Dynamic segregation phenomena during oxidation of titanium ferrites

Abstract

The cationic composition of three types of titanium ferrite Fe _2.5 Ti _0.5 O ₄ has been analyzed by XPS during their oxidation in order to reveal dynamic segregation phenomena. These samples included two pulverised materials obtained by high energy ball milling followed by a thermal treatment under a well controlled reducing atmosphere (I) and by a ceramic process followed by grinding (II), as well as a compact material obtained by a ceramic process (III). In each case, under pure oxygen and under a linear increase of the temperature, the material was subject to oxidation in the cation deficient phase i.e. without phase transformation below 350 °C. During this reaction, an important modification of the chemical composition of the near surface layers has been revealed: the titanium ferrite surface becomes richer in iron and poorer in titanium. For pulverised compounds, if the heating is extended above 400 °C, the oxidation in a cation deficient phase can proceed and some titanium can move back to the surface. Then, from this temperature, the amount of titanium detected by XPS increases. For samples obtained by high energy ball milling, this phenomenon can lead to a homogeneous compound. This is not so for the samples obtained by the ceramic process. For these, a phase transformation of the compound appears which generates α-Fe ₂ O ₃ at the surface of the material. The segregation phenomenon has been interpreted on the basis of the different mobilities of the species Fe ²⁺ , Fe ³⁺ , Ti ⁴⁺ and cation vacancies present in the material.