Teruyuki Ikeda

Rapid consolidation of powdered materials by induction hot pressing

A rapid hot press system in which the heat is supplied by RF induction to rapidly consolidate thermoelectric materials is described. Use of RF induction heating enables rapid heating and consolidation of powdered materials over a wide temperature range. Such rapid consolidation in nanomaterials is typically performed by spark plasma sintering (SPS) which can be much more expensive. Details of the system design, instrumentation, and performance using a thermoelectric material as an example are reported. The Seebeck coefficient, electrical resistivity, and thermal diffusivity of thermoelectric PbTe material pressed at an optimized temperature and time in this system are shown to agree with material consolidated under typical consolidation parameters.

High thermoelectric efficiency in lanthanum doped Yb14MnSb11

Lanthanum doping of the high-temperature p-type thermoelectric material Yb_(14)MnSb_(11) enhances the figure of merit zT through carrier concentration tuning. This is achieved by substituting La^(3+) on the Yb^(2+) site to reduce the free hole concentration as expected from the change in valence. The high-temperature transport properties (Seebeck coefficient, electrical resistivity, Hall mobility, and thermal conductivity) of Yb_(13.6)La_(0.4)MnSb_(11) are explained by the change in carrier concentration using a simple rigid parabolic band model, similar to that found in Yb_(14)Mn_(1−x)A_(lx)Sb_(11). Together, use of these two dopant sites enables the partial decoupling of electronic and structural properties in Yb_(14)MnSb_(11)-based materials.

Anisotropic elastic constants of lotus-type porous copper: measurements and micromechanics modeling

Abstract We studied the elastic constants of a lotus-type porous copper, regarding it as a composite material showing hexagonal elastic symmetry with the c-axis along the longitudinal direction of the pores. We used the combination of resonance ultrasound spectroscopy and electromagnetic acoustic resonance methods to determine the elastic constants of the composite. The resulting Young’s modulus E∥ decreases linearly and c33 does slowly with porosity, while E⊥ and c11 drop rapidly and then slowly. Micromechanics calculations considering the elastic anisotropy of the copper matrix can reproduce the measured anisotropic elastic constants. This indicates that the elastic properties of various types of porous metals can be predicted and designed with the present approach using micromechanics modeling.

Measurement and analysis of effective thermal conductivities of lotus-type porous copper

Lotus-type porous copper is a porous medium made of copper that contains many straight pores. To effectively employ lotus-type porous copper as a heat sink, it is necessary to clarify the pore effect on the thermal conductivity of lotus copper. This article describes an experimental and analytical investigation on the effective thermal conductivities of lotus copper parallel and perpendicular to the pores. The lotus copper displayed anisotropy of the effective thermal conductivity. The effective thermal conductivity keff⊥ perpendicular to the pores was lower than that of the parallel ones keff∥ and was 40% that of lotus copper material ks with porosity e of 0.4. Experimental data for keff∥ showed good agreement with analytical results derived from the assumption that heat flow through the cross-sectional area parallel to the pore axis is proportional to (1−e). Experimental data for keff⊥ showed good agreement with the analytical results derived from the assumption of orthorhombic symmetry and with the num...

Diffusion in Ni3Al, Ni3Ga and Ni3Ge

Abstract An overview is given on recent progress in the study of atomic diffusion in three nickel-based L1 2 -ordered intermetallic compounds, Ni 3 Al, Ni 3 Ga and Ni 3 Ge. Data of the diffusion of constituent species and those of chemical diffusion have been accumulated by radioactive tracer experiments and single-phase interdiffusion experiments. The tracer diffusion coefficients of the majority component, Ni, in the three compounds are of the same order of magnitude when normalized to the melting temperature, while in contrast, those of the minority component are widely different. Nevertheless, it appears possible to understand these common and distinctive features in terms of a simple model of diffusion, where both of the two species of atoms diffuse primarily over the sublattice of the majority component via the ordinary vacancy mechanism.

Fabrication of lotus-type porous iron and its mechanical properties

Abstract Porous iron whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen (nitrogen) and argon. Both hydrogen and nitrogen saturated in the molten iron are rejected at the solid–liquid interface during the solidification due to the difference of solubility between the liquid and the solid. The gas pores are evolved from the hydrogen (nitrogen) insoluble in the solid iron, which grow unidirectionally. The porosity is controlled by the partial pressures of hydrogen (nitrogen) and argon during melting and solidification. By increasing the partial pressure of argon gas the pore formation is suppressed, since the pressure and, therefore, the density of the hydrogen (nitrogen) gas in the growing pore are increased with the total pressure of the atmosphere. The nitrogen concentration in solid iron fabricated under nitrogen atmosphere increases linearly with partial pressure of nitrogen, leading to the improvement of mechanical properties of the porous iron. The ultimate tensile strength and the yield strength of the porous iron with the pore orientation parallel and perpendicular to the tensile direction are about twice as high as those under hydrogen atmosphere. Such superior strength is attributed to the solid-solution hardening due to solute nitrogen atoms in iron matrix.

Nanostructuring of thermoelectric Mg2Si via a nonequilibrium intermediate state

A new route to self-assembled nanocomposite thermoelectric materials is proposed. High-energy mechanical alloying brings materials into a nonequilibrium intermediate state, such as a solid solution with an extended solubility. The large driving force for the transformation to the equilibrium state leads to nanometer-scale microstructure formation, which is ideal for reducing lattice thermal conductivity.

Characteristics of sound absorption in lotus-type porous magnesium

Lotus-type porous magnesium with many unidirectional cylindrical pores was fabricated by unidirectional solidification of a melt dissolving hydrogen in a pressurized hydrogen atmosphere. The sound absorption coefficient of porous magnesium whose specimen face has many open pores was measured by the standing-wave method in the sound frequency range up to 4 kHz. The absorption coefficient increases with decreasing pore size, while it increases with porosity. Moreover, the peak value with a high absorption coefficient is shifted toward a higher sound frequency when the thickness of the porous magnesium specimen decreases. It is suggested that lotus-type porous magnesium exhibits excellent sound absorption characteristics.

Elastic constants of lotus-type porous magnesium: Comparison with effective-mean-field theory

Lotus-type porous (LTP) metals possess a markedly anisotropic porous structure, in which long straight pores are homogeneously aligned unidirectionally. In this paper, we first describe a procedure for the determination of reliable elastic constants of LTP metals with high porosity, and next apply the technique to LTP magnesium, and finally compare the measurement results with those from the effective-mean-field (EMF) theory that has been recently proposed by Tane and Ichitsubo [Appl. Phys. Lett. 85, 197 (2004)]. Young’s modulus in the direction parallel to the longitudinal axis of pores decreases virtually linearly with porosity p, i.e., E‖≈45.3(1−p)1.33, whereas the normal Young’s modulus E⊥, falls rapidly, i.e., E⊥≈45.3(1−p)2.64. The Mori-Tanaka mean-field theory yields values that are close to the measured elastic constants in a wide porosity range, but cannot explain this power-law behavior. In contrast, the EMF theory gives more accurate values in the high-porosity range, and can reproduce the power...

A combinatorial approach to microstructure and thermopower of bulk thermoelectric materials: the pseudo-ternary PbTe–Ag2Te–Sb2Te3 system

The microstructures and Seebeck coefficients of thermoelectric alloys in the pseudo-ternary PbTe–Ag2Te–Sb2Te3 system were examined using samples that were compositionally graded by unidirectional solidification by the Bridgman method and diffusion couples. At compositions near the middle of the pseudo-binary PbTe–AgSbTe2 line, a compositionally modulated microstructure has been found. From diffusion couple experiments, it is found that the PbTe–AgSbTe2 system exhibits a miscibility gap at low temperatures while it forms a complete solid solution at high temperatures; the critical temperature is between 400 °C and 450 °C. The modulated microstructure originates from the decomposition of the high-temperature solid solution during cooling. Scanning Seebeck coefficient measurement on these samples covers a wide compositional space of the pseudo-ternary system. The Seebeck coefficient transitions from positive values at AgSbTe2-rich compositions to negative values at PbTe-rich compositions on the pseudo-binary PbTe–AgSbTe2 line. Composition-graded samples prepared by the Bridgman method are thus useful to investigate thermoelectric materials in multi-component systems.