Enhanced thermoelectric performance driven by high-temperature phase transition in the phase change material Ge 4 SbTe 5

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Edgar Lara-Curzio, Ercan Cakmak, and Thomas Watkins Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA

Donald T. Morellia) Department of Chemical Engineering & Materials Science, Michigan State University; and Department of Physics & Astronomy, Michigan State University, East Lansing, Michigan 48824, USA (Received 27 January 2015; accepted 20 April 2015)

Phase change materials are identified for their ability to rapidly alternate between the amorphous and crystalline phases and have large contrast in the optical/electrical properties of the respective phases. The materials are not only primarily used in memory storage applications, but also recently they have been identified as potential thermoelectric materials [D. Lencer et al., Adv. Mater. 23, 2030–2058 (2011)]. Many of the phase change materials studied today can be found on the pseudo-binary (GeTe)1x(Sb2Te3)x tie-line. While many compounds on this tie-line have been recognized as thermoelectric materials, here we focus on Ge4SbTe5, a single phase compound just off of the (GeTe)1x(Sb2Te3)x tie-line, which forms in a stable rocksalt crystal structure at room temperature. We find that stoichiometric and undoped Ge4SbTe5 exhibits a thermal conductivity of ;1.2 W/m K at high temperature and a large Seebeck coefficient of ;250 lV/K. The resistivity decreases dramatically at 623 K due to a structural phase transition which leads to a large enhancement in both thermoelectric power factor and thermoelectric figure of merit at 823 K. In a more general sense, the work presents evidence that phase change materials can potentially provide a new route to highly efficient thermoelectric materials for power generation at high temperature.

I. INTRODUCTION

In an effort to improve and find new devices for solid state energy conversion, much research has recently been dedicated to thermoelectric materials. These materials possess a unique ability to directly convert thermal energy to electrical energy via solid state processes. In commercial applications, thermoelectric modules are used. These modules use n- and p-type thermoelectric materials wired electrically in series and placed thermally in parallel. As a temperature gradient is maintained, a voltage difference will be created across both of the module legs. The voltage difference generated can then be used to do electrical work. The efficiency by which a thermoelectric module generates electrical power from a temperature difference is constrained by the Carnot efficiency as well as the dimensionless figure-of-merit ZT, which is a quantity derived from the properties of fundamental materials: ZT 5 S2r/j. S2r is defined as the power factor, where S Contributing Editor: Ryoji Funahashi a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.124

is the Seebeck coefficient and r is the electrical conductivity, and j is the thermal conductivity of the material; consisting of both electronic and lattice contributions. The search for new high-e