Abstract:
A high-temperature furnace for melting materials, e.g. silicon, wherein the furnace comprises a vessel having a wall which defines an inner chamber. A crucible containing the material to be melted is positioned inside of the chamber and heating means is arranged around the crucible. An insulation package is provided in the chamber for protecting the vessel wall from the heat generated by said heating means. Specifically, the package comprises a support ring of insulation mounted in the chamber, a sleeve of insulation supported on the ring and extending upward between the heating means and the vessel wall, a catch tray adapted to receive any material which may leak from the crucible during heating mounted through the support ring, and a top and a lower layer of insulation to thermally seal the crucible within the package during heating.

Description:
[0001]    This application claims priority to and is a continuation of pending U.S. application Ser. No. 11/464,608 filed on Aug. 18, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a package or jacket for insulating between internal heating elements and the wall of a high temperature furnace and a method of installing the insulation package within the furnace. 
       BACKGROUND OF THE INVENTION 
       [0003]    In recent years, considerable advances have been made in photovoltaic (PV) cells or the like for directly converting solar energy into useful electrical energy. A typical PV cell is comprised of wafers or films of silicon which, in turn, are assembled into laminates or panels by various processes well known in the solar module art. The laminates or panels can then be installed onto the appropriate support structure (e.g. the roof of a house, building, or the like) or can be used directly as a building material, (e.g. “building integrated”) to provide electrical energy. 
         [0004]    Typically, the silicon wafers for PV cells are produced by melting and fusing pieces of silicon into a solid mass which then is cooled and crystallized before cutting into wafers of the required thickness. A well known system for melting silicon pieces into the desired mass is one in which a ceramic crucible, filled with the silicon pieces is placed into the chamber of a furnace. Electrical heating elements are positioned within the chamber so that they surround the crucible and lie between the crucible and the inner wall of the furnace. 
         [0005]    Since the crucible has to be heated to temperatures (e.g. about 1500° C.) which can cause serious damage to the wall of the furnace if left unchecked, (a) the furnace wall is water-cooled and (b) the gap between the heating elements and the furnace wall is filled with insulation so that the intense heat furnace wall is filled with insulation so that the intense heat generated by the heating elements does not bear directly on the furnace wall. This insulation also prevents the cooling from the water cooled furnace wall from affecting or lowering the heat conveyed to the crucible from the heating elements. 
         [0006]    In the known prior furnaces of this type, the insulation package is comprised of several large and many small individual pieces (e.g. 30 pieces or more) of insulation and support structure (e.g. layers of carbon or graphite) which have to be independently installed and fitted into the spaces between the heating elements and the furnace wall. This is both labor intensive and time consuming in that the individual pieces must be properly fitted together by the installers while they work within the furnace. Accordingly, this operation adds to the overall costs involved in the melting operation. Further, the interior surface of the insulation is prone to reaction when exposed to the melting atmosphere within the furnace and degrades over time that sometimes requires early replacement of the insulation package. As can be appreciated, this amounts to another expense which has to be considered in the production of PV cells. 
         [0007]    Still further and of major importance, if and when the crucible fractures during heating (not uncommon), liquid silicon comes into contact with portions of the insulation, especially that below the bottom of the crucible. This leaking liquid silicon can, and usually does, damage the insulation package to the point that the affected portions of the insulation package can not be reused. This also can add significantly to the costs of the operation since the type of insulation required is very expensive. Accordingly, having to replace a substantial portion of the insulation package whenever there is a spill, the overall costs are further increased. Since solar technology is extremely competitive with other, more conventional energy sources, keeping the costs of manufacture as low as possible is a very important consideration in the successful marketing of solar power to the public. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an insulation package for protecting the wall of a furnace used in melting materials, e.g. silicon, from the high temperature required for melting the material and a method for installing the insulation package into the furnace. The insulation package of the present invention is comprised of a relatively few pieces of insulation when considering the large number of individual, pieces required in known prior art furnaces. Further, the time needed to install the package is substantially reduced as are the fitting requirements of the installers. Further, the present insulation package includes a catch tray positioned below the crucible in the furnace so that if the crucible fractures or cracks during the melting operation, any leaking silicon will be caught in the tray without affecting other parts of the insulation package. 
         [0009]    More specifically, the present invention provides a furnace in which the present insulation package can be utilized. The high-temperature furnace is comprised of a vessel having a wall which defines a inner chamber. A crucible is positioned in the chamber and is filled with the material (e.g. silicon) to be melted. A heating means (e.g. a plurality of electrical heating elements) which is capable of generating the necessary heat is positioned in the chamber and surrounds the crucible. 
         [0010]    The insulation package of the present invention is positioned in the chamber so as to protect the wall of the vessel from the intense heat generated by the heating means. More specifically, the insulation package is comprised of a support ring that is mounted in the chamber below the crucible and has a central opening therethrough. A sleeve of insulation material is supported on the support ring and extends upward to effectively fill the gap between the wall of the vessel and the heating means. 
         [0011]    A catch tray also having a central opening therethrough is positioned within the central opening through the support ring and is supported on the ring. The catch tray is positioned so that it will catch any liquid material which may leak from the crucible during heating. The present insulation package further includes a top layer of insulation which extends over the top of the sleeve and a lower layer of insulation which effectively seals the opening through the catch ring during heating to thermally isolate the crucible within the insulation package. The support ring, sleeve, catch tray, upper layer, and lower layer are all comprised of any suitable insulation material which is capable of withstanding the high temperatures generated by the heating means and of sufficient strength to be self supporting, e.g. available insulation material comprised of carbon or graphite materials formed as rigidized felt, bonded fiber, or soft felt interspersed with structural carbon or carbon-fiber pieces. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The actual construction operation, and apparent advantages of the present invention will be better understood by referring to the drawings, not necessarily to scale, in which like numerals identify like parts and in which: 
           [0013]      FIG. 1  is a cross-section view of the furnace of present invention when in heat-up position without the outer water jacket; 
           [0014]      FIG. 2  is a cross-section view of the furnace of  FIG. 1 , when in the growth position; 
           [0015]      FIG. 3  is a sectional view taken along line  3 - 3  of  FIG. 2 ; and 
           [0016]      FIG. 4  is an enlarged perspective view, partly in section, of the insulation package or jacket of the present invention. 
           [0017]    While the invention will be described in connection with its preferred embodiments, it will be understood that this invention is not limited thereto. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents that may be included within the spirit and scope of the invention, as defined by the appended claims. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Referring now to drawings,  FIG. 1  illustrates the furnace  10  of the present invention when the furnace is in its “heat-up position”. The outer configuration and outer features of furnace  10  are basically the same as those of known, prior art furnaces of this type. That is, furnace  10  is comprised of a vessel  11  having a cylindrical wall  12  which forms an inner chamber  13 . Wall  12  and top closure or cap  14  are formed of any suitable material, e.g. stainless steel, that is capable of withstanding the temperature involved in the heating operation. The dimensions of a typical furnace  10  are relatively large in order to accommodate the large, square crucible  15  (e.g. 69 cm×69 cm×42 cm high) which are typically used in silicon melting operations of this type. One example of such a known furnace is GT-MX225 HEM Furnace, distributed by GT Equipment Technologies, Merrimack, N.H. 
         [0019]    In prior known furnaces of this type, heating means similar to the present heating means (e.g. electrical heating elements  16  in  FIG. 1 ) are positioned within chamber  12  so that they will encircle crucible  15  when the crucible is in place and provide the heat for melting the material in crucible  15 , e.g. silicon. However, the temperatures needed to melt materials such as silicon are so high (e.g. about 1500° C.) that the heat will damage the wall  12  of the vessel  11  if the wall is left unprotected. In addition to water-cooling the wall  12  of the vessel  11  (not shown), large amounts of insulation have to be provided between the heating elements  16  and the wall  12  of the vessel to further protect the wall from the excessive heat. 
         [0020]    In the known prior-art furnaces of this type, providing sufficient insulation requires hand wrapping the heating elements  16  (i.e. furnace liner) with carbon felt and securing the felt with graphite string before the elements are positioned in the chamber. Holes are cut through the felt wrapping to receive the electrodes which are necessary to supply power to the heating elements. Also additional insulation, (e.g. sets of graphite hearth plates, trimmed to fit, not shown) is required within the chamber vessel to protect vital areas of the chamber from severe heat damage. 
         [0021]    Providing the insulation necessary for such prior furnaces is both labor intensive and time consuming, hence expensive. For example, in one known furnace used for melting large volumes of silicon, the insulation package consists of twenty (20) lower support pieces, twenty-eight (28) lower insulation pieces, eight (8) center insulation pieces, a liner system that is hand-wrapped with loose insulation, nine (9) upper edge pieces, and a 15-piece upper insulation assembly; making a total of eighty-two (82) separate pieces. These pieces have to be installed individually and then fitted together once inside the chamber to form the required insulation package. It can readily be seen that this amounts to a substantial part of the overall cost of such an operation. 
         [0022]    In accordance with the present invention, a insulation package or jacket  20  (used interchangeably throughout), see  FIG. 4 , is provided which substantially replaces the individual pieces of the prior furnace&#39;s insulation package; i.e. reduces the required pieces from about eighty (80) pieces to about twelve (12)). By doing so, the time required in installing the required insulation is substantially reduced, making the melting of large volumes of silicon both less expensive and, by reducing the cost of manufacturing solar cells, the overall costs of producing solar cells and electrical energy from solar cells are reduced. 
         [0023]    Basically, insulation jacket  20  is comprised of insulation base ring  21  having a central opening  21   a,  catch tray  22  having a central opening  22   b,  insulation sleeve or cylinder  23 , top insulation layer or cover  24 , and a lower insulation layer  26 . The furnace also includes a crucible support means, i.e. support block  27  and support plate  28 , both being comprised of high temperature conductive material (e.g. graphite)(see  FIGS. 1 and 2 ). The insulation used in the support ring  21 , catch tray  22 , sleeve  23 , and top  24  and lower layers  26  in the present package  20  may be of any suitable material which is capable of withstanding the extreme temperatures involved and of sufficient strength to be self-supporting; e.g. carbon and/or graphite insulation such as that fabricated by Carbon Composites, Inc., Leominster, Mass. Also, while the various components, e.g. cylindrical sleeve  23 , top  24 , bottom insulation layer  26 , etc., are illustrated as being of a single thickness or integral composition, it should be understood that each of these components can also be formed from multi-sheets or stacked layers of insulation to build-up the desired thickness without departing from the present invention. 
         [0024]    In installing package  20  into vessel  11 , base ring  21  is installed onto its support (not shown for clarity) within the lower part of the vessel with the cylindrical sleeve  23  positioned on the ring  21  and extending upward between heating elements  16  and the wall  12  of vessel  11 , as shown in the FIGS.. As should be understood, sleeve  23  can be positioned onto the ring  21  and installed therewith or it may be positioned on the ring after the ring has been placed in the vessel. Holes  29  are then drilled or otherwise formed through insulation cylindrical sleeve  23  in alignment with openings  30  in the wall  12  of vessel  11  for a purpose explained below. Next, catch tray  22  is inserted through opening  21   a  in ring  21  so that a shoulder  22   a  on the tray  22  rests on and is supported by ring  21  (see  FIG. 4 ). 
         [0025]    Next the heating means, e.g. heating elements  16 , is installed within cylinder  23  and held in place by electrodes  32  inserted through aligned openings  29 ,  30  in the wall  12  of vessel and sleeve  23  and electrically connected to elements  16  to thereby provide the power for heating the furnace. Once this is done, the bottom insulation pack  26  is installed. 
         [0026]    Catch tray  22  has a central opening  22   b  therethrough which is approximately the same size and configuration as that of lower insulation layer  26  and support block  27 . As can be seen from comparing  FIGS. 1 and 2 , this allows both layer  26  and block  27  to move into and out of central opening  22   b  for a purpose described below. Lower insulation layer  26  is connected to movement arms (not shown) separate from support block  27 . Support block  27  is affixed to shaft  33  so that it will move in unison with the shaft as will be further explained below. Support plate  28  which is larger than opening  22   b  in tray  22  and slightly smaller or approximately the same size as crucible  15  is then positioned onto block  27  from within tray  22 . 
         [0027]    Crucible  15  is then positioned onto support plate  27  and is filled with the material to be melted (e.g. silicon). Top insulation cover  24  is installed and secured onto the top of sleeve  23  to effectively thermally isolate the crucible and heating elements  16  within the insulation package  20 . Vessel  11  is then closed by securing a cover  14  or the like in place and power is supplied to heating elements  16  through electrodes  32  to begin the heat-up operation; see  FIG. 1 . That is, shaft  33  is raised to position crucible  15  in the approximate center, both radially and longitudinally, of heating elements  16  to maximize the heat within the furnace. In this position, both support block  27  and more importantly, lower insulation layer  26  lie within opening  22   b  of catch tray  22 . Lower insulation layer  26  effectively acts as a “valve” to block the escape of heat from the heating elements during the heating operation, thereby maximizing the heat being transmitted to crucible  15  and protecting vessel  11 . 
         [0028]    Once the material in crucible  15  is melted, furnace  10  is moved to its “growth” or cooling position ( FIG. 2 ). That is, shaft  33  is moved downward to move the crucible  15  and block  27  downward to the position shown in  FIG. 2 . More importantly, lower insulation layer  26  is moved below and out of opening  22   b  in tray  22 . This now allows heat to flow or dissipate through the graphite block  27  (graphite being a good heat conductor) and out into the chamber  13  of vessel  11 . 
         [0029]    If crucible  15  should fracture during the heating or cooling operation (not uncommon under the extreme conditions involved), any leaking material (e.g. silicon) will be caught in the catch tray  22 . If a leak should happen, only the tray  22  is affected and only the tray will need to be discarded. The rest of the insulation package or jacket  20  can be reused thereby resulting in substantial savings since replacement of insulation is expensive. 
         [0030]    U.S. Provisional Patent Application 60/709,715 filed on Aug. 18, 2005, is incorporated herein by reference in its entirety.