WO2023089452A1

WO2023089452A1 - Spherical boron nitride particles having low surface roughness

Info

Publication number: WO2023089452A1
Application number: PCT/IB2022/060753
Authority: WO
Inventors: Toshihiro Kasai
Original assignee: 3M Innovative Properties Company
Priority date: 2021-11-22
Filing date: 2022-11-08
Publication date: 2023-05-25

Abstract

A composition comprising spherical hexagonal boron nitride particles having an average particle diameter (D50) in the range of 0.1 to 500 μm, a surface roughness characterized by an arithmetical mean deviation (ISO 4287:1997) Ra less than 10 nm, and a Brunauer-Emmett-Teller (BET) specific surface area less than 10 m2/gram. A method of making the composition. A thermally conductive composite comprising the composition and a resin.

Description

SPHERICAL BORON NITRIDE PARTICLES HAVING LOW SURFACE ROUGHNESS

FIELD OF INVENTION

The present invention relates to spherical hexagonal boron nitride particles having low surface roughness, a method of production thereof, and a thermally conductive composite comprising spherical hexagonal boron nitride particles.

BACKGROUND

Boron nitride has found diverse uses as a refractory material, heat conductor, lubricant and electrical insulator over the course of the last several decades. Developmental efforts in recent years have focused on the use of crystalline, hexagonal boron nitride (hBN) as a thermal interface material. hBN is a good heat conductor and electrical insulator, making it suitable for use in applications where heat generation occurs in small, confined spaces, such as in power semiconductors, high speed multicore chipsets, and electric vehicle batteries, where rapid heat dissipation is required.

Heat dissipation capabilities of hBN thermal compounds are associated with the morphological characteristics of individual hBN particles, which are directly influenced by the method of synthesis and conditions thereof. A well-established method of hBN synthesis involves nitriding boric acid with ammonia, which produces an amorphous boron nitride powder. Subsequent annealing of the powder at elevated temperatures forms crystalline hBN. This method yields hBN particles having an irregularly-shaped platelet structure and rheological properties characterized by poor flowability owing to relatively large frictional forces generated when the irregularly-shaped particles are moved over each other in a polymer resin. When hBN platelets are added directly as a filler to a thermal compound, the viscosity of the material increases significantly, making it unsuitable for syringe dispensing and spreading over small areas. To avoid an overly viscous composition, hBN loading is limited to low levels such that viscosity is not adversely affected, thereby limiting thermal conductivity.

In order to overcome the disadvantages of the platelet morphology, new synthesis methods have been developed to produce particles with improved morphology. US Patent No. 6,348,179 describes a production process involving aerosol assisted vapor synthesis, in which a solution of a boron compound is aerosolized and charged into a heat source, along with a gaseous nitriding agent, to form spherical boron nitride particles containing oxygen impurities. Subsequent calcining is carried out to eliminate oxygen in the form of water and volatile oxygen-containing species. US Patent No. 9,371,449 describes a powder manufacturing technique employed to agglomerate irregularly-shaped hBN powder particles into spheroidal or spherical agglomerates. hBN platelets are agglomerated with an inorganic binding material comprising a nitride and/or oxynitride of aluminum, silicon, or titanium, and then annealed in the presence of a nitriding source, to form agglomerated granules.

W02003013845 describes a production process which involves spray-drying a hBN slurry produced from hBN powder treated with a surfactant to enable high loading of hBN in the slurry. The spray drying process produces a powder with high hBN content. The powder is subsequently sintered in the presence of a nitriding source to form spherical agglomerates.

SUMMARY

Despite having an improved geometry, agglomerates formed by the above methods typically present high surface roughness. For example, the hBN agglomerates formed by compacting hBN flakes are characterized by the formation of platelets protruding from the surface of the agglomerate, presenting similar flowability issues. It is desirable that hBN particles have low surface roughness, as well as a spherical or spheroidal geometry, and high mechanical crush strength. It is also desirable that an efficient synthesis route be developed to produce such hBN particles with high yield.

The present disclosure addresses the object of providing spherical boron nitride particles having low surface roughness. Such particles can be used for various applications, such as a filler for thermal composites, where its low surface roughness and sphericity enables high loading levels of boron nitride material into polymers. The present disclosure furthermore addresses the object of providing a simple, cost effective method for producing these boron nitride particles having low surface roughness.

The present disclosure provides a composition comprising spherical hexagonal boron nitride particles having an average particle diameter (D50) in the range of 0.1 to 500 pm, a surface roughness characterized by an arithmetical mean deviation (ISO 4287: 1997) R_a less than 10 nm, and a Brunauer-Emmett-Teller (BET) specific surface area less than 10 m²/gram.

In some embodiments, the average particle diameter (D50) ranges from 2 to 30 pm.

In some embodiments, the particles have a Brunauer-Emmett-Teller (BET) specific surface area ranging from 0.5 to 3.0 m²/gram.

In some embodiments, the composition has an interparticle void volume ranging from 0.1 to 0.8 cm³/g, as measured by mercury intrusion porosimetry.

In some embodiments, the particles have a sphericity from 0.9 to 1.0. The present disclosure also provides a composite comprising a resin and the above composition.

In some embodiments, the resin is an epoxy resin.

In some embodiments, the amount of spherical hexagonal boron nitride particles present in the composite is at least 7.5 parts by weight per 2.7 parts by weight of the resin.

In some embodiments, the composite further comprises a cross-linking agent comprising dicyandiamide.

In some embodiments, the composite further comprises a curing accelerator comprising aromatic substituted urea.

The present disclosure further provides a method of making spherical hexagonal boron nitride particles comprising providing an aqueous precursor solution comprising a water- soluble boron compound and a water-soluble nitrogen-containing organic compound, spraydrying the precursor solution to form a boron nitride powder, subjecting the boron nitride powder to a first heat treatment at a temperature of at least 800°C in a nitriding atmosphere, and subjecting the boron nitride powder to a second heat treatment at a temperature of at least 1800°C in an inert atmosphere.

In some embodiments, spray-drying of the precursor solution is carried out with heated air at a temperature ranging from 180°C to 260°C.

In some embodiments, the nitrogen-containing organic compound has a boiling point above the heated air temperature.

In some embodiments, the spray -drying comprises atomizing or ultrasonic humidification of the precursor solution.

In some embodiments, the second heat treatment comprises a controlled increase in temperature at a rate ranging from 10°C to 20°C per minute until a temperature of 1800°C or more is reached.

In some embodiments, the boron nitride powder is heated on a flat plate during the second heat treatment.

In some embodiments, the water-soluble boron compound comprises boric acid.

In some embodiments, the water-soluble, nitrogen-containing organic compound comprises a nitrogen heterocycle.

In some embodiments, the nitrogen heterocycle is selected from the group consisting of triazole, thiazole, and imidazole. In some embodiments, the nitriding atmosphere comprises a gas mixture of ammonia and nitrogen gas.

The above summary of the present disclosure is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The description that follows more particularly exemplifies illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and IB are scanning electron microscope (SEM) images of spherical hexagonal boron nitride particles produced in Example 1 ;

FIGS. 2A and 2B are SEM images of spherical hexagonal boron nitride particles produced in Example 2;

FIGS. 3A and 3B are SEM images of spherical hexagonal boron nitride particles produced in Example 3;

FIGS. 4A and 4B are SEM images of boron nitride particles produced in Comparative Example 1; and

FIGS. 5 A and 5B are SEM images of boron nitride particles produced in Comparative Example 2.

DETAILED DESCRIPTION

In the following description of illustrative embodiments, reference is made to the accompanying figures of the drawing which form a part hereof, and in which are shown, by way of illustration, specific embodiments. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

As used herein:

The term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the description and claims. Such terms will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements. By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of.” Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present. By “consisting essentially of’ is meant including any elements listed after the phrase, and limited to other elements that do not interfere with or contribute to the activity or action specified in the disclosure for the listed elements. Thus, the phrase “consisting essentially of’ indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present depending upon whether or not they materially affect the activity or action of the listed elements.

The terms “a,” “an,” and “the” are used interchangeably with “at least one” to mean one or more of the components being described.

The term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements.

The term “some embodiments” means that a particular feature, configuration, composition, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of such phrases in various places throughout this specification are not necessarily referring to the same embodiment of the disclosure. Furthermore, the particular features, configurations, compositions, or characteristics may be combined in any suitable manner in one or more embodiments.

The terms “preferred” and “preferably” refer to embodiments of the disclosure that may afford certain benefits, under certain circumstances; however, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the disclosure.

All numbers are assumed to be modified by the term “about”. As used herein in connection with a measured quantity, the term “about” refers to that variation in the measured quantity as would be expected by the skilled artisan making the measurement and exercising a level of care commensurate with the objective of the measurement and the precision of the measuring equipment used.

The recitations of numerical ranges by endpoints include all numbers subsumed within that range as well as the endpoints (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). The phrase “up to” a number (e.g., up to 50) includes the number (e.g., 50).

Hexagonal boron nitride (hBN) particles of the present disclosure are spherical in shape and possess a high degree of surface smoothness that stands in sharp contrast to platelet agglomerates, which are typically presented with platelets protruding from the surface of the agglomerate across the entire particle, resulting in high surface roughness even if the particles have a generally spherical geometry. Spherical particles with low surface roughness can provide excellent fluidity and low viscosity in thermal materials, making it ideal for use as a filler for thermal management applications, e.g., in composites, polymers, and fluids, as described below. By minimizing its impact on viscosity, higher loading levels of hBN particles can be achieved in thermal interface materials while maintaining appropriate viscosity characteristics.

The process which has enabled the present production of spherical hBN particles with low surface roughness involves the use of an aqueous precursor solution that comprises a water-soluble boron compound (e.g., boric acid) and a nitrogen-containing organic compound (e.g., a nitrogen heterocycle). In some embodiments, the nitrogen-containing organic compound is triazole, thiazole or imidazole, more particularly thiazole. The inclusion of a nitrogen-containing organic compound, such as a nitrogen heterocycle, into the precursor hBN particles prior to heat treatment influences the surface morphology of the particles. It is thought that the presence of nitrogen-containing organic compound in the solid hBN precursor particles, as opposed to using the more conventional gaseous source of nitrogen, enables nitridation to take place much more uniformly across the particle surface during the first heat treatment, thereby producing a smooth particle surface.

Spherical hexagonal boron nitride particles according to the present disclosure comprise an average particle diameter (D50) in the range of 0.1 to 500 pm, a surface roughness characterized by an arithmetical mean deviation (ISO 4287: 1997) Ra less than 10 nm, and a Brunauer-Emmett-Teller (BET) specific surface area less than 10 m²/gram. In some embodiments, the average particle diameter (D50) is less than 50 pm, more preferably from 2 to 30 pm. The average particle diameter (D50) refers to the median size of a sample. In other words, in any given sample, the average particle diameter (D50) is the diameter at which there is 50% of particles with diameters smaller than the D50 value, and there is 50% of particles with diameters larger than the D50 value.

The arithmetical mean deviation, Ra, refers to the arithmetical mean of the absolute values of the profile deviations (Zi) from the mean line of the roughness profile. In the present embodiment, it is preferably less than 8 nm, or less than 6 nm, or less than 5 nm, or most preferably, less than 1 nm. The surface roughness as measured by ISO 4287 refers to the Geometrical Product Specifications (GPS) measurement method presented in the ISO 4287 standard established by the International Organization for Standardization.

In some embodiments, particularly those in which the spherical hBN particles are used in thermally conductive composites, it is preferable that the spherical hBN particles have a low specific surface area. This can be achieved if the particle does not have a porous surface. A highly porous surface presents diminished thermal conductivity across the particle, due to the presence of thermally insulating air spaces in the pores of the particles. Accordingly, in some embodiments, the hBN particles have a Brunauer-Emmett-Teller (BET) specific surface area of less than 10 m²/gram, or more preferably from 0.5 to 3.0 m²/gram. In particle morphology studies, the specific surface area can be estimated by the BET equation to yield an estimate of the surface area. Porous particles with high surface areas are typically in the range of several hundred to several thousand m²/gram, while particles with low porosity have specific surface areas of less than 100 m²/gram, or more preferably less than 10 m²/gram.

Spherical particles provide better close packing compared to particles with an irregular shape. Irregularly shaped particles are poorly packed, which gives rise to substantial void volume between particles. Similar to porosity, a high void volume results in air pockets which diminishes the thermal conductivity of the hBN particles. In an ideal cubic closest packed structure, the void volume of a hBN powder is at a numerical minimum. Void volume may be determined by mercury intrusion porosimetry. A porosimeter employs a pressurized chamber to force mercury to intrude into the voids in a porous substrate. As pressure is applied, mercury fills the larger pores first. As pressure increases, the filling proceeds to smaller and smaller pores. Both the inter-particle pores (between the individual particles) and the intra-p article pores (within the particle itself) can be characterized using this technique. In some embodiments, the interparticle void volume of the spherical hBN particles is from 0.1 to 0.8 cm³/g, as measured by mercury intrusion porosimetry.

In some embodiments, the hBN particles have a sphericity from 0.9 to 1.0, 0.95 to 1.0, 0.98 to 1.0, and 0.99 to 1.0. Sphericity is a measure of how closely the shape of an object resembles that of a perfect sphere. Sphericity can be defined as the ratio of the surface area of an equal-volume sphere to the actual surface area of the particle:

where V_P is the volume of the particle and A_p ss the surface area of the particle. A sphericity below 1 indicates a more ellipsoidal shape, which is still usable even though it may be less preferable compared to particles with sphericity of 1. For purposes of clarity, the term “spherical”, when used herein to describe hBN particles, means a particle having a sphericity from 0.9 to 1.0.

The spherical hBN particles may be incorporated into thermally conductive composites for use in a variety of applications, including thermal interface materials, such as heat sink compounds for electronic components and semiconductor components in digital and analogue devices. The thermally conductive composites typically comprise a matrix material and the spherical hBN particles disclosed herein. Exemplary matrix materials include thermosetting resins (e.g., polyamide resin, polyurethane resin, epoxy resin, silicone resin, polydicyclopentadiene, and poly(meth)acrylate) and thermoplastic resins (e.g., polyethylene, polypropylene, and poly(styrene-butadiene-styrene)). In some embodiments, the composite comprises 7.5 parts by weight of spherical hBN particles for every 2.7 parts by weight of resin. In some embodiments, the composite comprises a thermosetting resin, more particularly an epoxy resin. The composites may further include additives, such as cross-linking agents (e.g., dicyandiamide), curing accelerators (e.g., aromatic substituted urea), dispersing aids (e.g., ionic or nonionic surfactants), compatibilizing surface treatments (e.g., silane, aluminate, titanate, or zirconate coupling agents), or combinations thereof. In some embodiments, the composite further includes a cross-linking agent comprising dicyandiamide. In the same or different embodiments, the composite further includes a curing accelerator comprising aromatic substituted urea.

Another aspect of the present invention is directed to methods for making spherical hexagonal boron nitride particles. This method involves first forming a precursor solution of a water- soluble, boron compound (e.g., boric acid) and a water-soluble, nitrogen-containing organic compound. Subsequently, the precursor solution is spray dried to form boron nitride powder, which is in amorphous form. Heating is then carried out to convert the boron nitride powder into spherical hBN particles. The heating comprises a first heat treatment at a temperature of at least 800°C in a nitriding atmosphere, and a second heat treatment at a temperature of at least 1800°C, or preferably at least 2000°C, in an inert atmosphere, such as in nitrogen (N2) gas. In some embodiments, the nitriding atmosphere comprises a gas mixture of ammonia and nitrogen gas.

In some embodiments, the spray-drying of the precursor solution is carried out with heated air at a temperature ranging from 180°C to 260°C. Spray drying may be carried out with atomizing or ultrasonic humidification of the precursor solution. In order that the nitrogen-containing organic compound not boil or vaporize, the nitrogen containing compound preferably has a boiling point above the heated air temperature.

After spray drying is complete, a first heat treatment in a nitriding atmosphere is carried out. In some embodiments, the nitriding atmosphere comprises a mixture of ammonia and nitrogen gases. The nitrogen rich environment causes a reaction with the boric acid in which the boron atom bonds with the abundant nitrogen atoms, resulting in the formation of spherical hBN particles. A second heat treatment is carried out to remove impurities in the hBN particles. The second heat treatment preferably comprises a controlled increase in temperature at a rate ranging from 10°C to 20°C per minute, until a temperature of 1800°C or more is reached. In some embodiments, the second heating step is carried out by placing the precursor powder of boric acid and organic nitrogen compound on a flat graphite or BN plate.

In some embodiments, the water-soluble, nitrogen-containing organic compound comprises a nitrogen heterocycle. Preferably, the nitrogen heterocycle is selected from the group consisting of triazole, thiazole, and imidazole. In some preferable embodiments, the nitrogen heterocycle is thiazole, which has a boiling point above the spray drying temperature and contains a high number of nitrogen atoms per molecule that facilitate the nitriding reaction during the first heat treatment.

EXAMPLES

Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims.

The following abbreviations are used throughout: min = minutes; mL = milliliter; g = gram; nm = nanometer; pm = micrometer or micron; cm = centimeter; m = meter; N = newton; kHz = kilohertz; and °C = degrees Celsius.

Table 1. Materials Used in the Examples.

Example 1 (EX-1): Synthesis of Spherical hBN Particles

A precursor solution was made by sequentially adding the following to a 450 mL glass bottle: Boric acid (150 g); 3 -amino- 1,2, 4- triazole (100 g); and water (50 g). A polypropylene container was placed over the mouth of the glass bottle, where the diameter of the opening of the polypropylene container was slightly larger than the mouth of the glass bottle. The glass bottle (with cover) was placed in a microwave (JM-17B Haier Co.) and heated at 700W for about 6 minutes. The weight of the solution in the glass bottle was measured before and after heating. Any decrease in weight after heating was replenished to the pre-heated value by the addition of water.

The precursor solution was then spray dried using a spray dryer (DA2SW-16, Sakamoto Giken Co.) with gas atomizer nozzle, set at an inlet temperature of about 250°C. The resultant powder was further dried at 380°C in air.

The dried powder was then heated in a nitriding gas atmosphere of NH3/N2 (50%) at 980°C to produce a boron nitride powder.

The boron nitride powder was then place on a BN plate (without cover) and heat treated a second time at 2000°C for 1 hour in a N₂ atmosphere using an ultra high temperature furnace (SUN-VAC-13, Tokyo Vacuum Co.). More specifically, the second heat treatment involved heating the powder from room temperature to 2000°C at 15°C/min increments and holding at 2000°C for 1 hour. The powder was then cooled from 2000°C to 800°C in 15°C /min increments and further cooled to room temperature in furnace. The second heat treatment was carried out in N2 gas provided at a flow rate of 6 to 7 liters/min.

The resultant spherical hBN particles were characterized by X-ray diffractometry. Scanning electron microscopy (SEM) images of the hBN particles are provided in FIGS. 1A & IB. The diameter of the spherical hBN particles ranged from 2-30 pm in diameter, as determined from SEM images. The Brunauer-Emmett-Teller (BET) specific surface area was calculated to be 0.87 m²/g.

Example 2 (EX-2): Synthesis of Spherical hBN Particles

The synthesis of EX-2 followed the procedure set forth in EX-1, except that the maximum temperature for the second heat treatment was 2150°C instead of 2000°C. SEM images of the spherical hBN particles are provided in FIGS. 2A & 2B. The BET specific surface area was calculated to be 0.74 m²/g.

Example 3 (EX-3): Synthesis of Spherical hBN Particles

The synthesis of EX-3 followed the procedure set forth in EX-1, except that the boron nitride powder was placed on a BN plate (without cover) and heated to a maximum temperature of 2000°C and then placed in a BN container (with cover) for another heat treatment and heated to a maximum temperature of 2300°C. SEM images of the spherical hBN particles are provided in FIGS. 3A & 3B. The BET specific surface area was calculated to be 2.8 m²/g.

Comparative Example 1 (CE-1): Synthesis of Boron Nitride Particles

The synthesis for CE-1 followed the procedure set forth in EX-1, except that after the first heat treatment, the sample was placed inside a BN container (with cover) and the second heat treatment carried out under N2 atmosphere at a maximum temperature of 1850°C instead of 2000°C. SEM images of the synthesized particles are provided in FIGS. 4A & 4B. The SEM images revealed platelet crystals on the surface of the boron nitride particles. The BET specific surface area was calculated to be 22 m²/g.

Comparative Example 2 (CE-2): Synthesis of Boron Nitride Particles

The synthesis for CE-2 followed the procedure set forth in CE-1, except that during the second heat treatment, the samples were heated on a BN plate (without cover) in N2 atmosphere. SEM images of the synthesized particles are provided in FIGS. 5A & 5B. The SEM images show that the synthesized particles have a smooth surface. The BET specific surface area was calculated to be 270 m²/g, which indicates that the particles synthesized had high porosity.

Comparative Examples 3 through 5 (CE-3 through CE-5): Synthesis of Boron Nitride Particles

The synthesis of CE-3 and CE-4 followed the procedure set forth in EX-2, except samples were finally heated at 2000°C (CE-3) and 2150°C (CE-4), respectively, and boron nitride powder was heated in a BN container with lid during the second heat treatment.

The synthesis of CE-5 followed the procedure set forth in EX-2, except that the sample was heated on a BN plate during the first heat treatment and heated in a BN container with lid to a maximum temperature of 2300°C during the second heat treatment.

The synthesized CE-3 through CE-5 had boron nitride crystals (platelets) formed on the particle surface.

Examples 4 (EX4) and 5 (EX-5) and Comparative Example 6 (CE-6): Samples Annealed in a BN Container With and Without a Graphite Flat Plate

The synthesis of EX-4 followed the procedure set forth in EX-1, except that the sample was placed on a graphite plate inside a BN container during annealing.

The synthesis of EX-5 followed the procedure set forth in EX-2, except that the sample was placed on a graphite plate inside a BN container during annealing.

The synthesis of CE-6 followed the procedure set forth in CE-2, except that the sample was placed on a graphite plate inside a BN container during annealing.

Table 2 (below) compares six different samples in which a first heat treatment step was carried out either i) inside a BN container, or ii) on a graphite flat plate inside a BN container, at varying temperatures from 1850°C to 2300°C. Referring to Table 2 below, it can be seen that EX-4 and EX-5 have low specific surface area, and a very small volume of micropores and mesopores in the particles. In CE-6, the annealing temperature of 1850°C produced a highly porous surface having an area of 270 m²/g, which is usable for gas adsorption applications, but is not ideal as a thermally conductive filler. Table 2: Surface Area, Micro-Pore Volume and Meso-Pore Volume

Surface roughness measurements were taken multiple times per sample by atomic force microscopy. The equipment used was model Cypher S from Oxford Instruments Co. Probe: AC160TS (k=26 N/m, f=311 kHz, tip radius: 7 nm). Imaging size was 5x5 pm² and 1x1 pm² (Samples CE-6, EX-4, EX-6). The results are summarized in Table 3.

Table 3. R_a Surface Roughness (nm)

Referring to Table 3, it can be seen that EX-4 and EX-6, which correspond to embodiments of the invention, have low surface roughness values (Ra) - arithmetic mean deviation - of less than 5 nm, specifically between 0.5 nm to 2.8 nm. Although CE-6 also exhibits a low surface roughness value, as shown in Table 2, its surface area is relatively high, exhibiting a porous material that is less favorable for use in thermally conductive applications.

Thus, the present disclosure provides, among other things, spherical hBN particles, methods of making the spherical hBN particles, and thermally conductive hBN composites. Various features and advantages of the present disclosure are set forth in the following claims.

Claims

CLAIMS What is claimed is:

1. A composition comprising spherical boron nitride particles having an average particle diameter (D50) in the range of 0.1 to 500 pm, a surface roughness characterized by an arithmetical mean deviation (ISO 4287: 1997) Ra less than 10 nm, and a Brunauer-Emmett- Teller (BET) specific surface area less than 10 m²/gram.

2. The composition of claim 1, wherein the average particle diameter (D50) ranges from 2 to 30 pm.

3. The composition of claim 1 or claim 2, wherein the particles have a Brunauer- Emmett-Teller (BET) specific surface area ranging from 0.5 to 3.0 m²/gram.

4. The composition of any one of claims 1 to 3, wherein the composition has an interparticle void volume ranging from 0.1 to 0.8 cm³/g, as measured by mercury intrusion porosimetry.

5. The composition of any one of claims 1 to 4, wherein the particles have a sphericity from 0.9 to 1.0.

6. A composite comprising: a resin; and the composition of any one of claims 1 to 5.

7. The composite of claim 6, wherein the resin is an epoxy resin.

8. The composite of claim 6 or claim 7, wherein the amount of spherical hexagonal boron nitride particles present in the composite is at least 7.5 parts by weight per 2.7 parts by weight of the resin.

9. The composite of any one of claims 6 to 8, further comprising a cross-linking agent comprising dicyandiamide.

10. The composite of any one of claims 6 to 9, further comprising a curing accelerator comprising aromatic substituted urea.

11. A method for producing spherical hexagonal boron nitride particles, comprising providing an aqueous precursor solution comprising a water-soluble boron compound and a water-soluble nitrogen-containing organic compound, spray-drying the precursor solution to form a boron nitride powder, subjecting the boron nitride powder to a first heat treatment at a temperature of at least 800°C in a nitriding atmosphere, and subjecting the boron nitride powder to a second heat treatment at a temperature of at least 1800°C in an inert atmosphere.

12. The method of claim 11, wherein spray-drying of the precursor solution is carried out with heated air at a temperature ranging from 180°C to 260°C.

13. The method of claim 11 or claim 12, wherein the nitrogen-containing organic compound has a boiling point above the heated air temperature.

14. The method of any one of claims 11 to 13, wherein the spray-drying comprises atomizing or ultrasonic humidification of the precursor solution.

15. The method of claims 11 to 14, wherein the second heat treatment comprises a controlled increase in temperature at a rate ranging from 10°C to 20°C per minute until a temperature of 1800°C or more is reached.

16. The method of any one of claims 11 to 15, wherein the boron nitride powder is heated on a flat plate during the second heat treatment.

17. The method of claims 11 to 16, wherein the water-soluble boron compound comprises boric acid.

18. The method of any one of claims 11 to 17, wherein the water-soluble, nitrogen-containing organic compound comprises a nitrogen heterocycle.

19. The method of claim 18, wherein the nitrogen heterocycle is selected from the group consisting of triazole, thiazole, and imidazole.

20. The method of any one of claims 11 to 19, wherein the nitriding atmosphere comprises a gas mixture of ammonia and nitrogen gas.