Deposition processes mixed control

In the RSDT process, the steps for infroducing cafalysf, ionomer, and carbon into the gas mix are decoupled and can be independently controlled in such a manner that the Pt/C and ionomer/C ratios can be continuously modified during fhe deposition process. Reactive spray deposition technology has the capacity and flexibility required to produce compositionally and... 

Wet removal processes are further controlled by precipitation types and rates. Dry deposition processes on surfaces are affected by atmospheric transport rates that mix fresh pollutant into the surface boundary layers and by the physical properties of particles. For the Eastern U.S., the approximate annual deposition rates of sulfate can be compared as follows (Table III), considering that deposition flux is the product of a concentration and a velocity of deposition (Vd) (20) ... 

Figure 1 Conceptual model for the origin of mixed detrital-biogenic facies relating the three major inputs to the processes that control them. The major inputs are shown in boxes with bold-type labels. ControlUng factors are shown in italics. Large and medium scale arrows represent fluxes of key components involved in sedimentation and the biogeochemical cycles of carbon, sulfur, and oxygen. Thin arrows illustrate relationships between major controlling factors and depositional processes and/or feedback. Dashed thin arrows apply to major nutrient fluxes only. Dotted thin arrows apply to major authigenic fluxes only. See text for further explanation.

Metal deposition on the silicon surface may follow an instantaneous or a progressive nucleation process followed by a diffusion-limited growth of the nuclei. The growth of nuclei can be either kinetically limited, diffusion limited, or under a mixed control. The current transients measured by Oskam el at various potentials of... 

This concept can be also applied for the case of the electrodeposition of copper. As mentioned earlier, the morphology of the copper deposit obtained at cathodic potential of -500 mV/SCE under the parallel field was of cauliflower-like structure (Fig. 12b), while the morphology of the copper deposit obtained without the applied magnetic field had very dendritic structure (Fig. 12a). It is known that dendritic structures are main characteristic of electrodeposition in conditions of full diffusion control, while cauliflower-like structures are a characteristic of a dominant diffusion in mixed control of electrodeposition process.13... 

On the other hand, due to the overlapping of the nucleation exclusion zones,7,35,36 deposition on the partially covered graphite electrode is an excellent illustration of the above discussion. Namely, the diffusion layer on the inert electrode partially covered with grains of active metal can be formed and diffusion control established in the same way as on an electrode of massive active metal if the deposition process is characterized by a large jo/jh-1 If dendrites are formed on the grains, their tips enter the bulk solution and overall control of the deposition process becomes activation or mixed controlled. 

This raises some important possibilities, which have not escaped the attention of the electroplating community. For example, while metal deposition is conducted in fairly concentrated solutions of the metal being plated, and at current densities well below the mass-transport limit, additives acting as inhibitors for metal deposition are often introduced at concentrations that are several orders of magnitude lower, to ensure that their supply to the surface will be mass-transport limited. In this way, the tendency for increased rate of metal deposition on certain features on the surface, such as protrusions, will be moderated by the faster diffusion of the inhibitor to the very same areas. Furthermore, if deposition occurs in the region of mixed control, which is usually the case, it must be remembered that the relevant roughness factor is quite different for the charge-transfer and the mass-transport processes, and this may well be a function of current density, since the Faradaic resistance is inherently potential dependent. 

The effect of rotation rate was studied in the range of 2,000 to 5,000 rpm, which represents a 90% (= 2.5" ) increase in the rate of mass transport to a RCE. The effect of rotation rate on the deposition process is shown in Fig. 10. As the concentration of WO is increased tenfold, from 0.04 to 0.40 M, the current density increases by a factor of only two. The limiting current density, calculated on the basis of the concentration of WO4 in solution, is much higher than the partial current densities for deposition of this metal, so one would not expect a 40% increase of the rate of deposition of W with the increase of the rate of mass transport, as foimd experimentally. The explanation of these unexpected observations lies in the formation of the mixed-metal complex, as shown in Eq. (33). The concentration of this complex is low, and its rate of formation is also expected to be low. From the dependence of the partial current density for W deposition shown in Fig. 10a, the activation-controlled and the mass transport-limited current densities can be estimated, using the Levich equation, as applied to RCE experiments, namely... 

For the deposition of mixed films or of complex alloys from two or more evaporation sources, it is necessary to control the evaporation rate of each material independently with sufficient accuracy. A single quadrupole mass spectrometer can measure the evaporation rates of the different materials in a time-multiplex process. 

VLS siiicon carbide whiskers with diameters ranging from <3 to 11 jm, and lengths ranging from 5 jm to <10 cm are readily obtained by metal catalyzed chemical vapor deposition. This process facilitates an exacting control over whisker shape and dimension in a batch or a continuous process. The diameter of the catalyst determines that of the whisker. Less well-defined whiskers have been obtained by vapor deposition, chemical mixing and carbothermal processes with diameters ranging from <3 pim to >30 nm. 

The current density on the tip of a protrusion, i p, is determined by kp, hence by the shape of the protrusion. If p—>0, i p i (see the Eq. (1.13)) and if p oo, itip io (fc -fa) Th electrochemical process on the tip of a sharp needle-like protrusion can be under pure activation control outside the diffusion layer of the macroelectrode. Inside it, the process on the tip of a protrusion is under mixed control, regardless it is tmder complete diffusion control on the flat part of the electrode for 0 (see section Model of the spherical diffusion around the tip of a surface protmsion-deposition to the point ). If A p= 1, hence for hemispherical protrusion, iup will be somewhat larger than i, but the kind of control will not be changed. It is important to note that the current density to the tip of hemispherical protrusion does not depend on the size of it if A p = l. This makes a substantial difference between spherical microelectrodes in bulk solution and microelectrodes inside diffusion layer of the macroelectrode [3, 9,10]. In the first case, the hmiting diffusion current density depends strongly on the radius of the microelectrode. 

FIGURE 5.8 Summary of the key kinetic concepts associated with CVD under the surface reaction, diffusion, and mixed-control regimes, (a) Schematic illustration and deposition rate equation for CVD under surface reaction control, (b) Schematic illustration and deposition rate equation for CVD under reactant diffusion control, (c) Schematic iUusIration and deposition rate equation for CVD under mixed control, (d) Illustration of the crossover from surface-reaction-controlled behavior to diffusion-controlled behavior with increasing temperature. The surface reaction rate constant (k ) is exponentially temperature activated, and hence the surface reaction rate tends to increase rapidly with temperature. On the other hand, the diffusion rate increases only weakly with temperature. For CVD processes where the reactions become less thermodynamically favorable with increasing temperature (common), the rate will eventually fall at higher temperatures as the CVD process becomes unfavorable thermodynamically. The slowest process determines the overall rate. 

To improve the structure-dynamics relationships of CLs, the effects of applicable solvents, particle sizes of primary carbon powders, wetting properties of carbon materials, and composition of the catalyst layer ink should be explored. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules and, therefore, control the catalyst layer formation process. Mixing the ionomer with dispersed Pt/C catalysts in the ink suspension prior to deposition will increase the interfacial area between ionomer and Pt/C nanoparticles. The choice of a dispersion medium determines whether ionomer is to be found in the solubilized, colloidal, or precipitated forms. 

The schematic indicating the potential region where OPCD occurs is shown in Fig. 2 using the example of CoFe alloy. The schematic considers electrodeposition process from the solution which is at standard conditions (P°, Cqq2+ = Cpg2+ = 1 mol). In order to obtain desired composition of CoFe (50 50) alloy, the concentrations of Co and Fe ions in the solution have to be appropriately adjusted together with the potential (overpotential) or current at which the alloy deposition occurs. Typical approach towards the solution and deposition potential (current) design involves experiments where the concentration of more noble metal, Co, is such that C(-q2+ < Cp 2+, so that Co deposition occurs under mixed control for a... 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

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