Solid State Reactions in Functional Oxides

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Solid-state reactions for spinel nanotubes

H.J. Fan, M. Knez, R. Scholz, K. Nielsch, E. Pippel, D. Hesse, and M. Zacharias

(Top) Principle of the formation of ZnAl₂O₄ nanotubes from Al₂O₃-coated ZnO nanowires, using the Kirkendall effect. The latter occurs during the spinel formation due to the outdiffusion of ZnO and results in a corresponding formation of a hollow core in the previous ZnO nanowires, ending up with a spinel nanotube.

(Bottom) Initial and final stages of the solid state reaction:
(a) TEM image of an as-coated Al₂O₃/ZnO core-shell nanowire;
(b) SEM overview image and
(c) TEM detail image of ZnAl₂O₄ nanotubes grown by solid state reaction.

For details, see, e.g. Nature Materials 5 (2006) 627.

Solid-state reactions in complex tantalate and niobate systems

D.C. Sun, S. Senz, and D. Hesse

High-resolution transmission electron micrograph showing the reaction front Mg₄Ta₂O₉/MgO formed during the formation of Mg₄Ta₂O₉ from Ta₂O₅ and a MgO single crystal at 1100 °C. Beam direction is [1 -10] MgO || [1 -1.0] Mg₄Ta₂O₉. The (00.1) planes of the hexagonal tantalate are exactly parallel to the (111) planes of the cubic MgO, indicating a topotaxial reaction mechanism. The reaction front advances via movement of ledges, and shows strain contrast due to lattice misfit.

For details, see, e.g. J. Europ. Ceram. Soc. 24(2004) 2453,
and J. Europ. Ceram. Soc. 26 (2006) 3181.

Spinel-forming solid-solid reactions

H. Sieber, W. Blum, A. Graff, N.D. Zakharov, R. Scholz, S. Senz, and D. Hesse

High resolution transmission electron micrograph showing the reaction fronts Al₂O₃/MgAl₂O₄ and MgAl₂O₄/MgO formed during the spinel-forming solid-solid reaction between a sapphire (Al₂O₃) crystal and a thin MgO film. The arrows indicate particular features of the front. (Micrograph by R.Scholz).

For details, see, e.g. Z. Metallkunde 95 (2004) 252-257.

Spinel-forming gas-solid reactions

H. Sieber, A. Graff, S. Senz, and D. Hesse

Cross sectional 220-spinel dark field micrograph of a Mg₂SnO₄ island topotaxially grown on a free-standing MgO substrate surface during the spinel-forming gas-solid reaction between a SnO₂ vapour and the MgO substrate. (Micrograph by A.Graff).

For details, see, e.g. Ceramics International 26 (2000) 753-756.

Model experiments for solid-state reactions in electroceramics

A. Graff, A. Lotnyk, N.D. Zakharov, S. Senz, and D. Hesse

High resolution transmission electron micrograph (cross section) of the structure of the reaction fronts between a topotaxially grown Ba₃ Ti₁₁O₂₅ island and a BaTiO₃(001) single crystal substrate. (Micrograph by N.D.Zakharov).

For details, see, e.g. J. Europ. Ceram. Soc. 25 (2005) 2201-2206.

Model experiments for solid state reactions in solid oxide fuel cells

C.J. Lu, A. Schubert, S. Senz, and D. Hesse

Plan-view TEM dark field image of a La₂Zr₂O₇ pyrochlore island grown on a flat (001) surface of an yttria-stabilized ZrO₂ (YSZ) substrate. Four tilted domains (1 - 4), and networks of misfit dislocations are clearly visible. (Micrograph by C.J.Lu).

Cross-sectional scheme of two mechanisms of dislocation generation at the interface between a La₂Zr₂O₇ island (orange) and a YSZ substrate (blue), (a) on flat surface areas, and (b) near a pit rim. Black dislocation symbols - dislocations with Burgers vectors of type b = a_S/2 [101], green symbols - those with b = a_S/2 [100].

For details, see, e.g. Phil. Mag. A 81 (2001) 2705-2723.

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