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Journal of ELECTRONIC MATERIALS, Vol. 42, No. 7, 2013
DOI: 10.1007/s11664-012-2440-8
2013 TMS
High-Pressure Torsion to Improve Thermoelectric Efciency of Clathrates?
X. YAN,1,2,4 M. FALMBIGL,2 G. ROGL,2 A. GRYTSIV,2 A. PROKOFIEV,1E. BAUER,1 P. ROGL,2 M. ZEHETBAUER,3 and S. PASCHEN1,5
1.Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 810, 1040 Vienna, Austria. 2.Institute of Physical Chemistry, Vienna University, Wahringerstr. 42, 1090 Vienna, Austria. 3.Research Group Physics of Nanostructured Materials, Vienna University, Boltzmanngasse 5, 1090 Vienna, Austria. 4.e-mail: [email protected],5. e-mail: [email protected]
High-pressure torsion (HPT), as a technique to produce severe plastic deformation, has been proven effective to improve the thermoelectric performance of skutterudites. In this report, we present microstructural and thermoelectric properties of the clathrate Ba8Cu3.5Ge41In1.5 processed by HPT. The sample was synthesized from high-purity elements, subsequently annealed, ball milled, and hot pressed, and nally subject to HPT. Compared with the ball-milled and hot-pressed sample, the HPT-processed sample has higher electrical resistivity and Seebeck coefcient, and lower thermal conductivity, electron concentration, and mobility, which is attributed to the reduced grain size and increased density of dislocations, point defects, and cracks. No essential improvement of the dimensionless thermoelectric gure of merit is observed in the investigated temperature range, questioning the universal versatility of this technique for improvement of thermoelectric materials.
Key words: Thermoelectric materials, clathrates, high-pressure torsion, thermoelectric and transport measurements
INTRODUCTION
The efciency of a material for thermoelectric applications is characterized by the dimensionless thermoelectric gure of merit ZT = TS2/(qj), where
T, S, q, and j are the absolute temperature, Seebeck coefcient, electrical resistivity, and thermal conductivity, respectively. Materials with large values of ZT are necessary for efcient thermoelectric devices. Therefore, much effort has been devoted to the search for new materials and new fabrication techniques to enhance ZT.
Intermetallic type I clathrates, as promising thermoelectric materials for high-temperature applications, have been investigated extensively in recent years. With their unique structural characteristics, they are generally considered as a realization of the phonon glasselectron crystal (PGEC) concept:1 The atoms in the larger of the two cages of the crystal
structure are argued to reduce the lattice thermal conductivity by acting as additional phonon scattering centers15 and/or by modifying the phonon density of states.6 The charge transport, however, is assumed to be governed by...