High-Frequency Linear Array (20 MHz) Based on Lead-Free BCTZ Crystal

Centimeter-sized BaTiO3-based crystals grown by top-seeded solution growth from the BaTiO3–CaTiO3–BaZrO3 system were used to process a high-frequency (HF) lead-free linear array. Piezoelectric plates with (110)pc cut within 1° accuracy were used to manufacture two 1–3 piezo-composites with thicknesses of 270 and $78 \mu \text{m}$ for resonant frequencies in air of 10 and 30 MHz, respectively. The electromechanical characterization of the BCTZ crystal plates and the 10-MHz piezocomposite yielded the thickness coupling factors of 40% and 50%, respectively. We quantified the electromechanical performance of the second piezocomposite (30 MHz) according to the reduction in the pillar sizes during the fabrication process. The dimensions of the piezocomposite at 30 MHz were sufficient for a 128-element array with a 70- $\mu \text{m}$ element pitch and a 1.5-mm elevation aperture. The transducer stack (backing, matching layers, lens, and electrical components) was tuned with the characteristics of the lead-free materials to deliver optimal bandwidth and sensitivity. The probe was connected to a real-time HF 128-channel echographic system for acoustic characterization (electroacoustic response and radiation pattern) and to acquire high-resolution in vivo images of human skin. The center frequency of the experimental probe was 20 MHz, and the fractional bandwidth at −6 dB was 41%. Skin images were compared against those obtained with a lead-based 20-MHz commercial imaging probe. Despite significant differences in sensitivity between elements, in vivo images obtained with a BCTZ-based probe convincingly demonstrated the potential of integrating this piezoelectric material in an imaging probe.