EXAFS and XANES studies on spinel structure materials and application to humidity sensor

Deterioration of humidity sensors has been investigated by EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure). The humidity sensors were composed of spinel structure materials, Zn0.98Na0.02Fe2O4 and Mg0.98Na0.02Fe2O4. The former was excellent in durability, but the latter was poor. The experiment was carried out by a laboratory scale EXAFS equipment with a rotating anode type X-ray generator and a Si (220) Johansson type curved crystal. EXAFS and XANES spectra at the Fe K-absorption edge were measured in the transmisson mode. The spinel structure materials, ZnFe2O4, MgFe2O4 and related compounds were used as reference specimens. XANES spectra were derivated to enhance the difference between the specimens. Each spectrum from the two materials after a durability test was extremly diffe rent from each other. The spectrum obtained from Zn0.98Na0.02Fe2O4 was similar to that obtained from ZnFe2O4 in which Fe atoms occupy octahedral sites. In this specimen, no change was observed after the durability test. In the case of Mg0.98Na0.02Fe2O4, however, the spectrum varied from ZnFe2O4 type to MgFe2O4 type, in which Fe atoms exist in both octahedral and tetrahedral sites. It was sug g ested that the deterioration of Mg0.98Na0.02Fe2O4 is attributable to the movement of Fe atoms. It was proved that the laboratory scale EXAFS equipment is extremely useful for studying not only the atomic radial distribution but the coordination geometry around the excited atoms.