Stimulated Polarization Currents Research Articles

Universal installation for studying structural defects in electrical and optical fiber materials

A universal device for experimentally studying the migration of microscopic structure defects and the features of dielectric relaxation is proposed. it allows using the thermo stimulated depolarization method, in combination with the measurement of the tangent of the angle of dielectric losses and the thermo stimulated polarization current, to perform dielectric spectroscopy of hydrogen-bonded crystals and perform analysis of the properties and parameters of structure defects. A smaller (in comparison with the existing installation) additional compact device for measuring small values of electrical capacitance and the tangent of the angle of dielectric losses, including an electrometer B7-30, was designed, measurement was carried out using a q-factor meter VM 560. When measuringtgδ > 0.1, the VM-507 device was used. An experimental methodology is proposed that allows, in combination with the method of minimizing the comparison function (MFC - method), with a high degree of accuracy, to calculate the molecular characteristics of structural defects in composite materials based on semiconductors and dielectrics used in the electrical and optical fiber industry, electric power and insulation technology.

Electric characterization of LDPE films with TSC and dielectric spectroscopy

This contribution discusses thermally stimulated current (TSC) measurements on low density polyethylene (LDPE) in the form of thin films and virtually without additives. The TSC responses are divided into different parts attributed to capacitive currents, currents due to release of trapped carriers, and normal conduction currents. The three contributions dominate at low, intermediate, and high temperatures, respectively. Dielectric spectroscopy measurements at different temperatures indicate the relevance of the capacitive currents for the measurement of thermally stimulated polarization currents (TSPC). The separation of the conduction current, by subtracting reference curves from the total current, allowed the investigation of trap and conduction properties for different experimental parameters (e.g., excitation levels, fields, heating rates).

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.

A correlation of capacitive RF-MEMS reliability to AlN dielectric film spontaneous polarization

This paper investigates the effect of spontaneous polarization of magnetron-sputtered aluminum nitride on the electrical properties and reliability of Radio Frequency – Micro-Electro-Mechanical Systems capacitive switches. The assessment is performed with the aid of application of thermally stimulated polarization currents in metal-insulator-metal capacitors and temperature dependence of device capacitance. The study reveals the presence of a surface charge, which is smaller than that expected from material spontaneous polarization, but definitely is responsible for the low degradation rate under certain bias polarization life tests.

Electrical conduction and polarization in PbWO4 crystals

The regularities of ion-electron processes in an undoped PbWO4 single crystal upon transition to a quasi-equilibrium state in an external dc electric field with a linear variation in the temperature in the range 290–600 K are investigated using different methods. The total conductivity, thermally stimulated polarization current, and thermally stimulated depolarization current are measured. It is assumed that the temperature dependence of the conductivity can be described within the theory of small-radius polarons. The thermally stimulated polarization (depolarization) currents are interpreted in terms of the space-charge (peaks of the current in the range 400–550 K) and dipole (peaks of the current in the range 290–370 K) mechanisms of generation of a polarization charge in the sample. The inference is drawn that the dominant contribution to the dipole polarization is made by dipolons, namely, doubly charged (cation-anion) vacancy pairs coupled through electrostatic interaction. The basic parameters of relaxation phenomena and charge transfer are calculated.

Electrical transport in Gd2.6Ca0.4Mg0.25Zr0.65Ga4.1O12 crystalsMg0.25Zr0.65Ga4.1O12 crystals

The dc electrical conductivity of Gd2.6Ca0.4Mg0.25Zr0.65Ga4.1O12 garnet crystals was measured in the temperature range from 300 to 650 K in air. Thermally stimulated polarization currents were recorded in the range from 300 to 600 K. The activation energy for conductivity (10-16 to 10-17 S/cm at 295 K) and trap parameters were determined. Phenomenological analysis of the experimental results was performed. O- centers are assumed to be responsible for charge transport. The conduction mechanism can be understood in terms of the hopping of small-radius polarons. Heterovalent cation substitutions are likely to stabilize electrically active defects.

Glass transition studies in physically aged partially crystalline poly(ethylene terephthalate) by TSC

Physical ageing in poly(ethylene terephthalate) (PET) was studied by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and thermally stimulated polarization currents (TSPC) as a function of the material crystallinity degree. Calorimetric measurements confirm the presence of two separated glass transitions in partially crystallized samples (xc=27%), associated with the presence of two different amorphous regions: the interspherulitic region and the interlamellar region within the spherulites. The α peak obtained by TSDC shows the progressive decrease of the chain segments mobility in the amorphous fraction as ageing progresses. The increase in the crystallinity degree of the material also produces a decrease of the mobility in the amorphous fraction. In this case the decrease observed is associated with the interlamellar conformation of these regions. However, TSDC measurements do not show evidences of a double glass transition. TSPC measurements on aged samples show a current peak, which is directly related to the polarization of the amorphous regions when mobility is recovered above the glass transition. The study of these peaks in aged partially crystallized samples shows evidences of a double glass transition associated with theinterspherulitic and interlamellar amorphous regions.

Comparative TSPC, TSDC and DSC physical ageing studies on PET-a

In the present work a comparative study of physical ageing of poly(ethylene terephthalate) (PET) by thermally stimulated polarization currents (TSPC), thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC), is presented. In all cases, physical ageing was carried out by annealing the sample at 60°C for several ageing times (from ta = 0 to 264 h). Structural relaxation has been studied by DSC through the analysis of the endothermic peak superposed on the glass transition, which increases and shifts towards higher temperatures as the annealing time increases. Measured current spectra of annealed samples show the appearance of a relaxation peak which we named αa. The evolution of this peak with the annealing time is presented, and it has been related directly to the endothermic peak obtained by DSC, and therefore directly associated with the physical ageing of PET. Experiments carried out show the dipolar origin of the αa peak, related to the polarization of chains within the material when mobility is recovered above Tg.

The conduction and thermally stimulated polarization currents in (TGS-PE) particulate composites

Different concentrations of polyethylene (PE) are mixed with Triglycine Sulphate (TGS) powder. The temperature dependence of thermally stimulated polarization current (TSPC) was studied at different applied voltages and heating rates. The results were discussed and special stress was done on conduction mechanism, trapping, injection, cleaning of charge carriers and variations in bulk polarization. The conduction current was measured during cooling. It was found that this current is extremely lower than the TSPC current. Besides, the activation energy of the conduction process is lower than that calculated from TSPC process. The addition of PE is thought to be responsible for the observed phenomena.

Structural modifications of aluminum-polyethylene terephthalate-aluminum systems as evidenced by thermally stimulated polarization currents

The relaxation of thin aluminized polyethylene terephthalate (PET) films was investigated by means of the thermally stimulated polarization current method with the aim of evidencing structural modifications occurring during thermal treatment carried out under various conditions. The occurrence of a marked change in the charge exchange properties of the Al-PET interfaces upon annealing, previously advocated from thermally stimulated depolarization current measurements, has been confirmed. Molecular orientation and free-volume relaxation are shown to be determining factors governing the polarization processes in thin PET films.

Dielectric and conductivity relaxation in dry and humid solid PEO electrolytes

The interaction of water with uncomplexed poly(ethylene oxide) (PEO) is studied by dielectric relaxation spectroscopy techniques for water contents up to about 0.60(w/w). The techniques used include thermally stimulated depolarization currents, thermally stimulated polarization currents and dc conductivity measurements in the temperature range 77–300 K, as well as broadband ac techniques in the frequency range 10-10 10 Hz. The results are discussed in terms of molecular distribution and organization of water in a separate phase, restriction of the molecular mobility of water molecules, dipolar and space charge contributions to the main (α) relaxation and plasticizing action of water.

Grain size effects in barium titanate

Abstract Due to its high crystallinity and purity, hydrothermal powder was used to study the effects of grain size and density on the dielectric properties of BaTiO3. Curie-Weiss analysis was used to analyze the results as porosity increased. A correlation was found between the level of porosity and the difference between the Curie and Curie-Weiss temperatures, suggesting a limiting value of porosity above which insufficient flux continuity results in anomalous Curie-Weiss behavior. Thermally stimulated polarization currents (pyroelectric currents with biasing field) generated in poled samples were also studied with varying grain size and density and were found to produce ambiguous results for porous samples.

Dielectric Relaxation Phenomena in Butadiene-Acrylonitrile Copolymers

Abstract The polarization phenomena of the polybutadiene - acrylonitrile (PBAN) copolymers have been investigated in the vicinity of the glass transition region by methods of thermally stimulated depolarization currents (TSDC), thermally stimulated polarization currents (TSPC) and dc conductivity. Relaxation regions above the glass transition temperature Tg have been analyzed by thermal sampling and partial depolarization techniques in more detail. The effects associated with the cyclizing after pyrolysis in PBAN copolymers have been studied by thermally stimulated and viscoelastic measurements.

The application of thermally stimulated processes to the study of poly p-phenylene sulfide

The electrical properties of poly p-phenylene sulfide (PPS) samples sandwiched between metallic electrodes are studied as a function of the applied voltage, temperature, time, electrode materials, and sample thickness. Superlinear current-voltage characteristics are observed, which are explained in terms of Schottky effect and space-charge limited currents (SCLC). The conductivity data for variable-range hopping have also been studied, but the calculated values of density of states are approximately one order of magnitude higher than those obtained by SCLC measurements. From thermally stimulated polarization currents we observed a current peak around 80°C that was related with the glass transition temperature of PPS.

Charge storage in pure and titanium dioxide doped polyarylate, and conduction in pure polyarylate

Charge storage of cast, pure and titanium dioxide doped amorphous polyarylate films was studied using thermally stimulated charge decay and thermally stimulated currents (TSC). Optimum storage was obtained by corona charging at 180°C, using 10 wt% titanium dioxide. A half value charge decay temperature ( T 1 2 ) of 180°C at 30% R.H. was then obtained. This value is 54°C higher than the T 1 2 of 126°C obtained for room temperature charging. Pure polyarylate charged at 180°C had a T 1 2 of 126°C, the same as that for room temperature charging. Both dopant-matrix boundaries and/ or additives themselves may act as deep traps. Steady state current-field ( F) curves of pure polyarylate follow a sinh law in F/ kT in the temperatuer range 87–143°C, favouring carrier hopping as the conduction mechanism. An activation energy of 1.1 eV was obtained from thermally stimulated polarization currents, and agreed with that obtained from a hopping conduction theory.

The Effect of the Temperature Dependence of the Dielectric Constant on Thermally Stimulated Currents in Polymers

A dipolar theory of thermally stimulated currents taking into account the thermal dependence of the equilibrium polarization has been tested in the glass transition range of a series of polymers selected for their well-resolved relaxation spectra and the absence of any significant bulk conductivity (triblock, diblock, and star-shaped styreneisoprene copolymers). Systematic studies of the effect exercized on thermally stimulated polarization currents (TSPC) and thermally stimulated depolarization currents (TSDC) by varying parameters such as field strength, electrode material, field polarity, and type of heating-cooling sequence, have been carried out and full qualitative agreement with the predictions of the theory has been found. In particular, these results imply that the variations of equilibrium polarization occurring during the nonisothermal steps of TSDC and TSPC experiments cannot be a priori neglected, as is still commonly done in classical treatments of experimental data.

Thermally stimulated polarization current in sodium germanate glass

The electrical conductivity of XNa2O·(100−X)GeO2 glasses where X is 0.19 and 27.5 mol%, was studied by means of the technique of thermally stimulated polarization current. The compositional dependence of the parameter σ0T/n1/3, where σ0 is the preexponential facor for conductivity and n is the nominal concentration of sodium ions, suggests that these glasses contain some sort of inhomogeneities. This suggestion is supported by the phase diagram.

Simulation of field-induced thermally stimulated currents in dielectrics. Application to the dipolar case in polymeric systems

Numerical calculations of thermally stimulated depolarization currents (TSDC) and thermally stimulated polarization currents (TSPC) have been carried out by using the bistable model of Fröhlich and taking into account the temperature dependence of equilibrium polarization according to a Langevin function. For dipolar processes characterized by a single relaxation time as well as by a spectrum of relaxation times obeying classical distribution functions (Wagner, Cole-Cole, Fuoss-Kirkwood, Davidson-Cole, Havriliak-Negami), the following predictions have been obtained: (1) A TSDC peak must be always of larger magnitude than the corresponding TSPC peak as a result of an increase in polarization taking place during the cooling step; (2) For the same reason, the magnitude and area of a TSDC peak must be dependent on the cooling rate adopted in the poling process; it also follows that, unless an instantaneous cooling is postulated, such a peak can no more be considered as depicting the relaxation properties of the material at the temperature of polarization; (3) A TSPC peak must be followed by a significant current reversal as a necessary result of the tangential convergence of the transient polarization with its decreasing saturation limit in the high-temperature range. These predictions have been tested in a polymeric system selected for its well resolved relaxation spectrum (styrene-butadiene block copolymer). Full qualitative agreement between experiment and theory has been found as far as the behavior of the α relaxation (glass transition) is concerned. These results show that the variations of equilibrium polarization occuring during the nonisothermal steps of TSDC and TSPC experiments cannot be a priori neglected, as is still commonly done for the determination of dipolar relaxation parameters.

Thermally stimulated currents in dielectrics

The theory of thermally stimulated polarization currents (TSPC) is described and observation of TSPC due to the polarization of impurity-vacancy dipoles in alkali halides is reported. These TSPC results are compared with the well known ionic thermocurrents (ITC). A method is also described to distinguish between thermally stimulated currents which are due to dipole orientation and those due to the release of charge from localized states in the material. Analysis of these different types of thermally stimulated current is undertaken and values of trap depth and dipole parameters are obtained.