Superheating, crystal

The impossibility of realising a superheated crystal is ascribed by Tammann to the very large number of centres in the crystal in which fusion can commence, in contrast to the relatively small number of isolated points in the supercooled liquid where crystal-clusters begin to form. 

Finally, Tallon [13] has suggested another instability point where the entropy of the superheated crystal becomes equal to that for a superheated diffusionless liquid (a glass) rather than that of the liquid. Since the glass has lower entropy than the liquid, this instability temperature is lower than that predicted by Fetch and Johnson [12],... 

First-order solid-state amorphization occurs due to an entropy catastrophe [39] causing melting of superheated graphite and decompressed diamond below Pg when the entropy of the ordered crystal would exceed the entropy of the disordered liquid. This condition is resolved with the occurrence of a kinetic transition to a (supercooled) glass whereby the exact kinetic conditions during carbon transformation will be critically Pg-depen-dent [39]. It is important to consider the crystal to liquid transition and the effect of a superheated crystal whereof the ultimate stability is determined by the equality of crystal and liquid entropies [40]. When this condition is met, a solid below its Pg will melt to an amorphous solid, particularly... 

Keywords superheated crystals, molecular dynamics, defects, relaxation, cuboctahedral clusters... 

Crystals at T > T, are known as superheated crystals. To measure C, it is important to estimate the stability limit of crystals and melting must be avoided. Owing to n, surfaces and interfaces of crystals have a higher energetic state and melt first at T. Atoms in the crystals remain in metastable states. Hence if surfaces are avoided, alloys can be superheated. In order to avoid the contribution of interfaces and surfaces, single crystals, such as plating metals, are selected. With these methods, a superheated crystal is obtained and its Cjj can be measured. 

Boiling in the bulk of the fluid generally takes place at submicron nucleation sites as impurities, crystals, or ions. When there is a shortage of nucleation sites in the bulk of the liquid, its boiling point can be exceeded without boiling then the liquid is superheated. There is, however, a limit at a given pressure above which a liquid cannot be superheated, and when this limit is reached, microscopic vapor bubbles develop spontaneously in the pure liquid (without nucleation sites). 

Although Carnelley once thought he had been able to superheat ice ( hot ice ), it is almost certain that no solid can be maintained alone at a temperature higher than its melting-point. Tammann (Zeitschr. physik. Chem., 68, 257, 1910) finds, however, that a crystalline solid may, under certain circumstances, be superheated in the presence of its melt. This occurs when the supply of -heat to the crystal is sufficiently great in comparison with the linear velocity of crystallisation of the supercooled liquid (cf. Findlay Phase Hide). 

As with other properties of solids, the increased relative significance of surface energy in very small (i.e. micrometre-sized) crystals influenced the melting points [2,16,17] and diffusion at this temperature. Quantitative studies of rates of melting of solids are impracticable since superheating is effectively forbidden and the rate of the endothermic phase change is determined by the rate of heat supply and the thermal conductivity of the solid. 

The receiver is changed and the contents are now rapidly distilled, using superheated steam. Almost all of the biacetyl monoxime comes over in the first 5 1. of distillate. One to 1.5 kg. of salt is dissolved in the distillate, which is then cooled to o°. The solid biacetyl monoxime crystallizes out and may be filtered off. The yield is 480-520 g. The product can be further purified, if desired, by recrystallizing from water (Note 5). 

If the flask is not protected with an asbestos board or the equivalent, decomposition occurs where the substance is superheated on the side walls of the flask. If crystals of the cyanide are allowed to remain on the upper walls of the flask, they are not easily washed down and so are not hydrolyzed. 

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