Microstructure and electrical properties of Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics of (1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yLiNbO₃ (BNT–BKT–LN-x/y) have been fabricated by a conventional solid-state reaction method, and their microstructure and electrical properties have been investigated. The results of X-ray diffraction (XRD) measurement show that K⁺, Li⁺ and Nb⁵⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. The BKT and LN addition has no remarkable effect on the crystal structure. However, a significant change in grain size took place. Simultaneously, with increasing amount of LN, the temperature for a ferroelectric–antiferroelectric phase transition is clearly reduced. The temperature dependence of dielectric properties suggests that the ceramics have diffuse-type phase transition characteristics. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p of the ceramics attain maximum values of 195 pC/N and 0.336 at x=0.18 and y=0.01.     

Combined effects of Li content and sintering temperature on?polymorphic phase boundary and electrical properties of Li/Ta co-doped (Na, K)NbO3 lead-free piezoceramics

Crystallographic structure, phase transition and electrical properties of lead-free (Na_0.535K_0.485)_1−x Li _x (Nb_0.942Ta_0.058)O₃ (x=0.042–0.098) (NKL _x NT) piezoelectric ceramics were investigated. The experimental results show that both Li content and sintering temperature strongly affect the orthorhombic–tetragonal polymorphic phase boundary (PPB), which results in remarkable differences of the piezoelectric property and its temperature stability in the NKL _x NT ceramics. Chemical analysis indicates that sodium volatilizes more seriously than potassium and lithium with increasing sintering temperature. Due to the comprehensively optimized effects of Li content and sintering temperature, an enhanced piezoelectric constant d ₃₃ (276 pC/N) was obtained at room temperature in the ceramics with x=0.074 sintered at 1000°C. In the same composition, a further high d ₃₃ up to 354 pC/N was obtained at 43°C, which is close to its T _o−t temperature. Furthermore, better temperature stability can be obtained when x=0.082 sintered at 1000°C, whose piezoelectric constant d ₃₃ (236 pC/N) keeps almost constant from room temperature to 100°C. Such a temperature-independent piezoelectric property is available in the NKL _x NT ceramics with high Li content because its T _o−t was moved below room temperature.     

Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics

Lead-free 0.79(Bi_0.5Na_0.5)TiO₃–0.14[Bi_0.5(K_0.5−xLi_x)]TiO₃–0.07BaTiO₃ (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 μC/cm² and 157 pC/N of d₃₃ has been obtained when x = 0.3.     

Phase diagram of (1−x%)(0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3)–x%MnO2 lead-free anti-ferroelectric ceramics

A phase diagram for the lead-free ceramics in the (1?x%)(0.89Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–0.05K_0.5Na_0.5NbO₃)–x%MnO₂ (BNBKN-x%Mn) binary system is constructed for the first time based on the ferroelectric and dielectric measurements. The ferroelectric behaviors under different temperatures suggest that the ceramics are basically of relaxor anti-ferroelectric nature near room temperature. The temperature dependent dielectric properties show that when the addition of MnO₂ increases, the relaxor anti-ferroelectric phase can be stabilized to be close to the Curie point, which corresponds to a relaxor anti-ferroelectric to paraelectric phase transition.     

Dielectric,ferroelectric and piezoelectric properties of (1−x)[K0.5Na0.5NbO3]−x[LiSbO3] ceramics

Lead-free (1?x)[K_0.5Na_0.5NbO₃]?x[LiSbO₃] (x=0, 0.04, 0.05 and 0.06)/(KNN-LS) ceramics were prepared by conventional solid-state reaction route (CSSR). For dense morphology pure KNN ceramic was sintered at 1120 °C and LS modified KNN ceramics were sintered at 1080 °C for 4 h, respectively. The structural study at room temperature (RT) revealed the transformation of pure orthorhombic to tetragonal structure with the increase in LS content in KNN-LS ceramics. Temperature dependent dielectric study confirmed the increase of diffuse phase transition nature with the increase in LS content in KNN-LS ceramics. The presence of orthorhombic to tetragonal (T_O?T) polymorphic phase transition temperature (PPT) ～43 °C confirmed the presence of two ferroelectric (orthorhombic and tetragonal) phases in 0.95KNN-0.05LS ceramics at RT. 0.95KNN-0.05LS ceramics showed better ferroelectric and piezoelectric properties i.e., remnant polarization (P_r)～18.7 μC/cm², coercive field (E_c)～11.8 kV/cm, piezoelectric coefficient (d₃₃)～215 pC/N, coupling coefficient (k_p)～0.415 and remnant strain ～0.07% were obtained.     

Ferroelectric relaxor properties of (1 − x)K0.5Na0.5NbO3–xBa0.5Ca0.5TiO3 ceramics

The Ba_0.5Ca_0.5TiO₃ (BCT) composition dependent dielectric and structural properties of (1?x)K_0.5Na_0.5NbO₃–xBa_0.5Ca_0.5TiO₃ powders were investigated. Room temperature x-ray diffraction revealed the powder structure to transform from orthorhombic to cubic with increasing BCT composition. The frequency dependent dielectric constant measurements revealed a shift in the temperature of the maximum dielectric constant for at frequencies, suggesting that the system exhibits ferroelectric relaxor behavior. The system containing 15% BCT showed the closest calculated Curie–Weiss exponent to 2, which the exponent for a relaxor ferroelectric.     

Phase transition,dielectric and piezoelectric properties of NaNbO3–Bi0.5Li0.5TiO3 lead-free ceramics

Lead-free ceramics (1?x)NaNbO₃–xBi_0.5Li_0.5TiO₃ have been fabricated by an ordinary sintering technique, and their electric properties and temperature characteristics have been studied. All the ceramics possess a perovskite structure with orthorhombic symmetry, indicating that (Bi_0.5Li_0.5)TiO₃ diffuses into NaNbO₃ lattices to form a new solid solution. A low (Bi_0.5Li_0.5)TiO₃ doping level transforms the NaNbO₃ ceramics from antiferroelectric to ferroelectric. The ceramics with x ≤ 0.075 are normal ferroelectric, and the ferroelectric-paraelectric phase become diffusives with the doping level of Bi_0.5Li_0.5TiO₃ increasing. As x increases, the Curie temperature of the ceramics decreases linearly, while the relative permittivity ε_r increases. 0.925NaNbO₃–0.075(Bi_0.5Li_0.5)TiO₃ ceramic exhibits the relatively large piezoelectric constant (d₃₃ = 58 pC/N), high Curie temperature (T_C = 228 °C) and good temperature stability, suggesting that the ceramics are one of new possible candidates for lead-free piezoelectric materials.     

Impedance spectroscopy and piezoresponse force microscopy analysis of lead-free (1 − x) K0.5Na0.5NbO3 − xLiNbO3 ceramics

(1 ? x) K_0.5Na_0.5NbO₃ ? xLiNbO₃ (where x = 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) (KNLN) perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. X-ray diffraction patterns indicate that single phase was formed for pure KNN while a small amount of second phase (K₆Li₄Nb₁₀O₃₀, ～3%) was present in LN doped KNN ceramics. Phase analysis indicated the change in the crystal structure from orthorhombic to tetragonal with increase in LN content. The electrical behavior of the ceramics was studied by impedance spectroscopy technique in the high temperature range. Impedance analysis was performed using an equivalent circuit model. The impedance response in pure KNN and KNLN ceramics could be deconvoluted into two contributions, associated with the bulk (grains) and the grain boundaries. Activation energies for conductivity were found to be strongly frequency dependent. The activation energy obtained from dielectric relaxation data was attributed to oxygen vacancies. From PFM we found that the composition with 6.5 wt.% LN displays stronger piezocontrast as compared to pure KNN implying an evidence of a pronounced piezoelectric coefficient.     

Synthesis and electrical properties of Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics

Ceramic compositions of a combination between lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃, and lead titanate, PbTiO₃, were fabricated by using Mg₄Nb₂O₉ precursor technique. Their electrical properties with respect to temperature and frequency were examined and the effect of sintering conditions on phase formation, densification, microstructure and electrical properties of the ceramics were examined. It has been found that optimisation of sintering condition can lead to a highly dense and pyrochlore-free PMN–PT ceramics. The gradual decreasing of the physical properties of the sintered ceramics was related to the gradual decrease of density and inhomogeneous microstructure. The results also revealed that for the lower concentration of lead titanate a relaxor behavior is noticed with a high electrostrictive effect, which was almost hysteretic free. However, higher amount of lead titanate led to a normal ferroelectric behavior.     

Comparative study of the electrical and dielectric properties of PVA–PEG–Al2O3–MI (M=Na,K, Ag) complex polymer electrolytes

Plasticized nanocomposite polymer electrolytes (PNCPEs) based on poly(vinyl alcohol) (PVA)–MI (M=Na, K, Ag) dispersed with nanofillers Al₂O₃ and plasticized with poly(ethylene glycol) (PEG) have been prepared by solution cast technique. The structural properties of the samples have been characterized using various experimental techniques such as XRD, DSC, FTIR and B-G spectroscopy. The ionic conductivity and dielectric constant of the samples have been estimated using a LCZ meter in a wide temperature range, i.e. from 30 to 170 °C. It has been observed that the presence of water molecules in polymer electrolytes significantly affects the mobility of ionic species. The temperature dependent ionic conductivity shows the Arrhenius type behavior separated in three distinct regions. The ionic transference number for all PNCPE samples is found to be ≈1, which strongly suggests their ionic nature.     

Phase formation and electrical properties of BNLT–BZT lead-free piezoelectric ceramic system

Phase formation study in lead-free piezoelectric ceramics based on lanthanum doped bismuth sodium titanate (Bi_0.4871Na_0.4871La_0.0172TiO₃:BNLT) and zirconium doped barium titanate (BaZr_0.05Ti_0.95O₃:BZT), has been carried out in the system of (1−x)BNLT–xBZT where x = 0.0–1.0, by two-step mixed oxide method. It was observed that the addition of BZT in the BNLT ceramics developed the dielectric and piezoelectric properties of the ceramics with the optimum piezoelectric constant (d₃₃) and dielectric constant (ε_r) at room temperature of about 138 pC/N and 1651, respectively, from the 0.2 BNLT to 0.8 BZT ceramic sample. The Curie temperature (T_C) of this ceramic was found at 295 °C which is 195 °C higher than that of pure BZT ceramics, promising the use of this ceramic in a higher range of temperature.     

Enhanced electrical properties of lead-free Bi4−xSbxTi3O12 ceramics with high Tc

Bi_4?xSb_xTi₃O₁₂ (BST) ceramics were prepared and studied in this work in terms of Sb³⁺-modified microstructure and phase development as well as electrical response. By increasing Sb content, the phase structure of the ceramics changed from orthorhombic (Bi₄Ti₃O₁₂-like) to pyrochlore (Bi₂Ti₂O₇-like) phase. Raman spectroscopy showed that the intensities of the lattice vibration modes at frequencies decrease, whereas all bands and bandwidths in all frequency numbers also demonstrate changes. According to the results processed from SEM, Sb³⁺ into the Bi₄Ti₃O₁₂ (BIT) matrix had the effect of increasing the material grain size. ε_r, ε_m, P_r, d₃₃ and T_c of Bi_3.90Sb_0.10Ti₃O₁₂ ceramics are found to be 307, 3492, 8.09 μC cm^?2, 18 pC/N and 660 °C, respectively. And Bi_4?xSb_xTi₃O₁₂ ceramics have good temperature stability, which is very suitable for the practical high-temperature applications.     

The electronic structures,Born effective charge tensors,and phonon properties of cubic,tetragonal,orthorhombic,and rhombohedral K0.5Na0.5NbO3： A first-principles comparative study

The electronic structures, Born effective charges（BECs）, and full phonon dispersions of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5Nb O3 are investigated by the first principles method based on density functional theory.The hybridized states of Nb 4d and O 2p states are observed in the valence band, showing the formation of a strong Nb–O covalent bond which should be responsible for the displacement of Nb and O atoms. The abnormally large BECs of Nb and O indicate the possibility of phase instability induced by the off-center displacement of Nb and O atoms. The phonon dispersions reveal that the ferroelectric instability of K0.5Na0.5Nb O3 is dominated by Nb and O displacements with significant Na characteristics. In addition to the ferroelectric instability, there is also rotational instability coming from the oxygen octahedra rotation around one axis. Moreover, the Γ phonon properties of orthorhombic KNb O3, Na Nb O3, and K0.5Na0.5Nb O3 are also studied in detail.     

Raman spectra and anomalies of dielectric properties and thermal expansion of lead-free (1−x)Na0.5Bi0.5TiO3-xSrTiO3 (x = 0, 0.08 and 0.1) ceramics

ABSTRACT

Thermal expansion, Raman and dielectric properties of the lead-free (1?x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (x = 0, 0.08 and 0.1) ceramic solid solutions, fabricated by the conventional solid-state reaction method, were investigated. The Sr-doping results in an increase of the dielectric permittivity, broadening of the permittivity maximum, enhancement of the relaxation near depolarization temperature, broadening of the Raman bands and shift of all anomalies toward lower temperatures. The observed effects are attributed to an increase of the degree of cationic disorder and enhancement of the relaxor-like features. Anomalies in the thermal expansion strain were observed at the temperatures corresponding to the dielectric anomalies but not related to any phase transitions. These anomalies are supposed to follow changes of the averaged unit cell volume in the temperature interval of tetragonal and rhombohedral phase coexistence.     

Phase-transformation-induced microstructure in lead-free ferroelectric ceramics based on (Bi0.5Na0.5)TiO3–BaTiO3–(Bi0.5K0.5)TiO3

Lead-free ferroelectric ceramics with a morphotropic phase boundary (MPB) composition 85.4% (Bi_0.5Na_0.5)TiO₃–2.6%BaTiO₃–12.0% (Bi_0.5K_0.5)TiO₃ (BNT-BT-BKT at a molar ratio of 85.4: 2.6: 12.0) doped with 0.8?mol% Nb₂O₅ were studied for their crystalline phases and microstructure. The crystalline phases were identified using X-ray diffractometry (XRD) with the contents determined using the Rietveld refinement technique. The phase-transformation-induced microstructure was analyzed using transmission electron microscopy (TEM) and the crystal symmetries were determined using the convergent-beam electron diffraction (CBED) technique. Samples sintered at 1200°C contain a mixture of cubic (C-), tetragonal (T-) and rhombohedral (R-) phases at a ratio of C/T/R?=?56.6: 28.4: 15.0?wt%. Two types of grains are produced: one characterized by a featureless contrast consisting of nano-scale T-domains dispersed in a C-phase matrix; the other a core-shell structure with a shell containing twin and anti-phase-boundary (APB) domains coexisting with a (C?+?T)-phase mixture core. The T- and R-twin boundaries are determined to {111}_T and {110}_R, respectively, and the fault vector for T-APB to R?=?1/2?110]_T. The characteristic microstructure is discussed in terms of the reduction in the point group symmetry and changes in the unit cell volume or the Bravais lattice upon phase transformation among the C-, T- and R-phases. The twin and the APB domains are induced and explained.     

Effect of Li-substitution on piezoelectric and ferroelectric properties of (Bi0.92Na0.92−xLix)0.5Ba0.06Sr0.02TiO3 lead-free ceramics

New lead-free ceramics (Bi_0.92Na_0.92−xLi_x)_0.5Ba_0.06Sr_0.02TiO₃ have been fabricated by a conventional ceramic technique and their electrical properties have been studied. X-ray diffraction studies reveal that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure. The partial substitution of Li⁺ for Na⁺ increases the remanent polarization P_r of the ceramics. Because of the large P_r and low coercive field E_c, the ceramics with x = 0.075–0.125 exhibit excellent piezoelectric properties: d₃₃ = 189–235 pC/N, k_p = 33.6–36.3% and k_t = 51.6–54.3%. The ceramics exhibit relaxor behaviors after the substitution of Li⁺ for Na⁺. Our results also suggest that polar and non-polar phases may coexist in the ceramics at temperatures above the depolarization temperature T_d.     

Dielectric,thermal and Raman spectroscopy studies of lead-free (Na0.5Bi0.5)1−xSrxTiO3 (x = 0, 0.04 and 0.06) ceramics

ABSTRACT

Lead-free (Na_0.5Bi_0.5)_1?xSr_xTiO₃ (x = 0, 0.04 and 0.06) ceramics with relative densities above 97% were prepared by solid-state synthesis process. Their dielectric, thermal and Raman properties were studied. X-ray diffraction analysis shows perovskite structure with rhombohedral symmetry at room temperature. Sr doping of Na_0.5Bi_0.5TiO₃ (NBT) results in an increase of the dielectric permittivity, diffusing of the permittivity maximum and its shift toward lower temperatures. The temperature of the rhombohedral–tetragonal phase transition indicated by the differential scanning calorimetry (DSC) peak and relaxational dielectric anomaly near the depolarization temperature are also shifted toward lower temperatures. The observed increase and broadening of the permittivity maximum, enhancement of the dielectric relaxation near the depolarization temperature, broadening of the DSC anomaly related to the rhombohedral–tetragonal phase transition and broadening of the Raman bands with increasing Sr content are attributed to the increase of the degree of cationic disorder and evident enhancement of the relaxor-like features in NBT–xST. This enhancement could play a positive role in the improvement of the piezoelectric performance of NBT-based ceramics.     

Antiferroelectric–ferroelectric phase transition in lead zinc niobate modified lead zirconate ceramics: crystal studies, microstructure, thermal and electrical properties

The combination of antiferroelectric PbZrO₃ (PZ) and relaxor ferroelectric Pb(Zn_1/3Nb_2/3)O₃ was prepared via the columbite precursor method. The basic characterizations were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), linear thermal expansion, differential scanning calorimetry (DSC) techniques, dielectric spectroscopy, and hysteresis measurement. The XRD result indicated that the solid solubility limit of the (1−x)PZ–xPZN system was about x=0.40. The crystal structure of (1−x)PZ–xPZN transformed from orthorhombic to rhombohedral symmetry when the concentration of PZN was increased. A ferroelectric intermediate phase began to appear between the paraelectric and antiferroelectric phases of pure PZ, with increasing PZN content. In addition, the temperature range of the ferroelectric phase increased with increasing PZN concentration. The morphotropic phase boundary (MPB) in this system was located close to the composition, x=0.20.