Tomoya Nagira

Synthesis of NaxCo2O4 thermoelectric oxides by the polymerized complex method

Abstract The thermoelectric oxide Na x Co 2 O 4 was synthesized by the polymerized complex method, which provided a flaky powder. The sintered sample showed a high density, a fine microstructure and improved thermoelectric performance compared to a sample prepared by the conventional solid state reaction method.

Thermoelectric properties of (Na1-yMy)xCo2O4 (M=K, Sr, Y, Nd, Sm and Yb; y=0.01∼0.35)

Abstract Polycrystalline samples of (Na 1− y M y ) x Co 2 O 4 (M=K, Sr, Y, Nd, Sm and Yb; y =0.01∼0.35) were prepared by a solid state reaction method. In this study, in order to improve the thermoelectric properties of Na x Co 2 O 4 , the effects of partial substitution of other metals for Na on the thermoelectric properties of Na x Co 2 O 4 from room temperature to 1073 K were investigated. For M=Sr, the thermoelectric power and the electrical resistivity increased, and the electronic and lattice contribution to the thermal conductivity decreased compared to the non-substituted sample. These effects suggest that the carrier density was reduced by the substitution of Sr for Na. As a result, the figure of merit of the sample for M=Sr was improved. On the other hand, for other samples in spite of the increase in the electrical resistivity, the thermoelectric power decreased. These results are anomalous effects, which cannot be described merely by a change of the carrier density. For all samples, except for M=Y, the lattice contribution to the thermal conductivity decreased and for all samples, except for M=K, the electronic contribution slightly decreased.

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.

In situ observation of nucleation, fragmentation and microstructure evolution in Sn–Bi and Al–Cu alloys

Abstract This paper presents recent progress of in situ observation for the microstructure evolution during solidification. Nucleation and fragmentation of dendrite arms are important issues for controlling microstructure during solidification. However, there are few studies on in situ observation of nucleation and fragmentation in metallic alloys. Time resolved X-ray imaging technique has been developed to observe solidification of metallic alloy systems in situ. Fragmentation of dendrite arms often occurred at the root after growth velocity was reduced for the Sn–13 at.-%Bi alloys and the Al–15 mass%Cu alloys. In the Al–15 mass%Cu alloys, both of nucleation and fragmentation contribute to formation of grain structure. The result suggested that fragmentation should be considered for controlling grain structure.

Synthesis of NaxCo2O4 thermoelectric oxide with crystallographic anisotropy by chemical solution process

Abstract The synthesis of the NaxCo2O4 with a layered crystal structure was attempted by using the polymerized complex (PC) method or citric acid complex (CAC) method, both of which are a kind of chemical solution process. The effects of these chemical solution processes on the crystallographic anisotropy, microstructure and thermoelectric properties of the sintered NaxCo2O4 were investigated and compared to those of the conventional SSR method. In the case of the PC method, the sintered body with higher chemical homogeneity and finer microstructure than those of the SSR sample was obtained, resulting in the high thermoelectric performance. However, the crystallographic orientation was almost the same as that of the SSR sample. On the other hand, in the case of the CAC method, a flaky powder precursor was formed by grain growth of the NaxCo2O4 phase along the c-plane. The NaxCo2O4 sample sintered from this powder precursor showed a high crystallographic anisotropy and good chemical homogeneity, so the CAC process was found to be quite effective for synthesizing a crystallographically oriented NaxCo2O4 polycrystal with high thermoelectric performance.

In-situ observation of peritectic solidification in Sn-Cd and Fe-C alloys

Time-resolved absorption imaging using synchrotron radiation X-rays allows us to observe solidification of metallic alloys of interest. This paper presents peritectic solidification in Sn-Cd alloy and Fe-C alloys. In unidirectional solidification of Sn-Cd alloy, the formation of a banded structure, in which two phases were alternatively piled up in the growth direction, was clearly observed. Sequence of nucleation or fluctuation of triple junction (primary phase / secondary phase / liquid) resulted in the banded structure. Ease of nucleation for both phases contributed to the banded structure formation. In carbon steel (Fe-0.45mass%C), the transformation from δ phase to γ phase was observed. At lower cooling rates, γ phase was produced in semisolid state of δ phase and liquid, indicating the peritectic reaction occurred during solidification. In contrast, δ phase transformed into γ phase when solidification nearly completed at temperatures 100K below the liquidus temperature. Namely, the transformation seemed to be massive. The observation showed that two different transformation modes operated in Fe-C alloy.

Massive transformation from ? phase to ? phase in Fe?C alloys and strain induced in solidifying shell

Time-resolved in situ observations using synchrotron radiation X-rays showed that γ nucleation difficulties resulted in a massive transformation from the δ phase to the γ phase. The massive transformation occurred even in high-carbon steel (0.45 mass% C) at a cooling rate of 0.33 K/s or more. A crystallographic relationship in which the close-packed (111) plane of the γ phase tended to coincide with the close-packed (110) plane of the δ phase suggested that the γ phase nucleated in a favorable plane in the δ phase. Observations of the sequence in the initial stage of solidification showed that the induced strain and strain rate in the massive transformation were larger than those expected in peritectic solidification. These observations will help us to understand deformation of solidifying shells and to build a physical model.

Three-dimensional alignment of FeSi2 with orthorhombic symmetry by an anisotropic magnetic field

This paper presents the three-dimensional alignment of β-FeSi2 particles in a resin under an anisotropic magnetic field. The alignments obtained under a static magnetic field and a rotating magnetic field proved that the magnetic susceptibilities of β-FeSi2 are χc>χb>χa. The magnetic anisotropy was sufficiently large under a magnetic field of 10T for the crystallographic alignment. Under an anisotropic magnetic field (oscillating magnetic field), a-, b- and c-axes of β-FeSi2 particles suspended in a resin were aligned each other. A pseudo single crystal β-FeSi2 with the orthorhombic structure was fabricated by the oscillating magnetic field.

Synchrotron radiography of direct-shear in semi-solid alloys

Understanding phenomena occurring at the scale of the crystals during the deformation of semi-solid alloys is important for the development of physically-based rheological models. A range of deformation mechanisms have been proposed including agglomeration and disagglomeration, viscoplastic deformation of the solid skeleton, and granular phenomena such as jamming and dilatancy. This paper overviews in-situ experiments that directly image crystal-scale deformation mechanisms in equiaxed Al alloys at solid fractions shortly after the crystals have impinged to form a loose crystal network. Direct evidence is presented for granular deformation mechanisms including shear-induced dilation in both equiaxed-dendritic and globular microstructures. Modelling approaches suitable for capturing this behaviour are then discussed.

Formation and microstructure of Al2O3-YAG eutectic ceramics by phase transformation from metastable system to equilibrium system

Unidirectionally solidified Al2O3-YAG(Y3Al5O12: yttrium-aluminum-garnet) eutectic ceramic composites have been recognized as encouraging heat-resistance materials because of the superior mechanical properties at high temperatures. In addition to the excellent mechanical properties at high temperatures, some interesting solidification phenomena have been reported in the Al2O3-Y2O3 system. The Al2O3-YAG equilibrium eutectic at 2099 K and the Al2O3-YAP metastable eutectic at 1975 K exist in the Al2O3-Y2O3 system. The heating the metastable eutectic up to temperatures above the metastable eutectic temperature produced the undercooled melt. Solidification in the equilibrium path accompanied the melting of the metastable eutectic. The solidification process using undercooled melt resulted in the fine and uniform eutectic structure. In this study, the effect of the initial Al2O3-YAP particles size on the undercooled melt formation was examined. The Al2O3-YAP particles with diameters more than several μm resulted in the transformation through the undercooled melt. EBSD analysis showed that the domains of Al2O3 grains with same crystallographic orientation were observed and that their domain size depended on the Al2O3-YAP particles size. On the other hand, for the Al2O3-YAP particles with a diameter of 500 nm, the each Al2O3 grain with diameter of about 1 μm had the different crystallographic orientations, which suggested that the transformation from metastable eutectic to equilibrium eutectic occurred in the solid state. The increase in the Al2O3-YAP free surface area suppressed the undercooled melt formation.