Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Kohei Morishita Last modified date:2021.07.14

Associate Professor / Structural Materials Engineering / Department of Materials Science and Engineering / Faculty of Engineering


Papers
1. N. Nakaniwa, Y. Ezoe, M, Numazawa, M. Takeo, M. Ishida, K. Ishikawa, K. Morishita, K. Nakajima, Development of x-ray mirror foils using a hot plastic deformation process, Appiled Optics, 10.1364/AO.401644, 59, 28, 8793-8805, 2020.10.
2. Kiattisaksri Chatcharit, Akira Sugiyama, Kohei Morishita, Taka Narumi, Kentaro Kajiwara, Hideyuki Yasuda, Time Evolution of Solidification Structure in Ductile Cast Iron with Hypereutectic Compositions, International Journal of Metalcasting, 10.1007/s40962-020-00424-3, 2020.07, The microstructure evolution in ductile cast iron with magnesium addition was observed in situ by using X-ray radiography (two-dimensional observation) and time-resolved tomography (three-dimensional observation) in the BL20XU of a synchrotron radiation facility, SPring-8 (Hyogo, Japan). In the two-dimensional observation, graphite nodules nucleated in the melt and floated up immediately after nucleation. The floating was terminated by engulfment of graphite nodules into austenite dendrites. The radiography indicated that the average floating distance was shorter than the dendrite arm spacings in the 100-μm-thick specimen. Because the short distance could be influenced by the sample confinement, time-resolved tomography was performed by using a pink X-ray beam in the BL28B2 of SPring-8. Graphite nodules that nucleated in the melt (probably on magnesium–oxygen–sulfur inclusions in the melt) floated and were engulfed by austenite dendrites within several seconds, even in the bulk specimen. Although the average distance in the bulk specimen was approximately twice as large as that in the 100-μm-thick specimen, floating after nucleation and engulfment into austenite dendrites within a short duration were observed commonly from both techniques. The sequence of nucleation and engulfment had a critical effect on the number and size of the graphite nodules..
3. Yusuke Kobayashi, Kento Dobara, Hidekazu Todoroki, Cheolhee Nam, Kohei Morishita, Hideyuki Yasuda, In-situ measurements of solute partition coefficients between solid and liquid phases in Fe–Cr–Ni–Mo–Cu alloys during solidification, isij international, 10.2355/isijinternational.ISIJINT-2019-444, 60, 2, 276-285, 2020.01, The in-situ measurements of the solute partition coefficients, k, between the solid and liquid phases in Fe–Cr–Ni–Mo–Cu alloys were conducted using X-ray transmission imaging and X-ray fluorescence spectroscopy (EDS) in a synchrotron radiation facility, SPring-8. A nearly planar solid/liquid interface was achieved in a furnace with a temperature gradient (5–10 K/mm) using X-ray transmission imaging. The measurement points in the solid and liquid phases and close to the solid/liquid interface were determined by X-ray imaging. The compositions of the solid and liquid phases were measured by EDS. The solute partition coefficients along the solidification path in the Fe - 19.89–25.82mass% Cr - 24.73–34.81mass% Ni - 4.46–10.28mass% Mo - 1.47–5.79mass% Cu alloy were determined at 56 different liquid compositions. At the beginning of solidification, the partition coefficients of Cr, Ni, Mo and Cu were 0.96, 0.97, 0.70 and 0.86, respectively. The partition coefficients of Cr and Ni were almost constant during the unidirectional solidification. The partition coefficient of Mo gradually changed from 0.7 to 0.6, leading to a severe microsegregation at the end of solidification. In contrast, the partition coefficient of Cu was dispersed in the range from 0.8 to 0.9. This study demonstrated that the in-situ measurement was effective for systematic measurement..
4. Keita Nakano, Taka Narumi, Kohei Morishita, Hideyuki Yasuda, Characterization of growing dendrites in CrMnFeCoNi high-entropy alloy by time-resolved and in-situ tomography, Materials Transactions, 10.2320/matertrans.MT-MK2019006, 61, 4, 596-604, 2020.01, Time-resolved tomography (4D-CT) and X-ray diffraction (XRD) were combined to observe growing dendrites and to measure their crystallographic orientation in a CrMnFeCoNi high-entropy alloy with an FCC structure. The evolution of the dendritic grains cooling at 0.083 K/s was reconstructed using 200 projections over a 180° rotation every 4 s from 4D-CT and a phase field filter. The voxel size was a 6.5-µm cube. Simultaneously, the crystallographic orientations of the dendritic grains were measured by XRD. The dendrite arms grew preferentially along the.
5. Hideyuki Yasuda, Kohei Morishita, Noriaki Nakatsuka, Tomohiro Nishimura, Masato Yoshiya, Akira Sugiyama, Kentaro Uesugi, Akihisa Takeuchi, Dendrite fragmentation induced by massive-like δ–γ transformation in Fe–C alloys, Nature communications, 10.1038/s41467-019-11079-y, 10, 1, 2019.12, Dendrite arm fragmentation is considered in solidification structure tailoring. Time-resolved and in situ imaging using synchrotron radiation X-rays allows the observation of dendrite arm fragmentation in Fe–C alloys. Here we report a dendrite arm fragmentation mechanism. A massive-like transformation from ferrite to austenite rather than the peritectic reaction occurs during or after ferrite solidification. The transformation produces refined austenite grains and ferrite–austenite boundaries in dendrite arms. The austenite grains are fragmented by the liquid phase that is produced at the grain boundary. In unidirectional solidification, a slight increase in temperature moves the ferrite–austenite interface backwards and promotes detachment of the primary and secondary arms at the δ–γ interface via a reverse peritectic reaction. The results show a massive-like transformation inducing the dendrite arm fragmentation has a role in formation of the solidification structure and the austenite grain structures in the Fe–C alloys..
6. H. Yasuda, T. Kawarasaki, Y. Tomiyori, Y. Kato, K. Morishita, Characterization of dendritic growth in Fe-C system using time-resolved X-ray tomography and physics-based filtering, Joint 5th International Conference on Advances in Solidification Processes, ICASP 2019 and 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining, CSSCR 2019 IOP Conference Series: Materials Science and Engineering, 10.1088/1757-899X/529/1/012023, 529, 1, 2019.06, Time-resolved in situ tomography of dendritic growth in Fe-0.45 mass% C carbon steel was performed using synchrotron radiation X-rays at SPring-8 synchrotron radiation facility (Japan) with improvement of the image quality using a physics-based filter. The voxel size of the reconstructed image was approximately 6.5 μm × 6.5 μm × 6.5 μm, and the time resolution (duration of 360° rotation) was 4 s (0.25 rps). Three-dimensional images of the dendrites were reconstructed even without image processing; however, the low contrast resolution in Fe-C alloys led to poor image quality. Consequently, it was impossible to precisely track the solid/liquid interface or evaluate the average curvature. To improve the image quality, a physics-based filter (a PF filter) was developed using a phase-field model. In the PF filter, images were retrieved in terms of interface curvature. The PF filter significantly improved the computed tomography image quality. As a result, dendritic growth was clearly observed even in Fe-C alloys. Moreover, the average curvature of the solid/liquid interface was evaluated as a function of solidification time (solid fraction). The ability to systematically characterize growing dendrites will be beneficial for modeling and simulation of solidification phenomena..
7. Hideyuki Yasuda, Takahiro Hashimoto, Naoki Sei, Kohei Morishita, Masato Yoshiya, Investigation using 4D-CT of massive-like transformation from the δ to γ phase during and after δ-solidification in carbon steels, Joint 5th International Conference on Advances in Solidification Processes, ICASP 2019 and 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining, CSSCR 2019 IOP Conference Series: Materials Science and Engineering, 10.1088/1757-899X/529/1/012013, 529, 1, 2019.06, For a long time, the γ phase in metallic alloys has seemed to be produced through a peritectic reaction between the δ and liquid phases. However, direct observations have shown that a massive-like transformation, in which the δ phase transforms into the γ phase in the solid state, is dominant during or after solidification of the δ phase in carbon steels. To characterize such massive-like transformation, we use time-resolved tomography (4D-CT) to demonstrate the volume change during cooling from the melt and the crystallographic orientation relationship between the δ and γ phases. The volume changes from solidification and from the massive-like transformation from the δ to the γ phase were -3% and -0.5%, respectively. The transformation from the δ to the γ phase finished quickly, as demonstrated by the volume change. Fine γ grains were produced even in a single δ grain through the massive-like transformation. Also, the refined γ grains showed a wide crystallographic distribution..
8. Keizo Ono, Ayumu Hashimoto, Keiji Kurokawa, Noriaki Nakatsuka, Kohei Morishita, Hideyuki Yasuda, High-Density β-FeSi2 crystals with 3D alignment fabricated by an oscillating magnetic field, 9th International Symposium on Electromagnetic Processing of Materials, EPM 2018 IOP Conference Series: Materials Science and Engineering, 10.1088/1757-899X/424/1/012074, 424, 1, 2018.10.
9. Amanda Youssef, Jonas Schon, Tim Niewelt, Sebastian Mack, Sungeun Park, Kazuo Nakajima, Kohei Morishita, Ryota Murai, Mallory A. Jensen, Tonio Buonassisi, Martin C. Schubert, Swirl defect investigation using temperature- and injection-dependent photoluminescence imaging, 44th IEEE Photovoltaic Specialist Conference, PVSC 2017 2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017, 10.1109/PVSC.2017.8366265, 1-5, 2018.05.
10. H. Yasuda, A. Sugiyama, C. Kiattisaksri, Kohei Morishita, T. Nagira, M. Yoshiya, K. Uesugi, A. Takeuch, X-ray imaging of formation and growth of spheroidal graphite in ductile cast Iron, 11th International Symposium on the Science and Processing of Cast Iron, SPCI-XI 2017 Science and Processing of Cast Iron XI, 10.4028/www.scientific.net/MSF.925.104, 104-109, 2018.01.
11. Hideyuki Yasuda, Akira Sugiyama, Kiattisaksri Chatcharit, Kohei Morishita, Time-resolved and in-situ 2D / 3D imaging of solidification in ductile cast iron, 73rd World Foundry Congress: Creative Foundry, WFC 2018 73rd World Foundry Congress "Creative Foundry", WFC 2018 - Proceedings, 233-234, 2018.01.
12. Hideyuki Yasuda, Tomoya Nagira, Masato Yoshiya, Kohei Morishita, Kazuhiro Nogita, Time-resolved and in-situ observation of solidification phenomena, Yosetsu Gakkai Shi/Journal of the Japan Welding Society, 86, 5, 362-363, 2017.01.
13. Amanda Youssef, Jonas Schon, Tim Niewelt, Sebastian Mack, Sungeun Park, Kazuo Nakajima, Kohei Morishita, Ryota Murai, Mallory A. Jensen, Tonio Buonassisi, Martin C. Schubert, Swirl defect investigation using temperature- and injection-dependent photoluminescence imaging, 43rd IEEE Photovoltaic Specialists Conference, PVSC 2016 2016 IEEE 43rd Photovoltaic Specialists Conference, PVSC 2016, 10.1109/PVSC.2016.7749826, 1303-1307, 2016.11.
14. Sergio Castellanos, Maulid Kivambe, Mallory A. Jensen, Douglas M. Powell, Kazuo Nakajima, Kohei Morishita, Ryota Murai, Tonio Buonassisi, Exceeding 3 ms Minority Carrier Lifetime in n-type Non-contact Crucible Silicon, 6th International Conference on Crystalline Silicon Photovoltaics, SiliconPV 2016 Energy Procedia, 10.1016/j.egypro.2016.07.068, 92, 779-784, 2016.08.
15. Hideyuki Yasuda, Akira Sugiyama, Tomoya Nagira, Kohei Morishita, Masato Yoshiya, Tomohiro Nishimura, Koshiro Yamane, Solidification sequence of ductile cast iron - In-situ observation and modeling, 72nd World Foundry Congress, WFC 2016 72nd World Foundry Congress, WFC 2016, 257-258, 2016.01, Radiography was performed to examine solidification and melting behaviors in hypereutectic cast iron specimens with / without Mg addition. The specimens without Mg addition showed that graphite particles exhibited spherical shape at the beginning and then transformed to the flake-like shape. Growth of γ-Fe dendrites was followed by nucleation of graphite particles ahead of the eutectic front. In contrast, growth of graphite particles and γ-Fe dendrites occurred nearly at the same time in the specimens with Mg addition and the spherical graphite particles were engulfed by γ-Fe or eutectic. The addition of 0.05mass%Mg significantly reduced the temperature range in which the graphite particles grew as the primary phase. Image-based analysis of melting behavior clearly proved that even 0.05mass% addition modified the phase equilibrium of the liquid, γ-Fe and graphite phases..
16. Maulid Kivambe, Douglas M. Powell, Mallory Ann Jensen, Ashley E. Morishige, Kazuo Nakajima, Ryota Murai, Kohei Morishita, Tonio Buonassisi, >1.8 millisecond effective lifetime in n-type silicon grown by the noncontact crucible method, 40th IEEE Photovoltaic Specialist Conference, PVSC 2014 2014 IEEE 40th Photovoltaic Specialist Conference, PVSC 2014, 10.1109/PVSC.2014.6925560, 2988-2990, 2014.10.
17. Kazuo Nakajima, Kohei Morishita, Ryota Murai, Noritaka Usami, Formation process of Si3N4 particles on surface of Si ingots grown using silica crucibles with Si3N4 coating by noncontact crucible method, Journal of Crystal Growth, 10.1016/j.jcrysgro.2013.12.006, 389, 112-119, 2014.03, A noncontact crucible method was used to investigate the process by which a Si3N4 coating material forms Si3N4 particles or precipitates on the surface of Si melts and ingots. Si ingots were grown using crucibles with and without a mixture of α- and β-Si 3N4 particles. The oxygen and nitrogen concentrations in the ingots were measured by Fourier transform infrared spectrometry analysis. The nitrogen concentration in the ingots grown using crucibles with a Si 3N4 coating was significantly higher than that in ingots grown using crucibles without a Si3N4 coating because the nitrogen from the Si3N4 coating material dissolved into the Si melt. From orientation image maps analyzed using electron backscattering diffraction patterns of SixNy particles on the surface of the ingots, it was clarified that most of the SixNy particles were β-Si3N4. This was also confirmed by X-ray diffraction measurements. The Si3N4 particles on the surface of the ingots had several morphologies such as needle-like, columnar, leaf-like, and hexagonal structures. There were two cases in which floating Si3N4 particles were formed on the surface of the Si melts, i.e., the removal and dissolution of the Si3N4 coating material. The removed or dissolved Si3N4 coating materials, which consisted of a mixture of α- and β-Si 3N4 particles, are considered to have finally changed into β-Si3N4 in the form of transformers or precipitates on the surface of the Si melt, and these β-Si3N4 particles became attached to the surface of the ingots..
18. Kazuo Nakajima, Ryota Murai, Kohei Morishita, Growth of square Si single bulk crystals with large side-face widths using noncontact crucible method, Japanese journal of applied physics, 10.7567/JJAP.53.025501, 53, 2 PART 1, 2014.02, The noncontact crucible method was used to prepare square Si single bulk crystals. The size of the square part of the ingots was determined by the side-face width of the four-cornered pattern that appeared on the top surface. We obtained square Si single crystals with sizes of 9.4 ' 9.7 and 10.9 ' 11.0cm2 that had no fan-shaped {110} faces and had diagonal lengths of up to 91% of the crucible diameter. To obtain large square Si single bulk crystals with a large side-face width using the present method, the importance of establishing a larger low-temperature region in the Si melt while maintaining a smaller initial temperature reduction was considered..
19. Kazuo Nakajima, Ryota Murai, Kohei Morishita, Douglas M. Powell, Maulid Kivambe, Tonio Buonassisi, High speed growth of square-like Si single bulk crystals with a size of 23 × 23 cm2 for solar cells using the noncontact crucible method, 40th IEEE Photovoltaic Specialist Conference, PVSC 2014 2014 IEEE 40th Photovoltaic Specialist Conference, PVSC 2014, 10.1109/PVSC.2014.6924870, 3530-3533, 2014.01, A noncontact crucible method was proposed to obtain a crystal-diameter as large as a crucible-diameter. In this method, a Si melt used has a large low-temperature region in its central upper part to ensure Si crystal growth inside it. Therefore, the present method has several merits such as the convex shape of the growing interface in the growth direction, the possibility of growing large ingots even using a small crucible, and the growth of square-like single bulk crystals. In these ingots, dislocations in the ingot moved to the periphery of the ingot from its center during crystal growth, and the dislocation density was on the order of 102-103/cm2. The effective minority carrier lifetime was measured to be as high as 750 μs by the Quasi-Steady-State Photoconductance (QSSPC) method after phosphorus diffusion gettering and Al2O3 thin-film passivation. Especially, this method has a possibility to attain a high growth rate using a high cooling rate because the growth rate was determined by the expansion rate of the low-temperature region in Si melts. The growth rate increases as the cooling rate increases. At the cooling rate of 0.4 K/min, the horizontal growth rate became higher to 1.5 mm/min in the <110> direction. The vertical growth rate was determined as 0.3-0.6 mm/min, and it had a tendency to increase as the depth of Si melts increased. The diameter of ingots can be kept constant during crystal growth using a high cooling rate because the horizontal growth rate increases as the cooling rate increases. An ingot with a diagonal length of 24.5 cm was obtained using the high cooling rate of 0.4 K/min. The diagonal length was as large as 82% of the crucible diameter..
20. Yuichiro Ezoe, Tomohiro Ogawa, Masahiro Ikuta, Mayu Sato, Kazuhisa Mitsuda, Hitomi Yamaguchi, Yoshiaki Kanamori, Kohei Morishita, Kazuo Nakajima, Hiroshi Hiroshima, Ryutaro Maeda, MEMS-based X-ray optics for astronomy, planetary exploration, and earth observation, 2014 International Conference on Optical MEMS and Nanophotonics, OMN 2014 2014 International Conference on Optical MEMS and Nanophotonics, OMN 2014 - Proceedings, 10.1109/OMN.2014.6924553, 127-128, 2014.01, We have been developing novel ultra lightweight X-ray optics based on MEMS technologies, aiming at future space missions whose objectives include X-ray astronomy, planetary exploration, and Earth observation. Sidewalls of DRIE- micro pores through thin silicon wafers are used for X-ray mirrors. Recently, we built a prototype of a 4-inch Wolter type-I optic and fabricated a 12-inch single-stage op-tic. In this paper, we review our development and future space applications..
21. Kohei Morishita, Kazuo Nakajima, Takashi Fujii, Masakazu Shiinoki, Near-net shaping of silicon for optical lens by one-shot pressing at temperature just below silicon melting point and improvement of infrared transmittance by primary recrystallization, 8th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC 2013 THERMEC 2013, 2474-2479, 2014.01, Silicon is brittle and easily cracks even under a small load. The difficulty in shaping silicon has prevented breakthroughs in the mass production of silicon lenses for terahertz and infrared technology. We developed a novel method of deforming bulk single-crystal silicon into the required shape by one-shot pressing at a temperature just below the melting point of silicon, despite its brittleness and covalent nature, and realized the near-net shaping of the material into the plano-convex shape with the curvature radius R=7.5 mm for a infrared transmission lens. The crystallographic quality of the obtained lens could be improved by primary recrystallization. The simple method of 'pressing' will enable the mass production of not only silicon lenses but also lenses with a complex shape, such as aspherical lenses, and lens arrays by using dies with desired shape..
22. Tomohiro Ogawa, Yuichiro Ezoe, Takuya Kakiuchi, Masahiro Ikuta, Mayu Sato, Takaya Ohashi, Ikuyuki Mitsuishi, Kazuhisa Mitsuda, Kohei Morishita, Kazuo Nakajima, X-ray irradiation test of a MEMS-based X-ray optic, 2014 International Conference on Optical MEMS and Nanophotonics, OMN 2014 2014 International Conference on Optical MEMS and Nanophotonics, OMN 2014 - Proceedings, 10.1109/OMN.2014.6924555, 131-132, 2014.01, MEMS technolgies can provide future space missions with ultra light-wight and high resolution optics. We conducted an X-ray irradiation test of our first MEMS-based Wolter type-I optic. However, a focus was weak and wide spread. Hence, we improved fabrication processes and an alignment system. In this paper, we report on results of the latest X-ray irradiation test of our new Wolter type-I optic. The angular resolution and effective area were 4.1 arcmin and 32 mm2 both of which are 3 and 40 times better than those in the first test..
23. Ikuyuki Mitsuishi, Yuichiro Ezoe, Tomohiro Ogawa, Takuya Kakiuchi, Masahiro Ikuta, Takayuki Hayashi, Toshiki Sato, Takaya Ohashi, Kazuhisa Mitsuda, Kohei Morishita, Kazuo Nakajima, Assembly of a MEMS-based Wolter type-I X-ray optic toward a future planetary exploration mission, 2013 International Conference on Optical MEMS and Nanophotonics, OMN 2013 Optical MEMS and Nanophotonics 2013, OMN 2013 - Proceedings, 10.1109/OMN.2013.6659086, 115-116, 2013.12, We have been developing original MEMS-based X-ray optics toward future astronomical missions such as a planetary exploration mission. In this paper, we report results on the first X-ray irradiation test for a 2-stage MEMS-based optic. The obtained FWHM of the focusing image is 13±2 arcmin. However, an occultation effect and a deformation accuracy possibly affect a reduction of effective area..
24. Kazuo Nakajima, Ryota Murai, Kohei Morishita, Kentaro Kutsukake, Growth of Si single bulk crystals inside Si melts by the noncontact crucible method using silica crucibles without coating Si3N 4 particles, 39th IEEE Photovoltaic Specialists Conference, PVSC 2013 39th IEEE Photovoltaic Specialists Conference, PVSC 2013, 10.1109/PVSC.2013.6744123, 174-176, 2013.01, A noncontact crucible method for reducing stress in Si bulk crystals was proposed. In this method, the Si melt used has a large low-temperature region to ensure natural Si crystal growth inside it. Compared with the conventional growth methods, the present method has several merits such as the convex shape of the growing interface in the growth direction, the possibility of growing large ingots even with the use of a small crucible, and the low O concentration in the ingots because of the small convection in the Si melt due to existence of the large low-temperature region. We have confirmed that by using crucibles without coating Si3N4 particles, p-type Si single bulk crystals can be grown inside a Si melt without contacting with the crucible wall. The single bulk crystals grown had low dislocation densities (on the order of 103/cm2). The diameter of the ingot obtained using a crucible with a 30 cm diameter was 22cm. The O concentration in the present ingots was relatively lower than that in ingots grown by the CZ method. An n-type ingot was grown using a crucible without Si3N4 coating. Several Σ3 twin grain boundaries were observed in the cross section of the ingot. The average minority carrier lifetime of an n-type wafer was higher than that for p-type wafers..
25. Yuichiro Ezoea, Teppei Moriyama, Tomohiro Ogawa, Takuya Kakiuchi, Takaya Ohashi, Ikuyuki Mitsuishi, Kazuhisa Mitsuda, Mitsuhiro Horade, Susumu Sugiyama, Raul E. Riveros, Hitomi Yamaguchi, Yoshiaki Kanamori, Kohei Morishita, Kazuo Nakajima, Ryutaro Maeda, Progress with MEMS X-ray micro pore optics, Space Telescopes and Instrumentation 2012: Ultraviolet to Gamma Ray Space Telescopes and Instrumentation 2012 Ultraviolet to Gamma Ray, 10.1117/12.925811, 8443, 2012.12, Our development of ultra light-weight X-ray micro pore optics based on MEMS (Micro Electro Mechanical System) technologies is described. Using dry etching or X-ray lithography and electroplating, curvilinear sidewalls through a flat wafer are fabricated. Sidewalls vertical to the wafer surface are smoothed by use of high temperature annealing and/or magnetic field assisted finishing to work as X-ray mirrors. The wafer is then deformed to a spherical shape. When two spherical wafers with different radii of curvature are stacked, the combined system will be an approximated Wolter type-I telescope. This method in principle allows high angular resolution and ultra light-weight X-ray micro pore optics. In this paper, performance of a single-stage optic, coating of a heavy metal on sidewalls with atomic layer deposition, and assembly of a Wolter type-I telescope are reported..
26. Takuya Kakiuchi, Yuichiro Ezoe, Teppei Moriyama, Ikuyuki Mitsuishi, Tomohiro Ogawa, Kohei Morishita, Kazuo Nakajima, Kazuhisa Mitsuda, Assembly of a MEMS-based Wolter type-I x-ray optic, 2012 International Conference on Optical MEMS and Nanophotonics, OMN 2012 2012 International Conference on Optical MEMS and Nanophotonics, OMN 2012, 10.1109/OMEMS.2012.6318814, 85-86, 2012.11, Implementation of a MEMS-based Wolter type-I optic that is suitable for future planetary exploration missions is described. A devised alignment system permits independent adjustment of position and angular settings of x-ray mirrors..
27. Kazuo Nakajima, Ryota Murai, Kohei Morishita, Kentaro Kutsukake, Noritaka Usami, Growth of multicrystalline Si ingots for solar cells using noncontact crucible method without touching the crucible wall, 38th IEEE Photovoltaic Specialists Conference, PVSC 2012 Program - 38th IEEE Photovoltaic Specialists Conference, PVSC 2012, 10.1109/PVSC.2012.6317949, 1830-1832, 2012.11, Conventional crystal growth methods using crucibles cannot control the stress caused by expansion due to the solidification of the Si melt. We proposed a noncontact crucible method using a conventional crucible that reduces the stress in Si multicrystalline ingots. In this method, nucleation occurs on the surface of a Si melt using seed crystals, and crystals grow inside the Si melt without touching the crucible walls. Then, the ingots continue to grow while being slowly pulled upward to ensure that the crystal growth remains in the Si melt. A Si ingot with a diameter of 23 cm was obtained in a crucible with a diameter of 30 cm. The maximum solidification ratio in the growth was more than 80%. We have confirmed that such noncontact crucible growth was possible using a conventional crucible..
28. Ikuyuki Mitsuishi, Yuichiro Ezoe, Teppei Moriyama, Tomohiro Ogawa, Takuya Kakiuchi, Takaya Ohashi, Makoto Mita, Yoshitomo Maeda, Kazuhisa Mitsuda, Mitsuhiro Horade, Susumu Sugiyama, Raul Riveros, Hitomi Yamaguchi, Yoshiaki Kanamori, Kohei Morishita, Kazuo Nakajima, Ryutaro Maeda, MEMS-based novel X-ray optics for future astronomical missions, 2012 International Conference on Optical MEMS and Nanophotonics, OMN 2012 2012 International Conference on Optical MEMS and Nanophotonics, OMN 2012, 10.1109/OMEMS.2012.6318815, 87-88, 2012.11, We have been developing our original MEMS-based X-ray optics for future astronomical missions. To date, we verified the focusing of optical light and X-ray for the first time. The concept and recent advances are reviewed..
29. Yuichiro Ezoe, Teppei Moriyama, Tomohiro Ogawa, Takuya Kakiuchi, Ikuyuki Mitsuishi, Kazuhisa Mitsuda, Tatsuhiko Aoki, Kohei Morishita, Kazuo Nakajima, Large-aperture focusing of x rays with micropore optics using dry etching of silicon wafers, Optics Letters, 10.1364/OL.37.000779, 37, 5, 779-781, 2012.03.
30. I. Mitsuishi, Y. Ezoe, K. Ishizu, T. Moriyama, M. Mita, N. Y. Yamasaki, K. Mitsuda, M. Horade, S. Sugiyama, R. Riveros, H. Yamaguchi, Y. Kanamori, Kohei Morishita, K. Nakajima, R. Maeda, Novel ultra-lightweight and High-resolution MEMS X-ray optics for space astronomy, 2011 16th International Solid-State Sensors, Actuators and Microsystems Conference, TRANSDUCERS'11 2011 16th International Solid-State Sensors, Actuators and Microsystems Conference, TRANSDUCERS'11, 10.1109/TRANSDUCERS.2011.5969715, 566-569, 2011.09, We report a novel micromachined X-ray optics using DRIE formed micro mirrors for future space astronomical missions. We have fabricated a test optics and measured its imaging quality using X-rays. We have successfully verified the X-ray focusing using this type of optics for the first time in the world. The obtained angular resolution was about 20 arcmin and 3.1 mm. This result was consistent with the expected angular resolution based on the surface roughness of the DRIE formed side walls. To achieve a better angular resolution, we will condition annealing and magnetic field assisted finishing processes..
31. J. J. Sha, S. Ochiai, H. Okuda, S. Iwamoto, Kohei Morishita, Y. Waku, N. Nakagawa, A. Mitani, T. Ishikawa, M. Sato, Effects of microstructure on residual stresses in DSE Al2O3/YAG ceramic composite by experimental and numerical investigations, Ceramic and Polymer Matrix Composites Properties, Performance and Applications, 87-118, 2010.12, The effect of microstructure on the residual stresses in directionally solidified eutectic (DSE) Al2O3/Y3Al5O12 (YAG) ceramic composite were investigated by X-ray diffraction technique and finite element method (FEM). In the X-ray stress measurement, the YAG skeleton derived from the Al2O3/YAG composite by dioxidation of the Al2O3 phase was used as a reference specimen without thermally-induced stress, and the X-ray stress measurements with CuKα1 irradiation were performed on the two faces of a cubic specimen, namely, the faces parallel and perpendicular to the solidification direction, respectively. On the other hand, a numerical analysis using finite element method (FEM) which represents the actual microstructure features of the experimental specimen was carried out in different local regions with different morphologies to reveal the effect of microstructure on the distribution of residual stress in the composite. The distributions of residual stresses in both constituting phases were mapped by FEM calculation. Meanwhile, the mapping of residual stress indicated that the distribution of residual stress in the interior of each phase was not homogeneous being dependent on the solidification direction and local morphologies of constituting phases such as curvature of interfaces, array and volume fraction. The experimentally measured residual stresses were accounted for by the FEM analysis..
32. K. Kutsukake, T. Abe, N. Usami, K. Fujiwara, Kohei Morishita, K. Nakajima, Formation mechanism of twin boundaries in silicon multicrystals during crystal growth, 35th IEEE Photovoltaic Specialists Conference, PVSC 2010 Program - 35th IEEE Photovoltaic Specialists Conference, PVSC 2010, 10.1109/PVSC.2010.5617200, 810-811, 2010.12, Formation mechanism of twin boundaries in silicon multicrystals during crystal growth was investigated by using in-situ observation of the growth interface and following characterization of microstructures of the grown crystal. The crystal growth experiments were performed under two different conditions, which are constant cooling rate and intentionally changed crystal growth rate. Under both the conditions, twin boundaries were more frequently formed when the growth rate drastically increased than when the growth rate was constant in high value. Based on these new findings, formation mechanism of twin boundaries are discussed in terms of formation energy..
33. Yuichiro Ezoe, Ikuyuki Mitsuishi, Kensuke Ishizu, Teppei Moriyama, Kazuhisa Mitsuda, Noriko Y. Yamasaki, Takaya Ohashi, Mitsuhiro Horade, Susumu Sugiyama, Raul E. Riveros, Taylor Boggs, Hitomi Yamaguchi, Yoshiaki Kanamori, Nicholas T. Gabriel, Joseph J. Talghader, Kohei Morishita, Kazuo Nakajima, Ryutaro Maeda, MEMS-based X-ray optics for future astronomical missions, 2010 International Conference on Optical MEMS and Nanophotonics, Optical MEMS and Nanophotonics 2010 2010 International Conference on Optical MEMS and Nanophotonics, Optical MEMS and Nanophotonics 2010, 10.1109/OMEMS.2010.5672121, 191-192, 2010.12, X-ray optics based on MEMS technologies can provide future astronomical missions with ultra light-weight and high-performance optical systems. Curvilinear micropores vertical to a thin wafer are made by using DRIE (Deep Reactive Ion Etching) or X-ray LIGA. The side walls are smoothed by using magnetic field assisted finishing and annealing technologies in order that the walls can reflect X-rays. Two or four such wafers are bent to spherical shapes with different curvature of radii and stacked, to focus parallel X-rays from astronomical objects by multiple reflections. In this paper, the concept and recent advances of the MEMS X-ray optics are reviewed..
34. Ikuyuki Mitsuishi, Yuichiro Ezoe, Kensuke Ishizu, Teppei Moriyama, Yoshitomo Maeda, Takayuki Hayashi, Takuro Sato, Makoto Mita, N. Y. Yamasaki, K. Mitsuda, Mitsuhiro Horade, Susumu Sugiyama, Raul E. Riveros, Taylor Boggs, Hitomi Yamaguchi, Yoshiaki Kanamori, Kohei Morishita, Kazuo Nakajima, Ryutaro Maeda, X-ray imaging test for a single-stage MEMS X-ray optical system, 2010 International Conference on Optical MEMS and Nanophotonics, Optical MEMS and Nanophotonics 2010 2010 International Conference on Optical MEMS and Nanophotonics, Optical MEMS and Nanophotonics 2010, 10.1109/OMEMS.2010.5672138, 151-152, 2010.12, An X-ray imaging test for an X-ray optical system based on MEMS technologies was conducted at the ISAS 30 m beamline. An X-ray reflection and focusing were successfully verified at Al Kα 1.49 keV for the first time. The image quality estimated as a half power diameter was ∼20 arcmin. This was consistent with the angular resolution estimated from the surface roughness of 200 nm rms at 100 ìm scale. In this paper, the experimental setup and the result of X-ray imaging analysis are reported..
35. S. Ochiai, Kohei Morishita, S. Kuboshima, H. Okuda, T. Inoue, T. Ishikawa, M. Sato, Fracture toughness estimation of small diameter fibers, 12th International Conference on Fracture 2009, ICF-12 12th International Conference on Fracture 2009, ICF-12, 2, 1548-1554, 2009.12, In estimation of fracture toughness of small diameter (5∼15 um) fibers, the difficulty arises from the small physical dimensions, due to which proper method to introduce small notches is limited. Also, the diameter is usually different among the fiber test specimens. Accordingly, a proper method to introduce an artificial notch in the specimens is required, and the fracture toughness value shall be estimated for each fiber test specimen. In the present work, as the method to introduce artificial notch, focused-ion (Ga +)-beam was used, with which a mode I type straight-fronted edge notch was introduced in the fiber specimens. The notch-tip radius was around 25 nm. The fracture strength (af), notch depth (a) and fiber diameter (D) were measured for each fiber test specimen. The correction factor Y for the present straight-fronted edge notch, which is a function of a relative notch depth a/D was calculated by the finite element method. The fracture toughness KIc was estimated for each fiber specimen by substituting the measured values of a, D and σf and calculated value of Y into KIc=Yσf (πa)1/2. Such an approach was applied to the polycrystalline SiC fiber (Tyranno-SA® with diameter 5∼11μm) and amorphous SiC fiber (Tyranno-ZMI® with diameter 9∼14 um). The fracture toughness values of the polycrystalline and amorphous SiC fibers were estimated to be 2.7±0.4 and 1.8±0.3 Mpa *m1/2, respectively. The fracture toughness value was almost independent of the fiber diameter and notch depth in both fibers..
36. I. Mitsuishi, Y. Ezoe, U. Takagi, T. Hayashi, T. Sato, Kohei Morishita, K. Nakajima, N. Y. Yamasaki, K. Mitsuda, Optical image analysis of the novel ultra-lightweight and high-resolution MEMS X-ray optics, 2009 IEEE/LEOS International Conference on Optical MEMS and Nanophotonics, OPTMEMS 2009 2009 IEEE/LEOS International Conference on Optical MEMS and Nanophotonics, OPTMEMS 2009, 10.1109/OMEMS.2009.5338574, 123-124, 2009.12, We invented novel ultra-lightweight and high-resolution MEMS (Micro Electro Mechanical Systems) X-ray optics for space X-ray telescopes. As a first step of R&D, we conducted optical image analysis of a spherically-shaped test optic with a radius of curvature of 1000 mm. Focusing of the parallel light was verified with our optic for the first time..
37. Kazuo Nakajima, Kozo Fujiwara, Kohei Morishita, Systematic studies of Si and Ge hemispherical concave wafers prepared by plastic deformation, Journal of Crystal Growth, 10.1016/j.jcrysgro.2009.08.021, 311, 21, 4587-4592, 2009.10, Si and Ge hemispherical concave wafers can be prepared by plastic deformation using Si and Ge single- and polycrystal wafers. Deformation regions in which such Si and Ge hemispherical wafers can be obtained by high-temperature plastic deformation were systematically studied. The deformation regions in which well-shaped Si concave wafers can be obtained were studied for (1 0 0), (1 1 1), and polycrystal Si. It was found that Si (1 1 1) crystal wafers can be more easily deformed to a perfect hemispherical shape than (1 0 0) wafers because of the crystallographic symmetry. Si hemispherical wafers with a small radius of 25 mm can be perfectly deformed when 0.5-mm-thick Si (1 1 1) crystal wafers are used. Ge hemispherical wafers with a radius of 100 mm can be perfectly deformed when 0.5-mm-thick Ge crystal wafers are used. Ge hemispherical concave wafers with a perfect shape can be more easily obtained using Ge (1 1 1) wafers than (1 0 0) wafers. According to these results, the deformation behavior of Ge wafers is very similar to that of Si wafers at a normalized pressing temperature. As both the radius and the load on Si and Ge hemispherical wafers increase, thicker Si and Ge wafers can be used to obtain hemispherical wafers with a perfect shape. The dislocation density in the shaped wafers markedly decreases as the pressing temperature and hemisphere radius increase. Thus, it is suggested that Si and Ge shaped wafers are of sufficient quality and have the high potential for use as several types of lens such as those in Si and Ge X-ray monochromators..
38. Yuichiro Ezoe, Takayuki Shirata, Takaya Ohashi, Manabu Ishida, Kazuhisa Mitsuda, Kozo Fujiwara, Kohei Morishita, Kazuo Nakajima, Development of high-resolution and light-weight x-ray optics with deformed silicon wafers, EUV and X-Ray Optics: Synergy between Laboratory and Space EUV and X-Ray Optics Synergy between Laboratory and Space, 10.1117/12.823932, 7360, 2009.09, We report on our development of hot plastic deformation of silicon wafer for high-resolution and light-weight X-ray optics. The highly polished silicon wafer with an excellent flat surface is a promising candidate for the next generation space X-ray telescopes. Deformation accuracy and stability, especially if elastic deformation is used, are issues. The hot plastic deformation of the silicon wafer allows us 3-dimensional shaping without spring back after the deformation. As a first step of R & D, we conducted the hot plastic deformation of 4-inch silicon (111) wafers with a thickness of 300 μm by using hemispherical dies with a curvature radius of 1000 mm. The deformed wafer kept good surface quality but showed a slightly large curvature of 1030 mm. We measured the X-ray reflectivity of the deformed wafer at Al K α 1.49 keV. For the first time, we detected the total X-ray reflection on the deformed wafer. Estimated rms surface roughness was 0-1 nm and no significant degradation from the bare silicon wafers was seen..
39. Ikuyuki Mitsuishi, Yuichiro Ezoe, Utako Takagi, Makoto Mita, Raul Riveros, Hitomi Yamaguchi, Fumiki Kato, Susumu Sugiyama, Kouzou Fujiwara, Kohei Morishita, Kazuo Nakajima, Shinya Fujihira, Yoshiaki Kanamori, Noriko Y. Yamasaki, Kazuhisa Mitsuda, Ryutaro Maeda, Novel ultra-lightweight and high-resolution MEMS X-ray optics, EUV and X-Ray Optics: Synergy between Laboratory and Space EUV and X-Ray Optics Synergy between Laboratory and Space, 10.1117/12.823933, 7360, 2009.09, We have been developing ultra light-weight X-ray optics using MEMS (Micro Electro Mechanical Systems) technologies.We utilized crystal planes after anisotropic wet etching of silicon (110) wafers as X-ray mirrors and succeeded in X-ray reflection and imaging. Since we can etch tiny pores in thin wafers, this type of optics can be the lightest X-ray telescope. However, because the crystal planes are alinged in certain directions, we must approximate ideal optical surfaces with flat planes, which limits angular resolution of the optics on the order of arcmin. In order to overcome this issue, we propose novel X-ray optics based on a combination of five recently developed MEMS technologies, namely silicon dry etching, X-ray LIGA, silicon hydrogen anneal, magnetic fluid assisted polishing and hot plastic deformation of silicon. In this paper, we describe this new method and report on our development of X-ray mirrors fabricated by these technologies and X-ray reflection experiments of two types of MEMS X-ray mirrors made of silicon and nickel. For the first time, X-ray reflections on these mirrors were detected in the angular response measurements. Compared to model calculations, surface roughness of the silicon and nickel mirrors were estimated to be 5 nm and 3 nm, respectively..
40. Yuichiro Ezoe, Takayuki Shirata, Ikuyuki Mitsuishi, Manabu Ishida, Kazuhisa Mitsuda, Kohei Morishita, Kazuo Nakajima, Shaped silicon wafers obtained by hot plastic deformation
Performance evaluation for future astronomical x-ray telescopes, Applied Optics, 10.1364/AO.48.003830, 48, 19, 3830-3838, 2009.07, In order to develop lightweight and high angular resolution x-ray mirrors, we have investigated hot plastic deformation of 4 in. silicon (111) wafers. A sample wafer was deformed using hemispherical dies with a curvature radius of 1000mm. The measured radius of the deformed wafer was 1030mm, suggesting that further conditioning is indispensable for better shaping. For the first time to our knowledge, x-ray reflection on a deformed wafer was detected at Al K α 1:49keV. An estimated surface roughness of <1nm from the x-ray reflection profile was comparable to that of a bare silicon wafer without deformation. Hence, no significant degradation of the microroughness was seen..
41. M. S. Aly, Y. Fukasawa, Kohei Morishita, H. Okuda, S. Ochiai, K. Kato, K. Kita, K. Honma, Determination of the stress acting on the individual cells of open-cell stainless steel foams, 5th International Conference on Porous Metals and Metallic Foams, MetFoam 2007 MetFoam 2007 - Proceedings of the 5th International Conference on Porous Metals and Metallic Foams, 157-160, 2008.11, A series of tensile tests was performed on powder metallurgical processed open cell stainless steel foams with different densities and average pore sizes of 150, 300 and 600 μm, respectively to examine the effect of porosity as well as the cell wall thickness on the foams tensile mechanical properties. Moreover, a state of the art strain measuring device, which is not in touch with the sample's surface, was used to determine the strain and the other tensile properties accurately. The function of this device is based on the usage of a laser (CCD) camera. The laser (CCD) camera monitors the dimensional changes during loading by detecting the positional changes of special markers clipped on the sample's surface representing the gauge length. Thus we can avoid the constraint effects caused by using the conventional extensometers of contact type. By using the Weibull distribution function, the average fracture stress of the individual foam cells was statistically measured. The results showed a strong dependence of foams' mechanical properties on their porosity and thickness of their cell walls. Foams with small pores displayed higher mechanical properties than foams with larger ones. Foams with thick cell walls failed at larger stresses than foams having thin cell walls or struts..
42. Hiroshi Okuda, Kazuo Nakajima, Kozo Fujiwara, Kohei Morishita, Shojiro Ochiai, Point-focusing monochromator crystal realized by hot plastic deformation of a Ge wafer, Journal of Applied Crystallography, 10.1107/S0021889808016282, 41, 4, 798-799, 2008.07, Pre-polished Ge(111) single-crystal wafers were deformed just below the melting temperature to prepare point-focusing Johansson monochromator crystals. The (111) lattice plane had curvature 2R in the focusing plane and R perpendicular to it, with a hemispherical inner surface with a radius of R = 600 mm. By using Cu Kα radiation, the diverging X-ray beam was focused onto a small spot..
43. Hiroshi Okuda, Jae Kyong Shin, Sohei Iwamoto, Kohei Morishita, Yasuhiro Mukai, Hiroshi Matsubayashi, Shojiro Ochiai, Alex Otto, Edward J. Harley, Alexis P. Malozemoff, Masugu Sato, Assessment of strain of Bi2223 filaments in bent Ag-sheathed superconducting composites by synchrotron radiation, Scripta Materialia, 10.1016/j.scriptamat.2007.12.003, 58, 8, 687-690, 2008.04, The bending strains of Bi2223 filaments in Ag-sheathed superconducting composite tapes at the Bi2223/Ag sheath boundary have been evaluated by X-ray diffraction utilizing synchrotron radiation at room temperature. The strain in the Bi2223 filaments was found to be asymmetric between the sides under tension and compression, suggesting that the fracture strain is also asymmetric between tension and compression. The outermost part of the Bi2223 filaments is at the multiple fracture stage in the tension side..
44. J. J. Sha, S. Ochiai, S. Iwamoto, Kohei Morishita, H. Okuda, Y. Waku, N. Nakagawa, A. Mitani, M. Sato, T. Ishikawa, Experimental and numerical investigations on the residual strains of YAG phase in directionally solidified eutectic Al2O3/YAG ceramic composite, 16th International Conference on Composite Materials, ICCM-16 - "A Giant Step Towards Environmental Awareness: From Green Composites to Aerospace" Proceedings of the 16th International Conference on Composite Materials, ICCM-16 - "A Giant Step Towards Environmental Awareness From Green Composites to Aerospace", 2007.12, Because of the attractiveness in superior oxidation resistance, high stability of the microstructure and excellent high temperature mechanical properties in oxidative environment, directionally solidified eutectic (DSE) oxide/oxide ceramic composite have been considered as one of the most potential structural materials for advanced energy generation systems and space propulsion systems. However, due to the mismatch in coefficient of thermal expansion (CTE), the residual strains are always generated from the fabrication process and thermal cycling in use, which would have an influence on properties of DSE ceramic composites. Therefore, the investigation of magnitude, state and distribution of residual strains in oxide ceramic composites is a crucial work. In this study, the residual strains of YAG phase in directionally solidified eutectic Al2O3/Y3Al5O12 (YAG) ceramic composite were estimated by X-ray diffraction technique and finite element method (FEM). In the X-ray strain measurement, the YAG skeleton specimen derived from the composite by removing the Al2O3 phase was used as un-strained reference specimen. The X-ray strain measurements with CuKα irradiation were performed on the two faces: parallel and perpendicular to the solidification direction. The principal residual strains measured from the family of 888 diffraction in YAG phase indicated that YAG phase in composite is in compression. The measured residual strains are varied from -2.19±0.32 (×10-4) to -4.22±0.08 (×10-4) on faces parallel and perpendicular to the solidification direction respectively. The experimental results were accounted for by the FEM simulation..
45. S. Ochiai, H. Rokkaku, Kohei Morishita, J. K. Shin, S. Iwamoto, H. Okuda, M. Hojo, K. Osamura, M. Sato, A. Otto, E. Harley, A. Malozemoff, Thermally induced residual strain accumulation in Bi2223/Ag/Ag alloy composite superconductor, Superconductor Science and Technology, 10.1088/0953-2048/20/3/015, 20, 3, 202-210, 2007.03, A method to estimate the thermally induced residual strain accumulation under varying temperature in a Bi2223/Ag/Ag alloy composite superconductor was presented, in which the mechanical property values measured from the stress-strain curves of the samples with different residual strain states, the residual strain value of Bi2223 filaments in the composite tape at room temperature measured by x-ray diffraction and the reported coefficients of thermal expansion of the constituents (Bi2223, Ag and Ag alloy) in the relevant temperature range were incorporated. This method was applied to estimate the change of the residual strain of all constituents of the high critical current type composite tape fabricated by American Superconductor Corporation as a function of temperature. The residual strain value at 77K estimated by this method and the reported fracture strain of Bi2223 filaments accounted well for the measured strain tolerance of the critical current at 77K..
46. Kohei Morishita, Tasuku Matsumoto, Shojiro Ochiai, Hiroshi Okuda, Toshihiro Ishikawa, Mitsuhiko Sato, Degradation mechanism of amorphous silicon carbide fiber due to air-exposure at high temperatures, Materials Transactions, 10.2320/matertrans.48.111, 48, 2, 111-116, 2007.02, The degradation mechanism of the amorphous silicon carbide fiber, Tyranno-ZMI®, exposed in air at 1173-1873 K for 20ks were studied. The average strength of the bare fiber, which was prepared by etching away the oxidation layer on the fiber surface, decreased with increasing exposure temperature, especially when exposed at the temperature higher than 1673 K. The measurement of the crystallite size of β-SiC in the fiber with Sherrer method revealed that coarsening of the crystalline occurred in the fiber exposed at the temperatures higher than 1773K. The scanning electron microscope observation of the fiber surface showed that the many defects formed on the fiber surface. By introducing an artificial notch directly into the fiber specimens using a focused-ion(Ga+)-beam, the fracture toughness values of the as-supplied fiber and of the fiber exposed at 1673 and 1773 K were determined to be 1.8 ± 0.3, 1.9 ± 0.4 and 1.3 ± 0.4 MPa√m, respectively. Based on these results, the reason for the degradation of the fiber was attributed to the extension of the surface defect which was enhanced by the reduction in fracture toughness due to coarsening or the β-SiC crystalline..
47. H. Nakayama, Kohei Morishita, S. Ochiai, T. Sekigawa, K. Aoyama, A. Ikawa, Evaluation of environmental barrier coatings for SiC/SiC composites, Advanced Ceramic Coatings and Interfaces - 30th International Conference on Advanced Ceramics and Composites Advanced Ceramic Coatings and Interfaces - A Collection of Papers Presented at the 30th International Conference on Advanced Ceramics and Composites, 161-170, 2006.12, SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of the most promising materials for high temperature structural applications such as power generation and propulsion systems. SiC/SiC composites are, however, susceptible to accelerated attacks in water vapor environments through oxidation and volatilization reaction, so Environmental Barrier Coatings (EBCs) are indispensable. We have investigated some oxides and rare-earth silicates as topcoat candidates for EBCs. Topcoat materials must be stable in the high-water-vapor pressurized environments at high temperatures. Also, it is important that the thermal expansion coefficient of topcoat materials is similar to that of SiC/SiC composites. In this study, rare-earth silicates such as lutetium silicates, 8YSZ and ZrO2-SiO2 were selected as topcoat candidates. They were exposed in the water containing atmosphere at a temperature 1673K for 100 hr under a total pressure 0.96 MPa and under an atmospheric pressure. Mass changes, structure of crystals and microstructures were investigated after the exposure experiments in order to evaluate the thermal stability of those materials. After this evaluation, lutetium silicates were considered to be promising for topcoat materials. Lutetium silicates were coated on SiC/SiC composites, and their fracture toughness and microstructures were investigated and reported in this paper..
48. Hirosuke Rokkakku, Kohei Morishita, Dan Doko, Tomonobu Ishida, Shojiro Ochiai, Hiroshi Okuda, Kozo Osamura, Monte Carlo-shear lag simulation for fracture behavior of Bi2223 superconducting monofilament and its influence on critical current, Physica C: Superconductivity and its Applications, 10.1016/j.physc.2006.05.018, 445-448, 1-2, 756-761, 2006.10, The present work attempted to describe the tensile stress-strain behavior of the Bi2223 filaments in the composite tape and its influence on critical current by means of a Monte Carlo-shear lag simulation method, in which the distribution of strength within the filament was given by a Monte Carlo method and the stress distribution in the filament was calculated with a shear-lag analysis. With this approach, (a) the stress-strain curve of the Bi2223 filament in the composite tape measured by X-ray method, (b) the observed fracture morphology of the filament in which transverse (perpendicular to tensile axis) and longitudinal (parallel to tensile axis) fractures take place consecutively, and (c) the measured change of critical current with increasing applied tensile strain, could be described fairly well..
49. Kohei Morishita, Shojiro Ochiai, Hiroshi Okuda, Toshihiro Inshikawa, Mitsuhiko Sato, Tadanobu Inoue, Fracture toughness of a crystalline silicon carbide fiber (tyranno-SA3®), Journal of the American Ceramic Society, 10.1111/j.1551-2916.2006.01116.x, 89, 8, 2571-2576, 2006.08, The fracture toughness of a crystalline silicon carbide fiber, Tyranno-SA®, was determined by introducing an artificial notch directly into the fiber test specimens using a focused-ion (Ga +) beam. The relation between fracture strength and notch size was determined experimentally, while the relation among the notch size, fiber diameter, applied stress at infinity, and stress intensity factor was analyzed by the finite element method. By applying the analyzed result to the experimental results, the fracture toughness of the fiber was determined to be 2.7 ±0.4 MPa√m..
50. H. Nakayama, Kohei Morishita, S. Ochiai, T. Sekigawa, K. Aoyama, T. Oi, M. Yamamoto, K. Okamura, M. Sato, Quest and evaluation of topcoat materials for environmental barrier coatings of SiC/SiC composites, Key Engineering Materials, 317-318, 549-552, 2006.07, SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of the most promising materials for high temperature structural applications such as power generation and propulsion systems. SiC/SiC composites are, however, susceptible to accelerated attacks in water vapor environments through oxidation and volatilization reaction. For protection from such attacks, Environmental Barrier Coatings (EBCs) are indispensable. We have investigated some oxides and rare-earth silicates as topcoat candidate materials for EBCs. Topcoat materials must be stable in the high-water-vapor pressurized environments at high temperatures. Also, it is important that the thermal expansion coefficient of topcoat materials is similar to that of the SiC/SiC composites. In this study, first, zirconium oxides, lutetium silicates and yttrium silicates were selected as topcoat candidate materials. They were exposed in a water-containing atmosphere at a temperature of 1673 K for 100 h under a total pressure 0.96 MPa. Mass changes, structure of crystals and microstructures were investigated after the exposure experiments in order to evaluate the thermal stability of these materials. After their estimation, lutetium silicates were considered to be promising for topcoat materials. Then, lutetium silicates were coated as the topcoat of an EBC system on SiC/SiC composites, and their fracture toughness and microstructures were investigated after exposure to an oxidizing atmosphere. The evaluation results of the topcoat materials are reported in this paper..
51. H. Okuda, H. Rokkaku, Kohei Morishita, J. K. Shin, S. Iwamoto, S. Ochiai, M. Sato, K. Osamura, A. Otto, E. J. Harley, A. Malozemoff, In situ synchrotron-radiation measurements of axial strain in laminated Bi2223 superconducting composite tapes at room temperature, Scripta Materialia, 10.1016/j.scriptamat.2006.06.027, 55, 8, 691-694, 2006.01, Axial strain of Bi2223 superconducting filaments in Ag-sheathed superconducting composites reinforced by stainless steel lamination has been evaluated by in situ synchrotron-radiation diffraction. The Bi2223 filaments in the laminated composites were under 0.11% of compressive residual strain, whereas the residual strain of filaments in the composite after removing stainless steel layers was only 0.02% in compression. Under large tensile load, the composite showed a clear multiple fracture with an almost constant filament strain of about 0.11% in tension..
52. S. Ochiai, T. Ishida, D. Doko, Kohei Morishita, H. Okuda, S. S. Oh, D. W. Ha, M. Hojo, M. Tanaka, M. Sugano, K. Osamura, A Monte Carlo-shear lag simulation of tensile fracture behaviour of Bi2223 filament, Superconductor Science and Technology, 10.1088/0953-2048/18/12/004, 18, 12, 2005.12, The damage evolution in Bi2223 filaments and its influence on critical current was described by a Monte Carlo-shear lag simulation method. The experimentally observed zigzag crack propagation across aligned Bi2223 grains under tensile strain was effectively modelled by including transverse and longitudinal failure modes for individual grains. From the simulated stress-strain curve, the survival parameter (slope of the stress-strain curve normalized with respect to the original Young's modulus) was estimated with increasing applied strain. With this parameter combined with the strain sensitivity of the critical current, the measured change of critical current of the composite tape with applied strain could be described well..
53. Kohei Morishita, M. Hojo, H. Okuda, S. Ochiai, M. Sato, Fracture behavior of Tyranno-ZMI fiber exposed in air at 1173 ∼ 1673 K, 11th International Conference on Fracture 2005, ICF11 11th International Conference on Fracture 2005, ICF11, 788-793, 2005.12, The fracture behavior of the Tyranno-ZMI fiber exposed in air at 1173 ∼ 1673K for 2×104s was studied. The average strength of the fiber with SiO2 layer on its surface decreased and the scatter of strength increased with increasing exposure temperature. The fracture of the fiber was controlled by the propagation of the crack made by the premature fracture of the SiO2 layer into fiber. The average strength of the fiber without SiO2 layer prepared by etching away the SiO2 layer from the fiber exposed in air, decreased also with increasing exposure temperature, whereas it was far higher than that of the fiber with SiO 2 layer. The fracture toughness of the fiber, which was originally amorphous decreased with increasing exposure temperature due to the crystallization. In this way, the high temperature-exposure in air caused degradation of the fiber itself and the SiO2 layer-originated crack propagation..
54. S. Ochiai, Y. Sakai, K. Sato, T. Ueda, Kohei Morishita, H. Okuda, M. Tanaka, M. Hojo, Y. Waku, N. Nakagawa, S. Sakata, A. Mitani, T. Takahashi, Temperature- and displacement speed-dependence of notched strength at 1873-2023K of Alumina/YAG composite, 11th International Conference on Fracture 2005, ICF11 11th International Conference on Fracture 2005, ICF11, 2, 816-821, 2005.12, The temperature- and displacement speed-dependence of unnotched and notched strengths at 1873-2023 K of directionally solidified eutectic Al 2O3/YAG (Yttrium-Aluminum Garnet with the composition of Y3Al5O12) composite was investigated experimentally by tensile test and analytically by the finite element method. The notched specimens fractured in a brittle manner at low temperatures and at high displacement speeds, but in a ductile manner at high temperatures and at low displacement speeds. The notched strength increased, reaching maximum, and decreased with increasing temperature under a given displacement speed and with decreasing displacement speed under a given temperature. For the finite element analysis, the temperature dependence of the Young's modulus was measured to be 345-0.0361T (T: temperature, K) GPa and the temperature- and strain rate-dependence of the flows stress in the plastic deformation stage of unnotched specimens was estimated as {(strain rate)/(1.23×10 5)}1/5 exp{1.81×104/T} from the reported data. From the comparison of the experimental- with analytical results, the increase in the notched strength with increasing temperature and decreasing displacement speed up to the maximum notched-strength value was accounted for by the increase in plastic zone size ahead of the notch. Also the decrease in notched strength with further increasing temperature and decreasing displacement speed was accounted for by the loss of the stress carrying capacity of the yielded ligament due to the softening..
55. Kohei Morishita, H. Tanaka, S. Kimura, H. Okuda, S. Ochiai, M. Tanaka, M. Hojo, H. Nakayama, M. Sato, Fracture behavior and its shear lag - Monte Carlo simulation of SiC/SiC composite exposed in air at high temperatures, PRICM 5: The Fifth Pacific Rim International Conference on Advanced Materials and Processing Materials Science Forum, 475-479, II, 1101-1104, 2005.04, It has been reported that SiC/SiC composite has high strength and toughness, but is degraded when exposed in air at high temperatures due to the propagation of the crack made by the premature fracture of the SiO2 layer. The present work aimed to describe such a behavior with a computer simulation. For this aim, the shear lag - Monte Carlo simulation method was applied. The variation of strength of the composite as a function of thickness of the SiO2 layer and change of fracture morphology with progressing oxidation could be reproduced satisfactorily by this method..
56. S. Ochiai, Y. Sakai, K. Sato, T. Ueda, Kohei Morishita, H. Okuda, M. Tanaka, M. Hojo, Y. Waku, N. Nakagawa, S. Sakata, A. Mitani, T. Takahashi, Tensile and bending behavior of melt growth Al2O3/YAG composite at ultra high temperatures (1773-2023K), PRICM 5: The Fifth Pacific Rim International Conference on Advanced Materials and Processing Materials Science Forum, 475-479, II, 1091-1096, 2005.04, The deformation and fracture behavior at 1773-2023K of the unidirectionally solidified eutectic Al2O3/YAG (Yttrium-Aluminum Garnet with the composition of Y3A15O13) ceramic composite was investigated. The stress-stain curve and strength of unnotched and notched specimens, measured by bending and tensile tests, showed that (a) both unnotched and notched specimens fractured in a brittle manner at low temperatures and at high displacement speeds, but in a ductile manner at high temperatures and at low displacement speeds, and (b) the notched strength increased, reaching maximum, and decreased with increasing temperature and decreasing displacement speed. The increase in the notched strength with increasing temperature and decreasing displacement speed up to the maximum value was accounted for by the increase in plastic zone size ahead of the notch, and the decrease with further increasing temperature and decreasing displacement speed by the loss of the stress carrying capacity of the yielded ligament, based on the finite element analysis..
57. Kohei Morishita, S. Ochiai, H. Okuda, H. Nakayama, M. Sato, An attempt to recover strength of oxidized sic/sic composite by means of a deoxidation treatment, Composite Interfaces, 10.1163/156855405774984057, 12, 8-9, 853-858, 2005.01, An attempt was made to recover the strength of a PIP-processed SiC/SiC composite exposed in air at high temperatures by means of the deoxidization treatment with CO gas. By deoxidation treatment at 1723 and 1823 K for the composite specimen oxidized at 1573 K, the strength of deoxidized specimens became higher by around 4 and 1.5 times than that of oxidized one, respectively..
58. Kohei Morishita, S. Ochiai, H. Okuda, H. Nakayama, M. Sato, Simulation of fracture behavior of unidirectional sic/sic composite exposed in air at 823–1673 k, Composite Interfaces, 10.1163/156855405774984048, 12, 8-9, 693-706, 2005.01, It is known that SiC/SiC composite has high strength and toughness, but is degraded when exposed in air at high temperatures due to the propagation of the crack made by the premature fracture of the SiO2 layer. It is also known that the fracture morphology of this composite varies from fiber pull-out type to non pull-out type with progressing oxidation. The present work aimed to describe such a behavior with a computer-aided Monte Carlo shear lag simulation method. The experimentally measured variation of strength of the unidirectional composite as a function of thickness of the SiO2 layer and also as a function of exposure time and temperature, and the experimentally observed variation of fracture morphology with progressing oxidation, could be reproduced satisfactorily by this method..
59. Hiroshi Okuda, Kohei Morishita, Shojiro Ochiai, Dan Dokoh, Motohide Matsui, Hiroyuki Fujimoto, Masugu Sato, In situ strain measurements of Bi2223 superconducting filaments in multifilamentary Ag-sheathed Bi2223 superconducting tapes, Physica C: Superconductivity and its applications, 10.1016/j.physc.2004.05.019, 411, 3-4, 114-119, 2004.09, In situ strain measurement was made for Bi2223 superconducting filaments in Ag-sheathed Bi2223 composite tapes utilizing synchrotron radiation. The residual strain was 0.05 ± 0.01% in compression. A small deviation from linear relationship between strain of the Bi2223 filaments and the average strain of the sample was found, meaning a decrease in elastic modulus of the filament even below the irreversible strain..
60. S. Ochiai, H. Okuda, S. Kimura, Kohei Morishita, M. Tanaka, M. Hojo, M. Sato, Improvement of Young's modulus and tensile strength of polymer impregnation and pyrolysis processed SiC/SiC composite by improved continuity of matrix, Journal of Materials Research, 10.1557/JMR.2004.0290, 19, 8, 2377-2388, 2004.08, Influences of the continuity of the matrix on Young's modulus and tensile strength of unidirectional SiC/SiC mini-composite prepared by the polymer impregnation and pyrolysis method were studied experimentally by observation of appearance of matrix and tensile test and analytically by a shear lag-Monte Carlo simulation. The continuity of the matrix was improved by the addition of particles such as ZrSiO4, barium magnesium aluminosilicate, and Pyrex (borosilicate glass) into the matrix. The improved continuity of the matrix led to the increase in stress carrying capacity of the matrix and therefore to the increase in Young's modulus and tensile strength of the composite. Such a correlation between the continuity of the matrix and the property of the composite was verified numerically by the shear lag-Monte Carlo simulation..
61. S. Ochiai, H. Okuda, S. Kimura, Kohei Morishita, M. Tanaka, M. Hojo, M. Sato, Improvement of Young's modulus and tensile strength of polymer impregnation and pyrolysis processed SiC/SiC composite by improved continuity of matrix, Journal of Materials Research, 10.1557/JMR.2004.0290, 19, 8, 2377-2388, 2004.08, Influences of the continuity of the matrix on Young's modulus and tensile strength of unidirectional SiC/SiC mini-composite prepared by the polymer impregnation and pyrolysis method were studied experimentally by observation of appearance of matrix and tensile test and analytically by a shear lag-Monte Carlo simulation. The continuity of the matrix was improved by the addition of particles such as ZrSiO4, barium magnesium aluminosilicate, and Pyrex (borosilicate glass) into the matrix. The improved continuity of the matrix led to the increase in stress carrying capacity of the matrix and therefore to the increase in Young's modulus and tensile strength of the composite. Such a correlation between the continuity of the matrix and the property of the composite was verified numerically by the shear lag-Monte Carlo simulation..
62. S. Ochiai, S. Kimura, H. Tanaka, M. Tanaka, M. Hojo, Kohei Morishita, H. Okuda, H. Nakayama, M. Tamura, K. Shibata, M. Sato, Degradation of SiC/SiC composite due to exposure at high temperatures in vacuum in comparison with that in air, Composites Part A: Applied Science and Manufacturing, 10.1016/j.compositesa.2003.09.006, 35, 1, 33-40, 2004.01, Room temperature residual strength of the SiC/SiC composite exposed in vacuum at high temperatures (823-1673 K) was studied and compared with that exposed in air. The vacuum-exposed composite showed only the fiber-pullout type fracture, and the pullout length increased with increasing exposure temperature and time, while the fractured mode of the air-exposed one changed with progressing oxidation; from the fiber-pullout type to the nonfiber-pullout one characterized by the overall fracture perpendicular to the tensile axis without fiber-pullout. The reduction in residual strength in the case of vacuum exposure was attributed mainly to the extension of the decomposition-induced defects on the fiber surface into fiber, while that in the case of air exposure mainly to the extension of the crack made by premature fracture of the SiO 2 layer into the fiber. A simple model based on the kinetics of the growth of the defects and fracture mechanics was presented to describe the variation of composite strength as a function of exposure temperature and time for the vacuum exposure, which could describe the experimental results..
63. S. Ochiai, S. Kimura, H. Tanaka, M. Tanaka, M. Hojo, Kohei Morishita, H. Okuda, H. Nakayama, M. Tamura, K. Shibata, M. Sato, Residual strength of PIP-processed SiC/SiC single-tow minicomposite exposed at high temperatures in air as a function of exposure temperature and time, Composites Part A: Applied Science and Manufacturing, 10.1016/j.compositesa.2003.09.005, 35, 1, 41-50, 2004.01.