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The influence of stress on the small‐signal dielectric permittivity and piezoelectric coefficient of polycrystalline lead‐free perovskite 0.92(Na1/2K1/2)NbO3–(0.08 − x)Bi1/2Li1/2TiO3–xBaZrO3 (x = 0, 0.02, 0.04, 0.06, and 0.07) was characterized under different constant uniaxial stress up to −200 MPa within a temperature range of −150 to 450°C, revealing stress‐induced suppression of the electromechanical response as well as shifts in the phase boundaries. For all compositions, the interferroelectric and ferroelectric–paraelectric phase transitions were shifted to higher temperatures under the uniaxial compressive stress. Interestingly, the sensitivity to the applied stress was found to increase with increasing BZ/BLT ratio in the system. The origin of a different extent of stress‐sensitivity with BZ/BLT ratio is suggested to be related to the change in the crystal structure. Additionally, at temperatures below −50°C, the relative permittivity showed a significant increase under applied compressive stress. [ABSTRACT FROM AUTHOR]

The past two decades have seen a great enhancement of piezoelectric coefficients (d33) to higher than 570 pC/N in (K, Na)NbO3 (KNN) piezoelectrics, but one notoriously unresolved issue is their severe temperature instability, obstructing them toward practical applications. The present work demonstrates a facile approach to overcome this problem by introducing a layered distribution of key dopants (Li and Sb) in a monolithic ceramic, featuring stepwise varied polymorphic phase transition (PPT) temperatures along the thickness direction. The dopant‐graded ceramic exhibits an outstanding d33 of 508 pC/N and a very large piezoelectric strain (Suni, of 0.18%). More importantly, an excellent temperature stability (d33 variation within 13% over the temperature range of 25–150 °C) is achieved, which is superior to that of most state‐of‐art KNN counterparts. These are attributed to the construction of spatially diffused PPT in combination with enhanced polarization, permittivity, and piezoresponse through interfacial effect, including the Maxwell–Wagner effect and intergranular stress by gradient doping. The results offer an alternative strategy for designing high‐performance piezoelectric materials with desirable temperature reliability. [ABSTRACT FROM AUTHOR]

Here, we introduce phase change mechanisms in lead-free piezoceramics as a strategy to utilize attendant volume change for harvesting large electrostrain. In the newly developed (K,Na)NbO3 solid-solution at the polymorphic phase boundary we combine atomic mapping of the local polar vector with in situ synchrotron X-ray diffraction and density functional theory to uncover the phase change and interpret its underlying nature. We demonstrate that an electric field-induced phase transition between orthorhombic and tetragonal phases triggers a dramatic volume change and contributes to a huge effective piezoelectric coefficient of 1250 pm V−1 along specific crystallographic directions. The existence of the phase transition is validated by a significant volume change evidenced by the simultaneous recording of macroscopic longitudinal and transverse strain. The principle of using phase transition to promote electrostrain provides broader design flexibility in the development of high-performance piezoelectric materials and opens the door for the discovery of high-performance future functional oxides. Functional oxides with coexisting states of comparable energy typically exhibit extraordinary responses to external stimuli. Here, the authors demonstrate that coexisting phase structures provide large electric field-triggered volume change. [ABSTRACT FROM AUTHOR]

External thermal, electrical, and mechanical fields can induce structural phase transitions in lead‐free Li‐modified Na0.5K0.5NbO3 ferroelectrics, which significantly influence the macroscopic electromechanical response. In particular, the relative stability of the polar monoclinic (or orthorhombic) and tetragonal phases under temperature and stress is critical to realize the ferroelectric and piezoelectric response. In this study, the effect of mechanical and thermal fields on the local structure in the vicinity of the monoclinic‐tetragonal (M‐T) phase boundary was investigated using a novel in situ combined uniaxial compressive stress‐ and temperature‐dependent Raman spectroscopy experimental arrangement. Experiments were performed up to 300°C and −200 MPa, clearly demonstrating stress‐induced M‐T phase transition in Li‐modified Na0.5K0.5NbO3. A stress‐temperature phase diagram has been established based on the change in vibrational modes. It was possible to correlate the relative permittivity singularities previously observed to a given stage of the M‐T phase transition using ratio between characteristic Raman band areas. In addition, the measurement method reported here can be applied to other functional ceramics to investigate the influence of mechanical fields on local structure. [ABSTRACT FROM AUTHOR]

Robust resistivity against electrical cycling at not only ambient conditions but also enhanced temperatures is an essential requirement for high-end actuator applications. In this study, the temperature and electric field dependence of unipolar fatigue behaviors of CaZrO3-modfied (K, Na)NbO3 lead-free piezoceramics were investigated. The space charge accumulation during unipolar cycling is responsible for the build-up of internal bias field, the dynamics of which with respect to temperature and driving field can be described using a model based on Maxwell-Wagner relaxation process. Besides, clamping of domain walls can be inferred by comparing the large and small signal permittivity. Most intriguingly, the unipolar strain exhibits a fatigue-free behavior even at elevated temperatures, rendering the material exceptionally suitable for actuator applications. [ABSTRACT FROM AUTHOR]

Microscale 1–3-type piezoceramic/epoxy composites using Li/Ta-modified (Na,K)NbO3 (NKN-LT) lead-free piezoelectric ceramics were fabricated via a modified dice-fill method, by which the NKN-LT ceramic rods were miniaturized to a lateral width of 150 μm with an aspect ratio (height/width) higher than 3.5. The piezoelectric properties of the 1–3 composites as a function of ceramic volume fraction were measured and compared with theoretical predictions. In addition to a high electromechanical coupling coefficient (kt∼50%) and low acoustic impedances (Z∼10 Mrayls), the developed composites show a clear single thickness mode with a high resonance frequency over 3.5 MHz. The present Pb-free piezocomposites are suitable for medical imaging devices embedded in the human body. [ABSTRACT FROM AUTHOR]

1–3 connectivity lead zirconic titanate (PZT)/epoxy composite thick films containing fine-scale and high aspect ratio PZT pillars (40 μm sectional width, 40 μm spacing, and 5.5 aspect ratio) were obtained by a modified dice and fill method. The composite thick film works at a high frequency >7 MHz with a clear and uncoupled thickness mode, together with a relatively high electromechanical coupling coefficient (kt=56.4%), low acoustic impedance (Z=7.8 Mrayls), and low mechanical Qm (3.3). High piezoelectric coefficient (d33=332 pC N-1) along with reduced dielectric constant ([variant_greek_epsilon]r=105) endowed the composite films an enhanced piezoelectric voltage coefficient g33 value of 357×10-3 m2/C, which is required to increase the receiving voltage sensitivity. The present piezocomposite thick film with microscale ordered structure is promising for high-frequency ultrasonic transducers. [ABSTRACT FROM AUTHOR]

A pseudo-1-3-type multiferroic composite consisting of Pb(Zr,Ti)O3 (PZT) rod array (with base) and Terfenol-D/epoxy matrix was prepared by the dice-and-fill technique. Simple series and parallel mixture rules well described the measured dielectric and piezoelectric constants. Large magnetoelectric coefficients were observed in the pseudo-1-3-type composite, e.g., over 300 mV/cm Oe below 40 kHz and over 4500 mV/cm Oe at resonant frequency. The magnetoelectric response strongly depends on the magnetostrictive behavior of the matrix and the volume fraction of PZT rods, which gives us two convenient ways to modify their magnetoelectric response. For this pseudo 1-3-type multiferroic composite, the remarkable magnetoelectric response and well-developed fabrication technique are advantageous for their practical applications in piezoelectric-magnetoelectric multifunctional devices and large bandwidth magnetic sensors. [ABSTRACT FROM AUTHOR]

Abnormal grain growth (AGG) is frequently observed in sintered (K, Na)NbO3 (KNN)‐based piezoceramics. However, in the present study, abnormal grain growth was unexpectedly discovered in calcined KNN‐based powders. To explain the phenomenon, three well‐established models that account for the AGG in sintered ceramics were discussed, including (a) liquid‐phase‐assisted grain growth, (b) two‐dimensional nucleation grain growth, and (c) complexion coexistence. However, the AGG in calcined powders was concluded to be none of them, but a consequence of the A‐site compositional inhomogeneity in the K2CO3Na2CO3‐Nb2O5 ternary system. Since repeated calcination and ball milling have low efficiency on solving AGG and the accompanied compositional inhomogeneity, abnormal grains were found to coexist with normal grains at a very high calcination temperature, that is, 1000°C. The compositional inhomogeneity is believed to be remaining even after sintering and consequently deteriorate the comprehensive performances, which might be a determinant for the unstable reproduction of KNN‐based piezoceramics. [ABSTRACT FROM AUTHOR]

Abstract: During high‐temperature crystal growth, lattice defects will inevitably form inside piezoelectric materials, which can be a hindrance for performance optimization. Through appropriate atmosphere control during sintering, defect levels inside the piezoelectric material can be regulated. Herein, CaZrO3‐modified (K, Na)NbO3‐based lead‐free piezoelectric ceramics with a nominal composition of 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3‐0.05CaZrO3 are produced by sintering in an oxygen‐rich atmosphere. Compared with an air‐sintered sample, the piezoelectric constant of the oxygen‐sintered sample has greatly improved 15% up to 390 pC/N, which is comparable to commercial lead‐based counterparts. In addition, the planar electromechanical coupling factor kp is enhanced from 0.46 to 0.52. A qualitative model related to defect engineering is proposed to support the experimental observations. Our results indicate the feasibility of purposely optimizing the piezoelectric performance by sintering atmosphere control. [ABSTRACT FROM AUTHOR]

Monoclinic phases, as intermediate phases at the morphotropic phase boundary, have been considered as the origin of high piezoelectricity in various piezoelectric systems. However, those monoclinic phases have seldom been experimentally observed at the phase boundary in lead-free (K,Na)NbO3 (KNN)-based materials. This work shows that KNN epitaxial thin films demonstrate a monoclinic MC phase stabilized by elastic strain at room temperature, which is identified by both synchrotron X-ray reciprocal space mapping and piezoresponse force microscopy. It is revealed that the piezoelectricity is greatly enhanced at 170 °C due to the monoclinic MC-MA phase boundary. The discovery of monoclinic phases in this work may open an avenue for developing a new thermotropic phase boundary in KNN-based materials. The results also provide a guide for the exploration of high piezoelectricity driven by monoclinic phases in lead-free piezoelectric materials. [ABSTRACT FROM AUTHOR]

This work investigated the effect of MnO2 addition on the phase structure, microstructure, and electrical properties of AgSbO3-modified (Li,K,Na)(Nb,Ta)O3 (abbreviated as LKNNT-AS) lead-free piezoelectric ceramics with an optimized composition endowed with a state of two-phase coexistence. A small amount (0.1 wt%) of MnO2 can significantly further enhance the piezoelectric property of LKNNT-AS ceramics, whose piezoelectric constant d33 and converse piezoelectric coefficient d33* as well as planar electromechanical coupling factor kp reach 363 pC/N, 543 pm/V, and 0.49, respectively. The temperature stability of piezoelectricity in MnO2-modified LKNNT-AS samples also improved, which is associated with the more uniform and better thermally stable ferroelectric domains that were revealed by piezoresponse force microscopy. [ABSTRACT FROM AUTHOR]

(1 − x)Li(KNa)(NbTa)O- xAgSbO (abbreviated as LKNNT- xAS, 0.045 < x < 0.055) lead-free ceramics with a high relative density (~95 %) were successfully fabricated by the conventional method. Only a small AgSbO composition interval of 0.25 mol% was implemented for the purpose of property enhancement via the precise phase control. Room temperature XRD patterns confirmed that two phase coexistent structure of orthorhombic and tetragonal phases was obtained and the volume fraction of orthorhombic phase decreased as AgSbO increased from 0.045 to 0.055. A relatively high d value up to 289 pC/N was achieved in the LKNNT-0.0525AS ceramics. Meanwhile the planar electromechanical coupling factor k, the remnant polarization P and the relative dielectric constant ε reached 0.42, 16.3 μC/cm, 1967, respectively. Considering the electrical properties, AgSbO modified (Li,K,Na)(Nb,Ta)O ceramics may be a potential candidate of lead-free piezoelectric ceramics. [ABSTRACT FROM AUTHOR]

The temperature dependence of piezoelectric properties (direct piezoelectric coefficient d33, converse piezoelectric coefficient d33( E = 0), strain S and electromechanical coupling coefficient kp) for two niobate-based lead-free piezoceramics have been contrasted. 0.92(Na0.5K0.5)NbO3-0.02(Bi1/2Li1/2)TiO3-0.06BaZrO3 (6 BZ/2 BLT/92 NKN) has a morphotropic phase boundary ( MPB) between rhombohedral and tetragonal at room temperature and 0.92(Na0.5K0.5)NbO3-0.03(Bi1/2Li1/2)TiO3-0.05BaZrO3 (5 BZ/3 BLT/92 NKN) features an MPB engineered to be located below room temperature. At 30°C, d33, d33( E = 0), S (at 2 kV/mm), and kp are 252 pC/N, 230 pm/V, 0.069%, 0.51 for 5 BZ/3 BLT/92 NKN; and 348 pC/N, 380 pm/V, 0.106%, 0.57 for 6 BZ/2 BLT/92 NKN, respectively. With increasing temperature, the piezoelectric properties decrease. At 200°C, d33, d33( E = 0), S (at 2 kV/mm), and kp are 170 pC/N, 160 pm/V, 0.059%, 0.36 for 5 BZ/3 BLT/92 NKN; and 181 pC/N, 190 pm/V, 0.061%, 0.39 for 6 BZ/2 BLT/92 NKN. It is found that the electromechanical coupling coefficient has a better temperature stability than the piezoelectric coefficient in the studied system due to a large temperature-dependent compliance change. The results demonstrate that engineering an MPB is highly effective in tailoring temperature stability of piezoceramics. [ABSTRACT FROM AUTHOR]

The phase transition temperatures of CaZrO3-doped (Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3 lead-free piezoelectrics ( LKNNT- CZ5) have been studied with X-ray diffraction and Raman scattering. It is found that LKNNT- CZ5 ceramics and powders with the same crystallite size (~2-5 μm) exhibit distinctly different phase transition behaviors versus temperature. The ceramic bulk changes its orthorhombic phase to tetragonal structure abruptly after being pulverized to powders at room temperature( RT), and follows a phase transition sequence with increasing temperature: orthorhombic ( RT-65°C), orthorhombic and tetragonal (65°C-105°C), and tetragonal phase (105°C to ~220°C). Contrastively, the powders hold a tetragonal structure without any phase transition within the whole temperature region ( RT-220°C). These differences might be induced by the existing internal stress or residual strain, which could be released during the grinding process. [ABSTRACT FROM AUTHOR]

The growing environmental concerns have been pushing the development of viable green alternatives for lead-based piezoceramics to be one of the priorities in functional ceramic materials. A polymorphic phase transition has been utilized to enhance piezoelectric properties of lead-free ( K, Na) NbO3-based materials, accepting the drawbacks of high temperature and cycling instabilities. Here, we present that CaZrO3-modified ( K, Na) NbO3 piezoceramics not only possess excellent performance at ambient conditions benefiting from nanodomain engineering, but also exhibit superior stability against temperature fluctuation and electrical fatigue cycling. It was found that the piezoelectric coefficient d33 is temperature independent under 4 kV/mm, which can be attributed to enhanced thermal stability of electric field engineered domain configuration; whereas the electric field induced strain exhibits excellent fatigue resistance up to 107 sesquipolar cycles. These findings render the current material an unprecedented opportunity for actuator applications demanding improved thermal and cycling reliabilities. [ABSTRACT FROM AUTHOR]

The bipolar fatigue behavior of (K,Na)NbO3 (KNN)-based lead-free piezoceramics was investigated. A comparative analysis demonstrated that CaZrO3-modified KNN ceramics exhibited highly enhanced bipolar fatigue resistance due to the reduced lattice distortion (c/a ratio) and coexistence of orthorhombic and tetragonal phases. The hypothesis was verified by systematical studies of cycle-dependent large and small signal parameters and micro-morphologies. It was identified that mechanical stress induced microstructure damage beneath the electrodes renders the KNN ceramics to be vulnerable to bipolar cycling; while the mild fatigue behavior for CaZrO3-modified ones mainly originates from a thermally reversible domain wall pinning. [ABSTRACT FROM AUTHOR]

0.5Pb(NiNb)O-(0.5 − x)PbTiO- xPbZrO ( x = 0.10-0.20) polycrystalline ceramics were fabricated by solid-state reaction method. The phase structures, microstructure and temperature dependence of the electrical properties were investigated in detail. Both X-ray diffraction and Raman spectroscopy results indicated that the morphotropic phase boundary (MPB) separating tetragonal from rhombohedral phases located around the region with x = 0.14-0.16. Well-saturated ferroelectric P- E loops were obtained for all compositions and the compositions near MPB possessed the relative large remnant polarization P ~ 24 μC/cm and low coercive field E ~ 5 kV/cm. High performance of electric-induced-strain property with $$ d_{33}^{*} $$ ( S /E) up to 1,140 pm/V could be achieved in the composition with x = 0.15. When the content of PbZrO was 14 mol%, the ceramics showed slightly lowered piezoelectric coefficient $$ d_{33}^{*} $$ of 898 pm/V. But excellent temperature stability of piezoelectric property was exhibited that the $$ d_{33}^{*} $$ changed <10 % when the temperature increased from room temperature to 120 °C. The piezoelectric property of this solid solution makes it promising for application in multilayer piezoactuators. [ABSTRACT FROM AUTHOR]

The unipolar fatigue behavior of CaZrO3 and MnO2 co-modified (K,Na,Li)(Nb,Ta)O3 lead-free piezoceramics was investigated systematically. The well-known charge agglomeration model is shown to explain the overall changes observed during unipolar fatigue, such as the development of bias field as well as the anisotropy in bipolar strain hysteresis and field-dependent dielectric permittivity. In addition, it is found that the unipolar strain exhibits only small degradation within 3% at the field amplitude of 2 kV/mm up to 107 cycles. This exceptionally good fatigue resistance is identified due to the presence of additional process, assigned as a 'softening' effect that competes against the usual fatigue effect. [ABSTRACT FROM AUTHOR]

The development of lead-free piezoceramics has attracted great interest because of growing environmental concerns. A polymorphic phase transition (PPT) has been utilized in the past to tailor piezoelectric properties in lead-free (K,Na)NbO3 (KNN)-based materials accepting the drawback of large temperature sensitivity. Here a material concept is reported, which yields an average piezoelectric coefficient d33 of about 300 pC/N and a high level of unipolar strain up to 0.16% at room temperature. Most intriguingly, field-induced strain varies less than 10% from room temperature to 175 °C. The temperature insensitivity of field-induced strain is rationalized using an electrostrictive coupling to polarization amplitude while the temperature-dependent piezoelectric coefficient is discussed using localized piezoresponse probed by piezoforce microscopy. This discovery opens a new development window for temperature-insensitive piezoelectric actuators despite the presence of a polymorphic phase transition around room temperature. [ABSTRACT FROM AUTHOR]

AgSbO3 was doped into KNN-based lead-free piezoceramics with an optimized composition of Li0.02( Na0.53 K0.48)0.98 Nb0.8 Ta0.2 O3 (abbreviated as LKNNT) to further enhance its piezoelectric property. The doping of AgSbO3 was found to be effective in reducing the grain sizes, resulting in more uniform microstructure in AgSbO3-doped LKNNT ceramics. AgSbO3 lowers tetragonal-orthorhombic phase transition point ( TT-O), but with a more gentle rate as compared with other dopants. A large converse piezoelectric coefficient d33* up to 598 pm/V under a relatively low electric field of 1 kV/mm was obtained in the LKNNT-5 mol% AgSbO3 composition, whose tetragonal-orthorhombic phase transition point ( TT-O) was controlled near room temperature, but its Curie temperature was kept at 235°C. The d33* obtained in the present material is a very high value for nontextured KNN-based ceramics, which is attributed to the polymorphism phase transition effect and 'soft' behavior caused by the addition of AgSbO3. [ABSTRACT FROM AUTHOR]

Volatilization of alkali elements during sintering process is one of the major factors influencing the processing stability of lead-free (Na,K)NbO (KNN) piezoceramics. With a purpose to reduce sintering temperature by refining particle sizes, an extensive ball-milling step was intentionally introduced to the fabrication process of KNN-based ceramics with a nominal composition of Li(NaK)NbO by a solid-state powder synthesis and normal sintering method. It was revealed that prolonging the period for milling calcined powders before sintering process significantly reduced the particle sizes and hence sintering temperature. As a result, sintering at a reduced temperature of 1,025 °C resulted in a highly dense material with good piezoelectric and ferroelectric properties with converse piezoelectric coefficient d* and remnant polarization P reaching 266 pm/V and 15.7 μC/cm, respectively. This study provided a facile and effective method to enhance the piezoelectric properties in KNN-based ceramics with an optimized composition by reducing the particle sizes. [ABSTRACT FROM AUTHOR]

BiFeO-modified lead-free piezoceramics Li(NaK)NbTaO (LKNNT) were successfully fabricated by conventional method to investigate its influences on phase structure and electricity as well as mechanical properties. A tiny amount of BiFeO (BF) changes the phase structure of LKNNT to tetragonal and further to pseudocubic, and also effectively improves the sintering ability and densification of LKNNT which enhances the mechanical property. The LKNNT piezoceramics with the addition of 1 mol% BF has the bulk density of 5.02 g/cm and an enhanced fracture strength of 142 MPa which is even higher than that of the sample of similar composition prepared by spark plasma sintering, also possesses room temperature electric properties of d* ~ 225 pm/V, k ~ 36.3%, ε ~ 1140, tgδ ~ 0.018, T ~ 305 °C and Q ~ 80. The d* remains constant up to 150 °C and the change ratio of dielectric property largely decreases over the temperature range of −50-150 °C, which can be attributed to the temperature-independent phase structure of LKNNT piezoceramics, making the ceramics as a promising candidate for actual applications. [ABSTRACT FROM AUTHOR]

Abstract: Highly dense Li/Ta co-doped (K,Na)NbO3 lead-free ceramics were prepared by spark plasma sintering (SPS) in a relatively wide temperature window. The influences of post-annealing temperature on phase structure, electrical properties and grain growth were investigated. Through tailoring post-annealing temperature, relative dielectric constant ɛ r, and piezoelectric coefficient d 33 of SPSed KNN-based samples can be enhanced to 960 and 225pC/N for the same composition compared with normally sintered values (771 and 206pC/N). The enhanced electrical properties were discussed from the point view of microstructure (grain size and its distribution, porosity) and phase transition, which occurred when post-annealing temperature exceeded 1050°C. Also, with increasing post-annealing temperature, three grain growth behavior stages were investigated on the balance between the volatilization of alkali metal oxides and secondary grain growth. [Copyright &y& Elsevier]

The phase structure and electrical properties of (K,Na)NbO (KNN)-based lead-free piezoelectric ceramics with nominal composition Li (NaK)NbTaO (LKNNT, x = 0.50−0.55) were investigated with an emphasis on the influence of Na/K ratio. By XRD and Raman measurements, it was revealed that the phase transition from the co-existence of O and T to the co-existence of O and T occurs when x = 0.52. At this Na/K ratio, a peak of dielectric constants was obtained, which also corresponds to a morphotropic phase boundary between T and T besides that between O and O in LKNNT ceramics. All the Li (NaK)NbTaO ceramics with x = 0.50 − 0.55 show excellent piezoelectric performance, and the piezoelectric coefficient d* can reach as high as 335 pm/V. The room-temperature piezoelectric properties in the present LKNNT ceramics are not sensitive to the change of Na/K ratio, indicating that the orthorhombic to tetragonal phase co-existence dominates the enhancement of piezoelectric properties. However, the results on the piezoelectricity measurement at elevated temperature showed possible to further enhance piezoelectric properties by adjusting Na/K ratio in the tetragonal LKNNT ceramics. [ABSTRACT FROM AUTHOR]

Dense and fine-grained (Na0.535K0.485)1− xLi x(Nb0.8Ta0.2)O3 ( x=0.02–0.07) (abbreviated as NKL xNT) lead-free piezoceramics were fabricated by spark plasma sintering (SPS), followed by postannealing in air. All the NKL xNT ceramics showed single perovskite structures with a phase transition from an orthorhombic symmetry to a tetragonal one across a composition region of 0.04< x<0.05. The sample with a composition of x=0.05 had the maximum values of piezoelectric coefficient ( d33=243 pC/N) and planar electromechanical coupling coefficient ( kp=46.1%), and other good properties such as Qm=85, ℇr=1240, and tg δ=0.023. Because of the enhanced densification and refined microstructure, the SPS-processed NKL xNT ceramics also had better fracture strength than that prepared by normal sintering. [ABSTRACT FROM AUTHOR]

Abstract: A microfabrication method based on MEMS technology was developed to fabricate microscale multifunctional composites with 1–3 type patterned structure by combining silicon micromolding with sol-gel processing. By this method, a microscale rod array of lead-free Na0.5K0.5NbO3 piezoelectric ceramics was fabricated, which has a feature size of 20 µm lateral width and 30 µm height. Piezoceramic/epoxy composite thick films containing such an array as piezoelectric-active fillers were fabricated by vacuum infiltration, in which the piezoelectricity of the embedded Na0.5K0.5NbO3 rods was confirmed by a scanning probe system. [Copyright &y& Elsevier]

Microstructure characteristics, phase transition, and electrical properties of (Na0.535K0.485)0.926Li0.074(Nb0.942Ta0.058)O3 (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d33 is enhanced to 276 pC/N, which is a high value for the Li- and Ta-modified (Na,K)NbO3 ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor kp=46.2%, thickness electromechanical coupling factor kt=36%, mechanical quality factor Qm=18, remnant polarization Pr=21.1 μC/cm2, and coercive field Ec=1.85 kV/mm. [ABSTRACT FROM AUTHOR]

A microfabrication process combining sol–gel processing and silicon micromolding was proposed to fabricate microrod arrays of K0.5Na0.5NbO3 (KNN) lead-free piezoelectric ceramics. By ultraviolet lithography and inductively coupled plasma deep reactive ion etching, a Si mold with a regular array of columnar cavities was micromachined, and then filled with KNN sol, followed by pyrolysis and annealing treatment. By optimizing the processing conditions, KNN ceramic microrod arrays of 15 μm lateral width, 30 μm height, and 70 μm spacing were produced, which were identified as perovskite phase structure by X-ray diffraction. [ABSTRACT FROM AUTHOR]

Conventional sintering of (Na1− xK x)0.5Bi0.5TiO3 (abbreviated as NKBT x, x=18–22 mol%) lead-free piezoelectric ceramics was investigated to clarify the optimal sintering temperature for densification and electrical properties. Both sintered density and electrical properties were sensitive to sintering temperature; particularly, the piezoelectric properties deteriorated when the ceramics were sintered above the optimum temperature. The NKBT20 and NKBT22 ceramics synthesized at 1110°–1170°C showed a phase transition from tetragonal to rhombohedral symmetry, which was similar to the morphotropic phase boundary (MPB). Because of such MPB-like behavior, the highest piezoelectric constant ( d33) of about 192 pC/N with a high electromechanical coupling factor ( kp) of about 32% were obtained in the NKBT22 ceramics sintered at 1150°C. [ABSTRACT FROM AUTHOR]

Abstract: Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. (K, Na)NbO3-based ceramics are one of the most promising candidates. Normal sintering of un-doped and Li/Ta co-doped (K, Na)NbO3 ceramics was investigated to clarify the optimal sintering condition for densification, microstructure and electrical properties. It was found that density increased greatly within a narrow temperature range but turned to decrease when the sintering temperature slightly exceeded the optimal one. Piezoelectric properties also showed similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density. The grain growth and property change as a function of sintering temperature were discussed on basis of the formation of liquid-phase and the composition deviation caused by the volatilization of alkali components during sintering. [Copyright &y& Elsevier]

A unique core–shell structure was observed in coarse grains in (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics. It is morphologically different from the chemical inhomogeneity-induced core–shell grain structure reported previously in BaTiO3-based ceramics. The core region is composed of highly parallel nanosized subgrains, whereas the shell region consists of larger-sized but similar self-assembled subgrains. The electron-backscattered diffraction analysis and selected area electron diffraction pattern confirmed that coarse grains with a core–shell structure were single-crystalline-like grains. The formation process of such coarse grains was then discussed based on mesocrystal growth along with the classical theory of grain growth. The two studied KNN-based systems showed a similar grain growth transformation: from self-assembled aggregation clusters with nanosized subgrains to a typical core–shell grain structure when the sintering temperature was increased only by a range of 10°–20°C. The volatilized alkali metal oxides and liquid phase were supposed to accelerate such grain growth transformation. When abnormal grown grains with a core–shell structure occurred, both systems showed the highest densities and dielectric constants along with the lowest dielectric losses, while their piezoelectric properties tended to decline. [ABSTRACT FROM AUTHOR]

1-3 piezoceramic/epoxy composites containing 26 and 55 vol% potassium sodium niobate (KNN) ceramic phase were fabricated via a dice and fill method, respectively, by which fine-scale KNN ceramic rods with a high aspect ratio (height/width) of 8.5, a lateral width of 80 μ m and 70 μ m spacing were obtained. The microstructure observation confirmed that the ceramic rods and epoxy matrix were well bonded. The electrical properties, including some quaisistatic properties of piezoelectric coefficient d33 and dielectric constant ε33T/ε0, along with some dynamic properties such as mechanical quality factor Qm, coupling coefficient Kt and acoustic impedance Z were measured and compared with the theoretical values as well as those of the monolithic KNN ceramic. The present study revealed that KNN-based lead-free piezoelectric ceramics/epoxy composites have their advantages in the ultrasonic transducers applications. [ABSTRACT FROM AUTHOR]

The effects of sintering temperature on microstructure, density and electrical properties of Na0.5K0.5NbO3 prepared by normal sintering at 1020-1100°C were investigated. All the Na0.5K0.5NbO3 ceramics showed an orthorhombic structure similar to that of KNbO3 ceramics, while raising sintering temperature resulted in the shifting of diffraction peaks to a lower value and the formation of a second phase due to the excess volatilization of Na. Both density and d33 of the sintered samples increase with increasing sintering temperature. The density of the sample sintered at 1080°C reaches a maximum value (4.22 g/cm3, 92% of the theoretical density). The sample sintered at 1060°C showed a maximum d33 value of about 122 pC/N and the highest Tc of about 409°C, and the corresponding Ec and Pr were about 13 kV/cm and 15 μ C/cm2, respectively. [ABSTRACT FROM AUTHOR]

Lead-free piezoelectric ceramics have received attention because of increasing interest in environmental protection. Niobate ceramics such as NaNbO3 and KNbO3 have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na0.5K0.5NbO3 ceramics were prepared using spark plasma sintering (SPS). Although the SPS temperature was as low as 920°C, the density of the Na0.5K0.5NbO3 solid solution ceramics was raised to 4.47 g/cm3 (>99% of the theoretical density). After post-annealing in air, reasonably good ferroelectric and piezoelectric properties were obtained in the Na0.5K0.5NbO3 ceramics with submicron grains. The crystal phase of the Na0.5K0.5NbO3 has an orthorhombic structure. The Curie temperature is 395°C and the piezoelectric parameter ( d33) of the Na0.5K0.5NbO3 ceramics reached 148 pC/N. [ABSTRACT FROM AUTHOR]

Describes the fabrication of fine-grained piezoelectric lead zirconia titanate ceramics that show an enhanced photovoltaic effect by pressureless sintering of oxide powders treated by mechanical alloying. Fabrication procedure; Phase identification and microstructure characterization of sintered bodies; Photovoltage of the PLZT ceramics.

Porous ceramics of lead zirconate titanate (PZT) were prepared by sintering powder compacts consisting of PZT and stearic acid powders in an air atmosphere; stearic acid was added as a pore-forming agent (PFA). The dielectric, elastic and piezoelectric properties of uniformly porous PZT ceramics were investigated as a function of the porosity volume fraction. Furthermore, a beam-shaped PZT actuator sample with a graded porosity content across its thickness was fabricated by sintering PFA-graded powder compacts. The electric-fieldinduced bending displacement characteristics of the actuator samples were measured by using strain gauges and were found to be in good agreement with the theoretical predication based on a classical lamination theory. [ABSTRACT FROM AUTHOR]

A full set of elastic and electrical coefficients of (K,Na)NbO3-based lead-free piezoceramics with a nominal composition of Li0.03(K0.48Na0.52)0.97(Nb0.8Ta0.2)O3 (abbreviated as KNNLT) was evaluated by the standard resonance method, and compared to those of K4CuNb8O23 doped (K0.45Na0.55)NbO3 (KNN-KCN) as well as typical Pb(Zr,Ti)O3-based piezoceramics PZT5A. The measurement of intermediate elastic stiffness and compliance coefficients of KNNLT indicated that KNNLT is elastically 'softer' than KNN-KCN but 'harder' than PZT5A. An extremely high piezoelectric stiffness coefficient h33 = 68.8 × 108 V/m was obtained, due to an especially low clamped dielectric constant [variant_greek_epsilon]33S of 361. And a large longitudinal electromechanical coupling factor k33 = 57% was observed in the KNNLT system. Furthermore, the piezoelectric coefficient d33 of KNNLT was characterized via three distinctive methods: the resonance method, the Berlincourt method, and the converse piezoelectric strain measurement; while these three techniques yielded values of 174 pC/N, 195 pC/N, and 308 pm/V, respectively. The difference in d33 values determined by the three methods may be attributed to different measurement frequencies and extrinsic piezoelectric contributions related to domain wall vibration and domain switching. Besides, the piezoelectric performances of KNNLT piezoceramics were found insensitive to frequency but susceptible to temperature and electric field cycling. [ABSTRACT FROM AUTHOR]

Lead-free alkali niobate piezoelectric ceramics with nominal compositions [(Na0.535K0.480)0.942+xLi0.058](Nb0.90Ta0.10)O3 compensated with Na and K at a fixed and optimized ratio of 0.535:0.480 were prepared by normal sintering. The electrical properties were investigated with a special emphasis on the influence of compensating amount x of Na0.535K0.480. A phase transition from tetragonal to orthorhombic was observed as x was increased from 0 to 0.024, resulting in enhanced piezoelectric coefficient and planar electromechanical coupling coefficient from 216 and 38.1% to 268 pC/N and 46.2%, respectively. Our results indicate the importance of elaborate compositional control for enhancing piezoelectric properties in niobate-based ceramics. [ABSTRACT FROM AUTHOR]

LiNbO3-doped (Na,K)NbO3 lead-free piezoelectric ceramics were prepared by normal sintering, and the electrical properties were investigated with a special emphasis on the influence of sintering temperature. The ceramics synthesized at 1020–1080 °C showed a phase transition from orthorhombic to tetragonal symmetry, which is similar to the morphtropic phase boundary (MPB). Because of such MPB-like behavior, a high piezoelectric coefficient d33 (314 pC/N) was obtained in the nominal composition 0.058LiNbO3–0.942[(Na0.535K0.480)NbO3] ceramic sintered at 1060 °C; however, this high d33 value was reported previously only in the Li-modified (Na,K)NbO3-based ceramics with codopants of Ta and Sb to B site. [ABSTRACT FROM AUTHOR]

As a model for composite materials of piezoelectric ceramic and metal, lead zirconate titanate (PZT) and platinum (Pt) particulate composites were fabricated by power processing. The electrical and mechanical properties of the PZT–Pt composites were measured as a function of the Pt volume fraction. The relative dielectric constants of the PZT–Pt composites increased markedly, while the piezoelectric constants and electromechanical coupling coefficients decreased with increasing Pt content. When the Pt volume fraction exceeded 30%, the PZT–Pt composite became electrically conductive because of percolation of the Pt particles. The Pt-dispersed PZT composites enhanced the mechanical properties, particularly the high fracture resistance, compared to the monolithic PZT ceramics. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Because of the environmental concerns of eliminating lead from piezoelectric products, bismuth‐based perovskite is becoming one of the most potential candidates. However, its relatively low thermal depolarization temperature (Td) is still an imperative obstacle hindering implementation of this material for practical application. Here, an enhanced temperature stability of 0.94(Bi0.5Na0.5)TiO3‐0.06BaTiO3 (BNTBT6) piezoceramics is reported, which can be obtained by the effective quenching process. Quenching process enhances the depolarization temperature to 136 °C, which is 40 °C higher than that of normal sintered samples. By using X‐ray photoelectron spectroscopy and electron paramagnetic resonance methods, it is revealed that oxygen vacancy may exist in the quenched samples and consequently pins the domain walls, resulting in significant enhancement of depolarization temperature. Temperature‐dependent dielectric, piezoelectric, and ferroelectric behaviors are measured as criteria to evidence the enhanced temperature stability. A quenching and annealing process is used to systematically investigate the depolarization temperature of 0.94(Bi0.5Na0.5)TiO3‐0.06BaTiO3 lead‐free piezoceramics. Oxygen vacancy, which is proved by X‐ray photoelectron spectroscopy and electron paramagnetic resonance measurements, may mainly contribute to the enhanced depolarization temperature. Structure evolution is also investigated. This work definitely sheds light on possible alternatives of increasing temperature stability of (Bi0.5Na0.5)TiO3‐based piezoelectric ceramics. [ABSTRACT FROM AUTHOR]

Abstract A morphotropic phase boundary (MPB) between rhombohedral (R) and tetragonal (T) phases was identified in a few (0.9-x)NaNbO 3 -0.1BaTiO 3 -xABO 3 (x = 0–0.05) lead-free systems. Critical roles of R-phase inducers were specially evaluated in terms of phase boundary position, microstructure and piezoelectric responses. The results indicate not only the tolerance factor of the ABO 3 additive but also its ferroelectricity and corresponding volume change would influence the formation of phase boundary and further determine dielectric and ferroelectric responses. The piezoelectric coefficient d 33 of MPB compositions was compared with theoretically-calculated d 33-cal according to d 33 = 2P r ·ε 33 ·Q 33 , demonstrating that the piezoelectric response of these systems should be determined by combined effects of the phase coexistence, nano-scale domains and particularly enhanced dielectric responses. The largest d 33 ∼305 pC/N, the highest ε 33 T/ε o ∼2815 and the lowest P r ∼14.7 μC/cm2 were achieved in the MPB composition with 3.75% SrZrO 3. These experimental results provide a valuable reference for designing new NaNbO 3 -based lead-free piezoelectric materials. [ABSTRACT FROM AUTHOR]

Abstract: Microscale magnetoelectric composites with 1–3 connectivity structure were fabricated by a dice-and-fill method. Mechanically sliced microscale pillar arrays of (K,Na)NbO3-based ceramics were used as the piezoelectric phase, which were embedded in a hybrid matrix composed of ferromagnetic CoFe2O4 particles bonded with epoxy. Commercial CoFe2O4 nano-powder and CoFe2O4 particles derived from a sol–gel method during the filling process were used in two sets of samples, respectively. Obvious ME response signals were detected to be switched with the applied magnetic field. It was deduced that the different ME coefficient values of the two composites were mainly due to the interface morphology between the piezoelectric pillars and the matrix. [Copyright &y& Elsevier]

The origins of distinctive compositional dependence of relative permittivity, ɛ r , in the Pb-free piezoelectric system (1 − x ) Na 0.5 K 0.5 NbO 3 – x LiTaO 3 , x ⩽ 10 mol.% modified with BiScO 3 have been revealed using transmission electron microscopy with energy dispersive X-ray analysis (TEM-EDX). As the LiTaO 3 content increased the Curie peak at ∼370 °C in ɛ r – T plots became more diffuse, and at x = 5 mol.% an additional higher temperature peak occurred. TEM-EDX analysis showed the change in dielectric properties at x = 5 mol.% was due to a change in microstructure: micron-scale grains were replaced by submicron grains exhibiting core–shell chemical segregation. The outer shell was similar to the target solid solution composition, slightly enriched in Bi, Sc and Ta, whilst the core approximated to (Na, K, Li)NbO 3 and was responsible for the additional dielectric peak. Examples of a novel three-tier metastable grain structure were observed for certain compositions. [ABSTRACT FROM AUTHOR]

External thermal, electrical, and mechanical fields can induce structural phase transitions in lead‐free Li‐modified Na0.5K0.5NbO3 ferroelectrics, which significantly influence the macroscopic electromechanical response. In particular, the relative stability of the polar monoclinic (or orthorhombic) and tetragonal phases under temperature and stress is critical to realize the ferroelectric and piezoelectric response. In this study, the effect of mechanical and thermal fields on the local structure in the vicinity of the monoclinic‐tetragonal (M‐T) phase boundary was investigated using a novel in situ combined uniaxial compressive stress‐ and temperature‐dependent Raman spectroscopy experimental arrangement. Experiments were performed up to 300°C and −200 MPa, clearly demonstrating stress‐induced M‐T phase transition in Li‐modified Na0.5K0.5NbO3. A stress‐temperature phase diagram has been established based on the change in vibrational modes. It was possible to correlate the relative permittivity singularities previously observed to a given stage of the M‐T phase transition using ratio between characteristic Raman band areas. In addition, the measurement method reported here can be applied to other functional ceramics to investigate the influence of mechanical fields on local structure. [ABSTRACT FROM AUTHOR]

Piezoelectric materials are an important functional material in the modern world because they are capable of converting electrical energy into mechanical energy and vice versa. Lead-free potassium sodium niobate (KNN)-based piezoelectric materials have been intensively researched in the past decade for only one purpose: to replace the less environmentally friendly lead-based piezoelectric materials in commercial products. In this review, the historical research evolution of KNN-based piezoelectric ceramics is highlighted. Meanwhile, to evaluate the technology transfer status, we emphasize on the analysis of the patents applied within the past five years, as well as certain practical applications of KNN-based materials. This paper concludes with the prediction of remaining challenges and opportunities for adopting the lead-free KNN-based piezoelectric ceramics in practical applications. [ABSTRACT FROM AUTHOR]

The fracture toughness of unpoled and electrically poled lead-free KNN-based piezoelectric ceramics with the composition of 0.92KNN-0.02Bi 0.5 Li 0.5 TiO 3 -0.06BaZrO 3 was investigated. Results reveal that at room temperature, the intrinsic fracture toughness (K I0) of the unpoled samples, evaluated by the near-tip crack opening displacement (COD) technique, is the lowest with a value of 0.70 MPa ⋅ m0.5; the long (through-thickness) crack fracture toughness (K Ivnb), obtained by the single edge V-notch beam (SEVNB) technique, is the highest, with a value of 0.95 MPa ⋅ m0.5; intermediate short surface crack fracture toughness (K Isc) of 0.86 MPa ⋅ m0.5 was determined by the surface crack in flexure (SCF) technique. These results were rationalized by the toughening behavior of the material combined with the crack geometry-dependent stress intensity evolution during crack propagation. With increasing temperature, K Ivnb and K Isc decrease, and become nearly identical at 350 °C, suggesting an absence of toughening. For electrically poled samples, their room temperature fracture toughness was characterized by both SCF and SEVNB techniques, with values of 0.88 MPa ⋅ m0.5 and 0.99 MPa ⋅ m0.5, respectively, slightly larger than the values measured for unpoled samples. Nonlinear electric field-strain and stress-strain analysis of the material was also employed during electric field loading, mechanical compression and four-point bending in order to quantify crack tip shielding by domain switching and the actual stress at the point of instable crack propagation. (a) Comparison between the intrinsic fracture toughness and fracture toughness of KNN-BLT-6BZ measured by SCF and SEVNB technique. For comparison, the calculated fracture toughness and the K Isc obtained with the nonlinear stress-strain behavior considered were also plotted. (b) Crack growth resistance K IR curves with toughening effect and the evolution of the stress intensity factor as a function of crack length. The red tangent dots represent the point at which unstable crack growth begins. The part of the K IR curve with red color denotes the stage in which stable crack growth will happen with increasing applied load. Image 1 [ABSTRACT FROM AUTHOR]