EXAFS and XANES Studies on Spinel Structure Materials and Application to Humidity Sensor

Abstract

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, Zn_0.98Na_0.02Fe₂O₄ and Mg_0.98Na_0.02Fe₂O₄. 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 Zn_0.98Na_0.02Fe₂O₄ 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 Mg_0.98Na_0.02Fe₂O₄, 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 Mg_0.98Na_0.02Fe₂O₄ 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.