Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu Sol-gel synthesis of Mn 1.5 Co 1.5 O 4 spinel nano powders for coating applications S.T. Hashemi a, ⁎ , Amir Masoud Dayaghi b , M. Askari a , Paul E. Gannon c a Department of Material Science & Engineering, Sharif University of Technology, Tehran, 11155-9466, Iran b Fuel Cell Research Center/Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea c Chemical and Biological Engineering, Montana State University, Bozeman, MT 59717, USA ARTICLE INFO Keywords: Solid oxide fuel cell Nano powder Chelating agent ratio ABSTRACT Mn 1.5 Co 1.5 O 4 oxide spinels are widely used as protective coatings for stainless steel interconnects within planar solid oxide fuel cell stacks. Containing both cubic and tetragonal crystalline phases, these Mn/Co oxide spinels exhibit favorable thermal stability and electronic conductivity for the SOFC interconnect application. Slurry- based coating applications of Mn/Co oxides require precursor powders, which can benefit from being nano- structured. In this study, the sol-gel synthesis of nanocrystalline Mn 1.5 Co 1.5 O 4 spinel is investigated. The de- composition of sol-gel precursors, as well as the crystalline phase structures and microstructures of the product Mn 1.5 Co 1.5 O 4 are characterized by differential thermal and thermogravimetric (DTA/TG) analysis, X-ray dif- fraction (XRD), and transmission electron microscopy (TEM). The effects of various sol-gel annealing tem- peratures (T), treatment times (t), and citrate-to-metal ratios (Rc) are evaluated. Results suggest that nano- crystalline Mn 1.5 Co 1.5 O 4 spinel can be synthesized around 1050 ° C, and that T = 1050 ° C, t = 6 h and Rc = 2 are optimum conditions for producing the smallest grain size. Image analysis of TEM results shows that the size of Mn 1.5 Co 1.5 O 4 crystallites increases with increasing temperature, with average particle sizes ranging from ∼70 nm to ∼1 μm. Selected area diffraction pattern (SADP) of Mn 1.5 Co 1.5 O 4 spinel synthesized at 800 °C con- firms the dual (cubic/tetragonal) structure of Mn 1.5 Co 1.5 O 4 . 1. Introduction Special attention has been paid to solid oxide fuel cells (SOFCs) in comparison with other fuel cells due to their high efficiency and fuel flexibility (e.g., various hydrocarbon gases) [1–3]. Interconnects, which are one of the key components in SOFC stacks, play a pivotal role in SOFC system performance. The reduction of SOFC operation tempera- ture to < 800 °C permits the use of miscellaneous metal alloys for in- terconnects with advantages including easy fabrication, low material cost, high electric and thermal conductivity, and structure integrity [4–9]. Ferritic stainless steels (FSSs) are the most promising candidate among the candidate interconnect alloys due to their low cost and well- matched coefficient of thermal expansion (CTE = 11.5–14 ppm K -1 ) [4,10,11]. FSSs contain Cr which reacts with oxygen to form a dense and continuous surface oxide layer, protecting the bulk alloy against continued corrosion [12–14]. Despite these advantages, the growth of surface oxide layers in oxidizing atmospheres (humid H 2 or air) with time reduces electrical conductivity of the FSS interconnect [15–18]. Also, further oxidation of chromium-containing oxides in cathodic gas environments leads to vaporization of chromium in form of CrO 2 (OH) 2 (g), which can condense on the SOFC cathode and electrolyte, resulting in SOFC system performance degradation. As a result, various protec- tive surface oxide coatings such as common perovskites (ABO 3 ) and spinels (A 2 BO 4 ) have been developed to overcome these challenges [4,19–24]. Not only are spinel oxides being used as protective layers, but they could also be used as the electrodes and catalysts for oxygen reduction reactions in SOFCs [25–30]. Unlike traditional perovskite oxides, these electrodes do not react with neighboring layers as they don’t contain rare-earth or alkaline-earth elements. As a result, the chemical potential is low between the spinel electrode and its neigh- boring layers [31,32]. Larring and Norby [33] indicated that a (Mn,Co) 3 O 4 surface layer could be a good barrier for limiting oxide growth by preventing oxygen inward diffusion and Cr outward diffusion. This material also exhibits high electrical conductivity (∼60 S cm -1 ) and has a well-matched coefficient of thermal expansion (CTE = 10–12 ppm K -1 ) with FSS substrates [21,34,35]. As a result, the systems of MneCoeO spinels are promising coating materials [21,31,36]. https://doi.org/10.1016/j.materresbull.2018.02.040 Received 24 May 2017; Received in revised form 21 January 2018; Accepted 20 February 2018 ⁎ Corresponding author. E-mail address: s.tahereh.hashemi@gmail.com (S.T. Hashemi). Materials Research Bulletin 102 (2018) 180–185 Available online 21 March 2018 0025-5408/ © 2018 Elsevier Ltd. All rights reserved. T