Megumi Shimazu

Recent Achievements of NEDO Durability Project with an Emphasis on Correlation Between Cathode Overpotential and Ohmic Loss

Long-term performance testes by CRIEPI (Central Research Institute for Electric Power Industry) on six industrial stacks have revealed an interesting correlation between cathode polarization loss and ohmic loss. To make clear the physicochemical meaning of this correlation, detailed analyses were made on the conductivity degradation of YSZ electrolyte in button cells and then on the ohmic losses in the industrial cells in terms of time constants which are determined from speed of the tetragonal transformation through the Y diffusion from the cubic phase to the tetragonal phase. In some cases, shorter time constants (faster degradations) were detected than those expected from the two-time-constant (with and without NiO reduction effects) model, suggesting that additional ohmic losses after subtracting the contribution from the tetragonal transformation must be caused from other sources such as cathode-degradation inducing effects. Main cathode degradations can be ascribed to sulfur poisoning due to contamination in air in the CRIEPI test site. An important feature was extracted as this cathode degradations became more severe when the gadolinium-doped ceria (GDC) interlayers were fabricated into dense film. Plausible mechanisms for cathode degradations were proposed based on the Sr/Co depletion on surface of lanthanum strontium cobalt ferrite (LSFC) in the active area. Peculiar cathode degradations found in stacks are interpreted in term of changes in surface concentration by reactions with sulfur oxide, electrochemical side reactions for water vapor emission or Sr volatilization, and diffusion of Sr/Co from inside LSCF.

Evaluation of Linear Thermal Expansion Coefficients of Perovskite Oxides Using Ab-initio Molecular Dynamics with Small Cell Sizes for Materials Design

Calculations of linear thermal expansion coefficients have been carried out using ab-initio molecular dynamics with a small cell containing only ten atoms, which is a suitable cell size for materials design. The perovskite oxides of SrTiO3, BaZrO3, LaNiO3, CMgO3, CCaO3, and LaCoO3 were selected as the reference materials for comparison between calculated and experimental values. The calculated lattice constants were in good agreement with the experimental values. The thermal expansion coefficients of the six oxides, including LaCoO3 in the complex spin state, were evaluated from the temperature dependence of the lattice constants. There was reasonable agreement between the calculated and experimental values. Our results clearly proved that it is possible to obtain linear thermal expansion coefficients by ab-initio molecular dynamics, even with the use of a small cell for the calculations.

Evaluation of Thermal Expansion Coefficients Using Ab-initio Molecular Dynamics with Small Cell Sizes for Materials Design

The calculations of thermal expansion coefficients have been carried out using ab-initio molecular dynamics with a small cell containing only eight atoms that is a suitable calculation in size for materials design. Si, Ge, and GaP crystals were selected as the reference materials for comparison between calculation and experiment. The calculated lattice constants were in good agreement with experimental ones. The thermal expansion coefficients of three crystals were obtained from the temperature dependence of the lattice constants. There was reasonable agreement between the calculation and the experiment. Furthermore, even in use of the smaller cells containing two atoms for Ti, Cr, Fe, and four atoms for Al, Co, Ni crystals, the calculated thermal expansion coefficients were in good agreement with experiments. Our results proved firmly that there is a good possibility of obtaining the thermal expansion coefficients by ab-initio molecular dynamics even in use of a small cell for the calculations.