Patent Publication Number: US-2013239620-A1

Title: Directional Solidification Furnace Having Movable Insulation System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/534,571 filed on Sep. 14, 2011, the entire disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     This disclosure relates generally to multi-crystalline silicon ingots and, more specifically, to aspects of a directional solidification furnace used in the production of multi-crystalline silicon ingots. 
     BACKGROUND 
     Directional solidification furnaces are used, for example, to produce multi-crystalline silicon ingots. These furnaces have a crucible into which raw poly-crystalline silicon is placed. The crucible is supported by a structure that adds structural rigidity to the crucible. The crucible is disposed within a containment vessel that forms part of the furnace and seals the crucible from the ambient environment. 
     During use, the raw silicon is melted and then cooled at a controlled rate to achieve directional solidification within the resulting ingot. The controlled rate of cooling is established by any combination of reducing the amount of heat applied by the heaters, movement of or opening of insulation surrounding the crucible, and/or the circulation of a cooling medium through a heat exchanger disposed adjacent the crucible and/or the crucible support. The ingot solidifies in the region closest to the cooler side of the crucible and proceeds in a direction away from the cooler side of the crucible. 
     The size of silicon ingots produced in these furnaces has been increasing in order to improve efficiency and reduce the cost required to produce the ingots. However, previous attempts to increase the mass of the silicon ingots over about 600 kg have proved unsuccessful for a variety of reasons. There exists a need for a silicon ingot having greater mass (e.g., greater than about 600 kg) and furnaces capable of producing these larger ingots. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     SUMMARY 
     In a first aspect, a directional solidification furnace for producing a multi-crystalline silicon ingot is disclosed. The furnace comprises a crucible for containing a silicon charge, the crucible having a plurality of sides, a plurality of insulating members disposed adjacent the plurality of sides of the crucible, the insulating members movable between a first position where the insulating members restrict the flow of heat away from the sides crucible and a second position where the insulating members do not appreciably restrict the flow of heat away from the sides of the crucible, and an actuating system for moving the insulating members between the first position and the second position. 
     In another aspect, an insulation system for use in a directional solidification furnace for producing a multi-crystalline silicon ingot, the furnace having a crucible for containing a silicon charge is disclosed. The crucible has a plurality of sides. The furnace comprises a plurality of insulating members disposed adjacent the plurality of sides of the crucible, the insulating members movable between a first position where the insulating members restrict the flow of heat away from the sides crucible and a second position where the insulating members do not appreciably restrict the flow of heat away from the sides of the crucible, and an actuating system for moving the insulating members between the first position and the second position. 
     In still another aspect, a method for producing a multi-crystalline silicon ingot in a directional solidification furnace is disclosed. The method comprises charging a crucible in the furnace with poly-crystalline silicon, the mass of the poly-crystalline silicon being at least about 1000 kg, melting the poly-crystalline silicon, moving one or more insulating members disposed adjacent of the crucible from a first position where the insulating members restrict the flow of heat away from the sides crucible to a second position where the insulating members do not appreciably restrict the flow of heat away from the sides of the crucible, and cooling the molten silicon to form a multi-crystalline silicon ingot. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an example directional solidification furnace and heat exchangers; 
         FIG. 2  is a perspective view of an example insulation system for use in the furnace of  FIG. 1  with doors in a first position; 
         FIG. 3  is a perspective view of the insulation system of  FIG. 2  with the doors in the second position; 
         FIG. 4  is a front view of the insulation system of  FIG. 2 ; 
         FIG. 5  is a perspective view of lower insulating members in a second position for use with the furnace of  FIG. 1 ; 
         FIG. 6  is a perspective view of the lower insulating members of  FIG. 5  with the crucible support and other structures removed for clarity; 
         FIG. 7  is a perspective view of the lower insulating members of  FIG. 6  in a first position; 
         FIG. 8  is a perspective view of four heat exchangers for use in the furnace of  FIG. 1  and a lift mechanism to move the heat; 
         FIGS. 9-16  depict the lift mechanism of  FIG. 8  in various stages of assembly; 
         FIG. 17  is a perspective view of one of the heat exchangers of  FIG. 1 ; 
         FIG. 18  is a perspective view of a plate used in the heat exchanger of  FIG. 17 ; 
         FIG. 19  is an enlarged portion of  FIG. 18 ; 
         FIG. 20  is a perspective view of a portion of an inner conduit; 
         FIG. 21  is a cross-sectional view of the plate of  FIG. 18  and the inner conduit of  FIG. 20 ; 
         FIG. 22  is perspective view of a cover used in the heat exchanger of  FIG. 17 ; 
         FIG. 23  is a cross-sectional view similar to the  FIG. 21  with the cover of  FIG. 22  positioned atop the plate. 
         FIG. 24  is a perspective view of a connector; 
         FIG. 25  is a perspective view of the connector of  FIG. 24  in an inverted position; 
         FIG. 26  is a cross-sectional view similar to  FIG. 23  with the connector of  FIGS. 24 and 25  connected to the conduit; 
         FIG. 27  is a cross-sectional view similar to  FIG. 26  with an outer conduit connected to the connector; 
         FIG. 28  is a cross-sectional view of the terminal connector of  FIG. 17  taken along the  28 - 28  line; 
         FIG. 29  is a graph depicting the efficiency of photovoltaic devices made from ingots manufactured in different furnaces; 
         FIG. 30  is a box plot depicting the efficiency of photovoltaic devices made from ingots manufactured in different furnaces; and 
         FIG. 31  is a box plot comparing the density of dislocations in ingots manufactured in different furnaces. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Referring to the drawings, an exemplary directional solidification furnace is shown in  FIG. 1  and indicated generally at  100 . The furnace  100  is of the type used to melt poly-crystalline silicon and produce a multi-crystalline silicon ingot. Such an ingot may be used to manufacture photovoltaic devices, among other possible uses. The furnace  100  is operable to produce silicon ingots having a mass greater than about 1000 kg. 
     The directional solidification furnace  100  of  FIG. 1  comprises a crucible  102  having a base  106 . The crucible  102  and the base  106  are supported by a crucible support  103  having support walls  104  that add structural rigidity to the crucible. The crucible  102  is typically constructed of quartz, or another suitable material that can withstand high temperatures while remaining essentially inert. The crucible  102  is surrounded by a containment vessel  110 . Side insulation  109  is disposed around the crucible and may optionally be movable away from the crucible. In the example embodiment, upper insulation  111  is positioned vertically above the side insulation  109 . 
     Together with a lid  112 , the crucible  102  and crucible support  103  form an inner assembly  105  of the furnace  100 . In other embodiments, the furnace  100  may not include a lid. Heaters  108  are positioned around the walls  104  and within the containment vessel  110 . The heaters  108  may suitably be radiant heaters configured to apply the heat necessary to melt charge material within the crucible into a melt. The charge material of this embodiment is silicon, though other materials are contemplated. 
     A bottom  114  of the crucible support  103  may be positioned on support posts  115  ( FIGS. 6 and 7 ), broadly “supports” or “support structure”, in some embodiments. A heat exchanger, indicated generally at  200  and discussed in greater detail below, is positioned adjacent the bottom  114  of the crucible support  103  and proximate a lower surface  116  of the base  106  of the crucible  102 . Lower insulating members  400  and a cooling plate lift system  500  are shown schematically in  FIG. 1  and are described in greater detail below. 
     Two heat exchangers  200  (broadly, cooling plates) are shown in the cross-sectional schematic of  FIG. 1 , and two additional similarly sized and configured heat exchangers are omitted in  FIG. 1 , but shown in  FIG. 8 . Any number of heat exchangers  200  may be used without departing from the scope of the embodiments. The heat exchangers  200  are discussed in greater detail below in relation to  FIGS. 17-27 . 
     The heat exchanger  200  is used to transfer heat from the crucible  102  (and the melt contained therein) to a liquid coolant flowing through the heat exchanger. The heat exchanger  200  is supplied with “fresh” coolant from a source tank (indicated schematically at  150  in  FIG. 1 ). After flowing through the heat exchanger  200  the coolant is referred to as “spent” coolant and flows to a receiving tank (indicated schematically at  160  in  FIG. 1 ). The spent coolant may then be cooled (e.g., by a refrigeration or heat dissipation system) and flow back to the source tank  150 . The refreshed coolant can then flow again through the heat exchanger (i.e., be recycled). In other embodiments, the spent coolant may be disposed of and not reused after flowing to the receiving tank. 
     With reference to  FIGS. 2-4 , doors  300  (also referred to as louvers) are formed in the side insulation  109  surrounding the crucible  102 . In  FIGS. 2-4 , only the side insulation  109 , upper insulation  111 , and supporting structure  125  are shown and the other components of the furnace  100  are omitted for clarity. Moreover, the doors  300  are omitted in  FIG. 1  for clarity. 
     Each door  300  is sized to fit within a corresponding opening  302  (best seen in  FIG. 3 ) formed in the side insulation  109 . There are two doors  300  formed in each section of the side insulation  109  in the example embodiment, although other embodiments may use different numbers of doors. Moreover, other embodiments may use doors positioned differently in the side insulation  109  and/or doors positioned in the upper insulation  111 . For example, in other embodiments doors may be configured to rotate about a horizontal axis, rather than a vertical axis. Moreover, doors may be formed in a shape similar to slats or window blinds. 
     The doors  300  are connected to the side insulation  109  by hinges  304  disposed at longitudinal edges of the doors. The hinges  304  are in turn connected to the supporting structure  125 . In other embodiments, a rod (not shown) or other similar structure is connected to the doors  300  generally adjacent a centerline of the doors. Opposing ends of the rod are connected to the side insulation  109  adjacent the openings  302  and/or the supporting structure. The doors  300  in this embodiment are rotated about an axis parallel to the rod when opening or closing the doors. 
     The doors  300  are also connected to suitable actuators (not shown) that are operable to open and close the doors. In the example embodiment, two adjacent doors  300  are connected together by a linkage  306  such that the adjacent doors operate in unison and a single actuator is operable to operate both of the doors. 
     In a closed position (i.e., a first position) as seen in  FIGS. 2 and 4 , the doors  300  substantially restrict the flow of heat through the openings  302  formed in the side insulation  109 . Gaskets, lap-joints, or other structures (not shown) positioned at the edges of the doors  300  and/or openings  302  may be used to further restrict the flow of heat through any void that remains between the doors and the opening when the doors are closed. 
     In an open position (i.e., a second position) shown in  FIG. 3 , the doors  300  permit heat to flow through the exposed openings  302  in the side insulation  109 . According to some embodiments, the rotational position of the doors  300  may be adjusted to control the flow of heat through the side insulation  109 . For example, the doors  300  can be fully opened such that the doors are perpendicular to the side insulation  109  in order to permit more heat to pass through the openings  302 . 
     The doors  300  may alternatively be rotated such that they are disposed at an angle less than 90 degrees to decrease the amount of heat that can pass through the openings  302 . Such a position of the doors is referred to as an intermediate position. A control system (such as the controller  550  shown in  FIGS. 1 and 8 ) may be used to adjust the position of the doors  300  in the intermediate position to regulate the rate of heat transfer from the melt. 
       FIGS. 5-7  depict lower insulating members  400  disposed between the heat exchanger  200  ( FIG. 1 ) and the bottom  114  of the crucible support  103 . Other components of the furnace  100  have been omitted from  FIGS. 5-7  for clarity. Further, the bottom  114  of the crucible support  103  is omitted from  FIGS. 6 and 7 . 
     The lower insulating members  400  are laterally movable between a closed position (i.e., a first position) where they are disposed beneath the bottom  114  of the crucible support  103  ( FIG. 7 ) and an open positioned (i.e., a second position) where the members are disposed laterally outward and are not beneath the bottom of the crucible support ( FIGS. 1 ,  5  and  6 ). In the first position where the insulating members  400  are disposed beneath the crucible support  103 , the members substantially restrict the flow of heat into the heat exchanger  200  from the lower surface  116  of the base  106  of the crucible  102  and the bottom  114  of the crucible support  103 . In the second position, the members  400  permit heat to flow through the lower surface  116  of the base  106  of the crucible  102  and the bottom  114  of the crucible support  103  into the heat exchangers  200 . Further, the second position permits upward movement of the heat exchangers  200 . 
     While reference is made herein to positioning the members  400  in either the first position or the second position, they may instead be positioned in between these two positions during operation of the furnace  100 . For example, the members  400  may be positioned in an intermediate position to control the flow of heat from the melt in the crucible  102  through the crucible support  103 . In this intermediate position the members  400  restrict the flow of heat away from the crucible support  103  to a lesser extent than when in the first position. A control system (such as the controller  550  shown in  FIGS. 1 and 8 ) may be used to adjust the position of the members  400  in the intermediate position to regulate the rate of heat transfer from the melt through the crucible  102  and crucible support  103 , and into the heat exchangers  200 . Moreover, the intermediate position includes any position of the members  400  which is between the first position and the second position. 
     In the example embodiment, four insulating members  400  are provided and each has the shape of a quadrant of a circle or square. Accordingly, when in the first position the insulating members  400  have a generally circular or square shape and have a substantially contiguous surface. Other embodiments may use more or less members and/or different shaped members  400  without departing from the scope of the embodiments. This configuration of the four insulating members  400  in the example embodiment results in a relatively uniform rate of heat removal across the crucible support  103  when the members  400  are in an intermediate position. This relatively uniform rate is at least partially the result of the “X-shaped” symmetric opening formed between the edges  404  of the members when in an intermediate position. Contrastingly, if fewer insulating members (e.g., one or two) were used, such an “X-shaped” symmetric opening would not be formed between the members. The resulting asymmetric opening would result in an asymmetrical rate of heat removal across the crucible support  103  when the members are in an intermediate position. 
     In other embodiments, insulating members may be slats similar to window blinds that are configured to rotate between positions instead of moving laterally. These insulating members may be rotated to various positions to control the flow of heat therethrough. 
     As best seen in  FIG. 6 , edges  404  of each member  400  have an overlapping or “ship-lapped” configuration. When the members  400  are in the first position a portion of the edge  404  of one member overlaps a portion of the edge of an adjacent member. The overlapping configuration of the edges  404  reduces minimizes radiative heat transfer by reducing or eliminating the view factor of the heat exchanger  200  when the members  400  are in the first position. Moreover, any molten material which might spill from the crucible  102  would have to travel a more circuitous path to reach the heat exchangers  200 . This spilled material would thus be less likely to contact and possibly damage the heat exchangers  200 . 
     The lower insulating members  400  are each connected to an actuating system  402  that is operable to move the insulating members  400  between the first position and the second position. In the example embodiment, the actuating system  402  for each insulating member  400  comprises a nut  408  connected to a drive (broadly, power) screw  410 . The nuts  408  and corresponding drive screws  410  may have acme threads in some embodiments. The nuts  408  are in turn connected to carriages  420  onto which the insulating members  400  are mounted. In other embodiments, the nuts  408  and corresponding drive screws  410  may be ball screw systems and/or other types of actuators may be used. A radiation shield  422  is positioned vertically about the nuts  408  and screws  410  to shield the nuts and screws from radiative heat. 
     Each of the drive screws  410  is in turn connected to a single flexible drive shaft  412  by any suitable power transmission system (e.g., one or more gears). This drive shaft  412  is rotated by a suitable rotary actuator  414 . In the example embodiment, the power transmission system is a gearbox  416 . 
     Rotation of the drive shaft  412  results in rotation of each drive screw  410  and linear motion of each nut  408 . Linear motion of the nuts  408  results in corresponding linear motion of the insulating members  400  connected to each nut. This arrangement of a single rotary actuator  414  used to move each of the insulating members  400  ensures that the members move generally in unison. Other embodiments may use different systems of actuators or other mechanisms to move the members  400  between positions without departing from the scope of the embodiments. For example, each respective member  400  may be connected to a single actuator that is configured to move only the respective member between the positions. These single actuators may be connected to a suitable control system (such as the controller  550  shown in  FIGS. 1 and 8 ) that is operable to control their movement so that the actuators move in unison. Other embodiments may use a control system that permits the members  400  to be moved independently of each other. 
       FIGS. 8-16  depict a heat exchanger lift system  500  (broadly, a lift system). In the example embodiment, this lift system  500  is used in conjunction with the lower insulating members  400  and/or doors  300  described above. In other embodiments, the lift system  500  may be used in furnaces  100  that do not use movable lower insulating members  400  and/or doors  300 . 
     In  FIGS. 8 and 9 , a lower portion of the containment vessel  110  is shown and other components of the furnace  100  are omitted. In  FIGS. 9-16 , various components of the lift system  500  are shown in greater detail. 
     The lift system  500  is operable to move the heat exchangers  200  between a first position and a second position. In the first position, the heat exchangers  200  are spaced apart from the bottom  114  of the crucible support  103  by a sufficient gap such that the lower insulating members  400  can be disposed in their first position. Thus, the heat exchangers  200  are free from contact with the crucible support  103  in the first position. In the second position, the heat exchangers  200  are in contact with the bottom  114  of the crucible support  103 . When the heat exchangers  200  are in their second position, the lower insulating members  400  are in their second position as well. In the example embodiment, the heat exchangers  200  move between about ten inches to twenty inches when travelling between the first position and the second position, although they may travel greater or lesser distances without departing from the scope of the embodiments. 
     The heat exchangers  200  are movable between their first position and second position by an actuator  502 , as shown in  FIGS. 9 and 10 . The actuator  502  is connected at one end to a lower plate  504  ( FIG. 10 ) and at another, opposing to the containment vessel  110  ( FIG. 9 ). An upper plate  506  is connected to the lower plate  504  and springs  512  ( FIG. 14 ) are positioned between the two plates. Four collar clamps  508  are connected to the upper plate  506 , as shown in  FIG. 12 . The collar clamps  508  are operable to connect a conduit  250  of the heat exchangers  200  to the lift system  500  as best seen in  FIG. 13 . Bellows  510  ( FIG. 14 ) surround portions of these conduits  250  and are connected at one end to the upper plate  506  and at the other, opposing end to containment vessel  110 . 
     In the example embodiment, the actuator  502  (broadly, an actuating system) is a linear actuator that is operable to exert sufficient force on the heat exchanger  200  to press the heat exchanger against the bottom  114  of the crucible support  103  when in the second position. In another embodiment, the actuator  502  is a rotary actuator that is connected to a pinion gear. This pinion gear is in registry with a gear rack such that rotation of the pinion gear results in linear displacement of the gear rack. Other types of suitable actuators may be used without departing from the scope of the embodiments. 
     Helical compression springs  512  are disposed between the lower plate  504  and the upper plate  506 , as shown in  FIG. 14 . Eight springs  512  are used in the example embodiment, although the number of springs may be altered without departing from the scope of the embodiments. In one embodiment, a thumb screw  516  ( FIG. 16 ), a plunger  514 , the springs  512 , and a control system  550  ( FIGS. 1 and 8 ) are used to control the amount of force exerted by the lift system against heat exchangers  200 . The control system  550  can also be referred to as a force determination system in some embodiments. 
     The control system  550  is operable to receive communication from the plunger  514  (i.e., the two are communicatively coupled) when the plunger contacts the thumb screw  516  indicating as such. The plunger  514  and thumb screw  516  are referred to together as a limit switch. After the heat exchangers  200  have contacted the bottom of the crucible support  103 , additional upward movement of the heat exchangers  200  by the lift system  500  causes compression of the springs  512 . The control system  550  stops the lift system  500  from further raising the heat exchangers  200  when the plunger  514  communicates to the controller that the plunger has contacted the thumb screw  516 . 
     The distance between the plunger  514  and the thumb screw  516  (i.e., a set distance) may be adjusted in this embodiment by rotating the thumb screw  516  with respect to the upper plate  506 . A nut (not shown) may be used to prevent the thumb screw  516  from being further rotated once it is in a desired position. To increase the amount of force exerted by the lift system  500  against the heat exchangers  200 , the distance between the plunger  514  and the thumb screw  516  is increased such that lift system compresses the springs  512  to a greater degree. Conversely, the distance between the plunger  514  and the thumb screw  516  is decreased to reduce the amount of force exerted by the lift system  500  against the heat exchangers  200 . 
     Moreover the amount of force exerted by the lift system  500  against the heat exchangers can be calculated based on the displacement (i.e., compression) of the springs  512  and the spring constant k of the springs. In the example embodiment, this displacement is comprised of at least two components. The first is the distance between the plunger  514  and the thumbscrew  516  when the lift system  500  is in the first position as the springs  512  are displaced by this distance when the lift system  500  is in the second position. The second component is a preload compression caused when the lower plate  504  and the upper plate  506  are assembled together with fasteners. During this assembly, the springs  512  are compressed to some degree and this displacement can be measured. 
     The amount of force exerted by the actuator  502  on the heat exchangers  200  (and hence the force applied by the heat exchangers on the bottom  114  of the crucible support  103 ) is then defined by F=k*y, where y is the displacement of the springs  512 . As multiple springs  512  are used in the lift system  500 , the total force exerted by the lift system  500  against the heat exchangers  200  is determined by applying this equation to each of the springs. In the example embodiment where eight springs  512  are used and each have the same spring constant k and is displaced by the same amount, the force is defined by F=8*k*y. The above-described equation assumes that the springs  512  are linear springs. In embodiments using different types of springs (e.g., those which are not linear), the force may be calculated according to other suitable methods and/or equations. 
     In another embodiment, the plunger  514  or another suitable distance measuring device is used by the control system  550  to measure the distance between the plates  504 ,  506 , and a thumb screw is unnecessary. The measured distance and the displacement resultant from the preload compression of the springs  512  represent the total compression y of the springs. Alternatively, other suitable devices may be used to measure the compression of the springs  512  without departing from the scope of the disclosure. As described above, the amount of force exerted by the actuator  502  on the heat exchangers  200  (and hence the force applied by the heat exchangers on the bottom  114  of the crucible support  103 ) is thus defined by F=k*y. 
     The control system  550  in this embodiment is thus operable to adjust the amount of force exerted by the actuator  502  by changing the position of the heat exchangers  200  with the actuator. That is, the control system  550  is operable to receive an input (from a user or another computing system) of a desired amount of force to be exerted by the actuator  502  against the bottom of the crucible support  103 . The control system  550  can then monitor the exerted force and control the actuator  502  (and thus the position of the heat exchangers  200 ) such that the exerted force is equal to the desired amount of force or within a predetermined range of the desired amount (e.g., +/−5%). 
     Moreover, the control system  550  may also calculate the force exerted by the actuator  502  with one or more strain gauges and/or load cells. These strain gauges and/or load cells can be positioned between the bottom  114  of the crucible support  103  and the support posts  115  ( FIGS. 6 and 7 ) such that as the heat exchangers  200  apply force to the crucible support, the force exerted on the strain gauges and/or load cells decreases. Other embodiments may calculate the force by measuring the current draw of the actuator  502 , as the amount of current drawn by the actuator  502  increases as the amount of force exerted by the actuator increases. This increase in current draw correlates with the increase in force applied by the actuator  502  and lift system  500  against the heat exchangers  200 . 
     In the example embodiment, the force applied by the actuator  502  is about  800  lbs., although other embodiments may use different magnitudes of force without departing from the scope of the embodiments. The force applied by the heat exchangers  200  against the crucible support  103  ensures that substantially the entire outer surface  204  of the plate  202  of the heat exchangers is in contact with the crucible support  103 . This force also ensures that the outer surface  204  and/or the crucible support may deform slightly such that their surfaces are in contact. This contact between the crucible support  103  and the outer surface  204  increases the efficiency of heat transfer from the crucible support to the heat exchangers  200 . Moreover, the control system  550  may also be used to ensure that the actuator  502  does not exert a greater than specified force against the heat exchangers  200 . Forces greater than this specified force may damage the heat exchangers  200  and/or the crucible support  103  and/or lift the crucible support off of its support posts  115 . In the example embodiment, this specified force may be greater than about  3000  lbs and/or the mass of the crucible support  103 , crucible  102 , and the charge contained in the crucible. 
     In operation, the containment vessel  110  is opened and the crucible  102  is charged with pieces of poly-crystalline silicon (e.g., chunks, granules, dust, etc.). The lid  112  of the crucible  102  (assuming a lid is used) and the containment vessel  110  are then closed and the heaters  108  are used to melt the silicon. While the silicon is being melted, the doors  300  in the side insulation  109  are in the closed position and the lower insulating members  400  are in the first position where they are disposed beneath the bottom  114  of the crucible support  103 . Moreover, the heat exchangers  200  have been positioned in their first position by the lift system  500  such that they are spaced apart from the bottom  114  of the crucible support  103 . 
     After the silicon has melted, the heaters  108  cease operation or their heat output is reduced and the silicon melt begins to solidify into an ingot. The doors  300  are moved to their second position and the lower insulating members  400  are also moved to their second position such that they are not disposed beneath the bottom  114  of the crucible support  103 . Furthermore, the heat exchangers  200  are moved by the lift system  500  to its second position such that it is in contact with the bottom  114  of the crucible support  103 . In some embodiments the heat exchangers  200  may not be moved to their second position and remain in their first position during solidification of the melt. In these embodiments, the insulating members  400  and/or the doors  300  may be positioned in any of their first, second, or intermediate positions during solidification of the melt. 
     The opening of the doors  300  and the movement of the lower insulating members  400  and the heat exchanger  200  aid in increasing the flow of heat away from the melt and solidification of the melt into the ingot. Moreover, the position of the doors  300  may be adjusted to an intermediate position to further control the rate at which heat is transferred away from the crucible  102  and the melt/ingot. In the example embodiment, the position of the doors  300  is adjusted by rotating the doors about their vertical axis to control this rate of heat transfer away from the melt/ingot. This control of the rate of the heat transfer permits the control of the rate of solidification of the melt. In some embodiments, a quartz rod is inserted into the melt to probe the melt to determine the location of solidification front. 
     One of the heat exchangers  200  is shown in greater detail in  FIGS. 17-28  and is inverted from its position in  FIG. 1  to better show its internal structure. As shown in  FIG. 18 , the heat exchanger  200  includes a plate  202  having an outer surface  204  for positioning proximate the lower surface  116  of the crucible  102 . In the example embodiment, the outer surface  204  of the plate  202  is positioned adjacent the bottom  114  of the crucible support  103  and is substantially flat. The heat exchanger  200  is operable to transmit heat away from the lower surface  116  of the crucible  102  and silicon disposed in the crucible to a coolant. In other embodiments where the crucible support  103  is omitted, the outer surface  204  of the plate  202  is positioned adjacent the lower surface  116  of the crucible  102 . 
     The plate  202  has an inner surface  206  opposite the outer surface  204 . A cover  210  ( FIGS. 17 and 22 ) is positioned proximate the inner surface  206  of the plate  202  and is connected to the plate with any suitable fastening system (e.g., welding). 
     As shown in  FIG. 18 , a circuitous flow path  220  is formed in the plate  202  for directing a flow of coolant along the inner surface  206  of the plate  202 . The flow path  220  is defined by a channel including a plurality of members  222  extending from the inner surface  206  of the plate  202  to the cover  210  (the cover is omitted from  FIG. 18 ). The flow path  220  defined by the members  222  is circuitous such that coolant flows along substantially all of the inner surface  206 . The members  222  in the example embodiment extend generally perpendicularly from the inner surface  206  to the cover  210 . The members  222  extend to adjacent the cover  210  and thus prevent the flow of coolant between the members and the cover. The members  222  thus do not allow coolant to “short-circuit” between adjacent portions of the flow path  220 . 
     The flow path  220  has an inlet  224  for receiving a flow of fresh coolant and an outlet  226  through which coolant exits after it has flowed through the flow path. The inlet  224  and the outlet  226  are positioned adjacent each other. In some embodiments, the inlet  224  and the outlet  226  are coaxial with each other. A wall  230  ( FIG. 19 ) extending from the inner surface  206  to the cover  210  separates the inlet  224  from the outlet  226 . The wall  230  also aids in alignment of the other components of the heat exchanger  200 . The inlet  224  and the outlet  226  are shown in the example embodiment as being positioned generally at or near a center of the plate  202 . In other embodiments, the inlet  224  and the outlet  226  may be positioned differently (e.g., nearer a corner or a side of the plate  202 ). 
     The cover  210  ( FIG. 22 ) has an opening  232  formed therein that is in fluid communication with the inlet  224  and the outlet  226  of the flow path  220 . The opening  232  is positioned adjacent and/or coaxial both the inlet  224  and the outlet  226 . The opening  232  has an inlet portion  234  and a larger outlet portion  236 . 
     An inner conduit  240  ( FIGS. 20 ,  21 , and  23 ) is disposed within the inlet portion  234  of the opening  232  and is connected to the inlet  224  of the flow path  220 . An outer conduit  250  ( FIG. 28 ) is connected to the outlet  226  of the flow path  220 , as discussed below in greater detail. The term conduit as used herein includes pipes, hoses, tubes, or other structures operable to convey a flow of liquid from one point to another. The inner conduit  240  is connected to the inlet  224  of the flow path  220  and the inner surface  206  of the plate  202  by welding in the example embodiment. In other embodiments, the inner conduit  240  may be connected by any suitable fastening system (e.g., welding or mechanical fasteners). 
     The outer conduit  250  is connected to the outlet  226  of the flow path  220  by a connector  260  in the example embodiment. The connector  260 , as shown in  FIGS. 24 and 25 , has an inlet section  262  for connection to the cover  210  and an outlet section  264  for connection to the outer conduit  250 . The inlet section  262  of the connector  260  is connected to the cover  210  such that the inlet section is in fluid communication with the outlet  226  of the flow path  220 . As shown in  FIG. 26 , a portion  242  of the inner conduit  240  is disposed within a central opening  266  of the connector  260 . In other embodiments the connector  260  is omitted and instead the outer conduit  250  is connected directly to the cover  210  adjacent the outlet portion  236  of the opening  232  in the cover. 
     As shown in  FIG. 28 , the outer conduit  250  is concentric with the inner conduit  240  and the inner conduit is disposed within the outer conduit. The outer conduit  250  and the inner conduit  240  thus form a multi-lumen conduit structure. In some embodiments, insulation (not shown) may be disposed adjacent the inner conduit  240  to reduce heat transfer from coolant in the outer conduit  250  to coolant in the inner conduit. This insulation can be disposed on either or both of an inner surface  244  or an outer surface  246  of the inner conduit  240 . Moreover, all or a portion of the inner conduit  240  may be constructed from a material that has a lower thermal conductivity k compared to that of other components of the heat exchanger  200  to restrict the flow of heat through the inner conduit. 
     The conduits  240 ,  250  extend away from the cover  210  of the heat exchanger  200  and end at a terminal connector  270 . The terminal connector  270  has an inlet port  272  in fluid communication with the inner conduit  240  and a corresponding outlet port  274  (best seen in  FIG. 17 ) in fluid communication with the outer conduit  250 . A gasket-like member  276  disposed within the terminal connector  270  prevents coolant from travelling between the inlet port  272  and the outlet port  274 . The inlet port  272  is connected to the source tank  150  with the fluid communication system  170  (shown schematically in  FIG. 1 ). Likewise, the outlet port  274  is connected to the receiving tank  160  with the fluid communication system  170 . 
     In operation and as shown in  FIGS. 1 ,  17 , and  18 , fresh coolant is supplied to the inlet port  272  of the terminal connector  270  from the source tank  150 . The fresh coolant travels through the inner conduit  240  to the inlet  224  of the flow path  220  in the heat exchanger  200 . The fresh coolant then flows through the flow path  220  and heat is transferred from the inner surface  206  of the plate  202  to the coolant. The heat is transferred to the inner surface  206  of the plate  202  from the silicon with the crucible  102 . This heat transferred to the coolant causes the temperature of the coolant to increase. After flowing through the flow path  220 , the coolant exits the flow path through the outlet  226 . At this point, the coolant is referred to as spent coolant. The coolant flows through the outer conduit  250  to the terminal connector  270 . The coolant then flows through the outlet port  274  of the terminal connector  270  to the receiving tank  160 . The spent coolant may then be cooled by any suitable heat dissipation system that results in a reduction in the temperature of the coolant. The coolant may be transferred to the source tank  150  for subsequent reuse. Alternatively, the spent coolant may be disposed of after flowing from the outlet port  274  of the terminal connector  270 . 
     In the embodiments described herein, fresh coolant is supplied to the inlet  224  of the flow path  220  through the inner conduit  240 . In another reverse-flow embodiment, the flow of coolant through the flow path  220  may be reversed, such that fresh coolant is instead supplied to the outlet  226  of the flow path  220  from the outer conduit  250 . The spent coolant then exits the flow path  220  though the inlet  224  and into the inner conduit  240 . In this reverse-flow embodiment, the outlet port  274  of the terminal connector  270  is connected to the source tank  150  and the inlet port  272  is connected to the receiving tank  160 . 
     The components of the heat exchanger  200  are constructed from suitable materials that are resistant to corrosion. In the example embodiments, such materials include steel, alloys thereof (e.g., stainless steel), aluminum-bronze compounds, or synthetic materials (e.g., hydrocarbon-containing plastics) capable of withstanding elevated temperatures. 
     The heat exchangers  200  described herein have reduced complexity and increased efficiency compared to prior heat exchangers. As described above, the inner and outer conduits  240 ,  250  are in a multi-lumen configuration. In prior systems, separate, non-concentric conduits are used to supply and return coolant from heat exchangers. Moreover, such prior systems do not have a flow path with an inlet adjacent to an outlet. Instead, the inlet and the outlet are spaced-apart, resulting in a more complex and larger arrangement occupying more space. This larger arrangement may be even more problematic in the system described above that use four heat exchangers. 
     Furthermore, the use of prior systems having separate, non-concentric conduits results in the creation of bending moments at the junction of the conduits with the heat exchanger. Such bending moments cause significant stress at the junction that can eventually result in the formation of cracks at the junction due to fatigue. The arrangement of the inner and outer conduits  240 ,  250  and the connector  260  of the heat exchanger  200  strengthen and stiffen the junction of the conduits and the heat exchanger. Accordingly, the junction is able to withstand greater stresses and is significantly less likely to crack. 
     The furnace  100  and associated components described above permit the casting of an ingot having a mass of greater than about 1000 kg, greater than about 1200 kg, or greater than about 1600 kg. This ingot is also substantially free of other defects (such as dislocations). Defects can limit the efficiency of wafers formed from the ingots, and thereby negatively effect the photovoltaic devices formed using the wafers. The most prevalent types of intra-grain defect in these wafers (e.g., mc-Si wafers) are dislocations. The dislocations form clusters that initiate from grains of some orientations and may thereafter spread or fan out from the cluster. These dislocation clusters may be sites for the precipitation of impurities, which lower the efficiency of photovoltaic devices formed from the wafers. The presence of dislocation clusters affects material properties and performance properties of the photovoltaic devices. These dislocations are generated from thermal stresses in the melt and ingot during solidification of the ingot and growth of the crystal. 
     The furnace  100  and the associated components described above enable control of the thermal and growth profiles of the melt and ingot to minimize the thermal stresses imposed on the melt and ingot. This minimization of thermal stresses in the melt and ingot minimizes the formation of dislocations and increases the efficiency of wafers formed from the ingots which are used in photovoltaic devices or applications.  FIGS. 29 and 30  depict the efficiency of photovoltaic devices formed from ingots made with different furnaces. Data sets  1  and  2  depict efficiencies of devices made from ingots manufactured in the furnace  100 , while data set  3  depicts efficiencies of devices made in prior furnaces.  FIG. 29  represents the efficiency data as a probability plot, while  FIG. 30  represents the data as box plot. As clearly shown in these Figures, photovoltaic devices formed from ingots manufactured in the furnace  100  have greater efficiencies than those manufactured in prior furnaces. Furthermore,  FIG. 31  is a box plot comparing the density of dislocations for the three data sets in units of counts per square centimeter. Data sets  1  and  2  clearly have a significantly lower dislocation density than that of ingots manufactured in prior furnaces. Moreover, the dislocation density of data sets  1  and  2  is less than about 100,000 counts per square centimeter, while the dislocation density of data set  3  is greater than about 110,000 counts per square centimeter. Note that in some embodiments, the dislocation density of ingots manufactured in furnaces embodying this disclosure may be less than 95,000 counts per square centimeter, or less than 90,000 counts per square centimeter, or even less than 80,000 counts per square centimeter. 
     In some aspects of the disclosure, the ingot has a length and a width such that that the ingot is cut into pieces to form smaller bricks the resulting bricks each have a standard size. This standard size is substantially similar to that of bricks cut from ingots formed in standard furnaces. In the example embodiment, the ingot has a length and a width of about 1375 mm and a height of about 400 mm. This ingot may then be cut into 64 smaller bricks having equal length and width, e.g., of about 156 mm. In some embodiments, the ingot may first be cut into four smaller ingots before being cut into the eight smaller ingots with a length and width of about 156 mm. In other embodiments, the ingot may be cut into 36 smaller bricks having a length and a width of about 210 mm. In still other embodiments, the height of the ingot may be up to or greater than about 800 mm. 
     The furnace  100  and associated components described herein permit the rate of cooling of the silicon melt to be precisely controlled. Control of the rate of cooling of the silicon melt allows for the precise control of the rate of solidification of the melt. This precise control of the solidification rate results in the formation of a directional solidification front in the ingot. By controlling the solidification rate, this position and shape of the solidification front can be manipulated and/or controlled such that it progresses vertically upwards away from the heat exchangers  200  positioned beneath the furnace. Moreover, the systems described herein also permit the creation of a substantially horizontal solidification front with the silicon melt. Accordingly, substantially all locations within a given horizontal plane in the melt solidify at about the same point in time. 
     Moreover, in some embodiments the shape of the solidification front may be controlled such it curves slightly down at its edges when solidification nears completion. This downward curve captures or concentrates impurities or dislocations near the edges of the ingot. Accordingly, lesser amounts of material may be removed from the ingot in order to remove the impurities. Furthermore, the controlled solidification of the melt into an ingot also permits the capture or concentration of impurities or defects in a specific portion of the ingot. In the example embodiment, this portion of the ingot is disposed farthest away from the heat exchangers and is the last portion of the ingot to solidify. 
     This precise control of the solidification rate permits ingots having a mass of greater than about 1000 kg to be formed in the furnace described above. The precise control of the solidification rate also increases the throughput of the furnace by reducing the amount of time required to cast an ingot in the furnace. Previous known systems lacked the features described above that permit the control of the rate of cooling of the silicon melt between low to high levels. In such prior systems, the rate of solidification thus could not be precisely controlled over such a range. As a consequence, attempts to cast ingots larger than about 600 kg resulted in the ingots having dislocations and/or defects that rendered the ingots and wafers formed from the ingots unfit for end-use applications (e.g., the manufacture of photovoltaic cells). 
     When introducing elements of the present invention or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.