Thermally-stable large strain in Bi(Mn0.5Ti0.5)O3 modified 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3 ceramics

(1-x)[0.8Bi_0.5Na_0.5TiO₃-0.2Bi_0.5K_0.5TiO₃]-xBi(Mn_0.5Ti_0.5)O₃ (x = 0–0.06, BNKMT100x) lead-free ferroelectric ceramics were prepared via solid state reaction method. Bi(Mn_0.5Ti_0.5)O₃ induces a structure transition from rhombohedral-tetragonal morphotropic phases to pseudo-cubic phase. Moreover, the wide range of compositions within x = 0.03–0.055 exhibit large strain of 0.31%–0.41% and electrostrictive coefficient of 0.027–0.041 m⁴/C². Especially, at x = 0.04, the large strain and electrostrictive coefficient are nearly temperature-independent in the range of 25–100 °C. The impedance analysis shows the large strain and electrostrictive coefficient originate from polar nanoregions response due to the addition of Bi(Mn_0.5Ti_0.5)O₃.     

Effect of closed pores on dielectric properties of 0.8Na0.5Bi0.5TiO3-0.2K0.5Bi0.5TiO3 porous ceramics

Porous 0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃ ceramics are fabricated via the pore-forming agent method with polymethyl methacrylate (PMMA) and stearic acid (SA) as pore forming agents, and microstructure observations demonstrate that the porosity, pore shape, and pore sizes can be controlled by the synthesis technology. The dielectric properties of porous ceramics are found not only correlated to the pore-matrix composite model, but also have a significant grain-size effect. Based on the Zener Theory, pining forces exerted by pores on the grain boundary are calculated, to explain the shape effect of pores on grain boundary migration. A phase-field simulation is carried out to investigate pore shape effect on the grain size regulation in porous polycrystalline, and simulation results are in good agreements with experiential results as well as theoretical calculations. Thus, a modified equation is proposed to predict the effective permittivity of the porous piezoelectric ceramics by considering effects of porosity, pore shape and grain size.     

Enhanced piezoelectric performance in Na0.5Bi4.5Ti4O15-based bismuth-layered ceramics by Nb/Mn doping modification

In this work, Na_0.5Bi_4.5Ti_3.94–xMn_0.06Nb_xO_15+y bismuth-layered ferroelectric ceramics were prepared by a solid-state reaction method. The effect of Nb⁵⁺ content on crystal morphology, electrical properties, and piezoelectric performance were systematically investigated. The results show that the introduction of Nb⁵⁺ into Na_0.5Bi_4.5Ti_3.94–xMn_0.06Nb_xO_15+y ceramics to replace Ti⁴⁺ increases the ratio of b/a lattice parameter, leading to the TiO₆ octahedral distortion and the structural transformation tendency from the orthorhombic to tetragonal phase, which facilitates dipole movements of Na_0.5Bi_4.5Ti_3.94–xMn_0.06Nb_xO_15+y ceramics. Therefore, the ferroelectric properties of Na_0.5Bi_4.5Ti_3.94–xMn_0.06Nb_xO_15+y ceramics are improved, and an enhanced piezoelectric coefficient of 30 pC/N combining great temperature stability with d₃₃ value higher than 25 pC/N in the temperature range of 25°C–450°C has been realized in Na_0.5Bi_4.5Ti_3.94–xMn_0.06Nb_xO_15+y ceramics with x = 0.08 mol. Our work provides a good model for designing lead-free ultrahigh Curie temperature piezoelectric devices that can be practically applied in extremely harsh environments.     

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.     

Thickness-dependent switching behavior of 0.8(Bi0.5Na0.5)TiO3-0.2SrTiO3 lead-free piezoelectric thin films

0.8(Bi_0.5,Na_0.5)TiO₃-0.2SrTiO₃ (BNT-0.2ST) thin films, with thicknesses ranging from 90 to 364 nm, were fabricated on platinized silicon substrates by sol-gel method. These films were investigated by switching spectroscopy piezoresponse force microscope (SS-PFM) as a function of frequency at room temperature, revealing the enhanced ferroelectric response in ∼ 210 nm film at all frequencies (0.1 Hz - 1.5 Hz). This enhancement was ascribed to the largest thermally-activated stress at such thicknesses generated during film fabrications. As the temperature of the investigated films increases from room temperature to 200 ^oC, the piezoelectric parameters were obtained from SS-PFM, such as switching polarization (R_s), coercive bias (V₀), work of switching (A_s), maximum strain (S_max), and negative strain (S_neg), indicating an occurrence of phase transition from ferroelectrics to relaxors. This work revealed that thickness plays a crucial role for ferroelectric response and temperature-dependent phase transition in BNT-0.2ST films, since it affects the stress state and switching behavior.     

Microstructure and piezoelectric properties of textured (Na0.84K0.16)0.5Bi0.5TiO3 lead-free ceramics

Textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were fabricated by reactive-templated grain growth in combination with tape casting. The effects of sintering conditions on the grain orientation and the piezoelectric properties of the textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were investigated. The results show that the textured ceramics have microstructure with plated-like grains aligning in the direction parallel to the casting plane. The ceramics exhibit {h 0 0} preferred orientation and the degree of orientation is larger than 0.7. The degree of grain orientation increases with the increasing sintering temperature. The textured ceramics show anisotropy dielectric and piezoelectric properties in the directions of parallel and perpendicular to the casting plane. The ceramics in the perpendicular direction exhibit better dielectric and piezoelectric properties than those of the nontextured ceramics with the same composition. The optimized sintering temperature is 1150 °C where the maximum d₃₃ of 134 pC/N parallel to casting plane, the maximum k₃₁ of 0.31, and the maximum Q_m of 154 in perpendicular direction were obtained.     

(1-x)KNN-xNKBT无铅压电陶瓷的结构及其电性能研究

采用传统固相法制备了(1-x)K0.5Na0.5NbO3-x(Na0.8K0.2)0.5Bi0.5TiO3(x=0-5%)无铅压电陶瓷,研究了(Na0.8K0.2)0.5Bi0.5TiO3的不同引入量对其物相结构、显微形貌、介电性能以及压电性能的影响。结果表明:所有样品都具有钙钛矿结构;随着x的增加,室温下样品从正交相逐渐向四方相过渡并且居里温度向低温方向移动,样品的压电常数d33与机电耦合系数kp均先升高后降低。该体系多晶型转变PPT位于2%≤x≤3%,当x=3%时,样品的压电性能达到最佳,其中:d33=189pC/N,kp=41%,Qm=96,tanδ=0.028。     

Enhanced dielectric and energy-storage properties in BiFeO3-modified Bi0.5(Na0.8K0.2)0.5TiO3 thin films

Lead free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ thin films doped with BiFeO₃ (abbreviated as BNKT-xBFO) (x = 0, 0.02, 0.04, 0.08, 0.10) were deposited on Pt(111)/Ti/SiO₂/Si substrates by sol-gel/spin coating technique and the effects of BiFeO₃ content on the crystal structure and electrical properties were investigated in detail. The results showed that all the BNKT-xBFO thin films exhibited a single perovskite phase structure and high-dense surface. Reduced leakage current density, enhanced dielectric and ferroelectric properties were achieved at the optimal composition of BNKT-0.10BFO thin films, with a leakage current density, dielectric constant, dielectric loss and maximum polarization of < 2 × 10⁻⁴ A/cm³, ~ 978^, ~ 0.028 and ~ 74.13 μC/cm² at room temperature, respectively. Moreover, the BNKT-0.10BFO thin films possessed superior energy storage properties due to their slim P-E loops and large maximum polarization, with an energy storage density of 22.12 J/cm³ and an energy conversion efficiency of 60.85% under a relatively low electric field of 1200 kV/cm. Furthermore, the first half period of the BNKT-0.10BFO thin film capacitor was about 0.15 μs, during which most charges and energy were released. The large recoverable energy density and the fast discharge process indicated the potential application of the BNKT-0.10BFO thin films in electrostatic capacitors and embedded devices.     

Crystal structure refinement,enhanced magnetic and dielectric properties of Na0.5Bi0.5TiO3 modified Bi0.8Ba0.2FeO3 ceramics

(1-x) Na_0.5Bi_0.5TiO₃–x(Bi_0.8Ba_0.2FeO₃) (x=0.5, 0.6, 0.7, and 0.8) ceramics were synthesized via solid state reaction method. Powder X-ray diffraction investigations performed at room temperature along with Rietveld analysis show all the composites to exhibit a rhombohedral distorted perovskite structure, described by space group R3c. Rietveld refinement confirmed a good agreement between observed and calculated intensities and a low value of goodness of fit (χ²). Magnetic measurements were carried out at room temperature up to a field of 6 kOe. Magnetic properties of BBFO modified NBT ceramics are improved with a significant opening in the M–H hysteresis loop at room temperature. Remanent magnetization and coercive field increased with increase of BBFO concentration. The dielectric response of these samples was analyzed in the frequency range 10 Hz–7 MHz at different temperatures revealing a dispersion in dielectric constant (ε′) and in dissipation factor (tan δ) at lower frequencies. Both ε′ and tan δ increase with increase of BBFO content. The temperature dependence of frequency exponent ′s′ of power law suggests that quantum mechanical tunneling (QMT) model to be applicable at lower temperature and correlated barrier hopping (CBH) mechanism to be appropriate at higher temperature to describe the conduction mechanism in x=0.5 and x=0.6 samples. Further, with increase in BBFO content, the dielectric constant becomes more stable at higher frequencies and temperatures thereby improving the dielectric properties of the material.     

Thermal energy harvesting performance in 0.94Bi0.5Na0.5TiO3-0.06BaZr0.2Ti0.8O3: AlN composite ceramics based on the Olsen cycle

The pyroelectric effect provides an efficient route to convert low-grade waste heat into electricity. (Bi_0.5Na_0.5)TiO₃-based ceramic is one of the most attractive lead-free pyroelectric candidates due to its high polarization. However, the low breakdown strength (BDS) restricts its ability to achieve a high polarization, hindering the application for thermal energy harvesting based on the Olsen cycle. In this work, by incorporation of AlN into BNT-BZT ceramics to form the BNT-BZT: AlN composites, the BDS is enhanced from 160 kV cm⁻¹ to 260 kV cm⁻¹, leading to a high energy density of 1.54 J cm⁻³ which is ∼5 times that of the pristine BNT-BZT. The reason for the high energy harvesting performance is due to the addition of AlN that can depress the conductivity of the matrix, resulting in the high BDS that permits high electric fields to be applied on the samples to induce high polarization that changes pronouncedly with temperature.     

（1－x）Bi0．5（Na0．8K0．2）0．5TiO3－xKSbO3压电陶瓷的制备及性能研究

采用传统固相法制备了新型（1－x）Bi0．5（Na0．8K0．2）0．5TiO3－xKSbO3无铅压电陶瓷,利用XRD、 SEM等测试技术表征了该陶瓷的晶体结构、表面形貌、压电和介电性能。研究结果表明,在所研究的组成范围内陶瓷材料均能形成纯的钙钛矿固溶体。在室温下,当KSbO3的掺杂量为1％时,该体系表现出较好的介电性能：εr和tanδ分别为2231和0．055。     

Enhanced energy storage properties of {Bi0.5[(Na0.8K0.2)1-zLiz]0.5}0.96Sr0.04(Ti1-x-yTaxNby)O3 lead-free ceramics

Energy storage properties of {Bi_0.5[(Na_0.8K_0.2)_1-zLi_z]_0.5}_0.96Sr_0.04(Ti_1-x-yTa_xNb_y)O₃ (BNKLSTTN-x/y/z) lead-free ceramics are investigated. It is found that Ta performs better than Nb in the case of their energy storage density values, and the addition of optimum Li contents can enhance the energy storage properties by enhancing the dielectric breakdown strength (DBS). Enhanced energy storage density of 1.60 J/cm³ under a low electric field of 90 kV/cm is achieved in BNKLSTTN-0.025/0/0.10 samples, and the fatigue-free properties are also observed. In addition, the BNKLSTTN-0.025/0/0.10 samples show the enhanced temperature dependence of energy storage density. These results indicate that the BNKLSTTN-x/y/z ceramics are one of the most promising lead-free materials for energy storage applications.     

Enhanced energy density and thermal stability in relaxor ferroelectric Bi0.5Na0.5TiO3-Sr0.7Bi0.2TiO3 ceramics

Sr_0.7Bi_0.2TiO₃ (SBT) was introduced into Bi_0.5Na_0.5TiO₃ (BNT) via a standard solid-state route to modulate its relaxation behaviour and energy storage performance. With increasing SBT content, the perovskite structure of BNT transforms from a rhombohedral phase to a weakly polarized pseudo-cubic phase, and the relaxation behaviour is enhanced. In particular, the E_DBS is improved from 120 kV/cm of BNT to 160 kV/cm of 0.6BNT-0.4SBT, which displays a large recoverable energy storage density (W_rec = 2.20 J/cm³), implying a large potential ability of energy storage for the 0.6BNT-0.4SBT ceramic. Moreover, both dielectric properties (28–326 °C) and energy storage properties (20–140 °C) exhibit a good thermal stability for the same 0.6BNT-0.4SBT composition. These characteristics suggest 0.6BNT-0.4SBT ceramic could be a promising candidate to be applied in a pulse power system over a broad temperature range.     

Structural and electrical properties of La3+-doped Na0.5Bi4.5Ti4O15-Bi4Ti3O12 inter-growth high temperature piezoceramics

New lead-free inter-growth piezoelectric ceramics, Na_0.5Bi_8.5-xLa_xTi₇O₂₇ (NBT-BIT-xLa, 0.00≤x≤1.00), were prepared by the conventional solid-state method. Structural and electrical properties of NBT-BIT-xLa were studied. All the NBT-BIT-xLa samples exhibited a single inter-growth structured phase. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which strongly supports the variation of dielectric and ferroelectric properties. Plate-like grains were found to decrease with the increasing x contents. Impedance spectra analysis indicated that oxygen vacancy defects dominated the contributions to the electrical conductivity. The increased activation energies for dc conductivity evidenced the reduction of oxygen vacancy concentration after La substitution, inducing the enhancement in piezoelectric constant (d₃₃) and remanent polarization (2P_r). The studies of thermal depoling indicated that the optimal d₃₃ of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.     

Significantly enhanced breakdown field with high grain boundary resistance and dielectric response in 0.1Na0.5Bi0.5TiO3-0.9BaTiO3 doped CaCu3Ti4O12 ceramics

Electrical performances are strongly associated with the electrical heterogeneity of grains and grain boundaries for CaCu₃Ti₄O₁₂ (CCTO) ceramics. In this work, the dielectric ceramics of 0.1Na_0.5Bi_0.5TiO₃-0.9BaTiO₃ (NBT-BT) doped CCTO were fabricated by a conventional solid-state reaction method, and the ceramics were sintered at 1100 °C for 6 h. Relatively homogeneous microstructures are obtained, and the average grain sizes are characterized about 0.9∼1.5 μm. Impressively, a significantly enhanced breakdown field of 13.7 kV/cm and a noteworthy nonlinear coefficient of 19.4 as well as a lower dielectric loss of 0.04 at 1 kHz are achieved in the 0.94CCTO-0.06(NBT-BT) ceramics. It is found that the improved electrical properties are attributed to the increased grain boundary resistance of 3.7 × 10⁹ Ω and the Schottky barrier height of 0.7 eV. This is originated from the NBT-BT compound doping effect. This work demonstrates an effective approach to improve electrical properties of CCTO ceramics by NBT-BT doping.     

Large strain and low hysteresis in (0.64-x) Bi0.5Na0.5TiO3- 0.36Sr0.7Bi0.2TiO3- x(K0.5La0.5)(Ti0.9Zr0.1)O3 piezoceramics

In this work, (0.64-x)Bi_0.5Na_0.5TiO₃- 0.36Sr_0.7Bi_0.2TiO₃- x(K_0.5La_0.5)(Ti_0.9Zr_0.1)O₃ lead-free piezoceramics were designed and fabricated by a conventional solid-phase sintering process. It is found that large strains (0.33 %), low hysteresis coefficients (32 %), and large dynamic d₃₃* (367 p.m./V) were obtained at x = 0.01. The large strain originates from the reversible transition of the relaxor to the long-range ferroelectric order in the electric field. When the ferroelectric and relaxor phases coexist in a proper ratio, they can provide a favorable condition for the easier movement of the domains and improve the strain properties. In addition, after 10⁵ cycles, the bipolar strain loop of x = 0.01 content changed slightly, demonstrating excellent fatigue resistance. This work provides a new way to design piezoelectric ceramics with large strain and low hysteresis.     

BaTiO3 nanowires-induced phase transition and thermally stable strain in (Bi0.5Na0.5)TiO3 piezoelectric ceramics

Environment-friendly lead-free piezoceramics with high strain response and extremely excellent stability in a wide operating temperature range are critically important in practical actuator applications. Here, we develop a new strategy to tune the electrostrictive strain behavior in Bi_0.5Na_0.5TiO₃ (BNT)-based ceramics via using high aspect ratio BaTiO₃ nanowires (BT NWs) as a modifier. The addition of BT NWs generates a crossover from a typical ferroelectric (BT conventional spherical particles) to a complete ergodic relaxor (ER) phase at ambient temperature, accompanied by a large electrostrictive strain of ∼0.17% with d₃₃*(S_max/E_max) = 284 pm/V. Such a high electrostrictive strain is extremely thermally stable with only <7% fluctuation from 27 °C to 120 °C. In addition, the BT NWs-modified ceramics also exhibit acceptable fatigue endurance (<30% up to 10⁵ cycles) and frequency dependence (<20% at 10Hz–100Hz). These achieved exceptional performances can be ascribed to the BT NWs-driven complete ER phase at room temperature. The findings of this study can inspire enhanced interest in nanowires as a viable modifier to BNT-based materials due to promising potential for practical actuator applications in a wide temperature range.     

Enhanced energy-storage properties in Zr4+-modified (Bi0.4Ba0.2K0.2Na0.2)TiO3 high-entropy ceramics

Recently, high-entropy perovskite oxides (HEPOs) have received increasing interest for energy storage applications owing to their unique structure, huge composition space, and promising properties. However, designing HEPOs with improved energy storage performance remains a challenge. In this study, various HEPOs were designed by partially replacing Zr⁴⁺ for Ti⁴⁺ in (Bi_0.4Ba_0.2K_0.2Na_0.2)TiO₃ medium-entropy ferroelectric ceramics. The resulting ceramics exhibited a pseudo-cubic structure. With increasing Zr⁴⁺ content, the ceramics gradually transformed into relaxor ferroelectrics. The energy storage performance of the ceramics depended on the Zr⁴⁺ content. The sample with 20 mol% Zr⁴⁺ showed the best energy storage performance with a maximum reversible energy density of 2.47 J/cm³ and an energy storage efficiency of 82.3% at a low applied electric field (224 kV/cm). This study obtained a promising material for the new generation dielectric energy storage capacitors and provided a novel method for enhancing energy storage performance.     

Enhanced piezoelectric properties and low electrical conductivity of Ce-doped Bi7Ti4.5W0.5O21 intergrowth piezoelectric ceramics

New types of Ce-doped Ce_xBi_7-xTi_4.5W_0.5O₂₁ (BTW-BIT-xCe) Aurivillius intergrowth ceramics with high Curie temperatures were synthesized to improve the piezoelectric performances as well as the conduction behaviour, and these ceramics exhibit great potential for high-temperature lead-free piezoelectric applications. The crystal structure, electrical properties and conduction behaviour of BTW-BIT-xCe samples were analysed thoroughly. The XRD patterns combined with Rietveld refinements of the patterns showed that the crystal structure transformed from orthorhombic structure towards pseudo-tetragonal structure with increasing CeO₂ dopant, indicating that a higher symmetry was obtained. The dielectric properties of Ce-doped samples were improved, accompanied by a significant drop in the dielectric loss and a slight decreased Curie temperature (705 °C–683 °C). An enhanced piezoelectric constant d₃₃ of 25.3 pC/N was obtained in BTW-BIT-0.12Ce, which may be attributed to a common decrease in the electrical conductivity and coercive field. Besides, a low electrical conductivity of 2 × 10^-6 S/cm at 540 °C was achieved in the same component owing to a decreased concentration of the oxygen vacancies, which was verified by analyses on XPS spectra. The above results indicate that Ce-doped BTW-BIT samples have great development potential for high temperature piezoelectric applications.