Phase Transition and Large Electrostrain in Lead‐Free Li‐Doped (Ba,Ca)(Ti,Zr)O3 Ceramics

Large piezoelectric effect is achieved in Li‐doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃(BCTZ) ceramics by use of tuning the phase boundaries. Rhombohedral–orthorhombic (R–O) and orthorhombic–tetragonal (O–T) multiphase coexistence is constructed in the ceramics by changing Li contents. The high piezoelectric constant d₃₃ (493 pC/N) and large electrostrain (dS_max/dE_max = 931 pm/V) have been observed in the Li‐doped (Ba, Ca)(Ti, Zr)O₃ ceramics at low sintering temperature (1350°C/2 h). The significant enhancement in materials properties is ascribed to the multiphase region around room temperature induced by Li‐doped effect.     

Electrocaloric Effect in Ba(Zr,Ti)O3–(Ba,Ca)TiO3 Ceramics Measured Directly

In this paper, we report on studies of the electrocaloric (EC) effect in lead‐free (1?x)Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)TiO₃ ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, ΔT_direct = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase.     

Correlation Between Piezoelectric Properties and Phase Coexistence in (Ba,Ca)(Ti,Zr)O3 Ceramics

High piezoelectric properties are desired for lead‐free piezoelectric materials in consideration as a replacement for lead‐based materials in applications. Due to the high piezoelectric coefficient, (Ba_100?xCa_x) (Ti_100?yZr_y) O₃ (BCTZ) piezoelectric ceramics have been considered as a promising lead‐free alternate piezoelectric material. Here, six compositions were selected based on a prediction that all the compositions would have high piezoelectric coefficient at room temperature. The results confirmed all compositions exhibit well developed hysteresis loops and a large piezoelectric coefficient at room temperature. This is due to the coexistence of several phases where the major phase is likely to be orthorhombic and the second phase is proposed to be tetragonal. The phase transition was found to occur over a broad temperature range instead of at a specific temperature only. A relationship between the tetragonal–orthorhombic phase transition temperature and Ca²⁺ and Zr⁴⁺ content was proposed. This enables clear determination of BCTZ compositions with high piezoelectric coefficient at a desired operation temperature.     

Enhanced Piezoelectric Properties of Praseodymium‐Modified Lead‐Free (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 Ceramics

The role of Pr₆O₁₁ addition on the structure, microstructure, electrical, and electromechanical properties of lead‐free (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ piezoelectric ceramics has been systemically investigated. Addition of praseodymium (Pr) results in improved ferroelectric and piezoelectric properties. XRD analysis revealed the co‐existence of rhombohedral (R) and tetragonal (T) phases at room temperature. High remanent polarization values (2P_r ~17 μC/cm²) and loop squareness of nearly 0.87 were obtained for the BCZT‐0.04 wt%Pr ceramic, along with high piezoelectric coefficient (d₃₃ = 435 pC/N) and transduction coefficient [(d₃₃·g₃₃) = 11589 × 10^?15 m²/N]. Results are correlated with the crystal structure and microstructure that significantly influence the ferroelectric and piezoelectric properties near the R–T phase transition point. This material seems to be especially suitable for energy harvesting applications, exhibiting outstanding figure of merit.     

Dielectric nonlinear behavior of (Ba0.95Ca0.05)(Ti0.83Zr0.17)O3‐based multi‐layer ceramic capacitor

The influence of temperature on the variation in dielectric nonlinearity and domain structures was investigated for the (Ba_0.95Ca_0.05)(Ti_0.83Zr_0.17)O₃ (BCTZ)‐based multilayer ceramic capacitor that shows a diffuse phase transition. Whereas the dielectric constant (ε_r) vs temperature shows a broadened maximum peak at low ac driving field, such a peaked behavior disappears at high ac driving field due to an abrupt increase in dielectric constants at low temperatures. Such low temperature effect can be associated with an enhanced spontaneous polarization (P_S) and a significant increase in irreversible domain wall contribution to polarization representing normal ferroelectric behavior based on the Preisach analysis. No ferroelectric domain contrasts were observed at room temperature through transmission electron microscopy. However, they appeared and became more and more distinct with the decrease in temperature, and the crystal structure also changed from cubic to rhombohedral with increased lattice constants. It demonstrates that the dramatic increase in the dielectric nonlinearity with decreasing temperatures originates from the corresponding changes in domain and crystal structure, where the polar‐micro‐regions of BCTZ at room temperature change to normal ferroelectric domains at low temperatures.     

Influence of barium borosilicate glass on microstructure and dielectric properties of (Ba,Ca)(Zr,Ti)O3 ceramics

Barium borosilicate (BBS) glass was added as a sintering aid to (Ba_0.7Ca_0.3)TiO₃-Ba(Ti_0.8Zr_0.2)O₃ (BCZT) ceramics at levels from 2 to 15?wt%, yielding enhanced densification. The addition of BBS also induced changes in phase composition, from predominantly tetragonal to orthorhombic at room temperature. It is shown that the changes in phase content are caused by a shift of the orthorhombic to tetragonal phase transformation from below room temperature to ≈50?°C. An additional high temperature transition around 120?°C was also identified. These observations are interpreted in terms of the development of chemical heterogeneity associated with the redistribution of dopant elements (particularly Zr and Ca) through the liquid phase during sintering. The relative permittivity and electric field-induced polarisation values were generally degraded by the presence of the glass phase, but a reduction in ferroelectric hysteresis and improved densification behaviour have potential benefits in dielectric energy storage applications.     

A novel approach to sintering (Ba,Ca)(Ti,Zr)O3 multilayer ceramic capacitors with Ni electrodes

In this study, a novel sintering technique combining rapid heating and constrained sintering was adopted to fire multilayer ceramic capacitors (MLCCs). It was demonstrated that chamber development can be significantly minimized, leading to a small internal residual stress in MLCCs when they were fired by the novel sintering technique instead of free sintering. The magnitude of tensile stress was closely related to the heating rate and the thickness of the constraining layer. The presence of in-plane tensile stress resulted from the constrained sintering in the x–y plane of the MLCCs, which then modified both the densification rate of the dielectric materials and the inner electrode. The thin inner electrode (<1 μm) with high continuity (>98%) and the fine grain size (1.5 μm) with narrow distribution (±0.10 μm) of BCTZ-based MLCC with a concave-free morphology can be attained by using such a rapid constrained sintering technique when BT is used as a constraining layer laminated on both sides of the multilayer BCTZ-based MLCC.     

Dielectric Tunability,Dielectric Relaxation,and Impedance Spectroscopic Studies on (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 Lead‐Free Ceramics

Dielectric and impedance spectroscopies were employed to study the electrical behavior of (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ (abbr. BCTZ) lead‐free ceramic. The dielectric properties versus dc bias electric field experiment revealed high dielectric tunability (> 65%) as well as figure of merit (> 27) at 10 kHz and room temperature. At elevated temperature range, a dielectric loss peak was observed and verified to be correlate of oxygen vacancy relaxation. The impedance spectra studies indicate that the ceramic is a mixed ionic conductor of p‐type nature at the paraelectric phase and, the grain and total conductivity at 600°C reaches 6.0 × 10^?5 and 2.0 × 10^?5 S/cm, respectively.     

Dielectric Properties and Impedance Spectroscopy of MnCO3‐Modified (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 Lead‐Free Ceramics

A MnCO₃‐modified (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ system was synthesized by solid‐state reaction method. The phase structure, microstructure, electrical properties, and complex impedance were investigated in detail. The addition of small amount of MnCO₃ was quite effective to lower the sintering temperature. XRD analysis showed that the single perovskite phase was achieved in all compositions. XPS results demonstrated that the Mn ions were present in the mixed valence form of +2 and +4. The existence of Mn²⁺ ions gave rise to an increase in oxygen vacancies and suppressed the piezoelectric and ferroelectric properties of samples. The diffusive phase transition was enhanced while the Curie temperature was remained in the vicinity of 90°C. In addition, according to the impedance analysis, it was concluded that the dielectric relaxation behavior was non‐Debye type and the movements of singly ionized oxygen vacancies played an important role in the conduction behavior at higher temperatures.     

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.     

Ultrahigh and field-independent energy storage efficiency of (1-x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3-xBi(Mg0.5Ti0.5)O3 ceramics

Relaxor ferroelectrics are attracting an increasing interest in the application of pulse power systems due to their excellent energy storage performance. In this paper, the (1-x)(Ba_0·85Ca_0.15)(Zr_0·1Ti_0.9)O₃-xBi(Mg_0·5Ti_0.5)O₃ ((1-x)BCZT-xBMT, x ≤ 0.2) relaxor ceramics are prepared by the solid state method. The influence of BMT on the microstructure, dielectric and energy storage properties of the prepared ceramics is investigated. The XRD results show that the peak intensity of impurities (Bi₂O₃, TiO₂ and Ba₂Bi₄Ti₅O₁₈) is gradually stronger than that of BCZT phase with x increasing. Meanwhile, the grain size of (1-x)BCZT-xBMT ceramics gradually increases on account of the appearance of impurities Bi₂O₃. Influenced by the impurities and BMT, the dielectric constant of prepared ceramics gradually decreases with x increasing. A large W_rec value of 0.65 J/cm³ with an ultrahigh η value of 97.89% is achieved at x = 0.15 due to the high breakdown strength and slim P-E hysteresis loop. Meanwhile, the η is insensitive to the electric field. The ultrahigh η leads to lesser energy loss during the charge and discharge process. It makes the 0.85BCZT-0.15BMT ceramic more attractive in the application of pulse power systems.     

Polarization behavior of sol‐gel‐derived relaxor Ba(Zr,Ti)O3 films

Films of the relaxor ferroelectric BaZr_0.25Ti_0.75O₃ (0.25‐BZT) were synthesized via a sol‐gel route to investigate the effect of film thickness on the dielectric properties and for comparison with normal ferroelectric BaTiO₃ (BT). The as‐prepared films on Nb‐doped SrTiO₃ (Nb–ST) displayed a (100) orientation; thinner films had stronger (100) orientations. Microwave dielectric measurements up to a few GHz quantified the polarizations, that is, the dipole contribution, ε_dipole, the combination of the ionic and electronic polarizations, ε_ionic+el., and the total contribution, ε_total. The ε_dipole in the relaxors at a film thickness of t=630 nm was 360, which was double that for the normal ferroelectric BT (ε_dipole=180) at t=735 nm. The larger apparent permittivity of the BZT therefore originated from the larger ε_dipole of the polar nanoregions (PNRs), while the nanograins of BT with few domain walls led to a comparably smaller ε_dipole. The volume ratio of the surface and film‐substrate interface lacking the dipole interactions increased with the reduction in the film thickness, leading to the significant depression in the permittivity for both specimens. The difference in the thickness dependence of the dielectric properties of the sol‐gel derived relaxor BZT and the normal ferroelectric BT films was attributed to the different origins of their dipole contribution, that is, the PNRs and ferroelectric domains, respectively.     

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

A comprehensive study on energy harvesting characteristics as well as electro-mechanical properties of lead-free (1−x)(BaZr_0.2Ti_0.8)O₃—x(Ba_0.7Ca_0.3Ti)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 Q_m = 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 FOM_off = 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.     

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.     

Tuning of electrocaloric performance in (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 by induced relaxor-like behavior

Induced relaxor-like behavior is reported by addition of a sintering additive to the (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ solid solution. The effect of Bi₂O₃ sinter additive on microstructure is determined. The phase transition behavior is highlighted by dielectric permittivity measurements. The electrocaloric temperature change is directly measured and comparison with literature data is provided on basis of the material related cooling power. Addition of Bi₂O₃ drastically increases the temperature stability and an ultra-wide temperature range of over 100?K is achieved. The findings path a way to tune electrocaloric materials for optimization of properties for solid-state coolers based on the electrocaloric effect.     

Influence of Ca substitution on microstructure and electrical properties of Ba(Zr,Ti)O3 ceramics

In the present work, lead-free (Ba_1?xCa_x)(Zr_0.04Ti_0.96)O₃ (x=0.00–0.09) ceramics were fabricated via a solid-state reaction method. The microstructure and electrical properties of the ceramics were investigated. The microstructure of the BCZT ceramics showed a core shell structure at compositions of x=0.03 and 0.06. The substitution of small amount of Ba²⁺ by Ca²⁺ resulted in an improvement of the piezoelectric, dielectric and ferroelectric properties of the ceramics. The orthorhombic–tetragonal phase transition was found in the composition of x≤0.03. Piezoelectric coefficient of d₃₃～392 pC/N and lowest E_c～3.3 kV/cm with highest P_r～14.1 μC/cm² were obtained for the composition of x=0.03 while its Curie temperature (T_C) was as high as 125 °C. However, the ferroelectric to paraelectric transition temperature had slightly shifted towards room temperature with increasing Ca²⁺ concentration.     

Ca含量对(Ba1-xCax)(Ti0.9Zr0.1)O3无铅压电陶瓷性能的影响

采用传统的固相法制备(Ba1-xCax)(Ti0.9Zr0.1)O3(简称BCTZ)无铅压电陶瓷.借助扫描电镜(SEM)和X射线衍射仪(XRD)等测试方法研究了Ca含量对所制备BCTZ无铅压电陶瓷显微结构和压电介电性能的影响.结果表明:随着Ca含量的增加,(Ba1-xCax)(Ti0.9Zr0.1)O3无铅压电陶瓷的晶粒尺寸先增大后减小,所制备的BCTZ陶瓷的物相都是钙钛矿结构,没有杂相;随着Ca含量的增加,BCTZ陶瓷压电常数(d33),介电常数(εr),机电耦合系数(kp)分别先增加后降低,而介质损耗(tanδ)先减小后增大.当Ca含量(x)为0.15mol时,在1450℃烧结制得的(Ba1-xCax)(Ti0.9Zr0.1)O3无铅压电陶瓷性能最佳:压电常数(d33)为363pC/N,机电耦合系数(kp)为39.63％,介电常数(εr)为4184,介质损耗(tanδ)为1.08％.     

Improvement of dielectric loss of (Ba,Sr)(Ti,Zr)O3 ferroelectrics for tunable devices

(Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ (0.05 ≤ x ≤ 0.3) ferroelectric materials have cubic perovskite structure and show paraelectric properties at room temperature. Curie point shifted to a negative value as increasing Zr content in (Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ system. When Zr substituted 0.1 mol, the dielectric constant, dielectric loss, tunability, Curie point and FOM were 4500, 0.0005, 63%, −1.6 °C and 1260, respectively. This composition shows excellent microwave dielectric properties than those of (Ba_0.6Sr_0.4)TiO₃ ferroelectrics, which are limelight materials for tunable devices such as varactors, phase shifters and frequency agile filters, etc.     

Influence of [Ba + Ca]/[Ti + Zr] Ratio on the Interfacial Property of (Ba,Ca)(Ti,Zr)O3 (BCTZ) Powders in an Aqueous Medium

We report findings on the electrokinetic and solubility behaviors of (Ba,Ca)(Ti,Zr)O₃ (BCTZ) powders having three different [Ba + Ca]/[Ti + Zr] ratios: 0.995, 1.000, and 1.005. Electrokinetic and solubility properties of BCTZ powders in aqueous media are phenomenologically similar to BaTiO₃. Ba and Ca ions, occupying primarily A-sites on the perovskite lattice, dissolve during acid titration, which results in surface depletion of A-site cations in the surface region of BCTZ particles. The electrokinetics of colloidal BCTZ powders reflects changes in the surface chemistry that occur as a result of dissolution and adsorption/reprecipitation of surface ions. An increase in [Ba + Ca]/[Ti + Zr] ratio results in an increase in the dynamic mobility at all pH values, an increase in the titration hysteresis, and an increase in the isoelectric pH. Each of these effects can be attributed to Ba and Ca in the near-surface region of BCTZ.     

Creating adjoining polymorphic phase transition (PPT) regions in ferroelectric (Ba,Sr)(Zr,Ti)O3 ceramics

In this work, we report the polymorphic phase transitions(PPT) in ferroelectric Ba_0.95Sr_0.05Zr_xTi_(1-x)O₃ (BSZT, x = 0.01–0.10) ceramics synthesized by using a solid-state reaction method. The doping elements and composition ratios were selected to create adjoining PPT phase boundaries near room temperature, hence to achieve a broadened peak of piezoelectric performance with respect to composition. The temperature-composition phase diagram was constructed and the effects of PPT on the electromechanical and ferroelectric properties of the ceramics were investigated. It was revealed that the two adjacent PPT regions at room temperature showed different characteristics in property enhancement. However, due to the proximity of the phase boundaries, Ba_0.95Sr_0.05Zr_xTi_(1-x)O₃ ceramics in a fairly broad range of compositions (0.02 ≤ x ≤ 0.07) showed excellent piezoelectric properties, including a large piezoelectric constant (312 pC/N ≤ d₃₃ ≤ 365 pC/N) and a high electromechanical coupling coefficient k_p (0.42 ≤ k_p ≤ 0.49).