Piezoelectric ceramics, which enable the mutual conversion of mechanical and electrical energy via the piezoelectric effect, are indispensable for various industries, including robotics, communications, biomedicine engineering, etc. Currently, some piezoelectric ceramics face challenges such as difficulty in sintering and element volatilization. Pressure-assisted sintering (PAS) technology can promote the sintering process with the application of external pressure, thereby effectively engineering the electrical properties of piezoelectric ceramics. In this review, three commonly used pressure-assisted sintering techniques in the preparation of perovskite piezoelectric ceramics are highlighted, i.e., hot pressing sintering, hot isostatic pressing sintering, and spark plasma sintering.

Hot pressing sintering, which applies a single-axis pressure during the sintering phase, is particularly adept at grain alignment and the fabrication of textured ceramics with directional property enhancement. Hot isostatic pressing sintering applies pressure uniformly in all directions, ensuring homogenous densification and the precise maintenance of complex structure dimensions. This method is invaluable for the creation of precision components where dimensional integrity is non-negotiable. HIP further enables the sintering of ceramics at reduced temperatures and shortened durations, thereby curtailing energy expenditure and averting excessive grain coarsening. Spark plasma sintering is characterized by its expedited heating rates and abbreviated sintering cycles, facilitated by the passage of pulsed direct current through the powder compact. This approach not only catalyzes swift densification but also promotes the development of fine-grained microstructures by curbing grain expansion. SPS is especially beneficial for crafting ceramics with exceptional strength and electrical properties.

The advantages for enhancement of ceramic density, grain size, and defect engineering, texturing, as well as preparation of complex microstructures are demonstrated compared with conventional sintering. This review underscores the substantial benefits of PAS in bolstering ceramic density, a parameter of paramount importance to the mechanical and electrical properties of piezoelectric ceramics. The uniform density attainable through PAS is instrumental in enhancing performance and reliability. Moreover, the controlled sintering milieu in PAS is conducive to the precise manipulation of grain size, a factor with a significant bearing on piezoelectric performance. Fine-grained ceramics yielded via PAS have demonstrated superior functionality when juxtaposed with their coarse-grained analogs. Defect engineering represents another critical arena where PAS exerts a substantial influence. By moderating sintering temperatures and maintaining a controlled atmosphere, PAS minimizes the emergence of defects, such as vacancies and dislocations, which can diminish the efficacy of piezoelectric ceramics. The capacity to regulate defect concentration and type within the material is paramount for optimizing its electrical properties, including dielectric constant and piezoelectric coefficients. Furthermore, PAS has unveiled remarkable potential in the realm of complex microstructure fabrication, indispensable for state-of-the-art applications like high-frequency ultrasound transducers.

The review also probes potential future research trajectories for PAS in perovskite piezoelectric ceramics. There is an imperative need for deeper exploration into the fundamental mechanisms governing sintering behavior under pressure-assisted conditions. Gaining insights into these mechanisms is vital for devising more efficient sintering protocols and achieving material property enhancements. Additionally, there is a burgeoning interest in the development of in-situ monitoring techniques, capable of furnishing real-time sintering process feedback, thereby enabling more precise control over the final ceramic properties.

Despite its considerable merits, the scalability and economic implications of PAS must be deliberated to facilitate broader industrial implementation. Pressure-assisted sintering has validated its transformative potential in the realm of perovskite piezoelectric ceramic fabrication, offering meticulous control over microstructure and material properties. As investigative endeavors in this domain persist, PAS is poised to assume a central role in the genesis of next-generation piezoelectric materials.

Summary and prospects Pressure-assisted sintering effectively promotes ceramic densification by providing additional sintering driving force, offering a feasible method for the fabrication of piezoelectric ceramics with high density and performance. Furthermore, Pressure-assisted sintering allows for flexible control of sintering temperature and time, which enables the preparation of high-density ceramics with varying grain sizes. Additionally, pressure-assisted sintering can combine with sintering atmospheres, electric currents, and other physical fields to engineer the defects. Uniaxial pressure can also introduce texture in some piezoelectric ceramics. Therefore, pressure-assisted sintering is expected to achieve synergistic regulation of density, grain size, defects, and grain orientation in piezoelectric ceramics, thereby comprehensively enhancing the mechanical, dielectric, piezoelectric, and optical properties of the material.

However, precisely controlling the process of pressure-assisted sintering is challenging due to limitations in sintering equipment and mold materials. The issue of reproducibility caused by inhomogeneous pressure or current distribution deserves concern. For fundamental scientific research, pressure-assisted sintering can bring novel physical properties to piezoelectric ceramics. The clear demonstration of the sintering behavior of piezoelectric ceramics is the prerequisite for the in-depth understanding of these properties. Therefore, developing in-situ sintering characterization may assist in fully understanding the relationships among pressure, sintering behavior, and physical properties. For practical applications, high cost is the most significant barrier to scaling up pressure-assisted sintering for piezoelectric ceramics. The improvement in equipment and optimization of control systems are in high demand to overcome these technical challenges.

With the capability of interconversion between electrical and mechanical energy, piezoelectric materials have been revolutionized by the implementation of perovskite-piezoelectric-ceramic-based studies over 70 years. In particular, the market of piezoelectric ceramics has been dominated by lead zirconate titanate for decades. Nowadays, the research on piezoelectric ceramics is largely driven by cutting-edge technological demand as well as the consideration of a sustainable society. Hence, environmental-friendly lead-free piezoelectric materials have emerged to replace lead-based Pb(Zr,Ti)O₃ (PZT) compositions. Owing to the inherent high mechanical quality factor (Q_m) and low energy loss, (Li,Na)NbO₃ (LNN) materials have recently drawn increasing attention and brought advantages to high-power piezoelectric applications. Although the crystallographic structures of LNN materials were intensively investigated for decades, the technical strategies for electrical performance are still limited. As a result, the property enhancement appears to have approached a plateau. This review traces the progress in the development of LNN materials, starting from the polymorphism in terms of the crystal structures, phase transitions, and local structural distortions. Then, the key milestone works on the functional tunability of LNN are reviewed with emphasis on involved engineering approaches. The exceptional performance at a large vibration velocity makes LNN ceramics promising for high-power applications, such as ultrasonic welding (UW) and ultrasonic osteotomes (UOs). The remaining challenges and some strategic insights for synergistically engineering the functional performance of LNN piezoceramics are also suggested.