Conductive foam filled aramid honeycomb wave-absorbing composite material as well as preparation method and application thereof

Abstract

The invention provides a conductive foam filled aramid honeycomb wave-absorbing composite material as well as a preparation method and application thereof, and belongs to the technical field of advanced materials. The aramid fiber honeycomb wave-absorbing composite material is prepared by pressing a mixture obtained by mixing thermal expansion microspheres and multi-walled carbon nanotubes into aramid fiber honeycomb holes and foaming. The wave-absorbing composite material prepared by the invention has the characteristic of broadband absorption and is excellent in wave-absorbing effect; meanwhile, compared with an existing filling and dipping process, the preparation method does not involve addition and blending of any reagent, the production cost can be saved, and the preparation process can be simplified; the aramid honeycomb holes are completely and uniformly filled, the aramid honeycomb material is not processed and damaged in the preparation process, and the mechanical bearing capacity of the prepared wave-absorbing composite material is obviously enhanced compared with that of the wave-absorbing composite material prepared by a traditional dipping or filling process. Therefore, the wave-absorbing composite material and the preparation method thereof have wide application prospects in the field of electromagnetic wave stealth.

Description

Technical Field

[0001] The invention belongs to the technical field of advanced materials, and in particular relates to a conductive foam-filled aramid honeycomb wave-absorbing composite material, a preparation method thereof and uses thereof. Background Art

[0002] Nowadays, as a key tool for information transmission and radar detection, electromagnetic waves have made great progress in the fields of communication technology and radar exploration. However, electromagnetic wave technology will inevitably cause electromagnetic interference and electromagnetic pollution, which will have a negative impact on high-precision instruments and human health. In addition, in order to meet the needs of stealth technology, it is crucial to reduce the radar cross-section of equipment such as aircraft and ships. Therefore, in order to solve electromagnetic pollution and achieve the stealth performance of equipment, researchers have proposed a feasible solution, which is to design and develop electromagnetic wave absorbing materials to absorb excess electromagnetic wave energy. Although great progress has been made in the field of traditional microwave absorption, it still faces challenges such as broadband absorption and limited carrying capacity, which limits the development speed of practical applications. Developing and designing absorbing materials that are simple to prepare, broadband absorbing, and have a certain carrying capacity to meet the requirements of radar detection and equipment stealth has important research significance and application value.

[0003] New carbon-based materials such as graphene, carbon nanotubes, metal organic frameworks, etc. are different from traditional materials. They have the characteristics of light weight, large specific surface area, good chemical stability, and enhanced mechanical properties. Multi-walled carbon nanotubes are super fibers among carbon-based absorbing materials. They have outstanding advantages such as low density, high temperature resistance, and significant elastic deformation ability. Compared with other carbon-based materials, multi-walled carbon nanotubes have higher dielectric loss tangent, aspect ratio, mechanical strength and polarization effect, and can be better dispersed in the matrix. In addition, multi-walled carbon nanotubes also have good stability and electrical loss capacity, making them ideal microwave absorbing materials. However, when multi-walled carbon nanotubes are used as absorbing fillers to form absorbing coatings, the overall dielectric constant is too high, resulting in poor impedance matching of the material, affecting its microwave absorption performance.

[0004] In order to solve the problem that the multi-walled carbon nanotubes are not effective as wave-absorbing fillers, researchers have optimized a variety of solutions. For example, the patent with announcement number CN107365567B uses magnetic materials to reduce the dielectric constant, and the patent with announcement number CN111500255B uses barium ferrite to cover carbon tubes for good impedance matching. However, many optimization methods require complex preparation processes or the introduction of other materials for coating, which is not only difficult to accurately control but also puts great pressure on environmental protection. Therefore, it is necessary to further optimize the method of preparing wave-absorbing materials using multi-walled carbon nanotubes as wave-absorbing fillers. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the present invention provides a conductive foam filled aramid honeycomb wave absorbing composite material and a preparation method and use thereof.

[0006] The invention provides an aramid honeycomb wave-absorbing composite material, which is obtained by pressing a mixture of heat-expanded microspheres and multi-walled carbon nanotubes into aramid honeycomb holes for foaming.

[0007] Furthermore, the weight ratio of the thermally expandable microspheres to the multi-walled carbon nanotubes is 3:(3-7).

[0008] Furthermore, the weight ratio of the thermally expandable microspheres to the multi-walled carbon nanotubes is 3:5.

[0009] Furthermore, the particle size of the heat-expandable microspheres is 20 to 30 μm, the expansion ratio is 9 to 10 times, the initial foaming temperature is 125 to 135° C., and the optimal foaming temperature is 165 to 180° C.;

[0010] And / or, the multi-walled carbon nanotubes have a length of 15 to 30 μm, a tube diameter of 3 to 15 μm, and a resistivity of 1300-3000 μΩ·m.

[0011] Furthermore, the mixture of the heat-expandable microspheres and the multi-walled carbon nanotubes is ball-milled before being pressed into the aramid honeycomb holes, the ball-milling time is 5 to 15 minutes, and the rotation speed is 60 to 300 r / min.

[0012] Furthermore, the foaming method comprises the following steps:

[0013] The mixture of heat-expandable microspheres and multi-walled carbon nanotubes is pressed into the pores of the aramid honeycomb and then subjected to hot-pressing foaming;

[0014] Preferably, during the hot pressing foaming, the upper plate temperature of the hot pressing machine is 100-150°C, the lower plate temperature is 90-140°C, the pressure is 0.1-1.0 MPa, and the hot pressing time is 10-15 min;

[0015] More preferably, during the hot pressing foaming, the temperature difference between the upper and lower plates of the hot pressing machine is 2-10° C., and the hot pressing time is 15 minutes.

[0016] Furthermore, the density of the aramid honeycomb is 32-160 kg / m 3 , the side length is 1.83~5.50mm.

[0017] The present invention also provides a method for preparing the aforementioned aramid honeycomb wave absorbing composite material, which comprises the following steps:

[0018] (1) dry-mixing heat-expandable microspheres and multi-walled carbon nanotubes to obtain a mixture;

[0019] (2) The mixture obtained in step (1) is pressed into the pores of an aramid honeycomb for foaming.

[0020] The present invention also provides use of the aforementioned aramid honeycomb wave-absorbing composite material in preparing a wave-absorbing material.

[0021] Furthermore, the absorbing material is a stealth material;

[0022] Preferably, the stealth material is a stealth material used in the aerospace field.

[0023] Currently, there are two main preparation processes for the functionalization of aramid honeycomb cores: filling and impregnation. Although certain effects have been achieved in terms of wave absorption, the complex preparation process and the use of harmful reagents in existing research are two major problems that plague the large-scale preparation of functionalized aramid honeycombs.

[0024] The patent with announcement number CN109294519B uses multi-layer graphene sponge to gradient fill aramid honeycomb to achieve the preparation of composite materials with broadband absorption of electromagnetic waves and mechanical bearing function. The principle is that the reduced graphene sponge has a certain wave absorption ability, and the gradient effect is used to achieve broadband absorption of electromagnetic waves. However, the pre-steps for obtaining the graphene sponge include the preparation of graphene oxide hydrogel, the preparation of graphene oxide aerogel and the reduction of graphene oxide aerogel, which involve complicated processes and too many harmful reagents, making it difficult to control variables and difficult to carry out large-scale preparation. In addition, due to the limited mechanical properties of the graphene sponge, the overall composite material load-bearing capacity is also limited.

[0025] Patent No. CN109851995B uses a chemical reaction process to coat ferric chloride, ferrous chloride and polyaniline on the surface of expanded microspheres. This method gives the expanded microspheres magnetic loss and dielectric loss, making them have multiple loss mechanisms for electromagnetic waves. However, the preparation process of this method is complicated, including co-precipitation, in-situ polymerization, resin blending, curing, etc., involving too many reagent condition variables, which ultimately leads to poor and unstable wave absorption performance.

[0026] In addition, most traditional honeycomb fillings use a resin foaming system to obtain foam honeycomb composite materials, in which different amounts of absorbers added will inevitably have an adverse effect on the foaming system, leading to problems such as foam failure, deformation, and brittleness, limiting the further development of honeycomb composite materials.

[0027] The present invention first obtains a mixture by dry mixing heat-expandable microspheres and multi-walled carbon nanotubes, and then obtains an aramid honeycomb absorbing composite material by heating and limiting foaming the aramid honeycomb and the mixture for a certain period of time. The present invention adopts limited foaming based on heat-expandable microspheres, and foam with good mechanical properties can be obtained after heating and limiting foaming, and the aramid honeycomb core is filled in situ to enhance the bearing capacity of the overall composite material. The present invention uses multi-walled carbon nanotubes as absorbing fillers, which have better dispersibility when dry mixed with heat-expandable microspheres than other carbon-based absorbing materials, so that the prepared aramid honeycomb absorbing composite material has more stable absorbing performance.

[0028] When preparing the aramid honeycomb wave-absorbing composite material, the wave-absorbing performance is only related to the weight ratio of the selected heat-expandable microspheres and multi-walled carbon nanotubes, and has nothing to do with other variables in the process. Therefore, the present invention is suitable for aramid honeycomb cores of different densities, pore sizes, and sizes. The filling of aramid honeycomb cores of different properties and the reproduction of stable wave-absorbing capabilities can be achieved by simply controlling the weight ratio of the heat-expandable microspheres and multi-walled carbon nanotubes. Therefore, the present invention has wide applicability.

[0029] Therefore, the present invention solves the problems of complex preparation and abuse of harmful reagents of functionalized aramid honeycombs, realizes in-situ foaming and filling of aramid honeycomb cores, and the preparation process is simple and highly repeatable. No harmful reagents are used in the process, and no adhesives and foaming agents are added. Only thermal expansion microspheres and multi-walled carbon nanotubes are used to give the aramid honeycomb core wave absorption and stronger load-bearing capacity. This has not been reported in aramid honeycomb wave-absorbing composite materials and has significant advantages.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The preparation method of the present invention is simple, and the steps only include dry mixing of heat-expandable microspheres and multi-walled carbon nanotubes and filling of aramid honeycombs, and does not involve the addition and preparation of any reagents. Compared with traditional filling and impregnation processes, it can save production costs and simplify the preparation process.

[0032] 2. The mixture of heat-expandable microspheres and multi-walled carbon nanotubes of the present invention completely and uniformly fills the space inside the aramid honeycomb pores. The preparation process does not involve processing and destruction of the aramid honeycomb material. Compared with the traditional impregnation or filling process, the mechanical bearing capacity of the composite material is significantly enhanced.

[0033] 3. The broadband wave absorbing effect achieved by the present invention is only controlled by the weight ratio of the heat-expandable microspheres and the multi-walled carbon nanotubes and has nothing to do with other process conditions, thereby ensuring that the performance of the prepared composite material is stable and the process is repeatable; the functionalized aramid honeycomb material finally obtained by the present invention has excellent load-bearing and wave absorbing capabilities.

[0034] In summary, the present invention obtains a mixture by dry-mixing heat-expandable microspheres and multi-walled carbon nanotubes, and then heats the aramid honeycomb and the mixture and restricts foaming to obtain an aramid honeycomb absorbing composite material. The absorbing composite material prepared by the present invention has the characteristics of broadband absorption, and the absorbing effect is only controlled by the weight ratio of the heat-expandable microspheres and the multi-walled carbon nanotubes, ensuring that the prepared absorbing composite material has stable performance and the process is repeatable; at the same time, compared with the existing filling and impregnation processes, the preparation method of the present invention does not involve the addition and preparation of any reagents, which can save production costs and simplify the preparation process; and the present invention achieves complete and uniform filling of the aramid honeycomb holes, and the preparation process does not involve processing and destruction of the aramid honeycomb material. Compared with the traditional impregnation or filling process, the mechanical bearing capacity of the prepared absorbing composite material is significantly enhanced. Therefore, the absorbing composite material of the present invention and its preparation method have broad application prospects in the field of electromagnetic wave stealth.

[0035] Obviously, according to the above contents of the present invention, in accordance with common technical knowledge and customary means in the art, without departing from the above basic technical ideas of the present invention, other various forms of modification, replacement or change may be made.

[0036] The above contents of the present invention are further described in detail below through specific implementation methods in the form of embodiments. However, this should not be understood as the scope of the above subject matter of the present invention being limited to the following examples. All technologies realized based on the above contents of the present invention belong to the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is the reflection loss result of the wave-absorbing composite material obtained in Example 1.

[0038] Figure 2 This is the reflection loss result of the wave-absorbing composite material obtained in Example 2.

[0039] Figure 3 This is the reflection loss result of the wave-absorbing composite material obtained in Example 3.

[0040] Figure 4 This is the reflection loss result of the wave-absorbing composite material obtained in Comparative Example 1.

[0041] Figure 5 This is the reflection loss result of the wave-absorbing composite material obtained in Comparative Example 2.

[0042] Figure 6The optical pictures of the aramid honeycomb without any treatment and the aramid honeycomb of different sizes obtained by Example 1 are as follows: a-1 is a top view of the aramid honeycomb of length 5 cm×width 5 cm×height 1.5 cm; a-2 is a top view of the aramid honeycomb of length 5 cm×width 5 cm×height 1.5 cm; b-1 is a top view of the aramid honeycomb of length 18 cm×width 18 cm×height 1.5 cm; b-2 is a top view of the aramid honeycomb absorbing composite material of length 18 cm×width 18 cm×height 1.5 cm.

[0043] Figure 7 1 and 2 are stress-strain curves of the aramid honeycomb without any treatment and the wave-absorbing composite material obtained in Example 1: a is the aramid honeycomb without any treatment; b is the wave-absorbing composite material obtained in Example 1.

[0044] Figure 8 SEM images of the composite materials prepared in Examples 1 to 3: a-1, a-2 and a-3 are SEM images of the composite material obtained in Example 1 at different magnifications; b-1, b-2 and b-3 are SEM images of the composite material obtained in Example 2 at different magnifications; c-1, c-2 and c-3 are SEM images of the composite material obtained in Example 3 at different magnifications; the magnification of a-1, b-1 and c-1 is 100 times; the magnification of a-2, b-2 and c-2 is 500 times; the magnification of a-3 is 2000 times; the magnification of b-3 and c-3 is 10000 times. DETAILED DESCRIPTION

[0045] The raw materials and equipment used in the specific embodiments of the present invention are all known products and are obtained by purchasing commercially available products.

[0046] Thermal expansion microspheres are composed of a thermoplastic polymer shell and an internal hydrocarbon; the particle size is 20-30 μm, the expansion ratio is 9-10 times, the initial foaming temperature is 125-135°C, and the optimal foaming temperature is 165-180°C (Matsumoto Microspheres FN-100MD). The length of the multi-walled carbon nanotubes is 15-30 μm, the diameter of the carbon nanotubes is 3-15 μm, and the resistivity is 1500-2000 μΩ·m. Meta-aramid honeycomb core, density is 64 kg / m 3 , with a side length of 2.75 mm. All reagents were of analytical grade and used without purification.

[0047] The weight ratio of the thermal expansion microspheres to the multi-walled carbon nanotubes is 3:3 to 3:7.

[0048] When the present invention uses heat-expandable microspheres and multi-walled carbon nanotubes as a mixture, the weight ratio of the heat-expandable microspheres and the multi-walled carbon nanotubes is 3:3 to 3:7, and different mixtures are obtained by dry mixing. The wave-absorbing composite material obtained by foaming and filling the aramid honeycomb within this range has excellent broadband wave-absorbing effect and mechanical bearing capacity. It is worth pointing out that the use of the heat-expandable microspheres mixed with multi-walled carbon nanotubes filling foaming mode proposed by the present invention to improve the mechanical bearing and wave-absorbing performance of the aramid honeycomb core should be within the scope of protection of the patent authorization. As a new method, the same foaming filling mode with a weight ratio lower than this does not exceed the authorization scope of the present invention and should be within the scope of protection of the present invention.

[0049] The present invention tests the wave absorbing performance by using the bow method to test the reflectivity.

[0050] Example 1: Preparation of microwave-absorbing composite material by in-situ foaming and filling aramid honeycomb of the present invention

[0051] 1. Preparation of a mixture of thermally expandable microspheres and multi-walled carbon nanotubes

[0052] A certain amount of heat-expandable microspheres and multi-walled carbon nanotubes (weight ratio of 3:5) were weighed, poured into a beaker and mechanically stirred for 10 minutes for simple mixing, and then the mixture was placed in a planetary ball mill for ball milling. The ball milling time was 5 minutes and the rotation speed was 60r / min. After the ball milling, a mixture of heat-expandable microspheres and multi-walled carbon nanotubes was obtained.

[0053] 2. Process of preparing microwave-absorbing composite materials by in-situ foaming and filling aramid honeycomb

[0054] The above mixture was spread flat in a mold (length 18.5 cm × width 18.5 cm × height 3 cm) to make the surface as flat as possible to achieve uniform foaming, and then an aramid honeycomb (length 18 cm × width 18 cm × height 1.5 cm) was placed. Figure 6 b-1) slowly place the mixture on top and apply a certain pressure to press the mixture into the aramid honeycomb holes, transfer the material to a hot press for hot pressing, the upper plate temperature of the hot press is 120°C, and the lower plate temperature is 110°C to prevent the structural instability caused by the heat expansion microspheres below being heated for too long. The hot press is set to a pressure of 0.1MPa and the heating time is 15min. After limited foaming, an aramid honeycomb wave absorbing composite material ( Figure 6 b-2). The aramid honeycomb absorbing composite material obtained by the present invention has excellent broadband absorbing performance (EAB = 14.64 GHz) within 2-18 GHz, and the minimum reflection loss appears at a frequency of 2.8 GHz, which is -25.1 dB ( Figure 1 ).

[0055] Example 2: Preparation of microwave-absorbing composite material by in-situ foaming and filling aramid honeycomb of the present invention

[0056] 1. Preparation of mixture of thermally expandable microspheres and multi-walled carbon nanotubes

[0057] A certain amount of heat-expandable microspheres and multi-walled carbon nanotubes (weight ratio of 3:3) were weighed, poured into a beaker and mechanically stirred for 10 minutes for simple mixing, and then the mixture was placed in a planetary ball mill for ball milling. The ball milling time was 5 minutes and the rotation speed was 60r / min. After the ball milling, a mixture of heat-expandable microspheres and multi-walled carbon nanotubes was obtained.

[0058] 2. Process of preparing microwave-absorbing composite materials by in-situ foaming and filling aramid honeycomb

[0059] The above mixture is spread flat in a mold (length 18.5cm×width 18.5cm×height 3cm) to make the surface as flat as possible to achieve uniform foaming, and then an aramid honeycomb of length 18cm×width 18cm×height 1.5cm is slowly placed on top of the mixture and a certain pressure is applied to press the mixture into the aramid honeycomb holes, and the material is transferred to a hot press for hot pressing. The upper plate temperature of the hot press is 120°C and the lower plate temperature is 110°C to prevent the structural instability caused by the heat-expanding microspheres below being heated for too long. The set pressure of the hot press is 0.1MPa and the heating time is 15min. After limited foaming, an aramid honeycomb absorbing composite material is obtained. The aramid honeycomb absorbing composite material obtained by the present invention has excellent broadband absorbing performance (EAB=11.43GHz) within 2-18GHz, and the minimum reflection loss occurs at a frequency of 11.1GHz, which is -23.1dB ( Figure 2 ).

[0060] Example 3: Preparation of microwave-absorbing composite material by in-situ foaming and filling aramid honeycomb of the present invention

[0061] 1. Preparation of mixture of thermally expandable microspheres and multi-walled carbon nanotubes

[0062] A certain amount of heat-expandable microspheres and multi-walled carbon nanotubes (weight ratio of 3:7) were weighed, poured into a beaker and mechanically stirred for 10 minutes for simple mixing, and then the mixture was placed in a planetary ball mill for ball milling. The ball milling time was 5 minutes and the rotation speed was 60r / min. After the ball milling, a mixture of heat-expandable microspheres and multi-walled carbon nanotubes was obtained.

[0063] 2. Process of preparing microwave-absorbing composite materials by in-situ foaming and filling aramid honeycomb

[0064] The above mixture is spread flat in a mold (length 18.5cm×width 18.5cm×height 3cm) to make the surface as flat as possible to achieve uniform foaming, and then an aramid honeycomb of length 18cm×width 18cm×height 1.5cm is slowly placed on top of the mixture and a certain pressure is applied to press the mixture into the aramid honeycomb holes, and the material is transferred to a hot press for hot pressing. The upper plate temperature of the hot press is 120°C and the lower plate temperature is 110°C to prevent the structural instability caused by the heat-expanding microspheres below being heated for too long. The set pressure of the hot press is 0.1MPa and the heating time is 15min. After limited foaming, an aramid honeycomb absorbing composite material is obtained. The aramid honeycomb absorbing composite material obtained by the present invention has excellent broadband absorbing performance (EAB=10.25GHz) within 2-18GHz, and the minimum reflection loss occurs at a frequency of 10.7GHz, which is -22.5dB ( Figure 3 ).

[0065] Comparative Example 1: Preparation of microwave-absorbing composite materials by other methods

[0066] The weight ratio of the heat-expandable microspheres to the multi-walled carbon nanotubes in Example 1 was changed to 3:2, and a mixture was prepared according to the method described in Example 1. A aramid honeycomb (18 cm long×18 cm wide×1.5 cm high) was filled according to the method described in Example 1 to prepare a wave absorbing composite material.

[0067] The wave absorbing performance of the wave absorbing composite material prepared in Comparative Example 1 is as follows: Figure 4 As shown. Figure 4 It can be seen that when the weight ratio of thermally expandable microspheres and multi-walled carbon nanotubes is changed to 3:2, the reflection loss value of the absorbing composite material at 2-18 GHz is generally higher than -10 dB, showing poor electromagnetic wave absorption performance.

[0068] Comparative Example 2: Preparation of microwave-absorbing composite materials by other methods

[0069] The weight ratio of the heat-expandable microspheres to the multi-walled carbon nanotubes in Example 1 was changed to 3:8, and a mixture was prepared according to the method described in Example 1. A aramid honeycomb (18 cm long×18 cm wide×1.5 cm high) was filled according to the method described in Example 1 to prepare a wave absorbing composite material.

[0070] The wave absorbing performance of the wave absorbing composite material prepared in Comparative Example 2 is as follows: Figure 5 As shown. Figure 5 It can be seen that when the weight ratio of thermally expandable microspheres to multi-walled carbon nanotubes is changed to 3:8, the reflection loss values ​​of the absorbing composite material at 2-18 GHz are mostly higher than -10 dB, showing poor electromagnetic wave absorption performance.

[0071] According to the absorbing properties of the absorbing composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 2, it can be known that: when the sample thickness is determined, the weight ratio of heat-expanded microspheres to multi-walled carbon nanotubes is an important indicator affecting the final absorbing properties when preparing the aramid honeycomb absorbing composite material. When the weight ratio of heat-expanded microspheres to multi-walled carbon nanotubes is 3:3 to 3:7, the obtained composite material has good absorbing properties. Outside the above weight ratio range, too much or too little multi-walled carbon nanotubes will deteriorate the absorbing properties of the composite material.

[0072] The beneficial effects of the present invention are demonstrated below through specific test examples.

[0073] Test Example 1: Characterization of the compressive strength of the microwave-absorbing composite material of the present invention

[0074] 1. Experimental methods

[0075] In order to study the load-bearing capacity of the aramid honeycomb absorbing composite material prepared by the present invention, an aramid honeycomb absorbing composite material (5 cm long × 5 cm wide × 1.5 cm high) was prepared according to the process of Example 1 ( Figure 6 a-2), the sample was compressed using a universal tensile testing machine at a compression speed of 5 mm / min. A non-treated aramid honeycomb was used as a control.

[0076] 2. Experimental results

[0077] A sample of 5cm long × 5cm wide × 1.5cm high absorbing composite material was prepared to test its compressive strength. At the same time, an untreated aramid honeycomb ( Figure 6 a-1) Test under the same conditions to compare the changes in mechanical properties after filling ( Figure 7 ). Figure 7 a is the stress-strain curve of the untreated aramid honeycomb, and the maximum compressive strength is about 7.4 MPa. Figure 7 b The initial deformation is caused by the foam being higher than the aramid honeycomb core. It can be seen that the compression strength of the filled aramid honeycomb is increased to 13.2MPa compared to the original honeycomb core's 7.4MPa, and the compression resistance is increased by about 78%. First of all, the foam itself has a certain compression strength, and when it is under pressure, the filled foam will make the honeycomb wall evenly stressed, so the honeycomb core has a significant reinforcement effect.

[0078] Experimental Example 2: Micromorphology Changes of Wave-Absorbing Composite Materials Prepared with Different Weight Ratios of Thermally Expandable Microspheres and Multi-walled Carbon Nanotubes

[0079] In order to further study the micromorphology changes of the absorbing composite materials prepared with different weight ratios of thermal expansion microspheres and multi-walled carbon nanotubes, the microstructure of the absorbing composite materials was observed by scanning electron microscopy (SEM). Figure 8 ). Figure 8 a is the SEM image of Example 1, Figure 8 a-1 shows that the foam has a distinct pore structure with a size of about 150-200 μm; Figure 8 a-2 shows that the foam is a closed-cell structure with a relatively tight overall layout, and the multi-walled carbon nanotube residues on the pore walls are not obvious; Figure 8 a-3 shows the microscopic morphology of the foam pore wall, which is consistent with the above conclusion: only a small amount of inconspicuous multi-walled carbon nanotubes remain. Figure 8 b is the SEM image of Example 2, Figure 8 b-1 shows that the foam has a distinct pore structure with a size of about 150-200 μm; Figure 8 b-2 shows that the foam is a closed-cell structure as a whole, the inner pore wall is relatively smooth, and tubular objects are obviously gathered on the outer pore wall surface, which is consistent with the expansion principle of thermal expansion microspheres; Figure 8 b-3 shows the microscopic morphology of the foam pore wall, where it can be clearly seen that a large number of multi-walled carbon nanotubes are distributed and dispersed relatively well. Figure 8 c is the SEM image of Example 3, Figure 8 c-1 shows that the foam pore structure is not obvious, and a small amount of heat-expandable microspheres are not fully expanded; Figure 8 c-2 shows that the foam has a loose structure, the multi-walled carbon nanotube residues on the pore wall are obviously accumulated, and the inner pore wall cannot be seen; Figure 8 c-3 shows the microscopic morphology of the foam pore wall, in which a large number of multi-walled carbon nanotubes are accumulated, and the accumulation is relatively dense and the dispersion is poor.

[0080] It can be seen that the weight ratio of heat-expandable microspheres to multi-walled carbon nanotubes has a great influence on the microscopic morphology of the foam. The weight ratio of heat-expandable microspheres to multi-walled carbon nanotubes in Example 1 is most appropriate and has the best performance.

[0081] In summary, the present invention obtains a mixture by dry-mixing heat-expandable microspheres and multi-walled carbon nanotubes, and then heats the aramid honeycomb and the mixture and restricts foaming to obtain an aramid honeycomb absorbing composite material. The absorbing composite material prepared by the present invention has the characteristics of broadband absorption, and the absorbing effect is only controlled by the weight ratio of the heat-expandable microspheres and the multi-walled carbon nanotubes, ensuring that the prepared absorbing composite material has stable performance and the process is repeatable; at the same time, compared with the existing filling and impregnation processes, the preparation method of the present invention does not involve the addition and preparation of any reagents, which can save production costs and simplify the preparation process; and the present invention achieves complete and uniform filling of the aramid honeycomb holes, and the preparation process does not involve processing and destruction of the aramid honeycomb material. Compared with the traditional impregnation or filling process, the mechanical bearing capacity of the prepared absorbing composite material is significantly enhanced. Therefore, the absorbing composite material of the present invention and its preparation method have broad application prospects in the field of electromagnetic wave stealth.

Claims

1. An aramid honeycomb wave-absorbing composite material, characterized in that: The composite material is obtained by pressing a mixture of heat-expandable microspheres and multi-walled carbon nanotubes into aramid honeycomb holes for foaming.

2. The aramid honeycomb wave absorbing composite material according to claim 1, characterized in that: The weight ratio of the thermally expandable microspheres to the multi-walled carbon nanotubes is 3:(3-7).

3. The aramid honeycomb wave absorbing composite material according to claim 2, characterized in that: The weight ratio of the thermally expandable microspheres to the multi-walled carbon nanotubes is 3:

5.

4. The aramid honeycomb wave absorbing composite material according to any one of claims 1 to 3, characterized in that: The particle size of the heat-expandable microspheres is 20-30 μm, the expansion ratio is 9-10 times, the initial foaming temperature is 125-135° C., and the optimal foaming temperature is 165-180° C.; And / or, the multi-walled carbon nanotubes have a length of 15 to 30 μm, a tube diameter of 3 to 15 μm, and a resistivity of 1300-3000 μΩ·m.

5. The aramid honeycomb wave absorbing composite material according to any one of claims 1 to 3, characterized in that: The mixture of the heat-expandable microspheres and the multi-walled carbon nanotubes is ball-milled before being pressed into the aramid honeycomb holes. The ball-milling time is 5 to 15 minutes and the rotation speed is 60 to 300 r / min.

6. The aramid honeycomb wave absorbing composite material according to any one of claims 1 to 3, characterized in that: The foaming method comprises the following steps: The mixture of heat-expandable microspheres and multi-walled carbon nanotubes is pressed into the pores of the aramid honeycomb and then subjected to hot-pressing foaming; Preferably, during the hot pressing foaming, the upper plate temperature of the hot pressing machine is 100-150°C, the lower plate temperature is 90-140°C, the pressure is 0.1-1.0 MPa, and the hot pressing time is 10-15 min; More preferably, during the hot pressing foaming, the temperature difference between the upper and lower plates of the hot pressing machine is 2-10° C., and the hot pressing time is 15 minutes.

7. The aramid honeycomb wave absorbing composite material according to any one of claims 1 to 3, characterized in that: The density of the aramid honeycomb is 32-160 kg / m 3 , the side length is 1.83~5.50mm.

8. The method for preparing the aramid honeycomb wave absorbing composite material according to any one of claims 1 to 7, characterized in that: It includes the following steps: (1) dry-mixing heat-expandable microspheres and multi-walled carbon nanotubes to obtain a mixture; (2) The mixture obtained in step (1) is pressed into the pores of an aramid honeycomb for foaming.

9. Use of the aramid honeycomb wave-absorbing composite material according to any one of claims 1 to 7 in the preparation of wave-absorbing materials.

10. The use according to claim 9, characterized in that: The absorbing material is a stealth material; Preferably, the stealth material is a stealth material used in the aerospace field.

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