Manganese location and magnetic structure determination of SrMn2—W hexagonal ferrite by means of neutron diffraction

The Mn cation distribution in the seven sublattices of theW-type structure has been determined from high temperature neutron diffraction data of a SrMn₂Fe₁₆O₂₇ powder sample. The Mn²⁺ ions enter the tetrahedral sites of blockS with a preference for site 4e. Mn³⁺ ions in small amounts are found in the octahedral sites, substituting for Fe³⁺, while 0.3 Fe²⁺ has been found in site 6g. The sample has the formula SrMn_2.4Fe_15.6O₂₇. The crystal structure has been described in the non-centrosymmetric Pˉ62c space group, instead of the usual P6₃/mmc one. The magnetic structure, obtained from low temperature (4.2 K) data, is collinear, with the spins parallel to thec axis, according to the Gorter model. The magnetic moments of the sites give an experimental magnetization valueM _s=28.4(1.5) μ_B/f.u., in good agreement with magnetic measurements and the theoretical value.     

Lithium intercalation studies on MoO3 single crystals

The intercalation process of Li in molybdenum trioxide (MoO₃), a layered transition metal oxide that has been proposed as candidate cathode material for rechargeable batteries, is examined in relation to the crystallinity and the growth method of the pristine material. Atomic Force Microscopy (AFM) is used for surface roughness calculations. Roughness is enhanced with intercalation at the early stages and as the process proceeds, randomly scattered domains merge into ordered clusters, and finally a fragmented surface is formed. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Synthesis and electrochemical study of Li-Mn-Ni-O cathodes for lithium battery applications

Cathode powders of the Li–Mn–Ni–O system have been prepared at a Mn/(Mn+Ni) ratio varying from 0 to 1. The solid state reaction method was used to obtain the cathode materials by mixing MnO₂, LiCO₃ and NiO. A 20% excess of lithium was used in the precursors. The materials produced were examined by X-rays to identify their structure. Batteries were assembled by using these materials as cathode with a liquid electrolyte consisting of EC/DC 1:1, 1 LiPF₆ and Li anode. Their capacity, cycle fading and charge-discharge conditions were evaluated.Presented at the 3rd International Meeting "Advanced Batteries and Accumulators", June 16th–June 20th 2002, Brno, Czech Republic     

Time Evolution of Optical Emission Spectrum of a Hg-HID Lamp Exposed to X-ray

Time variation of optical emission lines for Hg, and other elements detected in a mercury HID lamp before and during X-ray irradiation were analyzed. Various irradiation conditions are causing differences in time dependencies of all Hg spectral line intensities (including that of 253.73 nm resonance line not completely self-absorbed). The X-ray optical depths are smaller in the case of Hg gas compared with burner wall glass. The free electrons generated during X-ray exposure (photoelectrons, Auger electrons) contribute to changes in plasma characteristics (e.g. plasma temperature, density, etc.) in transitory regime. In quasi-stable regime there are practically no differences between electron temperatures for normal and irradiated lamp. A qualitative explanation in the framework of Beer-Lambert law is given.     

Isothermal soot oxidation experiments with intermediate gas change in a Perkin-Elmer TGA6

Two isothermal soot oxidation protocols were tested in a Perkin-Elmer TGA6: (1) the sample was heated under N₂ and then the reaction gas was introduced and (2) the sample was introduced after the empty furnace was heated under the reaction gas. The first protocol is common in soot oxidation studies, it gives a measure of the volatiles and is easier to handle, but as it is shown the determined reaction rate may be falsified by the O₂ concentration. Using gas analysis it was found that ∼2000 s are necessary for the complete gas change in the instrument. An instrument specific correction function involving the O₂ concentration and reaction order n with respect to O₂ was developed which allowed the correlation of the rates measured with both protocols.     

Impact of Statistical Multiplexing on Voice Quality in Cellular Networks

This paper examines the quality of transmission of voice over cellular, packet-switched networks. The medium access mechanism in the uplink is simulated under various statistical multiplexing scenarios in order to assess the effect of front-end clipping on voice quality. Moreover, the simulation is implemented in a real-time demonstration platform utilized to acquire subjective indicators of voice quality by performing Mean Opinion Score (MOS) tests. Results from the MOS tests are reported, and an analysis of the obtained speech samples is presented. Finally, the results are summarized and potential further directions for the simulation tool and the speech models are discussed.     

Rotation des moments magnetiques dans BaLa2 Fe2 O7

In the compound BaLa₂Fe₂O₇, below 235 K, spins are oriented along the diagonal of the quadratic cell in an antiferromagnetic configuration. Above this temperature we observe by neutron diffraction a continuous rotation of the magnetic moments in the base plane xyO, the configuration remaining antiferromagnetic; the magnetic symmetry goes from I_pm′mm′ to P2′/m Shubnikov's group. At the transition temperature we observe a discontinuity of the thermal expansion coefficient. This phenomenon can be interpreted in agreement with Landau's theory of second order transitions.     

Simulation of an Ultrahigh-Pressure Short-Arc Xenon Discharge Plasma

Technical Physics - We have studied a high- (ultrahigh-) pressure short-arc discharge in xenon with thoriated tungsten cathodes. A system of equations formulated based on earlier experimental data...     

Chemometric analysis of high performance liquid chromatography-diode array detection-electrospray mass spectrometry of 2- and 3-hydroxypyridine

Triply coupled high performance liquid chromatography using diode array detection and positive ion electrospray mass spectrometry of 2- and 3-hydroxypyridine is presented. Considerations of the physical method for coupling the two detectors, the influence of pH on retention times, the cone voltage of the mass spectrometer and the linear concentration ranges are described. Data from both detectors are aligned and interpolated. The analyte mass spectra are reduced to 20 significant masses. Principal components plots on the raw, normalised and standardised data, derivatives to determine composition 1 regions, deconvolution and procrustes analysis to compare data from both detectors are discussed. Common trends in both mass spectral and diode array chromatograms are interpreted. This paper represents a new approach to common processing of chromatographic data from two detectors.     

Thermal Treatment of Lignite for Carbon Molecular Sieve Production

Carbon molecular sieves (CMS) were prepared from Greek lignite by a thermal treatment technique involving three sequential stages: carbonization, followed by activation with an oxidizing agent, and aperture modification by coke deposition. Adsorption of N₂ at 77 K and CO₂ at 298 K was used for the characterization of products. Molecular sieving properties were examined by measuring the adsorption kinetic curves of CO₂ and CH₄ at room temperature. Activated samples with the highest surface area were selected for CMS production by employing a propylene cracking technique. High temperatures resulted in the production of materials with large differences in their BET and CO₂ surface areas. CO₂-CH₄ selectivity ratios estimated from the adsorption kinetic curves were high for these samples. This revised version was published online in July 2006 with corrections to the Cover Date.