Thermally Stimulated Polarization Current Research Articles

Investigations of Current Generated in Construction of Polarization in Vectra A, Liquid Crystal Copolyester

<div><p><em>The Thermally Stimulated Polarization Current (TSPC) is used to study the dielectric behavior of Vectra A, liquid crystal copolyester. The TSPC spectra of samples of Vectra A for various polarizing fields (3.8 kV/cm- 19.2 kV/cm) have been investigated. A dipolar relaxation is observed in the low temperature region ~ 30 <sup>o</sup>C due to orientation of naphthalene groups present in Vectra A which is termed as peak-P<sub>1</sub>. In the higher temperature region above 110 <sup>o</sup>C, the dipolar and space charge peaks are in composite form. TSPC cycle is performed to distinguish the dipolar and space charge peaks. Also, the TSPC spectra of annealed and doped samples of Vectra A are studied. The results obtained from TSPC spectra are compared with TSDC spectra of samples of Vectra A.</em> </p></div>

Effects of crystallinity on dielectric properties of poly(L‐lactide)

AbstractPoly(L‐lactide) is attracting much attention as a biodegradable polymer. In this paper, effects of crystallinity on dielectric properties of PLLA are discussed. At 80 $C, which is above the glass transition temperature (= about 60 °C), the conductivity increases with a decrease in crystallinity. A thermally stimulated polarization current (TSPC) peak, which seems to be due to segmental‐mode relaxation, appears in all the samples at temperatures around 65 to 70 °C, and it becomes smaller as the crystallinity increases. All the samples show two thermally stimulated depolarization current (TSDC) peaks at around 65 °C and around 90 to 100 °C. The lower‐temperature peak seems to be due to the segmental‐mode relaxation, and the other due to the normal‐mode relaxation. Moreover, all the samples show a drastic increase in the real part of the complex permittivity ($\varepsilon_{\hbox{r}}^{\prime}$) and a peak in its imaginary part (ϵr.) at frequencies from 10−1 to 104 Hz, depending on the measurement temperature. The crystallinity dependencies of the two parts are similar to those of the TSPC and TSDC peaks at around 65 to 70 °C. Therefore, the increase in $\varepsilon_{\hbox{r}}^{\prime}$ and the peak in $\varepsilon_{\hbox{r}}^{\prime\prime}$ are ascribable to the segmental‐mode relaxation. © 2011 Wiley Periodicals, Inc. Electron Comm Jpn, 94(7): 1–8, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10348

Simulation of thermally stimulated polarization current (TSPC) based on the Frohlich two-state model and the complexity of the amorphous phase

Numerical simulations of thermally stimulated polarization current profiles have been performed using rate expressions based on the Frohlich two-state model. The qualitative behavior of simulations previously pub- lished by other researchers can be reproduced. The important feature examined is a peak in the evolution of polarization with temperature, which results in a change in sign for the polarization current profile. The previous researchers have assigned this to a crossover of the kinetic transient polarization with the equilibrium polarization described by the Langevin approximation. The origin of the peak in the evolution of polarization has been reexamined and found to be the kinetic consequence of the structure of the Frohlich model. When the field is applied, the two-state model requires that half the available dipoles are initially polarized. This high level of polarized dipoles contributes to an increased rate of the reverse process, depolarization, at high temperatures and results in the calculated decrease in polarization. The constraints of the Frohlich two-state model are too severe to represent the kinetics of a physi- cally plausible polar solid. Further the multiple modes of aggregation in the amorphous state impose complications on the computation of polarization current.

Dielectric Relaxation Investigations in Copolyesteramide (Vectra B950) Polymer Liquid Crystal: Thermally Stimulated Polarization Current Technique

Thermally stimulated polarization currents (TSPC) in copolyesteramide (Vectra B950) polymer liquid crystal have been investigated in the temperature region 20°–250°C under different polarizing fields ranging from 0.7 to 7 kV/cm and with different heating rates. The low temperature region of the TSP current spectra shows a pronounced peak (P1) around 30°C. This peak has been attributed to the dipolar relaxation process associated with ester group. The peak (P2) observed around 140°C has been attributed to a combination of two relaxation processes, the dipolar and space charge relaxation processes. A steady rise in the TSP currents by several orders of magnitude is observed above 150°C. Saturation in the TSP current around 220°C reveals the presence of a relaxation process owing to the movement of large segmental groups in LC phase.

Ｌ型ポリ乳酸の結晶化度が誘電特性に与える影響

Poly (L-lactide) (PLLA) is attracting much attention as a biodegradable polymer. In this paper, effects of crystallinity on dielectric properties of PLLA are studied. At 80°C, which is above the glass transition temperature (about 60°C), the conductivity increases with a decrease in crystallinity. A thermally stimulated polarization current (TSPC) peak, which seems to be due to segmental-mode relaxation, appears in all the samples at temperatures around 65 to 70°C, and it becomes smaller as the crystallinity increases. All the samples show two thermally stimulated depolarization current (TSDC) peaks at around 65°C and around 90 to 100°C. The lower-temperature peak seems to be due to the segmental-mode relaxation, while the other due to the normal-mode relaxation. Moreover, all the samples show a drastic increase in the real part of complex permittivity (er') and a peak in its imaginary part (er”) at frequencies from 10-1 to 104 Hz, depending on the measurement temperature. The crystallinity dependencies of the two parts are similar to those of the TSPC and TSDC peaks around 65 to 70°C. Therefore, the increase in er' and the peak in er” are ascribable to the segmental-mode relaxation.

Temperature Dependence of Complex Permittivity in Biodegradable Polybutylene Succinate

Polybutylene succinate, a kind of biodegradable polymer, shows thermally stimulated polarization current (TSPC) peaks at around -35°C and 50°C. The lower-temperature TSPC peak can be well explained by dipolar polarization. As for the higher-temperature TSPC peak, the permittivity increases more significantly with a decrease in frequency, and the increment in permittivity estimated from the TSPC area agrees with the difference in permittivity at a sufficiently low frequency and at a sufficiently high frequency. It is assumed that a hetero charge layer is formed in front of the two electrodes and that such space charge is responsible for both the higher-temperature TSPC peak and the permittivity increase. The dielectric loss factor also increases with a decrease in frequency, and the increment is in good agreement with the assumption that the dielectric loss is ascribed to conduction loss due to high conductivity at high temperatures. All the results indicate that mobile ions are dominant carrier species in polybutylene succinate.

High-temperature polarization and depolarization experiments on poly(ε-caprolactone)

Thermally stimulated depolarization current (TSDC) and thermally stimulated polarization current (TSPC) techniques are compared, and the results obtained for semicrytalline poly(ε-caprolactone) (PCL) are reported in this work. It is shown that concerning the glass transition relaxation, both techniques give the same information, provided that for the TSDC case the polarization temperature is above the temperature range where the labeled ρ peak occurs. The current reversal observed in TSPC spectrum of this polymer is originally reported here, and its presence predicted by a simple phenomenological model taking into account the T−1 dependence of the equilibrium polarization. The current reversal observed in the TSDC experiments when the sample is polarized at high temperatures is an evidence of the existence of an internal field applied during the heating cycle. Performing two consecutive TSPC experiments (TSPC2) two current reversal temperature zones were observed. The comparison of the dielectric spectra obtained in the same temperature zones by the different techniques envisages the possible presence of a dipolar mechanism contributing to the ρ peak. Further modeling and data treatment need to be done in order to corroborate this hypothesis.

A study of polarization mechanisms in sodium iron phosphate glasses by partial thermally stimulated depolarization current

The electrical polarization of sodium iron phosphate glasses, containing from 5.3 to 14.9 mol% Na 2O and with a constant Fe/P ratio of 0.67, was investigated by thermally stimulated polarization current (TSPC), and by depolarization current (TSDC) techniques. The partial TSDC measurements showed a group of distributed partial peaks, P 1, whose maximum ranged from 186 to 227 K. A group of higher temperature peaks, P 2, was found in a narrower temperature range from 257 to 264 K, with an additional process, P 2 ′, at about 240 K. The process P 1 was caused by a dipolar relaxation showing a compensation with the compensation point at T c=247 K. The physical meaning of the compensation point could not be related to analogous phenomena in organic polymers. The higher temperature peaks represent space charge relaxations greatly affected by the increase in Na 2O content and influenced by the remaining polarization and conductivity. The most likely cause for the electrical relaxation was an interaction between the Na + ions and the P–O − bonds. Increasing the Na + concentration caused an increase in the TSDC peaks and a slight increase in dc conductivity.

Charge transport properties for carbazolyl groups pendant poly(glutamate)

Carbazolyl groups pendant poly(glutamate) (PCLG) was prepared to analyze its charge-transport properties by employing mobility measurements and thermally-stimulated current (TSC) measurements. The mobility induced TSC (MITSC) model proposed by I.Chen was employed to evaluate the experimental TSC spectra with mobility results. Simulated MITSC spectra showed good agreement in its peak temperature with experimental TSC spectra for PCLG. This suggests that the carrier transport followed the trap-limited mechanism estimated by the mobility results. Further, the peaks in experimental TSC spectra appeared over the same temperature range as that in thermally-stimulated polarization current (TSPC) spectra. Since the TSPC spectra were found to be correlated with the dielectric tan δ spectra for PCLG, the peaks in TSPC spectra are attributed to the side-chain relaxation for PCLG. Therefore, the similarity between TSPC and TSC spectra indicates that the charge-transport mechanism for PCLG was considerably affected by side-chain relaxation for PCLG, which would vary the energy state of trap sites and effectively reduces the energy for the release of the carriers trapped on the illuminated surface. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 61–69, 1999

The conduction and thermally stimulated polarization currents in (TGS-PE) particulate composites

Different concentrations of polyethylene (PE) are mixed with Triglycine Sulphate (TGS) powder. The temperature dependence of thermally stimulated polarization current (TSPC) was studied at different applied voltages and heating rates. The results were discussed and special stress was done on conduction mechanism, trapping, injection, cleaning of charge carriers and variations in bulk polarization. The conduction current was measured during cooling. It was found that this current is extremely lower than the TSPC current. Besides, the activation energy of the conduction process is lower than that calculated from TSPC process. The addition of PE is thought to be responsible for the observed phenomena.

Electrical conduction in biaxially-oriented polypropylene

The electrical conduction in biaxially-oriented polypropylene (BOPP) has been studied. Space charge limited current (SCLC), Schottky and Poole-Frenkel effects and ionic conduction processes are discussed. Thermally stimulated polarization current (TSPC) studies showed a decrease in conductivity from 4.3 × 10 −16 to 3.0 × 10 −17 (ohm cm) −1 at 80°C after hexane treatment of BOPP. The decrease can be attributed to the removal of impurities by hexane treatment. The conduction is extrinsic in nature. Thermally stimulated current investigations show the presence of impurity traps in BOPP. A good fit of the ionic theory to be experimental data of the steady state currents in the temperature range 50–100°C suggests ion conduction to be predominant. Bulk electronic conduction also plays important role at high temperatures.

Dielectric relaxations in thermotropic liquid crystalline polypeptides

Thermally stimulated depolarization current (TSDC) and thermally stimulated polarization current (TSPC) measurements and the differential scanning calorimetry (DSC) method have been used for the investigation of thermal transitions in thermotropic poly( gamma -stearyl- alpha , L-glutamate). The beta peak (-80 degrees C) and alpha /sub 1/ and alpha '/sub 1/ peaks registered between -50 degrees C and 25 degrees C were attributed to movements of side chain dipoles. The alpha /sub 2/ peak, observed approximately=43 degrees C, was indicative of the melting of paraffinic side chains. The activation energies were determined for beta peaks and alpha /sub 2/ peaks at different heating rates. A good correlation between heating rate and the square of the maximum peak temperature was found, showing that these peaks are of dipolar origin. The alpha /sub 1/ and alpha /sub 2/ peaks were strongly influenced by thermal treatments. The position of alpha /sub 2/ was close to that determined by DSC. Depending on the thermal history of samples, the paraffinic side chain segments melted and crystallized over a large temperature range, suggesting the formation of crystallites with different dimensions. >

Study of the thermal relaxation effects in polymers by a conjugated use of thermally stimulated depolarization and polarization current methods

Among the various thermally stimulated current methods used for analyzing the relaxation behavior of polymers, the thermally stimulated polarization current (TSPC) technique is rarely used because the classical theory provides for obtaining spectra having relaxational characteristics similar to those obtained from the more familiar thermally stimulated depolarization current (TSDC) technique. In fact, a number of factors such as the temperature dependence of equilibrium polarization, the effect of thermal expansion, the annealing process necessarily associated with TSDC experiments, or the existence of a temperature-dependent structure, can lead to very dissimilar results. This is especially the case in polymers, where the use of TSDC and TSPC methods in conjunction is very useful, e.g., for differentiating between various polarization processes, for studying the physical aging phenomenon or the behavior of thermally unstable structures, and for revealing non-equilibrium intermediate relaxations.

A DLTS Study of InAs MIS Structures

Deep level transient spectroscopy (DLTS), capacitance–voltage (C–U) measurement, and thermally stimulated polarization current (TSPC) methods are used to study InAs MIS-structures in the temperature region from 5 to 400K. It is found that above 200K the measured DLTS signal is caused by charge accumulation in the insulator layer. The parameters of two electrically active defect centers in the semiconductor space charge region (SCR) Ec – 0.06 eV and Ec – (0.15–0.20) eV are determined. These levelsapear in the energy spectrum of the interface states density as two separate peaks. The DLTS data are complemented by and compared with the C–U data. [Russian Text Ignored].

A DLTS study of InAs mis structures

Deep level transient spectroscopy (DLTS), capacitance–voltage (C–U) measurement, and thermally stimulated polarization current (TSPC) methods are used to study InAs MIS-structures in the temperature region from 5 to 400K. It is found that above 200K the measured DLTS signal is caused by charge accumulation in the insulator layer. The parameters of two electrically active defect centers in the semiconductor space charge region (SCR) Ec – 0.06 eV and Ec – (0.15–0.20) eV are determined. These levelsapear in the energy spectrum of the interface states density as two separate peaks. The DLTS data are complemented by and compared with the C–U data. [Russian Text Ignored].

Sub-T/sub g/ relaxation in cold-drawn polymers: thermally-stimulated-current methods

Thermally stimulated depolarization current (TSDC), thermally stimulated polarization current (TSPC), and differential scanning calorimetry (DSC) methods have been used to study the intermediate relaxation induced in glassy polycarbonate (PC) by uniaxial cold-drawing at various drawing ratios, rates, and temperatures (below the glass transition temperature T/sub g/). A well-defined alpha ' peak has been observed in the intermediate temperature range located between the local mode beta transition and the alpha glass transition domains. The corresponding relaxation is very sensitive to physical aging effects, its characteristics differ markedly in TSPC and TSDC experiments, and they are also very different from those observed in PC subjected to other types of mechanical deformations such as cold rolling. It is a nonequilibrium relaxation, presumably resulting from local orientation (TSPC) or disorientation (TSDC) of chain segments made possible by the increase in free volume and molecular mobility subsequent to the drawing process and, as such, could be considered as a proximate precursor of the glass transition.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

TSDC and TSPC due to polarization by space charge migration over macroscopic distances

Most of the space charge in an electret is supposed to stay near its surface. The authors used two methods of investigating the charge near the surface of a polymer electret. One is the removal of a thin layer from the surface. The thermally stimulated depolarization current (TSDC) of such an electret is far smaller than before the removal. The other experiment is an investigation of stacked electrets. A stack of three sheets of polymer film, with or without metal foils between them, is poled to become a thermoelectret. After cooling, it is separated into three sheets. The author found that the TSDCs of the outer two sheets are larger than that of the middle one. This shows that space charge mainly gathers in the outer sheets. To inquire into the type of charge, the thermally stimulated polarization current (TSPC) was also measured. The results suggest that most of the space charge is ions. >

Ionic thermocurrents in sodium fluoride thin film

Ionic thermocurrent (ITC) in NaF thin film deposited between gold electrodes has been investigated in situ in the temperature region 300–500 K. The thermally stimulated polarization current (TSPC) and thermally stimulated depolarization current (TSDC) obtained under low electric field pf polarization ( E p) at a lower temperature of polarization ( T p) exhibited current-peak temperatures ( T M) about 420 K and 450 K. The activation energies associated with the first and the second current peaks were 1.15 + 0.05 eV and 0.65 + 0.02 eV, which are assigned to cation vacancy blocking at the grain boundary barrier and the electrode contact interface barrier respectively. Polarization under a strong electric field ( E p > MV m −1) at T p > 480 K causes a quasi-stable shift of the ITC peak positions to higher temperature. The current peak positions and magnitudes then depend upon the polarity of the electrode metal during polarization, and activation energies associated with the first peak positions are 0.90 + 0.02 eV and 0.79 + 0.02 eV for positive and negative bias at the counter electrode respectively, which may be attributed to the clustering of the cation vacancies and the dislocation networks at the interfaces.

Dislocation polarization and space-charge relaxation in solid solutions Ba1-xLaxF2+x.

Concentrated solid solutions ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{La}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$, x\ensuremath{\le}0.492, are studied by means of thermally stimulated depolarization [ionic thermal current (ITC)] or polarization [thermally stimulated polarization current (TSPC)] techniques. At high temperature two peaks are reported whose maxima shift to lower temperatures as x increases. The energies calculated for these relaxation processes are compared to the enthalpies deduced from conductivity measurements. The first high-temperature peak, D, has been already reported in a more restricted concentration range and has been attributed to either a space-charge accumulation near the electrodes or the polarization of the dislocations present in the crystal.We show here that it is the highest-temperature peak reported here for the first time and labeled as peak E which is best related to the displacement of free interstitial fluorines through the bulk of the crystal; that is, to the building up of a space charge near the electrodes. At low concentrations the behavior of peaks D and E is understood if the Debye-H\uckel interaction energy among unassociated defects is taken into account. For the more concentrated region (x\ensuremath{\gtrsim}0.05), the enhanced ionic motion model proposed to explain the variation of the ionic conductivity slopes as a function of x also applies here; the parameters present in the model are calculated and are found to be in agreement with those previously reported. The energy difference observed between peaks D and E is almost constant for x\ensuremath{\ge}0.05. This energy difference equal to 0.08 eV is attributed to the binding energy of the interstitial fluorines forming the dislocation charge cloud to the dislocation line. TSPC experiments are carried out on the same crystals and they show that peak D is present in all the TSPC spectra, thus confirming the localized displacement of the charges, while the highest-temperature peak E is replaced by a steep increase of the ionic current. Therefore, the origin of peak E observed in ITC must be attributed to the accumulation of charges near the electrodes; that is, the build up of the space-charge layer.

Effect of the internal field on polarization and depolarization experiments on SrF2:La3+

Thermally stimulated depolarization [ionic thermal current (ITC)] and thermally stimulated polarization current (TSPC) methods are compared, and the results obtained for ${\mathrm{SrF}}_{2}$ crystals doped with La are reported in this paper. It is shown that for normal dipolar peaks such as the low-temperature relaxation caused by the movement of the interstitial ${\mathrm{F}}_{\mathrm{i}}^{\mathrm{\ensuremath{-}}}$ bound to a ${\mathrm{La}}^{3+}$ cation to form a nearest-neighbor dipole, both techniques give the same information, provided that for the ITC case, the polarization temperature is below the temperature range where the very intense and somewhat anomalous high-temperature peak occurs. For an ITC experiment a current reversal is observed in the high-temperature tail of the dipolar peak if the sample is polarized at a temperature above the high-temperature peak. This current reversal, typical of a TSPC peak, is clearly observed and its height is useful in determining the effective polarization field applied to the sample when an ITC run is performed. The presence of the current reversal indicates the existence of a frozen-in polarization due to processes occurring in the high-temperature part of the relaxation spectrum. Also, information on the origin of the observed relaxation processes at high temperature can be deduced from a comparison of the two spectra.