F812 Journal of The Electrochemical Society, 162 (8) F812-F820 (2015) 0013-4651/2015/162(8)/F812/9/$33.00 © The Electrochemical Society Characterization of Terbium and Samarium Co-Doped Ceria Films Prepared Using Ultrasonic Spray Pyrolysis Mihkel Vestli, z Enn Lust, *, z and Gunnar Nurk *, z Institute of Chemistry, University of Tartu, 50411 Tartu, Estonia Tb and Sm cation binary co-doped ceria films were deposited using the ultrasonic atomizing spray pyrolysis method. Crack-free homogenous films with different dopant concentrations were deposited and thereafter annealed at fixed temperatures T = 900, 1200 and 1300 ◦ C, respectively. It was demonstrated that several microstructural parameters of oxide films are controlled by the sintering temperature. The Ce 0.9 Sm 0.1-x Tb x O 2-δ films formed were analyzed using X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, atomic force microscopy and a four probe DC technique at different pO 2 and temperature conditions. Based on the SEM analysis the average thickness of the Ce 0.9 Sm 0.1-x Tb x O 2-δ films was approximately 700 nm. The XRD patterns for the Ce 0.9 Sm 0.1-x Tb x O 2-δ films annealed at 1200 ◦ C indicated a high degree of crystallinity. Tb dopant ions influence the microstructural properties like median diameter of grains, microstrain, lattice parameter and electrical properties like activation energies of ionic and electronic part of conductivity for the Ce 0.9 Sm 0.1-x Tb x O 2-δ film. A significantly higher microstrain value for the lowest Tb dopant concentration with accompanied change in electrical properties was observed. © 2015 The Electrochemical Society. [DOI: 10.1149/2.0031508jes] All rights reserved. Manuscript submitted March 6, 2015; revised manuscript received April 22, 2015. Published May 5, 2015. Because of the high ionic conductivity at intermediate temper- atures (500–700 ◦ C) doped ceria oxides are of special interest as the promising electrolyte materials for intermediate temperature solid ox- ide fuel cells (IT-SOFC), oxygen separation membranes, electrolyz- ers, methane conversion reactors and therefore have been extensively studied during the last decades. 1,2 In addition, contrary to zirconia, ceria has good chemical stability with LSCO (La 1-x Sr x CoO 3-δ ) and LSCFO (La 1-x Sr x Co 1-y Fe y O 3-δ ) cathode materials. However, the main drawback of the ceria based solid electrolytes is the partial electronic conductivity at reducing conditions resulting in an internal partial short-circuiting of the cell, causing noticeable decrease of the cell voltage and thus, drop of the SOFC total efficiency. Efficiency losses up to 50% because of the shorting effect have been reported for sys- tems in the case of thin film (10–20 μm) ceria based electrolytes. 3 Deterioration of mechanical stability caused by chemical expansion due to partial reduction of ceria should also be mentioned here. 4 The electronic conductivity of ceria based electrolytes, caused by the hypostoichiometry-generated small polarons Ce Ce ’, increases due to the partial reduction of Ce 4+ to Ce 3+ at higher temperatures and lower oxygen partial pressures, 1,2 given as: O x O + 2Ce x Ce = 1/2O 2(g) + V •• O + 2Ce ′ Ce [1] To overcome this shortcoming, different approaches have been tested. It has been reported by Maricle et al. 5 that 3 mol% Pr to gadolinia-doped ceria (GDC) increases the membrane redox stabil- ity. Influence of Nd, La, Y, Sm and Pr ions as co-dopants for GDC have been studied comparatively with focus on the ionic and elec- tronic properties. 6 The formation of an electron blocking BaO-CeO 2 - Sm 2 O 3 ternary composite interlayer between the Ba containing anode and the samaria-doped ceria (SDC) electrolyte has been reported by W. Sun et al. 7 However, one promising approach to suppress the par- tial electronic conductivity of the ceria based electrolyte has been demonstrated using some 3d- and 4f-elements as electron traps. 8 The trapping of electrons by more easily reducible species D x Ce could be expressed as: Ce ′ Ce + D x Ce = Ce x Ce + D ′ Ce [2] In the case of Ti, Mn, Fe, Co and Cu (3d elements) and Eu and Yb (4f elements) no positive effect was observed. In equal proportion with Sm, in a way that their mean ionic radius is close to that of Ce 4+ , Tb was found to be an effective electron trap for the microcrystalline bulk specimen of ceria electrolytes. It was demonstrated by Yoo et al., that by co-doping of the SDC electrolyte with Tb, it is possible to sup- press the partial electronic conductivity by half an order of magnitude ∗ Electrochemical Society Active Member. z E-mail: mihkel.vestli@ut.ee; enn.lust@ut.ee; gunnar.nurk@ut.ee compared to the one establised for the 10 mol% Gd-doped ceria. 9 This could result in the lower bound of its electrolytic domain (temperature and oxygen partial pressure range at which the material is dominantly an ionic conductor) extended by two orders of magnitude to lower oxygen partial pressures. It has been suggested that Tb (existing as a mixture of Tb 4+ and Tb 3+ valence states) as dopant decreases the amount of Ce Ce ’ species in reducing conditions. It was concluded that excess electrons are trapped on Tb due to its tendency to reduce more easily. 9–11 Previous works have demonstrated that the magnitude of ionic and electronic conductivities are dependent on the Tb amount. In case of small Tb concentrations doped ceria is still a pure ionic conductor. Considerably larger amounts of Tb cause increase of elec- tronic conductivity. 12,13 Additionally it has been demonstrated that the reducibility of doped CeO 2 is enhanced with increasing dopant concentration at intermediate temperatures. 1,14 Improved properties of the doped ceria electrolyte raise a necessity for an economically reasonable preparation method of thin films with the discussed chemical composition. A number of techniques have been used for preparation of electrolyte layers with thickness smaller than 10 μm. 15–17 Spray pyrolysis has proven to be a time- and cost-efficient method for deposition of homogenous high quality thin films with rea- sonable cost compared to chemical or physical vapor deposition techniques. 18–20 In addition, the spray pyrolysis method is suitable for deposition of metal oxide films on objects with comparatively large areas such as planar SOFC components. 19 Spray pyrolysis involves forming small droplets by atomization of the precursor solution which are transported by carrier gas to the heated substrate. After solvent evaporation and thermal decomposi- tion the droplets of precursor solution form deposits on the surface of substrate. The film is built up by overlapping deposits during the spraying process for some period of time. In case of the ultrasonic spray pyrolysis method, the atomization of the precursor solution results in a finer mist with a relatively narrow droplet size distribu- tion compared to that formed using pressurized spray pyrolysis. As a result of the narrower droplet size distribution the formation of defects and porosity in the raw film caused by differences in evap- oration and deposition of the droplets with varying sizes noticeably can be suppressed. Therefore, the homogeneous thin films can be deposited in a more controlled manner. 19,21 However, the raw films obtained by the method under discussion are amorphous in nature. The annealing step results in a densified microstructure with a grain size up to 150 nm if sintering temperatures from 800 to 1100 ◦ C are applied. 18,22 It has been demonstrated that the pressurized spray pyrol- ysis method is a convenient way to deposit dense homogenous GDC thin films with thicknesses 100–800 nm using nitrates and chlorides as precursors 23,24 and the ultrasonic spray pyrolysis method has been ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.40.12.10 Downloaded on 2015-06-02 to IP