Gradient refractive index-based wide-band three-dimensional wave absorber and preparation method thereof

Abstract

This invention discloses a broadband three-dimensional microwave absorber based on gradient refractive index and its fabrication method, relating to the field of electromagnetic wave absorbing materials technology. The absorber is a three-dimensional structure containing at least one lossy filler. The volume fraction or concentration of the lossy filler within the absorber varies continuously or quasi-continuously along at least one spatial dimension, resulting in a gradient distribution of the equivalent complex refractive index of the absorber along the corresponding spatial dimension. This gradient distribution is configured to cause wavefront bending and propagation path extension of incident electromagnetic waves within the absorber, achieving efficient absorption of electromagnetic energy within a predetermined broadband band. This invention can be extended to integrate functional structures such as load-bearing, thermal management, or sensing into the model, enabling multifunctional integrated manufacturing.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic wave absorbing materials technology, and in particular to a broadband three-dimensional absorber based on gradient refractive index and its preparation method. Background Technology

[0002] With the rapid development of modern wireless communication, radar detection, and stealth technologies, high-performance electromagnetic wave absorbers play a crucial role in suppressing electromagnetic interference, improving equipment compatibility, and achieving low target detectability. An ideal absorber needs to efficiently absorb electromagnetic waves with various polarizations and incident angles across the widest possible operating frequency band, while simultaneously meeting the engineering application requirements of lightweight, thinness, and high environmental adaptability. Among numerous technical approaches, gradient refractive index absorbing structures based on impedance matching principles have become a research hotspot. One traditional method involves stacking multiple layers of homogeneous media with different electromagnetic parameters to form a stepped impedance gradient, thereby achieving wideband absorption. These multilayer absorbers guide electromagnetic waves through gradual transmission at the interlayer interfaces by progressively changing the dielectric constant or permeability layer along the perpendicular incident direction, reducing reflection and ultimately dissipating the waves in the loss layer. This technical approach is relatively mature, with an intuitive structural design, and can achieve good absorption performance in specific frequency bands by adjusting layer thickness and interlayer material parameters.

[0003] The aforementioned gradient refractive index design based on discrete layered structures still faces intrinsic challenges when dealing with broadband electromagnetic waves, especially those incident at large angles. Because the electromagnetic parameters within each layer are uniformly distributed, and clear physical interfaces exist between layers, electromagnetic waves undergo reflection and refraction when passing through these interfaces. Their propagation path is essentially a broken line governed by the discontinuous Snell's law. When electromagnetic waves are incident at large angles, this discrete impedance transition induces more significant reflections and causes a significant distortion in the standing wave field distribution within each layer compared to normal incidence, making the absorption performance extremely sensitive to the incident angle. To maintain broadband absorption, the number of layers is usually increased to smooth the impedance transition, but this inevitably increases the overall thickness and weight of the structure. Existing technologies have room for further optimization in coordinating broadband absorption, wide-angle stability, and a thinner structure. The core challenge lies in the difficulty of achieving continuous and smooth control of the propagation behavior of electromagnetic waves in three-dimensional space using discrete layered structures. Summary of the Invention

[0004] In view of the aforementioned existing problems, the present invention is proposed.

[0005] Therefore, this invention provides a broadband three-dimensional absorber based on gradient refractive index and its preparation method to solve the problem that it is difficult to balance performance and thinness due to the sudden change in interface impedance of discrete layered structures.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: In a first aspect, the present invention provides a broadband three-dimensional microwave absorber based on gradient refractive index and a method for preparing the same. The absorber is a three-dimensional structure containing at least one lossy filler. The volume fraction or concentration of the lossy filler within the absorber varies continuously or quasi-continuously along at least one spatial dimension, thereby causing the equivalent complex refractive index of the absorber to exhibit a gradient distribution along the corresponding spatial dimension. This gradient distribution is configured to cause wavefront bending and propagation path extension of incident electromagnetic waves within the absorber, achieving efficient absorption of electromagnetic energy within a predetermined broadband frequency band. As a preferred embodiment of the broadband three-dimensional absorber based on gradient refractive index according to the present invention, wherein: the at least one spatial dimension includes a depth direction (z direction) extending from the incident surface of the absorber inward, and the equivalent complex refractive index gradually increases in the depth direction from the near-air matching value at the incident surface inward.

[0007] Furthermore, the at least one spatial dimension includes a depth direction (z-direction) extending inward from the incident surface of the absorber, wherein the equivalent complex refractive index gradually increases in the depth direction from the near-air matching value at the incident surface towards the interior. This gradient refractive index distribution in the depth direction essentially constitutes a smooth impedance transition channel from free space to the high-loss medium region, its function being to minimize Fresnel reflection of incident electromagnetic waves at the absorber surface, ensuring that electromagnetic energy can be efficiently coupled into the interior of the absorber.

[0008] As a preferred embodiment of the broadband three-dimensional absorber based on gradient refractive index described in this invention, wherein: the at least one spatial dimension further includes at least one direction (x direction and / or y direction) in a transverse plane parallel to the incident surface, and the equivalent complex refractive index is distributed in an axisymmetric or centrosymmetric gradient in the transverse plane, configured to produce a focusing or deflection effect on obliquely incident electromagnetic waves.

[0009] Furthermore, the equivalent complex refractive index exhibits an axisymmetric or centrosymmetric gradient distribution within the transverse plane, configured to produce a focusing or deflecting effect on obliquely incident electromagnetic waves. This gradient refractive index distribution within the transverse plane functions similarly to a phase-tuning lens embedded within the absorber. It applies an additional phase delay distribution to the wavefront of electromagnetic waves incident at different angles, thereby guiding the wavefront to bend, converge, or propagate along a specific path within the absorber. This effectively extends the propagation path length of the obliquely incident electromagnetic wave within the loss region, enhancing the absorber's ability to capture and dissipate obliquely incident electromagnetic waves and strengthening its angular stability.

[0010] Secondly, the present invention provides a method for preparing a broadband three-dimensional absorber based on gradient refractive index, comprising: S1. Gradient design: designing the required three-dimensional gradient complex refractive index distribution function n(x,y,z) inside the absorber according to the target absorption frequency band and the incident angle range; S2. Material Mapping: Based on the effective medium theory, the three-dimensional gradient complex refractive index distribution function n(x,y,z) is mapped to the volume fraction spatial distribution function of at least two basic materials, wherein at least one basic material contains a lossy filler. S3. Digital Model Generation: Based on the volume fraction spatial distribution function, generate a three-dimensional voxelized digital model containing material composition information; S4. Gradient Additive Manufacturing: Based on the aforementioned three-dimensional voxelized digital model, a multi-material additive manufacturing device is used to deposit a composite material with spatial gradient components point by point by adjusting the supply ratio of two or more base materials in real time and mixing them online. After synchronous curing, the three-dimensional microwave absorber is formed.

[0011] As a preferred embodiment of the method for preparing a broadband three-dimensional absorber based on gradient refractive index according to the present invention, in step S2, the at least two basic materials include: a first matrix material having a low dielectric constant and loss tangent; and a second composite matrix material containing the lossy filler dispersed in the first matrix material or similar matrix materials, having a high dielectric constant and / or permeability and loss tangent.

[0012] Furthermore, in step S2, the at least two base materials include: a first matrix material with low dielectric constant and loss tangent; and a second composite matrix material containing the lossy filler dispersed in the first matrix material or a similar matrix material, having high dielectric constant and / or magnetic permeability and loss tangent. The core of this material system lies in the fact that, by precisely controlling the volume mixing ratio of the first matrix material and the second composite matrix material, a continuous and smooth transition of equivalent electromagnetic parameters from a pure low-loss matrix to a high-loss composite material can theoretically be achieved. This "matrix-filler" binary system provides a direct and flexible material realization path for physically realizing any designed gradient refractive index distribution, ensuring that the final absorber accurately reproduces the macroscopic gradient design in its microscopic material composition.

[0013] As a preferred embodiment of the method for preparing a broadband three-dimensional absorber based on gradient refractive index according to the present invention, wherein: the lossy filler is selected from at least one of carbon nanotubes, graphene, carbon fibers, silicon carbide powder, conductive polymers, carbonyl iron powder, ferrite powder, and magnetic metal alloy powder; the matrix of the first matrix material and the second composite matrix material is the same or different photocurable resin, thermosetting resin, or ceramic precursor polymer.

[0014] Furthermore, the lossy filler is selected from at least one of carbon nanotubes, graphene, carbon fibers, silicon carbide powder, conductive polymers, carbonyl iron powder, ferrite powder, and magnetic metal alloy powder; the matrix of the first matrix material and the second composite matrix material is the same or different photocurable resin, thermosetting resin, or ceramic precursor polymer. The above material combinations provide a wide range of electromagnetic parameter control and molding process adaptability. Among them, carbon-based fillers and ceramic fillers (such as SiC) mainly contribute to dielectric polarization loss; magnetic metal and ferrite fillers can simultaneously provide hysteresis loss and dielectric loss, which is beneficial for achieving impedance matching over a wider frequency band. The choice of matrix material directly determines the applicable additive manufacturing process and the usage environment of the final product. For example, photocurable resins are suitable for UV curing printing, thermosetting resins are suitable for thermally assisted extrusion, and ceramic precursor polymers are suitable for subsequent pyrolysis to obtain high-temperature resistant ceramic matrix composites.

[0015] As a preferred embodiment of the method for preparing a broadband three-dimensional absorber based on gradient refractive index according to the present invention, wherein: in step S4, the real-time control of the supply ratio of two or more basic materials and the online mixing specifically refers to: The multi-material additive manufacturing equipment obtains the corresponding target volume fraction from the three-dimensional voxelized digital model based on the coordinates of the current deposition point, and dynamically controls the flow rate or conveying speed of multiple independent feeding units, so that each base material is conveyed to a micro-mixing chamber in the target proportion to achieve uniform mixing, forming a mixed slurry with gradient components, which is then extruded.

[0016] Furthermore, in step S4, the real-time control of the supply ratio of two or more base materials and online mixing specifically involves: the multi-material additive manufacturing equipment obtaining the corresponding target volume fraction from the three-dimensional voxelized digital model based on the coordinates of the current deposition point, and dynamically controlling the flow rate or conveying speed of multiple independent feeding units, so that each base material is delivered to a micro-mixing chamber according to the target ratio to achieve uniform mixing, forming a mixed slurry with gradient components, which is then extruded. This technical step is the core of this preparation method to achieve precise control of spatial gradient components. Its working principle is similar to a "digital micro-palette," using a high-precision fluid control system (such as an injection pump or screw pump) to perform real-time, synchronous, and programmed control of the supply rate of multiple raw materials, thereby generating and extruding the specific material ratio required for each "voxel" according to a preset spatial function (i.e., gradient distribution) within the mixing chamber. This point-by-point programmable material supply method is a key guarantee for realizing complex three-dimensional gradient structures.

[0017] As a preferred embodiment of the method for preparing a broadband three-dimensional absorber based on gradient refractive index according to the present invention, in step S4, the synchronous curing is to apply an energy field to cure the gradient component material simultaneously with or immediately after its deposition; the energy field is one of an ultraviolet light field, a thermal field, or an infrared light field.

[0018] Furthermore, in step S4, the simultaneous curing involves applying an energy field to solidify the gradient component material simultaneously with or immediately after deposition; the energy field is one of an ultraviolet light field, a thermal field, or an infrared light field. The "deposition-simultaneous curing" strategy is crucial for maintaining the shape fidelity of the gradient structure. It instantly locks in the freshly deposited mixed slurry with a specific material ratio, preventing the diffusion, blurring, or destruction of the formed component gradient due to its own fluidity or the pressure of subsequent deposited materials. The selected energy field type (ultraviolet, thermal, infrared) matches the curing mechanism of the matrix material used (photosensitive resin, thermosetting resin, etc.) to ensure a rapid, efficient, and uniform curing reaction, thereby precisely "freezing" the pre-designed three-dimensional gradient information.

[0019] As a preferred embodiment of the method for preparing a broadband three-dimensional absorber based on gradient refractive index according to the present invention, in step S3, when generating a three-dimensional voxelized digital model, a three-dimensional model of a functional structure for bearing, thermal management or sensing is also integrated, so that a composite absorber with built-in functional structure is integrally formed in step S4.

[0020] Furthermore, in step S3, when generating the three-dimensional voxelized digital model, a three-dimensional model of the functional structure for load-bearing, thermal management, or sensing is also integrated, thereby integrally forming a composite absorber with built-in functional structures in step S4. This demonstrates the method's ability to extend to integrated structural and functional design while achieving the main function (wave absorption). By pre-designing and "implanting" load-bearing structures such as lightweight honeycomb or corrugated structures, microfluidic networks for heat conduction or liquid cooling, or reserved spaces and channels for components such as optical fibers and sensors for status monitoring in the digital model, synchronous and interface-free integral forming with the gradient absorber structure can be achieved in subsequent additive manufacturing processes. This avoids the connection interfaces and additional weight caused by traditional assembly methods, enabling the manufacture of multifunctional composite components integrating load-bearing, stealth, thermal management, and status sensing.

[0021] As a preferred embodiment of the method for preparing a broadband three-dimensional absorber based on gradient refractive index according to the present invention, in step S4, the multi-material additive manufacturing is a three-dimensional printing process based on extrusion molding; the two or more base materials are supplied in the form of viscous slurry.

[0022] Furthermore, in step S4, the multi-material additive manufacturing is a three-dimensional printing process based on extrusion molding; the two or more base materials are supplied in the form of a viscous slurry. Extrusion additive manufacturing is chosen because it has broad adaptability to material viscosity and morphology, and can effectively handle composite slurries with high solids content (especially those containing a high proportion of lossy fillers). This process, by precisely controlling the nozzle's movement path and the material extrusion flow rate, can layer-by-layer deposit parts with complex three-dimensional shapes. The viscous slurry form facilitates the aforementioned online continuous mixing and real-time component control, making it an effective and practical process path for achieving integrated material-structure-function gradient manufacturing.

[0023] The beneficial effects of this invention are as follows: Based on the target design, a three-dimensional gradient complex refractive index distribution function is used. Based on the effective medium theory, this function is mapped to a volume fraction spatial distribution function of at least two basic materials. A three-dimensional voxelized digital model containing material composition information is generated accordingly. Based on this model, a multi-material additive manufacturing device is used to deposit gradient component materials point-by-point by real-time controlling the supply ratio of various materials and mixing them online, followed by simultaneous curing, to integrally form the three-dimensional microwave absorber. This method can be extended to integrate functional structures such as load-bearing, thermal management, or sensing into the model, achieving multi-functional integrated manufacturing. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a flowchart of a method for fabricating a broadband three-dimensional absorber based on gradient refractive index. Detailed Implementation

[0026] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0027] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0028] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0029] Reference Figure 1 This is one embodiment of the present invention, which provides a broadband three-dimensional absorber based on gradient refractive index and its preparation method, including the following steps: Detailed Implementation

[0030] Example 1: This example provides a broadband absorber with a one-dimensional gradient refractive index, and its preparation method is as follows: Gradient design: The goal is to design a planar absorber with an absorption rate of over 90% in the 8-18 GHz frequency band and a thickness of 5 mm.

[0031] Using transmission line theory and optimization algorithms, the equivalent complex refractive index distribution function n(z) required to satisfy impedance matching and broadband absorption in the depth direction (z direction) is determined. This function is close to the free space refractive index (approximately 1) at the incident surface (z=0), and increases continuously inward (in the direction of z-increase) according to a specific law (such as a polynomial or exponential law), reaching its maximum value in the high-loss region inside.

[0032] Material mapping and slurry preparation: A transparent photosensitive resin (model: Formlabs Clear V4) was selected as the first matrix material (A) because it has a low dielectric constant (approximately 2.8, @10 GHz) and a very small loss tangent.

[0033] The above-mentioned photosensitive resin was mixed with multi-walled carbon nanotubes (MWCNTs) at a weight ratio of 10:1, and then dispersed by high-speed shearing and ultrasonication to prepare a uniform carbon nanotube / photosensitive resin composite slurry, which served as the second composite matrix material (B). Slurry B has a high dielectric constant and loss tangent.

[0034] Based on the Maxwell-Garnett effective medium theory formula, the designed n(z) function is mapped to the volume fraction f_B(z) of two slurries A and B at any z coordinate. f_B changes continuously from 0% at the incident surface to 100% at the back bottom (z=5mm).

[0035] Digital model generation: Create a cuboid digital model with dimensions of 100mm x 100mm x 5mm.

[0036] In the slicing software, based on the f_B(z) function, a unique mixing ratio of materials A and B is defined for each layer (with a layer thickness set to 50 micrometers) along the z-axis (thickness direction) of the model, thereby generating a voxelized digital model file. In this file, the proportion of material B increases continuously from the top layer (incident surface) to the bottom layer.

[0037] Gradient additive manufacturing: Equipment: A self-modified multi-material digital light processing (DLP) 3D printer. This equipment has two independent material tanks, one for material A and the other for material B. The bottom of the material tank is a light-transmitting window, above which are precision scrapers and a height-adjustable forming platform. The key innovation lies in the integration of a miniature dynamic liquid mixing and spreading unit, which can extract and mix material A and material B in real time and proportionally, achieving uniform online mixing.

[0038] Real-time blending printing: The printer reads the voxelized digital model file. During the printing of each layer, the mixing unit dynamically adjusts the volumetric flow rate of the slurry drawn from the A and B troughs in real time according to the f_B value corresponding to that layer, mixing the two evenly in the micro-chamber to form a uniform slurry with the specific components of that layer. The doctor blade then precisely spreads the slurry onto the forming surface to form a liquid film of uniform thickness.

[0039] Simultaneous curing: After the liquid film is spread, a high-resolution DLP projection system projects a specific pattern of ultraviolet light according to the two-dimensional cross-sectional shape of the layer, selectively exposing and curing the mixed slurry. After curing, the molding platform descends one layer and repeats the above "real-time mixing-spreading-curing" process until the entire gradient absorber is printed.

[0040] Post-processing: The printed components are cleaned with isopropyl alcohol to remove uncured resin and then placed under a UV lamp for post-curing.

[0041] Comparative Example 1: Traditional uniform thickness, discrete three-layer absorber To compare with Example 1, a discrete three-layer absorber with the same total thickness of 5 mm was prepared. The three layers from the incident surface to the back surface are as follows: First layer (impedance matching layer): pure photosensitive resin (material A), 1.7mm thick.

[0042] The second layer (transition layer) is a uniform composite material formed by pre-mixing slurries A and B in a 1:1 volume ratio and then curing it. The thickness is 1.6 mm.

[0043] The third layer (loss layer): pure B slurry (carbon nanotube / resin composite material), 1.7 mm thick.

[0044] Preparation method: The traditional process of "separate preparation - separate pouring / layout - separate curing - bonding" is adopted. That is, three layers of boards are prepared separately, and then the three layers are bonded together into a whole using transparent epoxy resin adhesive.

[0045] Comparison of beneficial effects: Absorption performance: Reflection loss was tested using the bow method in a microwave anechoic chamber. The gradient absorber of Example 1 exhibited a reflection loss of less than -10 dB in the 8-18 GHz frequency band. In contrast, the three-layer structure of Comparative Example 1 only achieved absorption below -10 dB in the 10-15 GHz band, with a sharp drop in absorption performance at the edges of low frequencies (8-10 GHz) and high frequencies (16-18 GHz). This demonstrates that a continuous gradient refractive index structure has a smoother impedance transition and a wider absorption bandwidth than a discrete layered structure.

[0046] Interface reflection: In Comparative Example 1, the presence of a distinct material interface and adhesive layer between layers introduces additional reflections at the interface, reducing energy coupling efficiency. In Example 1, due to the continuous variation in material composition and the absence of a physical interface, interface reflection is effectively suppressed.

[0047] Structural integrity: Example 1 is a one-piece molded structure with uniform mechanical properties. Comparative Example 1 relies on the adhesive layer, which poses a risk of interlayer debonding and has poor environmental adaptability (such as temperature cycling and vibration).

[0048] Example 2: This example provides a wave absorber with a three-dimensional gradient refractive index to improve the performance of large-angle incident light.

[0049] Gradient design: Design a hemispherical shell absorber with a radius of 30 mm and a wall thickness of 8 mm. The target frequency band is 2-18 GHz, and its performance at incident angles of 0-60 degrees should be optimized.

[0050] Design a three-dimensional gradient complex refractive index distribution function n(r, θ), where r is the radial distance to the center of the sphere and θ is the angle with the axis of the hemisphere. This function satisfies the following: in the radial direction (from the outer surface inward), n increases from the surface (matching air) towards the interior; on the same radius sphere, n also varies gradient along the θ direction (from the axis towards the edge), forming a refractive index distribution similar to a "non-uniform lens," aiming to guide and "bind" obliquely incident wavefronts to propagate a longer path within the shell wall.

[0051] Material mapping and slurry preparation: Matrix material: thermosetting epoxy resin system is selected.

[0052] Filler: Select sheet-like carbonyl iron powder that has both dielectric and magnetic loss properties.

[0053] Slurry preparation: Material A: Pure epoxy resin slurry (low viscosity).

[0054] Material B: Carbonyl iron powder / epoxy resin high solids content composite slurry (viscosity adjusted by rheology modifier, similar to slurry A).

[0055] The n(r, θ) function is mapped to the volume fraction spatial distribution f_B(r,θ) of slurries A and B through the effective medium theory.

[0056] Digital model generation: Create a 3D model of the hemispherical shell and perform voxelization (voxel size 0.1 mm).

[0057] For each voxel, a target f_B value is assigned based on its spatial coordinates (r, θ), generating a three-dimensional voxelized digital model containing material composition information.

[0058] Gradient additive manufacturing: Equipment: A multi-channel precision extrusion 3D printer. Equipped with two independent high-precision screw extrusion systems, connected to barrels A and B respectively. A static micro-mixer is integrated into the print head.

[0059] Real-time blending printing: The printer reads the voxelized model. During print path planning, the control system calculates the target f_B value in real time based on the coordinates of the current position of the printhead nozzle and dynamically adjusts the rotation speed of the two screws to precisely control the instantaneous extrusion flow ratio of slurries A and B. The two slurries are uniformly mixed in the mixer before ejection to form composite filaments with spatially continuously varying components.

[0060] Simultaneous curing: A near-infrared heating device is integrated behind the printhead. After the mixed material is extruded and deposited, near-infrared radiation immediately heats the material, triggering a thermosetting reaction of the epoxy resin, achieving immediate in-situ curing of the deposited material and fixing the gradient components.

[0061] One-piece molding: Through five-axis linkage, the print head is controlled to move along the normal and path of the hemispherical shell surface, and layers are stacked one by one to finally form a hemispherical shell absorber with a complex three-dimensional gradient distribution.

[0062] Comparative Example 2: Hemispherical absorber with uniform thickness and composition A hemispherical shell with the exact same dimensions as in Example 2 was prepared, but its material was a homogeneous composite material, meaning the entire shell was made of a single material formed by pre-mixing slurries A and B in a fixed ratio (e.g., a volume ratio of 1:1). The same multi-axis 3D printer was used, but a barrel containing only the single material was used for printing.

[0063] Comparison of beneficial effects: Angular stability: Tested using the free-space method in a microwave anechoic chamber. Within the 2-18 GHz frequency band, when the electromagnetic wave is incident at 0 degrees (normal), the absorption performance of Example 2 is similar to that of Comparative Example 2. However, when the incident angle increases to 60 degrees, the absorption performance of Example 2 (bandwidth with absorption rate >90%) decreases significantly less than that of Comparative Example 2. This is because the transverse (θ direction) gradient refractive index distribution in Example 2 produces effective phase compensation and wavefront bending effects for obliquely incident waves, guiding the electromagnetic wave to propagate more within the shell and be absorbed, while the homogeneous material of Comparative Example 2 does not possess this function.

[0064] Broadband performance: Example 2 exhibits superior absorption performance across a wide bandwidth (especially in the low-frequency range) compared to Comparative Example 2. This is attributed to its radial gradient design, which achieves broadband impedance matching from the surface to the interior, as well as the capture and path extension of electromagnetic wave energy. In contrast, the homogeneous material of Comparative Example 2 has a fixed impedance, making it difficult to achieve good matching with free space across the entire bandwidth.

[0065] Design freedom: Example 2 demonstrates the powerful ability of this method to manufacture complex-shaped, non-uniform functional components. Comparative Example 2, although also using 3D printing, can only manufacture components with "uniform material + complex shape," and cannot achieve the core function of "distributing material properties in space as needed."

[0066] In summary, this invention involves: designing a three-dimensional gradient complex refractive index distribution function based on the target, mapping this function to a volume fraction spatial distribution function of at least two basic materials based on effective medium theory, generating a three-dimensional voxelized digital model containing material composition information, and using multi-material additive manufacturing equipment to deposit gradient component materials point-by-point by real-time controlling the supply ratio of multiple materials and mixing them online, followed by simultaneous curing, to integrally form the three-dimensional microwave absorber. This method can be extended to integrate functional structures such as load-bearing, thermal management, or sensing into the model, achieving multi-functional integrated manufacturing.

[0067] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A broadband three-dimensional absorber based on gradient refractive index, characterized in that: include, The absorber is a three-dimensional structure containing at least one lossy filler. The volume fraction or concentration of the lossy filler inside the absorber varies continuously or quasi-continuously along at least one spatial dimension, thereby causing the equivalent complex refractive index of the absorber to have a gradient distribution in the corresponding spatial dimension. The gradient distribution is configured to cause the incident electromagnetic wave to bend at the wavefront and prolong its propagation path inside the absorber, and to achieve efficient absorption of electromagnetic energy within a preset wideband.

2. The broadband three-dimensional absorber based on gradient refractive index as described in claim 1, characterized in that: The at least one spatial dimension includes a depth direction (z-direction) extending inward from the incident surface of the absorber, wherein the equivalent complex refractive index gradually increases in the depth direction from the near-air matching value at the incident surface towards the interior.

3. The broadband three-dimensional absorber based on gradient refractive index as described in claim 2, characterized in that: The at least one spatial dimension also includes at least one direction (x-direction and / or y-direction) in a transverse plane parallel to the incident plane, wherein the equivalent complex refractive index is distributed in an axisymmetric or centrosymmetric gradient in the transverse plane and is configured to produce a focusing or deflection effect on obliquely incident electromagnetic waves.

4. A method for fabricating a broadband three-dimensional absorber based on gradient refractive index, based on the broadband three-dimensional absorber based on gradient refractive index as described in any one of claims 1 to 3, characterized in that: include, S1. Gradient Design: Based on the target absorption frequency band and incident angle range, design the required three-dimensional gradient complex refractive index distribution function n(x,y,z) inside the absorber; S2. Material Mapping: Based on the effective medium theory, the three-dimensional gradient complex refractive index distribution function n(x,y,z) is mapped to the volume fraction spatial distribution function of at least two basic materials, wherein at least one basic material contains a lossy filler. S3. Digital Model Generation: Based on the volume fraction spatial distribution function, generate a three-dimensional voxelized digital model containing material composition information; S4. Gradient Additive Manufacturing: Based on the aforementioned three-dimensional voxelized digital model, a multi-material additive manufacturing device is used to deposit a composite material with spatial gradient components point by point by adjusting the supply ratio of two or more base materials in real time and mixing them online. After synchronous curing, the three-dimensional microwave absorber is formed.

5. The method for preparing a broadband three-dimensional absorber based on gradient refractive index as described in claim 4, characterized in that: In step S2, the at least two base materials include: a first matrix material having a low dielectric constant and loss tangent; and a second composite matrix material containing the lossy filler dispersed in the first matrix material or similar matrix materials, having a high dielectric constant and / or magnetic permeability and loss tangent.

6. The method for preparing a broadband three-dimensional absorber based on gradient refractive index as described in claim 5, characterized in that: The destructive filler is selected from at least one of carbon nanotubes, graphene, carbon fiber, silicon carbide powder, conductive polymer, carbonyl iron powder, ferrite powder, and magnetic metal alloy powder; the matrix of the first matrix material and the second composite matrix material is the same or different photocurable resin, thermosetting resin, or ceramic precursor polymer.

7. The method for preparing a broadband three-dimensional absorber based on gradient refractive index as described in claim 4, characterized in that: In step S4, the real-time adjustment of the supply ratio of two or more basic materials and their online mixing specifically involves: The multi-material additive manufacturing equipment obtains the corresponding target volume fraction from the three-dimensional voxelized digital model based on the coordinates of the current deposition point, and dynamically controls the flow rate or conveying speed of multiple independent feeding units, so that each base material is conveyed to a micro-mixing chamber in the target proportion to achieve uniform mixing, forming a mixed slurry with gradient components, which is then extruded.

8. The method for preparing a broadband three-dimensional absorber based on gradient refractive index as described in claim 4, characterized in that: In step S4, the synchronous curing means applying an energy field to cure the gradient component material simultaneously with or immediately after its deposition; the energy field is one of an ultraviolet light field, a thermal field, or an infrared light field.

9. The method for preparing a broadband three-dimensional absorber based on gradient refractive index as described in claim 4, characterized in that: In step S3, when generating the three-dimensional voxelized digital model, a three-dimensional model of the functional structure for bearing, thermal management or sensing is also integrated, so that a composite absorber with built-in functional structure is integrally formed in step S4.

10. The method for preparing a broadband three-dimensional absorber based on gradient refractive index as described in claim 4, characterized in that: In step S4, the multi-material additive manufacturing is a three-dimensional printing process based on extrusion molding; the two or more base materials are supplied in the form of viscous slurry.

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