Full matrix electromechanical properties and applications to ultrasonic transducer of textured (Ba, Ca)(Zr, Ti)O<sub>3</sub> ceramics

<sec>Ultrasound diagnostic technology demonstrates unique clinical value in cardiovascular monitoring, precise ophthalmic diagnosis, and interventional therapy, and possesses the advantages of high efficiency, safety, non-invasiveness, and significant cost-effectiveness. The performance of transducer that is a core component of ultrasound imaging systems directly determines the image resolution. Piezoelectric materials, essential for the acoustic-to-electric energy conversion, exhibit piezoelectric and electromechanical properties that obviously affect the transducer sensitivity and bandwidth. Although commercial Pb(Zr,Ti)O<sub>3</sub> (PZT) ceramics offer excellent properties, the toxicity of the lead element in theentire material preparation, service life, and disposal process pose significant risks to human health and ecosystems.</sec><sec>The [001]<sub>C</sub>-textured lead-free (Ba,Ca)(Zr,Ti)O<sub>3</sub> (BCZT) ceramics are fabricated by the template grain growth (TGG) method. The materials demonstrate high piezoelectricity, elevated sound velocity, and low dielectric constant, making them highly suitable for developing high-sensitivity and large-bandwidth ultrasonic transducers. However, critical limitations are also existent: 1) the absence of full-matrix electromechanical properties such as dielectric constant <i>ε</i><sub><i>ij</i></sub>, piezoelectric coefficient <i>d</i><sub><i>ij</i></sub>, and elastic constant <i>s</i><sub><i>ij</i> </sub>essential for device design, and 2) the restriction of electromechanical coupling coefficient <i>k</i> calculations to extreme aspect ratios. The failure to accurately simulate the evolution of <i>k</i> under finite aspect ratio severely limits the practical applications.</sec><sec>To overcome such challenges, highly [00l]<sub>C</sub>-oriented textured BCZT ceramics (texture degree <i>f</i><sub>00l</sub>~98%) are synthesized via TGG. By combining resonance-antiresonance spectroscopy with pulse-echo ultrasonic measurements, the dataset of complete full-matrix electromechanical property is established for the first time. The textured BCZT ceramics exhibit strong anisotropic Poisson’s ratios. Their piezoelectric coefficient <i>d</i><sub>33</sub> (605 pC/N) and electromechanical coupling coefficient <i>k</i><sub>33</sub> (0.73) are comparable to those of PZT-5H ceramics, while the piezoelectric voltage constant <i>g</i><sub>33</sub> (23.6 × 10<sup>–3</sup> V·m<sup>–1</sup>·Pa<sup>–1</sup>) is 20 % higher than that of PZT-5H.</sec><sec>By using the piezoelectric constitutive equations, a theoretical model is developed to predict <i>k</i> at an arbitrary aspect ratio. Based on this model developed, the 1-3 type BCZT composite transducer with high sensitivity and wide frequency band is designed and fabricated, exhibiting a center frequency of ~3.0 MHz. The BCZT transducer achieves an insertion loss of –33.0 dB. The –6 dB bandwidth is as high as 107.1%, which is superior to the ultrasonic transducer made of PZT-5H composite reported in the literature. This work not only provides complete electromechanical parameters for lead-free piezoelectric device applications but also lays a theoretical and technical foundation for developing high-performance, eco-friendly ultrasonic diagnostic equipments.</sec>