Dielectric Relaxation

By using the thermally stimulated discharge currents (TSDC) technique and coupling it with the thermal step (TS) method, which allows accurate measurement of the space charge distribution (an outstanding electrical property), both the relaxation and the relative contribution of the intrinsic polarization charges and the extrinsic trapped charges have been analysed for a series of poly(vinyl chloride) (PVC) materials after partial hydrogenation as a precursor structure of polyethylene (PE). The a relaxation related to the release of frozen-in dipole orientation, and the r relaxation which is due to the trapping of extrinsic charges have been found to vary, both in intensity and in temperature, with the percentage of ethylene units. On the other hand, the TS measurement of the space charge distribution prior to and after destroying the polarization charges according to the TSDC behaviour, allows one to correlate, as a first approximation, those types of space charge with the molecular microstructure, of either PVC or PE parts. Even if further work remains to be done, the results, as discussed in the light of earlier work on tacticity-driven microstructure-PVC property relationships and on space charge distribution of PVC after hydrogenation, studied by the TS method, provide an original although still tentative interpretation of the space charge behaviour of PE.

It is shown that the temperature behavior of peaks fp, ε p of dielectric loss curves in the isotropic phase of n-cyanobiphenyls (n = 8, 9, 10) with isotropic-nematic and isotropic-smectic A transitions exhibits features characterisic for both supercooled, glass-forming liquids and critical, binary mixtures. The behavior of f p (T ) can be portrayed by the Vogel-Fulcher-Tamman relation and the "critical-like", modecoupling theory (MCT) equation. The latter is supported by the novel analysis of electric conductivity σ (T ). The obtained fp (T ) and σ (T ) dependencies can be related by using the fractional Debye-Einstein-Stokes law. For all tested mesogens the static dielectric permittivities ε (T ) and ε p (T ) are described by dependencies resembling those applied in the homogeneous phase of critical mixtures but with specific-heat critical exponent α ≈ 0.5. This behavior agrees with the novel fluidlike description for the isotropic-nematic transition (P.K. Mukherjee, Phys. Rev. E 51, 5745 (1995); A. Drozd-Rzoska, Phys. Rev. E 59, 5556 (1999)). The obtained glassy features of dielectric relaxation support the recent simulation analysis carried out by M. Letz et al. (Phys. Rev. E 62, 5173 (2000)).

Results of broadband dielectric studies in glass-forming liquid crystalline chiral isopentylcyanobiphenyl (5*CB) are presented. Tests conducted as a function of temperature and pressure revealed the coexistence of glassy and critical properties. The latter are associated with the isotropic-cholesteric phase transition at T I-Ch Ϸ250 K under atmospheric pressure. Dielectric loss curves in the isotropic liquid and in the cholesteric phase are clearly broadened on cooling and pressuring towards the glass transition. Although in the isotropic phase there is a single stretched loss curve, in the mesophase an additional relaxation process can be distinguished. The evolution of relaxation times is non-Arrhenius and can be portrayed by the Vogel-Fulcher-Tamman relation or its pressure counterpart. The glassy dynamics coexists with the critical-like behavior for the static dielectric permittivity and for the maxima of the dielectric loss curves. Their temperature and pressure dependences are associated with the critical exponent ϭ1Ϫ␣Ϸ1/2, where ␣Ϸ1/2 is the specific heat critical exponent. This behavior is associated with the continuous phase transition placed at ⌬TϷ1.5 K below the clearing temperature for Pϭ0.1 MPa. It has been found that 5*CB shows a unique pressure-temperature phase diagram. Pressure and temperature changes which begin in the isotropic liquid below at ca. TϷ265 K always result in the transition to the cholesteric phase which can be supercooled or superpressed. For TϾ265 K the phase transition to another phase, presumably a solid one, always occurs. However, a cholesteric-solid phase border seems to exist only in isothermal pressure tests. It does not appear in the temperature studies.

Linear and nonlinear dielectric measurements of liquid crystalline chiral isopentylcyanobiphenyl ͑5*CB͒ and n-pentylcyanobiphenyl ͑5CB͒, combined with viscosity ͑T͒ data, are presented. The 5 * CB compound glassifies on cooling in the cholesteric phase whereas 5CB crystallizes in the nematic phase. In both compounds the temperature evolution of dielectric relaxation times, the dc conductivity, and the viscosity are well described by the "critical-like" description from mode coupling theory ͑MCT͒. However, for 5 * CB a unique coincidence of the MCT "critical" temperature and extrapolated temperature of the hypothetical continuous isotropic-cholesteric ͑T * ͒ phase transition was found. The temperature dependence of the strong electric-fieldinduced changes of the dielectric permittivity exhibits a strong anomaly in the direction of negative values on approaching T*, not observed up to now. The anomaly is described by the susceptibility-related critical exponent ␥ = 1. The divergence of the "nonlinear" dielectric relaxation follows a power dependence described by the exponent y = 1. This paper recalls the recent discussions on the glassy dynamics of a "hard-ellipsoid" liquid and the possible relationship between the glass transition, critical phenomena, and isotropic-nematic transition.

We have studied the dielectric responses of the layered cobalt oxysulfide Sr2Cu2CoO2S2 with the CoO2 square-planes. With decreasing temperature below the Néel temperature, the resistivity increases like a semiconductor, and the thermopower decreases like a metal. The dielectric constant is highly dependent on temperature, and the dielectric relaxation is systematically changed with temperature, which is strongly correlated to the magnetic states. These behaviors suggest that carriers distributed homogeneously in the paramagnetic state at high temperatures are expelled from the antiferromagnetically ordered spin domain below the Néel temperature.

We have studied the dielectric responses of the layered cobalt oxysulfide Sr2Cu2CoO2S2 with the CoO2 square planes. With decreasing temperature below the Néel temperature, the resistivity increases like a semiconductor, and the thermopower decreases like a metal. The dielectric constant is highly dependent on temperature, and the dielectric relaxation is systematically changed with temperature, which is strongly correlated to the magnetic states. These behaviors suggest that carriers distributed homogeneously in the paramagnetic state at high temperatures are expelled from the antiferromagnetically ordered spin domain below the Néel temperature.

Polymer nanocomposites exhibit unique effective properties that do not follow conventional effective media approaches. The nanoparticle-polymer interphase has been shown to strongly influence the nanocomposites behavior due to its significant volume when the particles are nano-sized, affording an opportunity to tune the dielectric response of the resulting nanocomposite. In this study, we investigate the effects of TiO2 nanoparticles on the electrical properties and the charges distribution and transport in polydimethylsiloxane (PDMS) nanocomposites. Impedance spectroscopy shows suppression of interfacial Maxwell-Wagner-Sillars (MWS) polarization accompanied by a reduction in the low frequency dielectric permittivity and loss at high temperatures in the presence of the TiO2 nanoparticles. Thermally stimulated discharge current measurements confirm that the suppression of the interfacial polarization relaxations happens by redistributing or depleting the charges through the composite and hindering their mobility, potentially resulting in lower electrical conduction and higher breakdown strength. Although the model materials investigated here are TiO2 nanoparticles and Sylgard 184 PDMS, our findings can be extended to other nanoparticulate-filled elastomer composites to design lightweight dielectrics, actuators and sensors with improved capabilities.

Thermo-electrets of poly (vinyl chloride-co-vinyl acetate-co-2 hydroxy propyl acrylate) PVVH and its nanocomposite PVVH/ZnO were prepared using the thermal poling method. Thermally stimulated depolarization current TSDC technique used to study the alpha relaxation of the samples. The global spectra of the nanocomposites revealed that ZnO nanoparticles are completely compatible with the polymer matrix. The activation energy was calculated and found to be in the range of 0.69-1.25 eV. TS technique was used to resolve the complex TSDC behavior and thermodynamic parameters were obtained from Eyring relationship which verified the linear relation between the activation enthalpy ∆H and the activation entropy ∆S. The compensation phenomena had been studied and the degree of disorder DOD of all samples has been calculated and results values in the range 15 to 35.24 Cal degree -1 .

In this work it was demonstrated that the addition of Sn on CaCu3Ti4O12 material improved non-Ohmic behavior by suppressing dielectric properties. It was noted that the improvement of the varistor characteristics, monitored by the increase in nonlinear coefficient, occurs with the disappearance of the grain dielectric relaxation process with concomitant decreasing of both dielectric constant and dielectric loss values. By forming a solid solution, Sn4+ was incorporated into CaCu3Ti4O12 matrix deforming the crystal lattice and restricting the formation of polaronic stacking faults. Thus, the dielectric relaxation due to polaronic defects is believed to be the origin of the huge dielectric properties disappearance. This framework is in agreement with the nanosized barrier layer capacitor model.

In the present communication, by using dielectric spectroscopy measurement, the correlations between Nanosized Barrier Layer Capacitance (NBLC) (Bueno et al. (2009) [7]) and the high frequency polaronic near-Debye dipolar relaxation found in CaCu 3 Ti 4 O 12 compounds was discussed. The polaronic process was confirmed to be closely associated with the ultrahigh dielectric features of CaCu 3 Ti 4 O 12 materials and its concomitant dielectric loss. Herein, the shift in relaxation frequency as a function of temperature was used for calculating the activation energy for hopping electronic conduction. The value obtained was 33 meV, an energy whose magnitude is compatible and confirmed the hypothesis of polaronic features for this high frequency dipolar relaxation process. Furthermore, it is shown that the nanosized barrier inferred from the NBLC model has a polaronic feature with dielectric permittivity exiting orthogonally to dielectric loss, a phenomenological pattern that contradicts the normally observed behavior for traditional dielectrics but explain the dielectric and conductivity feature of CaCu 3 Ti 4 O 12 compounds.

Pr oceed in gs of t he I n t ern ati on al W or kshop N OA '00 , Gr yb §w 200 0 L I GH T P ROPA GA TIO N I N LI QU I D CRY STA L FI B E R STR UCTU RE S T . N asi ¤o ws ki a n d T .R. Wo l i ¥ski Facult y of Phy sics, Warsa w U ni versity of T echnolo gy K oszy kowa 75, 00-662 W arszaw a, Polan d T he pap er analyzes the light propagation in anisotropi c li qui d crystal Ùb er structures in order to Ùnd the p ossibil iti es to apply the m to control and sw itch the polarizati on state and mo dulate some parameters of the transmitted light . T he long term goal and motiv ation of the authors w as the suggestion to replace the light mo dulating liqui d crystal cell by a new generation of liquid crystal waveguides as an all-Ùber light mo dulator, as w ell as sensors for di ˜erent environmental parameters.

The electrical properties of natural cork, commercial cork agglomerates (for floor and wall covering) and a composite of cork/TetraPak® were studied. The composite was developed at LNEG/Portugal and is made of recycled cork and TetraPak® containers. Measurements ...

A detailed study is made of the low-temperature dielectric relaxation peak that develops in crys- talline quartz upon irradiation. A wide variety of crystals are employed: from diFerent sources, with diFerent purities, and with the Al + impurity compensated by a selected alkali-metal ion (Li+ or Na+) or by H+ through the use of the technique of "sweeping. " A comparison is made between the dielectric peak height and the intensity of the weB-known EPR spectrum due to the aluminum- hole (A1-h) center. It shows that the dielectric loss peak is due to the Al-h center, with the hole lo- cated on a "long-bond" oxygen atom Analysis of the relaxation rate as a function of temperature shows that, below -6 K the relaxation takes place by a one-phonon tunneling process, while above this temperature a multiphonon process occurs. %Rile the relaxation time in the one-phonon re- gime is unique, the relaxation spectrum and the mean relaxation time in the multiphonon regime are sample dependent, due to phonon scattering by crystal defects. Crystalline quartz has become the foremost material in ultrasonic devices and precision frequency standards. Accordingly, there is considerable interest in the nature of point defects in quartz crystals, particularly those that can alfect their frequency stability. Perhaps the most im- portant defects in quartz are those involving an A13+-ion impurity substituting for Sis+. Since the Al has a lower valence than Si, an additional positive defect is required for charge compensation. This is usually provided by an interstitial alkah ion (I. i+ or Na+ ) or by protons. The al- kali ion M+ becomes bound to the Al ion by Coulombic attraction to form an Al-M pair, while H+ forms an OH bond near the Al. The presence of the Al-OH de- fect is detectable by infrared measurements. ' The de- fect structure is changed by ionizing radiation in the vi- cinity of room temperature. Such changes under irradia- tion are of interest because they are accompanied by fre- quency drifts of quartz resonators.

Permanent magnets of varying shapes and sizes are increasingly produced. For hexaferrite magnets, it is challenging to incorporate polymers and a soft magnetic phase in the form of paste before injection molding or extrusion free-forming. In this study, hard magnetic barium hexaferrite/soft magnetic iron composites with a density of 2.28-2.34 g/cm 3 are obtained after paste-injection molding and subsequent sintering at 1150 • C for 5 h. Variations of the binder (143.5-287.0 mg poly(vinyl alcohol), PVA) and the plasticizer (75-150 mg poly(ethylene glycol), PEG-400) in the ceramic-polymer paste give rise to comparable remanent magnetization (33.10-33.63 emu/g) and coercivity (3854)(3855)(3856)(3857). Unlike all-ferrite systems, the presence of a soft magnetic metal phase is not detrimental to the coercivity. However, the remanent and saturation magnetizations are not substantially increased. The addition of 1% and 5% of iron oxide in the ceramic-polymer paste gives rise to hard/soft composites with lower densities of 2.11 and 2.14 g/cm 3 . The coercivity is increased to 3942-3945 Oe; however, the maximum energy product is reduced.

This work is related to the preparation of polymer-clay nanocomposites (PCNs) by the exfoliation-adsorption technique, from atactic poly(methyl methacrylate) (PMMA) in solution, using Bentonite (B) as a layered-silicate natural clay. To optimize the intercalation of B with PMMA, it has been organically modified (OB) with a quaternary ammonium salt that helps the interchange of cations, and thus converting its hydrophilic surface to an organophilic one. An investigation of the morphology and molecular motions or dynamics of the net polymer film as well as the PCN final films was performed by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Infra-Red Spectroscopy (IR), Thermally Stimulated Depolarization Currents (TSDC), and Differential Scanning Calorimetry (DSC) techniques. One of our interests was to discuss the solvent influence on the polymeric matrix. Herein, comparative studies of the effects of different solvents on the thermal and dielectric properties of the net PMMA films are reported. Four solvents were used to prepare the films, toluene, dichlorometane, tetrahydrofuran and acetone. 100 nm 100 nm 2 θ (d e g)

We investigate the effects of clay proportion and nanoscale dispersion in the dielectric response of poly͑vinyl alcohol͒-bentonite nanocomposites. The dielectric study was performed using the thermally stimulated depolarization current technique, covering the temperature range of the secondary and high-temperature relaxation processes. Important changes in the secondary relaxations are observed at low clay contents in comparison with neat poly͑vinyl alcohol͒ ͑PVA͒. The high-temperature processes show a complex peak, which is a combination of the glass-rubber transition and the space-charge relaxations. The analysis of these processes shows the existence of two segmental relaxations for the nanocomposites. Dielectric results were complemented by calorimetric experiments using differential scanning calorimetry. Morphologic characterization was performed by x-ray diffraction ͑XRD͒ and transmission electron microscopy ͑TEM͒. TEM and XRD results show a mixture of intercalated and exfoliated clay dispersion in a trend that promotes the exfoliated phase as the bentonite content diminishes. Dielectric and morphological results indicate the existence of polymer-clay interactions through the formation of hydrogen bounds and promoted by the exfoliated dispersion of the clay. These interactions affect not only the segmental dynamics, but also the secondary local dynamics of PVA.

Thermal sampling peaks recorded after windowing polarization are studied for the segmental mode in poly͑⑀-caprolactone͒. Also, numerical decompositions of the global thermally stimulated depolarization current peak into pure Debye contributions are performed with direct signal analysis ͑DSA͒ and simulated annealing direct signal analysis procedures for Arrhenius and Vogel-Tammann-Fulcher ͑VTF͒ temperature dependences, respectively. It is found that the results between the experimental and the numerical procedures agree very well and the approximations made in the analysis of the experimental curves are thus validated, despite the unphysical values for the relaxation parameters found by both methods when using Arrhenius relaxation times. On the contrary, when VTF relaxation times are used for the numerical decomposition, agreement is found with the results of isothermal dielectric absorption as a function of frequency, together with reasonable values for reorientation energies, pre-exponential factors and VTF temperature. Thermal sampling and DSA also compare well when studying the departure from the zero-entropy line which indicates the onset of a cooperative character in the dynamics of molecular motion. Compensation is found whenever the primary relaxation is analyzed with Arrhenius or Eyring relaxation times and does not appear when VTF relaxation times are used in the numerical decomposition.

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 occured 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.

A family of four thermotropic polyesters starting with the poly(tetramethylene terephthaloyl bis-4-oxybenzoate), substituting either asymmetrically in the tetramethylene spacer or incorporating polar substituents onto the aromatic units of the mesogen, is studied by the Thermally Stimulated Depolarization Currents (TSDC) technique. The results are compared to the Dynamic-Mechanical Analysis at frequencies ranging from 0.1 to 1 Hz from 123 to 423 K. The unsubstituted polymer has a complex low-temperature TSDC spectrum corresponding to local reorientation modes due to the motion of the COO groups with different locations along the main chain. By comparing the effect of the Cl and CH 3 substituents on the relative intensity and on the mean energies of the distribution of relaxation times determined by the DSA procedure, the lowest temperature mode is attributed to the COO peripheral groups and the modes located at higher temperatures, to the internal COO groups, which may be accompanied by adjacent segments. The mechanic and dielectric ␣-transitions are also very sensitive to the substituents, the addition of CH 3 in the 1,4 flexible spacer shifting the glass transition temperature above room temperature. The existence of the three-dimensional order that is present in the materials with a linear spacer significantly broadens the ␣-relaxation.

The effect of water sorption on the dielectric relaxation processes of nylon-6 samples with water concentrations ranging from the dry to the water-saturated polymer has been studied by thermally stimulated depolarization currents experiments in a broad temperature range, from 77 to 365 K. The strengths of the low-temperature modes, gand b-peaks, are affected in opposite ways by the water concentration, h, as the first one shows a decrease in intensity and the second one grows as h increases. The precise determination of the relaxation parameters is made by the decomposition in elementary Debye processes and best fitting to the experimental profile of the complex peak. For h õ 3%, the reorienting energies are almost independent of the water content, and the most significant intensity variations occur. The firmly bound water is held responsible for these effects. As for the higher temperature zone besides the apeak, which is the dielectric manifestation of the glass transition, intermediate temperatures modes are observed at high h values and are originated by the loosely bound water, while the highest temperature peak is attributed to a Maxwell-Wagner interfacial polarization. The characteristic parameters of the a-mode are determined and related to the plasticization effect of water.

In this paper we analyze the suitability and feasibility of the method proposed by Prakash for the determination of the dielectric relaxation parameters, namely activation energy and fundamental relaxation time, and order of kinetics, of a thermally stimulated depolarization (TSD) spectrum, also known as ionicthermocurrent (ITC) spectrum, by applying it both to numerically generated TSD spectrum of hypothetical system and experimentally reported TSD spectrums of AgCl:Pb2+ and KI:S2-. It is found that the method is simpler, convenient and can be used for the analysis of TSD spectrum or ITC spectrum involving any order of kinetics.

The effect of 10 wt% polypyrrole (PPy) incorporation on the evolution of conductivity, dielectric constant, and dielectric loss by varying the temperature and frequency of amine‐terminated poly(ethylene glycol)/randomly methylated β‐cyclodextrins (PEGBA/RAMEB) polyrotaxane cross‐linked with 1,6‐hexamethylene diisocyanate is reported. This composite material shows an enhanced dielectric constant value of 25.6 (at 10 Hz) compared with 8.3 of the pure cross‐linked PEGBA/RAMEB polyrotaxane. The values of the dielectric constant are consistent with the dielectric loss and conductivity results. The composite material shows that the direct current conductivity (σDC) at lower temperature is independent of frequency. In comparison, at higher temperature, σDC is dependent on frequency, denoting that the conductivity of the composite material is a thermally activated process. Moreover, the presence of PPy significantly influences the morphology of the cross‐linked PEGBA/RAMEB polyrotaxane. Add...

The dielectric spectroscopy of an antiferroelectric liquid crystal (S)-4-(1-methylheptyloxycarbonyl)phenyl-4 0 -(6-pentanoyloxyhex-1-oxy)biphenyl-4carboxylate for its homogeneous anchoring has been carried out for 10 lm and 4 lm cell thicknesses. This study suggests that the dielectric strength of Goldstone mode of the SmC Ã phase is significantly diminished from $195 to $69, whereas its relaxation frequency is increased from $2 kHz to $4 kHz from the higher to lower cell thickness. Alternatively, these parameters for the soft mode detected in the SmA Ã phase and for the two antiferroelectric relaxation modes detected in the SmC Ã A phase do not show any appreciable change with the cell thickness.

In this study, we report the results of an investigation into the sintering temperature dependence of magnetic and transport properties for GdBaCo 2 O 5+δ synthesized through a sol-gel method. The lowering of sintering temperature leads to the increase of oxygen content and the reduction of grain size. The increase of oxygen content results in the enhancement of magnetic interactions and the weakening of Coulomb repulsion effect, while the reduction of grain size improves the magnetoresistance effect. Metalinsulator transition accompanied with spin-state transition is observed in all samples.

an k o wsk i a , A . Ï l ¨za k a , W. K empi ¥ski a , B. A ndrze je ws k i a , B. R eic he l b an d G. Pl esch c a I n st i t ut e of Mo l ecul ar P hysi cs, Po l i sh Aca demy of Sciences Sm ol uchows kiego 17, 60-179 Po zna¥, Po l and b Facul t y of A ppl i ed Physi cs and Ma them ati cs, T echnical Uni versi ty of G da¥sk Naruto wi cza 11/ 12, 80 -952 G da¥sk, Po l and c D epartm ent of Ino rg ani c Chem i stry , Facult y of Natura l Sciences

The behavior of the Pm3 ¯m-R3 ¯c phase transition in LaAlO 3 ͑T C = 813 K from differential scanning calorimetry measurements͒ has been studied using temperature-dependent measurements of the crystal structure, dielectric relaxation, specific heat, birefringence, and the frequencies of the two soft modes ͑via Raman spectroscopy͒. While all these experiments show behavior near T C consistent with a second-order Landau transition, there is extensive evidence for additional anomalous behavior below 730 K. Below this temperature, the two soft mode frequencies are not proportional to each other, the spontaneous strain is not proportional to the square of the AlO 6 rotation angle, and anomalies are seen in the birefringence. Twin domains, which are mobile above 730 K, are frozen below 730 K. These anomalies are consistent with biquadratic coupling between the primary order parameter of the transition ͑AlO 6 rotation͒ and a second process. From the dielectric results, which indicate a smooth but rapid increase in conductivity in the temperature range 500-800 K, we propose that this second process is hopping of intrinsic oxygen vacancies. These vacancies are essentially static below 730 K and dynamically disordered above 730 K. The interaction between static vacancies and the displacive phase transition is unfavorable. A similar anomaly may be observed in other aluminate perovskites undergoing the same transition.

The polycrystalline bismuth strontium ferric titanate (Bi(Sr 0.25 Ti 0.25 Fe 0.5 )O 3 ), synthesized via a high-temperature solid-state reaction route, has been characterized by using different experimental procedures. Study of basic crystal structure with the help of X-ray diffraction data of the compound exhibits the rhombohedral crystal system. The quality of the sample was asserted by studying its microstructure and morphology. Detailed investigations of dielectric and electrical properties of Bi(Sr 0.25 Ti 0.25 Fe 0.5 )O 3 in a broad frequency (1 kHz-1 MHz) and temperature (25-450 °C) ranges have provided various important information on conduction mechanism, structure-properties correlationship, etc. The high dielectric constant and relativly low tangent loss with multiple phase transitions have been observed in the material I the experimental frequency and temperature ranges.

We report the results of a combined phenomenological analysis of the data from positron-annihilationlifetime spectroscopy ͑PALS͒ and the relaxation data from broadband dielectric spectroscopy ͑BDS͒ on diethyl phthalate ͑DEP͒. The ortho-positronium ͑o-Ps͒ lifetime, 3 , as a function of temperature over a temperature range from 67 K up to 300 K is compared with the spectral features and the relaxation parameters of the BDS spectra decomposed into the primary ␣ and the secondary ␤ processes in the temperature range from 140 K up to 380 K by using the Williams-Watts scheme. Phenomenological model-free analysis of the 3 -T plot provides the three characteristic PALS temperatures, where the two most pronounced ones at T g PALS = 185 K and T b2 = 245 K = 1.32T g PALS are related to the glass-liquid transition and the onset of a quasiplateau region, respectively. In the case of a weaker bend effect at T b1 = 210 K = 1.14T g PALS , a number of new coincidences with changes in the dielectric ␤ process have been found. They concern the changes in width parameter of the distribution function for the ␤ relaxation time and the activation energy of the ␤ eff process, a crossover from the Arrhenius to the non-Arrhenius type of temperature dependence as well as with the onset of a short-time tail of the ␤ relaxation time distribution and finally, with changes in the relaxation strength of the ␣ and ␤ eff processes. All these findings indicate a close connection of the o-Ps annihilation parameters and relaxation characteristics of BDS response for the DEP matrix.

The ultraviolet spectra of pure and lead salt-poly(viny1 alcohol) (PVA) composite films were studied at room temperature. Blending of PVA with lead acetate and lead nitrate cause a considerable change in the UV spectrum, indicating electronic structure modifications. The complex permittivity (c*) and the complex electric modulus ( M * ) of the pure PVA and the PVA-based composite films were investigated between 300 and 400 K in the 100 Hz-100 kHz frequency range. The frequency dependence of the permittivity is influenced by the space-charge polarization. The interfacial polarization is manifested a t high temperature and becomes important only above the PVA glass transition temperature and below 1 kHz. Dielectric properties of PVA-lead nitrate composite revealed that the salt is complexed with PVA through hydrogen bonding.

Intramolecular charge transfer (ICT) reaction in a newly synthesized molecule, of 4-(1-morpholenyl) benzonitrile (M6C), in AOT/water/heptane reverse micelles at different pool sizes has been studied by using steady-state and time-resolved fluorescence emission spectroscopy. The pool size dependences of the reaction equilibrium constant and reaction rate have been explained in terms of the average polarity of the confined solvent pools estimated from the fluorescence emission Stokes shift of a nonreactive probe, coumarin 153, dissolved in these microemulsions. The complex permittivity measurements in the frequency range 0.01 e ν/GHz e 2 for these microemulsions at different pool sizes (0 e w 0 e 40) and AOT concentrations (0.1 e c/M e 0.5) at 298.15 K have also been performed. At sufficient water content, a large dispersion with a relaxation time of ∼600 ps has been observed at ∼300 MHz and attributed to the average reorientation of water molecules residing in the close vicinity of the polar interface of the AOT headgroup and n-heptane. The reorientation of these interfacial water molecules is probably responsible for the nanosecond component observed in numerous polar solvation dynamics experiments in these reverse micelles. Subsequently, the estimated polarity and the measured reorientational time scale have been used to explain the dramatic slowing down of the ICT reaction rate and its dependence on pool size in these confined environments.

The universality of the ac electrical transport in granular metals has been scarcely studied and the actual mechanisms involved in the scaling laws are not well understood. Previous works have reported on the scaling of capacitance and dielectric loss at different temperatures in Co-ZrO2 granular metals. However, the characteristic frequency used to scale the conductivity spectra has not been discussed, yet. This report provides unambiguous evidence of the universal relaxation behavior of Pd-ZrO2 granular thin films over wide frequency (11 Hz–2 MHz) and temperature ranges (40–180 K) by means of Impedance Spectroscopy. The frequency dependence of the imaginary parts of both the impedance Z″ and electrical modulus M″ exhibit respective peaks at frequencies ωmax that follow a thermal activation law, ωmax ∝ exp(T1/2). Moreover, the real part of electrical conductivity σ′ follows the Jonscher’s universal power law, while the onset of the conductivity dispersion also corresponds to ωmax. ...

Broad-band dielectric spectroscopy (10-"106 Hz) was employed to investigate molecular dynamics in polyisoprene-polystyrene (PI-b-PS) diblock copolymers with different contents of PI varying from 27.9 to 50.4 w t 96 and having an identical molecular weight of the PS blocks (2830). The diblock copolymers studied were considered to be thermodynamically homogeneous concerning DSC measurements and having a value of xN from 5.5 to 8.9 at 25 O C , where x is the segment-segment interaction parameter and N the number of statistical segments per chain. Two relaxation processes are observed which sense the molecular dynamics of the PI chain in the diblock copolymer on two different length and time scales: The segmental relaxation is a local process corresponding to the primary relaxation of the PI chain in the PI-rich regions of the thermodynamically homogeneous phase. In contrast, the normal-mode relaxation is assumed to be mainly controlled by an effective friction coefficient reflecting the properties of both block components. The normal-model relaxation in its temperature dependence and in its distribution of relaxation times provides evidence for the coupling to composition fluctuations. Furthermore, there is a hint that the freezing-in of PS segmental fluctuations affects the normal-mode dynamics of the PI blocks. Our results are compared to similar investigations by Yao et on PI-b-PS under conditions of microphase separation. The findings are consistent with the fluctuation picture recently developed by Fredrickson et al. for diblock copolymers near the microphase-separation transition.

Dielectric spectra of dimethyl sulfoxide (DMSO)/water mixtures, over the entire concentration range, have been measured using the transmission line method at frequencies from 45 MHz to 26 GHz and at temperatures of 298-318 K. The relaxation times of the mixtures show a maximum at an intermediate molar fraction of DMSO. The specific structure of mixtures in different concentration regions was determined by the dielectric relaxation dynamics, obtained from the effect of temperature on the relaxation time. A water structure "breaking effect" is observed in dilute aqueous solutions. The average number of hydrogen bonds per water molecule in these mixtures is found to be reduced compared to pure water. The increase in the dielectric relaxation time in DMSO/water mixtures is attributed to the spatial (steric) constraints of DMSO molecules on the hydrogen-bond network, rather than being due to hydrophobic hydration of the methyl groups. The interaction between water and DMSO by hydrogen bonding reaches a maximum at a DMSO molar fraction of 0.33, reflected by the maximum activation enthalpy for dielectric relaxation in this concentration, suggesting the formation of a stoichiometric compound, H 2 O-DMSO-H 2 O. In highly concentrated solutions, negative activation entropies are observed, indicating the presence of aggregates of DMSO molecules. A distinct antiparallel arrangement of dipoles is obtained for neat DMSO in the liquid state according to the Kirkwood correlation factor (g K ) 0.5), calculated from the static permittivity. The similarity of the dielectric behavior of pure DMSO and DMSO-rich mixtures suggests that dipole-dipole interactions contribute significantly to the rotational relaxation process in these solutions.

Dielectric spectra of dimethyl sulfoxide (DMSO)/water mixtures, over the entire concentration range, have been measured using the transmission line method at frequencies from 45 MHz to 26 GHz and at temperatures of 298-318 K. The relaxation times of the mixtures show a maximum at an intermediate molar fraction of DMSO. The specific structure of mixtures in different concentration regions was determined by the dielectric relaxation dynamics, obtained from the effect of temperature on the relaxation time. A water structure "breaking effect" is observed in dilute aqueous solutions. The average number of hydrogen bonds per water molecule in these mixtures is found to be reduced compared to pure water. The increase in the dielectric relaxation time in DMSO/water mixtures is attributed to the spatial (steric) constraints of DMSO molecules on the hydrogen-bond network, rather than being due to hydrophobic hydration of the methyl groups. The interaction between water and DMSO by hydrogen bonding reaches a maximum at a DMSO molar fraction of 0.33, reflected by the maximum activation enthalpy for dielectric relaxation in this concentration, suggesting the formation of a stoichiometric compound, H 2 O-DMSO-H 2 O. In highly concentrated solutions, negative activation entropies are observed, indicating the presence of aggregates of DMSO molecules. A distinct antiparallel arrangement of dipoles is obtained for neat DMSO in the liquid state according to the Kirkwood correlation factor (g K ) 0.5), calculated from the static permittivity. The similarity of the dielectric behavior of pure DMSO and DMSO-rich mixtures suggests that dipole-dipole interactions contribute significantly to the rotational relaxation process in these solutions.

The use of electro-optic (EO) crystals for electromagnetic field detection is an attractive alternative to conventional techniques, due to their compactness, large intrinsic bandwidth, and ability to measure field amplitude and phase with minimum field perturbation. In our LiNbO3 sensors, anomalously large detection sensitivities were observed, which were found to be due to dielectric relaxation effects within the crystals. In this presentation, we demonstrate these effects, their impact on the EO sensor responsivity, and discuss implications for improving future EO field sensors.

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We report the modifications in the structural, morphological and dielectric properties of La 2 NiMnO 6 (LNMO) by doping Ba at La site for three different proportions. These ceramics were successfully synthesized by the solidstate route as revealed by the X-ray diffraction patterns of three compositions. The sample phases were confirmed by the Rietveld refinement which showed monoclinic symmetry having P2 1 /n space group. The scanning electron microscopy imaging of the pristine LNMO sample reveals ablated spheres, and this gets modified to sugar-cube like structures after the Ba substitution at La site. In addition to modification in morphology, a decrease in porosity is also noticed after Ba doping as calculated from the ratio of pore-pixels to the entire pixel-count of the image which reveals the percent porosity. The substitution of Ba 2þ in place of La 3þ is expected to bring about changes in the overall conduction mechanisms and also the dielectric relaxations in the LNMO. The temperature-dependent (100K-400K) and frequency-dependent (1 kHz-2 MHz) dielectric studies reveal that the Ba doping results in the decrease of dielectric constant and dielectric loss. This decrease is attributed to conversion between Ni 2þ to Ni 3þ and Mn 4þ to Mn 3þ due to Ba doping resulting in an increase of grain resistance as such decreasing possibility of most of the electrons to reach grain boundaries, which in turn causes a decrease in polarization and thus resulting in the decrement in the value of dielectric constant. Low dielectric materials are preferable and more reliable as they cause low heat dissipation and reduce any parasitic capacitance, thus allowing faster switching speeds in the fabricated devices.

We explore the dielectric relaxation properties of NiFe nanowires in a nanoporous silicon template. Dielectric data of the NiFe-silicon structure show a strong relaxation resonance near 30K. This system shows Arrhenius type of behavior in the temperature dependence of dissipation peaks vs. frequency. We report magnetic field dependence of dipolar relaxation rate and the appearance of structure in the dielectric spectrum related to multiple relaxation rates. A magnetic field affects both the exponential prefactor in the Arrhenius formula and the activation energy. From this field dependence we derive a simple exponential field dependence for the prefactor and linear field approximation for the activation energy which describes the data. We find a significant angular dependence of the dielectric relaxation spectrum for regular silicon and nanostructured silicon vs. magnetic field direction, and describe a simple sum-rule that describes this dependence. We find that although similar behaviour is observed in both template and nanostructured materials, the NiFe-silicon shows a more complex, magnetic field dependent relaxation spectrum.

Magneto-dielectric study of multiferroicity in biferroic YCrO 3 RELJA VASIC, HAIDONG D. ZHOU, CHRIS R. WIEBE, JAMES S. BROOKS, Florida State University -Dielectric measurements are used to characterize multiferroicity in the doped monoclinic ferroelectric oxides Y 1-x Ca x MnO 3 (x = 0, 0.15, 0.3). The focus of this study is on the effects of the magnetic field and Ca doping on the temperature-magnetic field dependent dielectric response of polycrystalline samples of rare earth chromates YCrO 3 . YCrO 3 shows dielectric relaxation around 150 K related to the weak ferromagnetic ordering in system. Dipolar response is activated following Arrhenius formalism in the frequency range of 0.01-100 kHz, but it is independent on magnitude of magnetic field. As frequency increases dielectric peaks become broader and smaller. Dielectric constant relaxation behavior and magnetic phase transitions are not coupled through lattice distortions in this ferroelectric ferromagnetic system. YCrO 3 is an example of system with dipolar response in magnetic field without coupling with magnetic structure. There is strong dispersion of Debye relaxation peaks but absence of influence of magnetic field on ferroelectric system YCrO 3 .

Dielectric relaxation is universal in characterizing polar liquids and solids, insulators, and semiconductors, and the theoretical models are well developed. However, in high magnetic fields, previously unknown aspects of dielectric relaxation can be revealed and exploited. Here, we report low temperature dielectric relaxation measurements in lightly doped silicon in high dc magnetic fields B both parallel and perpendicular to the applied ac electric field E. For B//E, we observe a temperature and magnetic field dependent dielectric dispersion ε(ω) characteristic of conventional Debye relaxation where the free carrier concentration is dependent on thermal dopant ionization, magnetic freeze-out, and/or magnetic localization effects. However, for B⊥E, anomalous dispersion emerges in ε(ω) with increasing magnetic field. It is shown that the Debye formalism can be simply extended by adding the Lorentz force to describe the general response of a dielectric in crossed magnetic and electric fields. Moreover, we predict and observe a new transverse dielectric response E H ⊥ B ⊥ E not previously described in magnetodielectric measurements. The new formalism allows the determination of the mobility and the ability to discriminate between magnetic localization/freeze out and Lorentz force effects in the magneto-dielectric response.

We explore the dielectric relaxation properties of NiFe nanowires in a nanoporous silicon template. Dielectric data of the NiFe-silicon structure show a strong relaxation resonance near 30K. This system shows Arrhenius type of behavior in the temperature dependence of dissipation peaks vs. frequency. We report magnetic field dependence of dipolar relaxation rate and the appearance of structure in the dielectric spectrum related to multiple relaxation rates. A magnetic field affects both the exponential prefactor in the Arrhenius formula and the activation energy. From this field dependence we derive a simple exponential field dependence for the prefactor and linear field approximation for the activation energy which describes the data. We find a significant angular dependence of the dielectric relaxation spectrum for regular silicon and nanostructured silicon vs. magnetic field direction, and describe a simple sum-rule that describes this dependence. We find that although similar behaviour is observed in both template and nanostructured materials, the NiFe-silicon shows a more complex, magnetic field dependent relaxation spectrum.

Studies of dielectric relaxation and ac conductivity have been made on three samples of sodium tungsten phosphate glasses over a temperature range of 77-420 K. Complex relative permittivity data have been analyzed using dielectric modulus approach. Conductivity relaxation frequency increases with the increase of temperature. Activation energy for conductivity relaxation has also been evaluated. Measured ac conductivity ´σm ´ω µµ has been found to be higher than σ dc at low temperatures whereas at high temperature σ m ´ω µ becomes equal to σ dc at all frequencies. The ac conductivity obeys the relation σ ac ´ω µ A ω S over a considerable range of low temperatures. Values of exponent S are nearly equal to unity at about 78 K and the values decrease non-linearly with the increase of temperature. Values of the number density of states at Fermi level ´N´E F µµ have been evaluated at 80 K assuming values of electron wave function decay constant α to be 0.5 ( Å) 1 . Values of N´E F µ have the order 10 20 which are well within the range suggested for localized states. Present values of N´E F µ are smaller than those for tungsten phosphate glasses.

In order to understand the relaxation processes responsible for thermally charged polyimide (PI) samples, the thermally stimulated discharge currents (TSDC) and fractional polarization thermally stimulated discharge current (FPTSDC) have been recorded. FPTSDC study is found to be having dipolar as well as space charge relaxation. The appearance of the second peak (i.e. β peak) at different poling field indicated that this peak is attributed to dipolar relaxation in polyimide. The peak position and activation energy do not change with the poling field (V p ), but it is very sensitive to the poling temperature (T p ). The magnitude of peak current has been found to vary with poling field and temperature. The poling field seems to change the reorientation of dipoles associated with β peak. The γ (first peak) and β (second peak) peaks were attributed to the motion of the side chain and the dipole-orientation associated with the residual reactive groups, respectively. The α peak (third peak) may be due to the space charge polarization or due to charge injected from the electrode. The activation energy and charge released have been found to vary with the poling field, poling temperature and poling time for this peak. FPTSDC spectra are showing two peaks which correspond to dipolar and space charge relaxation , respectively and are in good agreement with TSDC.

Single frequency concentration variational method of Gopala Krishna, Trans Faraday Soc, 33 (1957) 767., has been used to determine the dielectric relaxation times(τ) and the dipole moments(µ) of dilute solution of Nmethylacetamide(NMA), N,N-dimethylacetamide(DMA) and NMA+DMA binary mixtures in benzene solutions at 25, 30, 35C and 40°C by the standard standing wave techniques at 9.90GHz. Energy parameters ( H ε, F ε , S ε ) for the dielectric relaxation process of NMA+DMA binary mixtures containing 30% of DMA have been calculated at 25, 30, 35 and 40°C. Comparison has been made with the corresponding energy parameters for viscous flow. From the observations, it is found that the dielectric relaxation process like the viscous flow process can be treated as the rate process. On the basis of above studies, solute-solute and solute-solvent types of the molecular associations have been predicted.

The availability of large size langasite (La 3 Ga 5 SiO 14 ) wafers enables micromachining of this high-temperature stable piezoelectric material which has been shown to exhibit bulk oscillations at temperatures of up to at least 1400 °C. In particular, the realization of miniaturized resonant sensors becomes feasible. Such resonators are operated far above their dielectric relaxation frequency leading to relatively low losses. Our specific research is focused on the development of monolithic structures to overcome problems originating from thermal stress. The concept includes the local doping of langasite by niobium, strontium and praseodymium. Their chemical diffusion coefficients were determined and found to be fairly small. Field enhanced diffusion results in an increased doping depth and concentration as demonstrated for niobium. The effect is governed by local heating of the sample. Optimized process conditions lead potentially to a pronounced drift of the dopants. Strontium doping increases the conductivity of langasite by three to four orders in magnitude and enables the formation of monolithic electrodes for high-temperature resonators as demonstrated by operation of such devices at temperatures as high as 800 °C. Micromachined functional structures including planar and biconvex membranes, as well as cantilevers, are prepared and demonstrated to be operational up to 930 °C. The analysis of their resonance behavior shows high resonator quality factors, e.g. 190 for a 60 MHz bulk acoustic resonator at the above-mentioned temperature.

The polycrystalline bismuth strontium ferric titanate (Bi(Sr 0.25 Ti 0.25 Fe 0.5 )O 3 ), synthesized via a high-temperature solid-state reaction route, has been characterized by using different experimental procedures. Study of basic crystal structure with the help of X-ray diffraction data of the compound exhibits the rhombohedral crystal system. The quality of the sample was asserted by studying its microstructure and morphology. Detailed investigations of dielectric and electrical properties of Bi(Sr 0.25 Ti 0.25 Fe 0.5 )O 3 in a broad frequency (1 kHz-1 MHz) and temperature (25-450 °C) ranges have provided various important information on conduction mechanism, structure-properties correlationship, etc. The high dielectric constant and relativly low tangent loss with multiple phase transitions have been observed in the material I the experimental frequency and temperature ranges.