Tangle

If tlie level(s) associated witli tlie defect are deep, tliey become electron-hole recombination centres. The result is a (sometimes dramatic) reduction in carrier lifetimes. Such an effect is often associated witli tlie presence of transition metal impurities or certain extended defects in tlie material. For example, substitutional Au is used to make fast switches in Si. Many point defects have deep levels in tlie gap, such as vacancies or transition metals. In addition, complexes, precipitates and extended defects are often associated witli recombination centres. The presence of grain boundaries, dislocation tangles and metallic precipitates in poly-Si photovoltaic devices are major factors which reduce tlieir efficiency. 

The circular cross section of the polymer blobs does not prove that the polymer existed in solution as a tangled coil (although this is the case). The shape displayed by the particles in the photograph is probably due in part to surface tension occurring during the drying of the sample. 

At one time or another, all of us have tangled with problems of units, but generally these decrease in severity and frequency with experience. Advanced students juggle kilograms and grams, centimeters and angstroms, joules and calories, and rarely fumble in the process. Electrical units are sometimes more troublesome. 

Staple and Tow. The same extmsion technology that produces continuous filament yam also produces staple and tow. The principal difference is that spinnerets with more holes are used, and instead of winding the output of each spinneret on an individual package, the filaments from a number of spinnerets are gathered together into a ribbon-like strand, or tow. A mechanical device uniformly plaits the tow into a carton from which it can be continuously withdrawn without tangling. 

As a general rule, however, textile fibers do not wet out readily, are difficult to disperse, and tend to tangle with one another. Consequendy, large amounts of water are necessary to keep the fibers suspended. Further, if the slurry is not handled propedy, the fibers tangle and cause poor sheet formation. Two approaches to resolving these difficulties are increasing slurry—dilution ratio and controlling fiber orientation. 

This is because rubber, like many polymers, is composed of long spaghetti-like chains of carbon atoms, all tangled together as we showed in Chapter 5. In the case of rubber, the chains are also lightly cross-linked, as shown in Fig. 5.10. There are covalent bonds along the carbon chain, and where there are occasional cross-links. These are very stiff, but they contribute very little to the overall modulus because when you load the structure it is the flabby Van der Waals bonds between the chains which stretch, and it is these which determine the modulus. 

Slides Microstructures showing precipitates electron micrographs of dislocation tangles micrographs of polycrystalline metals. 

The glass temperature, T, you will remember, is the temperature at which the secondary bonds start to melt. Well below the polymer molecules pack tightly together, either in an amorphous tangle, or in poorly organised crystallites with amorphous... 

Seitz, F. and Einspruch, N.G. (1998) Electronic Genie The Tangled History of Silicon (University of Illinois Press, Urbana and Chicago). 

Figure 8.9. Diagram of the structure of a drawn polymer fibre. The Young s modulus of the crystallised portions is between 50 and 300 GPa, while that of the interspersed amorphous tangles will be only 0.1-5 GPa. Since the strains are additive, the overall modulus is a weighted average of...

FIGURE 9-3. These two workers appear to be working on a tank farm. One of the workers appears to be descending a caged ladder. Note how his airlines and apparatus are not likely to get tangled in the cage. Photo courtesy of DuPont Tyvelfl Tychenf protective apparel... 

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