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FF3.3.1

Synthesis and sintering of Ce1-xGdxO2-x/2 nanopowders via chemical routes. Agusti Sin, Antonino S. Aricò1, Massimo Seregni, Laura Gullo1, Daniela La Rosa1, Stefania Siracusano1, Vincenzo Antonucci1, Ana Tavares, Yuri Dubitsky, Antonio Zaopo Pirelli Labs, C.2172, Viale Sarca 222, I-20126 Milan, Italy 1 CNR-ITAE Via Salita Santa Lucia Sopra Contesse 5, Messina I-98125, Italy

ABSTRACT Gd-doped ceria (Ce0.80Gd0.20O1.90) obtained by several nanopowder synthesis processes are compared structurally, morphologically and electrochemically. The powders have been sintered into pellets and investigated by ac-impedance measurements. The electrochemical properties of the electrolytes have been correlated to the structural morphology of the sintered pellets. When the intergrain region exhibits an elevated interdiffusion, the observation of the grain boundaries becomes difficult while the electrochemical properties are improved. Acrylamide polymerization and oxalic co-precipitation techniques showed the best properties.

INTRODUCTION In the past years, CeO2-based materials have been intensively studied as catalysts, structural and electronic promoters for heterogeneous catalytic reactions and oxide ion conducting electrolytes for electrochemical cells. In all these applications, processes for obtaining fine powders to make bulk ceramics, coatings, films and composites are key points. Our efforts are addressed to develop ceria-based solid electrolytes for solid-state electrochemical devices, including solid oxide fuel cells (SOFCs). The ceria doped electrolytes have higher ion conductivity than conventional YSZ and may operate at lower temperatures (500 - 700°C)[1]. The most commonly used method to produce ceria doped electrolytes is a solid-state reaction, called the “ceramic route”. It involves intimate mechanical mixing of oxides, carbonates or nitrates and repeated grinding and heating cycles to achieve complete reaction between all reagents. Synthesis using wet chemistry, often called the “chemical route”, can overcome many of the disadvantages present in the ceramic route. The homogeneity of the product is expected to increase because mixing of the reagents occurs at the molecular level in solution. The resulting oxide powders have a high specific surface area and, consequently, a high reactivity, which decreases the final temperature treatment and time of synthesis. Different chemical routes are explored in this work for the preparation of fine ceramic powders, such as several co-precipitation methods and acrylamide gelification method. Their relative ability to achieve high density ceramics (ρrel > 95%) by sintering is examined by structural and electrical characterization.

FF3.3.2

EXPERIMENTAL The starting raw materials were Ce (NO3)3 xH2O (99.9%, Aldrich) and Gd(NO3)3 xH2O (99.9%, Aldrich), where x is determined by thermogravimetric analysis. Nitrates are easily dissolved in water in the stoichiometric ratio and the solution is treated as function of the selected chemical process. Oxalic co-precipitation The p