CN115626829B

CN115626829B - Boron nitride fiber and preparation method thereof

Info

Publication number: CN115626829B
Application number: CN202211564168.6A
Authority: CN
Inventors: 齐学礼; 徐浩南; 李茹; 丁伟宸; 王重海; 吕锋; 王玉娇; 陈勇
Original assignee: Shandong Industrial Ceramics Research and Design Institute Co Ltd
Current assignee: Shandong Industrial Ceramics Research and Design Institute Co Ltd
Priority date: 2022-12-07
Filing date: 2022-12-07
Publication date: 2023-03-21
Anticipated expiration: 2042-12-07
Also published as: CN115626829A

Abstract

The invention belongs to the technical field of ceramic fibers, and discloses a boron nitride fiber and a preparation method thereof. The preparation method of the boron nitride fiber comprises the following steps: carrying out melt drawing and heat treatment on the boron nitride precursor to obtain boron nitride fibers; the heat treatment comprises a first temperature rise stage, a second temperature rise stage and a third temperature rise stage which are sequentially carried out, wherein the temperature is raised to a first temperature in the first temperature rise stage, the temperature is raised to a second temperature in the second temperature rise stage, and the temperature is raised to a third temperature in the third temperature rise stage; wherein the first temperature is controlled to be 600 to 800 ℃, the second temperature is controlled to be 1000 to 1400 ℃, and the third temperature is controlled to be 1600 to 1800 ℃; the heat treatment atmosphere in the first temperature rise stage is ammonia gas, and the heat treatment atmosphere in the second temperature rise stage and the third temperature rise stage is nitrogen gas; and when the temperature is raised to a second temperature, carrying out fixed length treatment on the boron nitride precursor fiber. The preparation method of the invention can promote the crystallization and crystal orientation of the boron nitride fiber, thereby obtaining the boron nitride fiber with excellent mechanical property.

Description

Boron nitride fiber and preparation method thereof

Technical Field

The invention belongs to the technical field of ceramic fibers, and particularly relates to a boron nitride fiber and a preparation method thereof.

Background

The boron nitride fiber has the characteristics of high temperature resistance, corrosion resistance, excellent dielectric property, good electrical insulation property, good thermal conductivity and the like, and has good application prospect.

The organic precursor conversion method is the preferred scheme for preparing the high-performance continuous boron nitride fiber. However, the molecular weight and the polymerization degree of the boron nitride precursor are low, the length and the order degree of an internal molecular chain of a boron nitride precursor fiber obtained by melt-drawing the boron nitride precursor are low, the fiber is fragile in texture and extremely low in strength, and the requirements of conventional continuous drawing treatment cannot be directly met. Meanwhile, the boron-nitrogen bond has high energy and strong binding force, and the fiber material with high crystal orientation is difficult to prepare under the conventional heat treatment condition, so that the elastic modulus and the tensile strength of the boron nitride fiber are low.

In addition, the preparation of the boron nitride fiber by converting the organic precursor is a process of converting organic phase into inorganic phase, and a large amount of organic gas molecules are removed in the process, so that a large amount of defects such as cracks, holes and the like remain on the surface and inside of the fiber, and the mechanical property of the boron nitride fiber is further reduced.

Therefore, a method for improving the mechanical properties of boron nitride fibers is needed.

Disclosure of Invention

In order to solve the problems, the invention provides a boron nitride fiber and a preparation method thereof. According to the invention, the cohesive force generated by removing boron nitride precursor fiber heterogeneous elements (namely, other elements except nitrogen elements and boron elements in the boron nitride precursor fiber) and thermal sintering shrinkage is used as a tension source, the in-situ action is used for changing the microstructure of the fiber, promoting the crystallization orientation of the fiber and improving the mechanical property of the fiber.

The invention provides a preparation method of boron nitride fibers, which comprises the following steps:

carrying out melt drawing on the boron nitride precursor to obtain boron nitride precursor fiber;

carrying out heat treatment on the boron nitride precursor fiber to obtain a boron nitride fiber;

the heat treatment comprises a first temperature rise stage, a second temperature rise stage and a third temperature rise stage which are sequentially carried out, wherein the temperature is raised to a first temperature in the first temperature rise stage, the temperature is raised to a second temperature in the second temperature rise stage, and the temperature is raised to a third temperature in the third temperature rise stage;

wherein the first temperature is controlled to be 600-800 ℃, the second temperature is controlled to be 1000-1400 ℃, and the third temperature is controlled to be 1600-1800 ℃;

the heat treatment atmosphere of the first temperature rise stage is ammonia gas, and the heat treatment atmospheres of the second temperature rise stage and the third temperature rise stage are nitrogen gas;

and when the temperature is raised to the second temperature, carrying out fixed length treatment on the boron nitride precursor fiber.

The fixed-length treatment refers to that the boron nitride precursor fiber is tensioned to enable the length of the boron nitride precursor fiber to be fixed, and further the boron nitride precursor fiber can shrink directionally along the axial direction in the heat treatment process so that grain boundaries can slide to form crystal directional arrangement.

In one embodiment of the present invention, the boron nitride precursor is polyborazane.

In one embodiment of the present invention, the boron nitride precursor fiber is pretreated before being subjected to the heat treatment, and the pretreatment comprises the following steps: and (3) insulating the boron nitride precursor fiber for 1 to 5 hours at the temperature of 20 to 30 ℃ in an air atmosphere.

In one embodiment of the present invention, the relative humidity of air at the time of pretreatment is 1 to 5%. The relative humidity referred to herein is the percentage of the water vapor pressure in the air to the saturated water vapor pressure at the same temperature.

Through pretreatment of the boron nitride precursor fiber, a partial oxygen bridging structure can be introduced, so that the boron nitride precursor fiber can keep the fiber morphology in the heat treatment process, and the oxygen bridging structure is in a molten state at 1200 to 1400 ℃, so that the sliding of a crystal boundary is facilitated, and the crystal orientation is promoted.

In one embodiment of the present invention, the temperature increase rate in the first temperature increase stage is 10 to 30 ℃/h.

In one embodiment of the invention, the first temperature is kept for 1 to 5 hours, and the heat treatment atmosphere is kept as ammonia gas during the heat preservation.

Through the temperature rise in the first temperature rise stage and the heat preservation at the first temperature, ammonia gas can be used as reaction gas, organic elements in the boron nitride precursor fiber can be uniformly and effectively discharged, and appropriate oxygen and nitrogen elements are kept, so that the treated fiber has certain strength, and the fiber can not be broken in the subsequent fixed-length treatment.

In one embodiment of the present invention, the temperature increase rate in the second temperature increase stage is 30 to 180 ℃/h.

And when the temperature is increased to 1000-1400 ℃, fixing the length of the boron nitride precursor fiber so that the fiber can be directionally contracted in the subsequent heat treatment process, and promoting the directional arrangement of the fiber crystal.

In one embodiment of the present invention, the temperature increase rate in the third temperature increase stage is 60 to 300 ℃/h.

In one embodiment of the invention, the third temperature is kept for 0.5 to 3 hours, and the heat treatment atmosphere is kept to be nitrogen during the heat preservation process.

And raising the temperature to a third temperature through the temperature rise in a third temperature rise stage, and preserving the temperature for a certain time to obtain the boron nitride fiber.

The invention also provides a boron nitride fiber prepared according to the method.

Compared with the prior art, the invention has the following beneficial effects: during heat treatment, the temperature is firstly increased to a first temperature (600 to 800 ℃) for heat treatment, so that the boron nitride precursor fiber has certain strength; heating to a second temperature (1000 to 1400 ℃), carrying out fixed-length treatment on the boron nitride precursor fiber with certain strength, so that the boron nitride precursor fiber can directionally shrink along the axial direction in the subsequent heat treatment process, realizing in-situ hot drawing of the fiber based on the cohesive force generated by fiber heterogeneous element removal and hot sintering shrinkage, and promoting the crystal orientation of the fiber; and finally, raising the temperature to a third temperature (1600-1800 ℃) for final firing to obtain the boron nitride fiber with high tensile strength and high elastic modulus. In addition, the defects of cracks, air holes and the like on the surface of the fiber can be gradually healed under the action of in-situ hot drawing, and the mechanical property of the fiber is synchronously promoted to be improved.

Drawings

FIG. 1 is a surface Scanning Electron (SEM) and Transmission Electron (TEM) diffractogram of the boron nitride fiber prepared in example 1, wherein (a) is the SEM diffractogram, and (b) is the TEM diffractogram;

FIG. 2 is surface Scanning Electron (SEM) and Transmission Electron (TEM) diffractograms of the boron nitride fiber prepared in comparative example 1, wherein (a) is the SEM diffractogram, and (b) is the TEM diffractogram;

FIG. 3 is a surface Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) diffractogram of the boron nitride fiber prepared in comparative example 2, wherein (a) is the SEM diffractogram, and (b) is the TEM diffractogram;

FIG. 4 is a surface Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) diffractogram of the boron nitride fiber prepared in comparative example 3, wherein (a) is the SEM diffractogram, and (b) is the TEM diffractogram;

FIG. 5 is XRD spectra of boron nitride fibers prepared in example 1 and comparative examples 1 to 3, wherein the curves in the XRD spectra are from top to bottom of comparative example 3, comparative example 2, comparative example 1 and example 1;

FIG. 6 is a surface Scanning Electron (SEM) and Transmission Electron (TEM) diffractogram of the boron nitride fiber prepared in example 3, wherein (a) is the SEM diffractogram, and (b) is the TEM diffractogram;

FIG. 7 is a surface Scanning Electron (SEM) and Transmission Electron (TEM) diffractogram of the boron nitride fiber prepared in comparative example 4, wherein (a) is the SEM diffractogram, and (b) is the TEM diffractogram;

fig. 8 is an XRD spectrum of the boron nitride fibers prepared in example 3 and comparative example 4, wherein the curves in the XRD spectrum are the XRD spectra of comparative example 4 and example 3 from top to bottom.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example 1

The embodiment provides a preparation method of boron nitride fibers, which comprises the following steps:

(1) And carrying out melt drawing on the boron nitride precursor to obtain the boron nitride precursor fiber. Wherein the boron nitride precursor is polyborazane, and the melt wire drawing is carried out by adopting the process of the prior art.

(2) And carrying out heat treatment on the boron nitride precursor fiber to obtain the boron nitride fiber. The heat treatment comprises a first temperature rise stage, a second temperature rise stage and a third temperature rise stage which are sequentially carried out.

(1) A first temperature rise stage: the temperature was raised to 650 deg.c (i.e., the first temperature) at a ramp rate of 20 deg.c/h, during which the heat treatment atmosphere was ammonia gas.

(2) Keeping the temperature at 650 ℃ for 4h, wherein the heat treatment atmosphere is ammonia gas in the heat preservation process.

(3) A second temperature rising stage: the temperature was raised to 1000 c (i.e., the second temperature) at a ramp rate of 60 c/h, during which the heat treatment atmosphere was nitrogen.

(4) And (3) heating to 1000 ℃, and carrying out fixed length treatment on the boron nitride precursor fiber.

(5) A third temperature rise stage: the temperature was raised to 1700 c (i.e., the third temperature) at a ramp rate of 240 c/h, during which the heat treatment atmosphere was nitrogen.

(6) Keeping the temperature at 1700 ℃ for 2h, wherein the heat treatment atmosphere is nitrogen in the heat preservation process.

The present example also provides a boron nitride fiber prepared according to the above method. FIG. 1 (a) is the SEM surface morphology of the boron nitride fiber prepared in this example, the fiber surface is smooth and has no obvious defects; fig. 1 (b) is a diffraction pattern of a transmission electron microscope of the boron nitride fiber prepared in this example, and diffraction spots of (002) crystal planes are clear, which shows that the degree of crystal orientation is high. The fourth curve from top to bottom in fig. 5 is the XRD spectrum of the boron nitride fiber prepared in this example, the characteristic peak of the boron nitride (002) crystal plane corresponding to the diffraction angle of 25.9 °, and the orientation τ of the crystal plane is calculated to be 0.83. The boron nitride fiber prepared by the embodiment has the average monofilament tensile strength of 1495MPa and the average elastic modulus of 197GPa.

Comparative example 1

The comparative example is different from example 1 in that the boron nitride precursor fiber is not subjected to the fixed length treatment when the temperature of the comparative example is raised to 1000 ℃, and the other conditions are the same as those of example 1.

FIG. 2 (a) is an SEM surface morphology of boron nitride fibers prepared in this comparative example, with smooth fiber surfaces and no apparent defects; FIG. 2 (b) is a diffraction pattern of a transmission electron microscope of a boron nitride fiber prepared in this comparative example, and (002) crystal plane shows a diffraction ring and has a low degree of crystal orientation. The third curve from top to bottom in fig. 5 is the XRD spectrum of the boron nitride fiber prepared in this comparative example, and the characteristic peak orientation degree τ of the boron nitride (002) crystal plane calculated is only 0.34. The boron nitride fiber prepared by the comparative example has the average monofilament tensile strength of 960MPa and the average elastic modulus of only 34GPa.

Comparative example 2

This comparative example differs from example 1 in that the second temperature in example 1 was replaced with 1450 c, and the other conditions were the same as in example 1.

FIG. 3 (a) is an SEM surface morphology of boron nitride fibers prepared in this comparative example, with smooth fiber surfaces and no apparent defects; FIG. 3 (b) is a diffraction pattern of a transmission electron microscope of a boron nitride fiber prepared in this comparative example, and (002) crystal plane shows a diffraction ring and has a low degree of crystal orientation. The second curve from top to bottom in fig. 5 is the XRD spectrum of the boron nitride fiber prepared in this comparative example, and the characteristic peak orientation degree τ of the boron nitride (002) crystal plane calculated is only 0.36. The boron nitride fiber prepared by the comparative example has an average monofilament tensile strength of 912MPa and an average elastic modulus of only 52GPa.

Comparative example 3

This comparative example differs from example 1 in that the first temperature in example 1 was replaced with 500 ℃, and the other conditions were the same as in example 1.

FIG. 4 (a) is the SEM surface morphology of the boron nitride fiber prepared in this comparative example, the fiber surface is flat and has obvious crack defects; FIG. 4 (b) is a diffraction pattern of a transmission electron microscope of a boron nitride fiber produced in this comparative example, and (002) crystal plane shows a diffraction ring and has a low degree of crystal orientation. The first curve from top to bottom in fig. 5 is the XRD spectrum of the boron nitride fiber prepared in this comparative example, and the characteristic peak orientation degree τ of the boron nitride (002) crystal plane calculated is only 0.27. The boron nitride fibers prepared in this comparative example had an average monofilament tensile strength of 674MPa and an average modulus of elasticity of only 59GPa.

Example 2

(1) A first temperature rise stage: the temperature was raised to 750 c (i.e., the first temperature) at a ramp rate of 30 c/h, during which the heat treatment atmosphere was ammonia gas.

(2) Keeping the temperature at 750 ℃ for 2h, wherein the heat treatment atmosphere is ammonia gas in the heat preservation process.

(3) A second temperature rising stage: the temperature was raised to 1150 c (i.e., the second temperature) at a ramp rate of 80 c/h, during which the heat treatment atmosphere was nitrogen.

(4) And (3) heating to 1150 ℃, and carrying out fixed length treatment on the boron nitride precursor fiber.

(5) A third temperature rise stage: the temperature was raised to 1600 c (i.e., the third temperature) at a ramp rate of 180 c/h, during which the heat treatment atmosphere was nitrogen.

(6) Keeping the temperature at 1600 ℃ for 3h, wherein the heat treatment atmosphere is nitrogen in the heat preservation process.

The present example also provides a boron nitride fiber prepared according to the above method. The boron nitride fiber prepared by the embodiment has smooth surface and no obvious defect, the calculated orientation degree tau of the (002) crystal face of the fiber is 0.76, the average monofilament tensile strength of the boron nitride fiber is 1329MPa, and the average elastic modulus is 145GPa.

Example 3

(1) And carrying out melt wire drawing on the boron nitride precursor to obtain the boron nitride precursor fiber. Wherein the boron nitride precursor is polyborazane, and the melt wire drawing is carried out by adopting the process of the prior art.

(2) Pre-treating the boron nitride precursor fiber, the pre-treating comprising the steps of: and (3) preserving the heat of the boron nitride precursor fiber for 5 hours at 30 ℃ in an air atmosphere. Wherein the relative humidity of the air is 1%.

(3) And carrying out heat treatment on the pretreated boron nitride precursor fiber to obtain the boron nitride fiber. The heat treatment comprises a first temperature rise stage, a second temperature rise stage and a third temperature rise stage which are sequentially carried out.

(1) A first temperature rise stage: the temperature was raised to 700 c (i.e., the first temperature) at a ramp rate of 30 c/h, during which the heat treatment atmosphere was ammonia.

(2) Keeping the temperature at 700 ℃ for 3h, wherein the heat treatment atmosphere is ammonia gas in the heat preservation process.

(3) A second temperature rising stage: the temperature was raised to 1300 c (i.e., the second temperature) at a ramp rate of 100 c/h, during which the heat treatment atmosphere was nitrogen.

(4) And (3) heating to 1300 ℃, and carrying out fixed length treatment on the boron nitride precursor fiber.

(5) A third temperature rise stage: the temperature was raised to 1600 c (i.e., the third temperature) at a ramp rate of 300 c/h, during which the heat treatment atmosphere was nitrogen.

(6) Keeping the temperature at 1600 ℃ for 2.5h, wherein the heat treatment atmosphere is nitrogen in the heat preservation process.

The present example also provides a boron nitride fiber prepared according to the above method. FIG. 6 (a) is the SEM surface morphology of the boron nitride fiber prepared in this example, the fiber surface is smooth and has no obvious defects; fig. 6 (b) a diffraction pattern of a transmission electron microscope of the boron nitride fiber prepared in this example, and (002) crystal plane diffraction spots are clear, which indicates that the degree of crystal orientation is high. The second curve from top to bottom in fig. 8 is the XRD spectrum of the boron nitride fiber prepared in this example, the diffraction angle of 25.9 ° corresponds to the characteristic peak of the (002) crystal plane of boron nitride, and the orientation τ of the crystal plane is calculated to be 0.81. The boron nitride fiber prepared by the embodiment has the average monofilament tensile strength of 1410MPa and the average elastic modulus of 176GPa.

Comparative example 4

The comparative example is different from example 3 in that the boron nitride precursor fiber is not subjected to the fixed length treatment when the temperature of the comparative example is raised to 1300 ℃, and the other conditions are the same as those of example 3.

FIG. 7 (a) is an SEM surface morphology of boron nitride fibers prepared in this comparative example, the fiber surface was rough and had significant pore defects; FIG. 7 (b) is a diffraction pattern of a transmission electron microscope of a boron nitride fiber produced in this comparative example, and (002) crystal plane shows a diffraction ring and has a low degree of crystal orientation. In fig. 8, the first curve from top to bottom is an XRD spectrogram of the boron nitride fiber prepared in this comparative example, and the characteristic peak orientation degree τ of the (002) crystal plane of boron nitride is calculated to be only 0.21. The boron nitride fiber prepared by the comparative example has an average monofilament tensile strength of 977MPa and an average elastic modulus of only 26GPa.

Example 4

(2) Pre-treating the boron nitride precursor fiber, the pre-treating comprising the steps of: and (3) preserving the heat of the boron nitride precursor fiber for 3 hours at 20 ℃ in an air atmosphere. Wherein the relative humidity of the air is 5%.

(1) A first temperature rise stage: the temperature was raised to 600 c (i.e., the first temperature) at a ramp rate of 10 c/h, during which the heat treatment atmosphere was ammonia gas.

(2) Keeping the temperature at 600 ℃ for 1h, wherein the heat treatment atmosphere is ammonia gas in the heat preservation process.

(3) A second temperature rising stage: the temperature was raised to 1400 deg.c (i.e., the second temperature) at a ramp rate of 180 deg.c/h, during which the heat treatment atmosphere was nitrogen.

(4) And (3) heating to 1400 ℃, and fixing the length of the boron nitride precursor fiber.

(5) A third temperature rise stage: the temperature was raised to 1800 c (i.e., the third temperature) at a ramp rate of 300 c/h, during which the heat treatment atmosphere was nitrogen.

(6) Keeping the temperature at 1800 ℃ for 0.5h, wherein the heat treatment atmosphere is nitrogen in the heat preservation process.

The present example also provides a boron nitride fiber made by one of the above-described methods. The boron nitride fiber prepared by the embodiment has smooth surface and no obvious defect, and the calculated orientation degree tau of the (002) crystal face of the fiber is 0.85, the average monofilament tensile strength of the fiber is 1512MPa, and the average elastic modulus is 202GPa.

Example 5

(2) Pre-treating the boron nitride precursor fiber, the pre-treating comprising the steps of: and (3) preserving the heat of the boron nitride precursor fiber for 1h at 25 ℃ in an air atmosphere. Wherein the relative humidity of the air is 3%.

(1) A first temperature rise stage: the temperature was raised to 800 c (i.e., the first temperature) at a ramp rate of 25 c/h, during which the heat treatment atmosphere was ammonia.

(2) Keeping the temperature at 800 ℃ for 5h, wherein the heat treatment atmosphere is ammonia gas in the heat preservation process.

(3) A second temperature rising stage: the temperature was raised to 1330 deg.C (i.e., the second temperature) at a ramp rate of 30 deg.C/h, during which the heat treatment atmosphere was nitrogen.

(4) And heating to 1330 ℃, and fixing the length of the boron nitride precursor fiber.

(5) A third temperature rise stage: the temperature was raised to 1650 c (i.e., the third temperature) at a ramp rate of 60 c/h, during which the heat treatment atmosphere was nitrogen.

(6) The temperature is kept at 1650 ℃ for 1.5h, and the heat treatment atmosphere is nitrogen in the heat preservation process.

This embodiment also provides a boron nitride fiber made by one of the methods described above. The boron nitride fiber prepared by the embodiment has smooth surface and no obvious defect, and the calculated orientation degree tau of the (002) crystal face of the fiber is 0.87, the average monofilament tensile strength of the fiber is 1594MPa, and the average elastic modulus is 224GPa.

The degree of orientation τ is the degree of orientation of the crystal along a certain crystal plane, and can be calculated by XRD spectrogram, and is usually expressed by the ratio of XRD characteristic peak intensity of the crystal plane of the fiber to XRD characteristic peak intensity of the crystal plane after the fiber is pulverized. The orientation degree is the orientation of the crystal in the boron nitride fiber along the (002) crystal face (namely along the axial direction of the fiber), and is used for representing the orientation arrangement degree of the fiber crystal along the (002) crystal face promoted by the cohesive force generated by heterogeneous element removal and fixed-length treatment at different temperatures.

The process conditions for preparing boron nitride fibers in the above examples 1 to 5 and comparative examples 1 to 4, and the average tensile strength, the average elastic modulus, the degree of orientation τ and the fiber morphology of the prepared boron nitride fibers are shown in table 1.

TABLE 1 Process conditions and boron nitride fiber properties for the examples and comparative examples

As can be seen from Table 1, the tensile strength, elastic modulus and orientation degree tau of the boron nitride fiber prepared under the process conditions of the invention are obviously superior to those of the comparative example, and the fiber morphology has no obvious defects.

The above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art may still modify the technical solutions described in the foregoing embodiments, or may equally substitute some or all of the technical features; such modifications and substitutions do not depart from the spirit and scope of the present invention, and they should be construed as being included in the following claims and description.

Claims

1. A preparation method of boron nitride fiber is characterized by comprising the following steps:

carrying out melt wire drawing on a boron nitride precursor to obtain a boron nitride precursor fiber, wherein the boron nitride precursor is polyborozane;

the heat treatment atmosphere in the first temperature rise stage is ammonia gas, and the heat preservation is carried out for 1 to 5 hours at the first temperature;

the heat treatment atmosphere of the second temperature rise stage and the third temperature rise stage is nitrogen, and when the temperature rises to the second temperature, the boron nitride precursor fiber is subjected to fixed length treatment;

the fixed-length treatment refers to that the boron nitride precursor fiber is tensioned to enable the length of the boron nitride precursor fiber to be fixed, and then the boron nitride precursor fiber can shrink directionally along the axial direction in the heat treatment process, so that grain boundary sliding forms crystal directional arrangement.

2. The method of claim 1, wherein the boron nitride precursor fiber is pretreated before being subjected to the thermal treatment, and the pretreatment comprises the following steps: and (3) insulating the boron nitride precursor fiber for 1 to 5 hours at the temperature of 20 to 30 ℃ in an air atmosphere.

3. The method for preparing boron nitride fiber according to claim 2, wherein the relative humidity of air is 1 to 5% during pretreatment.

4. The method for preparing boron nitride fiber according to claim 1, wherein the temperature rise rate of the first temperature rise stage is 10 to 30 ℃/h.

5. The method for preparing boron nitride fiber according to claim 1, wherein the temperature rise rate of the second temperature rise stage is 30 to 180 ℃/h.

6. The method for preparing boron nitride fiber according to claim 1, wherein the temperature rise rate of the third temperature rise stage is 60 to 300 ℃/h.

7. The method for preparing the boron nitride fiber according to claim 1, wherein the temperature is kept at the third temperature for 0.5 to 3 hours.

8. A boron nitride fiber produced by the method for producing a boron nitride fiber according to any one of claims 1 to 7.