S. Nagata

Tohoku University, Japan

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Publications (246)304.08 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: In situ measurements of the volume electrical conductivities of chemical-vapor-deposited silicon carbide (CVD-SiC) samples were carried out under irradiation by 2.5- and 14-MeV fast-neutron beams in air at room temperature. A slight radiation-induced conductivity (RIC) was detected under fast-neutron irradiation. A radiation-induced electrical degradation (RIED)-like behavior was observed in the form of degradation of the base conductivity in the absence of irradiation with increase in the fast-neutron fluence. The SEM micrograph images and XPS analysis of the surface of the fast-neutron-irradiated CVD-SiC samples revealed that the dissociation of carbon from existing SiOxC3-x compositions did not only occur via displacement damage, but also via ionizing effects (radiolysis), thereby leading to the observed RIED-like behavior as well as RIC in the electrical properties of the CVD-SiC samples.
    Journal of Nuclear Materials 11/2014; 455(1-3). · 2.02 Impact Factor
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    ABSTRACT: Retention and release of ion implanted deuterium (D) and helium (He) in silicon carbide (SiC) were studied with respect to damage accumulation and annealing behavior using ion beam analysis techniques. α-SiC single crystals were irradiated by 10 keV D2+ and He+ at 300 and 770 K, and depth profiles of retained atoms and lattice disorder were measured during the implantation and successive heat treatments. For the sample pre-irradiated with He at a fluence of 1.0 × 1021 ions/m2, the thermal release of D atoms was completed at an annealing temperature of 1370 K, while the retained D atoms still remained in the sample without pre-irradiation. In contrast to the facilitated thermal release of D by He pre-irradiation, the He release temperature increased in the sample followed by D ion implantation. No significant difference of He retention and damage accumulation behavior was observed between the samples implanted at 300 and 770 K. The D retention and D-ion-induced disorder for 770 K implantation, was reduced to approximately 1/2 comparing to that of the sample implanted at 300 K. The D retention at 770 K was not affected by the He pre-implantation induced damage.
    Journal of Nuclear Materials 11/2014; 455(1-3). · 2.02 Impact Factor
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    ABSTRACT: Depth profiles of Li in a Li-ion battery system of Au/LiCoO2/LATP/Pt (LATP:Li1+X+YAlYTi2−YGeXP3−XO12) have been in situ measured at several stages of charging by means of the elastic recoil detection (ERD) and Rutherford backscattering (RBS) techniques with 9 MeV O4+ ion beam. The thicknesses of LiCoO2 and Au are estimated from the RBS spectrum for the as-received specimen to be 190 and 27 nm, respectively. From the ERD spectrum, it is also found that there exists impurity hydrogen of ~5% in LiCoO2. Moreover, it is found from the ERD data analysis after the first charging of 0.48 × 1016 e/cm2 to Au electrode that the fractional concentration of Li in LiCoO2 at the interface with Au electrode steeply decreases to 0.8, recovers with depth toward the interface with LATP and attains at 1.3 above the original value. With increasing the amount of charge supplied, the slope in the Li composition from Au electrode to LATP increases gradually. The data on the charging is concluded to include the irradiation effects due to the incident O ions, since the data on a non-irradiated part of the specimen obtained after finish of the charging shows that Li is almost completely reduced from LiCoO2. The dynamics of Li composition in LiCoO2 as a function of charging supplied is discussed in connection with hydrogen impurity and irradiation effect of the incident O ions. Copyright © 2014 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 08/2014; · 1.39 Impact Factor
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    ABSTRACT: 1-mol%-Yb-doped Gd3Al2Ga3O12 infra-red scintillator crystal has been studied as a novel implantable radiation monitor in radiation therapy. Powder X-ray diffraction measurement and chemical analysis with a field emission scanning microscope and wavelength dispersive spectrometer determined its garnet structure and average chemical composition of Yb0.03±0.01Gd2.99±0.07Al2.21±0.08Ga2.64±0.09O12.10±0.09. Transmittance measurements reached high values of approximately 70% in the human body transparency region between 650 to 1200 nm. Photoluminescence peaks were detected around 970 and 1030 nm under the 940 nm excitation with a Xe lamp. Infra-red scintillation emissions were clearly observed around 970 and 1030 nm due to Yb3+ 4f–4f transitions under X-ray excitation. Therefore, these results suggest that Yb-doped Gd3Al2Ga3O12 might be used as an infra-red scintillator material.
    Optical Materials 07/2014; · 2.08 Impact Factor
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    ABSTRACT: The present study reports, for the first time, the highly efficient formation of barrier-type anodic films, with flat and parallel metal/film and film/electrolyte interfaces, on magnesium in ethylene glycol electrolytes containing ammonium fluoride and water. The anodizing voltage increases linearly with time during galvanostatic anodizing at 10 A m− 2 up to 350 V. The anodic film formed to 200 V is 247 nm thick, containing a crystalline MgF2 phase. Analysis by Rutherford backscattering spectroscopy discloses the film composition of MgF1.8O0.1 and Pilling–Bedworth ratio (PBR) of 1.67. The PBR value greater than unity and the formation of chemically stable fluoride-based films may contribute to the film growth at high current efficiency.
    Electrochemistry Communications 01/2014; 46:30–32. · 4.29 Impact Factor
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    ABSTRACT: The degradation and recovery of the optical emission characteristics of polyethylene naphthalate (PEN) films were studied during irradiation with MeV protons and UV photons. The photo-stimulated luminescence (PL) consisting of two major peaks decreased with the incident energy fluence, particularly for ion irradiation. At the beginning of irradiation, the rate of reduction of the PL intensity in the UV-irradiated film was comparable to that for ion irradiation, but the residual PL intensity in the UV-irradiated film was considerably larger at higher fluences. In addition, no change in the PL characteristics of the UV-irradiated film was observed after stopping the UV irradiation, indicating that the damage caused by the UV photons was permanent. However, the PL intensity from the ion irradiated film increased immediately when the film was exposed to air. The recovery of the luminescence centers in the ion-irradiated PEN film is attributed to ion-induced surface modification, which plays a role in the enhancement of the dissociation of water molecules and the diffusion process for constituent atoms.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 11/2013; · 1.19 Impact Factor
  • K. Hoshi, S. Nagata, M. Zhao, T. Shikama
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    ABSTRACT: In order to examine the basic characteristics of light emission and damage accumulation in lithium tantalate (LiTaO3), ion-induced luminescence was measured. A broad peak centered at about 540 nm was observed under ion bombardment. The ion-induced luminescence was essentially the same as the luminescence under UV photon irradiation. The initial intensity of the luminescence was proportional to the projected range of the incident ions and independent of the electronic energy loss. The shape of the spectra was independent of the species, energy, and fluence of incident ions. The luminescence monotonically decreased with an increase in ion fluence. Assuming first-order kinetics in the annihilation and recovery process of the luminescent centers, the annihilation rate was proportional to the nuclear energy stopping power. It was considered that the luminescent center was mainly damaged by nuclear collisions, and the recovery of the damaged luminescent center was caused by local heating by the ion beam.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 11/2013; · 1.19 Impact Factor
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    ABSTRACT: Be-seeded, high-flux, deuterium/helium mixture plasma exposure experiments on tungsten target materials have been performed to simulate ITER all tungsten divertor erosion/modification and deposition phenomena. The exposure conditions are kept fixed at a typical low-ion-energy of 60 eV and a flux of 3-6 × 1022/m2/s. Sample temperature is 1123 K and plasma exposure times spanning 1050-10,100 s are explored. The typical ratio of He/D ions is 0.2 and Be content is 0.2%. A He-induced nanostructure layer is formed on the exposure surfaces of tungsten materials and the surface of the nanostructure is covered by a thin layer of Be and O. A fraction of the re-eroded Be from the target is deposited on a glassy carbon plate with line of sight to the tungsten target. Rutherford backscattering spectrometry analyses show that the Be redeposit layer is in the form of laminae. Small amounts of Mo, W and C are also found in the redeposited Be layer. Elastic recoil detection analyses show that D, He and H are also included in the redeposited Be layer.
    Journal of Nuclear Materials 11/2013; 442(1-3). · 2.02 Impact Factor
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    ABSTRACT: Toroidal profiles of the microscopic damage and erosion/deposition on the Large Helical Device (LHD) first-wall (SUS316L) were simultaneously evaluated. 10 pairs of SUS316L and Si specimens were mounted on 10 sets of special holders, and were located on the outer side of the first-wall surface in each 36° toroidal angle section (Nos. 1-10). For separate determination of the effects of glow discharge cleanings (GDCs) and main plasma discharges, two types of holders —"floating-potential" and "ground-potential"— were used in each toroidal section. The former was electrically insulated from the first wall; therefore, energetic ions could not be injected into the specimens during GDC. Hence, we could analyze two cases: with and without GDCs exposure. Sputtering erosion of the first-wall surfaces was mainly caused by GDCs and not main plasma discharges, and the erosion depths of each toroidal section were varied from 50 nm to 1 μm. Characteristics of the deposition layers and microscopic damages on the SUS316L matrix were different in each toroidal section.
    Journal of Nuclear Materials 11/2013; 442(1-3):873-. · 2.02 Impact Factor
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    ABSTRACT: The trapping of tritium in silicon carbide (SiC) injected from ceramic breeding materials was examined via tritium measurements using imaging plate (IP) techniques. Monolithic SiC in contact with ternary lithium oxide (lithium titanate and lithium aluminate) as a ceramic breeder was irradiated in the High Flux Isotope Reactor (HFIR) in Oak Ridge, Tennessee, USA. The distribution of photo-stimulated luminescence (PSL) of tritium in SiC was successfully obtained, which separated the contribution of 14C β-rays to the PSL. The tritium incident from ceramic breeders was retained in the vicinity of the SiC surface even after irradiation at 1073 K over the duration of ˜3000 h, while trapping of tritium was not observed in the bulk region. The PSL intensity near the SiC surface in contact with lithium titanate was higher than that obtained with lithium aluminate. The amount of the incident tritium and/or the formation of a Li2SiO3 phase on SiC due to the reaction with lithium aluminate under irradiation likely were responsible for this observation.
    Journal of Nuclear Materials 11/2013; 442(1-3). · 2.02 Impact Factor
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    ABSTRACT: Structural change in polyethylene naphthalate (PEN) foil during MeV proton beam irradiation was investigated by ion beam induced luminescence (IBIL). An aluminum thin film (thickness: 20 nm) was sputter-deposited on the upper surface of 4 μm thick PEN foil to evaluate how Al coating effects structural change. IBIL analysis reveals that the Al coating clearly reduces damage induced by ion beam irradiation, probably due to the enhancement of electrical conductivity of the foil.
    Vacuum. 03/2013; 89:153–156.
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    ABSTRACT: The dynamic behavior of water within ion beam (10 keV Ar+, 1.0 × 1016–1.2 × 1017 ions/cm2) modified perfluorosulfonic acid (PFSA) membranes was investigated at room temperature by combining direct-current (DC) resistance with alternative-current (AC) impedance methods under a water-saturated air atmosphere. The bulk impedance in existing surface sulfonate groups (SO3−) decreased approximately one order of magnitude as a result of Ar+ ion irradiation compared to the unirradiated membrane. The enhancement in the proton conductivity results in an improvement of the water absorption characteristics at the Ar+ ion-modified surface which showed large superficies as well as hydrophilic radicals. These results can be explained in base of a relative increase in both the water content of the membrane and the change in the interactions of water molecules with sulfonate group at the interface on the proton-transfer process.
    Vacuum. 03/2013; 89:225–228.
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    ABSTRACT: Changes in hydrogen distribution near the surface of non-stoichiometric titanium hydride (TiHx; x=0.49) thin films possessing an amorphous structure and approximately 170 and 400 nm thick, prepared by a magnetron ion-sputtering device, were investigated by combining an elastic recoil detection (ERD) technique with Rutherford backscattering spectrometry (RBS) one after isochronal annealing at different temperatures (300-500 K) for 10 min. The concentration of hydrogen atoms retained in TiHx thin films completely vanished at much lower temperatures of 350-400 K, as compared with the decomposition temperature (approximately 473 K) of δ-phase titanium hydride (δ-TiH1.92). In addition, the thickness of TiHx thin films also reduced with an increase in temperature. The rapid decrease of the hydrogen concentration in TiHx thin films below 400 K is caused by thermal desorption of hydrogen atoms, detrapped from the amorphous structure and associated, in addition, with Ti evaporation.
    Journal of Physics Conference Series 03/2013; 417(1):2027-.
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    ABSTRACT: The paper presents a brief overview of current research activities on functional ceramic materials for insulating components, tritium breeder and optical sensing systems, mainly carried out at Institute for Materials Research (IMR), Tohoku University. Topics include recent experimental results related to the electrical degradation and optical changes in typical oxide ceramics (e.g. Al2O3 and SiO2) concerning radiolytic effects. Hydrogen effects on the electrical conductivity in the Perovskite-type oxide ceramics and the interaction between hydrogen and irradiation induced defects in ternary Li oxides used as breeder materials, were dynamically observed under the irradiation environment. Further attention is focused on several challenging qualifications required for an advanced sensing system using optical characteristics (e.g., thermoluminescence in SiO2 core fiber, neutron-induced long lasting emission from oxides doped with rare-earth elements, and gasochromic coloration phenomenon of WO3).
    Journal of Nuclear Materials 01/2013; 442:S501. · 2.02 Impact Factor
  • Bun Tsuchiya, Kenji Morita, Shinji Nagata
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    ABSTRACT: Hydrogen storage and emission characteristics of Pt/Li4SiO4/Pt composite materials exposed to normal air at room temperature have been studied by means of elastic recoil detection analysis, RBS, and weight gain measurement techniques. The thermal annealing curves of Pt/Li4SiO4/Pt sandwich specimens with Pt layers of 1 and 10 nm in thickness measured by elastic recoil detection technique indicate that the hydrogen emission of residual hydrogen takes place in the two stages, and 70% of hydrogen retained is emitted at the first stage, which finishes at a temperature of ~350 K. The hydrogen storage curves at the first run indicate that the saturation storage levels of the specimens of 1 and 10-nm Pt layers pre-heated at 423 K are 12 wt.% and 14 wt.%, respectively. The hydrogen storage curve of the specimen with 10 nm Pt layers annealed at temperature of 773 K indicates that the initial slope of the weight gain is considerably larger than that at the first run. This fact seems to indicate that the high temperature annealing brings about enhancement in the absorption rate caused by formation of Pt-Si-Li-O complex oxides through the interfacial reaction between the Pt layer and Li4SiO4. Copyright © 2011 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 06/2012; 44(6). · 1.39 Impact Factor
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    ABSTRACT: Hydrogen trapping and its interaction with defects in Li2TiO3 and Li2ZrO3 that were irradiated with hydrogen isotopes and exposed to air were investigated using ion beam techniques and photo-stimulated luminescence (PSL) measurements. During irradiation with 10 keV H2+, the incident hydrogen was trapped in Li2TiO3 at the end of its trajectory until the local concentration reached 12.5 at.%, whereas an increase in the hydrogen retained in Li2ZrO3 was observed at a considerably greater depth as compared to the projected range. A broad PSL emission with two peaks at around 2.8 and 3.1 eV was observed under irradiation of Li2ZrO3 with 5.17 eV light. The PSL intensity of the band at 2.8 eV showed a remarkable decrease as compared to the peak at 3.1 eV upon irradiation with 10 keV D2+. The results of isochronal annealing subsequent to irradiation showed that the recovery of the luminescence intensity in the 2.8 eV luminescent centers and release of hydrogen occurred in the same temperature range of 350–450 K. In addition, the formation of a Li2CO3 layer at the surface of Li2TiO3 and Li2ZrO3 following exposure to air was subject to the humidity conditions and was considered to play an important role in the dissociation of water vapor and hydrogen trapping beneath the reaction layer in Li2TiO3 and Li2ZrO3.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 02/2012; 272:275–279. · 1.19 Impact Factor
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    ABSTRACT: Anodic oxide films with nanocrystalline tetragonal ZrO(2) precipitated in an amorphous oxide matrix were formed on Zr-Si and Zr-Al alloys and had significantly enhanced capacitance in comparison with those formed on zirconium metal. The capacitance enhancement was associated with the formation of a high-temperature stable tetragonal ZrO(2) phase with high relative permittivity as well as increased ionic resistivity, which reduces the thickness of anodic oxide films at a certain formation voltage. However, there is a general empirical trend that single-phase materials with higher permittivity have lower ionic resistivity. This study presents a novel material design based on a nanocrystalline-amorphous composite anodic oxide film for capacitor applications.
    ACS Applied Materials & Interfaces 06/2011; 3(7):2665-70. · 5.90 Impact Factor
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    ABSTRACT: The finite size effect of proton conductivity of amorphous silicate thin films, a-M(0.1)Si(0.9)O(x) (M = Al, Ga, Hf, Ti, Ta, and La), was investigated. The proton conductivity across films, σ, was measured in dry air by changing the thickness in the range of 10-1000 nm. σ of the films with M = Al, Ga, and Ta was elevated in a power law by decreasing thickness into less than a few hundred nanometers, and the increment was saturated at a thickness of several 10's of nanometers. On the other hand, σ of the films with M = Hf, Ti, and La was not related to the decrease of the thickness in the range of >10 nm. Thickness-dependent conductivity of the former could be numerically simulated by a percolative resistor network model that involves the randomly distributed array of two kinds of resistors R(1) and R(2) (R(1) > R(2)) in the form of a simple cubic-type lattice. High-resolution TEM clarified that a-M(0.1)Si(0.9)O(x) films involved heterogeneous microstructures made of the condensed domain and the surrounding uncondensed matrix due to the fluctuation of glass networks on the nanometer scale. The condensed domain had a wormlike shape with an average length of several 10's of nanometers and performed the role of the proton conduction pathway penetrating through the poorly conducting matrix. It was concluded that the thickness-dependent conductivity could be identical to finite-size scaling of the percolative network of the interconnected domains in the nanometer range.
    Journal of the American Chemical Society 02/2011; 133(10):3471-9. · 10.68 Impact Factor
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    ABSTRACT: The radiation-induced changes in the volume electrical conductivities of chemical vapor deposited silicon carbides (CVD-SiCs) were in-site investigated by performing irradiation using 1.17 and 1.33-MeV gamma-ray and 14-MeV fast neutron beams in air and vacuum. Under gamma-ray irradiation at ionization dose rates of 3.6 and 5.9 Gy/s and irradiation temperature of approximately 300 K, the initial rapid increase in electrical conductivity; this is indicative of radiation-induced conductivity (RIC), occurred due to electronic excitation, and a more gradual increase followed up to a dose of approximately 10–50 kGy corresponding to the results in base conductivity without radiation; this is indicative of radiation-induced electrical degradation (RIED). However, the radiation-induced phenomena were not observed at irradiation temperatures above 373 K. Under neutron irradiation at a further low dose rate below approximately 2.1 Gy/s, a fast neutron flux of 9.2 × 1014 n/m2 s, and 300 K, the RIED-like behavior according to radiation-induced modification of the electrical property occurred with essentially no displacement damage, but ionizing effects (radiolysis).
    Fusion Engineering and Design. 01/2011; 86:2487-2490.
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    ABSTRACT: Four graphite divertor tiles coated with a 100μm thick layer of tungsten by using a vacuum plasma spray (VPS) method were installed in the LHD (Large Helical Device) in the 2008FY campaign (a total of 1700 pieces of graphite tiles are used in LHD). Tungsten emission of the plasma and the reliability of the coated tungsten layer and graphite substrate have been investigated by using real-time spectroscopic analysis and post-mortem surface analysis, respectively. The tungsten coated tiles could adequately withstand for a single experimental campaign, the heat and particle loads from the divertor plasma which were highly localized at the strike points. Tungsten accumulation in the core plasma could not be seen for the operation schemes in this campaign, and sputtered tungsten impurities did not have any influence on plasma discharges.
    Journal of Nuclear Materials 01/2011; 415(1). · 2.02 Impact Factor

Publication Stats

540 Citations
304.08 Total Impact Points

Institutions

  • 1990–2014
    • Tohoku University
      • Institute for Materials Research
      Japan
  • 2009
    • Shimane University
      • Department of Physics and Material Science
      Matsu, Shimane Prefecture, Japan
  • 1993
    • The University of Tokyo
      • Department of Chemical System Engineering
      Tokyo, Tokyo-to, Japan
  • 1985
    • Nagoya University
      • Department of Energy Engineering and Science
      Nagoya, Aichi, Japan