WO2013024163A1 - A method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire and installation therefore - Google Patents

A method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire and installation therefore Download PDF

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Publication number
WO2013024163A1
WO2013024163A1 PCT/EP2012/066126 EP2012066126W WO2013024163A1 WO 2013024163 A1 WO2013024163 A1 WO 2013024163A1 EP 2012066126 W EP2012066126 W EP 2012066126W WO 2013024163 A1 WO2013024163 A1 WO 2013024163A1
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cold crucible
inductor
polycrystalline
cold
ingot
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PCT/EP2012/066126
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French (fr)
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Anatoly Shkulkov
Andrej MARKIEWICZ
Vladimir ONISHCHENKO
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Polycor Sp. z o.o.
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Publication of WO2013024163A1 publication Critical patent/WO2013024163A1/en

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    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/16—Oxides
    • C30B29/20—Aluminium oxides
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
    • C04B35/10—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on aluminium oxide
    • C04B35/107—Refractories by fusion casting
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
    • C30B11/001—Continuous growth
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
    • C30B11/002—Crucibles or containers for supporting the melt
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
    • C30B11/007—Mechanisms for moving either the charge or the heater
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27B—FURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
    • F27B14/00—Crucible or pot furnaces
    • F27B14/08—Details peculiar to crucible or pot furnaces
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D11/00—Arrangement of elements for electric heating in or on furnaces
    • F27D11/06—Induction heating, i.e. in which the material being heated, or its container or elements embodied therein, form the secondary of a transformer
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D11/00—Arrangement of elements for electric heating in or on furnaces
    • F27D11/12—Arrangement of elements for electric heating in or on furnaces with electromagnetic fields acting directly on the material being heated
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
    • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
    • C04B2235/3217—Aluminum oxide or oxide forming salts thereof, e.g. bauxite, alpha-alumina
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
    • C04B2235/40—Metallic constituents or additives not added as binding phase
    • C04B2235/402—Aluminium
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
    • C04B2235/656—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes characterised by specific heating conditions during heat treatment
    • C04B2235/6562—Heating rate
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
    • C04B2235/656—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes characterised by specific heating conditions during heat treatment
    • C04B2235/6565—Cooling rate
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
    • C04B2235/656—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes characterised by specific heating conditions during heat treatment
    • C04B2235/6567—Treatment time
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27B—FURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
    • F27B14/00—Crucible or pot furnaces
    • F27B14/08—Details peculiar to crucible or pot furnaces
    • F27B14/10—Crucibles
    • F27B2014/108—Cold crucibles (transparent to electromagnetic radiations)

Definitions

  • the invention relates to growing of sapphire single crystals by crystallization of a melt on a seed, and more specifically to synthesis of a ingoing billet in the form of polycrystalline corundum ingot, used for industrial process of growing sapphire single crystals.
  • the invention also relates to induction melting technology of refractory materials in the cold crucible. This invention can also be used for the synthesis of ingoing billet of other oxide materials.
  • a process is known of forming crystals of stabilized zirconium dioxide (fianite) using the induction melting in a cold crucible (described in [1], pp. 187- 189).
  • the crystals are created in a form of crystal druses forming a polycrystalline ingot that had been obtained by means of direct crystallization of oxide melt in a cold crucible .
  • This process comprises of the following technological phases: preparation of the fusion mixture, cleaning and placing the cold crucible in the working position (initial phase); loading a fusion mixture into a cold crucible placed in a working chamber; forming inside the fusion mixture a crystallization seed in the form of a metal cutting or small pieces (zirconium or yttrium); metal heating in the electromagnetic field of an inductor and then burning of metal (oxidation with the use of atmospheric oxygen) which takes place with heat emission; forming the starting melt bath of the oxide material and the induction heating of the melt in a high frequency field of the inductor; subsequently adding a fusion mixture into the cold crucible and its melting; crystallization of the melt in the form of polycrystalline ingot; cooling the formed polycrystalline ingot and its removal from the cold crucible.
  • This method is widely used in the industrial scale for synthesis of stabilized zirconium dioxide crystals by stabilization of the cubical structure through addition of Y 2 O 3 , CaO and other oxides in fusion mixture.
  • the drawback of this method is the lost time that is needed to cool down the formed polycrystalline ingot in the form of crystal druses. This drawback is not that critical for this method, since during the technological cycle, the phase of cooling the ingot takes definitely less time that the phases of melting and crystallization of the melt.
  • the closest method to the one according to the present invention is the process of synthesis of poly crystalline ingot of oxide materials by induction melting in a cold crucible under the conditions of periodic (two-stage) melting (described in [2], pp. 59-62) which has been taken as a prototype model for this invention.
  • the fusion mixture is loaded into a cold crucible placed within the working chamber; then the power source of the inductor is turned on for the electromagnetic field to be created, the starting melt bath of the oxide material is created, the melt is heated in the electromagnetic field of the inductor, the melting of the fusion mixture takes place, subsequently a new portion is added to the cold crucible and it is melted; the melted mass is crystallized into the form of a polycrystalline ingot under the process of vertical translation of the cold crucible, the power source is turned off, the synthesized polycrystalline ingot is cooled down and taken out from the cold crucible.
  • Figure 1 presents the sequence of technological phases performed in a well-known technological cycle of periodic synthesis of polycrystalline ingots of oxide materials, with the possibility of repeating it in industrial production processes.
  • Induction melting of oxide materials in a cold crucible is characterized by high intensiveness of fusion mixture melting due to the emission of electromagnetic field energy directly in the melt and intensive stirring of the melt mass.
  • This advantage of induction melting in a cold crucible is optimally used during synthesis of oxide materials being in the form of polycrystalline ingot.
  • the crystallization of the melt takes place at the high speed, since the formation of coarse crystal structure of the polycrystalline blocks is not necessary.
  • the ingot can be taken out only after its temperature has gone down to an ambient room temperature level.
  • the cooling time of the ingot depends on its mass and it is usually very similar to the time of the technological phases of loading the fusion mixture into the cold crucible, its melting and crystallization of the melted mass. Due to this fact, the time of technological phases of melting the fusion mixture and crystallization of the melted mass is of the same order of magnitude as the cooling phase of the formed ingot.
  • the basic technological systems namely: power source, system for sustaining the controlled environment in a working chamber and others, are not used.
  • This installation has a fusion mixture container equipped with input supply control system, high-frequency power source as well as a working chamber comprising: an inductor connected to a high-frequency power source, a cold crucible with a bottom, placed inside the inductor and capable of vertical movements via a translation mechanism.
  • the working chamber is linked to subsystems that ensure maintaining controlled environment in the chamber.
  • Low utilization is a disadvantage of the known installations because of the use of cold crucible during the cooling phase of the formed poly crystalline ingot in a periodic (two-stage) technological cycle, when all the other technical equipment is not used.
  • the closest solution to the one according to the present invention is the installation described in [5], designed for periodical (two-stage) synthesis of poly crystalline ingots of oxide materials.
  • This installation is equipped with a high-frequency power source and a working chamber comprising: an inductor connected to a high-frequency power source and a cold crucible placed inside the inductor.
  • the cold crucible is made of pipe sections, contains a bottom and is capable of vertical movements via a translation mechanism.
  • the working chamber is linked to subsystems that ensure maintaining controlled environment in the chamber.
  • the disadvantage of the installation is the lack of possibility of making use of the basic technical equipment during the cooling phase of the formed polycrystalline ingot of the oxide material placed in the cold crucible. This lowers down the performance ratio of the installation, at the same time raising the production cost, being an important factor in the process of production of corundum ingoing billet in large quantity manufacture of sapphire single crystals.
  • the purpose of this invention is to increase the utilization ratio of the basic technical equipment in the production process of the ingoing billet in the form of polycrystalline corundum ingot, using the method of periodic (two-stage) induction melting in a cold crucible.
  • Another purpose of this invention is the increase of productivity of the installation used for the synthesis of ingoing billet in the form of polycrystalline corundum ingot in the process of industrial production of sapphire crystals.
  • the method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire comprises introduction of a fusion mixture of aluminum oxide powder into a cold crucible, creating a high-frequency electromagnetic field with an inductor and initial melting of the fusion mixture, heating the melt in the high-frequency electromagnetic field of the inductor, feeding an additional portion of the fusion mixture in the cold crucible and its melting, crystallization the melt in the form of a polycrystalline ingot during a vertical shifting of the cold crucible with respect to the inductor, termination of the induction melting by switching off the high- frequency field of the inductor, cooling the polycrystalline ingot in the cold crucible, removing the polycrystalline ingot from the cold crucible and repeating the whole cycle from the introduction of the fusion mixture into the cold crucible and its initial melting.
  • the cold crucible with the polycrystalline ingot is taken out of the inductor and in its place a second identical cold crucible is introduced and the cycle is repeated to obtain another polycrystalline ingot in the second identical cold crucible, wherein the cooling of the previously produced polycrystalline ingot is carried out simultaneously with the synthesis of the next polycrystalline ingot and removing the polycrystalline ingot from the cold crucible is carried out after the induction melting of the next polycrystalline ingot has been terminated.
  • moving out the cold crucible with the polycrystalline corundum ingot from the inductor is performed simultaneously with putting in its place a second, identical cold crucible.
  • the simultaneous moving out the cold crucible with the polycrystalline corundum ingot and moving in the second, identical cold crucible is performed by rotation of both crucibles around a common vertical axis.
  • the synthesis of polycrystalline corundum ingots is carried out in vacuum.
  • the installation for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire comprises a high-frequency power source, batch container for fusion mixture comprising aluminum oxide powder, with controlled fusion mixture input connected to a working chamber, in which there is an inductor connected to a high-frequency power source, further comprising a cold crucible with a closed- bottom, placed within the internal space of the inductor, said crucible is capable of vertical shifting and is cooled by a flow of water, wherein the working chamber is connected to a unit for controlling atmosphere in the working chamber.
  • the installation is equipped with additional, identical cold crucible placed in the working chamber, capable of identical vertical shifting, wherein each of the cold crucibles is mounted on a horizontal shifting unit, allowing for introduction of subsequent cold crucibles into the internal space of the inductor.
  • the cold crucibles are connected mechanically with each other and form a unitary assembly with a common mechanism for vertical shifting and a common mechanism for horizontal shifting, where both mechanisms allow for simultaneous moving and consecutive placement of cold crucibles in the internal space of the inductor, and the horizontal shifting of the cold crucibles is performed by their rotation around a vertical axis.
  • the cold crucibles are connected in series with respect to the flow of the cooling water.
  • This method is characterized by high equipment utilization ratio by minimalization of non-productive time loss for cooling down the ingot.
  • This value is achieved by changing the order of technological phases of the process of synthesis of corundum ingoing billet in the form of polycrystalline ingots.
  • the technological phase of cooling a polycrystalline ingots is performed in parallel and simultaneously to other phases related to preparation of another, identical cold crucible and induction melting of the fusion mixture in this second crucible.
  • the change of the order of these technological phases is possible due to introduction of an additional cold crucible, creating the possibility of moving both crucibles as well as subsequent introduction of crucibles into the inductor.
  • Each crucible is used during the full cycle of forming the polycrystalline ingot, namely: induction melting of the fusion mixture, crystallization of the melt and cooling of the formed ingot.
  • the second crucible is used to create another polycrystalline ingot.
  • the increase of performance of the apparatus is achieved through shortening of the interruption time of the basic element of the apparatus, namely the inductor, which is powered by the high-frequency source, as well as auxiliary devices.
  • the very advantage of the solution according to the invention is ensured by placing an additional cold crucible, identical to the one already present therein, within the working chamber.
  • the two cold crucibles are merged into one unified system, capable of translations both in vertical as well as horizontal directions, and ensuring that each of the crucible is successively introduced into the inductor and thereby the performance ratio of the whole installation is increased.
  • Figure 1 presents the sequence of the technological phases in known technical solutions for the synthesis of polycrystalline blocks of oxide materials using the periodic (two-stage) melting in the cold crucible
  • Figure 2 presents the sequence of technological phases of the synthesis of ingoing billet in the form of polycrystalline corundum ingots using the periodic induction melting in a cold crucible to grow the sapphire crystals according to this invention
  • Figure 3 presents a scheme of the installation for synthesis of ingoing billet in the form of polycrystalline corundum blocks in the initial phase of the technological cycle
  • Figure 4 presents a scheme of the installation in the final phase of induction melting after the high-frequency electromagnetic field created by the inductor has been switched off, before the cold crucible have been shifted,
  • Figure 5 presents a scheme of the installation during loading the fusion mixture into the second identical cold crucible and simultaneous cooling of previously formed poly crystalline corundum ingot in the first cold crucible
  • Figure 6 presents a scheme of the installation in the final phase of induction melting of the next poly crystalline corundum ingot and taking the poly crystalline corundum ingot formed in the previous cycle in the first crucible out of this cold crucible, and
  • Figure 7 presents a scheme from Figure 6 in the transversal cross-section A- A during the final phase of induction melting of the next polycrystalline corundum ingot and taking the polycrystalline corundum ingot formed in the previous cycle out of the cold crucible.
  • the installation for the synthesis of the ingoing billet in the form of polycrystalline corundum ingots using the process of periodic (two-stage) induction melting in the cold crucible comprises: a working chamber 1 connected to the batch container 2 through a system for controlling the fusion mixture load 3 into the working chamber 1. Within the working chamber 1 there are an inductor 4 and two identical cold crucibles (5 and 6), mounted on a basis 7, having specified dimensions. Each of the cold crucibles 5 and 6 has a bottom. Both crucibles 5 and 6 are connected to the assembly coupled in series along a flow of cooling water system 8. The basis 7 is mounted on a rod 9, which allows for its vertical movement through a helical gear 10 and electrical driving gear 11.
  • the basis 7 can be moved in horizontal plane and can be put into one of the two possible fixed positions.
  • the fixing is enabled through a mechanism 12 when the rod 9 is turned around its axis. Shifting of the basis 7 in the horizontal plane is possible only when both cold crucibles 5 and 6 have been placed in their lowest positions.
  • the fixed position of the basis 7 also means that one of the cold crucibles is placed co-axially against the inductor 4.
  • the inductor 4 is connected to a high-frequency power source (not shown) through a high-frequency feeder 13.
  • the working chamber is linked with a system for maintaining a controlled atmosphere in the working chamber (not shown) through the connector 20. The installation works as follows.
  • the cold crucible 5 is introduced into the internal space of the inductor 4 and put into the initial position ( Figure 3). From the container 2, with the help of the system for controlled fusion mixture loading 3, a portion of the aluminum oxide powder (fusion mixture) 14 is loaded into the crucible. Then the high-frequency power source is turned on and inside the inductor, where the cold crucible had been placed, the electromagnetic field of high frequency is created. The initial melting of the aluminum oxide is induced, using the well-known procedures.
  • the initial melting shown on the presented embodiment of this invention comes out as a result of exothermic reaction of granules of the metallic aluminum 15, introduced into the aluminum oxide powder 14 after the initial portion of the fusion mixture has been loaded into the crucible.
  • the granules of the metallic aluminum 15 are heated within the electromagnetic field of the inductor and undergo rapid oxidation (they get burned).
  • the reaction of oxidations of the metal is accompanied by heat emission, which makes the surrounding granules of the fusion mixture 14 melt.
  • the melted mass gets heated within the electromagnetic field of the inductor, and subsequently the induction melting of the aluminum oxide takes place.
  • the aluminum oxide powder is added into the cold crucible 5 from the container 2 in a controlled way, through the fusion mixture dose control system 3.
  • the cold crucible 5 is shifted downwards by the rod 9 with the use of helical gear 10 and electrical gear 11. Simultaneously there is directional solidification of the melt and the synthesis of fused corundum in the form of an ingot.
  • Lowering speed of the crucible is 1-7 mm/min and is maintained such that the molten bath is in the zone of the inductor.
  • the melted mass undergoes the crystallization with the electromagnetic field of the inductor being turned off or then the power of the high-frequency source transferred to the melted mass is lowered and subsequently turned off.
  • both cold crucibles 5 and 6 are identical and joined together into a unified system, the aforementioned movement of the rod 9 creates the shifting of the cold crucible 5 with the polycrystalline corundum ingot 16 and another cold crucible 6 is being put onto the place of the first one.
  • the second crucible 6 is put into the inside of the inductor 4 and placed in the initial position as a result of vertical translation of rod 9 with the use of helical gear 10 and the electrical gear 11. Then the technological operations are repeated, starting from loading of aluminum oxide powder 14 into the cold crucible 6 and its initial melting through oxidizing of the granules of metallic aluminum 15, which, as the result, produces a similar polycrystalline ingot of corundum 17 in the second cold crucible 6.
  • the polycrystalline corundum ingot 16 is located in the first cold crucible 5 and gets cooled down to the ambient room temperature, so that it can be taken out of cold crucible 5.
  • the working chamber door 18 is open and the synthesized polycrystalline corundum ingot 16 is taken out of the cold crucible 5. This is the phase presented in Figure 6 (where the open door 18 is shown in a schematic way) and in Figure 7.
  • the polycrystalline corundum ingot taken out of the crucible is the product for the technological process, and it is subsequently used as the ingoing billet of the given weight to grow sapphire crystals.
  • the cold crucible 5 is then prepared for the next cycle. First of all it is being cleaned out of the remnants of the fusion mixture and, if necessary, its inside surface is cleaned with a piece of cloth immersed in alcohol. Subsequently, the door 18 of the working chamber 1 is closed, the cold crucible 5 is put in the initial position inside the inductor 4, as described above, and the technological phases are repeated, starting with the loading of the aluminum oxide powder into the cold crucible 5 and the initial melting. When the next polycrystalline corundum ingot is formed in the cold crucible 5, the previously formed polycrystalline ingot 17 gets cooled in the cold crucible 6. The door 19 of the working chamber 1 is open and the ingot 17 is taken out of the cold crucible 6 in a similar way, as for the polycrystalline corundum ingot 16 taken out of the cold crucible 5.
  • the process and the installation according to the invention are used in order to increase the density of the ingoing billet used for synthesis of corundum ( ⁇ - ⁇ 1 2 03), subsequently used to grow the sapphire crystals using the Kyropoulos- Musatov method, in an apparatus with a tungsten container of the diameter of 250 mm.
  • the internal shape of the container is similar to truncated cone with the smaller diameter of 212 mm and the depth of 340 mm.
  • the optimal weight of the ingoing billet is 30 - 35 kg.
  • the polycrystalline corundum ingots of high purity are obtained by the induction melting of the aluminum oxide powder in the cold crucible in the air atmosphere.
  • the fusion mixture is the aluminum oxide powder of high chemical purity produced by Sumitomo Chemicals Co Ltd.
  • the aluminum oxide powder is represented by the crystal phases of ⁇ - ⁇ 1 2 0 3 and ⁇ - ⁇ 1 2 0 3 .
  • the installation for the synthesis of the ingoing billet in the form of polycrystalline corundum ingots comprises of two identical cold crucibles with circular cross section and internal diameters of 210 mm.
  • Each crucible is made of copper tubes with square cross sections, with the side length of 12 mm, joined by pairs into sections with progressive-reversible flow of the cooling water. These sections are mounted onto a copper collector and form the melting space of the crucible with the depth of 350 mm.
  • the collector plays the role of the closed-bottom of the crucible and the cooling water separator between the cold crucible sections. Both crucibles are linked in a series in relation to the cooling water flow and form the unified assembly on one basis.
  • the inductor is a single-turn, with the internal diameter of 260 mm, height of 60 mm and is connected to the high-frequency power generator with the power of 60 kW and the frequency of 5.28 MHz.
  • the cold crucibles as well as the inductor are placed in the working chamber equipped with two doors.
  • the working chamber is linked with the fusion mixture container with a feeder that controls the amount of aluminum oxide powder that is added to the crucible.
  • the working chamber is connected to the atmosphere conditions control system, in this case: air cleaning system and working chamber ventilation.
  • the oxide materials are dielectric and their induction heating in the electromagnetic field of high frequency is not possible.
  • This phase of the induction melting is called the initial or start melting of the load.
  • the heat coming from exothermic reaction of metal oxidation is used.
  • cutting or granules of the metal are introduced into the inductor operating zone.
  • start melting of oxide materials for example: through an electric arch or through heating of some electric conductor in the electromagnetic field of the inductor.
  • the initial melting phase is a usual practice in the area of induction melting of oxide materials in the cold crucible and is realized with well-known methods and with the aid of well-known devices. The choice of a specific initialization method is determined by the type of the oxide material, the purpose of the technological process and the technological means available.
  • the fusion mixture in the form of aluminum oxide powder is loaded into the container.
  • the fusion mixture is represented by the crystal phases of ⁇ - ⁇ 1 2 0 3 and ⁇ - ⁇ 1 2 0 3 .
  • One of the cold crucibles is placed in the initial position inside the inductor.
  • the aluminum oxide powder with the weight of 5 - 6kg is introduced from the container into the cold crucible.
  • the working chamber is closed, the high-frequency power source is turned on.
  • the melting zone of the cold crucible a high-frequency electromagnetic field is created.
  • the aluminum granules are heated in the electromagnetic field of the inductor, then they get oxidized, the process being accompanied with the heat emission and the melting of the surrounding powder of the aluminum oxide takes place.
  • the initial bath melt is formed, which gets heated in the electromagnetic field of the inductor and increases its own volume. From that moment onwards, the induction heating of the melt as well as the induction melting of the aluminum oxide set off.
  • Another portion of aluminum oxide powder is introduced from the container into the crucible and the conditions of the power source are controlled such that the power transferred to the bath melt is in the range of 30 - 32kW. As the aluminum oxide powder melts, the upper level of the bath melt raises.
  • the cold crucible is translated downwards at the speed of 1 - 4 mm/min, so that the upper level of the bath melt stays fixed in relation to the inductor. Because the power of the electromagnetic field of the inductor is limited, when the aluminum oxide powder is melted and the crucible is translated, the crystallization of the lower parts of the bath melt takes place at the same speed as with which the crucible lowers down and at the weight gain speed equal to the weight addition speed of the load. After 35 kg of the aluminum oxide powder has been added to the crucible (including the initial portion of the load), the further addition stops and the crucible is no longer moved downwards.
  • the melting of the aluminum oxide powder on the upper level of the bath melt is finished and the power of the electromagnetic field transferred through the inductor to the melt is decreased to 12 - 15 kW within the period of 10 - 15 min through the change of parameters of the high-frequency power source. Finally the high-frequency power source is switched off which results in full crystallization of the bath melt.
  • the formed polycrystalline corundum ingot has a temperature of around 2000°C, thus it is dangerous to take it out of the crucible.
  • the cold crucible with a hot polycrystalline corundum ingot is translated vertically downwards along the inductor and then, through the rotation of the rod it is being moved out of the inductor.
  • the first polycrystalline ingot stays in the cold crucible and is cooled down to the ambient room temperature.
  • the working chamber is opened and the cooled polycrystalline ingot form the first cold crucible is taken out.
  • the weight of the formed polycrystalline corundum ingot is around 34.0 - 34.3 kg, its diameter 208 mm and the density in the range of 3.1 - 3.2 g/cm 3 .
  • the corundum is synthesized, that is, the phase of ⁇ - ⁇ 1 2 0 3 is formed.
  • the cold crucible is now prepared for the new melting cycle. It is cleaned out of the remnants of the fusion mixture (around 0.7 to 1.0 kg), its internal surface is cleaned with a piece of cloth immersed in alcohol and a new portion of fusion mixture is added into the container. After the preparations, the second cold crucible with a hot polycrystalline corundum ingot is taken out of the inductor internal space and moved horizontally. Simultaneously, the first crucible is moved into the initial position within the inductor. The new technological cycle of the installation can be started and the initial load of the aluminum oxide can be placed in the crucible.
  • the duration of consecutive technological phases of the installation for the synthesis of the ingoing billet in the form of polycrystalline corundum ingots with the diameter of 208 mm, height of 300 - 330 mm and the weight of 30 - 35 kg using the method of periodic (two-stage) induction melting in the cold crucible of the diameter of 210 mm is presented in the table below.
  • the fundamental technical equipment being the high-frequency power source, stands idle for the period of 0.72 hrs during one cycle (sum of duration of phases 1, 5, 6 and 7).
  • the full technological cycle time is 3.97 hrs., which gives the utilization rate of 0.82, while the cooling of the synthesized polycrystalline ingot takes 3.42 hrs.
  • ingot temperature goes down to 80 - 100°C, which allows for taking it out of the cold crucible.
  • the increase of the utilization rate of the technical equipment of the installation results in the increase of the efficiency of the modernized device in the range of 1.6 - 1.7 times compared to the known technological solutions.
  • the cooling of the block takes the same amount of time as in the case of making use of the solution according to this invention.
  • this is related to the fact that during that time the technological apparatus is not used, which increases the time of the technological melting cycle. Due to this reason, the utilization rate of the traditional technological installation is less than 0.5 which at the same time explains the lower capacity of the installation.
  • This process is used to receive the maximum volume density of the ingoing billet used to grow the crystals of sapphire using the same method as in the Example 1.
  • the synthesis of the ingoing billet in the form of polycrystalline corundum ingots is performed in vacuum.
  • the fusion mixture in this process is the same aluminum oxide powder as the one used in the Example 1.
  • the installation used in the Example 1 is used here. The difference is the additional equipment in the form of vacuum pump connected to the working chamber.
  • the fusion mixture in a form of aluminum oxide powder is loaded into the container.
  • One of the cold crucibles is placed in the initial position within the internal space of the inductor.
  • the working chamber is closed and the vacuum pump is turned on.
  • the pump sucks the air out of the working chamber and the connected container until the pressure of lxlO 4 Tor has been reached.
  • the aluminum oxide powder with the weight of 6 - 8 kg is added from the container to the cold crucible.
  • the high-frequency power source is turned on and the initial heating of the aluminum oxide is performed, using one of the well-known methods.
  • the initial melt is formed, it is heated in the electromagnetic field of the inductor and its volume grows as the load gets heated.
  • the induction heating of the melt as well as the induction melting of the aluminum oxide occurs - similar to the process described in the Example 1.
  • the cold crucible is translated downwards at the speed of 3 - 7 mm/min, so that the upper level of the bath melt stays fixed in relation to the inductor.
  • feeding the powder stops and stops the movement of crucible.
  • the melting process terminates in a similar way to the one described in the Example 1. As a result, the process of synthesis of the polycrystalline corundum ingot takes place.
  • the air is introduced into the working chamber, the door of the chamber is opened and the cooled polycrystalline ingot is taken out of the first cold crucible.
  • This cold crucible is prepared for the new cycle and a new portion of the fusion mixture is introduced into the container.
  • the second cold crucible with hot polycrystalline corundum ingot is taken out of the inductor and its place is taken by the previously prepared first cold crucible, which is put in the initial position.
  • the chamber door is closed.
  • the air is pumped out of the working chamber and the connected container until the pressure of lxlO 4 Tor has been reached.
  • the new technological cycle of the installation is initiated, starting from putting the fusion mixture of the aluminum oxide into the crucible and inducing the initial melting of this load.
  • the weight of the vacuum- synthesized ingoing billets in the form of polycrystalline corundum ingots is between 26 and 36 kg, depending on the production needs. Their density is between 3.83 and 3.88 g/cm 3 .
  • the high value of the density in this case is due to the lack of oxygen dissolved in the melt of aluminum oxide.
  • When performing the melting within the air atmosphere there is up to 30% of volume taken by the oxygen dissolved in the melt.
  • oxygen is collected on the border between the melt and corundum, bubbles are formed, which are preserved as pores in the ingot by quickly moving border layer of crystallization.
  • the polycrystalline corundum ingot contains less volatile admixtures such as Na, Mg, Ca and others.
  • the quality of the ingoing billets of corundum is higher.
  • the quality of sapphire crystals produced with these ingoing billets is elevated.

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Abstract

The invention relates to a method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire, comprising introduction of a fusion mixture of aluminum oxide powder into a cold crucible, creating a high-frequency electromagnetic field with an inductor and initial melting of the fusion mixture, heating the melt in the high-frequency electromagnetic field of the inductor, feeding an additional portion of the fusion mixture in the cold crucible and its melting, crystallization the melt in the form of a polycrystalline ingot during a vertical shifting of the cold crucible with respect to the inductor, termination of the induction melting by switching off the high-frequency field of the inductor, cooling the polycrystalline ingot in the cold crucible, removing the polycrystalline ingot from the cold crucible and repeating the whole cycle from the introduction of the fusion mixture into the cold crucible and its initial melting, characterized in that after termination of the induction melting and switching off the high-frequency field of the inductor, the cold crucible (5) with the polycrystalline ingot (16) is taken out of the inductor (4) and in its place a second identical cold crucible (6) is introduced and the cycle is repeated to obtain another polycrystalline ingot (17) in the second identical cold crucible (6), wherein the cooling of the previously produced polycrystalline ingot (16) is carried out simultaneously with the synthesis of the next polycrystalline ingot (17) and removing the polycrystalline ingot (16) from the cold crucible (5) is carried out after the induction melting of the next polycrystalline ingot (17) has been terminated. The invention covers also an installation for carrying out this method.

Description

A method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire
and installation therefore
The invention relates to growing of sapphire single crystals by crystallization of a melt on a seed, and more specifically to synthesis of a ingoing billet in the form of polycrystalline corundum ingot, used for industrial process of growing sapphire single crystals. The invention also relates to induction melting technology of refractory materials in the cold crucible. This invention can also be used for the synthesis of ingoing billet of other oxide materials.
A process is known of forming crystals of stabilized zirconium dioxide (fianite) using the induction melting in a cold crucible (described in [1], pp. 187- 189). The crystals are created in a form of crystal druses forming a polycrystalline ingot that had been obtained by means of direct crystallization of oxide melt in a cold crucible . This process comprises of the following technological phases: preparation of the fusion mixture, cleaning and placing the cold crucible in the working position (initial phase); loading a fusion mixture into a cold crucible placed in a working chamber; forming inside the fusion mixture a crystallization seed in the form of a metal cutting or small pieces (zirconium or yttrium); metal heating in the electromagnetic field of an inductor and then burning of metal (oxidation with the use of atmospheric oxygen) which takes place with heat emission; forming the starting melt bath of the oxide material and the induction heating of the melt in a high frequency field of the inductor; subsequently adding a fusion mixture into the cold crucible and its melting; crystallization of the melt in the form of polycrystalline ingot; cooling the formed polycrystalline ingot and its removal from the cold crucible.
This method is widely used in the industrial scale for synthesis of stabilized zirconium dioxide crystals by stabilization of the cubical structure through addition of Y2O3, CaO and other oxides in fusion mixture.
The drawback of this method is the lost time that is needed to cool down the formed polycrystalline ingot in the form of crystal druses. This drawback is not that critical for this method, since during the technological cycle, the phase of cooling the ingot takes definitely less time that the phases of melting and crystallization of the melt.
The closest method to the one according to the present invention is the process of synthesis of poly crystalline ingot of oxide materials by induction melting in a cold crucible under the conditions of periodic (two-stage) melting (described in [2], pp. 59-62) which has been taken as a prototype model for this invention.
In this process, the fusion mixture is loaded into a cold crucible placed within the working chamber; then the power source of the inductor is turned on for the electromagnetic field to be created, the starting melt bath of the oxide material is created, the melt is heated in the electromagnetic field of the inductor, the melting of the fusion mixture takes place, subsequently a new portion is added to the cold crucible and it is melted; the melted mass is crystallized into the form of a polycrystalline ingot under the process of vertical translation of the cold crucible, the power source is turned off, the synthesized polycrystalline ingot is cooled down and taken out from the cold crucible.
This process allows for creation of polycrystalline ingots of various oxide materials, including but not limited to: A1203, Y203, Sm203 and CaO. Figure 1 presents the sequence of technological phases performed in a well-known technological cycle of periodic synthesis of polycrystalline ingots of oxide materials, with the possibility of repeating it in industrial production processes.
Induction melting of oxide materials in a cold crucible is characterized by high intensiveness of fusion mixture melting due to the emission of electromagnetic field energy directly in the melt and intensive stirring of the melt mass. This advantage of induction melting in a cold crucible is optimally used during synthesis of oxide materials being in the form of polycrystalline ingot. The crystallization of the melt takes place at the high speed, since the formation of coarse crystal structure of the polycrystalline blocks is not necessary. The melting temperature of high-temperature oxide materials above 2000°C, in particular, for aluminum oxide, it is 2045°C, so after the melting has been finished and the power source of the inductor has been turned off, the polycrystalline ingot is very hot and it cannot be taken out of the cold crucible. The ingot can be taken out only after its temperature has gone down to an ambient room temperature level. The cooling time of the ingot depends on its mass and it is usually very similar to the time of the technological phases of loading the fusion mixture into the cold crucible, its melting and crystallization of the melted mass. Due to this fact, the time of technological phases of melting the fusion mixture and crystallization of the melted mass is of the same order of magnitude as the cooling phase of the formed ingot. During the cooling phase the basic technological systems, namely: power source, system for sustaining the controlled environment in a working chamber and others, are not used. This is exactly the very source of the main disadvantage of the well-known method of synthesis of the poly crystalline oxide material ingots with the periodic (two-stage) induction heating in the cold crucible: the main technological apparatus is not properly used because of nonproductive periods needed for cooling of the synthesized poly crystalline oxide material ingots. In other words: the well-known process is characterized by a low utilization factor of the essential equipment.
There exists a known installation used for synthesis of poly crystalline oxide materials with the method of periodic induction melting which consists of water-cooled (cold) crucible with a bottom and the inductor which surrounds the cold crucible [3]. The cold crucible is produced in the form of isolated tubular sections made of electricity- and heat-conducting material (usually copper), cooled by water. The inductor is connected to the high-frequency power source and the device is equipped with a batch box .
Another known induction-based installation used for the synthesis of polycrystalline ingots of the oxide materials, working in periodic (two-stage) cycles is described in [4]. This installation has a fusion mixture container equipped with input supply control system, high-frequency power source as well as a working chamber comprising: an inductor connected to a high-frequency power source, a cold crucible with a bottom, placed inside the inductor and capable of vertical movements via a translation mechanism. The working chamber is linked to subsystems that ensure maintaining controlled environment in the chamber. Low utilization is a disadvantage of the known installations because of the use of cold crucible during the cooling phase of the formed poly crystalline ingot in a periodic (two-stage) technological cycle, when all the other technical equipment is not used.
From the technical point of view, the closest solution to the one according to the present invention is the installation described in [5], designed for periodical (two-stage) synthesis of poly crystalline ingots of oxide materials. This installation is equipped with a high-frequency power source and a working chamber comprising: an inductor connected to a high-frequency power source and a cold crucible placed inside the inductor. The cold crucible is made of pipe sections, contains a bottom and is capable of vertical movements via a translation mechanism. The working chamber is linked to subsystems that ensure maintaining controlled environment in the chamber.
The disadvantage of the installation is the lack of possibility of making use of the basic technical equipment during the cooling phase of the formed polycrystalline ingot of the oxide material placed in the cold crucible. This lowers down the performance ratio of the installation, at the same time raising the production cost, being an important factor in the process of production of corundum ingoing billet in large quantity manufacture of sapphire single crystals.
The purpose of this invention is to increase the utilization ratio of the basic technical equipment in the production process of the ingoing billet in the form of polycrystalline corundum ingot, using the method of periodic (two-stage) induction melting in a cold crucible.
Another purpose of this invention is the increase of productivity of the installation used for the synthesis of ingoing billet in the form of polycrystalline corundum ingot in the process of industrial production of sapphire crystals.
According to this invention, the method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire, comprises introduction of a fusion mixture of aluminum oxide powder into a cold crucible, creating a high-frequency electromagnetic field with an inductor and initial melting of the fusion mixture, heating the melt in the high-frequency electromagnetic field of the inductor, feeding an additional portion of the fusion mixture in the cold crucible and its melting, crystallization the melt in the form of a polycrystalline ingot during a vertical shifting of the cold crucible with respect to the inductor, termination of the induction melting by switching off the high- frequency field of the inductor, cooling the polycrystalline ingot in the cold crucible, removing the polycrystalline ingot from the cold crucible and repeating the whole cycle from the introduction of the fusion mixture into the cold crucible and its initial melting. According to the invention, after termination of the induction melting and switching off the high-frequency field of the inductor, the cold crucible with the polycrystalline ingot is taken out of the inductor and in its place a second identical cold crucible is introduced and the cycle is repeated to obtain another polycrystalline ingot in the second identical cold crucible, wherein the cooling of the previously produced polycrystalline ingot is carried out simultaneously with the synthesis of the next polycrystalline ingot and removing the polycrystalline ingot from the cold crucible is carried out after the induction melting of the next polycrystalline ingot has been terminated.
Preferably, moving out the cold crucible with the polycrystalline corundum ingot from the inductor is performed simultaneously with putting in its place a second, identical cold crucible.
Preferably, the simultaneous moving out the cold crucible with the polycrystalline corundum ingot and moving in the second, identical cold crucible is performed by rotation of both crucibles around a common vertical axis.
In a particularly preferred embodiment of the present invention the synthesis of polycrystalline corundum ingots is carried out in vacuum.
According to this invention, the installation for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire, comprises a high-frequency power source, batch container for fusion mixture comprising aluminum oxide powder, with controlled fusion mixture input connected to a working chamber, in which there is an inductor connected to a high-frequency power source, further comprising a cold crucible with a closed- bottom, placed within the internal space of the inductor, said crucible is capable of vertical shifting and is cooled by a flow of water, wherein the working chamber is connected to a unit for controlling atmosphere in the working chamber. According to the invention, the installation is equipped with additional, identical cold crucible placed in the working chamber, capable of identical vertical shifting, wherein each of the cold crucibles is mounted on a horizontal shifting unit, allowing for introduction of subsequent cold crucibles into the internal space of the inductor.
Preferably, the cold crucibles are connected mechanically with each other and form a unitary assembly with a common mechanism for vertical shifting and a common mechanism for horizontal shifting, where both mechanisms allow for simultaneous moving and consecutive placement of cold crucibles in the internal space of the inductor, and the horizontal shifting of the cold crucibles is performed by their rotation around a vertical axis.
Preferably, the cold crucibles are connected in series with respect to the flow of the cooling water.
This method is characterized by high equipment utilization ratio by minimalization of non-productive time loss for cooling down the ingot. This value is achieved by changing the order of technological phases of the process of synthesis of corundum ingoing billet in the form of polycrystalline ingots. The technological phase of cooling a polycrystalline ingots is performed in parallel and simultaneously to other phases related to preparation of another, identical cold crucible and induction melting of the fusion mixture in this second crucible. The change of the order of these technological phases is possible due to introduction of an additional cold crucible, creating the possibility of moving both crucibles as well as subsequent introduction of crucibles into the inductor. Each crucible is used during the full cycle of forming the polycrystalline ingot, namely: induction melting of the fusion mixture, crystallization of the melt and cooling of the formed ingot. At the same time the second crucible is used to create another polycrystalline ingot. Through keeping this order of technological phases, one can ensure the full use of the fundamental technological equipment during the full production cycle, except for the operation of taking the polycrystalline ingot out and preparing the equipment for the next cycle, which takes a small part of the whole cycle of synthesis of the ingoing billet of corundum. Figure 2 presents the improved process of synthesis of ingoing billet in the form of polycrystalline ingots of corundum using the periodic induction melting in a cold crucible.
The increase of performance of the apparatus is achieved through shortening of the interruption time of the basic element of the apparatus, namely the inductor, which is powered by the high-frequency source, as well as auxiliary devices. The very advantage of the solution according to the invention is ensured by placing an additional cold crucible, identical to the one already present therein, within the working chamber. Moreover, the two cold crucibles are merged into one unified system, capable of translations both in vertical as well as horizontal directions, and ensuring that each of the crucible is successively introduced into the inductor and thereby the performance ratio of the whole installation is increased.
The invention has been introduced and described below, with reference to the enclosed drawings, however it is not limited to the solutions presented below.
Figure 1 (prior art) presents the sequence of the technological phases in known technical solutions for the synthesis of polycrystalline blocks of oxide materials using the periodic (two-stage) melting in the cold crucible,
Figure 2 presents the sequence of technological phases of the synthesis of ingoing billet in the form of polycrystalline corundum ingots using the periodic induction melting in a cold crucible to grow the sapphire crystals according to this invention,
Figure 3 presents a scheme of the installation for synthesis of ingoing billet in the form of polycrystalline corundum blocks in the initial phase of the technological cycle,
Figure 4 presents a scheme of the installation in the final phase of induction melting after the high-frequency electromagnetic field created by the inductor has been switched off, before the cold crucible have been shifted,
Figure 5 presents a scheme of the installation during loading the fusion mixture into the second identical cold crucible and simultaneous cooling of previously formed poly crystalline corundum ingot in the first cold crucible,
Figure 6 presents a scheme of the installation in the final phase of induction melting of the next poly crystalline corundum ingot and taking the poly crystalline corundum ingot formed in the previous cycle in the first crucible out of this cold crucible, and
Figure 7 presents a scheme from Figure 6 in the transversal cross-section A- A during the final phase of induction melting of the next polycrystalline corundum ingot and taking the polycrystalline corundum ingot formed in the previous cycle out of the cold crucible.
The installation for the synthesis of the ingoing billet in the form of polycrystalline corundum ingots using the process of periodic (two-stage) induction melting in the cold crucible comprises: a working chamber 1 connected to the batch container 2 through a system for controlling the fusion mixture load 3 into the working chamber 1. Within the working chamber 1 there are an inductor 4 and two identical cold crucibles (5 and 6), mounted on a basis 7, having specified dimensions. Each of the cold crucibles 5 and 6 has a bottom. Both crucibles 5 and 6 are connected to the assembly coupled in series along a flow of cooling water system 8. The basis 7 is mounted on a rod 9, which allows for its vertical movement through a helical gear 10 and electrical driving gear 11. The basis 7 can be moved in horizontal plane and can be put into one of the two possible fixed positions. The fixing is enabled through a mechanism 12 when the rod 9 is turned around its axis. Shifting of the basis 7 in the horizontal plane is possible only when both cold crucibles 5 and 6 have been placed in their lowest positions. The fixed position of the basis 7 also means that one of the cold crucibles is placed co-axially against the inductor 4. The inductor 4 is connected to a high-frequency power source (not shown) through a high-frequency feeder 13. The working chamber is linked with a system for maintaining a controlled atmosphere in the working chamber (not shown) through the connector 20. The installation works as follows.
In the chamber 1, as the result of vertical translation of the rod 9, fixed by the mechanism 12, the cold crucible 5 is introduced into the internal space of the inductor 4 and put into the initial position (Figure 3). From the container 2, with the help of the system for controlled fusion mixture loading 3, a portion of the aluminum oxide powder (fusion mixture) 14 is loaded into the crucible. Then the high-frequency power source is turned on and inside the inductor, where the cold crucible had been placed, the electromagnetic field of high frequency is created. The initial melting of the aluminum oxide is induced, using the well-known procedures. The initial melting shown on the presented embodiment of this invention comes out as a result of exothermic reaction of granules of the metallic aluminum 15, introduced into the aluminum oxide powder 14 after the initial portion of the fusion mixture has been loaded into the crucible. The granules of the metallic aluminum 15 are heated within the electromagnetic field of the inductor and undergo rapid oxidation (they get burned). The reaction of oxidations of the metal is accompanied by heat emission, which makes the surrounding granules of the fusion mixture 14 melt. The melted mass gets heated within the electromagnetic field of the inductor, and subsequently the induction melting of the aluminum oxide takes place. The aluminum oxide powder is added into the cold crucible 5 from the container 2 in a controlled way, through the fusion mixture dose control system 3. As the aluminum oxide powder gets melted, the cold crucible 5 is shifted downwards by the rod 9 with the use of helical gear 10 and electrical gear 11. Simultaneously there is directional solidification of the melt and the synthesis of fused corundum in the form of an ingot. Lowering speed of the crucible is 1-7 mm/min and is maintained such that the molten bath is in the zone of the inductor. After the pre-defined portion of the aluminum oxide powder has been added, the melted mass undergoes the crystallization with the electromagnetic field of the inductor being turned off or then the power of the high-frequency source transferred to the melted mass is lowered and subsequently turned off. In this way a polycrystalline ingot of corundum 16 is formed, with a pre-defined weight. The cold crucible 5 containing the polycrystalline corundum ingot 16 is shifted vertically and taken out of the inductor internal space 4 - this is the very phase presented in Figure 4. The rod 9 is turned around with the use of the mechanism 12 and the basis 7 is translated in the horizontal plane together with the cold crucibles 5 and 6, and it is finally placed in the second fixed position, again with the use of the mechanism 12. This phase has been presented in Figure 5. Because both cold crucibles 5 and 6 are identical and joined together into a unified system, the aforementioned movement of the rod 9 creates the shifting of the cold crucible 5 with the polycrystalline corundum ingot 16 and another cold crucible 6 is being put onto the place of the first one. The second crucible 6 is put into the inside of the inductor 4 and placed in the initial position as a result of vertical translation of rod 9 with the use of helical gear 10 and the electrical gear 11. Then the technological operations are repeated, starting from loading of aluminum oxide powder 14 into the cold crucible 6 and its initial melting through oxidizing of the granules of metallic aluminum 15, which, as the result, produces a similar polycrystalline ingot of corundum 17 in the second cold crucible 6. At the same time, the polycrystalline corundum ingot 16 is located in the first cold crucible 5 and gets cooled down to the ambient room temperature, so that it can be taken out of cold crucible 5. The working chamber door 18 is open and the synthesized polycrystalline corundum ingot 16 is taken out of the cold crucible 5. This is the phase presented in Figure 6 (where the open door 18 is shown in a schematic way) and in Figure 7. The polycrystalline corundum ingot taken out of the crucible is the product for the technological process, and it is subsequently used as the ingoing billet of the given weight to grow sapphire crystals.
The cold crucible 5 is then prepared for the next cycle. First of all it is being cleaned out of the remnants of the fusion mixture and, if necessary, its inside surface is cleaned with a piece of cloth immersed in alcohol. Subsequently, the door 18 of the working chamber 1 is closed, the cold crucible 5 is put in the initial position inside the inductor 4, as described above, and the technological phases are repeated, starting with the loading of the aluminum oxide powder into the cold crucible 5 and the initial melting. When the next polycrystalline corundum ingot is formed in the cold crucible 5, the previously formed polycrystalline ingot 17 gets cooled in the cold crucible 6. The door 19 of the working chamber 1 is open and the ingot 17 is taken out of the cold crucible 6 in a similar way, as for the polycrystalline corundum ingot 16 taken out of the cold crucible 5.
Then the whole technological cycle is repeated, starting from the loading of the initial portion of the aluminum oxide powder.
Example 1
The process and the installation according to the invention are used in order to increase the density of the ingoing billet used for synthesis of corundum (α-Α1203), subsequently used to grow the sapphire crystals using the Kyropoulos- Musatov method, in an apparatus with a tungsten container of the diameter of 250 mm. The internal shape of the container is similar to truncated cone with the smaller diameter of 212 mm and the depth of 340 mm. The optimal weight of the ingoing billet is 30 - 35 kg. The polycrystalline corundum ingots of high purity are obtained by the induction melting of the aluminum oxide powder in the cold crucible in the air atmosphere. The fusion mixture is the aluminum oxide powder of high chemical purity produced by Sumitomo Chemicals Co Ltd. The aluminum oxide powder is represented by the crystal phases of β-Α1203 and α-Α1203.
The installation for the synthesis of the ingoing billet in the form of polycrystalline corundum ingots comprises of two identical cold crucibles with circular cross section and internal diameters of 210 mm. Each crucible is made of copper tubes with square cross sections, with the side length of 12 mm, joined by pairs into sections with progressive-reversible flow of the cooling water. These sections are mounted onto a copper collector and form the melting space of the crucible with the depth of 350 mm. At the same time, the collector plays the role of the closed-bottom of the crucible and the cooling water separator between the cold crucible sections. Both crucibles are linked in a series in relation to the cooling water flow and form the unified assembly on one basis. This basis is mounted on a rod capable of making vertical shifts and horizontal rotations, which ensures consecutive positioning of the crucibles co-axially with the inductor. The inductor is a single-turn, with the internal diameter of 260 mm, height of 60 mm and is connected to the high-frequency power generator with the power of 60 kW and the frequency of 5.28 MHz. The cold crucibles as well as the inductor are placed in the working chamber equipped with two doors. The working chamber is linked with the fusion mixture container with a feeder that controls the amount of aluminum oxide powder that is added to the crucible. The working chamber is connected to the atmosphere conditions control system, in this case: air cleaning system and working chamber ventilation.
The oxide materials are dielectric and their induction heating in the electromagnetic field of high frequency is not possible. In order to initialize the induction melting of such materials, it is necessary to perform pre-melting of a part of the oxide material placed in the melting zone of the cold crucible using another heat source. This phase of the induction melting is called the initial or start melting of the load. Usually in order to start the initial melting of the load, the heat coming from exothermic reaction of metal oxidation is used. In order to do this, cutting or granules of the metal, usually the same as the one the oxide of which forms the load, are introduced into the inductor operating zone. There are also other alternatives for start melting of oxide materials, for example: through an electric arch or through heating of some electric conductor in the electromagnetic field of the inductor. The initial melting phase is a usual practice in the area of induction melting of oxide materials in the cold crucible and is realized with well-known methods and with the aid of well-known devices. The choice of a specific initialization method is determined by the type of the oxide material, the purpose of the technological process and the technological means available.
The fusion mixture in the form of aluminum oxide powder is loaded into the container. The fusion mixture is represented by the crystal phases of β-Α1203 and α-Α1203. One of the cold crucibles is placed in the initial position inside the inductor. The aluminum oxide powder with the weight of 5 - 6kg is introduced from the container into the cold crucible. There is a small portion of the metallic aluminum with the weight of 20g, comprised of granules of the size of 0.05 - 0.12mm, placed on the top of the load and covered with a layer of the aluminum oxide powder from the container of 5 - 10 mm high. The working chamber is closed, the high-frequency power source is turned on. As a result, in the melting zone of the cold crucible, a high-frequency electromagnetic field is created. The aluminum granules are heated in the electromagnetic field of the inductor, then they get oxidized, the process being accompanied with the heat emission and the melting of the surrounding powder of the aluminum oxide takes place. The initial bath melt is formed, which gets heated in the electromagnetic field of the inductor and increases its own volume. From that moment onwards, the induction heating of the melt as well as the induction melting of the aluminum oxide set off. Another portion of aluminum oxide powder is introduced from the container into the crucible and the conditions of the power source are controlled such that the power transferred to the bath melt is in the range of 30 - 32kW. As the aluminum oxide powder melts, the upper level of the bath melt raises. The cold crucible is translated downwards at the speed of 1 - 4 mm/min, so that the upper level of the bath melt stays fixed in relation to the inductor. Because the power of the electromagnetic field of the inductor is limited, when the aluminum oxide powder is melted and the crucible is translated, the crystallization of the lower parts of the bath melt takes place at the same speed as with which the crucible lowers down and at the weight gain speed equal to the weight addition speed of the load. After 35 kg of the aluminum oxide powder has been added to the crucible (including the initial portion of the load), the further addition stops and the crucible is no longer moved downwards. The melting of the aluminum oxide powder on the upper level of the bath melt is finished and the power of the electromagnetic field transferred through the inductor to the melt is decreased to 12 - 15 kW within the period of 10 - 15 min through the change of parameters of the high-frequency power source. Finally the high-frequency power source is switched off which results in full crystallization of the bath melt. The formed polycrystalline corundum ingot has a temperature of around 2000°C, thus it is dangerous to take it out of the crucible. The cold crucible with a hot polycrystalline corundum ingot is translated vertically downwards along the inductor and then, through the rotation of the rod it is being moved out of the inductor. Simultaneously there is another identical cold crucible put into the place of the first one, and it is placed co-axially in relation to the inductor. The rod is shifted up and the cold crucible is introduced into the internal space of the inductor and placed in the initial position. The installation is ready to begin a new induction melting cycle. The amount of 5 - 6 kg of the aluminum oxide powder is put into the cold crucible, the initial melting of the load is performed, then an additional amount of the load is placed from the container, the crystallization of the bath melt is performed, the poly crystalline corundum ingot is produced and the electromagnetic field of the inductor is turned off. In this way the full cycle of forming the second polycrystalline ingot has been done. During this production cycle of the second polycrystalline ingot, the first polycrystalline ingot stays in the cold crucible and is cooled down to the ambient room temperature. The working chamber is opened and the cooled polycrystalline ingot form the first cold crucible is taken out. The weight of the formed polycrystalline corundum ingot is around 34.0 - 34.3 kg, its diameter 208 mm and the density in the range of 3.1 - 3.2 g/cm3. Out of the fusion mixture, represented by the crystal phases of β-Α1203 and a- A1203, after the melting and crystallization, the corundum is synthesized, that is, the phase of α-Α1203 is formed.
The cold crucible is now prepared for the new melting cycle. It is cleaned out of the remnants of the fusion mixture (around 0.7 to 1.0 kg), its internal surface is cleaned with a piece of cloth immersed in alcohol and a new portion of fusion mixture is added into the container. After the preparations, the second cold crucible with a hot polycrystalline corundum ingot is taken out of the inductor internal space and moved horizontally. Simultaneously, the first crucible is moved into the initial position within the inductor. The new technological cycle of the installation can be started and the initial load of the aluminum oxide can be placed in the crucible.
The duration of consecutive technological phases of the installation for the synthesis of the ingoing billet in the form of polycrystalline corundum ingots with the diameter of 208 mm, height of 300 - 330 mm and the weight of 30 - 35 kg using the method of periodic (two-stage) induction melting in the cold crucible of the diameter of 210 mm is presented in the table below. As shown in the table, the fundamental technical equipment, being the high-frequency power source, stands idle for the period of 0.72 hrs during one cycle (sum of duration of phases 1, 5, 6 and 7). The full technological cycle time is 3.97 hrs., which gives the utilization rate of 0.82, while the cooling of the synthesized polycrystalline ingot takes 3.42 hrs. During that time ingot temperature goes down to 80 - 100°C, which allows for taking it out of the cold crucible. The increase of the utilization rate of the technical equipment of the installation results in the increase of the efficiency of the modernized device in the range of 1.6 - 1.7 times compared to the known technological solutions.
Table
Figure imgf000017_0001
In case of the synthesis of the polycrystalline corundum ingot of the similar dimensions performed using the known methods, the cooling of the block takes the same amount of time as in the case of making use of the solution according to this invention. In a well-known method this is related to the fact that during that time the technological apparatus is not used, which increases the time of the technological melting cycle. Due to this reason, the utilization rate of the traditional technological installation is less than 0.5 which at the same time explains the lower capacity of the installation.
Example 2
This process is used to receive the maximum volume density of the ingoing billet used to grow the crystals of sapphire using the same method as in the Example 1. The synthesis of the ingoing billet in the form of polycrystalline corundum ingots is performed in vacuum. Thus the quality parameters of the ingoing billets are increased. The fusion mixture in this process is the same aluminum oxide powder as the one used in the Example 1. The installation used in the Example 1 is used here. The difference is the additional equipment in the form of vacuum pump connected to the working chamber.
The fusion mixture in a form of aluminum oxide powder is loaded into the container. One of the cold crucibles is placed in the initial position within the internal space of the inductor. The working chamber is closed and the vacuum pump is turned on. The pump sucks the air out of the working chamber and the connected container until the pressure of lxlO 4 Tor has been reached. Then the aluminum oxide powder with the weight of 6 - 8 kg is added from the container to the cold crucible. The high-frequency power source is turned on and the initial heating of the aluminum oxide is performed, using one of the well-known methods. The initial melt is formed, it is heated in the electromagnetic field of the inductor and its volume grows as the load gets heated. From that moment, the induction heating of the melt as well as the induction melting of the aluminum oxide occurs - similar to the process described in the Example 1. The cold crucible is translated downwards at the speed of 3 - 7 mm/min, so that the upper level of the bath melt stays fixed in relation to the inductor. After the pre-defined quantity of the aluminum oxide powder has been added to the crucible (including the initial load), feeding the powder stops and stops the movement of crucible. The melting process terminates in a similar way to the one described in the Example 1. As a result, the process of synthesis of the polycrystalline corundum ingot takes place. The cold crucible with the hot polycrystalline corundum ingot leaves the internal space of the inductor and its place is taken by a second, identical cold crucible placed in the initial position and subsequently the new cycle of synthesis of the ingoing billet in the form of polycrystalline corundum ingot begins. After the processes of synthesis of polycrystalline corundum ingot in the second cold crucible have been finished and the high-frequency electromagnetic field of the inductor switched off, the air is introduced into the working chamber, the door of the chamber is opened and the cooled polycrystalline ingot is taken out of the first cold crucible. This cold crucible is prepared for the new cycle and a new portion of the fusion mixture is introduced into the container. After the preparation phase is completed, the second cold crucible with hot polycrystalline corundum ingot is taken out of the inductor and its place is taken by the previously prepared first cold crucible, which is put in the initial position. The chamber door is closed. The air is pumped out of the working chamber and the connected container until the pressure of lxlO 4 Tor has been reached. The new technological cycle of the installation is initiated, starting from putting the fusion mixture of the aluminum oxide into the crucible and inducing the initial melting of this load.
The weight of the vacuum- synthesized ingoing billets in the form of polycrystalline corundum ingots is between 26 and 36 kg, depending on the production needs. Their density is between 3.83 and 3.88 g/cm3. The high value of the density in this case is due to the lack of oxygen dissolved in the melt of aluminum oxide. When performing the melting within the air atmosphere, there is up to 30% of volume taken by the oxygen dissolved in the melt. During the crystallization, oxygen is collected on the border between the melt and corundum, bubbles are formed, which are preserved as pores in the ingot by quickly moving border layer of crystallization. As a result, in case of melting in the air atmosphere, there are pores formed in the polycrystalline corundum blocks and thus the volume density of the block is lowered. In case of melting in vacuum, apart from increased volume density, the polycrystalline corundum ingot contains less volatile admixtures such as Na, Mg, Ca and others. Thus the quality of the ingoing billets of corundum is higher. In the same way, the quality of sapphire crystals produced with these ingoing billets is elevated.
REFERENCES:
Kuzminov Yu.S. Tugoplavkie materialy iz kholodnogo
tiglya/Yu.S. Kuzminov, E.E. Lomonosova, V.V.Osiko. - M: Nauka, 2004, 369 pages.
Petrov, Yu.B. Indukcyonnaya plavka okislov / Yu.B.Petrov.- L:Energoatomizdat, 1983, 104 pages (pp.59-62).
French patent No 1492063, CI. int. F27b:C04b. Perfectionnment aux fours electriques haute frequence pour la fabrication en cobtinu de refractaires electrofondus.
Petrov Yu.B. Indukcyonnye pechi dla plavki oksidov (Bibloteka wysokochastotnika-termista)/Yu.B. Petrov, I.A.Kanaev. - L : Politekhnika, 1991, 56 pages.
Harrison, H.R. Skull melter growth of magnetite (FQ3O4)/ H.R. Harrison, R. Aragon// Mater. Res. Bull. -1978.- Vol. 13, No 11. - P. 1097-1104.

Claims

1. A method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire, comprising introduction of a fusion mixture of aluminum oxide powder into a cold crucible, creating a high-frequency electromagnetic field with an inductor and initial melting of the fusion mixture, heating the melt in the high-frequency electromagnetic field of the inductor, feeding an additional portion of the fusion mixture in the cold crucible and its melting, crystallization the melt in the form of a polycrystalline ingot during a vertical shifting of the cold crucible with respect to the inductor, termination of the induction melting by switching off the high-frequency field of the inductor, cooling the polycrystalline ingot in the cold crucible, removing the polycrystalline ingot from the cold crucible and repeating the whole cycle from the introduction of the fusion mixture into the cold crucible and its initial melting, characterized in that after termination of the induction melting and switching off the high-frequency field of the inductor, the cold crucible (5) with the polycrystalline ingot (16) is taken out of the inductor (4) and in its place a second identical cold crucible (6) is introduced and the cycle is repeated to obtain another polycrystalline ingot (17) in the second identical cold crucible (6), wherein the cooling of the previously produced polycrystalline ingot (16) is carried out simultaneously with the synthesis of the next polycrystalline ingot (17) and removing the polycrystalline ingot (16) from the cold crucible (5) is carried out after the induction melting of the next polycrystalline ingot (17) has been terminated.
2. The method according to claim 1, characterized in that moving out the cold crucible (5) with the polycrystalline corundum ingot (16) from the inductor (4) is performed simultaneously with putting in its place a second, identical cold crucible (6).
3. The method according to claim 1 or 2, characterized in that the simultaneous moving out the cold crucible (5) with the poly crystalline corundum ingot (16) and moving in the second, identical cold crucible (6) is performed by rotation of both crucibles (5, 6) around a common vertical axis.
4. The method according to any one of the preceding claims from 1 to 3, characterized in that the synthesis of polycrystalline corundum ingots (16, 17) is carried out in vacuum.
5. An installation for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire, comprising a high-frequency power source, batch container for fusion mixture comprising aluminum oxide powder, with controlled fusion mixture input connected to a working chamber, in which there is an inductor connected to a high-frequency power source, further comprising a cold crucible with a closed-bottom, placed within the internal space of the inductor, said crucible is capable of vertical shifting and is cooled by a flow of water, wherein the working chamber is connected to a unit for controlling atmosphere in the working chamber, characterized in that the installation is equipped with additional, identical cold crucible (6) placed in the working chamber (1), capable of identical vertical shifting, wherein each of the cold crucibles (5, 6) is mounted on a horizontal shifting unit (12), allowing for introduction of subsequent cold crucibles into the internal space of the inductor (4).
6. The installation according to claim 5, characterized in that the cold crucibles (5, 6) are connected mechanically with each other and form a unitary assembly with a common mechanism for vertical shifting (7, 9, 10, 11) and a common mechanism for horizontal shifting (12), where both mechanisms (7, 9, 10, 1 1, 12) allow for simultaneous moving and consecutive placement of cold crucibles (5, 6) in the internal space of the inductor (4), and the horizontal shifting of the cold crucibles (5, 6) is performed by their rotation around a vertical axis.
7. The installation according to claim 5 or 6, characterized in that the cold crucibles (5, 6) are connected in series with respect to the flow of the cooling water.
PCT/EP2012/066126 2011-08-17 2012-08-17 A method for synthesis of ingoing billets of corundum in the form of polycrystalline ingots for growing crystals of sapphire and installation therefore WO2013024163A1 (en)

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CN113930843A (en) * 2021-10-22 2022-01-14 中国电子科技集团公司第二十六研究所 Method for growing crystal based on horizontal directional solidification method

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