Microwave dielectric measurements

Shinyashiki, N., Yagihara, S., Arita, I. and Mashimo, S., Dynamics of water in a polymer matrix studied by a microwave dielectric measurement, /. Phys. Chem. B, 1998, 102, 3249. 

In microwave dielectric measurements (> 30 GHz) the dieleclric permittivity and dielectric losses for bound and free water show significantly different magnitudes. Thus, in measurements at high microwave frequencies the contribution from bound water in the dieleclric losses will be negligibly small, and the contribution from the free water fraction can be found. In contrast to the above-mentioned procedures used for calculation of bound water from the relaxation spectrum analysis, this approach will not involve analyses of overlapping relaxation processes and can thus easily be applied to microemulsions having a complex relaxation spectrum. 

Another method is the microwave dielectric measurements, in which the photopolymerization reaction occurs in a micro-wave cavity, and monomer concentration ([M]) is calculated by measuring the change in effeaive dielectric constant (Ae ) with the following relationship ... 

Microwave dielectric measurements confirm the ferroelectric nature of order phases, the dipoles being related to the orientation of the protonic species and not to the proton ordering, according to the small temperature shift on H/D substitution (Table 17.1). The extrapolated Curie-Weiss temperature of the first order I - II transition is located below the actual... 

Rolla et ah, used microwave dielectric measurements to monitor the polymerization process of mono functional n-butyl acrylate as well as 50/50 w/w blends with a difunctional hexane-diol diacrylate that gave highly cross-linked networks. In these real time cure experiments the decreasing acrylate monomer concentration was studied via a linear correlation with the dielectric loss index at microwave frequencies. This correlation is a result of the largely different time scales for dipolar polarization in the monomer on one hand and in the polymerized reaction product on the other hand. 

Complementary data on food and food constituents are obtained by other thermal analysis techniques, such as thermomanometry, thermogravimetry (TG), thermomicroscopy or hot stage microscopy (HSM), differential mechanical (thermal) analysis (DMA or DMTA), titration calorimetry, and microwave dielectric measurements during temperature scan. 

Around lOOX, thermal analysis measurements with open cracibles distinguish among adsorbed, absorbed, and crystal water. Thus, DSC curves, TG curves, or microwave dielectric measurements give information on water content or on the... 

M Ollivon, S Quinquenet, M Seras, M Dehnotte, C More. Microwave dielectric measurements during thermal analysis. Thermochim Acta 725 141-153 (1988). SD Holdsworth. Dehydration of food products. A review. J Food Technol 6 331-370 (1971). 

The microwave spectrum of isothiazole shows that the molecule is planar, and enables rotational constants and NQR hyperfine coupling constants to be determined (67MI41700>. The total dipole moment was estimated to be 2.4 0.2D, which agrees with dielectric measurements. Asymmetry parameters and NQR coupling constants show small differences between the solid and gaseous states (79ZN(A)220>, and the principal dipole moment axis approximately bisects the S—N and C(4)—C(5) bonds. 

Figure 3. Different geometries for microwave conductivity measurements, (a) Sample (black square) at end of microwave guide, (b) sample in microwave resonator, and (c) sample above dielectric microwave spiral. The electrical field E of the microwave is shown schematically.

Collision-induced microwave spectra. Measurements of the dielectric loss by resonant cavity techniques at 9 and 24 GHz were first reported by Birnbaum and Maryott [33], The cavity was at room temperature and filled with carbon dioxide gas at densities up to 100 amagat. The loss, which at not too low frequencies increases as the square of density,... 

Marand, E., Baker, K.R. and Graybeal, J.D., Comparison of reaction-mechanisms ofepoxy-resins undergoing thermal and microwave cure from in situ measurements of microwave dielectric-properties and infrared-spectroscopy, Macromolecules, 1992, 25, 2243. 

Figure 5.9 Planar nonlinear dielectric based thin film varactor structure (top) and planar microstrip resonator with capacitive gap. The microwave dielectric properties of the varactor structure are measured in a flipchip configuration of both substrates with the narrow (few microns) capacitive gap of the varactor structure being placed in the center of the large (500 microns) capacitive gap of the resonator structure (from [16]).

Microwave dielectric loss measurements in the region 1-9 GHz have been reported for beryllocene in different solvents (cyclohexane, decalin, benzene, and 1,4-dioxane) (279). The dipolar nature of beryllocene was confirmed. The dielectric absorption was interpreted to suggest rocking movements of the cyclopentadienyl groups synchronous with oscillation of the beryllium atom between two equivalent positions in an rj5, a structure, as indicated in CII. 

Microwave engineers find it helpful to characterize a microwave dielectric by the product Qf (/ measured in GHz) which, as a rough rule of thumb , is assumed to be constant. The theoretical justification for this is as follows. 

Light and potential modulated microwave reflectivity measurements offer a novel approach to the study of the semiconductor electrolyte interface. Perturbation of the density of electrons and holes in a semiconductor influences the conductivity and hence the imaginary component of the dielectric constant at microwave frequencies. For small perturbations, the change ARm in microwave reflectivity depends linearly on the change in conductivity [27, 28, 75). The application of frequency response analysis to light modulated microwave reflectance is relatively new [30]. Although the technique is analogous to IMPS, it provides additional information. 

Fessenden RW, Carton PM, Shimamori H, Scaiano JC. (1982) Measurement of the dipole moments of excited states and photochemical transients by microwave dielectric absorption. JPhys Chem 86 3803-3811. 

Prohling or imaging of a physical variable, e.g., monitoring of moisture diffusion process through an organic material is readily accomplished with low frequency dielectric sensors. Alternatively, a microwave probe can be used, but the sample penetration depth of the microwave probe would be limited, in many cases, to a few microns, whereas the penetration depth of low frequency dielectric measurements can be controlled by changing the separation between the sensor head electrodes. 

Experimental value (1) 1.4 D (from dielectric measurements).411 (2) 1.50 0.1 D (from Stark-effect measurements of its microwave spectrum).433... 

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