Space Charge Relaxation Processes Research Articles

Structural aspects of the high-temperature space charge relaxation in ferroelectric VDF/TFE 94/6 copolymer

Structural changes in isotropic films of ferroelectric vinylidene fluoride (VDF)/tetrafluoroethylene (TFE) 94/6 copolymers in the metastable state have been studied on heating. The metastable state was ascribed to γ-crystals. The dielectric relaxation spectroscopy was employed to study space charge relaxation processes. Two relaxations were observed at high temperatures. One of them was attributed to relaxation of space charge accumulated on the Al electrode which may be partially blocked due to new functional groups formed in the polymer surface. Recrystallization of the film from the melt is accompanied by small α-crystals formation. This results in a decrease in activation energy of the space-charge relaxation process.

Curie point and a space charge relaxation in ferroelectric poly(vinylidene fluoride‐trifluoroethylene) copolymers with different thermal history

ABSTRACTHigh‐temperature processes of space charge relaxation in films of poly(vinylidene fluoride‐trifluoroethylene) (70–30) have been investigated. The films were prepared by the crystallization from a solution in tetrahydrofuran at a room temperature. X‐ray data analysis showed that initial films crystallize in metastable state: a paraelectric phase is formed at fast crystallization below the Curie temperature. An exothermic process was observed at heating, indicating a solid‐state crystallization of metastable paraelectric regions. This process is controlled by melting of ordered regions formed at secondary crystallization. These transformations of structure affect characteristics of the high‐temperature space charge relaxation process. In particular, it was found that activation energy of extrinsic conductivity is reduced by several times in the region of ferroelectric‐paraelectric phase transition. An additional relaxation process was registered above the Curie temperature, which is caused by the relaxation of charges in double electric layer formed on an interface between an electrode (Au) and fluorine‐containing polymer. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46186.

Depolarization current relaxation process of insulating dielectrics after corona poling under different charging conditions

As an insulating dielectric, polyimide is favorable for the application of optoelectronics, electrical insulation system in electric power industry, insulating, and packaging materials in space aircraft, due to its excellent thermal, mechanical and electrical insulating stability. The charge storage profile of such insulating dielectric is utmost important to its application, when it is exposed to electron irradiation, high voltage corona discharge or other treatments. These treatments could induce changes in physical and chemical properties of treated samples. To investigate the charge storage mechanism of the insulating dielectrics after high-voltage corona discharge, the relaxation processes responsible for corona charged polyimide films under different poling conditions were analyzed by the Thermally Stimulated Discharge Currents method (TSDC). In the results of thermal relaxation process, the appearance of various peaks in TSDC spectra provided a deep insight into the molecular status in the dielectric material and reflected stored space charge relaxation process in the insulating polymers after corona discharge treatments. Furthermore, the different space charge distribution status under various poling temperature and different discharge voltage level were also investigated, which could partly reflect the influence of the ambiance condition on the functional dielectrics after corona poling.

Electric field induced fluorescence hysteresis of single molecules in poly(methyl methacrylate)

Single molecule (SM) chips could serve as the fundamental devices in quantum information processing. In this context, a chip with the non-polar SMs of squaraine-derived rotaxanes embedded in a polar poly(methyl methacrylate) matrix was realized and the SM fluorescence hysteresis induced by the electric field was observed at room temperature. Here, we presented a model considering both of the electron transfer and space charge relaxation processes to explain the fluorescence hysteresis effect, and the model-based simulations agreed reasonably well with the experimental results.

Injected charge as a cause of the slow dielectric relaxation in thin film Pt/(Ba,Sr)TiO3/Cu structures

Experimental studies of the response of Pt/Ba0.3Sr0.7TiO3/Cu capacitors to switching-off the bias voltage are reported. The remnant capacitance as a function of the bias voltage and the variation of this capacitance with time have been measured and analyzed in terms of the injection model of the space charge origin with consideration for the linear dielectric layer at the Pt/BSTO interface. The energy depths, 0.51 eV, and 0.65 eV, of two trap states controlling the slow (τ ≈ 100 s and τ ≈ 5 × 103 s) space charge relaxation process have been extracted from experiments.

Effect of recrystallization on the molecular mobility of a copolymer of vinylidene fluoride and hexafluoropropylene

AbstractRelaxation processes in copolymers of vinylidene fluoride with hexafluoropropylene (93/7) were studied by means of a dielectric method. The initial extruded film was recrystallized by free heating to temperatures above the melting point and by subsequent cooling. This increased both the perfection of the crystal phase and the degree of crystallinity. The impact of recrystallization on both the relaxation times (τ's) and the activation parameters of the local mobility (β process) and micro‐Brownian cooperative mobility in amorphous phase (αa process) was almost negligible, whereas the τ's of the αc relaxation were an order of magnitude higher after recrystallization. Qualitatively, it was predicted by the soliton model for the αc relaxation. The recrystallization affected characteristics of the transition at the highest temperature (α) registered in the region of the melting of the crystals even more. The process is related to the relaxation of the space charge formed by ionogenic impurities, which migrate through the amorphous phase with high free volume. It was shown that the dynamics in the amorphous phase controlled the drift mobility of free charge carriers and, by that, determined the τ of the space charge relaxation process. The structuring processes during the recrystallization also affected the parameters of the order–disorder transition in the low‐perfect ferroelectric (antiferroelectric) phase. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Dielectric Relaxation Investigations in Copolyesteramide (Vectra B950) Polymer Liquid Crystal: Thermally Stimulated Polarization Current Technique

Thermally stimulated polarization currents (TSPC) in copolyesteramide (Vectra B950) polymer liquid crystal have been investigated in the temperature region 20°–250°C under different polarizing fields ranging from 0.7 to 7 kV/cm and with different heating rates. The low temperature region of the TSP current spectra shows a pronounced peak (P1) around 30°C. This peak has been attributed to the dipolar relaxation process associated with ester group. The peak (P2) observed around 140°C has been attributed to a combination of two relaxation processes, the dipolar and space charge relaxation processes. A steady rise in the TSP currents by several orders of magnitude is observed above 150°C. Saturation in the TSP current around 220°C reveals the presence of a relaxation process owing to the movement of large segmental groups in LC phase.

Study of polar relaxation processes in Sr (1−1.5 x) La xTiO 3 ceramics by using field-induced thermally stimulated currents

Polar relaxation processes in Lanthanum doped SrTiO 3 (STO) ceramics, with general formulae Sr (1−1.5 x) La x TiO 3, were studied by undertaking field-induced thermally stimulated currents measurements below room temperature. The experimental results obtained for doped ceramic ( x = 0.0133) were analysed by using dipolar and space-charge relaxation thermally stimulated depolarization currents (TSDC) models in order to determine the nature of the relaxation processes involved. Our results reveal the existence of different relaxation processes in the temperature range 60–300 K. Whereas at low temperature, a relaxation mechanism of a dipolar type was disclosed within the temperature interval centred around 100 K, a space-charge relaxation process could be identified in the temperature range 120–300 K. The temperature dependence of the relaxation parameters will be also discussed in detail.

Multiple relaxation processes in high-energy ion irradiated kapton-H polyimide: Thermally stimulated depolarization current study

High-energy ion irradiation effects on the thermally stimulated depolarization current (TSDC) behaviour of kapton-H samples (12.5 μm) irradiated with 50 MeV Li ion (fluence 5 × 10 4, 10 5 and 5 × 10 5 ions/cm 2) have been investigated. The TSDC spectra of the irradiated samples reveal that the β-peak (appearing around 80–110 °C) associated with dipolar relaxation has been significantly affected owing to the demerization of carbonyl groups due to irradiation. The TSDC spectra also reveal a new relaxation process (termed as γ-relaxation) around 30 °C, due to increased water absorptivity in irradiated samples. The peak around 200 °C (α-peak) associated with space charge relaxation process also shows a behavioural change with ion irradiation. The peak not only shifts towards the higher temperature with increasing fluence but also show an increase in its activation energy (0.33–0.99 eV) with increasing polarizing field. The creation of new deep energy trap centers due to the formation of conjugated bonds after irradiation is responsible for this modification. The Cole–Cole distribution curves show the formation of new sub-polar group with different characteristic relaxation time.

Thermally stimulated discharge current studies of corona polarized pristine and swift heavy ion irradiated kapton-H polyimide

Thermally stimulated discharge (tsd) current technique has been used to investigate the corona polarized pristine and swift heavy ion irradiated kapton-H polyimide. The samples of thickness 25 μm were irradiated with 100 MeV Fe+ beam (fluence: 1.8 × 1011, 2.3 × 1012 and 1.4 × 1013 ions/cm2). Corona charged pristine kapton-H samples show two tsd current peaks (P1 and P2) ∼40–70 and 150–190 °C, respectively. The peak P1 is in the normal current direction and P2 is in the abnormal current direction. These peaks are attributed to space charge relaxation process owing to the surface and bulk energy traps, respectively. The x¯/d value (x¯ is the mean penetration depth and d is the thickness of the sample) estimated for pristine kapton-H polyimide comes out to be 0.32 and suggests that the space charge distribution is approximated by space charge limited current (SCLC) distribution. However, the SCLC distribution in irradiated sample is not confirmed by large x¯/d value (0.86). The activation energies estimated from Bucci plot method for peaks P1 and P2 vary from 0.66 to 0.97 eV and from 0.92 to 1.31 eV, respectively, depending on the corona voltage. The charge trapping density for surface and bulk traps comes out to be (0.3–10.2) × 1017 and (4.9–14.6) × 1017 m−3, respectively. In the irradiated samples, abnormal tsd currents dominate and there is an enhancement in the bulk charge trapping density (13.2 × 1018 m−3). This enhancement is due to the formation of double/triple bonds and free radicals in irradiated samples. The irradiated corona polarized samples after prolonged aging (10–30 days) show a high temperature maximum with average activation energy of 0.90 eV. This current maximum has been associated with the space charge relaxation process due to bulk deep charge trapping levels.

Sublinear dispersive conductivity in polymethyl methacrylate at temperatures above the glass transition

Dynamic electrical analysis shows that conductivity conditions the electrical properties of a polymer at high temperatures and low frequencies. In this paper we show the possibilities of the electric modulus formalism to study the properties of carrier transport and space charge relaxation processes in polymethyl methacrylate. Asymmetric Argand's plots are observed in the temperature range between 150 and 210 °C. This asymmetry is related to power-law dependencies in the real part of the conductivity, of the form ω n with n<1, as a result of correlated ion hopping. The complex part of the electric modulus exhibits a peak in the low frequency range that can be associated with these conductive processes. In the case of the relaxation time related to conductive processes, it has been observed that Maxwell time is higher than the characteristic time associated with the crossover frequency which determines the transition from dc to ac regime. This fact is explained by the presence of deep traps. Finally, a maximum in the value of n is observed between 180 and 190 °C which may be related to coupling between charge transport and chain segment motions.

Dispersive ionic space charge relaxation in solid polymer electrolytes. I. Experimental system polyethylene oxide

As a model system for solid polymer electrolytes thin polyethylene oxide (PEO) films (0.6–30 μm) are prepared as Al–PEO–Al and as Al–PEO–Si structures with blocking electrodes. Dielectric measurements (capacitance and loss angle) are carried out in a frequency range of 3 mHz⩽f⩽1 MHz in atmospheres of different relative humidities (0%–75%) and at different temperatures (293–313 K). The nominal permittivity varies from 4 at high frequencies to up to 150 000 at low frequencies. We find true volume polarization in the high frequency range and a thermally activated space charge relaxation process in the low frequency range. Its time constant exhibits a remarkable dependence on the sample thickness, the relative humidity, the ion concentration, and the temperature. Due to the absorbed dipolar water molecules the activation energy of this process is decreased, leading to higher mobility of the ions. An increase in ion concentration also leads to a lower activation energy. The ions are able to move through the sample to the electrode interfaces and build up ionic space charge. We assume that at the interfaces oxide layers are formed, and these are almost impermeable to these ions. The space charge relaxation process has a broad frequency spectrum. The imaginary part of the nominal permittivity εr″(ω)∝ωβ has a slope of β=−0.8 to 0.7 above the main relaxation frequency. In the time domain, after application of a voltage step a Kohlrausch behavior of the relaxation current is found. In the short time range we find a current of j∝t−0.3 and in the long time range we find j∝t−1.1. A comparison of the frequency and the time domain by Fourier transform shows that the space charge relaxation process is time invariant for small applied voltages. Nonlinear properties that appear at higher voltages are investigated in the frequency domain and in the time domain.