Mechanisms of deposition

Thin Film Coatings for Biomaterials and Biomedical Applications 

There are then three general growth modes to describe how thin films form based on the relative interaction strengths between the adparticles and the substrate, as shown in Fig. 2.4 (Ohring, 1992). 

Frank—van der Merwe When the interaction strength between the adparticles and the substrate is strongest, adparticles will tend to deposit in layers rather than islands. This results in smoother layers which are generally strongly adhered to the substrate. This is the case for ALD described previously, in which the interaction between the adparticles is essentially zero, but the substrate—adparticle interactions are strong. 

Stranski—Krastanov Often, growth of thin films occurs in the regime between the Volmer—Weber and Frank—van der Merwe mechanisms, in which an initial layer is formed on the substrate, but then 3D island structures form. 

---

Low heat-transfer rates at boiler surfaces generate only low steam outputs. Under these conditions, the mechanism of deposition is different from that of highly rated boilers. 

The mechanics of deposition in a boiler are often a cycle of cause and effect, wherein an initial low level of scale deposited on boiler surfaces causes a rapid localized rise in wall temperature. The temperature increase leads to localized steam blanketing, which in turn prevents the deposit from resolubilizing. Consequentially, conditions then exist for the further buildup of deposit on the heat transfer surface. 

Chemical treatment programs are often individually designed for particular boiler plant systems but usually contain oxygen scavengers, pH boosters, and corrosion inhibitors. In addition, the formulations employ materials specifically designed to limit the degree of deposition and control the mechanisms of deposition. 

A method to circumvent the problem of chalcogen excess in the solid is to employ low oxidation state precursors in solution, so that the above collateral reactions will not be in favor thermodynamically. Complexation strategies have been used for this purpose [1, 2]. The most established procedure utilizes thiosulfate or selenosulfate ions in aqueous alkaline solutions, as sulfur and selenium precursors, respectively (there is no analogue telluro-complex). The mechanism of deposition in such solutions has been demonstrated primarily from the viewpoint of chemical rather than electrochemical processes (see Sect. 3.3.1). Facts about the (electro)chemistry of thiosulfate will be addressed in following sections for sulfide compounds (mainly CdS). Well documented is the specific redox and solution chemistry involved in the formulation of selenosulfate plating baths and related deposition results [11, 12]. It is convenient to consider some elements of this chemistry in the present section. 

Munthe J, Hultberg H, Iverfeldt A. 1995b. Mechanisms of deposition of mercury and methyl-mercury to coniferous forests. Water Air Soil Pollut 80 363-371. 

The most significant information given in Table I is that the radon daughter ions do contribute to the total internal dose if only at about 3% in this study. The effect of free ions in contrast with daughters attached to aerosols is substantial when the mechanics of deposition in the bronchial tree and lungs are taken into account. The dose from "unattached" Po-218, which includes the Po-218 positive ions as a major component, can be from 3 to 40 times that of the attached Po-218 (NCRP, 1984). Current dosimetry models allow for the important role played by Po-218 in small-ion form. Hence, their effect is significantly greater than the 3% contribution to the PAEC (WL) shown in Table I. 

While the development of codeposition theories was essentially dormant, the understanding of the kinetics of particle deposition from suspensions was rapidly evolving. The omnipresence of the interaction of particles with surfaces [70] and the importance to deep-bed granular filtration, deposition of paints, fouling of coolant circuits, chemical reactors and membranes, led to careful theoretical and experimental investigations of the mechanism of deposition. The theory is most advanced in the area of filtration and a number of comprehensive reviews exist [71-73]. It is striking that of... 

The coevolution of H2 gas in electroless deposition processes is a phenomenon that needs to be understood not only to elucidate the mechanism of deposition, but also since it impacts the properties of deposits by H inclusion. Van den Meerakker [51] first proposed a correlation between simultaneous hydrogen evolution in electroless deposition and the heat of adsorption of hydrogen. In this useful endeavor, however, he has been criticized for erroneously calculating the heats of adsorption of H at Cu by Gottesfeld et al. [52], and Group I (or SP type) metals in general by Bindra and Tweedie [53]. 

If the dominant mechanism of deposition involves the formation of adatoms followed by surface diffusion to steps, the relation between current and electrode potential becomes complicated. The essential... 

Agnew, J.E. (1984). Physical properties and mechanisms of deposition of aerosols. In Aerosols and the Lung, Clinical Aspects (Clarke, S.W. and Pavia, D., Eds.). Butterworth, London, pp. 49-68. 

The initial theoretical treatment of these mechanisms of deposition was given by Lorenz (31-34). The initial experimental studies on surface diffusion were published by Mehl and Bockris (35, 38). Conway and Bockris (36, 40) calculated activation energies for the ion-transfer process at various surface sites. The simulation of crystal growth with surface diffusion was discussed by Gilmer and Bennema (43). 

Up to this point we have considered the mechanism of deposition of a single monoatomic layer (Section 7.6) and multilayers composed of a few monoatomic layers (Section 7.7). In Sections 7.8 to 7.12 we discuss how coherent electrodeposits develop. 

It will be obvious that the cluster mechanism of deposition is unlikely to lead to an oriented film, since the clusters would have to align themselves with the substrate lattice, either on adsorption or subsequently. Therefore an epitaxial film is highly suggestive of an ion-by-ion growth, which is more likely to be directed by the substrate. 

Equation (9) generalizes earlier porosity-Peclet number power-law correlations (Konstandopoulos et al., 2002) obtained at Pe > 0.3 down to the diffusion limited deposition limit. PeQ is a characteristic cross-over Peclet number defining the scale beyond which the convective mechanism will take over the diffusive mechanism of deposition and eK the large Peclet number asymptote of the porosity. K has a dependence on the aggregate size and it is described in a forthcoming publication (Konstandopoulos, 2007). Using Eq. (9) the experimental data of Fig. 9 can be collapsed on a single curve as shown in Fig. 10. 

A large number of commercially important plating processes occur from complex ion baths in which the metal is a constituent of an anionic complex, e.g. copper, zinc, cadmium, silver and gold are all commonly plated from cyanide baths, and tin plates from a stannate bath in which [SnIV(OH)6]2 is present. Chromium is commonly plated from a chromate bath although in this case the background medium is acid rather than alkaline. Thus the mechanism of deposition of metals from anionic complexes is of particular interest. It will be instructive to comment on two situations, one occurring in alkaline baths, the other in acidic baths. 

The mechanism of deposition is not well understood. The ion [Cu(CN)2] has been implicated but this may be subject to the same objections discussed in Section 57.2.3 when the deposition of zinc was considered. More free cyanide will polarize the cathode with the result that hydrogen... 

Zn(CN)4p-. It is believed that the mechanism of deposition involves the hydroxo complex,33 in which case Section 57.2.3 should be consulted. 

The mechanism of deposition and mobilization of iron in ferritin has been much discussed.1106,1112,1113 Ions and neutral molecules can all pass through the protein shell, due probably to the presence of entry channels of different character. The Cd2+ ions found in the hydrophilic channel in the crystal structure could be metal ions entering the shell. 

More interesting from a fundamental point of view is the observation that the efficiency of poisoning depends also on the physical structure of the deposited impurity this is in turn governed by the nature of chemical interaction between the impurity and the electrode surface. In other words, a different mechanism of deposition may result in a structure of the deposit which does not inhibit the active surface substantially. Thus, in the case of Pt electrodes, it has been observed that if the impurity content is around 14 ppm Fe, the cathode may even be activated since small crystallites of iron are formed [167]. 

Experiments on transfer of submicrometre radioactive particles to smooth surfaces (Wells Chamberlain, 1967 Chamberlain et al., 1984) have shown that the dependency of vg on D213 holds over many orders of magnitude of D. This means that the transport by Brownian diffusion becomes progressively less effective as the particle size increases. For example a particle of 0.1 pm diameter has a diffusivity of 6.8 x 10 10 m2 s 1, a factor 1.2 x 104 smaller than that of I2 vapour. Since D does not depend on the particle density, it is appropriate to discuss transport by Brownian motion in terms of the particle diameter. The aerodynamic diameter, dA, is equal to dppp2 where pp is the particle density in c.g.s. units (g cm-3) not SI units (kg m-3), and is the appropriate parameter for particles with dp> 1 pm, for which impaction and sedimentation are the mechanisms of deposition. 

The different mechanisms of deposition determine the shape of the curves in Fig. 6.9. For particles of diameter less than 0.2 /tm, Brownian diffusion is the dominant mechanism and vg varies according to Dm, as expected from equation (6.4). The minimum is at dp between 0.1 and 1 /un, where particles are too large to have appreciable Brownian motion but too small to impact. 

---