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Unformatted text preview: ABSORPTION AND DESORPTION CURRENTS The response of a linear system to a frequency dependent excitation can be transformed into a time dependent response and vice-versa. This fundamental principle covers a wide range of physical phenomena and in the context of the present discussion we focus on the dielectric properties s' and e". Their frequency dependence has been discussed in the previous chapters, and when one adopts the time domain measurements the response that is measured is the current as a function of time. In this chapter we discuss methods for transforming the time dependent current into frequency dependent e' and s". Experimental data are also included and where possible the transformed parameters in the frequency domain are compared with the experimentally obtained data using variable frequency instruments. The frequency domain measurements of & ' and &" in the range of 10~ 2 Hz-10 GHz require different techniques over specific windows of frequency spectrum though it is possible to acquire a 'single' instrument which covers the entire range. In the past the necessity of using several instruments for different frequency ranges has been an incentive to apply and develop the time domain techniques. It is also argued that the supposed advantages of the time domain measurements is somewhat exaggerated because of the commercial availability of equipments covering the range stated above 1 . The frequency variable instruments use bridge techniques and at any selected frequency the measurements are carried out over many cycles centered around this selected frequency. These methods have the advantage that the signal to noise ratio is considerably improved when compared with the wide band measurements. Hence very low loss angles of ~10 \JL rad. can be measured with sufficient accuracy (Jonscher, 1983). The time domain measurements, by their very nature, fall into the category of wide band measurements and lose the advantage of accuracy. However the same considerations of accuracy apply to frequencies lower than 0.1 Hz which is the lower limit of ac bridge techniques and in this range of low frequencies, 10" 6 </< 0.1 Hz, time domain measurements have an advantage. Use of time-domain techniques imply that the system is linear and any unexpected non-linearity introduces complications in the transformation techniques to be adopted. Moreover a consideration often overlooked is the fact that the charging time of the dielectric should be large, approximately ten times (Jonscher, 1983), compared with the discharging time. The frequency domain and time domain measurements should be viewed as complementary techniques; neither scheme has exclusive advantage over the other....
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This note was uploaded on 03/03/2010 for the course POWER 332 taught by Professor Dr during the Spring '10 term at Ain Shams University.

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