TiO3 Lead-free Ferroelectric Ceramics Research Articles

Structural and dielectric features of (Bi0.5Na0.5)1−xBaxTiO3 lead-free ferroelectric ceramics: An approach to the phase diagram

(Bi0.5Na0.5)1−xBaxTiO3 lead-free ferroelectric ceramics were synthesized via the conventional solid-state reaction method. Structural and dielectric properties were investigated as a function of the doping concentration, considering x = 0, 2, 5, 8, 10, 12, 16, and 18 at. % Ba. The structural analyses were carried out from the x-ray diffraction technique, including the Rietveld refinement method, and Raman spectroscopy. Results confirmed the formation of the perovskite structure, revealing different crystalline symmetries, depending on the Ba2+ concentration: the single rhombohedral ferroelectric phase (R3c) for x = 0 and 2 at. %; coexistence of both rhombohedral ferroelectric (R3c) and tetragonal antiferroelectric (P4bm) phases for x = 5 at. % Ba; the single tetragonal antiferroelectric phase (P4bm) for x = 8 at. % Ba; coexistence of two tetragonal phases (antiferroelectric P4bm and ferroelectric P4mm) for x = 10 at. % Ba; and the single tetragonal ferroelectric phase (P4mm) for x = 12, 16, and 18 at. % Ba. The characteristics of the phases’ transition, investigated from dielectric analysis, revealed the presence of two dielectric anomalies, which indeed have been associated to different phases’ transitions, one of them showing relaxor-like characteristics. The obtained results offer new insights for a better understanding on the features of the phase diagram for the studied ceramic system, according to the different observed crystalline symmetries (ferroelectric and antiferroelectric) in a very wide doping concentration. In the light of the obtained results, a new phase diagram has been proposed considering a wider compositional range than those reported in the literature.

Light-triggered “on-off” switchable dielectric constant in (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

Light control of dielectric properties is very significant to develop photo-dielectric devices. Here, light-triggered “on-off” switchable dielectric constant (εr) was observed in (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ferroelectric ceramics, exhibiting unconventional photo-dielectric effect (PDE). The effects of light switch period (2–165 min), light intensity (20–100 mW/cm2), and wavelength (450–940 nm) on PDE were studied in detail. Variation in εr exhibits remarkable switch effect with changing light on-off, while dielectric loss remains almost unchanged, and conductivity is still as low as 10−12 S/cm with excellent electrical insulation under light irradiation. The unconventional PDE exists in the high-frequency range ≥1 MHz and at the low-temperature of −100 °C. In addition to the thermal effect and light-induced change in domain behavior, it is believed that polarization of the trapped electron-hole pairs under an external electric field contributes to the increase in εr, and the low mobility of the pairs contributes to low dielectric loss and low photoconductivity. The results exhibit that dielectric constant of the lead-free ceramics can be regulated via light, whilst the ceramics keep excellent electrical insulation, endowing the lead-free ceramics potential applications in photo-dielectric devices.

Enhanced energy-storage properties of La2O3 doped (Na0.5Bi0.5)0.705Ba0.045Sr0.25TiO3 lead-free ferroelectric ceramics

The powder samples of (Na0.5Bi0.5)0.705Ba0.045Sr0.25TiO3-xLa2O3 (NBBSLT) have been prepared successfully by a wet solid-phase method. The effects of La doping on the morphology, ferroelectric and dielectric properties of NBBSLT ceramics were studied. XRD results show that the composite has perovskite structure. The dense phase morphology of NBBSLT was observed by SEM. Dielectric measurements show that depolarization temperature and Curie temperature move toward low temperature. With the increase of La content, the ferroelectric hysteresis loops at room temperature become thinner at first and then wider. The NBBSLT ceramics have excellent energy storage properties when the La content is 0.04.

Influence of Nb2O5 doping to energy-storage properties of (Na0.5Bi0.5)0.705Ba0.045Sr0.25TiO3 lead-free ferroelectric ceramics

The powder samples of (Na0.5Bi0.5)0.705Ba0.045Sr0.25TiO3-xNb2O5 (NBBSNT) were synthesized by a wet solid-phase method. The effects of Nb-doping on morphology, ferroelectric and dielectric properties of samples were investigated. Typical surface morphologies by scanning electron microscopy for the NBTBZN ceramics showed grain size decreasing. Dielectric measurements revealed that the depolarization temperature and Curie temperature shifted to lower temperature. With the increase of Nb content, the ferroelectric hysteresis loops became much slimmer, coupled with the reduction in remnant polarization and coercive field. Energy storage density of 0.855 J/cm3 and the energy storage efficiency of 74.26% were obtained for NBBSNT with x = 0.09.

Enhanced energy storage performance and fatigue resistance of Mn-doped 0.7Na0.5Bi0.5TiO3–0.3Sr0.7Bi0.2TiO3 lead-free ferroelectric ceramics

The validity of Mn element on enhanced energy storage performance and fatigue resistance of Mn-doped 0.7Na0.5Bi0.5TiO3–0.3Sr0.7Bi0.2TiO3 lead-free ferroelectric ceramics (BNT–BST–xMn) is certified by doping. The effects of Mn modification on the dielectric behavior, ferroelectric, energy storage properties, and AC impedance are comprehensively investigated. It is found that the average grain size of the ceramics modified by Mn additions is reduced slightly. Moreover, the relaxor properties are evidently enhanced with the increased Mn content. The AC impedance spectra can even better clarify the dielectric response and relaxor behavior. The results suggest that both of the dielectric response and relaxor behavior are determined by defects especially concentration of the oxygen vacancy. The superior energy storage properties are realized at x = 0.05 with an energy storage density (Wrec) of 1.33 J/cm3 as well as energy storage efficiency (η) of 86.2% at 100 kV/cm, accompanied with a superior thermal stability. BNT–BST–5Mn ceramics can maintain a stable energy storage performance within 106 fatigue cycles, indicating an excellent fatigue resistance.

Ferroelectric and dielectric properties of (Na0.47Bi0.47Ba0.06)0.99Ca0.01TiO3 ceramics sintered in different atmospheres

ABSTRACTThe (Na0.47Bi0.47Ba0.06)0.99Ca0.01TiO3 lead-free ferroelectric ceramics were prepared via the conventional solid-state reaction method. The atmospheres of nitrogen and oxygen were chosen during the sintering process, respectively. Both the ceramics sintered in nitrogen and oxygen atmospheres exhibit high relative density (>96%) and pure perovskite structure. The dielectric behaviors with diffusion phase transition were studied via the Curie–Weiss law and the modified Curie–Weiss law. The piezoelectric and ferroelectric properties of the two kinds of ceramics were studied comparatively. Compared to the O2 fired ceramics, the N2 fired samples exhibit higher depolarization temperature (Td = 117°C). The O2 fired ceramics show higher piezoelectric constant (d33 = 113 pC/N) and higher polarization values (Pm = 40.4 μC/cm2, Pr = 34.4 μC/cm2). The differences in the electrical properties of the ceramics sintered in the different atmospheres were discussed.

Sr2+ doping to enhanced energy-storage properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 lead-free ferroelectric ceramics

The powder samples of [(Na0.5Bi0.5)0.94Ba0.06]1−xSrxTiO3 (NBBST) have been prepared successfully by a wet solid-phase method. The effects of Sr-doping on morphology, ferroelectric and dielectric properties of NBBST ceramic samples have been investigated in detail. The XRD results show that the compounds possess perovskite phase structure. The dense phase morphology of NBBST is observed by scanning electron microscopy. Dielectric measurements show that the depolarization temperature and Curie temperature shift to low temperature. The ferroelectric hysteresis loops at room temperature become much slimmer with the increase of Sr content. When Sr-doping content is 0.30, the Ps of NBBSNT ceramic sample decreases distinctly. the Sr-25 ceramic samples have superior energy storage performance in the system, Which energy storage density is 0.318 J/cm3 at 70 kV/cm.

Large electrocaloric effect in (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ferroelectric ceramics by La2O3 addition

Large electrocaloric effect is observed in La doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ferroelectric ceramics synthesized by conventional solid state reaction. The dielectric permittivity and ferroelectric hysteresis loops are measured. The indirect method is used to calculate electrocaloric temperature change of BNBT6 doped by La. The introduction of La2O3 can enhance the electrocaloric effect of BNBT6. The largest temperature change |ΔT62|=2.61K and electrocaloric coefficient |ξmax|=0.052Kcm/kV is achieved in the 0.5wt% La doped BNBT6, which is significantly higher than that of BNT-based and lead-containing ferroelectric materials reported so far. For 1wt% ones, the |ΔT30|=1.954K and |ξmax|=0.039Kcm/kV is obtained in the room-temperature, which promises the environmental-friendly La doped BNBT6 is candidate for applications of solid state cooling devices.

Enhanced energy-storage properties of BaZrO3-modified 0.80Bi0.5Na0.5TiO3–0.20Bi0.5K0.5TiO3 lead-free ferroelectric ceramics

Large energy-storage density is observed in BaZrO3 (BZ)-modified 0.80Bi0.5Na0.5TiO3-0.20Bi0.5K0.5TiO3 (BNBK) lead-free ferroelectric (FE) ceramics synthesized by conventional solid-state reaction. The energy-storage property of (1 − x)BNBK–xBZ has been investigated. Certain content of BZ can enhance the energy-storage property of BNBK by enhancing the breakdown strength. The largest energy-storage density W1 = 0.73 J/cm3 and efficiency of energy storage η = 0.75 at E = 70 kV/cm are achieved in the 0.96BNBK–0.04BZ, which is significantly higher than that of BNT-based and lead-containing FE materials reported. Its energy-storage density exhibits the superior thermal stability with temperature range of 30–100 °C. Those properties promise that the environmental friendly (1 − x)BNBK–xBZ ceramics are candidate for applications of energy-storage devices.

Enhanced dielectric and piezoelectric properties of Ta-doped 0.50(Na0.5Bi0.5)TiO3–0.50(K0.5Bi0.5)TiO3 lead-free ferroelectric ceramics

Ta-doped 0.50(Na0.5Bi0.5)TiO3–0.50(K0.5Bi0.5)TiO3 piezoelectric ceramics were synthesized by solid state reaction and the effect of doping concentration on dielectric, ferroelectric and electrical properties were investigated. Ta-doping showed an increase in depolarization (Td) and phase transition (Tm) temperatures; thus increasing its usable temperature range. Ferroelectric properties were also enhanced in terms of increased remnant polarization and better squareness of P–E loop. In conductivity studies, activation energy was calculated and the contribution of grains to polarization mechanism was confirmed through Nyquist plots.

Enhancement of relaxor properties by Nb doping in Ba0.8Sr0.12Ca0.08TiO3 lead-free ferroelectric ceramics

Niobium-doped barium strontium calcium titanate (BSCTN) ceramics were prepared using a conventional solid-state reaction method. The effects of Nb contents on crystal structure, microstructure, dielectric properties and ferroelectric relaxor behavior of the BSCTN ceramics were investigated. The BSCTN ceramics showed dense microstructures with uniform crystal grains with Nb doping. It was demonstrated that Nb5+ entered the B-site of the perovskite BSCTN ceramic and substituted for Ti4+, which caused the expansion and distortion of crystal lattice of the tetragonal BSCTN ceramic. Doping of Nb resulted in more diffused phase transition and lower Curie temperature of the BSCTN ceramics. The diffuseness degree indicator γ increased until the addition of Nb dopant exceeded 1.5mol% where a maximum γ of 1.98 was achieved. Among the compositions assayed in this work, the BSCTN ceramics with Nb contents of approximately 1.0–1.5mol% yielded promising relaxor properties that made them alternative sources for development of environmental friendly lead-free relaxor ferroelectric materials.

Microstructure, dielectric and ferroelectric properties of (NaxBi0.5)0.94Ba0.06TiO3 lead-free ferroelectric ceramics: Effect of Na nonstoichiometry

(NaxBi0.5)0.94Ba0.06TiO3 lead-free ferroelectric ceramics with x=0.54, 0.50, 0.46, and 0.45 were prepared by solid state reaction of high energy ball milled powders. All ceramics exhibited typical diffraction peaks of ABO3 perovskite phase. A secondary phase peak was observed in the ceramic with x=0.45. Relative amount of tetragonal phase was related to the amount of Na. The Na nonstoichiometry produced an ignorable effect on the mean grain size of the ceramics. The deficiency of Na in the ceramics weakened their dielectric anomalies corresponding to the transition from ferroelectric phase to the so-called “intermediate phase”. For the ceramics with Na nonstoichiometry, the degree of diffuseness of the transition from the “intermediate phase” to paraelectric phase increased. With changing Na amount from excess to deficiency, the “depolarization temperature” increased. The difference in dielectric and ferroelectric properties among the ceramics with various Na amount was associated with the generation of Na and/or oxygen vacancies.

Influence of phase composition on electrostrains of doped(Bi0.5Na0.5)TiO3–BaTiO3–(Bi0.5K0.5)TiO3 lead-free ferroelectric ceramics

The actuating properties such as the electrostrain and piezoelectric charge coefficientd33 of thelead-free 0.854(Bi0.5Na0.5)TiO3–0.026BaTiO3–0.12(Bi0.5K0.5)TiO3 (abbreviated as BNBK 85.4/2.6/12) ferroelectric ceramic upon manganese (Mn) orcobalt (Co) doping are investigated. These dopants are chosen to improve thesinterability and ferroelectric characteristics of BNBK 85.4/2.6/12. Intricate strainingbehaviors are observed upon Mn doping—an electrostrain of about 0.1% can bemaintained when the Mn doping amount is in between 0.5 and 1.5 mol%. Once outsidethis doping range, the induced electrostrain is considerably smaller. In contrast,for the Co-doped BNBK 85.4/2.6/12 ceramics, the values of electrostrain andd33 stay relatively constant regardless of the Co doping level. By examining the evolution ofcrystalline phase composition with increasing doping concentration, the mole content of therhombohedral phase is shown to be a critical factor in deciding the straining behaviors ofthe doped BNBK 85.4/2.6/12 ceramics. At each increasing step in Mn or Co doping level,there is a marked similarity in the evolution of the induced electrostrain and rhombohedralphase content.

Intricate straining of manganese-doped (Bi0.5Na0.5)TiO3–BaTiO3–(Bi0.5K0.5)TiO3 lead-free ferroelectric ceramics

The strain hystereses of lead-free a(Bi0.5Na0.5)TiO3–bBaTiO3–c(Bi0.5K0.5)TiO3 (abbreviated as BNBK 100a/100b/100c) ferroelectric compositions inside and outside the morphotropic phase boundary (MPB) were investigated previously by the authors, and it was found that BNBK 85.4/2.6/12, a composition well within the MPB, possesses notable actuating properties such as an induced electrostrain of about 0.14%. In this study, BNBK 85.4/2.6/12 is doped with various amounts of manganese (Mn) to improve its sinterability and ferroelectric characteristics. Intricate strain hysteresis behaviours are observed upon Mn doping—an electrostrain of above 0.1% can be maintained when the Mn doping amount is between 0.5 and 1.5 mol%. Once outside this doping range, the induced electrostrain is considerably smaller. By examining the evolution of crystalline phase composition with Mn doping, the mole content of the rhombohedral phase is shown for the first time to be a critical factor in deciding the straining behaviours of the Mn-doped BNBK 85.4/2.6/12 ceramics. To each increasing step in Mn doping level, there is a marked similarity in the evolutions of the induced electrostrain and rhombohedral phase content. The rhombohedral ferroelectric phase possesses more equivalent crystallographic directions for non-180° polarization switching, and accordingly, the Mn-doped BNBK compositions with relatively higher rhombohedral phase contents exhibit larger electrostrains.

Determination of Thermal Diffusivity and Thermal Effusivity of the (Bi0.5Na0.5)0.935Ba0.065TiO3 Ferroelectric Ceramics by Photothermal Techniques

In this work, the thermal diffusivity and thermal effusivity properties of the (Bi0.5Na0.5)0.935 Ba0.065TiO3 lead-free ferroelectric ceramics are studied since it can be use as pyroelectric detectors. Photoacoustic and photothermal techniques were used to determine thermal diffusivity and thermal effusivity respectively. The preparation of the ceramics was perform in our laboratory by using the oxide-mixing conventional method. The ceramics present high value of thermal diffusivity compared with barium Titanate. All measurements were made at room temperature.