Hirotaka Odahara

Increase of the thermoelectric power factor in Cu∕Bi∕Cu,Ni∕Bi∕Ni, and Cu∕Bi∕Ni composite materials

The resultant thermoelectric power factors P of three types of Cu∕Bi∕Cu,Ni∕Bi∕Ni, and Cu∕Bi∕Ni composite materials welded with Bi were measured at 298 K as a function of relative thickness of Bi and compared with P values calculated by treating these devices as an electrical and thermal circuit. It was first demonstrated experimentally that the observed P values of composite devices have a local maximum at an optimum volume fraction (corresponding to the thickness) of Bi, as predicted theoretically. The maximum P values of composite materials were several times higher than those of pure Ni and Bi and were about 100 times larger than that of pure Cu. The dependence of P on the thickness of Bi was found to be explained roughly by introducing the modified thermal conductivity and an enhancement factor in the Seebeck coefficient to our simple model in which a device was treated as an electrical and thermal circuit.

Enhancement of the thermoelectric figure of merit in M∕T∕M (M=Cu or Ni and T=Bi0.88Sb0.12) composite materials

The resultant thermoelectric figure of merit ZT of M∕T∕M (M=Cu or Ni and T=Bi0.88Sb0.12) composite materials welded with Bi–Sb alloy was measured at 298K as a function of relative thickness of Bi–Sb alloy and compared with ZT values calculated by treating it as an electrical and thermal circuit. It was first clarified experimentally that the observed ZT values of composite materials have a local maximum at an optimum volume fraction (corresponding to the thickness) of Bi–Sb alloy in spite of macroscopic composite materials, owing to a significant enhancement in the Seebeck coefficient. It is sure that the enhancement in α is caused by the boundary Seebeck coefficient generated at the interface between Bi–Sb alloy and a metal. It was thus clarified that the interface effect appears clearly in macroscopic systems. The observed maximum ZT values at 298K reached a surprisingly great value of 0.44 at a relative thickness of approximately 0.7, which corresponds to approximately 1.7 times as large as 0.26 of Bi0...

Energy conversion efficiency of a thermoelectric generator under the periodically alternating temperature gradients

The thermo-emf ΔV and thermoelectric current ΔI generated by imposing the temperature gradient (TG) alternating at a period of T on a thermoelectric (TE) generator were measured as a function of t, where t is the lapsed time and T was varied from 8s to ∞. The alternating TG was produced by switching a voltage of 2.6V across two Peltier modules (20×20×4.0mm3) connected in series. Two different types of TE generators were employed as a generator and sandwiched between Peltier modules. The dimensions of the generators (1) and (2) were 15×15×4.0 and 20×20×3.8mm3, respectively. The input power Winput fed to two Peltier modules increases abruptly with an increase of 1∕T, but the effective temperature difference ΔTeff produced between two Peltier modules has a local maximum at 1∕T=1∕240s−1 for the generator (1) and at 1∕120s−1 for the generator (2), whose maximum values are 2.59 and 3.65 times as large as those obtained at 1∕T=0s−1. The local maxima of ΔVeff, ΔIeff, and ΔWeff, generated by a generator, appeared ...

Distribution of local Seebeck coefficient in M∕T∕M and M∕T∕M∕T∕M (M=Cu or Ni and T=Bi or Bi0.88Sb0.12) composite materials

The local Seebeck coefficient αl was measured at a step of 0.5mm along the cylindrical M∕T∕M and M∕T∕M∕T∕M (M=Cu or Ni and T=Bi or Bi0.88Sb0.12) composites welded with T, using two thermocouples set at an interval of 2mm, in order to investigate the reason why the resultant Seebeck coefficient α of these composites is enhanced significantly. When one probe is placed on T and another on metal M, αl of M∕T∕M has a local maximum much higher in absolute value than that of the intrinsic thermoelectric material T, while when both are put on T, αl was almost the same as those of thermoelectric materials T. As a result, the enhancement in the resultant α of composites was thus found to be attributed to an enhancement in αl in the region near the boundary of T, where the barrier thermo-emf is produced by a sharp temperature gradient generated at the interface. A similar phenomenon was also observed for touching M∕T composites. However, the resultant α of M∕T∕M∕T∕M was lower than that of M∕T∕M, so that the multilay...

NMR study on the magnetic properties and phase transformation of MnAl alloys

Abstract Magnetic properties of the τ-, e- and mixed phases in MnAl alloy and the formation mechanism of the τ-phase have been investigated using pulsed NMR. For the antiferromagnetic e-phase a sharp resonance peak has been observed at 120 MHz which is assigned to the 55 Mn nucleus. NMR resonance signals for the ferromagnetic τ-phase have been observed at 200, 170 and 100 MHz which are assigned to Mn atoms with different nearest-neighbor configurations and to the Al atom. NMR peak intensities have changed sensitively depending on heat treatments of the samples reflecting the phase transformation.

Hydrogen-doped Antiferromagnetic YBa2Cu3O7-δHx as Studied by Proton NMR and Cu NQR

Abstract of the C u(l) site have been measured on the antiferromagnetic phase of a powdered sample of hydrogen-doped tetragonal YBa2Cu3O6.1H0.14 between 5 and 300 K. The line width, Δω, of 1H NMR increases abruptly below 20 K. The enhancement of T2-1 and that of T1-1 of 63Cu (1) NQR occurs around 20 K and 40 K, respectively. The nuclear magnetization does not recover in a simple exponential manner below 80 K. The predominant mechanism governing T1 below 80 K was found to be the fluctuating magnetic field which originates from the staggered Cu2+ moments in either the Cu(l) oxygen-deficient layer or the Cu(2)O2 plane induced by the hole doping effect. These enhancements and the abrupt increase in Δω are attributed to this fluctuating magnetic field.