Temperature crystalline-amorphous transition

This reaction is irreversible. Epitaxial silicon is deposited. Polycrystalline silicon is obtained in the range of 610-630°C, which is close to the crystalline-amorphous transition temperature. 

The dissipation factor (the ratio of the energy dissipated to the energy stored per cycle) is affected by the frequency, temperature, crystallinity, and void content of the fabricated stmcture. At certain temperatures and frequencies, the crystalline and amorphous regions become resonant. Because of the molecular vibrations, appHed electrical energy is lost by internal friction within the polymer which results in an increase in the dissipation factor. The dissipation factor peaks for these resins correspond to well-defined transitions, but the magnitude of the variation is minor as compared to other polymers. The low temperature transition at —97° C causes the only meaningful dissipation factor peak. The dissipation factor has a maximum of 10 —10 Hz at RT at high crystallinity (93%) the peak at 10 —10 Hz is absent. 

The writing process, that is, the transition crystalline — amorphous, is caused by briefly (<50 100 ns) heating up the selected storage area (diameter (( )) ca 0.5—1 Hm) by a laser pulse to a temperature above the melting point of the memory layer (Eig. 15, Record), such that the film locally melts. When cooled faster than a critical quench rate (10 -10 ° K/s), the formation of crystalline nuclei is suppressed and the melted area sohdifies into the amorphous (glass-like) state. 

Table 3. Glass-Transition Temperature of Amorphous Poly(Vinyl Ether)s and Melting Points of Crystalline Poly(Vinyl Ether)s ...

Most PHAs are partially crystalline polymers and therefore their thermal and mechanical properties are usually represented in terms of the glass-to-rubber transition temperature (Tg) of the amorphous phase and the melting temperature (Tm) of the crystalline phase of the material [55]. The melting temperature and glass transition temperature of several saturated and unsaturated PHAs have been summarized in Table 2. 

Except for a lew thermoset materials, most plastics soften at some temperatures, At the softening or heat distortion temperature, plastics become easily deformahle and tend to lose their shape and deform quickly under a Load. Above the heat distortion temperature, rigid amorphous plastics become useless as structural materials. Thus the heat distortion test, which defines The approximate upper temperature at which the material can be Safely used, is an important test (4,5.7.24). As expected, lor amorphous materials the heat distortion temperature is closely related to the glass transition temperature, hut tor highly crystalline polymers the heat distortion temperature is generally considerably higher than the glass transition temperature. Fillers also often raise the heat distortion test well above... 

In contrast to the mature instrumental techniques discussed above, a hitherto nonexistent class of techniques will require substantial development effort. The new instruments will be capable of measuring the thermal (e.g., glass transition temperatures for amorphous or semicrystalline polymers and melting temperatures for materials in the crystalline phase), chemical, and mechanical (e.g., viscoelastic) properties of nanoscale films in confined geometries, and their creation will require rethinking of conventional methods that are used for bulk measurements. 

Although polymers exhibit both viscous and elastic responses at all temperatures, the elastic response is particularly strong at temperatures less than 50°C above the glass transition temperature, particularly for polymers well above their critical molecular weight. Polymers are often considered to have dominant viscous rheological responses if they are stressed at temperatures over 100 °C above the glass transition temperature for amorphous polymers or 100°C above the crystalline melting point for semicrystalline resins. 

Fig. 15. Time—temperature transformation in a ihin-pha.se change layer during recording/reading/erasing (3,105). C = Crystalline phase A = amorphous phase Tm = melting temperature Tg = glass-transition temperature RT = room temperature.

There are different criterion of how to classify solid-solid interfaces. One is the sharpness of the boundary. It could be abrupt on an atomic scale as, for example, in III-IV semiconductor heterostructures prepared by molecular beam epitaxy. In contrast, interdiffusion can create broad transitions. Surface reactions can lead to the formation of a thin layer of a new compound. The interfacial structure and composition will therefore depend on temperature, diffusion coefficient, miscibility, and reactivity of the components. Another criterion is the crystallinity of the interface. The interface may be crystalline-crystalline, crystalline-amorphous, or completely amorphous. Even when both solids are crystalline, the interface may be disturbed and exhibit a high density of defects. 

The structures of liquid crystals are intermediate between the amorphous and crystalline states. They have some short-range orientational order. Some also have positional order. Thousands of organic compounds exhibit liquid crystal structures. Most have molecules that are very long and thin, but some have molecules that are flat and pancake shaped. Many compounds may exist in more than one liquid crystalline state. Transitions from one state to another may be thermotropic (caused by temperature change) or lyotropic (caused by change of solute concentration). 

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