JP6690735B2 - Silicon nitride powder, release agent for polycrystalline silicon ingot, and method for producing polycrystalline silicon ingot - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a silicon nitride powder capable of forming a mold release layer having excellent adhesion to a mold and mold releasability on the mold, and particularly to a silicon nitride powder suitable as a mold release agent for a polycrystalline silicon ingot.

  A polycrystalline silicon substrate used for a solar cell is usually obtained from a polycrystalline silicon ingot produced by unidirectionally solidifying molten silicon using a vertical Bridgman furnace. High performance and low cost are required for polycrystalline silicon substrates, and in order to meet these demands, suppression of impurity mixing into polycrystalline silicon ingot during directional solidification of molten silicon and polycrystalline silicon ingot are required. It is important to improve the yield of In the unidirectional solidification of molten silicon by the vertical Bridgman method, a mold such as quartz is used, but in order to increase the yield of the polycrystalline silicon ingot, the mold has good releasability of the polycrystalline silicon ingot. Therefore, a mold in which a mold release agent containing silicon nitride powder is applied to the inner wall (the surface in contact with molten silicon) is generally used.

  Due to the structure of the vertical Bridgman furnace, heat escapes downward from the bottom surface of the mold, so that a large temperature gradient occurs in the vertical direction in the mold, and the temperature at the upper part of the mold becomes relatively high. In recent years, polycrystalline silicon ingots for solar cell substrates have tended to become larger and larger. If the temperature is raised until the silicon (melting point; 1414 ° C) at the bottom of the mold is sufficiently melted, depending on the structure of the Bridgman furnace, The temperature may be as high as 1500 ° C. or higher. In such a case, in the upper part of the mold where the temperature is high, there may be a problem that the releasability of the polycrystalline silicon ingot deteriorates, and the release layer peels from the mold and adheres to the polycrystalline silicon ingot. Therefore, even if the release layer of the polycrystalline silicon ingot is unidirectionally solidified at a high temperature, for example, 1500 ° C. or higher, the release property of the polycrystalline silicon ingot and the adhesion of the release layer to the mold are good. Is required.

  From such a background, even if the melting temperature of silicon during unidirectional solidification is increased to improve the yield of the polycrystalline silicon ingot applicable to the substrate of the solar cell, the releasability of the polycrystalline silicon ingot, and Development of a silicon nitride powder capable of forming a release layer having good adhesion to a mold is desired. Further, in order to obtain a long polycrystalline silicon ingot, when a mold having a large size in the vertical direction is used, the upper part of the mold is exposed to a high temperature for a particularly long time, so the melting time of silicon during unidirectional solidification It is desired to develop a silicon nitride powder capable of forming a release layer having a good releasability of a polycrystalline silicon ingot and a good adhesion to a mold even if the length is long.

  For example, Patent Document 1 discloses a silicon nitride powder having a 90% particle diameter of 3.0 to 10 μm by a laser diffraction scattering method, an α phase ratio of 20 to 60%, and an iron content of 100 ppm or less, and a particle size distribution. Has two local maxima, one is 0.2 μm or more and less than 1.0 μm (maximum 1), the other is 1.0 μm or more and 8.0 μm or less (maximum 2), and maxima 1 and 2 Of each frequency of {(frequency of maximum value 2) / (frequency of maximum value 1)} is 1.0 to 5.0, and the interval between maximum values 1 and 2 is 0.8 to 7.8 μm. When the powder is used as a release agent for the polycrystalline silicon ingot, workability during application is improved, and peeling of the release agent is suppressed, so that it is possible to reduce contamination of impurities in the polycrystalline silicon ingot. Have been described.

  Further, Patent Document 2 discloses a nitriding layer formed in a crucible used for solidifying a polycrystalline silicon ingot, which contains particles of 1 μm or less and particles of 2 μm to 50 μm, preferably 2 μm to 5 μm. It is described that the silicon-based release layer has high strength (can avoid peeling and flaking and falling) and excellent mechanical abrasion resistance.

JP, 2014-9111, A Japanese Patent Publication No. 2009-510387

  Patent Document 1 discloses a silicon nitride powder in which the frequency distribution curve obtained by particle size distribution measurement has two peaks and the peak top of the peak is in a specific range when the ratio of α phase is in a specific range. However, although it has been described that the release agent is prevented from peeling off and impurities are mixed into the polycrystalline silicon ingot, it is not effective when the peak top of the larger particle size is larger than 8.0 μm. Are also shown. Further, the crystallite diameter of silicon nitride and its ratio to the BET diameter are not described, and a polycrystalline silicon ingot obtained when the melting temperature of silicon is increased or the melting time of silicon is lengthened. Also, there is no description of the releasability of No. 1 and the adhesion of the release layer to the mold.

  Further, Patent Document 2 describes that when it contains particles of 1 μm or less and particles of 2 μm to 50 μm larger than that, the release layer is difficult to peel off, but the range of large particles is small. Only a wider range is described, the effect of using particles of two different sizes in combination is not shown, and the preferable range of large particles is only 2 μm to 5 μm. . Further, the crystallite diameter of silicon nitride and its ratio to the BET diameter are not described, and a polycrystalline silicon ingot obtained when the melting temperature of silicon is increased or the melting time of silicon is lengthened. Also, there is no description of the releasability of No. 1 and the adhesion of the release layer to the mold.

  Therefore, the present invention has a good releasability of a polycrystalline silicon ingot even when the melting temperature of silicon during unidirectional solidification is increased or when the melting time of silicon is lengthened. An object of the present invention is to provide a silicon nitride powder that can be suitably used as an agent.

  The present inventors have conducted extensive studies to solve the above problems, and have a specific specific surface area, a specific β-type silicon nitride ratio, a specific crystallite diameter and a ratio of the specific surface area equivalent diameter thereof, Specific particle size distribution, in particular, the frequency distribution curve has two peaks, when the release layer of the polycrystalline silicon ingot casting mold is formed using a silicon nitride powder in which the peak top of the larger peak is in a large range. The inventors found that even if the melting temperature of silicon during unidirectional solidification was increased, the releasability of the polycrystalline silicon ingot and the adhesion of the release layer to the mold were good, and the present invention was completed. It was That is, the present invention relates to the following matters.

(1) The specific surface area measured by the BET method is 2 m ² / g or more and 13 m ² / g or less, the proportion of β-type silicon nitride is 50% by mass or more, and the powder X-ray diffraction pattern of β-type silicon nitride the crystallite size of β-type silicon nitride which is calculated using the Williamson-Hall equation is taken as _{D C, D C} is not less 150nm or more, a specific surface area equivalent diameter calculated from the specific surface area was _{D BET} Sometimes D _BET / D _C (nm / nm) is 3 or less, and the frequency distribution curve obtained by the volume-based particle size distribution measurement by the laser diffraction scattering method has two peaks, and the peak top of the peak Is in the range of 0.5 to 2 μm and in the range of 6 to 30 μm, and the ratio of the frequency of the peak tops (frequency of peak tops in the range of particle size 0.5 to 2 μm / particle size in the range of 6 to 30 μm). Silicon nitride powder, wherein the frequency of Kutoppu) is 0.1 to 1.

(2) The crystal strain of β-type silicon nitride calculated from the powder X-ray diffraction pattern of β-type silicon nitride using the Williamson-Hall equation is 1.5 × 10 ⁻⁴ or less, (1) ) Silicon nitride powder.

  (3) The silicon nitride powder according to the above (1) or (2), wherein the peak tops are in the range of 0.5 to 2 μm and in the range of 9 to 30 μm.

  (4) The silicon nitride powder according to any one of (1) to (3) above, wherein the proportion of β-type silicon nitride is 70% by mass or more.

(5) The silicon nitride powder according to any one of (1) to (4) above, wherein the specific surface area is 2 m ² / g or more and 10 m ² / g or less.

  (6) The Fe content is 100 ppm or less, the Al content is 100 ppm or less, and the total content of metal impurities other than Fe and Al is 100 ppm or less. (5) Any of the silicon nitride powders.

  (7) A release agent for a polycrystalline silicon ingot, which contains the silicon nitride powder according to any one of (1) to (6) above.

  (8) A method for producing a polycrystalline silicon ingot, in which molten silicon contained in a mold is solidified, wherein the mold has silicon nitride powder on a contact surface with the molten silicon. A method for manufacturing a silicon ingot, which comprises using a mold coated with.

  According to the silicon nitride powder of the present invention, even if the melting temperature of silicon during unidirectional solidification is increased or the melting time of silicon is lengthened, the releasability of the polycrystalline silicon ingot and the release layer It is possible to provide a silicon nitride powder which can improve adhesion to a mold and is suitable as a mold release agent for a polycrystalline silicon ingot.

It is a schematic diagram of the combustion synthesis reaction apparatus used for manufacture of the silicon nitride powder of Examples 1-11, Comparative Examples 1-3, and Comparative Examples 6-13.

  An embodiment of the silicon nitride powder of the present invention will be described in detail.

(Silicon nitride powder)
The silicon nitride powder of the present invention has a specific surface area measured by the BET method of 2 m ² / g or more and 13 m ² / g or less, a proportion of β-type silicon nitride of 50% by mass or more, and a β-type silicon nitride powder. the crystallite size of β-type silicon nitride which is calculated using the Williamson-Hall type from X-ray diffraction pattern when the _{D C, D C} is not less 150nm or more, the specific surface area equivalent diameter calculated from the specific surface area the when the _{D BET, D BET / D C} (nm / nm) is 3 or less, the frequency distribution curve obtained by particle size distribution measurement of the volume value determined by a laser diffraction scattering method, has two peaks, The peak top of the peak is in the range of 0.5 to 2 μm and the range of 6 to 30 μm, and the frequency of the peak top in the range of particle size 0.5 to 2 μm / the peak in the range of particle size 6 to 30 μm. Tsu frequency of-flops) is characterized in that it is a 0.1 to 1. The ratio of the peak top frequencies may be less than 1.0, 0.95 or less, 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less.

The silicon nitride powder of the present invention has a specific surface area measured by the BET method of 2 m ² / g or more and 13 m ² / g or less. When the specific surface area is within this range, it is possible to form a release layer having good adhesion to the mold. From this viewpoint, the specific surface area is more preferably ² m ² / g or more and 10 m ² / g or less. The specific surface area of the silicon nitride powder measured by the BET method may be 8 m ² / g or less, 6 m ² / g or less, and 4 m ² / g or less.

  In the silicon nitride powder of the present invention, the proportion of β-type silicon nitride is 50% by mass or more. When the proportion of β-type silicon nitride is within this range, it is possible to form a release layer having excellent release properties of the polycrystalline silicon ingot and good adhesion to the mold. From this viewpoint, the proportion of β-type silicon nitride is more preferably more than 70% by mass. The proportion of β-type silicon nitride may be 60% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, and may be 100% by mass.

  Components other than silicon nitride are less than 3% by mass, more preferably less than 1% by mass, and particularly preferably less than 0.1% by mass. When a component other than silicon nitride is present, good releasability of the polycrystalline silicon ingot is obtained even when the melting temperature of silicon during unidirectional solidification as in the present invention is increased or when the melting time of silicon is lengthened. May not be obtained.

When the crystallite diameter of the β-type silicon nitride which is calculated using the Williamson-Hall type from powder X-ray diffraction pattern of β-type silicon nitride was _{D C,} silicon nitride powder of the present invention is a _{D C} is 150nm or more is there. When D _C is in this range, a release layer having good release properties of the polycrystalline silicon ingot and good adhesion to the mold can be obtained even if the melting temperature of silicon is increased or the melting time is lengthened. Can be formed. By the D _C and above 150 nm, even if prolonged contact with the molten silicon, for example 1500 ° C. or more high temperature, silicon nitride powder of the present invention is presumed to maintain the structural stability of the crystal. From this viewpoint, it is preferable that _{D C} is 300nm or more, and further preferably 500nm or more. D _C is, 180 nm or higher, 200 nm or higher, 250 nm or more, may be 400nm or more.

The silicon nitride powder of the present invention preferably has D _BET / D _C (nm / nm) of 3 or less, where D _BET is a specific surface area equivalent diameter calculated from the specific surface area. If D _BET / D _C (nm / nm) is in this range, a release layer having good release properties of the polycrystalline silicon ingot and good adhesion to the mold even if the melting temperature of silicon is increased is obtained. Can be formed. The reason is not clear, but the smaller the area of the interface of the crystallites in one particle of silicon nitride constituting the silicon nitride powder, the structure of the crystal of silicon nitride when it is in contact with molten silicon for a long time at high temperature. It is presumed that it will further improve the social stability. D _BET / D _C (nm / nm) can be 2 or less, 1.8 or less, and 1.5 or less.

  In the silicon nitride powder of the present invention, the frequency distribution curve obtained by the volume-based particle size distribution measurement by the laser diffraction scattering method has two peaks, and the peak top of the peak is in the range of 0.5 to 2 μm. 6 to 30 μm, and the ratio of the peak top frequencies (frequency of peak tops in the range of particle diameter 0.5 to 2 μm / frequency of peak tops in the range of particle diameter 6 to 30 μm) is 0.1 to 1. Is. If the frequency distribution curve has two peaks, the two peak tops are in each of the ranges, and the ratio of the frequencies is in the range, the release layer becomes dense and the release of the polycrystalline silicon ingot is released. And the adhesion of the release layer to the mold are also improved.

  Of the two peak tops, the particles having a peak top in the range of 0.5 to 2 μm have the effect of enhancing the adhesion between silicon nitride particles and the adhesion between the silicon nitride particles and the mold, and form a dense release layer. Have the effect of Therefore, when the peak top is in the range of 0.5 to 2 μm, it is possible to form a release layer having good release properties. On the other hand, particles having a peak top in the range of 6 to 30 μm have the effect of increasing the heat resistance of the release layer. Therefore, if the peak top is in the range of 6 to 30 μm, even if the silicon is melted at a high temperature of 1500 ° C. or higher, the release layer does not peel off and a good release layer can be formed. The peak top of the smaller particle size may be 1.5 μm or less, 1.0 μm or less, or 0.9 μm or less. The peak top of the larger particle size may be 9 μm or more, 10 μm or more, 11 μm or more, 13 μm or more, 15 μm or more, or 25 μm or less, 20 μm or less. Here, the range of the peak top of the smaller particle size and the range of the peak top of the larger particle size can be any combination of the various ranges described above. Moreover, if the ratio of the frequency of the peak tops (frequency of peak tops in the range of particle size 0.5 to 2 μm / frequency of peak tops in the range of particle size 6 to 30 μm) is 0.1 to 1, The effect of particles can be maximized, the adhesion between silicon nitride particles is good, the adhesion between silicon nitride particles and the mold is good, and a dense release layer can be formed. A release layer having good properties can be formed. The ratio of the frequency of peak tops (frequency of peak tops in the range of particle size 0.5 to 2 μm / frequency of peak tops in the range of particle size 6 to 30 μm) is less than 1.0, 0.95 or less, 0. It may be 9 or less, 0.8 or less, 0.7 or less, 0.6 or less. The peak top with the smaller particle size may be in the range of 0.6 μm to 1.5 μm, and the peak top with the larger particle size may be in the range of 11 μm to 29 μm.

In the silicon nitride powder of the present invention, the crystal strain of β-type silicon nitride calculated from the powder X-ray diffraction pattern of β-type silicon nitride using the Williamson-Hall equation is preferably 1.5 × 10 ⁻⁴ or less. . If the crystal strain of β-type silicon nitride is within this range, it is necessary to form a release layer that has good mold release properties of the polycrystalline silicon ingot and good adhesion to the mold even if the melting temperature of silicon is raised. You can By setting the crystal strain to 1.5 × 10 ⁻⁴ or less, it is presumed that the silicon nitride powder of the present invention can maintain the structural stability of crystals even when it is in contact with molten silicon at a higher temperature for a long time. It From this viewpoint, the crystal strain is more preferably 1.2 × 10 ⁻⁴ or less, and particularly preferably 1.0 × 10 ⁻⁴ or less. The crystal strain of β-type silicon nitride is 1.5 × 10 ⁻⁴ or less, 1.4 × 10 ⁻⁴ or less, 1.2 × 10 ⁻⁴ or less, 1.0 × 10 ⁻⁴ or less, 0.8 × 10 ^{−. 4} or less, it can be 0.7 × ^{10 -4} or less.

  The silicon nitride powder of the present invention has a Fe content of 100 ppm or less. When the content ratio of Fe is in this range, the mixing of Fe into the polycrystalline silicon ingot can be suppressed, and the yield of the polycrystalline silicon ingot applicable to the solar cell application is increased. The content ratio of Fe is preferably 50 ppm or less, 20 ppm or less, and particularly preferably 10 ppm or less. The silicon nitride powder of the present invention has an Al content of 100 ppm or less. When the content ratio of Al is within this range, the mixing of Al into the polycrystalline silicon ingot can be suppressed, so that the yield of the polycrystalline silicon ingot applicable to the solar cell application is increased. The content ratio of Al is preferably 50 ppm or less, 20 ppm or less, and particularly preferably 10 ppm or less. Further, the total content ratio of metal impurities other than Fe and Al is 100 ppm or less. When the content ratio of the metal impurities other than Fe and Al is within this range, it is possible to suppress the mixing of the metal impurities other than Fe and Al into the polycrystalline silicon ingot, and therefore the polycrystalline silicon applicable to the solar cell application Higher ingot yield. The content ratio of metal impurities other than Fe and Al is preferably 50 ppm or less, 20 ppm or less, and particularly preferably 10 ppm or less.

(Release agent for polycrystalline silicon ingot)
The release agent for a polycrystalline silicon ingot of the present invention contains the silicon nitride powder of the present invention. The release agent for a polycrystalline silicon ingot of the present invention may be the silicon nitride powder of the present invention as a main component, and may contain a component other than silicon nitride, but is composed only of the silicon nitride powder of the present invention. May be.

(Method for manufacturing polycrystalline silicon ingot)
The method for producing the polycrystalline silicon ingot of the present invention will be described below. The method for producing a polycrystalline silicon ingot of the present invention is a method for producing a polycrystalline silicon ingot in which molten silicon contained in a mold is solidified (particularly unidirectionally solidified), and as the mold, contact with the molten silicon is performed. It is characterized by using a mold whose surface is coated with the silicon nitride powder of the present invention.

(Method for producing silicon nitride powder)
An example of the method for producing the silicon nitride powder of the present invention will be described below. The silicon nitride powder of the present invention, for example, in a silicon nitride combustion synthesis process for synthesizing silicon nitride by a combustion synthesis method utilizing a self-heating and propagation phenomenon associated with a combustion reaction of silicon, using specific manufacturing conditions, Is mixed with silicon nitride powder as a diluent in a specific ratio to the raw material silicon powder, the content ratio of the metal impurities of the raw material silicon powder and the silicon nitride powder as the diluent is small, and the silicon powder and the silicon nitride powder Using a method of producing a combustion product with a small crushing strength by carrying out a combustion reaction with a small packing density of the mixture, and crushing the obtained combustion product with a small crushing strength with a small grinding energy and with which metal impurities are difficult to mix. By adjusting to specific grinding conditions and grinding, the content ratio of metal impurities is low and the content ratio of β-type silicon nitride is high. In a particular to the specific surface area and particle size distribution can be produced silicon nitride powder having a characteristic such as large crystal strains crystallite size is smaller. Hereinafter, an example of the manufacturing method will be specifically described.

<Preparation process of mixed raw material powder>
First, a silicon powder and a silicon nitride powder as a diluent are mixed to prepare a mixed raw material powder. Since the combustion synthesis reaction has a high temperature of 1800 ° C. or higher, silicon may be melted / welded in a portion where the combustion reaction occurs. For the purpose of suppressing this, it is preferable to add silicon nitride powder as a diluent to the raw material powder within a range that does not prevent self-propagation of the combustion reaction. The addition rate of the diluent is usually 10 to 50% by mass (the mass ratio of silicon: silicon nitride is 90:10 to 50:50), and further 15 to 40% by mass. Further, NH ₄ Cl, NaCl or the like may be added to adjust the proportion of β-type silicon nitride in the combustion product obtained by the combustion synthesis reaction. These additives have the effect of lowering the reaction temperature by sensible heat, latent heat and endothermic reaction. Here, the content ratio of Fe, the content ratio of Al, and the content ratio of metal impurities other than Fe and Al in the obtained mixed raw material powder are preferably 100 ppm or less, further preferably 50 ppμm or less and 10 ppm or less, respectively. Therefore, it is preferable to use a high-purity powder having a small content ratio of metal impurities as both the silicon powder and the silicon nitride powder as a diluent. In addition, it is preferable that the inner surface of the mixing container used for mixing the raw material powders and the portion such as the mixing medium which come into contact with the raw material powders are made of a non-metallic material having a small content ratio of Al and Fe. The method of mixing the raw material powders is not particularly limited, but when ball mill mixing is adopted, for example, the inner surface of the mixing container is preferably made of resin, and the outer surface of the mixing medium is preferably made of silicon nitride. Further, the bulk density of the mixed raw material powder is preferably less than 0.5 g / cm ³ . The bulk density of the mixed raw material powder to be less than 0.5 g / cm ^3, it is preferable that the bulk density is used 0.45 g / cm ³ or less of the silicon powder as the raw material powder. When the bulk density of the mixed raw material powder is less than 0.5 g / cm ³ , it is easy to set the crushing strength of the lumpy combustion product obtained in <Combustion synthesis reaction step> described below to 4 MPa or less.

<Combustion synthesis reaction process>
Next, the obtained mixed raw material powder is burned in a nitrogen-containing atmosphere to produce a lumpy combustion product made of silicon nitride. For example, the mixed raw material powder is housed together with an igniting agent in a container made of graphite, etc., the igniting agent is ignited in the combustion synthesis reaction device, and the nitriding reaction of silicon in the mixed raw material powder is performed by the nitriding combustion heat of the ignition agent When the reaction is started, the reaction is self-propagated through silicon to complete the combustion synthesis reaction, and a bulk combustion product made of silicon nitride is obtained.

  Here, the crush strength of the obtained combustion product is preferably 4 MPa or less. If the crush strength of the combustion product is 4 MPa or less, crushing that will increase the amount of metal impurities mixed in and will reduce the crystallinity of the silicon nitride powder in the <crushing and classifying process of combustion product> described below. It becomes easy to obtain a silicon nitride powder having a specific surface area or a particle size distribution having two peak tops specified in the present invention without performing pulverization with a large amount of energy.

<Crushing and classification process of combustion products>
The lumpy combustion product obtained is then coarsely crushed. The crushing means for coarse crushing is not particularly limited, but as the crushing medium, it is preferable to use a hard non-metallic material having a small content ratio of Al and Fe, and it is more preferable to use a crushing medium made of silicon nitride. . Since the combustion products are lumpy, the crushing by the roll crusher is efficient, and it is preferable that the roll crusher is provided with a roll made of ceramics such as silicon nitride.

  The silicon nitride powder obtained by the above coarse pulverization is sieved to remove particularly coarse particles. The sieve used for sieving is preferably made of a non-metal having a low content ratio of Al and Fe, and is preferably made of a resin.

  Next, the silicon nitride powder obtained by coarse pulverization is finely pulverized. The means for fine pulverization is not particularly limited, but pulverization with a vibration mill is preferable. In the case of pulverizing with a vibration mill, it is preferable that the inner surface of the pot for the vibration mill and the portion of the mixed medium or the like that comes into contact with the raw material powder are made of a non-metallic material having a small content ratio of Al and Fe. The inner surface of the pot is preferably made of resin, and the mixed medium is preferably made of silicon nitride. By adjusting the conditions (amplitude, frequency, crushing time) of the vibration mill appropriately, the specific surface area, the peak top of two peaks in the frequency distribution curve obtained by particle size distribution measurement, and the ratio of their frequencies can be adjusted. You can It is preferable to carry out fine pulverization under the condition that the breaking energy is relatively small, for example, under the condition that the vibration frequency and the pulverization time are short.

As described above, the silicon nitride powder of the present invention is a mixture of a silicon powder and a silicon nitride powder of a diluent, and the resulting mixed raw material powder is filled in a container to prevent self-heating and propagation phenomena associated with combustion reaction. In the method for producing a silicon nitride powder in which the silicon powder is burned by the combustion synthesis method used and the obtained combustion product is crushed, the mixed raw material powder contains Fe content ratio, Al content ratio, and Fe and Al content. It is preferable that the content of metal impurities other than is 100 ppm or less, and that the bulk density is less than 0.5 g / cm ³ be produced by a method for producing a silicon nitride powder. The pressure is preferably 4 MPa or less, and it is particularly preferable to use a grinding media made of silicon nitride for grinding the combustion products.

<Step of mixing powders having different average particle sizes>
The silicon nitride powder of the present invention can be obtained by coarsely pulverizing combustion products and finely pulverizing under conditions where the breaking energy is relatively small. However, it can be obtained by mixing silicon nitride powders having different average particle sizes. You can also For example, a silicon nitride powder obtained by coarsely pulverizing and classifying combustion products, a silicon nitride powder finely pulverized, or a finely pulverized silicon nitride powder classified and classified in particle size is mixed. You can also get it. In this case, the conditions for classification after coarse pulverization, the conditions for fine pulverization and the conditions for classification after fine pulverization, by appropriately adjusting the mixing ratio, the specific surface area, two peaks in the frequency distribution curve obtained by particle size distribution measurement The peak tops of, and their frequency ratio can be adjusted. Further, a silicon nitride powder obtained by coarsely pulverizing and classifying combustion products, a silicon nitride powder having a finely pulverized and classified particle size adjusted, or a silicon nitride powder having a finely pulverized and classified particle size adjusted, known It is also possible to adjust the specific surface area, peak top and their frequency ratio by mixing with the silicon nitride powder of.

  Hereinafter, the present invention will be described in more detail with reference to specific examples.

  Silicon nitride powder of the present invention, silicon powder used as raw material powder, raw material mixed powder and measurement of physical properties of combustion products, and release of polycrystalline silicon ingot when the silicon nitride powder of the present invention is applied as a mold release agent The moldability was evaluated by the following method.

(Method for measuring specific surface area of silicon nitride powder and method for calculating specific surface area equivalent diameter D _BET )
The specific surface area of the silicon nitride powder of the present invention was determined by measuring with a BET one-point method by nitrogen gas adsorption using Macsorb manufactured by Mountech.
Further, the specific surface area equivalent diameter D _BET was calculated from the following formula (1), assuming that all particles constituting the powder are spheres having the same diameter.
D _BET = 6 / (ρ _S × S) (1)

Here, ρ _S depends on the true density of silicon nitride (the true density of α-Si ₃ N ₄ is 3186 kg / m ³ , the true density of β-Si ₃ N ₄ is 3192 kg / m ³ and the ratio of α phase to β phase. The average true density was calculated and used as the true density.) And S is the specific surface area (m ² / g).

(Method for measuring the proportion of β-type silicon nitride in silicon nitride powder)
The proportion of β-type silicon nitride powder in the silicon nitride powder of the present invention was calculated as follows. With respect to the silicon nitride powder of the present invention, a target consisting of a copper tube and a graphite monochromator are used to step-scan an X-ray detector in a diffraction angle (2θ) range of 15 to 80 ° in 0.02 ° steps. X-ray diffraction measurement was performed by the regular step scanning method. When the silicon nitride powder contains components other than silicon nitride, the peaks of those components can be compared with the corresponding peaks of a standard sample of those components to determine the proportions of those components. In all of the following Examples and Comparative Examples, it was confirmed from the obtained powder X-ray diffraction patterns that the silicon nitride powder of the present invention was composed of only α-type silicon nitride and β-type silicon nitride. In addition, the proportion of β-type silicon nitride in the silicon nitride powder of the present invention is G. P. Gazzara and D.M. P. Messier, "Determination of Phase Content of Si3N4 by X-ray Diffraction Analysis", Am. Ceram. Soc. Bull. , 56 [9] 777-80 (1977) and by the method of Gazzara & Messier.

(Measurement method of crystallite diameter D _C and crystal strain of β-type silicon nitride)
The crystallite diameter D _C and crystal strain of β-type silicon nitride of the silicon nitride powder of the present invention were measured as follows. With respect to the silicon nitride powder of the present invention, a target consisting of a copper tube and a graphite monochromator are used to step-scan an X-ray detector in a diffraction angle (2θ) range of 15 to 80 ° in 0.02 ° steps. X-ray diffraction measurement was performed by the regular step scanning method. From the X-ray diffraction pattern of the obtained silicon nitride powder of the present invention, the respective integral widths of the (101), (110), (200), (201) and (210) planes of β-type silicon nitride were calculated, The integration width was substituted into the Williamson-Hall formula of the following formula (1). In the following equation (2), "2 sin θ / λ" is plotted as the x-axis and "β cos θ / λ" is plotted as the y-axis, and the least squares method is used to obtain the intercept and slope of the straight line obtained from the Williamson-Hall equation. It was Then, the crystallite diameter Dc of β-type silicon nitride was calculated from the intercept, and the crystal strain of β-type silicon nitride was calculated from the slope.
β cos θ / λ = η × (2 sin θ / λ) + (1 / Dc) (2)
(Β: integral width (rad), θ: Bragg angle (rad), η: crystal strain, λ: wavelength of X-ray source (nm), Dc: crystallite diameter (nm))

(Particle size distribution of silicon nitride powder and measuring method of peak top)
The particle size distributions of the silicon nitride powder of the present invention and the silicon powder used as a raw material in the present invention were measured as follows. The powder was put into a 0.2 mass% aqueous solution of sodium hexametaphosphate and dispersed for 6 minutes at an output of 300 W using an ultrasonic homogenizer equipped with a stainless steel center cone having a diameter of 26 mm to prepare a dilute solution. , And used as a measurement sample. The particle size distribution of the measurement sample was measured using a laser diffraction / scattering particle size distribution measuring device (Microtrac MT3000 manufactured by Nikkiso Co., Ltd.) to obtain a volume-based particle size distribution curve and its data. From the obtained particle size distribution curve and its data, the particle size at the peak top of the silicon nitride powder of the present invention and the frequency (volume%) were determined.

(Method for measuring the content ratio of Fe, Al and metallic impurities other than Fe and Al in silicon nitride powder, silicon powder and raw material mixed powder)
Content ratios of Fe and Al, and metal impurities other than Fe and Al in the silicon nitride powder of the present invention, the silicon powder used as a raw material in the present invention, and the raw material mixed powder were measured as follows. A container containing a liquid mixture of hydrofluoric acid and nitric acid was charged with the above powder, and the container was tightly sealed, and the container was irradiated with microwaves and heated to completely decompose silicon nitride or silicon, and the obtained decomposition was obtained. The solution was made up to volume with ultrapure water to obtain a test solution. Using ICP-AES (SPS5100 type) manufactured by SII NanoTechnology Inc., Fe, Al and Fe and metal impurities other than Al in the test solution were quantified from the detected wavelength and its emission intensity, and Fe, Al and The content ratio of metal impurities other than Fe and Al was calculated.

(Measurement method of bulk density of mixed raw material powder)
The bulk density of the mixed raw material powder obtained in the present invention was determined by a method in accordance with JIS R1628 "Method for measuring bulk density of fine ceramic powder".

(Method of measuring the crush strength of combustion products)
The crush strength of the combustion product obtained by the present invention was measured as follows. Five cubes each having a side of 10 mm were cut out from the combustion products to obtain measurement samples. The crushing strength of the measurement sample was measured using a manual crushing strength measuring device (Model 1334, manufactured by Aiko Engineering Co., Ltd.). A compression test was carried out by applying a load to the measurement sample placed on the pedestal, and the crush strength was calculated from the measured maximum load. The crush strength of the combustion product obtained in the present invention was an average value of the crush strength of five measurement samples.

(Evaluation method of releasability of polycrystalline silicon ingot)
In the present invention, a unidirectional solidification experiment of a polycrystalline silicon ingot is performed using a mold produced by applying the silicon nitride powder of the present invention as a release agent, and the polycrystalline silicon ingot is released from the mold. Thus, the silicon nitride powder of the present invention was evaluated. When the polycrystalline silicon ingot can be released from the mold and the release layer is not confirmed to be attached to the polycrystalline silicon ingot, ○, the polycrystalline silicon ingot can be released from the mold, but the release layer is attached to the polycrystalline silicon ingot. The case was confirmed as Δ, and the case where the polycrystalline silicon ingot could not be released from the mold, or the polycrystalline silicon ingot was cracked or chipped even if it could be released was taken as ×.

(Measuring method of metal impurities contained in polycrystalline silicon ingot)
Fe, Al and metallic impurities other than Fe and Al contained in the polycrystalline silicon ingot obtained by the directional solidification experiment were measured as follows. The obtained polycrystalline silicon ingot was divided into two parts such that the cut surface was parallel to the solidification direction, and the time of flight type two was set on the central axis of the cut surface as a measurement position 1 cm above the bottom. Surface analysis was performed by a secondary ion mass spectrometry method (manufactured by ULVAC-PHI, Inc. (TRIFT V nano TOF type)). Normalization of secondary mass spectra of Fe, Al, and metal impurities other than Fe and Al was detected when the secondary ion intensity was 1 × 10 ⁻⁴ or more, and undetected when it was less than 1 × 10 ⁻⁴ . Here, the normalized secondary ion intensity is the secondary ion intensity of each spectrum divided by the secondary ion intensity of all detected spectra.

(Example 1-1)
D50 is 4.0 μm, bulk density is 0.40 g / cm ³ , Fe content is 3 ppm, Al content is 4 ppm, and content of metal impurities other than Fe and Al is 3 ppm. As the silicon nitride powder (manufactured by Ube Industries, Ltd., product name “SN-E10” (Fe content ratio: 9 ppm, Al content ratio: 2 ppm, content ratio of metal impurities other than Fe and Al; 4 ppm)), A raw material powder was obtained by adding silicon nitride so that the addition rate was 20 mass% (the mass ratio of silicon: silicon nitride was 80:20). The raw material powder was housed in a nylon pot filled with balls of silicon nitride and having an inner wall surface lined with urethane, and a batch type vibrating mill was used to obtain a frequency of 1200 cpm and an amplitude of 8 mm for 0.5 hours. The mixture was mixed to obtain a mixed raw material powder.

FIG. 1 shows a combustion synthesis reaction apparatus 1 used in the combustion synthesis reaction of silicon in this example. The mixed raw material powder 2 obtained by mixing the raw material powders was housed in a square sheath-shaped graphite container 3 having a bottom surface of 200 × 400 mm, a depth of 30 mm, and a thickness of 10 mm. At this time, the bulk density of the mixed raw material powder was 0.45 g / cm ³ . Titanium powder and carbon powder were mixed and molded at a mass ratio of titanium: carbon of 4: 1 to prepare an igniting agent 4 used in a combustion synthesis reaction, and the igniting agent 4 was placed on the mixed raw material powder 2. . Next, the graphite container 3 accommodating the mixed raw material powder 2 and the igniting agent 4 is placed in a pressure resistant container 6 equipped with a carbon heater 5 for heating the igniting agent, and the carbon heater 5 is positioned directly above the igniting agent 4. So housed.

  After degassing the inside of the pressure-resistant container 6 using the vacuum pump 7, nitrogen gas was introduced from the nitrogen cylinder 8 into the reaction container to set the atmospheric pressure to 0.6 MPa. Next, the carbon heater 5 was energized to heat the igniter 4 to ignite the mixed raw material powder to start the combustion synthesis reaction. During the combustion synthesis reaction, the pressure of the nitrogen atmosphere in the pressure-resistant container 6 was 0.6 MPa, which was almost constant. When the inside of the pressure-resistant container 6 was observed through the viewing window 9, the combustion synthesis reaction was completed after continuing for about 20 minutes. After the reaction was completed, the graphite container 3 was taken out from the pressure resistant container 6 and the lumpy combustion product was collected.

  A portion near the igniting agent was removed from the obtained combustion product, and the remaining portion was coarsely pulverized with a roll crusher having an inner surface coated with urethane and equipped with a roll made of silicon nitride, and then a nylon sieve having an opening of 100 μm was used. It passed through a sieve, and the powder under the sieve was recovered. The obtained powder was placed in an alumina pot filled with silicon nitride balls and whose inner wall surface was lined with urethane, and a batch type vibrating mill was used to obtain a vibration frequency of 1200 cpm, an amplitude of 8 mm, and a volume of 0.25. After fine pulverization for a time, the silicon nitride powder of Example 1-1 was obtained. At the time of crushing with a batch type vibration mill, 1% by mass of ethanol was added to the powder as a crushing aid.

  Table 1 shows the physical property values of the silicon powder and the diluent used as the raw material powder, the physical property values of the mixed raw material powder, and the crush strength of the combustion product in Example 1-1, and the physical property values of the silicon nitride powder. It shows in Table 2.

  The evaluation of the silicon nitride powder of Example 1-1 as a mold releasing agent for the polycrystalline silicon ingot casting mold was performed as follows.

  The silicon nitride powder of Example 1-1 was housed in a polyethylene container capable of being hermetically sealed, and water was added to prepare a mixture ratio of the silicon nitride powder of 20 mass%. A silicon nitride ball was placed in a container containing silicon nitride powder and water, and the container was tightly stoppered and mixed with a batch type vibration mill at an amplitude of 5 mm and a frequency of 1780 cpm for 5 minutes to obtain a silicon nitride slurry.

  The obtained silicon nitride slurry of Example 1-1 was spray-applied to the inner surface of a quartz crucible having a square shape having a porosity of 16%, a bottom surface of 100 mm, and a depth of 100 mm, which was preheated to 90 ° C. It was dried at 90 ° C. for 15 hours. The thickness of the release layer at this time was about 0.2 mm. Further, using an air atmosphere furnace, the mold was held in air at 1100 ° C. for 3 hours for heat treatment to obtain a polycrystalline silicon ingot casting mold in which the silicon nitride powder of Example 1-1 was applied to the release layer.

  The mold was filled with 300 g of silicon granules having a purity of 7 N and a size of 2 to 5 mm and housed in a Bridgman furnace. Under argon flow at atmospheric pressure, the temperature in the furnace was raised to 1500 ° C. over 5 hours to melt the silicon granules. After holding at 1500 ° C. for 24 hours, the mold was pulled down at a pulling rate of 50 mm / h to unidirectionally solidify the molten silicon, and further cooled to room temperature. In addition, another method for producing a polycrystalline silicon ingot of Example 1-1 was prepared, and the holding temperature was changed to 1550 ° C. using the mold, by the same method as in the unidirectional solidification experiment. Unidirectional solidification experiments were conducted.

  The polycrystalline silicon ingot is released from the taken-out mold, and the polycrystalline silicon ingot casting mold and the polycrystalline silicon ingot of Example 1-1 are manufactured by the method described in "Evaluation method of polycrystalline silicon ingot casting mold". Was evaluated. The results are shown in Table 3.

(Examples 1-2 to 1-6)
The time of fine pulverization in Examples 1-2 to 1-6 was 0.30 hours, 1.50 hours, 2.50 hours, 4.00 hours, and 6.00 hours in order from Example 1-2. Except for the above, the silicon nitride powders of Examples 1-2 to 1-6 were obtained in the same manner as in Example 1-1. Then, using the obtained silicon nitride powder of each example as a mold release agent, two polycrystalline silicon ingot casting molds were produced in the same manner as in Example 1-1. In each example, unidirectional solidification experiments at two different furnace temperatures similar to those of Example 1-1 were performed in the same manner as in Example 1-1 using those molds. A polycrystalline silicon ingot casting mold was evaluated in the same manner as in.

(Example 1-7)
The silicon nitride powder of Example 1-4 was passed through a nylon sieve having an opening of 40 μm, and the powder under the sieve was recovered to obtain a silicon nitride powder of Example 1-7. Then, using the obtained silicon nitride powder of Example 1-7 as a mold release agent, two polycrystalline silicon ingot casting molds were produced in the same manner as in Example 1-1. Directional solidification experiments at two furnace temperatures of 1500 ° C. and 1525 ° C. were carried out in the same manner as in Example 1-1 using those molds, and polycrystals were performed in the same manner as in Example 1-1. A silicon ingot casting mold was evaluated.

(Example 1-8)
Ammonium chloride (manufactured by Wako Pure Chemical Industries, purity 99.9%) was added to the raw material powder as an additive at 12.4 mass% (the mass ratio of the mixed powder of silicon and silicon nitride and ammonium chloride was 87.6: 12. The silicon nitride powder of Example 1-8 was prepared in the same manner as in Example 1-3, except that it was further added. Then, using the obtained silicon nitride powder of Example 1-8 as a release agent, two polycrystalline silicon ingot casting molds were produced in the same manner as in Example 1-1. A directional solidification experiment at two different furnace temperatures similar to that of Example 1-1 was performed in the same manner as in Example 1-1 using those molds, and in the same manner as in Example 1-1. A casting mold for polycrystalline silicon ingots was evaluated.

(Example 1-9)
The silicon nitride powder of Example 1-2 was classified by using an air classifier (trade name “Turbo Classifier” manufactured by Nisshin Engineering Co., Ltd.) with a cut point set to 2 μm to classify the silicon nitride powder having a large particle diameter and small particles. A silicon nitride powder was obtained. The silicon nitride powder having the larger particle diameter is collected, and the silicon nitride powder and commercially available silicon nitride powder (manufactured by Ube Industries, Ltd., product name “SN-E10” (Fe content ratio; 9 ppm, Al content ratio) 2 ppm, content ratio of metal impurities other than Fe and Al; 4 ppm)) were mixed at a mass ratio of 1: 1 for 0.17 hours using a V-type mixer having an inner surface coated with urethane. Then, using the obtained silicon nitride powder of Example 1-9 as a mold release agent, two polycrystalline silicon ingot casting molds were produced in the same manner as in Example 1-1. Directional solidification experiments at two different furnace temperatures similar to those of Example 1-1 were performed in the same manner as in Example 1-1 using those molds, and in the same manner as in Example 1-1. A casting mold for polycrystalline silicon ingots was evaluated.

(Example 1-10)
After the coarse crushing, an air flow type crusher equipped with a silicon nitride liner in the kiss part (SJ-1500 type manufactured by Nisshin Engineering Co., Ltd.) was used, and the required air amount was 3.0 m ³ / min, the raw material supply rate. Silicon nitride powder obtained by pulverizing under a condition of about 250 g / min and commercially available silicon nitride powder (manufactured by Ube Industries, Ltd., product name "SN-E10" (Fe content ratio; 9 ppm, Al content ratio; 2 ppm, the content ratio of metallic impurities other than Fe and Al; 4 ppm)) in a mass ratio of 2: 1 for 0.17 hours using a V-type mixer having an inner surface coated with urethane. 1-10 silicon nitride powder was obtained. Then, using the obtained silicon nitride powder of Example 1-10 as a mold release agent, two polycrystalline silicon ingot casting molds were produced in the same manner as in Example 1-1. A directional solidification experiment at two different furnace temperatures similar to that of Example 1-1 was performed in the same manner as in Example 1-1 using those molds, and in the same manner as in Example 1-1. A casting mold for polycrystalline silicon ingots was evaluated.

(Example 1-11)
After the coarse crushing, an air flow type crusher equipped with a silicon nitride liner in the kiss part (SJ-1500 type manufactured by Nisshin Engineering Co., Ltd.) was used, and the required air amount was 3.0 m ³ / min, the raw material supply rate. Silicon nitride powder obtained by pulverizing under a condition of about 250 g / min and commercially available silicon nitride powder (manufactured by Ube Industries, Ltd., product name "SN-E10" (Fe content ratio; 9 ppm, Al content ratio; 2 ppm, the content ratio of metal impurities other than Fe and Al; 4 ppm)) were mixed in a mass ratio of 1: 1 for 0.17 hours using a V-type mixer whose inner surface was coated with urethane. 1-11 silicon nitride powder was obtained. Then, using the obtained silicon nitride powder of Example 1-11 as a mold release agent, two polycrystalline silicon ingot casting molds were produced in the same manner as in Example 1-1. Directional solidification experiments at two different furnace temperatures similar to those in Example 1-7 were performed in the same manner as in Example 1-1 using these molds, and in the same manner as in Example 1-1. A casting mold for polycrystalline silicon ingots was evaluated.

(Comparative Examples 1-1 and 1-2)
Comparative Examples 1-1 and 1-in the same manner as in Example 1-1 except that the fine grinding time was 0.16 hours in Comparative Example 1-1 and 12.00 hours in Comparative Example 1-2. 2 silicon nitride powder was obtained. As seen in Table 2, the silicon nitride powder of Comparative Example 1-1 is a powder having a small specific surface area of 1.8 m ² / g, and the silicon nitride powder of Comparative Example 1-2 has a specific surface area of 14.8 m ^2. The powder was as large as / g. Then, using the obtained silicon nitride powder of each comparative example as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. In each comparative example, the mold was used to evaluate the polycrystalline silicon ingot casting mold and the silicon ingot in the same manner as in Example 1-1.

(Comparative Example 1-3)
Ammonium chloride (manufactured by Wako Pure Chemical Industries, purity 99.9%) as an additive is added to the raw material powder at 16.7 mass% (mass ratio of mixed powder of silicon and silicon nitride to ammonium chloride is 83.3: 16.7) Comparative Example 1-3 was obtained in the same manner as in Example 1-3, except that the silicon nitride powder was further added. As shown in Table 2, the silicon nitride powder of Comparative Example 1-3 was a powder having a small proportion of β-type silicon nitride of 46%. Then, using the obtained silicon nitride powder of Comparative Example 1-3 as a release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. Using the mold, a polycrystalline silicon ingot casting mold and a silicon ingot were evaluated in the same manner as in Example 1-1.

(Comparative Examples 1-4, Comparative Example 1-5)
Silicon powder having an inner diameter of 30 mm and a D50 of 2.5 μm, a bulk density of 0.26 g / cm ³ , an Fe content of 2 ppm, an Al content of 3 ppm, and a metal impurity content of 3 ppm other than Fe and Al. The mold was filled and uniaxially molded at a pressure of 1500 kg / cm ² , to obtain a uniaxially molded body of silicon powder. The molded body was filled in a graphite container, housed in a batch-type nitriding furnace, the atmosphere in the furnace was replaced with a nitrogen atmosphere, and then the temperature was raised to 1450 ° C. in a nitrogen atmosphere and kept for 3 hours. After cooling to room temperature, the nitriding product was taken out. The obtained nitriding product was coarsely crushed with a roll crusher having a urethane nitride inner surface and equipped with a roll made of silicon nitride, and then sieved through a nylon sieve having an opening of 100 μm to collect powder under the sieve. . Next, the powder was placed in an alumina pot filled with silicon nitride balls and having an inner surface lined with urethane, and finely pulverized by a batch type vibration mill under the conditions of a frequency of 1200 cpm and an amplitude of 8 mm. The fine pulverization time was set to 0.33 hours in Comparative Example 1-4 and 2.00 hours in Comparative Example 1-5 to obtain silicon nitride powder of each Comparative Example. Silicon nitride powder of Comparative Example 1-4 and Comparative Example 1-5 by the direct nitriding method not combustion synthesis method, as seen in Table 2, respectively, the crystallite diameter D _c is as small as 50nm and 45 nm, the crystal strain The powder was as large as 2.55 × 10 ⁻⁴ and 2.20 × 10 ^−4, and had a large D _BET / D _c of 11.0 and 4.6. Then, using the obtained silicon nitride powder of each comparative example as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. In each comparative example, the mold was used to evaluate the polycrystalline silicon ingot casting mold and the silicon ingot in the same manner as in Example 1-1.

(Comparative Example 1-6)
The silicon nitride powders of Examples 1-4 were classified by using an air classifier (trade name "Turbo Classifier" manufactured by Nisshin Engineering Co., Ltd.) with a cut point set to 1 μm, and a silicon nitride powder having a large particle size and a small particle size. A silicon nitride powder was obtained. The silicon nitride powder with the larger particle diameter was recovered to obtain a silicon nitride powder of Comparative Example 1-6. As seen in Table 2, the silicon nitride powder of Comparative Example 1-6 was a powder having a large peak top with a smaller particle size of 3.5 μm. Then, using the obtained silicon nitride powder of Comparative Example 1-6 as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. Using the mold, a polycrystalline silicon ingot casting mold and a silicon ingot were evaluated in the same manner as in Example 1-1.

(Comparative Example 1-7)
A silicon nitride powder was obtained in the same manner as in Example 1-1, except that the pulverizing time was 0.2 hours. The obtained silicon nitride powder was classified by using an air classifier (trade name “Turbo Classifier” manufactured by Nisshin Engineering Co., Ltd.) with a cut point set to 2 μm, and a silicon nitride powder having a large particle diameter and a small silicon nitride powder were classified. And got. The silicon nitride powder having the larger particle diameter is collected, and the silicon nitride powder and commercially available silicon nitride powder (manufactured by Ube Industries, Ltd., product name “SN-E10” (Fe content ratio; 9 ppm, Al content ratio) 2 ppm, content ratio of metal impurities other than Fe and Al; 4 ppm)) in a mass ratio of 1: 1 using a V-type mixer having an inner surface coated with urethane for 0.17 hours. Thus, a silicon nitride powder of Comparative Example 1-7 was obtained. As seen in Table 2, the silicon nitride powder of Comparative Example 1-7 was a powder having a large peak top of 32.3 μm with a smaller particle size. Then, using the obtained silicon nitride powder of Comparative Example 1-8 as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. Using the mold, a polycrystalline silicon ingot casting mold and a silicon ingot were evaluated in the same manner as in Example 1-1.

(Comparative Example 1-8)
The silicon nitride powder of Example 1-3 was classified using an air classifier (trade name “Turbo Classifier” manufactured by Nisshin Engineering Co., Ltd.) with a cut point set to 12 μm, and a silicon nitride powder having a large particle size and a small particle size were used. A silicon nitride powder was obtained. The silicon nitride powder having the smaller particle diameter was recovered to obtain a silicon nitride powder of Comparative Example 1-8. As shown in Table 2, the silicon nitride powders of Comparative Examples 1-8 are powders having a large ratio of the peak top frequency of the smaller particle size to the peak top frequency of the larger particle size of 1.54. Met. Then, using the obtained silicon nitride powder of Comparative Example 1-8 as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. Using the mold, a polycrystalline silicon ingot casting mold and a silicon ingot were evaluated in the same manner as in Example 1-1.

(Comparative Example 1-9)
The silicon nitride powder of Comparative Example 1-8 was classified by using an air classifier (trade name “Turbo Classifier” manufactured by Nisshin Engineering Co., Ltd.) with a cut point set to 2 μm to classify the silicon nitride powder having a large particle diameter and a small particle size. A silicon nitride powder was obtained. The silicon nitride powder having the smaller particle diameter was recovered to obtain a silicon nitride powder of Comparative Example 1-9. The silicon nitride powder of Comparative Example 1-9 had a single particle size peak top. Then, using the obtained silicon nitride powder of Comparative Example 1-9 as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. Using the mold, a polycrystalline silicon ingot casting mold and a silicon ingot were evaluated in the same manner as in Example 1-1.

(Comparative Examples 1-10 to 1-12)
Silicon nitride powders of Comparative Examples 1-10 to 1-12 were obtained in the same manner as in Example 1-4, except that the powder shown in Table 1 was used as the raw material silicon powder. In the silicon nitride powders of Comparative Examples 1-10 to 1-12, the content of Fe, the content of Al, and the content of metal impurities other than Fe and Al are 159 ppm, 140 ppm, and 134 ppm, respectively. It was a lot of powder. Then, using the obtained silicon nitride powders of Comparative Examples 1-10 to 1-12 as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. In each of the examples, the template was used to evaluate the polycrystalline silicon ingot casting mold and the silicon ingot in the same manner as in Example 1-1.

(Comparative Example 1-13)
Example 1 except that a roll crusher equipped with an alumina roll was used for coarse pulverization of combustion products, and an alumina pot filled with alumina balls was used for fine pulverization by a batch type vibration mill. A silicon nitride powder of Comparative Example 1-13 was produced in the same manner as in -4. The silicon nitride powder of Comparative Example 1-13 was a powder having a large content of Fe, a content of Al, and a content of metal impurities other than Fe and Al of 240 ppm, 3700 ppm, and 129 ppm. Then, using the obtained silicon nitride powder of Comparative Example 1-13 as a mold release agent, one polycrystalline silicon ingot casting mold was produced in the same manner as in Example 1-1. Using the mold, a polycrystalline silicon ingot casting mold and a silicon ingot were evaluated in the same manner as in Example 1-1.

  In Examples 1-2 to 1-11 and Comparative Examples 1-1 to 1-13, the physical properties of the silicon powder used as the raw powder and the additive, the physical properties of the mixed raw powder, and the combustion products The crushing strength is shown in Table 1, and the physical properties of the silicon nitride powder are shown in Table 2. Table 3 shows the evaluation results of the polycrystalline silicon ingot casting molds and the polycrystalline silicon ingots of Examples 1-2 to 1-11 and Comparative examples 1-1 to 1-13.

(Example 2-1)
By the method described below, a polycrystalline silicon ingot casting mold having a release layer containing the silicon nitride powder of Example 1-1 was prepared, and the polycrystalline silicon ingot casting mold and the silicon ingot were evaluated. .

  The silicon nitride powder of Example 1-1 was housed in a sealable polyethylene container, and silica sol having a silica concentration of 20% by mass (Fuso Chemical Co., Ltd., product name "PL-3") and water were added. At this time, silicon nitride: silica sol: water were mixed in a mass ratio of 20: 8: 72. Next, a silicon nitride ball was placed in a container containing silicon nitride powder, silica sol, and water and sealed, and a batch type vibration mill was used to mix for 5 minutes at an amplitude of 5 mm and a frequency of 1780 cpm to obtain a silicon nitride slurry. Got

  The obtained silicon nitride slurry of Example 2-1 was spray-coated on the inner surface of a quartz crucible having a square shape having a porosity of 16%, a bottom surface of 100 mm, and a depth of 100 mm, which had been heated to 90 ° C. in advance. After drying at 90 ° C. for 15 hours, a mold for casting a polycrystalline silicon ingot having a release layer containing the silicon nitride powder of Example 2-1 was obtained. The thickness of the release layer at this time was about 0.2 mm.

  Using the obtained polycrystalline silicon ingot casting mold of Example 2-1, a directional solidification experiment was conducted in the same manner as in Example 1-1, and Example 2-1 was carried out in the same manner as in Example 1-1. The polycrystalline silicon ingot casting mold and the silicon ingot were evaluated. The results are shown in Table 4.

(Examples 2-2 to 2-11, Comparative examples 2-1 to 2-13)
A silicon nitride slurry was produced in the same manner as in Example 2-1 except that the silicon nitride powder shown in Table 4 was used, to produce a polycrystalline silicon ingot casting mold. Using the obtained polycrystalline silicon ingot casting molds of Examples and Comparative Examples, a unidirectional solidification experiment was conducted in the same manner as in Example 1-1, and the polycrystalline method was performed in the same manner as in Example 1-1. The silicon ingot casting mold and the silicon ingot were evaluated. The results are shown in Table 4.

  As described above, the silicon nitride powder of the present invention is that by applying a heat treatment at a high temperature after coating the mold, it is possible to form a mold release layer having good adhesion and releasability substantially by itself. It was also found that a mold release layer having good adhesion and mold releasability can be formed on a mold without heat treatment at high temperature by mixing with silica sol and applying it to the mold.

  INDUSTRIAL APPLICABILITY The silicon nitride powder of the present invention is useful as a mold release agent capable of forming a mold release layer having good adhesion to a mold and good mold releasability, and particularly high quality silicon substrates for solar cells It is useful as a release agent for polycrystalline silicon ingots that can be collected at a yield. Further, the silicon nitride powder of the present invention is capable of forming a dense release layer and has high crystallinity, and therefore is also useful as a raw material for a silicon nitride sintered body that exhibits high strength at high temperatures.

1 Combustion Synthesis Reaction Device 2 Mixed Raw Material Powder 3 Graphite Container 4 Ignition Agent 5 Carbon Heater 6 Pressure Resistant Container 7 Vacuum Pump 8 Nitrogen Cylinder 9 Peep Window

Claims (8)

The specific surface area measured by the BET method is 2 m ² / g or more and 13 m ² / g or less,
the proportion of β-type silicon nitride is 50% by mass or more,
The crystallite size of β-type silicon nitride which is calculated using the Williamson-Hall type from powder X-ray diffraction pattern of β-type silicon nitride is taken as _{D C,} and a _{D C} is 150nm or more,
When the specific surface area equivalent diameter calculated from the specific surface area is D _BET , D _BET / D _C (nm / nm) is 3 or less,
The frequency distribution curve obtained by the volume-based particle size distribution measurement by the laser diffraction scattering method has two peaks,
The peak top of the peak is in the range of 0.5 to 2 μm and the range of 6 to 30 μm,
Nitriding, characterized in that the ratio of the frequency of peak tops (frequency of peak tops in the range of particle diameter 0.5 to 2 μm / frequency of peak top in the range of particle diameter 6 to 30 μm) is 0.1 to 1. Silicon powder. The crystal strain of β-type silicon nitride calculated from the powder X-ray diffraction pattern of β-type silicon nitride using the Williamson-Hall equation is 1.5 × 10 ⁻⁴ or less, and the nitriding according to claim 1. Silicon powder.   The silicon nitride powder according to claim 1 or 2, wherein the peak tops are in the range of 0.5 to 2 µm and in the range of 9 to 30 µm.   The ratio of β-type silicon nitride is 70% by mass or more, and the silicon nitride powder according to claim 1. The silicon nitride powder according to any one of claims 1 to 4, characterized in that the specific surface area is less than ^2m 2 / g or more ^{10 m} 2 / g. Fe content ratio is 100 ppm or less,
The content ratio of Al is 100 ppm or less,
The silicon nitride powder according to any one of claims 1 to 5, wherein the total content of metal impurities other than Fe and Al is 100 ppm or less.   A mold release agent for a polycrystalline silicon ingot, which comprises the silicon nitride powder according to claim 1.   A method for producing a silicon ingot for solidifying molten silicon housed in a mold, wherein the silicon nitride powder according to any one of claims 1 to 6 is applied to the contact surface with the molten silicon as the mold. A method for manufacturing a silicon ingot, which comprises using a mold.

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