(K,Na)NbO3 lead-free piezoelectric ceramics synthesized from hydrothermal powders

Among various lead-free piezoelectric materials, (K,Na)NbO₃ is a very promising candidate. In this study, (K,Na)NbO₃ ceramics were sintered from mixed KNbO₃ and NaNbO₃ powders prepared by hydrothermal reaction. These two powders were mixed with distilled water in a KNbO₃/NaNbO₃ molar ratio of 1. After sintering the mixed powder, the solid solution of (Na,K)NbO₃ ceramics was obtained. The electrical properties such as the electromechanical coupling factors k_p and k₃₃, the mechanical quality factor, Q_m, and the piezoelectric constant d₃₃ of the sintered (K,Na)NbO₃ ceramics were 0.32, 0.48, 71 (radial mode), 118 ((33)mode), and 107 pC/N, respectively.     

Properties of (K,Na)NbO3-based lead-free piezoelectric films prepared by pulsed laser deposition

To investigate the properties of (K,Na)NbO₃-based lead-free piezoelectric films at the morphotropic phase boundary composition, we fabricated epitaxial [(K_0.5Na_0.5)_0.97Li_0.03] (Nb_0.8Ta_0.2)O₃ films on (001), (110) and (111)-oriented single crystal SrTiO₃ substrates by pulsed laser deposition. The structure and electrical properties of the films were studied. Dielectric constants of 540, 390 and 300 and remnant polarizations of 4.00, 1.05, and 0.35 μC/cm² were observed for the (001), (110) and (111) oriented films, respectively.     

Synthesis of (K, Na)NbO3 particles by high temperature mixing method under hydrothermal conditions

In order to prepare the pure (K, Na)NbO₃(KNN) particles with higher crystallinity, the high temperature mixing method (HTMM) under hydrothermal conditions was carried out in this work. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). The results indicate that the KNN particles size decrease gradually with the increase of mineralization concentration in the starting solution. The ratio of K⁺/(K⁺ + Na⁺) in the starting solution has a great effect on the phase of the products, and several phases coexist in the product when the ratio of K⁺/(K⁺ + Na⁺) in the starting solution is 0.7.     

Dielectric, ferroelectric and piezoelectric properties of Bi0.5Na0.5TiO3-(Ba0.7Ca0.3)TiO3 ceramics at morphotropic phase boundary composition

(1 − x)Bi_0.5Na_0.5TiO₃-x(Ba_0.7Ca_0.3)TiO₃ (BNT-xBCT, 0 ≤ x ≤ 0.15) solid solutions have been synthesized by a conventional solid state sintering method for obtaining a morphotropic phase boundary (MPB) with good piezoelectric properties. X-ray diffraction patterns reveal that a MPB of rhombohedral and tetragonal phases is formed at compositions 0.09 ≤ x ≤ 0.12. Addition of BCT into BNT greatly lowered coercive field E_c without degrading remanent polarization P_r. The specimen with x = 0.09 has the good piezoelectric properties: d₃₃ = 125 pC/N and k_p = 0.33. A modified Curie-Weiss law was used to fit the dielectric constant of BNT-xBCT ceramics, and a frequency dispersion was observed during the phase transitions from antiferroelectric to paraelectric in specimens with x exceeding 0.06.     

Effect of post-annealing treatment in oxygen on dielectric properties of K0.5Na0.5NbO3 thin films prepared by chemical solution deposition

K_0.5Na_0.5NbO₃ thin films were prepared on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method with different annealing temperatures of 550 °C, 600 °C, 700 °C. The post-annealing treatment was introduced at 550 °C for 3 min in oxygen ambient. It is found that the films were composed of pure provskite phase, and the post-annealing treatment promoted the crystallization and improved the quality of the films, which resulted in the enhancement of the dielectric property of the films. The effect of the post-annealing on the dielectric properties of the films was also discussed.     

Piezoelectric, ferroelectric and dielectric properties of La2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

La₂O₃ (0–0.8 wt.%)-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction. The influences of La₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties of the composites were investigated. X-ray diffraction (XRD) patterns indicate that 0.2-0.8 wt.% of La₂O₃ has diffused into the lattice of BNBT6 ceramics. Consequently, a pure perovskite phase is formed. SEM images show that the microstructure of the ceramics is changed with the addition of a small amount of La₂O₃. The temperature dependence of the relative dielectric constant shows that Curie point decreases with the increase of La₂O₃. At room temperature, the ceramics doped with 0.6 wt.% La₂O₃ show superior performance with high piezoelectric constant (d₃₃ = 167 pC/N), high planar electromechanical coupling factor (k_p = 0.30), high mechanical quality factor (Q_m = 118), high relative dielectric constant (ε_r = 1470) and lower dissipation factor (tanδ = 0.056) at a frequency of 10 kHz.     

SmAlO3-modified (K0.5Na0.5)0.95Li0.05Sb0.05Nb0.95O3 lead-free ceramics with a wide sintering temperature range

Lead-free ceramics (1 − x)(K_0.5Na_0.5)_0.95Li_0.05Sb_0.05Nb_0.95O₃-xSmAlO₃ (KNLNS-xSA) were prepared by conventional sintering technique. The phase structure, dielectric and piezoelectric properties of the ceramics were investigated. All compositions show a main perovskite structure, exhibiting room-temperature symmetries of tetragonal at x ≤ 0.0075, of pseudo-cubic at x = 0.0100. The Curie temperature of KNLNS-xSA ceramics decreases with increasing SmAlO₃ content. Moreover, the addition of SmAlO₃ can effectively broaden the sintering temperature range of the ceramics. The KNLNS-xSA ceramic with x = 0.0050 has an excellent electrical behavior of piezoelectric coefficient d₃₃ = 226 pC/N, planar mode electromechanical coupling coefficient k_p = 38%, dielectric loss tan δ = 3.0%, mechanical quality factor Q_m = 60, and Curie temperature T_C = 327 °C, suggesting that this material could be a promising lead-free piezoelectric candidate for piezoelectric applications.     

Enhanced dielectric,ferroelectric and optical properties of lead free (K0.17Na0.83)NbO3 ceramic with WO3 addition

Polycrystalline lead-free ceramics (K_0.17Na_0.83)NbO₃ + x wt.% WO₃; (x = 0, 1, 3 and 5) have been synthesized via solid state reaction method. X-ray diffraction pattern at room temperature indicates the formation of pure perovskite phase with monoclinic structure for all samples. Dielectric constant versus temperature measurements shows an increase in dielectric constant with a shift in Curie temperature (T_C) toward higher temperature side. Remnant polarization (P_r) is found to be enhanced and reached upto 24 μC/cm² for x = 5 wt.% WO₃ from 12.5 μC/cm² for pure (K_0.17Na_0.83)NbO₃ ceramic. The value of coercive field (E_c) decreases with increasing wt.% of WO₃. From optical band gap study, we found blue shift in the band gap of (K_0.17Na_0.83)NbO₃ with increasing concentration of WO₃.     

Dielectric and piezoelectric properties of LiSbO3 doped 0.995 K0.5Na0.5NbO3-0.005BiFeO3 piezoelectric ceramics

LiSbO₃ doped and undoped 0.995 K_0.5Na_0.5NbO₃-0.005BiFeO₃ piezoelectric ceramics with high properties have been fabricated in air by the conventional ceramic processing. By adding LiSbO₃ to K_0.5Na_0.5NbO₃-BiFeO₃ ceramics, the dielectric and piezoelectric properties evidently increase. The doped ceramics exhibit good electrical properties. The enhanced piezoelectric properties of the ceramics should be attributed to optimum LiSbO₃ substitution and better microstructure with high density. Results show that LiSbO₃ doped K_0.5Na_0.5NbO₃-BiFeO₃ lead-free piezoelectric ceramics are a promising lead-free piezoelectric material for applications in different devices.     

Piezoelectric and dielectric properties of K0.5Na0.5NbO3-LiSbO3-BiScO3 lead-free piezoceramics

(1 − x) (0.95K_0.5Na_0.5NbO₃-0.05LiSbO₃)-xBiScO₃ lead-free piezoceramics have been fabricated by an ordinary pressure-less sintering process. The relationship between the BS content, phase structure, density, and piezoelectric properties and their temperature stability was discussed particularly. All compositions show a main perovskite structure, showing room-temperature symmetries of orthorhombic at x = 0, of tetragonal at 0.002 ≤ x ≤ 0.01. When 0.002 ≤ x ≤ 0.008, the ceramics have excellent electrical properties of d₃₃ = 265-305 pC/N, k_p = 45-54%, ?_r = 1346-1638, Curie temperature T_c = 315-370 °C and depolarizing temperature T_d = 315-365 °C, comparable to that of other KNN-based piezoceramics. The results indicate that the ceramics are promising lead-free piezoelectric materials.     

Unique crystallization and formation of nonlinear optical (Na,K)NbO3 phases in (Na,K)NbGeO5 glasses

Crystalline phases and second-order optical nonlinearities in crystallized glasses of (Na,K)NbGeO₅ (i.e., xNa₂O · (25−x)K₂O · 25Nb₂O₅ · 50GeO₂) are examined. The molar volume of the glasses decreases with increasing Na₂O content, indicating that the glass structure of NaNbGeO₅ glass is more compact compared with KNbGeO₅ glass. The optical basicity of the glasses is 0.94–0.95, implying that chemical bonds among constituents ions are basically very ionic. (Na,K)NbGeO₅ glasses exhibit a unique crystallization depending on the Na₂O/K₂O ratio. The main crystalline phase in crystallized NaNbGeO₅ glass is the NaNbGeO₅ phase. KNbGeO₅ glass shows a prominent bulk nanocrystallization giving nanocrystals of the K_3.8Nb₅Ge₃O_20.4 phase. A glass with the composition of K_3.8Nb₅Ge₃O_20.4 also shows a bulk nanocrystallization. The crystallized glasses with K_3.8Nb₅Ge₃O_20.4 and NaNbGeO₅ crystalline phases do not show a second harmonic generation (SHG) at room temperature. The nonlinear optical (Na,K)NbO₃ phases showing a SHG are formed at the surface (5 μm) of crystallized glasses with x=15 and 20, giving an orientation of the (1 1 0) plane. The present study suggests the presence of mixed-alkali effect in the crystallization behavior of (Na,K)NbGeO₅ glasses.     

Dielectric and piezoelectric properties of Y2O3 doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free piezoelectric ceramics

Y₂O₃ doped lead-free piezoelectric ceramics (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (0-0.7 wt%) were synthesized by the conventional solid state reaction method, and the effect of Y₂O₃ addition on the structure and electrical properties was investigated. X-ray diffraction shows that Y₂O₃ diffuses into the lattice of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ to form a solid solution with a pure perovskite structure. The temperature dependence of dielectric constant of Y₂O₃ doped samples under various frequencies indicates obvious relaxor characteristics different from typical relaxor ferroelectric and the mechanism of the relaxor behavior was discussed. The optimum piezoelectric properties of piezoelectric constant d₃₃ = 137 pC/N and the electromechanical coupling factor k_p = 0.30 are obtained at 0.5% and 0.1% Y₂O₃ addition, respectively.     

Influence of sintering temperature on piezoelectric properties of (K0.5Na0.5)NbO3-LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ have been synthesized by traditional ceramics process without cold-isostatic pressing. The effect of the content of LiNbO₃ and the sintering temperature on the phase structure, the microstructure and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics were investigated. The result shows that the phase structure transforms from the orthorhombic phase to tetragonal phase with the increase of the content of LiNbO₃, and the orthorhombic and tetragonal phase co-exist in (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics when the content of LiNbO₃ is about 0.06 mol. The sintering temperature of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ decreases with the increase of the content of LiNbO₃. The optimum composition for (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics is 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃. The optimum sintering temperature of 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics is 1080 °C. Piezoelectric properties of 0.94 (K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics under the optimum sintering temperature are piezoelectric constant d₃₃ of 215 pC/N, planar electromechanical coupling factor k_p of 0.41, thickness electromechanical coupling factor k_t of 0.48, the mechanical quality factor Q_m of 80, the dielectric constant of 530 and the Curie temperature T_c = 450 °C, respectively. The results indicate that 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ piezoelectric ceramics is a promising candidate for lead-free piezoelectric ceramics.     

非化学计量比对(K0.465Na0.465Li0.07)(Nb0.95Sb0.05)O3无铅压电陶瓷结构及性能的影响

通过传统固相合成工艺制备了(K0.465+xNa0.465+yLi0.07)(Nb0.95-zSb0.05)O3(x,y,z=-0.01～0.02)无铅压电陶瓷,研究了非化学计量比对陶瓷结构及压电性能的影响。结果表明符合化学计量比的陶瓷具有四方钙钛矿结构;在实验范围内,K和Nb的过量或少量均不会改变体系的相结构,而Na过量将会导致体系正交-四方相变温度升高到室温以上,并且正交-四方相变温度随y的增加而升高;过量添加约0.5%(摩尔分数)的K或Na便能补偿在高温烧结时的碱金属元素的挥发损失,进而提高陶瓷的压电性能;该体系陶瓷的组分在较大范围内变化时(如当y=z=0时,x=0～0.02;当x=z=0时,y=0～0.01;以及当x=y=0时,z=-0.01～0.005),仍然能保持d33>200pC/N和kP>40%这样较好的性能。上述结果不仅有利于在研究中材料制备工艺的重复,而且有利于当材料在器件应用时所面临的规模化生产。     

Hydrothermal synthesis and characterization of (Bi,K)TiO3 ferroelectrics

A lead-free ferroelectric (Bi,K)TiO₃ (BKT) was synthesized by a hydrothermal process and characterized systematically at various temperatures. Well-crystallized BKT in the tetragonal phase was identified at a hydrothermal temperature over 220 °C. Small cubic particles were observed, regardless of hydrothermal temperature. The BKT sintered at 1050 °C was observed to be a typical relaxor behavior and very stable against frequency and temperatures, respectively. The sintered-BKT ceramics exhibited a high temperature of maximum dielectric permittivity (T_max = 356 °C at 10⁶ Hz) with piezoelectric constant (d₃₃ = 65 pC/N) and electromechanical coupling factors (k_p = 0.22, k_t = 0.43). Thus, the sintered-BKT showed excellent temperature stability with a high-T_max and piezoelectric properties.     

Fabrication of (K, Na)NbO3 glass-ceramics and crystal line patterning on glass surface

Crystallization behavior of (30−x)K₂O-xNa₂O-25Nb₂O₅-45SiO₂ (KNNS; x = 0, 5, 10, 20 and 30) (mol%) glasses was clarified and perovskite-type nonlinear optical (K, Na)NbO₃ (KNN) crystals were synthesized by using a conventional glass-ceramics method. It was found that Na₂O amounts over around x = 10 mol% were necessary to form perovskite-type KNN crystals showing second-harmonic generations. The substitution of K⁺ and Na⁺ ions was confirmed from X-ray diffraction (XRD) analysis. A continues-wave of Yb:YVO₄ fiber laser (wavelength: 1080 nm) was irradiated onto CuO doped KNNS; x = 10 (Cu-KNNS) surface. The absorption coefficient of this Cu-KNNS glass was determined to be α = 5.0 cm⁻¹. Perovskite-type KNN crystals were patterned in the condition of the laser power of >1.20 W and the laser scanning speed of S = 7 μm/s, and their structure was determined by Raman scattering spectra and XRD analysis.     

Modified (K0.5Na0.5)(Nb0.9Ta0.1)O3 ceramics with high Qm

Lead-free ceramics (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃ (KNNT) + x mol% K₄CuNb₈O₂₃ (KCN) + y mol% MnO₂ have been prepared using the conventional solid-state reaction technique. Crystalline structures and Microstructures were analyzed by X-ray diffraction and scanning electron microscope (SEM) at room temperature. The low dielectric loss tanδ and relatively high piezoelectric properties were obtained when KCN and MnO₂ were added into KNNT ceramics. The ceramics with x = 1.0, y = 0.50 exhibited excellent piezoelectric properties: high mechanical quality factor Q_m = 1563, piezoelectric coefficient d₃₃ = 96pC/N, electromechanical coupling coefficient k_p = 42.2%, k_t = 44.5%, k₃₃ = 58.4%, relative dielectric constant ε′ = 308, tanδ = 0.4%. This material is a promising candidate for the lead-free piezoelectric transformer applications.     

Highly enhanced mechanical quality factor in lead-free (K0.5Na0.5)NbO3 piezoelectric ceramics by co-doping with K5.4Cu1.3Ta10O29 and CuO

Effects of Cu doping on the ferroelectric and piezoelectric properties of 0.0038 mol K_5.4Cu_1.3Ta₁₀O₂₉ modified (K_0.5Na_0.5)NbO₃ ceramics have been investigated. On the basis of analyses on crystal structure and polarization hysteresis, it is suggested that Cu ions reveal amphoteric doping behavior in KNN ceramics. At doping levels up to 1 mol%, the Cu ions substitute pentavalent B-site cations, acting as acceptors that generate O-vacancies to resultantly harden the ceramics. At doping levels above 1.5 mol%, however, Cu ions play a role as donors by replacing monovalent A-site cations. A specimen doped with 0.5 mol% CuO shows an extremely high mechanical quality factor of 3053, which is higher than those of any other reports on KNN-based ceramics.     

Dielectric properties of low temperature sintered LiF doped BaFe0.5Nb0.5O3

Sintering of BaFe_0.5Nb_0.5O₃:BFN requires the use of high temperatures to achieve satisfactory densification of this material. The aim of this study is to determine the effect of LiF on the sintering and electrical properties of BFN ceramics (the LiF was added as a sintering agent). The results show that LiF lowers the sintering temperature by 150-200 °C without affecting the formation of BFN. Ceramics doped with 2-3% LiF show optimum densities of about 93-94% of the theoretical value when sintered at low temperatures (1000-1100 °C). Samples containing 2-3% LiF show the following dielectric behaviour. The dielectric constant curves are very broad over a wide temperature range, with room-temperature values of 7154 in the 2% LiF sample and 2527 in the 3% LiF sample. The dielectric constants gradually increase up to 300 °C to values of about 38,862 in the 2% LiF sample and 40,471 in the 3% LiF sample. Furthermore, the addition of 2-3% LiF to BFN causes a reduction in the room-temperature dielectric loss from 4.29 in undoped BFN to less than 1.2 for LiF-containing samples.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by Substituting Mg for Co

The microwave dielectric properties of La(Mg_0.5−xCo_xSn_0.5)O₃ ceramics were examined with a view to exploiting them for mobile communication. The La(Mg_0.5−xCo_xSn_0.5)O₃ ceramics were prepared using the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La(Mg_0.4Co_0.1Sn_0.5)O₃ ceramics revealed that La(Mg_0.4Co_0.1Sn_0.5)O₃ is the main crystalline phase, which is accompanied by small extent of La₂Sn₂O₇ as the second phase. Formation of this Sn-rich second phase was attributed to the loss of MgO upon ignition. Increasing the sintering temperatures seemed to promote the formation of La₂Sn₂O₇. An apparent density of 6.67 g cm⁻³, a dielectric constant (?_r) of 20.3, a quality factor (Q.F.) of 70,500 GHz, and a temperature coefficient of resonant frequency (τ_f) of −77 ppm °C⁻¹ were obtained for La(Mg_0.4Co_0.1Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.