Thermally Stimulated Discharge Current Research Articles

Field-induced effect in the -oriented 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 single crystals

Temperature-dependent dielectric constant, X-ray diffraction(XRD), thermally stimulated discharging current (TSDC) and polarization hysteresis loops for <111>-oriented 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 crystals after poling under different fields were measured. Comparing with unpoled sample during heating, except that there is a transition from the state of normal ferroelectric rhombohedral (macro-domain) to the state of relaxor ferroelectric rhombohedral (micro-domain) for the poled samples, it was found that no other phase transition was induced by poling fields. XRD showed that after poling the piezoelectric elongation of the unit cell occurred along the polar direction. It was suggested that an extra peak in the T–ε curve of poled sample around 105 °C is not only caused by a macro–micro domain transition but also caused by the depolarization process of 180° and 71° domain switching simultaneously.

Electric field effects on the domain structures and the phase transitions of0.62Pb(Mg1/3Nb2/3)O3–0.38PbTiO3 single crystals with different orientations

The domain states and phase transitions in0.62Pb(Mg1/3Nb2/3)O3–0.38PbTiO3 single crystals with different orientations were investigated through biased dielectricmeasurement, thermally stimulated discharging current (TSDC) and polarization–electric field(P–E) hysteresis loop measurements. The biased dielectric response shows that the crystals havea macrodomain tetragonal ferroelectric state in the ferroelectric phase under zero dc bias.An external bias field could modulate the domain state and induce a phase transition inthe crystals. Also, it is proposed that a bias field applied along the direction of the original tetragonal phase could induce triple likeP–E hysteresis loops and an orthorhombic ferroelectric phase in an intermediate temperaturerange.

Dielectric, Thermally Stimulated Discharge Current, Pyroelectric and Surface Morphology of Pmma Thin Films Prepared by Isothermal Immersion

Metal–Insulator–Metal structure was used to investigate the dielectric, thermally stimulated discharge current(TSDC)and pyroelectric behavior of poly methyl methacrylate(PMMA)thin films prepared by isothermal immersion method. The surface morphology and the structure of the above said films were studied by using atomic force microscopy(AFM)and X-ray diffraction(XRD)respectively. The XRD spectrum of as grown and films annealed at 513 K indicated the amorphous nature. No pits and dendritic features were observed from the AFM spectrum. The only important topographic features observed is the hillocks of about 10–12 nm large with a peak to valley vertical distance of about 0.5–1 nm. Both as grown and films annealed at 513 K showed very smooth surface and amorphous nature. From the TSDC spectrum activation energy(E), capture cross-section(σ), escape frequency(ν)and relaxation time(τ)were calculated. The observed amorphous phase, low loss, dielectric and thermal behavior imply the feasibility of utilizing PMMA thin films as gate dielectric insulator layer in organic thin film transistors(OTFTs), which can find application in flat panel display.

Charge trapping and dielectric relaxations of gamma irradiated radiolytically oxidized highly oriented LDPE

The influence of gamma irradiation in air on the effects of charge trapping in highly oriented LDPE has been studied through dielectric loss (tan δ) analysis in the temperature range from 25 to 325 K and using thermally stimulated discharge current (TSDC) measurements. The radiation induced oxidation was observed using IR spectroscopy. The intensity and the position of the γ and β dielectric relaxation maxima were correlated with maxima of TSDC measurements. It was found that the positions of the relaxation peaks are strongly dependent upon the changes in the microstructure of the amorphous phase and on the surface of the crystallites induced by orientation and gamma irradiation.

Thermally stimulated discharge current (TSDC) and dielectric constant of semiconducting glasses

In this paper the results of thermally stimulated discharge current (TSDC) and dielectric constant for 40PbO-60Bi2O3 glass thermoelectrets are presented. Measurements of TSDC and dielectric constant, ǵe, have been carried out in the temperature range 30–300°C. The thermoelectrets were prepared at different polarizing fields. The various observed peaks in the thermograms are discussed on the basis of space charge polarization. The trap energy is evaluated from the Garlick-Gibson plot of initial rise method. Similarly other parameters such as relaxation time, charge release etc are evaluated.

Scope of electron transport studies by thermally stimulated discharge current measurement

A logical approach to electron transport studies for barrier conduction in layered structures was adopted by thermally stimulated discharge current (TSDC) measurement. The scope and applicability of this technique to the evaluation of the thermoelectric parameters of relaxation time, detrapping energy and depolarization rates are demonstrated here. These are characterized by the controlling factors of layer resistance and the resultant thermal and voltage gradients which apply to the drift of electrons arising from both dipolar and interfacial charges. The methodologies used in this study are suitable for parametric evaluation of structured electronic devices.

Dielectric relaxation of vegetable-based polyurethane

Vegetable-based polyurethane (PU) was prepared in the thin film form by spin coating. This polymer is synthesised from castor oil, which can be extracted from the seeds of a native plant in Brazil called mamona. This polymer is biocompatible and is being used as material for artificial bone. The PU was characterised by dielectric spectroscopy in a wide range of frequency (10−5 Hz to 105 Hz) and by thermally stimulated discharge current (TSDC) measurements. The glass transition temperature (T g = 39°C) was determined and using the initial rise method the activation energy was found to be 1.58 eV.

Relaxation processes in aromatic polyimide

Electrical conduction in non-conjugated insulating polymers such as Kapton(R) films was studied as a function of temperature, T (15–500 K) and electric field F (⩽16 kV cm−1). The time dependent isothermal transient currents (ITC) were also studied at different temperatures (15 K<T<500 K). Thermally stimulated discharge current (TSDC) studies were carried out as a function of initial poling conditions. A distinct change in conduction mechanism as a function of T was observed at T⩾300 K from DC conductivity, ITC and TSDC measurements. The origin of this behaviour is discussed in terms of transition of dipolar at low T to space charge mechanism at high T which can be attributed to carrier trapping and release of charge carriers from the trap states. The effective barrier energy, Δ, for transport and hopping parameter λ(T) are estimated for Kapton on the basis of these studies. The results obtained for Kapton suggest that, for T<Tg at low or moderate fields, the transient currents are essentially governed by dipolar relaxation mechanisms.

Polarization and space charge analysis in thermally poled PVDF

The spatial distribution of the polarization and space charge in thermally poled poly (vinylidene fluoride) is studied using the laser intensity modulation method. Injected space charge, localized near the electrode polymer interface, tends to prevent the formation of uniform polarization in the polymer bulk. The actual amount of charge existing in the poled specimen is determined using hysteresis measurements and thermally stimulated discharge current (TSDC) measurements. By using the peak cleaning technique and by measuring the pyroelectric current during the cooling of the specimen, the contribution of depolarization current and space charge detrapping to the TSDC measurement is considered. From hysteresis measurements a relaxation process was observed around 65°C which was related to the dipolar relaxation in the crystalline phase known as the αc relaxation. A significant increase of the TSDC at temperatures higher than 130°C was observed meaning that the dipolar charge and the space charge are very stable up to high temperature. In this temperature range, the pyroelectric current is significant. Two relaxation processes were identified for a polarizing temperature lower than 120°C. One is centred around the polarizing temperature and is related to space charge release. The second is related to dipole relaxation in the crystalline phase. The position of the last peak is determined by interaction between the dipoles and the internal electric field, resulting from the charge stored in the sample. We propose to call this as the αcρ interaction. The higher temperature of the dipolar peak was identified as 86°C with an activation energy of 0.52±0.04 eV.

Comparative dielectric and TSDC studies of molecular mobility in liquid-crystalline side-chain poly(methacrylate)

The molecular mobility of a LC side-chain polymethacrylate (PM6) was carried out by dielectric spectroscopy and by thermally stimulated discharge current (TSDC) methods. Six relaxation processes γ2, γ1, β, β1, α, and δ at successively increasing temperatures were observed in PM-6. For each of them, a molecular interpretation was proposed. There is a good correlation between the peak temperature positions obtained from the global TSDC spectrum at an equivalent frequency and those obtained from the dielectric method. The TSDC thermal windowing experiments for PM-6 show the high ability to resolve the overlapping processes at low temperatures. Dielectric and TSDC methods giving different details of the observed relaxation processes, allow us to create the overall picture of molecular mobility in the system under investigation. These two methods revealing the differences and similarities of the obtained results could be considered as complementary to each other.

On the characterisation of electrically stressed polyethylene before and after chemical treatment

Low-density polyethylene (LDPE) was characterised for its dielectric and electrical properties before and after chemical treatment. A reduction in the permittivity and dielectric loss was observed in the polymer after treatment with hexane. The intensity in the Raman Spectrum in the disordered longitudinal acoustic mode region (DLAM) also was reduced due to a hexane treatment. Using thermally stimulated discharge current (TSDC) and laser-intensity-modulated method (LIMM) techniques it was observed that charge injection can be enhanced in the polymer matrix in the empty sites, created by the removal of the low molecular weight impurities with chemical treatment.

Thermally stimulated processes in heterocyclic materials suitable for heterolayer organic light emitting diodes

We studied ultraviolet photoelectron spectroscopy (UPS) and thermally stimulated processes in two types of phenylquinoxalines, and one type of starburst trisphenylquinoxaline in order to characterize the valence band structure and the localized states. An ionization potential of about 6.0 eV was determined for each material using UPS. Thermally stimulated discharge currents (TSDC) gave evidence of dipolar and/or charge transport mechanisms. We assigned each TSDC peak to the different elementary processes using in addition dielectric spectroscopy (DES), and from thermally stimulated luminescence (TSL) curves, performed for the first time on these materials, we estimated the mean trap depths.

Study of Dielectric Relaxation Processes in Poly(p-phenylene sulfide) Using a Thermally Stimulated Discharge Current Technique

The various dielectric relaxations in poly(p-phenylene sulfide) have been investigated using a thermally stimulated discharge current (TSDC) technique. The TSD current spectra in the temperature range 30 to 250°C was analyzed as a function of poling temperature (100–250°C), poling field (5–18 kV/cm), poling time (1–4 h), specimen thickness (0.8–1.6 mm), heating rate (1–4°C/min.) and storage time (2–720 h). From the variation of the properties of peaks appearing in the TSDC spectra, it is concluded that the peak appearing at around 110°C (β-relaxation) is a dipolar transition due to C–S linkage. Samples poled at higher (Tp > Tg) show a peak at around 170°C (β ′-relaxation), which has been associated with the formation of cross-linked ether linkages. The β ′-relaxation gives all the character of dipolar polarization. The peak at around 210°C (α-peak) has been ascribed to a space charge transition. A rigid amorphous phase (RAP) and unsaturated phenylene groups provide the deep charge trapping centers. The activation energy obtained from the Bucci plot method confirms the respective nature of these peaks. The observed dependence of peak temperature on poling temperature and poling time indicates the presence of continuous distributed relaxation.

Effect of high energy [formula omitted] ion-irradiation on the dielectric relaxation behaviour of Kapton-H polyimide film. Thermally stimulated depolarization current study

The effect of high-energy 238 U ions (11.6 MeV/n, flux 10 6 ions/cm 2, GSI facility, DARMSTADT) on the various dielectric relaxation processes in Kapton-H polyimide using thermally stimulated discharge (TSD) current technique has been investigated. The irradiated Kapton-H samples (25 μm thickness, area 1.13 cm 2) were polarized at different temperatures (363, 393, 423 and 473 K) under various biasing fields (140–600 kV/cm). TSD currents were obtained at the heating rate of 2 K/min. The peak appearing at low temperature and associated with dipolar polarization (β-transition) is largely affected in irradiated samples. This is mainly due to the demerization of the carbonyl groups. In the vicinity of β-transition, small multiple peaks in TSD current spectra are observed. These peaks have been associated with the cross-linking of imidic groups as a result of high-energy ion-irradiation. The same has been confirmed by the activation energy values of these peaks and also through the TSD current behaviour of repoled samples. Increase in activation energy of high-temperature peak (α-transition) associated with space charge relaxation is observed in irradiated samples. This is mainly due to the creation of new deep energy centers resulting from the formation of triple bonds and free radicals. The activation energy has been calculated using Bucci-plot method.

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

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 analysis 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).

Charge trapping in gamma irradiated low-density polyethylene

The influence of gamma irradiation in two different atmospheres, air and distilled water on charge trapping in LDPE has been studied through dielectric loss (tan δ) analysis in the temperature range from 25 to 325 K and using thermally stimulated discharge current (TSDC) measurements. The radiation induced oxidation was observed using IR spectroscopy. The intensity and the position of the γ and β dielectric relaxation maxima were correlated with maxima of TSDC measurements, and found to be strongly dependent upon the changes in the microstructure of the amorphous phase and on the surface of the crystallites induced by oxidation and crosslinking.

Thermal step and TSDC measurement in PVC

The space charge behavior of some polyvinylchloride (PVC) samples with different molecular microstructures, obtained through chemical modification of PVC has been analyzed by means of the thermal step (TS) and the thermally stimulated discharge current (TSDC) techniques. The magnitude of the space charge distribution has been shown to be lower as the degree of chemical modification increases. Combining the TS and the TSDC methods allowed us to study minority carriers, which could not be observed by a single TS measurement.

Poling and characterization of nonlinear polymer DCNP/PEK-c thin films

Polyetherketone (PEK-c) guest–host polymer thin films doped with 3-(1,1-dicyanothenyl)-1-phenyl-4, 5-dihydro-1H-pryazole (DCNP) were prepared. The poling profiles of the polymer film were investigated by using the in situ second-harmonic generation (SHG) measurement. The corona poling condition was optimized by the poling profiles. The second-order nonlinear optical (NLO) coefficient χ 33 (2)=20.86 pm/V at λ=1064 nm was determined by using the Maker fringe method after poling under the optimal poling condition. The temperature stability of the second-order optical properties of the poled polymer film was measured by the in situ SHG intensity probing. Moreover, the relaxation of the electric field-induced polar orientation of the chromophores in the poled polymer film was measured by the thermally stimulated discharge current (TSDC) technique.

TSDC in epoxy networks of well-controlled architecture: Experimental evidence for local and non-local relaxation process

Local and non-local relaxation processes were distinguished by investigating the heating rate dependence of the thermally stimulated discharge current (TSDC) line-shape in the case of three epoxy networks of well-controlled architecture. These networks were chosen because their chain dynamics differ by the relative extent of their local and short-scale cooperative molecular motions, as a consequence of marked differences in crosslink density. As far as local motions are concerned, the TSDC evolution measured at low temperature can be accounted for by using a model based on a distribution of activation energies. On the other hand, the higher temperature part of the spectrum of the densely crosslinked network, which mainly results from cooperative motions, shows a much different behavior which cannot be fitted by such a distribution model.

Space- and surface-charge analysis of plasma-preprocessed PTFE thin films

The space- and surface-charge behavior of corona-charged polytetrafluoroethylene thin films after plasma processing were studied experimentally by space-charge density distribution measurement and thermally stimulated discharge current (TSDC) measurement. The effect of the antistatic process using low-pressure discharge plasma and charge elimination process dipping in tap water was examined. The surface composition of the samples which were plasma- processed was analyzed with X-ray photoelectron spectroscopy (XPS) to study the mechanisms of antistatic process. It was found that charge elimination of plasma-processed samples was enhanced, independently of the kind of processing gas during the plasma processing. Oxygen and nitrogen atoms were observed from the XPS measurement of plasma-processed samples. In particular, a large amount of nitrogen atoms was found on the surface of the samples plasma processed in pure nitrogen gas. The samples plasma processed in pure nitrogen gas showed a large hetero TSDC peak at room temperature before the charge-elimination process. This might be due to nitrogen atoms on the sample surface that were generated during plasma processing.