Ken-ichi Kakimoto

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.

Solid-Solution Structure and Piezoelectric Property of KNbO3 Ceramics Doped with Small Amounts of Elements

The origin of the improvement of the ferroelectric and piezoelectric properties of KNbO3 ceramics by the addition of small amounts of La and Fe has been investigated. The La/Fe-codoped KNbO3 ceramics formed a single (K1-xLax)(Nb1-xFex)O3 solid solution, and their unit-cell volume decreased significantly with an x of 0.002 mol, while they recovered at x=0.004. In this region, the lattice parameters of the a-b basal plane decreased and the elongation in the c direction toward apical oxygen in NbO6 octahedra occurred. These results were correlated with the enhanced polarization and piezoelectric properties measured for the x=0.002 specimen. The x=0.002 specimen demonstrated a kt of 0.48 which corresponds to approximately 70% of the kt value predicted for a KNbO3 single crystal as its maximized property. Furthermore, La/Fe-codoped KNbO3 ceramics with MnO2 addition were prepared to observe the effect of multivalent ion (Fe and Mn) doping into the KNbO3 structure on the properties. Synchrotron near-edge X-ray absorption spectroscopy (XANES) study revealed a mixture of k-edge spectra consisting of Mn4+/Mn3+/Mn2+, while Fe3+ maintained its valence state. The influence of valence fluctuation on the ferroelectric and piezoelectric properties of KNbO3 ceramics was discussed.

Microwave-Millimeterwave Dielectric Materials

Development of microwave dielectric materials has been expected on the wireless communications in the high speed communication society. There are three important directions for research and development of microwave dielectric materials. We have been studied on these three directions based on the crystallography. First direction: tungstenbronze-type like compounds, Second direction: homologous compounds and third direction: silicate compound such as forsterite. INTRODUCTION Recently, microwave and millimeter-wave telecommunication has been developed for wide applications, such as mobile phone, wireless LAN, and intelligent transport system (ITS). Moreover, in the ubiquitous future system, everyone have computer such as IC-tag, which communicate each other using wireless antenna. Microwave and millimeter-wave dielectric materials are expected to be developed for variety of uses such as miniaturization for mobile phone, transmitter and receiver with high performance for base station, and millimeter applications for ultrahigh speed wireless LAN and ITS. The direction of development of microwave dielectric materials are shown in Fig.1, in which quality factors (Q·f) are shown as a function of dielectric constants (εr). Curve in the figure shows an outline the upper limit of Q·f obtained up to now for a given εr. Here, Q·f and εr are among the three important dielectric properties. Q is inverse of the dielectric loss (tanδ) and the effect of εr in shortening the wavelength is described by the relation λ=λ0/εr . Temperature coefficients of resonant frequency (τf) is also one of the three important properties. τf is expected to be close to zero. There are three directions for research and development of wireless communications. Properties demanded is high εr for the 1st direction, high Q and high εr for the 2nd one and extremely high Q and low εr for the 3rd one. The first direction are mainly on demand for miniaturization of mobile phone parts. The second one is on demand for increasing signal/noise ratio for the main application in mobile phone base station. The third direction is for devise working in millimeter-wave range. We have been studying microwave dielectric materials for all these directions. In this paper, we would like to show some examples in our results on the three directions. EXPERIMENTAL Crystal structure analyses were performed by using the four-circle X-ray diffractometer. The single crystals were grown by self-flux method. Full-matrix least-squares refinement was carried out using RADY program. Accurate lattice parameters were obtained 0 20 60 100 140 10

Low-temperature sintering of Ba6−3xSm8+2xTi18O54 microwave dielectric ceramics by B2O3 and GeO2 addition

Abstract Tungsten-bronze-type like Ba 6−3 x Sm 8+2 x Ti 18 O 54 solid solutions (BST) have been known to show relatively high dielectric constants ( e r =80), excellent quality factor ( Q · f =10,700 GHz) and good temperature coefficient of resonant frequency ( τ f =−15 ppm/K) at x =2/3. However, lower sintering temperature has been asked for the commercial request, since the sintering temperature is higher than 1733 K. In this study, we tried to lower the sintering temperature by several kinds of B 2 O 3 and GeO 2 additives. In the all additive systems investigated, the single addition of B 2 O 3 ceramic powders was most effective for lowering the sintering temperature of BST ( x =2/3). When 0.5 wt.% of B 2 O 3 ceramic powder was mixed with starting BST mixtures and sintered at 1473 K, the derived ceramics demonstrated dense microstructure with a large permittivity ( e r =76), high quality factor ( Q · f =10,500GHz) and low temperature coefficient of resonant frequency ( τ f =−19 ppm/K). It is noteworthy that the sintering temperature was significantly lowered by 260 K compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST ( x =2/3).

Controlled Temperature Coefficient of Resonant Frequency of Al2O3-TiO2 Ceramics by Annealing Treatment

The microwave dielectric properties of alumina (Al2O3) ceramics were studied. Our objective was to improve the large negative temperature coefficient of resonant frequency (τf = -60 ppm/°C) of Al2O3 ceramics by the adding rutile (TiO2) with a large positive τf (+450 ppm/°C). A near-zero τf (τf = +1.5 ppm/°C) with excellent microwave dielectric properties (Qf = 117,000 GHz, er = 12.4) was obtained in 0.9Al2O3-0.1TiO2 ceramics sintered at 1350°C for 2 h, followed by annealing at 1000°C for 2 h in air.

High-Q microwave dielectric SrTiO3-doped MgTiO3 materials with near-zero temperature coefficient of resonant frequency

The microwave dielectric properties of the (1-x)MgTiO3–xSrTiO3 system have been investigated. The objective of the present work is to compensate for the negative temperature coefficient of the resonant frequency (τf) of MgTiO3 by addition of SrTiO3. The variation in microwave properties of the system significantly depends on the amount of SrTiO3 present, and they can be matched using mixing rules. With increasing x, the dielectric constant (er) increased and the quality factor (Qf) decreased. A ceramic sample with x=0.036 of SrTiO3, sintered at 1270°C for 2 h, and having a near-zero temperature coefficient of resonant frequency (τf) of -1.3 ppm/°C can be obtained.

Synthesis and dielectric properties of SrxBa1-xNb2O6 formed by YAG laser ablation

Abstract Tungsten-bronze type tetragonal Sr x Ba 1– x Nb 2 O 6 (SBN) ceramics have been synthesized by pressure-less sintering at 1250°C through a mixed-oxide route. With increasing x value in the range from 0.3 to 0.6, the Curie temperature shifted to lower temperature, but the corresponding maximum dielectric constant increased. Further, Sr 0.3 Ba 0.7 Nb 2 O 6 thin films have been formed by pulsed YAG laser deposition (PLD) technique using a selected SBN bulk target with various choices of the deposition parameters. When Pt-coated Si substrates were used for PLD, a preferential c-axis-ordered structure was observed in the films deposited using an energy density above 2.0 J/cm 2 , at a substrate temperature above 650°C, and in oxygen partial pressure below 7 Pa. The thin film yielded a rhomboidal P – E hysteresis loop, and showing a remanent polarization ( P r ) of 3.1 μ C/cm 2 and coercive field ( E c ) of 1.28 kV/cm.

Microwave dielectric properties of textured BaLa4Ti4O15 ceramics with layered perovskite structure

Textured ceramics of BaLa4Ti4O15 (BaLT) with layered perovskite structure have been investigated by characterizing their dielectric properties at MHz and GHz frequencies. The highly grain-oriented ceramics were obtained by a templated grain growth technique. Hexagonal {001}-platelet BaLT particles prepared in molten salt were used as the template. The sintered ceramics containing template powders of 10 wt % exhibited a degree of -orientation of 80% for the casting plane, whereas the plane perpendicular to the casting direction showed a degree of -orientation of 76%. The effect of grain orientation on the dielectric properties was observed as an anisotropy between the directions of parallel and perpendicular to the c-axis. As a result of the anisotropy, enhanced microwave dielectric properties (er=53, Qf=41,400 GHz and τf=-1 ppm/°C) compared with those of a randomly oriented ceramics were obtained in the TE011 mode for a cylindrical specimen with a {001} grain-orientation in the circular plane.