Y. Kakudate

National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan

Are you Y. Kakudate?

Claim your profile

Publications (82)158.44 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The flame propagation and the blast wave behaviors of methane/oxygen gas mixtures have been investigated. The mixtures were charged into spherical transparent rubber balloons and ignited at its center. The flame propagation behaviors were recorded by a high-speed video camera and the blast wave pressures were measured by several piezoelectric sensors. Although different scale experiments were conducted, the flame propagation behaviors for the same equivalence ratio are similar to each other, and the velocity increased continuously during the flame propagation. The pressure-time histories showed a continuous rise of pressure immediately after the blast wave was formed in air. However, as the blast wave traveled, a discontinuity appeared which resulted in the formation of a shock wave. Eventually, the blast wave lost its discontinuity at long distances. Trinitrotoluene (TNT) equivalents for the mixtures were evaluated by comparing the peak pressures and the impulses with those for TNT. It was found that TNT equivalents near the explosion source are smaller than those at long distances, while they can be considered as constant if evaluated using the impulse at a sufficiently long distance.
    No preview · Article · Jan 2012 · Science and Technology of Energetic Materials
  • M Wakatsuki · Y Kakudate · K Yamamoto · S Usuba · D Kim · S Fujiwara
    [Show abstract] [Hide abstract]
    ABSTRACT: Melting method for synthesizing BCxN compounds has been improved, where the starting material is a mixture containing C and BN and wholly melted without reacting the container wall. Product is composed of two phases of BCxN compounds. Their structures are cleared to be of the hBN-type and of the graphite-type, respectively. Variation of lattice parameters of the both phases and their formation ratio are investigated in relation to the composition of the starting mixture. Existence of ordered arrangement of B, C and N atoms is suggested, too. A structure model is given for the phase of the hBN-type.
    No preview · Article · Apr 2010 · Journal of Physics Conference Series
  • [Show abstract] [Hide abstract]
    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · Feb 2010 · ChemInform
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have studied the external-field-induced orientation of fibriform polydiacetylene nanocrystals in dispersions that we fabricated through reprecipitation followed by orientational fixation. The polymer employed was poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (polyDCHD). An AC electric field and a magnetic field were both effective agents for controlling the orientation of the nanocrystals in the dispersions; in contrast, we observed no orientational effect when applying a DC electric field. The application of the AC electric and magnetic fields led to different anisotropic orientations of the same dispersed nanocrystals. The degree of orientation depended on the AC frequency and the strength of the AC electric or magnetic field.
    No preview · Article · Oct 2007 · Japanese Journal of Applied Physics
  • H Yokoi · Y Araki · N Kuroda · S Usuba · Y Kakudate
    [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic field effect on the growth of cobalt silicate tubes in aqueous solution of sodium silicate is studied. Cobalt silicate tubes grown from CoCl2 powders in 10wt.% sodium silicate solution under a horizontal magnetic field of 2.2 T and a field gradient of 20.3 T/m are found to tilt toward the higher field direction, which is attributed to the paramagnetic property of the Co2+ion. In addition, twisted structures are recognized on the tube walls. The direction of the twist is right-handed in the case that the higher field direction is parallel to the field direction and left-handed in the anti-parallel case. Observing the growth of the tubes in situ under magnetic fields has revealed that two cusps of silicate proceed revolving around at the growing end to form a tube wall double-helically. We have proposed a model for the mechanism of the helix formation that Lorentz force exerted on anions (Cl−) flowing through and out of the tubes would cause vortex around the growing ends and the vortex could strand the two cusps, which explains the twist direction with respect to the field direction reasonably.
    No preview · Article · Dec 2006 · Journal of Physics Conference Series
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a rational and general method to fabricate a high-densely packed and aligned single-walled carbon-nanotube (SWNT) material by using the zipping effect of liquids to draw tubes together. This bulk carbon-nanotube material retains the intrinsic properties of individual SWNTs, such as high surface area, flexibility and electrical conductivity. By controlling the fabrication process, it is possible to fabricate a wide range of solids in numerous shapes and structures. This dense SWNT material is advantageous for numerous applications, and here we demonstrate its use as flexible heaters as well as supercapacitor electrodes for compact energy-storage devices.
    Full-text · Article · Nov 2006 · Nature Materials
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a new form of carbon nanotubes (CNT) material where CNTs are aligned and packed densely in a bulk solid. This single-walled carbon nanotube (SWNT) solid is fabricated from liquid-induced collapse of the sparse as-grown SWNT forest synthesized by super-growth CVD [1], which results in a 20x increase in mass density and a 70x increase in the Vickers hardness while retaining alignment and high surface area. As such, SWNT solid is an ideal form of CNTs for material and energy storage. The high surface area and well-defined microscopic structure imply that SWNT solid can be regarded as a conductive and flexible mesoporous material. In addition, we can engineer shapes suitable to the application. These unique characters make SWNT solid as a valuable material for supercapacitor electrodes and flexible heaters. Furthermore, we show how partial shrinking of the as-grown material creates a handle for robust mechanical and electrical connection to demonstrate exceptional tribological character and wear rate. The SWNT solid promises to open new frontiers in within the carbon nanotube field. [1] K. Hata et al, Science, 306, 1241 (2004).
    No preview · Article · Mar 2006
  • [Show abstract] [Hide abstract]
    ABSTRACT: Lubricating abilities of diamond nanoparticles with size between 50 nm and 200 nm were studied in vacuum and in air to clarify the effective use of diamond fine powders for tribological purposes. Spraying of powders with a high-velocity argon gas jet was performed to form deposits on stainless steel (SUS304) substrates. For sliding in vacuum against SiC and Al2O3 balls under a 0.5 N applied load and 3.5 mm/s sliding speed, the deposits of microcrystalline diamond powders with a mean particle size of 50 nm and detonation nanodiamond with a mean aggregate size of around 75 nm demonstrated friction coefficients of less than 0.01 and 0.03, respectively, and ball wear rates of less than 2 · 10− 6 mm3/(Nm). This means that diamond fine powders smaller than 100 nm can be considered as good solid lubricants in vacuum, because they demonstrate not only a low friction coefficient, but also wear rate of SiC ball lower than non-lubricated SUS304 does. A C60 deposit, formed by the same method on the SUS304, was readily scratched from the substrate in vacuum; however, under open-air conditions, a friction coefficient of around 0.1 and a SiC ball wear rate of about 2 · 10− 6 mm3/(Nm) were observed. This fact calls attention to the influence of the deposition method on C60 frictional properties.
    No preview · Article · Sep 2005 · Diamond and Related Materials
  • Source
    Hiroyuki Yokoi · Noritaka Kuroda · Yozo Kakudate
    [Show abstract] [Hide abstract]
    ABSTRACT: Helical structure on freestanding metal silicate membrane tubes induced by a magnetic field is reported. The phenomenon is observed for iron silicate tubes and cobalt silicate tubes grown under a horizontal field of 2.2 T and a field gradient of 20.5 T/m at the room temperature in contrast to nonregular structure observed at the zero field. Direction control of helicity is achieved by changing the field gradient direction with respect to the field direction. The results are explained in terms of the magnetohydrodynamics effect on the flow of anion discharged from a metal salt solution at the growing end of the silicate tube.
    Preview · Article · May 2005 · Journal of Applied Physics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Spin-polarized photoluminescence spectra in tilted fields were measured to 60T at 1.5K in a modulation-doped n-CdTe/(Cd,Mg,Mn)Te single quantum well with the thickness of 10nm and the sheet electron density of 5.7×1011cm−2. A new σ−-polarized emission peak emerged on the higher energy side of a main peak in the σ− polarization around the Landau filling factor ν of 1. Magnetic field dependence of the main peak energy exhibited a novel two-step like behavior around ν of 0.9 and that of the peak intensity was observed to take local maximum there. With increasing the field angle, these features of the main peak disappeared. Temperature dependence of the profiles of these emission peaks was also investigated to 40T between 1.3 and 20K. Relative intensity of the new emission peak to the main one was found to decrease with increasing temperature. Taking into consideration that a larger Zeeman gap should be unfavorable to a spin-singlet charged exciton (X−) and degradation of quantum limit by thermal excitation could be unfavorable to a spin-triplet X−, the new peak is assigned to the spin-triplet X− and the main peak to the spin-singlet X−.
    No preview · Article · Apr 2004 · Physica E Low-dimensional Systems and Nanostructures
  • J. Kitamura · S. Usuba · Y. Kakudate · H. Yokoi · K. Yamamoto · A. Tanaka
    [Show abstract] [Hide abstract]
    ABSTRACT: Alumina (Al2O3) coatings on stainless steel substrates were fabricated by electromagnetically accelerated plasma spraying (EMAPS), which generated a high-velocity pulsed plasma jet of more than 2.5 km/ s with a high pressure of 1 MPa in peak. High amount of α-Al 2O3 crystalline phase of the raw powder was preserved in the EMAPS coatings where the ratio of α-Al2O3 and γ-Al2O3 was approximately 65:35, which were generally different from the γ-Al2O3-rich coatings formed from α-Al2O3 raw powders using conventional spraying methods, such as plasma spraying, detonation gun spraying and high-velocity oxygen-fuel (HVOF) spraying. The specific wear rate of the coating formed from 6.6 μm raw powder at a spray distance of 100 mm was 2.3 × 10-6 mm3/Nm, which was comparable to that of sintered bulk α-Al2O3. Morphological features of the coatings and the crystal structures suggested that the plastic deformation by the high-velocity impact of highly viscous raw powders due to insufficient heating was dominant in the formation of dense coatings with highly structural ratios of α-Al2O3.
    No preview · Article · Jan 2004
  • [Show abstract] [Hide abstract]
    ABSTRACT: Porosity control of boron carbide (B4C) coatings by changing both raw powder sizes and spray distances was investigated using the electromagnetically accelerated plasma spraying (EMAPS) method. Within the coating conditions using two differently sized raw powders (20 μm and 5 μm in median diameters) and different spray distances from the gun muzzle to the substrate surface (ranging from 10 to 50 mm), it was found that smaller powders and longer spray distances were more effective in forming a dense coating. The porosity in the coatings ranged from 7% under the conditions of 20 μm powder size and 10 mm spray distance to 2% under the conditions of 5 μm powder size and 50 mm spray distance. To understand the effects of these factors, the B4C particle velocity was measured using the rotating drum method. The results showed that a particle velocity of approximately 2.5 km/s, almost the same as that of the traveling plasma in the EMAPS method, was observed for the small-sized powder. The velocity of large-sized powder was found to be approximately 1.1 km/s. Such a large discrepancy in the powder velocities was considered to contribute to the difference in the densities of the B4C coatings from differently sized powders.
    No preview · Article · Jan 2004 · New diamond and frontier carbon technology: an international journal on new diamond, frontier carbon and related materials
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A deposit of carbon nanoparticles based on an onion-like structure was fabricated from detonation nanodiamond powders by a novel plasma spraying process, electromagnetically accelerated plasma spraying (EMAPS). EMAPS was able to transform nanodiamonds to onion-like structured carbon within 300 μs through a thermal graphitization process in which the temperature of the particles would be in the range of 2700–4500 K. Synthesized onion-like carbon nanoparticles were spherical or polyhedral. The G-band in the UV–Raman spectra of the produced deposits was found to be a superposition of a characteristic band of well-formed carbon onions at 1571 cm−1 and the G-band of defective carbon onions at 1592 cm−1. The availability of a plasma spraying process for developing solid lubricant coatings incorporating nanodiamond and onion-like carbon was demonstrated.
    Full-text · Article · Dec 2003 · Carbon
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Structural and mechanical properties are investigated on thick boron carbide (B4C) coatings, which are formed onto stainless steel substrates by using an electromagnetically accelerated plasma spraying. Hardness, porosity and surface roughness of the coatings show dependences on both the size of raw powder and the substrates distance. Within the coating conditions using two different sized raw powders; 30±10 μm and less than 10 μm in diameters, and different substrates distances ranging from 10 to 50 mm from the gun muzzle, smaller particle and longer substrates distance are found to be more effective in forming a coating with uniform hardness, low porosity and smooth surface. Results of Raman spectroscopy and X-ray diffraction suggest that the crystallite size of original raw B4C powder is significantly reduced during the process of coating formation.
    Full-text · Article · Oct 2003 · Diamond and Related Materials
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have synthesized B–C–N films by an electron beam excited plasma-CVD method and characterized structures and properties of the obtained films. In this paper, tribological properties of the films under dry-lubrication were investigated with a ball-on-disc type friction tester. The hardness of the films increases monotonically with increasing boron content and both the friction coefficient and the wear rate increase with increasing the hardness. From these results the low friction coefficient can be attributed to the formation of a sp2-bonded B–C–N structure. And it is suggested that the cause of the wear for the B–C–N films may be a microscopic peeling, because the film should become more brittle and the internal stress of the film should become larger for the harder film.
    No preview · Article · Jun 2003 · Surface and Coatings Technology
  • [Show abstract] [Hide abstract]
    ABSTRACT: A sputtering method enhanced by an electron-beam-excited plasma having a high dissociation efficiency of nitrogen was performed for the first time. Hydrogen-free carbon nitride films were deposited by sputtering a carbon target with N2 and Ar ions and evaluated for bonding states, composition and hardness. Results from Fourier transform infrared and X-ray photoelectron spectroscopy (XPS) suggested that the deposited carbon nitride films contained CN, CN and CN bonds. From XPS analysis, the N/C atomic ratios were found to be in the range of 0.38–0.46, and the films with higher N/C atomic ratios revealed lower dynamic hardnesses.
    No preview · Article · Jun 2003 · Surface and Coatings Technology
  • Source
    Takeshi Hasegawa · Kazuhiro Yamamoto · Yozo Kakudate
    [Show abstract] [Hide abstract]
    ABSTRACT: B–C–N films were deposited with electron beam excited plasma–chemical vapor deposition focusing on the structural stability where their compositions lie in the vicinity of BN. Film composition was controlled by varying process gas flow rates and was xs1.08–1.18 and ys0.01–0.34 for B C N. The dependence of Fourier transform infrared transmission spectra and hardness on x y composition show that the structure is very sensitive to film carbon content: a cubic phase is found only in a carbon-poor region and disappears abruptly when carbon content is increased. Results of X-ray diffraction and X-ray photoelectron spectroscopy measurement suggest that films involving a cubic phase of carbon-poor B–C–N compound were obtained. The disadvantage in forming a cubic B–C–N phase in a low carbon region is understood in terms of bonding energy and local charge neutrality, while the entropy of mixing should enable very low carbon solubility. 2003 Elsevier Science B.V. All rights reserved.
    Preview · Article · Mar 2003 · Diamond and Related Materials
  • [Show abstract] [Hide abstract]
    ABSTRACT: A new system of electromagnetically accelerated plasma spraying (EMAPS) consisting of a pulsed high-current arc-plasma gun and a large flow rate pulsed powder injector has been developed to synthesize a hard and dense coating of boron carbide (B4C) with a high adhesion. The plasma gun with a co-axial cylindrical electrode configuration generates electromagnetically accelerated arc plasma with a typical velocity and maximum pressure of 1.5–3.0 km/s and 1 MPa, respectively, by discharging a pulsed high current of about 100 kA in peak and about 300 µsec of duration. The heating and accelerating of source powder are accomplished by injecting it into the inter-electrode space of the gun prior to the plasma generation using a newly developed pulsed powder injector that enables a gram of powder to be injected within 1 ms with precisely controlled time delay. With this system, hard B4C coatings with a high adhesion and crystallinity were successfully formed on mirror-polished stainless (SUS304) substrates without a binder.
    No preview · Article · Feb 2003 · Journal of Thermal Spray Technology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Photoluminescence measurements have been conducted for a CdTe/Cd1-xMnx Te (x = 0.4) single-quantum-well structure at low temperatures under pressures to 0.49 GPa and magnetic fields to 60 T. At the ambient pressure, a new emission was induced by the application of a magnetic field. The emission has been assigned to exciton emission from the barrier layer, which is suppressed below 9 T due to the energy transfer from the exciton to local d electrons. At 0.49 GPa, the emission recovered at 44 T. In the field region where the energy transfer occurs, an anomalous red-shift of the exciton energy was observed clearly for the case of the ambient pressure. The alloy potential fluctuation effect and the magnetopolaron effect are examined as candidates for the mechanism to cause this phenomenon.
    No preview · Article · Oct 2002 · Journal of Physics Condensed Matter
  • Shu Usuba · Hiroyuki Yokoi · Yozo Kakudate
    [Show abstract] [Hide abstract]
    ABSTRACT: The dispersion process of typical carbon cluster synthesized by gas evaporation using (dc) arc was numerically investigated under normal gravity (1 G) and nongravity (0 G) conditions to clarify the effect of natural convection. Calculated pressure dependence of residence time of C <sub>60</sub> in the temperature state between 1000 and 2000 K under 1 G took a maximum value of about 260 ms at the pressure around 0.06 atm in helium, while under 0 G, it increased almost linearly with pressure. Such features in the pressure dependence of residence time of C <sub>60</sub> under both 1 and 0 G were related to its yield by a simple model based on an annealing of imperfect C <sub>60</sub> to perfect C <sub>60</sub> structure. According to this model, experimentally observed secondary increase of C <sub>60</sub> yield was consistently explained in terms of the effect of natural convection. © 2002 American Institute of Physics.
    No preview · Article · Jul 2002 · Journal of Applied Physics