Slow diffusion

Under equiUbrium conditions, magnesium can contain as much as 12.7% aluminum in soHd solution at the eutectic temperature. However, the slow diffusion of aluminum to the grain boundary leads to a coring effect in primary crystals and a hard-phase magnesium—aluminum compound(17 12)... 

C, k progressively increases because the formation rate of the smallest seed crystaUites is also increasing. However, at lower temperatures, the rapid crystallisa tion process is impeded by slow diffusion of polymer molecules from the overcooled melt to crystallisa tion sites. 

R = / -C H ), in low doses, exhibits the former behavior and is used primarily as an extradural agent in obstetrics. The lowest effective extradural concentration of etidocaine (21, X = CH, R = R = 2H, R = / -C H ), however, shows both adequate sensory and profound motor blockade so that it is useful in surgical situations where maximum neuromuscular blockade is necessary. In an isolated nerve preparation, bupivacaine blocks unmyelinated C fibers which are mainly responsible for pain perception at a much greater extent than the myelinated A fibers which carry motor impulses. It is postulated that absorption of bupivacaine by the vasculature at the site of injection, combined with the slow diffusion of this agent, results in an insufficient amount of the drug penetrating the large A fibers to cause motor conduction blockade. Clinically, motor block can be observed in some procedures. 

This slow diffusion of a crucial new technique can be compared with the invention of the scanning tunnelling microscope (STM) by Binnig and Rohrer, first made public in 1983, like X-ray diffraction rewarded with the Nobel Prize 3 years later, but unlike X-ray diffraction quickly adopted throughout the world. That invention, of comparable importance to the discoveries of 1912,now(2 decades later) has sprouted numerous variants and has virtually created a new branch of surface science. With it, investigators can not only see individual surface atoms but they can also manipulate atoms singly (Eigler and Schweitzer 1990). This rapid adoption of... 

Another subsidiary field of study was the effect of high concentrations of a diffusing solute, such as interstitial carbon in iron, in slowing diffusivity (in the case of carbon in fee austenite) because of mutual repulsion of neighbouring dissolved carbon atoms. By extension, high carbon concentrations can affect the mobility of substitutional solutes (Babu and Bhadeshia 1995). These last two phenomena, quenched-in vacancies and concentration effects, show how a parepisteme can carry smaller parepistemes on its back. 

There are a number of tissue adhesives currently approved as adjuncts to hemostasis. Each agent has specific properties which enhance intraoperative hemostasis in particular situations. All forms of surgical bleeding are not alike. Bleeding can be fast or slow, diffuse or punctate, arterial or venous, capillary or larger vessel. Thus one agent may be more efficacious than another in a specific clinical situation. 

Fuel matrix - Fission products, bound in a ceramic matrix, may escape only by slow diffusion or melting of the matrix. 

Fig. 3.9 Example of a slow diffusive- ke growth from a simple seed. I lie initial state a = ...0042400... evolves according to the k = 5, r = f totalistic rule T985707700. All sites with <7 > 0 are colored black.

For slow diffusion of organic ions, one of the most suitable methods for the determination of quasi-diffusion in the grain is the use of the calculation of mean sorption time t [108]... 

With porous materials, a slow diffusion in the pores can sometimes control the rate of desorption. This may give rise to complications because diffusion in the pores may be complex and difficult to treat mathematically. Cvetanovi6 and Amenomiya (48) gave a model treatment for their modification of the thermal desorption technique. 

In Gram-negative bacteria, diffusion of (3-lactam antibiotics into the periplasm (where the activity of PBPs takes place) occurs via the channels that porins create in the outer membrane. The number and properties of the porin molecules are such that diffusion is relatively rapid in E. coli but much slower in Enterobacter and Pseudomonas. Mutants can be selected after the permeability of porin channels or their number has been decreased. A slow diffusion into... 

It is therefore important to examine under what conditions the above criterion is met (i.e. fast ion backspillover relative to its desorption or consumption) for otherwise the promotional process will be internally diffusion limited not due to slow diffusion of the reactants but due to slow diffusion (backspillover) of the promoting species. 

To remove any metallic Au, the reaction mixture is evaporated to dryness and subsequently passed over a commercial terra infusorii column in a minimal volume of solvent. Crystallization may be performed by slow diffusion of a two-layer system. Yields are low (10-60%). The metal evaporation technique (see 8.2.1.2) gives higher yields than the sodium borohydride method. 

In Og (Figure 8.1.5b), xmlike in the Ig case, the fuel jet momentum dispersed and the centerline velocity decayed rapidly owing to the lack of buoyancy. As a result, the fuel molecules diffused in every direction and formed a quasi-spherical flame. The slow diffusion processes (1) limited the transport rates of the fuel and oxygen into the flame zone and (2) decreased... 

We have seen that particle sizes of 100-150 y in classical LC lead to slow and inefficient columns. The reason for this is related to the slow diffusion of molecules in the liquid state (e.g., gaseous diffusion coefficient 10 liquid diffusion coefficients). 

However, the depletion of stratospheric ozone will continue in spite of this prohibition. This is caused by slow diffusion of CFCs present in the troposphere to the stratosphere and... 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Consider the case when the equilibrium concentration of substance Red, and hence its limiting CD due to diffusion from the bulk solution, is low. In this case the reactant species Red can be supplied to the reaction zone only as a result of the chemical step. When the electrochemical step is sufficiently fast and activation polarization is low, the overall behavior of the reaction will be determined precisely by the special features of the chemical step concentration polarization will be observed for the reaction at the electrode, not because of slow diffusion of the substance but because of a slow chemical step. We shall assume that the concentrations of substance A and of the reaction components are high enough so that they will remain practically unchanged when the chemical reaction proceeds. We shall assume, moreover, that reaction (13.37) follows first-order kinetics with respect to Red and A. We shall write Cg for the equilibrium (bulk) concentration of substance Red, and we shall write Cg and c for the surface concentration and the instantaneous concentration (to simplify the equations, we shall not use the subscript red ). 

When it is limited by slow diffusion of the reactants, the mean-square amplitude is... 

In order to assess the role of the platinum surface structure and of CO surface mobility on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry experiments on a series of stepped platinum electrodes of [n(l 11) x (110)] orientation [Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because they adsorb OH at a lower potential than the (111) terrace sites, one would expect that for sufficiently wide terraces and sufficiently slow CO diffusion, the chronoamperometric transient would display a CottreU-hke tailing for longer times owing to slow diffusion of CO from the terrace to the active step site. The mathematical treatment supporting this conclusion was given in Koper et al. [2002]. 

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