Keisuke Kageyama

Microwave dielectric properties of lanthanide titanate ceramics

Microwave dielectric property changes with substitution of La3+ by Nd3+ and/or Sm3+ ions are studied for La2Ti2O7 and La4Ti9O24 ceramics. For the monoclinic La2Ti2O7, the substitution causes a considerable decrease in the permittivity (er) of the material from er=47 (La) to er=36 (Nd), with a simultaneous variation in the Qf value from 8500 to 16000 GHz. In La4Ti9O24-type dielectrics, only a slight change is observed in both the er(≈37) and Qf values (≈24000 GHz) by lanthanide substitution. The effects of mixing of different dielectric materials on the microwave properties are also examined for La2Ti2O7-La4Ti9O24 and Nd2Ti2O7-Nd4Ti9O24 composites. The permittivity and temperature coefficient of resonant frequency of the composite ceramics can be controlled using Lichtenekers rule, whereas the mixing of the different dielectrics results in the substantial lowering of the Qf value.

Dielectric Properties of Double-Oxide Ceramics in the System Ln2O3-TiO2 (Ln=La, Nd and Sm)

A systematic investigation on dielectric property changes with compositional variation was carried out for binary compound ceramics which were fabricated by the chemical powder preparation method in the identical systems Ln2O3-TiO2 (Ln=La, Nd and Sm). The temperature coefficient of permittivity (TCer) for those dielectrics was found to show a considerable deviation from a simple relation of TCer=-erαL. It also found that the solubility limits of Sm3+ into the Nd3+ lattice were greatly extended in the binary compounds up to ~80 or 90 mol%. Different dielectric property changes in the solid solutions with lanthanide substitution (La→Nd→Sm) were discussed on the basis of the different contributions of substituted ions to the polarization.

Formation and microwave dielectric properties of Sr(Ga0.5Ta0.5)O3-based complex perovskites

The Sr(Ga0.5Ta0.5)O3-based perovskites with O2- and/or Sr2+ vacancies were formed by changing the A-site and/or B-site cation ratios. The Sr-deficient perovskites with a limited composition of Sr0.86(Ga0.36Ta0.64)O3 could be obtained, whereas oxygen vacancies were hardly created. The B′-site Ga3+ cation could be replaced with large Sc3+, In3+, Y3+, Nd3+ and La3+. The crystal symmetry of the complex perovskites changed from cubic (B′=Ga3+, Sc3+, In3+) to tetragonal (B′=La3+) through rhombohedral (B′=Y3+, Nd3+). The dielectric permittivities of these ceramics indicated no distinct dependence on the B′-site cation species. The temperature coefficient of permittivity might be associated with the symmetry change of the perovskite phases. Higher microwave Qf values, >Qf=38 000 GHz, were obtained for our complex perovskite ceramics, except with B′=La3+.

Measurement of the Soft-Mode Dispersion in SrTiO3 by Terahertz Time-Domain Spectroscopic Ellipsometry

We have measured the soft-mode dispersion of SrTiO3 bulk single crystals by terahertz time-domain spectroscopic ellipsometry (THz-TDSE). The real and imaginary parts of the dielectric function of the sample are independently derived from the complex amplitude reflectivities of p- and s-polarized terahertz pulses. The TO1 soft-mode resonance is clearly observed at 2.62 THz (87 cm-1). The spectra of the complex dielectric function agree well with those predicted using the general harmonic oscillator model with parameters obtained from conventional Fourier-transform far-infrared (FT-FIR) reflection measurements. THz-TDSE developed in this study can be employed to investigate low-frequency dielectric dispersions.

Film Sensor Device Fabricated by a Piezoelectric Poly(L-lactic acid) Film

Synthetic piezoelectric polymer films produced from petroleum feedstock have long been used as thin-film sensors and actuators. However, the fossil fuel requirements for synthetic polymer production and carbon dioxide emission from its combustion have raised concern about the environmental impact of its continued use. Eco-friendly biomass polymers, such as poly(L-lactic acid) (PLLA), are made from plant-based (vegetable starch) plastics and, thus, have a much smaller carbon footprint. Additionally, PLLA does not exhibit pyroelectricity or unnecessary poling. This suggests the usefulness of PLLA films for the human–machine interface (HMI). As an example of a new HMI, we have produced a TV remote control using a PLLA film. The intuitive operation provided by this PLLA device suggests that it is useful for the elderly or handicapped.

Analysis of Dielectric Response of TiO2 in Terahertz Frequency Region by General Harmonic Oscillator Model

The terahertz range dielectric response of polycrystalline TiO2 (rutile) ceramics and rutile single crystals were investigated by analyzing infrared-active phonons by both Fourier-transform far-infrared (FT-FIR) reflectivity measurement and terahertz time-domain spectroscopy (THz-TDS). The dielectric functions at room temperature measured by THz-TDS are well expressed using the general harmonic oscillator model. We found that the imaginary part of the dielectric permittivity of the TiO2 ceramic is threefold higher than that of the rutile single crystal at 300 K. The large dielectric loss is attributed to the additional damping of the A2u mode or/and an increase in the phonon damping. The low-temperature dielectric loss of the rutile single crystals is exceedingly low. However, TiO2 ceramics shows little temperature dependence. These results indicate that extrinsic scattering significantly affects the dielectric loss of the TiO2 ceramics in the THz region.

Monolithic oxide–metal composite thermoelectric generators for energy harvesting

Monolithic oxide–metal composite thermoelectric generators (TEGs) were fabricated using multilayer co-fired ceramic technology. These devices consisted of Ni0.9Mo0.1 and La0.035Sr0.965TiO3 as p- and n-type thermoelectric materials, and Y0.03Zr0.97O2 was used as an insulator, sandwiched between p- and n-type layers. To co-fire dissimilar materials, p-type layers contained 20 wt. % La0.035Sr0.965TiO3; thus, these were oxide–metal composite layers. The fabricated device had 50 pairs of p–i–n junctions of 5.9 mm × 7.0 mm × 2.6 mm. The calculated maximum value of the electric power output from the device was 450 mW/cm2 at ΔT = 360 K. Furthermore, this device generated 100 μW at ΔT = 10 K and operated a radio frequency (RF) transmitter circuit module assumed to be a sensor network system.

Synthesis and microwave dielectric properties of xRe2O3-yB2O3 (Re = La, Nd, Sm, Dy, Ho and Y) compounds

The microstructure, crystal structure and microwave dielectric properties of xRe2O3-yB2O3 (Re = La, Nd, Sm, Dy, Ho and Y) were studied. Sintering temperature was increased while decreasing Re ion radius. The only compound well-sintered with porosity <1% was LaBO3 which had the highest fQ = 53000 GHz and dielectric constant (er) = 12.5 at 1200°C. With increases in A-site ion radii from Y to La, er increased from 5 to 12.5. The microstructure of LaBO3 ceramics was significantly different from that of other ReBO3 ceramics as seen by scanning electron microscope (SEM) and this will be effective for obtaining superior dielectric properties. (La1-xNdx)BO3 compounds were successfully synthesized and their microwave dielectric properties were also investigated.

Preparation of Highly Oriented Transparent (Sr,Ba)Nb2O6 Ceramics and Their Ferroelectric Properties

Highly oriented transparent (Sr,Ba)Nb2O6 (SBN) ceramics were prepared to obtain excellent electrooptic properties and a high spontaneous polarization, such as those observed in a Sr0.75Ba0.25Nb2O6 (SBN75) single crystal. As a result of slip casting at a high magnetic field of 12 T and firing at 1460 °C for 48 h in an oxygen atmosphere, highly oriented ceramics and strong ferroelectric properties were observed. The spontaneous polarization of highly oriented SBN ceramics was above 15 µC/cm2, which was close to that of the SBN75 single crystal (17 µC/cm2).

Preparation and Characterization of BaTiO3 Thick Films with Li2O-SiO2 Sintering Aid

The effect of adding a Li2SiO3 (LSO) sintering aid for low temperature fabrication of BaTiO3 (BT) thick films was investigated. LSO-added BT thick films were prepared by the screen printing method on Pt-coated ZrO2 substrates. Their sintering properties were significantly improved by the addition of LSO, and the relative density of the BT thick film fired in air at 1200°C was 96%. The secondary phase was not observed. The dielectric, ferroelectric, and piezoelectric properties of the LSO-added BT thick films were examined. A typical ferroelectric hysteresis loop was observed, and the remnant polarization (Pr) was 7.5 µC/cm2. The dielectric constant was 3200, and tan δ was 2.1%. The maximum piezoelectric constant d31 calculated from the displacement of the Pt/BT/Pt/ZrO2 monomorph cantilever was -140 pm/V, and d33 measured with an atomic force microscope was 320 pm/V.