Reaction interdiffusion

An inversion of these arguments indicates that release agents should exhibit several of the following features (/) act as a barrier to mechanical interlocking (2) prevent interdiffusion (J) exhibit poor adsorption and lack of reaction with at least one material at the interface (4) have low surface tension, resulting in poor wettabihty, ie, negative spreading coefficient, of the release substrate by the adhesive (5) low thermodynamic work of adhesion ... 

A limitation of CVI is the necessity of interdiffusion of reactants and reaction products through relatively long, narrow, and sometimes tortuous channels. To avoid rapid deposition and choking of the entrance end of the channels, conditions are chosen to ensure deposition in the kinetically limited regime. This is a slow process which may take as much as several weeks before densification is achieved. In fact, full densification is almost impossible to obtain due to the formation of closed porosity. [ " 1... 

As the density of devices placed on the silicon wafer increases, the problems of autodoping and interdiffusion become more acute and the high temperature limitation of the above reactions has prompted much experimental effort to develop epitaxial deposition at lower temperature. This has been accomplished in the following experimental developments ... 

Traditional solid-state synthesis involves the direct reaction of stoichiometric quantities of pure elements and precursors in the solid state, at relatively high temperatures (ca. 1,000 °C). Briefly, reactants are measured out in a specific ratio, ground together, pressed into a pellet, and heated in order to facilitate interdiffusion and compound formation. The products are often in powdery and multiphase form, and prolonged annealing is necessary in order to manufacture larger crystals and pure end-products. In this manner, thermodynamically stable products under the reaction conditions are obtained, while rational design of desired products is limited, as little, if any, control is possible over the formation of metastable intermediates. ... 

Lichtner, P. C., E. H. Oelkers and H. C. Helgeson, 1986, Interdiffusion with multiple precipitation/dissolution reactions transient model and the steady-state limit. Geochimica et Cosmochimica Acta 50, 1951-1966. 

Unlike premixed flames, which have a very narrow reaction zone, diffusion flames have a wider region over which the composition changes and chemical reactions can take place. Obviously, these changes are principally due to some interdiffusion of reactants and products. Hottel and Hawthorne [5] were the first to make detailed measurements of species distributions in a concentric laminar H2-air diffusion flame. Fig. 6.5 shows the type of results they obtained for a radial distribution at a height corresponding to a cross-section of the overventilated flame depicted in Fig. 6.2. Smyth et al. [2] made very detailed and accurate measurements of temperature and species variation across a Wolfhard-Parker burner in which methane was the fuel. Their results are shown in Figs. 6.6 and 6.7. 

Fig. 2.1. Interface bonds formed (a) by molecular entanglement (b) by electrostatic attraction (c) by interdiffusion of elements (d) by chemical reaction between groups A on one surface and groups B on the other surface (e) by chemical reaction following forming of a new compound(s), particularly in MMCs (0 by mechanical interlocking. After Hull (1981) and Naslain (1993).

When large spherical AP particles dg = 3 mm) are added, large flamelets are formed in the dark zone.Pl Close inspection of the AP particles at the burning surface reveals that a transparent bluish flame of low luminosity is formed above each AP particle. These are ammonia/perchloric acid flames, the products of which are oxidizer-rich, as are also observed for AP composite propellants at low pressures, as shown in Fig. 7.5. The bluish flame is generated a short distance from the AP particle and has a temperature of up to 1300 K. Surrounding the bluish flame, a yellowish luminous flame stream is formed. This yellowish flame is produced by in-terdiffusion of the gaseous decomposition products of the AP and the double-base matrix. Since the decomposition gas of the base matrix is fuel-rich and the temperature in the dark zone is about 1500 K, the interdiffusion of the products of the AP and the matrix shifts the relative amounts towards the stoichiometric ratio, resulting in increased reaction rate and flame temperature. The flame structure of an AP-CMDB propellant is illustrated in Fig. 8.1. 

Potassium salts are known to act as suppressants of spontaneous igmtion of hydrocarbon flames arising from interdiffusion with ambient air. It has been reported that potassium salts act to retard the chemical reaction in the flames of nitropolymer propellants. Two types of potassium salts used as plume suppressants are potassium mtrate (KNO3) and potassium sulfate (K2SO4). The concentration of the salts is varied to determine their region of effectiveness as plume suppressants. 

To describe the process of the formation of such structures, it is necessary to write down the equations for a component that may be composed of several species and consider reactions among the species (Fisher and Lasaga, 1981). For example, for the case of diffusion of silver ions into a gel containing chromate ions, there are two species of Ag one is Ag , which diffuses by interdiffusion with Na, and the other is Ag2Cr04 precipitation. The diffusivity of precipitated Ag2Cr04 is negligible. Therefore,... 

Ganguly J. andTazzoli V. (1994) Fe +-Mg interdiffusion in orthopyroxene retrieval from data on intracrystalline exchange reaction. Am. Mineral. 79, 930-937. 

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