Layers tapered

Tapered plates, prepared with a gradual increase in thiclcness of the layer from 0.3 nm to 1.7 am, can be used to improve resolution of the sample [215]. On the tapered layer the solvent front velocity decreases as the thickness of the layer increases. This results in the formation of a negative velocity gradient in the direction of solvent migration. As a result the lower portion of a zone moves faster than the top portion, keeping each component focused as a narrow band. Plates with concentrating zones are useful for optimizing sample application. 

In particular, the techniques based on the termination of certain plies within the laminate has also shown promise. Static tensile tests of [30°/-30°/30°/90°]s carbon-epoxy laminates containing terminals of [90°] layers at the mid-plane show that premature delamination is completely suppressed with a remarkable 20% improvement in tensile strength, compared to those without a ply terminal. Cyclic fatigue on the same laminates confirms similar results in that the laminate without a ply terminal has delamination equivalent to about 40% of the laminate width after 2x10 cycles, whereas the laminates with a ply terminal exhibit no evidence of delamination even after 9x10 cycles. All these observations are in agreement with the substantially lower interlaminar normal and shear stresses for the latter laminates, as calculated from finite element analysis. A combination of the adhesive interleaf and the tapered layer end has also been explored by Llanos and Vizzini, (1992). 

The concentration of precursor gases will decrease with respect to the flow direction over the susceptor due to the consumption of growth species, which results in a tapered layer thickness. This effect is known as depletion. To compensate for the depletion it is common to taper the susceptor such that the velocity of the gases increases along the flow direction over the susceptor and thus the boundary layer will be pushed downward, resulting in a shorter diffusion for the active species to the substrate. 

A unique plate for PLC combining a 700-pm-thick preadsorbent area and a wedge-shaped silica gel G layer is manufactured by Analtech (Uniplate-T Taper Plate). The silica gel layer is thin (300 pm) adjacent to the preadsorbent and varies uniformly to a thickness of 1700 pm at the top. Sample concentration occurs in the preadsorbent, and low-Rf zones are better separated on the thin, bottom area of the silica gel than they would be on a 1000-pm constant-thickness preparative layer. The solvent flow pattern in the tapered layer causes the lower portions of the zones to travel at a faster rate than the top portions, reducing vertical band spreading and leading to tighter, better resolved zones. Nyiredy... 

Preparative siJica gel plates are available precoated with a layer thickness of 500-2000 Use of these plates avoids the problems associated with preparing thick layers in the laboratory, most notably the formation of cracks during drying of the spread adsorbent slurry. Analtech offers a unique tapered layer for preparative separations (see Section V.D below). 

Fig. 17. Structural diagram (51) for sputtered layers. Zone 1 is a porous stmcture consisting of tapered crystallites separated by voids, Zone 2 shows...

The particle size and size distribution of adsorbents for preparative purposes are higher and wider, respectively, compared to analytical ones. In addition, the adsorbent layer is much thicker and effectively overloaded with the compoimds. These items make resolution difficult, which must even be better than for quantitative separations as discussed in Section 5.1. These facts necessitate an excellent and superior strategy to hud the best separation, i.e., the mobile phase with the best selectivity (see Chapter 4). It was also shown that plates with a thickness gradient, called Uniplate-T taper plate [5], could improve resolution in the lower-Mp range. 

Early investigators assumed that this so-called diffusion layer was stagnant (Nernst-Whitman model), and that the concentration profile of the reacting ion was linear, with the film thickness <5N chosen to give the actual concentration gradient at the electrode. In reality, however, the thin diffusion layer is not stagnant, and the fictitious t5N is always smaller than the real mass-transfer boundary-layer thickness (Fig. 2). However, since the actual concentration profile tapers off gradually to the bulk value of the concentration, the well-defined Nernst diffusion layer thickness has retained a certain convenience in practical calculations. 

Joel, V. Monzon Hernandez, D., Fast detection of hydrogen with nano fiber tapers coated with ultra thin palladium layers, Opt. Express 2005, 13, 5087 5092... 

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