of a ZnO block is very small compared with the physical thickness. Microscopic photography measurements showed that the ratio of sample thickness to dielectric thickness was about 1000. The direct effect of this reduced thickness would be the increase of the observed capacitance. Consequently, the permittivity of the material would not necessarily be as high as the calculated values from the experimental data. The nature of the additives is clearly shown to influence the measured values of both capacitance and dissipation factor [50, 115]. A more plausible explanation is based on the existence of depletion layers in the ZnO grains adjacent to the intergranular layer and the trapping of electrons at the interface. The net effect of the former is the rise of the material capacitance [38]. Extensive characterisation work [38, 42, 50, 115, 116] used bridges, Q-meters and
transmission line techniques over a wide range of frequency (30–108 Hz). In order to validate the application of dielectric theory, the measurements were performed at very low stresses, well below the knee of conduction, where the conduction in ZnO material is linear. A decreasing dielectric constant with increasing frequency was measured (Figure 5.17). Measurements at higher frequencies (1 MHz–1 GHz) on a sample 0.162 cm thick and 0.305 cm in diameter confirmed a relative permittivity for the material of about 900, and microwave transmission line techniques revealed an inductance value of about 0.35 nH associated with the body of the ZnO sample. The dielectric constant measured with polarisation current techniques [116] shows a decrease with increasing frequency and a strong temperature dependence at very low frequencies below 100 Hz. The dissipation factor, however, was found first to decrease with increasing frequency,to a minimum between 1–10 kHz, and then increased to reach a peak at around 300 kHz. Finally, it decreased for higher frequencies. The observed peak is reminiscent of a typical broadened Debye resonance [117]. In this range of frequency, tan δ
of a ZnO block is very small compared with the physical thickness. Microscopic photography measurements showed that the ratio of sample thickness to dielectric thickness was about 1000. The direct effect of this reduced thickness would be the increase of the observed capacitance. Consequently, the permittivity of the material would not necessarily be as high as the calculated values from the experimental data. The nature of the additives is clearly shown to influence the measured values of both capacitance and dissipation factor [50, 115]. A more plausible explanation is based on the existence of depletion layers in the ZnO grains adjacent to the intergranular layer and the trapping of electrons at the interface. The net effect of the former is the rise of the material capacitance [38]. Extensive characterisation work [38, 42, 50, 115, 116] used bridges, Q-meters and
transmission line techniques over a wide range of frequency (30–108 Hz). In order to validate the application of dielectric theory, the measurements were performed at very low stresses, well below the knee of conduction, where the conduction in ZnO material is linear. A decreasing dielectric constant with increasing frequency was measured (Figure 5.17). Measurements at higher frequencies (1 MHz–1 GHz) on a sample 0.162 cm thick and 0.305 cm in diameter confirmed a relative permittivity for the material of about 900, and microwave transmission line techniques revealed an inductance value of about 0.35 nH associated with the body of the ZnO sample. The dielectric constant measured with polarisation current techniques [116] shows a decrease with increasing frequency and a strong temperature dependence at very low frequencies below 100 Hz. The dissipation factor, however, was found first to decrease with increasing frequency,to a minimum between 1–10 kHz, and then increased to reach a peak at around 300 kHz. Finally, it decreased for higher frequencies. The observed peak is reminiscent of a typical broadened Debye resonance [117]. In this range of frequency, tan δ
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