Pre-oriented isothermal crystallized boron nitride fiber and preparation method thereof

Abstract

The invention belongs to the technical field of boron nitride fiber preparation, and particularly relates to a pre-oriented isothermal crystallization boron nitride fiber and a preparation method thereof.The preparation method comprises the following steps that melt spinning is conducted on a polyborazine organic precursor, and PBN fibril is obtained; pBN fibrils are placed in a reaction atmosphere furnace, shielding gas is introduced after the reaction atmosphere furnace is vacuumized, pre-orientation treatment is carried out on the PBN fibrils, and the pre-orientation treatment is carried out under the heating condition; sequentially carrying out first-stage heating treatment and second-stage heating treatment on the PBN fibril subjected to the pre-orientation treatment in an ammonia gas atmosphere to obtain the boron nitride fiber, through the pre-orientation treatment, an ordered region of a stretching chain is formed in the fiber before crystallization, and the pre-orientation degree, the crystallization rate and the crystallization activation energy of the PBN fibril can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of boron nitride fiber preparation, and particularly relates to a pre-oriented isothermal crystallization boron nitride fiber and a preparation method thereof. Background Art

[0002] The preparation methods of boron nitride fibers are divided into inorganic methods and organic methods. Boron nitride prepared by inorganic methods has the drawback of incomplete nitridation, while the organic method conversion can effectively avoid this problem. However, the crystallization degree in the process of preparing boron nitride fibers by the organic method is relatively low, resulting in poor mechanical properties of the fibers.

[0003] When preparing boron nitride fibers by the organic method, polyborazylene (PBN) is mostly selected as the precursor. The polyborazylene precursor is a six-membered ring structure with a B and N atomic ratio of 1:1. The groups such as methylamino groups connected to its side chains reduce the number of active groups B-H and N-H, control its crosslinking ability to a certain extent, lower the melting point of the precursor, and improve the spinnability. In the molten state, the spinning traction plays an orienting role on the boron nitride six-membered ring structure of the polyborazylene precursor, making the orientation of the six-membered ring plane network tend to be consistent. This orienting arrangement will be beneficial to the formation and crystallization of the BN layered structure. However, during the heat treatment process, the disorder degree in the glassy PBN structure increases, and the oriented molecular chains generated by the traction start to be disordered due to thermal motion, resulting in a decrease in the crystallization rate and crystallinity, and reducing the mechanical properties and thermal properties of the boron nitride fiber. Summary of the Invention

[0004] The present invention designs a preparation method of a pre-oriented isothermal crystallization boron nitride fiber, including the following steps:

[0005] Melting-spin the polyborazylene organic precursor at a spinning speed of 150 - 275 m / min to obtain PBN precursor fibers;

[0006] Place the PBN precursor fibers in a reaction atmosphere furnace, evacuate the reaction atmosphere furnace and then introduce a protective gas, and perform a pre-orientation treatment on the PBN precursor fibers. The pre-orientation treatment is carried out under heating conditions;

[0007] Under an ammonia atmosphere, perform a first-stage heat treatment and a second-stage heat treatment on the pre-oriented PBN precursor fibers in sequence to obtain boron nitride fibers.

[0008] Preferably, the protective gas is nitrogen or argon, and the flow rate of the introduced protective gas is 0.5 - 1.0 m 3 / h.

[0009] Preferably, the pre-orientation treatment is: apply a force to the PBN precursor fibers to make the PBN precursor fibers have a constant contraction length.

[0010] Preferably, the pre-orientation treatment is carried out under heating conditions of a temperature of 70-100 °C and a heat preservation time of 60-180 min.

[0011] Preferably, after the heat preservation is completed, it is cooled to room temperature in a nitrogen atmosphere.

[0012] Preferably, the first-stage heating treatment includes: a heating temperature of 130-220 °C and a heating time of 40-850 s.

[0013] Preferably, during the first-stage heating treatment, a traction force of 5-15 cN is applied to the PBN fibrils.

[0014] Preferably, the second-stage heating treatment includes a first heating stage and a second heating stage.

[0015] The first heating stage includes: at a heating rate of 2-5 °C / min, heating from the heating temperature of the first-stage heating treatment to 900-1100 °C and holding for 0.5-2.0 h.

[0016] The second heating stage includes: at a heating rate of 1-3 °C / min, heating from the holding temperature of the first heating stage to 1500-2000 °C and holding for 0.2-1.5 h.

[0017] Preferably, the second heating stage is carried out in a nitrogen atmosphere.

[0018] The present invention also designs a boron nitride fiber with pre-orientation isothermal crystallization, which is prepared by applying the above preparation method.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: By carrying out stretching pre-orientation on the PBN fibrils under conditions near the glass transition temperature of the PBN fibrils, pre-oriented fibers without crystallization can be obtained. The pre-orientation treatment enables the formation of an ordered region of straight chains inside the fibers before crystallization, which can effectively improve the pre-orientation degree, crystallization rate and crystallization activation energy of the PBN fibrils.

[0020] The first-stage heating treatment stage is an isothermal crystallization process. After the orientation degree of the fibers is improved in the pre-orientation treatment stage, the fibers grow and crystallize with the ordered region as the center, and the crystals grow and orient along the axial direction of the fibers, which can avoid the disordering of the organic PBN fibrils under low crystallinity.

[0021] In the second-stage heating treatment stage, the PBN fibrils undergo a transformation stage from organic fibers to inorganic fibers and ceramicization. Non-target elements in the fibers are removed, enabling the grains to further grow directionally, and finally forming inorganic boron nitride ceramic fibers with the crystal axis arrangement direction highly consistent with the fiber axial direction, effectively improving the fracture strength and fracture elongation of the fibers. Description of the Drawings

[0022] Figure 1 This is the X-ray diffraction pattern of the boron nitride fiber in the embodiment of the present invention.

[0023] Figure 2 This is the electron microscope image of the boron nitride fiber in the embodiment of the present invention.

[0024] Figure 3 This is the electron microscope image of the boron nitride fiber in Comparative Example 1 of the present invention. Detailed implementation manners

[0025] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0026] Example 1

[0027] This embodiment provides a method for preparing pre-oriented isothermal crystallization boron nitride fiber, including the following steps:

[0028] Step 1: Under nitrogen protection, melt-spin the polyboronazane organic precursor at a spinning speed of 150 m / min to obtain PBN precursor fiber.

[0029] Step 2: Place the PBN precursor fiber in a reaction atmosphere furnace, evacuate the reaction atmosphere furnace and replace the protective gas 2 times. The protective gas in this embodiment is nitrogen, and the flow rate is 0.5 m 3 / h.

[0030] Perform pre-orientation treatment on the polyboronazane precursor fiber, apply a force to the PBN precursor fiber to make its contraction length constant. The pre-orientation treatment is carried out under the heating condition of 70 °C and heat preservation for 60 min. After the heat preservation is completed, quickly cool to room temperature under nitrogen atmosphere.

[0031] Step 3: Under ammonia atmosphere, perform the first-stage heating treatment on the pre-oriented precursor fiber. The first-stage heating treatment is: apply a traction force of 5 cN to the PBN precursor fiber to keep the PBN precursor fiber taut, the heating temperature is 130 °C, and control the heat treatment crystallization time of the fiber by regulating the fiber residence time. The heating time in this embodiment is 40 s.

[0032] Step 4: Under ammonia atmosphere, perform the second-stage heating treatment on the precursor fiber after the first-stage heating treatment. The second-stage heating treatment includes a first heating stage and a second heating stage.

[0033] The first heating stage is: at a heating rate of 2 °C / min, raise the temperature from 130 °C to 900 °C and keep it warm for 0.5 h;

[0034] The second heating stage is as follows: under a nitrogen atmosphere, with a heating rate of 1 °C / min, heat from 900 °C to 1500 °C, and hold for 0.2 h to obtain boron nitride fibers. As Figure 1 shown, the boron nitride fibers prepared in this example have excellent crystallinity.

[0035] Comparative Example 1

[0036] The difference between this comparative example and Example 1 is that:

[0037] In Step 2, the polyboronazane precursor fibers were not pre-oriented.

[0038] As Figure 2 shown, the cross-section of the boron nitride fibers prepared in Example 1 exhibits a lamellar crystalline texture with a certain orientation, and both the crystallinity and density are good. As Figure 3 shown, the cross-section of the boron nitride fibers prepared in Comparative Example 1 is still in an amorphous state, and the crystallinity is poor.

[0039] Pre-orientation treatment can significantly increase the crystallization activation energy of the fibers. Under a constant tension condition, isothermal crystallization treatment of the pre-oriented precursor fibers can effectively avoid the disordering of the precursor fibers at low crystallinity during the heat treatment process. The fibers grow and crystallize with these ordered orientation regions as the centers, and finally form an inorganic boron nitride ceramic fiber in which the crystal axis arrangement direction of the lattice is highly consistent with the fiber axis. In terms of the mechanical and thermal properties of the boron nitride fibers, this is reflected in a significant increase in the fracture strength and fracture elongation.

[0040] Example 2

[0041] This example provides a method for preparing pre-oriented isothermal crystallization boron nitride fibers, including the following steps:

[0042] Step 1: Under nitrogen protection, melt-spin the polyboronazane organic precursor at a spinning speed of 200 m / min to obtain PBN precursor fibers.

[0043] Step 2: Place the PBN precursor fibers in a reaction atmosphere furnace, evacuate the reaction atmosphere furnace and replace the protective gas 2 times. The protective gas in this example is argon, and the flow rate is 0.7 m 3 / h.

[0044] Perform pre-orientation treatment on the polyboronazane precursor fibers, apply a force to the PBN precursor fibers to make their contraction length constant. The pre-orientation treatment is carried out under heating conditions of 85 °C and holding for 120 min. After the holding ends, quickly cool to room temperature under a nitrogen atmosphere.

[0045] Step 3: Under an ammonia atmosphere, perform a first-stage heating treatment on the prefetched fibrils. The first-stage heating treatment is as follows: Apply a traction force of 10 cN to the PBN fibrils to keep the PBN fibrils taut. The heating temperature is 170 °C. Control the heat treatment crystallization time of the fibrils by regulating the fiber residence time. In this embodiment, the heating time is 400 s.

[0046] Step 4: Under an ammonia atmosphere, perform a second-stage heating treatment on the fibrils after the first-stage heating treatment. The second-stage heating treatment includes a first heating stage and a second heating stage.

[0047] The first heating stage is as follows: Increase the temperature from 170 °C to 1000 °C at a heating rate of 3 °C / min and hold for 1 h.

[0048] The second heating stage is as follows: Under a nitrogen atmosphere, increase the temperature from 1000 °C to 1700 °C at a heating rate of 2 °C / min, hold for 1 h, and obtain boron nitride fibers.

[0049] Example 3

[0050] This embodiment provides a method for preparing pre-oriented isothermal crystallization boron nitride fibers, including the following steps:

[0051] Step 1: Under nitrogen protection, perform melt spinning on a polyboronazane organic precursor at a spinning speed of 275 m / min to obtain PBN fibrils.

[0052] Step 2: Place the PBN fibrils in a reaction atmosphere furnace, evacuate the reaction atmosphere furnace and replace the protective gas 2 times. The protective gas in this embodiment is nitrogen, and the flow rate is 1 m 3 / h.

[0053] Perform a pre-orientation treatment on the polyboronazane fibrils, apply a force to the PBN fibrils to make their contraction length constant. The pre-orientation treatment is carried out under heating conditions of 100 °C and holding for 180 min. After the holding ends, quickly cool to room temperature under a nitrogen atmosphere.

[0054] Step 3: Under an ammonia atmosphere, perform a first-stage heating treatment on the prefetched fibrils. The first-stage heating treatment is as follows: Apply a traction force of 15 cN to the PBN fibrils to keep the PBN fibrils taut. The heating temperature is 220 °C. Control the heat treatment crystallization time of the fibrils by regulating the fiber residence time. In this embodiment, the heating time is 850 s.

[0055] Step 4: Under an ammonia atmosphere, perform a second-stage heating treatment on the fibrils after the first-stage heating treatment. The second-stage heating treatment includes a first heating stage and a second heating stage.

[0056] The first heating stage is as follows: heating from 220°C to 1100°C at a heating rate of 5°C / min and holding for 2 h;

[0057] The second heating stage is as follows: under a nitrogen atmosphere, heating from 1100°C to 2000°C at a heating rate of 3°C / min and holding for 1.5 h to obtain boron nitride fibers.

[0058] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments or equivalently replace some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a pre-oriented isothermally crystallized boron nitride fiber, characterized in that, It includes the following steps: Melt-spin the polyboronazane organic precursor to obtain PBN precursor fibers; Place the PBN precursor fibers in a reaction atmosphere furnace. After evacuating the reaction atmosphere furnace, introduce a protective gas, and perform a pre-orientation treatment on the PBN precursor fibers. The pre-orientation treatment is carried out under heating conditions; Under an ammonia atmosphere, perform a first-stage heating treatment and a second-stage heating treatment on the pre-oriented PBN precursor fibers in sequence to obtain boron nitride fibers.

2. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 1, characterized in that, The protective gas is nitrogen or argon, and the flow rate of the introduced protective gas is 0.5 to 1.0 m 3 / h.

3. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 1, characterized in that, The pre-orientation treatment is: apply a force to the PBN precursor fibers to make the PBN precursor fibers have a constant shrinkage length.

4. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 1, characterized in that, The pre-orientation treatment is carried out under heating conditions with a temperature of 70-100 °C and a heat preservation time of 60-180 min.

5. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 4, characterized in that, After the heat preservation ends, cool down to room temperature under a nitrogen atmosphere.

6. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 1, characterized in that, The first-stage heating treatment includes: a heating temperature of 130-220 °C and a heating time of 40-850 s.

7. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 1, characterized in that, During the first-stage heating treatment, apply a traction force of 5-15 cN to the PBN precursor fibers.

8. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 1, characterized in that, The second-stage heating treatment includes a first heating stage and a second heating stage. The first heating stage includes: at a heating rate of 2-5 °C / min, raise the temperature from the heating temperature of the first-stage heating treatment to 900-1100 °C and keep it warm for 0.5-2.0 h; The second heating stage includes: at a heating rate of 1-3 °C / min, raise the temperature from the heat preservation temperature of the first heating stage to 1500-2000 °C and keep it warm for 0.2-1.5 h.

9. The preparation method of the pre-oriented isothermally crystallized boron nitride fiber according to claim 8, characterized in that, The second heating stage is carried out under a nitrogen atmosphere.

10. A pre-oriented isothermally crystallized boron nitride fiber, characterized in that, Prepared by using the preparation method according to any one of claims 1-9.

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