Dielectric constant 2 temperature variation

The values of (kA/D) and fi obtained from the temperature dependence of the pitch in the EDC-dioxane system show a linear change with the solvent composition in the same manner as the twisting power. For the EDC-m-cresol system which shows a quadratic variation of the twisting power with the solvent composition, a quadratic variation in kA/D and S is also observed. As S is related to the dielectric constant, the variation of the twisting power with solvent composition is related to the change of the dielectric constant with solvent composition in mixed solvents. A linear change of the dielectric constant will occur in the EDC-dioxane system, while a quadratic change will be expected in the EDC-m-cresol system. 

Furthermore, the dielectric constant is not really a constant. As just implied, the dielectric constant will vary with frequency. It will also vary with temperature and humidity. So besides test method, the frequency, temperature, and humidity conditions must also be considered. Last, even with the same material type, variations in resin content (resin-to-reinforcement ratio) also affect the dielectric constant.These variations will be further discussed in Chap. lO.Table 8.7 shows dielectric constants for some common fiberglass (E-glass) reinforced materials at 50 percent resin content. 

Figure 15.9. The variation of dielectric constant with temperature and frequency (Perspex) (the lines join points of equal dielectric constant). (Reproduced by permission of ICI)...

The satisfactory result shown in Table 12 suggests that one might give a more detailed and quantitative discussion of the variation with temperature. If we are to do this, we need some standard of comparison with which to compare the experimental results. Just as wq compare an imperfect gas with a perfect gas, and compare a non-ideal solution with an ideal solution, so we need a simple standard behavior with which to compare the observed behavior. We obtain this standard behavior if, supposing that. /e is almost entirely electrostatic in origin, we take J,np to vary with temperature as demanded by the macroscopic dielectric constant t of the medium 1 that is to say, we assume that Jen, as a function of temperature is inversely proportional to . For this standard electrostatic term we may use the notation, instead of... 

FIGURE 31.14 (a) Variation of the dielectric constant with temperature at different radiation doses, (b) Variation of the dielectric loss factor with temperature at different radiation doses. (From Banik, I., Chaki, T.K., Tikku, V.K., and Bhowmick, A.K., Angew. Makromol. Chem., 263, 5, 1998. With permission.)... 

The dipole moment of a molecule can be obtained from a measurement of the variation with temperature of the dielectric constant of a pure liquid or gaseous substance. In an electric field, as between the electrostatically charged plates of a capacitor, polar molecules tend to orient themselves, each one pointing its positive end toward the negative plate and its negative end toward the positive plate. This orientation of the molecules partially neutralizes the applied field and thus increases the capacity of the capacitor, an effect described by saying that the substance has a dielectric constant greater than unity (80 for liquid water at 20°C). The dipole moments of some simple molecules can also be determined very accurately by microwave spectroscopy. 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

Some important dielectric behavior properties are dielectric loss, loss factor, dielectric constant, direct current (DC) conductivity, alternating current (AC) conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties as a function of frequency, composition, voltage, pressure, and temperature. 

One of the important characteristics of ferroelectrics is that the dielectric constant obeys the Curie- Weiss law (equation 6.48), similar to the equation relating magnetic susceptibility with temperature in ferromagnetic materials. In Fig. 6.55 the temperature variation of dielectric constant of a single crystal of BaTiOj is shown to illustrate the behaviour. Above 393 K, BaTiOj becomes paraelectric (dipoles are randomized). Polycrystalline samples show less-marked changes at the transition temperature. 

The ion product of water is the product of the molality of the hydrogen and hydroxide ions, A",. = mH >ntjn The ion product increases with temperature to 2501C and then declines. The initial increase is the temperature effect, and the later decline is on account of the decline in the dielectric constant of water. This variation means that neutral pH, which is the square root of the ion product, varies with temperature. 

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