WO2012152773A1

WO2012152773A1 - Raw material input for manufacturing oxide crystals from melted load and method of its production

Info

Publication number: WO2012152773A1
Application number: PCT/EP2012/058405
Authority: WO
Inventors: Andrej MARKIEWICZ; Anatoly Shkulkov
Original assignee: Polycor Sp Z Oo
Current assignee: Polycor Sp Z Oo
Priority date: 2011-05-10
Filing date: 2012-05-07
Publication date: 2012-11-15
Anticipated expiration: 2013-11-10
Also published as: PL394818A1; PL224041B1

Abstract

The subject of the invention is a raw material input for manufacturing oxide crystals on a crystallization seed from melted load in a container, that had been initially synthesised from components to the form of a polycrystalline block, characterized in that said raw material input has the form of a melted polycrystalline block with the dimensions corresponding to the characteristic dimensions of the container and has the volume density not lower than 75% of the theoretical density of said oxide crystal and has the mass not lower than 75% of the full container load. This invention also covers a method of production of the raw material input for manufacturing oxide crystals on a crystallization seed from melted load in a container, consisting in liquid phase synthesis of the initial components by induction melting in a cold melting-pot, to form a melted polycrystalline block, characterized in that said liquid phase synthesis process is performed simultaneously with forming the polycrystalline block having the dimensions corresponding to the characteristic dimensions of the container.

Description

Raw material input for manufacturing oxide crystals from melted load and method of its production

The invention relates to a raw material input for manufacturing oxide crystals from melted load and a method of its production. The invention can be applied in the oxide crystals production technology, especially in industrial technology for production of sapphire single crystals.

The oxide single crystals play an important role in today's technology, e.g. in optical devices, lasers or microelectronic devices. The crystals of sapphire are somewhat special among contemporary crystals, first of all they are used in microelectronics for production of light-emitting diodes (LEDs).

It is a well-known technique to produce hard-melting single crystals from the crushed crystals of the same kind, as the ones being produced [1]. These crushed crystals which form the raw material input must have been previously created using another technique of crystal production. Particularly, in order to produce sapphire crystals, the crushed sapphire crystals are used that had been previously obtained using the Verneuil process. The raw material input in the form of crushed crystals is placed within a container, melted and a new crystal growth is evoked on a crystallization seed.

The drawback of such raw material input is that one cannot control the composition of contaminants during its preparation, since such an input is composed of the remnants (waste) from the production process of the sapphire crystals (either Verneuil or another one). Because the material input is a waste from another production, there is no possibility to ensure the high quality of crystals produced in the industrial conditions, especially for the crystals of sapphire.

The raw material input is known for the production process of lithium niobate single crystals from the melted load using the Czochralski process [2]. The raw material input is made of a piece of lithium niobate, synthesised to the polycrystalline form. This piece is placed within a container to get melted and conduct the crystal growth process on a crystallization seed. The density of the synthesized input lies between 50 and 70% of a theoretical density of crystals. Article [3] describes another known process of production of the lithium niobate being used as the raw material input. In this process the mixture of components, having been previously prepared in fixed proportion, namely: oxides and/or carbonates of lithium and niobium, undergoes for 2-3 hours the phases of pressing and annealing at the temperature of 1050°C up to 1100°C. In turns, the crystalline phases of lithium niobate are created out of the initial components and the raw material input is formed in the shape of a polycrysta l li ne block. The a mount of the crysta l line phase of lithi u m niobate i n t he polycrystalline block is not less than 99.5%.

The drawback of production of the raw material input here is that the process is based on crystalline structure synthesis in the solid state with the methodology used in the ceramic technology. This approach causes the contamination of the raw material input during the lengthy phases of mixing, pressing and annealing. In this way the quality of the raw material input is low, which in turn results in the low quality of the crystals, originated by the contamination.

Another known process of raw material input preparation, used in the production of sapphire crystals, has been presented in the patent specification [4]. The process comprises six subsequent technological operations, as follows: (1) grinding of aluminium oxide powder, (2) cleaning the powder particles. (3) spraying and drying of powder, (4) adding binding organic substance to the aluminium oxide powder, (5) pressing the aluminium oxide powder into a pre-defined shape (block), (6) initial annealing of the block within a container to half- sintered block, melting the block and growing the sapphire crystal.

The following operations, performed in the technological process of production of crystals, are regarded as disadvantages of this method: spraying of the aluminium oxide powder, introduction of binding organic substance and making use of the form to press the block of the raw material. During these operations the previously cleaned aluminium oxide powder is contaminated. At the same time, the presence of the binding organic material within the pressed block of raw material input creates the need to perform the annealing of this block and removal of the organic materia l directly within the container in which the crystal is supposed to be grown. All these factors lower the quality of the sapphire crystal, lengthen the growth cycle and consequently increase the production cost - and hence the price of crystals. In the course of patent-related research a technological solution has been found, related to the raw material input and the process of its production [5]. This solution has been found to be the closest - from the point of view of the number of important features - to the present invention and is considered the closest prior art for the present invention. In this case the raw material input used to grow oxide crystals on a crystallization seed within a container from melted mass, components of which are synthesised in order to receive a polycrysta lline block, has the form of bits of melted crystalline materia l that had been synthesised beforehand through melting the initial components (oxides).

The disadvantage of such a raw material input is its contamination during the process of breaking it into small bits as well as difficulties with filing the container up to the maximum possible degree, since bits of various sizes do not fit well into the container, leaving empty spaces in between. Apart from that, the space density of such raw input is smaller than the theoretical density of crystal. These shortcomings worsen the quality parameters of the initial raw material input, causing the contamination and lower quality of crystals. Application of such a raw input in the industrial conditions is not effective and raises the price of crystals.

Publication [5], related to the known technological solution, describes the production of the raw material input by induction melting of the initial components in a cold melting- pot, forming a melted polycrystalline block and crushing this block into small bits. The induction melting in a cold melting-pot is performed at the temperature exceeding the crystal melting point, which gives the possibility of leading the synthesis process in the liquid phase of the crystalline structure of a block and ensures 100% content of the crystalline phase.

The disadvantage of this production process of the raw material input is that an additional stage is necessary, introduced after the synthesis of the polycrystalline block, namely crushing the block into small bits before its introduction to a container that the crystal will be grown in. Thus the raw material input is contaminated and this lowers the quality of the crystals being grown. Apart from that, during the synthesis process in the liquid phase using the induction melting in the cold melting-pot, the melting products can be contaminated with the material of the melting-pot. Usually, the cold melting-pot is made of copper pipes with water cooling, hence the contamination with copper can take place. The purpose of this invention is to raise the quality of oxide crystals and ensure the repeatability of the quality parameters of the crystals grown in the industrial production conditions, which eventually leads to lowering their price.

According to the invention, this purpose is reached, because the raw material input for manufacturing oxide crystals on a crystallization seed from melted load in a container is initially subject to synthesis from components to the form of a polycrystalline block, wherein said raw material input has the form of a melted polycrystalline block with the dimensions corresponding to the characteristic dimensions of the container and has the volume density not lower than 75% of the theoretical density of said oxide crystal and has the mass not lower than 75% of the ful l conta iner load (ie. not lower tha n .75% of mass of the raw material input going into the container).

The invention covers also a method of production of such a raw material input, according to which liquid phase synthesis of the initial components by induction melting in a cold melting-pot to form a melted polycrystalline block is performed, while simultaneously (in one and the same technological operation) forming the polycrystalline block having the dimensions corresponding to the characteristic dimensions of the container.

Preferably, said liquid phase synthesis process is performed while shifting the melting zone of the initial components upwards in relation to the cold melting-pot, wherein a cold melting-pot is used having a full bottom and having the dimensions of the transversal cross- section corresponding to the characteristic dimensions of the transversal cross-section of the container.

Preferably, said liquid phase synthesis process is performed in vacuum.

In a particularly preferred embodiment of the present invention, said oxide crystal is sapphire or yttrium-aluminium garnet.

When carrying out the present invention, the raw material input must be synthesised beforehand from the initial components of the defined composition and desired chemical purity using the process of induction melting in a cold melting-pot in a form of polycrystalline shape. Simultaneously with melting, within one operation, the polycrystalline block is formed with the dimensions corresponding to the dimensions of the container. By "corresponding dimensions" or "similarity of dimensions" of the polycrystalline shape (block) it is understood that the block can be easily put into the container, having at the same time a similar shape. In order to achieve this, the cold melting-pot must be prepared in such a way that its transversal cross-section should have similar shape and dimensions to the cross- section of the container. For hard-melting oxide materia ls the ratio of density of melted mass to the theoretical crystal density usually lies within the range of 0.7 to 0.75. In order to maximise or optimise of filling the container with the melted mass, the volume density of the polycrystalline shape should not be lower than 75% of the theoretical crystal density. In such a way the effectiveness of crystal growing devices reaches its maximum, this leading to lowe ri ng of the price of crysta ls. Usi ng the raw materia l i n puts with homoge neous composition as well as eliminating of additional steps related to their preparation prevents them from getting contaminated. This ensures high purity rate of the raw material input, which, i n turns, gua ra ntees high qua lity of oxide crysta ls being produced in ind ustria l conditions. During the experiments it has been found that in order to safeguard the high quality of the grown crystals, the raw material input should consist of at least 75% of the polycrystalline shape, and the remaining part can be made of remnants from treatment of the same crystals re-directed back to the production phase or other raw materials.

Conducting the synthesis by induction melting in a cold melting-pot allows for performing at the same time the liquid phase synthesis, which enables creation of monophase polycrystalline structure of the raw material input, as well as enables formation of the raw material input having dimensions corresponding to those of the container. The induction melting in a cold melting-pot is considered to be a pure process of synthesis of crystalline materials, however during melting the products of the synthesis are contaminated with the material of the melting-pot, wh ich usua l ly is co p pe r. The contamination takes place as a result of high-temperature chemical reactions between active dopants being part of initial components, for example compounds of chlorine, sulphur or nitrogen, absorbed by the initial components during their own process of production in the chemical industry. During the melting of initial components, a dissociation of these dopants in the melting zone takes place. This results in emission of vapours of these dopants and their migration towards the walls of cold melting-pot. These dopants react with the material of the melting pot (i.e. with copper). Corresponding copper compounds are formed, which create a layer over the inner surface of the cold melting-pot. If the melting zone of the initial com ponents of the input is kept on a consta nt level - as ha ppens in the case of continuous induction melting in a cold melting-pot with movable bottom - then the layer of copper compounds on the melting-pot walls gets thick. Then the heat transfer conditions worsen, the layer of copper compounds gets partially melted and copper enters the melted load, thus conta minati ng it. It is possi ble to mi nimise or eli minate the tra nsfer of the melting-pot materia l into the melted load when the melti ng zone is consta ntly moved upwards in relation to the melting-pot. I n this case the copper compounds as well as said active dopants coming from original components of the raw material input, form a thin layer over the melting-pot walls and do not affect the heat transfer conditions of the melting-pot, which in turn limits or eliminates the transfer of melting-pot material to the melted load. The best way of shifting of the melting zone of initial components upwards in relation to the cold melting-pot walls it is to use a cold melting-pot with fixed bottom, where both the walls and the bottom are joined together and made as one piece of material. The aforementioned processes of copper transfer from a melting-pot to the product of synthesis are also known to happen for other metals of which the cold melting-pots are made.

It is known that during the melting process in open air atmosphere some oxide materials dissolve oxygen from air in their melted mass. Aluminium oxide is a particular example of such a material. As the result of this process, during the crystallization of the melted load of aluminium oxide, there appear pores in the polycrystalline block and its volume density can then be lower than 70% of the theoretical density of a crystal. In case of melting in the open air atmosphere, it is required to optimise technological conditions of the liquid phase synthesis of the polycrystal line block, so that its vol ume density does not become lower than 75% of the theoretical crystal density. However, when the polycrystalline block is introduced into a container, oxygen comes inside too, together with the block. This results in oxidising the container material, in increasing the amount of contaminants in the crystal and in lowering the crystal quality. In order to save the polycrystalline block of aluminium oxide (corundum) from such contamination, its liquid phase synthesis is performed in vacuum conditions, together with its shaping. Apart from eliminating the oxygen from the polycrystalline block of corundum, the melted mass is at the same time being cleaned from easily evaporating dopants which had been present in the original components. This additionally increases the quality of the raw material input.

The core effect of the invention lies in the fact that it is ensured that the raw material input used for growing of crystals is prepared in the optimal way. The raw material input in the process of liquid phase synthesis is transformed into a homogeneous form of polycrystalline block, with the dimensions corresponding to the dimensions of the container, as well as with the minimum amount of contaminants introduced during the process of synthesis and shaping. Such a preparation of the raw material guarantees production of homogeneous high-quality crystals in the large-scale industrial processing.

The invention is presented in a photograph in the attached drawing. The photograph shows polycrystalline blocks of corundum (aluminium oxide) with the diameter of 148 mm, intended to be used as the raw material input within a tungsten container having the diameter of 200 mm in the Kyropoulos-Musatov process for growing sapphire crystals.

Preferred embodiments of the invention

Example 1. Production of the raw material input for the Kyropoulos-Musatov process of growing the crystals of sapphire on a crystallization seed from melted load in a container.

The crystal growing device is equipped with a tungsten container having the diameter of 200 mm. The inner chamber of the container is in the shape of truncated cone. Its smaller diameter at the bottom of the container is 150 mm, its greater diameter is 155 mm, the working depth is 220 mm and the maximum depth is 240 mm. The characteristic cross- section shape of the container: circle. The characteristic dimensions: diameter at the bottom: 150 mm, depth: 240 mm. The optimum raw material mass lies between 12 and 12.2 kg. I n order to prod uce the raw materia l i n put of the best pa ra meters a nd with the maximum possible elimination of contaminants, its synthesis should be done in such a way that the block has dimensions corresponding to those of the container, i.e. it should be a melted cylindrical block having the diameter of 145-149 mm, the height of no more than 240 mm and the mass of 12 ± 0.1kg. The raw material input is synthesised from the initial component, being the aluminium oxide powder of high chemical purity, freely available in the market, branded SPA-AC and produced by Sasol North America INC Ceralox Division with the γ and a phases present. The synthesis is done through induction melting in a cold melting-pot. The full- bottom melting-pot is used, made of cooled copper tubes welded to the bottom, which in turn is sim ultaneously used as a cooling water separator. The melting zone of the cold melting-pot is com pa ra ble to the cha racteristic di mensions of the conta i ne r, i .e . the diameter of its circular cross-section is 150 mm. The working depth of the cold melting-pot is 300 mm.

The synthesis of the raw material input in the form of polycrystalline shape is done in the open air environment in the following way. A portion of the oxide powder with the mass of 12.5 kg is taken. A part of this portion placed in the cold melting-pot for initial heating, for example though creating an exothermic reaction of oxidising the shavings of metallic aluminium of high chemical purity. As the result of initial heating of the aluminium oxide, the initial melted mass load is formed, which is subsequently heated in the electromagnetic field of the inductor and occupies the whole cross-section of the cold melting-pot. The temperature of the melted mass lies within the range of 2100-2150°C. Next, the induction melting of the aluminium oxide is performed by introduction of the aluminium oxide powder to the melting zone of the cold melting-pot. I n the course of melting, the melting zone is shifted upwards in relation to the melting-pot. At the sa me time in the lower part of the melted mass the crystallization process of the melted mass as the corundum phase occurs and the polycrystalline block is formed, with the transversal cross-section corresponding to the cross-section of the cold melting-pot. I n this way the synthesis of the polycrystalline block is done and the block itself is formed into the shape of the container. After the whole portion of the aluminium oxide powder has been added, the induction melting process is finished and the melted mass gets crystallized.

As the result of the synthesis of the raw material input, a melted polycrystalline corundum block is produced, with the mass of 12.1 kg, cylindrical cross-section having the diameter of 148 mm and the height of 235 mm. The volume density of the polycrystalline corundum block is 3.0 g/cm³ , which is 76% of the theoretical density of the sapphire crystal. This polycrystalline block can be easily inserted into the container, ensuring at the same time the optimum filling of the container in a single operation. The attached photograph presents two identical polycrystalline corundum blocks with the diameter of 148 mm produced by the process described above. After the polycrystalline block has been introduced into the container and melted, the melted mass of the aluminium oxide is on the optimal level. The sapphire crystals grown of such a synthesised polycrystalline block are of high quality. They are transparent to the ultraviolet radiation, which qualifies them as crystals having minimum amount of contaminants.

Table 1 presents the results of the analysis of chemical composition and the amount of contaminants in source aluminium oxide powder as well as in two polycrystalline blocks of corundum produced - through the induction melting synthesis in a cold melting-pot having a full bottom - in two production cycles of the raw material input. As it can be seen, the raw material input has not been contaminated with copper from the cold melting-pot.

Example 2. Synthesis of the raw material input for the Kyropoulos-Musatov process of growing the single crystal of sapphire in a device having a tungsten container with the diameter of 300 mm.

The characteristic shape of the transversal cross-section of the container is the same as in the Example 1: cicular shape, diameter at the bottom: 260 mm, working depth: 260-280 mm. Optimum weight of the raw material input lies within the range of 60 to 66 kg.

The synthesis is performed using the induction melting process in a cold melting-pot. The melting-pot is of the same type as described in the Example 1, however the dimensions of the melting zone of the cold melting-pot correspond to the characteristic dimensions of the container, i.e. the diameter of this zone is 254 mm and its depth 450 mm.

In order to raise the quality of the polycrystalline block, the synthesis of the polycrystalline block is performed in vacuum. A portion of aluminium oxide powder, identical as in the Example 1, and having the same composition as in the Example 1, having the mass of 10 kg, is introduced into the cold melting-pot located in the vacuum working chamber, for performing the initial heating process. The necessary amount of the aluminium oxide is introduced into a tray, linked with the working chamber through a feeder. The air is pumped out from the working chamber and the tray until the residual pressure of lxl0^"4tor is achieved. Now the induction heating is turned on and the initial melting of the aluminium oxide powder is performed by a known process. The initial melted mass load is formed, which is then heated in the electromagnetic field of the inductor and which occupies the entire transversal cross-section of the cold melting-pot. Next, the induction melting of the oxide takes place by dosing the aluminium powder from the tray into the melting zone of the cold melting-pot. As the melting proceeds, the melting zone is shifted upwards in relation to the melting-pot. Crystallization of the melted mass occurs and q polycrystalline corundum block is formed, having the dimensions similar to the dimensions of the container, as in the Example 1. After the melting process has been finished and the synthesised polycrystalline block has cooled down, the working chamber is opened and the block is taken out. The block produced has the same dimensions as the container. Usually the mass of the synthesised block is in the range of 58-60 kg. The volume density of the block is 95% to 97% of the theoretical density of the sapphire crystal.

The crystals of sapphire produced with the use of raw material input having the form of polycrystalline corundum block obtained in the process of melting in vacuum conditions, have a higher quality than the ones grown on the raw material input that was produced by melting in open air conditions. The crystals grown as described in Example 2 have lower density of crystalline lattice defects, lower amount of contaminants and ensure higher productivity of the ready-made product that can be used for further processing.

Example 3. Synthesis of the raw material input for the horizontally-directed crystallization process (Bagdasarov process) of growing the yttrium-aluminium garnet crystals.

The device is equipped with a container of the "boat" shape, having the rectangular shape, having the dimensions of 40 mm of height and 90 mm of width. The length of the container is 250 mm. The crystallization seed is placed in the apex of the triangular part. The container is being moved in the horizontal direction with respect to a heater, made in the form of a rectangular tungsten coil surrounding the container and connected to the source of electrical current. The whole heat source is placed in a vacuum chamber. The synthesis of the raw material input is done by a process of induction melting in a cold melting-pot of the stoichiometric mixture of initial components: aluminium oxide powder and yttrium oxide powder. The cold melting-pot is produced with the full bottom and its transversal cross-section has di mensions com para ble to the dimensions of the container's cross-section, i.e. in this case it is a rectangle with the dimensions of 40 mm x 90 m m. I nitia l heating of the stoichiometric mixture is conducted i n course of isotherma l reaction to oxidation of meta llic yttrium . I n order to com pensate for the cha nge in the composition of the initial melted mass load, a pre-calculated amount of aluminium oxide is introduced into the mass load. The technological process of synthesis of the raw material input is performed as described in Example 1, however at the melted mass temperature of 2150-2200°C. Simulta neously with the synthesis of the crysta lline structure of ga rnet, a rectangular polycrystalline block is formed, with the di mensions correspondi ng to the characteristic dimensions of the container. The synthesis is terminated when the length of the block is equal to 180 m m. The structure of the synthesised polycrystalline block is a monophase structure of aluminium-yttrium garnet. Its volume density is about 90% of the theoretical crystal density. The polycrystalline block has dimensions corresponding to the di mensions of the conta i ner (the "boat) which a l lows to i ntrod uce the synthesized polycrystalline block into the container without any additiona l treatment. If needed, the tria ngu la r pa rt of the "boat" ca n be loaded with waste coming from treatment of the previously grown crystals, in order to ensure the optimal mass of the raw material input. The high quality of the raw input guarantees obtaining of high-quality single crystals.

Table 1

The amount of contaminants in two raw inputs of the initial aluminium oxide and in the produced polycrystalline blocks.

List of references

1. Avtorskoe svidetelstvo 1329208 (SSSR). Sposob vyrashchivaniya tugoplavkikh monokristalov. MPK S W 17/00.

2. Parfitt H.T., Robetson D.S. Domain structures in lithium niobate crystals./British J.

Applied Physics. Vol. 18 (1967), pp. 1709-1713.

3. Fedulov A.S., Shaporo Z.I., Lodyzhinskij I. B. Primenenie metoda Chokhralskogo dla vyrashchivaniya monokristallov LiNb0₃, LiTa0₃, NaNb0₃. A N S S S R . Kristallografiya, t.lO, No.2, 1965, pp. 268-270.

4. US Patent No 7381266 Bl. Int. CI. C03B 25/12. Sapphire crystal growth method.

Patent date: 03.01.2008.

5. Observation de I'effet laser con tinu dans I'aluminate Lao^Ndo MgAlu w monocristallin (LNA) elabore par la methode Crochralski./D. Vivien, A.M . Lejus,/ C.R. Acad. Sc. Paris. T.298, Serie II, Ne6, 1984, p. 195 - 198.

Claims

1. A raw material input for manufacturing oxide crystals on a crystallization seed from me lted load i n a conta i ner, that had bee n i nitia l ly synthesised from components to the form of a polycrystalline block, characterized in that said raw material input has the form of a melted polycrystalline block with the dimensions corresponding to the cha racteristic dimensions of the container a nd has the volume density not lower than 75% of the theoretical density of said oxide crystal and has the mass not lower than 75% of the full container load.

2. A method of production of the raw materia l in put for ma nufactu ri ng oxide crystals on a crystallization seed from melted load in a container, consisting in liquid phase synthesis of the initial components by induction melting in a cold melting-pot, to form a melted polycrystalline block, characterized in that said liquid phase synthesis process is performed simulta neously with forming the polycrystalline block having the dimensions corresponding to the characteristic dimensions of the container.

3. The method according to claim 2, characterized in that said liquid phase synthesis process is performed while shifting the melting zone of the initial components upwards in relation to the cold melting-pot, wherein a cold melting-pot is used having a full bottom and having the shape and dimensions of the transversa l cross-section corresponding to the characteristic sha pe and dimensions of the transversal cross-section of the container.

4. The method according to claim 2 or 3, characterized in that said liquid phase synthesis process is performed in vacuum.

5. The method according to claim 2, 3 or 4, characterized in that said oxide crystal is sapphire or yttrium-aluminium garnet.