Screening and Fabrication of Half-Heusler phases for thermoelectric applications
- PDF / 2,592,717 Bytes
- 6 Pages / 595 x 842 pts (A4) Page_size
- 61 Downloads / 256 Views
1128-U01-10
Screening and Fabrication of Half-Heusler phases for thermoelectric applications Wilfried Wunderlich1, Yuichiro Motoyama1 1) Tokai University, Fac. Eng, Materials Science Dept., Hiratsuka-shi, Japan
ABSTRACT Half-Heusler phases have gained recently much interest as thermoelectric materials. Screening of possible systems was performed by drawing their stability region in a three-dimensional Pettifor map. The fabrication of Half-Heusler phases requires three steps, surface activation of the raw material by ball milling, arc-melting of pressed pellets and finally long-term annealing treatment in a vacuum furnace. On doped TiCoSb specimens, Seebeck coefficients of 0.1 mV/K, on NiNbSn 0.16 mV/K were measured, although the microstructure was not yet optimized. INTRODUCTION Thermoelectric materials (TE) are considered as clean energy sources helping to solve the severe CO2- problem, but materials with higher efficiency need to be found. The figure-of-merit ZT=S2T/ requires a high Seebeck coefficient S and electric conductivity and low thermal conductivity . For increasing ZT several concepts for materials design of thermoelectrics have been introduced [1-2], such as phonon-glass, electron-crystal, (PGEC), heavy rattling atoms as phonon absorbers, high density of states at the Fermi energy, differential temperature dependence of density of states, high effective electron mass [3], superlattice structures with their confined two-dimensional electron gas [4] and electron-phonon coupling [5,6]. In this study we focus on the search and fabrication of Half-Heusler (HH) structures, which have been found as successful thermoelectric materials, like NiTiSn [7,8]. The reason why HH [7-9], perovskite [3-5] and Skutterudite [2] are successful is sketched in fig. 1. The phonon wave pushes the electron waves through the crystal, when the electron-phonon coupling has suitable interaction energy [6]. This can be successful, when the electron waves have enough freedom to vibrate. These three crystal structures have vacant lattice positions or force atoms to sit in larger atomic distances than according to their atomic spheres. The empty space is one of the necessities for good thermoelectric materials. In the first section the search for new Half-Heusler phases by three-dimensional Pettifor maps is described. The second and third sections describe the fabrication, experimentally obtained microstructures and the thermoelectric properties of different systems which were selected because they are possible candidates for HH alloys according to the 3- Pettifor maps. THREE-DIMENSIONAL PETTIFOR MAPS Drawing of Pettifor maps, see e.g. [10, 11] is the suitable method to display regions of element combinations, in which certain crystal structures are stable. To our best knowledge it is the first time to show such a map for ternary components XYZ. In HH-phases elements on each position of the three positions X1 Y Z come from different groups of the periodic table (fig. 2).
Figure 1. Sketch of the inter-action between phononand el
Data Loading...
