Dielectric permittivity versus temperature

The third relaxation process is located in the low-frequency region and the temperature interval 50°C to 100°C. The amplitude of this process essentially decreases when the frequency increases, and the maximum of the dielectric permittivity versus temperature has almost no temperature dependence (Fig 15). Finally, the low-frequency ac-conductivity ct demonstrates an S-shape dependency with increasing temperature (Fig. 16), which is typical of percolation [2,143,154]. Note in this regard that at the lowest-frequency limit of the covered frequency band the ac-conductivity can be associated with dc-conductivity cio usually measured at a fixed frequency by traditional conductometry. The dielectric relaxation process here is due to percolation of the apparent dipole moment excitation within the developed fractal structure of the connected pores [153,154,156]. This excitation is associated with the selfdiffusion of the charge carriers in the porous net. Note that as distinct from dynamic percolation in ionic microemulsions, the percolation in porous glasses appears via the transport of the excitation through the geometrical static fractal structure of the porous medium. 

The dielectric spectra were obtained using the samples with thicknesses ranging between 80 and 150 pm, and in the temperature -130°C to 150°C range, except for the PVDF-h-PVCN block copolymer (-130°C to 100°C) [89]. The values of the dielectric permittivities versus temperature (Figure 20.19) show the existence of two relaxations. At room temperature (100 Hz), the values for PVDF homopolymer, PVDF-h-PAN, PVDF-h-MAN, and PVDF-h-PVCN block copolymers are 6.3, 3.9, 5.0, and 5.5 respectively. 

Three-dimensional plots of both the measured real part s and the imaginary part s" of the complex dielectric permittivity versus frequency and temperature for 20-pm-thickness PS sample are shown in Fig. 17a,b. From the figure, one can identify three distinct processes, marked by I, II, and III, defined as follows ... 

The results are reported of a study of the effects of fillers and vulcanising agents on the physicomechanical and dielectric properties of EPDM. Fillers employed were kaolin, quartz, PVC and talc and the vulcanising agents were TMTD and sulphur/N-cyclohexyl-2-benzothiazyl sulphenamide. The permittivity and dielectric loss versus temperature for the vulcanisates are illustrated and the effects of thermal ageing on the physicochemical properties of the vulcanisates are discussed. 25 refs. EGYPT... 

Fig. 10.2. Dielectric permittivity of Ba Sr/TiOj films versus x and annealing temperature.

These books remain the reference works in measuring dielectric and magnetic constants of homogeneous materials. Methods used at that time were limited in frequency band measurements, permitting only the determination of the complex permittivity and permeability at fixed frequencies and sometimes versus temperature variations. 

Results of dielectric studies of a series of nanocomposites based on a semiaromatic PA-1 ITIO with HAp helped to explain the structure-property relationships [Sender, 2008], For dry, neat PA-llTlO, a symmetric depolarization peak, detected by TSC near Tg measured by DSC, was attributed to dielectric dynamic glass relaxation. By DDS, two distinct high-temperature processes were distinguished when plotting the experimental data points versus HT. The dependence is displayed in Figure 13.2, where the two dashed lines show the complex dielectric permittivity fitted by the Havriliak-Negami equation ... 

FIGURE 20.11 Dielectric spectra of the variation of permittivity (e ) (open symbols) and tan5 (closed symbols) versus temperature at different frequencies for (a) poly(AN-ifaf-MATRIF) (b) poly(AN-ifar-MATRIF) (c) poly(MVCN- faf-MATRIF) copolymers, and (d) poly(MATRIF) homopolymer [107]. 

FIGURE 20.16 Dielectric permittivity e (a) and dissipation factor tanS (b) versus temperature of P4FST homopolymer at various frequencies [88]. 

FIGURE 20.18 Dielectric spectra of the variation of permittivity e versus temperature at 18.8 Hz for P4FST homopolymer and at 12 Hz for poly(TSE-a/f-4FST) copolymer [88]. 

Figure 20.11 exhibits the plots of the permittivity (s ) and the dielectric loss (tanS) versus the temperature at different frequencies of such (co)polymers. 

Figure 20.12 shows a comparison of the permittivity values (e ) and the dielectric losses (tan5) of these materials (copolymers and homopolymer) versus the temperature at 120 Hz. 

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