FR2997419A1 - CREUSET INCORPORATING A SIALON COATING. - Google Patents

Abstract

Ceramic crucible for the manufacture of a silicon ingot, having a bottom and walls whose surfaces define an interior volume, said crucible being characterized in that it comprises: a core made of a material consisting of silica, nitride silicon, silicon oxynitride, silicon oxynitride and aluminum, graphite, silicon carbide or a mixture of at least two of these compounds, whose open porosity is between 15 and 60 % and the median pore diameter is between 0.1 and 10 microns and - a coating layer on said surfaces defining the interior volume of the crucible, said layer having a porosity of between 15 and 50% and consisting essentially of minus a crystalline phase of sialon type.

Description

BACKGROUND OF THE INVENTION The invention relates to a crucible that can be used in particular for obtaining silicon ingots by melting and crystallization techniques. Such ingots are particularly used in the field of solar energy, for obtaining polycrystalline silicon substrates used in photovoltaic cells. It has long been known to use crucibles for the manufacture of polycrystalline silicon ingots, as well as problems to be solved for obtaining such ingots. In particular, it is described in numerous patent applications the use of various refractory materials deemed inert for obtaining such crucibles. Among these materials, graphite, silicon oxide, in particular in quartz form, boron nitride or silicon nitride are most often mentioned. For example, it is described in US2009 / 0119882 or WO 2006/02779, the use of silicon oxide crucibles. The use of such crucibles however raises the problem of the reactivity of the molten silicon with the silica constituting the crucible. Such a reaction results after crystallization by areas of strong adhesion between the crucible and the ingot, complete destruction of the crucible may be necessary to recover the ingot. In addition, the difference in thermal expansion coefficients between the crucible material and the silicon may also cause stresses in the ingot, which may even crack during cooling. Also, the oxygen contained in the crucible can be a pollution factor for silicon ingots, the final purity of which is an essential element for their good use in the manufacture of photovoltaic cells. It is also described, for example in the application US 2009/0119882, the use of a coating comprising a mixture of silicon nitride and silicon oxide on the walls of the crucible, to avoid the appearance of bonding zones between the crystallized ingot and the crucible.

According to other publications, for example the applications WO2007 / 148986 or WO2004 / 016835, it is alternatively proposed to use silicon nitride crucibles, in particular obtained by reactive sintering (often called RBSN for Reaction Bonded Silicon Nitride).

To avoid the appearance of strong bonding zones between the crucible and the crystallized silicon ingot, it is described in the application WO 2004/016835 to affix a silicon nitride coating on the walls of the crucible, itself already being in silicon nitride. Deposition of such a coating of silicon nitride on a crucible made of the same material already shaped, however, poses problems of sintering and ultimately adhesion of the surface layer, as described in the publication WO 2004 / 053207 A1, paragraphs [0008] and [0009], again with a risk of pollution of the silicon ingot. To solve such a problem, the application WO 2004/053207 A1 proposes a thermal spraying of a mixture of silicon metal, silicon nitride and silica, supposed to lead to low final contamination of the ingot. However, this solution is relatively expensive and requires specific tools, difficult to implement and adjust on an industrial site of production.

U.S. 4,099,924 discloses a crucible for growing a silicon crystal in which a silicon oxynitride protective layer is deposited by CVD while the crucible is maintained at a high temperature, i.e. between 800 and 1500 ° C, for a period of the order of several hours and in the presence of ammonia NH3 and nitrous oxide N20 and under a stream of hydrogen. Such a process necessarily leads to contamination of the crucible itself by these different species, which in turn may contaminate the ingot, even in the presence of the silicon oxynitride barrier. U.S. Patent No. 6,110,853 discloses a plasma spraying thermal plasma coating made of Si3N4 particles in an at least partially amorphous nitride bonding matrix. In the specific case of the melting-recrystallization of a silicon ingot, a coating comprising such an amorphous or semi-amorphous phase is capable of reacting with the molten silicon to form intermediate phases at the coating / silicon interface, which in the end will lead to difficulties in removing the ingots. The publication of Sodoeka et al., J. Thermal Spray Technology, Vol. 1 (1992) describes the use of this same plasma-assisted thermal spraying technique for depositing a Sialon-type coating. The publication, however, indicates that the deposition results in partial decomposition of the Sialon phase and alumina formation, again with a reaction between the molten silicon during the preparation of the ingot and the formation of unwanted intermediate phases at the interface. coating / silicon. The coatings described in these publications thus remain perfectible, in particular with regard to the coating used: either they exhibit a still too strong reactivity with the molten silicon (with finally zones of strong adhesion between the coating and the silicon after crystallization) or they lead to oxygen contamination, even low, ingot finally obtained. Even if the most recent solutions offer improved compromises that are even acceptable with respect to the two preceding disadvantages with regard to a first melting-recrystallization of an ingot in a new crucible, they do not have sufficient performance, both at the level of the appearance of adhesion zones that ingot contamination, to allow possible reuse of the same crucible for obtaining a series of ingots.

The object of the present invention is therefore first of all to provide a crucible for the manufacture of a silicon ingot to find the best compromise between the economic profitability of said process, the purity of the silicon ingot finally obtained and the ease with which it is possible to extract the latter from the mold formed by said crucible. By economic profitability is meant that the crucible according to the invention is relatively simple to manufacture and does not require the implementation of specific tools, but also that it is reusable to ensure multiple cycles of melting-crystallization of silicon ingots. It is also understood by this term that the ingot manufacturing process does not require special equipment, expensive and / or complex to use in its implementation on an industrial scale. By ease of recovery, it is understood within the meaning of the present invention that the ingot finally obtained is easily "extractable" from the mold, that is to say that there is no or very few strong bonding areas between said mold and the solidified ingot, so that the ingot can be extracted without appreciable deterioration of the mold. In addition the surface of the crucible must have a high resistance to abrasion so as not to be damaged during the loading of the crucible with silicon granulate before heating the reactor. By purity of the silicon ingot, it is meant that the ingots obtained successively on different melting-crystallization cycles, and from the same crucible, have a purity sufficient for the desired use, that is to say the production of semiconductors for photovoltaic cells. To solve the technical problem as described above, the experiments conducted by the applicant company have shown that it was necessary to act on a series of characteristics relating both to the nature of the crucible and that of the coating disposed on said crucible and whose main function is to avoid the appearance of strong bond zones between the crucible and the solidified ingot. More particularly, the present invention aims to solve the problems previously exposed by the application on the walls of a "raw" crucible of a coating consisting essentially of a Sialon phase and having no or very few phases of another nature, in particular no or few oxide and / or amorphous phases. More specifically, the present invention thus relates to a ceramic crucible usable in particular for the manufacture of a silicon ingot, having a bottom and walls whose surfaces define an interior volume, said crucible being characterized in that it comprises a core (also called a "raw" crucible in the present description) made in a ceramic material, in particular a material consisting of silica, silicon nitride, silicon oxynitride, silicon oxynitride and aluminum, graphite, or silicon carbide, or constituted by a mixture of at least two of these compounds, material whose open porosity (by volume) is between 15 and 60% and the median pore diameter (in volume) is between 0.1 and 10 microns and a coating layer on said surfaces defining the interior volume of the crucible, said layer having a porosity of between 15 and 50%. t being essentially constituted by one or more crystallized phases of sialon type. According to preferred embodiments of the present invention, which may, if necessary and without any indication to the contrary, be of course combined with one another: The layer consists essentially of a crystalline phase of Sialon in IP form according to an Si6 2 formulation. , Aly8'0, with z greater than or equal to 1.0 and less than or equal to 4.2 and preferably with z greater than or equal to 1.5 and z is less than or equal to 3.2. According to an alternative method, the layer is essentially constituted by a crystalline phase of Sialon in O 'form, of formulation Si2'Alx1 \ 12-.01 + x, with x less than or equal to 0.4. According to another alternative method, the layer consists essentially of a crystalline phase of Sialon is in the form a 'and correspond to the formulation MxS i 12- (m + n) Alm + nOnNi6n in which is x less than 2 and M represents at least one element selected from the group consisting of Ca, Mg, Y or a rare earth. According to another alternative method, the layer consists of a mixture of crystallized phases in the forms a 'and / or Wet / or O'. The coating layer comprises less than 1% of amorphous phase and preferably wherein the coating layer is free of amorphous phase. said crystallized Sialon phase is present in the coating in the form of grains whose median size is between 0.5 and 30 microns, preferably between 0.5 and 5 microns. The core material consists essentially of silicon nitride grains, preferably of the RBSN type, preferably comprising less than 3% of oxygen. The core material consists essentially of grains of a silicon and aluminum oxynitride, in particular Sialon in the form a ', p', O 'or a mixture of at least two of these forms. - The porosity of the coating is between 20 and 40%. - The porosity of the core material is between 20 and 40%. - The average thickness of the coating layer is less than 1000 micrometers, preferably less than 500 micrometers. - The median size of sialon grains is between 0.5 and 5 times the median pore diameter of the material constituting the crucible. Importantly, the layer constituting the coating according to the invention is very essentially composed of crystallized phases, that is to say that at least 95% or even 99% by weight of the coating layer is in crystallized form. Preferably, the layer comprises less than 1% by weight of amorphous phase and is preferably free of amorphous phase.

The SiAlON coating material may, however, optionally include other elements in the form of unavoidable impurities, such as C, Cl, F, Fe, Cu, Ca, Na, K, Mg, Cr, Co, W, Ni, Ti , Zr, P, B these impurities being inherent to its manufacturing process. Preferably, the raw materials and the operating conditions making it possible to avoid contamination are chosen in such a way that the material comprises one or more following chemical characteristics (the contents being expressed relative to the total inorganic mass of the coating layer). ): the Fe content of the coating layer is less than 500 ppm or even less than 200 ppm. The boron content is less than 20 ppm, preferably less than 5 ppm. -The phosphorus content is less than 5ppm. The content of metallic silicon in the product is less than 1%, or even less than 0.5%, preferably less than 0.2%. The residual phase content of A1203 alumina, alpha or gamma in particular, is less than 5%, or even less than 2%, preferably less than 1%, or even undetectable by X-ray diffraction measurement. Aluminum metal is less than 0.2%, even less than 0.1%, or even less than 100 ppm or even less than 50 ppm of Aluminum Metal. By sialon (or SiAlON), we conventionally mean structures built from (Si, Al) (0, N) 4 tetrahedra, the vertices of which are connected together. For more details, reference may be made to the reference publication: KH Jack, "Sialons: a study in materials development, S. Hampshire (Ed.), Non-oxide Technical and Engineering Ceramics, Elsevier Applied Science, London (1986). ), pp. 130 ". The term SiAlON does not include, according to the present invention, the so-called Si 2 ON 2 oxynitride phase and without the element Al.

For the purposes of the present invention, the term "essentially constituted by at least one sialon-type crystallized phase" is intended to mean that said layer may comprise other materials or compounds but in such a limited quantity that its essential characteristics are not materially affected by their presence. By essential characteristics is meant in particular the properties sought and as described above. By way of example, in the context of the preceding definition, at least 90% of the total inorganic mass of the coating layer consists of sialon, especially in the form a ', IP, O', 15R or X, in particular p, preferably wherein z is between 1.5 and 3.2. More particularly, according to a very preferred embodiment of the invention, at least 95% of the total inorganic mass of the coating layer consists of sialon, especially in the form a ', 13 ,, 0', 15R or X, in particular p, preferably wherein z is between 1.5 and 3.2. Without departing from the scope of the invention, the layer may for example comprise, apart from the very majority phase (s) of the SiALON type, one or more crystallized phases, in particular an Si3N4 alpha and / or beta phase, and / or a 5iO2 phase. and / or an oxynitride phase not comprising the element A1, for example the Si2ON2 phase. These phases, however, preferably represent less than 5% or even less than 1% of the total inorganic mass of said layer, as determined for example by X-ray diffraction.

The median size of the particles constituting the coating layer can conventionally be measured by scanning electron microscope (SEM) analysis, preferably on a cross section of the coating. The determination of the median size of the particles constituting the coating layer is illustrative and comprises the following sequence of steps, typical in the field: A series of SEM images is taken of the observed coating material in a cross section (that is to say, in all its thickness). For more clarity, the pictures are taken on a polished section of the material. The acquisition of the image is performed on a cumulative length of the coating at least equal to 1.5 cm, in order to obtain values representative of the entire sample. The images are preferably subjected to binarization techniques, well known in image processing techniques, to increase the contrast of the particle contour. - For each particle constituting the coating, a measurement of its area is measured. An equivalent diameter is determined, corresponding to the diameter of a perfect disc of the same area as that measured for said particle (this operation possibly being carried out using a dedicated software, notably the Visiloe software marketed by the company Noesis ). a size distribution of particles or grains is thus obtained according to a conventional particle size distribution curve and a median size of the particles constituting the determined coating layer, this median size corresponding to the equivalent diameter dividing said distribution into a first population comprising no particles of median diameter greater than or equal to this median size and a second population comprising only particles of median diameter less than this median size.

It has been found according to the invention that a lower median size leads to a coating that is too reactive with respect to the molten silicon and poses problems of demolding after solidification of the silicon ingot. A median size that is too high leads to a coating that is too porous and too fragile to load the crucible. The porosity of the coating is preferably greater with respect to transport or at 15% and even more preferably above 20%. The porosity of the coating is preferably less than 45% and even more preferably less than 40%. The coating is thus sufficiently porous to allow easy demolding. According to the present invention, such porosity characteristics make the coating layer a "sacrificial" layer. It seems in particular under these conditions that it is not or very little impregnated by the molten silicon, thus avoiding the creation of strong adhesion zones hindering the demolding of the solidified silicon ingot after cooling. In other words, the porosity of this layer, driven by the size and the mass proportion of the SiAlON grains (or else by the size and the mass proportion of precursor powders of the SiAlON phase in the case of reactive sintering), is such that infiltration by molten silicon is reduced. The median pore diameter in said layer, as measured on electron micrographs, is between 0.1 and 20 microns, preferably between 0.2 and 15 microns, even more preferably between 0.5 and 10 micrometers, or even between 1 and 5 micrometers. Such a range of median pore diameter further reduces the wettability of the SiAlON type coating by the molten silicon. The measurements of the porosity and the median pore diameter of the coating layer are obtained by image analysis on cross sections of the layer, that is to say perpendicular to the plane formed by the coating layer, according to techniques similar to those already described in connection with measuring the particle size of the coating layer. Reference will also be made to the additional explanations given in Example 1 for the details of the measurement. The porosities and pore diameters for the core of the crucible (raw crucible) according to the present invention are measured by conventional mercury porosimetry techniques and are given by volume. The percentages of oxygen, in particular in the silicon nitride constituting the raw crucible, are measured after taking samples according to standard techniques and using in particular a device of LECO type TC436 (melting under an inert gas according to the well-known techniques of the invention. skilled person). The phases, in particular the crystalline phases present in the layer or in the crucible can be determined and quantified by X-ray diffraction and structural refinement according to the Rietveld method well known to those skilled in the art. The median diameter of the grain powders initially used for forming the coating according to the invention is obtained according to conventional techniques by a particle size distribution characterization by means of a laser granulometer. For the purposes of the present invention, the term "median diameter" of a powder consisting of a set of grains or particles, the percentile d50, that is to say the size dividing the particles into first and second equal populations in mass, these first and second populations comprising only particles having either a diameter greater than or equal to this median diameter, or a diameter less than this median diameter. According to a first possible mode, the SiAlON type crystallized phase or phases are present in the form of a matrix forming a continuous, possibly porous layer, in which may appear rather elongated, acicular or even fibrous structures, typically of 0.1 to 2 micrometers and of length up to 100, even 200 or even 500 micrometers. This first mode is in particular when the coating layer according to the invention is obtained by reactive sintering of precursors according to the first method described below. The term "reactive sintering" is conventionally referred to as a heat treatment whereby the powders constituting the layer before firing will react together to form a microstructure made of a material having a specific mechanical strength and forming a continuous material. In the particular case of reactive sintering under nitrogen, the atmosphere also reacts with the powders which can in particular nitride and form nitrogen phases, in particular at least one phase of the SiAlON type. The conditions of nitriding firing and / or the addition of elements such as Ca and / or Mg for example can influence the diameter, the length of these acicular structures and lead to crystalline structures with more massive crystals with length to diameter ratios. neighbors of 1.

According to a second possible mode, the crystalline phase or SiAlON type are present in the form of SiAlON particles whose population has a median size of between 0.1 to 20, preferably 0.5 to 10 microns. This mode is especially carried out when the coating layer according to the invention is obtained by depositing a mixture of SiAlON grains previously synthesized according to the second method described below.

In a third embodiment, the coating layer is formed of SiAlON particles in a SiAlON matrix. The coating of the invention according to this mode does not include intergranular phases whose formulation does not correspond to a sialon. Such a mode is notably achieved by the introduction, in step c) according to the first process for obtaining the coating layer obtained by reactive sintering, of presynthesized sialon grains, for example identical to those described in step a ) of the second method of obtaining the coating layer described above. Without departing from the scope of the invention, step a) of the first method of obtaining the coating layer described above is then optional as shown in Example 4.

According to a first method according to the invention for obtaining the coating on the raw crucible, the coating is obtained by reactive sintering on the raw crucible. More precisely, such a process comprises the following steps: a) Selection of the following inorganic powders: a silicon nitride powder, with a median diameter of between 0.1 and 10 microns, a silica powder, with a median diameter between 50 nanometers and 5 microns, preferably between 100 nanometer and 1 micron, aluminum metal powder, purity greater than 99.9% by weight, preferably greater than 99.99%, and more preferably greater than 99.999% % or alternatively an aluminum-silicon powder having a median diameter of between 3 and 20 microns and a mass composition close to the eutectic (approximately 13% by weight of silicon). optionally, in the case of the synthesis of a sialon of type a ', precursor powders in particular organic or organo-mineral elements Ca, Mg, Y or a rare earth, these elements being added preferably in oxide form. In this step a) the mass proportions of the inorganic powders are chosen so that relative to the total weight of the inorganic powders: the relative weight of Si 3 N 4 is between 0 and 80%, preferably between 20 and 60%, so more preferably 30 to 60%, - the relative weight of SiO 2 is between 10 and 55%, preferably between 20 and 40%, more preferably 23 to 37%, - The relative weight of Al is between 10 and 50%, preferably between 15 and 40%, more preferably 20 to 35%. In the case of the synthesis of a sialon of type a ', the relative weight of Ca, Mg, Y or of rare earth is less than 5% by weight, for example between 0.5 and 1.5%; The sum of the relative weights is obviously equal to 100%. By way of example, relative proportions of 30% Si3N4, 37% SiO2 and 33% Al are used to obtain a majority phase of SiAlON IP with z = 3. Proportions of 55% of Si3N4, 23% of SiO2 and 22% of Al are used to obtain a majority phase of SiAlON p, z = 2. Relative weight proportions of 53% SiO 2 to 47% Al are used to obtain a majority phase of SiAlON p, z = 4. b) Preparation of a suspension from the aluminum powder with tannic acid or a tannic acid derivative in a particular aqueous solvent, preferably in deionized water or in an alcohol, especially in ethanol. The amount of tannic acid is preferably between 15 and 35% of the weight of aluminum, preferably about 25% of the weight of aluminum. The tannic acid or its derivative and the aluminum powder are premixed so as to obtain a suspension of particles preferably comprising between 10 and 30% by weight of aluminum. The suspension is preferably homogenized in a jar spinner for at least 0.5 hours, preferably between 0.5 and 2 hours. C) Preparation of a suspension comprising the other powders in a solvent preferably identical to that used in step b), in which the inorganic filler formed represents at least 25% by weight of the suspension. The particles are preferably dispersed in a jar spinner for at least 2 hours, preferably 8 to 48 hours and more preferably 12 to 24 hours. d) Mixing suspension c) and suspension b). Preferably the mixture is homogenized by rotation in a jar spinner for at least 0.5 hours. Said mixture preferably comprises 10 to 60% by weight of inorganic or organo-mineral particles, a proportion of 20 to 30% being more preferred since it may not require preheating of the crucible. If the proportion of the particles in the suspension is greater than 30%, heating of the crucible between 50 and 90 ° C, preferably about 80 ° C, is carried out. e) Deposition of the mixture of suspensions on a ceramic raw crucible including a technique described below, the crucible consisting in particular of silica, silicon nitride, silicon oxynitride, silicon oxynitride and aluminum, graphite, silicon carbide or a mixture of at least two of these compounds. f) drying, preferably until the residual moisture is less than 0.5% by weight, preferably less than 0.3%. g) firing under a nitrogen atmosphere, preferably nitrogen of purity greater than 99.95% by weight and more preferably under atmospheric pressure at a temperature between 1100 ° and 1650 ° C, preferably between 1300 and 1550 ° C, and for a period of between 1 hour and 10 hours depending on the chemical composition of the finally obtained sialon. Optionally the process may comprise, before the firing step g), a debinding step preferably at a temperature below 550 ° C, under oxidizing atmosphere.

Preferably the Silicon, Silica and Aluminum Aluminum nitride powders are chosen so that their purity is compatible with the inorganic chemical composition of the desired coating. In particular, the aluminum powder preferably comprises less than 5 ppm of boron and or phosphorus. According to a second process for obtaining the coating on the raw crucible, the coating is applied to the crucible from a mixture of SiAlON grains previously synthesized. More precisely, according to this mode, the following procedure is carried out: a) preparation of an initial suspension comprising: a presynthesized SiAlON powder, with a median grain diameter of between 0.5 and 30 microns, preferably between 1 and and 20 microns, optionally dopants added during the preparation of the SiAlON powder or subsequently in the mixture, said doping elements containing the element Ca, Mg or a rare earth, in particular Y. - preferably a addition of inorganic particles of nitride and / or silicon oxynitride, preferably between 1 and 10%, preferably about 5%, optionally an organic binder in order to give sufficient mechanical strength to the layer before firing and increase the porosity of said layer after firing, - a solvent, preferably deionized water b) depositing the suspension on the raw ceramic crucible including a technique described below, the raw crucible being consisting in particular of silica, silicon nitride, silicon oxynitride, silicon oxynitride and aluminum, graphite, silicon carbide or a mixture of at least two of these compounds. c) drying, preferably so that the residual moisture is between 0 and 0.5, preferably less than 0.3%, d) consolidation heat treatment in an oxidizing or inert atmosphere at a temperature greater than 700 ° C and preferably less than or equal to 900 ° C under an oxidizing atmosphere or at a temperature below 1500 ° C under an inert atmosphere, each treatment being of a duration typically between 0.5 and 4 hours. Treatment in an inert or reducing atmosphere is preferred if the SiAlON powder has undergone prior oxidation treatment or if a pre-oxidized oxynitride or Si3N4 powder has been added. According to this second method, the SiAlON powder used during step a) can in particular be prepared according to the principles described above in relation to the first method for obtaining the coating according to the following protocol: implementation of steps a) to d) of the first method described above. A temporary binder for example polyethylene vinyl alcohol (PVA) and / or polyethylene glycol (PEG) is this time preferably added to the mixture of suspensions obtained at the end of step d). In one possible mode, this mixture optionally comprising the temporary binder is subjected to a granulation or atomization process well known to those skilled in the art. The size of the granules obtained is preferably between 50 and 500 microns. These granules can be dried under the same conditions already described in step f) of the first process. The granules are cooked under the same conditions already described in step g) of the first process. A grinding step may be necessary in order to obtain a powder of median desired diameter. According to a possible alternative method, to the suspension obtained in step b) are added the other powders so as to obtain a mixture comprising 3 to 5% of solvent, the solvent being preferably water, to which can be added 0, 5 to 3% by weight of a temporary binder relative to the mass of inorganic and optionally organo-mineral powders. The mixture is shaped in the form of briquettes for example of dimensions 220 * 110 * 30mm. These briquettes can be dried under the same conditions already described in step f) of the first process. The briquettes are then fired under the same conditions already described in step g) of the first process in order to obtain the composition of SiAlON type according to the invention. The briquettes are then ground so as to obtain a powder of median diameter desired, or possibly and additionally chemically treated to remove impurities particularly unwanted Fe. Optionally, prior to suspending, the SiAlON powder may have undergone oxidative post-treatment to increase its reactivity, i.e., improved sinterability without the addition of other inorganic particles, sintering or special binder. This oxidizing treatment may advantageously consist of an air baking of the powder, at a temperature preferably between 900 and 1400 ° C, preferably with a step of 1 to 4 hours. In the two processes described above, the initial powders are advantageously mixed with a solvent such as preferably deionized water, so as to obtain a slip generally comprising 10 to 60% by weight of the grains or particles constituting them, the rest being the solvent, said slip being then deposited on the surface of the bottom and the walls by a technique preferably selected from tempering, immersion, spin coating, brushing, spraying, scraping ( doctor blade), brushing (painting) and finally drying at a temperature evaporating the solvent, said temperature being below 200 ° C. In the case of spraying with a spray gun, those skilled in the art adapt, according to the usual techniques in the field, the different parameters of the device in order to obtain the thickness deposition and the quality of the desired surface finish, in particular by setting the propellant gas pressure (preferably air); slip or slurry flow rate for deposition (by adjustment of opening or nozzle); the concentration of the slip stream to spray by secondary gas to change the projection angle; gun-target distance. It is also possible to superpose thin layers of 10 to 20 microns by making different passes, since the lower layer is dried quickly. The invention also relates to a process for producing a Sialon powder which can be used as a coating in a crucible as previously described, the process comprising the following steps: a) Selection of the following inorganic powders: nitride powder of silicon, with a median diameter of between 0.1 and 10 microns, a silica powder with a median diameter of between 50 nanometers and 5 microns, and an aluminum metal powder with a purity of greater than 99.9% by weight b) Preparation of a suspension from the aluminum powder with tannic acid or a tannic acid derivative in a particular aqueous solvent, preferably in deionized water, the amount of tannic acid being preferably between 15 and 35% of the mass of aluminum, c) Preparation of a mixture comprising suspension b) and the other powders in a solvent which is preferably identical to that used in stage b), wherein the inorganic portion is at least 25% by weight of the mixture and to which is added a temporary binder, d) optionally granulation or atomization to obtain granules, preferably of size between 50 and 500 microns, or setting direct form of the mixture for obtaining briquettes, e) Drying, preferably until the residual moisture of the granules or briquettes is less than 0.5% by weight, f) Cooking under a nitrogen atmosphere at a temperature of between 11000 and 1650 ° C, so as to obtain granules or briquettes consisting essentially of at least one Sialon type phase, 25 g) grinding in order to obtain a SiAlON powder, whose median grain diameter is between 0.5 to 30 microns, preferably between 1 and 20 microns. Finally, the present invention relates to the manufacture of a silicon ingot, comprising the steps of melting high purity silicon at about 1500 ° C in a ceramic crucible as previously described, to solidify a silicon ingot and demolding said ingot of the crucible.

The examples which follow, given purely by way of illustration, illustrate the technical advantages related to the implementation of the present invention but in no way limit the scope of the present invention. Example 1 (comparative): A silicon nitride crucible is obtained by reactive sintering from a silicon powder whose median grain diameter is of the order of 30 microns. The powder is placed in a mold sized accordingly. The silicon metal powder is compacted by vibration in the mold. A circular crucible is obtained by reactive sintering of the compacted powder at 1400 ° C in a furnace maintained under a nitrogen atmosphere, to allow the conversion of silicon to silicon nitride. It has the following dimensions: - external diameter 280mm, - internal diameter 265mm, - internal height 200mm, - external height (bottom thickness included) 220mm. On the crucible thus obtained, an open porosity of the order of 30% by volume and a median pore diameter of about 1 micrometer are measured in the core of the material constituting the crucible. The surface of the walls and the bottom of the crude crucible thus obtained is spray-coated with SATAjet 1000 B RP Nozzle 1.0 0.6 / QCC of a mixture comprising a weight portion of a silicon nitride powder for a part weight demineralized water, in the presence of 0.5% by weight of dispersant Darvan CN grade of RI Vanderbilt compared to the introduced mass of silicon nitride. The silicon nitride powder used, sold by UBE under the reference SN-E05, is characterized by a median particle diameter of the order of 0.6 microns for a specific surface area of about 5 m 2 / gram. In order to obtain a homogeneous mixture before spraying, this mixture was homogenized and dispersed in a jar with a silicon nitride ball, the mass of balls being equal to the mass of the mineral powder of the mixture, at 100 rotations per minute for 24 hours. . The coated crucible is then air-dried for 4 hours at 110 ° C. and then heated at 1100 ° C. for 4 hours in air to obtain sufficient strength and adhesion of the layer to the walls of the crucible. The porosity characteristics of the coating layer are verified on images obtained by electron microscopy according to the following protocol: a polished cross-section of the coating on the crucible is practiced so as to obtain a sectional view and according to the entire thickness of the coating, - an image acquisition is performed by an SEM scanning electron microscope, to obtain an image of 300 microns (average coating thickness) over a cumulative length of about 1.5 cm, - the image Gross SEM is processed by a thresholding technique to obtain binarized images according to current image processing techniques, the area of each pore is measured in the image plane considered and an "equivalent diameter" is determined , on the basis of a perfect disc of the same area. a distribution curve of the pore area in the material can thus be obtained and a median pore diameter determined on the basis of all the data obtained by the SEM images and the distribution curve. The median diameter of pores is the median equivalent diameter dividing the pores into the first and second equal populations at the surface, these first and second populations having only pores having either an equivalent diameter greater than or equal to the median equivalent diameter, or a smaller equivalent diameter. at the median diameter. an overall porosity of the coating layer is also determined on the basis of the preceding measurements and measurements, by calculating the ratio between the sum of the areas of the pores observed on a plate and the total area of the plate. The coated crucible thus obtained is used for the melting of a powder and the crystallization of a silicon ingot under the usual conditions, comprising heating at 1500 ° C. under argon and then cooling slowly to a temperature of the order 1350 ° C. Very few areas of adhesion, relatively small, are observed and the ingot can finally be detached from the crucible without particular difficulties. The interstitial oxygen content of the ingot is also measured by measurement techniques using Infrared spectrophotometry, according to the SEMI standard MF1188-1107 and on the basis of ten samples taken at mid-height of the ingot. The value of oxygen retained is the average of these 10 samples. Oxygen contamination of the ingot is reported as a reference value 100. In parallel with these tests a coating layer sample was taken from a control crucible sample to determine its mineralogical composition prior to the silicon fusion test. An X-ray diffraction analysis with structural refinement using the Rietveld method makes it possible to identify the phases present.

EXAMPLE 2 (according to the invention): A second crucible is produced by repeating the procedure described in Example 1, but this time the spray mixture is produced according to the following protocol: A first suspension is prepared with a 0.05 micron median diameter silica nanometer powder (grade HDK 140 supplied by Wacker Chemie AG) and SN-E05 silicon nitride powder in deionized water. The mixture is placed in a silicon nitride ball jar rotator at 10Orpm for 23 hours. A second suspension is prepared from a 4 micron median diameter aluminum powder of 99.999% Al chemical composition provided by Ecka Granules with tannic acid supplied by Sigma Aldrich prepared at a concentration of 5mol / liter. in deionized water, the mass of tannic acid representing 25% of the mass of aluminum. The suspension comprises 20% by weight of aluminum. In order to obtain a homogeneous suspension, the mixture is placed in a Silicon nitride ball jar rotator at 100 rpm for 1 hour. The second suspension is added to the first suspension so that the mass proportions of SiO 2 N 4 and Al are respectively 23.6%, 55.2% and 21.2%, the inorganic filler representing 25% by weight of the solution. . In order to obtain a homogeneous suspension, the mixture is placed in a Silicon nitride ball jar turning at 100 rpm (rotation per minute) during the hour. In the hour following the surface of the walls and the bottom of the crucible of the second crucible are spray-coated according to the conditions similar to Example 1. The crucible thus coated is dried under air for 4 hours at 110 ° C. and then placed in a furnace to be fired at atmospheric pressure of nitrogen at a temperature of the order of 1500 ° C for a period of 4 hours (bearing temperature). Characterization of the porosity of the layer was performed according to the same procedure as in Example 1. X-ray diffraction analysis revealed that the coating consisted essentially of an SiAlON IP phase of z equal to 2. No amorphous phase nor alumina phase in crystalline form is detected. Although substantially more porous than that of Example 1, the coating of Example 2 leads to significantly lower contamination. Example 3 (according to the invention): A third crucible is manufactured using the procedure described in Example 1, but this time the silicon nitride crucible is coated with a mixture comprising a sialon powder. powder being obtained according to the following protocol: A first and a second suspension are prepared as in Example 2 but this time the amounts of nanometric nanoscale powder, silicon nitride powder and the amount of the second suspension to Aluminum base added to the first suspension are adapted so that the respective weight proportions of SiO 2, Si 3 N 4 and Al are 23.6%, 55.2% and 21.2%. The suspension is then atomized to obtain granules of average size of about 100 microns. The granules obtained are placed in an Si3N4 gasette and cooked under nitrogen at 1550 ° C. for 10 hours. The solid sample is then regrinded to obtain a SiAlON powder of about 5 microns in median diameter.

A suspension is prepared in deionized water comprising 50% by weight of a mineral filler consisting of 95% by weight of said SiAlON powder and 5% of Si3N4 powder and homogenized by rotation in a jar rotator with nitride ball. Silicon at 100 rpm for 24 hours. In the hour following this suspension is projected according to the same protocol as Example 1. Unlike Example 1 the crucible is heated to 900 ° C for 4 hours in air. A characterization of the porosity of the layer was carried out according to the same procedure as for example 1. The X-ray diffraction analysis reveals that the coating consists essentially of a SiAlON p phase, of z equal to 2. No amorphous phase nor alumina phase in crystalline form is detected. The presence of traces of Si3N4, 5iO2 and Si2ON2 phases is noted. The coating of Example 3 leads to a significantly lower contamination than for Example 1 but slightly higher than for Example 2. Example 4 (according to the invention): A fourth crucible is manufactured using the operating method described in Example 1, but this time the silicon nitride crucible is coated from a mixture comprising a sialon powder. Said sialon powder is obtained according to Example 3 and the mixture for the coating to be deposited on the crucible is produced according to the following protocol: A first suspension is prepared from an aluminum powder having a median diameter of 4 microns. chemical composition 99.999% of Al provided by Ecka Granules with tannic acid supplied by Sigma Aldrich prepared in a concentration of 5 mol / liter in deionized water, the mass of tannic acid representing 25% of the mass of 'Aluminum. The suspension comprises 20% by weight of aluminum. In order to obtain a homogeneous suspension, the mixture is placed in a Silicon nitride ball jar rotator at 100 rpm for one hour. A SiAlON powder as synthesized in the preceding example 3 is added to the first suspension and the deionized water is adjusted so that the respective respective proportions by weight of SiAlON powder and Al aluminum are 95 % and 5% and the inorganic filler represents 50% by weight of the solution. In order to obtain a homogeneous suspension, the mixture is placed in a jar rotator with silicon nitride balls for one hour. The crucible thus coated is dried under air for 4 hours at 110 ° C. and then placed in an oven for undergoing atmospheric nitrogen baking at a temperature of the order of 1500 ° C. for a period of 4 hours (bearing temperature). . A characterization of the porosity of the layer is carried out according to the same procedure as for example 1. The X-ray diffraction analysis reveals that the coating consists of a SiAlON phase p, substantially of z equal to 2.5 . No amorphous phase nor alumina phase in crystalline form is detected. The coating of Example 4 leads to a significantly lower contamination than Example 1 and similar to Example 2.

EXAMPLE 5 (Comparative) A fifth crucible is manufactured by using the procedure described in Example 1. The crucible is plasma-spraying with a SiAlON powder obtained beforehand from the synthesis of the nitriding of a mixture of powders of silicon nitride, alumina and aluminum nitride in respective proportions to form an IP type SiAlON of z equal to 3 according to the protocol described in the article "Structure and properties of plasma-sprayed SiAlON coatings ". A characterization of the porosity of the layer was carried out according to the same procedure as for example 1. The analysis of the coating layer deposited by plasma projection by X-ray diffraction with structural refinement using the Rietveld method reveals that the coating consists essentially of a phase SiAlON I 'of z equal to 3 but the important presence of an alumina phase as was observed in the publication cited above. Example 6 (Comparative): A sixth crucible is manufactured by using the procedure described in Example 1. The crucible is coated by plasma spraying from a SiAlON powder of z equal to 2 pre-synthesized according to the protocol described in FIG. article "Structure and properties of plasma-sprayed SiAlON coatings". The test confirms the observation of the publication cited above. The coating is not exploitable. A characterization of the porosity of the layer was carried out according to the same procedure as for example 1. The analysis of the coating layer deposited by plasma projection by X-ray diffraction with structural refinement using the Rietveld method reveals that the coating consists essentially of a SiAlON IP phase of z equal to 2 but the important presence of an alumina phase as has been observed in the publication cited above. The main characteristics of the examples and the results of the measurements made on the ingot are shown in Table 1 which follows: Example 1 2 3 4 5 6 (comparative) (invention) (invention) (invention) (comparative) (comparative) Crucible Type RBSN Type RBSN Type RBSN Type RBSN Type RBSN Type RBSN Initial Proportions of Powders 100% Si3N4 23.6% SiO2 95% SiAlON 95% SiAlON 5% Al 49.5% Si3N4 66.2% Si3N4 55.2% Si3N4 5% Si3N4 36% A1203 14.5% AIN 24.1% A1203 9.7% AIN 21.2% Al Technique for depositing the crucible layer Spray of a slip 50% mass of inorganic material Spray of a slip 25% by mass of inorganic material Spraying Spray Plasma spraying with Metco type 9MB H2 + 90% N2 power 42KW Plasma spraying with Metco type 9MB H2 + 90% N2 power 42KW of a slip of a slip 50% mass of 50% mass of inorganic inorganic material material Post heat treatment 1100 ° C / 4h / air 1500 ° C / 4h / N2 9 00 ° C / 4h / air 1500 ° C / 4h / N2 no No Result X-ray diffraction analysis: Si3N4 M: SiAlON M: SiAlON M: SiAlON M: SiAlON M: SiAlON M: Majority D: SiO2 (p-form, (form [3, (form p, (form 13, (form 13, D: detected with z = 2) with z = 2) with z = 2.5) with z = 3), with z = 2), D : A1203 D: A1203 Physical characteristics of the coating: -thickness pm -PO% 250pm 250pm 300pm 250pm 250pm No -D50 pores pm 30% 35% 25% 30% 55% coating 1pm 5pm 10pm 10pm 15pm mineable Large membership areas No No No No Yes NA Oxygen contamination halfway up the ingot after firing Base 100 -15% -10% -15% (not measured) NA O.) NA = not applicable Table 1

Claims (4)

REVENDICATIONS1. Ceramic crucible for the manufacture of a silicon ingot, having a bottom and walls whose surfaces define an interior volume, said crucible being characterized in that it comprises: a core made of a material consisting of silica, nitride silicon, silicon oxynitride, silicon oxynitride and aluminum, graphite, silicon carbide or a mixture of at least two of these compounds, whose open porosity is between 15 and 60 % and the median pore diameter is between 0.1 and 10 microns and a coating layer on said surfaces defining the interior volume of the crucible, said layer having a porosity of between 15 and 50% and essentially consisting of at least a crystalline phase of Sialon type. 2. Crucible according to one of the preceding claims, wherein the layer consists essentially of a crystallized phase of Sialon p-form, and corresponding to the formulation Si6_zAlzNa_z0z, with z greater than or equal to 1.0 and less than or equal to 4 2. 3. Crucible according to the preceding claim, wherein z is greater than or equal to 1.5 and wherein z is less than or equal to 3.2. 4. Crucible according to claim 1, wherein the layer consists essentially of a crystalline phase of Sialon in O 'form, formulation Si2, Al.N2,01 +., With x less than or equal to 0.4. . A crucible according to claim 1 wherein the layer consists essentially of a crystallized Sialon phase in the form a 'and corresponding to the formulation M'Si _n_ (m + n) Alm + nOnNi6n, where x is less than 2 and M represents at least one element selected from the group consisting of Ca, Mg, Y or a rare earth. 6. Crucible according to claim 1, wherein the layer consists essentially of a mixture of crystallized phases in the forms oe and / or Wet / or Cr. 7. The crucible according to one of the preceding claims, wherein the coating layer comprises less than 1% amorphous phase and preferably wherein the coating layer is free of amorphous phase. 8. Crucible according to one of the preceding claims wherein said crystallized Sialon phase is present in the coating in the form of grains whose median size is between 0.5 and 30 microns, preferably between 1 and 20 microns. 9. Crucible according to one of the preceding claims, wherein the core material consists essentially of silicon nitride grains, preferably of the RBSN type. 10. Crucible according to one of the preceding claims, wherein the porosity of the coating is between 20 and 40%. 11. Crucible according to one of the preceding claims, wherein the average thickness of the coating layer is less than 1000 micrometers, preferably less than 500 micrometers. 12. Crucible according to one of the preceding claims wherein the median size of sialon grains is between 0.5 and 5 times the median pore diameter of the material constituting the crucible. 13. A method of manufacturing a crucible incorporating a coating according to one of the preceding claims comprising the following steps: a) Selection of the following inorganic powders: a silicon nitride powder, having a median diameter of between 0.1 and 10; microns, - a silica powder, with a median diameter of between 50 nanometers and 5 microns, - an aluminum metal powder, with a purity greater than 99.9% by mass, b) Preparation of a suspension from the powder aluminum with tannic acid or a tannic acid derivative in a particularly aqueous solvent, preferably in deionized water, the amount of tannic acid being preferably between 15 and 35% of the aluminum mass c) Preparation of a suspension comprising the other powders in a solvent preferably identical to that used in step b), in which the inorganic part represents at least 25% by weight of the suspension. d) Mixing suspension c) and suspension b). e) Deposition of the mixture of suspensions on a raw crucible made in particular of a material consisting of silica, silicon nitride, silicon oxynitride, silicon oxynitride and aluminum, graphite, silicon decarbonide or a mixture of at least two of these compounds, f) Drying, preferably until the residual moisture is less than 0.5% by weight, g) Cooking under a nitrogen atmosphere at a temperature between 1100 ° and 1650 ° C. 14. A method of manufacturing a crucible incorporating a coating according to one of claims 1 to 12 comprising the following steps: a) preparation of a suspension comprising: - a SiAlON powder, with a median diameter of between 0.5 to 30 microns, - a solvent, preferably deionized water, b) depositing the suspension on a raw crucible made in particular of a material consisting of silica, silicon nitride, silicon oxynitride, silicon oxynitride and of aluminum, graphite, silicon carbide or a mixture of at least two of these compounds, c) drying, preferably so that the residual moisture is less than 0.5% by weight. mass, d) consolidation heat treatment in an oxidizing or inert atmosphere at a temperature greater than 700 ° C. 15. A method of manufacturing a crucible incorporating a coating according to the preceding claim, wherein the SiAlON powder is previously oxidized before suspending. 16. Process for producing a Sialon powder that can be used as a coating in a crucible according to one of claims 1 to 12, comprising the following steps: a) Selection of the following inorganic powders: a silicon nitride powder, of median diameter between 0.1 and 10 microns, - a silica powder, with a median diameter of between 50 nanometers and 5 microns, - an aluminum metal powder, with a purity greater than 99.9% by weight, b) Preparation of a suspension from the aluminum powder with tannic acid or a tannic acid derivative in a particularly aqueous solvent, preferably in deionized water, the amount of tannic acid being preferably comprised between 15 and 35% of the mass of aluminum, c) Preparation of a mixture comprising suspension b) and the other powders in a solvent which is preferably identical to that used in stage b), in which the inorganic part represents the Mo 25% by weight of the mixture and to which a temporary binder is added, d) optionally granulation or atomization to obtain granules, preferably of between 50 and 500 microns in size, or direct shaping of the mixture to obtain briquettes (e) Drying, preferably until the residual moisture of the granules or briquettes is less than 0.5% by weight, f) Cooking under a nitrogen atmosphere at a temperature between 11000 and 1650 ° C , so as to obtain granules or briquettes consisting essentially of at least one Sialon phase, g) grinding to obtain a SiAlON powder, whose median grain diameter is between 0.5 and 30 microns. . A method of manufacturing a silicon ingot, comprising the steps of melting high purity silicon at about 1500 ° C in a ceramic crucible according to one of claims 1 to 13, solidifying a silicon ingot and demolding said ingot of the crucible.