Testing underwater bottom-moored antenna arrays in the sea and in a man-made lake

Measurements of response, gain, and noise immunity are carried out for an underwater compensated additive receiving array with randomly spaced hydrophones that is moored at the bottom of a man-made lake with multimode sound propagation. The in-sea locating ability of a similar array is demonstrated with the sources of noiselike signals at frequencies of 5–100 Hz. A dedicated numerical processor is developed and tested for processing the signals received by a random underwater array.     

Atom transfer radical polymerization of methyl methacrylate by copper(I)‐pyridine‐2‐carboximidate catalysts

Pyridine‐2‐carboximidates [methyl ( 1a ), ethyl ( 1b ), isopropyl ( 1c ), cyclopentyl ( 1d ), cyclohexyl ( 1e ), n‐octyl ( 1f ), and benzyl ( 1g )] were prepared from the reaction of 2‐cyanopyridine with the corresponding alcohols. Cyclopentyl‐substituted 1d was found to be a highly effective ligand for copper‐catalyzed atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). For example, the observed rate constant for a CuBr/ 1d catalytic system was found to be nearly twice as high as the cyclohexyl‐substituted CuBr/ 1e catalytic system [k_obs = (1.19 vs 0.56) × 10^?4 s^?1). The effects of the solvents, temperature, catalyst/initiator, and solvent/monomer ratio on the ATRP of MMA were studied systematically for the CuBr/ 1d catalytic system. The optimum condition for the ATRP of MMA was found to be a 1:2:1:400 [CuBr]_o/[ 1d ]_o/[ethyl 2‐bromoisobutyrate]_o/[MMA]_o ratio at 60 °C in veratrole solution, which yielded well‐defined poly(MMA) with a narrow molecular weight distribution of 1.14. The catalytically active copper complex 2d was isolated from the reaction of CuBr with 1d . Narrow molecular weight distributions as low as 1.06 were achieved for the CuBr/ 1d catalytic system by employing 10% of the deactivator CuBr₂. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2747–2755, 2004     

Behavior and surface energies of polybenzoxazines formed by polymerization with argon,oxygen, and hydrogen plasmas

Polybenzoxazine (PBZZ) thin films can be fabricated by the plasma‐polymerization technique with, as the energy source, plasmas of argon, oxygen, or hydrogen atoms and ions. When benzoxazine (BZZ) films are polymerized through the use of high‐energy argon atoms, electronegative oxygen atoms, or excited hydrogen atoms, the PBZZ films that form possess different properties and morphologies in their surfaces. High‐energy argon atoms provide a thermodynamic factor to initiate the ring‐opening polymerization of BZZ and result in the polymer surface having a grid‐like structure. The ring‐opening polymerization of the BZZ film that is initiated by cationic species such as oxygen atoms in plasma, is propagated around nodule structures to form the PBZZ. The excited hydrogen atom plasma initiates both polymerization and decomposition reactions simultaneously in the BZZ film and results in the formation of a porous structure on the PBZZ surface. We evaluated the surface energies of the PBZZ films polymerized by the action of these three plasmas by measuring the contact angles of diiodomethane and water droplets. The surface roughness of the films range from 0.5 to 26 nm, depending on the type of carrier gas and the plasma‐polymerization time. By estimating changes in thickness, we found that the PBZZ film synthesized by the oxygen plasma‐polymerization process undergoes the slowest rate of etching in CF₄ plasma. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4063–4074, 2004     

Fracture toughening of epoxy matrices with blends of resins of different molecular weights and other modifiers

The microstructure and fracture behavior of epoxy mixtures containing two monomers of different molecular weights were studied. The variation of the fracture toughness by the addition of other modifiers was also investigated. Several amounts of high‐molecular‐weight diglycidyl ether of bisphenol A (DGEBA) oligomer were added to a nearly pure DGEBA monomer. The mixtures were cured with an aromatic amine, showing phase separation after curing. The curing behavior of the epoxy mixtures was investigated with thermal measurements. A significant enhancement of the fracture toughness was accompanied by slight increases in both the rigidity and strength of the mixtures that corresponded to the content of the high‐molecular‐weight epoxy resin. Dynamic mechanical and atomic force microscopy measurements indicated that the generated two‐phase morphology was a function of the content of the epoxy resin added. The influence of the addition of an oligomer or a thermoplastic on the morphologies and mechanical properties of both epoxy‐containing mixtures was also investigated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3920–3933, 2004     

On the Integrability Conditions for Some Structures Related to Evolution Differential Equations

Using the result by D. Gessler, we show that any invariant variational bivector (resp., variational 2-form) on an evolution equation with nondegenerate right-hand side is Hamiltonian (resp., symplectic).     

Three-dimensional structure of Saccharomyces cerevisiae inorganic pyrophosphatase complexed with cobalt and phosphate ions

Crystals of Saccharomyces cerevisiae inorganic pyrophosphatase suitable for X-ray diffraction study were grown by cocrystallization of the enzyme with cobalt chloride and imidodiphosphate. Saccharomyces cerevisiae is a metal-dependent enzyme which catalyzes hydrolysis of inorganic pyrophosphate to orthophosphate. The three-dimensional structure of this enzyme was solved by the molecular-replacement method and refined at 1.8 Å resolution to an R factor of 19.5%. Cobalt and phosphate ions were revealed in the active centers of both identical subunits (A and B) of the pyrophosphatase molecule. In subunit B, a water molecule was found between two cobalt ions. It is believed that this water molecule acts as an attacking nucleophile in the enzymatic cleavage of the pyrophosphate bond. It was demonstrated that cobalt ions and a phosphate group occupy only part of the potential binding sites (two chemically identical and crystallographically independent subunits have different binding sites). The arrangement of ligands and the structure of the nucleophile-binding site are discussed in relation to the mechanism of action of the enzyme and the nature of the metal activator.     

Energetics of an adiabatically-loaded excited anharmonic oscillator

The dependence of the energy characteristics on an adiabatically slowly increasing external force is determined analytically for an anharmonic oscillator. The analytical results are confirmed by a numerical calculation. The nature of the force dependences of the energy characteristics are determined and discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 153–157 (January 1997)     

Shell effects in the symmetric-modal fission of pre-actinide nuclei

Mass distributions of fragments in the low-energy fission of nuclei from ¹⁸⁷Ir to ²¹³At have been analysed. This analysis has shown that shell effects in symmetric-mode fragment mass yields from the fission of pre-actinide nuclei could be described if one assumes the existence of two strongly deformed neutron shells in the arising fragments with neutron numbers N₁ ≈ 52 and N₂ ≈ 68. A new method has been proposed for quantitatively describing the mass distributions of the symmetric fission mode for pre-actinides with A ≈ 180–220.     

On asymptotic approximation of a solution of a boundary-value problem for a nonlinear nonautonomous equation

On the basis of periodic Ateb functions, in the resonance and nonresonance cases, we construct the asymptotic approximation of one-frequency solutions of a boundary-value problem for a nonlinear nonautonomous equation.     

Thickness and temperature dependence of subsidiary absorption thresholds in yttrium-iron-garnet thin films

The microwave subsidiary absorption threshold in tangentially magnetized yttrium-iron-garnet (YIG) films was measured as a function of the static magnetic field, film thickness and decrease in the 0.3–3 μm range is observed. The effect is explained quantitatively by using modified Damon-Eshbach dispersion relations which take into account exchange interactions.

The effect of temperature on h_crit, the smallest threshold field amplitude, in the 250<T< 450 K temperature range can be expressed by an empirical relationship h_crit M_s(T)^-1.6 where M_s is the saturation magnetization. This h_crit vs M_s relationship is similar to that obtained for the magnetization dependence of parallel pump thresholds in Li-Ti ferrite compounds.