Yoshiaki Kogure

Embedded atom potentials in fcc and bcc metals

Abstract A new embedded atom potential has been proposed in this paper. The potential is expressed by simple functions and is applicable to the molecular dynamics simulations of large atomic systems. The potential parameters are determined from the experimental data using the cohesive energy, Born stability, elastic constants, C 11 , C 12 and C 44 , the formation energy of a vacancy. In case of fcc the stacking fault energy is also used to fit parameters. The potential functions for copper, silver and gold for fcc metals and for bcc metals Nb, Ta and Va are presented.

Lattice Thermal Conductivity of Crystals Containing Dislocations

General method to treat the scattering of lattice waves by static strain fields in anisotropic solids was applied to the calculation of thermal conductivity of crystals containing dislocations. The...

Scattering of Lattice Waves by Static Strain Fields in Crystals

On the basis of nonlinear elasticity theory, the scattering of lattice waves by a static strain field has been treated with special consideration of the elastic anisotropy of wave-propagating medium. A general form for the wave equation in strained medium was derived, and the scattered waves were determined by means of perturbation method by adopting the Born approximation. The Greens function method was conveniently used to determine the displacement of the scattered waves. The general method was applied to the case of scattering of lattice waves incident normally on a screw dislocation in a cubic crystal. The results of the calculation showed that the scattering was very anisotropic with regard to the incident direction and the scattering angle of the waves. It was found that, contrary to the dominance of purely forward scattering in isotropic medium, obliquely forward scattering occurred intensively in crystals.

Embedded atom potentials in fcc metals

Abstract A new embedded atom potential has been proposed in this paper. The potential is expressed by simple functions and is applicable to the molecular dynamics simulations of large atomic systems. The potential parameters are determined from the experimental data using the cohesive energy, Born stability, elastic constants, C 11 C 12 and C 44, the formation energy of a vacancy and the stacking fault energy. The potential functions for copper, silver and gold are presented.

Effect of Dislocations on Low-Temperature Thermal Conductivity and Specific Heat of Copper-Aluminum Alloy Crystals

Simultaneous measurements of the thermal conductivity and specific heat of copper-aluminum (2, 5, 8, and 12 at.%) alloy crystals have been done by the temperature-wave method at 1.4–7.5 K. Specimens containing dislocations of medium (10 10 cm -2 ) and high (10 12 cm -2 ) densities were used. In the case of crystals of medium dislocation density, the lattice thermal conductivity data were well explained by the mechanism of the strain-field scattering of phonons, and the numerical results agreed with those recently calculated by the authors. The lattice specific heat of the crystals was found to be strongly decreased by the introduction of dislocations, and the results were interpreted by considering the effect of atmosphere of alloying atoms. Anomalous behaviors of the specific heat and thermal conductivity were found in crystals of high dislocation density. The origin was assumed to be quasi-local phonon modes around dislocations, and the analysis of the data along this idea gave reasonable results.

Intense positron beam at KEK

Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e + /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

Simulation of Dislocation Configuration in Rare Gas Crystals

A simulation has been made for a screw dislocation in argon and xenon model crystals with various sizes. The number of atom rows in the crystal was 360-1400, and the Lennard-Jones (12-6) interatomic potential truncated at the third neighbors was adopted. The ordinary relaxation method was used to obtain the stable configuration of atoms in the crystal. It was found that the dislocation split into two Shockley partials, and the configuration of the partials depended on the boundary condition of the crystal surface. A method of modified boundary condition was proposed to control the effect of the surface. The split dislocations were able to stably exist when their separation was in a definite range, which was well understood by considering the balance of three forces: the interaction force between partials, the force due to stacking fault, and the image force from the crystal surface. By taking the limiting value of the separation distance for the crystal of infinite size, the stacking fault energy was esti...

Simultaneous Measurement of Low-Temperature Specific Heat and Thermal Conductivity by Temperature-Wave Method

An experimental procedure is proposed for a simultaneous measurement of thermal conductivity and thermal diffusivity of a specimen by a modified temperature-wave (Angstroms) method. The temperature wave is generated by an AC current in a heater attached to the specimen. The diffusivity can be obtained from the phase difference of the temperature wave between two positions of the specimen, and the conductivity is determined from the static temperature gradient produced by the DC component of input power. The temperatures and their variation at two positions are detected by carbon film resistance thermometers. The specific heat of the specimen is derived from the values of thermal diffusivity and conductivity. An example of experimental results is given for the case of Cu-Al alloy specimen. The reliability of the measured values of conductivity and diffusivity is estimated to be 5~8% in liquid-helium temperature range.

Low-temperature thermal diffusivity measurement by laser-flash method

The pulse heating method has been developed for measuring the thermal diffusivity of solids at low temperatures. The experimental procedures were such that the front surface of a disk-shaped specimen was irradiated by a ruby-laser pulse (pulse width< 1 msec), and the time variation of the temperature at the rear surface was detected by thin-film bolometers (sputtered gold film at higher temperatures and painted carbon film at lower temperatures) and was recorded by a sensitive high-speed electronic apparatus. The measurements were made at temperatures between 300 and 4.2 K, and the specimen used were polyethylene (60% crystallinity). The method to analyze the experimental data was precisely described. It was shown that the finite pulse-time effect could be ignored due to the short duration of the laser pulse, and that the consideration should be made on the heat loss from the specimen to its surroundings through the heat conduction, and also that an adequate choice of the bolometers was important especial...

Thermal Conductivity, Thermal Diffusivity and Specific Heat of Lithium Fluoride Crystals Containing Dislocations

The thermal conductivity and thermal diffusivity of lithium fluoride single crystals have been measured simultaneously by the temperature wave method at 1.5–20 K. The specimens were successively deformed by compression and dislocations with densities of 10 6 –10 8 cm -2 were introduced. The diffusivity data were used to analyze the scattering of phonons by dislocations. The analysis was made with the Callaways method, and the empirical representation of the relaxation rate for the dislocation scattering was found to be a sum of two rates with different dependences on phonon frequency: τ F -1 ∝ω -1 and τ A -1 ∝ω 2 . The former corresponds to the phonon scattering by the fluttering of dislocations, while the latter represents other unknown scattering mechanism effective at high temperatures. Specific heat of the crystals was also determined from the conductivity and diffusivity data. The Debye temperature was found to decrease or the specific heat to increase by fairly large amount when dislocations were i...