MATERIALS THERMODYNAMICS: ABRIDGE FROM FUNDAMENTALS TO APPLICATIONS

摘要

The applicability of the materials thermodynamics in terms of the correlation of the thermodynamic properties with other physicochemical properties and the bridge between the equilibrium thermodynamics and kinetic considerations through utilization of chemical potential diagram is discussed. Correlation of the enthalpies with the chemically important quantities such as valence, ionic size and coordination numbers provides fruitful results in applications of the thermodynamics. This makes it easy to estimate data for little known compounds and to adopt the materials design approach in which simple but powerful prediction methods are always required. Generalized chemical potential diagrams provide powerful tools to thermodynamically examine the stability of the interface with other solid materials or gaseous environments. This enables us to consider chemical reactions and diffusion simultaneously. Using these powerful tools, the materials problems in solid oxide fuel cells have been investigated to establish inexpensive fabrication techniques for such materials. The first was to resolve the interface stability between perovskite cathode and YSZ electrolyte in relation with the electrochemical performance. The second was to improve poor sinterability of the LaCrO_3-based interconnect. These materials problems have been resolved by adopting the materials thermodynamic approach based on the considerations on valence, ionic size and stoichiometry. As a result, the fundamental understanding on the target phenomena has been obtained leading successfully to appropriate solutions based on composition control. Further adoption of the present approach is discussed in relation with kinetic considerations that are needed to optimize the materials for the intermediate temperature SOFCs.