Kinetics, Stability, and Thermal Contact Resistance of Nickel–Ca 3 Co 4 O 9 Interfaces Formed by Spark Plasma Sintering T.C. HOLGATE, 1,3 N. WU, 1 M. SØNDERGAARD, 2 B.B. IVERSEN, 2 N.V. NONG, 1 and N. PRYDS 1 1.—Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Frederiksborgvej 399, Building 779, 4000 Roskilde, Denmark. 2.—Centre for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Aarhus C, 8000 Aarhus, Denmark. 3.—e-mail: timh@risoe.dtu.dk Incorporating oxide thermoelectric (TE) materials into TE power generation modules necessitates study of the interfaces between the oxide TE elements and the interconnect materials used to transfer current between them. In this study, interfaces between pure nickel and undoped calcium cobaltate (Ca 3 Co 4 O 9 ) have been formed directly by spark plasma sintering (SPS). An intermediate NiO phase is formed during the SPS processes, which grows during post-heating with Co entering from the cobaltate side to form a graded Ni 1x Co x O interfacial layer. The electrical and thermal transport across these interfaces, as well as the long-term chemical stability of the intermediate layers, have been studied and are discussed. Key words: Contact resistance, thermoelectric, metal–oxide interface, calcium cobaltate, spark plasma sintering Abbreviations TE Thermoelectric SPS Spark plasma sintering Co349 Ca 3 Co 4 O 9+d Co225 Ca 2 Co 2 O 5 SEM Scanning electron microscopy INTRODUCTION In order for high-temperature oxide thermoelectric (TE) modules to become a viable route for power generation, the overall efficiency of these devices must be improved. While most research currently focuses on enhancement of the TE properties of the p- and n-type elements of the module (an element meaning one bar-shaped sample of p- or n-type TE material; Fig. 1), it is also necessary to develop stable interconnects exhibiting low contact resistance. The majority of previous works have used silver foil and/ or conducting silver paste to connect the TE legs. The main issue with this approach, besides the cost, is that silver tends to flow at high temperatures and the contact area is reduced. Koumoto et al. 1 have shown that this can be mechanically stabilized and the contact resistance reduced by the addition of a few percent by weight of the TE oxide powder. There is much work to be done in reducing the electrical and thermal contact resistance (with the latter rarely even being discussed) in order to develop a suitable interconnect. A good interconnect candidate must be relatively cost effective, oxidation resistant in air at high temperatures (up to 800°C to 900°C), and able to form a well-contacted, stable interface with the TE oxide materials. The latter criterion must be met in terms of both the mechanical and the chemical stability. Mechan- ical stability implies that the interface between the interconnect and the TE elements will not form cracks under thermal cycling (generally caused by a large mismatch in thermal expansion), and that the adhesion between them will be sufficient to prevent delamination of the interconnect from the TE ele- ment. The chemical stability means that any inter- diffusion and intermediate phase growth at the (Received July 8, 2012; accepted November 8, 2012) Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-012-2363-4 Ó 2012 TMS