Hitoshi Ohsato

Phase transitional behavior and piezoelectric properties of (Na0.5K0.5)NbO3–LiNbO3 ceramics

Lead-free piezoelectric ceramics (1−x)(Na0.5K0.5)NbO3–xLiNbO3 {[Lix(Na0.5K0.5)1−x]NbO3} (x=0.04–0.20) have been synthesized by an ordinary sintering technique. The materials with perovskite structure is orthorhombic phase at x⩽0.05 and becomes tetragonal phase at x⩾0.07, a phase K3Li2Nb5O15 with tetragonal tungsten bronze structure begins to appear at x=0.08 and becomes dominant with increasing the content of LiNbO3. A morphotropic phase boundary between orthorhombic and tetragonal phases is found in the composition range 0.05<x<0.07. Analogous to Pb(Zr,Ti)O3, the piezoelectric and electromechanical properties are enhanced for compositions near the morphotropic phase boundary. Piezoelectric constant d33 values reach 200–235pC∕N. Electromechanical coefficients of the planar mode and the thickness mode reach 38%–44% and 44%–48%, respectively. The Curie temperatures (TC) of [Lix(Na0.5K0.5)1−x]NbO3 (x=0.04–0.20) are in the range of 452–510°C, at least 100°C higher than that of conventional Pb(Zr,Ti)O3. Our re...

Structure and Electrical Properties of Lead-Free (Na0.5K0.5)NbO3-BaTiO3 Ceramics

(Na0.5K0.5)NbO3 with 0–20 mol% BaTiO3 has been prepared following the conventional mixed oxide process. High-density samples were obtained through the addition of BaTiO3 into (Na0.5K0.5)NbO3 because of an increase in grain size. X-ray diffraction analysis revealed that, during sintering, all of the BaTiO3 diffuses into the lattice of (Na0.5K0.5)NbO3 to form a solid solution, in which orthorhombic symmetry changes to tetragonal symmetry at x≈0.06, and tetragonal symmetry changes to cubic symmetry at x=0.20 or higher. It was found that the samples with a low content of BaTiO3 exhibit relatively good ferroelectric and piezoelectric properties. For 0.98(Na0.5K0.5)NbO3-0.02 BaTiO3 ceramics, the remanent polarization (Pr) is 7.5 µC/cm2 with a coercive field (Ec) of 12 kV/cm. The piezoelectric constant d33 reaches 104 pC/N. The electromechanical coupling coefficients of the planar mode kp and the thickness mode kt reach ~29% and ~38%, respectively. Our results show that (Na0.5K0.5)NbO3-BaTiO3 is a good lead-free piezoelectric ceramic.

Raman Scattering Study of Piezoelectric (Na0.5K0.5)NbO3-LiNbO3 Ceramics

A Raman scattering study of (Na0.5K0.5)NbO3 (NKN)-LiNbO3 (LN) lead-free piezoceramics has been carried out on nominal compositions of (1-x)NKN-xLN (0 ≤x ≤0.70). The Raman spectra demonstrated a variety of changes with x, mainly classified as lattice translations involving motions of the alkaline cations and internal modes of NbO6 octahedra. At 0.05 ≤x ≤0.07, the broadening of the scattering peaks corresponding to the internal modes of the NbO6 octahedra occurred preferably, which is consistent with the evidence for a morphotropic phase boundary (MPB). Furthermore, an abrupt shift was observed in the peak position of the symmetric stretching mode v1 to a higher frequency, resulting from the distortion of O-Nb-O angles caused by incorporating small Li ions into the perovskite units. This is considered to be a prologue for the structural transformation in the NKN-LN solid solution from orthorhombic to tetragonal symmetry. At compositions in which x increases above the MPB, the translational mode of the Li+ cation emerged clearly, and the overall scattering pattern gradually changed to complex patterns caused by the formation of a tungsten-bronze K3Li2Nb5O15 (KLN) secondary phase and an increase in the number of distorted LiNbO3-type crystal units.

Science of tungstenbronze-type like Ba6-3xR8+2xTi18O54 (R = rare earth) microwave dielectric solid solutions

The crystallography of tungstenbronze-type like Ba6−3xR8+2xTi18O54 solid solutions for microwave dielectric materials applied for mobile phone and base station were studied. The crystal data and crystal structure of the fundamental lattice and supperlattice, and derivation of chemical formula and structural formula are summarized. Fourier map of the fundamental structure showed that oxygen ion positions of TiO6 octahedra were splitted. This fact proved the tilting of octahedra which is cause of superlattice. The mechanisms of microwave dielectric properties are summarized. Finally, guidelines for the design of the microwave dielectric materials are presented.

The effect of rare-earth (La, Sm, Dy, Ho and Er) and Mg on the microstructure in BaTiO3

Abstract The effect of rare-earth elements, La, Sm, Dy, Ho and Er, on the microstructure in BaTiO3(BT)–MgO–rare-earth oxide based system was studied. Larger amount of MgO was required to suppress the grain growth for the larger ionic radius rare-earth ion (La, Sm)-doped samples than for the smaller ion (Dy, Ho, Er)-doped samples. Also, substitution modes of rare-earth elements and Mg in BT lattice were investigated. The solubility of rare-earth ions in BT lattice and the substitution ratio of rare-earth ions into Ba-site decreased as ionic radius decreases. This suggests that the formation of core-shell structures in BT–MgO–rare-earth oxide based system is dependent on the substitution ratio of rare-earth ions into Ba-site.

The effect of MgO and rare-earth oxide on formation behavior of core-shell structure in BaTiO3

The formation mechanism of the core-shell structure of a BaTiO3(BT)–MgO–Ho2O3-based system was studied. Mg reacted with BT at low temperatures and showed poor diffusivity compared with Ho. The core-shell structure was considered to be formed by the suppression of the diffusion of Ho into the core region by Mg. Also, replacement modes of Mg and Ho in perovskite were investigated. Lattice parameters were determined at temperatures higher than the Curie temperature in order to avoid crystal structure change. It was confirmed that Mg dissolved in Ti site, and Ho dissolved in both Ba and Ti sites. This indicates that Mg ions act as acceptors and Ho ions act as both donors and acceptors in the shell phase.

Ferroelectric and piezoelectric properties of KNbO3 ceramics containing small amounts of LaFeO3

Dense KNbO3 ceramics have been successfully synthesized by pressure-less sintering under optimized heat-treatment conditions using a small amount of La2O3 and Fe2O3 additives. KNbO3 forms (K1-xLax)(Nb1-xFex)O3 solid solutions and changes in the crystal system, depending on the additive content, from orthorhombic to tetragonal at x of 0.020, and from tetragonal to cubic at x of 0.200 or higher. When only 0.002 mol of La2O3 and Fe2O3 (x=0.002) was added into KNbO3, the highest value (98.8%) of the theoretical density was obtained. This specimen showed orthorhombic symmetry with a high Curie temperature of 420°C, and demonstrated a well-saturated ferroelectric hysteresis loop with large remanent polarization (Pr) of 18 µC/cm2, which is comparable to the value reported for pure KNbO3 ceramics fabricated by hot pressing. Furthermore, the x=0.002 specimen showed a planar electromechanical coupling ratio (kp) of 0.17 and piezoelectric d33 constant of 98 pC/N, regardless of the unsaturated poling state.

Formation of Solid Solutions of New Tungsten Bronze-Type Microwave Dielectric Compounds Ba6-3xR8+2xTi18O54 (R=Nd and Sm, 0≤x≤1)

The formation of solid solutions of microwave dielectric compounds having compositions in the vicinity of Ba3.75-R9.5Ti18O54 and/or BaOR2O34TiO2 (R=rare earth) has been investigated. 3BaO2R2O39TiO2 (Ba6R8Ti18O54) solid solutions with the ideal formulae of Ba6-3xR8+2xTi18O54 (0.0≤x≤1.0) have been derived on the tie line with the ratio Ti:O=1:3 between perovskite and R2Ti3O9 based on the new tungsten bronze-type crystal structure with 2×2 perovskitelike unit cells. The type of solid solutions is substitutional with following the relation 3Ba2+\rightleftarrows2R3++\Box. Lattice parameters as a function of composition have been determined by the WPPD (whole-powder-pattern decomposition) method.17) The formation range is located at 0.3≤x≤0.7 compositions in the case of R=Sm and 0.0≤x≤0.7 in R=Nd. In the latter case, two kinds of substitutions of the solid solutions are considered.

Microwave Dielectric Properties of the Ba6-3x(Sm1-y,Ry)8+2xTi18O54 (R=Nd and La) Solid Solutions with Zero Temperature Coefficient of the Resonant Frequency.

Ba6-3x(Sm1-y,Ry)8+2xTi18O54 (R = Nd and La; x = 0.6, 0.0 ≤y ≤1.0) solid solutions in which Nd or La ions are substituted for Sm ions were studied from the point of microwave dielectric materials. In both Sm-Nd and Sm-La series, formation of solid solutions was found, which was deduced from linear change of the lattice parameters. We found high-quality resonator materials in the both series which show high e r of 84, Q f products of 9000 GHz, and temperature coefficient of resonance frequency τ f=0 ppm/\CC. The compositions of those materials in the Sm-Nd and the Sm-La series were determined to be y = 0.2 and 0.1, respectively. A relationship between dielectric constant and volume of octahedron is also stated.

Effect of (Na,K)-Excess Precursor Solutions on Alkoxy-Derived (Na,K)NbO3 Powders and Thin Films

Lead-free (Na0.5K0.5)NbO3 (NKN) powders and thin films were fabricated from stoichiometric (Na/K=50/50), 4 mol % excess (52/52, 53/51, and 55/49), and 10 mol % excess (55/55, 56/54, 58/52, and 60/50) precursor solutions by the sol–gel process. The NKN55/55 powder heat-treated at 800 °C showed an orthorhombic phase and rectangular grains whose size was estimated to be about 300 nm. With increasing Na/K ratio, grain size increased to about 1 µm and the cubic–tetragonal (Tc) phase transition peak emerged at 392 °C in a differential thermal analysis (DTA) curve. On the other hand, single-phase NKN thin films were fabricated at 600 °C from the (Na,K)-excess precursor solutions (58/52 and 60/50). In particular, the NKN58/52 thin film showed a low leakage current density (10-7 A/cm2 at 40 kV/cm), and the maximum polarization Pmax and dielectric constant er at 1 kHz were 9.1 µC/cm2 and 725, respectively.