Mixtures diffusion process

Carburization by Thermal Diffusion. Carburization of chemically processed metal or metal-compound powders is carried out through sohd-state, thermal diffusion processes, either in protective gas or vacuum. Carbide soHd solutions are prepared by the same methods. Most carbides are made by these processes, using loose or compacted mixtures of carbon and metal or metal-oxide powders. HaUdes of Group 5 (VB) metals recovered from ores by chlorination are similarly carburized. 

The diffusion process in natural and polychloroprene rubber adhesives can be explained by Campion s approach [1] which considers the concept of molecular free volume. This free volume is mainly affected by the solvent mixture of the adhesive (which will determine the degree of uncoiling of rubber chains) and by the ingredients in the formulation (mainly the amount and type of tackifier). 

The reduction diffusion process has also been used for the production of powders of the magnetic neodymium-iron-boron alloy (Nd15Fe77B8). The reaction involves use of a powder mix of neodymium oxide, iron, ferroboron and calcium. The reaction is conducted by heating the powder charge mixture at 1200 °C for 4 h under vacuum. Neodymium-iron-boron alloys are much more prone to oxidation than samarium-cobalt alloys and a proprietary leaching procedure is used for the separation of the alloy and calcium oxide. 

Various diffusion coefficients have appeared in the polymer literature. The diffusion coefficient D that appears in Eq. (3) is termed the mutual diffusion coefficient in the mixture. By its very nature, it is a measure of the ability of the system to dissipate a concentration gradient rather than a measure of the intrinsic mobility of the diffusing molecules. In fact, it has been demonstrated that there is a bulk flow of the more slowly diffusing component during the diffusion process [4], The mutual diffusion coefficient thus includes the effect of this bulk flow. An intrinsic diffusion coefficient, Df, also has been defined in terms of the rate of transport across a section where no bulk flow occurs. It can be shown that these quantities are related to the mutual diffusion coefficient by... 

To consider the control volume form of the conservation of mass for a species in a reacting mixture volume, we apply Equation (2.14) for the system and make the conversion from Equation (3.12). Here we select/ = pt, the species density. In applying Equation (3.13), v must be the velocity of the species. However, in a mixture, species can move by the process of diffusion even though the bulk of the mixture might be at rest. This requires a more careful distinction between the velocity of the bulk mixture and its individual components. Indeed, the velocity v given in Equation (3.13) is for the bulk mixture. Diffusion velocities, Vi, are defined as relative to this bulk mixture velocity v. Then, the absolute velocity of species i is given as... 

In equilibrium, impurities or vacancies wiU be distributed uniformly. Similarly, in the case of two gases, as above, once a thorough mixture has been formed on both sides of the partition, the diffusion process is complete. Also at that stage, the entropy of the system has reached its maximum value because the information regarding the whereabouts of the two gases has been minimized. In general, it should be remembered that entropy of a system is a measure of the information available about that system. Thus, the constant increase of entropy in the universe, it is argued, should lead eventually to an absolutely chaotic state in which absolutely no information is available. 

Wiistite is reduced to iron at temperatures greater than 700 °C in both CO/CO2 and H2/H2O mixtures. SEM examination of partly reduced crystals showed that the product could be porous iron, dense iron overlying porous wiistite or dense iron and wiistite together depending on the reaction conditions and their effect on the relative rates of the chemical and the diffusion processes (St. John et al., 1984,1984a). 

A solid or Hquid explosive is simply a chemical or a mixture of sohds that contains fuel and oxygen in the condensed state so that no diffusion processes slow the rate. The reachon of TNT can be written approximately as... 

First a derivative is given of the equations of change for a pure fluid. Then the equations of change for a multicomponent fluid mixture are given (without proof), and a discussion is given of the range of applicability of these equations. Next the basic equations for a multicomponent mixture are specialized for binary mixtures, which are then discussed in considerably more detail. Finally diffusion processes in multicomponent systems, turbulent systems, multiphase systems, and systems with convection are discussed briefly. 

In this text we are concerned exclusively with laminar flows that is, we do not discuss turbulent flow. However, we are concerned with the complexities of multicomponent molecular transport of mass, momentum, and energy by diffusive processes, especially in gas mixtures. Accordingly we introduce the kinetic-theory formalism required to determine mixture viscosity and thermal conductivity, as well as multicomponent ordinary and thermal diffusion coefficients. Perhaps it should be noted in passing that certain laminar, strained, flames are developed and studied specifically because of the insight they offer for understanding turbulent flame environments. 

From the standpoint of using multicomponent diffusion in a numerical simulation, it can be beneficial to pose the multicomponent diffusion in terms of an equivalent Fickian diffusion process [72,422]. To do this, imagine that a new mixture diffusion coefficient can be defined such that the first term (summation) in Eq. 12.166 can be replaced with the right-hand side of Eq. 12.162. An advantage of the latter is that the diffusion of the fcth species depends on its own mole fraction gradient, rather than on the gradients of all the other species the Jacobian matrix is more diagonally dominant, which can sometimes facilitate numerical solution. 

When we put two gases together, the molecules diffuse throughout the container, so that within a short time the mixture is homogeneous, or of uniform concentration throughout. Not all gases diffuse at the same rate, however the lighter the molecule, the more rapid the diffusion process. 

First we treat diffusion processes within the homogeneous phase. The presence of a temperature gradient in binary fluid mixtures and polymer blends requires an extension of Fick s diffusion laws, since the mass is not only driven by a concentration but also by a temperature gradient [76] ... 

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