Thin flexible sintered structures

Abstract: Thin inorganic sintered structures having strength and flexibility sufficient to permit bending without breakage in at least one direction to a radius of curvature of less than 20 centimeters, methods for making them, and products incorporating them, are described. Preferred sintered ceramic structures according to the invention can comprise zirconias, titanias, aluminas, silicas, rare earth metal oxides, alkaline oxides, alkaline earth metal oxides and first, second, and third transition series metal oxides and combinations thereof and therebetween. Sintered metal structures can also be provided.

Description: The present invention is directed toward flexible sintered structures. More specifically, the invention relates to flexible high strength inorganic structures such as inorganic sheets or tapes, made by combining powdered metallic, metalloid or, most preferably, oxide powders with appropriate liquid vehicle components and casting or otherwise shaping and sintering the resultant powder batches. Sheets, foils, ribbons, or other high aspect ratio products made in accordance with the invention can exhibit high hardness, flexibility, and toughness with excellent thermal stability over a wide range of temperatures. Thin flexible sintered structures are useful for a multitude of productive applications. They may be employed for electronic and/or electrooptic uses, such as waveguides, or as substrates for electronic coatings, superconductors, or high temperature superconductors. With improved mechanical properties, flexible ceramics could be useful as a protective layer for glass or other substrate materials where a layer of protection is needed to resist scratches. With sufficient structural flexibility in the flexible ceramic, the object to be protected could simply be wrapped for protection. Flexible inorganics, especially flexible ceramics, would offer unique advantages as chemically stable substrate materials. Porous ceramic materials are known to provide high surface areas. High surface area substrates provide desirable receiving surfaces for a variety of coatings. Alumina, for example, provides in its many crystalline forms an excellent surface for the application of catalysts. Porous or dense alumina which could be provided as a flexible ceramic foil and subsequently coated with a base or noble metal and/or oxide catalyst, or treated with zeolites, would have unique advantages for a variety of chemical applications. Sintered porous metallic foils, e.g., porous stainless steel foils, can be made and optionally oxidized or otherwise treated to provide high surface area metal-based substrates. Coated substrates of metallic or oxide type, formed into any desired honeycomb or other circular, laminar, and/or trapezoidal structures, would offer stable support in harsh environments where flexibility in combination with a specific substrate geometry would be particularly advantageous. Since the discovery of high temperature oxide superconductors, there has been widespread interest in combining these relatively brittle materials with strong flexible substrate materials to provide superconducting wires. Those skilled in the superconductor art have struggled to identify useful substrates for these superconductors. One suggestion has been to use metallic components to provide supporting substrates or jacketing for the superconductors. A particular disadvantage of metals, however, is the diffusivity of the metals at the sintering temperatures required for ceramic superconductor application, which could undesirably modify the compositions of the applied superconductor materials. Unlike metals, ceramic substrates are conventionally sintered at a higher temperatures than any of the yttrium barium copper oxide (YBCO), bismuth strontium copper oxide (BSCO) and/or thallium copper oxide families of high temperature superconductors, thus minimizing the diffusivity problem. Additionally, ceramics are more compatible with oxide superconductor coatings, due perhaps to improved wetting of the substrates by the coatings during coating application. Thus decreased interfacial discontinuities and increased substrate/layer stability are attainable. As those skilled in this art can appreciate, other metal and/or oxide and/or ceramic coatings would also benefit from this improved coating compatibility. Of course the production of thin and flexible ceramic fibers such as silicon carbide fibers and aluminosilicate fibers is well known. Ceramic fibers of these types are generally produced by spinning techniques or variations thereof. For example, Nicalon R (silicon oxycarbide) fibers, Nextel R (Al₂O₃-SiO₂-B₂O₃) fibers, and even Γ-alumina fibers are typically produced by spinning a fiber of a pyrolyzable precursor material and then pyrolyzing the spun fiber. Alternatively, fibers of alumina and zirconia can be produced by spinning a precursor material comprising fine oxide powder, followed by sintering to an integral oxide fiber product. Still other methods of fiber manufacture include the vapor deposition of precursors onto a starting or substrate filament and/or the spinning and optional heat treatment of glass fibers from molten glass. Although none of the fibers produced from precursors as above described are perfectly cylindrical, almost all are of very low aspect ratio, i.e., below 2:1. For a further discussion of the major fibers and their use in composites, reference may be made to Frank K. Ko, "Preform Fiber Architecture for Ceramic-Matrix Composites," Am. Ceram. Soc. Bull., 68 [2] 401-414 (1989). Unfortunately, while formed of inherently strong materials, long fibers of these ceramic materials are very weak. The weakness of fibers is simply due to the flaw populations in the fibers and the statistical laws which insure that most long fibers will include at least one defect of sufficient magnitude to cause failure at stress levels well below the inherent strength of the material. While the strength levels attainable depend of course on the number and size of the defects introduced into the fibers from batch or manufacturing process sources, the defect population needed to sustain successful production of strong long fibers is very small. Thus, for example, it can be calculated that, for fibers of 10 µm diameter comprising defect particles or voids of similar size, defect levels below 1 defect per each one hundred million parts of volume are needed to yield reasonable selections of strong kilometer-long lengths of fiber. Prior work in the field of thin film ceramics includes U.S. Patent No. 4,710,227 disclosing the preparation of thin flexible "green" (unfired) ceramic tapes from solutions, the tapes being coated and cut, stacked and fired to form thin-dielectric capacitors. This process is further described in published European applications EP 0302972 and EP 0317676. Capacitors with ceramic layers of 1-50 µm can be made; however, the capacitor fabrication process which is disclosed does not utilize the production or handling of sintered or fired (binder-free) flexible tapes in unstacked or unsupported form. In addition, the range of useful materials is limited by the ceramic process employed. Summary of the Invention The present invention solves many of the problems associated with prior applications and methodologies of flexible sintered structures. It provides an inorganic sintered ceramic sheet structure having a thickness not exceeding about 45 µm and a flexibility sufficient to permit bending without breakage in at least one direction to an effective radius of curvature of 1 cm or less, said sheet structure being obtainable by: (a) mixing inorganic constituents as a powder with one or more vehicle constituents to form a fluid batch; (b) forming the fluid batch on a fugitive polymer sheet into an elongated green sheet preform; (c) sintering the green sheet preform on the fugitive polymer sheet to provide a flat, smooth sintered inorganic ceramic sheet structure having a thickness not exceeding about 45 µm. The product of this invention is useful in any environment where a hard tough thin refractory flexible substrate and/or layer is needed. The flexibility will depend on layer thickness to a large measure and therefore can be tailored as such, for a specific use. Generally, the thicker the substrate the less flexible it becomes. Thinner substrates can be flexible to the point where toughened and hardened sintered materials may waft in a slight breeze, yet remain hard and tough to mechanical and/or thermal abuses. A use of this kind of strong flexibility could be as a diaphragm in a pump or valve. High surface areas can be created by manipulating porosity. Porosities are increased by manipulating sintering temperatures and/or including higher loadings of materials within the batch that burn out at firing temperatures. Porosities of the present invention can be as low as zero or as high as about 60%. Both porous and dense foils will maintain flexibility, due to the slight thickness of the product. Differential porosities in these materials can be useful for filtration and/or membrane operations. The chemical inertness as well as the surface morphology, thermal expansion, and flexibility of the present flexible inorganic substrates make them promising substrates for superconductor materials. The method of making the flexible inorganic ceramic sheet of the present invention comprises the steps of: (a) mixing the inorganic constituents as a powder with one or more vehicle constituents to form a fluid batch; (b) forming the fluid batch on a fugitive polymer sheet into an elongated green sheet preform; and (c) sintering the green sheet preform on the fugitive polymer sheet to provide a flat, smooth sintered inorganic sheet structure having a thickness not exceeding about 45 µm. Flexibility in the sintered material is sufficient to permit bending to an effective radius of curvature of 1 centimeter. or less, and preferably less than 0.5 centimeter or some equivalent measure. By an "effective" radius of curvature is meant that radius of curvature which may be locally generated by bending in a sintered body in addition to any natural or inherent curvature provided in the sintered configuration of the material. Thus the curved sintered ceramic products of the invention are characterized in that they can be further bent, straightened, or bent to reverse curvature without breakage. The cross-sectional thickness of the sintered structure on axes parallel to axes of applied force easily relieved by bending of the structure will not exceed about 45 µm, and preferably will not exceed about 30 µm. The lower limit of thickness is simply the minimum thickness required to render the structure amenable to handling without breakage. Sintered thicknesses of 4 µm can readily be achieved, and thicknesses on the order of 1 µm appear quite feasible. For thin sheet or tape structures, depending on the composition of the material, a single layer or a plurality of layers up to 500 µm in thickness can in some cases be made or assembled while still retaining some flexibility. However, for the desired low bending radius, sintered sheet or segment thicknesses will most preferably not exceed 30 µm, or even 10 µm, with thicker members being provided by layering the thin sintered sheets or other segments. Description of the Drawing The drawing is a plot of electrical resistivity versus temperature for a superconducting oxide coating disposed on a flexible ceramic substrate in accordance with the invention. Detailed Description Generally, the green material used in the invention is comprised of zirconias, aluminas, titanias, silicas (including zirconates, aluminates, titanates and silicates), rare earth metals and/or their oxides, alkalis and alkaline earth metals, and/or their oxides, steels, stainless steels, aluminides, intermetallics, aluminum and its alloys, the first, second, and third transition series of metals, their oxides, borides, nitrides, carbides, silicides, and/or combinations thereof and therebetween. Optional additions of sintering aids, dispersants, binders, plasticizers, toughening and hardening agents, and solvents can be advantageously present. The materials of interest especially include brittle materials. It is a particular advantage of the invention that structural flexibility can be achieved in sintered structures composed of materials which are normally considered to be brittle and inflexible. For the manufacture of the preferred green sheet or tape, a slurry or slip is preferably made from the green batch by the addition of sufficient solvent to obtain a fluid viscosity. The slurry or slip is then formed on a fugitive polymer sheet into a uniform thin sheet by a thin sheet forming means, for example doctor blading, rolling, mashing, extrusion or any means those skilled in the art use to make thin sheets or foils. The thin sheet is then heated to sintering temperatures. The resultant structure is a sintered strong material with multi-directional flexibility. Preferred ceramic compositions suitable for flexible substrate production in accordance with the invention include zirconia-based compositions. As is known, zirconia-based ceramic materials may optionally include the oxides of the transition series metals and the rare earth metal oxides. Stabilized zirconias, such as those stabilized by additions of alkaline earth metal oxides including for example magnesia and/or calcia, titanium and tin oxides, are preferred embodiments. Those compositions stabilized with yttria, are more preferred embodiments. Some other useful examples of stabilizers are those selected from indium oxide and the oxides of the rare earth metals such as lanthanum, cerium, scandium, praseodynium, neodynium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. The crystalline geometries of zirconia such as tetragonal, monoclinic, and/or cubic and their combinations are all important physical parameters of this structural material. Ceramic sheets or so-called ceramic foils can be made tougher by selecting certain toughening agents known to those skilled in this art. Particularly useful and preferred toughening agents are the oxides of tantalum and niobium which can be advantageously added to the above stabilizers. Reference to these toughening materials is made in published European patent application EP 0199459, published October 29, 1986, the substance of which is herein incorporated by reference in its entirety. That patent also discloses the properties of useful bulk materials such as α-alumina, β-alumina, β''-alumina, Al₂O₃-Cr₂O₃ solid solution, mullite, and spinel. These same materials can be usefully employed as ceramic body components and/or as companions to zirconia and the before-stated toughening agents. Combinations of titania and zirconia consisting essentially of 45 to 94.75 mole percent zirconia, 5 to 45 mole percent titania, and 0.25 to 10 mole percent rare earth metal oxides are found to be advantageous compositions for forming flexible substrates in accordance with the invention. Toughness and hardness properties are disclosed in U. S. Patent 4,753,902, the disclosure of which is incorporated herein by reference in its entirety. Combinations of molybdenum and tungsten oxides with magnesia, calcia, zirconia and rare earth metal oxides have also been found to provide useful ceramic materials. For instance, zirconia/hafnia-based compositions consisting essentially of about 79-99.5 mole percent of oxide components selected from the group consisting of ZrO₂, HfO₂, partially stabilized ZrO₂, partially stabilized HfO₂, ZrO₂-HfO₂ solid solution, and partially stabilized ZrO₂-HfO₂ solid solution, together with 0.25 to 15 mole percent of the before stated rare earth metal oxides and 0.25 to 6 mole percent of the oxides of molybdenum and/or tungsten, are found to be useful compositions. Optional supplemental additions of 0.5-10 mole percent of rare earth vanadates are useful in these formulations. The above zirconia/hafnia based compositions are disclosed in commonly assigned U. S. patent application Serial No. 07/245,523 now US-A-4,939,107, filed September 19, 1988, the disclosure of which, as filed, is herein incorporated by reference as filed. Further materials therein described include compositions consisting essentially of about 40 to 94.75 mole percent of the above zirconia/hafnia oxide components, 5 to 45 mole percent of SnO₂, and 0.25 to 15 mole percent of rare earth metal oxides, these providing particularly hard and tough ceramic materials. Also useful to provide flexible ceramics are compositions consisting essentially of about of 82 to 99 mole percent of one or more of oxides selected from the group consisting of ZrO₂, HfO₂, and ZrO₂-HfO₂ solid solutions, 0.5 to 10 mole percent of a stabilizer selected from the group yttria, scandia, rare earth metal oxides, ceria, titania, tin oxide, calcia, and magnesia, and 0.5-8 mole percent of toughening agents selected from the group consisting of yttrium and rare earth metal niobates, tantalates, and vanadates, and magnesium and calcium tungstates and molybdenates. These ceramics, characterizable as hard and tough ceramics exhibiting psuedo-plasticity, are disclosed in commonly assigned U. S. patent application Serial No. 328,532 filed March 24, 1989, now US-A-5,047,373. The invention also comprises thin flexible sintered ceramic structures as above described composed of certain recently developed hard refractory ceramic alloys. The ceramic alloys consist essentially of a novel zirconia alloy alone or in combination with a conventional refractory ceramic, the zirconia alloy constituting a least 5% and up to 100% by volume of the ceramic alloy. The conventional refractory ceramics for these alloys are selected from known materials. Typically, one or more ceramics selected from the group consisting of α-alumina, β-alumina, β''-alumina, alumina-chromia solid solutions, chromia, mullite, aluminum mullite-chromium mullite solid solutions, chromium mullite, sialon, nasicon, silicon carbide, silicon nitride, spinels, titanium carbide, titanium nitride, titanium diboride, zircon and/or zirconium carbide are used. The novel zirconia alloy, present alone or in a proportion of at least 5 volume percent as a toughening addition to the conventional refractory ceramic, will consist essentially of about: 35-99.75 mole % of oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solution and 0.25-45 mole % of one or more oxide additives selected in the indicated proportions from the following groups of additives. The first group consists of 5-45 mole % of titania and/or tin oxide. The second consists of 0-20 mole % total of metal oxides selected in the indicated proportions from the groups consisting of (i) 0-4 mole % of MoO₃ and/or WO₃, (ii) 0-10 mole % total of oxide compounds of the formula MM'O 4+/-δ wherein M' is V, Nb, Ta, or combinations thereof, M is Mg, Ca, Ti, Sn, Sc, Y, La, Ce, the rare earth metals, or combinations thereof, and δ is in the range of 0-1, and (iii) 0-6 mole % total of oxide compounds of the formula M''M'''O 4+/-δ wherein M''' is W and/or Mo, M'' is Mg, Ca, Ti, Sn, Sc, Y, La, Ce, the rare earth metals, or combinations thereof, and δ is in the range 0-1. In addition to one of the essential additives set forth above, the zirconia alloy may comprise, as optional additives, 0-20 mole % of cerium oxide, and 0-10 mole % total of oxides of one or more metals selected from the group consisting of Mg, Ca, Sc, Y, La, and the rare earth metals. In a more specific embodiment the zirconia alloy consists essentially of 35-94.75 mole % of oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solution, 5-45 mole % of titania and/or tin oxide, and 0.25-20 mole % total of oxides selected in the indicated proportions from the group consisting of 0-20 mole % cerium oxide and 0-10 mole % total of oxides of metals selected from the group of Mg, Ca, Sc, Y, La, and the rare earth metals. In yet another specific embodiment the zirconia alloy consists essentially of 70-99.5 mole % of oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solution, 0.5-10 mole % total of oxide compounds of the formula MM'O 4+/-δ wherein M' is selected from the group of V, Nb, Ta, and combinations thereof, M is selected from the group of Mg, Ca, Ti, Sn, Sc, Y, La, the rare earth metals, and combinations thereof, and δ is in the range of 0-1. Optional additions to these alloys include 0-20 mole % cerium oxide and 0-10 mole % of oxides of metals selected from the group of Mg, Ca, Sc, Y, La, the rare earth metals, and combinations thereof. In yet another specific embodiment the zirconia alloy consists essentially of 79-99.75 mole % of oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solutions, 0-15 mole % total of compounds selected in the indicated proportions from the group consisting of 0-7 mole % of oxides of Mg, Ca, Sc, Y, La, and the rare earth metals and 0-15 mole % of CeO₂, TiO₂, and/or SnO₂. The alloys further comprise one or more toughening agents selected from the group consisting of 0.25-6 mole % total of compounds of the formula M''M'''O 4+/-δ , wherein M''' is W and/or Mo, M'' is selected from the group consisting of Mg, Ca, Ti, Sn, Sc, Y, La, the rare earth metals and combinations thereof, and δ is in the range of 0-1, and 0.25-4 mole % of MoO₃ and/or WO₃. As noted, in addition to providing alloying additives to harden conventional ceramics as set forth above, the zirconia alloys alone may constitute the hard ceramic alloy material used to make flexible ceramic substrates in accordance with the invention. In the preferred method the invention takes compounds such as above described and produces thin flexible sintered sheets, foils, or ribbons therefrom. Of course, flexible whiskers and/or fibers may also be made from these materials, with good strength, but the very high strengths needed to provide strong, flexible ceramics providing dependable support properties in long lengths are not readily attainable in fiber and/or whisker configurations. In order to manipulate these compositions into flexible structures, novel processing methods are required. Heretofore, similar compositions were used for cutting tool inserts as disclosed in U. S. Patent 4,770,673, the disclosure of which is incorporated by reference. Due to their hardness and toughness after sintering, these compositions provided unlikely candidates for flexible ceramics. Nevertheless, it is now found that the following methodologies can successfully be used to embrace these materials and bodies within the family of thin sintered flexible materials. To provide high quality thin sheet materials, fine powders of the component composition are needed. Preferred particle sizes are less than 5 µm in diameter, most preferably less than 1.5 µm in diameter. The powder can be milled and separated to obtain the preferred powder size. To provide ceramic batches amenable to the appropriate forming techniques, the powdered batch materials are generally mixed with fugitive organic or inorganic vehicle formulations, most frequently formulations comprising one or more organic solvents. Examples of preferred organic solvents include mixtures of methanol and 2-methoxy ethanol. Other organic solvents that may be useful for this purpose are alcohols, ethers, aromatics, ketones, aldehydes, esters, alkanes, alkenes, alkynes, and or combinations thereof and therebetween. Inorganic solvents, particularly water, may additionally or alternatively be used as solvents. Also useful in the preparation of ceramic batches in accordance with the invention are powder dispersants. A large number of dispersants can be utilized for this purpose, including, for example, phosphate esters, polyether alcohols, polymeric fatty esters, polyelectrolytes, sulfonated polyesters, fatty acids and their alkali and ammonium salts, and combinations thereof and therebetween. An example of a specific and preferred dispersant is Emphos PS-21A dispersant, a phosphate ester dispersant commercially available from the Witco Chemical Co., New York, NY. Various plasticizers and binders known for use in the preparation of ceramic powder batches may also be included in the batch formulations of the invention. An example of a specific plasticizer which has been used is dibutyl phthalate, while a preferred binder is Butvar B-98 binder, a polyvinyl butyral binder commercially available from the Monsanto Company of St. Louis, Mo. Other binders that may be useful for this purpose include polyalkyl carbonates, acrylic polymers, alkyds, polyesters, cellulosic ethers, cellulosic esters, nitrocellulose, polyvinyl ethers, polyethylene glycol, polyvinyl butyral, polyvinyl alcohol, polyvinyl acetate, and slicones as well as copolymers, blends, or other combinations of the foregoing binder materials. When mixing with metals care must be taken to avoid pyrophoricity. Additionally, when sintering the metal compositions, an inert and/or reducing atmosphere, or a vacuum is necessary to enable the metals to sinter without oxidation. Advantageously, after sintering, the metals can then be oxidized as disclosed in U. S. patent application Serial No. 07/219,985 now US-A-4,912,077 filed July 15, 1988 the disclosure of which is herein incorporated by reference as filed. Once compounded and uniformly mixed, the batch is next formed on a fugitive polymer sheet into thin sheets or other preforms having thin flexible segments. This forming can be done by any means whereby a thin layer, sheet or web can be configured. Means such as doctor blading, pressing, rolling, extruding, printing, molding, casting, spraying, drawing, blowing, and combinations thereof and therebetween can provide green bodies incorporating thin segments or thin sheet configurations. Narrow ribbons or sheets many meters wide can be provided. Two methods have been found which improve the strength, formability, and handleability of the green structures. In the first, the extrusion and/or drawing of low viscosity slips is combined with immediate and direct contact between the thin extrudate and a gelling and/or drying liquid. This technique has been found to be advantageous for imparting green strength to the extruded or otherwise configured green bodies. Binder, solvent, and gelling liquid combinations may be chosen so that one or more of the solvents in the ceramic slip is highly miscible with the gelling and/or drying medium. Preferably, the binder employed for batches to be thus treated is not be miscible with the gelling and/or drying medium, to avoid binder loss during drying or gelling. Flocculation, gelation and/or drying are particularly useful for the extrusion of low-viscosity batch formulations. Slips with low initial viscosity can be extruded through fine orifices of complex shape into a gelling or drying liquid at relatively low extrusion pressures. With prompt gelling after extrusion, the extrudate gains strength and resists slumping and loss of shape definition. Thus shapes of complex configuration not otherwise extrudable, such as I-beam cross-sections or the like, can through rapid gelation be extruded with excellent shape retention in the green product. Gelation can be facilitated by pKa or pKb (pKs) adjustments of the slip or through the use of combinations of extrudate treating media and slip vehicle combinations which promote rapid gelation of the extrudate. Examples of suitable media/vehicle combinations include the following: Batch Vehicle Extrudate Treatment polyvinyl butyral/alcohol water polybutyl methacrylate/isopropanol methanol polymethyl methacrylate/tetrahydrofuran hexane polymethyl methacrylate/toluene hexane PKs adjustments can be effected by use of strong acids or bases and weak acids or bases, for example diethylamine. Weak acids such as propionic or acetic acid are preferred. The acid or base can be either organic or inorganic. A buffered system incorporated to adjust the pKs and/or maintain it within a certain range will also be effective. According to the invention the green material is formed on or in contact with one or more fugitive polymer layers or sheets. The processability and handleability of the green body are greatly enhanced through the support provided by such a polymer sheet. The material for the sheet or layer can be selected such that it provides initial support for the green body during subsequent sintering to a product, yet vaporizes without damage to the product in the same manner as the organic binders, dispersants and other organic constituents of the batch are vaporized. Vaporization of the fugitive polymer sheet or layer can occur before, during, or after other organic components of the green material are vaporized. Fugitive polymers which may be useful to provide such layers or sheets include acrylic polymers and co-polymers and polyalkyl carbonate polymers; optional sheet or layer components include plasticizers and waxes. These are generally though not necessarily free of inorganic powder additives. Green structures produced as described, whether provided in long continuous lengths or relatively short sheets, are typically sintered by treatment in a high temperature furnace. Long dwell times in the furnace are seldom required due to the low mass of green material present at any one time. For long continuous lengths of tape or ribbon configuration, the strength of the sintered material is frequently sufficient that the material itself can provide the drawing force needed to continuously draw unsintered green material through the furnace hot zone. As an aid to this process, it is useful to provide supporting setters within the sintering furnace which are angled downwardly in the direction of drawing. This provides a gravitational assist for the transport of the material through the furnace and reduces the draw tension required. The sintered structure of the invention can be used as a substrate for catalysis. Catalysts of interest for this purpose are the base metal and/or oxide catalysts, such as titanium, vanadium, chromium, cobalt, copper, iron, manganese, molybdenum, nickel, niobium, tantalum, tungsten, zinc, rare earth metals, alloys thereof and therebetween. Additionally, the noble metal catalysts, such as platinum, palladium, silver, rhodium, gold can be combined with the substrate. In combining the catalyst with the substrate, the combination can be by chemical vapor deposition, by coating with a high surface area base coating with a subsequent catalyst overcoat, by impregnating the substrate with the catalyst, or simply mixing the catalyst with the batch prior to sintering. The present invention can be incorporated as a structural material within other compositions as in a composite. For example, by drawing in narrow elongated form, the sintered material can be made part of another structural material, adding new strength and/or toughness to the material. Both metals and ceramic materials can be used in this manner. The following examples are illustrative of the various means to practice the invention herein disclosed, and are not intended to limit the scope of the invention. Example 1 To prepare a green ceramic material, 100 grams of yttria-stabilized zirconia powder (commercially available as TZ-2Y powder from the Tosoh Chemical Company of Tokyo, Japan and comprising 2 mole percent Y₂O₃ as a stabilizer) was milled with a mixture of 30 grams of methanol and 24 grams of 2-methoxy ethanol containing 0.25 grams of a phosphate ester dispersant. The dispersant is commercially available as Emphos PS-21A dispersant from the Witco Chemical Co. of New York, NY. This batch was designated as Batch A. Batch B was comprised of 100 grams of the zirconia powder, 16 grams of 2-methoxy ethanol, 20 grams of methanol, and 0.25 grams of Emphos PS-21A dispersant. The batches were milled with 1/2-inch zirconia balls. The milled batches were placed in 250 ml wide mouth Nalgene™ polyethylene bottles and then subsequently placed on a vibratory mill for 76 hours. The particle size distribution in the final batches averaged from about 0.88-1.20 µm, measured on a Leeds and Northrup Microtrac particle size analyzer. Similar particle size data obtained using a Horiba Capa-500 analyzer from Horiba LTD. of Kyoto, Japan suggest that particle sizes produced by the described milling procedure could be lower than the above reported values by a factor of from 3 to 5, but in any case particle sizes of the order of 0.1-1.2 µm predominate in these batches. The viscosity of the slip after milling ranged from about 4.2 mPa.s to 11.5 mPa.s at 39.2 sec⁻¹, with slips made from oven dried powders showing the lowest values. A viscosity within the range of 3 to 15 is preferred. Coarse particles were removed from the slip by settling for 3 days, then removing the fluid portion from the settled sludge. An alternative separation procedure is to centrifuge the slip at about 2000 rpm for 10 minutes. Next added to the slip of Batch A was 2.5 grams of glacial acetic acid, with 2.25 grams of glacial acetic acid being added to Batch B. The addition of the acid helped to develop a flocculated state which is evinced by thixotropy. The degree of thixotropy was dependent upon the amount of acid added. The slip of Batch A was then mixed with 6 grams of polyvinyl butyral binder, commercially available as Butvar B-98 binder from the Monsanto Company, and 3 grams of dibutyl phthalate. Batch B was mixed with 10 grams of methanol, 8 grams of 2-methoxy ethanol, 6 grams of the binder and 2.9 grams of dibutyl phthalate. The acid was added before the binder, making the binder easier to dissolve. The slips were shaken vigorously for 5 minutes, placed on a roller, and turned slowly for several hours to de-air. Some of the samples of the slip batch were further de-aired in vacuo. The final viscosity of the slips as measured on a Brookfield viscometer was within the range of 1500 to 5000 mPa.s at 8.7 sec⁻¹. Batch A, specifically, had a viscosity of 3470 mPa.s in this test. A fugitive polymer solution was prepared in a polyethylene bottle by dissolving 40 parts by weight polymethyl methacrylate (fugitive polymer) in 60 parts of ethyl acetate. The solution was placed on a roller mill to mix. The acrylic polymer solution thus provided was then cast onto a polyester substrate film using a doctor blade to form thin acrylic sheet. The polymer-coated substrate was then placed in a 60-70°C drying oven for 30 to 60 minutes. A slip containing yttria-stabilized tetragonal zirconia was then prepared utilizing the materials and procedures used to make zirconia Batch A. The ceramic slip was then cast over the acrylic layer using a doctor blade. The carrier film with the acrylic and ceramic layers of coating was transferred to a drying oven for 30 to 60 minutes. The thickness of the fired films was varied as a function of the height of the doctor blades. Thinner or thicker sheets were made by the proper choice of doctor blades and slip viscosity. The lower viscosity slips and smaller blade heights yielded thinner tapes. The thinnest sheets were produced by thinning a slip such as Batch A with solvents. To 10 grams of the slip were added 1.11 grams of methanol and 0.88 grams of 2-methoxyethanol. Slaps with a viscosity of 1500 mPa.s or less may be advantageously made by this technique. The thinned slip thus provided was cast with a 51 or 76 µm (2 or 3 mil) blade on top of a fugitive acrylic layer cast as above described with a 102 µm (4 mil) blade. This produced a composite tape with about a 6 µm green ceramic layer which sintered to around 5 µm. The firing schedule was as reported in Table 2 below: Start Temperature End Temperature Time Room Temperature 200 120 min 200 500 360 min 500 1420 375 min 1420 1420 120 min 1420 Room Temperature 120 min Data which were obtained for 1 to 2 mm wide ribbons of sintered ceramic made in accordance with the Example are given below in Table 3: Sample Thickness (10⁻⁶ m) Bend Radius (10⁻⁶ m) Strength (GPa) 1 6.0 362 1.77 2 5.2 344 1.61 3 11.5 710 1.72 4 11.5 725 1.69 5 11.5 914 1.34 6 11.5 850 1.44 7 16.0 1400 1.22 8 16.5 1520 1.15 Samples 9 and 10 above were made from a thinned slip, while the remaining samples were made with the standard Batch A slip. The scatter in the measurements increased for wider samples, such as the 3.5 mm wide sample as shown in Table 4. Sample Thickness (10⁻⁶ m) Bend Radius (10⁻⁶ m) Strength (GPa) 9 11.5 1020 1.20 10 11.5 737 1.66 11 11.5 1180 1.04 The thicknesses of the 5 to 6 µm ribbons were determined from optical micrographs. The other thicknesses were measured with a micrometer. Sintered 8 µm-thick sheets as large as 9.5 cm by 9.5 cm and a 4 µm-thick sheet 6 cm by 7 cm have been made with the fugitive polymer. Such sheets were transparent enough to read through. Even thinner sheets can be made by setting the doctor blade for the ceramic slip casting step at zero clearance. Under this condition only the bulk of the ceramic slip causes greater than zero clearance on the blade; thus a residual, very thin, slip layer is provided. Example 2 Small 1.8 mm x 4.8 mm x 8 µm rectangular pieces of ceramic tape were formed by screen printing an ink consisting of ceramic slip onto a fugitive polymer which had been cast on a Mylar R casting film. The ink used was prepared by mixing a slip conforming to Batch A of Example 1 with excess binder and enough 2-methoxyethanol to yield a thin consistency. After the printed images were dry, the printed images with fugitive polymer were released from the Mylar R casting film The images were then placed polymer side down onto a zirconia setter and sintered to 1420°C for 2 hours. The sintering schedule shown below in Table 5 was used: Start Temp. (°C) Stop Temp. (°C) Time (minutes) Room Temp. 150 60 150 500 420 500 1420 375 1420 1420 120 1420 Room Temp. 120 The ceramic tape samples produced by this process were strong and flexible. Example 3 Ceramic sheet samples approximately 1 cm wide were formed by printing with a rubber stamp onto a fugitive polymer as in Example 2 using the Batch A slip with enough additional t-butyl alcohol solvent to form a printable ink. After sintering using the schedule of Table 5, the resulting ceramic pieces accurately reflected the original image. These pieces retained image details with widths of 140 µm. Example 4 High temperature superconductor coatings were applied to flexible zirconia substrates using a laser ablation technique, known to those skilled in this art. Laser ablation targets of two distinct compositions were used, denominated Compositions X and U, the compositions being of the form Y₁Ba 2-x Ag x Cu₃O 7-δ wherein the mole proportions of the components were as follows: Composition X U (coefficient in formula) Y 1.0 1.0 Ba 2.0 1.85 Ag(x) 0.0 0.15 Cu 3.0 3.0 These compositions are disclosed in in U. S. Patent Application Serial No. 07/315,326, filed February 24, 1989, (EP-A-384,561) the disclosure of which, as filed, is herein incorporated by reference. These materials were deposited on both flexible ceramic tape and single crystal cubic zirconia using the laser ablation technique. The samples were then annealed according to the following schedule: Starting Set Temperature °C Next Set Temperature °C Time Minutes Room Temperature 600 60 600 600 60 600 700 15 700 700 60 700 850 15 850 850 360 850 Room Temperature 180 Following annealing, silver metal was evaporated onto the coated substrates in order to form electrical contacts for measurements and the coated substrates were heated again in oxygen to 300°C. Data indicating the electrical resistivities for Compositions X and U on tape and on single crystal cubic zirconia are shown in the Drawing. The Drawing plots the normalized resistance of the samples as a function of temperature over the temperature range from near 0° K to 100° K, setting unit resistance at the higher temperature. Curve A plots data for Composition U on a flexible ZrO₂ substrate, Curve B for Composition U on single crystal ZrO₂, Curve C for Composition X on flexible ZrO₂, and Curve D for Composition X on single crystal ZrO₂. As is clearly demonstrated by the data presented, continuous high temperature superconductor coatings have been provided on these flexible substrates. Example 5 Four narrow (1 mm to 2 mm wide) zirconia ribbons of 20 µm thickness were coated with Composition U of the previous example using laser ablation. The samples were annealed in flowing oxygen using the schedule of the previous example, except that temperatures measured were 661°C for the 600°C set, 761°C for the 700°C set, and 865 °C for the highest set temperature (for which the set point was 810°C). Silver electrodes were then laid down on the coatings by vacuum vapor deposition and the samples heated in oxygen to 300°C. One coated and annealed ribbon from this example was bent to test its strength after heat treatment. The thickness of the substrate was 20 µm, the sample was bent with the coating in compression to a radius of 1.9 mm without breaking. From these data the calculated strength was at least 1.11 GPa. No visible degradation occurred to the coating or the substrate. Example 6 A 0.5 µm coating Nb₃Sn coating was applied to zirconia ribbon substrates by co-sputtering Nb metal and Sn metal using a CVC rf sputtering apparatus. Niobium metal and tin metal were used for targets. The substrates were 25 µm x 1.65 mm x 2.5 cm zirconia ribbons which contained 2 mole % yttria. The coated ribbons were annealed at 960°C in vacuum for 1 hour in order to homogenize the alloy. The annealed alloy adhered to the substrates and could be bent to a radius of less than 5 mm in either direction (with the coating in either compression or tension) without either the coating or the substrate suffering visible damage. The x-ray diffraction pattern for a film produced concurrently on an alumina substrate showed a cubic material with lattice parameter of 53.4 nm (5.34 angstroms), close to the literature value of 52.91 (5.291) for Nb₃Sn. The film on the flexible zirconia substrate was found to superconduct with a Tc of 18K. Example 7 Flexible zirconia tapes were coated with silver metal by evaporative vapor deposition, followed by heating of the composite to 300°C. The product was a non-ductile, flexible conducting composite. Thus flexible conductors which may be fatigue resistant composites were provided. Example 8 A 1.7 mm wide sintered ribbon of ZrO₂ with 2 mole percent Y₂O₃ was adhered to a 3 mm thick 4.4 cm OD sintered zirconia ring using a zirconia slip having the composition of Batch A of Example 1. The body so formed was sintered to 1430°C for 2 hours. After sintering, the ribbon was affixed to the tube. Example 9 A slip was made with zirconia comprising 2 mole percent Y2O3 as a stabilizer. The initial batch was prepared in a 250 ml polyethylene bottle and contained 100 g of ceramic powder which had been dried in an oven at 400°C, 24 g of 2-methoxy ethanol, 28 g of methanol, 1.0 g of Emphos PS-21A, and 400 g of ZrO2 milling media. The batch was milled for 70 hours, poured into a 125 ml polyethylene bottle and left to settle for 168 hours. The batch was pipetted off the sediment into another 125 ml bottle and left to settle for an additional 24 hours. The batch was again pipetted off the sediment and into a 125 ml bottle. The twice-settled batch contained approximately 74.9 g of ceramic powder. The mean particle size was 0.38 µm as measured on a Horiba Capa-550. To this slip were added 1.69 g of acetic acid, 2.27 g of dibutyl phthalate, and 4.50 g of polyvinyl butyral. The slip was rolled on a ball mill to dissolve the binder and homogenize the slip. The final viscosity was 598 mPa.s at 8.7 sec-1. Two fugitive polymer solutions were prepared in 60 ml polyethylene bottles. One solution was prepared by first adding 0.05 g of water to 16 g of medium molecular weight polymethyl methacrylate from Aldrich Chemical Co., Inc. of Milwaukee, Wisconsin, rolling overnight, and warming to 60 °C in an oven. To this were added 32 g of ethyl acetate and 2 g of dibutyl phthalate. Likewise a second fugitive polymer solution was prepared but with 0.05 g of water, 15.6 g of polymer, 29.4 g of ethyl acetate, and 5 g of dibutyl phthalate. The first fugitive polymer solution was cast on a 51 µm (2 mil) Mylar R polymer carrier using a doctor blade with a 25 µm (1 mil) clearance. This was dried in an 60 °C oven for several minutes. The slip containing ceramic powder was then cast over the first fugitive polymer layer using a blade with a 25 µm (1 mil) clearance. This was also dried for several minutes in a 60°C oven. The second fugitive polymer solution was then cast over both of the previous layers, again with a blade with a 25 µm (1 mil) clearance, thus forming a 3-layer sandwich structure. The cast structure was released from the Mylar R carrier film, cut to size with a rotary blade and fired to 1450 °C for 2 hours. In this way strong, flexible, refractory 10 cm by 12 cm sheets were prepared which were 8 - 10 µm thick when measured with a micrometer. While the invention has been particularly described above with respect to specific materials and specific procedures, it will be recognized that those materials and procedures are presented for purposes of illustration only and are not intended to be limiting. Thus numerous modifications and variations upon the compositions and processes specifically described herein may be resorted to by those skilled in the art within the scope of the appended claims.

Claims: An inorganic sintered ceramic sheet structure having a thickness not exceeding about 45 µm and a flexibility sufficient to permit bending without breakage in at least one direction to an effective radius of curvature of 1 cm or less, said sheet structure being obtainable by: (a) mixing inorganic constituents as a powder with one or more vehicle constituents to form a fluid batch; (b) forming the fluid batch on a fugitive polymer sheet into an elongated green sheet preform; (c) sintering the green sheet preform on the fugitive polymer sheet to provide a flat, smooth sintered inorganic ceramic sheet structure having a thickness not exceeding about 45 µm. A sheet structure as claimed in claim 1 characterized in that it is in the form of a long, continuous sheet or tape, and wherein the step of sintering the preform comprises drawing a long continuous length of the preform through a hot zone in a sintering furnace. A sheet structure as claimed in claim 1 or claim 2 characterized in that it is a ceramic structure comprising zirconias, titanias, aluminas, silicas, rare earth metal oxides, alkaline oxides, alkaline earth metal oxides and optional additions of first, second and third transition series metal oxides and combinations thereof and therebetween, or a metallic structure comprising metals selected from the group consisting of rare earth metals, the first, second, and third transition series of the periodic chart, steels, stainless steels, aluminum, aluminides, intermetallics, alloys thereof and combinations therebetween, optionally further comprising one or more stabilizing agents selected from the group consisting of the oxides of magnesium, calcium, yttrium, titanium, scandium, tin, and rare earth metals selected from lanthanum, cerium, praseodynium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, and optionally further comprising one or more toughening agents selected from the group consisting of niobia, vanadia, tungstia, molybdena, and tantala. A structure as claimed in claim 3 which structure comprises a zirconia alloy and, optionally, a hard refractory ceramic, the ceramic alloy containing at least 5 volume percent of the zirconia alloy, wherein the zirconia alloy consists essentially of: 35-99.75 mole % of one or more oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solution, and 0.25-45 mole % of additives selected from the following groups: 5-45% mole % of titania and/or tin oxide, 0.25-20 mole % total of one or more oxide compounds selected in the indicated proportions from the groups consisting of (i) 0-10 mole % MM'O 4+/-δ wherein M' is selected from the group consisting of V, Nb, and Ta, M is selected from the group consisting of Mg, Ca, Ti, Sn, Sc, Y, La, and the rare earth metals, and δ is 0-1; (ii) 0-6 mole % M''M'''O 4+/-δ wherein M''' is W and/or Mo, M'' is selected from the group consisting of Mg, Ca, Ti, Sn, Sc, Y, La, and rare earth metals, and δ is 0-1; and (iii) 0-4 mole % MoO₃ and/or WO₃; and said alloy optionally additionally comprising 0-10 mole % of oxides of one or more metals selected from the group consisting of Mg, Ca, Sc, Y, La, and the rare earth metals, and 0-20 mole % of cerium oxide. A structure as claimed in claim 3 or claim 4 characterized in that the zirconia alloy consists essentially of: 35-94.75 mole % of oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solutions; 5-45 mole % titania and/or tin oxide; and 0.25-20 mole % total of oxides selected in the indicated proportions from the group consisting of 0-20 mole % cerium oxide and 0-10 mole % of oxides of one or more metals selected from the group of Mg, Ca, Sc, Y, La, and/or the rare earth metals. A structure as claimed in claim 3 or claim 4 characterized in that the zirconia alloy consists essentially of: 70-99.5 mole % of one or more oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solution; 0.5-10 mole % MM'O 4+/-δ wherein M' is one or more metals selected from the group consisting of V, Nb and Ta, M is one or more metals selected from the group consisting of Mg, Ca, Ti, Sn, Sc, Y, La, and the rare earth metals, and δ is 0-1; 0-20 mole % of cerium oxide; and 0-10 mole % of oxides of one or more metals selected from the group consisting of Mg, Ca, Sc, Y, La and the rare earth metals. A structure as claimed in claim 3 or claim 4 characterized in that the zirconia alloy consists essentially of: 79-99.75 mole % of oxides selected from the group consisting of zirconia, hafnia, and zirconia-hafnia solid solution; one or more toughening agents selected in the indicated proportions from the group consisting of: 0.25-6 mole % M''M'''O 4+/-δ wherein M''' is W and/or Mo, M'' is selected from the group consisting of Mg, Ca, Ti, Zn, Sc, Y, La, and the rare earth metals, and δ is 0-1; and 0.25-4 mole % MoO₃ and/or WO₃; and, optionally, 0-15 mole % total of one or more constituents selected in the indicated proportions from the following groups: 0-15 mole % CeO₂, TiO₂, and/or SnO₂; and 0-7 mole % of one or more of the oxides of Mg, Ca, Sc, Y, La, and the rare earth metals. A structure as claimed in any of claims 3 to 7 characterized in that the refractory ceramic is selected from the group consisting of α-alumina, β-alumina, β''-alumina, alumina-chromia solid solutions, chromia, mullite, aluminum mullite-chromium mullite solid solutions, chromium mullite, sialon, nasicon, silicon carbide, silicon nitride, spinels, titanium carbide, titanium nitride, titanium diboride, zircon and/or zirconium carbide. A structure as claimed in any of claims 3 to 7 characterized in that the oxide components are substituted at least in part by components selected from the group consisting of borides, nitrides, silicides, carbides and combinations thereof. A structure as claimed in any of claims 1 to 9 characterized in that it is a flexible ceramic sheet or tape, or a laminar structure comprising two or more layers of the flexible ceramic sheet or tapes. A structure as claimed in any of claims 1 to 10 characterized in that it has a porosity of up to about 60%. A structure as claimed in any of claims 1 to 11 characterized in that it is a substrate for a single and/or plurality of layers, wherein optionally one or more of said layers is electrically conducting, a superconductor, a high temperature superconductor, or which is a hard tough flexible layer in a layered composite. Use of the sintered inorganic structure of any one of the preceding claims as a catalyst carrier. Method of making a flexible inorganic sintered ceramic sheet as claimed in claim 1 comprising the steps of: a) mixing the inorganic constituents as a powder with one or more vehicle constituents to form a fluid batch; b) forming the fluid batch on a fugitive polymer sheet into an elongated green sheet preform; and c) sintering the green sheet preform on the fugitive polymer sheet to provide a flat, smooth sintered inorganic sheet structure having a thickness not exceeding about 45 µm. Method in accordance with claim 14 wherein the green sheet preform is a long, continuous length of preform, and wherein the step of sintering the preform comprises drawing the long continuous length of preform through a hot zone in a sintering furnace. A method as claimed in claim 14 or claim 15 wherein said vehicle constituents include one or more organic binder constituents selected from the group consisting of polyalkyl carbonates, acrylic polymers, copolymers and blends, polyethylene glycol, polyvinyl butyral, polyvinyl alcohol, polyvinyl acetate and silicones, optionally further one or more organic dispersants selected from the group consisting of phosphate esters, polyether alcohols, polymeric fatty esters, polyelectrolytes, sulfonated polyesters, and fatty acids and their alkali and ammonium salts, optionally further, one or more organic solvents selected from the group consisting of alcohols, ethers, aromatics, ketones, aldehydes, esters, alkanes, alkenes, and alkynes, and optionally further one or more flocculating agents selected from the group consisting of acids, bases, and/or buffer systems. A method as claimed in any of claims 14-16 characterized in that the green preform is formed in whole or in part by a process selected from the group consisting of doctor blading or printing. A method as claimed in any of claims 14-17 characterized in that the sintering comprises heating the green preform to a temperature not exceeding about 1600°C, optionally carried out in an inert or reducing atmosphere, or in a vacuum. A method as claimed in any of claims 14-18 characterized in that it includes the further step, before or after sintering, of combining the flexible sintered inorganic structure with at least one other structure element, at least one of which may be a flexible green preform or sintered organic structure. A method as claimed in any of claims 14-19 comprising the further step, before or after sintering, of providing at least one coating layer on the surface of the green preform or sintered inorganic structure. A method as claimed in any of claims 14-20 characterized in that the step of sintering the elongated green preform comprises continuously transporting the elongated green preform through a hot zone maintained at a temperature above the sintering temperature of the inorganic powder, which optionally may be in a generally downward direction through said hot zone. A method as claimed in any of claims 14-21 characterized in that the elongated green preform is formed by shaping against a rigid or plastic substrate. A method as claimed in any of claims 14-22 characterized in that the fluid batch is a gellable, low-viscosity slip, or is gelled and/or dried by contact with a gelling or drying liquid. A method as claimed in any of claims 14-23, comprising the further step of depositing a catalyst by chemical vapor deposition, surface coating, substrate impregnation, or admixture with the material of the substrate in particulate unsintered form. A method as claimed in claim 24 characterized in that the catalyst comprises a base metal and/or its oxide, or a noble metal.

Inventors: KETCHAM THOMAS DALE CORNING IN, SANDERSON WAYNE BRODIE CORNING, ST JULIEN DELL JOSEPH CORNING, WEXELL KATHLEEN ANN CORNING IN, KETCHAM, THOMAS DALE, CORNING INCORPORATED, SANDERSON, WAYNE BRODIE, CORNING INCORPORATED, ST. JULIEN, DELL JOSEPH, CORNING INCORPORATED, WEXELL, KATHLEEN ANN, CORNING INCORPORATED
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1990
Main classification: C04B35
IPC codes: B22F5, C04B35, B28B1, H01B12, C01B13, C01G1, C22C1, B01J35, H01L39