The boehmite material is particularly useful for making sintered ceramic articles such as fibers, catalyst supports, membranes, filters, capacitors, integrated circuit substrates, and abrasive grain.

Description of Related Art Boehmite materials are known in the art. Methods for making boehmite are reported, for example, in U. S. Pat. Nos. 4,202,870 (Weber et al.) and 4,676,928 (Leach et al.). Commercially available boehmites include those marketed under the trade designations"DISPERAL"from Condea Chemie, GmbH of Hamburg, Germany, and "DISPAL" (including"DISPAL 23480") and"CATAPAL" (including CATAPAL A", "CATAPAL B", and"CATAPAL D") from Condea Vista Chemical Company of Houston, TX;"HI-Q" (including"HI-Q 10","HI-Q 20","HI-Q 30", and"HI-Q 40") from Alcoa Industrial Chemicals; and"VERSAL (including"VERSAL 150","VERSAL 250", "VERSAL 450", VERSAL 700","VERSAL 850", and"VERSAL 900") from LaRoche Industries of Atlanta, GA.

Boehmite materials (although not necessarily each specific boehmite material listed above) have been used to make a variety of ceramic materials (typically sintered ceramic materials or articles). Such materials and articles include fibers, abrasive grain, catalyst supports, membranes, filters, capacitors, integrated circuit substrates, beads, spheres, and coatings.

The preparation of alpha alumina-based powders (see, e. g., European Pat.

Appl. No. 0 554 908 A1, published August 11,1993) and dense ceramic articles (including alpha alumina-based ceramic articles) (e. g., abrasive grain and fibers) derived from boehmite (although not necessarily using each specific boehmite material listed above) dispersions is known in the art (see, e. g., U. S. Pat. Nos. 4,314,827 (Leitheiser et al.), 4,770,671 (Monroe et al.), 4,744,802 (Schwabel), 4,881,951 (Wood et al.), and 5,139,978

(Wood), and 5,178,849 (Bauer), and European Pat. Appl. No. 0 168 606, published January 22,1986).

Boehmite dispersions are typically made by combining components comprising liquid medium (usually water), boehmite, generally a peptizing agent (usually nitric acid), and optional additives such as metal oxide (including silica, for example, colloidal silica) and/or metal oxide precursors (e. g., magnesium nitrate), followed by drying, calcining, and sintering steps (see, e. g., U. S. Pat. Nos. 4,314,827 (Leitheiser et al.), 5,178,849 (Bauer) 4,518,397 (Leitheiser et al.), 4,623,364 (Cottringer et al.), 4,744,802 (Schwabel), 4,770,671 (Monroe et al.), 4,881,951 (Wood et al.), 4,960,441 (Pellow et al.) 5,011,508 (Wald et al.), 5,090,968 (Pellow), 5,139,978 (Wood), 5,201,916 (Berg et al.), 5,227,104 (Bauer), 5,366,523 (Rowenhorst et al.), 5,429,647 (Larmie), 5,547,479 (Conwell et al.), 5,498,269 (Larmie), 5,551,963 (Larmie), 5,725,162 (Garg et al.), and 5,776,214 (Wood)).

One optional additive to the dispersion is a nucleating material. A nucleating material (in some instances referred to as a seed material) typically reduces the size of the alpha alumina crystallites, and enhances the density and hardness of the resultant ceramic material (e. g., the ceramic abrasive grain). Examples of nucleating materials include alpha-Al203, alpha-Fe203, and precursors of alpha-Fe203.

Although there are a number of boehmite materials that are currently available, there is a continuing need for additional forms of boehmite materials that offer one or more advantages, for example, in processing and/or in improved powders or articles made therefrom.

Summarv of the Invention The present invention provides a method for making a boehmite material, the method comprising dry milling (e. g., ball milling, attritor milling, jet milling, roll crushing, and combinations thereof) boehmite for a time sufficient to increase the average crystallite size of the boehmite by at least ten percent (preferably, at least 25 percent, more preferably, at least 50 percent).

Preferably, the milled boehmite material has a median particle size of less than 15 micrometers, preferably, less than 10 micrometers, more preferably, less than 5

micrometers, and even more preferably, less than 2 micrometers, as measured by the laser light scattering technique described herein. Preferably, at least 25 percent (more preferably at least 50 percent, or even 75 percent) by weight of the milled boehmite powder has a particle size less than 15 micrometers (more preferably, less than 10 micrometers or even 5 micrometers).

Boehmite according to the present can be used, for example, to make ceramic articles. For example, ceramic articles can be made by a method comprising: preparing a dispersion by combining components comprising a first liquid medium, peptizing agent, and the dry milled boehmite; and converting the dispersion to a ceramic article.

In another aspect, ceramic articles can be made by a method comprising: preparing a first dispersion by combining components comprising liquid medium and boehmite; drying the first dispersion to provide first precursor material; dry milling the first precursor material for a time sufficient to increase the average crystallite size of the boehmite by at least ten percent (preferably, at least 25 percent, more preferably, at least 50 percent); preparing a second dispersion by combining components comprising a first liquid medium, peptizing agent, and the dry milled precursor material; converting the second dispersion to second precursor material; and sintering the second precursor material to provide a ceramic article.

In this application: "Boehmite"as used herein refers to alpha alumina monohydrate and boehmite commonly referred to in the art as"pseudo"boehmite (i. e., Al203-xH20, wherein x=1 to 2).

"Milling"refers to any process, technique, or operation to crush or otherwise comminute a material.

"Dry", with regard to milling, means the milling environment is a non- liquid medium such as ambient air or other gaseous atmosphere (e. g., argon, nitrogen, and combinations thereof), and the material being milled contains less than 10 percent by

weight (preferably less 5 percent by weight, 3 percent, or even 1 percent by weight) liquid medium (including water), not including bound water.

"Converting", with regard to making the precursor material, refers to any step or series of steps that provide the precursor material, including deliquifying (typically drying).

"Ceramic precursor material"or"unsintered ceramic material"refers to deliquified or dried alumina-based dispersion (i. e., deliquified or dried ceramic precursor material) or calcined alumina-based dispersion (i. e., calcined ceramic precursor material), which typically has a density of less than 80% (typically less than 60%) of theoretical and are capable of being sintered and/or impregnated with an impregnation composition and then sintered to provide alpha alumina-based ceramic material.

"Alpha alumina-based ceramic material"as used herein refers to sintered, polycrystalline ceramic abrasive grain that have been sintered to a density of greater than 90% (preferably, at least 92%, more preferably, at least 94%, or even at least 95% or 97%) of theoretical, and contain, on a theoretical metal oxide basis, at least 60% by weight Al203, wherein at least 50% by weight of the Al203 is present as alpha alumina.

"Dispersion"refers to a solid-in-fluid (liquid and/or liquid and gas (e. g., air)) system wherein one phase comprises finely divided particles (in the colloidal size range) distributed throughout a fluid, and/or the fluid is distributed throughout the particles.

Boehmite material made according to the method of the present invention is useful for making ceramic articles, including fibers, catalyst supports, membranes, filters, capacitors, integrated circuit substrates, spheres, beads, coatings, and abrasive grain.

Brief Description of the Drawing FIG. 1 is a digital micrograph of a commercially available boehmite referred to in Comparative Examples A, B, F, and I ; FIG. 2 is a digital micrograph of the commercially available boehmite shown in FIG. 1 after being ball milled as described in Example 1; FIG. 3 is a digital micrograph of the commercially available boehmite shown in FIG. 1 after being attritor milled as described in Example 6; and

FIG. 4 is a digital micrograph of the commercially available boehmite shown in FIG. 1 after being jet milled as described in Example 9.

Detailed Description Boehmites suitable for making boehmite material according to the present invention include those commercially available under the trade designation"HIQ" (e. g., "HIQ-10,""HIQ-20,""HIQ-30,"and"HIQ-40") from Alcoa Industrial Chemicals, and those commercially available under the trade designations of"DISPERAL,"from Condea GmbH, Hamburg, Germany, and"DISPAL 23N480,"and"CATAPAL D"from Condea Vista Company, Houston, TX. These boehmites or alumina monohydrates are in the alpha form, and include relatively little, if any, hydrated phases other than monohydrates (although very small amounts of trihydrate impurities can be present in some commercial grade boehmite, which can be tolerated). They have a low solubility in water and have a high surface area (typically at least about 180 m2/g). Preferably the dispersed boehmite used to make abrasive grain according to the present invention has an average crystallite size of less than about 20 nanometers (more preferably, less than 12 nanometers). In this context,"crystallite size"is determined by the 120 and 031 x-ray reflections.

Applicant has discovered that the use of dry milled boehmite can provided a new form of boehmite. This new form of boehmite has been found to be useful, for example, in providing abrasive grain having improved grinding performance. Although not wanting to be bound by theory, it is believed that dry milling breaks up agglomerated boehmite particles thereby reducing the number of defects in the resulting abrasive grain.

Surprisingly, several dry milled boehmite powders, although decreasing in particle size when milled were found to decrease in surface area. This observation is contrary to what one of ordinary skill in the art would expect (see, e. g., Powder Surface Area and Porositv, S. Lowell and J. E. Shields, 2"d ed. 1984, p. 3, wherein it is taught that subdivided matter must be accompanied by an increase in surface area). Further, Applicant has discovered that the crystallite sizes of many of the dried milled boehmites, which were expected to subdivide with decreased milled particle size (see, e. g., Latin American Research, 21, pp.

63-68,1991 and Thermochimica Acta, 170, pp. 41-50,1990), were unexpectedly found to increase in size.

Under the present invention, boehmite is dry milled (e. g., ball milled, attritor milled, jet milled, roll crushed, and combinations thereof) for a time sufficient to increase the average crystallite size of the boehmite by at least ten percent (preferably, at least 25 percent, more preferably, at least 50 percent).

In another aspect, the surface area of boehmite dry milled according to the present invention typically has an average surface area of greater than 180 m2/g, preferably, greater than 200 m/g, and more preferably, greater than 220 m2/g.

Types of dry milling that have been found to be suitable in practicing the present invention include dry ball milling, dry jet milling, and dry attritor milling. Other types of dry milling that may be suitable include roll crushing, vibratory mills, and tumble millings (of which ball milling is one type) such as pebble milling, rod milling, tube milling, and compartment milling. Other suitable milling techniques may be apparent to those skilled in the art after reviewing the disclosure of the present invention.

Ball mills typically consist of a cylindrical jacket containing a charge of ball or cylindrical shaped media. The cylindrical jacket is rotated about its axis so that size reduction or pulverizing of the material to be milled is effected by the tumbling of the balls on the material between them.

Attritor mills typically comprise a cylindrical tank with a stirrer that mixes the milling media and material to be comminuted. Size reduction or pulverizing is achieved as the media hits and slides against the material to be milled during mixing.

Vibratory mills typically use a cylindrical shell containing a charge of milling media and the material to be ground. The milling action results when the shell is vibrated or oscillated.

The milling media is most commonly in the form of balls or cylinders. The media can be composed of any of a number of materials, including steel, glass, alumina, and zirconium silicate, and zirconia. The size of the media selected generally varies depending on the desired final particle size of the material to be comminuted. Smaller media will result in finer grinding due to greater surface area. Composition of media should be chosen carefully to minimize harmful contamination of the material to the ground. For example, Si contamination from glass media may have a detrimental effect on the densification of some sol-gel abrasive grain formulations.

Jet mills are generally of three types. The simplest form of jet mill is the spiral jet mill such as those available under the trade designations"MICRONIZER JET MILL"from Sturtevant, Inc. of Hanover, MA ;"MICRON-MASTER JET PULVERIZER" from The JET Pulverizer Co. of Mooretown, NJ; and"MICRO-JET"from Fluid Energy Aljet of Plumsteadville, PA. In a spiral jet mill a flat cylindrical grinding chamber is surrounded by a nozzle ring. The material to be ground is introduced inside the nozzle ring by an injector. The jets of compressed fluid expand through the nozzles and accelerate the particles, causing size reduction by mutual impact.

The second type of jet mill is known as a fluidized-bed jet mill. Such jet mills are available, for example, under the trade designations"CGS FLUIDIZED BED JET MILL"from Netzsch Incorporated of Exton, PA; and"ROTO-JET"from Fluid Energy Aljet of Plumsteadville, PA. This type of jet mill is equipped with mechanical classifier for finer products and allows for better control of the final particle size than does the spiral jet mill. The lower section of this type of machines is the grinding zone. A ring of grinding nozzles within the grinding zone is focused toward a central point, and the grinding fluid accelerates the particles. Size reduction takes place within the fluidzed bed of material, and this technique can greatly improve energy efficiency. The partially reduced product is carried with the expanded grinding fluid upward toward a classifier.

The oversize particles are rejected and the remaining fine particles leave the machine.

The third type is the opposed jet mill. Such jet mills are available under the trade designation"FLUIDIZED BED OPPOSED JET MILL AFG"from Hosokawa Alpine of Summit, NJ. This type of jet mills is similar to the fluidized-bed jet mill, except at least two opposed nozzles accelerate particles, causing them to collide at a central point.

A mechanical classifier is used to separate the oversize particles from particles with the right size.

Examples of other commercially available jet mills include those available under the trade designations"JET-O-MIZER"and"THERMAJET"from Fluid Energy Aljet.

Roll crushers provide size reduction by crushing agglomerates or particles between two counter-rotating rolls (i. e., the material is crushed between at least two surfaces). Typically, the rolls are either smooth or profiled. The latter is commonly used

with coarser materials. Examples of roll crushers include those available under the trade designations"CONCEPT 21 MODULAR ROLL CRUSHER"from Gundlach Crushing Equipment, Belleville, IL; and"MARCY 6000 6-1/2"X6"DOUBLE ROLL CRUSHER" from GENEQ Inc., Montreal, QC, Canada.

Boehmite milled according to the method of the present invention can be used, for example, to make ceramic articles, including sintered ceramic articles, such as fibers, catalyst supports, membranes, filters, capacitors, integrated circuit substrates, ceramic spheres, ceramic beads, ceramic coatings, and abrasive grain. Techniques for making such articles include those disclosed herein, as well as utilizing techniques known in the art for making such articles, wherein the use boehmite material according to the present invention apparent to those skilled in the art.

One common method for making many articles is to form a dispersion, sometimes referred to as a sol, which may be shaped and dried to form a boehmite article; shaped, dried, calcined to form a transitional alumina (including gamma-alumina, delta- alumina, and theta-alumina) article, shaped, dried, calcined, and sintered to form an alpha alumina-based article, although in some cases one or more of the dried, calcined, and/or sintered article may be shaped to provide the desired final article.

Ceramic fibers can be made, for example, by extruding a viscous concentrated boehmite dispersion (including boehmite according to the present invention) through a plurality of orifices resulting in fibers which can be mechanically draw or stretched in heated or room temperature air. Any forces exerted on the extruded, green fibers such as gravity, drawing or air streams typically cause attenuation or stretching of the fibers, reducing their diameter and increasing their length. The green fibers can be dried and calcined to form a transitional alumina (including gamma-alumina, delta- alumina, and theta-alumina) fibers, or dried, calcined, and sintered to form alpha alumina fibers. Oxide modifers and/or nucleating materials, as further discussed below, for example, may be included in the dispersion and/or impregnated into precursor material to modify the strength, microstructure, or sintering temperatures of the sintered, ceramic fibers. For additional details regarding fibers, including techniques for making fibers, see, for example, U. S. Pat. Nos. 3,795,524 (Sowman), 4,047,965 (Karst et al.), 4,954,462

(Wood), 5,090,968 (Pellow), 5,185,299 (Wood et al.), 5,217,933 (Budd et al.), and 5,348,918 (Budd et al.)).

Ceramic beads or spheres can be made, for example, by dispersing a boehmite dispersion (including boehmite according to the present invention) in the form of droplets in a dehydrating liquid having low water solubility (such as an alcohol). These dehydrating liquids are partly water immiscible allowing the water from the boehmite dispersion to be extracted at a slow enough rate to allow the spheres to solidify having uniform surface and internal structure. Ceramic beads or spheres can be made, for example, by spray-drying the dispersion in heated or room temperature air. Green beads or spheres can be dried and calcined to form a transitional alumina (including gamma- alumina, delta-alumina, and theta-alumina) beads or spheres, or dried, calcined, and sintered to form alpha alumina beads or spheres. Oxide modifers and/or nucleating materials, as further discussed below, for example, may be included in the dispersion and/or impregnated into precursor material to modify the strength, microstructure, or sintering temperatures of the sintered, ceramic spheres or beads.

Catalyst supports, filters, and filter materials can be made, for example, by mixing boehmite (including boehmite according to the present invention) with an acid solution, extruding the mixture, pressing or shaping the extrudate into the desired form (e. g., film, spheres, granules, powder, rods, discs, tubes, or cylinders). The shaped material is then dried, or dried and calcined, and optionally sintered to form boehmite, transitional alumina, or sintered alpha alumina bodies. Oxide modifers and/or nucleating materials, as further discussed below, for example, may be included in the boehmite mixture and/or impregnated into precursor material to modify the strength, microstructure, or calcining temperatures of the catalyst support, filter, or filter material. It is typically desirable for making catalyst support, filter, or filter material to include oxide modifers (e. g., rare earth oxides, alkali metal oxides, silica, and/or precursors thereof) in the boehmite mixture, and/or impregnate oxide modifers into precursor material to hinder the alpha alumina transformation temperature in order to provide a more porous article.

Further, to increase the porosity of the resulting article, it may be desirable to add filler materials (e. g., walnut shell flour,. polymers, or starch) to the boehmite mixture which will burn away during calcining or sintering. For additional details regarding catalyst supports,

including techniques for making catalyst supports, see, for example, U. S. Pat. Nos.

3,945,945 (Kiovsky et al.), 4,102,978 (Kiovsky), 4, 224,302 (Okamoto et al.), and 5,512,530 (Gerdes et al.)).

Sol coatings can be prepared on suitable substrates by spin coating, brushing, spraying, or dipping a boehmite dispersion (including boehmite according to the present invention) onto an appropriate substrate. Such coatings can be subsequently dried and calcined to transitional alumina or additionally sintered to alpha-alumina. Such coatings are useful for electronic applications such as electrical insulation, capacitors or corrosion protection. Oxide modifers and/or nucleating materials, as further discussed below, for example, may be included in the dispersion and/or impregnated into precursor material to modify the strength, microstructure, or sintering temperatures of the coating or film.

Typically, the (boehmite) dispersion is made by combining or mixing components comprising liquid medium, and dry milled boehmite (according to the present invention), and optionally unmilled boehmite and/or metal oxide and/or metal oxide precursor (including nucleating material). The components for making the dispersion also typically include a peptizing agent (e. g., an acid such as nitric acid). The liquid medium in which the boehmite is dispersed is typically water (preferably deionized water), although organic solvents, such as lower alcohols (typically C,-6 alcohols), hexane, or heptane, may also be useful as the liquid medium. In some instances, it is preferable to heat the liquid medium (e. g., 60-70°C) to improve the dispersibility of the boehmite.

The peptizing agent (s) is generally a soluble ionic compound (s) which is believed to cause the surface of a particle or colloid to be uniformly charged in a liquid medium (e. g., water). Preferred peptizing agents are acids or acid compounds. Examples of typical acids include monoprotic acids and acid compounds, such as acetic, hydrochloric, formic, and nitric acid, with nitric acid being preferred. The amount of acid used depends, for example, on the dispersibility of the boehmite, the percent solids of the dispersion, the components of the dispersion, the amounts, or relative amounts of the components of the dispersion, the particle sizes of the components of the dispersion, and/or the particle size distribution of the components of the dispersion. The dispersion

typically contains at least, 0.1 to 20%, preferably 1% to 10% by weight acid and most preferably 3 to 8% by weight acid, based on the weight. of boehmite in the dispersion.

In some instances, the acid may be applied to the surface of the boehmite particles prior to being combined with the water. The acid surface treatment may provide improved dispersibility of the boehmite in the water.

The boehmite containing dispersions typically comprise greater than 15% by weight (generally from greater than 20% to about 85% by weight; typically greater than 20% to about 80% by weight; more typically greater than 30% to about 80% by weight) solids (or alternatively boehmite), based on the total weight of the dispersion. Certain preferred dispersions, however, comprise 35% by weight or more, 45% by weight or more, 50% by weight or more, 55% by weight or more, 60% by weight or more and 65% by weight or more by weight or more solids (or alternatively boehmite), based on the total weight of the dispersion. Weight percents of solids and boehmite above about 80 wt-% may also be useful, but tend to be more difficult to process to make some articles (e. g., abrasive grain).

Optionally, a boehmite dispersion includes metal oxide (e. g., particles of metal oxide which may have been added as a particulate (preferably having a particle size (i. e., the longest dimension) of less than about 5 micrometers; more preferably, less than about 1 micrometer) and/or added as a metal oxide sol (including colloidal metal oxide sol)) and/or metal oxide precursor (e. g., a salt such as a metal nitrate, a metal acetate, a metal citrate, a metal formate, or a metal chloride that converts to a metal oxide upon decomposition by heating). The amount of such metal oxide and/or metal oxide precursor (that is in addition to the alumina provided by the boehmite) present in a dispersion or precursor (or metal oxide in the case of the abrasive grain) may vary depending, for example, on which metal oxide (s) is present and the properties desired for the sintered abrasive grain. For dispersions containing such metal oxides (and/or precursors thereof), the metal oxides (that are in addition to the alumina provided by the boehmite) are typically present, on a theoretical metal oxide basis, up to about 10 or even 15 percent by weight (preferably, in the range from about 0.1 to about 10 or even 15 percent; more preferably, in the range from about 0.5 to about 10 or even 15 percent by weight), based on

the total metal oxide content of the abrasive grain; although the amount may vary depending, for example, on which metal oxide (s) is present.

Examples of such other metal oxides include: praseodymium oxide, dysprosium oxide, samarium oxide, cobalt oxide, zinc oxide, neodymium oxide, yttrium oxide, ytterbium oxide, magnesium oxide, nickel oxide, manganese oxide, lanthanum oxide, gadolinium oxide, dysprosium oxide, europium oxide, hafnium oxide, and erbium oxide, as well as manganese oxide, chromium oxide, titanium oxide, and ferric oxide which may or may not function as nucleating agents.

Metal oxide precursors include metal nitrate salts, metal acetate salts, metal citrate salts, metal formate salts, and metal chloride salts. Examples of nitrate salts include magnesium nitrate (Mg (NO3) 2 6H2O), cobalt nitrate (Co (NO3)2#6H2O), nickel nitrate (Ni (NO3) 2-6H2O), lithium nitrate (LiNO3), manganese nitrate (Mn (NO3) 2 4H2O), chromium nitrate (Cr (NO3) 3 9H20), yttrium nitrate (Y (NO3) 3-6H20), praseodymium nitrate (Pr (NO3)3#6H2O), samarium nitrate (Sm (NO3)3#6H2O), neodymium nitrate (Nd (NO3)3#6H2O), lanthanum nitrate (La (NO3) 3 6H2O), gadolinium nitrate (Gd (NO3) 3-5H2O), dysprosium nitrate (Dy (NO3) 3 5H2O), europium nitrate (Eu (NO3) 3 6H2O), ferric nitrate (Fe (NO3) 3 9H2O), zinc nitrate (Zn (NO3)3#6H2O), erbium nitrate (Er (NO3) 3 5H2O), zirconium nitrate (Zr (NO3) 4 5H2O), and zirconium hydroxynitrate. Examples of metal acetate salts include zirconyl acetate (ZrO (CH 3COO) 2), magnesium acetate, cobalt acetate, nickel acetate, lithium acetate, manganese acetate, chromium acetate, yttrium acetate, praseodymium acetate, samarium acetate, ytterbium acetate, neodymium acetate, lanthanum acetate, gadolinium acetate, and dysprosium acetate. Examples of citrate salts include magnesium citrate, cobalt citrate, lithium citrate, and manganese citrate. Examples of formate salts include magnesium formate, cobalt formate, lithium formate, manganese formate, and nickel formate.

Colloidal metal oxides are discrete finely divided particles of amorphous or crystalline metal oxide typically having one or more of their dimensions within a range of about 3 nanometers to about 1 micrometer. The average metal oxide (including in this context silica) particle size in the colloidal is preferably less than about 150 nanometers, more preferably less than about 100 nanometers, and most preferably less than about 50

nanometers. In some instances, the particles can be on the order of about 3-10 nanometers.

Typically, the colloidal comprises a distribution or range of metal oxide particle sizes.

Preferably, the colloidal metal oxide sols are a stable (i. e., the metal oxide solids in the sol or dispersion do not appear by visual inspection to begin to gel, separate, or settle upon standing undisturbed for about 2 hours) suspension of colloidal particles (preferably in a liquid medium having a pH of less than 6.5). For additional information on silica sols see, for example, U. S. Pat. Nos. 5,611,829 (Monroe et al.) and 5,645,619 (Erickson et al.). For more information on ceria, silica, or zirconia sols, see, for example, U. S. Pat. Nos.

5,429,647 (Larmie), 5,498,269 (Larmie), 5,551,963 (Larmie), 5,611,829 (Monroe et al.), and 5,645,619 (Erickson et al.).

The use of a metal oxide modifier may decrease the porosity of the sintered material and thereby increase the density, although in some cases, for example, in making catalyst supports, metal oxide modifers can be used to desirably increase the porosity and decrease densification. Certain metal oxides may react with the alumina to form a reaction product and/or form crystalline phases with the alpha alumina which may be beneficial, for example, during use of abrasive grain in abrading applications. For example, the oxides of cobalt, nickel, zinc, and magnesium typically react with alumina to form a spinel, whereas zirconia and hafnia do not react with the alumina. Alternatively, the reaction products of dysprosium oxide and gadolinium oxide with aluminum oxide are generally garnet. The reaction products of praseodymium oxide, ytterbium oxide, erbium oxide, and samarium oxide with aluminum oxide generally have a perovskite and/or garnet structure. Yttria can also react with the alumina to form Y3AI5O, 2 having a garnet crystal structure. Certain rare earth oxides and divalent metal cations react with alumina to form a rare earth aluminate represented by the formula LnMAl"O, wherein Ln is a trivalent metal ion such as La3+, Nd3+, Ce3+, Pr3+, Sm3+, Gd3+, Er3+, or Eu3+, and M is a divalent metal cation such as Mg2+, Mn2+, Ni2+, Zn2+, or Co2+. Such aluminates have a hexagonal crystal structure. For additional details regarding the inclusion of metal oxide (and/or precursors thereof) in a boehmite dispersion see, for example, in U. S. Pat. Nos. 4,314,827 (Leitheiser et al.), 4,770,671 (Monroe et al.), 4,881,951 (Wood et al.) 5,429,647 (Larmie), and 5,551,963 (Larmie).

Optionally, the boehmite dispersion contains nucleating material (i. e., material that enhances the transformation of transitional alumina (s) to alpha alumina via extrinsic nucleation). The nucleating material can be a nucleating agent (i. e., material having the same or approximately the same crystalline structure as alpha alumina, or otherwise behaving as alpha alumina) itself (e. g., alpha alumina seeds, a-Fe203 seeds, or a-Cr203 seeds, Ti203 (having a trigonal crystal structure), MnO, (having a rhombic crystal structure), Li2O (having a cubic crystal structure), titanates (e. g., magnesium titanate and nickel titanate) or precursor thereof. Typically, nucleating material, if present, comprises, on a theoretical metal oxide basis (based on the total metal oxide content of the calcined precursor material before sintering (or the sintered material)), in the range from about 0.1 to about 5 percent by weight. Additional details regarding nucleating materials are also disclosed, for example, in U. S. Pat. Nos. 4,623,364 (Cottringer et al.), 4,744,802 (Schwabel), 4,964,883 (Morris et al.), 5,139,978 (Wood), 5,219,806 (Wood), and 5,611,829 (Monroe et al.) and 5,645,619 (Erickson et al.).

Additional details regarding sol-gel derived materials (e. g., abrasive grain) can be found, for example, in U. S. Pat. Nos. 4,518,397 (Leitheiser et al.), 4,770,671 (Monroe), 4,744,802 (Schwabel), 5,139,978 (Wood), 5,219,006 (Wood), and 5, (Monroe), and applications having U. S. Serial Nos. 09/407,672,09/406,952,09/407,671, and 09/407,781.

With regard to high solids dispersions (typically greater than about 50% by weight solids), such dispersion can be prepared, for example, by gradually adding a liquid component (s) to a component (s) that is non-soluble in the liquid component (s), while the latter is mixing or tumbling. For example, a liquid containing water, nitric acid, and metal salt may be gradually added to boehmite, while the latter is being mixed such that the liquid is more easily distributed throughout the boehmite. For further details regarding using high solids dispersions see U. S. Pat. Nos. 5,776,214 (Wood), 5,779,743 (Wood), and 5,893,935 (Wood).

In general, techniques for drying the dispersion are known in the art, including heating to promote evaporation of the liquid medium, or simply drying in air.

The drying step generally removes a significant portion of the liquid medium from the dispersion; however, there still may be a minor portion (e. g., about 10% or less by weight)

of the liquid medium present in the dried dispersion. Typical drying conditions include temperatures ranging from about room temperature to over about 200°C, typically between 50 to 150°C. The times may range from about 30 minutes to over days. To prevent salt migration, it may be desirable to dry the dispersion at low temperature.

In the case of abrasive grain, for example, after drying, the dried dispersion may be converted into precursor particles. One typical means to generate these precursor particles is by a crushing technique. Various crushing or comminuting techniques may be employed such as a roll crusher, jaw crusher, hammer mill, ball mill and the like. Coarser particles may be recrushed to generate finer particles. It is also preferred that the dried dispersion be crushed, as, for example, it is generally easier to crush dried gel versus the sintered alpha alumina based abrasive grain. For additional information regarding providing various sized particles and shapes, see, e. g., U. S. Pat. No. 5,201,916 (Berg et al.).

Typically, the dried dispersion is calcined, prior to sintering, although a calcining step is not always required. In general, techniques for calcining the dried dispersion or ceramic precursor material, wherein essentially all the volatiles are removed, and the various components that were present in the dispersion are transformed into oxides, are known in the art. Such techniques include using a rotary or static furnace to heat dried dispersion at temperatures ranging from about 400-1000°C (typically from about 450-800°C) until the free water, and typically until at least about 90 wt-% of any bound volatiles are removed.

It is also within the scope of the present invention to impregnate a metal oxide modifier source (typically a metal oxide precursor) into a calcined precursor material. Typically, the metal oxide precursors are in the form metal salts. These metal oxide precursors and metal salts are described above with respect to the boehmite dispersion. For additional information regarding impregnating precursor materials see U. S. Pat. Nos. 5,139,978 (Wood), 5,164,348 (Wood), and 5,893,935 (Wood).

After the precursor material is formed or optionally calcined, the precursor material may be sintered to provide a sintered article (e. g., a dense, ceramic alpha alumina based abrasive grain). In general ; techniques for sintering the precursor material, which include heating at a temperature effective to transform transitional alumina (s) into alpha

alumina, to causing all of the metal oxide precursors to either react with the alumina or form metal oxide, and increasing the density of the ceramic material, are known in the art.

The precursor material may be sintered by heating (e. g., using electrical resistance, microwave, plasma, laser, or gas combustion, on batch basis or a continuous basis).

Sintering temperatures are usually range from about 1200°C to about 1650°C, typically, from about 1200°C to about 1500°C. The length of time which the precursor material is exposed to the sintering temperature depends, for example, on the size of the ceramic article, composition of the ceramic article, and sintering temperature. Typically, sintering times range from a few seconds to about 60 minutes (preferably, within about 3-30 minutes). Sintering is typically accomplished in an oxidizing atmosphere, although inert or reducing atmospheres may also be useful.

With regard to abrasive grain, the longest dimension of the abrasive grain is typically at least about 1 micrometer. The abrasive grain described herein can be readily made with a length of greater than about 50 micrometers, and larger abrasive grain (e. g., greater than about 1000 micrometers or even greater than about 5000 micrometers) can also be readily made. Generally, the preferred abrasive grain has a length in the range from about 100 to about 5000 micrometers (typically in the range from about 100 to about 3000 micrometers), although other sizes are also useful, and may even be preferred for certain applications. In another aspect, abrasive grain according to the present invention, typically have an aspect ratio of at least 1.2: 1, or even 1.5: 1, sometimes, at least 2: 1, and alternatively, at least 2.5: 1.

Typically preferred ceramic fibers made according to the present invention have a diameter in the range from about 1 to about 50, more typically, in the range from about 7 to about 15 micrometers. The fibers can be continuous (e. g., have lengths greater than several times there diameter, including lengths of 50 meters, 100 meters, 500 meters, or even greater than 1000 meters). The fibers can also, for example, be provided as "chopped"fibers.

It is also within the scope of the present invention to recycle unused deliquified dispersion) material as generally described, for example, in U. S. Pat. No.

4,314,827 (Leitheiser et al.), the disclosure of which is incorporated herein by reference.

For example, a first dispersion can be made as described above, dried, crushed, and

screened, and then a second dispersion made by combining, for example, liquid medium (preferably, aqueous), boehmite (dry milled, unmilled or a combination thereof), and deliquified material from the first dispersion, and optionally metal oxide and/or metal oxide precursor. Alternatively, for example, the first dispersion can be made using unmilled boehmite, and the second dispersion made using, in part, dry milled boehmite.

Further, for example, the deliquified material from a first dispersion, which may include unmilled boehmite, can be dry milled as discussed above for boehmite, and such dry milled material used to prepare a second dispersion. The recycled material may provide, on a theoretical metal oxide basis, for example, at least 10 percent, at least 30 percent, at least 50 percent, or even up to (and including) 100 percent of the theoretical Al203 content of the dispersion which is deliquified and converted (including calcining and sintering) to provide the sintered alpha alumina-based material.

For additional details regarding the processing of fibers (including drying, calcining, and firing or sintering), see, e. g., U. S. Pat. Nos. 3,795,524 (Sowman), 4,047,965 (Karst et al.), 4,954,462 (Wood), and 5,348,918 (Budd et al.).

Methods for using abrasive grains, including making and using abrasive products (e. g., coated abrasives, bonded abrasive, abrasive brushes, and nonwoven abrasives) having abrasive grain are known in the art (see, e. g., U. S. Pat. Nos. 2,958,593 (Hoover et al.), 4,311,489 (Kressner), 4,652,275 (Bloecher et al.), 4,734,104 (Broberg), 4,737,163 (Larkey), 4,799,939 (Bloecher et al.), 4,997,461 (Markhoff-Matheny et al.), 4,898,597 (Hay et al.), 5,203,884 (Buchanan et al.), 5,378,251 (Culler et al.), 5,417,726 (Stout et al.), 5,436,063 (Follett et al.), 5,496,386 (Broberg et al.), 5,520,711 (Helmin), and 5,679,067 (Johnson et al.).

Fibers made according to the method of the present invention can be useful, for example, as thermal insulation, as a filter component, and as a reinforcement for structural composites.

Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. All parts and percentages are by weight unless otherwise indicated.

Examples Any reference to the percent solids levels of the dispersion used in the following examples are the approximate solids levels, as they do not take into account the 2-6% water commonly found on the surface of boehmite, nor the solids provided by any non-boehmite additives.

The following designations are used in the examples: DWT deionized water that was at a temperature of 20-25°C, unless otherwise specified HNO3 nitric acid, 70% concentrated H-20 an alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial Chemicals, Houston, TX, under the trade designation"HIQ-20") H-30 an alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial Chemicals, under the trade designation"HIQ-30") H-40 an alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial Chemicals, under the trade designation"HIQ-40") AAMH-R an alpha-alumina monohydrate (boehmite) (obtained from Condea Chemie, Hamburg, Germany, under the trade designation"DISPERAL RTM") MEM a rare earth nitrate solution prepared by mixing a lanthanum, neodymium, and yttrium nitrate (having, on a theoretical metal oxide basis, 23% rare earth oxide (i. e., La203, Nd203, and Y203) ; available from Molycorp of Lourviers, CO) with a sufficient amount of magnesium nitrate (Mg (N03) 2-6H2O) solution (having, on a theoretical metal oxide basis, 11% MgO; available from Mallinckrodt Chemical of Paris, KY) and cobalt nitrate (Co (NO3) 2 6H2O) solution (having, on a theoretical metal oxide basis 19% CoO; available from Hall Chemical of Wickliffe, OH) to provide a solution containing, on a theoretical metal oxide basis 5.8% La (N03) 3#6H2O, 5.8% Nd (N03) 3-6H20, about 7.1% Y (NO3) 3-6H2O, about 14.4% Mg (N03) 2#6H2O, about 0.4% Co (N03) 2 6H2O, and the balance deionized water.

Example 1 and Comparative Example A Approximately 1200 grams of H-40 powder was ball-milled at 70-80 rpm in a polyurethane lined vessel (30 cm (12 inch) ball mill jar obtained from Paul Abbe, Inc.) containing about 8000 grams of 1.27 cm (0.5 inch) zirconia media (obtained from U. S.

Stoneware, East Palatine, OH) for 24 hours. After recovering the ball-milled powder, 1000 grams of the powder was mixed with an acid-water solution (prepared by mixing 60 grams of HNO3 with 607.6 grams of DWT) by hand in a plastic wash-tub. The resulting powder material was allowed to stand over a weekend in sealed-plastic bags. The powdered material was then extruded through a single screw extruder containing a die with thirty 2.54 mm (0.1 inch) openings. The extrudate was dried through a tunnel oven at a rate of 1 m/min. The tunnel oven had first zone, 2.43 meters in length, set at 71°C (160°F), and the second zone, 2.43 meters in length, set at 82°C (180°F). The dried extrudate as collected in aluminum pans and allowed to stand for 2 hours in a forced air oven set at 80°C.

The resulting dried material was crushed into particles using a pulverizer (having a 1.1 mm gap between the steel plates; obtained under the trade designation "BRAUN"Type UA from Braun Corp., Los Angeles, CA) and screened to sizes +30-16 mesh (+0. 68 mm-1.44 mm) using a conventional screener (obtained under the trade designation"EXOLON SCREENER"from Exolon-ESK, Tonawanda, NY).

The retained particles were fed into a calcining kiln to provide calcined abrasive grain precursor material. The calcining kiln consisted of a 15 cm inner diameter, 1.2 meter in length, stainless steel tube having a 0.3 meter hot zone. The tube was inclined at a 3.0 degree angle with respect to the horizontal. The tube rotated at about 3.5 rpm, to provide a residence time in the tube of about 4-5 minutes. The temperature of the hot zone was about 650°C.

The calcined particles were impregnated with MEM, wherein the ratio of solution to particles was 60 ml of solution to 100 grams of particles. The impregnated particles were dried using a blow gun. The dried, impregnated particles were then calcined again at 650oC as described above to provide abrasive grain precursor particles.

The calcined abrasive grain precursor particles were fed into a rotary sintering kiln. The sintering kiln consisted of an 8.9 cm inner diameter, 1.32 meter long

silicon carbide tube inclined at 4.4 degrees with respect to the horizontal and had a 31 cm hot zone. The heat was applied externally via SiC electric heating elements. The sintering kiln rotated at 5. 0 rpm, to provide a residence time in the tube of about 7 minutes. The sintering temperature was about 1400°C. The sintered abrasive grain exited the kiln into room temperature air where it was collected in a metal container and allowed to cool to room temperature.

The density of the abrasive grain was determined with a helium gas pycnometer (obtained under the trade designation"MICROMERITICS ACCUPYC 1330" from Micromeritics Instruments Corp., Norcross, GA). The density of the Example abrasive grain was 3.93 g/cm2. Comparative Example A abrasive grain were prepared as described for Example 1, except the H-40 powder was not ball milled. The density and bulk density of the Comparative Example A abrasive grain were determined as described for Example 1 to be 3.90 g/cm2 and 1.54 +/-0.01 g/cm2, respectively.

Example 2 and Comparative Example B Example 2 was prepared as described for Example 1 except two identical alumina ball mills (17.78 cm in height and 15.24 cm in diameter), each with 1500 grams 0.635 cm (0.25 inch) alumina media, were used, and the samples were milled for 18 hours. Each mill contained 600 grams of H-40 powder. The milled powder was combined into one sample.

The density and bulk density of the Example 2 abrasive grain were determined as described for Example 1 to be 3.89 g/cm2 and 1.60 +/-0.01 g/cm2, respectively.

Comparative Example B abrasive grain were prepared as described for Example 2 except the H-40 was not ball milled. The density and bulk density of the Comparative Example B abrasive grain were determined as described for Example 1 to be 3.87 g/cm2 and 1.53 +/-0.01 g/cm2, respectively.

Example 3 and Comparative Example C An alumina ball mill, measuring 17.78 in height (without cover) and 15.24 cm in diameter (obtained from U. S. Stoneware), was filled with 1200 grams of H-30 powder and 3300 grams of 0.635 cm (0.25 inch) alumina media (U. S. Stoneware). The sample was milled at 80 rpm for 48 hours. The milled powder was recovered from the mill. The median particle size of the milled powder was measured using a laser scattering particle size analyzer (obtained under the trade designation"HORIBA LA-910"from Horiba Laboratory Products, Irvine, CA) and found to be 7. 3 micrometers.

A sol was prepared by combining 338 grams of the milled powder, 31 grams of HNO3 and 1550 grams of DWT together in a conventional 4 liter, food grade blender (Model 34BL22 (CB6), Waring Products Division, Dynamics Corp. of America, New Hartford, CT). The contents of the blender were mixed at low speed for one minute.

The resulting sol was poured into a 23 cm (9 inch) x 30 cm (12 inch) glass pan (obtained under the trade designation"PYREX"). The sol was dried overnight in a forced air oven at 93°C (200F). The resulting friable material was crushed into particles using the pulverizer, screened, calcined, impregnated, re-calcined, sintered, and used to make coated abrasive discs as described in Example 1.

The density and bulk density of the Example 3 abrasive grain were determined as described for Example 1 to be 3.93 g/cm2 and 1.89 +/-0.01 g/cm2, respectively.

Comparative Example C abrasive grain were prepared as described for Example 3 except the H-40 was not ball milled. The milled powder was recovered from the mill. The median particle size of the milled powder was measured as described in Example 3 and found to be 31.7 micrometers.

The density and bulk density of the Comparative Example C abrasive grain were determined as described for Example 1 to be 3.91 g/cm2 and 1.78 +/-0.01 g/cnr', respectively.

Example 4 and Comparative Example D An 18.9 liter metal pail with a polyethylene liner was charged with 2600 ml of DWT, 48 grams of HN03, and 800 grams of AAMH powder. The charge was dispersed

at high speed for five minutes using a conventional homogenizer (obtained from under the trade designation"GIFFORD-WOOD HOMOGENIZER MIXER"from Greeco Corp., Hydson, NH). The resulting sol was poured into a 46 cm x 66 cm x 5 cm polyester-lined aluminum tray and dried in an air oven at 150°C, to provide to a friable solid. The dried material was crushed using the pulverizer described in Example 1. The crushed material was screened to-150 mesh using a USA Standard Testing Sieves.

An alumina-lined mill (24.13 cm (9.5 inch) in height without cover and 25.4 cm (10 inch) in diameter; obtained from U. S. Stoneware) was charged with 1,200 grams of the-150 mesh fines and 3,300 grams of 0.635 cm (0.25 inch) alumina media (U. S. Stoneware). The charge was ball-milled at 80 rpm for 26 hours. A thousand grams of the milled powder was mixed by hand with 575 grams of DWT into the milled powder in a plastic tub. The resulting mixture was extruded using a 5.1 cm (2 inch) extruder (obtained under the trade designation"BONNET EXTRUDER"from Bonnet Co., Uniontown, OH) using a die which produced extrudate approximately 0.159 cm (1/16 inch) in diameter. The extrudate was allowed to dry overnight at room temperature in aluminum trays.

The dried extrudate was crushed using a roll crusher (obtained from Allis- Chalmers, Milwaukee, WI). About 400 grams of the-16+46 mesh (+0.68 mm-1.44 mm) material collected was screened ("EXOLON SCREENER") through five 20 mesh screens (U. S. A. Standard Testing Sieves) for 5minutes using a mechanical siever (obtained under the trade designation"ROTOTAP RX29"from U. S. Tyler, Inc., Mentor, OH. The rods retained on the bottom 20 mesh screen were retained. The retained rod-shaped particles were then calcined, impregnated, calcined, and sintered as described for Example 1.

The density of the Example 4 abrasive grain was determined as described for Example 1 to be 3.88 g/cm2.

Comparative Example D abrasive grain were prepared as described for Example 4 except the-150 mesh fines were not ball milled. The density of the Comparative Example A abrasive grain were determined as described for Example 1 to be 3.84 g/cm.

Example 5 and Comparative Example E An alumina lined mill, measuring 24. cm (9.5 inch) in height without cover and 25.4 cm (10 inches) in diameter (U. S. Stoneware), was filled with 1000 grams of H-20 and 3,300 grams of 0.635 cm (0.25 inch) alumina media (U. S. Stoneware). The material was ball-milled for 8 hours. After recovering the ball-milled powder, 1000 grams of the powder was mixed with an acid-water solution (prepared by mixing 60 grams of HN03 with 650 grams of DWT) by hand in a plastic wash-tub.

The resulting mixture was extruded, dried, crushed, screened crushed, calcined, impregnated, calcined, and sintered as described for Example 4.

The density of the Example 5 abrasive grain was determined as described for Example 1 to be 3.82 g/cm2.

Comparative Example E abrasive grain were prepared as described for Example 4 except the H-20 was not ball milled. The density of the Comparative Example E abrasive grain was determined as described for Example 1 to be 3.80 g/cm2.

Example 6 and Comparative Example F An attritor mill (obtained under the trade designation"SZEGVARI ATTRITOR MILL", Model HSA1, from Union Process, Akron, OH) containing a polymer coated impellor and 3.8 liter (1 gallon) sized polymer-lined sample vessel was filled with 3000 grams of 0.635 cm (0.25 inch) cylindrically shaped zirconia milling media (obtained from U. S. Stoneware) and 600 grams of H-40 powder. The H-40 powder was milled by rotating the impellor at 250 rpm for 20 minutes. The resulting milled powder was separated from the media.

The surface area of the milled (Example 6) and unmilled (Comparative Example F) H-40 powder were determined using a nitrogen gas-sorption analyzer (obtained under the trade designation"NOVA 1000"from Quantachrome Corp. Boynton Beach, FL) and determined to be 196.3 m2/g and 228.4 m2/g, respectively. The median particle size of the milled and unmilled H-40 powders were measured using a laser scattering particle size analyzer ("HORIBA LA-910") and found to be 4.5 micrometers and 46.6 micrometers, respectively.

Further, the average crystallite size for the milled and unmilled boehmite powders were determined as follows. The boehmite powder to be evaluated was placed on a glass microscope slide using a double sided adhesive tape. The excess powder was blown or brushed off to provide a monolayer of powder on the glass slide. The sample was then placed in an x-ray diffractometer (obtained under the trade designation"INEL CPS 120"from Inel of Stratham, NJ). The x-rays were generated using a conventional x- ray generator (obtained under the trade designation"PHILLIPS 3100"from Phillips of Mahwah, NJ). The average crystallite size of a sample was measured using the 120 peak for boehmite located at 26 = 28. and the 031 peak located at 20 = 38. This peak was scanned at a sufficiently slow rate to obtain greater than 10,000 counts under the 120 peak, or the 031 peak. The average crystal size of the boehmite was calculated using the formula: D = k X coso, wherein k is a constant = 0.89 X is the wavelength of the x-rays used to irradiate the sample (i. e. copper x-rays = 1. 5418 Angstroms) 6 corresponds to the 120 peak for boehmite, 29 = 28. = 14. or, for the 031 peak, 29 = 38. 5°, 9 = 19. and ß=(ß2 measured - ß2 instrument)0.5 where ß measured is the full width half maximum of the 120 peak, or the 031 peak, for the boehmite sample and ß nstrument is a constant unique to each x-ray diffractometer calculated using a known standard sample of known crystal size. All P values are measured in radians.

The average crystallite size for the unmilled powder was 61 Angstroms; the milled powder, 99 Angstroms.

Sols were prepared of the milled and unmilled H-40 powders by mixing 200 grams of the respective powders, 12 grams of HN03, and 1000 grams of DWT. The ingredients were thoroughly mixed using a blender (Waring Model 34BL22 (CB6) at low

speed for one minute. The sols were poured into 23 cm (9") x 30 cm (12") glass ("PYREX") trays and placed in a forced air oven at 93 °C (20QOF) overnight. The resulting friable materials were crushed, screened, calcined, impregnated, re-calcined, and sintered as described in Example 1, except the crushed material was screened to sizes +46- 14 mesh (+0.68 mm-1.44 mm), and the ratio of MEM to particles was 70 ml of solution to 100 grams of particles.

The density of the resulting abrasive grains were as described in Example 1.

The density of the Example 6 and Comparative Example F abrasive grains were 3.85 g/cm2 and 3.87 g/cm2, respectively.

Example 7 and Comparative Example G -150 mesh dried gel material as described in Example 4 was jet milled (using a jet mill obtained under the trade designation"CCE FLUID BED JET MILL"from CCE Technologies, Mendota Heights, MN) at a feed rate of 204 kg/hr (450 lb./hr.). The median particle size of the milled (Example 7) and unmilled (Comparative Example G) materials were measured using a laser scattering particle size analyzer ("HORIBA LA- 910") and found to be 2.2 micrometers and 69.4 micrometers, respectively.

The average crystallite size for the unmilled and milled powders were determined as described in Example 6, and found to be 123 Angstroms and 130 Angstroms, respectively.

Sols were prepared of the milled and unmilled material by mixing control by mixing 280 grams of the respective materials, 8 grams of HN03, and 1000 grams of DWT. The ingredients were thoroughly mixed using a blender (Waring Model 34BL22 (CB6) at low speed for one minute. The sols were poured into 23cm (9") by 23 cm (9") glass ("PYREX") trays and placed in a forced air oven at 93 °C (20QOF) overnight.

The resulting friable materials were crushed, screened, calcined, impregnated, re-calcined, and sintered as described in Example 6.

The density of the Example7 and Comparative Example G abrasive grains, as determined according to Example 1 were 3.89 g/cm2 and 3.85 g/cm2, respectively.

Example 8 and Comparative Example H About 1000 grams of C-D powder was ball milled with 6000 grams of zirconia media (U. S. Stoneware, East Palestine, OH) in a plastic 3.8 liter (1 gallon) 25.4 cm high, 15 cm diameter jar for 24 hours at a rotation rate of 80 rpm. The average crystallite size for the unmilled powder (Comparative Example H) was 66 Angstroms; the milled powder (Example 8), 100 Angstroms.

The surface area of the milled (Example 8) and unmilled (Comparative Example H) were analyzed by nitrogen gas-sorption analyzer (obtained under the trade designation"NOVA 1000"from Quantachrome Corporation, Boynton Beach, FL).

Surface areas were measured to be 244.6 m2/g for the milled powder and 218. 7 m2/g for the unmilled powder. The median particle size of the milled and unmilled powders were measured using the laser scattering particles size analyzer ("HORIBA LA-910") and found to be 11.4 micrometers for the milled powder, and 51. 8 micrometers for the unmilled powder.

Example 9 and Comparative Example I H-40 powder was jet milled by the manufacturer, Alcoa Industrial Chemicals. The average crystallite size for the unmilled powder (Comparative Example I) was 68 Angstroms; the milled powder (Example 9), 82 Angstroms. The average particle size of the milled and unmilled powders were 1.6 micrometers and 46.6 micrometers, respectively.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.