Advances in Science and Technology Vol. 74

Abstract: Thermal fluids are served to give the greater ΔT to the thermoelectric (TE) junctions. For the larger power generation using a limited number of modules and the limited amounts of thermal fluids, the multi-layered TE panels can save the occupied space effectively. Because the temperatures of the fluids vary along the planer TE panels due to heat exchange through the panels, the directions of fluids is important to obtain the larger output power, P, from a TE power generator. The methods to stack a few long planer TE panels and to determine the fluid directions are mathematically studied from the steady-state heat exchange. P for various kinds of fluid directions are calculated. P commonly shows the maximum against the panel length because of the balance between the internal resistance and ΔT. This maximum of P can be further maximized by choosing the counter flow. The non-dimensional analysis predicts that two fluids should flow in counterwise. When two series of fluids are used, the circulation method is another key issue for the optimal TE design.

Abstract: The theory of thermal matching of a thermoelectric generator with the environment has been applied in this work to a wearable thermoelectric generator. This enabled evaluation of its top performance characteristics in typical environmental conditions. To correctly perform the modeling, the relevant properties of the human body as a heat generator for a small-size thermoelectric generator have been studied and presented in the paper as well. The results have been practically validated in different wearable thermoelectric generators. In particular, a power over 1 mW per square centimeter of the skin has been practically demonstrated on a walking person at ambient temperature of –2 °C. The comparison with wearable photovoltaic cells shows that in typical situations thermoelectric generators provide at least ten times more power.

Abstract: Recently it was reported that AgxTeyTz shows extremely low thermal conductivity, and high power generating efficiency as a thermoelectric conversion material[1]. We evaluate the seebeck coefficient on basis of the first principles calculations. The electronic band structure calculation is performed using all-electron full-potential linearized augmented plane-wave method(FLAPW) within the local density approximation(LDA). The seebeck coefficent is analyzed by Bloch-Boltzmann equation. In this paper, we find that AgTeTl and AgTe2Tl3 are better thermoelectric material among AgxTeyTlz.

Abstract: A partially cobalt-substituted solid solution of Nowotny chimney-ladder phase, (Mn1-xCox)Si􀀂, has been prepared using a tetra-arc-type furnace and a subsequent annealing process. The compounds consist of two tetragonal subsystems of [Mn1-xCox] and [Si], with an irrational c-axis ratio 􀀂 = cMn/cSi ~ 1.7. The crystal structure and thermoelectric properties of (Mn1-xCox)Si􀀂 solid solution were compared with those of the Fe-substituted solid solution, (Mn1-xFex)Si􀀂. In the case of Co-series, extra valence electrons are introduced relative to Fe-series, since the valence electron count is 3d74s2 for Co but 3d64s2 for Fe, respectively. It was naturally expected that the Feand Co-substituted MnSi􀀁 becomes n-type conductor from the p-type one at x > 0.23(5) and x > 0.06(1), respectively. Experimentally, the Fe-substituted samples become n-type at x > 0.28 but it is not the case for the Co-substituted ones. It is thus evident that there is an unknown factor which controls the thermoelectric properties of Co-substituted samples.

Abstract: Sb-doped Heusler Fe2VAl alloy was synthesized to evaluate the effect of heavy element doping on thermoelectric properties of the Heusler alloy. For the sample preparation, a pulse-current sintering technique with fine powder prepared by mechanical alloying was used. By using the powder metallurgy technique, a heavy element with a low melting point can be stably doped, compared to the melting process. Thermal conductivity was effectively-reduced by a small amount of the substitution because of the large atomic mass of Sb compared to the constituent elements of Fe2VAl alloy. Conduction type was controlled by adjusting the valence electron density. For the p-type conduction, Ti substitution for the V site was examined and large positive Seebeck coefficient was obtained while maintaining the low thermal conductivity.

Abstract: The evolution of designed [(Ti-Te)]x[(Sb-Te)]y, [(Bi-Te)]x[(Sb-Te)]y, [(Ti-Te)]w[(Bi-Te)]x[(Sb-Te)]y and [(Ti-Te)]w[(Bi-Te)]x[(Ti-Te)]y[(Sb-Te)]z precursors were followed as a function of annealing temperature and time using both low and high angle x-ray diffraction techniques to probe the self assembly into nanolaminate materials. The [(Bi-Te)]x[(Sb-Te)]y precursors were found to interdiffuse at low temperatures to form a (BixSb1-x)2Te3 alloy. The [(Ti-Te)]x[(Bi-Te)]y and [(Ti-Te)]x[(Sb-Te)]y precursors formed ordered nanolaminates [{(TiTe2)}1.35]x[Bi2Te3]y and [{(TiTe2)}1.35]x[Sb2Te3]y respectively. The [(Ti-Te)]w[(Bi-Te)]x[(Sb-Te)]x precursors formed [{(TiTe2)}1.35]w[(Bi0.5Sb0.5)2Te3]2x nanolaminates on annealing, as the bismuth and antimony layers interdiffused. Over the range of TiTe2 thicknesses used in [(Ti-Te)]w[(Bi-Te)]x[(Ti-Te)]y[(Sb-Te)]z precursors, Bi and Sb were found to interdiffuse through the 2-4 nm thick Ti-Te layers, resulting in the formation of (BixSb1-x)2Te3 alloy layers as part of the final nanolaminated products. When the Bi-Te and Sb-Te thicknesses were equal in the amorphous precursors, symmetric [{(TiTe2)}1.35]m[(Bi0.5Sb0.5)2Te3]n nanolamiantes were formed. When the thicknesses of Bi-Te and Sb-Te layers were not equal in the amorphous precursor, asymmetric [(TiTe2)1.35]m[(BixSb1-x)2Te3]n[(TiTe2)1.35]m[(BixSb1-x)2Te3]p nanolaminates were formed. These results imply that to form (A)w(B)x(C)y nanolaminates using designed layered precursors all three components must be immiscible. To form (A)x(B)y(A)x(C)z nanolaminates, the components must be immiscible or the precursor to the A component and the A component itself must be an effective interdiffusion barrier preventing B and C from mixing.

Abstract: In this work, nano-crystalline Mg2Si powder was prepared by ball milling and structural studies vs ball milling time are presented. The identification of the phases of the materials and the evaluation of their purity were performed using Powder X-ray diffraction (PXRD). Crystallite size evolution during ball milling was followed by PXRD and single line analysis, based on Scherrer equation. Transmission Electron microscopy (TEM) observations and IR Reflectivity measurements were used for the investigation of nano-features and confirmation of the PXRD results.