X-ray powder diffraction study of fine-grained ZrO2-Sc2O3 dense ceramics

P.M. ABDALA; D.G. LAMAS; M.C.A. FANTINI; A.F. CRAIEVICH

2009 Activity Report of the Brazilian Synchrotron Light Laboratory

LNLS

Año: 2010 p. 1562 - 1562

1518-0204

Fuel cells are promising devices for environmentally clean energy production by directly converting chemical energy into electricity. Among them, solid oxide fuel cells (SOFCs) havethe unique capability to use different fuels such as hydrocarbons or hydrogen. However, several issues have to be solved in order to improve their efficiency at moderate temperatures and in terms of costs of operation. The reduction of the typically high working temperature, which is usually around 900 to 1000°C, is one of the most important ones. The development of intermediate temperature solid oxide fuel cells (ITSOFCs), operating between 600 and 800°C, requests electrolytes with higher oxide-ion conductivity than that of traditional materials used for this purpose, typically ZrO2-8 mol% Y2O3.Scandia-stabilized zirconia (ScSZ) electrolytes exhibit the highest ionic conductivity among all ZrO2-based materials. For this reason, ScSZ ceramics are considered as promising candidates for solid electrolytes in IT-SOFCs. The optimum ionic conductivity corresponds to Sc2O3 contents between 9 and 11 mol%. Unfortunately, a rhombohedral phase (known as the beta-phase) of poor electrical properties exists at lowtemperatures for this compositional range. The beta-phase transforms to the high-conductivity cubic phase on heating at temperatures above 600°C, and the reverse transformation occurs on cooling at about 500°C. This transition is undesirable for applications because it causes a loss in the ionic conductivity and also the volume change involved can produce the deterioration of the device. Therefore, it is important to avoid this transition. For this purpose, the introduction of co-dopants that promote the retention of the cubic phase, such as Y2O3 or CeO2, has been proposed. On the other hand, our researchgroup has recently demonstrated that the beta-phase can be avoided in nanocrystalline ZrO2-Sc2O3 solid solutions.We found that the t"-form of the tetragonal phase and/or the cubic one are retained in these nanomaterials, with a critical crystallite size of 35 nm. The aim of the present work was to investigate the conditions for the retention of the high-conductivity tetragonal or cubic phases in fine-grained ScSZ dense ceramics with different average grain size. For purpose of comparison, we also analyzed ScSZ ceramics co-doped with Y2O3 (Y-ScSZ).