Mid-infrared laser-absorption diagnostic for vapor-phase fuel mole fraction and liquid fuel film thickness

A novel two-wavelength mid-infrared laser-absorption diagnostic has been developed for simultaneous measurements of vapor-phase fuel mole fraction and liquid fuel film thickness. The diagnostic was demonstrated for time-resolved measurements of n-dodecane liquid films in the absence and presence of n-decane vapor at 25°C and 1 atm. Laser wavelengths were selected from FTIR measurements of the C–H stretching band of vapor n-decane and liquid n-dodecane near 3.4 μm (3000 cm⁻¹). n-Dodecane film thicknesses <20 μm were accurately measured in the absence of vapor, and simultaneous measurements of n-dodecane liquid film thickness and n-decane vapor mole fraction (300 ppm) were measured with <10% uncertainty for film thicknesses <10 μm. A potential application of the measurement technique is to provide accurate values of vapor mole fraction in combustion environments where strong absorption by liquid fuel or oil films on windows make conventional direct absorption measurements of the gas problematic.     

Electron mobility in compensated VPE GaAs films

Electron Hall mobilities were measured on a series of intentionally compensated vapor phase epitaxy (VPE) GaAs layers. Using Sn and Zn as dopants, compensation ratiosK=(N_D+N_A)/(N_D-N_A) as high as 50 were obtained. Already for samples with the lowestK values the 300 K mobilities are higher than the 77 K values. In the range 20<T<100 [K] the data may be represented by μ∼T ^α with α increasing from 0.6 to 1.1 with compensation. The experimental μ values are smaller than those predicted from current models in all cases. It appears that scattering at ionized impurities is the dominant process also at temperatures well above 77 K, and that this scattering process is quantitatively underestimated in current models.     

Small-angle light scattering studies of dense AOT-water-decane microemulsions

Summary We have performed extensive studies of a three-component microemulsion system composed of AOT-water-decane (AOT=sodium-bis-ethylhexyl-sulfosuccinate is an ionic surfactant) using small-angle light scattering (SALS). The small-angle scattering intensities are measured in the angular interval 0.001–0.1 radians, corresponding to a Bragg wave number range of 0.14 μm⁻¹<Q<<1.4 μm⁻¹. The measurements were made by changing temperature and volume fraction ϕ of the dispersed phase (water + AOT) in the range 0.05<ϕ<0.75. All samples have a fixed water-to-AOT molar ratio,w=[water]/[AOT]=40.8, in order to keep the same average droplet size in the stable one-phase region. With the SALS technique, we have been able to observe all the phase boundaries of a very complex phase diagram with a percolation line and many structural organizations within it. We observe at the percolation transition threshold, a scaling behavior of the intensity data. This behavior is a consequence of a clustering among microemulsion droplets near the percolation threshold. In addition, we describe in detail a structural transition from a droplet microemulsion to a bicontinuous one as suggested by a recent small-angle neutron scattering experiment. The loci of this transition are located several degrees above the percolation temperatures and are coincident with the maxima previously observed in shear viscosity. From the data analysis, we show that both the percolation phenomenon and this novel structural transition are derived from a large-scale aggregation between microemulsion droplets.     

Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

The diffusivityD _μ of positive muons (μ⁺) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ⁺SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsD _μ through the variation of the magnetic field felt by the μ⁺ during their diffusion through the crystals. For μ⁺ near the centres of the flux lines inNb it givesD _μ(4.6 K)=(8±2)·10⁻¹¹m²S⁻¹. The positive temperature coefficient ofD _μ indicates that at liquid-helium temperatures the diffusivity of μ⁺ inNb is mainly due to phonon-assisted tunnelling processes.     

Search for D+→φl+ν and measurement of the branching fraction for D+→φπ+

Using a data sample of integrated luminosity of about 33 pb^-1 collected around 3.773 GeV with the BESII detector at the BEPC collider, the semileptonic decays D⁺→φe⁺ν_e, D⁺→φμ⁺ν_μ and the hadronic decay D⁺→φπ⁺ are studied. The upper limits of the branching fractions are set to be BF(D⁺→φe⁺ν_e)<2.01% and BF(D⁺→φμ⁺ν_μ)<2.04% at the 90% confidence level. The ratio of the branching fractions for D⁺→φπ⁺ relative to D⁺→K^-π⁺π⁺ is measured to be 0.057±0.011±0.003. In addition, the branching fraction for D⁺→φπ⁺ is obtained to be (5.2±1.0±0.4)×10^-3.     

On the enzymatic formation of platinum nanoparticles

A dimeric hydrogenase enzyme (44.5 and 39.4 kDa sub units) was isolated in a 39.5% yield from the fungus Fusarium oxysporum and purified 4.64-fold by ion exchange chromatography on Sephacryl S-200. Characterisation of the enzyme afforded pH and temperature optima of 7.5 and 38 °C, respectively, a half-life stability of 36 min and a V _max and K _m of 3.57 nmol min⁻¹ mL⁻¹ and 2.25 mM, respectively. This enzyme was inhibited (non-competitively) by hydrogen hexachloroplatinic acid (H₂PtCl₆) at 1 or 2 mM with a K _i value of 118 μM. Incubation of the platinum salt with the pure enzyme under an atmosphere of hydrogen and optimum enzyme conditions (pH 7.5, 38 °C) afforded <10% bioreduction after 8 h while at conditions suitable for platinum nanoparticle formation (pH 9, 65 °C) over 90% reduction took place after the same length of time. Cell-free extract from the fungal isolates produced nearly 90% bioreduction of the platinum salt under both pH and temperature conditions. The bioreduction of the platinum salt by a hydrogenase enzyme takes place by a passive process and not an active one as previously understood.     

Porous silicon based β-FeSi2 and photoluminescence

Highly-pure iron powder was covered on porous silicon for fabricating semiconducting β-FeSi₂ structures. X-ray diffraction and Raman scattering results confirm the formation of pure-phase β-FeSi₂ after high-temperature annealing at 1100°C and then long-time persistence at 900°C. Scanning electron microscope observations reveal that large-size (>μm) β-FeSi₂ grains mainly form in the pores of porous silicon and some nanocrystals grow on local surfaces. The temperature-dependent photoluminescence spectra disclose that the observed ∼1.54 μm emission arises from free exciton recombination, which is confirmed via the activation energy (0.25 eV) measurement. Our method provides a way to synthesize single-phase β-FeSi₂ materials.     

μ + Knight shift and spin relaxation in indium single crystalKnight shift and spin relaxation in indium single crystal

μ ⁺ SR measurements have been performed in a single crystal indium sample between 12 K and 300 K with a stroboscopic μSR spectrometer. The muonic Knight shiftK _μ and the muonic depolarization rate σ were obtained for various angles θ between the tetragonal crystallinec-axis and the direction of the external field. The isotropic part ofK _μ is only weakly temperature dependent and is consistent with the estimated Pauli spin susceptibility value. At a temperature of 12 K the angular dependence ofM ₂ (the second moment of the field distribution at the muon, obtained from the measured σ(θ) values) allows a clear determination of the muon location — the symmetric tetrahedral site. The observed anisotropicK _μ cannot be explained by the dipoles at the In atoms responsible for the bulk magnetic susceptibility but probably originates from an anisotropic Pauli spin susceptibility.     

Sensitive detection of temperature behind reflected shock waves using wavelength modulation spectroscopy of CO2 near 2.7?μm

Tunable diode-laser absorption of CO₂ near 2.7 μm incorporating wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) is used to provide a new sensor for sensitive and accurate measurement of the temperature behind reflected shock waves in a shock-tube. The temperature is inferred from the ratio of 2f signals for two selected absorption transitions, at 3633.08 and 3645.56 cm⁻¹, belonging to the ν ₁+ν ₃ combination vibrational band of CO₂ near 2.7 μm. The modulation depths of 0.078 and 0.063 cm⁻¹ are optimized for the target conditions of the shock-heated gases (P∼1–2 atm, T∼800–1600 K). The sensor is designed to achieve a high sensitivity to the temperature and a low sensitivity to cold boundary-layer effects and any changes in gas pressure or composition. The fixed-wavelength WMS-2f sensor is tested for temperature and CO₂ concentration measurements in a heated static cell (600–1200 K) and in non-reactive shock-tube experiments (900–1700 K) using CO₂–Ar mixtures. The relatively large CO₂ absorption strength near 2.7 μm and the use of a WMS-2f strategy minimizes noise and enables measurements with lower concentration, higher accuracy, better sensitivity and improved signal-to-noise ratio (SNR) relative to earlier work, using transitions in the 1.5 and 2.0 μm CO₂ combination bands. The standard deviation of the measured temperature histories behind reflected shock waves is less than 0.5%. The temperature sensor is also demonstrated in reactive shock-tube experiments of n-heptane oxidation. Seeding of relatively inert CO₂ in the initial fuel-oxidizer mixture is utilized to enable measurements of the pre-ignition temperature profiles. To our knowledge, this work represents the first application of wavelength modulation spectroscopy to this new class of diode lasers near 2.7 μm.     

K<Subscript><Emphasis Type="Italic">α</Emphasis></Subscript> x-ray emission characterization of 100 Hz, 15 mJ femtosecond laser system with high contrast ratio

We report K _α x-ray production with a high energy (110 mJ per pulse at 800 nm before compression/15 mJ at 400 nm after compression), high repetition rate (100 Hz), and high pulse contrast (better than 10⁻⁹ at 400 nm) laser system. To develop laser-based x-ray sources for biomedical imaging requires to use high-energy and high-power ultrafast laser system where compression is achieved under vacuum. Using this type of laser system, we demonstrate long-term stability of the x-ray yield, conversion efficiency higher than 1.5×10⁻⁵ with a Mo target, and the x-ray spot size close to the optical focal spot. This high-repetition K _α x-ray source can be very useful for x-ray phase-contrast imaging. S. Fourmaux and C. Serbanescu contributed equally to this work.     

Muonic Molecule Formation in Condensed Deuterium

Resonant formation of the muonic molecule ddμ in dμ atom scattering in condensed deuterium is considered. In particular, ddμ formation in D₂ solid targets containing different ortho-D₂ concentration is discussed, and the respective time spectra of the dd fusion products are shown. The results of the first calculation of the resonant ddμ formation rate in liquid deuterium are presented. At large momentum transfers the ddμ formation rate takes the Doppler form, similar to that obtained for a dilute gas target. A condition of validity of this approximation is also discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Persistent spin splitting of a two-dimensional electron gas in tilted magnetic fields

The effect of atomic disorder on the electron transport and the magnetoresistance (MR) of Co₂CrAl Heusler alloy (HA) films has been investigated. We show that Co₂CrAl films with L2₁ order exhibit a negative value for the temperature coefficient of resistivity (TCR) in a temperature range of 10 < T < 290 K, and the temperature dependence of electric conductivity varies as T ^3/2 similarly to that of the zero-gap semiconductors. The atomic or the site disorder on the way of L2₁ → B2 → A2 → amorphous state in Co₂CrAl HA films causes the deviation from this dependence: reduction in the absolute value of TCR as well as decrease in the resistivity down to ϱ(T = 293 K) ∼ 200 μΩ cm in comparison to ϱ(T = 293 K) ∼ 230 μΩ cm typical for the Co₂CrAl films with L2₁ order. The magnetic-field dependence of MR of the Co₂CrAl films with L2₁ order is determined by two competing contributions: a positive Lorentz scattering and a negative s-d scattering. The atomic disorder in Co₂CrAl films drastically changes MR behavior due to its strong influence on the magnetic properties.     

Wavelet analysis of learning and forgetting of photostimulation rhythms for a nonstationary electroencephalogram

Quantitative parameters characterizing transient processes of mastering and forgetting of photostimulation (PST) rhythms for a nonstationary electroencephalogram (EEG) are developed on the basis of a continuous wavelet transformation. Nonstationarity factor K _nst(μ), as well as rhythm mastering K _M (μ) and confinement K _C (μ) factors are calculated for various spectral ranges μ. Photoflash mastering time τ _M = τ _S + τ _I , which is the sum of latent silence period τ _S after PST actuation and the rhythm increasing period τ _I is calculated. In the case of PST, the EEG rhythm retardation time τ _R relative to the beginning of PST is calculated. Rhythm forgetting time τ _F = τ _P + τ _D after PST actuation is the sum of the preservation time τ _P of the corresponding rhythm over a certain time interval and its decay period τ _D . The lag time τ _L of the EEG signal relative to the PST signal after its removal is determined. The proposed method is used in quantitative analysis and classification of transient processes characterizing the properties of the central nervous system. Possible applications of the method in analysis of various nonstationary signals in physics are discussed.     

Non-adiabatic small-polaron hopping conduction in VN–PbO–TeO2 glasses

The dc conductivity of VN–PbO–TeO₂ glasses with different mole percentages of VN, PbO and TeO₂ has been measured in the temperature range 125–450?K. The conductivity of the glasses increases with increasing VN content for a fixed mole percentage of PbO. Neither Mott's variable-range hopping (VRH) model at low temperatures (T<Θ_D/4, where Θ_D is the Debye temperature) nor Greaves’ VRH model at intermediate temperatures (Θ_D/?4<T<Θ_D/2) describe the dc conductivity data for these glasses. Multiphonon tunnelling transport of strongly coupled electrons is also unable to account for the carrier transport. However, at high temperatures (T?>?Θ_D/2), conduction is shown to be due to small-polaron hopping in the non-adiabatic regime. Alteration of the VN content causes a change in the model parameters achieved from best-fitting curves for the glasses. Modulated differential scanning calorimetry analysis shows that the glass transition temperatures T _g in this system vary from 269 to 302°C.     

Spatiotemporal spectroscopic diagnostics of a pulse-periodic strontium vapor laser

Results of spectroscopic investigations into plasma of a pulse-periodic strontium vapor laser operating in the superradiance mode on the infrared transition at λ = 6.45 μm are presented. The method of determining the electron temperature and concentration as well as the gas temperature – T _e , n _e , and T _g – based on measuring the absolute intensities of some SrI and SrII and buffer gas (helium or neon) spectral lines is used. Time dependences of the line intensities during a current pulse (τ = 150 ns) and near afterglow (up to 3 μs) are obtained under conditions of non-equilibrium plasma ionization and recombination. The optical system collects radiation from the entire length of the plasma column by means of separating radial volume zones, includingthe central zone and the zone closer to the walls, with the monochromator slit. The results obtained allow us not only to calculate T _e , n _e , and T _g values, but also to trace the spatiotemporal plasma evolution.     

Laser light scattering technique for non-invasive<Emphasis Type="Italic">in situ</Emphasis> simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals

The laser light scattering technique for non-invasivein situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals is introduced. By measuring the autocorrelation function of the scattered light from nematic liquid crystals at different scattering angles, the splay and twist elastic constantsK ₁₁ andK ₂₂ are obtained from the amplitudes of the autocorrelation function, and the viscosity coefficients ofη _Splay andη _Twist are determined using the viscoelastic ratiosK ₁₁/η _Splay andK ₂₂ η _Twist from the telaxation parameters of the two modes.     

Electro- and magnetotransport in La0.67Ba0.33MnO3 nanosized films coherently grown on a vicinally polished (LaAlO3)0.29 + (SrAl0.5Ta0.5O3)0.71 substrate

The structure, orientation, and the response of electroresistance to magnetic field H and varying temperature T have been studied for 30-nm-thick La_0.67Ba_0.33MnO₃ (LBMO) films. The deviation of the [001] direction in manganite layers from the normal to the plane of the (LaAlO₃)_0.29 + (SrAl_0.5Ta_0.5O₃)_0.71 substrate strictly corresponds to the vicinal angle of the latter. The minimum yield determined from 227-keV proton scattering spectra is 0.025, signifying a high order of the cationic sublattice in the films. The biaxial compression of stable nuclei of the manganite phase affects their stoichiometry, thus contributing to the depletion of LBMO films in the alkaline-earth element. The maximum electroresistance values have been observed in the films grown at T _max ≈ 320 K, a temperature about 20 K lower than the Curie temperature of the corresponding bulk single crystals, and the maximum magnetoresistance (MR ≈ −0.42, μ₀ H = 2 T) occurs at T ≈ 300 K. At low temperatures (T < T _max/3) and μ₀ H < 0.45 T, the electroresistance response of LBMO films to a magnetic field materially depends on the anisotropic magnetoresistance and the intensity of hole scattering from domain walls; when μ₀ H > 0.5 T, the major current-carrier relaxation mechanism is the interaction with magnons.     

Pulsed laser ablation and deposition of silicon

A KrF laser was used to ablate a polycrystalline Si target for deposition of Si on MgO and GaAs substrates at room temperature. The deposition was performed in 10⁻⁸ mbar, with two types of laser beams: a homogeneous beam being imaged onto the target (2.9 J/cm²), and a non-homogeneous which is nearly focused (2 J/cm², 6.5 J/cm²). In both cases, the beam was scanned over an area of 1 cm². For the homogenous beam, we observed only a limited number of droplets (<0.1 μm). A high number of micron-sized (<5 μm) droplets were observed on the film by the higher fluence nonhomogeneous laser beam. Raman spectroscopy showed that the micron-sized droplets are crystalline while the film is amorphous. The generation of the large droplets is most likely related to the cone structures formed on the ablated target. We also compared cone formation on a polycrystalline Si target and a single crystalline Si wafer, using multiple laser pulses onto a single spot.     

One-pot fabrication and enhanced thermoelectric properties of poly(3,4-ethylenedioxythiophene)-Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanocomposites

Bi₂S₃ nanotubes and de-doped poly(3,4-ethylenedioxythiophene) (PEDOT) composite nanopowders were synchronously synthesized by a one-pot self-assembly method. The powders were characterized by X-ray powder diffraction, infrared spectroscopy, and transmission electron microscopy, respectively. Thermoelectric properties of the Bi₂S₃–PEDOT composite nanopowders with different Bi₂S₃ contents after being cold pressed into pellets were measured at room temperature. The sample with 36.1 wt% Bi₂S₃ has a highest power factor of 2.3 μWm⁻¹K⁻², which is higher than that of both pure PEDOT (0.445 μWm⁻¹K⁻²) and Bi₂S₃ (1.94 μWm⁻¹K⁻²).