Doppler power spectrum from a Gaussian sample volume

A closed-form expression for the Doppler power spectrum due solely to the range of blood velocities passing through a Gaussian sample volume placed anywhere in a vessel under conditions of axisymmetric flow, uniform backscatter and negligible intrinsic spectral broadening has been derived. The formulation presented here allows the independent specification of the sample volume position and width, in the three dimensions, and enables simple estimations of spectral shape for pulsed wave Doppler systems. Simpler expressions were derived for the cases of symmetric sample volume projections onto the vessel cross-section and/or sample volumes centred in the vessel. Closed form expressions were derived for mean frequency and spectral width in the case of a symmetric sample volume projection centred in the vessel. The effects of sample volume size and position on the Doppler spectral width and mean frequency are shown for a range of velocity profiles.     

Toeplitz Operators with Symbols Generated by Slowly Oscillating and Semi-Almost Periodic Matrix Functions

We develop the Fredholm theory for Toeplitz operators, withsymbols in the C*-algebra D = [SO, SAP]_{n, n} generated by allslowly oscillating (SO) and semi-almost periodic (SAP) n x nmatrix functions, on the Hardy spaces (with 1 < p < ) over the upper half-plane.Using limit operator techniques, we get necessary Fredholm conditionsfor any operator in the Banach algebra alg(S, D) of singularintegral operators with coefficients in D on the space [L^p (R)]_n.Applying the Allan–Douglas local principle and the theoryof Toeplitz operators with SAP matrix symbols, we also establishFredholm criteria for Toeplitz operators with matrix symbolsg D on the space . An index formula for Fredholm Toeplitz operators with matrix symbolsin D is obtained on the basis of an appropriate approximationof slowly oscillating components of the symbols. 2000 MathematicsSubject Classification 47B35 (primary), 47A53 (secondary).     

Evaporation of Electrolyte during SVET Measurements: The Scale of the Problem and the Solutions

The objective of this work is to investigate the scale of the effect of spontaneous solution evaporation during SVET (Scanning Vibrating Electrode Technique) measurements and demonstrate how it biases the final results. When SVET maps are continuously acquired for more than several hours, the measured currents are smaller than expected. This is attributed to solvent (typically water) evaporation which leads to an increase in solution conductivity over time. If this is not considered when converting the measured potential differences into the local current densities, the SVET results display currents smaller than the true ones. Here, this effect is studied with a platinum disk electrode as source of a constant current and a model corroding system consisting of the AA2024/CFRP galvanic couple. Corrective actions are proposed to mitigate the problem, either in the experimental set‐up or as numerical correction.     

Wiener-Hopf operators with oscillating symbols and convolution operators on a union of intervals

The present work was sponsored by JNICT (Portugal) under grant n. 87422/MATM and Programa Ciência.     

Convolution equations of the first kind on a finite interval in Sobolev spaces

The study of a class of operators associated with convolution equations of the first kind on a finite interval is reduced to the study of Wiener-Hopf operators with piecewise continuous symbol on R. Fredholm properties and invertibility conditions for this class of operators are investigated. An example from diffraction theory is considered.Sponsored by J.N.I.C.T. (Portugal) under grant n ^o 87422/MATM.     

Preparation, structural analysis and dielectric properties of Bi x La1−x FeO3 perovskite

Rare earth element substituted bismuth ferrites (BiFeO₃) are of enormous importance as magnetoelectric materials. The polycrystalline samples of Bi _x La_1−x FeO₃ (x=0, 0.2, 0.4, 0.6, 0.8) were prepared by solid-state reaction using standard ceramic method. The single-phase formation of these compounds was confirmed by X-ray diffraction (XRD) studies. The samples with x=0, 0.2, 0.4, 0.6 are found to be orthorhombic while the sample with x=0.8 is triclinic. The dielectric constant (ε′) and dissipation factor (tan δ) were measured in the frequency range 100 Hz to 1 MHz at room temperature and as a function of temperature at certain fixed frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz). All the samples showed dielectric dispersion. The dielectric constant with temperature shows a broad peak; the peak temperature shifts with frequency which reflects the relaxor-type behavior. The peak above 600 K in the measured temperature range corresponds to antiferromagnetic ordering temperature (Néel temperature). The broadness of the peak changes with composition. The ac conductivity as well as ε′ are found to be maximum for the sample x=0.2 at room temperature.     

Production of no carrier added residue free22Na

A method for the production of no carrier added and residue free²²Na, was developed via the²⁴Mg/d, /²²Na nuclear reaction applying 14 MeV deuterons and a high current Mg–Cu target. The yield for the reaction target. The yield for the reaction was measured and the chemical separation performed using a Dowex 50WX8 resin in H⁺ form. The overall yield was higher than 98%. Residue could not be detected. All experiments were carried out at IEN's CV-28 cyclotron.     

Polypropylene reactor mixture obtained with homogeneous and supported catalysts

Propylene polymerizations were performed with homogeneous ?₂C(Flu)(Cp)ZrCl₂ and SiMe₂(Ind)₂ZrCl₂ catalyst mixtures and with mixtures supported on the zeolite acid mordenite. The polymerizations were performed in toluene and hexane/triisobutylaluminum at different temperatures and Al(MAO)/Zr concentration ratios. The effects of these variables on the catalyst activity were investigated with statistical experimental planning. The average molecular weights, molecular weight distributions, melting temperatures, and crystallinities of the obtained polymers were examined. The results showed lower activities for the homogeneous catalyst mixture than for the isolated systems. On the other hand, high activities were obtained for the syndiospecific heterogeneous system, but very low values were obtained for the supported isospecific metallocene, although both catalysts were prepared under the same conditions. The supported binary system showed intermediary catalyst activity in comparison with the syndiospecific and isospecific supported catalysts. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 263–272, 2005     

Fragmentation of diketopiperazines from Aspergillus fumigatus by electrospray ionization tandem mass spectrometry (ESI-MS/MS)

Diketopiperazines (DKPs) corresponding to cyclic dipeptides have been reported to exhibit antimicrobial, antitumor, antimutagenic and antiviral properties. These compounds are commonly isolated from microorganisms and sponges and from a variety of tissues and body fluids. In this work, we used electrospray ionization tandem mass spectrometry (ESI-MS/MS) to investigate the fragmentation of a series of DKPs previously isolated from Aspergillus fumigatus, which exhibit the same structural core. Loss of CO directly from the protonated molecule was found to be a fragmentation process common to all the compounds analyzed. However, our results revealed a series of ions that are diagnostic for the substituents at C(4) and C(9). In order to rationalize the differences in the fragmentation pathways of substituted and nonsubstituted DKPs, the relative Gibbs energies (DeltaG) of the product ions and intermediate ions were estimated using the B3LYP/6-31 + + G(d,p) model. The data reported here can be used for the structural elucidation of DKPs from low sample amounts, as an alternative to NMR.