High-temperature gases structural changes

Figure 5.11 Variation in the catalytic activity of an Mg(0001) surface when exposed to a propene-rich propene- oxygen mixture at room temperature. The surface chemistry is followed by XPS (a), the gas phase by mass spectrometry (b) and surface structural changes by STM (c, d). Initially the surface is catalytically active producing a mixture of C4 and C6 products, but as the surface concentrations of carbonate and carbonaceous CxHy species increase, the activity decreases. STM images indicate that activity is high during the nucleation of the surface phase when oxygen transients dominate. (Reproduced from Ref. 39).

Such changes in the defect population can be critical in device manufacture and operation. For example, a thin him of an oxide such as SiO laid down in a vacuum may have a large population of anion vacancy point defects present. Similarly, a him deposited by sputtering in an inert atmosphere may incorporate both vacancies and inert gas interstitial atoms into the structure. When these hlms are subsequently exposed to different conditions, for example, moist air at high temperatures, changes in the point defect population will result in dimensional changes that can cause the him to buckle or tear. 

No epitaxy could be obtained by reaction of the metal films with reactive gas for short reactions times. This is understandable as the carburization and nitridation reactions progress from the surface of the metal films to the substrate and occur with a change in crystal structure of the film (for instance bcc to hex). So even if the starting metal film is epitaxial, the final carbide or nitride compound could be polycrystalline. For high temperatures and for long time treatments (>15 h), however, perfect epitaxial Y Mo2N films could be obtained on MgO (100).17 In this last case, the crystalline state of the precursor metal film had no effect on the final parallel orientation of the nitride. 

It is apparent that introducing the inert gas does induce a local structural change in the water. At the same time lines are still observed in these solutions, characteristic of pure water. Recall again our claim that the temperatures of the kinks are largely unaffected by the presence of solute even in a moderately high concentration (of both electrolytes and nonelectrolytes.)... 

Figure 2.11 Very high pressure (0.3-2.1 GPa) structural changes of gas hydrates at room temperature. Numerical values (adjacent to square boxes) indicate transition pressures. Hexagonal (sH ) and tetragonal (sT ) hydrate phases are distinct from sH and sT hydrate structures found at normal pressures. (Modified and redrawn from Hirai, H., Tanaka, H., Kawamura, K., Yamamoto, Y., Yagi, T., J. Phys. Chem. Solids, 65, 1555 (2004). With permission from Elsevier.)...

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