Electrical conductivity enhancement in an eutectic system containing dispersed second phase particles

The electrical conductivities of the eutectic composition 35.2 m/o LiNO₃-39.8 m/o KNO₃-25.0 m/o CsNO₃ with various amounts of γ-Al₂O₃ have been measured. A conductivity enhancement of three orders of magnitude has been observed at 92°C. The maximum of conductivity appears for about 35 m/o γ-Al₂O₃. The results are discussed within the percolation theory approach. The pellets of the eutectic mixture with suitable amounts of γ-Al₂O₃ were found to keep their shape during a 24 h thermal treatment at 150°C, far above the melting point of the matrix phase, under spring loaded condition. The behavior of this so-called semi-solid state has been investigated.     

Frequency dispersion of effective conductivity of high-temperature superconductors in the normal phase

The paper is devoted to the presently widely discussed question of the electrodynamic properties of high-temperture superconducting ceramics in the normal phase. The effective-dielectric-constant method is used to take into account the influence of the soft phonon modes connected with the structural phase transition accompanying the superconducting transition. The presence of soft phonons gives grounds for assuming that there exists a frequency region in which the contributions of the electron and phonon subsystems can be comparable. The frequency dependences of the effective conductivity and of the dielectric constant are investigated. Estimates show that an anomalous decrease of the effective conductivity should be observed, as was recently done in a number of experiments, in the microwave region. The influence of the dynamic character of the soft modes is also discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 39–42, December, 1989.     

Electrical conductivity of polycrystalline alkali nitrates in stable states and at phase transitions

The temperature dependence of the electrical conductivity of polycrystalline alkali nitrates between room temperature and their melting temperatures has been studied. The lg =f(1/T) dependence is a discontinuous line consisting of regions of impurity and intrinsic conductivity. The conductivity changes discontinuously at a first-order phase transition, during the transition from a structure with lower symmetry to one with higher symmetry, and vice versa. The activation energy for ionic migration in the series of alkali nitrates is at a minimum for potassium nitrate.     

Ionic conductivity of alkali metal chloroaluminates

The electrical conductivity of three alkali metal chlorluminates has been investigated from room temperature to above the melting point. Ionic conductivities at 25°C are 1.2 × 10^?6, 3.5 × 10^?7 and 3.2 × 10^?9omh^?1 cm^?1, and activation enthalpies are 0.47, 0.46 and 0.53 eV for LiAlCl₄, NaAlCl₄ and KAlCl₄.     

Space-time dispersion of graphene conductivity

We present an analytic calculation of the conductivity of pure graphene as a function of frequency ω, wave-vector k, and temperature for the range where the energies related to all these parameters are small in comparison with the band parameter γ≃3 eV, but much larger than the collision rate τ^-1. The simple asymptotic expressions are given in various limiting cases. For instance, the conductivity for kv₀≪ T≪ω is equal to σ(ω,k)=e²/4ħ and independent of the band structure parameters γ and v₀. Our results are also used to explain the known dependence of the graphite conductivity on temperature and pressure.     

Interfacial enhancement of flexible PVC-silica composites by silane coupling agents

Enhancement of a flexible poly(vinyl chloride) (PVC)-silica composite interface was studied by the application of γ-aminopropyltrimethoxysilane on silica. Composites containing silica and silanized silica up to 25.6 phr (per hundred resin) and prepared by sol-gel technology were subjected to water and water vapor attacks similar to that in their daily use. Silane application resulted in diminishing liquid water and water vapor sorption by about 24.0% and 11.9%, respectively. Equilibrium weight gain values of the composites having different amounts of silica correlated well with a peak at 3400 cm^-1 in the IR spectra which was attributed to the stretching vibration of the O-H group of water. Liquid water and water vapor diffusivities in composites determined by the evaluation of weight gain against time data were about 0.4 x 10^-13 and 0.4 x 10^-12 m² s^-1, respectively. Inhibition of dioctyl phthalate (DOP) migration from composites by silane application was also determined as 24% using UV measurements. The most impressive merit of silane enhancement was observed as the retention of ultimate tensile strength (UTS) under wet conditions. While the untreated silica composite reduced its UTS by about 21.2%, silanized silica composite reduced its UTS by only about 13.6%, on wetting.     

Silver ionic conductivity enhancement by network former mixed in oxide-based glasses

Glasses in the binary system xAg₂P₂O₆ − (1 − x)Ag₂Te₂O₅ have been prepared for 0 x 1. For each composition only one glass transition temperature is observed in the temperature range of 180–220 °C. All glasses appear homogeneous considering their optical and electrical properties. Nevertheless, in SEM observations, some glass compositions appear to be heterogeneous after decoration following short nitric acid etching. For each composition, conductivity data obtained in the temperature range of 25–200 °C using impedance techniques obey an Arrhenius relationship with a composition independent pre-exponential term. Variation of the conductivity activation energy with x induces correlative variations of isothermal conductivity curves leading to an increase of the ionic conductivity of about one order of magnitude compared with linearity at 25 °C. This behaviour is discussed with respect to the thermodynamic properties of the glassy solutions.     

Measurement of electrical conductivity of low-temperature alkali plasma by weak probing signals

A method for measuring the real component of the electrical conductivity in a plasma diode is described. The phase — sensitive detection enables to use extremely low probing signal down to 10 V.     

Photoconductivity enhancement in alkali metal doped multiwall carbon nanotubes

Photoconductivity effects in pristine and alkali-metal (K, Li) doped multiwalled carbon nanotubes (CNTs) were studied under xenon (100 mW) and also halogen (10 mW) light continues sources. To perform the measurements, the pristine and alkali doped CNTs were deposited into pores of a silver foam plate with nano-metric porosity by electrophoresis technique. The foam acted as a conducting frame for sweeping the photo-induced electrons to prevent rapid local electron–hole recombination in the CNTs. The radiation spectrum of the xenon source was similar to the Sun light spectrum and under normal ambient condition the photocurrents in the alkali doped samples were enhanced noticeably in comparison with the pristine CNTs. These results present a functional photoconductive performance of a heap of as-prepared alkali-metal doped CNTs that would be applicable as a light sensor without the necessity of separation between metallic and semiconducting CNTs (m- and s-CNTs).     

Nonlinear phase dispersion spectroscopy

Nonlinear phase dispersion spectroscopy is introduced as a means to retrieve wideband, high spectral resolution profiles of the wavelength-dependent real part of the refractive index. The method is based on detecting dispersion effects imparted to a light field with low coherence transmitted through a thin sample and detected interferometrically in the spectral domain. The same sampled signal is also processed to yield quantitative phase maps and spectral information regarding the total attenuation coefficient using spectral-domain phase microscopy and spectroscopic optical coherence tomography (SOCT), respectively. Proof-of-concept experiments using fluorescent and nonfluorescent polystyrene beads and another using a red blood cell demonstrate the ability of the method to quantify various absorptive/dispersive features. The increased sensitivity of this method, novel to our knowledge, is compared to intensity-based spectroscopy (e.g., SOCT), and potential applications are discussed.     

The non-pair forces and phonon dispersion in heavy alkali metals

Two types of the non-pair forces, one from the Born-Mayer and other from the Morse potential, are derived to discuss the response of the electrons in heavy alkali metals, i.e. rubidium and caesium. These potentials are added to the two-body potential of Morse to account for the ion-ion interactions as well. The potentials so obtained are employed to predict the phonon dispersion relations in the bcc metals, which are also compared with the recent precise neutron scattering data.     

Anomalous dispersion due to TI+ presence in alkali halides

We report on the optical rotatory power of TI⁺ ions randomly embedded in an alkali halide lattice when in an external magnetic field. The theoretical analysis allows us to examine closely the nature of interactions between energy levels through the magnetic field hamiltonian. The experimental curves, analyzed in order to estimate the square modules of the electric dipole moment matrix elements, are shown to support the theory proposed by Shen.     

Fourth-order dispersion suppression of ultrashort optical pulses by second-order dispersion and cosine phase modulation

A technique for significantly suppressing severe fourth-order dispersion of ultrashort optical pulses in long-distance transmission is analyzed by use of the quadratic phase from the excess second-order dispersion of the fiber link in conjunction with synchronously applied cosine phase modulation of temporally stretched pulses. Numerical simulation predicts much improved transmission of 250-fs pulses at 5-10-GHz repetition rates over ~100 km of fiber by pi-3.5pi phase modulation.     

Characterization of the Suzuki phase in doped alkali halides by Raman spectroscopy

The symmetries and frequencies of the Raman active modes of the Suzuki phase in the systems 6NaCl:CdCl₂; and 6NaCl:MnCl₂, have been calculated. Three of the four peaks allowed by symmetry (A_1g and 2F_g) agree to within 25% in position with the experimental values presented here and with other experimental results. The fourth peak (E_g) seems to be associated with a very soft mode, which explains why this peak has not been observed. The features of the Raman spectrum depend mainly upon the chlorine ions.     

FCC → SC phase transition in alkali halides

The NaCl → CsCl structural pressure transformation is studied using a new interionic potential. The theoretical transition pressures are in good agreement with experiment.     

Remarkable enhancement of terahertz conductivity of graphene tuned by periodic gate voltage

Optical conductivity of a graphene sheet subject to a one-dimensional cosinusoidal potential is studied. We find that the optical conductivity can be tuned by the strength of the superlattice potential. When this strength is within a certain range the terahertz conductivity can be enhanced by two orders of magnitude.