WO2014094157A1

WO2014094157A1 - A method and apparatus for melting aluminum oxide

Info

Publication number: WO2014094157A1
Application number: PCT/CA2013/050978
Authority: WO
Inventors: Scott Nichol
Original assignee: Polar Sapphire Ltd.
Priority date: 2012-12-17
Filing date: 2013-12-17
Publication date: 2014-06-26

Abstract

A vessel comprises a bottom wall having a perimeter and an internal region, one or more side walls having a top and a bottom, wherein the perimeter of the bottom surface is in contact with the bottom of the one or more side walls to define a vessel interior, and a passageway from the vessel interior through the bottom wall at the internal region, wherein the passageway comprises an entrance that is vertically spaced from the bottom wall. A system can include the vessel and a mould located substantially below the passageway of the vessel.

Description

A METHOD AND APPARATUS FOR MELTING ALUMINUM OXIDE

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of U.S. Provisional

Patent Application Serial No. 61/737,929, filed on December 17, 2012, which application is incorporated by reference herein in its entirety.

BACKGROUND

[0002] High-purity aluminum oxide powder can be melted using the

Verneuil process to form "crackle," which can be used as feedstock and melted in a furnace using a crystal growing process such as the Czochralski process, the Bridgman-Stockbarger technique, the float-zone technique, the zone refining technique, the Kyropulos Method, the heat exchanger method, the Stepanov/ Edge-Defined Film-Fed Growth (EFG), or other methods known in the art to make a sapphire ingot. These ingots can be cored and wafered to make substrates, for example to be used in the manufacture of light-emitting diodes (LEDs). The Verneuil process, also called flame fusion, can involve melting a finely powdered substance using an oxyhydrogen flame, and crystallizing the melted droplets into a boule.

[0003] The Verneuil process is capital intensive and is expensive to run.

In addition, due to its use of hydrogen in the process, the Verneuil process generally leaves a higher hydrogen concentration in the sapphire ingot, resulting in undesirably porous sapphire ingots. Also, crackle is generally used in the form of small rocks or chunks. These chunks can tend to leave a lot of space between each other in the crucible, reducing the packing density compared to a solid.

[0004] One method that has been attempted to increase packing density for aluminum oxide comprises compressing the aluminum oxide powder into pucks or pellets. However, compressing aluminum oxide powder generally does not sufficiently improve the packing density. Another problem with powder, partially melted powder, or compressed pucks or pellets is the formation of excessive porosity in the resulting sapphire ingots. The excessive porosity is likely caused by the high surface area of the powder. Aluminum oxide can also have pores in its structure that can contain oxygen and water, which can contaminate the bath when melted directly in a crystal-growing furnace.

OVERVIEW

[0005] The present disclosure describes an apparatus and process for melting aluminum oxide, such as aluminum oxide powder, to form sapphire. The apparatus and process of the present disclosure can provide for a lower cost of manufacturing of sapphire ingots. The process can also provide for a higher packing factor of the aluminum oxide used to produce the sapphire, and can result in less porosity in the sapphire. The apparatus and process can also result in processing of aluminum oxide powder into a solid sapphire form that has a lower hydrogen level compared to sapphire produced by the conventional Verneuil process.

[0006] In an example, the present disclosure describes a system for melting aluminum oxide, the apparatus comprising:

a) a crucible for melting aluminum oxide, the crucible comprising a

passageway at or proximate to a bottom of the crucible, the passageway configured to allow molten aluminum oxide to drip or flow from the crucible;

b) a furnace environment for heating the crucible, the furnace environment comprising at least one of an inert atmosphere and a vacuum atmosphere; c) a heat source configured to heat the furnace environment to a temperature sufficient to melt the aluminum oxide;

d) a feed system configured to control addition of aluminum oxide to the crucible; and

e) a mould under the passageway of the crucible to collect the molten

aluminum oxide.

[0007] In another example, the present disclosure describes a method for processing aluminum oxide, the method comprising: a) Continuously or semi-continuously feeding aluminum oxide to a crucible comprising at least one of tungsten, iridium, rhenium, graphite, and molybdenum, or an alloy comprising at least one of tungsten, iridium, rhenium, graphite;

b) melting the aluminum oxide in the crucible in a furnace environment comprising at least one of an inert atmosphere and a vacuum atmosphere, to provide molten aluminum oxide;

c) flowing or dripping the molten aluminum oxide from the crucible

through a passageway at or proximate to a bottom of the crucible into a mould positioned below the passageway;

d) directionally solidifying the molten aluminum oxide in the mould.

[0008] The present disclosure also describes a system for melting aluminum oxide, the apparatus includes:

a) a crucible comprising a refractory metal, the crucible being configured for melting solid aluminum oxide, the refractor metal crucible comprising a passageway extending upward from a bottom of the crucible so that an entrance into the passageway is spaced from the bottom of the crucible, wherein the passageway is configured to allow molten aluminum oxide to flow or drip from the crucible and to allow a molten bath of aluminum oxide to form within the crucible;

b) a furnace configured for heating the refractory metal crucible and the solid aluminum oxide in order to melt the solid aluminum oxide to provide the molten aluminum oxide, wherein the furnace is configured to be back-filled with argon during melting of the solid aluminum oxide; c) a heat source configured to heat the furnace above a melting point of aluminum oxide;

d) a feed system configured to continuously or semi-continuously feed the aluminum oxide solid into the crucible to provide for a continuous or semi-continuous drip or flow of the molten aluminum oxide through the passageway;

e) a mould positioned under the passageway to collect the molten aluminum oxide, the mould being lined with at least one of aluminum oxide, molybdenum, and tungsten with a Zr0₂ back up insulation, wherein the mould is shaped to provide for directional solidification from a bottom of the mould to a top of the mould, wherein heat from the crucible can be used to at least partially heat a top portion of the molten aluminum oxide in the mould;;

f) a cooling apparatus configured to cool a bottom portion of the mould; g) a refractory or refractory metal supporting crucible configured to support the mould.

BRIEF DESCRIPTION OF THE DRAWING

[0009] In the drawings, like numerals can be used to describe similar elements throughout the several views. Like numerals having different letter suffixes can be used to represent different views of similar elements. The drawings illustrate generally, by way of example, but not by way of limitation, various examples discussed in the present document.

[0010] FIG. 1 is a schematic diagram of an example system processing aluminum oxide, such as for the production of sapphire.

[0011] FIG. 2 is a schematic diagram of an example crucible that can be used for melting aluminum oxide in the example system of FIG. 1.

[0012] FIG. 3 is a schematic diagram of another example crucible that can be used for melting aluminum oxide in the example system of FIG. 1.

[0013] FIG. 4 is a flow diagram of an example process of processing aluminum oxide, such as for the production of sapphire.

DETAILED DESCRIPTION

[0014] In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof. The drawings show, by way of illustration, a specific example of an apparatus and a process for converting aluminum oxide into sapphire. The examples are described in sufficient detail to enable those skilled in the art to practice, and it is to be understood that other embodiments can be utilized and that changes can be made without departing from the scope of the present disclosure. Therefore, the following Detailed Description is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims and their equivalents. [0015] The following describes an apparatus and process for processing aluminum oxide to produce sapphire. The apparatus and process can provide for lower-cost manufacturing of sapphire ingots. The process can provide for a higher packing factor of the aluminum oxide used to produce the sapphire, which can result in less porosity in the sapphire. The apparatus and process can also result in processing of aluminum oxide into a solid sapphire form having lower hydrogen level compared to sapphire produced by the conventional Verneuil process.

[0016] The apparatus and process of the present disclosure can also provide for processing of different forms of aluminum oxide, such as alpha aluminum oxide powder, scrap sapphire pieces, top and bottoms from sapphire boules or ingots, sapphire pot scrap, aluminum oxide beads, and the like. FIG. 1 shows an example system 10 for melting aluminum oxide. An aluminum oxide (AI₂O₃) feedstock 12, such as alumina powder, can be loaded into a hopper 14 with a metering system 16 to meter the powder into the furnace system. The hopper 14 can be sealed and a vacuum can be applied to the hopper 14. The hopper 14 can also be back filled with argon (Ar) to create an inert atmosphere in the hopper 14. The hopper 14 can be connected to a crucible 18 positioned within a furnace 20, such as via a high-temperature resistant feed tube 22. The high-temperature resistant tube 22 can be made out of zirconia (Zr0₂), alumina (AI2O3), sapphire, molybdenum, tungsten, iridium, or an alloy of one or more of these metals. A high-temperature resistant lid 24 can be placed over top of the crucible 18 with the high temperature tubes going through it. A pyrometer 26 can be used to measure a temperature of molten aluminum oxide 28 inside the crucible. A separate pyrometer (not shown) can be used to measure a temperature of the material of the crucible 18 itself. A back-up refractory 30 can be packed around the crucible 18 and induction coils (not shown) of the furnace 20. In an example, a stabilized zirconium oxide grout or grog can be used for the backup refractory 30. .

[0017] The crucible 18 can comprise a material that is inert or substantially inert to the aluminum oxide 12, 28 that is to be melted inside the crucible 18. The crucible 18 can also comprise a material that can withstand the temperatures produced by the furnace 20 in order to melt the solid aluminum oxide 12 to provide the molten aluminum oxide 28, which can be from about 2030 °C to about 2200 °C. The crucible 18 can also be configured so that the crucible 18 does not contaminate the molten aluminum oxide 28 by allowing impurities to melt or leech into the molten aluminum oxide 28, also referred to as being a "non-contaminating crucible." Examples of materials from which the crucible 18 can be made include, but are not limited to, one or more of aluminum oxide, zirconium oxide, molybdenum, tungsten, iridium, rhenium, graphite coated with molybdenum, titanium nitride, zirconium carbide, tantalum, a tantalum-tungsten alloy, tantalum carbide, or an alloy made from one or more of these materials. In an example, the crucible 18 comprises a high-purity tungsten.

[0018] The crucible 18 can be made by vapor deposition from a metal halide or an organometallic compound on graphite at 600°C in a hydrogen atmosphere with WF₆ or by electrochemical deposition, using a mould as one electrode in an electrolyte-containing salt bath comprising borates and tungstic oxide. The crucible can be made with Two layer coating with Iridium or rhenium 0.2 mm thick with 1 mm molybdenum or tungsten backup coating

[0019] The furnace 20 can comprise a heat source that is capable of producing temperatures in the crucible 18 that can melt the aluminum oxide 12 to provide the molten aluminum oxide 28. In an example, the heat source can be an induction heating source or a resistance heating source (such as a graphite or tungsten resistance heating elements). In an example, the heat source can be sufficient to provide temperatures within the furnace 20 of from about 2030 °C to about 2200 °C. The furnace 20 can be maintained at a temperature that is sufficiently high to melt the aluminum oxide powder 12 as it is added into the crucible 18.

[0020] In an example, the furnace 20 can comprise an induction furnace.

When the induction furnace 20 is turned on, an initial portion of aluminum oxide powder 12 can be melted through use of the crucible 18 as the scepter and the induction coils of the induction furnace 20. Once the aluminum oxide 28 in the crucible 18 gets to a sufficiently high temperature, it can couple with induction coils of the furnace 20 and melt. In an example, the induction furnace 20 can melt the aluminum oxide powder 12 using a scepter made of high-purity aluminum, graphite, or tungsten placed away from the walls of the crucible 18. The scepter material can be mixed with the aluminum oxide 12 being added to the crucible 18. The walls of the crucible 18 can be kept below the melting point of the crucible 18. A copper, water-cooled induction coil can be used to cool the crucible 18. In an example, once some of the aluminum oxide is melted to form molten aluminum oxide 28, the scepter can be removed. A molten pool of aluminum oxide 28 can then be formed in the center of the crucible 18. The induction furnace 20 can couple with the molten aluminum oxide 28 and start to melt the surrounding aluminum oxide powder 12. As more of the aluminum oxide powder 12 melts, some of the molten aluminum oxide can be allowed to drip out of the crucible 18 into the mould 34. By maintaining the molten pool of aluminum oxide 28 and adding new aluminum oxide powder 12, as molten aluminum oxide 28 drips or flows out of the crucible 18, a balance can be reached (described in more detail below). Argon can be flushed through the furnace 20 during the process. The molten aluminum oxide 28 can also be poured out of the crucible 18 in a batch process into the mould 34, then new aluminum oxide powder 12 can be added to the crucible 18 and the scepter can be used again to get the molten aluminum oxide bath 28 started again

[0021] The environment within the furnace 20 can comprise an inert gas, such as CO, C0₂, N₂, or argon. The environment within the furnace 20 can also comprise a vacuum. In an example, a pressure in the furnace 20 is less than 1 x 10^"3 atmosphere (atm) (less than about 1 torr), for example, less than about 6 x 10^"4 atm (less than about 0.5 torr).

[0022] In an example, the furnace environment is vacuum backed and filled with argon. In an example, the whole system 10 can be exposed to a vacuum and then refilled with argon. Pump down can be performed several times to reduce oxygen and impurities in the furnace 20.

[0023] In an example, the crucible 18 can comprise a passageway 32 at or proximate to a bottom of the crucible 18. As described in more detail below, the passageway 32 can allow molten aluminum oxide 28 to flow or drip from the crucible 18 and into a mould 34. The passageway 32 can be configured so that an entrance 36 into the passageway 32 is higher than the bottom of the crucible 18, which can allow a pool of the molten aluminum oxide 28 to form at the bottom of the crucible 18 below the level of the entrance 36 before the molten aluminum oxide 28 can begin to flow through the passageway 32. The height of the entrance 36 above the bottom of the crucible 18 can be selected so that the molten aluminum oxide 28 can accumulate to a predetermined level before the molten aluminum oxide 28 can begin to flow or drip from the crucible 18 through the passageway 32. In an example, one or more sapphire pieces can be added to the passageway 32 to plug the passageway 32 so that flow of the molten aluminum oxide 28 is prevented or limited until the sapphire is melted.

[0024] In an example, the passageway 32 can extend upward from a bottom wall of the crucible 18. As shown in FIG. 1, the passageway 32 can comprise a tube 38 extending through and upward from a bottom wall of the crucible 18. The tube 38 can comprise a material that is inert relative to aluminum oxide and that can withstand the high temperatures of the molten aluminum oxide 28. In an example, the tube 38 comprises a tungsten tube extending upward from the bottom wall of the crucible 18. The crucible 18 with the tube 38 can be made by machining or sintering the crucible 18 to include a cylinder 39 extending upward from the bottom wall of the crucible 18 with a bore through the cylinder 39 to provide the passageway 32. The bore can be formed by drilling through the cylinder 39. The tube 38, such as a tungsten tube 38, can be placed into the bore in the cylinder 39 in a tight-fitting relationship with the cylinder 39. In an example, a small groove or channel can be added at the top of the tube 38 and the cylinder 39.

[0025] The mould 34 can be placed under an outlet of the passageway 32, for example by placing the mould 34 directly under the crucible 18. The mould 34 can be positioned under the passageway 32 so that molten aluminum oxide that flows from the crucible 18 through the passageway 32 can fall into the mould 34. The mould 34 can comprise a crucible 40.

[0026] The crucible 40 of the mould 34 can be made from a material capable of withstanding the temperature of the molten aluminum oxide 28. In an example, the portion of the mould that is exposed to the molten aluminum oxide 28 (e.g., the crucible 40) can comprise at least one of aluminum oxide, zirconium oxide, molybdenum, tungsten, iridium, rhenium, graphite coated with molybdenum, titanium nitride, zirconium carbide, tantalum, a tantalum-tungsten alloy, tantalum carbide, or an alloy made from one or more of these materials. The crucible 40 of the mould 32 can be surrounded with a backup insulation 42. The backup insulation 42 can comprise a stabilized zirconium oxide, such as a stabilized zirconium oxide grog. The mould 34 can be placed on a bottom block or ring 44 that can withstand the temperature of the mould 34. In an example, the block or ring 44 can comprise a molybdenum or a refractory metal on a tungsten stand or graphite block. A high-temperature resistant coating can be applied to the block or ring 44 to prevent or reduce sticking between the block or ring 44 and the mould 34.

[0027] The mould 34 can be configured to be a directional solidification mould 34 for the directional solidification of the molten aluminum oxide 28. As the molten aluminum oxide 28 drips or flows into the mould 34, the molten aluminum oxide 28 can slowly freeze from the bottom up in the mould 34. As the aluminum oxide freezes in the mould 34, solid crystals 46 can form in the pool of molten aluminum oxide 28 in the mould 34. The crystals 46 can grow until all of the molten aluminum oxide 28 in the mould 34 have solidified to form an aluminum oxide ingot.

[0028] The crystals 46 can be multi-crystals or mono-crystals 46, with the crystals 46 forming from the bottom of the mould 34 up so that the molten pool of aluminum oxide 28 is on top of the crystals 46. Heat can be slowly added to the center and top of the growing crystals 46 from the continuous dripping of recently melted aluminum oxide 28 from the crucible 18. The rate of melting, induction furnace power, and metering rate of the aluminum powder 12 can be adjusted to help control the freezing rate of the molten aluminum oxide 28 into sapphire crystals 46 in the mould 34. The growing crystals 46 can be monocrystalline or multicrystalline sapphire grown on any acceptable plane, such as in the a-axis, r-axis, m-axis, or c-axis. In an example, a seed crystal (not shown) can be used in the bottom of the mould 34. The seed crystal can be partially melted by the molten aluminum oxide 28 dripping onto the seed crystal so that a desired crystal axis and monocrystalline crystals 46 can form.

[0029] In an example, the bottom of the upper crucible 18 can radiate heat down onto a top surface of the crucible 40 of the mould 34, which can improve the directional solidification. The top surface can also optionally be heated with a tungsten or graphite heater (not shown). A bottom of the mould 34 can be cooled by using the block or ring 44 under the crucible, which can be cooled by gas exchange. Gases that can be used for cooling of the mould 34 or the block or ring 44, or both, can include, but are not limited to, air, argon, and helium. Alternatively, the bottom block or ring 44 can be cooled by a doubled walled shell of the furnace 20. In an example, the maximum temperature gradient in the bottom mould is -25 °C/mm. In an example, the velocity of the solidification front of the crystals 46 in the mould 34 is from about 1 mm/hour to about 10 mm/hour.

[0030] In an example, the mould 34 can comprise a high-purity aluminum oxide powder formed into a desired shape of the mould 34, such as outer walls and a bottom. The example shaped aluminum oxide powder mould 34 can be backed up with a high-temperature resistant material, such as stabilized zirconium oxide. The temperature of the molten aluminum oxide 28 and the rate of addition of the molten aluminum oxide 28 can be controlled so that the molten aluminum oxide 28 does not melt a significant amount of the aluminum oxide powder that forms the mould 34, while still allowing a molten pool of aluminum oxide 28 to be maintained while the molten aluminum oxide 28 is being added to the mould 34. In an example, sapphire can be placed in the bottom of the mould 34 so that the sapphire partially melts in order to protect the aluminum oxide powder of the mould 34 to prevent or reduce melting of the aluminum oxide powder forming the bottom and walls of the mould 34. The sapphire can also act as a seed crystal for the formation of the crystals 46.

[0031] The mould 34 can comprise any shape that is desirable for the final shape of the aluminum oxide ingot that will form therein. In an example, the mould 34 comprises a round shape, a square shape, or a rectangular shape. In an example, the mould 34 can have a slight draft on it.

[0032] The mould 34 can be configured with a shape and size similar to known crucibles for sapphire ingot growth for the Czochralski (Cz) method, the heat-exchanger method (HEM), the Kyropolous (Ky) method, edge-defined film growth (EFG), the gradient-freeze (GF) method, the Bridgeman method,

Bridgeman variant methods, or other methods know in the art. In an example, the mould 34 can have the same shape as a crucible from one of these methods, but with a slightly smaller size. These shapes can allow a sapphire ingot grower to maximize the packing factor in the mould 34. Space can be left in the shape for a seed crystal or a bottom section of a feedstock boule can be cut off. In an example, the packing factor of a sapphire resulting from the systems or methods of the present disclosure is at least 90% of solid sapphire density for a given crucible volume. [0033] In an example, the flow of the molten aluminum oxide 28 from the crucible 18 into the mould 34 via the passageway 32 can be controlled in order to control the directional solidification of the aluminum oxide in the mould 34. For example, the flow rate of the molten aluminum oxide 28 from the crucible 18 to the mould 34 can be varied and control so that the aluminum oxide 28 does not completely freeze until the mould 34 is full. In an example, a feed system, such as the metering system 16 and the hopper 14, can provide for control of the rate of addition of the solid aluminum oxide powder 12 into the crucible 18. The feed system can vary the rate of addition of the aluminum oxide powder 12 during the filling of the mould 34 to control directional solidification in the mould 34.

[0034] Once the mould 34 is full, the mould 34 can be exchanged for a new mould 34 and the process can be continued. In an example, the mould 34 can hold 400kg or up to 1000 kg of sapphire. In an example, argon can be flushed through the system during the changing of bottom moulds 34. In an example, the furnace 20 can be cooled before switching the bottom mould 34. Additional aluminum oxide powder 12 can be loaded into the hopper 14 by closing an on/off value on the hopper 14 and applying a vacuum to the hopper 14, or flushing the hopper with argon gas, or both.

[0035] The resulting sapphire boule can have the top cut off then can be machined, milled, etched, blasted or ground to remove surface contamination and used as a feedstock for a mono-ingot growing process. The top section can be used or sold for lower grade sapphire production, it also can be recycled. The resulting feedstock will fit into the mono-sapphire ingot grower's mould with about 3mm of extra space between the sapphire and the mould wall to maximize the packing density.

[0036] The resulting aluminum oxide ingot can be used to produce sapphire, for example for use in mobile device window or as a substrate for light-emitting diodes (LEDs). The mould can be configured so that the ingot is square, which can maximize the yield of the process. In an example, the ingot is grown in the a-axis with a seed crystal oriented so that the c-axis is orthogonal to a flat side of the mould.

[0037] In an example, the apparatus 10 and methods described herein can result in the formation of sapphire having a lower hydrogen level than crackle made from the conventional Verneuil process. In an example, the hydrogen level in the sapphire is less than 500 ppm, for example, as low as 100 ppm (using NMR analysis).

[0038] FIG. 2 shows another example configuration of a melting crucible 50 that can provide an alternative passageway 52 through which molten aluminum oxide 54 can flow or drip from the crucible 50 and into a mould, similar to the arrangement between the crucible 18 and the mould 34 described above with respect to FIG. 1. The example melting crucible 50 can comprise a first, smaller crucible 56 that is placed inside of a second, larger crucible 58. The inner crucible 56 can have a height that is lower than a height of the outer crucible 58 so that the molten aluminum oxide 54 can fill up the inner crucible 56 and flow over the edge of the inner crucible 56 and into the outer crucible 58 as additional solid aluminum oxide is added to the inner crucible 56. The outer crucible 58 can include a hole 60 through which the molten aluminum oxide 54 can flow out of the melting crucible 50. The inner crucible 56 can also include one or more legs 62 so that the inner crucible 56 is not flat against the outer crucible 58, e.g., so that the inner crucible 56 is spaced from the outer crucible 58 to allow a pathway for the molten aluminum oxide 54 to flow to the hole 60 to exit the melting crucible 50 by dripping of flowing through the hole 60.

[0039] FIG. 3 shows another example melting crucible 70. The example melting crucible 70 includes a sloped crucible bottom 72 that slopes upwardly from the outer walls 74 of the crucible 70 toward a center passageway 76. This can allow molten aluminum oxide 78 to pool around an outside portion of the crucible 70 until a level of the molten aluminum oxide 78 is high enough so that the molten aluminum oxide 78 can flow through the center passageway 76.

[0040] Yet another alternative crucible (not shown) can include a hole in the crucible that is sufficiently small so that molten aluminum oxide will not freely flow out of the crucible. In such a configuration, as the feed of the solid aluminum oxide is started and melted, the flow of the molten aluminum oxide will be limited through the small hole size to allow the molten aluminum oxide to build up until the head pressure of the molten aluminum oxide in the crucible is sufficiently high so that the flow rate of the molten aluminum oxide through the small hole will equal the feed rate of the solid aluminum oxide being fed to the crucible. [0001] FIG. 4 shows a flow diagram of an example process 100 for melting aluminum oxide, for example to form sapphire from the melted aluminum oxide. An aluminum oxide feedstock 102 can be provided, such as the aluminum oxide powder 12 shown in FIG. 1. The aluminum oxide feedstock 102 can be metered 104 into a melting apparatus 106, such as the crucible 18, 50, 70 (FIGS. 1-3) and the furnace 20. The aluminum oxide feedstock 102 can be metered 104 continuously of substantially continuously, for example with the metering system 16 and the hopper 14 shown in FIG. 1. As described in more detail below, the continuous or substantially continuous metering of the solid aluminum oxide feedstock 102 can provide for a continuous or semi-continuous flow of molten aluminum oxide from the melting apparatus 106 into a directional solidification apparatus 114, such as the mould 34 shown in FIG. 1. In an example, the aluminum oxide feedstock 102 can be metered 104 to the melting apparatus in a continuous, semicontinuous, or intermittent fashion. The metering 104 of the aluminum oxide feedstock 102 can be from a vacuum atmosphere, an inert atmosphere, or both (e.g., a vacuum atmosphere that is inert).

[0002] In an example, before melting the aluminum oxide, the aluminum oxide feedstock 102 can be put under a vacuum and heated to a temperature below the melting point of aluminum oxide in order to reduce impurities within the aluminum oxide feedstock 102. The application of a vacuum to the aluminum oxide feedstock 102 can reduce impurities and remove other gases while the aluminum oxide feedstock 102 is heating up to the melting point. The aluminum oxide feedstock 102 can be held under vacuum close to the melting point for a period of time before melting. The addition of the aluminum oxide feedstock 102 can be stopped and the furnace can be put under vacuum to remove impurities. Then, the furnace can be back filled with argon and the addition of the aluminum oxide feedstock 102 can continue.

[0003] The aluminum oxide feedstock 102 can then be melted 108 in the melting apparatus 106 to provide a molten aluminum oxide 110. The aluminum oxide feedstock 102 can be melted 108 at a temperature of from about 2030 °C to about 2200 °C. The melting 108 can be accomplished via induction heating, such as with an induction furnace, as described above. The melting 108 of the aluminum oxide feedstock 102 can be performed under a vacuum to reduce impurities through evaporation. The furnace can also be operated in vacuum while melting 108 instead of with argon to reduce volatile impurities.

[0004] After melting 108 the aluminum oxide feedstock 102 to provide the molten aluminum oxide 110, the molten aluminum oxide 110 can be flowed 112 out of the melting apparatus 106 and into a directional solidification apparatus 114, such as a directional solidification mould 34. The flowing 112 of the molten aluminum oxide 110 can be done actively (e.g., with pumps or other fluid moving apparatus) or passively (e.g., via gravity flow).

[0005] The flowing 112 of the molten aluminum oxide 110 can be such that the molten aluminum oxide 110 will flow from the melting apparatus 106 into the directional solidification apparatus 114 as new aluminum oxide feedstock 102 is added to the melting apparatus. In an example, the flowing 112 of the molten aluminum oxide 110 can be through a passageway in the melting apparatus 106, such as the passageway 32 described above with respect to crucible 18. An entrance into the passageway can be spaced upward from a bottom of the melting apparatus 106 so that a pool of the molten aluminum oxide 110 can form in the melting apparatus 106 before flowing through the passageway. The formation of the pool of the molten aluminum oxide 110 can allow for continuous or semi-continuous flow 112 of the molten aluminum oxide 110 out of the melting apparatus 106 through the passageway and into the directional solidification apparatus 114.

[0006] After flowing 112 into the directional solidification apparatus

114, the molten aluminum oxide 110 can be directionally solidified 116 in the directional solidification apparatus 114 to provide an aluminum oxide ingot 118.

Claims

CLAIMS What is claimed is:

1. A vessel comprising:

a bottom wall having a perimeter and an internal region;

one or more side walls having a top and a bottom;

wherein the perimeter of the bottom surface is in contact with the bottom of the one or more side walls to define a vessel interior; and

a passageway from the vessel interior through the bottom wall at the internal region, wherein the passageway comprises an entrance that is vertically spaced from the bottom wall.

2. The vessel of claim 1 , wherein the perimeter of the bottom wall is integrally bonded to the bottom of the one or more side walls.

3. The vessel of either one of claims 1 or 2, wherein the bottom wall and the one or more side walls are configured to withstand temperatures of at least about 2000 °C.

4. The vessel of any one of claims 1-3, wherein each of the bottom wall and the one or more side walls comprise at least one aluminum oxide, zirconium oxide, molybdenum, tungsten, iridium, rhenium, graphite coated with molybdenum, titanium nitride, zirconium carbide, tantalum, a tantalum-tungsten alloy, tantalum carbide, or an alloy thereof.

5. The vessel of any one of claims 1-4, wherein the passageway comprises an open cylinder or a hollow tube, positioned substantially perpendicular to the bottom surface.

6. The vessel of claim 5, wherein a ratio of a length of the cylinder or hollow tube to a diameter of the passageway is from about 1 : 1000 to 1000: 1.

7. The vessel of any one of claims 1-6, wherein the bottom wall is not planar.

8. A system comprising:

(a) the vessel of any one of claims 1-7; and

(b) a mould located substantially below the passageway of the vessel.

9. The system of claim 8, configured such that heat from the vessel heats the mould.

10. The system of either one of claims 8 or 9, further comprising a lid configured to integrally reside on top of the vessel.

1 1. The system of any one of claims 8-10, wherein the mould comprises at least one of aluminum oxide, zirconium oxide, molybdenum, tungsten, iridium, rhenium, graphite coated with molybdenum, titanium nitride, zirconium carbide, tantalum, a tantalum-tungsten alloy, tantalum carbide, or an alloy made from one or more of these materials.

12. A method comprising:

(a) heating solid aluminum oxide contained with the vessel of any one of claims 1-7, wherein the heating is sufficient to form molten aluminum oxide;

(b) passing the molten aluminum oxide passes through the passageway into a mould; and

(c) allowing the molten aluminum oxide to cool in the mould.

13. The method of claim 12, which is a method of producing sapphire.

14. The method of claim 13, wherein the sapphire has a hydrogen level of less than 500 ppm.

15. The method of either one of claims 13 or 14, wherein the sapphire has a packing factor of at least 90%.

16. The method of any one of claims 13-15, wherein the sapphire comprises at least one of mono-crystalline sapphire and multi-crystalline sapphire.

17. The method of any one of claims 12-16, further comprising feeding the solid aluminum oxide into the vessel continuously, semi-continuously, or in one or more batches.

18. The method of any one of claims 12-17, wherein the molten aluminum oxide passes to the mould continuously or semi-continuously.

19. The method of any one of claims 12-18, wherein allow the molten aluminum oxide to cool comprises directionally solidifying the molten aluminum oxide from a bottom of the mould to top of the mould.

20. The method of claim 19, wherein directionally solidifying the molten aluminum oxide comprises heating the top of the mould.

21. The method of either one of claims 19 or 20, wherein directionally solidifying the molten aluminum oxide comprises cooling the bottom of the mould.

22. The method of any one of claims 12-21, further comprising at least one of applying a vacuum, applying an inert atmosphere, and applying a gas comprising at least one of CO and C0₂ during one or more of the heating, the passing the molten aluminum oxide, and the allowing to aluminum oxide to cool.