Backing effects on the underwater acoustic absorption of a viscoelastic slab with locally resonant scatterers

Backing effects on the underwater acoustic absorption of a viscoelastic polymer slab embedded with locally resonant scatterers are reported. The polymer slab is embedded with two layers of locally resonant scatterers, i.e. Al spheres coated by soft silicon rubber. Theoretical absorption coefficients of the polymer slab under different backings using a layer multiple scattering method show good agreement with the experimental results, which supports unambiguously the experimental observation. Then relations between the resonance modes and the low-frequency absorption peaks of the composite slab are clarified to address the absorption mechanisms. It shows that the mass of the steel backing affects evidently the low-frequency absorption, the absorption peak shifts to lower frequency range while increasing the backing mass.     

A comparison of automated and manual techniques for measurement of electrospun fibre diameter

Electrospinning is a fibre manufacturing process, and fibre diameter is a fundamental property. We compare diameter measurements made by human operators against two automated algorithms (FibreQuant™ and SEMAnalyser™). The effects of scanning electron microscopy preparation by iridium, gold and carbon coating on fibre diameter are also examined.A human takes 2.2 h to make 150 measurements. Automated analysis produces 9000 measurements less than 5 minutes. The automated method produces results without researcher bias and with greater consistency, but will occasionally include incorrect measurements because of the simple heuristics used. The manual method used by human operators shows larger variation in reported averages and is labour intensive.Before obtaining scanning electron microscopy images, the fibre samples require a conductive coating to prevent charging and burning of the fibres; the effects of SEM preparation methods such as iridium, gold and carbon coating showed that iridium coating had the least impact on fibre diameter.     

Fibre products, non-positive curvature, and decision problems

We give a criterion for fibre products to be finitely presented and use it as the basis of a construction that encodes the pathologies of finite group presentations into pairs of groups where G is a product of hyperbolic groups and P is a finitely presented subgroup. This enables us to prove that there is a finitely presented subgroup P in a biautomatic group G such that the generalized word problem for is unsolvable and P has an unsolvable conjugacy problem. An additional construction shows that there exists a compact non-positively curved polyhedron X such that is biautomatic and there is no algorithm to decide isomorphism among the finitely presented subgroups of . Received: October 7, 1999.     

Evolution of the degree of co-polarization in high-birefringence fibers

An analytical model accurately describing the evolution of the relative state of polarization between two signals in high-birefringence fibers is presented. The degree of co-polarization is calculated as a function of the angle between the principal axes of the fiber and the state of polarization of the two input signals, for two different input polarization schemes. It is shown that for small fiber lengths or narrow wavelength separation, launching the two signals with orthogonal polarizations can present a higher degree of co-polarization along the propagation; for large wavelength separation or long distances, the degree of co-polarization for the parallel scheme is always equal or higher than for the orthogonal case. The model for the degree of co-polarization is compared with theoretical and experimental four-wave mixing results in a dispersion shifted high-birefringence fiber.     

Determination of Copper and Aluminium in Steel by Means of Hollow Cathode and Glow Discharge Light Sources: A Comparative Study

Abstract

The applicability of hollow cathode and glow discharge light sources to the determination of minor constituents in steel was tested in the case of copper (0.033 ? 0.34[wt]%) and aluminium (0.015 ? 0. 16%). The linearity of the calibration graphs, reflected by the Bravais-Pearson coefficient, was found to be slightly improved in all the cases investigated when operating with hollow cathode. Furthermore, results showed that blackening values of the analytical lines can be plotted directly without resorting to reference lines.     

Study of mass transfer in a dynamic hollow‐fibre liquid phase microextraction system

The extraction characteristics of a dynamic hollow‐fibre liquid phase microextraction system were investigated by studying the mass transfer and diffusion rates of dinitrophenols from plasma samples over the liquid membrane (dihexylether). The measured diffusion coefficients were compared with theoretical values calculated from Stokes diameters. The diffusion mechanism was simulated by computer and the most polar compounds, 2,4‐dinitrophenol and 4,6‐o‐dinitrocresol, had associated diffusion coefficients that were close to the calculated theoretical values. 2‐sec‐Butyl‐4,6 dinitrophenol and 2‐tert‐butyl‐4,6‐dinitrophenol, the compounds with the highest log P values, were retained by the polypropylene membrane, which reduced the experimentally observed diffusion rates to about half of the theoretical values. The retention was most likely due to dispersive forces interacting with the pore inner walls. Extraction was linearly correlated with time for all compounds and the repeatability was high (RSDs 7–11%), even for the shortest extraction times. Method LOD as the amount injected ranged between 0.3 and 3.1 ng for an extraction cycle of 213 s.     

Sulphur speciation of PM10 samples by XANES spectroscopy

Sulphur K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to identify sulphur compounds in PM10 samples collected simultaneously at two sites with different environmental situations in the province of Trieste (NE of Italy), during summer and winter seasons respectively. The first site is an industrial site located near a steel mill plant and the second is a coastal site. The sulphur speciation at the industrial site has shown the presence of the following sulphur compounds in both seasons: organic compounds (thiophenes or Polycyclic Aromatic Sulphur Heterocycles — PAHs) and sulphates while in the winter season sulphites were also present. In the coastal site organic compounds (thiophenes or PASHs) and sulphate were found during winter season, moreover bisulphates were found during summer season. Relative percentages of the different sulphur compounds have shown that sulphate is the most abundant form of sulphur in the industrial site samples during both seasons and in the coastal site sample collected during the winter season (> 98%); in the coastal site sample collected during the summer season the relative percentages of bisulphate and sulphate were around 40% and 60% respectively.      

Fast steel-cleanness characterization by means of laser-assisted plasma spectrometric methods

Laser-assisted plasma spectrometry is a palette of analytical techniques (L-OES, LA-ICP-MS) capable of fast spatially-resolved elemental analysis in the micrometer range. For fast estimation of the occurrence in steel samples of non-metallic inclusions, which degrade the material's technical properties, simultaneous OES detection and sequential ICP-MS detection were compared. Histograms were obtained for the intensity distribution of the acquired signals (laser pulse statistics). The skewness coefficient of the histograms for Al (indicator of non-metallic inclusions) was found to be clearly dependent on the fraction of non-metallic inclusions in the case of scanning L-OES. For LA-ICP-MS less clear dependence was observed, which was influenced by the acquisition characteristics. In fact, less measurement throughput limited for LA-ICP-MS the counting statistics to an extent that overrides the benefit of higher detection power as compared to L-OES.     

Methane oxidation in a mixed ionic–electronic conducting ceramic hollow fibre reactor module

A reactor module, consisting of six gas-tight hollow fibre membranes made of the mixed ionic–electronic conducting perovskite, , has been tested for oxygen permeation and stability during methane oxidation in the temperature range of 540 to 960°C. Rigorous leak testing was undertaken and it was demonstrated that the module could be adequately sealed. Oxygen permeation fluxes were similar to those reported by previous workers. At higher temperatures of operation, it appeared that mass transfer limited the oxygen flux, as this flux was dependent upon the flow rates on either side of the membrane. In this way, reactant flow rates could be used to manipulate the transmembrane oxygen flux. It was found that the product distribution on the methane side was dependent upon this flux, with carbon monoxide and hydrogen production being favoured at low fluxes and carbon dioxide and water production being favoured at higher fluxes. Furthermore, at low oxygen flow rates, periodic increases in the transmembrane oxygen flux were observed. The cause of this behaviour is unclear but may be as a result of phase/stoichiometric changes associated with the membrane material.     

An efficient step method for a system of differential equations with delay

Using the step method, we study a system of delay differential equations and we prove the existence and uniqueness of the solution and the convergence of the successive approximation sequence using the Perov''s contraction principle and the step method. Also, we propose a new algorithm of successive approximation sequence generated by the step method and, as an example, we consider some second order delay differential equations with initial conditions.