Reaction rate film growth

Thermodynamics indicates whether a particular chemical reaction is feasible. However, it will not provide any information about the speed of the reaction and film growth rates. Even if a reaction is thermodynamically possible, if... 

In interfacial polycondensation, the hydrated diamine diffuses through the continuously thickening film and reacts with the diacyl chloride on the organic solvent side. The reaction is obviously diffusion controlled since the diffusion rate is an order of magnitude smaller than the reaction rate. The growth rate of the film dLjdt decreases as the thickness of the film increases, i.e., dLjdt = k cJL). The proportionality constant k includes the diffusion coefficient of the diamine in the film. 

Because the film growth rate depends so strongly on the electric field across it (equation 1.115), separation of the anodic and cathodic sites for metals in open circuit is of little consequence, provided film growth is the exclusive reaction. Thus if one site is anodic, and an adjacent site cathodic, film thickening on the anodic site itself causes the two sites to swap roles so that the film on the former cathodic site also thickens correspondingly. Thus the anodic and cathodic sites of the stably passive metal dance over the surface. If however, permanent separation of sites can occur, as for example, where the anodic site has restricted access to the cathodic component in the electrolyte (as in crevice), then breakdown of passivity and associated corrosion can follow. 

The terms hot corrosion or dry corrosion are normally taken to apply to the reactions taking place between metals and gases at temperatures above 100 C i.e. temperatures at which the presence of liquid water is unusual. The obvious cases of wet corrosion at temperatures above 100 C, i.e. in pressurised boilers or autoclaves, are not considered here. In practice, of course, common metals and alloys used at temperatures above normal do not suffer appreciable attack in the atmosphere until the temperature is considerably above 100 C. Thus iron and low-alloy steels form only the thinnest of interference oxide films at about 200 C, copper shows the first evidence of tarnishing at about 180 C, and while aluminium forms a thin oxide film at room temperature, the rate of growth is extremely slow even near the melting point. 

Logani and Smeltzer " have observed that, for Fe-1.5%Si at 1 000°C in CO/CO2, the initial slow reaction rate was followed by regions of linear behaviour due to the amorphous Si02 film being consumed by the growth of wustite-fayelite nodules during the early stages. These wustite-fayelite... 

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

All reactions involved in polymer chain growth are equilibrium reactions and consequently, their reverse reactions lead to chain degradation. The equilibrium constants are rather small and thus, the low-molecular-weight by-products have to be removed efficiently to shift the reaction to the product side. In industrial reactors, the overall esterification, as well as the polycondensation rate, is controlled by mass transport. Limitations of the latter arise mainly from the low solubility of TPA in EG, the diffusion of EG and water in the molten polymer and the mass transfer at the phase boundary between molten polymer and the gas phase. The importance of diffusion for the overall reaction rate has been demonstrated in experiments with thin polymer films [10]. 

UV/visible absorbance measurements of H2S-exposed M-FA films (M = Cd or Hg) have provided information about reaction rates. Delineation of the two processes depicted in Eq. (7) and (8) is difficult, however, as they can occur simultaneously. The reactions of LB films (CdAr) and cast self-assembled films of the cadmium salt of ditetradecyl-iY-[4-([6-/V, Ar, Ar -trimelhylethylenediamino)-hexyI oxy) benzoyl]-L-glulamate (Cd(DTG)2) with H2S have been followed by time-resolved UV/visible absorbance (85). The most profound observation from this investigation is with respect to the effect that water has on the rate of growth of the CdS particles... 

The deposition rate, or film growth rate, is usually expressed in mn/s, as is the reaction rate constant, ks. Deposition rates can be expressed in mol/m s by multiplying by the molar density of solid germanium. 

As pointed out earlier, CVD is a steady-state, but rarely equilibrium, process. It can thus be rate-limited by either mass transport (steps 2, 4, and 7) or chemical kinetics (steps 1 and 5 also steps 3 and 6, which can be described with kinetic-like expressions). What we seek from this model is an expression for the deposition rate, or growth rate of the thin film, on the substrate. The ideal deposition expression would be derived via analysis of all possible sequential and competing reactions in the reaction mechanism. This is typically not possible, however, due to the lack of activation or adsorption energies and preexponential factors. The most practical approach is to obtain deposition rate data as a function of deposition conditions such as temperature, concentration, and flow rate and fit these to suspected rate-limiting reactions. 

With this electric potential Poisson equation (A

el = net charge density) to eventually obtain the concentration of electrons at the film surface (A ). It further follows that Ne(A ) varies with the film layer thickness as A -2. If we now assume that the (catalyzed) rate of dissociation of the adsorbed X2 molecules is proportional to the surface concentration of electrons, and that this dissociation process is rate determining, a cubic rate law for the film growth can be expected (A — At 2 At - t in). In fact, during the oxidation of Ni at temperatures between 250 and 400 °C, an approximately cubic rate law has been experimentally observed. We emphasize, however, that the observed cubic oxidation rate does not prove the validity of the proposed reaction mechanism. Different models and assumptions concerning the atomic reaction mechanism may lead to the same or similar dependences of the growth rate on thickness. 

For ideal solutions, the partial pressure of a component is directly proportional to the mole fraction of that component in solution and depends on the temperature and the vapor pressure of the pure component. The situation with group III-V systems is somewhat more complicated because of polymerization reactions in the gas phase (e.g., the formation of P2 or P4). Maximum evaporation rates can become comparable with deposition rates (0.01-0.1 xm/min) when the partial pressure is in the order of 0.01-1.0 Pa, a situation sometimes encountered in LPE. This problem is analogous to the problem of solute loss during bakeout, and the concentration variation in the melt is given by equation 1, with l replaced by the distance below the gas-liquid interface and z taken from equation 19. The concentration variation will penetrate the liquid solution from the top surface to a depth that is nearly independent of zlDx and comparable with the penetration depth produced by film growth. As result of solute loss at each boundary, the variation in solute concentration will show a maximum located in the melt. The density will show an extremum, and the system could be unstable with respect to natural convection. 

The adsorbed species, which are considered to be adatoms, can diffuse to favorable low-energy sites and react, or they can be emitted into the gas phase. At sufficiently low temperatures, adatoms may have insufficient energy to diffuse and react or to be emitted into the gas phase. These adatoms will be codeposited with the compound film as crystal defects or as a second solid phase. As a result of these competing processes in the surface reaction zone, the growth rate and film composition depend on the flux and energy of the incident species and on the substrate temperature. 

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