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A temperature-insensitive FOM was obtained in lead-free system with diffuse multiphase coexistence, and a record power density of 865 μW cm−3 at 80 °C was measured for cantilever beam-type PEH.

The rapid development of high technology—such as space exploration and electric vehicles—urgently requires ultra-wide temperature multilayer ceramic capacitors (UWT MLCCs) to achieve reliable operation of electronic circuits in harsh environments. However, simultaneously achieving high dielectric permittivity, low dielectric loss, and ultrahigh thermal stability has been a major challenge for practical dielectric ceramics. The co-firing matching of the internal electrode and the dielectric ceramic is also an important factor that affects the reliability of UWT MLCCs. Herein, through multifaceted modification—i.e., composition design related to the modulation of the local polar nanoregions (PNRs) and optimizing device sintering in the context of the compatibility of the heterogeneous interface—these concerns have been well-addressed. A new lead-free dielectric system (1-x) (0.56Na0.5Bi0.5TiO3-0.14K0.5Bi0.5TiO3-0.3NaNbO3)-xCaZrO3 (NKBTNN-xCZ) dominated by P4bm PNRs was designed and corresponding UWT MLCCs with reliable Pt internal electrode interface bonding were fabricated by optimizing the sintering temperature. A record-high dielectric permittivity (er = 839 ± 15 %) and low dielectric loss (tanδ ≤0.02) was achieved over an ultra-wide temperature range from -70 °C to 337 °C for NKBTNN-0.063CZ UWT MLCCs. This work suggests that multifaceted modification should be generalized for construction of high-performance UWT MLCCs.

An excellent power generation performance with good anti-vibration fatigue performance is achieved in the 5BCZT/0.1CNTs/PVDF piezocomposite by the synergistic design of the piezoelectric phase and conductive phase.

By introducing K0.5Bi0.5(Zn1/3Nb2/3)O3, the [AO12] cages are enlarged, which enhances the local polarization; meanwhile, the long-range correlations are disrupted, which renders PNRs percolate to room temperature. Finally, a high recoverable energy density of 5.1 J cm−3 and a high efficiency over 80% are obtained.

A novel strategy of combining the PPT structure design with ZrO2-composite toughening has been proposed to build a lead-free KNN-based composite to realize excellent power generation in environment-friendly PEHs with enhanced mechanical properties.

A large increase in the output power density of PENGs has been realized by adopting a strategy that rationally tunes the orientation of the three-dimensional (3D) interconnected piezoceramic skeletons in the polymer matrix.

Lead-free piezoceramics are international research frontier in the fields of functional materials. In this work, high-quality Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZ) ceramics were prepared by using ultrafine powder synthesized by the molten salt method. The results show that the enhanced piezoelectric properties are obtained at intermediate grain size of 14.7 μm. Furthermore, by the optimization of the DC poling parameters, excellent d33 ~ 600 pC/N and kp ~ 0.50 values of BCTZ lead-free ceramic can be obtained, while d33 ~ 420 pC/N and kp ~ 0.35 were obtained under the optimal AC poling parameters. The property difference is mainly due to that AC poling does not give enough time for the internal stress to relax, allowing the switched ferroelastic domains to accommodate each other, so it is difficult to achieve saturated poling. Though, AC poling method still provides a new insight for the optimization of the performance of piezoelectric ceramics.

The rapid development of flexible micropower electronics has aided the opportunity for the broader application of flexible piezoelectric composites (PCs) but has also led to higher requirements for their power generation. Among them, 0-3 PCs with embedded zero-dimension piezoparticle fillers, although low cost and easy to prepare, suffer from suboptimal output performance because of inherent structural defects. In this work, the voltage output was increased from 3.4 to 12.7 V under a force of 7 N, through first-step regulation by aligning the KNbO3 (KN) particles in the polydimethylsiloxane (PDMS) matrix; then, a significantly enhanced current output (from 0.7 to 4.5 μA) through second-step regulation by introducing copper nanorods (Cu NRs) interspersed in the gaps between the KN chains. Consequently, the proposed PC exhibits much higher power density, 37.3 μW/cm2, than that of random KN/PDMS and thus shows good potential for high-performance, flexible piezoelectric energy harvesters.

Quenching alkaline bismuth titanates from sintering temperatures results in increased lattice distortion and consequently higher depolarization temperature. This work investigates the influence of quenching on the ergodicity of relaxor Na1/2Bi1/2TiO3-BaTiO3-K0.5Na0.5NbO3. A distinct departure from ergodicity is evidenced from the increase in remanent polarization and the absence of frequency dispersion in the permittivity response of poled samples. Further, the samples exhibit enhanced negative strain upon application of electric field, indicating proclivity towards correlated polar nanoregions, corroborated by the enhanced tetragonal distortion. As a result, ergodic relaxor Na1/2Bi1/2TiO3-6BaTiO3- 3K0.5Na0.5NbO3 exhibits a depolarization temperature of 85°C with a 60% increase in remanent polarization and approximately a threefold increase in remanent strain upon quenching. Quenching-induced changes in the local environment of Na+ and Bi3+ cations hinder the development of ergodicity promoted by the A-site disorder. These results provide new insight into tailoring ergodicity of relaxor ferroelectrics.

Due to the self-supplied energy requirements of wearable electronic devices, flexible piezoelectric energy harvesters (FPEHs) that can convert the vibration energy in the environment into electrical energy have received widespread attention in recent years. The key to increasing the power generation capacity of FPEHs is to increase the piezoelectric charge constant d33 while reducing the internal resistance, which is still a huge challenge for ceramic/polymer composites. In this work, a synergistic effect is used to construct FPEHs with high power generating capacity. On the one hand, a highly electroactive 0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr1/2Ti1/2)O3 (PZN–PZT) ceramic skeleton is constructed through a freeze-casting method to greatly increase the d33 of a PDMS based composite. On the other hand, multi-walled carbon nanotubes (MW-CNTs) are introduced as a bridging phase to reduce the internal resistance of the composite. The 2–2 type PZN–PZT/MW-CNTs/PDMS piezocomposite prepared using this strategy has a d33 as high as 144 pC N−1, which is significantly better than those of previously reported composites. The FPEH with dimensions of 1.5 × 1.5 × 0.1 cm3 can light 22 LEDs through the rectifier circuit without any storage units, showing an important application prospect for self-supply of energy in wearable electronic devices.

The utilization of relaxor-based ferroelectrics with high piezoelectricity is considered to be an effective way to enhance the power generation capacity of piezoelectric energy harvesters (PEHs). However, the severe depolarization behavior in the high-temperature region is the main issue that hinders the application of relaxor-based ferroelectrics in high-temperature PEHs. Here, the goal of obtaining a high piezoelectric charge coefficient (d33) together with excellent temperature stability in relaxor-based ferroelectrics is achieved through a lattice distortion design strategy combined with domain configuration matching. Based on this concept, a new relaxor-based ferroelectric xPb(Zn1/3Nb2/3)O3–(1 − x)Pb(HfyTi(1−y))O3 (xPZN–(1 − x)PHyT(1−y)) is investigated and the corresponding high-temperature PEHs are fabricated. A high Curie temperature of 319 °C and a large d33 of 540 pC N−1 are achieved at the optimal composition (x/y = 0.075/0.49) simultaneously, which are superior to those of the existing commercial PZT-based piezoceramics. The enhanced lattice angle distortion (|β-90°|) with a hierarchical domain configuration balances the piezoelectricity and thermal stability in relaxor-based ferroelectrics. Furthermore, the newly developed PZN–PHT PEH exhibits a stable output current over a broad temperature range (25–275 °C) and satisfactory cycling reliability (up to 200 000 cycles without degradation) at 250 °C. These characteristics demonstrate that the PZN–PHT relaxor-based ferroelectric is a promising candidate for high-temperature PEH applications.

By maintaining high piezoelectric coefficients (d33), increasing the depolarization temperature is the key to constructing high-performance high-temperature piezoceramics. Unfortunately, so far, no piezoceramic has been found that still has a d33 value above 700 pC N−1 by in situ testing at a high-temperature of 400 °C. For popular 0.36BiScO3-0.64PbTiO3 ceramics with a MPB structure, the in situ quasi-static d33 value at 400 °C is only 405 pC N−1. Herein, a new strategy to enhance perovskite lattice distortion to obtain oxides with excellent high-temperature piezoelectricity has been proposed. By introducing Bi(Zn0.5Hf0.5)O3 to enhance lattice distortion of a (1 − x)BiScO3-xPbTiO3 matrix, a ternary system zBiScO3-xPbTiO3-yBi(Zn0.5Hf0.5)O3 (zBS-xPT-yBZH) was designed. A record-high in situ quasi-static d33 value of 726 pC N−1 at 400 °C is achieved in a 0.355BS-0.635PT-0.01BZH composition. Structural analysis confirmed that the introduction of highly tetragonal Bi(Zn0.5Hf0.5)O3 can enhance the lattice distortion and the sample annealed at 400 °C still maintains a stable domain configuration. Moreover, a high-temperature piezoelectric energy harvester is manufactured from the optimal material, and exhibits excellent high-temperature power generation capacity, and a 10 μF commercial electrolytic capacitor can be easily charged to 0.9 V in 40 s at 400 °C. This work demonstrates that zBS-xPT-yBZH ceramics have great potential for application in extreme high temperature environments, and pave the way for obtaining high-quality high-temperature piezoelectric materials.

The assembly of high-temperature electronic equipment places great demands on ultra-high temperature multilayer ceramic capacitors (UHT-MLCCs). However, the relatively low dielectric constant and inferior reliability associated with the protruding ferroelectric phase transition for existing dielectric materials have hindered the development of UHT-MLCCs. Here, these concerns have been addressed by the strategy of composition modulation of different types of polar nanoregions (PNRs) in bismuth-based perovskite relaxor. A new lead-free dielectric system (1-x)(0.8Na0.5Bi0.5TiO3-0.2K0.5Bi0.5TiO3)-xBi(Mg2/3Nb1/3)O3 (NBT-KBT-xBMN) dominated by P4bm PNRs has been built and the corresponding UHT-MLCCs with 70Ag/30Pd inner electrodes are fabricated by tape casting and cofiring processes. A record-high dielectric constant (er = 1800 ± 15%) together with low dielectric loss (tanδ ≤ 0.025) is achieved over a wide range of 100 °C–440 °C for NBT-KBT-0.2BMN MLCC, which consists of nine dielectric layers with the single-layer thickness of 70 μm. The excellent results suggest that NBT-KBT-0.2BMN MLCC can provide realistic solutions for next-generation UHT-MLCC applications and the material design strategy can be generalized for the construction of more high performance capacitor dielectrics.

Grain size effect is one of the most important issues to develop next-generation multilayer microdevices. In this work, the tetragonal 0.4Ba(Zr0.2Ti0.8)O3–0.6(Ba0.7Ca0.3)TiO3 (BZT–60BCT) ceramics with a wide grain size from 2.1 to 24 μm were successfully prepared by using ultrafine nano powder and two-step sintering. The results demonstrate that critical/intermediate grain size of dielectric constant er and piezoelectric constant d33 appears at ∼12.9 μm. It was found that the presence of large lattice tetragonality, and enhanced domain wall motion induced by domain coexistence between submicron and nano size in sample with a grain size of ∼12.9 μm, resulting in the superior dielectric and piezoelectric properties. These findings and analyses of the origin of superior dielectric and piezoelectric properties at intermediate grain size have important practical implications in the design of high-performance piezoelectric ceramics.

Piezoelectric materials, which convert mechanical vibration energy into electric energy, are essential for vibration energy harvester. Nevertheless, the piezoelectric energy harvesting also encounters the bottleneck that piezoelectric materials generally produce only micro scale current due to its high impedance. In this work, the developed lead-free (Ba0.85Ca0.15)0.9985Sm0.001(Ti0.9Zr0.1)O3 (BCSm0.001TZ) piezoelectric ceramic possesses both high piezoelectric charge constant and low impedance, boosting its energy harvesting performance: an extremely high short-circuit current (60 µA) and large power density (2.1 µW/mm3) in a cantilever-type energy harvester, which is superior to those of other reported ones. Due to the giant current performance, the related charging speed of BCSm0.001TZ energy harvester possessed ~100% enhancement relative to the non-doped counterpart. Electrical property measurement and nanostructure observation revealed that the outstanding electromechanical properties as well as energy harvesting performances in BCSm0.001TZ ceramic benefits from a complex domain architecture and low concentration of defect, which is closely associated with the local structural heterogeneity induced by the rare-earth Sm doping. This work provides a new important paradigm for developing high-performance viable green energy materials and devices, especially for the fast recharging microelectronic storage devices in wireless sensor network.

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.

Based on the strong demand for self-powered wearable electronic devices, flexible piezoelectric energy harvesters (FPEHs) have recently attracted much attention. A polymer-based piezocomposite is the core of an FPEH and its transduction coefficient (d33 × g33) is directly related to the material's power generation capacity. Unfortunately, the traditional 0–3 type design method generally causes a weak stress transfer and poor dispersion of the filler in the polymer matrix, making it difficult to obtain a high d33 × g33. In this work, a unique interconnected skeleton design strategy has been proposed to overcome these shortcomings. By using the freeze-casting method, an ice-templated 2–2 type composite material has been constructed with the popular piezoelectric relaxor 0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr1/2Ti1/2)O3 (PZN–PZT) as the filler and PDMS as the polymer matrix. Both the theoretical simulation and the experimental results revealed a remarkable enhancement in the stress transfer ability and piezoelectric response. In particular, the 2–2 type piezocomposite has an ultrahigh transduction coefficient of 58 213 × 10−15 m2 N−1, which is significantly better than those of previously reported composite materials, and even textured piezoceramics. This work provides a promising paradigm for the development of high-performance FPEH materials.

Ferroelectrics are a class of functional materials that have spontaneous polarization and whose direction can be modulated by external electric fields. The improvement of the electrical properties of ferroelectrics is closely related to the control of their crystal structure and morphology. The molten salt method is a chemical synthesis method in which a molten salt functions as a high-temperature liquid solvent. The special molten salt reaction environment is conducive to the dissolution of raw materials, the transport of reactants, and the directional assembly of basic units to prepare scale- and shape-controllable target materials. In particular, ferroelectrics synthesized by the molten salt method have a variety of technical advantages, such as controllable morphology (0D, 1D and 2D), high dielectric constant, high piezoelectric performance, and strong sintering activity. Therefore, ferroelectrics synthesized by this process have been used in important applications in the energy field in different forms, such as single crystals, textured ceramics, piezoelectric composites, oriented composites, multilayer composites and ferroelectric ceramics. This review article highlights the critical issues and the recent progress of molten salt preparation of ferroelectrics and their application from the viewpoint of synthesis strategies, reaction mechanisms, and energy device assembly. It is anticipated that this review can serve as an overview and evaluation of the state-of-the-art molten salt synthesis of ferroelectrics and their design for applications in energy conversion and energy storage devices.

High-performance lead-free piezoelectric materials have become a hot topic in the field of advanced functional materials. In 2009, the (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT–BCT) system with surprisingly high piezoelectric properties was first proposed by Liu and Ren; as a milestone discovery in the field of ferroelectrics, it has aroused extensive attention. In this review, initially, we highlight the remarkable electrical properties of the BZT–BCT system, and then summarize the advances in the BZT–BCT material based on five aspects including the origin of its high properties, preparation and poling, grain size control, chemical modification and texture design. We focus on a systematic discussion of its phase boundaries, domain structure, and defect chemistry in relation to improved dielectric and piezoelectric properties. In addition, the device performance of BZT–BCT materials is also evaluated. Due to their excellent comprehensive properties, BZT–BCT materials have been widely studied in various applications such as energy harvesters, flexible sensors and energy storage capacitors. Finally, based on the state-of-the-art knowledge, we present an outlook on the challenges and application prospects of BZT–BCT materials. This review will help promote the practical applications of BZT–BCT materials, and accelerate the development of lead-free ferroelectric materials in specific fields.

Improving the operating characteristics of piezoelectric devices in high temperature environments urgently requires the development of piezoceramics with both high piezoelectric coefficient and excellent temperature stability. However, it is difficult for existing piezoceramics to take care of both at the same time. Generally, high piezoelectricity can be obtained at morphotropic phase boundary (MPB) in the binary systems, such as PbZrO3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3, but the temperature stability is unsatisfactory, which seriously restricts the practical application. Here, an optimum composition having excellent comprehensive properties is constructed by designing multiple MPBs in the novel xPb(In1/2Nb1/2)O3-yBiScO3–zPbTiO3 ternary system. When x = 0.04, y = 0.345 and z = 0.615, the specimen has a high piezoelectric coefficient d33 of 478 pC/N at 200 °C, meanwhile, the fluctuation of d33 is less than ± 10% over an ultra-broad temperature range of 50–350 °C. Combined with a variety of in situ analysis techniques, it can be determined that the temperature-insensitive high piezoelectric coefficient is related to the multiple MPBs design, which is beneficial to the optimization of the hierarchical domain configuration. The developed phase boundary design strategy paves a new way to building next generation high performance high temperature piezoceramics.

Relaxor ferroelectrics, with polar nanoregions (PNRs), are good candidates for electrocaloric (EC) refrigeration. This study investigates the EC performance of the Pb1−1.5x La x Zr0.8Ti0.2O3 relaxor ferroelectric ceramics by the direct measurement using a modified differential scanning calorimeter. All the ceramics exhibit pure perovskite phase with a dense microstructure. The results indicate that the increasing La substitution strengthens the relaxor behaviour, promotes the transition from non-ergodic to ergodic state, and enhances the thermal stability of EC property (instability η

Here, we show the large electrocaloric effect near room temperature in ergodic relaxor (Bi 1/2 Na 1/2 ) 0.93 Ba 0.07 (Zn 1/3 Nb 2/3 ) 0.01 Ti 0.99 O 3 (BNBZN) observed by a modified differential scanning calorimeter. Microstructure analysis revealed that BNBZN sample demonstrates global pseudocubic structure with locally distorted structures. Electric measurements showed that the sample coexists of two types of polar nanoregions (PNRs) with different coupling ability, and behaves as ergodic relaxor near room temperature. Due to the transition between two types of PNRs around 40 °C, large adiabatic temperature change of 1.04 °C with high electrocaloric strength of 2.1 × 10 −7 °C·m/V was recorded, indicating its promising prospect for solid-state refrigeration application.

The lead-free (1−x)(0.8Na0.5Bi0.5TiO3-0.2 K0.5Bi0.5TiO3)-xBi(Mg2/3Nb1/3)O3 ceramics were prepared using pressureless solid-state sintering for compositions x ≤ 0.4. At 120 °C, the recoverable energy storage density (Wrec) of x = 0.2 sample reached as high as 0.80 J/cm3 under the electric field of 90 kV/cm, significantly better than the other literature reported Na0.5Bi0.5TiO3 based samples. The fine-grained sample maintained high insulation resistivity (1011 Ω·m order) from 25 °C to 200 °C, which contributes to large breakdown electric field at high temperatures and leads to the excellent energy storage characteristics. This work could broaden the applications of Na0.5Bi0.5TiO3 based materials in building high-temperature multi-layer energy storage capacitor.

Designing piezoceramics with a large transduction coefficient (d33×g33) is key to improving the power generation of piezoelectric energy harvesters (PEHs). However, in popular PEH materials such as Pb(Zn1/3Nb2/3)O3-Pb(Zr1/2Ti1/2)O3 (PZN-PZT), the same trend in the variation of d33 and er makes it difficult to achieve a large d33 × g33. In this work, we introduced low-er Pb(In1/2Nb1/2)O3 (PIN) into the PZN-PZT matrix to pull down the er of the quaternary system. This simultaneously stabilized d33 in a certain range due to the refinement of the domain structure and increased the electrostrictive coefficient (Q33), and consequently the d33 × g33 value increased to 16081 × 10−15 m2/N for the optimal composition. We assembled a cantilever beam type PEH from the preferred material with a power density up to 3.37 μW/mm3 at 1 g acceleration, and demonstrated its use to charge an electrolytic capacitor to drive commercial blue LEDs and a speed sensor.

Bi0.5Na0.5TiO3 (BNT)-based lead-free materials are important for piezoelectric actuator, and several researchers have studied the effect of B-site complex ion doping on strain in (Bi0.5Na0.5)TiO3–SrTiO3. In this work, a paraelectric perovskite Sr(Zn1/3Nb2/3)O3 (SZN) with B-site complex structure was introduced into 0.80(Bi0.5Na0.5)TiO3–0.20SrTiO3 (BNTST) to investigate the phase structure and electrical properties as well as the field-induced strain behavior. The results showed that SZN substitution decreases the rhombohedrality 90-γ and induces the transition from dominant ferroelectric to nonergodic relaxor by shifting its TF-R to lower temperatures. Moreover, the field-induced ferroelectric domains cannot remain stable at room temperature when SZN substitution is large than 1.0 mol%. These behaviors induced the transition between nonergodic relaxor and ergodic relaxor, which contributed to its large strain and related properties. In this work, this material gave the largest bipolar strain of 0.43% and large normalized unipolar strain of 505 pm/V at the SZN content of 2 mol% under 8 kV/mm, and showed good temperature stability up to 100 °C. The above encouraging results may be helpful for further investigation of BNTST-based ternary systems in search of a potential Pb-free piezoelectric material.

A giant field-induced strain of 0.65% with excellent temperature insensitivity and fatigue resistance was achieved by introducing 1.5 mol.% B-site complex cations (Zn1/3Nb2/3)4+ into the 0.93(Bi1/2Na1/2)TiO3-0.07BaTiO3 lead-free ferroelectric ceramics. (Bi1/2Na1/2)0.93Ba0.07(Zn1/3Nb2/3)xTi1-xO3 (0 ≤ x ≤ 0.025) ceramics were prepared by the columbite route, their microstructure, electric properties, and field-induced strain behavior were systematically investigated. XRD analysis reveals that the B-site substitution by complex cations (Zn1/3Nb2/3)4+ induces the phase transition from R3c to pseudocubic structures, and demonstrates different phase evolution under field applied. At the composition x = 0.015, a composition-driven boundary between the nonergodic state and ergodic state is observed. Meanwhile, the substitution by complex cations (Zn1/3Nb2/3)4+ reduces the dimension of polar nanoregions (PNRs) due to the destruction of the long-range ferroelectric ordering. A phenomenological illustration based on Bokov's suggestion and Landau-Devenshire theory suggests the relationship between the size of PNRs and field-induced strain behavior.

One dimensional ferroelectric materials have important applications in the field of micro-nanoelectronic devices. In this work, three kinds of ferroelectric niobates (KNbO3, KSr2Nb5O15, K0.5Bi2.5Nb2O9) with different structures (perovskite, tetragonal tungsten bronze-TTB, bismuth niobates layer structure-BNLS) have been synthesized by using the one dimensional Nb2O5 as precursor through topochemical molten salt synthesis (TMSS) method. Among them, perovskite-KNbO3 can maintain the one dimensional morphology of the Nb2O5 template. TTB-KSr2Nb5O15 partly inherited the template morphology, but BNLS-K0.5Bi2.5Nb2O9 had the totally different morphology with the Nb2O5 precursors. Through the comparison and analysis of the crystal structure of the three target noibates, it indicated that the inheritance of template’s morphology depended on the product’s reassembly modes of the [NbO6] octahedron. Perovskite type ANbO3 (A = K, Na, Li) with the corner-shared [NbO6] octahedron was most conducive to the succession of the morphology of the Nb2O5 template in the three products. Relevant results can help us to choose templates and target products more rationally for synthesis of one dimensional nanomaterials through TMSS method.

In this work, we reported a large electrocaloric effect (ECE) in tetragonal perovskite 0.03 Bi(Mg1/2Ti1/2)O3–0.97(0.875Bi1/2Na1/2TiO3–0.125BaTiO3) ceramic. It was found that the composition demonstrates a coexisting state of the ferroelectric P4mm and relaxor P4bm. Due to the field-induced phase transition from relaxor with P4bm symmetry to ferroelectric with P4mm symmetry, a positive ECE is observed in the temperature range of 25–130 °C. Under an electric field of 4 kV/mm, a large temperature change ΔT of 0.75 °C was achieved at 80 °C, and the EC responsivity ΔTmax/ΔE of 0.19 K·cm/kV, indicating a promising lead-free material for potential application in solid-state refrigerators.

Thermal depolarization is an insurmountable obstacle to the application of piezoceramics and there is still controversy about its physical origin. In this work, taking four representative piezoceramics (0.94(Na0.5Bi0.5)TiO3-0.06BaTiO3, 0.2Pb(Zn1/3Nb2/3)O3-0.8Pb(Zr0.5Ti0.5)O3, 0.36BiScO3-0.64PbTiO3 and Ba0.85Ca0.15Ti0.9Zr0.1O3) as an example, by using a designed in-situ Berlincourt-type high-temperature d33 meter, combined with variable temperature X-ray diffraction and dielectric temperature spectrum test, it was confirmed the depolarization behavior is directly related to the structural phase transition, meanwhile it is reasonable to apply the peak value of piezoelectric coefficient with temperature, Tdp, as the depolarization characteristic temperature. More importantly, the original in-situ varied temperature d33 meter design helps to investigate the temperature dependent piezoelectric mechanism of various piezoceramics.

Based on the strong requirement for self-powered devices, energy harvesting by utilizing piezoelectric materials has recently attracted extensive attention. Transduction coefficient (d33·g33) is the core parameter of the piezoelectric energy harvesting materials, which is directly determined by the ratio of the piezoelectric charge constant (d33) to the dielectric constant (er). Unfortunately, traditional solid solution design method generally causes a simultaneous increase or decrease in both d33 and er, making it difficult to obtain a high d33·g33. In this work, a composite design strategy was proposed to separate the synergistic change of d33 and er. This was achieved by introducing lower-er ZnAl2O4 secondary phase into the popular 0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr1/2Ti1/2)O3 (PZN–PZT) perovskite matrix. Encouragingly, the er value rapidly decreased, while d33 value improved within a certain range compared to those of pure PZN–PZT. This was ascribed to the formation of fine domain structures in composites caused by the heterogeneous interfacial effect. Subsequently, the cantilever beam type piezoelectric energy harvesters (PEHs) made from the optimal composites exhibited a high power density of 4.0 μW mm−3 at 1 g acceleration. More importantly, PEHs could harvest vibrational energy from the operating motor to charge a capacitor and instantly drive wireless micro-sensors, demonstrating their potential application in self-powered electronics. Under the guide of the composite design strategy proposed in this work, more high performance piezoelectric energy harvesting materials can be built in the future.

With the rapid development of wearable devices, a highly sensitive flexible piezoelectric sensor shows tremendous potential for future demands. Here, we report a novel lead-free flexible piezoelectric sensor fabricated by a composite with chain-distributed (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 (BCZT) particles in a polydimethylsiloxane (PDMS) matrix. The BCZT particles were prepared by a conventional solid oxide process and then dispersed in PDMS oriented by the application of a dielectrophoretic alignment field. The fabricated aligned flexible composite with 8 vol% BCZT addition had a high open circuit voltage of 28.8 V (peak-to-peak), which was nearly twice as high as that of the randomly dispersed composite. This new mechanically reinforced flexible piezoelectric composite was demonstrated to resolve the problems of the weak stress transfer to piezoelectric elements and the poor dispersion of fillers in the polymer matrix. The 8 vol% BCZT/PDMS composite was incorporated into a flexible piezoelectric touch sensor, which possesses excellent sensitivity in a wide pressure range and an extraordinary cycling stability. These results indicate that the fabricated flexible piezoelectric sensor has great potential in the wireless sensor network and wearable electronic fields.

BaTiO3 microcrystals with special two-dimensional morphology have important applications in the field of piezoelectric devices. They are not only an ideal template for constructing textured ceramic...

The sol-gel process has become a rapidly growing research area in materials science. A variety of materials prepared via sol-gel routes have shown unique properties and characteristics that are difficult to achieve using conventional methods. In recent years, tremendous progress in sol-gel RD has been made not only in the world but also in China. Here, this review provides a retrospective overview of the sol-gel history in China and summarizes recent progress and applications of sol-gel research in Chinese universities, institutes, and industries. It highlights some of the recent developments published by Chinese researchers in the last 5 years, ranging from the sol-gel synthesis of nanomaterials, bulk materials, and functional coatings, to their applications in the fields of energy conversion, energy storage, photocatalysis, etc. It is evident that sol-gel technology nowadays in China has evolved into a vibrant research area both in academia and industry. Graphical abstract.

Piezoelectric energy harvesters (PEHs) with piezoceramics as the core can convert low-frequency vibration energy that is ubiquitous in the environment into electrical energy and are at the frontier of research in the field of energy. The high piezoelectric charge coefficient (d) together with the large piezoelectric voltage coefficient (g) are essential for enhancing the energy harvesting performances of PEHs working on a nonresonant state. However, conventional doping and solid solution design strategies lead to the same increase or decrease trend of d and dielectric permittivity e, making it difficult to obtain a high g value because g = d/e. Herein, exceptionally well-balanced performances of high d and large g are achieved simultaneously in modified Pb(Zr, Ti)O3(PZT)-based ceramics via a multiscale heterogeneity strategy, which involves coordination among the defect dipole, hierarchical domain, and composite. The electromechanical parameters of the optimal specimen are not only superior to those of many state-of-the-art commercial counterparts but also exhibit good thermal stability. Most importantly, the assembled PEH with the optimal specimen shows excellent variable temperature power generation characteristics. This work provides a paradigm for building PEH material through a multiscale heterogeneity strategy, expected to benefit a wide range of electromechanical coupling materials.