Abstract:
Apparatus for the producing workpieces and blocks from meltable materials in which a liquid starting material is solidified in a directional manner in a casting mold using a cooling device. For the controlled guiding of the solidification front during cooling of the molten material a cooling structure having a plurality of heat conducting bodies is introduced from below into a corresponding recesses of a body assigned to the bottom of the casting mold.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for manufacturing workpieces or blocks from meltable materials, where the material, starting as a liquid is solidified in a directed manner in a casting mold by using a cooling device. 
     The invention additionally relates to a device for manufacturing workpieces or blocks from meltable materials using a casting mold that is heatable using a heating device and where a cooling device is assigned to the base of the casting mold. 
     Subsumed under the term “meltable materials”, as it is used here, are materials made of ceramics, including sapphires, rubies, spinels, etc., metals, metal alloys, or materials from the group of semiconductors with an oriented multi-crystalline or mono-crystalline structure. 
     With such methods as relate to the invention, as well as the respective devices, the starting material is either provided to the casting mold in the liquid phase or is melted in the casting mold and thereafter solidified in a directed manner in the casting mold. 
     Such a type of solidification guiding is known in various prior known methods. According to one method, or the corresponding device, respectively, described in GB-A-2 279 585, the casting mold with the melt is pulled downwards out of a heating furnace. This results in the solidification front progressing from bottom to top. For long components or materials with a low thermal conductivity, the influence of an applied cooling plate becomes insignificant after just a few centimeters. After that, heat removal occurs basically on the sides via the chill surface, which in practical applications does not enable setting an essentially plane phase boundary between already solidified and liquid melt material. This method is not suitable for manufacturing large-surface blocks solidified in a directed manner because with large cross-sections, the heat conducting paths from the center of the block to the heat removing surfaces on the sides are too long making it impossible to achieve plane phase boundaries in connection with sufficiently high temperature gradients. 
     In the Technical Digest of the International PVSEC-9, Miyazaki, Japan 1996, Ritsua Kawamura et al. show in “Recent Progress in Electromagnetic Casting for Polycrystalline Silicone Ingots” that the phase boundary between solid and liquid silicon has a significant concave shape. Parallel radial crystalline structures cannot be achieved using this method. The maximum block size is described as 22 cm×22 cm. 
     Larger silicon blocks solidified in a directed manner are manufactured according to the state-of-the-art in blocks of 66 cm×66 cm and a height of 2.5 cm using the HEM Method (Heat Exchanger Method). With the HEM Method, according to the state-of-the-art, the energy required to maintain the solidification speed and the temperature gradient is removed across a central area of the chill bottom. With a constant temperature of the heater located above the melt surface, the heat transfer coefficient between the base of the chill and the cooling plate essentially determines the heat removal flow, and thus, the speed of growth of the crystalline block. 
     SUMMARY OF THE INVENTION 
     It is the objective of the present invention, beginning with the state-of-the-art described above, to develop a method and device with the features mentioned above, such that the solidification of the melt can be guided in a defined manner and where the initiation of the cooling phase can be continuously advanced from the heating phase to the cooling phase. In addition, the device and the method shall offer a broad variation with simple means of design with regard to this definably guided solidification in a defined manner. 
     This objective is accomplished with the above mentioned method such that a cooling structure with at least one thermal conductor is inserted in a least one corresponding recess from the bottom side into the body assigned to the base of the casting mold in order to accomplish a defined guiding of the solidification front during the cooling phase of the molten material. 
     With regard to a device, the objective is accomplished in that the above mentioned device is characterized in that the cooling device contains a cooling structure with at least one thermal conductor that can be inserted from the bottom into at least one corresponding recess in a body assigned to the base using a sliding mechanism. 
     Using the described method and device, the solidification of the material starting as a liquid that was poured into the casting mold can be carried out in a defined guided manner beginning at the base of the casting mold by inserting the thermal conductor in different positions into the recess of the body assigned to the base of the casting mold. By adjusting the at least one thermal conductor in the at least one recess assigned to this body, the heat transfer, and thus the cooling performance, can be set in a defined manner and can be changed as well. Through the respective geometry of the thermal conductor and its corresponding recess, it is additionally possible to influence the solidification front that moves from the base upwards. Depending on the number of thermal conductors and the corresponding recesses of the used cooling structure, crystallization speeds of 0.2 mm/min to 2 mm/min can be achieved with cooling performances in a range from 10 to 150 kW per m 2 . 
     In order to change the amount of removed heat per unit of time in addition to adjusting the thermal conductor in the corresponding recess, it may also be advantageous to maintain a gaseous atmosphere with an adjustable pressure surrounding the cooling structure. By lowering the gas pressure to a few mbar, the performance density can be controlled more sensitively. Furthermore, a gaseous atmosphere of Argon should be maintained around the heat conducting body in such a case, where continued purging is carried out with such a gas because, especially with Argon, additional contamination can be removed from the heating space. 
     As already indicated, the cooling structure may contain several thermal conductors that can be inserted in slots and/or blind holes of the body that is assigned to the base of the casting mold. Suitable thermal conductors are plates, bolts and/or bars that may additionally be designed with different cross-sectional geometries. In an embodiment that should be emphasized especially, a heating device is positioned underneath the base of the casting mold such that the one or more thermal conductors in the inserted position penetrate through the heating device into the body that is assigned to the bottom side of the base. With such an arrangement, the transition between heating and cooling of the casting mold can not only be determined by the insertion of the thermal conductors into the recess(es), but also through additional control of the heating device, since it also essential to heat the base of the casting mold for maintaining the liquid phase of the starting material. For this purpose, the heating device may be placed in a carrier plate that is assigned to the base of the casting mold and that carries the casting mold. The carrier plate is then provided with holes or recesses that serve the purpose of changing the outer surface that is available overall for heat transfer in a broader range than would be possible with the bottom surface of the base of the casting mold alone. 
     Preferred dimensions of such thermal conductors are diameters or thicknesses and/or width of 5 mm to 20 mm, preferably of 10 mm to 14 mm. Furthermore, the width of the web remaining between neighboring recesses in the body in which the thermal conductors are inserted should be between 5 and 20 mm. Furthermore, the insertion depth of the thermal conductors into the body should be at least 20 mm to be able to adjust the cooling performance over a sufficient range. However, the individual thermal conductors may have a significantly greater length than corresponds to an insertion depth of 50 mm, that is, the height of the thermal conductor may be between 100 and 150 mm, preferably about 130 mm. 
     In the simplest case, the thermal conductors are designed as round pins. For reasons of stability, the diameter of such a thermal conductor in its design as a round bolt should not be selected to be less than 10 mm. The ratio between the effective exchange surface and the flat surface with a remaining web width of 10 mm with a bolt diameter range of 10 mm to 20 mm is practically independent of the selected bolt diameter. To further increase the cooling performance, the individual thermal conductors may have a cross section in the shape of a cross or star. Such thermal conductors then enter recesses in the body assigned to the base of the casting mold where the shape of the cross-section of said recesses is adapted to the shape of the element, such that large areas are provided both in the recesses and on the cooling elements. To have as large a range of cooling performance as possible, with the cooling power being adjustable, the ratio of the sum of the cross-sectional areas of the thermal conductors to the sum of the cross-sectional areas of the recesses should be between 1.5:1 and 5.5:1. This results in possible cooling powers of about 10 to 150 kW/m 2 . 
     The movement of the thermal conductors in the recesses of the body that is assigned to the casting mold can be easily realized, technically, with a lifting mechanism. With a stroke of 50 mm and a thermal conductor made of copper with a diameter of 12 mm and an effective height of the thermal conductor of 130 mm and a hole distance of 26 mm and a hole diameter of 14 mm, a heat transfer coefficient of about 10 W/(m 2 ×K) to about 240 W/(m 2 ×K) can be set for a 1000 mbar Argon atmosphere between carrier plate and thermal conductor at a carrier plate temperature of 1400° C. These values correspond to about 1400 to 1500 thermal conductors per square meter. 
     The thermal loss through the thermal insulation is negligible due to the small ratio of diameter to hole length, such that, with a retracted cooling structure, the thermal losses through the open penetrations are tolerable. 
     It is further possible to control the removed energy density even more sensitively by reducing the gas pressure to a few mbar. For this purpose, the entire cooling structure can be arranged in a chamber with an adjustable pressure. For effective heat removal, it is particularly advantageous if the body is an integral part of the base of the casting mold and furthermore, if this base is textured, for example with elevations and depressions, where the respective thermal conductors are moved into or are retracted from the bottom in respective holes in the elevations of the base of the casting mold. 
     As already mentioned above, the device subject to the invention enables setting a temperature profile directly above the chill or casting mold base surface. Through this special design of the chill or casting mold base, the radial crystallization can be influenced in the area of the depressions, viewed from the base area. The lowest points of these individual depressions are aligned with the respective thermal conductors such that crystallization starts at the lowest (coldest) points of the chill base. In this manner, a slightly planar or slightly convex phase boundary between solid and liquid material can be set intentionally to achieve a certain objective, for example, to initiate a thermal convection. Research has shown that, especially when taking the objective of cleaning into account, a slightly curved phase boundary is advantageous in directed (controlled) solidification. 
     For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-section of a melt device according to the invention, where the cooling structure is presented with thermal conductors retracted from the recesses. 
     FIG. 2 shows the device presented in FIG. 1, however with the thermal conductors inserted in the recesses. 
     FIGS. 3A to  3 C show three different possible shapes of cross-sections for the thermal conductors as can be employed in the devices according to FIGS. 1 and 2. 
     FIG. 4 shows the schematic design of a device, where the thermal conductors can be moved in recesses that are formed directly in the base of the casting mold and where furthermore the base of the casting mold is textured. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to FIGS. 1-4 of the drawings. Identical elements in the various figures are identified by the same reference numerals. 
     As FIGS. 1 and 2 show, the melt device consists of an oven with an upper oven chamber la and a lower oven chamber  1   b , in which a casting mold or a chill mold  15 , provided on its outer side with thermal insulation  2 , is held by suitable supports  7 . The thermal insulation  2  is provided with side thermal padding  14 , bottom thermal padding  16  and top thermal padding  20  such that the chill is surrounded on all sides with this thermal insulation  2 . The upper oven chamber  1   a  is connected to the lower oven chamber  1   b  with a flange connector  12 , which contains a gasket  12   a , such that the oven chamber  1   a ,  1   b  can be opened by removing the upper oven chamber la and can again be closed tightly. A lower heating device  3  is located underneath the base  19  of the chill  15 . Additionally, an upper heating device  4  is located above the chill. The two heating devices  3  and  4  are supplied with electric current via current supply leads  5  and  6  to enable setting of the respective heating power  3 ,  4 . The space between the upper and the lower heating chamber la and  1   b  and the chill  15 , or the thermal insulation  2  surrounding the chill  15 , respectively, can be evacuated via an evacuation connector  11  to change the pressure within this chamber  1   a ,  1   b.    
     As mentioned above, the chill  15 , or the chill together with the thermal insulation  2 , is kept on supports  7  such that sufficient space is kept between the base of the lower oven chamber  1   b  and the chill base  15 . A cooling structure  26  is located in this area, that is, underneath the base of the chill  15 , where said cooling structure contains a cooling plate  9  from which protrude individual thermal conductors  10  that are at a certain distance from one another. Recesses  17  are assigned to these individual thermal conductors  10 , where said recesses pass through both the lower thermal padding  16  and the carrier plate  13 , which supports the base of the chill  15 . 
     Furthermore, these recesses  17  are aligned in relation to the lower heating device  3  located in the area of the chill carrier plate  13  chill such that they pass between the individual coils of the heating device  3  and reach into the chill carrier plate  13  in the shape of blind holes  13 a. 
     The cooling plate  9  is held by a lifting piston  8  such that it can move up in the direction of the arrow  27  shown in FIG. 1, such that the individual thermal conductors  10  can be inserted into the corresponding recesses  17 . In addition, the lifting piston  8  has a cooling water feed and drain  18  allowing forced cooling of the cooling plate  9 , which has a respective hollow space  28  for the cooling medium. 
     To manufacture a workpiece or a block from a meltable material, the molten liquid material is poured into the casting mold or chill  15 , which has been pre-heated to the melting temperature, or it is melted in the chill. Thereafter, the pouring hole is closed, for example with a lid placed on the chill  15 , and the melt is left alone for a preset time to allow for floating or sedimentation of contaminants. Thereafter, the lower heating device  3  is switched off and the cooling structure  26 , or the thermal conductors  10  assigned to it, are moved into the recesses  17  in the lower thermal padding  16  and the chill carrier plate  13  with a fixed preset speed. Alternatively to a preset speed, the position control of the respective position of the cooling structure  26  in the recesses  17 , or the blind holes  13   a  in the chill carrier plate  13  can be performed in relation to the cooling power to be removed. During the time of cooling with the cooling structure  26 , the cooling medium is continuously forced to the cooling plate  9  via the cooling medium feed and drain pipes  18 . Via the evacuation connector  11 , the oven chamber can be evacuated if required, which is always necessary or advantageous when oxidationsensitive materials are used. 
     FIG. 2 shows the assembly of FIG. 1 with the thermal conductors  10  of the cooling structure  26  completely moved into the chill carrier plate  13 . In this position, the lower heating device  3  is switched off and the upper heating device  4  is still operated and set or controlled to a temperature that keeps the surface of the melt  21  above the melting point. Heat removal required for crystallization is carried out via the already solidified portion of the block  23  and the chill base and from there to the chill carrier plate  13 . From the chill carrier plate  13 , the heat flows via the gap between holes/recesses  17 ,  13   a  and the thermal conductors  10  to the cooling plate  9 , where it is transferred to the cooling medium. It is apparent that the amount of heat to be removed can be set or controlled very sensitively through the insertion depth of the thermal conductors  10  into the chill carrier plate  13 . In this manner, the solidification of the block and the formation of radial crystals can be set and guided very precisely beginning at the base of the chill. 
     After the block  23  is solidified, the cooling structure  26  is moved downwards in the direction of the arrow  24  shown in FIG. 2 such that is completely disengaged from the chill carrier plate  13  as well as the lower thermal padding  16 . Then, the heating temperature of the upper heating device  4  is reduced to a temperature below the solidus temperature. Now, the lower heating device  3  is switched on and its temperature is set to the temperature of the bottom of the block. The heating temperature is increased in a controlled manner to the value of the upper heating device  4 . After temperature equalization in the oven chamber, the temperature in the oven chamber is kept for a preset holding time. Thereafter, the heating temperature of the upper and lower heating devices  3 ,  4  is reduced in a programmed manner. 
     FIGS. 3A to  3 C show three different shapes of cross-sections for thermal conductors  10 , as can be used in the assembly described above using FIGS. 1 and 2. FIG. 3A shows as an example of a field with a total of  9  thermal conductors  10  exhibiting a cross-section in the shape of a cross. The recesses  13   a  in the chill carrier plate  13   a  are, as indicated on the upper right side of FIG. 3A, shaped correspondingly to the cross-section of the thermal conductors  10 , such that a small gap remains between the wall of the recesses  13   a  in the chill carrier plate  13  and the respective inserted thermal conductor  10 . By using this cross shape, the thermal conductors  10  can be provided with large surfaces to achieve a great thermal transfer via these thermal conductors  10 . 
     FIG. 3B shows an arrangement of nine thermal conductors  10 , each with a circular cross-section. Such thermal conductors  10  then enter into recesses  13   a  (not shown) with a corresponding cross-sectional shape, such that again a small gap remains as shown in FIG. 3A. A third cross-sectional shape for the thermal conductors  10  is shown in FIG. 3C, where the cross-sections are in the shape of stars. Using this star shape, an even greater surface area can be achieved than in the arrangement shown in FIG. 3A depending on the number of points or fins. 
     The specific surfaces according to the respective shapes of the cross-sections in FIGS. 3A,  3 B and  3 C should be selected taking into account the temperature, the thermal conductivity, the length of the thermal conductors  10  and the mechanical stability. For example, the thermal conductors  10  should have a thickness and/or width, designated in FIG. 3B with the reference number  30 , of 5 to 20 mm, preferably 10 to 14 mm. Neighboring thermal conductors  10  should be at a distance of about 50 mm, or the width of the web remaining between neighboring thermal conductors, with the width designated in FIG. 3B with reference No.  29 , should be 50 mm. The length, or height, of the thermal conductors, that is, in the direction perpendicular to the drawing plane in FIGS. 3A to  3 C should be in a range of 100 to 150 mm, preferably about 130 mm. 
     As already indicated above, the oven chamber may be filled with gas, preferably Argon, and the pressure in the oven chamber can be controlled during the cooling phase or during the movement of the cooling structure  26  in the direction of the chill carrier plate  13 . The pressure is set such that the entire stroke height of the thermal conductors is utilized to achieve a very sensitive control behavior. 
     FIG. 4 is the schematic presentation of a chill  15  with a thermal insulation  2 . In this embodiment, no particular chill carrier plate  13   a  for holding the base of the chill is present, such as is the case in the devices shown in FIGS. 1 and 2; instead, the base of the chill itself, designated in FIG. 4 with the reference number  33 , is provided with holes or recesses  13   a , where again the respective thermal conductors  10  of the cooling structure  26  enter. Furthermore, the chill base  33 , assigned to the melt, is textured such that individual depressions  25  and elevations  35  are provided with, for example, a triangular cross-section to increase the thermal exchange surface. As is apparent in FIG. 4, here the respective recesses  13   a  are arranged such that they each are assigned to a respective elevation  35  in the texture of the chill base  33 . This texture of the chill base with the depressions  25  is also advantageous to provide starting points for the crystalline growth at the respective depressions. It is clear that the side walls  19  of the chill  15  are tightly connected with the chill base  33 . 
     With the devices presented in FIGS. 1 and 2 as well as in FIG. 4, taking into account the respective cross-sectional shapes of the thermal conductor  10 , cooling powers can be achieved in a range from 10 to 150 kW/m 2  due to different positioning of the thermal conductors  10  in the respective recesses  13   a , such that the respective solidification speed can be set in a defined manner. Furthermore, in a continued development of the device as presented, the individual thermal conductors can be moved differently to one another to remove different amounts of heat at different places of the chill base through different positions in the respective recesses  13   a . For example, in a particular embodiment, the outer thermal conductors  10  can be inserted into the respective recesses  13   a  sooner or later than the thermal conductors  10  closer to the center to adjust the solidification profile or the solidification front; for this purpose, the lifting mechanism, or the lifting piston  8 , respectively, shown in the Figures would have to be divided into several individual lifting pistons assigned to the respective thermal conductors. 
     There has thus been shown and described a novel method and device for manufacturing workpieces or blocks from meltable materials which fulfills all the objects and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is to be limited only by the claims which follow.