Development of an improved devitrifiable fuel cell sealing glass - II. Crystallization behavior and structures of crystalline phases

The crystallization behavior of the BCAS-type sealants presented in Part I of this series was studied using in situ diffraction and by quantitative diffraction after annealing for 12 or 200 h in air. The baseline sealant, EG9207, is surface nucleated and a kinetics analysis provides the activation energy for crystallization as 281(21) kJ mol(-1). EG9207 crystallizes fully in several minutes at 900 degrees C, while the PNNL G18 composition and several of the new sealants require between 2 and 12 h to crystallize fully. Two phases encountered as crystallization products, Ba1.4Ca1.52SiO4 and Ba0.48Ca1.52SiO4, were isolated and the crystal structures were solved using ab initio simulated annealing methods from powder X-ray diffraction data. The two materials crystallize in the Pnma (62) and P3m1(164) space groups, respectively, and Ba0.48Ca1.52SiO4 undergoes a slow phase transition between similar to 900 and 1140 degrees C. The structures of Sr5Ta4O15, Ba3Ta5O15, and Ba2MgSi2O7, were also solved and refined from powder X-ray data using isostructural compounds. The thermal expansion of each phase was measured using in situ diffraction from room temperature to 1300 degrees C. The two silicates have large thermal expansion coefficients (RT-900 degrees C), 13.0 for Ba1.4Ca1.52SiO4 and 13.9 ppm/K for Ba0.48Ca1.52SiO4. The CTE values for Ba3Ta5O15, and Ba2MgSi2O7, and Sr5Ta4O15, are 7.0, 8.2, and 10.2 ppm/K (RT-900 degrees C), respectively. (C) 2007 The Electrochemical Society.