Yasutoshi Noda

Bismuth-telluride/iron-disilicide segmented thermoelectric elements: patterning, preparation and properties

We report on the development of a stacked thermogenerator element consisting of Bi-Te based materials for the low temperature and FeSi/sub 2/ for the high temperature stage. In order to determine the optimum segment length, a new numerical method was applied which maximizes the differential, i.e. local electrical power output. The resulting segment length for the Bi-Te stage is smaller than predicted by locally maximizing the figure of merit. For the practical preparation of the anticipated structure, powdered semiconductors and metals were consolidated by means of a uniaxial hot-pressing method, i.e. So-called plasma activated sintering (PAS). It was also employed in directly forming the material junctions. Due to the large differences in thermophysical and mechanical properties, Bi-Te and FeSi/sub 2/ cannot be directly joined in a single processing step. We employed Ni as a suitable common interface material with intermediate coefficient of thermal expansion. The joint to the low yield strength FeSi/sub 2/ requires however an additional layer, for which NiFe alloys were identified to be applicable. From the standpoint of electrical and thermal transport, all junctions exhibit reliable properties. Problems occur at the FeSi/sub 2//NiFe interface under high atmospheric temperatures due to the development of corrosion products and mechanical mismatch of the reaction layer.

Ni/n-PbTe and Ni/p-Pb/sub 0.5/Sn/sub 0.5/Te joining by plasma activated sintering

In order to prepare the thermoelectric element, Ni electrodes were joined with n-PbTe or p-Pb/sub 0.5/Sn/sub 0.5/Te by plasma activated sintering (PAS). To reduce the interface resistivity between Ni and p-Pb/sub 0.5/Sn/sub 0.5/Te, a buffer layer of p-SnTe was introduced into the Ni/p-Pb/sub 0.5/Sn/sub 0.5/Te joint. The characterization of the Ni/n-PbTe and Ni/p-SnTe/p-Pb/sub 0.5/Sn/sub 0.5/Te junctions was carried out by measuring voltage distribution around the joint boundary. For Ni/n-PbTe joint, no potential voltage gap was found at the interface between Ni and n-PbTe. In the case of Ni/p-Pb/sub 0.5/Sn/sub 0.5/Te joint, large potential gap was found at the interface, where the intermediate layer with complicated composition was formed. With insertion of p-SnTe between Ni and p-Pb/sub 0.5/Sn/sub 0.5/Te, low resistivity joint of metal/semiconductor was formed. These results indicate that the thermoelectric elements were successfully formed by the PAS process.

Thermoelectric and transport properties of semi-conducting Bi88Sb12 alloy

Abstract Homogenized sample of Bi88Sb12 was prepared by quenching and annealing at 523 K for 200 days. It is an n-type semiconductor, and its properties show different temperature dependences between the low-temperature (T K ) and high-temperature (T>70 K ) regions. Chemical potential was calculated from the measured carrier concentration, and found to change abruptly at 70 K , increasing rapidly with temperature. This explains the above differences in the two temperature regions in the framework of the Boltzmann theory, indicating Bi88Sb12 is a strongly degenerate semiconductor.

Evaluation of monolithic and segmented thermoelectric materials by using a large-temperature-span apparatus

In order to evaluate the thermoelectric properties and power output performance of a gradient TE material under a wide working temperature range, a large-temperature-span apparatus has been developed. However, an accurate evaluation is not so easy to put into practice on the gradient TE material because a lot of difficulties are associated with the heat flow control and the reducing of electrical resistance in the measurement circuit. The present work is a practice to characterize monolithic and segmented thermoelectric materials by the new developed apparatus and approach the method to evaluate a gradient TE material accurately and effectively. This work was carried out for the evaluation of a stacked monolithic (SiGe, PbTe and Bi/sub 2/Te/sub 3/) TE leg and a segmented thermoelectric branch (PbTe/Bi/sub 2/Te/sub 3/), and we confirm their thermoelectromotive force, electrical resistance and temperature distribution both for the apparatus and for specimen stacks. We also made a trial power output evaluation using an electronic load and explore some technical problems in the evaluation of TE materials under a large temperature span. The results of evaluation on power output by electronic load indicate much lower value than the calculation using the real electrical resistance in TE materials. We discuss some problems for the evaluation of stacked thermoelectric materials under a large temperature difference condition.

Preparation of p- and n-type SiC-based thermoelectric materials by spark plasma sintering

We report on the preparation of Si/sub 3/N/sub 4/ or Al/sub 4/C/sub 3/ added SiC by spark plasma sintering (SPS) and their thermoelectric properties. The relative densities of all the sintered materials were more than 80% of the theoretical value. The conduction types were controlled by the addition of Si/sub 3/N/sub 4/ for n-type and Al/sub 4/C/sub 3/ for p-type. The electrical resistivity decreased and the Seebeck coefficient increased with temperature. We also found that the power factor depends on the composition of the Si/sub 3/N/sub 4/ or Al/sub 4/C/sub 3/ and takes the maximum value at 7wt% addition at 973K.

Approach to Optimum Layer Structure of Functionally Graded Materials

Minimum interlayer numbers of functionally graded materials (FGMs) are studied based on the empirical analysis of thermal stress due to the differences in the thermal expansion coefficient  and temperature T between sintering and room temperatures. It is found the maximum ,   and the minimum interlayer number necessary to produce a NiCr / Fly ash FGM structure without interface cracking were 4.0×10-6 K-1, 1080 K, 0.043 and 2, respectively. The condition  < 0.043 was derived and confirmed to be valid for available FGM systems.

Surface texture of Bi 2 Te 3 -based materials deformed under pressure-current heating

Deformation of Bi₂Te₃-based materials was performed. The source disks with nominal compositions of (Bi_0.25Sb_0.75)₂Te₃, (Bi_0.25Sb_0.75)₂(Te_0.95Se_0.05)₃ and (Bi_0.90Sb_0.10)₂(Te_0.95Se_0.05)₃ were cut from the ingots grown by the vertical Bridgman method (VBM). The disks were deformed by either cold press (CP) or pressure current heating (PCH). The crystal structures of the deformed materials were identified to be hexagonal by X-ray diffraction. By using the diffraction intensities, the degree of the preferred orientation was estimated, where the texture of the (00middot <i>l</i> ) plane was detected in the thin surface layer regions of the (PCH) materials. The thermal stress acting to the Bi₂Te₃-based material was estimated by using physical properties at the heterogeneous contact between the graphite and the Bi₂Te₃-based material at PCH temperature. The extension stress was found to be large enough to deform the Bi₂Te₃-based material at the contact surface. Therefore, it can be concluded that the origin of the surface texture is attributed to the thermal stress at the heterogeneous contact. The resistivity measured parallel to the pressing axis was 1/6 less than that measured perpendicular to the axis. However the anisotropies in Seebeck coefficient and carrier concentration were not so clearly detected in these two directions. The power factors of the PCH deformed materials were comparable to those of the original as-grown ingot.

Mechanism of thermopower maximum of Bi-Sb semiconducting alloys

Temperature dependence of the Seebeck coefficient of Bi-Sb alloy, which is known to be an efficient thermoelectric material, is investigated in terms of the multi-carrier Boltzmann transport theory. The chemical potential is calculated self-consistently with the measured Hall coefficient, by solving an integral equation. The calculated chemical potential shows a clear indication of the extrinsic-to-intrinsic transition, and determines the temperature dependence of all the transport properties. In particular, maximum thermopower is bound to occur in the vicinity of the transition temperature. It is confirmed that the electrons in the L-point conduction band act as dominant carriers, whereas the valence bands at the L, T and H points serve primarily as carrier reservoirs. The electrons are sufficiently degenerate that the conventional analysis is misleading, corresponding to the unphysical solution of the equation.

Thermoelectric properties of semiconducting Bi-rich Bi-Sb alloys

Bi/sub 100-x/Sb/sub x/ (x=8-17) alloys were prepared by direct melting of constituent elements, which was followed by quenching and annealing. The semiconducting and thermoelectric properties of the samples were investigated by. measuring Hall coefficient, electrical resistivity and Seebeck coefficient in the temperature range from 20 to 300K. The properties change gradually with the Sb concentration x, which is attributed to the variation of the energy gap. In all samples, the electrical resistivity decreases with increasing temperature and the absolute value of the Seebeck coefficient takes a maximum value around 70K. The large power factor was resulted in the temperature range from 100 to 200K. The Hall mobility was enhanced by annealing, which leads to a small electrical resistivity and a large Seebeck coefficient.. Consequently, a large power factor about 8.5W/mK/sup 2/ was obtained in the annealed Bi/sub 88/Sb/sub 12/ alloy.

Preparation and thermoelectric properties of AgSbTe/sub 2/

Melt-grown and sintered materials of AgSbTe/sub 2/ were prepared by Bridgman method and plasma-activated sintering, respectively. In case of low growth rate of 10 mm/d, the ingot was coarse-grain polycrystal and the EPMA maps showed a 80% uniformity along the growth axis, while high growth rate of 10 mm/h caused non-uniformity with Ag/sub 2/Te precipitation in a whole ingot. The thermoelectric properties were characterized by high carrier concentration in the order of magnitude of 10/sup 25/ m/sup -3/ and low mobility of 10/sup -3/ m/sup 2/ V/sup -1/ s/sup -1/. The microstructures of the sintered materials reflected whether their powder source materials were obtained from the ingot with or without Ag/sub 2/Te-precipitation.