Surface crystallisation

Attempts to use selenium either in place of or in conjunction with sulphur in the vulcanisation of rubber do not appear to be completely successful,2 although it is claimed that rubber vulcanised with sulphur and selenium in the presence of an organic accelerator exhibits enhanced rigidity and resistance to abrasion.3 The incorporation of powdered selenium in ordinary rubber for vulcanisation by sulphur is said to provide crystalline selenium nuclei which induce the internal crystallisation of any superfluous sulphur and so prevent the undesirable surface crystallisation (or bloom ). The effect is attributed to the isomor-plious character of selenium and sulphur. 

Ding, Y. et al. (1995) Second order optical non-linearity of surface crystallised glass with lithium niobate, J. Appl. Phys., 77, 2208-10. 

This type is similar in principle to the tank type, but the cooling surfaces are continually scraped or agitated to prevent the fouling by deposited crystals and to promote heat transfer. They are suitable for processing high-viscosity liquors. Scraped-surface crystallisers can be operated batchwise, with recirculation of the mother liquor, or continuously. A disadvantage of this type is that they tend to produce very small crystals. A typical unit is the Swenson-Walker crystalliser shown in Volume 2. 

The polymer film may have resulted from cross-linking of the anionic units via calcium ions. Outside the region of the polymer film there was a mixture of calcite and vaterite crystals, as in the control experiment indicating that the surface film was influencing the surface crystallisation. The SEM image shows calcite crystals with roughened faces, the crystals on the surface of the polymer film displayed a relatively narrow distribution of shape and size as compared to the blank experiment. 

Kinetic studies of alkanethiol adsorption from ethanol solutions onto Au(lll) surfaces have shown that at relatively low concentrations (10 M) two distinct adsorption kinetic steps can be observed a very fast step, which takes a few minutes and leads to about 80-90% of maximal coverage and a slow step, which lasts several hours and at the end of which the thickness and contact angles reach their final values [110]. The initial step, described by the diffusion-controlled Langmuir adsorption, was found to be strongly dependent on the thiol concentration. At 1 mM solution the first step was over after 1 min, while it required over 100 min at 1 xM concentration. The second step can be described as a surface crystallisation process where the alkyl chains move from disordered states into unit cells, thus forming a two-dimensional crystal [110]. A more detailed study of Buck et al. involved different types of adsorption sites and showed that the rate-determining step strongly depends on the type of solvent [111-113]. 

The photostructuring of diffusion-modified FS21 glasses is limited with a depth of 400 pm because of the long diffusion time required, the onset of surface crystallisation and additionally an angle of wall inclination that exceeds 10°. Using this method it is impossible to fabricate holes, trenches and structures of a definedly varying depth in the same substrate. 

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