Thermally Stimulated Depolarization Current Spectra Research Articles

Comparison of pulsed electroacoustic and thermally stimulated depolarization current measurements of thermally poled PET electrets

We have compared measurements of a set of polyethylene terephthalate (PET) electret samples by means of pulsed electroacoustic method (PEA) and thermally stimulated depolarization current (TSDC) techniques. Samples where the α or the ρ relaxation is activated show uniform polarization and image charge at the electrodes. Different electrode configurations have been tested when external charge carriers are injected into the sample. In these experiments, differences in TSDC spectra are explained by the different blocking behavior of the electrodes. All in all, PEA in combination with TSDC turns out to be a useful technique in the study of thermally poled electrets.

Thermally-stimulated defect relaxations and microwave/terahertz dielectric response of La,Al co-doped (Ba,Sr)La4Ti4O15 ceramics

The thermally-stimulated relaxation of defects in La,Al co-substituted (Ba,Sr)La4Ti4O15 dielectric ceramics with hexagonal perovskite structure were investigated systematically with the thermally stimulated depolarization current (TSDC) measurements. The ion substitution decreased the lattice parameters and the bond lengths of the cations and O2−. TSDC spectra indicated the main extrinsic defects in the ceramics were oxygen vacancies and the concentration of oxygen vacancies decreased with the increasing ion substitutions, which was derived from the decrease in the lattice parameters. The terahertz time-domain spectra indicated the damping factor of lattice vibration was reduced with the decline of the concentration of oxygen vacancies, which decreased the microwave/terahertz dielectric loss. High-performance (Ba0.2Sr0.8)0.75La4.25Ti3.75Al0.25O15 (εr = 43.9, Q × f = 84300 GHz, τf = -18.0 ppm/°C) ceramics were obtained. The correlations between crystal structures, defect behaviors and dielectric properties discussed in this work could provide guidance for the modification of microwave dielectric ceramics.

TSDC of Irradiated and Non-Irradiated Cellulose Acetate

Films of cellulose acetate (CA) have been prepared by casting method using tetrahydrofuran (THF). CA films are γ-irradiated with varying radiation doses of 10, 20, 30, 40, 50 and 60 kGy using cobalt-60 (60Co) source. Global thermally stimulated depolarization current (TSDC) of non-irradiated and irradiated CA samples has been investigated under the effect of various poling electric field (Ep) in a temperature range from 300 K to 440 K. It is observed that, global TSDC spectra of non-irradiated and irradiated CA samples are characterized by two relaxation peaks. One in the low temperature range ~321 K and the other in the high temperature range ~ 376-383 K are observed for non-irradiated sample. On the other hand, these temperatures are shifted towards lower temperature for irradiated samples to be located at 317 K and ~371 K. These relaxations are assigned as  and -relaxation and attributed to molecular motion of the polar acetate groups, C2H3O2 and polarization of the space charges, respectively. TS-technique has been carried out to decompose global TSDC spectra of all samples into its elementary peaks and the molecular parameters such as, activation energy and pre-exponential factor are calculated for each TS peak. Relaxation map (RM) of all samples has been analyzed using Eyring transformation and thermodynamic parameters such as, enthalpy activation (H), entropy activation (S) and Gibbs free energy (G) are estimated. The compensation phenomenon was verified by the linear relationship between both enthalpy and entropy.

Mechanism of Thermal Charge Relaxation in Poled Silicate Glasses in a Wide Temperature Range (From Liquid Nitrogen to Glass Melting Temperature).

Data on thermally stimulated depolarization current (TSDC) study of the same poled glass in the temperature range 100-1000 K are analyzed. Four specific temperature ranges in the TSDC spectrum of this glass are identified, with each range being attributed to the charge relaxation processes of different natures. During linear heating in the temperature range 100-250 K, charge relaxation is related to the adsorption/desorption of particles from the atmosphere, supposedly water cluster ions H+(H2O)n. The next TSDC band, which is observed at room temperature and above, is related to the disordering of the polar structural entities. The TSDC band in the temperature range 500-750 K is attributed to the relaxation of spatial charge by the diffusion mechanism. The TSDC band in the temperature range 750-1000 K is attributed to the relaxation of spatial charge by the viscous flow mechanism. All these data allowed drawing a schematic TSDC spectrum of silicate glasses in the full temperature range.

Polarization induced dielectric and electrical response of electrovector hydroxyapatite and ferroelectric sodium potassium niobate ceramics

The ability of hydroxyapatite (HA) to convert readily into an electret under low polarizing fields (⩽1 kV cm−1) makes it as an excellent substitute for electro-active prosthetic implants. In the present study, compacts of electrovector HA and ferroelectric Na0.5K0.5NbO3 (NKN) were developed via the spark plasma sintering route. The compacts were then polarized at various polarizing fields of 30, 50, and 90 kV cm−1, respectively, at 150 °C, for 1 h. Thereafter, samples were depolarized at heating rates of 1, 5, and 10 °C min−1, respectively. The thermally stimulated depolarization current (TSDC) measurement reveals that the charge density in HA increases from ∼0.2 nC cm−2 to ∼2 µC cm−2 with an increase in the polarizing field from 30 to 90 kV cm−1. The dielectric (ϵ), as well as the ac conductivity behaviour of unpolarized and polarized (90 kV cm−1) HA, are compared. The activation energy is measured to be in the range of 0.6–0.7 eV, which is mainly associated with proton (H+) migration. For NKN, TSDC spectra reveal a slight shift in the phase transition temperatures (TO−T, TC) towards higher temperatures, with an increase in the polarizing field (30–90 kV cm−1) as well as the heating rate. The dielectric and ac conductivity measurements suggest that the polarization of NKN slightly decreases TO−T and TC, as compared to its unpolarized counterpart. The conductivity in NKN is mainly attributed to the migration of oxygen vacancies and n-type charge carriers. Furthermore, x-ray photoelectron spectroscopy analyses suggest that the polarization of HA at such a high field (90 kV cm−1) does not alter its surface chemistry.

Loss evolution of hexagonal Ba0.6Sr0.4La4Ti4O15 microwave dielectric ceramics

In the present work, Ba0.6Sr0.4La4Ti4O15 (BSLT) ceramics with and without the sintering aid (1 wt% ZnO + 0.25 wt% WO3) were synthesized via the solid-state reaction method. Microwave loss evolutions of Ba0.6Sr0.4La4Ti4O15 were investigated emphatically utilizing thermally stimulated depolarization current (TSDC). The sintering behavior, crystal structure and microwave dielectric properties were contrasted mutually. X-ray diffraction confirms that single phase solid solutions are achieved with a hexagonal perovskite structure. And the addition of the sintering aid causes the movement of diffraction peaks to low angles. Due to the incorporation into lattice, the sintering characteristic has not been improved by the sintering aid. With ZnO and WO3, BSLT sintered at 1500 °C demonstrates microwave dielectric properties of er = 46, Qf = 21,100 GHz and τf = − 4.6 ppm/°C. Especially, the Qf value displays an obvious drop from 48,600 to 21,100 GHz, thus a near zero τf value may be achieved. The extrinsic loss mechanism associated with defects chemistry is explored and TSDC spectra indicate a distinct difference induced by the sintering aid. It can be manifested that the Qf variation originates in the different concentration of oxygen vacancies in host dielectrics.

Thermally stimulated depolarization current study on barium titanate single crystals

Thermally stimulated depolarization current (TSDC) measurements were applied to investigate BaTiO3 single crystals with <100>, <110>, and <111> orientations, and annealed under different atmospheres, respectively. The TSDC peaks related to ferroelectric domain, ferroelectric–paraelectric phase transition, TiTi′−VÖ dipole relaxation and oxygen vacancy migration were observed from the TSDC spectra. The formation of oxygen vacancy clusters in BaTiO3 single crystals leads to a high dipole activation energy. The <111> crystalline face of BaTiO3 is more stable under external field than others.

Thermally stimulated depolarization currents and dielectric properties of Mg0.95Ca0.05TiO3 filled HDPE composites

Mg0.95Ca0.05TiO3 (MCT) filled high density polyethylene (HDPE) composites were prepared by twin-screw extrusion followed by hot pressing technique. The thermally stimulated depolarization current (TSDC) measurement was performed to analyze the contribution of charge distribution and interfacial characteristics to the dielectric loss. TSDC spectra under different polarization conditions show that the introduction of ceramic fillers engenders shallow traps in the vicinity of ceramic-polymer interface, which hinders the injection of space charge from the electrode into the polymer matrix. In the composite materials applied to an external field, charges tend to be captured by these traps. The temperature dependence of relative permittivity and dielectric loss of the composites was measured, and a strong reliance of dielectric loss on temperature was observed. In the heating process, the release of charges accumulating at interfacial region is considered to contribute to the rise in dielectric loss with the increase of temperature.

Study of Trapping Phenomena in SrTiO3 by Thermally Stimulated Techniques

Thermally stimulated current (TSC), thermally stimulated depolarization current (TSDC), and thermally stimulated luminescence (TSL) spectroscopies were combined to study trapping phenomena in undoped bulk SrTiO3 crystals. Electrical measurements were also performed and showed that the crystals are highly resistive in the dark but exhibited an unusually high photocurrent upon 400-nm illumination. Several traps were revealed in both TSC and TSDC spectra between 83 K and 450 K in such a broad temperature range and their activation energies were extrapolated from the trap positions (peaks). TSL spectra demonstrate similar characteristics comparable to TSC and TSDC spectra, though there are some differences because of different excitation and recombination mechanisms. This work reveals the presence of large number of traps in SrTiO3 single crystals, which are most likely the source of many of the interesting phenomena in SrTiO3 such as transient and persistent photoconductivity.

Effect of nano‐TiO2 surface modification on polarization characteristics and corona aging performance of polyimide nano‐composites

ABSTRACTUsing coupling agent isocyanatopropyltriethoxysilane (ICTOS) to modified nano‐TiO2, the polyimide (PI) with different titanium dioxide (TiO2) contents (0, 1, 2, 3, 4, and 5 wt %) doped nano‐composites were prepared by sol–gel method (PI/TiO2 ICTOS composites). The effect of ICTOS modification on polarization and time‐to‐breakdown properties of composites were investigated by thermally stimulated depolarization current (TSDC) method, dielectric, and Corona aging measurements. The TSDC spectra show that ICTOS modification enhanced α‐peak intensity and make β‐peak disappear in composites. Relevant trap parameters were calculated by an approximate model, and the results indicate that introduction of ICTOS is effective for the charge carrier traps, activation energy distribution in composites. Corona aging measurement show corona resistance was also sufficient improved in PI/TiO2 ICTOS composites. The changes of activation energy and intensity of traps in composites may be responsible for the corona resistance. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45101.

Microwave dielectric properties and thermally stimulated relaxations of Ba0.6Sr0.4La4Ti4O15−TiO2 composite ceramics by flowing oxygen sintering

Phase structure, sintering behavior, microwave dielectric properties and thermally stimulated relaxations of the 0.93Ba0.6Sr0.4La4Ti4O15−0.07TiO2 composite ceramics were investigated through using flowing oxygen atmosphere sintering. Ba0.6Sr0.4La4Ti4O15 (BSLT) sintered at 1500 °C possesses microwave dielectric properties of er = 44, Q × f = 45,000 GHz and τ f = −7.5 ppm/°C. Rutile TiO2 with er = 105, Q × f = 46,000 GHz, τ f = +465 ppm/°C was combined with the BSLT to compensate the negative τ f value in current research. XRD reveals that the ceramic samples possess a hexagonal perovskite structure and a secondary phase of TiO2−δ. The 0.93BSLT−0.07TiO2 compounds sintered in flowing oxygen exhibit small grain sizes and some pores, which are less compactly sintered compared with the ones sintered in air. The densification, grain growth and microwave dielectric properties of 0.93BSLT−0.07TiO2 have a clear dependence on sintering temperature. The 0.93BSLT−0.07TiO2 composite ceramics sintered at 1500 °C possess good microwave dielectric properties: er = 47.2, Q × f = 32,400 GHz and τ f = −3.3 ppm/°C. The extrinsic loss mechanism associated with defects was explored by relying on thermally stimulated depolarization current (TSDC) measurement. It can be manifested that TSDC relaxations are mostly derived from oxygen vacancies and TSDC spectra are evidently affected by the sintering temperature.

Charge Carrier Relaxation Study in Glass-Added Barium Titanate Ceramics Using Thermally Stimulated Depolarization Current

The depolarization process of glass-added barium titanate (BaTiO3) ceramics with two different glass concentrations was investigated using a thermally stimulated depolarization current (TSDC) technique. The TSDC spectra of the glass-added BaTiO3 ceramics show three peaks. The first sharp peak near the Curie temperature is due to pyroelectric current associated with ferroelectric-paraelectric phase transition. The middle temperature peak at about 200°C showed no dependence on the depolarization current peak position in the polarization field, and the activation energies of this peak were between 0.43 eV and 0.55 eV, which are attributed to the behavior of defect dipoles related to oxygen vacancies within the BaTiO3 grains. Moreover, the high temperature peak at around 300°C indicated that the depolarization current peak position depends on the polarization temperature and decreases with increasing polarization field. The activation energy of this high temperature peak was between 0.78 eV and 0.98 eV, which is similar to the activation energy for the motion of oxygen vacancies in perovskite oxides. The high temperature peak could be attributed to the migration of oxygen vacancies across grain boundaries. In this work we developed a model in which oxygen vacancies that originated from the defect within grains migrated from the anode to the cathode and some were trapped at the grain boundaries. It is presented here and successfully interprets the appearance and behavior of these peaks.

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.

Study of electrical properties of polymethylmethacrylate treated in aqueous and saline environments

Polymethylmethacrylate (PMMA) is one of the most studied polymers in the last years due to its outstanding mechanical and physical-chemical properties. This material is used in various applications in which it can be subjected to different constraints. In this work, PMMA samples were submitted to thermal treatment (at 100 °C for 20 min) in aqueous and saline environments. We studied the hydrothermal effect as well as the saline environment effect on electrical properties of PMMA by means of thermal step method (TSM) and thermally stimulated depolarization current (TSDC) technique. It was found that polarization was significantly influenced by the hydrothermal treatment realized particularly in a saline medium. α - and ρ -relaxations were observed in all TSDC spectra. For each treated sample, ρ -relaxation peak is more intensive than the dipolar one and the corresponding releasing charge quantity increases.

Structure, Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1 − x)Ba0.6Sr0.4La4Ti4O15–xBa5Nb4O15 Solid Solutions

(1 − x)Ba0.6Sr0.4La4Ti4O15–xBa5Nb4O15 (x = 0.05, 0.1, 0.15 and 0.2, BSLT–BN) ceramic samples were prepared by co‐firing the mixtures of Ba0.6Sr0.4La4Ti4O15 and Ba5Nb4O15 powders. Crystal structure, microwave dielectric properties and thermally stimulated depolarization currents (TSDC) of the BSLT–BN series ceramics were investigated. X‐ray diffraction patterns reveal that all the samples exhibit a hexagonal perovskite structure, which implies that the BSLT–BN mixtures form solid solutions. With increasing Ba5Nb4O15 content, the diffraction peaks shift to low angles and the sintering temperature of BSLT–BN decreases. Raman spectra analysis reveals the shifting and splitting of the vibration modes. The microwave dielectric properties of the well‐sintered (1 − x)BSLT–xBN ceramics vary with Ba5Nb4O15 content. The dielectric permittivity of the ceramics exhibits a slight decreasing trend. The quality factor varies in the range of 45 000–11 200 GHz, whereas near‐zero temperature coefficients of the resonant frequency may be achieved by changing the Ba5Nb4O15 content. TSDC was utilized to explore the extrinsic loss mechanism associated with defects. TSDC relaxation peaks are mainly generated by oxygen vacancies, and the Ba5Nb4O15 content has a significant influence on the TSDC spectra.

Low-Temperature TSDC Studies of Molecular Dynamics in Pristine and O7+ Ion–Modified Liquid Crystalline Polyurethane

The molecular dynamics of pristine and 100 MeV O7+ ion–modified liquid crystalline polyurethane (LCPU) was carried out using the thermally stimulated depolarization currents (TSDC) method. The TSDC spectra of pristine LCPU samples consist of current peaks appearing around 110,160, 210, and 260 K and are called the γ, β, α, and δ peaks respectively. The γ-peak is attributed to the crankshaft motion of the spacer group (but-2-ene group). The β-peak is due to the localized rotational fluctuations of C˭O groups. The α-peak is associated with the dipolar orientation of imines group present in the urethane linkage, and the δ-peak has its origin in the space charge trapping mechanism. In the TSDC spectra of O7+ ion–irradiated LCPU samples the γ-peak almost disappears, which shows that the ion induced modification in the methylene linkage. A relaxation process owing to new sub-polar groups formed due to dehydrogenation of CH2 groups and occurring in the vicinity of β-relaxation is discussed. The radiation-induced chain scissioning at NH groups in LCPU is also confirmed from the fluence effect on the α-peak. The irradiation results in the production of a large number of defect sites and an increase in the same results in the enhancement of δ-relaxation.

Annealing Effect on Chain Segment Motion and Charge Trapping and Detrapping in Nylon 1010

The chain segment motion and charge trapping and detrapping in nylon 1010 films were investigated by means of thermally stimulated depolarization current (TSDC). There were three current peaks (named α, ρ1, and ρ2 peaks, respectively) in the experimental TSDC spectra above room temperature. The α peak is attributed to a background dipole relaxation by the motion of chain segments and space charge contribution, the ρ1 peak is originated from a space charge trapped in the bulk amorphous regions and the interphase between crystalline and amorphous regions, the ρ2 peak is originated from space charge trapped in crystalline regions. By analyzing the characteristic parameters of these peaks, it was found that with in increase of the degree of crystallinity the activation energy of the a peak increased from 1.12 to 1.22 eV and the trap depth of the ρ2 peak increased from 2.70 to 2.82 eV, while the trap depth of the ρ1 peak decreased from 1.50 to 1.29 eV. Annealing induced a decrease of the chain segment mobility and promoted the creation of traps in nylon 1010. Annealing also decreased the stability of the trapped charges in the bulk amorphous and the interphase regions and increased the stability of the trapped charges in the crystalline regions.

Study of polarization parameters effect on dipolar relaxation in epoxy-based polymer using thermally stimulated depolarization current

In this paper, dipolar relaxation processes in epoxy-based polymer were investigated under various polarization conditions, using thermally stimulated depolarization current (TSDC) technique. The TSDC spectrum of un-poled epoxy polymer reveals an unusual negative current, associated with a thermally generated charge carriers, which were due to impurities ionization and structural changes mainly at high temperature (above T g ). β and α dipolar relaxations were detected respectively around 115 °C and 154 °C in conventionally poled sample at T P =110 °C and E P = 3 kV/mm. Dipolar relaxation parameters were evaluated using two techniques: (1) theoretical decomposition of global complex TSDC spectrum, using Bucci-Fieschi expression based on single Debye process, (2) experimental procedure based on selective polarization, commonly known as windowing polarization (WP). The obtained values are T P dependent: relaxation time decreases and activation energy increases when polarization temperature increases. An ω relaxation peak due to water molecule departure was detected around 135 °C, using WP techniques for T P =100 °C Unlike dipolar relaxations, its position is independent of polarization temperature. Complementary thermal analysis investigations by differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA), were made to support some given conclusions.

Thermally Stimulated Depolarization Currents and Thermal Sampling Technique of γ-Irradiated Gelatin and PVA Homopolymers and 50/50 (wt/wt %) Blend Sample

Thermally stimulated depolarization current (TSDC) and thermal sampling (TS) method were used to systematically characterize the α-relaxation process in gelatin and PVA homopolymers and their blend sample of 50/50 (wt/wt %) composition. In addition, γ-irradiation effect on TSDC spectra of samples under investigation was studied. Measurements of TSDC at different polarizing field strengths, polarization temperatures and times made it possible to obtain a complete picture of kinetic transitions, local modes of motion and space charge polarization in one heating cycle. On γ-irradiation, the change in shape, position and area of the α-relaxation peak of the samples were attributed to the variation of distribution function of associated relaxation times. The thermal sampling procedure was applied to decompose the complex relaxations into their narrowly distributed components. The molecular parameters such as activation energy (Ea) and pre-exponential factor (τo) for TS processes have been estimated. A linear relationship between the activation energy and logarithm of pre-exponential factor confirms the existence of a compensation behavior. The compensation temperature Tc and compensation time τc for the present samples have been determined.

Structural features and functional properties of water in model DMPC membranes: thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) studies

Thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) spectroscopies were employed to investigate the state of water incorporated in a model DMPC (dimyristoyl-phosphatidylcholine) membrane. The lipid multilayers, highly inhomogeneous from the dielectric point of view, originate complex TSDC spectra critically dependent on the sample water content and thermal history. Different temperature ranges were chosen to polarize the sample, i.e. 100–300 K (type I) and 100–285 K (type II). The purpose of the latter choice was to avoid any sample heating above the DMPC phase transition temperature (295 K) along the sample polarization. According to the results, water in a fully hydrated system (aw = 0.92) (1) is ordered around the hydrophilic head molecular groups, (2) is layered in the interbilayer space and (3) penetrates among the hydrocarbon chains. It can assume different local structural configurations depending on the lipid packing. Irreversible conformational transitions in the lipid array system were monitored as a consequence of different dehydration treatments.FTIR absorption measurements were performed to study the water sorption kinetics into a DMPC thin film. The water related OH band was decomposed into three components, describing three water states, with different propensity to the H-bond formation. The changes of the lipid characteristic groups (CH2/CH3, and C=O) absorption bands as a function of increasing hydration level were monitored and discussed.