As a kind of important functional material, flexible piezoelectric materials can realize the effective conversion between mechanical energy and electrical energy, with the advantages of good toughness, high plasticity and light weight. Therefore, they can be attached to the human body to obtain human or environment information in real time, which is widely used in the fields of motion detection, health monitoring, and human-computer interaction. Due to high requirements of various three-dimensional (3D) structures of the flexible piezoelectric materials, additive manufacturing has been extensively utilized to fabricate different kinds of piezoelectric materials. This technology is expected to break the bottleneck of traditional processing of piezoelectric material by improving the structural design freedom and the performance of flexible piezoelectric materials, and provides enormous potential and opportunities for the application of flexible piezoelectric materials. Based on the introduction of the classification and features of flexible piezoelectric materials, this paper explained the main additive manufacturing technologies, including fused deposition modeling, direct ink writing, selective laser sintering, electric-assisted direct writing, stereolithography, and inkjet printing that commonly used in processing these materials. Then, various structural designs, such as multi-layer structure, porous structure, and interdigital structure for the flexible piezoelectric materials produced by different additive manufacturing approaches were reviewed. Moreover, the applications of additive manufactured flexible piezoelectric materials in energy harvesting, piezoelectric sensing, human-computer interaction, and bioengineering were introduced. Finally, the challenges faced by additive manufacturing on processing flexible piezoelectric materials and the development trends in the future were summarized and prospected.

Wearable instruments are functional devices that can be worn on human body, sensing, transmitting and processing body or environmental information in real time, and show broad application prospects in medical health, especially artificial intelligence, sports and entertainment. With the development of wearable instruments, various flexible sensors have emerged. Flexible mechanical sensors based on piezoelectric effect have attracted much attention because of their advantages of wide sensing frequency, fast response, good linearity, and self-power supply. However, traditional piezoelectric materials are mostly brittle ceramics and crystalline materials, which limit their application in flexible devices. With the deepening of research, more and more flexible piezoelectric materials and piezoelectric composites continue to emerge, injecting new development vitality into flexible wearable mechanical devices. This article mainly summarizes the cutting-edge progress of flexible wearable piezoelectric devices, including piezoelectric principle, preparation and performance improvement methods of flexible piezoelectric materials. In addition, the main application directions of flexible wearable piezoelectric devices, including medical health and human-computer interaction, as well as the challenges and opportunities encountered, are summarized.

Calcium bismuth niobate (CaBi₂Nb₂O₉) is a typical bismuth layered structure piezoelectric material with high Curie temperature (about 943 ℃) and high stability, which is an important candidate functional element for high temperature vibration sensors above 600 ℃. However, its low piezoelectric coefficient and high temperature resistivity seriously limit the signal acquisition of high-temperature piezoelectric vibration sensor. To improve the comprehensive performance, in this work, W/Cr co-doped CaBi₂Nb_1.975W_0.025O₉-x%Cr₂O₃ (CBNW-xCr, 0<x≤0.2) Aurivillius phase ceramics were prepared via conventional solid-state sintering route. The effects of W/Cr co-doping on the crystal structure and electrical properties of CBN piezoelectric ceramics were investigated. The results show that co-doping of W/Cr elements transforms crystal structure of the ceramics from orthorhombic to tetragonal crystal system, enhances distortion of the crystal structure, and significantly improves piezoelectric and insulating properties of the piezoelectric ceramics. When x=0.1, the Curie temperature is 931 ℃, the piezoelectric coefficient is 15.6 pC/N, the resistivity reaches the order of 10⁶ Ω∙cm at 600 ℃, and the dielectric loss is only 0.029, which endows the system an important potential application in the field of high-temperature piezoelectricity.

Antiferroelectric (AFE) materials exhibit great potential in the application of high-performance dielectric energy storage capacitors due to their electric field-induced AFE-ferroelectric (FE) phase transition. However, the large hysteresis of field-induced phase transition makes it difficult to simultaneously achieve high energy-storage density (W_rec) and efficiency (η) for AFEs. This work improved the energy-storage performance of NaNbO₃-based lead-free AFE ceramics by introducing the third group Bi(Mg_0.5Ti_0.5)O₃ into 0.76NaNbO₃-0.24(Bi_0.5Na_0.5) TiO₃ to regulate its relaxation characteristics. Novel lead-free AFE ceramics, (0.76-x)NaNbO₃-0.24(Bi_0.5Na_0.5)TiO₃-xBi(Mg_0.5Ti_0.5)O₃, were prepared by a traditional solid-state reaction method. Their phase structure and microstructure as well as dielectric, energy-storage, and charge-discharge characteristics were studied. The results indicated that introduction of Bi(Mg_0.5Ti_0.5)O₃ obviously enhanced the dielectric relaxor behavior of the matrix without changing its AFE R-phase structure, which led to the significantly reduced polarization hysteresis. Especially, a linear-like polarization-field hysteresis loop with extremely-low hysteresis was obtained in the composition of x=0.050. At the same time, microstructure of the ceramic was effectively optimized, its dielectric constant decreased, and its breakdown strength had significant enhanced. As a result, a high W_rec=3.5 J/cm³ and a high η=93% were simultaneously achieved under a moderate electric field of 30 kV/mm in the x=0.050 ceramic. Moreover, the x=0.050 ceramic also exhibited excellent charge-discharge characteristics with a high-power density P_D=131(1±1%) MW/cm³, a high discharge energy density W_D=1.66(1±6%) J/cm³ and a fast discharge rate t_0.9<290 ns at 20 kV/mm. The charge-discharge properties maintained good stability within a wide temperature range of 25-125 ℃. These results indicate that 0.71NaNbO₃-0.24(Bi_0.5Na_0.5)TiO₃-0.050Bi(Mg_0.5Ti_0.5)O₃ ceramics can be expected to be applied in high-power energy-storage capacitors.

Dielectric thin film, one of the materials of which storage energy in the form of electrostatic field via dielectric polarization, can be widely used in electric equipment, due to their high power density and high charge/ discharge efficiency. Currently, the dielectric energy storage films perform lower energy density and weak temperature stability. In this work, 0.9BaTiO₃-0.1Bi(Mg_1/2Ti_1/2) O₃(0.9BT-0.1BMT) ferroelectric thin films were prepared via a Sol-Gel method on Pt/Ti/SiO₂/Si substrates and annealed in the range of 700-900 ℃ to realize high energy storage density and wide-temperature stability by introducing BMT. The effect of annealing temperature on phase composition and microstructure was investigated. The results show that denseness of thin films reduce obviously when the annealing temperature is over 750 ℃ and their grain size increases gradually with the increase of treatment temperature. Additionally, the thin films annealed at 750 ℃ display optimized comprehensive feature: room-temperature dielectric constant of ~399, loss tangent of ~5.79% at 1 kHz, and ∆C/C_{25 ℃} ratio only within ±13.9%. Meanwhile, relaxor value, γ≈1.96 calculated according to Currie-Weiss law consolidates that the thin films possess obvious relaxor characteristics. Results of energy storage shows that the max value of W_rec is ~ 51.9 J/cm³, and the τ_0.9 is below 15 μs at pulse charge measure. Moreover, results of temperature stability measurement show W_rec>20 J/cm³, η>65% (1600 kV/cm) and τ_0.9<7.2 μs from room temperature to 200 ℃, demonstrating that the film still exists high and stable energy storage under high temperature. Therefore, the ferroelectric thin film 0.9BT-0.1BMT prepared in this work has a promising applications in energy storage under high temperature environment.

In recent years, pressure sensors have been widely applied in the fields of smart wearable textile, health detection, and electronic skin. The emergence of the two-dimensional nanomaterial MXene has brought a brand-new breakthrough for pressure sensing. Ti₃C₂T_x is the most popular studied MXene in the field of pressure sensing and shows good mechanical, electrical properties, excellent hydrophilicity, and extensive modifiability, enabling it an ideal material for pressure sensing. Hence, researchers have conducted a lot of explorations and studies on design and application of MXene in pressure sensors in recent years. Herein, the preparation technologies and antioxidant methods are summarized. Design of MXene-based microstructures is also introduced, including aerogels/porous structural materials, hydrogels, flexible substrates, and films, which are beneficial to improve the response range, sensitivity, and flexibility of pressure sensors, and promote the rapid development of pressure sensors. The mechanisms of MXene pressure sensors are further broached, including piezoresistive, capacitive, piezoelectric, triboelectric, battery typed and nanofluidic. MXene has been applied in a wide range of sensors for various mechanisms due to its excellent characteristics. Finally, the chance and challenge in the synthesis, properties, and pressure sensing performance of MXene materials are prospected.

Development of 5 G wireless communication and low-frequency radar detection has made low- frequency electromagnetic wave radiation a serious problem today. Although research on medium and high frequency band absorbing materials is now relatively mature, designing low frequency band absorbing materials remains a major challenge. Here, we designed a low-band composite absorbing material of 0.5-3 GHz based on the quarter-wavelength cancellation mechanism. A CIPs@Mn_0.8Zn_0.2Fe₂O₄-CNTs ternary composites were prepared by using a simple one-step hydrothermal method, which involved growing ferrite on the surface of carbonyl iron powder and carbon nanotubes. The influence of carbon nanotube content on the absorption peak frequency of the material was investigated. Experimental results show that carbon nanotubes enhances the material's attenuation coefficient by introducing additional interfacial polarization, dipole polarization and other loss mechanisms. Furthermore, coupling of high dielectric and high permeability enables the material to achieve better impedance matching in the low frequency band based on the quarter-wavelength cancellation mechanism. At a thickness of 4 mm, the reflection loss of the samples was obtained at 2.11 GHz and 1.75 GHz, with a -10 dB bandwidth of 1.70-2.70 GHz and 1.40-2.20 GHz, respectively. The composites exhibit excellent low-frequency absorption performance, endowing it highly suitable for applications helped by its simple preparation process and well low-frequency absorption. This research provides a new method for developing more effective low-frequency absorbing materials.

Antiferroelectric materials have been extensively studied in the field of dielectric energy storage due to their ultra-high power density. Lead zirconate (PbZrO₃, PZO), as a prototype of antiferroelectric material, has been one of the most studied antiferroelectric materials, and research on enhancing energy storage performance of PZO-based materials is a hotspot of the current study. In this work, further improvement of the energy storage performance of PZO-based antiferroelectric thin films was realized by further doping small-radius Sr²⁺ into the A-site of the PZO perovskite structure on the basis of La³⁺-doped PZO. A series of antiferroelectric thin films of A-site La/Sr co-doped Pb_0.94-xLa_0.04Sr_xZrO₃ (Sr-PLZ-x, x = 0, 0.03, 0.06, 0.09, 0.12) were prepared by Sol-Gel method, and the effects of Sr²⁺ doping on the crystal structure and electrical properties such as ferroelectricity, energy storage, and fatigue properties of Sr-PLZ-x antiferroelectric films were systematically investigated. The results show that with the doping of Sr²⁺, the lattice constants are gradually reduced, and the saturation polarization of the films is first slightly increased and then maintained, but finally gradually decreased. The tolerance factors of Sr-PLZ-x films are reduced with increasing Sr²⁺ doping content, while the antiferroelectricities of the films are enhanced. Both the switching field and the breakdown strength are increased, resulting in an improved energy storage performance of Sr-PLZ-x films. At x=0.03, the energy storage performance of Sr-PLZ-x antiferroelectric film reaches the highest, with the energy storage density and efficiency are 31.7 J/cm³ and 71%, respectively. Meanwhile, the doping of Sr²⁺ also makes the fatigue characteristics of Sr-PLZ-x antiferroelectric films further improved. The x=0.12 antiferroelectric film exhibits only 3.4% and 2.7% degradation in energy storage density and energy storage efficiency after 10⁷ cycles. In summary, the method of A-site La/Sr co-doping can effectively improve the energy storage performance of PZO-based antiferroelectric films.

Wireless passive devices based on surface acoustic wave (SAW) technology are the firstly selected sensors in extreme conditions, and high temperature stability of piezoelectric substrates is the key factor limiting the performance of SAW devices. Langatate (LGT) crystal is an ideal high temperature piezoelectric substrate for SAW devices due to high resistivity and stability. The high temperature resistivity of pure LGT and aluminum- doped langatate (LGAT) crystals in oxygen, nitrogen and argon atmosphere were characterized, and the high temperature full matrix material coefficient of pure LGT crystal was characterized by ultrasonic resonance spectroscopy (RUS) technology. The results show that conductive behavior of LGT crystal under high temperature were significantly varied when tested in different atmospheres. The pure LGT crystal in nitrogen has the highest resistivity in the temperature range of 400-525 ℃, and in argon has highest resistivity between 525 ℃ and 700 ℃, with resistivity up to 2.05×10⁶ Ω·cm at 700 ℃. However, LGAT crystal in nitrogen has the highest resistivity in the whole test temperature range, with a resistivity of 1.12×10⁶ Ω·cm at 700 ℃, compared to pure LGT crystal. The elastic and piezoelectric properties of LGT crystal are very stable from room temperature to 400 ℃ according to RUS analysis results. As the temperature rises, the elastic coefficient decreases slightly, while the piezoelectric coefficient d₁₁ is remained almost unchanged. In conclusion, LGT crystal has very high resistivity and stability at high temperature so that it is suitable to be used as piezoelectric substrate for fabricating high temperature piezoelectric devices, shedding light on the design and fabrication of LGT-based high temperature piezoelectric devices.

BiFeO₃-BaTiO₃ (BF-BT) ceramics possess both high Curie temperature and excellent piezoelectric properties, and have a quite wide application prospects in high-temperature piezoelectric sensors and actuators. However, the resistivity of BF-BT ceramics is too low at high-temperature, which can lead to deterioration or even failure of the device's high-temperature performance. Therefore, improving the resistance performance of BF-BT ceramics is the key issue that must be addressed before its application. However, as a type of ferrite, it is difficult to improve resistivity through conventional methods, such as doping modification and optimizing sintering system. In this work, an abnormal increase in resistivity was discovered in BF-BT ceramics, which was confirmed to be related to the second phase Bi₂₅FeO₄₀. Microstructural analysis shows that the second phase has a special layered periodic structure, in which every three rows of atoms constitute a period, and most defects concentrate in one layer of atoms. The pure Bi₂₅FeO₄₀ was successfully synthesized using traditional solid phase method and introduced as an additive into the 0.70BF-0.30BT component, which can increase the resistivity at 300 ℃ from 1.03 MΩ·cm to 4.33 MΩ·cm. In addition, the results of COMSOL simulation confirm that introducing this second phase can increase the resistivity of the 0.67BF-0.33BT component by one order of magnitude. According to the energy filtering effect, this special structure with high energy barriers can prevent carrier migration and improve the resistivity of BF-BT ceramics. This work provides a practical and feasible method for improving the resistivity of BF-BT ceramics.

The perovskite-type oxynitride with AB(O,N)₃ formula is a new type of functional ceramic materials, which have unique dielectric/magnetic/photocatalytic properties and prospective applications in the field of energy storage and conversion. However, the traditional preparation process takes a long time and the product purity is low. In this study, SrTa(O,N)₃ ceramic powder was synthesized and densified by a pressureless spark plasma sintering equipment with urea as nitrogen source and metal oxides as precursors. Effects of heating rate and synthesis temperature on the composition and microstructure of the powder were deeply investigated, and the dielectric properties of the optimized ceramic bulks were characterized. The results show that higher heating rate and moderate synthesis temperature are beneficial to sufficient nitridation, while the SrTa(O,N)₃ powder prepared at 100 ℃/min and 1000 ℃ possesses the highest purity (~97% oxynitride phase content) with a particle size distribution of 100-300 nm. Elements of Sr, Ta, O and N are evenly distributed. The optimized densification process is firstly sintering at 1300 ℃, with heating rate of 300 ℃/min, and dwelled for 1 min. After sintering, the density of SrTa(O,N)₃ ceramic pellet can reach >94% with a high purity. Dielectric constant and loss tangent of the material are 8349 and 10^-4 level at 300 Hz, respectively, which are superior to that reported in the literature. The high dielectric constant prepared in this study is closely related to standard density and purity, because the existence of pores and impurities can reduce the dielectric constant of materials. Therefore, high density and purity are the key factors to obtain excellent dielectric properties of SrTa(O,N)₃ oxynitride ceramics.

The fracture properties of ferroelectrics directly determine their processability and reliability of devices made of them. However, both experimentally and theoretically reported fracture toughness of piezoelectric ceramic materials remains nearly the same as that reported 30 years ago, limiting the application of piezoelectric devices in situation where high reliability is required. Here, we try to reveal the parameters that could be used to optimize the fracture performance of ferroelectrics. Specifically, stress-strain curves, intrinsic fracture toughness and long-crack fracture toughness of three typical PZT ceramics were measured by uniaxial compression method, crack-tip opening displacement (COD) technique and single-side V-notch beam (SEVNB) technique, respectively. It is shown that the intrinsic fracture toughness is positively correlated with the Young’s modulus of the material, which suggests that improving the Young’s modulus of ferroelectrics is an effective way to improve their intrinsic fracture toughness. The long-crack fracture toughness is related to the intrinsic toughness and extrinsic ferroelastic domain switching/phase transformation toughening, which also suggests that optimizing the ferroelastic switching behavior of piezoelectric ceramics can improve their extrinsic effect. Compared to the hard doped PZT, the soft doped PZT has low coercive stress, high remanent strain and high shielding toughness. The fracture patterns observed in different PZT materials are related to the different ferroelastic switching behavior of the materials. Soft PZT ceramics exhibit intergranular fracture, while hard PZT with weak ferroelastic switching behavior exhibits transgranular fracture. In conclusion, fracture toughness of ferroelectrics is enhanced by optimizing Young’s modulus and toughening of ferroelastic switching.

Ferroelectric superlattices are artificial film materials with layered periodic structure formed by an alternate growth of two or more ferroelectric materials or non-ferroelectric materials at unit cell scale. Ferroelectric superlattices can exhibit excellent ferroelectric, piezoelectric, dielectric, and pyroelectric properties due to the existence of a large number of heterogeneous interfaces and the remarkable interface effect, and even show new functional properties that are not available in their constituent materials. Therefore, ferroelectric superlattices not only provide an ideal platform for studying interactions between charges and lattices at the interface of complex oxide materials, but also play an indispensable role in the next generation of integrated ferroelectric devices. With the development of preparation and characterization methods, researchers can design and control the microstructure and chemical composition at atomic scale to improve the functional properties of ferroelectric superlattice thin films. Ferroelectric polarization is the most basic property of ferroelectric film materials. In addition to being used for information storage devices, ferroelectric polarization also plays an important role in regulating the energy conversion performance of integrated ferroelectric devices such as piezoelectric devices, photovoltaic devices and electrocaloric devices. Therefore, the ferroelectric polarization intensity of ferroelectric superlattices directly determines their functional characteristics and practical application value of integrated ferroelectric devices composed of them. In this short review paper, we firstly introduced the structural characteristics, classification and several typical functional characteristics of ferroelectric superlattices, and then focused on several factors affecting the polarization performance of ferroelectric superlattices based on recent research results, including strain effect, electrostatic coupling effect, defect effect, and period thickness. Finally, we looked forward to the future research directions in ferroelectric superlattices to provide reference for the research in this field.

Film capacitors are the core electronic components of modern power devices and electronic equipment. However, due to the low dielectric constant, it is difficult to obtain high energy storage density (effective energy storage density or discharged energy density) for present film capacitors, leading to a large device size and high application cost. To improve the energy storage density of film capacitors, a nanocomposite approach is an effective strategy via combining high dielectric constant of the ceramic nanoparticles with high breakdown strength of the polymer matrix. Nevertheless, for single-layer structure of 0-3 polymer/ceramic composites, the dielectric constant and breakdown strength are difficult to be effectively enhanced at the same time, which limits the further improvement of energy storage density. To solve this contradiction, researchers have combined the composite film with high dielectric constant and high breakdown strength in a superposition to prepare 2-2 type multilayer composite dielectrics, which can achieve synergistic regulation of polarization strength and breakdown strength to obtain high energy storage density. The optimization of electric field distribution and the synergistic regulation of dielectric constant and breakdown strength can be achieved through mesoscopic and microstructural modulation of multilayer composite dielectrics. In this paper, the research progress of multilayer polymer-based composite dielectrics including ceramic/polymer multilayer structure and all-organic polymer multilayer structure in recent years is reviewed. Effect of multi-layer structure control strategy on the improvement of energy storage performance is emphasized. Moreover, enhancement mechanism of energy storage performance of polymer-based multilayer structure composite dielectric is summarized. Finally, challenges and development directions of multilayer composite dielectrics are discussed.

Compared with other electric energy storage devices, dielectric capacitors made of dielectric composites have great advantages in fast charging and discharging capacity with high power density. A dilemma of improving the energy density of dielectric composites and synchronous optimizing their breakdown performance is becoming an intriguing research direction. To further adjust the contradiction between dielectric constant and dielectric breakdown performance, here a finite element numerical simulation based on dielectric breakdown model (DBM) was proposed to study the effect of the distribution of inorganic fillers on the electric field and breakdown damage morphology in flexible polydimethylsiloxane(PDMS) based dielectric composite system. The results show that a large dielectric difference is observed between filler and matrix, which indicates that polymer matrix with a large dielectric constant or inorganic filler with a small dielectric constant can realize reducing the size of the high electric field area at the interface and improving the breakdown resistance of the material. This study further reveals that the more dispersed structure of inorganic fillers, the more likely its dendritic damage channels tend to branch, indicating that this situation is conducive to the increase of damage sites of dielectric breakdown dendritic damage channels, the decrease of damage rate, and the improvement of breakdown resistance of materials. All above data demonstrate that this study provides certain guidance for the development of organic-inorganic dielectric composites with both high energy storage and excellent breakdown performance.

PbTiO₃ (PTO) is an important ferroelectric functional material, but its structure, stability, mechanical property, and thermodynamic property under pressure is still unknown, leading to restriction in applying in the field of electronic communication. Here, first-principles calculations based on density functional theory was performed to study the structure and thermal properties of pre-perovskite phase PbTiO₃ (PP-PTO), ferroelectric tetragonal phase PbTiO₃ (TP-PTO), and paraelectric cubic phase PbTiO₃ (CP-PTO) under pressure. It is found that their compressibility in descending order is PP-PTO>TP-PTO>CP-PTO. Under considered pressure, three PTO phases have not undergone a phase transition analyzed by band structure and density of states, and their band gap gradually decreases with increasing pressure. Among them, the TP-PTO changes from an indirect to a direct band gap semiconductor at 20 GPa, while the others remain a direct band gap semiconductor. Those PTO phases are mechanically stable and anisotropy from 0 to 30 GPa. Furthermore, their comprehensive mechanical properties increase and anisotropy firstly decreases and then increases with increasing pressure. Analysis based on quasi- harmonic Debye approximation theory was performed to study the influence of temperature and pressure on Debye temperature, entropy and heat capacity. The results illuminate that Debye temperature decreases with temperature increase, nevertheless, pressure has the opposite effect, which elucidates that the order of covalent bond from strong to weak is CP-PTO>TP-PTO>PP-PTO. Entropy and heat capacity of PTO increase with rising temperature, but decrease with the increase of pressure.

BaTiO₃ has a wide range of applications in microelectromechanical systems and integrated circuits due to its excellent dielectric, ferroelectric, piezoelectric, and pyroelectric properties. For the applied research and device applications of BaTiO₃ films, reducing its deposition temperature to be compatible with the CMOS-Si technology is an important Challenge. Here, with the help of a LaNiO₃ buffer layer which has a closely-matched lattice with BaTiO₃, (001)-textured BaTiO₃ films were sputter-deposited at 450 ℃ on single crystalline Si(100) substrates, which consisting of well-cryotallized, evenly-distributed columnar nanograins with an average grain size of 27 nm. Our result showed that this deposition temperature can maintain the columnar nanograin structure with a relatively large grain size, leading to a good ferroelectric performance. In addition, a small residual strain on Si was also helpful to improve its ferroelectric and dielectric properties. The remnant polarization and saturated polarization of these BaTiO₃ films reached 7 and 43 μC·cm^-2, respectively, while leakage current densities were as low as 10^-5 A·cm^-2 at an applied electric field of 0.8 MV·cm^-1. These BaTiO₃ films also displayed excellent frequency stability with a low dielectric loss in which relative dielectric constant measured to be ~155 at 1 kHz, slightly being reduced to ~145 after increasing the frequency to 1 MHz. Meanwhile, the dielectric loss slightly increased from 0.01 at 1 kHz to 0.03 at 1 MHz. Lastly, through capacitance-voltage (C-V) tests, these films exhibited a large dielectric tunability of~51% and a figure of merit (FOM) of ~17 (@1 MHz). These films have a good potential for applications in tunable dielectrics.

Piezoelectric actuators have advantages of fast response, high positioning accuracy, small size, and have received widespread attention in the field of precision drives. Lead-based piezoelectric actuators occupy the main commercial market. To avoid the use of the harmful element lead, a lead-free piezoelectric materials and actuators must be developed. Among them, bismuth sodium titanate (Bi_0.5Na_0.5)TiO₃ (BNT), was reported but it has some disadvantages of higher driving voltage, larger hysteresis, and poor temperature stability. To optimize the strain performance of the lead-free actuator, this study adopted the solid-phase reaction method to prepare (1-x) {0.76(Bi_0.5Na_0.5)TiO₃- 0.24SrTiO₃}-xNaNbO₃ (BNT-ST-xNN, x=0-0.03) lead-free ferroelectric ceramics, systematically studied its field- induced strain performance. The results show that when x=0.01, the strain value of the ceramic can reach 0.278% under a low electric field (E = 4 kV/mm), and the piezoelectric coefficient d^*₃₃ is as high as 695 pm/V. Meanwhile, the ceramic is at the non-ergodic/ergodic relaxation phase boundary, and the electric field induced relaxor-ferroelectric phase transition leads to large field-induced strain. Compared with x=0.01, the strain value at x=0.02 is 0.249%, which is slightly reduced, but the hysteresis is significantly reduced to 43% of the comparator. In addition, the strain remains stable in the temperature range of 25-100 ℃. This study shows that introduction of SrTiO₃ and NaNbO₃ into BNT can increase the field-induced strain value while maintaining a low driving voltage and good temperature stability, indicating suitable for the development of piezoelectric actuators.

0.96NaNbO₃-0.04CaZrO₃(NNCZ) ceramic shows stable double hysteresis loops at room temperature, but the property of energy density, energy storage efficiency and breakdown strength of NNCZ are terrible, which limit NNCZ to be used as energy storage materials. In this work, Fe₂O₃ was chosen to modify the energy storage property of NNCZ. (0.96NaNbO₃-0.04CaZrO₃)-xFe₂O₃ (NNCZ-xFe) antiferroelectric ceramics were prepared by traditional solid reaction method. The phase, morphology, dielectric property and energy storage property of NNCZ-xFe were characterized. The results indicated that the crystal structures of NNCZ-xFe ceramics were pure perovskite structure. The sintering temperature of NNCZ ceramic was decreased with addition of Fe₂O₃. With the increase of Fe₂O₃ content, the grain size of NNCZ-xFe were decreased firstly and then raised. The NNCZ-0.02Fe ceramic obtained the smallest grain size (5.04 μm) and the best energy storage property. The breakdown strength of NNCZ-0.02Fe was 230 kV/cm at room temperature (RT). The recoverable energy density and energy storage efficiency before breakdown were 1.57 J/cm ³and 55.74% respectively. At 125 ℃ and 180 kV/cm, the energy density of NNCZ- 0.02Fe was 4.53 J/cm ³. Fe₂O₃ doping decreased the sintering temperature of NNCZ ceramics, reduced the the migration rate of oxygen vacancies and inhibited the growth of grains. At the same time, it reduced the dielectric loss and improved the breakdown strength. The oxygen vacancies pinning made antiferroelectric phase switch to ferroelectric phase harder, avoided appearance dumbbell-shaped double hysteresis loops, so the energy storage efficiency was improved. This research shows that NNCZ-xFe has a good potential application in the field of dielectric energy storage.

Zr and Ca co-doped brium titanate (BCTZ) based ceramics have attracted much attention due to their excellent dielectric and piezoelectric properties, which are potential candidates for lead-free piezoelectric materials. However, the piezoelectric property of BCTZ-based ceramics are still lower than that of commercial lead-based ceramics. Therefore, Sn elements are doped in (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1-xSn_x)O₃ (x = 0.02-0.07) ceramics to improve their piezoelectric property in this work. First, all ceramics exhibit a pure perovskite structure without any secondary phase. Furthermore, orthorhombic and tetragonal phases coexist at room temperature for all counterparts. Second, all ceramics display a dense microstructure. And the grain size gradually increases with Sn content increasing. The crystal structure of x=0.04 ceramic locates in morphotropic phase boundary. Meanwhile, the x=0.04 ceramic exhibits the most dense microstructure. Hence, x=0.04 ceramic has the best piezoelectric property with d₃₃ = 590 pC•N ^-1, k_p = 52.2%, tanδ = 0.016, ε ^T₃₃= 5372, d ^*₃₃= 734 pm•V ^-1, and IR = 57.8 GΩ•cm. This work demonstrates that the Sn-doped BCZT based lead-free piezoelectric ceramics could be applied in transducers, electromechanical sensors and actuators.

Potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) based ceramics can be widely used for pulsed power systems due to their fast charge-discharge rate, high transparency, wide range of working temperature, and long cycle life. Improving the electrical and optical property of KNN-based ceramics through modification is a research hotspot in this field. 0.825(K_0.5Na_0.5)NbO₃-0.175Sr_1-3x/2La_x(Sc_0.5Nb_0.5)O₃ (x=0, 0.1, 0.2, 0.3) (0.825KNN- 0.175SLSN) ceramics were synthesized by solid state method. The effect of La₂O₃ doping on the phase structure, microstructure, optical property, dielectric property, ferroelectric property and energy storage property of the ceramic was studied. The results indicated that the structure of 0.825KNN-0.175SLSN ceramics is pseudo-cubic phase with high symmetry. With increment of La₂O₃ content, the average grain size of 0.825KNN-0.175SLSN ceramics decreased, and the phase transition temperature (T_m) and saturation polarization intensity (P_max) increased and then decreased. 0.825KNN-0.175SLSN ceramics exhibit excellent transparency at x=0.3, the transmittance in the visible wavelength (780 nm) and near-infrared wavelength (1200 nm) ranges reaches 65.2% and 71.5%, respectively. The dielectric breakdown strength of 310 kV/cm and a recoverable energy density of 1.85 J/cm ³ are achieved at x=0.3.

LaNiO₃ (LNO), as a promising material in ferroelectric super lattices, super conductive heterostructures and catalysts has recently attracted great interest. Herein, a facile and low-cost polymer assisted deposition (PAD) method is established to prepare epitaxial LNO thin films on (001) orientated SrTiO₃ (STO) with excellent conductivity. Various structural and electrical characterizations of the film were investigated. The film has good crystallinity with a full-width at half-maximum value of 0.38° from the rocking curve for the (002) reflection. High resolution XRD φ-scans further confirmed the heteroepitaxial growth of LNO film on STO substrate. There are four peaks separated by 90°, showing that the LNO thin film is cubic-on-cubic grown on STO substrate. In-situ high temperature XRD measurement showed epitaxial growth of LNO thin film on STO substrate. Metal cations could be released orderly on the monocrystalline substrate and epitaxial crystallization occurs after decomposition of polymer. XPS results indicated that LaNiO₃ thin film fabricated by PAD was stoichiometric without oxygen vacancy. The atomic force microscopy analysis showed that the smooth surface with root-mean-square surface roughness was 0.67 nm. The resistivity as functions of temperature revealed that it has good conductivity from 10 K to 300 K. All results demonstrate that the LaNiO₃ thin films deposited by PAD have better comprehensive performance, indicating that PAD method has great potential for preparing epitaxial functional thin film materials.

The electrocaloric effect refers to the phenomenon of the temperature change of a material caused by the application or removal of an electric field, and includes two types, positive electrocaloric effect and negative electrocaloric effect. As a high-efficiency, noise-free and environment-friendly refrigeration effect, the electrocaloric effect shows promising application in the field of solid-state refrigeration, especially in integrated circuit refrigeration, and has attracted extensive research interests over the past decades. It is reported that the cooling capacity of the electrocaloric effect can be significantly improved by combining the positive and negative electrocaloric effects. However, different from the widely studied positive electrocaloric effect, the research on negative electrocaloric effect is relatively rare due to its complex physical origin. This article focuses on the latest research progress of the negative electrocaloric effect in antiferroelectric materials. The specific content includes the following four parts. Firstly, starting from the research history of the electrocaloric effect, the principle of refrigeration of the electrocaloric effect is introduced, as well as a typical dual refrigeration cycle that can combine positive and negative electrocaloric effect. Secondly, the indirect measurement method of negative electrocaloric effect based on Maxwell relationship, and several direct measurement methods of negative electrocaloric effect, and the suitable applying conditions as well as the advantages and disadvantages of different methods are discussed. Thirdly, taking antiferroelectric which is a typical negative electrocaloric effect materials as an example, the physical origin of negative electrocaloric effect is narrated. Then the recent progress of negative electrocaloric effect in the antiferroelectric film and antiferroelectric bulk materials is reviewed. In addition, the negative electrocaloric effect in some other ferroelectric materials is also briefly introduced. Finally, a summary and prospect of the research on the negative electrocaloric effect are put forward.

Multiferroic material is one of hot spots in the materials research area which can be widely used in many new functional devices. Barium titanate (BaTiO₃, BTO) has attracted many interests for its multiferroic properties, such as ferroelectricity, high dielectric constant and electro-optical properties at room temperature. The BaTi_0.94(TM_1/2Nb_1/2)_0.06O₃ (TM=Mn/Ni/Co) ceramic samples were prepared by solid state reaction method, and their structure, electrical, magnetic, and optical properties were systematically studied. The crystal structure of all doped samples changes from tetragonal to cubic phase without any hexagonal phase depending on ionic radius. Weakening of Raman scattering peaks of BTO tetragonal phase further proves the phase transition to cubic phase caused by doping. The Curie temperature (T_C) has a dramatic decrease with the dopant as the phase transition from tetragonal phase to cubic phase. Although the ferroelectricity is weakened, it is still existed. The magnetic measurement suggests that Ni-Nb doped sample has the strongest ferromagnetism among different dopants which can be deduced by the F-center exchange (FCE) theory. Furthermore, energy gaps of BaTi_0.94(TM_1/2Nb_1/2)_0.06O₃ are obviously reduced compared to that of BTO, which can be reasonably explained by impurity level and band theory. These results indicate that BTO based multiferroic ceramics with ferroelectric and ferromagnetic coexisting at room temperature can be obtained by B-site co-doping, which can be expected to be widely used in multiferroic functional devices.