Dielectric measurement, room-temperature

Values of tan can be calculated from conductance measurements or obtained directly using modern capacitance bridges. Polymers are comparatively lossy dielectrics. Typical room-temperature values of tan dg for PVDF range from 0.015 to 0.02 at 1 kHz and for VDF TrFE from 0.015 to 0.025, compared with values of 10 for some ferroelectric ceramics such as lithium tantalate and lead germanate. 

The specimen may be a sheet of any size convenient to test, but should have uniform thickness. The test may be run at standard room temperature and humidity, or in special sets of conditions as desired. In any case, the specimens should be preconditioned to the set of conditions used. Electrodes are applied to opposite faces of the test specimen. The capacitance and dielectric loss are then measured by comparison or substitution methods in an electric bridge circuit. From these measurements and the dimensions of the specimen, dielectric constant and loss factor are computed. 

The absorption of moisture critically affects other important resin properties, particularly those associated with low-dielectric and thermomechanical applications. Results of a 96-h boiling water immersion test are presented in Table 2.2. The moisture absorbed decreased substantially with fluoromethylene chain length from n = 3 to n = 6, followed by only modest decreases for n = 8 and 10. This latter behavior was somewhat unexpected and may be the effect of decreased cross-link density counteracting the increased fluorine content. These 100°C measurements are just above the glass transition and the situation may be different at room temperature. These measurements are in progress. 

In order to prepare the samples for dielectric-constant measurements, etherdiacrylate 6 was mixed with a trace amount of AIBN at room temperature in a cylindrical donut mold made from General Electric RTV 11 silicon molding compound. The donuts had an outer diameter of 7.0mm, an inner diameter of 3.0 mm, and a thickness of 3.0 mm the semisolid triacrylate 3 was mixed with a... 

The temperature changes of the bulk chip upon microhotplate heating were assessed. The chip was mounted in a standard ceramic DIL package. The discrepancy between ambient temperature and the bulk-silicon chip temperature was measured as a function of the microhotplate temperature and is shown in Fig. 5.20. The measurement was done at room temperature, and the control voltage was increased in steps of 25 mV thus heating the membrane from room temperature to 500 °C. The maximum discrepancy between bulk chip temperature and ambient temperature was less than 4 °C, which demonstrates the excellent thermal isolation between the microhotplate on the dielectric membrane and the bulk substrate. 

To measure the equilibrium in low-dielectric medium requires quite a strong base. Unless this condition is met, measurable fractions of the octahedral form are not seen. Thus, for example, CoCl2 in quinoline shows only the spectrum of the tetrahedral configuration, although pyridine solvent shows both octahedral and tetrahedral CoCl2 spectra (6). Primarily for this reason, the bases most used have been pyridine and substituted pyridines. The equilibrium has been studied in pyridine as a function of temperature for CoCl2 (8, 39). In pure pyridine the octahedral form predominates at room temperature, and only at temperatures approximating 60°C are the two forms equally abundant (8). 

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,... 

G. R. Fleming In simple liquids such as methanol and ethanol there is no evidence for relaxation times slower than expected from dielectric measurements. Glasses at room temperature clearly show time scales that are infinite on our measurement time scale. In complex liquids such as glycerol-water mixtures and ethylene glycol, we may observe time scales that are longer than dielectric relaxation, but further studies are required to confirm this. 

Hydrated Zeolites. The zeolitic pellets are hydrated by equilibration at atmospheric moisture content. The cell is immersed in liquid air, and a minimum equilibrium temperature of — 120°C was obtained. At that temperature the conductivity and capacity of the samples are measured over the frequency range 200-107 Hz. After eliminating the cooling liquid, the temperature rises slowly (0.5°C/min). Measurements are performed continuously in the same frequency range during the. temperature rise up to room temperature. The results are near-equilibrium values, and the errors are assumed to be the same over the complete temperature range. The same procedure was applied by Mamy for dielectric measurements on montmorillonite 11). 

Butadiene and isoprene have also been studied in tetrahydrofuran (72). At 0° the rates are close to first order in polyisoprenyl or poly-butadienyllithium concentration which indicates that the rate constant for the ion-pair is being measured in the concentration range studied (> 10-s molar). The rate constants at 30°, k9 (butadiene) =1.8 litre/mole sec, kj, (isoprene) = 0.13 litre/mole sec, are appreciably lower than for styrene. For butadiene at —39° a kp value of 6 X 10 2 litre/mole sec can be derived from the results of Spirin (98). This value checks well with that extrapolated from Morton s data. The observed propagation constant for isoprene is rather low and is in fact equal to that of the mono-etherate in solvent mixtures of appreciably lower dielectric constant. At room temperature there is evidence for isomerization of polyisoprenyl-lithium in tetrahydrofuran which becomes particularly marked as the... 

The temperature dependence of the dielectric properties of foods has been extensively measured and reviewed by Bengtsson and Risman (1971) and Buffler (1993). Mudgett et al. (1977) has pioneered the prediction of dielectric properties of foods as a function of constituency and temperature. Prediction of the temperature behavior of dielectric properties is crucial for accurate mathematical modeling of foods. Many workers today still use constant room temperature values or a look-up table at best. In the author s opinion, dielectric prediction of food properties is still a very fertile and useful research field. 

The y Relaxation. In common with many other polymers (8) both PPO and PS display significant loss maxima below room temperature at the frequencies under consideration. Whereas the process responsible for the a loss is at least qualitatively understood in terms of a main chain relaxation associated with the glass transition, y losses can often only tentatively be attributed to specific mechanisms. In PPO, for example, it does not seem unreasonable to propose that the y loss is associated with librations in the two pendant methyl groups this view is somewhat reinforced by the observation that in the dielectric measurements the relaxational strengths of the y and a loss processes are comparable. As the latter can be well interpreted (6) in terms of a dipolar relaxation of the main chain in which the entire dipolar contributions arise from the methyl groupings, it seems plausible to assume that the same dipoles are responsible for the y loss mechanism. In polystyrene there is a similar... 

The preceding discussion assumed a pure liquid was used for the measurement. Most molecules of interest, however, are not in the liquid state at room temperature. In this case it is common to dissolve the compound in an appropriate solvent and conduct the measurement. Contributions to the second harmonic signal are therefore obtained from both the solvent and solute. Since r and the local field factors that are related to e and n, (the dielectric constant and refractive index respectively) are concentration dependent, the determination of p for mixtures is not straightforward. Singer and Garito (15) have developed methods for obtaining r0, eQ, and nQ, the values of the above quantities at infinite dilution, from which accurate values for p can be obtained in most cases. 

In other words, since for solvents with H-bonding ability (methanol and water) the aN of the nitroxide radical is shifted to higher values because of the influence of one or more hydrogen bonds between the solute and the solvent, it becomes necessary to build a model in which nonspecific effects are described in terms of continuum polarizable medium with a dielectric constant typical of the protic solvent under study, whereas specific effects are taken into account through an explicit hydrogen-bonded complex between the radical and some solvent molecules. Figure 2.6 reports the aN values for the complexes formed by TEMPO with phenol, methanol, and water measured experimentally at room temperature, and computed in the gas phase and in solution. The values computed in solution fit the experimental data quite well. 

Below we give the dielectric characteristics of polymethyldimethylsi-lazane film dried for 24 hours at room temperature (the measurements... 

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