RU2419689C2 - Apparatus for growing sapphire monocrystals - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: apparatus has a vacuum chamber 1, an electric heater 4, a crucible for melting raw material 6, inside the heater 4, a holder 7 and heat shields 8, lying between the electric heater 4 and water-cooled walls 1a, 1b of the vacuum chamber 1. Part of the shields 8 has a mirror surface facing the heater, which lies in a sealed cavity formed by welding butt-ends of cowlings of neighbouring shielding sheets in spaces between which separators are fitted and that cavity is then evacuated.

EFFECT: maximum coefficient of reflection of heat flux, low heat loss, improved parametres of the temperature field and quality of articles, low electrical energy and power of the heaters.

8 cl, 6 dwg

Description

The invention relates to vacuum furnaces for growing sapphire single crystals and other similar materials, and can also be used for other high-temperature technological operations in vacuum.

Usually in vacuum furnaces, where radiation is the main type of heat transfer, screens made of metal sheets are used as thermal insulation / 1 /.

A device for growing sapphire single crystals, containing a vacuum chamber, an electric heater inside which is installed a crucible, a reflector of sheets and rods of molybdenum and tungsten located around the heater, as well as thermal screens of sheets of molybdenum, installed between the reflector and the walls of the chamber / 2 /.

The disadvantage of this device is that the sheets-screens, with which thermal insulation of the walls of the vacuum chamber is carried out, have a low reflectivity, due to which a significant amount of heat is transferred to the walls of the chamber. Therefore, the chamber has to be made water-cooled, consisting of the inner and outer walls, and a significant amount of heat is carried away with water cooling the inner wall of the chamber. For the same reason, it is necessary to use a large number of sheet screens (usually closed cylinders) or a multilayer roll screen, high-power heaters (up to 100 kW) and spend a significant amount of electricity on melting raw materials and growing a single crystal. In addition, a significant amount of energy is consumed by devices that cool the water discharged from the furnace for its multi-turn use.

Another disadvantage of the prototype design is the difficulty of creating such a temperature field inside the chamber, which provides the beginning of the formation of a single crystal in the center and in the upper part of the crucible, so that the outer layers of the single crystal farthest from the walls of the crucible crystallize last. This nature of the temperature field makes it possible to remove impurities and possible defects of the crystal lattice onto the surface of the single crystal, from where they can be easily removed, and obtain a single crystal of high quality and the required shape.

Attempts to use sheets with a mirror surface that returns almost all the radiant energy incident on it for the reflector and screens in known devices have been ineffective due to oxidation and contamination of the mirror surfaces when air enters the chamber when loading the charge and unloading the products. In addition, a decrease in the reflectivity of the screens was due to the interaction at high temperature of the surface of the screens with the emissions of ceramic reflectors commonly used in the design of such furnaces, atmospheric residues, and also with inert gas impurities during cooling of the obtained single crystal.

The objective of the invention is to reduce heat loss and energy consumption for the production of single crystals, improving their quality, reducing the power of the heater and the dimensions of the device.

This problem is solved due to the fact that in the known device for growing sapphire single crystals, including a vacuum chamber, an electric heater, a crucible for melting the feedstock for the grown single crystal, a reflector mounted outside the heater, and also screen sheets located between the reflector and the chamber walls, part of the screens located behind the reflector near the internal water-cooled walls of the chamber are made of stainless steel sheets. What is new is that at least two consecutive sheet-screens are connected at the edges by welds with the formation of a sealed cavity between them, which communicates with the channel for preliminary evacuation. In this case, the surface of the sheets facing the center of the chamber inside this sealed internal cavity is made mirrored.

In addition, the gaps between the welded sheet-screens are equipped with distance elements that prevent the sheets from closing under the influence of atmospheric pressure and the transfer of significant heat flux through the mutual contact surfaces of adjacent screens.

In a modification of the invention, the sheet-screen with the mirror surface farthest from the heater is simultaneously the inner wall of the chamber.

In addition, the distance elements along the edges of the sheets can be made in the form of flanges on one of the sheets, and in another modification of the invention, in the form of wire coils installed between the sheets.

Throughout the rest of the surface of the sheet-screens, the distance elements are made in the form of periodically repeating protrusions on the sheets that prevent them from closing.

In addition, the channel for evacuating the airtight cavity formed by the sheets-screens welded together is made in the form of a tube, one end of which is welded with these sheets, and the other end of the tube has a weld formed after evacuation by flattening and welding of the tube walls.

In another modification of the invention, the channel for evacuating the airtight cavity formed by the sheets-screens welded together is equipped with a shut-off valve that can be connected to a vacuum pump.

When conducting patent research from the prior art, no solutions are identified that are identical to the claimed, and therefore, the claimed invention meets the condition of patentability “novelty”.

The information set forth in the application materials is sufficient for the practical application of the invention

The invention is illustrated by drawings, in which:

figure 1 presents a section along the axis of the working chamber of the device;

figure 2 presents on an enlarged scale the place And in figure 1;

figure 3 presents on an enlarged scale the place In figure 1;

figure 4 is an enlarged view of the place C in figure 1;

figure 5 presents the same place before evacuation formed by sheets of screens of a sealed cavity;

figure 6 presents on an enlarged scale the place D in figure 1.

A device for growing sapphire single crystals includes (Fig. 1) a vacuum working chamber 1 with water-cooled inner 1a and outer 1b walls, which is closed on top by large 2 and small 3 covers, and an electric heater 4 from tungsten rods. A crucible 6 is installed inside the heater on the stand 5, and a reflector 7 is installed around the heater, which receives the heat flux of maximum intensity emitted by the heater. The reflector is made of refractory ceramic materials, for example zirconia or tungsten-molybdenum compositions.

Behind the reflector are metal sheets-screens 8 of molybdenum and stainless steel (more distant). Screens can be either in the form of separate shells or in the form of sheets rolled into a spiral. Since in a vacuum heat transfer is carried out by radiant heat transfer, the number of sheet screens or the number of turns of sheets rolled into a spiral is determined by the magnitude of the emission and absorption coefficients of the surface of the sheet screens that perform the function of thermal insulation. Theoretically, a screen having a reflection coefficient close to unity can minimize the size of the chamber, the consumption of expensive refractory materials, and heat loss. However, in practice, up to 20 sheets of screens have to be used, and a significant amount of radiant energy falls on the water-cooled walls of the chamber and is carried away with water into heat exchangers, where additional energy has to be used to cool the water and reuse it. This is due to oxidation and contamination of the surface of the sheet-screens during loading and unloading of products, interaction with the excretion of reflectors and impurities of argon, which is used to accelerate the cooling of the thermal unit and the grown crystal after the end of the growing process. In addition, the instability of the absorption and reflection coefficients of the sheet-screens affects the instability of the thermal field, the solidification process of the single crystal and its quality.

To eliminate these shortcomings facing the center of the camera 1, the surface 9-11 of the device screens (figure 2) are made mirrored. In this case, the screen furthest from the center of the camera is aligned with the inner wall 1a of the camera. A screen 8a is installed in front of the camera’s center in front of it, having spacing elements at the ends, made in the form of a flange 12, and over the rest of the sheet surface, in the form of periodically repeating protrusions 13. The presence of protrusions allows the use of screens of very thin sheets without fear of them deformation and abutment under atmospheric pressure. The spherical shape of the protrusions provides point contact of the sheets and minimal heat transfer by thermal conductivity. A slightly larger amount of heat transfer between adjacent sheets of screens will be in the case of the protrusions of a different shape, however, in this case, greater rigidity and manufacturability of the manufacture of sheet screens can be provided.

The inner wall of the chamber 1a may be made of a thick-walled stainless sheet (4-6 mm) with a mirror surface facing the center of the chamber, or lined with a mirror foil or a thin-walled mirror sheet.

The mirror surfaces 9 and 10 of the screens 8b and 8c facing the center of the chamber are located inside the sealed chamber and are separated from each other by distance elements in the form of a wire 14 and spherical protrusions. In this case, the screen sheets 8b, 8c and 8d are connected by welding to form a sealed cavity 15 between the sheets, which communicates with the channel for preliminary evacuation, made in the form of a tube 16 (Figs. 4 and 5). At the welding site, the tube 16 is flattened and has an oval section adjacent to the ends of the sheets 8b and 8d, and a gap is made in the remote elements 14 to connect the sealed cavity 15 with the inner channel of the tube. Between the flat screens 17a and 17b, which serve to insulate the water-cooled bottom 18 of the chamber (FIGS. 2 and 3), distance elements are installed in the form of wire rings 19a and 19b at the edges of the sheets and protrusions 20a and 20b in the gap between them. The screens 17a and 17b are welded together with the formation of a sealed cavity 21 between them, which communicates with the channel for its preliminary evacuation, similar to the tube 16 (not shown), or with the cavity 15. Through these channels, the sealed cavities 15 and 21 are pumped out during the manufacture of the installation air using a vacuum pump connected to the tube by a hose. This ensures the absence of airtight cavities of convective heat transfer and the preservation of minimum values of the absorption coefficients of the heat flux at the mirror surfaces of the screens. The surface of the screen 17b facing the heater is also mirrored. The weld seam 22, which ensures the tightness of the cavity 15, is made, for example, by diffusion or resistance welding by heating and compression at the end of the evacuation of the tube 16 connected to the pump. An additional seam 23 can be obtained by cutting a flattened tube with fusion and welding of the cutting line or additionally applied for reliability on the end of the tube 16 cut off after diffusion or resistance welding.

The vertical screen sheets 8b, 8c and 8d, together with the horizontal sheets 17a and 17b welded with them, form a container 24, inside which other screen sheets 8 and a reflector 7 are placed. In the upper part of this container, lifting grips 25 are welded to the vertical sheets for lifting and Installing screens in the camera.

In the embodiment of the invention (Fig.6), the sealed cavity 26 is connected by a channel 27 made in the flange of the chamber 1 of the furnace with the highway 28 and the valve 29 (Fig.1). A vacuum sensor 30 is installed on the line 28, which makes it possible to control the presence of vacuum in the cavity 26. If necessary, the cavity 26 can be connected through a flexible hose and valve 29 to the vacuum pump 31 for preliminary pumping of air from it before starting the installation, as well as in case of increasing pressure in the cavity 26 due to leakage through welds or pipe joints. A similar solution can be used for the container 24. It is also possible to use for evacuation of the cavity 26 of the vacuum system used to pump air from the working chamber.

When the device is operating, the feedstock is loaded through an open small cover 3 into a crucible 6, after which the small cover is closed and air is pumped out of the inner cavity of the chamber 1 by vacuum pumps through a vacuum line (not shown). After reaching a pressure of 10 ^-1 -10 ^-3 Pa, a voltage is applied to the heater 4 through the water-cooled current leads 32 installed in the large cover 2. Under the action of the current passing through the heater, the tungsten rods are gradually heated to a temperature of 2100-2200 ° C. Due to the presence of mirror surfaces 9-11, almost all the thermal radiation of the heated tungsten rods in the radial direction by the reflector 7 and screens 8 is directed to the crucible and goes to heat the crucible and melt the charge. A similar effect is achieved in the axial direction of the mirror surface of the screen 17b facing the heater. This reduces the energy consumption, allows the use of heaters of lower power and reduces heat loss through the water-cooled side walls and the bottom of the chamber 1.

After the charge is melted using a water-cooled rod 33, a seed (sapphire crystal plate) is lowered into the melt and the heater current is reduced. Moreover, due to the presence of mirror screens and a large number of tungsten rods, a temperature field is formed under the crucible with minimum temperatures on the axis of the crucible in the zone of lowering the seed, where the crystal begins to grow. Then, as the temperature decreases further and the seed rises, all lower central and bottom layers crystallize, where the heat transfer by heat conductivity through the support 5 is large, and the formed crystal is lifted by the rod through the seed over the crucible walls. Thanks to the screens, the sections of the forming single crystal, which are the most distant from the center in the radial direction, cool down most slowly, due to which the less refractory impurities of the initial charge remain in these areas and can then be easily removed.

To accelerate the cooling process of the obtained single crystal, an inert gas (usually argon) is introduced into the chamber through a shut-off valve mounted on a vacuum system (not shown). As a result, convective heat transfer becomes the predominant type of heat transfer instead of radiation, and the presence of mirror surfaces of the screens does not significantly affect the cooling time and furnace productivity. After cooling, the small furnace lid is opened and the crystal is removed from the furnace, then the cycle repeats.

The described technical solution can be used both in new and in existing devices for growing crystals. The number of sheet-screens with a mirror surface used for thermal insulation placed inside sealed vacuum cavities is determined by the specific conditions of use of the device and technological parameters. The container 24, formed by screens with mirror surfaces located in an airtight cavity, can be used both in combination with a screen with a mirror surface aligned with the chamber wall, and independently of it, and vice versa.

The invention can also be used to obtain single crystals having a shape different from the most common cylindrical. In this case, it is possible to obtain the desired configuration of the temperature field by changing the reflection coefficient located in the sealed cavities of the mirror surfaces of the screens in accordance with the desired shape of the single crystal.

Information sources

1. RF patent No. 2338140. IPC F27B 5/04 (2006.01).

2. RF patent No. 2227821. IPC С30В 15/34, С30В 15/14 (prototype).

Claims (8)

1. A device for growing sapphire single crystals, comprising a vacuum chamber, an electric heater, a crucible for melting raw materials, a reflector and heat shields from gap-separated metal sheets, characterized in that at least two consecutive metal sheet-screens are connected along the edges by welds with the formation between them of a sealed cavity, which is connected to the channel for evacuation, facing the axis of the chamber, the surfaces of the sheets inside the sealed cavity are mirrored, and between heel between screens sheets are provided with spacers. 2. The device according to claim 1, characterized in that the distance elements at the ends of the sheets are made in the form of a flanging. 3. The device according to claim 1, characterized in that the distance elements at the ends of the sheets are made in the form of a wire. 4. The device according to claim 1, characterized in that the distance elements are made in the form of periodically repeating protrusions on the sheets-screens. 5. The device according to claim 1, characterized in that the sheet-screen with the mirror surface most remote from the heater is simultaneously the inner wall of the vacuum chamber. 6. The device according to claim 1, characterized in that the channel for evacuating the airtight cavity between the sheet screens is made in the form of a tube, one end of which is welded with sheets forming the airtight cavity, and the other end of the tube has a weld formed after evacuation by flattening and welding the walls of the tube. 7. The device according to any one of paragraphs.2, 3, 4, characterized in that the vertical sheets of screens forming an airtight cavity are equipped with lifting grippers and welded with horizontal sheets of screens so that they form a container for loading and unloading from a vacuum reflector cameras and sheet screens. 8. The device according to claim 5, characterized in that the channel for evacuating the airtight cavity between the sheet-screens is equipped with a shut-off valve installed with the ability to connect to a vacuum pump.

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