Diffusion barriers for CeFe 4 Sb 12 /Cu thermoelectric devices
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Diffusion barriers for CeFe4Sb12/Cu thermoelectric devices Laetitia Boulat, Romain Viennois, Didier Ravot, Nicole Fréty Université Montpellier 2, Institut Charles Gerhardt, UMR 5253 CNRS-UM2-ENSCM-UM1, cc 1504, Place E. Bataillon, 34095 Montpellier Cedex 5, France ABSTRACT The efficiency of a tantalum nitride interlayer as a diffusion barrier for CeFe4Sb12 thermoelectric material against electrode copper material has been investigated. The thermal stability of CeFe4Sb12/TaN/Cu stackings has been investigated after annealing at 600°C from a microstructural study. CeFe4Sb12 and Cu appear to chemically react through the formation of CeCu2 and Cu2Sb phases whereas no reaction is observed for CeFe4Sb12 with TaN. This study showed that the TaN interlayer cannot inhibit the diffusion of Sb from the skutterudite substrate to the copper electrode but prevents the diffusion of Ce and consequently the formation of the CeCu2 phase. INTRODUCTION The conversion of waste heat generated during industrial processes to electricity by use of thermoelectric devices is of particular interest [1-2]. Skutterudite compounds have been reported to be promising thermoelectric materials for applications in the [400°C-600°C] intermediate temperature range [3]. Doped or filled CoSb3 and CeFe4Sb12-based skutterudites have been shown to be good candidates, respectively, for n and p-type legs of thermoelectric devices. However the performance of thermoelectric devices is strongly dependent on the joining of thermoelectric couples with metal electrodes as the conversion efficiency is greatly influenced by the contact resistance [4, 5]. A high electrical and thermal conductivity is required associated with a high interfacial mechanical strength [4]. Moreover the joining material has to be selected to avoid any interfacial reaction occurring during the device fabrication and use. The joining material has to play a role of diffusion barrier to limit the interfacial reactions which may be detrimental to the thermoelectric device performance. The aim of this work is to study joining of CeFe4Sb12 material with Cu as electrode material. TaN is selected as the interlayer between CeFe4Sb12 and Cu due to its high thermal stability and electrical conductivity [6, 7] and due to its efficiency against Cu diffusion [8, 9]. This study focuses on the thermal stability of the CeFe4Sb12/TaN/Cu stacking after annealing at 600°C, which is reported to be the highest temperature of CeFe4Sb12 use for thermoelectric applications [3]. The efficiency of TaN as a diffusion barrier is determined from the evolution of the microstructure after annealing as a result of interfacial reactions. Interfacial reactions in CeFe4Sb12/Cu, CeFe4Sb12/TaN and CeFe4Sb12/TaN/Cu systems have been studied for this purpose. EXPERIMENTAL DETAILS CeFe4Sb12 skutterudite ingots were synthesized by direct reaction of stoichiometric amounts of Ce, Fe and Sb elements (Ce 99.9 % - Aldrich, Fe 99.9999 % - Alfa, Sb 99.9999 % Alfa). The mixing was sealed in a carbon-coated silica tube under vacuum and heate
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