second heat shrinking treatment, and as shown in FIG. 7B, mandrel 47 undergoes another reduction in size, i.e., to the second reduced size of mandrel 76 in which external surface 77 thereof completely separates from interior wall 75 of tubular anode structure 11 allowing the anode to be readily separated from mandrel 76 without incident and thereafter subjected, if desired, to one or more further production operations such as the formation thereon of one or more additional layers, interlayer thin film(s), electrolyte, cathode, etc., burning out of organics, sintering, and so forth. It is also within the scope of the invention to form a tubular structure upon mandrel 47 which is equal in length to several lengths of tubular anode 11 and thereafter to subdivide the tubular structure into the desired lengths of tubular anode 11.
The process of the invention is generally applicable to the production of all manner of tubular ceramic structures of non-circular cross section including, without limitation, tubular anode components of a SOFC unit encompassing a wide range of lengths, outside diameters and wall thicknesses. For example, the process can be used to provide a tubular ceramic structure, e.g., a tubular anode, possessing one of the following sets of dimensions:















External



Set of

Cross-Sectional
Wall


Dimensions
Length (mm)
Area (mm2)
Thickness (mm)







A
20 to 1000
1 to 50
0.100 to 5


B
50 to 500 
2 to 30
0.200 to 3


C
100 to 250 
5 to 20
 0.25 to 2









In the perspective and plan views of tubular SOFC unit 80 shown, respectively, in FIGS. 8A and 8B, blower system 81 provides a flow of gaseous fuel, e.g., hydrogen, to manifold 82 for introduction into and through passageway 14 of each tubular SOFC unit 10 constituting an SOFC array 20B of FIG. 2B. Second blower system 83 provides a flow of air, initially at ambient temperature, to manifold 84 from which it exits through outlets 85 to provide a source of oxygen for cathode component 12 of each tubular SOFC unit 10 of an SOFC array 20B. The air entering manifold 84 gains heat from the hot combustion gases exiting tail burner 86 into heat exchanger 87. The dotted lines show the flow path of the heated air exiting outlets 85 of manifold 84, and into, through and radially outwardly from central lateral passageway 22 of SOFC arrays 20B and into tail burner 86 where it provides oxygen to support combustion of unspent fuel present in the exhaust gas emerging from the tubular SOFC elements into exhaust manifold 88 and from there into the tail burner. Finally, the hot combustion gases enter heat exchanger 87 where they serve to preheat incoming air provided by second blower system 83 as previously indicated. Blower systems 81 and 83, which are of similar construction, are described in detail in copending, commonly assigned U.S. patent application Ser. No. 13/168,280, filed Jun. 24, 2011, the entire contents of which are incorporated by reference herein.
The following example is illustrative of the process of the invention for producing a triangularly shaped tubular anode support component of an SOFC unit in the green state, i.e., the state in which the anode is self-supporting but still contains organic components such as residual solvent, dispersant, binder, etc.
EXAMPLE
A green state tubular anode having the cross section of an equilateral triangle with rounded vertices is produced possessing the following dimensions: length of 230 mm, cross-sectional area of about 31.5 mm2 and wall thickness of 0.50 mm.
An anode-forming composition in the form of an organic solvent slurry is provided by combining the following ingredients in the indicated amounts:














Component
Amount (g)



















8-mol % yttrium zirconium oxide powder
2.10



NiO powder
3.90



methylethylketone (MEK)
10.0



polyvinylpyrrolidone (PVP) powder
2.00










The tubular anode is produced from the foregoing anode-forming composition employing the following operations.
(a) Forming the Triangularly Shaped Mandrel-Spindle Assembly
Stock heat-shrinkable polyethylene terephthalate (PET) cylindrical tubing having an outside diameter of 7.6 mm is divided into 230 mm lengths with each tubular section being weighed to within ±0.01 g accuracy. A triangularly shaped spindle of 305 mm length clad with a friction-reducing layer of polytetrafluoroethylene (PTFE) and possessing a cross sectional geometry corresponding to the triangularly shaped bore of the tubular anode structure to be produced is inserted into the bore of a PET tubular section to provide a pre-shrunk oversized mandrel-spindle subassembly. The subassembly is heated in a convection oven to 110° C. for 10 min. to shrink the PET tubular section (the oversized mandrel component of the subassembly) to the point where the mandrel becomes closely fitted to the spindle with the spindle being slidably removable therefrom. The thus-formed mandrel-spindle assembly is now ready for use in the spraying operation.
(b) Spraying the Anode-Forming Composition Upon the Surface of the Mandrel-Spindle Assembly to Form a Triangularly Shaped Anode in the Green State
The triangularly shaped mandrel-spindle assembly is installed in the traveling support frame of a FlexiCoat ultrasonic spray coating apparatus (Sono-Tek Corporation, Milton, N.Y.). The mandrel-spindle assembly is rotated about its longitudinal axis at a rate of 125 r.p.m. during the spraying operation. The surface of the rotating mandrel is maintained at a constant distance from the tip of the ultrasonic spray nozzle by the camming modification of the apparatus described infra. The ultrasonic spray nozzle delivers approximately 0.5 ml/sec of anode-forming composition in a slightly bowed-shape spray pattern of microdroplets to the surface of the rotating mandrel. By the time the spray impinges on the surface of the rotating mandrel, sufficient evaporation of the volatile component of the anode-forming composition, namely, its methylethylketone (MEK) slurry-forming component, has taken place so that the sprayed material, now semi-dry, adheres to the mandrel as a substantially uniform coating or layer thereon. Continuous back-and-forth (reciprocal) motion of the traveling support frame of the spraying apparatus results in the increasing accumulation of anode-forming composition on the surface of the mandrel. After a predetermined period of time (or number of spraying cycles), spraying is discontinued, the spindle is removed from the coated mandrel to provide a triangularly shaped mandrel-tubular anode assembly and the latter is weighed to within ±0.01 g from which it is calculated that 5.6 g anode-forming composition, now substantially devoid of its volatile MEK component, has been deposited on the mandrel as a tubular anode structure of generally triangular cross section having the dimensions indicated above.
(c) Heat-Treating the Mandrel-Tubular Anode Assembly to Further Heat Shrink the Mandrel
The triangularly shaped mandrel-tubular anode assembly is vertically supported upon a ceramic pin having a diameter that is smaller, e.g., 20-30% smaller, than the interior diameter of the mandrel. The supported mandrel is placed in a convection oven and heated to the target temperature of 250° C. at a heating and cooling rate of 1° C./min. and a dwell time at the target temperature of 60 minutes. As a result of this heat treatment, the mandrel shrinks and separates from the interior surface of the tubular anode from which the triangularly shaped mandrel is now readily removed.
Although the invention has been described in detail for the purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention which is defined in the claims.

## Claims

1. A process for producing tubular ceramic structures of non-circular cross section is provided which comprises:
a) rotating a mandrel-spindle assembly having a non-circular external cross section corresponding to the non-circular internal cross section of the tubular ceramic structure to be produced, the mandrel-spindle assembly comprising a mandrel component and a spindle component, the mandrel component being a heat shrinkable polymeric tube of non-circular cross section the external surface of which corresponds to the internal surface of the tubular ceramic structure of non-circular cross section to be produced and the internal surface of which defines a bore, the spindle component having a non-circular cross section corresponding to that of the bore of the mandrel and being in close fitting but slidably removable contact therewith;
b) applying a ceramic-forming composition to the external surface of the mandrel component of the rotating mandrel-spindle assembly to produce a tubular ceramic structure of non-circular cross section the internal surface of which is in contact with the external surface of the mandrel;
c) removing the spindle from the bore of the mandrel to provide a mandrel-tubular ceramic structure assembly in which the interior surface of the tubular ceramic structure of non-circular cross section remains in contact with the external surface of the mandrel; and,
d) heat shrinking the mandrel component of the mandrel-tubular ceramic structure assembly to cause the mandrel to undergo shrinkage to a reduced size in which the external surface of the mandrel separates from the interior surface of the tubular ceramic structure of non-circular cross section facilitating removal of the mandrel therefrom.

2. The process of claim 1 wherein the tubular mandrel is fabricated from shape memory polymer selected from the group consisting of polyethylene terephthalate, block copolymer of polyethylene urephthalate, block copolymer of polystyrene and poly(1,4-butadiene) and blends and alloys thereof.

3. The process of claim 1 wherein the mandrel-spindle assembly possesses the cross sectional geometry of a convex or non-convex polygon having rounded angles.

4. The process of claim 1 wherein the ceramic-forming composition is an anode-forming composition for producing the tubular anode component of a tubular solid oxide fuel cell.

5. The process of claim 4 wherein the anode-forming composition is applied to the external surface of the mandrel by ultrasonic spraying.

6. The process of claim 5 wherein a substantially constant distance is maintained between the tip of the spraying nozzle and the surface of the rotating mandrel-spindle assembly.

7. The process of claim 4 wherein the anode-forming composition comprises:
(i) particulate ceramic;
(ii) particulate source of metal;
(iii) slurry-forming liquid;
(iv) dispersant;
(v) where dispersant (iv) does not function as a binder, a binder and/or binder-forming material(s); and, optionally,
(vi) one or more additives selected from the group consisting of plasticizer, surfactant, foaming agent, defoaming agents and wetting agents.

8. The process of claim 7 wherein:
(i) particulate ceramic (i) is at least one member selected from the group consisting of yttrium oxide, zirconium oxide, cerium oxide, lanthanum oxide, gallium oxide, strontium oxide, magnesium, scandium oxide, samarium oxide, praseodymium oxide and mixtures thereof;
(ii) particulate source of metal (ii) is at least one member selected from the group consisting of nickel, copper, silver, platinum, ruthenium, rhodium, palladium, compounds and mixtures thereof;
(iii) slurry-forming liquid is at least one member selected from the group consisting of water, organic solvent and mixtures thereof;
(iv) dispersant or suspending agent (iv) is at least one member selected from the group consisting of polymer electrolyte, organic acid, copolymer of butadiene, isobutylene and/or styrene with maleic anhydride, phthalate ester and mixtures thereof; and,
(v) binder (v) is at least one member selected from the group consisting of gellable or swellable polymer, polymeric binder-forming mixture providing polymeric binder in situ, and mixtures thereof.

9. The process of claim 4 wherein the anode-forming composition possesses a viscosity of from 1 to 100 cP at 20° C.

10. The process of claim 4 wherein the formulation of the anode-forming composition is changed or modified during the applying step.

11. The process of claim 10 wherein the anode-forming composition is changed or modified during the applying step for a selected section or region of the tubular anode component.

12. The process of claim 1 wherein the ceramic-forming composition is applied to the external surface of the mandrel by spraying.

13. The process of claim 12 wherein a substantially constant distance is maintained between the tip of the spraying nozzle and the surface of the rotating mandrel-spindle assembly.

14. The process of claim 1 wherein the ceramic-forming composition is applied to the external surface of the mandrel by ultrasonic spraying.

15. The process of claim 1 wherein the ceramic-forming composition possesses a viscosity of from 1 to 500,000 cP at 20° C.

16. The process of claim 1 wherein the formulation of the ceramic-forming composition is changed or modified during the applying step.

17. The process of claim 1 wherein the external surface of the spindle possesses an anti-friction cladding or lubricious coating to facilitate removal of the spindle from the mandrel-spindle assembly.

18. The process of claim 1 wherein the tubular ceramic structure possesses one of the following sets of dimensions:















External



Set of

Cross-Sectional
Wall


Dimensions
Length (mm)
Area (mm2)
Thickness (mm)







A
20 to 1000
1 to 50
0.100 to 5


B
50 to 500 
2 to 30
0.200 to 3


C
100 to 250 
5 to 20
 0.25 to 2

19. Apparatus for producing tubular ceramic structures of non-circular cross section which comprises:
a) a support track;
b) a traveling frame movably mounted upon the support track;
c) a mandrel having a non-circular cross section and an external surface for supporting ceramic-forming material deposited thereon, the mandrel being rotatably mounted within the traveling frame;
d) a first driver for moving the traveling frame in back-and-forth cycles upon the support track;
e) a second driver for rotating the mandrel, the second driver being flexibly coupled to the mandrel to allow movement of the mandrel in a direction perpendicular to its axis of rotation; and,
f) a stationary spray nozzle disposed proximate to the surface of the mandrel and adapted to deposit a ceramic-forming composition thereon, the distance between the spray nozzle and the surface of the mandrel remaining substantially constant throughout a complete rotation of the mandrel.

20. The apparatus of claim 19 wherein the mandrel includes first and second cylindrical end portions and cams in spring-loaded tension contact therewith, the cams being configured to maintain a substantially constant distance between the spray nozzle and the external surface of the mandrel throughout a complete rotation of the mandrel.

21. The apparatus of claim 20 wherein the spray nozzle is an ultrasonic spray nozzle.

22. The apparatus of claim 20 wherein the spray nozzle is an ultrasonic spray nozzle and the second driver is flexibly coupled to the mandrel by a flexible shaft.

23. The apparatus of claim 19 wherein the spray nozzle is an ultrasonic spray nozzle.

24. The apparatus of claim 19 wherein the second driver is flexibly coupled to the mandrel by a flexible shaft.