Diffusion, consolidant

The attack of most glasses in water and acid is diffusion controlled and the thickness of the porous layer formed on the glass surface consequently depends on the square root of the time. There is ample evidence that the diffusion of alkali ions and basic oxides is thermally activated, suggesting that diffusion occurs either through small pores or through a compact body. The reacted zone is porous and can be further modified by attack and dissolution, if alkali is still present, or by further polymerisation. Consolidation of the structure generally requires thermal treatment. 

Turner (T14) has proposed two detailed models of packed beds which try to closely approximate the true physical picture. The first model considers channels of equal diameter and length but with stagnant pockets of various lengths opening into the channels. There is no flow into or out of these pockets, and all mass transfer occurs only by molecular diffusion. The second model considers a collection of channels of various lengths and diameters. We will briefly discuss each of these models, which are probably more representative of consolidated porous materials than packed unconsolidated beds. 

The process by which a thermoplastic matrix composite consolidates to form a laminated structure has been attributed to autohesive bond formation at the ply interfaces. Autohesive bond formation is controlled by two mechanisms (1) intimate contact at the ply interfaces, and (2) diffusion of the polymer chains across the interface (healing). The rate of autohesive bond formation and hence the speed of the composite consolidation process is directly related to the temperature-pressure-time processing cycle. 

Consolidation and development of interlaminar bond strength for thermoplastic matrix composites have been modeled by two mechanisms intimate contact and autohesion. Intimate contact describes the process by which two irregular ply surfaces become smooth (Fig. 13.10). In areas in which the ply surfaces are in contact, autohesion occurs, and the long thermoplastic polymer chains diffuse across the ply boundaries. Filament winding with thermoplastic matrix materials is considered an on-line consolidation process in that local... 

The basic reaction underlying the combustion of many gasless delay formulations is the Goldschmidt or thermite reaction where a metal powder and a metallic oxide interact in an oxidation-reduction reaction manner with the evolution of a large amount of heat but very little or no gas. Consequently, these formulations are used where no vent or very little vent is provided in the ammunition. Gasless delay formulations tend to burn faster under higher consolidation as the points of contact of fuel and oxidizer increase. This is because the reaction in this case is a solid state reaction by diffusion. 

The experiments demonstrate the development of a streaming potential in consolidated bentonite clay when flushed by a NaCl-solution of either low or high concentration. The streaming potential measured in our experiments is at least 5 to 10 times larger than values reported for bentonite in the literature. Apparently this is caused by a very low electric conductivity of the bentonite samples studied. This low conductivity might be ascribed to overlapping diffuse double layers on the clay particles, caused by the high compaction and the presence of monovalent ions in the equilibrium solution. The bentonite, thus compacted, will be a very effective medium for active application of electroosmosis. Compared with electrically shorted conditions, chemical osmosis will be reduced when the clay is not short-circuited. 

Now the technique provides the basis for simulating concentrated suspensions at conditions extending from the diffusion-dominated equilibrium state to highly nonequilibrium states produced by shear or external forces. The results to date, e.g., for structure and viscosity, are promising but limited to a relatively small number of particles in two dimensions by the demands of the hydrodynamic calculation. Nonetheless, at least one simplified analytical approximation has emerged [44], As supercomputers increase in power and availability, many important problems—addressing non-Newtonian rheology, consolidation via sedimentation and filtration, phase transitions, and flocculation—should yield to the approach. 

Hydraulic cements are excellent examples of accelerated chemical bonding. Hydrogen bonds are formed in these materials by chemical reaction when water is added to the powders. These bonds are distinct from the bonds in ceramics in which high temperature interparticle diffusion leads to consolidation of powders. 

When the electrode is completely immersed in the electrolyte solution, only a two-phase interface (i.e., liquid-solid) is present in the electrode structure. In form it may be either a consolidated powdered active carbon or a confined but unconsolidated bed of carbon particles. These are u.sed for flow-through porous electrodes in many electrochemical systems. The other mode of operation is the gas-diffusion electrode, in which the electrode pores contain both the electrolyte solution and a gaseous phase. Numerous publications [29-31] have reported on a theoretical analysis of flow-through porous electrodes and gas-diffusion electrodes, which takes into account the physicochemical characteristics of carbon electrode materials. There does not seem to be a uniform explanation for the effects of structural and chemical heterogeneity in carbons. 

---