Room temperature constructing rhombohedral-tetragonal phase boundary in novel (Bi,Na)(Zr,Ti)O3 modified (K,Na)(Nb,Sb)O3 ceramics: Phase structure,defect and piezoelectric performance

Lead-free (1-x)(K_0.5Na_0.5)(Nb_0.96Sb_0.04)O₃-x(Bi_0.5Na_0.5)(Zr_0.8Ti_0.2)O₃ ceramics (abbreviated as (1-x)KNNS-xBNZT, x = 0, 0.01, 0.02, 0.03, 0.035 and 0.04) were synthesized by the solid-state method, and the dependence of phase evolution, microstructure, oxygen vacancy defect and electrical properties on compositions were carefully investigated. All ceramics had a pure perovskite structure and a dense microstructure. The phase transition temperatures (T_R-O and T_O-T) of the ceramics were adjusted by adding BNZT, and the rhombohedral-tetragonal (R-T) phase coexistence boundary was successfully constructed at room temperature when x = 0.03, the excellent piezoelectric performance (d₃₃ ～ 323 pC/N, k_p ～ 0.372) and high Curie temperature (T_C ～ 276 °C) have been achieved at this time. The grain size of the ceramics showed a strong difference on x content, and the maximum relative density value of 95.42% was obtained. The domain structure characterized by PFM confirmed that the ceramics possess small-sized nano-domains and complex domains at x = 0.03, which are the origin of enhanced piezoelectric properties. Moreover, the oxygen vacancy defect that can pin the domain walls was increased with the addition of (Bi_0.5Na_0.5)(Zr_0.8Ti_0.2)O₃. As a result, the doping with BNZT can significantly affect the phase structure and electrical properties of the ceramics, indicating that the (1-x)KNNS-xBNZT ceramics system with a R-T phase boundary is a promising lead-free piezoelectric material.     

Piezoelectric properties of (1-x)BZT-xBCT system for energy harvesting applications

In this study, we investigated (1-x)Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)TiO₃ lead-free piezoelectric ceramics for energy harvester applications. The (1-x)BZT-xBCT ceramic is a promising lead-free piezoelectric material in the field of piezoelectric energy harvesting. Piezoelectric and energy properties of (1-x)BZT-xBCT ceramics were analyzed to confirm the possibility of using them as energy-harvesting materials. Especially, the vicinity of the phase convergence region was investigated to improve their piezoelectric properties. In the phase convergence region, cubic, rhombohedral, orthorhombic, and tetragonal regions co-exist within the narrow region. Near the phase transition region between the orthorhombic and tetragonal phase, the highest piezoelectric property d₃₃?=?464 pC/N and the highest energy density of 158.5 μJ/cm³ were observed. This output energy density of 158.5 μJ/cm³ is the recorded highest value among lead-free ceramics. We found that the optimal sintering temperature was 1475?°C and the optimal composition was BZT-0.5BCT.     

Anomalously large lattice strain contributions from rhombohedral phases in BiFeO3-based high-temperature piezoceramics estimated by means of in-situ synchrotron x-ray diffraction

Thermally-stable (0.75-x)BiFeO₃-0.25PbTiO₃-xBa(Zr_0.25Ti_0.75)O₃ (0.1?≤?x?≤?0.27) piezoelectric ceramics were reported to have excellent dielectric and electromechanical properties of d₃₃～405 pC/N, k_p～46%, ε₃₃^T/ε₀～1810, tanδ～3.1% and T_c～421?°C close to tetragonal (T)-rhombohedral (R) morphotropic phase boundary. The dielectric measurement indicates that R ferroelectric phase is gradually transformed into relaxor ferroelectric across the phase boundary due to the substitution of BZT for BF. The transmission electron microscopy and convergent beam electron diffraction provide clear evidences that both the R-T phase coexistence and polar nanodomains contribute to enhanced piezoelectric properties at x?=?0.19 through cooperatively facilitating polarization orientation. In combination with the macroscopic piezoelectric coefficient measurement, the quantitative analysis of synchrotron diffraction data under electric fields suggests that extremely large lattice strain contribution predominantly from R phases plus little extrinsic domain switching contribution should dominate the piezoelectric response of the x?=?0.19 sample, mainly owing to both irreversible field-induced T to R phase transition and irreversible non-180° domain switching.     

Phase transition and electrical properties of Bi0.5(Na0.8K0.2)0.5ZrO3 modified (K0.52Na0.48)(Nb0.95Sb0.05)O3 lead-free piezoelectric ceramics

In this work, (1−x)(K_0.52Na_0.48)Nb_0.95Sb_0.05O₃−xBi_0.5(Na_0.8K_0.2)_0.5ZrO₃ [abbreviated as (1−x)KNNS−xBNKZ, x=0–0.06] lead-free ceramics were fabricated using solid-state reaction method. The effects of BNKZ contents on the phase structure, piezoelectric and ferroelectric properties were investigated. The phase boundaries including orthorhombic-tetragonal (O-T) and rhombohedral-tetragonal (R-T) multiphase coexistence were identified by XRD patterns and temperature-dependent dielectric constant by adding different content of BNKZ. A giant field induced strain (~0.25%) along with converse piezoelectric coefficient d₃₃^* (~629.4 pm/V) and enhanced ferroelectricity P_r (~38 μC/cm²) were obtained when x=0.02, while the specimen with x=0.03 presented the optimal piezoelectric coefficient d₃₃ of 215 pC/N, due to the O-T or R-T phase coexistence near room temperature respectively. These results show that the introduction of Bi_0.5(Na_0.8K_0.2)_0.5ZrO₃ is a very effective way to improve the electrical properties of (K_0.52Na_0.48)(Nb_0.95Sb_0.05)O₃ lead-free piezoelectric ceramics.     

Phase structure and enhanced piezoelectric properties in (1-x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-x(Bi0.5Na0.42Li0.08)0.9Sr0.1ZrO3 lead-free piezoelectric ceramics

The piezoelectric properties of KNN lead-free piezoelectric ceramics could be greatly enhanced by forming multiphase coexistence. In this work, binary system (1-x)(K_0.48Na_0.52)(Nb_0.95Sb_0.05)O₃-x(Bi_0.5Na_0.42Li_0.08)_0.9Sr_0.1ZrO₃ [(abbreviated as (1-x)KNNS-xBNLSZ] ceramics with rhombohedral-tetragonal (r-T) phase boundary was designed and synthesized using the conventional solid-state sintering method, and effects of BNLSZ contents on their micrograph, phase structure and electrical properties were also investigated. According to phase diagram from the results of temperature-dependent capacitance and dielectric constant, the ceramics exhibit the R-T phase coexistence in the composition range of 3.5%≤x<4.5%, and an enhanced dielectric, ferroelectric, and piezoelectric behavior was obtained at such a phase boundary zone. As a result, the ceramics with x=0.04 exhibit optimum electrical properties of d₃₃~461 pC/N, k_p~46%, tan δ~0.03, P_r~16.9 μC/cm², and E_c ~9 kV/cm, together with a Curie temperature (T_C) of ~228 °C. Such a good comprehensive performance obtained in this present work is due to the R-T phase transition and enhanced ɛ_rP_r. It was believed that this ceramic system would promote the development of KNN-based lead-free ceramics.     

Orientation-dependent,field-induced phase transitions in soft lead zirconate titanate piezoceramics

In situ high-energy X-ray diffraction (XRD) was performed on lead-zirconate-titanate-based ferroelectric materials with composition near the morphotropic phase boundary (MPB). The utilization of the two-dimensional area detector in in situ field-dependent experiments enables the complete analysis of the material response with respect to all azimuthal angles at each field amplitude. The studies reveal that the field-induced phase transition from tetragonal to rhombohedral is dependent on crystal orientation in Nb-doped PbZr_0.53Ti_0.47O₃ that is in close compositional proximity to the MPB. However, only domain wall motion is activated in Nb-doped PbZr_0.50Ti_0.50O₃, which is further in composition from the MPB. This synchrotron-based XRD characterization approach illustrates the importance in evaluating the orientation-dependence of phase transitions in piezoelectric and ferroelectric polycrystalline materials.     

The high nano-domain improves the piezoelectric properties of KNN lead-free piezo-ceramics

Due to their high Curie temperature and large dielectric constant, potassium sodium niobate-based lead-free piezo-ceramics (KNN) are regarded as one of the most hopeful piezo-ceramics candidate materials. Herein, (1-x) (K_0.5Na_0.5)(Nb_0.96Sb_0.04)O₃ - x (Ba_0.5Sr_0.25)ZrO₃ [abbreviated as (1-x) KNNS - x BSZ, x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05] lead-free piezo-ceramics are prepared through chemical doping using the traditional solid phase method. The phase structure, domain structure, and microstructure of KNN ceramics have been thoroughly examined. Doping of BZS causes the formation of R-O-T phase boundaries and increases the proportion of polar nano-domains within the crystals, thus increasing the rate of motion of the domain walls and making the domains more easily deflected. The piezoelectric and dielectric properties of the material are improved simultaneously. When x = 0.04, the piezoelectric properties of ceramics reach the optimal value (d₃₃ = 351 pC/N, T_C = 305 °C, K_p = 43% and ε_r = 41267). This work offers a fresh concept for enhancing the overall performance of lead-free piezo-ceramics and aids in understanding the nature of doping modification of lead-free piezo-ceramics.     

Grain size engineered Ba0.9Sr0.1Ti0.9Hf0.1O3-Na0.5Bi0.5TiO3 relaxor ceramics with improved energy storage performance

Novel lead-free diphasic (1-x)Ba_0.9Sr_0.1Ti_0.9Hf_0.1O₃-xNa_0.5Bi_0.5TiO₃ (BSTH-NBT) ceramic nanocomposites were synthesized via an economically viable modified mechano-chemical activation technique. In the present investigation, we have developed an energy storage composite material by systematically optimizing the charge transport behavior and charge storage characteristics between the ferroelectric BSTH and piezoelectric NBT phase. The composite with x = 0.09 NBT concentration has shown the best energy storage properties with 1.61 J/cm³ discharge energy density along with 80.1% energy efficiency. The BSTH and NBT had a synergetic effect on the ferroelectric properties of the composites. The improvement in ferroelectric and piezoelectric properties along with excellent aging characteristics in composite materials is mainly attributed to enhancement in microstructural density, grain boundary interface, and stress effects. The improved dispersibility and excellent compatibility between BSTH and NBT phase have resulted in approximately 20% enhancement in breakdown strength of composite compared to pure BSTH ceramic.     

Enhanced thermal stability of piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O3 systems through tailoring phase transition behavior

Good thermal stability in lead-free BaTiO₃ ceramics is important for their applications above room temperature. In this study, thermal stable piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O₃ ceramics was enhanced by tailoring their phase transition behaviors. Comparison between (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ and (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ revealed that latter system at y?=?0.80 had much better thermal stable piezoelectric coefficient than the former at x?=?0.45. Both systems crystalized in tetragonal to orthorhombic phase boundary at room temperature. The phase transition temperature and degree of diffusion were adjusted by Ca and Zr ions contents and demonstrated great influence on temperature dependent dielectric permittivity, hysteresis loops, and in-situ domain structures. The improved thermal stability of (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ prepared at y?=?0.80 was linked to its higher paraelectric to ferroelectric phase transition temperature (T_m?=?115.7?°C) and less degree of diffusion (degree of diffusion constant γ?=?1.35). By comparison, (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ prepared at x?=?0.45 revealed T_m?=?81.3?°C and γ?=?1.65. Overall, these findings look promising for future stimulation of phase transition behaviors and design of piezoelectric materials with good thermal stabilities.     

Tetragonal tungsten bronze phase potential in increasing the piezoelectricity of sol-gel synthesized (K0.5Na0.5)1-xLixNbO3 ceramics

(K,Na)NbO₃ (KNN)-based ceramics have been proven to be formidable candidates among lead-free piezoelectric materials, yet poor reproducibility always hinders their progress. In the present study, the effects of low lithium substitution on the electrical properties and microstructure of (K_0.5Na_0.5)_1-xLi_xNbO₃ (KNLN) ceramics were investigated. All samples were synthesized by the sol-gel method. The Curie temperature (T_C) of the ceramics shifted to higher temperature and gradually decreased the monoclinic-tetragonal (T_M-T) phase transition. Li⁺ substitution had a prominent effect on the ferroelectric properties and improved the piezoelectric coefficient (d₃₃) up to 181 pC/N. X-Ray Diffraction (XRD) studies and Field Emission Scanning Electron Microscopy (FESEM) images revealed an inevitable tetragonal tungsten bronze (TTB) secondary phase, which was formed during the preparation process. It was demonstrated that the volatilization of Li⁺ cations facilitated TTB growth. The coexistence of two different phase structures proved to enhance the KNN piezoelectric performance.     

Small Reduction of the Piezoelectric d33 Response in Potassium Sodium Niobate Thick Films

We have investigated the electromechanical response of potassium sodium niobate (K_0.5Na_0.5NbO₃ or KNN) thick films. The high‐field strain hysteresis loops and weak‐field converse piezoelectric d₃₃ coefficient of the films were measured and compared with those of KNN bulk ceramics under the same electric field conditions. The converse d₃₃ values of the thick films and bulk ceramics were equal to 82.5 and 138 pm/V, respectively, at 0.4 kV/mm. The fundamental difference between the piezoelectric response of the KNN films and the ceramics was studied in terms of the effective (“clamped”) piezoelectric d₃₃ coefficient. The reduction in the piezoelectric d₃₃ coefficient of the KNN films, resulting from the clamping by the substrate, was compared to lead‐based ferroelectric thick films, including Pb(Zr,Ti)O₃ (PZT) and (1 ? x)Pb(Mg_1/3Nb_2/3)O₃?xPbTiO₃ (PMN‐PT). We propose a possible explanation, based on the particular elastic properties of KNN, for the small relative difference observed between the “clamped” and “unclamped” (“bulk”) d₃₃ of KNN, in comparison with lead‐based systems.     

Full characterization for material constants of a promising KNN-based lead-free piezoelectric ceramic

Potassium-sodium niobate (K_1-xNa_xNbO₃, referred to as KNN) solid solutions, which are an important type of lead-free piezoelectric materials possessing environmentally friendly features, good piezoelectric response and high Curie temperature, have attracted considerable attention in replacing lead-based ceramics. In order to promote the application of KNN-based ceramics in piezoelectric devices, we characterized a complete set of material constants of a high performance KNN-based ceramic, that is 0.965(K_0.48Na_0.52) (Nb_0.96Sb_0.04)O₃-0.035Bi_0.5Na_0.5Zr_0.15Hf_0.75O₃ (KNNS-BNZH), whose Curie temperature is 235 °C, piezoelectric coefficient d₃₃ is 380 pC/N and electromechanical coupling factor k₃₃ is 70%. These results will benefit the design of piezoelectric transducers and actuators using lead-free piezoelectric ceramics.     

High piezoelectric performance and temperature dependence of ferroelectric and piezoelectric properties of Bi(Mg0.5Zr0.5)O3–PbTiO3 near morphotropic phase boundary

According to consideration on the average radius of B-site cation of BiMeO₃, we reported that the Bi(Mg_0.5Zr_0.5)O₃–xPbTiO₃ compound at the morphotropic phase boundary (MPB) of x=0.58 possesses a piezoelectric coefficient d₃₃ as high as 306 pC/N. The optimal piezoelectric and ferroelectric properties near the MPB might be attributed to its lower lattice distortion, as described by change of FWHM value for {1 1 1}_PC peaks. Furthermore, Bi(Mg_0.5Zr_0.5)O₃–xPbTiO₃ displayed stable ferroelectric and piezoelectric properties over a temperature range from ambient temperature to above 160 °C, as exhibited by temperature dependence polarization and strain versus electric field curves and thermal depoling process.     

Ferroelectric‒to‒relaxor crossover in KNN‒based lead‒free piezoceramics

This study investigated the phase transition behavior and electrical properties of (K_0.5Na_0.5)(Nb_1-xZr_x)O₃ (KNN?100xZ) and (K_0.5Na_0.5)NbO₃–yBaZrO₃ (KNN–100yBZ) lead–free piezoelectric ceramics. The phase transitions in crystal structures were compared in KNN ceramics between single Zr⁴⁺ doping and Ba²⁺Zr⁴⁺ co?doping. Piezoelectric properties such as the piezoelectric constant (d₃₃) and electromechanical coupling factor (k_p) are optimized for KNN?6BZ ceramics and were clarified via the polymorphic phase transition from the orthorhombic to pseudocubic phase. The fitted degree of diffuseness (γ) for a phase transition from the modified Curie–Weiss law indicated that KNN ceramics as ferroelectrics are gradually transformed through BaZrO₃ modification. Accordingly, the enhanced strain properties at y = 0.08 consist of coexisting ferroelectric domains and polar nanoregions that are supported by ferroelectric–to–relaxor crossover in KNN?100BZ ceramics.     

High‐energy storage density and excellent temperature stability in antiferroelectric/ferroelectric bilayer thin films

The antiferroelectric/ferroelectric (PbZrO₃/PbZr_0.52Ti_0.48O₃) bilayer thin films were fabricated on a Pt(111)/Ti/SiO₂/Si substrate using sol‐gel method. PbZr_0.52Ti_0.48O₃ layer acts as a buffered layer and template for the crystallization of PbZrO₃ layer. The PbZrO₃ layer with improved quality can share the external voltage due to its smaller dielectric constant and thinner thickness, resulting in the enhancements of electric field strength and energy storage density for the PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer thin film. The greatly improved electric breakdown strength value of 2615 kV/cm has been obtained, which is more than twice the value of individual PbZr_0.52Ti_0.48O₃ film. The enhanced energy storage density of 28.2 J/cm³ at 2410 kV/cm has been achieved in PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer film at 20°C, which is higher than that of individual PbZr_0.52Ti_0.48O₃ film (15.6 J/cm³). Meanwhile, the energy storage density and efficiency of PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer film increase slightly with the increasing temperature from 20°C to 120°C. Our results indicate that the design of antiferroelectric/ferroelectric bilayer films may be an effective way for developing high power energy storage density capacitors with high‐temperature stability.     

Electrical property optimization of BaTiO3-based lead-free piezoceramics by chemical doping

Pb-free piezoceramics have become an inevitable trend in the development of piezoelectric materials due to their non-toxic and environmentally friendly characteristics. In this work, theoretical designs combined with phase-field simulations were used to prepare a novel lead-free system, that is, a type of A-site donor and B-site donor-acceptor compound dopant (Nd_0.5Li_0.5)(Li_0.25Nb_0.75)O₃, which was added to the matrix of Ba(Hf_0.02Ti_0.98)O₃ to tune its composition and optimize its electrical properties. The experimental results showed that the phase transition temperature between orthorhombic and tetragonal (T_O-T) was regulated near room temperature (RT), benefitting their electrical properties. Furthermore, improvements in piezoelectricity were related not only to the O-T phase boundary constructed around RT but also to the enhancements in dielectricity and ferroelectricity. Excellent integrated electrical properties were obtained, which were accompanied by an increase in Curie temperature (T_C) in the 0.10 mol% sample.     

Microstructure and Optical Property in an Irregular Multilayer Comprising Ferroelectric and Paraelectric Materials

A special sequence of multilayer, consisting of PbZr_0.5Ti_0.5O₃ and SrTiO₃ films, was fabricated using a simple chemical solution deposition. X‐ray diffractometer (XRD) measurement reveals that each film in this multilayer has been crystallized into the single perovskite phase. The high‐angle annular dark‐field scanning transmission electron microscopy (STEM) image shows that the obtained SrTiO₃/PbZr_0.5Ti_0.5O₃ multilayer contains three components with different optical thicknesses: dense and porous PbZr_0.5Ti_0.5O₃ layers, together with dense SrTiO₃ layers. This multilayer system exhibits superior optical performance, with a peak reflectivity of～95% and a bandwidth of ～113 nm, rendering its promising candidate as dielectric mirrors, optical cavities, and selective filters.     

High piezoelectricity in CuO-modified Ba(Ti0.90Sn0.10)O3 lead-free ceramics with modulated phase structure

High piezoelectricity was achieved in Ba(Ti_0.90Sn_0.10)O₃ lead-free ceramics by optimizing CuO addition and sintering temperature. The phase structure of 1.0 mol% CuO-doped Ba(Ti_0.90Sn_0.10)O₃ ceramic is coexisting rhombohedral and tetragonal phases as sintered at 1300 °C. The coexistence of rhombohedral, tetragonal and orthorhombic phases appears in 1.0 mol% CuO-doped Ba(Ti_0.90Sn_0.10)O₃ ceramics as sintered at 1350–1450 °C, which leads to highly enhanced d₃₃ up to 650pC/N. This work demonstrates that high piezoelectric property (d₃₃ = 650pC/N) can be obtained in BaTiO₃-based lead-free piezoceramics with a simple composition modification by modulating phase structures, which also indicates that Ba(Ti,Sn)O₃ is a promising candidate to replace the lead-based piezoceramics.     

Field-insensitive giant dynamic piezoelectric response and its structural origin in (Ba,Ca)(Ti,Zr)O3 tetragonal-orthorhombic phase-boundary ceramics

BaTiO₃ (BT)-based ceramics usually exhibit superior quasi-static piezoelectric response but relatively low electrostrain, which limits their actuator applications. In this study, lead-free (Ba_0.835+xCa_0.165-x)(Ti_0.91Zr_0.09)O₃ (x = 0–0.06) (Ba_xCTZ) ceramics with the compositions close to the tetragonal (T)-rich side of orthorhombic (O)-T polymorphic phase boundary (PPB) were reported to exhibit a field insensitive giant dynamic piezoelectric response (d₃₃* >1050 pm/V) over a wide electric field range up to 2 kV/mm, resulting in the large strain value of ∼0.21 %. Detailed structural investigations combined with various electrical properties measurements reveal that the superior dynamic piezoelectric response is attributed to the combination of piezoelectric effect and domain switching behavior due to the chemical modulated O-T PPB, and the field induced partially irreversible T-O phase transition. The results demonstrate that the studied compositions have great potential for applications of lead-free actuator piezoceramics.     

A new method to improve the electrical properties of KNN-based ceramics: Tailoring phase fraction

Although both the phase type and fraction of multi-phase coexistence can affect the electrical properties of (K,Na)NbO₃ (KNN)-based ceramics, effects of phase fraction on their electrical properties were few concerned. In this work, through changing the calcination temperature of CaZrO₃ powders, we successfully developed the 0.96 K_0.5Na_0.5Nb_0.96Sb_0.04O₃-0.01CaZrO₃-0.03Bi_0.5Na_0.5HfO₃ ceramics containing a wide rhombohedral-tetragonal (R-T) phase coexistence with the variations of T (or R) phase fractions. It was found that higher T phase fraction can warrant a larger piezoelectric constant (d₃₃) and d₃₃ also showed a linear variation with respect to tetragonality ratio (c/a). More importantly, a number of domain patterns were observed due to high T phase fraction and large c/a ratio, greatly benefiting the piezoelectricity. In addition, the improved ferroelectric fatigue behavior and thermal stability were also shown in the ceramics containing high T phase fraction. Therefore, this work can bring a new viewpoint into the physical mechanism of KNN-based ceramics behind R-T phase coexistence.