Thermally Stimulated Depolarization Current Data Research Articles

Rectifying behavior of the GaAs / Er-doped SnO2 heterostructure: Interface dipole role and monochromatic light influence

The characteristic current-voltage (I x V) curves are measured across the interface of a GaAs/SnO2 heterostructure, where the top layer (tin dioxide) is doped with 1 at% of the rare-earth Er. It leads to a diode-like behavior, which is associated with the presence of electric dipoles mainly at interface. Thermally stimulated depolarization current (TSDC) technique is also applied to this heterostructure in the dark and in the presence of monochromatic light from a He–Cd laser or a InGaN LED. The effect of monochromatic light on the possible formation or destruction of dipoles in this heterostructure system is analyzed by the I x V curves as well as by the TSDC data, considering the main types of defects in this structure: EL2 in the GaAs side, or oxygen vacancies and ionized Er ions in the SnO2 side. Photo-induced TSDC (PTSDC) points to the destruction of bands previously obtained in the dark, suggesting a large difference on the thermal and optical activation energies of defects. This effect is stronger with the illumination of a InGaN LED (2.76 eV) compared to He–Cd laser (3.80 eV) irradiation.

The determination of the glass transition temperature by thermally stimulated depolarization currents. Comparison with the performance of other techniques

ABSTRACTSeveral experimental techniques are currently used for the determination of the glass transition temperature, Tg. Thermally stimulated depolarization currents (TSDC) is a thermal analysis technique whose experimental results display a very clean glass transition signature and that, nevertheless, is seldom used as a technique for Tg determination. In the present work we explain how to get the glass transition temperature from TSDC data, and we compare the values obtained for a vast number of glass forming systems (with Tgs in a wide range between −145 and +180 °C and fragilities between m = 15 and m = 100), with the values obtained by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). We conclude that the Tg determination by TSDC is direct, accurate and reproducible and that the obtained values correlate very well with those obtained by DSC and DRS. This general survey thus suggests TSDC as a valuable alternative technique for determining Tg.

The slow relaxation dynamics in active pharmaceutical ingredients studied by DSC and TSDC: Voriconazole, miconazole and itraconazole

The slow molecular mobility of three active pharmaceutical drugs (voriconazole, miconazole and itraconazole) has been studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). This study yielded the main kinetic features of the secondary relaxations and of the main (glass transition) relaxation, in particular their distribution of relaxation times. The dynamic fragility of the three glass formers was determined from DSC data (using two different procedures) and from TSDC data. According to our results voriconazole behaves as a relatively strong liquid, while miconazole is moderately fragile and itraconazole is a very fragile liquid. There are no studies in this area published in the literature relating to voriconazole. Also not available in the literature is a slow mobility study by dielectric relaxation spectroscopy in the amorphous miconazole. Apart from that, the results obtained are in reasonable agreement with published works using different experimental techniques.

Thermally stimulated depolarization current and dielectric spectroscopy used to study dipolar relaxations and trap level distribution in PMMA polymer

In this work, thermally stimulated depolarization current (TSDC) and dielectric relaxation spectroscopy (DRS) techniques were performed in complementarities to characterize the dielectric relaxation processes in poly(methylmethacrylate) (PMMA) polymer. Three relaxations were highlighted. Based on correlation between TSDC and DRS data, they were attributed respectively to dipolar β and α relaxations as well as interfacial ρ relaxation. New experimental procedure of fractional polarization was used to investigate space charge trap depth distribution within the band-gap of PMMA. Then the individual peaks were fitted to Bucci–Fieschi model, to evaluate the relaxation parameters. The obtained values were discussed in correlation to those evaluated from DRS data fitted to Havriliak–Negami and Arrhenius models. The various observed peaks in the thermograms are discussed on the basis of space charge polarization. The trap energies were evaluated from the fit to the Bucci–Fuschi model. Similarly other parameters such as relaxation time and charge release are evaluated.

Dielectric relaxation of 2-ethyl-1-hexanol around the glass transition by thermally stimulated depolarization currents

We explore new routes for characterizing the Debye-like and α relaxation in 2-ethyl-1-hexanol (2E1H) monoalcohol by using low frequency dielectric techniques including thermally stimulated depolarization current (TSDC) techniques and isothermal depolarization current methods. In this way, we have improved the resolution of the overlapped processes making it possible the analysis of the data in terms of a mode composition as expected for a chain-like response. Furthermore the explored ultralow frequencies enabled to study dynamics at relatively low temperatures close to the glass transition (Tg). Results show, on the one hand, that Debye-like and α relaxation timescales dramatically approach to each other upon decreasing temperature to Tg. On the other hand, the analysis of partial polarization TSDC data confirms the single exponential character of the Debye-like relaxation in 2E1H and rules out the presence of Rouse type modes in the scenario of a chain-like response. Finally, on crossing the glass transition, the Debye-like relaxation shows non-equilibrium effects which are further emphasized by aging treatment and would presumably emerge as a result of the arrest of the structural relaxation below Tg.

Photo-induced dipole relaxation current in natural Amethyst

Thermally stimulated depolarization current (TSDC) measurements were carried out for SiO2 in the amethyst form, aiming to investigate the relationship of observed current with relaxation phenomena related to quartz impurities. In addition to TSDC conventional dark procedure, photo-induced TSDC was also carried out, where the exciting light came from an Ar+ laser, tuned either at 488 nm or at 541 nm. X-ray diffraction and optical absorption measurements were used as complement for the interpretation of TSDC data. Optical absorption data, mainly in the range 400-700 nm, allow identifying the characteristic bands of amethyst as well as to relate them with TSDC and photo-induced TSDC data, leading to a relationship between absorption bands and light irradiation with selected wavelengths. These results allow determining how the formation of a TSDC band in the range 220-260 K, is affected by the light absorption, modifying the formation and the dipole orientation distribution in the samples. Results also help the verification of defects formed by Fe3+ or Fe4+ ions in the amethyst structure, as well as suggest that these defects, besides the participation in the amethyst structure as color centers, also play a role in the formation of TSDC bands, contributing for the observed effect of monochromatic light irradiation on these bands.

Influence of electric fields on the depolarization temperature of Mn-doped (1-x)Bi1/2Na1/2TiO3-xBaTiO3

The transition between induced long-range order and relaxor-like behavior upon heating is investigated in lead-free (1-x)Bi1/2Na1/2(Ti0.995Mn0.005)O3-xBa(Ti0.995Mn0.005)O3 piezoceramics with x = 0.03, 0.06, and 0.09 (BNT-100xBT:Mn). Temperature-dependent permittivity ɛ′(T) and thermally stimulated depolarization currents (TSDC) of poled samples were measured under identical heating conditions to clarify the depolarization mechanism. In both methods, the influence of electric bias fields on the transition temperature was investigated. Fields applied in the poling direction shift the transition to higher temperatures, with corresponding results in ɛ′(T) and TSDC measurements. While the response of transition temperature to external fields displays a similar trend in all investigated compositions, the shape of TSDC is clearly connected with the composition and, hence, the crystal symmetry of the sample. Furthermore, the comparison of ɛ′(T) and TSDC data reveals a systematic shift between transition temperatures obtained with the two different methods.

Electret effect in Pb5Ge3O11 crystals

The short-circuit thermally stimulated depolarization current (TSDC) through lead germanate single crystals has been measured as a function of temperature (100–800 K) and time. The results indicate that poling of the crystals at elevated temperatures (t ≥ 150°C) produces an electret state related to the spacecharge mechanism of polarization. Relatively mild poling conditions (E = 0.75 kV/cm, t = 150°C) produce a well-defined, long-lived electret with an abnormally large electret charge (1400 μC/cm2). The electret discharge leads to heating of the crystal, in the form of a peak in the range 520–570 K, whose height exceeds that of the pyroelectric peak near T C by two to three orders of magnitude. From the TSDC data, we evaluated the stored electret charge, its density, and the activation energy of traps for electret charges, and made preliminary conclusions as to the nature of the traps. We demonstrate that neglect of the electret discharge current (which depends on a number of factors) when determining the pyroelectric coefficient p σ above 280 K leads to a significant scatter in data.

Slow Molecular Mobility in the Amorphous Solid State of Fructose: Fragility and Aging

The molecular mobility of beta-D-fructose was studied by thermally stimulated depolarization currents (TSDC) in the amorphous solid state. The amorphous solid samples were prepared in such a way that the tautomeric mixture was near the equilibrium composition. A broad secondary relaxation was observed, that merges, at high temperatures, with the alpha relaxation. The alpha relaxation temperature provided by the TSDC technique is T(g) = 13 degrees C (at 4 K min(-1)). The fragility index calculated from TSDC data is m = 34, significantly lower when compared with the values reported in the literature obtained from Dielectric Relaxation Spectroscopy (DRS). The physical significance of the fragility obtained by the 2 dielectric techniques is discussed. The influence of physical aging on the secondary relaxation in amorphous fructose was analyzed as the glass structurally relaxes. A complex behavior was observed such that the faster components (lower temperature) of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes (higher temperature) show a significant dependence on aging and correspond to the genuine Johari-Goldstein beta-relaxation.

Effect of Acceptor (Mg) Concentration on the Resistance Degradation Behavior in Acceptor (Mg)‐Doped BaTiO3 Bulk Ceramics: II. Thermally Stimulated Depolarization Current Analysis

Thermally stimulated depolarization current (TSDC) of acceptor (Mg)‐doped BaTiO3 ceramics was analyzed with different acceptor concentrations for coarse‐grained specimens with uniform grain sizes. In all specimens, the increase of the polarization temperature (TP) for a fixed condition of polarization field (EP) and polarization time (tP) increased TSDC peak associated with an oxygen vacancy (VO••) relaxation. When the acceptor concentration is increased, both TP to generate same magnitude of TSDC and the relaxation temperature (Tm) of the TSDC peak systematically decreased. On the other hand, the activation energy of the oxygen vacancy relaxation showed roughly constant values of ∼0.9 eV, irrespective of acceptor concentration. Such behavior can be explained by a decrease in the relaxation time constant (τ0), which is in turn associated with the shape of oxygen vacancy profile in the specimen after polarization. The decrease of TP, Tm, τ0, and the little change of activation energy from the TSDC data with the increase of acceptor concentration implies an increase in the oxygen vacancy concentration C(VO••). The experimentally observed behavior of C(VO••) vs acceptor concentration could be explained by the defect chemical model, and from these results, the acceptor ionization energy EA was estimated to be about 1.0 eV.

Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques

We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at Tg, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (Tg) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10K as compared to the pure matrix).

Molecular Mobility in Raffinose in the Crystalline Pentahydrate Form and in the Amorphous Anhydrous Form

The aims of the study are to characterize the slow molecular mobility in solid raffinose in the crystalline pentahydrate form, as well as in the anhydrous amorphous form (Tg = 109 degrees C at 5 degrees C/min), and to analyze the differences and the similarities of the molecular motions in both forms. Thermally stimulated depolarization current (TSDC) is used to isolate the individual modes of motion present in raffinose, in the temperature range between -165 and +60 degrees C. From the experimental output of the TSDC experiments, the kinetic parameters associated with the different relaxational modes of motion were obtained, which allowed a detailed characterization of the distribution of relaxation times of the complex relaxations observed in raffinose. The features of the glass transition relaxation in raffinose were characterized by differential scanning calorimetry (DSC). A complex mobility was found in the crystalline form of raffinose. From the analysis of the TSDC data, we conclude that these molecular motions are local and noncooperative. A sub-Tg relaxation, or secondary process, was also detected and analyzed by TSDC in the amorphous phase. It has low activation energy and low degree of cooperativity. The glass transition was studied by DSC. The fragility index (Angell's scale) of raffinose obtained from DSC data is m = 148. TSDC proved to be an adequate technique to study the molecular mobility in the crystalline pentahydrate form of raffinose. In the amorphous form, on the other hand, the secondary relaxation was analyzed by TSDC, but the study of the glass transition relaxation was not possible by this experimental technique as a consequence of conductivity problems. The DSC study of the glass transition indicates that raffinose is an extremely fragile glass former.

Effect of high-energy heavy ion irradiation on dielectric relaxation behaviour of kapton-H polyimide

High-energy ion irradiation effects on the thermally stimulated depolarization current (TSDC) behaviour and dielectric constant (ε′)/loss factor have been investigated. Kapton-H samples of 25 μm thickness were irradiated with 50 MeV Si + beam (fluence 2.3×10 12 and 1.38×10 13 ions/cm 2). The TSD currents and dielectric constant/loss factor (frequency 1 and 100 kHz) have been investigated from room temperature to 250 °C, following the usual method. The TSDC spectrum of the irradiated samples reveal that the β-peak (appearing at ≈80 °C) associated with dipolar relaxation has been largely affected due to irradiation. At higher fluence, β-peak almost vanishes. The peak appearing at approximately 180 °C (α-peak) is due to space charge relaxation. The peak magnitude increases in samples irradiated with higher fluence. This is mainly due to the creation of new energy traps by irradiation. The ion irradiation results in a large increase in low temperature dielectric constant (30–70 °C) ascribed to the increase in water absorption capacity. The dielectric constant in temperature range 70–180 °C is mainly governed by dipolar relaxation and space charge relaxation. It is suggested that though the space charge relaxation tends to increase ε′, a major loss in carbonyl groups due to high-energy ion irradiation results in a decrease in ε′. In high temperature region (180–250 °C) the dominance of interfacial polarization increases the ε′ value. The loss peaks appearing at approximately 50, 180 and 250 °C are representative of the various relaxation processes. The findings of the TSDC data are in conformity with the dielectric constant data.

The activation energy at Tg and the fragility index of indomethacin, predicted from the influence of the heating rate on the temperature position and on the intensity of thermally stimulated depolarization current peak.

The purpose of this study was to estimate the activation energy at the glass transition temperature (and the fragility index) of amorphous indomethacin from the influence of heating rate on the features of the relaxation peaks obtained by thermally stimulated depolarization currents (TSDC) and to compare the obtained results with those obtained by other procedures based on TSDC data. The glass transition temperature region of amorphous indomethacin was characterized at different heating rates by TSDC in a way similar to that used to determine the kinetics of the glass transition relaxation by differential scanning calorimetry. The features of a thermal sampled TSDC peak, namely the temperature location and the intensity, depend on the heating rate. The activation energy for structural relaxation (directly related to glass fragility) was estimated from the heating rate dependence of the TSDC peak location, T(m), and of the maximum intensity of the TSDC peak, I(T(m)). The methods for determining the activation energy for structural relaxation and fragility of indomethacin from TSDC data obtained with different heating rates were compared with other procedures previously proposed. TSDC, which is not a very familiar technique in the community of pharmaceutical scientists, proved to be a very convenient technique to study molecular mobility and to determine the fragility index in glass-forming systems. The value of approximately 60 appears as a reasonable value of the fragility index of indomethacin.

Physical aging of poly(vinyl acetate). A thermally stimulated depolarization current investigation

The influence of physical aging on the dielectric α-relaxation of poly(vinyl acetate) (PVAc) is investigated by means of the thermally stimulated depolarization currents (TSDCs) technique. The obtained TSDC data are analyzed in terms of the Kohlrausch–Williams–Watts (KWW) equation for an isothermal decay of a stored charge. Assuming that at temperatures less than the glass transition temperature the shape of the KWW function depends neither on the temperature nor on the thermal history, the generalized Bucci and Fieschi equation allows us to obtain the dependence of the KWW time. The results obtained are interpreted in the Adam and Gibbs theory framework.

Secondary dielectric β-relaxation in amorphous poly(ethylene terephthalate): combined thermally stimulated and isothermal depolarization current investigations

The complex β dielectric relaxation process in amorphous poly(ethylene terephthalate) (PET) is investigated by thermally stimulated depolarization currents (TSDC) and isothermal depolarization currents (IDC) techniques. TSDC and IDC data are analyzed in the framework of the Ngai coupling model in the [−150; −40°C] temperature range using the Marchal method and involving a Kohlrausch–Williams–Watts (KWW) functional form of the polarization time decay. The temperature dependence of the characteristic τ KWW relaxation time and of the β KWW stretching parameter of the KWW relaxation function derived from both depolarization current methods for the β relaxation of PET are determined and then compared. The very good agreement observed between the two sets of experimental stretching parameters determined by TSDC and IDC confirms the high potentiality of the TSDC technique to study complex relaxation processes as such as the β relaxation of PET. The two different overlapped relaxation processes involved in the β relaxation have been confirmed throughout the determination of two different temperature behaviors of the β KWW parameter. Each process of complex β relaxation is characterized by a low and nearly temperature-independent KWW stretching parameter. We show that the β KWW value of the high-temperature component, involving phenyl group motions, is slightly lower than the one corresponding to the low-temperature process assigned to the motions of carbonyl groups.

Molecular mobility and fragility in indomethacin: a thermally stimulated depolarization current study.

To show that thermally stimulated depolarization currents (TSDC), which is a dielectric experimental technique relatively unknown in the pharmaceutical scientists community, is a powerful technique to study molecular mobility in pharmaceutical solids, below their glass transition temperature (Tg). Indomethacin (Tg = 42 degrees C) is used as a model compound. TSDC is used to isolate the individual modes of motion present in indomethacin, in the temperature range between -165 degrees C and +60 degrees C. From the experimental output of the TSDC experiments, the kinetic parameters associated with the different relaxational modes of motion were obtained, which allowed a detailed characterization of the distribution of relaxation times of the complex relaxations observed in indomethacin. Two different relaxational processes were detected and characterized: the glass transition relaxation, or alpha-process, and a sub-Tg relaxation, or secondary process. The lower temperature secondary process presents a very low intensity, a very low activation energy, and a very low degree of cooperativity. The fragility index (Angell's scale) of indomethacin obtained from TSDC data is m = 64, which can be compared with other values reported in the literature and obtained from other experimental techniques. TSDC data indicate that indomethacin is a relatively strong glass former (fragility similar to glycerol but lower than sorbitol, trehalose, and sucrose). The high-resolution power of the TSDC technique is illustrated by the fact that it detected and characterized the secondary relaxation in indomethacin, which was not possible by other techniques.

A thermally stimulated depolarization current study of polymers in the glass transition region

AbstractThe low‐frequency dielectric properties of a number of polymers, composites and blends have been studied using a thermally stimulated depolarization current (TSDC) apparatus that was designed and constructed in‐house. The TSDC technique can be used to determine the glass transition of a polymer sample. This TSDC glass transition temperature has been shown to be very similar to that obtained from differential scanning calorimetry (DSC). The actual difference between these two values depends on the heating rates used with each technique, however. TSDC data can also be combined with AC dielectric data to produce a data set, which possesses a very wide frequency range. Finally, individual TSDC relaxation peaks can be fit with the Williams‐Walts distribution function to obtain an estimate of their distributions. This is especially useful when studying polymer blends, but could also be utilized in the study of other systems.

Effect of parasitic galvanic current on the thermally stimulated depolarization current of polymers

The parasitic galvanic current I in the observed thermally stimulated depolarization current (TSDC) at high temperature was found to obey the equation I = A exp(-E/kT) + C. The pure TSDC signal was obtained by deducting this parasitic conduction current from the experimental TSDC data. Using this method, the pure TSDC spectra of polythiourea, polyvinylidene fluoride-trifluoroethylene (VDF/TrFE) copolymer and polyethylene terephthalate were obtained.

Glass transition relaxation and fragility in the molecular glass forming m-toluidine: A study by thermally stimulated depolarization currents

The glass transition relaxation in m-toluidine was studied by thermally stimulated depolarization currents (TSDC). The departure from the zero entropy prediction, also called compensation behavior, was analyzed and the fragility index of the glass-former m-toluidine was calculated from the TSDC data according to a recently proposed procedure. The obtained result was compared with literature values. A particular attention was focused on the physical aging process of the nonequilibrium glass, which was followed easily because of its slow evolution. The influence of the aging temperature on the aging rate was analyzed. The significance of the so-called compensation point was discussed on the basis of the results obtained in this study of structural aging.