Effective microwave index

The electrical signal propagates partly in air. This reduces the effective microwave index and brings it closer to the optical index. 

The rate of displacing atoms and electrons within a molecule corresponds to optical frequencies, and hence no dielectric dispersion of Mm is normally encountered in or below the microwave range (v < 10 GHz). Since all other polarization effects are not that fast, the polarizabilities a, determine exclusively the electric permittivity at optical frequencies, eop. Thus at sufficiently high frequencies the relations (12) and (13) may be combined with (1) to evaluate a,. For the pure species s consisting of isotropic spherical molecules, a, can be given by the refractive index n, if Maxwell s relation... 

Tab. 6.8. Polyethers from isoidide and 1,8-dimesyloctane. Effect of reaction time on the yields of high-molecular-weight fraction (FP MeOH), low molecular weight fraction (FP Hex), and molecular weight distribution. Mn and Mw are, respectively, the number average and weight average molecular weights of the FP MeOH fraction and the ratio Mw/Mn is the polydispersity index (monomode microwave reactor, 300 W).

The effect of microwave irradiation on chemical reactions is usually described by comparing time needed to obtain a desired yield of final products compared with conventional thermal heating. Research in the area of chemical synthesis has shown potential advantages in the ability not only to drive chemical reactions but to perform them more quickly. In polymer synthesis other factors can be considered, for example molecular weight, polydispersity index, crystallinity, mechanical properties (i.e. strength, elongation, modulus, toughness), and thermal properties... 

D18.3 Dipole moments are not measured directly, but are calculated from a measurement of the relative permittivity, (dielectric constant) of the medium. Equation 18.IS implies that the dipole moment can be determined from a measurement of function of temperature. This approach is illustrated in Example 18.2. In another method, the relative permittivity of a solution of the polar molecule is measured as a function of concentration. The calculation is again based on the Debye equation, but in a modified form. The values obtained by this method are accurate only to about 10%. See the references listed under Furiher reading for the details of this approach. A third method is based on the relation between relative permittivity and refractive index, eqn 18.17, and thus reduces to a measurement of the refractive index. Accurate values of the dipole moments of gaseous molecules can be obtained from the Stark effect in their microwave spectra. 

Such a technique has been shown [26] to work well, for example, for the computation of the refractive index of CH3CN in the microwave and far-infrared. Crawford and co-workers [22-24] have also advocated using a correlation function based on the complex part of the local susceptibility, (introduced to correct for dielectric effects on the optical constants). In that case. 

The electron density in the path of a microwave adsorbs energy and attenuates the transmitted signal. This microwave attenuation can be used to analyze the plasma density. A plasma has an effective index of refraction for microwave radiation. By measuring the phase shift of transmitted/received microwave radiation as it passes through the plasma, the charge density can be determined. Generally the phase shift is determined by interferometric techniques. 

Figure 14.6 Simulated microwave effective index of the coplanar waveguide as a function of substrate dielectric constant. The intersection of the simulated and measimed indices occiu s at 2.72 and corresponds to the dielectric constant of PDMS.

---