Molecular motion and phase transitions of clathrate hydrates**

Abstract

Heat capacities and complex dielectric permittivities of three clathrate hydrates of type II, encaging tetrahydrofuran (THF), acetone (Ac), and trimethylene oxide (TMO), were measured at low temperatures. The heat capacity measurement was done in the temperature range 13–300 K by using an adiabatic calorimeter with a built-in cryorefrigerator. The permittivities were measured in the temperature range 20–260 K and in the frequency range 20 Hz-1 MHz. For pure samples, with a glass transition due to freezing out of water, reorientational motion of the host lattice was observed calorimetrically at 85 K for THF and at 90 K for Ac hydrates, respectively. Spontaneous temperature drift rates of the calorimetric cell were measured under adiabatic conditions to derive the characteristic time for enthalpy relaxation. The enthalpy relaxation times thus derived were well correlated in an Arrhenius plot with the dielectric relaxation times derived from the dielectric relaxation of orientation polarization. The situation is the same as hexagonal ice which has a similar four co-ordinated hydrogen-bonded network.     

Collective and molecular dynamics in ferroelectric liquid crystals: from low molar to polymeric and elastomeric systems

Broadband dielectric spectroscopy delivers in the frequency range from 10 Hz to 10¹⁰ Hz two collective dielectric loss processes (soft and Goldstone modes) and one molecular relaxation (β-relaxation). The soft mode and Goldstone mode are assigned to the fluctuation of the amplitude and the phase of the helical superstructure. The β-relaxation corresponds to the libration (hindered rotation) of the mesogene around its long molecular axis. At the SmA–SmC^* phase transition this process does not split or broaden, and the temperature dependence of its relaxation rate does not show any deviation from an Arrhenius-like behavior. Its dielectric strength does not decline at the SmA–SmC^* phase transition. These experimental findings are in contrast to the common explanation of the origin of the saturation polarization (“induced spontaneous polarization”), which is based on the existence of a “free” rotation inside the SmA phase and its strong hindrance in the ferroelectric SmC^* phase. Furthermore, the high frequency results require a reformulation for the (generalized) Landau theory as applied to the SmA–SmC^* phase transition. In comparing low molar mass and polymeric (elastomeric) FLC, the collective and molecular dynamics are qualitatively similar, independent of the molecular architecture (e.g. side-chain, combined main- and side-chain or crosslinked systems).     

Electrical properties of carbon black in an SBR–wax matrix

The resistivity and dielectric constant for carbon black (5–12 vol-%) in a nonconducting matrix have been measured over the range 20–90°C and dc to 2–10⁵ Hz. Styrene–butadiene rubber dissolved in tetracosane (22 parts SBR to about 70 parts wax) constituted the matrix. Wide variations in resistivity and dielectric constant with temperature and/or frequency were observed. A theory is presented to explain the experimental observations. It correctly predicts the qualitative features of the experimental results but its predictions are not quantitatively accurate.     

Dielectric relaxation study of gamma irradiated oriented low-density polyethylene

The influence of drawing, gamma irradiation and accelerated aging on the dielectric relaxation of low-density polyethylene has been studied using dielectric loss tangent measurements in the temperature range from 25 to 325 K and in the frequency range from 10³ to 10⁶ Hz. The intensity, position and activation energy of the γ- and β-dielectric relaxations were found to be strongly dependent upon the changes in the microstructure of the amorphous phase induced by uniaxial orientation, oxidation and crosslinking.     

Colossal dielectric constant in PrFeO3 semiconductor ceramics

The perovskite PrFeO₃ ceramics were synthesized via sol–gel method. The dielectric properties and impedance spectroscopy (IS) of these ceramics were studied in the frequency range from 100 Hz to 1000 kHz in the temperature range from 80 K to 300 K. These materials exhibited colossal dielectric constant value of ∼10⁴ at room temperature. The response is similar to that observed for relaxorferroelectrics. IS data analysis indicates the ceramics to be electrically heterogeneous semiconductor consisting of semiconducting grains with dielectric constant 30 and more resistive grain boundaries with effective dielectric constant ∼10⁴. We conclude, therefore that grain boundary effect is the primary source for the high effective permittivity in PrFeO₃ ceramics.     

Ferroelectric polarization switching in nylon-11

Cold-drawn Nylon-11 films, which were prepared by stretching the melt-quenched films to a draw ratio of 2.8: 1 at room temperature, were found to exhibit an electric displacement versus electric field hysteresis loop. The results confirmed that the Nylon-11 film exhibited ferroelectric behavior at or below room temperature. The coercive fields were 65, 98, 125, 160, and 215 MV/m at 20, 0, ?20, ?40, and ?60°C and the remanent polarization at ?20°C (where there was considerably less dc conduction) was 56 mC/m². Switching of the polarization was almost completed within 20 ms.     

Combustion synthesis and thermal expansion of RE6UO12 (RE = Dy and Tb)

Citrate–nitrate combustion method was adopted for the synthesis of RE₆UO₁₂ (RE = Dy and Tb). These compounds were characterized by X-ray diffraction. Thermal expansion coefficient of these compounds were measured in the temperature range of 298–1,273 K by high temperature X-ray powder diffractometry (HT-XRD) and compared with other rare earth compounds reported in the literature. There was no observed phase transition in Dy₆UO₁₂, but Tb₆UO₁₂ showed a second-order phase transition at 670 K which was confirmed using differential scanning calorimeter. The average volume thermal expansion coefficient of Dy₆UO₁₂ in the temperature range of 298–1,273 K is (29.82 ± 4.02) × 10^?6 and that of Tb₆UO₁₂ in the temperature range of 298–673 K is (13.76 ± 2.64) × 10^?6 K^?1.     

Dielectric properties of moist cellulose

The dielectric constant ?′ and the dielectric loss ?″ for cellulose fiber were measured over a frequency band 0.2 to 10 Mc/sec and a temperature range from ?20 to 80°C. Also, the variation of the dielectric behavior with relative humidity was measured at 25°C. From these data, both the specific resistivity R_s and the dissociation energy U₀ were calculated. The results showed that the dielectric constant increased with frequency and temperature. This may be due to the increase in the rotation and the polarization of the flexible part in the fiber. The variation of the dielectric loss with temperature showed a maximum absorption corresponding to the β-relaxation. For the moist fiber, it is found that as the relative humidity increases, the dielectric constant and the dielectric loss increase. This increase may be due to the presence of polar water molecules, to the freeing of the polar groups, and to the freeing of the ions in the fiber molecule as well as to the increase in the number of OH^? and H⁺ ions resulting from the ionization of water. A relation between the dielectric constant and resistivity at different humidities is represented graphically. From this relation, it is found that the dissociation energy is equal to 0.318 × 10^?12 and 5.46 × 10^?12 erg below and above 52% RH, respectively.     

Hyperelectronic polarization in macromolecular solids

Hyperelectronic polarization may be viewed as the electrical polarization in external fields due to the pliant interaction with the charge pairs of excitons, in which the charges are molecularly separated and range over molecularly limited domains. It is particularly pronounced in molecular solids composed of long polymeric molecules having extensive regions of electronic orbital delocalization. Hyperelectronic polarization is regarded as the principal contributor to the high polarizabilities of the following five macromolecular solids: four polyacene quinone radical polymers formed by condensation of aromatic hydrocarbon derivatives with aromatic acids and poly(Cu(II)-N,N′- dimethyl rebeanate). The first four polymers at 100 hz had dielectric constants of 1800–2400, decreasing to about 58–100 at 100,000 hz, with relaxation times of 10^?3 to 10^?4 sec. A sixth polymer, a Schiff-base condensate of 1,4-napthaquinone and p-toluene diisocyanate, however, showed little hyperelectronic polarization, displaying a dielectric constant of 10, constant over 100–100,000 hz. By simple arguments it is shown that the relaxation times and the frequency response of conduction are consonant with the proposed model of charges roaming over long molecular domains (up to 4000 A.) but restricted by molecular boundaries.     

Electrochemical impedance spectroscopic study of perfluorovinyl ether copolymer with tetrafluoroethylene in the swollen state

The temperature and frequency dependences of the dielectric impedance of the perfluorovinyl ether copolymer with tetrafluoroethylene have been studied. The impedance spectra of the films swollen in water have been measured in the frequency range of 10^?1–10⁷ Hz at 213–278 K. The model of equivalent electric chains has been suggested for analysis of the frequency dependence of dielectric impedance. The parameters of this model have been calculated. The correlation of these parameters with effects in a polymer matrix is described. Specifically, the pure ohmic conductivity is determined for the copolymer under study.     

Preparation and dielectric properties of highly preferred-(100) orientation (Pb,La)(Zr,Ti)O<Subscript>3</Subscript> antiferroelectric thick films by sol–gel processing

Pb_0.97La_0.02Zr_0.95Ti_0.05O₃ (PLZT) antiferroelectric thick films of highly preferred-(100) orientation with different thickness were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates depending on the sol–gel process technique. The effects of the PLZT thick films in the preparation and electric properties are investigated. The films show polycrystalline perovskite structure with a (100) preferred orientation by X-ray diffractometer analyses. The antiferroelectric nature of the thick films is demonstrated by P (polarization)–E (electric field). The temperature dependence of the dielectric constant and dielectric loss displays the similar behavior in both cases at 100 kHz while the values of polarization characteristic are decreased with the increase of the film thickness. The phase switching current are studied as a function of a gradually change dc electric field and the voltage dependent current density of the most highly (100)-oriented PLZT film is 1.49 × 10⁻⁸ A/cm² over electric field range from 0 to ±261 kV/cm. The film at 2,498 nm exhibits excellent dielectric properties and highly preferred-(100) orientation.     

The dielectric properties of dry and water-saturated nylon-12

The dielectric relaxation of 30% crystalline Nylon-12 of molecular weight 20,000 has been studied at frequencies from 12 Hz to 0.1 MHz and temperatures from 77 to 375 K, and the effect of water on the relaxation spectra has been investigated. Absorbed water increases both the rate and the intensity of both its α-and β-relaxation processes and increases the rate but decreases the intensity of its γ-relaxation process. These results are interpreted in terms of the hydrogen bonding effects of water on localized motions of dipoles in Nylon-12. The relatively large half-width of the α-relaxation, which becomes better resolved at high temperatures, is attributed mainly to the random distribution of crystalline regions in the polymer. It is suggested that water lowers the steric hindrance for the localized mode of dipolar reorientation and causes a redistribution of local sites such that the β-relaxation process grows at the expense of the γ-process.     

Glass transition in polyacrylonitrile: Analysis of dielectric relaxation data

Dielectric relaxation of polyacrylonitrile (PAN) samples, in which the presence of an amorphous phase is evidenced by x-ray diffraction, has been studied over the temperature range 30–150°C and frequency range 10²–10⁵ Hz. These data as well as those reported by other authors, reveal several points useful to the understanding of the dielectric relaxations of PAN in relation to its structure. A glass transition in PAN is evidenced by at least two of the four data sets investigated; the third shows combined effect of two relaxations, whereas the fourth clearly shows a relaxation process different from the glass transition. The glass-transition behavior of the dielectric relaxation data is confirmed by Williams-Landel-Ferry theory and a recent theory of Phillips, both of which lead to consistent conclusions.     

AC Electrical Properties of Plasma Polymerized <Emphasis Type="Italic">o</Emphasis>-Methoxyaniline Thin Films

Alternate current (ac) electrical properties of the plasma polymerized o-methoxyaniline (PPOMA) thin films synthesized in the glow discharge plasma using a capacitively coupled reactor are studied. Measurement revealed that the ac electrical conductivity varies with frequency ω as ω ⁿ , where the exponent n is less than unity in the range 0.1 to 2.0 kHz, indicating the Debye type conduction mechanism in the PPOMA thin films, while above this frequency range the exponent is become greater than unity indicating non-Debye type conduction. At low frequencies the conduction is considered to be due to hopping of carriers between the localized states. The PPOMA thin films of thicknesses 100?250 nm possesses dielectric constant <10, which remains static in the range 0.1?10 kHz, and decreases at higher temperature due to the orientation polarization. The dielectric loss increases with the increase in frequency having a peak around or above 10 kHz for all the PPOMA films of different thicknesses. Cole-Cole plot between the real and imaginary dielectric constant exhibits single relaxation mechanism in the PPOMA thin films.     

Synthesis,crystal structure and properties of a quaternary oxide with a new structure type,BiGaTi4O11

A new quaternary oxide, BiGaTi₄O₁₁ (bismuth gallium tetratitanium undecaoxide), was prepared by heating a mixture of the binary oxides at 1373 K in air. BiGaTi₄O₁₁ melts at 1487 K and prismatic single crystals were obtained from a sample melted at 1523 K and solidified by furnace cooling. The structure of BiGaTi₄O₁₁ was analyzed using single‐crystal X‐ray diffraction to be of a new type that crystallized in the space group Cmcm. A Bi³⁺ site is coordinated by nine O^2? anions, and three oxygen‐coordinated octahedral sites are statistically occupied by Ga³⁺ and Ti⁴⁺ cations. A relative dielectric constant of 46 with a temperature coefficient of 57 ppm K^?1 in the temperature range 297–448 K was measured for a polycrystalline ceramic sample at 150 Hz–1 MHz with a dielectric loss tan δ of less than 0.01. Electrical resistivities measured at 1073 K by alternating‐current impedance spectroscopic and direct‐current methods were 1.16 × 10^?4 and 1.14 × 10^?4 S cm^?1, respectively, which indicates that electrons and/or holes were conduction carriers at high temperature. The optical band gap estimated by the results of diffuse reflectance analysis was 2.9–3.0 eV, while the band gap obtained from the activation energy for electrical conduction was 3.5 eV.     

Dielectric studies of photodegradation and photooxidation of polystyrene

The effects of vacuum photolysis at 254 nm and the short-(λ = 254 nm) and long-wave (λ > 300 nm) photooxidations on the dielectric constants (?′) and dielectric losses (?″) of polystyrene have been investigated at 25 ± 1°C. Dielectric constants generally increase on photodegradation, but more pronounced increases occur in the low-frequency region on vacuum photolysis. It is suggested that such increases are associated with Maxwell-Wagner-Sillars polarization. Increases in dielectric losses are observed in three main frequency regions: around 10² Hz, 10⁴ Hz, and at 3 × 10⁶ Hz. The lowest frequency loss which occurs in both the vacuum-irradiated and in the photooxidized samples is attributed to a combination of the effects of interfacial polarization and of the increased direct-current conductivity of the polymer which occurs as a result of these reactions. The 10⁴ Hz dispersion associated only with photooxidation is related to the presence of small, volatile, polar oxidation products, like ketones. The loss peak overlaps the intrinsic γ-relaxation of polystyrene, and it appears that the motions of small molecules, or of relaxing dipoles in these, are coupled to the phenyl group rotational vibrations. The high-frequency losses (3 × 10⁶ Hz) are ascribed to orientation polarization of carbonyl dipoles attached to chain ends, these compounds being produced as a result of hydroperoxide decompositions. A good correlation between the carbonyl dipole relaxation strength and carbonyl concentration is observed. It would appear that relaxation strength measurements could provide quantitative kinetic information for polystyrene oxidation.     

Dielectric spectroscopy study of cyanate ester oligomer curing kinetics

The evolution of dielectric properties during the cyanate ester oligomer curing process was studied in the electric field frequencies range of 10^–2—10⁵ Hz. The kinetic of the curing process was investigated using the acquired data for complex dielectric permittivity and conductivity. It is demonstrated that, along with the formation of the polymer network, a microphase separation of the reaction intermediate, namely carbamate, takes place during the trimerization reaction. It is manifested in the frequency spectra as the Maxwell—Wagner polarization.     

Li7PS6 and Li6PS5X (X: Cl,Br, I): Possible Three‐dimensional Diffusion Pathways for Lithium Ions and Temperature Dependence of the Ionic Conductivity by Impedance Measurements

Possible three‐dimensional diffusion pathways of lithium ions in crystalline lithium argyrodites are discussed based on earlier studies of local dynamics and site preferences. The specific Li‐ionic conductivities of the lithium argyrodites Li₇PS₆ and Li₆PS₅X (X: Cl, Br, I) and their temperature dependences are measured by impedance spectroscopy using different electron‐blocking and ion‐blocking electrode systems. Measurements were carried out between 160 K and 550 K depending on the respective sample. Bulk and grain boundary contributions and the influence of sample preparation are discussed. Typical values for the ionic conductivities at room temperature are in the range 10^–7 to 10^–5 S ·  cm^–1 and at 500 K between 10^–6 and 10^–3 S ·  cm^–1. Thermal activation energies are in the range 0.16 to 0.56 eV. The electronic conductivity at room temperature was measured by polarization measurements for the samples Li₆PS₅X (X: Cl, Br) and was shown to be in the order of magnitude of 10^–8 S ·  cm^–1. Chemical diffusion coefficients of lithium were calculated based on the polarization measurements. For Li₆PS₅Br a high value of 3.5 × 10^–6 cm² · s^–1 was found.     

Dielectric properties of polyester resins—1

The dielectric properties of unsaturated polyester resins mixed with styrene have been investigated in the microwave 1–12 GHz range. The frequency dependence of the permittivity (a_r^′anda_r^′′) is typical of an orientational polarization relaxation process. A Davidson-Cole like model fits the experimental results reasonably well. A hypothesis on the molecular process is discussed, correlating the influence of temperature and sample composition.     

Calculating the polarizability and dielectric permittivity of liquid helium

The molar polarization and dielectric permittivity of liquid helium ⁴He and the polarizability of a helium atom from 0 K to the precritical range are calculated.