Phase stability and oxygen non-stoichiometry of Gd-doped ceria during sintering in reducing atmosphere

Abstract

Firing of Gd-doped ceria (GDC) powder compact in reducing atmosphere leads to reduction and subsequent decomposition of GDC, and cracking of the pellet upon cooling, depending upon the firing temperature T and oxygen partial pressure Po-2. In this study, GDC powder compacts were fired in various reducing atmosphere by controlling the ratio of H-2 and H2O mixing (H2O content: 0.28, 1.3, 2.7, 3 %) at temperatures T = 703, 835, 975, 1088, 1180, 1262 and 1354 A degrees C. The phase and oxygen non-stoichiometry of GDC, respectively, were examined by using X-ray diffraction (XRD) and measuring the weight change of GDC pellet after cooling with fixed cooling rate to room temperature. Po-2 below which GDC decomposes was determined as a function of temperature for a fixed cooling rate (5 A degrees C/min). GDC pellets were stable at T <= 1088 A degrees C regardless of Po-2, and decomposed at T = 1354 A degrees C in all tested Po-2. A single-phase GDC without decomposition may be obtained at T = 1180 or 1262 A degrees C with appropriate Po-2. Both the degree of nonstoichiometry and the temperature are important factors to determine the stability of GDC pellet. To avoid cracking of GDC pellets due to chemical expansion during sintering in reducing atmosphere, GDC may be sintered at low T or high Po-2 or both, or it may be sintered to result in low porosity with small grain size for increased fracture strength.