Low-Temperature Sintering of K0.5Na0.5NbO3 Ceramics

The objective of this work was to lower the sintering temperature of K_0.5Na_0.5NbO₃ (KNN) without reducing its piezoelectric properties. The KNN was sintered using 0.5, 1, 2, and 4 mass% of (K, Na)-germanate. The influence of the novel sintering aid, based on alkaline germanate with a melting point near 700°C, on the sintering, density, and piezoelectric properties of KNN is presented. The alkaline-germanate-modified KNN ceramics reach up to 96% of theoretical density at sintering temperatures as low as 1000°C, which is approximately 100°C less than the sintering temperature of pure KNN. The relative dielectric permittivity (ɛ/ɛ₀) and losses (tanδ), measured at 10 kHz, the piezo d ₃₃ coefficient, the electromechanical coupling and mechanical quality factors ( k _p, k _t, Q _m) of KNN modified with 1 mass% of alkaline germanate are 397, 0.02, 120 pC/N, 0.40, 0.44, and 77, respectively. These values are comparable to the best values obtained for KNN ceramics sintered above 1100°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.     

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.     

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.     

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.     

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.     

Cryogenic Microwave Dielectric Properties of Sintered (Zr0.8Sn0.2)TiO4 Doped with CuO and ZnO

The effect of CuO on the sintering temperature, microstructure, and the microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) has been investigated. The microwave dielectric properties of the ZST ceramics have been measured at cryogenic temperatures (15–290 K). The crystallite sizes of the sintered ZST ceramics are in the 30–50-nm range. The addition of CuO effectively reduced the sintering temperature to 1300°C due to the liquid-phase effects. The addition of CuO did not cause any secondary phases up to 1.5 wt% of CuO. It is found that the quality factor ( Q ) of the sample without CuO decreased with an increase in temperature, whereas the samples with the addition of CuO up to 1.0 wt% showed less dependence on temperature at cryogenic temperatures. The microwave dielectric properties of the ZST ceramics measured at cryogenic temperatures exhibited a Q factor of 15 000 for pure ZST and 11 800 for ZST with 0.5 wt% of CuO at 15 K. The increase in Q factor at cryogenic temperatures can be attributed to the reduction in both intrinsic and extrinsic losses in the ZST ceramics.     

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 Electrical Properties of MnO-Doped (Na0.5Bi0.5)0.92Ba0.08TiO3 Lead-Free Piezoceramics

Microstructure and electrical properties of manganese oxide (MnO)-doped (Na_0.5Bi_0.5)_0.92Ba_0.08TiO₃ (NBBT) piezoceramics were investigated in this work. X-ray diffraction analysis shows that the suitable substitution of Mn ion into the B site induces the lattice distortion of perovskite NBBT: the solution limit is at 0.3 wt% MnO. Besides, it is observed that the sintering properties can be improved by adding a small amount of MnO, thus increasing the grain size and the relative density. Further, the temperature dependence of the dielectric permittivity of NBBT ceramics indicates that the MnO addition reconstructs the disorder array destroyed by joining BaTiO₃ in the Na_0.5Bi_0.5TiO₃ system due to the sizable radius of the B-site cations. Combining these effects of MnO addition, the optimal electrical properties were acquired for NBBT ceramic with addition of 0.30 wt% MnO. The excellent electrical properties of MnO-doped NBBT ceramics indicate its promising application in large displacement actuators.     

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.     

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.     

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.     

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.     

Effect of Simultaneous Addition of BiFeO3 and Ba(Cu0.5W0.5)O3 on Lowering of Sintering Temperature of Pb(Zr,Ti)O3 Ceramics

Sintering of 0.5-wt%-MnO₂-added Pb(Zr_0.53Ti_0.47)O₃ ceramics progresses at 935°C for 50 min by the addition of complex oxides of perovskite-type crystal structure, BiFeO₃ and Ba(Cu_0.5W_0.5)O₃. In order to elucidate the low-temperature sintering mechanism of Pb(Zr,Ti)O₃ ceramics, the shrinkage and the evolution of the microstructure of a compacted body during heating were studied. It has been shown that the densification process was separated into the following three stages: the rearrangement of grains, the grain boundary diffusion of atoms, and then grain growth. Also, microstructural and elemental analyses of the ceramics revealed the existence of an amorphous phase at the grain boundaries predominantly composed of lead and copper oxides. Consequently, this process can be facilitated by the occurrence of a transient liquid phase corresponding to the above amorphous phase.     

Characteristics of High-Q Microwave Dielectric Ceramics Nd(Co1/2Ti1/2)O3 With CuO Addition

We report the microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by the conventional solid-state route. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibits a mixture of Co and Ti showing a 1:1 order in the B site. Lowering the sintering temperature (as low as 1260°C) and promoting the densification of Nd(Co_1/2Ti_1/2)O₃ ceramics could be effectively achieved by adding CuO (up to 0.75 wt%). At 1350°C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% CuO addition possess a dielectric constant (ɛ_r) of 27.6, a Q × f value of 165 000 GHz (at 9 GHz), and a temperature coefficient of resonant frequency (τ_f) of −20 ppm/°C. By comparing with pure Nd(Co_1/2Ti_1/2)O₃ ceramics, incorporating additional CuO helps to render a dielectric material with a higher dielectric constant, a smaller τ_f value, and a 20% dielectric loss reduction, which makes it a very promising candidate for applications requiring low microwave dielectric loss.     

Fabrication of Textured Bi3NbTiO9 Ceramics

Highly textured Bi₃NbTiO₉ ceramics are fabricated by normal sintering from molten salt-synthesized plate-like crystallites. Fine Bi₃NbTiO₉ plate-like crystallites (∼1 μm) not only facilitate the densification, but also enhance texture in Bi₃NbTiO₉ ceramics. Weak-agglomerated platelets exhibit higher sinterability and can be densified at a temperature as low as 1000°C, which is about 100°C lower than that of equiaxed powders prepared by directly calcining Bi₃NbTiO₉ precursor. Meanwhile, the orientation degree of textured Bi₃NbTiO₉ ceramics increases with sintering temperature. Highly oriented Bi₃NbTiO₉ (orientation degree of ∼0.91) ceramic with a relative density of ∼92% is obtained at 1150°C. Because of the oriented grain microstructure, textured Bi₃NbTiO₉ ceramic exhibits anisotropic electrical properties.     

Influence of SiO2 Addition on the Dielectric Properties and Microstructure of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 Ceramics

In this study, the effect of SiO₂ doping on the sintering behavior, microstructure, and dielectric properties of BaTiO₃-based ceramics was investigated. Silica was added to (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ (BCTZ) powder prepared using the solid-state method. SiO₂-doped BCTZ ceramics with a high density and a uniform grain size were obtained and sintered at 1220°C in a reducing atmosphere. A second phase (BaTiSiO₅) existed in samples when SiO₂ was added in excess of 1%. The amount of the second phases was observed to increase as the number of SiO₂ additives increased. It was found that BCTZ ceramics sintered with SiO₂ are helpful in reducing the sintering temperature for a typical thick film and MLCC applications. However, there were disadvantageous effects on the dielectric properties with mere addition of SiO₂ addition (3% and 5%) due to higher formation of BaTiSiO₅. Doping with a small amount of silica can improve the sintering and dielectric properties of BCTZ ceramics. In addition, to understand the effect of the BaTiSiO₅ phase on the dielectric properties of BCTZ ceramics, the BaTiSiO₅ composition was synthesized from individual BaCO₃, TiO₂, and SiO₂ powders using conventional solid-state methods. X-ray diffraction results show the presence of mainly the crystalline phase, BaTiSiO₅, in the sintered ceramics.     

Low temperature sintering and microwave dielectric properties of 0.5LaAlO3–0.5SrTiO3 ceramics using copper oxide additions

The microwave dielectric properties and the microstructures of 0.5LaAlO₃–0.5SrTiO₃ ceramics with CuO addition prepared with conventional solid-state route have been investigated. Doping with CuO (up to 1 wt.%) can effectively promote the densification and remain comparable dielectric properties of 0.5LaAlO₃–0.5SrTiO₃ ceramics. It is found that 0.5LaAlO₃–0.5SrTiO₃ ceramics can be sintered at 1400 °C due to the sintering aid effect resulted from CuO as addition observed by scanning electron microscopy. The dielectric constant as well as the Q×f value decreases with increasing CuO content. At 1460 °C, 0.5LaAlO₃–0.5SrTiO₃ ceramics with 0.25 wt.% CuO addition possess a dielectric constant (_r) of 35.2, a Q×f value of 24 000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −13.5 ppm/°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.