Dislocations in a Partially-Filled Skutterudite
- PDF / 320,020 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 97 Downloads / 224 Views
Dislocations in a Partially-Filled Skutterudite Jennifer S. Harper and Ronald Gronsky Department of Materials Science and Engineering University of California Berkeley, California 94720-1760 ABSTRACT
The partially filled skutterudite structure is a candidate thermoelectric material with the capacity for phonon scattering by the decoupled rattling of filling ions. In this transmission electron microscopy investigation of a 1.6%Ce, 1.6%Ni, 4.9%Ge, 22.8%Co, and 69.1%Sb alloy, the structure is confirmed to be of a partially-filled skutterudite, but with a number of defects. Isolated dislocation dipoles, arrays of dislocation dipoles, and a subgrain boundary are observed and characterized. An atomic structure of a dislocation dipole is also proposed, and the effect of such dislocations on thermoelectric properties is discussed. INTRODUCTION A new effort to pursue better thermoelectrics began in the early 1990’s, based upon the concept of a "phonon glass electron crystal.1" Examples included the clathrates with atoms "rattling" in atomic cages, such as the filled skutterudites that are under intense scrutiny today. It was also theorized about the same time that quantum confinement would improve the thermoelectric properties of materials, and current studies of quantum wells2 and quantum wires3 cover a number of different material systems, including Bi4, and Bi2Te35. The exploration of ternaries and quaternary compounds are also being explored, extending Ioffe’s concepts from binary compounds developed in the 1950’s, to include the chalcogenides and ’normal’ broadband semiconductors.6 Finally, the exploration of "correlated" metals and semiconductors, in which fband conduction provides non-parabolic bands and an unusually high density of states near the Fermi energy are also under way, in pursuit of Mahan and Sofo’s7 theory for the ideal thermoelectric material. The skutterudites, based upon the original compound CoAs3, are at the heart of this investigation. A binary skutterudite has a body-centered unit cell with 32 atoms per unit cell and space group Im 3 (see Figure 1). Both its complex structure and its ability to be alloyed are a continuation of Ioffe’s concept. The thermoelectric properties of the skutterudites can be described by the dimensionless figure of merit, ZT, defined as ZT =
S2σ T κ
(1)
where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity comprised of two components, κe the electronic thermal conductivity, and κg the lattice thermal conductivity, and T is the absolute temperature. In this context, the skutterudites have favorable electronic properties due to very high hole mobilities producing a high power factor (S2σ), but Y8.5.1
they also have a relatively high thermal conductivity, resulting in a relatively low figure of merit (ZT
Data Loading...
