Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Sato Yukio Last modified date:2021.06.10

Associate Professor / Material process engineering / Department of Materials Science and Engineering / Faculty of Engineering


Papers
1. Yoshihiro Kuroiwa, Sangwook Kim, Ichiro Fujii, Shintaro Ueno, Yuki Nakahira, Chikako Moriyoshi, Yukio Sato, Satoshi Wada, Piezoelectricity in perovskite-type pseudo-cubic ferroelectrics by partial ordering of off-centered
cations, Communications Materials, 10.1038/s43246-020-00072-4, 1, 2020.10.
2. Hiroki Moriwake, Rie Yokoi, Ayako Taguchi, Takafumi Ogawa, Craig A. J. Fisher, Akihide Kuwabara, Yukio Sato, Takao Shimizu, Yosuke Hamasaki, Hiroshi Takashima, and Mitsuru Itoh, A computational search for wurtzitestructured ferroelectrics with low coercive voltages, APL Materials, 10.1063/5.0023626, 8, 121102, 2020.12.
3. Kaita Fukao, Yukio Sato, Ryo Teranishi, Kenji Kaneko, Scanning transmission electron microscopy simulation for multi-domain barium titanate, Journal of the Physical Society of Japan, 10.7566/JPSJ.90.034803, 90, 3, 2021.02.
4. Syota Fujinaka, Yukio Sato, Ryo Teranishi, Kenji Kaneko, Understanding of scanning-system distortions of atomic-scale scanning transmission electron microscopy images for accurate lattice parameter measurements, Journal of Materials Science, 10.1007/s10853-020-04602-w, 55, 19, 8123-8133, 2020.07, Atomic-scale scanning transmission electron microscopy (STEM) imaging has opened up the possibility of studying the local lattice parameters of crystalline materials. To ensure more accurate measurements, low-frequency distortions of the images should be properly calibrated, which requires a better understanding of their causes. Although the major possible causes are sample drift and the scanning systems of microscopes, their effects are intricate because the rates of sample drifts differ in respective measurements. In the present study, low-frequency distortions of STEM images and their dependence on scan rotations were evaluated by measuring the lattice parameters of a reference specimen, strontium titanate. The distortions due to sample drifts and the scanning system of a microscope were separately calculated and corrected using affine transformations. In the as-observed images, the length scales in the x and y directions were underestimated by 0.4–1.2% and 2.7–3.6%, respectively, with shear distortions of 0.6°–1.2°, and the magnitudes of the underestimation and shear distortions were dependent on the scan rotations. On the basis of these findings, a methodology was proposed for the correction of distortions for accurate measurement of the lattice parameters of materials..
5. Yukio Sato, Ryuki Miyauchi, Mai Aoki, Syota Fujinaka, Ryo Teranishi, Kenji Kaneko, Large Electric-Field-Induced Strain Close to the Surface in Barium Titanate Studied by Atomic-Scale In Situ Electron Microscopy, Physica Status Solidi - Rapid Research Letters, 10.1002/pssr.201900488, 14, 1, 2020.01, Measuring the lattice parameters of crystalline materials under an electric field at the nanoscale improves the understanding of how electronics operate. Herein, atomic-scale in situ electron microscopy is used to measure the lattice parameters near the surface of barium titanate under an electric field, which is varied to understand how it changes the lattice parameters. Applying an electric field along the a axis does not cause domain switching. Instead, as the electric field increases, so as well does the c-axis lattice parameter (c), whereas the a-axis lattice parameter (a) remains nearly constant. The c value increases as much as 4.5 − 7.0 pm at ±13.8 kV cm<sup>−1</sup>, which is beyond the measurement precision. The increases are localized near the surface and the c value recovers slowly, which suggests that the observed phenomena are related to surface effects and slow processes. These results cannot be explained by electrostrain, even when considering additional effects such as mechanical constraint, inclination of electric field, and surface and size effects. Instead, the results are explained using a combination of Joule heating, vacancy introduction, and surface effects..
6. Yukio Sato, Mai Aoki, Ryo Teranishi, Kenji Kaneko, Masaki Takesada, Hiroki Moriwake, Hiroshi Takashima, Yukiya Hakuta, Atomic-Scale Observation of Titanium-Ion Shifts in Barium Titanate Nanoparticles
Implications for Ferroelectric Applications, ACS Applied Nano Materials, 10.1021/acsanm.9b01221, 2, 9, 5761-5768, 2019.09, Ferroelectric nanoparticles (NPs) have attracted considerable attention owing to their size effect on the ferroelectricity and their possible application toward future electronic devices such as multilayer ceramic capacitors and ferroelectric random access memory. The ferroelectricity disappears for NPs smaller than the critical size, which has been an obstacle for the development of materials. Although the fundamental mechanisms of the size effect should be clarified to overcome this problem, the understanding has been made ambiguous by the fact that NPs of different morphologies prepared by different methods exhibit various critical sizes, which indicates that more investigations should be conducted on the appearance/disappearance of ferroelectricity in NPs. To gain insight into the appearance of ferroelectricity, atomic-scale characterizations are beneficial because the ferroelectricity is closely related to the atomistic structures. In the present study, atomic-scale scanning transmission electron microscopy (STEM) observations were conducted for a barium titanate NP prepared by a hydrothermal method, using a supercritical continuous-flow reaction system. Two STEM images were obtained with different foci: one was observed by focusing an electron probe on the top surface of the NP and the other on the middle. Different directions of titanium-ion shifts were observed near the top surface and in the middle of the NP, which could be explained by STEM image simulations using structural models with the presence of an additional region with a different titanium-ion-shift direction. The present findings imply that this NP should exhibit the ferroelectricity and contains two regions of different polarization directions..
7. Ryo Teranishi, Kazuya Hiramatsu, Syotaro Yasuyama, Tomohiro Miyajima, Yukio Sato, Kenji Kaneko, Satoshi Awaji, Akiyoshi Matsumoto, Masayoshi Inoue, Superconducting Joint of GdBa2Cu3Oy Coated Conductors by Crystallization of an Additionally Deposited Precursor Layer, IEEE Transactions on Applied Superconductivity, 10.1109/TASC.2019.2902693, 29, 5, 2019.08, A superconducting joint of REBa2Cu3Oy (REBCO) coated conductors (CCs) has been demanded strongly to fabricate long length CCs for high field magnet applications such as nuclear magnetic resonance and magnetic resonance imaging. In the previous reports of superconducting joint, specimens of REBCO CCs were jointed via melting REBCO phases or via solid state diffusion of REBCO phases. In our study, we propose a new method of joint for REBCO CCs. A precursor layer is additionally deposited on GdBCO CC by a metal organic deposition process, and then two pieces of them are stuck together face-To-face and crystallized the precursor to form 123 phase under mechanical pressure in an oxygen atmosphere. The microstructures and temperature dependence of resistance of the jointed sample are characterized by a cross-sectional transmission electron microscopy (TEM) and a four-probe method, respectively. As a result, TEM observation reveals that two CCs are jointed together without formation of secondary phases at the joint interface. Also, temperature dependence of resistance shows Tc onset and Tc zero of 93 K and 82 K, respectively. Consequently, a superconducting joint has been completed successfully. The concept of this method is combining film growth and solid-state diffusion for the additionally deposited precursor layers..
8. Minoru Ochi, Kousei Sato, Ryo Teranishi, Yukio Sato, Jun Ichi Hamada, Chikako Takushima, Toru Hara, Kenji Kaneko, Nanostructural analyses of Intra- And intergranular precipitates in high-temperature heat-treated nitrogen-added austenitic stainless steel, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.TETSU-2018-067, 105, 4, 8-15, 2019.04, More than one thousand intragranular and intergranular precipitates of nitrogen-added austenitic stainless steel, SUSXM15J1, were characterized by FIB-SEM serial-sectioning tomography, by conventional transmission electron microscopy (TEM) and by scanning transmission electron microscopy (STEM). All of intragranular precipitates were found nitrided to form dichromium nitride, Cr2N, Two types of intergranular precipitates were found Cr2N and Cr3Ni2Si(N), and some of them were grown and jointed due to the additional heat treatment during the tensile test at 1173 K. which probably contributed to its high-temperature strength..
9. Tomohiro Miyajima, Ryo Teranishi, Syotaro Yasuyama, Yukio Sato, Kenji Kaneko, Valery Petrykin, Sergey Lee, Tatsunori Okada, Satoshi Awaji, Akiyoshi Matsumoto, Influence of joint pressure on superconducting and mechanical properties for jointed GdBa2Cu3Oy coated conductors via precursor films, Japanese Journal of Applied Physics, 10.7567/1347-4065/ab0727, 58, 5, 2019.01, There has been demand to lengthen superconducting joints of REBa2Cu3Oy coated conductors to realize magnet applications. In this work, jointed samples of GdBa2Cu3Oy coated conductors were prepared by crystallizing precursor films deposited by a pulsed laser deposition method under mechanical pressures, and influence of the pressures on superconducting and mechanical properties were investigated. While areal fraction and shearing stress of the joints were increased by increasing the pressure, superconducting properties were degraded. X-ray diffraction analysis indicated that the degradation was due to the formation of a secondary phase and decrease of oxygen contents in GdBa2Cu3Oy.
10. Tomohiro Miyajima, Ryo Teranishi, Syotaro Yasuyama, Yukio Sato, Kenji Kaneko, Takuya Maeda, Minoru Ochi, Kazuya Hiramatsu, Miyuki Nakamura, Valery Petrykin, Sergey Lee, Tatsunori Okada, Satoshi Awaji, Microstructures of superconducting joint between GdBa2Cu3Oy-coated conductors via additionally deposited precursor films, Japanese Journal of Applied Physics, 10.7567/1347-4065/ab0f23, 58, 5, 2019.01, Superconducting joints have been demanded to lengthen REBa2Cu3O y -coated conductors for electrical power applications. In this study, we propose a new joint method for GdBa2Cu3O y -coated conductors via crystallizing precursor films additionally deposited on the conductors. Two GdBa2Cu3O y -coated conductors with precursor films were placed in a face-to-face manner in a furnace, then they were crystallized under mechanical pressure to joint them. A superconducting joint was achieved with a high critical temperature of 90.8 K. Microstructural observation by a transmission electron microscope revealed that there were only a few pores at the joint boundary without a secondary phase having c-axis-oriented GdBa2Cu3O y.
11. Takuya Maeda, Kenji Kaneko, Takuya Namba, Yuki Koshino, Yukio Sato, Ryo Teranishi, Yasuhiro Aruga, Structural and compositional study of precipitates in under-aged Cu-added Al-Mg-Si alloy, Scientific reports, 10.1038/s41598-018-35134-8, 8, 1, 2018.12, Atomic scale characterization of fine precipitates in an under-aged Cu added Al-Mg-Si alloy was carried out by combination of atomically-resolved annular dark-field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. Two types of precipitates were observed in the alloy. In the case of ordered β” precipitates, β” was proposed as Mg5-xAl2+xSi4 (x ≈ 1) with solute Cu atoms replacing Al site of β” precipitate. In the case of disordered precipitates, the precipitates were found to consist of β” sub-unit cells, three-fold symmetric structure without Cu atoms, Cu containing structures termed as “Cu sub-unit cluster”, and Q’ sub-unit cells. Among these structures, the morphologies of three-fold symmetric structure without Cu atoms, Cu sub-unit cluster, and Q’ sub-unit cell were almost the same, so that these structures should be the clusters of Q’ phase. Since the areal density, length and diameter of precipitates were almost equal between Cu free Al-Mg-Si alloy and Cu added Al-Mg-Si alloy, the increase of hardness by Cu addition should be due to the precipitation of Cu related precipitates, such as Cu sub-unit clusters and Q’ sub-unit cells..
12. Minoru Ochi, Kousei Sato, Ryo Teranishi, Yukio Sato, Jun Ichi Hamada, Chikako Takushima, Toru Hara, Kenji Kaneko, Nanostructural analyses of intra- and intergranular precipitates in high-temperature heat-treated nitrogen-added austenitic stainless steel, isij international, 10.2355/isijinternational.ISIJINT-2017-756, 58, 8, 1459-1466, 2018.08, More than one thousand intragranular and intergranular precipitates of nitrogen-added austenitic stainless steel, SUSXM15J1, were characterized by FIB-SEM serial-sectioning tomography, by conventional transmission electron microscopy (TEM) and by scanning transmission electron microscopy (STEM). All of intragranular precipitates were found nitrided to form dichromium nitride, Cr2N, two types of intergranular precipitates were found Cr2N and Cr3Ni2Si(N), and some of them were grown and jointed due to the additional heat treatment during the tensile test at 1 173 K, which probably contributed to its high-temperature strength..
13. Ryo Teranishi, Kazuki Konya, Masayoshi Inoue, Yukio Sato, Kenji Kaneko, Teruo Izumi, Satoshi Awaji, Study of growth process for YBa2Cu3Oy Coated Conductors with BaZrO3 Flux Pinning Centers by Monitoring Electrical Conductivity, IEEE Transactions on Applied Superconductivity, 10.1109/TASC.2018.2818752, 28, 4, 2018.06, YBa2Cu3Oy (YBCO) coated conductors were prepared by a metal organic deposition method using a chemical solution with tri-fluoroacetate salts. A salt of Zr was also added into the solution to introduce BaZrO3 (BZO) flux pinning centers into the YBCO films. The solution was coated on CeO2/LaMnO3/MgO/Gd2Zr2O7/Hastelloy substrates, and the coated films were crystallized at 1053 K. In this work, an additional holding temperature step was introduced during the crystallization of the films at 873 K to miniaturize the size of BZO nanoparticles, and then the effect of the heat treatment on the electrical conductivity of the film during the crystallization was examined. It was found that the conductivity of the film with the additional holding temperature step was approximately 1.7 times higher than that of the film fabricated without the one when the film temperature reached the crystallization temperature of 1053 K. In addition, the period until the conductivity saturated was three times smaller for the film fabricated with the additional holding temperature step. These results indicate that the growth temperature of YBCO shifts down and the holding period for YBCO crystallization can be shorter by introducing an additional holding temperature step..
14. Yudai Miyake, Yukio Sato, Ryo Teranishi, Kenji Kaneko, Effect of heat treatments on the microstructure and formability of Al–Mg–Mn–Sc–Zr alloy, Micron, 10.1016/j.micron.2017.07.003, 101, 151-155, 2017.10, Microstructures and formability of scandium and zirconium added Al-Mg-Mn alloy sheets with various heating conditions were examined to improve their mechanical properties. Formability of these samples were judged by the Lankford value, r-value. It was possible to fabricate mechanically balanced Al-Mg-Mn-Sc-Zr alloy with high hardness 76.2 Hv and with high formability with r = 1.2, by not only adding scandium and zirconium but also optimizing the heat treatment conditions..
15. Yukio Sato, Takashi Gondo, Hiroya Miyazaki, Ryo Teranishi, Kenji Kaneko, Electron microscopy with high accuracy and precision at atomic resolution
In-situ observation of a dielectric crystal under electric field, Applied Physics Letters, 10.1063/1.4986361, 111, 6, 2017.08, Measuring atomic positions in-situ under an external electric field can provide important insights into the structure-property relationship of electronic materials. In this paper, we demonstrate picometer level accuracy and precision of atomic positions in single-crystalline SrTiO3 under an electric field through annular dark-field scanning transmission electron microscopy. By carrying out electrical biasing in-situ electron microscopy at the atomic scale, the lattice constant was measured with a precision of 9.0 pm under an electric field of ±0.57 kV/cm. In addition, the Ti position in the SrTiO3 unit cell was measured with an accuracy of 20.0 pm at a confidence level of greater than 93%. This opens up a possibility of characterizing functional electronic devices at atomic resolution under operative conditions..
16. T. Maeda, K. Kaneko, K. Yamada, A. Roy, Y. Sato, R. Teranishi, T. Kato, T. Izumi, Y. Shiohara, Nanostructural characterization of artificial pinning centers in PLD-processed REBa2Cu3O7-δ films, Ultramicroscopy, 10.1016/j.ultramic.2016.11.015, 176, 151-160, 2017.05, In the context of high temperature superconductors, pulsed laser deposition derived GdBa2Cu3O7-δ sample with BaHfO3 nanoparticles has been reported to achieve high current density and good IC-B-θ characteristics at high temperatures. Herein, we have carried out a thorough nanostrucural characterization of BaHfO3 nanoparticles embedded in GdBCO matrix using scanning transmission electron microscopy, with an emphasis on the dispersion behavior, morphologies and nanostrain, to understand the role of BaHfO3 nanoparticles..
17. Minoru Ochi, Rika Kawano, Takuya Maeda, Yukio Sato, Ryo Teranishi, Toru Hara, Masao Kikuchi, Kenji Kaneko, Three-dimensional studies of intergranular carbides in austenitic stainless steel, Microscopy, 10.1093/jmicro/dfw103, 66, 2, 89-94, 2017.04, A large number of morphological studies of intergranular carbides in steels have always been carried out in two dimensions without considering their dispersion manners. In this article, focused ion beam serial-sectioning tomography was carried out to study the correlation among the grain boundary characteristics, the morphologies and the dispersions of intergranular carbides in 347 austenitic stainless steel. More than hundred intergranular carbides were characterized in three dimensions and finally classified into three different types, two types of carbides probably semi-coherent to one of the neighboring grains with plate-type morphology, and one type of carbides incoherent to both grains with rod-type morphology. In addition, the rod-type carbide was found as the largest number of carbides among three types. Since large numbers of defects, such as misfit dislocations, may be present at the grain boundaries, which can be ideal nucleation sites for intergranular rod-type carbide precipitation..
18. Ryosuke Urakami, Yukio Sato, Masayoshi Ogushi, Takeshi Nishiyama, Aoi Goto, Kazuhiro Yamada, Ryo Teranishi, Kenji Kaneko, Mikito Kitayama, Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive, Journal of the American Ceramic Society, 10.1111/jace.14663, 100, 3, 1231-1240, 2017.03, Microstructure and mechanical property of silicon nitride (Si3N4) ceramic are strongly dependent on the selection of sintering additives. When rare-earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si3N4 grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α-β phase transformation and (ii) segregation behavior at the interface were studied for Si3N4 ceramics sintered using mixture of La2O3 and Lu2O3 as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α-β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β-Si3N4/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth..
19. H. Horita, R. Teranishi, Kazuhiro Yamada, K. Kaneko, Y. Sato, K. Otaguro, T. Nishiyama, T. Izumi, S. Awaji, Miniaturization of BaHfO3 nanoparticles in YBa2Cu3O y -coated conductors using a two-step heating process in the TFA-MOD method, Superconductor Science and Technology, 10.1088/1361-6668/30/2/025022, 30, 2, 2017.02, The critical current density (J c) of YBa2Cu3Oy (YBCO)-coated conductors in a magnetic field can be enhanced by the doping of flux-pinning centers in a metal organic deposition (MOD) process with trifluoroacetates (TFA). The size of these flux-pinning centers should be less than 10 nm to achieve commercial use due to the coherence length of YBCO at 77 K. In this paper, BaHfO3 (BHO) nanoparticles were introduced into YBCO films using the TFA-MOD method. Microstructures and the J c properties of the films prepared using a two-step heating process at crystallization were compared with film prepared using a conventional one-step heating process. The two-step heating process produced 15 nm average-sized BHO nanoparticles in the film compared to 19 nm nanoparticles in a film prepared using a one-step process. It was revealed that the size of nanoparticles in the films could be miniaturized by improving the heating processes in the MOD method, and the miniaturized nanoparticles could contribute to increased J c in magnetic fields. The mechanism of miniaturization is also discussed based on microstructure observations of quenched films..
20. Yusuke Kasuya, Yukio Sato, Ryosuke Urakami, Kazuhiro Yamada, Ryo Teranishi, Kenji Kaneko, Electron microscopic analysis of surface damaged layer in Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal, Japanese Journal of Applied Physics, 10.7567/JJAP.56.010312, 56, 1, 2017.01, Single crystals of lead magnesium niobate-lead titanate, Pb (Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), have superior dielectric and piezoelectric properties suitable for medical ultrasound imaging. Imaging devices with superior performance can be manufactured from thinner PMN-PT single crystals by mechanical dicing and/or polishing. Although it is often a concern that a damaged layer may form during the mechanical dicing and/or thinning process, the microscopic characteristics of the damaged layer have not yet been investigated in detail. In this study, the microstructural characterization of a damaged layer was investigated by transmission electron microscopy. It was found that mechanical polishing introduced dislocation near the surface of the crystal. It was also found that the domain structure was affected by the introduction of dislocation..
21. Susumu Shiraki, Yoshitaka Takagi, Ryota Shimizu, Tohru Suzuki, Masakazu Haruta, Yukio Sato, Yuichi Ikuhara, Taro Hitosugi, Orientation control of LiCoO2 epitaxial thin films on metal substrates, Thin Solid Films, 10.1016/j.tsf.2016.01.032, 600, 175-178, 2016.02, We demonstrate the deposition of LiCoO2(11-20) epitaxial thin films on chemically etched Pt(110) substrates, and compare their electrochemical properties with those of LiCoO2(10-14) epitaxial thin films on sputter-annealed Pt(110) reconstructed substrates. Cyclic voltammogram measurements of LiCoO2(11-20) epitaxial films exhibit stable charge and discharge operations, despite the application of a compressive strain on the films..
22. K. Hiramatsu, R. Teranishi, K. Yamada, Y. Sato, K. Kaneko, Joint of REBa2Cu3O7-δ Coated Conductors Using Metal Organic Deposition, 28th International Symposium on Superconductivity, ISS 2015 Physics Procedia, 10.1016/j.phpro.2016.04.003, 81, 109-112, 2016.01, Joint techniques connecting REBa2Cu3O7-δ(REBCO) coated conductors (CCs) are required to fabricate long length CCs and to repair locally damaged one. Two pieces of REBCO CC were attempted to be jointed using a metal organic deposition (MOD) method. The starting solution for YBCO layer was coated on GdBCO layer of CCs and calcined to fabricate precursor films, two of which were stuck together in a face to face manner, and then these films were pressurized and crystallized to joint them. Two CCs were successfully jointed together with c-axis oriented YBCO without pores and reacted phases at the joint interface..
23. Ji Young Roh, Yukio Sato, Yuichi Ikuhara, Grain boundary plane effect on Pr segregation site in ZnO Σ13 [0001] symmetric tilt grain boundaries, Journal of the American Ceramic Society, 10.1111/jace.13536, 98, 6, 1932-1936, 2015.06, As grain boundary (GB) and GB segregation often have significant impact on various properties of polycrystalline materials, their atomic structures as well as the location of segregated dopant should be intensively investigated. We have thus reported several papers about segregation of Pr (praseodymium) in ZnO (zinc oxide) GBs for case study. In this study, we study the atomic structure of Pr-doped ZnO [0001]/(134¯0) Σ13 symmetric tilt GB, and the results are compared with that for the (257¯0) GB [Sato et al., Phys. Rev. B, 87,140101 (2013)]. Although atomic arrangements of these GBs can be characterized using the same kinds of structural units (SUs), Pr segregation sites relative to the SU vary with GBs. It is suggested that change in strain distribution for different GBs would cause the variation in the segregation sites and Pr would prefer the Zn site of locally highest tension..
24. H. Yang, H. S. Lee, P. G. Kotula, Y. Sato, Y. Ikuhara, N. D. Browning, Amphoteric doping of praseodymium Pr3+ in SrTiO3 grain boundaries, Applied Physics Letters, 10.1063/1.4916223, 106, 12, 2015.03, Charge compensation in rare-earth Praseodymium (Pr3+) doped SrTiO3 plays an important role in determining the overall photoluminescence properties of the system. Here, the Pr3+ doping behavior in SrTiO3 grain boundaries (GBs) is analyzed using aberration corrected scanning transmission electron microscopy. The presence of Pr3+ induces structural variations and changes the statistical prevalence of the GB structures. In contrast to the assumption that Pr3+ substitutes on the Sr site in the bulk, Pr3+ is found to substitute on both Sr and Ti sites inside the GBs, with the highest concentration at the Ti sites. This amphoteric doping behavior in the boundary plane is further confirmed by first principles theoretical calculations..
25. Rika Kawano, Kenji Kaneko, Toru Hara, Kazuhiro Yamada, Yukio Sato, Kenji Higashida, Masao Kikuchi, Decorated dislocations with fine precipitates observed by FIBSEM slice-sectioning tomography, isij international, 10.2355/isijinternational.55.858, 55, 4, 858-862, 2015.01, Dispersion behavior of intragranular NbC precipitates in Nb added austenitic stainless steel were investigated via nanoscopic characterization in detail, FIB-SEM slice-sectioning tomography, orientation image microscopy, energy-dispersive X-ray spectrometry (EDS), selected area electron diffraction pattern (SAEDP) and transmission electron microscopy (TEM). The heterogeneous dispersion of fine intragranular NbC precipitates were visualized, and in particular, it was found that they were on the {111} slip plane and associated with dislocations..
26. Rika Kawano, Kenji Kaneko, Toru Hara, Kazuhiro Yamada, Yukio Sato, Kenji Higashida, Masao Kikuchi, Decorated dislocations with fine precipitates observed by FIB-SEM slice-sectioning tomography, Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 10.2355/tetsutohagane.TETSU-2015-004, 101, 8, 422-425, 2015.01, Dispersion behavior of intragranular NbC precipitates them in Nb added austenitic stainless steel were investigated via nanoscopic characterization, FIB-SEM slice-sectioning tomography, energy-dispersive X-ray spectrometry (EDS), selected area electron diffraction pattern (SAEDP) and transmission electron microscopy (TEM). The heterogeneous dispersion of fine intragranular NbC precipitates were visualized, and in particular, it was found that they were on the {111} slip plane and interacted with <110> dislocations..
27. Susumu Shiraki, Hideki Oki, Yoshitaka Takagi, Tohru Suzuki, Akichika Kumatani, Ryota Shimizu, Masakazu Haruta, Takeo Ohsawa, Yukio Sato, Yuichi Ikuhara, Taro Hitosugi, Fabrication of all-solid-state battery using epitaxial LiCoO2 thin films, Journal of Power Sources, 10.1016/j.jpowsour.2014.05.133, 267, 881-887, 2014.12, We demonstrate the orientation control of LiCoO2 epitaxial thin films deposited on (110)-2 × 1 reconstructed surfaces of Au and Pt by using pulsed laser deposition. The epitaxial LiCoO2 thin films have CoO2 layers tilted with respect to the surface normal, which is suitable for Li-ion insertion/extraction reactions in LiCoO2. We show the successful operation of all-solid-state thin-film Li-ion batteries by employing the epitaxial LiCoO2 thin films as cathodes. The electrochemical properties of epitaxial films in all-solid-state batteries are improved compared to the batteries using liquid electrolyte..
28. K. Yamada, T. Nishiyama, K. Kaneko, Y. Sato, R. Teranishi, T. Kato, A. Ibi, M. Yoshizumi, T. Izumi, Y. Shiohara, Microstructural characterization of GdBa2Cu3O7-δ superconductor films with BaHfO3 artificial pinning centers by scanning transmission electron microscopy, Microscopy, 63, i27, 2014.11.
29. Akiho Nakamura, Sho Furutsuki, Shin Ichi Nishimura, Tetsuya Tohei, Yukio Sato, Naoya Shibata, Atsuo Yamada, Yuichi Ikuhara, Phase boundary structure of LixFePO4 cathode material revealed by atomic-resolution scanning transmission electron microscopy, Chemistry of Materials, 10.1021/cm5024986, 26, 21, 6178-6184, 2014.11, A variety of cathode materials in lithium ion batteries exhibit phase separations during electrochemical reactions, where two phases with different Li compositions are in equilibrium across the phase interface. Because of the lattice mismatch between these phases, large structural distortions are introduced around the interface region. To characterize their potential effect upon the Li migration behavior, the phase interface structure should be determined accurately. In this study, we perform sophisticated structural analyses for phase interfaces in the well-known cathode material LixFePO4, using atomic resolution scanning transmission electron microscopy. The lattice deformation behavior and Li composition gradient are separately measured across the interface and superimposed after spatial calibrations. The combined result reveals that their relationship significantly deviates from simple models, such as Vegard's law or other higher order interpolations. Notably, the interface region has small lattice sizes comparable to the FePO4 phase, while having intermediate Li compositions. The origin of observed structure is discussed considering the local phase stability by estimating the pair distance variations of dominant attractive/repulsive ionic couples. Because of the nonlinear variations of each structural parameter, well-optimized experiments with high spatial resolutions and sufficient accuracies are required to correctly understand the phase interface structures..
30. Ji Young Roh, Yukio Sato, Yuichi Ikuhara, Atomistic structure and segregation behavior in secondary structure and facet of Pr-doped ZnO ∑13 27.8° [0001] tilt grain boundary, Journal of the Ceramic Society of Japan, 10.2109/jcersj2.122.381, 122, 1426, 381-385, 2014.06, Much attention has been paid to grain boundaries (GBs) in ceramics owing to the impact on material properties. GB atomic scale investigations have so far mostly focused on the major structures. However, actual GB structure is more complex; there could be multiple types of atomistic structure and different morphology such as step and facet. As a case study to characterize these, we report extensive scanning transmission electron microscopy observations for a zinc oxide (ZnO) [0001] 27.8° (13, in the framework of coincidence site lattice theory) tilt GB doped with praseodymium (Pr) in this paper. In addition to the major structure that covers most of the (257̄0) GB plane area [Sato et al., Phys. Rev. B, 87, 140101 (2013)], two types of metastable atomistic structure are found. One is the secondary structure for the (257̄0) GB plane area, which is mostly found near facets. The other is a different type of atomistic structure formed in facets. Pr concentration is lower in the secondary structure than in the major structure. It is thus demonstrated that there is a variety in the atomistic structure and chemical composition within a single GB..
31. K. Takehara, Y. Sato, T. Tohei, N. Shibata, Y. Ikuhara, Titanium enrichment and strontium depletion near edge dislocation in strontium titanate [001]/(110) low-angle tilt grain boundary, Journal of Materials Science, 10.1007/s10853-014-8034-5, 49, 11, 3962-3969, 2014.06, Dislocations are linear lattice defects in a crystalline solid. Since the unusual atomistic environment of the dislocation may greatly influence various material properties, control of the composition would offer more opportunities to obtain unique one-dimensional structures. In the present study, we have characterized the structure of dislocations in a low-angle tilt grain boundary of strontium titanate (SrTiO3). High-spatial resolution elemental mapping by electron energy loss spectroscopy combined with scanning transmission electron microscopy has enabled visualization of the enrichment of titanium (Ti) and the depletion of strontium (Sr) near the dislocation cores. The Ti enrichment and the Sr depletion have been observed at all of the dislocations, and the grain boundary is considered to be Ti excess. The extra Ti ions are located on the positions different from the normal perovskite lattice, suggesting that the local structure is largely reconstructed. It has been proposed that tensile strain at the dislocations may be a cause of the Ti enrichment..
32. T. Saito, X. Gao, K. Fukunaga, T. Hirayama, S. D. Findlay, N. Shibata, Y. Sato, and Y. Ikuhara, Direct Observation of Light Elements in Crystalline Materials by ABF-STEM Method, AMTC Letters, 4.0, 280-281, 2014.05.
33. A. Kumamoto, N. R. Lugg, S. D. Findlay, N. Shibata, Y. Sato, and Y. Ikuhara, Atomic Resolution EDS Mapping in Oxide Materials, AMTC Letters, 4.0, 270-271, 2014.05.
34. B. Feng, H. Hojo, Y. Sato, H.Ohta, N. Shibata, and Y. Ikuhara, Atomic Structure and Energetics of Oxygen Vacancy in CeO2 Grain Boundaries, AMTC Letters, 4.0, 98-99, 2014.05.
35. Y. Sato, T. Hirayama, and Y. Ikuhara, Transmission Electron Microscopy and Electron Diffraction Study on Nanodomain in Morphotropic Phase Boundary Ferroelectrics, AMTC Letters, 4.0, 40-41, 2014.05.
36. J. -Y. Roh, Y. Sato, and Y. Ikuhara, Structural Multiplicity in Pr-doped ZnO [0001] 27.8° and 30.0° Tilt Grain Boundaries, AMTC Letters, 4.0, 26-27, 2014.05.
37. Y. Ikuhara, Y. Sato, and N. Shibata, Grain Boundary Structure Transformation and Reconstruction in Oxide Ceramics, AMTC Letters, 4.0, 4-5, 2014.05.
38. Daichi Oka, Yasushi Hirose, Hideyuki Kamisaka, Tomoteru Fukumura, Kimikazu Sasa, Satoshi Ishii, Hiroyuki Matsuzaki, Yukio Sato, Yuichi Ikuhara, Tetsuya Hasegawa, Possible ferroelectricity in perovskite oxynitride SrTaO2N epitaxial thin films, Scientific reports, 10.1038/srep04987, 4, 2014.05, Compressively strained SrTaO2N thin films were epitaxially grown on SrTiO3 substrates using nitrogen plasma-assisted pulsed laser deposition. Piezoresponse force microscopy measurements revealed small domains (101 - 102 nm) that exhibited classical ferroelectricity, a behaviour not previously observed in perovskite oxynitrides. The surrounding matrix region exhibited relaxor ferroelectric-like behaviour, with remanent polarisation invoked by domain poling. First-principles calculations suggested that the small domains and the surrounding matrix had trans-type and a cis-type anion arrangements, respectively. These experiments demonstrate the promise of tailoring the functionality of perovskite oxynitrides by modifying the anion arrangements by using epitaxial strain..
39. Ji Young Roh, Yukio Sato, Yuichi Ikuhara, Atomic structure of ZnO Σ13 [0001]/{13 4 ̄ 0} symmetric tilt grain boundary, Journal of the American Ceramic Society, 10.1111/jace.12666, 97, 2, 617-621, 2014.02, Zinc oxide (ZnO) is used for a wide range of electrical applications, where its grain boundaries (GBs) possibly influence the physical properties. It is important to determine the GB structure in atomic scale to understand GB effects on the electrical properties. In this study, the atomic structure of a ZnO GB is investigated in detail by scanning transmission electron microscopy and theoretical calculations. It is shown that the atomic structure of ZnO Σ13 [0001]/{134̄0} 32.2° symmetric tilt GB is constructed as an array of structural units (SUs) of six- and eight-membered rings. The GB has two different SU alignments. The dominant structure of the GB is the zig-zag SU alignment and the secondary one is the straight SU alignment. The relation between the SU alignment and the rotation angle has been determined..
40. Yukio Sato, Yuichi Ikuhara, Role of grain boundaries in ZnO, 5th Annual Oxide Based Materials and Devices Conference Oxide-Based Materials and Devices V, 10.1117/12.2042752, 8987, 2014.02, ZnO is used in a wide variety of applications owing to the electrical properties. Polycrystalline ZnO ceramics have long been used such as varistor, and ZnO films are currently intensively studied for transparent conductor applications. Grain boundary (GB) in ZnO varistor is believed to be the origin of nonlinear current-voltage characteristics, and GB in ZnO films possibly affects the electrical conductivity. It is therefore important to understand the role of ZnO GB on the electrical properties, which should be closely related with the structure in atomic scale. With these viewpoints, we have studied the atomistic structure of ZnO GBs, where the orientation relations of adjacent crystals are well defined. Single GBs studied were obtained by fabricating ZnO bicrystals and the GBs were characterized by scanning transmission electron microscopy (STEM) and theoretical calculations. It is found that coordination number of ions change in ZnO GBs; there are underfold or overfold coordinated ions that are unusual in bulk inside. It is calculated that these atomistic structures alters the electronic structure but would not create deep states in the band gap. On the other hand, when praseodymium (Pr), which is known to be a key dopant element to obtain nonlinear (I-V) characteristics, is added to the GBs, Pr strongly localizes to the GBs and occupies specific atomic sites. Pr facilitates the formation of the acceptorlike defects such as zinc vacancies, which we think that is an important role of Pr on generation of nonlinear (I-V) characteristics. Furthermore, atomic arrangement and localization behavior of Pr are studied for several GBs to obtain fundamental understanding about GB structure formation..
41. Yukio Sato, T. Hirayama, Y. Ikuhara, Monoclinic nanodomains in morphotropic phase boundary Pb(Mg 1/3Nb2/3)O3-PbTiO3, Applied Physics Letters, 10.1063/1.4866791, 104, 8, 2014.01, Crystalline structure is a fundamental characteristic of many materials, and drastic changes in properties may accompany crystal phase transitions. A prominent example of this is the morphotropic phase boundary of (Pb(Mg 1/3Nb2/3)O3-PbTiO3) single crystal, a region that exhibits a high piezoelectric effect. Although the highest piezoelectricity is often attributed to a monoclinic crystal phase, formation of ferroelectric nanodomains (NDs) complicates understanding of this crystal structure. In this Letter, we report dedicated transmission electron microscopy and electron diffraction analysis to understand the crystal structure at the ND level. Splitting of diffraction spots, caused by very small lattice distortion in the NDs, is important to understanding crystal structure and has been unambiguously observed. The results can be explained by monoclinic phase NDs. Combining these results with our previous findings on ND dynamics [Sato et al. Phys. Rev. Lett. 107, 187601 (2011)], monoclinic NDs can potentially make a considerable contribution to the piezoelectricity in these materials..
42. Hao Yang, Paul G. Kotula, Yukio Sato, Miaofang Chi, Yuichi Ikuhara, Nigel D. Browning, Segregation of Mn2+ dopants as interstitials in SrTiO3 grain boundaries, Materials Research Letters, 10.1080/21663831.2013.856815, 2, 1, 16-22, 2014.01, Mn-doped SrTiO3 shows promising magnetic and electrical properties, but the doping mechanism remains unclear. In this research Mn4 + is found to substitute Ti in bulk SrTiO3, but Mn2 + segregates inside grain boundaries at both Sr and interstitial sites. Mn interstitial doping has never been reported, but is found possible with the formation of Sr vacancies. This finding is significantly different from the amphoteric doping of Mn2 + substituting Sr and Mn4 + substituting Ti sites, therefore leads to different understanding on the defect mediated electrical and magnetic properties of transition metal-doped perovskites..
43. Yukio Sato, Investigation of microstructure and its impact on physical property in electroceramics, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.121.825, 121, 1417, 825-830, 2013.09, Microstructure often has a significant impact on the physical properties of electroceramics, and such a view is being widespread. In this article, two of our recent microstructure investigations in electroceramics are reviewed. In the first example, atomic-scale structure of grain boundaries in zinc oxide, which plays important role in generation of the electrical function, was analyzed. It was suggested that a role of praseodymium doping is to facilitate the formation of acceptorlike defects such as zinc vacancy. In the second case, dynamics of nanodomains in piezoelectric single crystals was investigated. In-situ transmission electron microscopy observations revealed that reorientation of nanodomains is the dominant behavior of polarization reversal. I believe that such microscopic viewpoints help us better understand the origin of intriguing physical properties in electroceramics..
44. H. Yang, Y. Sato, H. S. Lee, Y. Ikuhara, P. Moeck, P. G. Kotula, and N. D. Browning,, Atomic Scale Manipulation of Grain Boundary Structures Through Doping and in-situ Gas Reduction, Microscopy and Microanalysis, 19.0, suppl. 2, 1946-1947, 2013.07.
45. Y. Sato, J. Y. Roh, and Y. Ikuhara,, Grain Boundary Phase Transformation of ZnO:Pr System, Microscopy and Microanalysis, 19.0, suppl. 2, 1944-1945, 2013.07.
46. Yukio Sato, Ji Young Roh, Yuichi Ikuhara, Grain-boundary structural transformation induced by geometry and chemistry, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.87.140101, 87, 14, 2013.04, Since the presence of dopant at grain boundaries (GBs) has a big impact on physical properties of crystalline solids, GB atomic structure and dopant location have been clarified in detail in some particular cases. However, further studies are required to reveal the underlying structure-formation mechanisms, because there are huge numbers of possible structures due to a wide variety in GB geometry, such as crystallographic orientation relationships and GB planes. Although an ultimate goal of the GB-structure study is to understand all GBs with different geometries, it is quite unrealistic to investigate large numbers of GBs on the atomic scale. Alternatively, a realistic and important step toward this goal is to clarify a GB structure-geometry-chemistry relationship. In this Brief Report we report that change of GB geometry together with doping of praseodymium induces structural transformation in a zinc oxide GB. This demonstrates that geometrical and chemical effects cooperatively play a crucial role in determining the GB atomic-scale structure..
47. H. Yang, H. S. Lee, M. C. Sarahan, Y. Sato, M. Chi, P. Moeck, Y. Ikuhara, N. D. Browning, Quantifying stoichiometry-induced variations in structure and energy of a SrTiO3 symmetric Σ13 {510}/<100 > grain boundary, Philosophical Magazine, 10.1080/14786435.2012.746479, 93, 10-12, 1219-1229, 2013.04, Grain boundaries (GBs) in complex oxides such as perovskites have been shown to readily accommodate nonstoichiometry changing the electrostatic potential at the boundary plane and effectively controlling material properties such as capacitance, magnetoresistance and superconductivity. Understanding and quantifying exactly how variations in atomic scale nonstoichiometry at the boundary plane extend to the practical mesoscale operating length of the system is therefore critical for improving the overall properties. Bicrystals of SrTiO3 were fabricated to provide the model GB model structures that are analysed in this paper. We show that statistical analysis of aberration-corrected scanning transmission electron microscope images acquired from a large area of GB is an effective routine to understanding the variation in boundary structure that occurs to accommodate nonstoichiometry. In the case of the SrTiO3 22.6° ∑13 (510)/[100] GB analysed here, the symmetric atomic structures observed from a micron-long GB can be categorized as two different competing structural arrangements, with and without a rigid-body translation along the boundary plane. How this quantified experimental approach can provide direct insights into the GB energetics is further confirmed from the first principles density functional theory, and the effect of nonstoichiometry in determining the GB energies is quantified..
48. Takao Morimura, Yukio Sato, Yuichi Ikuhara, Takahiro Yamaguchi, Masayuki Hasaka, Scanning transmission electron microscopy and thermoelectric properties of Sr-doped misfit cobalt oxide, Japanese journal of applied physics, 10.7567/JJAP.52.071101, 52, 7 PART 1, 2013.01, In order to clarify the role of Sr doping in the themoelectric properties of Ca3Co4O9, Ca3-x SrxCo 4O9 samples were fabricated by uniaxial compression molding and sintering in atmosphere. Their thermoelectric properties and atomic-scale structures were investigated. Forx < 0:3, the dimensionless figure of merit (ZT) increases with x through a decrease in the thermal conductivity despite the nearly constant Seebeck coefficient and electrical resistivity. Scanning transmission electron microscopy images and energy-dispersive X-ray spectroscopy maps have revealed that Sr mainly substitutes in the Ca site in the rocksalt layer. These results suggest that Sr doping enhances ZT by decreasing in the rocksalt layer, but it does not significantly affect electrical properties because it is an isovalent substitution..
49. Miao Zhong, Yukio Sato, Mario Kurniawan, Aleksandra Apostoluk, Bruno Masenelli, Etsuo Maeda, Yuichi Ikuhara, Jean Jacques Delaunay, ZnO dense nanowire array on a film structure in a single crystal domain texture for optical and photoelectrochemical applications, Nanotechnology, 10.1088/0957-4484/23/49/495602, 23, 49, 2012.12, A single crystal domain texture quality (a unique in-plane and out-of-plane crystalline orientation over a large area) ZnO nanostructure of a dense nanowire array on a thick film has been homogeneously synthesized on a-plane sapphire substrates over large areas through a one-step chemical vapor deposition (CVD) process. The growth mechanism is clarified: a single crystal [] oriented ZnAl2O4 buffer layer was formed at the ZnO film and the a-plane sapphire substrate interface via a diffusion reaction process during the CVD process, providing improved epitaxial conditions that enable the synthesis of the high crystalline quality ZnO nanowire array on a film structure. The high optoelectronic quality of the ZnO nanowire array on a film sample is evidenced by the free exitonic emissions in the low-temperature photoluminescence spectroscopy. A carrier density of ∼1017 cm-3 with an n-type conductivity of the ZnO nanowire array on a film sample is obtained by electrochemical impedance analysis. Finally, the ZnO nanowire array on a film sample is demonstrated to be an ideal template for a further synthesis of a single crystal quality ZnO-ZnGa2O4 core-shell nanowire array on a film structure. The fabricated ZnO-ZnGa2O4 sample revealed an enhanced anticorrosive ability and photoelectrochemical performance when used as a photoanode in a photoelectrochemical water splitting application..
50. Yukio Sato, Yasuhisa Yamamura, Kazuya Saito, Yuichi Ikuhara, Real-space distribution of local WO 4 ordering in negative thermal expansive ZrW 2O 8, Journal of the American Chemical Society, 10.1021/ja305456j, 134, 34, 13942-13945, 2012.08, Solids usually expand when they are heated. This is quite common behavior of solids; however, there are some exceptions. Zirconium tungstate (ZrW 2O 8) is a prototype material among them, because it has the highest degree of negative thermal expansion (NTE) over broad temperature range. Intensive investigation of NTE mechanisms has suggested the importance of metal-oxygen polyhedra. However, most of the studies have been done with volume-averaged techniques, and microscopic information has been lacking. Here, our electron microscopy observations have unraveled the real-space distribution of local WO 4 tetrahedra ordering for the first time. We have found that (i) the WO 4 ordering is partly inverted; (ii) WO 4 is disordered on the nanoscale; and (iii) doping with scandium enhances the WO 4 disordering. These findings led to construction of a microstructure model for ZrW 2O 8, providing a new structural perspective for better understanding of local structure and its role in phase transitions..
51. A. Nakamura, S. Furutsuki, S. Nishimura, Y. Sato, N. Shibata, A. Yamada, and Y. Ikuhara,, Phase boundaries in the delithiated LiFePO4 crystals, AMTC Letters, 3.0, 232-233, 2012.07.
52. T. Saito, K. Fukunaga, T. Hirayama, S. D. Findlay, N. Shibata, Y. Sato, J. Matsuda, K. Asano, E. Akiba, and Y. Ikuhara,, Direct Observations of Hydrogen in Crystalline Materials, AMTC Letters, 3.0, 176-177, 2012.07.
53. Y. Sato, T. Hirayama, and Y. Ikuhara,, In-Situ Transmission Electron Microscopy Study on Nanodomains in PMN-PT (Pb(Mg1/3Nb2/3)O3 - PbTiO3) Single Crystals, AMTC Letters, 3.0, 86-87, 2012.07.
54. J. -Y. Roh, Y. Sato, and Y. Ikuhara,, Atomic Structure Characterization of ZnO S13 (1340) Symmetric Tilt Grain Boundaries, AMTC Letters, 3.0, 48-49, 2012.07.
55. T. Yamamoto, A. Fukumoto, H. S. Lee, T. Mizoguchi, N. Shibata, Y. Sato, and Y. Ikuhara,, Grain Boundary Atomic Structure of Asymmetric S3 Tilt Boundaries in SrTiO3 Bicrystal, AMTC Letters, 3.0, 42-43, 2012.07.
56. H. Yang, Y. Sato, H. S. Lee, Y. Ikuhara, and N. D. Browning,, Understanding Structural Variability Induced by Grain Boundary Doping in SrTiO3, AMTC Letters, 3.0, 34-35, 2012.07.
57. H. Yang, Y. Sato, H. S. Lee, Y. Ikuhara, N. D. Browning, Understanding Structural Variability Induced by Pr segregation in SrTiO3 Grain Boundaries, Microscopy and Microanalysis, 10.1017/S1431927612003984, 18, 426-427, 2012.07.
58. Shunsuke Kobayashi, Scott D. Findlay, Naoya Shibata, Teruyasu Mizoguchi, Yukio Sato, Eiji Okunishi, Yuichi Ikuhara, Takahisa Yamamoto, Simultaneous visualization of oxygen vacancies and the accompanying cation shifts in a perovskite oxide by combining annular imaging techniques, Applied Physics Letters, 10.1063/1.4714920, 100, 19, 2012.05, The defect structures around oxygen vacancies in perovskite-type oxides play very important roles in determining material properties through the change of the electronic state. Therefore, from the viewpoint of developing condensed matter physics and technological applications, it is crucial to obtain direct images of these structures. We report the simultaneous visualization of oxygen vacancies and the cation shifts around them in perovskite-type manganite by scanning transmission electron microscopy. The defective structure of the material makes it amenable to using Fourier analysis to obtain important structural information like the oxygen vacancy locations and the shift of nearby cation columns..
59. Yukio Sato, Tsukasa Hirayama, Yuichi Ikuhara, Evolution of nanodomains under DC electrical bias in Pb(Mg 1/3Nb 2/3)O 3-PbTiO 3
An In-situ transmission electron microscopy study, Applied Physics Letters, 10.1063/1.4705418, 100, 17, 2012.04, Domain structure and its evolution under electrical biases play important roles in piezoelectrics. The effects could be more significant in morphotropic phase boundary piezoelectrics, such as Pb(Mg 1/3Nb 2/3) O 3-PbTiO 3 (PMN-PT), since domains are miniaturized down to nanometer scale. Here, we report in-situ transmission electron microscopy observations for evolution of nanodomains under DC electrical biases in PMN-PT. We find that nanodomains switch with redistributions of invariant nanodomains population for the unpoled crystals, which is consistent with our previous results for the poled cases. This suggests that response of nanodomains for the unpoled and poled crystals can be understood in similar ways..
60. B. Feng, Hajime Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, Yukio Sato, N. Shibata, T. Yamamoto, Y. Ikuhara, Atomic structure of a σ3 [110]/(111) grain boundary in CeO 2, Applied Physics Letters, 10.1063/1.3682310, 100, 7, 2012.02, The atomic structure of a 3 [110]/(111) grain boundary in CeO 2 was studied by scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principles calculations. It was revealed that this grain boundary does not promote the formation of oxygen vacancies and keeps oxygen stoichiometry, which is different from that of 5 CeO 2 grain boundary studied previously [H. Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, N. Shibata, T. Yamamoto, and Y. Ikuhara, Nano Lett. 10, 4668 (2010)]. It was found that the difference in grain boundary oxygen stoichiometry is correlated with the grain boundary atomic structure..
61. Yukio Sato, Tsukasa Hirayama, Yuichi Ikuhara, Real-time direct observations of polarization reversal in a piezoelectric crystal
Pb(Mg1/3Nb2/3)O3-PbTiO3 studied via in situ electrical biasing transmission electron microscopy, Physical Review Letters, 10.1103/PhysRevLett.107.187601, 107, 18, 2011.10, The ultrahigh piezoelectricity of lead magnesium niobate-lead titanate (PMN-PT) single crystals is utilized for medical imaging and ultrasonic devices. It has been revealed that the domain structure is miniaturized down to the nanometer scale, indicating that the domain structure and its response to electric fields could play more roles. In this Letter, we report real-time direct observations of polarization reversals in PMN-PT with the use of in situ electrical biasing transmission electron microscopy. We find that a dominant behavior is a reversible response composed of the reorientation of nanoscale non-180° domain walls (DWs) and movement and/or elimination of microscale DWs. These results basically agree with some predictions made by adaptive phase theory (Jin et al.) which gives explanations of extrinsic contributions to the high piezoelectricity of this material..
62. S. Kobayashi, Y. Tokuda, T. Ohnishi, T. Mizoguchi, N. Shibata, Y. Sato, Y. Ikuhara, T. Yamamoto, Cation off-stoichiometric SrMnO3-δ thin film grown by pulsed laser deposition, Journal of Materials Science, 10.1007/s10853-010-5103-2, 46, 12, 4354-4360, 2011.06, The laser energy density (laser fluence) dependency of the Sr/Mn ratio was investigated for SrMnO3-δ (SMO) thin films grown by pulsed laser deposition (PLD). It was found that the Sr/Mn ratio showed a steep increase followed by a gradual increase as the laser fluence was increased. However, the Sr/Mn ratio always showed Mn-excess under the present laser fluence condition as long as stoichiometric SrMnO3 targets were used. In order to obtain cation stoichiometric SMO films, it was necessary to use Sr-excess SrMnO3 targets in addition with laser fluence tuning. The crystal quality of the SMO thin film was found to vary with the Sr/Mn ratio. In stoichiometric or Sr-excess SMO thin films, epitaxial thin films could be obtained, whereas Mn-excess thin films showed very low crystallinity. Sr-excess films were also found to have some extra SrO planes. In addition, they exhibited out-of-plane lattice expansion which electron energy loss spectroscopy analysis revealed was due to Mn vacancies. The variation of film growth was closely related to point defects due to excess cations included in growing thin films..
63. Hiromichi Ohta, Yukio Sato, Takeharu Kato, Sungwng Kim, Kenji Nomura, Yuichi Ikuhara, Hideo Hosono, Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal, Nature communications, 10.1038/ncomms1112, 1, 8, 2010.12, Water is composed of two strong electrochemically active agents, H + and OH- ions, but has not been used as an active electronic material in oxide semiconductors. In this study, we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from insulator to metal. We fabricated a field-effect transistor structure on an oxide semiconductor, SrTiO3, using water-infiltrated nanoporous glass-amorphous 12CaO·7Al2O3 -as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at room temperature. This leads to the formation of a thin (∼3 nm) metal layer with an extremely high electron concentration (1015-10 16cm-2), which exhibits exotic thermoelectric behaviour. The electron activity of water as it infiltrates nanoporous glass may find many useful applications in electronics or in energy storage..
64. S. Kobayashi, Y. Tokuda, T. Mizoguchi, N. Shibata, Y. Sato, Y. Ikuhara, T. Yamamoto, Quantitative analyses of oxidation states for cubic SrMnO3 and orthorhombic SrMnO2.5 with electron energy loss spectroscopy, Journal of Applied Physics, 10.1063/1.3520655, 108, 12, 2010.12, The oxidation state of Mn in cubic SrMnO3 and orthorhombic SrMnO2.5 was investigated by electron energy loss (EEL) spectroscopy. Change in the oxidation state of Mn produced some spectral changes in the O-K edge as well as in the Mn-L2,3 edge EEL spectra. This study demonstrated that the oxidation ssstate of Mn and the amount of oxygen vacancies in cubic SrMnO3 and orthorhombic SrMnO2.5 could be quantified by analyzing the features of the O-K edge spectrum and the Mn L 3 / L2 ratio in the Mn-L2,3 edge spectrum. Our quantitative analysis showed that the spectral changes in the Mn-L2,3 edge were mainly caused by the oxidation state of Mn, whereas those in the O-K edge could be sensitive to both the oxidation state of Mn and to lattice distortions..
65. Scott D. Findlay, Tomohiro Saito, Naoya Shibata, Yukio Sato, Junko Matsuda, Kohta Asano, Etsuo Akiba, Tsukasa Hirayama, Yuichi Ikuhara, Direct imaging of hydrogen within a crystalline environment, Applied Physics Express, 10.1143/APEX.3.116603, 3, 11, 2010.11, Techniques capable of localized imaging of the structure of hydrogen-bearing crystalline materials at the atomic scale have been conspicuously absent. We show that annular bright field scanning transmission electron microscopy enables the direct imaging of hydrogen atomic columns within a sample of vanadium dihydride, VH2. The suitability of this technique for quantification of the hydrogen content is discussed..
66. Y. Ikuhara, Y. Sato, S. D. Findlay, T. Mizoguchi, N. Shibata, and T. Yamamoto,, Understanding Atomic-Scale Phenomena in Functional Materials by using STEM, ELNES, and Theoretical Calculations, Microscopy and Microanalysis, 16.0, suppl. 2, 1466-1467, 2010.07.
67. T. Mizoguchi, T. Tohei, Y. Sato, T. Yamamoto, Y. Ikuhara, H. Ohta, H. Hiramatsu, and H. Hosono,, Understanding Atomic-Scale Phenomena in Functional Materials by using STEM, ELNES, and Theoretical Calculations, Microscopy and Microanalysis, 16.0, suppl. 2, 84-85, 2010.07.
68. T. Mizoguchi, M. Imaeda, Y. Sato, H. S. Lee, T. Yamamoto, and Y. Ikuhara,, First principles and HAADF-STEM study on [001](310) S5 Grain Boundaries of perovskite oxides, AMTC Letters, 2.0, 46-47, 2010.07.
69. Y. Sato, T. Mizoguchi, N. Shibata, T. Yamamoto, T. Hirayama, and Y. Ikuhara,, Structure of a Praseodymium-Doped Zinc Oxide Grain Boundary: An Atomic Level Study, AMTC Letters, 2.0, 24-25, 2010.07.
70. Yukio Sato, Teruyasu Mizoguchi, Naoya Shibata, Takahisa Yamamoto, Tsukasa Hirayama, Yuichi Ikuhara, Atomic-scale segregation behavior of Pr at a ZnO [0001] Σ49 tilt grain boundary, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.80.094114, 80, 9, 2009.09, The atomic arrangement and segregation of Pr dopants at a ZnO [0001] Σ49 grain boundary were investigated by Z -contrast scanning transmission electron microscopy and first-principles calculations. Scanning transmission electron microscopy revealed that Pr selectively segregated at three kinds of the grain boundary atomic sites. It was found that atomic arrangement of the Pr-doped ZnO [0001] Σ49 grain boundary was similar to that of the undoped GB, and Pr substituted at three kinds of Zn sites at the grain boundary. Comparison of the experimental image with the structure of the undoped grain boundary revealed that the Zn-O interatomic distances are the longest at these Zn sites. There was a tendency for lengths and electronic structures of Pr-O bonds at the Pr-doped ZnO Σ49 grain boundary, when compared with those in the Pr-doped ZnO crystal bulk, to be closer to those in the stable Pr2 O3 crystal phases..
71. Yukio Sato, Teruyasu Mizoguchi, Naoya Shibata, Hiroki Moriwake, Tsukasa Hirayama, Yuichi Ikuhara, Transmission electron microscopy and scanning transmission electron microscopy study on B -site cation ordered structures in a (1-x) Pb (Mg 1/3Nb2/3) O3 -x PbTiO3 single crystal, Applied Physics Letters, 10.1063/1.3183508, 95, 2, 2009.07, Transmission electron microscopy and scanning transmission electron microscopy were performed to investigate B -site cation-ordered structures in a (1-x) Pb (Mg1/3 Nb2/3) O3 -x PbTiO3 (x∼0.30) single crystal. Selected-area diffraction patterns showed presence of F -spots corresponding to 1:1 order. Besides, the F -spots were elongated to be " X "-like in shape. Transmission electron microscopy dark-field images revealed that size of ordered domains was about a few nanometers, and they were dispersed within disordered matrix. Scanning transmission electron microscopy indicated the presence of 1:1 ordered domains. Presence of 1:2 ordered structures was also indicated. These are consistent with features of the selected-area diffraction patterns..
72. M. Imaeda, T. Mizoguchi, Y. Sato, H. S. Lee, S. D. Findlay, N. Shibata, T. Yamamoto, Y. Ikuhara, Atomic structure, electronic structure, and defect energetics in [001](310)Σ5 grain boundaries of SrTiO3 and BaTiO3, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.78.245320, 78, 24, 2008.12, In order to find relationships among the atomic structure, electronic structure, and defect energetics, [001] (310)Σ5 grain boundaries (GBs) of SrTiO3 and BaTiO3 were investigated by using first-principles calculations and high-resolution scanning transmission electron microscopy. It was found that the rigid-body translations of one grain with respect to the other are indispensable to obtain the stable GB structure, and the rigid-body translation plays an important role to reduce the structural distortions such as dangling bonds and strains. It was clearly demonstrated that a fit of calculated structures with microscopy images is not enough to determine the GB structure unless O columns can be seen in the microscopy image. Although the vacancy formation energy depends on the atomic site, the defect energetics at the GB was found to be similar to that in the bulk. It was also found that Ti vacancy is more sensitive to the structural distortions than Sr(Ba) and O vacancies. This would be caused by the difference in the bonding character of Ti-O and Sr(Ba)-O. Through this study, the atomic structures of the [001] (310) Σ5 GBs of SrTiO3 and BaTiO3 were determined, and the characteristic electronic structures and defect energetics of those GBs were identified..
73. Y. Sato, T. Mizoguchi, J. P. Buban, N. Shibata, T. Yamamoto, T. Hirayama, and Y. Ikuhara,, Atomic structure and dopant segregation of [0001] tilt grain boundaries in ZnO bicrystals, Proceeding of the 14th European Microscopy Congress, 667-668, 2008.07.
74. Y. Sato, T. Mizoguchi, J. P. Buban, N. Shibata, T. Yamamoto, T. Hirayama, and Y. Ikuhara,, Atomic structures and electrical properties of ZnO grain boundaries, AMTC Letters, 1.0, 18-19, 2008.07.
75. T. Mizoguchi, Y. Sato, K. Matsunaga, T. Yamamoto, Y. Ikuhara, C. Pickard, and I. Tanaka,, Experimental and theoretical ELNES from crystal interface, Microscopy and Microanalysis, 13.0, suppl. 2, 1284-1285, 2007.06.
76. Y. Ikuhara, J. P. Buban, Y. Sato, N. Shibata, and T. Yamamoto,, STEM Characterization of Ceramic Grain Boundaries, Microscopy and Microanalysis, 13.0, suppl. 2, 1172-1173, 2007.06.
77. T. Yamamoto, T. Mizoguchi, S.Y. Choi, Y. Sato, N. Shibata, and Y. Ikuhara,, Grain Boundary Atomic Structures in SrTiO3 and BaTiO3, Materials Science Forum, 558-559, 851-856, 2007.06.
78. Yukio Sato, Takahisa Yamamoto, Yuichi Ikuhara, Atomic structures and electrical properties of ZnO grain boundaries, Journal of the American Ceramic Society, 10.1111/j.1551-2916.2006.01481.x, 90, 2, 337-357, 2007.02, Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain-boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic-structure-property relationships, well-defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain-boundary orientation relationship and grain-boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current-voltage (I-V) characteristics across ZnO grain boundaries. High-resolution transmission electron microscopy (HRTEM) observations and lattice-statics calculations revealed the atomic structures of the undoped ZnO [0001] Σ7 and Σ49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The Σ49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I-V measurements of the undoped ZnO bicrystals showed linear I-V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I-V characteristics of ZnO ceramics. On the other hand, the nonlinear I-V characteristic appeared when doping the boundaries with Pr. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Pr-doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I-V characteristics, as the Pr existed in the three-plus state and would not produce acceptor states in the boundary. First-principles calculations revealed that Pr doping instead promotes the formations of acceptor-like native defects, such as Zn vacancies. We believe that such acceptor-like native defects are microscopic origin of the nonlinear I-V characteristics. Investigations of various types of grain boundaries in the Pr and Co-codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I-V characteristics significantly depend on the grain-boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I-V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I-V characteristics. Finally, a Pr and Co-codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I-V characteristic. This result indicates that ZnO single-grain-boundary varistors can be designed by controlling grain-boundary atomic structures and fabrication processes. Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I-V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single-grain-boundary varistors..
79. T. Yamamoto, T. Mizoguchi, S. Y. Choi, Yukio Sato, N. Shibata, Y. Ikuhara, Grain boundary atomic structures in SrTiO3 and BaTiO3, 3rd International Conference on Recrystallization and Grain Growth, ReX GG III Recrystallization and Grain Growth III - Proceedings of the Third International Conference on Recrystallization and Grain Growth, ReX and GG III, 558-559, 851-856, 2007.01, SrTiO3 bicrystals with various types of grain boundaries were prepared by joining two single crystals at high temperature. By using the bicrystals, we examined their current-voltage characteristics across single grain boundaries from a viewpoint of point defect segregation in the vicinity of the grain boundaries. Current-voltage property in SrTiO3 bicrystals was confirmed to show a cooling rate dependency from annealing temperature, indicating that cation vacancies accumulate due to grain boundary oxidation. The theoretical results obtained by ab-initio calculation clearly showed that the formation energy of Sr vacancies is the lowest comparing with Ti and O vacancies in oxidized atomosphere. The formation of a double Schottky barrier (DSB) in n-type SrTiO3 is considered to be closely related to the accumulation of the charged Sr vacancies. Meanwhile, by using three types of low angle boundaries, the excess charges related to one grain boundary dislocation par unit length was estimated. In this study, we summarized our results obtained in our group..
80. Yuki Tokumoto, Teruyasu Mizoguchi, Yukio Sato, Naoya Shibata, Takahisa Yamamoto, Yuichi Ikuhara, Atomic structure and relaxation behavior at AlN (0001 )/Al 2O3 (0001) interface, Journal of the Ceramic Society of Japan, 10.2109/jcersj.114.1018, 114, 1335, 1018-1021, 2006.11, The atomic structure of AIN (0001)/Al2O3 (0001) Interface, which is a model of a large lattice-mismatched film/substrate interface, was investigated by using static lattice calculations. Detailed analysis of the calculated structure revealed that the atomic relaxation behavior strongly depends on local atomic configurations of the interface. Interface distance became larger when Al atoms of AIN side and O atoms of Al2O3 side at the interface are in on-top configurations, while it became smaller when they are not in on-top configurations. The calculated structure qualitatively reproduced the experimental high-resolution transmission electron microscopy (HRTEM) image, which was obtained in our previous study. Such varieties of the atomic relaxation can be concluded to play one of the important roles for the formation of stable hetero-interface structures..
81. Yukio Sato, James P. Buban, Teruyasu Mizoguchi, Naoya Shibata, Masatada Yodogawa, Takahisa Yamamoto, Yuichi Ikuhara, Role of Pr segregation in acceptor-state formation at ZnO grain boundaries, Physical Review Letters, 10.1103/PhysRevLett.97.106802, 97, 10, 2006.09, The role of Pr doping on double Schottky barrier formations at ZnO single grain boundaries was investigated by the combination of current-voltage measurements, atomic-resolution Z-contrast scanning transmission electron microscopy, and first-principles calculations. Although Pr segregated to the specific atomic site along the boundaries, it was found not to be the direct cause of nonlinear current-voltage properties. Instead, under appropriate annealing conditions, Pr enhances formations of acceptor-type native defects that are essential for the creation of double Schottky barriers in ZnO..
82. M. Igarashi, Y. Sato, N. Shibata, T. Yamamoto, Y. Ikuhara, HRTEM study of [001] low-angle tilt grain boundaries in fiber-textured BaTiO3 thin films, Journal of Materials Science, 10.1007/s10853-006-0447-3, 41, 16, 5146-5150, 2006.08, Low-angle tilt grain boundaries in [001] fiber-textured BaTiO3 thin films were investigated by high-resolution transmission electron microscopy. Extensive observation revealed a very high density of low-angle tilt grain boundaries in the film. The low-angle tilt grain boundaries can be described as periodical arrays of dislocations on {100} and {110} boundary planes. The boundaries with (100) plane on {100} planes are composed of perfect dislocations with Burgers vectors b = a < 100 > (a = lattice constant of BaTiO3: 0.3992 nm), while the boundaries with (110) plane on {110} planes are composed of the dissociated dislocations with Burgers vectors a/2 < 110 >. It was thus found that the difference in the boundary plane leads to different dislocation structures along the low-angle grain boundaries..
83. J. P. Buban, Y. Sato, K. Matsunaga, N. Shibata, T. Yamamoto, Y. Ikuhara, Observing impurity doping in oxide grain boundaries using STEM, Microscopy and Microanalysis, 10.1017/S143192760606507X, 12, SUPPL. 2, 138-139, 2006.08.
84. Y. Tokumoto, Y. Sato, T. Yamamoto, N. Shibata, Y. Ikuhara, Atomic structure of AlN/Al2O3 interfaces fabricated by pulsed-laser deposition, Journal of Materials Science, 10.1007/s10853-006-7767-1, 41, 9, 2553-2557, 2006.05, The atomic structure of AlN/Al2O3 interface fabricated by pulsed laser deposition is characterized by high-resolution transmission electron microscopy (HRTEM) combined with systematic multi-slice HRTEM image simulations. It is found that the AlN film deposited on a (0001) Al2O3 substrate grows epitaxially with the orientation relationship of (0001)AlN//(0001) Al2O3 and [1100 ]AlN//[11 20 ]Al2O3, with an atomically sharp interface. The observed interface showed best correspondence with the rigid structural model that AlN is terminated by Al at the interface, while the Al 2O3 substrate is terminated by O. Detailed structural analysis indicates that Al sites at the interface are coordinated by both oxygen and nitrogen in this model, with similar coordination environment in AlN. This favored coordination state at the interface may stabilize the AlN/Al 2O3 interface..
85. Teruyasu Mizoguchi, Yukio Sato, James P. Buban, Katsuyuki Matsunaga, Takahisa Yamamoto, Yuichi Ikuhara, Sr vacancy segregation by heat treatment at SrTiO3 grain boundary, Applied Physics Letters, 10.1063/1.2146051, 87, 24, 1-3, 2005.12, Electron energy loss near-edge structures (ELNES) and first-principles band structure calculations were combined in order to identify native defects which are segregated by heat treatments at the vicinity of SrTiO3 grain boundary (GB). Spectral differences between the bulk and the vicinity of GB mainly appear around the second peak of O K ELNES. The spectral differences can be reproduced by calculating the theoretical O K ELNES from the first-nearest-neighbor oxygen to a Sr vacancy. It is therefore concluded that the Sr vacancies are concentrated in the vicinity of GB by the heat treatment..
86. Yukio Sato, Fumiyasu Oba, Masatada Yodogawa, Takahisa Yamamoto, Yuichi Ikuhara, Current-voltage characteristic and grain boundary Structure in undoped and Pr and Co doped ZnO bicrystals, Materials Science Forum, 475-479, V, 3867-3870, 2005.12, Our recent studies on current-voltage characteristic and grain boundary structure in ZnO bicrystals are reviewed in this paper. All types of undoped ZnO bicrystals showed Ohmic characteristics. This indicates that nonlinear current-voltage characteristic cannot be generated solely by the atomic disarrangement in undoped ZnO. On the other hand, co-doped ZnO bicrystals showed nonlinear current-voltage characteristics, which depended on the types of grain boundary. It is considered that the nonlinearity depends on the Pr concentrations at the respective grain boundaries..
87. Teruyasu Mizoguchi, Masaki Sakurai, Atsutomo Nakamura, Takeo Sasaki, Yukio Sato, Katsuyuki Matsunaga, Takahisa Yamamoto, Yuichi Ikuhara, Theoretical and experimental Ti-K NEXAFS of various Ti-oxides, Materials Science Forum, 475-479, IV, 3119-3122, 2005.12, Near-edge structure of X-ray absorption spectrum (NEXAFS) of various Ti-oxides were investigated by combined with first principles orthogonalized linear combinations of atomic orbitals (OLCAO) method. From experimental and theoretical studies on the NEXAFS of the tetravalent and trivalent Ti-oxides, including rutile, anatase, brookite, columbite, and Ti2O3, it was found that the valence state of Ti can be identified by regarding the positions of the spectral onset and the shoulder in the main-peak of Ti-K NEXAFS..
88. Yukio Sato, Teruyasu Mizoguchi, Fumiyasu Oba, Yuichi Ikuhara, Takahisa Yamamoto, Arrangement of multiple structural units in a [0001] Σ49 tilt grain boundary in ZnO, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.72.064109, 72, 6, 2005.08, Arrangement of structural units in a ZnO [0001] Σ=49 symmetric tilt boundary was investigated using a combination of high-resolution electron microscopy and atomistic calculation in detail. The boundary is found to be described by the combination of two different dislocationlike structural units and a bulklike structural unit. The two dislocationlike units were very similar to structural units found in a Σ=7 boundary. One of the dislocationlike units contains threefold-coordinated atoms and the other one has fivefold-coordinated atoms in contrast to fourfold coordination in ZnO bulk. Interestingly, the Σ=49 boundary dominantly consists of a straight alternate array of dislocationlike units and a bulklike unit. A zigzag array of the units partly appeared, which can be related to the straight array via atom flipping at the boundary core. It is considered that the alternate array of dislocationlike units effectively relaxes local strain in the vicinity and, hence, minimizes the boundary energy..
89. Yukio Sato, Masatada Yodogawa, Takahisa Yamamoto, Naoya Shibata, Yuichi Ikuhara, Dopant-segregation-controlled ZnO single-grain-boundary varistors, Applied Physics Letters, 10.1063/1.1899762, 86, 15, 1-3, 2005.07, A ZnO single-grain-boundary varistor was designed using a bicrystal. Pr and Co dopant cosegregation at the boundary, key to obtain high varistic property, is optimized by controlling grain-boundary misorientation and, hence, grain-boundary atomic structure. Thus obtained single grain boundary exhibited sufficiently high varistic property. The present result opens up the possibility of single-grain-boundary varistors required for future nanoscale electronic devices..
90. Y. Sato, T. Mizoguchi, F. Oba, M. Yodogawa, T. Yamamoto, Y. Ikuhara, Atomic and electronic structure of [0001]/(1̄2̄30) ∑7 symmetric tilt grain boundary in ZnO bicrystal with linear current-voltage characteristic, Journal of Materials Science, 10.1007/s10853-005-2665-5, 40, 12, 3059-3066, 2005.06, The atomic and electronic structures of [0001]/(1̄2̄30) ∑ 7 symmetric tilt grain boundary in an undoped ZnO bicrystal were investigated by high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. HRTEM imaging and atomistic calculations revealed that the grain boundary was composed of at least two types of structural units. It was also found that one of the structural units has two threefold-coordinated atoms per a unit and the other has two fivefold-coordinated atoms. First-principles calculations indicated that these atoms with various coordination numbers do not form deep unoccupied electronic states in the band gap of ZnO, which is in consistency with a linear current-voltage characteristic observed for the bicrystal with the ∑ 7 boundary..
91. F. Oba, Y. Sato, T. Yamamoto, H. Ohta, H. Hosono, Y. Ikuhara, Effect of boundary plane on the atomic structure of [0001] ∑ 7 tilt grain boundaries in ZnO, Journal of Materials Science, 10.1007/s10853-005-2666-4, 40, 12, 3067-3074, 2005.06, The atomic structure of [0001] ∑ 7 tilt grain boundaries with {123̄0}∥{123̄0}, {145̄0}∥{145̄0} and {101̄0}∥{358̄0} boundary planes in ZnO was investigated through high-resolution transmission electron microscopy observation of fiber-textured thin films and atomistic calculations. These boundaries were found to comprise three kinds of common structural units that are characterized by fourfold- to eightfold-coordinated channels along the [0001] direction in contrast to sixfold-coordinated channels in wurtzite structure. The boundary structural units are very similar to the multiple core structures of edge dislocations with Burgers vectors of 1/3 <112̄0> . Transformation between two of the three configurations can easily occur through an atom flipping corresponding to dislocation glide. Depending on the orientation of boundary planes with respect to the Burgers vectors, the dislocation-like units exhibit straight or zigzag arrangements with periodicities corresponding to the ∑ 7 misorientation..
92. Takahisa Yamamoto, Yukio Sato, Tomohito Tanaka, Katsuro Hayashi, Yuichi Ikuhara, Taketo Sakuma, Electron transport behaviors across single grain boundaries in n-type BaTiO3, SrTiO3 and ZnO, Journal of Materials Science, 10.1007/s10853-005-6505-4, 40, 4, 881-887, 2005.02, In some electroceramic materials, their unique electrical properties are due to potential barriers, i.e., double Schottky barriers (DSBs), formed at grain boundaries. So far, some researchers have revealed that the electrical properties of DSB are closely related to grain boundary characters, especially grain boundary coherency. For example, highly coherent boundary does not give PTCR or varistic property, while random types exhibit clear resistivity jump or abrupt current increment. Therefore, a concept of grain boundary design will be required for future device manufacturing, even in bulk materials. But it has not been clarified yet why the electron transport behaviors depend on them. In order to address this question, it is necessary to carry out a systematic experiment focusing on single grain boundaries using well-defined bicrystals. In the present study, we have summarized our studies with a special interest in electron transport behavior across single grain boundaries for n-type BaTiO 3, SrTiO3 and ZnO..
93. Fumiyasu Oba, Hiromichi Ohta, Yukio Sato, Hideo Hosono, Takahisa Yamamoto, Yuichi Ikuhara, Atomic structure of [0001]-tilt grain boundaries in ZnO
A high-resolution TEM study of fiber-textured thin films, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.70.125415, 70, 12, 2004.09, The atomic structure of [0001]-tilt grain boundaries in ZnO was investigated using high-resolution transmission electron microscopy (HRTEM) and atomistic calculations. HRTEM observation was conducted for [0001] fiber-textured ZnO thin films grown on quartz-glass substrates by the pulsed-laser deposition. The [0001]-tilt boundaries observed in the films can be classified into three types: low-angle boundaries composed of irregular dislocation arrays, boundaries with {1010} facet structures, and near-low ∑ boundaries represented by symmetric periodicity units. The atomic structure of the boundaries is discussed with a focus on a ∑=7 boundary in conjunction with atomistic calculations and HRTEM image simulations. The ∑=7 boundary consists of multiple structural units that are very similar to the core structures of edge dislocations. Straight or zigzag arrangements of the dislocationlike structural units constitute other high-angle boundaries with symmetric and {1010} facet structures as well. It is suggested that [0001]-tilt boundaries in ZnO are generally described as an array of the dislocationlike units..
94. Yukio Sato, Teruyasu Mizoguchi, Fumiyasu Oba, Masatada Yodogawa, Takahisa Yamamoto, Yuichi Ikuhara, Identification of native defects around grain boundary in Pr-doped ZnO bicrystal using electron energy loss spectroscopy and first-principles calculations, Applied Physics Letters, 10.1063/1.1766078, 84, 26, 5311-5313, 2004.06, Electron energy loss spectroscopy and first-principles calculations were used to analyze the native defects and Pr dopant around grain boundaries in Pr-doped and undoped ZnO bicrystals. It was observed that the Pr-doped bicrystal exhibited a nonlinear current-voltage characteristic, whereas the undoped bicrystal shows an ohmic characteristic. Pr was found to be present within 8 nm around the grain boundary in the Pr-doped bicrystal. The results show that the EELS investigation of native defects combined with first-principles calculations indicate the presence of zinc vacancies in the vicinity of the Pr-doped grain boundary..
95. Yukio Sato, Fumiyasu Oba, Masatada Yodogawa, Takahisa Yamamoto, Yuichi Ikuhara, Grain boundary dependency of nonlinear current-voltage characteristics in Pr and Co doped ZnO bicrystals, Journal of Applied Physics, 10.1063/1.1636816, 95, 3, 1258-1264, 2004.02, The I-V characteristics and grain boundary structures of Pr and Co doped boundaries were investigated using ZnO bicrystals. As such, transmission electron microscopy (TEM) observations and TEM-EDS analysis revealed that atomically joined boundaries were predominantly fabricated and Pr6O11 existed partially at all the boundaries. Although existence of Pr6O11 indicates that Pr was saturated at all boundaries, no Pr was detected on the Σ1 boundary..
96. Yukio Sato, Tomohito Tanaka, Fumiyasu Oba, Takahisa Yamamoto, Yuichi Ikuhara, Taketo Sakuma, Non-linear current-voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals, Science and Technology of Advanced Materials, 10.1016/j.stam.2003.10.031, 4, 6, 605-611, 2003.11, SrTiO3 and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current-voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO 3, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current-voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current-voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current-voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent Σ1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping..
97. Y. Sato, F. Oba, M. Yodogawa, T. Yamamoto, Y. Ikuhatra, Al-doped Zno ceramics fabricated by mechanical alloying and high-pressure sintering technique, Journal of Materials Science Letters, 10.1023/A:1025392315864, 22, 17, 1201-1204, 2003.09, Al-doped ZnO polycrystals were fabricated using a combined mechanical alloying and ultra high-pressure sintering process. Fully densified materials with an Al content of 1 at.% were obtained using the process. An electrical resistivity of 1.2 × 10-3 ωcm was achieved by sintering at 900 °C for 0.5 hours..
98. Fumiyasu Oba, Yukio Sato, Takahisa Yamamoto, Yuichi Ikuhara, Taketo Sakuma, Current-voltage characteristics of cobalt-doped inversion boundaries in zinc oxide bicrystals, Journal of the American Ceramic Society, 10.1111/j.1151-2916.2003.tb03526.x, 86, 9, 1616-1618, 2003.01, Undoped and cobalt-doped basal inversion boundaries were fabricated in ZnO bicrystals to investigate their current-voltage characteristics. High-resolution transmission electron microscopy observations and energy-dispersive X-ray spectroscopy analyses for a cobalt-doped bicrystal revealed that the boundary was highly coherent and free from intergranular phases and precipitates, but a certain amount of cobalt was present near the boundary. The cobalt-doped bicrystals exhibited nonlinear characteristics that depend on cooling rates from annealing temperature, in contrast to linear characteristics of the undoped bicrystals. It is suggested that the presence of cobalt impurities enhances the formation of acceptor-like native defects near the boundaries to generate electrostatic potential barriers..
99. Yukio Sato, Fumiyasu Oba, Takahisa Yamamoto, Yuichi Ikuhara, Taketo Sakuma, Current-voltage characteristics across [0001] twist boundaries in zinc oxide bicrystals, Journal of the American Ceramic Society, 10.1111/j.1151-2916.2002.tb00424.x, 85, 8, 2142-2144, 2002.08, Current-voltage (I-V) characteristics across [0001] twist boundaries with various misorientation angles were investigated in undoped ZnO bicrystals fabricated by a hot-joining technique. It was confirmed by high-resolution transmission electron microscopy that the boundaries were perfectly joined without intergranular phase. None of the bicrystals prepared in this study exhibited nonlinear I-V characteristics irrespective of coherency at the boundaries. Therefore, grain-boundary atomic configuration had no relation to the formation of double Schottky barriers at the [0001] twist boundaries in ZnO..