Key Engineering Materials Vol. 617

Abstract: An efficient synthesis route is proposed to obtain single powder CaFe₅O₇ ceramic. This complex structure can be described as an intergrowth between one CaFe₂O₄ unit and n= 3 slices of FeO Wustite-type structures. A fine structural study has been carried out at room temperature by transmission electron microscopy (TEM) observations (ED and HREM). The analysis of these data has revealed a supercell with a monoclinic symmetry associated to some twinning phenomena. From the hkl conditions deduced to electron diffraction study, the centrosymmetric P2₁/m setting can be proposed. This monoclinic cell exhibit close relationships with the previous one reported as orthorhombic (a_o =3.05 Å, b_o = 10.05 Å and c_o = 18 Å) according to the following metric a_m=2c_o, b_m=c_o, c_m=b_o/2sinβ and β=106.74°.

Abstract: The influence of heat treatments in air on the surface and thermoelectric properties of polycrystalline silicon clathrate of nominal composition Ba₈Al₁₅Si₃₁ was investigated. The samples were prepared by combining arc melting and spark plasma sintering methods. Heat treatments were conducted in air at 873 K for 0-480 h. The surface oxidation was investigated by bulk surface X-ray diffraction (XRD) measurements, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The Seebeck coefficient and the electrical conductivity of samples were measured before and after heat treatments. XRD, SEM, EDS, and XPS results revealed that the clathrate phase near the surface was changed to Al-poor clathrate phases and an oxide phase (probably barium aluminate: BaAl₂O₄). However, the thermoelectric properties of samples were almost unchanged by the heat treatment in air at 873 K for up to 480 h because the chemical composition of the inner part of sample was almost stable.

Abstract: A water lens was used to concentrate sunlight on the surface of a thermoelectric (TE) module in order to heat it. The shape of this water lens could be flexibly adjusted to compensate for solar altitude changes. The light condensed by the water lens produced a large energy distribution on the top of the module. In this study, we simulated the power generation properties when the top of the module was heated by light with a certain condensing distribution. The simulation results revealed that the energy distribution had little effect on the TE generation if the solar light was effectively condensed on a plate with a condensing width of a few millimeters.

Abstract: Heat and electric charge transfer through the thermoelectric (TE) module were analyzed numerically. The thermal heat can expand or shrink in the trapezoid shape in comparison with the conventional Π type module, and the output and efficiency from the module with trapezoid TE elements was examined from the view of shape optimization. The temperature profile and some thermoelectric properties were calculated using the infinite volume method and the original code. The temperature profile in the module showed a complex distribution in the TE elements, however, the efficiency in power generation did not change from that of the rectangular TE module.

Abstract: Perovskite-type A_xCa_1-xB_yMn_1-yO_3-δ(A; La, Bi, Y, Sr) (B; Ni, Ti, V) thin films were prepared by the conventional electrostatic spray deposition (ESD) method in order to study which kind of substitution element show better thermoelectric property than A_xCa_1-xB_yMn_1-yO_3-δ. Electrical conductivity of the single-doped compound was increased when the substituted element was higher valence than Ca or Mn ion. PF value also showed similar tendency. Thermoelectric property of all compounds obtained the ESD method were consistent with the report of the solid-state reaction. Also, co-doped Bi_0.02Ca_0.98Mn_0.98V_0.02O_3-δ and Bi_0.02Ca_0.98Mn_0.98Ni_0.02O_3-δ showed relatively higher conductivity and PF value.

Abstract: Thermoelectric (TE) phenomenon of a helical generator is numerically analyzed by using the finite-volume method in combination with a three-dimensional finite-element (FE) model. The distributions of temperature and current density are significantly influenced by the generator dimension. The output power of helical generator is also affected by the geometric parameter, such as the helical pitch. The output power and conversion efficiency of helical generator are better than those of straight generator where all the TE elements aligned in a straight line. The helical geometry has a satisfactory potential to be a good TE generator.