Thermally Stimulated Depolarization Current Measurements Research Articles

Electrovector Effect of Polarized and Chemically Treated Na<sub>2</sub>O-CaO-P<sub>2</sub>O<sub>5</sub>-SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub> Bioglasses

Na2O-CaO-P2O5-SiO2-Al2O3 (NCPSA) glass was investigated in its electrical polarizability by complex impedance and thermally stimulated depolarization current (TSDC) measurements. Moreover, the NCPSA glass was also subjected to a chemical treatment for bioactivity improvement. The effects of the electrical polarization and the chemical treatment on apatite formability of NCPSA glass were investigated by immersion test using simulated body fluid. From the result of complex impedance measurements, the activation energies for electrical conductivities NCPSA glass was 1.1eV. From the TSDC measurements, the polarized NCPSA glass showed monotonically increasing curves in depolarization current density spectra. The stored electrical charge calculated from TSDC measurements of the NCPSA glass polarized with electrical dc field of 2kV･cm-1 at 600°C for 1h was 4.91 mC･cm-2. The NCPSA glass treated with 1M NaOH indicated apatite formability in SBF and exhibited bioactivity. We disclosed that the combination of electropolarization and chemical treatment changed the apatite formability of the NCPSA glass in SBF, consequently, improved the bioactivity of the glass.

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.

The use of windowing polarization for the study of multi-component glass

An attempt was made to investigate the possibility of applying thermally stimulated depolarization current (TSDC) techniques to the analysis of multi-component glasses with different relaxation mechanisms. Therefore, multi-component borosilicate glass containing the addition of Li 2O, Na 2O, MgO, BaO or in some cases Fe 2O 3, was investigated by integral and by partial TSDC measurements. Except for the usual partial peaks, which follow the applied poling window Δ T p, some additional peaks appeared independently of Δ T p. By the consideration of temperature of the partial peaks maxima versus temperature of poling it was possible to map and separate different and overlapping events. Generally, in the range 170–320 K several processes were observed. P 0 with maximum at 198±2 K caused by the space charge, P 1 at 228±3 K and P 2 at 263±4 K caused by dipolar relaxations. The windowing technique shows a higher temperature process P 3 at about 285–330 K attributed to the Maxwell–Wagner effect. Special attention was paid to the appearance of a number of single-relaxation time processes in the range of P 2 caused by the interaction between the divalent ions and vacancies. Glass with Fe 2O 3 showed a decrease in the distribution width in P 1 and P 2 ranges, which corresponds to an increase in the homogeneity of the system. The addition of Fe 2O 3 also increased the magnitude of the depolarized current. An additional heat treatment of the glass increased the number of the single-relaxation time processes.

Dielectric Relaxation Behavior Studies in Amorphous Terpolymer of Poly(Vinyl Chloride-co-Vinyl Acetate-co-2-Hydroxypropyl acrylate) Using Dielectric Spectroscopy and Thermally Stimulated Current Techniques

Dielectric studies on random terpolymer of poly(vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) (PVVH) have been carried out. The dielectric techniques used include broadband ac dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) techniques. TSDC technique enables us to observe the primary α-relaxation, which is associated with the glass transition temperatutre, and the space charge relaxation. The peak parameters value of α-relaxation, namely, the activation energy E a and the preexponential factor τ 0, have been evaluated and found to be 1 eV and 3 × 10−11 s, respectively. The variation of the dielectric properties with frequencies and temperatures has been related mainly to two processes: (i) the α-relaxation and (ii) the conductivity relaxation. Electric modulus representation shows two peaks, namely, the α-relaxation and conductivity relaxation. The dielectric data were fitted to Havriliak–Negami (HN) relaxation model, and the shape parameter values suggest a broad and an asymmetric distribution of relaxation time. The activation energy value (˜1 eV) of the α-relaxation, obtained from electric modulus representation, was found to be the same as that obtained from TSDC measurements.

INFLUENCE OF POLARIZING ELECTRIC FIELD ON ELECTRICAL AND OPTICAL PROPERTIES OF PDLC FILMS

We obtained polymer dispersed nematic liquid crystal films E7/PMMA. The thermally stimulated depolarization current (TSDC) measurements indicate a space charge limited current. Conduction of electronic type was proven and we measured the value of the activation energy, specific to the conduction process. Optical measurements have been performed simultaneously with TSDC measurements and the phase transition nematic – isotrop was detected by the switch of the optical transmission.

Structural changes and phase behavior of electron-irradiated poly(vinylidene-trifluoroethylene) copolymers

The effect of high-energy electron irradiation on poly(vinylidene-trifluoroethylene) P(VDF-TrFE), copolymer of 56/44 comonomer molar ratio is studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermally stimulated depolarization current (TSDC). DSC results show that the energy necessary to process ferroelectric transition decreases after irradiation, which indicates that high-energy electron irradiation makes ferroelectric phase less stable, causing transition to paraelectric phase at a temperature below melting point. XRD data show that the peak at 2 θ=19° gradually disappears with the increase of absorbed dose, which indicates that the transition from all-trans ferroelectric state to a trans-gauche amorphous conformation takes place. It is found that both the peak temperature and released charge carrier density decrease with the irradiation doses in TSDC spectra, demonstrating less dipoles and crystalline regions in the irradiated copolymer films during ferro-paraelectric (F–P) transition. The results of TSDC measurements are related to the morphological changes induced by electron beam (EB) irradiation as well as the presence of local polar ordering and decrease of crystallinity revealed in DSC and XRD results.

Electrical polarization of bioactive glass and assessment of their in vitro apatite deposition.

We evaluated the electrical polarizability of a bioactive glass of the 45S5 type (BG) by thermally stimulated depolarization current (TSDC) measurement based on the effects of the surface charges generated by an electrical polarization treatment on the growth of calcium phosphate crystals in simulated body fluid (SBF). From the results of the TSDC measurements, the polarized BG showed broad peaks in the depolarization current density spectra. The stored electrical charges calculated from the TSDC measurements of the BG polarized with an electrical dc field of 10 V x cm(-1) at 500 degrees C for 1 h was 8.5 mC x cm(-2) and depended on the electrical dc field intensity. The depolarization of the BG was assumed to be due to a migration of sodium ions and influenced by transitions of the BG structure. The immersion tests with the SBF showed that the effects of the electrical polarization produced morphologic changes in the calcium phosphate deposit grown on the BG after the short period of 2 h. We expected the effects to be caused by a change in the ion concentrations in the surrounding SBF due to the surface charges of the polarized BG. The change in the ion concentrations induced the difference in the calcium phosphate deposition. Consequently, we confirmed that BG had an electrical polarizability, a possibility of storing large charges, and differences in the growth of the calcium phosphate deposit due to the effects of the surface charges.

Water sorption and water-induced molecular mobility in dental composite resins.

Water sorption in two resin composites, Kulzer's Solitaire (S) and SDI's Wave (W), and in a polyacid-modified composite resin, 3M's F2000 (compomer F), was investigated by means of equilibrium sorption isotherms (ESI) and of dynamic sorption (DS) measurements. Molecular mobility in these materials was studied by means of dielectric relaxation spectroscopy (DRS) and of thermally stimulated depolarization currents (TSDC) measurements. The results of ESI measurements show that at equilibrium, water is molecularly distributed in the materials and the effects of hydrophilic sites and clustering are negligible. Hysteresis effects in sorption-desorption cycles are larger in the resin composites than in the compomer. Equilibrium water uptakes in both ESI and DS conditions are rather low, in the range 1-2%. Diffusion coefficients of water are about 1x10(-8) cm(2)/s in the resin composites and by a factor of about 2 smaller in the compomer. Molecular mobility increases with hydration, as suggested by preliminary DRS and TSDC measurements. Detailed dielectric measurements may give important information for understanding, at the molecular level, water-induced degradation in dental materials.

Electrical conductivity and TSDC study of the thermal aging in conductive polypyrrole/polyaniline blends

The thermal aging of conducting polypyrrole/polyaniline (PPy/PA) blends heated at 70°C for up to about 600 hours was studied by d.c. conductivity measurements and by thermally stimulated depolarization current (TSDC) spectroscopy in the temperature range from 80 to 300 K. The composition of the samples varied from pure polypyrrole to pure polyaniline with the PA content increasing in steps of 10%. Although the σ = σ(T) curves seem smooth, the corresponding Δσ/ΔT = f(T) curves exhibit systematically ascattering of points in the temperature ranges from 100 to 140 K and from 225 to 320 K approximately for all the samples before and after the heat treatment. TSDC measurements with the MISIM (metal-insulator-sample-insulator-metal) configuration, show a low temperature peak at 100–120 K and a high temperature peak at 280–300 K. The first disappears when the polypyrrole content exceeds 70% and for all the samples after heat treatment, although the high-temperature peak persists. The explanation given to this correspondence between d.c. conductivity and TSDC signals is based on the destruction of conformons and the mobility change of the polymer chains with rising temperature.

Analysis by thermally stimulated currents of the frequency power-lawdomains of the dielectric loss

The analysis of complex peaks in the technique of thermallystimulated depolarization currents (TSDC) often makes use of a distribution ofrelaxation times. Here we suggest a different approach that emphasizes themanifestation, in the TSDC measurement, of the asymptotic power-law domains ofthe relaxation function in the frequency measurement. Calculations for theCole-Cole response function show that the results of a typical TSDC experimentfor systems showing time-temperature superposition contain information aboutthe frequency response on both sides of the spectrum. The reason for this isthat the equivalent frequency of TSDC measurements is only low in absoluteterms, but not with respect to the system's relaxation time, which decreasesrapidly during the temperature scan of a TSDC experiment. The analysis of thestretched exponential and Havrilak-Negami functions indicates that the TSDCtechnique is able to provide an accurate determination of the fractionaryexponents of the loss peak.

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.

A Modified SBN System for Pyroelectric Sensors

AbstractThe Thermally Stimulated Depolarization Current (TSDC) and pyroelectric properties of the modified SBN ferroelectric ceramic system were studied for different lanthanum and titanium doping concentrations. The TSDC measurements show the pyroelectric peak for all compositions while a second smaller peak at higher temperature, possibly associated to induced vacancy-impurity dipoles, is also observed in all cases. The second peak contribution was experimentally and mathematically eliminated to determine the remanent polarization and pyroelectric coefficient, both associated only to permanent ferroelectric dipoles. The figures of merit for sensor devices are determined for all compositions and compared with those of other pyroelectric systems. The La0.03Sr0.255Ba0.7 Nb1.95Ti0.05O5.975 sample, in particular, has excellent pyroelectric response, making this material very suitable for pyroelectricity-derived applications.

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.

Open- and short-circuit thermally stimulated currents in ethyl cellulose (EC): polymethyl methacrylate (PMMA) blend

Mechanisms of charge generation and its persistence in one and both-side vacuum-aluminized ethyl cellulose (EC):polymethyl methacrylate (PMMA) blend thermoelectrets, prepared under different fields (10, 25, 50 and 100 kV/cm) and temperatures (40, 60, 80 and 100°C), have been analysed using short- and open-circuit thermally stimulated depolarization current (TSDC) technique. The TSDCs were recorded by reheating the samples at a linear heating rate of 4°C/min. The TSDC thermograms of polyblends containing EC:PMMA in different weight ratio are, in general, characterized with two peaks in lower and higher temperature regions. However, the polarity of the peaks was found to be just opposite in short- and open-circuit TSDC measurements. Moreover, results on 97:3, 93:7 and 90:10 EC:PMMA polyblends indicated that the current increases with concentration of PMMA. The results indicate the existence of heterocharge due to dipole orientation and ionic charge drift together with the injection of charge carriers from electrodes with their subsequent localization in surface and bulk traps. Further, the chances of charge trapping in polyblends, at the interfaces are greater than in the individual polymers.

Polymer–water interactions in PAA hydrogels

Polymer–water interactions in poyacrylamide (PAA) hydrogels were investigated by means of broad-band dielectric relaxation spectroscopy (DRS, 10 −2–10 6 Hz) and of thermally stimulated depolarization currents (TSDC) techniques (77–300 K) at several hydration levels. The critical water content for the saturation of primary hydration sites was determined to be h c=0.15 (dry basis). For water contents h> h c plasticization effects dominate the behaviour of hydrogels. Two further critical water contents were determined from TSDC measurements at subzero temperatures related to freezing of water and to reorganization of water in the hydrogels, respectively. The activation energy of DC protonic conductivity was determined to be 0.8±0.1 eV at h=0.25.

Comparative study of mechanical and electrical relaxations in poly(etherimide). Part 1

Relaxations in poly(etherimide) PEI Ultem 1000 have been analysed by differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) measurements. DMTA and DRS results show three distinct relaxations γ, β and α in the temperature range -140 to 250°C. The first one depends strongly on the water content in the sample as will be discussed in more detail in the second paper of this series. These results are in good agreement with those observed by TSDC of conventionally polarized electrets. In addition to these three relaxations, TSDC measurements show: (1) a peak (ρ) at which is attributed to space charge temperatures above the α relaxation, (2) indications of structure in the β relaxation zone. In the case of electrets formed by the windowing polarization method, the resulting TSDC spectra allow us to discern the fine structure of the β relaxation, which is formed by three sub-relaxations. In this work, the activation energies calculated by the different techniques are compared, and a molecular origin for each relaxation is proposed. © 1998 SCI.

Comparative study of mechanical and electrical relaxations in poly(etherimide). Part 1

Relaxations in poly(etherimide) PEI Ultem 1000 have been analysed by differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) measurements. DMTA and DRS results show three distinct relaxations γ, β and α in the temperature range -140 to 250°C. The first one depends strongly on the water content in the sample as will be discussed in more detail in the second paper of this series. These results are in good agreement with those observed by TSDC of conventionally polarized electrets. In addition to these three relaxations, TSDC measurements show: (1) a peak (ρ) at which is attributed to space charge temperatures above the α relaxation, (2) indications of structure in the β relaxation zone. In the case of electrets formed by the windowing polarization method, the resulting TSDC spectra allow us to discern the fine structure of the β relaxation, which is formed by three sub-relaxations. In this work, the activation energies calculated by the different techniques are compared, and a molecular origin for each relaxation is proposed. © 1998 SCI.

An Electrical Model of Summation of Thermally Stimulated Depolarization Current by Means of a Ps-Pice Software

In this paper we present an electrical model of simulation. This model, used to simulate thermally stimulated depolarization current (TSDC) measurements, yields the same behaviour for its time constant as that observed for amorphous materials in a large temperature interval. This model, whose main device is a current source controlled by the exponential of a voltage with linear temperature dependence could be considered as a generalization of the classical model generally used in TSDC interpretation attempts. Its response is very close to that observed in various TSDC measurements carried out on different polymers.

Water binding to biopolymers in different cereals and legumes: proton NMR relaxation, dielectric and water imbibition studies

Proton NMR relaxation time (T1), dielectric properties by means of the thermally stimulated depolarization currents (TSDC) method, and water imbibition were measured in cereal and legume grains (wheat, triticale, maize, pea, chick pea, horsebean, white lupin, lentil and beans) having different chemical composition (proteins, carbohydrates, lipids). T1 versus water content in the range 0.05–1.40 g water/g dry matter showed characteristic V-shaped curves with a sharp or a broad minimum depending on the species. Water content at T1 min was in high positive correlation with protein content of the grains (r=0.90) and in high negative correlation with soluble carbohydrates (r=−0.92), while lipids gave a very low correlation (r=0.38). The water content at T1 min (0.18–0.47 g water/g dry matter) was assigned to a primary hydration sphere around the macromolecules, since, when T1 was plotted versus per cent maximum hydration, the T1 min values for all grains fell between 25–30% of maximum hydration. The extrapolated value for zero protein content was 0.08 g water/g dry matter, which coincided with data in the literature for the water monolayer on starch. The TSDC measurements enabled us to determine the amount of tightly (irrotationally) bound water at primary hydration sites to 0.18±0.02 g water/g dry matter for beans, pea and chickpea, and, tentatively, to about 0.10 g water/g dry matter for wheat. Water imbibition data for 11 cereal and legume species gave total water hydration capacity in the range a=0.44–1.82 g water/g dry matter. This value divided by the water content of the primary hydration sphere (swelling index) was also in high positive correlation with the protein content of the grains (r=0.84).

On the interpretation of the TSDC results in the study of the α-relaxation of amorphous polymers

Abstract The α-relaxation around the glass transition of poly(vinyl acetate) (PVAc) is investigated by combining several dielectric techniques, namely frequency and time domain dielectric spectroscopies, as well as thermally stimulated depolarization current (TSDC). The results obtained by means of both the isothermal methods and the TSDC technique are interpreted on the basis of a Kohlrausch-Williams-Watts functional form of the time decay function. Both isothermal and TSDC measurements show that the dynamics of the α-relaxatioin in the glassy state has a non-Debye character which is similar to the one observed in the supercooled melt. From the comparison between the obtained results, a new way of analysis and interpretation of the TSDC results in the α-relaxation range is proposed. Moreover, from a set of TSDC spectra obtained at different heating rates, both β ( T ) and τ ( T ) are calculated in the whole temperature range covered by TSDC, i.e. below and across the glass transition.