Tetragonal BF-xPT-0.1BZT ceramics with high Curie temperature and large piezoelectric constant

Obtaining both high Curie temperature and large piezoelectric constant simultaneously is of great significance for the application of actuators and sensors. In this research, the piezoelectric ceramics of (0.9-x)BiFeO₃-xPbTiO₃-0.1Ba(Zr_0.5Ti_0.5)O₃ (BF-xPT-0.1BZT, x = 0.29, 0.30, 0.31 and 0.32) were fabricated by the traditional solid-state reaction method. BF-xPT-0.1BZT ceramics exhibit the tetragonal perovskite structure without detectable second phases, and the tetragonality c/a ratio and the tolerance factor of ceramics are about 1.03 and 0.98, respectively. SEM images reveal that ceramics are well densified with the average grain size of 5–12 μm. The Curie temperature T_C of BF-xPT-0.1BZT ceramics is about 490–509 °C, slightly changing with the variation of PT content. The excellent comprehensive dielectric and piezoelectric properties are achieved for BF-0.31PT-0.1BZT ceramics with dielectric constant ε_r (1 kHz), tanδ, piezoelectric constant d₃₃ and T_C of 1001, 0.008, 230 pC/N and 509 °C, respectively. The significant enhancement of piezoelectric constant in the x = 0.31 sample is attributed to the enlargement of grain size. Moreover, the d₃₃ and the planar coupling coefficient k_p remain stable in the elevated temperature range of 200–400 °C, and the fluctuations are only 2% /℃ and 0.007% /℃, respectively, both of which are superior to that of BS-PT based piezoelectric ceramics. Our results indicate that BF-0.31PT-0.1BZT ceramics with high T_C, large d₃₃ and good thermal stability possess great potential for high temperature piezoelectric applications.     

High-temperature BiFeO3–PbTiO3-Ba(Zr,Ti)O3 ternary ceramics with excellent piezoelectricity

Ternary ceramics of (0.87−x)BiFeO₃–xPbTiO₃–0.13Ba(Zr_0.5Ti_0.5)O₃ (BF–xPT–0.13BZT, 0.27 ≤ x ≤ 0.37) were prepared by the traditional solid state reaction methods. X-ray diffraction results display that BF-xPT-0.13BZT ternary ceramics of x ≥ 0.29 exhibit the perovskite structure with dominant tetragonal (T) phases mixed with a small amount of rhombohedral (R) phases. Scanning electron microscopy (SEM) images reveal that the average grain size of BF-xPT-0.13BZT ternary ceramics is in a range of 10–11 μm, increasing first and then decreasing with the increase of PbTiO₃ (PT) content. The low tanδ of about 0.015 and high Curie temperature T_c of above 450°C were obtained for BF-xPT-0.13BZT ternary ceramics. Moreover, the fluctuation of piezoelectric coefficient d₃₃ is less than ±10% over a broad temperature range of 30°C–400°C. BF-xPT-0.13BZT ternary ceramics for x = 0.33 possess the maximum T_c and d₃₃ of 470°C and 320 pC/N respectively, with the room temperature resistivity of about 10¹¹ Ω·cm. These results indicate that BF-xPT-0.13BZT ternary ceramics for x = 0.33 with both excellent piezoelectric properties and high Curie temperature have promising applications in high-temperature piezoelectric devices.     

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.     

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.     

Enhanced piezoelectric properties in BF-BT based lead-free ferroelectric ceramics for high-temperature devices

Due to the high Curie temperature (T_C), BiFeO₃–BaTiO₃ (BF-BT) ceramics have been broadly investigated in high-temperature piezoelectric devices. The piezoelectric constant is one of the most significant factors in determining the sensitivity and reliability of piezoelectric functional components. However, the poor piezoelectric constant (d₃₃) of BF-BT ceramic has prevented the practical application of the material. In this work, we innovatively introduce the 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ (0.93NBT-0.07BT) component to 0.7BF-0.3BT ceramic, to build a morphotropic phase boundary (MPB) for enhancing d₃₃. The XRD analysis shows that the (0.7BF-0.3BT)-x(0.93NBT-0.07BT) ceramics are still in the MPB region with R–PC phases coexistence, and exhibits a homogeneous solid solution. Moreover, the introduction of 0.93NBT-0.07BT ceramic suppresses the generation of defects and facilitates grain growth, thus enhancing piezoelectric property. In consequence, an optimum piezoelectricity d₃₃ = 213 pC/N along with T_c～450 °C was obtained in (0.7BF-0.3BT)-0.01(0.93NBT-0.07BT). This research provides a new idea for the application of BF-BT ceramics in high-temperature piezoelectric devices.     

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.     

Large piezoelectricity and high Curie temperature in novel bismuth ferrite-based ferroelectric ceramics

The requirement for ferroelectric ceramics with a high Curie temperature and a high piezoelectric coefficient remains an important research task for high-temperature sensors and actuators applications. (0.76-x)BiFeO_3-0.24PbTiO_3-xBa(Sn_0.2Ti_0.8)O₃ (BF-PT-BST) piezoelectric ceramics were fabricated using the solid-state reaction method. XRD analysis indicated that the incorporation of large ionic radius Ba²⁺ at A-site and nonferroelectric-active Sn⁴⁺ at B-site generated a decrease in the tetragonality degree c/a. A wide multiphase coexistence region was formed with the content of BST ranging from 0.13 to 0.28. The enhanced piezoelectric coefficient (d₃₃ ~ 200pC/N) was achieved while maintaining a high Curie temperature (T_C ~ 500°C) and a high depolarization temperature (T_d ~ 450°C) for the composition of 0.6BF-24PT-0.16BST. TEM patterns provided clear evidence for the presence of nanodomains (~2nm) would be the predominant source for the enhanced piezoelectricity for the composition x = 0.20. The designed BF-PT-BST ternary system provides great potential for high-temperature piezoelectric applications.     

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)(K_0.40Na_0.60)(Nb_0.955Sb_0.045)O₃–0.006BiFeO₃–x(Bi_0.50Na_0.50)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, T_R-O, is almost independent of the (Bi_0.50Na_0.50)ZrO₃ content, while orthorhombic-tetragonal phase transition temperature, T _O-T, decreases largely with increasing the (Bi_0.50Na_0.50)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.     

Ferroelectric,upconversion emission and optical thermometric properties of color-controllable Er3+-doped Pb(Mg1/3Nb2/3)O3-PbTiO3-Pb(Yb1/2Nb1/2)O3 ferroelectrics

The (0.98-x)(0.6Pb(Mg_1/3Nb_1/3)O₃-0.4PbTiO₃)-xPb(Yb_1/3Nb_1/3)O₃-0.02Pb(Er_1/2Nb_1/2)O₃ ((0.98-x)(PMN-PT)-xPYN:Er³⁺) ceramics were prepared through a solid-state reaction method. The phase structure, piezoelectric response, ferroelectric performance and upconversion emission of the ceramics were systematically investigated. The phase structure, the electrical and optical properties are strongly related to the content of PYN. The optimized piezoelectric response and upconversion emissions of the ceramics were achieved near x = 0.12, which locates in the morphotropic phase boundary (MPB) composition. Furthermore, the temperature sensing behaviors of the resultant compounds based on the thermally coupled levels of ²H_11/2 and ⁴S_3/2 of Er³⁺ ions in the temperature range of 133–573 K were studied by utilizing the fluorescence intensity ratio technique. Additionally, the thermal effect, which is induced by the laser pump power, of the studied ceramics is also investigated and the produced temperature is enhanced from 268 to 348 K with the pump power rising from 109 to 607 mW.     

Structure,microstructure, dielectric and piezoelectric properties of (1-x-y)BiFeO3-xPbFe0.5Nb0.5O3-yPbTiO3 ceramics

Ceramics of seven quasi-binary concentration sections of the ternary solid solution system (1-x-y)BiFeO₃-xPbFe_0.5Nb_0.5O₃-yPbTiO₃ were prepared by the conventional solid-phase reaction method in the range of 0.05 ≤ x ≤ 0.325; 0.05 ≤ y ≤ 0.325. By using x-ray diffraction technique, the phase diagram of the system was constructed which was shown to contain the regions of tetragonal and rhombohedral symmetry and the morphotropic phase boundary between them. Grain morphology, dielectric and piezoelectric properties of selected solid solutions were investigated. The highest piezoelectric coefficient d₃₃ = 50 pC/N was obtained. Dielectric characteristics of ceramics revealed ferroelectric relaxor behavior and region of diffuse phase transition from the paraelectric to ferroelectric phase in the temperature range of 600–800 K.     

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.     

Giant strain response with low hysteresis in potassium sodium niobate based lead-free ceramics

In this work, the relationships between the composition-driven phase boundary, ferroelectricity and strain properties of the (1-x)(K_0.48Na_0.52)(Nb_1-ySb_y)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃ (abbreviated as (1-x)KNN_1-yS_y-xBNKZ) ceramics were investigated. A giant electric field-induced strain of 0.3% (d₃₃¹ = 750 p.m./V) and a low hysteresis (16.4%) were obtained in the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics. The giant strain is attributed to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-field-induced phase transition from the relaxor phase to the ferroelectric phase. Furthermore, the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics exhibit good thermal stability in the temperature range from 25 °C to 150 °C. Hence, this work can promote the practical applications of KNN-based lead-free piezoelectric ceramics in highly sensitive and precise piezoelectric actuators.     

Coexistence of excellent piezoelectric performance and thermal stability in KNN-based lead-free piezoelectric ceramics

With close attention being paid to environmental issues and more legislation coming into force to limit the application of Pb-based materials, accelerating research on lead-free piezoelectric ceramics has become increasingly requisite and urgent. Herein, we have devised and synthesized (1-x)(K_0.5Na_0.5)_0.98Ag_0.02(Nb_0.96Sb_0.04)O₃-x(Bi_0.5Na_0.5)ZrO₃ [abbreviated as (1-x)KNANS-xBNZ, x = 0.01, 0.02, 0.03, 0.035, 0.04, 0.045, 0.05, 0.06] Pb-free ceramics. Phase transition, microstructure, electrical properties, and temperature stability of the ceramics have been comprehensively investigated. The findings illustrate that optimizing BNZ content can give rise to a rhombohedral-tetragonal (R-T) phase boundary when x = 0.04, 0.045, 0.05. The specimens with x = 0.04 show improved piezoelectric properties (d₃₃ ~ 440 pC/N, k_p ~ 53%, T_C ~ 250 °C, d₃₃* ~ 553 pm/V) and good temperature stability. The overall performance is excellent and indicates that (1-x)KNANS-xBNZ ceramics have great potential for replacing their lead-based counterparts.     

Effects of sintering temperature and KBT content on microstructure and electrical properties of (Bi.5Na.5)TiO3-BaTiO3-(Bi.5K.5)TiO3 Pb-free ceramics

A route exploring the morphotropic phase boundaries (MPB) region in (Bi_.5Na_.5)TiO₃-BaTiO₃-(Bi_.5K_.5)TiO₃ ternary system has been designed based on the phase diagram. X-ray diffraction (XRD) has been performed to determine the phases of the prepared samples. The dielectric, ferroelectric, and piezoelectric properties of [(1-x) 0.9363(Bi_.5Na_.5)TiO₃–0.0637BaTiO₃]-x(Bi_.5K_.5)TiO₃ (BNKBT100x) ternary lead-free piezoelectric ceramics are investigated as the functions of x and sintering temperature. When x was varied from 0 to 0.11, the BNKBT100x ceramics show single perovskite structure sintered at 1130–1210?°C. These ceramics show large dielectric permittivity, small dielectric loss, and diffused phase transition behavior. Well-defined ferroelectric polarization-electric field (P-E) hysteresis loop and relative large piezoelectric and electromechanical coefficients are also found in these ceramics. When increasing x, the electrical performances first increase, then decrease. The same rule is found when varying the sintering temperature. The optimized composition and sintering temperature are finally obtained.     

High energy-storage density under low electric fields and improved optical transparency in novel sodium bismuth titanate-based lead-free ceramics

A novel (1-x)Na_0.5Bi_0.5TiO₃-xBaHfO₃ (abbreviated as (1-x)NBT-xBH) transparent ceramic was fabricated by the solid state reaction method. X-ray diffraction analysis showed that NBT-based transparent ceramics exhibit a cubic-like perovskite structure and the solid solubility of BH in NBT reached to 0.15. The Landau-Devonshire theory and I-E curves revealed that the transition between the antiferroelectric like phase and the ferroelectric phase deeply relies on the variation of composition and free energy. One sample (x = 0.15) was found to show a high dielectric constant (˜2418±10%) over the temperature range 57–400 °C. These ceramics also exhibited a high discharge energy density (W_d) of 2.1 J/cm³ and a high maximum polarization P_m of 34 μC/cm² under relatively low electric fields which were less than 175 kV/cm. There was also high transparency in the visible spectra (more than 0.5) when the sample thickness was 250 μm.     

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.     

A dispersed polycrystalline phase boundary constructed in CaZrO3 modified KNN based ceramics with both excellent piezoelectric properties and thermal stability

In this paper, the ceramics with composition of (0.98-x)(K_0.5Na_0.5)(Nb_0.96Sb_0.04)O₃-0.02(Bi_0.5Na_0.5)(Zr_0.8Ti_0.2)O_3-xCaZrO₃ (abbreviated as (0.98-x)KNNS-0.02BNZT-xCZ, x = 0, 0.01, 0.015, 0.02, 0.025, 0.03) were prepared by a traditional solid-state reaction method. The effect of the additional amount of CaZrO₃ on the phase structure, microstructure, dispersion index, domain structure and piezoelectric properties of ceramics was systematically studied. Finally, the piezoelectric properties and thermal stability of ceramics could be controlled by adding different amounts of CaZrO₃. The addition of CaZrO₃ transferred the phase structure of the ceramics from orthogonal-tetragonal (O-T) coexistence phase to rhombohedral-orthogonal (R–O) coexistence phase, which could be demonstrated by XRD test, temperature-dependent Raman spectra and ε_r‐T plot analysis. And when x = 0.02, the ceramics possessed the best piezoelectric and dielectric properties (d₃₃ = 253 pC/N, ε_r = 1185, tanδ = 0.044). Such excellent electrical properties could be originated from the heterogeneous domain structure and small-size nano-domains of the ceramics. Moreover, with the increase of CaZrO₃ doping amount, the dispersion index of ceramics gradually increased from 1.404 to 1.871, which showed more obvious dispersion phase transition characteristics and improved the thermal stability of ceramics. Particularly, when x = 0.02, after annealing at a high temperature of 220 °C (close to its Curie temperature), the d₃₃ tested at room temperature remained above 85% of that without annealing. The results indicated that (0.98-x)KNNS-0.02BNZT-xCZ ceramic was a promising lead-free piezoelectric ceramic system.     

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.     

Achieving both large piezoelectric constant and high Curie temperature in BiFeO3-PbTiO3-BaTiO3 solid solution

The high-temperature and high-performance piezoelectric ceramics are required urgently in the petrochemical, automotive, and aerospace industries. In this work, the (0.85-x)BiFeO₃-xPbTiO₃-0.15BaTiO₃ (BF-PT-BT, x = 0.21, 0.22, 0.23, 0.24 and 0.25) piezoelectric ceramics with both high Curie temperature and large piezoelectric constant d₃₃ were presented. X-ray diffraction analysis shows that BF-PT-BT ceramics exhibit dominant perovskite structure with the coexistence of tetragonal (T) and rhombohedral (R) phases. The c/a ratio, Curie temperature, piezoelectric constant, dielectric constant and loss of the BF-PT-BT ceramics for x = 0.23 are 1.06, 546 °C, 222 pC/N, 545 and 0.013, respectively. Room temperature piezoelectric constant of BF-PT-BT ceramics is much higher than those of PbTiO₃, PbNb₂O₆ and other ABO₃ perovskite compounds (BaZrO₃, Bi(Zn, Ti)O₃, PbZrO₃ and Pb(Mg, Nb)O₃) modified ternary BiFeO₃-PbTiO₃ ceramics with similar Curie temperatures. The piezoelectric constant is almost unchanged after BF-PT-BT ceramics was annealed at 450 °C for 30 min, which is due to the stable switched non-180° domain and transformed R phase by annealing treatment.     

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.