Solid State Reactions in Functional Oxides



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 = aS/2 [101], green symbols - those with b = aS/2 [100].


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

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