Relaxor-PbTiO3 (PT) based ferroelectric crystals with the perovskite structure have been investigated over the last few decades due to their ultrahigh piezoelectric coefficients (d33 > 1500 pC/N) and electromechanical coupling factors (k33 > 90%), far outperforming state-of-the-art ferroelectric polycrystalline Pb(Zr,Ti)O3 ceramics, and are at the forefront of advanced electroacoustic applications. In this review, the performance merits of relaxor-PT crystals in various electroacoustic devices are presented from a piezoelectric material viewpoint. Opportunities come from not only the ultrahigh properties, specifically coupling and piezoelectric coefficients, but through novel vibration modes and crystallographic/domain engineering. Figure of merits (FOMs) of crystals with various compositions and phases were established for various applications, including medical ultrasonic transducers, underwater transducers, acoustic sensors and tweezers. For each device application, recent developments in relaxor-PT ferroelectric crystals were surveyed and compared with state-of-the-art polycrystalline piezoelectrics, with an emphasis on their strong anisotropic features and crystallographic uniqueness, including engineered domain - property relationships. This review starts with an introduction on electroacoustic transducers and the history of piezoelectric materials. The development of the high performance relaxor-PT single crystals, with a focus on their uniqueness in transducer applications, is then discussed. In the third part, various FOMs of piezoelectric materials for a wide range of ultrasound applications, including diagnostic ultrasound, therapeutic ultrasound, underwater acoustic and passive sensors, tactile sensors and acoustic tweezers, are evaluated to provide a thorough understanding of the materials’ behavior under operational conditions. Structure-property-performance relationships are then established. Finally, the impacts and challenges of relaxor-PT crystals are summarized to guide on-going and future research in the development of relaxor-PT crystals for the next generation electroacoustic transducers.
Single crystal relaxor ferroelectrics of PZN-8%PT were investigated for potential application in ultrasound transducers. The full set of electromechanical properties was determined using combined resonance and laser interferometry techniques. Ultra-high length extensional coupling (k(33)) of 0.94 was observed, a 25% increase over Navy Type VI PZT ceramics. The thickness extensional coupling (k(t)) of 0.48 was comparable to PZT compositions, and the compliance S(33)(E) was a factor of six greater. To maximize height extensional coupling (k'(33)), while minimizing length extensional coupling k(31) in array elements, it was necessary to align the elements along the 100 crystallographic direction in the x-y plane. Mode coupling plots and test samples for array elements determined that width-to-height ratios of less than 0.5 were desired, similar to the requirement for polycrystalline PZT ceramics. Modeling of 1-3 composites and experimental results demonstrated that thickness coupling greater than 0.80 could be achieved with a 40% to 70% volume fraction of PZN-PT. Although this is a substantial increase over PZT 1-3 composites, with a thickness coupling coefficient of 0.66, it represents a smaller fraction of the length extensional coupling k(33). This reduction may be a consequence of the increased compliance of PZN-PT, which results in significant clamping by the polymer matrix. Ultrasonic transducers fabricated using PZN-8%PT 1-3 composites achieved experimental bandwidths as high as 141%. The pulse-echo responses displayed good agreement with modeled results using the Redwood equivalent circuit.
The hydrostatic piezoelectric properties of [011] poled Pb(MgNb)O-PbTiO (PMN-PT) crystals and corresponding 2-2 crystal/epoxy composites were investigated. The crystal volume ratio and compositional dependencies of the hydrostatic charge and voltage coefficients ( and ) and hydrostatic figure of merit (FOM) were determined, where large FOM value of 3.2 pm/N with high stability as a function of hydrostatic pressure was achieved for rhombohedral crystal composites. In addition, the stress amplification effects of the face-plate and different epoxy matrixes were investigated, with maximum FOM value being on the order of 92 pm/N, indicating that 2-2 crystal/epoxy composites are promising materials for hydrostatic applications.
High temperature Pb(Zr,Ti)O3 /epoxy 1-3 composites were fabricated using the dice and fill method. The epoxy filler was modified with glass spheres in order to improve the thermal reliability of the composites at elevated temperatures. Temperature dependent dielectric and electromechanical properties of the composites were measured after aging at 250°C with different dwelling times. Obvious cracks were observed and the electrodes were damaged in the composite with unmodified epoxy after 200 hours, leading to the failure of the composite. In contrast, composites with >12 vol% glass sphere loaded epoxies were found to exhibit minimal electrical property variation after aging for 500 hours, with dielectric permittivity, piezoelectric coefficient and electromechanical coupling being on the order of 940, 310pC/N and 57%, respectively. This is due to the improved thermal expansion behavior of the modified filler.
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