Crystallographic texture and phase structure induced excellent piezoelectric performance in KNN-based ceramics

It is difficult to maintain strong piezoelectric properties over a wide temperature range in (K,Na)NbO₃ (KNN)-based ceramics owing to the polymorphic phase boundary (PPB). Here, we propose advantageously utilizing the synergistic effect of crystal orientation and phase structure to address this issue. The 〈0 0 1〉_pc textured (1 − x)(K_0.48Na_0.52)(Nb_0.96Sb_0.04)O₃–x(Bi_0.5Ag_0.5)ZrO₃ (KNNS–xBAZ) ceramics with different phase structures were synthesized via the templated grain growth method. A high piezoelectric coefficient (d₃₃) of 505 ± 25 pC/N, an electric field-induced strain of 0.21%, and a superior temperature stability (d₃₃ exhibited a high retention of ≥78% at the temperature up to 200°C; strain maintained within 5.7% change over a temperature range of 30–150°C) were simultaneously achieved in textured KNNS–0.03BAZ ceramics. The flattened Gibbs free energy induced by the R–O–T multiphase coexistence, the strong anisotropy of crystals, and the abundant nanodomains contributed to the enhanced piezoelectric properties. The contribution of the strong anisotropy of crystals in 〈0 0 1〉_pc textured ceramics compensates for the deterioration of the piezoelectric properties caused by the phase structure deviation from the PPB with increasing temperature, which benefits the superior temperature stability of the textured KNNS–0.03BAZ ceramics. The previous merits prove that utilizing the synergistic effect of crystal orientation and phase structure is an effective strategy to boost the piezoelectricity and their temperature stability of KNN-based ceramics.     

Enhanced temperature stability in the O–T phase boundary of (K,Na)NbO3-based ceramics

In order to obtain excellent electrical properties and its temperature stability of KNN-based ceramics to meet the practical applications, a new lead-free material system of (1−x)K_0.5Na_0.5Nb_0.96Sb_0.04O₃-xBi_0.5Na_0.5Zr_0.8Sn_0.2O₃ (KNNS-xBNZS, 0 ≤ x ≤ 0.060) was designed, and the enhanced electrical properties (eg, d₃₃ ~ 465 pC/N, ε_r ~ 3318, S_uni ~ 0.133%) is obtained in the ceramics with x = 0.04. The physical origin of enhanced electric properties should be ascribed to the phase instability of R-T, resulting in a low-energy barrier, which can greatly facilitate the polarization switching. Moreover, the temperature stable of piezoelectric constant (d₃₃ or d₃₃*) is measured by three distinctive methods in different phase boundaries (O, O-T, R-T) at the temperature range 20°C-180°C, revealing that the O-T phase boundary has a relatively good temperature stability. A mode is used to show how to effectively modified the piezoelectric constant (d₃₃ or d₃₃*) and its temperature stability, we believe that such a strategy may further improve the temperature stability of d₃₃ or d₃₃* value in KNN-based ceramics.     

High mechanical quality factor and piezoelectricity in potassium sodium niobate ceramics

Plenty of works have done to enhance the piezoelectricity of potassium-sodium niobate (KNN), aiming to replace lead-zirconate titanate (PZT) in the consideration of eco-friendly requirement. However so far, KNN ceramics with high piezoelectric performances tend to have a low mechanical quality factor (Q_m), which could result in excessive dielectric loss, especially when working in high frequencies. Thus, increasing Q_m is a crucial task in KNN-based ceramics. By constructing phase boundaries together with inducing oxygen vacancies, a new KNN ceramic system is built by using conventional solid-state method with high Q_m (>250), high piezoelectric performance as well as outstanding temperature stability. Optimum overall properties of KNN-based ceramics can be as large as d₃₃ = 231 pC/N, Q_m = 355, T_C = 366 °C. This work provides a deeper insight to KNN-based ceramics with high mechanical quality factor and makes a progress on the high frequency application of lead-free ceramics.     

(Bi0.5Na0.5)ZrO3 modified KNN-based ceramics: Enhanced electrical properties and temperature insensitivity

To further improve the properties of KNN-based lead-free ceramics, a new ceramic system, (0.98-x)K_0.525Na_0.475Nb_0.965Sb_0.035O₃-0.02 BaZr_0.5Hf_0.5O₃-x(Bi_0.5Na_0.5)ZrO₃(KNNS-BZH-xBNZ) was designed, the relevant properties such as piezoelectricity, strain, and temperature stability were analysed in detail. It was found that the R-T phase boundary can be successfully constructed when x=0.030, and this two-phase coexistence shows relatively good comprehensive properties (d₃₃~410 pC/N, T_C~255 °C, S_uni~0.132%, and d₃₃*~441 pm/V). Meanwhile, its strain property also shows good temperature stability from room temperature to 180 °C (S_uni100°C/S_uniRT~97.5% and S_uni180°C/S_uniRT~83.9%), which is comparatively superior to many KNN-based ceramics and some lead-based ceramics. Therefore, KNNS-BZH-xBNZ ceramics may broaden the practical application of lead-free ceramics.     

Modified Pb(Mg1/3Nb2/3)O3-PbZrO3–PbTiO3 ceramics with high piezoelectricity and temperature stability

There is a great demand to develop ferroelectric ceramics with both high piezoelectric coefficient and broad temperature usage range for emerging electromechanical applications. Herein, a series of Sm³⁺-doped 0.25Pb(Mg_1/3Nb_2/3)O₃-(0.75−x)PbZrO₃-xPbTiO₃ ceramics were fabricated by solid-state reaction method. The phase structure, dielectric and piezoelectric properties were investigated, where the optimum piezoelectric coefficient d₃₃ = 745 pC/N and electromechanical coupling factor k₃₃ = 0.79 were obtained at the morphotropic phase boundary composition x = 0.39, with good Curie temperature T_C of 242°C. Of particular importance is that high-temperature stability of the piezoelectric and field-induced strain was obtained over the temperature range up to 230°C for the tetragonal compositions of x = 0.40. The underlying mechanism responsible for the high piezoelectricity and temperature stability is the synergistic contribution of the MPB and local structural heterogeneity, providing a good paradigm for the design of high-performance piezoelectric materials to meet the challenge of piezoelectric applications at elevated temperature.     

Effect of a lattice distortion strategy on the phase transition and properties in KNN-based ceramics

High performance lead-free piezoelectric ceramics are of great importance to the sustainable development of the environment. To obtain excellent comprehensive performance KNN-based lead-free piezoelectric ceramics, a lattice distortion strategy combined with domain configuration was designed in (1 − x)K_0.5Na_0.5Nb_0.95Sb_0.05O₃–xCaHfO₃ ((1 − x)KNNS–xCH) system by introduced Ca²⁺ into the A-site and Hf⁴⁺ into the B-site. The results demonstrated that the rhombohedral–orthorhombic–tetragonal polymorphic phase boundary (PPB) was constructed in 0.02 ≤ x ≤ 0.04 and significant lattice distortion occurred in R- and T-phase. Moreover, the 0.97KNNS–0.03CH sample exhibited excellent electrical performance (e.g., k_p ∼ 43.8%, d^*₃₃ ∼ 478.6 pm/V, and d₃₃ ∼ 392 pC/N) together with a high Curie temperature (T_C ∼ 295°C) profited from the PPB and domain configurations. The ceramics also showed the optimal thermal stability, which was beneficial to promote the development of KNN-based ceramics.     

A simple Bi3+ self-doping strategy constructing pseudo-tetragonal phase boundary to enhance electrical properties in CaBi2Nb2O9 high-temperature piezoceramics

Calcium bismuth niobate (CaBi₂Nb₂O₉, CBN)-based ceramics are promising candidates for high temperature application, the electrical properties of which are commonly enhanced by complex ion substitution or texture processes. Here, we report that high piezoelectricity and high resistivity were achieved in Ca_1-xBi_2+xNb₂O₉ by constructing pseudo-tetragonal boundary through a simple strategy of Bi³⁺ self-doping. At the pseudo-tetragonal boundary, Ca_0.96Bi_2.04Nb₂O₉ ceramics maintain high Curie temperature T_c = 942 °C, and show high piezoelectric coefficient d₃₃ = 15.1 pC/N and high resistivity ρ_dc = 2 × 10⁶ Ω cm (@600 °C). It is proved that the good piezoelectric property mainly originates from the increase of domain density. In addition, Ca_0.96Bi_2.04Nb₂O₉ ceramics reveal good thermal depoling performance, remaining 90% of piezoelectricity after thermal depoling at 900 ℃, which is due to small thermal expansion and structural distortion. Our work provides a promising candidate for high temperature applications and an easy way to improve the performance of Aurivillius-type piezoelectric ceramics.     

Piezoelectricity,thermal stability,and fatigue resistance in Nb and Ta-doped Bi4Ti3O12 high-temperature piezoceramics

The effect of Nb/Ta donor doping on the piezoelectricity, thermal stability, and fatigue resistance of bismuth titanate Bi₄Ti₃O₁₂ (BIT) ceramics was investigated in relation to their structural and oxygen vacancy-related electrical properties. As the Nb/Ta doping amount increased, the activation energy of oxygen vacancy conduction increased, indicating a reduction in the concentration of oxygen vacancies. The improved electrical insulating properties of the Nb/Ta-doped Bi₄Ti₃O₁₂ ceramics (BTNT) with fewer oxygen vacancies, contributed to their effective poling and strong piezoelectricity. Outstanding piezoelectric performance with high piezoelectric constant (39 pC/N) and Curie temperature (690 °C) could be achieved in the 0.005 mol Nb/Ta-doped BTNT ceramic with high density and anisotropic grain growth. The BTNT ceramics exhibited superior thermal aging stability and fatigue resistance compared to the BIT ceramic, suggesting that the reduction of oxygen vacancy defects plays a decisive role in enhancing elevated-temperature-induced and electric-field-induced degradation stabilities.     

Giant strain response with low hysteresis in potassium sodium niobate based lead-free ceramics

In this work, the relationships between the composition-driven phase boundary, ferroelectricity and strain properties of the (1-x)(K_0.48Na_0.52)(Nb_1-ySb_y)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃ (abbreviated as (1-x)KNN_1-yS_y-xBNKZ) ceramics were investigated. A giant electric field-induced strain of 0.3% (d₃₃¹ = 750 p.m./V) and a low hysteresis (16.4%) were obtained in the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics. The giant strain is attributed to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-field-induced phase transition from the relaxor phase to the ferroelectric phase. Furthermore, the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics exhibit good thermal stability in the temperature range from 25 °C to 150 °C. Hence, this work can promote the practical applications of KNN-based lead-free piezoelectric ceramics in highly sensitive and precise piezoelectric actuators.     

Textured CaBi4Ti4O15 ceramics with large piezoelectricity,excellent thermal stability and high resistivity

The bismuth layer-structured ferroelectrics (BLSF) are promising high-temperature piezoelectric materials, in which large piezoelectricity, good thermal stability and high electrical resistivity are desired. Here highly textured CaBi₄Ti₄O₁₅ BLSF ceramics with orientation factor of 82% have been fabricated by spark plasma sintering technique. The piezoelectric coefficient d₃₃ is significantly enhanced by 250%, from 7.2 pC/N for the texture-less sample to 25.3 pC/N for the textured one, accompanied by a high Curie temperature T_C= 788 °C. The variation of d₃₃ is below 5% in the temperature range of 25–500 °C, showing excellent thermal stability. The textured sample exhibits high electrical resistivity ρ = 2.1 × 10¹¹ Ω·cm, an order of magnitude larger than that of the texture-less sample. At the temperature as high as 500 °C, the textured sample still maintains excellent electrical properties of d₃₃ = 24.2 pC/N, tanδ = 9.9% and ρ = 2.7 × 10⁶ Ω·cm, suggesting that the textured CaBi₄Ti₄O₁₅ ceramics could be a potential candidate for high-temperature piezoelectric sensor or detector applications.     

Temperature stability and electrical properties in La‐doped KNN‐based ceramics

To improve the temperature stability and electrical properties of KNN‐based ceramics, we simultaneously consider the phase boundary and the addition of rare earth element (La), 0.96K_0.5Na_0.5Nb_0.96Sb_0.04O₃‐0.04(Bi_1‐xLa_x)_0.5Na_0.5ZrO₃ (0 ≤ x ≤ 1.0) ceramics. More specifically, we investigate how the phase boundary and the addition of La³⁺ affect the phase structure, electrical properties, and temperature stability of the ceramic. We show that increasing the La³⁺ content leads to a change in phase structure, from a rhombohedral‐tetragonal (R‐T) phase coexistence to a cubic phase. More importantly, we show that the appropriate addition of La³⁺ (x = 0.2) can simultaneously improve the unipolar strain (from 0.127% to 0.147%) and the temperature stability (i.e., the unipolar strain of 0.147% remains unchanged when T is increased from 25 to 80°C). In addition, we find that the ceramics with x = 0.2 exhibit a large piezoelectric constant (d₃₃) of ~430 pC/N, a high Curie temperature (T_C) of ~240°C and a fatigue‐free behavior (after 10⁶ electric cycles). The enhanced electrical properties mostly originate from the easy domain switching, whereas the improved temperature stability can be attributed to the R‐T phase boundary and the appropriate addition of La³⁺.     

Large electric‐field‐induced strain and enhanced piezoelectric constant in CuO‐modified BiFeO3‐BaTiO3 ceramics

In this work, we fabricated the (1‐x)BiFeO₃‐xBaTiO₃+y‰ mol CuO ceramics by the modified thermal quenching technique. The pure perovskite phase was formed and a morphotropic phase boundary (MPB) was observed in the ceramics with x = 0.30‐0.33. The addition of CuO can significantly enhance the density of the BiFeO₃‐BaTiO₃ material. Importantly, an enhanced piezoelectric constant (d₃₃=165 pC/N), a large electric‐field‐induced strain (?S = 0.54%: peak to peak strain) and a large piezoelectric actuator constant (d₃₃*=449 pm/V) together with a high Curie temperature (T_C) of 503°C were observed in the ceramics with x = 0.30 and y = 5. As a result, the enhanced piezoelectricity and large electric‐field‐induced strain could significantly stimulate further researches in BFO‐based ceramics.     

Enhanced pyroelectric performance in potassium sodium niobate-based ceramics via multisymmetries coexistence design

Achieving excellent pyroelectric performance remains a challenge for lead-free piezoelectric ceramics. To meet the requirements of both an enhanced pyroelectric coefficient at room temperature and good thermal stability during the encapsulation of pyroelectric devices, (1–x)K_0.48Na_0.52NbO₃–xBi_0.5Ag_0.5ZrO₃–0.2%Fe₂O₃ (KNN–BAZ–Fe) lead-free ferroelectric ceramics with high Curie temperatures were prepared to obtain improved pyroelectric performance via the coexistence of multiple symmetries. The variation of BAZ content led to the formation of rhombohedral–orthorhombic–tetragonal phase boundary and promoted grain growth, resulting in the best pyroelectric coefficient (p = 5.09 × 10⁻⁴ C m⁻²°C⁻¹) and enhanced figures of merit (F_i = 0.2084 × 10⁻⁹ (m V⁻¹), F_v = 0.0142 m² C⁻¹, F_d = 0.0947 × 10⁻⁴ Pa^−1/2, and F_e = 17.66 J m⁻³ K⁻²) for infrared (IR) detection when x = 0.05. The room-temperature pyroelectric coefficient obtained in this study is approximately four times that of the pure KNN ceramic and is the maximum value reported for niobate-based piezoelectric ceramics. Moreover, compared with the poor thermal stability of barium titanate- and bismuth sodium titanate-based ceramics because of their ultralow Curie temperature or thermal depolarization temperature, the ceramics investigated here exhibit much better thermal stability because of their high Curie temperature (T_C > 300°C) and diffused phase-transition behavior, making them more adaptable for practical applications. These results suggest that KNN–xBAZ–Fe ceramics are attractive candidates for applications in the field of IR sensors.     

Diffused and successive phase transitions of (K,Na)NbO3-based ceramics with high strain and temperature insensitivity

Phase boundaries realize enhanced piezoelectricity in lead-free (K, Na)NbO₃-based ceramics but suffer from the weakness of undesirable temperature sensitivity. Here, an effective method is designed to develop temperature-insensitive piezoelectricity (small signal piezo-coefficient [d₃₃] and large signal piezo-coefficient []) in KNN-based piezoceramics by constructing the diffused and successive phase transitions, which results in a broadness of the optimal temperature range of the electrical properties. The room-temperature value in KNN-based ceramics modified with BaZrO₃ and (Bi_0.5Na_0.5)HfO₃ reaches as high as 540 (±10) pm V⁻¹, which is higher than PZT-5H and most reported KNN-based systems. Notably, superior temperature insensitivity of the and P_r values is also observed among the diffused and successive phase transitions region (20-100°C), with <5% fluctuation. In addition, the in situ temperature-dependent d₃₃ measurement shows a high-temperature reliability and less fluctuation (<15%) in a wide temperature range (20-120°C). These results open a new window for further development of highly temperature-insensitive lead-free piezoceramics.     

Low-temperature sintering of (K,Na)NbO3-based lead-free piezoceramics with addition of LiF

Dense Li/Ta-codoped KNN-based piezoceramics with d₃₃^* up to 375 pm/V were successfully fabricated by conventional sintering at a temperature as low as 900 °C by using LiF as a sintering additive. The reduction of densification temperature up to 200 °C was realized by a transient liquid phase sintering mechanism, consequently no grain boundary phase was observed in the sintered samples. It was found that the addition of LiF could further shift down the tetragonal–orthorhombic transition point (T_T-O), indicating that a small amount of Li⁺ could diffuse into the A-site of KNN matrix. The introduction of LiF enhanced the linearity of strain curves of the ceramics, which is unambiguously in favor for the actuator application. The present work reveals that low-temperature sintered LiF-doped KNN-based piezoceramics demonstrates promising potential in multilayer-structured actuator applications.     

Relaxor behavior of potassium sodium niobate ceramics by domain evolution

Relaxor behavior is proved to be responsible for high piezoelectricity in piezoelectric materials due to the promoted polarization rotation, and an ultrahigh piezoelectricity can be also induced in potassium sodium niobate [(K,Na)NbO₃, KNN]-based ceramics by dielectric relaxation at the multiphase coexistence region. However, it is still absent of the association study between domain evolution and relaxor behavior created by nanoscale multiphase coexistence. Herein, the frequency-dependent domain response, dwell-time dependence of domain radius and voltage-dependent domain switching are characterized in KNN-based ceramics with relaxor R–T phase boundary. These novel domain evolutions further illustrate the contribution of relaxor behavior on high piezoelectricity, yielding to easy polarization rotation based on low energy barrier of polar nanoregions merging into long-range ordering states. And an improved temperature stability is observed at 30?60 °C for relaxor R–T due to the unusual domain evolution. This study affords a new perspective in the mechanism of high-performance lead-free piezoelectrics.     

Tailoring the chemical heterogeneity of Mn-modified 0.75BiFeO3-0.25BaTiO3 ceramics for piezoelectric sensor applications

The Mn-modified 0.75BiFeO₃-0.25BaTiO₃ (75BFBTMn) piezoelectric ceramic possesses a high depolarization temperature of 500 °C and a large piezoelectric coefficient of 110 pC/N, showing the potential for high temperature piezoelectric sensors. However, 75BFBTMn ceramic usually suffers dielectric degradation and abrupt drop of piezoelectric coefficient in the range of 300 °C to 500 °C. Combined the high-energy synchrotron X-ray diffraction analysis with Backscatter-SEM results, it is demonstrated that the electrical thermal instability is owing to the existence of chemical inhomogeneity. The Air-annealing treatment is able to decrease the volume fraction of pseudo-cubic phase and the lattice distortion, removes the chemical inhomogeneity in the grain and free Bi₂O₃ at grain boundary, and then eliminates dielectric anomalies and piezoelectric degradation with temperature. These results indicate that air-annealing is a simple but effective method to eliminate the chemical inhomogeneity in 75BFBTMn ceramics, thereby improving the property thermal stability for high temperature piezoelectric sensor applications.     

Attaining excellent piezoelectric properties and thermal stability in PIN-PHT ceramics by integrating tetragonal phase and relaxor ferroelectrics

Herein, Nd₂O₃-doped 0.11PIN-0.89PHT (PIN-PHT) single-phase tetragonal piezoelectric ceramics are prepared by traditional solid-phase method. In addition, impacts of Nd-doping on crystal structure and electrical performance for 0.11PIN-0.89PHT ceramics are systematically investigated. Based on Landau theory, we propose a novel strategy for obtaining high-performance ceramics by combining tetragonal phase and relaxor ferroelectrics. Results reveal that the introduction of polar nano-regions in tetragonal phase ceramics by doping with rare-earth ions to convert normal ferroelectrics into relaxed ferroelectrics is responsible for excellent properties of 0.11PIN-0.89PHT-xNd ceramics. The optimized comprehensive performance is obtained at x = 0.9 mol%, where d₃₃ = 670 pC/N, S_max = 0.29% (45 kV/cm), strain hysteresis = 8.68% (45 kV/cm), d₃₃* = 736 p.m./V (30 kV/cm), T_C = 312.6 °C, ε_r = 3234, k_p = 0.62, tanδ = 0.014, and excellent high-temperature stability in temperature range of 20–240 °C. After 10⁶ cycles, electrical properties and strain remain unchanged, showing excellent anti-fatigue behavior. This work provides a novel approach for the development of ceramics with outstanding piezoelectric response, high strain, low strain hysteresis, excellent anti-fatigue resistance and thermal stability, and is expected to realize practical applications of piezoelectric ceramics.     

Simultaneous realization of high transparency and piezoelectricity in low symmetry KNN-based ceramics

Transparent piezoelectric ceramic, as a lead-free multifunctional ceramic, is in dire need of development for future high-tech industries. However, excellent piezoelectricity and high transmittance are usually hard to achieve simultaneously, mainly due to the two mutual restricting factors (phase structure and grain size). In this work, we report that high piezoelectricity and transmittance can be obtained simultaneously in K_0.5Na_0.5NbO₃ ceramics via Sr(Sc_0.5Nb_0.5)O₃ (SSN) modification. The superior piezoelectric performance comes from the retain of orthorhombic phase structure at room temperature (RT); while the high transparency (>70% at 780 nm) can be attributed to the improved relative density and reduced grain size via SSN modification. Remarkably, in the sample with 0.05SSN modification, we realized a comprehensively high transmittance (73% at 780 nm) accompanied by a superior piezoelectric constant (d₃₃ = 101 pC/N), which outperform other reported KNN-based transparent ceramics to our best knowledge. Our results may provide insight for further developing the transparent piezoelectric ceramics by controlling the grain size and phase structure.     

Design on improving piezoelectric strain and temperature stability of KNN-based ceramics

Lead-free 0.99(0.96K_0.46Na_0.54Nb_1-xTa_xO₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃)-0.01CaZrO₃ (0.99(0.96KNNTax-0.04BNZ)-0.01CZ) ceramics were prepared by a solid-state sintering method. Ta₂O₅ doped in the 0.99(0.96KNNTax-0.04BNZ)-0.01CZ ceramics results in a phase structure transition from the orthorhombic (O)/tetragonal (T) phase to the rhombohedral (R)/T phase. The Ta₂O₅ dopant induces a decrease in the average grain size from ~1.70 to ~0.69 μm. At x = 0.02 and 0.04, the ceramics have a high reverse piezoelectric coefficient (~500 pm/V under 25 kV/cm). The ceramics with x = 0.04 show an optimal level of unipolar strain, reaching 0.17% under 35 kV/cm at room temperature, and their field-induced strain varies <10% in the temperature range from 25 to 135°C. The presence of the O phase in the polymorphic phase boundary (PPB) improves the temperature stability the reverse piezoelectric coefficient (). Obtaining KNN-based ceramics with good piezoelectric properties and weak temperature sensitivity by designing a R/O/T phase boundary and controlling the average grain size to the submicrometer level is highly feasible.