http://journals.cambridge.org Downloaded: 11 Dec 2014 Username: timholgate IP address: 192.88.94.1 Effects of conducting oxide barrier layers on the stability of CroferÒ 22 APU/Ca 3 Co 4 O 9 interfaces Tim C. Holgate, a),b) Li Han, NingYu Wu, Ngo Van Nong, and Nini Pryds Department of Energy Conversion and Storage, Technical University of Denmark—Risø Campus, DK4000 Roskilde, Denmark (Received 29 April 2014; accepted 8 October 2014) Practical implementation of oxide thermoelectrics on an industrial or commercial scale for waste heat energy conversion requires the development of chemically stable interfaces between metal interconnects and oxide thermoelements that exhibit low electrical contact resistances. A commercially available high-chrome iron alloy (i.e., CroferÒ 22 APU) serving as the interconnect metal was spray coated with LaNi 0.6 Fe 0.4 O 3 (LNFO) or (Mn,Co) 3 O 4 spinel and then interfaced with a p-type thermoelectric material—calcium cobaltate (Ca 3 Co 4 O 9 )—using spark plasma sintering. The interfaces have been characterized in terms of their thermal and electronic transport properties and chemical stability. With long-term exposure of the interfaced samples to 800 °C in air, the cobalt–manganese spinel acted as a diffusion barrier between the Ca 3 Co 4 O 9 and the CroferÒ 22 APU alloy resulting in improved interfacial stability compared to that of samples containing LNFO as a barrier layer, and especially those without any barrier. The initial area specific interfacial resistance of the Ca 3 Co 4 O 9 /(Mn,Co) 3 O 4 /CroferÒ 22 APU interface at 800 °C was found to be ;1mXÁcm 2 . I. INTRODUCTION Thermoelectric power generation has garnered in- terest over the past decade as a clean and reliable technology for recovering unused heat energy from industrial processes, automotives, and nature. The attrac- tiveness of oxide thermoelectrics in lieu of their alloy counterparts are the relative low cost, low toxicity, and natural abundance of their constituent elements, as well as their superior high-temperature stability in air. 1 Standard power generation modules consist of an array of p- and n-type thermoelectric legs (thermoelements) connected thermally in parallel but electrically in series. The latter is generally achieved by bridging neighboring thermo- elements with precious metal foil interconnects (usually silver) and/or conducting pastes (again, usually silver- based). 2–4 In an effort to not mitigate the low-cost advantages oxide thermoelectrics, a cheaper intercon- nect material was investigated. A high-chrome iron alloy (CroferÒ 22 APU, ThyssenKrupp VDM GmbH; denoted Cr22A) developed for solid-oxide fuel cell applications was chosen for its oxidation resistance and a thermal expansion coefficient close to that of Ca 3 Co 4 O 9 (henceforth referred to as Ca349). In a previous investigation 5 of the interface between Ca349 and a custom high-chrome iron alloy without any barrier layer, the interfaces degraded at high temperatures due to a reaction between the Ca in the Ca349 and the protective chromia-containing scale of the alloy to form the poorly conducting CaCrO 4 . It was the intent of this investigation to prevent this reaction by introducing a barrier layer consisting of a relatively well-conducting oxide. This technique has been widely used in the solid oxide fuel cell (SOFC) community with some success. 6–10 Komatsu et al. 6 demonstrated that La(Ni,Fe)O 3 resists the uptake of Cr from the chromia scale of nickel alloys such as Inconel 600, and Yang et al. 7 have shown that (Mn,Co) 3 O 4 -coated Cr22A can success- fully be applied to a SOFC cathode material ((La,Sr)FeO 3 ) to create a stable and conducting interface. The specific purpose of this study was to investigate the effectiveness of these barrier materials when applied to the interface between the thermoelectric material Ca 3 Co 4 O 9 and the CroferÒ 22 APU interconnect alloy. Pre-selection of candidates for a barrier layer material involved consideration of the thermal expansion, electronic conductivity, and chemical stability in air as well as when interfaced with a chromium containing compound. As thermoelectric power generation modules are generally used in “one-way applications” where there is a definitive “hot-side”, the application of the barrier layer is only necessitated at this side of the module. Therefore only the thermal, electrical, and thermomechanical properties of the barrier layer a) Address all correspondence to this author. e-mail: timholgate@hotmail.com b) Current affiliation: Teledyne Energy Systems, Inc. (TESI), Hunt Valley, MD, USA DOI: 10.1557/jmr.2014.320 J. Mater. Res., Vol. 29, No. 23, Dec 14, 2014 Ó Materials Research Society 2014 2891