Structural health monitoring technology can assess the status and integrity of structures in real time by advanced sensors, evaluate the remaining life of structure, and make the maintenance decisions on the structures. Piezoelectric materials, which can yield electrical output in response to mechanical strain/stress, are at the heart of structural health monitoring. Here, we present an overview of the recent progress in piezoelectric materials and sensors for structural health monitoring. The article commences with a brief introduction of the fundamental physical science of piezoelectric effect. Emphases are placed on the piezoelectric materials engineered by various strategies and the applications of piezoelectric sensors for structural health monitoring. Finally, challenges along with opportunities for future research and development of high-performance piezoelectric materials and sensors for structural health monitoring are highlighted.

Piezoelectric materials are capable of converting between mechanical and electrical energy, and are suitable for sensing, actuating and energy harvesting. While most conventional piezoelectric materials are brittle solids, flexible piezoelectric materials (FPM) retain functionality even under bending and stretching conditions. This characteristic has garnered increasing attention in recent years, particularly for wearable devices, where the ability to adapt to dynamic human movements is essential. In addition, wearable devices also demand excellent conformability, durability, and adaptability to miniaturization. FPM emerge as a promising solution that meet all these requirements. This review thus aims to offer a comprehensive summary of recent advances in the field of FPM, including piezoelectric polymers, composites, and inorganic flexible films. We introduce and categorize the specific features of these materials and highlight their emerging applications in electronic devices, and comment on the prospect of FPM as well as their potential challenges.

Here, we introduce phase change mechanisms in lead-free piezoceramics as a strategy to utilize attendant volume change for harvesting large electrostrain. In the newly developed (K,Na)NbO₃ solid-solution at the polymorphic phase boundary we combine atomic mapping of the local polar vector with in situ synchrotron X-ray diffraction and density functional theory to uncover the phase change and interpret its underlying nature. We demonstrate that an electric field-induced phase transition between orthorhombic and tetragonal phases triggers a dramatic volume change and contributes to a huge effective piezoelectric coefficient of 1250 pm V⁻¹ along specific crystallographic directions. The existence of the phase transition is validated by a significant volume change evidenced by the simultaneous recording of macroscopic longitudinal and transverse strain. The principle of using phase transition to promote electrostrain provides broader design flexibility in the development of high-performance piezoelectric materials and opens the door for the discovery of high-performance future functional oxides.

(K,Na)NbO₃ (KNN) is considered as one of the most promising candidates for lead-free piezoelectric ceramics, due to its large piezoelectric response as well as high Curie temperature. In the present study, the effect of various poling conditions on CaZrO₃-modified KNN-based lead-free piezoceramics was investigated. Featured by a polymorphic phase transition around ambient temperature, the samples differ from conventional Pb(Zr,Ti)O₃-based (PZT) ceramics in phase constitution, which inevitably influences the optimal poling conditions. It is found that the KNN ceramics can reach a saturated polarization state under reduced poling field and at short times, compared with PZT. Besides, poling at a relatively high temperature of 120 °C can yield an enhanced piezoelectric constant d₃₃ of up to 345 pC N⁻¹. The poling behavior of CaZrO₃-modified KNN-based ceramics is rationalized by the competition between domain reorientation and space charge accumulation.

Despite the pivotal role of stannum doping in achieving ultrahigh piezoelectric performance in barium titanate-based ceramics, the fundamental mechanisms underlying this enhancement remain elusive. Here, we introduce a single variable nonstoichiometric stannum strategy in lead-free barium titanate-based ceramics with giant piezoelectricity, revealing that stannum doping contributes intrinsically and extrinsically to enhance piezoelectricity. Density functional theory calculations elucidate the intrinsic enhancement of polarization arising from lattice distortion and increased space for titanium-oxygen bonds induced by optimal stannum doping, which is corroborated by Rayleigh analysis. A phase transition from ferroelectric multiphase coexistence to paraelectric phase is observed, alongside a rapid miniaturized and eventually disappeared domains with increasing stannum doping. This evolution in phase structure and domain configuration induces a nearly vanishing polarization anisotropy and low domain wall energy, facilitating easy polarization rotation and domain wall motion, thereby significantly contributing to the extrinsic piezoelectric response. Consequently, the origins of ultrahigh performance can be attributed to the synergistic effect of stannum-induced intrinsic and extrinsic contributions in barium titanate-based ceramics. This study provides fundamental insights into the role of doping elements and offers guidance for the design of high-performance piezoelectrics.