Dielectric absorber

What are the typical materials used to create the color-shifting flakes A symmetrical layering pattern of absorber/dielectric/reflector/dielectric/absorber is... 

Figure 12.7.2 Layering patterns of absorber (partially reflected)/dielectric (low re-fractive)/reflector (inner reflector)/dielectric/absorber used to create the optical effect of color-shifting ink.

The effect of active spots on the polarization of adsorbed molecules by a dielectric absorbent (Sec. V,6) is very great. The nature of the active spots is the same as of those which affect the attraction of ions or dipoles. Edges or corners of crystals, other crystallographic faces, and especially those places where the growth of individual crystal faces stopped, as well as lattice disturbances in the surface, will be active. 

Lemarquis, F. Marchand, G. (1999). Analytical achromatic design of metal-dielectric absorbers, Appl, Opt. Vol. 38, p>p. 4876 —4884. 

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. 

S. M. Abbas, M. Chandra, A. Verma, R. Chatterjee, T. C. God, Complex Permittivity and Microwave Absorption Properties of a Composite Dielectric Absorber. Compos Part A 2006,37, 2148-2154. 

Tables of computed characteristic functions of scattering on dielectric, absorbing, and polydisperse particles have been compiled (van de Hulst, 1957 Shifrin and Telmanovich,...

Electrical Properties. Polytetrafluoroethylene is an excellent electrical insulator because of its mechanical strength and chemical and thermal stabihty as well as excellent electrical properties (Table 6). It does not absorb water and volume resistivity remains unchanged even after prolonged soaking. The dielectric constant remains constant at 2.1 for a temperature range of —40 to 250°C and a frequency range of 5 Hz to 10 GHz. 

The dielectric constant of a compound is increased by small amounts of absorbed water hence wire insulation for communications generally must have a dielectric constant as stable as possible in the presence of water or moisture. 

The dielectric properties of most foods, at least near 2450 MH2, parallel those of water, the principal lossy constituent of food (Fig. 1). The dielectric properties of free water are well known (30), and presumably serve as the basis for absorption in most foods as the dipole of the water molecule interacts with the microwave electric field. By comparison, ice and water of crystaUi2ation absorb very Httie microwave energy. Adsorbed water, however, can retain its Hquid character below 0°C and absorb microwaves (126). 

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). 

The electrical-insulating and dielectric properties of the pure EPM/EPDM are excellent, but in compounds they are also strongly dependent on the proper choice of fillers. The electrical properties of vulcanizates are also good at high temperatures and after heat-aging. Because EPM/EPDM vulcanizates absorb Htde moisture, their good electrical properties suffer minimally when they are submerged in water. 

Electrical and Mechanical Properties. Electrical properties include dielectric strength, dielectric constant, dissipation factor, and volume resistivity these properties can change with temperature and absorbed water. 

This gas is electronegative and its molecules quickly absorb the free electrons in the arc path between the contacts to form negatively charged ions. This apparent trapping of the electrons results in a rapid build-up of dielectric strength after a current zero. The detailed. sequence of arc extinction may be summarized as follows. 

In terms of environmental exposure, water and humidity must be carefully evaluated in electrical applications. In general, if a plastic absorbs a significant amount of water, the electrical resistivity drops. As examples this is the case for nylons and phenolic. Care must be used in selecting a dielectric to insure that the electrical properties such as the insulation resistance and dielectric strength, as well as other electrical properties are adequate under the conditions of field use, particularly if this involves exposure to high humidity conditions. Temperature also causes changes in most electrical products. 

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