Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Investigation of the electrical conductivity of sintered monoclinic zirconia (ZrO 2 ) Oh Hyun Kwon a,b , Changheui Jang b, ⁎ , Junho Lee b , Hu Young Jeong c , Young-il Kwon d , Jong Hoon Joo d , Hongjin Kim a a KEPCO Nuclear Fuel, Co., Ltd., Daejeon 34057, Republic of Korea b Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea c UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea d Chungbuk National University, Chungbuk 28644, Republic of Korea ARTICLE INFO Keywords: Monoclinic ZrO 2 Electrical conductivity Impedance Undoped zirconia ABSTRACT High-density monoclinic ZrO 2 was manufactured through sintering at ~1200 °C by using nanosized powders. Then, the electrical conductivity was measured at a range of high temperatures (700–900 °C) by electrical impedance spectroscopy (EIS). For the as-sintered monoclinic ZrO 2 , the measured electrical conductivity was 3.2×10 -5 s/cm (for 80% TD) and 4.4×10 -5 s/cm (for 89% TD) at 900 °C. After aging at 900 °C for 100 h, the electrical conductivity of the monoclinic ZrO 2 of 80%-TD decreased by more than 50%. However, after reheating at 1200 °C for 1 h, approximately 80% of the conductivity was recovered compared to the value of the as- sintered monoclinic ZrO 2 . The pure monoclinic crystal structure was retained despite the aging and reheating treatment. Based on microstructural observations of the aged and reheated monoclinic ZrO 2 , the changes in electrical conductivity after aging and reheating were explained by the formation and recovery of micro-cracks, respectively. 1. Introduction Zirconia (ZrO 2 ) is widely known as a solid electrolyte because ZrO 2 - based systems attain superior ionic conductivity when acceptor-type dopants, such as Y 2 O 3 and Sc 2 O 3 , form oxygen vacancies [1]. In particular, solid oxide fuel cells require high-performance zirconia electrolytes that must retain sufficient oxygen-ion conductivity at high temperature ranges, without degradation in extremely oxidizing or reducing atmospheres [2]. Because of its superior properties, the electrical conductivity of ZrO 2 has been extensively studied from the perspective of doping elements and their effects to achieve better electrolytic performance [1]. However, fundamental studies on monoclinic ZrO 2 without dopants have rarely been reported. The lack of research on monoclinic ZrO 2 may be attributed to difficulties in the manufacturing process and the mechanical instability of ZrO 2 to temperature variations [3,4]. Usually, sintering of ZrO 2 requires a temperature higher than 1400 °C [5,6]. When the bulk is cooled down after sintering at a high temperature (~1400 °C or more), it goes through a phase transformation from the tetragonal to the monoclinic phase at approximately 1200 °C. Local stress produced during the phase transformation can cause fracturing of the sintered ZrO 2 [3,8]. Therefore, applications of monoclinic ZrO 2 have thus far been impractical in industry; and electrical conductivity studies on monoclinic ZrO 2 seem to have received less attention due to the difficulty of manufacturing [5,7]. However, several studies of the electrical conductivity of monoclinic ZrO 2 have been reported, which do not face the manufacturing problem mentioned above. By fabrication through hot-pressing of powders, it has been reported that monoclinic ZrO 2 has extremely low electrical conductivity ranging from ~2×10 -6 s/cm to ~6×10 -5 s/cm at 990 °C [9–12]. However, there has yet been no report on sintered monoclinic ZrO 2 from the perspective of high density, crystal structure and microstructure. Thus, in the present study, pure monoclinic ZrO 2 was manufactured with a relatively high density (~80% or more) compared to the theoretical density, by sintering nanosized powders at a relatively low temperature of 1200 °C. The sintering temperatures and time were optimized to obtain specimens with sufficient fracture resistant to the large temperature changes associated with cooling from sintering temperature, heat-treatment, and electrical property measurement. Because of the extremely low electrical conductivity and performance of the electrolyte at the operation temperature, the electrical conduc- http://dx.doi.org/10.1016/j.ceramint.2017.03.152 Received 9 February 2017; Received in revised form 24 March 2017; Accepted 24 March 2017 ⁎ Corresponding author. E-mail address: chjang@kaist.ac.kr (C. Jang). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2017 Published by Elsevier Ltd. Please cite this article as: Kwon, O.H., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.03.152