Ceramic disk and rod, methods of manufacture therefor and articles comprising the same

Abstract

To provide a ceramic disk or a rod provided with a high-density shell and a porous core for providing a uniform gas flow for various uses such as a semiconductor manufacturing operation.SOLUTION: The present disclosure discloses a double density disk provided with a high-density outside tube including alumina with a purity of over 99% and a porous core including alumina with a density lower than a density of the high-density outside tube, wherein the porous core has an alumina purity of over 99%. A method is also disclosed that includes arranging a slurry including an alumina powder and a fine pore-forming agent in a high-density outside tube, heating the high-density outside tube arranged with the slurry disposed therein at a temperature of 300-600°C for activation of the fine pore-forming agent, preparing a porous core in the high-density outside tube, and sintering the high-density outside tube having the porous core in one or more stages at a temperature of 800-2000°C.SELECTED DRAWING: Figure 1(B)

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to a provisional application filed on November 27, 2019, and assigned Serial No. 62 / 941,241, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] Disclosed herein are ceramic discs and rods, methods for their manufacture, and articles comprising the same. More specifically, disclosed herein are alumina discs and rods having a dense shell with a porous core. Summary of the Invention

[0003] Disclosed herein is a dual density disc comprising a high density outer tube comprising alumina having a purity greater than 96% and a porous core comprising alumina of a lower density than the density of the high density outer tube, wherein the porous core has an alumina purity greater than 99%.

[0004] Also disclosed herein is a method that includes disposing a slurry containing alumina powder and a pore former in a densified outer tube, heating the densified outer tube with the slurry disposed therein to a temperature of 300-600°C to activate the pore former, creating a porous core in the densified outer tube, and sintering the densified outer tube with the porous core in one or more stages at a temperature of 800-2000°C.

[0005] In an embodiment, a method for manufacturing an alumina disk includes disposing an alumina powder in a dense outer tube. The powder is free of pore formers and / or solvents. The dense outer tube with the alumina powder disposed therein is heated to a temperature of 800-2000°C in one or more steps. This creates a porous core in the dense outer tube having a purity greater than 99%. Both the porous core and the dense outer tube have a purity greater than 99%.

[0006] Disclosed herein is a dual density disk comprising a high density outer tube comprising a first ceramic having a purity greater than 96% and a porous core comprising a second ceramic having a density lower than that of the high density outer tube, the porous core having a purity greater than 99%. In embodiments, the first ceramic may be the same as the second ceramic. In other embodiments, the first ceramic may be different from the second ceramic. [Brief explanation of the drawings]

[0007] [Figure 1(A)] 1A-1C are schematic diagrams of top and side views of dual density alumina disks and rods. [Figure 1(B)] FIG. 1 is a cross-sectional view of a dual density alumina disk having a dense shell and a porous core. [Figure 2(A)] 1 is a photomicrograph of the porous core taken at a lower magnification. [Figure 2(B)] FIG. 2(B) is another micrograph of the porous core taken at a higher magnification than FIG. 2(A). [Figure 3] 1 is a graph from energy dispersive X-ray analysis showing that the core contains only aluminum and oxygen in the form of alumina, the alumina being greater than 99.3 percent pure. DETAILED DESCRIPTION OF THE INVENTION

[0008] Disclosed herein is a dual-density ceramic disc or rod (hereinafter referred to as a "disc") comprising a dense shell and a porous core (of lower density than the shell) used to provide uniform gas flow for various applications, such as in semiconductor manufacturing operations. The rod may be sliced ​​into several smaller slices called discs. The disc has high strength provided by the dense ceramic shell (hereinafter referred to as the dense shell). The dense shell surrounds a mostly porous ceramic core (hereinafter referred to as the porous core), which allows uniform gas flow during manufacturing operations (such as the fabrication of semiconductor wafers). In addition, the disc contains a dense ceramic shell with a purity greater than 96%, thereby preventing contamination of semiconductor components during manufacturing operations in which the ceramic disc is deployed. The porous core contacts a dense outer tube (shell) on its inner surface, and the outer tube continuously contacts the porous core along the entire circumference of the porous core. In embodiments, the porous core contains the same chemical composition as the dense shell, except that the core is porous while the shell is dense. The density of the shell is greater than that of the porous core.

[0009] In another embodiment, the porous core contains a ceramic having a chemical composition different from that of the dense shell. Disclosed herein is a dual density disk comprising a dense outer tube containing a first ceramic having a purity greater than 96% and a porous core containing a second ceramic having a density lower than that of the dense outer tube, the porous core having a purity greater than 99%. In an embodiment, the first ceramic may be the same as the second ceramic. In another embodiment, the first ceramic may be different from the second ceramic.

[0010] The ceramics used in the disks include metal oxides, carbides, oxycarbides, nitrides, oxynitrides, borides, borocarbides, boronitrides, silicides, iodides, bromides, sulfides, selenides, tellurides, fluorides, or borosilicates. Suitable metals are aluminum, titanium, zirconium, silicon, cerium, etc., or combinations thereof.

[0011] In embodiments, the ceramic is preferably a metal oxide. Preferred metal oxides are titania, silica, alumina, zirconia, ceria, etc., or combinations thereof. A preferred metal oxide for use in the disk is alumina. While the article and manufacturing method detailed below are directed to alumina disks, they are equally applicable to any of the ceramics listed above. The temperatures and atmospheres listed below for manufacturing alumina disks work equally well for any of the ceramics listed above, and therefore there is no duplication of the annealing temperature, sintering temperature, or atmosphere and pressure used to manufacture the ceramic disks.

[0012] Also disclosed herein are dual-density alumina disks or rods (hereinafter referred to as "disks") that include a dense shell and a porous core (of lower density than the shell) used to provide uniform gas flow for semiconductor manufacturing operations. The rods can be sliced ​​into several smaller slices called disks. The disks have high strength provided by the dense shell. The dense shell surrounds a mostly porous core that allows uniform gas flow (for manufacturing semiconductors) during manufacturing operations. In addition, the disks contain high-purity alumina (greater than 96%), thereby preventing contamination of semiconductor components during manufacturing operations in which the alumina disks are deployed. The porous core contacts a dense outer tube (shell) on its inner surface, and the outer tube is in continuous contact with the porous core along its entire circumference. This core-shell dual-density structure provides radial hermiticity (i.e., prevents leakage from the sides of the porous body) and allows flow only through the longitudinal direction. It should be noted that the outer shell is referred to herein as the "alumina shell," the "alumina tube," the "density shell," and the "density outer tube."

[0013] Also disclosed herein is a method for producing an alumina disk or rod. The method involves filling a high-density alumina tube with an alumina slurry. The alumina slurry contains alumina powder (of high purity) and a pore former. The tube containing the slurry is then fired to produce a porous core in the high-density alumina tube. The alumina tube with the porous core can then be subjected to finishing operations such as slicing, lapping, grinding, etc. to produce an alumina disk.

[0014] FIG. 1 shows a ceramic disc 100 comprising a ceramic shell 102 and a porous ceramic core 104 contained therein.

[0015] The ceramic tube (also referred to herein as the ceramic shell) used to manufacture the ceramic disc has an outer diameter (D) of 1.0 to 75 millimeters, preferably 3.5 to 12.6 millimeters. o ) and an inner diameter (D i These diameter dimensions also apply to ceramic discs that can be obtained by slicing the ceramic tube. The ceramic tube has a length (L) of 1 to 120 millimeters, preferably 3.5 to 25.4 millimeters. The ceramic tube has a purity of more than 96%, preferably more than 99%, and a density (g / cm) of 3.0 to 3.95 grams per cubic centimeter. 3 The dimensions shown in FIG. 1 apply to an alumina tube with an alumina dense shell and a porous alumina core.

[0016] The porous core is produced by filling the hollow center of an alumina tube with an alumina slurry or powder, and then heating the tube containing the slurry or powder to form an alumina disk. The slurry contains alumina powder with high purity, an optional solvent, and a pore former, which are described in more detail below.

[0017] Porous cores can also be formed without the use of pore formers and solvents. A distribution of particle sizes can produce porous cores when sintered. Adjacent portions of the particles undergo necking as they bond together to form the porous core.

[0018] Alumina powders having either a narrow or broad particle size distribution are formed into a slurry with a solvent and a pore former. The alumina powders can have particle sizes between 10 nanometers and 500 micrometers, preferably between 100 nanometers and 150 micrometers, and more preferably between 150 nanometers and 100 micrometers. The alumina powders can have a unimodal particle size distribution, or can have a bimodal or higher particle size distribution.

[0019] The alumina powder also has a purity of greater than 99%, preferably greater than 99.3%. The alumina powder may be present in the slurry in an amount of 10 to 90% by weight, preferably 20 to 60% by weight, based on the total weight of the slurry.

[0020] The solvent is optional. The solvent used to form the slurry can include a polar solvent or a non-polar solvent. The solvent can be protic or aprotic. Liquid aprotic polar solvents such as propylene carbonate, ethylene carbonate, butyrolactone, acetonitrile, benzonitrile, nitromethane, nitrobenzene, sulfolane, dimethylformamide, N-methylpyrrolidone, etc., or combinations thereof, are generally desirable. Polar protic solvents, such as, but not limited to, water, methanol, acetonitrile, nitromethane, ethanol, propanol, isopropanol, butanol, or the like, or combinations thereof, can be used. Other non-polar solvents, such as benzene, toluene, carbon tetrachloride, hexane, diethyl ether, tetrahydrofuran, etc., or combinations thereof, can also be used. Preferred solvents are water, alcohol, or combinations thereof.

[0021] When present, the solvent may be used in an amount of 5 to 80% by weight, preferably 15 to 60% by weight, based on the total weight of the slurry.

[0022] The pore former is optional. It may or may not be mixed into the slurry to promote the formation of a porous core in the alumina tube. As mentioned above, the particle size distribution can result in porosity present in the core (if no solvent or pore former is used in the slurry). The pore former can be a gas, liquid, or solid. In embodiments, the pore former is organic and can decompose to liberate a gas when heated to high temperatures. Examples of such pore formers include solids such as AIBN (azobisisobutyronitrile), an organic compound with the formula [(CH3)2C(CN)]2N2. It is a white powder and is soluble in alcohol and common organic solvents.

[0023] In another embodiment, the pore former can be an organic polymer. The polymer, typically in powder form, is mixed with the alumina powder to form a mixture (or slurry, if a solvent is also used) that is then placed in the alumina tube. When the alumina tube is heated to a temperature of 300-600°C, the polymer decomposes, promoting the formation of pores in the alumina powder.

[0024] The organic polymer used in the spaced features and / or surfaces can be selected from a wide variety of thermoplastic polymers, blends of thermoplastic polymers, thermosetting polymers, or blends of thermoplastic and thermosetting polymers. The organic polymer can also be a polymer, copolymer, terpolymer, or a blend of combinations comprising at least one of the aforementioned organic polymers. The organic polymer can also be an oligomer, homopolymer, copolymer, block copolymer, alternating block copolymer, random polymer, random copolymer, random block copolymer, graft copolymer, star block copolymer, dendrimer, polyelectrolyte (polymer with some repeating groups containing electrolytes), polyampholite (polyelectrolyte with both cationic and anionic repeating groups), ionomer, etc., or a combination comprising at least one of the aforementioned organic polymers. The organic polymer has a number average molecular weight greater than 10,000 grams per mole, preferably greater than 20,000 g / mole, and more preferably greater than 50,000 g / mole.

[0025] Exemplary organic polymers include polyacetals, polyacrylic acids, polycarbonates, poly(meth)acrylates, polyalkyds, polystyrenes, polyolefins, polyesters, polyamides, polyaramids, polyamideimides, polyarylates, polyurethanes, epoxies, phenolics, silicones, polyarylsulfones, polyethersulfones, polyphenylene sulfides, polysulfones, polyimides, polyetherimides, polytetrafluoroethylenes, polyetherketones, polyetheretherketones, polyetherketoneketones, polybenzoxazoles, polyoxadiazoles, polybenzothiazinophenothiazines, polybenzothiazoles, polypyrazinoquinoxalines, polypyromellitimides, polyguinoquinoxalines, polyguinoquinoxalines, polypyromellitimides ... xaline), polybenzimidazole, polyoxindole, polyoxoisoindoline, polydioxoisoindoline, polytriazine, polypyridazine, polypiperazine, polypyridine, polypiperidine, polytriazole, polypyrazole, polycarborane, polyoxabicyclononane, polydibenzofuran, polyphthalide, polyacetal, polyanhydride, polyvinyl ether, polyvinyl thioether, polyvinyl alcohol, polyvinyl ketone, polyvinyl halide, polyvinyl nitrile, polyvinyl ester, polysulfonate, polysulfide, polythioester, polysulfone, polysulfonamide, polyurea, polyphosphazene, polysilazane, polypropylene, polyethylene, polyethylene terephthalate, polyvinylidene fluoride, polysiloxane, and the like, or a combination thereof.

[0026] In another embodiment, the pore former can be a gas that is soluble in the alumina powder or the solvent used in the slurry. The gas can then phase separate (by binodal decomposition) from the slurry (upon changes in pressure and / or temperature) to form a porous phase (which is the pores) in the slurry. Examples of such gases include carbon dioxide, argon, hydrogen, nitrogen, or combinations thereof.

[0027] In yet another embodiment, blowing agents such as chlorofluorocarbons, hydrochlorofluorocarbons (HCFCs), and / or hydrofluorocarbons (HFCs) may be used to form the pores. Chlorofluorocarbons (CFCs) are derived from methane and ethane, and these compounds have the formula CCl m F 4-m and C2Cl m F 6-m where m is not zero. Hydrochlorofluorocarbons (HCFCs) are also derived from methane and ethane, and these compounds have the formula CCl m F n H 4-m-n and C2Cl x F y H 6-x-y where m, n, x, and y are not zero. Hydrofluorocarbons (HFCs) can also be derived from methane, ethane, propane, and butane, and these compounds have the formula CF m H 4-m , C2F m H 6-m , C3F m H 8-m , and C4F m H 10-m where m is not zero.

[0028] The pore-forming agent may also be used in an amount of 5 to 50% by weight, preferably 10 to 50% by weight, based on the total weight of the slurry. Combinations of the aforementioned pore-forming agents may also be used.

[0029] A slurry is prepared by mixing alumina powder, an optional solvent, and a pore former. The slurry is then introduced into the hollow core of the alumina tube. The slurry may be compacted or uncompacted.

[0030] The alumina tube containing the slurry is subjected to temperatures of 300 and 600°C for a period of 10 minutes to 12 hours to form pores in the core of the alumina tube. This step promotes activation of the pore former. The high temperature causes the pore former to decompose and liberate gases, which promote pore formation in the core of the alumina tube.

[0031] The alumina tube containing the porous alumina is then subjected to one or more sintering steps. In embodiments, the first sintering step is performed at a temperature of 800-1600°C in air or a controlled atmosphere. This first sintering step is optional and is performed to improve handling (i.e., to ensure that the porous core remains intact within the dense alumina shell). The first sintering step is performed in a vacuum, atmospheric pressure, and / or a controlled pressure environment where the pressure is greater than atmospheric pressure, either in vacuum, air, oxygen, argon, nitrogen, natural gas, hydrogen, carbon dioxide, or a combination thereof.

[0032] The alumina tube with the porous core is then subjected to a second sintering step, carried out at temperatures between 1500 and 2000°C in either vacuum, air, oxygen, argon, nitrogen, natural gas, hydrogen, carbon dioxide, or a combination thereof, to obtain an alumina disk with a porous center containing open-cell pores that allow gas to flow through the alumina disk from one end to the other.

[0033] In an embodiment, a method for manufacturing an alumina disk includes disposing an alumina powder in a dense outer tube. The powder is free of pore formers and / or solvents. The dense outer tube with the alumina powder disposed therein is sintered in one or more steps at a temperature of 800-2000°C. The two-step sintering process described above can also be used here. This creates a porous core in the dense outer tube. The porous core is directly covalently bonded to the dense outer tube. This creates a porous core in the dense outer tube with a purity greater than 99%. Both the porous core and the dense outer tube have a porosity greater than 99%. In other words, no adhesives or additives are used to facilitate bonding of the porous core to the dense outer tube.

[0034] In both of the sintering methods disclosed above, the porous core has a higher purity than the dense outer tube. This applies to all ceramic discs.

[0035] The sintered alumina disk is now a dual-density part having a first, higher-density shell and a second, lower-density core. The part may or may not then be subjected to further finishing operations. The density of the alumina shell is greater than the density of the porous alumina core. In embodiments, finishing operations can involve slicing the disk into several smaller disks. In other embodiments, the alumina disk can have features such as radii at the edges, steps, or chamfers that can be machined into the shell before the porosity (interior) is formed or after the pores are formed. The porous portion in the outer alumina shell can be made flush with the shell wall by machining, including milling. The outer diameter can be ground to the desired dimensions. Machining can be performed using an aqueous cooling water to flush away any remaining post-machining impurities and ensure high purity of the part.

[0036] Dual-density alumina disks allow fluids to flow from one side to the other and can be used as flow controllers and filters fabricated with a variety of pore size distributions. High crush strength due to the high-density alumina shell and high flow rates due to the porosity within the shell (formed without compaction) are key features of dual-density parts. Pore sizes greater than 0.5 micrometers can be controlled for flow control and filtration applications. Strong bonds (via covalent and / or ionic bonds) between the porous alumina and the alumina tube (shell) are created during the sintering process.

[0037] Dual-density alumina disks comply with semiconductor industry cleanliness standards, as evidenced by low particulate shedding under ultrasonic exposure in deionized water. The crush strength of dual-density alumina disks can exceed 20,000 pounds per square inch, and the crush strength of the porous portion can exceed 1,000 pounds per square inch, preferably 2,000 pounds per square inch, when tested as detailed below. The crush strength and flow of gas within the pores depend on the initial particle size distribution of the alumina powder used, the amount (i.e., weight percent) of pore-former in the blend, and the sintering conditions. Furthermore, flow increases with increasing inner diameter and decreasing length of the alumina disk. Flow rate, crush strength (of the porous region within the dense shell), and density data for dual-density disks obtained using different blend compositions are shown in Table 1 below. Table 1 contains exemplary data from tested samples. Flow rate was determined at a gauge pressure of 30 pounds per square inch.

[0038] [Table 1]

[0039] Table 1 shows the material composition of the porous core after sintering. The flow rate through the alumina disk can vary over a wide range depending on the application. It is noted that the nitrogen flow rate through the alumina disk varies from 5,000 to 30,000 standard cubic centimeters per minute, preferably from 10,000 to 22,000 standard cubic centimeters per minute. The bulk density of the porous core varies from 1.00 to 2.50 g / cc, preferably from 1.05 to 1.30 g / cc, and preferably from 1.08 to 1.27 g / cc, and the crush strength within the alumina disk is 1,800 to 2,500 pounds per square inch. Figure 1(B) shows a cross section of an alumina disk having a dense skin and a porous core. In an embodiment, when present within a dense outer tube, the porous core has a bulk density of 1.00 to 1.30 g / cc and a crush strength of greater than 2,000 pounds per square inch.

[0040] The porous core has a porosity of greater than 30 volume percent, preferably greater than 50 volume percent, preferably greater than 70 volume percent, preferably greater than 80 volume percent, and more preferably greater than 90 volume percent, based on the total volume of the core. The ceramic disc also exhibits radial Hermiticity, i.e., fluids transported through it do not leak out the sides of the porous body. Thus, flow is only possible in the longitudinal direction (length L) (see FIG. 1(A)).

[0041] Figures 2(A) and 2(B) show scanning electron micrographs of the porous region of an alumina disk, with Figure 2(A) showing the porous region at low magnification and Figure 2(B) showing the porous region at high magnification.

[0042] Energy dispersive X-ray analysis (EDAX) performed on the core indicates that it is highly pure (>99.3%). This can be seen in Figure 3, which shows a graph of the aluminum signal of the alumina disk. Figure 3 shows the EDAX spectrum of the dual-density part, showing the presence of aluminum and oxygen as the only elements, demonstrating the high purity of the dual-density alumina disk. The porous section has very high purity, having been fabricated using alumina powder with a purity of over 99.9%, and the alumina tube (shell) has a purity of over 99.3%. The porous section is covalently bonded directly to the alumina tube (shell).

[0043] Test Method Porous crushing strength test To reach the crushing strength of the porous core, a method was developed to apply force independently to the porous core, rather than to both the porous core and the dense shell. This method involves clamping the part in a fixture that allows a gauge pin with a diameter equal to the diameter of the porous core to rest perpendicular to the surface of the porous core. The opening in the fixture that accommodates the gauge pin is made 0.005 inches wider than the diameter of the gauge pin to minimize friction. The part is fixed in this fixture so that the porous core can displace under sufficient load, while the shell is pinned to the internal surface of the fixture and cannot displace.

[0044] To test for crush strength, the part is placed into an MTS or Instron compression strength tester. The fixture is placed on a flat, fixed bottom plate. A gauge pin is loaded into the top of the fixture. The flat top plate is then set to displace downward at a rate of 0.001 in / sec, loading the gauge pin and, in turn, the part. Part failure is recorded mechanically and manually / visually characterized by two failure modes. The first failure mode involves the extrusion of the porous material from the shell. The second failure mode involves the fracture of the porous material. Whatever failure mode occurs first, the load at this failure mode is converted to compressive strength.

[0045] Density determination test Shell: The outer diameter and length of the shell are sized through the use of a drop gauge. The inner diameter is measured through imaging and analysis. The mass is recorded through the use of an analytical balance. The density is recorded as the measured mass divided by the calculated volume.

[0046] Porous core: The mass is taken before and after processing the porous core. The difference between the measured mass of the shell only and the measured mass of the finished product (with shell and porous core) is converted to the mass of the porous core. The dimensions of the porous core are taken: a drop gauge measures the length of the porous core, and optical imaging provides the diameter. The density is the differential mass divided by the calculated volume.

[0047] Shell + Porous: The finished part is sized with a drop gauge. The mass is recorded via analytical balance. The density of the bulk part is calculated as the measured mass over the calculated volume.

[0048] While the present invention has been described with reference to several embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is not intended that the invention be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but the invention is intended to include all embodiments within the scope of the appended claims.

Claims

1. A dual density disc, a dense outer tube comprising alumina having a purity greater than 96%; a porous core comprising alumina of a density lower than that of the dense outer tube, the porous core having an alumina purity greater than 99%.

2. 10. The dual density disc of claim 1, wherein the porous core, when present within the dense outer tube, has a bulk density of 1.00 to 2.5 g / cc and a crush strength of greater than 2000 pounds per square inch.

3. 2. The dual density disk of claim 1, wherein the porous core contacts the dense outer tube on its inner surface and the dense outer tube contacts the porous core continuously around the entire circumference of the porous core.

4. 10. The dual density disk of claim 1, wherein the nitrogen flow rate at 30 pounds per square inch gauge pressure through said dual density disk ranges from 5,000 to 30,000 standard cubic centimeters per minute.

5. 10. The dual density disk of claim 1, having radial Hermitianity that prevents leakage out the sides of the porous body and allows flow only in the longitudinal direction.

6. 10. The dual density disk of claim 1, wherein the high density outer tube has an outer diameter of 1 to 75 millimeters and an inner diameter of 0.8 to 70 millimeters.

7. 10. The dual density disc of claim 1, wherein the high density outer tube has an initial length of 1 to 120 millimeters.

8. 10. The dual density disk of claim 1, wherein the porous core is made from an alumina powder having an initial purity of greater than 99%.

9. 10. The dual density disk of claim 1, wherein the porous core is made from an alumina powder having an initial purity of greater than 99.3%.

10. 1. A method comprising: High density outer tube disposing a slurry comprising alumina powder and a pore former; heating the dense outer tube with the slurry disposed therein to a temperature of 300-600°C to activate the pore former; forming a porous core in the dense outer tube; sintering the dense outer tube with the porous core at a temperature of 800-2000°C in one or more steps.

11. The method of claim 10, wherein the sintering operation is carried out in air, nitrogen, natural gas, argon, hydrogen, or a combination thereof.

12. The method of claim 10, wherein the pore-forming agent is an organic pore-forming agent.

13. The method of claim 10 , wherein the slurry further comprises a solvent.

14. The method of claim 10, wherein the sintering is preceded by a first sintering step at a temperature of 1200-1600°C.

15. The method of claim 10, wherein the pore-forming agent is azobisisobutyronitrile.

16. The method of claim 10, wherein the pore former is a gas that is soluble in the alumina slurry.

17. The method of claim 10 wherein the pore-forming agent is a liquid.

18. The method of claim 10 wherein the pore-forming agent is an organic polymer.

19. The organic polymer may be selected from the group consisting of polyacetal, polyacrylic acid, polycarbonate, poly(meth)acrylate, polyalkyd, polystyrene, polyolefin, polyester, polyamide, polyaramid, polyamideimide, polyarylate, polyurethane, epoxy, phenols, polyarylsulfone, polyethersulfone, polyphenylene sulfide, polyimide, polyetherimide, polytetrafluoroethylene, polyetherketone, polyetheretherketone, polyetherketone, polybenzoxazole, polyoxadiazole, polybenzothiazinophenothiazine, polybenzothiazole, polypyrazinoquinoxaline, polypyromellitimide, polyginoxaline, polybenzimidazole, polyoxazole, 19. The method of claim 18, wherein the polyisoindoline is selected from the group consisting of sindole, polyoxoisoindoline, polydioxoisoindoline, polytriazine, polypyridazine, polypiperazine, polypyridine, polypiperidine, polytriazole, polypyrazole, polycarborane, polyoxabicyclononane, polydibenzofuran, polyphthalide, polyacetal, polyanhydride, polyvinyl ether, polyvinyl thioether, polyvinyl alcohol, polyvinyl ketone, polyvinyl halide, polyvinyl nitrile, polyvinyl ester, polysulfonate, polysulfide, polythioester, polysulfone, polysulfonamide, polyurea, polyphosphazene, polysilazane, polyvinylidene fluoride, polysiloxane, and combinations thereof.

20. 1. A method comprising: disposing pore-former-free alumina powder in a dense outer tube; sintering the dense outer tube with the alumina powder disposed therein at a temperature of 800-2000°C; forming a porous core in said dense outer tube having a purity greater than 99%.

21. 21. The method of claim 20, further comprising heating the densified outer tube with the alumina powder disposed therein to a temperature of 1200 to 1600°C.

22. A dual density disc, a dense outer tube comprising a first ceramic having a purity greater than 96%; a porous core comprising a second ceramic having a density lower than that of the dense outer tube, the porous core having a purity greater than 99%; A dual density disc comprising:

23. 23. The dual density disk of claim 22, wherein the first ceramic is the same as the second ceramic.

24. 23. The dual density disk of claim 22, wherein the first ceramic is different from the second ceramic.

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