Elastic-stiffness coefficients of a silicon carbide fibre at elevated temperatures: Acoustic spectroscopy and micromechanics modelling

We have determined all five independent elastic-stiffness coefficients C_ij of a silicon carbide fibre with transverse isotropy from room temperature up to 873 K. Firstly, we measured the C_ij of a titanium-alloy-matrix composite reinforced unidirectionally with the fibres and the matrix alloy alone. Electromagnetic acoustic resonance detected the free-vibration resonance frequencies of the specimens to determine their C_ij . Secondly, we applied a micromechanics calculation to deduce the fibre C_ij from the measured composite and matrix C_ij . The resulting fibre C_ij shows strong anisotropy in the temperature derivatives of the C_ij ; the temperature derivatives for the fibre-axis-direction C_ij are much smaller than the others.     

On the spectral function of a hole in a one-dimensional quantum antiferromagnet

We discuss the spectral function of a single hole moving in a one-dimensional quantum antiferromagnet. The latter is described by an anisotropic version of thet-J model, wheret is the hopping matrix element. We introduce two independent coupling parametersJ andJ for the Ising and the transverse part of the Heisenberg exchange. Strong electronic correlations which are incorporated in the model prevent the use of usual diagrammatic techniques for dynamic Green functions based on Wick's theorem. For that reason a new projection technique for general correlation functions in terms of cumulants is used. We consider the case of max. For the case of small transverse coupling relative to the Ising part, we give exact expressions for the one hole correlation function. In the limit of vanishing spin fluctuations our result reduces to earlier calculations of the motion of a hole in a one-dimensional Néel state. However, the inclusion of the spin fluctuations leads to drastic modifications of the spectral function.     

The synthesis of boron-containing nanoparticles inert at room temperature

Boron-containing (C₂B₄H₂) _n nanoparticles with size 6.7, 7.8, and 10.8 nm inert at room temperature were synthesized. The synthesis was performed by the pyrolysis of gaseous carborane C₂B₄H₆ at 1200–1280 K and the initial carborane pressure (5–25) × 10⁻³ MPa. An analytic dependence relating the size of nanoparticles to the temperature and initial carborane pressure was obtained.     

Dependence of the superconducting transition temperature of quench-condensed aluminium-films on the oxygen content

Al-films were evaporated from Al₂O₃-crucibles or W-boats onto substrates held at 4.2 K or at room temperature (R.T.). The superconducting transition temperatureT _c was measured in situ, while the oxygen contentC _o of the films was determined by Rutherford-backscattering. Layers evaporated at R.T. from Al₂O₃-crucibles exhibit aT _c vs.C _o-relation like granular Al-films. For layers quench-condensed from W-boats without an external oxygen source typicalT _c -values of 2.5 K and oxygen concentrations of 10 at % were found. In case of quench-condensation from Al₂O₃-crucibles it is concluded from theT _c vs.C _o-relation that oxygen is incorporated into the films in the form of Al₂O-molecules. The different behavior of theT _c vs.C _o-curves will be discussed.     

Energy filtration of charge carriers in a nanostructured material based on bismuth telluride

The dependences of the electrical conductivity and thermopower on the size of grains in a nanocrystalline material based on Bi₂Te₃-Sb₂Te₃ solid solutions of the p type have been investigated theoretically and experimentally. The relaxation time in the case of hole scattering by nanograin boundaries in an isotropic polycrystal has been calculated taking into account the energy dependence of the probability of tunneling of charge carriers and the dependence of the scattering intensity on the nanograin size L _n . A decrease in the probability of boundary scattering with an increase in the energy of charge carriers leads to an increase in the thermopower. The dependences of the thermopower and electrical conductivity on the nanograin size, which have been obtained taking into account the boundary scattering and scattering by acoustic phonons, are in good agreement with experimental data. For the material under consideration, the thermopower coefficient increases by 10–20% compared to the initial solid solution at L _n = 20–30 nm. This can lead to an increase in the thermoelectric figure of merit by 20–40%, provided that the decrease in the electrical conductivity and the decrease in the lattice thermal conductivity compensate each other. Despite the absence of a complete compensation, it has been possible to increase the thermoelectric figure of merit for the samples under investigation to ZT = 1.10–1.12.     

富勒烯掺杂NPB空穴传输层的有机电致发光器件

报道了不同掺杂浓度NPB：C₆₀（富勒烯）作为空穴传输层对有机电致发光器件性能的影响.采用真空热蒸镀方法,制作了ITO/ NPB：C₆₀ （x % ）/Alq₃/LiF/Mg:Ag结构的四种有机电致发光器件.当NPB：C₆₀的掺杂浓度是15％时,器件的启亮电压是4 V,最大亮度是11000 cd/m².然而,当NPB：C₆₀的掺杂浓度是20％时,器件的最大亮度降     

Measuring the solubility of benzoic acid in room temperature ionic liquids using chronoamperometric techniques

The electrochemical reduction of benzoic acid (BZA) has been studied at platinum micro‐electrodes (10 and 2 µm diameters) in acetonitrile (MeCN) and six room temperature ionic liquids (RTILs): [C₂mim][NTf₂], [C₄mim][NTf₂], [C₄mpyrr][NTf₂], [C₄mim][BF₄], [C₄mim][NO₃] and [C₄mim][PF₆] (where [C_nmim]⁺ = 1‐alkyl‐3‐methylimidazolium, [NTf₂]^? = bis(trifluoromethylsulphonyl)imide, [C₄mpyrr]⁺ = N‐butyl‐N‐methylpyrrolidinium, [BF₄]^? = tetrafluoroborate, [NO₃]^? = nitrate and [PF₆]^? = hexafluorophosphate). Based on the theoretical fitting to experimental chronoamperometric transients in [C₄mpyrr][NTf₂] and MeCN at several concentrations and on different size electrodes, it is suggested that a fast chemical step preceeds the electron transfer step in a CE mechanism (given below) in both RTILs and MeCN, leading to the appearance of a simple one‐electron transfer mechanism. The six RTIL solvents and MeCN were saturated with BZA, and potential‐step chronoamperometry revealed diffusion coefficients of 170, 4.6, 3.2, 2.7, 1.8, 0.26 and 0.96 × 10^?11 m² s^?1 and solubilities of 850, 75, 78, 74, 220, 2850 and 48 mM in MeCN and the six ionic liquids, respectively, at 298 K. The high solubility of BZA in [C₄mim][NO₃] may suggest a strong interaction of the dissolved proton with the nitrate anion. Although there are relatively few literature reports of solubilities of organic solutes in RTILs at present, these results suggest the need for further studies on the solubilities of organic species (particularly acids) in RTILs, because of the contrasting interaction of dissolved species with the RTIL ions. Chronoamperometry is suggested as a convenient methodology for this purpose. Copyright © 2008 John Wiley & Sons, Ltd.     

Approximate analytical expressions of apertured broadband beams in the far field

The approximate analytical expressions of the apertured broadband beams in the far field with Gaussian and Laguerre-Gaussian spatial modes are presented.For the radially polarized Laguerre-Gaussian beam,the result reveals that the electromagnetic field in the far field is transverse magnetic.The influences of bandwidth(Γ) and truncation parameter(C 0) on the transverse intensity distribution of the Gaussian beam and on the energy flux distribution of radially polarized Laguerre-Gaussian beam are analysed.     

Evolution of heterophase structures and elastic effects in twinned crystals (Bi1 ? zPbz)FeO3 (0 ≤ z ≤ 0.100)

The specific features of the phase coexistence in crystals of (Bi_{1 − z} Pb _z )FeO₃ multiferroic solid solutions with a significantly distorted perovskite unit cell below the Curie temperature T _C have been investigated. Some examples of the evolution of the Pm[`3]m = R3cPm\bar 3m = R3c, R3c-C2/m, and R3c-Pbnm heterophase structures and different scenarios of mechanical stress relaxation in the presence of several types of twin domains of the R3c phase have been analyzed. The Pm[`3]m = R3cPm\bar 3m = R3c heterophase structures in the temperature intervals [T _C-150 K; T _C] (z = 0.025) and [T _C-185 K; T _C] (z = 0.050) have been interpreted. The results have been systematized using the diagrams relating twin structures and phase boundaries at 0 ≤ z ≤ 0.100.     

Microwave spectrum and conformation of 6-thiabicyclo[3.1.0]hexane

The microwave spectrum of 6-thiabicyclo[3.1.0]hexane (cyclopentene sulfide) has been measured in the region 26,500-40,000 MHz. The experimental data are consistent with a single stable conformation. Furthermore, these data can only be satisfactorily explained by assuming that this conformation is the boat form. Rotational constants were obtained, both for the ground state and two excited vibrational states, while centrifugal distortion coefficients were obtained for the ground state and one excited vibrational state. The ground state rotational constants found were A₀ = 5026.243 ± 0.003 MHz, B₀ = 2833.813 ± 0.003 MHz, and C₀ = 2411.679 ± 0.03 MHz. For the ground state of the molecule, the electric dipole moment components were found to be μ_a = 1.800 ± 0.012 D and μ_c = 1.155 ± 0.024 D, yielding a total dipole moment μ = 2.139 ± 0.027 D.     

Effects of internal viscous damping on the stability of a rotating shaft driven through a universal joint

A rotating flexible shaft, with both external and internal viscous damping, driven through a universal joint is considered. The mathematical model consists of a set of coupled, linear partial differential equations with time-dependent coefficients. Use of Galerkin's technique leads to a set of coupled linear differential equations with time-dependent coefficients. Using these differential equations some effects of internal viscous damping on parametric and flutter instability zones are investigated by the monodromy matrix technique. The flutter zones are also obtained on discarding the time-dependent coefficients in the differential equations which leads to an eigenvalue analysis. A one-term Galerkin approximation aided this analysis. Two different shafts (“automotive” and “lab”) were considered. Increasing internal damping is always stabilizing as regards to parametric instabilities. For flutter type instabilities it was found that increasing internal damping is always stabilizing for rotational speeds v below the first critical speed, v₁. For v>v₁, there is a value of the internal viscous damping coefficient, C_iv, which depends on the rotational speed and torque, above which destabilization occurs.The value of C_iv (“critical value”) at which the unstable zone first enters the practical range of operation was determined. The dependence of C_iv critical on the external damping was investigated. It was found for the automotive case that a four-fold increase in external damping led to an increase of about 20% of the critical value. For the lab model an increase of two orders of magnitude of the external damping led to an increase of critical value of only 10%.For the automotive shaft it was found that this critical value also removed the parametric instabilities out of the practical range. For the lab model it is not always possible to completely stabilize the system by increasing the internal damping. For this model using C_iv critical, parametric instabilities are still found in the practical range of operation.     

Line Shift Investigations for Different Isotopomers of Carbon Monoxide

Line shift coefficients for five lines of five different isotopomers in the fundamental band of CO in the spectral region near 2058 cm⁻¹were measured using a three channel lead salt diode laser spectrometer. The study includes the linesP(3) of¹³C¹⁷O,R(3) of¹³C¹⁸O,P(9) of¹²C¹⁸O,P(10) of¹³C¹⁶O, andP(21) of¹²C¹⁶O, and covers collisions with N₂, O₂, H₂, D₂, He, Ne, Ar, Kr, and Xe. Line shifts of the isotopomers¹³C¹⁶O,¹²C¹⁸O,¹³C¹⁸O, and¹³C¹⁷O were determined for the first time. Within the experimental uncertainty no significant dependence of the shift effect on the isotopomer was found. TheR-branch line under study shows a smaller line shift coefficient than aP-branch line with a similar rotational quantum number. With increasing mass of the noble gas perturber the absolute size of the shift coefficient increases. Moreover self- and nitrogen-broadening coefficients for the isotopomer lines were determined. Compared to previous measurements no significant deviations between different isotopomers were observed.     

A muon spin rotation study of hydrogen in YBa2Cu3O y

Superconducting YBa₂Cu₃O _y samples were doped with hydrogen and investigated by the muon spin rotation technique. For hydrogen concentrations above a thresholdx, which depends on the oxygen stoichiometry, we find a well defined precession signal in zero external field. This is a clear indication of magnetic ordering in these samples. In samples with a smaller hydrogen content no magnetic ordering was found. For these samples however, the depolarization rate σ(T→0) as measured in a transverse external field depends strongly on the hydrogen content. Our data are consistent with the assumption that hydrogen acts as an electron donor, filling the hole states in YBa₂Cu₃O _y .     

Hydrogen-abstraction reactions of fully hydrogenated fullerene cages with the amino radical: a density functional study

We have applied density functional theory calculations to study the reactions of NH₂ + C_nH_n (n = 20, 40, 50, 60, 70 and 80). Due to the hard curvature in C₂₀ cage, the NH₂? + C₂₀H₂₀ → NH₃ + C₂₀H₁₉? reaction is nearly thermoneutral with a high potential barrier height. For the C_nH_n fulleranes with n > 20 the transition states appear earlier on the reaction paths, as can be anticipated for exothermic reactions. Using the spherical excess parameter, we distinguished different curvatures on the surfaces of fullerane cages. The reaction enthalpies ΔH°₂₉₈ and potential barrier heights ΔE^TS of the considered reactions indicate good correlation with the values of ?_i parameter, showing an upward trend with the curvature increasing at carbon sites. We have also investigated the H-abstraction of the chemical derivatives of the C₂₀H₂₀ cage (C₂₀H₁₉–CH₃, C₂₀H₁₉–CH₂CH₃ and C₂₀H₁₉–CH₂CH₂CH₃) in comparison to the corresponding isolated alkanes (CH₄, C₂H₆ and C₃H₈). Overall, it could be inferred that the H-abstraction from the primary and secondary C–H bonds of isolated alkanes could occur more easily than fullarane derivatives.     

The influence of intermolecular interactions on the Kerr effect in gases

Expressions are derived for the second and third Kerr virial coefficients, B _K and C _K, of spherical top molecules in terms of irreducible cluster polarizabilities, and values are calculated using the dipole-induced dipole model for argon, krypton, xenon, methane, tetrafluoromethane, neopentane and sulphur hexafluoride. For mixtures of rare gases it is shown that the collision-induced dipole moment makes a negligible contribution to B _K. The effect of the choice of intermolecular potential function on the calculated second Kerr virial coefficients is also demonstrated. It is found that the predominant contributions to C _K arise from the pair polarizability, and that the triplet polarizability is only of minor importance.     

Depletion-induced sphere-cylinder transition in C12E5 microemulsion: a small-angle X-ray scattering study

Small-angle X-ray scattering was used to study the mixture of C₁₂E₅ (pentaethylene glycol monododecyl ether)/H₂O/n-decane microemulsion and polyethylene glycol (PEG). The size, shape and the structure factor of the microemulsion were investigated by adding the polymer (PEG) to the mixture. Attractive depletion potential was induced between the microemulsion droplets by the non-adsorb polymer. The range and strength of the attractive potential were changed by varying the molecular weight and concentration of PEG. The forward scattering, S(0), of the spherical microemulsion, declined gradually as the polymer concentration decreased. For PEG with the molecular weight of M_n = 285?315, the microemulsion morphology remained spherical, but the main peak of the structure factor moved towards a bigger q. When PEG with molecular weights of M_n = 2200 and M_n = 6000 were used, a shape transition from spherical to cylindrical was induced in line with increasing polymer concentration.     

Propagation of ultrasonic waves and anomalies nearT c in single crystalline YBa2Cu3O7−x high-T c superconductors

The attenuation and the velocity of ultrasonic waves in single crystalline high-T _c YBa₂Cu₃O_7–x have been measured in the temperature range from 3 to 280 K for the C₁₁-, C₃₃-, C₄₄- and C₆₆-mode. At the superconducting transition a pronounced peak in the absorption is observed for the longitudinal C₁₁-mode. The longitudinal C₁₁- and C₃₃-modes and the transverse C₄₄-mode exhibit a plateau in the temperature dependence of the velocity. The temperature dependence of the absorption of the transverse C₄₄-mode reveals a change in the slope atT _c. The data are compared with the predictions derived from a thermodynamic treatment and with the results of measurements on coarse grain and fine grain sintered and sinter-forged materials.     

Reverse phenomena of magnetic field effects and time-resolved EPR spectra in the photogenerated biradical from intramolecular electron-transfer in a phenothiazine–C60 linked compound with a semi-rigid spacer

Photoinduced electron-transfer reactions and magnetic field effects (MFEs) on the decay rates of the photogenerated biradical in a phenothiazine (Ph)–C₆₀ linked compound with a biphenyl group (Ph(BP)C₆₀) were examined in benzonitrile and benzene. Fluorescence and transient absorption spectra indicate that the intramolecular electron-transfer for Ph(BP)C₆₀ from the Ph to the singlet or triplet excited state of C₆₀ was suppressed by the biphenyl group. The decay rates of the photogenerated biradical decreased in the 0–0.2 T magnetic field range and increased in the 0.2–1 T magnetic field range. The reverse phenomena of the MFEs in Ph(BP)C₆₀ were strongly enhanced with increasing temperature and similar to those in Ph(n)C₆₀ (n = 6?12). The MFEs in Ph(BP)C₆₀ can be governed by spin-lattice relaxation and/or spin-spin relaxation mechanisms as observed in Ph(n)C₆₀ (n = 6?12). Time-resolved EPR spectra of Ph(BP)C₆₀ showed absorption, emission, absorption and emission patterns, and are quite different from those in Ph(n)C₆₀ (n = 4?12). The result indicates that the magnitude and distribution of the exchange interaction |2J| in Ph(BP)C₆₀ are smaller than those in Ph(n)C₆₀ (n = 4?12) and charge recombination occurs in the inverted region because the sign of the J is positive.     

Use of Cm and cross product coefficients in influence coefficient algorithms for quantitative XRF analysis

The use of influence coefficients in algorithms for the correction of matrix effects in quantitative XRF analysis is discussed. In principle, multi‐element influence coefficients should be used. In practice, many algorithms are based on the use of binary influence coefficients, even when dealing with multi‐component specimens. This involves approximations which may affect the accuracy of the analysis. It is shown that the effects of this approximation can be reduced, provided that the binary influence coefficients are calculated for the correct composition (this involves the use of C_m, where C_m is the sum of the concentrations of the matrix elements, i.e. C_m = 1 ? C_i) and by applying cross‐product coefficients. Copyright © 2004 John Wiley & Sons, Ltd.     

Intramolecular Diels–Alder reaction in enyne–allenes: a computational investigation and comparison with the Myers–Saito and Schmittel reactions

The reaction mechanisms as well as substituted effect and solvent effect of the enyne–allenes are investigated by Density Functional Theory (DFT) method and compared with the Myers–Saito and Schmittel reactions. The Myers–Saito reaction of non‐substituted enyne–allenes is kinetically and thermodynamically favored as compared to the Schmittel reaction; while the concerted [4 + 2] cycloaddition is only 1.32 kcal/mol higher than the C₂? C₇ cyclization and more exothermic (Δ_RE = ?69.38 kcal/mol). For R₁ = CH₃ and t‐Bu, the increasing barrier of the C₂? C₇ cyclization is higher than that for the C₂? C₆ cyclization because of the steric effect, so the increased barrier of the [4 + 2] cycloaddition is affected by such substituted electron‐releasing group. Moreover, the strong steric effect of R₁ = t‐Bu would shift the C₂? C₇ cyclization to the [4 + 2] cycloaddition. On the other hand, for R₁ = Ph, NH₂, O^?, NO₂, and CN substituents, the barrier of the C₂? C₆ cyclization would be more diminished than the C₂? C₇ cyclization due to strong mesomeric effect; the reaction path of C₂? C₇ cyclization would also shift to the [4 + 2] cycloaddition. The solvation does not lead to significant changes in the potential‐energy surface of the reaction except for the more polar surrounding solvent such as dimethyl sulfoxide (DMSO), or water. Copyright © 2009 John Wiley & Sons, Ltd.