Papers by Author: Tadachika Nakayama

Abstract: A mechanical model is proposed to estimate internal stress during sintering of ceramic multiphase laminates. A symmetrical multi-layered laminate is assumed, and one-dimensional elastic analysis is carried out on the change in stress of each layer during sintering, based on the differences in sintering strain, thermal expansion strain and phase transformation strain between the layers. By taking a limit such that the thickness of each layer approaches infinitesimally small, the internal stress expression can be extended into the case of the materials with continuous compositional change (viz. functionally gradient materials).

Abstract: Copper nanoparticles have been prepared by pulsed wire discharge (PWD) using copper wire in deionized water at various relative energy (K) from 0.8 to 5.5, which is ratio of the charged energy of capacitor in the electrical circuit to the vaporization energy of the wire. From the X-ray diffraction analysis of the prepared nanoparticles in deionized water at various K, the nanoparticles prepared at all conditions were identified as Cu and Cu₂O. The Cu content in prepared nanoparticles was increased with the decrease in K, and changed from 77 to 95 [%]. Additionally, the deposited energy in the arc discharge after the wire heating was decreased with decreasing K. From these results, we considered that the Cu content was increased with decreasing the deposited energy in the arc discharge during particle formation.

Abstract: Cr-Me-N-O (Me; Ni, Cu and Mg) thin films have been designed and successfully prepared by the pulsed laser deposition (PLD) method. It was found that Me, which form the monoxide MeO, are effective for hardening the Cr(N,O) thin films.

Abstract: 3Y-TZP/TiNiCo composites have been successfully fabricated by three step heating (dehydrogenation, hydrogen reduction, hot-pressing) of 3Y-TZP /NiO/TiH2/CoO powder mixtures. XRD analysis revealed that TiNi-base intermetallic compounds such as Ni4Ti3, NiTi, Ni3Ti had formed. The bending strength of 3Y-TZP/TiNiCo composites (~650 MPa) were much higher than those of 3Y-TZP monolith (
350 MPa) sintered at the same condition. The electrical resistivity characteristics indicated that 3Y-TZP/30 vol%TiNiCo composites were good electrical conductors. Cobalt addition to TiNi phase influenced on electrical properties of final composites, while their fracture strength was unchanged by the Co addition.

Abstract: Multi Wall Carbon Nanotubes (MWCNTs) with a diameter of 20-30 nm were used as a conductive phase to add electric conductivity to yttria stabilized tetragonal zirconia (3Y-TZP). Almost fully dense 3Y-TZP/MWCNTs nanocomposite was obtained by pressureless sintering under inert atmosphere and Hot Isostatic Pressing (HIP) treatment. The conductivity of the nanocomposites increased with increasing content of MWCNTs. Moreover, the fracture toughness increment of the composite was confirmed at 0.5 wt% addition. Scanning electron microscopy and transmission electron microscopy observation of the microstructures showed that MWCNTs were fairly homogeneously dispersed in the 3Y-TZP matrix.

Abstract: Si3N4 ceramics with V2O5 based glasses as sintering additives were successfully fabricated by a powder mixing process and rapid sintering by the PECS method. The fabricated materials by Pulsed Electric Current Sintering (PECS) exhibited very fine microstructure with α and β grains. Electric conductivity for the Si3N4/(V2O5-B2O3) and the Si3N4/(V2O5-B2O3-Al2O3) at room temperature were four and six orders of magnitude higher at room temperature, and two and three orders of magnitude higher at 1000 oC than the conventional Si3N4, respectively

Abstract: Mullite-based iron nanocomposites were prepared by the reduction of a mullite-iron oxide solid solution and successive hot pressing. The solid solution was obtained from the heat treatment of diphasic gel by sol-gel method. Some of the α-iron nanoparticles have an intra-granular structure just after reduction. Mechanical properties are strongly affected by the content of iron. Low iron content is beneficial to strengthening while high iron content can improve the fracture toughness. Furthermore, the nanocomposites also behave ferromagnetic properties at room temperature.

Abstract: A novel transparent polycrystalline silicon nitride was fabricated by hot-press sintering with MgO and AlN as additives. The mixed powder with 3 wt.% MgO and 9 wt.% AlN was sintered at 1900oC for 1 hour under 30 MPa pressure in a nitrogen gas atmosphere. Transparent polycrystalline silicon nitride was successfully fabricated. The mechanical properties such as density, hardness, young’s modulus, fracture strength and fracture toughness were evaluated. The effect of α/β phase on the mechanical properties of transparent polycrystalline silicon nitride was investigated. The properties were changed depending on the amount of α/β phase. The hardness and Young's modulus increased with increasing the volume fraction of α-phase fraction as a reflection of the higher hardness of α-phase Si3N4. The fracture toughness and fracture strength decreased with decreasing the volume fraction of β-phase Si3N4.

Abstract: We have investigated a synthesis of metal (Nb, V, Cr, Mn, Co) -doped titania nanotubes using a solution chemical processing in order to control optical and electrical properties. Titania nanotubes doped with a small amount of cations up to 1 wt% exhibited similar morphology and XRD pattern as the pure titania nanotubes, however, color of nanotubes was changed depending on the dopants. It was found that Cr, Mn and Co doped titania nanotubes formed new absorption bands in UV spectra. On the other hand, electrical resistivity of doped titania nanotubes was lower than that of pure titania nanotubes.

Abstract: Copper nanosized powders were prepared by pulsed wire discharge (PWD) in N2 gas at low pressure (130 Pa) with high-speed gas puff. The pressure around one of the electrodes was changed to investigate the effect of electrical discharge prevention by gas puff. The peak pressure was changed from 22 kPa to 10 kPa in the gas puff apparatus by changing the distance from an acrylic resin tube and a bottom plate. The energy deposition in the copper wire at 22, 15 and 10 kPa was about 40, 37 and 33 J, respectively. The energy deposition in the wire increased with the increase in pressure around the electrode.