Solute distribution in KNbO3 melt-solution and its effect on dendrite growth during rapid solidification

This paper reports that the rapid solidification of mixed Li 2 B 4 O 7 and KNbO 3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO 3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO 3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions: in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.     

相场晶体法模拟过冷熔体中的晶体生长

利用相场晶体(phase-field crystal)模型,采用有限差分法,模拟了过冷熔体中晶体生长过程,研究了不同相区、不同过冷度对晶体生长过程的影响.结果表明,在共存区中,随着演化的进行,晶体生长被抑制,液-晶边界层逐渐变厚;在单相区中,随着过冷度的增大,晶态区面积显著增加,液-晶边界层逐渐变薄.晶体生长速度和过冷度成近似线性的关系. 关键词： 相场晶体 自由能函数 过冷熔体 晶体生长     

Nonlinear aspects of the Cahn-Hilliard equation

This paper treats phase separation within the context of the phenomenological Cahn-Hilliard equation, c_t = · [M(c)(∂f/∂c - K²c)], where c is the concentration, M(c) is the mobility, and f(c) is homogeneou s free energy, which is assumed here to be a fourth degree polynomial. Natural boundary conditions are introduced. The full set of equilibrium solutions is specified. A comparison theorem for stability criteria which was postulated by Langer is proved here within the framework of the natural boundary conditions. Energy methods are used to define and estimate the limit of monotonic global stability. It is pointed out that within the parameter region where the uniform homogeneous state is the only equilibrium solution, there may still exist some internal “excitable” region in which the homogeneous solution possesses growing fluctuations. Furthermore a periodic instability is shown to exist in the metastable region in addition to the well-known nucleation instability.     

Interference effect in the photoproduction of electron-positron pairs on a nucleus in the field of two light waves

In the general relativistic case, the nonresonance photoproduction of electron-positron pairs on a nucleus in the field of two circularly polarized light waves propagating in one direction is theoretically investigated. It is shown that there are two essentially different kinematical regions: the noninterference region, where the Bunkin-Fedorov quantum parameters play the role of multiphoton parameters, and the interference region, where interference effects are important and where quantum interference parameters are treated as multiphoton parameters. The interference effect is found in the photoproduction of an electron-positron pair on a nucleus in the field of two light waves. This effect occurs in the interference region and is due to the production of an electron-positron pair in the plane spanned by the light wave vector and the incident-photon momentum and to stimulated correlated emission and absorption of photons of the two waves. The cross sections for pair photoproduction on a nucleus in the above kinematical regions are determined for moderately strong fields. It is shown that, in the interference region, the partial cross sections for the case where there is a correlated emission (absorption) of the same number of photons of the two waves can significantly exceed the corresponding cross sections in any other geometry.     

Dynamics of entanglement in quantum computers with imperfections

The dynamics of the pairwise entanglement in a qubit lattice in the presence of static imperfections exhibits different regimes. We show that there is a transition from a perturbative region, where the entanglement is stable against imperfections, to the ergodic regime, in which a pair of qubits becomes entangled with the rest of the lattice and the pairwise entanglement drops to zero. The transition is almost independent of the size of the quantum computer. We consider both the case of an initial maximally entangled and separable state. In this last case there is a broad crossover region in which the computer imperfections can be used to create a significant amount of pairwise entanglement.     

Radon measurements over a natural-gas contaminated aquifer

Radon and thoron concentrations in soil pores in a gas production region of the Anzoategui State, Venezuela, were determined by active and passive methods. In this region, water wells are contaminated by natural gas and gas leaks exist in the nearby river. Based on soil gas Radon data surface hydrocarbon seeps were identified. Radon and thoron concentration maps show anomalously high values near the river gas leaks decreasing in the direction of water wells where natural gas is also detected. The area where the highest concentrations of ²²²Rn were detected seems to indicate the surface projection of the aquifer contaminated with natural gas. The Radon/Thoron ratio revealed a micro-localized anomaly, indicating the area where the gas comes from deep layers of the subsoil. The radon map determined by the passive method showed a marked positive anomaly around abandoned gas wells. The high anomalous Radon concentration localized near the trails of ascending gas bubbles at the river indicates the zone trough where natural gases are ascending with greater ease, associated with a deep geological fault, being this the main source of methane penetration into the aquifer. It is suggested that the source of the natural gas may be due to leaks at deep sites along the structure of some of the abandoned wells located at the North-East of the studied area.     

Polariton reflectance spectra from thin ZnSxSe1−x layers

A study of reflectance spectra from thin ZnS_xSe_1−x solid-solution layers in the region of excitonic resonances is reported. It has been found that an increase in sulfur concentration in the layers increases the inhomogeneous broadening of the quantized polariton lines. It has been established that the inhomogeneous line broadening in a reflectance spectrum depends on the magnitude of exciton-photon mixing; it is small in the long-wavelength region where the photon component of the polariton is large, and large at short wavelengths where the mechanical component dominates. Fiz. Tverd. Tela (St. Petersburg) 40, 867–868 (May 1998)     

Dust grains in plasma with coulomb collisions

Dust-grain charging in a dense plasma with electron and ion concentration up to 10¹⁶cm^?3, where the electron transport cross section is dominated by Coulomb collisions, was investigated. It was established that the charge of an isolated dust grain increases with electron concentration, whereas its potential is almost constant in the range 10¹⁴–10¹⁶cm^?3 of electron concentrations studied. It is shown that the self-consistent electric-field potential at moderate and large distances from the dust grain is well approximated by the Debye expression, but the screening radius is appreciably larger than even the electron Debye radius. The region of parameters of a dense photoresonance sodium plasma, where the dust-grain ensemble may crystallize, was established.     

Power-law slip profile of the moving contact line in two-phase immiscible flows

Large-scale molecular dynamics (MD) simulations on two-phase immiscible flows show that, associated with the moving contact line, there is a very large 1/x partial-slip region where x denotes the distance from the contact line. This power-law partial-slip region is verified in large-scale adaptive continuum calculations based on a local, continuum hydrodynamic formulation, which has proved successful in reproducing MD results at the nanoscale. Both MD simulations and numerical solutions of continuum equations indicate the existence of a universal slip profile in the Stokes-flow regime.     

Nanoindentation properties and the microstructure of grain boundary precipitate-free zones (PFZs) in an AlCuSiGe alloy

We have characterized the nanoscale mechanical properties of grain boundary precipitate-free zones (PFZ's) in an AlCuSiGe alloy, using combined nanoindentation and in-situ atomic force microscopy (AFM). These mechanical properties were then correlated to the composition, precipitate distribution and, indirectly, to the vacancy concentration within these regions, as analyzed by transmission electron microscopy and spectroscopy. Using these results we constructed a structure-zone map of the area adjacent to the grain boundary, which relates the reduced elastic modulus and nanoindentation hardness of the alloy to its graded microstructure. Our analysis indicates that the lowest hardness was found in the region where no precipitates are present at all, regardless of solute concentration. In regions where precipitation is different from that of the bulk, somewhat inferior mechanical properties are achieved.     

NMR and magnetization study of the mixed systems (Pd1-xCux)2 MnIn and (Pd1-xNix)2 MnIn

Systematic NMR and magnetization data with concentration show that for decreasing x these systems go from ferromagnetic (F) to anti-ferromagnetic (AF) order by passing a mixed phase region where both magnetic orders coexist in different domains. The magnetic phase diagrams for both alloy series are nearly coincident. The NMR spectra show that the Cu or Ni atoms substitute randomly onto Pd sites and we find no evidence that the participation of Mn atoms in F or AF domains is at all correlated with their nearest neighbour environments. In the limit where the AF order becomes largely dominant the samples exhibit a magnetic anisotropy after field cooling.     

Electron transport in dirty multi-channel systems

We study the temperature dependence of the conductance and of the conductivity of a doped two-chain ladder system in the presence of a barrier or of a low impurity concentration, respectively, focusing on the effects of electronelectron interaction. Like in the purely 1-D case, the conductance vanishes at low temperature when the Luttinger-liquid exponent Kρ+ (for even-charge modes) is smaller than unity, despite the presence of dominant superconducting correlations. However, there is a region of repulsive interaction where perfect transmission across the barrier occurs. This is due to a renormalization of Kρ+ produced by the difference in Fermi velocities of the bonding and antibonding bands.     

Excitation spectrum and phase structure of the chiral Potts model

The spectrum of low lying excitations of the integrable chiral Potts model is computed. It is shown that there is a region in the parameter space where the excitation energies become negative thus indicating that a level crossing transition to a new incommensurate ground state has occurred.     

精密表面疵病检测美标数字化评价实现算法

采用美国军用标准MIL-PRF-13830B（简称美标）对光学元件表面疵病进行数字化评价,其难点主要在于美标中表面疵病密集度的判定。针对该难点,提出了一种表面疵病密集度判定算法。该算法采用权重域叠加的方法,通过为表面疵病赋予相应的权重域来确定疵病密集圆域,并将权重域间的叠加转换成了矩阵间位置关系的判断和计算,有效的利用了矩阵运算的便利性。实验中通过对已知表面疵病信息的石英标准板进行判定,其得到的圆域即为疪病密集圆域,与预期结果一致,验证了该算法的正确性。目前该算法已经运用于光学车间精密光学元件表面疵病的美标数字化评价。     

Method for solving the nonlinear diffusion equation

We discuss the properties of the nonlinear diffusion equation which was presented for the approximation of high-concentration arsenic and boron profiles. Typical features of the profile are a flat region near the surface, a decreasing intermediate region and a tail region dropping sharply down compared with an error function complement. Earlier the nonlinear equation of the diffusion where the diffusion coefficient is directly proportional to the concentration of the impurities has been proposed. This proposal has been made on the theoretical assumption that diffusion is the result of Brownian movement and must occur with a finite velocity.     

Pomor Polymath: The Upbringing of Mikhail Vasilyevich Lomonosov, 1711–1730

The life story of Mikhail Vasilyevich Lomonosov (1711–1765) opens a window onto Russian science, politics, language, and social advancement in the era of Peter the Great (1672–1725). We cover Lomonosov’s background and upbringing, from his birth in 1711 near Kholmogory until his departure for Moscow on foot in 1730. The special character of the Pomor region, in Russia’s north, where Lomonosov was born and raised, is important for understanding his character, upbringing, and subsequent career trajectory. This character sprang from four overlapping factors: the isolation of the region, the political and religious tolerance that mainly prevailed there, the trade that brought the region into contact with foreigners, and the hardy lifestyle.     

In and Si distribution in synthetic opals

An X-ray photoelectric absorption by samples of synthetic opals (SO) loaded by In and Si has yielded filler distribution profiles over thickness. The SO+In sample exhibited a uniform filling of SO voids throughout the sample thickness (on average, 16.9% of each large void). The SO+Si sample, besides the near-surface region where large voids are completely filled, revealed a region with variable Si concentration in the SO, where the Si content decreases linearly down to a depth where pure SO is found. Fiz. Tverd. Tela (St. Petersburg) 40, 1373–1375 (July 1998)     

Combining macroscopic and microscopic diffusion studies in zeolites using NMR techniques

In this study the zero length column (ZLC) technique is extended to the case where the decay of the adsorbed phase concentration is observed directly by nuclear magnetic resonance (NMR). An adsorption-desorption apparatus compatible with a 400-MHz NMR spectrometer was developed. It operates with nitrogen or helium as the inert purge gas. The column of the adsorbent material is placed in the sensitive region of the superconducting magnet and the rf coil of the NMR spectrometer. The time scales of the adsorption and desorption processes depend on concentration, temperature and crystal shape and are found to be in the range of 1-10 min. From the desorption branch, the non-equilibrium ZLC-NMR measurements yield intracrystalline diffusion coefficients in the range of 10(-13) to 10(-11) m2/s for different alkanes in silicalite-1. These values are always found to be smaller than the values measured by pulsed field gradient NMR under equilibrium condition indicating that there must be additional transport resistance at the external surface of these silicalite-1 zeolite crystals.     

Zero infrared reflectance anomaly in doped silicon lamellar gratings. I. From antireflection to total absorption

Zero-reflectance phenomenon for a binary lamellar grating on n-Si substrate irradiated by normally incident TE polarized plane electromagnetic wave of wavelength 10.6 μm is studied. The treatment is performed in the strong diffraction regime, where the structural dimensions and the wavelength are of the same order of magnitude, using data on the IR dielectric function of bulk doped silicon and a version of rigorous coupled-wave analysis. The evolution of normal reflectance zeros with increasing electron concentration from dielecric to metallic-like n-Si is traced. It is shown that the groove height undergoes sharp increase and the period shrinks when plasma wavelength becomes equal to the radiation wavelength. This marks the transition from the antireflection to the total absorption regime where most of incident power is absorbed in the grating region. The cavity-resonance origin of total absorption and satellite peaks in the spectral response are discussed.     

Three dimensional stagnation flow of Casson nanofluid through Darcy-Forchheimer space: A reduction to Blasius/Sakiadis flow

Heat and mass transport properties in the neighbourhood of stagnation region are analyzed in this paper where the region is created by the motion of Casson nanofluid inside a porous space. Porous space bounded by a sheet from one side is modeled by Darcy-Forchheimer rule, and is under the influence of heat source/sink and normal magnetic field. The developed mathematical model is solved for numerical solutions by the application of Runge-Kutta scheme due to Fehlberg along with Newton Raphson Shooting approach. This model is further reduced to Blasius and Sakiadis type flows to correlate them. For the validation of code and results, a comparison graph is prepared. Solutions depicted by figures and tables, reveal that velocity is declined in Sakiadis flow with Casson parameter however opposite nature is noted for Blasius flow. In Blasius flow, it is observed that temperature is greater and concentration is lower if compared with Sakiadis flow.