Hard epitaxy

This work was followed by a number of researchers who confirmed the experimental results on various metallic multilayer systems (e.g. Cu/Ni [78-80] as well as on hard epitaxial and polycrystalline superlattices of nitrides, such as TiN/VN... 

PTFE is a special case. It has been shown that rubbed PTFE layers induce the oriented crystallization of a number of materials crystalline and liquid-crystalline polymers as well as low molecular weight liquid-crystalline materials [55]. The underlying orientation process is not elucidated in every case. In many instances, it seems that some form of graphoepitaxy may be involved since no clear-cut structural matching can be dehned. Another indicator is the parallel orientation of the deposited chain with the PTFE substrate, which would be compatible with some form of milder interaction— but, of course, does not rule out hard epitaxy. One example of hard epitaxy, which also results in a tilted orientation of the chain axis relative to the PTFE substrate chain orientation, will be analyzed shortly. 

Finally, several liquid-crystalline (EC) polymers have been used as nucleating agents for crystalline polymers that crystallize below the LC transition temperature. As far as the present authors are aware, there have been no detailed reports on the exact structural relationship between these LC polymers and the crystalline polymers, except to mention that the chain axes are parallel, which takes us back to the situation analyzed for PTFE. This limitation also stems from the fact that in the LC state, no clear-cut crystallographic organization in the exposed faces can be dehned, which precludes the type of analysis that needs be developed in order to define hard epitaxy. 

Epitaxy between Linear and Helical Polymers Epitaxy between linear and helical polymers may be of particular interest when the contact plane of the helical polymer is build up with isochiral helices, and when the chain axis of the deposited linear polymer is oriented parallel to the helical path. This provides a direct means to determine the exposed helical hand of the substrate polymer. The tilted orientation of chain axes when a helical polymer is involved is therefore an unmistakable indicator of specific interactions, that is, of hard epitaxy [11,29,30]. 

Several analyses of polymer/nucleating agent interactions have been performed over the years. As indicated, when the analysis is made at a sufficient level of detail and when the partners in the epitaxy have well-defined crystal structures, (that is, excluding, e.g., liquid-crystalline phases), it appears that most epitaxial interactions belong to the class of hard epitaxy. However, a major characteristic of these analyses is that they deal with polymer/nucleating agent systems that have been, for the most part, discovered empirically. 

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

The films are epitaxial in the sense that the lattice constant is intermediate between those of copper and nickel. As indicated above, that modulated strain is probably responsible for the increased hardness. Other authors (5) have tried to explain similar effects by stating that the layers were specifically oriented. Our example (6) demonstrates that these considerations must be reexamined since it was possible to achieve the effect in a crystalline multilayer deposited on an amorphous nickel-phosphorus underlayer. It appears that layer thickness is the important parameter here. 

Whereas the disclination picture provides a convincing explanation of the properties of multilayers of fatty acids, being consistent with both the electron diffraction and optical evidence, it is not yet proven that, at room temperature, such systems are really in the hexatic state. It is equally probable that the existence of an initial hexatic monolayer on which subsequent layers are grown by epitaxy produces a material which is far from thermal equilibrium and has more of the nature of a glass state rather than of a mesophase. Indeed, the relative hardness of multilayers and their resemblance to true three-dimensional crystals of fatty acids tends to support this view. 

Hoogenboom JP, van Langen-Suurling AK, Romijn J, van Blaaderen A (2003) Hard-sphere crystals with hep and non-close-packed structure grown by colloidal epitaxy. Phys Rev Lett 90 1-4... 

The mechanical properties of materials involve various concepts such as hardness, shear and bulk modulus. The group III nitrides are now mostly used as fihns or layers grown by metal organic vapour phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) on sapphire, GaAs or SiC. The lattice parameters of the substrate do not generally match those of the deposited layer, and therefore, stresses appear at the interface and in the layer and modify its physical properties. Hence, it is necessary to have a good knowledge of these properties. 

It would be hard to find a clearer proof of epitaxial attachment. In the end of these considerations of epitaxial attachment, one is left with a certain appreciation for the comic figure of a befuddled man standing in the rain with a large ring of keys and trying to open his front door. When the right key (molecule) is found for the particular door (substrate), the result is most gratifying. 

Hard. brittle, garnet-type crystals. Hardness about equal to that ot quartz Melts above I 040 . The melt is easily amenable to doping and crystal nucleation by epitaxy. Crystals may be oriented along the simple axes (100), (110), and (ill) using the natural (110) face of the crystal. 

For properly chosen conditions the epitaxial layer growth rate is limited only by the rate of vapour transport from source to substrate and it is hardly affected by surface phenomena kinetics [30-33]. Since the silicon vapour pressure sufficiently exceeds the pressures of carbon-containing species, the vapour flow from source to substrate is limited by the transport of carbon, and the flux qsiC is given by ... 

---