Thermally Stimulated Depolarization Current Measurements Research Articles

Oxygen vacancy migration and impact on high voltage DC polarization in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3

AbstractElectrical polarization and defect transport are examined in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3, an attractive capacitor material for high power electronics. Oxygen vacancies are suggested to be the majority charge carrier at or below 250°C with a grain conduction hopping activation energy of 0.97 eV and 0.92 eV for thermally stimulated depolarization current (TSDC) and impedance spectroscopy measurements, respectively. At higher temperature, thermally generated electronic conduction with an activation energy of 1.6 eV is dominant. Significant oxygen vacancy concentration is indicated (up to ∼1%) due to cation vacancy formation (i.e., acceptor defects) from observed Bi (and likely Zn) volatility. Oxygen vacancy diffusivity is estimated to be 10−12.8 cm2/s at 250°C. Low diffusivity and high activation energies are indicative of significant defect interactions. Dipolar oxygen vacancy defects are also indicated, with an activation energy of 0.59 eV from TSDC measurements. The large oxygen vacancy content leads to a short lifetime during high voltage (30 kV/cm), high temperature (250°C) direct current (DC) electrical measurements.

Thermally stimulated relaxation and behaviors of oxygen vacancies in SrTiO3 single crystals with (100), (110) and (111) orientations

The strontium titanate (SrTiO3) single crystals with different orientations of (100), (110) and (111) were investigated using thermally stimulated depolarization current (TSDC) measurements, which has been proved to be an effective strategy to fundamentally study the relationship between relaxation phenomena and defect chemistry in dielectrics. The origins of different relaxations in SrTiO3 crystals were identified and the activation energy of oxygen vacancies was estimated from TSDC measurements. It was further found that oxygen-treated SrTiO3 crystals exhibit different relaxation behaviors. Noticeable changes of thermal relaxation associated with oxygen vacancies have taken place in relation to the crystalline anisotropy. The SrTiO3 (110) samples display higher concentration and activation energy of oxygen vacancies. First-principles calculations were carried out on SrTiO3 (110) crystals to study the effect of oxygen vacancy on different surface microstructure. From the resulting minimum formation energy of 0.63 eV, it demonstrates that the oxygen vacancies tend to form on the TiO-terminated surfaces. Considering the band structure, oxygen vacancies near the surface contribute to the transition of crystal from insulator to metallic characteristic.

TSDC and impedance spectroscopy measurements on hydroxyapatite, β-tricalcium phosphate and hydroxyapatite/β-tricalcium phosphate biphasic bioceramics

Bone grafting and surgical interventions related with orthopaedic disorders consist in a big business, generating large revenues worldwide every year. There is a need to replace the biomaterials that currently still dominate this market, i.e., autografts and allografts, due to their disadvantages, such as limited availability, need for additional surgeries and diseases transmission possibilities. The most promising replacement materials are biomaterials with bioactive properties, such as the calcium phosphate-based bioceramics group. The bioactivity of these materials, i.e., the rate at which they promote the growth and directly bond with the new host biological bone, can be enhanced through their electrical polarization.In the present work, the electrical polarization features of pure hydroxyapatite (Hap), pure β-tricalcium phosphate (β-TCP) and biphasic hydroxyapatite/β-tricalcium phosphate composites (HTCP) were analyzed by measuring thermally stimulated depolarization currents (TSDC). The samples were thermoelectrically polarized at 500°C under a DC electric field with a magnitude of 5kV/cm. The biphasic samples were also polarized under electric fields with different magnitudes: 2, 3, 4 and 5kV/cm. Additionally, the depolarization processes detected in the TSDC measurements were correlated with dielectric relaxation processes observed in impedance spectroscopy (IS) measurements.The results indicate that the β-TCP crystalline phase has a considerable higher ability to store electrical charge compared with the Hap phase. This indicates that it has a suitable composition and structure for ionic conduction and establishment of a large electric charge density, providing great potential for orthopaedic applications.

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.

Phase Transitions and Molecular Mobility in 5CB and CE8 Studied by Dielectric Techniques

Two complementary dielectric techniques, broadband dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC), covering together broad ranges of frequency and temperature, were employed to study molecular mobility in the molecular liquid crystal 5CB. Three relaxations were observed and analyzed with respect to time scale, strength and shape, a sub-glass β relaxation and the α and δ relaxations in the supercooled nematic phase. In addition, TSDC and isochronal DRS measurements were used to follow phase transitions in 5CB and CE8 in comparison with DSC. The results demonstrate the power of dielectric techniques, in particular TSDC, for monitoring phase transitions in liquid crystals.

Effect of lanthanum modification on dielectric relaxation behavior in lead zirconate stannate titanate antiferroelectric ceramics

The effect of lanthanum concentration on dielectric relaxation behavior in lead zirconate stannate titanate antiferroelectric ceramics was investigated by X-ray diffraction technique, dielectric spectroscopy, Sawyer–Tower polarization methods, and thermally stimulated depolarization current (TSDC) measurements. The tetragonal phase was found to be stabilized from the rhombohedral phase by lanthanum substitution. As the lanthanum content was increased, the dielectric constant decreased gradually over the measuring temperature range. As a result of polarization–electric field hysteresis loops, the maximum polarization decreased and the switching field increased with the increase of lanthanum content. Investigations of TSDC measurements revealed that the peak is due to space-charge polarization. It is suggested to be associated with the relaxation of oxygen vacancies.

Crystallinity effect on the structural relaxation of polyethylene naphtalate (PEN) by TSDC and DSC experiments

The effect of physical ageing process on the electrical and thermal properties of amorphous and semi-crystalline PEN was investigated by means of the thermally stimulated depolarization currents (TSDC) and the differential scannig calorimetry (DSC). The TSDC measurements revealed a significant decrease in the mobility of the molecular chains in the amorphous PEN (PENa), aged at a temperature Tg-10K (Tg is the glass transition temperature). This mobility is directly related to the polarisation corresponding to the main relaxation of the material. This phenomenon was also observed in the case of biaxially stretched PEN (PENbiax). However, it is much less important in this case. The calorimetric technique (DSC) showed, in the case of PENa, that the effect of ageing is particularly important when the ageing time increases with a maximum effect for ageing temperature equal to Tg-10 K. In the other side, no effect was observed on the semi crystalline PENbiax by the DSC technique unlike the TSDC technique. These results confirm again the high resolving power of this latter. Also, the effect of crystallinity on the structural relaxation of PEN was highlighted and is summeryzed as following : firstly, we have observed a decrease in the TSDC peak's intensity of the α-relaxation and in the recovery enthalpy of the structural relaxation when the crystallinity rate increases. Secondly, an increase in the activation energy Δh* with the increase in the crystallinity rate and a decrease in the limit fictive temperature T'f were observed.

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.

Thermally Stimulated Depolarization Current analysis to the determination of polarization and relaxation parameters in aged PMMA

Thermally Stimulated Depolarization Current (TSDC) technique has been extensively used to study the main and secondary relaxations process in various materials, such as polymers.In this work, samples of amorphous polar poly(methyl methacrylate) (PMMA) are submitted to a particular thermal aging with different durations, and then TSDC measurements are made. Theoretical model is applied for the analysis of complex TSDC spectra for aged PMMA samples. The polarization was determined for each aged PMMA sample, as a sum of single depolarization process contribution.The different relaxation mechanisms are recognized and their relative contributions to the polarization of polymer are evaluated. The obtained results show that the thermal aging affects slightly the primary and the secondary, dipolar relaxations and more strongly the space charges relaxation.

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.

Impact of the CMP process on the electrical properties of ultra low k porous SiOCH

The impact of chemical mechanical polishing (CMP) on SiOCH films (thickness = 300 nm) for the 32–45 nm node Cu-interconnect process is investigated by low-frequency dielectric spectroscopy and thermally stimulated depolarization current (TSDC). After CMP process, the dielectric permittivity is degraded of about 25% in the whole range of the investigated frequency (10 −1 Hz–100 kHz). In a same way, the dielectric losses tan δ increase at the lowest frequencies. An annealing (300 °C during 20 min) carried out after CMP induces a reduction of the dielectric permittivity without however reaching the value of initial as-deposited material. In agreement with other published papers focusing on the damage caused by the CMP, OH bonding and water adsorption due to surfactants explain the degradation of these dielectric properties. The identification of OH bonds and an increase in the intensity of CH x in the 2800–3050 cm −1 range after CMP seems to confirm this point. The moderate temperature of annealing, used to restore layers and to avoid the degradation of copper lines, suppresses the physisorbed water but not the chemisorbed water. TSDC measurements confirm that dipolar relaxation, due to water in the material, result in a peak of relaxation at a temperature around 175 °C.

Control of interfaces on electrical properties of SiO 2–Parylene-C laminar composite dielectrics

The interfacial interactions and their effects on electrical properties are reported for thermally grown SiO 2–Parylene-C laminar composites. The segmental chain mobility of the interfacial Parylene-C was found to be enhanced in the composites. TSDC (thermally stimulated depolarization current) measurements showed the presence of dipolar interfacial energy states in these laminar composites. Functionalization of the oxide surface with 3-aminopropyltriethoxysilane (APTES) created dipolar energy states with larger activation energy at the interface between Parylene-C and SiO 2. In addition to the modification of interfacial states, incorporation of APTES interfacial layer also increased polarizability (real part of permittivity) and reduced the quasi-DC conductivity of laminar composites. The systematic changes in interfacial characteristics with thickness of APTES layer were explained based on structural orientation and cross linking of silane molecules.

Bioactivity of electro-thermally poled bioactive silicate glass

A 45S5 bioactive glass (nominal composition: 46.1 mol.% SiO 2, 2.6 mol.% P 2O 5, 26.9 mol.% CaO, 24.4 mol.% Na 2O) was electrothermally poled by applying voltages up to 750 V for 45 min at 200 °C, and the thermally stimulated depolarization currents (TSDCs) were recorded. Changes in chemical composition and electrical properties after poling were investigated by TSDC measurements, impedance spectroscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). The poling led to the formation of interfacial layers underneath the surface in contact with the electrodes. Under the positive electrode, the layer was characterized by Na + ion depletion and by a negative charge density, and the layer was more resistive than the bulk. The influence of poling on the bioactivity was studied by immersion of samples in simulated body fluid (SBF) with subsequent cross-sectional SEM/EDX and X-ray diffraction analysis. It was found that poling leads to morphological changes in the silica-rich layer and to changes in the growth rate of amorphous calcium phosphate and bone-like apatite on the glass surface. The bone-like apatite layer under the positive electrode was slightly thicker than that under the negative electrode.

Thermally Stimulated Relaxation in Fe‐Doped SrTiO3 Systems:I. Single Crystals

Thermally stimulated depolarization current (TSDC) measurements were carried out on 0.1% and 1% iron (Fe)‐doped SrTiO3 crystals to study various effects that are not detected from other electrical characterization methods such as impedance spectroscopy. Using this TSDC technique, the origins of different relaxations in Fe‐doped SrTiO3 crystals were identified, and activation energy of the orientated dipole, energy depth of trap site, and trap density in the polarized samples were then estimated from TSDC measurements. From the resulting activation energy of 0.65±0.08 eV of trap charges, these traps are assigned to oxygen vacancy‐iron defect complexes. The high‐temperature peak at temperature ∼190°C is assigned to the relaxation current of oxygen vacancies and the calculated activation energy of 0.91±0.05 eV is in good agreement with the literature data.

Thermally Stimulated Relaxation in Fe‐Doped SrTiO3 Systems: II. Degradation of SrTiO3 Dielectrics

Thermally stimulated depolarization current (TSDC) measurements were carried out on degraded SrTiO3 crystals and ceramics to investigate the relationship between depolarization effects and degradation phenomena. Using this technique, the origins of dielectric relaxations in Fe‐doped SrTiO3 crystals and ceramics have been verified; the physical origins of relaxation mechanisms were later linked to the transient leakage behavior of the samples undergoing degradation from oxygen vacancy migration. In the course of degradation in a single crystal system, the migration of oxygen vacancies, , and the build‐up of an internal bias are detected through the TSDC measurements on samples with different degradation levels. Using a curve fitting method, trap charge concentration on the order of 1014 cm−3 is obtained in the single crystal system. Other than those relaxation mechanisms identified in the single crystal system, extra relaxation mechanism was found in the polycrystalline systems and was attributed to the relaxation of oxygen vacancies across grain boundaries. Using the initial rise method, the activation energies estimated for the relaxation of defect dipoles, the in‐grain oxygen vacancies pile up at grain boundaries, and relaxation of oxygen vacancies across grain boundaries are 0.73±0.03, 0.86±0.07, and 1.1±0.09 eV, respectively. An ionic demixing model is applied to account for the evolution of TSDC spectra and to bridge changes to the leakage behavior of the degraded samples. In the case of the polycrystalline system, it is suggested that a strong degradation to the insulation resistance occurs when oxygen vacancies migrate across grain boundaries and start to pile up at the cathode region of metallic electrodes. Before that point, the vacancies accumulate at partial blocking grain boundaries in each of the crystallites.

Heat treatment effects on dielectric and physico-chemical properties of an epoxy polymer

Infrared (IR) spectroscopy, dielectric spectroscopy (DS), and thermally stimulated depolarization current (TSDC) have been used to study heat treatment effects on an epoxy-based polymer. Variations in physico-chemical and dielectric properties were examined for annealing temperatures between 55 and 170 °C. IR results have shown that heating causes both chain scission and thermal oxidation of the polymer, increasing thus the amount of trapped charges. The complex dielectric permittivity and the dielectric modulus have been analyzed, by means of DS, to highlight and separate charge relaxation phenomena from conduction contributions. Results indicate structural rearrangements, leading to a decrease of dipolar relaxation frequency (from 16 to 13.5 kHz) and an increase of the relaxation strength (around 20%). TSDC measurements have shown a current peak shift towards higher temperatures, and a significant intensity decrease, which is proportional to the quantity of released charges.

Thermally Stimulated Depolarization Current and Thermal Analysis Studies of Gamma Irradiated Lithium‐Salt/Polymer Electrolyte Blends

Thermally stimulated depolarization current (TSDC) and thermal analysis studies of gamma irradiated LiOH/PVA blends were done. To study the mechanisms of conduction and TSDC in poly(vinyl alcohol) (PVA) and LiOH/PVA blends, short circuit TSDC at a polarizing temperature 353 K with a polarizing field of 3 kV cm−1 have been analyzed in the temperature range 300–410 K. Two peaks are evident from the global TSDC measurements on the pure PVA homopolymer. Meanwhile, in all blended samples; there is only one broad peak with a shoulder on the high temperature side due to the relaxation of the poly‐blend system. The temperature dependence, 300–408 K, of the current density (J) for pure PVA and its blended samples has been studied. It was observed that J values increase dramatically with increasing temperature (in the low temperature region from 300–340 K) owing to the formation of local ordered regions in the otherwise disordered amorphous matrix of PVA. Further increase in the temperature caused a marginal increase in J values. The temperature dependence of the specific heat for all samples was measured. A linear increase of C p was observed with an increase in temperature, which is ascribed to the increase in lattice vibration of linear macromolecules and consequently, increases in the number of internal degree of freedom of phonons.

Study of thermal aging effect on space charge in poly(methyl methacrylate)

Charges evolution in poly(methyl methacrylate) (PMMA) samples under thermal aging effect has been studied by means of two complementary techniques, thermal step method (TSM) and thermally stimulated depolarization current (TSDC). For the first method, measurements reveal that injected charges, whose quantity is found depending on the number of applied temperature cycles, remain close to the surface sample. TSDC measurements have been carried out for different temperatures ranging from 25 °C to 140 °C. Three distinguishable dipolar relaxations ( β 1 , β 2 and α) have been highlighted. In the same way, the presence of polarization and injected charges has been confirmed. In support of electric characterization, X-ray reflectometry has been used. The obtained results equally emphasized the thermal aging effect on the material.

Simultaneous space charge and TSDC measurements on polarized polymer

The present work is concerned with the development of an experimental device for simultaneous measurements of space charge profiles and thermally stimulated depolarization currents (TSDC) on polymer sheets. Space charge measurements are nowadays realized by nondestructive methods but TSDC are still a destructive one. However coupling the results from the two techniques allows to watch the evolution of the charges and finally their disappearance. The space charge measurement is based on the current response to a low frequency thermal wave applied on the sample by using the alternating thermal wave method (ATWM). Basically the TSDC require to heat the sample slowly and linearly. So, a controlled thermo-electrical module is used to generate the appropriate thermal profile and a temperature cycle including both the linear temperature rises and low frequency thermal waves around a chosen temperature level is generated. Both measurements are made on one and the same sample without handling it during the procedure. The space charge measurement accuracy is improved by varying the thermal wave frequencies from large to short wavelengths. The purpose is to scan successively the different layers in the sample. The use of an adapted mathematical model for the electric field representation and space charge distribution is helpful for the signal processing. Experimental results obtained on polymer materials are given and discussed to illustrate the change in density and position of the remaining space charge due to the TSDC effect

Simultaneous space charge and TSDC measurements on polarized polymer

The present work is concerned with the development of an experimental device for simultaneous measurements of space charge profiles and thermally stimulated depolarization currents (TSDC) on polymer sheets. Space charge measurements are nowadays realized by nondestructive methods but TSDC are still a destructive one. However coupling the results from the two techniques allows to watch the evolution of the charges and finally their disappearance. The space charge measurement is based on the current response to a low frequency thermal wave applied on the sample by using the alternating thermal wave method (ATWM). Basically the TSDC require to heat the sample slowly and linearly. So, a controlled thermo-electrical module is used to generate the appropriate thermal profile and a temperature cycle including both the linear temperature rises and low frequency thermal waves around a chosen temperature level is generated. Both measurements are made on one and the same sample without handling it during the procedure. The space charge measurement accuracy is improved by varying the thermal wave frequencies from large to short wavelengths. The purpose is to scan successively the different layers in the sample. The use of an adapted mathematical model for the electric field representation and space charge distribution is helpful for the signal processing. Experimental results obtained on polymer materials are given and discussed to illustrate the change in density and position of the remaining space charge due to the TSDC effect