High Temperature Thermoelectric Properties of LnPdX (Ln = lanthanide; X = Sb, Bi) Ternary Compounds
- PDF / 135,434 Bytes
- 6 Pages / 595 x 842 pts (A4) Page_size
- 23 Downloads / 257 Views
0886-F08-04.1
High Temperature Thermoelectric Properties of LnPdX (Ln = lanthanide; X = Sb, Bi) Ternary Compounds Takeyuki Sekimoto, Ken Kurosaki, Hiroaki Muta, Shinsuke Yamanaka Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT Ternary compounds LnPdX (Ln = lanthanide elements of La, Gd, Er; X = Sb, Bi) were prepared by a spark plasma sintering (SPS) technique. The crystal structure of LaPdSb and GdPdSb was confirmed to be a hexagonal ZrBeSi-type structure and different from the other compounds with a MgAgAs-type structure. The electrical resistivities ρ of LaPdSb and GdPdSb indicate the metallic or semimetallic characteristics, while those of ErPdSb and LnPdBi indicate semiconductor characteristics. From the ln ρ – 1/T plot, the band gap energies Eg were estimated to be 0.28, 0.053, 0.081, and 0.049 eV for ErPdSb, LaPdBi, GdPdBi, and ErPdBi, respectively. All the samples have positive thermoelectric powers S above room temperature. The largest power factor S2/ρ was obtained as 49.5 µW/K2 cm at 327 K for LaPdSb. From the Hall effect measurements on ErPdX, the carrier concentration n of ErPdSb and ErPdBi were obtained as 5.9x1018 and 3.21x1019 cm-3 at room temperature, respectively. It is considered that the difference of n at room temperature is mainly due to the magnitude of the band gap energy. INTRODUCTION It has been known that ternary compounds LnPdBi (Ln = lanthanide) have a cubic MgAgAs -type (half-Heusler) structure with space group F4/3m (No. 216). On the other hand, ternary compounds LnPdSb crystallize in a variety of structure types depending on lanthanide atoms: hexagonal CaIn2 or ZrBeSi-type [1] (P63/mmc, No. 194) for La-Sm and Gd-Dy, orthorhombic TiNiSi-type for Eu, MgAgAs-type for Dy-Tm, and so on. Recently, thermoelectric properties below room temperature on the half-Heusler compounds LnPdSb (Ln = Y, Ho, Er) and LnPdBi (Ln = Nd, Y, Dy, Ho, Er) have been reported [2, 3]. Before these reports, (Dy, Ho, Er)PdSb [4] and (La, Nd, Gd, Dy, Er, Lu)PdBi [5] systems were reported. These compounds have a positive thermoelectric power. Mastronardi et al. have measured the thermoelectric properties on the samples of the different nominal composition of Pd and/or Sb on (Dy, Ho, Er)PdSb [4]. They have found that the samples of Sb-rich nominal compositions show large electrical resistivity and thermoelectric power, while those of Pd-rich nominal compositions show small electrical resistivity and thermoelectric power. It has been known that the properties of half-Heusler compounds depend on the deviation from the stoichiometric composition and
0886-F08-04.2
annealing condition. However, the thermoelectric properties at high temperature of these half-Heusler and ZrBeSi-type LnPdX have not been reported until now.
Fig. 1. Crystal structure of the cubic MgAgAs-type (left) and the hexagonal ZrBeSi-type (right) ternary compounds LnPdX (Ln = lanthanide; X = Sb, Bi).
EXPERIMENTAL DETAILS Ingots of LnPdX
Data Loading...
