CN114956830B - Boron nitride-coated carbon nanotube-reinforced polymer conversion ceramic-based wave-absorbing material and preparation method - Google Patents

Abstract

The invention relates to a boron nitride-coated carbon nanotube-reinforced polymer conversion ceramic-based wave-absorbing material and a preparation method, wherein pretreated carbon nanotubes (CNTs) are added to boron nitride (BN) prepared by boric acid and urea Ultrasonic dispersion treatment in the precursor solution, and then through multiple vacuum filtration-drying-heat treatment methods to obtain BN-coated CNTs nanopowders (BN-CNTs); uniformly disperse the above nanopowders into liquid polycarbosilane (PCS ), polymer-converted silicon carbide (PDC-SiC) ceramic composites reinforced with BN-CNTs were prepared by low-temperature crosslinking and high-temperature pyrolysis heat treatment. The introduction of BN-CNTs has improved the current situation of impedance mismatch and insufficient loss capability when PDC-SiC ceramics are used as absorbing materials, optimized the dielectric constant of PDC-SiC, and improved its absorbing performance.

Description

Boron nitride-coated carbon nanotube-reinforced polymer-converted ceramic-based wave-absorbing material and its fabrication preparation method

technical field

The invention belongs to the technical field of wave-absorbing materials, and relates to a boron nitride-coated carbon nanotube-reinforced polymer-converted ceramic-based wave-absorbing material and a preparation method.

Background technique

As the detection and tracking capabilities of defense systems around the world become stronger and stronger, the survivability of military targets and the penetration capabilities of weapon systems are increasingly threatened. Therefore, the development of high-performance microwave-absorbing stealth materials has become a very important task in modern defense systems. Important and critical directions. Polymer converted silicon carbide (PDC-SiC) ceramics have excellent high temperature creep resistance and chemical stability, and the preparation process is simple and adjustable, due to its special dielectric and electrical properties (changes with the cracking temperature) , making it a promising absorbing material. However, when pure PDC-SiC ceramics are used as absorbing materials, their dielectric loss capability is weak and the real part of relative permittivity is too high, and the impedance matching with air is poor, which also greatly reduces its absorbing properties. wave capability. So how to improve the impedance matching of PDC-SiC ceramics and improve its absorbing performance is urgently needed.

Document 1 "Li Q, Yin X, Duan W, et al. Electrical, dielectric and microwave-absorption properties of polymer derived SiC ceramics in X band[J]. Journal of alloys and compounds, 2013, 565:66-72." Publication Research on the dielectric, electrical and microwave properties of polymer-converted silicon carbide ceramics in the X-band. The ceramics were prepared at different cracking temperatures (1100-1600°C). The research found that in the range of 8.2-12.4GHz (X-band), As the cracking temperature increases, the content of silicon carbide nanocrystals and free carbon in PDC-SiC ceramics gradually increases, and the resulting grain boundaries produce space charge polarization and interface relaxation under the action of electromagnetic waves, thus consuming electromagnetic waves. However, due to the poor impedance matching of the material and insufficient loss capability, the average reflectance of the sample cracked at 1400°C is only -9.97dB, which cannot reach the standard of -10dB reflectivity under effective loss. How to improve this One problem is worth thinking about and solving.

Document 2 "Hong W, Dong S, Hu P, et al.In situ growth of one-dimensional nanowires on porous PDC-SiC/Si ₃ N ₄ ceramics with excellent microwave absorption properties[J].Ceramics International,2017,43(16) :14301-14308." discloses a method for the in-situ growth of Si ₃ N ₄ nanowire-modified porous PDC-SiC/Si ₃ N ₄ ceramics, in which Si ₃ N ₄ NWs are in situ by a gas-solid (VS) mechanism Formed in the pores, as the content of Si ₃ N ₄ NWs increases, the microstructure and mechanical properties of PDC-SiC/Si ₃ N ₄ porous ceramics also change, and the minimum reflection coefficient of the entire composite material also increases with the increase of the content of PDC-SiC The improvement is achieved with the increase of the content of Si 3 N 4 NWs, which is mainly due to the enhanced electron dipole polarization and interfacial scattering by the different interfaces between the in situ formed PDC-SiC nanoparticles, nanocarbons, and Si ₃ N ₄ NWs. Although this technology improves the microwave-absorbing performance of PDC-SiC to a certain extent, for the single-layer thickness of the material, its microwave-absorbing frequency band is narrow, and its practical application ability is weak.

Carbon nanotubes (CNTs) are high-performance absorbing materials with low density, good stability, large specific surface area, and high conductivity, and are often selected as nanofill phases to improve the dielectric loss capability of the matrix.

Document 3 "Zhang Y, Yin X, Ye F, et al. Effects of multi-walled carbonnanotubes on the crystallization behavior of PDCs-SiBCN and their improved dielectric and EM absorbing properties [J]. Journal of the European Ceramic Society, 2014, 34( 5): 1053-1061." discloses a method for the preparation of polymer conversion-derived silicon boron carbonitride ceramics (PDC-SiBCN) containing multi-walled carbon nanotubes, in which multi-walled carbon nanotubes are used as nucleating agents to promote The heterogeneous nucleation reduces the crystallization temperature of SiC in SiBCN, and the A(SiBCN matrix)+B(SiC)+C(MWCNTs) structure formed in MWCNTs-SiBCN is beneficial to improve the dielectric properties and Electromagnetic absorption properties. However, the high electrical conductivity of MWCNTs intensifies the impedance matching between the entire composite material and free space, which also makes the effective bandwidth of the material in the X-band only 3 GHz, limiting its application prospects. Therefore, the degree of impedance matching between CNTs and polymer-converted ceramics needs to be improved.

As a traditional two-dimensional material, hexagonal boron nitride (h-BN) has good oxidation resistance and low relative permittivity, and is an excellent candidate material for improving polymer conversion ceramics. The introduction of BN phase can reduce the composite The real part of the relative permittivity of the material, so as to meet the requirements of impedance matching and improve the absorbing performance of the material. Therefore,

Contents of the invention

technical problem to be solved

In order to avoid the deficiencies of the prior art, the present invention proposes a boron nitride-coated carbon nanotube-reinforced polymer-converted ceramic-based wave-absorbing material and its preparation method to solve the problem of weak dielectric loss and impedance matching of PDC-SiC problem of poor sex.

The invention provides a method for preparing a boron nitride-coated carbon nanotube-reinforced polymer conversion ceramic-based wave-absorbing material. First, boric acid and urea were used as reaction raw materials to prepare a BN precursor solution, and then the BN-coated CNTs nanophase (BN@CNTs) was prepared through the steps of ultrasonic dispersion-multiple vacuum filtration-drying-heat treatment. PDC-SiC ceramic composites with uniform distribution of BN@CNTs were obtained by cross-linking and pyrolysis heat treatment. The BN@CNTs-enhanced PDC-SiC obtained in the present invention can effectively improve the current situation of impedance mismatch and insufficient loss capability when PDC-SiC ceramics are used as wave-absorbing materials.

Technical solutions

A boron nitride-coated carbon nanotube-reinforced polymer-converted ceramic-based wave-absorbing material is characterized in that polymer-converted silicon carbide ceramics are used as a matrix and combined with BN@CNTs nanopowders to form a composite material containing SiC, A multiphase composite material of free carbon, BN and CNTs; the mass percentage of BN@CNTs nanopowder is 1% to 5%; BN@CNTs is uniformly distributed in the SiC ceramic matrix.

A method for preparing the boron nitride-coated carbon nanotube-reinforced polymer-converted ceramic-based wave-absorbing material, characterized in that the steps are as follows:

Step 1: In an Ar atmosphere, heat-treat CNTs at 200-600°C for 2-5 hours, then add the heat-treated CNTs into concentrated nitric acid solution and sonicate for 0.5-2 hours, and then wash the CNTs until neutral;

Step 2: Mix and disperse boric acid and urea in deionized water, stir magnetically for 10-15 hours until the solution becomes transparent; add the CNTs pretreated in step 1 into the solution and ultrasonically disperse for 30-90 minutes, use a vacuum filtration device to collect CNTs and dry Dry;

Step 3: In a flowing _N2 atmosphere, raise the furnace temperature from room temperature to 800-1200 °C at a rate of 3-10 °C/min and keep it warm for 3-7 hours; turn off the power and naturally cool to room temperature to obtain heat treatment After the CNTs;

Repeat the above steps 2 and 3 for 2 to 6 times for the heat-treated CNTs to obtain BN@CNTs;

Step 4: ultrasonically disperse and mix BN@CNTs with a mass fraction of 0% to 20% and liquid polycarbosilane PCS for 1 to 4 hours; in a flowing Ar atmosphere, raise the furnace temperature from Rise the room temperature to 100-300°C and keep it warm for 1-3 hours; turn off the power and naturally cool to room temperature to obtain a cross-linked sample;

The cross-linked sample is fully ground and sieved to obtain the precursor powder, and the powder is pressed into a solid block;

Step 5: Put the solid block into a heat treatment furnace with resistance wire as the heating element, in a flowing Ar atmosphere, raise the furnace temperature from room temperature to 800-1500°C at a heating rate of 3-10°C/min, and keep it warm 1-4h; turn off the power and cool down to room temperature naturally to obtain PDC-SiC ceramics reinforced by BN@CNTs.

The step 1, step 3 and step 4 are heated in a heat treatment furnace using a resistance wire as a heating element.

The concentration of the concentrated nitric acid solution is 12mol/L.

The molar ratio of boric acid to urea in step 2 is 1:1-10.

The precursor powder in step 4 is passed through a 100-400 mesh sieve; the pressure of the tablet press is 5-20KN.

Beneficial effect

The present invention proposes a boron nitride-coated carbon nanotube-reinforced polymer-converted ceramic-based wave-absorbing material and a preparation method thereof, in which pretreated carbon nanotubes (CNTs) are added to boron nitride (BN) prepared from boric acid and urea ) in the precursor solution for ultrasonic dispersion treatment, and then through multiple vacuum filtration-drying-heat treatment methods to obtain BN-coated CNTs nanopowders (BN@CNTs); the above nanopowders were uniformly dispersed into liquid polycarbosilane ( In PCS), polymer-converted silicon carbide (PDC-SiC) ceramic composites reinforced by BN@CNTs were prepared by low-temperature crosslinking and high-temperature pyrolysis heat treatment. The introduction of BN@CNTs has improved the current situation of impedance mismatch and insufficient loss capability when PDC-SiC ceramics are used as wave-absorbing materials, optimized the dielectric constant of PDC-SiC, and improved its wave-absorbing performance.

In the present invention, CNTs are added to the BN precursor solution prepared with boric acid and urea for ultrasonic dispersion, and after suction filtration, the obtained product is dried and heat-treated at high temperature to obtain BN@CNTs powder; the BN@CNTs powder is evenly dispersed into the liquid polycarbosilane, BN@CNTs reinforced ceramic matrix composites were prepared by low temperature crosslinking and high temperature heat treatment. The material is based on PDC-SiC ceramics and combined with BN@CNTs nanopowder to form a multi-phase composite material containing SiC, free carbon and BN@CNTs. The introduction of BN@CNTs not only improves the electron transfer capability of the entire PDC-SiC ceramic structure, but also enhances the dielectric loss capability of PDC-SiC, and the existence of the BN phase increases the interfacial layer inside the composite material, improving the PDC-SiC The matching impedance with the free space of electromagnetic waves provides the microwave absorbing ability of the material. At the same cracking temperature, the minimum reflection coefficient of BN@CNTs-enhanced PDC-SiC dropped to -49.47dB compared to -40.32dB of PDC-SiC, and the maximum effective absorption bandwidth (<-10dB) increased from 1.9GHz to 4.0 GHz. In addition, the PDC-SiC prepared by the present invention has the characteristics of low raw material input and equipment cost and high output, is suitable for large-scale production, and has good application prospects.

Description of drawings

Figure 1 shows the scanning electron micrographs of the as-prepared BN@CNTs (b) and pristine CNTs (a), respectively. It can be clearly seen that the diameter of the original carbon nanotubes is between 40 and 50 nm, and the length reaches the micron level. After BN coating, a uniform coating layer is formed on the surface of CNTs.

Figure 2 shows the transmission electron micrographs of the prepared BN@CNTs (a) (b) and pristine CNTs (c), respectively. It can be seen that the thickness of the coated BN phase is about 10nm.

Fig. 3 is the SEM images of PDC-SiC (a) and BN@CNTs reinforced PDC-SiC (b) ceramic materials prepared by the present invention, respectively. It can be seen that the particle size of SiC ceramic particles is about 10 μm, and BN@CNTs are evenly distributed on the ceramic surface.

Fig. 4 is a diagram of the microwave absorption properties of the prepared pure PDC-SiC ceramics (a) and the BN@CNTs reinforced PDC-SiC ceramics (b) prepared by the present invention. Taking -10dB as the standard for material to achieve effective absorption, it can be seen that the minimum reflectance of pure PDC-SiC ceramics is -40.32dB, and the effective absorption bandwidth is 1.9GHz, while the minimum reflectance of PDC-SiC ceramics enhanced by BN@CNTs is reduced to - 49.47dB, the effective absorption bandwidth is increased to 4.0GHz.

Detailed ways

Now in conjunction with embodiment, accompanying drawing, the present invention will be further described:

Example 1:

(1) First put the CNTs into a heat treatment furnace with resistance wire as the heating element, heat treat the CNTs at 400°C for 3.5 hours in an Ar atmosphere, and then add the heat-treated CNTs to a 12mol/L concentrated nitric acid solution for ultrasonic treatment for 0.5 h, and then wash the CNTs to neutrality;

(2) Mix and disperse boric acid and urea in a molar ratio of 1:1 to 10 in 200ml of deionized water, stir magnetically for 12 hours until the solution becomes transparent; add the pretreated CNTs to the solution for ultrasonic dispersion treatment for 60 minutes, and use vacuum pumping The filter device collects CNTs and dries them for later use;

(3) Put the dried CNTs into a heat treatment furnace with resistance wire as a heating element, in a flowing _N2 atmosphere, raise the furnace temperature from room temperature to 800-1200 °C at a heating rate of 5 °C/min, and keep warm 5h; turn off the power and cool down to room temperature naturally to obtain heat-treated CNTs.

Repeat the above steps 2 and 3 4 times for the heat-treated CNTs to obtain BN@CNTs;

(4) Ultrasonic dispersion and mixing of BN@CNTs with a mass fraction of 3% and liquid polycarbosilane (PCS) for 2 hours; the uniformly mixed sample was placed in a heat treatment furnace with a resistance wire as a heating element, and was heated in a flowing Ar atmosphere. In the process, the furnace temperature was raised from room temperature to 100-400 °C at a heating rate of 5 °C/min, and kept for 2 hours; the power was turned off and naturally cooled to room temperature to obtain a cross-linked sample;

Fully grind and sieve the cured sample to obtain a precursor powder of 100-400 meshes, and use a pressure of 5-20KN to press the powder into a square sample with a size of 22.86mm×10.16mm×2.00mm;

(5) Put the square sample into a heat treatment furnace with resistance wire as the heating element, in a flowing Ar atmosphere, raise the furnace temperature from room temperature to 800-1500 °C at a heating rate of 5 °C/min, and keep it warm for 2 hours; Turn off the power and cool down to room temperature naturally to obtain PDC-SiC ceramics reinforced by BN@CNTs.

Example 2

(1) First put the CNTs into a heat treatment furnace with resistance wire as the heating element, heat treat the CNTs at 400°C for 3.5 hours in an Ar atmosphere, and then add the heat-treated CNTs to a 12mol/L concentrated nitric acid solution for ultrasonic treatment for 0.5 h, and then wash the CNTs to neutrality;

(2) Mix and disperse boric acid and urea in a molar ratio of 1:1 to 10 in 200ml of deionized water, stir magnetically for 12 hours until the solution becomes transparent; add the pretreated CNTs to the solution for ultrasonic dispersion treatment for 60 minutes, and use vacuum pumping The filter device collects CNTs and dries them for later use;

(3) Put the dried CNTs into a heat treatment furnace with resistance wire as a heating element, in a flowing _N2 atmosphere, raise the furnace temperature from room temperature to 800-1200 °C at a heating rate of 5 °C/min, and keep warm 5h; turn off the power and cool down to room temperature naturally to obtain heat-treated CNTs.

Repeat the above steps 2 and 3 4 times for the heat-treated CNTs to obtain BN@CNTs;

(4) Ultrasonic dispersion and mixing of BN@CNTs with a mass fraction of 5% and liquid polycarbosilane (PCS) for 2 hours; the uniformly mixed sample was placed in a heat treatment furnace with a resistance wire as a heating element, and was heated in a flowing Ar atmosphere. In the process, the furnace temperature was raised from room temperature to 100-400 °C at a heating rate of 5 °C/min, and kept for 2 hours; the power was turned off and naturally cooled to room temperature to obtain a cross-linked sample;

Fully grind and sieve the cured sample to obtain a precursor powder of 100-400 meshes, and use a pressure of 5-20KN to press the powder into a square sample with a size of 22.86mm×10.16mm×2.00mm;

(5) Put the square sample into a heat treatment furnace with resistance wire as the heating element, in a flowing Ar atmosphere, raise the furnace temperature from room temperature to 800-1500 °C at a heating rate of 5 °C/min, and keep it warm for 2 hours; Turn off the power and cool down to room temperature naturally to obtain PDC-SiC ceramics reinforced by BN@CNTs.

Example 3

(1) First put the CNTs into a heat treatment furnace with resistance wire as the heating element, heat treat the CNTs at 400°C for 3.5 hours in an Ar atmosphere, and then add the heat-treated CNTs to a 12mol/L concentrated nitric acid solution for ultrasonic treatment for 0.5 h, and then wash the CNTs to neutrality;

(2) Mix and disperse boric acid and urea in a molar ratio of 1:1 to 10 in 200ml of deionized water, stir magnetically for 12 hours until the solution becomes transparent; add the pretreated CNTs to the solution for ultrasonic dispersion treatment for 60 minutes, and use vacuum pumping The filter device collects CNTs and dries them for later use;

(3) Put the dried CNTs into a heat treatment furnace with resistance wire as a heating element, in a flowing _N2 atmosphere, raise the furnace temperature from room temperature to 800-1200 °C at a heating rate of 5 °C/min, and keep warm 5h; turn off the power and cool down to room temperature naturally to obtain heat-treated CNTs.

Repeat the above steps 2 and 3 4 times for the heat-treated CNTs to obtain BN@CNTs;

(4) Ultrasonic dispersion and mixing of BN@CNTs with a mass fraction of 10% and liquid polycarbosilane (PCS) for 2 hours; the uniformly mixed sample was placed in a heat treatment furnace with a resistance wire as a heating element, and was heated in a flowing Ar atmosphere. In the process, the furnace temperature was raised from room temperature to 100-400 °C at a heating rate of 5 °C/min, and kept for 2 hours; the power was turned off and naturally cooled to room temperature to obtain a cross-linked sample;

Fully grind and sieve the cured sample to obtain a precursor powder of 100-400 meshes, and use a pressure of 5-20KN to press the powder into a square sample with a size of 22.86mm×10.16mm×2.00mm;

(5) Put the square sample into a heat treatment furnace with resistance wire as the heating element, in a flowing Ar atmosphere, raise the furnace temperature from room temperature to 800-1500 °C at a heating rate of 5 °C/min, and keep it warm for 2 hours; Turn off the power and cool down to room temperature naturally to obtain PDC-SiC ceramics reinforced by BN@CNTs.

Claims (5)

1. A boron nitride coated carbon nano tube reinforced polymer conversion ceramic-based wave absorbing material is characterized in that polymer conversion silicon carbide ceramic is used as a matrix to be compounded with BN@CNTs nano powder, so that a multiphase composite material containing SiC, free carbon, BN and CNTs is formed; wherein the mass percentage of BN@CNTs nano powder is 1% -5%; BN@CNTs are uniformly distributed in the SiC ceramic matrix; the preparation method of the boron nitride coated carbon nanotube reinforced polymer conversion ceramic-based wave-absorbing material comprises the following steps:

step 1: performing heat treatment on CNTs for 2-5 hours at 200-600 ℃ in Ar atmosphere, adding the heat-treated CNTs into concentrated nitric acid solution, performing ultrasonic treatment for 0.5-2 hours, and washing the CNTs to be neutral;

step 2: mixing boric acid and urea, dispersing in deionized water, and magnetically stirring for 10-15 h until the solution is transparent; adding the CNTs pretreated in the step 1 into the solution, carrying out ultrasonic dispersion treatment for 30-90 min, collecting the CNTs by using a vacuum suction filtration device, and drying;

step 3: dried CNTs in flowing N ₂ In the atmosphere, the furnace temperature is raised from room temperature to 800-1200 ℃ at a heating rate of 3-10 ℃/min, and the temperature is kept for 3-7 h; closing a power supply, and naturally cooling to room temperature to obtain CNTs after heat treatment;

repeating the steps 2 and 3 for 2 to 6 times to obtain BN@CNTs;

step 4: mixing BN@CNTs with mass fraction of 0% -20% with liquid phase polycarbosilane PCS in an ultrasonic dispersion way for 1-4 h; in flowing Ar atmosphere, raising the furnace temperature from room temperature to 100-300 ℃ at a heating rate of 3-10 ℃/min, and preserving heat for 1-3 h; naturally cooling the sample to room temperature after the power supply is turned off, so as to obtain a crosslinked sample;

fully grinding and screening the crosslinked sample to obtain precursor powder, and pressing the powder into solid blocks;

step 5: placing the solid block into a heat treatment furnace taking a resistance wire as a heating body, heating the furnace from room temperature to 800-1500 ℃ at a heating rate of 3-10 ℃/min in flowing Ar atmosphere, and preserving heat for 1-4 h; and (3) closing the power supply, and naturally cooling to room temperature to obtain the PDC-SiC ceramic reinforced by BN@CNTs.

2. The boron nitride coated carbon nanotube reinforced polymer converted ceramic based wave absorbing material of claim 1, wherein: the heating in the step 1, the step 3 and the step 4 is performed in a heat treatment furnace adopting a resistance wire as a heating body.

3. The boron nitride coated carbon nanotube reinforced polymer converted ceramic based wave absorbing material of claim 1, wherein: the concentration of the concentrated nitric acid solution is 12mol/L.

4. The boron nitride coated carbon nanotube reinforced polymer converted ceramic based wave absorbing material of claim 1, wherein: and 2, the molar ratio of boric acid to urea in the step is 1:1-10.

5. The boron nitride coated carbon nanotube reinforced polymer converted ceramic based wave absorbing material of claim 1, wherein: the precursor powder in the step 4 is sieved by a 100-400 mesh sieve; the pressure of the tablet press is 5-20 KN.