JP2005152987A - Mold for casting silicon and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mold for casting a silicon with which the drops of the characteristic in the silicon ingot and the yield caused by a parting agent are not brought about. <P>SOLUTION: In the mold for casting the silicon provided with a mold vessel 1 holding and solidifying molten silicon 3 in the inner part, the inner surface of the mold vessel 1 is coated with the parting agent 2 mainly containing silicon nitride and also, the parting agent 2 has the higher hardness at the molten silicon side 2b than that at the vessel side 2a. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a silicon casting mold particularly suitable for casting polycrystalline silicon for forming solar cells and the like, and a method for manufacturing the same.

  Conventionally, polycrystalline silicon has been used as a kind of semiconductor substrate for forming solar cells. Such polycrystalline silicon is usually poured into a mold in which a release material is applied using a brush or spatula to the inner surface of a separable graphite container, and a silicon melt heated and melted at a high temperature is poured. It is formed by solidifying the silicon raw material, or once the silicon raw material placed in the mold is dissolved and then solidified again.

  In general, powder such as silicon nitride, silicon carbide and silicon dioxide is mixed and stirred in a solution composed of an appropriate binder and solvent to form a slurry, which is coated on the inner wall of the container by means such as coating or spraying. It is known as a known technique. (For example, refer nonpatent literature 1).

  However, when silicon nitride is applied to the inner surface of a graphite mold by the above-described means and silicon is cast, the silicon nitride film is fragile, so when pouring silicon melt and during subsequent solidification. The silicon nitride film is broken, the silicon melt contacts the mold, and the silicon ingot is chipped when the mold adheres to the silicon ingot and is removed from the mold. Further, there is a problem that the silicon nitride film is broken when the silicon raw material put in the mold is melted.

  It has also been proposed to cast silicon by applying silicon dioxide to the inner surface of a graphite mold. However, when silicon dioxide is used as a release material, silicon dioxide has good adhesion to graphite, and silicon dioxide and Because silicon ingots also have good adhesion, silicon dioxide adheres to the mold, making it impossible to reuse the mold, or the mold adheres to the silicon ingot via a mold release material and part of the silicon ingot is removed There was a problem that chipping occurred.

  In order to solve such problems, silicon dioxide is applied to the first layer to ensure adhesion, a mixture of silicon dioxide and silicon nitride is applied to the second layer, and silicon nitride is applied to the third layer. Has been proposed (see, for example, Patent Document 1). However, when the release material is applied to the three-layer structure in this way, it is necessary to prepare and apply the release materials corresponding to the respective layers, and there is a problem that it takes time to apply and prepare the release material.

  In addition, in order to prepare a slurry-like release material and apply it to a mold, a solvent such as water or alcohol, a binder for coating molding, and an additive for increasing fluidity are appropriately mixed and stirred. It is normal. PVA (polyvinyl alcohol) is the most used substance among the molding binders. PVA is suitable for adhesion and bonding of powders because of its excellent adhesiveness.

  After molding (coating), degreasing is usually performed at a temperature of about 600 ° C. in an oxidizing atmosphere in order to prevent thermal decomposition products from being mixed into the melt during subsequent heating or contact with the melt. It has been broken. PVA undergoes rapid thermal decomposition near 300 ° C and is gasified to CO and the like. As a result, PVA can be rapidly removed up to about 90%, but the remaining 10% can be easily heated to a temperature of 500 ° C or higher. In many cases, carbon residue is not removed.

  In addition, when a mold release material is applied to a graphite-based mold material, if high temperature degreasing is performed in an oxidizing atmosphere, the mold material will oxidize, resulting in a decrease in durability, resulting in an increase in the production cost of the silicon ingot. There is a problem.

  There are various organic binders that can replace PVA, but there is no such thing that has both coatability and adhesiveness.

In view of the above circumstances, the present inventors have used a plasma spraying machine in which silicon nitride and silicon dioxide powder are mixed in a silicon casting method in which a mold release material is applied to the inner surface of a mold and silicon melt is poured. By coating the silicon ingot, it is possible to prevent silicon chipping caused by the mold adhering to the silicon ingot, and to eliminate the conventionally used debinding step for removing the organic binder. A silicon casting method that can reduce the manufacturing cost has been proposed (see Patent Document 2).
Japanese Patent Laid-Open No. 7-206419 JP 2002-292449 A 15TH PHOTOVOLTAIC SPESIALISTS CONF. (1981), P576 ~ P580, “A NEW DIRECTIONAL SOLIDIFICATION TECHNIQUEFOR POLYCRYSTALLINE SOLAR GRADE SILICON”

  However, thermal spraying with a plasma spraying machine is a coating that forms a film by melting or softening the coating material by heating, accelerating it into fine particles, colliding with the surface of the object to be coated, and solidifying and depositing flattened particles Although it is a technology, it is possible to form a denser layer compared to a release material layer formed by brush coating or spray coating, but due to the density of the formed sprayed coating layer being too high, There is a problem that the thermal spray release material layer may be peeled off due to a difference in thermal expansion coefficient between a certain side part and bottom part mold material.

  Further, when spraying a thermal spray powder composed of silicon nitride and silicon dioxide, since silicon nitride does not have a liquid phase, the two powders are bonded together by softening at a low temperature of about 1000 ° C. The silicon dioxide to be formed is only a physical bonding force that is softened and liquidized during thermal spraying and solidifies while including silicon nitride. Silicon dioxide is again formed under a high temperature environment during the solidification of silicon in the mold. Along with a decrease in bonding strength due to softening, there has been a problem that the release material layer powder falls off and is easily mixed into the melt.

  In addition, when silicon dioxide and silicon nitride are used in combination, the adhesion between the mold material and the mold container is increased by mixing silicon dioxide. However, when the weight ratio of silicon dioxide increases, the mold release material and the mold container adhere to each other. As a result, the mold cannot be reused, and when a mold release material is applied to the carbon-based mold container, performing high temperature degreasing in an oxidizing atmosphere will cause the mold container to oxidize and wear out, resulting in reduced durability. As a result, there is a problem that the manufacturing cost of the silicon ingot is increased.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a silicon casting mold that suppresses a decrease in characteristics and yield of a silicon ingot caused by a release material.

  In order to achieve the above object, a silicon casting mold according to claim 1 of the present invention is a silicon casting mold including a container for holding and solidifying a silicon melt therein, the inner surface of the container. Is covered with a release material mainly composed of silicon nitride, and the release material is characterized in that the hardness on the side in contact with the silicon melt is higher than the side in contact with the container. Thus, the side of the mold release material that contacts the container is low in hardness and fragile, so that it is easy to take out the solidified silicon ingot and the side of the mold release material that contacts the silicon melt is high in hardness. Therefore, it is possible to avoid the problem that the release material is broken and mixed into the silicon melt.

  The silicon casting mold according to claim 2 of the present invention is the silicon casting mold according to claim 1, wherein at least a part of the silicon nitride powder constituting the release material is necked on the side in contact with the silicon melt. It is characterized by that. Since it did in this way, silicon nitride powder couple | bonds with powder, the hardness improves markedly, and generation | occurrence | production of the problem that a mold release material is damaged can be suppressed further.

  A silicon casting mold according to a third aspect of the present invention is the silicon casting mold according to the first or second aspect, wherein the container is graphite. A silicon casting mold according to claim 4 of the present invention is the silicon casting mold according to claim 3, wherein the graphite is a carbon fiber reinforced material. Such a graphite mold is suitable for repeated use. However, since it is vulnerable to oxidation, a conventionally used release material containing silicon dioxide has a problem in that oxidation proceeds from a portion in contact with the graphite portion. However, in the present invention, the silicon oxide and the graphite part do not come into contact with each other, so that it can be suitably used. A graphite carbon fiber reinforced material is preferable because it has high strength and is not easily damaged.

  A method for producing a silicon casting mold according to claim 5 of the present invention is a method for producing a silicon casting mold in which an inner surface of a container for holding and solidifying a silicon melt is coated with a release material. A slurry in which silicon and a binder are mixed is applied to the inner surface of the container, and then heat-treated at 1600 ° C. or higher in a nitrogen atmosphere at 1 atm. According to this manufacturing method, the container side of the release material is in contact with the container, and the silicon melt side is in contact with the heated nitrogen atmosphere, so that the temperature on the silicon melt side is higher than that on the container side. Therefore, the silicon casting mold of the present invention can be easily produced on the silicon melt side, and the silicon melt side has higher hardness than the container side.

  A method for producing a silicon casting mold according to claim 6 of the present invention is a method for producing a silicon casting mold in which an inner surface of a container for holding and solidifying a silicon melt is coated with a release material. A slurry in which silicon and a binder are mixed is applied to the inner surface of the container, and then a powder containing silicon nitride is coated thereon by a thermal spraying method using a plasma spraying machine. According to this manufacturing method, the side of the release material that comes into contact with the silicon melt becomes a release material that is appropriately sintered with each other by plasma spraying. Therefore, the silicon melt side has higher hardness than the container side. A silicon casting mold can be easily produced.

  In the present invention, the hardness of the release material is used as a standard indicating the strength of bonding between the release materials, and the absolute value is not particularly significant. Therefore, any known hardness measurement method may be used, but the hardness of the release material on the silicon melt side and the container side may be measured and compared under the same conditions and the same conditions.

  For example, using a Vickers hardness tester, Knoop hardness tester, etc., indentation is indented with an indenter while applying a predetermined load to the container side and silicon side of the release material, and the magnitude of each indentation is compared to compare the magnitude of each can do. Moreover, you may evaluate the hardness of a mold release material by a scratch test. The scratch test is a method in which the object is scratched while applying a predetermined load to a needle tip having a predetermined shape, and the strength of the binding force and hardness of the object are compared depending on the load and its state when destruction occurs. By freely changing the tip diameter of the needle and the applied load, it is possible to compare the difference in minute bonding force between the films.

  For the sample for measuring the hardness of the mold release material, for example, the mold release material formed on the graphite mold is mechanically ground and polished to remove the mold part, or kept in an oxidizing atmosphere at 600 ° C. or higher. What is necessary is just to remove a graphite part chemically. It is efficient to obtain a mold release material by mechanically removing a part of the mold to some extent, and then finally chemically removing the mold at a high temperature in an oxidizing atmosphere. Alternatively, the mold itself may be broken, and the respective hardnesses of the silicon melt side and the container side may be measured from the cross-sectional portion of the release material. Once the relationship between the forming conditions of the release material and the hardness of the release material on the silicon melt side and the container side is obtained, it is sufficient to form the release material under the same conditions from the next time. There is no need to obtain hardness.

  According to the silicon casting mold according to claim 1 of the present invention, since the side of the release material that contacts the container is low in hardness and brittle, it is easy to take out the solidified silicon ingot and Since the side in contact with the silicon melt has high hardness, it is possible to avoid the problem that the release material is broken and mixed into the silicon melt. Also, since a mold release material composed mainly of silicon nitride powder is used, the mold cannot be reused because of the adhesion between the mold release material and the mold container that occurs when silicon dioxide is mixed with the mold release material. In addition, when a mold release material is applied to a graphite mold container, if high temperature degreasing is performed in an oxidizing atmosphere, the mold container will oxidize, resulting in a decrease in durability, resulting in an increase in silicon ingot production costs. The problem of end will not occur.

  According to the silicon casting mold according to claim 2 of the present invention, since the silicon nitride powder is bonded to each other, the hardness thereof is remarkably improved, and the problem of breakage of the release material is further increased. It is possible to obtain a silicon casting mold that can be suppressed and that does not cause deterioration of the characteristics and yield of the silicon ingot caused by the release material.

  According to the silicon casting molds according to claims 3 to 4 of the present invention, conventionally, the mold release material containing silicon dioxide has a problem that it is vulnerable to oxidation, such as oxidation proceeds from a portion in contact with the graphite portion. A graphite mold, particularly a carbon fiber reinforced material having high strength, can be suitably used, and can be repeatedly used to increase productivity.

  According to the method for producing a silicon casting mold according to claim 5 of the present invention, the container side of the release material is in contact with the container, and the silicon melt side is in contact with the heated nitrogen atmosphere. The temperature on the side is higher than that on the container side. Therefore, the silicon casting mold of the present invention can be easily produced on the silicon melt side, and the silicon melt side has higher hardness than the container side. Since a mold release material having high hardness can be produced without using silicon dioxide or oxygen, the problem of reduction in the life of the mold container due to oxidation, which has been a problem in the past, is solved, and as a result, the manufacturing cost of the silicon ingot is reduced. Will be able to.

  According to the method for manufacturing a silicon casting mold according to claim 6 of the present invention, the side of the release material that comes into contact with the silicon melt is a release material that is appropriately sintered with each other by plasma spraying. Thus, the silicon casting mold of the present invention having higher hardness than the container side can be easily produced. A silicon casting mold capable of easily demolding the solidified silicon ingot and preventing the release material from being mixed into the silicon ingot and suppressing the deterioration of the characteristics and yield of the silicon ingot caused by the release material. Can be obtained.

  Hereinafter, the invention according to each claim will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a view showing a silicon casting mold of the present invention, and FIG. 1A is a view showing an embodiment of a mold container 1 which is a container according to the silicon casting mold of the present invention. FIGS. 1B and 1C are views showing the action of the release material applied to the silicon casting mold of the present invention.

  The mold container 1 for holding and solidifying the silicon melt inside is made of, for example, graphite and is configured in the form of a divided mold container that can be divided and assembled by combining one bottom member 1a and four side members 1b. . The bottom member 1a and the side member 1b are assembled so as to be separable by fixing with bolts (not shown) or the like, or fixed with a frame member (not shown) in which the bottom member 1a and the side member 1b just fit. Is assembled in a separable manner.

  The inner surface of the mold container 1 is covered with a release material 2 containing silicon nitride as a main component so that the bottom member 1a and the side member 1b can be repeatedly used. The release material 2 is configured such that the hardness of the release material 2 is higher on the side in contact with the silicon melt than on the side in contact with the mold container 1.

  FIG. 1B and FIG. 1C show the action of the release material according to the present invention. FIG. 1 (b) shows a state when the silicon melt 3 is poured into the mold container 1 provided with the release material 2 according to the present invention, and FIG. 1 (c) shows that the silicon melt 3 is cooled and solidified. The state when the silicon ingot 4 obtained in this way is removed from the mold container 1 is shown. Thus, since the container side 2a of the release material 2 is low in hardness and fragile, when the solidified silicon ingot 4 is taken out, it can be easily broken and the silicon ingot 4 can be smoothly demolded. . On the other hand, since the silicon melt side 2b has high hardness and is more strongly bonded to each other, it is damaged when the silicon melt 3 is poured or cooled, so that the component of the mold release material 2 becomes the silicon melt. The problem of being mixed in can be avoided.

  Conventionally, since the release material 2 made of silicon nitride is fragile, the problem that it breaks and melts into the silicon melt 3 is caused by the fact that everything from the container side 2a to the silicon melt side 2b of the release material 2 occurs. This is a problem that occurs because it is fragile, and this problem can be solved by increasing the hardness of the release material 2 on the surface portion corresponding to the silicon melt side 2b as in the present invention. On the contrary, if the hardness of all of the mold release material 2 from the container side 2a to the silicon melt side 2b is increased, the solidified silicon ingot 4 cannot be demolded, and the original purpose of the mold release material 2 cannot be achieved. As a result, the mold container 1 cannot be reused, which increases the manufacturing cost of the silicon ingot 4.

  Further, in the present invention, since the release material mainly composed of silicon nitride powder is used, the release material 2 and the mold container 1 that are generated in the release material that has been used by adding conventional silicon dioxide are adhered. Thus, there is no problem that the mold cannot be reused.

  As for the silicon nitride powder which comprises the mold release material 2 concerning the casting mold for silicon casting of the present invention, it is desirable that at least a part thereof is necked on the side in contact with the silicon melt.

  FIG. 2 is a schematic diagram for explaining necking. When the powder is heated, a phenomenon occurs in which the particles 5 are bonded to each other on the contact surface. This phenomenon is called necking, and the connecting portion is called neck 6. The force to reduce the surface energy of the particles 5 becomes a driving force, causing mass transfer to the neck 6, and the neck 6 gradually becomes thicker as shown by the block arrows, resulting in a coupled state. In other words, necking refers to a pre-sintered state where bonding between powders has started.

  In the conventional method, the slurry in which the silicon nitride powder and the binder are mixed is applied to the inner surface of the mold container 1 and dried, so that the powder is not necked and the particles are in a disjointed state. For this reason, the release material 2 is fragile, and may be broken or cracked, resulting in problems such as the release material 2 being mixed into the silicon ingot or the silicon melt adhering to the mold container 1 and being broken. there were.

  However, in the mold release material 2 according to the mold for casting silicon according to the present invention, the silicon nitride powder as the main component is necked and bonded to each other, whereby the hardness is remarkably improved. Further, when the neck becomes fat and joined, the hardness is further increased. Accordingly, it is possible to suppress the occurrence of the problem that the release material 2 is damaged, and it is possible to obtain a silicon casting mold that does not cause deterioration of the characteristics and yield of the silicon ingot caused by the release material 2.

  Moreover, it is desirable to use graphite for the mold container 1 according to the silicon casting mold according to the present invention, and it is desirable to use a carbon fiber reinforced material among the graphite. Such a graphite mold is suitable for repeated use. However, since it is vulnerable to oxidation, a mold release material containing silicon oxide, which has been used in the past, is oxidized from the point where the graphite part and silicon oxide (silicon dioxide) are in contact with each other. There was a problem such as progress. On the other hand, when the release material 2 according to the present invention is used, the silicon dioxide and the graphite portion do not come into contact with each other. Further, a graphite carbon fiber reinforced material is preferable because it has high strength and is not easily damaged.

  Next, a method for producing a silicon casting mold will be described.

  Fig.1 (a) is a figure which shows one Example of the casting_mold | template used for the casting_mold | template for silicon casting concerning this invention. The mold container 1 is made of, for example, graphite, and is composed of a divided mold that can be assembled and assembled by combining one bottom member 1a and four side members 1b.

  The bottom member 1a and the side member 1b are assembled so as to be separable by fixing with bolts (not shown) or the like, or fixed with a frame member (not shown) in which the bottom member 1a and the side member 1b just fit. Is assembled in a separable manner.

  A mold release material 2 is coated on the inner surface of the mold container 1 so that the bottom member 1a and the side member 1b can be repeatedly used. Specifically, at least a predetermined amount of silicon nitride is weighed on the side that becomes the inner surface of the mold, mixed with 5 to 15% by weight of a PVA (polyvinyl alcohol) aqueous solution as a molding binder, and these are powders if stirred. The raw material can be made into a slurry and can be easily applied to the mold container 1. The mold release material 2 is formed by applying and drying the mold release material slurry thus prepared on the inner surface of the mold container 1. As the molding binder, it is desirable to use PVA. PVA is suitable for adhesion and bonding of powders because of its excellent adhesiveness. In addition, additives for increasing fluidity may be appropriately mixed with the slurry.

  As a coating method, a method of applying to a member of the mold container 1 with a brush or spatula and drying on a hot plate may be used, but for example, applying to the inner surface of the mold container 1 using a spray or the like, A method of forming by drying and a method of thermocompression bonding using a laminating apparatus provided with a heating plate / silicon rubber diaphragm are also possible. In addition, when the mold container 1 is assembled by a detachable side member 1b and a bottom member 1a having a substantially planar shape, it is preferable because many coating methods can be applied. It is desirable to apply and form a slurry of the mold release material 2 by using it. If the release material 2 is applied to the inner side surface portion and the inner bottom surface portion using this screen printing, the uniformity of the release material 2 can be remarkably improved. By forming the silicon ingot 4 inside, the yield of the silicon ingot 4 can be increased.

  The release material 2 is desirably formed to have a thickness in the range of 0.3 mm to 1.2 mm. If the thickness is smaller than this range, the silicon melt 3 penetrates the mold release material 2 and is fused to the mold container 1, and the cast silicon ingot 4 is cracked or chipped. The mold release material 2 is not sufficiently fused to the mold container 1 but has a sufficient thickness. However, it takes time to form the mold release material 2, and if the mold release material 2 is too thick, the mold release material 2 is damaged or peeled off. This is because the release material 2 peeled off from the mold container 1 is likely to be generated and mixed as a foreign substance in the silicon during the process of melting silicon and cooling and solidifying the silicon melt 3, thereby causing a defect in the silicon ingot 4 and reducing the yield. .

  In the method for producing a silicon casting mold of the present invention, the release material 2 formed by coating is heat-treated at 1600 ° C. or higher in a nitrogen atmosphere at 1 atmosphere, or is nitrided from above the release material 2 formed by coating. It is characterized by coating silicon-containing powder by a thermal spraying method using a plasma spraying machine. Details will be described later.

  After the mold release material 2 is formed in this way, the mold is placed in an argon (Ar) atmosphere whose pressure is reduced to 70 to 90 Torr, and heated to the same temperature as or slightly lower than the silicon melt 3 to inject the silicon melt 3. I boil water. Alternatively, the silicon raw material may be placed in the mold container 1 having the release material 2 applied to the inner surface and dissolved directly. Thereafter, the temperature is gradually lowered from the bottom of the mold container 1 to gradually solidify the silicon melt 3 from the bottom of the mold. Finally, the mold container 1 is divided and the silicon ingot 4 is taken out to complete the silicon ingot 4. The mold container 1 from which the silicon ingot 4 has been removed can be reassembled and used repeatedly.

  In the method for producing a silicon casting mold according to claim 5 of the present invention, the mold release material 2 formed by coating is heat-treated at 1600 ° C. or higher in a nitrogen atmosphere at 1 atm. According to this manufacturing method, since the container side 2a of the release material 2 is in contact with the mold container 1, and the silicon melt side 2b is in contact with the heated nitrogen atmosphere, the silicon melt side 2b is closer to the container side 2a. The temperature becomes higher than. Therefore, the silicon casting mold according to the present invention can be easily produced by sintering on the silicon melt side 2b more easily than the container side 2a, and the silicon melt side 2b having higher hardness than the container side 2a. Can do.

  Here, this mechanism will be described in more detail. A natural oxide film around the silicon nitride powder is sintered by heat-treating the mold release material 2 made of slurry mixed with silicon nitride and binder coated on the inner surface of the mold container 1 at 1600 ° C. or higher in a nitrogen atmosphere. It plays the role of an auxiliary agent, and necking between the particles progresses, and a pre-sintered state is obtained.

  Usually, the surface of the silicon nitride raw material contains about 1 to 3% as an oxide layer, which can be confirmed by observation with a TEM (transmission electron microscope). Here, as described above with reference to FIG. 2, the necked state is a temporarily sintered state in which the particles 5 are slightly fused to each other in the initial stage of sintering and the neck 6 is generated. As described above, since the release materials 2 are appropriately bonded by the necking of the particles 5 generated by the heat treatment in the nitrogen atmosphere, the amount taken into the silicon melt during pouring or solidification is reduced. Can do.

  Since the necking of the silicon nitride powder occurs rapidly from about 1600 ° C. or higher in a nitrogen atmosphere at 1 atm, the heat treatment is performed at a temperature of 1600 ° C. or higher. Necking occurs even at 1600 ° C. or lower, but the sintering rate is slow and not realistic. Moreover, since it sublimes and decomposes at 1800 ° C. or higher, the processing temperature is 1800 ° C. or lower. In the case of 1750 ° C., sufficient strength as the release material 2 according to the present invention can be obtained by heat treatment for 1 hour.

  When all of the release material 2 is completely sintered and becomes a plate-like strong layer, the partially infiltrated silicon melt 3 is solidified, and when the solidified solidified layer is cooled, the silicon ingot 4 and the release material 2 Although the problem that the silicon ingot 4 breaks due to the difference in thermal expansion coefficient and the problem that the silicon ingot 4 cannot be demolded arises, in the present invention, the hardness of the container side 2a of the release material 2 and the silicon melt side 2b are different. The above problems will not occur. Further, when a slurry made of silicon nitride and polyvinyl alcohol (not containing silicon dioxide) is used, a natural oxide film mainly serves as a sintering aid, so that a complete sintered body is not obtained. Further, according to this method, since the mold release material 2 does not contain silicon dioxide, the problem of deterioration of the mold container 1 due to silicon dioxide contained in the mold release material 2, which has been a conventional problem, does not occur.

  Furthermore, in the present invention, since heating is performed in a nitrogen atmosphere, there is no problem that the high temperature degreasing is performed in an oxidizing atmosphere as in the prior art, and the mold is oxidized and consumed. Therefore, durability can be ensured even if relatively inexpensive graphite is used as the mold container 1, and the manufacturing cost of the silicon ingot can be reduced. Here, as the graphite, durability is further enhanced by using a carbon fiber reinforced material.

  In the method for manufacturing a silicon casting mold according to claim 6 of the present invention, the release material 2 formed by coating is coated with a powder containing silicon nitride thereon by a thermal spraying method using a plasma spraying machine. According to this manufacturing method, the silicon melt side 2b of the mold release material 2 becomes the mold release material 2 appropriately sintered by plasma spraying, so that the silicon melt side 2b has higher hardness than the container side 2a. The silicon casting mold of the present invention can be easily produced.

  The plasma spraying machine used here is an apparatus for forming a film by sending various powder materials into a plasma flow and melt-spraying them. The thermal spraying temperature uses a temperature range of about 10,000 ° C. in a plasma airflow extending to 32000 K, and the spray speed of the molten particles reaches Mach 1. As a result, an extremely high quality and denser film layer can be formed.

At this time, it is desirable to mix silicon dioxide forming a glass layer at a relatively low temperature as a raw material powder as a sintering aid. The reason is that silicon nitride undergoes sublimation decomposition at about 1800 ° C. (1 atm, in N ₂ ), so that it is difficult to form a coating layer by thermal spraying without forming a liquid phase by itself. Here, by mixing silicon dioxide as a sintering aid, the silicon nitride powder is fused in the molten silicon dioxide and fused to the base material and rapidly cooled to be fixed in a necked state. . The release material 2 formed on the surface portion by plasma spraying should have a mixing ratio of silicon nitride and silicon dioxide of 1: 9 to 9: 1. With such a mixing ratio, it is easy to remove the mold and the effect of preventing the silicon melt 3 from being dissolved or mixed can be further enhanced.

  At this time, the mold container 1 has already been formed with a mold release material 2 mainly composed of silicon nitride, and the mold release material 2 containing silicon dioxide is not in direct contact with the mold container 1, so that there has been a problem in the past. There is no problem of deterioration of the mold container 1 due to silicon dioxide contained in the release material 2.

  It should be noted that the embodiment of the present invention is not limited to the above-described example, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

  In the present invention, if the hardness of the silicon melt side 2b is higher than that of the container side 2a of the release material 2, the effect of the invention is achieved. For example, it may be a multilayer structure having two or more layers of a high hardness portion and a low hardness portion, or a structure in which the hardness gradually increases from the container side 2a toward the silicon melt side 2b.

  Here, when the release material has a multilayer structure of two layers or more, it is preferable that the thickness of the portion having high hardness in contact with the silicon melt 3 is 5 μm or more. This is because if the thickness is 5 μm or less, the release material 2 may not be able to sufficiently exhibit the effect of preventing the melt into the silicon melt 3. Further, the thickness of the low hardness portion of the container side 2a in contact with the mold container 1 is set to about 50 to 200 μm, thereby preventing the silicon melt 3 from adhering to the mold container 1 and coagulating inside the mold container 1. The silicon ingot 4 thus made can be easily removed.

It is a figure which shows an example of the casting mold for silicon casting of this invention, (a) is an example of a casting_mold | template container, (b) and (c) are figures which show the effect | action of the mold release material concerning this invention. It is a figure which shows the state of necking.

Explanation of symbols

1: Mold container 1a: Bottom member 1b: Side member 2: Release material 2a: Container side 2b: Silicon melt side 3: Silicon melt 4: Silicon ingot 5: Particle 6: Neck

Claims (6)

A mold for silicon casting provided with a container for holding and solidifying a silicon melt therein, and the inner surface of the container is covered with a release material mainly composed of silicon nitride, A silicon casting mold characterized in that the hardness on the side in contact with the silicon melt is higher than that on the side in contact with the container. 2. The silicon casting mold according to claim 1, wherein at least a part of the silicon nitride powder constituting the release material is necked on the side in contact with the silicon melt. The silicon casting mold according to claim 1, wherein the container is graphite. The silicon casting mold according to claim 3, wherein the graphite is a carbon fiber reinforced material. A method for producing a silicon casting mold in which an inner surface of a container for holding and solidifying a silicon melt is coated with a release material, and after applying slurry mixed with silicon nitride and a binder to the inner surface of the container A method for producing a silicon casting mold, wherein heat treatment is performed at 1600 ° C. or higher in a nitrogen atmosphere at 1 atmosphere. A method for producing a silicon casting mold in which an inner surface of a container for holding and solidifying a silicon melt is coated with a release material, and after applying slurry mixed with silicon nitride and a binder to the inner surface of the container A method for producing a silicon casting mold, wherein a powder containing silicon nitride is coated thereon by a thermal spraying method using a plasma spraying machine.

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