Evaporation, bulk element

Creation of Metal Vapors by Bulk Element Evaporation. Another chemical method for the generation of free atoms, the creation of atomic vapors by heating of the bulk element, has been used to generate Group IV atoms as well as a wide variety of metal atoms. The work on carbon using a carbon arc has recently been reviewed by Shevlin 111) together with his strictly chemical method for the generation of carbon atoms 112). Skell has also reviewed his work with the carbon arc (IIS). 

The need to prepare very thin sample sources for counting necessitates a lengthy radiochemical separation procedure to remove bulk elements efficiently, as well as alpha emitters that will interfere with the spectra (Table 1). Radiochemical procedures and sample mounting onto a planchet are variable in efficiency and can be affected by sample-related factors. It is therefore imperative to use a yield monitor for the procedure if quantitation is required. After radiochemical separation the solution is prepared for alpha counting using electrodeposition, co-precipitation and filtration as a thin source, direct evaporation, electrospraying, or vacuum sublimation. If the planchet is contaminated with polonium, the planchet is heated to remove the volatile polonium. 

The smallest metal particle is a single atom. In recent years atoms of metallic elements have attracted a great deal of interest. First, their formation by evaporation of the bulk element has been studied extensively. It is possible to evaporate metals that vaporize under vacuum at temperatures below 2000°C by using electrical resistive heating of ceramic crucibles (such as aluminium oxide coating on a tungsten wire basket). The metals listed below all evaporate nicely using this method. All but Sn, As,... 

As mentioned, samples are usually produced by evaporation of fiL volumes of the sample solution on the surface of a TXRF sample carrier resulting in a residue of small sample amounts. This is illustrated in Fig. 12-25. Application of this procedure directly to seawater would lead to the formation of a large amount of salt crystals on the surface of the sample carrier during the evaporation process. This would give rise to a strong increase in scattered radiation from the sample matrix. Moreover, the presence of the bulk elements Na, K, Mg... 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

Finally, it is to be expected that the evaporation coefficient of a very stable compound, such as alumina, which has a large heat of sublimation resulting from the decomposition into the elements, will be low. Since the heat of evaporation must be drawn from the surface, in die case of a substance widr a low thermal conductivity such as an oxide, the resultant cooling of the surface may lead to a temperature gradient in and immediately below the surface. This will lower die evaporation rate compared to that which is calculated from the apparent, bulk, temperature of the evaporating sample as observed by optical pyromeuy, and thus lead to an apparently low free surface vaporization coefficient. This is probably die case in the evaporation of alumina in a vacuum. 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

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