Broadband efficient electromagnetic shielding composite material based on gradient foamed aluminum structure and preparation method of broadband efficient electromagnetic shielding composite material

Abstract

The invention relates to the technical field of electromagnetic protection materials, in particular to a broadband efficient electromagnetic shielding composite material based on a gradient foamed aluminum structure and a preparation method of the broadband efficient electromagnetic shielding composite material. The broadband efficient electromagnetic shielding composite material sequentially comprises a metasurface resonance layer, a multi-stage impedance matching layer, a gradient foamed aluminum core layer and a conductive backing layer from top to bottom. According to the invention, the gradient foamed aluminum core layer is combined with enhancement of the ferroferric oxide and carbon nanotube composite coating through multi-layer cooperation of bottom layer reflection, transition layer eddy current loss and a surface resonant cavity, so that the shielding effectiveness of the composite material is relatively high, complex electromagnetic interference from low frequency to high frequency is effectively dealt with, meanwhile, a heat conduction path is further optimized, and the service life of the composite material is prolonged. The heat conductivity coefficient of the material is high, and performance degradation caused by heat accumulation is avoided; when electromagnetic waves are incident, the I-shaped units generate strong resonance attenuation on the electromagnetic waves in a target frequency band through an inductance-capacitance resonance effect, the electromagnetic shielding effectiveness in the frequency band is remarkably improved, and the gradient foamed aluminum core layer is matched, so that the frequency band range of efficient shielding is further widened.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic shielding material preparation technology, specifically to a broadband high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure and its preparation method. Background Technology

[0002] Broadband high-efficiency electromagnetic shielding composite material is a functional composite material that achieves high-efficiency shielding of wide-band (such as DC-40GHz) electromagnetic waves through multi-layer structure synergy and functional gradient design. As electronic devices develop towards higher frequencies and integration, the demand for broadband high-efficiency electromagnetic shielding materials in aerospace, new energy and other fields is becoming increasingly urgent, and the materials are required to be lightweight and have good thermal conductivity to adapt to complex working conditions.

[0003] In existing technologies, aluminum foam is a single-layer structure rather than a gradient multilayer structure. Due to its uniform porosity and pore size, the electromagnetic wave reflection and loss mechanism is fixed, and it can only effectively shield in a specific frequency band, resulting in insufficient broadband performance. The single composition leads to limited conductivity and magnetic loss capability, and it cannot simultaneously absorb and reflect high and low frequency electromagnetic waves. Furthermore, the wave impedance has poor matching with air or adjacent layers, which easily generates additional reflection loss at the interface, reducing the overall shielding efficiency. Traditional multi-level impedance matching layers are mainly composed of epoxy resin / vinyl ester resin and tungsten powder, etc. Traditional materials have poor environmental friendliness, limited impedance adjustment range, and the fillers are prone to oxidation and agglomeration, while also having low processing efficiency.

[0004] Based on this, the present invention provides a broadband high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure and its preparation method. Summary of the Invention

[0005] The purpose of this invention is to provide a broadband high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure and its preparation method. The electromagnetic shielding composite material prepared by this invention has good shielding effectiveness, thermal conductivity and moderate areal density, ensuring the material is lightweight and has a balanced performance.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a broadband high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure, comprising, from top to bottom, a metasurface resonant layer, a multi-level impedance matching layer, a gradient aluminum foam core layer, and a conductive backing layer; The gradient foam aluminum core layer is divided into three regions along the thickness direction: the bottom layer, the transition layer, and the surface layer. The multi-level impedance matching layer is composed of three raw materials: waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate.

[0007] Preferably, the bottom layer of the gradient foam aluminum core layer is high-purity aluminum with a pore size of 0.05-0.1 mm and a porosity of 40%, forming a high-conductivity reflective layer; the transition layer is an alloy layer formed by adding 1-2% copper by mass to the aluminum substrate, with a pore size of 0.2-0.5 mm and a porosity of 60%, forming an eddy current loss layer; the surface layer is an alloy layer formed by adding 0.5-1% silicon by mass to the aluminum substrate, with a pore size of 0.8-1.2 mm and a porosity of 75%, forming a multi-level resonant cavity layer.

[0008] Preferably, the thickness of the gradient aluminum foam core layer is 4-10 mm, and its direction is a vertical gradient structure; in the gradient aluminum foam core layer, the thickness of the iron oxide + carbon nanotube composite coating is 100-500 nm; the metasurface resonant layer is a patterned metal mesh composed of an I-shaped metal unit array.

[0009] Preferably, the raw material pretreatment method for the multi-level impedance matching layer is as follows: First, sieve out 5-20μm silver-coated aluminum flake particles, place them in an ethanol solution for ultrasonic cleaning for 15-20min, and then dry them in an oven at 80-100℃ for 30-40min before taking them out for use; grind barium strontium titanate dielectric particles to a particle size of 1-5μm by ball milling, place them in an ethanol solution for ultrasonic cleaning for 10-15min, and then dry them in an oven at 80-100℃ for 30-40min before taking them out for use; stir the waterborne polyurethane at 25-30℃ for 10-15min to adjust its viscosity to 500-800mPa·s.

[0010] The preparation method of broadband high-efficiency electromagnetic shielding composite material based on gradient aluminum foam structure includes the following steps: Step 1: Preparation of gradient foam aluminum core layer: Foaming is controlled by a spatial temperature field. The bottom temperature of the mold is set to 600℃ and the top temperature is set to 400℃. A preform is prepared by mixing titanium hydride + calcium carbonate composite foaming agent with aluminum powder. At the same time, 3-7wt% of bismuth trioxide-zinc oxide piezoelectric phase is added to the aluminum powder. The piezoelectric phase is uniformly dispersed in the mixture of aluminum powder and composite foaming agent by stirring. The differential decomposition rate of composite foaming agent in spatial temperature field is used to form a vertical pore size gradient. CVD pore wall modification is then performed on the gradient foam aluminum core layer. Step 2: Preparation of multi-level impedance matching layer: Waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate dielectric particles are mixed evenly and coated on one side of the gradient aluminum foam core layer, with the thickness controlled at 50-200μm; Step 3: Fabrication of the metasurface resonant layer: I-shaped Ag meshes are fabricated on the PET release film by laser direct writing; Step 4: Conductive backing layer lamination: A 0.1 mm thick aluminum foil is used as a conductive backing layer and laminated onto the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer.

[0011] Preferably, the preparation steps of the composite foaming agent are as follows: first, screen titanium hydride and calcium carbonate particles to 65–165 μm, mix them in a ratio of 30–50% titanium hydride and 50–70% calcium carbonate, add 3–5% ethanol, and mix in a planetary ball mill at a speed of 200–300 r / min for 1–2 h to obtain a uniform composite foaming agent.

[0012] Preferably, the CVD modification method for preparing the gradient foamed aluminum core layer is as follows: the foamed aluminum obtained by gradient foaming is placed in a CVD reactor and a reaction gas is introduced. The carbon source in the reaction gas is ethylene with a flow rate of 50-100 sccm, and the iron source is iron pentacarbonyl, which is carried into the reactor by argon gas and the flow rate is maintained at 30-50 sccm. The temperature in the reactor is controlled at 400-500℃, the pressure is maintained at 0.08-0.12 MPa, and deposition is carried out at a deposition rate of 45-55 nm / min to form a network structure of Fe3O4 nanoparticles embedded in carbon nanotubes.

[0013] Preferably, the preparation method of the multi-level impedance matching layer is as follows: waterborne polyurethane, silver-coated aluminum sheet and barium strontium titanate dielectric particles are weighed in a mass ratio of 60:30:10, and the three are mixed in a planetary ball mill and mixed at a speed of 200-300 r / min for 1-2 hours to prepare a slurry; the slurry is coated on one side of the modified gradient aluminum foam core layer by spraying, and the spraying pressure is controlled at 0.3-0.5 MPa and the spraying distance is 15-20 cm. After coating, it is dried at 80-100℃ for 30-60 min, and then placed at room temperature for 1-2 hours for natural cooling and stress release.

[0014] Preferably, the method for preparing the metasurface resonant layer is as follows: an I-shaped Ag grid is prepared on the surface of a PET release film by laser direct writing. The laser direct writing uses a pulsed laser with a wavelength of 532nm, a pulse width of 10-20ns, a pulse frequency of 20-50kHz, a laser power of 10-15W, and a scanning speed of 50-80mm / s. Then, the prepared I-shaped Ag grid is transferred to the side of the multi-level impedance matching layer away from the gradient aluminum foam core layer using heat-release tape, thus completing the composite of the metasurface resonant layer.

[0015] Preferably, the method for composite of the conductive backing layer is as follows: select an aluminum foil with a thickness of 0.1 mm as the conductive backing layer, cover it on the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer, and composite it by hot pressing process, controlling the hot pressing temperature at 200-250℃, the pressure at 10-15MPa, and the hot pressing time at 15-20min, so that the aluminum foil and the gradient foam aluminum core layer are tightly bonded.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. In this invention, the gradient foam aluminum core layer is divided into three layers: a bottom layer as a high-conductivity reflective layer, which uses high-purity aluminum to efficiently reflect incident electromagnetic waves and block direct penetration of electromagnetic waves; a transition layer as an eddy current loss layer, in which copper is added to the aluminum matrix to improve the material's conductivity and magnetic loss capability, and the medium-diameter, medium-porosity structure allows electromagnetic waves to form eddy currents within it, converting electromagnetic energy into heat energy through Joule heating, thus achieving absorption and loss of electromagnetic waves; and a surface layer as a multi-level resonant cavity layer, in which the large-diameter, high-porosity structure combined with the dielectric properties of silicon allows electromagnetic waves to be reflected and interfered with multiple times within the pores, forming a resonance effect, further attenuating the energy of high-frequency electromagnetic waves.

[0017] 2. In this invention, Fe3O4 is a magnetic material that can efficiently absorb high-frequency electromagnetic waves through mechanisms such as hysteresis loss and domain wall resonance. CNTs have excellent conductivity and can convert electromagnetic energy into Joule heat dissipation using the eddy current effect. The two work together to form a composite structure of magnetic particles and conductive network, which greatly improves the absorption and attenuation ability of the pore walls for electromagnetic waves. At the same time, the bismuth trioxide-zinc oxide piezoelectric phase added during the modification process can convert electromagnetic energy into mechanical energy dissipation. This forms a multi-mechanism synergy with the magnetoelectric loss of iron tetroxide + carbon nanotubes, further broadening the frequency range of efficient shielding. Combined with the multi-layer shielding mechanism of gradient aluminum foam's own reflection-loss-resonance, the electromagnetic shielding effectiveness of the composite material is significantly enhanced in a wide frequency band.

[0018] 3. In this invention, the metasurface resonant layer is prepared using laser direct writing technology to fabricate an I-shaped Ag grid. Leveraging the micron-level processing precision of laser direct writing, the grid line width, unit size, and array spacing can be adjusted to ensure structural consistency and avoid resonant frequency drift. The I-shaped units, through the inductor-capacitor structure formed by the horizontal and vertical arms, can construct a dual-band or wideband magnetic resonant system. Combined with the shielding mechanism of the gradient aluminum foam core layer, this broadens the efficient shielding frequency band and enhances the electromagnetic attenuation effect. The high conductivity of silver enables the grid to form a low-resistance conductive network, reducing surface current transmission loss and dissipating electromagnetic energy through the eddy current effect. The patterned design significantly reduces the areal density compared to a solid metal layer, helping the overall material achieve a balance between efficient shielding and lightweight construction, making it suitable for lightweight applications. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that all raw materials used in the following experiments are commercially available.

[0021] Example 1: A broadband high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure, comprising, from top to bottom, a metasurface resonant layer, a multi-level impedance matching layer, a gradient aluminum foam core layer, and a conductive backing layer; The gradient foam aluminum core layer is divided into three zones along the thickness direction: the bottom layer, the transition layer, and the surface layer. The multi-level impedance matching layer is composed of three raw materials: waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate.

[0022] The bottom layer of the gradient foam aluminum core is high-purity aluminum with a pore size of 0.05 mm and a porosity of 40%, forming a high-conductivity reflective layer; the transition layer is an alloy layer formed by adding 1% copper by mass to the aluminum substrate, with a pore size of 0.2 mm and a porosity of 60%, forming an eddy current loss layer; the surface layer is an alloy layer formed by adding 0.5% silicon by mass to the aluminum substrate, with a pore size of 0.8 mm and a porosity of 75%, forming a multi-level resonant cavity layer.

[0023] The gradient aluminum foam core layer has a thickness of 4 mm and its direction is a vertical gradient structure; in the gradient aluminum foam core layer, the composite coating of iron oxide + carbon nanotubes has a thickness of 100 nm; the metasurface resonant layer is a patterned metal mesh composed of an array of I-shaped metal units.

[0024] The pretreatment method for the raw materials of the multi-level impedance matching layer is as follows: First, 5μm silver-coated aluminum flake particles are sieved out, and then ultrasonically cleaned in ethanol solution for 15 minutes to remove surface oil and oxide impurities. After that, they are dried in an 80℃ oven for 30 minutes and then taken out for use. The barium strontium titanate dielectric particles are ground to a particle size of 1μm by ball milling, and then ultrasonically cleaned in ethanol solution for 10 minutes to remove fine dust generated during the grinding process. After that, they are dried in an 80℃ oven for 30 minutes and then taken out for use. The waterborne polyurethane is stirred at 25℃ for 10 minutes to adjust its viscosity to 500mPa·s.

[0025] The preparation method of broadband high-efficiency electromagnetic shielding composite material based on gradient aluminum foam structure includes the following steps: Step 1: Preparation of gradient foam aluminum core layer: Foaming is controlled by a spatial temperature field. The bottom temperature of the mold is set to 600℃ and the top temperature is set to 400℃. A preform is prepared by mixing titanium hydride + calcium carbonate composite foaming agent with aluminum powder. At the same time, 3wt% of bismuth trioxide-zinc oxide piezoelectric phase is added to the aluminum powder. The piezoelectric phase is uniformly dispersed in the mixture of aluminum powder and composite foaming agent by stirring. The differential decomposition rate of composite foaming agent in spatial temperature field is used to form a vertical pore size gradient. CVD pore wall modification is then performed on the gradient foam aluminum core layer. Step 2: Preparation of multi-level impedance matching layer: Waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate dielectric particles are mixed evenly and coated on one side of the gradient aluminum foam core layer, with the thickness controlled at 50 μm; Step 3: Fabrication of the metasurface resonant layer: I-shaped Ag meshes are fabricated on the PET release film by laser direct writing; Step 4: Conductive backing layer lamination: A 0.1 mm thick aluminum foil is used as a conductive backing layer and laminated onto the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer.

[0026] The preparation steps of the composite foaming agent are as follows: First, screen the particles of titanium hydride and calcium carbonate to 65μm respectively, mix them in a ratio of 30% titanium hydride and 70% calcium carbonate, add 3% ethanol, and mix in a planetary ball mill at a speed of 200r / min for 1h to obtain a uniform composite foaming agent.

[0027] The CVD modification method for preparing gradient aluminum foam core layers is as follows: aluminum foam obtained through gradient foaming is placed in a CVD reactor and a reaction gas is introduced. The carbon source in the reaction gas is ethylene with a flow rate of 50 sccm, and the iron source is iron pentacarbonyl, which is carried into the reactor by argon gas and kept at a flow rate of 30 sccm. The temperature in the reactor is controlled at 400℃, the pressure is maintained at 0.08 MPa, and deposition is carried out at a deposition rate of 45 nm / min to form a network structure of Fe3O4 nanoparticles embedded in carbon nanotubes.

[0028] The preparation method of the multi-level impedance matching layer is as follows: waterborne polyurethane, silver-coated aluminum sheet and barium strontium titanate dielectric particles are weighed at a mass ratio of 60:30:10, and the three are mixed in a planetary ball mill and mixed at a speed of 200 r / min for 1 hour to make a slurry. The slurry is coated on one side of the modified gradient aluminum foam core layer by spraying process, and the spraying pressure is controlled at 0.3 MPa and the spraying distance is 15 cm. After coating, it is dried at 80℃ for 30 min to ensure that the impedance matching layer thickness is 50 μm, thereby realizing the gradual change of air-material interface wave impedance from 377Ω to 1.5Ω. Then, it is placed at room temperature for 1 hour for natural cooling and stress release.

[0029] The method for preparing the metasurface resonant layer is as follows: I-shaped Ag grids are prepared on the surface of a PET release film using a laser direct writing process. The laser direct writing uses a pulsed laser with a wavelength of 532nm, a pulse width of 10ns, and a pulse frequency of 20kHz. The laser power is set to 10W and the scanning speed to 50mm / s, ensuring that the size of the I-shaped metal unit is 0.3×0.1mm and the unit spacing is 0.05mm. The unit period is λ / 8 at a frequency of 40GHz to form a dual-band magnetic resonance. Then, the prepared I-shaped Ag grids are transferred to the side of the multi-level impedance matching layer away from the gradient aluminum foam core layer using heat-release tape, thus completing the composite of the metasurface resonant layer.

[0030] The method for composite conductive backing layer is as follows: A 0.1 mm thick aluminum foil is selected as the conductive backing layer and covered on the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer. The composite is carried out by hot pressing process, controlling the hot pressing temperature at 200℃, the pressure at 10 MPa, and the hot pressing time at 15 min, so that the aluminum foil and the gradient foam aluminum core layer are tightly bonded together, resulting in an electromagnetic shielding composite material with shielding effectiveness ≥90 dB, areal density ≤2.8 kg / m², and thermal conductivity ≥95 W / (m·K) in the DC-40 GHz wideband.

[0031] Example 2: A broadband high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure, comprising, from top to bottom, a metasurface resonant layer, a multi-level impedance matching layer, a gradient aluminum foam core layer, and a conductive backing layer; The gradient foam aluminum core layer is divided into three zones along the thickness direction: the bottom layer, the transition layer, and the surface layer. The multi-level impedance matching layer is composed of three raw materials: waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate.

[0032] The bottom layer of the gradient foam aluminum core is high-purity aluminum with a pore size of 0.07 mm and a porosity of 40%, forming a high-conductivity reflective layer; the transition layer is an alloy layer formed by adding 1.5% copper by mass to the aluminum substrate, with a pore size of 0.3 mm and a porosity of 60%, forming an eddy current loss layer; the surface layer is an alloy layer formed by adding 0.7% silicon by mass to the aluminum substrate, with a pore size of 1.0 mm and a porosity of 75%, forming a multi-level resonant cavity layer.

[0033] The gradient aluminum foam core layer has a thickness of 7 mm and its direction is a vertical gradient structure; in the gradient aluminum foam core layer, the composite coating of iron oxide + carbon nanotubes has a thickness of 300 nm; the metasurface resonant layer is a patterned metal mesh composed of an array of I-shaped metal units.

[0034] The pretreatment method for the raw materials of the multi-level impedance matching layer is as follows: First, sieve out the 12μm silver-coated aluminum flake particles, place them in an ethanol solution for ultrasonic cleaning for 17min to remove surface oil and oxide impurities, and then dry them in a 90℃ oven for 35min before taking them out for use; grind the barium strontium titanate dielectric particles to a particle size of 3μm by ball milling, place them in an ethanol solution for ultrasonic cleaning for 12min to remove the fine dust generated during the grinding process, and then dry them in a 90℃ oven for 35min before taking them out for use; stir the waterborne polyurethane at 27℃ for 12min to adjust its viscosity to 650mPa·s.

[0035] The preparation method of broadband high-efficiency electromagnetic shielding composite material based on gradient aluminum foam structure includes the following steps: Step 1: Preparation of gradient foam aluminum core layer: Foaming is controlled by a spatial temperature field. The bottom temperature of the mold is set to 600℃ and the top temperature is set to 400℃. A preform is prepared by mixing titanium hydride + calcium carbonate composite foaming agent with aluminum powder. At the same time, 5wt% of bismuth trioxide-zinc oxide piezoelectric phase is added to the aluminum powder. The piezoelectric phase is uniformly dispersed in the mixture of aluminum powder and composite foaming agent by stirring. The differential decomposition rate of composite foaming agent in spatial temperature field is used to form a vertical pore size gradient. CVD pore wall modification is then performed on the gradient foam aluminum core layer. Step 2: Preparation of multi-level impedance matching layer: Waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate dielectric particles are mixed evenly and coated on one side of the gradient aluminum foam core layer, with the thickness controlled at 125μm; Step 3: Fabrication of the metasurface resonant layer: I-shaped Ag meshes are fabricated on the PET release film by laser direct writing; Step 4: Conductive backing layer lamination: A 0.1 mm thick aluminum foil is used as a conductive backing layer and laminated onto the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer.

[0036] The preparation steps of the composite foaming agent are as follows: First, screen the particles of titanium hydride and calcium carbonate to 115μm, mix them in a ratio of 40% titanium hydride and 60% calcium carbonate, add 4% ethanol, and mix in a planetary ball mill at a speed of 250r / min for 1.5h to obtain a uniform composite foaming agent.

[0037] The CVD modification method for preparing gradient foamed aluminum core layer is as follows: The foamed aluminum obtained by gradient foaming is placed in a CVD reactor and a reaction gas is introduced. The carbon source in the reaction gas is ethylene with a flow rate of 75 sccm, and the iron source is iron pentacarbonyl, which is carried into the reactor by argon gas and kept at a flow rate of 40 sccm. The temperature in the reactor is controlled at 450℃, the pressure is maintained at 0.10 MPa, and deposition is carried out at a deposition rate of 50 nm / min to form a network structure of Fe3O4 nanoparticles embedded in carbon nanotubes.

[0038] The preparation method of the multi-level impedance matching layer is as follows: waterborne polyurethane, silver-coated aluminum sheet and barium strontium titanate dielectric particles are weighed at a mass ratio of 60:30:10, and the three are mixed in a planetary ball mill and mixed at a speed of 250 r / min for 1.5 h to prepare a slurry. The slurry is coated on one side of the modified gradient foam aluminum core layer by spraying. The spraying pressure is controlled at 0.4 MPa and the spraying distance is 17 cm. After coating, it is dried at 90℃ for 45 min to ensure that the impedance matching layer thickness is 125 μm, thereby realizing the gradual change of air-material interface wave impedance from 377Ω to 1.5Ω. Then, it is placed at room temperature for 1.5 h for natural cooling and stress release.

[0039] The method for preparing the metasurface resonant layer is as follows: I-shaped Ag grids are prepared on the surface of a PET release film using a laser direct writing process. The laser direct writing uses a pulsed laser with a wavelength of 532nm, a pulse width of 15ns, and a pulse frequency of 35kHz. The laser power is set to 12W and the scanning speed to 65mm / s, ensuring that the size of the I-shaped metal unit is 0.3×0.1mm and the unit spacing is 0.05mm. The unit period is λ / 8 at a frequency of 40GHz to form a dual-band magnetic resonance. Then, the prepared I-shaped Ag grids are transferred to the side of the multi-level impedance matching layer away from the gradient aluminum foam core layer using heat-release tape, thus completing the composite of the metasurface resonant layer.

[0040] The method for composite conductive backing layer is as follows: A 0.1 mm thick aluminum foil is selected as the conductive backing layer and covered on the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer. The composite is carried out by hot pressing process, controlling the hot pressing temperature at 225℃, the pressure at 12 MPa, and the hot pressing time at 17 min, so that the aluminum foil and the gradient foam aluminum core layer are tightly bonded together, resulting in an electromagnetic shielding composite material with shielding effectiveness ≥90 dB, areal density ≤2.8 kg / m², and thermal conductivity ≥95 W / (m·K) in the DC-40 GHz wideband.

[0041] Example 3: A broadband high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure, comprising, from top to bottom, a metasurface resonant layer, a multi-level impedance matching layer, a gradient aluminum foam core layer, and a conductive backing layer; The gradient foam aluminum core layer is divided into three zones along the thickness direction: the bottom layer, the transition layer, and the surface layer. The multi-level impedance matching layer is composed of three raw materials: waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate.

[0042] The bottom layer of the gradient foam aluminum core is high-purity aluminum with a pore size of 0.1 mm and a porosity of 40%, forming a high-conductivity reflective layer; the transition layer is an alloy layer formed by adding 2% copper by mass to the aluminum substrate, with a pore size of 0.5 mm and a porosity of 60%, forming an eddy current loss layer; the surface layer is an alloy layer formed by adding 1% silicon by mass to the aluminum substrate, with a pore size of 1.2 mm and a porosity of 75%, forming a multi-level resonant cavity layer.

[0043] The gradient aluminum foam core layer has a thickness of 10 mm and its direction is a vertical gradient structure; in the gradient aluminum foam core layer, the composite coating of iron oxide + carbon nanotubes has a thickness of 500 nm; the metasurface resonant layer is a patterned metal mesh composed of an array of I-shaped metal units.

[0044] The pretreatment method for the raw materials of the multi-level impedance matching layer is as follows: First, 20μm silver-coated aluminum flake particles are sieved out, placed in an ethanol solution and ultrasonically cleaned for 20 minutes to remove surface oil and oxide impurities, and then dried in a 100℃ oven for 40 minutes before being taken out for use; the barium strontium titanate dielectric particles are ball-milled to a particle size of 5μm, placed in an ethanol solution and ultrasonically cleaned for 15 minutes to remove fine dust generated during the grinding process, and then dried in a 100℃ oven for 40 minutes before being taken out for use; the waterborne polyurethane is stirred at 30℃ for 15 minutes to adjust its viscosity to 800mPa·s.

[0045] The preparation method of broadband high-efficiency electromagnetic shielding composite material based on gradient aluminum foam structure includes the following steps: Step 1: Preparation of gradient foam aluminum core layer: Foaming is controlled by a spatial temperature field. The bottom temperature of the mold is set to 600℃ and the top temperature is set to 400℃. A preform is prepared by mixing titanium hydride + calcium carbonate composite foaming agent with aluminum powder. At the same time, 7wt% of bismuth trioxide-zinc oxide piezoelectric phase is added to the aluminum powder. The piezoelectric phase is uniformly dispersed in the mixture of aluminum powder and composite foaming agent by stirring. The differential decomposition rate of composite foaming agent in spatial temperature field is used to form a vertical pore size gradient. CVD pore wall modification is then performed on the gradient foam aluminum core layer. Step 2: Preparation of multi-level impedance matching layer: Waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate dielectric particles are mixed evenly and coated on one side of the gradient aluminum foam core layer, with the thickness controlled at 200 μm; Step 3: Fabrication of the metasurface resonant layer: I-shaped Ag meshes are fabricated on the PET release film by laser direct writing; Step 4: Conductive backing layer lamination: A 0.1 mm thick aluminum foil is used as a conductive backing layer and laminated onto the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer.

[0046] The preparation steps of the composite foaming agent are as follows: First, screen the particles of titanium hydride and calcium carbonate to 165μm, mix them in a ratio of 50% titanium hydride and 50% calcium carbonate, add 5% ethanol, and mix in a planetary ball mill at a speed of 300r / min for 2h to obtain a uniform composite foaming agent.

[0047] The CVD modification method for preparing gradient foamed aluminum core layer is as follows: The foamed aluminum obtained by gradient foaming is placed in a CVD reactor and a reaction gas is introduced. The carbon source in the reaction gas is ethylene with a flow rate of 100 sccm, and the iron source is iron pentacarbonyl, which is carried into the reactor by argon gas and kept at a flow rate of 50 sccm. The temperature in the reactor is controlled at 500℃, the pressure is maintained at 0.12 MPa, and deposition is carried out at a deposition rate of 55 nm / min to form a network structure of Fe3O4 nanoparticles embedded in carbon nanotubes.

[0048] The preparation method of the multi-level impedance matching layer is as follows: waterborne polyurethane, silver-coated aluminum sheet and barium strontium titanate dielectric particles are weighed at a mass ratio of 60:30:10. The three are mixed and placed in a planetary ball mill and mixed at a speed of 300 r / min for 2 hours to prepare a slurry. The slurry is coated on one side of the modified gradient foam aluminum core layer by spraying. The spraying pressure is controlled at 0.5 MPa and the spraying distance is 20 cm. After coating, it is dried at 100℃ for 60 min to ensure that the impedance matching layer thickness is 200 μm, thereby realizing the gradual change of air-material interface wave impedance from 377Ω to 1.5Ω. Then, it is placed at room temperature for 1-2 hours for natural cooling and stress release.

[0049] The method for preparing the metasurface resonant layer is as follows: I-shaped Ag grids are prepared on the surface of a PET release film using a laser direct writing process. The laser direct writing uses a pulsed laser with a wavelength of 532nm, a pulse width of 20ns, and a pulse frequency of 50kHz. The laser power is set to 15W and the scanning speed to 80mm / s, ensuring that the size of the I-shaped metal unit is 0.3×0.1mm and the unit spacing is 0.05mm. The unit period is λ / 8 at a frequency of 40GHz to form a dual-band magnetic resonance. Then, the prepared I-shaped Ag grids are transferred to the side of the multi-level impedance matching layer away from the gradient aluminum foam core layer using heat-release tape, thus completing the composite of the metasurface resonant layer.

[0050] The method for composite conductive backing layer is as follows: A 0.1 mm thick aluminum foil is selected as the conductive backing layer and covered on the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer. The composite is carried out by hot pressing process, controlling the hot pressing temperature at 250℃, the pressure at 15 MPa, and the hot pressing time at 20 min, so that the aluminum foil and the gradient foam aluminum core layer are tightly bonded together, resulting in an electromagnetic shielding composite material with shielding effectiveness ≥90 dB, areal density ≤2.8 kg / m², and thermal conductivity ≥95 W / (m·K) in the DC-40 GHz wideband.

[0051] Comparative Example 1: The difference between this comparative example and Example 1 is that the composite foaming agent titanium hydride + calcium carbonate is not used in this comparative example, that is, a gradient aluminum foam structure cannot be formed.

[0052] Comparative Example 2 differs from Example 1 in that the graded foamed aluminum in this comparative example is not modified.

[0053] Comparative Example 3 differs from Example 1 in that it does not contain the bismuth trioxide-zinc oxide piezoelectric phase.

[0054] Comparative Example 4 differs from Example 1 in that the metasurface in this comparative example deviates from the I-shaped unit array.

[0055] Performance testing: Performance tests were conducted on the products prepared in Examples 1, 2, 3, Comparative Examples 1, 2, 3, and 4. Shielding effectiveness (DC-40GHz) test: Low frequency band (DC-1GHz): The sample is installed in a flange coaxial test device. The transmitting end emits electromagnetic waves of known intensity. After passing through the sample, the receiving end receives the electromagnetic wave signal that has passed through the sample. By comparing the electromagnetic wave power intensity at the transmitting and receiving ends, the shielding effectiveness is calculated according to the formula. High frequency band (1-40GHz): The test is conducted in a microwave anechoic chamber using a far-field test system. The transmitting antenna emits electromagnetic waves of a specific frequency and power, so that the electromagnetic waves are incident perpendicularly on the sample surface. The receiving antenna receives the electromagnetic waves after reflection, absorption and scattering by the sample. Similarly, by comparing the power of the transmitted and received electromagnetic waves, the shielding effectiveness is obtained. The two sets of data are combined. Test standard: GB / T30148-2013. Areal density test: Cut at least three square specimens with dimensions of 100mm × 100mm from the prepared composite material sample. Weigh each specimen using an electronic balance with an accuracy of 0.001g. At the same time, measure the actual length and width of each specimen using a vernier caliper with an accuracy of 0.01mm. Calculate the area of ​​each specimen and calculate the areal density of the specimen according to the formula. The final result is the arithmetic mean of the three specimen test values ​​as the areal density of the composite material. Test standard: GB / T4456-2008. Thermal conductivity test: A circular sample was cut, and the thickness of the sample at different locations was measured using a thickness gauge with an accuracy of 0.001 mm and the average value was taken. Then, the sample was placed on the sample stage of the laser flare instrument. Under the protection of inert gas, the lower surface of the sample was instantaneously heated by a short-duration high-energy laser pulse emitted by the instrument. At the same time, the temperature change curve of the upper surface of the sample over time was recorded in real time using an infrared detector. Based on the core principle of the laser flare method, combined with the thermal diffusivity directly measured by the instrument, and the material density and specific heat capacity at constant pressure measured by a densitometer and a differential scanning calorimeter, respectively, the thermal conductivity of the composite material was obtained. The test was conducted in accordance with ISO13826:2021 and GB / T22588-2008. The obtained test data are recorded in Table 1 below:

[0056] By comparing and analyzing the relevant data in Table 1, it can be seen that the electromagnetic shielding composite material prepared by the method of the present invention based on the gradient aluminum foam structure not only has good shielding effectiveness and thermal conductivity, but also has a moderate areal density. This indicates that the method of the present invention for preparing the broadband high-efficiency electromagnetic shielding composite material based on the gradient aluminum foam structure has a broader market prospect and is more suitable for widespread application.

[0057] In the description of this specification, references to terms such as "an embodiment," "an example," and "a specific example" indicate that a particular feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0058] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A wideband high-efficient electromagnetic shielding composite material based on gradient aluminum foam structure, characterized in that: From top to bottom, it includes a metasurface resonant layer, a multi-level impedance matching layer, a gradient aluminum foam core layer, and a conductive backing layer; The gradient foam aluminum core layer is divided into three regions along the thickness direction: the bottom layer, the transition layer, and the surface layer. The multi-level impedance matching layer is composed of three raw materials: waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate.

2. The wideband high-efficient electromagnetic shielding composite material based on gradient aluminum foam structure according to claim 1, characterized in that: The bottom layer of the gradient foam aluminum core is high-purity aluminum with a pore size of 0.05-0.1 mm and a porosity of 40%, forming a high-conductivity reflective layer; the transition layer is an alloy layer formed by adding 1-2% copper by mass to the aluminum substrate, with a pore size of 0.2-0.5 mm and a porosity of 60%, forming an eddy current loss layer; the surface layer is an alloy layer formed by adding 0.5-1% silicon by mass to the aluminum substrate, with a pore size of 0.8-1.2 mm and a porosity of 75%, forming a multi-level resonant cavity layer.

3. The wideband high-efficient electromagnetic shielding composite material based on gradient aluminum foam structure according to claim 1, characterized in that: The thickness of the gradient aluminum foam core layer is 4-10 mm, and its direction is a vertical gradient structure; in the gradient aluminum foam core layer, the thickness of the iron oxide + carbon nanotube composite coating is 100-500 nm; the metasurface resonant layer is a patterned metal mesh composed of an I-shaped metal unit array.

4. The wideband high-efficient electromagnetic shielding composite material based on gradient aluminum foam structure according to claim 1, characterized in that, The pretreatment method for the raw materials of the multi-level impedance matching layer is as follows: First, sieve out the 5-20μm silver-coated aluminum flake particles, place them in an ethanol solution for ultrasonic cleaning for 15-20min, and then dry them in an oven at 80-100℃ for 30-40min before taking them out for use; grind the barium strontium titanate dielectric particles to a particle size of 1-5μm by ball milling, place them in an ethanol solution for ultrasonic cleaning for 10-15min, and then dry them in an oven at 80-100℃ for 30-40min before taking them out for use; stir the waterborne polyurethane at 25-30℃ for 10-15min and adjust its viscosity to 500-800mPa·s.

5. The preparation method of the wideband high-efficiency electromagnetic shielding composite material based on the gradient aluminum foam structure, according to any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Preparation of gradient foam aluminum core layer: Foaming is controlled by a spatial temperature field. The bottom temperature of the mold is set to 600℃ and the top temperature is set to 400℃. A preform is prepared by mixing titanium hydride + calcium carbonate composite foaming agent with aluminum powder. At the same time, 3-7wt% of bismuth trioxide-zinc oxide piezoelectric phase is added to the aluminum powder. The piezoelectric phase is uniformly dispersed in the mixture of aluminum powder and composite foaming agent by stirring. The differential decomposition rate of composite foaming agent in spatial temperature field is used to form a vertical pore size gradient. CVD pore wall modification is then performed on the gradient foam aluminum core layer. Step 2: Preparation of multi-level impedance matching layer: Waterborne polyurethane, silver-coated aluminum sheet, and barium strontium titanate dielectric particles are mixed evenly and coated on one side of the gradient aluminum foam core layer, with the thickness controlled at 50-200μm; Step 3: Fabrication of the metasurface resonant layer: I-shaped Ag meshes are fabricated on the PET release film by laser direct writing; Step 4: Conductive backing layer lamination: A 0.1 mm thick aluminum foil is used as a conductive backing layer and laminated onto the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer.

6. The method for preparing a broadband, high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure according to claim 5, characterized in that, The preparation steps of the composite foaming agent are as follows: First, screen titanium hydride and calcium carbonate particles to 65–165 μm, mix them in a ratio of 30–50% titanium hydride and 50–70% calcium carbonate, add 3–5% ethanol, and mix in a planetary ball mill at a speed of 200–300 r / min for 1–2 h to obtain a uniform composite foaming agent.

7. The method for preparing a broadband, high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure according to claim 5, characterized in that, The CVD modification method for preparing the gradient foamed aluminum core layer is as follows: the foamed aluminum obtained by gradient foaming is placed in a CVD reactor and a reaction gas is introduced. The carbon source in the reaction gas is ethylene with a flow rate of 50-100 sccm, and the iron source is iron pentacarbonyl, which is carried into the reactor by argon gas and the flow rate is maintained at 30-50 sccm. The temperature in the reactor is controlled at 400-500℃, the pressure is maintained at 0.08-0.12 MPa, and deposition is carried out at a deposition rate of 45-55 nm / min to form a network structure of Fe3O4 nanoparticles embedded in carbon nanotubes.

8. The method for preparing a broadband, high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure according to claim 5, characterized in that, The preparation method of the multi-level impedance matching layer is as follows: water-based polyurethane, silver-coated aluminum sheet and barium strontium titanate dielectric particles are weighed at a mass ratio of 60:30:10, and the three are mixed and placed in a planetary ball mill. The mixture is stirred at a speed of 200-300 r / min for 1-2 hours to prepare a slurry. The slurry is coated on one side of the modified gradient aluminum foam core layer by spraying. The spraying pressure is controlled at 0.3-0.5 MPa and the spraying distance is 15-20 cm. After coating, the slurry is dried at 80-100℃ for 30-60 minutes and then placed at room temperature for 1-2 hours for natural cooling and stress release.

9. The method for preparing a broadband, high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure according to claim 5, characterized in that, The method for preparing the metasurface resonant layer is as follows: an I-shaped Ag grid is prepared on the surface of a PET release film by laser direct writing. The laser direct writing uses a pulsed laser with a wavelength of 532nm, a pulse width of 10-20ns, a pulse frequency of 20-50kHz, a laser power of 10-15W, and a scanning speed of 50-80mm / s. Then, the prepared I-shaped Ag grid is transferred to the side of the multi-level impedance matching layer away from the gradient aluminum foam core layer using heat-release tape, thus completing the composite of the metasurface resonant layer.

10. The method for preparing a broadband, high-efficiency electromagnetic shielding composite material based on a gradient aluminum foam structure according to claim 5, characterized in that, The method for bonding the conductive backing layer is as follows: select an aluminum foil with a thickness of 0.1 mm as the conductive backing layer, cover it on the side of the gradient foam aluminum core layer away from the multi-level impedance matching layer, and use a hot pressing process to bond the aluminum foil and the gradient foam aluminum core layer.

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