Silicon nitride powder and method for producing silicon nitride sintered body

Abstract

Provided are: a silicon nitride powder having an isoelectric point at a pH of 7-9; and a method for producing a silicon nitride sintered body wherein a sintering raw material containing the silicon nitride powder is molded and sintered. In the silicon nitride powder, the alpha-phase percentage is preferably 90-97%, the pH at which positive zeta potential becomes the maximum is preferably 2-4, and the zeta potential at a pH of 1.5 is preferably 30-45 mV. The present invention makes it possible to provide a silicon nitride powder which enables production of a sintered body having excellent insulating properties and heat dissipation properties. The present invention also makes it possible to provide a silicon nitride sintered body having excellent insulating properties and heat dissipation properties.

Description

Method for producing silicon nitride powder and silicon nitride sintered body

[0001] The present invention relates to a silicon nitride powder and a method for producing a silicon nitride sintered body. This application claims priority to Japanese Patent Application No. 2024-057571, filed on March 29, 2024, the contents of which are incorporated herein by reference.

[0002] Silicon nitride sintered bodies are materials with excellent strength, hardness, toughness, heat resistance, corrosion resistance, thermal shock resistance, etc., and are therefore used in various industrial parts such as die-casting machines and melting furnaces, as well as insulating substrates for automotive parts, etc. As the silicon nitride powder that is the raw material for silicon nitride sintered bodies, silicon nitride powder with a high degree of alpha conversion is used in order to obtain high-quality sintered bodies.

[0003] Silicon nitride substrates are also being considered for use as insulating substrates for power modules in automobiles, machine tools, etc. For example, Patent Document 1 proposes using silicon nitride substrates in aluminum-ceramic bonding substrates. For such applications, high insulation and heat dissipation properties are required.

[0004] JP 2011-077546 A

[0005] An object of the present invention is to provide a novel silicon nitride powder that can be used to produce a sintered body having excellent insulating and heat dissipating properties, and a method for producing a silicon nitride sintered body using the same.

[0006] The present invention has the following aspects. [1] Silicon nitride powder having an isoelectric point at pH 7 to 9. [2] The silicon nitride powder according to [1], having a gelatinization rate of 90 to 97%. [3] The silicon nitride powder according to [1] or [2], having a pH of 2 to 4 at which the positive zeta potential is maximized. [4] The silicon nitride powder according to [3], having a zeta potential of 30 to 45 mV at pH 1.5. [5] A method for producing a silicon nitride sintered body, comprising molding and firing a sintering raw material containing the silicon nitride powder according to any one of [1] to [4] above. [6] A silicon nitride sintered body produced by the method according to [5] above. [7] The silicon nitride sintered body according to [6], having a breakdown voltage of 5 kV or more, preferably 9 kV or more, measured by the method described in the Examples below. [8] The silicon nitride sintered body according to [6] or [7], having a thermal conductivity of 50 W / (m K) or more, preferably 70 W / (m K) or more, measured by the method described in the Examples below.

[0007] According to the present invention, it is possible to provide a silicon nitride powder that can be used to produce a sintered body having excellent electrical insulation and heat dissipation properties. Also, according to the present invention, it is possible to provide a silicon nitride sintered body having excellent electrical insulation and heat dissipation properties.

[0008] The meanings and definitions of terms used in this specification are as follows. A numerical range expressed as "to" means a numerical range in which the numerical values ​​before and after "to" are the lower and upper limits. In this specification, the upper and lower limits can be combined arbitrarily. The "isoelectric point" is the pH at which the zeta potential becomes zero. The "zeta potential" is a value measured by the electroacoustic method (ESA (electrokinetic sonic amplitude) method). The electroacoustic method applies an AC voltage to a dispersion of silicon nitride powder, and measures the zeta potential from the ESA signal of ultrasonic waves generated by particle vibration.

[0009] The "average particle size" is the 50% cumulative volume particle size (hereinafter referred to as "D ５０"The particle size distribution can be obtained in accordance with JIS R 1629:1997 "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction / scattering method." The "specific surface area" is a value measured by the BET single-point method using nitrogen gas in accordance with JIS R 1626:1996 "Method for measuring specific surface area of ​​fine ceramic powder by gas adsorption BET method."

[0010] The "alpha phase ratio" refers to the phase ratio of alpha silicon nitride to the total silicon nitride, and is a value calculated by the following formula 1: alpha phase ratio (%) = [I α（１０２） +I α（２１０） ] / [I α（１０２） +I α（２１０） +I β（１０１） +I β（２１０） ] × 100 ... Formula 1 In the above Formula 1, I α（１０２） is the intensity of the diffraction peak of the (102) plane of the α phase of silicon nitride, I α（２１０） is the intensity of the diffraction peak of the (210) plane of the α phase of silicon nitride, I β（１０１） is the intensity of the diffraction peak of the (102) plane of the β phase of silicon nitride, I β（２１０） is the intensity of the diffraction peak of the (210) plane of the β phase of silicon nitride. Each diffraction peak can be obtained from an XRD pattern obtained by powder X-ray diffraction measurement.

[0011] The iron content of the silicon nitride powder is a value measured using an X-ray fluorescence analyzer (Rigaku Corporation, device name: Primas II). The chlorine content and fluorine content of the silicon nitride powder are based on values ​​measured by heating the silicon nitride powder, dissolving the generated gas in water, and measuring the amounts of Cl ions and F ions in the water using an ion chromatograph, and are sometimes referred to as the "chlorine (Cl) content" and the "fluorine (F) content," respectively.

[0012] The oxygen content of the silicon nitride powder is the total amount of oxygen obtained by heating the silicon nitride powder to 2000° C. in a helium atmosphere and detecting the amount of oxygen in the released gas.

[0013] <Silicon Nitride Powder> The silicon nitride powder of this embodiment (hereinafter also referred to as "this powder") has an isoelectric point at pH 7 to 9. That is, the pH at which the zeta potential of this powder becomes zero is at least within the range of pH 7 to 9. Furthermore, the powder may have an isoelectric point outside the range of pH 7 to 9. For example, the powder may have an isoelectric point at pH 1 or less. The powder preferably has an isoelectric point within the range of pH 7.2 to 8.8, and more preferably within the range of pH 7.4 to 8.6. The isoelectric point of the silicon nitride powder can be controlled by the method and conditions for producing the silicon nitride powder.

[0014] The zeta potential of the present powder is preferably maximized at a pH of 2 to 4. That is, in a graph in which the zeta potential of the present powder is plotted on the vertical axis and pH on the horizontal axis, the maximum value preferably exists within a pH range of 2 to 4. The zeta potential of the present powder is more preferably maximized at a pH of 2 to 4 and has a zeta potential of 30 to 45 mV at pH 1.5.

[0015] The specific surface area of ​​this powder is 5 to 15 m ２ / g is preferred, and 6 to 14 m ２ / g is more preferable, and 7 to 13 m ２ The specific surface area of ​​the silicon nitride powder can be controlled, for example, by adjusting the grinding conditions during the production of the silicon nitride powder.

[0016] D of this powder ５０ is preferably 1 μm or less, more preferably 0.9 μm or less, and even more preferably 0.8 μm or less. ５０ The lower limit of the D of the powder is not particularly limited, but may be, for example, 0.4 μm or more, 0.5 μm or more, or 0.6 μm or more. The upper and lower limits can be appropriately combined. For example, ５０ The D of the powder may be 0.4 μm or more and 0.9 μm or less, 0.5 μm or more and 0.9 μm or less, or 0.6 μm or more and 0.8 μm or less. ５０ When the D of the silicon nitride powder is equal to or less than the upper limit, it becomes easier to produce a silicon nitride sintered body. ５０can be controlled, for example, by adjusting the pulverization method, pulverization conditions, classification conditions, etc. during the production of silicon nitride powder.

[0017] The content of iron element (Fe content) relative to the total mass of the present powder is preferably 300 ppm by mass or less, more preferably 200 ppm by mass or less, and even more preferably 150 ppm by mass or less. The lower limit of the content of iron element relative to the total mass of the present powder is not particularly limited, but may be, for example, 10 ppm by mass or more, 30 ppm by mass or more, 50 ppm by mass or more, or 100 ppm by mass or more. The upper and lower limits can be appropriately combined. For example, the Fe content may be 10 ppm by mass or more and 300 ppm by mass or less, 30 ppm by mass or more and 200 ppm by mass or less, 50 ppm by mass or more and 150 ppm by mass or less, or 100 ppm by mass or more and 150 ppm by mass or less. The Fe content may also be 120 to 130 ppm by mass. When the iron content relative to the total mass of the present powder is equal to or less than the above upper limit, the thermal conductivity and mechanical properties of the silicon nitride sintered body produced using the present powder are likely to be improved. The iron content in the silicon nitride powder is, for example, an impurity introduced by wear of the components of the pulverizer used in the silicon nitride powder production process. The iron content in the silicon nitride powder can be controlled by the treatment method and conditions used in the treatment to remove impurities (e.g., acid treatment process, washing process, etc.).

[0018] The gelatinization rate of the present powder is preferably 88% or more, more preferably 89% or more, and even more preferably 90% or more. The upper limit of the gelatinization rate of the present powder is not particularly limited, but may be, for example, 97% or less, 95% or less, or 93% or less. The upper and lower limits can be appropriately combined. For example, the gelatinization rate of the present powder may be 88% to 97%, 89% to 95%, or 90% to 93%. When the gelatinization rate of the present powder is equal to or greater than the lower limit, grain growth during sintering can be promoted when a sintered body is produced using the present powder. As a result, a sufficiently densified silicon nitride sintered body can be produced. Therefore, a silicon nitride sintered body with even superior high-temperature strength and high thermal conductivity can be obtained. For example, when silicon nitride is synthesized by a direct nitriding method, the gelatinization rate of the silicon nitride powder is essentially determined by the nitriding reaction conditions of the silicon powder.

[0019] The oxygen content of the present powder is preferably 0.5 to 1.5 mass%, more preferably 0.6 to 1.4 mass%, and even more preferably 0.7 to 1.3 mass%. It is preferable for the oxygen content of the present powder to be within the above range, as this enhances the effects of the present invention. The oxygen content of the silicon nitride powder is derived from oxides present in the primary particles of silicon nitride. The oxygen content of the silicon nitride powder can be controlled by adjusting the conditions of the step in which oxidation occurs during the production process of the silicon nitride powder. For example, it can be controlled by adjusting the component composition of the firing atmosphere in the firing step, the concentration of the acidic substance used in the acid treatment step, etc.

[0020] The present powder may contain elements other than silicon, nitrogen, and oxygen. Examples of the other elements include calcium and halogen elements derived from the fluorite, and iron and metal elements other than silicon derived from the metal powder, the metal compound powder, and the pulverizing apparatus. The total content of the other elements relative to the total mass of the present powder is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and even more preferably 0.5 mass% or less.

[0021] <<Method for Producing Silicon Nitride Powder>> The method for producing the present powder is not particularly limited. The present powder is preferably a silicon nitride powder produced, for example, by a process of pulverizing silicon nitride obtained by a direct nitriding method. The pulverization of silicon nitride is broadly divided into dry pulverization and wet pulverization, with wet pulverization being easier to produce fine powder. The present powder is preferably a silicon nitride powder produced by a process of nitriding metal silicon and a pulverization process of pulverizing the silicon nitride obtained in the nitriding process. The pulverization process preferably includes a coarse pulverization step to obtain a coarse powder, followed by a fine pulverization step to pulverize the silicon nitride by wet pulverization. Hereinafter, "coarse powder" refers to a powder having an average particle size of more than 1 μm.

[0022] Although wet milling produces silicon nitride powder with a small particle size, it can wear down the components of the wet mill. This can result in the inclusion of impurities (e.g., iron) derived from the components of the wet mill. Furthermore, wet milling is performed in water, whereby a portion of the silicon nitride reacts with water to produce silica. These impurities degrade the quality of the silicon nitride sintered body produced from the silicon nitride powder. Therefore, it is preferable to perform an acid treatment step in which the silicon nitride powder obtained by wet milling is immersed in an acidic solution to dissolve the impurities, followed by a washing step in which the dissolved impurities are removed by solid-liquid separation. The powder is preferably silicon nitride powder produced through the acid treatment and washing steps. In the washing step after the acid treatment, the solution containing the silicon nitride powder, acidic substances, and impurities is fed into a solid-liquid separator to obtain a silicon nitride powder-containing composition with reduced acidic substances and impurities. The resulting silicon nitride powder-containing composition is dried to obtain the acid-treated silicon nitride powder. The powder is preferably a silicon nitride powder produced through a coagulation and filtration process in which a filtration device is used as the solid-liquid separator, a coagulant is added to the liquid to be treated, and the liquid is then filtered.

[0023] (Metallic silicon nitriding process) In the metallic silicon nitriding process, raw material powder containing metallic silicon powder is fired in a nitrogen-containing atmosphere to nitride it. As the metallic silicon powder, pulverized metallic silicon lumps or particles can be used. Examples of pulverizing devices include a hammer mill, a pin mill, a ball mill, a vibration mill, and a jet mill.

[0024] The purity of the metal silicon powder is preferably 98% by mass or more, more preferably 99% by mass or more. The metal silicon powder may contain impurities derived from metal silicon agglomerates or particles and impurities derived from the grinding device.

[0025] The metallic silicon powder may be used as the raw material powder as it is, or the raw material powder may be prepared by blending metallic silicon powder, fluorite, and other metal powders or metal compound powders (e.g., chromium compounds, nickel compounds). The content of metallic silicon powder per 100 parts by mass of the raw material powder is preferably 92 parts by mass or more, more preferably 95 parts by mass or more, and even more preferably 97 parts by mass or more.

[0026] The raw material powder is fluorite (CaF ２ ), the content of fluorite per 100 parts by mass of metal silicon powder is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.8 parts by mass or more. If the content of fluorite is above the lower limit, nitridation of the metal silicon is likely to proceed. The content of fluorite per 100 parts by mass of metal silicon powder is preferably 3.0 parts by mass or less, more preferably 2.0 parts by mass or less, and even more preferably 1.5 parts by mass or less. If the content of fluorite is below the upper limit, the contents of calcium element and fluorine element in the resulting silicon nitride powder can be reduced.

[0027] In the nitriding process for metal silicon, the raw material powder may be fired as is, or the raw material powder may be formed into a compact and then fired. Nitriding is performed by firing the raw material powder or a compact of the raw material powder (hereinafter also referred to as "raw material powder, etc.") in a firing furnace under a nitrogen-containing atmosphere. The firing furnace may be a continuous furnace or a batch furnace. Examples of batch furnaces include electric furnaces and rotary kilns. Examples of continuous furnaces include container-transporting tunnel-type pusher furnaces and roller hearth kilns. When firing the raw material powder as is, a continuous furnace is preferred. A continuous furnace allows the container containing the raw material powder to be continuously heated within the furnace. Since the raw material powder can be fired without being molded, the nitriding reaction proceeds efficiently. This allows silicon nitride to be obtained in a short period of time.

[0028] (Coarse pulverization step) In the coarse pulverization step, the molded or powdered silicon nitride obtained in the metal silicon nitriding step is pulverized. The pulverization may be carried out using, for example, a coarse pulverizer, a ball mill, a vibration mill, etc. The coarse pulverization is preferably carried out in a dry state.

[0029] (Fine Pulverization Step) In the fine pulverization step, the coarse powder is supplied to a media agitation mill and wet-pulverized. Wet pulverization means that the coarse powder is mixed with a solvent, i.e., pulverized in a slurried state. Water, for example, can be used as the solvent.

[0030] The media in a media agitation mill may be either metallic or ceramic. From the viewpoint of sufficiently pulverizing the coarse powder, metallic media are preferred. As the metal, iron and steel containing iron as the main component are preferred. Only one type of media may be used, or two or more types may be used in combination.

[0031] The coarse powder and the solvent may be directly fed to the media agitation mill, or a slurry may be prepared in advance and then fed to the media agitation mill.

[0032] D of silicon nitride powder obtained by the fine grinding process ５０ The D of the silicon nitride powder obtained in the fine grinding process remains almost unchanged even after the acid treatment and washing processes. ５０The preferred size of D of the powder is ５０ is the same as the preferred size of

[0033] (Acid Treatment Step) An acid treatment is carried out by adding an acidic solution to the silicon nitride powder obtained through wet milling. By adding the acidic solution to the silicon nitride powder, impurities such as metals and silica contained in the silicon nitride powder can be dissolved and removed.

[0034] Examples of acidic substances contained in the acidic solution include inorganic acids such as hydrogen fluoride, hydrogen chloride, nitric acid, and sulfuric acid. Hydrochloric acid (aqueous hydrogen chloride solution) is preferably used to dissolve metals contained in the silicon nitride powder after wet pulverization. Hydrofluoric acid (aqueous hydrogen fluoride solution) is preferably used to dissolve silica contained in the silicon nitride powder after wet pulverization. That is, the acid treatment step is preferably carried out using a mixed solution of hydrofluoric acid and hydrochloric acid. The acid treatment step may be carried out by stirring an acid treatment slurry containing silicon nitride powder and an acidic solution, or by allowing the mixture to stand without stirring.

[0035] The acid-treated slurry contains the silicon nitride powder after the acid treatment, acidic substances, and impurities. After the acid treatment, the acid-treated slurry is used as the liquid to be treated, and the acidic substances and impurities are removed in the subsequent washing step.

[0036] The total concentration of acidic substances such as hydrofluoric acid and hydrogen chloride relative to the total mass of the acid-treated slurry is preferably 7.0 to 10.0 mass%, more preferably 7.5 to 9.5 mass%, and even more preferably 8.0 to 9.0 mass%. When the concentration of the acidic substances is equal to or greater than the lower limit, dissolution of impurities such as iron and silica contained in the silicon nitride powder is promoted. When the concentration of the acidic substances is equal to or less than the upper limit, the acidic substances are easily removed by washing.

[0037] The solids concentration of the acid-treated slurry used as the liquid to be treated is preferably 10 to 45% by mass, more preferably 15 to 40% by mass, and even more preferably 20 to 35% by mass. The pH of the acid-treated slurry (i.e., the liquid to be treated) after completion of the acid treatment is preferably 0 to 2.0, more preferably 0 to 1.5, and even more preferably 0 to 1.0.

[0038] (Washing process) In the washing process, the liquid to be treated is fed to a solid-liquid separator, concentrated, and then the washing liquid is added after concentration, followed by another concentration. This washing process is carried out continuously. This results in a silicon nitride powder-containing composition in which the acidic substances and impurities in the liquid to be treated have been reduced. It is preferable to use a continuous solid-liquid separator. Examples of continuous solid-liquid separators include a cross-flow filtration device and a continuous centrifuge.

[0039] Examples of cross-flow filtration devices include filter-type cross-flow filtration devices equipped with a filtration membrane as a filter medium and rotary cross-flow filtration devices equipped with a filter cloth as a filter medium. The materials of the components that come into contact with the liquid to be treated, such as the filter medium, may be any material that is acid-resistant. Examples include ceramics and acid-resistant resins such as polyvinylidene fluoride (PVDF) and polypropylene (PP).

[0040] In the cross-flow method, the liquid to be treated flows parallel to the filtering surface of the filter medium, and pressure is applied from the inside to the outside of the filtering surface. This prevents solid matter from accumulating inside the filtering surface, and the liquid passes through the filtering medium, concentrating the liquid to be treated. The pore size of the filtration membrane is determined by the D of the silicon nitride powder in the liquid to be treated. ５０ The membrane pore size is preferably 0.01 to 1.00 times, more preferably 0.02 to 0.50 times, and even more preferably 0.03 to 0.30 times, relative to the filter cloth. When the membrane pore size is equal to or greater than the lower limit, a sufficient filtration flux is easily obtained, while when it is equal to or less than the upper limit, the ultrafine silicon nitride powder is less likely to pass through the filtration membrane, making it easier to reduce the amount of solids in the waste liquid. The transmembrane pressure difference of the filtration membrane is, for example, preferably 0.10 to 0.50 MPa, more preferably 0.15 to 0.40 MPa, and even more preferably 0.20 to 0.30 MPa. The air permeability of the filter cloth is 0.10 to 0.50 cc / cm ２ ・s is preferred, 0.15 to 0.45 cc / cm ２ s is more preferably 0.20 to 0.40 cc / cm ２ When the membrane pore size is equal to or larger than the above lower limit, a sufficient filtration flux is easily obtained, whereas when the membrane pore size is equal to or smaller than the above upper limit, the ultrafine powder of silicon nitride is less likely to pass through the filtration membrane, making it easier to reduce the amount of solids in the waste liquid.

[0041] A continuous centrifuge is equipped with a mechanism for continuously supplying the liquid to be treated to a rotating separator, applying centrifugal force to separate the solid and liquid, and discharging the concentrated liquid and separated liquid from their respective outlets. Commercially available continuous centrifuges can be used. They may be horizontal or vertical. The material of the continuous centrifuge that comes into contact with the liquid to be treated may be any material as long as it is acid-resistant. For example, an acid-resistant alloy may be used, with Hastelloy being preferred. The centrifugal force is preferably set so that the solids concentration of the precipitate formed by centrifugation is 65 to 80% by mass, preferably 68 to 75% by mass. Water is added at the outlet for discharging this precipitate, and the concentrated liquid is discharged from the separator in a state with a solids concentration of 20 to 45% by mass, preferably 30 to 40% by mass. When the solids concentration of the concentrated liquid is equal to or less than the upper limit, the viscosity of the concentrated liquid is reduced, improving its transportability to the next process. The centrifugal force (centrifugal acceleration) of the separator is, for example, preferably 1000 to 3000 G, and more preferably 2000 to 2500 G.

[0042] The flocculant used in the flocculation / filtration step is preferably an inorganic flocculant, and the inorganic flocculant is preferably at least one selected from the group consisting of ammonium ammonium sulfate, ammonium acetate, aluminum sulfate, iron sulfate, and the like.

[0043] The preferred method of adding the coagulant is to add an aqueous solution of the coagulant to the liquid to be treated. If the concentration of the coagulant relative to the mass of the aqueous coagulant solution (unit: mass %, hereinafter also referred to as "addition concentration") is too high, the coagulant will remain in the silicon nitride powder after drying, and if it is too low, sufficient coagulation effect will not be obtained. Therefore, it is preferable to set the concentration within a range that does not cause these problems. For example, 0.005 to 0.15 mass % is preferred, 0.01 to 0.14 mass % is more preferred, and 0.05 to 0.13 mass % is even more preferred.

[0044] The pH of the liquid to be treated in the coagulation and filtration process preferably includes at least a portion of the pH range in which the dispersibility of the silicon nitride powder is increased. When the dispersibility of the silicon nitride powder is increased in the liquid to be treated, clogging occurs in the filter medium with a small pore size, reducing the filtration flux, while when the pore size of the filter medium is large, the silicon nitride powder passes through the filter medium, increasing the solid content of the waste liquid. By adding a coagulant to the liquid to be treated in a pH range in which the dispersibility is increased, and promoting coagulation, it is possible to obtain an increase in the filtration flux, a reduction in the solid content of the waste liquid, or both of these effects. The pH range in which the dispersibility of the silicon nitride powder is increased varies depending on the method for producing the silicon nitride powder.

[0045] In the washing step of this embodiment, it is preferable to carry out a first step using water as the washing liquid, followed by a second step (coagulation / filtration step) using an aqueous coagulant solution as the washing liquid, followed by a third step using an alkaline solution as the washing liquid for washing and neutralization. Examples of alkaline solutions include aqueous ammonia, aqueous sodium hydroxide, and aqueous calcium hydroxide. Among these, aqueous ammonia is preferred because it leaves little residue on the silicon nitride powder after drying. The pH of the alkaline solution is preferably 9 to 12, more preferably 10 to 11.

[0046] The pH of the liquid to be treated at the start of the first step may be the same as the pH of the slurry at the end of the acid treatment step. The pH of the liquid to be treated at the end of the first step is the same as the pH of the liquid to be treated at the start of the second step, and is preferably in the same range as the pH of the liquid to be treated at the start of the above-mentioned coagulation and filtration step. The pH of the liquid to be treated at the end of the second step is the same as the pH of the liquid to be treated at the start of the third step, and is preferably in the same range as the pH of the liquid to be treated at the end of the above-mentioned coagulation and filtration step. The pH of the liquid to be treated at the end of the third step (end point) is preferably 7 to 10, more preferably 8 to 9.

[0047] After the second step, instead of the third step using an alkaline solution, it is possible to supply only water and wash until the pH reaches the end point. However, providing the third step is preferable in terms of shortening the treatment time and reducing the amount of waste liquid.

[0048] The solids concentration of the silicon nitride powder-containing composition after the washing step is preferably 15 to 40 mass %, more preferably 20 to 35 mass %. A concentration equal to or greater than the lower limit is preferred in terms of reducing the heat energy required for drying, while a concentration equal to or less than the upper limit is preferred in terms of ease of transport to the next step.

[0049] <Drying Step> The silicon nitride powder-containing composition obtained in the washing step can be dried to obtain silicon nitride powder. The drying temperature is preferably 100 to 140°C, more preferably 110 to 130°C. The drying time is preferably 40 to 180 minutes, more preferably 60 to 120 minutes.

[0050] <Applications> The silicon nitride powder of this embodiment has a small average particle size and therefore excellent sinterability. Therefore, the silicon nitride powder can be used as a sintering material for silicon nitride sintered bodies. As shown in the examples described below, sintered bodies with excellent electrical insulation and heat dissipation properties can be produced.

[0051] <Method for manufacturing silicon nitride sintered body> One embodiment of the method for manufacturing a silicon nitride sintered body includes a step of molding a sintering raw material containing the present powder to produce a molded body, and a step of firing. In addition to the silicon nitride powder (the present powder), the sintering raw material may also contain an oxide-based sintering aid. Examples of oxide-based sintering aids include yttrium oxide, magnesia, and alumina. The content of the oxide-based sintering aid in the sintering raw material is, for example, 3 to 10 mass%.

[0052] In the step of producing the compact, for example, a compact is obtained by pressing at a molding pressure of 3.0 to 30 MPa. The compact may be produced by uniaxial pressing or by CIP (Cold Isostatic Pressing) molding. Furthermore, the compact may be fired while being molded by hot pressing. The compact may be fired in an inert gas atmosphere such as nitrogen or argon. The pressure during firing may be 0.7 to 1 MPa. The firing temperature is preferably 1860 to 2100°C, more preferably 1880 to 2000°C. The firing time at the firing temperature is preferably 6 to 20 hours, more preferably 8 to 16 hours. The rate of temperature rise to the firing temperature is preferably, for example, 1.0 to 10.0°C / min.

[0053] Silicon nitride sintered bodies produced using this powder have excellent insulating properties and heat dissipation. For example, silicon nitride sintered bodies having a breakdown voltage of 5 kV or more, preferably 9 kV or more, as measured by the method described in the Examples below, can be obtained. For example, silicon nitride sintered bodies having a thermal conductivity of 50 W / (m K) or more, preferably 70 W / (m K) or more, as measured by the method described in the Examples below, can be obtained.

[0054] The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

[0055] (Zeta potential, isoelectric point) The zeta potential was measured by electroacoustic method using a high-concentration particle size / zeta potential measuring device (Colloidal Dynamics product name "Acousto Sizer IIx"), and the isoelectric point was determined. Specifically, a dispersion containing silicon nitride powder was prepared by the following method, and this was set in the measuring device as a sample, and the zeta potential was measured while changing the pH. [Sample preparation] Water was added to the powder to be measured to prepare a 5 mass% slurry, which was then dispersed using a commercially available dispersing device and then measured. [Measurement conditions] · Measurement environment: room temperature (23.0°C), humidity 34% · Measurement mode: polar mode · Analysis mode: lognormal · Measurement method: potentiometric titration (pH = 1, 1.5, 2, 2.5, 3, 4, 5, 6) · Stirring speed: 110 rpm · Circulation speed: 140 rpm · Titrant: 1 M hydrochloric acid · Other: The sample was stirred before shaking before measurement.

[0056] (D ５０ ) D ５０ The D of the silicon nitride powder after the washing step was determined by measuring the particle size distribution by a laser diffraction / scattering method. The measurement was carried out in accordance with the method described in JIS R 1629:1997 "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction / scattering method." ５０was measured by the above-mentioned method. In the measurement, 60 mg of the target powder was weighed into a 500 mL container. 2 mL of a 20 mass % aqueous solution of sodium hexametaphosphate and 200 g of water were added as a dispersant to prepare a dispersion. This container was set in an ultrasonic disperser (manufactured by Sharp Corporation) so that the entire portion containing the dispersion was immersed, and ultrasonic dispersion was performed for 1 minute. The above-mentioned particle size distribution measurement was performed using the sample after ultrasonic dispersion. The D of the silicon nitride powder contained in the liquid to be treated was ５０ is the silicon nitride powder D after the above-mentioned washing process after drying and crushing the silicon nitride slurry (silicon nitride slurry after wet pulverization) before adding the acidic solution. ５０ was measured by the same method.

[0057] (Specific Surface Area) The BET specific surface area of ​​the silicon nitride powder was measured by the single-point BET method using nitrogen gas in accordance with JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method." The silicon nitride slurry after wet pulverization was thoroughly dried in a tray dryer, then crushed in a mortar and measured by the above method. The silicon nitride powder-containing composition after acid treatment and washing was dried in a spray dryer and then measured by the above method.

[0058] (Contents of chlorine, fluorine, oxygen, and iron) The chlorine content and fluorine content of the silicon nitride powder were measured by the method described above. Specifically, the silicon nitride powder was heated using an automatic sample combustion apparatus (manufactured by Mitsubishi Chemical Corporation, apparatus name: AQF-2100H type), and the generated gas was dissolved in water. The Cl ions and F ions in the water were measured using an ion chromatograph (manufactured by Thermo Fisher Scientific, apparatus name: ICS-2100). Based on the obtained measured values, the chlorine (Cl) content and fluorine (F) content of the silicon nitride powder were calculated. The oxygen content of the silicon nitride powder was determined as the total oxygen amount using the method described above. Specifically, using an oxygen / nitrogen analyzer (manufactured by Horiba, Ltd., model EMGA-920), silicon nitride powder was heated in a helium atmosphere from 20°C to 2000°C at a heating rate of 8°C / sec, the amount of oxygen released was quantified, and the oxygen content (mass%) of the entire silicon nitride powder was determined. The iron content of the silicon nitride powder was measured using an X-ray fluorescence analyzer (manufactured by Rigaku Corporation, model Primas II). The iron content in the treated liquid was measured by drying and crushing the silicon nitride slurry (silicon nitride slurry after wet pulverization) before the addition of the acidic solution, using the same method as for the iron content of the silicon nitride powder described above.

[0059] (α-phase ratio) The α-phase ratio was measured by the method described above. XRD patterns were obtained by powder X-ray diffraction measurement. Specifically, an XRD pattern was obtained by using a powder X-ray diffractometer (manufactured by Rigaku Corporation, device name: Ultima IV), filling a dedicated substrate with the target powder, and performing measurement using a Cu-Kα radiation source under conditions of a diffraction angle 2θ = 10 to 80°, a sampling width of 0.02°, and a scan speed of 10° / min. Next, using integrated powder X-ray analysis software "Rietveld method software (manufactured by MDI, integrated powder X-ray software Jade+9)," the α-phase ratio was calculated according to Equation 1 from the diffraction peak intensity of the (102) plane of the α-phase of silicon nitride, the diffraction peak intensity of the (210) plane of the α-phase of silicon nitride, the diffraction peak intensity of the (102) plane of the β-phase of silicon nitride, and the diffraction peak intensity of the (210) plane of the β-phase of silicon nitride in the XRD pattern.

[0060] (Solid Content) The solid content concentration (unit: mass %) of the liquid to be treated was calculated using the formula: (input weight of silicon nitride powder subjected to wet pulverization) ÷ (weight of silicon nitride slurry obtained by wet pulverization + weight of mixed solution of hydrofluoric acid and hydrochloric acid added to the silicon nitride slurry) × 100.

[0061] The amount of solids in the wastewater (unit: mg / L) was measured using a method conforming to the method for measuring the concentration of suspended solids specified in JIS K 0102: 2019 "Testing Methods for Industrial Wastewater." Specifically, the wastewater to be measured was filtered through filter paper with a pore size of 1 μm and a diameter of 25 to 50 mm, and the solids amount (unit: mg / L) was calculated by dividing the dry weight of the material remaining on the filter paper by the volume of the filtered liquid.

[0062] <Production Example 1: Preparation of treated liquid> The treated liquid used in each example was prepared by producing silicon nitride by a direct nitriding method, wet-pulverizing it to obtain a slurry containing silicon nitride powder (silicon nitride slurry), and then treating it with acid. Specifically, a molded body (bulk density: 1.4 g / cm) was formed using metallic silicon powder. ３ The obtained compact was placed in an electric furnace and fired at 1400°C for 60 hours to obtain a fired body containing silicon nitride. The firing atmosphere was a mixed gas of nitrogen and hydrogen (N ２ and H ２ A gas mixture (a mixture of silicon nitride and argon in a volume ratio of 80:20 under standard conditions) was supplied to the furnace. The obtained sintered body was crushed using a dry jaw crusher and then crushed in a dry ball mill to obtain a coarse powder containing silicon nitride. The obtained coarse powder was supplied to a media-agitation mill and subjected to wet crushing using water as a solvent to obtain a fine powder, thereby obtaining a silicon nitride slurry containing silicon nitride powder. The media material of the media-agitation mill was an iron-containing alloy. The obtained silicon nitride slurry contained at least iron as an impurity in addition to the silicon nitride powder.

[0063] The silicon nitride slurry obtained by wet grinding was placed in an acid treatment tank, a mixture of hydrofluoric acid and hydrochloric acid was added, and the mixture was stirred at room temperature for 6 hours to perform acid treatment, obtaining a solution to be treated. The solution to be treated contains the silicon nitride powder after acid treatment, as well as the acidic substance used in the acid treatment and at least iron as an impurity. The solids concentration, hydrochloric acid content, hydrofluoric acid content, iron content (Fe content), and D of the silicon nitride powder in the solution to be treated used in each example were measured. ５０ are shown in Table 1. The hydrochloric acid content and hydrofluoric acid content of the liquid to be treated were calculated from the content of acidic substances in the acid solution used in the acid treatment step.

[0064] In this example, a cross-flow filtration device using a ceramic membrane as a filter medium was used to separate the liquid to be treated into solid and liquid, and a cleaning process was also carried out. The ceramic membrane was a tubular membrane made of alumina and having a pore size of 0.1 μm. Specifically, a 1 m ３The liquid to be treated was stored in the storage tank, and the liquid in the storage tank was continuously supplied to the filter medium. The permeated liquid that permeated the filter medium was removed as waste liquid, and the concentrated liquid that did not permeate the filter medium was returned to the storage tank, circulating the liquid. Simultaneously with the start of supplying the liquid to the filter medium, water was supplied to the storage tank so that the liquid volume in the storage tank remained constant. That is, water was added to the concentrated liquid returned to the storage tank, and the operation of concentrating the liquid again using the filter medium was continuously performed. In this manner, washing with water was performed until the pH of the liquid supplied to the filter medium reached 1 (Step 1). The average filtration flux in Step 1 is shown in Table 1 (the same applies hereinafter). Next, instead of water, an aqueous ammonium sulfate solution with a concentration of 0.05% by mass was supplied to the storage tank as a flocculant solution so that the liquid volume in the storage tank remained constant. In this manner, washing was performed while adding the flocculant until the pH of the liquid supplied to the filter medium reached 3 (Step 2). The average filtration flux in Step 2 is shown in Table 1 (the same applies hereinafter). Next, instead of the aqueous coagulant solution, an aqueous ammonia solution with a concentration of 0.08% by mass was supplied to the storage tank so that the amount of liquid in the storage tank remained constant. In this way, neutralization and washing using the aqueous ammonia solution were carried out until the pH of the liquid supplied to the filter medium reached 9 (Step 3). The average filtration flux in Step 3 is shown in Table 1 (the same applies below). When the pH of the liquid supplied to the filter medium reached 9, the supply of liquid to the storage tank was stopped, and a silicon nitride powder-containing composition after washing was obtained. The solids concentration of the silicon nitride powder-containing composition was the same as that of the liquid to be treated. The silicon nitride powder-containing composition obtained in the washing step was dried at 300 ° C. to obtain silicon nitride powder after the washing step.

[0065] The total amount of waste liquid, the amount of solids in the waste liquid, the overall average of the filtration flux, and the treatment capacity in all steps are shown in Table 1 (the same applies below). The amount of solids in the waste liquid was measured by the method described above. The total amount of waste liquid was the sum of the total amount of water supplied in the first step, the total amount of the aqueous coagulant solution supplied in the second step, and the total amount of the aqueous ammonia solution supplied in the third step. The treatment capacity was calculated as follows: ３ The total treatment time is the sum of the treatment times for the first, second, and third steps. The overall average filtration flux was calculated by dividing the total amount of waste liquid by the filter media area and the total treatment time.

[0066] The zeta potential of the obtained silicon nitride powder was measured by the above method, and the isoelectric point, the zeta potential at pH 1.5 (referred to in the table as "zeta potential (pH 1.5)"), the pH at which the positive zeta potential was maximized (referred to in the table as "pH at maximum potential"), and the maximum value of the positive zeta potential (referred to in the table as "maximum positive potential") are shown in Table 2. The D of the obtained silicon nitride powder ５０ The specific surface area, iron content (Fe content), oxygen content, and alpha conversion rate are shown in Table 2 (the same applies below). ５０ is the D of silicon nitride powder in the liquid to be treated ５０ It was confirmed that iron, an impurity in the treated liquid, was highly removed.

[0067] In this example, a cross-flow filtration device with a resin membrane filter was used to separate the liquid to be treated and to carry out a cleaning process. The resin membrane was made of polyvinylidene fluoride (PVDF), had a pore size of 0.1 μm, an inner diameter of 1.4 mm, and a filter area (effective filtration area) of 7.9 m. ２ A hollow fiber membrane of this type was used. Steps 1, 2, and 3 were carried out in the same manner as in Example 1 to obtain a silicon nitride powder-containing composition after washing. The obtained silicon nitride powder-containing composition was dried in the same manner as in Example 1 to obtain silicon nitride powder after the washing step.

[0068] Example 3 In this example, a rotary cross-flow filtration device was used to separate the liquid to be treated into solid and liquid, and a cleaning step was also carried out. ３ / cm ２ A polypropylene filter cloth (double woven) of s was used. Steps 1, 2, and 3 were carried out in the same manner as in Example 1 to obtain a silicon nitride powder-containing composition after washing. The obtained silicon nitride powder-containing composition was dried in the same manner as in Example 1 to obtain a silicon nitride powder after the washing step.

[0069] Comparative Example 1 A commercially available silicon nitride powder was used as the measurement subject, and the items listed in Table 2 were measured. The results are shown in Table 2.

[0070] <Evaluation of Silicon Nitride Sintered Body> [Production of Silicon Nitride Sintered Body] 91.4 parts by mass of the silicon nitride powder produced in each example was mixed with Y as a sintering aid. ２ O ３ 6.0 parts by mass of and 1.5 parts by mass of MgO were added, and an organic solvent, an organic binder, a plasticizer, etc. were added, and the mixture was uniformly mixed in a ball mill to prepare a raw material slurry. The raw material slurry was degassed and thickened, and then formed into a sheet using a doctor blade method to obtain a molded body. The obtained sheet molded body was cut, degreased at 500°C in an air atmosphere, and then fired in a firing furnace at 1850°C in a nitrogen atmosphere for 5 hours to produce a silicon nitride sintered body.

[0071] [Evaluation of silicon nitride sintered body: insulating properties] The silicon nitride sintered body was ground to prepare a rectangular parallelepiped of 50 mm x 50 mm x 0.3 mm for measuring the breakdown voltage. The breakdown voltage was measured at room temperature in accordance with JIS C 2110-1:2016. The results are shown in Table 2. The evaluation criteria for insulating properties are as follows: A: Breakdown voltage is 9 kV or more. B: Breakdown voltage is 5 kV or more but less than 9 kV. C: Breakdown voltage is less than 5 kV.

[0072] The silicon nitride sintered body was ground to prepare a 50 mm x 50 mm x 1.0 mm rectangular parallelepiped for measuring thermal conductivity. Thermal diffusivity and specific heat capacity were measured using the laser flash method (in accordance with JIS R 1611:2010), and the product of the density, thermal diffusivity, and specific heat capacity of the sintered body was calculated to determine the thermal conductivity at room temperature. The results are shown in Table 2. The thermal conductivity evaluation criteria are as follows: A: Thermal conductivity of 70 W / (m K) or more; B: Thermal conductivity of 50 W / (m K) or more but less than 70 W / (m K); C: Thermal conductivity of less than 50 W / (m K).

[0073]

[0074]

[0075] In all of the filtration devices of Examples 1 to 3, no corrosion was observed in the components that came into contact with the liquid to be treated, and the acid resistance was good. As shown in Table 2, the silicon nitride sintered bodies produced using the silicon nitride powders of Examples 1 to 3 had excellent insulating properties and heat dissipation properties. On the other hand, the silicon nitride powder of Comparative Example 1 had an isoelectric point of 3, and the silicon nitride sintered body produced using this powder had poor insulating properties and heat dissipation properties.

Claims

1. Silicon nitride powder with an isoelectric point between pH 7 and 9.

2. The silicon nitride powder according to claim 1, wherein the gelatinization rate is 90 to 97%.

3. The silicon nitride powder according to claim 1, wherein the pH at which the positive zeta potential is maximized is 2 to 4.

4. The silicon nitride powder according to claim 3, which has a zeta potential of 30 to 45 mV at pH 1.

5.

5. A method for producing a silicon nitride sintered body, which comprises molding and firing a sintering raw material containing the silicon nitride powder according to any one of claims 1 to 4.

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