Yoshiaki Ishikawa

Magneto-Seebeck coefficient of a bismuth microwire arrayin a magnetic field

The enhancement of the magneto-Seebeck coefficient of a bismuth microwire array under a magnetic field is measured at temperatures from 45to295K. The measured magneto-Seebeck coefficient exhibits a peak at a certain magnitude of magnetic field, with the peak shifting to higher magnetic fields and becoming broader with increasing temperature. The results show that the magneto-Seebeck coefficient can be improved by approximately 20% by applying an appropriate external magnetic field and temperature. The Boltzmann equation with a relaxation-time approximation is solved numerically to determine the magnetic field and temperature dependences of the magneto-Seebeck coefficient for the bismuth microwire array. The experimental results are compared with calculations, and the two sets of results are shown to be in very good agreement, clarifying the mechanisms contributing to the magneto-Seebeck coefficient for bismuth. The wire array structure is thus suitable for enhancing the thermoelectric properties of materi...

Electronic transport properties of a bismuth microwire array in a magnetic field

The magneto-Seebeck coefficient and magneto-resistivity of a polycrystalline bismuth microwire array were measured under magnetic fields of 0 to 2 Tesla and at temperatures of 50 to 300 K. To avoid the influence of contact resistance between the wire array and the electrodes, bulk bismuth was used for the electrodes. In the absence of a magnetic field, the Seebeck coefficient and resistivity were -76 /spl mu/V/K and 1.8 /spl mu//spl Omega/m at 300 K, respectively. The magneto-Seebeck coefficient for the wire array increased with the application of an external magnetic field, attributable to the precise control of impurities and carrier scattering process in the fabrication of the wire array. The phonon drag effect was observed below 100 K, with a corresponding increase in the magneto-Seebeck coefficient under high magnetic fields. However, the magneto-resistivity was also raised under higher magnetic fields, detracting from the thermoelectric properties. Through analysis of the power factor, the optimum magnetic field was determined for each temperature, revealing a trend for the optimum magnetic field to increase with temperature. The power factor was improved by a maximum factor of 1.12, achieved at 200 K and 0.25 Tesla. Further improvements appear to be possible by eliminating the bulk bismuth employed for the electrodes.

Reduction of contact resistance at terminations of bismuth wire arrays

Contact resistance at the terminations of bismuth wire array of 25 mum diameter is reduced by ion plating of a titanium interlayer 100 nm upon the wire ends. 1,000 nm-thick copper electrodes are ion plated upon the titanium. Copper probe electrodes are then attached using Pb-Sn solder. The temperature dependence of the Seebeck coefficient and resistance are measured upon heating from 25 K to 300 K and the results compared with those for the polycrystalline bulk bismuth sample. The resistivity of the micro-wire array is found to be 1.37 muOmega m at 300 K. Based on the similarities between the temperature dependences of resistivity and Seebeck coefficient for the wire and bulk samples, contact resistance of the wires is considered to have been completely eliminated. This technique makes it possible to simultaneously estimate the resistivity and Seebeck coefficient for nano-wire arrays of thermoelectric materials

Investigation of physical and electric properties of silver pastes as a binder for thermoelectric materials

The physical and electric properties of several silver pastes, Pb-Sn solder, and indium were systematically investigated as potential binders for attaching electrodes to bismuth-based bulk or nanowire array thermoelectric elements. Scanning electron microscopy observations and x-ray diffraction surface measurements of the silver pastes were performed and the temperature and magnetic field dependence of the resistance were measured to characterize their electrical properties from 300K down to 15K and from 0to1T at 15K, respectively. The silver pastes were not reactive with bismuth and hence were found to be potentially useful as electrically conductive adhesives with bismuth-based materials. The properties of each of the binders were quite different; the typical size of the particles in the silver pastes before solidification were distributed in the range from 3nmto20μm, and the size strongly affected the resistances of the silver pastes, which were distributed over 2 and 3 orders of magnitude at 300 and 1...

Numerical calculation of magneto-seebeck coefficient of bismuth under a magnetic field

Seebeck and Nernst coefficients were numerically calculated by solving the Boltzmann equation with relaxation time approximation for bismuth under a magnetic field as functions of the products of cyclotron frequency (ωc) and relaxation time (τ0), taking into consideration the scattering process of carriers as a function of energy. The relationship between ωcτ0 and magnitude of the magnetic field was derived from the definition of mobility, and each coefficient was estimated as a function of the magnetic field. The magneto-Seebeck coefficient was estimated by the addition of the Seebeck coefficient to the Nernst coefficient, and the contribution of thermoelectric effect in the presence of the magnetic field was dominant, being derived from the Seebeck effect. The magnetic field and temperature dependences of the magneto-Seebeck coefficient were evaluated by the use of a two-carrier model and mobility of single-crystal bismuth. The results show that the magneto-Seebeck coefficient can be improved by a factor of 1.3 to 1.4 in the presence of a magnetic field to control the scattering process of the carriers.

Geometrical Dependence of Magnetoresistivity in Bismuth Microwire Arrays

Geometrical dependence of the magnetoresistivity using polycrystalline bismuth microwire array was measured under magnetic fields of 0-2 T at temperatures of 50-300 K, and its diameter of prepared microwire arrays were 10 and 25 mum. Aspect ratio, defined by wire length divided by the diameter, was determined for the wire array since both edges of all wires were attached to Ti thin film layer in order to avoid contact resistance. The attachment of the layer makes it possible to estimate the resistivity of the wire arrays. Both resistivities of the wire arrays at 300 K were equal to approximately 1.35 muOmegam and were good agreement not only at 300 K but also in temperature region of 50-300 K in the absence of a magnetic field. Although the magnetoresistivities of both samples increased in the presence of the magnetic field, the behavior of magnetoresistivity depended on wire diameter. The magnetoresistivity of the wire array with higher aspect ratio was suppressed even if the resistivities of both samples without magnetic field are equal. The difference of magnetoresistivities was remarkable in high magnetic field and low temperature region. Therefore, the thermoelectric element with high aspect ratio such as micro- and nano-wire array leads suppression of magnetoresistivity increase more than that of bulk material

Numerical analysis of thermoelectric properties of bismuth under magnetic field

We study the anisotropic thermoelectric properties of bismuth associated with many-valley bandstructure in magnetic field. In order to obtain the thermoelectric properties of anisotropic materials under a magnetic field, we numerically solved the Boltzmann equation in tensor form. The magnetic field dependences of Seebeck coefficient tensor and conductivity tensor are calculated in various directions of magnetic field. The ratio of effective mass along the longest axis and the shortest axis of each ellipsoid is sensitive for the magnetic field effect of thermoelectric properties. The thermoelectric properties of bismuth under magnetic field are calculated varying relaxation time and Fermi energy, which concern purity of bismuth sample. The lower relaxation time, which corresponds to neutral impurity concentration, makes the optimum strength of magnetic field higher. The higher Fermi energy corresponding to doping impurity causes larger magneto-Seebeck coefficient.

Seebeck coefficient and resistivity measurement of polycrystalline Bi in a magnetic field

Seebeck coefficient and resistivity of a polycrystalline Bi sample were measured from 15 to 300 K in a magnetic field. The measured Seebeck coefficient at 15 K was -10 /spl mu/V/K at 0 Tesla; however, the Seebeck coefficient enormously varied as a function of magnetic field and was over -1 mV/K at 1.5 Tesla. The Hall coefficient was measured by Van der Pauw method in order to evaluate the carrier density for estimating the mechanism of varying Seebeck coefficient at 15 K, and the carrier density decreased by a factor of 1/30 at 1.5 Tesla. It was shown that one reason for the variation of the Seebeck coefficient was the influence of phonon drag effect, even for polycrystalline Bi. A large Seebeck coefficient was observed below the Debye temperature of Bi in the magnetic field, The resistivity at 15 K also enormously increased in the transverse magnetic field due to the decreased carrier density, and the ratio of the increase of the resistivity compared to the case for no magnetic field was approximately 1000 times at 1.5 Tesla and decreased with increasing temperature. The increase in the resistivity influenced the phonon drag effect and increased the Seebeck coefficient.

Investigation on binder for thermoelectric module

Physical and electric properties of several silver pastes were investigated to replace conventional Pb-Sn solder used in thermoelectric module. The melting point of the conventional solder is relatively low and its temperature stability is insufficient for use in a high temperature region. Therefore silver paste (epoxy) is a candidate instead of the Pb-Sn solder. A copper block was glued to electrodes using several silver pastes to measure the electric property, and the resistances were measured by four-probe method from 15K to 300K, and the magnetic field dependence was also investigated. The results showed that the resistance of a silver paste adopted nanoparticles (called silver nano paste) was lower than that of any binders used. We concluded that the nano paste is a candidate as a. binder for conventional thermoelectric modules. Besides the nano paste has a possibility to be used on thermoelectric modules in the high temperature region.