This article presents results on the DC conductivity of moist pressboard impregnated with insulating oil and its dependence on the moisture and temperature of the samples. The transfer of charges was found to take place because of electron hopping between potential wells produced by water nanodrops. The average dimension of the nanodrops was around 2.32 nm, forming approximately 220 water molecules.
The present paper investigates conductivity r for a wide temperature range of 2 K < T < 300 K in the nanocomposite (FeCoZr) x (Al 2 O 3 ) 1Àx films (1-6 lm thicknesses, 0.30 < x < 1.00) which were sputtered on glass-ceramic substrates from the compound target in vacuum chamber filled with Ar (set 1 films) or Ar + O 2 gas mixture (set 2 films). TEM and HRTEM images revealed that granular films of set 1 containing nanosized crystalline metallic bcc a-FeCo-based nanoparticles embedded in the amorphous Al 2 O 3 matrix. In case of Ar + O atmosphere of deposited a-FeCo-based crystalline ''cores'' with stabilized sizes were covered with an amorphous FeCo-based oxide ''shells''. The r(T) dependences were found to be in agreement with this phase structure. They show transition of the set 1 films r(T) curves with x from tunneling (hopping) behavior below the percolation threshold x < x C % 0.46 to metallic one for x > x C . Moreover, as opposed to such behavior, for the set 2 films hopping mechanism of r(T) was revealed far beyond x C due to the formation of semiconducting FeCo-based oxide ''shells'' separating electrically contacting FeCoZr cores.
The paper presents an analysis of test results by Frequency Domain Spectroscopy (FDS) of the loss tangent (tgd) in electrotechnical pressboard impregnated with insulating oil and containing nanoparticles of water depending on the frequency of alternating current, sample temperature and degree of moisture. We found tgd reduction in areas of low and ultra-low frequency. It is associated with relaxation due to the hopping conductivity (tunnelling) of electrons between moisture nanoparticles occurring in the cellulose impregnated with insulating oil. In frequency areas close to the local minimum of tgd and higher, relaxation does not depend on the moisture content and is associated with other polar molecules which are part of the cellulose. We developed a new method converting experimental frequency dependence of the loss angle tangent, measured by the FDS method to the reference temperature of 293 K (20°C), using the exponential dependence of the relaxation time on the temperature, was developed. The activation energy of the relaxation time was determined based on the loss angle tangent of moist electrotechnical pressboard impregnated with insulating oil. It was found that the variation in moisture content in the composite cellulose-mineral oil-water nanoparticles do not cause changes in the activation energy of the relaxation time. The conversion of experimental frequency dependence of the loss angle tangent determined by the FDS method to the reference temperature of 293 K (20°C) eliminates the temperature dependence in the areas of ultra-low and low frequencies occurring in the runs made directly on the basis of the measurement results. After calculating the frequency dependence of the loss angle tangent to the reference temperature, all that remains is its dependence on the moisture content.
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