Diffusion coefficient, apparent

South East UK coast, splash zone (Folkestone), 6 y [23, 32]  

Under other environmental conditions, Q can have different values. Bamforth [33] has reported on exposure of concrete blocks made with similar compositions and binders as described above along a motorway in the UK, subjected to de-icing salt application. The results (Table 6.1) show that Q increases over several years, taking values between 0.3 % and 0.6% chloride by mass of concrete (corresponding to 3 to 4% by mass of cement) after 9 y. These values are similar to those found after marine splash exposure and they depend in a similar way on the type of binder [23]. 

The exposure of concrete blocks made with different binders in the marine-splash zone or in a motorway environment described above show a wide range of apparent diffiision coefficients after 6 and 9 y, respectively, as shown in Table 6.1, ranging from 0.3 to 14 X 10 m /s, with lower values for fly ash and slag containing binders and higher values for pure Portland cement [23, 33]. 

From Eq. (1) it is clear that the penetration depth x for a certain content Q depends on the product D pp t. Consequently, other factors being constant, reducing the apparent diffusion coefficient by a factor of 10 increases the initiation time by a factor of 10. Advantages of using mineral additives to obtain so-called blended cements in the design of durable structure will be illustrated in Section 12.6.1. 

When the apparent diffusion coefficient is used for prediction purposes, it should be considered that it tends to reduce in time [9, 23], especially for blended 

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The above data relate to very pure iron samples with low dislocation densities. In real steels the trapping effects result in much lower apparent diffusivities, which are dependent on the metallurgical state of the steel, as well as its chemical composition. Typical values for the apparent diffusion coefficient of hydrogen in high-strength alloy steel at room temperature are in the region of 10" mVs. 

Figure 48. Evolution of the apparent diffusion coefficient (V as a function of solution ionic conductivity (x ) (Reprinted from H.-J. Grande, T. F. Otero, and 1. Cantero, Conformational relaxation in conducting polymers Effect of the polymer-solvent interactions. J. Non-Cryst. Sol. 235-237, 619, 1998. Figs. 1-3, Copyright 1998. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

The permeability tests for alkali metal ions in the aqueous solution were also conducted. When an aqueous salt solution moves to cell 2 through the membrane from cell 1, the apparent diffusion coefficient of the salt D can be deduced from a relationship among the cell volumes Vj and V2, the solution concentration cx and c2, the thickness of membrane, and time t6 . In Table 12, permeabilities of potassium chloride and sodium chloride through the 67 membrane prepared by the casting polymerization technique from the monomer solution in THF or DMSO are compared with each other and with that the permeability through Visking dialyzer tubing. The... 

Ratio of apparent diffusion coefficient of 67 membrane to the cellulosic membrane. Visking dialyzer tubing. 

Besides establishing the diagnosis of ischemic stroke, DWI also offers the capability of measuring the approximate age of infarcts. The apparent diffusion coefficient (ADC) of water, a measure of diffusion that can be derived easily from DWI images, follows a typical sequence of changes in evolving infarcts. °" ADC... 

Grant PE, He J, Halpern EF, Wu O, Schaefer PW, Schwamm LH, Budzik RF, Sorensen AG, Koroshetz WJ, Gonzalez RG. Frequency and chnical context of decreased apparent diffusion coefficient reversal in the human brain. Radiology 2001 221 43-50. 

Schlaug G, Siewert B, Benfield A, Edelman RR, Warach S. Time course of the apparent diffusion coefficient (ADC) abnormality in human stroke. Neurology 1997 49 113-119. 

Yang Q, Tress BM, Barher PA, Desmond PM, Darhy DG, Gerraty RP, Li T, Davis SM. Serial study of apparent diffusion coefficient and anisotropy in patients with acute stroke. Stroke 1999 30 2382-2390. 

Lansberg MG, Thijs VN, O Brien MW, Ali JO, de Crespigny AJ, Tong DC, Moseley ME, Albers GW. Evolution of apparent diffusion coefficient, diffusion-weighted, and T2-weighted signal intensity of acute stroke. Am J Neuroradiol 2001 22 637-644. 

Copen WA, Schwamm LH, Gonzalez RG, Wu O, Harmath CB, Schaefer PW, Koroshetz WJ, Sorensen AG. Ischemic stroke effects of etiology and patient age on the time course of the core apparent diffusion coefficient. Radiology 2001 221 27-34. 

Fiebach JB, Jansen O, Schellinger PD, Heiland S, Hacke W, Sartor K. Serial analysis of the apparent diffusion coefficient time course in human stroke. Neuroradiology 2002 44 294-298. 

NMR Self-Diffusion of Desmopressin. The NMR-diffusion technique (3,10) offers a convenient way to measure the translational self-diffusion coefficient of molecules in solution and in isotropic liquid crystalline phases. The technique is nonperturbing, in that it does not require the addition of foreign probe molecules or the creation of a concentration-gradient in the sample it is direct in that it does not involve any model dependent assumptions. Obstruction by objects much smaller than the molecular root-mean-square displacement during A (approx 1 pm), lead to a reduced apparent diffusion coefficient in equation (1) (10). Thus, the NMR-diffusion technique offers a fruitful way to study molecular interactions in liquids (11) and the phase structure of liquid crystalline phases (11,12). 

More than one point defect species may be present in a crystal at any temperature, and the amount of matter transported by diffusion will depend upon the number of each defect type present. In general, therefore, the overall apparent diffusion coefficient, D, will be the sum of the individual contributions, for example ... 

In the absence of field effects, equations (4.22), (4.23), (4.25), and (4.26) amount to Fick s second and first laws of diffusion with an apparent diffusion coefficient given by equation (4.24). 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

From a theoretical point of view, charge propagation in films containing space-distributed redox centers can be achieved either by the physical displacement of the sites or by the transference (hopping) of electrons from neighboring reduced to oxidized sites or by the combination of both processes. In the case of free diffusing couples immobilized in oppositely charged polyelectrolytes, both processes occur and an apparent diffusion coefficient can be defined and measured [136, 142, 143] ... 

Relaxation times, MT ratios, and diffusion properties allow insight into the microstructure of various tissues. Determination of these parameters is possible by recording and analysing of a series of volume selective spectra, even for metabolites with relatively low concentrations in vivo. For recording series of spectra usually one parameter is changeable (e.g., inversion time TI for Ti measurements, echo time TE for T2 measurements, MT preparation for assessment of spin transfer and chemical reaction rates, or diffusion sensitizing gradients for assessment of apparent diffusion coefficients or even diffusion... 

Further, the electron-transport properties of the polymer 7 were enhanced by extending the separation between the redox center and backbone from a single Os—amino linkage to one that extends over 17 bonds. The goal was to provide mobility of the redox center independently of backbone motion, which is necessarily restricted by cross-linking. The mobility of the redox center can be characterized by an apparent diffusion coefficient, Z app- According to the relation proposed by Blauch and Saveant ... 

The long linkage in compound 7 led to an apparent diffusion coefficient, Dapp, that was a factor of 10 higher than the " 10 cm /s observed for compound... 

In partially saturated media, the diffusion coefficient also is a function of the volumetric water content, 9. Calvet (1984) showed that the variation in soil water content influences the apparent diffusion coefficient for organic contaminants in two ways by changing the ratio of gas diffusion of volatilizable pollutants to liquid diffusion, because the air-filled porosity is affected, and by modifying pollutant... 

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