Epitaxial crystallisation

Single-crystal spinel ferrite films can be epitaxially grown on MgO substrates. The ferrite, in a melt with Na2C03 as solvent, is deposited on the substrate at temperatures of 1100-1250 °C. Crystallisation of the... 

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. " Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

A strategy, which is perhaps more aligned with experiment, is simulated templated crystallisation. Here, one uses a (crystalline) substrate material to template the crystallisation of an overlying amorphous thin film. In particular. Fig. 5.9 shows the structure of a thin film of MgO supported on BaO. Such simulations, similar to atom deposition, attempt to simulate processes that occur during, for example, chemical vapour deposition or molecular beam epitaxy, where at some point the substrate will help nucleate and template the crystallisation of the amorphous thin film deposited thereon. 

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