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Recently, NaNbO3-based ceramics with excellent energy storage performances (ESPs) and fast charge-discharge characteristics have attracted great attention. In this case, BaTiO3 modified 0.85NaNbO3-0.15Bi(Zn2/3Nb1/3)O3 (0.85NN-0.15BZN-yBT) ceramics were designed and successfully fabricated using solid state synthesis. The discharge storage density of the ceramics increases from 1.22 J/cm3 at y = 0–1.84 J/cm3 at y = 8%, which is 1.5 times higher than pure 0.85NN-0.15BZN ceramics. This ceramic reveals excellent temperature stability up to 150 °C, good discharge cycling after 105 cycles and high discharge speed of 2.0 μs, broadening the scope of NaNbO3-based ceramics for energy storage applications.

There is a general observation that the Curie temperature and piezoelectric property of the ferroelectric ceramics can be enhanced only at the expense of each other, i.e., higher piezoelectricity, lower Curie temperature, thus, limits their applications over broad temperature range. In this research, Sm-modified 0.15 Pb(Mg1/3Nb2/3)O3-(0.85-x)PbZrO3-xPbTiO3 ceramics have been studied, where excellent piezoelectric coefficients d33 = 720 pC/N, d33∗ = 950 pm/V and high Curie temperature TC = 293 °C were simultaneously achieved for x = 0.42 composition by designing the morphotropic phase boundary (MPB) with local structural heterogeneity. Of particular significance is that a high thermal stability was observed with piezoelectric variation below 20% with temperature up to 280 °C, demonstrating that the x = 0.42 composition is a good candidate for piezoelectric application over broad temperature range where high temperature stability is required. This work provides a good paradigm for designing high-performance piezoelectric ceramics with high thermal stability via a combination of MPB and local structural heterogeneity.

Based on the thermal-elastic-plastic theory, a three-dimensional finite element (FE) model for residual stress in multi-layer and multi-pass repair welding of P91 steel pipe with wall thickness of 60 mm is established, which considers the sequential coupling among thermal field and microstructure and mechanical properties. The arc heat input is described as a double-ellipsoid heat source, and the solid-state phase transformation (SSPT) concerning austenite and martensite during welding of P91 steel is modeled through allowing for the changes of material volume and yield stress as well as phase transformation plasticity in the finite element analysis. Using the established model, the residual stresses in repair welding of P91 steel pipe with similar and dissimilar filler metals are calculated through SYSWELD software, which are also compared with experimental results to validate the accuracy of the numerical model. The distribution features of residual stresses under the two conditions are comparatively investigated, and the influence of different filler metals on them is also analyzed. When using similar filler metal, the stress components and equivalent stress have strong fluctuations on the weld surface due to martensitic transformation, and there exists relatively wide compressive stress region on the weld surface. At the fusion zone (FZ) near the weld surface, a serious stress concentration appears. The tensile stress peak can reach 813 MPa, close to the yield stress of untempered martensite. While using Ni-based alloy as filler metal, the peak values of tensile stress components and equivalent stresses in the HAZ of weldment surface are higher than those for the former owing to lack of relaxation by phase transformation. In the interior of weldment, the region with high stress in the FZ is much smaller, and the welding stress in the HAZ is also lower compared with those for the former.

Nb-containing antiferroelectric materials have recently attracted great research interest as energy storage materials for pulsed power capacitors due to their extraordinary energy storage performances. In this case, the optimization of the energy storage performance is obtained by a compositional modulation of NaNbO3-Bi(Zn2/3Nb1/3)O3 bulk ceramics. An optimal energy performance can be obtained with a composition of 0.85NaNbO3-0.15Bi(Zn2/3Nb1/3)O3, which is accompanied by a stable charge energy density in temperatures up to 150 °C owing to its relaxor characteristics and excellent cycling stability after 105 cycles. This work further broadens the scope of NaNbO3-based ceramic applications in the area of pulsed power sources.

High-performance piezoelectric materials are pivotal to many electromechanical applications including piezoelectric actuators, sensors, and transducers. However, the general approach to achieve high piezoelectric properties by establishing morphotropic phase boundary (MPB) has limitation due to the weak anisotropy of the Gibbs free energy profile at the MPB region. Here, aliovalent Sm3+-doped 0.4Pb(Mg1/3Nb2/3)O3-(0.6-x)PbZrO3-xPbTiO3 piezoelectric ceramics were fabricated by a solid-state method, where the optimized piezoelectric coefficient d33 = 910 pC/N, dielectric constant er = 4090, and Curie temperature TC = 184 °C were obtained at x = 0.352, being attributed to the synergistic contributions from the MPB and enhanced local structural heterogeneity. Rayleigh analysis was adopted to study the intrinsic and extrinsic contributions in Sm-doped PMN-PZ-PT ceramics, where the extrinsic contribution was found to be on the order of 25-67% at 4 kV/cm. Of particular significance is that a large signal d33* = 820 pm/V (at 20 kV/cm) with a minimal strain variation of 5% was achieved for a composition of x = 0.372 over the temperature range of 20-160 °C, being superior to those previously reported piezoelectric ceramic materials. This work offers a good paradigm to simultaneously achieve high piezoelectric properties with good temperature stability in ferroelectric ceramics, which have great potential for piezoelectric application at elevated temperatures.

Repair welding is a popular method to repair the cracked zones of pipes in power plants. However, residual stresses induced by repair welding are inevitably generated and have a significant effect on the performance of repair-welded components during service. The purpose of this research is to investigate the effects of solid-state phase transformation (SSPT) on the repair welding residual stress distribution of multi-layer and multi-pass welding in P91 steel. Based on thermal elastic-plastic theory, a three-dimensional thermal–metallurgical–mechanical finite element model considering the SSPT is established, and shielded metal arc welding (SMAW) heat input is modelled as one double elliptical heat source. The effect of phase transformation on residual stress is investigated by considering effects with and without phase transformation in the database of materials. The results show that transverse compressive stress and a low longitudinal stress area are generated in the final pass due to martensite transformation. Compared with different zones along the thickness direction, the effect of SSPT is greater around the outer surface than around the internal fusion zone. The rationality of the developed finite element model is verified by the experimental method, and good agreement is obtained between those results from the numerical analysis and experimental methods.

(K,Na)NbO3 based ceramics are considered to be one of the most promising lead-free ferroelectrics replacing Pb(Zr,Ti)O3. Despite extensive studies over the last two decades, the mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics has not been fully understood. Here, we combine temperature-dependent synchrotron x-ray diffraction and property measurements, atomic-scale scanning transmission electron microscopy, and first-principle and phase-field calculations to establish the dopant–structure–property relationship for multi-elements doped (K,Na)NbO3 ceramics. Our results indicate that the dopants induced tetragonal phase and the accompanying high-density nanoscale heterostructures with low-angle polar vectors are responsible for the high dielectric and piezoelectric properties. This work explains the mechanism of the high piezoelectricity recently achieved in (K,Na)NbO3 ceramics and provides guidance for the design of high-performance ferroelectric ceramics, which is expected to benefit numerous functional materials., The mechanism for the enhanced piezoelectricity in (K,Na)NbO3 based ceramics has not been fully understood. Here, the authors find that the dopants induced tetragonal phase and the accompanying high-density nanoscale heterostructures are responsible for the high dielectric and piezoelectric properties.

A three-dimensional transient model is proposed to investigate the weld formation during laser + metal inert gas (MIG) hybrid welding of aluminum alloy for horizontal fillet joint, which considers the coupling of keyhole, droplet, and liquid metal pool as well as the influence of joint geometry feature and welding position. An asymmetrical double-elliptic heat source is utilized to model the arc heat input and a conic heat source with increased peak heat density is used to describe the laser energy. The coordinate rotation is used to take into account the inclination of heat source and the related forces. According to the experimental and simulated results, weld formation mechanism of horizontal fillet joint is studied. At a lower laser power, the fluid flow pattern in hybrid welding is similar to that in MIG welding and a strong downward flow caused by both gravity and arc pressure occurs at the liquid metal zone of the vertical plate, leading to the accumulation of molten metal at the horizontal plate and the resultant weld asymmetry. At high laser power, a clockwise vortex appears above the keyhole. In this case, the sagging of liquid metal near weld pool surface due to effect of gravity is an important factor responsible for the asymmetry of fillet weld. Besides, with increasing laser power, the tendency of weld pool free surface sagging is enhanced to some extent and weld bead surface becomes concave.

The research on Na0.5Bi0.5TiO3 based lead-free piezoelectric ceramics attracted widely attention during last decades. However, the electrical performance is still much inferior to that of lead-based counterparts, which limits its practical applications. Based on this situation, a ternary system of Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–BaTiO3 [(1 − 2x)NBT–xKBT–xBT] is constructed in this work, hoping to further improve its piezoelectric response. The ceramics were fabricated by the traditional solid state reaction approach, and their structure and electrical performance were studied systematically. Excellent piezoelectric properties with d33 = 146 and kp = 26.5% were obtained at x = 0.08, which was supposed to be related with the coexistence of tetragonal and rhombohedral phases and appropriate grain size. Besides these, a relatively high dielectric constant (er = 2250), a high remnant polarization (Pr = 31.5 µC/cm2) and low coercive field (Ec = 1.54 kV/mm) were also obtained at this composition. This work provides a new paradigm for the further optimization of NBT–KBT–BT based lead-free piezoelectric ceramics.

The Bi1 + xFe0.95Cr0.05O3 (BFCO) (x = 0, 5, 10, 15 and 20%) thin films are fabricated on FTO/glass substrate using a chemical solution deposition method and sequential-layer annealing process. The effects of the excess Bi content on crystalline structure, morphology, and electrical performance of BFCO thin films are investigated. All the BFCO thin films are crystallized into polycrystalline perovskite structure and belonging to the space group of R3c. The BFCO thin films with 5 and 10% excess Bi contents possess no impurity phase. Especially, a dense surface morphology and columnar crystal structure can be obtained for the film with 5% excess Bi content. Especially, the one possesses superior ferroelectricity with a relative high remnant polarization (P r) of 69.8 µC/cm2 and low coercive electric field (E c) of 291 kV/cm at 1 kHz due to the relatively low leakage current density of 3.04 × 10− 5 A/cm2 at 200 kV/cm.

Bi2Fe4O9 (BFO) is one of the most important photocatalyst materials and its composition with graphene may leave an optimizing effect on the photocatalytic performance. In this paper, reduced graphene oxide (RGO) with various contents is selected to be composited with BFO successfully via one-step hydrothermal method. A series of BFO-xRGO (x=0, 1.25, 2.50, 3.75, 5, 6.25, and 7.50 wt.%) were prepared and the effects of RGO content on crystalline, light absorption, impedance, and photocatalytic degradation rate of methyl violet (MV) solution are characterized. The entire film samples exhibit enhanced photocatalytic efficiency. Especially, with 5 wt.% RGO content added, the film sample shows the best photocatalytic degradation efficiency with a MV solution degradation rate of 95%. This implies that the composition of RGO allows BFO-based thin film as an efficient photocatalyst candidate, and as well, the BFO/RGO composite possesses the potential for better use in the related photocatalyst applications.

High-performance piezoelectric materials are pivotal to many electromechanical applications including piezoelectric actuators, sensors, and transducers. However, the general approach to achieve high piezoelectric properties by establishing morphotropic phase boundary (MPB) has limitation due to the weak anisotropy of the Gibbs free energy profile at the MPB region. Here, aliovalent Sm

Piezoelectric actuators are essential for numerous electromechanical applications, because of their advantages of high resolution, large generated forces, no friction, low heat dissipation and fast response. The design of the next-generation piezoelectric actuators is desired to meet the complex needs of advanced machines and instruments, where the high-performance and stable output under the combined thermal and mechanical loads are expected. However, simultaneously achieving high-performance and good temperature stability of piezoelectric actuators is a challenge, which hinders the use temperature range in practical applications. Here, in the studied Sm-doped Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 system, the high piezoelectric strain coefficient d33* = 870 pm/V with superior temperature stability (below 7% variation over the temperature of 20°C to 280°C), together with negligible property degradation up to 106 cycles was successfully achieved. The outstanding blocking stress of 37.2 MPa and mechanical work of 6.4 kJ/m3 were obtained under the driving electric field of 15 kV/cm. This work provides a paradigm to achieve high piezoelectric strain properties with good temperature stability, where the studied materials are expected to greatly benefit the development of high temperature piezoelectric actuator applications.

Compositional design by multiphase coexistence is regarded as a guiding rule for material design to develop high-performance piezoelectric materials. An open question is whether a certain PZT composition outside of the morphotropic phase boundary (MPB) region can obtain high piezoelectric response like that close to MPB region. Here, a non-MPB composition, rhombohedral Pb(Zr0.54Ti0.46)O3 was selected as the parent material. The optimized electrical properties were obtained by Sm doping with a large piezoelectric coefficient d33 of 590 pC/N, a high planar electromechanical coupling factors kp of 57.1%, a large piezoelectric strain S of 0.31% (d33∗ = 632 pm/V), and a promising Curie temperature of 335 °C, compared to the commercial PZTs. The enhancement of piezoelectric response can be ascribed to the donor characteristics by Sm replacement, which contribute to polarization orientation of ferroelectric dipoles. This work broadens the domain of high-performance piezoelectrics by developing novel solid solutions beyond the MPB framework.

The numerical simulation on laser trepanning of square-slotted hole and hole array in stainless steel sheets with and without ultrasonic assistance was reported for analyzing the stress fields on the workpieces trepanned. Based on reasonable modification and verification for the established models using corresponding experimental results, the transient and/or residual stress distribution characteristics, which were induced by single hole trepanning and hole array trepanning with and without ultrasonic assistance, were analyzed systematically and numerically using finite element method (FEM) coupled with self-developed subprograms after predicting the transient temperature fields. The influence of ultrasonic assistance on stress reduction was also discussed. It was shown that the distribution characteristics of equivalent von-Mises stress fields on the top surface of laser trepanned workpiece were different from those on its corresponding bottom surface. For laser-trepanned workpieces, the residual stress distribution characteristics predicted after sufficient cooling were different from the corresponding transient stress characteristics simulated without sufficient cooling. As a whole, the ultrasonic assistance resulted in a small change for the distribution trend of the transient/residual stress field predicted, while the influence of ultrasonic-induced softening effect on laser trepanned workpiece was primarily demonstrated by the effective reduction of the stress values.

The keyhole induced porosity in laser-MIG hybrid fillet welding of aluminum alloy in the horizontal position was investigated with simulation and experimental methods. A three dimensional model is developed, which takes into account geometric feature of horizontal filled joint and the coupling of keyhole, droplet and molten pool. Meanwhile, the major forces are also incorporated in the model. The inclination of heat source is dealt with through rotation of coordinate system. This model is able to calculate the keyhole dynamic behavior and the formation process of gas bubble as well as keyhole induced porosity directly. Also, it can describe the merging and disappearing of bubbles. The weld porosity was examined by the X-ray non-destructive testing. Large weld pool size provides more time for bubble to escape from the molten pool, which is the major factor responsible for the reduction in hybrid welding. Besides, the forward flow caused by the clockwise vortex is weakened, which also plays a positive role in improving the stability of back keyhole wall. The keyhole collapses at the middle part easily in horizontal fillet welding, raising the possibility of forming large pore. Bubble is easy to be captured by the upper weld pool boundary when floating up. With enhancing the welding current, the stability of keyhole is enhanced to some extent and the keyhole induced porosity is reduced.