Correlation between structural,ferroelectric, piezoelectric and dielectric properties of Ba0.7Ca0.3TiO3-xBaTi0.8Zr0.2O3 (x = 0.45, 0.55) ceramics

We report on the correlation between structural, ferroelectric, piezoelectric and dielectric properties of the (1-x)Ba_0.7Ca_0.3TiO₃-xBaTi_0.8Zr_0.2O₃ (x?=?0.45, 0.55; abbreviated as 55BCT30 and 45BCT30) ceramics close to morphotropic phase boundary (MPB) region. The 55BCT30 and 45BCT30 ceramics were synthesized by the standard, high-temperature solid state ceramic method. X-ray diffraction (XRD) along with Rietveld refinement indicate that the 55BCT30 ceramics exhibit rhombohedral (R, space group R3m), orthorhombic (O, space group Amm2) and tetragonal (T, space group P4mm) phases while 45BCT30 ceramics exhibit only T and O phases. The temperature dependent Raman spectroscopy measurements confirm the structure and phase transformations observed from XRD. All the ceramics are chemically homogeneous and exhibit a dense microstructure with a grain size of 5–7?µm. The presence of polarization-electric field and strain-electric field hysteresis loops confirm the ferroelectric and piezoelectric nature of the ceramics. The polarization current density-electric field curves show the presence of two sharp peaks in opposite directions indicating the presence of two stable states with opposite polarity. Higher values of direct piezoelectric coefficient (d₃₃ ~?360 pC/N) were observed due to the existence of low energy barrier near MPB region and polymorphism. The 55BCT30 ceramics exhibit a higher value of electrostrictive coefficient (Q₃₃ ~?0.1339?m⁴/C²) compared to the well-known lead-based materials. The temperature dependent dielectric measurements indicate the O to T phase transition for 55BCT30 and 45BCT30. These ceramics exhibit a Curie temperature (T_c) of 380?K with a dielectric maximum of ~ 4500.     

Phase transition,microstructure, and dielectric properties of Li/Ta/Sb co-doped (K,Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−xTa_xSb_0.07)O₃ (abbreviated KNLNST_x) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 °C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNST_x ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST_0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d₃₃=313 pC/N, k_p=47%, and ε_r=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d₃₃=200 pC/N and ε_r=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.     

Structural and electrical properties of Na0.47K0.47Li0.06NbO3 lead-free piezoelectric ceramics modified by AgSbO3

(1?x)Na_0.47K_0.47Li_0.06NbO₃ (NKLN)–xAgSbO₃ lead-free piezoelectric ceramics were prepared using a reaction sintering method. The effects of AgSbO₃ doping on the structural and electrical properties of NKLN ceramics sintered at 1000–1040 °C were studied. The dopant affected densification, phase content, sintering temperature, microstructure and electrical properties. Variations in the relative intensity of X-ray diffraction peaks were consistent with Ag⁺ and Sb⁵⁺ ions substituting on the perovskite lattice to produce a change in the proportions of co-existing tetragonal and orthorhombic phases. Grain growth during secondary re-crystallization was also affected. The temperature of the orthorhombic–tetragonal (O–T) phase transition and the Curie temperature (T_C) decreased as a result of AgSbO₃ modifications. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic–tetragonal polymorphotropic phase boundary. The 0.92Na_0.47K_0.47Li_0.06NbO₃–0.08AgSbO₃ ceramics exhibited optimum electrical properties (d₃₃=252 pC/N, ε_r=1450, tan δ=0.02, and T_C=280 °C). These results reveal that (1?x)Na_0.47K_0.47Li_0.06NbO₃–xAgSbO₃ ceramics are promising materials for lead-free piezoelectric application.     

Compositional dependence of phase structure and electrical properties in (K0.50Na0.50)0.97Bi0.01(Nb1−xZrx)O3 lead-free ceramics

(K_0.50Na_0.50)_0.97Bi_0.01(Nb_1-xZr_x)O₃ (KNBNZ) lead-free ceramics were prepared by the conventional solid-state sintering process. Their phase structure is dependent on the Zr content in the investigated range, and the ceramics endure a phase transition from pseudocubic to orthorhombic with increasing Zr content. Improved piezoelectric properties have been observed when the poling temperature is located at ~100 °C because of the coexistence of orthorhombic and tetragonal phases. Their dielectric and piezoelectric properties were enhanced by doping Zr, the ceramic with x=0.02 showing optimal electrical properties, i.e., d₃₃~161 pC/N, k_p~0.41, Q_m~81, T_c~370 °C, and T_o−t~130 °C. These results show that the KNBNZ ceramic is a promising lead-free piezoelectric material.     

Phase transitions in (1−x)BaZr0.2Ti0.8O3−xBa0.7Ca0.3TiO3 powders and ceramic pellets

The phase transitions in (1−x)BaZr_0.2Ti_0.8O₃−xBa_0.7Ca_0.3TiO₃ (BZT-xBCT) powders and ceramic pellets were studied. It is found that the phase compositions in the pellets are different from that in the powders, which may be caused by the stress in the pellets. The monoclinic phase exists near the morphotropic phase boundary (MPB) in the ceramics. The piezoelectric coefficient (d₃₃) measurement of the ceramics shows that the higher piezoelectric properties are corresponding to higher content of monoclinic phase.     

Investigation of enhanced performance in BF-xPT-0.05BZ ternary ceramics for high temperature applications

(0.95-x)BiFeO₃-xPbTiO₃-0.05BaZrO₃ (BF-xPT-0.05BZ) ternary ceramics were synthesized by the solid-state reaction method, which present a perovskite structure without detectable second phase. BF-0.32PT-0.05BZ ceramics display the dominant rhombohedral structure with a small amount of tetragonal phase, revealing the characteristics of morphotropic phase boundary (MPB). The well-densified ceramics possess the average grain size of 8–15 μm with different PT contents. The ferroelectric-paraelectric phase transition of BF-xPT-0.05BZ ceramics shows the characteristic of the first-order transition, which occurs at around 550 °C. It is noticeable that BF-0.32PT-0.05BZ ceramics exhibit the giant remnant polarization of 60 μC/cm² under the field of 175 kV/cm. Moreover, BF-0.32PT-0.05BZ ceramics show the highest piezoelectric constant d₃₃ of 105 pC/N and Q_m of 501. The depolarization temperatures of BF-xPT-0.05BZ ceramics are much close to their Curie temperature T_C, proving the extensive prospect for high temperature piezoelectric applications.     

Dielectric,ferroelectric, and piezoelectric properties in potassium sodium niobate ceramics with rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries

A new phase boundary with rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries is designed in (K_0.48Na_0.52)NbO₃ by adding Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5ZrO₃ (BNKLZ), where Zr⁴⁺ and (BNKL)²⁺ are respectively used to improve the temperature of a rhombohedral phase and to decrease the temperature of an orthorhombic–tetragonal phase coexistence. These ceramics endure several continuous phase transitions with increasing BNKLZ content, i.e., an orthorhombic phase (0≤x<0.03), orthorhombic–tetragonal phases (x=0.03), orthorhombic–tetragonal and rhombohedral–orthorhombic (O–T and R–O) phase existence (0.03<x≤0.05), a rhombohedral phase (0.05<x≤0.07). The ceramics with O–T and R–O have a better piezoelectric behavior as compared with other phases because of more polarization states, enhanced ε_r and P_r, and a dense microstructure. Moreover, piezoelectric properties could be further optimized by modifying their sintering and poling temperatures. As a result, the construction of O–T and R–O phase coexistence benefits the improvement of piezoelectric properties in KNN-based ceramics.     

Simultaneously enhanced energy storage density and efficiency in novel BiFeO3-based lead-free ceramic capacitors

In this work, a series of novel lead-free (1-x)Bi_0.83Sm_0.17Fe_0.95Sc_0.05O₃-x(0.85BaTiO₃-0.15Bi(Mg_0.5Zr_0.5)O₃) [(1-x)BSFS-x(BT-BMZ), x = 0.45?0.85] relaxor ceramics were prepared by solid phase sintering, and their dielectric properties and energy storage performances were explored. It was revealed that all the samples have a dense structure with pure pseudo-cubic phase. With the increase of x, the ferroelectric hysteresis loop is gradually slimmed accompanied by a decreasing polarization, indicating an enhanced relaxor behavior. Moreover, the electric breakdown strength increases linearly with x due to the fine grain size and enhanced relative density. Interestingly, a large recoverable energy density (～3.2 J/cm³) with an outstanding efficiency (～92 %) is achieved under an electric field ～206 kV/cm for the optimized component x = 0.75, which is superior to other reported lead-free ceramic systems. Moreover, the optimized ceramics of 0.25BSFS-0.75(BT-BMZ) show good thermal stability (25?100 °C) and excellent fatigue endurance (cycle number: > 10⁵) in energy storage performances. This work opens up a new route to tailor lead-free dielectric ceramics with high energy storage properties.     

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.     

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.     

Improved Li and Sb doped lead-free (Na,K)NbO3 piezoelectric ceramics for energy harvesting applications

Piezoelectric energy harvesting is the most widely investigated technology for renewable energy applications. In this work, (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ piezoelectric ceramics were prepared through conventional mixed oxide fabrication methods with different sintering temperatures. Although the (Na_0.5K_0.5)NbO₃ piezoelectric material is representative among the lead-free ceramics, it is difficult to densify by typical sintering techniques owing to its easy evaporation properties of potassium (K⁺) and sodium ion (Na⁺). Hence, lithium (Li⁺) and antimony ion (Sb⁵⁺) were used for the partial substitution of (Na_0.5K_0.5)NbO₃. With the optimized sintering temperature, Li⁺ and Sb⁵⁺ are expected to be crucial in increasing the density and enhance the piezoelectric and ferroelectric properties. In this study, the phase, microstructure, and dielectric and electrical properties of (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ ceramics depending on the sintering temperature is examined by employing X-ray diffraction, field emission scanning electron microscopy, impedance analyzer, and mechanical force system for energy harvesting.     

Structure,dielectric and piezoelectric properties of Ba0.90Ca0.10Ti1−xSnxO3 lead-free ceramics

Lead-free piezoelectric ceramics Ba_0.90Ca_0.10Ti_1−xSn_xO₃ have been prepared by a conventional ceramic fabrication technique and the effects of Sn⁴⁺ on the structure, dielectric and piezoelectric properties of the ceramics have been investigated. All the ceramics exhibit a pure perovskite structure. After the substitution of Sn⁴⁺, the crystal structure of ceramics is transformed gradually from a tetragonal to an orthorhombic phase, and becomes a pseudo-cubic phase at x≥0.14. The substitution also decreases the Curie temperature greatly from 138 °C at x=0 to 33 °C at x=0.12, and shifts the orthorhombic–tetragonal phase transition to higher temperatures. Coexistence of the orthorhombic and tetragonal phases is formed in the ceramic at x=0.10, leading to significant improvements in the piezoelectric properties: d₃₃=521 pC/N and k_p=45.5%. Our results also reveal that the ceramics sintered at higher temperatures contain larger grains, and thus exhibit more noticeable tetragonal–orthorhombic phase transition and enhanced ferroelectric and piezoelectric properties.     

Improved ferroelectric properties of BiFeO3-based piezoelectric ceramics through morphotropic phase boundary construction

The ceramic (1-x)BiFe_0.985Sc_0.015O₃-xBaZr_0.2Ti_0.8O₃ + 1mol% MnO₂ (x = 0.20, 0.25, 0.30, 0.35) (BFS-xBZT) was synthesized using the traditional ceramic sintering method. The components of the ceramic were determined by constructing a morphotropic phase boundary (MPB) which consists of rhombohedral (R) and tetragonal (T) phases (0.24≤ x ≤ 0.26). With the accumulation of BZT content, relax behaviors were observed by dielectric properties measurements. At the MPB, the crystal structures of the R phase and T phase change abruptly. The distortion degree of the R phase increases, and the differences between a and c of the T phase decrease. An enhanced ferroelectricity Pr of ~27.8 μC/cm² and apex piezoelectric coefficient d₃₃ of 131 ± 4 pC/N are obtained near the MPB (x = 0.25), due to the R-T phase coexistence near room temperature. The results show that BFS-xBZT ceramics could be a candidate for lead-free piezoelectric ceramics at high operating temperatures.     

Ferroelectric,dielectric, and impedance properties of Sm-modified Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

To investigate the effect of Sm doping on the electrical properties of Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) (x = 40, 50, 60) ceramics, three Sm-modified ceramics were prepared using the conventional solid-state reaction method. Related electrical measurements, including ferroelectric and dielectric investigations and impedance spectroscopy, were recorded for these ceramics. It was found that a tilted morphotropic phase boundary resulted from the addition of Sm, which induced the best piezoelectric properties and insulating behaviour in the Sm-BZT-60BCT sample. An abnormal P-E loop shrinkage appeared in the Sm-BZT-50BCT sample but not in the other two samples. This could be attributable to the different electronegativities between Ca²⁺ and Ba²⁺ and between Zr⁴⁺ and Ti⁴⁺, whose contents are different in varied samples and have an effect on defect-dipole alignment as well as spontaneous polarization. The activation energies for the bulk conductivity in the three composites were calculated to be 0.28 ± 0.01, 0.08 ± 0.01, and 0.36 ± 0.01 eV, confirming the existence of oxygen vacancies in our samples. The Sm dopant is responsible for the oxygen vacancies. This also leads to an increased Curie temperature in the three composites.     

Domain structure of nonstoichiometric sodium potassium niobate-based ceramics for piezoelectric acoustic actuators

In this work, three piezoelectric acoustic actuators are prepared by using multi-layer technology for lead-free (Na_0.48-xK_0.48-xLi_0.04)Nb_0.89-xTa_0.05Sb_0.06O₃-xSrTiO₃ (NKLNTS-xST) ceramics, x=0 and 0.007, and commercial lead-based ceramic. The phase and domain structure are also investigated by using the XRD patterns and TEM images of the NKLNTS-xST ceramics. The (Na_0.473K_0.473Li_0.04Sr_0.007)Nb_0.883Ti_0.007Ta_0.05Sb_0.06O₃ piezoelectric acoustic actuator has the best sound pressure level and d₃₃ value of 650 pCN^?1. The response mechanisms suggest that the piezoelectric ceramic vibration amplitude was increased and the sound pressure level improved since the orthorhombic and tetragonal phases were found to coexist and the nanoscale domain increased for the (Na_0.473K_0.473Li_0.04Sr_0.007)Nb_0.883Ti_0.007Ta_0.05Sb_0.06O₃ ceramic.     

Room temperature phase superposition as origin of enhanced functional properties in BaTiO3 - based ceramics

BaSn_xTi_1-xO₃ (x = 0, 0.05) ceramics with orthorhombic/tetragonal phases at room temperature were comparatively investigated to understand the role of phase composition on their functional properties. With respect to the values of BaTiO₃, the switching polarization, permittivity peak, tunability and piezoelectric coefficients are enhanced by doping with 5 % Sn onto Ti⁴⁺ sites. The orthorhombic polymorph amount is larger in the doped ceramic and explains its higher switching polarization. High field poling favors the orthorhombic phase in both compositions; this polymorph becomes predominant in the BaSn_0.05Ti_0.95O₃ ceramic. Landau-based calculations developed for ceramics with randomly oriented grains predicted the stability of variable amounts of orthorhombic/tetragonal phases around room temperature and explain the field-induced predominant orthorhombic state, mostly in BaSn_0.05Ti_0.95O₃. Due to the twelve allowed spontaneous polarization directions, the orthorhombic state is responsible for the enhanced polarization, tunability and piezoelectric properties with respect to the tetragonal state with six polarization possible orientations.     

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.     

Excellent energy-storage properties of NaNbO3-based lead-free antiferroelectric orthorhombic P-phase (Pbma) ceramics with repeatable double polarization-field loops

The energy-storage performance of stable NaNbO₃-based antiferroelectric (AFE) ceramics was for the first time reported in (0.94-x)NaNbO₃-0.06BaZrO₃-xCaZrO₃ lead-free ceramics. A gradual evolution from an instable AFE phase (x≤0.01) to an orthorhombic AFE P phase (Pbma) (0.01<x≤0.05) was found to accompany the appearance of repeatable double-like polarization versus electric field loops although poled samples (x<0.01) own an AFE monoclinic phase (P2₁). Interestingly, compared with x≤0.01 samples with instable antiferroelectricity, a relatively high recoverable energy storage density W_rec ? 1.59 J/cm³ (@ 0.1 Hz) and a storage efficiency η of ?30% were achieved in the x = 0.04 ceramic. Moreover, a high W_rec of > 1.16 J/cm³ and an outstanding charge-discharge performance with fast discharge rate (t_0.9 < 100 ns) were generated in the temperature range from room temperature to 180 °C in the x = 0.04 ceramic. These results suggest that NaNbO₃-based AFE P-phase ceramics could be new potential dielectric materials for high-energy storage capacitors.     

Dielectric and impedance spectroscopy analysis of lead-free (1-x)(K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3-xBaTiO3 ceramics

(1-x)(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃-xBaTiO₃ (labeled as (1-x)KNLNTS- xBT, x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) lead-free ceramics were prepared by a solid-state sintering method. As the BT content increased, the phase of ceramics changed from orthorhombic (0.00 ≤ x ≤ 0.02) to orthorhombic-tetragonal (0.02 < x < 0.06) structure, and finally turned into tetragonal (0.06 ≤ x ≤ 0.10) structure. The Curie-Weiss law and modified Curie-Weiss law were applied to analyzing dielectric properties. With the increase of BT content, the relaxation degree increased, which indicated that the ceramics shown a excellent relaxation behavior. For 0.9KNLNTS- 0.1BT ceramics, the dispersion coefficient γ reached the maximum of 1.73, which is hugely attractive for lead-free relaxor ferroelectrics. From its variation of impedance spectroscopy with temperature, it was found that the relaxation and conduction behavior were associated with the thermal activation, and the oxygen vacancies were the potential ionic carriers. Moreover, through Arrhenius fitting, the activation energy of 0.9KNLNTS- 0.1BT ceramic was 0.82(6) eV, indicating that the oxygen vacancy concentration for the ceramics was high.     

Phenomenological modeling of phase transitions and electrocaloric effect in Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3

Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) is a promising lead-free ferroelectric system. In this paper, we present two sets of free energy coefficients and carry out phenomenological modeling to study the phase transition and electrocaloric effect. The calculated phase diagram is in excellent quantitative agreement with experiments. Furthermore, we propose a new method based on effective internal electric field to simulate polarization in the macroscopic paraelectric state of ferroelectric relaxor. The computed composition and temperature-dependent entropy and temperature change induced by electrocaloric effect are in good agreement with the measured data available for single crystal.