Sublimation advantages

The advantages of this method are twofold (i) It is possible to observe minute changes in colour and structure before and during the process of melting. (2) It is possible to use a single crystal which, e.., is often obtained from a semi-micro sublimation. 

Dyes for WORM-Disks. Regarding their memory layer, dye-in-polymer systems show advantages over metal layers in their higher stabiHty, lower toxicity, lower heat conductivity, lower melting and sublimation temperature, and simpler manufacturing technique (substrate coating by sublimation or spincoating). 

Freeze-drying, like all drying processes, is a method to separate liquid water from a wet solid product or from a solution or dispersion of given concentration. However, the main difference is that the liquid water is separated by solidification (i.e., the formation of ice crystals) and subsequent vacuum sublimation instead of evaporation. This allows a drying at subzero temperatures which can be advantageous in case of heat-sensitive products. There are two general applications... 

Volatile substances of which the vapours, on cooling, condense directly to crystals without passing through the liquid phase are sometimes advantageously purified by sublimation, particularly when solubility relations render recrystallisation difficult. The purification of iodine is a well-known case in point. In organic chemistry this process is particularly suitable for quinones. 

Although the application of fluidisation techniques to sublimation-desublimation processes was first proposed by Matz" 11, the technique has not yet been widely adopted for large-scale commercial use, despite its obvious advantage of improving both heat and mass transfer rates. G aiko 112 1 has, however, reported on a fluidised-bed de-sublimation unit operating in the United States for the production of aluminum chloride at the rate of 3 kg/s (11 tonne/h). 

Lyophilization, although a relatively expensive proeess, can offer advantages for relatively imstable eompounds. In lyophilization, the solvent (usually water) is frozen and then removed by sublimation in a vaeuum environment. The low temperature maintained during the entire process minimizes thermal degradation of the drug compound. 

In the Langmuir free-evaporation method, the sample is suspended freely in a vacuum system with no container sunounding it. As very low levels of vapour pressure can be measured it has advantages over the Knudsen method where the lower limit is about 10" atm. (Kubaschewski et al. 1993). It is therefore more usefril in materials with high sublimation energies and therefore inherently low vapour pressiues. It has a further advantages in that there is no container with which to react, but there are more significant problems associated with temperature measurement. 

One of the prime advantages of the HTCVD approach is the resulting crystal properties. Due to the high purity of the gases, the material comes out intrinsically semi-insulating. Also, since the source material is produced on demand, the stoichiometry can always be kept the same, unlike the case with seeded sublimation growth. This will improve the yield of the grown material. 

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