S. Nagata

Tohoku University, Miyagi, Japan

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Publications (280)429.21 Total impact

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    ABSTRACT: Hydrogen (H)- and water (H2O)-storage and desorption characteristics of 25 nm thick Pt films onLi2ZrO3composite materials, exposed to normal air at room temperature, have been investigated by means of elastic recoil detection (ERD), Rutherford backscattering spectrometry (RBS), weight gain measurement (WGM), and thermal desorption spectroscopy (TDS) techniques. It was found by the ERD and TDS that H and H2O were absorbed into the Pt-coated Li2ZrO3 in air at room temperature and desorbed from it in vacuum at much low temperatures of approximately 317 and 309 K, respectively. In addition, the WGM and TDS spectra revealed that the absorption and desorption characters ofsome gases such as CH4, CO, and CO2including H as well as H2Ointo the Li2ZrO3 bulk were improved by Pt deposition.
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    ABSTRACT: This study demonstrates the formation of uniform barrier-type anodic films on magnetron-sputtered magnesium films at high current efficiency in ethylene glycol electrolytes containing 0.1 mol dm−3 NH4F and various concentrations (0.1–28 mol dm−3) of H2O. The anodic films containing a crystalline MgF2 phase develop both at the metal/film and film/electrolyte interfaces due to simultaneous migrations of anions inwards and cations outwards, respectively. When a Mg −1.2 at% Au/Mg bilayer film is anodized, initial prior oxidation of magnesium proceeds with gold atoms accumulating in a thin layer beneath the anodic film. The accumulated gold atoms are incorporated into the anodic film as a band when the alloy layer is completely anodized. Fluoride-containing gold species are formed by the incorporation and the gold species migrate outwards at a rate of 0.4 times the rate of Mg2+ ions. The addition of phosphate in the electrolyte results in the formation of an amorphous anodic film, and the phosphate incorporated into the anodic film is distributed throughout the film thickness. The transport number of cations is also influenced by the phosphate incorporation.
    Electrochimica Acta 02/2015; DOI:10.1016/j.electacta.2015.02.168 · 4.50 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 12/2014; 46(12-13). DOI:10.1002/sia.5620 · 1.39 Impact Factor
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    ABSTRACT: Ultra-long-pulse helium discharge with ion and electron cyclotron heating (ICH + ECH) in the Large Helical Device (LHD) was achieved in a 48 min plasma (ne ∼ 1.2 × 1019 m−3, Ti,e ∼ 2 keV) with an average heating power of 1.2 MW. The temperature of the first-wall surface during discharges remained at nearly room temperature. However, even in ultra-long-pulse helium discharge, the discharge conditions cannot be said to be in a steady-state, because of two major issues interrupting the steady-state condition. One is the “dynamic change of the wall pumping rate” and the other is the “termination of the discharge with the exfoliation of the mixed-material deposition layers.” Microscopic modifications, such as helium radiation damage and the formation of the mixed-material deposition layers composed of C (∼98%) and Fe (∼2%), on the plasma facing components (PFMs) were clarified to possibly influence the major issues.
    Journal of Nuclear Materials 12/2014; 463. DOI:10.1016/j.jnucmat.2014.12.062 · 2.02 Impact Factor
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    Yunosuke Ishikawa · Shinji Nagata · Ming Zhao · Tatsuo Shikama
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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). DOI:10.1016/j.jnucmat.2014.08.015 · 2.02 Impact Factor
  • B. Tsuchiya · T. Shikama · S. Nagata · K. Saito · T. Nozawa
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    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). DOI:10.1016/j.jnucmat.2014.08.057 · 2.02 Impact Factor
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    ABSTRACT: Magnetron-sputtered iron films were potentiodynamically anodized at two different sweep rates to 50 V in an ethylene glycol electrolyte containing ammonium fluoride and water. At a high sweep rate of 1.0 V s−1, a barrier-type anodic film was formed even though the current efficiency was as low as ∼50%. In contrast, a nanoporous anodic film developed at a low sweep rate of 0.05 V s−1, and the film-formation efficiency reduced to 37%. The main part of the anodic films consists of iron (III) hydroxyfluoride with a thin inner layer composed of FeF3. The inner fluoride layer is formed owing to the faster inward migration of fluoride ions compared to that of the oxygen species. During immersion or re-anodizing of the iron specimen with an approximately 100-nm-thick, barrier-type anodic film at and below 15 V, thinning of the anodic film proceeded uniformly and film dissolution was enhanced by applying an electric field. The impact of the electric field on film formation and dissolution is discussed.
    Electrochimica Acta 11/2014; 151. DOI:10.1016/j.electacta.2014.10.132 · 4.50 Impact Factor
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    ABSTRACT: An electrochemical analysis was conducted with respect to a hydrogen membrane fuel cell (HMFC) comprising proton-conducting, amorphous zirconium phosphate, a-ZrP2.5O8.9H1.3, thin film electrolyte supported on a dense Pd anode. The HMFC gave rise to an OCV of 1.0 V, but the maximum power density was limited and was about 1 mW cm(-2) at 400 degrees C. The impedance spectroscopy revealed that the interfacial polarizations were decreased by two orders of magnitude when the cell configuration was changed from the fuel cell setup to the hydrogen concentration cell with an anode symmetric configuration. These results indicated that the polarization losses at the solid-solid anode interface are not a main contribution to the voltage loss of the HMFC. The large cathode polarization might be attributed to the lessened conductivity of amorphous zirconium phosphate electrolyte thin film formed on a precious metal electrode.
    Electrochemistry -Tokyo- 10/2014; 82(10):859-864. DOI:10.5796/electrochemistry.82.859 · 0.98 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 09/2014; 46:30–32. DOI:10.1016/j.elecom.2014.06.011 · 4.85 Impact Factor
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    ABSTRACT: We examined the migration of uranium atoms in uranium glasses after applying the electric field of 5-15kV/mm at RT and at 373K using the technique of Rutherford backscattering (RBS). After applying the electric field of 10kV/mm between the front surface and the back surface of the glasses (1mm in thickness) at 373K for 2 hours, there was neither enhancement nor depression in the concentration of uranium atoms at both surfaces of the negative charge or of the positive charge. In addition, we examined some optical properties such as optical absorption, photoluminescence, photoluminescence excitation and the decay properties of the photoluminescence of the uranium glasses. The optical absorption showed intense absorptions below 345nm and three peaks at 416nm, 482nm and 706nm. The luminescence excited by 416nm light showed four peaks at 512nm, 52Snm, 556nm and 584nm. A decay time determined by the forth harmonic light (266nm) of Nd:YAG laser is about 250 μs.
    Journal of Nuclear Science and Technology 08/2014; 39(sup3):497-499. DOI:10.1080/00223131.2002.10875515 · 1.45 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; 36(9). DOI:10.1016/j.optmat.2014.04.007 · 2.08 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). DOI:10.1016/j.jnucmat.2012.12.021 · 2.02 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). DOI:10.1016/j.jnucmat.2013.05.006 · 2.02 Impact Factor
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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 11/2013; 442(1-3):S501. DOI:10.1016/j.jnucmat.2013.05.039 · 2.02 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; 315:157-160. DOI:10.1016/j.nimb.2013.03.027 · 1.12 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-. DOI:10.1016/j.jnucmat.2013.03.019 · 2.02 Impact Factor
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    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; 315:149-152. DOI:10.1016/j.nimb.2013.05.066 · 1.12 Impact Factor
  • International Journal of Oral and Maxillofacial Surgery 10/2013; 42(10):1374. DOI:10.1016/j.ijom.2013.07.730 · 1.36 Impact Factor
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    ABSTRACT: This paper describes the results of characterization of high heat and particle fluxes produced at the end-cell of the large tandem mirror GAMMA 10 and of the initial plasma-irradiation experiments. In the case of ICRF plasmas, the heat flux of 0.8 MW/m(2) and the particle flux of 4 x 10(22)/m(2) s were achieved at the end-mirror exit. The heat flux increases with the ICRF power and has a linear relationship with the stored energy. Direct ion energy analysis clarified that the parallel ion temperature can be controlled from 100 eV to 400 eV by changing the ICRF power. Additional plasma heating using another ICRF system in the anchor-cell significantly increases the particle flux, which gives a clear prospect of generating the higher particle flux by applying additional ICRF heating in the neighboring cells. The initial results of the plasma-gas-material interactions on a new V-shaped tungsten target were also reported.
    Journal of Nuclear Materials 07/2013; 438:S738-S741. DOI:10.1016/j.jnucmat.2013.01.157 · 2.02 Impact Factor
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    ABSTRACT: Novel proton-conducting ZrO2-WO3-SiO2 nanofilms of various compositions and thicknesses (similar to 50 to similar to 300 nm) have been prepared by anodizing of magnetron-sputtered Zr-W-Si alloys in 0.1 mol dm(-3) phosphoric acid electrolyte at 20 degrees C. All the anodic oxide nanofilms examined reveal efficient proton conductivity after post-annealing at 250 degrees C. Further increase in the post-annealing temperature results in the conductivity degradation for the anodic oxide nanofilms on the alloy containing only 5 at% silicon, while the high conductivity is maintained even after post-annealing at 300 degrees C for those containing 15 at% or more silicon. The proton conductivity is dependent upon tungsten content; the conductivity of 5 x 10(-6) S cm(-1) for the similar to 100 nm-thick films on the Zr31W55Si14 at 100 degrees C is approximately 10 times that on the Zr48W37Si15. The anodic oxide nanofilms consist of two layers, comprising a thin outer ZrO2 layer and an inner ZrO2-WO3-SiO2 layer. Both layers show thickness-dependent conductivity and the proton conductivity of the two-layer anodic films is enhanced one order of magnitude by reducing the film thickness from similar to 300 nm to similar to 100 mu. Different mechanisms are proposed for the thickness dependence of the conductivity of the outer and inner layers.
    Journal of The Electrochemical Society 06/2013; 160(9):F1096-F1102. DOI:10.1149/2.120309jes · 2.86 Impact Factor