Surface premelting

One of the consequences of SFG s ability to distinguish surface layers is connected with the debate concerning surface premelting. Premelting temperatures have been variously identified as 261 K (prism face), 260K (basal face) and 200 a considerable discrepancy. We suggest resolution of this issue as follows. A liquid has no surface anisotropy in the absence of an applied field. Ice, however, is anisotropic The prism face can be oriented either with the c-axis in the plane of incidence (p) or... 

Another important feature of the SPC prepared according to this combined approach as compared with common composites, is the amount of the matrix and the material s resulting density. Since the material is created during compression molding only as a result of partial surface premelting, the quantity of isotropic matrix amounts to only 10 to 20 wt% compared to the typical 50 to 70 wt% required in common composite materials. The SEM observations presented in Figure 19.8 demonstrate that a PET nanofibrillar single-polymer composite (PET-NSPC) has indeed been produced. 

Fig. 12.5 SEM micrographs of the various stages of pieparation of nanofibrillar SPCs a ciyo actured surface made parallel to the draw direction of nanofibrillar polymer-polymer composite based on LLDPE/PVDF highly drawn blend subjected to compression molding, b neat PVDF nanofibrils after extraction with selective solvent of IXDPE from the IXDPEfPVDF polymer-polymer composite (a), c cryofractured cross section of nanofibrillar single polymer composite prepared via compression molding of the parallel aligned neat PVDF nanofibrils (b), d surface of the nanofibrillar SPC shown in (c) demonstrating the adherence of PVDF nanofibrils due to surface premelting only...

Thermal or Flame Spray Process. The earliest experiments in metal spray used molten metal fed to a spray apparatus, where it was dispersed by a high speed air jet into tiny droplets and simultaneously blown onto the surface of the part to be covered. The metal solidified on contact. Modem processes use a more convenient source than premelted metal. Spray heads using a flame or an electrical arc to melt metal wires or powders directly are much more convenient. These are the only types used on a large scale in the United States. 

These contact angles can be related to the physical state of the surface. The 100 facet is better wetted than the 111 one because the 100 surface is partly premelted. But, the liquid-like disordered monolayer is too thin to have the properties of the macroscopic liquid, and this "adsorbed liquid layer" coexists with a non-wetting macroscopic liquid. This so-called "incomplete surface melting" has also been observed on a pure single crystal of ice. ... 

In both eases, the anisotropy of wetting is driven by the anisotropy of surface energy of the solid. The degree of wetting is shown to be strongly correlated to the degree of premelting at the solid surfaee. 

A similar reversible thermal disordering of the surface layer has also been observed in extended-chain alkanes, starting from the early work on /7-C94H190,93 where the effect has been compared to the premelting phenomenon in polymers.94 This is illustrated in Figure 15 where the temperature-... 

Empirical tight-binding calculation of silicon and carbon surfaces can be traced back to the early work of Chadi [19] and Mele et al. [106,107]. Recently, several newly developed models have also been applied to the studies of silicon, germanium, and diamond surfaces [22,108-110,112]. Almost all of these studies were zero-temperature statie calculations except for the work of Stokbro et al., who applied the newly developed effective-medium TB model to study surface melting and defect-indueed premelting behavior on the Si(lOO) surface [112]. 

---