Yi Quan

Since the advent of ultrasonic testing, piezoelectric ultrasonic transducers have been widely applied in nonintrusive inspection across aerospace, nuclear energy, and petrochemical industries, owing to their compact structure, low cost, and broad applicability. However, as service temperatures increase, issues such as the degradation of piezoelectric material properties and mechanical failure of transducer components become increasingly pronounced, highlighting the need for further optimization to enhance transducer performance under high-temperature conditions. This paper reviews the recent research progress on high-temperature piezoelectric ultrasonic transducers (HTPUTs) from the perspectives of materials, structures, and applications. Specifically, commonly used piezoelectric material systems for high-temperature environments and their performance characteristics are systematically summarized, with particular emphasis on degradation mechanisms at elevated temperatures. Design optimization strategies for transducers operating under high-temperature conditions are discussed in detail, including the selection of matching layers and backing materials, acoustic coupling optimization, and the development of novel structural configurations. Representative applications of HTPUTs in the energy and manufacturing sectors are also highlighted, together with the key challenges encountered. Finally, future development directions for HTPUT research are outlined.

Electromechanical film (EMFi)-based air-coupled ultrasonic transducers are recognized for their significant potential in non-destructive testing (NDT) applications, primarily due to their large piezoelectric sensitivity and ultra-low acoustic impedance. However, challenges such as high electrical impedance and phase misalignment, have significantly limited their practical application. In this work, the five-layer co-extruded cellular polypropylene (cPP) with an acoustic impedance of 0.029 MRayls were utilized for the EMFi-based air-coupled ultrasonic transducers (EAUT). To address electrical impedance issues, we employed an electrical matching optimization method using the Smith chart and Butterworth-Van Dyke (BVD) model. Specifically, a series inductor of 921 μH was integrated into the electrical matching circuit. Consequently, the matched EAUT demonstrates a signal intensity exceeding that of the non-matched configuration by more than 100 times. Ultimately, the ultrasound imaging capabilities of the matched EAUT underscore its substantial potential for practical applications.

High-frequency ultrasound imaging, due to its penetrability and micron-level resolution, is a critical modality for non-destructive testing (NDT) of integrated circuit (IC) packaging. However, the substantial acoustic impedance mismatch between the coupling medium (water) and the silicon substrate creates a severe measurement bottleneck: strong surface reflections drastically reduce the energy transmitted into the chip, degrading the signal-to-noise ratio (SNR) and resolution of internal interconnects. To overcome this physical limitation and enhance the measurement capability of scanning acoustic microscopy (SAM), we have designed a precise anti-reflection coating based on the theory of the evolution from electromagnetic transmission lines to acoustic transmission lines. By modeling the acoustic propagation path as an equivalent circuit, we designed and fabricated a three-layer composite structure (Parylene C/Au/Parylene C) specifically optimized for the particular frequency (70 MHz in this study). Experimental validation on commercial IC chips demonstrates that this "instrumentation conditioning" approach significantly improves signal integrity. Specifically, the coating yielded a 52% increase in the amplitude of internal echoes (from 142 mV to 216 mV). Furthermore, C-scan imaging revealed a 93% average improvement in the mean gradient of internal metal wires across multiple layers, enabling high-fidelity 3D reconstruction of interconnection structures that were previously indistinguishable from noise. This method offers a robust, low-cost solution to the intrinsic attenuation problem in high-frequency acoustic measurement, significantly extending the detection depth and clarity for IC reliability analysis.

Ultrahigh frequency (UHF) ultrasonic transducers are widely used in the fields of biomedical microscopy and nondestructive testing. However, the operating frequency and electro-acoustic conversion efficiency of UHF transducers have potential for further improvement. This work presents design, simulation, and fabrication of focused 300 MHz ultrahigh frequency (UHF) ultrasonic transducers. The UHF ultrasonic transducers were designed and simulated by equivalent circuit model and finite element analysis. Furthermore, the effect of lens aperture and ultrasonic frequency on the distribution of sound field were studied. To improve their performance, the UHF ultrasonic transducers were fabricated by using lithium niobate piezoelectric material and quartz glass focal lens. As a result, The pulse-echo measurements show the center frequency of the echoes were 238.7 MHz, 119.8 MHz and 73.1 MHz from the lens, near field and the focal point, respectively. According to the axial sound pressure distribution of the focusing transducer and the echoes, the ultrasound attenuation in water was calculated of 1.74 (± 0.16) × 10^-3 × f² (dB/cm). In addition, the image of 10 μm tungsten wire target illustrated the lateral and axial resolutions of the UHF ultrasonic transducers were 28.9 μm and 31.4 μm, respectively. This study highlights strategies for optimizing transducer performance and improving imaging resolution in Scanning Acoustic Microscopy (SAM) systems.

Ultrasonic attenuation affects the quality of ultrasonic imaging and material characterization significantly, especially at ultrahigh frequencies (>100 MHz), where higher attenuation bring about additional challenges. However, the reliable ultrasonic attenuation coefficients are scarce in this frequency range. In this study we propose a method of measuring ultrasound attenuation coefficient at ultrahigh frequencies. By the spectral analysis of multiple ultrasonic echoes ,the aggregate attenuation coefficient was extracted. Ultrasonic diffraction attenuation and transmission loss at the material interface are calibrated by using circular flat transducer acoustic fields and acoustic transmission line theory, respectively. The attenuation coefficient of silicon at approximately 150 MHz was measured to be 10.52 dB/cm. This article presents a novel approach for evaluating ultrasonic attenuation in the ultrahigh frequency range, reducing experimental errors and equipment costs. The proposed methodology also contributes to the development and characterization of ultrahigh frequency ultrasonic transducers.

Ultrasound is one of the most promising methods for blood pressure monitoring due to its harmless, non-invasive, and high-precision characteristics. To further enhance the biocompatibility of ultrasound blood pressure monitors, this work reports wearable ultrasonic patches for blood pressure monitoring based on lead-free KNN (potassium sodium niobate)-based materials. The patches are designed and fabricated with a center frequency of 5 MHz and dimensions of 2.8 mm × 2.8 mm, optimized for both electrical impedance matching and vascular detection. Moreover, biocompatible silicone rubber is used for the packaging. The wearable ultrasonic patches are demonstrated to effectively transmit and receive signals. The diameter of artificial blood vessels is measured to validate the vascular diameter detection capability of the patches. The relationship between blood pressure and vascular diameter is then calculated. A radial artery vascular system platform is built to simulate changes in human blood pressure. Finally, the patches are shown to successfully measure the variation in vessel diameters on this platform. These patches exhibit sufficient detection ability, good biocompatibility, and can adhere tightly to human skin without coupling agents, providing the possibility for safe, sustainable, comfortable, and wearable long-term blood pressure monitoring.

Acoustic holography technology is widely used in the field of ultrasound due to its capability to achieve complex acoustic fields. The traditional acoustic holography method based on single-phase holograms is limited due to its inability to complete acoustic field control with high dynamics and accuracy. Here, we propose a method for constructing an acoustic holographic model, introducing an ultrasonic array to provide dynamic amplitude control degrees of freedom, and combining the dynamically controllable ultrasonic array and high-precision acoustic hologram to achieve the highest acoustic field accuracy and dynamic range. This simulation method has been proven to be applicable to both simple linear patterns and complex surface patterns. Moreover, it is possible to reconstruct the degree of freedom of the target plane amplitude effectively and achieve a breakthrough in high information content. This high-efficiency acoustic field control capability has potential applications in ultrasound imaging, acoustic tweezers, and neuromodulation.

The development of high-temperature nondestructive testing (NDT) requires ultrasonic transducers with good temperature resistance and high sensitivity for improved detection efficiency. Piezoelectric composite can improve the performance of transducers because of its high electromechanical coupling coefficient and adjustable acoustic impedance. In this study, 1-3-2 composites and 1-3-2 high-temperature composite ultrasonic transducers (HTCUTs) based on 0.36BiScO ₃ -0.64PbTiO ₃ (BSPT), which are preferred piezoelectric materials at 200°C-300°C, and high-temperature epoxy with a center frequency of 6 MHz were designed and fabricated. From 25°C to 250°C, 1-3-2 composites show a higher electromechanical coupling coefficient k_t especially at high temperatures (~0.53 at 25°C, and ~0.64 at 250°C) than monolithic BSPT (~0.5). The signal of the pulse-echo response of 1-3-2 HTCUTs is distinguishable up to 250 °C and remains stable ( V_pp ~500 mV) below 150°C, exhibiting higher sensitivity (improved by 7 dB) than that of monolithic BSPT high-temperature ultrasonic transducers (HTUTs). Bandwidth has been greatly enhanced especially at high temperatures (~103250°C) compared with that of monolithic BSPT HTUTs(~30250°C). To verify the excellent performance, B-mode scanning imaging measurement of a stepped steel block and defect location detection of a steel block were performed, showing the potential for high-temperature NDT applications.