Atmosphere bulk composition

During vaporization of non-stoichiometric refractory carbides each element vaporizes at a different rate which is dependent on surface composition or relevant activities at the surface. When the initial bulk composition is near C/M = 1, the vaporization of C is much greater than that of M. As a result, the surface C content decreases and eventually approaches a constant value, which we will call the steady-state CVC (ssCVC). At the ssCVC, the vapor composition is nearly equal to the initial bulk composition. As C diffusion to the vaporizing surface reduces the C content of the bulk material, the surface composition asymptotically approaches the equilibrium CVC (eCVC). The rate at which eCVC is approached depends on the relative magnitudes of C vaporization and diffusion. When the eCVC has been reached, the surface and bulk C/M ratios are equal to the vapor composition. The intersection of the solid eCVC map with the solidus boundary of the monocarbide phase determines where melting occurs under equilibrium conditions for a particular atmosphere. 

Aerosol delivery of the precursors was chosen because it was expected that although the species were monomeric, their volatility was likely to be low based on the TGA results described earlier, and that the crown ether might dissociate on heating for extended periods. The compound Ca(SOCMe)2(15-crown-5) was dissolved in ethanol and delivered in a nitrogen gas stream to a cold-wall atmospheric pressure CVD reactor where the substrate temperature was varied between 300 and 400°C. Analysis of films deposited at 310°C revealed that crystalline CaS was deposited, approximately 100 nm thick, with a deposition rate of 10 nm/min. The X-ray powder pattern indicates that CaS was formed with preferential (200) orientation (see Fig. 63). The SEM data showed that the films comprised cubic crystallites with dimensions consistent with the linewidth of the peaks observed by X-ray diffraction. The bulk composition corresponded to CaS as determined by AES. 

Table 3.1 Bulk composition of unpolluted air. These are the components that provide the background medium in which atmospheric chemistry takes place. From Brimblecombe (1986).

Bulk Composition, Physical Structure, and Dynamics of the Atmosphere... 

SNC-based geochemical model of Mars bulk composition which calls for a -40% mass fraction of volatile-rich, oxidized Cl-like material in the planet. However subsequent expansion and recalculation of Xe data from the SNC meteorites now suggest that solar-wind Xe is a viable alternative to Cl-Xe as the principal atmospheric constituent on early Mars (see Primordial Xenon section below), raising the challenging question of how Mars could have acquired a large solar Xe component while Earth apparently did not. 

Thermogravimetry (or thermogravimetic analysis, TGA) is one of the oldest thermo-analytical procedures and has been used extensively in the study of polymeric systems. The technique involves monitoring the weight loss of the sample in a chosen atmosphere (usually nitrogen or air) as a function of temperature. It is a popular technique for the evaluation of the thermal decomposition kinetics of polymeric materials and hence provides information on thermal stability and shelf life. However, it is well known for its ability to provide information on the bulk composition of polymer compounds. 

The surface of a metallic object can be easily reacted in normal atmospheres making it behave quite differently from a piece of unreacted metal, often resulting in a situation resembling dissimilar metal corrosion. A film may be formed which is invisible, actually only a few molecules in thickness, but which may have a potential as much as 0.3 V different from the unfilmed metal. Naturally, such a potential difference is enough to create an active corrosion cell. Steels in soil have a tendency to film with time. "Old" steel, that is, steel which has been in the ground for several years may then become cathodic with respect to "new" steel, even when the two are identical in bulk composition (Fig. 7.37). It is thus strictly a surface phenomenon [21]. 

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