Undercooled melt

A. Milchev, I. Gutzow. Temperature dependence of the configurational entropy of undercooled melts and the nature of the glass transition. J Macromol Sci B 22 583-615, 1982. 

In this section we discuss the basic mechanisms of pattern formation in growth processes under the influence of a diffusion field. For simphcity we consider the sohdification of a pure material from the undercooled melt, where the latent heat L is emitted from the solidification front. Since heat diffusion is a slow and rate-limiting process, we may assume that the interface kinetics is fast enough to achieve local equihbrium at the phase boundary. Strictly speaking, we assume an infinitely fast kinetic coefficient. 

To be specific, we consider the two-dimensional growth of a pure substance from its undercooled melt in about its simplest form, where the growth is controlled by the diffusion of the latent heat of freezing. It obeys the diffusion equation and appropriate boundary conditions [95]... 

Here U = T — T )Cp/L is the appropriately rescaled temperature field T measured from the imposed temperature of the undercooled melt far away from the interface. The indices L and 5 refer to the liquid and solid, respectively, and the specific heat Cp and the thermal diffusion constant D are considered to be the same in both phases. L is the latent heat, and n is the normal to the interface. In terms of these parameters,... 

Non-Equilibrium Solidification of Metastable Materials from Undercooled Melts... 

Fig. 35 Changes in R2n, the averaged distance between atoms n-bonds apart along the chain, during crystallization at 330 K. In the initial state of undercooled melt ( ), R2n depends linearly on n showing the random coil nature of the melt chains. With the onset of crystallization, the functional form of R2n changes considerably (o) after 6.4 ns, (A) after 12.8 ns, and (A) after 24.32 ns...

The microscopic structure of the undercooled melt has been a subject of great interest in studies of polymer crystallization. There have been long arguments in favor of the presence of mesoscopic local order in the melt or at the crystal-... 

We also examined the fold statistics in this Ciooo system. The distribution of the inter-stem vectors connecting stems linked by the loops, and their radial distribution function again indicated that about 60-70% of the folds are short loops connecting the nearest or the second and third nearest stems, though the crystallization did not complete. The presence of local order in the under cooled melt in the present Ciooo system is also examined through the same local order P(r) parameter, the degree of bond orientation as a function of position r, but again we did not detect any appreciable order in the undercooled melt. 

Vol. 9 The Local Chemical Analysis of Materials by J. W. Martin Vol. 10 Metastable Solids from Undercooled Melts by D. M. Herlach, P. Galenko and D. Holland-Moritz... 

Fig. 3. Phase diagram of a portion of ihe Sitb-AbO% system, showing regions of phase separation of undercooled melts. The projected composition of Class-F fly ash glasses lies in the range of 10-4051 mol /t AI,Ot. Adapted from Roth et al. (1987).

Lamellae obtained by direct crystallization from the melt into stable crystalline modifications at temperatures of 60-80 °C have thicknesses which differ only marginally from those of the fully mesomorphic samples. Such a finding, if confirmed, is highly relevant because it suggests that the lamellar organization depends only to a very limited extent on the particular crystal form while, to a first approximation, the pre-existing arrangement in the undercooled melt appears to play a key role. 

In the undercooled melt far from Tg, ED is constant for a given polymer. Mandelkern determined activation energies for a series of polymers and found that ED for different polymers increases monotonically with Tg, in first approximation. Table 19.4 gives a survey of the data. Mandelkern stated that ED increases "monotonically" with Tg. This correlation... 

Trinitrotoluene (also known as y-TNT) is one of the main impurities in military and commercial grades of TNT. Chick and Thorpe (1971) characterized two polymorphs. Form I (mp 376.2 K) may be obtained by recrystallization from alcohol or solidification of the melt. Form II (mp 347.2 K) is produced in small quantities with difficulty from an undercooled melt. It readily converts to Form I by mechanical perturbation or even spontaneously. Chick and Thorpe also determined latent heats of fusion, entropies of fusion, specific heats, IR spectra. Due to the conversion induced by grinding no X-ray data were presented for either form. No crystal structures have been reported. 

Fig. 5.2. SEM image of Si faceted dendrite grown from undercooled melt. EBSP analysis was performed to find parallel twins (shown by white lines). Parallel twins exist at the center of the faceted dendrite [16]...

Fig. 5.3. Growth process of Si crystal from undercooled melt [16]. (a) Crystal is growing with faceted interface, (b)-(d) Faceted dendrite grows from a part of faceted interface. The direction of the growth of faceted dendrite is parallel to the 111 facet face on the interface [16]...

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