Heterogeneous deposition

Homogeneous deposition of ultrafine metal particles on the surfaces of fine powder is not easy using PVD. A device for stirring the powder support in a vacuum chamber is needed to avoid heterogeneous deposition. Sputter deposition units equipped with stirring powder supports have already been adapted for the industrial production of Ti02 and carbon-supported gold catalysts by 3M [35]. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

The ionization of a molecule and the rupture of a chemical bond by ionizing radiation necessarily result in the pairwise formation of radical species. The pairwise correlation of radical species will be more or less retained in solid polymers where the radical migration is restricted. This heterogeneity of spatial distribution of radical species affects the radiation chemistry of polymers. Another source of spatial heterogeneity is the heterogeneous deposition of radiation energy [6, 7]. Low LET radiations such as y-rays produce an ensemble of isolated spurs. Each spur is composed of a few ion-pairs and/or radical... 

The final step in removal of any species from the atmosphere involves heterogeneous deposition to the Earth s surface. Removal processes include wet deposition via rain-out (following uptake into tropospheric clouds) and dry deposition to the Earth s surface, principally to the oceans. The rates of these processes are largely determined by the species chemistries in aqueous solution. Heterogeneous lifetimes of the parent HFCs, HCFCs and HFEs are of the order of hundreds of years because of their low aqueous solubility and reactivity. 

This patent describes a different application of the properties of supercritical fluids, i.e., using the supercritical fluid as a medium from which to grow crystals. Whereas the nucleation process discussed in Chapter 12 involved homogeneous nucleation during the pressure reduction, (Krukonis, 1984d) this invention involves heterogeneous deposition from an expanding gas solution. 

B]t) are conveniently situated in concentration windows between 10 " to 10 M (i.e. 10 to 10 " for K ). To obtain specifically the heterogeneous growth we have seen in the previous section that the supersaturation must remain sufficiently low. This allows to define deposition concentration windows from the solubility diagrams. As shown in figure 3b, in the case of CdS without added ligands, only one window appears in acidic situation. Heterogeneous deposition in acidic conditions has been indeed realized for CdS and PbS [40]. 

Very small nuclei that might form in the gas phase can deposit on the substrate and contribute to heterogeneous deposition and accelerate growth. This accounts for blanket deposition (irrespective of substrate) at high supersaturation and also explains the morphological features of high-temperature deposition discussed below. 

Second, when multiple metals are deposited simultaneously, as is the case in a real stripping voltammetric measurement, not only is their interaction with the diamond surface important, but equally critical is their interaction with each other. There is a possibility of intermetallic compounds or alloys forming, both of which will affect the oxidation or stripping potential for each. When these heterogeneous deposits form, the oxidation of a particular metal can occur from different sites on the diamond surface or from another metal surface. Oxidation from these multiple sites leads to peak broadening due to a spread in reaction kinetics. Ideally, for this application, highly dispersed metal deposits of low volume, without any intermetallic interactions, are desired. Even with these complexities, it is supposed that diamond will become a useful electrode for the determination of trace metal ions via anodic-stripping voltammetry. 

The polyion complex LB technique is the most precise fabrication method of layer-by-layer polymer assemblies because the lateral molecular ordering and layer structure (X-, Y-, and Z-type deposition and heterogeneous deposition) of the polymer films can be controlled. The molecular assembly in polyion complex monolayers can be switched by pH change [105] or photochemical reaction [106]. Film uniformity of the polyion complex monolayer was improved by in situ ther-... 

In addition to this gas phase reaction with subsequent homogeneous nucleation, heterogeneous deposition of monosilane at the surface of the Si nanoparticles as well as condensation of free monomer on the surface of existing particles was included to the population balance (PB) model (Peev et al, 1991) ... 

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