Time domain methods (ow and high frequency approaches) stormer ratio arm bridge Q-meter method Quasi Broad band t Optical methods and interferometers Log(f) 止的 12 ITHz 300 FIGURE 55.3 Frequency range of various dielectric test methods [Bartnikas, 198 resonance that arises from a displacement and vibration of atoms relative to each other, while an electronic resonance absorption effect occurs over the ultraviolet frequencies as a consequence of the electrons being orced to execute vibrations at the frequency of the external field. The characterization of dielectric materials must be carried out in order to determine their properties for various applications over different parts of the electromagnetic frequency spectrum. There are many techniques and methods available for this purpose that are too numerous and detailed to attempt to present here even in a cursory manner. However, Fig. 55. 3 portrays schematically the different test methods that are commonly used to carry out the characterization over the different frequencies up to and including the optical regime. a direct relationship exists between the time and frequency domain test methods via the Laplace transforms The frequency response of dielectrics at the more elevated frequencies is primarily of interest in the electrical communications field. In contradistinction for electrical power generation, transmission, and distribution, it is the low-frequency spectrum that constitutes the area of application. Also, the use of higher voltages in the electrical power area necessarily requires detailed knowledge of how the electrical losses vary as a function of the electrical field. Since most electrical power apparatus operates at a fixed frequency of 50 or 60 Hz, the main variable apart from the temperature is the applied or operating voltage At power frequencies the dipole losses are generally very small and invariant with voltage up to the saturation fields which exceed substantially the operating fields, being in the order of 107 kV cm-l or more. However, both the space charge polarization and onic losses are highly field-dependent. As the electrical field is increased, ions of opposite sign are increasingly <s gregated; this hinders their recombination and, in effect, enhances the ion charge carrier concentration. As dissociation rate of the ionic impurities is further augmented by temperature increases, combined rises in temperature and field may lead to appreciable dielectric loss. Thus, for example, for a thin liquid film bounded by two solids, tan8 increases with voltage until at some upper voltage value the physical boundaries finally limit the amplitude of the ion excursions, at which point tan& commences a downward trend wit (Boning-Garton effect). The interfacial or space charge polarization losses may evince a rather intricate field dependence, depending upon the manner in which the discrete conductivities of the contiguous media change c 2000 by CRC Press LLC© 2000 by CRC Press LLC resonance that arises from a displacement and vibration of atoms relative to each other, while an electronic resonance absorption effect occurs over the ultraviolet frequencies as a consequence of the electrons being forced to execute vibrations at the frequency of the external field. The characterization of dielectric materials must be carried out in order to determine their properties for various applications over different parts of the electromagnetic frequency spectrum. There are many techniques and methods available for this purpose that are too numerous and detailed to attempt to present here even in a cursory manner. However, Fig. 55.3 portrays schematically the different test methods that are commonly used to carry out the characterization over the different frequencies up to and including the optical regime. A direct relationship exists between the time and frequency domain test methods via the Laplace transforms. The frequency response of dielectrics at the more elevated frequencies is primarily of interest in the electrical communications field. In contradistinction for electrical power generation, transmission, and distribution, it is the low-frequency spectrum that constitutes the area of application. Also, the use of higher voltages in the electrical power area necessarily requires detailed knowledge of how the electrical losses vary as a function of the electrical field. Since most electrical power apparatus operates at a fixed frequency of 50 or 60 Hz, the main variable apart from the temperature is the applied or operating voltage. At power frequencies the dipole losses are generally very small and invariant with voltage up to the saturation fields which exceed substantially the operating fields, being in the order of 107 kV cm–1 or more. However, both the space charge polarization and ionic losses are highly field-dependent. As the electrical field is increased, ions of opposite sign are increasingly segregated; this hinders their recombination and, in effect, enhances the ion charge carrier concentration. As the dissociation rate of the ionic impurities is further augmented by temperature increases, combined rises in temperature and field may lead to appreciable dielectric loss. Thus, for example, for a thin liquid film bounded by two solids, tand increases with voltage until at some upper voltage value the physical boundaries begin to finally limit the amplitude of the ion excursions, at which point tand commences a downward trend with voltage (Böning–Garton effect). The interfacial or space charge polarization losses may evince a rather intricate field dependence, depending upon the manner in which the discrete conductivities of the contiguous media change FIGURE 55.3 Frequency rangse of various dielectric test methods [Bartnikas, 1987]