Diffusion, surface deterioration

Degradation of Polymers. From the results of dynamic analysis, the degree of the deterioration of polymers varies with time and the depth from the surface, and is influenced by the photochemical reaction constants (ki, k2) and diffusion coefficient (D.). Figure 9 shows the theoretical result of the influence of photo-oxidation on the distribution of oxygen in polymers. Deriving the time dependence of surface deterioration, we obtain equation 13. 

It is contended that the renal slice technique measures primarily basolateral uptake of substrates or nephrotoxins, based on histological evidence of collapsed tubular lumens. This results in the inaccessibility of brush-border surfaces for reabsorptive transport (Burg and Orloff, 1969 Cohen and Kamm, 1976). This observation limits the ability of this model to accurately reflect reactions to nephrotoxins that occur as the result of brush-border accumulation of an injurious agent. Ultrastructurally, a number of alterations, particularly in the plasma membrane and mitochondrial compartments, have been shown to occur over a 2-h incubation period (Martel-Pelletier et al., 1977). This deterioration in morphology is very likely a consequence of the insufficient diffusion of oxygen, metabolic substrates, and waste products in the innermost regions of the kidney slice (Cohen and Kamm, 1976). Such factors also limit the use of slices in studying renal metabolism and transport functions. 

Membrane deterioration may be merely caused by decrease of acetyl content(C ) in the active surface layer as a result of hydrolysis or oxidation, not by structure change. Analysis was carried out based on solution-diffusion model proposed by Lonsdale etal( ), using their measured values of solute diffusivity and partition coefficient in homogeneous membrnaes of various degree of acetyl content and also using those values of asymmetric membranes heat treated at various temperatures measured by Glueckauf(x) ... 

Laboratory measurements on stored samples are common, but many precautions must be observed. Samples must be representative of the propellant grain, enclosed in its case-liner insulation envelope. Since diffusion paths and exposed surface area may be germane to aging, these characteristics require duplication in aging samples. There is considerable evidence also that samples stored in sealed containers deteriorate much more rapidly and by different mechanisms than those exposed to the atmosphere. 

Raney-nickel catalysts are barely sensitive to catalyst poisoning (as are Pt-activated cathodes), e.g., by iron deposition, but they deteriorate due to loss of active inner surface because of slow recrystallization—which unavoidably leads to surface losses of 50% and more over a period of 2 years. A further loss mechanism is oxidation of the highly dispersed, reactive Raney nickel by reaction with water (Ni + 2H20 — Ni(OH)2 + 02) under depolarized condition, that is, during off times in contact with the hot electrolyte after complete release of the hydrogen stored in the pores by diffusion of the dissolved gas into the electrolyte. 

Agglomeration of Pt crystallites due to Brownian motion can really be observed and it can also be shown that, indeed, the interaction between the Pt particles and the supporting soot in the presence of the electrolyte, phosphoric acid, is weak enough to allow for relatively free movement of the Pt particles. This fast process obviously is also the reason for the nonobservability of slower surface diffusion-induced Ostwald ripening. Fortunately alloy catalysts composed of platinum and nonnoble metals seem to show a reduced tendency to agglomeration as their deterioration and activity loss is much slower than that of the pure platinum catalyst. 

Si doping of GaN by plasma assisted MBE was reported with a dopant concentration up to 2 x 1019 cm 3 measured both electrically and by SIMS [2]. SIMS measurement of the Si profiles in GaN shows that the dopant concentration was constant throughout the layers, and there was no evidence for segregation or diffusion in the layers for the range of concentrations considered. The doped layers were smooth and did not show any deterioration of the surface morphology. The chemical concentration of Si was the same in both wurtzite and zincblende GaN for a fixed Si flux. The Si concentration varied with cell temperature in the expected manner [3],... 

Insulation suited to cryogenic equipment are characterized by multiple small spaces or pores that occlude more or less stagnant air of comparatively low thermal conductivity. Table 8.19 lists the most common of these materials. In application, vapor barriers are provided in the insulating structure to prevent inward diffusion of atmospheric moisture and freezing on the cold surface with resulting increase in thermal conductivity and deterioration of the... 

Liquid scrubbers remove contaminants quite effectively. They can be made even more effective by breaking up the gas into smaller bubbles with screens or diffusor/air-stones. These devices expose a greater surface area of the gas to the liquid removal agent and thus make the scrubbing more efficient. Use a larger pore diffuser for this - finer pores take more pressure and clog up faster. Better quality glass bonded silica diffusers have better resistance to KOH and do not deteriorate as quickly as other types. 

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