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.     

Fabrication of Transparent Tellurite Glasses Containing Potassium Niobate Crystals by an Incorporation Method

To fabricate transparent oxide glasses containing ferroelectric KNbO₃ crystals, a new method in which KNbO₃ particles are directly incorporated into TeO₂─K₂O─Nb₂O₅ glasses has been developed. Transparent TeO₂-based glasses containing KNbO₃ crystals with a diameter of ∼ 10 μm have been first successfully fabricated by adjusting temperature and time for incorporation. A small difference in the refractive indexes, n , between TeO₂-based matrix glasses ( n = 2.0) and incorporated KNbO₃ crystals ( n = 2.21) is a significant reason for the transparency. This new method is applicable for the fabrication of new transparent glasses containing other functional materials with high refractive indexes.     

Synthesis and Dielectric Characterization of Potassium Niobate Tantalate Ceramics

The preparation and dielectric properties of potassium niobate tantalate (KTN) have been investigated with the aim of exploring the material's potential for ferroelectric tunable applications. The samples were prepared both by conventional sintering in air and by uniaxial hot pressing. A relative average density greater than 92% was obtained with both methods. An inhomogeneous Nb/Ta distribution was found in the samples prepared by both methods, but the inhomogeneity extent was lower in the hot-pressed samples. While both sintering processes resulted in ceramics of lower transition temperatures in comparison with the reported results on single crystals, a relative up shift of the temperature ( T _max) at which the dielectric constant is maximum was found for hot-pressed samples compared with that of samples sintered in air. All the samples exhibited strong frequency dispersion in dielectric properties. The effect of DC bias at room temperature was measured and modeled using the Landau–Devonshire model. It was found that the nonlinear coefficient β of KTN, which is important for tunable applications, is comparable with the value reported on KNbO₃ and SrTiO₃ single crystals. A dielectric tunability of 16% and 42% at room temperature was demonstrated under 20 kV/cm for the sintered and hot-pressed ceramics, respectively. The loss tangent, low at megahertz frequencies, was augmented to 9%–17% at low gigahertz frequencies. This is believed to be related to the frequency dispersion observed in the ceramics because of charged defects.     

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.     

Phase Transitions in the System BaTiO3—KnbO3

Solid solution formation in the system BaTiO₃—KnbO₃ was established by X-ray diffraction and dielectric measurements. Solid solutions with cubic symmetry were observed in the composition range from 4 to 90 mole % KnbO₃ at room temperature. The lattice parameter for the BaTiO₃ solid solutions increased with increasing KNbO₃; that for the KnbO₃ solid solutions decreased with the addition of BaTiO₃. A distinct discontinuity in lattice parameter was observed at the composition containing about 65 mole % BaTiO₃. Dielectric measurements were made from-195° to 400°C. The cubic-tetragonal transition temperature of BaTiO₃ was rapidly lowered with increasing addition of KNbO₃, whereas the two lower phase transition temperatures were raised. All three phase transitions of KnbO₃ were rapidly lowered with increasing addition of BaTiO₃. The observed phase transitions, lattice parameters, and electron probe data suggest a complex region in the subsolidus which extends from 35 to about 75 mole % KNbO₃.     

Dielectric Properties of Ceramics in Ba (Co1/3 Nb2/3)O3–Ba(Zn1/3Nb2/3)O3 Solid Solution

The dielectric properties of the Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ system were determined. Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ has a complex perovskite structure, a high dielectric constant, a low dielectric loss, and a low temperature coefficient of the resonant frequency. A solid-solution ceramic with 0.7Ba (Co_1/3 Nb_2/3)O₃·0.3 Ba(Zn_1/3Nb_2/3)O₃ has a dielectric constant of K=33.5, Q=11000 at 6.5 GHz, and a temperature coefficient of the resonant frequency of τ_f=0 ppm/°C. The temperature coefficient of resonant frequency can be varied by changing the composition. The Q values of the ceramics can be increased by annealing in a nitrogen atmosphere. These ceramics can be used for resonant elements and stabilized oscillators.     

Dielectric and Ferroelectric Properties of Ceramics in the Pb(Zn 1/3Nb2/3)O3-BaTiO3-PbTiO3 System

The use of Pb(Zn_1/3Nb_2/3)O₃ ceramics is restricted by the formation of a pyrochlore phase detrimental to both dielectric and piezoelectric properties. Recently it has been shown that a 6 mol% addition of BaTiO₃ to PZN suppresses the formation of pyrochlore phase. Phase relations and dielectric properties of ceramics in the PZN-BT-PT system are reported here. Compositions with the perovskite structure, having high dielectric constant and low temperature coefficient of capacitance, have been identified.     

Microwave Dielectric Properties and Low-Temperature Sintering of Ba0.60Sr0.40TiO3 Ceramics

In the present work, the sintering behaviors and dielectric properties of Ba_0.60Sr_0.40TiO₃ (BST) ceramics with the addition of BaCu(B₂O₅) were investigated in detail. The results indicated that the addition reduced the sintering temperature of BST by about 500°C. It was suggested that a liquid phase BaCu(B₂O₅) assisted the densification of BST ceramics at lower temperatures. For a low-level BaCu(B₂O₅) addition (2.0 mol%), the BST sample sintered at 950°C for 5 h displayed good dielectric properties, with a moderate dielectric constant (ɛ=2553) and a low dielectric loss (tan δ=0.00305) at room temperature and at 10 kHz. The sample showed 45.9% tunability at 10 kHz under a dc electric field of 30 kV/cm. At the frequency of 0.984 GHz, BST-added 2.0 mol% BaCu(B₂O₅) possessed a dielectric constant of 2204 and a Q value of 146.7.     

Crystal Growth and Electrical Properties of Pb((Zn1/3Nb2/3)0.91Ti0.09)O3 Single Crystals Produced by Solution Bridgman Method

Piezoelectric Pb((Zn_1/3Nb_2/3)_0.91Ti_0.09)O₃ (PZNT 91/9) single crystals 40 mm in diameter were successfully grown from solution by the Bridgman method with a PbO flux. The crystals were grown in a platinum crucible heated to 1130°C. Growth rate was 0.35 mm/h. The obtained crystals were ~40 mm in diameter 20 mm in length and were a rust-brown color. The Curie temperature, T _C, ranged from 175° to 185°C, and the dielectric constant before poling at room temperature was 2000-8900 within a wafer. After electrical poling, specimens had electromechanical coupling coefficients in rectangular bar mode, k _33´, of 79%-88%, which were larger than for PZT ceramics ( k _33´ < 70%). These PZNT 91/9 single crystals grown by the Bridgman process satisfy the requirements for array-type transducers used in echocardiographic equipment. Results confirm that the Bridgman method is useful for mass-producing large crystals of PZNT 91/9.     

Compositional Dependence of Piezoelectric Properties in NaxK1−xNbO3 Lead-Free Ceramics Prepared by Spark Plasma Sintering

Lead-free piezoelectric Na _x K_{1− x} NbO₃ ( x =20–80 mol%) ceramics were fabricated using spark plasma sintering at a low temperature (920°C). All the Na _x K_{1− x} NbO₃ ceramics showed a similar orthorhombic phase structure, while the corresponding lattice parameters decreased from the KNbO₃ side to the NaNbO₃ side with increasing Na content. A discontinuous change in lattice parameter close to composition of 60 mol% Na indicated the presence of a transitional area that is similar to the morphotropic phase boundary (MPB) in Na _x K_{1− x} NbO₃ ceramics. The sintered density of the Na _x K_{1− x} NbO₃ ceramics decreased with increasing Na content, from a relative density of 99% for the K-rich side to 92% for the Na-rich side. The piezoelectric constant d ₃₃ and planar mode electromechanical coupling coefficient k _p showed a maximum value of 148 pC/N and 38.9%, respectively, due to the similar MPB effects in the PZT system.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

Influence of Processing Parameters on the Sintering and Electrical Properties of Pb(Zn1/3Nb2/3)O3-Based Ceramics

Pb(Zn_1/3Nb_2/3)O₃-based ceramics have been prepared by two different processing methods: conventional (PZN-C) and reaction-sintering (PZN-RS). The conventionally prepared PZN-based ceramics densified at lower temperatures (950°C) than the reaction-sintered samples (1100°C), but the perovskite/pyrochlore ratio was always higher in PZN-RS. The presence of a substantial amount of pyrochlore phase in PZN-C ceramics caused a decrease in the electrical properties. The maximum dielectric constant values in PZN-C ceramics were 10%–15% lower than those of PZN-RS, despite a similar average grain size, 7 ± 0.2 μm. The temperature of the maximum of the dielectric constant ( T _max) was lower than that expected from the mixing rule because of the possible formation of Ba–Nb clusters. The higher chemical homogeneity in PZN-RS ceramics is the main reason for the higher dielectric constant, T _max and electromechanical response, as well as for the lower difference between T _max and the depolarization temperature ( T _d) and the lower diffusiveness parameter (δ).     

A Polyethylene Glycol-Modified Solid-State Reaction Route to Synthesize Relaxor Ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT)

By introducing polyethylene glycol (PEG) to the conventional simultaneously mixed oxide reaction route, the 0.65Pb(Mg_1/3Nb_2/3)–0.35PbTiO₃ (0.65PMN–0.35PT) powders and ceramics with pure perovskite phase have been successfully synthesized. It is found that PEG interacts with PbO oxide in a way favoring the formation of the desired perovskite phase. As a result, pyrochlore-free 0.65PMN–0.35PT powders are synthesized at a low temperature of 850°C. The ceramics sintered at 1000°C show uniform grains with the size ranging from 1 to 3 μm. The room temperature dielectric constant is 3440. The maximum dielectric constant is 16 220 at 1 kHz. This method can be applied to the synthesis of other Pb-containing and Bi-containing ferroelectric materials, especially the relaxor-type ferroelectrics in which the pyrochlore phase is difficult to eliminate.     

Microwave Dielectric Properties of a New A5B4O15-Type Cation-Deficient Perovskite Ba2La3Ti3TaO15

High dielectric constant and low loss ceramics with composition Ba₂La₃Ti₃TaO₁₅ have been prepared by a conventional solid-state ceramic route. This compound adopts A₅B₄O₁₅ cation-deficient hexagonal perovskite structure. The dielectric properties of dense ceramics sintered in air at 1520°C have been characterized at microwave frequencies. It shows a relative dielectric constant of ∼45, quality factor Q _u× f of ∼26 828 GHz and temperature variation of resonant frequency of −0.97 ppm/°C.     

Intrinsic Size Effects in a Barium Titanate Glass-Ceramic

A series of glass ceramics have been synthesized to produce bulk materials with nanometer-sized barium titanate (BaTiO₃) crystals grown in a residue glass matrix. Structure-property relations have been made to determine the size distribution and the dielectric temperature dependence of the ceramics. Through dielectric and density mixing laws, it has been inferred that depolarization fields limit the dielectric polarizability of the particles and influence the transition temperature. The transition temperature, dielectric anomaly broadening, and peak dielectric constant all scale systematically with the mean size of the BaTiO₃ crystals, which is consistent with an intrinsic size effect. In addition, scaling the transition temperature with the Ishikawa relation predicts a critical size of 17 nm, for which BaTiO₃ cannot support a ferroelectric transition. These results are discussed in relation to other size studies on ferroelectric materials.     

Influence of Stress and Chemical Homogeneity on Dielectric Properties of BaTi0.9Zr0.1O3

The influence of mechanical stress and chemical homogeneity on the permittivity of BaTi_0.9Zr_0.1O₃ ceramics prepared from mixed-oxide and hydrothermal powders was studied. To reduce stress, liquid-phase sintering was applied in conjunction with a low heating rate to stimulate the formation of large grains. The influence of chemical homogeneity was studied by variations in sintering temperatures and times. For both types of ceramics, the dielectric constant at the Curie temperature was influenced by both factors, but to a different extent. In the mixed oxide ceramic, chemical homogeneity played a more prominent role, while internal stress appeared to exert a larger influence in the hydrothermal ceramics. The dielectric constant at the Curie temperature could be increased by 5%–10% by an annealing treatment at 200°C, followed by slow cooling.     

Influence of V2O5 Additions to 5Li2O–1Nb2O5–5TiO2 Ceramics on Sintering Temperature and Microwave Dielectric Properties

The effects of the addition of V₂O₅ on the sintering behavior, microstructure, and microwave dielectric properties of 5Li₂O–1Nb₂O₅–5TiO₂ (LNT) ceramics have been investigated. With low-level doping of V₂O₅ (≤3 wt%), the microstructure of the LNT ceramic changed from a special two-level intergrowth structure into a two-phase composite structure with separate grains. And the sintering temperature of the LNT ceramics could be lowered to around 900°C by adding a small amount of V₂O₅ without much degradation in microwave dielectric properties. Typically, better microwave dielectric properties of ɛ_r=41.7, Q × f =7820 GHz, and τ _f =45 ppm/°C could be obtained for the 1 wt% V₂O₅-doped ceramics sintered at 900°C.     

Ferroelectric Phase Transitions in the System PbTiO3,-KNbO3,

The formation of a complete series of solid solutions in the system PbTiO₃-KNbO₃ was established by X-ray and dielectric studies. The room-temperature symmetry of the entire system was tetragonal except for compositions containing more than 96 mole % KNbO₃, which had orthorhombic symmetry. The axial ratio, c/a , and the ferroelectric Curie temperature decreased from both the end-members. The lowest Curie temperature observed in the system was 175°C for the composition with 80 mole % KNbO₃. A generalization has been made on the variation of ferroelectric Curie temperatures with compositions in binary systems of perovskitetype solid solutions with and without a common cation.     

Piezoelectric Properties and Phase Relations of Pb(Mg1/3Nb2/3)O3-PbTiO3−PbZrO3 Ceramics with Barium or Strontium Substitutions

When a small amount of Ba or Sr is substituted for Pb in Pb(Mg_1/3 Nb_2/3)O₃-PbTiO₃-Pb2rO 3 , the morphotropic boundary and the compositions which show the highest planar coupling coefficient and dielectric constant shift slightly toward the decreasing PbTiO₃ content. The tetragonality of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ and Pb(Mg_1/2 Nb_2/3)-O₃-PbTiO₃-PbZrO₃ ceramics decreased with increasing Ba or Sr content. The lattice parameter (α axis) in the rhombohedral or pseudocubic phase increased with the increase of Ba but decreased with the increase of Sr substitution. Although the Curie temperature was lowered with the increase of Ba or Sr, the dielectric constants of the ceramics were increased. The dielectric and piezoelectric properties of the ternary compositions near the morphotropic boundary were improved through selection of sub-stituent and base composition. A planar coupling coefficient of 0.66 and a low Young's modulus were obtained with substitution of 5 mole % Ba. A dielectric constant greater than 3500 and a planar coupling of 0.63 can be obtained by substituting 5 mole % Sr.     

Grain Growth-Controlled Giant Permittivity in Soft Chemistry CaCu3Ti4O12 Ceramics

We report a dielectric constant of up to 5.4 × 10⁵ at room temperature and 1 kHz for CaCu₃Ti₄O₁₂ (CCTO) ceramics, derived from multiphase powders (coprecipitation products), made by a "chimie douce" (coprecipitation) method, and then sintered in air. The sintered products are pure-phase CCTO ceramics. The high dielectric constant is achieved by tuning the size of grains and the thickness of grain boundaries. The grain growth is controlled by varying the concentration of excess CuO in the initial powder (calcined coprecipitation products) between 1 and 3.1 wt%. The dielectric constant of pure CCTO ceramics increases with the initial CuO concentration, reaching its maximum at 2.4 wt% of CuO. A further increase of excess CuO in powders results in a permittivity decrease, accompanied by the formation of CuO as a separate phase in the sintered products. The unusual grain growth behavior is attributed to a eutectic reaction between CuO and TiO₂ present in the initial powder.