Multilayer structures, design

The current trend in photoreceptor design is to use multilayer structures of various Se-based compositions, hi particular, certain additives, e.g.. As, Te, and Bi, have been shown to extend the spectral sensitivity of selenium to longer wavelengths [17, 20, 21]. Arsenic has been particularly effective in providing increased thermal stability and resistance to crystalhzation. 

Tubes and blown Aims can be produced as multilayer structures by employing multiple extruders and coextrusion manifolds and dies. Figure 12.44 is a schematic representative of a conventional and new spiral coextrusion die. The designs can be used for both blown-film and blown-molding parison dies. In the extrusion of tubes, such as rigid PVC or PE pipe, the extrudate passes over a water-cooled mandrel and enters a cold-water bath whose length depends on the tube thickness the tube leaves the bath well below its Tm (if it is crystalline) or Tg (if it is amorphous) and is sectioned to the desired lengths. 

The electrochemical design of special nanocompositions can be achieved both by creating the multilayer structures and by locally modifying the surfaces. These common approaches are used for both the HTSC and or their precursors. 

Use of optical design to optimise the position of the active layers within the multilayer structure (Penmans et al, 2000). 

With a proper understanding of the transport properties in such multilayered structures, the containers for use in sterilization processes can be appropriately designed. As an example in designing for such a situation, a mass balance on water permeating into or out of a multilayered structure can be made to estimate the amount of water which can affect the hydrophilic polymer. With this analysis, the time required to dry the water sensitive layer and return the container to ambient oxygen permeability values can be estimated. Assuming the transport of water in the multilayer configuration is limited by the outer layers, the water content (M) of the inner layer in a prescribed container can be described by ... 

It is now possible to design materials possessing permittivities which have low variation with the frequency (e"a(w ) or high (e"ao) ) directly correlated to the intrinsic conductivity of the polymer. To obtain the best performance (large bandwidth) these materials have to be associated in multilayer structures. Moreover, the use of structural laminates allows the integration of two functions (stealth and mechanical). 

More complex multilayer structures also exhibit interference enhancement phenomena that are manifested by a dependence of Raman band intensities on excitation wavelength and the appearance of new features in measured spectra (37). Studies in progress are designed to model the scattered Raman intensity as a function of probe wavelength for a particular mu 1ti1ayer stack design based on the optical properties of the muHilayer. Figure 15 presents some... 

This work demonstrates that these new materials have the potential to provide enhanced optical limiting by the more efficient use of nonlinear refractive processes. Clearly, it is necessary to understand in more detail how the magnitude of the different components, real and imaginary (absorptive), of the nonlinearities in the layers and the distribution of layer thickness influence the nonlinear response. Experiments to this end are in progress, however, a better theoretical model for the response of multilayer structures with both nonlinear absorption and refraction is desirable. Such a model would aid in the design of more efficient layer structures. 

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