Influences of Sintering Temperature on Piezoelectric, Dielectric and Ferroelectric Properties of Li/Ta-Codoped Lead-Free (Na,K)NbO3 Ceramics

Li/Ta-codoped lead-free (Na,K)NbO₃ ceramics with a nominal composition of [(Na_0.535K_0.480)_0.942Li_0.058](Nb_0.90Ta_0.10)O₃ were synthesized normally at 1070°–1100°C. The XRD patterns of all samples show a single pervoskite structure with tetragonal symmetry. Although MPB separating the orthorhombic and tetragonal symmetries was absent, the maximum piezoelectric coefficient ( d ₃₃), electromechanical coupling coefficient ( k _p), Curie temperature ( T _c), and remanent polarization ( P _r) were optimized as 216 pC/N, 38.1%, 445°C, and 8.73 μC/cm², respectively.     

Compositional Inhomogeneity in Li- and Ta-Modified (K, Na)NbO3 Ceramics

Compositional inhomogeneity is examined in Li- and Ta-modified potassium sodium niobate (K,Na)NbO₃ perovskite ceramics. The inhomogeneous distribution of the A-site (K and Na) and B-site (Nb and Ta) cations is found to be correlative. The inhomogeneity could not be eliminated by prolonged high-temperature annealing in samples prepared by direct mixing of alkaline carbonates and Ta and Nb oxides. In contrast, the precursor method, where a (Nb _x Ta_{1− x} )₂O₅ solid solution was first formed, led to a considerably improved compositional homogeneity and appreciably enhanced dielectric, ferroelectric, and piezoelectric properties. Our results suggest that more attention needs to be paid toward controlling the compositional fluctuation of this complex solid solution system.     

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.     

BiScO3 Doped (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

Lead-free potassium sodium niobate-based piezoelectric ceramics (1− x )(Na_0.5K_0.5)NbO₃– x BiScO₃ (KNN–BS) ( x =0∼0.05) have been prepared by an ordinary sintering process. Single perovskite phase of KNN–BS exhibits an orthorhombic symmetry at x <0.015 and pseudocubic symmetry at x >0.02, separating by a MPB at 0.015≤ x ≤0.02. Piezoelectric and ferroelectric properties are significantly enhanced in the MPB, which are as follows: piezoelectric constant d ₃₃=203 pC/N, planar coupling coefficient k _p=0.36, remnant polarization P _r=24.4 μC/cm². These solid solution ceramics look promising as a potential lead-free candidate materials.     

Microstructure and Electrical Properties of (K, Na, Li)NbO3–Pb(Zr,Ti)O3 Piezoelectric Ceramics

A new type (1− x )(K_0.485Na_0.485Li_0.03)NbO₃– x Pb(Zr_0.53Ti_0.47)O₃ piezoelectric ceramics was fabricated by conventional ceramics sintering technique. Their microstructure and electrical properties of the ceramics were also studied. X-ray diffraction and scanning electron microscopy patterns indicate that all ceramics samples exhibit a pure perovskite and highly dense structure, and the coexistence of the tetragonal and orthorhombic phases is formed; The ceramic with x =0.75 exhibits the following excellent properties: d ₃₃=363 pC/N, k _p=63%, Q _m=142, ɛ_r=1590, tan δ=1.70%, P _r=28.6 μC/cm², E _c=0.89 kV/mm, T _c=295°C. These results indicate that the ceramic is a promising candidate for piezoelectric ceramics in practical applications.     

Influence of Sintering Temperature on Grain Growth and Phase Structure of Compositionally Optimized High-Performance Li/Ta-Modified (Na,K)NbO3 Ceramics

Microstructure characteristics, phase transition, and electrical properties of (Na_0.535K_0.485)_0.926Li_0.074(Nb_0.942Ta_0.058)O₃ (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d ₃₃ is enhanced to 276 pC/N, which is a high value for the Li- and Ta-modified (Na,K)NbO₃ ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k _p=46.2%, thickness electromechanical coupling factor k _t=36%, mechanical quality factor Q _m=18, remnant polarization P _r=21.1 μC/cm², and coercive field E _c=1.85 kV/mm.     

Hydrothermal Synthesis and Spark Plasma Sintering of (K, Na)NbO3 Lead-Free Piezoceramics

A facile hydrothermal route was adopted for synthesis of lead-free piezoceramics (K, Na)NbO₃ powders. The influences of temperature and KOH/NaOH concentration on the resultant powders were investigated. Although two similar perovskite phases appeared when K/Na ratio tended toward 1:1, the two-phase coexistence tendency was weakened by increasing hydrothermal reaction temperature, and consequently only one phase could be obtained after spark plasma sintering. Reasonably good ferroelectric and piezoelectric properties were obtained for the samples after postannealing, whose piezoelectric constant ( d ₃₃) reached 135 pC/N. The optimal remnant polarization ( P _r) and mechanical quality factor ( Q _m) were 26.2 μC/cm² and 164, respectively, which were both twice as much as those of the samples using powders prepared from solid-state reaction.     

Normal Sintering of (K,Na)NbO3-Based Ceramics: Influence of Sintering Temperature on Densification, Microstructure, and Electrical Properties

Normal sintering of Li-doped and Li/Ta-codoped potassium sodium niobate (KNbO₃–NaNbO₃, KNN)-based ceramics was investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. It was found that density increased greatly within a narrow temperature range but tended to decrease when the sintering temperature slightly exceeded the optimal one, accompanied by the appearance of abnormal grain growth, which was considered to be due to the intensified volatilization of alkali metal oxides. Piezoelectric and dielectric properties also showed a similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density, because both densification and composition affect the electrical properties. The highest d ₃₃ value of 206 pC/N was obtained for the Li- and Ta-codoped KNN ceramics prepared at 1090°C.     

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.     

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.     

Core–Shell Structure Analysis of BaTiO3 Ceramics by Synchrotron X-Ray Diffraction

The shell thickness of a BaTiO₃ ceramic with a core–shell structure has been measured by means of synchrotron X-ray diffraction (XRD). BaTiO₃ ceramic is known from transmission electron microscope (TEM)/energy dispersive X-ray spectroscopy (EDS) observations to have an inhomogeneous microstructure with cores of a pure BaTiO₃ and shells doped with additives. It is also known, from XRD observations, that the BaTiO₃ cores have a tetragonal lattice structure and the shells are pseudocubic. We have estimated the shell thickness d from the full-width at half-maximum (FWHM) of the cubic ( 400 )_c peak, using Scherrer's equation. The shell thickness d _cal was also evaluated from the volume fraction of tetragonal BaTiO₃ using a spherical core–shell model. The two values thus determined agree well, confirming that the BaTiO₃ ceramic specimens have a core–shell structure. Our results show that synchrotron XRD is a simple and effective tool for quantitative analysis of the core–shell structure. It enables us to understand quantitatively the relationship between the microstructure and the dielectric properties of BaTiO₃ ceramics.     

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.     

Dielectric and Electromechanical Properties of (Li, Na)NbO3 Ceramics

Permittivity, dielectric loss tangent, radial coupling coefficient, and radial frequency constant have been measured as a function of composition for pressure-sintered Li._rNa_1-xNbO₃ in the range 0.015≤x≤0.15. An anomaly in each of these properties when plotted as a function of composition is associated with a structural transition between 2 ferroelectric phases; this transition lies near room temperature for compositions where x∼0.12. Compositions near this phase boundary exhibit room-temperature properties which are of interest for high-frequency filter applications.     

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.     

High-Temperature Instability of Li- and Ta-Modified (K,Na)NbO3 Piezoceramics

This paper addresses the high-temperature instability of Li- and Ta-modified (K,Na)NbO₃ piezoceramics. The grains with abnormal size evolve out of the fine matrix grains during high-temperature annealing. They are found to be precipitates with a tetragonal tungsten bronze structure, which result from the volatilization and segregation of the alkali metal elements. With the growth of the abnormal grains the composition of the perovskite matrix phase also changes remarkably, as has been suggested by EDX analysis (for Na) and electric measurements (for Li). These variations lead to a large increase in the tetragonal/orthorhombic phase transition temperature and appreciable variations in the dielectric, ferroelectric, and piezoelectric properties of the ceramic samples. Control of the volatilization of the alkali metal elements can efficiently depress the abnormal grain growth and the compositional segregation.     

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.     

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.     

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.