Demarcation energy

The concept of a time-dependent demarcation energy, defined by... 

The time dependenee of the average trap energy of the carriers is given by the demarcation energy... 

Ej is a demarcation energy, similar to that defined in the analysis of dispersive transport (see Section 3.2.1). It is assumed that all carriers which are thermally excited recombine non-radiatively, but the same result is obtained if some fraction are subsequently retrapped and recombine radiatively. The luminescence efficiency is given by the fraction of carriers deeper than E, . An exponential band tail density of states proportional to exp (E/kf,) results in a quantum efficiency of... 

Recombination is evidently controlled by trapping into defect states, consistent with the other recombination measurements. The recombination transitions through defects with two gap states are illustrated in Fig. 8.24, with electrons and holes captured into either of the two states. This type of recombination is analyzed by the Shockley-Read-Hall approach which distinguishes between shallow traps, for which the carrier is usually thermally excited back to the band edge, and deep traps, at which the carriers recombine. A demarcation energy, which is usually close to the quasi-Fermi energy, separates the two types of states. The occupancy of the shallow states is determined by the quasi-equilibrium and that of the deep states by the recombination processes. No attempt is made here at a comprehensive analysis of the photoconductivity, which rapidly becomes complicated. Instead some approximate solutions are derived which illustrate the processes. 

This simple model is not exact because it takes no account of the occupancy of the defects, which is different from the equilibrium occupancy. The time-of-flight measurement of the trapping rates is performed with few excess carriers, so that the trap occupancy is the same as in equilibrium, but in a steady state photoconductivity experiment, and the demarcation energies are often far from midgap. The trap occupancy is calculated from the rate equations for band tail electrons and holes... 

The density of holes at the surface can be described conveniently by the quasi-Fermi level for holes, Ef,p(x = 0), given by Ey - ksTin[p 0)/Ny]. The states are empty above a demarcation energy, E , defined by ... 

Refractories are materials that resist the action of hot environments by containing heat energy and hot or molten materials (1). There is no weU-estabhshed line of demarcation between those materials that are and those that are not refractory. The abiUty to withstand temperatures above 1100°C without softening has, however, been cited as a practical requirement of industrial refractory materials (see Ceramics). The type of refractories used in any particular apphcation depends on the critical requirements of the process. For example, processes that demand resistance to gaseous orHquid corrosion require low permeabihty, high physical strength, and abrasion resistance. Conditions that demand low thermal conductivity may require entirely different refractories. Combinations of several refractories are generally employed. 

To ensure that both concentration and time constraints are met, this analysis should be applied in each of the concentration intervals. It would be expected that a similar approach to the Problem Table Algorithm (Linnhoff and Flower, 1978) should be applied in order to ensure that a specific minimum concentration difference holds in each of the intervals. This would entail shifting of the inlet and outlet concentrations for the process streams as it was done in setting the energy targets. However, the fact that the limiting concentration constraints have been built into the problem makes the shifting of concentrations irrelevant. The concentration intervals are demarcated by the inlet and outlet concentrations as shown in Fig. 12.3. 

Fig. 18 The equilibrium temperatures (circles) and the heights of the free-energy barrier at these temperatures (triangles) for a single 512-mer as a function of B/Ep. The dashed line shows the demarcation for the occurrence of a prior collapse transition (Hu and Frenkel, unpublished results)...

Generally, reactions are considered to be nonadiabatic when the effective splitting, IVyp, is smaller than the thermal energy. This is, however, an inexact prediction—polaron theory provides a more complete set of demarcations between the two limits [86]. 

One way of doing this is to assume that the demarcation between stable and unstable colloids occurs at the value of k for which the height of the barrier is zero. Physically, this is a somewhat arbitrary choice Thermal energy is sufficient to allow particles to overcome a barrier of low but nonzero height. Mathematically, however, the assumption that the maximum in the potential energy curve occurs at zero permits us to write... 

In Ihe food processing field, dehydration is sometimes described as the removal of 95 f or more of the water from a food substance, by exposure to thermal energy by various means. The aims of dehydration are reduction in volume of the product, increase in shelf life, and lower transportation costs, among other factors. There is no clearly defined line of demarcation... 

By definition, yield stress demarcates the point beyond which the material looses its ability to elastically return as shown in Fig. 1(a). Thus, beyond this point the material is plastically deformed. From the electronic bonding point of view, this demarcation correspond to the upper limit of the energy potential well where the energy curve begins to deviate from symmetric shape (see Fig. 3). It must be emphasized that it is not the mechanical strength required to break the covalent bond completely but rather the stress required to move beyond the symmetric portion of the potential curve. 

In food processing, dehydration is the removal of more than 95% of the water by use of thermal energy. However, there is no clearly defined fine of demarcation between drying and dehydrating, the latter sometimes being considered as a supplement of drying. 

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