Thermodynamic study of NaFe complex oxides. High temperature properties of Na5FeO4 and Na3FeO3

In order to contribute the investigation into corrosion mechanism of the structural materials by leakage sodium, thermodynamic study of Na-Fe complex oxides formed by the reactions was carried out. Na5FeO4 and Na3FeO3 were used as the sample. Its high temperature properties (i.e. melting, solidification and transformation) were observed by Differential Scanning Calorimetry, DSC. Moreover, the original test named 'melting point confirmation test' was performed for the observation of traces of melting and solidification after the tests. Following contents have been obtained by this study. (1) Na5FeO4 was stably as the solid without phase transformation and melting until 800degC. However, the compound was showing a tendency to change into Na4FeO3 with temperature increasing under the low oxygen potential. (2) The stability of Na3FeO3 is the same as that of Na5FeO4 until 700degC. Over the temperature, the compound was changed differential compound registered as Na31Fe8O29 or Na8Fe2O7 in X-ray powder diffraction data base, and then it was melted. (3) The stability region on the oxygen and sodium partial pressures was examined for the compound. As the results, it was considered that the compound was Na8Fe2O7. (author)