Abstract:High piezoelectricity of (K,Na)NbO 3 (KNN) lead-free materials benefits from a polymorphic phase transition (PPT) around room temperature, but its temperature sensitivity has been a bottleneck impeding their applications. We find that good thermal stability can be achieved in CaZrO 3 -modified KNN lead-free piezoceramics, in which the normalized strain d 33 * almost keeps constant from room temperature up to 140 o C. In situ synchrotron X-ray diffraction experiments combined with permitivity measurements disclose the occurrence of a new phase transformation under an electrical field, which extends the transition range between tetragonal and orthorhombic phases. It is revealed that such an electrically-enhanced diffused 2 polymorphic phase transition (EED-PPT) contributed to the boosted thermal stability of KNN based lead-free piezoceramics with high piezoelectricity. The present approach based on phase engineering should also be effective in endowing other lead-free piezoelectrics with high piezoelectricity and good temperature stability. IntroductionPiezoelectricity, a phenomenon whereby materials become electrically polarized upon the application of stress or deform in response to electrical stimuli, has been an active research topic since its discovery in 1880 by Pierre and Jacques Curie, because of its scientific interests and abundant applications. For the last half-century, the lead-contained materials, e.g., Pb(Zr,Ti)O 3 (PZT) and Pb(Mg,Nb)O 3 -PbTiO 3 (PMN-PT), have been the icons of piezoelectrics, exhibiting a morphotropic phase boundary (MPB), where plural phases with negligible difference in free energy coexist and strongly enhanced functional properties arise.[1] However, a possible toxicity of lead in PZT and PMN-PT has been raising intense health and environmental concerns; thus, the last decade has witnessed the surging dedication to viable lead-free alternatives. [2][3][4][5] Resembling the principle characteristics of MPB, [4][5][6][7][8][9][10] polymorphic phase transition (PPT) boundary has also been extensively pursued. [2, 11,12] Unfortunately, contrary to the nearly vertical MPB in the well-known PZT and PMN-PT systems, [1] the PPT in lead-free piezoelectrics is always tilted, resulting in unavoidable thermally unstable electromechanical properties. [13][14][15] Weak thermal stability is unacceptable for many industrial applications, even though lead-free piezoelectrics have competitive performance at ambient conditions. To address the issue, two approaches have been adopted so far, i.e., fabricating textured samples, [2] or shifting the PPT temperature T O-T well below room temperature. [13] However, the former confronts the poor reproducibility due to an excessively complex synthesis procedure; while the latter would inevitably sacrifice a large 3 portion of piezoelectric activity. Consequently, a barrier still exists in developing reliable lead-free piezomaterials as alternatives to currently market-dominating lead-based materials.Inspired by the nature of MPB in PZT and PMN-PT...

With the rapid development of wearable devices, a highly sensitive flexible piezoelectric sensor shows tremendous potential for future demands.

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Ceramic capacitors with upper operating temperatures far beyond 200°C are essential for high-temperature electronics used in deep oil drilling, aviation, automotive industry and so on. Recent advances in existing lead-free dielectrics for potential high-temperature capacitor applications are reviewed and grouped into three categories according to the parent component of the solid solution. Their desirable temperature stabilities were summarised comprehensively. However, there are still some limitations in the current research, such as achieving low loss in a wide temperature range and maintaining stable dielectric properties with different frequencies or at different voltages. Furthermore, the successful implementation of multilayer ceramic capacitors is one of the biggest challenges, which will have far-reaching impacts on the realisation of high-temperature capacitor application in the future.

Dielectric composites boost the family of energy storage and conversion materials as they can take full advantage of both the matrix and filler. This review aims at summarizing the recent progress in developing high‐performance polymer‐ and ceramic‐based dielectric composites, and emphases are placed on capacitive energy storage and harvesting, solid‐state cooling, temperature stability, electromechanical energy interconversion, and high‐power applications. Emerging fabrication techniques of dielectric composites such as 3D printing, electrospinning, and cold sintering are addressed, following by highlighted challenges and future research opportunities. The advantages and limitations of the typical theoretical calculation methods, such as finite‐element, phase‐field model, and machine learning methods, for designing high‐performance dielectric composites are discussed. This review is concluded by providing a brief perspective on the future development of composite dielectrics toward energy and electronic devices.

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In order to achieve a high-performance flexible piezoelectric energy harvester (FPEH), a unique sandwich structure, that is, a PVDF film filled with FeTiNbO6 (FTN) semiconductor particles as an intermediate layer and a pure PVDF film as an upper and lower barrier layer, has been designed, and the corresponding PVDF-FTN/PVDFx-PVDF (P-FTNx-P) compact composite has been prepared by hot-pressing technology. The special sandwich structure combined with the introduction of FTN particles is beneficial to enhance the interfacial polarization and the content of the electroactive phase in PVDF. Together with the maximum piezoelectric voltage coefficient and the moderate Young’s modulus, the P-FTN15%-P FPEH exhibited the optimal energy-harvesting performance with a high power density of 110 μW/cm3 and a large charge density of 75 μC/m2 in cantilever mode. The outstanding design in this work is expected to provide a new way for the development of high-performance FPEH materials.

An ultrahigh g33 of 600 × 10−3 V m N−1 is achieved by a composite of BCZT particles aligned in a PDMS matrix by dielectrophoresis.