Trapping effect

The inhomogeneous field produces a trapping effect for ions so that, if an ion moves close to a pole, the field pushes it away and back toward the central axis. 

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. 

Umsov VS, Dudnikova VB (1998) The trace component trapping effect experimental evidence, theoretical interpretation, and geochemical applications. Geochim Cosmochim Acta 62 1233-1240 Van Orman JA, Grove TL, Shimizu N (1998) Uranium and thorium diffusion in diopside. Earth Planet Sci Lett 160 505-519... 

In desert areas of southern California fruit are often injured but leaves are seldom injured by sulfur dust. In coastal areas fruit burn is less marked but leaf burn may be acute. Where the air-vapor density is high, leaf temperatures in the sun may sometimes become higher than fruit temperatures. The leaf, a better absorber of radiation and a better radiator than the fruit, has a higher surface-mass ratio and appears to be very sensitive to the heat trap effect of high vapor density its temperature changes with great rapidity, but fruit temperature may lag until the danger period is passed (18). 

Here (g)T = (e/m)Tf2/(r( + Tt) is called the ballistic mobility and (/t)H = + Tt) is the usual trap-controlled mobility. (q)F is the applicable mobility when the velocity autocorrelation time ( 1) is much less than the trapping time scale in the quasi-free state (fTf l). In the converse limit, (jj)t applies, that is—trapping effectively controls the mobility and a finite mobility results due to random trapping and detrapping even if the quasi-free mobility is infinite (see Eq. 10.8). 

Yang et al. [2] have compared sorbent trapping with solvent trapping after the supercritical fluid extraction of volatile petroleum hydrocarbons in soil. Sorbent trapping yielded quantitative collections of n-alkanes as volatile as n-hexane, while solvent trapping effectively collected w-alkanes as volatile as n-octane. 

Both Chou (1974) and Davis and Friedman (1976) have shown that sulfenic acids can also be trapped effectively by (CH3)3SiCl to afford trimethylsilyl sulfenate esters, RSOSi(CH3)3. 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

In upwelling areas, the resupply of DSi via Ekman transport leads to a silica trapping effect as illustrated in Figure 16.4. Under these conditions, growth rates in the surface waters are controlled by the supply rate of DSi via upwelling. An example of nutrient... 

The DSi stripped out of the Southern Ocean s surfece waters at the site of SAMW formation is converted to BSi. This BSi eventually sinks into the deep waters (CDW), where it is remineralized to DSi and driven back south to be upwelled again into the surface waters. This trapping effect is a large part of why half of the global marine sedimentary sink of BSi is located in the Southern Ocean. South of 55°S, iron limitation is so severe, as compared to the rate of upweiiing supply of DSi, that the diatoms are not able to reduce silicic acid concentrations to zero. 

Because of this trapping effect, estuarine sediments can provide excellent chronological records of contamination histories. An example is provided in Figure 28.24 for sediment collected from two sites located in two watersheds in the Elizabeth River, VA, USA. In general, sedimentary trace metals tend to be closely related to the transport and deposition of fine-grained and organic-rich sediments. The downcore profiles presented... 

If adatom-impurity atom interaction is attractive, then the impurity atom can act as a trapping center. A diffusing adatom may be trapped. In heterogeneous catalysis, the reaction rate may be changed by the trapping effect of impurities as also by lattice defects and lattice steps and so on. 

Diethylcarbamazine, a synthetic piperazine derivative, is marketed as a citrate salt. It is rapidly absorbed from the gastrointestinal tract after a 0.5 mg/kg dose, peak plasma levels are reached within 1-2 hours. The plasma half-life is 2-3 hours in the presence of acidic urine but about 10 hours if the urine is alkaline, a Henderson-Hasselbalch trapping effect (see Chapter 1). The drug rapidly equilibrates with all tissues except fat. It is excreted, principally in the urine, as unchanged drug and the N-oxide metabolite. Dosage may have to be reduced in patients with persistent urinary alkalosis or renal impairment. 

We also had to overcome radiation trapping effects by monitoring the Na(3p) — Na(4s) emission off line-center. The results shown in Figs. 2 and 4 are obtained by subtracting the contribution of these processes from the observed Na(3p) signal. To do so we calibrated the contribution from the Na(3d) — ... 

Assuming that the deposition process is dominated by equilibration rather than kinetic trapping effects, Rivetti et al. have performed statistical analysis of the chain conformation in terms of the mean-square end-to-end distance, , and the mean-square bend angle, <02> [493]. Both values are well described theoretically for both two-dimensional and three-dimensional states [494-498]. For a two dimensional system, the following formula can be used ... 

Few reliable measurements of thermoelectric power in homomolecular crystals are available. The data on the phthalocyanines appear to be the most representative, and in the studies of Hamann (1972 and references therein) and co-workers trapping effects and variation between differently prepared samples of the same material have been taken into consideration. Since the validity of a... 

---