Effects of sintering temperature and poling conditions on the electrical properties of Ba0.70Ca0.30TiO3 diphasic piezoelectric ceramics

The effects of sintering temperature and poling conditions on the electrical properties of tetragonal and orthorhombic diphasic Ba_0.70Ca_0.30TiO₃ (BCT) lead-free ceramics have been systematically investigated. On the one hand, with increasing sintering temperature from 1270 °C to 1400 °C, the bulk density increases monotonically and the Curie temperature keeps almost constant with the value of ∼120 °C, whereas the grain size, the maximum relative dielectric constant, room temperature polarization reach the maximum values for samples sintered at 1340 °C. On the other hand, it is found that the piezoelectric property depends on poling electric field and poling temperature significantly. An enhanced piezoelectric behavior of d₃₃=126 pC/N, k_p=0.29, and Q_m=588 is obtained for the BCT ceramics poled at 100 °C with 30 kV/cm field for 20 min. The aging behavior of the piezoelectric property is also investigated.     

Enhancement of the dielectric,piezoelectric, and ferroelectric properties in BiYbO3-modified (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free ceramics

Lead-free (1−x)(Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃−xBiYbO₃ [(1−x)BCTZ−xBYO] piezoelectric ceramics in the range of BYO concentrations were prepared by the conventional oxide-mixed method, and the effect of BYO content on their microstructure, crystalline structure, density and electrical properties was investigated. A dense microstructure with large grain was obtained for the ceramics with the addition of BYO. The ceramics with x=0.1% exhibit an optimum electrical behavior of d₃₃~580 pC/N, r~10.9 Ω, k_p~56.4%, and tan δ~1.12% when sintered at a low temperature of ~1350 °C. When the measuring electric field is 40 kV/cm, the well-saturated and square-like P–E loops for the ceramics were observed with P_r~12.2 μC/cm² and E_c~1.83 kV/cm.     

High piezoelectric coefficient of Pr2O3-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics

Pr₂O₃-doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ-xPr) ceramics were prepared by the conventional solid-state method. A tetragonal phase is only observed in these ceramics, and the introduction of Pr₂O₃ decreases their sintering temperature without affecting negatively the piezoelectric constant. Enhanced ferroelectric properties were obtained in these BCTZ-xPr ceramics. The ceramic with x=0.06 wt% exhibits a good electrical behavior of d₃₃∼460 pC/N, k_p∼47.6%, ε_r∼4638, and tan δ∼0.015 when sintered at a low temperature of ∼1400 °C. As a result, the BCTZ-xPr ceramic is a promising candidate for lead-free piezoelectric ceramics.     

Compositional dependence of phase structure and electrical properties in (K0.50Na0.50)0.97Bi0.01(Nb1−xZrx)O3 lead-free ceramics

(K_0.50Na_0.50)_0.97Bi_0.01(Nb_1-xZr_x)O₃ (KNBNZ) lead-free ceramics were prepared by the conventional solid-state sintering process. Their phase structure is dependent on the Zr content in the investigated range, and the ceramics endure a phase transition from pseudocubic to orthorhombic with increasing Zr content. Improved piezoelectric properties have been observed when the poling temperature is located at ~100 °C because of the coexistence of orthorhombic and tetragonal phases. Their dielectric and piezoelectric properties were enhanced by doping Zr, the ceramic with x=0.02 showing optimal electrical properties, i.e., d₃₃~161 pC/N, k_p~0.41, Q_m~81, T_c~370 °C, and T_o−t~130 °C. These results show that the KNBNZ ceramic is a promising lead-free piezoelectric material.     

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.     

Low temperature synthesis and dielectric, ferroelectric and piezoelectric study of microwave sintered BaTiO3 ceramics

Barium titanate (BaTiO₃/BT) ferroelectric system was synthesized in single perovskite phase at low temperature by using powders derived from modified solid state reaction (MSSR) and sintered by microwave (MW) processing routes. Conventional calcination temperature was optimized at 900 °C for 4 h. MW sintering of BT samples was carried out at 1100 °C for 30 min to get dense (98% density) ceramics. Room temperature (RT) dielectric constant (?_r) and dielectric loss (tan δ) at 1 kHz frequency of MW sintered BT samples was found to be ∼2500 and 0.03, respectively. Saturated polarization vs. electric field (P-E) loops with remnant polarization (P_r) ∼6 μC/cm² and coercive field (E_c) ∼1.45 kV/cm confirmed the ferroelectric nature of MW sintered BT samples. Piezoelectric coefficient from strain vs. electric field (S-E) loops study was found to be 335 pm/V.     

Synthesis and properties of (BaxPb1−x)(Zn1/3Nb2/3)O3 relaxor ferroelectric ceramics using a reaction-sintering process

(Ba_xPb_1−x)(Zn_1/3Nb_2/3)O₃ (BPZN; x = 0.06–0.1) relaxor ferroelectric ceramics produced using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. BPZN ceramics of 100% perovskite phase were obtained. Highly dense BPZN ceramics with a density higher than 98.5% of theoretical density could be obtained. Maximum dielectric constant K_max 13,500 (at 75 °C), 19,600 (at 50 °C) and 14,800 (at 28 °C) at 1 kHz could be obtained in 6BPZN, 8BPZN and 10BPZN, respectively. Dielectric maximum temperature (T_max) in BPZN ceramics via reaction-sintering process is lower than BPZN ceramics prepared via B-site precursor route.     

Microwave dielectric properties of Ba3Ti4−x(Zn1/3Nb2/3)xNb4O21 for low temperature co-fired ceramics

The effects of substitution of (Zn_1/3Nb_2/3) for Ti on the sintering behavior and microwave dielectric properties of Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (0 ≤ x ≤ 4) ceramics have been investigated. The dielectric constant (?_r) and the temperature coefficient of the resonant frequency (τ_f) of Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ ceramics decreased with increasing x. However, the Q × f values enhanced with the substitution of (Zn_1/3Nb_2/3) for Ti. It was found that a small amount of MnCO₃-CuO (MC) and ZnO-B₂O₃-SiO₂ (ZBS) glass additives to Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (x = 2) ceramics lowered the sintering temperature from 1250 to 900 °C. And Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (x = 2) ceramics with 1 wt% MC and 1 wt% ZBS sintered at 900 °C for 2 h showed excellent dielectric properties: ?_r = 53, Q × f = 14,600 GHz, τ_f = 6 ppm/°C. Moreover, it has a chemical compatibility with silver, which made it as a promising material for low temperature co-fired ceramics technology application.     

Structural and electrical properties of 0.56Pb(Ni1/3Nb2/3)O3-0.10Pb(Zn1/3Nb2/3)O3-0.34PbTiO3 ceramics prepared by different ceramic processings

The ternary system of 0.56Pb(Ni_1/3Nb_2/3)O₃-0.10Pb(Zn_1/3Nb_2/3)O₃-0.34PbTiO₃ (0.56PNN-0.10PZN-0.34PT) ceramics were prepared by conventional solid-state reaction method via straight mixed oxide method, columbite precursor method and B-site oxide mixing route. X-ray diffraction (XRD) measurement demonstrated that both the tetragonal and rhombohedral phases coexist in the B-site oxide mixing route prepared ceramics accompanied by the largest content of perovskite phase of 95.18%. The 0.56PNN-0.10PZN-0.34PT ceramics prepared by the straight mixed oxide method and the B-site oxide mixing route exhibit rather homogeneous microstructure. As a comparison, in the columbite precursor method prepared ceramics nebulous granules and octahedral or other polyhedral morphology grains are observed. All the sintered ceramics exhibit diffused ferroelectric phase transition where the dielectric response peaks are broad, diffused and strongly frequency dependent. However, the temperature of dielectric maximum (T_m) increases greatly from 398.0 K of the 0.56PNN-0.10PZN-0.34PT ceramics prepared by the B-site oxide mixing route to 423.3 K of the ones prepared by the straight mixed oxide method. Saturated and symmetric P-E hysteresis loops are observed in all the sintered ceramics, where the B-site oxide mixing route prepared ceramics exhibit large value of remanent polarization (P_r) of 17.13 μC/cm² and the least value of coercive field (E_c) of 11.99 kV/cm. Piezoelectric constant (d₃₃) exhibits the largest value of 449 pC/N for the ceramics prepared by the B-site oxide mixing route. Such results are related to the phase composition, density and porosity of the ceramics.     

Enhanced ferroelectric properties in (Ba1−xCax)(Ti0.94Sn0.06)O3 lead-free ceramics

Lead-free (Ba_1−xCa_x)(Ti_0.94Sn_0.06)O₃ (BCST) (x = 0.01-0.04) ceramics were prepared using a solid-state reaction technique. The effects of Ca content on the phase structure and electrical properties of the BCST ceramics were investigated. High piezoelectric coefficient of d₃₃ = 440 pC/N, planar electromechanical coupling factor of k_p = 45% and dielectric constant ?_r = 6900 were obtained for the samples at x = 0.03. At room temperature, a polymorphic phase transition (PPT) from orthorhombic phase to tetragonal phase was identified in the composition range of 0.02 < x < 0.04.     

High energy-storage properties of [(Bi1/2Na1/2)0.94 Ba0.06] La(1−x) ZrxTiO3 lead-free anti-ferroelectric ceramics

Energy-storage properties of [(Bi_1/2Na_1/2)_0.94Ba_0.06]La_(1−x)Zr_xTiO₃ (BNT-BLZT, x=0, 0.02, 0.04, and 0.06) lead-free anti-ferroelectric ceramics fabricated via the conventional sintering technique were first investigated. Calculation from the X-ray diffraction results reveals that BNT-BLZT ceramic possesses a single perovskite structure phase. In addition, the P–E hysteresis loops measured at room temperature show that the BNT-BLZT (x=0.02) ceramics obtain the maximum P value of 37.5 μC/cm² and the largest energy-storage density W_max is 1.58 J/cm³. The temperature dependence of dielectric permittivity ε_r and dielectric loss tanδ illustrate that the addition of Zr can improve the piezoelectric properties of BT-BLZT ceramics. These properties indicate that BNT-BLZT ceramics might be a promising lead-free anti-ferroelectric material for energy storage application.     

Sintering and electrical properties of La-modified (Na0.52K0.45Li0.03)1−3xLax(Nb0.88Sb0.09Ta0.03)O3 lead-free ceramics

(Na_0.52K_0.45Li_0.03)_1−3xLa_x(Nb_0.88Sb_0.09Ta_0.03)O₃ (NKLL_xNST) lead-free ceramics were prepared by normal sintering and their dielectric and piezoelectric properties were investigated. The X-ray methods indicate that the NKLL_xNST ceramics with x≤0.003 present a pure perovskite phase at room temperature. The bulk density of NKLL_xNST ceramics increases with proper amount of La₂O₃ contents, and reaches its highest value of 4.544 g/cm³ with the addition of 0.3 mol% La₂O₃. At x=0.003, remnant polarization P_r, piezoelectric constant d₃₃ and planar mode electromechanical coupling factor k_p of NKLL_xNST ceramics reach the highest values of 37.80 μC/cm², 346 pC/N and 40%, respectively, exhibiting excellent “soft” piezoelectric characteristics, demonstrating a tremendous potential of the compositions studied for device applications.     

Preparation and microwave dielectric properties of 3ZnO·B2O3 ceramics with low sintering temperature

The preparation and dielectric properties of 3ZnO·B₂O₃ ceramics were investigated. Dense 3ZnO·B₂O₃ ceramics were obtained as sintered in the temperature range from 950 to 1000 °C for 3 h. The X-ray diffraction showed that the obtained ceramics were of a monoclinic 3ZnO·B₂O₃ structure. The ceramic specimens fired at 955 °C for 1 h exhibited excellent microwave dielectric properties: ?_r ∼ 6.9, Q × f ∼ 20,647 GHz (@6.35 GHz), and τ_f ∼ −80 ppm/°C. The dependences of relative density, ?_r, and Q × f of ceramics sintered at 955 °C on sintering soaking time showed that they all reached their plateaus as the soaking time was up to 60 min. Meanwhile, 3ZnO·B₂O₃ ceramics had no reaction with silver during cofiring, indicating it is a potential candidate for low-temperature cofired ceramic (LTCC) substrate.     

Enhanced piezoelectric properties of (K0.40Na0.60)0.94Li0.06Nb0.94SbO3 lead-free ceramics by optimizing the sintering temperature

Effects of sintering temperature on the microstructure and electrical properties of (K_0.40Na_0.60)_0.94Li_0.06Nb_0.94SbO₃ (KNLNS) lead-free ceramics are investigated. The grain size gradually becomes larger with increasing sintering temperature from 1055 °C to 1105 °C, and the piezoelectric property could be enhanced by optimizing their sintering temperature. The ceramic sintered at 1075 °C has optimum electrical properties, i.e., d₃₃~272 pC/N, k_p~43.5%, ε_r~1152, tan δ~0.026, and T_C~346 °C. These results show that the sintering temperature can optimize electrical properties of KNLNS ceramics.     

Microstructure and piezoelectric properties of CuO added (K, Na, Li)NbO3 lead-free piezoelectric ceramics

Lead-free Na_0.5K_0.5NbO₃ (NKN) and Na_0.475K_0.475Li_0.05NbO₃ (NKLN) ceramics doped with CuO were prepared by the mixed oxide route. The powders were calcined at 850-930 °C and sintered at 850-1100 °C. Small additions of CuO reduced the sintering temperature and increased the density to 96% theoretical. Cu first appears to enter the A site then the B site. In NKLN the orthorhombic-tetragonal and tetragonal-cubic phase transitions are approximately 150 °C lower and 50 °C higher, respectively than in NKN. With increasing addition of Cu to NKN and NKLN the remanent polarization (P_r) increased and coercive field (E_c) decreased. NKLN prepared with 0.4 wt% CuO exhibited a saturation polarization (P_sat) of 30 μC/cm², remanent polarization (P_r) of 27 μC/cm² and coercive field (E_c) of 1.0 kV/mm. CuO caused the NKLN ceramics to harden considerably; the mechanical quality factor (Q_m) increased from 50 to 260, d₃₃ ∼ 285 and piezoelectric coupling factors were >0.4.     

Effect of Eu doping on structure and electrical properties of lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

Eu-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT6-xEu, x=0.00–2.00 at%) lead-free piezoelectric ceramics have been synthesized by the solution combustion method. The effect of Eu doping concentration on the phase structure, microstructure and electrical properties of BNBT6 ceramics has been investigated. The XRD analysis confirms that the europium additive incorporates into the BNBT6 lattice and results in a phase transition from the coexistence of rhombohedral and tetragonal phases to a more symmetric pseudocubic phase. The SEM images indicate that the europium additive has little effect on the ceramic microstructure and the average grain size is about 2.0 μm. The electrical properties of BNBT6 ceramics can be improved by appropriate Eu doping. The 0.25 at% Eu doped BNBT6 ceramic presents excellent electrical properties: piezoelectric constant d₃₃=149 pC/N, remnant polarization P_r=40.27 μC/cm², coercive field E_c=2.95 kV/mm, dielectric constant ε_r=1658 and dissipation factor tan δ=0.0557 (10 kHz).     

Effect of HfO2 content on the microstructure and piezoelectric properties of (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃–xHfO₂ [BNBT–xHfO₂] lead-free ceramics were prepared using the conventional solid-state reaction method. Effects of HfO₂ content on their microstructures and electrical properties were systematically studied. A pure perovskite phase was observed in all the ceramics with x=0–0.07 wt%. Adding optimum HfO₂ content can induce dense microstructures and improve their piezoelectric properties, and a high depolarization temperature was also obtained. The ceramics with x=0.03 wt% possess optimum electrical properties (i.e., d₃₃~168 pC/N, k_p~32.1%, Q_m~130, ε_r~715, tan δ~0.026, and T_d~106 °C, showing that HfO₂-modified BNBT ceramics are promising materials for piezoelectric applications.     

Synthesis,structural analysis and dielectric properties of Ba8(Mg1−xZnx)Nb6O24 hexagonal perovskites

Ba₈(Mg_1−xZn_x)Nb₆O₂₄ (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered at 1375–1425 °C. The structure of the system was analyzed using X-ray diffraction and vibrational spectroscopic studies. The microstructure of the sintered pellet was analyzed using scanning electron microscopy. The dielectric constant (ε_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) were measured in the microwave frequency region. The τ_f values of the compositions were reduced by varying the value of x from 0 to 1. The dielectric responses to frequency were also studied in the radio frequency region. The compositions have good microwave dielectric properties and hence are suitable for dielectric resonator applications.     

Influence of cooling mode on the electrical properties and microstructure of Ba1.022−xSmxTiO3 ceramics sintered in a reducing atmosphere

We investigated the effects of the Sm-dopant content and the cooling rate on the electrical properties and microstructure of Ba_1.022−xSm_xTiO₃ (BST) ceramics, which were sintered at 1200 °C for 30 min in a reducing atmosphere and then reoxidized at 800 °C for 1 h. The results indicated that the cooling rate affected the electrical properties and the microstructure of the BST samples, whose room-temperature resistivity increased with increasing cooling rate. The semiconducting BST ceramics showed a pronounced positive temperature coefficient of resistivity effect, with a resistance jump greater by 3.16 orders of magnitude, along with a low room-temperature resistivity of 157.4 Ω cm at a cooling rate of 4 °C/min. The room-temperature resistivity of the specimen was lower after sintering for 30 min at 1150 °C during cooling.