ALPHA PHASE SEEDING OF TRANSITION ALUMINA USING CHROMIUM OXIDE-BASED NUCLEATING AGENTS

Abstract: This invention provides an improved alpha alumina-based ceramic and methods to make the same. The improvement consists of the addition of a chromium-based nucleating agent. Particularly useful embodiments of the improved alpha alumina-based ceramic include abrasive particles and fibers.

Description: A PHA PHASE SEEDING OF TRANSITION ALUMINA USING CHROMIUM OXIDE-BASED NUCLEATING AGENTS Field of Invention This invention provides an alpha alumina-based ceramic article seeded with a chromium-based nucleating agent. In another aspect, this invention relates to 10 methods for preparing an alpha alumina-based ceramic by alpha phase seeding transition alumina using a chromium oxide-based nucleating agent. Background of the Invention 15 The preparation of dense alumina-based ceramic articles via a sol-gel process comprising an alpha alumina or alpha ferric oxide nucleating material is known in the art. For example, M. Kumagai and G. L. Messing spoke at the American Ceramic Society meeting of May 2, 1984 on 2Q this subject and later caused the publishing of a paper in November, 1984 in Communications of the American Ceramic Society entitled "Enhanced Densification of Boehmite Sol-Gels by Alpha Alumina Seeding". Other references on this subject include the following publications: - A. "Microstructural Evolution in Sintering of ALOOH Gels", by Yarbrough et al., J. Mater. Res., 3 , (4), 1987, pp. 494-515; "Enhancing Densification of 93% Al 2 0 3 - 7% MgO Triphasic Zerogels with Crystalline α-Al 2 0 3 and MgAl 2 0 4 Seeds," Komareni et al., J. Mater. Sci. Lett., ιS, 1987, 30 pp. 525-27; "Sapphire Whiskers From Boehmite Gel Seeded with α-Alumina, "Jagota et al., 5, 1987, pp. 527-34; "Ceramics By The Solution-Sol-Gel Route", Roy, Science, 238, December 18, 1987, pp. 1664-69; "Solid-Phase Epitaxy of Boehmite-Derived from Alumina on Hematite Seed - Crystals," Messing et al., J. Am. Ceramic Soc, 72, (5), 1989, pp. 864-67; "Nucleation of Alpha Alumina in Boehmite Gel," Pach et al., J. Mater. Res., 5_, (2), February 2, 1990, pp. 278-85; U.S. Patent Nos. 4,744,802 (Schwabel), 4,770,671 (Monroe et al.), 4,623,364 (Cottringer et al. ), 4,829,031 (Roy et al. ) , and 4,797,139 (Bauer), and European Patent Application Nos. 0293163 (Wood), published November 30, 1988; and 0294208 (Wood et al.), published December 7, 1988. Although both Monroe and Cottringer et al. 10 disclose the use of chromium oxide as a modifying additive in a sol-gel derived alpha alumina-based ceramic, they neither suggest nor teach its use as a nucleating agent. Although Schwabel indicates that materials crystallographically similar to alpha alumina, in addition , c to alpha ferric oxide, are contemplated as alpha alumina nucleating agents, he neither specifies the use of alpha chromic oxide nor teaches the use of alpha chromic oxide as a nucleating agent. Further, he discloses the use of chromium oxide as a modifying additive, but does not 2Q suggest its use as a nucleating agent. Several of the publications cited above suggest the use of chromium oxide (Cr 2 0 3 ) as a nucleating agent for the growth of alpha alumina from boehmite. None of these publications, however, disclose a method for the 25 effective use of such a chromium-based seed or nucleating agent. For example, Bauer suggests the use of chromium oxide (Cr 2 0 3 ) as a nucleating agent for the growth of alpha alumina crystals from boehmite under appropriate conditions, he fails, however, to disclose or suggest such ,- conditions. Roy et al. attempted to use Cr 2 0 3 as a nucleating agent and concluded that a Cr 2 0 3 seed has "hardly any effect" on the alpha alumina transformation temperature, because it appears that such a seed needs to exactly match the alpha alumina lattice. 35 Yarbrough et al. attempted to seed an alpha alumina precursor by introducing a high purity chromia powder into a xerogel. Diffuse reflectance infrared spectra of the calcined gel suggests that the oxidation c state of all or all but a small amount of the chromium was +6, not +3. Further, the authors concluded that alpha Cr 2 0 3 does not increase nucleation frequency as much as might be expected. This article includes a photomicrograph of their best example which, reveals 10 domains having an average diameter of about 3 micrometers. No density data were reported. Wood et al. teach the use of an amorphous iron containing oxy-hydroxy polymer as a nucleating material for the alpha transformation in basic aluminum salt 15 derived aluminas. Although the inventors disclose that nucleating materials prepared by the cohydrolysis of iron and chromium salts can be as effective at nucleating the alpha alumina transformation in basic aluminum salt derived alumina as the hydrous iron complex they do not - z. teach or suggest the use of alpha chromic oxide (Cr 2 0 3 ) as an alpha phase nucleating agent. Wood teaches a method for preparing abrasive grits by a boehmite-baεed sol-gel process, wherein an aqueous solution comprising a precursor of a modifying 25 additive (i.e., a metal oxide) is impregnated into the calcined sol-gel. Although the inventor suggests impregnating nucleating material into the calcined boehmite-based gel, he does not suggest or teach the use of chromium-based nucleating material. Further, patentee 30 fails to provide any examples of seeding by impregnation of a nucleating material. U.S. Patent No. 3,717,497 discloses a hard, abrasion resistant refractory ceramic comprising chromium oxide, prepared by impregnating a porous, partially fired __ alumina structure with a chromic acid solution and heating to a temperature in the range of about 600 to 1000°C, repeating the impregnation and heating step at least 5 times, and then heating to a temperature in the range of about 1000 to 1600°C. No microstructural data were given. Patentees do not suggest or teach the use of alpha chromic oxide (Cr 2 0 3 ) as an alpha phase nucleating agent. 5 Summary of the Invention The present invention provides alpha alumina-based ceramic having a seeded microstructure comprising alpha alumina domains nucleated with alpha 10 chromic oxide. Preferably, the alpha alumina-based ceramic has a well-seeded microstructure. More . preferably, the average diameter of the alpha alumina domains is less than about 0.5 micrometer, and most preferably their average diameter is less than about 0.25 ,c micrometer. Typically, the ceramic has a chromium content, derived from alpha chromic oxide, in the range of about 0.085 to about 20 weight percent, based on the amount of alpha alumina present. Preferably, chromium is present in the range of about 0.085 to about 7 weight 20 percent, more preferably it is present in the range of about 0.3 to about 3.5 weight percent, even more preferably, it is present in the range of about 0.7 to about 2.75 weight percent, and most preferably, it is present in the range of about 1 to about 2.4 weight 25 percent. The terms "seed" and "nucleate" are used herein interchangeably to refer to the providing of multiple externally introduced alpha alumina crystal growth sites. The term "domain" as used herein refers to an _- identifiable, generally equiaxed region comprised of alpha alumina microcrystallites or a single alpha alumina microcrystallite, wherein the lattices of the alpha alumina microcrystallites are similarly oriented. The term "nucleating agent" as used herein refers to an externally introduced alpha phase crystal growth site. The term "nucleating material" as used herein refers to a nucleating agent or a precursor thereof. The term "well-seeded microstructure" as used herein refers to a ceramic, which after sintering, comprises domains of less than about 1 micrometer average diameter nucleated with a nucleating agent, and having a ς density of greater than 95 percent of theoretical, wherein substantially all porosity is located at the microcryεtallite boundaries or triple points. The term "seeded microstructure" as used herein refers to a ceramic, which after sintering, comprises domains of less ιn than about 2 micrometers average diameter nucleated with a nucleating agent, and having a density of greater than 90 percent of theoretical, wherein substantially all porosity is located at the microcrystallite boundaries or triple points. The term "non-seeded microstructure" as used A g. herein refers to a ceramic, which after sintering, comprises domains of greater than about 2 micrometers average diameter or a density of less than 90 percent theoretical, wherein the microstructure may be further characterized as being vermicular or wormy. The term "aqueous mixture" as used herein refers 20 to a dispersion-based sol, a solution-based sol, or the like. The term "immobilize" as used herein refers to either gelling the dispersion-based sol or rigidifying the solution-based sol. 5 The term "transition alumina" as used herein means any crystallographic form of alumina which exists after heating alumina to remove any water of hydration prior to the production of alpha alumina. Examples of transition alumina include the eta, theta, delta, chi, iota, kappa, and gamma forms and any intermediate 0 combinations of such forms. In another aspect, this invention provides two methods for making an alpha alumina-based ceramic. 5 Method I Method I comprises the steps of: a) preparing an aqueous mixture ( e.g., a ς dispersion or a solution) comprising: i) an alpha alumina precursor; and ii) a sufficient amount to nucleate during the following process steps said alpha alumina precursor to alpha alumina having a seeded 10 microstructure, of at least one compound selected from the group consisting of an alpha chromic oxide precursor, alpha chromic oxide, and mixtures thereof; b) immobilizing the aqueous mixture; l ._b c) drying the immobilized aqueous mixture to produce a dry body; d) calcining the body at a temperature and in an atmosphere sufficient to provide substantially all the chromium derived from the compound within the body in an oxidation state of not greater than +3; 20 and e) sintering said calcined body at a temperature and in an atmosphere sufficient to provide an alpha alumina-based ceramic having a seeded microstructure. 5 Two important aspects of Method I which are not taught in the art are the preferred particle size of the nucleating material and the preferred oxidation state of the nucleating material during the making of the alpha alumina-based ceramic. Preferably, the particle size of 0 the nucleating material is generally less than about 0.5 micrometer. The oxidation state of the alpha chromic oxide (Cr 2 0 3 ) nucleating agent of the present invention is +3. Except for when the nucleating material is acting as an 5 alpha phase crystal growth site (i.e. as a nucleating agent), the preferred oxidation state of the chromium comprising the nucleating material may not necessarily be +3. For example, it is thought that the dispersibility of the inventive nucleating material in the sol is enhanced if at least some of the surface chromium comprising the c nucleating material is in an oxidation state higher than +3, but lower than +6. The more preferred oxidation state of the surface chromium comprising the chromium-based nucleating material introduced into the sol is believed to be +4. Preferably, substantially all the chromium 0 comprising the nucleating material during calcining is converted to or maintained in an oxidation state of not greater than +3. Further, it is thought that during sintering substantially all the chromium should have an oxidation state of not greater than +3. Preferably, the chromium content of the ceramic prepared by Method I (derived from alpha chromic oxide) is in the range of about 0.065 to about 7 weight percent, based on the amount of alpha alumina present. Method II Method II comprises the steps of: a) preparing a liquid admixture which comprises a sufficient amount of a precursor of alpha chromic oxide that when converted to alpha chromic oxide nucleates the transformation of transition alumina to alpha alumina, wherein the alpha alumina has a seeded microstructure; b) impregnating the admixture of step (a) into a transition alumina-based body; c) drying the impregnated transition alumina body; d) calcining the dried impregnated transition alumina body at a temperature and in an atmosphere sufficient to substantially remove volatile materials and convert the precursor to its nucleating phase; and -8- e) sintering the calcined-impregnated transition alumina body at a temperature and in an atmosphere sufficient to provide an alpha alumina-based ceramic having a seeded microstructure comprising alpha alumina nucleated with alpha chromic oxide. The preferred liquid admixture is a molten metal salt (e.g., chromic nitrate). More preferably, the liquid admixture is aqueous. The preferred nucleating agent is alpha chromic oxide. Preferably, the chromium content of the ceramic prepared by Method II (derived from alpha chromic oxide) is in the range of about 4 to about 20 weight percent, based on the amount of alpha alumina present. More preferably, chromium is present in the range of about 5 to about 8.5 weight percent. The introduction of an alpha chromic oxide nucleating material into a transition alumina body via impregnation of a liquid admixture provides a novel method of preparing an alpha alumina-based ceramic having a seeded or well-seeded microstructure. Method II eliminates the need for milling and classifying particulate alpha phase nucleating materials which are added to a sol comprising an alpha alumina precursor. The methods of the present invention teach means for providing the nucleating material in the desired oxidation state during processing. For example, during calcining the oxidation state of the chromium may be controlled by the presence of internal reducing materials (e.g., organic material), the calcining atmosphere, or combinations thereof. During sintering, the oxidation state of the chromium may be controlled by the sintering atmosphere. Particularly useful embodiments include abrasive grain (sometimes referred to as abrasive particles) and fibers. Abrasive grain according to the present invention exhibit increased grinding performance over a comparable ceramic without nucleating sites. Preferred abrasive products comprising the abrasive grain include coated, bonded, and low density, three-dimensional products. The preferred fiber according to the present invention is flexible, has an average surface roughness of less than 0.100 micrometer, and has a chromic content, derived from alpha chromic oxide, in the range from about 0.085 to about 7 weight percent, based on the alpha alumina present. Such fibers are useful in the manufacture of woven refractory products such as fabric, batting, mats, roving, and thread. These fibers are also useful as a reinforcement and as a toughening phase in composites, such as ceramic composites or metal-matrix composites. The art teaches neither the alpha alumina-based ceramic of the present invention, nor a method to make the same. Detailed Description of the Preferred Embodiments Alpha phase nucleating agents useful in preparing the ceramic of the present invention are crystallographically similar to alpha alumina and include alpha chromic oxide. The presence of the nucleating material during the thermal processing of the sol gel results in a ceramic material which has smaller domains and a higher density than the ceramic without the nucleating material, producing a more durable ceramic. The nucleating agent is thought to provide a growth site for alpha alumina. Method I Chromium-based alpha phase nucleating materials useful in the present invention include alpha chromic oxide or a precursor thereof, such as Cr0 2 or HCrθ 2 . Surprisingly, the most preferred nucleating material is Cr0 2 , wherein the chromium is in the +4 oxidation state. The amount of chromium-based nucleating material useful in the present invention can vary considerably. Preferably, the sintered ceramic comprises in the range of about 0.085 to about 7 percent by chromium, based on the amount of alpha alumina present, although amounts outside of this range are also thought to be useful. - The particle size of the alpha chromic oxide or alpha chromic oxide precursor nucleating material which is useful in the practice of the present invention may vary considerably. Preferably, the chromium-based nucleating material has an average particle size of less than about A ,. 0.5 micrometer in the primary dimension, wherein the primary dimension is the largest measurement of length of a particle where the line of length runs though the center of the particle. More preferably, the average particle size is less than about 0.25 micrometer, and most . .. preferably it is less than about 0.1 micrometer. The particle size distribution of the nucleating material is thought to be less important than the size of the particles, although coarse particles which would tend to settle from the dispersion are preferably removed. Preferably, the chromium-based nucleating material should 0 either remain in the alpha form during thermal processing or convert to alpha chromic oxide at a temperature below the temperature at which aluminum oxide monohydrate would transform to alpha alumina. ς Useful chromium-based nucleating materials are commercially available. For example, alpha chromic oxide is available from CMC Cometals, Inc., New York, NY. HCr0 2 may be prepared by techniques known in the art including, for example, autoclaving a chromium salt solution, such as chromium nitrate or chromium chloride. Cr0 2 may be prepared by thermally decomposing Cr0 3 , as described in U.S. Patent No. 3,117,093. The particle size of the nucleating material may be reduced using techniques known in the art, including, for example, ball milling, jet milling, and the like. Preferably, the surface of the chromium-based nucleating material is hydrated. By hydrated it is meant that the surface of the particle has reacted with water to form surface hydroxyl groups. Generally, the degree of surface hydration may be increased by exposing chro ia to moisture or by aging it in water over a period of time (e.g., several hours to several months). It is believed that hydration of the surface of the nucleating material enhances its dispersibility in a sol as well as promotes the interaction of the nucleating material with the alpha alumina precursor. Although not wanting to be bound by theory, it is believed that it may be desirable for the surface of the chromium-based nucleating material as added to the sol to have at least some of the chromium in an oxidation state higher than +3, but lower than +6. The more preferred oxidation state of the surface chromium is believed to be +4. It is thought that chromium in the +3 oxidation state is inert and may not hydrate as rapidly or interact as readily with the alumina precursor as chromium in other oxidation states. It is also thought that raising the oxidation state of the surface chromium may change the electro-kinetic properties of the chromium-based nucleating material. Such a change in the electro-kinetic properties may enhance the interaction between the chromium-based nucleating material and the alumina precursor. For example, at a pH of about 3 to 4, boehmite particles are positively charged, whereas Cr0 2 particles are negatively charged. Chromium on the surface of alpha chromic oxide particles may be raised to a higher oxidation state by techniques known in the art including comminution in the presence of an aqueous solution containing an oxidizing agent, such as chromic acid, nitric acid, atmospheric oxygen, the like, or combinations thereof. in a preferred method, the ceramic of the present invention is prepared by a sol-gel process, wherein at least one chromium-based nucleating material is added to an aqueous mixture comprising an alumina c precursor. More preferably, the ceramic of the present invention is prepared using a dispersion-based sol-gel process, wherein the alpha alumina precursor is preferably aluminum oxide monohydrate (boehmite) or a solution-based sol-gel process, wherein the alpha alumina precursor is A z. preferably an aluminum carboxylate or an aluminum nitrate. A process utilizing a mixture of the dispersion- based sol and the solution-based sol may also be useful in preparing the ceramic of the present invention. 5 -A Preferred Dispersion-Based Sol-Gel Process A sol-gel process containing boehmite as the alpha alumina precursor usually begins with the preparation of a dispersion comprising from about 2 to almost 60 weight percent aluminum oxide monohydrate in the boehmite crystalline form. The boehmite can either be 0 prepared from various techniques well known in the art or can be acquired commercially from any of a number of suppliers. For example, boehmite is commercially available under the trade designation "DISPERAL" from ς Condea Chemie, GMBH or "CATAPAL D" from Vista Chemical Company. These aluminum oxide monohydrates are in the boehmite form, are relatively pure (including relatively little, if any, hydrate phases other than monohydrates), and have a high surface area. The physical properties of the final fired product will generally depend upon the 0 type of alumina monohydrate chosen for'the dispersion. Preferably, the chromium-based nucleating material is added to the dispersion as an aqueous slurry. The dispersion may also contain a modifying additive or a precursor thereof which can be added to 5 enhance some desirable property of the finished product or increase the effectiveness of the sintering step. These additives are generally in the form of colloidal particles or soluble salts (typically water soluble) and typically consist of a metal-containing oxide or a precursor of a metal oxide, wherein preferably the metal is selected from the group consisting of magnesium, zinc, cobalt, nickel, zirconium, hafnium, yttrium, scandium, gadolinium, silicon, lanthanum, praseodymium, cerium, dysprosium, neodynium, samarium, ytterbium, erbium and titanium. The exact proportions of these components that are present in the dispersion can vary to convenience. A peptizing agent is usually added to the boehmite dispersion to produce a more stable hydroεol or colloidal dispersion. Monoprotic acids or acid compounds which may be used as the peptizing agent include acetic, hydrochloric, formic, and nitric acid. Although nitric acid is a preferred peptizing agent, it is preferable to minimize the amount of nitric acid used. The thermal degradation of nitric acid can generate N0 2 which can oxidize and affect the oxidation state of the chromium comprising the chromium-based nucleating material. Multiprotic acids are normally avoided since they rapidly gel the dispersion making it difficult to handle or mix in additional components. Some commercial sources of boehmite contain an acid titer (such as absorbed formic or nitric acid) to assist in forming a stable dispersion. Organic additives which are easily oxidized, such as sugars, starches, organic polymers, and the like may be added to the dispersion to inhibit oxidation of the chromium-based nucleating material. The dispersion can be formed by any suitable means which may simply be the mixing of aluminum oxide monohydrate with water containing a peptizing agent or by forming an aluminum oxide monohydrate slurry to which the peptizing acid is added. Once the dispersion is formed, it preferably is then immobilized. The immobilized dispersion or gel can be formed by any conventional technique such as the addition of a dissolved or dispersed -14- metal containing modifying additive, e.g., magnesium nitrate or zirconium nitrate, the addition of a salt such as ammonium acetate or ammonium nitrate, the removal of water from the dispersion, or some combination of such techniques. Once the gel has formed, it may be shaped by any convenient method such as pressing, molding or extrusion and then carefully dried to produce an uncracked body of the desired shape. If an abrasive material is desired, the gel can be extruded or simply spread out to any convenient shape and dried, typically at a temperature below the frothing temperature of the gel. Any of several dewatering methods, including solvent extraction, can be used to remove the free water of the gel to form a solid. Preferably, the gel is dried by heating in a forced air convection oven with an air temperature between about 70 and about 99 β C. After the solid is dried, it can be cut or machined to form a desired shape or crushed or broken by an suitable means, such as a hammer, roll crusher, or ball mill, to form particles. Any method for comminuting the solid can be used and the term "crushing" is used, to include all such methods. After shaping, the dried gel can then be calcined to remove essentially all volatiles and transform the various components of the dried gel into ceramics (metal oxides). Preferably, the dried gel is calcined by heating to a temperature in the range of about 400 to about 1000 β C and held within this temperature range until the free water and over 90 weight percent of any bound water is removed. Preferably, the calcining atmosphere should be such that substantially all the chromium derived from the chromium-based nucleating material is in an oxidation state not greater than +3. If an internal reducing material such as a sugar, a starch, an organic polymer, or the like is present, the dried gel may be calcined in air or in a mixture of air and an inert gas (i.e., argon, nitrogen, helium, carbon dioxide, the like, or mixtures thereof). In the absence of such an internal reducing 5 material, the calcining atmosphere preferably comprises an inert gas. More preferably, the calcining atmosphere comprises an inert gas and a reducing gas (i.e., hydrogen, carbon monoxide, the like, or mixtures thereof), and most preferably, it comprises a reducing gas. 0 The calcined material is then sintered by heating to a temperature of between about 1200 and about 1650°C and held within this temperature range until substantially all of the alpha alumina precursor is converted to alpha alumina. More preferably, the - sintering temperature is in the range from between about 1300 and about 1450°C, and most preferably, it is between about 1350 and about 1450°C. The length of time to which the ceramic must be exposed to the sintering temperature to achieve this level of conversion will depend upon 0 various factors but usually from about 1 to about 60 minutes is sufficient. The sintering atmosphere is such that substantially all the chromium comprising the nucleating agent is in an oxidation state of not greater than +3. It ς is believed, although not confirmed, that having a relatively small amount of the chromium in the +2 oxidation state may enhance the sintering process. The preferred sintering atmosphere is dependent on the amount of nucleating material present, or more specifically on the chromium content of the calcined body. 0 For a calcined body comprising more than about 0.7 weight percent chromium, based on the alumina ' content of the calcined body, the preferred sintering atmosphere comprises an inert gas. More preferably, the sintering atmosphere comprises an inert gas and a reducing gas, and most preferably it comprises a reducing gas. For*a calcined body comprising less than about 0.7 weight percent chromium, the preferred sintering atmosphere comprises a gas selected from the group consisting of air, inert gas, reducing gas, and compatible 5 mixtures thereof. Other steps can be included in this process, such as rapidly heating the material from the calcining temperature to the sintering temperature, sizing granular material, centrifuging the dispersion to remove sludge 10 waste, etc. Moreover, this process can be modified by combining two or more of the individually described steps, if desired. It is within the scope of this invention to use other nucleating materials known in the art, including ις alumina, ferric oxide, and ferric oxide precursors, provided such nucleating materials are not added in an amount sufficient to allow conversion to an alpha alumina-based ceramic having a seeded or well-seeded microstructure, in the absence of the nucleating material 0 of the present invention. These process steps and materials are more fully described in U.S. Patent Nos. 4,314,827 and 4,744,802. -A Preferred Solution-Based Sol-Gel Process _ A solution-based sol can be prepared by techniques known in the art. Typical preparation techniques include dissolving an alumina-based salt or complex in water; or diluting or concentrating a solution comprising an alumina-based salt or complex. Preferably, the solution-based sol comprises in the range of about 5 0 to 45 weight percent of an alpha alumina precursor. Preferably, the solution-based sol-gel comprises a soluble aluminum salt or other soluble aluminum-based complex. More preferably, the solution-based sol-gel comprises at least one of the following alpha alumina 5 precursors: a basic aluminum carboxylate, a basic aluminum nitrate, and a partially hydrolyzed aluminum alkoxide. Preferred solution-based sols include those comprising basic aluminum salts with carboxylate or nitrate counterions or mixtures thereof. The preferred aluminum carboxylates are 5 represented by the general formula, Al(OH) y D 3 _ y , wherein y can range from between about 1 and about 2, preferably between about 1 and about 1.5, and D (the carboxylate counterion) is formate, acetate, propionate, 0 oxalate, the like, and combinations thereof. Aluminum carboxylates can be prepared by techniques known in the art including the methods described in U.S. Patent No. 3,957,598, wherein aluminum metal is digested in a carboxylic acid solution and U.S. Patent No. 4,798,814, _ wherein aluminum metal is dissolved in a hot aqueous solution comprising formic acid and acetic acid. The preferred basic aluminum nitrates are represented by the general formula. Al(OH) z (N0 3 ) 3 _ z , 0 wherein Z can range from about 0.5 to about 2.5. The preparation of basic aluminum nitrates is known in the art and includes the methods taught in U.S. Patent No. 3,340,205 and British Patent No. 1,139,258, wherein aluminum metal is digested in a nitric acid solution. ς Basic aluminum nitrates may also be prepared according to U.S. Patent No. 2,127,504, wherein aluminum nitrate is thermally decomposed. Preferably, the chromium-based nucleating material is added to the solution as an aqueous slurry. Modifying additives as well as organic additives as described in the preferred dispersion-based sol-gel process may be added to the solution-based sol. For example, organic additives such as, sugars, starches, organic polymers, or hydroxy organic polymer acids may be admixed with the sol comprising the basic aluminum nitrate to facilitate removal of the nitrate during calcining. The ' solution-based sol may be immobilized (or rigidified) by any conventional technique including dehydrative rigidification. The immobilized sol may be dried by known techniques including those described above for drying a gelled dispersion-based sol. An abrasive material or other shaped article may be prepared as described above in the preferred dispersion-based sol-gel process. In a preferred method for forming a fiber, the 10 solution-based sol generally contains about 5 to about 20 weight percent equivalent oxide solids. The weight percent equivalent oxide solids can be determined, for example, by calcination. The sol is then concentrated or viscosified in order to convert it to a viscous fluid 15 concentrate which will readily solidify when the concentrate is extruded and drawn in air to form fibers. The concentration step can be carried out by techniques known in the art, see e.g., U.S. Patent No. 3,795,524. Sufficient concentration will be obtained when the - equivalent oxide solids content is generally in the range of 20 to 40, typically less than 35 weight percent, and viscosities (Brookfield at ambient room temperature) are in the range of 15,000 to 1,000,000 centipoise, preferably 45,000 to 600,000 centipoise, depending on the type of 2ς fiberizing dehydrative solidifying (or solution immobilization) technique and apparatus used and the desired shape of the green (uncalcined) fiber. High viscosities tend to result in fibers which are circular in cross-section whereas low viscosities (e.g., less than 50,000 centipoise) tend to result in fibers which are oval 0 in cross-section. In making continuous fibers, ' the viscous concentrate can be extruded through a plurality of orifices (e.g., up to 400 or more) from a stationary head and the resulting green fibers allowed to fall in air by 5 the force of gravity or drawn mechanically in air by means of drawing rolls or a drum or winding devices rotating at a speed faster than the rate of extrusion. The concentrate can also be extruded through orifices from a stationary or rotating head and blown by parallel, oblique or tangential streams of air such as in the making of cotton candy. The resulting blown green fibers being in staple form or short form with lengths generally 25 cm or less (rather than in long or continuous form) and collected on a screen or the like in the form of a mat. Any of these forces exerted on the extruded green fibers, e.g., gravity, drawing, or air streams, cause attenuation or stretching of the fibers, reducing their diameter by about 50 to 90 percent or more and increasing their length by about 300 to 10,000 percent or more and serving to hasten or aid the drying of the green fibers. The dehydrative solidifying of the green fibers can be carried out in ambient air or heated air can be used if desirable or necessary to obtain fast drying. The rate assists in controlling of the shape of the fiber. The relative humidity of such air should be controlled since large amounts of moisture will cause the solidified or shaped green fibers to stick together and excessively dry atmosphere can lead to fiber breakage. Generally, air with relative humidity in the range of 20 to 60 percent can be used at temperatures of 15 to 30° C, though such air can be heated subsequently to about 70° C or higher. In some cases, for example, where continuous green fibers are made and gathered together in parallel alignment or juxtaposition in the form of a multi-fiber strand, the fibers or strand can be treated with a size to prevent the fibers from sticking together. The process steps for preparing fibers are more fully described in European Patent Application No. 0294208, published December 7, 1988. The process steps for preparing other preferred embodiments (i.e., a bead, a microsphere, a flake, an uregul rly shaped particle, a film, or a microcapsule are desc ibed in the same European Patent Application. The dried solution may be calcined and sintered as described above in the preferred dispersion-based sol-gel process except the preferred calcination atmosphere is dependent on the alpha alumina precursor 5 used. If the alpha alumina precursor is a basic aluminum carboxylate, the preferred calcining atmosphere is related to the carbon content of the precursor. For a basic aluminum carboxylate having a relatively high carbon content (typically greater than . z, about 15 to 20 weight percent), the preferred calcining atmosphere comprises air or a mixture of air and an inert gas. For a basic aluminum carboxylate having a relative low carbon content (typically less than about 10 ,5 to 15 weight percent), the calcining atmosphere preferably comprises an inert gas. More preferably, the calcining atmosphere comprises a mixture of an inert gas and a reducing gas. Most preferably, the calcining atmosphere comprises a reducing gas. z - If the alpha alumina precursor is a basic aluminum nitrate, the preferred calcining atmosphere comprises an inert gas. More preferably, the calcining atmosphere comprises a mixture of an inert gas and a reducing gas. Most preferably, the calcining atmosphere 2ς comprises a reducing gas. Other steps can be included in this process, such as rapidly heating the material from the calcining temperature to the sintering temperature, sizing granular material, centrifuging the dispersion to remove sludge 0 waste, etc. Moreover, this process can be modified by combining two or more of the individually described steps, if desired. Method II Preferably, an aqueous admixture is prepared by dissolving a precursor of alpha chromic oxide in water. The aqueous admixture may contain a cationic or anionic chromium-based specie. Preferably, the chromium-based specie comprising the aqueous admixture is anionic. The aqueous admixture may comprise other ionic species, provided they do not substantially interfere with the function of the nucleating agent. The preferred alpha phase nucleating material is a chromium-based salt such as chromic nitrate or Cr0 3 . The size of the nucleating material in the liquid admixture must be small enough to allow it to pass through the pores of the transition alumina during impregnation. Preferably, the concentration of the alpha phase nucleating material in the liquid admixture is sufficient to provide the desired amount of nucleating agent after calcination. Where a single step impregnation is desired, the concentration of nucleating material in the liquid admixture should be high enough to provide, after calcination, a sufficient amount of chromium oxide to seed the alpha alumina transformation. Preferably, the specific surface area, as determined by the BET nitrogen absorption method, of the transition alumina body is greater than about 20 m 2 /g. More preferably, the specific surface area is in the range of about 50 to about 600 m 2 /g, and most preferably, it is about 180 m 2 /g. The transition alumina body may be prepared by techniques known in the art. Preferred methods of preparing transition alumina include a dispersion-based sol-gel process, wherein, preferably, the transition alumina precursor is aluminum oxide monohydrate (boehmite) or a solution-based sol-gel process, wherein, preferably, the transition alumina precursor is an aluminum carboxylate or an aluminum nitrate. -A Preferred Dispersion-Based Sol-Gel Process A sol-gel process containing boehmite as the alpha alumina precursor usually begins with the preparation of a dispersion comprising from about 2 to almost 60 weight percent aluminum oxide monohydrate. The boehmite can either be prepared from various techniques well known in the art or can be acquired commercially from any of a number of suppliers. Boehmite is commercially 5 available under the trade designation "DISPERAL" from Condea Chemie, GMBH or "CATAPAL D" from Vista Chemical Company. These aluminum oxide monohydrates are in the alpha-form, are relatively pure (including relatively little, if any, hydrate phases other than monohydrates), and have a high surface area. The physical properties of 10 the final fired product will generally depend upon the type of alumina monohydrate chosen for the dispersion. The dispersion may also contain a modifying additive or a precursor thereof which can be added to z. _ enhance some desirable property of the finished product or increase the effectiveness of the sintering step. These additives are in the form of colloidal particles or soluble salts (typically water soluble), and typically consist of a metal-containing oxide or a precursor of a 20 metal oxide, wherein preferably the metal is selected from the group consisting of magnesium, zinc, cobalt, nickel, zirconium, hafnium, yttrium, scandium, gadolinium, silicon, lanthanum, praseodymium, cerium, dysprosium, neodynium, samarium, ytterbium, erbium, and titanium. The exact proportions of these components that are present in 5 the dispersion can vary to convenience. A peptizing agent is usually added to the boehmite dispersion to produce a more stable hydroεol or colloidal dispersion. Monoprotic acids or acid compounds which may be used as the peptizing agent include acetic, 0 hydrochloric, formic, and nitric acid. The preferred peptizing agent is nitric acid. Multiprotic acids are normally avoided since they rapidly gel the dispersion making it difficult to handle or mix in additional components. Some commercial sourceε of boehmite contain 5 an acid titer (εuch as absorbed formic or nitric acid) to assist in forming a stable dispersion. The disperεion can be formed by any εuitable means which may simply be the mixing of aluminum oxide monohydrate with water containing a peptizing agent or by forming an aluminum oxide monohydrate slurry to which the peptizing acid is added. Once the dispersion is formed, it preferably is then gelled. The gel can be formed by any conventional technique such as the addition of a dissolved or diεperεed metal containing modifying additive, e.g., magneεium nitrate or zirconium nitrate, the addition of a εalt εuch aε ammonium acetate or ammonium nitrate, the removal of water from the dispersion, or some combination of such techniques. Once the gel haε formed, it may be shaped by any convenient method εuch aε pressing, molding or extrusion and then carefully dried to produce an uncracked body of the desired shape. If an abrasive material is desired, the gel can be extruded or simply spread out to any convenient shape and dried, typically at a temperature below the frothing temperature of the gel. Any of several dewatering methods, including solvent extraction, can be uεed to remove the free water of the gel to form a εolid. Preferably, the gel iε dried by heating in a conventional forced air convection oven with an air temperature between about 70 and about 99 β C. After the εolid iε dried, it can be cut or machined to form a deεired εhape or crushed or broken by an suitable means, such as a hammer, roll crusher, or ball mill, to form particles. Any method for comminuting the solid can be used and the term "crushing" is used, to include all such methods. After shaping, the dried gel ' can then be calcined to remove essentially all volatiles and transform the various components of the dried gel into ceramics (metal oxides). Preferably, the dried gel is calcined by heating to a temperature in the range of about 400 to about 1000°C and held within this temperature range until -24- the free water and over 90 weight percent of any bound water is removed. Generally it iε preferred that the calcined material have a high εurface area. Other εtepε can be included in this proceεε, such as rapidly heating the material from the calcining temperature to the sintering temperature, sizing granular material, centrifuging the disperεion to remove εludge waεte, etc. Moreover, thiε proceεε can be modified by combining two or more of the individually described steps, if desired. These procesε steps and materials are more fully described in U.S. Patent Nos. 4,314,827 and 4,744,802. -A Preferred Solution-Based Sol-Gel Procesε A solution-baεed εol can be prepared by techniqueε known in the art. Typical preparation techniqueε include dissolving an aluminum-based salt or complex in water; or diluting or concentrating a solution comprising an aluminum-based salt or complex. Preferably, the solution-baεed sol compriseε in the range of about 5 to about 45 weight percent of an alpha alumina precurεor. Preferably, the εolution-baεed εol-gel comprises a soluble aluminum salt or other soluble aluminum-based complex. More preferably, the εolution-baεed εol-gel co priεeε at leaεt one of the following alpha alumina precurεorε: a baεic aluminum carboxylate, a baεic aluminum nitrate, and a partially hydrolyzed aluminum alkoxide. Preferred εolution-based sols include those comprising baεic aluminum εaltε with carboxylate or nitrate counterions or mixtures thereof. Preferred aluminum carboxylates are represented by the general formula, wherein y can range from between about 1 and about 2, preferably between about 1 and about 1.5, and D (the carboxylate counterion) iε formate, acetate, propionate, oxalate, the like, and combinationε thereof. Aluminum carboxylateε can be prepared by techniques known in the art including the methods described in U.S. Patent No. 3,957,598, wherein aluminum metal is digested in a carboxylic acid solution and U.S. Patent No. 4,798,814, wherein aluminum metal is dissolved in a hot aqueous solution comprising formic acid and acetic acid. Preferred basic aluminum nitrates are represented by the general formula. Al(OH) z (N0 3 ) 3 _ z , wherein Z is in the range of about 0.5 to about 2.5. The preparation of basic aluminum nitrates iε known in the art and includeε the methodε taught in U.S. Patent No. 3,340,205 and Britiεh Patent No. 1,139,258, wherein aluminum metal iε digeεted in a nitric acid εolution. Baεic aluminum nitrateε may alεo be prepared according to U.S. Patent No. 2,127,504, wherein aluminum nitrate iε thermally decomposed. Modifying additiveε aε deεcribed in the preferred dispersion-based sol-gel proceεε may be added to the εolution-based sol. The solution-baεed sol may be immobilized (or rigidified) by any conventional technique including dehydrative rigidification. The immobilized sol may be dried by known techniqueε including thoεe deεcribed above for drying a gelled diεperεion-baεed εol. An abraεive material or other εhaped article may be prepared as described above in the preferred dispersion-based sol gel process. The process steps for preparing other preferred embodiments (i.e., a bead, a microsphere, a flake, an irregularly εhaped particle, a film, or a microcapsule) are deεcribed in European Patent Application No. 0294208, published December 7, 1988. The dried εolution may be calcined as described above in the preferred disperεion-based sol-gel process. Other steps can be included in this procesε, such as sizing granular material, centrifuging the disperεion to remove εludge waεte, etc. Moreover, thiε process can be modified by combining two or more of the individually described εtepε, if desired. It is within the scope of this invention to prepare a transition alumina-based body by making a transition alumina-based powder according to a sol-gel method described above or by other techniques known in the art including, for example, calcining alumina trihydrate, conventionally forming the powder into a shaped, green article, and then calcining. Such conventional forming techniques are known in the art and include, for example, dry pressing, slip casting, tape casting, etc. •A Preferred Impregnation Method A liquid admixture according to the present invention may be impregnated into a transition alumina body using impregnation techniques known in the art, including, for example, allowing a liquid to wick into a porous body, forcing a liquid into a porous body by the application of presεure, or using an aεpirator to pull a partial vacuum to allow air trapped within the poroεity of a body to eεcape, and a liquid to more infiltrate the poroεity. Impregnation of the transition alumina may be accomplished in a single impregnation (i.e., using only one impregnation liquid a single time) or it may be accomplished in several impregnation steps (i.e., the single impregnation step may be repeated). Preferably, the number of impregnation εtepε is not greater than 3. The precursor and concentration thereof may be the same or different for each impregnation. The impregnation liquid may, if desired, comprise of more than one alpha phase nucleating material. Between each impregnation step, excess liquid may be removed, if necesεary, by filtering or centrifuging, and the impregnated material iε then dried and calcined, aε described in the preferred c dispersion-based sol-gel proceεε above, to remove volatile species introduced during impregnation, except that the calcination atmosphere depends on the nucleating agent chosen. The atmosphere should be εuch that the impregnated precursor convert to its nucleating phase during calcining. If the nucleating material iε Cr0 3 the preferred calcining atmoεphere comprises air. More preferably, the calcining atmosphere compriseε a mixture of air and an inert gas (i.e., argon, nitrogen, helium, carbon dioxide, 15 the like, or mixtures thereof). Even more preferably, the calcining atmosphere compriseε an inert gaε. Still more preferably, the calcining atmoεphere compriεeε a mixture of an inert gaε and a reducing gaε (i.e., hydrogen, carbon monoxide, the like, or mixtureε thereof), and most preferably it compriεeε a reducing gaε. 20 If the nucleating material iε chromic nitrate the preferred calcining atmoεphere comprises an inert gas. More preferably, the calcining atmosphere comprises a mixture of an inert gas and a reducing gas. Most 2 __b preferably, the calcining atmosphere compriεeε a reducing gas. After the final impregnation and calcining steps the material is sintered by heating to a temperature sufficient to provide an alpha alumina-based ceramic having a seeded or well-seeded microstructure. 30 Preferably, the sintering temperature is in the range of about 1200 to about 1650°C. fiore preferably, the sintering temperature is in the range of about 1300 to about 1450°C, and most preferably about 1350 to about 1450°C. 5 -28- The sintering atmosphere should be such that substantially all the chromium comprising the nucleating material iε in an oxidation εtate of not greater than +3. It is believed, although not confirmed, that having a c relatively small amount of chromium in the +2 oxidation state may enhance the sintering procesε. Preferably, the εintering atmosphere compriεes a gas selected from the group consisting of air, inert gas, reducing gas, and compatible mixtures thereof. 10 Optionally, the calcined material may be rapidly heated from the calcining temperature to the sintering temperature. The sintered ceramic may be coated with a powder or powder-like coating. In applications requiring good A - adhesion to a ceramic according to the present invention, it is preferably that this coating be removed. Such a coating may be removed by techniques known to one skilled in the art including washing with water, sand milling, vibro milling, the like, and combinations thereof. 20 It is within the scope of this invention to use a transition alumina body comprising nucleating agentε or precurεorε thereof known in the art which have been introduced prior to impregnating according to the preεent invention, provided such nucleating agents are not added c in a sufficient amount to provide an alpha alumina-based ceramic having a seeded or well seeded microstructure, in the abεence of the impregnated nucleating agent according to the preεent invention. It iε alεo within the εcope of thiε invention to add the modifying additives described above using an 0 impregnation procesε. The εtepε of impregnating a tranεitional alumina are more fully deεcribed in European Patent Application No. 0293163. 5 The alpha alumina-baεed ceramic of the present invention may consist esεentially of alpha alumina having the chromium-baεed nucleating material diεperεed therein. Preferably, the total amount of alumina and chromium oxide preεent in the alpha alumina-baεed ceramic of the present invention is at least 60 weight percent, based on the total ceramic composition. More preferably, the amount of alumina and chromium oxide present is at least 80 weight percent of the ceramic composition, and even more preferably alumina and chromium oxide comprise at least 90 weight percent. The improved ceramic may be substantially void free or it may be characterized by including porosity. Typically equaxial pores which are for the most part on the interior of the ceramic with a minor part of the poreε extending to the εurface. Preferably, however, the porosity is closed. Preferably, the improved ceramic has a hardnesε greater than 18 GPa. More preferably, the hardness is greater than 20 GPa, even more preferably greater than 24 GPa, and most preferably, greater than 25 GPa. The ceramic material of the present invention may be used in areas where conventional ceramics are used and particularly where a more durable ceramic is needed. Particularly useful embodiments include articles in the shape of an abrasive grain, a fiber, a bead, a microsphere, a flake, an irregularly εhaped particle, a film, a microcapεule, a thread guide, a bearing, or mill media. An abrasive grain according to the preεent invention may be used in conventional abraεive productε. Preferred abrasive products are coated abrasives, bonded abrasiveε, such as abrasive grinding wheels or cut-off wheels, and low density, three-dimensiβnal nonwoven abraεiveε. In a preferred embodiment the abraεive grains according to the preεent invention iε blended with less expensive conventional mineral such as fused aluminum oxide, silicon carbide, garnet, fused alumina-zirconia, and the like. The 'methods of making such abrasive products are well-known to those skilled in the art. A coated abrasive product includes a backing, for example, formed of fabric (e.g., woven or non-woven fabric such as paper) which may ,- be saturated with a filled binder material, a polymer film such aε that formed of oriented heat-set polypropylene or polyethylene terephthalate which may be first primed, if needed, with a priming material, or any other conventional backing material. The coated abraεive also includes a binder material, typically in layers including a make or maker coat, a size or εizing coat and poεsibly a εupersize coat. Conventional binder materialε include phenolic resins. Low-denεity, three-dimenεional, non-woven z, - abraεive productε typically include an open porous lofty polymer filament structure having the ceramic abrasive grain distributed throughout the structure and adherently bonded therein by an adhesive material. Bonded abrasive productε typically conεist of a shaped masε of abraεive grain held together by an organic 20 or ceramic binder material. The εhaped maεs is preferably in the form of a grinding wheel. The preferred binder materials for the ceramic abrasive grain of the invention are organic binderε. Ceramic or vitrified binderε may be used if they are curable at temperatures and under 5 conditions which will not adversely affect the ceramic abrasive grain of the present invention. A preferred embodiment of the present invention is a fiber containing lesε than 0.1 weight percent iron equivalence, based on the total alpha alumina content of 0 the fiber. More preferably, the fiber contains lesε than 0.05 weight percent iron equivalence, όased on the total alpha alumina content of the fiber. Thiε invention iε further illuεtrated by the following exampleε, but the particular materials and 5 amounts thereof recited in these examples, as well as other conditions and detailε, εhould not be construed to unduly limit this invention. All parts and percentages are by weight unlesε otherwiεe indicated. Examples Control A Example Control A, which haε a non-seeded microstructure, illustrates an alpha alumina-based ceramic prepared without the inventive alpha alumina nucleating material of the present invention. A 1 liter "Waring" type blender (Waring Products Div., Dynamics Corp. of America; New Hartford, CT) was charged with 420 grams of deionized water, while rapidly stirring, 168.5 grams of boehmite (commercially available under the trade designations "DISPERAL" from Condea Chi ie; Hamburg, West Germany) were added followed by 5.6 ml of 16 N HN0 3 . An ammonium acetate solution was prepared by dissolving 2.2 grams of ammonium acetate in 10 grams of deionized water. To induce gelation, the ammonium acetate εolution waε added to the boehmite dispersion, while rapidly stirring. The dispersion was transferred to a shallow aluminum tray and allowed to gel. The gel was dried overnight in a conventional forced air convection oven at 95°C. The dried gel was calcined in a tube furnace with a flowing 95% N 2 /5% H 2 atmosphere by heating to 650°C at a rate of about 3.5 β C/minute. The calcined gel was sintered in a conventional box furnace (Rapid Temp; CM, Inc., Bloomingfield, NJ) in air for 5 minutes at a temperature of 1400 β C. The εintered ceramic waε white. Several pieceε of the sintered ceramic were crushed using a tungsten carbide mortar and pestle. Optical microscopy of the crushed ceramic at a magnification of 500x, using cross-polarized light, indicated domainε having a diameter in the range of 5 to 12 micrometers. The average diameter of the domains was 6 to 8 micrometers. SEM (scanning electron microscopy) examination of a fracture surface of the sintered ceramic revealed a porous, vermicular microεtructure with non-faceted domainε. The denεity of the ceramic waε eεtimated to be less than 80 percent of theoretical. 10 Control B Example Control B illustrateε that the mere addition of a chro ium-baεed material does not necesεarily produce an alpha alumina-baεed ceramic having a εeeded or well-εeeded microstructure, nor does its presence necessarily have a significant effect on the 15 microstructure of an alpha alumina-baεed ceramic. Control B was prepared in the same manner as Control A except 17.6 grams of basic chromium acetate (Cr 3 (OH) 2 (CH 3 C0 2 ) ) 7 ; Matheson Coleman Bell Manufacturing Chemists; Norwood, OH) was added to the dispersion prior 20 to the addition of the nitric acid. The color of the sintered ceramic, which waε not homogeπouε, waε pink-purple with εome εmall green regionε. The εintered ceramic was examined with an 2 -_b optical microscope and an SEM as described in Control A. The microstructure of the Control B ceramic was εimilar to the microstructure of the Control A ceramic. Example 1 Example 1 illustrateε the uεe of Cr0 2 to produce 30 a boehmite-derived alpha alumina-based-ceramic having a well-seeded microstructure. A 4 liter "Waring" industrial type blender was charged with 850 grams of deionized water. While stirring at high speed, 336.9 grams of boehmite were added followed 5 by 11 ml of 16N HN0 3 and 14.7 grams of Cr0 2 powder (having an average primary dimension of about 0.3 micrometer; commercially available under the trade designation DUPONT D-500-02 from E. I. DuPont de Nemours & Co.; Wilmington, DE). An ammonium acetate solution was prepared by dissolving 4.6 grams of ammonium acetate in 10 grams of deionized water. To induce gelation, the ammonium acetate solution was added to the boehmite dispersion, while rapidly stirring. The gel was transferred to a shallow aluminum tray and allowed to dry in air for 3 days. The gel was then dried in a forced air convection oven overnight at 90°C. The dried gel waε calcined and sintered as described in Control A. The calcined material was green. The sintered material was a deep pink-purple. SEM examination, as described in Control A, revealed a dense microstructure (i.e., greater than about 95 percent of theoretical density) comprising domains having an average diameter of less than about 0.8 micrometer. The size of the pores, which were present at triple point junctions of the faceted domains, were lesε than about 0.2 micrometer. Example 2 Example 2, which haε the εame Cr 2 0 3 equivalent as the ceramic of Example 1, illustrateε the superior εeeding capability of a Cr0 2 nucleating material added to the boehmite sol over the addition of comparatively sized Cr 2 0 3 particles. Example 2 was prepared in the same manner as Example 1 except 13.3 grams of alpha chromia powder (having an average primary dimension of about 0.3 micrometer; Grade GR-99; CMC Cometals, -Inc; New York, NY) was added to the dispersion in place of the Cr0 2 powder. SEM examination, as described in Control A, revealed porous vermicular domains and a microstructure which was about 80 to 85 percent of theoretical density. Few well-defined faceted domains were present. -34- Although the ceramic of thiε example had the same composition as the ceramic of Example 1, the Example 2 ceramic was significantly lighter in color than the Example 1 ceramic. It is believed that the difference in color is due to the difference in density, wherein the higher the density (i.e., less porous), the darker the color. Example 3 Example 3 further illustrates the superior seeding capability of a Cr0 2 nucleating material added to the boehmite sol over the addition of Cr 2 0 3 particles. Example 3 was prepared in the εame manner aε Example 1, except the "Waring" type blender waε charged with 650 gramε of deionized water rather than 850 gramε, 204 gramε of an alpha chromia εol (prepared as described below) was substituted for the Cr0 2 powder, and a solution of 6 grams of ammonium acetate dissolved in 15 grams of deionized water was added to induce gelation. The alpha chromia sol was prepared by charging a "Waring" type blender with 500 grams of deionized water and 100 grams of alpha chromia (Grade GR-99; CMC Cometals, " Inc.). While εtirring rapidly, 1.75 ml of a diεpersing aid (commercially available under the trade designation CORCAT P-600 polyethyleneimine from Virginia Chemicals; Portsmith, VA) were added to the alpha chromia diεperεion. The alpha chromium diεperεion waε allowed to εtand undiεturbed for 2 days. The supernatant was separated by decantation. The concentration of alpha chromia preεent in the supernatant, as determined by calcination was about 1.6 percent. SEM examination, as described in Control A, revealed a microstructure similar to the ceramic of Example 2 except the density was about 85 to 90% of theoretical. Example 4 In Example 4, t e particle εize of a Cr 2 0 3 nucleating agent was reduced by ball milling. The Cr 2 0 3 nucleating material having the reduced particle size was used in preparing a boehmite-derived alumina-based ceramic which was poorly seeded (i.e., approaching a seeded microstructure) . A 2-liter, rubber-lined mill jar waε charged with 4600 gramε of zirconia mill media (1.3 cm(l/2 inch) diameter cylinderε; Stoneware Corp.; Mahwah, NJ), 1 liter of deionized water, 300 grams of chromium oxide (having an average particle size of about 0.6 to 0.8 micrometer; Chrome Oxide 112; American Chrome and Chemical Inc.; Corpus Christi, TX), and 100 ml of 16N hydrochloric acid. The jar was rotated at 70 rp for 72 hours. The resulting slurry was eentrifuged at 5000 rpm for 40 minutes. The εupernatant waε removed by decantation. The eentrifuged solid was redispersed in 400 ml of deionized water and then allowed to stand undisturbed for about 4 weeks at a temperature of about 25°C. The solid material which had settled was redisperεed by εtirring and by the application of sonic energy provided by a Fisher Sonic Dismembrator (Model 300; Fisher Scientific; Pittsburgh, PA), wherein the sonic dismembrator was set to provide intermediate tip at 60% power for 4 minutes. The disperεion waε then allowed to stand undisturbed for about 12 hours. The resulting supernatant was removed by decantation. The concentration of chromium oxide present in the supernatant was determined by calcination to be about 7 percent. Two hundred grams of the supernatant containing 7 percent chromium oxide was allowed to stand undiεturbed for about 1 week. The resulting supernatant was removed by decantation and determined by calcination to contain about 0.54 percent chromium oxide. A "Waring" type blender waε charged with 20 grams of the 0.54 percent chromium oxide sol deεcribed above. While rapidly εtirring, 5 grams of deionized water, 7 grams of boehmite and 0.3 ml of 16N HN0 3 were added to the supernatant containing 0.54 percent chromium oxide. Three drops of a 1 molar ammonium acetate solution were then added to the dispersion, while εtirring, to induce gelation. The diεperεion gelled within about 5 minuteε. The gel was transferred to a porcelain dish and dried in a forced air convection oven for 4 hours at a temperature of 95 β C. The dried gel was calcined as described in Example 1. The calcined material was green. The calcined gel was sintered as described in Example 1. Optical microscopy examination, as described in Control A, indicated domains having a diameter of lesε than about 2 micrometerε. SEM examination, aε deεcribed in Control A, revealed domains having a diameter of less than about 2 micrometers, wherein regions of the ceramic had greater than about 80 to 85% of theoretical density as well aε porous regions having a vermicular microstructure. The ceramic exhibited both trans- and intra-granular fracture. Example 5 Example 5 illustrateε the uεe of a Cr 2 0 3 nucleating material to produce boehmite-derived alpha alumina ceramic having a seeded microstructure by using glacial acetate acid as a disperεing aid rather than nitric acid. A "Waring" type blender was charged with 275 grams of deionized water. While stirring rapidly, the following were added in sequence: 156.2 grams of boehmite, 10 ml of glacial acetic acid, 50 gramε of the εupernatant deεcribed in Example 4 (containing 7 percent chromium oxide) and 1 ml of a 2 molar ammonium acetate solution. The resulting gel was transferred to aluminum trays and dried in a forced air convection oven for 72 hourε at a temperature of 75° C. About 10 gramε of the dried gel waε calcined in a tube furnace with flowing 95%Ar/5% H 2 atmoεphere by heating to 550° C at a rate of about 4.4°C/minute. The calcined material waε green. The calcined material waε sintered aε deεcribed in Example 1. Optical microεcopy examination, aε deεcribed in Control A, revealed domainε having an average diameter of leεε than about 2 micrometers. SEM examination, aε deεcribed in Control A, revealed εub-micrometer domainε, wherein the denεity of the ceramic waε about 80% of theoretical. The ceramic exhibited both trans- and intra-granular fracture. The density of the ceramic of this example was increased to about 90% of theoretical by further sintering for about 10 minutes at about 1460 β C. The average diameter of the domains comprising the re-sintered ceramic was about 1 micrometer. Example 6 Example 6 illustrateε that the alpha alumina ceramic described in Example 5 can be prepared by sintering the calcined material from Example 5 in air. About 10 grams of the dried gel described in Example 5 were calcined in a tube furnace in air by heating the material to 600°C at a rate of about 9.6°C/ minute and then holding the material at 600°C for 30 minutes. The calcined material was green. The calcined material was sintered in a box furnace in air for 5 minutes at a temperature of 1400°C. Optical microscopy examination, as described in Control A, revealed domains having a diameter of less than about 2 micrometers. SEM Examination, as described in Control A, revealed εub-micrometer domainε and a denεity of about 80% of theoretical. The ceramic exhibited both trans- and intra-granular fracture. - The density of the ceramic of this example increaεed to about 90% of theoretical by re-εintering at 1460 β C for 10 minuteε. The average diameter of the domains comprising the re-sintered ceramic was about 1 micrometer. 0 Example 7 Example 7 illustrateε the uεe of a Cr 2 0 3 nucleating material to produce a εolution-derived alpha alumina-baεed ceramic having a well-εeeded microstructure. _ A basic aluminum carboxylate solution was prepared according to the procedure described in Example 1 of U.S. Patent No. 4,798,814, except the ratio of total carboxylic acid to aluminum ratio was about 1:1 and the amount of aluminum present waε about 9 percent. 0 Twenty gramε of a diεperεion containing 7 percent chromium oxide, prepared aε deεcribed in Example 4, waε added to 25 grams of the basic aluminum carboxylate solution described above, while stirring. The mixture was autoclaved at 235°C for 1 hour. The mixture was cooled and then eentrifuged at a speed of 5000 rpm for 10 b minutes. The turbid supernatant sol was dried at 95°C. The resulting solid was calcined in a tube furnace with a flowing N 2 atmosphere by heating it to 600° C at a rate of about 4.8°C/ minute. The calcined material was green. The calcined gel was sintered as described in Example 1. The sintered ceramic was pink. SEM examination, as described in Control A, revealed an extremely uniform microstructure wherein most of the domains had a diameter in the range of about 0.2 to 0.4 micrometer. The largest domain observed had a diameter of less than about 0.5 micrometer. The density of the ceramic was greater than about 99% of theoretical. Exa ple 8 Example 8 further illustrates the use of a Cr0 2 nucleating material to produce a boehmite-derived alpha alumina ceramic having a εeeded microεtructure. A "Waring" type blender was charged with 850 grams of deionized water. While rapidly εtirring, the following were added in sequence: 336.9 grams of boehmite, 14.7 grams of Cr0 2 powder (commercially available under the trade designation "DUPONT" D-500-02), and 11 ml of 16N nitric acid. A 2 molar ammonium acetate solution was added dropwise, while rapidly stirring, until gelation occured. The gel was transferred to a thick walled filter flask and degassed in a vacuum. The degassed gel waε dried at 75 β C in a forced air convection oven for 72 hours. The dried gel was calcined in a tube furnace as described in Example 1. The color of the calcined material was green. Ten grams of the calcined gel were sintered as described in Example 1. Optical examination, as described in Control A, indicated domains having a diameter of lesε than about 1 micrometer. SEM examination, as described in Control A, revealed a microstructure compriεing domainε having an average diameter of about 0.4 to 0.9 micrometer, wherein the median diameter was about 0.7 to 0.9 micrometer. The density of the ceramic was observed to be about 90 to 95 percent of theoretical. Example 9 Example 9 demonstrateε the uεe of the ceramic according to Example 8 as an abraεive grain. Abraεive grain were prepared using the proceεε described in Example 8, except the size of the batch was increased by a factor of four, the dried gel was crushed with a roll crusher (Laboratory Model; Allis Chalmers; Milwaukee, WI) and then screened to about -20+54 mesh -40- (about 295 to about 833 micrometerε) uεing U.S. Standard Screens. The screened material was calcined as described in Example 1 except the atmosphere was 95% Ar/5%H 2 . The color of the calcined material was green. The abrasive grain were used to make 17.75 cm (7-inch) diameter coated abrasive discε having a 2.2 cm (7/8 inch) centerhole. The abraεive grain for each diεc conεisted of a 1:1 by weight mixture of -30 + 35 mesh (average diameter of about 550 micrometers) and -35 + 40 mesh (average diameter of about 460 micrometers) screen cuts obtained using U.S. Standard Screens. The discs were prepared using conventional coated abrasive making procedures, conventional 0.76 mm vulcanized fiber backings and conventional calcium carbonate-filled phenolic make and size resins, without adjusting for mineral density differences. The make resin waε precured for 75 minuteε at 88°C. The εize reεin was precured for 90 minutes at 88°C followed by a final cure of 100°C for 10 hours. Conventional one-trip coating techniques and curing in a forced air oven were employed. The coating weights (wet basiε) were aε follows: Coating Coating Weight (g/cm, ) make 0.017 size 0.052 mineral 0.073 The resultant cured discε were firεt conventionally flexed to controllably crack the hard bonding reεinε, mounted on a beveled aluminum back-up pad, and uεed to grind the face of 1.25 cm x 18 cm 1018 cold rolled steel workpiece. The disc was driven at 5000 rpm while the portion of the disc overlying the beveled edge of the back-up pad contacted the workpice at a presεure of 0.91 kg/cm 2 , generated a disc wear path of about 140 cm 2 . Each disc was used to grind 12 separate workpieces for 1 minute each. Three diεcs were prepared and tested according c to the procedures described above, except the abraεive material waε brown fuεed alumina. Using the average cumulative cut of 3 discs made with brown fused alumina abrasive grain as 100%, the average relative cumulative cut of the 12 cutε for 3 diεcε having the abraεive 0 grain of Example 9 waε 239%. Three diεcε were prepared and tested according to the procedures described above, except the abraεive material waε abraεive grain aε taught by Schwabel in U.S. Patent No. 4,744,802 (commercially available under the 5 trademarked deεignation CUBITRON from 3M; St. Paul, MN), the diεcloεure of which iε incorporated herein by reference. Uεing the average cumulative cut of 3 diεcε made using "CUBITRON" abrasive grain as 100%, the average relative cumulative cut of the 12 cutε for 3 discs having Q the abrasive grain of Example 9 was 94%. Example 10 Example 10 demonstrateε the εuperior grinding performance of the abraεive grain according to Example 9 when εintered in a 95% N 2 /5%H 2 atmoεphere rather than in air. Abrasive grain were prepared in the same manner as described in Example 9, except the calcined gel was sintered in a 95%N 2 /5% H 2 atmosphere. Coated abraεive discε having the abraεive grain of thiε example were prepared and teεted aε deεcribed in Example 9. The average relative cumulative cutε of the coated abraεive diεcε having the abraεive grain of Example 10 waε 102% of diεcε having "CUBITRON" abraεive grain. The hardneεs of seven Example 10 abrasive particles were determined by a standard Knoop indentation technique (Leitz Miniload 2; Ernst Leitz Wetzlar GMBH; West Germany) where the applied load was 500 grams. The average hardness of 5 indents per particle ranged from 23 to 26 GPa. 5 Example 11 Example 11 illustrate the use of a Cr0 2 nucleating material to produce a solution-derived alpha alumina-based ceramic comprising domains having an average 10 diameter of lesε than 1.0 micrometer. A 2 liter, rubber lined ball mill waε charged with 4600 gramε of zirconia mill media (U.S. Stone Corp.; Mahwah, NJ), 600 gramε of a baεic aluminum carboxylate εolution (prepared aε deεcribed in Example 7), and 75 15 grams of Cr0 2 powder (commercially available under the trade designation "DUPONT" D-500-02). The jar was milled at 70 rpm for 24 hours. Ten grams of the slurry were removed from the mill jar and diluted with 50 grams of the basic aluminum carboxylate solution described above, while stirring. The 20 resulting disperεion waε allowed to εtand undiεturbed for 72 hourε. The clear εupernatant waε removed, dried in a convection oven at 85° C for 12 hourε, and calcined in air by heating to 650° C at a rate of about 10.4° C/minute. - j . The calcined material was sintered aε deεcribed in Example 1. The sintered ceramic was a very light pink. Optical examination, as described in Control A, revealed a microstructure having a clear almoεt glass-like appearance, wherein the average diameter of the domains was less than about 1 micrometer. 30 The slurry remaining in the mill jar was milled for an additional 48 hours. The slurry was removed and allowed to stand undisturbed for 12 hours. The clear supernatant was removed by decantation. About 119.5 grams of the εupernatant diεperεion 5 juεt deεcribed waε added to 4.9 gramε of a 88% lactic acid, while εtirring. The mixture waε filtered through a fine filter paper (Whatman No. 5; W&K Balston; England). The filtered liquid waε concentrated to about 24 percent εolids, based on the equivalent oxide, by roto-evaporation (commercially available under the trade designation "ROTOVAPOR" flask from Buchi/Brinkmann Rotary Evaporators; Westbury, NJ). Fibers were formed by extruding this concentrate through a spinnerette having 40 to 76 micrometer diameter holes. The fibers were drawn down, while partially drying in air having 35% relative humidity, and collected onto a rotating 58.4 cm (23 inch) diameter wheel. The fibers were calcined in air by heating to 650° C at a rate of about 10.4°C/minute. The calcined fiberε were green in color. The calcined fiberε were εintered in a conventional box furnace in air for 5 minuteε at 1400 β C. The diameter of the εintered fiberε waε about 10 to 40 micrometerε. SEM examination of a fiber surface indicated the average surface roughness height was less than about 0.100 micrometer. SEM examination of a fiber, as deεcribed in Control A, revealed a microstructure having a uniform distribution of 0.15 to 0.3 micrometer diameter domainε and a density of about 95 to about 100% of theoretical. Example 12 Example 12 illustrateε the use of a chromium-based nucleating material prepared by hydrolyzing a chromium salt in the preεence of a reducing material to produce a boehmite-derived alpha alumina-baεed ceramic having a well-εeeded microεtructure. A 3-necked, 1000 ml round bottom flaεk waε fitted with a reflux condenser and a rubber septum, and then connected to an oil filled gas bubbler. The flask was charged with 79.9 gramε of CrCl 3 -6H 2 0 (J.T. Baker Chemical Co.; Phillipsburg, NJ) and 500 ml of deionized water. While εtirring, the εolution waε deaerated by vigorouεly bubbling nitrogen gaε through the solution for about 30 minutes. About 2.7 grams of aluminum powder (Fisher Scientific Co.; Fair Lawn, NJ) was introduced into the solution under a positive preεεure of nitrogen gaε. The mixture waε gently heated with a heating mantle for 30 minutes. The mixture was then refluxed for 30 minutes. About 1 gram of chromium powder (Lunex Co.; Davenport, IA) was then added to the mixture. The mixture was refluxed for 16 additional hours and then cooled. The mixture was eentrifuged at 5000 rpm for 50 minutes. The εupernatant , 0 was decanted and the solid residue redispersed in 250 ml of deionized water by stirring and by the application of sonic energy for 90 εecondε. Gravimetric evaluation of the resulting disperεion, aε deεcribed in Example 4, determined it contained 3 percent chromium oxide. 5 A "Waring" type blender waε charged with about 154.6 gramε of the diεperεion just described 800 gramε of deionized water. While rapidly εtirring, the following were then added in sequence: 200 grams of boehmite and 3 ml of 16N nitric acid. A 2 molar ammonium acetate Q solution was added drop-wiεe, while rapidly εtirring, until gelation occurred. The gel waε tranεferred to aluminum trayε and dried at 95°C in a forced air convection oven for 12 hourε. The dried gel waε calcined aε deεcribed in Example 1. The calcined material waε 5 green. The calcined material was εintered as described in Example 1. SEM examination, as described in Control A, revealed a microstructure comprising domains having an average diameter in the range of about 0.7 to 1 0 micrometer. The density of the ceramic was about 95 to 99 percent of theoretical. Example 13 Example 13 illustrates the use of a chromium-based nucleating material prepared by hydrolyzing a chromium salt in the presence of a reducing material to produce a solution-derived alpha alumina-based ceramic having a seeded microstructure. A 3-neck, 250 ml round bottom flask was fitted with a rubber septum through which gas could be introduced with a needle, a pressure release valve, and a reflux condenser attached to an oil bubbler. The flask was charged with 20 grams of CrCl 3 .6H 2 0 and 50 ml of deionized water. While stirring, the solution waε deaereated by pasεing N 2 through the εolution for about 0.5 hour. Under a poεitive preεsure of N 2 gaε, 1.7 gramε of aluminum powder were introduced into the εolution. The mixture waε 10 gently heated, aε deεcribed in Example 13, for about 20 minutes. The mixture was refluxed for about 1 hour and allowed to cool. Seven millimeters of the cooled mixture was extracted by syringe and transferred to a small autoclave. The mixture was autoclaved for about 3 hours ις at 250 β C. The resulting dispersion waε analyzed using conventional X-ray diffraction techniqueε wherein the cryεtalline phaεeε identified were boehmite and HCr0 2 . About 3 gramε of the diεperεion waε eentrifuged at 5000 rpm for 15 minutes. The supernatant was removed by 2 Q decantation and discarded. The reεidue waε dispersed in 5 grams of deionized water. About 1.5 grams of this dispersion was added to 50 grams of a basic aluminum carboxylate solution, (prepared aε described in Example 11), while stirring. 5 The resulting sol was dried in a forced air convection oven for 4 hourε at 85 β C. The dried material waε calcined in air for 1 hour at 500°C. The color of the calcined material was green. The calcined material was sintered in a box furnace for 1 to 2 minutes at 1400°C in air. The εintered ceramic was pink. 0 SEM examination, as described in Control A, revealed a microstructure having a bi-modal distribution comprised of faceted domains having diameters in the 0.2 to 0.4 micrometer range and spherical domains having diameters in the 0.05 to 0.1 micrometer range. The 5 density of the ceramic was about 90 to 95 percent of theoretical. Example 14 Example 14 illustrates the use of a chromium-based nucleating material prepared by autoclaving a chromium salt to produce a boehmite-derived alpha c alumina-based ceramic having a well-seeded microstructure. A "Waring" type blender was charged with 100 ml of deionized water, 12 gramε of Cr(N0 3 ) 3 -ΘH J O, and 4.1 gramε of boehmite. The contentε of the blender were εtirred rapidly for about 5 minuteε. About 7 ml of thiε 0 diεperεion waε placed in a εmall autoclave and heated for 1 hour at 220 β C. The resulting pink-purple sol, 35 ml of deionized water, and 8.2 grams of boehmite were placed in a "Waring" type blender and stirred rapidly for 2 minutes, during which time gelation occurred. The gel was tranεferred to a glass tray and dried in a forced air convection oven for 120 minutes at 95°C. The dried gel waε calcined in a tube furnace with a flowing 95%Ar/5% H 2 atmoεphere by heating to 600° C in 2.5 hourε, and then holding the material at 600° C for 20 minuteε. The calcined material waε εintered in a box furnace for 10 minuteε in air at 1400°C. SEM examination, aε deεcribed in Control A, revealed domains having an average diameter of about 0.4 micrometer, wherein the maximum domain diameter observed was lesε than about 0.5 micrometer. The denεity of the ceramic was about 95 to 99 percent of theoretical. Microstructural data for each Example is summarized in Table 1, below. -48- Examples 15 and 16 Control C The Control C Example illustrates an alpha alumina-based ceramic having a non-seeded microstructure. Particles having a size of -24 +50 meεh (i.e., about 300 to about 735 micrometerε) were prepared according to the general procedure aε deεcribed in Examples 1-32 of European Patent Application No. 0293163. The (calcined) particles were porous εuch that 500 grams of the calcined material was capable of absorbing 225 ml of deionized water. The crushed, calcined material was identified by conventional x-ray diffraction techniques as transition alumina. The calcined material waε εintered in a conventional box furnace (Rapid-Temp Furnace; CM, Inc.; Bloomingfield, NJ) in air for 5 minuteε at a temperature of 1400 β C. Several pieceε of the εintered ceramic were cruεhed using a tungsten carbide mortar and pestle. Optical microscopy of the crushed ceramic at a magnification of 500X using crosε polarized light indicated domains having a diameter of about 5 to about 12 micrometers. The average diameter of the domains was about 6 to about 8 micrometers. Example 15 A chromic acid solution was prepared by dissolving 41.6 grams of Cr0 3 (J.T. Baker; Phillisburg, NJ) in 67.5 gramε of deionized water. About 200 gramε of the chromic acid εolution were added to a glaεε beaker charged with 200 gramε of the calcined ' material described in the Control Example. The contents of the beaker waε gently mixed with a εpatula. The reεulting impregnated material waε dried in a forced air convection oven for 15 hourε at 95°C. The material was then calcined in a tube furnace in air by heating it to 650° C in 1 hour, and then holding the material at this temperature for 1 hour. About 5 grams of this re-calcined material was sintered as described in the Control Example to provide Sample 1. _b Optical microscopy of the Sample 1 ceramic, as described in the Control Example, revealed that portions of the ceramic had domains having diameters of lesε than about 3 micrometerε. Sample 2 waε prepared by re-impregnated about 200 gramε of the re-calcined-impregnated material, described above, with a chromic acid solution prepared by dissolving 41.6 grams of Cr0 3 in 50 grams of deionized water. This re-impregnation did not completely saturate the Sample 1 material. The reimpregnated Sample 1 ._ material was impregnated again with a chromic acid solution prepared by dissolving 25 grams of Cr0 3 in 20 gramε of deionized water. The material waε then dried, calcined, and εintered aε deεcribed for Sample 1 to provide Sample 2. The appearance of the εintered particleε of Sample 2 were very dark red. 20 SEM examination of fracture εurfaceε of Sample 2 revealed a uniform microεtructure compriεed domainε having a diameter of about 0.4 to about 0.7 micrometer. The denεity of the ceramic, however, waε not uniform. Some 2.b pieceε of the ceramic had a denεity in the range of about 95 to 99 percent of theoretical, while otherε had a density less than about 95 percent of theoretical. Example 16 Example 16 was prepared in the same manner aε 30 Sample 1 of Example 15, except the chromic acid εolution was prepared by diεεolving 100 gramε of Cr0 3 in a sufficient amount of deionized water to provide a total volume of 90 ml and the impregnated material was calcined by heating it to 600°C in 1 hour. 5 -50- SEM examination, as described in Example 15, revealed the microεtructure of Example 16 waε more uniform and slightly denser than Sample 1 of Example 15. The hardness of 4 Example 16 particles were - determined by a standard Knoop hardness technique (Leitz Miniload 2; Ernst Leitz Wetzlar GMBH; West Germany) where the applied load was 500 grams. The average hardnesε of 2-5 indentε per particle ranged from 19 to 30 GPa. The resulting ceramic of Example 16 was used as 10 abraεive grain to make a 17.75 cm (7-inch) diameter coated- abrasive diεc having a 2.2 cm (7/8 inch) centerhole. The abrasive grain for the diεc conεiεted of a 1:1 by weight mixture of -30 + 35 eεh (average diameter of about 550 micrometers) and -35 + 40 mesh (average diameter of about ις 460 micrometers) screen cutε obtained uεing U.S. Standard Screenε. The disc was prepared uεing conventional coated abrasive making procedureε, conventional 0.76 mm vulcanized fiber backingε and conventional calcium carbonate-filled phenolic make and εize reεinε, without 20 adjusting for mineral denεity differences. The make resin was precured for 75 minutes at 88°C. The size resin was precured for 90 minutes at 88°C followed by a final cure of 100°C for 10 hours. Conventional one-trip coating -_ techniques and curing in a forced air oven were employed. The coating weights (wet basis) were as follows: Coating Coating Weight (g/cm-) make 0.017 size 0.052 0 mineral 0.073 The resultant cured disc was first conventionally flexed to controllably crack the hard bonding resinε, mounted on a beveled aluminum back-up pad, 5 and used to grind the face of 1.25 cm x 18 cm 1018 cold rolled steel workpiece. The disc was driven at 5000 rpm while the portion of the disc overlying the beveled edge of the back-up pad contacted the workpiece at a pressure of 0.91 kg/cm 2 , generated a disc wear path of about 140 cm 2 . Each disc was used to grind 12 separate workpieces c for 1 minute each. Uεing the average cumulative cut of 3 diεcε made uεing brown fused alumina abraεive grain as 100%, the average relative cumulative cut of the 12 cutε for 1 disc having the abrasive grain of Example 16 was 188%. 0 Microstructural data for each Example is summarized in Table 2, below. Table 2 *after impregnation Variouε modifications and alterations of this invention will become apparent to thoεe εkilled in the art without departing from the εcope and εpirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims: -54- CLAIMS : 1. Alpha alumina-based ceramic having a seeded microstructure comprising alpha alumina domains nucleated 5 with alpha chromic oxide. 2. The alpha alumina-based ceramic according to Claim 1, said ceramic having a well-seeded microstructure. 10 3. The alpha alumina-based ceramic according to claim 1 alεo including a modifying additive. 4. The alpha alumina-baεed ceramic according to claim 1 conεiεting eεεentially of alpha alumina having . - chromium-baεed nucleating material diεperεed therein. 5. The alpha alumina-baεed ceramic according to claim 1 having a content of chromium derived from εaid alpha chromic oxide of about 0.085 to about 20 weight _- percent, baεed on the amount of alumina preεent. 6. The alpha alumina-baεed ceramic according to claim 1 wherein the total amount of alumina pluε chromia iε at least 60 weight percent, based on the total ceramic 2 5 composition. 7. The alpha alumina-based ceramic according to claim 1 wherein said ceramic is a shape selected from a fiber, a bead, a microsphere, a flake, an irregularly shaped particle, a film, abrasive grain, or a microcapsule. 8. An abrasive article comprising abrasive 5 grain at least a portion of which comprises εaid abrasive grain according to claim 7. 9. The fiber according to claim 7 having less than 0.1 weight percent iron equivalence, based on the total alpha alumina content of said ceramic. 5 10. A method of making an alpha alumina-based ceramic having a seeded microstructure comprising alpha alumina domains nucleated with an alpha alumina chromic oxide nucleating agent, εaid method comprising the steps of Method I or Method II 0 Method I a) preparing an aqueous mixture comprising: i) an alpha alumina precursor; and ii) a sufficient amount to nucleate 5 during the following process steps said alpha alumina precursor to alpha alumina having a seeded microstructure of at least one compound selected from the group consiεting of an alpha chromic oxide _ precurεor, alpha chromic oxide, and mixtures thereof; b) immobilizing said aqueous mixture; c) drying said immobilized aqueous mixture to produce a dry body; 5 d) calcining said body at a temperature and in an atmosphere sufficient to provide substantially all the chromium derived from said compound within εaid body in an oxidation state of not greater than +3; and e) sintering said calcined body at a temperature and in an atmosphere sufficient to provide alpha alumina-based ceramic having a seeded microstructure. Method II a) preparing a liquid admixture which comprises a sufficient amount of a precursor of alpha chromic oxide that when converted to alph chromic oxide nucleates the transformation of transition alumina to alpha alumina, wherein said alpha alumina has a seeded microstructure; b) impregnating εaid admixture of εtep (a) into a tranεition alumina-baεed body; c) drying said impregnated transition alumina body; d) calcining said dried impregnated transition alumina body at a temperature and in an atmosphere sufficient to subεtantially remov volatile materialε and convert εaid precurεor t itε nucleating phaεe; and e) εintering εaid calcined-impregnated tranεition alumina body at a temperature and in an atmosphere εufficient to provide an alpha alumina-baεed ceramic having a seeded microstructure comprising alpha alumina domainε nucleated with alpha chromic oxide. 11. The method according to claim 10, εaid alpha alumina-based ceramic having a well-seeded microstructure. 12. The method according to claim 10 wherein said aqueous mixture is one of a dispersion and a solution, wherein said disperεion iε immobilized by gelling and εaid solution is immobilized by rigidification. 13. The method according to claim 10 wherein a least one of said calcining atmosphere and sintering atomsphere compriseε at leaεt one of air, a mixture of air and an inert gaε, an inert gaε, a mixture of an inert gaε and a reducing gas, and a reducing gas. 14. The method according to claim 10 wherein said alpha chromic oxide precursor for Method I is a compound selected from the group consisting of Cr0 2 , HCr0 2 , and a mixture thereof; and wherein said alpha - chromic oxide precursor for Method II is selected from the group consisting of a chromium-based salt, Crθ 3 , and combinations thereof. 15. The method according to claim 10 wherein 0 steps (b), (c), and (d) are repeated in sequence up to 3 times. 16. The method according to claim 10 wherein said immobilized aqueous εolution iε formed aε an abrasive grain, a fiber, a bead, a microsphere, an irregularly εhaped particle, a flake, a film, or a microcapεule.

Application year: 1991
Main classification: C04B35
IPC codes: B24D3, C04B35, C09C1, C09K3