Developing mechanicaland chemical stable contacts for thermoelectric oxide materials

D. Muchilo¹, A. Mrotzek¹, E.Müller¹, K. Kozlowska², J. Plewa², H.Altenburg²

¹GermanAerospace Center (DLR), Institute of Materials Research, D-51170 Cologne,Germany

²Univ. of Applied Sciences, Departmentof Chemical Engineering, D-48565 Steinfurt, Germany

Oxide thermoelectric materials are characterisedby a high thermal stability in air, are non-toxic and exhibit promisingthermoelectric properties. Materials as Na_xCoO₂ are favourable candidates forthermoelectric energy conversion up to high temperature. One of the maintechnological problems of oxide thermoelectric devices is to providemechanically and chemically stable contacts at the hot side. In this study,joints of the thermoelectric oxide Na_xCoO₂ with metals prepared by hightemperature brazing and diffusion welding have been investigated. The use ofcommercially available activated brazes like B-Ag96Ti 970 and B-Ag72,5CuInTi730/760 (according to ISO 3677) is limited because they require processingtemperature as high as 800-1000 °C leading to decomposition of Na_xCoO₂ under vacuum and argon, respectively.However, employing metallic contacts of Ni, Au, Pt, and Pd, respectively, whichpossess high thermal stability is feasible. Sufficient adhesion between metaland oxide materials can be achieved by adding active metal components as Zr orTi.

A thin layer (5 µm) of Ni, Au, and Pt,respectively, was deposited on Na_xCoO₂ by sputtering. In order to study diffusion between metaland thermoelectric material in air (an atmosphere where Na_xCoO₂ remains stable) the metallised samplewas covered by a protective coating of Al₂O₃. Thereaction between metal and thermoelectric material was investigated fordifferent annealing periods at high temperature in air by means of REM and XRDrevealing little diffusion between Na_xCoO₂ and Au and Ni, respectively.

This work is supported by theDeutsche Forschungsgemeinschaft, No. KA 664/10-1