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In this work, (Bi0.5Na0.5)0.94Ba0.06TiO3 (BN6BT) lead‐free ceramics were prepared via cold‐sintering followed by annealing, and effects of grain size of precursor powders and annealing temperature on microstructure, dielectric, and ferroelectric properties of the ceramics were studied. The precursor powders for cold sintering were obtained by mixing the powders with sub‐micrometer (average grain size, AGS = 0.26 μm) and nanometer (AGS = 73 nm) grains in various mass ratios (1:0, 1:1, 1:2, and 1:4). The precursor powders were mixed with deionized water and cold sintered at 180°C for 1 h under 500 MPa. The cold‐sintered samples were then annealed at temperatures between 800 and 1000°C for 2 h. For the ratios of 1:2 and 1:4, the corresponding ceramics annealed at 950°C exhibit dielectric and ferroelectric properties similar to those of the BN6BT ceramics sintered at 1170°C via the solid‐state sintering method. The present BN6BT ceramics exhibit dense microstructure with finer grains with AGS = 280 nm, higher breakdown strength (BDS = 160 kV· cm−1) compared to the solid‐state sintered BN6BT ceramics with AGS = 1.6 μm and BDS = 75 kV· cm−1. The results demonstrate that the cold‐sintering followed by annealing is effective in preparing BN6BT ceramics with fine grains and excellent dielectric and ferroelectric properties at relatively low temperatures. [ABSTRACT FROM AUTHOR]

The lead-free ceramics (1-x)(0.84Bi 0.5 Na 0.5 TiO 3 -0.16Bi 0.5 K 0.5 TiO 3)-xBiMg 2/3 Nb 1/3 O 3 (denoted as (1-x)BNKT-xBMN) and (1-y)(0.85BNKT-0.15BMN)-ySr 0.7 La 0.2 TiO 3 with y = 0.3 (denoted as BNKMN-0.3SLT) were prepared by means of a conventional solid state sintering method. Effects of introduction of BiMg 2/3 Ni 1/3 O 3 and Sr 0.7 La 0.2 TiO 3 on phase structure, microstructure, and dielectric, ferroelectric and energy storage properties of the ceramics were studied in detail. The ceramics (1-x)BNKT-xBMN show pure perovskite structure, while BNKMN-0.3SLT appears a secondary phase. All ceramics exhibit dense microstructures with mean grain sizes between 0.56 μm and 1.36 μm. The introduction of SLT into BNKMN causes obvious decrease in dielectric constant around Curie temperature, inducing the widest temperature window of 263 °C from 29 °C to 292 °C for TCC≤±15 %. The ceramic BNKMN-0.3SLT shows high electric breakdown strength (E b ∼ 300 kV/cm), large maximum polarization (P m ∼ 36.48 μC/cm2), and low remanent polarization (P r ∼ 1.70 μC/cm2), inducing high energy storage density of 4.03 J/cm3 as well as high energy storage efficiency of 85.2 %. [ABSTRACT FROM AUTHOR]

Dense K0.5Na0.5NbO3 lead‐free ceramics were successfully fabricated by combining cold sintering process (CSP) with annealing in air atmosphere or low pO2 atmosphere. Acetic acid was used as transition liquid. By optimizing the liquid content and pressure during CSP, the CSP samples with relative densities 74.2% were obtained with adding 15 wt% acetic acid under 800 MPa. The CSP samples were then annealed in air atmosphere between 1000 and 1100°C. The ceramics annealed at 1050°C exhibit excellent electrical properties with piezoelectric constant d33 = 125 pC/N, dielectric constant εr = 509, dielectric loss tan δ = 0.04, and remanent polarization Pr = 38.17 μC/cm2. Additionally, the mixture of K0.5Na0.5NbO3 and base metal Ni powders was also cold‐sintered and then annealed in the low pO2 atmospheres in order to detect whether Ni was oxidized or not. The optimized low pO2 atmospheres to protect the Ni from oxidation are pO2 = 10−10 atm followed by reoxidizing in pO2 = 10−6 atm. The K0.5Na0.5NbO3 fabricated by CSP with annealing in the optimized low pO2 atmospheres exhibits comparable electrical properties of the ceramics annealed in air atmosphere, providing an optional approach for using base metals as electrodes. [ABSTRACT FROM AUTHOR]

The lead-free ceramics (1 − x)(0.94 Bi 0. 4 7 Na 0. 4 7 Ba 0. 0 6 TiO3-0.06BiAlO3)- x AgNbO3 (denoted as BNBTA- x AN) were synthesized via a solid-state sintering method. The effect of AgNbO3 doping amount on dielectric properties of the ceramics was studied systematically. X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscope were used to detect the structure of the ceramics. Temperature-dependent dielectric spectra, frequency-dependent dielectric constant and alternating current (ac) electric conductance at various temperatures were measured. The doping of AgNbO3 greatly reduces dielectric constant around Curie temperature and thus enhances the temperature stability of the dielectric constant. The ceramic BNBTA-0.03AN exhibits excellent temperature-stable dielectric properties with temperature coefficient of capacitance (TCC) ≤ ± 15% between 55∘C and 418∘C with temperature window 363∘C and small changes of dielectric constant and dielectric loss from 100 Hz to 1 MHz at different temperatures. The obtained ceramics are expected to be used in high-temperature capacitors due to its excellent temperature stability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Lead-free ceramics 0.98(Bi0.5Na0.42K0.08)0.96Sr0.04Ti0.975Nb0.025O3-0.02BiAlO3 were sintered in a wide temperature range between 1115 and 1160 °C. All ceramics have dense microstructures with relative densities higher than 96%. With an increase in sintering temperature from 1115 to 1160 °C, the average grain size of the ceramics increases from 0.8 to 1.3 μm. Dielectric, ferroelectric, and electrostrain behavior of the ceramics sintered at the different temperatures were studied. By optimizing the sintering temperatures, it is found that the ceramic sintered at 1145 °C shows improved dielectric and ferroelectric properties. For the ceramics sintered at 1145 °C, Curie temperature, maximum dielectric constant, maximum polarization, coercive field, and maximum unipolar strain are 299 °C, 3822, 43.7 μC/cm2, 14 kV/cm, and 0.29%, respectively. The related mechanism was also discussed. [ABSTRACT FROM AUTHOR]

Scaling behavior of dynamic hysteresis of (Na0.47Bi0.47Ba0.06)0.95A0.05TiO3 (A = Sr2+/Ca2+) (expressed as BNBT-Sr and BNBT-Ca, respectively) lead-free ceramics was comparatively studied. BNBT-Ca exhibits typical ferroelectric hysteresis (polarization vs electric field, P–E) loops, while BNBT-Sr shows pinched P–E loops. The two samples' electric field and frequency-dependent dynamic hysteresis behavior were systematically investigated. The dopants difference (Sr2+/Ca2+) produces different scaling behavior of dynamic hysteresis, which is related to the electric field-induced phase transition and domain size between BNBT-Sr and BNBT-Ca. [ABSTRACT FROM AUTHOR]

Lead-free ceramics (Bi0.5Na0.5)0.94-δBa0.06Ti1−xNbxO3 (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) were prepared via a solid-state sintering method. The ceramics exhibit pure perovskite structure as x ≤ 0.04, while the ceramic with x = 0.05 has a secondary phase. The rhombohedral (R)–tetragonal (T) morphotropic phase boundary exists in all the samples. The relative content of the T phase increases due to the Nb5+ doping. The ceramics show dense microstructures with mean grain sizes around 1–2 μm. The changes of dielectric constant and dielectric loss as a function of temperature for the unpoled and poled samples were compared. Two dielectric anomalies appear on the dielectric curves around the temperatures denoted as TRE and Tm. With the increase of Nb5+ amount, the values of TRE decrease and Tm increase. The Nb5+-doped ceramics exhibit better temperature stability of dielectric constant during a wide high-temperature window. The polarization hysteresis (P–E) loops change from typical ferroelectric loops for the sample with x = 0 to constricted P–E loops with increasing Nb5+ amount. The ceramic with x = 0.03 shows a maximum strain (Smax) of 0.55% and piezoelectric strain constant (d33*) of 675 pm/V. [ABSTRACT FROM AUTHOR]

Microstructure and dielectric properties of the lead-free ferroelectric ceramics (Bi0.5Na0.5)0.94Ba0.06Ti1−xAlxO3−δ (abbreviated as BNBTA/x, x = 0, 0.04, 0.06, 0.08) obtained via a solid-state reaction method were studied. The ceramics with x≤0.06 exhibit pure structure without detectable secondary phase by X-ray diffraction measurement. There appears a second phase in the ceramic BNBTA/0.08. With an increase in the Al3+ amount, dielectric constant of the ceramics decreases, and the change in dielectric constant with increasing temperature between the two dielectric characteristic temperatures (TRE and Tm) is gradual. The poled ceramic BNBTA/0 shows a steep increase in permittivity at the temperature denoted as Td around 100 °C. But, the poled ceramics with x > 0 do not exhibit the Td anomaly on the permittivity spectra. The diffuse dielectric behavior around Tm was studied via the Curie–Weiss law and modified Curie–Weiss law. Compared to the ceramic BNBTA/0, the Al3+ -doped ceramics show slim ferroelectric hysteresis loops. The temperature-dependent ferroelectric properties and energy-storage behavior were studied. [ABSTRACT FROM AUTHOR]

The (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. [ABSTRACT FROM AUTHOR]

The dense ceramics (Na0.47Bi0.47Ba0.06)0.97Sr0.03TiO3 were sintered in nitrogen atmosphere. The weight loss during sintering is 0.2%. The mean size of grains is about 1.5 μm. The dielectric permittivity and dielectric loss of the ceramics at room temperature and 1 kHz are 1836 and 0.05, respectively. The dielectric spectra of the unpoled and poled ceramics were measured. The diffuse dielectric behavior was studied via the Curie-Weiss law and modified Curie-Weiss law. The ceramics exhibit excellent ferroelectric properties. At room temperature, the d33 is as high as 157 pC/N. The ceramics exhibit excellent temperature stability of d33 between room temperature and 115 °C. [ABSTRACT FROM AUTHOR]

Lead-free ceramics K0.5Na0.5NbO3–0.5 mol%Al2O3 with simple composition were prepared via the hot-press sintering. The ceramics exhibit dense microstructure, fine grains, and pure orthorhombic structure. Via the hot-press sintering as well as adding 0.5 mol%Al2O3 into K0.5Na0.5NbO3, piezoelectric and ferroelectric properties of the obtained ceramics were obviously enhanced. The ceramics exhibit high remnant polarization Pr = 26.8 μC/cm2 and piezoelectric constant d33 = 137 pC/N. The maximum unipolar strain is 0.17%, and the converse piezoelectric coefficient d33* is 212 pm/V under electric field E = 80 kV/cm. Under E = 50 kV/cm, the Pr increases from 21.6 to 22.2 μC/cm2, and the d33* increases from 223 to 322 pm/V as the temperature increases from room temperature to 170 °C. The ex situ measurement result demonstrates that d33 still keeps a high value of 97 pC/N as the annealing temperature increases to 380 °C, implying excellent thermal stability. [ABSTRACT FROM AUTHOR]

The ceramics (Na0.47Bi0.47Ba0.06)1−xSrxTiO3 were prepared by means of a solid-state sintering method. Effects of Sr2+-doping on microstructure, ferroelectric, impedance, dielectric, and piezoelectric properties of the ceramics were investigated. Variation of electrical properties of the ceramics with the change of the mean radii of the A-site cations was shown. All ceramics exhibit dense microstructures and pure perovskite structure. Dielectric behavior is related to the electric poling and Sr2+ amount. Compared to the ceramic with x = 0, the values of coercive field of the ceramics with x = 0.03 and 0.05 decrease from 34.7 to 27.6 kV/cm and 20.4 kV/cm, respectively; the corresponding remnant polarization values increase from 28.5 to 32.8 μC/cm2 and 31.2 μC/cm2, respectively. The ceramics with x = 0, 0.01, 0.02, 0.03, 0.05, and 0.08 exhibit the piezoelectric constant (d33) values of 119, 123, 138, 169, 187, and 91 pC/N, respectively. With the increase of annealing temperature, the samples with x = 0.01, 0.02, and 0.03 show good temperature stability of d33. [ABSTRACT FROM AUTHOR]

The lead-free piezoelectric ceramics (Na .47 Bi .47 Ba .06 ) 1-x Ca x TiO 3 (x = 0, 0.01, 0.02, 0.03, 0.05, and 0.08, abbreviated as BNBTC/0, BNBTC/1, BNBTC/2, BNBTC/3, BNBTC/5, and BNBTC/8, respectively) were obtained using the solid-state reaction method. The structure, electric conductivity, and dielectric, ferroelectric, and piezoelectric properties of the Ca 2+ -doped (Na .47 Bi .47 Ba .06 )TiO 3 ceramics were thoroughly investigated. The ceramics sintered at 1200 °C exhibit dense microstructures, having relative densities higher than 96%. The X-ray diffraction results demonstrate that all ceramics have a pure perovskite structure. The mean grain sizes of the ceramics are related to the Ca 2+ quantity. A small quantity of Ca 2+ ions (x ≤ 0.03) improves the piezoelectric and ferroelectric properties of the samples. The dielectric behavior of the samples is sensitive to the Ca 2+ content and electric poling. The results demonstrate that the electrical properties of the (Na .47 Bi .47 Ba .06 )TiO 3 lead-free ceramics can be well tuned by varying the Ca 2+ quantity. [ABSTRACT FROM AUTHOR]

Two different sintering methods, i.e., a hot-press sintering method and a conventional sintering method, were utilized to prepare (KNa)NbO lead-free piezoelectric ceramics. Microstructure, dielectric, ferroelectric, and piezoelectric properties of the ceramics obtained via the two methods were comparatively studied. Both the ceramics exhibit a single orthorhombic phase. The ceramic via the conventional sintering method shows micron-sized grains and poor densification with many pores. By means of the hot-press sintering, the mean size of grains decreases to 0.1 μm and relative density increases to 96%. The evolution of domains with electrical field was detected via a scanning probe microscope, indicating field-induced domain switching on the microscale. Compared to the conventional sintering, the hot-press sintering causes an increase in the dielectric constant from 183 to 830, remnant polarization from 8.5 to 17.3 μC cm, piezoelectric constant from 60 to 120 pC N, and helps a decrease in the dielectric loss from 0.46 to 0.02, coercive field from 20 to 16 kV cm. The hot-press sintering facilitates improving the electrical properties of the (KNa)NbO ceramic. [ABSTRACT FROM AUTHOR]

In this work, 0.99(0.94Bi 0.47 Na 0.47 Ba 0.06 TiO 3 -0.06BiAlO 3)-0.01NaNbO 3 lead-free ceramics were synthesized by means of different methods including conventional solid state sintering (SSS), cold-sintering-assisted sintering (CSS), and spark plasma sintering (SPS) methods. Their microstructure and dielectric properties were studied comparatively. The ceramics via SPS and SSS exhibit pure structure, while a secondary phase exists in the ceramics via CSS. Compared to SSS, CSS and SPS can obviously reduce the processing temperatures for obtaining dense ceramics. The ceramics via SPS have the smallest mean grain size of 80 nm, while those via CSS and SSS exhibit increased grain sizes. The dielectric measurement demonstrates that temperature stability in dielectric constant at high temperatures is affected by sintering methods. The SPS sample shows the poorest temperature stability of permittivity. The ceramics post-annealed at 1050 °C via CSS exhibits the widest temperature range between 49 °C and 412 °C for temperature coefficient of capacitance ≤±15%, which can be expected to be used in the field of high-temperature capacitors. • BNBTA-0.01NN ceramics were prepared via the three methods including SPS, CSS, SSS. • Processing temperatures for obtaining dense ceramics are reduced via CSS and SPS. • Ceramics annealed at 1050 °C via CSS show TCC ≤±15% between 49 °C and 412 °C. • Temperature window of 363 °C is much higher than the requirement of X9R standard. [ABSTRACT FROM AUTHOR]

The lead-free ceramics (1-x)(0.94Bi 0.47 Na 0.47 Ba 0.06 TiO 3 -0.06BiAlO 3)-xNaNbO3 were prepared by means of a traditional solid-state sintering method. All ceramics have pure perovskite structure and average grain sizes around 1 μm. Their dielectric and energy storage behavior were studied. The NaNbO 3 -doped ceramics display drastically reduced temperature coefficient of capacitance (TCC) compared to the ceramics without doping. The samples with x = 0.02 exhibit high permittivity and excellent temperature stability between 67 °C and 380 °C, meeting the requirement of TCC ≤ ±15%. The energy storage density was greatly improved to 1.71 J/cm3 as x = 0.03, showing excellent frequency stability from 0.5 Hz to 100 Hz, great temperature stability from 25 °C to 130 °C, and excellent fatigue resistance within 105 fatigue cycles. • Dense BNBTA-xNN lead-free ceramics were prepared. • BNBTA-0.02NN has high ε r and excellent temperature stability from 67 to 380 °C. • BNBTA-0.03NN has excellent stability of energy storage density from 0.5 to 100 Hz and 25–130 °C. • Excellent fatigue resistance in ferroelectric behavior within 105 cycles was obtained. [ABSTRACT FROM AUTHOR]

The (1−x)[0.8Bi 0.5 Na 0.5 TiO 3 -0.2Bi 0.5 K 0.5 TiO 3 ]-xBiCoO 3 (x = 0, 0.02, 0.05, abbreviated as BNKT, BNKT-002Co, BNKT-005Co, respectively) lead-free ferroelectric ceramics were prepared via the solid state reaction method. The phase structure, microstructure, dielectric, ferroelectric, pyroelectric, and piezoelectric properties of the ceramics were investigated comparatively by using a combination of characterization techniques. All the samples exhibit typical X-ray diffraction peaks of ABO 3 perovskite structure. The doping of BiCoO 3 causes a decrease in lattice parameters and an increase in grain size of the ceramics. The Raman spectroscopy results suggest a lattice distortion due to the doping. It is found that BNKT-002Co and BNKT-005Co have higher depolarization temperatures compared with BNKT. The Curie-Weiss law and modified Curie-Weiss law explored a diffuse phase transition character for all the samples. The results of ultraviolet–visible diffuse reflectance suggests that BiCoO 3 -doped ceramics possess higher defect concentration. The impedance analysis shows a temperature dependent relaxation behavior, and the activation energy for the electrical responses varies with the change of BiCoO 3 amount. The ferroelectric and piezoelectric properties of the ceramics decrease due to the doping of BiCoO 3 . Based on the results of the Rayleigh analysis, it was suggested that the differences in the electrical properties among the ceramics are closely related to the change in oxygen vacancy concentration. [ABSTRACT FROM AUTHOR]

Lead-free ceramics, 0.97[(Na 0.5 Bi 0.5 ) 1−1.5 x La x ]TiO 3 –0.03BaTiO 3 (NBTB-xLa, x = 0, 0.01, 0.02, 0.03, 0.05 and 0.08), were prepared by using a conventional solid state reaction method. The ceramics with x ⩽ 0.05 have pure perovskite, while a secondary phase appears in the sample with x = 0.08. The doping of La 3+ decreases grain size of the ceramics. For both the unpoled and poled NBTB-xLa ceramics, there exist two dielectric anomalies. For the poled ceramics with x = 0, 0.01, 0.02 and 0.03, a new dielectric anomaly appears at depolarization temperature ( T d ). The T d peak is not observed in the ε r –T curves for the ceramics with x = 0.05 and 0.08. The phase transition temperature ( T m ) increases with increasing amount of La 3+ , but the values of T d and maximum dielectric constant ( ε m ) decrease gradually . Piezoelectric constants ( d 33 ) of the ceramics with x = 0, 0.01, 0.02, 0.03, 0.05 and 0.08 are 91, 87, 77, 56, 7 and 2 pC/N, respectively. The change in ferroelectric properties of the samples with increasing content of La 3+ is also discussed. [ABSTRACT FROM AUTHOR]