Thermally Stimulated Depolarization Research Articles

ВЛИЯНИЕ ТОЛЩИНЫ ПЛЕНОК ПОЛИДИФЕНИЛЕНФТАЛИДА НА СПЕКТР ТОКОВ ДЕПОЛЯРИЗАЦИИ МНОГОСЛОЙНЫХ СТРУКТУР

The depolarization currents of multilayer polymer films of polydiphenylene phthalide were measured using the thermally stimulated depolarization (TSD) method in the range from 170 to 400 K. The solution concentration was 2.5 and 5 wt.%. Each layer of polymer film was pre-dried at room temperature, then annealed in an oven. The number of layers of polymer film was 1, 2, 3 and 7 layers for 2.5 wt.%. and 1, 2, 3, 5 and 7 layers for 5 wt.%. The polarizing electric field strength was 2.5 × 105 V/cm. The rate of temperature change was 5 K/min. Based on experimental data, the dependence of the temperature of the position of the maximum peak of the depolarization current, the maximum values of the peaks of the depolarization currents, the values of the released charge and the activation energy of relaxation processes on the number of layers of the polymer film is shown. The temperature dependences of depolarization currents revealed two regions for a solution concentration of 2.5 wt.%. and one area for 5 wt.%. The first region is interpreted in the dipole charge approximation. The second region is explained by the presence of a wide distribution of traps occupied by injected charges. Interlayer polarization is observed in inhomogeneous dielectrics and is caused by differences in the electrical conductivity and dielectric constant of individual particles (or microparticles) of the dielectric. In such dielectrics, when an electric field is applied at the interface between its various components, charge will accumulate, which is equivalent to the creation of polarization. The results for the temperature dependences of the TSD currents of the samples showed that the total accumulated charge in the volume increases as the number of polymer layers increases. The reason for such a high concentration of trapped charges is most likely due to the presence of a wide distribution of traps occupied by injected charges. Moreover, the magnitude of the released charge is greater for samples prepared from a polymer solution with 2.5 wt. % This effect depends on the thickness of the polymer films. Namely, the thinner the films, the more charged particles penetrate the interface between the films.

Crosslinkable benzocyclobutene based host polymer for high temporal stability of organic second-order nonlinear optical materials

Second-order nonlinear optical (NLO) materials have become a pivotal core for photonic devices with the development of the information globalization. The alignment stability of chromophores is the key factor affecting the services life of materials. Benzocyclobutene (BCB)-based resin, as a good encapsulating materials, exhibited excellent thermal stability after cross-linking reaction. BCB remains a challenge if used directly in NLO materials due to its high curing temperature. Benzocyclobutene (BCB)-based co-polymer have been synthesized and systematically investigated for improving temporal stability of NLO materials, and bismaleimide (BMI) had been introduced as curing agent for further tuning the curing temperature of BCB in this paper. As differential scanning calorimetry (DSC) curves analysis, introduction of curing agent BMI could efficiently improve reactivity of the ring-opening and then cycloaddition of BCB group leading to lower curing temperature (the peak temperature decreased from 194 °C to 160 °C), which prevent the decomposition of doped chromophore during cross-linking. Thermal stimulated depolarization (TSD) results indicated that the temporal stability of NLO materials was remarkably increased due to the cross-linked network structure.

Effect of LiCoO 2 on the positive temperature coefficient effect (PTC) properties of semiconductive composites and its ability to inhibit space charge injection

The semiconductive shield layer is between the conductive core and the insulation layer of high voltage direct current transmission cables and plays an important role in suppressing the accumulation of space charge in the insulation layer. Since the prevalent positive temperature coefficient effect (PTC) of the semiconducting layer leads to cable ageing and failure, this paper proposes the use of lithium cobaltate to modify the semiconducting shield to suppress its PTC effect and improve its ability to inhibit space charge injection. The ionic conductor LiCoO2 was prepared by the sol–gel method. LiCoO2 particles with particle sizes ranging from tens to hundreds of nanometres were obtained by ball milling. The prepared LiCoO2 was used as a filler to modify the carbon black (CB)/ethylene vinyl acetate copolymer (EVA)/low density polyethylene (LDPE) composites. After melt blending and moulding cross-linking, LiCoO2/CB/EVA/LDPE semiconductive shielding specimens were successfully fabricated. Resistivity, Thermally Stimulated Depolarisation Currents (TSDC), and pulsed electro-acoustic measurements were conducted for the prepared specimens. The PTC effect, which is common in polymer composites, was successfully suppressed, and the PTC strength was reduced by 17%. The resistivity of the semiconductive shielding layer was significantly reduced. The ability of the semiconducting shield to inhibit space charge injection from the conductive core to the insulation layer was significantly improved. When LiCoO2 content is 0.5 wt%, the semiconductive shielding layer has the best performance, and the space charge in the insulation layer was reduced by 70% compared to the undoped semiconducting shield.

A Dual-Phase Approach to Reveal the Presence and Impact of the TLL Transition in Polymer Melts. Part II. The Theoretical and Practical Relevance of the TLL Existence and Characteristics on the Understanding of the Melt Thermal Analysis and Rheological Properties of Polymers: Duality and Cross-Duality of the Interactions

In Part I of these 2 parts article, we derived mathematically the existence of a unique state for polymeric melts, occurring at a specific temperature above Tg, which we recognized to be the liquid–liquid transition, TLL, observed and described by Boyer and others. TLL is the temperature at which the melt is in an iso-free-volume and iso-enthalpic state independent of the molecular weight. It is a fundamental property of the material. The purpose of part II is to examine and explain the following: 1) the elusive character of TLL (at the origin of the controversy about the existence of TLL in the past), 2) the increase of free volume at TLL, and 3) the endothermal change of heat capacity on heating across TLL. Finally, our objective is to provide an explanation of TLL and emphasize its importance as an example of a self-dissipative dynamic process that converts, at TLL, into a classical thermally activated process. In this article, the experimental evidence found in the literature for TLL is critically examined to point out the often biased reviews offered by the antagonistic authors of a controversy, here the pros and cons TLL. We propose a Dual-Phase origin of the interactions in polymers to explain the weak and elusive manifestations of TLL and show, by DSC, that the TLL manifestations are made much more visible and prominent when the samples’ state has been brought out of equilibrium. We analyze, in detail, the thermally activated depolarization of samples which have been submitted to a polarization stage by a voltage field. The experimental technique of thermal stimulated depolarization (TSD), and its sister derivative the thermal windowing deconvolution (TWD), are unique and powerful analytical tools that can experimentally characterize “interactive coupling”, the factor that we have assumed is quantitatively responsible for the behavior of polymers and in particular of TLL. The existence and the characteristics of TLL were understood and predicted in Part I from rheological results by the use of the Thermo–Vogel–Fulcher equation whose thermo-kinetic terms, ΔH, ΔS, and T∞, could be interpreted by the interactive coupling of the local free volume and the rotational isomeric conformational state of dual-conformers belonging to the macromolecules, themselves embedded in a collective dissipative system of interactions. The statistics controlling the interactive coupling parameters was described by the Dual-Phase and Cross-Dual-Phase models. In Part II, the same models are used to explain the interactive coupling manifestations specific to the TSD and TWD results. We show that certain characteristics of the TSD and TWD results are directly related to specific parameters of the Dual-Phase model. It is the case for the transitions visible by TSD, such as Tg,ρ related to space charges and local free volume (F-conformers), and TLL marking the end of the specific impact of the Dual-Phase statistics on the properties. It is also the case when interactive coupling is analyzed by TWD: the compensation of the enthalpy and entropy of activation of the relaxations taking place at various polarization temperatures only occurs below TLL, permitting its specific determination. We conclude pointing out the perhaps crucial importance of TLL in establishing the distinct role of thermal energy in structuring or modulating the dynamics of the interactions. The Dual-Phase view of the interactions in polymers suggests that the local density difference between the b-grains and the F-conformers is “time-averaged” by the constant wiping (above Tg) of an “elastic dissipative wave” having a frequency, ωo, i.e., a function of temperature and molecular weight, and thus is different from the Brownian dissipation, i.e., the thermal fluctuation characteristic of the Boltzmann’s mean field (the classical kT/h term). The elastic dissipative wave kinetically loses its collective modulation role and becomes the thermal wave at TLL.

Technology of thermally stimulated diagnostics of anisotropy and optical axes in crystals

For implementing the technology of thermally stimulated diagnostics of anisotropy and optical axes in crystals, the sample is thermostated at a temperature not exceeding the melting point, an electric field not exceeding the breakdown field is applied to the sample and polarization is produced for a time greater than the relaxation time at this temperature. After that, without switching off the electric field, the sample is cooled to the liquid nitrogen temperature, following which the field is switched off, the sample is linearly heated to a temperature above the polarization temperature and the resultant thermally stimulated depolarization (TSD) spectra taken along and perpendicular to the optical axis of the crystal are examined. When comparing the spectra the presence of anisotropy is detected and the direction of the optical axes is determined from the magnitude and presence of the TSD maxima.

Technology for thermostimulated diagnostics of anisotropy and optical axes of crystals

To implement the technology of thermally stimulated diagnostics of anisotropy and optical axes of crystals, the sample is thermostated at a temperature not exceeding the melting point, an electric field not exceeding the breakdown field is applied to the sample, polarization is produced for a time greater than the relaxation time at this temperature. After that, without disconnecting the electric field, cooling to the temperature of liquid nitrogen is performed, then the field is switched off, the sample is linearly heated to a temperature above the polarization temperature and the obtained thermally stimulated depolarization (TSD) spectra taken along and perpendicular to the optical axis of the sixth order C6 crystal are examined. When comparing the obtained spectra, the presence of anisotropy is determined, and the exact direction of the optical axes is determined by the magnitude and presence of the TSD maxima.

External impacts on structure and relaxation properties of thermotropic liquid crystal copolyester

The methods of thermally stimulated depolarization (TSD) and X-ray diffraction analysis were used to study the impact of electric and temperature fields on the relaxation properties, structure, and molecular mobility of a liquid crystal (LC) copolymer based on polyethylene terephthalate (PET) and n-hydroxybenzoic acid (n – HBA). It was shown that polarization in the LC state besides the main relaxation transition leads to the formation of additional high-temperature maximum on current thermograms of thermally stimulated depolarization (TSD). The data of X-ray diffraction analysis indicate a change in the orientation of mesogenic units after polarization in an electric field. The results were explained by the combined action of electric and temperature fields on the conformational structure of the liquid crystal copolyester, namely, a change in the degree of crystallinity of the studied copolyester under the influence of external fields.

The Effect of Thickness of Submicron Films of Electroactive Polymers on Thermally Stimulated Depolarization Currents

The results of studying the thermally stimulated depolarization currents in thin films of polydiphenylenephthalide (PDP) with a thickness of 20 nm to 1.4 μm under a constant stress in a structure with an insulating dielectric layer are presented. Nonmonotonic behavior of the dependence of the activation energy of charge carrier trapping sites and the relaxation of the space charge on the thickness of PDP polymer films is found. A correlation between the change in the supramolecular structure of the films and the above parameters is established. The obtained results are interpreted using the theory of thermally stimulated discharge in short-circuit heterogeneous structures.

Features of the Charge State of UHMWPE + α-SiO2 Nanocomposites

This paper presents the results into the study of electret properties of composite materials based on ultrahigh molecular weight polyethylene (UHMWPE) and nano-dispersed amorphous silica filler α-SiO2 (aerosil) depending on the volume content of silicon dioxide and the absorbed dose D γ-irradiation at room temperature using the method of thermally stimulated depolarization (TSD). It was found that introducing up to 5% of spherical nanoparticles of amorphous α-SiO2 silicon dioxide with a diameter of 20 nm in UHMWPE first increases and then reduces the electret characteristics of the composites. UHMWPE + 1% α-SiO2 has the maximum value of the electret potential difference and the stability of the electret state. It was shown that preliminary γ-irradiation affects the electret properties of the studied composite material, which is associated with its increased electrical conductivity and a high charge relaxation rate. The observed increase in the initial surface charge density σel of UHMWPE compositions is associated with the presence of energy traps of injected charge carriers. Based on the analyzing the spectra of TSD, it was shown that the thermal stability of electrets from the UHMWPE composite + 1% α-SiO2 is improved. The nature of the TSD current of pre-γ-irradiated films once again proves that, after γ-irradiation, the injected charges from the corona zone mainly accumulate in the near-surface layers of the irradiated samples. The results of TSD suggest that the relaxation of the electret state in the materials under study occurs due to the bulk conductivity.

Quantitative and qualitative analysis of segmental dielectric relaxations and space charge by TSDC in nanocomposites of natural rubber/clay

The effect of adding three different layered clays, a sodium montmorillonite and two commercial modified montmorillonites, on the morphology and molecular dynamics of natural rubber characterized by Transmission Electron Microscopy and Thermally Stimulated Depolarization Currents (TSDC) was studied. Carbon black was employed as reinforcing filler in a standard compound prepared and used for comparison purposes. The morphological results revealed that the sample with Cloisite\circledR 15A displays the highest degree of exfoliation, which suggests a stronger compatibility between the organic and inorganic phases. When the dispersion degree increases, a decrease of the activation energy was found from the quantitative analysis of the space charge dielectric relaxations. From the qualitative analysis of the dipolar dielectric relaxations around $T_{g}$, changes in the dielectric relaxation profile and in the peak localization were attributed to probable interactions between the nanofillers and the elastomer in the glass transition region of the NR.

Effect of Semi-Conductive Layer Modified by Magnetic Particle SrFe12O19 on Charge Injection Characteristics of HVDC Cable.

For high voltage direct current (HVDC) cable, a semi-conductive layer lies between the conductor and the insulation layer; as the charge migrates the path from the conductor to the insulation material, it will affect space charge injection. In this work, the research idea of changing the injection path of moving charges within semi-conductive layer by magnetic particles was proposed. Semi-conductive composites with different SrFe12O19 contents of 1 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.% were prepared, and the amount of injected charges in the insulation sample was characterized by space charge distribution, polarization current, and thermally-stimulated depolarization current. The experimental results show that a small amount of SrFe12O19 can significantly reduce charge injection in the insulation sample, owing to the deflection of the charge migration path, and only part of the electrons can enter the insulation sample. When the content is 5 wt.%, the insulation sample has the smallest charge amount, 0.89 × 10−7 C, decreasing by 37%, and the steady-state current is 6.01 × 10−10 A, decreasing by 22%. When SrFe12O19 content exceeds 10 wt.%, the charge suppression effect is not obvious and even leads to the increase of charge amount in the insulation sample, owing to the secondary injection of charges. Most moving charges will deflect towards the horizontal direction and cannot direct access to the insulation sample, resulting in a large number of charges accumulation in the semi-conductive layer. These charges will seriously enhance the interface electric field near the insulation sample, leading to the secondary injection of charges, which are easier to inject into the insulation sample.

TSDC measurements to analyze the electrical ageing state of large generator stator bar insulation

Abstract The electrical ageing status of stator bar epoxy-mica insulation is studied for the potential application in the large generators. Insulation testing specimens of four stator bars from the same manufacturer at different electrical ageing time were all investigated by TSDC (Thermal Stimulated Depolarization Current) at the polarization time of 10mins, the polarization temperature of 60 °C and the polarization field strength of 600V/mm. SEM (Scanning Electron Microscope) and FTIR (Fourier Transform Infrared Spectroscopy) were also used as the auxiliary methods to analyze the insulation in different ways. The results indicated that the insulation were subjected to microcosmic structural change during the electrical ageing, also indicated that some of the polymer molecule chains of epoxy resin were disconnected and the polar functional groups were produced. The trap level and charge quantity of the epoxy-mica insulation generally increased with the electrical ageing time, but both of them were non-linear.

Analysis of Performance Instabilities of Hafnia‐Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents

AbstractThe discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high‐endurance memory applications like nonvolatile random‐access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field‐induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake‐up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance.

Spontaneous polarization of a nonstoichiometric ferroelectric NaNO2at low temperatures

We report measurements of the temperature dependence of the pyroelectric charge in a nonstoichiometric z-cut sample of sodium nitrite in the temperature range 4.2–300 K. The obtained data are supplemented by the measurements of thermally stimulated depolarization (TSD) in electric fields of different magnitudes and polarities. All the experimental results serve as the basis for constructing the temperature dependence of the spontaneous polarization ΔPs(T,U) and the pyroelectric coefficient γ(U,T) for fixed external stimuli. It is found that the value of ΔPs in the sample of sodium nitrite is negligible at low temperatures, up to 40 K. This indicates (for known experimental values of the linear expansion coefficients) a minor contribution of the sample piezoelectricity to the spontaneous polarization in this temperature range. Starting from T = 75 K, an exponential increase in the contribution to the total charge of the TSD is found only in the defect subsystem of the crystal. Based on the data of physical studies, a crystal-physics model is proposed, which differs from the model presented by Lines and Glass. Due to the splitting of ion positions in all three sublattices upon the transition to the paraelectric phase, NaNO2 crystals can be assigned to the three-dimensional type of ferroelectrics, according to the classification by Abrahams and Keve. This agrees with the estimate of the spontaneous polarization Ps ∼ 0.01 C/m2 given in the paper. This value is significantly different from the data obtained by repolarization of nonstoichiometric samples in strong electric fields up to the phase transition temperature.

Influence of Polarization Charges on the Photoluminescence Properties of Nanocomposites Based on Polyvinylidene Fluoride and Titanium Dioxide Nanoparticles

The purpose of this study is to investigate the electret properties and charge states of the nanocomposites based on the polyvinylidenefluoride and titanium dioxide nanoparticles. It has been found that, large amount of charges are accumulated in the interface as a result of migration polarization and these charges create rather large internal local field in which the polarization of titanium-dioxide nanoparticles occurs. Calculated, the amount of the charges accumulated in the interfacial layer of nanocomposites, the intensity of the field created by them and the value of charge activation energy of PVDF/TiO2 nanocomposites after polarization from thermally stimulated depolarization (TSD) curves. As well as the thickness of the interphase layer of PVDF/TiO2 nanocomposites was calculated and it has been revealed that, as titanium dioxide content increases in the polymeric matrix, the thickness of the interphase layer increases and reaches at its highest value at 5% volume content of TiO2 nanoparticles, after this it commences to decrease.

Photo-Induced Thermally Stimulated Depolarization Current (TSDC) in Natural and Synthetic Alexandrite (BeAl<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup>)

The investigation of electrical properties in alexandrite (BeAl2O4:Cr3+) in synthetic and natural forms is presented in this paper. Alexandrite is a rare and precious mineral that changes color according to the light incident on it. In the synthetic form, it is used technologically as an active laser medium. The electrical characterization was obtained using the Thermally Stimulated Depolarization Current (TSDC) technique, an interesting tool to study the behavior of impurities in insulators. Alexandrite presented the electric dipole relaxation phenomenon, both in natural and in synthetic samples. It was possible to observe TSDC bands for the synthetic sample at around 170 K, and at around 175 K for the natural sample. Besides, photo-induced TSDC measurements were performed through the excitement of the samples by using a continuous wave argon laser. In addition, photoluminescence measurements were performed to verify in advance whether the laser light would be absorbed by the sample, and in order to complement the photo-induced TSDC measurements analysis. The results of photo-induced TSDC experiments have contributed to the understanding of the TSDC bands behavior: the results obtained with the technique suggest that there is an effective participation of Cr3+ ions in the formation of TSDC bands because they were more intense when the sample was exposed to the argon laser beam.

Characterization of deep level defects and thermally stimulated depolarization phenomena in La-doped TlInS2 layered semiconductor

Lanthanum-doped high quality TlInS2 (TlInS2:La) ferroelectric-semiconductor was characterized by photo-induced current transient spectroscopy (PICTS). Different impurity centers are resolved and identified. Analyses of the experimental data were performed in order to determine the characteristic parameters of the extrinsic and intrinsic defects. The energies and capturing cross section of deep traps were obtained by using the heating rate method. The observed changes in the Thermally Stimulated Depolarization Currents (TSDC) near the phase transition points in TlInS2:La ferroelectric-semiconductor are interpreted as a result of self-polarization of the crystal due to the internal electric field caused by charged defects. The TSDC spectra show the depolarization peaks, which are attributed to defects of dipolar origin. These peaks provide important information on the defect structure and localized energy states in TlInS2:La. Thermal treatments of TlInS2:La under an external electric field, which was applied at different temperatures, allowed us to identify a peak in TSDC which was originated from La-dopant. It was established that deep energy level trap BTE43, which are active at low temperature (T ≤ 156 K) and have activation energy 0.29 eV and the capture cross section 2.2 × 10−14 cm2, corresponds to the La dopant. According to the PICTS results, the deep level trap center B5 is activated in the temperature region of incommensurate (IC) phases of TlInS2:La, having the giant static dielectric constant due to the structural disorders. From the PICTS simulation results for B5, native deep level trap having an activation energy of 0.3 eV and the capture cross section of 1.8 × 10−16 cm2 were established. A substantial amount of residual space charges is trapped by the deep level localized energy states of B5 in IC-phase. While the external electric field is applied, permanent dipoles, which are originated from the charged B5 deep level defects, are aligned in the direction of the applied electric field and the equilibrium polarization can be reached in a relatively short time. When the polarization field is maintained, while cooling the temperature of sample to a sufficiently low degrees, the relaxation times of the aligned dipoles drastically increases. Practically, frozen internal electric field or electrets states remain inside the TlInS2:La when the applied bias field is switched off. The influence of deep level defects on TSDC spectra of TlInS2:La has been revealed for the first time.

Thermally Activated Processes in CaMoO4 Crystals

The effect of annealing in the oxygen flow atmosphere of calcium molybdate crystals on their absorption spectra and thermally stimulated luminescence (TSL) glow curve was studied. The thermally stimulated depolarization (TSD), polarization (TSP) currents and temperature dependence of X-ray luminescence of CaMoO4were investigated in the temperature range of 100-320 K. One of the trap detected by TSL was found to disappear after high-temperature annealing of calcium molybdate single crystals. The nature of the absorption bands is discussed. The values of activation energy were calculated for the detected traps.

Absorption spectra of ZnO:Li thin films in the region of a phase transition

We have studied the temperature variation of the intrinsic absorption edge for ZnO:Li films obtained by high-frequency magnetron sputtering. We have observed anomalous behavior of the optical bandgap width in the vicinity of the temperature of a ferroelectric phase transition at T = 327 K. For the first time, we have observed the appearance of this phase transition on the thermally stimulated depolarization (TSD) current curves.

Dipole defects in beryl

Dipole defects in gamma irradiated and thermally treated beryl (Be3Al2Si6O18) samples have been studied using the Thermally Stimulated Depolarization Currents (TSDC) technique. TSDC experiments were performed in pink (morganite), green (emerald), blue (aquamarine) and colourless (goshenite) natural beryl. TSDC spectra present dipole peaks at 190K, 220K, 280K and 310K that change after gamma irradiation and thermal treatments. In morganite samples, for thermal treatments between 700K and 1100K, the 280K peak increase in intensity and the band at 220K disappears. An increase of the 280K peak and a decrease of the 190K peak were observed in the TSDC spectra of morganite after a gamma irradiation of 25kGy performed after the thermal treatments. In the case of emerald samples, thermal treatments enhanced the 280K peak and gamma irradiation partially destroyed this band. The goshenite TSDC spectra present only one band at 280K that is not affected either by thermal treatments or by gamma irradiation. All the observed peaks are of dipolar origin because the intensity of the bands is linearly dependent on the polarization field, behaviour of dipole defects. The systematic study, by means of TSDC measurements, of ionizing irradiation effects and thermal treatments in these crystals makes possible a better understanding of the role played by the impurities in beryl crystals.