Poly (vinylidene fluoride) (PVDF) is a kind of semicrystalline organic polymer piezoelectric material. Adopting processes such as melting crystallization and solution casting, and undergoing post-treatment processes such as annealing, stretching, and polarization, PVDF films with high crystallinity and high piezoelectric response level can be realized. As a polymer material, PVDF shows excellent mechanical properties, chemical stability and biocompatibility, and is light in weight, easily prepared, which can be designed into miniaturized, chip-shaped and integrated devices. It has a wide range of applications in self-powered equipment such as sensors, nanogenerators and currently is a research hotspot for use as flexible wearable or implantable materials. This article mainly introduces the crystal structures, piezoelectric properties and their applications in flexible piezoelectric devices of PVDF materials.

High-temperature vibration sensors are needed for precise monitoring of dynamic mechanical conditions in aerospace, automotive, and energy-generation-related systems. In this article, a compression-mode vibration sensor was designed and fabricated using a Ba ₂ TiSi ₂ O ₈ single crystal for structural health monitoring at elevated temperatures (650 °C). The fundamental performance of the sensor was simulated, and the effect of pretightening torques on sensor performance was studied. Sensor performance was characterized as a function of temperature up to 650 °C in a frequency range of 100—600 Hz. The sensitivity of the sensor was found to be on the order of 2.6–2.7 pC/g over the tested frequency and temperature ranges, with variation below 2%. In addition, the sensor was found to maintain ultrastable sensitivity at 650 °C. Thus, it exhibits good sensing capability and high reliability at elevated temperatures.

Crystal materials with integrated functionalities are desirable for the miniaturization of future optical devices. In this paper, nonlinear optical crystals of Ca₃TaGa₃Si₂O₁₄ (CTGS) and Nd³⁺ doped CTGS (Nd:CTGS) were grown using the Czochralski pulling method and explored for self-frequency-doubling applications. Based on the refractive indexes, the optimum phase matching directions for CTGS crystals were determined for type I (38.7°, 30.0°) and type II (61.1°, 0°) orientations, with respective effective nonlinear optical coefficients on the order of 0.44 pm V⁻¹ and 0.34 pm V⁻¹. Self-frequency-doubling was realized in Nd:CTGS crystals [(38.7°, 30.0°) (type I)] for the first time, and 18.8 mW laser power at 533 nm was achieved.

Rare-earth calcium oxyborate crystals, ReCa₄O(BO₃)₃ (ReCOB, Re = Er, Y, Gd, Sm, Nd, Pr, and La ), are potential piezoelectric materials for ultrahigh temperature sensor applications, due to their high electrical resistivity at elevated temperature, high piezoelectric sensitivity and temperature stability. In this paper, different techniques for ReCOB single-crystal growth are introduced, including the Bridgman and Czochralski pulling methods. Crystal orientations and the relationships between the crystallographic and physical axes of the monoclinic ReCOB crystals are discussed. The procedures for dielectric, elastic, electromechanical and piezoelectric property characterization, taking advantage of the impedance method, are presented. In addition, the maximum piezoelectric coefficients for different piezoelectric vibration modes are explored, and the optimized crystal cuts free of piezoelectric cross-talk are obtained by rotation calculations.