Yukio Higuchi

(K,Na)NbO3-Based Multilayer Piezoelectric Ceramics with Nickel Inner Electrodes

A lead-free (K,Na)NbO3-based multilayer piezoelectric ceramic with nickel inner electrodes was obtained. Its normalized electric-field-induced thickness strain (Smax/Emax) was 360 pm/V, where Smax denotes the maximum strain and Emax denotes the maximum electric field. This value of 360 pm/V is about half that for the Pb(Zr,Ti)O3-based ceramics that are widely used. However, the (K,Na)NbO3-based multilayer ceramic can achieve comparable displacement to that attainable with Pb(Zr,Ti)O3-based multilayer ceramics by stacking more ceramic layers with smaller thicknesses. Additionally, it is easier to reduce the ceramic-layer thickness by using nickel electrodes. The developed (K,Na)NbO3-based multilayer ceramic is thus considered a good candidate for lead-free piezoelectric actuators.

Recent progress on the dielectric properties of dielectric resonator materials with their applications from microwave to optical frequencies

The dielectric properties at microwave and millimeter-wave frequency are reported for practical dielectric resonator materials. The dielectric materials suitable for each application such as antenna duplexers for mobile phones, filters for cellular base station, millimeter-wave applications, and high temperature superconductors are introduced with their dielectric properties. Recently, valuable optical properties have been obtained by altering specific dielectric resonator materials.

Relationship between Vibration Direction and High-Power Characteristics of -Textured SrBi2Nb2O9 Ceramics

The effect of the angle between the vibration direction and the oriented direction on high-power characteristics was investigated for -textured SrBi2Nb2O9 (SBN) ceramics in the longitudinal mode (33 mode). Specimens with their long side formed at angles of 90 and 45° to the -oriented direction were fabricated from a 95%-textured SBN ceramic. Their high-power characteristics were compared with those of a randomly oriented specimen. In the case of the specimen with a 90° angle, the resonant frequency was almost constant during high-vibration-velocity driving. On the other hand, the resonant frequency of the specimen with a 45° angle decreased with increasing vibration velocity as did that of the randomly oriented SBN specimen. Therefore, it is considered that the excellent high-power characteristics of highly textured ceramics depend on the vibration direction for the -textured ceramics. The angle between the vibration direction and the -oriented direction is important in the excellent high-power characteristics of textured SBN ceramics.

High-Power Characteristics of Thickness Shear Mode for Textured SrBi2Nb2O9 Ceramics

The high-power piezoelectric characteristics of the thickness shear mode for oriented ceramics of bismuth layer structured ferroelectrics (BLSF), SrBi2Nb2O9 (SBN), were studied by the constant current driving method. These textured ceramics were fabricated by the templated grain growth (TGG) method, and the Lotgering factor was 95%. The vibration of the thickness shear mode in the textured SBN ceramics was stable at the vibration velocity of 2.0 m/s. The resonant frequency was almost constant with increasing vibration velocity in the textured SBN ceramics, however, it decreased with increasing vibration velocity in the randomly oriented SBN ceramics. In the case of Pb(Mn,Nb)O3–Pb(Zr,Ti)O3 ceramics, the vibration velocity of the thickness shear mode was saturated at more than 0.3 m/s, and the resonant frequency decreased at lower vibration velocity than in the case of SBN ceramics. The dissipation power density of the textured SBN ceramics was the lowest among those of the randomly oriented SBN and Pb(Mn,Nb)O3–PZT ceramics. The thickness shear mode of textured SBN ceramics is a good candidate for high-power piezoelectric applications.

Piezoelectric Ceramics for High Temperature Applications

Ceramic materials based on lead titanate, lead niobate and bismuth layer-structured ferroelectrics (BLSF) were studied to develop piezoelectric ceramics for high temperature sensor applications. Compositional modification enabled lead titanate and lead niobate type ceramics to exhibit good piezoelectric properties at 500°C . The Curie temperature for one BLSF, CaBi4Ti4O15 was close to 800°C, though the piezoelectric constant was smaller than those of lead titanate and lead niobate ceramics. These ceramics seem to be good candidates for use as high temperature sensor materials. In addition, textured SrBi2Nb2O9 (SBN), another BLSF, ceramics with various orientation factors were fabricated through the templated grain growth (TGG) method. The resonant frequency of 76% textured SBN varied linearly with temperature and exhibited stable temperature characteristics. The temperature coefficient of the resonant frequency was –0.85 ppm/°C from –50 to 250°C, and was smaller than that of a quartz oscillator. Therefore, textured SBN ceramics are suitable for use as a resonator material when stable resonant frequency is needed in a high temperature range.

Ferromagnetic resonance and spin pumping efficiency for inverse spin-Hall effect normalization in yttrium-iron-garnet-based systems

The inverse spin Hall effect has made its way from being a peculiar phenomenon predicted and discovered in semiconductors more than 30 years ago to being an extremely important effect in modern spintronics. It allows for the detection and conversion of spin current, along with the probing of the spin–orbit interaction of materials. However, the discrepancy between theoretical and experimental results has slowed down the progress of this branch of spintronics and its applications. In this study, we analyzed yttrium-iron-garnet-based systems highly exploited in spin pumping experiments and demonstrated the correct normalization procedure for the inverse spin Hall effect in such systems.

Temperature Dependence of Piezoelectric Properties for Textured SrBi 2 Nb 2 O 9 Ceramics

Temperature dependences of piezoelectric properties were studied from -50 to 250degC for the textured ceramics of bismuth layer-structured ferroelectrics, SrBi2Nb2O9. Excellent stability of resonance frequency was obtained over a wide temperature range for 76% textured ceramics. The temperature variation of resonance frequency was slight even in a high temperature region above 150degC, and was smaller than that of typical quartz crystal oscillator. Textured SBN ceramics are major candidate materials for resonator applications in a high temperature range.

Temperature dependence of piezoelectric properties for textured SBN ceramics

Temperature dependences of piezoelectric properties were studied for (001) textured ceramics of bismuth layer-structured ferroelectrics, SrBi2Nb2O9 (SBN). The textured ceramics with varied orientation degrees were fabricated by templated, grain-growth method, and the temperature dependences of resonance frequency were estimated. Excellent temperature stability of resonance frequency was obtained for the 76% textured ceramics. The resonance frequency of the 76% textured specimens varied almost linearly over a wide temperature range. Therefore, the variation was slight, even in a high temperature region above 150degC. Temperature stability of a quartz crystal oscillator is generally higher than that of a ceramic resonator around room temperature. The variation of resonance frequency for the 76% textured SrBi2Nb2O9 was larger than that of oscillation frequency for a typical quartz oscillator below 150degC also in this study. However, the variation of the textured SrBi2Nb2O9 was smaller than that of the quartz oscillator over a wide temperature range from -50 to 250degC. Therefore, textured SrBi2Nb2O9 ceramics is a major candidate material for the resonators used within a wide temperature range.

Significant reduction in spin pumping efficiency in a platinum/yttrium iron garnet bilayer at low temperature

The temperature evolution of a direct-current electromotive force (EMF) generated by spin pumping and the inverse-spin Hall effect in a platinum (Pt)/yttrium iron garnet (YIG) bilayer was investigated down to 80 K. The magnitude of the EMF decreased significantly with decreasing temperature and disappeared at approximately 80 K. 40-nm-thick YIG films fabricated by a metal organic decomposition method exhibited single-peak ferrimagnetic resonance (FMR) spectra without any spin wave resonance, which allowed us to precisely analyze the FMR spectra. We determined that the temperature evolution of the Gilbert damping constant is the dominant factor in the temperature dependence of the EMF. The comparison of the FMR linewidth between the X- and Q-bands revealed that an increase in Gilbert damping constant at low temperatures is not due to the enhancement of the spin pumping efficiency but due to an additional spin relaxation in the YIG film itself, which reduces the precession angle of the magnetization under the FMR conditions.

Textured PbTiO3 Based Ceramics Fabricated by Slip Casting in a High Magnetic Field

Textured PbTiO3 (PT) ceramics were fabricated by slip casting in a high magnetic field of 12 T. The sample obtained from the powder calcined at 1100 °C was (100), (010), and (001)-oriented, and the orientation factor was 90 %. On the other hand, the sample obtained from the powder calcined at 900 °C was not oriented. Aggregability and crystallinity of powder are an important factor when textured ceramics are fabricated by slip casting in a high magnetic field.