Boron nitride fiber, thermal grease and thermal sheet

Abstract

To provide a fibrous insulating inorganic filler having good thermal conductivity, and to provide a heat dissipating grease and a heat dissipating sheet using the filler.SOLUTION: A boron nitride fiber has an average fiber diameter of 1 to 30 μm, is solid, and has orientation in the fiber axis direction. There is provided a heat dissipating grease and a heat dissipating sheet which contain this boron nitride fiber.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to boron nitride fibers and thermally conductive greases and thermally conductive sheets containing the boron nitride fibers. [Background technology]

[0002] In heat generating elements such as power devices, transistors, thyristors, and CPUs, how to efficiently dissipate heat generated during use is an important issue. Conventionally, as a heat dissipation measure, it has been common to dissipate heat generated from the heat generating elements by conducting it to a heat dissipation component such as a heat sink. In order to efficiently conduct heat generated from the heat generating element to the heat dissipation component, it is desirable to fill the air gap at the contact interface between the heat generating element and the heat dissipation component with a heat dissipation material. As such a heat dissipation material, for example, heat dissipation grease using boron nitride as an inorganic filler is known because it is easy to handle (for example, see Patent Document 1). Boron nitride has high thermal conductivity and is an insulating material, so it is possible to increase the thermal conductivity of the heat dissipation grease and ensure the insulation of the heat dissipation grease.

[0003] When a heat dissipating grease is interposed between a heat generating element and a heat dissipating part, the heat dissipating grease may experience a pump-out phenomenon (a phenomenon in which the heat dissipating grease flows out from the mounting part). As a heat dissipating grease capable of suppressing the pump-out phenomenon, for example, a grease composition containing carbon nanotubes described in Patent Document 2 is known as a conventional technology (Patent Document 2). In the grease composition described in Patent Document 2, the entangled structure of the carbon nanotubes acts to keep the components of the grease layer within the grease layer, thereby suppressing the components of the grease layer from flowing and pumping out. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2002-194379 A [Patent Document 2] JP 2018-104651 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, although carbon nanotubes have good thermal conductivity, they also have good electrical conductivity, and therefore the insulating properties of the grease composition described in Patent Document 2 are considered to be poor. For this reason, a fibrous insulating inorganic filler having good thermal conductivity has been desired as an inorganic filler for obtaining a heat dissipating grease having good thermal conductivity, capable of suppressing the pump-out phenomenon, and having good insulating properties.

[0006] Therefore, the present invention aims to provide a fibrous insulating inorganic filler having good thermal conductivity and a thermal grease using the filler. In addition, by using a fibrous insulating inorganic filler having good thermal conductivity as the inorganic filler of a thermal dissipation sheet, a network structure of the inorganic filler that serves as a heat transfer path can be formed inside the thermal dissipation sheet. This may result in a thermal dissipation sheet with high thermal conductivity. Therefore, the present invention also aims to provide a thermal dissipation sheet using the above filler. [Means for solving the problem]

[0007] As a result of extensive research, the present inventors have discovered that the above-mentioned problems can be solved by forming boron nitride, which has been used as an inorganic filler in thermal grease, into fibers. The present invention is based on the above findings and has the following gist. [1] Boron nitride fibers have an average fiber diameter of 1 to 30 μm, are solid, and exhibit orientation in the fiber axis direction. [2] The boron nitride fiber according to the above [1], having an average fiber length of 50 to 2000 μm. [3] The boron nitride fiber according to [1] or [2] above, having an aspect ratio (average fiber length / average fiber diameter) of 10 to 1,000. [4] A boron nitride fiber according to any one of the above [1] to [3], wherein a cross section approximately perpendicular to the fiber axis direction has a concentric structure. [5] The boron nitride fiber according to any one of the above [1] to [4], wherein the porosity (area ratio) in a cross section approximately perpendicular to the fiber axis direction is 5% or less. [6] A thermal grease containing the boron nitride fiber according to any one of the above items [1] to [5]. [7] A heat dissipation sheet comprising the boron nitride fiber according to any one of the above [1] to [5]. [8] The heat dissipation sheet according to the above [7], having a thickness of 200 μm or less. [9] The heat dissipation sheet according to the above [7] or [8], wherein the content of the boron nitride fibers is 5 to 40 mass %. Effect of the Invention

[0008] According to the present invention, it is possible to provide a fibrous insulating inorganic filler having good thermal conductivity, and a thermally conductive grease and a thermally conductive sheet using the filler. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of a reaction apparatus for producing the boron nitride fibers of the examples. [Diagram 2] FIG. 2 is a micrograph of the boron nitride fiber of the embodiment. [Diagram 3] FIG. 3 shows X-ray diffraction patterns of the boron nitride fiber and boron nitride powder of the examples. [Figure 4] FIG. 4 is a micrograph of an example of a mirror-polished cross section of a boron nitride fiber of the embodiment. [Diagram 5] FIG. 5 is an SEM photograph of an example of a fracture surface of a boron nitride fiber of an example. [Figure 6] FIG. 6 is an SEM photograph of an example of a boron nitride fiber of the embodiment. [Figure 7] FIG. 7 is an SEM photograph of an example of a boron nitride fiber of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [Boron nitride fiber] The boron nitride fiber of the present invention has an average fiber diameter of 1 to 30 μm, is solid, and exhibits orientation in the fiber axis direction. By using this boron nitride fiber in a thermal grease, it is possible to obtain a thermal grease that has good thermal conductivity, can suppress the pump-out phenomenon, and has insulating properties. In addition, by using this boron nitride fiber in a thermal sheet, it is possible to obtain a thermal grease that has good thermal conductivity and insulating properties.

[0011] The average fiber diameter of the boron nitride fiber of the present invention is 1 to 30 μm. If the average fiber diameter of the boron nitride fiber is less than 1 μm, the boron nitride fiber may become difficult to handle. If the average fiber diameter of the boron nitride fiber is greater than 30 μm, the boron nitride fibers may not be sufficiently entangled when used in a thermal grease, and the pump-out phenomenon of the thermal grease may not be sufficiently suppressed. From this viewpoint, the average fiber diameter of the boron nitride fiber is preferably 1 to 20 μm, more preferably 2 to 15 μm. The average fiber diameter of the boron nitride fiber can be measured by the method described in the Examples below.

[0012] The boron nitride fiber of the present invention is solid and exhibits orientation in the fiber axis direction. This allows the thermal conductivity of the boron nitride fiber to be increased. Whether the boron nitride fiber of the present invention is solid or hollow can be determined by observing the cross section of the boron nitride fiber with an electron microscope. In addition, whether the boron nitride fiber exhibits orientation in the fiber axis direction can also be determined by observing the cross section of the boron nitride fiber with an electron microscope. Specifically, when the cross section of the boron nitride fiber that is approximately perpendicular to the fiber axis direction has a concentric structure (annular structure), it can be determined that the boron nitride fiber exhibits orientation in the fiber axis direction.

[0013] The average fiber length of the boron nitride fiber of the present invention is preferably 50 to 2000 μm. When the average fiber length of the boron nitride fiber is 50 μm or more, when the boron nitride fiber is used in a thermal grease, the boron nitride fibers can be sufficiently entangled with each other, and thus the pump-out phenomenon of the thermal grease can be sufficiently suppressed. When the average fiber length of the boron nitride fiber is 50 μm or more, when the boron nitride fiber is used in a thermal sheet, a network structure of boron nitride fibers that serves as a heat transfer path can be formed inside the thermal sheet, and thus the thermal sheet can efficiently conduct heat, and the thermal conductivity of the thermal sheet can be improved. When the average fiber length of the boron nitride fiber is 2000 μm or less, the coating property of the slurry used to prepare the thermal grease and the thermal sheet is improved. From this viewpoint, the average fiber length of the boron nitride fiber of the present invention is more preferably 100 to 900 μm, and further preferably 200 to 800 μm. The average fiber length of the boron nitride fibers can be measured by the method described in the Examples below.

[0014] The aspect ratio (average fiber length / average fiber diameter) of the boron nitride fiber of the present invention is preferably 10 to 1000. When the aspect ratio of the boron nitride fiber is 10 or more, when the boron nitride fiber is used in a thermal grease, the boron nitride fibers can be sufficiently entangled with each other, and thus the pump-out phenomenon of the thermal grease can be sufficiently suppressed. When the aspect ratio of the boron nitride fiber is 10 or more, when the boron nitride fiber is used in a thermal sheet, a network structure of boron nitride fibers that serves as a heat transfer path can be formed inside the thermal sheet, and thus the thermal sheet can conduct heat more efficiently, and the thermal conductivity of the thermal sheet can be improved. When the aspect ratio of the boron nitride fiber is 1000 or less, the coating property of the slurry used to prepare the thermal grease and the thermal sheet is improved. From this viewpoint, the aspect ratio of the boron nitride fiber of the present invention is more preferably 30 to 800, and further preferably 40 to 600. The aspect ratio of the boron nitride fibers can be measured by the method described in the Examples below.

[0015] In the boron nitride fiber of the present invention, the cross section substantially perpendicular to the fiber axis direction preferably has a concentric structure. When the cross section substantially perpendicular to the fiber axis direction of the boron nitride fiber of the present invention has a concentric structure, the boron nitride fiber exhibits sufficient orientation in the fiber axis direction. This can further improve the thermal conductivity of the boron nitride fiber in the fiber axis direction.

[0016] The porosity (area ratio) in a cross section substantially perpendicular to the fiber axis direction of the boron nitride fiber of the present invention is preferably 5% or less. If the porosity in a cross section substantially perpendicular to the fiber axis direction of the boron nitride fiber is 5% or less, the thermal conductivity of the boron nitride fiber can be further improved. From this viewpoint, the porosity in a cross section substantially perpendicular to the fiber axis direction of the boron nitride fiber of the present invention is more preferably 3% or less, further preferably 1% or less, and particularly preferably 0%. The porosity in a cross section substantially perpendicular to the fiber axis direction of the boron nitride fiber can be measured by the method described in the Examples below.

[0017] The boron nitride fiber of the present invention preferably has a branched structure. This allows the boron nitride fibers to be more easily entangled with each other. As a result, when the boron nitride fiber of the present invention is used in a thermal grease, the pump-out phenomenon of the thermal grease can be further suppressed. Furthermore, when the boron nitride fiber of the present invention is used in a thermal sheet, a more complex network structure of boron nitride fibers that serves as a heat transfer path can be formed inside the thermal sheet, and this allows the thermal sheet to conduct heat more efficiently, thereby further improving the thermal conductivity of the thermal sheet. Furthermore, since the boron nitride fiber has a branched structure, the boron nitride fibers are strongly entangled with each other, so that a sheet made only of boron nitride fibers can be produced. Then, by impregnating this sheet with a resin, a thermal sheet with a high boron nitride content can be produced.

[0018] It is preferable that the needle-like fibers protrude from the side of the boron nitride fiber of the present invention. This makes it easier for the boron nitride fibers to be entangled with each other. As a result, when the boron nitride fiber of the present invention is used in a thermal grease, the pump-out phenomenon of the thermal grease can be further suppressed. Furthermore, when the boron nitride fiber of the present invention is used in a thermal sheet, a more complex network structure of boron nitride fibers that serves as a heat transfer path can be formed inside the thermal sheet, and this allows the thermal sheet to conduct heat more efficiently, thereby further improving the thermal conductivity of the thermal sheet. Furthermore, since the needle-like fibers protrude from the side of the boron nitride fiber, the boron nitride fibers are strongly entangled with each other, so that a sheet made only of boron nitride fibers can be produced. Then, by impregnating this sheet with a resin, a thermal sheet with a high boron nitride content can be produced. Note that the branched structure is a structure in which the boron nitride fiber is branched, and the needle-like fibers are, so to speak, like thorns protruding from the side of the boron nitride fiber.

[0019] [Manufacturing method of boron nitride fiber] Boron nitride fibers can be produced, for example, by heating boric acid, and nitriding the dehydrated and vaporized boron oxide on a boron nitride substrate with a mixed gas of nitrogen and ammonia. For example, a boron nitride substrate is placed in a tubular furnace, and an alumina boat is placed thereon. Here, the purity of boron nitride in the boron nitride substrate may be, for example, 95% by mass or more, or may be 100% by mass (an embodiment in which the substrate is substantially made of boron nitride). The size of the boron nitride substrate may be appropriately set according to the size of the alumina board and the tubular furnace, etc. The thickness of the boron nitride substrate may be, for example, 5 mm or more, or may be 20 mm or less. The purity of alumina in the alumina board may be, for example, 95% by mass or more, or may be 100% by mass (an embodiment in which the container is substantially made of alumina). The tubular furnace has an inlet and an outlet for passing nitrogen gas and ammonia gas. This tubular furnace may be made of, for example, alumina. The purity of this alumina may be, for example, 95% by mass or more, or may be 100% by mass (an embodiment in which the container is substantially made of alumina). The size of the tubular furnace may be any size that allows a boron nitride substrate or the like to be placed therein. The volume of the tubular furnace may be, for example, 0.2 L or more, 1 L or more, or 5 L or more, and may be 30 L or less, 20 L or less, or 10 L or less. The cross-sectional area of ​​the tubular furnace (the area of ​​the cross section perpendicular to the flow direction of the nitrogen gas and ammonia gas) is 3 cm or less. 2 More than 15cm 2 or more than 30cm 2 More than 180cm 2 Below, 100cm 2 Less than or equal to 50cm 2 It may be the following: Then, boric acid (powdered) is placed on an alumina boat, and the boric acid is heated in a nitrogen and ammonia mixed gas flow at a heating temperature of, for example, 1450 to 1800°C for a heating time of 0.5 to 5 hours. The flow rate of the nitrogen gas is, for example, 0.5 to 5 L / min per 1 g of boric acid. The flow rate of the ammonia gas is, for example, more than 1.0 L / min and 5 L / min or less per 1 g of boric acid. The boric acid is heated, and the dehydrated and vaporized boron oxide is nitrided by a nitrogen and ammonia mixed gas. The boron nitride obtained by nitriding boron with a nitrogen and ammonia mixed gas grows on a boron nitride substrate to become boron nitride fibers. Note that there is a tendency for the branched structure to be reduced by increasing the flow rate of ammonia gas relative to the amount of boric acid.

[0020] The above-mentioned resin composition can be used, for example, as a heat dissipation material. The heat dissipation material can be produced, for example, by curing the resin composition. The method for curing the resin composition is appropriately selected depending on the type of resin (and the curing agent used as necessary) contained in the resin composition.

[0021] [Thermal grease] The thermal grease of the present invention contains the boron nitride fiber of the present invention, which makes it possible to suppress the pump-out phenomenon of the thermal grease.

[0022] The thermal grease of the present invention is, for example, a paste obtained by kneading a liquid polymer and the boron nitride fiber of the present invention. Examples of the liquid polymer include hydrocarbon oils such as polyolefins, alkyl aromatics, and alicyclic compounds, polyethers such as polyglycols and phenyl ethers, esters such as diesters and polyol esters, phosphorus compounds such as aromatic phosphate esters, silicon compounds such as silicone, halogen compounds such as fluorinated polyethers, mineral oils, fluorosilicones, acrylic resins, and urethane resins. Among these liquid polymers, silicone is preferred from the viewpoints of heat resistance, weather resistance, electrical insulation, and chemical stability.

[0023] The content of boron nitride fiber in the thermal grease of the present invention is preferably 5 to 40% by mass. When the content of boron nitride fiber is 5% by mass or more, the thermal conductivity of the thermal grease can be increased. In addition, the occurrence of the pump-out phenomenon of the thermal grease can be further suppressed. When the content of boron nitride fiber is 40% by mass or less, the coatability of the thermal grease can be improved. From this viewpoint, the content of boron nitride fiber in the thermal grease of the present invention is more preferably 10 to 35% by mass, and further preferably 15 to 30% by mass.

[0024] [Heat dissipation sheet] The heat dissipation sheet of the present invention contains the boron nitride fiber of the present invention, which allows a network structure of the boron nitride fiber to be formed inside the heat dissipation sheet as a heat transfer path, and the heat dissipation sheet can efficiently conduct heat, thereby improving the thermal conductivity of the heat dissipation sheet.

[0025] The heat dissipation sheet of the present invention can be produced, for example, by forming a slurry containing a resin component and boron nitride fiber into a sheet by a doctor blade method. Examples of the resin of the resin component include epoxy resin, silicone resin (including silicone rubber), acrylic resin, phenolic resin, melamine resin, urea resin, unsaturated polyester, fluororesin, polyamide (e.g., polyimide, polyamideimide, polyetherimide, etc.), polyester (e.g., polybutylene terephthalate, polyethylene terephthalate, etc.), polyphenylene ether, polyphenylene sulfide, wholly aromatic polyester, polysulfone, liquid crystal polymer, polyethersulfone, polycarbonate, maleimide-modified resin, ABS resin, AAS (acrylonitrile-acrylic rubber-styrene) resin, AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resin, etc. Among these, silicone resin is preferred from the viewpoints of heat resistance, flexibility, and adhesion to heat sinks, etc.

[0026] The content of boron nitride fiber in the heat dissipation sheet is preferably 5 to 40% by mass. When the content of boron nitride fiber is 5% by mass or more, the thermal conductivity of the heat dissipation sheet can be further increased. When the content of boron nitride fiber in the heat dissipation sheet is 40% by mass or less, the coating properties of the slurry used to prepare the heat dissipation sheet are good, making it easier to manufacture the heat dissipation sheet. From this perspective, the content of boron nitride fiber in the heat dissipation sheet is more preferably 10 to 35% by mass, and even more preferably 15 to 30% by mass.

[0027] The thickness of the heat dissipation sheet of the present invention is preferably 200 μm or less. When the thickness of the heat dissipation sheet is 200 μm or less, the heat dissipation property of the heat dissipation sheet can be further improved. In addition, since the heat dissipation sheet of the present invention uses boron nitride fiber as an inorganic filler, it is easy to make the thickness of the heat dissipation sheet 200 μm by orienting the boron nitride fiber in the in-plane direction. From this viewpoint, the thickness of the heat dissipation sheet is more preferably 180 μm or less, and even more preferably 150 μm or less. In addition, from the viewpoint of the strength of the heat dissipation sheet, the thickness of the heat dissipation sheet of the present invention is preferably 80 μm or more, more preferably 100 μm or more, and even more preferably 120 μm or more.

[0028] In order to improve the strength of the heat dissipation sheet of the present invention, the heat dissipation sheet of the present invention may have an intermediate reinforcing layer. Examples of the intermediate reinforcing layer include paper, cloth, film, nonwoven fabric, and metal foil. Among these, cloth is preferred from the viewpoint of suppressing the inhibition of heat conduction by the intermediate reinforcing layer by providing an opening portion, glass cloth and polyamide-imide fiber cloth are more preferred, and glass cloth is even more preferred. In addition, a film is preferred from the viewpoint of ensuring high insulation even when the heat dissipation sheet is thin, and a polyimide film is more preferred from the viewpoint of heat resistance and thermal conductivity. EXAMPLES

[0029] The present invention will be described in detail below with reference to examples, although the present invention is not limited to the following examples.

[0030] [Production of Boron Nitride Fibers] Place the boron nitride substrate 2 inside the alumina furnace tube 5 of the tubular furnace 4 shown in Fig. 1. Arrange two alumina boats 1 on the boron nitride substrate 2 in the direction in which the mixed gas described later flows, with the longitudinal direction of the alumina boat 1 parallel to the direction in which the mixed gas described later flows. Place a boron nitride substrate 3 on each alumina boat 1 so as to block a part of the opening of the alumina boat. Place 2 g of boric acid on the upstream alumina boat with respect to the flow of the mixed gas described later, and place 1 g of boric acid on the downstream alumina boat 1. Supply nitrogen gas from the nitrogen gas cylinder 8 at a flow rate of 2 L / min and ammonia gas from the ammonia gas cylinder 9 at a flow rate of 4 L / min to the gas mixer 10 to produce a mixed gas of nitrogen gas and ammonia gas, and supply it into the alumina furnace tube 5. Heat the heating element 6 to heat the alumina boat 1 at a heating temperature of 1600 °C and a heating time of 1 hour. Generate boron nitride fibers on the boron nitride substrate near the upstream alumina boat 1 to obtain the boron nitride fibers of the example. A micrograph of the boron nitride fibers of the example formed on the boron nitride substrate is shown in Fig. 2.

[0031] [Average Fiber Diameter, Average Fiber Length, and Aspect Ratio (Average Fiber Length / Average Fiber Diameter) of Boron Nitride Fibers] Sprinkle the obtained boron nitride fibers of the example on the sample stage of a digital microscope (trade name "VHX-7000", manufactured by Keyence Corporation). Then, using the digital microscope, measure the fiber diameter and fiber length of 50 boron nitride fibers, and take their average values as the average fiber diameter and average fiber length of the boron nitride fibers of the example. Then, divide the average fiber length by the average fiber diameter to calculate the aspect ratio (average fiber length / average fiber diameter) of the boron nitride fibers of the example. As a result, the average fiber diameter of the example was 10 μm, the average fiber length was 481 μm, and the aspect ratio was 48.

[0032] [Elements Contained in Boron Nitride Fibers] The elements contained in the boron nitride fiber of the example were examined using a scanning electron microscope (SEM-EDS) equipped with an EDS (energy dispersive X-ray analyzer) (product name "JSM-7001F", manufactured by JEOL Ltd.) The results showed that the main elements contained in the boron nitride fiber of the example were boron and nitrogen.

[0033] [X-ray powder diffraction analysis of boron nitride fiber] An X-ray powder diffraction analysis of the boron nitride fiber of the example was performed using an X-ray diffractometer (product name "Ultima IV", manufactured by Rigaku Corporation). For reference, a powder X-ray diffraction analysis of boron nitride powder (product name "Denka Boron Nitride GP", manufactured by Denka Co., Ltd.) was also performed. The X-ray diffraction patterns of the boron nitride fiber and boron nitride powder of the example are shown in FIG. 3. The main peak of the X-ray diffraction pattern of the boron nitride fiber of the example coincided with the main peak of the X-ray diffraction pattern of the boron nitride powder.

[0034] [Porosity (area ratio) in the cross section of boron nitride fiber] The mixture obtained by mixing the boron nitride fiber of the embodiment with the epoxy resin was cured at 25°C for 12 hours. Thereafter, any cross section including the cross section of the boron nitride fiber of the embodiment filled in the resin was mirror-polished. Then, a scanning electron microscope (SEM) (trade name "JSM-7001F", manufactured by JEOL Ltd.) was used to photograph the mirror-polished cross section of the boron nitride fiber of the embodiment. Using image analysis software (trade name "Mac-View", manufactured by Mountec Co., Ltd.), the photographed image was binarized so that the solid part and the void part could be extracted in the cross section of the boron nitride fiber of the photographed image, and the porosity (area ratio) in the cross section of the boron nitride fiber was measured. Then, the porosity (area ratio) in the cross section of 10 boron nitride fibers was measured, and the average value was taken as the porosity (area ratio) in the cross section of the boron nitride fiber of the embodiment. A micrograph of an example of a mirror-polished cross section of the boron nitride fiber of the embodiment is shown in FIG. 4. As a result, it was found that the boron nitride fibers of the examples were solid, and the porosity (area ratio) in the cross section of the boron nitride fibers of the examples was 5% or less.

[0035] [Cross-sectional structure of boron nitride fiber] A scanning electron microscope (SEM) (product name "JSM-7001F", manufactured by JEOL Ltd.) was used to observe the fracture surface of the boron nitride fiber of the Example, and the structure of the cross section substantially perpendicular to the fiber axis direction of the boron nitride fiber was examined. FIG. 5 shows an SEM photograph of an example of the fracture surface of the boron nitride fiber of the Example. As a result, it was found that the cross section substantially perpendicular to the fiber axis direction of the boron nitride fiber of the Example has a concentric structure. And, since the cross section of the boron nitride fiber of the Example has a concentric structure, it was found that the boron nitride fiber of the Example exhibits orientation in the fiber axis direction. And, since the boron nitride fiber of the Example exhibits orientation in the fiber axis direction, and the boron nitride fiber of the Example is solid, it is found that the boron nitride fiber of the Example has good thermal conductivity.

[0036] [Structure of boron nitride fiber] The boron nitride fiber of the Example was observed using a scanning electron microscope (SEM) (product name "JSM-7001F", manufactured by JEOL Ltd.) to examine the structure of the boron nitride fiber. SEM photographs of an example of the boron nitride fiber of the Example are shown in Figs. 6 and 7. As a result, as shown in the SEM photograph of Fig. 6, it was found that the boron nitride fiber of the Example has a branched structure. In addition, as shown in the SEM photograph of Fig. 7, it was found that needle-like fibers protrude from the side of the boron nitride fiber of the Example. [Explanation of symbols]

[0037] 1 Alumina boat 2,3 Boron nitride substrate 4 tube furnace 5 Alumina furnace tube 6 Heating element 7 Thermocouples 8,9 Gas Cylinders 10 Gas Mixer

Claims

1. The average fiber diameter is 1 to 30 μm; It is solid and It shows the orientation in the fiber axis direction. A boron nitride fiber with a fracture surface roughly perpendicular to the fiber axis direction, as examined using a scanning electron microscope, that has a concentric structure.

2. 2. The boron nitride fiber according to claim 1, having an average fiber length of 50 to 2000 μm.

3. 3. The boron nitride fiber according to claim 1 or 2, having an aspect ratio (average fiber length / average fiber diameter) of 10 to 1,000.

4. 4. The boron nitride fiber according to claim 1, wherein the porosity (area ratio) in a cross section approximately perpendicular to the fiber axis direction is 5% or less.

5. A thermal grease comprising the boron nitride fiber according to any one of claims 1 to 4.

6. A heat dissipation sheet comprising the boron nitride fiber according to any one of claims 1 to 4.

7. The heat dissipation sheet according to claim 6, having a thickness of 200 μm or less.

8. The heat dissipation sheet according to claim 6 or 7, wherein the content of the boron nitride fibers is 5 to 40 mass %.

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