Diffusion studies

If a fluid is placed between two concentric cylinders, and the inner cylinder rotated, a complex fluid dynamical motion known as Taylor-Couette flow is established. Mass transport is then by exchange between eddy vortices which can, under some conditions, be imagmed as a substantially enlranced diflfiisivity (typically with effective diflfiision coefficients several orders of magnitude above molecular difhision coefficients) that can be altered by varying the rotation rate, and with all species having the same diffusivity. Studies of the BZ and CIMA/CDIMA systems in such a Couette reactor [45] have revealed bifiircation tlirough a complex sequence of front patterns, see figure A3.14.16. 

Lahajnar, G., Zupancic, L, Rupprecht, A. Proton NMR relaxation and diffusion study of water sorbed in oriented DNA and hyaluronic acid samples. In Biophysics of Water (Franks, F., Mathias, S., eds) Wiley, New York (1982) 231-234... 

This assumes that the gas-solid exchange kinetics at the interface is rapid. When this process affects the exchange kinetics significantly dieii analysis of concentrations layer by layer in die diffused sample is necessaty. This can be done by the use of SIMS (secondary ion mass spectrometry) and the equation used by Kihier, Steele and co-workers for this diffusion study employs a surface exchange component. 

E Garrett, P Chemburkar. Evaluation, control and prediction of drug diffusion through polymeric membranes I. Methods and reproducibility of steady-state diffusion studies. J Pharm Sci 57 944, 1968. 

GM Zentner, JR Cardinal, SW Kim. Progestin permeation through polymer membranes, I Diffusion studies on plasma-soaked membranes. J Pharm Sci 67(10) 1347-1351, 1978. 

Chemicals. Purified, P Cj-labelled alachlor (specific activity = 17 mCi/mM), butylate (specific activity = 2.54 mCi/mM) and metolachlor (specific activity = 4.5 mCi/mM) were used in the leaching, adsorption, and diffusion studies. The radiopurity of these compounds was greater than 95% as determined by thin-layer chromatography. All other studies were conducted using analytical grade, non-radioactive material (purity 5 95%). 

Schwille, P., Bieschke, J. and Oehlenschlager, F. (1997). Kinetic investigations by fluorescence correlation spectroscopy The analytical and diagnostic potential of diffusion studies. Biophys. Chem. 66, 211-28. 

Thomas, D.D., Dalton, L.R., and Hyde, J.S. 1976. Rotational diffusion studied by passage saturation transfer electron paramagnetic resonance. The Journal of Chemical Physics 65 3006-3024. 

Large zeolite crystals with dimensions of tens and hundreds of micrometers have proven to be irreplaceable as model materials for reactivity and diffusion studies in the field of zeolite science and heterogeneous catalysis [1-3], These large crystallites often possesses complex structures consisting of several intergrown subunits and since the pore orientations of the different elements are not always aligned, this phenomenon can have a considerable effect on the accessibility of the pores in different crystallite regions [4]. 

Other supportive evidence for a specific water-solid interaction is available from thermal studies showing the amount of nonfreezable water [57-59], nuclear magnetic resonance [29,60-66], and diffusion studies [67,68]. The evidence is less clear, however, concerning whether there is distinct binding of water to... 

As for silicon, secondary ion mass spectrometry (SIMS) is the most widely used profiling analysis technique for deuterium diffusion studies in III-V compounds. Deuterium advantageously replaces hydrogen for lowering the detection limit. The investigations of donor and acceptor neutralization effects have been usually performed through electrical measurements, low temperature photoluminescence, photothermal ionization spectroscopy (PTIS) and infrared absorption spectroscopy. These spectroscopic investigations will be treated in a separated part of this chapter. 

Substitutional impurities can move by way of a number of mechanisms. The most usual is the vacancy mechanism described above. Diffusion studies on semiconductors have suggested that a number of additional mechanisms might hold. As well as vacancy diffusion, an impurity can swap places with a neighboring normal atom, exchange diffusion, while in ring diffusion cooperation between several atoms is... 

Comparisons with high temperature diffusion studies... 

The preceding data, though limited in nature, represent one of the first attempts to measure solid state diffusion rates of alkali elements into the near-surface region of feldspars and natural glasses at low temperature. As such, interesting comparisons can be made with diffusion coefficients and activation energies calculated from numerous high temperature isotope and tracer diffusion studies f 11-181. 

The activation energies calculated for Rb, Cs and Sr in the present study (Table III and Figure 8) are considerably lower than those calculated for high temperature diffusion in both crystalline and glass silicates. This discrepancy in the latter case implies that the glass matrix may be significantly different in high and low temperature diffusion studies. 

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