Wedge effect

Vermiculite is also known for the so-called wedge effect (Fig. 4.12), which may explain certain cation replaceability behavior in the interlayer. For example, cations such as Ca2+ may expand the outermost portion of the interlayer, but because Ca2+ is held strongly in such sites, K+ would be trapped deeper in the interlayer. On the other... 

Type A plasma polymers have the characteristic internal stress built in the film, and the plasma polymerization coating acts as the tempered ultrathin layer on the substrate. The internal stress is caused by the wedging effect of the deposition process, and the total stress increases linearly with the thickness. As the thickness increases, the internal stress reaches the critical point beyond which the internal stress becomes greater than the cohesive force or the adhesive force of the plasma polymerized coating. Above the critical thickness, therefore, the coating cracks (not necessarily in macroscopic sense) or delaminates (buckles) from the substrate. Consequently, there exists a thickness limit of plasma polymerization coating. The tighter the structure, the smaller is the thickness limit. 

Most of the applications and methods developed by a variety of laboratories and regulatory organizations are applicable to specimens of a specified nature and matrix. Some of the more versatile procedures include a method to account for differences in matrix between standard reference materials and unknowns. One of the effects that is usually neglected in this case is the wedge effect, which is caused by the fact that the volume that is excited by the beam and the volume from which characteristic radiation can be observed are not the same and vary (for a given spectrometer system) depending upon the density and the quantity of the sample. This effect will be illustrated and discussed. 

Another effect compounding the situation is the wedge effect. The algorithms developed for specimens of less-than-infinite thickness are usually derived with the assumption that the thickness is small compared to the surface area. This implies that the surface of any layer within the specimen is constant. This assumption is no longer justified when the thickness (or rather the path length) is comparable to the diameter of the specimen see Fig. 5. 

The data in Fig. 6 and Table 3 show that the measured intensity reaches its maximum value for a thickness of 20 mm. A sample with a thickness of 8 mm tdready produces 85 % of the maximum reachable intensity. Also, from Fig. 7 it can be concluded that one way to avoid the wedge effect is to analyze small amounts of sample if the thickness of the specimen is small (e.g., below 1 mm, although this depends on the energy of the analyte s radiation), the effect becomes negligible. When this approach is taken, however, it must be clear that always the same mass of specimens must be used for analysis, or a finite thickness correction must be used. 

Methods based on an internal standard (such as ISO 14596, which uses Zr as an internal standard for the determination of S in oil) will implicitly correct for this wedge effect and for the matrix effect because the effect is the same for both the internal standard and the analyte lines. 

FIG. 6—Measured intensity (solid line) and calculated intensities of Sn Ka. Dashed line calculated without wedge correction dotted line calculated with wedge correction of samples with different thicknesses. Note that the wedge effect is significant for Sn Ka in butanol (refer to the penetration depth of Sn Ka in H2O in Table 2). 

But for methods without internal standard, care must be taken to matrix-match the standards and the unknowns, and to work with constant weight in the sample cups. The alternative procedure is to account for the wedge effect and to use a full fundamental parameter approach to the analysis. 

The wedge effect is significant for high energetic radiation in low-Z matrices. The effect can be modelled adequately by taking it into account in the Fundamental Parameter equations. 

When analyzing heavy elements in low-Z matrices, two scenarios can be used either measure only a small amount of sample to reduce the wedge effect, or take the wedge effect into account if larger amounts are measured. In both cases, finite thickness correction is required. Measuring only small amounts will not affect the sensitivity drastically. 

The ratio NIP represents the magnification of the friction capacity due to the curvature and depth of the groove. This wedging effect is given by... 

Figure 8.20 Schematic of alignment with tilted fibrous grains. Because ofthe wedge effects, tilted fibrous grains give rise to improved properties both in the strong (parallel to the alignment) and weak (perpendicular to the alignment) directions.

YOUNG, 3, "Thermal wedge effect in hydrodynamic lubrication", the Engineering 3ournal, 1962, 46-54. 

Apparently, a significant decrease in the neodymium chloride particle size in the first 4-5 h of the synthesis is not due to the occurrence of the complexation on the particle surface (contracting sphere model), because the yield of the complex in this period is low. Most probably, a decrease in the particle size of the initial NdCl3 is associated with disintegration of the solid phase under the action of mechanical stirring, with the wedging effect produced by solvent molecules solvating the particle surface. This... 

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