Classifications

• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
  • C03B5/16—Special features of the melting process; Auxiliary means specially adapted for glass-melting furnaces
  • C03B5/225—Refining
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/02—Silicon
  • C01B33/037—Purification
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
  • C03B5/02—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture in electric furnaces, e.g. by dielectric heating
  • C03B5/021—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture in electric furnaces, e.g. by dielectric heating by induction heating
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B14/00—Crucible or pot furnaces
  • F27B14/06—Crucible or pot furnaces heated electrically, e.g. induction crucible furnaces with or without any other source of heat
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B14/00—Crucible or pot furnaces
  • F27B14/08—Details peculiar to crucible or pot furnaces
  • F27B14/10—Crucibles
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
  • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/547—Monocrystalline silicon PV cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention concerns the manufacture of sili ⁇ cium to constitute cells for generating electrical energy by photovoltaic effect.
• This silicon of superior quality to metallurgical silicon is generally referred to as photovoltaic silicon (Solar Grade or SoG).
• Photovoltaic silicon Solar Grade or SoG
• the silicon for photovoltaic techniques is essentially made up of the scrap of the microelectronics industry, because the silicon used for photovoltaic applications can contain a proportion of impurities (of the order of one part per million) less critical than the level of impurities (of the order of a part per billion) that is generally required in microelectronics.
• EP-A-0 459 421 discloses a method for purifying silicon of directing an arc plasma to the surface of a silicon bath contained in a hot silica-walled crucible (SiC> 2). high speed of the plasma causes a bath movement whose intensity depends on the power of the plasma.
• a hot crucible with refractory wall is a form of industrial crucible commonly used in the metallurgical industry.
• a disadvantage of this technique is that the silicon already heated by the electromagnetic excitation of the coil surrounding the hot crucible undergoes additional heating due to the plasma. This additional heating is typically several hundred degrees and causes the silicon bath to reach the melting temperature of the silica wall. Indeed, the melting temperature of the silica is of the order of 200 0 C higher than that of silicon. Under the effect of the melting of the walls, there is therefore a risk from the point of view of the safety of the installation due to the possible leakage of liquid metal.
• a hot crucible has a wall thickness limit of less than a few centimeters.
• the number of turns of the inductive winding around the crucible is relatively small.
• the number of turns of the inductive winding around the crucible is relatively small.
• the turns are spaced apart from one another in the height of the crucible, again for reasons of homogeneity of the field, and also for reasons of electrical insulation. Therefore, even if the winding is itself cooled (for example, by circulating water inside the turns), this is not sufficient to cool the outer wall of the crucible, if only by because of the gap between the different towers in the height of it.
• the present invention aims to propose a silicon purification plant, in particular intended for applications photovoltaic systems, using a cold crucible and not having the disadvantages of a conventional inductive cold crucible installation.
• the invention also aims to propose a solution compatible with the use of a plasma torch directed towards the surface of the bath to remove impurities.
• a facility for refining a silicon charge comprising a crucible comprising at least one hearth consisting of at least a first refractory material good conductor the heat ; a cooling means of the sole; a protective element consisting of at least a second refractory material which is poor in heat conductor and intended to be interposed between the crucible and the charge; and charge induction heating means comprising a coil disposed in or under the hearth.
• the sole is traversed by a pipe in which is intended to circulate a cooling fluid, said pipe being in the second refractory material, a third refractory material or an electrically conductive material.
• the winding corresponds to a hollow tube in which is intended to circulate a cooling fluid.
• the protection element corresponds to a powder comprising at least the second refractory material, the protection element having a pocket-shaped face and being intended to contain the charge.
• the protection element further comprises carbon at least at the level of said face.
• the sole comprises a rounded side on the load side.
• the protec ⁇ tion element comprises a portion covering the rounded face, said portion having a constant thickness to 10%.
• the winding conforms to the shape of the rounded face.
• the installation further comprises a plasma torch intended to be directed towards the free surface of the load.
• the crucible further comprises a metal side wall at the periphery of the hearth, the installation comprising a cooling means of the side wall.
• the side wall corresponds to a one-piece metal part having a cavity in which is intended to circulate a cooling fluid ⁇ disse.
• a method of refining a silicon charge comprising the steps of providing a crucible comprising at least one hearth of at least a first refractory material good conductor of heat; disposing in the crucible a protective element consisting of at least a second refractory material bad conductor of heat; placing the load on the protection element; to cool the sole; and heating the charge by induction heating means comprising a coil disposed in or under the hearth.
• the protective element corresponds to a powder comprising at least the second refractory material, the process consisting in distributing the powder in the crucible by forming a pocket-shaped face intended to contain the charge.
• FIGS. 1 and 2 are diagrammatic sections of exemplary embodiments. of a silicon refining plant according to the invention. detailed description
• a feature of the present invention is to provide a crucible comprising a cooled hearth, also called bottom or soleplate, of a refractory material and to provide an inductive heating means of the silicon bath comprising a coil which is disposed in the hearth or under the sole.
• the cooled side wall of the crucible when present, can then be made in a non-sectoral manner, which simplifies the construction of the crucible.
• Another charac ⁇ teristic of the present invention is to interpose a shielding member made of a refractory material poor conductor of heat between the cold crucible and the silicon melt. This keeps the silicon bath at a high temperature.
• FIG. 1 schematically represents an exemplary embodiment of a refining installation comprising a crucible 5 containing a silicon bath s.
• the crucible 5 comprises a cylindrical sidewall 10 with a circular base of a metal material, for example copper or steel inoxy ⁇ dable.
• the wall 10 contains a cavity 12 in which circulates a cooling fluid (for example water or air).
• the installation comprises an element 14 intended to organize the circulation of the cooling fluid in the cavity 12.
• the crucible 5 further comprises a sole 20, also called bottom or sole, made of a refractory material, good conductor of heat, for example refractory cement based on silicon carbide, comprising upper surfaces 21 and lower 22 opposite and planar.
• the upper face 21 is located on the side of the silicon bath s and the lower face 22 is located on the opposite side to the silicon bath s.
• the winding 23 is fed by a generator 24 (G) low frequency (typically a few tens to tens of thousands of hertz).
• G generator 24
• Induction heating of the silicon bath is obtained.
• Providing the winding 23 directly at the sole 20 makes it possible to obtain an effective electromagnetic coupling between the winding 23 and the silicon bath s.
• the hearth 20 may be traversed by a cooling pipe 26 in which a cooling fluid (for example water) circulates.
• the cooling duct 26 can be interposed between the upper face 21 and the winding 23 to reduce the heat flow reaching the winding 23.
• the pipe 26 is, for example, circular or square section and can be made in a refractory material that is a good conductor of heat, for example a material based on silicon carbide.
• the pipe 26 is then, advantageously, substantially transparent to the electromagnetic field emitted by the coil 23. This improves the efficiency of the refining process.
• the pipe 26 can be made of a good material conducts heat and electricity, for example copper or stainless steel.
• the refining installation comprises an element 28 intended to organize the circulation of the cooling fluid in the pipe 26.
• the winding 23 can be arranged on the side of the upper face 21, that is to say interposed between the upper face 21 and the cooling pipe 26.
• the winding 23 may correspond to a hollow tube in which circulates a cooling fluid, for example water.
• a cooling fluid for example water.
• the sole 20 can be directly cooled by the cooling fluid circulating in the winding 23.
• the cooling pipe 26 may not be present.
• the winding 23 can be placed under the hearth 20 near the lower face 22 of the hearth 20.
• the driving refroi ⁇ disse 26 may be disposed in the floor 20 so as to project at least partially protruding at the upper face 21.
• a protective member 30 of a refractory material which is poor in heat conductance is interposed between the crucible 5 and the silicon bath s.
• the material constituting the protective element 30 is chosen so as not to react chemically or weakly to react with the molten silicon. This is for example a powder of a refractory material, such as alumina, quartz, zirconia or silica, or a mixture of two or more of these materials.
• the powder is then packed so as to be as compact as possible and define a pocket-shaped face 32 containing the silicon bath s, for example conical, spherical or elliptical, as continuous as possible.
• a very fine particle size powder can be used, for example a powder having a particle size of less than 10 microns.
• the fact that the protective element 30 consists of a non-sintered powder facilitates the realization of the face 32 of the protective element 30 containing the silicon bath s. Indeed, once the powder disposed in the crucible 5, the face 32, for example pocket-shaped, can be made very simply for example by pressing the powder through a punch.
• the thickness of the protection element 30 is sufficient to limit the heat flow of the silicon bath s towards the sole 20 and the side wall 10.
• the minimum thickness of the protective layer 30 is greater at least one millimeter, preferably greater than 5 millimeters.
• the protective element 30 further prevents direct contact between the molten silicon bath and the side wall 10 and the hearth 20 of the crucible 5. This enables the side wall to be made.
• the protective element 30 provides protection in case of incidental cooling of the molten silicon. Indeed, in case of solidification, the silicon tends to increase in volume and exert pressure on the protection element 30.
• the protective element 30, which has a powdery consistency, tends to deform easily, reducing the forces exerted on the side wall 10 and the sole 20 of the crucible 5.
• the protective element 30 also comprises a carbon powder, for example graphite, which may be mixed with the remainder of the protective element 30 or which may correspond to a layer of a pure carbon powder disposed at the face 32 of the protection element 30.
• the carbon may be used to trap by capillarity certain impurities of the molten silicon (in particular iron and / or boron) of the silicon bath which tend to react with the carbon.
• certain impurities of the molten silicon in particular iron and / or boron
• the carbon is arranged in the form of a layer covering the face 32 of the protective element 30, it is even possible, in use, to observe the formation of a layer of carbide silicon at the face 32 of the protection element 30.
• the silicon bath may not be in direct contact with the protective element 30.
• the silicon bath may be contained in an intermediate crucible made of a refractory material, for example silica, the intermediate crucible being disposed in contact with the protective element 30.
• the intermediate crucible may be integral or consist of several pieces connected to each other.
• the protective element 30 may be rigid and correspond to a single piece or consist of several pieces connected to each other.
• the protective element 30 is, for example, obtained by sintering a powder of a refractory material.
• the protective element 30 is then placed in the crucible 5 in contact with the side wall 10 and the hearth 20 and defines an internal volume receiving the silicon bath s.
• 2 shows another embodiment of the crucible 5 in which the upper face 21 of the sole 20 has a rounded shape, corresponding, for example, to a portion of ellipsoid, a spherical portion, a cone, etc.
• the protective element 30 may then correspond to a layer of a powder of a refractory material or of several materials refractory, this layer being uniformly disposed on the upper face 21 of the sole 20.
• the thickness of the protective layer 30 may be constant to within 10% and greater than at least one millimeter, and preferably , greater than 5 millimeters. This has the advantage of allowing better control of heat exchange between the molten silicon bath and the sole 20.
• the hearth 20 may have a constant thickness so that the lower face 22 of the hearth 20 also has a rounded shape which reproduces the shape of the upper face 21.
• the coil 23 is disposed under the lower face 22 of the sole 20 of which it advantageously conforms to the shape. According to a variant, the coil 23 is disposed in the hearth 20, for example near the upper face 21 of the hearth 20 whose shape it matches.
• the curvature of the sole 20 may be sufficient for the side wall 10 is not present.
• the holding of the crucible 5 is then directly carried out at the sole 20.
• the dimensions of the crucible 5, in particular the dimensions of the protection element 30 are such that the silicon bath is globally contained in a cylindrical volume of diameter D and height h such that the ratio between the height h and the diameter D is less than 0.5, preferably less than 0.1.
• an inductive plasma torch 35 is provided which is placed so that the flame f of the plasma licks the free surface of the silicon bath s.
• the holding device of the plasma torch 35 is not shown.
• the function of the plasma is to create a medium formed of free radicals and ions of the plasma gas or gases in the vicinity of the free surface of the bath.
• the atmosphere thus created is extremely reactive and the impurities present on the surface of the bath combine with the reactive gas of the plasma and become volatile (or, conversely solids) at the surface temperature of the bath.
• the entire installation is maintained under a controlled atmosphere, which allows the molecules containing the impurities to be evacuated as and when they occur.
• the plasma torch 35 comprises, for example, a feed 36 of reactive gas gr in the center of the torch, a concentric feed 37 of an auxiliary gas ga (for example, argon).
• a plasma gas gp (for example, also argon) is further conveyed concentrically to the auxiliary gas ga.
• An induction coil 38 surrounds the free end of the torch 35 so as to create the inductive plasma.
• the coil 38 is generally excited by an alternating current at a frequency of the order of one megahertz by a generator 39.
• different reactive gases can be injected into the plasma, either simultaneously or successively for their selective actions on the undesirable elements. .
• the crucible 5 of the embodiments described above may comprise a casting device 40 located for example at the bottom and in the center of the hearth 20.
• the casting device 40 is, for example, consisting of an orifice initially closed using a valve or a slide valve placed beneath the protection element 30.
• the protection element 30 advantageously protects the casting device 40 from direct contact with the silicon in the melting and heating phases. Silicon purification.
• the casting device 40 may also include a stopper device or a sintered silica washer. Alternatively, the casting device 40 may not be present.
• the crucible 5 can then be mounted on a rotary element, not shown, to reverse the contents.
• the protective element 30 is placed in the crucible 5 and shaped in the case where the element of protection 30 has a powdery consistency.
• the protective element 30 is then filled with a silicon charge consisting of powder, chips, or silicon debris.
• a load of 200 to 400 kg can be disposed in the protection element 30.
• the silicon being semiconductor, it must be preheated before becoming progressively conductive (around 800 0 C) and then be able to be induction heated by means of the coil 23.
• the plasma torch 35 is first actuated to preheat the solid silicon charge and bring it to the temperature that makes it possible to obtain a coupling with the low-frequency field created by the coil 23 of the crucible 5.
• the gas used in this preheating phase is preferably argon. If necessary, hydrogen is introduced as a reactive gas to increase the thermal conductivity of the plasma and thus accelerate the preheating of the silicon charge.
• a second phase promotes mixing tur ⁇ bulent bath in the direction of arrows in Figures 1 and 2 and is introduced into the plasma, simultaneously or sequentially, one or more suitable reactive gases to eliminate impurities which, when combined with a reactive gas on the surface of the bath, form volatile species which are vaporized.
• the silicon thus purified can be doped with elements promoting the photovoltaic power of the polycrystalline silicon by passivating the defects, for example by doping it with hydrogen.
• the silicon once refined and optionally doped, is emptied from the crucible 5 by means of the casting device 40 or by inclination of the crucible 5. Part of the molten silicon may be left in the crucible 5 for to promote the melting of solid silicon pieces added to the crucible 5 for the treatment of a new charge of silicon.
• Forming an electromagnetic field into the bath of liquid silicon s having high fluidity allows an efficient stirring of the molten silicon that promotes purification by authorized ⁇ gation impurities then "skimming" thereof on the surface of the bath.
• the applicant has demonstrated by simulation that the electromagnetic forces in the s silicon melt are essentially vertical, which is favorable for brewing This rise is also favored by the low relative depth of the bath due to the low shape of the crucible 5.
• the diameter D of the silicon bath s (associated with its low height h) allows the obtaining an effective surface "evaporation" treatment while allowing the treatment of a large quantity of silicon for each silicon charge to be treated, Moreover, the small relative depth of the crucible 5 makes it possible to e easily achieve almost complete evacuation of molten silicon by moderate tilting of the crucible.
• the present invention is susceptible of various variations and modifications which will be apparent to those skilled in the art.
• the gas used will be selected func ⁇ impurities to be removed.
• the dimensions of the various elements of the installation are within the reach of those skilled in the art from the functional indications given above and from the application.
• a cylindrical crucible with a circular base has been described, provision may be made for the use of a frustoconical crucible or a crucible with a square or rectangular base.
• the purification of the molten silicon can be carried out by any suitable means.
• a system for injecting reactive gas bubbles directly into the molten silicon can be used.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• General Engineering & Computer Science (AREA)
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• Silicon Compounds (AREA)

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• 2007
  • 2007-02-14 FR FR0753256A patent/FR2912397B1/fr not_active Expired - Fee Related
• 2008
  • 2008-02-12 AU AU2008220638A patent/AU2008220638B2/en not_active Ceased
  • 2008-02-12 EP EP08762072A patent/EP2118005A2/fr not_active Withdrawn
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  • 2008-02-12 US US12/527,036 patent/US20130133373A1/en not_active Abandoned
  • 2008-02-12 WO PCT/FR2008/050220 patent/WO2008104702A2/fr active Application Filing
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