High-performance KNN piezoceramics with reproducibility and chemical stability. Part I: Solid-state reaction route

(K,Na)NbO₃-based compounds are attracting much attention for their high potential as Pb-free piezoelectric ceramics. They have, however, three major drawbacks to overcome in order to achieve a mature development: densification, reproducibility, and chemical stability. In this first part, based on standard solid-state reaction, the way to synthesize high-density homogeneous ceramics with high piezoelectric performance and chemical stability is elucidated. It relies on a low-temperature spark-plasma sintering process, after the raw materials have been finely pulverized prior to reaction. The reproducibility, however, remains a pending issue due to the unavoidable random formation of parasitic phases.     

Simultaneously enhancing the Td and the thermal stability of the real-time d33 in BNT-based piezoceramics

One of the inherent disadvantages of Bi_0.5Na_0.5TiO₃-based (BNT-based) piezoceramics is that an increment of the depolarization temperature (T_d) is generally accompanied by a deterioration of the temperature stability of the real-time piezoelectric constant (d₃₃), which severely restricts the practical applications of the materials. Herein, we propose a new strategy to mitigate the conflict between the elevation of T_d and the temperature stability of the real-time d₃₃ in BNT-based ceramics via integrating the pressure-assisted sintering and quenching process. By this strategy, we not only increased the T_d from 109 to 166°C but also improved the temperature stability of the real-time d₃₃ (the temperature coefficient T_tc = ± 13 pC/N) in 0.94Bi_0.5Na_0.5TiO₃–0.06BaTiO₃ ceramic. Both the built-in field and the increment of the field-induced R3c phase fraction play the significant role in enhancing the T_d and the temperature stability of the real-time d₃₃. Thus, our work provides a simple and effective approach for designing piezoceramics with high T_d and a superior temperature stability of the real-time d₃₃.     

Defect engineering on phase structure and temperature stability of KNN‐based ceramics sintered in different atmospheres

Lead‐free MnO‐doped 0.955K_0.5Na_0.5NbO₃‐0.045Bi_0.5Na_0.5ZrO₃ (Abbreviated as KNN‐0.045BNZ) ceramics have been prepared by the conventional solid‐state sintering method in reducing atmosphere ( = 1 × 10^?10 atm) and air. For ceramics sintered in reducing atmosphere, only Mn²⁺ ions exist in ceramics who preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4, whereas Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site to form defects () at x > 0.4. For ceramics sintered in air, mixed Mn²⁺, Mn³⁺, and Mn⁴⁺ ions coexist here. The Mn²⁺ ions preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4 and then Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site at x > 0.4. Meanwhile, the Mn³⁺ ions and Mn⁴⁺ ions substitute for Nb⁵⁺ ions in B‐site to form defects () at x = 0.2‐0.8. The (, , and ) dipolar defects show a positive dipolar defect contribution (DDC) to the , whereas the dipolar defects () show a negative DDC to the . The dipolar defects ( ‐ and ) can help improve the temperature stability of . The 0.4% MnO‐doped KNN‐0.045BNZ ceramics sintered in reducing atmosphere show excellent piezoelectric constant d₃₃ = 300 pC/N and 0.2% MnO‐doped KNN‐0.045BNZ ceramics sintered in air possess optimal piezoelectric constant d₃₃ = 290 pC/N.     

Piezoelectric properties of [001]-textured high-power PMnN-PZT piezoceramics sintered at a low temperature

The 1.0 mol% CuO-added 0.14Pb(Mn1/3Nb2/3)O₃-0.86Pb(Zr_0.55Ti_0.45)O₃ (CPMnN-PZT) piezoceramics were synthesized at low temperatures (≤ 950 °C) by the columbite method to remove the pyrochlore phase. It exhibited a high Q_m of 3016 and a high T_C of 309 °C. However, it showed small d₃₃ and k_p values of 143 pC/N and 0.43, respectively. The CPMnN-PZT piezoceramic was textured along the [001] direction using BaTiO₃ (BT) templates to enhance its soft piezoelectric properties. The [001]-textured CPMnN-PZT + y vol% BT piezoceramics (1.0 ≤ y ≤ 5.0) were densified at a low temperature of 950 °C with a high Lotgering factor (≥ 92%). The textured sample (y = 1.0) showed a high Q_m of 1400 with increased d₃₃ and k_p values of 278 pC/N and 0.58, respectively, and its piezoelectricity showed excellent temperature stability up to 225 °C. Therefore, the textured CPMnN-PZT sample can be used for developing high-power piezoelectric devices.     

Densification behavior of flash sintered boron suboxide

A study to quantify the flash sintering kinetics of boron suboxide (B₆O) under various electric field strengths and cut‐off amperages is presented. B₆O is conventionally sintered at a prolonged temperature above 1800°C, near its thermal decomposition temperature, with an overpressure >3 atm. By applying a direct current (DC) electric field across a green powder compact, B₆O can be sintered at 1000°C at atmospheric pressure. During the flash sintering process, an intensive radiation was emitted (electroluminescence), which is distinct from the thermal radiation (thermoluminescence) that is expected in conventional sintering. It was observed that the degree of sintering of the large B₆O specimen was heterogeneous due to apparent localization of electrically conducting paths. The material near the surface was sintered, but the core of the specimen was not. It was found that the flash event occurred at a critical temperature, which was obtained by combining external heating via ambient furnace conditions and internal Joule heating. The progressive densification behaviors of the B₆O are also presented.     

Piezoelectric and ferroelectric properties of (Bi,Na)TiO3–(Bi,Li)TiO3–(Bi,K)TiO3 ceramics for accelerometer application

The piezoelectric and ferroelectric properties of 0.76(Bi_0.5Na_0.5)TiO₃–0.04(Bi_0.5Li_0.5)TiO₃–0.2(Bi_0.5K_0.5)TiO₃ (abbreviated as 0.76BNT–0.04BLT–0.2BKT) ceramics were investigated to clarify the optimal sintering temperature, and the vibration characteristics were examined for a compression‐mode accelerometer assembly in which 0.76BNT–0.04BLT–0.2BKT ceramics sintered at the optimized temperature served as the piezoelectric elements. The increase in the grain size of the 0.76BNT–0.04BLT–0.2BKT ceramics with the sintering temperature provides a beneficial contribution to the piezoelectric coefficient; however, it detrimentally contributes to the depolarization temperature. The charge sensitivity of the prototype accelerometers was evaluated with changes in the seismic mass and the layer number of the piezoceramics. The deviation between the theoretical and measured values of charge sensitivity was less than 10%.     

Thermally stable large strain in low-loss (Na0.2K0.8)NbO3-BaZrO3 for multilayer actuators

CuO-doped (1–x)(Na_0.2K_0.8)NbO₃-xBaZrO₃ ceramics (0.0 ≤ x ≤ 0.06) were densified at 960°C. The ceramic with x = 0 exhibited a large sprout-shaped strain vs electric-field (S-E) curve and a double polarization vs electric-field (P-E) hysteresis curve, owing to the defect polarization (P_D) developed between Cu²⁺ ions at Nb⁵⁺ sites and oxygen vacancies. The sizes of the S-E and P-E loops decreased with increasing x, owing to the decrease in the number of P_Ds. The ceramic with x = 0.04 displayed small S-E and P-E curves, indicating its small dielectric loss. It exhibited large strain (0.19% at 8.0 kV/mm) at room temperature, which was maintained at 200°C. A similar strain was observed after applying 10⁶ cycles of an electric field (3.0 kV/mm). Hence, this specimen exhibited large strain with excellent thermal and fatigue properties. Moreover, the synthesized multilayer actuator using the ceramic with x = 0.04 showed excellent vibrational properties, making it promising for applications in multilayer piezoelectric actuators.     

Thermally stable high strain and piezoelectric characteristics of (Li,Na, K)(Nb,Sb)O3-CaZrO3 ceramics for piezo actuators

The 0.97(Na_0.5K_0.5)(Nb_1−xSb_x)O₃-0.03CaZrO₃ ceramic with x = 0.09 exhibits a high d₃₃ of 518 pC/N and a strain of 0.13% at 4.0 kV/mm owing to its orthorhombic-pseudocubic polymorphic phase boundary (PPB) structure. However, these values decreased considerably above 90°C owing to its low Curie temperature (T_C), indicating that its thermal stability is not sufficient for practical applications. Li₂O was added to the specimen with x = 0.11 to improve its thermal stability of the strain and d₃₃ by increasing the T_C without degrading the actual d₃₃ and strain values. The 0.97(Li_0.04Na_0.46K_0.5)(Nb_0.89Sb_0.11)O₃-0.03CaZrO₃ ceramic, having an orthorhombic-tetragonal PPB structure, exhibits a d₃₃ of 502 pC/N and a strain of 0.16%. This large strain was maintained up to 150°C and the d₃₃ slightly decreased to 475 pC/N at 130°C. Therefore, this lead-free ceramic displays excellent piezoelectric characteristics with improved thermal stability, indicating that it can be applied to piezoelectric actuators.     

Development of a lead‐free copper co‐fired BNT‐based piezoceramic sintered at low temperature

Compatibility of Bi‐based piezoelectric ceramic and copper electrodes is demonstrated by co‐firing 0.88Bi_1/2Na_1/2TiO₃–0.08Bi_1/2K_1/2TiO₃–0.04BaTiO₃ (BNKBT88) with copper. A combination of Bi₂O₃, CuO, ZnO, Li₂CO₃, and B₂O₃ are used as additives to reduce firing temperature to 900°C with minimal effect on the electromechanical properties compared to sintering at 1150°C without additives. Co‐firing with copper electrodes requires controlled oxygen sintering at low temperature. The atmosphere is controlled using carbon dioxide and hydrogen gas to maintain an oxygen partial pressure of 6.1 × 10^?8 atm, which is necessary for the coexistence of Cu metal and Bi₂O₃. The thermodynamic activity of bismuth oxide in BNKBT88 is calculated to be 0.38. BNKBT88 ceramics were successfully co‐fired with internal as well as surface Cu metal electrodes. The copper co‐fired ceramics were successfully polarized and the dielectric and piezoelectric properties are evaluated.     

Defect suppression in CaZrO3‐modified (K,Na)NbO3‐based lead‐free piezoceramic by sintering atmosphere control

During high‐temperature crystal growth, lattice defects will inevitably form inside piezoelectric materials, which can be a hindrance for performance optimization. Through appropriate atmosphere control during sintering, defect levels inside the piezoelectric material can be regulated. Herein, CaZrO₃‐modified (K, Na)NbO₃‐based lead‐free piezoelectric ceramics with a nominal composition of 0.95(Na_0.49K_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃‐0.05CaZrO₃ are produced by sintering in an oxygen‐rich atmosphere. Compared with an air‐sintered sample, the piezoelectric constant of the oxygen‐sintered sample has greatly improved 15% up to 390 pC/N, which is comparable to commercial lead‐based counterparts. In addition, the planar electromechanical coupling factor k_p is enhanced from 0.46 to 0.52. A qualitative model related to defect engineering is proposed to support the experimental observations. Our results indicate the feasibility of purposely optimizing the piezoelectric performance by sintering atmosphere control.     

Structure and electrical properties of perovskite layer (1−x)Sr2Nb2O7‐x(Na0.5Bi0.5)TiO3 high‐temperature piezoceramics

The structure and electrical properties of perovskite layer structured (PLS) (1?x)Sr₂Nb₂O₇‐x(Na_0.5Bi_0.5)TiO₃ (SNO‐NBT) prepared by solid‐state reaction method are investigated. The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering, leading to better sinterability and higher bulk density up to 96.8%. The solid solution limit x in the SNO‐NBT system is below 0.03, over which Ti⁴⁺ is preferable to aggregate and results in the generation of secondary phase. After the modification by NBT, all SNO‐NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0 pC/N. The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density. This study not only demonstrates how to improve piezoelectricity by NBT addition, but also opens up a new direction to design PLS piezoceramics by introducing appropriate second phase.     

Effect of Li2O on the defect polarization in CuO‐added (K0.9Na0.1)NbO3 piezoelectric ceramics

CuO‐added (Li_xK_0.9?xNa_0.1)NbO₃ [C(L_xK_0.9?xN_0.1)N] ceramics with 0.0≤x≤0.05 were well‐sintered at 960°C for 6 hours. The lattice parameters of the specimens decreased with the addition of Li₂O. Defect polarization (P_D) formed between Cu²⁺ions and oxygen vacancies. Double polarization vs electric field (P‐E) hysteresis and sprout‐shaped strain vs electric field (S‐E) curves were observed in these specimens with a large strain of 0.16% at 7.0 kV/mm, possibly owing to the presence of P_D. When the P‐E curve was measured at temperatures higher than 75°C, the C(K_0.9N_0.1)N ceramic exhibited a normal P‐E hysteresis curve, whereas the C(L_0.04K_0.86N_0.1)N ceramic maintained the double P‐E hysteresis curve up to 125°C, indicating that Li₂O increased the thermal stability of P_D. The latter specimen also showed the sprout shaped S‐E curve with a strain of 0.15% at 7.0 kV/mm after 10⁴ cycles of a high electric field of 7.0 kV/mm.     

Surface structure and quenching effects in BiFeO3-BaTiO3 ceramics

The structure and properties of Mn-doped 0.67BiFeO₃-0.33BaTiO₃ ceramics are systematically investigated with respect to the effects of annealing prior to rapid cooling by quenching in air. Air-quenching induces a change in crystal structure from pseudo-cubic to rhombohedral, with higher quenching temperatures leading to an increased rhombohedral distortion. These structural changes are correlated with the appearance of more well-defined ferroelectric domain configurations. It is shown that the surface preparation procedures for XRD measurements can induce significant changes in the peak profiles, indicating differences in crystal structure between the surface and bulk regions. Frequency dispersion in the temperature-dependent relative permittivity for the as-sintered sample is significantly reduced after quenching, accompanied by enhancement of the Curie point and improved temperature-stability of piezoelectric properties. It is proposed that the formation of defect clusters by A-site cation diffusion during cooling is circumvented by quenching, leading to the observed modification of structural distortion and ferroelectric properties.     

Combining effects of TiO6 octahedron rotations and random electric fields on structural and properties in Na0.5Bi0.5TiO3

The structural and dielectric properties of Na_0.5Bi_0.5TiO₃ (NBT) ceramics and crystals have been investigated and are compared to that of Pb(Zr_0.55Ti_0.45)O₃ (PZT55/45) and Pb(Mg_1/3Nb_2/3)_0.72Ti_0.28O₃ (PMNT 72/28) ceramics. X-ray diffraction (XRD) profiles for (100), (110), (111), (200), (220), and (222) (referred to cubic structure) reveal that the monoclinic structure with Cc space group exists both in the NBT single crystal and ceramics. The diffraction profile obtained with high resolution laboratory XRD for the NBT single crystal can be well described, using Cc model instead of R3c model. The dielectric constant of NBT below T_hump shows some similarity to that of PZT45/55 ceramics below 50°C in which oxygen octahedron rotations cause the frequency dispersion of the dielectric constant. The temperature-dependent dielectric constant for NBT can be deconvolved into two independent processes. The lower temperature process shows a typical relaxor characteristic and follows the Vogel-Fulcher relationship. The other process at higher temperature shows less frequency-dependent behavior. Comparing the dielectric constant of NBT with that of PZT55/45 and PMNT72/28 reveals that both oxygen octahedral rotations and random electric fields play an important role in the frequency dispersion of the dielectric constant for NBT relaxor feroelectric.     

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.     

Improving piezoelectric properties and temperature stability for KNN‐based ceramics sintered in a reducing atmosphere

Lead‐free 0.955K_0.5Na_0.5Nb_1‐zTa_zO₃‐0.045Bi_0.5Na_0.5ZrO₃+0.4%MnO ceramics (abbreviated as KNNTaz‐0.045BNZ+0.4Mn) were prepared by a conventional solid‐state sintering method in a reducing atmosphere (oxygen partial pressure of 1 × 10^?10 atm). All ceramics with a pure perovskite structure show the two‐phase coexistence zone composed of rhombohedral and tetragonal phase. Ta⁵⁺ ions substitute for Nb⁵⁺ ions on the B‐site, which results in a decrease in the R phase fraction in the two‐phase coexistence zone. The R‐T phase transition temperature moves to room temperature due to the substitution of Nb⁵⁺ ions by Ta⁵⁺ ions. A complex domain structure composed of small nano‐domains (~70 nm) formed inside large submicron domains (~200 nm) exists in KNNTa0.02‐0.045BNZ+0.4Mn ceramics, which can induce a strong dielectric‐diffused behavior and improve the piezoelectric properties. The temperature stability for the reverse piezoelectric constant for the KNNTaz‐0.045BNZ+0.4Mn ceramics can be improved at z = 0.02. Excellent piezoelectric properties (d₃₃ = 328 pC/N, and  = 475 pm/V at E_max = 20 kV/cm) were obtained for the KNNTa_0.02‐0.045BNZ+0.4Mn ceramics.     

Utilizing the Cold Sintering Process for Flexible–Printable Electroceramic Device Fabrication

Conventional thermal sintering of ceramics is generally accomplished at high temperatures in kilns or furnaces. We have recently developed a procedure where the sintering of a ceramic can take place at temperatures below 200°C, using aqueous solutions as transient solvents to control dissolution and precipitation and enable densification (i.e., sintering). We have named this approach as the “Cold Sintering Process” because of the drastic reduction in sintering temperature and time relative to the conventional thermal process. In this study, we fabricate basic monolithic capacitor array structures using a ceramic paste that is printed on nickel foils and polymer sheets, with silver electrodes. The sintered capacitors, using a dielectric Lithium Molybdenum Oxide ceramic, were then cold sintered and tested for capacitance, loss, and microstructural development. Simple structures demonstrate that this approach could provide a cost‐effective strategy to print and densify different materials such as ceramics, polymers, and metals on the same substrate to obtain functional circuitry.     

Effect of degree of crystallographic texture on ferro‐ and piezoelectric properties of Ba0.85Ca0.15TiO3 piezoceramics

Crystallographic texturing is a promising approach to reduce the performance gap between randomly oriented polycrystalline piezoelectrics and perfectly oriented single crystals. Here, the influence of the degree of crystallographic texture on the electromechanical properties and their temperature stability of the lead‐free perovskite ferroelectric Ba_0.85Ca_0.15TiO₃ is investigated. Samples with a broad range of (100),(001) crystallographic texture (Lotgering factor 26%‐83%) were prepared by the reactive templated grain growth method. Crystallographic and microstructural analysis have been carried out using X‐ray diffraction and scanning electron microscopy, while the temperature‐dependent electromechanical properties were characterized by dielectric, piezoelectric, polarization, and strain measurements. It was revealed that the total bipolar strain and the coercive field are linearly dependent on the Lotgering factor. The total bipolar strain increased by 80%, whereas the coercive field decreased by 18% due to crystallographic texturing. Likewise, the temperature stability of the electromechanical properties of the samples was found to be dependent on the degree of texture. A sample with a high degree of texture exhibited a Curie temperature of 117°C, which is 21% higher compared to a counterpart with a low degree of texture. This was related to chemical inhomogeneity and a modified internal mechanical stress state.     

Temperature stability and electrical properties of MnO‐doped KNN‐based ceramics sintered in reducing atmosphere

Lead‐free MnO‐doped 0.955K_0.5Na_0.5NbO₃‐0.045Bi_0.5Na_0.5ZrO₃ (abbreviate as KNN‐0.045BNZ) ceramics have been prepared by a conventional solid‐state sintering method in reducing atmosphere. The MnO addition can suppress the emergence of the liquid phase and improve the homogenization of grain size. All ceramics sintered in reducing atmosphere show a two‐phase coexistence zone composed of rhombohedral (R) and tetragonal (T) phase. MnO dopant results in the content increase in R phase and slight increase in Curie temperature T_C. For KNN‐0.045BNZ ceramics, Mn²⁺ ions preferentially occupy the cation vacancies in A‐site to decrease oxygen vacancy concentration for 0.2%‐0.4% MnO content, whereas Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site to form oxygen vacancies at x ≥ 0.5. The defect dipole is formed at the moderate concentration from 0.5 to 0.6, which can provide a preserve force to improve the temperature stability of piezoelectric properties for k_p and . The Mn0.4 ceramics show excellent electrical properties with quasistatic piezoelectric constant d₃₃ = 300 pC/N, electromechanical coupling coefficient k_p = 51.2%, high field piezoelectric constant  = 430 pm/V (at E_max = 25 kV/cm) and T_C = ~345°C, insulation resistivity ρ  =  6.13 × 10¹¹ Ωcm.     

PNN‐PZN‐PMN‐PZ‐PT Multilayer Piezoelectric Ceramic with Low Sintering Temperature

Multilayer piezoelectric ceramic material with a composition of 0.1Pb(Ni_1/3Nb_2/3)O₃‐0.35Pb(Zn_1/3Nb_2/3)O₃‐0.15Pb(Mg_1/3Nb_2/3)O₃‐0.1PbZrO₃‐0.3PbTiO₃‐4 mol% excess NiO (0.1PNN‐0.35PZN‐0.15PMN‐0.10PZ‐0.3PT‐0.04NiO) was fabricated by a roll‐to‐roll tape casting process and co‐fired with Ag/Pd electrode at low temperature of 950°C. Their dielectric, piezoelectric, and ferroelectric properties were evaluated. The effective piezoelectric coefficient d₃₃ of the obtained multilayer piezoelectric material was 412 pm/V, while d₃₃ for the ceramic pellet was 503 pm/V. Piezoelectric displacement measurements revealed small displacement hysteresis for the multilayer material. The combined characteristics of the multilayer piezoelectric material using the selected composition showed the potential for high power, high strain, and high force actuation applications. In addition, as the composition had a tetragonal phase, which substantially deviated from morphotropic phase boundary (MPB), the excellent properties may be more tolerant to stoichiometric fluctuation, which can allow larger processing and composition window as desired for scalable production.