Yoshio Akimune

Piezoelectric Properties of Spark-Plasma-Sintered (Na0.5K0.5)NbO3–PbTiO3 Ceramics

In the search for high-performance piezoelectrics, (1-y)(Na0.5K0.5)NbO3–yPbTiO3 (y≤0.50) solid solution has been prepared using the spark-plasma-sintering method. Dielectric investigation reveals that both the ferroelectric phase transition temperatures and the peak value of the dielectric constant are strongly suppressed in the low y range. However, improved electromechanical coupling constants are observed at the same y range. These properties are explained by considering factors such as composition, crystal structure, ferroelectric domain microstructure, and poling conditions.

Dielectric and ferroelectric properties of tetragonal tungsten bronze Sr2−xCaxNaNb5O15 (x=0.05–0.35) ceramics

This letter reports the dielectric and ferroelectric properties of tungsten bronze Sr2−xCaxNaNb5O15 (SCNN, x=0.05–0.35) ceramics. Two dielectric anomalies and a diffuse ferroelectric transition behavior were appreciably observed in the compositions of x=0.05–0.25. The incorporation of smaller calcium cations into the crystal structure resulted in an increase in the Curie temperature, from 279 (x=0.05) to 297 (x=0.35), and a decrease in the permittivity, from 1353 to 543, at their respective Curie temperatures. Ferroelectricity was observed in the compositions with x=0.05–0.25, but absent in the compositions with x=0.30 and 0.35 at room temperature. The maximum spontaneous polarization Ps of 9.1 μC/cm2 and remanent polarization Pr of 3.0 μC/cm2 were achieved in the composition of x=0.15.

Optical properties of β-Si3N4 single crystals grown from a Si melt in N2

Large colored β-Si3N4 single crystals were successfully grown from a Si melt in N2. The transmission optical absorption of coloring β-Si3N4 single crystal shows that impurities introduce a midgap level of ∼2.4 eV into the wide band gap of ∼5.3 eV in nondoped Si3N4. The infrared transmission spectrum and electron probe x-ray microanalysis of β-Si3N4 samples show that the solution of the Al element affects the silicon–nitrogen molecular vibration and the states within the band gap of β-Si3N4. The obtained results mean that the Al impurity acts as the radiative center and is the origin of the color in the β-Si3N4 single crystal.

Lead-free piezoelectric ceramics in the (1 - x)Sr2NaNb5O15-xCa2NaNb5O15 (0.05 ≤ x ≤ 0.35) system

The study investigating the electrical properties of dense lead-free piezoelectric ceramics in the (1 − x)Sr2NaNb5O15–xCa2NaNb5O15 (SCNN) system with x ranging from 0.05 to 0.35 is reported here. X-Ray diffraction patterns all showed a single tetragonal tungsten bronze phase, peak broadening being seen on increasing the Ca content. The lattice constant calculation indicated distortion and shrinkage of the crystal structure with Ca substitution. Dielectric spectra of each composition displayed two dielectric anomalies and diffusive phase transitions. The Curie temperature, Tc, shifted upward whereas the relative permittivity at Tc declined monotonically as the Ca content increased. Ferroelectric and piezoelectric properties of SCNN greatly depended on the Ca content. The composition with x = 0.15 exhibited the greatest polarization with Pr = 3.0 μC cm−2 and piezoelectric constant of d33 = 96 pC N−1, whereas the compositions with x = 0.30 and 0.35 almost lost their ferroelectricity and piezoelectricity. This study apparently indicates that lead-free SCNN piezoelectric ceramics have potential for electromechanical applications.

Dielectric and Piezoelectric Properties of Barium-substituted Sr1.9Ca0.1NaNb5O15 Ceramics

Highly dense piezoelectric ceramics of tungsten bronze-type (Sr1.9Ca0.1)1-0.5xBaxNaNb5O15 (where x=0.1–0.8) were prepared by spark plasma sintering. The crystallographic parameters, dielectric behaviors and piezoelectric properties of the sintered ceramics were investigated, and the effects of the Ba substitution on these electrical properties were discussed. The structural analysis and the electrical property measurements indicate a morphotropic phase boundary (MPB)-like phenomenon at x=0.4–0.5. In all compositions, a diffuse phase transition and a relaxor behavior are observed. The electrical properties are found to be crystallographically dependent.

EFFECT OF SINTERING ADDITIVE COMPOSITION ON THE THERMAL CONDUCTIVITY OF SILICON NITRIDE

The thermal conductivity of silicon nitride prepared with varying sintering additive compositions was studied. Samples of Si 3 N 4 + 0.5 mol% Y 2 O 3 + 0.5 mol% Nd 2 O 3 and a further additional agent were gas pressure sintered at 2173 K. MgO or Al 2 O 3 was employed as the additional agent. While both agents improved sinterability, the former promoted grain growth and the latter suppressed it. Thermal conductivity increased with increasing MgO content, and a maximum value of 128 Wm -1 K -1 was attained when 2 mol% MgO was added. In contrast, addition of Al 2 O 3 degrades thermal conductivity. This is probably due to the suppression of grain growth and the dissolution of Al 2 O 3 into Si 3 N 4 grains.

Damage behaviour of silicon nitride for automotive gas turbine use when impacted by several types of spherical particles

Spherical particle impact damage and strength degradation phenomena of silicon nitride by several types of spheres were analysed in comparison with chipping fracture behaviour reported in the literature. It was found that steel and partially stabilized zirconia (PSZ) particles caused Hertzian cone cracks, resulting from the elastic response of the material in accordance with the Hertzian cone crack theory. In contrast, alumina and sialon particles induced both median-radial crack systems at low impact velocity range and Hertzian cone cracks at high impact velocity range. The critical impact velocity for strength degradation,Vc, correlated with the hardness of the spheres and target ceramics, andVc, for Hertzian cracks and median cracks were higher than that for chipping fracture.

Mechanical properties of SiC-particle/sialon composites

On a utilise des particules de carbure de silicium pour renforcer des sialons. On examine la microstructure et les proprietes mecaniques des composites obtenus

Apodized fibre Bragg grating acousto-ultrasonic sensor under arbitrary strain using dual Fabry-Perot filters

This paper describes a novel acousto-ultrasonic (AU) sensing system in which a fibre Bragg grating (FBG) is permanently built in a structure for in situ health monitoring. The AU wave detection using an FBG can be realized by a demodulation technique using an optical filter or light source narrower than the FBG spectrum. However, since the spectral wavelength shift induced from structural strain is much larger than the spectral oscillation induced from the AU wave, it is difficult for the demodulation wavelength to follow the spectral shift. For this work, we introduce a passive demodulation method regardless of the spectral shift based on an apodized FBG (AFBG) and dual fibre Fabry–Perot (FFP) filters. The free spectral range (FSR) of a high-finesse FFP filter is matched to the full-bandwidth of the AFBG without side lobes. Therefore, a part of the AFBG spectrum is always filtered by one of the multiple interferometric peaks. However, the wavelength regime near the maximum or minimum reflectivity of the AFBG provides insufficient sensitivity with respect to the wave. Therefore, another 1/4 FSR-biased FFP filter with the same FSR is simultaneously operated.

Dielectric Properties of Spark-Plasma-Sintered (Na 0.5 K 0.5 )NbO 3 -PbTiO 3 Ceramics

Dielectric properties of spark-plasma-sintered solid solution (1 m x )(Na 0.5 K 0.5 )NbO 3 m x PbTiO 3 were studied. Both the cubic-tetragonal and the tetragonal-orthorhombic phase transitions become more and more diffuse with increasing x . The strong deviation of the cubic-tetragonal phase transition temperature versus composition curve from the Vegards law is observed. It is considered that (1) the introduction of heterovalence ions to both the perovskite A and B sites, and (2) the change of the mass difference between the perovskite B- and A-site, m B - m A, are two main factors responsible for the observations.