Dynamic segregation phenomena during oxidation of titanium ferrites

• Published in: Journal of materials chemistry Vol. 9; no. 5; pp. 1179 - 1183
• Main Authors: Domenichini, Bruno, Perriat, Pascal, Merle, Jérôme, Basset, Karine, Guigue-Millot, Nadine, Bourgeois, Sylvie
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 1999
• Subjects: Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Materials science; Oxidation; Physics; Solid-fluid interfaces; Solubility, segregation, and mixing; phase separation; Surface treatments; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Inorganic compounds; Segregation; Ternary compounds; Transition element compounds; Surfaces; Experimental study; Photoelectron spectroscopy; Ferrites; Titanium oxides; Ion mobility; Dynamics; Reaction mechanism; X radiation; Oxidation; Iron oxides
• ISSN: 0959-9428; 1364-5501
• DOI: 10.1039/a808732f

The cationic composition of three types of titanium ferrite Fe2.5Ti0.5O4 was analysed by XPS during oxidation in order to reveal dynamic segregation phenomena. These samples included two pulverised materials obtained by high energy ball milling followed by heat treatment under a well controlled reducing atmosphere and by a ceramic process followed by grinding, as well as a compact material obtained by a ceramic process. In each case, under pure O2 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 was revealed: the Ti ferrite surface becomes richer in Fe and poorer in Ti. For pulverised compounds, if the heating is extended above 400 C, the oxidation in a cation deficient phase can proceed and some Ti can move back to the surface. Then from this temperature, the amount of Ti can move back to the surface. Then, from this temperature, the amount of Ti 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 alpha-Fe2O3 at the surface of the material. The segregation phenomenon has been interpreted on the basis of the different mobilities of the species Fe2+, Fe3+, Ti4+ and cation vacancies present in the material. 34 refs.