Shigenobu Ogata

Ab initio calculation of the crystal structure of the lanthanide Ln2O3 sesquioxides

The Ln2O3 rare-earth oxides, sesquioxides, in the lanthanide series are a group of compounds of particular importance, which are the most widespread lanthanide compounds generally used as a catalyst for the synthesis of many other 4f-materials. In this study, density functional theory was employed to calculate the equilibrium crystal lattice dimensions for most of the rare-earth sesquioxides. The results were found to be in reasonable agreement with experimental data given in the literature.

First-principles approaches to intrinsic strength and deformation of materials: perfect crystals, nano-structures, surfaces and interfaces

First-principles studies on the intrinsic mechanical properties of various materials and systems through ab initio tensile and shear testing simulations based on density-functional theory are reviewed. For various materials, ideal tensile and shear strength and features of the deformation of bulk crystals without any defects have been examined, and the relation with the bonding nature has been analyzed. The surfaces or low-dimensional nano-structures revealpeculiarstrengthanddeformationbehaviorduetolocaldifferentbonding nature. For grain boundaries and metal/ceramic interfaces, tensile and shear behaviors depend on the interface bonding, which impacts on the research of real engineering materials. Remaining problems and future directions in this research field are discussed. (Some figures in this article are in colour only in the electronic version)

Mechanical integrity of carbon nanotubes for bending and torsion

The mechanical integrity of single-walled carbon nanotubes for bending and torsional deformations is evaluated using molecular dynamics simulations. A cantilever type nanotube loaded by a follower lateral force at the free end of the tube exhibits an unstable local buckling near the fixed end. A hysteresis loop is observed, accompanied by an irreversible thermodynamically stable topological transition from four hexagons to two pairs of pentagons and heptagons. The finite deformation can be described by a linear elastic solution almost up to the point of buckling. The initial torsional deformation gives a linear relationship between the torsional moment and the angle of twist per unit length. After buckling, an anti-plane distorted structure is generated, whose rigidity is weaker that of the original structure. The critical bending stress and the shear stress in torsion depend not only on the geometry of the cross section but also on the tube length.

Controlled rejuvenation of amorphous metals with thermal processing.

Rejuvenation is the configurational excitation of amorphous materials and is one of the more promising approaches for improving the deformability of amorphous metals that usually exhibit macroscopic brittle fracture modes. Here, we propose a method to control the level of rejuvenation through systematic thermal processing and clarify the crucial feasibility conditions by means of molecular dynamics simulations of annealing and quenching. We also experimentally demonstrate rejuvenation level control in Zr55Al10Ni5Cu30 bulk metallic glass. Our local heat-treatment recipe (rising temperature above 1.1Tg, followed by a temperature quench rate exceeding the previous) opens avenue to modifying the glass properties after it has been cast and processed into near component shape, where a higher local cooling rate may be afforded by for example transient laser heating, adding spatial control and great flexibility to the processing.

Chirality dependence of mechanical properties of single-walled carbon nanotubes under axial tensile strain

The chiral dependence of the axial tensile strain versus stress characteristics for single-walled carbon nanotubes has been investigated by numerical simulation using a tight binding calculation and a first-principles density functional theory calculation. It is found that the tight binding calculation adopted is accurate. Both calculations show that nanotubes have a Youngs modulus of 1.0 TPa and a maximum tensile strength of 0.10 TPa with no chiral dependence. However, the critical tensile strain for breaking has a chiral dependence. This is discussed from the viewpoint of the deformation of six-membered rings.

Hydrostatic compression and high-pressure elastic constants of coesite silica

Using density-functional theory, we computed all the independent elastic constants of coesite, a high-pressure polymorph of silica, as functions of pressure up to 15 GPa. The results are in good agreement with experimental measurements under ambient conditions. Also, the predicted pressure-dependent elastic properties are consistent with x-ray data in the literature concerning lattice strains at high pressures. We find that coesite, like quartz, exhibits a gradual softening of a shear modulus B44 with increasing pressure, in contrast to the rising bulk modulus.

Ab-initio simulation of thermal properties of AlN ceramics

Aluminum nitride (AlN) ceramics are prospective materials for electronic parts, particularly LSI substrates, because of their excellent mechanical, dielectrical and thermal properties. Their high thermal conductivity is estimated to be about 320 W m-1 K-1 from some theoretical and experimental considerations. However, existence of crystalline imperfections such as impurities, lattice defects or a different phase (zincblende phase) practically reduces it to only two-thirds of the theoretically predicted one. In this paper an interatomic potential of AlN which is adequate for molecular dynamics MD calculation is established on the basis of ab initio molecular orbital MO analysis of an AlN cluster. Then the thermal conductivity of AlN is estimated by equilibrium MD simulation, and its dependence on crystallographic structure and lattice defects is examined. We find that the energy difference between two kinds of phase structure in AlN (the wurtzite and zincblende) is so small that they are able to coexist in the realistic material. The thermal conductivity of the zincblende is also found to be much lower than that of the wurtzite.

Toughness Scale from First Principles

We correlate the experimentally measured fracture toughness of 24 metals and ceramics to their quantum mechanically calculated brittleness parameter. The brittleness parameter is defined as the ratio of the elastic energy density needed to spontaneously break bonds in shear versus in tension, and is a primitive-cell property. Under 300 GPa hydrostatic pressure, the model predicts that diamond has smaller brittleness than molybdenum at zero pressure, and thus should deform plastically without cracking at room temperature.

An ab initio study of the ideal tensile and shear strength of single-crystal β–Si3N4

In this study, the ideal tensile and shear strength of single-crystal β-Si 3 N 4 was calculated using an ab initio density functional technique. The stress-strain curve of the silicon nitride polymorph was calculated from simulations of uniaxial strain deformation. In particular, the ideal strength calculated for an applied ∈ 1 1 tensile strain was estimated to be approximately 57 GPa. Recently, a good correlation was reported between the shear modulus of high-strength materials and the experimentally determined Vickers indentation hardness value. Using the reported correlation an estimate was made of the Vickers indentation hardness of single-crystal β-Si 3 N 4 : approximately 20.4 GPa.

Ab initio tensile testing simulation of aluminum and aluminum nitride ceramics based on density functional theory

The ideal strength and mechanical behavior of aluminum, aluminum nitride, and their composites at zero temperature are investigated using ab initio norm conserving pseudopotential methods based on local density approximation. The total energy and stress of supercells which contain these atomistic models are calculated under uniaxial tensile strain. Estimated various physical properties of aluminum and aluminum nitride at equilibrium state are in good agreement with the experiment. The tensile test of the composite model shows a failure of the composite in the aluminum matrix. The ideal strength of these models is also estimated.