04_dielectrci_spacechg - Montanari: Dielectric Material...

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Montanari: Dielectric Material Properties In © estigated Through Space 56 Dielectric Material Properties Investigated Through Space Charge Measurements Gian Carlo Montanari Department of Electrical Engineering High Voltage and Material Engineering Laboratory University of Bologna, 40136 Bologna, Italy ABSTRACT This paper investigates means of exploitation of space charge measurement results in order to achieve information about chemical, physical and microstructural properties of dielectric materials. The derivation of quantities as mean volume density of space charge, threshold for dc space charge accumulation, apparent-trap controlled mobility is discussed and various applications to dielectric materials are shown. In particular, the differences of trapping behavior of polyethylene-based materials, as low-density polyethylene, cross-linked polyethylene and high-density polyethylene, the effect of additives containing polar groups, the evaluation of cel - lular polymeric electrets are discussed on the basis of the results of space charge measurements and the relevant extracted quantities. Index Terms — Space charge measurements, material evaluation, dielectric properties. 1 INTRODUCTION LECTRICAL measurements have shown to be E able to provide useful information on physical, chem- ical and microstructural properties of dielectric materials. In particular, charging-discharging current measurements have been performed for many years to characterize injec- wx tion and transport processes of many materials 1 ± 7. Direct or indirect indications about material morphol- ogy and chemical properties were obtained from conduc- tion and discharging current. For example, theories based, Ž e.g. on space charge accumulation space charge limited . current, SCLC 8,9 allow deduction of apparent mobility of charge carriers and trap depth distribution. Further in- formation about morphology can be achieved from dielec- tric spectroscopy, that can derive from bridge measure- ments and r or from frequency transformation of discharg- Ž ing current characteristics using, e.g. Fourier, Hamon, . Kramers-Kronig transforms 10 ± 14 . In such a way, indi- rect indications about the extent of crystalline-amorphous interfaces, the effect of contaminants, additives or fillers can be guessed. Arrhenius or Eyring models applied to temperature dependence of conduction current or polar- ization peaks lead to estimation of the activation energy Manuscript recei ® ed on 20 September 2002, in final form 6 May 2003. of reactions associated to charge conduction, thus provid- ing indirect insights into the behavior of chemical species present in the investigated dielectrics 15 ± 17 . The quantities thus derived, e.g. conduction current, apparent mobility, polarization peaks are obviously sensi- tive to chemical-physical and microstructural changes, so that they can become also useful ageing markers for elec- trical insulation 7,18,19 .
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04_dielectrci_spacechg - Montanari: Dielectric Material...

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