Thermally Stimulated Depolarization Current Results Research Articles

Solution Modified Fumed Silica and Its Effect on Charge Trapping Behavior of PP/POE/Silica Nanodielectrics

Various dielectric nanocomposite materials are studied in the frame of the European Commission funded project GRIDABLE. This project has the aim to develop DC cable extruded insulation and medium and low voltage DC capacitor films exhibiting enhanced performance with respect to presently used materials. The nanocomposites intended for cable applications are polypropylene (PP)/polyolefin elastomer (POE) blends filled with surface modified nano-silica particles. The surface modification is carried out via the state-of-the-art solution method using a polar silane as the modifying agent. Thermally Stimulated Depolarization Current (TSDC) measurements were carried out in order to study the charge trapping behavior of the nanocomposite samples. TSDC results indicate that the addition of the treated nano-silica, for most cases, reduces the density of the deep traps significantly. The effect of the addition of silica nanoparticles - both modified and unmodified - on the crystallinity of the samples was studied using X-ray Diffraction (XRD). This is important as the charge trapping properties of the nanodielectrics can be affected by the degree of crystallinity. While more detailed studies are necessary, these results imply that the depth and the density of the deep trap states is profoundly influenced by the level of the silica modification i.e. the amount of the grafted silane on the silica surface.

Surface Modification of Fumed Silica by Dry Silanization for PP/POE-based Dielectric Nanocomposites

A systematic study of the influence of surface modification of nanosilica with coupling agents having different polarities on the dielectric properties of a PP/POE/silica blend was performed. The main goal of this investigation was to tailor the chemical composition of the silica surface, in order to modify the charge trapping properties of the nanocomposites. For the modification of the silica surface, a “green” approach was utilized: a dry silanization method, which is performed without the need of a solvent. The results of the thermogravimetric analysis (TGA) indicate that the dry process is an effective method to perform silica surface modification using alkoxysilanes.The charge trapping properties were studied by Thermally Stimulated Depolarization Current (TSDC) measurements. The obtained TSDC results show significant differences in charge trapping properties of PP/POE composites filled with differently modified silicas. Polar functional groups attached to the surface of the silica appear to have a strong effect on the charge trapping properties: The trap depth distribution becomes deeper and the trap density decreases to significantly lower levels.All results show that incorporation of surface modified nanosilica into a PP/POE matrix is a promising approach to tailor its dielectric properties. Further development of these composites may lead to benefits for application in high-voltage cable and capacitor applications.Keywords: silica surface modification, dry silanization, charge trapping, PP, POE, TSDC, high voltage cable.

Influence of Temperature on Charge Accumulation in Low-Density Polyethylene Based on Depolarization Current and Space Charge Decay.

Temperature is one of the key factors affecting space charge accumulation in high voltage direct current (HVDC) cable insulation material. The influence of temperature on charge accumulation in low density polyethylene (LDPE) has been investigated with a combined thermally stimulated depolarization current (TSDC) method and pulsed electro-acoustic (PEA) method. The experimental results indicate that there exists a transition temperature region of charge accumulation around 50 °C. The total accumulated charges all firstly increase and then decrease with the increasing polarization temperature under three typical polarization electric fields, and they have more accumulated charges in LDPE around 50 °C. The phenomenon has a close link with the dynamic processes of charge trapping and de-trapping, which were verified by TSDC results. At room temperature, the trapped charges are difficult to release from the traps, and these homocharges near the cathode can depress the further injection of the charges. More charges can be injected from the electrodes with the increase of temperature, while the charge migration is relatively lower before 50 °C, leading to more accumulated charges. When the temperature exceeds around 50 °C, the molecular movement is accelerated which can enhance the hopping probability of charges between the adjacent traps, resulting in few accumulated charges.

Influence of trap characteristics on DC surface flashover performance of low density polyethylene in vacuum

Surface flashover is the primary limitation to the development of power system and the increase of voltage level. Previous work has proved that the trap can greatly influence flashover performances, but the relationship between trap parameters and surface flashover voltage is not clear. In the paper, we study the effects of crystallization behavior, microstructure, and trap parameters on DC surface flashover performance of semi-crystallinity polymer through adding phenolphthalein which is regarded as nucleating agent in low density polyethylene (LDPE). Micro-IR spectroscopy result proves that phenolphthalein exactly exists in LDPE/phenolphthalein composite. Differential scanning calorimeter (DSC) and scanning electron microscope (SEM) are used to investigate the effect of nucleating agent (phenolphthalein) on crystallinity behavior and microstructure of LDPE, and their results indicate that the phenolphthalein modification greatly changes the crystallization behavior of LDPE. The SEM results show that the spherulite size of LDPE decreases and is distributed more uniformly with the increase of phenolphthalein concentration. The DSC results show that the crystallinity and lamella thickness increase. Thermally stimulated depolarization current (TSDC) is used to characterize the trap parameters of LDPE/phenolphthalein composites. The TSDC results indicate that the shallow trap level (<i>γ</i> peak) increases from 0.19 eV to 0.65 eV and the deep trap (<i>α</i> peak) increases from 0.81 eV to 0.99 eV with the increase of phenolphthalein concentration. Relationship between microstructure and trap parameters shows that the smaller spherulite size indicates the deeper trap level (for LDPE, the trap level increases from 0.81 eV to 0.99 eV when the spherulite size decreases from 23.2 μm to 14.9 μm), and larger crystallinity means smaller trap density (for LDPE, the trap density decreases from 1404 pC to 612 pC when the crystallinity increases from 34.51% to 43.25%). The DC surface flashover performance increases with the increase of phenolphthalein concentration, and reaches a highest value: when the concentration is 1 wt%, the highest value is increased by 48.42%. Finally, it is concluded that the microstructure of semi-crystallinity polymerinfluences the trap parameters, which affects the surface flashover performance through affecting the carrier transport process in the development process of surface flashover. The trap level and trap density play complementary, cooperation and mutual transformation roles in improving the surface flashover performances as indicated by the analysis of the " U-shaped” relationship between trap parameters and flashover voltage.

Effects of grain size and temperature on the energy storage and dielectric tunability of non-reducible BaTiO3-based ceramics

BaTiO3-based ceramics with various grain sizes (136–529 nm) are prepared through a chemical coating method followed by sintering in a reducing atmosphere. Effects of grain size and temperature on electric properties, energy-storage properties, and dielectric tunability are studied via Current-Field (J-E) curves, ferroelectric hysteresis loops, Capacitance-Voltage (C–V) curves and Thermally stimulated depolarization currents (TSDC). At all temperatures, fine-grain ceramics yield a lower energy density but a higher energy efficiency under the same electric field, owing to a lower ferroelectric contribution. Meanwhile, fine-grain ceramics exhibit a higher maximum energy density due to their higher breakdown strength. Fine-grain ceramics with the grain size of 136 nm have the maximum energy density of 0.41 J/cm3 under the breakdown strength of 75 kV/cm, the corresponding efficiency is 81%. C–V curves show that fine-grain ceramics have better bias-field stability. According to TSDC results, fine-grain ceramics exhibit fewer oxygen vacancies and a higher relaxation activation energy.

High electric field conduction in low-alkali boroaluminosilicate glass

Electrical conduction in silica-based glasses under a low electric field is dominated by high mobility ions such as sodium, and there is a transition from ionic transport to electronic transport as the electric field exceeds 108 V/m at low temperatures. Electrical conduction under a high electric field was investigated in thin low-alkali boroaluminosilicate glass samples, showing nonlinear conduction with the current density scaling approximately with E1/2, where E is the electric field. In addition, thermally stimulated depolarization current (TSDC) characterization was carried out on room-temperature electrically poled glass samples, and an anomalous discharging current flowing in the same direction as the charging current was observed. High electric field conduction and TSDC results led to the conclusion that Poole-Frenkel based electronic transport occurs in the mobile-cation-depleted region adjacent to the anode, and accounts for the observed anomalous current.

Study of an initial transient relaxation in XLPE cable insulation by TSDC and PEA

Space charge profiles in electrets of cable XLPE insulation have been determined by pulsed electroacustic method (PEA). A single positive peak close to the cathode, which decreases with annealing time, is observed. Typically, thermally stimulated depolarization currents (TSDC) of those electrets show heteropolar peaks initially. A new transient homopolar peak emerges in TSDC profiles at 99°C with annealing at high temperatures. No direct correlation can be established between PEA and TSDC results, since the case of the largest value of the maximum of this TSDC peak corresponds to the smallest charge peak observed in PEA profile. The transient behavior is explained by means of charge trapped on crystals surfaces due to the differences between the conductivities of amorphous and crystalline phases. The homoplar peak is associated with dipolar contaminants located in the interlamellar amorphous phase, which are reoriented by the field produced by charge trapped on crystal surfaces, during the TSDC discharges. Detrapping of charge trapped on crystal surfaces occurs when lamellae melt, which contributes homopolarly to the current. Both contributions result in the peaks observed in the TSDC discharge.

Secondary molecular mobility in amorphous ethyl cellulose: Aging effects and degree of co‐operativity

AbstractThe secondary relaxations in amorphous ethyl cellulose are studied using thermally stimulated depolarization currents (TSDCs) in the temperature region from 165 °C (108 K) up to 145 °C (418 K). The influence of aging on the current peaks of the secondary relaxation is discussed, and it is concluded that some modes of motion of this mobility are aging independent, while others are affected by aging. A particular attention is focused on the discussion of the degree of co‐operativity of the motional modes of the secondary relaxation. The conclusion to be driven from the obtained TSDC results is that the aging‐independent motional modes, as well as those that are aging dependent, do not show any appreciable co‐operative character. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 820–829, 2009

In Vitro Deposition of Hydroxyapatite on Cortical Bone Collagen Stimulated by Deformation-Induced Piezoelectricity

In the present work, we have studied the effect of the piezoelectricity of elastically deformed cortical bone collagen on surface using a biomimetic approach. The mineralization process induced as a consequence of the piezoelectricity effect was evaluated using scanning electron microscopy (SEM), thermally stimulated depolarization current (TSDC), and differential scanning calorimetry (DSC). SEM micrographs showed that mineralization occurred predominantly over the compressed side of bone collagen, due to the effect of piezoelectricity, when the sample was immersed in the simulated body fluid (SBF) in a cell-free system. The TSDC method was used to examine the complex collagen dielectric response. The dielectric spectra of deformed and undeformed collagen samples with different hydration levels were compared and correlated with the mineralization process followed by SEM. The dielectric measurements showed that the mineralization induced significant changes in the dielectric spectra of the deformed sample. DSC and TSDC results demonstrated a reduction of the collagen glass transition as the mineralization process advanced. The combined use of SEM, TSDC, and DSC showed that, even without osteoblasts present, the piezoelectric dipoles produced by deformed collagen can produce the precipitation of hydroxyapatite by electrochemical means, without a catalytic converter as occurs in classical biomimetic deposition.

TSDC study of XLPE recrystallization effects in the melting range of temperatures

The electrical properties of crosslinked polyethylene (XLPE), employed in mid-voltage cable insulation are studied using thermally stimulated depolarization currents (TSDC), differential scanning calorimetry (DSC) and x-ray diffraction. A complex heteropolar peak appears by TSDC between 50 and 110 °C, with a maximum at 105 °C. These measurements reveal that there is an optimal polarization temperature (Tpo) around 90 °C. For this polarization temperature, the measured discharge peak area is maximum. Although the presence of a Tpo is common in the study of relaxations by TSDC, in this case one would expect a monotonic decrease in the TSDC response with increasing polarization temperatures due to the decrease in the total crystalline fraction. In this paper, TSDC curves obtained under several conditions are interpreted in terms of recrystallization processes in XLPE during the polarization stage, if the sample is polarized in the melting temperature range. In this case, the recrystallization of a fraction of the material molten at this temperature promotes the formation of more stable and defect-free crystals. The presence of recrystallization processes is detected by DSC and confirmed by x-ray diffractometry. TSDC measurements have been performed with samples polarized at several temperatures (Tp) cooling from the melt or heating from room temperature. Also, TSDC results are obtained with previous annealing or with several cooling rates. These results allow us to infer that crystalline material grown from recrystallization processes that take place in the polarization stage attains a particularly stable polarization. Possible microscopical causes of this effect are discussed.

Characteristics of Corona-Charged PET Sandwich Electrets

Polyethylene terephthalate (PET) is one of the major electret materials which has been studied and widely used in charge storage applications for a long time. The homo-charge and hetero-charge formation responsible for the electret effect of PET can be explained by the dipolar polarization, space charge separation and charge injection respectively as a result of contact poling or corona charging. These processes are also verifiable from studies of surface charges and thermally stimulated depolarization current (TSDC) of the PET films. Now consider a stack of PET films charged under the same condition as a single film. It would be intuitively expected that the electret effects of the individual films of the stack should be different due to their different positions in the stack. However, in a recent study on charged PET stacks, it was found that the electret effects of the individual films are quite similar. The surface charges of the individual films are about the same and they sum to make same total as a single charged PET film. On the other hand, the TSDC thermograms of individual films show similar characteristics in the thermal relaxation processes where homo- and hetero- charge effects are still observed. From the TSDC results, especially for the film in the middle of a triple layer sandwich, it shows clearly dipolar polarization and charge injection related peaks. These findings agree with the results of surface charge studies of the individual films.

Investigation of the Microphase Separation in Blends of Polyurethane-Based Ionomers

Blends of an anion containing polyurethane (PU1) and polyaminourethane (PU2) were investigated. Dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS) were used in a study of the molecular mobility and microphase morphology and their dependence on the composition of these ionomers. Dielectric thermally stimulated depolarization current (TSDC) measurements of the ionomer blends showed four relaxation mechanisms. The subglass secondary γ- and β-relaxations are related to local motions; the α-relaxation is related to the glass transition of the amorphous soft phase of the ionomers, and the Maxwell-Wagner-Sillars (MWS) relaxation is related to interfacial polarization caused by the motion of ions released during the glass transition. The α- and MWS TSDC relaxations are strongly affected by the degree of microphase segregation (DMS) of the ionomer blends. Based on TSDC measurements, the parameter m TSDC, a criterion expressing a relative degree of phase mixing, is introduced. According to this parameter, the classification of the investigated ionomer blends, in order of decreasing phase mixing (PU1/PU2 in %) is 50/50 (mixed) > 0/100 > 5/95 > 10/90 > 100/0 > 30/70 (high separation). It appears that, as a result of the complexity of the systems, their structures and their properties are not simple functions of their composition. SAXS measurements show that pure PU1 ionomer has a high degree of microphase separation. The introduction of PU2 in the blend causes defects in the hard domains and leads to a structure with mixing of the soft and hard microphases. SAXS measurements concerning DMS are in good agreement with the TSDC results and support the introduced criterion of the relative degree of phase mixing.

Application of the Electronegativity Equalization Method to the Interpretation of TSDC Results: Case of a Mordenite Exchanged by Na+ and Li+ Cations

The work presented here proposes a new interpretative method for thermally stimulated depolarization current (TSDC) measurements. By applying the concept of electronegativity equalization to TSDC results, we show that it is possible to obtain an experimental evaluation of the chemical potential of the electrons of the exchanged cation and of the host site in zeolites. This step leads to an evaluation of the fundamental parameters, such as the effective hardness and electronegativity of the sites of zeolites. Such an approach gives an evaluation of the heterogeneity of the aluminosilicate surface and is applied, for the first time, to an exchanged hydrogen mordenite containing various amounts of substituted lithium ions or sodium ions. A relation betwen electronegativity and hardness and the acid−base properties of the zeolite is made possible.

Rock magnetic and dielectric characterizations of a formational contact in cretaceous strata (eastern Venezuela)

Rock magnetic and dielectric studies have been conducted in sedimentary rocks close to a lithological contact (Chimana and Querecual formations) at the Pertigalete sedimentary sequence (Cretaceous northeastern Venezuela). Rock magnetic results suggest that the presence of secondary magnetic minerals has been conditioned by the transitional zone between Chimana and Querecual. In fact, this contact might have acted as a conduit for fluid circulation and a focus of chemical alteration on most of its adjacent strata. Some of the secondary magnetic minerals (e.g. magnetite) must be the outcome of the anoxic conditions that prevailed during times when the sedimentary basin reached its maximum temperatures when oil and gas were generated (Lower Miocene). Some others (e.g. hematite) are the likely byproduct of alteration dating from times when oxidizing conditions were dominant. The Thermally Stimulated Depolarization Current (TSDC) technique is used here to dielectrically characterize the same contact. The complex TSDC spectra obtained in the temperature range from 77 to 230 K for the samples studied, indicate a notable difference between the Chimana (CH) and Querecual (QC) samples. Drying and rehydration treatments performed on these samples show that the whole depolarization spectrum is mainly due to moisture trapped in the microporosity of the material. The broad spectra, separated by using Direct Signal Analysis, reveal the presence of four overlapping peaks. TSDC results indicate a progressive change in the mean re-orientation energy with the lithology for the four relaxations. This variation, due to the change in the environment seen by the water dipoles hydrogen-bonded to different sites in the material, is observed for the same lithologies where magnetic anomalies have been reported. Thus, rock magnetic analyses, combined with the TSDC technique, seem to serve as a sensitive and alternative means of characterizing lithologies and stratigraphic discontinuities in a sedimentary sequence.

Correlation between dipolar TSDC and AC dielectric spectroscopy at the PVDF glass transition

The αa-mode (associated to the dynamic glass transition) in PVDF-α has been studied by Thermally Stimulated Depolarization Currents (TSDC) and Dielectric Spectroscopy (DS) techniques. The distribution of relaxation parameters, reorientation energies, characteristic temperature, and preexponential factors of the Vogel–Tammann–Fulcher relaxation times have been precisely determined by using the Simulated Annealing Direct Signal Analysis applied to a partially discharged TSDC αa peak. This distribution has been used to predict the variation of the dielectric loss, e″(ω, T), in the temperature and frequency range where the DS measurements were made on the same material. The simulated e′(T, ω) for various ω, are compared to the experimental values. The width of the peak is always too low, due to the restricted distribution used for the generation of the curves. A relaxation map including the TSDC results is used to determine the relaxation time variation. In the limited frequency range where the AC DS experiments are performed (102 ≤ f ≤ 105 Hz) a master curve is drawn and the exponents of the frequency dependence are found at low and high frequency; also, a fitting to the Havriliak–Negami distribution is successfully performed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2483–2493, 1997