3He Anomalously High Spin Diffusion in Solid Para-Hydrogen

NMR measurements of ³He spin diffusion coefficient in solid para-H₂ are carried out at the temperatures 0.45–1.5 K. The crystals have been grown under constant pressure 20–130 bar. The ³He concentrations in the initial para-H₂–³He gas mixtures were 0.1% and 0.3%. It is found out that the decay of echo amplitude vs both magnetic field gradient G and time interval τ between RF pulses is of non-exponential character, typical of one-dimensional diffusion in restricted geometry. The values of true spin-diffusion coefficient D _S measured are found to be ∼10⁻⁴ cm²/s at 20 bar. At 108 bar D _S value is one order of magnitude less. D _S does not depend on temperature. Such spin diffusion coefficient values seem to be anomalously high in comparison with well-known values of D _S =10⁻⁵ cm²/s for bulk liquid ³He at 27 bar and D _S =10⁻⁸ cm²/s for bulk solid ³He at 108 bar. The special experiments with the crystal annealing make it clear that the high spin diffusion here is connected with fast diffusion along dislocation lines.     

Computation of phonon dynamics of Mg<Subscript>70</Subscript>Zn<Subscript>30</Subscript> metallic glass

In the present paper, the computation of the phonon dynamics of binary Mg₇₀Zn₃₀ metallic glass is reported using the well-recognized model potential of Gajjar et al. The present study includes the phonon dispersion curves (PDC), elastic and thermodynamic properties such as longitudinal sound velocity υ_L, transverse sound velocity υ_T, Debye temperature θ_D, isothermal bulk modulus B _T, modulus of rigidity G, Poisson’s ratio σ and Young’s modulus Y and specific heat capacity C _V of the glass. Three theoretical models given by Hubbard–Beeby (HB), Takeno–Goda (TG) and Bhatia–Singh (BS) are used to compute the PDC. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are employed for the first time to study the effect of exchange and correlation in the aforesaid properties. The pseudo-alloy-atom (PAA) model is applied for the first time instead of Vegard’s Law.     

Photochemical modification of single-walled carbon nanotubes using HPHMP photoinitiator for enhanced organic solvent dispersion

Photochemical modification of single-walled carbon nanotubes (SWCNTs) was carried out by covalent attachment of 2-propanol-2-yl radicals on the surface of SWCNTs, which were engendered by the photolysis of 1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (HPHMP) under ultraviolet (UV) light. Pristine single-walled carbon nanotubes (p-SWCNTs) were dispersed in acetone along with HPHMP photoinitiator. After that, the mixture was irradiated by UV light to generate the free radicals which were introduced onto the surface of SWCNTs. The modification of SWCNTs was supported by UV/visible spectroscopy, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, thermal gravimetric analysis–mass spectrometry (TGA–MS), and transmission electron microscopy (TEM). UV/visible results revealed the loss of van Hove singularities due to covalent modification. The modification was further verified by FT-IR showing the signals at 3421 and 1100 cm⁻¹ due to stretching and bending of O–H group, respectively. Moreover, other peaks at 2927 and 2858 cm⁻¹ indicated the asymmetric and symmetric stretching modes of aliphatic C–H bond, respectively. Raman spectra illustrated that the intensity ratio of the tangential mode to the disorder mode (I _G/I _D), for modified SWCNTs (F-SWCNTs), decreased nearly four times than p-SWCNTs. TGA–MS also evidenced the signal corresponding to m/z 59 at 400 °C indicating the presence of 2-propanol-2-yl groups. TEM and dispersibility data demonstrated that the sidewall modification detached the bundled structure, enhanced the dispersion in common organic solvents and retained the original size of SWCNTs without hefty modification, which could cut or damage the nanotubes.     

Density of states effective mass of SnBi<Subscript>4</Subscript>Se<Subscript>7</Subscript> deduced from the temperature dependence of electrical conductivity in the activation regime

Current–voltage (I–V) measurements on polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compound in the quasi-binary system of SnSe–Bi₂Se₃ at different temperatures in the vicinity of room temperature have been performed. Also, temperature dependence of electrical conductivity has been measured. From the analysis of the temperature dependence of electron concentration in the activation regime above room temperature, the density of states effective mass, m*, has been determined. Some intrinsic and contact properties such as barrier heights, ideality factors, and carriers concentrations have been investigated using I–V characteristics. It has been found that all samples exhibit ohmic and space charge limited conduction at low and high fields, respectively.     

Infinite-Dilution Binary Diffusion Coefficients of 2-Propanone, 2-Butanone, 2-Pentanone, and 3-Pentanone in CO2 by the Taylor Dispersion Technique from 308.15 to 328.15 K in the Pressure Range from 8 to 35 MPa

Infinite-dilution binary diffusion coefficients of 2-propanone, 2-butanone, 2-pentanone, and 3-pentanone in carbon dioxide were measured by the Taylor dispersion method at temperatures from 308.15 to 328.15 K and pressures from 7.60 to 34.57 MPa. The D ₁₂ values were obtained from the response curves by the method of fitting in the time domain. The accuracy in the fitting error was examined for each measurement. The measured D ₁₂ data were found to be well correlated by the Schmidt number correlation, with AAD%=3.74% for all solutes.     

Electron-beam enhanced fusion of binary metals and alloying in melts

Binary mixing and alloying during electron-beam fusion in bi-metal joints have been investigated and presented in this paper. In case of beam-power densities greater than about 10¹⁰ Wm⁻², rapid fusion of metals is accompanied by violent agitation of the melts. The resulting mass transport in the melt is described here from an enhanced mass diffusion coefficientD =D _s(W′/a ₀ η)^1/2, whereD _s is mass diffusivity in solid,W′ is beam-power density,a ₀ is acceleration in melt growth andη is the coefficient of viscosity. Moreover, a uniformly mixed central-widthw′ is predicted in the fusion zone (total widthw), where,w′ is significantly dependent onη andw throughW′. For these conditions, the continuity of mass transport in stationary and travel modes of the source has been solved using line and plane source models and conservation principles. The predictions from these calculations have been validated through quantitative analyses of experiments.     

Optimization of Taylor Dispersion Technique for Measurement of Mutual Diffusion in Benchmark Mixtures

The Taylor dispersion technique has been used for measuring binary mutual diffusion coefficients for mixtures of 1,2,3,4-tetrahydronaphthalene (THN), isobutylbenzene (IBB) and dodecane (C₁₂ H ₂₆) at 0.5:0.5 mass fraction symmetric points, and for 0.9:0.1 mass fraction in IBB- C₁₂ H ₂₆. From the Stokes–Einstein equation and our experimental results, the limiting diffusion coefficients, D ⁰, and the equivalent solvated radii, R _s, have been estimated at infinitesimal concentration of these species (TNH, IBB and C₁₂ H ₂₆). The measured diffusion coefficients are used to estimate activity coefficients of the components in the mixture, contributing to a better understanding of the structure of such systems and of their thermodynamic behaviour at different concentrations. We have also investigated the diffusion properties for a ternary system containing equal mass fractions of all the components (0.33THN: 0.33IBB: 0.33C₁₂ H ₂₆) and at 298.15 K.     

Cd-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yCdyO10?δ (y=0,0.5,1.0,1.5,2.0) Superconductors

Superconducting properties of cadmium doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) samples have been studied using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. In X-ray diffraction studies these samples have shown to have tetragonal structure. The zero resistivity critical temperature and magnitude of diamagnetism are suppressed with the increased incorporation of Cd in the final compound. A change in the shape of FTIR absorption spectra, after doping, has shown the incorporation of Cd in the unit cell. A systematic hardening of the apical oxygen modes and softening of the CuO₂ planar modes of vibration with increased Cd doping have shown that it is incorporated in the unit cell of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) superconductors. The FTIR absorption measurements of these samples have shown that hardening of the apical oxygen modes of types Cu(1)–O(2)–Tl and Cu(1)–O(2)–Cu(2)/Cd _y (y=0,0.5,1.0,1.5,2.0) increases with the increase of Cd doping in the samples. A softening of the CuO₂ planar oxygen mode Cu(2)–O–Cu(2) is also observed with the increased Cd doping in the final compound. It is most likely that hardening of the apical oxygen modes and the softening of the planar modes of vibration are associated damped harmonic oscillations produced by heavier Cd atoms in the CuO₂ planes, which suppress the phonon population from a desired level, reducing the magnitude of superconductivity in the final compound.     

Transport Properties of Nonelectrolyte Liquid Mixtures. XI. Mutual Diffusion Coefficients for Toluene+n-Hexane and Toluene+Acetonitrile at Temperatures from 273 to 348 K and at Pressures up to 25 MPa

Mutual diffusion coefficients,D ₁₂, have been measured at pressures up to 25 MPa using the chromatographic peak broadening technique (Taylor dispersion method) forxtoluene+(1–x)n-hexane in the temperature range 298 to 348 K and forxtoluene+(1–x) acetonitrile in the temperature range 273 to 348 K. The estimated uncertainty is ±4%. Both systems show negative deviations from straight-line behavior. The fractional decrease inD ₁₂is about 0.8% per MPa. Hard-sphere theory is applied under limiting conditions where one of the components is present in a trace amount. It is shown that the diffusion coefficients can be estimated by the Dullien method from a knowledge of the viscosity and density under the same conditions.     

Soft-Mode Behavior in Ternary Silicides MAlSi (M=Ca, Sr, Ba)

We present a combined experimental and theoretical investigation of the lattice dynamics and electron–phonon coupling of the ternary silicides MAlSi (M = Ca, Sr, Ba). Temperature dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states provide evidence for a soft-mode behavior for CaAlSi which is absent for the other two compounds. Density-functional-based calculations of phonon spectra and electron–phonon coupling support this interpretation and identify the soft mode as strong coupling out-of-plane Al vibrations, in agreement with previous theoretical work. Observed differences in superconducting properties, in particular in T _c , among the investigated compounds can be explained by the presence of the soft mode.     

Pre-exponential factor of Arrhenius equation for the isothermal crystallization of some Se–Ge,Se–In and Se–Te chalcogenide glasses

Compensation law or Meyer–Neldel rule is observed in many activated phenomena, including solid state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers, and electronic conduction in amorphous semiconductors. In the present paper, we have reported the compensation effect for the isothermal crystallization of some Se–Ge, Se–In and Se–Te chalcogenide glasses. We have observed Meyer–Neldel rule between pre-exponential factor K ₀ and activation energy of crystallization E _c in the present case.     

NMR Evidence for C<Subscript>60</Subscript> Configurational Fluctuations Around Na Sites in Na<Subscript>2</Subscript>CsC<Subscript>60</Subscript>

The ²³Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin–spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na₂CsC₆₀ in the temperature range of 10–300 K. The NMR line associated with the tetrahedral sodium site is split below 170 K (T and T′ lines) similarly to Rb₃C₆₀ although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T′ sites are microscopically close. The merger of the two lines at about 170 K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C₆₀ molecules. The exchange dynamics inferred from the spectra, spin–spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T′ splitting originates from different local fullerene configurations around the tetrahedral alkaline sites.     

Superconducting Properties of Zn-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yZnyO10?δ Superconductors

A reproducible synthesis and characterization of Zn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Zn _y O_12−δ (y=0, 0.5, 1.0, 1.5) superconductors at a relatively lower synthesis temperature of 840°C are studied by using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. The X-ray diffraction (XRD) studies of these samples have shown a tetragonal structure in which the c-axis length has been found to decrease with increased Zn doping. The critical temperature and magnitude of diamagnetism have not been significantly affected with the doping of Zn at this synthesis temperature. The magnitude of diamagnetism in the as-prepared undoped samples is decreased, whereas it remains stable (unchanged) in oxygen post-annealed samples. The apical oxygen phonon’s modes of type Tl–O_A–M(2) and Cu(1)–O_A–M(2) {where M=Cu/Zn} and the planar oxygen phonon modes of type M(2)–O_P–M(2) are also softened with the increase of Zn doping. We interpreted the softening of these oxygen related phonon modes linked with the decreased c-axis length, reduced John–Teller distortions and increased mass of Zn (65.38 amu) as compared to that of Cu (63.54 amu) (Kaplan et al., Phys. Rev. B 65, 214509, 2002).     

An automated setup for investigation of the thermal diffusion and mutual diffusion of gas mixtures

We present a block diagram of a setup for determining the thermal-diffusion factor and the coefficient of mutual diffusion of gas mixtures. The procedure of the experiment is described. Experimental results on the temperature and concentration dependence of α _t and D₁₂ for He-Ar and Ar-CO mixtures are given. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 1, pp. 182–188, January–February, 1998.     

Polarization Induced Spin Wave Damping in Spin Polarized Liquid 3He–4He

We have measured the temperature and polarization dependence of Silin spin wave spectra in a saturated ³He–⁴He mixture with a concentration of 9.4% at a pressure of 8 bars. The mixture has been cooled and polarized by a Leiden dilution refrigerator to temperatures in the range 10–15 mK and polarizations as high as 9.2% corresponding to 3.4 times the equilibrium polarization of 2.7% in the external magnetic field of 11.36 T. The analysis takes into account the dipolar interactions and results in the relaxation time τ _⊥ and spin diffusion constant D _⊥ . We find that τ _⊥ and D _⊥ are proportional to where T is the temperature, is the polarization enhancement factor and T _a0 is the anisotropy temperature for the mixture at equilibrium in the external field. Our result T _a0=3.66±0.14 mK is 30% higher than the theoretical prediction for very dilute mixtures and is evidence for the existence of polarization induced relaxation of transverse spin currents.     

Electron–Phonon Interaction and Phonon Renormalization in the Lamellar Cobaltate Na x CoO2

We study theoretically the electron–phonon interaction in Na _x CoO₂. For the A _1g and E _1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω, we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E _1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A _1g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.     

Infinite Dilution Binary Diffusion Coefficients of Benzene in Carbon Dioxide by the Taylor Dispersion Technique at Temperatures from 308.15 to 328.15 K and Pressures from 6 to 30 MPa

Infinite dilution binary diffusion coefficients, D ₁₂, of benzene in carbon dioxide were measured by the Taylor dispersion technique at temperatures from 308.15 to 328.15 K and pressures from 6 to 30 MPa. The diffusion coefficients were obtained by the method of fitting in the time domain from the response curves measured with a UV–vis multidetector by scanning from 220 to 280 nm at increments of 1 or 4 nm. The wavelength dependences on the binary diffusion coefficient and the uncertainty were examined. The detector linearity, in terms of the relationship between the absorbance intensity and the product of the peak area of the response curve and CO₂ velocity, was found to fail at some characteristic absorption wavelengths such as 243, 248, 253, and 259 nm, even when the maximum absorbance intensities of the response curves were less than 0.5 and the fits were good. Although the D ₁₂ values obtained from the response curves measured at 253 nm were almost consistent with some literature data, the D ₁₂ values measured at wavelengths showing the detector linearity to be satisfactory, i.e., at 239 nm, were higher than those at 253 nm. The present D ₁₂ data at 239 nm were well represented by the Schmidt number correlation, except for those showing the anomalous decrease in a plot of D ₁₂ vs density in the density range from 250 to 500 kg·m^–3.     

Growth rate of Nb<Subscript>3</Subscript>Sn for reactive diffusion between Nb and Cu–9.3Sn–0.3Ti alloy

In order to examine experimentally the growth behavior of Nb₃Sn during reactive diffusion between Nb and a bronze with the α + β two-phase microstructure, a sandwich (Cu–Sn–Ti)/Nb/(Cu–Sn–Ti) diffusion couple was prepared from pure Nb and a ternary Cu–Sn–Ti alloy with concentrations of 9.3 at.% Sn and 0.3 at.% Ti by a diffusion bonding technique. Here, α is the primary solid-solution phase of Cu with the face-centered cubic structure, and β is the intermediate phase with the body-centered cubic structure. The diffusion couple was isothermally annealed at temperatures between T = 923 and 1,053 K for various times up to 843 h. Owing to annealing, the Nb₃Sn layer is formed along each (Cu–Sn–Ti)/Nb interface in the diffusion couple, and grows mainly into Nb. Hence, the migration of the Nb₃Sn/Nb interface governs the growth of the Nb₃Sn layer. The mean thickness of the Nb₃Sn layer is proportional to a power function of the annealing time. The exponent of the power function is close to unity at T = 923 K, but takes values of 0.8–0.7 at T = 973–1,053 K. Consequently, the interface reaction at the migrating Nb₃Sn/Nb interface is the rate-controlling process for the growth of the Nb₃Sn layer at T = 923 K, and the interdiffusion across the Nb₃Sn layer as well as the interface reaction contributes to the rate-controlling process at T = 973–1,053 K. Except the effect of Ti, the growth rate of the Nb₃Sn layer is predominantly determined by the activity of Sn in the bronze and thus the concentration of Sn in the α phase. As a result, the growth rate is hardly affected by the volume fraction of the β phase, though the final amount of the Nb₃Sn layer may depend on the volume fraction.     

A relationship between diffusion and charge transfer parameters in a binary melt formed during contact melting

Theoretical and experimental data on the mass and charge transfer, charge transfer during contact melting, and hydrodynamic mixing of inert labels introduced into a contact between two dissimilar metals are analyzed and an integral criterion for the charge and mass transfer direction in this system is formulated. According to this criterion, which is valid in all the known binary systems of this type, (Ω₁–Ω₂)(D ₁–D ₂)]=−z ₁*–z ₂*, where Ω_i, D _i , and z _i * are the atomic volumes, partial diffusion coefficients, and effective charges of light (i=1) and heavy (i=2) components.