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OwnerTed A. Loxley, John F. Blackmer +1inventors
InventorsTed A. Loxley, John F. Blackmer, Klaus-Markus Peters
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Cup-shaped porous silica preforms suitable for manufacture of large 24-inch crucibles used in Czochralski crystal-growing furnaces are produced by a unique electrophoretic casting process using a high-purity aqueous silica slip or slurry having a predetermined particle-size distribution, an average particle size of from 6 to 10 microns and a solids content of from 80 to 85 percent by weight. The slurry contains an electrolyte, such as ammonium hydroxide, has a pH of from 7.5 to 8.5, and can be wet milled at a pH of at least 7 in such manner as to provide the micronized silica particles with excellent electrophoretic mobility, thereby providing a superb process for economical mass production of large pure silica preforms using safe voltages, such as 20 to 40 volts. The electrophoretic casting apparatus can be of the type shown in FIGS. 1 and 2 and includes a permeable porous cup-shaped carbon-graphite mold (2) that serves as a positive anode and an internal reticulate or perforated cathode (3) of similar shape having a thin pervious cover means, such as the shroud 25, to prevent local dilution of the slurry at the cathode.
An electrophoretic deposition process for making quartz glass products comprising preparing an aqueous slurry consisting essentially of high-purity micronized silica particles and water, providing said silica particles with a negative electrical surface charge sufficient to provide electrophoretic mobility, providing negative and positive electrodes in contact with said slurry, the positive electrode having an electrically conductive portion providing a shaping surface, said slurry being electrically conductive and substantially free of contaminating metallic ions, maintaining said particles in suspension while controlling the water content of the slurry adjacent to the electrodes, causing the charged silica particles to be deposited at the positive electrode to form a porous silica body of substantial thickness, on said shaping surface, separating the silica body from the shaping surface, and drying and firing the same.
A process for making molded quartz glass articles from at least one aqueous slurry consisting essentially of a suspension of high-purity silica particles and water, providing the silica particles of each slurry with a negative surface charge sufficient to provide electrophoretic mobility, providing a positive anode and a negative cathode in contact with the slurry and applying a voltage to cause electrophoretic deposit of the charged particles at the anode, said anode comprising a mold with an electrically conductive shaping surface, causing the charged silica particles to be deposited on the shaping surface and to form a porous silica preform of substantial thickness, drying and firing the preform, and thereafter sintering the preform at a temperature above 1500° C. to form a dense quartz glass article.
An electrophoretic casting process for making porous silica preforms suitable for use in the manufacture of sintered high-purity quartz glass receptacles, said process comprising preparing a basic aqueous slurry containing an electrolyte and consisting essentially of high-purity micronized silica particles and water, furnishing said silica particles with a negative surface charge sufficient to provide electrophoretic mobility, providing a cup-shaped carbonaceous mold with an internal conductive shaping surface that serves as a positive anode, locating within the mold a hollow electrically-conductive cathode having a pervious cover means that permits movement of hydroxyl ions from the slurry to the cathode, supplying slurry to the interior of the mold to cover the anodic shaping surface and the cathode cover means, applying a direct current with a voltage sufficient to move the charged silica particles toward the anode and to cause them to be deposited at the shaping surface and to form a porous cup-shaped silica preform, separating the wet preform from the mold, drying the preform, and thereafter firing the preform.
In a process of the character described in U.S. Pat. No. 4,072,489 for making transparent quartz glass receptacles from a cup-shaped porous silica preform wherein an aqueous slurry is prepared consisting essentially of at least 80 percent by weight of high-purity micronized silica particles and up to about 20 percent by weight of water, the silica particles are wet milled to provide an average particle size up to 10 microns, the slurry is poured into a cup-shaped casting mold, the silica is deposited on the internal shaping surface of the mold to form a wet porous silica preform with a wall thickness of at least about 0.2 inch, the preform is separated from the mold, dried and fired at a temperature of from 1000° to 1200° C., and the resulting fired preform with a porosity of at least 15 volume percent is sintered in helium in a furnace to a temperature of at least 1700° C. to produce a full-density quartz glass receptacle, such as a Czochralski crucible, the improvement wherein the slurry has a pH of at least 7 with a predetermined particle size distribution, at least 40 percent by weight of the silica particles having a particle size of from 6 to 20 microns, the average particle size is from 6 to 10 microns, the particles have a negative electrical surface charge sufficient to provide superior electrophoretic mobility, the slurry is electrically conductive and substantially free of objectionable contaminating metallic ions, the mold is an electrically-conductive positive electrode or anode having an internal negative electrode or cathode, means are provided for controlling the water content of the slurry adjacent to the electrodes, and a direct current is imposed to attract the silica particles to the anodic shaping surfaces of the mold and thereby form a porous silica preform by electrophoretic deposition.
In a process for making high-purity quartz glass receptacles wherein a cup-shaped preform formed of compacted micronized particles of vitreous silica and having a porosity of at least 15 volume percent is dried, fired and sintered to a temperature above 1700° C., the improvement wherein an aqueous slurry consisting essentially of charged high-purity micronized silica particles and water and having a pH of from 7 to 9 is prepared by wet milling a silica composition having a predetermined particle size distribution to provide the particles with a substantial surface charge and superior electrophoretic mobility, an electrophoretic casting means is provided including a cup-shaped conductive mold serving as a positive anode and an internal shaped cathode having means for controlling the water content of the slurry and for containing and resisting local dilution of the slurry due to rapid formation of water at the cathode, the solids content of the slurry adjacent the mold being maintained substantially uniform at 80 to 85 percent by weight.
Dependencies are inferred from the claim text.