Multilayer absorbers

So, it is difficult to achieve the width bandwidths generally desired for radar absorbers by using a thin single-layer absorber. Therefore, much work has been done in extending the bandwidth of absorbers by the use of multiple layers. The approach consists of slowly changing the effective impedance with distance into the material to minimise reflections. Two important types of multilayer absorbers have been developed, Jaumann absorbers and graded magnetico-dielectric absorbers. 

Polypyrrole or polyaniline loaded fabrics have been studied as microwave absorbers either in the form of pure fabric or composites [84,80-92]. The application involves their use either under the form of multilayer absorbers [86], camouflage nets [93], or absorber with continuous variation of conductivity inside the plane [94] (edge cards for use in low observable technologies). An example of microwave properties which can be achieved with this type of material is given in Figure 8.6. 

Multilayer absorbers of silica aerogel. Proceedings of Forum Aucusticum, Sevilla, September 2002. 

Naishadham [475] carried out comprehensive microwave studies of the CPs P(Ac) and P(BT), as cited earlier in this chapter. Fig. 19-1 above has shown the correlation he observed between the EMI-SE and the Reflection Factor for a multilayer absorber consisting of several CPs. As predicted, due to less resonance in multilayer configuration, the EMI-SE is more broad band. 

As a general rule, adsorbates above their critical temperatures do not give multilayer type isotherms. In such a situation, a porous absorbent behaves like any other, unless the pores are of molecular size, and at this point the distinction between adsorption and absorption dims. Below the critical temperature, multilayer formation is possible and capillary condensation can occur. These two aspects of the behavior of porous solids are discussed briefly in this section. Some lUPAC (International Union of Pure and Applied Chemistry) recommendations for the characterization of porous solids are given in Ref. 178. 

Because of the high functional values that polyimides can provide, a small-scale custom synthesis by users or toU producers is often economically viable despite high cost, especially for aerospace and microelectronic appHcations. For the majority of iudustrial appHcations, the yellow color generally associated with polyimides is quite acceptable. However, transparency or low absorbance is an essential requirement iu some appHcations such as multilayer thermal iusulation blankets for satellites and protective coatings for solar cells and other space components (93). For iutedayer dielectric appHcations iu semiconductor devices, polyimides having low and controlled thermal expansion coefficients are required to match those of substrate materials such as metals, ceramics, and semiconductors usediu those devices (94). 

Using this strategy, constmction of multilayer films of - O. fim thickness by self-assembly of methyl 23-ttichlorosilyltticosanoate (MTST) on siUcon substrates has been demonstrated (Fig. 9) (165). The linear relationship between the film thickness and the layer number showed a slope of 3.5 nm /layer. Filipsometry data, absorbance intensities, and dichroic ratios for the multilayers all suggest that the samples were composed of distinct monolayers. However, ir data indicated that there maybe more tilting or disordering of the alkyl chains in the seven-layer sample than for the monolayer samples. 

Structured laundry liquids are currently available in Europe and were recently introduced in the United States [50,51]. These products typically contain high levels of surfactants and builder salts, as well as enzymes and other additives. In the presence of high ionic strength, the combination of certain anionic and nonionic surfactants form lamellar liquid crystals. Under the microscope (electron microscope, freeze fracturing) these appear as round droplets with an onion-like, multilayered structure. Formation of these droplets or sperulites permits the incorporation of high levels of surfactants and builders in a pourable liquid form. Stability of the dispersion is enhanced by the addition of polymers that absorb onto the droplet surface to reduce aggregation. 

Since natural Au consists solely of Au, the interface-selective enrichment technique cannot be applied in Au studies. The absorber thickness for Au is required to be large and therefore multilayered samples of Au layers/3r/ metal layers have to be prepared. The spectra for Au/Fe with varying Au-layer thickness are shown in Fig. 7.83 [437]. The results were interpreted as follows large magnetic hyperfine fields at Au sites exist only within two monolayers at the interface region, which are supposed to be induced by direct coupling with anti-ferromagnetically oriented Fe 3d atoms. 

From optical point of view, an OLED structure can be considered as a multilayer thin-film system composed of absorbing and nonabsorbing materials, as shown in Figure 6.27. Therefore, the optical properties and optimal structure of such a multilayer device can be investigated by applying thin-film optical analysis techniques. Based on the theory of optical admittance analysis for analyzing the optical properties of a thin-film system [92], the optical properties of an OLED thin-film system can be simulated to reduce the ambient reflection. 

Multilayer-metal tandem solar absorbing surface, 23 11... 

Films of the NA-EBDMA mixture absorbed light strongly only at wavelengths below about 300 nm and appropriate dyes would have to be added if films of these materials were used as bottom layers in multilayer-resist structures to be patterned at 366 or 436 nm. 

At low gas concentrations, whilst the monolayer is still incomplete, the absorbed molecules are relatively immobile. In the multilayer region, the adsorbed molecules behave more like a liquid film. The amount of capillary condensation that occurs depends on the pore sizes and their distribution, as well as on the concentration in the gas phase. 

Figure 14. A formation of protein multilayers as monitored by UV absorbance.

Air is drawn thru meihanol at such a rate that a mixture conrg 30—50% methanol is obtd. The alcohol-air mixture passes thru a preheater (10 0—30 0°) in order to eliminate any alcohol in liquid form, and then goes to the reactor, where it remains only 0.01 second. The reactor contains either Ag or Cu multilayer gauze or screen, maintained at 450—60 0° The resulting gaseous product is absorbed in water to give the commercial product... 

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