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Kinetic unmixing and kinetic decomposition of oxides in external electric fields

Synthesis and Phase Control

Kinetic unmixing and kinetic decomposition of oxides in external electric fields

Prof. Dr Manfred Martin
RWTH Aachen University
Institute of Physical Chemistry

Landoltweg 2
52056 Aachen
Tel: +49-241-8094712
Fax: +49-241-8092128

Proj.-Nr. MA 1090/28-1

Ternary oxides AmBnOx are being used in numerous devices that are exposed to various thermodynamic forces, such as oxygen chemical potential gradient (solid oxide fuel cells), electric potential gradient (or electric field) (multi-layered ceramic capacitors), thermal gradient (thermoelectrics), stress gradient (piezoelectrics), etc. The materials range from purely ionic conductors (e.g. electrolytes for solid oxide fuel cells or lithium-ion batteries) to mixed conductors and semiconductors (e.g. interconnects and electrodes in solid oxide fuel cells). When these ternary oxides are exposed to external driving forces (or put under operation) especially at high temperatures where the ions that compose the oxide are more mobile, the ions migrate, in general with different mobilities. As a result, concentration gradients are built up in opposite directions, i.e. the faster cation will be enriched in the direction of transport and vice versa (see Fig. 1). This phenomenon is called kinetic unmixing, and if the extent of unmixing becomes large enough, the original oxide will undergo kinetic decomposition, i.e. new phases will form. In addition to these unmixing and decomposition phenomena morphological changes of the surfaces may appear.

Our two recent investigations on BaTiO3 and NiTiO3 [1,2] show, however, that the phenomenon of kinetic decomposition of a ternary or higher oxide in an external electric field is not understood yet and needs more detailed experimental and theoretical studies.