Das kubische Tantalmononitrid (B 1-Typ) und seine Mischbarkeit mit den isotypen Übergangsmetallnitriden und-carbiden

The position of the recently detected cubic -TaN_1–x phase (B1 type) with respect to the other tantalum nitride phases in the system Ta–N is discussed and a tentative phase diagram for the partial system Ta–N is proposed. -TaN_1–x , a high temperature phase, is completely miscible with all isotypic mononitrides and monocarbides of the 4 a and 5 a group metals. Lattice parameters of the solid solutions are given.

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Mit 4 Abbildungen

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Herrn Prof. Dr.O. Hromatka zum 70. Geburtstag gewidmet.     

Beitrag zur Kenntnis des Systems La−C−N

An isothermal section at 1,200°C of the partial system La–LaN–LaC₂ was examined by X-ray and metallographic techniques. There is no evidence of an appreciable solid solubility of carbon in LaN or of nitrogen in La₂C₃ or LaC₂ at 1,200°C. There are indications however that at higher temperatures, especially that of the electric arc, the solubility of carbon in LaN might be appreciable or that a carbonitride phase of Lanthanum might exist. LaC₂ reacts with nitrogen under pressure at 1,200°C to form a new carbonitride phase with the composition La₁C_0,88N_0,77.

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Mit 2 Abbildungen     

Zur Existenz des Seandiummonoxides

Zusammenfassung Durch Reduktion von Sc₂O₃ mit Silicium oder Magnesium bildet sich ein Produkt mit B1-Typ und einer Gitterkonstantea=4,45 Å. Die früher bestimmten Gitterparameter im Bereich 4,48–4,54 Å gelten für Produkte, die etwas Kohlenstoff enthalten, d. h. für SxO_xC_y mitx=0,95–1,03 undy=0,03–0,05.

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Existence of scandium oxide

The reduction of Sc₂O₃ with silicon or magnesium yields a product of B1-type, lattice-constanta=4,45 Å. Lattice parameters, obtained before, at 4,48–4,54 Å are typical for substances containing some carbon, i.e. for ScO_xC_y, withx=0,95–1,03 andy=0,03–0,05.

     

Calculation of electric hyperfine interaction parameters in solids

A theoretical approach for the calculation of hyperfine parameters in solids from first principles is presented. These calculations are based on the full potential linearized-augmented-plane-wave (LAPW) band structure method, which is currently one of the most accurate schemes to determine the electronic structure in ordered solids. Exchange and correlation is treated within density functional theory using the generalized gradient approximation. Once the electron density is calculated self-consistently with high accuracy, quantities like electric field gradients (EFG), isomer shifts or hyperfine fields can easily be obtained from this density without further approximations. Using this approach we have studied various systems including metals, insulators, ionic compounds or the highT _c superconductors. In general we find good agreement between theory and experiment, which proves that our method is very accurate. Having these results in mind we are confident that this method is accurate enough to determine the value of the nuclear quadrupole momentQ, provided experimental measurements of the quadrupole coupling constant are available. This procedure is demonstrated forQ of⁷⁷Se and¹⁰⁰Rh, two nuclei recently used in PAC measurements. An extensive study of EFGs at Fe sites in various Fe-compounds has been performed leading to a very reliable quadrupole moment ofQ(⁵⁷Fe)=0.16 b, a value twice as large as that deduced from recent HF calculations but back to older estimates.     

Über Sauerstoff-haltige Verbindungen vom Strukturtyp B1 der ersten vier Übergangsmetalle

Zusammenfassung Es werden die Herstellungsbedingungen und einige Eigenschaften der Monooxide der ersten vier Übergangsmetalle durch Reduktion ihrer höheren Oxide angeführt. Neben der geeigneten Auswahl von Glühtemperaturen in Wasserstoff-Atmosphäre ist auch die Anwesenheit einer gewissen CO-Menge notwndig, deren Grenzen bisher nicht bestimmt wurden.Die entstandenen pulverförmigen Produkte wurden einer GTA-, DTA-, Rö- und IR-spektroskopischen Analyse unterzogen; die gewonnenen Resultate werden mit bisherigen Kenntnissen verglichen.

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The conditions of preparation and some properties of monooxides of the first four transition-metals through reduction of their higher oxides are described. In addition to suitable choice of heating temperatures in a hydrogen atmosphere the presence of certain carbon-monoxide-amounts is necessary, the limits of which were not yet identified.The powdered products were examined by GTA-, DTA-, Rö, and IR-spectroscopical analysis and the obtained results were compared with existing knowledges.

     

Suspended load and bed-load transport of particle-laden gravity currents: the role of particle–bed interaction

The development of particle-enriched regions (bed-load) at the base of particle-laden gravity currents has been widely observed, yet the controls and relative partitioning of material into the bed-load is poorly understood. We examine particle-laden gravity currents whose initial mixture (particle and fluid) density is greater than the ambient fluid, but whose interstitial fluid density is less than the ambient fluid (such as occurs in pyroclastic flows produced during volcanic eruptions or when sediment-enriched river discharge enters the ocean, generating hyperpycnal turbidity currents). A multifluid numerical approach is employed to assess suspended load and bed-load transport in particle-laden gravity currents under varying boundary conditions. Particle-laden flows that traverse denser fluid (such as pyroclastic flows crossing water) have leaky boundaries that provide the conceptual framework to study suspended load in isolation from bed-load transport. We develop leaky and saltation boundary conditions to study the influence of flow substrate on the development of bed-load. Flows with saltating boundaries develop particle–enriched basal layers (bed-load) where momentum transfer is primarily a result of particle–particle collisions. The grain size distribution is more homogeneous in the bed-load and the saltation boundaries increase the run-out distance and residence time of particles in the flow by as much as 25% over leaky boundary conditions. Transport over a leaky substrate removes particles that reach the bottom boundary and only the suspended load remains. Particle transport to the boundary is proportional to the settling velocity of particles, and flow dilution results in shear and buoyancy instabilities at the upper interface of these flows. These instabilities entrain ambient fluid, and the continued dilution ultimately results in these currents becoming less dense than the ambient fluid. A unifying concept is energy dissipation due to particle–boundary interaction: leaky boundaries dissipate energy more efficiently at the boundary than their saltating counterparts and have smaller run-out distance.

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