Thermally Stimulated Depolarization Research Articles

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.

Molecular dynamics of an epoxy resin studied by Thermally Stimulated Depolarization Currents

The Thermally Stimulated Depolarization Current (TSDC) technique has been used to study the molecular mobility of diglycidyl ether of bisphenol-A (DGEBA) in the temperature range from −160 °C up to 10 °C. The molecular mobility in the glassy state and in the supercooled liquid was then characterized. Two secondary relaxations were detected, one of them partially disguised by the main α-process. Differential Scanning Calorimetry (DSC) was used to analyse the dynamics of the structural relaxation around T g (from the heating rate dependence of the onset temperature of the glass transition signal). The values of the fragility index obtained in this work by TSDC and DSC are compared with the values reported in the literature obtained from other experimental techniques. A relaxation above the glass transition temperature was also observed. The controversy about the attribution of this upper- T g mobility at the molecular level is reported.

The effect of contact junctions on thermally stimulated depolarization current in thin semiconductor films

The effect of interfacial conditions on the behaviour of thermally stimulated depolarization (TSD) currents in thin-film metal–semiconductor–metal structures has been investigated. It is revealed that at blocking junctions additional TSD peaks can appear. The appearance of TSD peaks is related to the injection of free charge carriers into the semiconductor from an electrode through the energy barrier. It is shown that such an injection can cause a change in the current sign at the TSD peak. This current inversion is determined by the relation between the energy barriers at both electrodes and by the positions of donor levels in the semiconductor.

Pyroelectricity in polyvinylidene fluoride: Influence of polarization and charge

Fractional thermally stimulated depolarization (TSD) currents and temperature-dependent pyroelectricity are experimentally studied in polyvinylidene fluoride samples poled at high direct current field at room temperature. It is demonstrated by a detailed charge balance that the generally assumed proportionality between pyrocoefficients and residual polarization cannot be observed in the low temperature range where polarization irreversibly decreases during heating, while the pyrocoefficient remains constant up to about 60 °C. It is assumed that a portion of the residual polarization results from alignment of the dipoles in the amorphous phase of the polymer. This portion is not related to pyroelectricity and irreversibly vanishes during heating. For lack of screening charges at the crystallite boundaries, an irreversible back switching of the polarization occurs at temperatures higher than 80 °C. Since the relaxation of the compensating charges and that of the ferroelectric polarization are interrelated, it is not possible to distinguish between contributions from polarization and compensation charge to the measured TSD current and to decide which component is thermally more stable. All TSD current cycles show almost the same activation energy of 2.7±0.3 eV. The total released charge during the fractional TSD current experiments amounts to about 30–40% of the residual polarization, indicating that the TSD current due to charge motion partly compensates the depolarization current caused by the dipole realignment. Therefore, the integral of the measured TSD current is always smaller than the corresponding decrement of the residual polarization.

Carrier transport and capture in GaN single crystals and radiation detectors and effect of the neutron irradiation

Carrier transport and trapping were investigated in GaN single crystals and semi-insulating epitaxial MOCVD layers by thermally stimulated current (TSC) and thermally stimulated depolarization (TSD) spectroscopy. Effect of irradiation by 100 keV reactor neutrons with fluencies of up to 10 16 n/cm 2 was identified. We demonstrate that in the unirradiated samples TSC spectra might be caused not by carrier generation from traps, but it rather originates from the thermal mobility variation. The numerical analysis revealed that mobility limited by ionized impurities varies as ∼ T 2.8 and lattice scattering causes the dependence ∼ T −3.5. The highest mobility values were up to 1550 cm 2/Vs at 148–153 K, indicating relatively high quality of the unirradiated samples. The irradiation, depending on the doses, induced complex changes of the carrier transport. The growth of the sample resistivity took place because of the more intense carrier trapping and scattering accompanied by appearance of the percolation transport phenomena. The whole ensemble of the defect traps was identified by the TSC in multiple heating regime and by measurements at different applied biases. In the samples irradiated by 10 16 n/cm 2 the following activation energy values were found: 0.16–0.2, 0.27–0.32, 0.36–0.45 and 0.73–0.74 eV. Effects associated with the percolation transport in a disordered media were observed in the samples irradiated by 5×10 14–10 16 n/cm 2, e.g., current instabilities and decrease of the effective thermal activation energy values by heating in the temperature region below 150 K. A model of carrier percolation transport is presented, in which effect of potential fluctuations due to crystal inhomogeneities is involved. The TSD measurements had confirmed the intensifying effect of inhomogeneities upon irradiation.

Characterisation of the nanoporous structure of collagen-glycosaminoglycan hydrogels by freezing-out of bulk and bound water

The nanoporous structure of collagen-glycosaminoglycan (CG) hydrogels was studied using 1H NMR spectroscopy and thermally stimulated depolarisation (TSD) current with layer-by-layer freezing-out of bulk and interfacial water. The depression of the freezing point of water is related to the size of the nanopore, to which it is confined. Changes in the Gibbs free energy of the unfrozen interfacial water are related to the amount of bound water in the hydrogel matrix and to the re-arrangement of the 3D network structure of the biopolymer. Analysis of the thermodynamic properties of bulk and interfacial water using the layer-by-layer freezing-out technique combined with NMR and TSDC provides valuable information about the structural features of CG hydrogels that can be used for characterisation of different types of hydrogels and soft tissue scaffolds, artificial skin substitutes and other biomaterials.

Thermally stimulated and photoinduced depolarization processes in PbI2 crystals

The thermally stimulated depolarization (TSD) of photo-, thermo-, and electroelectret states in undoped lead iodide crystals was studied in the temperature range 85–330 K. The results demonstrate that the centers responsible for the electret charge localization in PbI2 crystals are related to structural defects. The peak observed in the PbI2 thermoelectret TSD current at 170 K arises from the dipole relaxation of donor-acceptor pairs formed by Pb+ ions and anion vacancies Va, and the peak at 295–305 K is due to the formation of a space charge during poling owing to the displacement of anion vacancies. The electroelectret formation during poling of PbI2 near 85 K in the dark is probably associated with the electric-field ionization of Pb+ and Pb0 centers and the formation of electronic F-centers, responsible for the IR sensitivity of PbI2. The main peak (130–135 K), present in both the thermo-and electroelectret TSD current curves, arises from the thermal dissociation of those centers. IR irradiation of crystals photopolarized at 85 K leads to electron release from trapping centers, accompanied by electret field decay and an increase in Va concentration.

Dielectric relaxation in piezo-, pyro- and ferroelectric polyamide 11

Ferroelectric polyamide 11 films were prepared by melt-quenching, cold-drawing and electrical poling. Their ferroelectricity was studied by means of dielectric-hysteresis measurements. A remnant polarisation of up to 35 mC/m/sup 2/ and a coercive field of 75 MV/m were obtained. The piezoelectric d/sub 33/ coefficient and the pyroelectric coefficient of the films are reduced by annealing just below the melting region, but remain at about 3 pC/N and 8 /spl mu/C/(m/sup 2/K), respectively, during further heat treatment. Differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarisation (TSD) were applied for investigating the conformational changes induced by melt-quenching, cold-drawing and annealing. The results indicate that the cold-drawn film mainly consists of a rigid amorphous phase which exhibits considerably lower conductivity, no glass transition and consequently no dielectric /spl alpha/ relaxation. Instead, an /spl alpha//sub r/ relaxation is found, which is related to chain motions in regions of the rigid amorphous phase where the amide-group dipoles are not perfectly ordered. Annealing removes imperfectly ordered structures, but does not affect the ferroelectric polarisation. Therefore, it may be concluded that essentially the /spl alpha//sub r/ relaxation causes the thermally nonstable part of the piezo- and pyroelectricity in polyamide 11.

Interaction of poly(vinyl pyrrolidone) with fumed silica in dry and wet powders and aqueous suspensions

Abstract Interaction of poly(vinyl pyrrolidone) (PVP) with fumed silica is of interest because of possible medicinal applications of their appropriate mixtures. The PVP/silica blends were studied in the form of dry and wet powders and aqueous suspensions by means of the adsorption, 1 H NMR, infrared spectroscopy, thermally stimulated depolarisation (TSD), electrophoresis, photon correlation spectroscopy, and rheology methods. Mechanical mixing of dry polymer and dry silica powder does not lead to the distribution of PVP on the silica surface. However, the diffusion of PVP molecules at a wet silica surface is promoted by water or ethanol adsorbed from saturated vapour. The polymer forms relatively uniform and dense adsorption layer with no the use of the solution, which is of importance from the practical point of view. Adsorbed PVP adds the effective hydrodynamic diameter of small particles and reduces the size of large swarms of silica particles, affects their ζ potential, modifies the hydrogen bond network in the interfacial water and its Gibbs free energy. These effects appear in the thermally stimulated depolarisation and 1 H NMR spectra. The viscosity (η) increases with increasing concentrations of oxide and polymer at their constant ratio γ=CPVP/CSiO2, but at a constant CSiO2, η depends nonlinearly on CPVP. Dried PVP/silica powders possess the pore size distribution depending on a treatment technique. Obtained results are useful for deeper understanding of regularities in the system PVP/fumed silica in liquids and dried powders.

Thermally Stimulated Depolarization Currents on Colloidal Liquid Crystal Composite System

In this paper we examined the electrical and optical properties of the pure liquid crystal 8CB and in a composite system obtained by surface modified particles ZrO2and the liquid crystal 8CB. Using the Thermally Stimulated Depolarisation Current(TSDC)method we obtained information regarding the conduction mechanism, the activation energy and the pre-exponential frequency factor in the Arrhenius diagrams. We study the dependence of phase transition temperatures on the magnitude of the polarizing d.c. electric field previously applied on the sample. The optical transmission is slightly dependent on the polarization field and the thermal switch behavior was obtained at the nematic–isotropic transition. The TSDC and optical results are compared with the ones obtained by rheology.

Physical and chemical effects on the dynamics of the β-relaxation of PMMA in Rhodamine 6G+PMMA+SiO2 matrices

The Thermally Stimulated Depolarization (TSD) Current technique has been used to reveal confinement effects on the β-relaxation process of PMMA polymerized in-situ SiO2 in the presence of the laser dye Rhodamine 6G/Cl- (Rh6G). The TSD current band of the β process, associated with the rotational relaxation of the mobile carboxymethyl side groups of PMMA, presents a drastic low temperature shift with the introduction of Rh6G or the confinement of PMMA in the nanometric silica substrate. By using the fractional polarization technique we have decomposed this band into single current signals. Irrespective of the initiator type used in the polymerization process, the broad activation energy distribution associated with the β process is nearly identical in the Rh6G+PMMA+SiO2 materials and bulk PMMA. The increase of the dye concentration enlarges the band shift, without any significant modification in the TSD current spectrum (i.e. distinct bands from the polar chromophore). The changes in the molecular dynamics of the polar carboxymethyl side groups of PMMA are discussed in terms of the hydrogen-bond interactions between the silicic acid pore surface and the basic side-groups of PMMA and the decrease of the polymer chain entanglements due to pore directed propagation of polymerization. The relative importance of the various types of interactions between the two chemically identifiable phases in the presence of the dye is discussed.

Physical and chemical effects on the dynamics of the β-relaxation of PMMA in Rhodamine 6G+PMMA+SiO2 matrices

The Thermally Stimulated Depolarization (TSD) Current technique has been used to reveal confinement effects on the β-relaxation process of PMMA polymerized in-situ SiO2 in the presence of the laser dye Rhodamine 6G/Cl– (Rh6G). The TSD current band of the β process, associated with the rotational relaxation of the mobile carboxymethyl side groups of PMMA, presents a drastic low temperature shift with the introduction of Rh6G or the confinement of PMMA in the nanometric silica substrate. By using the fractional polarization technique we have decomposed this band into single current signals. Irrespective of the initiator type used in the polymerization process, the broad activation energy distribution associated with the β process is nearly identical in the Rh6G+PMMA+SiO2 materials and bulk PMMA. The increase of the dye concentration enlarges the band shift, without any significant modification in the TSD current spectrum (i.e. distinct bands from the polar chromophore). The changes in the molecular dynamics of the polar carboxymethyl side groups of PMMA are discussed in terms of the hydrogen-bond interactions between the silicic acid pore surface and the basic side-groups of PMMA and the decrease of the polymer chain entanglements due to pore directed propagation of polymerization. The relative importance of the various types of interactions between the two chemically identifiable phases in the presence of the dye is discussed.

The Impact of the Meyer-Neldel Rule on the Non-Isothermal Observation of Thermally Stimulated Processes in Solids

The impact of the Meyer–Neldel rule (MN rule) on the response of thermally stimulated processes in solids, from which the thermally stimulated depolarisation (TSD) was chosen as a formal representative, is analysed. The current arising in the external circuit was calculated for both cases: validity and invalidity of the MN rule. The results obtained for a set of discrete energy levels and for continuously distributed relaxation times demonstrate that the presence of the MN rule deforms the response of the thermally stimulated process which can lead to its erroneous interpretation. The possible way how to minimise the influence of the MN rule as well as how to attest its presence is suggested.

Study of relaxation mechanisms in structurally related biomaterials by thermally stimulated depolarization currents

Thermal-stimulated depolarization current (TSDC) experiments were performed on a series of tyrosine-derived polyarylates with different number of methylene groups in the backbone. The low temperature TSDC spectra (80–240K) were analyzed at two different degrees of hydration to assess the effects of small amounts of loosely and tightly bound water in the observed complex β band. Higher temperature TSDC spectra (250–350K) provided the α relaxation peaks associated with the glass transition temperature. The direct signal analysis (DSA) method allowed the deconvolution of the peaks into elementary processes and the determination of their mean relaxation parameters. The variations in the low and high temperature polarization with the number of backbone methylene groups were interpreted as a combination of flexibility and entanglement phenomena. The dielectric manifestation of the disentanglement effects was observed for the case of the longest polymer.

Electrical conduction of thin 1,4-cis-polybutadiene films

This work contains results on the internal structure and dc conduction of 1,4-cispolybutadiene. X-ray and thermally stimulated depolarization (TSD) studies have been a source of information on phase transitions in the investigated material. The conduction measurements have been performed on thin films of thickness varying from 1 to 10 /spl mu/m. Gold and aluminum have been applied as electrodes. The temperature range changed from 13 to 325 K. The investigated material was 1,4-cis polybutadiene of the Philips Petroleum Co, It contained 96% cis and 4% of the trans form of polybutadiene. It was observed that the internal structure of the polymer had a strong influence on its electrical properties. Electric conduction resulted from non-activated or thermally activated hopping with some contribution of the Poole-Frenkel effect. Electrode contact phenomena, such as thermionic emission and field emission (tunneling), are very likely responsible for charge carrier injection into the investigated material. The obtained activation energies range from 0.002 to N 0.3 eV.

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.

Dielectric characterization of the water-matrix interaction in porous materials by thermal depolarization spectroscopy

We investigate the dielectric behavior of sandstone, which consists of a porous matrix with a small amount of inherent humidity, by the thermal-stimulated depolarization current technique. Nine different relaxation mechanisms are detected by the thermal sampling scheme, and are characterized. The activation energy distribution and the pre-exponential factor are obtained by analyzing the signals under the constraint of a normal distribution in the activation energy. The drying of the specimen at elevated temperature under dynamic vacuum affects some of the relaxation mechanisms. The model of freely rotating dipoles may not account for all the drying-sensitive mechanisms. It is probable that water molecules are organized in a way that provide either conductive layers over the surface of the grains or for conductive inclusions inside the bulk. Long-distance charge-transport mechanisms are also affected by the removal of the humidity.

Electrical Study of Pr3+ Ion Environment in Ca1?xPrxF2+x Thin Films

The environment of Pr3+ ions and their spatial distribution in Ca1—xPrxF2+x thin films epitaxially grown on silicon substrates were studied by means of complex admittance spectroscopy (CAS) and thermally stimulated depolarization (TSD) studies. Contrary to the case of bulk single crystals, these techniques reveal only one type of praseodymium-fluoride compensating ion centers in the layers, whatever x varying from 0.001 to 0.10. These centers correspond to isolated substituted Pr3+ ions compensated by interstitial Fi— ions in nearest-neighbor (nn) sites of C4v symmetry. Moreover, it is demonstrated that for these praseodymium concentrations, the Fi— ions can be transferred between two neighboring centers. The corresponding dipolar characteristics show that the nn pairs are homogeneously distributed into the bulk of the layers. These results are of great interest. They indicate that energy transfers between excited rare-earth ions, which arise in bulk single crystals for high concentrations, must be considerably reduced in thin films. Consequently the self-quenching phenomena, which rapidly annihilate the luminescence, must be minimized.

Thermally Stimulated Depolarization in Bi4Ge3O12 Single Crystals and Thin Films

Thermally stimulated depolarization (TSD) in Bi 4 Ge 3 O 12 single crystals and thin films was investigated. The TSD spectra were found to contain bands at 185, 230, 270, 290 and 325 K. Concurrent homo- and heterocharges at 185 and 210 K and at 270 and 290 K, respectively, in Bi 4 Ge 3 O 12 generation occurs through accumulation of carriers at traps of different energy.

Dehydration‐induced structural relaxation effects in poly(methyl methacrylate)

AbstractThe Thermally Stimulated Depolarization (TSD) dielectric technique and Dielectric Relaxation Spectroscopy (DRS) have been used in order to investigate aging phenomena in poly(methyl methacrylate) (PMMA). Earlier TSD studies on amorphous PMMA report peculiar dielectric relaxation signals within the range of the glass transition (at ∼378 K) and the secondary relaxation (∼230 K). In the present study, an intense TSD current relaxation band maximizing around 310 K is tentatively attributed to the molecular mobility due to a residual free volume below the glass transition temperature, Tg, that allows structural recovery at the free volume released from the desorption of H2O molecules during evacuation. Limited motions in the main backbone provoke dipole (re)orientation of the ester carbonyl pendant groups with an activation energy E=0.85±0.05 eV, being responsible for the latter dielectric relaxation effect. Alternative attributions based on the short‐range jump relaxation of electric charges and boundary effects are also discussed.