Depolarization Current Measurements Research Articles

Relaxations in lamellar polystyrene–polybutadiene diblock copolymer. Thermally stimulated depolarization current study

The samples of the polystyrene-polybutadiene (PSPB) diblock copolymers with lamellar structure were investigated by integral and partial thermally stimulated depolarization current (TSDC) measurements in the 160–303 K range, and compared to the analogous investigation performed on a homopolymer sample of polystyrene (PS). The current peak, which appeared at 187 K, was found to be a dipolar relaxation α peak caused by the glass transition in the PB phase. The space-charge relaxation, distributed in the 220–260 K range and split into two parts – denoted as β 1 and β 2, was attributed to the possible local motion of the phenyl rings in the PS phase. The current maxima of the β relaxations significantly increased and shifted to the lower temperatures in the copolymer samples due to the PS–PB interfaces. The occurrence of doubleness and the relative distance between β 1 and β 2 was discussed. The relative numbers of the relaxing dipoles ( N) vs. E a were approximated. The energy width ( ω) was significantly increased in the copolymers due to the released motions at the glass–liquid PS–PB interfaces.

Comparative study of mechanical and electrical relaxations in poly(etherimide). Part 1

Relaxations in poly(etherimide) PEI Ultem 1000 have been analysed by differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) measurements. DMTA and DRS results show three distinct relaxations γ, β and α in the temperature range -140 to 250°C. The first one depends strongly on the water content in the sample as will be discussed in more detail in the second paper of this series. These results are in good agreement with those observed by TSDC of conventionally polarized electrets. In addition to these three relaxations, TSDC measurements show: (1) a peak (ρ) at which is attributed to space charge temperatures above the α relaxation, (2) indications of structure in the β relaxation zone. In the case of electrets formed by the windowing polarization method, the resulting TSDC spectra allow us to discern the fine structure of the β relaxation, which is formed by three sub-relaxations. In this work, the activation energies calculated by the different techniques are compared, and a molecular origin for each relaxation is proposed. © 1998 SCI.

Comparative study of mechanical and electrical relaxations in poly(etherimide). Part 1

Relaxations in poly(etherimide) PEI Ultem 1000 have been analysed by differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) measurements. DMTA and DRS results show three distinct relaxations γ, β and α in the temperature range -140 to 250°C. The first one depends strongly on the water content in the sample as will be discussed in more detail in the second paper of this series. These results are in good agreement with those observed by TSDC of conventionally polarized electrets. In addition to these three relaxations, TSDC measurements show: (1) a peak (ρ) at which is attributed to space charge temperatures above the α relaxation, (2) indications of structure in the β relaxation zone. In the case of electrets formed by the windowing polarization method, the resulting TSDC spectra allow us to discern the fine structure of the β relaxation, which is formed by three sub-relaxations. In this work, the activation energies calculated by the different techniques are compared, and a molecular origin for each relaxation is proposed. © 1998 SCI.

A new method of data analysis in thermally stimulated depolarisation current experiments (αrelaxation in amorphous polymer)

In this paper we propose a new method of data analysis to interpret results obtained by Thermally Stimulated Depolarisation Current (TSDC) measurements on glassy materials. We give a new law to link the evolution of the relaxation time τ with the temperature T . While the classical law (compensation law) is only convenient in a small range of temperatures (around the compensation temperature T c ), a calculation of the depolarisation current, taking into account this new law, leads to a curve I vs. T very close to the real depolarisation current measured on a polymer in the whole range of temperatures. In addition this new law allows the determination of the compensation temperature T c from the complex spectrum which needs single scan of measurements. To our knowledge it is the first time that this determination is proposed from the complex spectrum.

Thermally Stimulated Depolarization Current Measurements

A law is proposed to link the evolution of the relaxation time τ with the temperature T in thermally stimulated depolarization current (TSDC) experiments (complex spectrum and elementary spectra). This model is equivalent to the classical one (compensation law) around T c. Its response, obtained by simulation of the complex and elementary spectra, is very close to the real depolarization current curves throughout the whole range of temperatures. Experimental results were obtained on an amorphous copolymer: PETN8 (92% of poly (ethylene terephthalate) (PET); 8% poly(ethylene naphthalene) (PEN).

An Electrical Model of Summation of Thermally Stimulated Depolarization Current by Means of a Ps-Pice Software

In this paper we present an electrical model of simulation. This model, used to simulate thermally stimulated depolarization current (TSDC) measurements, yields the same behaviour for its time constant as that observed for amorphous materials in a large temperature interval. This model, whose main device is a current source controlled by the exponential of a voltage with linear temperature dependence could be considered as a generalization of the classical model generally used in TSDC interpretation attempts. Its response is very close to that observed in various TSDC measurements carried out on different polymers.

The thermally induced phase transition in 2,3,7,8‐tetramethoxythianthrene

Abstract2,3,7,8‐Tetramethoxythianthrene is known to crystallize in orthorhombic and monoclinic polymorphs, depending on the solvent used for crystallization. Upon heating the orthorhombic form to ca. 150°C, an enantiotropic (reversible) phase transition into the monoclinic phase occurs. This phase transition has been characterized by different techniques: differential scanning calorimetry (DSC), temperature‐resolved X‐ray powder diffractometry (TXRD), and thermally stimulated depolarization current measurements (TSDC). A mechanism for this solid‐state transition is proposed based on single‐crystal X‐ray structures.

Formation and relaxation of poled order in dye doped polystyrene probed by isothermal and nonisothermal current measurements

Experimental results are presented on isothermal and nonisothermal electric current measurements obtained with a polystyrene and disperse red 1 guest–host system containing different percentages of the dye. The superposition principle holds for isothermal measurements and polarization currents seem to be almost independent of temperature in the range from 30 to 70 °C, below the glass transition temperature Tg. Thermally stimulated depolarization (TSD) current measurements show that the polarization induced in the samples depends strongly on poling conditions. Samples poled at temperatures above Tg have a lower polarization than those poled just below Tg. If the poling temperature Tp is far below Tg, the TSD current peak appears at much higher temperatures than Tp. The TSD curves differ somewhat from corresponding curves obtained during linear heating under applied voltage.

Electrical simulation of Thermally Stimulated Depolarization Current Experiments

In this paper we present an electrical model of simulation. This model is used to simulate Thermally Stimulated Depolarization Current measurements. The time constant of the model shows the same temperature behaviour as that observed for amorphous materials. The computations by means of SPICE software give results which are very close to be behaviour laws used in the theory of relaxation in amorphous materials.

Thermally stimulated depolarisation current measurements and interpretation

The fictive thermodynamic equilibrium temperature Tfi determined from thermal investigation (differential scanning calorimetry, for example) and the compensation temperature Tc introduced in thermally stimulated depolarisation current measurements are compared. A brief description is given of the experimental apparatus of TSDC measurements built in the authors' laboratory, and the main parameters and results obtained from the two methods (DSC and TSDC) are given. An interpretation of the experimental equality observed between the experimental values of the thermodynamic equilibrium temperature Tfi obtained from DSC investigations and the temperature compensation Tc introduced in TSDC measurements is proposed.

Thermally stimulated depolarization current of novolac resin

A novolac phenol-formaldehyde resin was investigated via the thermally stimulated depolarization current, using integral and partial measurements in the temperature range from 137 to 270 K. Tow broadened peaks, assigned as Β1, and Β2, appeared at about 160 and 190 K. The influence of water and ¯Mn was investigated. The activation energy Ea vs. T relationship was analysed, and a search was made for compensation phenomena. The distribution of the relaxing dipolesN vs. Ea was approximated. Three different relaxation ranges were distinguished. All the motions were attributed to the rotation of the phenyl rings. The differences found are due to the heterogeneity in the resin. Contamination with water increases the polarizability, decreases the structure differences and relieves the motions in the resin. A resin with a smaller ¯Mn exhibits a higher polarizability and a decreases in structure variety.

The onset of an electric field-induced ferroelectric-like phase in the perovskite

We study the effect of DC electric field application on a ceramic by impedance spectroscopy and depolarization current measurements. The results indicate the onset of a true ferroelectric phase, characterized by long-range order, instead of a glassy one. The `freezing' temperature deduced from the Vogel - Fulcher relationship coincides with the phase transition one, whereas the fitted relaxation frequency displays a minimum, as in usual ferroelectrics. The chemical and physical structure of the material is the key parameter to understand the dielectric response which results both from the clusters' dynamics and from their development.

Thermally stimulated depolarization current of monovalent copper ions in calcium fluoride

Abstract Thermally stimulated depolarization current (TSDC) measurements of CaF2:Cu+ samples show three different dipolar relaxation bands, with temperature peaks at 52, 144 and 187 K. The two higher temperature bands are more intense and reveal the presence of Cu+ in probably two different lattice sites. The band at 144 K suggests it is related to the Cu+ off-center effect, observed through an optical absorption at 328 nm, and it is likely temperature independent. To this Cu+ off-center effect it is also associated an impurity-vacancy defect, that result from needed charge compensation in CaF2. The observed TSDC band at 187 K is attributed to Cu+ i—Fi − pair, both ions in interstitial positions. The weakest TSDC at 52 K is assumed to be from a low un-intentional Mn2+ doping, sitting in an off-center substitutional position. It is proposed a model to explain the several positions available for the substitutional Cu+ that response for the TSDC result.

CU+ high doping effects in KCl and KBr films

Highly Cu + doped KCl and KBr polycrystalline films were investigated in the range of 10 20 -10 21 cm -3 Cu + concentration. Several techniques were used to obtain the optical and structural properties, as optical absorption and transmittance, electronic microscopy (SEM), X-ray diffraction and ellipsometry. The Cu + off-center effect is still present in the films, as determined by optical and thermally stimulated depolarization current measurements (TSDC). The films were obtained by thermal evaporation, and the corresponding doping Cu-halide is introduced in the same evaporating crucible. Samples with diverse concentrations, ranging from nominal 1 to 10% mole, were prepared. The increase of Cu + concentration results in an improved cyrstallinity of the grains, with higher density and a better transmittance of the film above 350 nm. Optical application as UV filter device can be considered.

Structural modifications of aluminum-polyethylene terephthalate-aluminum systems as evidenced by thermally stimulated polarization currents

The relaxation of thin aluminized polyethylene terephthalate (PET) films was investigated by means of the thermally stimulated polarization current method with the aim of evidencing structural modifications occurring during thermal treatment carried out under various conditions. The occurrence of a marked change in the charge exchange properties of the Al-PET interfaces upon annealing, previously advocated from thermally stimulated depolarization current measurements, has been confirmed. Molecular orientation and free-volume relaxation are shown to be determining factors governing the polarization processes in thin PET films.

Polymer-water interactions in poly(hydroxyethyl acrylate) hydrogels studied by dielectric, calorimetric and sorption isotherm measurements

Abstract Poly(hydroxyethyl acrylate) hydrogels with water contents up to 0.96 (g water/g dry material) are studied with equilibrium and dynamic water sorption isotherm measurements, differential scanning calorimetry, thermally stimulated depolarization current measurements and broadband a.c. dielectric relaxation spectroscopy. Our main interest is focused on comparing results on the state of water in the hydrogel obtained with the different techniques on the same sample and on discussing in a quantitative way the relations among the different descriptions imposed by the different techniques. The results show that for water contents lower than about 0.20-0.30 the hydrogel is a homogeneous system in the temperature range down to −100 °C. At higher water contents phase separation occurs. Several critical water contents have been obtained by means of the different techniques used.

Vulcanization influence on tsd currents and wettability of room temperature vulcanized silicone rubber hv insulator coatings

AbstractThis paper deals with the Thermally Stimulated Depolarization (TSD) current and contact angle measurements on Room Temperature Vulcanized (RTV) silicone rubber High‐Voltage (HV) insulator coatings taken recently by Dr. S. M. Gubanski and Dr. M.‐S. E. Wang.

Thermally stimulated relaxations in DNA.

The thermally stimulated depolarization current (TSDC) measurements in frozen aqueous solutions, gels and solid layers of NaDNA show typically up to three dipolar overlapping peaks in the low-temperature range of 80-150 K. Up to four discrete relaxation peaks have been observed at higher temperatures above 150 K. The low-temperature TSDC peaks are due to the dipolar relaxations of free and loosely bound water which crystallizes. Part of bound water especially in the first hydration shell of DNA molecule is at low temperatures in the form of glass. The transition of this glass from solidlike behavior to liquidlike behavior observed mainly in gels and solid samples is associated with a previously founded TSDC relaxation peak. The peak is at its maximum at 165-250 K depending on the sample humidity. Existence of this relaxation in the samples with water contents in a broad range confirms, that the slowly relaxing shell (minimally 5-7 water molecules/nucleotide) closely associated with DNA double helix retains its characteristics. Also another peak of the high-temperature band at 180-205 K which was observed in the samples at hydration 2-1800 g H2O/g dry NaDNA is due to a relaxation in the sample volume. At the highest temperatures relax the space charges trapped at the electrodes.

Thermally stimulated polarization and depolarization processes in Al2O3:Mg.

Thermally stimulated polarization (TSP) and depolarization current measurements in as-cut as well as in oxidized magnesium-doped aluminum oxide show a huge peak at around 265 K, another one at 245 K, and a broadband at higher temperatures. They are related to the presence of ${\mathrm{Mg}}_{\mathrm{Al}}^{\mathit{x}}$ centers (substitutional magnesium ions with a hole trapped at a neighboring oxygen ion) and originate from the thermally activated ionization of these centers, which release holes. The current-voltage curves and the TSP results related to the main polarization process show that this is due to the formation of blocking contacts at the sample-electrode interfaces.

Investigation of β relaxation in novolac phenol-formaldehyde resin by use of thermally stimulated depolarization current

The β relaxations of novolac phenol-formaldehyde (NPF) resin were investigated by whole and partial thermally stimulated depolarization current (t.s.d.c.) measurements in the temperature region from 137 to 270 K. The whole measurements showed two peaks with maximum current at 158 and 187 K, respectively. The peaks obtained by partial polarization were characterized by the activation energy ( E a ) and relaxation time at the temperature of the peak maximum. The relaxation map and the compensation diagram were drawn. The mean E a for both relaxations is 0.51 eV. The initial part of β 2 obeys the compensation law with T c =308 K, 26 K below T g . The appearance of extremely low convergence of the Arrhenius lines and thus irrational compensation points due to the narrow width of the activation energy (Δ E a ) were discussed. The relaxations are supposed to be caused by the motions of the phenyl rings around the methylene links.