Low-Temperature Sintering and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

(Na_0.5K_0.5)NbO₃ (NKN) ceramic with 1.5 mol% CuO added (NKNC) was well sintered even at a low temperature of 900°C with the addition of ZnO. Most of the ZnO reacted with the CuO and formed the liquid phase that assisted the densification of the specimens at 900°C. A few Zn²⁺ ions entered the matrix of the specimens and increased the coercive field ( E _c) and Q _m values of the specimens. High-piezoelectric properties of k _p=0.37, Q _m=755, and ɛ₃ ^T /ɛ₀=327 were obtained from the NKNC ceramics containing 1.0 mol% ZnO sintered at 900°C for 2 h.     

Effect of CuO on the Sintering Temperature and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

When a small amount of CuO was added to (Na_0.5K_0.5)NbO₃ (NKN) ceramics sintered at 960°C for 2 h, a dense microstructure with increased grains was developed, probably due to liquid-phase sintering. The Curie temperature slightly increased when CuO exceeded 1.5 mol%. The Cu²⁺ ion was considered to have replaced the Nb⁵⁺ ion and acted as a hardener, which increased the E _c and Q _m values of the NKN ceramics. High piezoelectric properties of k _p=0.37, Q _m=844, and ɛ₃ ^T /ɛ₀=229 were obtained from the specimen containing 1.5 mol% of CuO sintered at 960°C for 2 h.     

Effect of BiScO3 and LiNbO3 on the Piezoelectric Properties of (Na0.5K0.5)NbO3 Ceramics

Lead-free potassium sodium niobate-based piezoelectric ceramics (1− y )(Na_{0.5−0.5 x} K_{0.5−0.5 x} Li _x )NbO₃− y BiScO₃ (  y =0.01, x= 0–0.06) have been prepared by an ordinary sintering process. The XRD analysis showed that the structure changes from orthorhombic to tetragonal with the increase of x (at y =0.01, abbreviated as KNNBSL100 x ). At room temperature, the polymorphic phase transition from the orthorhombic to the tetragonal phase was identified at approximately 0.02≤ x ≤0.04. The piezoelectric and ferroelectric properties were significantly enhanced. The temperature dependences of the relative permittivity revealed that the Curie temperature was increased with the addition of LiNbO₃. These solid solution ceramics are promising as potential lead-free candidate materials.     

Phase Transitional Behavior and Piezoelectric Properties of Lead-Free (Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃–(Bi_0.5K_0.5)TiO₃ solid solution ceramics were successfully fabricated, exhibiting a continuous phase transition with changing x at room temperature from orthorhombic, to tetragonal, to cubic, and finally to tetragonal symmetries. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was found at 2–3 mol% (Bi_0.5K_0.5)TiO₃ (BKT), which brings about enhanced piezoelectric and electromechanical properties of piezoelectric constant d ₃₃=192 pC/N and planar electromechanical coupling coefficient k _p=45%. The MPB composition has a Curie temperature of 370°–380°C, comparable with that of the widely used PZT materials. These results demonstrate that this system is a promising lead-free piezoelectric candidate material.     

Piezoelectric Properties of (1−x)(Na0.5K0.5)NbO3–xAgSbO3 Lead-Free Ceramics

(1− x )(Na_0.5K_0.5)NbO₃– x AgSbO₃ lead-free piezoelectric ceramics were prepared by normal sintering. The effects of the AgSbO₃ on the phase structure and piezoelectric properties of the ceramics were systematically studied. These results show that the AgSbO₃-modified (K_0.50Na_0.50)NbO₃ lead-free piezoelectric ceramics form stable solution with orthorhombic structure, and the Curie temperature and the polymorphic phase transition of the ceramics decreased with increasing AgSbO₃. The result shows that the piezoelectric properties of the ceramics strongly depend on the AgSbO₃. The ceramics with x =0.05 possess optimum properties ( d ₃₃=192 pC/N, k _p=43%, T _c=348°C, T _o−t =145°C, ɛ_r∼632, and tan δ∼3.5%). These results indicate that the ceramic is a promising candidate material for lead-free piezoelectric ceramics.     

Sintering and Electrical Properties of Lead-Free Na0.5K0.5NbO3 Piezoelectric Ceramics

Lead-free Na_0.5K_0.5NbO₃ (NKN) piezoelectric ceramics were fairly well densified at a relatively low temperature under atmospheric conditions. A relative density of 96%–99% can be achieved by either using high-energy attrition milling or adding 1 mol% oxide additives. It is suggested that ultra-fine starting powders by active milling or oxygen vacancies and even liquid phases from B-site oxide additives mainly lead to improved sintering. Not only were dielectric properties influenced by oxide additives, such as the Curie temperature ( T _c) and dielectric loss ( D ), but also the ferroelectricity was modified. A relatively large remanent polarization was produced, ranging from 16 μC/cm² for pure NKN to 23 μC/cm² for ZnO-added NKN samples. The following dielectric and piezoelectric properties were obtained: relative permittivity ɛ ^T ₃₃/ɛ ₀ =570–650, planar mode electromechanical coupling factor, k _p=32%–44%, and piezoelectric strain constant, d ₃₃=92–117 pC/N.     

Microstructure and Piezoelectric Properties of 0.95(Na0.5K0.5)NbO3–0.05SrTiO3 Ceramics

The 0.95(Na_0.5K_0.5)NbO₃–0.05SrTiO₃ (0.95NKN–0.05ST) ceramics formed in this study had a porous microstructure with small grains and low piezoelectric properties due to their low density. However, when a small amount of Na₂O was intentionally subtracted from the 0.95NKN–0.05ST ceramics, a liquid phase was formed, which led to increased density and grain size. Piezoelectric properties were also improved for the Na₂O-subtracted 0.95NKN–0.05ST ceramics. The increased density and grain size were responsible for the enhancement of the piezoelectric properties. In particular, the 0.95(Na_0.49K_0.5)NbO_2.995–0.05ST ceramics showed high piezoelectric properties of d ₃₃=220, k _p=0.4, Q _m=72, and ɛ₃^T/ɛ_o=1447, thereby demonstrating their promising potential as a candidate material for application to lead-free piezoelectric ceramics.     

Low-Temperature Sintering and Piezoelectric Properties of CuO-Added 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

The sintering temperature of 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ (NKN–BT) ceramics needs to be decreased below 1000°C to prevent Na₂O evaporation, which can cause difficulties in poling and may eventually degrade their piezoelectric properties. NKN–BT ceramics containing CuO were well sintered at 950°C with grain growth. Poling was easy for all specimens. Densification and grain growth were explained by the formation of a liquid phase. The addition of CuO improved the piezoelectric properties by increasing the grain size and density. High piezoelectric properties of d ₃₃=230 pC/N, k _p=37%, and ɛ₃^T/ɛ₀=1150 were obtained from the specimen containing 1.0 mol% of CuO synthesized by the conventional solid-state method.     

Ferroelectric and Piezoelectric Properties of Fine-Grained Na0.5K0.5NbO3 Lead-Free Piezoelectric Ceramics Prepared by Spark Plasma Sintering

Lead-free piezoelectric ceramics have received attention because of increasing interest in environmental protection. Niobate ceramics such as NaNbO₃ and KNbO₃ have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na_0.5K_0.5NbO₃ ceramics were prepared using spark plasma sintering (SPS). Although the SPS temperature was as low as 920°C, the density of the Na_0.5K_0.5NbO₃ solid solution ceramics was raised to 4.47 g/cm³ (>99% of the theoretical density). After post-annealing in air, reasonably good ferroelectric and piezoelectric properties were obtained in the Na_0.5K_0.5NbO₃ ceramics with submicron grains. The crystal phase of the Na_0.5K_0.5NbO₃ has an orthorhombic structure. The Curie temperature is 395°C and the piezoelectric parameter ( d ₃₃) of the Na_0.5K_0.5NbO₃ ceramics reached 148 pC/N.     

Microstructure and Piezoelectric Properties of (1−x)(Na0.5K0.5)NbO3–xLiNbO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃– x LiNbO₃ [(1− x )NKN– x LN] ceramics were produced by the conventional solid-state sintering method, and their microstructure and piezoelectric properties were investigated. The formation of the liquid phase and K₆Li₄Nb₁₀O₃₀ second phase that were observed in the (1− x )NKN– x LN ceramics was explained by the evaporation of Na₂O during the sintering. A morphotropic phase boundary (MPB) was observed in the specimens with 0.05< x <0.08. Promising piezoelectric properties were obtained for the specimens with x =0.07. Therefore, the piezoelectric properties of this 0.93NKN–0.07LN ceramic were further investigated and were found to be influenced by their relative density and grain size. In particular, grain size considerably affected the d ₃₃ value. Two-step sintering was conducted at different temperatures to increase the grain size. Piezoelectric properties of d ₃₃=240 (pC/N) and k _p=0.35 were obtained for the 0.93NKN–0.07LN ceramics sintered at 1030°C and subsequently annealed at 1050°C.     

Transmission Electron Microscopy Microstructure of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

Microstructural characterizations using transmission electron microscopy on 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ ceramics sintered at 1030°–1150°C for 2 h were carried out. The liquid phase was found at the triple junction of the grains in all specimens and abnormal grain growth occurred in the presence of the liquid phase. Abnormally grown grains whose shapes were cuboidal were well developed. Anisotropically faceted amorphous liquid phase pockets were observed inside the grain in a specimen sintered at 1060°C for 2 h. The interface between the grain and the liquid matrix was flat and some were identified to be {100} planes of the grains. A certain amount of liquid at the sintering temperature of 1060°C enhanced the abnormal grain growth and contributed to the improvement of the piezoelectric properties.     

Microstructures and Piezoelectric Properties in the Li2O-Excess 0.95(Na0.5K0.5)NbO3–0.05LiTaO3 Ceramics

As a candidate for lead-free piezoelectric materials, Li₂O-excess 0.95(Na_0.5K_0.5)NbO₃–0.05LiTaO₃ (NKN–5LT) ceramics were developed by a conventional sintering process. The sintering temperature was lowered by adding Li₂O as a sintering aid. Abnormal grain growth in NKN–5LT ceramics was observed with varying Li₂O content. This grain-growth behavior was explained in terms of interface reaction-controlled nucleation and growth. In the 1 mol% Li₂O excess NKN–5LT samples sintered at 1000°C for 4 h in air, the electromechanical coupling factor and the piezoelectric constant of NKN–5LT ceramics were found to reach the highest values of 0.37 and 250 pC/N, respectively.     

Conventionally Sintered (Na0.5,K0.5)NbO3 with Barium Additions

The effect of Ba additions on sodium potassium niobate (NKN) ceramic bodies was investigated with regard to sintering behavior, densification, and electrical properties. Small additions of Ba retard grain growth and increase density. The approximate solubility limit of Ba is 1.5 mol%. During firing the loss of Na and K was very low. Both electrical phase transitions in NKN are lowered with Ba additions. The Ba-containing bodies show higher permittivity values than hot-pressed materials, while radial coupling and d₃₃ were similar to or higher than those of the conventionally sintered pure materials.     

Microstructure, Piezoelectric, and Ferroelectric Properties of Bi2O3-Added (K0.5Na0.5)NbO3 Lead-Free Ceramics

Lead-free piezoelectric (K_0.5Na_0.5)NbO₃– x wt% Bi₂O₃ ceramics have been synthesized by an ordinary sintering technique. The addition of Bi₂O₃ increases the melting point of the system and improves the sintering temperature of (K_0.5Na_0.5)NbO₃ ceramics. All samples show a pure perovskite phase with a typical orthorhombic symmetry when the Bi₂O₃ content <0.7 wt%. The phase transition temperature of orthorhombic–tetragonal ( T _{O − T} ) and tetragonal–cubic ( T _C) slightly decreased when a small amount of Bi₂O₃ was added. The remnant polarization P _r increased and the coercive field E _c decreased with increasing addition of Bi₂O₃. The piezoelectric properties of (K_0.5Na_0.5)NbO₃ ceramics increased when a small amount of Bi₂O₃ was added. The optimum piezoelectric properties are d ₃₃=140 pC/N, k _p=0.46, Q _m=167, and T _C=410°C for (K_0.5Na_0.5)NbO₃–0.5 wt% Bi₂O₃ ceramics.     

Enhanced Piezoelectric Properties in Mn-Doped 0.98K0.5Na0.5NbO3–0.02BiScO3 Lead-Free Ceramics

Mn-doped 0.98K_0.5Na_0.5NbO₃–0.02BiScO₃ (0.98KNN–0.02BS) lead-free piezoelectric ceramics have been prepared by a conventional sintering technique and the effects of Mn doping on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of Mn can improve the densification of the ceramics effectively. Because of the high densification, fine grain, and Mn doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. Very good piezoelectric and dielectric properties of d ₃₃=288 pC/N, k _p=0.46, ɛ_r=1591, and T _C=328°C were obtained for the 0.98KNN–0.02BS ceramics doped with 0.8 mol% Mn. Therefore, the 0.98KNN–0.02BS ceramics containing a small amount of Mn are a good candidate material for lead-free piezoelectric ceramics.     

Processing and Piezoelectric Properties of (Na0.5K0.5)0.96Li0.04(Ta0.1Nb0.9)1−xCuxO3−3x/2 Lead-Free Ceramics

The Cu-modified (Na_0.5K_0.5)_0.96Li_0.04Ta_0.1Nb_0.9O₃ lead-free piezoelectric ceramics were fabricated at relatively low temperatures by ordinary sintering. The results indicate that the addition of copper oxide (CuO) does not change the crystal structure and the dielectric–temperature characteristic, but tends to slightly increase the loss tangent and significantly modify the ferroelectric and electromechanical properties. Moreover, the grains get clearly coarsened with increasing CuO content. When doped with <0.25% CuO, the materials get softer with slightly decreased coercive fields ( E _c) and increased maximum electric field-driven strains ( S _m), and thus own enhanced piezoelectric properties; however, as the doping level becomes higher, the materials get harder, possessing larger E _c and reduced remanent polarization and S _m. The change in the electrical properties can be attributed to both the formation of oxygen vacancies by Cu²⁺ replacing Nb⁵⁺ and the modification of densification.     

Microstructural Evolution of Crystalline-Oriented (K0.5Na0.5)NbO3 Piezoelectric Ceramics with a Sintering Aid of CuO

The microstructural development of crystalline-oriented (K_0.5Na_0.5)NbO₃ (KNN)-based piezoelectric ceramics during sintering was investigated. The addition of CuO as a sintering aid was found to be effective for fabricating highly oriented and dense KNN ceramics. KNN specimens containing 0.5–1.0 mol% CuO sintered at 1100°C for 1 h were found to have relative densities and pseudo-cubic {100} orientation degrees of 95% or higher. In the early stages of sintering, KNN is formed in the reaction between complementary reactants NaNbO₃ and KNbO₃, after which oriented grain growth proceeds at a relative density of more than 90%. In addition, the results of transmission electron microscopy observation showed that textured KNN ceramics have a unique pectinate-like domain structure with domain walls consisting of {101} planes.     

Nanopowders of Na0.5K0.5NbO3 Prepared by the Pechini Method

A Pechini-based chemical synthesis route was used to produce powders of Na_0.5K_0.5NbO₃ (NKN). The thermochemistry of the gel was investigated using thermogravimetric analysis-fourier transform infrared (TGA-FTIR) evolved gas analysis; in addition, powder FTIR was used to analyze the gel residues after different heat treatments. The final decomposition of the organic components occurred at ∼650°C. However, hydrated–carbonated secondary phase(s) were detected by FTIR in powders that had been heated at 700°C, indicating that the NKN nanopowders are susceptible to a reaction with atmospheric moisture and carbon dioxide. The NKN particle sizes were in the range 50–150 nm after decomposition at 700°C.     

Synthesis of Na0.5K0.5NbO3 Piezoelectrics by a Solution Coating Approach

The solution coating method is shown to provide more homogeneous mixing of the starting powders and purer (Na_0.5K_0.5)NbO₃ (NKN) powder than the conventional method thereby rendering cold isostatic pressing unnecessary. Sintering NKN with potassium copper niobate (KCN) by uniaxial pressing gave a relative bulk density of 92%, d ₃₃ coefficient of 112 pC/N and a loss factor of 1% after 6 h at 1100°C. In contrast, the conventional method yielded 84% relative bulk density and a 31% loss factor after 6 h at 1100°C and the sample was too porous to allow for dielectric and piezoelectric measurements.     

Effect of Particle Sizes of Starting Materials on Microstructure Development in Textured Bi0.5(Na0.5K0.5)0.5TiO3

Microstructure development in Bi_0.5(Na_0.5K_0.5)_0.5TiO₃ prepared by a reactive-templated grain growth process was dependent on the sizes of platelike Bi₄Ti₃O₁₂ (BiT) and equiaxed TiO₂ particles used as starting materials. Calcined compacts were composed of large, platelike template grains and small, equiaxed matrix grains, the sizes of which were determined by those of the BiT and TiO₂ particles, respectively. Texture was developed by the growth of template grains at the expense of matrix grains during sintering, and a new mechanism of grain growth was proposed on the basis of microstructure observation. The grain growth rate was determined by the template and matrix grain sizes, and a dense ceramic with extensive texture was obtained using small BiT and TiO₂ particles.