KNN-based Ceramics Research Articles

Multidimensional synergy-induced high piezoelectricity and reliability KNN piezoceramics for high-frequency ultrasonic transducers

High-frequency ultrasonic transducers have been implemented as a powerful tool for modern medical diagnosis and therapy. Most current transducers use piezoelectric ceramics for electromechanical couplings, which will undergo periodic electro-acoustic transitions at high frequencies (≥20 MHz), thereby requiring high reliability. This work presents a structural engineering strategy to improve the piezoelectricity and reliability of potassium sodium niobate (KNN)-based ceramics with potential high-frequency ultrasonic imaging transducers. The KNN-based ceramics exhibit fatigue-free behavior with enhanced piezoelectricity (d33 ∼550 ± 20 pC N−1), and the mechanisms are discussed in view of multi-dimensions from macroscopy to microscopy. Considering the synergistic effect of multi-phase coexistence, well-kept microstructure, and flexible domain rotation, property-worsening cracks during fatigue are restrained; furthermore, the aggregation of space charges is hindered, thereby reducing the pinned domain and improving the fatigue resistance. Based on the high-performance piezoceramics, the fabricated ultrasonic transducer has high reliability with a steady sensitivity and an unchanged broad bandwidth (∼76%) from room temperature to 80°C. In addition, images of a tilapia eyeball are scanned to confirm the imaging capability of transducers. We expect that the novel approach to achieving high performance through the structural strategy promotes KNN-based ultrasonic devices for biomedical imaging.

Superior energy storage efficiency through tailoring relaxor behavior and band energy gap in KNN-based ferroelectric ceramic capacitors

With the increasing demand of high-power and pulsed power electronic devices, environmental-friendly potassium sodium niobate ((Na[Formula: see text]K[Formula: see text])NbO3, KNN) ceramic-based capacitors have attracted much attention in recent years owning to the boosted energy storage density ([Formula: see text]). Nevertheless, the dielectric loss also increases as the external electric field increases, which will generate much dissipated energy and raise the temperature of ceramic capacitors. Thus, an effective strategy is proposed to enhance the energy storage efficiency ([Formula: see text]) via tailoring relaxor behavior and bad gap energy in the ferroelectric 0.9(Na[Formula: see text]K[Formula: see text])-NbO3–0.1Bi(Zn[Formula: see text](Nb[Formula: see text]Ta[Formula: see text])[Formula: see text])O3 ceramics. On the one hand, the more diverse ions in the B-sites owing to introducing the Ta could further disturb the long-range ferroelectric polar order to form the short–range polar nanoregions (PNRs), resulting in the high [Formula: see text]. On the other hand, the introduction of Ta ions could boost the intrinsic band energy gap and thus improve the [Formula: see text]. As a result, high [Formula: see text] of 3.29 J/cm3 and ultrahigh [Formula: see text] of 90.1% at the high external electric field of 310 kV/cm are achieved in [Formula: see text] = 0.5 sample. These results reveal that the KNN-based ceramics are promising lead-free candidate for high-power electronic devices.

Effects of Sb Doping on Electrical Conductivity Properties in Fine-Grain KNN-Based Ferroelectric Ceramics

In this work, the effects of Sb doping on the electrical conductivity of fine-grain 0.9(K0.5Na0.5)(Nb1−xSbx)O3-0.1Bi(Ni2/3Nb1/3)O3 (KNNSx-BNN) ceramics were systemically investigated. It was found that the grain size decreases from ~900 nm (x = 0) to ~340–400 nm (x = 0.06–0.08), and then increases again to ~700 nm (x = 0.10). This is because the solubility limit of Sb doping is about 0.08 in this ceramic, and more Sb doping will facilitate the grain growth as the sintering aids. Impedance and conductivity analyses reveal that the grain resistance and its activation energy show a similar changing tendency with grain size, while grain boundary conductivity steadily increases after Sb doping. In this process, the grain contribution on ceramic conductivity changes with grain size variation, and grain boundary contribution becomes more obvious with increasing doping content. The reduction in grain size, improvement in grain boundary density and doping ions entering into the grain boundary should contribute to the evolution of electrical conductivity properties after Sb doping in KNN-based ferroelectric ceramics.

Coexisting multi-phase and relaxation behavior in high-performance lead-free piezoceramics

Phase boundary engineering (PBE) has remarkably enhanced the piezoelectric properties of potassium sodium niobate {(K, Na)NbO3, KNN} piezoceramics, yet the physical mechanisms need to be further understood. Here we outline a new physical phenomenon to describe piezoelectricity enhancement in KNN-based ceramics with PBE. We propose that the enhancement is due to the multi-phase coexistence featured with strong relaxation behavior. The strong relaxation behavior was unambiguously revealed by cryogenic experiments and originated from the polar nanoregions (PNRs) exhibiting a scale of 2.1 nm and a weak tetragonality (c/a = 1.0040). in situ temperature-dependent experiments uncovered the thermal evolution of the ferroelectric matrix and PNRs in both unpoled and poled samples, the first report in KNN-based ceramics. Our experiments combined with phenomenological theory revealed that ultra-fine nanodomains, PNRs, and easy polarization rotation together promote macro dielectric and piezoelectric properties in the relaxation-featured multi-phase coexistence. This work reveals the physical mechanism from different levels (e.g., local-mesoscopic-macroscopic), thus providing a new systematic understanding of the observed enhancement of piezoelectricity.

(K, Na)NbO3 lead-free piezoceramics prepared by microwave sintering and solvothermal powder synthesis

Low temperature and short processing time are favorable for fabricating (K, Na)NbO3 (KNN) lead-free piezoelectric ceramics with a drawback of easy alkali metal volatilization during the powder synthesis and sintering processes. This study revealed that microwave sintering (MS) in addition to microwave solvothermal synthesis (MSS) of powder can lead to short-time fabrication of KNN ceramics with high density and fine grains. It was found that when the concentration of KOH/NaOH exceeds 1 M, KNN powder with single perovskite structure can be synthesized successfully by MSS method, which can be densified by MS to a high relative density close to 97% at 1050 °C even using 2/9 shorter time than conventional sintering (CS) at the same temperature. Importantly, the MSed KNN ceramics show better piezoelectric and ferroelectric properties than CSed counterparts. Without any dopants, the piezoelectric coefficient (d33) of the MSed pure KNN ceramics is 132 pC/N, and the remnant polarization (Pr) is 26.3 μC/cm2. The results show that the MS method is a promising method for preparing KNN-based ceramics.

Low-temperature dielectric relaxation associated with NbO6 octahedron distortion in antimony modified potassium sodium niobate ceramics

Although the antimony (Sb) has been widely used to modify potassium sodium niobate (KNN) ceramics for tailoring the phase structure and performance, the role of Sb still remains insufficiently understood, consequently hindering the understanding of the physical origin of high-performance KNN-based ceramics. Here, we combine the experiments and first-principles calculations to deeply reveal the effects of Sb on KNN ceramics. Our results reveal a re-entrant-like relaxation behavior near the rhombohedral-orthorhombic (R-O) phase transition at the low content of Sb, which transforms into a canonical one at higher content of Sb. First-principles calculations show a significantly decreased difference in the bond length of six B-O bonds of NbO6 octahedral in Sb-modified KNN ceramics compared to pristine KNN ceramics, responsible for the low-temperature re-entrant-like dielectric relaxation. Furthermore, the addition of Sb would soften the B-O repulsion and gradually break the long-range ferroelectric ordering, resulting in the occurrence of nanoscale domains and enhanced local heterogeneity. Finally, we find that the optimized piezoelectric properties are the trade-off between the long-range ferroelectric ordering and the local heterogeneity. Therefore, this work not only deeply reveals the effects of Sb on KNN ceramics from multi-scale perspectives but also helps the future composition design for achieving high piezoelectricity.

Enhancing densification and electrical properties of KNN-based lead-free ceramics via two-step sintering

Enhancing densification and electrical properties of KNN-based lead-free ceramics via two-step sintering

Tailoring micro-structure of eco-friendly temperature-insensitive transparent ceramics achieving superior piezoelectricity

Transparent ferroelectrics with high piezoelectricity are highly desirable for acousto-optic-electrical multifunctional coupling devices. However, it is challenging to simultaneously achieve high piezoelectricity and perfect transparency in lead-free piezoelectric ceramics because of the contradiction between the inherent characteristics of traditional high piezoelectricity and the transparency. Here, we demonstrate an efficient method to tailoring the microstructure in originally poor piezoelectricity fine-grain KNN-BNN ceramics to simultaneously exhibit excellent transparency, an superior piezoelectric coefficient (∼185 pC/N, highest in KNN-based transparent ceramics), an excellent EO coefficient (∼84 pm/V), and high Tc (∼350 °C). Combining the multiple in-situ characterizations and DFT simulations, we found that both the piezoelectricity and EO effect of fine-grain KNN transparent ceramics can be greatly improved without greatly reducing the transparency via tailoring the structure of phase and domains to enhance local inhomogeneity, enabling prominent acousto-optic-electrical multifunctional coupling in vivid potential applications, such as self-energy-harvesting touch screens, invisible robotic devices and electro-optic (EO) devices.

Excellent fatigue resistance in Sb nonstoichiometric KNN-based ceramics by engineering relaxor multiphase state

Potassium sodium niobate (KNN)-based ceramics were considered to be a promising candidate for actuators, and the long-term reliability, as a major consideration in practical applications, should be given adequate attention. Here, lead-free KNN-based ceramics with antimony (Sb) nonstoichiometric have been designed to improve both the piezoelectricity and electric fatigue resistance. The variation of Sb not only benefits for the piezoelectricity, but also greatly affects their electric fatigue behaviors. That is, the ceramic with excess Sb has excellent fatigue resistance while the ceramic with deficient Sb has pronounced fatigue. This new phenomenon is attributed to the local polar-structure states: The relaxor multiphase coexistence state, reduced orthorhombic phase fraction and refined domain size in the ceramic with excess Sb, together contribute to the enhancement of fatigue resistance. We believe that the simple approach of Sb nonstoichiometric adjustment has provide an important route to simultaneously achieve excellent piezo-response and fatigue resistance.

A new family of high temperature stability and ultra-fast charge–discharge KNN-based lead-free ceramics

A new family of high temperature stability and ultra-fast charge–discharge KNN-based lead-free ceramics

Effect of microwave sintering on KNN-based lead free ceramics

Microwave sintering has been known as the effective method to prepare ceramics. In this study, the microwave sintering method was used to synthesize 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3-0.04CaZrO3 ceramics. The effect of holding time on the properties of ceramics was investigated. With the holding time of 30 min, the excellent piezoelectric properties (d 33 = 286 pC/N) can be achieved.

Simultaneously achieving high performance of energy storage and transparency via A-site non-stoichiometric defect engineering in KNN-based ceramics

Ceramic-based transparent dielectric materials are regarded as the best candidates for advanced energy storage and conversion materials because of their outstanding optical and electric properties. Nevertheless, due to the presence of low density, low band gap energy and large grain size, it is difficult to simultaneously obtain high energy storage density and high optical transmittance in lead-free based ceramics, which limiting their further development of practical applications. In this work, the relaxor ferroelectric ceramics of x mol% Er3+-doped 0.91 K0.5Na0.5NbO3-0.09BamSrnTiO3 (xEr-SrmBan) were constructed via A-site non-stoichiometric defect engineering. It is worth noting that excessive addition of Sr and Ba, especially Sr, can significantly refine the grain size and domain size on account of vacancy-related defect pinning. Finally, high energy storage density (W = 6.39 J/cm3, Wrec = 3.42 J/cm3) together with high optical transmittance (∼72% at 900 nm) can be achieved simultaneously in 0.25Er-Sr1Ba0.5 due to the exist of dense structure, ultrafine grain size (<100 nm), small-sized nano-microdomains (i.e., PNRs, < 50 nm) and large bandgap energy (∼3.10 eV). Furthermore, a fast discharge time of 42 ns can also be realized. The above results indicate that the present study helps to promote the development of advanced transparent energy storage and conversion materials.

High-temperature workable flexible piezoelectric energy harvester comprising thermally stable (K,Na)NbO3-based ceramic and polyimide composites

Composite-based piezoelectric devices have been widely investigated to realize a semi-permanent power supplier or self-powered sensors with high flexibility; however, their working temperatures are mostly restricted below 200 °C. Herein, we report high-temperature and flexible composite-based piezoelectric energy harvesters (f-PEHs) comprising lead-free (K,Na)NbO 3 (KNN)-based ceramic dispersants and polyimide matrix. The KNN-based ceramics with different BiScO 3 (BS) compositions of 0.0025 and 0.01 were synthesized via a solid-state reaction and characterized under high-temperature conditions. Thereafter, the output voltage, current signals, and effective power of f-PEHs with 0.0025BS- and 0.01BS-doped KNN-based particles were measured at the temperature range from room temperature (RT) to 300 °C. The 0.0025BS KNN-adopted f-PEH not only generated 130% higher output voltage (∼4 V) and 137% higher current (∼0.24 μA) than the device with 0.01BS-doped KNN at RT, but also has superior electrical outputs compared to those harvested from the 0.01BS KNN-adopted f-PEH over all temperature ranges. These measurement results corresponded well with the trend of change in piezoelectric voltage constants and piezoelectric figure of merit characterized from ceramic discs, respectively. Moreover, theoretical and quantitative analyses using finite element method simulations proved that 0.0025BS is more suitable for the fabrication of high-temperature f-PEH compared to 0.01BS. This work can serve as a guide for developing a fully flexible and high-temperature workable f-PEH to realize self-powered sensor systems in high-temperature environments. • High-temperature workable flexible piezoelectric energy harvesters (f-PEHs) were fabricated using piezo-ceramic and polyimide. • Temperature-dependent piezoelectric properties of (K,Na)NbO 3 -based ceramics were characterized under high-temperature. • A fabricated energy harvester generated an output voltage of ∼4 V and a current of ∼0.24 μA under repeated bendings. • A fabricated energy harvester exhibits exceptional high-temperature stability over the temperature range from 30 to 300 °C.

Dielectric performance and conductive mechanism of KNN-based ceramics prepared via sol–gel core–shell process

Dielectric performance and conductive mechanism of KNN-based ceramics prepared via sol–gel core–shell process

Simultaneous high transmittance and large tunability of up-conversion photoluminescence in Er3+ doped KNN-based ceramics

0.98(0.94K0.51Na0.5NbO3-0.06SrZrO3)-0.02Li0.5La0.5TiO3+x mol Er3+ ceramics were prepared using a conventional solid-state reaction method. The transmittance of the sample x = 0.5% sintered at 1210 °C reaches 56% in at the wave length of 780 nm and 73% at the wave length of 2000 nm (the thickness of the sample is 0.3 mm). Under 980 nm excitation, two typical emission bands are obtained, which are the green emission band at 510 nm–580 nm and the red emission band at 645–695 nm. Under the irradiation of visible light, the up-conversion luminescence intensity of the sample is significantly reduced, showing a luminescence quenching characteristic. The PL quenching degree (ΔR) of up-conversion emission is up to 78.2% for the sample x = 0.5% sintered at 1210 °C. Besides, a behavior of photosensitive resistance is achieved at room temperature and 350 °C, which suggests that this system has a great application prospect in optical information storage.

Effect of template amounts on the orientation degree and electrical properties of lead-free piezoelectric textured KNN-based ceramics

Textured process is considered an effective way to enhance the performance of piezoelectric ceramics. To obtain lead-free piezoelectric KNN-based ceramics with high performance, ⟨001⟩c-textured lead-free piezoelectric 0.915(K0.45Na0.5Li0.05)–0.075BaZrO3–0.01Bi0.5Na0.5TiO3 (KNLN–BZ–BNT) ceramics with the addition of 5 mol. % NaNbO3 templates were prepared, and the improved piezoelectric properties and thermal stability were expectedly achieved. For textured ceramics, the textured degree is one of the very important parameters and strongly depends on the amount of templates. Therefore, in this work, the effect of the amount of templates on the textured degree and electrical properties of ⟨001⟩c-textured KNLN–BZ–BNT ceramics were investigated in detail. It was found that the templates take a positive effect when the template amount is low, in which the textured degree and electrical properties increased with the amount of templates. In contrast, excess templates could induce a severe shift of stoichiometry within the textured ceramics, accompanied by degraded performances. As a result, for the ceramics with template amount less than 4 mol. %, enhanced textured degree and electrical properties were obtained. When the template amount exceeds 5 mol. %, the textured degree and electrical properties become degraded. The textured ceramic with the 4 mol. % template shows the highest textured degree of 93% and the highest piezoelectric constants of d33 = 360 pC/N and d33∗ = 615 pm/V. Combining a vertical morphotropic phase boundary with an appropriate template amount of 4 mol. %, the ceramics exhibit superior thermal stability within the temperature range of 30–200 °C.

Influence of annealing environments on the conduction behaviour of KNN-based ceramics

The electrical conduction behaviour of the lead-free (1-x)K0·5Na0·5Nb0·95Sb0·05O3-xBi0.5Na0·41K0·09ZrO3 (x = 0.05) ceramics annealed in air, oxygen, and argon environments at various temperatures was investigated. Post-sintering annealing in different atmospheres affected the temperature of dielectric phase transition and electrical conductivity values significantly. The ceramic sample annealed in an oxygen environment showed the lowest conductivity for grain (σg ∼ 1.22 × 10−5 S/m) and grain boundary (σgb ∼ 1.56 × 10−6 S/m) regions at 250 °C which is due to the reduction in oxygen vacancy defects. The activation energy Ea for DC conduction in this sample obeyed the Arrhenius relation and was found to be ∼ 0.99 ± 0.03 eV. An additional anomaly observed at T ∼ 250 °C dielectrics vs. temperature plots for the as-sintered and argon-annealed samples is ascribed to defect relaxation, which was inconspicuous in the samples annealed in air and oxygen. In addition, the in-situ impedance measurement was performed to analyze the impact of argon atmosphere on the electrical conductivity of the O2-annealed samples with high-temperature electrode curing.

Single-Beam Acoustic Tweezer Prepared by Lead-Free KNN-Based Textured Ceramics.

Acoustic tweezers for microparticle non-contact manipulation have attracted attention in the biomedical engineering field. The key components of acoustic tweezers are piezoelectric materials, which convert electrical energy to mechanical energy. The most widely used piezoelectric materials are lead-based materials. Because of the requirement of environmental protection, lead-free piezoelectric materials have been widely researched in past years. In our previous work, textured lead-free (K, Na)NbO3 (KNN)-based piezoelectric ceramics with high piezoelectric performance were prepared. In addition, the acoustic impedance of the KNN-based ceramics is lower than that of lead-based materials. The low acoustic impedance could improve the transmission efficiency of the mechanical energy between acoustic tweezers and water. In this work, acoustic tweezers were prepared to fill the gap between lead-free piezoelectric materials research and applications. The tweezers achieved 13 MHz center frequency and 89% −6 dB bandwidth. The −6 dB lateral and axial resolution of the tweezers were 195 μm and 114 μm, respectively. Furthermore, the map of acoustic pressure measurement and acoustic radiation calculation for the tweezers supported the trapping behavior for 100 μm diameter polystyrene microspheres. Moreover, the trapping and manipulation of the microspheres was achieved. These results suggest that the KNN-based acoustic tweezers have a great potential for further applications.

Ultrahigh energy harvesting properties in temperature-insensitive eco-friendly high-performance KNN-based textured ceramics

Through the simultaneous use of composite design and template grain growth technology, the comprehensive performance of KNN-based piezoelectric ceramics has been significantly improvedviathe synergy of the textured structure and multiphase coexistence.

Large electrocaloric effect under electric field behavior in potassium sodium niobate ceramics with incompletely overlapped phase boundaries

An enhancement of the electrocaloric effect with incomplete overlap of the FE–FE and FE–PE phase boundaries in KNN-based ceramics.