Eutectic melt crystallization

Figure 4. Thermobarogram on heating for the mixture MET/MNT with 11.9 wt% MET. Sample weight was 1 mg. Sequence of phase transitions eutectic melting, crystal dissolution of excess MNT, nematic/ isotropic phase transition. Reproduced from [12] by permission of Gordon and Breach Science Publishers, Inc. and OPA Ltd...

Recovery by melt crystallization also is limited by the eutectic composition. 

Column crystallizers of the end-fed type can be used for purification of many eutectic-type systems and for aqueous as well as organic systems (McKay, loc. cit.). Column crystallizers have been used for xylene isomer separation, but recently other separation technologies including more efficient melt crystallization equipment have tended to supplant the Phillips style crystallizer. 

Top Bridgman procedure for the production of large single crystals. From a eutectic melt one can obtain a single crystal with embedded wires. Bottom possible subsequent steps of processing... 

A higher temperature, Tc l 10K, phase in the Bi-Sr-Ca- Cu-O system has also been reported (78-80). This phase contains three infinite two-dimensional Cu-O layers. However, lead appears to play an important role in stabilizing this compound in phase pure form (75)(79)(80). Single crystals grown by eutectic melt growth techniques have been reported (77) but the phase purity of these crystals with respect to intergrowths containing various numbers of Cu-O planes is a concern. 

Production rates of melt crystallization of organic materials usually are low enough to warrant the use of scraped surface crystallizers like that of Figure 16.10(a). A major difficulty in the production of crystals is the occlusion of residual liquor on them which cuts the overall purity of the product, especially so because of low temperatures near the eutectic and the consequent high viscosities. Completeness of removal of occluded liquor by centrifugation or filtration often is limited because of the fragility and fineness of the organic crystals. 

Complete recovery of dissolved solids is obtainable by evaporation, but only to the eutectic composition by chilling. Recovery by melt crystallization also is limited by the eutectic composition. 

Up to now the so-called catalytic process is the only way to produce c-BN on an industrial scale. However, catalytic is not the correct scientific term, because the activation energy for transformation is not decreased by these substances. The substances which are used have the function of a solvent, and are responsible for the formation of c-BN. This method is successful because of the different solubilities of c-BN and h-BN in the flux. The precursor substances form a eutectic melt with the h-BN [152]. If the reaction conditions are in the domain of stable c-BN, spontaneous crystallization takes place and the c-BN growth rate is relatively high. 

Thermal transitions which are commonly observed in frozen systems are illustrated in Figure 7 where, for the sake of this discussion, a deflection upward indicates an endothermic transition. The glass transition is a shift in the baseline toward higher heat capacity. Crystallization during the DSC experiment is observed as an exothermic event, and eutectic melting is an endothermic transition which preceeds the melting of ice. 

Fig. 1. 28.1. DTA measurement of a 10% sucrose-10% NaCI solution during slow rewarming after quick (200 °C/min) freezing. 1, Glass transition at = 93 °C 2, crystal growth (exothermic) at -65 °C 3, significant exothermic event, crystallization of NaCI at =44 °C 4, eutectic melting at =22 °C ...

Fig. 1.29. Behavior of a su-crose-NaCI solution at different sucrose—NaCI concentrations and temperatures after quick freezing (200°C/min) during slow rewarming (Figure 8 from [1.25]). 1, Eutectic melting temperature of NaCI 2, crystallization temperature of NaCI 3, temperature at which the glass phase starts to soften 4, glass transition temperature...

Fig. 6 C. Crystals of molybdenum sesquisulfide (46.5 at. % S) displaying distinct cleavage, partly with liquid inclusions, floating in the binary Mo-S eutectic melt, chilled from 1630 °C x 400, in air...

As an example, we will consider the molecular dynamical behavior of egg white lysozyme. The temperature dependence of mobility of fluorescence, spin and Mossbauer labels attached to lysozyme was found to be similar to other investigated proteins the monotonic increase typical for rigid polymers in dry states and in samples with water content (wt) was less than the critical value (wtcr) and drastically burst when wt > wtcr at T > 200 K took place (Frolov et al., 1978 Likhtenshtein, 1979). At similar conditions, experiments on the temperature dependence of heat capacity indicated only a monotonic steady increase for rigid organic material. Recently, in the fully dried lysozyme crystal, similar monotonic behavior of heat capacity was observed in temperatures between 8 and 30°C. At D20 content more than 24 wt %, a slight deviation from the monotony was observed at temperatures above approximately 185 K, which most probably is due to the eutectic melting of NaCl/2H20 present in the samples to prevent water crystallization (Miyazaki et al., 2000). 

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