Zur ma?analytischen Bestimmung von Wolfram und Uran

Zusammenfassung Die für die technische Bestimmung von Molybdän brauchbare fällungsanalytische Titration mit Silbernitrat und potentiometrischer Endpunktsanzeige ist für Wolfram und Uran wegen der ungünstigeren Löslichkeitsverhältnisse nicht brauchbar.Die Reduktion im Silberreduktor und anschließende Titration mit Dichromat anstatt mit dem bisher verwendeten Cer(IV)-sulfat mit Endpunktsanzeige durch Farbumschlag (Diphenylamin) oder auf potentiometrischem Wege ist für Uran bei Einhaltung entsprechender Arbeitsbedingungen (Reduktion in 4 n Salzsäure bei 60°C, 2 · 10^–3 Mol Uran/l als obere Grenze bei der Titration) gut verwendbar. Der Silberreduktor hat jedoch keine erkennbaren Vorteile gegenüber dem Bleireduktor.Für Wolfram ist diese Methode wegen der ungünstigen Lage der Redoxpotentiale nicht brauchbar.     

Synthese,Charakterisierung und EPR-spektroskopische Untersuchung von Heteropolyverbindungen mit tetraedrisch bzw. oktaedrisch koordiniertem Eisen(III)

Synthesis, Characterization, and EPR Studies of Heteropoly Compounds with Iron(III) in Tetrahedral and Octahedral Coordination The heteropoly compounds H₅[FeO₄W₁₂O₃₆] · 6 H₂O (a₀ = 1216 pm), H₃[Fe(OH)₆Mo₆O₁₈] · 4 H₂O, Na₅[FeO₄W₁₂O₃₆] · nH₂O and FeH₂[FeO₄W₁₂O₃₆] · 17 H₂O, for the first time obtained in this work by freeze-drying and characterized by means of chemical analysis, i.r. and u.v. spectroscopy, X-ray powder-photographs, and magnetic measurements, appear as suitable model systems for EPR investigations. They contain, like a number of known Fe^III-heteropoly compounds, Fe^III in FeO₄ or/and FeO₆ units, which are isolated from each other by structural reasons. In the Keggin-compounds M₅[E^IIIO₄W₁₂O₃₆] · nH₂O ( I ) (M = Na, Rb, TMA, TEA; E = Fe, Al, B) Fe^III occupies slightly distorted tetrahedral positions (g′ ≈? 2), which are characterized by zfs-values of ≈? 10 mT and line widthes ΔB of 2.0 ?15 mT. Unlike as for I cations with different physico-chemical characteristics have only little effect on the Fe^III-zfs. This holds for the Anderson-complexes M₃[Fe(OH)₆Mo₆O₁₈]·nH₂O, (M = H, K, NH₄, TMA; g′ ≈? 4.3 ΔB ≈? 67 mT) and for M5[SiO₄W₁₁O₃₅FeO₅(OH₂)]·nH₂O, (M = K, TMA; g′ = 4.3 ΔB = 26.5 mT). The FeO₆ octahedra are more distorted than the FeO₄ tetrahedra in I and therefore less susceptible for structural changes.     

Calcium oxalate crystals in tomato and tobacco plants: morphology and in vitro interactions of crystal-associated macromolecules

Plants form calcium oxalate crystals with unique morphologies under well-controlled conditions. We studied the morphology of single calcium oxalate monohydrate (whewellite) crystals extracted from tomato and tobacco leaves. These crystals have a pseudotetrahedral shape. We identified the (101), (101) or (102), (110), and (hk0) faces as stable faces. The morphology is chiral with unique handedness. We also show that calcium oxalate monohydrate crystals isolated from tomato, tobacco, and bougainvillea leaves contain macromolecules rich in Gly, Glx, and Ser. Crystal-associated macromolecules extracted from tomato and tobacco influence the morphology of calcium oxalate monohydrate crystals grown in vitro, promoting preferential development of the [120] faces. Furthermore, crystal-associated macromolecules from tobacco promote nucleation of calcium oxalate monohydrate crystals, whereas model polypeptides do not have any significant effect on nucleation. These results imply an active role of the crystal-associated macromolecules in the formation of pseudotetrahedral shapes in vitro, and these properties may in part be responsible for the unique chiral morphology of the natural pyramidal-shaped crystals.     

Optimization of Graphene Layers Grown on Pt/Ti/SiO2 by Hot Filament Chemical Vapor Deposition

Large area single and bilayer graphene are grown on Pt/Ti/SiO₂ substrates by hot filament chemical vapor deposition (HFCVD) with and without the assistance of Cu foil. The quality and number of graphene layers deposited on the substrate are assessed by Raman Spectroscopy. Atomic Force Microscopy (AFM) is used for assessing the surface topography of the graphene films grown on the Pt/Ti/SiO₂ substrates. The microstructure and elemental analyses are performed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The results show that bilayer graphene growth is facilitated by a copper foil placed nearby Pt/Ti/SiO2 substrate and by a high filament temperature in the HFCVD reactor. Monolayer graphene grows only when no copper foil is placed near the Pt/Ti/SiO₂ substrate at a low filament temperature. The approach paves a novel pathway towards the layer-controlled growth of graphene on Pt/Ti/SiO₂ substrates by HFCVD for frontier applications.     

Die titanometrische Bestimmung von Chlorat

Ohne Zusammenfassung     

Fast wavelength-parallel polarimeter for broadband optical networks

We describe a novel wavelength-parallel polarimeter operating in the light-wave band that measures the complete state of polarization of 256 wavelengths in parallel within 20 ms (software-limited), with the potential for submillisecond operation. By use of fast switching ferroelectric liquid crystals in conjunction with an InGaAs arrayed detector, selection and wavelength-parallel detection of individual polarization components can be achieved within approximately 150 microseconds. This instrument offers unprecedented sensing capability that is relevant to the compensation of polarization-related impairments in high-speed light-wave communications.     

Broadband polarization correction with programmable liquid-crystal modulator arrays

We demonstrate a novel method of parallel, multiwavelength state-of-polarization (SOP) correction. Using a new liquid-crystal modulator array design, we are able to rotate the distorted input SOP spectrum to a fixed linear state on a wavelength-by-wavelength basis. We report experimental correction of up to 25.5-dB polarization-dependent loss over a 13-nm bandwidth around 1550 nm.     

Hadron masses and screening from anti-de Sitter space Wilson loops

We show that in strongly coupled N=4 supersymmetric Yang-Mills conformal theory the binding energy of a heavy and a light quark is independent of the strength of the coupling constant. As a consequence we are able to show that in the presence of light quarks the analog of the QCD string can snap and color charges are screened. The resulting neutral mesons interact with each other only via pion exchange and we estimate the masses of those states.     

Multi-Implicit Peer Two-Step W-Methods for Parallel Time Integration

Peer two-step W-methods are designed for integration of stiff initial value problems with parallelism across the method. The essential feature is that in each time step s ‘peer’ approximations are employed having similar properties. In fact, no primary solution variable is distinguished. Parallel implementation of these stages is easy since information from one previous time step is used only and the different linear systems may be solved simultaneously. This paper introduces a subclass having order s−1 where optimal damping for stiff problems is obtained by using different system parameters in different stages. Favourable properties of this subclass are uniform stability for realistic stepsize sequences and a superconvergence property which is proved using a polynomial collocation formulation. Numerical tests on a shared memory computer of a matrix-free implementation with Krylov methods are included. AMS subject classification (2000) 65L06, 65Y05.Received June 2004. Revised January 2005. Communicated by Timo Eirola.Helmut Podhaisky: The work of this author was supported by the German Academic Exchange Service, DAAD.     

Neutrino mass anarchy

What is the form of the neutrino mass matrix which governs the oscillations of the atmospheric and solar neutrinos? Features of the data have led to a dominant viewpoint where the mass matrix has an ordered, regulated pattern, perhaps dictated by a flavor symmetry. We challenge this viewpoint and demonstrate that the data are well accounted for by a neutrino mass matrix which appears to have random entries.