Rhombohedral-orthorhombic-tetragonal Research Articles

Energy harvesting properties for potassium-sodium niobate piezoceramics through synergistic effect of phase structure and texturing engineering

In order to address the energy dilemma and meet environmental protection requirements, it is an urgent need to develop lead-free piezoelectric energy harvesters (PEHs). However, the low output power density has seriously hindered the application of lead-free piezoceramics as high efficiency energy harvesters. To solve above question, the combination of phase structure regulation and texturing technique in the K0.5Na0.5NbO3-xBi0.5Na0.5ZrO3-0.5%wtBiFeO3 (KNN-xBNZ) ceramics was adopted in this work. Because of both the Rhombohedral-Orthorhombic-Tetragonal (R-O-T) multiphases coexistence and texturing engineering, an ultrahigh piezoelectric activity quality factor d33×g33 ∼ 18324 ×10-15 m2·N-1 was achieved in the textured KNN-0.05BNZ (T-KNN-0.05BNZ) ceramics, which is about third times higher than that of random KNN-0.05BNZ (R-KNN-0.05BNZ) (d33×g33 ∼ 6672 ×10-15 m2·N-1) ones. A high output power of ∼ 0.32 mW was also obtained in the T-KNN-0.05BNZ ceramics by cantilever beam-based energy harvester devices under the resonance frequency of 75 Hz and matched RL of 1.2 MΩ, suggesting that the T-KNN-0.05BNZ ceramics have a great potential for application in the PEHs devices. This result also reveals that the combination of phase structure regulation and texturing technique is an effective way to achieved a high energy harvesting performances.

Revealing multiphase coexistence of R-O-T phases in BaTiO3-CaTiO3-BaSnO3 electroceramics for energy harvesting and storage response with piezo actuation

The (Ba1-xCax)(SnyTi1-y)O3 piezoelectric ceramics (where x=0.016, y=0.024; x=0.032, y=0.048; x=0.048, y=0.072; x=0.064, y=0.096; x=0.08, y=0.12) were designed by conventional solid state reaction method. The crystal structure for the composition of x=0.064 and y=0.096 (abbreviated as BCST-4) possesses the coexistence of non-centrosymmetric rhombohedral-orthorhombic-tetragonal (R-O-T) tri-lattice symmetries at room temperature, as demonstrated by the structural Rietveld refinement, Raman analysis, and temperature dependence of dielectric study. Because of the R-O-T multiphase coexistence, BCST-4 possesses a superior electromechanical and piezoelectric property viz. kp ∼ 0.45, strain ∼ 0.100 %, d33* ∼ 649 pm/V, and an improved d33 ∼ 452 pC/N, which is comparable to commercially available soft PZT ceramics (d33 ⁓ 370 pC/N). For BCST-4 ceramics, an exceptional electrostriction coefficient Q33 ∼ 0.0434 m4/C2 value was attained. The intrinsic piezo-actuation DC strain was observed to be 130 microstrain (με) and 188 με with ε33 and ε31 modes respectively. The BCST-4 ceramic exhibits a maximum output power of 1.03 mW, a power density of 13.5 µW/mm3, a maximum output current of 88 µA, and an open circuit voltage Vpp of 28 V which successfully glowed ‘SPPU’ panel having 40 red commercial light-emitting diodes (LEDs). The energy storage study reveals that BCST-4 ceramics exhibit a maximum energy storage density (Wrec) of 165.87 mJ/cm3 with efficiency (ƞ) 68.00 %. Therefore, the improvement in electrostriction coefficient, piezoelectric charge coefficient, and energy storage response indicates that BCST-4 ceramic has the potential for actuator, energy harvesting, and energy storage applications.

Effect of the Crystal Structure on the Piezoelectricity of [001]-Textured (Na, K)(Nb, Sb)O3-SrZrO3-(Bi, Ag)ZrO3 Lead-Free Piezoelectric Thick Film

An amount of 3.0 mol% NaNbO3 seeds was used to align the grains of 0.96(Na0.5K0.5)(Nb0.93Sb0.07)O3-(0.04−x)SrZrO3-x(Bi0.5Ag0.5)ZrO3 [NKNS-(0.04−x)SZ-xBAZ] thick films (0.0 ≤ x ≤ 0.04) along the [001] direction. All the textured thick films had large Lotgering factors (>95%). The textured NKNS-0.02SZ-0.02BAZ thick film has a rhombohedral-orthorhombic-tetragonal (R-O-T) structure with a large proportion of the R-O structure (>80%) and nanodomains (0.7 nm in width and 6 nm in length). This thick film exhibited a large d33 value (760 ± 20 pC/N), kp value (0.58) and strain (0.16% at 4.0 kV/mm), with good temperature stability and fatigue properties. The high piezoelectricity of this thick film can be attributed to its high degree of texturing, optimized domain configuration, and the presence of nanodomains. The piezoelectric ceramic with a large d15/d33 value showed a large d33 value after [001] texturing because of the easy rotation of the spontaneous polarizations. Hence, the d15/d33 value can be used to select piezoelectric ceramics with large d33 values after [001] texturing.

Multiple stability of electrical properties for oxides-modified (K,Na)NbO3-based ceramics

Lead-free potassium sodium niobate (KNN) ceramics have become crucial advanced functional materials with conspicuous performance. Although continuous improvements of electrical properties were achieved via various strategies these years, the poor stability leads to the bottleneck for application. Herein, the stability of electrical properties under different conditions (i. e. electric field, frequency and temperature) were emphasized on in KNN-based ceramics with addition of oxides based on the availability for optimizing properties. With addition of Fe2O3, high ferroelectric, piezoelectric and dielectric properties remain due to the maintained rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary at room temperature and little changed Curie temperature (TC), while deterioration happens with excess Fe2O3 attributing to the pinning effect from replacement of Fe3+ in crystal lattice. Furthermore, the elevated beginning electric field for stable ferroelectricity and negative strain under increasing electric field are gained with elevating Fe2O3 along with little changed piezoelectricity. Stable strain maintains with increasing frequencies when Fe2O3 content changes, as an enhancement are discovered for residual polarization (Pr) and coercive electric field (EC) at 0.5–1 Hz for x ≥ 1.0%. Moreover, improved temperature stability for piezoelectricity and strain as well as remained high ferroelectricity is presented with increasing Fe2O3, originating from the increasing diffusion behavior of R-O-T phase boundary and non-declined TC, respectively. Thus, this work provides an effectively strategy to optimize the stability of electrical properties for application of KNN-based materials.

Improvement of piezoelectricity of (Na, K)Nb-based lead-free piezoceramics using [001]-texturing for piezoelectric energy harvesters and actuators

0.96(Na0.5K0.5)(Nb0.93Sb0.07)O3−(0.04−x)BaZrO3−x(Bi0.5Ag0.5)ZrO3[NKNS−(0.04−x)BZ−xBAZ] ceramics are well textured along the [001] direction using 3.0 mol% NaNbO3 seeds. The textured-NKNS−0.02BZ−0.02BAZ thick film has a rhombohedral-orthorhombic-tetragonal (R-O-T) structure with a large proportion of the O-R structure (> 80%). This specimen exhibits the largest values for d33 (805 pC/N) and d33 ×g33 (29.5 ×10−12 m2/N), which are the largest d33 and d33 ×g33 values of NKN-based piezoceramics to date. It exhibits a large strain (0.17% at 4.0 kV/mm). Therefore, it is an outstanding piezoceramic material for piezoelectric energy harvesters (PEHs) and actuators. A PEH and actuator are fabricated using this specimen. The PEH shows a large power density (4.3 mW/cm3), which is the largest value among the PEHs produced by lead-free piezoceramics. The actuator exhibits a large acceleration (50.8 G) and displacement (3.9 mm), which are the best actuating properties among the actuators produced by lead-free piezoceramics. Therefore, texturing is an excellent technique for improving the piezoelectricity of NKN-based piezoceramics.

Modulating polarization rotation to stimulate the high piezocatalytic activity of (K, Na)NbO3 lead-free piezoelectric materials

The strategy of modulating polarization rotation of potassium sodium-niobate ((K0.5Na0.5)NbO3, KNN) lead-free piezoelectric materials was reported to boost piezoelectric properties and resultant piezocatalytic activities. The effectiveness of modulating polarization rotation was proved by degrading Rhodamine B (RhB) using three KNN-based samples with different phase structures (i.e., orthorhombic (O) phase, orthorhombic-tetragonal (O-T) coexistence phase, and rhombohedral-orthorhombic-tetragonal (R-O-T) coexistence phase). Poled samples with the R-O-T coexistence phase show a reaction rate constant of 0.091 min−1 owing to the easiest polarization rotation, 2.12 times more than that of poled O-phase featured samples with the most difficult polarization rotation. Enhanced piezocatalytic activities primarily originate from the easier polarization rotation and improved carrier concentration, accompanied by the trace of the mechano-charge generation. Therefore, modulating polarization rotation effectively boosts piezocatalysis of KNN-based materials, promising for harnessing natural energy and disease treatment.

Feasible Way to Achieve Multifunctional (K, Na)NbO3-Based Ceramics: Controlling Long-Range Ferroelectric Ordering.

It is challenging to achieve highly tunable multifunctional properties in one piezoelectric ceramic system through a simple method due to the complicated relationship between the microscopic structure and macroscopic property. Here, multifunctional potassium sodium niobate [(K, Na)NbO3 (KNN)]-based lead-free piezoceramics with tunable piezoelectric and electrostrictive properties are achieved by controlling the long-range ferroelectric ordering (LRFO) through antimony (Sb) doping. At a low Sb doping, the slightly distorted NbO6 octahedron and the softened B-O repulsion well maintain the LRFO and induce plenty of nanoscale domains coexisting with a few polar nanoregions (PNRs). Thereby, the diffused rhombohedral-orthorhombic-tetragonal (R-O-T) multiphase coexistence with distinct dielectric jumping is constructed near room temperature, by which the nearly 2-fold increase in the piezoelectric coefficient (d33 ∼ 539 pC/N) and the temperature-insensitive strain (the unipolar strain varies less than 8% at 27-120 °C) are obtained. At a high Sb doping, the LRFO is significantly destroyed, leading to predominant PNRs. Thus, a typical relaxor is obtained at the ferroelectric-paraelectric phase transition near room temperature, in which a large electrostrictive coefficient (Q33 = 0.035 m4/C2), independent of the electric field and temperature, is obtained and comparable to that of lead-based materials. Therefore, our results prove that controlling the LRFO is a feasible way to achieve high-performance multifunctional KNN-based ceramics and is beneficial to the future composition design for KNN-based ceramics.

Investigations on phase coexistence and functional properties of BCZT lead-free piezoceramic

Large piezoelectric properties was observed in (1-x)BZT-(x)BCT where x=0.5 or Ba0.85Ca0.15Ti0.9Zr0.1O3 (denoted as BCZT), leading to a promising candidate for lead-free piezoelectric materials. However, phase formation of the BCZT is controversial and still unclear since various phase coexistences were identified in the literatures, for instances, the mixed phases of rhombohedral-tetragonal (R-T), ortho-rhombic-tetragonal (O-T) or rhombohedral-orthorhombic-tetragonal (R-O-T). Additionally, it is well known that the crystal structure plays a crucial role on the occurrence of polarization in the piezoceramics. Therefore, this work aims to investigate the coexistence of phase formation at room temperature for the BCZT powder and ceramic. Moreover, the electrical properties as a function of temperature, frequency and electric field were observed in order to evaluate the extrinsic contribution of piezoelectric response. It was found that, according to the results from temperature-dependent dielectric properties as well as Rietveld refinement of XRD profiles, the coexistence of O-T phase was observed in the BCZT powder and ceramic. Furthermore, the enhancement of Ca2+ substitution into Ba2+ site in BCZT ceramic caused the shrinkage of unit cell, leading to the shift of XRD profile and Raman spectra. In addition, it was found that the applications of frequency and electric field can influence on changes of domain-wall motion and micro-polar cluster in the BCZT piezoceramic.

Excellent piezoelectric properties of (K, Na)(Nb, Sb)O3-CaZrO3-(Bi, Ag)ZrO3 lead-free piezoceramics

Fe2O3-containing 0.96(K0.5Na0.5)(Nb0.93Sb0.07)-(0.04-x)CaZrO3-x(Bi, Ag)ZrO3 piezoceramics with 0.0 ≤ x ≤ 0.04 fabricated at 1090 °C show a dense microstructure without any secondary phases. The piezoceramic with x = 0.03 has an ideal rhombohedral–orthorhombic–tetragonal (R-O-T) multi-structure because its constituent structure proportions are similar. Its R-O-T phase transition temperature (TR-O-T) increases with increasing measuring frequency, indicating that this piezoceramic exhibits relaxor properties which develop near its ferroelectric-to-ferroelectric phase transition temperature, that is, its TR-O-T. This piezoceramic has nanodomains that are approximately 3 nm × 20 nm in size. The associated low boundary energies of these domains are expected to facilitate domain rotation. Furthermore, the piezoceramic has an extremely high piezoelectric constant (d33) of 680 ± 10 pC/N, which exceeds the maximum d33 reported in the literature for similar materials. This high d33 is attributed to the existence of an ideal R-O-T multi-structure and nanodomains.

Piezoelectric properties, phase transitions and domain structure of (Bi,Na)HfO3−modified (K,Na)(Nb,Sb)O3 ceramics

Piezoelectric performances of (K,Na)NbO3-based ceramics are often greatly affected by phase transitions and domain structure, which are in turn substantially determined by chemical compositions. Systematical studies from this perspective are important for the good understanding about piezoelectric characters. Further, whether rhombohedral-tetragonal (R-T) or rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence is really the essential precondition for obtaining high piezoelectric coefficient d33 is also a fundamental issue. A series of (1-x)(K0.48Na0.52)Nb0.955Sb0.045O3–x(Bi0.50Na0.50)HfO3 ceramics with 0.01≦x≦0.04 were chosen in this work to examine the compositional effects on piezoelectric properties, phase transitions and domain structure. It was found that piezoelectric coefficient d33, orthorhombic-tetragonal (O-T) phase transition and domain structure are sensitive to the chemical composition. The d33 increases first, reaches a maximum of 512 pC/N at x = 0.037 and then decreases with x. Dielectric measurement showed that the O-T phase transition temperature TO-T decreases monotonically with x while the rhombohedral-orthorhombic phase transition temperature TR-O remains almost unchanged when x ≥ 0.03. Domain structure was investigated by acid etching. All the compositional ceramics exhibit quite complex domain patterns consisting of long parallel stripes and irregularly-shaped domain boundaries before poling. Domain patterns become less complicated upon poling due to the collective polarization reversals of domain clusters. The ceramic with x = 0.037 differs significantly from the others, showing its TO-T close to room temperature and possessing the unique domain structure that contains a large amount of hierarchical nanodomain structure. Its outstanding piezoelectric properties should be ascribed to the O-T phase coexistence and the characteristic domain structure.

Composition driven (Ba,Ca)(Zr,Ti)O3 lead‐free ceramics with large quality factor and energy harvesting characteristics

AbstractA comprehensive study on energy harvesting characteristics as well as electro‐mechanical properties of lead‐free (1−x)(BaZr0.2Ti0.8)O3—x(Ba0.7Ca0.3Ti)O3 ceramics were systematically carried out. Raman and Rietveld analyses show a formation of rhombohedral‐orthorhombic‐tetragonal (R‐O‐T) phase boundary region between 4/6 BZT/BCT and 6/4 BZT/BCT compositional range. Raman modes shift toward lower frequencies with increased Zr/Ca stoichiometric ratio attributed to asymmetric Ti‐O phonon vibrations, which caused local disorder, widening of energy band and reduced Curie temperature. The large mechanical quality factor Qm = 556 is related to the hardening effect and significantly high energy conversion efficiency η = 96% was discovered for 3/7 BZT/BCT composition. Largely, the noblest electro‐mechanical properties were retrieved for 5/5 BZT/BCT ceramics, in which d33 = 500 pC/N (from quasi‐static d33 meter), d33 = 540 pm/V (from field‐dependent d33 curves) indicating that the both methods are analogous with a deviation of 8%. The outstanding energy harvesting characteristics such as voltage constant g33 = 27 × 10−3 Vm/N, transduction coefficient d33 × g33 = 13 301 × 10−15 m2/N, figure of merit under off‐resonance conditions FOMoff = 12.1 × 10−10 m2/N and fairly large η of 94% were attained again for 5/5 BZT/BCT ceramics. These outstanding characteristics were ascribed to the R‐O‐T phase boundary region that comprises a low energy barrier, consequently facilitated easy polarization rotation and triggered an increased electro‐mechanical conversion. These characteristics outperform other lead‐free and even most commercially available lead‐based ceramics, and thus suitable for sensors, actuators, resonators, and energy harvesting applications.

Rhombohedral-orthorhombic-tetragonal multiphases coexist in (Ba0.85Ca0.15)(Ti0.9Zr0.08Sn0.02)O3-SrTiO3 piezoelectric ceramics prepared by microwave sintering techniques

To upgrade integrated piezoelectric properties of lead-free ceramics, (Ba0.85Ca0.15)(Ti0.9Zr0.08Sn0.02)O3-SrTiO3 ceramics were successfully fabricated by the tape casting method and microwave sintering (MWS) techniques. The experimental results show that rhombohedral-orthorhombic-tetragonal(R-O-T) multiphases coexist in the samples, and excellent piezoelectric performance of the ceramics (d33 = 398 pC/N, Pr = 3.90 µC/cm2, Ec = 3.43 kV/cm, tanδ = 4.8%, er = 7791, and Tc = 82 °C, respectively) is obtained under 1300 °C for 1 h. The template of ST, prepared through molten-salt two-step method, can guide the grains to grow in the direction.

Exploring the high-performance (1-x)BaTiO3-xCaZrO3 piezoceramics with multiphase coexistence (R-O-T) from internal lattice distortion and domain features

Although the multiphase coexistence has made great achievements in improving the piezoelectric properties of barium titanate-based (BT) ceramics, the problems such as internal lattice distortion and external physical mechanism at the phase boundary still remain. Here, we have studied the phase structure, atomic parameters and domain structure of (1-x)BaTiO3-xCaZrO3 (abbreviated as BT-xCZ) ceramics. By regulating the phase transition temperatures, the rhombohedral-orthorhombic-tetragonal (R-O-T) multiphase coexistence (x = 0.06) is realized at room temperature. Notably, high piezoelectric constant of d33 ∼ 445 ± 20 pC/N and large electromechanical coupling factor of kp = 55% are obtained at x = 0.06. With the doping of CaZrO3 (CZ), the lattice of ceramics is distorted, thereby increasing the ferroelectricity and piezoelectricity. Importantly, the domain structure of BT-xCZ ceramics observed by the acid etching technology further proves that the high piezoelectricity is attributed to the multiphase coexistence and reduction of the domain wall energy density.

Coexistence of excellent piezoelectric performance and high Curie temperature in KNN-based lead-free piezoelectric ceramics

A new KNN-based lead-free piezoelectric material system of (1-x)K0.4Na0.6Nb0.96Sb0.04O3-x(Bi0.45Sm0.05)Na0.5ZrO3 (abbreviated as KNNS-xBSNZ, 0.00 ≤ x ≤ 0.06) is designed to obtain high performance to meet the requirement of next generation electronic devices. The relevant electric properties such as piezoelectricity, strain, and temperature stability are analyzed in detail. An excellent comprehensive property (d33∼508 pC/N, TC∼268 °C) is obtained in ceramic with x = 0.04 through the construction of rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary. In addition, a good temperature stability of strain for KNNS-0.04BSNZ ceramic is also obtained within the range from room temperature to 175 °C (Suni100°C/SuniRT∼97.81%, Suni150°C/SuniRT∼84.67%), which is comparatively superior to some reported KNN-based ceramics and lead-based ceramics. Therefore, we believe that this new material system would be a promising lead-free candidate for the practical applications.

Role of Extrinsic Contribution to Temperature Stability of Piezoelectricity for (K,Na)NbO3-(Bi,M)ZrO3 Ceramics with Morphotropic Phase Boundary

Temperature stability of dielectric, ferroelectric, and piezoelectric properties were investigated in situ by choosing (K,Na)NbO3-(Bi,K)ZrO3 (KNN-BKZ), (K,Na)NbO3-(Bi,Na,K,Li)ZrO3 (KNN-BNKLZ), and (K,Na)NbO3-(Bi,Li)ZrO3-(KNN-BLZ) as representative ceramics with rhombohedral-orthorhombic-tetragonal (R-O-T), R-T, and enriched T phase boundaries, respectively. The KNN-BNKLZ ceramics, which have an R-T phase boundary, showed the highest piezoelectricity but the worst temperature stability. On the other hand, the KNN-BLZ ceramics, which have an enriched T-phase, were slightly worse in terms of piezoelectricity compared to the R-O-T or R-T phase boundary, but their thermal stability was the best. From analyses of an extrinsic contribution by a difference between the small signal d33 and εrPr and an intensity variation of the (002) and (200) X-ray diffraction peaks for the KNN-based ceramics, it was suggested that increasing the extrinsic contribution in the morphotropic phase boundary region improves the piezoelectricity but decreases the thermal stability.

Remarkably strong piezoelectricity, rhombohedral-orthorhombic-tetragonal phase coexistence and domain structure of (K,Na)(Nb,Sb)O3–(Bi,Na)ZrO3–BaZrO3 ceramics

Demands for lead-free piezoelectric ceramics increase rapidly owing to environmental concerns. Strong piezoelectricity in (K,Na)Nb-based ceramics was usually realized by taking the advantage of rhombohedral-tetragonal (R-T) phase coexistence associating with the R-T phase transition. In this paper, we report the study results on a series of dense (0.96-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3−0.04(Bi0.50Na0.50)ZrO3−xBaZrO3 ceramics (abbreviated as KNNS-BNZ-xBZ ceramics), in which the one with x = 0.01 shows a giant piezoelectric coefficient d33 of 610 pC/N with quite large electromechanical coupling coefficients of kp = 0.58 and k33 = 0.69. Dielectric measurement indicated that the rhombohedral-orthorhombic (R–O) phase transition temperature TR-O, orthorhombic-tetragonal (O-T) phase transition temperature TO-T and Curie temperature TC of this ceramic are −29 °C, 43 °C and 241 °C, respectively. Meanwhile, XRD analysis unveiled that this ceramic is in rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence at room temperature. Acid etching revealed that domain structure inside the polycrystalline grains consists of lamellar domain stacks with a large amount of hierarchical nanodomain structure existing in part of the lamellar domains. The observed strong piezoelectricity is ascribed to the R-O-T phase coexistence and the corresponding characteristic domain structure.

Second‐order‐transition like characteristic contributes to strain temperature stability in (K, Na)NbO3‐based materials

AbstractHigh strain and good temperature stability are contradictory properties in (K, Na)NbO3 (KNN)‐based materials. Herein, good temperature stability with high strain is obtained in a multiphase coexistent [ie, orthorhombic‐tetragonal (O‐T) and rhombohedral‐orthorhombic‐tetragonal (R‐O‐T)] KNN. A second‐order transition‐like characteristic should contribute to the temperature stability, in which an intrinsic lattice structure forms a bridge between them. The observed second‐order transition‐like characteristic is due to the reduced discrepancy among different lattice symmetries and a broadened temperature region for the phase transition. These integrated factors can slow the latent heat in a first‐order transition and extend it over a wide temperature region, thereby exhibiting second‐order transition‐like behavior. Correspondingly, the abrupt increase in strain near the phase transition temperature significantly slows. In addition, the appearance of pure tetragonal symmetry (P4mm) is deferred to a much higher temperature than TO‐T, in which the strain will inevitably decrease. As a result, good temperature stability with a high strain response can be realized in multiphase coexistent KNN materials, including d33*=448 pm/V, ‐27.5%≤fluctuation≤4.2% for O‐T, and d33*=446 pm/V, ‐17.5%≤fluctuation≤7.6% for R‐O‐T, over the whole temperature range 25 °C‐190 °C.

Giant piezoelectricity, rhombohedral-orthorhombic-tetragonal phase coexistence and domain configurations of (K,Na)(Nb,Sb)O3–BiFeO3–(Bi, Na)ZrO3 ceramics

Research on lead-free piezoelectric ceramics has been an important subject in recent years due to increasingly strong environmental concerns. In this paper, we report the piezoelectric performance, phase transitions and domain structure for a series of dense (0.994-x)(K0.40Na0.60)(Nb0.955Sb0.045)O3–0.006BiFeO3–x(Bi0.50Na0.50)ZrO3 ceramics prepared by two step-sintering through solid-state reaction. Among these ceramics, the one with x = 0.03 shows a giant piezoelectricity with remarkably high piezoelectric coefficient d33 of 550 pC/N at room temperature. Concurrent measurements of ε′ and ε′′ vs. temperature dependences and X-ray diffraction analysis showed that ceramics with x ≤ 0.04 are in rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence at room temperature. More intriguingly, the rhombohedral-orthorhombic phase transition temperature, TR-O, is almost independent of the (Bi0.50Na0.50)ZrO3 content, while orthorhombic-tetragonal phase transition temperature, TO-T, decreases largely with increasing the (Bi0.50Na0.50)ZrO3 content. Domain configurations of the ceramic with x = 0.03 were investigated by acid-etching. Complicated domain patterns consisting of watermark-shaped domains of long parallel stripes separated by irregularly shaped boundaries are seen before poling. In contrast, relatively simple domain patterns of long parallel stripes with some nanodomains appearing in a part of broad stripes are observed after poling. The obtained excellent piezoelectric properties are ascribed to the R-O-T phase coexistence and the corresponding characteristic domain structure.

Boosting piezoelectric response of KNN‐based ceramics with strong visible‐light absorption

AbstractA series of (1 − x)(K0.48Na0.52)NbO3‐x(Bi0.5Na0.5)(Zr0.55Ni0.45)O3‐δ (KNN‐BNZN) ceramics are designed to achieve excellent piezoelectric response along with narrow bandgap. The ceramics with x = 0.04 exhibit unprecedented piezoelectric coefficient d33 ~ 318±10pC N−1 in comparison with all reported narrow bandgap ceramics. A rhombohedral‐orthorhombic‐tetragonal (R‐O‐T) phase boundary is observed, indicating the formation of defect dipoles (Ni2+‐) at morphotropic phase boundary region is desirable for piezo‐/ferroelectric properties. In addition, a narrow bandgap ~2.5 eV along with gap states (~0.9 eV and ~1.6 eV) is obtained from the ceramics when x > 0.02, which can be persuasively explained by the schematic plot of bandgap splitting mechanism proposed in this work, where Ni 3d energy state plays a role as a scaffold in the process of electron transition. More importantly, largely enhanced photovoltaic performance of the ceramics is achieved under AM 1.5 irradiation. The NIR photoresponse property (maximum current density of ~100 nA cm−2) indicates such KNN‐based ceramics with sub bandgap ~ 0.9 eV may even have potential to be applied in NIR light‐activated devices. Our findings might pave way for the further development of piezoelectric/photoresponsive multifunctional devices.

High-performance potassium-sodium niobate lead-free piezoelectric ceramics based on polymorphic phase boundary and crystallographic texture

Lead-free piezoelectric ceramics are urgently needed in the field of electromechanical conversion devices due to the restriction on the use of lead-based ceramics. In this study, the polymorphic phase boundary (PPB) were tuned by incorporating different concentration of (Bi0.5K0.5)HfO3 into the matrix (K0.5Na0.5)(Nb0.965Sb0.035)O3CaZrO3, and the <00l>c crystallographic texture was realized by templated grain growth method. The maximal d33 (∼550 pC/N) and kp (∼72%) were achieved in the <00l>c textured ceramics with composition around rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary. It is proposed that the enhanced piezoelectricity should be ascribed to several combined effects, which primarily contain the R-O-T phase boundary facilitating polarization rotation, the crystallographic orientation induced intrinsic piezoelectric anisotropy, electric-field-induced lattice distortion and phase transitions, and NaNbO3 seed-crystal-driven nanodomain structures. This work provides an effective solution to enhance piezoelectric properties by simultaneous tailoring polymorphic phase boundary and using crystallographic texture in potassium-sodium niobate based piezoelectric ceramics. We believe that the simple solution and design principle can also be applied to other piezoelectric ceramic systems, no matter lead-based or lead-free.