*5.3. Effect of Bonding Layer on Transducer Performance*

The acoustic impedances of each layer with different bonding systems are shown in Table 4, and the amplitude signals are provided in Figure 7. Without the bonding layer, the 1-3 piezoelectric composite, first matching layer, and second matching layer are connected by the air, and the transducer has an amplitude of 111.2 mV, as demonstrated in Figure 7a. This amplitude is only 11.5% of the transducer bonded by the epoxy, as provided in Figure 6b. After changing the bonding material to the hollow glass microsphere/epoxy composite, the amplitude increased to 1283 mV, 33% higher than that of the epoxy bonding. This result can be explained by the acoustic impedance distribution in Table 4.

When the acoustic wave propagates from one homogenous material to another, both the reflection and refraction happen in the interface. The reflected acoustic wave returned to the first material, and the refracted one prorogates into the second material. The larger discrepancy between the two materials, the smaller percentage of the acoustic can propagate into the second material. For the transducer with two matching layers, there are five interfaces to be penetrated. When connected by air, most acoustic waves from the 1-3 piezoelectric reflect the composite, and only a few propagate into the air. When the acoustic wave in the air propagates to the first matching layer, most of the energy reflects into the air again, which happens several times in the air bonded transducer. Thus, little energy can

pass through the transducer and be received by the receiving transducer. For the transducer bonded by the epoxy, the discrepancy is reduced, especially between the 1-3 composite and the first matching layer. This reduced acoustic impedance difference allows more energy to pass through the transducer. For the transducer bonded by the hollow glass microsphere and epoxy composite, even if the difference between the 1-3 piezoelectric composite and the first matching layer increased, the interface layer decreased to three to reduce the energy dissipation. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 13 of 18

**Figure 6.** Time–domain filtered and enveloped signal of (**a**) BR20‐15‐15, (**b**) BR20‐19‐15, (**c**) **Figure 6.** Time–domain filtered and enveloped signal of (**a**) BR20-15-15, (**b**) BR20-19-15, (**c**) BR20-15-15, and (**d**) transducer from Japan Probe Co., LTD.

The acoustic impedances of each layer with different bonding systems are shown in Table 4, and the amplitude signals are provided in Figure 7. Without the bonding layer, the 1‐3 piezoelectric composite, first matching layer, and second matching layer are connected by the air, and the transducer has an amplitude of 111.2 mV, as demonstrated in Figure 7a. This amplitude is only 11.5% of the transducer bonded by the epoxy, as provided in Figure 6b. After changing the bonding material to the hollow glass micro‐ sphere/epoxy composite, the amplitude increased to 1283 mV, 33% higher than that of the epoxy bonding. This result can be explained by the acoustic impedance distribution in

Table 4.

BR20‐15‐15, and (**d**) transducer from Japan Probe Co., LTD.

*5.3. Effect of Bonding Layer on Transducer Performance*

**Figure 7.** Time–domain filtered and enveloped signal. (**a**) Without bonding layer, (**b**) with the hol‐ low glass beads/epoxy resin system. **Figure 7.** Time–domain filtered and enveloped signal. (**a**) Without bonding layer, (**b**) with the hollow glass beads/epoxy resin system.


**Table 4.** Acoustic impedance distribution of the transducer with three different bonding materials. **Table 4.** Acoustic impedance distribution of the transducer with three different bonding materials.

### When the acoustic wave propagates from one homogenous material to another, both the reflection and refraction happen in the interface. The reflected acoustic wave returned *5.4. Practical Application Test of Self-Developed Air-Coupled Ultrasonic Transducer*

to the first material, and the refracted one prorogates into the second material. The larger discrepancy between the two materials, the smaller percentage of the acoustic can propagate into the second material. For the transducer with two matching layers, there are five interfaces to be penetrated. When connected by air, most acoustic waves from the 1‐3 piezoelectric reflect the composite, and only a few propagate into the air. When the acoustic wave in the air propagates to the first matching layer, most of the energy reflects into the air again, which happens several times in the air bonded transducer. Thus, little energy can pass through the transducer and be received by the receiving transducer. For the transducer bonded by the epoxy, the discrepancy is reduced, especially between the 1‐3 composite and the first matching layer. This reduced acoustic impedance difference allows more energy to pass through the transducer. For the transducer bonded by the hollow glass microsphere and epoxy composite, even if the difference between the 1‐3 piezoelectric composite and the first matching layer increased, the interface layer de‐ creased to three to reduce the energy dissipation. The received signals from the air-coupled ultrasonic transducer after penetrating the testing plate are shown in Figure 8. When an ultrasonic wave propagates in PVC foam and CFRP plate without defects, the peak-to-peak amplitudes of received signals are respectively 363.2 and 83.6 mV, as seen in Figure 8a,b. When the defects in PVC foam and CFRP plates locate in the ultrasonic propagation path, the peak-to-peak amplitude of received signals were 155.8 and 54.5 mV, which decreased by about 57% and 35%, respectively. Due to the interaction between defects and ultrasonic waves, the energy of the transmitted wave was reduced, which led to the change in the shape and amplitude of the received wave. Due to the different types, sizes, and locations of defects, the received acoustic wave may change into various shapes, but this will not affect the detection of defects. By comparing the waveform amplitude of the received signals with or without defects, the defects are easy to detect. Further, the defect information can be directly distinguished by scanning and detecting the area to be inspected. The air-coupled ultrasonic transducer developed in this study can be used in practical testing applications.

**Figure 8.** Time–domain filtered and enveloped signal of (a) PVC foam, (b) CFRP plates with or without defects. **Figure 8.** Time–domain filtered and enveloped signal of (**a**) PVC foam, (**b**) CFRP plates with or without defects.

### **6. Conclusions 6. Conclusions**

This paper introduces a methodology to model, design, fabricate, and optimize air‐coupled ultrasonic transducers matching system, which includes the matching layer This paper introduces a methodology to model, design, fabricate, and optimize aircoupled ultrasonic transducers matching system, which includes the matching layer and

bonding layer. The matching layer modeling process and evaluation methods are demonstrated and verified by the self-developed air-coupled ultrasonic transducer with double matching layers. The mechanism of the matching layer components affecting the transducer performance is explained. Additionally, the bonding layer effects on the transducer performance are described. The testing results indicate that the self-developed air-coupled ultrasonic transducer in this study has a 20% higher amplitude than the product on the market and can easily identify defects in non-metallic materials. This study provides the foundation for air-coupled ultrasonic transducer modeling and development.

**Author Contributions:** Conceptualization, J.Z. and Y.L.; methodology, J.B. and Y.L.; validation, J.B. and Y.L.; investigation, J.B.; data curation, J.B. and Y.L.; writing—original draft preparation, J.B. and Y.L.; writing—review and editing, J.Z.; project administration, J.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** Supported by the Opening Project of Shanxi Key Laboratory of Advanced Manufacturing Technology (No. XJZZ201903).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

