On the effect of the width of the scattering indicatrix on the dispersion of photon density waves in a strongly scattering medium

The dispersion of photon density waves in strongly scattering media with different widths of the scattering indicatrix is studied by the spherical harmonics method using approximations of various orders (up to the P ₇ approximation inclusive). It is shown that, beginning from the P ₃ approximation, the reduction in the velocity of photon density waves that is characteristic of the P ₁ approximation is eliminated and, independently of the width of the scattering indicatrix in the region of modulation frequencies exceeding 10¹⁰ Hz, the velocity of photon density waves asymptotically approaches the speed of light. Our study of the damping of photon density waves has shown that the formula obtained previously for the calculation of the damping coefficient (Imk(μ _s ^′ , ω)) as a function of the transport scattering coefficient and the velocity is valid at Imk ≤ μ_s (μ_s is the light scattering coefficient). The maximum growth in the damping coefficient of photon density waves with a further increase in the frequency is limited by the value of the light scattering coefficient Imk _max ≈ μ_s.     

Front shock behavior of stable curved detonation waves in rectangular-cross-section curved channels

The propagation of curved detonation waves of gaseous explosives stabilized in rectangular-cross-section curved channels is investigated. Three types of stoichiometric test gases, C₂H₄ + 3O₂, 2H₂ + O₂, and 2C₂H₂ + 5O₂ + 7Ar, are evaluated. The ratio of the inner radius of the curved channel (r_i) to the normal detonation cell width (λ) is an important factor in stabilizing curved detonation waves. The lower boundary of stabilization is around r_i/λ = 23, regardless of the test gas. The stabilized curved detonation waves eventually attain a specific curved shape as they propagate through the curved channels. The specific curved shapes of stabilized curved detonation waves are approximately formulated, and the normal detonation velocity (D_n)?curvature (κ) relations are evaluated. The D_n nondimensionalized by the Chapman–Jouguet (CJ) detonation velocity (D_CJ) is a function of the κ nondimensionalized by λ. The D_n/D_CJ?λκ relation does not depend on the type of test gas. The propagation behavior of the stabilized curved detonation waves is controlled by the D_n/D_CJ?λκ relation. Due to this propagation characteristic, the fully-developed, stabilized curved detonation waves propagate through the curved channels while maintaining a specific curved shape with a constant angular velocity. Self-similarity is seen in the front shock shapes of the stabilized curved detonation waves with the same r_i/λ, regardless of the curved channel and test gas.     

X-ray investigation of the mean square thermal displacements in VDx, NbHx and TaHx interstitial alloys

X-Ray measurements of the integrated Bragg intensities from V, Nb and Ta single crystals as a function of hydrogen (D) concentration and temperature have been carried out. Two different methods were applied: (i) the usual angular dispersive method using MoK_α1 characteristic radiation; and (ii) the energy dispersive method using the white spectrum of the Mo tube and an intrinsic Ge detector. From the results the thermal Debye-Waller factor for the three metals and its change with hydrogen concentration are determined. The mean square of the thermal displacements (u²) decreases with hydrogen (D) concentration. For NbH_x(D_x) this agrees well with (u²) values as determined from measured phonon density of states spectra. The results are also given in terms of a Debye temperature depending on H(D) concentration.     

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH _x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ～ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH _x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH₂. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH₂ are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH₂. To attain high diffusion rates, the structures of Mg₅₃Sc and Mg₅₃Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg₅₃ScH₁₀₈ near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH₂, while the frequency of jumps in Mg₅₃TiH₁₀₈ near Ti atoms is higher by approximately a factor of 2.5 × 10⁶. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.     

Visible-light photocatalytic activity of nitrogen-doped TiO2 thin film prepared by pulsed laser deposition

Nitrogen-doped titanium dioxide (TiO_2−xN_x) thin films have been prepared by pulse laser deposition on quartz glass substrates by ablated titanium dioxide (rutile) target in nitrogen atmosphere. The x value (nitrogen concentration) is 0.567 as determined by X-ray photoelectron spectroscopy measurements. UV-vis spectroscopy measurements revealed two characteristic deep levels located at 1.0 and 2.5 eV below the conduction band. The 1.0 eV level is attributable to the O vacancy state and the 2.5 eV level is introduced by N doping, which contributes to narrowing the band-gap by mixing with the O2p valence band. The enhanced degradation efficiency in a broad visible-light range was observed from the degradation of methylene blue and methylene orange by the TiO_2−xN_x film.     

Crystallization and melting of spin-polarized<Superscript>3</Superscript>He

The melting and growth of³He crystals, spin-polarized by an external magnetic field, are different in nature depending on whether the temperature is higher or lower than the characteristic ordering temperatures in the crystal (the Neel temperatureT _N ) and in the liquid (the superfluid transition temperatureT _c ). In the high-temperature region (T≥T _N ,T _c ) the liquid which appears upon melting has a high nonequilibrium spin density. In the low-temperature region (T?T _N ,T _c ) the melting and growth are accompanied by spin supercurrents both in the liquid and in the crystal in addition to mass supercurrents in the liquid. The crystallization waves at the liquid-solid interface should exist in the low-temperature region. With increasing magnetic field the waves change in nature, because the spin currents begin to play a dominant role. The wave spectrum becomes linear with a velocity inversely proportional to the magnetic field. The attenuation of the waves at low enough temperatures is mainly due to the interaction of the moving crystal-liquid interface with thermal spin waves in the crystal. The waves could be weakly damped at temperatures below a few hundreds microkelvins.     

Application of PIV to over-expanded supersonic flows: Possibilities and limits

Two-dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using a digital particle im age velocimetry (PIV). Mean velocities , vor ticity field and volume dilatation field were obtained from PIV images using 0 .33 μm titanium dioxide (TiO₂) particle. The seeding particle of larger size , 1.4 μrn Ti0₂, was also used for the experimental comparison of velocity lag downstream of shock waves. The results have been compared and analyzed with schlieren photographs for the locations of shock waves and over-expanded shock structure to inspect possibilities and limits of a PIV technique to over-expanded supersonic flows. It is found that although the quantitative velocity measurement using PIV on over-expanded supersonic flows with large velocity and pressure gradients is limited, the locations of normal shock and oblique shock waves can be resolved by the axial/radial velocity fields, and over-expanded shock structure can be predicted by vorticity field and volume dilatation field which are acquired from the spatial differential of the velocity field.     

Pure and Nb-doped TiO1.5 films grown by pulsed-laser deposition for transparent p-n homojunctions

Pure and Nb-doped titanium oxide thin films were grown on sapphire substrates by pulsed-laser deposition in vacuum (10⁻⁷ mbar). The PLD growth leads to titanium oxide thin films displaying a high oxygen deficiency (TiO_1.5) compared with the stoichiometric TiO₂ compound. The structural and electrical properties (phase, crystalline orientation, nature and concentration of charge carriers, etc.) of these titanium oxide films were studied by XRD measurements and Hall effect experiments, respectively. The undoped TiO_1.5 phase displayed a p-type semiconductivity. Doping this titanium oxide phase with Nb⁵⁺ leads to an n-type behaviour as is generally observed for titanium oxide films with oxygen deficiency (TiO_x with 1.7 < x < 2). Multilayer homojunctions were obtained by the stacking of TiO_1.5 (p-type) and Nb-TiO_1.5 (n-type) thin films deposited onto sapphire substrates. Each layer is 75 nm thick and the resulting heterostructure shows a good transparency in the visible range. Finally, the I-V curves obtained for such systems exhibit a rectifying response and demonstrate that it is possible to fabricate p-n homojunctions based only on transparent conductive oxide thin films and on a single chemical compound (TiO_x).     

Structural phase transitions in semiconducting SrTiO3

From ultrasonic velocity measurements we found that in SrTiO₃ the temperature for the structural phase transition is lowered in samples reduced in hydrogen, and increased in Nb-doped samples. In the reduced samples the shift in T_c is proportional to the concentration of oxygen vacancies. The results can qualitatively be understood in terms of local changes in the d_?/p_σ-hybridization of Ti-3d and 0–2p orbitals.     

The effect of concentration of <Emphasis Type="Bold">H</Emphasis> <Subscript> <Emphasis Type="Bold">2</Emphasis> </Subscript> physisorption on the current–voltage characteristic of armchair BN nanotubes in CNT–BNNT–CNT set

In this research, we have studied physisorption of hydrogen molecules on armchair boron nitride (BN) nanotube (3,3) using density functional methods and its effect on the current–voltage (I–V) characteristic of the nanotube as a function of concentration using Green’s function techniques. The adsorption geometries and energies, charge transfer and electron transport are calculated. It is found that H₂ physisorption can suppress the I–V characteristic of the BN nanotube, but it has no effect on the band gap of the nanotube. As the H₂ concentration increases, under the same applied bias voltage, the current through the BN nanotube first increases and then begins to decline. The current–voltage characteristic indicates that H₂ molecules can be detected by a BN-based sensor.     

General Schrödinger equation for whistler waves propagating along a magnetic field

The general Schrödinger equation (GSE) for whistler waves with their group velocity directed along an external magnetic field is derived. The “mean” wave vector of the wave beam may be parallel to or have an angle Θ = arccos(2ω/ω_c) with the magnetic field. Applications of GSE to the whistler propagation in density ducts are considered. The results are important for the problem of the self-focusing of whistler waves.     

Acoustic properties of ZrS3−xSex compounds

In the anisotropic ZrSe_3?xSe_x compounds, the velocity of acoustic waves propagating in a direction parallel to the fibers decreases linearly as the Selenium content is progressively increased for 0 < x < 2; in this region of composition two acoustic waves with different velocities are observed. For x > 2, following an increase of velocity, only one acoustic wave is likely to be propagated with a nearly constant velocity. This behavior is tentatively explained by a possible structural transformation taking place at x ～ 2.     

Soft X-ray absorption spectroscopy of titanium dioxide nanopowders with cobalt impurities

The charge states of the cobalt ions in TiO₂ nanopowders with the anatase lattice are studied by soft X-ray absorption spectroscopy. It is found that, at a low cobalt impurity concentration (1.8 at %), the cobalt ions with an oxidation state 2+ are mainly located in the tetrahedral (T _d ) environment of oxygen ions. Amorphous titanium dioxide exists on the sample surface before heat treatment. Annealing in vacuum or hydrogen leads to the enrichment of the nanoparticle surfaces with Co²⁺ ions, a change in the coordination of the remaining part of cobalt ions from octahedral to tetrahedral, stabilization of the anatase structure, and the disappearance of the amorphous phase. The crystal lattice of the samples with a relatively high cobalt concentration (12 at %) is distorted, and annealing does not cause the disappearance of the amorphous phase of TiO₂. Cobalt is reduced to its metallic state upon hydrogen annealing of the samples with a high cobalt concentration.     

Two- and three-dimensional waves in falling film flow in the nonlaminar flow regime: an NMR study

Nuclear magnetic resonance (NMR) velocity-encoded imaging results as well as propagators are presented for the nonlaminar flow regime of falling films. The film is generated by a continuous flow of silicon oil along a vertical poly(methyl methacrylate) plate. While the film remains purely laminar for a film Reynolds number Re_f=0.5, it exhibits laminar-wavy behavior for 1.0≤Re_f≤2.5. In this range, a laminar residual film can be distinguished from averaged waves near the surface of the film from measurements of the flow velocity ν₂ along the direction of gravity as a function of the coordinatex normal to the plate. The perpendicular velocity components, ν _x and ν _y are zero within the accuracy of the measurement, indicating that the wave motion is two-dimensional in the laminar-wavy case. For higher Re_f the waves are found to be three-dimensional, a straightforward division into a residual film and waves in ν _z is lost and the waves extend over the whole thickness of the film.     

The influence of molecular hydrogen on the conductivity of magnesium oxide containing paramagnetic surface defects

MgO with a high concentration of defects was produced by the dehydration of high-purity Mg(OH)₂ and the conductivity was studied in the temperature range 20–250°C. An atmosphere of hydrogen increased the conductivity by up to a factor of 10³ and produced a characteristic conductivity maximum when the sample was heated. The hydrogen interacts with the sample at V¹_i-centers which are located at the surface of the dehydrated material. These centers are supposed to have a catalytic activity, i.e. they decompose hydrogen molecules into atoms and further into electrons and protons which contribute to the current.     

Refinement of the structure of hydrogen–vacancy complexes in titanium by the Rietveld method

The VT1-0 titanium alloy (phase α-Ti) with various hydrogen and hydrogen-vacancy concentrations has been studied. The stability of the 32-atom Ti–nV–mH supercell (n is the number of the V vacancies, and m is the number of hydrogen atoms H) with varying numbers of vacancies and hydrogen atoms has been calculated from the first principles. The structural state of the α-Ti phase has been identified by the Rietveld method based on the calculations of the supercell stability and the data on the defect concentration obtained using positron spectroscopy. The complete structural information on the considered states of the α-Ti phase (the lattice parameters, spatial distribution of titanium and hydrogen atoms and vacancies) has been obtained.     

Microwave heating of electrons of a dense plasma column at frequencies higher than electron cyclotron frequency

In the overdense collisionless plasma column inserted through the narrow sides of a rectangular waveguide, the excited electron cyclotron harmonic waves (CHWs) are studied by means of two movable probes and a phase interferometer in the range of parameters of 2 >ω/ω _ce >1; 0·5 < (ω _pe /ω)² < 15. Two kinds of CH waves have been found in the dipole mode:

1. Stable backward CH waves with the phase velocity in the direction from the axis to the periphery.
2. Unstable backward CH waves with the phase velocity in the opposite direction.

From the correlation measurements and amplitude distribution of the CH waves at the applied frequency and its second harmonic frequency it has been found that in the range of 2 >ω/ω _ce > >1·5 efficient nonlinear resonant interaction of CH waves takes place. The position of the loci of resonant interaction inside the plasma column has been found both experimentally and by a simple new graphical procedure for the resonant conditions of the formω ₂=2ω ₁; k₂=2¦k₁¦. In absence of this condition, no effective generation of the second harmonic frequency has been found. The resonant interaction of CHWs atω/ω _ce =1·85 is the cause of self-trapping of CH waves between the zones of resonant interaction in radial direction and of the anomalous heating rate of electrons.     

Low temperature thermal properties of PbI2

The heat capacity and thermal conductivity of a large (56.5 g) crystal of PbI₂ have been measured in the temperature region 0.5 < T < 3.9°K. Analysis of the heat capacity data yields a value of the limiting apparent Debye characteristic temperature θ₀^c = 99.4 (±0.3)°K, which corresponds to an average lattice wave velocity of 1.151 (±0.005) × 10⁵ cm sec^?1. It is consistent with a wave velocity estimated from neutron scattering experiments, but not with one determined from Brillouin spectra. The heat capacity data also show that dispersion of the low frequency waves is not unusual, as might have been expected for a layer-type crystal. The apparent thermal conduction is found to be surprisingly small in the crystal.     

Hydrogen behavior in Pd0.75Ag0.25H x studied by PAC and PAS

Hydrogen behavior in Pd_0.75Ag_0.25H _x has been investigated by both perturbed angular correlation (PAC) and positron annihilation lifetime spectroscopy (PAS) techniques. The PAC measurements with the ¹¹¹Ag/¹¹¹Cd probe nuclei were conducted at room temperature as a function of the hydrogen concentration x up to 0.35. At x = 0 the probe nuclei experienced no quadrupole perturbation, while a distinct perturbation was observed after hydrogen charging. The quadrupole interaction frequency ω ₁ and its distribution width σ ₁ and relative fraction f₁ all decrease with increasing hydrogen concentration, indicating size growth and concentration reduction of the hydrogen induced vacancy clusters. The measured frequency distribution width σ ₁ reveals hydrogen diffusion. The PAS measurements were also performed at room temperature. The obtained lifetimes and their variation with hydrogen concentration strongly support the PAC results. The experimental results can be explained by the appearance of the β-phase in the Pd_0.75Ag_0.25H _x samples. The α-phase is the major phase in the Pd_0.75Ag_0.25H _x samples. Charging of the hydrogen leads to the formation of the β-phase and the fraction of the β-phase increases with increasing hydrogen concentration. The present PAC and PAS results show that as the fraction of the β-phase increases, hydrogen induced vacancy clusters are formed and become larger and larger, while the cluster density reduces due to the combination of the vacancy clusters into larger size vacancy clusters.