Papers by Author: Tsutomu Mashimo

Abstract: Homogeneous Bi-Pb alloy samples in the ε-phase are treated under mega-gravity centrifugal acceleration field. Atomic sedimentation in solid and associated partial crystallization occurs during the treatment. Small differences of densities in the ε-phase are enhanced under the mega-gravity, and they cause the separation of the ε-phase between fully packed hcp structure of the Bi₃Pb₇ and partially defected hcp structure of the Bi₃Pb_7-δ. The compositional graded superconducting alloy is obtained in the latter. Partially melt-growth occurs in the latter and microstructure of Pb is formed along the gravity field associating with the formation of highly oriented the (211) phase of Bi₃Pb_7-δ. The superconductivity parallel to gravity is Pb, and perpendicular to gravity is Bi₃Pb_7-δ as the result. Superconducting properties differs between the separated two phases.

Abstract: Through the measurement of Hugoniot parameters, we can get useful information about high-pressure phase transitions, equations of state (EOS), etc. of solids, without pressure calibration. And, we can discuss the transition dynamics, because the relaxation times of phase transition and compression process are of the same order. We have performed the Hugoniot-measurement experiments on various kinds of compound materials including oxides, nitrides, borides and chalcogenides by using a high time-resolution streak photographic system combined with the propellant guns. The structure-phase transitions have been observed for several kinds of inorganic materials, TiO2, ZrO2, Gd3Ga5O12, AlN, ZnS, ZnSe, etc. The phase transition pressures under shock and static compressions of metals, ionic materials, semiconductors and some ceramics are consistent with each other. Those are not consistent for strong covalent bonding materials such as C, BN and SiO2. Here, the Hugoniot compression data are reviewed, and the shock-induced phase transitions and the dynamics are discussed, as well as the EOS of the high-pressure phase up to evem 1 TPa.

Abstract: Ultracentrifuge experiments were performed on the twinned Y1Ba2Cu3O7-x (Y123) single crystal at much lower temperatures than the melting point. Two layers structure with slightly different compositions was observed in the sample ultracentrifuged at 250°C(380,000 G), which might be due to the sedimentation of atoms. In the strong gravity layer, it was found that the Y123 phase disappeared, and unknown XRD peaks appeared. Decomposition occurred in the sample ultracentrifuged at 400°C.

Abstract: A visible four-layers structure with anomalous nano-sturucture was formed from a homogeneous -phase Bi3Pb7 intermetallic compound under a strong gravitational field (1.02x106 G, 130°C, 100 hours). In the 4th layer (lowest-gravity region), pure Bi particles precipitate. In the 2nd 3rd layers, composition graded structures, where Pb content increased along the gravity direction, were formed. It was found that the very broad XRD peak appeared in the 2nd layer, which indicated that an amorphous structure was contained.

Abstract: An atomic-scale graded structure has been formed by sedimentation of substitutional atoms under an ultra-strong gravitational field of 1 million G level in alloys and compounds. In this study, we investigate the sedimentation of impurity atoms in semiconductor materials under a strong gravitational field. High-temperature ultracentrifuge experiments (0.59×106 G, 400°C, 60 hours) have been performed on an InSb single crystal wafer which surface was coated with Ge by means of Physical Vapor Deposition (PVD). It was observed that the penetration depth of diffused Ge atoms under the gravitational field was several times larger than under terrestrial field at the same temperatures.

Abstract: The atomic-scale graded structure of In-Pb alloy was formed by an ultracentrifuge under a gravitational field of 0.81 x 106 g for 100 hours at 150 °C in solid state. The isotope ratio measurements were performed on the centrifuged sample with secondary ion mass spectrometer (SIMS, CAMECA IMS-6f). 206Pb/208Pb and 207Pb/208Pb isotope ratio changed with negative gradient in the direction of centrifugal force approximately 1.5% and 0.8%, respectively. There was a tendency that the heavy 208Pb isotope abundance increased and the light 206Pb isotope abundance decreased in the direction of centrifugal force. Three-isotope diagram of 206Pb/208Pb versus 207Pb/208Pb proved that the isotope fractionation depends on the isotopic mass difference. These results showed that a strong gravitational field not only affected the inter-diffusion but also the self-diffusion in this alloy by causing isotope fractionation effect, which was dependent on the mass-difference.

Abstract: Mechanical alloying (MA), super cooling process, etc. have been used to prepare amorphous phases, metastable solid solutions, nanocrystals, and so on. It is important to consolidate these powders for evaluating the physical properties, and for applications. On the other hand, shock compression can be used as an effective consolidation method for metastable material powders without recrystallization or decomposition. We had prepared metastable transition-metal system bulk alloys and compounds (Fe-Co, Fe-Cu, Fe-W, Co-Cu, Sm-Fe-N systems, etc) by using MA and shock compression. The Fe-Cu and Co-Cu metastable solid solutions showed a fit curve to the Slater-Pauling one. The Co-Cu metastable solid solution bulk alloy showed a magneto-resistance. The Fe-Co fine-grained bulk alloys show the higher coeicivity than that of molten alloy. In this paper, the preparation and magnetic properties of the metastable alloys (Fe-Co, Fe-Cu, Co-Cu systems) are reviewed, and the applications to materials science and engineering are discussed.

Abstract: In this study, we investigate the crystalline states and conditions for the grain-refinement of Bi70Sb30 (at.%) alloy. It was considered under an ultra strong gravity field, the crystals were fine-grained from the primary grain sizes of several mm to several tens of mm, and the crystal growth followed with formation of graded-composition structure due to sedimentation of atoms along the direction of gravity. It was found that for the crystal-grain refinement in Bi70Sb30 alloy the minimum gravitational field and the minimum time duration were at least 160,000 G and 10 hours, respectively at about 200 °C.

Abstract: We developed a synthesis method of nanomaterials by the impulse plasma in liquid. The method is based on the low voltage pulsed plasma. The apparatus is very simple and does not require vacuum system, high-energy, cooling system, but can evaporate even refractory metals. Preparation experiments of nanomaterials by using Impulse Plasma in Liquid method were performed. We succeeded in synthesis of nanocrystals of some metals, TiO2, and fullerene C60. The synthesized TiO2 powder consists of fine-dispersed particles of rutile and anatase phases with 5-15 nm grain size. Pure fullerene C60 was prepared by dispersion of graphite electrodes by Impulse Plasma in toluene. It was suggested that the present method can be effectively used for nanomaterials preparation.