Phase Stability of 10mol%Sc 2 O 3 -1mol%CeO 2 -ZrO 2 Ceramics
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1074-I03-66
Phase Stability of 10mol%Sc2O3-1mol%CeO2-ZrO2 Ceramics Sergey Yarmolenko1, Svitlana Fialkova2, Devdas M. Pai2, and Jag Sankar2 1 Center for Advanced Materials and Smart Structures, North Carolina A&T State University, 1601 E. Market St., 242 IRC, Greensboro, NC, 27411 2 Mechanical Engineering, North Carolina A&T State University, 1601 E. Market St., 242 IRC, Greensboro, NC, 27411 ABSTRACT Scandia-doped zirconia is a very promising material for solid oxide fuel cells due to its high oxygen conductivity in the 700-850ºC temperature range. 10 mol% Sc2O3 - 1 mol% CeO2 - ZrO2 ceramics were sintered at temperatures 1100-1600ºC using different heating rates and dwell times. Ceramics sintered at temperatures higher 1300ºC were found to exist in cubic phase at room temperature and exhibit slow phase transformation from cubic (c) to rhombohedral (β) phase between 330 and 400ºC. Analysis of c-β phase transition efficiency in the ceramics shows a strong correlation between the transition rate and sintering temperature. Kinetics of phase transitions was studied by high temperature X-ray diffractometry (HTXRD) and differential scanning calorimetry methods. The reversible c-β phase transition was found to have very wide hysteresis (45-70ºC), which depends on sintering temperature and density. Coefficients of thermal expansion of c- and β-phases were calculated from temperature dependence of lattice parameters obtained by HTXRD in the temperature range of 25-800ºC. Microstructural changes on the surface of the cubic phase due to c-β phase transition studied by SEM and AFM. INTRODUCTION Zirconia doped with rare-earth ions has high oxygen conductivity and is considered among the most promising materials for SOFCs [1, 2]. The cubic phase of 10% Sc2O3 doped ZrO2 (ScSZ) has high oxygen conductivity in the temperature range 700-800ºC [2, 3].There is a cubicrhombohedral phase transition in the range 500-600ºC which causes abrupt decrease of electrical conductivity of ScSZ upon cooling [4]. Phase transitions also can decrease the mechanical stability of solid oxide fuel cells (SOFC). Several recent works have focused on phase transformations and phase stability of ScSZ [5, 6]. A common approach to stabilize the cubic phase without compromising ionic conductivity is doping the system ZrO2-RO1.5 (R=rare-earth elements) with 0.5-1.0 mol% of stabilizing oxides such as CeO2, HfO2, Yb2O3, Bi2O3, Y2O3, etc. It has been shown that when zirconia is stabilized with CeO2 and Sc2O3, it no longer exhibits an unfavorable phase transition, making this material an extremely promising option for intermediate temperature SOFC electrolytes [7, 8]. In this work, we have studied the effect of sintering parameters on the phase stability of 10 mol% Sc2O3 – 1 mol% CeO2 – 89 mol% ZrO2 ceramics prepared from nanocrystalline powder manufactured by Daiichi Kigenso Kagaku Kogyo (DKKK, Japan). EXPERIMENTAL Ceramics were sintered from uniaxially pressed pellets at temperatures ranging between 1100º and 1600ºC in air (using sintering times of 2 and 10 hrs, a
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