Ferroelectric materials toward next-generation electromechanical technologies.

Ferroelectric materials have been widely used in various electromechanical devices, from ultrasonic transducers and actuators to mechanical energy harvesters. The key performance metrics of these devices, such as sensitivity, efficiency, and bandwidth of ultrasonic transducers, are largely determined by the piezoelectric properties. This Review highlights recent research progress in improving the piezoelectricity of ferroelectric materials and offers potential strategies for further enhancement to meet the ever-increasing demands for high-performance piezoelectric devices and systems. It provides insights into the future development of ferroelectrics to address the increasing demands of emerging applications, including photoacoustic imaging and piezoelectric fans and motors in integrated circuit–enabled electronic devices. Additionally, it emphasizes the need to consider environmental impacts across the entire life cycle of ferroelectrics, from sourcing and manufacturing to usage and disposal. Editor's summary: Many small-scale electromechanical devices, such as the speakers, fans, and camera motors in a cell phone, as well as larger devices such as medical ultrasound imagers, rely on piezoelectric materials that change shape in response to an applied electric field. Li et al. reviewed the development of these materials and discuss directions for improvements. Not only are materials with larger piezoelectric coefficients and electromechanical coupling factors needed, but also ones with improved parameters such as dielectric constant, dielectric loss, coercive field, and mechanical loss. —Phil Szuromi BACKGROUND: Ferroelectrics are widely recognized for their exceptional functional properties, particularly piezoelectricity, which enables the efficient conversion of electrical energy into mechanical energy and vice versa. This ability has positioned ferroelectric materials at the forefront of numerous modern technologies. During the past 100 years, a wide array of ferroelectric materials with excellent piezoelectric properties have been developed, including lead zirconate titanate ceramics, lead-free ceramics, scandium-doped aluminum nitride thin films, polyvinylidene fluoride–based ferroelectric polymers, and relaxor ferroelectric crystals. These innovations have expanded the scope of advanced ferroelectric applications and provided greater flexibility in device design. They have also notably benefited numerous piezoelectric devices. Their functions in cell phones, for example, are many and include piezoelectric cooling fans that cool integrated circuit chips and maintain device performance. Ultrasonic transducers, which have facilitated advancements in critical areas such as medical imaging, where high-resolution ultrasound is essential, are another example. Nonetheless, there remains a pressing need for ferroelectric materials with even better performance to address the challenges posed by emerging technologies. For example, to facilitate the application of piezoelectric fans and motors, piezoelectric ceramics with even higher piezoelectric coefficients are required for reducing the driving voltage and energy consumption. ADVANCES: The primary approach to achieving high piezoelectricity in ferroelectrics has focused on inducing instability in the orientation of spontaneous polarization, which involves flattening the free-energy profile that connects various stable and metastable polar states, as described in phenomenological theory. Numerous theoretical and experimental studies have demonstrated that such a flat free-energy profile can be achieved through the design of a compositionally driven ferroelectric phase boundary, particularly a morphotropic phase boundary. This strategy has gained popularity in the wake of the development of the phase diagram for lead zirconate titanate solid solution. Recently, researchers have identified approaches to further tailor the free-energy profile by carefully arranging long-range ferroelectric domains and introducing local structural heterogeneity at the nanoscale. These innovations have led to the highest piezoelectricity recently observed in rare-earth element–doped relaxor ferroelectric crystals. Alternative strategies for achieving even higher piezoelectricity are being actively explored, including engineering the grain orientation in ceramics, facilitating ion redistribution under electric fields, and incorporating the flexoelectric effect at the nanoscale. These diverse strategies, when combined, hold great promise for pushing the boundaries of piezoelectricity in ferroelectrics and expanding their applicability in advanced technologies. OUTLOOK: New high-performance ferroelectric materials have emerged, such as ferroelectric ceramics with a piezoelectric coefficient >2000 pC N−1 (4 to 10 times higher than currently used materials) and ferroelectric crystals simultaneously possessing high piezoelectricity and high light transparency. These materials hold immense potential in consumer and health care electronics, playing a crucial role in next-generation integrated circuit chip cooling devices, adaptive zoom systems, and wearable, implantable, and self-powered medical diagnostic devices. Looking ahead, research is increasingly focused on developing lead-free or biodegradable ferroelectrics to address environmental concerns. In parallel, environmentally friendly manufacturing techniques, such as low-temperature sintering, aerosol deposition, and bioinspired methodologies, are being explored to reduce energy consumption and carbon emissions, aligning with global sustainability and carbon neutrality goals. Additionally, textured ferroelectric ceramics have emerged as a promising alternative to crystal counterparts, offering good piezoelectric properties along with the robust mechanical strength of ceramics, while reducing overall costs. In the search for new materials, advances in computational methods, such as first-principles calculations and phase-field simulations, along with progress in material characterization technologies, such as in situ high-resolution atomic-scale imaging, are enabling more accurate predictions of material behavior and properties. These developments are undoubtedly accelerating the progress of high-performance ferroelectrics. Currently used and emerging piezoelectric devices in the field of 3C (computer, communication, and consumer electronics).: The smartphone is used as an example of a typical consumer electronic product to illustrate the diversity of piezoelectric applications. MEMS, microelectromechanical system. [ABSTRACT FROM AUTHOR]