Preparation method of boron nitride / carbon nanotube electromagnetic wave absorbing material

Abstract

The invention relates to a preparation method of a boron nitride / carbon nanotube electromagnetic wave absorbing material. The invention belongs to the technical field of electromagnetic wave absorbing materials. The bottleneck problems that existing boron nitride is insufficient in dielectric loss and single in function are solved. The method comprises the following steps: preparing a melamine diborate precursor solution from boric acid and melamine, adding nickel nitrate hexahydrate and melamine, stirring, cooling, standing, carrying out suction filtration, washing and freeze-drying to obtain boron nitride / carbon nanotube composite precursor powder, and calcining at a constant temperature to finish the preparation. The obtained material has excellent thermal stability and chemical inertness of BN and high conductivity of CNTs, forms a unique three-dimensional porous fiber-spiny heterostructure, has the characteristic of light weight (low density), shows excellent electromagnetic wave absorption performance (broadband and strong loss), does not need complex equipment in the preparation process, adopts cheap and easily available raw materials, has strong controllability of process parameters, and is suitable for industrial production. The method is suitable for large-scale production and has wide application prospects in the fields of electromagnetic shielding, stealth coatings and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of electromagnetic wave absorbing materials, and specifically relates to a method for preparing a boron nitride / carbon nanotube electromagnetic wave absorbing material. Background Art

[0002] Ceramic-based materials represented by boron nitride (BN) have advantages such as excellent high-temperature oxidation resistance (decomposition temperature>800℃), chemical stability and low dielectric constant (ε"≈0.02), but their loss mechanism is single, which limits their application in the field of electromagnetic wave absorption. In contrast, carbon nanotubes (CNTs) have outstanding performance in electromagnetic wave energy conversion due to their high conductivity and abundant interface polarization sites, but excessively high dielectric constants can easily cause impedance mismatch, resulting in reflection loss (RL) that is difficult to exceed -30dB. Therefore, constructing boron nitride / carbon nanotube (BN@CNTs) heterostructure composites can synergistically optimize impedance matching and enhance electromagnetic loss performance. At present, the preparation methods of BN@CNTs composites mainly include physical blending and chemical vapor deposition (CVD). However, physical blending (such as ball milling) is difficult to precisely control the distribution of CNTs, which can easily lead to agglomeration, reducing the interface polarization effect; and although the CVD method can achieve in-situ growth of CNTs, the process is complex, the cost is high, and it is difficult to be compatible with the BN substrate. The existing technology generally has problems such as weak heterogeneous interface bonding, poor microstructure controllability, and difficulty in balancing lightweight and strong absorption performance. In response to the above challenges, it is urgent to develop a new in-situ synthesis strategy to achieve molecular-scale hybridization by precisely regulating the interface bonding of BN and CNTs, so as to optimize the dielectric parameter gradient distribution and enhance the synergistic effect of multiple loss mechanisms. In addition, the introduction of porous boron nitride can further improve the lightweight and thermal insulation properties of the material, providing a theoretical basis and technical support for a new generation of multifunctional intelligent absorbing materials. Therefore, studying a method for preparing BN@CNTs absorbing materials with controllable structure, simple process and excellent performance has important application prospects in the fields of aerospace stealth coatings, electromagnetic wave absorption of electronic equipment, etc. Summary of the invention

[0003] The purpose of the present invention is to solve the bottleneck problems of insufficient dielectric loss and single function of existing boron nitride, and to provide a method for preparing a boron nitride / carbon nanotube electromagnetic wave absorbing material.

[0004] A method for preparing a boron nitride / carbon nanotube electromagnetic wave absorbing material is achieved by the following steps:

[0005] 1. Add boric acid to deionized water, heat to 90°C and stir at constant temperature with magnetic force until it is completely dissolved to form a transparent solution, then add melamine, maintain 90°C and stir at constant temperature for 0.5h to 3h to obtain a uniform and transparent solution, which is the melamine diborate (M·2B) precursor solution;

[0006] II. Nickel nitrate hexahydrate and melamine are successively added to the above-mentioned melamine diborate precursor solution, and the temperature is maintained at 90 °C with constant stirring for 0.5 h to 3 h, then it is naturally cooled to room temperature and left standing for 24 h to 36 h, and then the precipitate is collected through a vacuum filtration device. The precipitate is washed and freeze-dried to obtain a fluffy powder, which is the boron nitride / carbon nanotube (BN@CNTs) composite precursor powder;

[0007] III. The boron nitride / carbon nanotube composite precursor powder is evenly spread on a corundum boat and then placed in the constant temperature zone of a tube furnace. Under the protection of an inert gas, it is heated to 1000 °C and calcined at a constant temperature for 1 to 2 h. After calcination, it is naturally cooled to room temperature to obtain a black porous material, which is the boron nitride / carbon nanotube electromagnetic wave absorption material, thus completing the preparation method.

[0008] Further, the molar ratio of melamine to boric acid in the melamine diborate precursor solution in step I is 1:3.

[0009] Further, the washing and freeze-drying in step II: Wash 3 to 4 times with deionized water, and then perform freeze-drying at -50 °C to -80 °C for a duration of 12 to 24 h.

[0010] Further, the addition amount of nickel nitrate hexahydrate in step II is 0.1 to 0.3 times the molar amount of boric acid in step I.

[0011] Further, the molar ratio of melamine in step II to melamine in step I is 1:1, 2:1, 3:1 or 4:1.

[0012] Further, the inert gas in step III is nitrogen or argon.

[0013] Further, the heating rate in step III is 2 °C / min to 10 °C / min.

[0014] Advantages of the present invention:

[0015] The boron nitride / carbon nanotube electromagnetic wave absorbing material prepared by the present invention is an aerogel composite material, which has significant process and performance advantages: through a simple solution self-assembly combined with a controllable pyrolysis process, the molecular-level composite of boron nitride and carbon nanotubes is realized, solving the problems of weak interfacial bonding in the traditional physical mixing method and complex process in the chemical vapor deposition method; the obtained material combines the excellent thermal stability, chemical inertness of BN and the high conductivity of CNTs, forming a unique three-dimensional porous "fiber-spinous" heterostructure, which not only has the characteristics of light weight (low density), but also exhibits excellent electromagnetic wave absorption performance (wide frequency band, strong loss). At the same time, the preparation process does not require complex equipment, the raw materials are cheap and easy to obtain, the process parameters are highly controllable, suitable for large-scale production, and has broad application prospects in the fields of electromagnetic shielding, stealth coatings, etc.

[0016] Mechanism of the present invention:

[0017] The present invention realizes the synergistic composite of boron nitride and carbon nanotubes through the molecular self-assembly and in-situ catalytic growth mechanisms. The invention mechanism lies in: using melamine diborate (M·2B) formed by boric acid and melamine in an aqueous solution at 90 °C as a precursor template, introducing nickel nitrate hexahydrate as a catalyst and using additional melamine as a carbon source to uniformly construct a metal-organic coordination network in the solution phase; when the three-dimensional porous precursor obtained after freeze-drying is pyrolyzed in an inert atmosphere at 1000 °C, the M·2B precursor decomposes to form a boron nitride skeleton, while nickel nanoparticles catalyze the pyrolysis of melamine and in-situ grow carbon nanotubes on the surface of BN, finally forming a BN@CNTs heterostructure with strong interfacial bonding; in this process, the insulating property of BN optimizes the impedance matching of the material, while the conductive network and heterointerfacial polarization effect of CNTs enhance the electromagnetic loss performance, and the synergistic effect of the two enables the composite material to have the characteristics of light weight and porosity and excellent electromagnetic wave absorption ability.

[0018] Electromagnetic performance research shows that the wave absorption characteristics of the boron nitride / carbon nanotube wave absorbing material prepared by the present invention present a significant non-linear optimization law. The boron nitride / carbon nanotube wave absorbing material of the BN-M4 system achieves an effective absorption bandwidth of 4.8 GHz at a characteristic thickness of 1.3 mm, and its performance improvement is due to the synergistic optimization mechanism of dielectric loss and impedance matching. CST full-wave simulation confirms that the material maintains a stable scattering suppression performance with RCS ≤ -30 dB·m2 in a wide angular range of θ = 60° to 120°.

[0019] The present invention is applicable to the preparation of boron nitride / carbon nanotube (BN@CNTs) electromagnetic wave absorbing materials. Brief Description of the Drawings

[0020] Figure 1 It is a scanning electron microscope image of the boron nitride / carbon nanotube electromagnetic wave absorbing material in Example 1;

[0021] Figure 2 It is the XPS diagram of the boron nitride / carbon nanotube electromagnetic wave absorbing material in Example 1;

[0022] Figure 3 It is the two-dimensional RCS curve diagram of the boron nitride / carbon nanotube in Example 1 and the comparative example;

[0023] Figure 4 It is the two-dimensional projection diagram of the RL value of the boron nitride / carbon nanotube in Example 1 changing with frequency. Specific implementation manners

[0024] The technical solution of the present invention is not limited to the following specific implementation manners listed, and also includes any combination between the specific implementation manners.

[0025] Specific implementation manner one: A preparation method of a boron nitride / carbon nanotube electromagnetic wave absorbing material in this implementation manner is realized according to the following steps:

[0026] I. Add boric acid to deionized water, heat to 90 °C and keep stirring magnetically at a constant temperature until completely dissolved to form a transparent solution, then add melamine, keep at 90 °C and stir at a constant temperature for 0.5 h to 3 h to obtain a homogeneous and transparent solution, which is the melamine diborate (M·2B) precursor solution;

[0027] II. Add nickel nitrate hexahydrate and melamine to the above melamine diborate precursor solution in sequence, keep at 90 °C and stir at a constant temperature for 0.5 h to 3 h, then naturally cool to room temperature and stand for 24 h to 36 h, and then collect the precipitate through a vacuum filtration device. The precipitate is washed and freeze-dried to obtain a fluffy powder, which is the boron nitride / carbon nanotube (BN@CNTs) composite precursor powder;

[0028] III. Evenly spread the boron nitride / carbon nanotube composite precursor powder on a corundum boat, then place it in the constant temperature zone of a tube furnace, under the protection of an inert gas, heat to 1000 °C and keep calcining for 1 to 2 h, and naturally cool to room temperature after calcination to obtain a black porous material, which is the boron nitride / carbon nanotube electromagnetic wave absorbing material, and the preparation method is completed.

[0029] The purpose of keeping stirring at 90 °C for 0.5 h to 3 h in step II of this implementation manner is to ensure the full complexation of the metal salt and the organic ligand.

[0030] The purpose of naturally cooling to room temperature and standing in step II of this implementation manner is to ripen at room temperature to promote crystal growth.

[0031] The purpose of washing in step II of this implementation manner is to remove free ions.

[0032] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that the molar ratio of melamine to boric acid in the melamine diborate precursor solution in Step 1 is 1:3. Other steps and parameters are the same as those in Specific Embodiment 1.

[0033] Specific Embodiment 3: The difference between this embodiment and Specific Embodiment 1 is that in Step 1, it is maintained at 90 °C and stirred constantly for 1 h. Other steps and parameters are the same as those in Specific Embodiment 1.

[0034] Specific Embodiment 4: The difference between this embodiment and Specific Embodiment 1 is that the addition amount of nickel nitrate hexahydrate in Step 2 is 0.1 - 0.3 times the molar amount of boric acid in Step 1. Other steps and parameters are the same as those in Specific Embodiment 1.

[0035] Specific Embodiment 5: The difference between this embodiment and Specific Embodiment 1 is that the molar ratio of melamine in Step 2 to melamine in Step 1 is 1:1, 2:1, 3:1, or 4:1. Other steps and parameters are the same as those in Specific Embodiment 1.

[0036] In this embodiment, the molar ratio of melamine in Step 2 to melamine in Step 1 is 1:1, 2:1, 3:1, or 4:1, that is, the total molar ratio of all the melamine and boric acid added in Step 1 and Step 2 is 2:3, 3:3, 4:3, or 5:3, and the obtained boron nitride / carbon nanotube electromagnetic wave absorbing materials in subsequent preparation are named BN, BN-M2, BN-M3, BN-M4, or BN-M5 respectively.

[0037] Specific Embodiment 6: The difference between this embodiment and Specific Embodiment 1 is that in Step 2, it is maintained at 90 °C and stirred constantly for 2 h, and then naturally cooled to room temperature and left standing for 28 h. Other steps and parameters are the same as those in Specific Embodiment 1.

[0038] Specific Embodiment 7: The difference between this embodiment and Specific Embodiment 1 is that for the washing and freeze-drying in Step 2: it is washed 3 - 4 times with deionized water, and then freeze-dried at -50 °C to -80 °C for 12 - 24 h. Other steps and parameters are the same as those in Specific Embodiment 1.

[0039] Specific Embodiment 8: The difference between this embodiment and Specific Embodiment 1 is that in Step 3, it is heated to 1000 °C and calcined constantly for 1.5 h. Other steps and parameters are the same as those in Specific Embodiment 1.

[0040] Specific Embodiment 9: The difference between this embodiment and Specific Embodiment 1 is that the inert gas in Step 3 is nitrogen or argon. Other steps and parameters are the same as those in Specific Embodiment 1.

[0041] Embodiment 10: The difference between this embodiment and Embodiment 1 is that the heating rate in Step 3 is 2°C / min to 10°C / min. Other steps and parameters are the same as those in Embodiment 1.

[0042] The beneficial effects of the present invention are verified through the following examples:

[0043] Example:

[0044] A preparation method of a boron nitride / carbon nanotube electromagnetic wave absorption material is realized according to the following steps:

[0045] I. Add boric acid to deionized water, heat to 90°C and keep stirring magnetically at a constant temperature until completely dissolved to form a transparent solution, then add melamine, keep at 90°C and stir at a constant temperature for 0.5 h to obtain a homogeneous and transparent solution, which is the melamine diborate (M·2B) precursor solution;

[0046] II. Add nickel nitrate hexahydrate and melamine to the above melamine diborate precursor solution in sequence, keep at 90°C and stir at a constant temperature for 1 h, then naturally cool to room temperature and stand for 26 h, and then collect the precipitate through a vacuum filtration device. The precipitate is washed and freeze-dried to obtain a fluffy powder, which is the boron nitride / carbon nanotube (BN@CNTs) composite precursor powder;

[0047] III. Spread the boron nitride / carbon nanotube composite precursor powder evenly in a corundum boat, then place it in the constant temperature zone of a tube furnace, and under the protection of an inert gas, heat to 1000°C and keep calcining for 1.5 h. After calcination, naturally cool to room temperature to obtain a black porous material, which is the boron nitride / carbon nanotube electromagnetic wave absorption material, and the preparation method is completed.

[0048] The purpose of stirring at a constant temperature for 0.5 h at 90°C in Step II of this example is to ensure the full complexation of the metal salt and the organic ligand.

[0049] The purpose of naturally cooling to room temperature and standing in Step II of this example is to ripen at room temperature to promote crystal growth.

[0050] The purpose of washing in Step II of this example is to remove free ions.

[0051] In Step I of this example, the molar ratio of melamine to boric acid in the melamine diborate precursor solution is 1:3.

[0052] The washing and freeze-drying in Step II of this example: Wash 4 times with deionized water, and then perform freeze-drying at -70°C for 16 h.

[0053] The addition amount of nickel nitrate hexahydrate in Step II of this example is 0.2 times the molar amount of boric acid in Step I.

[0054] In step two of this embodiment, the molar ratio of the melamine described to the melamine described in step one is 3:1.

[0055] In step two of this embodiment, the molar ratio of the melamine described to the melamine described in step one is 3:1, that is, the total molar ratio of all the melamine and boric acid added in step one and step two is 4:3, and the obtained boron nitride / carbon nanotube electromagnetic wave absorption material prepared subsequently is named BN-M4.

[0056] The inert gas described in step three of this embodiment is nitrogen.

[0057] The heating rate described in step three of this embodiment is 2 °C / min.

[0058] Comparative example:

[0059] The difference between the comparative example and Example 1 is that no melamine is added in step two; others are the same as in Example 1.

[0060] The material obtained in the comparative example is named BN.

[0061] Results:

[0062] For the boron nitride / carbon nanotube electromagnetic wave absorption material obtained in Example 1:

[0063] Its scanning electron microscope image, as Figure 1 shown, the original boron nitride fiber presents a typical micro-nano fiber-like structure, with a uniform diameter distribution and a smooth surface. This high aspect ratio feature provides an ideal carrier substrate for the heterogeneous growth of subsequent carbon nanotubes. By adding melamine pyrolysis carbon source for the second time, the controllable growth of carbon nanotubes on the surface pores of BN fibers is successfully realized.

[0064] Its XPS test results are as Figure 2 shown, the characteristic peaks of B1s (190.5 eV), N 1s (398.2 eV), C1s (284.8 eV), O 1s (532.1 eV) and Ni 2p (854 eV) can be clearly identified. The appearance of oxygen element is due to surface adsorption of the material and partial oxidation during the preparation process.

[0065] Its RCS simulation analysis is as Figure 3 shown, in the single scattering mode (θ = 0° - 180°), BN-M4 shows a stable RCS value of -25 to -35 dB·m2 in the characteristic angular range of 60° - 120°, which is about 45 dB lower than the peak value on the surface of the ideal conductor, confirming its effective suppression of the specular reflection of electromagnetic waves; while the electromagnetic wave attenuation ability of the material BN obtained in the comparative example is poor, and its RCS simulation results are equivalent to those of the PEC group (blank group).

[0066] In this embodiment, the prepared boron nitride / carbon nanotube electromagnetic wave absorbing material is used to prepare test samples by the coaxial ring method. The outer diameter of the coaxial ring is 7 mm, the inner diameter is 3.04 mm, and the thickness does not affect the test results. After testing, the two-dimensional projection diagram of the reflection loss (RL) value changing with frequency is as follows Figure 4 shown. When the material thickness is 1 - 2 mm, the effective absorption bandwidth is 4.8 GHz, showing a relatively wide effective absorption bandwidth.

[0067] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a boron nitride / carbon nanotube electromagnetic wave absorption material, characterized in that It proceeds as follows:

1. Add boric acid to deionized water, heat to 90 °C and stir magnetically at a constant temperature until completely dissolved to form a transparent solution. Then add melamine, keep at 90 °C and stir at a constant temperature for 0.5 h to 3 h to obtain a homogeneous transparent solution, which is the melamine diborate precursor solution; 2. Sequentially add nickel nitrate hexahydrate and melamine to the above melamine diborate precursor solution, keep at 90 °C and stir at a constant temperature for 0.5 h to 3 h. Then naturally cool to room temperature and let stand for 24 h to 36 h. Then collect the precipitate through a vacuum filtration device. The precipitate is washed and freeze-dried to obtain a fluffy powder, which is the boron nitride / carbon nanotube composite precursor powder; 3. Evenly spread the boron nitride / carbon nanotube composite precursor powder on a corundum boat, then place it in the constant temperature zone of a tubular furnace. Under the protection of an inert gas, heat to 1000 °C and calcine at a constant temperature for 1 to 2 h. After calcination, naturally cool to room temperature to obtain a black porous material, which is the boron nitride / carbon nanotube electromagnetic wave absorption material, thus completing the preparation method.

2. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorption material according to claim 1, characterized in that In step 1, the molar ratio of melamine to boric acid in the melamine diborate precursor solution is 1:

3.

3. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorbing material according to claim 1, characterized in that In step 1, keep at 90 °C and stir at a constant temperature for 1 h.

4. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorption material according to claim 1, wherein In step 2, the washing and freeze-drying: Wash 3 to 4 times with deionized water, and then carry out freeze-drying at -50 °C to -80 °C for 12 to 24 h.

5. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorption material according to claim 1, characterized in that In step 2, the added amount of nickel nitrate hexahydrate is 0.1 to 0.3 times the molar amount of boric acid in step 1.

6. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorbing material according to claim 1, characterized in that In step 2, the molar ratio of melamine to the melamine in step 1 is 1:1, 2:1, 3:1 or 4:

1.

7. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorbing material according to claim 1, characterized in that In step 2, keep at 90 °C and stir at a constant temperature for 2 h, then naturally cool to room temperature and let stand for 28 h.

8. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorption material according to claim 1, characterized in that In step 3, heat to 1000 °C and calcine at a constant temperature for 1.5 h.

9. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorbing material according to claim 1, characterized in that In step 3, the inert gas is nitrogen or argon.

10. The preparation method of a boron nitride / carbon nanotube electromagnetic wave absorption material according to claim 1, characterized in that In step 3, the heating rate is 2 °C / min to 10 °C / min.

Images