Nonuniform coating

If the sample is an insulator there are still methods by which it can be studied in the instrument. The simplest approach is to coat it with a thin (10-nm) conducting film of carbon, gold, or some other metal. In following this approach, care must be taken to avoid artifacts and distortions that could be produced by nonuniform coatings or by agglomeration of the coating material. If an X-ray analysis is to be... 

In addition to flow maldistribution, small deviations in the channel diameter (which typically originate from imperfect manufacturing) cause a broadening of the RTD. The deviations may also be the result of nonuniform coating of the channel walls with catalyst material (Section 5). If the number of parallel channels is large (i.e., N > 30), a normal distribution of the channel diameters with a standard deviation a can be assumed. The effect of the diameter variation on the pressure drop in the microreactor can be estimated on the basis of the relative standard deviation, G = ffd/ t [81 ] ... 

Reactors for conventional thermally activated CVD are of two types cold-wall and hot-wall reactors, respectively internally and externally heated. The disadvantage of a hot-wall reactor is deposition on the wall and partial depletion of reactants leading to nonuniform coatings. A correct reactor geometry and gas inlet manifold can compensate for gas depletion in hot-wall reactors. There is no limit to the form of the objects to be coated, but sizes are restricted. In a cold-wall reactor the substrates to be coated are heated by a graphite susceptor that is inductively heated by an rf generator. Only the hot parts are coated and not the reactor walls, which remain relatively cold. 

Note that the length scale in the definition of the Thiele mod-tdus in Equation 8.46 is now as given in Equation 8.42a. The extension of this result to nonlinear kinetics and nonuniform coating geometries was also discussed in Lopes et al. [94]. Reaction rate expressions other than first order can be treated using the concept of generalized Thiele modulus O, where the previously defined parameter is modified so that its introduction in an expression such as... 

Mesh Tension Mesh sticking to screen Poor quality printing Low coating thickness Nonuniform coating across panel... 

FIGURE 3.2 Scanning electron photomicrographs of (a) nonuniform coating of 20% Carbowax 20M on Chromosorb W HP, 80/100 mesh note the stationary-phase pooling in left-hand portion of photograph and (b) a more uniform coating of Carbowax 20M. (Reference 7.)... 

There remain problems also with the collection of experimental data at high-concentration intervals, and at very high concentrations the behavior of the interaction parameter of polymer-solvent systems tends to be unsatisfactory [84], Another source of error might be the nonuniform coating of the stationary phase on the support, as well as nonuniform packing throughout the column [76]. 

Some bubbles can be expected to form during the mixing. Therefore, mixing should be performed under vacuum. If there are bubbles in the slurry, their release at elevated temperatures will lead to nonuniform coating. 

Formation of stagnation zones in the case of the incorrect particle dynamics, stagnation zones can be formed in the bottom part of the annulus, which can lead to nonuniform coating. 

In the left part of Fig. 20, the bottom zone of apparatus is shown. Here, the particles are sucked into Wurster tube and their recirculation occurs. To analyze the efficiency of the process, it is important to analyze the dynamics of particles in the bottom zone of apparatus. Here, the stagnation zones, where particles building almost fixed bed regions, can appear. As a consequence, this can lead to the nonuniform coating of material or to undesired agglomeration. 

Increase of the gap distance between Wurster tube and perforated bottom plate leads to a decrease of the overspray fraction. Therefore a smaller amount of suspension droplets hits the apparatus walls and leaves the FB as dust. This happens because an increase of gap distance leads to a larger volume fraction of particles in the Wurster tube. However, such an increase of the gap distance leads to larger stagnation zones in the comers of the apparatus. As a consequence, nonuniform coating of pellets can occur. 

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