A method for preparing high and low dielectric composite broadband absorbing material

Abstract

The invention discloses a preparation method of a high-low dielectric composite broadband absorbing material, comprising the following steps: step 1, dissolving Co(NO3)2·6H2O and dimethylimidazole in a molar ratio of 1:4 in a methanol solution, mixing, and performing coordination reaction to obtain a ZIF-67 precursor; step 2, dispersing the ZIF-67 precursor in a mixed solvent of deionized water and methanol, adding dimethylimidazole to the mixed solvent, stirring at a temperature of 25°C to 45°C to disperse the dimethylimidazole in the ZIF-67 precursor, and then adding La(NO3)3·6H2O, wherein the molar ratio of La(NO3)3·6H2O to dimethylimidazole is 1:(4-9); performing coordination reaction at a temperature of 25°C to 45°C to obtain a ZIF-67@La(OH)3 precursor; step 3, thermally decomposing the ZIF-67@La(OH)3 precursor to obtain an absorbing material. The La2O3@Co / C composite material prepared by this method is a broadband and strong electromagnetic wave absorber.

Description

Technical Field

[0001] The invention belongs to the technical field of functional materials, and in particular relates to a method for preparing a high-low dielectric composite broadband wave absorbing material. Background Art

[0002] The upgrade of radar detection and defense systems poses a severe test to the survival of aircraft. The stealth and detection accuracy of aircraft are of great significance to their penetration and attack. Advanced absorbing materials are an important foundation. The poor absorbing effect of traditional absorbing materials for meter-wave and decimeter-wave low-frequency stealth materials has become a problem. In particular, the expansion of broadband absorption at low frequencies is a bottleneck problem, and there is a great demand for low-frequency broadband absorbing materials.

[0003] In recent years, new high-performance absorbers represented by the pyrolysis conversion of metal / carbon nanocomposites into metal-organic framework (MOF) materials have attracted extensive attention from researchers at home and abroad because they can simultaneously meet the advantages of "thin thickness, low density, wide bandwidth and strong absorption" and have the potential to be applied to high-performance absorbing materials.

[0004] However, the absorption bandwidth of most MOF derivative microwave absorbers is still concentrated in the range of 4-18 GHz, while there are few reports on broadband (>6.0 GHz) MOF derivatives. Moreover, after the derivatives are pyrolyzed, the impedance does not match. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a high-low dielectric composite broadband absorbing material. The La2O3@Co / C composite material prepared by the method is a broadband and strong electromagnetic wave absorber.

[0006] The present invention adopts the following technical scheme: a method for preparing a high-low dielectric composite broadband absorbing material, the preparation method comprising the following steps:

[0007] Step 1: Dissolve Co(NO3)2·6H2O and dimethylimidazole in a methanol solution at a molar ratio of 1:4, mix them, and react them to obtain a ZIF-67 precursor;

[0008] Step 2: Disperse the ZIF-67 precursor in step 1 in a mixed solvent of deionized water and methanol, add dimethylimidazole to the mixed solvent, stir at a temperature of 25°C to 45°C to disperse the dimethylimidazole in the ZIF-67 precursor, and then add La(NO3)3·6H2O, wherein the molar ratio of La(NO3)3·6H2O to dimethylimidazole is 1:(4-9);

[0009] The coordination reaction was carried out at a temperature of 25°C to 45°C to obtain the ZIF-67@La(OH)3 precursor;

[0010] Step 3: thermally decompose the ZIF-67@La(OH)3 precursor in step 2 to obtain a microwave absorbing material.

[0011] Furthermore, in step 2, the dimethylimidazole and La(NO3)3·6H2O undergo a sol-gel reaction, and the obtained La(OH)3 is a nanorod structure.

[0012] Furthermore, in step 2, the molar ratio of deionized water to methanol is 1:1.

[0013] Furthermore, the pyrolysis treatment process in step three is as follows: heating to 700°C at a heating rate of 2K / min, pyrolyzing at 700°C, and after pyrolysis, cooling to 400°C at a cooling rate of 5K / min, and then cooling naturally.

[0014] The beneficial effects of the present invention are: 1. The impedance matching of electromagnetic waves on the surface of the absorber is achieved by adopting a high and low dielectric matching design strategy. By introducing La2O3 nanorods, the impedance mismatch problem after the thermal decomposition of the MOF precursor material is solved. 2. When La2O3 nanorods are used to interact with electromagnetic waves, the electromagnetic waves can have sufficient scattering characteristics. Due to the anisotropy of their interface, the prepared La2O3@Co / C is guaranteed to be a broadband and strong electromagnetic wave absorber. 3. Compared with the ZIF-67 precursor derivative, its absorption bandwidth is wider and the absorption intensity is higher. Compared with the Co / C composite material, its wave absorption performance far exceeds that of the traditional MOF precursor material, and it is a broadband and strong electromagnetic wave absorber. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the SEM image of La(OH)3 nanorods prepared in Example 1 of the present invention.

[0016] Figure 2 This is the SEM image of the ZIF-67@La(OH)3 composite structure prepared in Example 2 of the present invention.

[0017] Figure 3 This is the reflection loss curve of ZIF-67@La(OH)3 at the pyrolysis temperature of 700℃.

[0018] Figure 4 This is the change graph of the real part of the complex dielectric constant of La2O3 nanorods and La2O3@Co / C-600 / 700 / 800 at different temperatures.

[0019] Figure 5 The curves showing the change of the imaginary part of the complex dielectric constant of La2O3 nanorods and La2O3@Co / C-600 / 700 / 800 at different temperatures.

[0020] Figure 6Impedance diagrams of La2O3 nanorods and La2O3@Co / C-600 / 700 / 800 at different pyrolysis temperatures. DETAILED DESCRIPTION

[0021] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Example 1

[0023] The present invention provides a method for preparing a high-low dielectric composite broadband wave absorbing material, comprising the following steps:

[0024] Step 1: Dissolve Co(NO3)2·6H2O and dimethylimidazole in 20 mL of methanol solution in a molar ratio of 1:4, mix them, and react them to obtain a ZIF-67 precursor with a regular dodecahedron; wherein the molar concentration of Co(NO3)2·6H2O is 5 mmol / L, and the amount is 10 mL; the molar concentration of dimethylimidazole is 20 mmol / L, and the amount is 10 mL.

[0025] Step 2: Disperse the ZIF-67 precursor in step 1 in a mixed solvent of deionized water and methanol, the molar ratio of deionized water to methanol is 1:1, the mixed solvent is 30 mL, add dimethylimidazole to the mixed solvent, stir at 30°C for 0.5 hours to disperse the dimethylimidazole in the ZIF-67 precursor, and then add La(NO3)3·6H2O, the molar ratio of La(NO3)3·6H2O to dimethylimidazole is 1:4, the molar concentration of La(NO3)3·6H2O is 5 mmol / L, the dosage is 10 mL, and the molar concentration of dimethylimidazole is 20 mmol / L, the dosage is 10 mL.

[0026] At a temperature of 25°C to 45°C, stir for about 1 hour and let stand for 24 hours for coordination reaction to obtain ZIF-67@La(OH)3 precursor; the La(OH)3 obtained by vegetation is a nanorod structure.

[0027] Step 3: thermally decompose the ZIF-67@La(OH)3 precursor in step 2 to obtain a wave absorbing material, wherein the wave absorbing material is a La2O3@Co / C wave absorbing material.

[0028] The pyrolysis process is as follows: ZIF-67@La(OH)3 is placed in a tube furnace, heated to 700°C at a heating rate of 2K / min, pyrolyzed at 700°C, cooled to 400°C at a cooling rate of 5K / min, and then naturally cooled to room temperature. Room temperature refers to the temperature of the environment in which the product is prepared, which is generally 25°C in the laboratory.

[0029] Example 2

[0030] A method for preparing a high-low dielectric composite broadband absorbing material comprises the following steps:

[0031] Step 1: Dissolve Co(NO3)2·6H2O and dimethylimidazole in 30 mL of methanol solution at a molar ratio of 1:9, mix them, and react them to obtain a ZIF-67 precursor with a regular dodecahedron; wherein the molar concentration of Co(NO3)2·6H2O is 5 mmol / L, and the amount is 10 mL; the molar concentration of dimethylimidazole is 45 mmol / L, and the amount is 10 mL.

[0032] Step 2: Disperse the ZIF-67 precursor in step 1 in a mixed solvent of deionized water and methanol, the volume ratio of deionized water and methanol is 1:1, take 10 ml of each, add dimethylimidazole to the mixed solvent, stir for 0.5 hours at 30°C to disperse the dimethylimidazole in the ZIF-67 precursor, and then add La(NO3)3·6H2O, the molar ratio of La(NO3)3·6H2O to dimethylimidazole is 1:4, the molar concentration of La(NO3)3·6H2O is 5 mmol / L, and the dosage is 10 mL. The molar concentration of dimethylimidazole is 20 mmol / L, and the dosage is 10 mL.

[0033] At a temperature of 25°C to 45°C, stir for about 1 hour and let stand for 24 hours for coordination reaction to obtain ZIF-67@La(OH)3 precursor; the La(OH)3 obtained by vegetation is a nanorod structure.

[0034] Step three: thermally decompose the ZIF-67@La(OH)3 precursor in step two to obtain a wave absorbing material, wherein the wave absorbing material is a La2O3@Co / C wave absorbing material.

[0035] The pyrolysis process is as follows: ZIF-67@La(OH)3 is placed in a tube furnace, heated to 700°C at a heating rate of 2K / min, pyrolyzed at 700°C, cooled to 400°C at a cooling rate of 5K / min, and then naturally cooled to room temperature. Room temperature refers to the temperature of the environment in which the product is prepared, which is generally 25°C in the laboratory.

[0036] Example 3

[0037] The preparation method of La(OH)3 nanorods includes the following:

[0038] La(NO3)3·6H2O and dimethylimidazole were added to a beaker at a molar ratio of 1:4, dissolved and mixed, and then stirred for 0.5 hours. The beaker containing the mixed solution was then sealed and left to stand at 25°C for 24 hours. The white precipitate was finally washed out with ethanol and deionized water. The molar concentration of La(NO3)3·6H2O was 5mmol / L, and the dosage was 10mL. The molar concentration of dimethylimidazole was 20mmol / L, and the dosage was 10mL.

[0039] In this embodiment, the mixture after standing for 24 hours is in a white gel state. After drying, the white gel is La(OH)3 nanorods, and its microstructure is as follows: Figure 1 As shown, La(OH)3 nanorods can be obtained by the sol-gel method, and the aspect ratio of the nanorods is 15:1, the nanorods have a structure with uniform size distribution.

[0040] Example 4

[0041] Take 25 g of the ZIF-67@La(OH)3 precursor prepared in Example 1, place it in a tubular furnace, set the temperature program to 2K / min, the protective gas to nitrogen atmosphere, the pyrolysis temperature to 700°C, and after pyrolysis, naturally cool to room temperature to obtain a La2O3@Co / C composite material.

[0042] The La2O3@Co / C absorbing material and paraffin were ultrasonically heated and mixed at a mass ratio of 3:7, with an ultrasonic power of 60 kW and a temperature of 65°C to obtain a uniform mixture. The mixture was pressed into a coaxial ring with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm through a mold for testing.

[0043] The La2O3 nanorods produced by pyrolysis in Example 3 and the La2O3@Co / C composite material prepared in this example were tested. The dielectric parameters are as follows: Figure 4 and 5 As shown by Figure 4 It can be seen that as the frequency increases, the dielectric real part shows obvious dispersion characteristics. For lanthanum oxide nanorods, its dielectric real part is in the lowest range (2.78-2.64), while for La2O3@Co / C-700, its dielectric real part is second only to La2O3@Co / C-800, between 6.62-3.0. Since the dielectric real part of La2O3@Co / C-800 (15.4-8.4) is too high, the impedance matching for electromagnetic waves is weak. Figure 5 It can be seen that the imaginary part of lanthanum oxide nanorods is at the lowest value, and its imaginary part is distributed between 0.04-0.04. For La2O3@Co / C-700, its dielectric imaginary part is second only to La2O3@Co / C-800, and is between 4.75-1.82.

[0044] Depend on Figure 6 It can be seen that at 700°C, the value of the La2O3@Co / C sample is closer to 0.8-1, while compared with other samples, it is far from this range, indicating that its impedance matching with electromagnetic waves is poor. Impedance matching is calculated according to formula (2). It can be seen that the impedance matching characteristics can be adjusted by matching the low dielectric and low loss of lanthanum oxide with the high dielectric and high loss of ZIF-67. Generally speaking, the value of impedance Z is between 0.8-1, indicating that it has good impedance characteristics.

[0045] The La2O3@Co / C composite material prepared in the present invention is used as an absorbent, such as Figure 3 As shown, it can be concluded from this curve that at different matching thicknesses, the calculated reflection loss (RL) values ​​all show strong absorption performance and wide absorption bandwidth. At 2.5mm, the effective absorption bandwidth (EAB) is 4.6GHz, and the RL is -29.8dB. At 3.0mm, the EAB value is 6.9GHz, and the RL value is -48.9dB. When the matching thickness is 3.37mm, its EAB reaches the maximum, which is 8.56GHz (9.44-18GHz), almost covering the X and Ku bands. When the matching thickness is 5.0mm, the RL value is -63.0dB.) It is generally believed that when RL is less than -10dB, it means that 90% of the electromagnetic waves are absorbed, and when RL is less than -20dB, it means that 99% of the electromagnetic waves are absorbed. Among them, RL is calculated by the following formula:

Claims

1. A method for preparing a high and low dielectric composite broadband absorbing material, characterized in that: The preparation method comprises the following steps: Step 1: Dissolve Co(NO3)2·6H2O and dimethylimidazole in a methanol solution at a molar ratio of 1:4, mix them, and react them to obtain a ZIF-67 precursor; Step 2: Disperse the ZIF-67 precursor in step 1 in a mixed solvent of deionized water and methanol, add dimethylimidazole to the mixed solvent, stir at a temperature of 25°C to 45°C to disperse the dimethylimidazole in the ZIF-67 precursor, and then add La(NO3)3·6H2O, wherein the molar ratio of La(NO3)3·6H2O to dimethylimidazole is 1:(4-9); The coordination reaction was carried out at a temperature of 25°C to 45°C to obtain the ZIF-67@La(OH)3 precursor; Step 3: thermally decompose the ZIF-67@La(OH)3 precursor in step 2 to obtain a microwave absorbing material.

2. The method for preparing a high and low dielectric composite broadband absorbing material according to claim 1, characterized in that: In the step 2, the dimethylimidazole and La(NO3)3·6H2O undergo a sol-gel reaction, and the obtained La(OH)3 is a nanorod structure.

3. The method for preparing a high-low dielectric composite broadband absorbing material according to claim 1 or 2, characterized in that: In the step 2, the molar ratio of the deionized water to methanol is 1:

1.

4. The method for preparing a high and low dielectric composite broadband absorbing material according to claim 1, characterized in that: The pyrolysis treatment process in step three is as follows: heating to 700° C. at a heating rate of 2 K / min, pyrolysis at 700° C., after pyrolysis, cooling to 400° C. at a cooling rate of 5 K / min, and then cooling naturally.

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