Second-order-transition like characteristic contributes to strain temperature stability in (K,Na)NbO3-based materials

High strain and good temperature stability are contradictory properties in (K, Na)NbO₃ (KNN)-based materials. Herein, good temperature stability with high strain is obtained in a multiphase coexistent [ie, orthorhombic-tetragonal (O-T) and rhombohedral-orthorhombic-tetragonal (R-O-T)] KNN. A second-order transition-like characteristic should contribute to the temperature stability, in which an intrinsic lattice structure forms a bridge between them. The observed second-order transition-like characteristic is due to the reduced discrepancy among different lattice symmetries and a broadened temperature region for the phase transition. These integrated factors can slow the latent heat in a first-order transition and extend it over a wide temperature region, thereby exhibiting second-order transition-like behavior. Correspondingly, the abrupt increase in strain near the phase transition temperature significantly slows. In addition, the appearance of pure tetragonal symmetry (P4mm) is deferred to a much higher temperature than T_O-T, in which the strain will inevitably decrease. As a result, good temperature stability with a high strain response can be realized in multiphase coexistent KNN materials, including d₃₃*=448 pm/V, -27.5%≤fluctuation≤4.2% for O-T, and d₃₃*=446 pm/V, -17.5%≤fluctuation≤7.6% for R-O-T, over the whole temperature range 25 °C-190 °C.     

Abnormal grain growth in (K,Na)NbO3-based lead-free piezoceramic powders

Abnormal grain growth (AGG) is frequently observed in sintered (K, Na)NbO₃ (KNN)-based piezoceramics. However, in the present study, abnormal grain growth was unexpectedly discovered in calcined KNN-based powders. To explain the phenomenon, three well-established models that account for the AGG in sintered ceramics were discussed, including (a) liquid-phase-assisted grain growth, (b) two-dimensional nucleation grain growth, and (c) complexion coexistence. However, the AGG in calcined powders was concluded to be none of them, but a consequence of the A-site compositional inhomogeneity in the K₂CO₃-Na₂CO₃-Nb₂O₅ ternary system. Since repeated calcination and ball milling have low efficiency on solving AGG and the accompanied compositional inhomogeneity, abnormal grains were found to coexist with normal grains at a very high calcination temperature, that is, 1000°C. The compositional inhomogeneity is believed to be remaining even after sintering and consequently deteriorate the comprehensive performances, which might be a determinant for the unstable reproduction of KNN-based piezoceramics.     

High Qm values and humidity effect on the electrical properties of (K,Na)NbO3 lead‐free piezoceramics doped with B2O3–CuO mixed oxides

To greatly enhance the mechanical quality factor (Q_m) of piezoceramic materials, B₂O₃–CuO mixed oxides were added to a K_0.48Na_0.52NbO₃‐based lead‐free piezoceramic (abbreviated as BC‐KNN). The results suggest that the B₂O₃–CuO additives effectively improved the sinterability and Q_m value of the piezoceramic. An optimal Q_m value as high as 2128 was obtained, which is 35 times higher than that of pure KNN ceramic. Interestingly, we found that the Q_m value was sensitive to humidity of the surrounding environment. As the relative humidity (RH) increased from 25% RH to 78% RH, the Q_m value of the BC‐KNN ceramics decreased from 2128 to 267. We found that the dependence of the Q_m value on humidity was closely related to the instability of the relative dielectric constant (?_r). Our results show that a dense microstructure is critical for maintaining a stable high Q_m performance in a humid environment.     

Composition segregation and grain growth in (K0.5Na0.5)NbO3 piezoceramics prepared by two-step cold sintering process

Cold sintering process (CSP) is a new method to prepare ceramics under quite low temperature. In this work, two-step CSP under different pressures was employed to prepare (K_0.5Na_0.5)NbO₃ (KNN) ceramics. The density of KNN green pellets can be raised by enhancing the pressure of second-step CSP. Energy-dispersive spectroscopy reveals the composition segregation of A-site cations in large grains. The dissolution rate of K⁺ in an aqueous medium is faster than Na⁺, and high pressure can accelerate K⁺ dissolution, resulting in more Na⁺ in some grains. Besides, the diffusion rate of Na⁺ in grains is better than K⁺, which promote the grains growth. Finally, the piezoelectric property is improved even with low ceramic density due to the larger grains, which possess the higher performance composition. This result demonstrates that the pressure and inhomogeneous dissolution of alkali metal ions among CSP play an important role in grain growth and piezoelectric enhancement.     

K0.5Na0.5NbO3 ceramics fabricated by combining cold sintering with annealing in air atmosphere or low pO2 atmosphere

Dense K_0.5Na_0.5NbO₃ 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 ε_r = 509, dielectric loss tan δ = 0.04, and remanent polarization P_r = 38.17 μC/cm². Additionally, the mixture of K_0.5Na_0.5NbO₃ 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 K_0.5Na_0.5NbO₃ 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.     

Ho3+-doped (K,Na)NbO3-based multifunctional transparent ceramics with superior optical temperature sensing performance

Ho³⁺-doped (K_0.5Na_0.5)NbO₃-based transparent ceramics have been prepared via pressureless solid-state method. The ceramics possess moderate optical transparency with the energy band gap of ~2.9 eV and submicron-sized grains (<500 nm). The temperature-dependent dielectric properties and ferroelectric polarization-electric field hysteresis loops demonstrate that the ceramics own relaxor-like characteristics. The up-conversion photoluminescence and optical temperature sensing properties of the ceramics have been investigated. The temperature dependence of photoluminescence provides a fluorescent method to detect phase transitions, which can be expanded to other ferroelectric systems. The outstanding optical temperature sensitivity (~0.0075/K at 430 K) of the ceramic is higher than many other rare-earth-doped ceramics or glasses. These results suggest that the Ho³⁺-doped (K_0.5Na_0.5)NbO₃-based transparent ceramics are promising lead-free transparent materials for multifunctional applications, especially in temperature sensing devices.     

Electrocaloric effect and pyroelectric performance in (K,Na)NbO3-based lead-free ceramics

The issue of how to achieve an electrocaloric effect (ECE) and pyroelectric effect in a material simultaneously remains to be a challenge for developing practical solid-state cooling devices and RF-detectors. Here, we structure a polymorphic phase transition (PPT) region by doping modification in KNN-based ceramics, which are developed to achieve the ECE. The direct measured ECE and pyroelectric properties are investigated in lead-free (1-x)K_0.5Na_0.5NbO₃-xBi_0.5Na_0.5ZrO₃ (KNN-xBNZ) ceramics. The adiabatic temperature change (∆T) of 0.22 K at 100°C, 0.14 K at 70°C and 0.16 K at 30°C can be obtained under an electric field of 35 kV cm^–1 for x = 0.03, 0.04 and 0.05, respectively. In addition, the temperature dependence of pyroelectric coefficient (p) is established for all compositions via the Byer-Roundy method. A large p of 454.46 × 10^–4 C m^–2 K^–1 is detected at Curie temperature (T_C) in the ceramics with x = 0.03. Achieving electrocaloric effect and pyroelectric performance simultaneously may shed light and provide a feasible design scheme for developing practically useful electrocaloric and pyroelectric materials.     

(K,Na) (Nb,Ta,Sb)O3粉体制备及陶瓷介电性能研究

Ta/Sb掺杂的K0.5Na0.5Nb0.7TaxSb0.3-xO3(KNNSTx,x＝0.06,0.09,0.12,0.15,0.18,0.21, 0.24)粉体经水热合成,Ta、Sb对粉体物相、微观结构的影响被系统研究,烧结后陶瓷的微观结构、介电性能表明:陶瓷物相均具有纯钙钛矿结构;随着Ta含量的增加陶瓷晶粒尺寸逐渐增大,x＝0.09、0.12时较小粒径颗粒均匀分布在大颗粒的空隙之间,陶瓷密度增加;样品的介电常数随着Ta含量的增加x＝0.06～0.12逐渐降低,x＝0.15～0.24逐渐增加,居里温度均在350 ℃左右,且x＝0.09、0.12时陶瓷介电损耗较小.     

Piezoelectric voltage constant and sensitivity enhancements through phase boundary structure control of lead-free (K,Na)NbO3-based ceramics

The piezoelectric voltage constant (g₃₃) is a material parameter critical to piezoelectric voltage-type sensors for detecting vibrations or strains. Here, we report a lead-free (K,Na)NbO₃ (KNN)-based piezoelectric accelerometer with voltage sensitivity enhanced by taking advantage of a high g₃₃. To achieve a high g₃₃, the magnitudes of piezoelectric charge constant d₃₃ and dielectric permittivity ε_r of KNN were best coupled by manipulating the intrinsic polymorphic phase boundaries effectively with the help of Bi-based perovskite oxide additives. For the KNN composition that derives benefit from the combination of ε_r and d₃₃, the value of g₃₃ was found to be 46.9 × 10^?3 V·m/N, which is significantly higher than those (20 – 30 × 10^?3 V·m/N) found in well-known polycrystalline lead-based ceramics including commercial Pb(Zr,Ti)O₃ (PZT). Finally, the accelerometer sensor prototype built using the modified KNN composition demonstrated higher voltage sensitivity (183 mV/g) when measuring vibrations, showing a 29% increase against the PZT-based sensor (142 mV/g).     

Electrical properties and temperature stability of SrTiO3-modified (Bi1/2Na1/2)TiO3-BaTiO3-(K1/2Na1/2)NbO3 piezoceramics

SrTiO₃-modified lead-free piezoelectric ceramics, (0.93-x)Bi_0.5Na_0.5TiO₃-xSrTiO₃-0.06BaTiO₃-0.01 K_0.5Na_0.5NbO₃ [(BNT-xST)-BT-KNN, x = 0-0.06], were prepared using a conventional solid-state reaction method. The XRD structure analysis and electric properties characteristics revealed the ST-induced phase transformation from the ferroelectric phase to the relaxor phase and their coexistence state. Benefiting from the ST-destructed ferroelectric long-range orders, the high normalized strain value of 600 pm/V was obtained in the (BNT-0.02ST)-BT-KNN ceramic at 5 kV/mm. The ST-generated relaxor phase was found to have a constructive effect on improving the temperature stability and restraining the hysteresis of the electric-field-induced strain. The normalized strain of (BNT-0.06ST)-BT-KNN ceramics could be kept at a high value ~337 pm/V at elevated temperature up to 120°C.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

(K0.5Na0.5)(TaxNb1-x)O3无铅压电陶瓷的性能特征

采用传统电子陶瓷制备工艺制备了(K0.5Na0.5)(TaxNb1-x)O3无铅压电陶瓷。研究了不同Ta含量下(K0.5Na0.5)(TaxNb1-x)O3陶瓷的晶相组成及性能特征。结果表明,(K0.5Na0.5)(TaxNb1-x)O3陶瓷在低Ta含量时形成单一斜方相固溶体,但Ta含量达到0.08mol后则有K6Ta10.8O30次晶相产生。随着Ta的加入,陶瓷的体积密度逐渐增大,居里温度(Tc)逐渐降低。当Ta含量为0.08mol时陶瓷具有良好的铁电、压电性能和介电稳定性能,其压电常数d33为76pC/N。     

水热法合成(K,Na)NbO_3陶瓷粉体及其压电性能研究

利用水热法在碱性溶液中合成了具有钙钛矿结构的(K,Na)NbO_3无铅压电陶瓷粉体,研究了初始溶液中K+的含量对产物晶相、形貌以及化学组成的影响。采用X射线衍射仪以及扫描电镜对产物的结构和形貌进行了表征,利用X荧光分析仪对粉体的化学组成进行了精确分析。实验结果表明:随着初始溶液中K~+含量的变化,有两相钙钛矿(K,Na)NbO_3粉体生成,在溶液中K~+的反应活性低于Na~+。然后,将水热合成的K_(0.58)Na_(0.42)NbO_3粉体采用传统固相烧结制备陶瓷材料,其结构较致密,压电常数d_(33)达到94 pC/N。     

The effect of pre-annealing on the microstructure of (K,Na)NbO3 ceramics

The effects of pre-annealing on the microstructure development and piezoelectric properties for 0.95(K_0.5Na_0.5)NbO₃–0.05LiSbO₃ (0.95KNN–0.05LS) ceramics were investigated. The pre-annealing suppressed the abnormal grain growth in both the undoped and Mn-doped 0.95KNN–0.05LS ceramics. The pre-annealed samples possessed smaller abnormal grains, larger matrix grains, and a broader grain size distribution compared to the samples sintered without a pre-annealing step. The pre-annealed samples presented better dielectric and piezoelectric properties, a larger dielectric constant (ε_r) and electromechanical coupling factor (k_p), and a smaller dielectric loss factor (tan δ).     

(K,Na)NbO3-based ceramics with excess alkali oxide for piezoelectric energy harvester

(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ (KNLNTS) ceramics were prepared by a solid-state reaction. The effect of excess (K,Na)₂O alkali oxide on the densification, phase evolution, microstructure development, and piezoelectric properties was investigated. The figure of merit (FOM) (d₃₃·g₃₃) for piezoelectric energy harvesting applications was also compared between the samples with and without excess alkali oxide. The addition of the excess alkali oxide changed the tetragonal crystal structure to orthorhombic and decreased the sintering temperature by about 100 °C. The dielectric constant of the orthorhombic phase is much lower than that of the tetragonal phase. The orthorhombic sample with excess alkali oxide sintered at 1020 °C demonstrated higher FOM in spite of having a smaller piezoelectric constant (d₃₃) than the stoichiometric sample sintered at 1100 °C. This result indicates that a KNN ceramic with an orthorhombic composition near the MPB with a moderate piezoelectric constant and smaller permittivity is more advantageous for an energy harvesting application than that with a morphotropic phase boundary (MPB) or a tetragonal composition.     

High-energy storage density in NaNbO3-modified (Bi0.5Na0.5)TiO3-BiAlO3-based lead-free ceramics under low electric field

Ceramic-based dielectric capacitor are highly suitable for pulsed power applications due to their high power density and excellent reliability. However, the ultrahigh applied electric field limit their applications in integrated electronic devices. In this work, (1−x){0.96(Bi_0.5Na_0.5)(Ti_0.995Mn_0.005)O₃-0.04BiAlO₃}-xNaNbO₃ (BNT-BA-xNN, x = 0, 0.04, 0.08, 0.12, and 0.16) ternary ceramics were designed to achieve excellent energy storage properties. It was found that the introduction of NaNbO₃ (NN) effectively increase the difference (ΔP) between P_max and P_r, resulting in an obvious enhancement of the energy storage properties. High recoverable energy storage density, responsivity, and power density, that is, W_rec = 2.01 J/cm³, ξ = W_rec/E = 130.69 J/(kV⋅m²), and P_D = 25.59 MW/cm³, accompanied with superior temperature stability were realized at x = 0.14 composition. In addition, the thermal stable dielectric properties of the sample can be prominently improved with increasing NN content. The temperature coefficient of capacitance (TCC) of x = 0.16 composition is lower than 15% over the temperature range from 49°C to 340°C, with a high dielectric permittivity of 1647 and a low dielectric loss (0.0107) at 150°C. All these features show that the BNT-BA-xNN ceramics are promising materials for energy storage application.     

Low-temperature sintering and electrical properties of (K,Na)NbO3 based lead-free ceramics with high Curie temperature

Lead-free ceramics (1 ? x)(K_0.48Na_0.52)NbO₃–(x/5.15)K_2.9Li_1.95Nb_5.15O_15.3 (x = 0.3–0.6, KNN–KLN100x) were prepared by conventional sintering technique at a low temperature of 960 °C. The effects of KLN contents on microstructure, dielectric, and piezoelectric properties were investigated. After the addition of KLN, the sintering performance and Curie temperature of the ceramics were markedly improved. The ceramics with x = 0.3 exhibited very good piezoelectric properties: d₃₃ = 138 pC/N, k_p = 45.03%, T_c = 495 °C, the dielectric constant at room temperature ?_r (RT) = 478 and the maximum dielectric constant ?_r (max) = 5067. These results indicated that the KNN–KLN100x lead-free ceramics sintered at low temperatures are promising for high temperature piezoelectric applications.     

Buffered template strategy for improving texture quality and piezoelectric properties of heterogeneous templated grain growth (K,Na)NbO3-based ceramics through interface engineering

Notwithstanding the advances in improving piezoelectric properties through templated grain growth, insights into the mechanical stress and microstructure of such materials are necessary. This is because the properties can be significantly varied depending on these parameters. Constructing heterostructure templates (²D–⁰D), in which the 0D nanoparticles have compositions similar to that of matrix powders, has significant potential in improving the above aspects. Here, BaTiO₃ (BT) templates and elements-doped (K,Na)NbO₃ (KNN) matrix powders were selected. Heterostructure BT (h-BT) templates were prepared by nucleation and growth of dopant-free KNN nanoparticles on bare BT. h-BT enabled a reduction in the mechanical stress at the interfaces within textured ceramics (h-BT-KNN), leading to larger grain growth and a higher texturing degree than those of textured ceramics (BT-KNN) fabricated using bare BT. The h-BT-KNN exhibited the enhanced piezoelectric constant (d₃₃) by ～170% and ～600% compared to those of the BT-KNN and nontextured ceramics, respectively.     

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.     

Investigations of domain switching and lattice strains in (Na,K)NbO3-based lead-free ceramics across orthorhombic-tetragonal phase boundary

Various strain contributions of (Na_0.52K_0.48 − x)(Nb_0.92 − xSb_0.08)O₃− xLiTaO₃ ceramics in the proximity of orthorhombic (O) and tetragonal (T) polymorphic phase boundary (PPB) were quantitatively resolved by means of synchrotron x-ray diffraction together with macroscopic strain measurements. Compared with O-rich compositions with a governing mechanism of intrinsic lattice strains, T-rich compositions exhibited a dominant strain mechanism from reversible domain switching. Quantitative analysis of diffraction data suggested that extrinsic strain contributions should depend on not only the lattice distortion δ, but also the poling texture Δf, phase content (for PPB compositions) and domain types. Smaller lattice distortion and higher poling texture tended to enhance the number of irreversible domain switching in O-rich compositions, thus leading to larger fraction of intrinsic lattice strain contribution. The calculated results demonstrated that the product of two parameters Δf and δ would give a reliable estimation of domain-switching strains for T-phase compositions but an overestimation for O-phase compositions.