Method for producing a monolithic compression alumina material for single crystal growth

Abstract

This application relates to the manufacture of compressed alumina green compacts and monoliths. More specifically, this application relates to a process for the manufacture of compressed alumina green compacts, including injecting a slurry containing micron-sized particles of high-purity alumina and a solvent into a mold and drying the slurry. More specifically, this application relates to an improved process that does not include a mechanical compression step to provide a high-density monolith for use in the single crystal growth industry.

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims the priority of U.S. Patent Application No. 63 / 353,834, filed on June 20, 2022. This document is hereby incorporated by reference in its entirety into this specification.

[0002] (Field of the Invention) This application relates to the field of the production of alumina. More specifically, this application relates to a process for the production of compressed alumina materials.

Background Art

[0003] Aluminum oxide (Al2O3) is one of the most widely used ceramic materials in the advanced ceramic industry. Alumina is extracted from bauxite using the Bayer process. This material is suitable for numerous applications in various industrial, technical, and military uses due to its high thermal, electrical, and physical properties. Today, alumina is used in several modern industries such as synthetic sapphire, light - emitting diodes (LEDs), semiconductors and lithium - ion batteries (LIBs), automotive and aerospace industries, wear protection, dental and orthopedic implants.

[0004] One particular application is the single - crystal growth industry, such as for synthetic sapphire. In the case of sapphire production, high - purity alumina (HPA) is required, which is mainly obtained by the calcination of aluminum salts. Such aluminum salts can be, but are not limited to, aluminum chloride, aluminum nitrate, aluminum sulfate, ammonium aluminum sulfate and ammonium aluminum hydroxycarbonate, or their hydrates; organic aluminum salts, such as aluminum oxalate, aluminum acetate, aluminum stearate, aluminum lactate, aluminum laurate, etc.

[0005] The main drawback of alumina derived from aluminum salts for sapphire industry use is its low bulk density, which reduces the efficiency and capacity of sapphire furnaces. For example, alumina derived from aluminum chloride hexahydrate has a loose bulk density of about 0.4 g / cm 3 , which is one tenth of the true density of alumina (3.96 g / cm 3 ). The cycle time for sapphire production is about three weeks and involves heating the alumina to its melting temperature (>2000 °C) and then controlled cooling under vacuum. Thus, if a low density feedstock (e.g., alumina derived from aluminum chloride) is used in such a furnace, only 10% of the furnace capacity is utilized. Furthermore, the low density is a sign of porosity that can adsorb a significant amount of gas (e.g., oxygen, water vapor), which causes bubbles and other defects in the crystal and significantly shortens the lifespan of crucibles, heating elements, and other components of the crystal growth furnace.

[0006] Therefore, HPA powders must be pretreated to significantly increase their density and reduce the amount of gas trapped. To obtain higher efficiency in sapphire furnaces, typically, the feed alumina is compressed and sintered before being fed into the sapphire furnace. Compression produces a green body with acceptable strength and sintering increases the density of the green body. Generally, for single crystal growth, a compressed material with a density higher than 3.2 g / cm 3 is required. Some of the main methods of compression / molding of ceramic powders are freeze casting, slip casting, powder injection molding, cold isostatic pressing, uniaxial die pressing, and extrusion additive manufacturing.

[0007] The flowchart shown in FIG. 1 depicts a process typically used to manufacture compression materials for the sapphire industry. Commercially, these compression materials are referred to as "packs". Generally, the supplied HPA powder has a large particle size distribution, and it is not feasible to compress / sinter such large granules. Therefore, the HPA particles must pass through a grinding step. A typical grinding technique is wet grinding, in which the powder is ground using a ceramic grinding medium. The product of the wet grinder is a slurry of alumina, which must be dried. A typical device for drying the alumina slurry is a spray dryer. In most cases, an organic binder is added to the slurry to increase the mechanical strength of the green body after the compression step. Since α-alumina does not easily sinter together, a binder is used. Many sapphire manufacturers have experienced problems associated with the compression materials achieved using binders. Such organic binders are usually burned during the sintering process, but there is a possibility that the binder may be trapped within closed pores, which increases the impurity level of the material. Even a small amount of impurities (on the order of ppm) can affect the quality of the resulting single crystal, resulting in an opaque crystal. Therefore, an increasing number of suppliers are developing aqueous slurries for the sintering process. Overall, a very small number of suppliers are able to provide high-density (>3.2 g / cm 3 , high-purity binder-free materials).

[0008] There are challenges in the operation of spray dryers, and it is difficult to dry the powder without forming aggregates. The formation of aggregates usually affects the quality of the compression step. Compression is usually carried out under high pressure using a uniaxial press to form a green body. The required pressure is high (10,000 - 50,000 psi). The use of such presses has several drawbacks as follows. · High cost of automated equipment or low production capacity of manual equipment. · HPA has poor fluidity, which makes it impossible or unreliable to use automatic feeding of alumina powder. As a result, manual feeding is used, which affects the production speed. · The materials of the die and punch of the press can be made from tool steel, and such materials may increase the impurity level of the HPA powder, especially considering that alumina is an abrasive material. Another option for the materials of the die and punch is ceramic (e.g., carbide), in which case the fine HPA powder may flow into the gap between the die and the punch, potentially damaging the parts. · Alumina powder tends to adhere to the punch or die in the alumina compression process. To solve this problem, lubricants can be used on the surfaces of the die and punch, but this may also increase the impurities in the powder. · Since high pressure is required to compress the HPA powder into a green body, the production of larger packs requires higher forces, and most dies cannot withstand very high loads, resulting in die breakage in some cases. This mainly occurs in square dies where high stress exists at the corners of the mold. To reduce stress, cylindrical molds can be used, but this is not preferred as it reduces the capacity of the sapphire furnace by π / 4. · The produced compressed materials have limited shapes and limited dimensions. Next, for space optimization, the compressed materials must be strategically placed in the furnace for single crystal growth. In addition, when placed inside the furnace, if the mold is not square or rectangular, there will be gaps between the compressed materials, which reduces the efficiency / capacity of the furnace.

[0009] Korean Patent No. 20130022616(A) proposes manufacturing a larger compression material. However, this technology still uses a compression step, which increases the chance of cross-contamination and involves higher capital and operating costs as described above. Furthermore, the compression of alumina particles pushes the particles in random directions, leaving gaps between the particles. As a result, the density of the green body obtained by such a process is 1.9 - 2.4 g / cm 3 . Furthermore, there is a maximum amount of water that can be used in this process. Therefore, the pulverized material needs to be spray-dried to prepare a slurry, and thus still involves a spray-drying step.

[0010] Therefore, in order to overcome at least some of the drawbacks of existing methods, there is a need to provide an improved process for the preparation of compressed alumina. Summary of the Invention

[0011] Surprisingly, it is shown herein that the process of the present application can produce compressed alumina monoliths and provide larger monoliths with higher densities suitable for use in single crystal growth industries such as synthetic sapphire. Therefore, the process of the present application provides the manufacture of compressed monoliths with reduced impurity levels, and at the same time reduces operational challenges and costs. The present application further provides the use of these processes for the manufacture of compressed alumina monoliths and the monoliths obtained therefrom. Comparable processes do not exhibit the same characteristics, highlighting the surprising results obtained with the process of the present application.

[0012] Therefore, the present application includes a process for the manufacture of a compressed alumina green body, comprising injecting a slurry containing micron-sized particles of high-purity alumina and a solvent into a mold, and drying the slurry to obtain a compressed alumina green body.

[0013] In some embodiments, the high-purity alumina is selected from alpha alumina, transitional alumina, amorphous alumina, and mixtures thereof, or the high-purity alumina is doped with at least one element selected from Mg, Ba, Si, Ti, Zr, Fe, W, Zn, and rare earth elements, or the high-purity alumina is a mixture of high-purity alumina and metal oxides.

[0014] In some embodiments, the process further includes grinding the high-purity alumina to obtain micron-sized particles. In some embodiments, the grinding is performed using a wet grinder, a dry grinder, a ball mill, an air jet mill, or a steam jet mill.

[0015] In some embodiments, the micron-sized high-purity alumina particles have a particle size distribution of from about 0.5 microns to about 100 microns. In some embodiments, the micron-sized high-purity alumina particles have a particle size distribution of from about 0.5 microns to about 50 microns. In some embodiments, the micron-sized high-purity alumina particles have a particle size distribution of from about 1 micron to about 5 microns.

[0016] In some embodiments, the solvent is present in an amount of from about 5 wt% to about 75 wt% based on the total weight of the slurry. In some embodiments, the solvent is present in an amount of from about 10 wt% to about 40 wt% based on the total weight of the slurry. In some embodiments, the solvent is present in an amount of from about 10 wt% to about 20 wt% based on the total weight of the slurry.

[0017] In some embodiments, the slurry is obtained by mixing the high-purity alumina and the solvent. In some embodiments, the solvent is selected from water, methanol, ethanol, isopropanol, acetone, and mixtures thereof. In some embodiments, the solvent is water.

[0018] In some embodiments, the slurry further comprises an organic binder, a dispersant, or a mixture thereof. In some embodiments, the organic binder is a polymer selected from carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyethylene glycol (PEG). In some embodiments, the dispersant is selected from disodium 4,5-dihydroxy-1,3-benzenedisulfonate, carbonate, ammonium polymethacrylate, carbonate ester, sodium pyrophosphate, diammonium hydrogen citrate, triammonium salt of aurintricarboxylic acid, Darvan C-N, sodium pyrophosphate, diammonium hydrogen citrate, citric acid, nitric acid, ascorbic acid, ammonium polyacrylate, and polycarbonate.

[0019] In some embodiments, the process further comprises a sedimentation period prior to drying the slurry to allow sedimentation of micron-sized particles. In some embodiments, the sedimentation period further comprises vibrating the mold. In some embodiments, the sedimentation period is carried out for about 0.5 hours to about 24 hours. In some embodiments, the sedimentation period is carried out for about 1 hour to about 12 hours. In some embodiments, the sedimentation period is carried out for about 1 hour to about 5 hours.

[0020] In some embodiments, the drying is carried out in an oven, a gas combustion dryer, an electric dryer, or a microwave oven. In some embodiments, the drying is carried out at a temperature of about 15°C to about 150°C. In some embodiments, the drying comprises heating at a temperature of about 30°C to about 150°C. In some embodiments, the drying comprises heating at a temperature of about 50°C to about 150°C.

[0021] In some embodiments, the drying is carried out for about 0.5 hours to about 1 week. In some embodiments, the drying is carried out for about 1 hour to about 48 hours. In some embodiments, the drying is carried out for about 12 hours to about 24 hours.

[0022] In some embodiments, the process further includes removing the compressed alumina green body from the mold and subjecting the compressed alumina green body to a heat treatment, thereby providing a single unitary compacted alumina piece. In some embodiments, the heat treatment includes heating at a temperature of about 150°C to about 1200°C. In some embodiments, the heat treatment includes heating at a temperature of about 200°C to about 1200°C. In some embodiments, the heat treatment includes heating at a temperature of about 250°C to about 1200°C. In some embodiments, the heat treatment is performed for about 0.5 hours to about 24 hours. In some embodiments, the heat treatment is performed for about 1 hour to about 12 hours. In some embodiments, the heat treatment is performed for about 2 hours to about 5 hours.

[0023] In some embodiments, the process further includes removing the compressed alumina green body from the mold and subjecting the compressed alumina green body to sintering, thereby providing a single unitary compacted alumina piece. In some embodiments, the sintering includes heating at a temperature of about 1200°C to about 1800°C. In some embodiments, the sintering includes heating at a temperature of about 1400°C to about 1800°C. In some embodiments, the sintering includes heating at a temperature of about 1600°C to about 1800°C. In some embodiments, the sintering is performed for about 0.5 hours to about 24 hours. In some embodiments, the sintering is performed for about 2 hours to about 12 hours. In some embodiments, the sintering is performed for about 2 hours to about 8 hours.

[0024] In some embodiments, the mold is made of foamed plastic, Styrofoam (trademark), polytetrafluoroethylene (PTFE), silicone polymer, or polystyrene. In some embodiments, the mold is rectangular, square, circular, or oval. In some embodiments, the mold has a thickness of about 50 mm to about 200 mm. In some embodiments, the mold has a thickness of about 100 mm to about 200 mm. In some embodiments, the mold has a thickness of about 100 mm to about 150 mm.

[0025] In some embodiments, the compressed alumina green body has a bulk density of about 2.0 g / cm 3 to about 3.3 g / cm 3 . In some embodiments, the compressed alumina green body has a bulk density of about 2.3 g / cm 3 to about 3.0 g / cm 3 . In some embodiments, the compressed alumina green body has a bulk density of about 2.5 g / cm 3 to about 2.8 g / cm 3 .

[0026] In some embodiments, a single integral compressed alumina piece has a bulk density of about 3.0 g / cm 3 to about 3.7 g / cm 3 . In some embodiments, a single integral compressed alumina piece has a bulk density of about 3.1 g / cm 3 to about 3.6 g / cm 3 . In some embodiments, a single integral compressed alumina piece has a bulk density of about 3.2 g / cm 3 to about 3.5 g / cm 3 .

[0027] In some embodiments, the process does not include a mechanical compression step.

[0028] Further provided is a compressed alumina green body produced by the process of the present application.

[0029] The present application further includes a compressed alumina green body comprising compressed micron-sized particles of high-purity alumina.

[0030] Further provided is a single integral compressed alumina piece produced by the process of the present application.

[0031] The present application further includes a single integral compressed alumina piece comprising compressed micron-sized particles of high-purity alumina.

[0032] In some embodiments, the compressed alumina green body and the single integral compressed alumina piece have the properties defined above.

[0033] Furthermore, the use of the alumina green body or the single integral compressed alumina piece of the present application in the single crystal growth process is also included.

[0034] Furthermore, the use of the alumina green body or the single integral compressed alumina piece of the present application in a single crystal growth furnace is also provided.

[0035] Furthermore, the use of the alumina green body or the single integral compressed alumina piece of the present application in the production of sapphire is included.

[0036] Other features and advantages of the present application will become apparent from the following detailed description. However, it should be understood that the detailed description and specific examples showing embodiments of the present application are given by way of illustration only, and the scope of the claims should not be limited by these embodiments, but should be given the broadest interpretation consistent with the overall description.

Brief Description of the Drawings

[0037] Next, embodiments of the present application will be described in more detail with reference to the accompanying drawings.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Best Mode for Carrying Out the Invention

[0038] I. Definitions Unless otherwise indicated, the definitions and embodiments described in this section and other sections are intended to apply to all embodiments and aspects of this application described in this specification as are suitable, as understood by those skilled in the art.

[0039] As used in this disclosure and the claims, the terms “comprising” (and any form of “comprise” and “comprises”), “having” (and any form of “have” and “has”), “including” (and any form of “include” and “includes”), or “containing” (and any form of “contain” and “contains”) are inclusive or open-ended and do not exclude additional, unrecited elements or process steps.

[0040] The term “consisting of” and its derivatives as used herein are intended to be closed-ended terms that specify the presence of the recited features, elements, components, groups, integers, and / or steps and exclude the presence of other unrecited features, elements, components, groups, integers, and / or steps.

[0041] The term “consisting essentially of” as used herein is intended to specify the presence of the recited features, elements, components, groups, integers, and / or steps, as well as the presence of those that do not substantially affect the basic and novel characteristics of those features, elements, components, groups, integers, and / or steps.

[0042] As used herein, the terms "about," "substantially," and "approximately" mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree are to be construed to include at least a ±5% deviation of the modified term, where such deviation does not negate the meaning of the word it modifies or unless the context suggests otherwise to one of ordinary skill in the art.

[0043] As used in this application, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0044] In embodiments that include "additional" or "second" components, the second component as used herein is chemically different from the other components or the first component. The "third" component is different from the other components, the first component, and the second component, and further enumerated components or "additional" components are similarly different.

[0045] As used herein, the term "and / or" means that the listed items are present or used individually or in combination. In effect, this term means "at least one of" or "one or more of" the listed items are used or present.

[0046] As used herein, the term "suitable" means that the selection of a particular composition or condition depends on the particular step being performed, the identity of the component being transformed, and / or the particular use of the composition, but the selection is well within the skill of one of ordinary skill in the art.

[0047] The term "smelting grade alumina" or "SGA" as used herein refers to a grade of alumina that may be useful for processes for preparing aluminum metal. Smelting grade alumina typically contains α-Al2O3 in an amount of less than about 5% by weight based on the total weight of the smelting grade alumina.

[0048] As used herein, the term "high-purity alumina" or "HPA" refers to a grade of alumina that contains alumina in an amount of 99 wt% or more based on the total weight of the high-purity alumina.

[0049] As used herein, the expression "transition alumina" refers to polymorphic forms of alumina other than α-alumina. For example, transition alumina can be χ-Al2O3, κ-Al2O 3、 γ-Al2O3, θ-Al2O 3、 δ-Al2O 3、 η-Al2O3, ρ-Al2O3, or a combination thereof.

[0050] As used herein, the expression "amorphous alumina" refers to an amorphous polymorph of alumina that lacks the long-range order characteristics of a crystal.

[0051] As used herein, the term "sintering" generally refers to a thermal process that converts loose fine particles into a coherent mass of an individual by heat and optionally pressure without completely melting the particles to the melting point.

[0052] As used herein, the term "single crystal growth" generally refers to the growth of an inorganic bulk single crystal in which the crystal lattice of the entire sample is continuous up to the edge of the sample and is a solid without grain boundaries.

[0053] As used herein in the context of alumina, the term "green body" generally refers to a mass of material that typically needs to be further processed, e.g., by firing, sintering, etc., before use.

[0054] As used herein, the term "monolith" in the context of alumina refers to a single massive organized block, and is also interchangeably referred to as a single piece of integrally compressed alumina.

[0055] As used herein, the terms "kiln," "furnace," and "oven" are used interchangeably and refer to a heat-insulated chamber that generates a temperature sufficient to complete a process of converting a material into a different form or a chemically different material, such as by curing, drying, or calcining.

[0056] II. Methods and Uses of the Present Application Surprisingly, it is shown herein that the process of the present application can produce compressed alumina monoliths and provide larger monoliths with higher density suitable for use in single crystal growth industries such as synthetic sapphire. Thus, the process of the present application provides for the production of compressed monoliths with reduced impurity levels while simultaneously reducing operational challenges and costs. The present application further provides for the use of these processes for the production of compressed alumina monoliths and the monoliths obtained therefrom. Comparable processes do not exhibit the same characteristics, highlighting the surprising results obtained with the process of the present application.

[0057] Accordingly, the present application includes a process for the production of a compressed alumina green body, the process comprising injecting a slurry comprising micron-sized particles of high-purity alumina and a solvent into a mold and drying the slurry to obtain a compressed alumina green body.

[0058] In some embodiments, the high-purity alumina (HPA) is selected from alpha alumina, transitional alumina, amorphous alumina, and mixtures thereof. In some embodiments, the HPA is doped with at least one element or is a mixture of HPA and other metal oxides. In some embodiments, the dopant is selected from Mg, Ba, Si, Ti, Zr, Fe, W, Zn, and rare earth elements. Those skilled in the art will understand that the composition of the starting material can be varied depending on the different intended uses of the final compressed material.

[0059] In some embodiments, the solvent is selected from water, methanol, ethanol, isopropanol, acetone, and mixtures thereof. In some embodiments, the solvent is water.

[0060] In some embodiments, the process further includes grinding high-purity alumina to obtain micron-sized particles. In some embodiments, the grinding is performed using a wet grinder, a dry grinder, a ball mill, an air jet mill, a steam jet mill, etc. Selecting appropriate techniques and apparatuses for grinding the material to an appropriate size is within the knowledge of those skilled in the art.

[0061] In some embodiments, the micron-sized high-purity alumina particles have a particle size distribution of about 0.5 microns to about 100 microns. In some embodiments, the micron-sized high-purity alumina particles have a particle size distribution of about 0.5 microns to about 50 microns. In some embodiments, the micron-sized high-purity alumina particles have a particle size distribution of about 1 micron to about 5 microns.

[0062] In some embodiments, the solvent is in an amount of about 5 wt% to about 75 wt% based on the total weight of the slurry. In some embodiments, the solvent is in an amount of about 10 wt% to about 40 wt% based on the total weight of the slurry. In some embodiments, the solvent is in an amount of about 10 wt% to about 20 wt% based on the total weight of the slurry.

[0063] In some embodiments, the slurry is obtained by mixing high-purity alumina and a solvent.

[0064] In some embodiments, the slurry further comprises an organic binder, a dispersant, or a mixture thereof. In some embodiments, the organic binder is a polymer selected from carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyethylene glycol (PEG). In some embodiments, the dispersing medium is selected from disodium 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron™), carbonate (Dolapix™ CE 64), ammonium polymethacrylate (Darvan™ C), carbonate ester (Dolapix ET 85), sodium pyrophosphate, diammonium hydrogen citrate, triammonium salt of aurintricarboxylic acid (Aluminon™), Darvan C-N, sodium pyrophosphate, diammonium hydrogen citrate, citric acid, nitric acid, ascorbic acid, ammonium polyacrylate (Seruna™ D-305), and polycarbonate (Dolapix PC33). In some embodiments, the process does not include a binder.

[0065] In some embodiments, the process further includes a sedimentation period prior to drying the slurry to allow for sedimentation of micron-sized particles. In some embodiments, the sedimentation period further includes vibrating the mold. In some embodiments, the sedimentation period is performed for about 0.5 hours to about 24 hours. In some embodiments, the sedimentation period is performed for about 1 hour to about 12 hours. In some embodiments, the sedimentation period is performed for about 1 hour to about 5 hours.

[0066] In some embodiments, drying is performed in an oven, a gas combustion dryer, an electric dryer, or a microwave oven. In some embodiments, drying is performed at a temperature of about 15°C to about 150°C. In some embodiments, drying includes heating at a temperature of about 30°C to about 150°C. In some embodiments, drying includes heating at a temperature of about 50°C to about 150°C. In some embodiments, drying is performed for about 0.5 hours to about 1 week. In some embodiments, drying is performed for about 1 hour to about 48 hours. In some embodiments, drying is performed for about 12 hours to about 24 hours.

[0067] In some embodiments, the method further includes removing the compressed alumina green body from the mold and subjecting the compressed alumina green body to a heat treatment, thereby providing a single integral compressed alumina piece. In some embodiments, the heat treatment includes heating at a temperature of about 150°C to about 1200°C. In some embodiments, the heat treatment includes heating at a temperature of about 200°C to about 1200°C. In some embodiments, the heat treatment includes heating at a temperature of about 250°C to about 1200°C. In some embodiments, the heat treatment is performed for about 0.5 hours to about 24 hours. In some embodiments, the heat treatment is performed for about 1 hour to about 12 hours. In some embodiments, the heat treatment is performed for about 2 hours to about 5 hours.

[0068] In some embodiments, the process further includes removing the compressed alumina green body from the mold and subjecting the compressed alumina green body to sintering, thereby providing a single integral compressed alumina piece. In some embodiments, sintering includes heating at a temperature of about 1200°C to about 1800°C. In some embodiments, sintering includes heating at a temperature of about 1400°C to about 1800°C. In some embodiments, sintering includes heating at a temperature of about 1600°C to about 1800°C. In some embodiments, sintering is performed for about 0.5 hours to about 24 hours. In some embodiments, sintering is performed for about 2 hours to about 12 hours. In some embodiments, sintering is performed for about 2 hours to about 8 hours.

[0069] In some embodiments, the mold is made of foamed plastic, Styrofoam (trademark), polytetrafluoroethylene (PTFE - Teflon (trademark)), silicone polymer, or polystyrene. In some embodiments, the mold is rectangular, square, circular, elliptical, or the like. In some embodiments, the mold has a thickness of about 50 mm to about 200 mm. In some embodiments, the mold has a thickness of about 100 mm to about 200 mm. In some embodiments, the mold has a thickness of about 100 mm to about 150 mm. One of ordinary skill in the art will readily understand that the size and shape of the mold can be easily adjusted according to the desired final product and its use.

[0070] In some embodiments, the compressed alumina green body has a bulk density of about 2.0 g / cm 3 to about 3.3 g / cm 3 . In some embodiments, the compressed alumina green body has a bulk density of about 2.3 g / cm 3 to about 3.0 g / cm 3 . In some embodiments, the compressed alumina green body has a bulk density of about 2.5 g / cm 3 to about 2.8 g / cm 3 .

[0071] In some embodiments, the compressed alumina monolith (a single integral piece), i.e., after heat treatment / sintering, has a bulk density of about 3.0 g / cm 3 to about 3.7 g / cm 3 . In some embodiments, the compressed alumina green body has a bulk density of about 3.1 g / cm 3 to about 3.6 g / cm 3 . In some embodiments, the compressed alumina green body has a bulk density of about 3.2 g / cm 3 to about 3.5 g / cm 3 .

[0072] In some embodiments, the process does not include a mechanical compression step.

[0073] This application further provides a compressed alumina green body and a single integral compressed alumina piece produced by the method of this application.

[0074] Also included are alumina green bodies or single integral compressed alumina pieces containing compressed micron-sized particles of high-purity alumina.

[0075] In some embodiments, the compressed alumina green body and the single integral compressed alumina piece have the shape and dimensions of the mold as defined above. Again, one of ordinary skill in the art will readily understand that the size and shape of the mold can be easily adjusted to provide compressed alumina green bodies and single integral compressed alumina pieces of the desired size and shape according to the desired application.

[0076] This application further provides the use of the compressed alumina green body and the single integral compressed alumina piece for use in single crystal growth.

[0077] This application further provides the use of the compressed alumina green body and the single integral compressed alumina piece in a single crystal growth furnace.

[0078] In some embodiments, the process of this application can be used to produce compressed materials for use as feedstock for the synthetic sapphire industry. The main applications of sapphire are, for example, but not limited to, wafers for semiconductors and camera lenses, gemstones, medical, defense, aerospace, watches, display covers, and LEDs.

Examples

[0079] The following non-limiting examples are for the purpose of illustrating this application.

[0080] General method The flowchart of the process of this application is shown in Figure 2. The feedstock (HPA powder) was ground in a wet grinder to produce micron-sized powder. The output of the grinder was a slurry of alumina. Optionally, the feedstock may already be micron-sized, in which case the grinding step can be omitted. In this case, a slurry can be made by adding deionized water in an amount of 10-30% by weight to the alumina powder. Next, the slurry was poured / injected into a mold. Before drying, the cast material was allowed to stand for several hours to allow the time for the alumina particles to settle (sedimentation period). Without being bound by theory, during this step, the alumina particles settled in an organized manner, minimizing the gaps between the particles and maximizing the green density. When the sedimentation of the material was performed, the excess water can optionally be removed from the surface of the sedimented portion. Then, the mold was placed in an oven to remove further free water and dry the slurry. The oven temperature can be varied between room temperature and 150 °C. After evaporating and removing the free water and taking the green body material out of the mold, the green density was 2.0-2.8 g / cm 3 was. In some applications of the single crystal growth operation, the green body may be used as it is. Further heat treatment such as sintering may be performed as needed.

[0081] Example 1 4.00 kg of micron-sized high-purity alumina (99.999%) supplied from the decomposition of aluminum chloride hexahydrate was mixed with deionized water to produce a slurry of alumina. The concentration of water in the slurry was 20% by weight. This slurry was poured into a rectangular Styrofoam (trademark) mold having dimensions of 195 mm × 145 mm. The material was left overnight in a laboratory environment (sedimentation period). Then, it was dried in an oven at about 80 °C. The material was taken out of the mold, and its green density was >2.6 g / cm as shown in Figure 3 3 was. Next, the block was sintered at a high temperature of 1600 °C for 5 hours. The obtained sintered monolith is shown in Figure 4, and its density was 3.3 g / cm 3 was.

[0082] Example 2 Micron-sized alpha high-purity alumina (99.999%) supplied from the decomposition of aluminum chloride hexahydrate was mixed with deionized water to produce a slurry of alumina. The concentration of water in the slurry was 15 wt%. This slurry was poured into a cylindrical styrofoam mold having a diameter of 241.3 mm. The material was left overnight in a laboratory environment (sedimentation period). Then, it was dried in an oven at 70-80 °C for 48 hours. The material was taken out of the mold, and the green body is shown in Figure 5, and its density was 2.639 g / cm 3 and it was 97 mm in height, 11.2 kg in weight, and 237 mm in diameter. Then, the green body was sintered at about 1600 °C for about 5 hours to obtain a monolith having a density of 3.2 g / cm 3 with a height of 86.9 mm and a diameter of 212-218 mm.

[0083] Results As shown in the above examples, the process of the present application produces a monolithic compression material that can fit inside the crucible of a single crystal growth furnace. Compared with the general method of manufacturing small rectangular compressed materials, the gap between the compressed materials is reduced, and thus more materials can be loaded into the furnace. This process of the present application provides a simple, reliable, flexible, cost-effective, and contaminant-free process.

[0084] The compression material can be manufactured in any shape and any size according to the shape and dimensions of the single crystal growth furnace, while in the general method, only a small rectangular shape can be manufactured.

[0085] The method of the present application also eliminates the uniaxial compression step, reduces the capital cost and operating cost, and avoids cross-contamination from the die and punch of the press.

[0086] The method of the present application replaces the complex spray drying step with normal drying, and also reduces the capital cost and operating cost.

[0087] Since the addition of an organic binder is also optional in the process of the present application, potential contamination from the binder can be avoided. Needless to say, there is the advantage of avoiding the purchase cost of a high-purity binder.

[0088] Finally, the density of the obtained green body is very high (2.7 - 2.8 g / cm 3 ), which can be directly used in a single crystal growth furnace, thus avoiding the need for an expensive and high-maintenance sintering furnace.

[0089] The teachings of the applicant described herein are related to various embodiments for purposes of illustration, but since the embodiments described herein are intended to be examples, it is not intended that the teachings of the applicant be limited to such embodiments. On the contrary, the teachings of the applicant described and illustrated herein include various alternative forms, modifications, and equivalents without departing from the embodiments described herein, and its general scope is defined in the appended claims.

Claims

1. A method for producing an unsintered compressed alumina body, The slurry, comprising micron-sized particles of high-purity alumina having a volume-based particle size distribution of 1 to 5 microns and which may be doped with at least one element optionally selected from Mg, Ba, Si, Ti, Zr, Fe, W, Zn, and rare earth elements, and a solvent in an amount of 5% to 20% by weight based on the total weight of the slurry, is injected into a mold. The slurry is dried to a concentration of 2.5 g / cm³. ３ ~2.8 g / cm ３ To obtain the compressed alumina unsintered body having the following bulk density, Includes, A method for producing an unsintered compressed alumina, wherein the high-purity alumina contains alumina in an amount of 99% by weight or more based on the total weight of the high-purity alumina.

2. The method according to claim 1, wherein the high-purity alumina is selected from alpha-alumina, transition alumina, and amorphous alumina, or the high-purity alumina is doped, or is a mixture of high-purity alumina and a metal oxide.

3. The method according to claim 1 or 2, further comprising grinding the high-purity alumina to obtain the micron-sized particles.

4. The method according to claim 1 or 2, wherein the amount of the solvent is 10% to 20% by weight based on the total weight of the slurry.

5. The method according to claim 1 or 2, wherein the solvent is selected from water, methanol, ethanol, isopropanol, acetone, and mixtures thereof.

6. The method according to claim 1 or 2, further comprising a settling period before drying the slurry in order to allow the micron-sized particles to settle.

7. The method according to claim 1 or 2, wherein the drying is performed in an oven, a gas combustion dryer, an electric dryer, or a microwave oven.

8. The method according to claim 1 or 2, further comprising removing the compressed unsintered alumina body from the mold, heat-treating the compressed unsintered alumina body, thereby providing a single, integral compressed alumina piece.

9. The method according to claim 8, wherein the heat treatment includes heating at a temperature of 250°C to 1200°C.

10. The method according to claim 1 or 2, further comprising removing the compressed unsintered alumina body from the mold, subjecting the compressed unsintered alumina body to sintering, thereby providing a single, integrated compressed alumina piece.

11. The method according to claim 10, wherein the sintering is performed by heating at a temperature of 1400°C to 1800°C.

12. The aforementioned single, integrally compressed alumina piece has a density of 3.1 g / cm³. ３ ~3.6 g / cm ３ The method according to claim 8, having the bulk density of

13. The aforementioned single, integrally compressed alumina piece contains 3.2 g / cm³ ３ ~3.5 g / cm ３ The method according to claim 8, having the bulk density of

14. The method according to claim 1 or 2, wherein the mechanical compression step is not included.

15. The method according to claim 1 or 2, which does not contain a binder.

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