CA2024143A1 - Thin flexible sintered structures - Google Patents

Abstract

Abstract of the Disclosure 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

Ketcham-sanderson~st. Julien~Wexell g-l-2-2A

~ ~ c3 ~ ,,3 THIN FLEXIB1E SXNTERED STRUC~URES

This application is a Continuation-In-Part application of Serial No. 07/393,53?, filed August 11, 1989.

Background of the Invention 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 v~hicle 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 tempera-tures.
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 unigue advantages as chemically stable substrate -2- ~ t~

materials. Porous ceramic materials are known to provide high suf~ace areas. High surface area substrates provide desirable receiving surfaces for a variety of coatings.
Alumina, for example, provides in its many crystalline S 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 uni~ue advantages for a varie~y of chemical applications.
Sintered porous metallic foils, e.g., porous stainless steel foils, can be made and optionally oxidized or other-wise treated to provide high surface area metal-based substrates. Coated substrates of metallic or oxid~ type, formed into any desired honeycomb or other circular, laminar, and/or trapezoidal structures, would offer stable support in harsh environments where flexibility in combina tion with a specific substrate geometry would be particu-larly advantageous.
Since the discovery of high temperature oxide super~
conductors, there has ~een widespread interest in combining these relatively brittle materials with strong flexible substrate materials to provide supexconducting 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 super-conductors. A particular disadvantage of metals, however, is the diffusivity of the metals at th~ sintering tempera-tures 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 o high tem~erature superconductors, thus minimizing thP diffusivity -3~ 3~ ~

problem. Additionally, ceramics are more compatible with oxide s~erconductor 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 ibers such as silicon carbide fibers and aluminosilicate fibers ls well known. Ceramic fibers of these types are generally produced by spinning techniques or variations thereof f For example, NicalonR (silicon oxycarbide) fibers, NextelR (Al2O3-SiO2-B2O3) fibers, and even r-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 o fiber manufacture include the vapor deposition of precursors onto a starting or substrate filament and/ox the spinning and optional heat treatment of glass fibers ~rom 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)o Unfortunately, while formed of inherently strong materials, long fibers of these cer~mic materials are very weak. The weakness of fibers is ~imply due to the flaw populations in the fibers and the statistical laws which insure that most long fibers will include at least one --4-- ~ f ~ 3 defect of sufficient magnitude to cause failure at stress levels ~ll 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 microns 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 'igreen" (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 microns 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 Eorm. 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. The present invention provides a thin f lexible sintered structure for a wide field of uses, herebefore deprived of a suitable sintered structural flexible material.
The product of this invention is useful in any envi-ronment where a hard tough thin refractory flexible substrate and/or layer is needed. The flexibility will -5~

depend on layer thickness to a large measure and therefore can be ~ilored 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 S toughened and hardened sintered materials may waff in a slight breeze, yet remain hard and tough to mechanical and/or thermal abuses. A use of this kind or strong flexibili~y 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 tempera-tures. 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 thick-ness of the product. Differential porosities in these materials can be useful for flltration and/or membrane operations.
The chemical inertness as well as the surface morphol-ogy, thermal expansion, and flexibility of the presentflexible inorganic substrates make them promising substrates for superconductor materials.
This invention thus provides a means for applying ceramic process technology to a wide variety of materials for the production of flexible inorganic, preferably ceramic, products. Thin materials can be formed in the green state in a molded configuration and subsequently sintered to a dense or porous structure with a large measure of flexibilityO
In the method of the present invention a thin preform, for example a thin sheet or layer comprising the green material, is first produced. The material is then sintered to provide a thin sintered structure with a flexibility sufficient to permit a high degree of bending without breakage under an applied force. Flexibility in the sintered material is sufficient to permit bending to an effective radius of curvature of less than 20 centimeters -6- ~t) ~

ox some Pquivalent measure, preferably less than 5 centi-meters ~ some equivalent measure, more preferably less than 1 centimeter or some equivalent measure, and most 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 furthex bent, straightened, or hent 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 preferably will not exceed about 45 microns, and most preferably will not exceed about 30 microns. The lower limit of thickness is simply the minimum thickness required to render the struc~ure amenable to handling without breakage. Sintered thicknesses of 4 microns can readily be achieved, and thicknesses on the order of 1 micron 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 ~m 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 ~m, or even 10 ~m, with thicker members being provided by layering the thin sintered sheets or other segments~

Description of the Drawinq 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 _escr~ption Generally, the green material used .in the invention is comprised of zirconias, aluminas, titanias, silicas (including æirconates, 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 therPbetween.
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.
Utilizing extrusion, tape casting or other known ceramic batch shaping technology, a selected combination of the above components is mixed into a plastic batch, formed into an elongated green body of any desired cross-sectional shape, and sintered. While the preferred cross-sectional shape of the sintered structure is linear (as for thin sheet or tape), other shapes including rectangular, cylin drical ~tubular), trapezoidal, I-shapes, H-shapes, or dumbbell shapes may be provided. In each case, however, the cross-sectional shape is characterized by at least one high-aspect-ratio segment, such as a straight or curved web or connecting segment or an extending fin or other protru-ding segment, which is suf f iciently thin to be flexible in sintered form. By a high-aspect-ratio segment is meant a segment having an aspect ratio ~segment length to thickness) of at least 2:1, more preEerable at least 3:1.
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 ~hen formed into a uniform~hin 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 tempera-tures. The r~sultant 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 ~he oxides of the transition series metals and the rare earth metal oxides. Stabilized zirconias, such as those stabllized 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 2S, 198~, the subs~ance of which is herein incorporated by re~erence in its entirety.
That patent also discloses the properties of useful bulk materials such as a-alumina, ~-alumina, ~"-al~ina, g ~ ~ 2 ~

A12O3-Cr2O3 solid solution, mullite, and spinel. These same ma~erials 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 rnetal 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 HO2, 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 sxides 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 ahove zirconia/hafnia based compositions are disclosed in commonly assigned U. S. patent application Serial NoO 07/245,523, filed Septembar 19, 1988, the disclosure of which, as filed, is herein incorporated by reference as filed. Further materials therein descrlbed 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 matexials.

Also useful to provide flexible ceramics are composi-tions c~sisting 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 Parth metal niobates, tantalates, and vanadates, and magnesium and calcium tungstates and molybdenates. These ceramics, characteriz-able as hard and tough ceramics exhibiting psuedo-plasticity, are disclosed in commonly assigned U. S. patent application Serial No. 3~8,532 filed March 24, 1989.
The invention also comprises thin flexible sintered ceramic structures as above described composed of certain recently developed hard refractory ceramic alloysO 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 a-alumina, ~-alumina, ~ " -alumina, alumina-chromia solid solutions, chromia, mullite, aluminum mulllte-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 r 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 yroup consist~ 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 5 consisting of ~i) 0-4 mole % of MoO3 and/or WO3, (ii) 0-10 mole % ~o~al of oxide compounds of the formula MM'O4+/ ~
whexein 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 ~ is in the range of 0-1, and (iii) 0-6 mole % total of oxide compounds of the formula M''M'''O4~/ ~ wherein M''' is W and/or Mo, M'' is Mg, Ca, Ti, Sn, Sc, Y, La, Ce, the rare earth metals, or combina-tions thexeof, and ~ 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 zi~conia, ha~nia, 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.
~n 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'O4+/ ~ 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 ~ is in the range of 0-1. Optional additions to these alloys -12- ~ ~ r~

include 0-20 mole % cerium oxide and 0-10 mole ~ of oxides of meta~s selected from the group of Mg, Ca, Sc, Y, La, the rar~ 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 CeOz, 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'''O4+/ ~, 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 ~ 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 zlrconia 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 thererom. 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 rererence. Due to their hardness and toughness ater sintering, these composi-tions provided unlikely candidates for flexible ceramics.

- 1 3 ~ l 3 Nevertheless, it is now found that the following methodolo-gies 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. Pre~erred particle sizes are less than 5 ~m in diameter, most prefer-ably less than 1.5 ~m 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 ~he 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 usef~l 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 silicones 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 filed July 15, 1988 the disclosure of which is herein incorporated by reference as filed.
Once compounded and uniformly mlxed, the batch is next formed into thin sheets or other preforms having thin flexible segments. This for~ing 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 configura-tions. Narrow ribbons or sheets many meters wide can beprovided.
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 vlscosity 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 50 that one or more of the solvents in the ceramic slip is highly miscible ~ith the gelling and/or drying medium. Preferably, the binder ~15~ s~

employed for batches to be ~hus treated is not be miscible with th~gelling and/or drying medium, to avoid binder loss during drying or gelling.
Flocculation, gelation and/or drying are particularly s useful for the extrusion of low-viscosity batch formula-tions. 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 pRa 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 reatment polyvinyl butyral/alcohol water polybutyl methacrylate/
/isopropanol methanol polymethyl methacrylate/
/tetrahydrofuran hexane ~5 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.
It has also found particularly useful to form the green material on or in contact with one or more fugitive polymer layers or sheets. The processability and handle-ability of the green body are greatly enhanced through the support provided by such a polymer shee~. The material for the she~ or layer can if desired be selected ~uch that it provides initial support for the green body during subse-~uent sintering to a product, yet vaporizes without damage S to the product in the same manner as the organic binders, dispersants and other organic constituents of the batch are vapori~ed.
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 lony continuous lengths of tape or ribbon configur-ation, the strength of the sintered material is frequently suf f icient 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 me~al and/or oxide catalysts, such as titanium, vanadium, chromium, cobalt, copper, iron, manga-nese, molybdenum, nickel, niobium, tantalum, tungsten, zinc, rare earth metals, alloys thereof and therebetween.

-17~ 3 Additionally, the noble metal catalys~s, such as platinum, palladi~, 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 struc-tural material within other compositions as in a composite.For example, by drawing in narrow elongated form, the sintered material can be made part sf 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 Zro2 as a stabilizer) was milled with a mixture of 30 grams of methanol and 24 grams of 2-methoxy ethanol containing 0.25 grams o~ 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. Th~ batches were milled with 1/2-inch zirconia balls.
~5 The milled batches were placed in 250 ml wide mouth NalgeneTM polyethylene bottles and then subsequently placed on a vibratory mill for 76 hours. The particle size 1 8 ~ ~ t..~ ~ -J ~ 3 distribution in the final batches averaged from about O.88~ , as measured on a Leeds and Northrup Microtrac particle size analyzer. Similar particle size data obtained using a ~oriba 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 repor~ed values by a factor of from 3 to 5, but in any case particle sizes of the order of 0.1-1.2 ~m predominate in these batches~
The viscosity of the slip after milling ranged from about 4.2 cps to 11.5 cps at 39.2 sec 1, with slips made from oven dried powders showing ~he 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 ~dded to Batch B. The addition of the acid h~lped 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 gra~s o~
dibutyl phthalate. ~atch 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 bind~r 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 cps at 8.7 sec 1. Batch A, specifically, had a viscosity of 3470 cps in this test.

19 2 ~

Tapes were cast from the batches produced a~ described using a ~tandard 2, 4, or 6 mil doctor blade to cast onto a 2 mil Mylar~ polyester carrier film. In general, the smoothness of the substrate can determine the smoothness of the casting. Thus plastic-coated paper can alternatively be used, but typically provides a surface roughness similar to paper fibers. Smoother substrates including polyester, fluorocarbon, polyethylene and/or polypropylen~ films are therefore used when a smooth product surface is desired.
The cast tapes thus provided were then allowed to air dry from 5 minutes to several hours, then placed in a drying oven at about 70~C and/or 90C for 5 minutes to an hour. The tape was less brittle and the adhesion to the carrier film lessened after oven drying.
The dried green tape was next released from the carrier film by pulling the film over a sharp edge.
Removal of the tape by this or equivalent means prior to cutting of the tape is preferred. The tape was then cut into strips 0.5 to 100 mm in width. The cut tape was then placed on a flat setter plate for sintering, oriented so that the portion of the tape which had contacted the film was facing toward the setter. Alumina and zirconia setters were used.
The tapes were then fired according to the following schedule:
Room temperature to 200C in 1 hour 200C to 500C in 1 hour 500C to 1450C in 3 hours 1450C hold for 2 hours 1450C to room temperature for 5 hours The heating rates used were not critical; both faster heat-up and faster cool down rates were successfully tried.
However, uniform heating of the tape is preferred to avoid warping during the binder burnout or sintering.
Properties of the tape products thus provided are reported below in Table 1. The table includes a number of -20~ Ç~

samples and their geometrical dimensions produced from Batch A~ Once fired the tapes were strong. This was demonstrated by the bend radius achievable for the sintered ribbons. The actual strengths may be calculated from the bending radius attainable without breakage using the bend radius equation, known to those skilled in this art. The accepted elastic modulus of 200 GPa and and Poisson's ratio of 0.25 for this zirconia material were used in the equation. The porosity of the sintered tape samples was less than 5 volume percent.

Table_1 Width Thickness Bend Radius Strength 15Sample (10 3 m) (10-6 )~10 3 m~ (GPa) 1 1.80 20 1.70 1.25 2 1.80 18 1.78 1.07 3 1.80 18 1.54 1.24 4 1.32 18 1.70 1.12 1.32 18 1O71 1.11 6 1.02 23 2.10 1.1~
7 1.02 ~3 2.36 1.03 8 0.99 20 1.83 1.16 The as-fired surfaces of the tapes which had been in contact with the carrier film were very flat and smooth, providing an excellent surface for coating. The averaye surface roughness of these as-fired tape surfaces was 8.99 nrn for the tape cast on MylarR polyester film, as measured by WYKO surface analysisO

-21- "" ,' ExamPle 2 In Example 2, ribbon samples of green material from Batch A were sintered in accordance with a process of continuously firing the green ribbon. Green ceramic ribbons with lengths up to 30 centimeters were fired by drawing the ribbons through a platinum wound furnace heated to 1350C. The furnace had a small hot zone. The support-ing surface for the ribbon within the furnace was set at an incline of between 12 and 20 degrees downwardly from the entrance toward the exit end of the furnace, to provide a gravitational assist for the drawing process.
As the green tape was drawn through the hot zone, the tape sintered to a dense structure that could be easily manipulated. The time in the hot zone was less than 5 minutes, with a rate of sintering of about 2 cm of ribbon length per minute. ~igher sintering rates can be achieved by increasing the sintering temperature, e.g., to about 1500C.

Example 3 The use of a fugitive polymer base layer in the tape casting procedures of Examples 1 and 2 is advantageous because it makes the thin green material easier to handle.
To provide such a layer, 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 ~o mix.
The acrylic pol~mer solution thus provided was then cast onto a polyester substrate ~ilm using a doctor blade to form thin acrylic sheet. The polymer-coated substrate was then placed in a 60-70C drying oven for 30 to 60 minutes.
A slip containing yttria-stabilized tetragonal zirconia was then prepared utilizing the materials and procedures -22- f~

used to make zirconia Batch A of Example 1. The ceramic slip wa~'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 ~heets 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 thinniny a slip such as Batch A of Example 1 with solvents. To 10 grams of the slip were added 1.11 grams of methanol and 0.88 grams of 2-methoxyethanol. Slips with a viscosity of 1500 cps or less may be advantageously made by this technique.
The thinned slip thus provided was cast with a 2 or 3 mil blade on top of a fugitive acrylic layer cast as above described with a 4 mil blade. This produced a composite tape with about a 6 ~m green ceramic layer which sintered to around 5 ~m. The firing schedule was as reported in Table 2 below:

Table 2 25 Start Temperature End Temperature Time Room Temperature 200 120 min 200 500 360 min 500 1420 375 min 1420 1420 120 min 301420 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:

-23~

Table 3 e~ ~
ThicknessBend Radius Strength Sam~le (10-6 ) (10-6 ) (GPa?
9 6.0 362 1.77 1~ 5.2344 1.61 11 11.5710 1.72 12 11.5725 1.69 13 11.591~ 1.34 10 1~ 11.5 850 1.4 15 16.01400 1.22 16 16.51520 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.

Table 4 ThicknessBend Radius Strength Sam~ (1o-6 ) _ tlO 6 m) (GPa~
17 11.5 1020 1.20 18 11.5 737 1.66 19 11.5 1180 1.04 The thicknesses of the 5 to 6 ~m ribbons were deter-mined from optical micrographs. The other thicknesses were measured with a micrometer.
Sintered eight-micron-thick sheets as large as 9.5 cm by 9.5 cm and a four-micron-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 -24~

clearance on the blade; thus a residual, very thin, slip layer i~ provided.

Example 4 Small 1.8 mm x 4.8 mm x 8 ~m rectangular pieces of ceramic tape were formed by screen printing an ink consist-ing of ceramic slip onto a fugitive polymer which had been cast on a MylarR 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 rele~sed from th~
MylarR casting film. The images were then placed polymer side down onto a zirconia setter and sintered to 1420C for 2 hours. The sintering schedule shown below in Table 5 was used:

Ta~le 5 Start Temp. Stop Temp. Time (C) (C) (minutes) . .
Room Temp. 150 60 1~20 1420 120 1420 Room Temp. 120 The ceramic tape samples produced by this process were strong and flexible.

25~ 2,~

~xample 5 Ceramic sheet samples approximately 1 cm wide were formed by printing with a rubber stamp onto a fugitive polymer as in Example 4, 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 ~m.

Example 6 Narrow tapes of yttria-stabilized Zro2 were formed from a slip of the composition of Batch A of Example 1 utilizing a combination bead extrusion/doctor blading method. This method was as follows:
a) A doctor blade was brought into contact with a polymer sheet carrier;
b) A narrow continuous bead of slip was placed before the advancing blade ~nd the blade spread the extruded bead to form a narrow tape from the slip;
c) The narrvw tape was dried and removed from the carrier film; and, d) The tape was sintered in the manner described in Example 1 above.
The polymer sheet carrier in this Example was a 2-mil MylarR carrier film. The bead was extruded using a 10 cc syringe fitted wi~h a 21 gauge needle. Either ~ mil or 6 mil blades were used to spread the beads into narrow thin tapes. The tapes had a high degree of transparency indicating little porosity.
Results from the bend-testing of the ceramic tapes produced in accordance with the Example are set forth in Table 6 below-~26~ '3 Table 6 c WidthThickness Bend Radius Strength Sample(10-3 (1o-6 (10-3 (GPa) 18 1 25 3.0 0.89 19 3 33 3.5 1.00 Thus high strength in combination with good flexibility in the tape samples were achieved.
To provide cross-sectional configurations other than thin sheet or tape, extrusion processes can be used. Using extrusion, it is possible to provide low aspect ratio ceramic products, even including ceramic fibers. As previously noted, fibers do not exhibit the strength and flexibility of products with high aspect ratio cross-sections or segments. Nevertheless the following example illustrates that fibers can be successfully formed by this technique.

Exam~le 7 .

Ceramic fibers were prepared by extruding a ceramic slip into a gelling liquid. The slip had the composition of Batch A of Example 1, and was used to fill a syringe fitted with a stainless steel 21, 25, and/or 26 gauge needle. The needle ~nd of the syringe was subm~rged in a gelling liquid, in this case cold water being the preferred agent, and was extruded out through the needle.
The slip gelled upon contact with the water and formed a gelled fiber which reflected the shape of the needle orifice. Details of orifice configuration as fine as 5 microns have been produced. The gelled fiber was draw through 5 to 30 cm of cold water and was then pulled from the water bath and dried by exposure to air.
Green fiber was made by this process at a rate of 2 to 20 cm per second and could be made in very long lengths.
Smaller diameter green fiber was produced by using a smaller diameter orifice in combination with lower viscosity slips. The process can be operated in a continuous fashion by wrapping the extruded material on a rotating spool placed approximately one meter above the gelling liquid.
In the process as described, the diameter of the fiber is determined in part by the relative rates at which the material is pushed through the orifice and/or the rate at which it is drawn from the orifice. If material is drawn slowly, a larger diameter is achieved, while if the material is drawn more quickly a thinner diameter is achieved.
The dried fiber samples thus provided were finally fired for 2 houxs at 1430C to yield sinter d fiber from ~5 to 150 ~m in diameter.

Example 8 Tape 50 ~m thick by 250 ~m wide was produced by extrusion through an approximately rectangular orifice into a gelling liquid as in Example 7. The orifice was prepared by compressing the end of a syringe needle perpendicular to the needle's long axis and then b~ grinding the needle tip flat. The slip and gelling liquid of Example 7 may be used. Tape as thin as 5 ~m could be made by this method using slip thinned to an appropriate consistency.
Ceramic formulations similar in composition to Batch A
of Example 1 but comprising other zirconia powders are also preferred materials for making strong flexible tape, as illustrated in the following Examples.

-28~

Example 9 e;
A slip was prepared from a ceramic powder using zirconia comprising 4 mole percent Y2O3 as a stabilizer.
The powder was first dried in a vacuurn furnace for 90 minutes at 200C, and then combined into a formulation containing the following ingredients:
Ceramic powder 40 g Ethanol 9.2 g 2-Methoxy ethanol 6.0 g Methyl isobutyl ketone4.0 g Di-butyl phthalate 3.9 ~
Emphos PS 21A dispersant 3.0 g ~illing media 39 g The above mixture was milled in a SPEX 8000 Miller/Mixer for 45 minutes. To the resulting mixture was then added 3.0 g of Butvar B-98 binder, with continued milling for an additional 45 minutes.
Tape was cast from the resulting slip onto a plastic coated paper film carrier using a 6-mil doctor blade clearance. The tape was next dried and ribbon was cut from the dried tape using a razor blade. The tape was sintered between zirconia or alumina setter sheets to 1450C.
A tape 7.3 mrn wide, 43 ~m thick and 8 cm long produced as described could be bent to a curvature radius of 8~5 rnm, for a calculated strength of 538 MPa. Other structures made from slips of this zirconia powder included sintered ribbon 35 ~m thick by 1 cm wide by 10 cm long, and a 3 cm x 3 cm by 75 ~m square zirconia sheet.

-29~ 3 Exam~le 10 A slip was made containing ZrO2 comprising 6 mole percent Y2O3. The slip was prepared following the procedure used to make Batch ~ of Example 1. The mean particle size of the zirconia after milling was 1.1 ~m with 50% of the particles finer than 0.91 ~m.
Tapes were then prepared from the slip following the procedures of Example 1. The properties of the tapes thus provided are reported below in Table 7.

Table 7 WidthThickness Bend Radius Strength Sample(10 3 m)(10-6 ) (10 3 m) ~MPa) 22 1.1 33 8.0 439 23 2.5 33 8.5 409 The average strength for these samples was 424 MPa, and the tapes exhibited transparency indicative of low porosity.

Example 11 A slip was made comprising ceramic powder containing Zr2 with 2 mole percent Y2U3 and 2 mole percent Y~bO4.
The composition of the slip was as follows:
Ceramic powder 61 g Methanol 40 g 2-methoxy ethanol 32 g Emphos PS21A dispersant 1.0 g Acetic acid 1O6 g Milling media 450 g The mix was vibramilled overnight, and an additional 2 g of methanol was added. To 139.8 g of the resulting slip the following binder components were added:

-30- ~ t~

Poly vinyl butyral 4.86 g Di-butyl phthalate 2.43 g Tapes were cast from the resulting slip with a 4 mil doctor blade clearance, cut into strips, and the cut samples fired for 2 hours to 1390C, 1420C or 1500C.
Additional tape was made by casting with a 2 mil blade over an acrylic layer which had been cast with a 4 mil blade.
The ~ollowing properties were obtained:
Table 8 Firing Bend Temperature Thickness Radius Strength 15 Sample (C) (1o-6 ) (10 m) (GPa) 24 1390 10 0O26 *
1420 30 2.0 1.60 26 1420 36 3.1 1.23 27 1420 6 1.7 0.375**
28 1500 33 3.5 1.00 * This sample demonstrated porosity and the elastic modulus used in calculating the strength (200 GPa) would tend to overestimate the actual strength.
** These data are for a 2 cm x 2 cm sheet. The remaining data are for 1 to 2 mm wide ribbons.

To account for the bend radius observed in sample 24 above and still have an inherent material strength of order 1.6 GPa, an effective elastic modulus of 80 GPa could be ~ used in the bend radius equation.
In addition to flexibility, the cut sample of the above tape fired at 1500C exhibited a high degree of transformation (psuedo-) plasticity, as evidenced by transformation bands both alon~ the fracture surface and a~
probable areas of stress concentration away from ~he fracture surface. These transformation bands have been associated with transforma~ion plasticity in materials of -31~ 2~

this and similar compositions. The present invention thus combines both flexibility and transformation plasticity in one ceramic body.

Example 12 A slip was made containing alumina powder with the following ingredients:
Alumina 67 y Methanol 30 g 2-methoxy ethanol 24 g Acetic acid 0.3 g Milling media 450 g (zirconia balls) The alumina powder used was Alcoa A-1000 SG, lot 4BD
6742. The above mixture was vibramilled for 3 days, with the mean particle size after milling being 1.05 ~m as measured on a Microtrac analyzer. After milling, the following constituents were added to the slip:
Acetic acid 1.08 g Poly vinyl butyral 1.30 g Di-butyl phthalate 0.65 g Tape was then cast from this slip, dried, and sintered at 1600C for two hours. The sintered tape was sufficiently transparent to serve as a clear overlay through which printed material could easily be read. The following data were obtained from bend tests of the tape, using an elastic modulus of 380 GPa for the sintered alumina:

Table 9 Width Thickness Bend Radius Strength Sam~le(10-3 ) (lO _ m) (10~3 ) (MPa) 35 29 1.0 38 25 307 1.3 36 13 535 -32- ~3r~

~= ~
A slip comprisiny a mixture of alumina and yttria-stabilized zirconia was made from the following ingredients:
Batch A zirconia slip16.66 g Alumina slip ~Example 12) 4~55 g Acetic acid 0.19 g Butvar B-98 0.22 g Di-butyl phthalate 0.11 g This mixture was milled on a SPEX mill for 10 minutes, and tapes were then cast from the mixture onto MylarR
polymer sheets using 4-mil and 6-mil doctor blade clearances.
Strips were cut from the cast tapes and the cut samples fired to about 1430C for 2 hours.
The following strengths were calculated from tape bend tests using an elastic modulus value of 230 GPa.

Table 10 WidthThickness Bend Radius Strength Sample (10-3 )(10 6 m) ~10-3 ) (MPa) 31 1.2 36 4.24 1030 32 1.3 20 2.95 829 33 1.3 20 2.89 8 Thus high levels of strength and flexibility in the ceramic tape were achieved.

_33_ ~ 7~ ~ 3 ~ ExamPle 14 A slip containing mullite powder was prepared from the following ingredients:
Mullite powder 55 g Methanol 30 g 2-methoxy ethanol 24 g Milling media 450 g The mullite powder used was Baikowski 1981 Ref 193 Mullite Powder Deagglomerated l~pe CR. The mixture was vibramilled for 3 days, after which the mean particle size of the mullite powder was determined to be 1.36 ~m as measured on a Microtrac analyzer.
The milled mixture was allowed to settle overnight and the supernatant slip, retaining about 49 weight percent of dispersed mullite, was recovered by decantation. To 25.01 g of this slip were added:
Acetic acid 0.57 g Poly vinyl butyral 1.37 g Di-butyl phthalate 0.69 g The resulting slip was thoroughly mixed, and tapes were cast and subsequently fired to about 1600C for 2 hours. Mullite ribhon 38 microns in thickness prepared from this slip could be bent to a radius of curvature below 3.0 cm without breakage.

Ex~le 15 A slip was made containing magnesium aluminate spinel powder with the following ingredients:
Spinel 61 g Methanol 30 g 2-methoxy ethanol24 g Acetic acid 0.4 g Milling media 450 g ~0 The spinel powder used was Baikowski 8293264 log 822 8/82 powder with an alumina content of 72.75%. The mixture of ingredients was vibramilled for 3 days, following which an additional 7.5 g of methanol and 6 g of 2-methoxyethanol were added. The mean particle size after milling was 1.78 ~m as measured on a Microtrac analyzer.
To a 20 g portion of the spinel slip thus prepared the following were added:
Methanol 3.00 g 2-methoxy ethanol2.40 g Acetic acid 0.43 g Poly vinyl butyral 1.04 g Di-butyl phthalate0.52 g Tapes were cast from this slip using a 6-mil blade clearance, and the cast tape was then cut into ribbon and fired to 1600C. The fired ribbon had a thickness of 43 ~m and could be bent to a radius of curvature of 2.5 cm without breakage.

Example 16 A laminar structure was formed by casting a tape of yttria-stabilized ZrO2 comprising 2 mole percent Y2O3 over a tape of stabilized ZrO2 comprising 6 mole percent Y2O3.
The 6 mole percent Y2O3 slip was cast on a MylarR carrier film using a 2 mil doctor blade~ The tape was allowed to dry for about 5 minutes. Next a slip containing zirconia with 2 mole percent Y2O3 was cast on top of th~ first tape using a 6 mil doctor blade. The laminar tape was allowed to dry in an oven at 90C, released from the carrier film, cu~ into ribbons and fired at about 1430C for 2 hours. It was apparent from a cross-sectional photomicrograph that the sintered laminar ribbons had two distinct layers which exhibited different fracture behavior and scattered light differently. The 6 mole percent Y2O3 layer of one sintered ribbon was 12 ~m thick while the 2 mole percent Y203 layer was 25 ~m thick.
Results for bend tests conducted on the laminated samples are reported in Table 11 below. Included in Table 11 for each of the samples tested are the dimensions and properties of the tapes, as well as an indication, by composition, of which surface or side of each tape sample was the side in tension in the bend test.
Table 11 Bend Side in Width Thickness Radius Strength 3~ Sample Tension10 3 m) (10_ m) (10_ m) (MPa) 34 6 mol/o0.8g 42 9 495 2 molto0.89 42 6.5 687 6 mol/o0.86 41 7.25 602 2 mol/o0.86 41 6.5 671 36 6 mol/o1.8 43 12.5 366 -36- 2~ 3 As the data indicate, the laminar structures of this Example exhibited a strength anisotropy. Hence, strength values obtalned with the 6 mole percent Y2O3 layer in tension (averaging 466 MPa) were generally lower than those obtained with the 2 mole percent Y2O3 layer in tension (averaging 679 MPa). These values can be compared with the average values obtained for 6 mole percent Y2O3 ribbons (424 MPa) and for similar 2 mole percent Y2O3 ribbons (1.11 GPa).
Example 17 Flexible ceramic tapes for composite superconducting wires have been produced. In one procedure, a zirconia lS ribbon substrate was coated with a slip containiny Y1Ba2Cu3O7 O (where ô equals 0 to 1.0, the compound being referred to as a 123 superconductor)~ The sintered tape had a thickness of approximately 75 ~m and was coated with approximately 30 ~m of superconductor-containing slip, and then fired. Firing was for a period of 4 hours at 940C to sinter the superconductor and give good adhesion between the coating and the substrate.
X-ray diffraction patterns indicated a high degree of orientation for the coating, with the crystallographic b axis being perpendicular to the plane of the ribbon. The degree of orientation was indicated by the enhanced in-tensity of the lin~s with Miller Indices 010, as shown in Table 12 below which reports X-ray line intensities for the 123 superconductor sintered on the ZrO2 tape substrate, and 3~ for the bulk 123 powder. The x-ray pattern was indicative of a reasonably good superconducting material, orthorhombic Y1Ba2Cu3O7 ~. Flexibility was also evident after sintering of the coating onto the substrate.

~s~ t''3 Table 12 Relative Intensities Miller 123 on sulk Lines with Index* Zirconia 123 Enhanced IntensitY

020 14 - ~
001, 030 49 9 +

021, 120 5 031 _# 28 130, 110 100 100 041, 140, 050 79 14 002~ 060 92 30 +

161, 132 28 48 231 5 _ 21 _ -0 * The indexin~ scheme used designates the lon~ axis as the b axis.
_ Indicates line not discerned by computer program used.
# This line appears as a shoulder and was not given an intensity value by the computer program used.

Thick superconductor films of 123 composition tend to react with the zirconia substrate to form a thin layer of barium zirconate at the interface. A sheet of ZrO2 was coated with a slurry containing 123 powder. When this coated zirconia was heated to 950C for 20 minutes, the thick film of 123 sintered to itself but did not adhere well to the zirconia. Reaction ~or sintering) times of 2 hours at 927C were acceptable, improving adherence.

-38~

Example 18 Fluoride-enhanced thick film superconductor composi-tions also form superconduc~ing coatings on the flexible ceramic substrates of the invention. Compositions success-fully applied include the fluorine-containing materials disclosed in U. S. patent application Serial No. 07/207,170 filed June 15, 1988 the disclosure of which, as filed, is incorporated herein by reference.
As an illustrative procedure, a slurry containing ethyl acetate and a powdered ceramic superconductor consisting essentially of Y1Ba2Cu3O7 ~F was prepared by mixing 3 g of powder with 3 g of ethyl acetate. The slurry was pipetted onto a flexible zirconia sheet and the excess slurry allowed to run off. The substrate used was zirconia comprising 4 mole percent of a yttria stabilizer and having a sheet thickness of approximately 70 ~m. After firing onto the substrate, the coating demonstrated superconductive behavior; the resistivity of the coating at 7iK decxeased at least 3 orders of magnitude from the room temperature resistivity. A 50% reduction in resistivity is more typical of con~entional conductive materials.
Voltage/current data for the superconducting coating of the Example is set forth in Table 13 below. The Table records the voltages needed to induce cuxrent flow over a range of current magnitudes (0.1-10000 mAl for the sample at room temperature (25 C) and at 77 K.

Table 13 _ _ .
Current(mA) 0.1 1 10 100 1000 2000 5000 10000 Voltages (mV)at 25 C
0.000 0.006 0.047 0.~62 4.625 - - -35 at 77 K
0.000 0.000 0.000 0.000 __0.000 0.011 0.194 0.943 - indicates not measured -39- 2 ~

Example 19 High temperature superconductor coating 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 YlBa2-xAgxCU37-~ wherein the mole proportions of the components were as follows:
lO 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 U. S. Patent Application Serial No. 07/315,326, filed February 24, 1989, 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:

Table 14 Starting Set Next Set Time Temperature C Temperature C Minutes Room Temperature 600 60 850 Room Temperature 180 4 o Following annealing, silver metal was evaporated onto the ~c coated substrates in order to form electrical contacts for measurements and the coated substrates were heated again in oxygen to 300C.
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 o~ the samples as a ~unction 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 tempera-ture superconductor coatings have been provided on these flexible substrates.

Example 20 Four narrow (1 mm to 2 mm wide) zirconia ribbons of 20 ~m 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 661C for the 600C
set, 761C for the 700C set, and 865 C for the highest set temperature (for which the set point was 810C).
Silver electrodes were then laid down on the coatings by vacuum vapor deposition and the samples heated in oxygen to 300C.
one coated and annealed ribbon from this example was bent to test its strength after heat treatment. The thickness of the substrate was 20 ~m~ the sample was bent with the coating in compression to a radius of 1.9 mm without breakingO From this data the calculated strengthwas at least 1.11 GPa. No visible degradation occurred to the coating or the substrate.

-41~ L ~.
Exam~le 21 A 0.5 ~m 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 ~m x 1.65 mm x 2.5 cm zirconia ribbons which contained 2 mole %
yttria.
The coated ribbons were annealed at 960C 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 ~han S mm in either direc~ion (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 5.34 angstroms, close to the literature value of 5.291 for Nb3Sn. The film on the flexible zirconia substrate was found to superconduct with a Tc of 18K.
ExamPle 22 Flexible zirconia tapes were coated with silver metal by evaporative vapor deposition, followed by heatin~ of the composite to 300C. The product was a non-ductile, flexible conductiny composite. Thus flexible conductors which may be fatigue resistant composites were provided.

A 1.7 mm wide sintered ribbon of ZrO2 with 2 mole percent Y~O3 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 1430C for 2 hours. After sintering, the ribbon was affixed to the tube.

42 ~ . t . . A .

ExamPle 24 Thin green metal tape was prepared by a tape casting process. 167 grams of stainless steel powder was mixed with 42.4 grams of a binder, 10.0 grams of a plasticizer, and 54.5 grams of 1~1,1 trichloroethane. The binder was commercially available 5200 MLC binder made by the E. I.
duPont Company and the plasticizer was Santicizer 150 commercially available from the Monsanto Company. These materials were mixed and cast onto MylarR polymer film to form a green stainless steel tape layer about 28 mils thick.
To form a covering ce.ramic layer, 100 grams of ZrO2 powder comprising 2 mole percent of a Y2O3 stabilizer, 6.48 grams of Butvar B-98 binder, 19.52 grams of ethanol, 31.24 grams of xylene, and 8.24 grams of dibutyl phthalate were mixed into a slurry and applied over the surface of the green stainless steel to provide a green laminar tape configuration. The laminar green tape was then sintered for 2 hours at 1300C in a vacuum furnace to provide an strong, flexible integral metal/ceramic composite tape.
Composite sintered tapes of less than 30 ~m thickness may be made by this process.

Example 25 c A slip was made with zirconia comprising 2 mole percent Y203 as a stahilizer. The initial batch was pr~pared in a 250 ml polyethylene bottle and contained 100 g of ceramic powder which had be~n dried in an oven at 400C, 24 g of 2-methoxy ~thanol, 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 ~m 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 homogeni~e the slip. The final vis~osity was 598 cps 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 was 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 q of polymer, 29.4 g of ethyl acetate, and 5 g of dibutyl phthalate.
The first fugitive polymer solution was cast on a 2 mil Mylar~ polymer carrier using a doctor blade with a 1 mil clearance. This was dried in an 60 C oven or several minutes. The slip containing ceramic powder was then cast over the first fugitive polymer layer using a blade with a 1 mil clearance. This was also dried for several minutes in a 60C oven. The second fugitive polymer solution wa~

-44~ ~ ~J~ L~

then cast over both of the previous layers, again with a .~
blade with a 1 mil clearance, thus forming a 3-layer sandwich structure.
The cast structure was released from the MylarR 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 ~m 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 modifi-cations 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 (59)

1. An inorganic sintered structure comprising at least one high-aspect-ratio segment having flexibility sufficient to permit bending without breakage in at least one direction to an effective radius of curvature of less than 20 centimeters.

2. The structure of claim 1 which 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.

3. The structure of claim 1 which is 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.

4. The structure of claim 2 comprising one or more stabilizing agents selected from the group consisting of the oxides of magnesium, calciun, yttrium, titanium, tin, and rare earth metals selected from lanthanum, cerium, praseodynium, neodynium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

5. The structure of claim 2 comprising one or more toughening agents selected from the group consisting of niobia, vanadia, tungstia, molbdena, and tantala.

6. The structure of claim 2 comprising one or more alkaline earth metal oxides selected from the group consisting of magnesium, calcium, strontium, and barium.

7. A structure according to claim 1 which consists essentially of zirconia alone or in combination with one or more stabilization agents selected from the group consisting of yttria, calcia, magnesia, rare earth metal oxides, titania, scandia, and tin oxide.

8. A structure according to claim 2 consisting essentially of one or more alumina compositions selected from the group of .alpha.-alumina, .beta.-alumina, .beta."-alumina, Al2O3-Cr2O3 solid solution, mullite, and spinel.

9. The structure of claim 1 which is a substrate for a single and/or plurality of layers.

10. The structure of claim 9 wherein one or more of said layers is electrically conducting.

11. The structure of claim 9 wherein one or more of said layers is a superconductor.

12. The structure of claim 9 wherein one or more of said layers is a high temperature superconductor.

13. The structure of claim 1 which is a hard tough flexible layer in a layered composite.

14. The structure of claim 1 having at least one dimension of cross-sectional thickness not exceeding 45 µm.

15. The structure of claim 14 wherein said thickness not exceed about 30 µm.

16. The structure of claim 1 wherein said radius of curvature is less than 5 centimeters.

17. The structure of claim 1 wherein said radius of curvature is less than 1 centimeter.

18. A thin flexible sintered structure having a flexibility at least sufficient to permit bending in one or more directions to a radius of curvature of less than 20 centi-meters without breakage, the sintered structure comprising a ceramic alloy comprising 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 follow-ing 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'O4+/-.delta.
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 .delta. is 0-1; (ii) 0-6 mole % M''M'''O4+/ .delta. 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 .delta. 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.

19. The structure of claim 18 wherein the refractory ceramic is selected from the group consisting of .alpha.-alumina, .beta.-alumina,.beta.''-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.

20. The structure of claim 18 wherein 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.

21. The structure of claim 20 wherein the refractory ceramic consists essentially of one or more compounds selected from the group consisting of .alpha.-alumina, .beta.-alumina, .beta.''-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.

22. The structure of claim 18 wherein said 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'O4+/.delta. 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 .delta. 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.

23. The structure of claim 22 wherein the refractory ceramic is one or more compounds selected from the group consisting of .alpha.-alumina, .beta.-alumina, .beta.''-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 zirconium carbide.

24. The structure of claim 18 wherein 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 groups consisting of:
0.25-6 mole % M''M'''O4+/-.delta. wherein M''' is W
and/or Mo, M'' is selected from the group consisting of Mg, Ca, Ti, Sn, Sc, Y, La, and the rare earth metals, and .delta. 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.

25. The structure of claim 24 wherein the hard refractory ceramic consists essentially of at least one member selected from the group consisting of .alpha.-alumina, .beta.-alumina, .beta.''-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 diborde, zircon and/or zirconium carbide.

26. The structure of claim 1 wherein said structure is laminar.

27. The structure in accordance with claim 1 which is a flexible ceramic sheet or tape.

28. A laminar structure comprising two or more layers of the flexible ceramic sheet or tape of claim 27.

29. The structure of claim 2 wherein said oxide components are substituted at least in part by components selected from the group consisting borides, nitrides, silicides, carbides and combinations thereof.

30. A structure in accordance with claim 1 having a porosity of up to about 60%.

31. A method for making a flexible inorganic sintered structure comprising the steps of:
a) mixing an inorganic powder with one or more vehicle constituents to form a fluid batch;
b) forming the fluid batch into an elongated green preform having a cross-sectional configuration comprising at least one segment of aspect ratio greater than 2:1; and c) sintering said elongated green preform to provide a sintered inorganic structure of sufficient strength and flexibility to survive bending in at least one direction to a radius of curvature not exceeding about 20 centimeters without breakage.

32. The method of claim 31 wherein said inorganic powder is of metal or metal oxide composition.

33. The method of claim 31 wherein said vehicle constitu-ents 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.

34. The method of claim 31 wherein said vehicle constitu-ents include 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.

35. The method of claim 31 wherein said vehicle constitu-ents include one or more organic solvents selected from the group consisting of alcohols, ethers, aromatics, ketones, aldehydes, esters, alkanes, alkenes, and alkynes.

36. The method of claim 31 wherein said vehicle constitu-ents include one or more flocculating agents selected from the group consisting of acids, bases, and/or buffer systems.

37. The method of claim 31 wherein said elongated green preform is formed in whole or in part by a process selected from the group consisting of doctor blading, printing, extrusion, pressing, rolling, molding, casting, spraying, drawing, and blowing.

38. The method of claim 31 wherein sintering comprises heating the green preform to a temperature not exceeding about 1600°C.

39. The method of claim 31 wherein said sintering is carried out in an inert or reducing atmosphere.

40. The method of claim 31 wherein said sintering is carried out in a vacuum.

41. The method of claim 31 wherein the segment thickness does not exceed about 45 µm.

42. The method of claim 31 wherein the inorganic powder is a ceramic powder comprising zirconia alone or in combination with one or more toughening agents selected from the group consisting of niobia, tantala, vanadia, tungstia, and molbdena.

43. The method of claim 31 comprising the further step, before or after sintering, of combining the flexible sintered inorganic structure with at least one other structural element.

44. A method in accordance with claim 43 wherein at least one of said other structural elements is a flexible green preform or sintered inorganic structure.

45. A method in accordance with claim 31 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.

46. A method in accordance with claim 31 wherein the step of sintering the elongated green preform comprises continu-ously transporting the elongated green preform through a hot zone maintained at a temperature above the sintering temperature of the inorganic powder.

47. A method in accordance with claim 45 wherein the elongated green preform is transported in a generally downward direction through hot zone.

48. The method of claim 31 wherein the elongated green preform is formed by shaping against a rigid or plastic substrate.

49. A method in accordance with claim 48 wherein the substrate includes a separable surface layer composed of a polymer.

50. A method in accordance with claim 49 wherein the polymer is selected from the group consisting of polymeth-acrylates and polyalkyl carbonates.

51. The method of claim 31 wherein said fluid batch is a gellable, low-viscosity slip.

52. The method of claim 51 wherein said slip is gelled and/or dried by contact with a gelling or drying liquid.

53. A method in accordance with claim 45 wherein the coating layer is electrically conducting.

54. The method of claim 53 wherein the coating layer is a superconducting layer.

55. The method of claim 54 wherein the superconducting layer is a high temperature superconductor.

56. A sintered inorganic structure comprising a flexible ceramic or porous metallic substrate on which is disposed a catalyst, the structure exhibiting a flexibility sufficient to permit bending without breakage to a radius of curvature not exceeding about 20 centimeters.

57. The structure of claim 56 wherein said catalyst comprises a base metal and/or a base metal oxide.

58. The structure of claim 56 wherein said catalyst comprises a noble metal.

59. The structure of claim 56 wherein said catalyst is disposed on the substrate by a process selected from the group consisting of chemical vapor deposition, surface coating, substrate impregnation, and admixture with the material of the substrate in particulate unsintered form.