Optical matching layer and method for producing same

Abstract

The application discloses an optical matching layer and a preparation method thereof, and the optical matching layer is composed of an epoxy resin matrix and surface-modified glass micro-powder fillers dispersed in the epoxy resin matrix; the modified glass micro-powder fillers are bismuthate glass micro-powder treated by a silane coupling agent; and the chemical composition of the bismuthate glass micro-powder is 54‑56mol%Bi2O3 、 28‑32mol%B2O3 , 14-16 mol% ZnO. By using the bismuthate glass matched in refractive index, light scattering is fundamentally reduced. Optical simulation (based on Mie scattering theory) shows that the light transmittance of the composite material is as high as 86.5% at 30% volume fraction, which is significantly better than that of a silica composite material with a light transmittance of 58.2% under the same conditions.

Description

Technical Field

[0001] This invention relates to the field of medical imaging and nondestructive testing, specifically to an optical matching layer and its preparation method. Background Technology

[0002] In photoacoustic imaging transparent ultrasonic transducers, an acoustic matching layer must be placed at the front end of the transducer to achieve efficient transmission of ultrasonic waves from a piezoelectric element (such as lithium niobate, acoustic impedance ~34 MRayl) to a load (such as human tissue, acoustic impedance ~1.5 MRayl). An ideal matching layer must simultaneously meet the requirements of high acoustic impedance matching and high optical transparency.

[0003] Traditional matching layer materials are mostly opaque, such as composite materials made of epoxy resin and ceramic powders such as tungsten powder and alumina. Although these materials can adjust acoustic impedance, their opacity brings difficulties to the assembly, debugging and performance evaluation of transducers.

[0004] Existing transparent matching layers often use epoxy resin doped with silicon dioxide ( Nanoparticle powder and other materials are used as matching layer materials. By adjusting... The acoustic impedance can be controlled within a certain range by adjusting the filling ratio. However, due to the refractive index mismatch, this method causes severe Mie scattering when light passes through the composite material, resulting in increased optical haze and decreased transmittance, which significantly degrades the signal-to-noise ratio and resolution of photoacoustic imaging. Furthermore, the upper limit of the acoustic impedance of the epoxy resin-silica composite system is relatively low (usually <6.5 MRayl), making it difficult to achieve ideal broadband matching for high acoustic impedance piezoelectric materials such as lithium niobate.

[0005] Existing technologies have mentioned the use of bismuth oxide powder or glass microspheres as fillers, but these materials either become opaque due to refractive index mismatch (such as bismuth oxide) or cannot simultaneously provide sufficiently high acoustic impedance and optical transparency. Therefore, there is an urgent need in the field to develop a novel matching layer material that can provide a wide range of tunable acoustic impedance while maintaining high optical transparency, in order to solve the problems in existing technologies. Summary of the Invention

[0006] To address the challenge that existing matching layers cannot simultaneously provide sufficiently high acoustic impedance and optical transparency, this invention provides a transparent epoxy resin-bismuthate composite matching layer. Through innovative material selection, this matching layer has been proven, through simulation and theoretical verification, to simultaneously achieve high acoustic impedance (7-8 MRayl) and high optical transparency (transmittance >85%).

[0007] The first aspect of the present invention provides an optical matching layer, the optical matching layer being composed of an epoxy resin matrix and a surface-modified glass micropowder filler dispersed therein; The modified glass micropowder filler is bismuthate glass micropowder surface-treated with silane coupling agent; The chemical composition of the bismuthate glass micropowder is 54-56 mol% Bi2O3, 28-32 mol% B2O3, and 14-16 mol% ZnO.

[0008] The core discovery of this invention lies in the fact that not all bismuth-based fillers can resolve the technical contradictions. While bismuth oxide (Bi₂O₃) powder has high acoustic impedance, its refractive index (~2.4) is much higher than that of epoxy resin (~1.55-1.60), resulting in a composite material that is essentially opaque. However, the bismuthate glass (Bi₂O₃-B₂O₃-ZnO system) selected in this invention, through compositional design, allows for precise control of its refractive index between 1.6 and 1.7, achieving refractive index matching with epoxy resin—a key to achieving high transparency. Simultaneously, this glass system inherits the high-density characteristics of bismuth, achieving an acoustic impedance as high as 8-12 MRayl. This matching layer not only possesses excellent optical transparency, facilitating transducer assembly and testing, but also enables precise control of acoustic impedance over a wide range, simultaneously meeting the requirements of high acoustic impedance and high transparency.

[0009] Preferably, the absolute value of the difference between the refractive index of the bismuthate glass micropowder and the refractive index of the epoxy resin matrix is ​​less than 0.1. By carefully selecting bismuthate glass with a refractive index matching the epoxy resin matrix (absolute difference less than 0.1) and combining it with its high acoustic impedance characteristics, and through acoustic and optical simulation verification, this composite material can simultaneously achieve a high acoustic impedance of 7-8 MRayl and a high optical transmittance of greater than 85%, effectively solving the technical contradiction that traditional matching layers cannot simultaneously achieve high acoustic impedance and high transparency. The epoxy resin matrix is ​​composed of epoxy resin and a curing agent, with a mass ratio of epoxy resin to curing agent of 100:(30-40).

[0010] Preferably, the volume fraction of the modified glass micropowder filler is 10%-60%; more preferably, the volume fraction of the modified glass micropowder filler is 30%-60%; and even more preferably, the volume fraction of the modified glass micropowder filler is 30%.

[0011] Preferably, the D50 particle size of the bismuthate glass micropowder is less than 1 μm.

[0012] Preferably, the chemical composition of the bismuthate glass micropowder is 55 mol% Bi2O3, 30 mol% B2O3, and 15 mol% ZnO.

[0013] The acoustic impedance of the optical matching layer of the present invention is 7 to 8 MRayl, which is adjustable within this range, and the optical transmittance in the visible light band is greater than 85%.

[0014] A second aspect of the present invention provides a transparent ultrasonic transducer, the structure of which includes the aforementioned optical matching layer.

[0015] The transparent ultrasonic transducer of the present invention includes a piezoelectric element, and the acoustic matching layer is disposed on the acoustic radiation surface of the piezoelectric element; the piezoelectric element is a lithium niobate crystal or a transparent piezoelectric ceramic.

[0016] A third aspect of the present invention provides a photoacoustic imaging probe, wherein the photoacoustic imaging probe integrates the aforementioned transparent ultrasonic transducer.

[0017] A fourth aspect of the present invention provides a method for fabricating the aforementioned optical matching layer, the method comprising the following steps: The modified glass micropowder filler is obtained by mixing, stirring, filtering, washing, and drying a silane coupling agent solution with bismuthate glass micropowder. The modified glass micro powder filler is mixed with epoxy resin and curing agent to obtain a slurry; The slurry is poured into a mold and cured to obtain the optical matching layer.

[0018] Preferably, the silane coupling agent solution is a KH-550 ethanol aqueous solution; more preferably, the mass ratio of KH-550, deionized water and anhydrous ethanol is 1:(8-10):(85-95); even more preferably, the mass ratio of KH-550, deionized water and anhydrous ethanol is 1:9:90.

[0019] Preferably, the solid-liquid ratio of the bismuthate glass micropowder to the silane coupling agent solution is 1 g: (8-12) mL.

[0020] Preferably, the mass ratio of the epoxy resin to the curing agent is 100:(30-40).

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. Synergistic achievement of high transparency and high acoustic impedance: By utilizing bismuthate glass with refractive index matching, light scattering is fundamentally reduced. Optical simulations (based on Mie scattering theory) show that, at a volume fraction of 30%, the transmittance of the composite material of this invention reaches 86.5%, significantly better than the transmittance of 58.2% for silica composite material under the same conditions.

[0022] 2. Wide-range adjustable acoustic impedance and performance breakthrough: Bismuthate glass itself has high acoustic impedance (8-12 MRayl). Acoustic simulation (based on the Bruggeman model and transmission line theory) confirms that by adjusting the filler volume fraction (30%-60%), the composite material of this invention can easily achieve an acoustic impedance of 7-12 MRayl, breaking through the upper limit of the epoxy resin-silica system (approximately 6.5 MRayl). This provides near-ideal impedance matching for high acoustic impedance piezoelectric materials.

[0023] 3. This invention overcomes technical bias: While existing technologies generally mention bismuth oxide powder and glass microspheres, they do not provide any technical insights that can simultaneously solve the problems of optical transparency and acoustic impedance improvement. This invention, by selecting specific types of bismuthate glass and limiting the matching relationship between its refractive index and the matrix, achieves a superior combination of performance that was previously impossible to obtain simultaneously.

[0024] 4. Excellent Comprehensive Performance: This composite material combines the flexibility and ease of processing of epoxy resin with the high density and high sound velocity characteristics of bismuthate glass. The resulting mating layer exhibits excellent mechanical properties and strong adhesion to piezoelectric elements. A lower filler ratio means lower viscosity and better flowability of the composite material. This significantly improves the material's casting and molding performance, reduces the generation of bubbles and internal defects, and enhances manufacturing yield, consistency, and reliability of the mating layer.

[0025] 5. Simple process and controllable cost: The raw materials used are readily available, the preparation method is compatible with traditional composite material processes, no special equipment is required, and it is suitable for large-scale production. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the ultrasonic transducer assembled with the composite matching layer of the present invention.

[0027] Explanation of reference numerals: 1-backing layer, 2-brass tube, 3-ITO, 4-piezoelectric material, 5-fixing ring, 6-brass ring, 7-ITO, 8-matching layer, 9-conductive silver strip. Detailed Implementation

[0028] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0029] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0030] Example 1 This embodiment provides a method for fabricating a matching layer with an acoustic impedance of 7.1 MRayl and high optical transparency, specifically including the following steps: Raw material preparation: 1. Epoxy Resin Matrix: An optically transparent two-component epoxy resin is selected, which is composed of component A (resin) and component B (curing agent) mixed at a mass ratio of 100:35. This ratio is determined based on stoichiometric calculations and rheological optimization experiments to ensure complete curing and minimal air bubbles. The epoxy resin system with this ratio has an acoustic impedance of 3MRayl and a refractive index of 1.57 (at a wavelength of 589nm) after curing.

[0031] 2. Bismuthate glass filler: Composed of a Bi₂O₃-B₂O₃-ZnO system, this system comprises 55 mol% Bi₂O₃, 30 mol% B₂O₃, and 15 mol% ZnO. It has a refractive index of 1.6 and an acoustic impedance of 10 MRayl, exhibiting good refractive index matching with the epoxy resin matrix. It is suitable for preparing low-scattering, high-acoustic-impedance matching layers. The average particle size (D50) is less than 1 μm.

[0032] 3. Surface treatment agent: γ-aminopropyltriethoxysilane (KH-550).

[0033] Preparation process: (1) Surface treatment of the filler: Prepare a KH-550 ethanol-water solution (prepared by mixing KH-550, deionized water and anhydrous ethanol at a mass ratio of 1:9:90). Weigh bismuthate glass micro powder and add it to the above KH-550 solution at a solid-liquid ratio of 1g:10mL. Stir mechanically (400rpm) for 3 hours in a water bath at 50℃. After the reaction is complete, filter the solution and wash it three times with anhydrous ethanol. Dry the filter cake in a vacuum drying oven at 60℃ to obtain the surface-modified hydrophobic glass filler.

[0034] (2) Vacuum mixing and degassing: According to the Bruggeman mixing model theory, 30 vol% of modified glass micropowder filler is required to achieve the target acoustic impedance of 7.1 MRayl. Based on the required total volume of the final composite material, the treated glass filler and epoxy resin component A are accurately weighed.

[0035] The resin and curing agent are premixed at 25°C; after adding bismuthate filler, the system temperature is raised to 32°C for planetary centrifugal dispersion; after dispersion, the slurry is cooled to below 28°C and then vacuum degassing is performed. The temperature is controlled within the range of 25-35°C throughout the process to ensure uniform dispersion and avoid pre-curing, ultimately obtaining a slurry with moderate viscosity, uniformity, and no visible bubbles.

[0036] (3) Molding and curing: Use a smooth and glossy glass plate or polytetrafluoroethylene plate as the mold. Clean it with acetone and spray it with a release agent before use. Slowly pour the degassed slurry into the mold and gently vibrate the mold to help the slurry flow smoothly and remove large air bubbles on the surface.

[0037] The coated mold was placed in a programmable temperature-controlled oven for stepped curing: 40℃ / 2h → 80℃ / 4h → 100℃ / 2h, with the heating rate controlled at 0.5℃ / min. After curing, the mold was cooled to below 50℃ in the oven and then removed. This process aims to ensure that the composite material is fully cured while minimizing internal stress and optical defects.

[0038] (4) Post-processing: After demolding, the acoustic matching layer can be obtained. As needed, finished products of specific shapes and sizes can be obtained by mechanical processing such as cutting and polishing.

[0039] The matching layer of the present invention is in accordance with the appendix Figure 1 The structure shown is assembled into an ultrasonic transducer.

[0040] Performance verification (based on COMSOL simulation): Optical performance simulation: Optical simulations were performed on the prepared composite material (30 vol% bismuthate glass, refractive index 1.6; epoxy resin matrix, refractive index 1.57) using a model based on Mie scattering theory. The simulation results show that the material exhibits excellent optical transparency with a transmittance greater than 85% in the visible light band (550 nm).

[0041] Acoustic performance simulation: The above-mentioned matching layer (acoustic impedance 7.1 MRayl) was tested using a pulse-echo simulation model. The simulation results confirm that when applied to a lithium niobate transducer (acoustic impedance 34 MRayl) with a center frequency of 30 MHz, it can effectively broaden the operating bandwidth, and its -6dB bandwidth is also improved compared to the traditional silicon dioxide matching layer (acoustic impedance 5.8 MRayl).

[0042] The simulation results above show that this embodiment successfully fabricated a composite matching layer with both high acoustic impedance and high optical transparency.

[0043] Comparative Example 1 This comparative example prepares an epoxy resin-alumina composite matching layer. The purpose of this comparative example is to illustrate that if traditional fillers with high acoustic impedance but mismatched refractive index are used, optical transparency cannot be achieved.

[0044] Preparation process reference: Except for replacing the filler with the same volume fraction (30 vol%) of alumina (Al2O3) micro powder (refractive index ~1.76, average particle size ~1μm), the other raw materials, surface treatment and preparation process are exactly the same as in Example 1.

[0045] Performance verification (based on COMSOL simulation): Optical performance simulation: Optical simulation based on Mie scattering theory shows that light scattering is extremely severe due to the high refractive index difference (0.19) between alumina and the epoxy resin matrix. Simulation results indicate that the composite material is completely opaque in the visible light band, with a transmittance approaching 0%, making it unsuitable for transparent transducers requiring light penetration.

[0046] Acoustic performance simulation: Its acoustic impedance simulation value is 8.5 MRayl, which can meet the high acoustic impedance requirements, but it is completely opaque.

[0047] Comparative Example 2 This comparative example prepares an epoxy resin-silica composite matching layer. The purpose of this comparative example is to illustrate that the performance of traditional transparent matching layer materials is far inferior to that of the present invention due to their low upper limit of acoustic impedance and incomplete refractive index matching.

[0048] Preparation process reference: Except for replacing the filler with silica (SiO2) micro powder (refractive index ~1.46, average particle size ~1μm) with the same volume fraction (30 vol%), the other raw materials and preparation process are exactly the same as in Example 1.

[0049] Performance verification (based on COMSOL simulation): Optical performance simulation: Optical simulation of the matching layer was performed using a model based on Mie scattering theory. Despite a refractive index difference of 0.11 between silica and epoxy resin, significant Mie scattering still occurred. Simulation results showed that its transmittance was only 58.2%, significantly inferior to Example 1 (transmittance >85%).

[0050] Acoustic performance simulation: The acoustic impedance of this composite material is 5.8 MRayl. When applied to the same transducer model of Example 1 for simulation, its -6dB bandwidth is reduced compared to the matching layer bandwidth of this invention. This demonstrates poor acoustic matching, limiting transducer performance.

[0051] Comparative Example 3 This comparative example prepares an epoxy resin-bismuth oxide powder composite matching layer. This comparative example aims to demonstrate that its technical effect is fundamentally different from the bismuthate glass of the present invention.

[0052] Preparation process reference: Except for replacing the filler with the same volume fraction (30 vol%) of bismuth oxide (Bi2O3) powder (refractive index ~2.4, average particle size ~1μm), the other raw materials and preparation process are exactly the same as in Example 1.

[0053] Performance verification (based on COMSOL simulation): Optical performance simulation: Optical simulation shows that the large difference in refractive index between bismuth oxide and epoxy resin (0.83) leads to strong light scattering. The simulation results show that the composite material is completely opaque, with a transmittance of less than 5%, which completely fails to meet the optical requirements of a transparent transducer.

[0054] Acoustic performance simulation: Its acoustic impedance simulation value is 9.0 MRayl.

[0055] This comparative example demonstrates a significant difference in the technical effects achieved between bismuth oxide powder and the bismuthate glass micropowder of this invention. Although both contain bismuth, bismuth oxide powder, due to its inherently high refractive index, cannot achieve optical transparency. This invention, by selecting bismuthate glass micropowder with specific components and adjusting the refractive index to match that of epoxy resin, unexpectedly overcomes both acoustic and optical challenges simultaneously.

[0056] The performance test comparisons of the embodiments and comparative examples are shown in Table 1 below.

[0057] Table 1 Performance Comparison of Examples and Comparative Examples

[0058] The above clearly demonstrates that by carefully selecting bismuthate glass micropowders with matching refractive indices and optimizing their volume fraction and preparation process, this invention has successfully obtained an excellent matching layer material with adjustable acoustic impedance in the range of 7 to 8 MRayl and optical transmittance exceeding 80%. This method effectively resolves the technical contradiction between high performance and high transparency, providing crucial material support for the design and manufacturing of next-generation ultrasonic transducers.

[0059] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. An optical matching layer, characterized in that, The optical matching layer is composed of an epoxy resin matrix and surface-modified glass micropowder filler dispersed therein; The modified glass micropowder filler is bismuthate glass micropowder surface-treated with silane coupling agent; The chemical composition of the bismuthate glass micropowder is 54-56 mol% Bi2O3, 28-32 mol% B2O3, and 14-16 mol% ZnO.

2. The optical matching layer according to claim 1, characterized in that, The absolute value of the difference between the refractive index of the bismuthate glass micropowder and the refractive index of the epoxy resin matrix is ​​less than 0.

1.

3. The optical matching layer according to claim 1, characterized in that, The volume fraction of the modified glass micropowder filler is 10%-60%.

4. The optical matching layer according to claim 1, characterized in that, The D50 particle size of the bismuthate glass micropowder is less than 1 μm.

5. The optical matching layer according to claim 1, characterized in that, The chemical composition of the bismuthate glass micropowder is 55 mol% Bi2O3, 30 mol% B2O3, and 15 mol% ZnO.

6. A transparent ultrasonic transducer, characterized in that, The structure of the transparent ultrasonic transducer includes the optical matching layer as described in any one of claims 1-5.

7. A photoacoustic imaging probe, characterized in that, The photoacoustic imaging probe integrates the transparent ultrasonic transducer as described in claim 6.

8. A method for preparing an optical matching layer according to any one of claims 1-5, characterized in that, The preparation method includes the following steps: The modified glass micropowder filler is obtained by mixing, stirring, filtering, washing, and drying a silane coupling agent solution with bismuthate glass micropowder. The modified glass micro powder filler is mixed with epoxy resin and curing agent to obtain a slurry; The slurry is poured into a mold and cured to obtain the optical matching layer.

9. The method for preparing the optical matching layer according to claim 8, characterized in that, The silane coupling agent solution is a KH-550 ethanol aqueous solution; the solid-liquid ratio of the bismuthate glass micropowder to the silane coupling agent solution is 1g:(8-12)mL.

10. The method for preparing the optical matching layer according to claim 8, characterized in that, The mass ratio of the epoxy resin to the curing agent is 100:(30-40).

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