Masafumi Senoo

Preparation of bulk amorphous alloys by high temperature sintering under a high pressure

Abstract Until now, there have been no reports in which bulk amorphous alloys of the same densities as those of the corresponding ribbons had been successfully prepared from the amorphous alloy powders by static pressure. Because of the fact that the crystallization temperature Tx of amorphous alloys increased under a high pressure, measurements of Tx on Fe78B13Si9 alloy under pressure were carried out, and it was confirmed that Tx for the alloy rose at a rate of about 10 K GPa−1. Using this phenomenon, amorphous alloy powder of Fe78B13Si9 was compacted at temperatures just below Txh (Tx under a high static pressure) at 5.4 GPa. As a result, we could prepare bulk amorphous alloys with the same densities as those of the corresponding ribbons; it was not possible to prepare these alloys by high pressure compaction below Txo (Tx under ordinary pressure). Most of the physical properties of the compacts were similar to those of the ribbons.

Evaluation of thermal rectification at the interface of dissimilar solids by phonon heat transfer

Many investigations have suggested that the heat transfer coefficient at the interface between two dissimilar solids depends on the direction of heat flow across the interface. Although many factors that affect heat transfer across the interface are reasonably well understood, the directional dependence of the heat transfer coefficient, called thermal rectification, has not yet been completely explained. In this paper, we evaluate the thermal rectification from the results of linear response theory by considering scattering and transmission of phonons across the interface, which expresses the dependence of the temperature at the interface. This model explains the reported behavior of thermal rectification.

Elastic and superconducting properties of supersaturated Al-Si and Al-Ge solid-solution alloys treated under a 5.4 GPa pressure

Supersaturated Al-Si and Al-Ge solid solutions having up to a 10 at% solute concentration were treated at 5.4 GPa. The variations in the elastic moduli, superconducting transition temperature Tc and other physical properties were obtained. Using the resonance method the moduli of the solid solutions in both alloy systems decreased with increasing solute concentration, in contrast with those of the two-phase states which increased. The maximum value of Tc was determined as 6.6 K for an Al-15 at% Si solid solution. It was confirmed that the dependence of Tc on the valence electron concentration was remarkably greater than that of other non-transition and noble metal alloys. Using low-temperature specific-heat experiments as a basis, the superconducting properties were discussed in terms of the electronic specific heat coefficient and the Debye temperature in comparison with other metallic superconductors.

Consolidation of C60 powder by a high-pressure technique up to 5.4 GPa and its mechanical properties

Since the synthesis of C 60 polymers at high pressure [1], intense interest has been shown in the structure, the phase instability, the physical properties and the mechanical properties. A lot of the structural and physical properties have been reported [1 −4], but mechanical properties of the polymerized bulk samples have not been investigated because of an inability to obtain adequate macroscopic samples. In this study, we investigated the phase stability under high-pressure and high-temperature conditions up to 5.4 GPa and 800 ◦C, and determined the suitable consolidation conditions of C60 bulk samples for both the polymerized and pristine fcc-phase. Mechanical properties of obtained bulk samples such as elastic moduli, strength and tribological properties were clarified, and a comparison of the properties between both phases was investigated. The pristine fcc C60 powder employed as a sample was 99.98% pure and was previously heated in a vacuum at 400 ◦C for 2 h in order to minimize the amount of solvent present in the sample before experiments. High-pressure experiments were carried out by a cubic-type multianvil press [5]. The experiments were performed under pressure of 3 and 5.4 GPa at 20– 800◦C. The C60 sample was encapsulated in a boron nitride container. The temperature was measured by a chromel-alumel thermocouple attached to the sample. A bulk sample ofφ6× 6 mm was obtained after high-pressure sintering. The X-ray diffraction method (XRD) with Cu-Kα radiation and Fourier-transform infrared (FTIR) spectroscopy were employed to investigate the structure and the chemical bonding states of the samples. To investigate the microstructures, transmission electron microscopy (TEM) observation at an operating voltage of 200 kV and scanning electron microscopy observation (SEM) were performed. Thermal analysis was carried out using a differential scanning calorimetry (DSC) apparatus at a heating rate of 10◦C min−1. Measurements of elastic moduli were performed by the three-dimensional resonance method developed by the present authors [6]. The compressive strength was deduced from the stress–strain curve at room temperature with a strain rate of 6 × 10−4 s−1. Tribological behavior was measured by a disk-on-flat type tribometer with a steel SKD11 disk in an ambient. Wear scars of the disc were measured by the volume of material lost through wear using stylus profilometry. Fig. 1 shows XRD patterns of a consolidated sample at 5.4 GPa as a function of synthesizing temperature. The diffraction patterns of the samples treated at 200, 400 and 600◦C show the appearance of (2 0 0) reflections, as shown by the arrows in Fig. 1, and the powder patterns can be indexed as fcc structures. The peak shifts to a higher diffraction angle could be observed, and the lattice parameter decreased gradually from a= 1.42 nm to 1.32 nm as the treating temperature increased, as shown in Fig. 2. However, the C 60 phase is reduced to the graphite phase after treating at 800 ◦C. The XRD patterns of the samples treated at 3 GPa showed a similar tendency with the results of 5.4 GPa. In our experiments, the rhombohedral structure reported by Iwasaet al. [1] could not be observed.

Elastic moduli of Al-Li alloys treated at a high pressure of 5.4 GPa

The elastic moduli of high pressure treated supersaturated Al-Li solid solutions were measured. An interesting elastic behaviour was observed, in that, the bulk modulus decreased with an increase in the lithium content, whereas the Youngs modulus and shear modulus increased. In order to clarify this property, we investigated the compressibility of the Al-Li supersaturated solid solutions prepared by high-pressure solid-solidification by high-pressure Synchrotron X-ray diffraction. The obtained pressure-volume relations were fitted to Birchs equation of state. The calculated bulk moduli were lower than those of pure Al at a reduction of about 0.6 GPa per mol% Li. The temperature dependence of the elastic modulus for those supersaturated solid solutions was also measured, and it was found that the trend in the variation of the elastic modulus against the lithium concentration was maintained in the temperature range of 5–290 K. Therefore the attractive relationship between the bulk modulus and Youngs modulus was demonstrated to be intrinsic.

High-pressure phase diagram of an aluminium-rich Al-Li alloy at a pressure of 5.4 GPa

The high-pressure phase diagram of an aluminium-rich Al-Li alloy at a 5.4 GPa pressure was investigated. To determine the equilibrium state under high-pressure and high-temperature conditions, the quenching method was applied and a phase analysis of the sample was performed using X-ray diffraction and microscopic observations. The experimental results were then compared with a thermodynamic theoretical calculation and good agreement was found. The resultant solid solubility of lithium in aluminium was subsequently increased up to 20 at % and the eutectic temperature increased up to 800 °C.

MICROSTRUCTURAL OBSERVATION OF BORON NITRIDE FILMS SYNTHESIZED BY ION IMPLANTATION

We report on the synthesis of boron nitride (BN) films by ion implantation and demonstrate the results of microstructural observation of the films. Boron films were initially prepared on single-crystal Si(100) substrates by rf sputtering. Then, 30 keV N2+ was implanted at doses ranging from 1×1017 ions/cm2 to 6× 1017 ions/cm2 to synthesize BN. No substrate heating was used in the sputtering and implantation processes. Chemical composition analysis of the films was carried out by Auger electron spectroscopy. Transmission electron microscopy was employed for microstructural observation. Mixed-phase BN, including t-BN, c-BN, w-BN and B25N, was identified in the implanted films. The BN formation mechanism was discussed and compared with theoretical models. The results of this work are consistent with the stress-induced BN formation model in several aspects.

Variations of elastic moduli during ageing process of Al-Li solid solutions treated at high pressure

Supersaturated Al-10 and 15 mol% Li solid solutions were prepared by means of heat treatments under high pressure and high temperature, and the variations of elastic modulus, electrical resistivity and micro-Vickers hardness in these solid solutions during isochronal ageing up to 673 K were investigated. The maximum shear modulus was observed during the precipitation of the δ′ phase at 453 K ageing. The difference in the decomposition behaviour of several specimens obtained by high-pressure solid solutioning and the conventional one is discussed based on the results of elastic moduli measurements.

Catastrophic Transformation of Electron Stress and Electron Stiffness Parameter on Metal and Semiconductor

In this study, we calculated the quantum stress of electrons, called electron stress (ES), in a metal and a semiconductor under uniaxial tensile and compressive deformation (−50% to 50%) by using the pseudopotential method. Since the positions of atoms are fixed with external forces, the ES of a stationary electron shows the internal stress among atoms. From the derivative of ES with strain, we defined a “quantum stiffness parameter (ESP)”. We calculated ES and ESP in Al, K and Si during deformation. From the results of the calculation, we consider the following points: (i) Change of ES and ESP during deformation, (ii) ES and ESP demonstrate the difference between a metal and a semiconductor. During uniaxial tensile deformation, alter showing a gradual decrease, ESP shows a sudden divergence from positive to negative values at a certain critical strain. Under compression, a merely gradual increase of ESP was observed. Strain at the critical transformation of ESP, termed “critical strain (e cr)”, shows a material dependence; 20% in Al, 30% in K, but only 2.5% in Si. The result of a low e cr in Si and high e cr values in metals corresponds to the general knowledge on mechanical properties of materials; metals are more ductile than semiconductors. The critical transitions of ES and ESP in solids could be explained from the electric properties in solids. These parameters might provide a key to understand ideal fractural properties in solid.

Evaluation of Sound Velocity and Temperature Dependence of Electrical Resistivity using Pseudopotential Method.

There are two main methods to calculate the temperature dependence of electrical resistivity. The first one known as the Ziman method is based on relaxation time calculations. The second, known as the correlation function method, is one of many methods used to arrange Kubo expressions into a new formula is very simple to use to calculate transport properties. In this work, we applied these two methods to electron-ion interactions of simple metals which can be described by a local pseudopotential. There the temperature dependence of the electrical resistivity of the simple metals was calculated. The results were compared with experimental data over a wide range of temperatures and agreement was found to be good for both methods. Compared with Ziman method, correlation function method is more appropriate for calculating the electrical resistivity of high Debye temperature metals. Also, the relationship between the electrical and mechanical properties of metals is discussed. Particularly, we outline a method to evaluate the temperature dependence of sound velocity and elastic constant by calculating electrical resistivity.