Sintering and electrical properties of La-modified (Na0.52K0.45Li0.03)1−3xLax(Nb0.88Sb0.09Ta0.03)O3 lead-free ceramics

(Na_0.52K_0.45Li_0.03)_1−3xLa_x(Nb_0.88Sb_0.09Ta_0.03)O₃ (NKLL_xNST) lead-free ceramics were prepared by normal sintering and their dielectric and piezoelectric properties were investigated. The X-ray methods indicate that the NKLL_xNST ceramics with x≤0.003 present a pure perovskite phase at room temperature. The bulk density of NKLL_xNST ceramics increases with proper amount of La₂O₃ contents, and reaches its highest value of 4.544 g/cm³ with the addition of 0.3 mol% La₂O₃. At x=0.003, remnant polarization P_r, piezoelectric constant d₃₃ and planar mode electromechanical coupling factor k_p of NKLL_xNST ceramics reach the highest values of 37.80 μC/cm², 346 pC/N and 40%, respectively, exhibiting excellent “soft” piezoelectric characteristics, demonstrating a tremendous potential of the compositions studied for device applications.     

Phase transition,microstructure, and dielectric properties of Li/Ta/Sb co-doped (K,Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−xTa_xSb_0.07)O₃ (abbreviated KNLNST_x) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 °C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNST_x ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST_0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d₃₃=313 pC/N, k_p=47%, and ε_r=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d₃₃=200 pC/N and ε_r=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.     

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.     

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.     

Domain structure of nonstoichiometric sodium potassium niobate-based ceramics for piezoelectric acoustic actuators

In this work, three piezoelectric acoustic actuators are prepared by using multi-layer technology for lead-free (Na_0.48-xK_0.48-xLi_0.04)Nb_0.89-xTa_0.05Sb_0.06O₃-xSrTiO₃ (NKLNTS-xST) ceramics, x=0 and 0.007, and commercial lead-based ceramic. The phase and domain structure are also investigated by using the XRD patterns and TEM images of the NKLNTS-xST ceramics. The (Na_0.473K_0.473Li_0.04Sr_0.007)Nb_0.883Ti_0.007Ta_0.05Sb_0.06O₃ piezoelectric acoustic actuator has the best sound pressure level and d₃₃ value of 650 pCN^?1. The response mechanisms suggest that the piezoelectric ceramic vibration amplitude was increased and the sound pressure level improved since the orthorhombic and tetragonal phases were found to coexist and the nanoscale domain increased for the (Na_0.473K_0.473Li_0.04Sr_0.007)Nb_0.883Ti_0.007Ta_0.05Sb_0.06O₃ ceramic.     

Improved Li and Sb doped lead-free (Na,K)NbO3 piezoelectric ceramics for energy harvesting applications

Piezoelectric energy harvesting is the most widely investigated technology for renewable energy applications. In this work, (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ piezoelectric ceramics were prepared through conventional mixed oxide fabrication methods with different sintering temperatures. Although the (Na_0.5K_0.5)NbO₃ piezoelectric material is representative among the lead-free ceramics, it is difficult to densify by typical sintering techniques owing to its easy evaporation properties of potassium (K⁺) and sodium ion (Na⁺). Hence, lithium (Li⁺) and antimony ion (Sb⁵⁺) were used for the partial substitution of (Na_0.5K_0.5)NbO₃. With the optimized sintering temperature, Li⁺ and Sb⁵⁺ are expected to be crucial in increasing the density and enhance the piezoelectric and ferroelectric properties. In this study, the phase, microstructure, and dielectric and electrical properties of (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ ceramics depending on the sintering temperature is examined by employing X-ray diffraction, field emission scanning electron microscopy, impedance analyzer, and mechanical force system for energy harvesting.     

Phase transition behavior and temperature-stable piezoelectric properties of new quaternary (K,Na)NbO3 based ceramics

(K, Na)NbO₃-based lead free materials have been found to exhibit good piezoelectric properties due to the orthorhombic–tetragonal polymorphic phase transition (PPT) temperature compositionally shifted downward to near room temperature. However, this transition correspondingly results in a strong temperature dependence of the dielectric and piezoelectric properties. In this work, new quaternary (1?x) (K_0.4425Na_0.52Li_0.0375)(Nb_0.8925Sb_0.07Ta_0.0375)O₃ (KNLNST)–xSrTiO₃ (ST) lead-free piezoelectric ceramics were fabricated by a conventional ceramic technique and their structure and piezoelectric properties were also studied. The results of X-ray diffraction reveal that SrTiO₃ diffuses into the KNLNST lattices to form a new solid solution with a perovskite structure. After the addition of SrTiO₃, tetragonal–orthorhombic phase transition shifts to lower temperatures. The good piezoelectric properties of 0.995 KNLNST–0.005 ST material were found to be d₃₃～295 pC/N, k_p～42%, and ε_r～1902, with greatly improved temperature stability over the temperature range of 0–100 °C, demonstrating practical potential for actuator and ultrasonic transducer applications.     

Low Temperature Sintering and Enhanced Piezoelectricity of Lead‐Free (Na0.52K0.4425Li0.0375)(Nb0.86Ta0.06Sb0.08)O3 Ceramics Prepared from Nano‐Powders

(Na_0.52K_0.4425Li_0.0375)(Nb_0.86Ta_0.06Sb_0.08)O₃ powders were synthesized via sol–gel and solid‐state reaction methods as a raw material for the preparation of the ceramics. Dependence of piezoelectric properties and microstructure on sintering temperatures was investigated in this study. Sol–gel‐derived nano‐powders could be densified at a lower temperature of 940°C and exhibited excellent electrical properties after sintering at 1020°C (d₃₃ = 424 pC/N, d₃₃^* = 780 pm/V, k_p = 52.1%, and T_c = 265°C). The enhanced electric properties were most likely due to the coexistence of orthorhombic and tetragonal phase in the samples at room temperature, homogenous microstructure with fine grain and high density.     

CuO掺杂对(K0.46Na0.54)(Nb0.93Sb0.03Ta0.04)O3无铅压电陶瓷结构和性能的影响

采用传统陶瓷制备方法制备了(K_0.46Na_0.54)(Nb_0.93Sb_0.03Ta_0.04)O₃-x mol% CuO(简记为:KNNST- xCuO)陶瓷,探讨了CuO含量对该陶瓷材料的相结构、显微结构和电学性能的影响.实验结果表明,CuO的掺入使陶瓷材料的机械品质因数Q_m得到了显著的提高.当CuO含量为3mol%时,陶瓷样品的综合性能最佳:d₃₃=124pC/N ,k_p=0.41,Q_m=1054。     

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.     

Nanodomains in KNN‐Based Lead‐Free Piezoelectric ceramics: Origin of Strong Piezoelectric Properties

Excellent piezoelectric properties of d₃₃* = 768 pm/V and strain = 0.07% at 1 kV/mm, were obtained by lead‐free in (Na_0.52K_0.4425Li_0.0375)(Nb_0.86Ta_0.06Sb_0.08)O₃ ceramics. They displayed good temperature stability up to 200°C. Significantly enhanced piezoelectricity originated from nanodomains of width 20–30 nm, and the result was confirmed by transmission electron microscopy. The above‐mentioned nanodomains emerged because of low domain wall energy near polymorphic phase transition regions and insignificant differences in cell parameters between orthorhombic and tetragonal phases. The nanodomain configuration easily responded to an external electric field, leading to high electric field‐induced strain.     

Relaxor behavior and energy storage density induced by B-sites substitutions in (Ca0.28Ba0.72)2.1Na0.8Nb5O15 Tungsten bronze ceramics

Lead-free relaxor ferroelectrics (Ca_0.28Ba_0.72)_2.1Na_0.8Nb_5-xSb_xO₁₅ (CBNNS) and (Ca_0.28Ba_0.72)_2.1Na_0.8Nb_5-yTa_yO₁₅ (CBNNT) with tungsten bronze structure were fabricated via solid-state reactions. The obtained CBNNS and CBNNT ceramics showed different dielectric behaviors. Only the CBNNS ceramics revealed an intensified diffusion and relaxor-like characteristics, which could be verified by the modified Curie–Weiss law. The relaxor behaviors in CBNNS were attributed to the radii difference between Sb⁵⁺ and Nb⁵⁺ ions co-occupying in B-sites. For the substitution of Nb⁵⁺ by Sb⁵⁺ in CBNNS ceramics, the change from macroscopic polarization to local polarization could also give rise to the obvious relaxor behavior. The Raman spectra verified a larger off-centering of the cation and a higher distortion degree for BO₆ octahedron in the ab plane for CBNNS ceramics when compared with those of CBNNT. In addition, the ferroelectric properties of CBNNS ceramics further indicated the relaxor ferroelectric nature, and also confirmed that the relaxor behavior helped to improve the energy-storage performance.     

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.     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

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.     

Compositional dependence of phase structure and electrical properties in (K0.50Na0.50)0.97Bi0.01(Nb1−xZrx)O3 lead-free ceramics

(K_0.50Na_0.50)_0.97Bi_0.01(Nb_1-xZr_x)O₃ (KNBNZ) lead-free ceramics were prepared by the conventional solid-state sintering process. Their phase structure is dependent on the Zr content in the investigated range, and the ceramics endure a phase transition from pseudocubic to orthorhombic with increasing Zr content. Improved piezoelectric properties have been observed when the poling temperature is located at ~100 °C because of the coexistence of orthorhombic and tetragonal phases. Their dielectric and piezoelectric properties were enhanced by doping Zr, the ceramic with x=0.02 showing optimal electrical properties, i.e., d₃₃~161 pC/N, k_p~0.41, Q_m~81, T_c~370 °C, and T_o−t~130 °C. These results show that the KNBNZ ceramic is a promising lead-free piezoelectric material.     

Phase transition and electrical properties of Bi0.5(Na0.8K0.2)0.5ZrO3 modified (K0.52Na0.48)(Nb0.95Sb0.05)O3 lead-free piezoelectric ceramics

In this work, (1−x)(K_0.52Na_0.48)Nb_0.95Sb_0.05O₃−xBi_0.5(Na_0.8K_0.2)_0.5ZrO₃ [abbreviated as (1−x)KNNS−xBNKZ, x=0–0.06] lead-free ceramics were fabricated using solid-state reaction method. The effects of BNKZ contents on the phase structure, piezoelectric and ferroelectric properties were investigated. The phase boundaries including orthorhombic-tetragonal (O-T) and rhombohedral-tetragonal (R-T) multiphase coexistence were identified by XRD patterns and temperature-dependent dielectric constant by adding different content of BNKZ. A giant field induced strain (~0.25%) along with converse piezoelectric coefficient d₃₃^* (~629.4 pm/V) and enhanced ferroelectricity P_r (~38 μC/cm²) were obtained when x=0.02, while the specimen with x=0.03 presented the optimal piezoelectric coefficient d₃₃ of 215 pC/N, due to the O-T or R-T phase coexistence near room temperature respectively. These results show that the introduction of Bi_0.5(Na_0.8K_0.2)_0.5ZrO₃ is a very effective way to improve the electrical properties of (K_0.52Na_0.48)(Nb_0.95Sb_0.05)O₃ lead-free piezoelectric ceramics.     

Effect of oriented defect-dipoles on the ferroelectric and piezoelectric properties of CuO-doped (K0.48Na0.52)0.96Li0.04Nb0.805Ta0.075Sb0.12O3 ceramics

The orientation characteristics of the defect dipoles formed by acceptor ions (${\mathrm{Cu}}_{\mathrm{Nb}}^{″\prime }$) and oxygen vacancies ($V_{\mathrm{O}}^{??}$) in CuO-doped (K_0.48Na_0.52)_0.96Li_0.04Nb_0.805Ta_0.075Sb_0.12O₃ piezoceramics were investigated. The ferroelectric and piezoelectric properties of the ceramics were obviously affected by the defect dipoles which are oriented along with the domains when poled under a dc electrical field of 3.5?kV/mm at the temperature near T_C. The poled ceramics with 1.0–3.0?mol% CuO displayed strong asymmetric P-E loops and a large internal bias field (E_i), 3.89–4.57?kV/cm, when polarized at the temperature near T_C. The enhanced piezoelectric coefficient d₃₃, 186–192?pC/N, was obtained for the ceramics poled near T_C when 0.02?≤?x?≤?0.03. The ceramics with oriented defect dipoles showed a relatively good thermal stability of d₃₃ until 180?°C.     

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).     

Sb doping effects on the piezoelectric and ferroelectric characteristics of lead-free Na0.5K0.5Nb1−x SbxO3 piezoelectric ceramics

The piezoelectric, phase structure and ferroelectric properties of lead-free Na_0.5K_0.5Nb_1−xSb_xO₃ piezoelectric ceramics were systematically investigated. (Na_0.5K_0.5)Nb_1−xSb_xO₃ ceramics were synthesized using the conventional solid-state reaction method. The effect of B-site Sb⁵⁺ substitution for Nb⁵⁺ on the structural characteristics and piezoelectric properties of samples was determined. According to XRD patterns, the phase gradually changes from tetragonal to orthorhombic with increasing Sb content. At x = 0.03, the tetragonal and orthorhombic phases coexist and relatively high piezoelectric properties were obtained. The relatively high Q_m value obtained after substitution with Sb⁵⁺ ions might be due to the increased electronegativity and decreased ionic radius at the morphotropic phase boundary (MPB) at x = 0.03. However, the Q_m value decreases with further Sb addition. Relatively high piezoelectric properties at x = 0.03 of k_p = 0.42, k_t = 0.48, ?_r = 446, Q_m = 143, tan δ = 2.3%, and density = 4.31 g/cm³, d₃₃ = 123 pC/N, were obtained.