Dynamic segregation phenomena during oxidation of titanium ferrites

The cationic composition of three types of titanium ferrite Fe2.5Ti0.5O4 ha s been analyzed by XPS during their oxidation in order to reveal dynamic se gregation phenomena. These samples included two pulverised materials obtain ed by high energy ball milling followed by a thermal treatment under a well controlled reducing atmosphere (I) and by a ceramic process followed by gr inding (II), as well as a compact material obtained by a ceramic process (I II). In each case, under pure oxygen and under a linear increase of the tem perature, the material was subject to oxidation in the cation deficient pha se i e. without phase transformation below 350 degrees C. During this react ion, an important modification of the chemical composition of the near surf ace layers has been revealed: the titanium ferrite surface becomes richer i n iron and poorer in titanium. For pulverised compounds, if the heating is extended above 400 degrees C, the oxidation in a cation deficient phase can proceed and some titanium can move back to the surface. Then, from this te mperature, the amount of titanium detected by XPS increases. For samples ob tained by high energy ball milling, this phenomenon can lead to a homogeneo us compound. This is not so for the samples obtained by the ceramic process . For these, a phase transformation of the compound appears which generates alpha-Fe2O3 at the surface of the material. The segregation phenomenon has been interpreted on the basis of the different mobilities of the species F e2+, Fe3+,Ti4+ and cation vacancies present in the material.