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This research explored the influence of Nb5+ doping on the electrical properties and microstructure of [0.852(Bi0.5Na0.5)-0.11(Bi0.5K0.5)-0.038Ba]Ti(1-x)NbxO3 (designated as BNKT-BT). The BNKT-BT samples were fabricated and the XRD results showed that the doping of Nb5+ ions induced a transformation from the coexistence of both rhombohedral and tetragonal phase to a single pseudo cubic phase. Also, the doping of Nb5+ ions caused the temperature of ferroelectric state-ergodic relaxor state transition to decrease below the room temperature (RT). The SEM observations showed that the Nb5+ doping had a minor effect on the grain size of BNKT-BT samples. In addition, the samples achieved a large electrostrain of 0.43% (d*33 = Smax/Emax = 614 pm/V) under 7 kV/mm at RT. Moreover, the aging process employed at 80 °C for two weeks in air could effectively increase the electrostrain from 0.38% to 0.41% under 6 kV/mm (d*33 = 683 pm/V) and reduce the strain hysteresis from 57% to 34%. These results indicated that the B site doping, combined with the aging process, could provide an effective way to enhance the electrostrain performance of lead-free piezoelectric ceramics.

A series of HfO2-modified (Bi0.475Na0.405K0.055Li0.015)0.95Ba0.05Ti(1-x)HfxO3 (x = 0–0.015) lead-free ceramics were prepared. Results of detailed characterizations showed that the addition of HfO2 had little influence on the grain size, and all of the samples showed the pure perovskite structure. As the Hf4+ content increases, the samples changed from the normal ferroelectric phase to the ergodic relaxor state at room temperature, and the ceramics with x = 0.01 showed a high electro-strain of 0.295% at 6 kV/mm. Therefore, this study provides a new insight into of how the HfO2 addition influences the ferroelectric property of the BNT-based ceramics by B-site substitution.

Defect dipoles in acceptor-doped perovskite-based ferroelectric ceramics play an important role in piezoelectric properties. In this work, the acceptor, such as Fe2+ and Cu+, -doped (K0.5N0.5)NbO3 (KNN) ceramics are fabricated in reduced atmosphere by the conventional solid-state reaction method. By tailoring the strongly polarized defect dipoles through poling and aging processes, ultrahigh electro-strain and large signal piezoelectric d33* values can be achieved. The electro-strains of 0.41% and 0.50% with d33* ~820 pm/V and ~1000 pm/V are obtained in Fe2+-doped and Cu+-doped KNN samples, respectively, which are 4-fold and 5-fold larger than that of pure KNN. Of particular interest is that the electro-strain and d33* values are found to improve with increasing temperature up to 140°C, accompanied with low strain hysteresis of 10%-20%. A detailed mechanism considering the interaction between defect dipole polarization and spontaneous polarization after poling and aging processes is proposed to explain the observed phenomena, which provides a good paradigm for achieving high piezoelectric response by defect engineering in perovskite-based ferroelectric ceramics.

In this study, plate-like Na0.5Bi0.5TiO3 (BNT) templates with perovskite structure were obtained by two-step molten salt synthesis (MSS) method at a low temperature. Firstly, Bi4Ti3O12 precursors were synthesized at 1030 °C in NaCl–KCl molten salt. Secondly, plate-like Na0.5Bi0.5TiO3 particles with perovskite structure were obtained from plate-like layer-structured ferroelectric ceramic of Bi4Ti3O12 by topochemical microcrystal conversion method. Result showed that excessive Na2CO3 was beneficial to facilitate the low temperature synthesis. In the case of an excess of 30 mol% Na2CO3, plate-like BNT particles could be obtained by synthesis at temperatures ranging from 760 °C to 800 °C, which indicated a flexible processing route. Also, it has been observed that plate-like BNT particles show a high aspect ratio with 1 μm in thickness and 10–20 μm in length. These Na0.5Bi0.5TiO3 plate-like particles can be good candidates for the preparation of lead-free BNT-based piezoelectric ceramics with oriented grain microstructure.

Flexible and free-standing electrode materials are prerequisite and key components for next-generation flexible energy storage and conversion devices. However, it is still a chanllenge to fabricate these materials from a continuous, straightforward, and facile method. Herein, we report a flexible composite film with silicon oxides decorated on few-walled carbon nanotubes, which can be continuously fabricated and directly drawn from the hot zone of the reactor. The composite film can be readily used for electrochemical lithium ion storage with high and reversible specific capacity, good rate capability, and excellent cycling performance. These exceptional characteristics make it very promising for flexible energy storage and other emerging applications.

A unique flexible NiFe2O4 hollow sphere@S/rGO–CNT (NiFe2O4@S/C) cathode was rationally designed and synthesized to tackle the issues of lithium–sulfur batteries. In this strategy, the introduced rG...

As a new-era of energy harvesting technology, the enhancement of triboelectric charge density of triboelectric nanogenerator (TENG) is always crucial for its large-scale application on Internet of Things (IoTs) and artificial intelligence (AI). Here, a microstructure-designed direct-current TENG (MDC-TENG) with rationally patterned electrode structure is presented to enhance its effective surface charge density by increasing the efficiency of contact electrification. Thus, the MDC-TENG achieves a record high charge density of ~5.4 mC m−2, which is over 2-fold the state-of-art of AC-TENGs and over 10-fold compared to previous DC-TENGs. The MDC-TENG realizes both the miniaturized device and high output performance. Meanwhile, its effective charge density can be further improved as the device size increases. Our work not only provides a miniaturization strategy of TENG for the application in IoTs and AI as energy supply or self-powered sensor, but also presents a paradigm shift for large-scale energy harvesting by TENGs., Low charge density is the bottleneck for the applications of triboelectric nanogenerator (TENG). Here, the authors demonstrate a microstructure-designed direct-current TENG with rationally patterned electrode structure to enhance its effective charge density to a new milestone.

The large temperature dependence of electro-strain for Bi0.5Na0.5TiO3 (BNT)-based ceramics limits their practical applications. In this work, Li-doped BNT-based lead-free multi-layer piezoelectric ceramic actuators are prepared by solid-state reaction synthesis method. The multi-layer actuator structure is fabricated by gradient doping, in which each layer corresponds to different compositions, with enhanced temperature stability compared to the single-layer one. For specific composition, it could achieve maximum electro-strain value at a certain temperature. Therefore, the electro-strain of each layer in this BNT-based multi-layer actuator could compensate each other at different temperatures with improved temperature stability. Consequently, the electro-strain of the five-layer structure varies less than ∼12% when the temperature changes from 50 °C to 130 °C, and a large electro-strain of 600 pm/V could be obtained at 70 °C. This work provides an effective approach by gradient doping to broaden the use temperature range for BNT-based actuators with improved temperature stability.

Rational design of electrode materials for sodium-ion batteries with high flexibility is still challenging. Here, superelastic 3D few-layer MoS2/carbon framework heterogeneous electrodes (abbreviated as, MoS2@CF) are fabricated by a simple vacuum infiltration and heating process. The interconnected ultrathin MoS2 network filled with carbon framework forms a 3D heterogeneous network with hierarchical pore structure. The unique structure of the electrodes results in excellent mechanical and electrochemical performances. The MoS2@CF electrodes exhibit superior elasticity and recoverability. After 180° bending repeatedly, the electrodes still can recover their initial sizes. Moreover, the electrodes show outstanding cycling stabilities with high reversible capacities reaching up 240 mA h g−1 after 500 cycles at 1 A g−1 (capacity retention of ~ 99%). Full-cells assembled with MoS2@CF anodes and Na3V2(PO4)3 cathodes show high reversible capacity (~ 252 mA h g−1 at 0.5 A g−1). Overall, the superior mechanical properties and high electrochemical performances indicate the promising potential of MoS2@CF electrodes in large-scale flexible sodium-ion storage devices.

With the fast development of the Internet of Things, the requirements of system miniaturization and integration have accelerated research on multifunctional sensors. Based on the triboelectric nanogenerator, a self-powered multifunctional motion sensor (MFMS) is proposed in this work, which is capable of detecting the motion parameters, including direction, speed, and acceleration of linear and rotary motions, simultaneously. The MFMS consists of a triboelectric nanogenerator (TENG) module, a magnetic regulation module, and an acrylic shell. The TENG module is formed by placing a free-standing magnetic disk (MD) on a polytetrafluorethylene (PTFE) plate with six copper electrodes. The movement of the MFMS causes the MD to slide on the PTFE plate and hence excites the electrodes to produce a voltage output. The carefully designed six copper electrodes (an inner circle electrode, an outer circle electrode, and four arc electrodes between them) can distinguish eight directions of movement with the acceleration and determine the rotational speed and direction as well. Besides, the magnetic regulation module is applied here by fixing a magnetic cylinder (MC) in the shell, right under the center of the PTFE plate. Due to the magnetic attraction applied by the MC, the MD will automatically return to the center to prepare for the next round of detection, which makes the proposed sensor much more applicable in practice.

By the traditional solid-state reaction method, Bi0.5(Na0.8K0.2)0.5Ti1−x(Fe0.5Nb0.5) x O3 (marked as BNKT-xFN; x = 0.00, 0.01, 0.02, 0.03 and 0.04) piezoelectric ceramics were fabricated, and the effects of FN substitution on the dielectric, ferroelectric, piezoelectric and electric field-induced strain performance were investigated. The relative dielectric permittivity and loss tangent reveal that the phase transition temperature between ferroelectric and ergodic relaxor phase is reduced from 90 °C to room temperature (RT), even below RT, with increasing FN content. Temperature dependence of polarization–electric field loops and strain-electric field curves exhibits that the ferroelectric order is disturbed gradually, and the ergodic relaxor phase forms with increasing FN content. A large unipolar strain of 0.42% and corresponding d33* (= Smax/Emax) of 642 pm/V for the sample with x = 0.04 are obtained under 6.5 kV/mm due to the phase transition from ergodic relaxor to ferroelectric. These results indicate that the (Fe0.5Nb0.5)4+ complex ion-modified BNKT-based ceramics would have great potentials for lead-free electromechanical actuator applications.

Highly 〈001〉 textured (~81%) ternary BNT-BKT-BT ceramics are prepared by reactive templated grain growth (RTGG) method with plate-like Bi4Ti3O12 particles as templates. Outstanding electric properties, which are both higher than state of art BNT-based ceramics, emerge in the aforementioned sample due to the grain orientation. For the textured ceramics, a high piezoelectric constant d33 of 314 pC/N could be obtained, which is ~1.4-fold larger than that of a random one (221 pC/N). A high electro-strain response (~0.41%) is obtained at a low applied electric field (4 kV/mm) in textured ceramics, with the normalized strain d33⁎ (Smax/Emax) = 1030 pm/V near the TF-R (the temperature of ferroelectric to ergodic relaxor state transition). Moreover, the evolution process of relaxor phases is studied. The texture technique also could improve the TF-R from 70 °C to 100 °C, enhancing the temperature stability. In addition, the polarization switching process in ferroelectric phase and ergodic relaxor state are analyzed in this work, respectively. Keywords: Lead-free ceramics, Piezoelectric properties, Ferroelectric properties, Strain response, Phase transition

The temperature dependent properties of (K,Na)NbO3-based piezoelectric materials enlightened us that the combination of compositions gradient could remedy the fluctuation of the electrical properties induced by the temperature variation. Then Li modified (K,Na)NbO3 ceramics was chosen as target materials and a multi-layered structure containing a series of composition gradient layers was prepared via a conventional solid state reaction. The results of relative permittivity against temperature demonstrated that the as-called relaxor feature was observed in this proposed multi-layered structure and the change rate of the dielectric anomaly peak got evidently reduced. Besides, the field-induced strain varying less than 10% from the room temperature to 100 °C was also achieved in this layered-structure ceramics. Moreover, the investigation here could render us an important reference for designing ferroelectric materials to satisfy the diversified requirements in the functional device.

The instability and high resistance of metallic Na/solid-state electrolyte (especially oxide-based electrolyte) interface are still challenges for all-solid-state sodium batteries. Herein, we propose a grain-boundary engineering strategy to stabilize the Na/Na3Zr2Si2PO12 interface and improve the capability of sodium ion transfer at the interface. The chemical composition at the grain boundary of Na3Zr2Si2PO12 is mediated via the addition of sintering additive Na2B4O7 to facilitate the densification sintering at relatively low temperature and boost sodium ion migration across the grain boundary. Na3Zr2Si2PO12-10 wt% Na2B4O7 demonstrates an optimized conductivity of 1.72 mS cm−1 at room temperature and the corresponding symmetric sodium cells exhibit ultra-stable sodium plating/stripping cycling under a current density of 0.3 mA cm−2 for over 2500 h at room temperature. Analysis reveals that a kinetically stable interphase forms between electrolyte and metallic Na, reducing the interfacial resistance from 90 Ω cm2 for Na3Zr2Si2PO12 to 36 Ω cm2 for Na3Zr2Si2PO12-10 wt% Na2B4O7. Cycling at stepwise changing temperature reconfirms the super stability of Na/Na3Zr2Si2PO12-10 wt% Na2B4O7 interface. All solid-state batteries based on the Na3Zr2Si2PO12-10 wt% Na2B4O7 demonstrates excellent cycling performance for over 200 cycles with limited capacity degradation. Our findings here open up a fertile avenue of exploration for all-solid-state sodium batteries utilizing NASICON-type electrolytes.

La3+ and Nb5+ co-doped 0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 (BNT-BKT-BT) samples were prepared through conventional solid oxide route, and the effects of doping ions on the phase structure, electrical properties, and electro-strain behavior were investigated. The XRD results showed that La3+ and Nb5+ co-dopant induced a phase transition from coexistence of ferroelectric tetragonal and rhombohedral phase to a pseudocubic phase. With increasing dopant content, the normal ferroelectric-to-ergodic relaxor (FE-to-ER) transition temperature (TF-R) decreased down from 120 °C to room temperature (RT), even below RT. A large unipolar strain of 0.34% (d33∗ = Smax/Emax = 680 pm/V) was achieved under 5 kV/mm and the largest electro-strain of 0.46% was achieved under an applied electric field of 7.5 kV/mm at RT for x = 0.0050. Temperature dependence of both polarization loops and bipolar strain curves from room temperature to 160 °C were also studied, and the results suggested that the origin of the large strain is attributed to a reversible nonpolar isotropic ergodic relaxor state to polar anisotropic ferroelectric phase transition.

0.8Bi0.5Na0.5Ti(1-x)NbxO3−0.2Sr0.85Bi0.1TiO3 (BNT-SBT-xNb, x = 0.00, 0.01, 0.02, and 0.03) piezoelectric ceramics were prepared by traditional solid state reaction and the influence of Nb substitution on the phase structure, ferroelectric, piezoelectric, and electric-field-induced strain properties in BNT-SBT ceramics were studied. XRD results exhibited that Nb5+ ions could fully diffuse into BNT-SBT structure to form a solid solution when x = 0.01. P-E loops and S-E curves suggested that the ferroelectric phase transformed to ergodic relaxor state (FE-to-ER) with the increasing the amount of Nb additive, indicating the ferroelectric long-ranged order was disturbed by the excess of Nb. With increasing Nb doping, phase transition temperature from normal ferroelectric to ergodic relaxor (short for TF-R) could be reduced from 120 °C to 40 °C. Furthermore, for sample with x = 0.01, the normalized strain d33* got a maximum value ~571 pm/V due to the phase transition from ergodic relaxor to ferroelectric (ER-to-FE) under electric field.

Rapid advancements in stretchable and multifunctional wearable electronics impose a challenge on corresponding power devices that they should have comparable portability and stretchability. Here, we report a highly stretchable and washable all-yarn-based self-charging knitting power textile that enables both biomechanical energy harvesting and simultaneously energy storing by hybridizing triboelectrical nanogenerator (TENG) and supercapacitor (SC) into one fabric. With the weft-knitting technique, the power textile is qualified with high elasticity, flexibility, and stretchability, which can adapt to complex mechanical deformations. The knitting TENG fabric is able to generate electric energy with a maximum instantaneous peak power density of ∼85 mW·m–2 and light up at least 124 light-emitting diodes. The all-solid-state symmetrical yarn SC exhibits lightweight, good capacitance, high flexibility, and excellent mechanical and long-term stability, which is suitable for wearable energy storage devices. The ...

A nanogenerator, as a self-powered system, can operate without an external power supply for energy harvesting, signal processing, and active sensing. Here, near-infrared (NIR) photothermal triggered pyroelectric nanogenerators based on pn-junctions are demonstrated in a p-Si/n-ZnO nanowire (NW) heterostructure for self-powered NIR photosensing. The pyroelectric-polarization potential (pyro-potential) induced within wurtzite ZnO NWs couples with the built-in electric field of the pn-junction. At the moment of turning on or off the NIR illumination, external current flow is induced by the time-varying internal electric field of the pn-heterostructure, which enables a bias-free operation of the photodetectors (PDs). The NIR PD exhibits a high on/off photocurrent ratio up to 107 and a fast photoresponse component with a rise time of 15 μs and a fall time of 21 μs. This work provides an unconventional strategy to achieve active NIR sensing, which may find promising applications in biological imaging, optoelect...

Although silicon (Si) devices are the backbone of modern (opto-)electronics, infrared Si-photosensing suffers from low-efficiency due to its limitation in light-absorption. Here, we demonstrate a large improvement in the performance, equivalent to a 366-fold enhancement in photoresponsivity, of a Si-based near-infrared (NIR) photodetector (PD) by introducing the piezo-phototronic effect via a deposited CdS layer. By externally applying a −0.15‰ compressive strain to the heterojunction, carrier-dynamics modulation at the local junction can be induced by the piezoelectric polarization, and the photoresponsivity and detectivity of the PD exhibit an enhancement of two orders of magnitude, with the peak values up to 19.4 A/W and 1.8 × 1012 cm Hz1/2/W, respectively. The obtained maximum responsivity is considerably larger than those of commercial Si and InGaAs PDs in the NIR waveband. Meanwhile, the rise time and fall time are reduced by 84.6% and 76.1% under the external compressive strain. This work provides ...

Achievement of p–n homojuncted GaN enables the birth of III-nitride light emitters. Owing to the wurtzite-structure of GaN, piezoelectric polarization charges present at the interface can effectively control/tune the optoelectric behaviors of local charge-carriers (i.e., the piezo-phototronic effect). Here, we demonstrate the significantly enhanced light-output efficiency and suppressed efficiency droop in GaN microwire (MW)-based p–n junction ultraviolet light-emitting diode (UV LED) by the piezo-phototronic effect. By applying a −0.12% static compressive strain perpendicular to the p–n junction interface, the relative external quantum efficiency of the LED is enhanced by over 600%. Furthermore, efficiency droop is markedly reduced from 46.6% to 7.5% and corresponding droop onset current density shifts from 10 to 26.7 A cm–2. Enhanced electrons confinement and improved holes injection efficiency by the piezo-phototronic effect are revealed and theoretically confirmed as the physical mechanisms. This stud...

Defect dipoles in acceptor-doped perovskite-based ferroelectric ceramics play an important role in piezoelectric properties. In this work, the acceptor, such as Fe2+ and Cu+, -doped (K0.5N0.5)NbO3 (KNN) ceramics are fabricated in reduced atmosphere by the conventional solid-state reaction method. By tailoring the strongly polarized defect dipoles through poling and aging processes, ultrahigh electro-strain and large signal piezoelectric d33* values can be achieved. The electro-strains of 0.41% and 0.50% with d33* ~820 pm/V and ~1000 pm/V are obtained in Fe2+-doped and Cu+-doped KNN samples, respectively, which are 4-fold and 5-fold larger than that of pure KNN. Of particular interest is that the electro-strain and d33* values are found to improve with increasing temperature up to 140 °C, accompanied with low strain hysteresis of 10%-20%. A detailed mechanism considering the interaction between defect dipole polarization and spontaneous polarization after poling and aging processes is proposed to explain the observed phenomena, which provides a good paradigm for achieving high piezoelectric response by defect engineering in perovskite-based ferroelectric ceramics.

The piezo-phototronic effect, as a three-way coupling effect of piezoelectricity, semiconductor and optical excitation in piezoelectric semiconductors to improve the performance of optoelectronic devices, has a wide range of applications in various fields. However, under different light illumination conditions, the piezo-phototronic effect would have different effects on the optoelectronic performance due to the different photo-generated carrier excitation regions. Here, the piezo-phototronic effect is utilized to modulate the carrier transport behavior of a p-Si/n-CdS heterojunction PD during the optoelectronic process in a broadband range from visible to near-infrared light. The strain-induced piezo-charges significantly improve the photoresponse performance of the heterojunction PD. The photoresponsivity increases by 1867% under −0.35‰ strain under 808 nm light illumination, with a remarkable reduction in the rise and fall times to ∼2.1 ms (reduced by 83.3% and 50.0%, respectively). However, since the photo-generated carriers are produced only at the n-CdS side under 442 nm light illumination, the photoresponse performance is greatly weakened by the piezo-phototronic effect. The corresponding working mechanism of the piezo-phototronic effect on the heterojunction photodiode under different light illumination conditions is proposed in detail. These results provide an in-depth understanding about the piezo-phototronic effect, which would enable more efficient utilization of the piezo-phototronic effect in optoelectronic devices.

The siliconboron carbonitride/graphene (SiBCN/GN) composites were designed and synthesized by the insertion of GN sheets into the ceramic network of polymer-derived SiBCN via liquid dispersion (SiBCN/GN-ld) and solid phase blending (SiBCN/GN-sp) at 1100 °C. The composites were used as anode materials in lithium-ion batteries, and their electrochemical performances were investigated. Electrochemical measurements showed that the SiBCN/GN-ld exhibited a first cycle discharge capacity of 844.2 mA h g −1 at a current density of 80 mA g −1 , which was higher than those of SiBCN/GN-sp, SiBCN, and GN anodes. The discharge capacity reduced to 347 mA h g −1 and remained in this range over 30 cycles. SiBCN/GN-ld anode contained Si 3 N 4 , SiC, BNC, and free carbon nanocrystals sites, and GN acted as layered base supporting the SiBCN cluster, thus resulting in improved chemical stability and chemical electronic conductivity. Therefore, the SiBCN/GN-ld showed the improved electrochemical performances that were attributed to the presence of GN. GN layers acted as the supporting layers to stabilize the SiBCN matrix and alleviate the expansion of material structure during charge–discharge cycle. Dependence of the electrochemical capacities of the SiBCN/GN composites on their compositions and structures indicated the promising potential to enhance the electrochemical performances of the materials through molecular design and/or the control of the material structure.

By conventional ceramics sintering technique, the lead-free 0.85Bi0.5Na0.5(1−x)Li0.5xTiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 (x =0–0.15) piezoelectric ceramics were obtained and the effects of Li dopant on the piezoelectric, dielectric, and ferroelectric properties were studied. With increasing Li addition, the temperature-dependent permittivity exhibited the normal ferroelectric-to-ergodic relaxor (FE-to-ER) transition temperature (TFE-ER, abbreviated as TF-R) decreasing down to room temperature. The increasing Li content also enhanced the diffuseness of the FE-to-ER transition behavior. For composition with x = 0.15, a large unipolar strain of 0.37% (d33∗ = Smax/Emax = 570 pm/V) was achieved under 6.5 kV/mm applied electric field at room temperature. Both unipolar and bipolar strain curves related to the temperature closely, and when the temperature reached the TF-R, the normalized strain d33∗ achieved a maximum value (e.g., for x = 0.10, d33∗ = 755 pm/V) owing to the electric-field-induced ER-to-FE state transition.

Oxidation behaviors of SiC in the presence of some impurities are very important for the performance of SiC used in high temperature environments such as aerospace vehicles and nuclear reactors. In this study, the effects of NaF on the oxidation behaviors of SiC powder with grain sizes from 0.3~2 μm were investigated. NaF with contents of 0.2~10 wt.% was added to the 6H-SiC powder (~ 1 μm) and the mixtures were oxidized at 500~800 °C for 30 min in air atmosphere. Compared with pure SiC, enhanced oxidation was observed with the NaF addition as low as 0.2 wt.%. When the content of NaF was increased to 2.0%, the weight gain ratio of SiC reached 23.0% at the oxidation temperature of 700 °C, which was only 0.6% for pure SiC powder. In the TG analysis, the onset oxidation temperature for the SiC-2.0%NaF sample was significantly decreased to 570 °C. The weight gain was increased with the decrease of SiC grain size. The intensive oxidation was confirmed by SEM, TEM, XRD and XPS characterizations. The formation of free carbon during the oxidation process was demonstrated by XRD and Raman spectroscopy. New polytype of SiC of 15R and defects were formed in the 6H-SiC matrix after oxidation. The results showed that NaF could greatly accelerate the oxidation of SiC and played a very important role in the microstructure evolution of SiC. Possible mechanism for the effects of NaF on the oxidation behaviors of SiC was discussed.

The introduction of a built-in electric field by using ferroelectric materials is an efficient way to modulate the carrier generation and transportation behavior of photocatalysts. Here, we propose a hydrothermal-synthesized Ag2O/BaTiO3 heterostructure composite, whose photocatalytic performance can be largely enhanced by a four-way coupling effect among piezoelectricity, pyroelectricity, semiconductor and photoexcitation, referred as the piezo/pyro-phototronic synergistic effect. The photoelectrochemical (PEC) test of Ag2O/BaTiO3 shows a high photocurrent density of 3.48 mA/cm2 at 1.5 VSCE, which is 1.5 and 1.3 times as high as that of pristine BaTiO3 and Ag2O, respectively, and the transient photocurrent density of the composite at 0.5 VSCE is 20 times higher than that of the pure BaTiO3. Under varying temperature field and ultrasonic excitation condition, the photocatalytic degradation efficiency of the Ag2O/BaTiO3 composite is enhanced greatly when compared to that under only light illumination condition. This enhancement can be attributed to the attraction and release process of photogenerated carriers during the varying temperature field and ultrasonic excitation cycling. These results indicate that the ferroelectric heterostructure composites can act as a promising photocatalyst in wastewater treatment field under different environmental conditions.

〈001〉 textured Ba 0.85 Ca 0.15 Ti 0.90 Zr 0.10 O 3 (BCZT) lead-free ceramics were fabricated by reactive-templated grain growth (RTGG) method using plate-like BaTiO 3 templates. Aligned plate-like BaTiO 3 particles grew at the expense of equiaxed matrix powder with random orientation with increasing sintering temperature, resulting in development of texture. Highly textured (texture fraction f =0.98) BCZT ceramics were prepared at first with a RTGG process. Development of texture was investigated during sintering. The influence of templates content on texture fraction and electrical properties of the textured BCZT ceramics were also studied. The textured ceramics shows high piezoelectric coefficient ( d 33 =455 pC/N) and dielectric properties ( e r ~3500, tan δ ~0.010).

Piezoelectric ceramics doped with small amounts of elements usually cause significant improvement in piezoelectric properties. In this study, we prepared Ba0.85Ca0.15Ti0.90Zr0.10O3–x wt% ZnO (0 ⩽ x ⩽ 0.16) ceramics via a conventional solid-state reaction method. The evolution of domain structure, relaxor behavior, ferroelectric and piezoelectric properties with Zn doping was systematically investigated. The ceramics with x = 0.08 possess slender domain patterns and high domain wall density. The Ba0.85Ca0.15Ti0.90Zr0.10O3–8 wt% ZnO ceramics show maximum remnant polarization and spontaneous polarization (Pr = 10.14 μC/cm2, Ps = 19.68 μC/cm2). A giant piezoelectric coefficient of d33 = 603 pC/N and high planar electromechanical coupling factor of kp = 0.56 were obtained for the sample of x = 0.08, showing the potential possibilities for lead-free piezoelectric application.

The textured Ca0.85(LiCe)0.075Bi4Ti4O15 (LCCBT) ceramics were fabricated by templated grain growth method, with the piezoelectric constant (d 33) around 29.5 pC/N. Owning to the orientation of the grains, the properties of the textured ceramics have been improved. The mechanism of texture development was explored for textured LCCBT ceramics. The factors contributing to the texture evolution include the templates growth, the morphological change of the matrix grains and the grains group formation. The change of the matrix grains goes through four stages: the equiaxed grains, the square tabular grains, the pyramid-like terraces, the platelike sheets.

A long debate on the charge identity and the associated mechanisms occurring in contact-electrification (CE) (or triboelectrification) has persisted for many decades, while a conclusive model has not yet been reached for explaining this phenomenon known for more than 2600 years! Here, a new method is reported to quantitatively investigate real-time charge transfer in CE via triboelectric nanogenerator as a function of temperature, which reveals that electron transfer is the dominant process for CE between two inorganic solids. A study on the surface charge density evolution with time at various high temperatures is consistent with the electron thermionic emission theory for triboelectric pairs composed of Ti-SiO2 and Ti-Al2 O3 . Moreover, it is found that a potential barrier exists at the surface that prevents the charges generated by CE from flowing back to the solid where they are escaping from the surface after the contacting. This pinpoints the main reason why the charges generated in CE are readily retained by the material as electrostatic charges for hours at room temperature. Furthermore, an electron-cloud-potential-well model is proposed based on the electron-emission-dominatedcharge-transfer mechanism, which can be generally applied to explain all types of CE in conventional materials.

Cadmium sulfide (CdS) has received widespread attention as the building block of optoelectronic devices due to its extraordinary optoelectronic properties, low work function, and excellent thermal and chemical stability. Here, a self-powered flexible photodetector (PD) based on p-Si/n-CdS nanowires heterostructure is fabricated. By introducing the pyro-phototronic effect derived from wurtzite structured CdS, the self-powered PD shows a broadband response range, even beyond the bandgap limitation, from UV (325 nm) to near infrared (1550 nm) under zero bias with fast response speed. The light-induced pyroelectric potential is utilized to modulate the optoelectronic processes and thus improve the photoresponse performance. Lasers with different wavelengths have different effects on the self-powered PDs and corresponding working mechanisms are carefully investigated. Upon 325 nm laser illumination, the rise time and fall time of the self-powered PD are 245 and 277 µs, respectively, which are faster than those of most previously reported CdS-based nanostructure PDs. Meanwhile, the photoresponsivity R and specific detectivity D* regarding to the relative peak-to-peak current are both enhanced by 67.8 times, compared with those only based on the photovoltaic effect-induced photocurrent. The self-powered flexible PD with fast speed, stable, and broadband response is expected to have extensive applications in various environments.

The rechargeable sodium-ion battery has become a research hotspot due to low cost and a large abundance of sodium resources. In this work, a novel nano-architecture of Cu2Se nanoparticles imbedded in ordered mesoporous carbon (Cu2Se@ CMK-3) was synthesized. Benefiting from the high conductivity and special nano-architecture of CMK-3 matrix, the as-synthesized Cu2Se@ CMK-3 composite delivers a high specific discharge capacity (231 mA h g−1) and superior cycling stability (a capacity decay of 0.33% per cycle during 100 cycles at 0.1 C) when used as cathode in sodium-ion batteries.

Ferroelectric materials have exhibited immense promise for the control of optoelectronic processes in emerging devices. However, the utilization of ferroelectric materials to directly modulate the energy band at the junction interface has rarely been investigated. Here, a ferroelectric V-doped ZnO nanosheets/p-Si heterojunction photodetector (VZnO/Si PD) has been prepared, and the effect of ferroelectricity on the photoresponse performance of the VZnO/Si PD is studied deeply. Due to the existence of aligned ferroelectric spontaneous polarization charges under applied electrical field, the junction interface energy band can be modulated directly and effectively, which greatly improves the generation, separation and transportation efficiency of photogenerated electron-hole pairs. In contrast with the non-ferroelectric ZnO/Si PD, the VZnO/Si PD has a large enhancement in photoresponse performance with a twelve-fold increase in the photoresponsivity (R) under +1 V bias, accompanied by fast response speed in a broad spectral range. Interestingly, even under the −1 V bias voltage, the reverse ferroelectric polarizations also can improve the photoresponse behavior. These results prove the feasibility of direct modulation on energy band structure at the junction interface by ferroelectricity, which provides a new perspective for energy band engineering.

As one of the most crucial intrinsic contributions, defect dipoles show unique and prominent influence on the responsive behavior of piezoelectric ceramics. Here, we present the formation of the defect dipoles and the state-of-the-art effect on lead-free piezoelectric ceramics in detail. First, the formation of defect dipoles in ceramics are demonstrated. The coupling effect between defect dipole polarization and normal ferroelectric spontaneous polarization, which results in the unusual ferroelectric properties (e.g., “pinched” or asymmetric polarization hysteresis loops) and high electric field induced-strain performance in piezoelectric ceramics, is elaborated. Thereafter, we systematically introduce the switching and decoupling process of defect dipoles under different external conditions. Finally, the researches about the effect of defect dipoles on various lead-free piezoelectric ceramics are presented herein, with special attention on the three systems: the barium titanate, sodium-bismuth titanate and potassium sodium niobate. The remaining challenges, however, such as weak temperature/cycling stability, still need further effort for the extensive applications of these materials.

Emerging 2D electronic materials have shown great potential for regulating and controlling optoelectronic processes. A 2D ferroelectric semiconductor coupled with the piezo‐phototronic effect may bring unprecedented functional characteristics. Here, a heterojunction photodetector made of p‐Si/V‐doped‐ferroelectric‐ZnO 2D nanosheets (FESZ‐PD) is fabricated, and the ferroelectricity‐enhanced piezo‐phototronic effect on the photoresponse behavior of the FESZ‐PD is carefully investigated. By introducing the ferroelectricity and the piezo‐phototronic effect, improved current rectification performance is achieved and the photoresponse performance of the heterojunction is enhanced in a broad spectral range. The applied voltage bias during measurement naturally causes ferroelectric spontaneous polarizations to align, resulting in a change in band structure near the interface and the local piezo‐phototronic effect. The modulated energy band promotes the generation, separation, and transportation efficiency of photogenerated carriers greatly. Compared with the Si/ZnO 2D nanosheets photodetector without ferroelectricity under strain‐free conditions, the photoresponsivity R of the FESZ‐PD increases by 2.4 times when applying a −0.20‰ compressive strain at +1 V forward bias. These results confirm the feasibility of coupling the ferroelectricity with the piezo‐phototronic effect in 2D ferroelectric materials to enhance the photoresponse behavior, which provides a good way to enable the development of high‐performance electronic and optoelectronic devices.

This paper investigates a system of 0.93[Formula: see text][Formula: see text]TiO3–0.06BaTiO3–0.01[Formula: see text][Formula: see text]NbO3–[Formula: see text]CuO (BNT–BT–KNN–[Formula: see text]CuO, [Formula: see text][Formula: see text]mol.%) ceramics, which were fabricated by the conventional solid-state process through the granulation of vacuum freeze drier. The results show that the CuO doping made a significant enhancement on the piezoelectric properties of the BNT–BT–KNN ceramics. With the doping of CuO, the transition temperature between ferroelectric phase and ergodic relaxor state is reduced to near room temperature, resulting in pinched [Formula: see text]–[Formula: see text] loops and “sprout” shape [Formula: see text]–[Formula: see text] curves. For the composition with [Formula: see text], a high unipolar strain of 0.39% under 5[Formula: see text]kV/mm contributes a large [Formula: see text][Formula: see text]pm/V at room temperature, which is competitive with the other BNT-based ceramics.

0.85Na0.5Bi0.5TiO3–0.04BaTiO3–0.11K0.5Bi0.5TiO3 (BNBK) lead-free piezoelectric ceramics with extensive [001]pc (pc: pseudo cubic) texture were fabricated by the reactive-templated grain growth method using anisotropic Bi4Ti3O12 (BIT) particles as templates. The degree of grain orientation increased with increasing heat treatment temperature (600–1,200 °C). The obtained textured ceramics showed dense and brick-wall like microstructure, giving a Lotgering factor of 0.6. A physical understanding of interaction between BIT templates and matrix powders and the mechanism of texture evolution were proposed and confirmed by experimental evidences of X-ray diffraction patterns, scanning electron microscope images and density measurements. The piezoelectric response was enhanced by the grain orientation, and the piezoelectric constant (d33) of the textured ceramics sintered at 1,170 °C attained a value of 254 pC/N, which was 41 % higher than random ceramics (180 pC/N).

(1 − x)(0.85Bi0.5Na0.5TiO3–0.11Ba0.5K0.5TiO3–0.04BaTiO3)- xK0.5Na0.5NbO3 lead-free piezoelectric ceramics with x = 0.00, 0.02, 0.03, 0.04, 0.05, and 0.10 were prepared by a conventional solid state method. A coexistence of rhombohedral (R) and tetragonal (T) phases was found in the system, which tended to evolve into pseudocubic symmetry when x increases. The x = 0.04 sample exhibited improved electrical properties: the dielectric constant er = 1900 with the low loss tangents 0.06, the Smax/Emax of ~400 and ~460 pm/V under unipolar and bipolar electric field, respectively. Meanwhile, piezoelectric constant d33 still maintained ~160 pC/N. These could be owed to the formation of polar nanoregions for relaxor phase.

〈001〉 textured Bi4Ti3O12 platelets with micro scale size were synthesized by a facile molten salt method. The photocatalytic activities of the as-prepared samples were measured with the photodegradation of methylene blue at room temperature under visible light irradiation. The Bi4Ti3O12 with the aspect ratio of 35 exhibited good absorption in the visible light region and the photodegradation against methylene blue was higher than that of anatase TiO2 reference, showing that the high degree of preferred {001} facets on the plate surface benefits the electronic transmission. In addition, the layer-pervoskite structure facilitates the mobility of the photogenerated carriers and hampers their recombination. The above results indicated that the large specific surface area of the as-prepared samples could attribute to the presence of a number of oxygen vacancies and then lead to the good photo-electric property. This work proposed an alternative way to tailor the structure of micro-sized platelets to get excellent properties comparable to the nano materials.

Ferroelectric Ba0.55Sr0.4Ca0.05TiO3 (BSCT) ceramics with nano-MgO additive have been prepared by typical electrical ceramic technology sintered at 950°C for 2 h. The influence of the additive content on the sintering behaviors of BSCT ceramics is investigated. The results show that the interactions of MgO with BSCT and B2O3-Li2O liquid sintering additive help to form wetting boundaries for accelerating densification process, promote the grain growth of BSCT ceramic. The BSCT ceramic with 20 wt% nano-MgO possesses uniform microstructure, high densification, and superior dielectric properties. The modification mechanism of nano-MgO on BSCT ceramic is summarized as “MgO assisted wetting sintering model.”

Self-powered photodetectors (PDs) have long been realized by utilizing photovoltaic effect and their performances can be effectively enhanced by introducing the piezo-phototronic effect. Recently, a novel pyro-phototronic effect is invented as an alternative approach for performance enhancement of self-powered PDs. Here, a self-powered organic/inorganic PD is demonstrated and the influences of externally applied strain on the pyro-phototronic and the photovoltaic effects are thoroughly investigated. Under 325 nm 2.30 mW cm-2 UV illumination and at a -0.45% compressive strain, the PD's photocurrent is dramatically enhanced from ≈14.5 to ≈103 nA by combining the pyro-phototronic and piezo-phototronic effects together, showing a significant improvement of over 600%. Theoretical simulations have been carried out via the finite element method to propose the underlying working mechanism. Moreover, the pyro-phototronic effect can be introduced by applying a -0.45% compressive strain to greatly enhance the PD's response to 442 nm illumination, including photocurrent, rise time, and fall time. This work provides in-depth understandings about the pyro-phototronic and the piezo-phototronic effects on the performances of self-powered PD to light sources with different wavelengths and indicates huge potential of these two effects in optoelectronic devices.

In this study, 0.95(Na0.52K0.44Li0.04)(Nb0.88Sb0.08Ta0.04)O3–0.05KNbO3 (NKLNST–0.05KN) ceramics were prepared by two-step sintering method. The effects of KN and sintering temperature on the microstructures and piezoelectric properties of NKLNST–0.05KN were discussed. The addition of KN aids plays an important role in grain growth behavior and allows lower first-step sintering temperature. And with the extension of the second-step sintering time, the liquid phase with low Na content forms solid solution with the NKLNST ceramics and the nano-sized white spots with high Na content crystallize on the surface of grains. The P–E hysteresis loops of the ceramics are well-saturated, and NKLNST–0.05KN ceramics demonstrate good piezoelectric property d33=328 pC/N under 1130/950 °C sintered by two-step sintering.

(1 − x)Bi0.5Na0.5TiO3–x(Na0.53K0.44Li0.04)(Nb0.88Sb0.08Ta0.04)O3 (BNT–xNKLNST) with x = 0–0.10 lead-free piezoelectric ceramics were prepared by a solid state method, and the structure and electrical properties were investigated in this study. It is found that a morphotropic phase boundary (MPB) of rhombohedral (R) and tetragonal (T) phase exists in the range of 0.03 ≤ x ≤ 0.05 and the structure changes to paraelectric phase when x > 0.07. The samples with x = 0.05 exhibit improved electrical properties owing to the formation of MPB, which are as follows: piezoelectric constant d33 = 120 pC/N, remnant polarization Pr = 39.4 μC/cm2 and coercive field Ec = 3.6 kV/mm. These results indicate that the enhanced piezoelectric properties for BNT can be achieved by forming the coexistence of R and T phase.

Lead-free piezoelectric ceramics of an a(Bi0.5Na0.5)TiO3–b(Bi0.5K0.5)TiO3–cBaTiO3 (BNBK) ternary system close to the composition of MPB have been widely investigated in recent decades. In this research, the MPB Linear Combination Rule (LCR) is introduced to explore the ternary system of lead-free piezoelectric ceramics for the first time, and the best properties of d33=220 pC/N, kp=0.31, kt=0.476, Td=157°C, and ɛr=1648 are obtained for a composition with a/b/c=85.25/10.995/3.755. It is suggested that BNBK with the composition (85.25/10.995/3.755) is a candidate material for lead-free applications. Moreover, the LCR of MPB is an effective method, applicable to lead-free ceramics to search for an optimal composition with excellent properties.

The piezoelectric and ferroelectric properties of 0.852[Bi 0.5 (Na (1-x) Li x ) 0.5 ]TiO 3 -0.110(Bi 0.5 K 0.5 )TiO 3 -0.038BaTiO 3 (x = 0, 0.02, 0.05, 0.10, 0.15, and 0.30, abbreviated as BNKLBTx) lead-free piezoelectric ceramics were prepared by conventional ceramic processing. X-ray diffraction on the samples revealed that Li + ions diffused into the Bi 0.5 Na 0.5 TiO 3 lattices (A-site) and formed a solid solution with a single perovskite structure. All the samples could be sintered at 1160°-1180°C, with the density being on the order of > 95% of the theoretical. The ceramics with x = 0.10 exhibited the highest properties, with the piezoelectric coefficient being on the order of d 33 = 235 pC/N, while the electromechanical coupling factors were found to be k p = 0.30 and k t = 0.45, respectively. The dielectric, pyroelectric, electromechanical, and ferroelectric behaviors were investigated as a function of temperature. It was found that the depolarization temperature T d showed a strong dependence on the content of Li + addition, shifting to a lower temperature with increasing Li + dopant.

Lead-free piezoelectric ceramics (1 − x )(0.98K 0.5 Na 0.5 NbO 3 –0.02LiTaO 3 )– x (0.96Bi 0.5 Na 0.5 TiO 3 –0.04BaTiO 3 ) (KNN–LT–BNT–BT) with x = 0–0.10 have been synthesized by a conventional sintering technique. All samples possess pure perovskite structure, showing room temperature symmetries of orthorhombic at x x ≤ 0.10. A coexistence of orthorhombic and tetragonal phases in the composition range of 0.02 ≤ x d 33 = 155 pC/N, remnant polarization P r = 24.2 μC/cm 2 , and coercive electric field E c = 2 kV/mm. The results indicate that although this kind of ceramics displays good properties, further study is needed to promote the stabilities of the ceramics in order to utilize them in varying temperature environments.

In this study, we used conventional ceramics techniques to prepare lead-free ceramics (K0.5Na0.5)NbO3-x mol CuO (KNN- xCu) sintered in air atmosphere and argon atmosphere, respectively, and investigated the corresponding hardening characteristics. The microstructure, P–E hysteresis loops, and X-ray photoelectron spectroscopy results indicate that the Cu2+ ions can play different roles on the hardening mechanism in air atmosphere and argon atmosphere. The Cu2+ ions are considered to replace the Nb5+ ions for forming O2− vacancies, and act as a hardener when sintered in air. However, when sintered in argon atmosphere, a large portion of Cu2+ ions could be reduced into Cu+ ions and then replaced by Na+.