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.     

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.     

Effect of CuO on the sintering temperature and piezoelectric properties of lead-free 0.95(Na0.5K0.5)NbO3-0.05CaTiO3 ceramics

The addition of a small amount of CuO to the 0.95(Na_0.5K_0.5)NbO₃-0.05CaTiO₃ (0.95NKN-0.05CT) ceramics sintered at 960 °C for 10 h produced a dense microstructure with large grains due to the liquid phase sintering. Due to the negligible Na₂O evaporation, poling was easy for all specimens sintered at 960 °C. The piezoelectric properties of the specimens were considerably influenced by the relative density, grain size and liquid phase amount. The high piezoelectric properties of d₃₃ = 200 pC/N, k_p = 0.37, and Q_m = 350 were obtained for the 0.95NKN-0.05CT ceramics containing 2.0 mol% CuO sintered at 960 °C for 10 h. Therefore, the 0.95NKN-0.05CT ceramics containing a small amount of CuO are a good candidate material for lead-free piezoelectric ceramics.     

Co2O3 doped (Na0.65K0.35)NbO3 piezoceramics

Lead-free (Na_0.65K_0.35)NbO₃ + x wt.% Co₂O₃ (KNN-xCo) piezoceramics were synthesized by conventional ceramic processing and the effects of low Co₂O₃ concentration on the microstructure and electrical properties were investigated. The experimental results show that the orthorhombic-tetragonal phase transition temperature (T_O-T) decreases slightly whereas the tetragonal-cubic phase transition temperature (T_C) and crystal structure keep unchanged with the increasing of Co₂O₃ content. The doping of Co₂O₃ facilitates the grain growth and improves the density and piezoelectric properties of the ceramics. The sample of x = 0.2 exhibits good piezoelectric properties of piezoelectric coefficient d₃₃ = 127pC/N and electromechanical coupling coefficient k_p = 35.1% with density of ρ = 4.31/cm³. These results strongly suggest that the composition of (Na_0.65K_0.35)NbO₃ is another promising lead-free candidate for investigation besides (Na_0.5K_0.5)NbO₃ system.     

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.     

Dielectric and piezoelectric properties of alkaline-earth titanate doped (K0.5Na0.5)NbO3 ceramics

(K_0.5Na_0.5)NbO₃ (KNN) and 0.995(K_0.5Na_0.5)NbO₃-0.005AETiO₃ (AE = Mg, Ca, Sr, Ba) were successfully prepared by conventional ceramic processing and without the cold-isostatic-pressing (CIP) process. The effects of low AETiO₃ (AET) concentration on crystal structure, density, dielectric and piezoelectric properties of the KNN based ceramics were evaluated. The results show that adding MgTiO₃(MT) and BaTiO₃(BT) to KNN can lead to the appearance of a trace amount of second phase(s), reduced density and deteriorated electrical properties. Adding CaTiO₃(CT) and SrTiO₃(ST) to KNN can promote densification and optimize electrical properties. Two phase transitions at T_t-o ( the temperature at which the phase transition from orthorhombic to tetragonal occurs) and T_c (the Curie temperature) were observed in KNN and all KNN-AET ceramics, by using differential scanning calorimetry (DSC) analysis and dielectric characterization. Adding AET to KNN caused the variations of T_t-o and T_c.     

Impedance spectroscopy and relaxation phenomena of (Na,K) excess Na0.5K0.5NbO3 thin films grown by chemical solution deposition

Na_0.5K_0.5NbO₃ (NKN) and 10 mol% (Na,K) excess Na_0.5K_0.5NbO₃ (NKN10) thin films on Pt/Ti/SiO₂/Si substrate were prepared by chemical solution deposition. Crystallization of NKN10 thin films was confirmed by X-ray diffraction. The (Na,K) excess Na_0.5K_0.5NbO₃ thin film shows a ferroelectric P-E hysteresis loop. Dielectric properties and impedance spectroscopy of thin films were investigated in the frequency range from 0.1 Hz to 100 kHz and the temperature range of 25 ~ 500 °C. By analyzing the complex impedance relaxation with Cole-Cole plots, we found impedance relaxations for the thin film. The contribution of electrical conduction is discussed in relation to grain, grain boundary, and interface effects.     

KxNa1 − xNbO3 powder synthesized by molten-salt process

K_xNa_1 − xNbO₃ ceramic powders have been successfully synthesized in different salts (NaCl, KCl, NaCl-KCl). Our results reveal that K_xNa_1 − xNbO₃ powders with single-phase perovskite structure can be formed at a low temperature such as 750 °C. The type of salts has significant effects on the morphology and chemical composition of the powders. As Na⁺ has a higher diffusing rate and occupies the A-site in the perovskite structure more easily as compared to K⁺, the powder contains only a small amount of K⁺ (x ∼ 0.10) when it is synthesized according to formula K_0.5Na_0.5NbO₃ and in a flux containing the same molar content of Na⁺ and K⁺. By using a NaCl or KCl salt, the K⁺ concentration x can be adjusted to almost 0 and 0.77, respectively.     

Dielectric and ferroelectric properties of (1-x)(Na0.5K0.5)NbO3-xBaTiO3 ceramics

(1-x)(Na_0.5K_0.5)NbO₃-xBaTiO₃ ceramics were prepared by a solid state reaction approach, and their dielectric and ferroelectric properties were evaluated together with the crystal structure. Three phase transitions at T_t1, T_t2 and T_t3 were observed by the combination of DTA analysis and dielectric characterization. These phase transitions corresponded to those of (Na_0.5K_0.5)NbO₃, and they were greatly pulled down by forming solid solution with BaTiO₃. The phase transition around T_t1 was incompletely diffusive and the appearance of diffusiveness of non ferro-paraelectric phase transition was an exception. The hysteresis loops changed their shapes from “square” into “thin square” with increasing x.     

Ferroelectric properties of lithia-doped (Bi0.95Na0.75K0.20)0.5Ba0.05TiO3 ceramics

Lead-free piezoelectric (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics have been prepared by conventional process for different lithium substitutions. The SEM images show that the ceramics are well sintered at 1428 K. Dielectric and ferroelectric measurements have been performed. With the increasing of lithium substitution, the Curie temperature of the (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics shifts from 570 K to 620 K, but the maximum value of the dielectric constant decreases from 6700 to 4700 correspondingly. A relatively larger remanent polarization of 36.8 μC/cm² has been found in the x = 0.05 sample. The coercive field decreases as the lithium substitution amount increases. An optimized d₃₃ = 194 × 10^− 12 C/N and a relative dielectric constant ε_r = 1510 have been obtained in (Bi_0.95Na_0.75K_0.15Li_0.05)_0.5Ba_0.05TiO₃.     

Effects of BiAlO3 on structure and electrical properties of K0.5Na0.5NbO3-LiSbO3 lead-free piezoceramics

(1−x)(0.948 K_0.5Na_0.5NbO₃-0.052LiSbO₃)-xBiAlO₃ (KNNLS-xBA) lead-free piezoceramics were synthesized by conventional solid state reaction method. The compositional dependence of phase structure and electrical properties of the ceramics was systemically studied. XRD patterns revealed that all the ceramic samples possessed pure perovskite structure. In addition, polymorphic phase transition (PPT) for the ceramics with BA doping could not be observed in the measuring range from room temperature to 500 °C. Within the studied range of BA addition, the ceramics with x = 0.002 represented a relatively desirable balance between the degradation of the piezoelectric properties, improvement in temperature stability and mechanical quality factor. It was found that the KNNLS-0.002BA ceramics exhibited optimum overall properties (d₃₃ = 233 pC/N, k_p = 35%, tanδ = 0.047, P_r = 27.3 μC/cm², Q_m = 56 and T_c = 349 °C), suggesting that this material should 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.     

Sintering, microstructure and piezoelectric properties of CuO and SnO2 co-modified sodium potassium niobate ceramics

The CuO and SnO₂ co-modified Na_0.52K_0.48NbO₃ ceramics were prepared by a conventional mixed oxide method. Densification can be further improved but the grain growth is inhibited as a small amount of SnO₂ is added into 1% CuO doped Na_0.52K_0.48NbO₃. The results indicate that the physical and various electrical properties of CuO and SnO₂ doped Na_0.52K_0.48NbO₃ ceramics significantly depend on sintering conditions and the content of dopants. The ceramics doped with 1 mol% CuO and 1 mol% SnO₂ sintered at 1070 °C for 3 h show improved dielectric and piezoelectric properties: d₃₃ = 120 pC/N, k_p = 0.38, Q_m = 1040, ?_r = 710 and tanδ = 0.013 (1 kHz), in comparison with un-doped or CuO doped compositions.     

Dielectric properties of (1 − x)(Mg0.95Zn0.05)TiO3-x(Na0.5Nd0.5)TiO3 ceramic system at microwave frequencies

Ceramics in the system (1 − x)(Mg_0.95Zn_0.05)TiO₃-x(Na_0.5Nd_0.5)TiO₃ were prepared by the conventional mixed oxide route. It shows a two-phase system of an ilmenite structured (Mg_0.95Zn_0.05)TiO₃ and a perovskite structured (Na_0.5Nd_0.5)TiO₃, which were confirmed by XRD and EDX. In addition, (Mg_0.95Zn_0.05)Ti₂O₅ was identified as a second phase. It was also responsible for a rapid drop in the Q × f value. The temperature coefficient of resonant frequency was a function of compositional ratio. Specimen with x = 0.16 possessed an excellent combination of microwave dielectric properties: ε_r ~ 24.27, Q × f ~ 82,000 GHz (at 9 GHz) and τ_f ~ 0 ppm/°C.     

Effects of CuO doping on the electrical properties of 0.98K0.5Na0.5NbO3-0.02BiScO3 lead-free piezoelectric ceramics

CuO-doped 0.98K_0.5Na_0.5NbO₃-0.02BiScO₃ (0.98KNN-0.02BS-xCu) lead-free piezoelectric ceramics have been fabricated by ordinary sintering technique. The effects of CuO doping on the dielectric, piezoelectric, and ferroelectric properties of the ceramics were mainly investigated. X-ray diffraction reveals that the samples at doping levels of x ≤ 0.01 possess a pure tetragonal perovskite structure. The specimen doped with 1 mol% CuO exhibits enhanced electrical properties (d₃₃ ~ 207 pC/N, k_p ~ 0.421, and k_t = 0.424) and relatively high mechanical quality factor (Q_m = 288). These results indicate that the 0.98KNN-0.02BS-0.01Cu ceramic is a promising candidate for lead-free piezoelectric ceramics for applications such as piezoelectric actuators, harmonic oscillator and so on.     

Structure and electrical properties of (1 − x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 ceramics near morphotropic phase boundary

The binary lead-free piezoelectric ceramics with the composition of (1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ were synthesized by conventional mixed-oxide method. The phase structure transformed from rhombohedral to tetragonal phase in the range of 0.16 ≤ x ≤ 0.20. The grain sizes varied with increasing the Bi_0.5K_0.5TiO₃ content. Electrical properties of ceramics are significantly influenced by the Bi_0.5K_0.5TiO₃ content. Two phase transitions at T_t (the temperature at which the phase transition from rhombohedral to tetragonal occurs) and T_c (the Curie temperature) were observed in all the ceramics. Adding Bi_0.5K_0.5TiO₃ content caused the variations of T_t and T_c. A diffuse character was proved by the linear fitting of the modified Curie-Weiss law. Besides, the ceramics with homogeneous microstructure and excellent electrical properties were obtained at x = 0.18 and sintered at 1170 °C. The piezoelectric constant d₃₃, the electromechanical coupling factor K_p and the dielectric constant ?_r reached 144 pC/N, 0.29 and 893, respectively. The dissipation factor tan δ was 0.037.     

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.     

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.     

Characterization of (K0.5Na0.5)NbO3 powders and ceramics prepared by a novel hybrid method of sol–gel and ultrasonic atomization

(K_0.5Na_0.5)NbO₃ powders and ceramics were prepared by a novel hybrid method of sol–gel and ultrasonic atomization, in which Nb₂O₅ was used as the niobium source to replace those expensive soluble niobium salts. X-ray diffraction and thermal analysis were performed to investigate the synthesis process and phase transformation behavior of (K_0.5Na_0.5)NbO₃ powders. The results showed that (K_0.5Na_0.5)NbO₃ powders with a reasonably fine particle size and single-phase perovskite structure were formed at a temperature as low as 650 °C. Dense (K_0.5Na_0.5)NbO₃ ceramics with a relative density of 93% were obtained using the refined powders. The (K_0.5Na_0.5)NbO₃ ceramics prepared by the novel hybrid method exhibited relatively good properties (d₃₃ = 90 pC/N, k_p = 0.32, P_r = 20.6 μC/cm², T_c = 405 °C, ε_r = 712), suggesting that this novel hybrid method might be a promising method for the powders and ceramics preparation.     

The synthesis of lead-free ferroelectric Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 thin films by sol-gel method

(1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ [BNT-BKT-100x] thin films have been successfully deposited on Pt/Ti/SiO₂/Si substrates by a sol-gel process together with rapid thermal annealing. A morphotropic phase boundary (MPB) between Bi_0.5Na_0.5TiO₃ and Bi_0.5K_0.5TiO₃ was determined around x ∼ 0.15. Near the MPB, the film exhibits the largest grain size, the highest ε value (360) and the largest P_r value (13.8 μC/cm²). The BNT-BKT thin film system is expected to be a new and promising candidate for lead-free piezoelectric applications.