The structure and electrical properties of Ca0.6(Li0.5Bi0.5-xPrx)0.4Bi2Nb2O9 high-temperature piezoelectric ceramics

Ca_0.6(Li_0.5Bi_0.5-xPr_x)_0.4Bi₂Nb₂O₉ ceramics were prepared via a solid-state reaction method. The effect of the Pr content on the structural and electrical properties was systematically investigated. X-ray diffraction (XRD) combined with Rietveld refinement and X-ray photoelectron spectroscopy (XPS) demonstrated that a moderate amount of Pr³⁺ can be incorporated into the NbO₆ octahedra, while excess Pr³⁺ ions probably enter into the (Bi₂O₂)²⁺ layers, thus resulting in an increase in the tetragonality of the crystal structure. The introduction of Pr suppressed the generation of oxygen vacancies and improved the preferential grain growth along the c-axis, which might be responsible for enhancing the resistivity (ρ ~ 10⁶ Ω cm at 600°C). The replacement of Pr³⁺ for A-site Bi³⁺ enhanced the piezoelectric property, and the piezoelectric constant d₃₃ increased from 13.8 pC/N to 16.3 pC/N. The high depolarization temperature (up to 900°C) implied that CBN-LBP100x ceramics are promising candidates for ultrahigh-temperature application.     

Significantly enhanced dc electrical resistivity and piezoelectric properties of Tb-modified CaBi2Nb2O9 ceramics for high-temperature piezoelectric applications

Bismuth layer–structured ferroelectric calcium bismuth niobate (CaBi₂Nb₂O₉, CBN) is considered to be one of the most potential high-temperature piezoelectric materials due to its high Curie temperature T_c of ∼940°C, but the drawbacks of low electrical resistivity at elevated temperature and low piezoelectric performance limit its applications as key electronic components at high temperature (HT). Herein, we report significantly enhanced dc electrical resistivity and piezoelectric properties of CBN ceramics through rare-earth element Tb ions compositional adjustment. The nominal compositions of Ca_1−xTb_xBi₂Nb₂O₉ (abbreviated as CBN-100xTb) have been fabricated by conventional solid-state reaction method. The composition of CBN-3Tb exhibits a significantly enhanced dc electrical resistivity of 1.97 × 10⁶ Ω cm at 600°C, which is larger by two orders of magnitude compared with unmodified CBN. The donor substitutions of Tb³⁺ ions for Ca²⁺ ions reduce the oxygen vacancy concentrations and increase the band-gap energy, which is responsible for the enhancement of dc electric resistivity. The temperature-dependent dc conduction properties reveal that the conduction is dominated by the thermally activated oxygen vacancies in the low-temperature region (200–350°C) and by the intrinsic conduction in the HT region (350–650°C). The CBN-3Tb also exhibits enhanced piezoelectric properties with a high piezoelectric coefficient d₃₃ of ∼13.2 pC/N and a high T_c of ∼966°C. Moreover, the CBN-3Tb exhibits good thermal stabilities of piezoelectric properties, remaining 97% of its room temperature value after annealing at 900°C. These properties demonstrate the great potentials of Tb-modified CBN for high-temperature piezoelectric applications.     

Enhanced electrical properties related to structural distortion of CaBi2Nb2O9-based piezoelectric ceramics

CaBi₂Nb₂O₉ (CBN)-based high Curie temperature piezoelectric ceramics with formula Ca_0.8-xSr_x(Li_0.5Ce_0.15Bi_0.35)_0.2Bi₂Nb_1.94Ta_0.04W_0.02O₉ were prepared by conventional solid-state reaction method. The effects of strontium substitution for calcium in CBN pseudo-perovskite structure A-site were systematically studied. Results showed that the addition of Sr²⁺ ions lead to an improvement of the tetragonality of lattice structure, which resulted in an enhancement of piezoelectric and ferroelectric properties together with high Curie temperature T_C and good resistance to thermal depolarization. The analysis of dielectric spectrums revealed that the space charge polarization induced an additional dielectric anomaly occurred below T_C. The composition with x = 0.025 showed good integrated performance, the piezoelectric coefficient d₃₃ and T_C were ~17.5 pC/N and ~917°C, respectively. Even though the as-studied ceramics underwent high depolarizing temperature reached up to 875°C, d₃₃ decreased by 8% merely. The remanent polarization 2P_r and the resistivity ρ at 650°C were on the order of ~10 μC/cm² and 3 × 10⁵ Ω cm, respectively.     

Enhanced piezoelectric properties and electrical resistivity in W/Cr co-doped CaBi2Nb2O9 high-temperature piezoelectric ceramics

W/Cr co-doped Aurivillius-type CaBi₂Nb_2-x(W_2/3Cr_1/3)_xO₉ (CBN) (x?=?0.025, 0.050, 0.075, 0.100, and 0.150) piezoelectric ceramics were prepared by the conventional solid-state reaction method. The crystal structure, microstructure, dielectric properties, piezoelectric properties, and electrical conductivity of these ceramics were systematically investigated. After optimum W/Cr modification, the CBN ceramics showed both high d₃₃ and T_C. The ceramic with x?=?0.1 showed a remarkably high d₃₃ value of ~15 pC/N along with a high T_C of ~931?°C. Moreover, the ceramic also showed excellent thermal stability evident from the increase in its planar electromechanical coupling factor k_p from 8.14% at room temperature to 11.04% at 600?°C. After annealing at 900?°C for 2?h, the ceramic showed a d₃₃ value of 14?pC/N. Furthermore, at 600?°C, the ceramic also showed a relatively high resistivity of 4.9?×?10⁵ Ω?cm and a low tanδ of 9%. The results demonstrated the potential of the W/Cr co-doped CBN ceramics for high-temperature applications. We also elucidated the mechanism for the enhanced electrical properties of the ceramics.     

A realization of excellent piezoelectricity and good thermal stability in CaBi2Nb2O9: Pseudo phase boundary

Phase boundaries (PBs) are known to contribute to the outstanding performances of lead-based and lead-free materials. However, a lack of PBs restricts the promotion of piezoelectric performance in bismuth layer-structured ferroelectrics (BLSFs). In this work, a pseudo PB, ie, pseudotetragonal distortion (regulated by Ce), is proposed to promote the piezoelectric properties of CaBi₂Nb₂O₉-based ceramics, and an excellent piezoelectric constant (d₃₃) of 20.2 pC/N with a high Curie temperature of 923°C is obtained. Verified Ce incorporation into the (Bi₂O₂)²⁺ layer alters the environment of the (Bi₂O₂)²⁺ layer, thereby influencing the atomic displacement in the Nb-O octahedron and modulating the theoretical spontaneous polarization (P_s). Strengthening of the pseudotetragonal distortion is favorable to the polarization switching, and maintains the theoretical P_s of ceramics at a high level, thus realizing the promotion of d₃₃. Furthermore, pseudotetragonal distortion guarantees good thermal depoling performance of the ceramic, which remains at 89.6% (18.1 pC/N) of its initial d₃₃ after depoling at 875°C. This work provides clear guidance on obtaining high d₃₃ and good thermal stability in BLSFs.     

Enhanced electrical properties of Cr2O3 addition NBT-based high-temperature piezoelectric ceramics

Owing to industrial and technological developments, there has been an increasing demand for piezoelectric ceramics that can function at temperatures of 500°C or higher. Na_0.5Bi_4.5Ti₄O₁₅ (NBT) with its high Curie temperature (T_C) of 650°C is a typical bismuth layer–structured ferroelectric. However, its relatively low piezoelectric coefficient (d₃₃ ∼ 16 pC/N) hinders its potential application at high temperatures. In this study, compositions of Ca_0.05(Na_0.5Bi_0.5)_0.95Bi₄Ti₄O₁₅ with different additions of Cr₂O₃ (CNBT–Cr100x) were designed based on previous studies on Ca²⁺-doped NBT piezoceramics, and the effects of the addition on the structural and electrical properties were investigated. The d₃₃ value of CNBT–Cr20 was as high as 29 pC/N, almost twice higher than that of pure NBT ceramics. This increase was investigated in depth using X-ray diffraction refinement and piezoelectric force microscopy in terms of intrinsic and extrinsic contributions. The P_s values of CNBT and CNBT–Cr20 were almost equal. The density of the domain walls of CNBT–Cr20 was significantly higher than that of CNBT, indicating that the increase of d₃₃ of CNBT–Cr20 is mainly due to the increase in the extrinsic contribution. The CNBT–Cr20 ceramic exhibited excellent properties with a high T_C of 655°C, a high d₃₃ of 29 pC/N, and a resistivity high than 10⁶ Ω cm at 500°C, demonstrating its potential for applications at high temperatures such as 500°C.     

Tailoring structure and piezoelectric properties of CaBi2Nb2O9 ceramics by W6+-Doping

Calcium bismuth niobate (CaBi₂Nb₂O₉) is a typical bismuth-layer structured piezoelectrics (BLSPs) with a high Curie temperature (T_C) of ～943 °C, but it has low piezoelectric coefficient and high-temperature resistivity which severely limits signal acquisition in the high-temperature piezoelectric vibration sensors. Ion-doping modification is regarded as an effective way to enhance electrical properties. In this work, W⁶⁺ donor-doping at Nb⁵⁺ site in the CaBi₂Nb_2-xW_xO₉ (x = 0, 0.020, 0.025, 0.030, 0.035 and 0.040) piezoelectric ceramics with T_C of 931 ± 2 °C were fabricated by the conventional solid-state reaction method. The effects of W⁶⁺-doping on crystal structure of CaBi₂Nb_2-xW_xO₉ as well as microscopic morphology and electrical properties of ceramics were investigated systematically. The tetragonality, isotropy and electrical properties of the ceramics were enhanced with the introduction of W⁶⁺ dopant. It was found that x = 0.025 was the optimal W⁶⁺-doping ratio that yielded remnant polarization of 8.0 μC/cm², electrical resistivity of 3.0 × 10⁶ Ω cm at 600 °C, piezoelectric coefficient (d₃₃) of 14.4 pC/N, and good thermal depoling property. Our work has established a feasible approach to tune the structure of CaBi₂Nb₂O₉ to improve piezoelectric properties for potential applications in high-temperature piezoelectric vibration sensors.     

Large enhancement of piezoelectric properties and resistivity in Cu/Ta co-doped Bi4Ti3O12 high-temperature piezoceramics

In this study, the electrical properties of Bi₄Ti₃O₁₂-based Aurivillius-type ceramics were tailored by a B-site co-doping strategy combining high valence Ta⁵⁺ and low valence Cu²⁺. A series of Bi₄Ti_3−x(Cu_1/3Ta_2/3)_xO₁₂ (BTCT) (x = 0, 0.005, 0.01, 0.015, 0.02, 0.025, and 0.03) ceramics were prepared by the conventional solid-state reaction method. The effect of Cu/Ta co-doping on the crystal structure, microstructure, dielectric properties, piezoelectric properties, ferroelectric properties, and electrical conductivity of these ceramics was systematically investigated. Co-doping significantly enhanced the piezoelectric properties and DC electrical resistivity of the resulting composites. The optimized comprehensive performances were obtained at x = 0.015 with a large piezoelectric coefficient (34 pC/N) and a relatively high resistivity of 9.02 × 10⁶ Ω cm at 500°C. Furthermore, the ceramic also exhibited stable thermal annealing behaviors and excellent fatigue resistance. The results of this study demonstrated great potential of the Cu/Ta co-doped Bi₄Ti₃O₁₂ ceramics for high-temperature piezoelectric device applications.     

Doping level effects in Nb self-doped Bi3TiNbO9 high-temperature piezoceramics with improved electrical properties

Nb self-doped Bi₃Ti_1-xNb_1+xO₉ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) high-temperature piezoelectric ceramics were fabricated through the conventional solid-state sintering method. The effects of different Nb self-doping levels on the microstructure, piezoelectric activities, and electrical conduction behaviors of these Nb self-doped Bi₃Ti_1-xNb_1+xO₉ ceramics were studied in detail. Large doping level effects on piezoelectric activity and resistivity were confirmed, which might be ascribed to the evolution of the crystal structure and the variations of the oxygen vacancy concentration and the grain anisotropy induced by Nb doping. An optimized piezoelectric coefficient (d₃₃) of 11.6 pC/N was achieved at x = 0.04 with a Curie temperature of 906°C. Additionally, an improved DC resistivity of 6.18 × 10⁵ Ω·cm at 600°C was acquired in this ceramic. Furthermore, the ceramic exhibited excellent thermal stability with the d₃₃ value maintaining 95% of its initial value after being annealed at 850°C for 2 hours. These results showed that Nb self-doped Bi₃Ti_1-xNb_1+xO₉ ceramics might have great potentials for high-temperature piezoelectric applications.     

Nb5+掺杂改性CaBi4Ti4O15压电陶瓷的研究

采用传统固相烧结法,制备了CaBi4Ti(1-x)NbxO1(5x=0.00-0.05,CBT-N)系铋层状结构无铅压电陶瓷。研究了Nb5+掺杂对CBT压电陶瓷压电与介电性能的影响。研究结果表明:添加Nb5+离子,改善了CBT陶瓷的烧结特性,提高了瓷体的致密度。Nb2O5的引入降低了CBT系列陶瓷的介质损耗,改善了陶瓷的压电与介电性能。当掺入量x=0.04(CaBi4Ti0.96Nb0.04O15)时制备的CBT基铋层状压电陶瓷具有优异的压电性能:d33=14pC/N,Qm=3086,εr=212,tanδ=0.0041,kt/kp=1.681。     

Ba(Mg1/3Nb2/3)O3-Mg4Nb2O9复相陶瓷微波介电性能研究

采用传统固相法制备了(1?x)Ba(Mg1/3Nb2/3)O3?xMg4Nb2O9 [(1?x)BMN?xM4N2,x = 0.003 ~ 0.125] 微波介质陶瓷,研究了相结构、烧结性能与介电性能随 x 的变化规律。结果表明： BMN 与 M4N2 可以两相共存,且二者间存在有限固溶,BMN 的烧结温度及高温稳定性有所降 低。随着 x 的增大,介电常数 εr和谐振频率温度系数 τf逐渐减小,Q × f 值的变化易受到 BMN 有序参数 S 的影响,高度 1:2 有序的 x = 0.026 陶瓷获得了最大 Q × f 值 125000 GHz。综合来看, 在 1320°C 下保温 4 h 烧结的 x = 0.125 样品表现出最佳的微波介电性能：εr = 26.6,Q × f = 111000 GHz,τf = 5 ppm/ºC。     

Relaxor behavior and photocatalytic properties of BaBi2Nb2O9

Lead-free Aurivillius phase BaBi₂Nb₂O₉ powders were prepared by solid-state reaction. Ferroelectric measurements on BaBi₂Nb₂O₉ (BBNO) ceramics at room temperature provided supporting evidence for the existence of polar nanoregions (PNRs) and their reversible response to an external electric field, indicating relaxor behavior. The photocatalytic degradation of Rhodamine B reached 12% after 3 hours irradiation of BBNO powders under simulated solar light. Silver (Ag) nanoparticles were photochemically deposited onto the surface of the BBNO powders and found to act as electron traps, facilitating the separation of photoexcited charge carriers; thus, the photocatalytic performance was significantly improved. The present study is the first examination of the photochemical reactivity of a relaxor ferroelectric within the Aurivillius family with PNRs.     

Improving the piezoelectric properties of CBN-based ceramic by a Ce ion

CaBi₂Nb₂O₉ (CBN), one of the bismuth-layered structural ferroelectrics, with high Curie temperature (T_C), has great potential in high-temperature applications. In this work, high Curie temperature and piezoelectric constant (d₃₃) are realized in modified CaBi₂Nb₂O₉ ceramics with Ce-substitution. Ce-substitution changes the crystal structures and domain structures of CBN-based ceramics, so as to improve the piezoelectric properties. The optimal performances are obtained with a high d₃₃ value (∼18.0 pC/N) and a T_C value (∼930°C), together with a low tan δ value (∼0.028 at 500°C). Moreover, the thermal stability is also enhanced, where the d₃₃ value maintains 93.9% of its original value after annealing at 900°C for 2 h. Thus, these findings play a meaningful role in devices manufacturing, where the apply temperature is often more than 500°C.     

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.     

Low‐temperature sintering and electrical properties of Sr2Nb2O7 piezoceramics by CuO addition

Effects of 0.5 wt% CuO addition on the sintering, structural and electrical properties of perovskite layer structured (PLS) Sr₂Nb₂O₇ ceramics prepared by solid‐state reaction method are investigated. The addition of CuO is beneficial to the liquid phase bridge formation at sintering process, leading to lower sintering temperature of 1180°C and larger bulk density up to 98%. Meanwhile, CuO modified Sr₂Nb₂O₇ ceramics show a remarkable d₃₃ of (1.1 ± 0.1) pC/N while still with a very high T_c of (1340 ± 2)°C. Raman spectra indicate that the improvement of piezoelectricity could be attributed to the rotation and/or distortion of oxygen octahedron caused by possible Cu²⁺ substitution at the A‐sites of Sr₂Nb₂O₇.     

Influence of Co/W co-doping on structural and electrical properties of Na0.5Bi2.5Nb2O9 piezoelectric ceramics

Co/W co-doped Na_0.5Bi_2.5Nb_2-x(Co_1/3W_2/3)_xO₉ (NBNCW-x) ceramic samples were prepared by the conventional solid state reaction method. The electrical properties and crystal structure of the NBNCW-x ceramic samples were analyzed in detail. The XRD and Rietveld refinement results showed that the samples lattice distortion decreased with the increment of Co/W doping. The XPS results showed that the number of oxygen vacancies in the Na_0.5Bi_2.5Nb₂O₉ ceramics could be reduced by the substitution of a small amount of Co/W. The weakened lattice distortion and reduced number of oxygen vacancies of the Na_0.5Bi_2.5Nb₂O₉ ceramics synergistically contributed to its improved electrical properties. In particular, the Na_0.5Bi_2.5Nb_1.97(Co_1/3W_2/3)_0.03O₉ ceramic exhibited the best performance, and its T_c, d₃₃ and P_r were 780 °C, 24.9 pC/N and 12.6 μC/cm², respectively. The dielectric loss was only 3.3% at 550 °C. In addition, this ceramic exhibited excellent thermal stability, with the d₃₃ value of the ceramic being 95.2% of its original value when annealed at 750 °C. These properties indicate that the Co/W co-doped Na_0.5Bi_2.5Nb₂O₉-based ceramics have potential application in the high-temperature field.     

Simultaneous enhancement of electrical and mechanical properties in CaBi2Nb2O9-based ceramics

CaBi₂Nb₂O₉ (CBN) with Aurivillius phase has an enormous potential in high-temperature piezoelectric devices due to their high Curie temperature and excellent free-fatigue characteristics. Nevertheless, simultaneous enhancement of electrical and mechanical properties in CBN-based ceramics are still a great challenge because of the trade-off between the electrical and mechanical properties. Herein, a strategy, the synergy effect of lattice distortion and oxygen vacancy, is designed to realize the enhanced electrical and mechanical properties of CBN-based ceramics via the domain structure and grain size engineering. The materials can simultaneously deliver a high piezoelectric property of 17.3 pC/N, large hardness of 4.68 GPa, and intensive bending strength of 113.07 MPa, which are enhanced by 346%, 197%, and 141% over those of unmodified CBN ceramics. We believe that the founding of this research opened up a novel and efficient guideline for exploring new bismuth-layered structure ceramics with excellent electrical and mechanical properties.     

Effects of cerium on structures and electrical properties of (Nb,Ta) modified Bi4Ti3O12 piezoelectric ceramics

Bi₄Ti₃O₁₂ high-temperature piezoelectric ceramics composed of 0.03 mol (Nb, Ta)⁵⁺ substituting B site and x mol CeO₂ (x = 0–0.05, abbreviated as BCTNT100x) substituting A site were synthesized by the conventional solid-state reaction method. The effects of Ce additive on the structures and electrical properties of resulting Bi₄Ti₃O₁₂-based ceramics were systematically investigated. In-situ temperature-dependent X-ray diffraction (XRD) confirmed that the phase structure of BCTNT100x ceramics change from orthorhombic structure to tetragonal structure as temperature increased. The ceramics at Ce content x = 0.03 illustrated optimal performances with superior piezoelectric constant (d₃₃ = 36.5 pC/N), high Curie temperature (T_C = 649 °C), and large remanent polarization (2P_r = 21.6 μC/cm²). BCTNT3 ceramics also possessed high d₃₃ of 32.5 pC/N at an annealing temperature of 600°C, with electrical resistivity preserved at 10⁶ Ω cm at 500 °C. These results demonstrate that BCTNT100x ceramics can be used as high-temperature piezoelectric devices.     

Crystal distortion and electrical properties of Ce-doped BIT-based piezoelectric ceramics

Cerium (Ce)-modified Bi₄Ti_2.94W_0.03Ta_0.03O₁₂ (BITWT) high Curie temperature ceramics (abbreviated as BITWT-xCe) were fabricated by a conventional solid-state sintering method. All BITWT-xCe ceramics had an orthogonal phase, but the structural distortion of the Ce-doped BITWT ceramics was higher than that of BITWT ceramics, which reduced symmetry and improved piezoelectric performance. The relative density (ρ_r) of BITWT-xCe ceramics was greater than 97%. Under the same conditions, the hysteresis loop of BITWT-0.04Ce ceramics had higher saturation than that of BITWT ceramics. The piezoelectric constant (d₃₃) was enhanced, and the highest d₃₃ of 24.7 pC/N at x = 0.04 was obtained, which was 25% higher than that of BITWT ceramics (d₃₃ = 19.8 pC/N). In addition, the tentative conduction mechanism of BITWT-xCe ceramics was also discussed. Two oxidations (Ce³⁺ and Ce⁴⁺) were present in the Ce-doped BITWT ceramics.