Kenta Takagi

Electrical and mechanical properties of piezoelectric ceramic/metal composites in the Pb(Zr, Ti)O3/Pt system

As a model for composite materials of piezoelectric ceramic and metal, lead zirconate titanate (PZT) and platinum (Pt) particulate composites were fabricated by power processing. The electrical and mechanical properties of the PZT–Pt composites were measured as a function of the Pt volume fraction. The relative dielectric constants of the PZT–Pt composites increased markedly, while the piezoelectric constants and electromechanical coupling coefficients decreased with increasing Pt content. When the Pt volume fraction exceeded 30%, the PZT–Pt composite became electrically conductive because of percolation of the Pt particles. The Pt-dispersed PZT composites enhanced the mechanical properties, particularly the high fracture resistance, compared to the monolithic PZT ceramics.

Fabrication of a three-dimensional terahertz photonic crystal using monosized spherical particles

Three-dimensional artificial crystals with periodicity corresponding to terahertz wave lengths were fabricated by self-assembling monosized metal spherical particles. The metal crystals were weakly sintered to utilize them as templates. The metal templates were inverted to air spheres crystal embedded in dielectric resin though infiltration and etching. The resulting resin inverted crystals clearly presented the photonic stop gaps within terahertz wave region and the frequencies of the gaps were confirmed to agree well with calculation by plane wave expansion method.

Fabrication of three-dimensional terahertz photonic crystals with diamond structure by particle manipulation assembly

We reported the fabrication of terahertz photonic crystals by three-dimensional (3D) particle manipulation assembly. Our method, which is based on pick-and-place manipulation and interparticle laser welding, enabled accurate assembling of an arbitrary 3D structure, regardless of particle polydispersity. By using this method, we fabricated a diamond crystal from ZrO2/polyethylene composite particles (diameter of 400 μm). The obtained crystal exhibited a photonic stop gap in the ⟨111⟩ direction; this result was in good agreement with the theoretical result, suggesting that the crystal has a full photonic bandgap at around 0.2 THz.

High-pressure sintering behavior of α″-Fe16N2 nanopowder

α″-Fe16N2 nanopowder was sintered at high-pressure and low temperature in order to explore its feasibility as a bulk magnet. TEM observation confirmed that the nanopowder was densely consolidated by sintering at a pressure of 1.2 GPa and temperatures around 200 °C. Increasing the sintering temperature enhanced the densification, leading to a suppression of oxidization. However, XRD analysis revealed that sintering at 222 °C caused the decomposition α″ → α-Fe + e-Fe3N, which reduced the saturated magnetization (Ms) and coercivity (Hc). Consequently, sintering at 190 °C produced a dense magnet that retained the Ms of the raw powder due to the avoidances of oxidization and decomposition. On the other hand, it was found that Hc degrades after sintering, which is believed to be due to magnetic interaction between the particles.

A microsphere assembly method with laser microwelding for fabrication of three-dimensional periodic structures

The orderly build-up of monosized microspheres with sizes of hundreds of micrometres enabled us to develop three-dimensional (3D) photonic crystal devices for terahertz electromagnetic waves. We designed and manufactured an original 3D particle assembly system capable of fabricating arbitrary periodic structures from these spherical particles. This method employs a pick-and-place assembling approach with robotic manipulation and interparticle laser microwelding in order to incorporate a contrivance for highly accurate arraying: an operation that compensates the size deviation of raw monosized particles. Pre-examination of particles of various materials revealed that interparticle laser welding must be achieved with local melting by suppressing heat diffusion from the welding area. By optimizing the assembly conditions, we succeeded in fabricating an accurate periodic structure with a diamond lattice from 400 µm polyethylene composite particles. This structure demonstrated a photonic bandgap in the terahertz frequency range.

Design, Processing and Evaluation of Graded Piezoelectric Ceramic Bending Actuators

This paper introduces the development of two kinds of graded piezoceramic bending actuators. For designing the graded compositional profiles, classical lamination theory (CLT) is used to explore the optimal gradient that generates large electromechanical response. As the first model material, a laminated piezoceramic bimorph actuator was designed and fabricated with a graded compositional distribution of PZT and Pt, and its electric-induced bending characteristics were evaluated and compared with the analytical results by CLT. Furthermore, porosity-graded PZT ceramics were developed, which may be used as the preforms for the infiltration process to fabricate bending actuators with graded ceramic/metal and/or ceramic/polymer interfaces. The fabrication process and property evaluation of homogenously porous and porosity-graded PZT ceramics were introduced.

Fabrication and modeling of porous FGM piezoelectric actuators

The elastic behavior of porous piezo-laminates actuators is developed using modified classical lamination theory (CLT). The curvature is obtained for porous piezoelectric laminate with functionally graded microstructure (FGM) under applied voltage throughout its thickness. The porous FGM system consists of multi porous piezoelectric layers where the porosity gradient increases in the thickness direction. The porous FGM actuator is fabricated by co-sintering powder compacts of PZT and stearic acid in air. The electroelastic properties of each layer in the FGM systems were measured and used as input data in the analytical model to predict the FGM actuator curvature. Two optimization techniques are employed to enhance the performance of the porous FGM actuators: (1) Thickness of each layer in the porous FGM actuator, (2) Number of layer in the porous FGM actuator. The thickness of each layer in the FGM system is made to vary in a linear or non-linear manner by changing the FGM thickness exponent. Two, three, and five layer porous FGM systems are investigated to obtain the maximum curvature. The analytical predictions are found to agree well with the experimental measurements.

Anisotropic Sm2Fe17N3 sintered magnets without coercivity deterioration

In order to solve the problem of coercivity decrease during sintering, we developed a low-oxygen process capable of producing sintered compacts while avoiding oxidization. This study proved that Sm2Fe17N3 sintered compacts produced by the low-oxygen process maintained the coercivity of the raw powder. Scanning transmission electron microscopy observation found no obvious formation of an oxide layer at the sintered interfaces. Magnetic measurements revealed that the coercivity of the sintered magnets decreased as the oxygen concentration increased. In summary, this study demonstrated that Sm2Fe17N3 sintered magnets having the same coercivity as the raw powder can be produced by avoiding oxidization.

Advanced Nanostructure-Controlled Functionally Graded Materials Employing Carbon Nanotubes

Capability of multiwalled carbon nanotubes (CNTs) to create in-depth gradients in properties and functionalities of conventional materials has been investigated for the first time. Functionally graded material (FGM) concept has also been employed for the first time to bridge conventional materials to their advanced nanocomposites containing a high concentration of CNTs, which is promising for unexplored yet novel structural, electronic and biomaterial applications. In this study, α-alumina ceramics considered as the most challenging case has been used as the matrix. Bulk, layered, nanostructure-controlled, CNT-based, functionally graded α-alumina ceramics have been fabricated employing a recently established powder processing technology. In-depth gradients in microstructure, grain size and hardness have been successfully achieved in alumina ceramic without cracking, delamination or warping, after homogeneous and gradual incorporation of the CNTs within the alumina ceramic matrix. The FGM approach showed promise to successfully bridge conventional ceramics to their nanocomposites containing a high concentration of CNTs.

Effect of Polydispersity on Photonic Band Gap of Terahertz Photonic Crystals Fabricated by Particle Assembly

To fabricate terahertz photonic crystals via a particle assembly technique, sufficient understanding of the effect of particle size dispersity on the photonic band gap may aid the development of new processes and structures. In this study, a combination of experimental and numerical approaches was used to investigate the substantial effect of polydispersity. Polymer-inverted fcc crystals with structural errors were prepared from self-assembled and intentionally polydispersed metal particles. As the polydispersity increased, the band gap reduced and then steeply decreased above a particle size distribution of 4%. Comparison of this experimental spectrum with an analytical spectrum showed that the gap reduction originated due to the displacement of particles from ideal lattice points produced by the polydispersity but not due to the polydispersity itself. This result will aid the development of a new assembly technique for three-dimensional terahertz photonic crystal devices.