Showing papers by "Vienna University of Technology published in 1985"

From Hamiltonians to Phase Diagrams

Abstract: The ab-initio calculation of phase diagrams for metals and alloys using electronic and thermodynamic perturbation theories is discussed.

First-Principles Calculation of the Electric Field Gradient of Li 3 N

Abstract: The electric field gradient can be obtained from self-consistent energy-band calculations by the linearized-augmented-plane-wave method provided that a general potential is used. This first-principles method, which does not rely on any Sternheimer antishielding factor, is tested for ${\mathrm{Li}}_{3}$N and yields electric field gradients for Li(1), Li(2), and N in excellent agreement with NMR experiments. The electric field gradient is mainly determined by local distortions of the electronic charge density, especially in the case of the polarizable ${\mathrm{N}}^{3\ensuremath{-}}$ ion.

Determination of the nonlinear optical susceptibility χ(2) of surface layers by sum and difference frequency generation in reflection and transmission

Abstract: The theoretical investigation of sum and difference frequency generation in thin surface layers with rotational symmetry leads to formulas which connect the generated light intensities to the surface second order nonlinear susceptibility tensor. A maximum of seven tensor components can be determined in the case of lowest symmetry. Measurements in transmission should be especially useful since they allow easy variation of both polarization and angle of incidence. On the other hand, large signal enhancements are expected for total internal reflection geometries. A consistent set of χ(2) tensor components for a thin layer of rhodamine-6G adsorbed on fused silica is found based on data from reflection and transmission measurements.

Measuring the dimension of space time.

Abstract: Operationalistic definition of the dimension of space-time leads to the possibility of its experimental determination. Several reasons may be given for the fractional dimension of space-time to be slightly smaller than four, yielding a finite quantum electrodynamics. Comparison between the best experimental values for the electron $g\ensuremath{-}2$ factor and theoretical prediction gives the value 4- (5.3\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ for the dimension of space-time.

Decay of momentum flux in submerged jets

Abstract: Slender laminar and turbulent, plane and axisymmetric jets emerging from orifices in plane or conical walls are studied at large distances from the orifices. The entrainment of momentum coupled with the entrainment of volume into a jet is determined, and its effect on the flow field is analysed by combining inner and outer expansions with a multiple scaling approach.In turbulent (plane or axisymmetric) jets, the axial velocity decreases more rapidly than predicted by classical boundary-layer solutions, and the momentum flux vanishes as the distance from the orifice tends to infinity. The analysis unveils a source of discrepancies in previous experimental data on turbulent jets.In a laminar plane jet, the momentum flux changes but little. In a laminar axisymmetric jet, the momentum flux changes slowly, yet considerably. When a critical distance from the orifice is approached, the momentum flux in the jet becomes very small, the jet diameter very large, and a toroidal viscous eddy is predicted. The structure of the flow field is briefly discussed.

Formation and location of glucose oxidase in citric acid producing mycelia of Aspergillus niger

Abstract: The formation and location of glucose oxidase was studied in Aspergillus niger, which was pregrown under citric acid producing conditions. Glucose oxidase could be “de novo” induced by a shift in pH from 1.7 to 5.5. The induction required the intracellular presence of either glucose or glucose-6-phosphate. Glucose oxidase so produced was rapidly secreted into the medium, which was not due to autolysis.

Stochastical approximation of convex bodies

Abstract: Enveloppe convexe de points aleatoires sur la sphere. Enveloppe convexe de points aleatoires sur la frontiere d'un corps convexe lisse

On the efficiency of subsumption algorithms

Abstract: The costs of subsumption algorithms are analyzed by an estimation of the maximal number of unification attempts (worst-case unification complexity) made for deciding whether a clause C subsumes a clause D. For this purpose the clauses C and D are characterized by the following parameters: number of variables in C, number of literals in C, number of literals in D, and maximal length of the literals. The worst-case unification complexity immediately yields a lower bound for the worst-case time complexity.First, two well-known algorithms (Chang-Lee, Stillman) are investigated. Both algorithms are shown to have a very high worst-case time complexity. Then, a new subsumption algorithm is defined, which is based on an analysis of the connection between variables and predicates in C. An upper bound for the worst-case unification complexity of this algorithm, which is much lower than the lower bounds for the two other algorithms, is derived. Examples in which exponential costs are reduced to polynomial costs are discussed. Finally, the asymptotic growth of the worst-case complexity for all discussed algorithms is shown in a table (for several combinations of the parameters).

Binding mechanism and itinerant magnetism of ZrFe2 and YFe2

Abstract: Self-consistent spin-polarized energy band calculations for the two Laves phases show that a ferrimagnetic state is more stable than the paramagnetic one, but at a slightly larger volume. For ZrFe2 the interaction between the low lying Zr and the exchange-split Fe states leads to covalent magnetism which is discussed using site- and spin-projected densities of states. The different interaction between Fe and Zr for majority and minority spin is responsible for the ferrimagnetic ordering which is analyzed in terms of spin densities. The resulting magnetic moment of about 1.9 μB for Fe is spatially localized near the Fe site, while around Zr a small but extended negative spin density causes (through the large volume) a moment of about -0.56 μB a prediction which should be verified experimentally. YFe2 is qualitatively similar, but contrasts YCo2 which is near a possible metamagnetic transition.

The use of X-ray diffraction peak-broadening analysis to characterize ground Al2O3 powders

Abstract: X-ray peak broadening has been used to study the milling behaviour of a number of commercial alumina powders. It is shown that the milling behaviour is dependent upon the original particle size, internal defects in particles and the milling liquid used. Peak-broadening studies allow the effects of milling upon reduction of crystallite size and increase in stored energy to be separated. The effect of these two parameters was separated using the Cauchy correction method. Measurement of the particle size of the unmilled alumina powders in the transmission electron microscope was used to determine that the Cauchy method gave the most correct estimation of crystallite size. Both alumina crystallite size and stored energy are expected to enhance sintering of the powder to a high density. Attempts are made to predict the sintering ability of the materials studied in terms of the above parameters.

Least‐squares fits of fundamental parameters for quantitative x‐ray analysis as a function of Z (11 ≤ Z ≤ 83) and E (1 keV ≤ E ≤ 50 keV)

Abstract: A presentation by useful approximations of fundamental parameters needed for XRF and EPMA is given. The set of fundamental parameters include: critical energy or edge energy, energy of characteristic x-rays, edge-jump, fluorescence yield, transition probability, mass absorption coefficient, backscatter coefficient, mean ionisation potential and atomic weight.

Thermodynamic variational method for liquid alloys with chemical short-range order.

Abstract: We present a first-principles study of chemical short-range ordering in liquid (s,p)-bonded alloys. Our approach is based on an optimized pseudopotential technique for the construction of the interatomic potentials and a thermodynamic variational technique based on the Gibbs-Bogoliubov inequality and hard-sphere Yukawa reference potentials (we use the analytical solution of the mean spherical solution for the equal-diameter case). The analysis of the redistribution of the valence electrons upon alloying allows us to elucidate the electronic origin of the ordering potential. In the case of a moderately strong ordering interaction, the application of the Gibbs-Bogoliubov variational technique yields a reasonably accurate prediction of the structure factors and of the thermodynamic excess functions. For very strong ordering potentials, a free minimization of the variational upper bound to the exact free energy gives unrealistic results. This is a consequence of the complete decoupling of number-density and concentration fluctuations in the mean-spherical approximation to the equal-diameter hard-sphere Yukawa mixture. We find that realistic solutions may be found by imposing the condition that the exact and the reference-system ordering potentials be the same at the mean effective atomic diameter. This constrained minimization of the variational free energy yields good results for the structure factors, but rather bad ones for the thermodynamic excess functions. We are able to show that this is due to a neglect of the finite electronic mean free path of the electrons in those concentration regions where it is comparable to the mean interatomic distance.

Metabolic effects of manganese deficiency in Aspergillus niger: evidence for increased protein degradation

Abstract: The effect of manganese deficiency on macromolecule synthesis has been studied in a citric acid producing strain of Aspergillus niger: pulse labelling experiments showed that the synthesis of both protein and RNA was not influenced by the presence of manganese; however, increased protein degradation occurred under manganese deficiency. This was also reflected by the increased activity of an intracellular proteinase activity under these conditions. In replacement cultures addition of inhibitors of RNA, DNA or protein synthesis revealed that only emetine and cycloheximide (which both act at the ribosome) successfully antagonized the adverse effect of manganese ions on citric acid accumulation. Manganese deficiency was also characterized by a decreased portion of polysomes and 80 S ribosomes.

NADPH-specific and NADH-specific xylose reduction is catalyzed by two separate enzymes in Pachysolen tannophilus

Abstract: Pachysolen tannophilus contains — in addition to an NADPH-linked xylose reductase — a separate NADH-linked one, in this respect differing from the yeast Pichia stipitis. Both enzyme proteins can conveniently be separated from each other by either ion exchange chromatography or chromatofocusing.

Phase transitions between the quasicrystalline, crystalline, and amorphous phases in Al16 at% V

Abstract: By means of electron irradiation the amorphous state can be produced in quasicrystalline and crystalline Al14 at% Mn. The transitions between these three phases are studied in situ in a high-voltage electron microscope. The behaviour of the quasicrystalline phase exhibits features distinctly different from those of the crystalline phase. Under certain conditions the phase transition quasicrystalline–amorphous is reversible and the quasicrystalline phase develops from the amorphous state. From the temperature dependence of the transformation kinetics conclusions can be drawn on the temperature dependence of atom transport in the phases investigated. Mit Hilfe von Elektronenbestrahlung last sich in quasikristallinem und kristallinem Al14 at% Mn der amorphe Zustand einstellen. Die Ubergange zwischen diesen drei Phasen werden in situ in einem Hochspannungselektronenmikroskop untersucht. Die quasikristalline Phase zeigt dabei ein Verhalten, das deutlich verschieden ist von dem der kristallinen Phase. Der Ubergang quasi-kristallin–amorph kann reversibel gefuhrt werden, d. h. unter bestimmten Bedingungen bildet sich aus dem amorphen wieder der quasikristalline Zustand. Aus der Temperaturabhangigkeit der Kinetik der Phasenubergange last sich auf die Temperaturabhangigkeit des Atomtransportes in den untersuchten Phasen schliesen.

Resistivity of liquid, supercooled liquid and amorphous alloys under elevated pressure

Abstract: We present a calculation of the electrical resistivity of liquid, supercooled liquid and amorphous Mg 7 Zn 3 and Ca 7 Mg 3 alloys as a function of temperature and pressure on the basis of the generalized Faber-Ziman theory. The calculation has been performed from first principles, without any adjustable parameter. The theory successfully explains the variation of the electrical resistivity with temperature [positive temperature coefficient (TCR) for CaMg, a generally negative TCR in MgZn with shallow resistivity minima in the liquid and small resistivity maxima and minima in the amorphous phase] and with pressure [positive pressure coefficient (PCR) for MgZn, essentially zero PCR for CaMg] in terms of the partial structure factors, ion-electron potentials and vibrational spectra. We demonstrate the important influence of the thermal expansion upon the calculated resistivity isobars.

Visualization of the viscous flow induced by a round jet

Abstract: The flow induced by a laminar round glycerol jet emerging from a plane wall is investigated The flow field (jet region as well as outer flow field) is visualized by means of very small bubbles suspended in the fluid For slender jets the observed flow pattern disagrees with both potential-flow theory and Squire's ‘exact’ solution, but it is in excellent agreement with results of an asymptotic analysis by Schneider (1981) At a downstream distance that depends exponentially on the square of the Reynolds number, the jet develops into a toroidal eddy as predicted by Schneider (1985) In an apparatus of reasonable size the eddy can be observed only in a very limited Reynolds-number regime

Quantitative electron microprobe analysis without standard samples

Abstract: This review of the theory of pure element count rates measured by means of an electron microprobe analysis system with energy-dispersive x-ray detection defines the fundamental parameters needed for an analytical algorithm. Additionally, a presentation of these parameters by useful approximations is given. This is a basic requirement for the evaluation of theoretical expressions valid for characteristic x-ray intensities of multielement samples. The derivation of these expressions is also given and from the results of numerous experiments it is concluded that (a) there is no systematic deviation of measured concentrations when compared with chemical analysis, (b) the quality expressed by absolute and relative errors of the analytical results is as good as that obtained by known methods that require standard samples, (c) the presented computer program, written for an HP 75C portable computer, gives the results of analysis within a calculation time of about 1 min and (d) the program is applicable to EDS and WDS both energy- and wavelength-dispersive spectrometers and the range of application covers elements of Z ≥ 12.

Total single-electron-capture cross sections for impact of H+, H2 +, He+, and Ne+ (2-20 keV) on Li.

Abstract: Total cross sections for single-electron capture from Li(2s) by ${\mathrm{H}}^{+}$, ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$, ${\mathrm{He}}^{+}$, and ${\mathrm{Ne}}^{+}$ have been measured at impact energies between 2 and 20 keV. All data sets show maximum cross sections of about 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ and are compared with other available measurements as well as calculations.

Results of Baire Category Type in Convexity

Abstract: In the following, there is first a short survey of Bake category results in convexi ty (Section 2), then two new results are presented: one on minimal ellipsoids (Section 3), and one on lattice packings of convex bodies (Section 4). A convex body in d-dimensional Euclidean space Ed is a compact convex subset of Ed with nonempty interior. Denote by V the space of all convex bodies in Ed endowed with its “natural” topology. This topology is induced by, e.g., the Hausdorff metric or the symmetric difference metric dS, defined by

D-amino acid oxidase from Trigonopsis variabilis : Comparison of enzyme specificity “in vivo” and “in vitro”

Abstract: A method is described for permeabilization of intact cells of the yeastTrigonopsis variabilis with respect toin vivo measuring D-amino acid oxidase activity. The kinetic results so obtained differ from those obtained with the purified enzyme, pointing to the advantage of using the purified enzyme or the permeabilized cells in the oxidative deamination of different D-amino acids.

Effect of manganese deficiency on plasma-membrane lipid composition and glucose uptake in Aspergillus niger

Abstract: Plasma membranes were isolated by means of the concanavalin A technique from protoplasts of manganese deficient (< 10−8 M Mn2+) and sufficient (10−5 M Mn2+) grown mycelium. The membranes differed with respect to their quantitative contents of fatty acids, sterols and phospholipids. These changes did not influence the glucose transport system, as shown by kinetic investigations using intact mycelia.