Phase Transfers Including Sublimation

Tetrazoles are usually prepared by the reaction of an azide with a nitrile, or an activated amide tri-n-butyltin azide and trimethylsilyl azide are more convenient and safer reagents than azide anion is some cases. The second example shown illustrates the use of a cyanoethyl group as a removable protecting group for amide nitrogen. Other variations on this method from nitriles include the use of triethylammonium chloride (instead of ammonium chloride) to avoid the possible sublimation of potentially explosive azides, and the use of micelles as reaction media. Amides can be activated with trifluoromethanesulfonic anhydride, or via formation of the thioamide, or by the use of triphenylphosphine with diethyl azodicarboxylate the equivalent imidochloride will react under phase transfer conditions. ... 

Sublimation. During sublimation, the lattice constituents of the solid are directly transferred to the gas phase without the intervention of liquefaction, though there may be mobile intermediates at the surface of the heated solid. Various features of the sublimation process have been reviewed by Somorjai [18] and by Rosenblatt [19] who included consideration of kinetic aspects. Rhead [ 20] has discussed diffusion processes at surfaces. 

Some of the convective heat and mass transfer processes with phase change that we will deal with in the following have already been explained in the previous chapters. This includes the evaporation of a liquid at the interface between a gas and a liquid or the sublimation at a gas-solid interface. They can be described using the methods for convective heat and mass transfer. 

Entrapment pumps are self-contained units (these do not require any other pumps) capable of producing a vacuum down to 10 ° Torr. As the name suggests, entrapment pumps operate by trapping or condensing gas phase molecules into the solid state or by ionization and accelerating these molecules into a solid. Ion Pumps with Titanium sublimation pumps are more commonly included on state-of-the-art SIMS instruments. Ion pumps are operated continuously to sustain UHV conditions. Sublimation pumps, on the other hand, are operated in an intermittent maimer to allow improvement in vacuum conditions. As such pumps should not be operated at pressures greater than 10 Torr, initial pumpdown of these chambers is carried out through a combination of displacement and momentum transfer pumps. 

There are a number of other screen process formulations, including the use of sublimation dyes, printable resists, and printable liquid adhesives, which can also benefit from substrate pretreatment. When heat and pressure convert sublimation dyes into a gas phase decoration that is typically transferred from a paper carrier to the sublimate (material to be decorated), the hydrophilicity of the receiving sublimate is key. The same can be said about the need to remove low molecular weight organics to heighted the hydrophilicity of metal surfaces which must accept screen-printable resists prior to the final acid etching process. And, if adhesive patterns are required for bonding two similar or dissimilar materials, the need for... 

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