Electric aging behavior of lead-free Li0.06(K0.48Na0.52)0.94(Nb0.86Ta0.08Sb0.06)O3 piezoelectric ceramics improved by pre-calcined method

In order to improve the piezoelectric and aging properties of the lead-free Li_0.06(K_0.48Na_0.52)_0.94(Nb_0.86Ta_0.08Sb_0.06)O₃ piezoelectric ceramics, the conventional solid-state reaction method and the B-side pre-calcined method were achieved and compared in this paper. The physical and electrical properties of the lead-free Li_0.06(K_0.48Na_0.52)_0.94(Nb_0.86Ta_0.08Sb_0.06)O₃ piezoelectric ceramics material were investigated and discussed. For the B-side pre-calcined method, the ceramic material exhibited the excellent electrical and piezoelectric parameters. Finally, the electromechanical coupling factors, the resonance frequencies, and the resonance resistances of the lead-free ceramic materials were also discussed.     

Technology and electrophysical properties of the (K0.44Na0.52Li0.04)(Nb0.9–xTa0.1Sbx)O3 ceramics

Compounds based on (K_zNa_1–z)NbO₃ (KNN) are promising lead-free ferroelectric materials that reveal good electrophysical properties. In the present work, we report the results of the study influence of the doping effect of antimony on the technology, microstructure and electrophysical properties of potassium sodium niobate ceramics modified by lithium and tantalum ions (K_0.44Na_0.52Li_0.04)(Nb_0.9–xTa_0.1Sb_x)O₃ (KNLNTSbx). The four KNLNTSbx ceramic compositions were designed:

1. (K_0.44Na_0.52Li_0.04)(Nb_0.9Ta_0.1)O₃ (for x_Sb?=?0),

2. (K_0.44Na_0.52Li_0.04)(Nb_0.88Ta_0.1Sb_0.02)O₃ (for x_Sb?=?0.02),

3. (K_0.44Na_0.52Li_0.04) (Nb_0.87Ta_0.1Sb_0.03)O₃ (for x_Sb?=?0.03),

4. (K_0.44Na_0.52Li_0.04) (Nb_0.86Ta_0.1Sb_0.04)O₃ (for x_Sb?=?0.04).

All ceramic powders were synthesised by the standard solid-state reaction method from the mixture of oxides and carbonates. The paper presents the technology and results of crystal structure, microstructural, dielectric properties, as well as DC electrical conductivity of the KNLNTSbx ceramics. The conducted research proved that suitable doping of the KNN materials improves the sinterability of the ceramic compositions and positively influences the useful electrophysical properties thereof.     

Effects of Sb content on electrical properties of lead-free piezoelectric (K0.4425Na0.52Li0.0375) (Nb0.9625−xSbxTa0.0375)O3 ceramics

Lead-free (K_0.4425Na_0.52Li_0.0375) (Nb_0.9625−xSb_xTa_0.0375)O₃ piezoelectric ceramics were prepared by the conventional sintering method. The effects of the Sb content on the phase structure, microstructure, dielectric, piezoelectric, and ferroelectric properties of the (K_0.4425Na_0.52Li_0.0375) (Nb_0.9625−xSb_xTa_0.0375)O₃ ceramics were investigated. The much higher Pauling electronegativity of Sb compared with Nb makes the ceramics more covalent. By increasing x from 0.05 to 0.09, all samples exhibit a single perovskite structure with an orthorhombic phase over the whole compositional range, and the bands in the Raman scattering spectra shifted to lower frequency numbers. The grain growth of the ceramics was improved by substituting Sb⁵⁺ for Nb⁵⁺. Significantly, the (K_0.4425Na_0.52Li_0.0375) (Nb_0.8925Sb_0.07Ta_0.0375)O₃ ceramics show the peak values of the piezoelectric coefficient (d₃₃), electromechanical coupling coefficient (k_p), and dielectric constant (?), which are 304 pC/N, 48% and 1909, respectively, owing to the densest microstructure of typical bimodal grain size distributions. Besides, the underlying mechanism for variations of the electrical properties due to Sb⁵⁺ substitution was explained in this work.     

Investigation of fracture toughness of modified (KxNa1 − x)NbO3 lead-free piezoelectric ceramics

While most of the previous studies have focused on the processing and electrical properties of KNN-based ceramics, very little research has been carried out to evaluate their mechanical behavior. This work presents for the first time an examination of the fracture toughness, K_IC, of the most widely studied (K_xNa_1 ? x)NbO₃ (KNN)-based lead-free ceramics modified with lithium, tantalum and antimony. The samples were produced through the conventional mixed-oxide route and the K_IC values were measured using the single edge V-notched beam (SEVNB) method under four-point bending. The mean K_IC values were determined to be 0.48 ± 0.18 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)NbO₃, 0.8 ± 0.18 MPa m^1/2 for (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃, 0.86 ± 0.04 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.9Ta_0.1)O₃ and 1.06 ± 0.21 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ compositions. The microstructure, phase structure and dielectric constant values of the samples have been used to correlate the results of the K_IC values.     

Dielectric and impedance spectroscopy analysis of lead-free (1-x)(K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3-xBaTiO3 ceramics

(1-x)(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃-xBaTiO₃ (labeled as (1-x)KNLNTS- xBT, x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) lead-free ceramics were prepared by a solid-state sintering method. As the BT content increased, the phase of ceramics changed from orthorhombic (0.00 ≤ x ≤ 0.02) to orthorhombic-tetragonal (0.02 < x < 0.06) structure, and finally turned into tetragonal (0.06 ≤ x ≤ 0.10) structure. The Curie-Weiss law and modified Curie-Weiss law were applied to analyzing dielectric properties. With the increase of BT content, the relaxation degree increased, which indicated that the ceramics shown a excellent relaxation behavior. For 0.9KNLNTS- 0.1BT ceramics, the dispersion coefficient γ reached the maximum of 1.73, which is hugely attractive for lead-free relaxor ferroelectrics. From its variation of impedance spectroscopy with temperature, it was found that the relaxation and conduction behavior were associated with the thermal activation, and the oxygen vacancies were the potential ionic carriers. Moreover, through Arrhenius fitting, the activation energy of 0.9KNLNTS- 0.1BT ceramic was 0.82(6) eV, indicating that the oxygen vacancy concentration for the ceramics was high.     

Effect of ZnO on the structure,microstructure and electrical properties of KNN-modified piezoceramics

The effects of ZnO addition on structure, microstructure and dielectric properties of (K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃ ceramics (KNL–NTS) are investigated. Doping provokes phase segregation, evidenced by the appearance of different types of grains and a tetragonal tungsten–bronze secondary phase at high ZnO doping levels. A clear increase of ferroelectric–paraelectric and orthorhombic-to-tetragonal phase transition temperatures with ZnO content is observed. This produces a reduction of the room temperature piezoelectric constants due to the reduction of the tetragonality (c/a) ratio of the KNL–NTS structure.     

Piezoelectric and dielectric properties of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3–PVDF composites

(Li, Ta, Sb) modified sodium potassium niobate/poly(vinylidene fluoride) [(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃–PVDF] 0-3 composites were prepared by a cold press technique, and their piezoelectric and dielectric properties were characterized. All composites exhibited good dispersion of ceramic particles in the polymer matrix. The piezoelectric and dielectric constants were found to be enhanced as the concentration of sodium potassium niobate increases. Even though the process is simple, the composite prepared in this study showed better piezoelectric and dielectric properties than PZT–polymer composites. At room temperature, a (K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃–PVDF (7:3) composite revealed a relative dielectric constant, ?_r = 166, piezoelectric constant, d₃₃ = 33 pC/N and coercive field, E_c = 5 kV/cm.     

[Bi0.5(Na0.4-xLixK0.1)]0.96Sr0.04Ti0.975Ta0.025O3 lead-free RELAXOR ceramics with the enhanced recoverable energy density

The high recoverable energy density (W_rec) of 1.83 J/cm³ was achieved at 120 kV/cm by simply introducing 0.050 mol. % Li⁺ ions into the A-site of Bi_0.5(Na_0.4K_0.1)]_0.96Sr_0.04Ti_0.975Ta_0.025O₃ lead-free relaxor ceramics. Bi_0.5(Na_0.4-xLi_xK_0.1)]_0.96Sr_0.04Ti_0.975Ta_0.025O₃ (BNKSTT-100xLi) ceramic samples were prepared by solid-state reaction method. The influence of Li⁺ ions on structure and electrical properties was analyzed in detail. XRD patterns and Raman spectra of these samples were revealed a pure perovskite-type structure. BNKSTT-100xLi ceramics exhibited an obvious relaxor characteristic with the downshift of T_F-R to ambient temperature. J - E loops confirmed the relaxor ferroelectric like nature. Leakage current density was proved to decrease with the increasing Li⁺ ions concentration. At the composition x = 0.050, remnant polarization (P_r) kept steady while maximum polarization (P_max) and breakdown strength (DBS) increased obviously, which was beneficial to the enhancement of recoverable energy density.     

Poling and depoling influence on the micro-stress states and phase coexistence in KNN-based piezoelectric ceramics

In this work a microstructural qualitative and quantitative study of spatial stress distributions in modified KNN ceramics (K_0.44Na_0.52Li_0.04)_1-xCo_x/2 (Nb_0.86Ta_0.10Sb_0.04)O₃, according to the polarization state is shown. X-ray diffraction reflects a perovskite crystalline structure with coexistence of Tetragonal and Orthorhombic phases (T/O). Confocal Raman microscopy shows that these crystalline phases are distributed in randomly micrometric regions through the ceramic volume. Tetragonal regions show higher piezoelectric coefficient and exhibit a higher micro-stress that hardens the ferroelectric response. By the contrary, the occurrence of orthorhombic micro-regions softened the ferroelectric behavior and reduced their piezoelectric coefficients. The ferroelectric response of ceramics is studied, where poling is also shown as a factor that affects the spatial micro-stress distributions. Finally, a model that relates the results obtained by Raman characterization with the ferroelectric properties and stress states is proposed.     

The effect of B-site substitution on the structural evolution and electrical properties of lead-free (K,Na)NbO3 ceramics

Lead-free (K_0.49Na_0.51)(Nb_1−xSb_x)O₃ piezoelectric ceramics were prepared via the conventional sintering method and the effect of the substitution of Nb with Sb on the phase structure, microstructure and electrical properties of the prepared (K_0.49Na_0.51)(Nb_1−xSb_x)O₃ ceramics was systematically investigated. The prepared ceramics exhibited a single-phase perovskite structure which changed from a standard orthorhombic structure to a pseudocubic structure as x was increased from 0 to 0.1. X-ray diffraction patterns and Raman spectra obtained for the prepared ceramics clearly revealed that the degree of structural symmetry increased with x. Substituting an appropriate amount of Sb⁵⁺ for Nb⁵⁺ was found to improve the microstructure and thereby enhance the piezoelectric/ferroelectric properties. Further increasing the Sb content resulted in a decrease of the average grain size and a deterioration of the performance. The peak values of the piezoelectric constant d₃₃ (182 pC/N) and the electromechanical coupling coefficient k_p (41%) were obtained for the ceramic with x=0.06.     

An enhanced mechanical quality factor and a low dielectric loss in lithium sodium niobate lead-free ceramics

(Li_0.12Na_0.88)(Nb_0.96?xSb_0.04Ta_x)O₃ (LNNST) ceramics were fabricated by the normal sintering. These LNNST ceramics endure a phase transition from an orthorhombic phase, a coexistence of orthorhombic and tetragonal phases, to a tetragonal phase with increasing Ta content. Dense microstructure has been developed for all ceramics. The T_c decreases and the ?_r increases with increasing Ta content, together with a very low dielectric loss of less than 1.3%. A high Q_m value of ～1230 is demonstrated for the ceramic with x = 0.06. Enhanced piezoelectric properties are also demonstrated for the ceramic with x = 0.03 because of a coexistence of two phases. Therefore, this ceramic is a promising candidate for the transducer and transformer applications.     

Understanding the piezoelectric properties in potassium-sodium niobate-based lead-free piezoceramics: Interrelationship between intrinsic and extrinsic factors

Lead zirconate titanate (PZT) based ceramics are currently enjoying a wide use in piezoelectric devices despite lead toxicity. Due to growing environmental concerns, the attention on piezoelectric ceramics has been moving to lead-free materials, in particular to (K,Na)NbO₃-based ceramics. Here we report a systematic evaluation of the effects of the compositional modifications on [(K_0.44Na_0.52Li_0.04)[(Nb_0.86Ta_0.10Sb_0.04)_1-xZr_5x/4]O₃ lead–free piezoceramics. We show that an interrelationship between the intrinsic and extrinsic factors is the linchpin for the development of good piezoelectric properties. Hence, the stabilization of the tetragonal symmetry on the orthorhombic-tetragonal polymorphic phase boundary facilities the poling process of the system, thereby enhancing the piezoelectric response. Additionally, the microstructure appears to be related to the piezoelectric properties; i.e., the improved piezoelectric properties correlate to the increase in grain size. The results of this work could help to understand the origin of piezoelectricity in potassium–sodium niobate-based ceramics.     

Full characterization for material constants of a promising KNN-based lead-free piezoelectric ceramic

Potassium-sodium niobate (K_1-xNa_xNbO₃, referred to as KNN) solid solutions, which are an important type of lead-free piezoelectric materials possessing environmentally friendly features, good piezoelectric response and high Curie temperature, have attracted considerable attention in replacing lead-based ceramics. In order to promote the application of KNN-based ceramics in piezoelectric devices, we characterized a complete set of material constants of a high performance KNN-based ceramic, that is 0.965(K_0.48Na_0.52) (Nb_0.96Sb_0.04)O₃-0.035Bi_0.5Na_0.5Zr_0.15Hf_0.75O₃ (KNNS-BNZH), whose Curie temperature is 235 °C, piezoelectric coefficient d₃₃ is 380 pC/N and electromechanical coupling factor k₃₃ is 70%. These results will benefit the design of piezoelectric transducers and actuators using lead-free piezoelectric ceramics.     

Feasible acid-etching method for investigating temperature-dependent domain configurations of ferroelectric ceramics

Domain structure often plays significantly important roles in determining both the piezoelectric properties and the piezoelectric temperature stabilities of ferroelectric ceramics. However, a convenient and reliable technique for studying temperature-dependent domain configurations is greatly desired. Inspired by “burning CDs”, we proposed here a temperature-variable acid-etching method (abbreviated hereafter as VTAE) for such purpose. Utilizing this method, domain configurations of the poled 0.96(K_0.48Na_0.52)(Nb_0.96Sb_0.04)O₃–0.04(Bi_0.50Na_0.50)ZrO₃ (abbreviated hereafter as KNNS-4%BNZ)lead-free ceramic were investigated at different temperatures. Diversified domain patterns that consist of parallel-stripe domains, herringbone domains and wedge-shaped domains are recognized in different phases, respectively. It is found from the evolution of domain configurations with temperature that nanodomain structure is closely related to the high piezoelectric responses and temperature instabilities in the KNNS-4%BNZ ceramic. The result proves that this VTAE method is a feasible method for studying the temperature-dependent domain configurations of ferroelectric ceramics.     

A study on epoxy‐based 1–3 piezoelectric composites using finite element method

The article deals with finite element study of 1–3 composites made of piezoceramic fibers embedded in epoxy. The study focuses on evaluation of effective properties of piezoelectric fiber composites using representative volume element method. The physical properties deduced are further used for analyzing sensing and actuation characteristics of unimorph cantilever beam. K_0.5Na_0.5NbO₃‐LiSbO₃ (KNN‐LS), KNN‐LS doped with 1 wt% CaTiO₃ [KNN‐LS‐CT (1 wt%)], K_0.475Na_0.475Li_0.05(Nb_0.92Ta_0.05Sb_0.03)O₃ (KNLNTS), 0.885 (Bi_0.5Na_0.5)TiO₃−0.05(Bi_0.5K_0.5)TiO₃−0.015 (Bi_0.5Li_0.5) TiO₃ −0.05 BaTiO₃ (BNKLBT) and lead zirconate titanate (Pb [Zr_xTi_1−x] O₃) (PZT) are embedded in epoxy matrix to form 1–3 piezocomposites. K_0.475Na_0.475Li_0.05(Nb_0.92Ta_0.05 Sb_0.03)O₃ (KNLNTS) shows excellent performance than other composites under study. It can be concluded that KNLNTS (lead‐free piezoelectric material) is a potential candidate which can replace lead‐based PZT for smart structure applications. POLYM. COMPOS., 37:1895–1905, 2016. © 2015 Society of Plastics Engineers     

Hydrothermal synthesis and piezoelectric property of Ta-doping K0.5Na0.5NbO3 lead-free piezoelectric ceramic

Ta-doping K_0.5Na_0.5Nb_1−xTa_xO₃ (x = 0.1, 0.2, 0.3, 0.4) powder was synthesized by hydrothermal approach and its ceramics were prepared after sintering and polarizing treatment in this work. The K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics near morphotropic phase boundary (MPB), which exhibited optimum piezoelectric properties of d₃₃ = 210 pC/N and good electromechanical coupling factors of K_p = 0.3. The domain structure has been observed from TEM images which indicates that the K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics have good piezoelectric and ferroelectric properties for it is near the MPB.     

Investigation of phase boundaries in the system (KxNa1−x)1−yLiy(Nb1−zTaz)O3 using high-throughput experimentation (HTE)

A high-throughput experimental (HTE) approach starting from dry, fine-grained powders was used to synthesize bulk samples in the system (K_xNa_1?x)_1?yLi_y(Nb_1?zTa_z)O₃, a doped variant of the piezoelectric (K_0.5Na_0.5)NbO₃ (KNN). Starting from the system (K_0.5Na_0.5)_1?yLi_y(Nb_1?zTa_z)O₃ known from the works of Saito et al. an effort was made to establish a higher order phase diagram. Special emphasis was put on expanding the known morphotropic phase boundary that constitutes a region of special interest for electroceramic materials as it features maximum piezoelectric properties. Analyses were performed using a HTE-compatible technique, namely automated powder X-ray diffraction (XRD).     

Influence of B-site compositional homogeneity on properties of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3-based piezoelectric ceramics

The effect of B-site compositional homogeneity on microstructure, piezoelectric properties and dielectric behaviour of lead-free piezoelectric ceramics, (K_0.44Na_0.52Li_0.04) (Nb_0.86Ta_0.10Sb_0.04)O₃, is investigated. The B-site compositional homogeneity is evaluated by using an intermediate precursor obtained by solid state reaction between adequate amounts of Nb₂O₅, Ta₂O₅ and Sb₂O₅, calcined at 1350 °C and attrition milled. The B-site precursor powder is mixed with alkaline carbonates to synthesize perovskite powders and, finally, sinter piezoceramics. X-ray diffraction and Raman spectroscopy reveal the formation of a perovskite phase, although tetragonal tungsten-bronze structure is detected as minor secondary phase. Ceramics processed by using B-site precursor show different crystalline structure as a function of sintering conditions or K/Na ratio. The B-site precursor route produces thus lower piezoelectric properties, but the control of alkali volatilization by using sintering powder bed resulted in a relevant decrease of dielectric losses that favours the d₃₃ enhancement.     

Improvement of Physical Properties for KNN‐based Ceramics by Modified Two‐Step Sintering

A modified two‐step sintering (MTSS) technique was attempted to fabricate dense (K,Na)NbO₃‐based lead‐free ceramics through the solid‐state reaction route. (K_0.50Na_0.50)_0.98Li_0.02(Nb_0.795Ta_0.18Sb_0.025)O₃ and (K_0.45Na_0.55)_0.98Li_0.02Nb_0.77Ta_0.18Sb_0.05O₃ were chosen as the test chemical compositions, and high relative density values over 97.5% were successfully obtained in their corresponding ceramics. Consequently, the d₃₃ values were enhanced considerably and reached 353 and 436 pC/N, respectively. The latter d₃₃ value is a record one ever achieved in the (K,Na)NbO₃‐based ceramic materials so far. Furthermore, observation analyses of microstructure and domain patterns were carried out and showed that partial depoling occurs more easily in large grains than in small ones. In particular, it was found that the (K_0.50Na_0.50)_0.98Li_0.02(Nb_0.795Ta_0.18Sb_0.025)O₃ ceramic densified by MTSS has a quite uniform grain‐size distribution with comparatively small grains and exhibits the very excellent time‐aging stability. The study results suggest that microstructure with high relative density and suitably suppressed grain sizes is desirable for further acquiring the superior KNN‐based ceramics of both excellent piezoelectric properties and good depoling stabilities.     

Phase structure and enhanced piezoelectric properties in (1-x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-x(Bi0.5Na0.42Li0.08)0.9Sr0.1ZrO3 lead-free piezoelectric ceramics

The piezoelectric properties of KNN lead-free piezoelectric ceramics could be greatly enhanced by forming multiphase coexistence. In this work, binary system (1-x)(K_0.48Na_0.52)(Nb_0.95Sb_0.05)O₃-x(Bi_0.5Na_0.42Li_0.08)_0.9Sr_0.1ZrO₃ [(abbreviated as (1-x)KNNS-xBNLSZ] ceramics with rhombohedral-tetragonal (r-T) phase boundary was designed and synthesized using the conventional solid-state sintering method, and effects of BNLSZ contents on their micrograph, phase structure and electrical properties were also investigated. According to phase diagram from the results of temperature-dependent capacitance and dielectric constant, the ceramics exhibit the R-T phase coexistence in the composition range of 3.5%≤x<4.5%, and an enhanced dielectric, ferroelectric, and piezoelectric behavior was obtained at such a phase boundary zone. As a result, the ceramics with x=0.04 exhibit optimum electrical properties of d₃₃~461 pC/N, k_p~46%, tan δ~0.03, P_r~16.9 μC/cm², and E_c ~9 kV/cm, together with a Curie temperature (T_C) of ~228 °C. Such a good comprehensive performance obtained in this present work is due to the R-T phase transition and enhanced ɛ_rP_r. It was believed that this ceramic system would promote the development of KNN-based lead-free ceramics.