MATERIALS SCIENCE AND ENGINEERING

QUANTITATIVE ASSESSMENT OF FUEL CELL CATALYST NANOPARTICLE SIZE DISTRIBUTIONS

Metal–oxide interfaces play a major role in a variety of applications, such as packaging for electronic devices, oxide-dispersion strengthening of alloys, or thermal barrier coatings. These applications require an appropriate adhesion between metals and oxides. It is therefore important to understand the physics of metal–oxide bonding. In recent studies on metal–oxide interfaces aluminum–spinel composites (Al–MgAl₂O₄ ), Raj et al. [1] have obtained evidence for a micromechanism that could play an important role for establishing as well as adjusting metal–oxide adhesion: ion exchange. More precisely, the experimental observations on the Al–MgAl₂O₄ composites seemed to indicate that Al^3+, ions diffuse from the matrix into the spinel particles of the composite, while Mg^2+, ions from the spinel diffuse into the Al matrix.

Studies in our group [2] have shown that Mg transport also – and predominantly – occurs away from the Al–MgAl₂O₄ interface, deeper into the spinel. In any event, the incorporation of aluminum from the metal side involves charged point defects in the spinel. This is of great significance for two reasons: (i)  the presence of charged point defects with relatively long-reaching electric fields should have an impact on metal–oxide adhesion via formation of "image charges" in the metal [3]. (ii) Applied electric fields can influence the spatial distribution of the charged point defects and therefore the at the region between the metal and oxide.

In recent work, therefore, we have begun to explore the effect of applied electric fields on planar Al–MgAl₂O₄ interfaces using highly sophisticated techniques of microcharacterization and mechanical testing (collaboration with Prof. Raj, Materials Engineering Department, University of Colorado). Our results show that applied electric fields indeed have a profound impact on the interface microstructure, the spatial distribution of atomic species, and the mechanical properties. This may provide entirely new opportunities for controlling the mechanical properties of metal–oxide joints.

Fig. 1. TEM and dark-field image showing the formation of a reaction phase (arrowed) at a Al–MgAl₂O₄ interface upon annealing under an applied electric field.

1. R. Raj, A. Saha, L. An, D.P.H. Hasselman, and F. Ernst: Ion exchange at a metal/ceramic interface. Acta Materialia 50 (2002) 1165.

2. Y. Yu: Diffusion Reactions at Metal–Oxide Interfaces and the Effect of an Applied Electric Field Doctoral Thesis, Case (2005).

3. A.M. Stoneham and P.W. Tasker: Metal–non-metal and other interfaces: The role of image interactions Journal of Physics C: Solid State Physics 18 (1985) L543.

This material is based upon work supported by the National Science Foundation under Grant No. DMR0208008. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.