272 research outputs found
Evaluation of anisotropic thermoelectric power of ReSi1.75
No full textThe highly anisotropic transport properties of ReSi1.75 single crystals have been evaluated with the Boltzmann transport equation under the assumption that the deformation potential acoustic phonon scattering is dominant. There is a large difference between the calculated transport properties and the measured properties along [100], while there is little difference between the calculated and the measured along [001]. The intervalley scattering mechanism is proposed as the reason of the discrepancy in the transport properties along [100]. It is also shown that the large effective mass of holes along [001] restrict the contribution of the holes in transporting the energy induced by the Seebeck effect, resulting in good thermoelectric conversion efficiency. (c) 2006 Elsevier B.V. All rights reserved
水素吸蔵合金における水素侵入機構-水素吸蔵過程における格子欠陥の生成に注目して-
No full text平成12-13年度科学研究費補助金(基盤研究(B)(2))研究成果報告書 課題番号：12450282 研究代表者：乾晴行(京都大学工学研究科助教授
The Plaston Concept
No full textThis open access book presents the novel concept of plaston, which accounts for the high ductility or large plastic deformation of emerging high-performance structural materials, including bulk nanostructured metals, hetero-nanostructured materials, metallic glasses, intermetallics, and ceramics. The book describes simulation results of the collective atomic motion associated with plaston, by computational tools such as first-principle methods with predictive performance and large-scale atom-dynamics calculations. Multi-scale analyses with state-of-the art analytical tools nano/micro pillar deformation and nano-indentation experiments are also described. Finally, through collaborative efforts of experimental and computational work, examples of rational design and development of new structural materials are given, based on accurate understanding of deformation and fracture phenomena. This publication provides a valuable contribution to the field of structural materials research
The Plaston Concept
No full textThis open access book presents the novel concept of plaston, which accounts for the high ductility or large plastic deformation of emerging high-performance structural materials, including bulk nanostructured metals, hetero-nanostructured materials, metallic glasses, intermetallics, and ceramics. The book describes simulation results of the collective atomic motion associated with plaston, by computational tools such as first-principle methods with predictive performance and large-scale atom-dynamics calculations. Multi-scale analyses with state-of-the art analytical tools nano/micro pillar deformation and nano-indentation experiments are also described. Finally, through collaborative efforts of experimental and computational work, examples of rational design and development of new structural materials are given, based on accurate understanding of deformation and fracture phenomena. This publication provides a valuable contribution to the field of structural materials research
Plasticity of hard and brittle materials at micron-meter size scales
No full textThere are many hard materials that are considered to be candidates for structural applications under extreme conditions such as very high temperatures. This stems from the fact that many of them possess peculiar properties such as high hardness, high melting temperature, and so on. But, one of the common characteristics for these hard materials is their brittleness. They usually fail in cleavage without showing any plastic deformation at ambient temperature. So, even, fundamentals for plasticity such as operating slip systems and their CRSS values have yet to be known for many of them. If we assume that fracture in these hard materials occurs in a brittle manner at a pre-existing micro-crack, the effective defect size of the microcrack to cause fracture is believed to vary with the fracture toughness (KIC) at a given fracture stress. If the fracture stress is fixed at 1 GPa, the effective defect size of the micro-crack is calculated to be 320 nm for KIC of 1 MPam-1/2, but this value increases to about 8 mm if the KIC values is increased to 5 MPam-1/2. Then, there is a chance for these hard materials to plastically deform in the form of micropillars of the micron-meter size even at ambient temperature.
We have investigated the compression deformation behavior of transition-metal silicides as typical examples of hard materials such in the micropillar form with the specimen size ranging from 0.5 to 10 mm at room temperature. Those hard materials include transition-metal (M) silicides of the M5Si3-type such as Mo5Si3, Nb5Si3 and Mo5SiB2 and those of the MSi2-type such as MoSi2, VSi2, CrSi2, NbSi2 and TaSi2. Although none of them listed above deform plastically at room temperature in the bulk form, plasticity is clearly observed at room temperature for all of them in the micropillar forms. This is very surprising in particular for transition-metal silicides of the M5Si3-type, since they usually need more than 1300°C for their plastic deformation to occur in the bulk form. Because of such a high temperature, slip systems have never been identified with confidence for these transition-metal silicides of the M5Si3-type. However, plasticity observed in the micropillar form at room temperature has made us to clearly identify their operative slip systems with their CRSS (critical resolved shear stress) values. For transition-metal silicides of the MSi2-type, slip systems operative at high temperatures in the bulk form are observed also to operate in the micropillar form at room temperature. The room-temperature bulk CRSS values for these slip systems can be obtained by extrapolating the power-law of the CRSS-specimen size dependence to the bulk size, which can be estimated to be 30-50 m. The room-temperature bulk CRSS values thus estimated are on the extension of the CRSS-temperature curve of the corresponding slip system for some silicides but not for other silicides. The origin of the latter behavior is proved to be due to a transition of deformation mechanisms
Rhenium silicide as a new class of thermoelectric material
No full textABSTRACTThe microstructure, defect structure and thermoelectric properties of binary and some ternary Re silicide have been investigated as a new class if thermoelectric material. Binary Re silicide is identified to contain many Si vacancies, which are arranged in an ordered manner in the underlying tetragonal C11b structure so that the silicide is formulated to be ReSi1.75 with a monoclinic unit cell and contains four differently oriented domains accompanied by the twinned microstructure. The density and arrangement of Si vacancies can be controlled by ternary alloying. When the number of valence electrons of a ternary element is smaller than that for Re, the density of Si vacancies decreases with ternary additions, whereas the density of Si vacancies increases with ternary additions when the number of valence electrons of a ternary element is larger than that for Re. For both cases, the variation of the density of Si vacancies upon ternary alloying is accompanied by the introduction of the so-called shear structure.Binary ReSi1.75 exhibits nice thermoelectric properties as exemplified by the high value of dimensionless figure of merit (ZT) of 0.70 at 800 °C when measured along [001], although the ZT value along [100] is just moderately high. The ZT value is further increased to 0.8 with a small amount (2% substitution for Re) of Mo addition, by which an incommensurate microstructure is formed as result of extensive shear operation on the nano-scale.</jats:p
Improvement of Thermoelectric Properties of Chimney-ladder Compounds through the Introduction of PBET Interfaces
No full textPlaston—Elemental Deformation Process Involving Cooperative Atom Motion
No full textThe concept of ‘plaston’ that involves cooperative atom motion under shear stress is discussed as a deformation carrier that nucleates and moves in the deformation front under shear stress in many different materials in general. The selection of a plaston of a particular type among many different plastons depends on stress level/state, crystallographic orientation, specimen size (grain size) and so on. The importance of the understanding of the activation of various plastons is discussed for the improvement of mechanical properties of existing structural materials and the development of new structural materials with high strength and high ductility
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