Thermally Stimulated Discharge Current Research Articles

Changing of HDPE film due to surface effects during fabrication

Abstract High density polyethylene (HDPE) films have been fabricated using a presse-sheet system. The results of thermally stimulated discharge current (TSDC) analysis on these films are presented. The relationship between TSDC results and film manufacturing parameters are also presented. The following are the parameters investigated; cooling rate, film forming pressure and surface roughness. The surface roughness is shown to be dependent on the “forming plate” material. These materials investigated are Teflon TM , stainless steel and aluminum. Using Teflon TM as the forming plate causes the HDPE film to display a large TSDC homocurrent peak at 120°C. However, using stainless steel as the forming plate causes the HDPE film to display no TSDC peaks. Results of the characterization of HDPE surfaces immediately following the peeling of the forming plate materials are presented. Results of the characterization of plasma treated surfaces are presented before and after a second treatment using a low pressure glow discharge.

Molecular motions in a side‐chain liquid‐crystalline polymethacrylate. A thermally stimulated currents study of the dipolar relaxations in the vitreous and liquid‐crystalline phases and at the glass transition

AbstractIn the present work we report the results of a Thermally Stimulated Discharge Currents (TSDC) study of a side‐chain liquid‐crystalline polymer. The different discharges observed in the TSDC spectrum have been analysed by the thermal sampling technique and have been attributed to specific relaxation mechanisms at the molecular level. A particular feature of the polymer studied in this work is that the mesogenic side group presents both longitudinal and transverse components of the dipole moment, and this seems to be at the origin of the presence of two separated relaxations in the liquid‐crystalline phase.

Relaxation of Polar Order in Poled Polymer Systems: A Comparison between an Isothermal and a Thermally Stimulated Experiment

The relaxation of polar order in the poled polymer [OPh(R)OCOPh(Pr) 2 CO] m [OPh(R)OCOPh(OBz) 2 CO] n (R= (CH )6 OPHCH=CHPhNO 2 ) system which had been designed in the context of nonlinear optical (NLO) materials was investigated. The isothermal relaxation of the Pockels effect as measured by the modulation of waveguide modes for different temperatures are best described by a symmetric Havriliak-Nagami relaxation time distribution. The width of the distribution increases with decreasing temperature. The temperature dependence of the mean relaxation time followed a modified version of the Vogel-Tamann-Fulcher equation. Thermally stimulated discharge current (TSDC) experiments were carried out in order to verify the proposed time-temperature behavior. The modified Vogel-Fulcher description of relaxation times describes the TSDC spectra well using the same relaxation time distributions as found for the isothermal relaxation

Preliminary results of combined thermo-luminescence and thermally stimulated current measurements on an additive free polyethylene

Samples of additive free low density polyethylene have been irradiated at − 180 and − 130 C with 1 keV electrons. The sample arrangement was such that simultaneous measurements of thermally stimulated discharge currents (TSDC) and luminescence (TL) were possible and these techniques were used to observe effects of surface charge relaxation. The resulting spectra were similar in some respects to those obtained by other workers using either higher electron energies, which create bulk charges, and higher irradiation temperatures, or using X- or γ-radiation to create internal charge. In particular some of the TL peaks in the spectra obtained by the present authors were observed in similar regions and a current reversal seen in the TSDC spectrum obtained at the higher irradiation temperature (− 130°C) was also observed by another worker but for material irradiated at above room temperature. The current reversal is most likely to be associated with charge movement in the bulk rather than across the sample surface.

Molecular motions in a rigid backbone polymer: poly(n-hexyl isocyanate). A Study by thermally stimulated currents

The molecular motions of poly(n-hexyl isocyanate) have been studied systematically by the technique of thermally stimulated discharge currents (TSDC) in the temperature range –160–120 °C. Owing to the thermal decomposition of the substance occuring at temperatures near 120 °C, it was not possible to extend the study to higher temperatures and, particularly, it was not possible to study the molecular dynamics in the liquid-crystalline phase. In the studied temperature range, a complex pattern of relaxation mechanisms was observed, which included a low-temperature relaxation ascribed to the mobility of side-chain alkyl groups, the glass-transition relaxation (Tg=–50 °C) and a higher temperature relaxation ascribed to the worm-like motions of the polymer backbone.

Thermally stimulated discharge currents in corona-charged aramid paper

Thermally stimulated discharge (TSD) currents were measured in corona-charged aramid paper to study the mechanisms of charge storage and its subsequent release from the bulk of the material. Studies were carried out on paper thicknesses of 76 and 127 μm using a point-plane gap in air at atmospheric pressure. TSD currents were measured over a temperature range of 0–200 °C and the influence of various parameters, such as the poling voltage, rate of heating and effect of electrode materials, were investigated. Corona-charged aramid paper with positive polarity voltage showed that the TSD current is of either polarity depending upon the temperature range. Three distinct peaks were observed, one in the low-temperature range, 20–25 °C, and the other two peaks at higher temperatures. The low-temperature peak was considered to be due to an abnormal TSD current, whereas the currents at higher temperatures were normal TSD currents. Activation energies were determined using the low-temperature tail of the TSD curves and were found to be dependent on extrinsic parameters such as the thickness of the sample. The activation energy for aluminium electrodes was observed to be in the range of 0.5–2.0 eV. The TSD currents for the low-temperature peak was considered to be electronic. It is postulated that the charge carriers are generated within the material by the intense electric field due to corona.

Dielectric and thermally stimulated discharge current studies of rubber-modified epoxy resins

Abstract Comparison is drawn between data obtained from dielectric relaxation and thermally stimulated discharge current (TSDC) measurements on blends of carboxyl-terminated butadiene acrylonitrile (CTBN) with an epoxy resin (ER) obtained by reaction of the diglycidyl ether of bisphenol A (DGEBA) with either triethylenetetramine (TETA) or diamino-diphenylmethane (DDM). The effect of varying the composition of CTBN on the nature of the phase separation was examined using electron microscopy and was also evident from changes in the dielectric spectrum and the calculated activation energy for the molecular relaxation process. The morphology consists of spherical rubber particles dispersed in an epoxy matrix at low levels of CTBN, and at higher concentrations epoxy resin is observed to be precipitated as spheres within the rubbery phase. Detailed analysis of the dipolar spectra indicates that at low concentrations there appears to be some cosolution of the epoxy in the CTBN.

Thermally stimulated discharge of electron-beam-and corona-charged polypropylene films

Studies of short-circuit and open-circuit thermally stimulated discharge currents (TSDC) for electron-beam- and corona-charged polypropylene (PP) samples are reported. The experimental results indicate that open-circuit TSDC peaks at about 70 degrees C and 100 degrees C are caused by the detrapping of charges from surface and bulk traps respectively, while the origin of a peak at 150 degrees C is presently not clear. The short-circuit TSDC shows the existence of a positive-current peak between two negative peaks when the sample has been irradiated with a large dose. This current-reversal phenomenon verifies the fact that the charge layer in electron-beam-charged PP broadens towards the rear electrode when the injected charge density is increasing. A qualitative model to describe the charge dynamics of PP is suggested.

Thermally Stimulated Discharge Currents in Pure and Doped Sodium Yttrium Fluoride

AbstractThe thermally stimulated discharge currents (TSDC) of pure NaYF4, and NaYF4 doped with 0.5 mole% Sm (in the form of SmF3) have been studied as function of polarizing temperature and polarizing field strength at a constant heating rate (β) = 0.05 Ks−1. The activation energies, relaxation parameters and charge were evaluated. The origin of TSDC spectra in NaYF4: Sm is attributed to space charge polarization. Whereas both dipolar and space charge process are found to operate in NaYF4 sample.

Thermally stimulated discharge currents in potassium yttrium fluoride

Thermally stimulated discharge currents of potassium yttrium fluoride pellets have been studied as a function of polarizing temperature and polarizing field strength at a constant heating rate of 0.09 K s −1. The activation energies, relaxation parameters and charge were evaluated. The origin of the thermally stimulated discharge current (TSDC) peak was attributed to the space charge process.

Thermally stimulated discharge current studies on styrene-methyl methacrylate copolymer films

Thermally stimulated discharge current (TSDC) studies on thin films of styrene-methyl methacrylate copolymer in the ratio 45:55 are reported. Films of thickness 7.36 μm were grown by the isothermal immersion technique. Only one current maximum was observed in the TSDC spectrum for polarizing fields < 20 × 10 5 V/m and two current maxima were observed at higher fields for all polarizing temperatures. Relaxation times and activation energies were evaluated using existing theories. The low-temperature peak was attributed to dipolar orientation and the high-temperature peak to glass transition relaxation.

Conduction and thermally stimulated discharge currents in aramid paper

Conduction currents and thermally stimulated discharge (TSD) currents were studied in aramid paper at various field strengths and temperatures. At low field strengths the conductivity is ohmic, and at higher field strengths (up to 30 MV/m), ionic conduction is a possible mechanism. The activation energy of low field conduction is determined and at high electric fields the hopping distance for ions is in the range of 4.4 to 8.0 nm. TSD currents are also measured at various poling fields, formation and storage times, and rates of heating. The TSD current shows a peak at 135 degrees C for a rate of heating of 1 K/min and shifts to a higher value when the rate of heating is increased. The peak current and the charge released to the external circuit is proportional to the poling field. The observed peak is attributed to the depolarization of permanent dipoles with a room temperature relaxation time of 5.3*10/sup 5/ s. >

Dopant concentration effect on the thermal discharge behaviour of Fe-doped polyvinyl acetate films

The thermal discharge behaviour of pure and Fe-doped polyvinyl acetate films under different conditions has been studied using the TSDC (thermally stimulated discharge current) technique. The peak positions, peak currents, stored charge and activation energies associated with the TSDC peaks are strongly affected by the concentration of the dopant. The results have been explained in terms of the interaction between the dopant (Fe) and acetate group in the polymer matrix.

Tunnelling and Thermally Stimulated Phenomena in Highly Filled PMMA Composites

Abstract Films of polymethylmethacrylate composites, filled with high volume fractions (P ≥ 26%) of Fe fine powder, were prepared by casting method. The electrical resistivity was measured in the temperature range of 305–406 K. Composites of P = 26–34% exhibited a temperature (T) independent resistivity in the range of T = 305–400 K indicating the predominance of a tunnelling conduction. Relatively sharp resistivity peaks were observed at T > 400K, due to the thermally stimulated discharging current (TSDC). Composites of P > 34% were characterised by broad TSDC peaks without tunnelling region. The results were interpreted on the basis of a previously suggested theoretical model which considers the discharge process in a dielectric with two bipolar deeply trapped charge layers. It is implied that the present system exhibits a high effective surface charge density and a low value of the average electric field inside the layers. These results ruled out the application of simple percolation models and confirm...

Thermally stimulated discharge current studies in pure and fe‐doped polystyrene films

AbstractStudies were made on the thermally stimulated discharge currents (TSDCs) in pure (undoped) and Fe‐doped polystyrene films as a function of polarizing field, polarizing temperature and dopant concentration. While undoped films exhibited a single peak, doped films showed two peaks one at low temperatures and another at high temperatures. The low temperature peak, which exhibits a shift towards lower temperatures with increasing dopant concentration, is attributed to the relaxation of the main chain, while the high temperature peak, which shows a tendency to shift towards higher temperatures with dopant concentration, is due to space charge polarization. The TSDCs were higher for low dopant concentrations than their undoped counterparts, while for high concentrations of the dopant, the TSDCs decreased. Formation of charge transfer complexes at low dopant concentrations and molecular aggregates at higher dopant concentrations are suggested as the possible reasons for this behaviour.

On the nature of AC field aging of cross-linked polyethylene using liquid electrodes

The nature of AC field (6 kV mm-1 and 1 kHz) aging of cross-linked polyethylene (XLPE) with (i) NaCl and (ii) CuSO4 (0.1 M) liquid electrodes has been studied with thermally stimulated discharge current (TSDC), thermal transient current (TTC) and Fourier transform infrared spectroscopic (FTIR) techniques. The behaviour of thermal transient current appears to be related to the electrical aging time and consists primarily of two components, i.e. dipolar and space-charge contributions. The magnitude of the TTC is related to that of the integrated charge released in a TSDC thermogram. FTIR measurements show that oxidation of stabilized XLPE is significantly lower than that of the unstabilized polymer. No noticeable increase in bound water content in the polymer was observed as measured by IR absorbance at approximately 3370 cm-1. Progressive enhancement of absorbance at approximately 1715 cm-1 and the TSDC peak at 70 degrees C with increasing field aging time appear to have the same origin.

Low temperature relaxation of DGEBA epoxy resins: A thermally stimulated discharge current (TSDC) study

AbstractThe effect of curing conditions on the low temperature relaxation behavior of catalyst‐cured epoxy systems based on digliycidyl ether of bisphenol A (DGEBA) has been characterized by the thermally stimulated discharge current (TSDC) technique. In these chemically relatively simple epoxy systems, five relaxation processes, designated as γ, β, β′, βOH, and β″, have been observed: Their molecular origins are discussed in detail. The results are in agreement with prior suggestions of an inhomogeneous cross‐link density morphology.

Thermally stimulated discharge current studies of corona-charged Tefzel

Thermally-stimulated-discharge current (TSDC) of corona-charged Tefzel has been studied. The activation energy of the trapping carriers was observed to increase with increasing charging voltage. The activation energy for the positively charged Tefzel was observed to be greater than that for the negatively charged samples. The behavior of the TSDC thermogram was found to be dependent on the annealing time. It is suggested that there occurs a change in the trapped carrier location from the shallower into the deeper traps during the annealing process.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electrification and Its Evaluation of Polymers by Using the Mini-Scanning Electron Microscope

The electron beam charging is the most suitable technique to inject the charge deeply into the dielectric material, such as polymers of high resistivity where the mean depth of the charge is easily estimated from the energy of the beam. The authors attempted to realize this electron beam injection by using the mini-scanning microscope (Akashi: Mini-SEM type S4). The beam energy values are 15, 25 and 35 ke V with beam current of a few na and the injection area is only about 1 cm2. Very low surface potential of-9KV was observed in the vacuum although the average potential profile measured in the air was only less than-KV and so on. FEP teflon sheets with thickness of 100μm and polypropylene sheets with thickness of 60μm were charged by that method and their charging characteristics were examined by the Thermally Stimulated Discharge Currents (TSDC). A homo-current peak is observed at low temperature (40-80°C) and a hetero-current peak is also recognized in most samples (teflon and polypropylene). Some new results for the polypropylene sheet were obtained that the annealed sample shows hetero-current at a little bit above 75°C where the nonannealed sample does homo-current peak at 95°C. Typical charging time was 10 minutes which was in good agreement of the estimation from the saturation of surface potential and its charging current, but longer charging caused the new hetero-current peak at less than 70°C. Metalization of one or two side of the sheet is found to be also effective to the TSDC characteristics.

Cold crystallization effect on α and ρ amorphous poly(ethylene terephthalate) relaxations by thermally stimulated discharge currents

The thermally stimulated discharge currents (TSDC) of crystalline poly(ethylene terephthalate) (PET-c) show two clearly differentiated relaxations, α c and ρ c, which could result from the α and ρ relaxations of amorphous PET as a consequence of the crystallization process. The evolution of α and ρ relaxations of amorphous PET has been studied by the application of a method which we have called thermal stimulation by steps (TSS) and by isothermal crystallization of amorphous samples for different times. The ρ relaxation increases due to progressive nucleation and subsequent growth of the nuclei until a maximum value is reached; from then on until extinction ρ is mainly affected by growth. The α relaxation decreases progressively until its total extinction following the increase of restrictions. For a certain degree of crystallinity, by applying the TSS method, simultaneous occurrence of α, ρ, α c and ρ c is achieved, which implies that α c and ρ c do not proceed from α and ρ and, consequently, that their origin is different.