Effects of firing conditions and addition of Co on bulk and grain boundary properties of CGO D. Pe ´rez-Coll a , P. Nu ´n ˜ez a, * , J.C.C. Abrantes b , D.P. Fagg c , V.V. Kharton c , J.R. Frade c a Departamento de Quı ´mica Inorga ´nica, Universidad de La Laguna, E-38200 La Laguna, Tenerife, Spain b ESTG, Instituto Polite ´cnico de Viana do Castelo, 4900 Viana do Castelo, Portugal c Departamento de Engenharia Cera ˆmica e do Vidrio, CICELO Universidade de Aveiro, 3810-193 Aveiro, Portugal Received 23 September 2004; received in revised form 9 February 2005; accepted 24 June 2005 Abstract Commercial ceria – gadolinia powders were used to obtain bulk CGO samples, by sintering at 900 to 1500 -C with Co nitrate additions, or at temperatures in the range 1500 – 1600 -C, without Co. These samples were characterized by impedance spectroscopy in air, at temperatures in the range 150–600 -C, to distinguish the bulk and grain boundary behaviour. Addition of Co nitrate allows densification at lower temperatures and plays significant effects on both microstructural contributions of impedance spectra, enhancing the bulk and grain boundary conductivities and lowering their activation energy. Typical values of activation energy of bulk conductivity vary from 0.77 to 0.94 eV, and the activation energy of grain boundary conductivity were in the range 0.96 –1.05 eV. The effects of sintering additive are spoilt on raising the sintering temperature, due to depletion of Co content in grain boundaries of samples fired at high temperatures. These observations indicate that grain boundary behaviour may be determined by segregation of Co and/or Gd and the corresponding space charge layers, at least for materials prepared from high purity precursor powders. The p-type electronic conductivity is also enhanced for samples fired at relatively low temperatures with addition of Co, thus indicating that significant changes in defect chemistry occur. D 2005 Elsevier B.V. All rights reserved. PACS: 81.05 Je; 66.30 Jt Keywords: Ceria electrolytes; Grain boundaries; Sintering additives; Segregation; Impedance spectroscopy 1. Introduction Partial substitution of Ce 4+ by suitable trivalent cations such as Gd 3+ , Sm 3+ ,Y 3+ or La 3+ enhances the chemical stability, increases the ionic conductivity and suppresses the reducibility of ceria-based materials, up to a certain maximum [1–3]. Sintering additives have been proposed to obtain samples with sub-micrometer grain sizes, at relatively low temperatures [4]. The most effective additives are Gd 2 O 3 and Sm 2 O 3 [1,5,6]. Significant differences between results for CSO and CGO reported by different authors may be due to differences in powder preparation and sintering schedule [7–9] and the corresponding effects on the relative role of the resistive grain boundary. Differences were also found between the grain boundary conductivities of CGO and 2%Co–CGO samples prepared from commer- cial powders and from freeze-dried precursor powders [4,10,11]. It was also reported that addition of small amounts of cobalt to CGO enhanced the grain boundary conductivity compared to undoped CGO [10,12] and this was attributed to the presence of the amorphous cobalt oxide thin film at the grain boundaries, acting as a cleaner of impurities segregated at the grain boundary. In contradiction with these results, Lewis et al. [13] reported lower grain boundary conductivity for 2%Co – CGO than for undoped CGO and attributed this to differences in purity of different CGO powders. A proper interpretation of the grain boundary behaviour should also take into account microstructural differences, as reported by different authors for solid electrolytes [14–21], 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.06.023 * Corresponding author. Solid State Ionics 176 (2005) 2799 – 2805 www.elsevier.com/locate/ssi