Foil thickness

With a thermionic electron source, and a foil thickness of 100 nm, the volume of specimen excited is of the order 10-5 pm3. With a FEG source in a dedicated STEM, however, with a foil of 1 nm thickness this specimen-beam interaction volume can be as small as 10 8pm3. Very small signal levels are thus to be expected in AEM, hence the importance of employing higher brightness sources and the need to modify the specimen-detector configuration to maximize the collection angle. 

Fig. 10.11. Maximal experimental energies and number of particles for protons (a) and (c) and carbon ions (b) and (d) as a function of foil thickness. Open and closed black circles and squares are experimental data. The solid lines are the estimates from the analytical model. Closed diamonds are 2D PIC code results...

In 1984, the AMPTE mission launched the first carbon-foil TOF-MS into space, which would have been the second, had the Challenger shuttle disaster not delayed the Ulysses launch until 1991 (Fig. 11.2) [23]. The photons were filtered out by a traditional blackened deflection system, which directed the ions toward the 2 p,g/cm2 thick foil mounted on an 85% transparent grid almost a square centimeter in area. The grid provided the support needed to survive the launch. The foil thickness permitted >2keV/nuc ions to pass through and hit a SSD some 10 cm away. To ensure that the ions made it through the foil and also through the dead layer on the SSD (caused by the upper electrode), the foil and the entire TOF section were floated at 20 kV to post-accelerate the ions. Electrons sputtered off the carbon foil became the start, whereas electrons sputtered off the SSD became the stop pulse for the TOF. 

Crystal defects, crystal orientations, accurate lattice parameter measurements, local strain, thin foil thickness can be identified from CBED and LACBED [7],... 

Materials. Low density polyethylene (LDPE) foils (thickness 37 im) were purchased from a commercial supplier. Some LDPE samples were cross-linked using electron beam irradiation (dose 250 kGy) under inert gas conditions. S02 of > 99.98% purity as well as compressed air were received from Linde AG. 

This method has the advantage of not requiring a knowledge of the foil thickness t, but it becomes very difficult to count surface intersections for dislocation densities higher than about 10 cm". Clearly, measurements of the number-density of small dislocation loops or small inclusions (such as bubbles or voids) requires a knowledge of thickness t. 

Figure 8.53. Typical [0101 image of cylindrite, with corresponding diffraction pattern (A). The inset B is a simulated image (foil thickness r = 2 nm, defect of focus A/=40 nm). (From Williams and Hyde 1988.)...

Figure 11 Linear variation of geometric surface area and nonlinear pressure drop across a metal monolith as a function of metal foil thickness for three cell densities 200,400, and 600 cells inch. ...

To illustrate the effect on Prox performance by changing the substrate from a monolith to an SCT, Figure 7.10 shows the conversion versus selectivity for the exact same Pt/y-Al203 formulation. The SCT reactor was a woven mesh type where a number of interlaced wires are woven together to form a screen. The wire diameters were 0.003 in. and made of Inconel. The monolithic reactor was constructed of 400 cells in. 2 with a foil thickness of 0.003 in. and 0.3-in. thick. The material was made from Hastelloy-X, which is a high-temperature metal alloy. The catalyst formulation was identical on both reactors with a nominal loading per piece of 0.5244 g or 178.1 mg/in. 3. 

Figure 3-4 Nomogram for Selecting Wires and Foils Thicknesses, Based on Skin Depth Considerations...

If we slowly increase the thickness of the foil, once the foil thickness exceeds 28, the ac resistance won t change any further, since the cross-sectional area available for the high-frequency current remains confined to 8 on each side of the foil. But the dc resistance continues to decrease as per 1/h — and as a result Fr will increase. So the relationship between Rac and Fr is not necessarily obvious. Therefore, since Fr = Rac/Rdc, with Rdc oc 1/h, we get Rac oc Fr/1i. And this is what we really need to minimize (for a foil). Further, since we always like to write any frequency-dependent dimension in reference to the skin depth, we have plotted not Fr/1i, but Fr/X, versus X, in Figure 3-10. 

For the secondary, we have finally chosen copper foil of thickness 0.4625 mm (i.e. 0.4625 x 39.37 = 18 mil). Let us assume each foil is covered on both sides by a 2 mil thick mylar tape. Since 1 mil is 0.0254 mm, we have effectively added 4 x 0.0254 mm to the foil thickness. In addition there will be three layers of tape between each of the two primary-secondary boundaries. So in all, the thickness occupied by the secondary and the insulation, hs, is... 

However, the equivalent foil thickness is now half of what it was — 28 (i.e. X = 2). And we also now have two layers per portion from Dowell s standpoint. Consulting Dowell s curves, we get an Fr of about 5 now (marked B ). Since we are keeping Rdc fixed in the process, Rac oc Fr. Therefore now, decreasing Fr is a sure way to go, to decrease Rac- So an Fr of 5 is decidedly worse than an FR of 4. We now go ahead and subdivide once more, in a similar manner. So we then get four layers per portion, each with X = 1, and Fr has gone down to about 2.6 (points marked C ). We subdivide once more, and we get eight layers per portion, with X = 0.5. This gives us an Fr of about 1.5 (marked D ). This is an acceptable value for Fr. 

For example, after four subdivision steps, the foil thickness will drop by a factor of 16, and the number of layers will increase by the same factor. We can then look at Dowell s curves to find out the new Fr. 

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