CN114956830A - Boron nitride coated carbon nanotube reinforced polymer conversion ceramic-based wave-absorbing material and preparation method thereof - Google Patents

Abstract

The invention relates to a boron nitride coated carbon nanotube enhanced polymer conversion ceramic-based wave-absorbing material and a preparation method thereof, wherein the method comprises the steps of adding pretreated Carbon Nanotubes (CNTs) into Boron Nitride (BN) precursor solution prepared from boric acid and urea for ultrasonic dispersion treatment, and then obtaining BN coated CNTs nano powder (BN-CNTs) through a mode of multiple vacuum filtration, drying and heat treatment; uniformly dispersing the nano powder into liquid Polycarbosilane (PCS), and performing low-temperature crosslinking and high-temperature cracking heat treatment to prepare the BN-CNTs reinforced polymer-converted silicon carbide (PDC-SiC) ceramic composite material. The introduction of BN-CNTs improves the current situations of impedance mismatch and insufficient loss capacity when the current PDC-SiC ceramic is applied as a wave-absorbing material, optimizes the dielectric constant of the PDC-SiC and improves the wave-absorbing performance of the PDC-SiC.

Description

Boron nitride-coated carbon nanotube-reinforced polymer-converted ceramic-based microwave absorbing material and 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 the defense systems of various countries in the world are getting stronger and stronger, the survivability of military targets and the penetration capability of weapon systems are increasingly threatened. Therefore, the development of high-performance absorbing stealth materials has become a very important part of the modern defense system. 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 tunable, due to its special dielectric and electrical properties (varies with the cracking temperature) , making it a promising absorbing material. However, when pure PDC-SiC ceramic is used as a wave absorbing material, its dielectric loss capability is weak, the real part of the relative permittivity is too high, and the impedance matching with air is poor, which also greatly reduces its absorption. 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." published In this paper, the dielectric, electrical and microwave properties of polymer-converted silicon carbide ceramics were studied in the X-band. The ceramics were prepared at different pyrolysis temperatures (1100-1600 °C). With the increase of the cracking temperature, the content of silicon carbide nanocrystals and free carbon in PDC-SiC ceramics gradually increases, and the generated grain boundaries generate 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 capacity, the average reflectivity 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 A problem is worth thinking about and solving.

Literature 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 absorptionproperties[J]. Ceramics International, 2017, 43(16) : 14301-14308." discloses a method for preparing porous PDC-SiC/Si ₃ N ₄ ceramics modified by in situ growth of Si ₃ N ₄ nanowires, wherein Si ₃ N ₄ NWs are in situ by gas-solid (VS) mechanism The microstructure and mechanical properties of PDC-SiC/Si ₃ N ₄ porous ceramics also changed with the increase of Si ₃ N ₄ NWs content, and the minimum reflection coefficient of the whole composite also increased with the increase of PDC-SiC. This is mainly due to the enhanced electronic dipole polarization and interface scattering due to the different interfaces between the in _- situ formed PDC _- SiC nanoparticles, nanocarbons, and Si3N4 NWs. Although this technology improves the wave-absorbing performance of PDC-SiC to a certain extent, for the material with a single-layer thickness, its wave-absorbing frequency band is narrow and its practical application ability is weak.

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

Reference 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 preparing a polymer-converted-derived silicon boron carbonitride ceramic (PDC-SiBCN) containing multi-walled carbon nanotubes, wherein the multi-walled carbon nanotubes act as a nucleating agent to promote 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 aggravates the impedance matching between the entire composite material and free space, which also limits the effective bandwidth of the material in the X-band to only 3 GHz, limiting its application prospects. Therefore, the degree of impedance matching between CNTs and polymer-converted ceramics still 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 for improving polymer-converted 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,

SUMMARY 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 a preparation method, which can solve the problem of the weak PDC-SiC dielectric loss and impedance matching. Sexual issues.

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

Technical solutions

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

A preparation method of 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 the CNTs at 200-600°C for 2-5 hours, then add the heat-treated CNTs to a concentrated nitric acid solution and ultrasonicate for 0.5-2 hours, and then wash the CNTs to neutrality;

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

Step 3: In a flowing N ₂ atmosphere, the oven temperature is raised from room temperature to 800-1200°C at a heating rate of 3-10°C/min for 3-7h; the power is turned off to cool down to room temperature naturally to obtain heat treatment after the CNTs;

Repeat the above steps 2 and 3 for 2 to 6 times on 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, increase the furnace temperature from 3 to 10 °C/min at a heating rate. The room temperature was raised to 100-300 °C, and the temperature was kept for 1-3 hours; the power was turned off and cooled to room temperature naturally to obtain the cross-linked sample;

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

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

The heating in the steps 1, 3 and 4 is performed in a heat treatment furnace using a resistance wire as a heating element.

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

In the step 2, the molar ratio of boric acid and urea is 1:1～10.

In the step 4, the precursor powder 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. The pretreated carbon nanotubes (CNTs) are added to boron nitride (BN) prepared from 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 nano-powders (BN-CNTs); the above-mentioned nano-powders are uniformly dispersed into liquid polycarbosilane ( In PCS), BN-CNTs reinforced polymer-converted silicon carbide (PDC-SiC) ceramic composites were prepared by low-temperature cross-linking and high-temperature pyrolysis heat treatment. The introduction of BN-CNTs improves the current situation of impedance mismatch and insufficient loss capability when PDC-SiC ceramics are used as absorbing materials, optimizes the dielectric constant of PDC-SiC, and improves its absorbing properties.

In the present invention, CNTs are added to a BN precursor solution prepared with boric acid and urea for ultrasonic dispersion, and after suction filtration, the obtained product is dried and subjected to high temperature heat treatment to obtain BN-CNTs powder; the BN-CNTs powder is uniformly dispersed into liquid polycarbosilane, BN-CNTs reinforced ceramic matrix composites were prepared by low-temperature cross-linking and high-temperature heat treatment. The material is based on PDC-SiC ceramics and is compounded with BN-CNTs nano-powder to form a multiphase composite material containing SiC, free carbon and BN-CNTs. The introduction of BN-CNTs not only improves the electron transfer ability of the entire PDC-SiC ceramic structure and enhances the dielectric loss ability of PDC-SiC, but also the existence of BN phase increases the interfacial layer inside the composite material and improves the PDC-SiC The matched impedance with the free space of electromagnetic waves provides the microwave absorption capability of the material. At the same pyrolysis temperature, the minimum reflection coefficient of BN-CNTs-enhanced PDC-SiC is reduced to -49.47dB compared to -40.32dB of PDC-SiC, and the maximum effective absorption bandwidth (<-10dB) is 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 microscope images 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 micrometer level. After BN coating, a uniform coating layer is formed on the surface of the CNTs.

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

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 the SiC ceramic particles is about 10 μm, and the BN-CNTs are uniformly distributed on the ceramic surface.

4 is a graph showing the wave absorbing 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 material to reach the standard of effective absorption, it can be seen that the minimum reflectivity of pure PDC-SiC ceramics is -40.32dB, and the effective absorption bandwidth is 1.9GHz, while the minimum reflectivity of BN-CNTs-enhanced PDC-SiC ceramics is reduced to - 49.47dB, the effective absorption bandwidth is increased to 4.0GHz.

Detailed ways

The present invention will now be further described in conjunction with the embodiments and accompanying drawings:

Example 1:

(1) First put the CNTs into a heat treatment furnace with a resistance wire as a heating element, heat the CNTs at 400 °C for 3.5 h in an Ar atmosphere, and then add the heat treated CNTs to a 12 mol/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 200ml deionized water in a molar ratio of 1:1～10, stir magnetically for 12h until the solution becomes transparent; add the pretreated CNTs to the solution for ultrasonic dispersion treatment for 60min, 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 a resistance wire as a heating element, and in a flowing _N2 atmosphere, the furnace temperature is raised from room temperature to 800-1200 °C at a heating rate of 5 °C/min, and the temperature is kept warm. 5h; turn off the power and naturally cool to room temperature to obtain CNTs after heat treatment.

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

(4) BN-CNTs with a mass fraction of 3% and liquid polycarbosilane (PCS) were ultrasonically dispersed and mixed for 2 h; the uniformly mixed sample was placed in a heat treatment furnace with a resistance wire as a heating element, and was placed 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 h; the power was turned off and cooled to room temperature naturally to obtain a cross-linked sample;

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

(5) Put the square sample into a heat treatment furnace with a resistance wire as a 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 the temperature for 2 hours; Turn off the power and naturally cool to room temperature to obtain BN-CNTs-enhanced PDC-SiC ceramics.

Example 2

(1) First put the CNTs into a heat treatment furnace with a resistance wire as a heating element, heat the CNTs at 400 °C for 3.5 h in an Ar atmosphere, and then add the heat treated CNTs to a 12 mol/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 200ml deionized water in a molar ratio of 1:1～10, stir magnetically for 12h until the solution becomes transparent; add the pretreated CNTs to the solution for ultrasonic dispersion treatment for 60min, 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 a resistance wire as a heating element, and in a flowing _N2 atmosphere, the furnace temperature is raised from room temperature to 800-1200 °C at a heating rate of 5 °C/min, and the temperature is kept warm. 5h; turn off the power and naturally cool to room temperature to obtain CNTs after heat treatment.

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

(4) BN-CNTs with a mass fraction of 5% and liquid polycarbosilane (PCS) were ultrasonically dispersed and mixed for 2 h; the uniformly mixed sample was placed in a heat treatment furnace with a resistance wire as a heating element, and was placed in a flowing Ar atmosphere 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 h; the power was turned off and cooled to room temperature naturally to obtain a cross-linked sample;

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

(5) Put the square sample into a heat treatment furnace with a resistance wire as a 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 the temperature for 2 hours; Turn off the power and naturally cool to room temperature to obtain BN-CNTs-enhanced PDC-SiC ceramics.

Example 3

(1) First put the CNTs into a heat treatment furnace with a resistance wire as a heating element, heat the CNTs at 400 °C for 3.5 h in an Ar atmosphere, and then add the heat treated CNTs to a 12 mol/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 200ml deionized water in a molar ratio of 1:1～10, stir magnetically for 12h until the solution becomes transparent; add the pretreated CNTs to the solution for ultrasonic dispersion treatment for 60min, 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 a resistance wire as a heating element, and in a flowing _N2 atmosphere, the furnace temperature is raised from room temperature to 800-1200 °C at a heating rate of 5 °C/min, and the temperature is kept warm. 5h; turn off the power and naturally cool to room temperature to obtain CNTs after heat treatment.

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

(4) The BN-CNTs with a mass fraction of 10% were ultrasonically dispersed and mixed with liquid polycarbosilane (PCS) for 2 hours; the uniformly mixed samples were placed in a heat treatment furnace with a resistance wire as a heating element, and the samples were placed in a flowing Ar atmosphere 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 h; the power was turned off and cooled to room temperature naturally to obtain a cross-linked sample;

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

(5) Put the square sample into a heat treatment furnace with a resistance wire as a 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 the temperature for 2 hours; Turn off the power and naturally cool to room temperature to obtain BN-CNTs-enhanced PDC-SiC ceramics.

Claims (6)

1. A boron nitride coated carbon nanotube reinforced polymer conversion ceramic-based wave-absorbing material is characterized in that polymer conversion silicon carbide ceramic is taken as a matrix and is compounded with BN-CNTs nano powder to form a multiphase composite material containing SiC, free carbon, BN and CNTs; wherein the mass percentage of BN-CNTs nano powder is 1% -5%; BN-CNTs are uniformly distributed in the SiC ceramic matrix.

2. A preparation method of the boron nitride coated carbon nanotube reinforced polymer transition ceramic matrix wave-absorbing material according to claim 1 is characterized by comprising the following steps:

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

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

and step 3: dried CNTs in flowing N ₂ In the atmosphere, raising the furnace temperature from room temperature to 800-1200 ℃ at a temperature raising rate of 3-10 ℃/min, and keeping the temperature for 3-7 h; turning off a power supply, and naturally cooling to room temperature to obtain the CNTs subjected to heat treatment;

repeating the step 2 and the step 3 for 2-6 times to obtain BN-CNTs;

and 4, step 4: carrying out ultrasonic dispersion and mixing on 0-20% by mass of BN-CNTs and liquid-phase polycarbosilane PCS for 1-4 h; in a flowing Ar atmosphere, heating the furnace temperature from room temperature to 100-300 ℃ at a heating rate of 3-10 ℃/min, and keeping the temperature for 1-3 h; turning off the power supply, and naturally cooling to room temperature to obtain a cross-linked sample;

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

and 5: putting the solid block into a heat treatment furnace with a resistance wire as a heating body, heating the furnace to 800-1500 ℃ from room temperature at a heating rate of 3-10 ℃/min in a flowing Ar atmosphere, and keeping the temperature for 1-4 h; and turning off the power supply, and naturally cooling to room temperature to obtain the BN-CNTs reinforced PDC-SiC ceramic.

3. The method of claim 2, wherein: the heating in the steps 1, 3 and 4 is carried out in a heat treatment furnace using a resistance wire as a heating body.

4. The method of claim 2, wherein: the concentration of the concentrated nitric acid solution is 12 mol/L.

5. The method of claim 2, wherein: in the step 2, the molar ratio of the boric acid to the urea is 1: 1-10.

6. The method of claim 2, wherein: sieving the precursor powder in the step 4 by using a sieve of 100-400 meshes; the pressure of the tablet press is 5-20 KN.

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