Surface migration, high-temperature depositions

Experiments on vapor-deposited iron films also point to an extent of adsorption of carbon monoxide of about 1.0 x 10 molecules per cm. For example, the effect of carbon monoxide on the electrical conductance of an iron film is represented in Fig. 5.6. When the level of adsorption reaches 0.7 to 1.0 x 10 molecules per cm, the conductance is unaffected by further addition of carbon monoxide. It appears that the smaller ratios measured on iron catalysts after more severe reduction treatments arise from segregation of oxides to the iron surface. The oxides are present initially within the iron and migrate to the iron surface at high temperatures. 

The deposition of particles on macroscopic surface is the primary goal in CVD processes, bnt rednces the efficiency of vapor phase particle synthesis. Particles can deposit by Brownian motion, bnt in high-temperature reactors, thermophoretic deposition often dominates. Thermophoresis is the migration of small aerosol particles as a resnlt of a temperatnre gradient. It causes particles carried in a hot gas to deposit on a cool surface. Eor small particles, Kn 1, a dimensionless group can be created to describe thermophoresis, Th ... 

Immobile is a solid or liquid organic matter with kinematic viscosity greater than 1,000 cm c E This is first of all dispersed organic matter of rocks, humus of soil, peat, etc., or very viscous or solid bitumens and petroleum products. Bitumens in conditions of high temperature are capable of slowly migrating. In particular, bitumens emerge on the surface in the Dead Sea, for which it was called Asphalt Sea. Nevertheless, these organic solvents is convenient to consider a component of rocks or deposits, and their absorption capability to consider as property of rock as a whole. 

Frank (28) measured the changes in photoelectric emissivity of tungstic oxide (p. 358), as caesium in measured quantity was deposited, and then allowed to diffuse away. He found that at high temperatures the deposit decayed more slowly, but this result was due in some way to the caesium which liad collected below the surface by migration, during the deposition, or in earlier experiments. 

There are two procedures used to prepare dealuminated zeolites. One procedure involves a rapid hydrothermal dealumination at a relatively high temperature of 500°C (10,11). The resulting material contains a network of larger zeolitic mesopores within the crystal and connecting directly to the zeolitic pore system. Another procedure uses silicon hexafluoride at a relatively low temperature, <100°C (12). The result is a slow dealumination with the formation of much less mesopore structure. The two procedures result in zeolites that differ in the distribution of active framework aluminum sites as well as in the presence or absence of mesoporosity. The liquid phase dealumination results in a zeolite with the active aluminum sites preferentially removed from the outside of the zeolite particle or crystal. Further, during the dealumination process, additional silicon is deposited on the zeolite surface (13). The exact opposite happens during hydrothermal dealumination. While the framework is more or less uniformly dealuminated, the aluminum atoms removed from the framework do not remain within the pore system, but are observed to migrate to the outside of the zeolite particle (14). Consequently, unless the alumina is removed, the outside of the zeolite particle is alumina rich. 

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