Deposition processes diffusion control

Beyond the double layer, there is a depleted region named the diffusion layer with a thickness of microns, much wider than the double layer, formed during deposition by the consumption of a particular species. Fig. 7A is a plot of the concentration of an ionic species as a function of the distance from the surface of the electrode, showing the diffusion layer. The consumption of ions because of metal deposition generates a concentration gradient that, in steady-state conditions, is essentially determined by the redox reaction rate. If the consumption of ions arriving at the surface by diffusion is very high, the concentration of ions at the surface Cs is effectively zero, and the deposition process is controlled by diffusion. If the consumption is low, then the ion concentration at the surface is different from zero and the deposition is controlled by kinetics, i.e., by the velocity of the reaction. 

If the gas has the correct composition, the carbon content at the surface increases to the saturation value, ie, the solubiUty limit of carbon in austenite (Fig. 2), which is a function of temperature. Continued addition of carbon to the surface increases the carbon content curve. The surface content is maintained at this saturation value (9) (Fig. 5). The gas carburizing process is controlled by three factors (/) the thermodynamics of the gas reactions which determine the equiUbrium carbon content at the surface (2) the kinetics of the chemical reactions which deposit the carbon and (J) the diffusion of carbon into the austenite. 

Figure 19.2 shows, at a microscopic level, what is going on. Atoms diffuse from the grain boundary which must form at each neck (since the particles which meet there have different orientations), and deposit in the pore, tending to fill it up. The atoms move by grain boundary diffusion (helped a little by lattice diffusion, which tends to be slower). The reduction in surface area drives the process, and the rate of diffusion controls its rate. This immediately tells us the two most important things we need to know about solid state sintering ... 

The kinetics and mechanisms of the displacement deposition of Cu on a Zn substrate in alkaline media was studied by Massee and Piron (5). They determined that at the beginning of the deposition process, the rate is controlled by activation. The activation control mechanism changes to diffusion control when the copper covers enough of the Zn surface to facilitate further deposition of copper. This double mechanism can explain the kinetic behavior of the deposition process. 

Properties of thin layers of lead electrodeposited on vitreous carbon have been found identical with that of metallic lead [304]. Therefore Pb and Pb02 coated reticulated vitreous carbon (RVC) electrodes [185] can be applied as electrodes in lead-acid batteries, as reviewed in [305]. The deposition of lead on carbon is through the diffusion-controlled process with instantaneous or progressive nucleation, for high and low Pb + concentration, respectively, and three-dimensional growth mechanism. The number of nucleation sites increases with deposition overpotential, as shown for vitreous [306] and glassy carbon [307] electrodes. The concentration dependence of the nucleation... 

Electrodeposition on transparent material such as indium tin oxide (ITO) can be used for electrochromic applications [328]. Pb deposition on indium-tin oxide electrode occurs by three-dimensional nucle-ation with a diffusion-controlled growth step for instantaneous nucleation [329], and the electrode process has also been studied using electrochemical impedance spectroscopy [328]. 

Heat transfer is an extremely important factor in CVD reactor operation, particularly for LPCVD reactors. These reactors are operated in a regime in which the deposition is primarily controlled by surface reaction processes. Because of the exponential dependence of reaction rates on temperature, even a few degrees of variation in surface temperature can produce unacceptable variations in deposition rates. On the other hand, with atmospheric CVD processes, which are often limited by mass transfer, small susceptor temperature variations have little effect on the growth rate because of the slow variation of the diffusion with temperature. Heat transfer is also a factor in controlling the gas-phase temperature to avoid homogeneous nucleation through premature reactions. At the high temperatures (700-1400 K) of most... 

In the final scenario, the species in solution is adsorbed in the forward scan and desorbs electrochemically in the reverse scan. This case is frequently observed in the deposition of metals. In the forward scan, a common diffusion-controlled voltammetric wave is observed, but in the reverse scan a sharp return peak is obtained because of the fixed amount of metal deposited in the forward scan (Fig. 2.4b). The metal is already present at the electrode when the reverse scan is started therefore no diffusion process is observed, and the current drops to zero after the peak because of the fixed and limited amount of deposited metal present at the electrode surface. 

In the mid-1970s, it was realized that low-pressure CVD processing could have significant advantages over atmospheric pressure systems. By reducing the pressure, it was found that the diffusion coefficient was sufficiently enhanced that deposition became surface controlled (see Chapter 1). In this case, wafers could be stacked closely and placed in a diffusion furnace to be processed... 

In Chapter 2, we reviewed the concept of carrying out CVD processes at low pressure so that deposition becomes surface controlled. When the only thing controlling the uniformity of deposition is the temperature of the wafer surface, all we have to do is ensure that the wafer is in a uniform temperature furnace. Again, at low pressures, the diffusion coefficient is so large that we can stack wafers up next to each other so 50 to 100 can be placed in a long tubular furnace. 

Lay and Skyllas-Kazacos were the first to describe a deposition from imidazolium-based tetrachloroaluminate ionic liquid [8], On glassy carbon, aluminum was deposited at —0.2 V (instead of—0.43 V for the pyridinium-based system of Osteryoung and Welch). Furthermore, they were able to show that the deposition process has complicated kinetics and is not simply controlled by diffusion. Using a tungsten electrode they were able to demonstrate in chronopotentiometric measurements that initially a potential of—0.65 V is necessary due to the nucleation process, but after reaching the barrier the potential drops below —0.2 V. 

In our hands, EQCM studies of this system have confirmed previous reports of y-FeOOH deposition kinetics and the chemical reaction of ferrous ions with this film after a current interruption step. Figure 12.1 depicts the simultaneous transients of anodic current (Fig. 12.1(b)) and frequency shift (A/) when a potential step (Fig. 12.1(c)) is applied from a potential where there is no reaction on gold to a potential where diffusion controlled oxidation of ferrous ions takes place. The current transient shown in Fig. 12.1(b) can be described by a diffusion process since a linear dependence of the anodic current density with t 1/2 was found as predicted by the Cottrell equation ... 

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