Sublimation process

Chemical products are produced from technical-grade oxide in two very different ways. Molybdenum trioxide can be purified by a sublimation process because molybdenum trioxide has an appreciable vapor pressure above 650°C, a temperature at which most impurities have very low volatiUty. The alternative process uses wet chemical methods in which the molybdenum oxide is dissolved in ammonium hydroxide, leaving the gangue impurities behind. An ammonium molybdate is crystallized from the resulting solution. The ammonium molybdate can be used either directly or thermally decomposed to produce the pure oxide, MoO. ... 

In all of these processes it is possible to increase the yield of whiskers by a dding metallic impurities, and the sublimation process requires such additions. The vapor—Hquid—sohd (VLS) growth mechanism is often thought to be involved. 

A high percentage 01 water remains after the sublimation process, present as adsorbed water, water of hydration or dissolved in the diy amorphous solid this is difficult to remove. Usually, shelf-temperature is increased to 25 to 40°C and chamber pressure is lowered as far as possible. This stiU does not result in complete diying, however, which can be achieved only by using even higher temperatures, at which point thermally induced product degradation can occur. 

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. 

Fig. 17. Unified reaction scheme for the thermal decomposition of ammonium perchlorate, proposed by Jacobs et al. [59,925,926], In the low temperature reaction, the interaction occurs between adsorbed species (a) whereas the high temperature reaction and sublimation process involved volatilization intermediates (g). X] and X2 represent mixtures of intermediates.

Vacuum sublimation is a very popular method for purification of organometallic compounds, because it is so convenient and easy. The sublimation process is not very selective, however, so that it is seldom possible to separate cleanly more than one or perhaps two compounds from a mixture, while in many cases several compoimds occur simultaneously in the irradiated targets. Moreover, annealing may be induced by the heating of the sample for sublimation, although this can be minimized by prior dissolution of the sample to release reactive atoms and... 

A sublimation process is controlled primarily by the conditions under which phase equilibria occur in a single-component system, and the phase diagram of a simple one-component system is shown in Figure 15.30 where the sublimation curve is dependent on the vapour pressure of the solid, the vaporisation curve on the vapour pressure of the liquid, and the fusion curve on the effect of pressure on the melting point. The slopes of these three curves can be expressed quantitatively by the Clapeyron equation ... 

Vacuum sublimation is a development of simple sublimation, which is particularly useful if the pressure at the triple-point is lower than atmospheric, where the transfer of vapour from the vaporiser to the condenser is enhanced by the increased driving force attributable to the lower pressure in the condenser. Iodine, pyrogallol, and many metals have been purified by vacuum sublimation processes in which the exit gases from the condenser are usually passed through a cyclone or scrubber to protect the vacuum equipment and to minimise product loss. 

In entrainer sublimation, an entrainer gas is blown into the vaporisation chamber of a sublimer in order to increase the vapour flowrate to the condensing equipment, thereby increasing the yield. Air is the most commonly used entrainer, though superheated steam can be employed for substances such as anthracene that are relatively insoluble in water. If steam is used, the vapour may be cooled and condensed by direct contact with a spray of cold water. Although the recovery of the sublimate is efficient, the product is wet. The use of an entrainer gas in a sublimation process also provides the heat needed for sublimation and an efficient means of temperature control. If necessary, it may also provide dilution for the fractional condensation at the desublimation stage. Entrainer sublimation, whether by gas flow over a static bed of solid particles or through a fluidised bed, is ideally suited to continuous operation. 

The product yield from an entrainer-sublimation process may be estimated as follows. The mass flowrate G of the inert gas and the mass sublimation rate S are related by ... 

The theoretical maximum yield from an entrainer sublimation process is the difference between the calculated sublimation rates corresponding to the conditions in the vaporisation and condensation stages. 

Subcritical Mass when the quantity of radioactive fuel is insufficient to produce a self-sustaining chain reaction Sublimation process where a substance passes directly from the solid to gaseous phase without going through the liquid phase... 

We point out here that the colloid prepared by these methods is very clean, because the carrier gas used is usually high-purity grade at six-nine, the chamber is once evacuated to depress the extent of contaminating oxygen and moisture, and the liquids themselves are always purified by sublimation process except for the solution trap method. To transfer the colloidal suspension after preparation, a specially designed stock bottle with a Luer-lock syringe is normally used in order to enable the operations under Ar flow to avoid unexpected air contamination. Therefore, we can carry the suspension liquid away from the production chamber without exposure to air, which means that the surface of colloidal metal is very clean if it does not react with suspension liquids. 

From this point on, the solid materials are pyrophoric, especially the residual solids from the sublimation process. However, the ruthenocene obtained by sublimation is not pyrophoric. The checkers found that careful addition of the sublimation residues to water under nitrogen destroys their pyrophoric character. 

Valve I is opened and the trimethylindium is sublimed, under static vacuum, into the 2-L flask (trap-trap distilled). A hot-air gun is used to assist this sublimation process and to ensure that all the trimethylindium is collected on, or near, the surface of the cold, or frozen, amine/petroleum spirit suspension. When all the trimethylindium is trapped into the 2-L flask, valve D is closed to seal the 2-L flask and the rest of the system is filled with air through valve H (make sure valve D is closed ). The liquid nitrogen Dewar is removed and the 2-L flask is disengaged from the rest of the system by careful disconnection between the adaptor housing valve D and the U tube. The top of this adaptor is plugged with a stopper. 

Sublimation (or Primary Drying). For the sublimation phase of the process, the frozen material usually is subjected to a vacuum of about 4.6 millimeters of mercury. The ice-crystal sublimation process can be regarded as comprised of two basic processes (l)Heal transfer, and (2) mass transfer. In essence, heat is furnished to the ice crystals to sublime them he generated waler vapor resulting is transferred out of the sublimation interface. Thus, it is evident thal sublimation will be rare-limited by both resistances to heat and mass transfer as they occur within the material. 

The theory of the sublimation process has been discussed in detail elsewhere41 the following describes the practical aspects of this technique which is applicable down to a few milligrams of material. 

With care a mixture may be fractioned by sublimation when the amount of sublimate formed at a particular temperature no longer seems to increase, the sublimation process should be stopped and the sublimate removed. The cleaned cold finger is then reintroduced and the sublimation is continued at a higher... 

Equation (9) is valid for evaporation and sublimation processes, but not valid for transitions between solids or for the melting of solids. Clausius-Clapeyron equation is an approximate equation because the volume of the liquid has been neglected and ideal behaviour of the vapour is also taken into account. 

At higher pressures the iodide undergoes partial decomposition during the sublimation process. 

Evidently, the dissolved fullerene molecules in toluene reaching the plasma zone with glowing nickel particles, undergo the crystallization and sublimation processes. These molecules become both nuclei and building material for the formation of carbon nanotubes of small and large diameters (>50 nm) on the surface of metallic spheres 15-30 pm in diameter (Fig. 11), forming the metal-carbon composite. 

This sublimation process should be repeated once more to produce what is called double sublimed reagent grade iodine , which will have a purity of no less then 98% iodine. 

Figure 2.5. Schematic of three common methods used to grow single crystals. Shown is (a) diffusion where vapors from a volatile nonsolvent meet the crystallization solvent (b) interfacial where the nonsolvent is layered on top of the crystalhzation solvent (c) sublinration where the solid mixture is heated and the vapors form crystalhtes on the surface of a cold finger. For this latter technique, the crystallization flask may be opened to vacuum throughout the sublimation process (dynamic vacuum), or closed after maintaining initial vacuum to allow slower crystal growth (static vacuum).

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