Impedance-tapered wave-absorbing material based on fused deposition modeling process and preparation method thereof

Abstract

The application provides an impedance-graded wave-absorbing material based on a fused deposition modeling process and a preparation method thereof, which comprises a polymer matrix and a wave-absorbing agent distributed in the polymer matrix; along an electromagnetic wave incident direction, the content of the wave-absorbing agent is continuously graded from the surface layer to the bottom layer without obvious steps; the mass fraction of the wave-absorbing agent is 1-5% in the surface layer of the impedance-graded wave-absorbing material, and the mass fraction of the wave-absorbing agent is 30-70% in the bottom layer of the impedance-graded wave-absorbing material. The wave-absorbing agent comprises 20-60 wt% of a first component, 0-40 wt% of a second component and 20-70 wt% of a third component; the first component comprises two-dimensional transition metal carbide and / or nitride MXene nanosheets; the second component comprises liquid metal microspheres; and the third component comprises carbonyl iron powder and / or ferrite micro powder. The wave-absorbing material in the application has the electromagnetic wave absorption performance of wide frequency band, strong absorption and low reflection.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic absorbing materials technology, and particularly relates to an impedance-gradient absorbing material based on fused deposition modeling process and its preparation method. Background Technology

[0002] With the rapid development of modern radar detection technology, electronic countermeasures technology, and precision-guided weapons, the battlefield survivability of military equipment faces unprecedented threats. Developing high-performance radar-absorbing materials and structures to achieve radar stealth for weaponry is a major strategic requirement in modern defense science and technology. Current military equipment demands increasingly urgent requirements for radar-absorbing structures, including wideband stealth, thin and lightweight design with integrated structural load-bearing capacity, conformal design for complex curved surfaces and manufacturability, intelligent operation, maintainability, and low cost.

[0003] The performance of microwave absorbing materials depends primarily on two key factors: impedance matching characteristics and electromagnetic attenuation capability. Impedance matching determines whether electromagnetic waves can effectively penetrate the material's interior, while electromagnetic attenuation capability determines whether the electromagnetic waves that have entered the material can be effectively dissipated. An ideal microwave absorbing material should possess both excellent impedance matching characteristics and strong electromagnetic attenuation capability. However, these two requirements are often contradictory: high electromagnetic attenuation capability usually requires a high dielectric constant or high permeability, which leads to a severe mismatch between the material's surface impedance and free-space impedance, causing electromagnetic waves to be strongly reflected at the material's surface and making it difficult for them to penetrate the material's interior. Summary of the Invention

[0004] The purpose of this invention is to provide an impedance-gradient absorbing material based on fused deposition modeling (FDM) and its preparation method. The absorbing material of this invention has electromagnetic wave absorption performance with wide bandwidth, strong absorption, and low reflection.

[0005] This invention provides an impedance gradient absorbing material based on a fused deposition modeling process, comprising a polymer matrix and a microwave absorbing agent distributed within the polymer matrix;

[0006] Along the incident direction of the electromagnetic wave, the content of the absorbing agent exhibits a continuous gradient distribution without abrupt changes from the surface layer to the bottom layer.

[0007] In the surface layer of the impedance-gradient absorbing material, the mass fraction of the absorbing agent is 1~5%, and in the bottom layer of the impedance-gradient absorbing material, the mass fraction of the absorbing agent is 30~70%.

[0008] The microwave absorbing agent comprises 20-60 wt% of a first component, 0-40 wt% of a second component, and 20-70 wt% of a third component;

[0009] The first component includes MXene nanosheets;

[0010] The second component includes liquid metal microspheres;

[0011] The third component includes carbonyl iron powder and / or ferrite micro powder.

[0012] Preferably, the polymer matrix comprises one or more of polylactic acid, acrylonitrile-butadiene-styrene copolymer, thermoplastic polyurethane, polycarbonate, polyetheretherketone or polyetherimide.

[0013] Preferably, the MXene nanosheets are Ti3C2T. x Ti2CT x Mo2CT x Nb2CT x and Ti4N3T x One or more of the following; surface functional groups T x Including one or more of -O, -OH, and -F;

[0014] The MXene nanosheets have a thickness of 1~10 nm and a width of 0.5~10 μm.

[0015] Preferably, the liquid metal microspheres are eutectic gallium-indium alloy and / or eutectic gallium-indium-tin alloy;

[0016] The liquid metal microspheres have a particle size of 30~50μm.

[0017] Preferably, the carbonyl iron powder includes flake carbonyl iron powder and / or spherical carbonyl iron powder, wherein the thickness of the flake carbonyl iron powder is 0.1~1μm and the particle size of the spherical carbonyl iron powder is 1~10μm;

[0018] The ferrite micro powder is one or more of NiZn ferrite, MnZn ferrite and Ba ferrite, and the particle size of the ferrite micro powder is 0.5~50 μm.

[0019] Preferably, the impedance-gradient absorbing material has an effective absorption bandwidth greater than 8 GHz in the 2~18 GHz frequency band with a reflection loss RL≤-10 dB and a minimum reflection loss RLmin≤-20 dB; the density of the impedance-gradient absorbing material is 1.2~3.5 g / cm³. 3 The thickness is 2~20 mm.

[0020] This invention provides a method for preparing the impedance-gradient absorbing material based on the fused deposition modeling process described above, comprising the following steps:

[0021] A) Based on the impedance gradient distribution requirements of the target absorbing material, design the mass fraction distribution function of the absorbing agent at each position along the thickness direction of the target absorbing material. Based on the 3D printing process parameters and filament diameter, combined with the designed 3D printing path, deduce the mass distribution function of each component in the absorbing agent at different lengths in the filament.

[0022] B) The first component, the second component, and the third component are respectively added to the independent side feeding device of the twin-screw extruder, and the polymer matrix is ​​added to the main feeding device of the twin-screw extruder. Based on the mass distribution function of the microwave absorber at different lengths in the filament obtained in step A), the feeding rate of the main feeding device and the side feeding device is set to prepare a composite filament with a continuously varying microwave absorber along the length of the filament.

[0023] C) The composite filament obtained in step B) is melt-deposited according to a preset three-dimensional model and a designed printing path to obtain an impedance-gradient absorbing material.

[0024] Preferably, the twin-screw extruder is provided with a main feeding device, a side feeding device for the second component, a side feeding device for the third component, and a side feeding device for the first component in sequence in the material flow direction.

[0025] Preferably, the length-to-diameter ratio of the twin-screw extruder is 36:1 to 60:1, and the screw speed is 200 to 400 rpm.

[0026] Preferably, the twin-screw extruder has at least four temperature zones along the length of the screw, which sequentially include a first temperature zone, a second temperature zone, a third temperature zone, and a die head temperature zone along the material flow direction.

[0027] The side feeding device for the second component is located in the first temperature zone, the side feeding device for the third component is located in the second temperature zone, and the side feeding device for the first component is located in the third temperature zone.

[0028] This invention provides an impedance-gradient microwave absorbing material based on a fused deposition modeling process, comprising a polymer matrix and an absorbing agent distributed within the polymer matrix; along the incident direction of the electromagnetic wave, the content of the absorbing agent exhibits a continuous gradient distribution without significant abrupt changes from the surface layer to the bottom layer; in the surface layer of the impedance-gradient microwave absorbing material, the mass fraction of the absorbing agent is 1-5%, and in the bottom layer, the mass fraction of the absorbing agent is 30-70%. The absorbing agent comprises 20-60 wt% of a first component, 0-40 wt% of a second component, and 20-70 wt% of a third component; the first component comprises two-dimensional transition metal carbide and / or nitride MXene nanosheets; the second component comprises liquid metal microspheres; and the third component comprises carbonyl iron powder and / or ferrite micropowder.

[0029] Compared with the prior art, the present invention has the following significant advantages:

[0030] (1) To truly achieve a continuous impedance gradient and eliminate interlayer interface reflections

[0031] This invention achieves continuous variation of the microwave absorbing agent content along the filament length through precise control of a multi-path independent feeding system in a twin-screw extruder. This, in turn, allows for the generation of a continuous impedance gradient without significant abrupt changes along the thickness direction via FDM molding. Compared to traditional multilayer laminated structures, this invention significantly reduces the concentration abrupt changes of the microwave absorbing agent between layers through a continuously varying filament melt deposition molding process. This avoids impedance abrupt changes and enhanced reflection at the interface, significantly improving the energy incident efficiency of electromagnetic waves.

[0032] (2) Magnetism-dielectric cooperative loss mechanism to achieve broadband strong absorption

[0033] This invention employs a composite microwave absorption system of MXene (dielectric loss), liquid metal (conductive network enhancement), and carbonyl iron powder / ferrite (magnetic loss), broadening the absorption bandwidth through the synergistic effect of multiple mechanisms. The synergistic effect among the three is as follows: MXene two-dimensional transition metal carbide or nitride nanosheets serve as the dielectric loss component. Their unique two-dimensional layered structure provides a huge specific surface area and abundant surface functional groups, generating strong interfacial polarization and dipole polarization losses under the action of alternating electromagnetic fields, especially prominent in the mid-to-high frequency range. However, the pure MXene system suffers from an excessively high dielectric constant, leading to impedance mismatch. Liquid metal microspheres, as a conductive network reinforcing component, complement MXene with their extremely high conductivity. When constructing a three-dimensional conductive network in the polymer matrix, liquid metal microspheres can fill the gaps between MXene sheets to form conductive bridges, significantly reducing the percolation threshold of the composite system and allowing the dielectric loss factor to reach its peak even with low filler content. Simultaneously, the self-limiting oxide layer on the surface of the liquid metal microspheres acts as an insulating interface, inducing multiple electromagnetic scattering and extending the propagation path of electromagnetic waves within the material. Furthermore, the lubricating effect of the liquid metal preferentially mixing with the molten polymer during twin-screw extrusion improves the dispersion uniformity of subsequently added solid fillers. Carbonyl iron powder or ferrite micropowder, as a magnetic loss component, contributes primarily to the absorption of electromagnetic waves in the mid-to-low frequency band due to its high permeability and natural resonance characteristics. The anisotropic shape of the sheet-like carbonyl iron powder can be adjusted by regulating the resonant frequency to the target frequency band, overlapping with the dielectric loss band of MXene, achieving strong magnetic and dielectric losses in a specific frequency band. Ferrite micropowder, on the other hand, fills the low-frequency absorption gap through magnetic domain wall resonance and natural resonance mechanisms. The three components exhibit significant frequency band complementarity in their electromagnetic properties: MXene covers the dielectric loss in the mid-to-high frequencies, ferrite covers the magnetic loss in the mid-to-low frequencies, and the liquid metal acts as a conductive network enhancer and scatterer across the entire frequency band. This combination of magnetic-dielectric synergistic loss mechanism and conductive network enhancement effect, along with a continuous impedance gradient design, enables the absorbing structure to achieve an effective absorption bandwidth greater than 8 GHz with RL ≤ -10 dB in the 2–18 GHz frequency band, and a minimum reflection loss RL_min ≤ -20 dB.

[0034] (3) Optimize the feeding sequence to solve the problem of multi-component dispersion.

[0035] This invention innovatively designs the arrangement sequence of the side feed inlets: liquid metal → magnetic loss powder → MXene. This sequence fully considers the physical properties and processing sensitivity of each component: the liquid metal is first mixed with the molten polymer, utilizing the good fluidity of the melt in its low viscosity stage to achieve uniform dispersion; the magnetic loss powder is then added to avoid agglomeration problems when added simultaneously with the liquid metal; MXene is added last to minimize shear breakage and maintain the integrity of the two-dimensional sheets.

[0036] (4) Advantages of additive manufacturing: achieving structural and functional integration

[0037] Based on FDM technology, this invention can fabricate microwave absorbing components with complex geometries and internal structures, realizing the synergistic design of structure and function, such as honeycomb sandwich, lattice structure, and conformal curved surface, to meet the lightweight and integrated design requirements of aerospace, weaponry and other fields. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0039] Figure 1 This is a schematic diagram of the cross-section of the impedance-gradient absorbing material in this invention;

[0040] Figure 2 This is a schematic diagram of the multi-feeding system of the twin-screw extruder of the present invention;

[0041] Figure 2 In the diagram, ① is the main feeding device, ② is the second component feeding device, ③ is the third component feeding device, ④ is the first component feeding device, and ⑤ is the die head; A is the first temperature zone, B is the second temperature zone, C is the third temperature zone, and D is the die head temperature zone.

[0042] Figure 3 The electromagnetic wave reflection loss curves are shown for the absorbing material in Embodiment 1 of the present invention and the absorbing materials in Comparative Examples 1 and 2. Detailed Implementation

[0043] This invention provides an impedance gradient absorbing material based on a fused deposition modeling process, comprising a polymer matrix and a microwave absorbing agent distributed within the polymer matrix;

[0044] Along the direction of electromagnetic wave incidence, the content of the absorbing agent exhibits a continuous gradient distribution without significant abrupt changes from the surface layer to the bottom layer.

[0045] In the surface layer of the impedance-gradient absorbing material, the mass fraction of the absorbing agent is 1~5%, and in the bottom layer of the impedance-gradient absorbing material, the mass fraction of the absorbing agent is 30~70%.

[0046] The microwave absorbing agent comprises 20-60 wt% of a first component, 0-40 wt% of a second component, and 20-70 wt% of a third component;

[0047] The first component comprises two-dimensional transition metal carbides and / or nitride MXene nanosheets;

[0048] The second component includes liquid metal microspheres;

[0049] The third component includes carbonyl iron powder and / or ferrite micro powder.

[0050] In this invention, the polymer matrix includes one or more of polylactic acid (PLA), acrylonitrile-butadiene-styrene copolymer (ABS), thermoplastic polyurethane (TPU), polycarbonate (PC), polyetheretherketone (PEEK), and polyetherimide (PEI).

[0051] In this invention, the microwave absorbing agent comprises a first component, a second component, and a third component, which together constitute the magnetic-dielectric synergistic composite microwave absorbing system of this invention.

[0052] In this invention, the first component is a dielectric loss component, comprising MXene nanosheets, wherein the MXene nanosheets are preferably two-dimensional transition metal carbides and / or two-dimensional transition metal nitrides, and the MXene nanosheets are preferably Ti3C2T. x Ti2CT x Mo2CT x Nb2CT x and Ti4N3T x One or more of the following; surface functional groups T x It includes one or more of -O, -OH, and -F; the thickness of the MXene nanosheets is 1~10 nm, such as 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any of the above values ​​as the upper or lower limit; the width, i.e., the lateral dimension, of the MXene nanosheets is preferably 0.5~10 μm, such as 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any of the above values ​​as the upper or lower limit.

[0053] In this invention, the first component utilizes its high conductivity and layered structure to provide strong dielectric loss (conductivity loss + polarization loss), while the abundant surface functional groups (-O, -OH, -F) generate dipole polarization under the action of alternating electromagnetic field, which enhances dielectric loss. In addition, the layered structure is conducive to multiple reflections and scattering of electromagnetic waves between layers, which prolongs the propagation path; (4) it works with liquid metal to construct a multidimensional conductive network and optimize electromagnetic parameters.

[0054] In this invention, the mass fraction of the first component in the microwave absorber is preferably 20-60%, more preferably 30-50%, such as 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or any of the above values ​​as the upper or lower limit.

[0055] In this invention, the second component is a conductive network enhancement component, preferably liquid metal microspheres, which are eutectic gallium-indium alloys and / or eutectic gallium-indium-tin alloys; the particle size of the liquid metal microspheres is 30~50μm, such as 30μm, 35μm, 40μm, 45μm, 50μm, or any of the above values ​​as the upper or lower limit; the surface of the liquid metal microspheres has a self-limiting oxide layer, the main component of which is Ga2O3, and the thickness of the self-limiting oxide layer is preferably 1~3nm, such as 1 nm, 2 nm, 3 nm, or any of the above values ​​as the upper or lower limit.

[0056] In this invention, the extremely high conductivity of the second component provides strong conductive loss; and the fluidity of the liquid metal allows it to deform into a flat or fibrous shape during extrusion, forming anisotropic conductive pathways. In addition, the surface oxide layer (Ga2O3) of the liquid metal microspheres serves as an insulating dielectric layer, preventing excessive connectivity of the conductive network, avoiding electromagnetic shielding effects, and generating interfacial polarization. The surface oxide layer also allows the microspheres to maintain their spherical shape in the polymer melt shear environment, preventing aggregation. Furthermore, the oxide layer improves the interfacial bonding between the liquid metal and the polymer matrix, improving dispersibility. At the same time, the addition of liquid metal can reduce the problem of increased brittleness and severe agglomeration of the composite material caused by the addition of a large amount of solid microwave absorbing filler.

[0057] In this invention, the mass fraction of the second component in the microwave absorber is preferably 0 to 40%, such as 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or any of the above values ​​as the upper or lower limit.

[0058] In this invention, the third component is preferably a magnetic loss component, preferably carbonyl iron powder and / or ferrite micro powder. The carbonyl iron powder includes flake carbonyl iron powder and / or spherical carbonyl iron powder. The thickness of the flake carbonyl iron powder is 0.1~1μm, such as 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, or any of the above values ​​as the upper or lower limit. The particle size of the spherical carbonyl iron powder is 1~10μm, such as 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, or any of the above values ​​as the upper or lower limit. The ferrite micro powder is preferably one or more of NiZn ferrite, MnZn ferrite and Ba ferrite, and the particle size of the ferrite micro powder is preferably 0.5~50 μm, such as 0.5μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, or any of the above values ​​as the upper or lower limit.

[0059] In this invention, the third component provides a magnetic loss mechanism (hysteresis loss, natural resonance, exchange resonance, eddy current loss) that synergizes with dielectric loss; the introduction of the magnetic component reduces the overall dielectric constant of the composite material and improves impedance matching characteristics; the high permeability of carbonyl iron powder and the low dielectric constant of ferrite are beneficial for achieving broadband impedance matching; the heterogeneous interface between the magnetic particles and the MXene / liquid metal conductive network generates strong interfacial polarization, further enhancing dielectric loss.

[0060] In this invention, the mass fraction of the third component in the microwave absorber is preferably 20-70%, more preferably 30-60%, such as 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any of the above values ​​as the upper or lower limit.

[0061] In this invention, the surface layer of the absorbing material is close to the incident direction of the electromagnetic wave and is an impedance matching layer, while the bottom layer of the absorbing agent is far away from the incident direction of the electromagnetic wave and is a strong absorption layer. There is a gradient transition layer between the surface layer and the bottom layer, that is, the electromagnetic wave first passes through the surface layer, then through the gradient transition layer, and finally reaches the bottom layer.

[0062] In this invention, along the incident direction of electromagnetic waves, the content of the absorbing agent exhibits a continuous gradient distribution without abrupt changes from the surface layer to the bottom layer, forming a continuous impedance gradient distribution; that is, the mass fraction of the absorbing agent in the absorbing material exhibits a continuous gradual distribution without abrupt changes along the incident direction of electromagnetic waves. In a plane perpendicular to the incident direction of electromagnetic waves, the proportion of the three components in the absorbing agent can remain constant or may vary at different points in the plane.

[0063] In this invention, the mass fraction of the absorbing agent on the surface of the absorbing material is preferably 1-5%, more preferably 2-4%, such as 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any of the above values ​​as the upper or lower limit; the mass fraction of the absorbing agent in the bottom layer of the absorbing material is preferably 30-70%, more preferably 40-60%, such as 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any of the above values ​​as the upper or lower limit.

[0064] This invention provides a method for preparing the impedance-gradient absorbing material based on the fused deposition modeling process described above, comprising the following steps:

[0065] A) Based on the impedance gradient distribution requirements of the target absorbing material, design the mass fraction distribution function of the absorbing agent at each position along the thickness direction of the target absorbing material. Based on the 3D printing process parameters and filament diameter, derive the mass distribution function of each absorbing agent at different lengths in the filament.

[0066] B) The first component, the second component, and the third component are respectively added to the independent side feeding device of the twin-screw extruder, and the polymer matrix is ​​added to the main feeding device of the twin-screw extruder. Based on the mass distribution function of the microwave absorber at different lengths in the filament obtained in step A), the feeding rate of the main feeding device and the side feeding device is set to prepare a composite filament in which the microwave absorber changes continuously along the length of the filament.

[0067] C) The composite filament obtained in step B) is melt-deposited according to a preset three-dimensional model to obtain an impedance-gradient absorbing material.

[0068] This invention designs a mass fraction distribution function of the absorbing agent at different positions along the thickness direction of the target absorbing material (i.e., along the electromagnetic wave incident direction), based on the impedance gradient distribution requirements of the target absorbing material. This function represents the mass fraction of the absorbing agent in the target absorbing material as a function of its thickness. Then, based on 3D printing process parameters (including nozzle diameter, printing speed, layer thickness, and infill rate) and filament diameter, the mass distribution function of each component in the absorbing agent at different lengths within the fused deposition modeling composite filament is derived. This invention does not impose any special restrictions on the form of this mass fraction function, as long as it satisfies the structural characteristics of the target absorbing material.

[0069] Specifically, in some embodiments of the present invention, if the total thickness of the target absorbing material is set to H, and the position coordinate along the electromagnetic wave incident direction (i.e., the structural thickness direction) is z (0≤z≤H), then the absorbing agent mass fraction distribution function W(z) can be expressed as:

[0070] Linear function: W(z) = W0 + (W H -W0)·z / H;

[0071] Power function: W(z) = W0 + (W H -W0)·(z / H) n , where n>0;

[0072] Exponential function: W(z) = W0·exp(λz), where λ = ln(W H / W0) / H.

[0073] In the formula, W0 is the mass fraction of the surface absorbing agent (1wt%~5wt%), W H The mass fraction of the bottom layer absorber is 30wt%~70wt%.

[0074] Based on the FDM printing parameters, the position z in the thickness direction is converted into the filament length coordinate L, and the mass distribution function W(L) of the mass fraction of the absorbing agent in the filament as a function of the filament length is established, which serves as the input signal for feeding control.

[0075] In this invention, the first, second, and third components are respectively added to the independent side feeding device of a twin-screw extruder, and the polymer matrix is ​​added to the main feeding device of the twin-screw extruder. Based on the mass distribution function W(L) of the microwave absorber in the filament as a function of the filament length obtained above, the feeding rates of the main feeding device and the side feeding device are set to prepare a composite filament in which the microwave absorber changes continuously along the filament length.

[0076] In this invention, the main feeding device and the side feeding device are equipped with a loss-in-weight metering feeding system driven by a continuously variable speed motor, with a metering accuracy of not less than ±0.1%. The raw material addition rate can be independently and accurately controlled by controlling the feeding screw or metering pump.

[0077] In this invention, the twin-screw extruder is provided with a main feeding device, a side feeding device for the second component, a side feeding device for the third component, and a side feeding device for the first component in sequence along the material flow direction. Preferably, the twin-screw extruder is provided with at least four temperature zones along the screw length direction, which are provided with a first temperature zone, a second temperature zone, a third temperature zone, and a die head temperature zone in sequence along the material flow direction. The side feeding device for the second component is located in the first temperature zone, the side feeding device for the third component is located in the second temperature zone, and the side feeding device for the first component is located in the third temperature zone.

[0078] In this invention, the second component, liquid metal microspheres, is first shear-mixed with the freshly molten polymer matrix. The high fluidity of the melt promotes the uniform dispersion of the liquid metal, forming a preliminary conductive network framework. Subsequently, the third component, magnetic loss absorbing agent powder (carbonyl iron powder / ferrite), is added to avoid the blending and agglomeration problem caused by the simultaneous addition of solid powder and liquid metal. The first component, dielectric loss component, is added last to shorten its residence time and shearing process in the twin-screw extruder, preventing excessive shearing time from causing the two-dimensional sheets to break and maintaining its complete layered structure and dielectric loss characteristics.

[0079] In this invention, the twin-screw extruder is a co-rotating twin-screw extruder, and the length-to-diameter ratio is preferably 36:1 to 60:1, such as 30:1, 35:1, 40:1, 45:1, 50:1, 55:1, 60:1, or any of the above values ​​as the upper or lower limit. The screw speed is preferably 200 to 400 rpm, such as 200 rpm, 250 rpm, 300 rpm, 350 rpm, 400 rpm, or any of the above values ​​as the upper or lower limit.

[0080] In this invention, the temperature settings of the four temperature zones of the twin-screw extruder are related to the type of polymer matrix, as shown in Table 1.

[0081] Table 1. Melt extrusion temperature range for different polymer-based microwave absorbing filaments

[0082]

[0083] The diameter of the filament obtained by extrusion through a twin-screw extruder is 1.75±0.05 mm or 2.85±0.05 mm.

[0084] After obtaining the above-mentioned filament, the present invention performs melt deposition molding of the filament according to a preset three-dimensional model to obtain a microwave absorbing material with a continuous impedance gradient.

[0085] In this invention, the printing temperature needs to be determined according to the type of polymer matrix. The printing nozzle temperature needs to be increased by 10~30°C compared to the printing nozzle temperature of pure matrix material to compensate for the increase in melt viscosity caused by the addition of microwave absorber. The printing speed is preferably 10~60 mm / s, such as 10 mm / s, 15 mm / s, 20 mm / s, 25 mm / s, 30 mm / s, 35 mm / s, 40 mm / s, 45 mm / s, 50 mm / s, 55 mm / s, 60 mm / s, or any of the above values ​​as the upper or lower limit. The layer thickness is 0.1~0.3 mm, such as 0.1 mm, 0.15 mm, 0.2 mm, 0.25 mm, 0.3 mm, or any of the above values ​​as the upper or lower limit.

[0086] This invention provides an impedance-gradient microwave absorbing material based on a fused deposition modeling process, comprising a polymer matrix and an absorbing agent distributed within the polymer matrix; along the incident direction of the electromagnetic wave, the content of the absorbing agent exhibits a continuous gradient distribution without significant abrupt changes from the surface layer to the bottom layer; in the surface layer of the impedance-gradient microwave absorbing material, the mass fraction of the absorbing agent is 1-5%, and in the bottom layer, the mass fraction of the absorbing agent is 30-70%. The absorbing agent comprises 20-60 wt% of a first component, 0-40 wt% of a second component, and 20-70 wt% of a third component; the first component comprises two-dimensional transition metal carbide and / or nitride MXene nanosheets; the second component comprises liquid metal microspheres; and the third component comprises carbonyl iron powder and / or ferrite micropowder.

[0087] Compared with the prior art, the present invention has the following significant advantages:

[0088] (1) To truly achieve a continuous impedance gradient and eliminate interlayer interface reflections

[0089] This invention achieves continuous variation of the microwave absorbing agent content along the filament length through precise control of a multi-path independent feeding system in a twin-screw extruder. This, in turn, allows for the generation of a continuous impedance gradient without abrupt changes along the thickness direction via FDM molding. Compared to traditional multi-layer microwave absorbing agent gradient printing structures, which use filaments with a single microwave absorbing agent content for each layer, the preparation of multiple microwave absorbing filaments is cumbersome and costly. Changing filaments during printing is complex, and the resulting printing interruptions can easily lead to internal defects. Therefore, even if the microwave absorbing agent content in different filaments can vary almost continuously, the intermittent defects introduced by changing filaments cannot be eliminated. This invention avoids impedance abrupt changes and enhanced reflection at the interface, significantly improving the energy incident efficiency of electromagnetic waves.

[0090] (2) Magnetism-dielectric cooperative loss mechanism to achieve broadband strong absorption

[0091] This invention employs a composite microwave absorption system of MXene (dielectric loss), liquid metal (conductive network enhancement), and carbonyl iron powder / ferrite (magnetic loss), broadening the absorption bandwidth through the synergistic effect of multiple mechanisms. The synergistic effect among the three is as follows: MXene two-dimensional transition metal carbide or nitride nanosheets serve as the dielectric loss component. Their unique two-dimensional layered structure provides a huge specific surface area and abundant surface functional groups, generating strong interfacial polarization and dipole polarization losses under the action of alternating electromagnetic fields, especially prominent in the mid-to-high frequency range. However, the pure MXene system suffers from an excessively high dielectric constant, leading to impedance mismatch. Liquid metal microspheres, as a conductive network reinforcing component, complement MXene with their extremely high conductivity. When constructing a three-dimensional conductive network in the polymer matrix, liquid metal microspheres can fill the gaps between MXene sheets to form conductive bridges, significantly reducing the percolation threshold of the composite system and allowing the dielectric loss factor to reach its peak even with low filler content. Simultaneously, the self-limiting oxide layer on the surface of the liquid metal microspheres acts as an insulating interface, inducing multiple electromagnetic scattering and extending the propagation path of electromagnetic waves within the material. Furthermore, the lubricating effect of the liquid metal preferentially mixing with the molten polymer during twin-screw extrusion improves the dispersion uniformity of subsequently added solid fillers. Carbonyl iron powder or ferrite micropowder, as a magnetic loss component, contributes primarily to the absorption of electromagnetic waves in the mid-to-low frequency band due to its high permeability and natural resonance characteristics. The anisotropic shape of the sheet-like carbonyl iron powder can be adjusted by regulating the resonant frequency to the target frequency band, overlapping with the dielectric loss band of MXene, achieving strong magnetic and dielectric losses in a specific frequency band. Ferrite micropowder, on the other hand, fills the low-frequency absorption gap through magnetic domain wall resonance and natural resonance mechanisms. The three components exhibit significant frequency band complementarity in their electromagnetic properties: MXene covers the dielectric loss in the mid-to-high frequencies, ferrite covers the magnetic loss in the mid-to-low frequencies, and the liquid metal acts as a conductive network enhancer and scatterer across the entire frequency band. This combination of magnetic-dielectric synergistic loss mechanism and conductive network enhancement effect, along with a continuous impedance gradient design, enables the absorbing structure to achieve an effective absorption bandwidth greater than 8 GHz with RL ≤ -10 dB in the 2–18 GHz frequency band, and a minimum reflection loss RL_min ≤ -20 dB.

[0092] (3) Optimize the feeding sequence to solve the problem of multi-component dispersion.

[0093] This invention innovatively designs the arrangement sequence of the side feed inlets: liquid metal → magnetic loss powder → MXene. This sequence fully considers the physical properties and processing sensitivity of each component: the liquid metal is first mixed with the molten polymer, utilizing the good fluidity of the melt in its low viscosity stage to achieve uniform dispersion; the magnetic loss powder is then added to avoid agglomeration problems when added simultaneously with the liquid metal; MXene is added last to minimize shear breakage and maintain the integrity of the two-dimensional sheets.

[0094] (4) Advantages of additive manufacturing: achieving structural and functional integration

[0095] Based on FDM technology, this invention can fabricate microwave absorbing components with complex geometries and internal structures, realizing the synergistic design of structure and function, such as honeycomb sandwich, lattice structure, and conformal curved surface, to meet the lightweight and integrated design requirements of aerospace, weaponry and other fields.

[0096] To further illustrate the present invention, the following detailed description of an impedance-gradient absorbing material based on fused deposition modeling process and its preparation method is provided in conjunction with embodiments, but it should not be construed as limiting the scope of protection of the present invention.

[0097] In the following examples and comparative examples, the following methods are used. Figure 2 The apparatus shown is prepared.

[0098] Example 1: Ternary Composite Gradient Absorption Structure Based on PLA MXene / Liquid Metal / Carbonyl Iron Powder

[0099] S1: Gradient Distribution Function Design

[0100] The total thickness of the structure is designed to be H=10 mm, using a linear gradient distribution. The surface layer (z=0) has a microwave absorber mass fraction W0=3wt%, and the bottom layer (z=H) has a microwave absorber mass fraction W... H =50wt%.

[0101] The microwave absorber formulation is MXene nanosheets (Ti3C2T). x The absorber comprises 40% liquid metal microspheres (EGaIn, 40 μm in diameter) and 40% carbonyl iron powder (flakes, 5 μm in diameter and 0.5 μm in thickness).

[0102] The distribution function is W(z) = 3 + 47z / 10 (wt%). Based on the FDM printing parameters (printing speed 30 mm / s, layer thickness 0.2 mm, fill rate 80%, filament diameter 1.75 mm), the correspondence between the filament length coordinate L and the microwave absorber mass fraction is obtained by inversion and input into the feeding control system.

[0103] S2: Multi-component precision feeding

[0104] PLA granules were added to the main feeding device, while MXene nanosheets, EGaIn microspheres, and carbonyl iron powder were added to three separate side feeding devices. Each feeding device was equipped with a loss-in-weight metering system with a metering accuracy of ±0.1%.

[0105] Side feed port arrangement: The EGaIn microsphere feed port is located at the end of zone one, the carbonyl iron powder feed port is located in the middle of zone two, and the MXene nanosheet feed port is located at the front of zone three.

[0106] S3: Continuous gradient filament extrusion

[0107] A co-rotating twin-screw extruder (L / D=48:1, screw speed 300 RPM) was used, with temperature settings as follows: Zone 1 150℃, Zone 2 180℃, Zone 3 200℃, and die head 210℃. The control system adjusted the speed of the feeding devices on each side in real time according to the W(L) function to produce continuous gradient filaments with a diameter of 1.75±0.05 mm.

[0108] S4: Fused Deposition Modeling

[0109] Printing temperature: 210℃ (20℃ higher than pure PLA), printing speed: 30 mm / s, layer thickness: 0.2 mm, infill rate: 80%. Printing is performed according to the preset 3D model printing path to obtain a 10 mm thick gradient absorbing structure.

[0110] Performance testing:

[0111] Electromagnetic wave reflection loss was tested using the bow-shaped method. Results showed that within the 2–18 GHz frequency band, the effective absorption bandwidth (RL ≤ -10 dB) was 10.5 GHz, and the minimum reflection loss RL... min = -35.2 dB (occurring at 12.3 GHz), with a structure density of 2.1 g / cm³.

[0112] Example 2: MXene / ferrite binary composite gradient absorbing structure based on PEEK matrix

[0113] S1: Gradient Distribution Function Design

[0114] The total structural thickness is designed to be 15 mm, using a power-law gradient distribution (n=2). W0=2wt%, W H =60wt%.

[0115] Microwave absorber formulation: MXene nanosheets (Mo2CT) x The composition is 50% NiZn ferrite powder (10 μm particle size). It contains no liquid metal, and its distribution function is W(z) = 2 + 58·(z / 15). 2 (wt%).

[0116] S2~S4: Preparation and Molding

[0117] PEEK pellets are the main feed, with MXene and NiZn ferrite fed separately from the side feeders. Side feed port arrangement: NiZn ferrite in front, MXene behind. Twin-screw extruder parameters: L / D=52:1, screw speed 350 RPM. Temperature settings: Zone 1 350℃, Zone 2 370℃, Zone 3 380℃, die head 390℃. Printing temperature: 390℃ (20℃ higher than pure PEEK), printing speed 20 mm / s, layer thickness 0.15 mm.

[0118] Performance testing:

[0119] The effective absorption bandwidth (RL≤-10 dB) is 9.2 GHz, and the minimum reflection loss RL min =-32.8 dB, with a structure density of 2.8 g / cm³. Due to the use of PEEK matrix, the structure has good high temperature resistance (long-term operating temperature >250℃), making it suitable for radar stealth of components with temperature resistance requirements.

[0120] Example 3: Flexible gradient absorbing structure based on TPU matrix

[0121] S1: Gradient Distribution Function Design

[0122] The total thickness of the structure is H=5 mm, with an exponential function distribution, W0=5wt%, W H =40wt%.

[0123] Absorber ratio: MXene (Ti2CT) x 30%, EGaIn 30%, carbonyl iron powder 40%.

[0124] Distribution function: W(z) = 5·exp(0.833z) (wt%)

[0125] S2~S4: Preparation and Molding

[0126] TPU granules are the main feeder. The three-zone temperature settings are: Zone 1 170℃, Zone 2 195℃, Zone 3 215℃, and the print head 225℃. The printing temperature is 225℃, and the printing speed is 40 mm / s.

[0127] Performance testing:

[0128] Effective absorption bandwidth 8.5 GHz, RL min =-28.6 dB. Due to the excellent flexibility of the TPU matrix, it can be fitted to curved surfaces, making it suitable for electromagnetic protection of wearable devices or flexible electronic systems.

[0129] Example 4: Thin broadband absorbing structure based on PC substrate

[0130] S1: Gradient Distribution Function Design

[0131] The total thickness of the structure is H = 3 mm, linearly distributed, W0 = 1wt%, W H =35wt.

[0132] Absorber ratio: MXene (Nb2CT) x 25%, EGaIn 15%, MnZn ferrite 60%.

[0133] S2~S4: Preparation and Molding

[0134] PC pellet main feeder, temperature settings: Zone 1 240℃, Zone 2 270℃, Zone 3 280℃, die head 290℃.

[0135] Printing temperature: 290℃, printing speed: 50 mm / s, layer thickness: 0.1 mm.

[0136] Performance testing:

[0137] The effective absorption bandwidth is 8.2 GHz in the 2~18 GHz frequency band, RL min =-31.5 dB. Despite a thickness of only 3 mm, broadband absorption was achieved, demonstrating the advantages of continuous gradient design in terms of thinness.

[0138] Comparative Example 1: Uniform Component Absorbing Structure

[0139] Using the same microwave absorber ratio and total content (average 26.5 wt%) as in Example 1, but preparing filaments with uniform microwave absorber concentration through uniform mixing, and then FDM forming the same size structure.

[0140] Test results show that the effective absorption bandwidth is only 3.2 GHz, RL min =-13.5 dB. Due to severe impedance mismatch and strong surface reflection, the absorption performance is significantly worse than that of Example 1.

[0141] Comparative Example 2

[0142] Using the same gradient range as in Example 1 (surface layer (z=0) absorber mass fraction W0=3wt%, bottom layer (z=H) absorber mass fraction W... H =50wt%), but five different absorbing agent contents were used (absorbing agent contents of 3wt%, 14.75wt%, 26.5wt%, 38.25wt%, and 50wt%), respectively. The concentration of each filament was uniform, and the prepared absorbing material was divided into five layers (2 mm each) in the thickness direction. The absorbing agent content in each layer was uniform, and the layers showed a uniform step change.

[0143] Test results show: effective absorption bandwidth 6.8 GHz, RL min =-22.3 dB. The impedance abrupt change at the interlayer interface leads to enhanced reflection, and the absorption performance is between that of a homogeneous structure and a continuous gradient structure.

[0144] Comparative Example 3

[0145] The same formulation and process as in Example 1 were used, but the side feed port arrangement was MXene → carbonyl iron powder → liquid metal (opposite to Example 1).

[0146] Test results show: effective absorption bandwidth 7.5 GHz, RL min =-24.6 dB. Premature addition of MXene leads to severe shear breakage, and the liquid metal added last is difficult to disperse uniformly in the already highly viscous melt, resulting in a smaller absorption bandwidth.

[0147] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An impedance-gradient microwave absorbing material based on a fused deposition modeling process, comprising a polymer matrix and a microwave absorbing agent distributed within the polymer matrix; Along the incident direction of the electromagnetic wave, the content of the absorbing agent exhibits a continuous gradient distribution without abrupt changes from the surface layer to the bottom layer. In the surface layer of the impedance-gradient absorbing material, the mass fraction of the absorbing agent is 1-5%, and in the bottom layer of the impedance-gradient absorbing material, the mass fraction of the absorbing agent is 30-70%. The microwave absorbing agent comprises 20-60 wt% of a first component, 0-40 wt% of a second component, and 20-70 wt% of a third component; The first component includes MXene nanosheets; The second component includes liquid metal microspheres; The third component includes carbonyl iron powder and / or ferrite micro powder.

2. The impedance-tapered metamaterial absorber based on a fused deposition modeling process of claim 1, wherein, The polymer matrix includes one or more of polylactic acid, acrylonitrile-butadiene-styrene copolymer, thermoplastic polyurethane, polycarbonate, polyetheretherketone or polyetherimide.

3. The impedance-tapered metamaterial absorber based on a fused deposition modeling process of claim 1, wherein, The MXene nanosheets are Ti3C2T x Ti2CT x Mo2CT x Nb2CT x and Ti4N3T x One or more of the following; surface functional groups T x Including one or more of -O, -OH, and -F; The MXene nanosheets have a thickness of 1~10 nm and a width of 0.5~10 μm.

4. The impedance-tapered metamaterial absorber based on a fused deposition modeling process of claim 1, wherein, The liquid metal microspheres are eutectic gallium-indium alloy and / or eutectic gallium-indium-tin alloy; The liquid metal microspheres have a particle size of 30~50μm.

5. The impedance-tapered metamaterial absorber based on a fused deposition modeling process of claim 1, wherein, The carbonyl iron powder includes flake carbonyl iron powder and / or spherical carbonyl iron powder, wherein the thickness of the flake carbonyl iron powder is 0.1~1μm and the particle size of the spherical carbonyl iron powder is 1~10μm; The ferrite micro powder is one or more of NiZn ferrite, MnZn ferrite and Ba ferrite, and the particle size of the ferrite micro powder is 0.5~50 μm.

6. The impedance-tapered metamaterial absorber based on a fused deposition modeling process of claim 1, wherein, The impedance-gradient absorbing material has an effective absorption bandwidth greater than 8 GHz in the 2–18 GHz frequency band with a reflection loss RL ≤ -10 dB and a minimum reflection loss RLmin ≤ -20 dB; the density of the impedance-gradient absorbing material is 1.2–3.5 g / cm³. 3 The thickness is 2~20 mm.

7. A method for preparing an impedance-gradient absorbing material based on fused deposition modeling as described in any one of claims 1 to 6, comprising the following steps: A) Based on the impedance gradient distribution requirements of the target absorbing material, design the mass fraction distribution function of the absorbing agent at each position along the thickness direction of the target absorbing material. Based on the 3D printing process parameters and filament diameter, combined with the designed 3D printing path, deduce the mass distribution function of each component in the absorbing agent at different lengths in the filament. B) The first component, the second component, and the third component are respectively added to the independent side feeding device of the twin-screw extruder, and the polymer matrix is ​​added to the main feeding device of the twin-screw extruder. Based on the mass distribution function of the microwave absorber at different lengths in the filament obtained in step A), the feeding rate of the main feeding device and the side feeding device is set to prepare a composite filament with a continuously varying microwave absorber along the length of the filament. C) The composite filament obtained in step B) is melt-deposited according to a preset three-dimensional model and a designed printing path to obtain an impedance-gradient absorbing material.

8. The preparation method according to claim 7, characterized in that, The twin-screw extruder is provided with a main feeding device, a side feeding device for the second component, a side feeding device for the third component, and a side feeding device for the first component in sequence in the material flow direction.

9. The preparation method according to claim 7, characterized in that, The twin-screw extruder has a length-to-diameter ratio of 36:1 to 60:1 and a screw speed of 200 to 400 rpm.

10. The preparation method according to claim 7, characterized in that, The twin-screw extruder has at least four temperature zones along the length of the screw, which, in sequence along the material flow direction, include a first temperature zone, a second temperature zone, a third temperature zone, and a die head temperature zone. The side feeder for the second component is arranged in the first temperature zone, the side feeder for the third component is arranged in the second temperature zone, and the side feeder for the first component is arranged in the third temperature zone.

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