Lead-free Potassium Sodium Niobate Research Articles

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 × 10-4Cm-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.

Effect of different donor dopants on the seed-free solid-state crystal growth of potassium sodium niobate

Seed-free solid-state crystal growth (SSCG) is a developing technique to produce lead-free potassium sodium niobate (KNN) single crystals with the advantage of expeditious growth rates. The fundamental mechanism for seed-free SSCG is abnormal grain growth, which can be promoted through the inclusion of a precise concentration of an A-site donor dopant in KNN. To date, Ba2+ and Bi3+ have been utilised to grow centimetre-sized KNN single crystals with piezoelectric properties equivalent to traditional melt growth techniques. This research explores a broad range of potential donor dopants in (K0.5Na0.5Li0.006Dx)NbO3 (where D = Ba2+, Ca2+, Sr2+, Pb2+, Bi3+, La3+, Ce3+,4+ and Pr3+,4+) to determine disparities in abnormal grain growth behaviour and the critical parameters that enable substantial single crystal growth. The incorporation of different donor dopants influences the grain growth behaviour, single crystal sizes and preferential crystallographic orientations. This inspires future research to explore different compositions and techniques to improve the KNN crystal sizes and piezoelectric properties and addressing other challenges. These efforts are essential to improve the maturity of seed-free SSCG, making it industrially viable and a genuine alternative to costly single crystal growth techniques.

A phase-field simulation of easily switchable vortex structure for multilevel low-power ferroelectric memory

Ferroelectric materials with multi-directional polarization orders can form a variety of special domain structures, which have a wide range of applications. In this work, lead-free potassium sodium niobate ferroelectric thin films were simulated by solving the Time-Dependent-Landau-Ginzburg (TDLG) equation. Based on vortex topological structure characteristic of perovskite-type ferroelectric, domain types are simplified from 26 to 9 through virtual polarization vectors. By temperature selection, we accomplished a large easily switchable vortex structure with 20% side length. Based on this, as low as 0.04V/nm electric-field pulse was applied to explore vortex domain structures change. Under different low external electric-field pulses, conductance states change in vortex have been calculated and discussed systematically. Free energy computation results reveal that the domain wall (DW) conductance states primarily originate from the competition between electrostatic energy, gradient energy, and elastic energy. Finally, we propose the multilevel data ferroelectric memory through easily switchable vortex topological structures for the first time. Under electric-field pulses, diverse storage states were presented through 0/1 measurement of 8 positions. By achieving an easy-to-distinguish and robust-to-switch data map at the vortex structure, this work might provide a significant idea of vortex structure on low-power ferroelectric memory applications.

Optimizing Output Power Density in Lead-Free Energy-Harvesting Piezoceramics with an Entropy-Increasing Polymorphic Phase Transition Structure.

It is an urgent need to develop lead-free piezoelectric energy harvesters (PEHs) to address the energy dilemma and meet environmental protection requirements. However, the low output power densities limit further promotion of lead-free PEHs for use in daily life. Here, an entropy-increasing strategy is proposed to achieve an increased output power density of 819 μW/cm3 in lead-free potassium sodium niobate (KNN)-based piezoceramics by increasing the configuration entropy and realizing nearly two times the growth compared with low-entropy counterparts. Evolution of the energy-harvesting performance with increasing configuration entropy is demonstrated systematically, and the excellent energy-harvesting properties achieved are attributed to the enhanced lattice distortion, the flexible polarization configuration, and the high-density randomly distributed nanodomains with the entropy-increasing effect. Moreover, excellent vibration fatigue resistance and variable temperature output power characteristics were also realized in the PEH prepared by the proposed entropy-increasing material. The significant enhancement of the comprehensive energy-harvesting performance demonstrates that the construction of KNN-based ceramics with high configuration entropy represents an effective and convenient strategy to enable design of high-performance piezoceramics and thus promotes the development of advanced PEHs.

Stress dynamics during O-T phase transitions in lead-free KNN-based piezoelectric ceramics

In this study, we have investigated the stress dynamics arising from the Orthorhombic-Tetragonal Polymorphic Phase Boundary (PPB) in lead-free potassium sodium niobate (KNN)-based ceramics and its temperature dependence, as a means to elucidate the characteristics of PPBs in lead-free piezoelectric oxides. The dynamics are dictated by the phase transitions that occur upon cooling from the cubic phase and the coexistence of different crystal structures. Our findings reveal that the growth of Orthorhombic phases is constrained by the high-temperature tetragonal phase distributions. In particular, it is evidenced that two main mechanisms regulate the decrease of stress processes in which structural and microstructural effects are correlated; the first one is associated to a purely microstructural effect in which bimodal grain distribution hinders the formation of non-180° domains. By contrast, the second is mainly governed by ferroelectric domain distribution and the occurrence of pseudo-cubic regions around room temperature associated with local structural heterogeneity (that is, polar nano regions, PNRs). Specifically, these mechanisms generate regions with different stress state and explain the widening of the phase transition due the phase coexistence.

Potassium sodium niobate-based transparent ceramics with high piezoelectricity and enhanced energy storage density

Lead-free potassium sodium niobate (KNN)-based transparent ceramics are highly desirable owing to their excellent piezoelectricity, and recoverable energy storage density (Wrec) especially for optoelectronic devices. However, it is challenging to achieve all parameters such as efficient light transmittance and excellent piezoelectricity or energy storage performance in a single device. Herein, we report a facile fabrication of transparent ceramic composed of (1 – x)[(Na0.57K0.43)0.94Li0.06][(Nb0.94Sb0.06)0.95Ta0.05]O3-xCaZrO3 (x = 0, 0.01, 0.03, 0.05, 0.07, 0.09 and 0.10) ((1 – x)KNLNST-xCZ) via conventional solid-state reaction method. It has been found that ceramics can retain good light transmission and maintain their piezoelectric properties by adjusting the phase structure and refining the grains to a certain extent. In particular, KNLNST-based transparent ceramic with 0.07CZ modifications demonstrates a high transmittance (T = 73.5% at 1800 nm) and exceptional piezoelectric constants (d33 = 130 pC/N), which is more efficient than reported KNN-based transparent ceramics. More importantly, a significant improvement in grain size refinement is also achieved through the integration of CZ into KNLNST, which allow us to increase the breakdown strength of the ceramics while improving their light transmittance, which result in a high density of energy storage. The highly efficient energy storage performance (Wrec = 4.88 J/cm3) and the higher transparency (T = 75% at 1800 nm) at 0.91KNLNST-0.09CZ ceramic. We believe that this work will provide useful strategies for the development of KNN-based functional ceramics.

Enhanced thermal stability by short-range ordered ferroelectricity in K0.5Na0.5NbO3-based piezoelectric oxides.

Industrial application of lead-free piezoelectric ceramics is prevented by intrinsic thermal instability. Herein, we propose a method to achieve outstanding thermal stability of converse piezoelectric constant () in lead-free potassium sodium niobate (KNN)-based ceramics by inducing a synergistic interaction between the grain size and polar configuration. Based on computational methods using phase-field simulations and first-principles calculations, the relationship between the grain size and polar configuration is demonstrated, and the possibility of achieving improved thermal stability in fine grains is suggested. A set of KNN systems is presented with meticulous dopant control near the chemical composition at which the grain size changes abnormally. Comparing the two representative samples with coarse and fine grains, significant enhancement in the thermal stability of is exhibited up to 300 °C in the fine grains. The origin of the thermal superiority in fine-grained ceramics is identified through an extensive study from a microstructural perspective. The thermal stability is realized in a device by successfully demonstrating the temperature dependence of piezoelectricity. It is notable that this is the first time that lead-free piezoelectric ceramics are able to achieve exceptionally stable piezoelectricity up to 300 °C, which actualizes their applicability as piezoelectric devices with high thermal stability.

Multiple stability of electrical properties for oxides-modified (K,Na)NbO3-based ceramics

Lead-free potassium sodium niobate (KNN) ceramics have become crucial advanced functional materials with conspicuous performance. Although continuous improvements of electrical properties were achieved via various strategies these years, the poor stability leads to the bottleneck for application. Herein, the stability of electrical properties under different conditions (i. e. electric field, frequency and temperature) were emphasized on in KNN-based ceramics with addition of oxides based on the availability for optimizing properties. With addition of Fe2O3, high ferroelectric, piezoelectric and dielectric properties remain due to the maintained rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary at room temperature and little changed Curie temperature (TC), while deterioration happens with excess Fe2O3 attributing to the pinning effect from replacement of Fe3+ in crystal lattice. Furthermore, the elevated beginning electric field for stable ferroelectricity and negative strain under increasing electric field are gained with elevating Fe2O3 along with little changed piezoelectricity. Stable strain maintains with increasing frequencies when Fe2O3 content changes, as an enhancement are discovered for residual polarization (Pr) and coercive electric field (EC) at 0.5–1 Hz for x ≥ 1.0%. Moreover, improved temperature stability for piezoelectricity and strain as well as remained high ferroelectricity is presented with increasing Fe2O3, originating from the increasing diffusion behavior of R-O-T phase boundary and non-declined TC, respectively. Thus, this work provides an effectively strategy to optimize the stability of electrical properties for application of KNN-based materials.

Isolated-Oxygen-Vacancy Hardening in Lead-Free Piezoelectrics.

Defect engineering is a well-established approach to customize the functionalities of perovskite oxides. In demanding high-power applications of piezoelectric materials, acceptor doping serves as the state-of-the-art hardening approach, but inevitably deteriorates the electromechanical properties. Here, a new hardening effect associated with isolated oxygen vacancies for achieving well-balanced performances is proposed. Guided by theoretical design, a well-balanced performance of mechanical quality factor (Qm ) and piezoelectric coefficient (d33 ) is achieved in lead-free potassium sodium niobate ceramics, where Qm increases by over 60% while d33 remains almost unchanged. By atomic-scale Z-contrast imaging, hysteresis measurement, and quantitative piezoresponse force microscopy analysis, it is revealed that the improved Qm results from the inhibition of both extrinsic and intrinsic losses while the unchanged d33 is associated with the polarization contributions being retained. More encouragingly, the hardening effect shows exceptional stability with increasing vibration velocity, offering potential in material design for practical high-power applications such as pharmaceutical extraction and ultrasonic osteotomes.

Compositionally Graded KNN-Based Multilayer Composite with Excellent Piezoelectric Temperature Stability.

The inherent disadvantage of lead-free potassium sodium niobate (KNN)-based ceramics is the severe temperature instability of piezoelectric charge coefficient (d33 ) caused by the polymorphic phase boundary. Herein, a new concept of structural gradient is proposed by designing compositionally graded multilayer composites with multiple successive phase transitions, to solve the challenge of the inferior temperature stability. The structural gradient ceramics exhibit a superior temperature reliability (d33 remains almost unchanged in the temperature range of 25-100 °C), far outperforming the previously reported KNN counterparts with d33 variation above 27% over the same temperature range. The synergistic contribution of the continuous phase transition, the strain gradient, and the complementary effect of each constituent layer leads to the excellent temperature stability, which is also confirmed by phase-field simulation. These findings are expected to provide a new paradigm for functional material design with outstanding temperature stability.

Realizing High Comprehensive Energy Storage and Ultrahigh Hardness in Lead-Free Ceramics.

Due to the presence of pores and low density, a high recoverable energy density (Wrec) value is usually obtained at the cost of energy storage efficiency (η) in lead-free potassium sodium niobate [(K, Na)NbO3, KNN] based ceramics, which also affects the hardness of ceramics, finally limiting the further development of practical applications. A high Wrec (∼3.60 J/cm3 ) and a high η (∼74.2%) are obtained in 0.975K0.5Na0.5NbO3-0.025LaBiO3 (0.975KNN-0.025LB) ceramics simultaneously under a high dielectric breakdown strength (DBS) of 340 kV/cm, together with a fast discharge rate (t0.9 ∼ 46 ns) and high power density (PD ∼ 49.4 MW/cm3). Further analysis of the intrinsic electronic structure is carried out via the first-principles calculation based on the density functional theory (DFT). An ultrahigh hardness (H) of 6.63 GPa can be accordingly obtained. This work combines excellent energy storage properties and ultrahigh hardness, which provides significant guidelines for applications in pulsed-power systems.

The origin of chemical inhomogeneity in lead-free potassium sodium niobate ceramic: Competitive chemical reaction during solid-state synthesis

Excellent electromechanical properties have been reported in lead-free potassium sodium niobate (KNN)-based piezoelectric ceramics over the past 15 years. However, poor reproducibility of the electromechanical properties of KNN has been a major barrier to industrial development, due to a lack of full understanding of some of the processing aspects. Chemical inhomogeneity is one of the most critical challenges in processing, as the properties of KNN-based ceramics are strongly composition-dependent. In the present study, in-situ temperature-dependent X-ray diffraction and thermogravimetric analysis were employed to investigate the chemical reactions that occur during solid-state synthesis. Chemical homogeneity was found to be sensitive to the competition among reactants during synthesis. The phenomenon can reasonably explain the chemical inhomogeneity in KNN and possibly other lead-free piezoelectric ceramic systems prepared via solid-state synthesis.

Synthesis of KNN powders: Scaling effect of the milling step

The scaled-up production of lead-free potassium sodium niobate (KNN)-based piezoceramics still suffers from poor reliability and reproducibility issues that hinder their penetration into the piezoelectric market. To address this issue, pure KNN ceramics were prepared in this work by mechanochemical activation of the powders, followed by calcination at 700 °C, obtaining high density sintered pellets featuring fine and uniform-grain-size. Results show that both cumulative kinetic energy released in the system and hindrance factor, the latter being paramount, must be properly taken into account when the synthesis is scaled-up. Mechanochemical activation strongly effects both stoichiometry and morphology of the final KNN powders during the milling step, which in turn affects the piezoelectric properties of the sintered pellets. This work definitely proves that proper design and control of the powder synthesis process is a valuable tool in the scaled-up production of KNN powders for the production of reliable and performant KNN ceramics.

High-performance (K,Na)NbO3-based binary lead-free piezoelectric ceramics modified with acceptor metal oxide

Environmental awareness has led to legislation that restricts lead-containing piezoelectric material use; consequently, lead-free alternatives are urgently sought. Chemical modification of the multiphase piezoelectric system can be used to develop electrical properties of lead-free ceramics with thermal stability. This paper reports lead-free potassium sodium niobate (K, Na) NbO3 (KNN)-based piezoelectric ceramics modified with Fe2O3 and prepared via the solid-state route. The results demonstrate that via the partial substitution of Fe3+ for Nb5+ and Ti4+ ions in the B-sites of the perovskite structure, the polymorphic phase transition (PPT) region at the orthorhombic–tetragonal (O–T) phase boundary can be moved to approximately room temperature and the mass transportation during sintering can be enhanced, thereby promoting homogenous grain growth and increased densification. The substantially diminished polarization anisotropy between the O-T phase boundaries promotes domain wall mobility, which can substantially enhance the piezoelectric properties. An optimal piezoelectric coefficient value of d33 = 246 pC/N, an electromechanical coupling coefficient value of kp = 49%, a dielectric permittivity of εr = 1420, a remnant polarization of Pr = 23 μC/cm2 and a high Curie temperature of TC = 343 °C were realized at x = 0.08 mol% of Fe2O3. Due to their enhanced electrical properties and high thermal stability, KNN-based lead-free ceramics doped with low-cost Fe2O3 can replace commercial high-lead-content piezoelectric ceramics.

Lead Free Multilayer Piezoelectric Actuators by Economically New Approach

The replacement of lead zirconate titanate ceramics (PZTs), with a lead-oxide (PbO)-free alternative is the subject of intense investigation worldwide. In this research, a cheap reliable methodology for the fabrication of multilayers (MLs) of lead-free potassium sodium niobate (KNN)-based ceramics is presented without the need for vacuuming and/or hot isostatic pressing of the tape. The thickness per active layer was 193 μm and 102 μm for the 10 and 16 layer MLs actuators, respectively. Effective d33 (piezoelectric coefficient), effective d33∗ (electrostrain coefficient), bi-polar strain (Smax), max displacement, dielectric constant (ϵr) and loss (tanδ) are 2500 pC/N, 4604 pm/V, 0.17%, 2.2 μm, 1812 and 2% at 1 kHz, respectively, where the effective d33 or d33∗ is the total output of all layers together. The ultimate target materials for potential substitution with KNN-based ceramics and MLs are commercial PZT-4 and PZT-8.

Spark plasma texturing: A strategy to enhance the electro-mechanical properties of lead-free potassium sodium niobate ceramics

Controlling the sintering and microstructure of lead-free potassium sodium niobate ((K1-xNax)NbO3, KNN) ceramics is of primary importance to optimize its piezoelectric / ferroelectric properties. However, sintering dense and monophasic KNN remains a challenge. Here, we prepare KNN ceramics using spark plasma texturing (SPT), a modified spark plasma sintering (SPS) technique, in which uniaxial pressure is applied in an edge-free configuration, allowing ceramics to deform in the radial direction. Densification at low temperatures (1000 °C) and for short times (20 min) is achieved by SPT accompanied by constrained grain growth (average grain size = 1.4 μm), resulting in enhanced piezoelectric properties (d33 = 108 pC N−1 and g33 = 21.2 × 10−3 Vm N−1). In addition, and of relevance, SPT KNN ceramics reveal a more homogeneous electrical microstructure postulated to be related with a reduced diffusion and local segregation of defects, resulting in grain cores and shells with more similar capacitances and conductivities. Our work brings new practical understanding to sintering of KNN and demonstrates the potential of alternative densification strategies for improved lead-free dielectrics.

Ultrahigh Performance in Lead-Free Piezoceramics Utilizing a Relaxor Slush Polar State with Multiphase Coexistence.

Owing to growing environmental concerns, the development of lead-free piezoelectrics with comparable performance to the benchmark Pb(Zr,Ti)O3 (PZT) becomes of great urgency. However, a further enhancement of lead-free piezoelectrics based on existing strategies has reached a bottleneck. Here we achieve a slush polar state with multiphase coexistence in lead-free potassium-sodium niobate (KNN) piezoceramics, which shows a novel relaxor behavior, i.e., frequency dispersion at the transition between different ferroelectric phases. It is very different from the conventional relaxor behavior which occurs at the paraelectric-ferroelectric phase transition. We obtain an ultrahigh piezoelectric coefficient (d33) of 650 ± 20 pC/N, the largest value of nontextured KNN-based ceramics, outperforming that of the commercialized PZT-5H. Atomic-resolution polarization mapping by Z-contrast imaging from different orientations reveals the entire material to comprise polar nanoregions with multiphase coexistence, which is again very different from conventional ferroelectric relaxors which have polar domains within a nonpolar matrix. Theoretical simulations validate the significantly decreased energy barrier and polarization anisotropy, which is facilitated by the high-density domain boundaries with easy polarization rotation bridging the multiphase-coexisting nanodomains. This work demonstrates a new strategy for designing lead-free piezoelectrics with further enhanced performance, which should also be applicable to other functional materials requiring a slush (flexible) state with respect to external stimulus.

Technology transfer of lead-free (K, Na)NbO3-based piezoelectric ceramics

Piezoelectric materials are an important functional material in the modern world because they are capable of converting electrical energy into mechanical energy and vice versa. Lead-free potassium sodium niobate (KNN)-based piezoelectric materials have been intensively researched in the past decade for only one purpose: to replace the less environmentally friendly lead-based piezoelectric materials in commercial products. In this review, the historical research evolution of KNN-based piezoelectric ceramics is highlighted. Meanwhile, to evaluate the technology transfer status, we emphasize on the analysis of the patents applied within the past five years, as well as certain practical applications of KNN-based materials. This paper concludes with the prediction of remaining challenges and opportunities for adopting the lead-free KNN-based piezoelectric ceramics in practical applications.

Effects of surface modification on electrical properties of KNN nanorod-incorporated PVDF composites

The demand of self-powered electronic devices has stimulated a great interest in daily life. To fulfill this, thrust researchers are engaged to develop different piezoelectric self-powered-based devices. The piezoelectricity is broadly dependent on the electroactive phase of the polymer structure. Herein, lead-free potassium sodium niobate (KNN) nanorods-incorporated PVDF polymer-based nanocomposite films have been developed wherein KNN nanorods have been surface-modified by three diverse surface modifiers such as 3-aminopropyltrimethoxysilane (APS), polyaniline (PANI) and polyvinylpyrrolidone (PVP) for resolving the agglomeration problem of the nanorods and to look into their effect on the microstructural growth including nucleation of polar crystals. The nanocomposite films have been developed simply by solution cast method. The growth of electroactive phases (β and γ) of the PVDF polymer has been observed to be improved significantly by incorporation of surface-modified KNN nanorods. The calculated beta fraction (F(β)) and gamma fraction (F(γ)) as calculated by the FTIR spectrum (98% and 99%) are maximum for the silane- and PVP-modified KNN nanorods-incorporated PVDF polymer, because of the higher molecular weight and homogeneous distribution of the nanorods throughout the PVDF matrix. The total crystallinity evaluated by XRD patterns, specifically β crystal part for the surface-modified KNN nanorods-based film, has been improved (17% for pure PVDF, 45% for untreated KNN-based film and more than 50% for the surface-modified KNN-based films). The remnant polarization values are also remarkably higher (0.49 μC/cm2 for silane, 0.022 μC/cm2 for PVP and 0.015 μC/cm2 for PANI-modified KNN nanorods-based composites, respectively) for surface-modified KNN nanorods-based composite with contrast to pure PVDF (0.001 μC/cm2) and untreated KNN-based film (0.002 μC/cm2). The dielectric constant values for modified KNN nanorods-incorporated PVDF polymer composites have also been demonstrated as substantial enhancement which are 68, 71 and 70, respectively, for silane-, PANI- and PVP-modified samples, whereas the values are 2 and 38 for pure PVDF and untreated KNN-based films. This study clearly defines that the selection of a suitable surface modifier (interface) can play a noteworthy role in exploitation of excelling electroactive phases of PVDF to the maximum extent to enhance its dielectric, ferroelectric as well as piezoelectric response with low level of loading.

Mechanism of densification in low-temperature FLASH sintered lead free potassium sodium niobate (KNN) piezoelectrics

Lead-free potassium sodium niobate (K0.5Na0.5NbO3, KNN) piezoelectric ceramics have been densified at temperatures lower than 300 °C using atmosphere-water assisted FLASH sintering.