Crystal showing amorphous layer

In conditions of high viscosity of the metastable phase the time of establishment of a stationary concentration of nuclei becomes longer. The process of non-stationary nucleation may be characterized by the stationary nucleation rate J and the lag time. Non-stationary nucleation shows up in the crystallization of amorphous layers of water. The crystallization of such layers proceeds during continuous heating or an isothermal allowance after a stepwise rise in the temperature. 

The concentration of interlamellar tie chains in a given sample depends on molar mass, which determines the spatial distribution of the chains, and long period, i.e. the sum of crystal and amorphous layer thicknesses. Figure 7.33 shows two very... 

Crystalline solids other than metals examined, by electron diffraction, show some disturbance of the normal structure on polishing, but do not usually form a completely amorphous layer. Polishing tends to disintegrate the crystals at the surface, but not to the same extent as in metals and it often happens that continued polishing removes the... 

The second condition to validate the scheme B is that embrittlement must correspond to a critical morphological state that is the only approach to explain its sudden character. The extensive and careful work of Kennedy et al. (//) on relationships between fracture behavior, molar mass and lamellar morphology, shows that this condition is fulfilled in the case of PE. Comparing various samples of different molar masses with different thermal histories, they found that the thickness of the amorphous layer (la) separating two adjacent lamellae is the key parameter (Fig. 6). As a matter of fact, there is a critical value lac of the order of 6-7 nm. For la > lac the samples are always ductile whatever their molar mass, whereas for U < laC the samples are consistently brittle. As a result, lac appears to be independent of the molar mass. Indeed, there is a specific molar mass, probably close to 70 kg.mof for PE below which crystallization is so fast that it is impossible to have la values higher than lac whatever the processing conditions. 

In view of the multitude of observed deformation mechanisms it is useful at this point to examine the effects of external variables, especially that of ambient temperature, on the deformation behavior of semi-crystalline thermoplastics. At room temperature many of these polymers are above their glass transition point and owe their strength and stiffness to the crystalline phases. The first displacements start in the relatively soft amorphous layers, but the stress-strain curve is largely determined by the presence and arrangement of the crystals. Interlamellar slip has been identified as an important mechanism, but, in addition, crystalline deformation mechanisms occur at moderate strains The corresponding stress-strain curve shows an... 

The H profile in the y direction supports the presence of an outer softer amorphous layer (H = 120 MPa) in the samples with Tmouid below 120 °C (Pig. 7.8). However, on increasing Tmouid above 120 °C, the amorphous layer crystallizes and hardens. It should be noted that for the samples prepared at Tmouid > 120 °C, H shows a distinct maximum value at both surfaces, suggesting an enhancing nucleation effect of the metallic walls of the mould on the crystallizing material. 

Figure 17- High resolution image of a crystal of YBa2Cu306.5 showing the amorphous layer (some tens A) which systematically appears in that sample.

Fig. 8.8. Backscattering and channeling spectrum for amorphous Si layer on (100) Si crystal, showing dechanneled levels below the amorphous layer...

When crystallized from the melt, most polymers show a spherulitic texture (Figure 3.2). The spher-ulites then consist of lamellar stacks of alternating crystalline and amorphous layers, radiating from the center (the primary nucleus). 

Fig. 6.66 High-resolution transmission electron micrograph of a grain boundary in ABC-SiC after high-temperature annealing (1400 C/840 h), showing that the amorphous layer has become fully crystallized [41], With kind permission of Elsevier...

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