Temperature dependence of macroscopic and microscopic PZT properties studied by thermo-mechanical analysis,dielectric measurements and X-ray diffraction

The main objective of the paper is to point out the influence of composition and of poling state (poled and unpoled samples) on the evolution of various parameters (?_r, dilatation coefficients, lattice parameters) at different scales (lattice, bulk) using X-ray diffraction (XRD), thermo-mechanical analysis (TMA) and dielectric methods. The transition temperatures have been determined from the above measurements and compared. The composition has an influence on the transition temperature and not on the dilatation coefficient value. The poling state influences only the macroscopic dilatation evolution and not the value of the transition temperature. A scale effect is only observed on majoritary tetragonal compositions which show different values of transition temperatures from microscopic and macroscopic methods.     

Interactions between PVC and solvents by longitudinal nuclear relaxation times

PVC/aliphatic ketone systems were investigated by NMR relaxation times such as proton and carbon-13 spin-lattice relaxation time (T1) and proton spin-lattice relaxation time in the rotating frame (T1). The proton T1 results showed that these systems are homogeneous, while proton T1 indicates the existence of more than one domain.     

Local dynamics in epoxy coatings containing iron oxide nanoparticles by dielectric relaxation spectroscopy

Nanocomposites of photocurable epoxy resin and epoxy‐modified iron oxide magnetic nanoparticles were analyzed by dielectric relaxation spectroscopy to study the local dynamics at temperatures well below the glass‐transition temperature. Two secondary processes were detected, β and γ processes, but the second one was just detected at lower temperatures in the high‐frequency part of the spectra and moved out of the frequency range at higher temperatures. Data were fitted to the Havriliak–Negami and Arrhenius models, and the obtained parameters were analyzed. Relaxation times of the β secondary relaxation did not change with the nanoparticle content, but the relaxation strength increased. The increase could not be explained when we took into account the molecular origin of the relaxation. The presence of ferromagnetic nanoparticles enhanced the internal field and increased the relaxation strengths. Transmission electron microscopy images showed that the nanoparticles were well dispersed in the matrix, without magnetic agglomerates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Relation between dielectric property and desulphurization of coal by microwaves

Dielectric properties of various coals and aqueous NaOH solutions were determined to study thermochemical coal desulphurization by microwave irradiation. Aqueous NaOH solutions were used as media for irradiation. The coal samples were placed in a rectangular cavity and irradiated with 2450 MHz microwaves, resulting in 70–80% desulphurization efficiency.     

Thermal dynamics of side-chain copolymethacrylates as studied by the dielectric spectroscopy and relaxation of second-harmonic generation

A series of copolymethacrylates with different contents of tolane-based mesogenic groups have been synthesized. The mesogenic group content was characterized with ¹H-n.m.r.. The phase behaviours were determined using a differential scanning calorimeter and optical polarizing microscopy. A smectic A phase was obtained when the mesogenic group content was increased up to 80 mol.%. Dielectric relaxation results indicated that the amplitude of the -relaxation was suppressed significantly due to the presence of the liquid crystalline phase. The reduction of the molecular motion is beneficial to the enhancement of the temporal stability of the effective second-harmonic coefficient for the polymer with a higher mesogenic group content. Moreover, the second harmonic coefficient is enhanced as the mesogenic group content increases. The self-alignment nature of the liquid crystal phase is favourable for alignment of the NLO-active mesogenic group under an applied electric field and preserving such alignment after removal of the electric field. The relationship between thermal dynamic behaviour and second-order nonlinear optical properties is also discussed.     

Relation between low-frequency dielectric behavior of cellulosic materials and their inorganic contaminations

Results are given for dielectric constant and loss factor of some cellulosic materials of different fine structure free of and contaminated with water-soluble inorganic salts in the frequency range 0.1–100 kc/s and at temperatures 283.15–333.15 K. The analysis of the results reveals that the dielectric dispersion and loss peak found before with medicated cotton and viscose rayon, of ash contents of 0.3 and 1.4% respectively, disappear when these fibers have been “cleaned up” from inorganic impurities by acid washes. These anomalous dielectric behaviours reappear again with “cellulose-inorganic salts” admixtures. Based on these findings, it has been concluded that the secondary relaxation process observed in the low-frequency region 0.1–1 kc/s with cellulosic materials contaminated with inorganic impurities is not intrinsic but extrinsic in nature due to the displacement of ions adsorbed in the inner surfaces of such interstitial conduction system.     

Cofiring characteristics and dielectric properties for dielectric composites with macroscopic inhomogeneity and X7R characteristics

A multilayer dielectric composite with a compositional gradient and temperature-stable characteristic was successfully prepared through the conventional tape casting technique. The cofiring behaviors and dielectric properties of the composite were investigated. It was shown that processing parameters not only affected the physical integrity of the composites, but also altered their final dielectric properties. The processing sensitivity was attributed to the interfacial interdiffusion and mismatched sintering shrinkage. Microstructural observation and compositional analysis indicated that the sintered dielectric composite had macroscopic inhomogeneity and layer-like structure. By suitable structural and processing designs, the X7R EIA specification of the composite was obtained, together with low-sintering temperature and high dielectric constant.     

Correlation of dielectric and visco-elastic relaxation in polymer solutions

The interpretation of studies of dielectric relaxation in polymers is often restricted by ignorance of the mode of motion responsible for dipole orientation in an applied field. Relevant information can be drawn from studies of visco-elastic relaxation, since the visco-elastic relaxation time is comparable with the dielectric relaxation time when the latter requires some ‘whole molecule’ mode of motion involving simultaneous movement of many chain segments. The dielectric and visco-elastic relaxation times are expected to differ when the former involves a localised segmental motion. The dielectric relaxation process is shown to involve a localised segmental motion in acrylic polymers, but involves a ‘whole molecule’ rotation in low molecular weight poly(N-vinylcarbazole). Poly(p-substituted phenyl acetylenes) exhibit two dielectric relaxation processes. A low frequency relaxation comparable with the visco-elastic relaxation is ascribed to the backbone double bonds being polarised unidirectionally along the chain, and a high frequency process is ascribed to a localised segmental motion of the substituted phenyl moiety.     

Relationship between melt viscosity and dielectric relaxation time for a series of epoxide oligomers

Melt viscosity has been investigated for a series of bisphenol-A type epoxide oligomers with different weight-average mol wts (M?_w), ranging from 388 to 2640. The temperature dependence of the melt viscosity is described by the Williams–Landel–Ferry (WLF) equation. The melt viscosity η is correlated with both the direct current (dc) conductivity σ and the dielectric relaxation time τ. The two relationships between these three properties, σ·η^κ = const (0.63 ≦ κ ≦ 1.12) and η/τ^? = const (0.73 ≦ ? ≦ 1.06), are experimentally derived. Both exponents, κ and ?, depend on the M?_w of the oligomer. The lower M?_w oligomer has the larger value of κ. The κ value is close to unity for the low M?_w oligomer, which agrees with Walden's rule, σ·.η = const, applicable to most low mol wt liquids. The ? value is near unity for the epoxide oligomer with higher M?_w than 2000, which means that the melt viscosity is proportional to the dielectric relaxation time. The low M?_w oligomer (M?_w < 2000), on the other hand, has a smaller value of ? below unity. The result indicates that the melt viscosity is not proportional to the dielectric relaxation time for the low M?_w epoxide oligomer, whose dielectric α-relaxation is not governed by the Debye equation. © 1993 John Wiley & Sons, Inc.     

Universal dielectric relaxation induced giant dielectric permittivity in Mn-doped PbZrO3 ceramics

Manganese-doped lead zirconate ceramics were prepared by a conventional solid reaction method. Although the doping content varies from 5% (mole) to 20% (mole), significantly enhanced universal dielectric relaxation occurs only in 5% Mn-doped lead zirconate. The behavior cannot be properly explained by Maxwell-Wagner relaxation, which is commonly used for explaining giant dielectric permittivity in electroceramics. It is found that the enhanced giant dielectric permittivity is related to hetero-valence cation doping induced universal dielectric relaxation.     

Modification of calcium carbonate surface properties: macroscopic and microscopic investigations

Understanding the wettability of mineral powders and its modification by the addition of various surface active agents is crucial for many industrial applications. In many cases, wettability is investigated by macroscopic characterization techniques. In this framework, we decided to study non-porous calcium carbonate powders coated by known amounts of water-repellent molecules. A detailed characterization of the interface, focusing on the analysis of surface heterogeneity, was carried out using water vapor, nitrogen, and argon adsorption. It clearly reveals specific adsorption sites for water-repellent molecules. Wettability and immersion enthalpy measurements show that saturation of the carbonate surface by water-repellent molecules is not necessary for obtaining maximum hydrophobicity. It is reached for approximately one-third of surface saturation; at that point, some high-energy surface sites are still available for water and nitrogen adsorption. This suggests that wettability is not only linked to the availability of surface sites for water molecules.     

Correlation between microscopic structural parameters and macroscopic flow behaviour of some polymer-surfactant systems suitable for tertiary oil recovery

Some materials, suitable for application in polymer-surfactant flooding used in tertiary oil recovery (EOR), have been characterized by static and dynamic light scattering and studied in respect to their suitability in various model brines by measuring their rheological and dynamic-rheological behaviour. Xanthan and polyacrylamides of different degree of hydrolysis have been used as polymers, an ethylene glycol sulfonate as surfactant. Correlations have been established between microscopic structural parameters (Hydrodynamic radius, radius of gyration, molar mass, shape of the polymers) and the macroscopic behaviour, such as viscosity and elasticity, being important rheological parameters for EOR application. Additional investigations on surface tension and long term viscosity study lead then to a judgement for the efficiency and long term stability of polymer-surfactant slugs in various reservoirs of different salinity.     

Co-substitution tailored dielectric relaxation and electrical conduction in lanthanum orthoferrite

Effect of co-substitution on structural, dielectric and electrical conduction property of LaFeO₃ is reported. Partial co-substitution of La by Na and Fe by Mn has been observed to cause substantial modification in structural property including lattice distortion in LaFeO_3. Evidence of Jahn teller distortion has been noticed in Raman analysis due to presence of Mn³⁺ Jahn teller active ion at Fe lattice site. Raman analysis also indicated Fe–O–Fe bond weakening in LaFeO₃ on co-substitution. Dielectric response provides evidence of temperature dependent polydispersive relaxations contributed by combined effect of dipolar and space charge relaxations. Temperature response of dielectric parameter (ε_r and tan δ) provided indication of phase transition that is similar to found in ferroelectric materials. The dielectric relaxations are studied in the framework of complex impedance and electrical modulus spectrum. The electrical response comprises both grain and grain boundary contribution in the reported polycrystalline sample and it seems consistent with brick layer model of electrical equivalent circuit. It is evident from the activation energy estimation from modulus and conductivity plot that the small polarons hoping are the key factor in both dielectric ordering as well as electrical conduction mechanism in co-substituted LaFeO₃.     

Molecular dynamics and NMR spin relaxation in proteins

Molecular dynamics simulations often play a central role in the analysis of biomolecular NMR data. The focus here is on NMR spin-relaxation, which can provide unique insights into the time-dependence of conformational fluctuations, especially on picosecond to nanosecond time scales which can be directly probed by simulations. A great deal has been learned from such simulations about the general nature of such motions and their impact on NMR observables. In principle, relaxation measurements should also provide valuable benchmarks for judging the quantitative accuracy of simulations, but there are a variety of experimental and computational obstacles to making useful direct comparisons. It seems likely that simulations on time scales that are just now becoming generally feasible may provide important new information on internal motions, overall rotational diffusion, and the coupling between internal and rotational motion. Such information could provide a sound foundation for a new generation of detailed interpretation of NMR spin-relaxation results.     

Multiple dielectric relaxation processes in an unequilibrated bisphenol-A polycarbonate

Complex relative permittivity data within the ranges 0·1 Hz–3 MHz and −175°C to +200°C are presented for an unequilibrated compressionmoulded bisphenol-A polycarbonate of commercial origin (LEXAN 141). Comparisons are drawn with the nitrogen-equilibrated material in which eight distinct absorption regions have been identified. One low-frequency region common to both materials at low temperature is shown to be caused directly by a processing stabiliser. A second at high temperature is though to originate in part from an interfacial polarisation associated with a second phase of polycarbonate. An absorption observed only in the unequilibrated polycarbonate is attributed to bound water. Two intermediate-temperature losses are differentiated and delineated for the first time. Their origins and reason for diminution by annealing are examined. The multiplicity of overlapping absorptions, including a low-temperature loss region which is almost continuous at higher frequencies, is consistent with the high impact strength of polycarbonate over an extended temperature range.     

Complex permittivities and dielectric relaxation of granular activated carbons at microwave frequencies between 0.2 and 26 GHz

Carbonaceous materials are amenable to microwave heating to varying degrees. The primary indicator of susceptibility is the complex permittivity (?*), of which the real component correlates with polarization and the imaginary term represents dielectric loss. For a given material, the complex permittivity is dependent upon both frequency and temperature. Here we report the complex permittivities of three activated carbons of diverse origin over the frequency range from 0.2 to 26 GHz. Dielectric polarization-relaxation phenomena for these materials are also characterized. Measurements were made using a coaxial dielectric probe and vector network analyzer based system across the temperature region between 22 and 190 °C.     

Role of coupling microscopic and macroscopic phenomena during the crystallization of semicrystalline polymers

In this work a microscopic cell model of crystallization incorporating solute rejection is coupled with the macroscopic heat transfer using a finite element procedure to study the evolution of crystallinity. The role of coupling is analyzed using two contrasting polymers, polyethylene and polystyrene. Simulations of the solidification were carried out using both the coupled and the uncoupled formulations to elucidate the effect of coupling. Both polymeric systems considered were affected by this coupling on the evolution of the crystallinity; however, the initial solute content has more effect on the slower crystallizing system. An incremental stress model is also applied to predict the residual stresses generated during the crystallization of the two polymeric systems. It was found that the predicted residual stresses are significantly affected by coupling. Good agreement with experimental results was observed for crystallization half‐time results from the coupled solution by incorporating solute segregation effects for high undercooling, but the deviation was more significant at lower undercooling. For the slow‐crystallizing polymer, fair agreement was observed irrespective of the degree of undercooling. The results show that the solute segregation has only a secondary influence on these systems, especially compared to the effect of undercooling.     

High-pressure studies of dielectric relaxation in ferroelectric pyridinium tetrafluoroborate crystals

Dielectric relaxation of ferroelectric pyridinium tetrafluoroborate single crystal was measured as a function of hydrostatic pressure and temperature. High-pressure caused a shift the ferroelectric-paraelectric phase transition at T₁=238.7 K as well as the non-ferrolectric phase transition at T₂=204 K towards higher temperatures. The influence of pressure on the critical slowing down process at the ferroelectric-paraelectric phase transition was analyzed. Moreover, in the low temperature phase (below 204 K) a relaxation process, characterized by an enthalpy of 13.2 kJ/mole, was observed. The activation enthalpy of the process was found to increase linearly with pressure.     

Anomalous dielectric relaxation in poly(dimethyl siloxane) polymers

The dielectric relaxation in poly(dimethylsiloxane) of viscosity 10⁵ CSt at around the melting point is found to deviate from the existing theory. Although the position of maximum loss is insensitive to changes in temperature in the range ?40°C to ?54°C, the magnitude of the loss changes by about 50%.