Conventional and field-assisted sintering of nanosized Gd-doped ceria synthesized by co-precipitation M. Biesuz a , G. Dell'Agli b,d,n , L. Spiridigliozzi b , C. Ferone c,d , V.M. Sglavo a,d a Department of Industrial Engineering, University of Trento, Via Sommarive 9, 38123 Trento, Italy b Department of Civil and Mechanical Engineering and INSTM Research Unit, University of Cassino and Southern Lazio, Via G. Di Biasio 43, 03043 Cassino, FR, Italy c Department of Engineering, Parthenope University of Naples, Centro Direzionale, Is. C4, 80143 Napoli, Italy d INSTM – National Interuniversity Consortium of Materials Science and Technology, Via G. Giusti 9, 50121 Firenze, Italy article info Article history: Received 16 March 2016 Received in revised form 16 April 2016 Accepted 18 April 2016 Keywords: Sintering GDC Flash sintering Co-precipitation abstract Gadolinium-doped ceria is an attractive electrolyte for potential application in SOFCs operating at in- termediate temperature; for such use, the fundamental compositions typically contain 10–20 mol% Gd 2 O 3 . In this work, we produced nanosized 10 mol% gadolinium-doped ceria powder by co-precipita- tion, starting from Ce and Gd nitrate solutions and using ammonia solution as precipitating agent. The co-precipitate was characterized by DTA-TG, TEM, XRD and nitrogen adsorption analyses. We studied the behavior of the nanopowder under both conventional and Flash sintering. Very different behavior was seen: the conventional sintering cycle produced a poorly densified material, while Flash sintering al- lowed production of a perfectly densified material, with uniform sub-micrometric grain size. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 1. Introduction Solid Oxide Fuel Cells (SOFCs) are very promising energy-con- version devices characterized by high efficiency, modularity and noiselessness. They can use various fuels (hydrogen and methane, biogas and natural gas) at very limited emission levels [1,2]. Today the SOFCs as developed must operate at temperatures above 800 °C, due to the use of yttria-stabilized zirconia (YSZ) for the electrolyte. This leads to high costs for the materials and to limited durability. Lowering the service temperature of SOFCs is a major target for researchers operating in this field, who aim to exploit IT-SOFCs (Intermediate Temperature SOFCs), operating at temperatures between 500 and 800 °C [3,4]. One of the require- ments for the success of IT-SOFCs is the availability of ceramic electrolytes characterized by ionic conductivity higher than YSZ in the specific temperature range. From this point of view, ceria- based ceramics have been identified as the fundamental candidate materials [5]. Characterized by relatively high abundance, cerium oxide (CeO 2 ) is a technologically important material. Applications are as catalysts, ionic conductors, oxygen sensors, oxygen per- meation membranes, fuel cells, glass-polishing materials, electro- chromic thin films, ultraviolet absorbents; and also in biotechnology, environmental chemistry and medicine [6]. The oxygen vacancy concentration and concomitant ionic conductivity can be increased by the substitution of cerium with a lower-valence metal ion (i.e., R 3 þ ions – R ¼ Gd, Sm, Nd, Y, Pr etc.); the fluorite-type crystal lattice of CeO 2 lets us replace a relatively large amount of cerium cations with rare-earth cations, which can greatly influence ionic conductivity [1]. Among ceria-based cera- mics, gadolinium-doped ceria (GDC) is an attractive electrolyte for potential application in SOFCs operating at intermediate tem- perature, typically with 10–20 mol% substitution of Ce þ 4 by Gd þ3 [7–10]. For ionic conductivity enhancement of ceria, atoms must be used with ionic radius close to that of the cerium cation. The perfect coincidence between the host (Ce þ 4 cation) and the do- pant (Gd þ 3 cation) radius in GDC accounts for high ionic con- ductivity and low activation energy, due to the low binding energy between oxygen vacancies and Gd þ3 [4]. We note that in addition to the intrinsic properties of the material, ionic conductivity depends strongly on the micro- structure (porosity, density, grain size, grain boundaries etc.) and is therefore influenced by materials processing. Moreover, a densi- fied ceramic with no open porosity is essential for use as the electrolyte in SOFCs. Nevertheless, one of the main drawbacks of ceria-based materials is the high sintering temperature required to obtain full densification [2,11]. Such high sintering temperatures also lead to large grain sizes that cause poor mechanical properties [12–14]. Therefore, to improve the sintering behavior of ceria- based ceramics, a first strategy is the synthesis of more reactive Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ceramint Ceramics International http://dx.doi.org/10.1016/j.ceramint.2016.04.097 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author at: Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Via G. Di Biasio 43, 03043 Cassino, FR, Italy. E-mail address: dellagli@unicas.it (G. Dell'Agli). Please cite this article as: M. Biesuz, et al., Conventional and field-assisted sintering of nanosized Gd-doped ceria synthesized by co- precipitation, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.097i Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎