Void formation, time dependence

Besides the total shrinkage, shrinkage differences are of importance. The shrinkage may vary from place to place, dependent on the time scale of solidification. The simplest example is a solid block, which solidifies rapidly at its outside the hard skin formed this way prevents further shrinkage of the inner part. Hence the density decreases from the outside to the inside, resulting in internal stresses which may even lead to tearing and void formation. 

The time-dependence of void formation in Inconel, as observed both in thermal-convection and forced-circulation systems, indicates that the attack is initially quite rapid but that, it then decreases until a straight-line relationship exists between depth of void formation and time. This effect can 1)0 explained in terms of the corrosion reactions discussed above. The initial rapid attack found for both types of loops stems from the reaction of cliromium with impurities in the molt [reactions (13-1) and (13-2)] and with the FF4 constituent of the salt [reaction (13-3)] to establish a quasi-etiuilibrium amount of CrF2 in the salt. At this point attack proceeds linearly with time and occurs by a mass-transfer mechanism which, although it arises from a different cause, is similar to the phenomenon of temperature-gradient mass transfer observed in liquid metal corrosion. 

Once the criterion for craze initiation is satisfied, localized plastic flow produces microcavities whose rate of formation at a given temperature depends on the value of the applied stress and increases with increasing stress. This is a slow, time-dependent process that results in an interconnected void network. The subsequent growth of the craze is thought to occur by the repeated breakup of the concave air-polymer interface at the craze tip [39], as shown schematically in Figure 12.23 [40]. The process is similar to what happens when two flat plates... 

The Kirkendall effect (8) is time and temperature dependent, and with some metal couples, it takes place even at room temperature. For instance, adhesion of solder to gold is damaged by heating to about 150°C for about 5 minutes, due to the formation of Kirkendall voids. Naturally, the formation of Kirkendall voids is accelerated by increased temperature and dwelling time. 

The diffusion and permeabihty are closely interconnected with the solubility of a polymer. The permeation of the permeants through polymeric membrane film occurs in three stages (1) Sorption includes the initial adsorption, absorption, penetration, and dispersal of penetrant into the voids of the polymer membrane surface and cluster formation. The distribution of permeant in the membrane may depend on penetrant size, concentration, temperature, and swelling of the matrix as well as on time. The extent to which permeant molecules are sorbed and their mode of sorption in the polymer depends upon the enthalpy and entropy of permeant-polymer mixing, i.e., upon the activity of the permeant within the polymer at equilibrium. When both polymer-permeant and permeant-permeant interactions are weak relative to polymer-polymer interactions, i.e., dilute solution occurs, Henry s law is obeyed. The solubihty coefficient S is a constant independent of sorbed concentration at a given temperature. (2) Diffusion includes the transfer of the penetrant through the polymer membrane which depends on penetrant concentration that leads to a plasticization effect, penetrant size and shape, polymer Tg, time, and temperature. The diffusion coefficient is determined by Pick s first law of diffusion. (3) Desorption includes release of the penetrant from the opposite side of the membrane face. 

For the mobilization of silica the basicity of the synthesis solution is important. In the absence of mineral bases the basicity of the synthesis solution depends only on the Bronsted basicity of the SDA. The higher Ae basicity of the SDA in solution, the higher is the concentration of polysilicate anions of different molecular weight and, hence, the higher is the reaction rate of porosil formation. In order to enhance the reaction rate, basic help molecules (Table 6) have been used extensively together with the SDAs. The help base promotes the mobilization of silica, but does not act as SDA in the formation of porosils. In the presence of only ethylenediamine (e ), for example, no porosils crystallize even after 9 months of reaction time. When en is used in combination with an SDA, porosils crystallize faster, but only the SDA is occluded in the structure-determining porosil void. As minor component the help bases might also be included in other voids of the silica frameworks [50]. 

Nanoshells, produced in the diffusive reactions of nanoparticles within the ambient phase with a simultaneous formation of Kirkendall voids inside, are unstable in principle, but the shrinkage time can be very large due to their cubic dependence on the radius of the nanoparticle. The mechanism of shrinkage is the out-diffusion of vacancies from the void due to the curvature effect. 

Often, water reacts with the polymer matrix and causes irreversible chemical changes and diminishes performance. The process of moisture absorption and desorption on the surface layers takes place almost immediately on contact with the environment, but moisture diffusion into the bulk material is usually a slow process. It takes weeks to months for a substantial amount of moisture to be absorbed by the composite and long time (i.e., 1-2 years) before the material reaches saturation. The rate of moisture uptake by a composite laminate depends on the relative humidity, temperature, exposure time and mechanical load. Elevated temperatures accelerate the rate of moisture uptake and material degradation. Similarly, tensile loads accelerates moisture uptake by opening existing internal cavities or voids, and by micro-crack formation. The effect of moismre on polymer composites is potentially... 

One of the most thorough comparisons of formaldehyde formation in the presence of oxide catalysts and in a void reactor was performed in [57]. Figure 2.5 displays the dependences of the methane conversion, selectivity of CH2O formation, and the yield of CH2O in an empty quartz reactor on the reaction temperature for a 1 5 methane—air mixture at a pressure of P = 5 atm and a reaction time of = 2.3 s. 

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