KNN-based Ceramics Research Articles

Improved energy-harvesting performance of (K, Na)NbO3-based ceramics through the synergistic effect of texture and defect engineering

Textured CuO-doped KNN-based ceramics with [001]C orientation were developed to enhance energy harvesting performance. Textured ceramics doped with 0.3 mol% CuO exhibit remarkable piezoelectric coefficients d33 of 347 pC/N and g33 of 96 × 10−3 Vm/N, resulting in a higher figure of merit (d33 × g33 ∼ 33 × 10−12 m2/N) for evaluating energy conversion. The d33 and g33 increased by 105 % and 269 %, respectively, compared to those of randomly oriented ceramics. The piezoelectric energy harvester fabricated using the textured ceramic achieved an output power density of 3 μW/mm3, comparable to PZT-based energy harvesters. The enhancement in d33 is mainly due to favorable [00 l]C orientation. The orientation, along with the hardening effect caused by CuO doping, contributes to decreased dielectric coefficient εTr3, resulting in increased g33. The synergistic strategy is expected to facilitate the design high-performing, eco-friendly piezoelectric ceramics.

Optimized energy storage performances via high-entropy design in KNN-based relaxor ferroelectric ceramics

Dielectric capacitors play an irreplaceable role in complex and integrated electronic systems. However, attaining ultrahigh recoverable energy storage density (Wrec) alongside energy storage efficiency (η) poses a formidable challenge, impeding the advance towards the miniaturization and integration of cutting-edge energy storage devices. In this work, a high-entropy strategy with a viscous polymer process is adopted, bringing about ultrafine grains and polar nanoregions (PNRs) accompanied by enhanced electrical homogeneity and polarization relaxation characteristics. As a result, an ultrahigh Wrec ∼ 7.51 J/cm3 is achieved in a high-entropy KNN-based energy storage ceramic with a high η ∼ 88.4% at 750 kV/cm. Moreover, the ceramic capacitor exhibits a colossal power density reaching approximately 420 MW/cm3 and a discharge energy density of approximately 4.85 J/cm3 at 160 ℃. Impressively, it maintains low variation rates of less than 4% across a broad temperature range from 20 ℃ to 160 ℃. The new approach opens avenues for the design and development of superior dielectric materials used in next-generation high-power pulse devices.

The impact of chemical inhomogeneity in calcinated powders on grain growth behavior and piezoelectric properties of lead-free KNN-based ceramic

Despite outstanding electromechanical properties are achieved in KNN-based ceramics, unstable reproducibility of piezoelectric properties remains a challenge to industrial production, owing to a lack of in-depth understanding of grain growth behavior and chemical homogenization throughout the overall solid-state synthesis process. Herein, a significant chemical heterogeneity behavior was found in calcinated 0.957K0.48Na0.52Nb0.96Sb0.04O3-0.04Bi0.5Na0.5ZrO3-0.003CaZrO3 powders owing to non-uniform diffusion and competitive chemical reaction among multicomponent reactants. The chemical heterogeneity is significantly alleviated by modulating calcination temperatures and ball-milling the calcinated powders. Furthermore, it was demonstrated that the heterogeneous calcinated powder with different particle size distribution plays a significant role in grain growth behavior and chemical homogenization of bulk ceramics. Finally, we obtained uniform grain size distribution and preferable chemical homogeneity in ceramics with the calcination of 900 oC and undergone twice ball milling, which exhibit superior piezoelectric properties (d33=368 pC/N, kp=0.52). This work provides an effective paradigm to ameliorate the inferior reproducibility of KNN-based ceramics by effectively regulating the processing condition during solid-state synthesis.

High performance (K,Na)NbO3-based textured ceramics for piezoelectric energy harvesting

Piezoelectric energy harvesters (PEHs), which can provide sustainable green energy for low-power electronics have received extensive attention in the viewpoint of science and industrialization. In order to achieve large output power of PEHs, high-performance piezoelectric materials are extremely desired. Herein, excellent electromechanical conversion properties of d33 ∼ 447 pC/N, g33 ∼ 30 × 10-3 Vm/N, k33 ∼ 47% and Suni ∼ 0.18% were achieved in <001> grain aligned KNN-based lead-free ceramics with a favorable rhombohedral (R), orthorhombic (O) and tetragonal (T) multiphase structure. The superior electromechanical conversion properties can be attributed to grain orientation and crystal symmetry controlled anisotropy, and multiphase coexistence significantly decreased poling energy barrier, as well as the flexible response to external stimuli of the nanometer domains. The PEHs fabricated using the optimal performance KNN-based textured ceramics exhibit high output power of 0.46 mW and power density of 6.7 mW/cm3 at a resonant frequency of 41 Hz, which is associated with the large k33 (∼ 47%) and figure-of-merit (d33×g33 ∼ 13.4×10-12 m2/N) values. The output power can actually light up more than 145 commercial LEDs, demonstrating great potential of KNN-based PEHs to power wireless sensors.

Improved transparency and conferred luminescence property via Er3+/Ho3+ co-doped in KNN-based ceramics

Improved transparency and conferred luminescence property via Er3+/Ho3+ co-doped in KNN-based ceramics

High Piezoelectric Performance of KNN-Based Ceramics over a Broad Temperature Range through Crystal Orientation and Multilayer Engineering.

Uninterrupted breakthroughs in the room temperature piezoelectric properties of KNN-based piezoceramics have been witnessed over the past two decades; however, poor temperature stability presents a major challenge for KNN-based piezoelectric ceramics in their effort to replace their lead-based counterparts. Herein, to enhance temperature stability in KNN-based ceramics while preserving the high piezoelectric response, multilayer composite ceramics were fabricated using textured thick films with distinct polymorphic phase transition temperatures. The results demonstrated that the composite ceramics exhibited both outstanding piezoelectric performance (d33~467 ± 16 pC/N; S~0.17% at 40 kV/cm) and excellent temperature stability with d33 and strain variations of 9.1% and 2.9%, respectively, over a broad temperature range of 25-180 °C. This superior piezoelectric temperature stability is attributed to the inter-inhibitive piezoelectric fluctuations between the component layers, the diffused phase transition, and the stable phase structure with a rising temperature, as well as the permanent contribution of crystal orientation to piezoelectric performance over the studied temperature range. This novel strategy, which addresses the piezoelectric and strain temperature sensitivity while maintaining high performance, is well-positioned to advance the commercial application of KNN-based lead-free piezoelectric ceramics.

Simultaneous Improvement of Energy Storage Characteristics and Temperature Stability in K0.5Na0.5NbO3-Based Ceramics via LiF Modification.

The splendid energy storage performances with eminent stability of dielectric ceramics utilized in pulsed power devices have been paid more attention by researchers. This scheme can be basically realized through introducing Li+, Bi(Mg2/3Ta1/3)O3, NaNbO3, and LiF into KNN-based ceramics. Under the breakdown strength (BDS) of 460 kV/cm, an outstanding energy storage density (W) of 6.05 J/cm3 with a high energy efficiency (η) of 85.9% is implemented. Within the broad temperature range from 20 to 140 °C, the numerical fluctuations of energy storage characteristics can be maintained at a relatively stable level (ΔWrec ≈ 3.5%, Δη ≈ 2.8%). As for the charging-discharging performances, this component possesses a fast discharging speed (t0.90 ≈ 51 ns) and remarkable temperature stability (the variations are smaller than 3.5%). Additionally, the internal mechanisms of outstanding energy storage properties can be confirmed via crystal structures and domain structures, the content of oxygen vacancies, dielectric and impedance spectra, and phase simulation. Hence, the combination of outstanding energy storage with remarkable thermal stability can be fulfilled in one ceramic system according to this discovery, providing a research thought of developing the materials for dielectric capacitors.

Enhanced piezoelectric properties of KNN-based ceramics by synergistic modulation of phase constitution, grain size and domain configurations

Despite significant advancements in KNN-based piezoelectric ceramics, controlling their grain size remains a challenge, impacting reproducibility of piezoelectric properties. Here, we address this issue by initially investigating the defect chemistry of a ternary system 0.96 K0.48Na0.52Nb0.96Sb0.04O3-0.04Bi0.5Na0.5ZrO3-ABO3 (KNNS-BNZ-ABO3). The ABO3 dopant not only influences the phase constitution, but also induces different charge compensation mechanisms, affecting the grain size and domain configurations of ceramics. Furthermore, the relationship between the grain size and the piezoelectric properties is investigated. The superior piezoelectric charge coefficient (d33) of 435 pC/N, ultra-high planar mode electromechanical coupling factor (kp) of 0.62 and preferable temperature stability are obtained in KNN-based ceramics with large grain size (∼50 μm) and hierarchical domain structures. Finally, we elucidated the intrinsic and extrinsic contribution of the grain size on piezoelectric properties of KNN-based piezoceramics based on Rayleigh analysis. Our work provides an effective paradigm for the development of lead-free piezoelectric ceramics for industrial applications.

KNN composite ceramics with superior pyroelectric performance for self-powered thermal detector

The thermal detector based on pyroelectric effect has attracted widespread attention due to its self-powering and rapid response. Although phase boundary engineered lead-free potassium sodium niobate [K0.5Na0.5NbO3, KNN] based ferroelectrics exhibit promising for pyroelectric sensor application, the high dielectric constants (εr) of these materials significantly constrain their pyroelectric figures of merit (FOMs), owing to the inverse correlation between FOMs and εr. To address this problem, we innovatively introduce perovskite bismuth sodium titanate [Bi0.5Na0.5TiO3, BNT] with low εr and high pyroelectric coefficient as the second phase to construct a KNN/BNT composite. The introduction of BNT can not only remain the high pyroelectric coefficient (p=7.12 × 10−4 C m−2 K−1) of KNN matrix, but also result in superior FOMs (Fv increased by 74 %), outperforming previous KNN-based pyroelectric ceramics. The excellent pyroelectric performance originates from the unique composite microstructure with compositional heterogeneity. That is, both the low εr BNT enriched phase and refined grain size as well as the promoted vibration of Nb–O octahedron contribute to the superior pyroelectric performance. A self-powered thermal detector with great thermal response has been developed, which also exhibits potential for detecting ultraviolet energy and infrared radiation.

High energy storage performance of KNN-based relaxor ferroelectrics in multiphase-coexisted superparaelectric state

Although K0.5Na0.5NbO3 (KNN) possesses large maximum polarization and relatively high breakdown strength, the large remnant polarization constrains their practical applications as energy storage materials. In this work, through multi-element doping, (K0.5−0.5xNa0.5−0.5xBix)(Nb1−xSn0.2xZn0.6xTa0.2x)O3 relaxor ferroelectrics were prepared. As the superparaelectric states (SPE) were adjusted to room temperature, orthorhombic, tetragonal, and cubic phases coexisted, accompanied by the highly dynamic polar nanoregions (PNRs). In particular, a high recoverable energy storage density of 4.5 J/cm3 and an energy storage efficiency of 83% were achieved for the x = 0.125 ceramic, with the variations less than 11% and 4%, respectively, in the wide temperature range of 20–180 °C. These results demonstrate that the multiphase PNRs in the SPE state is an effective strategy for improving the energy storage performance of KNN-based ceramics.

Defect engineering by annealing: Managing the trade-off relationship between photochromic and electrical properties in KNN-based translucent ceramics

Rare-earth-doped transparent ferroelectrics exhibit outstanding optical transparency, photoluminescence (PL), photochromic (PC) and various electrical properties, showing potential for use in optoelectronic applications. However, there is an inevitable trade-off relationship between optical and electrical coupling during the integration of multifunctionality. In this work, Ba2+, Ho3+ and Yb3+ were codoped in KNN translucent-ferroelectric ceramics (as a model material) to overcome the contradiction between optical (transparency/PC) and electrical properties (resistivity/ferroelectricity). The defects (mainly oxygen vacancies) within the ceramics can be effectively regulated through annealing under different atmospheres. At the cost of a slight decrease in PC performance, the electrical properties significantly improved; i.e., for the optimal ceramic annealed in an oxygen atmosphere, both the maximum and remnant polarizations nearly doubled, and the resistivity significantly decreased compared to that of the original sample. A mechanism for regulating these properties was proposed. This research serves as a valuable example for adjusting the optical and electrical properties of other ferroelectrics through annealing.

Thermal and electromechanical performance in BaSnO3-modified potassium sodium niobate piezoceramics via two-step sintering

In view of industrial applications requiring high piezoelectric activity, KNN-based ceramics face two key constraints: insufficient density and temperature sensitivity. To address these challenges, this study first introduces BaSnO3 and employs two-step sintering method (TSS) for fabrication, thereby synthesizing ternary 0.975K0.5Na0.5NbO3-0.025Bi0.5K0.5TiO3-xBaSnO3 (KNN-1000xBaSn; x = 0, 0.3 %, 0.6 % and 0.9 %) with superior piezoelectricity (d33 = 217–257 pC/N; d33* = 265–330 pm/V; S = 0.16–0.21 %), high Curie temperature (TC = 325–349 °C) and densification (4.3–4.5 g/cm3; relatively density ∼96 %). In one respect, the TSS allows minimal volatilization of alkali metals while promoting grain homogenization. In another respect, the introduction of BaSnO3 enhances the mobility of domain walls, meanwhile fosters diffuse phase transitions (DPT) and widens dielectric peaks without significantly impairing TC. Moreover, the KNN–6BaSn obtain not only reinforced d33 (256 pC/N) with high TC (325 °C) and ε′ (1016), but also fatigue resistance and extreme-temperature endurance with excellent d33* (445 pm/V at 90 °C; 300 pm/V at 180 °C). This work provides a resultful dual-strategy for the emergence of lead-free piezoelectric products.

Adaptive ferroelectric states in KNN-based piezoceramics: Unveiling the mechanism of enhancing piezoelectric properties through multiple phase boundary engineering

Developing high-performance lead-free piezoceramics, such as (K,Na)NbO3 (KNN) material, is critical to achieving environmental sustainability in next-generation electromechanical devices. Although substantial advancements have been made in KNN-based ceramics, a general coherent framework is lacking that links microscopic structure to the enhancement of macroscopic properties. Our findings indicate that the enhanced performance of KNN-based ceramics in multiphase coexistence boundaries can be attributed to the formation of self-adjusting nanodomains in response to strong local structural heterogeneity. The self-adjusting behavior of domain structure is an outcome of minimizing the local stress generated by the lattice mismatch within KNN-based ceramics, which conforms to the thermodynamic principles that favor minimizing total free energy. Guided by this strategy, the present work achieves a significant improvement of electromechanical properties, including a piezoelectric coefficient (d33) of ∼585 pC N−1, an electromechanical coupling factor (kp) of ∼62 %, and a figure of merit (d33 × g33) of ∼12.78 ×10−12 m2 N−1. This study offers a new strategy for developing lead-free piezoceramics through dedicated design of novel phase boundaries.

Effect of different rare-earth dopings of KNN-based transparent energy storage ceramics

Rare-earth elements [Formula: see text]-, [Formula: see text]-, [Formula: see text]- and [Formula: see text]-doped ([Formula: see text][Formula: see text])[Formula: see text][Formula: see text][Formula: see text][Formula: see text]O3 ceramics (abbreviated as KNLNB-0.1%RE) were prepared by conventional solid-phase sintering method. The structure, transparency, energy storage and photoluminescence properties of the samples are investigated. All ceramics have the pseudo-cubic phase structure without the impurity phase at room temperature. KNLNB-0.1%RE ceramics exhibit excellent optical transmittance, with KNLNB-0.1%Ho achieving 71.8% transmittance in the visible wavelength range (780[Formula: see text]nm) and largest effective energy storage density of 1.45[Formula: see text]J/cm3. In our experiments, rare-earth-doped KNLNB ceramics exhibit photoluminescence effects. This work facilitates the development of transparent energy storage ceramics with fluorescent effects.

Optimizing electromechanical properties of KNN-based piezoelectric ceramics through regulation of lattice distortion

With the rapid development of broadband transducers, there is an increasing demand for enhanced electromechanical properties in lead-free piezoelectric ceramics. In this study, the strategies to enhance the electrical properties of ceramics were explored by doping BaZrO3 into 0.96(K0.48Na0.52)Nb0.96Sb0.04O3-0.04(Bi0.5Ag0.5)ZrO3 ceramics, inducing lattice distortion (c/a) and regulating the phase structure. The ceramics doped with 1.5 mol% BaZrO3 demonstrate excellent electrical performance, with a piezoelectric coefficient (d33) of 545 pC/N, a relative dielectric constant (εr) of 4126, and an electromechanical coupling factor (kp) of 54.1 %. The reduced c/a ratio and rhombohedral-orthorhombic-tetragonal phase structure reduce the polarization energy barrier and promote the ordered arrangement of ferroelectric dipoles, thereby enhancing the comprehensive performance of the ceramics. Lastly, the electrical impedance of ceramics is studied in detail and reveals the main conduction mechanism in the microscopic region. This research provides new insights into optimizing the electrical properties of ceramics through microstructural manipulation and demonstrates the potential application of KNN-based ceramics in high performance piezoelectric transducers.

Superior energy storage properties with prominent thermal stability in lead-free KNN-based ceramics through multi-component optimization strategy

The advancement of high energy storage properties and outstanding temperature stability ceramics plays a decisive role in the field of pulsed power systems. The multi-component optimization strategy is conducted by introducing Li+, Bi(Ni1/2Zr1/2)O3 and NaNbO3 into KNN-based ceramics. An excellent energy storage (W) of 7.82 J/cm3 along with a large efficiency (η) of 81.8 % is achieved at the breakdown strength (BDS) of 500 kV/cm for the ceramics. Simultaneously, the remarkable energy storage thermal stability (ΔWrec: ∼ 2.9 %, Δη: ∼ 3.9 %) is acquired in the range from 20 ℃ to 100 ℃. The splendid charging-discharging performances (discharge energy density Wd = 1.78 J/cm3, current density CD = 1376 A/cm2, power density PD = 124 MW/cm3) are also realized in the ceramics after doping. By investigating the evolution of crystal structure and domain structure, complex impedance and first-principle calculations, the internal mechanism of obtaining superior energy storage properties is analyzed. Thus, this research proves that the ceramics can effectively broaden the temperature range of the applications for the pulsed power systems while maintaining high energy storage properties, which offers a valuable approach for the improvement of dielectric capacitors.

Contribution of (Bi0.45Y0.05)Na0.5ZrO3 to induced multiphase coexistence and enhanced piezoelectric properties of K0.5Na0.5NbO3 lead-free ceramics

In this study, new lead-free piezoelectric ceramics (1 − x)(K0.5Na0.5)NbO3 (KNN)–x(Bi0.45Y0.05)Na0.5ZrO3 (BYNZ) were prepared using the conventional solid-state method, and the relation between the doping amount of (Bi0.45Y0.05)Na0.5ZrO3 and its structural and electrical properties was investigated. Material characterization and structural analyses conducted using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy revealed that KNN–BYNZ ceramics formed single perovskite structures at 0 ≤ x ≤ 0.05 and a polycrystalline phase boundary (PPB) for the coexisting orthorhombic and tetragonal phases at 0.03 ≤ x ≤ 0.05. The ceramics exhibited desirable electrical properties at x = 0.04 (d33 = 305 pC/N, kp = 45.8 %, εr = 1316, tanδ = 2.4 %, Tc = 332 °C, Pr = 28.2 μC/cm2, and Ec = 1.07 kV/mm). These findings reveal that the introduction of BYNZ enables the effective formation of a PPB in the KNN-based ceramics to improve their electrical properties, and they exhibit potential for replacing lead-based piezoelectric ceramics in the market.

Fabrication and physical properties of Na(Nb1-xSbx)O3 templates for the [001]-texturing of KNN-based ceramics

Na(Nb1-xSbx)O3 [N(N1-xSx)] templates were fabricated to texture the 0.96(Na0.5K0.5)(Nb1-ySby)O3-0.01BaZrO3-0.03(Bi0.5Ag0.5)ZrO3 [KN(N1-ySy)-BZ-BAZ] piezoceramics along the [001] direction. To the best of our knowledge, this is the first study that fabricated these templates. Thus far, NaNbO3 (NN) templates have been utilized to texture KNN-modified piezoceramics; however, they produce many holes in the piezoceramics because of the dissolution of the NN templates during sintering. N(N0.9S0.1) templates with an average size of ∼18 μm were produced by the molten-salt and topochemical methods utilizing Bi2.5Na3.5(Nb0.9Sb0.1)5O18 precursors. Due to their high melting temperature, the N(N0.9S0.1) templates did not dissolve during sintering. Therefore, no holes were present in the KN(N0.95S0.05)-BZ-BAZ piezoceramic textured using the N(N0.9S0.1) templates. The piezoceramic textured using the N(N0.9S0.1) templates showed a large hardness (7.56 ± 0.26 GPa) and an elastic modulus (154 ± 5 GPa) similar to those of the untextured piezoceramic. However, the piezoceramic textured using the NN templates showed reduced hardness (5.94 ± 0.58 GPa) and elastic modulus (127 ± 14 GPa) because of the presence of the holes. The KN(N0.95S0.05)-BZ-BAZ piezoceramics textured using the N(N0.9S0.1) templates provided large d33 (740 pC/N) and kp (0.62) values. Cantilever-type piezoelectric energy harvesters (CPEHs) were produced using the KN(N0.95S0.05)-BZ-BAZ piezoceramic textured with N(N1-xSx) templates and the CPEH produced using the piezoceramic textured by N(N0.9S0.1) templates generated the largest power of 2.92 mW. Therefore, N(N0.9S0.1) is a promising template for the [001] texturing of KNN-based piezoceramics with good mechanical properties.

Superior energy-storage density and ultrahigh efficiency in KNN-based ferroelectric ceramics via high-entropy design

The rapidly advancing energy storage performance of dielectric ceramics capacitors have garnered significant interest for applications in fast charge/discharge and high-power electronic techniques. Simultaneously improving the recoverable energy storage density Wrec and efficiency η becomes more prominent at the present time for their practical applications. Herein, a high-entropy concept is implemented on the (K0·5Na0.5)NbO3 (KNN)-based ferroelectric ceramics to design the high-performance dielectric capacitors. First, the strong lattice distortion can absorb some electric energy during the electrical loading process and result in the delayed polarization saturation. Additionally, the large composition fluctuations induce the weak correlation between polar nanoregions and enhance the η. Finally, the high-entropy design and viscous polymer processing method reduce the grain size and improve the Eb. In consequence, excellent Wrec of 11.14 J/cm3 with high η of 87.1% are achieved under an electric field of 750 kV/cm in the high-entropy component. These results demonstrate that the high-entropy concept is a potential avenue to design the KNN-based high-performance dielectric energy storage capacitors.

Mechanism and application of lead-free KNN-based ceramics with superior piezoelectricity

Piezoelectric ultrasonic energy harvesters (PUEHs) have recently attracted increasing attention. For minimizing the energy losses during energy transfer, it is crucial for core piezoelectric materials to maintain high piezoelectric coefficient (d33), mechanical quality factor (Qm), and piezoelectric voltage coefficient (g33). Herein, a simple way of hardening strategy improves the piezoelectric properties of KNN-based ceramics, combined with optimizing grain growth. The enhanced d33 (∼340 pC/N), high Qm (∼211) and TC (∼305 °C) are obtained simultaneously in this system, accompanied by superior d33×g33=8770×10−15 m2/N. Through variable temperature resonance test, the polarization rotation diversity caused by phase structure difference is found to closely related to the temperature stability of Qm value. A prototype of PUEH device was further designed and fabricated, with adjustable ultrasound-induced output voltage up to 8.2 V. Our work exhibits an effective way to improve performance of PUEHs by optimizing piezoelectric materials, which is helpful for further developing PUEHs.