Ultrasonic transducer, multi-layer gradient acoustic impedance matching layer and preparation method of multi-layer gradient acoustic impedance matching layer

Abstract

The invention belongs to the technical field of acoustic devices and ultrasound, and discloses an ultrasonic transducer, a multi-layer gradient acoustic impedance matching layer and a preparation method of the multi-layer gradient acoustic impedance matching layer, and the multi-layer gradient acoustic impedance matching layer comprises a plurality of acoustic impedance layers which are arranged in sequence and have different acoustic impedances; in the direction from the first acoustic impedance layer to the last acoustic impedance layer, the acoustic impedance of the acoustic impedance layer is in a decreasing trend; the surface of the previous acoustic impedance layer is provided with a micro-columnar structure, the micro-columnar structure extends into the next acoustic impedance layer, a mechanical interlocking interface is formed between the previous acoustic impedance layer and the next acoustic impedance layer through the micro-columnar structure, and the micro-columnar structure is gradually thinned from the bottom to the tip. According to the invention, the gradient impedance design of the acoustic impedance layers significantly reduces the acoustic reflection between the acoustic impedance layers, improves the acoustic energy transmission efficiency, and widens the working bandwidth of the ultrasonic device; the micro-columnar structure not only maintains the stability of interlayer adhesion in a high vibration or thermal stress environment, but also further smooths gradient transition and reduces interface reflection.

Description

Technical Field

[0001] The present invention belongs to the technical field of acoustic devices and ultrasonic technology, and particularly relates to an ultrasonic transducer, a multi-layer gradient acoustic impedance matching layer and a preparation method thereof. Background Art

[0002] An ultrasonic transducer relies on an acoustic impedance matching layer to achieve efficient transmission of acoustic energy from its excitation unit (such as piezoelectric ceramics, CMUT, PMUT or other advanced ultrasonic transducer materials) to a target medium (such as water, tissue or various solid materials). In the absence of a suitable matching layer, there is usually a large acoustic impedance mismatch between the transducer and the external medium, resulting in strong reflection of sound waves at the interface and limited available bandwidth. Theoretically, multi-layer or graded impedance matching can significantly broaden the working bandwidth and improve the sensitivity, while reducing the reflected energy loss. However, in practical applications, such multi-layer or gradient structures often encounter challenges due to poor interlayer adhesion, high porosity, interface defects or uneven dispersion of nano-fillers. The above defects may cause additional scattering and reflection of sound waves, thereby offsetting the acoustic performance advantages that should have been obtained.

[0003] Recent research has explored acoustic impedance gradient materials as matching layers, where the impedance varies continuously (or in fine steps) in thickness. Such a gradient matching layer can provide a broadband transmission window and near-perfect acoustic energy transmission between the probe and the medium. In particular, the gradient following an exponential impedance distribution has been shown to minimize reflections over a wide frequency range, superior to linear or other gradient forms.

[0004] However, manufacturing the gradient layer presents challenges. One method is to layer composites with different filler ratios: for example, depositing mixtures of high-impedance particles (such as tungsten) in epoxy resin in multi-layers with a gradually changing concentration. Under carefully controlled deposition (even centrifugal sedimentation), the filler content can be smoothly reduced along the propagation direction, thus approaching the ideal exponential impedance distribution. Although this multi-step layering is effective, interface problems may be encountered - if not fully connected, each layer interface will become a potential point for delamination or acoustic impedance mismatch. Uniform particle distribution within each layer is also crucial; any agglomeration of heavy particles or uneven dispersion will lead to local impedance fluctuations and increased scattering.

[0005] Despite these advancements, there is still a need for an optimized acoustic matching layer structure that combines smooth impedance grading with strong interlayer adhesion and uniform material distribution. In particular, ensuring the uniform dispersion of nanoparticles in the matrix (to achieve the desired impedance curve without local defects) and the strong bonding between gradient sub-layers (to avoid delamination) are key technical hurdles. Existing methods, such as simple mechanical stirring or layering without surface texture, may not fully address these issues, especially when the number of layers or gradient complexity increases. Therefore, improving the manufacturing process of gradient matching layers is crucial for realizing their full performance potential. Summary of the Invention

[0006] To address the deficiencies of the acoustic impedance matching layer in bandwidth, adhesion, and uniform particle dispersion in the prior art, the object of the present invention is to provide an ultrasonic transducer, a multi-layer gradient acoustic impedance matching layer, and a preparation method thereof, which achieve broadband low reflection and stable interlayer adhesion through the novel manufacturing process and structural design of the present invention, thereby significantly improving the acoustic energy transmission efficiency and device performance.

[0007] To achieve the above object, the present invention adopts the following technical solutions: A multi-layer gradient acoustic impedance matching layer includes a plurality of acoustic impedance layers with different acoustic impedances arranged in sequence; Among them, in the direction from the first acoustic impedance layer to the last acoustic impedance layer: the acoustic impedance of the acoustic impedance layer shows a decreasing trend; the surface of the previous acoustic impedance layer has a micro-columnar structure, and this micro-columnar structure extends into the next acoustic impedance layer. A mechanical interlock interface is formed between the previous acoustic impedance layer and the next acoustic impedance layer through the micro-columnar structure, and the micro-columnar structure gradually tapers from its bottom to its tip.

[0008] Preferably, the thickness of each acoustic impedance layer is 1 / 4 of the acoustic wave wavelength; from the first acoustic impedance layer to the last acoustic impedance layer, an exponential acoustic impedance distribution is satisfied.

[0009] Preferably, the exposed surface of the multi-layer gradient acoustic impedance matching layer is conformally coated with a parylene coating.

[0010] Preferably, each acoustic impedance layer includes a polymer matrix and high acoustic impedance nanoparticles uniformly dispersed in the polymer matrix.

[0011] Preferably, the high acoustic impedance nanoparticles include at least one of tungsten nanoparticles, barium titanate nanoparticles, PZT nanoparticles, and magnetite nanoparticles.

[0012] Preferably, the micro-columnar structure is a structure that protrudes from the surface of the previous acoustic impedance layer into the interior of the next acoustic impedance layer. The shape of the micro-columnar structure is a protruding columnar structure or a periodic microcavity concentric with the previous acoustic impedance layer. The micro-columnar structure is made of the same material as the previous acoustic impedance layer.

[0013] The present invention also provides a method for preparing the multi-layer gradient acoustic impedance matching layer as described above, including the following processes: Disperse high acoustic impedance nanoparticles in a polymer matrix to obtain a nanoparticle-polymer suspension. For different acoustic impedance layers, the concentration of the high acoustic impedance nanoparticles is different; Apply a rotating magnetic field to the nanoparticle-polymer suspension while performing ultrasonic cavitation to obtain a uniformly dispersed mixture; Deposit the uniformly dispersed mixture layer by layer to form a multi-layer gradient acoustic impedance matching layer; wherein, before the previous acoustic impedance layer is cured, a micro-columnar structure is processed on the surface of this layer of acoustic impedance layer. After the previous acoustic impedance layer is cured, repeat the processing process of the previous acoustic impedance layer to prepare the next acoustic impedance layer until all the acoustic impedance layers are prepared; and during the curing process of each layer of acoustic impedance layer, continuously apply a rotating magnetic field.

[0014] Preferably, before the previous acoustic impedance layer is cured, a micro-columnar structure is processed on the surface of this layer of acoustic impedance layer, including: using imprinting or photolithography to pattern micro-scale columns, or pre-forming periodic microcavities on the surface of the previous acoustic impedance layer.

[0015] Preferably, the above preparation method of the present invention further includes the following process: After obtaining the multi-layer gradient acoustic impedance matching layer, deposit it by chemical vapor deposition using parylene to conformally coat the multi-layer gradient acoustic impedance matching layer with a parylene layer, and then cure or anneal the multi-layer gradient acoustic impedance matching layer conformally coated with the parylene layer to completely harden the polymer layer.

[0016] The present invention also provides an ultrasonic transducer, the surface of which is provided with the multi-layer gradient acoustic impedance matching layer as described above in the present invention.

[0017] Compared with the prior art, the present invention has at least the following beneficial technical effects: In the multi-layer gradient acoustic impedance matching layer of the present invention, the gradient impedance design of several acoustic impedance layers with different acoustic impedances significantly reduces the acoustic reflection between the acoustic impedance layers, improves the acoustic energy transmission efficiency and broadens the working bandwidth of the ultrasonic device; the micro-columnar structure not only maintains the firmness of the interlayer adhesion in a high-vibration or high-thermal stress environment, but also further smooths the gradient transition and reduces the interface reflection.

[0018] Furthermore, the exposed surface of the multi-layer gradient acoustic impedance matching layer is conformally coated with a parylene coating, which strengthens the mechanical properties of the overall structure by filling the voids around the micro-columnar structure and can finely tune the local acoustic impedance as required. Description of the Drawings

[0019] Figure 1 Schematic diagram of the preparation of a nanoparticle-polymer mixture assisted by a rotating magnetic field and ultrasonic oscillation in an embodiment of the present invention; Figure 2 Schematic diagram of the effects of ultrasonic cavitation and acoustic streaming on particle distribution during ultrasonic vibration in an embodiment of the present invention; Figure 3 Schematic diagram of the movement of particles under the action of a rotating magnetic field in an embodiment of the present invention; Figure 4 Schematic diagram of an acoustic impedance multi-layer stacked structure obtained by combining micro-columns and a parylene coating in an embodiment of the present invention.

[0020] In the figure, 1 - ultrasonic generator, 2 - PID temperature controller, 3 - cooling gas, 4 - ultrasonic speaker, 5 - ultrasonic transducer, 6 - welding head, 7 - vibrator support, 8 - slide bar, 9 - heating furnace, 10 - container, 11 - base, 12 - AC coil, 13 - signal acquisition instrument, 14 - PC terminal, 15 - ultrasonic transmitting head, 16 - ultrasonic shock wave, 17 - cluster, 18 - cavitation bubble, 19 - rotating magnetic field, 20 - nanoparticle, 21 - gradient acoustic impedance matching layer, 22 - parylene coating, 23 - micro-columnar structure. Detailed Embodiments

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, it should be understood that these descriptions are merely exemplary and do not limit the scope of the present invention. In the following detailed description, for the sake of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present invention. However, obviously, one or more embodiments can also be implemented without these specific details.

[0022] The terms used herein are merely for describing specific embodiments and are not intended to limit the present invention. The terms "including", "comprising", etc. used herein indicate the presence of the described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0023] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0024] The present invention introduces a highly optimized acoustic impedance matching layer for ultrasonic applications. The core innovation lies in the combination of a hierarchical composite structure and advanced manufacturing techniques. Through the rotation magnetic field and ultrasonic agitation, highly uniform dispersion of nanoparticles in the polymer matrix is achieved, thus avoiding spatial irregularities and defects in the impedance gradient. Based on the dual mixing mode of magnetic force and cavitation, even high-density nano-fillers can maintain stable dispersion, avoiding sedimentation and agglomeration. In terms of structural design, by setting micro-columnar structures at the interface of the acoustic impedance layer and tightly bonding each sub-layer (i.e., the acoustic impedance layer) with a parylene conformal coating, a solid integrated gradient transition is realized, significantly enhancing the inter-layer adhesion and suppressing the generation of interface weaknesses, thereby overcoming the problem of poor bonding in traditional multi-layer matching. The low-impedance medium is formed by filling parylene around each micro-columnar structure, enabling the composite matching layer (i.e., the multi-layer gradient acoustic impedance matching layer of the present invention) to exhibit continuous impedance changes at the microscale, greatly reducing internal reflection and energy loss. Due to the uniform nanoparticle distribution and customizable gradient profile, the multi-layer gradient acoustic impedance matching layer of the present invention can effectively broaden the working bandwidth of ultrasonic devices and reduce reflection, improving imaging resolution and signal intensity. At the same time, its enhanced mechanical stability also ensures reliability under long-term use. In addition, by adjusting the nanoparticle content, the geometric structure of the micro-columnar structure, and the coating thickness, it can be flexibly adapted to different transducer materials and target media, achieving the maximum acoustic transmission efficiency and being applicable to various acoustic application scenarios.

[0025] See Figure 1 , the construction of the optimized matching layer in the present invention involves a series of material processing steps combining magnetic field-assisted mixing and ultrasonic agitation as follows: First, high acoustic impedance nanoparticles (such as tungsten nanoparticles, barium titanate nanoparticles, PZT nanoparticles, or magnetite nanoparticles) are dispersed in a polymer matrix (such as epoxy resin or thermosetting polymer) to obtain a nanoparticle-polymer suspension, and different filler concentrations are customized for each sub-layer (i.e., the acoustic impedance layer) to form the required gradient. To maintain the high uniformity of the nanoparticle-polymer suspension at both the macroscopic and microscopic scales, the present invention applies a rotation magnetic field through an external AC coil and uses an ultrasonic speaker to provide high-frequency mechanical vibration. The magnetic nanoparticles (i.e., high acoustic impedance nanoparticles) continuously tumble in the rotation magnetic field, preventing their aggregation in the xy cross-section. With the cooperation of a heating furnace and a PID temperature controller, the temperature of the mixture (i.e., the nanoparticle-polymer suspension) can be stably controlled and the curing process can be realized when necessary, avoiding defects caused by overheating or temperature fluctuations. At the same time, key parameters such as temperature, vibration frequency, and pressure are monitored in real time by a signal acquisition instrument and a PC, further precisely controlling the preparation conditions, thereby ensuring the initial uniformity and stability of the material during subsequent layer-by-layer deposition and micro-structure setting.

[0026] See Figure 2 , the ultrasonic speaker generates strong cavitation effects and acoustic streaming in the fluid, subjecting the mixture to high-frequency mechanical vibrations, breaking up the high acoustic impedance nanoparticle aggregates and maintaining a uniform distribution in the yz and xz cross-sections; the cavitation microbubbles capture and carry out the gas in the matrix, thus contributing to the degassing of the entire composite material; during this process, the ultrasonic action and the rotating magnetic field complement each other: the magnetic force maintains stable stirring at the macroscopic scale, while the ultrasonic vibration breaks up the particle aggregates at the microscopic scale, and the two work together to effectively prevent particle sedimentation or caking, ensuring that the prepared suspension is dense inside and has a low defect rate.

[0027] See Figure 3 , under the action of the rotating magnetic field, the high acoustic impedance nanoparticles are continuously subjected to torque and perform circular motions. This magneto-driven convection maintains the dispersion and suspension of the particles in the xy cross-section, thus further avoiding particle deposition in local areas; the suspension treated by magnetic-ultrasonic coupling has good uniformity, laying a foundation for subsequent layer-by-layer deposition, micro-structure forming, and parylene encapsulation, enabling the prepared matching layer to exhibit excellent acoustic impedance gradient characteristics and internal density during final forming.

[0028] See Figure 4 , on the premise of achieving uniform dispersion of the nanoparticle-polymer mixture and obtaining good fluidity, the present invention constructs a multi-layer stacked structure with a smooth acoustic impedance transition through steps such as gradient-controlled layer-by-layer deposition, micro-columnar structure formation, and parylene coating and curing.

[0029] In the above design of the present invention, the structure of the matching layer makes its acoustic impedance gradually decrease from the transducer side to the load (target medium) side, preferably following an exponential gradient to achieve the best broadband performance. This is achieved by changing the concentration of high acoustic impedance nanoparticles (such as tungsten nanoparticles, ferrite nanoparticles, or PZT nanoparticles) that vary along the thickness in the polymer or elastomeric binder. Different from traditional single-layer or double-layer matching, the proposed structure effectively behaves as a continuum of many sub-layers, eliminating large impedance jumps at the interfaces. The result is an almost continuous impedance gradient, greatly reducing the ultrasonic reflections at the probe / layer and layer / media boundaries and promoting almost complete transmission of acoustic energy. By customizing the gradient profile, the present invention can broaden the working bandwidth of the ultrasonic transducer and improve the signal fidelity, solving the long-existing bandwidth limitation of traditional matching layers.

[0030] A key aspect of the present invention is the method of nanoparticle dispersion. To create a uniform gradient material, the nanoparticles must be evenly distributed and prevented from aggregating or settling during the manufacturing process. The present invention achieves this by applying a rotating magnetic field and ultrasonic vibrations simultaneously before and during curing to the particle-polymer mixture. The rotating magnetic field applies a torque on the magnetoresponsive nanoparticles (or additional magnetic tags / particles attached to the high-impedance particles), thereby causing a continuous stirring motion in the fluid. This magnetically driven convection keeps the particles suspended and evenly distributed, counteracting the effects of gravity or density segregation. As ferromagnetic hydrodynamic studies have shown, a rotating external magnetic field can move ferromagnetic fluids, effectively acting as a non-contact mixer in the context of the present invention. Superimposed on this, ultrasonic oscillations generate intense microscale stirring through acoustic streaming and cavitation. The ultrasonic energy breaks up the particle aggregates and distributes the particles evenly. Nanoparticle aggregates are dispersed into a uniform distribution within minutes under ultrasonic treatment. The magnetic-ultrasonic combination method ensures that even very dense nanoparticles, which might otherwise settle or agglomerate on their own, remain evenly dispersed in the matrix. This results in a smooth impedance gradient material without large aggregates or voids, directly addressing the uniformity challenge.

[0031] Another innovative feature is the micro-columnar interface structure between the gradient sub-layers. Instead of stacking planar layers that meet at a planar interface, the present invention uses a microscopic column structure (also known as a micro-columnar structure) that extends from one acoustic impedance layer to the next. In practice, this can be achieved by forming a series of tiny columns or protrusions of the high-impedance phase (e.g., "columns" rich in cured nanoparticles) that project into the subsequent layer of the low-impedance material. These interpenetrating micro-columns (i.e., the micro-columnar structure) greatly increase the contact area and create a mechanical interlock between the layers. Subsequent material deposition surrounds and engulfs these micro-columns, eliminating any distinct boundary at the interface. This structuring technique enhances adhesion and reduces the risk of delamination or debonding under thermal stress or acoustic stress. Additionally, the microstructural transition (i.e., the micro-columnar structure) itself acts as a fine-scale impedance gradient: the volume fraction of the high-impedance material tapers gradually from the base to the tip of each micro-column (i.e., the micro-columnar structure) (see Figure 4 ), thereby providing a smooth acoustic transition at the microscale. Thus, the micro-columnar structure not only strengthens the assembly but also supplements the impedance grading, effectively creating a hierarchical gradient (both a macro-gradient and a micro-gradient across the layer thickness within each interface region).

[0032] To further strengthen the structure, the present invention applies a parylene coating, allowing parylene to penetrate around the micro-columnar structure and the surface of the composite material. Parylene (a polymer with low acoustic impedance) has multiple functions: it encapsulates the micro-columnar structure and fills any tiny gaps, thereby bonding the components into a single integrated layer and providing a controlled low-impedance matrix that, together with the high-impedance columns, forms a composite material with spatially varying impedance. By adjusting the thickness of the parylene coating, the local impedance gradient around the micro-columns can be fine-tuned (since a thicker polymer coating increases the polymer fraction in the upper region). Parylene also improves the environmental stability and biocompatibility of the layer, and its excellent conformal adhesion ensures that there are no air gaps around the micro-structures. Essentially, the nanoparticle-filled layer, the micro-columnar structure, and the parylene binder are all integrated together to form a matching layer with an impedance that gradually transitions from one side to the other and has strong internal adhesion.

[0033] Specifically, the preparation method of the multi-layer gradient acoustic impedance matching layer of the present invention includes the following process: A composite material is formed by combining nanoparticles (i.e., high acoustic impedance nanoparticles) with a polymer matrix, and a continuous or multi-stage gradient reduction of acoustic impedance is achieved from one surface to another in the composite material; wherein, the high-impedance nanoparticles are embedded in the polymer matrix with spatially varying concentrations, and the volume fraction of the nanoparticles is the highest at one surface and gradually decreases towards the other surface, generating a corresponding gradient in the acoustic impedance through this gradient distribution, so as to achieve broadband acoustic impedance matching between the piezoelectric transducer and the target medium. The preparation process adopts a combination of a rotating magnetic field and ultrasonic oscillation to uniformly disperse the nanoparticle-polymer mixture before curing; wherein, the rotating magnetic field prevents the precipitation of nanoparticles in the cross-section through convective motion, and the ultrasonic oscillation destroys particle agglomeration and evenly distributes the nanoparticles in the longitudinal section through cavitation effect, so as to obtain a matching layer with uniform nanoparticle distribution and conforming to the gradient design after curing. The matching layer is formed by bonding multiple sub-layers, and adjacent sub-layers are interconnected through micro-columnar structures extending from the upper sub-layer to the lower sub-layer; the micro-columnar structures are composed of resin protrusions rich in nanoparticles or similar materials, and are embedded in the matrix of adjacent sub-layers to form a mechanical interlocking interface, thereby expanding the effective contact area and enhancing the inter-layer adhesion force, and avoiding delamination under high-frequency vibration; at the same time, the micro-columnar interface forms a fine impedance transition zone acoustically, reducing the internal reflection at the inter-layer boundary. A parylene coating is further coated outside the multi-layer structure and the micro-columnar interface; the parylene coating can fill the voids around the micro-columns and form a continuous matrix around the high-impedance columns, so that the micro-columns and the surrounding parylene jointly constitute a local gradient acoustic impedance composite region, thereby providing chemical and mechanical stability while maintaining the overall gradient distribution, and significantly reducing the acoustic reflection at each interface. The gradient structure satisfies an exponential or its approximate acoustic impedance distribution to significantly reduce acoustic reflection in a wide frequency band range; through the synergistic effect of the uniform gradient dispersion of nanoparticles in the polymer matrix and the micro-column enhanced structure, the output energy of the ultrasonic transducer can obtain higher coupling efficiency and wider bandwidth during transmission in the medium, thereby improving the resolution of acoustic imaging and maximizing energy transfer.

[0034] Example This example provides a preparation method for a multi-layer gradient acoustic impedance matching layer of the present invention, specifically including the following steps: Step 1. Preparation of nanoparticle-polymer mixture: Select high acoustic impedance fillers (such as tungsten, barium titanate, PZT, or magnetite nanoparticles) and a polymer matrix (such as epoxy resin or thermosetting polymer). Disperse the nanoparticles into the liquid polymer to produce a suspension. The filler concentration can be customized for each sublayer. For example, the load in the first layer (near the sensor) is higher, and the load in subsequent layers gradually decreases to achieve a gradient. To facilitate magnetic mixing, a fraction of the filler or the filler itself should be magnetically responsive (ferromagnetic or superparamagnetic). This can be inherent (such as magnetite Fe3O4 nanoparticles) or achieved by coating high impedance particles or combining high impedance particles with magnetic materials.

[0035] Step 2. Magneto-ultrasonic dispersion: Subject the suspension obtained in Step 1 to the influence of a rotating magnetic field while applying ultrasonic vibrations. The rotating field (e.g., using a rotating permanent magnet or electromagnetic array) causes the magnetic nanoparticles to circulate continuously, preventing precipitation and creating a stirring effect in the fluid. At the same time, an ultrasonic transducer (or an immersion ultrasonic horn) introduces high-frequency mechanical oscillations into the mixture. The ultrasonic vibrations generate cavitation microbubbles and acoustic streaming in the fluid, thus strongly agitating the mixture. As a result, any nanoparticle aggregates will be decomposed by the cavitation force, and the particles become uniformly dispersed throughout the resin. This dual-field treatment is maintained until the mixture reaches a homogeneous and uniform state, with the nanoparticles uniformly suspended in the polymer (verified, for example, by sampling or real-time monitoring of the mixture homogeneity). The combined action is more effective than using either method alone: magnetic stirring distributes the particles throughout the volume and keeps them suspended, while ultrasonic energy ensures deagglomeration down to the nanoscale and promotes particle interaction across the upper and lower sections.

[0036] Step 3. Gradient-controlled layer-by-layer deposition: Coat the transducer surface (or substrate) with the dispersed mixture treated in Step 2 in a series of layers to form an impedance gradient. The first layer (closest to the transducer) contains the highest fraction of nanoparticles to closely match the impedance of the transducer. Subsequent layers use gradually decreasing filler content to gradually reduce the impedance approaching the medium. Each layer is deposited with a controlled thickness (the deposition thickness of all layers is near 1 / 4 of the acoustic wavelength (i.e., the theoretical thickness of each layer is 1 / 4 of the acoustic wavelength)) to achieve the theoretically optimal acoustic impedance thickness, and is partially cured or gelled before adding the next layer. The rotating magnetic field can be maintained during the curing process to keep the nanoparticles uniformly distributed until the matrix solidifies. This discrete layer-by-layer method mimics a continuous exponential impedance distribution through small incremental changes in composition. The number of layers and their composition are selected according to the design (e.g., an exponential decrease in the filler volume fraction) to minimize acoustic reflection. Under optimal conditions (e.g., using gentle centrifugation or field gradients), it is even possible to fine-tune the particle distribution in each layer to approach a continuous gradient without distinct layer boundaries.

[0037] Micro-columnar structure formation: During the process of step 3, before each subsequent layer is fully cured, micro-column features spanning the interface are created. One method is to imprint or lithographically pattern micro-scale columns in the semi-cured layer (e.g., using a micro-mold or by laser ablation of small holes and then allowing some uncured material to protrude). Alternatively, periodic micro-cavities can be pre-formed on the surface so that when the layer is deposited, it fills these cavities and forms a micro-column array after curing. These micro-columns are composed of the nanoparticle-rich material of the layer and will be embedded in the next layer. When the lower filling material of the next layer is coated, it flows around the exposed micro-columns and wraps them. This produces an interdigitated interface where the columns of the first layer extend into the matrix of the second layer. The dimensions (diameter, height, spacing) of these micro-columns are designed such that they do not produce acoustic scattering at the operating frequency (usually their dimensions are much smaller than the ultrasonic wavelength), but rather act as a graded interface. The micro-structured interface significantly increases the adhesion force through mechanical interlocking and an enlarged contact area. It also creates a micro-scale impedance gradient: the high-impedance columns gradually taper into the low-impedance surroundings with a smooth transition. If multiple layers are used, this process can be repeated for each interface.

[0038] Step 4. Parylene coating and curing: Once the layered structure with micro-columns is in place (i.e., after the end of step 3), a parylene coating is deposited over the entire matching layer. Parylene is deposited through a chemical vapor deposition process, which will conformally coat all exposed surfaces, penetrate into the gaps between the micro-columns, and cover all interfaces. It wraps the micro-columns and fills any micro-voids, effectively bonding the layers together at the molecular level. As a result, the high-impedance columns and the parylene matrix form a continuous medium with a gradient acoustic impedance. The thickness of the parylene can be controlled to adjust the amount of low-impedance material added to the top region of the layer; for example, a thicker parylene deposition will further reduce the top surface impedance, resulting in a slower impedance drop from bottom to top. After coating, the entire structure is cured or annealed as needed (to fully harden the polymer layer). The final product is a solid acoustic matching layer integrated on the sensor with a smooth impedance transition and a robust structure.

[0039] By the above method, the present invention produces a multi-layer gradient acoustic impedance matching layer that is durable both functionally (in terms of acoustic transmission) and structurally. The use of a rotating magnetic field and ultrasonic waves during the mixing stage ensures a uniform nanoparticle distribution in each layer, which is crucial for predictable acoustic behavior. The micro-column / parylene enhancement solves the classical bonding problem in multi-layer matching layers by effectively eliminating discrete weak interfaces - each interface blurs into a tightly contacting gradient region. The resulting impedance curve can be designed to closely follow an ideal curve (such as an exponential decay).

[0040] In summary, in the process of preparing the multi-layer gradient acoustic impedance matching layer, the present invention applies a rotating magnetic field and ultrasonic vibration simultaneously, which can effectively avoid the agglomeration and sedimentation of nanoparticles and achieve a highly uniform dispersion of gradient materials throughout the whole layer; the gradient impedance design in the form of multi-layers or continua significantly reduces the acoustic reflection between layers, improves the acoustic energy transmission efficiency and broadens the working bandwidth of ultrasonic devices; the micro-column reinforcement structure not only maintains the stability of inter-layer adhesion in high-vibration or thermal stress environments, but also further smoothes the gradient transition and reduces the interface reflection; the parylene coating strengthens the mechanical properties of the overall structure by filling and encapsulating the voids around the micro-columns and can fine-tune the local acoustic impedance according to requirements. Combining the above technical solutions, the present invention provides a simple and reliable solution for the manufacture of multi-layer gradient acoustic impedance matching layers, with excellent acoustic performance, and is applicable to various fields such as ultrasonic imaging, non-destructive testing and underwater acoustics.

[0041] The above description is only one implementation mode of the present invention, not all or the only implementation mode. Any equivalent transformation of the technical solution of the present invention by those of ordinary skill in the art after reading the specification of the present invention shall be covered by the claims of the present invention.

Claims

1. A multi-layer gradient acoustic impedance matching layer, characterized in that It includes several impedance layers with different acoustic impedances arranged in sequence; Among them, in the direction from the first impedance layer to the last impedance layer: the acoustic impedance of the impedance layer shows a decreasing trend; the surface of the previous impedance layer has a micro-columnar structure, and this micro-columnar structure extends into the next impedance layer. A mechanical interlocking interface is formed between the previous impedance layer and the next impedance layer through the micro-columnar structure, and the micro-columnar structure gradually tapers from its bottom to the tip.

2. The multi-layer gradient acoustic impedance matching layer according to claim 1, wherein The thickness of each impedance layer is 1 / 4 of the acoustic wave wavelength; from the first impedance layer to the last impedance layer, an exponential acoustic impedance distribution is satisfied.

3. The multi-layer gradient acoustic impedance matching layer according to claim 1, wherein The exposed surface of the multi-layer gradient acoustic impedance matching layer is conformally coated with a parylene coating.

4. A multi-layer gradient acoustic impedance matching layer according to claim 1, wherein, Each impedance layer includes a polymer matrix and high-acoustic-impedance nanoparticles uniformly dispersed in the polymer matrix.

5. The multi-layer gradient acoustic impedance matching layer according to claim 4, wherein, The high-acoustic-impedance nanoparticles include at least one of tungsten nanoparticles, barium titanate nanoparticles, PZT nanoparticles, and magnetite nanoparticles.

6. A multi-layer gradient acoustic impedance matching layer according to claim 4 or 5, characterized in that, The micro-columnar structure is a structure that extends and protrudes from the surface of the previous impedance layer into the next impedance layer. The shape of the micro-columnar structure is a protruding columnar structure or a periodic micro-cavity concentric with the previous impedance layer, and the micro-columnar structure is made of the same material as the previous impedance layer.

7. The preparation method of the multi-layer gradient acoustic impedance matching layer according to any one of claims 1-6, characterized in that, It includes the following process: Disperse the high-acoustic-impedance nanoparticles in the polymer matrix to obtain a nanoparticle-polymer suspension. For different impedance layers, the concentration of the high-acoustic-impedance nanoparticles is different; Apply a rotating magnetic field to the nanoparticle-polymer suspension while performing ultrasonic cavitation to obtain a uniformly dispersed mixture; Deposit the uniformly dispersed mixture layer by layer to form a multi-layer gradient acoustic impedance matching layer; among them, before the previous impedance layer is cured, a micro-columnar structure is processed on the surface of this impedance layer. After the previous impedance layer is cured, repeat the processing process of the previous impedance layer to prepare the next impedance layer until all impedance layers are prepared; and during the curing process of each impedance layer, a rotating magnetic field is continuously applied.

8. The preparation method of the multi-layer gradient acoustic impedance matching layer according to claim 7, wherein, Before the previous impedance layer is cured, a micro-columnar structure is processed on the surface of this impedance layer, including: using imprinting or photolithography to pattern micro-scale columns, or pre-forming periodic micro-cavities on the surface of the previous impedance layer.

9. The preparation method of the multi-layer gradient acoustic impedance matching layer according to claim 7, characterized in that, It also includes the following process: After obtaining the multi-layer gradient acoustic impedance matching layer, use parylene and deposit it through a chemical vapor deposition process to conformally coat the multi-layer gradient acoustic impedance matching layer with a parylene layer, and then cure or anneal the multi-layer gradient acoustic impedance matching layer conformally coated with the parylene layer to make the polymer layer completely hardened.

10. An ultrasonic transducer, characterized in that, Its surface is provided with the multi-layer gradient acoustic impedance matching layer according to any one of claims 1-6.

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