Eugen R. Neagu

Electrical conductivity studies in nylon 11

The results of electrical and dielectric studies in semicrystalline nylon 11 (degree of crystallinity 62%) by means of dielectric relaxation spectroscopy (DRS) in the frequency range 10−2–106 Hz at temperatures between the glass transition temperature of about 40 °C and the melting temperature of about 190 °C are reported. Emphasis is put on the investigation of electrical conductivity and conductivity effects. Additional information on dc conductivity is obtained from triangular voltage measurements. The results of DRS were analyzed within the formalism of permittivity (ac conductivity), impedance, and electric modulus. At temperatures higher than about 100 °C the spectra are dominated by dc conductivity and conductivity current relaxation. Despite high values of dc conductivity (σdc=10−6–10−5 S/m at 170 °C), electrode effects and space charge polarization, giving rise to high values of e′ at low frequencies, are negligible in the whole temperature range of measurements. This feature, as well as the rath...

Analysis of the thermally stimulated discharge current around glass-rubber transition temperature in polyethylene terephthalate

The nature of the thermally stimulated discharge current (TSDC) for polyethylene terephthalate samples in the temperature range from room temperature to above glass-rubber transition temperature of the amorphous phase is analyzed. The well conditioning of the sample is strictly necessary in order to have a good reproducibility and accuracy of results. A main peak was observed whose maximum temperature moves towards a lower value with the decreasing of the amount of charge that flows through the sample during polarization. The peak position changes as well, if the sample is polarized in air or in oxygen and the nature of change is more important in the case of oxygen. The shape of the peak is complex and at least four shoulders have been identified around 85, 90, 105, and 125 °C using the cleaning technique. The activation energy tends to increase with repetition of the TSDC runs, in the glass-rubber transition temperature range, in the case when the cleaning technique is used for the peaks separation. For...

Dielectric studies of dipolar relaxation processes in nylon 11

The dielectric properties of semicrystalline nylon 11 (mixture of γ and α phase, degree of crystallinity 62%) were investigated in wide ranges of frequency and temperature by broadband dielectric relaxation spectroscopy, thermally stimulated depolarization current techniques and triangular voltage measurements. The main interest was focused on characterizing the relaxation processes in nylon 11, so measurements were, in general, limited to temperatures lower than about 100 °C. The secondary γ and β relaxations, in order of increasing temperature in isochronal measurements, the primary α relaxation, associated with the glass transition of the amorphous polymer, and a process at about 95 °C, related to a structural phase transformation from triclinic α phase to pseudo-hexagonal γ phase, were observed and studied in detail. Absorption of water was found to have a significant effect on the secondary relaxations, giving, in particular, rise to splitting of the β relaxation into two relaxations, β1 and β2 in order of decreasing temperature in isochronal measurements. The γ relaxation is assigned to the motion of methylene sequences involving adjacent dipolar amide groups and the β relaxation to motion of water-polymer complexes of different molecular configurations.

Evaluation of the dielectric parameters from TSDC spectra: application to polymeric systems

Abstract The Thermally Stimulated Discharge Current (TSDC) technique is widely used for the study of main and secondary dielectric relaxations in polymers. The TSD current is described by different equations that can be arranged in a unique three-parameters (the activation energy W, A and B) general form. The physical meaning of A and B depends on the origin of the discharge currents. In this paper a method is proposed to obtain these parameters by fitting the experimental data with the analytical expression of the current, in the range around the maximum. Simulations were carried out to underline the relative importance of the parameters. A method is proposed for the decomposition of experimentally determined complex bands into a limited number of elementary peaks, each of them characterized by average values for W and B. The errors resulting from different approximations used in the analytical current expression or by the utilization of various expressions for the relaxation time are analyzed. The method is applied for the analy-sis of the TSDC spectra in the glass-rubber transition temperature regions of PET and PMMA, yielding several peaks characterized by narrow distributions of W (ΔW≈± 0.06 eV).

Nonisothermal and isothermal discharging currents in polyethylene terephthalate at elevated temperatures

The thermally stimulated discharge current and the isothermally final discharging current have been measured, in vacuum and in different ambient gases for “as-received” polyethylene terephthalate specimens, in order to understand the nature of the origin of the released current in the temperature range from glass–rubber transition temperature up to 220 °C. The behavior of the samples thermally treated in oxygen, in nitrogen and in ambient air was analyzed, the gases have been used for detecting the localized states in the material. The current spectrum is determined by the space-charge existing in the as-received sample, and by the adsorbed and/or absorbed gases and water vapors. The movement of the ions, resulting from the interaction of the adsorbed and absorbed gases with the parasitic space charge, in the field produced by the space charge, is responsible for observed change in polarity of the current during nonisothermal and/or isothermal measurements and for the appearance of the ρ or space-charge p...

Combined Isothermal and Non-Isothermal Techniques to Analyze Charge Trapping and Stability in Insulating Materials

Taking into account the time-consuming trapping and detrapping of electric charge in insulating materials, a sequential protocol for the investigation the trapping sites and trapped charge stability is proposed. The method is demonstrated for Teflon FEP-A. Elemental peaks are obtained whose maxima shift with increasing field from 456 to 486 K and are characterized by activation energies that increase from 1.1 to 1.9 eV. The relaxation time of the trapped charge, at 523 K, increases as the amount of charge stored in the material decreases and explains the good charge stability for Teflon FEP-A electrets.

A new method for analysis of isothermal discharging current

Abstract Significant results concerning very low frequency dielectric relaxation are obtained by transforming the measured time data into the frequency domain. The key point of this transformation is the relation between the time range and the frequency range. An accepted relation is that proposed by Hamon, for the case when the experimental data can be approximated by the Curie–von Schweidler function, although there are disagreements between the theory and the calculated results. The theory and the experimental results are in good agreement if the isothermal current is approximated by an exponential decay function or by the Kohlrausch–Williams–Watts function. A complete analysis of the data can be performed only if the isothermal current can be approximated with one or a sum of two exponential decay functions and a new method for evaluating experimental data is proposed. The new method allows the separation of relaxation processes and the data can be analyzed using a simple model and/or the Fourier transform. The experimental data obtained for polyethylene terephthalate are analyzed and two relaxation processes at very low frequency are shown. The results are in good agreement with those obtained from alternating current or from thermally stimulated discharge current measurements. By using the analytical expression of the current and the Fourier transform the errors determined by the fact that any experiment will cover a finite range of time are estimated. In order to obtain a correct characterization of the relaxation processes the measuring range of time has to be strictly correlated with the relaxation time of the processes.

Electrical conduction and space charge trapping in highly insulating materials

The electrical conduction in low-density polyethylene (LDPE) was investigated in the temperature range from 20?C to 95?C and for applied fields up to 12?MV?m?1. The objective was to study the effects of the space charge (SC) accumulated in the material on the low and medium field quasi-steady-state conductivity in LDPE. Very long time (a week or longer) isothermal current measurements were carried out to focus on the situation when the SC effects are dominant. Complementary results were obtained using the isothermal discharge current, the final thermally stimulated discharge current and the isothermal final discharge current measurements. The results are explained by taking into account the constraint imposed by the trapped SC on charge injection, trapping and transport processes.

Space-charge-controlled conductivity in low-density polyethylene

We studied the dc conductivity of low-density polyethylene in the temperature range from 20 to 90 °C for electric fields from 4 kV m−1 to 20 MV m−1. The isochronal data measured after 1 h are in agreement with the literature. For a long time (6 days), the conduction mechanism is dominated by the space charge trapped in the material. The current does not attain a steady-state value after 29 days at 50 °C and 8 MV m−1. It oscillates continuously, the variations are less regular and the conductivity decreases significantly as the sample thickness increases. The activation energy decreases from 0.8 to 0.58 eV when determined from long time measurements. An explanation is proposed considering the constraints imposed by the trapped space charge on charge injection and transport. We propose to call the observed mechanism space-charge-controlled conductivity.We studied the dc conductivity of low-density polyethylene in the temperature range from 20 to 90 °C for electric fields from 4 kV m−1 to 20 MV m−1. The isochronal data measured after 1 h are in agreement with the literature. For a long time (6 days), the conduction mechanism is dominated by the space charge trapped in the material. The current does not attain a steady-state value after 29 days at 50 °C and 8 MV m−1. It oscillates continuously, the variations are less regular and the conductivity decreases significantly as the sample thickness increases. The activation energy decreases from 0.8 to 0.58 eV when determined from long time measurements. An explanation is proposed considering the constraints imposed by the trapped space charge on charge injection and transport. We propose to call the observed mechanism space-charge-controlled conductivity.

The determination of relaxation parameters and their distributions using thermally stimulated discharge current measurements

For any elemental peak obtained by the thermally stimulated depolarization current technique, there is a relationship between the activation energy W, the pre-exponential factor τ0, the temperature of the maximum current Tm and the heating rate b. This relationship can yield useful information concerning the values and the distribution of the relaxation parameters. Numerical simulations, using concrete experimental data obtained for nylon 11, are used to demonstrate the analysis for organic dielectrics. Lower limits for the incertitude intervals of W and τ0 represent the natural or minimum incertitude intervals expected for an elemental relaxation process. Taking advantage of the fact that the natural incertitude interval in activation energies is ΔWkTm (k is Boltzmanns constant), the natural incertitude interval for τ0 is deduced as Δτ0τ0. For example, assuming Tm = 300 K and Δτ0 = 0, the interval ΔTm for two resolved neighbour elemental peaks, in other words the interval in which Tm can have values as W varies in the limits on the natural incertitude interval, increases from 7.1 to 11.9 K as W decreases from 1.05 to 0.59 eV. An experimental thermogram can be decomposed into a limited number of elemental peaks having W and τ0 distributed in the limits of the natural or minimum incertitude intervals (ΔWkTm and Δτ0τ0). The distribution function for a relaxation parameter cannot be determined unambiguously for the case when the width of the distribution is comparable with the natural standard deviation for the given conditions. Only one parameter or only one distribution must be avoided considering any analysis as variable.