Transport-interactive traps

Trap-Controlled Hopping. In trap-controlled hopping, the scenario described for trap-controlled band mobility applies. However, the microscopic mobility is associated now with carriers hopping in a manifold of localized states. Overall temperature and field dependence reflects the complicated convolution of the temperature and field dependence of both the microscopic mobility and the trap kinetic processes. Glearly, the observed behavior can now range from nondispersive to anomalously dispersive behavior as before, depending on the energy distribution of transport-interactive traps. 

At very high dopant concentrations, transport occurs direcdy between the dopant molecules. The polymer acts only as a binder in most cases. Taking TPD-doped PVK as an example, at low TPD concentrations the hole mobihty first decreases from 3 x 10 cm /Vs to 10 cm /Vs with increasing TPD concentration, because TPD molecules act as hole traps (48,49). At higher TPD concentrations, new direct transport channels between the TPD molecules open up and the hole mobihty increases to lO " cm /Vs for ca 60% TPD doping (Table 1, entries 9—11) (48,49). In this case, there is no evidence for unusual interaction between TPD and PVK that affects the hole transport process. 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

Smith AJ, van Helvoort A, van Meer G, Szabo K, Welker E, Szakacs G et al. MDR3 P-glycoprotein, a phosphatidylcholine translocase, transports several cytotoxic drugs and directly interacts with drugs as judged by interference with nucleotide trapping. J Biol Chem 2000 275(31) 23530—23539. 

In discussing low temperature-dependent mobility, we should mention charge transport by polarons, an intermolecular phonon-assisted hopping process 24>25>. Polarons (charge carriers trapped in their polarization field) arise from a strong electron-phonon interaction where there is a weak overlap of wave functions of... 

The phenomenon of metal transport via the creation of volatile metal carbonyls is familiar to workers using carbon monoxide as a reactant. It is often found that carbon monoxide is contaminated with iron pentacarbonyl, formed by interactions between carbon monoxide and the walls of a steel container. Thus, it is common practice to place a hot trap between the source of the CO and the reaction vessel. Iron carbonyl decomposes in the hot trap and never reaches the catalyst that it would otherwise contaminate or poison. Transport of a number of transition metals via volatile metal carbonyls is common. For example, Collman et al. (73) found that rhodium from rhodium particles supported on either a polymeric support or on alumina could be volatilized to form rhodium carbonyls in flowing CO. 

Electric fields use in soil restoration has been focused on contaminant extraction by their transport under electroosmosis and ionic migration. Contaminant extraction by electric fields is a successful technique for removal of ionic or mobile contaminants in the subsurface. However, this technique might not be effective in treatment of soils contaminated with immobile and/or trapped organics, such as dense non aqueous phase liquids (DNAPLs). For such organics, it is possible to use electric fields to stimulate in situ biodegradation under either aerobic or anaerobic conditions. It is necessary to evaluate the impact of dc electric fields on the biogeochemical interactions prior to application of the technique. It is not clear yet how dc electric fields will impact microbial adhesion and transport in the subsurface. Further, the effect of dc fields on the activity of microorganisms in a soil matrix is not yet well understood. 

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