다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

The great variability in the electrical properties of multinary oxide materials, ranging from insulating, through semiconducting to metallic behaviour, has given rise to the idea of modulating the electronic properties on a nanometre scale for high-density electronic memory devices. A particularly promising aspect seems to be the ability of perovskites to provide bistable switching of the conductance between non-metallic and metallic behaviour by the application of an appropriate electric field. Here we demonstrate that the switching behaviour is an intrinsic feature of naturally occurring dislocations in single crystals of a prototypical ternary oxide, SrTiO(3). The phenomenon is shown to originate from local modulations of the oxygen content and to be related to the self-doping capability of the early transition metal oxides. Our results show that extended defects, such as dislocations, can act as bistable nanowires and hold technological promise for terabit memory devices.

Switching the electrical resistance of individual dislocations in single-crystalline SrTiO3

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-t...

Defects in ZnO

Epitaxial oxide thin films are at the heart of new “oxide electronic” applications, such as excitonic ultraviolet light-emitting diodes and resistive switching memories Complex oxide films are often grown by pulsed laser deposition (PLD) because the technique is believed to be material agnostic Here, we show that one of the fundamental premises used to justify the use of PLD, that material is transferred from an ablation target to the film without stoichiometry deviations, is incorrect even when no volatile elements are involved Even more importantly, the commonly used solution of increasing the laser energy density above a material-specific threshold value to obtain stoichiometric films cannot be used in the case of low carrier density systems such as SrTiO3, where even minute 1018 cm−3 order cation nonstoichiometry can have a dramatic effect on transport Lattice parameter deviations in oxide films, which are often incorrectly ascribed to oxygen loss, correlate very well with cation nonstoichiometry

Defects and transport in complex oxide thin films

First-principles plane-wave pseudopotential calculations were performed to study electronic structures, structural relaxation, and energetics of intrinsic vacancies and interstitials in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ In the presence of the intrinsic point defects, extra levels appeared in the band gap. Considering various charge states for the intrinsic point defects, it was found that each point defect is most stable in its fully ionized state. From the formation energies of individual point defects, Schottky, O Frenkel, and Al Frenkel energies were also evaluated and were compared with previous results by experiment and static lattice calculations. Although previous static lattice calculations showed different relative stabilities of Schottky and Frenkel formation, depending on the choice of interatomic potentials, our calculations revealed that the relative values of formation energies are in the order of $\mathrm{Schottky}l\mathrm{Al}$ $\mathrm{Frenkel}l\mathrm{O}$ Frenkel, which is in good agreement with experimental data.

First-principles calculations of intrinsic defects in Al 2 O 3

Pulsed laser deposition (PLD) is a good method for growing high-quality functional oxide thin films because of the technical simplicity and the ease with which deposition can be switched from one material to another. However, the repeatability of film quality is often hard to achieve, especially when using several different PLD systems. Here we report the steps that we have taken to grow nearly bulk-equivalent defect-free thin films, with SrTiO3 as an example, by using PLD in a reproducible fashion. The ablation laser fluence was found to have a very strong effect on the lattice constant and defect structure of the films. Nonstoichiometric transfer of material from the ablation target was observed when either the laser fluence or the beam spot area was inadequate.

Improved stoichiometry and misfit control in perovskite thin film formation at a critical fluence by pulsed laser deposition

We have performed first-principles plane-wave pseudopotential calculations to study the electronic structures, structural optimization, and formation energies of intrinsic vacancies in bulk ${\mathrm{SrTiO}}_{3}.$ The anion and cation vacancy-induced levels appeared near the valence- and conduction-band edges in the band gap. The formation energies of isolated vacancies with different charge states were obtained, and the defect reaction energies, such as Sr partial Schottky ${(V}_{\mathrm{Sr}}^{2\ensuremath{-}}{+V}_{\mathrm{O}}^{2+}),$ Ti partial Schottky ${(V}_{\mathrm{Ti}}^{4\ensuremath{-}}{+2V}_{\mathrm{O}}^{2+}),$ and full Schottky ${(V}_{\mathrm{Sr}}^{2\ensuremath{-}}{+V}_{\mathrm{Ti}}^{4\ensuremath{-}}{+3V}_{\mathrm{O}}^{2+})$ were also evaluated. It was found that depending on the atomic chemical potentials, the relative stability of the defect species or reactions is different. The overall trend of the stable defect structures can explain the electrical conductivity of ${\mathrm{SrTiO}}_{3}$ for different chemical environments experimentally observed.

First-principles study on structures and energetics of intrinsic vacancies in SrTiO 3

For the present study, 1.1-at.%-Nd-doped YAG ceramics with controlled amounts of grain-boundary phase were fabricated by a solid-state reaction method using high-purity powders. The optical scattering loss of the Nd:YAG ceramics, obtained from Fresnels equation, increased simply with increased amounts of grain-boundary phase. The continuouswave laser output power of the Nd:YAG ceramics clearly was related to the scattering loss coefficients of the specimens that, in turn, were affected by the amount of grain-boundary phase. Although the scattering loss coefficients of Nd:YAG ceramics with grain-boundary-free structure and a lower pore volume (}150 vol ppm) were almost equivalent to those of a 0.9-at.%-Nd-doped YAG single crystal grown by the Czochralski method, the laser output power of the Nd:YAG ceramics exceeded that of the Nd:YAG single crystal with increased exciting power under excitation with an 808 nm diode laser because of the large amount of neodymium additives. Lasing performance was not affected by the existence of grain boundaries in the polycrystalline specimen.

Takahisa Yamamoto

Papers

Defects and transport in complex oxide thin films

First-principles calculations of intrinsic defects in Al 2 O 3

Improved stoichiometry and misfit control in perovskite thin film formation at a critical fluence by pulsed laser deposition

First-principles study on structures and energetics of intrinsic vacancies in SrTiO 3

Optical Scattering Centers in Polycrystalline Nd:YAG Laser