Piezoelectric enhancements in K0.5Na0.5NbO3-based ceramics via structural evolutions

The current work aims to compare the effect of systematic A-site and B-site substitutions on the piezoelectricity of Ka_0.5Na_0.5NbO₃ (KNN)-based perovskite ceramics. The A-site elements was replaced by Li⁺ while Nb5⁺ was substituted by Sb⁵⁺ to form (K_0.4675Na_0.4675Li_0.065)NbO₃ (KNLN) and (K_0.4675Na_0.4675Li_0.065)(Nb_0.96Sb_0.04)O₃ (KNLNS) respectively. The ceramics were prepared using solid-state sintering method. The density of the ceramics steadily improved with the substitutions while the crystal structure evolved from monoclinic (in KNN) to the coexistence of monoclinic and tetragonal (in KNLN) and finally tetragonal in KNLNS. Distinct variations on size and morphology were recorded. Although density, crystal structure and morphology have minor effect on the E_c, they imposed considerable influences on P_r, d₃₃ and k_p. Despite relatively lower density, KNLN exhibited the highest P_r, d₃₃ and k_p at 9.80 μC/cm²,185 pC/N and 0.43 respectively signifying the positive enhancement brought by the co-existence of monoclinic and tetragonal crystal structures. More importantly, this work systematically proved that the co-existence of both structures signified the morphotropic phase boundary (MPB) composition as the primary factor for the enhancement of KNN piezoelectric properties.     

Reversible and color controllable emissions in Er3+/Pr3+-codoped K0.5Na0.5NbO3 ceramics with splendid photochromic properties for anti-counterfeiting applications

It is highly significant to develop multi-mode optical anti-counterfeiting materials to efficiently fight against counterfeit products. In this study, we chose ferroelectric K_0.5Na_0.5NbO₃ (KNN) with excellent photochromism properties as the host and rare-earth Er³⁺ and Pr³⁺ ions as dopants to prepare the Er³⁺/Pr³⁺-codoped KNN ceramics. The color-tunable emissions can be obtained from red-orange-yellow to green by controlling the excitation wavelength. Upon 980 nm excitation, the synthesized ceramics does not only have superior upconversion (UC) emission behaviors but also have good luminescence modulation properties based on the photochromism properties. It is found that the KNN:0.003Er³⁺/0.003Pr³⁺ sample with the optimal UC emission features shows a highest ΔR_t value of 74.52% when irradiated by 390 nm light for 5 min, whereas the KNN:0.005Er³⁺/0.003Pr³⁺ ceramics also exhibit a high ΔR_t value of 66.81% under 395 nm light irradiation. According to the XPS and EPR results, one knows that the mechanism of luminescence modulation is closely related to defects and traps caused by the oxygen vacancies. Furthermore, the optical information writing and erasing test is conducted, exhibiting a good reproducibility and fatigue resistance. These results reveal that the designed ceramics are appropriate for the anti-counterfeiting applications.     

Enhanced electromechanical properties of CaZrO3-modified (K0.5Na0.5)NbO3-based lead-free ceramics

Pairing of large strain response and high d₃₃ with high T_c in (K_0.5Na_0.5)NbO₃-based materials is of high significance in practical applications for piezoelectric actuators. Here, we report remarkable enhancement in the electromechanical properties for (1-x)(K_0.52Na_0.48) (Nb_0.95Sb_0.05)O₃-xCaZrO₃ (KNNS-xCZ) lead-free ceramics through the construction of a rhombohedral (R)-tetragonal (T) phase boundary. We investigated the correlation between the composition-driven phase boundary and resulting ferroelectric, piezoelectric, and strain properties in KNNS-xCZ ceramics. The KNNS-xCZ ceramics with x=0.02 exhibited a large strain response of 0.23% while keeping a relatively large d₃₃ of 237pC/N, which was mainly ascribed to the coexistence of R and T phases confirmed by the XRD and dielectric results. It was found that pairing of large strain response and high d₃₃ in KNN-based materials was achieved. As a consequence, we believe that this study opens the possibility to achieve high-performance lead-free electromechanical compounds for piezoelectric actuators applications.     

Preparation and characterization of (K0.5Na0.5)NbO3 ceramics

In this paper the preparation and characterization of the ceramic material (K_0.5Na_0.5)NbO₃ (KNN) has been studied. Although conventional processing of KNN is often reported to result in sintered bodies lacking sufficient density, samples produced in this work exhibit theoretical density over 95% and yield superior piezoelectric properties than those obtained by the same method and reported previously. The electromechanical coupling coefficient in the thickness direction, k_t, is found to reach 45%. Apart from k_t, the piezoelectric coefficients in longitudinal and planar directions (d₃₃ of 100pC/N and d₃₁ of 43pC/N), hysteresis loop, pyroelectric coefficient measurements and dielectric properties are presented.     

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%.     

Phase-composition dependent domain responses in (K0.5Na0.5)NbO3-based piezoceramics

Lead-free (K_0.5Na_0.5)NbO₃-based (KNN) piezoceramics featuring a polymorphic phase boundary (PPB) between the orthorhombic and tetragonal phases at room temperature are reported to possess high piezoelectric properties but with inferior cycling stability, while the ceramics with a single tetragonal phase show improved cycling stability but with lower piezoelectric coefficients. In this work, electric biasing in-situ transmission electron microscopy (TEM) study is conducted on two KNN-based compositions, which are respectively at and off PPB. Our observations reveal the distinctive domain responses in these two ceramics under cyclic fields. The higher domain wall density in the poled KNN at PPB contributes to the high piezoelectric properties. Upon cycling, however, a new microstructure feature, “domain intersection”, is directly observed in this PPB composition. In comparison, the off-PPB KNN ceramic develops large domains during poling, which experience much less extent of disruption during cycling. Our comparative study provides the basis for understanding the relation between phase composition and piezoelectric performance.     

Optical and electrical properties of pressureless sintered transparent (K0.37Na0.63)NbO3-based ceramics

Lead-free (1−x)(K_0.37Na_0.63)NbO₃-xCa(Sc_0.5Nb_0.5)O₃ (x=0.050, 0.070, 0.090, 0.095 and 0.100) transparent ferroelectric ceramics have been fabricated by pressureless sintering procedure. Transmittance of 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics sintered in sealed alumina crucible was 15% higher than those sintered unsealed in air. By increasing the content of Ca(Sc_0.5Nb_0.5)O₃, the phase structure of (K_0.37Na_0.63)NbO₃ ceramics transformed from orthorhombic to tetragonal symmetry first and then to pseudo cubic symmetry. The 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics exhibited high density (98%), high transmittance (60%) in the near-IR region and relatively good electrical properties (ε_r=1914, tanδ=0.037, T_c=147 °C, P_r=6.88 μC/cm², E_c=8.49 kV/cm). Meanwhile, the introduction of Ca(Sc_0.5Nb_0.5)O₃ induced a composition fluctuation in the (K_0.37Na_0.63)NbO₃ lattice and made the ceramics more relaxor-like, which would lead to a further reduction of light scattering. These results demonstrated that 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ could be promising lead-free transparent ferroelectric ceramics.     

Enhanced energy storage properties of Bi0.5Na0.5TiO3-based ceramics via regulating the doping content and sintering temperature

Eco-friendly lead-free energy-storage ceramics featuring high energy storage properties and ultra-high stability have been regarded to be one of the most potential materials in the field of energy storage. In this work, a new element system, (1-x)(0.6Bi_0.5Na_0.5TiO₃-0.4SrTiO₃)-xBi[Zn_2/3(Nb_0.5Ta_0.5)_1/3]O₃ ((1-x)BNST-xBZNT) lead-free ceramics, were synthesized via a conventional solid-state sintering technology. And the phase structure, microstructure and energy storage properties of the (1-x)BNST-xBZNT ceramics were comprehensively studied. After the introduction of BZNT, the average grain size of the materials is greatly decreased, thereby enhancing the dielectric breakdown strength (DBS). Additionally, the thermal stability of the ceramics is significantly improved via regulating the doping content and sintering temperature. Furthermore, the ferroelectric long-range order of the ceramics is decomposed into randomly-oriented polar nano-domains (PNRs) after introducing BZNT, leading to strong relaxor behavior and significantly reducing remanent polarization (P_r). As a result, even under a relatively low electric field of 139 kV/cm, the 0.98BNST-0.02BZNT ceramic sintered at 1150 °C possesses high values of energy storage efficiency (η) value of 92.78% and total energy storage density (W_tot) of 1.67 J/cm³ as well as remarkable thermal stability (25–175 °C), frequency stability (20–70 Hz) and fatigue resistant stability (10⁰-10⁵ cycles). This investigation provides a useful reference for developing advanced energy storage ceramics by regulating the doping content and sintering temperature.     

Efficient photochromic self-recovery behavior in Tm/Yb co-doped K0.5Na0.5NbO3 thick films

Photochromic (PC) materials have potential applications in the fields of memory, anti-counterfeiting due to their reversible writable/erasable properties. The common organic PC films are restricted during applications owing to the strict requirements of thermal stability and waterproof of devices. Additionally, most of the PC materials strongly rely on external conditions (irradiation/heat) to realize reversible PC regulations, which may cause irreversible damage to devices (reduce the performance and service life). Hence, it is imperative to develop new inorganic membrane materials with PC self-recovery behavior. Here, a novel strategy to achieve completely reversible PC regulation based on the K_0.5Na_0.5NbO₃ (KNN) based thick films is reported. It is worth noting that the PC self-recovery behavior can be optimized via forming the inter-electronic-levels in the band gap via Tm/Yb co-doping in KNN, alleviating the barrier of carrier transition from valence band to conduction band and heightening the recombination efficiency of carriers. And the obtained thick films manifest a reversible PC self-recovery reaction within 6 min. This work is expected to provide the effective guidance for facilitating the further utilization of PC materials.     

Donor doping of K0.5Na0.5NbO3 ceramics with strontium and its implications to grain size,phase composition and crystal structure

In this study, the particular effects of A-site donor doping, such as crystal-structure change, the secondary-phase formation and the grain-size decrease, in a lead-free piezoceramic material K_0.5Na_0.5NbO₃ (KNN) doped with Sr²⁺, were investigated. Extended X-ray absorption fine structure (EXAFS) analyses proved that Sr occupies the perovskite A-sublattice, and locally modifies the KNN monoclinic structure to cubic. Introducing Sr into the A-sublattice, as well as accounting for the charge-compensating A-site vacancies in the starting composition, causes increasing lattice disorder and microstrain, as determined from a Rietveld refinement of the synchrotron X-ray diffraction data. Above 2% Sr the system segregates the A-site vacancies in a secondary phase in order to release the chemical pressure, as revealed by Raman spectroscopy. All these effects result in an increasing number of low-angle grain boundaries that limit the grain growth and finally lead to a significant grain-size decrease.     

Duplex structure in K0.5Na0.5NbO3-SrZrO3 ceramics with temperature-stable dielectric properties

Temperature-stable dielectric properties have been developed in the 0.86 K_0.5Na_0.5NbO₃-0.14SrZrO₃ solid solution system. High dielectric permittivity (ε′ = 2310) with low loss sustained in a broad temperature range (−55–201 °C), which was close to that of the commercial BaTiO₃-based high-temperature capacitors. Transmission electron microscopy with energy dispersive X-ray analysis directly revealed that submicron grains exhibited duplex core-shell structure. The outer shell region was similar to the target composition, whilst a slightly poor content of Sr and Zr presented in the core region. Based on Lichtenecker’s effective dielectric function analysis along with Lorentz fit of the temperature dependence of dielectric permittivity, a plausible mechanism explaining the temperature-stable dielectric response in present work was suggested. These results offer an opportunity to achieve the X8 R specification high-temperature capacitors in K_0.5Na_0.5NbO₃ based materials.     

Highly responsive photochromic behavior with large coloration contrast in Ba/Sm co-doped (K0.5Na0.5)NbO3 transparent ceramics

Ferroelectrics that simultaneously possess optical transparency and photochromic (PC) behavior have attracted extensive attention for multi-functionality. However, inability to achieve both rapid and large coloration contrast in photo-stimulated ferroelectrics limits their practical application. In this work, we propose a new strategy for realizing rapid photochromism by constructing the intermediate trap level (T₂) in Ba/Sm co-doped (K_0.5Na_0.5)NbO₃ (KNN) ferroelectric transparent ceramics. Specifically, rapid photo-response time of about 2 s was achieved, and the modulation ratios of transmittance and luminescence intensity were 37.6% and 72.6% within 2 s for the ceramics. This highly responsive PC behavior with large coloration contrast is expected to broaden the application of PC materials in optical devices, e.g. photo-sensitive glasses and rewritable information displays.     

Microstructural changes in (K0.5Na0.5)NbO3 ceramics sintered in various atmospheres

(K_0.5Na_0.5)NbO₃ is a potential lead-free piezoelectric ceramic, but often suffers from abnormal grain growth. Previous work on BaTiO₃ and SrTiO₃ has shown that abnormal grain growth can be suppressed by controlling the sintering atmosphere. In the present work, (K_0.5Na_0.5)NbO₃ was sintered in atmospheres ranging from O₂ to H₂ and the effect on grain growth behaviour studied. Sintering in reducing atmospheres causes a delay in the onset and a reduction in the amount of abnormal grain growth. The effect of sintering atmosphere on grain growth behaviour can be explained using the 2D nucleation-controlled theory of grain growth. Changes in the grain shape during sintering in reducing atmospheres indicate a reduction in the edge free energy of (K_0.5Na_0.5)NbO₃ caused by an increase in the concentration of oxygen vacancies. This decreases the critical driving force necessary for rapid grain growth and causes a transition from abnormal to pseudo-normal followed by abnormal grain growth.     

Synergistic enhancement of piezoelectricity and thermal stability in AlN-doped Bi0.5Na0.5TiO3-based ceramics

The inverse relationship between piezoelectric coefficient (d₃₃) and depolarization temperature (T_d) in Bi_0.5Na_0.5TiO₃-based ceramics is a longstanding obstacle for their applications. In this work, synergistically enhanced d₃₃ and T_d is achieved in AlN-modified 0.84Bi_0.5Na_0.5TiO₃-0.11Bi_0.5K_0.5TiO₃-0.05BaTiO₃ ceramics. Addition of 1 mol% AlN, increases both d₃₃ from 165 to 234 pC/N and T_d by ～50 °C. Rietveld analysis of X-ray powder diffraction (XRD) data reveals an increase in the proportion of the tetragonal phase at 1 mol% AlN incorporation. Moreover, at this composition the modified ceramics exhibit larger grains and high-density lamellar nanodomains with sizes of 30–50 nm. Polarization reversal and domain mobility are thus significantly enhanced, contributing to the large d₃₃. Temperature-dependent dielectric and XRD data revealed that the delayed thermal depolarization is attributed to the improved and poling-field stabilized tetragonality in the modified ceramics.     

Grain size control of lead-free Li0.06(Na0.5K0.5)0.94NbO3 piezoelectric ceramics by Ba and Ti doping

In this study, Ba- and Ti-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ [(1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07)] ceramics were prepared by using conventional solid state reaction method, and the microstructure and electric properties of these samples were investigated. The grain size distribution of non-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ ceramics was relatively wide. The microstructure was composed of grains ranging 1.1–5.0 μm in size. However, with increasing Ba and Ti content, the grain size distribution became narrow and the average grain size decreased from 2.0 to 0.9 μm in size. In particular, the microstructure of x = 0.07 sample was composed of grains ranging 0.5–2.2 μm in size. As a result, the frequency dispersion of dielectric constant for the (1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07) ceramics was reduced and the mechanical quality factor Q_m was enhanced with increasing Ba and Ti content.     

Large electrostrictive effect and dielectric properties of (K0.5Na0.5)NbO3-BaZrO3 ceramics

Electrostrictive materials are sought after in high-precision displacement actuators due to their ultra-low hysteresis electrostrain. Here we report a very promising electrostrictive material by doping barium zirconate into a sodium potassium niobate matrix. Also, the dielectric behaviors and electrostrictive properties of (1-x)K_0.5Na_0.5NbO₃-xBaZrO₃ (x = 0.10, 0.12, 0.14 and 0.16) ceramics are investigated in details. The doping of barium zirconate allows the diffuse phase transition to occur near room temperature. Diffuse dielectric anomalies at high temperatures are observed in poled ceramics, which are closely related to the activation of defects. High electrostrain (～0.1 %) with ultra-low strain hysteresis (< 12 %) are obtained at x = 0.14. Moreover, electrostrain exhibits excellent stability over a wide temperature range from 30 ℃ to 120 ℃.Further, the electrostrictive coefficient of ～0.066 m⁴/C² for the component was determined at high temperatures and high electric fields, demonstrating the excellent potential of the K_0.5Na_0.5NbO₃-BaZrO₃ solid solutions as electrostrictive materials.     

Crystal growth and ferroelectric property of Na0.5K0.5NbO3 and Mn-doped Na0.5K0.5NbO3 crystals grown by floating zone method

Na_0.5K_0.5NbO₃ (NKN) and Mn-doped NKN crystals, which are one of the promising candidates of lead-free piezoelectric materials, were grown by using a floating zone (FZ) method. The resulting crystal growth was compared with crystal growth that resulted from using a flux method in a previous study. In the crystal grown by FZ method under where the growth rate was controlled to 3 mm/h, thin layers formed parallel and perpendicular directions to the growth direction. In the crystal grown by FZ method, the crystal structure could not be classified as having the orthorhombic lattice of Amm2, which was observed in the crystal grown using a flux method. It was found that doped Mn was substituted in the perovskite-type lattice of NKN. Pure NKN crystals showed 90° domains that had a zig-zag shape, whereas Mn-doped NKN crystals were aligned to the domain layers in straight lines. It was confirmed that Mn-doped NKN crystal showed a square P–E hysteresis loop.     

Dielectric,ferroelectric and field-induced strain behavior of K0.5Na0.5NbO3-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

Lead-free piezoelectric (1 ? x)Bi_0.5(Na_0.78K_0.22)_0.5TiO₃–xK_0.5Na_0.5NbO₃ (BNKT–xKNN, x = 0–0.10) ceramics were synthesized using a conventional, solid-state reaction method. The effect of KNN addition on BNKT ceramics was investigated through X-ray diffraction (XRD), dielectric, ferroelectric and electric field-induced strain characterizations. XRD revealed a pure perovskite phase with tetragonal symmetry in the studied composition range. As the KNN content increased, the depolarization temperature (T_d) as well as maximum dielectric constant (?_m) decreased. The addition of KNN destabilized the ferroelectric order of BNKT ceramics exhibiting a pinched-type hysteresis loop with low remnant polarization (11 μC/cm²) and small piezoelectric constant (27 pC/N) at 3 mol% KNN. As a result, at x = 0.03 a significant enhancement of 0.22% was observed in the electric field-induced strain, which corresponds to a normalized strain (S_max/E_max) of ～434 pm/V. This enhancement is attributed to the coexistence of ferroelectric and non-polar phases at room temperature.     

Thermal stability and local atomic structure of (Na0.5K0.5)NbO3 doped BaTiO3 ceramics

Dielectric properties of x(Na_0.5K_0.5)NbO₃–(1−x)BaTiO₃ (x=0.00 and 0.06) specimens were investigated in terms of changes in local atomic structure, according to the phase transition by elevating the overall temperature. A 0.06(Na_0.5K_0.5)NbO₃–0.94BaTiO₃ (NKN–BT) specimen exhibited enhanced temperature stability along with an increased dielectric constant. The degree of reduction in tetragonality (c/a) at the Curie temperature was smaller in NKN–BT compared to that in pure BaTiO₃, as calculated by Rietveld refinement. From a comparison of the pre-edge region in the Ti K-edge, it was determined that the off-center displacement of the Ti atom was also raised to 13.4% through NKN substitution, with a change in local orientation from the [001] to the [111] directions. The substitution by NKN, which has a different ionic radius and electrical charge compared with BaTiO₃, causes structural distortion of the TiO₆ octahedra in the NKN–BT lattice, resulting in local polarization. These structural changes lead to the temperature stability of the dielectric constant and an overall improvement in the electrical properties of BaTiO₃.     

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.