CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 40 (2014) 3847–3853 Microstructure and electrical conductivity of Mo/TiN composite powder for alkali metal thermal to electric converter electrodes Sun-Dong Kim n , Se-Young Kim, Jong Hoon Joo, Sang-Kuk Woo Energy Materials and Convergence Division, Korea Institute of Energy Research, 152 Gajeongro, Yuseonggu, Daejeon 305–343, Republic of Korea Received 11 June 2013; received in revised form 6 August 2013; accepted 6 August 2013 Available online 15 August 2013 Abstract A Mo/TiN composite powder has been synthesized by a sol–gel method to improve the electrical performance and microstructural stability of the alkali metal thermal to electric converter electrode. The core (TiN)–shell (Mo) structure of the composite powder is confirmed by energy- dispersive X-ray spectroscopy and scanning electron microscopy. The composite powder is primarily composed of submicron (400–800 nm) particles that are coated on a core ( 43–5 μm) particle. The Mo/TiN composite electrode exhibits high electrical conductivities of 1000 Scm À1 at 300 1C and 260 Scm À1 at 700 1C in an Ar atmosphere. The electrode exhibits excellent tolerance against grain growth during thermal cycling tests (R.T.2800 1C), where the average growth rate of Mo grains in the Mo/TiN composite electrodes is controlled less than 0.5%/time (0.62- 0.65 μm), while the growth rate in Mo electrodes is 306.7%/time (0.24-3.92 μm). It can be concluded that the Mo/TiN composite powder will suppress the degradation of the electrode and enhance the performance and durability of the unit cell at elevated temperatures. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Grain growth; Alkali metal thermal to electric converter; Mo/TiN composite; Core-shell; Thermal cycling 1. Introduction The alkali metal thermal to electric converter (AMTEC) is one of the most promising energy conversion devices that generate electricity from heat energy. Theoretically, an AMTEC system operates continuously without refueling using the sodium circulation in a beta”-alumina solid electrolyte (BASE) and a porous wick. The driving forces of this sodium circulation are the capillary force of the porous wick and the sodium pressure/activity difference between the Na evapora- tion region (600–800 1C) and the Na condensation region (200–400 1C). Due to the high efficiency, low manufacturing cost, light weight and static operation, the AMTEC system was once considered for use as a space power source [1–3]. However, there have been some problems associated with its durability during continuous operation. Causes of power loss in an AMTEC system include the increase in electrode polarization due to grain growth and the evaporation of the electrode material at high temperatures [4–8]. Thus, the development of a high performance, highly durable electrode is very important for AMTEC research. Up till now, thin and porous Mo electrodes have shown the best performance as AMTEC electrodes [9–11]. This result is due to the formation of Na–Mo–O compounds, which might be responsible for the enhancement of Na transport through the Mo electrode. However, from the work of Williams et al. [11], it is confirmed that this compound evaporated at temperatures above 800 1C, resulting in the degradation of the electrode performance. To develop better electrode materials for use at high temperatures, Nakata et al. [12] investigated the reactivity of a ceramic electrode with liquid sodium, and TiN, TiC, NbN and NbC were considered as candidates for the AMTEC electrode material [12–14]. Although TiN and TiC can be used for the AMTEC electrode, Mo is the best material by far in terms of the electrochemical performance and the activity with a Na vapor [12]. Thus, the aim of this research is to achieve an AMTEC electrode with high electrochemical performance and microstruc- tural stability at high temperatures using a Mo/TiN composite www.elsevier.com/locate/ceramint 0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.08.025 n Corresponding author. Tel.: þ82 42 860 3328; fax: þ 82 42 860 3133. E-mail address: amastra@kier.re.kr (S.-D. Kim).