JP2017039614A - Silicon nitride powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon nitride powder suitable for a mold release agent, suppressing detachment of a mold release agent from a crucible, capable of preventing the mold release agent from contaminating in a polycrystalline silicon ingot as an impurity.SOLUTION: A silicon nitride powder has the total number of an imino group (-NH) and an amino group (-NH) of 1 to 10 pieces/nmper unit surface of the silicon nitride. A mold release agent uses the silicon nitride powder.SELECTED DRAWING: None

Description

  The present invention relates to silicon nitride powder.

  Solar cells are expected to be put to practical use in a wide range of fields from small households to large-scale power generation systems as a clean alternative energy source for oil. These are classified into crystalline solar cells, amorphous solar cells, compound solar cells and the like according to the type of raw materials used, and most of those currently on the market are crystalline silicon solar cells. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. The single crystal silicon solar cell has the advantage that it is easy to increase the conversion efficiency because the quality of the single crystal silicon substrate is good, but has the disadvantage that the manufacturing cost of the single crystal silicon substrate is high. .

  On the other hand, polycrystalline silicon solar cells have been distributed in the market for a long time, but in recent years, as interest in environmental issues has increased, it has become the current mainstay in terms of total power generation. Therefore, there is a demand for higher conversion efficiency at lower cost. In order to cope with such a demand, it is necessary to reduce the cost and quality of the polycrystalline silicon substrate, and it is required to manufacture a high-purity silicon ingot with a high yield.

  A polycrystalline silicon substrate used for a polycrystalline silicon solar cell is generally manufactured by a method called a casting method. This casting method is a method in which raw material silicon is put into a quartz crucible or a graphite crucible and heated and melted at about 1500 ° C. in an inert atmosphere to form a polycrystalline silicon ingot. And the edge part of this silicon ingot is removed, it cut | disconnects by cutting to a desired magnitude | size, The sliced silicon ingot is sliced to desired thickness, and the polycrystal silicon substrate for forming a solar cell is obtained.

  The power generation efficiency of a solar cell using this polycrystalline silicon substrate depends on impurities present inside the polycrystalline silicon substrate. It is known that metal impurities such as iron and aluminum, particularly iron, is an element that reduces power generation efficiency as a lifetime killer.

  A method of forming a polycrystalline silicon ingot by melting raw material silicon by heating at a high temperature easily takes in impurities from peripheral members such as a crucible due to the high activity of molten silicon. For this reason, in the method of forming a polycrystalline silicon ingot, measures for preventing contamination with impurities are taken.

  When melting raw material silicon to form a polycrystalline silicon ingot, quartz crucibles and graphite crucibles are used as melting crucibles, but the polycrystalline silicon ingot and crucible stick together, causing damage to the polycrystalline silicon ingot. For the purpose of preventing the above, and for the purpose of preventing impurities contained in the crucible itself from diffusing into the molten silicon, a release agent is applied to the inner wall of the crucible in advance (Patent Document 1). ).

  The mold release agent is preferably a powder containing silicon as a main component and a chemically stable component at a high temperature from the viewpoint of preventing impurities from being mixed. A representative material that satisfies this condition is silicon nitride.

  The release agent is mixed with a solvent and applied to the crucible. In a slurry obtained by mixing a release agent with a solvent, the release agent is likely to aggregate, and a gap is likely to occur in the release agent layer applied to the crucible. Therefore, a release agent slurry with high dispersibility is required.

  In order to prevent the release agent from aggregating in the slurry solution, for example, a slurry may be prepared using about 0.01 to 2% by mass of a polycarboxylic acid type anionic surfactant dispersant ( Patent Document 2). However, since the metal compound and the carbon compound generated from the dispersant may be incorporated as impurities into the polycrystalline silicon ingot, there is a concern about deterioration of the characteristics of the solar cell.

  In the crucible, since the molten silicon comes into contact with the silicon nitride powder as a release agent, a high-purity silicon nitride powder with a small impurity content, for example, about 20 ppm of iron impurities, is used for the purpose of preventing impurity diffusion into silicon. It has been proposed to improve the characteristics of solar cells by using as a release agent (Patent Document 3).

  D50 and D90 are particle sizes that serve as an index of the particle size distribution (volume distribution) of the powder. The smaller and larger sides of D50 are the same, and the cumulative distribution of D90 is the smaller particle size. Is 90%, and the cumulative distribution on the larger side is 10%.

There has been proposed a method of producing high-purity silicon nitride fine powder by nitriding using high-purity silicon processing scraps of D50 of 1.1 μm or less and D90 of 4 μm or less (Patent Document 4). Although the silicon nitride powder in Patent Document 4 defines D50, D90, and D100, the release agent is applied to the polycrystalline silicon ingot for solar cells as an impurity by applying the release agent to the inner surface of the quartz crucible. There is no description of what can be prevented from mixing. In addition, such a manufacturing method has a problem that a release agent using silicon nitride adheres to the silicon ingot because imino groups (—NH) and amino groups (—NH ₂ ) do not exist on the surface of the silicon nitride particles. It was.

Special table 2009-510387 JP 2002-239682 A JP 2007-261832 A JP 2011-051856 A

The present invention has been invented to cope with such a problem, and can release the release agent from the crucible and prevent the release agent from being mixed into the polycrystalline silicon ingot as an impurity. A silicon nitride powder suitable for the above is provided.

The inventors of the present invention have made extensive studies to achieve the above object, and have found a silicon nitride powder that achieves this. The present invention is based on such knowledge and has the following gist.
(1) A silicon nitride powder, wherein the total number of imino groups (—NH) and amino groups (—NH ₂ ) per unit surface area of the silicon nitride powder is 1 to 10 / nm ² .
(2) A mold release agent using the silicon nitride powder according to (1).

  When the silicon nitride powder of the present invention is used as a release agent, peeling of the release layer from the crucible can be suppressed, so that the release agent is not mixed in the silicon ingot, and a polycrystalline silicon ingot with few impurities is obtained.

An example of the manufacturing apparatus of silicon nitride powder is shown. Sectional drawing of the quartz crucible which has a mold release layer which used the silicon nitride powder of this invention as a mold release agent.

Hereinafter, the present invention will be described in detail.
In the silicon nitride powder of the present invention, the total number of imino groups (—NH) and amino groups (—NH ₂ ) per unit surface area is 1 to 10 / nm ² .
The imino group (—NH) and amino group (—NH ₂ ) on the surface of the silicon nitride particles are obtained by using a release agent layer, a silicon ingot layer, and a release agent layer when silicon nitride powder is used as a release agent. And affects the adhesion of the quartz crucible layer.
When the total number of imino groups (—NH) and amino groups (—NH ₂ ) per unit surface area is less than 1 / nm ² , the releasability from the silicon ingot is deteriorated, and when the total number is more than 10 / nm ^2. The release agent layer is easily peeled off from the quartz crucible.

Examples of the method for producing the silicon nitride powder include an imide pyrolysis method and a direct nitridation method. Although there is no restriction | limiting in particular in the manufacturing method of a silicon nitride powder, Since imino group and an amino group cannot be provided to the silicon nitride surface in a manufacturing process in an imide thermal decomposition method, surface modification is needed after manufacture. On the other hand, in the direct nitridation method, imino vessels and amino groups can be imparted to the surface of silicon nitride particles within the production process by producing in a non-oxidizing atmosphere containing ammonia. It is preferable to do.

As the metal silicon used as the raw material for the direct nitriding method, it is necessary to use a high-purity silicon raw material with a small amount of iron and aluminum mixed therein. The amount of iron / aluminum mixed is 100 ppm or less, and preferably 10 ppm or less. For example, silicon scrap obtained during processing and polishing from silicon for semiconductors can be used.

There is no particular limitation on the reaction furnace used for the direct nitriding method. For example, a batch furnace, a vertical continuous reactor, a fluidized bed reactor, etc. are used, but in the present invention, productivity is high and a pulverization step after the nitriding reaction is required. It is preferable to use a vertical continuous reactor that does not.

In the case of using a vertical continuous reaction furnace, for example, an apparatus as shown in FIG. 1 comprising a raw material supply unit, a reaction unit, and a collection unit can be used. The raw material supply unit is composed of a raw material tank 1 and a raw material supply machine 2, and after introducing metal silicon as a raw material into the raw material quantity tank 1, a furnace of a vertical reactor together with nitrogen gas as a carrier gas from the raw material supply machine 2 Supplied to the top. There is no restriction | limiting in particular in the raw material supply machine used here, A screw feeder, a table feeder, a circle feeder, etc. are used.

  A raw material supply pipe 3 for spraying metallic silicon and nitrogen gas into the furnace and an ammonia gas supply pipe 4 for supplying ammonia gas as a reaction gas are installed at the top of the vertical continuous reaction furnace.

  Metallic silicon and nitrogen gas supplied to the top of the furnace are sprayed by a raw material supply pipe 3 installed at the top of the furnace onto a vertical continuous reactor heated to 1200 to 1800 ° C. at a gas flow rate of 25 to 140 m / s. This spray gas flow velocity is calculated from the flow rate of nitrogen gas as the carrier gas and the inner diameter of the raw material supply pipe 3. If the spray gas flow rate is less than 5 m / s (Normal), adhesion is likely to occur in the furnace, and if it exceeds 30 m / s (Normal), sufficient reaction time cannot be obtained and non-nitridation occurs. Further, when the temperature in the furnace is lower than 1200 ° C., the nitriding reaction does not proceed sufficiently, and when it exceeds 1800 ° C., the generated silicon nitride powder is decomposed. Here, Normal represents a standard state (0 ° C., 101.325 kPa).

Ammonia gas, which is a reaction gas, is supplied into the furnace at a rate of 1.5 to 3.0 m ³ (Normal) with respect to 1 kg of metal silicon from an ammonia gas supply pipe 4 installed at the top of the furnace. If the ratio of ammonia gas to 1 kg of metal silicon is less than 1.5 m ³ (Normal), the nitridation reaction of metal silicon will be difficult to proceed, and if it exceeds 3.0 m ³ (Normal), the reaction field in the furnace will be decomposed by decomposition of ammonia gas. It is cooled and the nitriding reaction is difficult to proceed.

  The reaction part has a heating element 6 made of isotropic graphite at the outer periphery of the center of the reaction tube 5 made of, for example, boron nitride, and a branch pipe 7 made of, for example, isotropic graphite is installed on the upper and lower outer periphery. Yes. The periphery is covered with a heat insulating material 8 made of, for example, porous carbon beads or alumina fibers. The heating element 6 is heated by the high-frequency coil 9 installed on the outer periphery of the furnace body to control the temperature of the reactor reaction field.

  In the collection unit, the silicon nitride powder conveyed from the furnace is separated and collected from nitrogen gas or ammonia gas. A bug filter, a cyclone or the like is used as the collecting device 10.

As ammonia gas which is a reaction gas, liquefied ammonia having a purity of 99.8% by mass or more can be used, and as nitrogen gas, nitrogen cylinder gas or liquefied nitrogen having a purity of 99.9% by volume or more can be used.

An imino group and an amino group are present on the surface of the silicon nitride particles produced by the above method. The number of imino groups and amino groups on the surface of silicon nitride particles varies depending on the ratio of ammonia gas and nitrogen gas. In the present invention, by controlling the volume ratio of ammonia gas and nitrogen gas (ammonia gas: nitrogen gas, normal) supplied into the furnace to 1: 5 to 1:15, imino groups per unit surface area of the silicon nitride powder. And the total number of amino groups can be controlled to 1 to 10 / nm ² .

The particle size distribution of the silicon nitride powder suitable for the present invention is preferably such that D50 is 0.8 to 2.7 μm, D90 is 2.0 to 14.0 μm, and D100 is 8 to 68 μm. When the particle size distribution is in this range, the dispersibility of the aqueous slurry is good, the coating property is good, and the release agent layer is not peeled off from the quartz crucible.

The aqueous slurry using the silicon nitride powder of the present invention is produced by mixing and stirring the silicon nitride powder and water. The water is preferably pure water or ion-exchanged water, and the conductivity is preferably 1.0 (μS / cm) or less.
The slurry concentration is preferably 10 to 50% by mass, and more preferably 30 to 50% by mass. When the slurry concentration is less than 10% by mass, the release agent is easily peeled off from the quartz crucible, and when it exceeds 50% by mass, the dispersibility of the slurry is lowered and the applicability is deteriorated.

  The aqueous slurry using the silicon nitride powder of the present invention needs to have a pH of 7 or more, more preferably 9 or more. When the pH is lower than 7, the silicon nitride powder in the slurry is agglomerated and it is difficult to uniformly apply it to the quartz crucible. In the above slurry concentration range, the pH may be lower than 7 on the low concentration side. In such a case, the pH can be adjusted by adding a basic substance such as aqueous ammonia to the slurry.

  The aqueous slurry using the silicon nitride powder of the present invention is applied to the inner wall of the crucible by application using a spatula or a brush, or spraying. In consideration of uniform application to the inner wall of the crucible and workability of application work, application by spraying is preferable.

Hereinafter, the present invention will be described in detail with reference to examples and comparative examples.
(Examples 1-3)
A silicon nitride powder was produced using metal silicon having a D50 of 1.0 μm as a starting material and using the apparatus shown in FIG. The raw material charged in the raw material tank 1 is 1 kg / h by the screw feeder of the raw material supply machine 2 and is transported to the feed pipe 3 installed at the top of the furnace together with nitrogen gas, and the vertical temperature is maintained at 1500 ° C. The continuous reactor was sprayed at a flow rate of 50 to 140 m / s. Simultaneously, it was supplied into the furnace at a rate of 2.0 m ³ / h (Normal) and a flow rate of 30 m / s from an ammonia gas supply pipe installed at the top of the furnace. At this time, the ratio of ammonia gas to nitrogen gas was controlled between 1: 5 and 1:15. These conditions are as shown in Table 1. The manufactured silicon nitride powder was mixed and stirred in pure water to prepare a slurry having a concentration of 50% by mass.

The total number of imino groups and amino groups per unit area of the silicon nitride powder was determined by the following analysis. 0.24 g of benzoic acid (Wako Pure Chemical Industries, Ltd., reagent grade, molecular weight 122.12 g / mol) was added to the prepared slurry, and stirring was performed with a stirrer at 300 rpm for 1 minute. After completion of stirring, the silicon nitride powder slurry is allowed to stand, and 2 mL of the supernatant liquid is extracted after 1, 2, 3, 5, 8 hours from the end of stirring, and an ultraviolet-visible spectrophotometer (trade name “V” manufactured by JASCO Corporation) -630 UV-visible spectrophotometer ") was used to measure the concentration of benzoic acid in the supernatant solution using 251 nm UV light.
By comparing the transmittance at this time with a calibration curve prepared in advance, the amount of benzoic acid adsorbed on the silicon nitride powder was calculated, and the imino group adsorbed on the silicon nitride powder and the amino acid were calculated from the amount of benzoic acid. The total number of units per unit surface area was calculated. The results are shown in Table 1.

  The produced slurry was applied to the inner wall of the quartz crucible by spraying, and baked in an air atmosphere at 1000 ° C. for 3 hours to produce a release layer as shown in FIG. Solar grade polycrystalline silicon was charged and held at 1500 ° C. for 10 hours under an argon atmosphere. The formed silicon ingot was taken out and the state of the release layer was observed.

The peel rate was measured as follows. The same quartz crucible as that used in the production of the polycrystalline silicon ingot was cut, and a silicon nitride slurry was sprayed onto the quartz plate and dried to produce a release layer. A commercially available cellophane tape was adhered to the release layer, and the peeled rate of the silicon nitride powder was measured by measuring the weight of the silicon nitride powder adhered to the tape. It has been found that when the peeling rate exceeds 10%, the yield of the polycrystalline silicon ingot and the performance as a solar power generation device are lowered.

(Comparative Examples 1-3)
It implemented like Example 1-5 except the ratio of ammonia gas and nitrogen gas being outside the range of 1: 5 to 1:15.

(Comparative Example 4)
It carried out similarly to Examples 1-3 and Comparative Examples 1-3 except having used the commercially available high purity silicon nitride powder (grade name E10) by Ube Industries, Ltd. for silicon nitride powder.

By comparing the example and the comparative example, if the total number of imino groups and amino groups per unit surface area is less than 1 / nm ² , the releasability of the release agent and the silicon ingot deteriorates and 10 / nm. ^{When the} number is more than ^2, it can be seen that the peeling rate from the quartz crucible increases.

The silicon nitride powder of the present invention controls the amount of imino group (-NH) and amino group (-NH2), which are functional groups on the surface of silicon nitride particles, and will be used as a filler and surface modifier in the future. It can be used widely.

DESCRIPTION OF SYMBOLS 1 Raw material tank 2 Raw material supply machine 3 Raw material supply pipe 4 Ammonia gas supply pipe 5 Reaction pipe 6 Heat generating body 7 Branch pipe 8 Thermal insulation material 9 High frequency coil 10 Collection apparatus 11 Nitrogen gas 12 Ammonia gas 13 Exhaust gas 14 Silicon nitride powder 15 Silicon casting Mold (quartz crucible)
16 Silicon nitride release agent layer

Claims (2)

A silicon nitride powder, wherein the total number of imino groups (—NH) and amino groups (—NH ₂ ) per unit surface area of the silicon nitride powder is 1 to 10 / nm ² . A mold release agent using the silicon nitride powder according to claim 1.