Study of acoustic parameters of binary mixtures of a nonpolar liquid with polar liquid at different frequencies

The densities (ρ) and ultrasonic velocities (C) of binary mixture of diisopropyl ether (DIPE) and bromobenzene (BB) have been measured at different frequencies (1 MHz, 3 MHz and 5 MHz) over the entire range of mole fraction of diisopropyl ether (DIPE) at temperature 303 K. The intermolecular free length (L _f ), isentropic compressibility (β), acoustic impedance (Z) and excess values of isentropic compressibility (β ^E ) and acoustic impedance (Z^E) have been computed using values of ultrasonic velocity (C) and density (ρ). The ultrasonic velocity, intermolecular free length are positive whereas the excess values of isentropic compressibility and acoustic impedance are negative over the entire composition range of DIPE which indicates presence of specific interactions between unlike molecules. The results are discussed in the light of intermolecular interactions occurring in the mixtures.      

Effect of frequency on acoustic parameters in a binary mixture of polar liquids

The densities (α) and ultrasonic velocities (C) of binary mixtures of a polar liquid like acetone and toluene have been measured at different frequencies (lMHz, 3MHz and 5MHz) over the entire range of mole fraction of acetone at temperature 303.16k. The intermolecular free length (L _f ), isentropic compressibility (β), acoustic impedance (Z), excess values of isentropic compressibility (β ^E ) and acoustic impedance (Z ^E ) have been computed using values of ultrasonic velocity (C) and density (α). The ultrasonic velocity (C), intermolecular free length (L _f ) and excess values of isentropic compressibility are positive whereas excess values of acoustic impedance is negative for the entire composition range which indicates the specific interaction between unlike molecules. The results are discussed in the light of intermolecular interactions occurring in the solutions.     

Excess molar volumes and isentropic compressibilities of binary liquid mixtures containing n-alkanes at 298.15 K

Excess molar volumes (V ^E) and deviation in isentropic compressibilities (Δβ _s) have been investigated from the density ρ and speed of sound u measurements of six binary liquid mixtures containing n-alkanes over the entire range of composition at 298.15 K. Excess molar volume exhibits inversion in sign in one binary mixture, i.e., n-heptane + n-hexane. Remaining five binary mixtures, n-heptane + toluene, cyclohexane + n-heptane, cyclohexane + n-hexane, toluene + n-hexane and n-decane + n-hexane show negative excess molar volumes over the whole composition range. However, the large negative values of excess molar volume becomes domainant in toluene + n-hexane mixture. Deviation in isentropic compressibility is negative over the whole range of composition in the case of all the six binary mixtures. Existence of specific intermolecular interactions in the mixtures has been analyzed in terms of excess molar volume and deviation in isentropic compressibility.     

Ultrasonic study of binary mixtures of acetylacetone with polar diluents at 303.16 K vis-a-vis molecular interaction

The ultrasonic velocity (C) and density (p) of binary mixture of acetylacetone with polar diluents have been measured at temperature 303.16 K and frequency of 2 MHz. The data of C and p have been used to evaluate, isentropic compressibility (β), acoustic impedence (Z) and intermolecular free length (L _f ) to study the molecular interaction. The study reveals that interaction is maximum in bromo-benzene acetylacetone mixture.      

Microwave dielectric characterization of binary mixture of formamide with <Emphasis Type="Italic">N,N</Emphasis>-dimethylaminoethanol

Dielectric relaxation measurements of formamide (FMD)-N,N-dimethylaminoethanol (DMAE) solvent mixtures have been carried out over the entire concentration range using time domain reflectometry technique at 25, 35 and 45°C in the frequency range of 10 MHz to 20 GHz. The mixtures exhibit a principle dispersion of the Davidson-Cole relaxation type at microwave frequencies. Bilinear calibration method is used to obtain complex permittivity ɛ*(ω) from complex reflection coefficient ρ*(ω) over the frequency range of 10 MHz to 10 GHz. The excess permittivity (ɛ ^E), excess inverse relaxation time (1/τ)^E, Kirkwood correlation factor (g ^eff), activation energy and Bruggeman factor (f _B) are also calculated to study the solute-solvent interaction.      

Dark Energy with Generalized Equation of State in Bianchi Type-I Cosmological Model

Dark energy with the usually used equation of state p=γρ, where γ=const<0 is hydrodynamically unstable. To overcome this drawback we consider the cosmology of a perfect fluid with a linear equation of state of a more general form p=α(ρ−ρ ₀), where the constants α and ρ ₀ are free parameters. The anisotropic Bianchi type-I cosmological model filled with dark energy has been considered. A generalized equation of state for the dark energy component of the universe has been used. The exact solutions to the corresponding Einstein field equations and the statefinder diagnostic pair i.e. {r,s} parameters have been obtained in three interesting cases (i) when ρ _Λ>0 and A>0 (ii) when ρ _Λ>0 and A<0 and (iii) when ρ _Λ<0 and A>0 at the singularities i.e. t→0 and t→±∞.     

Charged sphere of shear-free fluids with $$\frac{\partial}{\rho} \partial r \leq 0$$

The Einstein-Maxwell equations for non-static charged shear-free spherically symmetric perfect fluid distribution reduce to a second-order non-linear differential equation in the radial parameter. Several solutions of this equation have been obtained in earlier work without considering the general requirement for physical relevance of the solutions. Generally physically acceptable relativistic fluid models demand that the solutions satisfy the reality conditions ρ ≥ 0, p ≥ 0, ρ _r ≤ 0, etc. throughout the fluid model. In this article the expression for density gradient ρ _x (or ρ _r ) has been utilized to produce charged shear-free relativistic fluid models with non-positive density gradient (NDG)ρ _r ≤ 0. Eventually, we have found that none of the Riccati solutions have NDG including Vaidya metric. Also, the solutions with NDG neither possess Lie-symmetries nor Painlevé property. Further, it is observed that the solutions with NDG have no uncharged analogue.     

Ultrasonic study of molecular interaction in binary liquid mixtures at 30°C

Densities ρ and ultrasonic speeds u of the binary mixtures of tetrahydrofuran (THF) with 1-butanol and tert-butanol, at 30°C, over the entire composition range were measured. From these data isentropic compressibility, K _s, intermolecular free length L _f, relative association R _A, acoustic impedance Z, molar sound speed R _m, deviations in isentropic compressibility ΔK _s, and excess volume V ^E were calculated. The variation of these parameters with composition of the mixture helps us in understanding the nature and extent of interaction between unlike molecules in the mixtures. Further, theoretical values of ultrasonic speed were evaluated using theories and empirical relations. The relative merits of these theories and relations were discussed.     

Adiabatic evaporation of binary liquid mixtures on the porous ball surface

Measured data for the temperature of a porous spherical surface to which an evaporating binary liquid mixture was supplied are reported. In the experiments, solutions of ethyl and methyl alcohols in water, and also solutions of acetone in water, were used. The concentration of mixture components was varied throughout the widest possible range of X _L = 0–1, and the temperature of dry air flow past the sphere was in the range t ₀ = 15–300 °C. In the present study, a strong influence of the composition of the mixtures on their adiabatic evaporation temperature was established. In the heat- and mass-transfer process, the air temperature is also of paramount importance. An experimental correlation is obtained which generalizes data on adiabatic evaporation temperature in a broad range of component concentrations and temperatures for the experimentally examined binary liquid mixtures. This work was supported by the Russian Foundation for Basic Research (Grant No. 06-08-39002—GFEN).     

Magnetoresistance and magnetic ordering in praseodymium and neodymium hexaborides

The magnetoresistance Δρ/ρ of single-crystal samples of praseodymium and neodymium hexaborides (PrB₆ and NdB₆) has been measured at temperatures ranging from 2 to 20 K in a magnetic field of up to 80 kOe. The results obtained have revealed a crossover of the regime from a small negative magnetoresistance in the paramagnetic state to a large positive magnetoresistive effect in magnetically ordered phases of the PrB₆ and NdB₆ compounds. An analysis of the dependences Δρ(H)/ρ has made it possible to separate three contributions to the magnetoresistance for the compounds under investigation. In addition to the main negative contribution, which is quadratic in the magnetic field (−Δρ/ρ ∝ H ²), a linear positive contribution (Δρ/ρ ∝ H) and a nonlinear ferromagnetic contribution have been found. Upon transition to a magnetically ordered state, the linear positive component in the magnetoresistance of the PrB₆ and NdB₆ compounds becomes dominant, whereas the quadratic contribution to the negative magnetoresistance is completely suppressed in the commensurate magnetic phase of these compounds. The presence of several components in the magnetoresistance has been explained by assuming that, in the antiferromagnetic phases of PrB₆ and NdB₆, ferromagnetic nanoregions (ferrons) are formed in the 5d band in the vicinity of the rareearth ions. The origin of the quadratic contribution to the negative magnetoresistance is interpreted in terms of the Yosida model, which takes into account scattering of conduction electrons by localized magnetic moments of rare-earth ions. Within the approach used, the local magnetic susceptibility χ_loc has been estimated. It has been demonstrated that, in the temperature range T _N < T < 20 K, the behavior of the local magnetic susceptibility χ_loc for the compounds under investigation can be described with good accuracy by the Curie-Weiss dependence χ_loc ∝ (T − Θ _p )⁻¹.     

Electrical properties of thin films of Eu2O3 substituted compounds

Europium oxide (Eu₂O₃) substituted compound has been prepared by solid-solid reaction of the powders of Eu₂O₃, BaCO₃ and CuO at 950°C for 16 hours. The thin films have been deposited by high vacuum evaporation technique (vacuum ≈ 10⁻⁶ torr). The variation of current (I) with voltage (V) at room temperature (RT) i.e. 294 K and in ice (273 K) are found to be linear. The variation of electrical resistivity (ρ) with temperature (T) by heating the sample above RT has been determined. Resistivity is found to decrease with increase in temperature. Further the variation of electrical resistivity (ρ) with temperature (T) from 77 K, liquid nitrogen temperature (LNT), to 270 K has also been determined. It is observed that resistivity suddenly becomes zero at around 87 K. Thus the prepared material has superconducting properties with superconducting transition temperature, T _c at 87 K.      

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (T _C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for f_NRM, δε^NRM and distribution parameter α_NRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT _C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.     

Higher Dimensional Bianchi Type-V Cosmological Models with Perfect Fluid and Dark Energy

We have studied the Bianchi type-V cosmological models with binary mixture of perfect fluid and dark energy in five dimensions. The perfect fluid is obeying the equation of state p=γρ with γ∈[0,1]. The dark energy is considered to be either the quintessence or the Chaplygin gas. The exact solutions of the Einstein’s field equations are obtained in quadrature form.     

Ion conduction and space-charge polarization processes in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The electrical properties of a lithium heptagermanate (Li₂Ge₇O₁₅) crystal have been studied in DC and AC measuring fields at temperatures from 500 to 700 K. In a DC field, a substantial decrease of electrical conductivity σ with time has been detected. On the basis of kinetic dependences σ(t), estimates of the charge carrier diffusion coefficient D have been obtained. In the frequency range 10¹–10⁵ Hz, the spectra of complex impedance ρ*(f) have been measured. The analysis of diagrams in the complex plane (ρ″–ρ′) has been performed within the equivalent circuit approach. It has been shown that, in the considered temperature and frequency intervals, the electrical properties of Li₂Ge₇O₁₅ crystals have been determined by the hopping conduction of interstitial lithium ions A _Li and accumulation of charge carriers near the blocking Pt electrodes.     

Influence of grain boundaries on resistivity of manganites La1 − x K x MnO3

Results of investigation of resistivity and magnetoresistance of manganites La_{1 − x} K _x MnO₃ (x = 0.050–0.175) are presented. Behavior of resistivity ρ(T) in the paramagnetic and ferromagnetic phases has been described. To describe ρ(T) near the phase-transition temperature, notions of the percolation theory have been used. Two maxima have been found in the dependence ρ(T); their appearance has been attributed to the ceramic nature of the studied samples. The observed increase in magnetoresistance with a decrease in temperature is caused by intergranular spin-polarized tunneling of charge carriers.     

Bianchi type-V model with a perfect fluid and A-term

A self-consistent system of gravitational field with a binary mixture of perfect fluid and dark energy given by a cosmological constant has been considered in Bianchi Type-V universe. The perfect fluid is chosen to be obeying either the equation of state p=γρ with γ ε |0,1| or a van der Waals equation of state. The role of A-term in the evolution of the Bianchi Type-V universe has been studied.     

Pressure-induced metamagnetic transition in the Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript> ferromagnetic semiconductor

The magnetic susceptibility χ/χ₀ and the longitudinal Δρ _zz /ρ₀ and transverse Δρ _xx /ρ₀ magnetoresistances have been measured as functions of the hydrostatic pressure P ≤ 7 GPa at room temperature in the high-temperature ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ with a chalcopyrite structure and the Curie temperature T _c = 355 K. A pressure-induced metamagnetic transition from the low-magnetization state to the high-magnetization state has been observed in Cd_0.7Mn_0.3GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of the magnetic susceptibility and magnetoresistance.     

Dispersion of cerebral on-resonanceT 1 in the rotating frame (T 1ρ) in global ischaemia

T ₁ in the rotating frame (T _1ρ) has been recently shown to be an early magnetic resonance imaging index of ongoing pathology in vivo. Dispersion ofT _1ρ at aB ₁ range from 620 to 160500 rad/s was quantified in the rat brain to study the effects of ischaemia onT _1ρ relaxation and to elude the role of physico-chemical mechanisms underlying the relaxation changes. The results show a biphasic effect of the dispersion ofR _1ρ (1/T _1ρ) in ischaemic brain. In the lowB ₁ region (less than 5000 rad/s), an increase was detected, while at the highB ₁ end of the experiments,R _1ρ was reduced. This was accompanied by a change in the fitted values describing the dispersion characteristics. Injection of a superparamagnetic contrast agent, AMI-227, before ischaemia shifted theR _1ρ curve upwards by 4% but showed no effect on the shape of theR _1ρ dispersion curve. The present results argue for a specific effect of endogenous susceptibility on cerebralR _1ρ in the early moments of ischaemia due to a relaxation enhancement in the blood pool. Our results confirm that quantitativeR _1ρ dispersion nuclear magnetic resonance (NMR) improves the sensitivity ofR _1ρ NMR to detect ischaemia in vivo. Simple empirical linear or exponential fits proved to be efficient ways to describe dispersion data in the limitedB ₁ range, as more detailed models may not offer additional information in a physically relevant manner for complex systems, such as the brain.     

Electronic topological transition in an <Emphasis Type="Italic">n</Emphasis>-BiSb semiconductor alloy in the quantum limit range of magnetic fields for H‖<Emphasis Type="Italic">C</Emphasis><Subscript>2</Subscript>

The galvanomagnetic properties of single-crystal samples of the Bi_0.93Sb_0.07 semiconductor alloy with the electron density n = 1.6 × 10¹⁷ cm⁻³ in magnetic fields up to 14 T at T = 1.6 K have been investigated. The resistivity ρ and Hall coefficient R have been measured as functions of the magnetic field directed along the binary axis of a crystal for a current flowing through a sample along the bisector axis; i.e., the components ρ₂₂ and R _{32, 1} have been measured. The strong anisotropy of the electron spectrum of the samples makes it possible to separately observe quantum oscillations of the magnetoresistance ρ₂₂(H) for H ‖ C ₂ in low magnetic fields for two equivalent ellipsoids with small extremal cross sections (secondary ellipsoids) and in high magnetic fields for electrons of the ellipsoid with a large extremal cross section (main ellipsoid). An increase in the energy of the electrons of secondary ellipsoids in the quantum limit magnetic fields is accompanied by the flow of electrons to the main ellipsoid; i.e., an electronic topological transition occurs from the three-valley electron spectrum to the single-valley one. After the flow stops, the Fermi energy E _F increases from 18 meV to 27.8 meV. With an increase in the quantizing magnetic field, the Fermi energy of the electrons decreases both in the region of quantum oscillations of the resistance that are attributed to the electrons of the secondary ellipsoids and in the region of oscillations associated with the electrons of the main ellipsoid. The Hall coefficient R _{32, 1} decreases in high magnetic fields; this behavior indicates the absence of the electron magnetic freezing effect.     

Exact Results for Thermodynamics of the Hydrogen Plasma: Low-Temperature Expansions Beyond Saha Theory

We study hydrogen in the Saha regime, within the physical picture in terms of a quantum proton-electron plasma. Long ago, Saha showed that, at sufficiently low densities and low temperatures, the system behaves almost as an ideal mixture made with hydrogen atoms in their groundstate, ionized protons and ionized electrons. More recently, that result has been rigorously proved in some scaling limit where both temperature and density vanish. In that Saha regime, we derive exact low-temperature expansions for the pressure and internal energy, where density ρ is rescaled in units of a temperature-dependent density ρ ^* which controls the cross-over between full ionization (ρ ≪ ρ ^* ) and full atomic recombination (ρ≫ρ ^* ). Each term reduces to a function of ρ/ρ ^* times temperature-dependent functions which decay exponentially fast when temperature T vanishes. Scaled expansions are ordered with respect to the corresponding decay rates. Leading terms do reduce to ideal contributions obtained within Saha theory. We consistently compute all corrections which are exponentially smaller by a factor exp (β E _H ) at most, where E _H is the negative groundstate energy of a hydrogen atom and β=1/(k _B T). They include all effects arising from both the Coulomb potential and the quantum nature of the particles: excitations of atoms H, formation of molecules H ₂, ions H ₂⁺ and H ⁻, thermal and pressure ionization, plasma polarization, screening, interactions between atoms and ionized charges, etc. Scaled low-temperature expansions can be viewed as partial resummations of usual virial expansions up to arbitrary high orders in the density.