Low-frequency Dielectric Loss Research Articles

Evidence for helical structures in poly(l-olefin sulfones) by TEM

Advances in the techniques of TEM, including the preparation of ultrathin vertically shadowed platinum-carbon (Pt-C) replicas, have allowed the direct recognition of single-structural features, such as DNA , pectin helices or polysiloxane polymers. Here we report some observations on the chain conformations of serval poly (olefin sulfones). We find strong evidence for the presence of some helical conformations, which had been inferred for over a decade but could not so far be established by other methods.Unusual conformational behavior of poly(olefin sulfones) was indicated over twenty-five years ago. Dilute solutions of several poly (1-olefin sulfones), [-CH2-CHR-S02-]X, in non-polar solvents exhibited unexpectedly strong low-frequency dielectric dispersion, with relaxation times typical of rigid tumbling or of the terminal normal mode. In contrast, polysulfones made from olefins with internal double bonds show no low-frequency dielectric loss. To explain these facts, we refer to the crucial experiments of Fawcett and Fee (1982), who prepared terpolymers of 1-hexene,

Ion exchange of Rb, Ba, and Sr in KTiOPO4

The potassium ions of potassium titanyl phosphate (KTiOPO4, or KTP) are relatively mobile and may be exchanged with other ions from molten salts. Distribution coefficients for potassium, rubidium, barium, and strontium between nitrate melts and titanyl phosphate were measured between 350 and 425 °C. At 425 °C, using melts containing K and a dopant chosen from Rb, Ba, Sr, and with dopant cation mole fractions in the melt of 0.1, Rb is incorporated in the crystal at a site fraction of 0.014, Ba at 0.007, and Sr at 0.002. Introducing 0.008 fraction of Ba increases the low-frequency dielectric constant and loss of KTP several orders of magnitude, giving conductivities (ωε0ε″) of 4×10−5 Ω−1 cm−1 at 200 kHz. The results support a potassium vacancy mechanism for ionic conductivity in KTP. Using low concentrations of divalent ions in the melt, flux-grown KTP has a lower uptake of divalent ions than does hydrothermally grown KTP, an effect attributed to the presence of potassium vacancies created in the high-temperature flux growth process. Ba or Sr incorporation results in a contraction of the lattice constants of the KTP structure.

Electrical properties of ceramic/polymer composites

Three types of ceramic/polymer composites were prepared and their electrical, dielectric and pyroelectric properties analyzed. The composite PZT5/VDF-TrFE shows a significantly higher pyroelectric figure of merit than PZT (VDF=vinylidine fluoride, TrFE=trifluoroethylene). The dielectric loss of the ceramic/polymer composites is observed to be dominated by those of the polymer, whereas the ceramic phase has a significant contribution on the steady-state electrical conduction and low-frequency dielectric loss at high temperatures. >

Hydrogen and carbon in solid solution in oxides and silicates

The dissolution of H2O and CO2 in structurally dense, nominally anhydrous and non-carbonate oxide matrices such as MgO and CaO is reviewed. H2O and CO2 are treated as gaseous oxide components which enter into solid solution with the refractory oxide hosts. They form anion complexes associated with cation vacancy sites. Evidence is presented that OH− pairs which derive from the dissolution of H2O are subject to a charge transfer (CT) conversion into peroxy moieties and molecular hydrogen, O 2 2− ... H2. Because the O 2 2− moiety is small (O−-O− distance ≈ 1.5 A) high pressure probably favors the CT conversion. Mass spectroscopic studies show that molecular H2 may be lost from the solid which retains excess oxygen in the form of O 2 2− , leading to the release of atomic O. The dissociation of O 2 2− moieties into a vacancy-bound O− state and an unbound O− state can be followed by measuring the internal redox reactions involving transition metal impurities, the transient paramagnetism of the O− and their effect on the d.c. conductivity. Evidence is presented that CO2 molecules dissolve dissociatively in the structurally dense oxide matrix, as if they were first to dissociate into CO+O and then to form separate solute moieties CO 2 2− and O 2 2− , both associated with cation vacancy sites. In the CO 2 2− moiety (C-O− distance 1.2–1.3 A, OCO angle ≈ 130°) the C atom probably sits off center. The transition of the C atom into interstitial sites is accompanied by dissociation of the CO 2 2− moiety into CO− and O−. This transition can be followed by infrared spectroscopy, using OH− as local probes. Further support derives from magnetic susceptibility, thermal expansion, low frequency dielectric loss and low temperature deformation measurements. The recently observed emission of O and Mg atoms besides a variety of molecules such as CO, CO2, CH4, HCN and other hydrocarbons during impact fracture of MgO single crystals is presented and discussed in the light of the other experimental data.

Explanation for the universal low-temperature and dynamical properties of a particular glass.

We present new measurements of the time-dependent specific heat of cubic (KBr${)}_{0.5}$(KCN${)}_{0.5}$. Using a microscopic model of the tunneling centers (180\ifmmode^\circ\else\textdegree\fi{} ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ reorientations) we provide a unified explanation of this specific heat and the low-frequency dielectric loss. Thus, in a crystal exhibiting the universal low-temperature thermal properties and the broad low-frequency dielectric relaxations characteristic of glasses, we have explained both using a microscopic tunneling model.

NMR relaxation study of the H-bonded glassRb1−x(NH4)xH2PO4

The existence of a minimum in the temperature dependence of the $^{87}\mathrm{Rb}$ spin-lattice relaxation time ${T}_{1}$ in ${\mathrm{Rb}}_{1\ensuremath{-}x}{(\mathrm{N}{\mathrm{H}}_{4})}_{x}{\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}$ with $x=0.35$ indicates a tremendous progressive slowing down of the proton intrabond (O-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}O) jump motion from ~ ${10}^{\ensuremath{-}11}$ to ~ ${10}^{\ensuremath{-}7}$ sec not found in ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ or other members of the ${\mathrm{KH}}_{2}$${\mathrm{PO}}_{4}$ family exhibiting ferro- or antiferroelectric transitions. The occurrence of a linewidth broadening together with the ${T}_{1}$ minimum exhibits a wide distribution of correlation times that develops near the onset of the low-frequency dielectric dispersion and loss characteristic of the glass state. The results can be described in terms of a cluster freeze-out model originally developed for spin-glasses. The ${\mathrm{NH}}_{4}$ reorientations freeze out much before the low-temperature glass state has been reached.

Low frequency dielectric loss measurements on Al 2O 3 films

Measurements of the dielectric loss in thin Al 2O 3 films at low and very low frequencies are reported. A very simple but efficient experimental technique for measuring the loss tangent was used in the study. A Debye-type relaxation peak was observed and this is discussed.

A study of absorption currents in polystyrene and polystyrene-chloranil complex

The absorption currents in polystyrene (PS) and its donor-acceptor complex with chloranil (PS-CA) have been analysed in the temperature range 323–413 K. Low frequency (10-s-10-3 Hz) dielectric loss has been computed utilizing Haman's Fourier conversion of the absorption currents. A relaxation peak has been observed in the isochronal current and dielectric loss thermograms and identified with the a relaxation process of these polymers. The activation energy of the relaxation process has been evaluated from the Arrhenius shift of the relaxation peak. The effects of complex formation on absorption currents and the relaxation process have been discussed.

Dielectric properties of phase separated polymer solids: 1. Styrene-butadiene-styrene triblock copolymers

Theories currently applicable to the description of the low frequency dielectric loss, associated with trapping of charge carriers at interphase boundaries in polymers, are presented. Experimental observations on styrene-butadiene-styrene triblock copolymers are compared with theory. The interfacial polarization process in a film-cast copolymer with 30 wt % styrene was found to be in reasonable agreement with predictions for a morphology of parallel cylinders oriented at random, whereas that for a 50 wt % styrene copolymer was compatible with prediction for a random arrangement of partly parallel lamellae. These structures correspond to morphologies observed by electron microscopic observation. No evidence was found for a dielectric effect specifically requiring a diffuse conducting interfacial zone between the polystyrene and polybutadiene domains. The bulk d.c. conductivity also correlated with predictions made using the same parameters as those involved in calculation of the relaxation effects.

A digital approach to low-frequency dielectric loss measurement

A small, on-line, calculator-based measurement system is described for determining capacitance C, conductance G and dielectric loss, tan delta , in the frequency range 0.4 Hz to 2.0 kHz. The voltage across the sample under test and the current through it are multiplexed into a transient-recorder memory and subsequently analysed using least-squares curve fitting to yield the amplitudes and phases of these waveforms. Amplitude accuracy is +or-0.5% and the maximum error in phase angle difference between current and voltage is +or-0.1 degrees . The system allows large-amplitude AC voltages ( approximately 100 V peak-to-peak) to be applied to a sample under study, resulting in increased sensitivity at the lower frequencies. The current is recorded using an operational amplifier connected as a current-to-voltage converter with virtual ground input.

The Temperature Dependence of the Dielectric and Conductivity Anisotropies of Several Liquid Crystalline Materials

Abstract The temperature dependence of the conductivity and dielectric anisotropies are reported for several liquid crystalline materials of varying structure. The types of compounds studied are Schiff bases, p-phenyl benzoates, and phenyl-p-benzoyloxy benzoates, having either carbonate or alkyl terminal substituents. The anisotropies are computed from the bulk conductance and capacitance measured both parallel and perpendicular to an external magnetic field. The sign and magnitude of the anistropies are sensitive to changes in molecular structure as well as to phase transitions among the mesophases. Changes in sign of the conductivity anisotropy, dielectric anisotropy, or both, were found to occur with changing temperature in the alkoxy-carbonate Schiff bases, whereas no such anomaly was observed for the carbonate-alkoxy homologs. In such anisotropy sign reversals, there occurs a depression in ∊‖ which is frequency-independent from 10–105 Hz. A sign reversal in the dielectric anisotropy also occurs in the phenyl-p-benzoyloxy benzonates, because of an unusually low frequency dielectric loss in ‖. The loss appears to occur as a single relaxation process with a characteristic frequency of 10 kHz at about 50°C. The dielectric behavior of the p-phenyl benzoates shows a nearly constant anisotropy throughout their nematic range.

SPIN–LATTICE INTERACTION EXPERIMENTS ON COLOR CENTERS IN QUARTZ

The paramagnetism of the color centers in smoky quartz is caused by the electron which is missing from a nonbonding oxygen orbital near an aluminum impurity. The effects produced by externally introduced microwave phonons on the spin-resonance signal from these color centers have been studied in detail as a function of the relative phonon and spin-resonance frequencies, the phonon power, the spectrometer power, the magnetic field angle, and the phonon polarization. As has been noted previously, these interactions do not conform with those observed in other paramagnetic crystals. Effects of an applied electric field on the resonance lines and various further experiments on the color centers in smoky quartz are reported; two in particular suggest that the "hole", the missing electron, makes thermally excited transitions between two different sites adjacent to a given impurity atom. One of the experiments is a measurement of the cross relaxation which takes place between the various lines of the spectrum, while the other is a measurement of the low-frequency dielectric loss found at liquid helium temperatures. An attempt is made to discuss the spin-phonon results in terms of such transitions for the holes.

Electrical Conductivity of Certain Crystalline Materials after Grinding

IT was recently reported1 that quinol shows a large increase in low-frequency dielectric loss on grinding. The effect is temporary and decreases with time. It has now been found that this phenomenon is more general and occurs with many inorganic salts and some crystalline organic compounds.