Journal of Alloys and Compounds 486 (2009) 747–753 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Physical properties of alumina/yttria-stabilized zirconia composites with improved microstructure R.H.L. Garcia, V. Ussui, N.B. de Lima, E.N.S. Muccillo, D.R.R. Lazar ∗ Center of Materials Science and Technology, Energy and Nuclear Research Institute-IPEN, Av. Prof. Lineu Prestes, 2242, S. Paulo, 05508-000, SP, Brazil article info Article history: Received 16 January 2009 Received in revised form 26 June 2009 Accepted 29 June 2009 Available online 16 July 2009 Keywords: Ceramics Chemical synthesis Ionic conduction Mechanical properties Microstructure abstract Cubic stabilized zirconia is the preferred material for application as solid electrolyte in solid oxide fuel cells. However, this material has low fracture toughness, which can lead to formation of cracks during long-term operation. Moreover, increase of mechanical as well as electrical properties would be useful for cost-effectiveness of this type of device. In this context, addition of alumina to zirconia-based solid electrolyte can be an interesting option to accomplish that purpose. In this work, ceramic composites containing various amounts of alumina in a 9 mol% yttria-stabilized zirconia matrix were synthesized by the coprecipitation route using a low-silica zirconium precursor. Characterization techniques included scanning electron microscopy, X-ray diffraction, Vickers hardness and impedance spectroscopy. As a con- sequence of optimization of the synthesis route a homogeneous dispersion of the additive along with good densification was obtained. Although alumina addition to stabilized zirconia exerts a deleterious effect on the electrical conductivity, it improves the Vickers hardness and fracture toughness of the composite materials. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Solid oxide fuel cells (SOFCs) are devices that generate electric- ity from the electrochemical reaction of oxidation of a combustible [1]. The most important characteristics of SOFCs are the highest theoretical efficiency of chemical/electrical energy conversion, low noise level, almost no emission of toxic gases and fuel flexibility. Nowadays, attention is focused in cost reduction for commercial purposes [1] and one of the main challenges is the selection of materials due to the high operation temperature (700–1000 ◦ C). The most studied material for use as solid electrolyte is the cubic stabilized zirconia (CSZ), due to its high ionic conductivity, chemical stability in oxidizing and reducing atmospheres, and low electronic conductivity. However, CSZ shows low fracture tough- ness, which can lead to crack formation thereby compromising the cell efficiency due to the combination of the reagent gases. To overcome this problem, alumina may be added to CSZ matrix enhancing the hardness, bending strength and fracture toughness of the ceramic [2–4]. Several publications in this subject reveal that the experimental results are still a controversial issue [5–8], although many recent reports have demonstrated the advantageous properties of zirconia–alumina composite solid electrolytes [9–12]. Therefore, additional studies are necessary to correlate the elec- trical and mechanical properties with the amount of alumina in ∗ Corresponding author. Tel.: +55 11 3133 9224; fax: +55 11 3133 9276. E-mail address: drlazar@ipen.br (D.R.R. Lazar). low-silica zirconia solid electrolyte. Moreover, there is a consensus in the literature that the synthesis route of ceramic powders plays a key role on the definition of the microstructure, and consequently, on the mechanical and the electrical properties. Among various chemical methods of ceramic powder synthesis, the coprecipitation route produces ceramic powders with excellent physical and chemical characteristics, by a simple procedure with inexpensive equipments [13]. This method consists in the prepa- ration of a salt solution of the metallic precursors with a defined stoichiometry, and precipitation of metallic ions by a reaction with a precipitant solution. In general, the reagents are mixed together in an atomic scale ensuring high homogeneity, high purity and well defined stoichiometry [13–15]. Optimization of calcination and sintering conditions of the synthesized powders is of prime impor- tance for the attainment of a homogeneous ceramic microstructure. Low calcination temperatures promote the formation of highly reactive powders, but a high temperature is often necessary for oxide formation and to eliminate organic residues [16]. Zirconia-based ceramics can be stabilized in tetragonal, cubic or monoclinic phases [17] depending on dopant concentration and on the temperature of thermal treatments. Alumina can be found as -phase, which is stable at room temperature, or many other metastable forms as registered in the literature [18]. The variety of alumina phases is a function of the employed synthesis route, of the thermal decomposition of aluminum salts and hydroxides, and of the presence of impurity ions [18,19]. The crystallization pro- cesses of zirconia and alumina occur at distinct temperatures, and are mutually inhibited when they are mixed together [14,20]. Most 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.204