Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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Wolfgang Wisniewski, Christian Rüssel, 'Oriented surface nucleation in inorganic glasses – A review', Prog Mater Sci. 118 100758 (2021) doi:10.1016/j.pmatsci.2020.100758

Heating inorganic glasses above their respective glass transition temperature leads to surface crystallization in many cases. Analyzing the immediate surface of the resulting glass-ceramics using electron backscatter diffraction (EBSD) has shown that the topmost layer of crystals already shows orientation preferences, aka textures. As the information depth of EBSD is less than 100 nm, these textures can only result from oriented nucleation. This is noteworthy, considering that random nucleation is assumed in the classic nucleation theory for glasses. Since the effect was first described in 2010, oriented nucleation has been proven for 13 crystal phases including cases from all seven crystal symmetries ranging from cubic to triclinic. Of the 13 confirmed cases, six phases show a single texture while seven show up to three coexisting textures at the immediate surface. The texture intensities range from weak to extreme. It has been shown that modifying the crystallization temperature can affect oriented nucleation as well as modifying the chemical composition of the glass by e.g. systematically increasing the amount of the network forming component. Of the described textures, at least 20 indicate a low indexed lattice plane being aligned parallel to the sample surface while eight are more complex. In all cases, the orientation alignment is not strict: while most show tolerances of less than ±15°, some cases also feature a preference of much wider orientation domains. A fundamental explanation for the occurrence of oriented nucleation has not been proposed so far. While varying surface energies for different lattice planes of a phase could explain the effect, surface energies are usually unknown and hard to measure. Hence they cannot be used to predict the orientation preference at the surface at the current time. Considering the crystal structure of the phases, however, is an alternative and plausible approach. A detailed analysis of the cases analyzed so far shows that the smallest low indexed lattice plane of the crystal unit cell is often aligned parallel to the surface. This approach, however, can only explain the orientation preference, if an a-, b- or c-axis is perpendicular to the surface because higher indexed lattice planes usually have larger areas. Considering the smallest area per atom of the least mobile species being oriented parallel to the surface leads to an even better correlation between the approach and the experimental observations. This might be explained as follows: During nucleation, in a first step, an arrangement of atoms is formed at the immediate surface of the glass, which is similar to a certain plane of the unit cell of the formed crystal. For this purpose, the diffusion of atoms is necessary. Hence it seems plausible that first a layer rich in less mobile atoms should be formed and subsequently the more mobile atoms are added. As the least mobile atoms in the glass structure are the network formers, the first step of nucleation should then be that the lattice plane with the highest number of network formers per area is formed at the immediate glass surface. This lattice plane should initially remain parallel to the surface as further atoms are added. The orientation of the crystals below the immediate surface may change during crystal growth into the bulk so that the fastest growing crystallographic direction becomes oriented perpendicular to the surface. Here we review the work concerning oriented nucleation in glasses and present two models addressing the possible cause of the effect. Both a critically discussed and compared to the experimentally observed cases of oriented nucleation.