Piezoelectric ceramics generate strain through the intrinsic piezoelectric effect, the motion of ferroelectric domain walls, or through field-induced phase transitions. The enhanced piezoelectric properties observed in morphotropic solid solutions arise from several distinct, but interrelated, mechanisms associated with the near degeneration of the energy surface from cubic to spherical symmetry. The phenomenological theory of ferroelectricity is used to explain the thermodynamic origins of strain generation mechanisms in these solid solutions. The displacement of ferroelectric domain walls is an extrinsic contribution to the piezoelectric response that can be controlled by modifying the host material with small concentrations of dopants. The concept of “hardening” is introduced; hardening can be useful in applications where piezoelectric energy conversion and low energy loss are more important than large strain. The operative mechanisms of strain generation and energy conversion in technologically important lead-based and lead-free piezoelectric materials are summarized.

The piezoelectric properties of lead-free ferroelectric materials have been dramatically improved over the past two decades. For some limited applications, their properties have reached the same levels or have even surpassed the properties of the benchmark lead-based material Pb(Zr,Ti)O3 (PZT). Initial commercial lead-free products, including powders, ceramic components, films, and devices (e.g., ultrasonic cleaner, knocking sensor), are now available on the market. Several prototype devices, such as inkjet printheads, ultrasonic motors, angular sensors, and energy harvesters, have been developed. Their overall performance is still inferior to that of PZT-based devices; however, these prototypes and products point the way for future applications. Here, we provide an overview of recent industrial developments in the field and discuss the main advantages and disadvantages of lead-free piezoceramics for individual applications.

The focus on piezoelectric ceramics based on the potassium sodium niobate system began in 2004. After years of dedicated research, these materials can be considered one of the most promising lead-free piezoceramics with comprehensive performance. While their structure–property relationships are still not completely understood, the thermal stability issue is partly resolved, which leaves further room for phase-boundary engineering. Technological advancement has recently focused on using base metals as inner electrodes for multilayer actuators, which provides cost benefits as compared to lead zirconate titanate devices. The remaining challenges, however, such as poor sinterability and weak reproducibility of functional properties, still hinder extensive applications of these materials.

The field of lead-free piezoceramics, which aims to replace lead zirconate titanate (PZT) and related perovskite materials, has been vibrant for almost 15 years. Once the science in this field attained a certain stage of maturity, materials with properties better than PZT have appeared, and the first products are about to reach the marketplace. This article describes the three most promising lead-free piezoceramics currently under discussion to replace PZT. Each has a pronounced property profile geared for specific applications. Guidelines for directions for fundamental future research on as well as technology transfer to industry of lead-free piezoceramics are provided.

Sodium bismuth titanate (NBT) and its solid solutions with other ABO3 perovskites are of great interest for lead-free ferroelectric and piezoelectric applications. In this article, we provide an introduction to the complex structure of NBT, including atomic displacements and nanoscale defects. We also review poling effects and properties as well as NBT-ABO3 phase equilibria. The interesting relaxor properties, frequency dispersion in dielectric permittivity, and field-induced structural phase transitions of these systems are discussed. Finally, we describe other functional, mechanical, and electrical properties of NBT.

This article reviews a grounding in thin-film science and technology, an interest in combining materials science with applied physics and electrical engineering, and the active pursuit of collaborations with experts in other disciplines. That basis has enabled participation in the beginnings of integrated-circuit technology, the invention of new solar cells, the understanding of hydrogenated amorphous silicon for solar cells and thin-film transistors, the development of the principles of flexible, conformable, and stretchable electronics, and the devising and demonstration of large-area electronic systems.

Coherent cuboidal B2 nanoprecipitation in body-centered cubic (BCC)-based high-entropy alloys (HEAs) is important for the improvement of mechanical strength. The present work primarily investigated the effect of Ti substitution for Al on the cuboidal B2 nanoprecipitates in BCC Al0.7NiCoFeCr2 HEAs. A series of (Al,Ti)0.7NiCoFeCr2 HEAs with different Al/Ti ratios were prepared by suction-cast processing, and their microstructures and mechanical properties were then characterized comprehensively. It was found that the substitution of Ti for Al can change the phase structures of ordered precipitation, from the B2-AlNi to a highly ordered L21-Ni2AlTi phase. Especially, a small amount addition of Ti (≤4.2 at.%, Al/Ti ratio ≥2/1) renders the HEAs with cuboidal L21 nanoparticles coherently precipitated into the BCC matrix, which is attributed to the moderate lattice misfit (ε = 0.5–0.6%) between BCC and L21 phases. HEAs with such coherent microstructures exhibit high compressive yield strength of about 1700–1800 MPa. When the Ti content reaches up to 6.25 at.%, the matrix of the alloy will be turned into the σ phase, rather than BCC, leading to a heavy brittleness.