Fe-doped BNKBT lead-free piezoceramics with enhanced electromechanical performance for 10 MHz ultrasonic imaging

Ultrasound imaging is widely used in medical diagnosis because it is real-time and non-invasive. The piezoelectric element at the heart of an ultrasonic transducer converts electrical energy into acoustic waves and receives returning echoes, so its electromechanical properties directly affect transducer sensitivity and image quality. Lead zirconate titanate and other lead-based piezoelectrics remain dominant because of their strong performance, but concerns over lead have created a strong need for environmentally friendly lead-free alternatives. Bismuth sodium titanate (BNT)-based ceramics are promising candidates because of their relatively low dielectric constant and acoustic impedance. However, pure BNT has a high coercive field and limited piezoelectric response. The Xidian University team had previously developed a BNT-BKT-BT composition near a morphotropic phase boundary and demonstrated a 10 MHz lead-free ultrasonic transducer. The challenge was to further improve the ceramic's electromechanical response and, importantly, to show that the materials-level improvement could translate into better transducer performance for imaging.

In the new study, the researchers introduced Fe³⁺ into the B site of the BNKBT perovskite lattice as an acceptor dopant and systematically varied the Fe content. Rietveld refinement, PFM, ferroelectric measurements and related electrical characterization showed that moderate Fe doping modified the coexistence of the R3c and P4bm phases and changed local polarization switching. At higher Fe contents, oxygen-vacancy-related defects increasingly restricted domain-wall motion. The best balance occurred at x = 0.01, where the ceramic reached a piezoelectric coefficient d₃₃ of 180 pC/N and a thickness-mode electromechanical coupling factor k_t of 0.42. The k_t/k_p ratio also reached 3.22, indicating particularly prominent thickness-mode coupling within the composition series.

“The key point is not simply that Fe doping changes one material parameter. At an appropriate concentration, it creates a favorable balance between phase-structure modification and defect-related pinning, which improves the electromechanical response needed for thickness-mode ultrasonic operation.” said Dong.

The team then fabricated planar 10 MHz ultrasonic transducers using the optimized BNKBT-Fe(0.01) ceramic. The measured center frequency was about 10.7 MHz and the -6 dB bandwidth was 54%. To isolate the contribution of the ceramic itself, the researchers compared transducers based on undoped BNKBT and Fe-modified BNKBT under the same device design. The insertion loss decreased from 23 dB to 16 dB after Fe modification, demonstrating higher echo sensitivity and energy-conversion efficiency without relying on an acoustic lens. After establishing this material-related advantage, the researchers added an epoxy acoustic lens as a device-level focusing strategy. The focused transducer produced a peak-to-peak echo voltage of 2168 mV at a center frequency of 10.72 MHz. In tungsten-wire imaging tests, the focused device achieved axial and lateral resolutions of 182 and 264 μm, respectively. It also clearly visualized major structures in the anterior segment of a porcine eyeball and reproduced surface patterns on a commemorative coin using C-mode scanning.

“By separating the planar-transducer comparison from the later focusing step, the study directly links the ceramic improvement to transducer sensitivity and then shows how acoustic focusing can further improve imaging resolution.” said Dong.

The work was made available online in Journal of Advanced Ceramics on September 4, 2026.

The results indicate that Fe-modified BNKBT ceramics are promising for environmentally friendly high-frequency ultrasonic transducers. Around 10 MHz, such devices can provide a practical balance between penetration depth and spatial resolution for ophthalmic imaging, superficial-tissue examination and small anatomical structures, while also being relevant to high-resolution nondestructive inspection of small components and surface features. Future work may further optimize backing, acoustic matching and focusing structures to broaden bandwidth while retaining the improved sensitivity of the lead-free ceramic.

Other contributors include Bilin Zhang, Yifan Zhang, Ning Yang and Xinyang Guo from the School of Advanced Materials and Nanotechnology at Xidian University; Yi Quan, Chunlong Fei and Yintang Yang from the Faculty of Integrated Circuit at Xidian University; and Haitao Huang from the Department of Applied Physics and Research Institute for Smart Energy at The Hong Kong Polytechnic University.

About Author

Guangzhi Dong is an associate professor and doctoral supervisor at the School of Advanced Materials and Nanotechnology, Xidian University, China. He received his PhD degree from Northwestern Polytechnical University and has conducted visiting fellow research at University of Wollongong, Australia. His research interests include functional ceramic, piezoelectric/ferroelectric functional materials, and related devices. ORCID: 0000-0002-2627-8567.

Funding

This work was supported by the Key Research and Development Program of Shaanxi (2025GH-YBXM-047).

DOI Link: 10.26599/JAC.2026.9221372

About Journal of Advanced Ceramics

Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen. JAC’s 2025 IF is 14, ranking in Top 1 (1/34, Q1) among all journals in “Materials Science, Ceramics” category, and its 2025 CiteScore is 24.6 (6/133) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508