A mixed hybrid formulation based on oscillated finite element polynomials for solving Helmholtz problems

A mixed-hybrid-type formulation is proposed for solving Helmholtz problems. This method is based on (a) a local approximation of the solution by oscillated finite element polynomials and (b) the use of Lagrange multipliers to “weakly” enforce the continuity across element boundaries. The computational complexity of the proposed discretization method is determined mainly by the total number of Lagrange multiplier degrees of freedom introduced at the interior edges of the finite element mesh, and the sparsity pattern of the corresponding system matrix. Preliminary numerical results are reported to illustrate the potential of the proposed solution methodology for solving efficiently Helmholtz problems in the mid- and high-frequency regimes.     

Structural study by X-ray diffraction and Mössbauer spectroscopy of a new synthetic iron phosphate: K4MgFe3(PO4)5

A new iron phosphate K₄MgFe₃(PO₄)₅ has been synthesized by the flux method and characterized by single-crystal X-ray diffraction and Mössbauer spectroscopy. It crystallizes in the tetragonal system with the space group and the unit cell parameters a=9.714(3) Å and c=9.494(5) Å. The crystal structure is of a new type. It exhibits a three-dimensional framework built up from corner-sharing MO₅ (M=0.75Fe+0.25Mg) trigonal bipyramids and PO₄ tetrahedra. The K⁺ ions are occupying large eight-sided tunnels running along c. A room temperature Mössbauer study confirmed the +3 valence state of iron and its five-coordination.     

Understanding the nucleation mechanisms of carbon nanotubes in catalytic chemical vapor deposition

The nucleation of carbon caps on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. It takes place in a well defined carbon chemical potential range, when a critical concentration of surface carbon atoms is reached. The solubility of carbon in the outermost Ni layers, that depends on the initial, crystalline or disordered, state of the catalyst and on the thermodynamic conditions, is therefore a key quantity to control the nucleation.     

Solvent-free synthesis of some new 2-（3,5-dimethoxy-4-methylphenyl）-5-aryl-1,3,4-oxadiazoles

An expeditious microwave-accelerated one-step synthesis of some new 2-（3,5-dimethoxy-4-methylphenyl）-5-aryl-1,3,4- oxadiazoles by reaction of 3,5-dimethoxy-4-methyl hydrazide with different carboxylic acids in presence of thionyl chloride under neat conditions,has been achieved.     

Microwave-accelerated synthesis of some 1-aroyl-3,5-dimethylpyrazoles

Rapid and highly efficient synthesis of some 1-aroyl-3,5-dimethylpyrazoles is achieved by the cyclocondensation of corresponding aroyl hydrazides with 2,4-pentanedione in the absence of solvent and catalyst, under microwave irradiation. The reaction proceeds rapidly and is completed within 40-70 s giving a series of 1-aroyl-3,5-dimethylpyrazoles derivatives in high yields. All of the compounds have been characterized by FTIR, ^1H NMR, and mass spectroscopy.     

Studies of inhibitor binding to the [4Fe-4S] cluster of quinolinate synthase

Stop for NadA! A [4Fe-4S] enzyme, NadA, catalyzes the formation of quinolinic acid in de?novo nicotinamide adenine dinucleotide (NAD) biosynthesis. A structural analogue of an intermediate, 4,5-dithiohydroxyphthalic acid (DTHPA), has an in?vivo NAD biosynthesis inhibiting activity in E. coli. The inhibitory effect can be explained by the coordination of DTHPA thiolate groups to a unique Fe site of the NadA [4Fe-4S] cluster.     

Sturm theory for the equation of vibrating beam

In this paper we develop an extension of the classical Sturm theory [C. Sturm, Sur une classe d'equations à derivée partielle, J. Math. Pures Appl. 1 (1836) 373-444], to study the oscillation properties for the eigenfunctions of some fourth-order two point boundary value problems on the interval [0,1]. We are mainly interested in the case when these problems have negative eigenvalues induced by the sign of the parameters in the boundary conditions. In particular, we give an asymptotic estimate of the number of zeros in (0,1) of the first eigenfunction in terms of the variation of parameters in the boundary conditions.