STRUCTURE,MAGNETIC, AND MAGNETOCALORIC PROPERTIES OF SUBMICRONIC YTTRIUM IRON GARNET PARTICLES

Journal of Structural Chemistry - Scanning electron microscopy, X-ray diffraction, Raman and Mössbauer spectroscopies, field and temperature dependences of magnetic saturation and...     

Slow and Fast Oscillations in a Model of an Optoelectronic Oscillator with Delay

Doklady Mathematics - The local dynamics of a well-known model of an optoelectronic oscillator with delayed feedback is studied. In a neighborhood of zero equilibrium, normalized equations are...     

Reversible Modulation of Surface Plasmons in Gold Nanoparticles Enabled by Surface Redox Chemistry

Switchable surface redox chemistry is demonstrated in gold@iron/iron oxide core–shell nanoparticles with ambient oxidation and plasmon‐mediated reduction to modulate the oxidation state of shell layers. The iron shell can be oxidized to iron oxide through ambient oxidation, leading to an enhancement and red‐shift of the gold surface plasmon resonance (SPR). This enhanced gold SPR can drive reduction of the iron oxide shell under broadband illumination to reversibly blue‐shift and significantly dampen gold SPR absorption. The observed phenomena provide a unique mechanism for controlling the plasmonic properties and surface chemistry of small metal nanoparticles.     

Features of the composition and structural transformations of manganese minerals of the lake Bol’shoe Miassovo (South Ural)

The mineral and microelement composition of natural manganese minerals from iron-manganese concretions of the Bol’shoe Miassovo lake (South Ural) is investigated. The structural transformations of the samples under the effect of different temperatures are studied. Apart from layered 9.6 Å and 7.2 Å structures of manganese minerals the lake samples often contain various manganese-containing carbonates. For carbonate minerals by IR spectroscopy the dependence of the wavenumbers of ν₂ and ν₄ vibrations of the CO₃ group on the mineral composition is analyzed. The composition of macro- and microelements of iron-manganese concretions of Bol’shoe Miassovo lake varies in wide ranges not only in different samples, but also in different parts of the same sample. An analysis of the microstructure of lake manganese concretions revealed their biomorphic nature that is exhibited in globe-shaped to filament organic bacterial forms.     

Automatic generation of linear programming problems for computer aided instruction

The paper presents a method for generating random linear programming problems with a previously selected type of solution. The user can choose a problem whose solution is unbounded, bounded for minima, maxima or both, unique or multiple, with given structure, at wish. Initially, the feasible solution of the LPP is generated as the sum of a linear space, a cone, and a polytope, depending on the desired properties of the solution. With the aim of obtaining a simple set of constraints, the generators of these three structures are selected as random vectors with integer simple components, the range of which can be given. Next, an objective function that satisfies the required conditions, i.e. leads to a solution of the desired type, is obtained. The generating algorithms have been implemented in Mathematica and some illustrative examples are given to clarify the generation process. With this tool, a LPP can be generated, according to the instructor requirements, where this is a human or an expert system. They can control student progress and generate a sequence of problems covering all possible cases, in steps of increasing difficulty. Combining this tool with another (also produced by the same authors) that solves the problems and explains the whole process, step by step, a computer aided module for learning LPP, which is completely autonomous, can be easily obtained.     

Conjugate convective heat transfer of a crystal with an ambient medium under the conditions of thermal gravitation convection in Czochralski method

Results on conjugate convective heat transfer in a system geometrically similar to the system “crystal-gas-growing vessel shell” of a growing vessel obtained by the Czochralski method are presented. Equations of thermal gravitation convection in Boussinesq approximation are solved by the finite element method in the following variables: temperature, stream function, and vorticity in cylindrical coordinates. Heat transfer from the crystal was studied within the range of Grashof number 100 ≤ Gr ≤ 10⁴ for the fixed configuration of calculation domain. Dependence of the temperature field in the silicon crystal on convection intensity in gas (argon) was examined.     

Manganese minerals from the Miassovo freshwater lake: Composition and structure

The mineral, chemical, and trace element composition and the microstructures of manganese minerals in bottom nodules and crusts collected from the Bolshoe Miassovo freshwater lake (South Urals) were studied. Freshwater manganese species were shown to have more diversified mineral compositions than in marine species. In addition to (9.6- and 7.2 Å) layered manganese mineral structures, various manganese-containing carbonates were frequently present in lacustrine samples. As opposed to marine samples, lacustrine samples were poor in trace elements, in particular rare-earth elements, but have higher contents of Sr, As, Ti, and V. A structural analysis of lacustrine manganese species showed their biomorphic nature, which manifested itself as organic bacterial species ranging from spherulitic to filamentary ones.     

Interfering modes and an axial wave in an underwater sound channel

Experiments on long-range propagation of low-frequency sound that were conducted starting from the mid-1980s indicate a complex character of propagation in an underwater sound channel, in which a source and a receiver are located close to the channel axis. A burst of energy propagating along the axis follows early arrivals, which are well described by the formulas of geometrical acoustics, in plots of acoustic intensity as a function of propagation time and hydrophone depth. This energy burst cannot be described using geometrical acoustics because of caustics with caustic beaks located near the channel axis. Very complex interference processes occur near these caustics. As the distance from the source grows, the dimensions of the interference vicinity increase and start to overlap producing a peculiar “axial wave.” For an arbitrary two-dimensional underwater sound channel, the axial wave can be represented as a sum of the first normal modes and a residue. This conclusion is based on the use of two representations for an acoustic field. The first of them includes the sum of ray components and an axial wave. The second representation consists of ray addends, the sum of the first normal modes, and a residue. Numerical results are obtained for a canonical profile of sound velocity at the frequency of 200 Hz for the distances of 1600–1650 km.     

Fabrication and characterization of TEOS-based silica aerogels prepared using rapid supercritical extraction

Silica aerogels were prepared using the precursor tetraethylorthosilicate (TEOS) via a rapid supercritical extraction (RSCE) method. Multiple consistent batches of monolithic TEOS-based aerogels were fabricated via an 8-h RSCE process. Fabricating TEOS-based aerogels with an RSCE method offers some distinct advantages. One advantage is the relative simplicity of the RSCE approach: liquid precursors are mixed and poured into a metal mold in a hydraulic hot-press, where gelation, aging and extraction of liquid from the pores occur. The precursor recipe employs TEOS, ethanol, water, oxalic acid to catalyze hydrolysis, and ammonia to catalyze the subsequent polycondensation reactions. Another advantage is that reaction of TEOS to form sol gels yields ethanol as a byproduct. A process that releases ethanol, rather than methanol (as in tetramethylorthosilicate (TMOS)-based aerogels) may be more appealing for commercial applications, involving scale-up of the process. The significantly lower cost of TEOS, compared to TMOS, is a considerable advantage. The TEOS-based RSCE aerogels are mesoporous and optically translucent, have bulk densities of 0.099(±0.003) g/cm³ and surface areas of 460(±10) m²/g. Signals observed in infrared and Raman spectra of the aerogels are consistent with Si–O framework bonds. Using scanning electron microscopy imaging, the surface morphology of the aerogel samples was imaged at magnifications up to 150 kX.