JP4889306B2 - Silicone coagulation mold with mold release material - Google Patents

Description

  The present invention relates to a mold used for solidifying silicon, and is intended to release a solidified silicon mass very well without damaging the mold. The obtained silicon lump can be used as a raw material for solar cells or an ingot for cutting out wafers for solar cells.

  Silicon used for solar cells generally requires a purity of about 99.9999%, various metal impurities are 0.1 mass ppm or less, and B is at least 0.3 mass ppm or less, preferably 0.1 mass. It is required to be ppm or less. As silicon satisfying this purity, there is silicon for semiconductors, that is, high-purity silicon obtained by pyrolyzing silicon chloride after distillation. However, this Siemens method is expensive and unsuitable for solar cells that require a large amount of silicon.

  Therefore, various techniques for producing inexpensive silicon that can be used for solar cells have been studied. Various metal impurities other than B and P, such as Fe, Al, and Ca, can be removed by a unidirectional solidification method. It is common. That is, when the silicon melt is solidified, it is a purification method using the phenomenon that a large amount of metal impurities are distributed to the coexisting melt silicon and only a small amount is taken into the solidified silicon. In various technologies for manufacturing inexpensive silicon that can be used for solar cells, including this unidirectional solidification method, a process for melting and solidifying silicon is often indispensable.

  As a mold for solidifying molten silicon, generally, a quartz mold or a carbon mold is often used. If these molds are used as they are, solidified silicon lump is used as a mold. It adheres and the yield of silicon recovery decreases. There is also a problem that the mold cannot be reused and is not practical. In order to solve these problems, a method has been developed in which a mold release material is applied to the inner surface of a mold made of carbon that can be assembled and disassembled, and after the silicon is solidified, the carbon mold is disassembled to extract a silicon lump.

  (Patent Document 1) and (Patent Document 2) describe a method of covering a silicon assembly mold with silicon oxide, nitride, and carbide and taking out a silicon lump without damaging the mold. However, there is no description of a specific method for coating a carbon mold with silicon oxide, nitride, or carbide.

  (Patent Document 3) describes a method of coating the inner surface of a mold with a slurry in which silicon nitride powder and an organic binder are dissolved in a solvent. However, the organic binder generally desorbs from the release material at high temperatures. In fact, in column 4, lines 58 to 60 of this publication, it is described that the organic binder can be selected from polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyrate. It will be detached from.

  Furthermore, column 5 lines 9 to 11 of this publication state that polyvinyl alcohol is preferably desorbed at a low temperature. However, if the organic binder is detached from the release material at high temperature, only the silicon nitride powder remains, and these powders are sintered very loosely to form a release material. Is low and there is a risk of the release material being damaged. As a matter of course, when the release material is broken, there is a problem that silicon adheres to the mold in the region.

  In order to increase the strength of the release material, it is only necessary to select a powder that sinters more strongly than the silicon nitride powder. In this case, shrinkage occurs due to sintering, and cracks occur in the release material. Therefore, the fact is that powders that are easily sintered cannot be used.

  (Patent Document 4) to (Patent Document 12) describe further advanced methods. In other words, it describes the improvement of the particle size of the powder in the release material, the improvement of the composition ratio, the addition of a dispersant, etc., but essentially uses powder and an organic binder, and the organic binder used is at a high temperature. In this case, a material that is detached from the release material is selected.

  For example, (Patent Document 11) is characterized in that the binder is degreased at 450 to 600 ° C. as described in the claims, and that the binder is made of polyvinyl alcohol. Furthermore, as described in paragraphs [0007] to [0011] of (Patent Document 11), for the purpose of preventing carbon uptake into silicon, the binder is not completely left in the form of carbon or the like, but as completely as possible from the release material. It is supposed to be removed.

  However, the more the binder is removed, the lower the strength of the release material and the possibility of breakage of the release material. Specifically, cracks occur in the release material, and molten silicon penetrates into the back side of the release material from the part, resulting in a problem that the solidified silicon and the mold are fixed. If this happens, naturally, the mold cannot be reused, and the economy is lost.

  For example, when molten silicon is poured into a mold and solidified, the molten silicon collides with the mold release material in the mold, so that the mold release material itself is strong and firmly fixed to the mold. There is a high possibility that it will be destroyed or peeled off.

  Even if the binder remains in the mold release material, if the binder is a general organic substance, the binder softens or melts at a high temperature of about 500 to 1000 ° C. and loses its strength, so it retains its strength as a mold release material. I can't do it. As a result, the release material is easily peeled from the mold. If part of the mold release material is peeled off, that part of the mold is fixed to the solidified silicon, and the mold cannot be reused.

  In order to solve the above problems, the inventors of the present invention have previously described that in a mold for solidifying silicon in which a mold release material is held on the inner surface of the mold, the mold release material has a melting point substantially equal to or higher than the melting point of silicon (1414 ° C.). It has been found that these problems can be solved by using a silicon coagulation mold characterized by comprising a powder and a resin that produces carbon at a temperature of 100 ° C. or higher and below the melting point of silicon.

  However, in this method, if the thickness of the release material is 1 mm or more, the total amount of the release material to be used increases, so the cost is accordingly generated. If the thickness of the release material is less than 1 mm in order to reduce the cost, In rare cases, the mold release material on the inner surface of the mold in the area where the upper surface of the molten silicon touches, or the mold release material in the area where the plates that make up the mold meet, peel off, and the solidified silicon mass sticks to the mold. Therefore, further improvement of the release material has been desired.

JP 62-108515 A JP 62-260710 A US Pat. No. 5,431,869 JP-A-6-144824 Japanese Patent Laid-Open No. 7-206419 JP-A-9-175809 Japanese Patent Laid-Open No. 10-182133 JP 2001-198648 A JP 2002-239682 A JP 2002-292449 A Japanese Patent Laid-Open No. 2002-321037 JP 2003-64388 A

  The present invention solves the above problems, reduces the total amount of release material on the inner surface of the mold for solidifying silicon as much as possible, eliminates breakage and peeling of the release material, and ensures solidified silicon and the mold on the entire surface of the mold. An object of the present invention is to provide a silicon coagulation mold that is released from the mold.

Means constituting the present invention are as follows.
(1) In a silicon solidification mold in which a release material is held on the inner surface of the mold, the thickness of the release material on the inner surface of the mold in the region where the upper surface of the molten silicon touches is 1 mm or more, and the inner surface of the mold other than the above region is separated. The thickness of the mold material is less than 1 mm, and the mold inner surface of the area where the mold release material is applied with a thickness of 1 mm or more is recessed according to the thickness of the mold release material to be applied. A mold for solidifying silicon, characterized in that the surface is flat .

(2) the release material is substantially a powder having a melting point of melting point (1414 ° C.) or more silicon, characterized in that it consists of a resin which generates carbon at a temperature below the silicon 100 ° C. higher than the melting point (1) 2. A mold for solidifying silicon as described in 1.

( 3 ) The mold for solidifying silicon according to ( 2 ), wherein the resin is heat-cured in the mold at 30 ° C. or more and 500 ° C. or less in advance.

( 4 ) The mold for solidifying silicon according to ( 2 ), wherein the resin is preliminarily heated and carbonized at 200 ° C. or higher and 1000 ° C. or lower in the mold.

( 5 ) The mold for silicon coagulation according to any one of ( 2 ) to ( 4 ), wherein the resin is 1 part by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the powder.

( 6 ) The mold for silicon coagulation according to any one of ( 2 ) to ( 5 ), wherein the resin is a phenol resin.

( 7 ) The powder for melting the silicon according to any one of ( 2 ) to ( 6 ), wherein the powder having a melting point equal to or higher than the melting point of silicon is at least one of silica sand, silicon nitride, and silicon carbide. template.

( 8 ) The mold for solidifying silicon according to ( 7 ), wherein the silica sand has an average particle size of 0.1 mm or less.

( 9 ) The silicon coagulation mold according to ( 7 ), wherein the silicon nitride has an average particle size of 50 μm or less.

(10) The silicon solidification mold according to ( 7 ), wherein the silicon carbide has an average particle size of 1 mm or less.

  According to the present invention, it becomes possible to increase the strength of the mold release material on the inner surface of the mold for solidifying silicon, and the possibility of breakage of the mold release material is further reduced. As a result, the solidified silicon and the mold can be reliably released. It has become possible. In particular, the problem that the solidified silicon lump in the mold inner surface in the area where the upper surface of the molten silicon touches or in the area where the plates constituting the mold are brought together is fixed to the mold. In addition, the total amount of release material used can be reduced, and the cost of the release material can be reduced.

  The main configuration of the present invention is as follows.

  In a mold for solidifying silicon in which a release material is held on the inner surface of the mold, the thickness of the release material on the inner surface of the mold in the region where the upper surface of the molten silicon touches is 1 mm or more, and the thickness of the release material on the inner surface of the mold in other regions This is a silicon coagulation mold having a thickness of less than 1 mm. Further, in the assembly mold for silicon solidification in which the release material is held on the inner surface of the mold, the thickness of the release material on the inner surface of the mold in the area where the upper surface of the molten silicon touches and the area where the plate constituting the assembly mold is brought together This is a silicon coagulation mold in which the thickness of the release material is 1 mm or more and the thickness of the release material on the inner surface of the mold in the other region is less than 1 mm.

  On the inner surface of the mold that touches the upper surface of the molten silicon, there is a greater risk of the release material peeling off due to the undulation of the molten silicon than in other areas. It is effective to increase the thickness. Specifically, it is sufficient that the release material thickness is 1 mm or more.

  Naturally, if the release material is peeled off, the solidified silicon and the mold are fixed, making it difficult to reuse the mold. In the solidification of silicon, unidirectional solidification often occurs from the lower side to the upper side. However, as silicon solidifies, the volume of the upper surface of the molten silicon gradually rises as it solidifies. In the region where the thickness of the release material is 1 mm or more, the upper surface of the molten silicon has a width corresponding to the rise.

  In the present invention, the region including the increased amount is called a region where the upper surface of the molten silicon touches. Further, since the density of molten silicon at each temperature and the density of solidified silicon are known, this region can be calculated from the amount of silicon in the mold.

  In order to solidify the silicon, a mold made of a rectangular parallelepiped shape is often used by combining carbon plates, but the mating parts of the plates that make up the assembly mold may be peeled off due to thermal expansion of the plates. The risk of cracks in the release material is greater than in other flat areas. For this reason, it is effective to make the release material thickness of the mating portion of the plate thicker than other regions. Also in this case, it is sufficient that the thickness of the release material is 1 mm or more.

  Furthermore, for the purpose of flattening each surface of the mold inner surface after applying the mold release material, it is also effective to indent the mold inner surface of the area where the mold release material is applied with a thickness of 1 mm or more according to the thickness of the mold release material to be applied. is there. By doing this, although the thickness of the release material on the inner surface of the mold is different in each part, after applying the release material, each surface of the inner surface of the mold becomes flat, and as a result, a silicon solidified mass having a flat surface without unevenness can be obtained. This is advantageous for the subsequent handling of the silicon mass.

  The release material may be substantially composed of a powder having a melting point not lower than the melting point of silicon (1414 ° C.) and a resin that generates carbon at a temperature not lower than 100 ° C. and not higher than the melting point of silicon. That is, a powder having a melting point equal to or higher than the melting point of silicon (1414 ° C.) and a resin that generates carbon at a high temperature are mixed and applied to the inner surface of the silicon solidification mold, and this is used as the silicon solidification mold.

  The carbon produced from the resin firmly binds the powder and improves the strength of the release material. Since carbon has a sufficient strength up to about 2000 ° C., it is possible to firmly hold a powder having a melting point equal to or higher than that of silicon, and to firmly adhere to a mold. As a result, a good release material can be formed.

  Strictly speaking, the components of the release material are not limited to powders having a melting point higher than that of silicon and resins that generate carbon at high temperatures. Ultimately, it is only necessary to form a film in which a powder having a melting point equal to or higher than the melting point of silicon is firmly bonded with carbon, and this prevents the formation of such a film in the powder and resin to be applied initially. If there are no mixed substances, there is no particular problem.

  For example, even if the powder contains a powder having a melting point of about 5% by mass or less of silicon, it reacts with carbon generated from the resin to form a carbide, resulting in a substance having a melting point higher than that of silicon. Then there is no problem. Even if the resin contains about 10% by mass of a resin that does not generate carbon at a high temperature, the resin generally vaporizes or decomposes and vaporizes at a high temperature near the melting point of silicon, so there is no problem.

  In this sense, the release material may be substantially composed of a powder having a melting point equal to or higher than the melting point of silicon (1414 ° C.) and a resin that generates carbon.

  The proportion of carbon produced from the resin is meaningless if it is too small, and it is preferable that 20% by mass or more of the resin becomes carbon and remains in the release material at a high temperature. More preferably from the practicality, it is preferable that 40% by mass or more of the resin becomes carbon and remains in the release material.

  Generally, when the temperature of the mold is raised, the temperature is often raised to a temperature higher than the melting point of silicon without maintaining at a specific temperature at a rate of several to ten and several degrees per minute. When using the mold which has, it may be better to hold | maintain at the specific temperature between 30 degreeC or more and 500 degrees C or less, for example at 200 degreeC for about 0.1 to 10 hours, More preferably, it is 1-5. Sometimes it is better to hold for about an hour. Thereby, hardening of resin can be advanced further and the intensity | strength of a mold release material can be raised more as a result.

  Moreover, when using the casting_mold | template which has a mold release material of this invention, it is better to hold | maintain at the specific temperature in the temperature range of about 200 degreeC-about 1000 degreeC, for example, for about 0.1 to 10 hours at 700 degreeC. In some cases, it may be better to hold for about 1 to 5 hours. As a result, most of the resin can be carbonized, and as a result, the amount of carbon generated from the resin when reaching the melting point of silicon or higher can be increased.

  The amount of resin in the mold release material is preferably 1 part by mass or more of resin with respect to 100 parts by mass of powder in order to bind and hold the powder. It is preferable that resin is 200 mass parts or less with respect to the mass part. More preferably, the resin is 5 parts by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the powder.

  A phenol resin can be used as the type of resin. The phenol resin includes a thermoplastic resin and a thermosetting resin, but both can be used. In some cases, it is more convenient to add a curing agent in advance to cure. As the curing agent, for example, hexamethylenetetramine or a certain kind of acid, for example, a sulfonic acid organic acid or the like can be used. Moreover, as a kind of resin, an epoxy resin, an acrylic resin, a furan resin, a pitch etc. other than a phenol resin can be used.

  As the powder having a melting point equal to or higher than the melting point of silicon, a silicon compound can be used from the viewpoint of reducing contamination, and silica sand, silicon nitride, silicon carbide and the like are preferable. However, the present invention is not particularly limited to powders of these silicon compounds.

  As the shape of the powder, for example, silica sand having an average particle diameter of 0.1 mm or less and about 1 μm or more can be used. For example, silicon nitride having an average particle diameter of 50 μm or less and about 1 μm or more can be used. Further, for example, silicon carbide having an average particle diameter of 1 mm or less and about 1 μm or more can be used, but both are not limited to these average particle diameters.

  Up to now, as the mold used in the present invention, an assembly mold made mainly of carbon has been considered, but the present invention is not particularly limited to this type of mold. For example, an integral mold made of carbon can be sufficiently used. Also, the material is not limited to carbon. For example, it is possible to solidify silicon by applying the release material of the present invention to the inner surface of a stack of refractory bricks and forming a mold shape.

  Next, the best mode for carrying out the present invention will be described in detail.

  When the thickness of the release material applied to the inner surface of the mold is less than 1 mm, it is preferably 10 μm or more and less than 1 mm. When the thickness of the release material is 1 mm or more, 1 mm or more and 50 mm or less is preferable. The thickness of the release material is less than 10 μm. Then, even if there is no problem in mold release, the mold may be damaged.

  Further, it is sufficient that the thickness of the release material is about 50 mm, and even if there is a slight crack in the release material, it hardly penetrates 50 mm. As a result, the molten silicon does not penetrate the release material, and solidified silicon and mold Can be released very well.

  When the former release material has a thickness of 10 μm or more and less than 1 mm, for example, silica sand having an average particle size of about 5 μm and silicon nitride powder having an average particle size of 1 μm or less has a mass ratio of 1: 5 to 5: Mix in a range of about 1, and add to this a phenol resin of about 5 to 100% of the mass of the powder. In the case of a thermoplastic phenol resin, a predetermined amount of the aforementioned curing agent may be added. This may be applied to the inner surface of the carbon assembly mold with a thickness of less than 1 mm.

  When the silicon raw material lump is melted at room temperature in the mold, it is sometimes preferable to cure the release material in advance before putting the silicon raw material lump in order not to damage the release material. For this purpose, it may be held at about 200 ° C. for about 1 hour after the release material is applied. Since this step is sufficiently low for carbon, it can be performed in an air atmosphere even when a carbon mold is used. Of course, it is also possible to carry out in an inert atmosphere such as argon or nitrogen.

  Further, after applying the release material, it may be cured at room temperature only, and held at about 200 ° C. for about 1 to 2 hours during the temperature rise after the silicon raw material lump is put in the mold. While the mold is heated, the silicon raw material lump inside moves in many cases and damages the release material and the inner surface of the mold. However, if the release material is sufficiently cured at about 200 ° C., this damage can be prevented.

  After the above operation, the temperature is raised above the melting point of silicon in an inert atmosphere or in a vacuum, and after melting the silicon, the temperature is lowered to solidify the silicon. At this time, the lower part of the mold is cooled, the silicon melt is given a temperature distribution, the silicon is unidirectionally solidified from the bottom to the top, various metal impurities are collected above the silicon, and most of the silicon is generally purified. Is.

  Unlike the above-described operation, when the silicon melt is poured into the mold and solidified, the meaning of maintaining the temperature at about 200 ° C. is lower than when the silicon raw material lump is initially placed in the mold and dissolved.

  Further, as described above, during the temperature increase, for example, by holding for about 1 hour at about 700 ° C., the residual carbon ratio of the phenol resin can be increased, and as a result, the amount of phenol resin used can be reduced. Economical.

  As described above, when the silicon is solidified and the assembly mold made of carbon is disassembled after cooling, the obtained silicon lump and the mold do not adhere to each other, and the mold can be reused any number of times.

  Next, the case where the thickness of the release material is 1 mm or more and 50 mm or less will be described. In this case, since the release material is thick and the amount of powder used is increased, it is more practical to use a cheaper powder. From this viewpoint, silica sand is preferably used as the powder. For example, phenol resin having a mass of about 10 to 100% is added to and mixed with silica sand having an average particle size of about 60 μm.

  What is necessary is just to affix this mixture on the inner surface of a carbon assembly mold with a thickness of about 5 mm, for example. The subsequent handling is essentially the same as when the release material is less than 1 mm. Further, as described above, it is advantageous to indent the inner surface of the mold in the region where the release material is thickly applied for the purpose of flattening the inner surface of the mold after the release material is applied.

Example 1
After mixing 70 parts by mass of silicon nitride having an average particle diameter of 0.7 μm and 30 parts by mass of silica sand having an average particle diameter of 3 μm with 100 parts by mass of methanol, 50 parts by mass of phenol resin and 5 parts by mass of a curing agent were added and mixed. And a release material stock solution (hereinafter referred to as release material stock solution 1). Next, when 30 parts by mass of methanol was added to and mixed with 100 parts by mass of silica sand having an average particle size of 60 μm, 50 parts by mass of phenol resin and 5 parts by mass of the curing agent, a viscous clay-like mixture was obtained. (Hereinafter referred to as mold release material 2).

  The mold release material 2 was applied at a thickness of 5 mm to a region having a height of 240 to 275 mm from the lower surface of the inner surface of the cube-shaped carbon assembly mold having an inner side and a depth of 400 mm. This region is obtained by adding a slight margin in the vertical direction to the value calculated from the amount of silicon introduced into the mold and the density of molten silicon and solidified silicon. Moreover, the mold release material stock solution 1 was applied to the entire surface having a mold inner surface height of less than 240 mm with a thickness of 0.2 mm. Thereafter, the release material 2 and the release material stock solution 1 were cured at 20 ° C.

  This mold is placed in a chamber having an induction heating type melting furnace, the inside of the chamber is replaced with argon 0.1 MPa, 100 kg of silicon is melted at 1550 ° C. in the melting furnace, and the mold is heated by a resistance heater. The sample was heated to 1550 ° C. at a heating rate of 5 ° C. per minute. Next, 100 kg of molten silicon in the melting furnace was poured into the mold, and then the temperature of the mold was gradually lowered to gradually solidify the silicon from below. After the entire silicon was solidified, the heater and other power sources were turned off and the furnace was cooled to room temperature.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. In addition, the mold and the silicon lump were easily peeled off, and a silicon lump having a corner of 400 mm and a height of about 265 mm could be taken out, but the portion of about 240 to 265 mm from the bottom corresponds to the mold release material thickness, It was 390 mm. Each part of the dismantled mold was not damaged and could be used again.

(Example 2)
After mixing 70 parts by mass of silicon nitride having an average particle diameter of 0.7 μm and 30 parts by mass of silica sand having an average particle diameter of 3 μm with 100 parts by mass of methanol, 50 parts by mass of phenol resin and 5 parts by mass of a curing agent were added and mixed. And a release material stock solution (hereinafter referred to as release material stock solution 1). Next, when 30 parts by mass of methanol was added to and mixed with 100 parts by mass of silica sand having an average particle size of 60 μm, 50 parts by mass of phenol resin and 5 parts by mass of the curing agent, a viscous clay-like mixture was obtained. (Hereinafter referred to as mold release material 2).

  The mold release material 2 was applied at a thickness of 5 mm to a region having a height of 242 to 277 mm from the lower surface of the inner surface of the cube-shaped carbon assembly mold having an inner side and a depth of 400 mm. This region is obtained by the same method as in Example 1. Furthermore, the mold release material 2 was applied to each corner portion in an area below 242 mm on the inner surface of the mold with a maximum thickness of 5 mm. Moreover, the mold release material stock solution 1 was applied to the entire surface having a mold inner surface height of less than 242 mm with a thickness of 0.2 mm.

  Thereafter, the release material 2 and the release material stock solution 1 were cured at 20 ° C. This mold is placed in a chamber having an induction heating type melting furnace, the inside of the chamber is replaced with argon 0.1 MPa, 100 kg of silicon is melted at 1550 ° C. in the melting furnace, and the mold is heated by a resistance heater. The sample was heated to 1550 ° C. at a heating rate of 5 ° C. per minute. Next, 100 kg of molten silicon in the melting furnace was poured into the mold, and then the temperature of the mold was gradually lowered to gradually solidify the silicon from below. After the entire silicon was solidified, the heater and other power sources were turned off and the furnace was cooled to room temperature.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. Also, the mold and the silicon lump were easily peeled off, and a silicon lump with a corner of 400 mm and a height of about 267 mm could be taken out, but the portion of about 242 to 267 mm from the bottom corresponds to the release material thickness, It was 390 mm. In addition, the corners below about 242 mm on the bottom and side surfaces of the silicon lump were rounded according to the thickness of the release material. Each part of the dismantled mold was not damaged and could be used again.

(Example 3)
After mixing 70 parts by mass of silicon nitride having an average particle diameter of 0.7 μm and 30 parts by mass of silica sand having an average particle diameter of 3 μm with 100 parts by mass of methanol, 50 parts by mass of phenol resin and 5 parts by mass of a curing agent were added and mixed. And a release material stock solution (hereinafter referred to as release material stock solution 1). Next, when 30 parts by mass of methanol was added to and mixed with 100 parts by mass of silica sand having an average particle size of 60 μm, 50 parts by mass of phenol resin and 5 parts by mass of the curing agent, a viscous clay-like mixture was obtained. (Hereinafter referred to as mold release material 2).

  A cube-shaped carbon assembly mold with an inner side of 400 mm and a depth of 400 mm was used, but the inner surface height of 240 to 275 mm was recessed by about 5 mm, and the inner surface of the mold was lower than 240 mm. A mold with a maximum of about 5 mm in each side area was also used. The mold release material 2 was applied at a maximum thickness of 5 mm to the indentation in the region where the height of the inner surface was 240 to 275 mm and the indentation on each side below this.

  Furthermore, the mold release material stock solution 1 was applied at a thickness of 0.2 mm to an area having no depression below 240 mm on the inner surface of the mold. As a result, the finished mold inner surface with the release material became a substantially complete rectangular parallelepiped shape with a corner of 400 mm, although the thickness of the release material was different in each part.

  Thereafter, the release material 2 and the release material stock solution 1 were cured at 20 ° C. This mold is placed in a chamber having an induction heating type melting furnace, the inside of the chamber is replaced with argon 0.1 MPa, 100 kg of silicon is melted at 1550 ° C. in the melting furnace, and the mold is heated by a resistance heater. The sample was heated to 1550 ° C. at a heating rate of 5 ° C. per minute. Next, 100 kg of molten silicon in the melting furnace was poured into the mold, and then the temperature of the mold was gradually lowered to gradually solidify the silicon from below. After the entire silicon had solidified, the heater and other power sources were turned off and the furnace was cooled to room temperature.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. Moreover, the mold and the silicon lump were easily peeled off, and an almost perfect rectangular parallelepiped shaped silicon lump having a corner of 400 mm and a height of about 265 mm could be taken out. Each part of the dismantled mold was not damaged and could be used again.

Example 4
An experiment similar to Example 1 was performed, except that 70 parts by mass of silicon carbide having an average particle diameter of 0.3 μm was used instead of 70 parts by mass of silicon nitride having an average particle diameter of 0.7 μm.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. Further, the mold and the silicon lump were easily peeled off, and the same silicon lump as in Example 1 could be taken out. Each part of the dismantled mold was not damaged and could be used again.

(Example 5)
An experiment similar to Example 1 was performed except that 100 parts by mass of silicon carbide having an average particle diameter of about 100 μm was used instead of 100 parts by mass of silica sand having an average particle diameter of 60 μm.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. Further, the mold and release material, and the release material and silicon lump were easily peeled off, and the same silicon lump as in Example 1 could be taken out. Each part of the dismantled mold was not damaged and could be used again.

(Example 6)
The same experiment as in Example 1 was performed except that the amount of phenol resin was 40 parts by mass.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. Further, the mold and the release material were easily peeled off, but the release material and the silicon lump were easily peeled off for the most part, but some of the molds were impregnated with silicon. This was not a problem, and the release material could be removed from the silicon mass with some work. As a silicon lump, the same silicon lump as in Example 1 could be taken out. Each part of the dismantled mold was not damaged and could be used again.

(Example 7)
An experiment similar to that of Example 6 was performed, except that the mold was held at 700 ° C. for 1 hour during the heating with a resistance heater at a heating rate of 5 ° C. per minute.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. Further, the mold and the release material, and the release material and the silicon lump were easily peeled off, and the release material was not impregnated with silicon.

  The shape, characteristics, and the like of the extracted silicon lump were the same as in Example 6.

  Although there was no problem in Example 6, it can be seen from Example 7 that when the amount of phenol resin is reduced, the mold is not impregnated with silicon when held at 700 ° C. during the temperature rise. This is considered to be due to the fact that the amount of carbon produced is recovered by maintaining at 700 ° C. during the temperature rise of the mold, although the amount of carbon produced is reduced when the phenol resin is reduced. As a matter of course, it is advantageous for handling the silicon mass after solidification that the mold release material is not impregnated with silicon.

(Example 8)
After mixing 70 parts by mass of silicon nitride having an average particle diameter of 0.7 μm and 30 parts by mass of silica sand having an average particle diameter of 3 μm with 100 parts by mass of methanol, 50 parts by mass of phenol resin and 5 parts by mass of a curing agent were added and mixed. And a release material stock solution (hereinafter referred to as release material stock solution 1). Next, when 30 parts by mass of methanol was added to and mixed with 100 parts by mass of silica sand having an average particle size of 60 μm, 50 parts by mass of phenol resin and 5 parts by mass of the curing agent, a viscous clay-like mixture was obtained. (Hereinafter referred to as mold release material 2). The mold release material 2 was applied at a thickness of 5 mm to a region 140 to 165 mm in height from the lower surface of the inner surface of the cube-shaped carbon assembly mold having an inner side and a depth of 400 mm. This region was calculated from the amount of silicon in the same manner as in Example 1. Further, the release material stock solution 1 was applied to a whole surface having a mold inner surface height of less than 140 mm with a thickness of 0.2 mm.

  Thereafter, the release material 2 and the release material stock solution 1 were cured at 20 ° C. This mold was placed in a chamber, and 60 kg of a silicon raw material block having a size of about 5 cm was inserted into the mold. After replacing the inside of the chamber with 0.1 MPa of argon, the mold was heated to 1550 ° C. at a heating rate of 5 ° C. per minute with a resistance heater and held for 1 hour. Thereafter, the temperature of the mold was gradually lowered to gradually solidify the silicon from below. After the entire silicon had solidified, the heater and other power sources were turned off and the furnace was cooled to room temperature.

  After cooling to room temperature, the mold was taken out and disassembled. The silicon lump was peeled off from the mold, and a silicon lump having a corner of 400 mm and a height of about 160 mm could be taken out, but the portion of about 140 to 160 mm from the bottom corresponded to the release material thickness and was approximately 390 mm in corner. It was. In addition, there were several slight scratches on the inner surface of the mold, but the mold could be reused and was not particularly problematic.

Example 9
An experiment similar to that of Example 8 was performed except that the mold was held at 200 ° C. for 1 hour during the temperature rising at a rate of 5 ° C. per minute with a resistance heater.

  When the mold was taken out after cooling to room temperature, the mold was not damaged and the mold could be easily disassembled. Further, the mold and the silicon lump were easily peeled off, and the same silicon lump as in Example 8 could be taken out. Each part of the dismantled mold was not damaged and could be used again.

  Compared with Example 8, when inserting a silicon raw material lump into a casting_mold | template from the beginning, it turns out that it is more preferable to hold | maintain at 200 degreeC for 1 hour.

  By this method, the solidified silicon lump and the mold can be reliably released, and the number of repeated use of the mold can be dramatically increased. Furthermore, leakage of molten silicon out of the mold can be completely prevented, and the possibility of damaging the apparatus body is greatly reduced. By using this method and the unidirectional solidification method, it is possible to produce a polycrystalline silicon ingot for cutting a wafer for solar cells at a very low cost.

Claims (10)

In a mold for silicon solidification in which a mold release material is held on the inner surface of the mold, the thickness of the mold release material on the inner surface of the mold in an area where the upper surface of the molten silicon touches is 1 mm or more, and the thickness of the mold release material on the mold inner surface other than the above area The inner surface of the mold is indented to match the thickness of the release material to be applied, so that each surface of the inner surface of the mold is flat after application of the release material. A mold for solidifying silicon, characterized in that 2. The mold release material according to claim 1, comprising: a powder having a melting point substantially equal to or higher than a melting point of silicon (1414 ° C.) and a resin that generates carbon at a temperature not lower than 100 ° C. and not higher than the melting point of silicon. Silicone solidification mold. 3. The silicon coagulation mold according to claim 2 , wherein the resin is heat-cured in the mold at 30 ° C. or more and 500 ° C. or less in advance. 3. The silicon coagulation mold according to claim 2 , wherein the resin is preliminarily heated and carbonized at 200 ° C. or more and 1000 ° C. or less in the mold. The silicon solidification mold according to any one of claims 2 to 4 , wherein the resin is 1 part by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the powder. The silicon coagulation mold according to any one of claims 2 to 5 , wherein the resin is a phenol resin. The silicon solidification mold according to any one of claims 2 to 6 , wherein the powder having a melting point equal to or higher than the melting point of silicon is at least one of silica sand, silicon nitride, and silicon carbide. 8. The silicon coagulation mold according to claim 7 , wherein the silica sand has an average particle size of 0.1 mm or less. 8. The silicon coagulation mold according to claim 7 , wherein the silicon nitride has an average particle size of 50 [mu] m or less. The silicon solidification mold according to claim 7 , wherein the silicon carbide has an average particle size of 1 mm or less.