Crystalline-to-amorphous transition

A disadvantage of amorphous alloys is their metastable character which makes them transform into the stable crystalline state as a function of temperature and time. In calorimetric experiments the amorphous-to-crystalline transition is revealed by an exothermic heat effect. Typical traces obtained using a differential scanning calorimeter are shown for amorphous Gd064Co0 36 in fig. 51. The dependence of the crystallization temperature Tx on the heating rate s implies that there is a risk of crystallization taking place even at room temperature after an extended period (s - 0). This is particularly likely when Tx is rather low, and it may have consequences for practical applications. 

Cefazolin sodium is capable of existing in a number of crystalline modifications, but the amorphous state is produced by the lyophilization process [38]. The amorphous form retained its nature at relative humidities less than 56% but converted completely to the pentahydrate above 75% RH. Interestingly, the hygroscopicity of the amorphous form was essentially similar to that exhibited by the dehydrated monohydrate or dehydrated a-form. Lamotrigine mesylate was also found to exhibit a moisture-dependent amorphous-to-crystalline transition [39]. The transition was found to be independent of the presence of mannitol, whether the amorphous solid was produced by lyophilization or by spray-drying. 

Explain why Si NMR can be used to study the amorphous-to-crystalline transition in a silica gel. [Note You may find the following references of use Brown, I.D. and Shannon, R.D. (1973) Empirical bond-strength bond-length curves for oxides, Acta Crystallogr. A 29, 266 Smith, K.A., Kirkpatrick, R.J., Oldfield, E., and Henderson, D.M. (1983) High-resolution Si nuclear magnetic-resonance spectroscopic study of rockforming silicates, Am. Mineral. 68, 1206.]... 

Included in this section is a description of the various transformations taking place in amorphous alloys, such as structural relaxation, glass transition and amorphous-to-crystalline transition. The main techniques employed to study these transitions are described in more detail and further conclusions are given that can be drawn from the results of these studies. 

A.C. Martin, N. Lakhera, A.L. DiRienzo, D.L. Safranski, A.S. Schneider, C.M. Yakacki, et al., Amorphous-to-crystalline transition of polyetherether-ketone-carbon nanotube composites via resistive heating. Composites Science and Technology, 89,110-119, 2013. 

However, the most intensively studied system with Hquid precursors is still CaCOs. If PILP droplets are deposited on a substrate, they coalesce and form a coating, which subsequently transforms into patchwork-Hke calcite films with different single crystalUne domains via an amorphous to crystalline transition [229]. 

Fig. 21 a Schematic illustration of the new approach for the formation of microperfo-rated single crystals deposition of the ACC mesh from the CaCOs solution, oriented nucleation at the imprinted nucleation site, and the amorphous-to-crystalline transition of the ACC film on the engineered 3D templates. (Reproduced from [247], 2003, American Association for the Advancement of Science)... 

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