Method for manufacturing boron nitride powder, boron nitride powder and resin sealing material

Abstract

This method for producing boron nitride powder involves a decarburization crystallization step for producing boron nitride primary particles by baking a raw material mixture including boron carbonitride powder, a boron source, and a carbonate to obtain powder including aggregated particles formed from aggregation of primary particles, wherein, in the decarburization crystallization step, the amount of the boron source used is 55% by mass or more based on the total mass of the raw material mixture, and the amount of the carbonate used is 4% by mass or more based on the total mass of the raw material mixture.

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing boron nitride powder, the boron nitride powder, and a resin encapsulant. [Background technology]

[0002] Boron nitride powder, an aggregate of boron nitride particles, has lubricity, high thermal conductivity, insulating properties, and is widely used as a solid lubricant, thermally conductive filler, insulating filler, and other applications.

[0003] Boron nitride particles (especially hexagonal boron nitride particles) have large anisotropy in thermal conductivity. Therefore, it has been studied to suppress the anisotropy in thermal conductivity due to the orientation of primary particles by aggregating primary particles of boron nitride to form boron nitride agglomerated particles (for example, Patent Document 1, etc.). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-135731 Summary of the Invention [Problem to be solved by the invention]

[0005] One example of a material that requires improved thermal conductivity is the resin encapsulant for semiconductor memory devices. Resin encapsulants for semiconductor memory devices must be made of a material that is less likely to emit alpha rays to prevent soft errors in the semiconductor memory devices. It is particularly desirable to use such materials with a low uranium content.

[0006] On the other hand, the agglomerated particles disclosed in Patent Document 1 generally contain trace amounts of uranium. Furthermore, due to their structure, the uranium trapped inside the agglomerated particles is difficult to remove. Therefore, it is difficult to reduce the uranium content in the agglomerated particles after their formation.

[0007] Therefore, one aspect of the present disclosure aims to provide a boron nitride powder containing agglomerated particles formed by agglomeration of primary particles of boron nitride, the boron nitride powder having a reduced uranium content, and a method for producing the same. Another aspect of the present disclosure aims to provide a boron nitride powder containing agglomerated particles formed by agglomeration of primary particles of boron nitride, the boron nitride powder having a sufficiently low uranium content. Another aspect of the present disclosure aims to provide a resin encapsulant for a semiconductor memory element containing the boron nitride powder of the above aspect. [Means for solving the problem]

[0008] The present disclosure provides at least the following [1] to [9].

[0009] [1] A method for producing boron nitride powder, comprising a decarburization / crystallization step of generating primary particles of boron nitride by firing a raw material mixture containing boron carbonitride powder, a boron source, and a carbonate, and obtaining a powder containing agglomerated particles formed by agglomeration of the primary particles, wherein in the decarburization / crystallization step, the amount of the boron source used is 55 mass% or more, based on the total mass of the raw material mixture, and the amount of the carbonate used is 4 mass% or more, based on the total mass of the raw material mixture.

[0010] [2] The method for producing boron nitride powder according to [1], wherein the firing temperature in the decarburization and crystallization step is 1800°C or higher and the firing time is 8 hours or longer.

[0011] [3] The method for producing boron nitride powder according to [1], wherein the firing temperature in the decarburization and crystallization step is 2000°C or higher and the firing time is 4 hours or longer.

[0012] [4] The method for producing boron nitride powder according to any one of [1] to [3], further comprising a preparation step of preparing the boron carbonitride powder, the preparation step comprising: an acid treatment step of bringing boron carbide powder into contact with an acid solution containing hydrofluoric acid to acid-treat the boron carbide powder; and a pressure nitridation step of firing the acid-treated boron carbide powder in a pressurized nitrogen atmosphere.

[0013] [5] The method for producing a boron nitride powder according to [4], wherein the acid solution is a mixed acid containing the hydrofluoric acid, and the proportion of hydrofluoric acid in all acid components in the mixed acid is 10 mass% or more.

[0014] [6] The method for producing a boron nitride powder according to [4] or [5], wherein the treatment temperature in the acid treatment step is 60°C or higher, the treatment time in the acid treatment step is 3 hours or longer, and the acid concentration of the acid solution is 20 mass% or higher.

[0015] [7] A boron nitride powder containing agglomerated particles formed by agglomeration of primary particles of boron nitride, having an orientation index of 15 or less and a uranium content of 20 ppb by mass or less.

[0016] [8] The boron nitride powder according to [7], having a purity of 98.5% by mass or more, an average particle size of 10 to 90 μm, and a crushing strength of the agglomerated particles of 5 MPa or more.

[0017] [9] A resin encapsulant for a semiconductor memory element, comprising the boron nitride powder according to [7] or [8]. [Effects of the Invention]

[0018] According to one aspect of the present disclosure, there is provided a method for producing a boron nitride powder containing agglomerated particles formed by agglomeration of primary particles of boron nitride, the boron nitride powder having a reduced uranium content. According to another aspect of the present disclosure, there is provided a boron nitride powder containing agglomerated particles formed by agglomeration of primary particles of boron nitride, the boron nitride powder having a sufficiently low uranium content. According to another aspect of the present disclosure, there is provided a resin encapsulant for a semiconductor memory element containing the boron nitride powder of the above aspect. DETAILED DESCRIPTION OF THE INVENTION

[0019] Unless otherwise specified, the materials exemplified herein can be used singly or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of the multiple substances present in the composition unless otherwise specified. In this specification, numerical ranges indicated using "to" indicate ranges that include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. Furthermore, unless otherwise specified, the units of the numerical values ​​before and after "to" are the same. In the numerical ranges described herein, the upper or lower limit of the numerical range may be replaced with the value shown in the examples. Furthermore, the individually described upper and lower limits can be arbitrarily combined.

[0020] Hereinafter, embodiments of the present disclosure will be described. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents.

[0021] <Method for manufacturing boron nitride powder> A method for producing boron nitride powder according to one embodiment includes a decarburization and crystallization step of producing primary particles of boron nitride by firing a raw material mixture containing boron carbonitride powder, a boron source, and a carbonate, and obtaining a powder containing agglomerated particles constituted by agglomeration of the primary particles.

[0022] The inventors have found through their investigations that the uranium content in the finally obtained boron nitride powder can be reduced by using a boron source and a carbonate in the decarburization crystallization step in amounts equal to or greater than predetermined values. Specifically, the uranium content can be reduced when the amount of boron source used is 55% by mass or more, based on the total mass of the raw material mixture, and the amount of carbonate used in the crystallization step is 4% by mass or more, based on the total mass of the raw material mixture.

[0023] The method for producing boron nitride powder according to one embodiment may further include a preparation step of preparing boron carbonitride powder to be used in the decarburization crystallization step. The preparation step may include, for example, a pressure nitridation step of firing boron carbide powder in a pressurized nitrogen atmosphere. The preparation step may further include an acid treatment step (first acid treatment step) of contacting the boron carbide powder with an acid solution containing hydrofluoric acid to acid-treat the boron carbide powder. When the preparation step includes the first acid treatment step, the acid-treated boron carbide powder obtained through the acid treatment step is used in the pressure nitridation step. The preparation step may further include an acid treatment step (second acid treatment step) of contacting the boron carbonitride powder with an acid solution containing hydrofluoric acid to acid-treat the boron carbonitride powder. The boron carbonitride powder used in the second acid treatment step may be the powder obtained through the pressure nitridation step. When the preparation step includes a second acid treatment step, the decarburization crystallization step uses acid-treated boron carbonitride powder obtained through the acid treatment step. By performing the first acid treatment step and / or the second acid treatment step, the uranium content in the finally obtained boron nitride powder can be reduced. Therefore, by setting the amounts of the boron source and carbonate used in the decarburization crystallization step within the above-mentioned predetermined ranges, and then performing the first acid treatment step and / or the second acid treatment step, the uranium content in the finally obtained boron nitride powder can be further reduced.

[0024] Each step in the method for producing boron nitride powder will be described in detail below, except that the second acid treatment step is the same as the first acid treatment step except that boron carbonitride powder is used instead of boron carbide powder, and therefore a description of the second acid treatment step will be omitted.

[0025] (preparation process) [First acid treatment step] In the first acid treatment step, the boron carbide powder (BC powder) is brought into contact with an acid solution containing hydrofluoric acid to dissolve the boron carbide particles in contact with the acid solution, thereby removing at least a portion of the uranium present in the boron carbide powder (particularly the uranium present near the surfaces of the boron carbide particles).

[0026] Boron carbide powder is an aggregate of boron carbide particles. The purity of the boron carbide powder (boron carbide content) is preferably, for example, 97% by mass or more. Commercially available boron carbide powder or separately prepared boron carbide powder may be used as the boron carbide powder. For example, the boron carbide powder can be obtained by a method including the steps of: mixing boric acid and acetylene black, heating the mixture in an inert gas atmosphere at 1800 to 2400°C for 1 to 10 hours to obtain a boron carbide block; and pulverizing the obtained boron carbide block, sieving, washing, removing impurities, drying, and other appropriate procedures to prepare boron carbide powder. From the viewpoint of improving the efficiency of uranium removal during acid treatment, it is preferable to use boron carbide powder with an average particle size of 10 μm or more (e.g., 10 to 50 μm). From the same perspective, the specific surface area of ​​1m 2 / g or less (e.g., 0.05 to 1m 2 It is preferable to use boron carbide powder with a content of 0.1g / g.

[0027] An acid solution is an aqueous solution containing hydrofluoric acid as an acid component. An aqueous solution of hydrogen fluoride is sometimes called hydrofluoric acid, but in this specification, the compound represented by HF among the acid components contained in the aqueous solution (acid solution) is referred to as hydrofluoric acid. An acid component is also defined as a substance that dissolves in water and releases hydrogen ions.

[0028] From the viewpoint of improving the uranium removal efficiency, the concentration of hydrofluoric acid in the acid solution may be 0.1% by mass or more, or may be 1% by mass or more, 2% by mass or more, 3% by mass or more, or 10% by mass or more. From the viewpoint of production cost and safety, the concentration of hydrofluoric acid in the acid solution may be 50% by mass or less, or may be 45% by mass or less, 40% by mass or less, or 30% by mass or less. From these viewpoints, the concentration of hydrofluoric acid in the acid solution may be, for example, 0.1 to 50% by mass, 1 to 50% by mass, 2 to 45% by mass, 3 to 40% by mass, or 10 to 30% by mass. Note that the concentration of hydrofluoric acid refers to the content of hydrofluoric acid (HF) based on the total mass of the acid solution.

[0029] The acid solution may be a mixed acid further containing an acid component other than hydrofluoric acid. In this case, the proportion of hydrofluoric acid in the total acid components in the acid solution may be 4% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more, from the viewpoint of improving the uranium removal efficiency. The proportion of hydrofluoric acid in the total acid components may be 80% by mass or less, 70% by mass or less, or 60% by mass or less, from the viewpoints of production cost and safety. From these viewpoints, the proportion of hydrofluoric acid in the total acid components may be, for example, 4 to 80% by mass, 10 to 70% by mass, 20 to 60% by mass, 30 to 60% by mass, or 40 to 60% by mass.

[0030] Examples of acid components other than hydrofluoric acid that can be contained in the acid solution include hydrochloric acid (HCl), nitric acid (HNO), sulfuric acid (HSO), etc. Among these, hydrochloric acid is preferably used from the viewpoint of improving the efficiency of uranium removal.

[0031] From the viewpoint of efficient removal, the concentration of hydrochloric acid in the acid solution may be 2% by mass or more, 3% by mass or more, 5% by mass or more, 7% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more. From the viewpoints of production cost and safety, the concentration of hydrochloric acid in the acid solution may be 60% by mass or less, 50% by mass or less, 35% by mass or less, 30% by mass or less, or 25% by mass or less. From these viewpoints, the concentration of hydrochloric acid in the acid solution may be, for example, 2 to 60% by mass, 3 to 60% by mass, 5 to 60% by mass, 7 to 60% by mass, 10 to 60% by mass, 20 to 60% by mass, 30 to 60% by mass, 40 to 60% by mass, or 5 to 35% by mass, 7 to 30% by mass, or 10 to 25% by mass. Note that the concentration of hydrochloric acid refers to the content of hydrochloric acid (HCl) based on the total mass of the acid solution.

[0032] From the viewpoint of improving the uranium removal efficiency, the acid concentration of the acid solution may be 2% by mass or more, 3% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more. From the viewpoint of production cost and safety, the acid concentration of the acid solution may be 50% by mass or less, 45% by mass or less, or 40% by mass or less. From these viewpoints, the acid concentration of the acid solution may be, for example, 2 to 50% by mass, 3 to 45% by mass, 5 to 40% by mass, 10 to 50% by mass, 20 to 50% by mass, 30 to 50% by mass, or 40 to 50% by mass. Note that the acid concentration refers to the content of all acid components based on the total mass of the acid solution.

[0033] The method for contacting the boron carbide powder with the acid solution is not particularly limited, and examples thereof include a method of mixing the boron carbide powder and the acid solution by, for example, introducing the boron carbide powder into the acid solution, and a method of continuously injecting the acid solution into the boron carbide powder. In the method of mixing the boron carbide powder and the acid solution, the mixture containing the boron carbide powder and the acid solution may be stirred to improve the uranium removal efficiency. Stirring may be performed using, for example, a stirrer, a magnetic stirrer, a disperser, or the like. When acid treating the boron carbide powder by mixing the boron carbide powder with the acid solution, the acid treatment of the boron carbide powder may be repeated multiple times. For example, a series of operations may be repeated, in which the boron carbide powder and the acid solution are mixed, the boron carbide powder is separated from the acid solution after a certain period of time has elapsed, and the separated boron carbide powder is mixed again with a new acid solution. The amount of the acid solution used may be, for example, 80 parts by mass or more, 500 parts by mass or less, or 80 to 500 parts by mass, relative to 100 parts by mass of the boron carbide powder.

[0034] The treatment temperature in the acid treatment step may be 40°C or higher, or may be 60°C or higher, or 75°C or higher, from the viewpoint of improving the uranium removal efficiency. The treatment temperature in the acid treatment step may be 95°C or lower, or may be 92°C or lower, or 90°C or lower, from the viewpoint of preventing a decrease in treatment efficiency due to volatilization of hydrogen fluoride and from the viewpoint of safety. From these viewpoints, the treatment temperature in the acid treatment step may be, for example, 40 to 95°C, 60 to 92°C, or 75 to 90°C. Note that the above treatment temperature refers to the temperature of the acid solution brought into contact with the boron carbide powder during the acid treatment. When the acid treatment of the boron carbide powder is performed by mixing the boron carbide powder with an acid solution, the above treatment temperature may be referred to as the temperature of the mixed solution containing the boron carbide powder and the acid solution.

[0035] The treatment time in the acid treatment step may be 1 hour or more, or may be 2 hours or more, 3 hours or more, or 5 hours or more, from the viewpoint of increasing the amount of uranium removed. The treatment time in the acid treatment step may be 15 hours or less, or may be 12 hours or less, or 10 hours or less, from the viewpoint of production costs and safety. From these viewpoints, the treatment time in the acid treatment step may be, for example, 1 to 15 hours, 2 to 12 hours, 3 to 10 hours, or 5 to 10 hours. Note that the above treatment time indicates the contact time between the boron carbide powder and the acid solution.

[0036] From the viewpoint of further increasing the amount of uranium removed, it is preferable that the treatment temperature in the acid treatment step be 60°C or higher and the treatment time in the acid treatment step be 3 hours or longer, and it is more preferable that the treatment temperature in the acid treatment step be 60°C or higher, the treatment time in the acid treatment step be 3 hours or longer, and the acid concentration of the acid solution be 20 mass% or higher.

[0037] [Pressure nitriding process] In the pressure nitriding step, boron carbide powder is sintered under a pressurized nitrogen atmosphere to nitride the boron carbide and obtain a sintered product containing boron carbonitride. The obtained sintered product tends to contain hexagonal boron carbonitride at a high purity. Examples of boron carbide powder include the same boron carbide powders as those exemplified as the boron carbide powder used in the first acid treatment step. When the first acid treatment step is performed, the acid-treated boron carbide powder obtained through the first acid treatment step is used.

[0038] The firing temperature in the pressure nitriding step is preferably higher than the firing temperature in the decarburization crystallization step. The firing temperature in the pressure nitriding step may be, for example, 1900 to 2200°C, 2000 to 2200°C, or 2100 to 2200°C. By setting the firing temperature within the above range, the crystallinity of boron carbonitride can be improved, and the proportion of hexagonal boron carbonitride can be increased. The firing time in the pressure nitriding step is not particularly limited as long as nitriding proceeds sufficiently, and may be, for example, 6 to 30 hours, 8 to 25 hours, or 10 to 20 hours.

[0039] The pressure (atmospheric pressure) in the pressure nitriding step may be, for example, 0.6 to 1 MPa, 0.7 to 1 MPa, or 0.8 to 1 MPa. By setting the pressure within the above range, it is possible to suppress the production cost and to efficiently and sufficiently proceed with the nitriding of boron carbide. Note that the above pressures are gauge pressures.

[0040] The nitrogen gas concentration in the pressurized nitrogen atmosphere in the pressurized nitriding step may be, for example, 95% by volume or more, 98% by volume or more, or 99.9% by volume or more. The upper limit of the nitrogen gas concentration is 100% by volume. By setting the nitrogen gas concentration within the above range, the nitriding of boron carbide can be carried out under milder conditions. Note that the above nitrogen gas concentration is a concentration based on volume under standard conditions.

[0041] The fired product obtained in the pressure nitriding step may be used directly in the decarburization crystallization step, or may be subjected to appropriate treatments such as pulverization, classification, washing, and heat treatment (e.g., oxidation treatment in an oxygen-containing atmosphere) before being used in the decarburization crystallization step. The pulverization can be carried out using a general pulverizer or crusher such as a ball mill, vibration mill, or jet mill. In this specification, "pulverization" also includes "crushing."

[0042] (Decarburization crystallization process) In the decarburization and crystallization process, boron carbonitride powder (BCN powder) is calcined together with a boron source and carbonate to decarburize the boron carbonitride and increase the crystallinity of the boron nitride. As a result, boron nitride powder (BN powder) containing agglomerated particles formed by the aggregation of primary particles of boron nitride is obtained. The boron nitride in the boron nitride powder obtained by this method is usually hexagonal boron nitride, and contains agglomerated particles formed by the aggregation of scaly hexagonal boron nitride primary particles.

[0043] The boron carbonitride powder is an aggregate of boron carbonitride particles. The purity of the boron carbonitride powder (boron carbonitride content) is preferably, for example, 98 mass% or more. As the boron carbonitride powder, commercially available boron carbonitride powder may be used, or one prepared in the above-mentioned preparation step may also be used. From the viewpoint of increasing the proportion of hexagonal boron nitride in the obtained boron nitride powder, it is preferable to use boron carbonitride powder with a high proportion of hexagonal boron carbonitride, and it is more preferable to use boron carbonitride powder obtained through the above-mentioned pressure nitriding step.

[0044] Examples of boron sources include boric acid and boron oxide. These may be used alone or in combination of two or more. Boric acid is preferably used as the boron source from the viewpoint of easily achieving the effect of promoting the growth of primary particles and easily reducing the uranium content.

[0045] Examples of carbonates include sodium carbonate, calcium carbonate, and strontium carbonate. These may be used alone or in combination of two or more. As the carbonate, sodium carbonate is preferably used from the viewpoint of easily achieving the effect of promoting the growth of primary particles and easily reducing the uranium content.

[0046] The amount of the boron source used may be 55% by mass or more, or may be 57% by mass or more, or 59% by mass or more, from the viewpoint of reducing the uranium content. The amount of the boron source used may be 70% by mass or less, 65% by mass or less, or 60% by mass or less, from the viewpoint of increasing the crushing strength of the agglomerated particles. The amount of the boron source used may be less than 55% by mass. However, if the amount of the boron source used is 50% by mass or more, the growth of primary particles of boron nitride is likely to be promoted, making it easier to obtain boron nitride powder having an average particle size of 10 to 90 μm and a crushing strength of 5 MPa or more. From the above viewpoints, the amount of the boron source used may be 50 to 70% by mass, 55 to 70% by mass, 57 to 70% by mass, 59 to 70% by mass, 55 to 65% by mass, 57 to 65% by mass, 59 to 65% by mass, 55 to 60% by mass, 57 to 60% by mass, or 59 to 60% by mass. The above-mentioned amount used is based on the total mass of the raw material mixture, but the amount used based on the total amount of the boron carbonitride powder, boron source, and carbonate may also be within the above range.

[0047] The amount of carbonate used may be 4% by mass or more, 5% by mass or more, or 6% by mass or more, from the viewpoint of reducing the uranium content. The amount of carbonate used may be 10% by mass or less, 8% by mass or less, or 6% by mass or less, from the viewpoint of increasing the crushing strength of the agglomerated particles. The amount of carbonate used may be less than 4% by mass. However, if the amount of carbonate used is 1% by mass or more, the growth of primary particles of boron nitride is likely to be promoted, making it easier to obtain boron nitride powder having an average particle size of 10 to 90 μm and a crushing strength of 5 MPa or more. From this viewpoint, the amount of carbonate used may be 1 to 10% by mass, 4 to 10% by mass, 5 to 10% by mass, 6 to 10% by mass, 4 to 8% by mass, 5 to 8% by mass, 6 to 8% by mass, 4 to 6% by mass, or 5 to 6% by mass. The above amounts are based on the total mass of the raw material mixture, but the amount used may be within the above range based on the combined amount of the boron carbonitride powder, boron source, and carbonate.

[0048] In the decarburization and crystallization step, materials other than the boron carbonitride powder, the boron source, and the carbonate may be used, or only the boron carbonitride powder, the boron source, and the carbonate may be used.

[0049] The firing temperature in the decarburization and crystallization step is preferably 1800°C or higher, and may be 1900°C or higher or 2000°C or higher. By setting the firing temperature to 1800°C or higher, the growth of primary particles can be more sufficiently promoted. The firing temperature in the decarburization and crystallization step is preferably 2400°C or lower, and may be 2200°C or lower or 2100°C or lower. By setting the firing temperature to 2400°C or lower, yellowing of the boron nitride powder can be suppressed. From the above viewpoint, the firing temperature in the decarburization and crystallization step may be, for example, 1800 to 2400°C, 1900 to 2200°C, or 2000 to 2100°C. The firing temperature refers to the temperature maintained during heating (firing). The heating start temperature is not particularly limited, but may be room temperature (e.g., 25°C). When heating is started from a temperature lower than the holding temperature, the rate of temperature increase up to 1000°C may be, for example, 1 to 10°C / min, and the rate of temperature increase above 1000°C may be, for example, 0.1 to 5°C / min.

[0050] The firing time in the decarburization and crystallization step is preferably 4 hours or more, and may be 6 hours or more or 8 hours or more. By setting the firing time to 4 hours or more, the growth of primary particles can be more sufficiently promoted. The firing time in the decarburization and crystallization step is preferably 40 hours or less, and may be 30 hours or less or 20 hours or less. By setting the firing time to 40 hours or less, an increase in manufacturing costs can be suppressed. From the above viewpoint, the firing time in the decarburization and crystallization step may be, for example, 4 to 40 hours, 6 to 30 hours, or 8 to 20 hours. The firing time means the holding time at the firing temperature (holding temperature).

[0051] According to the studies of the present inventors, the uranium content can also be reduced by changing the firing conditions in the decarburization crystallization step. From the viewpoint of further reducing the uranium content, it is preferable that the firing temperature is 1800°C or higher and the firing time is 8 hours or longer, or that the firing temperature is 2000°C or higher and the firing time is 4 hours or longer, and it is more preferable that the firing temperature is 2000°C or higher and the firing time is 8 hours or longer.

[0052] The firing atmosphere in the decarburization crystallization step may be, for example, air or vacuum, or may be an inert gas atmosphere such as nitrogen gas or argon gas. The firing in the decarburization crystallization step may be performed under normal pressure (atmospheric pressure) or under a pressure higher than atmospheric pressure. The pressure (atmospheric pressure) in the decarburization crystallization step may be, for example, 10 kPa or more, 15 kPa or more, or 20 kPa or more. Setting the pressure to 10 kPa or more prevents the boron source, carbonate, and other auxiliary agents from being removed from the system, making the reaction field of the boron nitride particles even more uniform. The pressure (atmospheric pressure) in the decarburization crystallization step may be, for example, 80 kPa or less, 60 kPa or less, or 40 kPa or less. Setting the pressure to 80 kPa or less further prevents the collapse of aggregated particles during the decarburization crystallization step. From the above viewpoint, the pressure (atmospheric pressure) in the decarburization and crystallization step may be 10 to 80 kPa, 10 to 60 kPa, or 20 to 40 kPa. Note that the pressure indicated above is a gauge pressure.

[0053] The boron nitride powder after the decarburization and crystallization step may be appropriately subjected to pulverization, classification, washing, etc. The pulverization can be performed using a general pulverizer or crusher such as a ball mill, a vibration mill, or a jet mill.

[0054] The method for producing boron nitride powder described above can produce boron nitride powder containing agglomerated particles formed by agglomeration of primary particles of boron nitride, and having an orientation index of 15 or less. Furthermore, the method for producing boron nitride powder described above can reduce the uranium content while forming agglomerated particles formed by agglomeration of primary particles of boron nitride, by using a boron source and carbonate in the decarburization crystallization step at 55% by mass or more and 4% by mass or more, respectively, and by treating the boron carbide powder with an acid solution containing hydrofluoric acid. It is not necessary to implement all of these methods for reducing the uranium content, but by combining multiple methods, it is possible to provide boron nitride powder with an even lower uranium content. In this specification, the orientation index of boron nitride powder refers to a value measured according to the following method.

[0055] X-ray diffraction measurements are performed on boron nitride powder to obtain an X-ray diffraction spectrum of the boron nitride powder, and the peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained from the X-ray diffraction spectrum. The obtained peak intensities are used to calculate the orientation index [I(002) / I(100)] of the boron nitride powder. An X-ray diffraction device such as the "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used.

[0056] <Boron nitride powder> A boron nitride powder according to one embodiment includes agglomerated particles formed by agglomeration of primary particles of boron nitride, has an orientation index of 15 or less, and a uranium content of 20 mass ppb or less. The boron nitride powder may include primary particles in addition to the agglomerated particles. The primary particles of boron nitride may be, for example, scaly hexagonal boron nitride particles.

[0057] The boron nitride powder has a uranium content of 20 ppb by mass or less, making it a material that is unlikely to emit alpha rays. Furthermore, because the boron nitride powder has an orientation index of 15 or less, even if at least a portion of the aggregated particles disintegrate during kneading with a resin, increasing the orientation, the boron nitride powder can prevent significant anisotropy in the heat dissipation properties of the resin encapsulant. Therefore, the boron nitride powder is suitable for use in resin encapsulants for semiconductor memory devices, which require high heat dissipation properties.

[0058] The boron nitride powder can be obtained, for example, by using the boron source and carbonate in the decarburization and crystallization step in an amount of 55 mass % or more and 4 mass % or more, respectively, in the manufacturing method according to the embodiment.

[0059] The uranium content of the boron nitride powder may be, for example, 18 mass ppb or less, 16 mass ppb or less, 14 mass ppb or less, 12 mass ppb or less, 10 mass ppb or less, 7 mass ppb or less, or 5 mass ppb or less. Boron nitride powder with such a uranium content can be obtained by, for example, combining the use of a boron source and a carbonate in the decarburization crystallization step at 55 mass% or more and 4 mass% or more, respectively, with an acid treatment step (first acid treatment step and / or second acid treatment step), changing the acid treatment conditions in the acid treatment step, changing the firing conditions in the decarburization crystallization step, etc. The lower limit of the uranium content may be, for example, 5 mass ppb. The uranium content may be, for example, 5 to 20 mass ppb, 5 to 18 mass ppb, 5 to 16 mass ppb, 5 to 14 mass ppb, 5 to 12 mass ppb, 5 to 10 mass ppb, or 5 to 7 mass ppb. The above content is based on the total mass of the boron nitride powder.

[0060] The uranium content of the boron nitride powder in this specification means a value measured according to the following method. According to the JCRS108 method for chemical analysis of boron nitride powder, 0.5 g of sample (boron nitride powder) is subjected to pressure acid decomposition in a mixture of nitric acid, sulfuric acid, and hydrofluoric acid at 180°C for 18 hours. The solution obtained by pressure acid decomposition is then solidified by drying on a hot plate, and the resulting solid is mixed with nitric acid to form a mixed solution. The resulting mixed solution is then heated on a hot plate to dissolve the solid, yielding a nitric acid solution, after which the amount of uranium in the nitric acid solution is measured using an ICP-MS (inductively coupled plasma mass spectrometry) system. The ICP-MS measurement conditions are as follows: (ICP-MS measurement conditions) Plasma mode: low matrix mode, Tune mode: He, Spectrum mode Peak pattern: 1 point, Repetition: 3, Number of sweeps: 100, Integration time: 1 second

[0061] From the viewpoint of further reducing the influence of the collapse of the agglomerated particles, the orientation index may be 12 or less, or 10 or less. The orientation index may be 3 or more, 4 or more, or 6 or more, and may be 3 to 15, 4 to 12, or 6 to 10. The orientation index of the boron nitride powder can be adjusted, for example, by controlling the growth of primary particles when producing the boron nitride powder.

[0062] The purity of the boron nitride powder is preferably 98.5% by mass or more, and may be 99% by mass or more or 99.5% by mass or more. The upper limit of the purity of the boron nitride powder is not particularly limited, and may be 100% by mass or 99.5% by mass.

[0063] The purity of boron nitride powder in this specification refers to a value determined by titration, which will be described later. First, a sample of boron nitride powder is alkaline decomposed with sodium hydroxide, and ammonia is distilled from the decomposition liquid by steam distillation and collected in an aqueous boric acid solution. This collected liquid is then titrated with a normal sulfuric acid solution. The nitrogen atom (N) content in the sample is calculated from the titration results. The boron nitride content in the sample is determined based on the obtained nitrogen atom content and formula (I), and the purity of the boron nitride powder can be calculated. The formula weight of boron nitride is 24.818 g / mol, and the atomic weight of nitrogen atoms is 14.006 g / mol. Boron nitride content in sample [mass%] = nitrogen atom (N) content [mass%] × 1.772 (I)

[0064] The agglomerated particles contained in the boron nitride powder preferably have a crushing strength of 5 MPa or more, from the viewpoint of being less likely to collapse when kneaded with a resin. The crushing strength of the agglomerated particles may be 8 MPa or more or 10 MPa or more. The crushing strength of the agglomerated particles may be 20 MPa or less, 15 MPa or less, or 12 MPa or less. When the crushing strength of the agglomerated particles is 20 MPa or less, at least a portion of the agglomerated particles collapses appropriately when kneaded with a resin, making it easier to suppress the generation of voids. As a result, the resulting resin encapsulant has higher insulating properties. From the above viewpoint, the crushing strength of the agglomerated particles may be, for example, 5 to 20 MPa, 8 to 15 MPa, or 10 to 12 MPa. The crushing strength of the agglomerated particles can be adjusted, for example, by changing the firing conditions, etc.

[0065] The crushing strength in this specification refers to a value measured in accordance with the description in JIS R 1639-5:2007 "Fine ceramics - Measurement methods for granule characteristics - Part 5: Single granule crushing strength." The crushing strength σ (unit: MPa) of a single agglomerate particle is calculated from the dimensionless number α (α=2.48), which varies depending on the position within the agglomerate particle, the crushing test force P (unit: N), and the particle diameter d (unit: μm), as follows: σ=α×P / (π×d 2) is calculated using the formula. Measurements were performed on 20 or more agglomerated particles, and the value at the cumulative destruction rate of 63.2% was calculated. A micro-compression tester can be used for the measurement. For example, the "MCT-W500" (product name) manufactured by Shimadzu Corporation can be used as a micro-compression tester.

[0066] The average particle diameter of the boron nitride powder may be, for example, 90 μm or less, 80 μm or less, or 70 μm or less. An average particle diameter of 90 μm or less allows the sealing portion formed by the resin sealing material to be thinner. The average particle diameter of the boron nitride powder may be, for example, 10 μm or more, 20 μm or more, or 30 μm or more. An average particle diameter of 10 μm or more allows the thermal conductivity of the resin sealing material to be further improved. From these viewpoints, the average particle diameter of the boron nitride powder may be, for example, 10 to 90 μm, 20 to 80 μm, or 30 to 70 μm.

[0067] In this specification, the average particle size refers to the 50% cumulative diameter (median diameter) in the volume-based cumulative particle size distribution. More specifically, it refers to the particle size (D50) at which the cumulative value reaches 50% in the volume-based cumulative particle size distribution obtained by laser diffraction scattering for a powder. The laser diffraction scattering method is measured in accordance with the method described in ISO 13320:2009. A laser diffraction scattering particle size distribution analyzer, such as the "LS-13 320" (product name) manufactured by Beckman Coulter, Inc., can be used for the measurement. The measurement is performed without homogenization, in the presence of aggregated particles.

[0068] From the viewpoint of the insulating properties of the resin sealing material, the boron nitride powder preferably has a purity of 98.5 mass % or more, an average particle size of 10 to 90 μm, and a crushing strength of agglomerated particles of 5 MPa or more.

[0069] <Resin sealing material> A resin encapsulant according to one embodiment is a resin encapsulant for semiconductor memories, and contains the boron nitride powder according to the above embodiment.

[0070] The resin contained in the resin encapsulant may be any known resin used in resin encapsulants, such as liquid crystal polymers, fluororesins, silicone resins, silicone rubber, acrylic resins, polyolefins (such as polyethylene), epoxy resins, phenolic resins, melamine resins, urea resins, unsaturated polyesters, polyimides, polyamideimides, polyetherimides, polybutylene terephthalate, polyethylene terephthalate, polyphenylene ether, polyphenylene sulfide, wholly aromatic polyesters, polysulfones, polyethersulfones, polycarbonates, maleimide-modified resins, ABS (acrylonitrile-butadiene-styrene) resins, AAS (acrylonitrile-acrylic rubber-styrene) resins, and AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resins.

[0071] The resin content may be, for example, 15% by volume or more, 20% by volume or more, or 30% by volume or more, based on the total volume of the resin encapsulant. The resin content may be, for example, 60% by volume or less, 50% by volume or less, or 40% by volume or less, based on the total volume of the resin encapsulant. The resin content may be, for example, 15 to 60% by volume, 20 to 50% by volume, or 30 to 40% by volume, based on the total volume of the resin encapsulant.

[0072] The content of the boron nitride powder may be, for example, 30 volume % or more, 40 volume % or more, 50 volume % or more, or 60 volume % or more, based on the total volume of the resin encapsulant. The content of the boron nitride powder may be, for example, 85 volume % or less, 80 volume % or less, or 70 volume % or less, based on the total volume of the resin encapsulant. The content of the boron nitride powder may be, for example, 30 to 85 volume %, 40 to 85 volume %, 40 to 80 volume %, 50 to 80 volume %, 50 to 70 volume %, or 60 to 70 volume %, based on the total volume of the resin encapsulant.

[0073] In addition to the resin and boron nitride powder, the resin encapsulant may further contain a curing agent that cures the resin. The curing agent can be appropriately selected depending on the type of resin. When the resin is an epoxy resin, examples of the curing agent include phenol novolac compounds, acid anhydrides, amino compounds, and imidazole compounds. The content of the curing agent may be, for example, 0.5 parts by mass or more or 1 part by mass or more, 15 parts by mass or less, 10 parts by mass or less, or 0.5 to 15 parts by mass or 1 to 10 parts by mass, relative to 100 parts by mass of the resin. [Example]

[0074] The present disclosure will be described in more detail with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0075] Example 1 [Preparation of boron carbide powder] 100 parts by mass of orthoboric acid manufactured by Nippon Denko Corporation and 35 parts by mass of acetylene black (product name: HS100L) manufactured by Denka Co., Ltd. were mixed using a Henschel mixer. The resulting mixture was loaded into a graphite crucible and heated in an arc furnace under an argon atmosphere at 2200°C for 6 hours to obtain lumped boron carbide (B4C). The resulting lumps were coarsely crushed using a jaw crusher to obtain coarse powder. The resulting coarse powder was further crushed using a ball mill equipped with silicon carbide balls (diameter: 10 mm) to obtain crushed powder. Crushing using the ball mill was carried out at a rotation speed of 25 rpm for 60 minutes. The crushed powder was then classified using a vibrating sieve with 63 μm openings to produce boron carbide powder (B4C powder) with an average particle size of 20 μm. The specific surface area of ​​the boron carbide powder was 0.4 m 2 / g and the purity was 98% by mass.

[0076] The average particle size of the boron carbide powder was measured using a Beckman Coulter laser diffraction particle size analyzer (instrument name: LS-13 320) in accordance with ISO 13320:2009. The boron carbide powder was not homogenized. When measuring the particle size distribution, water was used as the solvent to disperse the boron carbide powder, and hexametaphosphoric acid was used as the dispersant. The refractive index of water was 1.33, and the refractive index of the boron carbide powder was 2.6.

[0077] The specific surface area of ​​the boron carbide powder was calculated using the BET single-point method using nitrogen gas in accordance with JIS Z 8830:2013, "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption." The specific surface area was measured using a specific surface area measuring device (device name: Cantorsorb) manufactured by Yuasa Ionics Co., Ltd. The measurement was performed after drying and degassing the boron carbide powder at 300°C for 15 minutes.

[0078] The purity of the boron carbide powder was calculated from the sum of the carbon and boron contents. The carbon content was calculated using the combustion infrared absorption method, and the boron content was calculated using ICP atomic emission spectrometry.

[0079] [Pressure nitriding process] The prepared boron carbide powder was sintered in a carbon resistance heating furnace for 12 hours. The sintering atmosphere was a nitrogen gas atmosphere, the sintering temperature was 2050°C, and the atmospheric pressure was 0.90 MPa. In this way, boron carbonitride powder was obtained. The boron carbonitride powder was analyzed by powder X-ray diffraction (XRD), and the disappearance of boron carbide and the formation of hexagonal boron carbonitride were confirmed.

[0080] [Decarburization crystallization process] The fired product (boron carbonitride powder) obtained in the pressure nitriding step, boric acid, and sodium carbonate were mixed in a Henschel mixer to obtain a raw material mixture. The amount of boric acid used was 60 mass% based on the total mass of the raw material mixture, and the amount of sodium carbonate used was 5 mass% based on the total mass of the raw material mixture. Next, the resulting raw material mixture was filled into a boron nitride crucible and fired in a resistance heating furnace under a nitrogen gas atmosphere at a firing temperature of 1950°C and atmospheric pressure for 5 hours. Specifically, the temperature was raised from room temperature to 1000°C at a rate of 10°C / min, then raised from 1000°C to 1950°C at a rate of 2°C / min, and held at 1950°C for 5 hours. The resulting powder was then crushed to 20°C in a Henschel mixer, and the crushed material was then classified by passing it through a sieve with 75 μm openings to obtain the boron nitride powder of Example 1, which contained agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride. The resulting boron nitride powder had a uranium content of 19 ppb by mass, an orientation index of 7, a purity of 99% by mass, an average particle size of 40 μm, and a crushing strength of the agglomerated particles in the boron nitride powder of 12 MPa.

[0081] The uranium content of the boron nitride powder was determined by the following method. According to the JCRS108 method for chemical analysis of boron nitride powder, 0.5 g of a sample (boron nitride powder) was subjected to pressure acid decomposition in a mixture of nitric acid, sulfuric acid, and hydrofluoric acid at 180°C for 18 hours. The solution obtained by pressure acid decomposition was then solidified by drying on a hot plate, and the resulting solid was mixed with nitric acid to form a mixed solution. The resulting mixed solution was then heated on a hot plate to dissolve the solid, yielding a nitric acid solution, after which the amount of uranium in the nitric acid solution was measured using an ICP-MS (inductively coupled plasma mass spectrometry). The ICP-MS measurement conditions were as follows: (ICP-MS measurement conditions) Plasma mode: low matrix mode, Tune mode: He, Spectrum mode Peak pattern: 1 point, Repetition: 3, Number of sweeps: 100, Integration time: 1 second

[0082] The orientation index of boron nitride powder was determined from the results of measurements using powder X-ray diffraction. First, boron nitride powder was filled into the recess of a glass cell with a 0.2 mm depth recess attached to an X-ray diffractometer (Rigaku Corporation, product name: ULTIMA-IV). The measurement sample was prepared by compacting the powder sample at a set pressure M using a powder sample molding machine (Amenatec Corporation, product name: PX700). If the surface of the packed material compacted by the molding machine was not smooth, it was smoothed manually before measurement. The measurement sample was irradiated with X-rays, and after baseline correction, the peak intensity ratio of the (002) and (100) planes of boron nitride was calculated. The orientation index [I(002) / I(100)] was determined based on this value.

[0083] The purity of the boron nitride powder was determined using the following method. First, the boron nitride powder was alkaline decomposed using sodium hydroxide, and ammonia was distilled from the decomposition liquid using steam distillation and collected in an aqueous boric acid solution. This collected liquid was then titrated with a normal sulfuric acid solution. The nitrogen atom (N) content in the boron nitride powder was calculated from the titration results. The boron nitride content in the boron nitride powder was determined using the obtained nitrogen atom content based on equation (1), and the purity of the boron nitride powder was calculated. The formula weight of boron nitride was 24.818 g / mol, and the atomic weight of the nitrogen atom was 14.006 g / mol. Boron nitride (BN) content in boron nitride powder [mass%] = nitrogen atom (N) content [mass%] × 1.772 (1)

[0084] The average particle size of the boron nitride powder was measured in accordance with ISO 13320:2009 using a Beckman Coulter laser diffraction particle size analyzer (instrument name: LS-13 320). The boron nitride powder was not homogenized. When measuring the particle size distribution, water was used as the solvent to disperse the boron nitride powder, and hexametaphosphoric acid was used as the dispersant. The refractive index of water was 1.33, and the refractive index of the boron nitride powder was 1.80.

[0085] The crushing strength of the agglomerated particles was measured in accordance with JIS R 1639-5:2007 "Fine ceramics - Measurement methods for granule characteristics - Part 5: Single granule crushing strength." A microcompression tester (Shimadzu Corporation, product name "MCT-W500") was used for the measurement. The measurement was performed on 20 or more agglomerated particles, and the value at a cumulative fracture rate of 63.2% was calculated.

[0086] <Examples 2 to 4> Boron nitride powders of Examples 2 to 4 were obtained in the same manner as in Example 1, except that the firing temperature and / or firing time in the decarburization crystallization step was changed as shown in Table 1. The uranium content, orientation index, purity, and average particle size of the boron nitride powder, as well as the crushing strength of agglomerated particles in the boron nitride powder, were measured in the same manner as in Example 1. The results are shown in Table 1.

[0087] <Example 5> The boron nitride powder of Example 5 was obtained in the same manner as in Example 1, except that in the "Preparation of boron carbide powder" the pulverization using a ball mill was carried out at 100 rpm for 100 minutes and subsequent classification was carried out using a vibrating sieve with 34 μm openings, and in the "Decarburization and Crystallization Step" classification after crushing was carried out using a sieve with 45 μm openings. The uranium content, orientation index, purity, and average particle size of the boron nitride powder, as well as the crushing strength of the agglomerated particles in the boron nitride powder, were also measured in the same manner as in Example 1. The results are shown in Table 1.

[0088] <Comparative Examples 1 and 2> Boron nitride powders of Comparative Examples 1 and 2 were obtained in the same manner as in Example 1, except that the amounts of boric acid and sodium carbonate used in the decarburization crystallization step (based on the total mass of the raw material mixture) were changed to the values ​​shown in Table 1. The uranium content, orientation index, purity, and average particle size of the boron nitride powder, as well as the crushing strength of agglomerated particles in the boron nitride powder, were measured in the same manner as in Example 1. The results are shown in Table 1.

[0089] [Table 1]

[0090] Example 6 [Acid treatment process] Boron carbide powder prepared in the same manner as in Example 1 was subjected to an acid treatment. Specifically, first, boron carbide powder was prepared in the same manner as in Example 1. Next, an acid solution was prepared by mixing a hydrofluoric acid solution and a hydrochloric acid solution. The acid concentration of the acid solution was 40 mass %, and the mass ratio of hydrofluoric acid to hydrochloric acid (the mass ratio of HF to HCl) in the acid solution was 1:1 (i.e., the hydrofluoric acid concentration and hydrochloric acid concentration in the acid solution were both 20 mass %). Next, the acid solution was heated and held at 80°C, and boron carbide powder was added thereto and stirred at 80°C for 5 hours. In this way, the boron carbide powder was brought into contact with the acid solution, and the boron carbide powder was subjected to an acid treatment. After stirring, decantation was performed, and a new acid solution was added and the acid treatment was performed 15 times. The slurry was then dried to obtain an acid-treated boron carbide powder. The average particle size and purity of the obtained boron carbide powder were measured by the methods described in Example 1, and it was found that the average particle size was 20 μm and the purity was 99% by mass.

[0091] Next, the pressure nitriding step and decarburization crystallization step were carried out in the same manner as in Example 1, except that the acid-treated boron carbide powder was used as the boron carbide powder, thereby obtaining boron nitride powder of Example 6. The uranium content, orientation index, purity, and average particle size of the boron nitride powder, as well as the crushing strength of agglomerated particles in the boron nitride powder, were measured in the same manner as in Example 1. The results are shown in Table 2.

[0092] Example 7 The boron nitride powder of Example 7 was obtained in the same manner as in Example 6, except that the firing temperature and firing time in the decarburization crystallization step were changed as shown in Table 2. The uranium content, orientation index, purity, and average particle size of the boron nitride powder, as well as the crushing strength of agglomerated particles in the boron nitride powder, were measured in the same manner as in Example 1. The results are shown in Table 2.

[0093] Example 8 The boron nitride powder of Example 8 was obtained in the same manner as in Example 7, except that the concentration of the acid solution, the treatment temperature (temperature of the mixed solution of boron carbide powder and acid solution), and the treatment time (stirring time) in the acid treatment step were changed to the values ​​shown in Table 2. The uranium content, orientation index, purity, and average particle size of the boron nitride powder, as well as the crushing strength of agglomerated particles in the boron nitride powder, were measured in the same manner as in Example 1. The results are shown in Table 2. When analyzed in the same manner as for uranium, the thorium content was found to be 4 ppb by mass.

[0094] Example 9 The boron nitride powder of Example 9 was obtained in the same manner as in Example 6, except that the concentration of the acid solution, the composition of the acid solution, the treatment temperature (temperature of the mixed solution of boron carbide powder and acid solution), and the treatment time (stirring time) in the acid treatment step were changed to the values ​​shown in Table 2. The uranium content, orientation index, purity, and average particle size of the boron nitride powder, as well as the crushing strength of the agglomerated particles in the boron nitride powder, were measured in the same manner as in Example 1. The results are shown in Table 2.

[0095] [Table 2]

Claims

1. A method for producing boron nitride powder using a raw material mixture containing uranium, comprising: a decarburization and crystallization step of generating primary particles of boron nitride by firing the raw material mixture containing boron carbonitride powder, a boron source, and a carbonate, and obtaining a powder containing agglomerated particles constituted by agglomeration of the primary particles, a method for producing a boron nitride powder, wherein in the decarburization crystallization step, an amount of the boron source used is 55 mass% or more, based on the total mass of the raw material mixture, and an amount of the carbonate used is 4 mass% or more, based on the total mass of the raw material mixture.

2. 2. The method for producing boron nitride powder according to claim 1, wherein the firing temperature in the decarburization and crystallization step is 1800°C or higher and the firing time is 8 hours or longer.

3. 2. The method for producing boron nitride powder according to claim 1, wherein the firing temperature in the decarburization and crystallization step is 2000° C. or higher and the firing time is 4 hours or longer.

4. The method further includes a preparation step of preparing the boron carbonitride powder, 4. The method for producing boron nitride powder according to claim 1, wherein the preparation step includes: an acid treatment step of bringing boron carbide powder into contact with an acid solution containing hydrofluoric acid to acid-treat the boron carbide powder; and a pressure nitridation step of firing the acid-treated boron carbide powder in a pressurized nitrogen atmosphere.

5. the acid solution is a mixed acid containing the hydrofluoric acid, 5. The method for producing boron nitride powder according to claim 4, wherein the proportion of hydrofluoric acid in the total acid components in the mixed acid is 10 mass % or more.

6. The treatment temperature in the acid treatment step is 60°C or higher, The treatment time in the acid treatment step is 3 hours or more, The method for producing a boron nitride powder according to claim 4, wherein the acid concentration of the acid solution is 20 mass % or more.

7. The boron nitride powder contains agglomerated particles formed by agglomerating primary particles of boron nitride, The orientation index is 15 or less, A boron nitride powder having a uranium content of 1 ppb by mass or more and 20 ppb by mass or less.

8. The purity is 98.5% by mass or more, and the average particle size is 10 to 90 μm, 8. The boron nitride powder according to claim 7, wherein the agglomerated particles have a crushing strength of 5 MPa or more.

9. A resin encapsulant for a semiconductor memory element, comprising the boron nitride powder according to claim 7 or 8.

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