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Oh-ishi, K.,Zhang, H.W.,Ohkubo, T.,Hono, K. Elsevier Sequoia 2007 Materials science & engineering. properties, micro Vol.456 No.1
The microstructure of bulk nanocrystalline Fe-C alloy produced by mechanical milling (MM) and spark plasma sintering (SPS) has been characterized using transmission electron microscopy (TEM) and a three-dimensional atom probe (3DAP) to understand the origin of the unusually high yield strength and plastic strain observed from the sample. A bimodal grain structure consisting of fine (∼250nm) and coarse (∼900nm) grains was observed as a result of partial recrystallization. The fine grained region was found to be a duplex phase structure comprised of ferrite and cementite grains. From TEM and 3DAP analyses, the presence of fine oxide particles containing chromium was confirmed. The presence of the recrystallized coarse grains is attributed to the large plastic strain in compression.
Blistering kinetics in H-implanted 4H-SiC for large-area exfoliation
Sharma Madan,Soni K.K.,Kumar Ashutosh,Ohkubo Tadakatsu,Kapoor A.K.,Singh Rajendra 한국물리학회 2021 Current Applied Physics Vol.31 No.-
Layer transfer of single-crystalline SiC based on layer splitting and wafer bonding on an alternate substrate is a viable approach to fabricate SiC power devices at a lower cost. Dependence of implantation-induced surface blistering and exfoliation of thin layers, on the implantation parameters and subsequent annealing conditions, for large area exfoliation of 4H-SiC have been studied. We report the optimization of the annealing process (one-step or two-step) and annealing parameters like temperature and time to achieve large area exfoliation of SiC. Using the F¨oppl-von Karman (FvK) theory, the pressure inside the blister cavities and implantation-induced stress inside the damage region have been studied. We deduce that pressure inside the blister depends primarily on the blister size. HRXRD analysis has been used to determine the strain relaxation behavior as a function of annealing conditions. Raman spectroscopy and STEM measurements revealed the formation of defects inside the material due to ion implantation. AFM, Nomarski, and SEM were used to measure the size and depth details of the blisters as a function of annealing temperature and time. We have successfully exfoliated large area (up to 500 μm) in a two-step annealing process. The results reported in this paper are useful for SiC layer transfer on an arbitrary substrate by the Smart Cut process.
Characteristics of second harmonic ECCD on CHS
yasuo Yoshimura,H. Matsushita,H. Igami,K. Nagasaki,K. Matsuoka,K. Ohkubo,K. Nagaoka,M. Isobe,S. Kobayashi,S. Okamura,S. Kubo,S. Nishimura,T. Minami,T. Akiyama,T. Notake,T. Shimozuma,A. Shimizu,C. Suzu 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.III
Second harmonic electron cyclotron current drive (ECCD) has been investigated on CHS (Compact Helical System) by using a 2-D steerable EC-beam injection system. The direction of the plasma current, which is the sum of bootstrap current and EC-driven current, is clearly reversed by a reversal of the EC-beam injection direction. The direction of driven current is consistent with expectation in the case of low-field side injection of EC wave. So far, a driven current up to 6 kA has been obtained when bootstrap current is less than 2 kA. It is confirmed that right-hand polarization and low density are effective for ECCD.
Microstructures and mechanical properties of extruded and heat treated Mg-6Zn-1Si-0.5Mn alloys
Mendis, C.L.,Oh-ishi, K.,Ohkubo, T.,Shin, K.S.,Hono, K. Elsevier Sequoia 2012 Materials science & engineering. properties, micro Vol.553 No.-
Si-rich intermetallic particles stabilize the fine grained microstructure in extruded Mg-6Zn-1Si-0.5Mn alloy, resulting in a yield strength (YS) of ∼200MPa, ultimate tensile strength (UTS) of ∼310MPa with an elongation of ∼20%. Ca additions further refine the grain size leading to an increase in UTS by 30MPa with no decrease in YS and elongation. The extruded Mg-6Zn-1Si-0.5Mn alloy can be age-hardened with the precipitation of fine [0001]<SUB>Mg</SUB> rod-like β<SUB>1</SUB>' (Mg<SUB>4</SUB>Zn<SUB>7</SUB>) particles and YS∼290MPa and UTS∼320MPa were achieved in the peak aged condition. The mechanical properties were further enhanced by pre-aging the alloy at 70<SUP>o</SUP>C due to the refinement of the precipitate microstructure with YS∼350MPa and UTS∼360MPa.
Das, Sushanta K.,KC, Chandra B.,Ohkubo, Kei,Yamada, Yusuke,Fukuzumi, Shunichi,D'Souza, Francis The Royal Society of Chemistry 2013 Chemical communications Vol.49 No.20
<P>Graphene oxide decorated with an electron donor, zinc(<SMALL>II</SMALL>) phthalocyanine, and an electron acceptor, fullerene, has been synthesized, and photoinduced electron transfer leading to charge-separation is reported.</P> <P>Graphic Abstract</P><P>Immobilized on the graphene oxide surface, an electron donor, zinc phthalocyanine, and an electron acceptor, fullerene, are shown to undergo photoinduced electron transfer. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cc38898k'> </P>
Superconducting Strip Ion Detectors for Time-of-flight Mass Spectrometer
N. Zen,K. Suzuki,S. Shiki,M. Ukibe,M. Koike,A. Casaburi,M. Ejrnaes,R. Cristiano,M. Ohkubo 한국초전도학회 2012 Progress in superconductivity Vol.14 No.2
Superconducting detectors are promising as ion detectors for time-of-flight mass spectrometers (TOF MS). They can achieve mass-independent detection efficiency even for macromolecular bombardments, because output signals are produced through the deposited kinetic energy at ion impact instead of secondary electron emission that is the ion detection mechanism of conventional microchannel plate (MCP) detectors or secondary electron multipliers (SEM). Among the superconducting detectors, the superconducting strip ion detectors (SSIDs), which consist of several hundreds of superconducting lines with a width of a few hundreds nm and a thickness of a few tens of nm, have a fast response time of less than 1 ns. Inherently, the response time of SSIDs is determined by kinetic inductance, so that it was difficult to realize a fast SSID with a large detection area. However, we succeeded in realizing the detector size up to 5×5 mm2 without response time degradation by using a parallel configuration.
Sasaki, T.T.,Ohkubo, T.,Hono, K. Elsevier 2009 Acta materialia Vol.57 No.12
<P><B>Abstract</B></P><P>Nanocrystalline Al–5at.% Fe alloy powders produced by mechanical alloying were consolidated by spark plasma sintering. The sintered sample showed high strength >1000MPa with a large plastic strain of 15% at room temperature and 500MPa at 350°C. Microstructure characterizations by transmission electron microscopy and atom probe tomography revealed that the sintered samples are composed of α-Al and Al<SUB>6</SUB>Fe nanocrystalline regions with 90nm in diameter and a minor fraction of Al<SUB>13</SUB>Fe<SUB>4</SUB> phase and coarsened 0.5–1μm α-Al grains. This bimodally grained feature is attributed to the relatively large plastic strain for the strength level of 1000MPa at room temperature.</P>