Diffusion and dissolution

For instance, doped phosphosilicate glasses used in planarization cannot be heated above their flow temperature of 725°C. Likewise, after a layer of aluminum is deposited, subsequent temperatures cannot exceed 380°C because spiking and the formation of hillocks would occur rapidly (see diffusion barrier in the following chapter). The factor of time at a given temperature is just as important, as it will influence phenomena, such as diffusion and dissolution. In the planning of a CVD process, the sequence of events and the thermal budget are essential considerations. 

Modeling the behavior of bioerodible polyanhydrides is complicated by the many phenomena contributing to release profiles described in the previous section. The degradation kinetics may be coupled to other processes, such as diffusion and dissolution, and the overall erosion kinetics represent the sum of all of these multiple processes (Goepferich, 1996a). 

Lee, K., Ishihara, A., Mitsushima, S., Kamiya, N. and Ota, K. 2004. Effect of recast temperature on diffusion and dissolution of oxygen and morphological properties in recast Nafion. Journal of the Electrochemical Society 151 A639-A645. Vebrugge, M. R. 1989. Journal of the Electrochemical Society 136 417. 

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

Initially the free C02 is distributed in radially decreasing concentrations in zones around the injection site (Fig. 2a van der Meer 1996). Nearest the injection site lies a zone of near completely saturated pores, containing isolated beads of trapped brine, some of which evaporate into the C02 (Pruess etal. 2003). The middle zone contains mixed brine and C02 (Saripalli McGrail 2002 Pruess et al. 2003). In the outer zone C02 is present only as aqueous species. Following injection, C02 saturations around the injection site are predicted to decrease over tens of years as the free C02 rises buoyantly, spreads laterally, and dissolves into the brine (Weir et al. 1995). Over time-scales of hundreds of years, dispersion, diffusion, and dissolution can reduce the concentration of both free and aqueous C02 to near zero (McPherson Cole 2000). 

Although lipids and lipid-based formulations cannot promote drug association with intestinal lipoproteins in the absence of the requisite physicochemical dmg properties, lipid-based delivery systems can have an appreciable effect on the extent of drug absorption into the enterocyte as described in the previous section. The eventual extent of lymphatic drug transport therefore is the product of the sequential processes of drug diffusion and dissolution in the GIT, drug absorption and metabolism within the enterocyte, and partition of the drug mole-... 

Recent research has shown that HOCs, particularly aromatic compounds, may be biodegraded by microorganisms to a residual concentration that no longer decreases or which decreases only very slowly over years [ 18]. It is widely believed that further reductions are limited by the availability of hydrocarbons to microorganisms and all the more so for aged contaminants [19, 20]. The bioavailability of a chemical is controlled by a number of physical-chemical processes such as sorption and desorption, diffusion, and dissolution [21]. In particular, for aged soils a fraction of the contaminant appears to be inaccessible for biodegradation. The pollutant, not nutrient availability, seems to be the... 

The Monte Carlo model is based on a technique originally used to model the leaching of silver from a Au - 50 at.% Ag alloy, examining the evolution of nano-porosity [10], For NiAl alloys two key processes are assumed to be taking place during leaching, namely adatom diffusion (Ni or Al) and A1 dissolution. These processes are assumed to occur at known rates that serve as the inputs for the model. Both diffusion and dissolution of adatoms are assumed to proceed at a rate described by the following Arrhenius relationships respectively ... 

Since diffusion and dissolution of filamentous carbon in Ni particles are less well known, the diffusivity of carbon through Ni and the saturation concentration of filamentous carbon in Ni were estimated from experimental data. The estimated diffusivity is 1.3-10 mVs, which is rather close to the experimental value of 8.6-10 m /s at 923 K (17). The experimental (TEOM) and simulated carbon deposition are presented in Fig. 4. 

Because the solubility of B is very low in the situation considered, the reaction plane tends to coincide with the macrointerface (x = 0). This implies that now the rate is controlled only by the diffusion and dissolution of B. For this to happen, the condition... 

For the calculation of the rate constants of olefin polymerization as well as the constants of copolymerization, it is necessary to know the actual concentration of monomer near the active centers [56]. According to the known schemes [57-59], polyolefin is formed on the surface of the catalyst particles as a polymer shell, and monomer access to the active centers is by diffusion through this polymer shell. As shown [60], the crystallites in polyethylene are impenetrable and are randomly distributed on a macroscopic scale with respect to the diffusion and dissolution processes the amorphous phase of polymer behaves as a homogeneous liquid. That is, monomer access to the active centers occurs by monomer dissolution in... 

Mullin, J.W. and Cook, T.P. (1965) Diffusion and dissolution of the hydroxybenzoic acids in water. Journal of Applied Chemistry, 15, 145 151. 

A liquid membrane (as in a multiple emulsion), in which case chemical release is controlled by diffusion and dissolution. 

Diffusion and dissolution of substances chromatographed on stationary phases are determined by the mass transfer resistance, which affects the efficiencies of chromatographic columns. Efficiencies of columns with LCSPs are generally lower than those of columns with conventional stationary phases. Therefore, mixtures of LCSPs with conventional stationary phases can be advantageous. The mixture of a liquid crystal with a conventional stationary phase increases the efficiency of a column by improving the homogeneity of the stationary phase film on the wall of a capillary column. The mixture of two hquid crystals can also prove advantageous. The properties of combined stationary phases are described elsewhere. ... 

And, thus, the macroscopic diffusivity can be obtained from a consideration of the random atomic motions. The importance of this and the previous derivation is that uq and t are both parameters that can be easily extracted from a KMC simulation and thus diffiisivities can be obtained that can be used to compare with experimentally determined values or that can be used to calibrate less easily measured parameters, such as atom-electrolyte interactions or interspecies bonding, that are used to determine the energy barriers in the local bond-breaking model for diffusion and dissolution. 

In a technical process, the rate of extraction determines how closely and rapidly we reach the equilibrium solubility. This rate is the result of the interplay of mass transfer from the bulk phase of the solvent to the particle, pore diffusion, and dissolution into the solvent. Hence, depending on the value of the respective rate constants of these three processes, external and/or internal mass transfer may influence the effective extraction rate. 

A different ignition temperature for pruticular binary (Ti-B, Nb-B or Cr-B) systems as well as high adiabatic temperature and various melting points of reaction products indicate a complex reaction mechanism, including both diffusion and dissolution-precipitation. 

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