Showing papers by "Vienna University of Technology published in 1983"

Approximation of convex bodies

Abstract: Approximation of convex bodies by either smooth convex bodies or polytopes has been considered frequently in convexity. There are two incentives for it. On the one hand approximation is used as a tool for investigations ranging from classical results on mixed volumes to questions concerning the e-entropy of spaces of convex bodies, on the other hand there is an intrinsic geometric interest in the approximation problem itself.

Electron densities and chemical bonding in TiC, TiN, and TiO derived from energy band calculations

Abstract: The chemical bonding in the interesting class of refractory transition metal compounds is illustrated for TiC, TiN, and TiO. Self-consistent augmented plane wave (APW) calculations are already available for these compounds. Using the respective potentials we have repeated the band calculations on a finer k grid with the linearized APW method to obtain accurate densities of states (DOS). These DOS can be divided into local partial contributions to characterize the bonding. Further information can be obtained from a decomposition of the metal dDOS into t2g and eg symmetry components. These partial local DOS are compared with the LCAO counterpart and give a first picture of the chemical bonding in these compounds. The electron densities corresponding to the occupied valence states are obtained from the LAPW calculations. They provide further insight into characteristic trends in the series from TiC to TiO: around the nonmetal site the density shows increasing localization; around the metal site the deviation from spherical symmetry changes from eg to t2g. These effects can be traced back to the three types of valence bands. Electron density plots of characteristic band states (all energies of a selected k point in the Brillouin zone) will be shown. These plots can describe the different types of bonding occurring in these systems.

The aliasing problem in discrete-time Wigner distributions

Abstract: There is no straightforward way to proceed from the continuous-time Wigner distribution to a discrete-time version of this time-frequency signal representation. A previously given definition of such a function turned out to yield a distribution that was periodic with period π instead of 2π and this caused aliasing to occur. Various alternative definitions are considered and compared with respect to aliasing and computational complexity. From this comparison it appears that no definition leads to a function that is optimum in all respects. This is illustrated by an example.

Finite boxes—A generalization of the finite-difference method suitable for semiconductor device simulation

Abstract: A two-dimensional numerical device-simulation system is presented. A novel discretization scheme, called "finite boxes," allows an optimal grid-point allocation and can be applied to nonrectangular devices. The grid is generated automatically according to the specified device geometry. It is adapted automatically during the solution process by equidistributing a weight function which describes the local discretization error. A modified Newton method is used for solving the discretized nonlinear system. To achieve high flexibility the physical parameters can be defined by user-supplied models. This approach requires numerical calculation of parts of the coefficients of the Jacobian. Supplementary algorithms speed up convergence and inhibit the commonly known Newton overshoot. The advantages and computer resource savings of the new method are described by the simulation of a 100-V diode. We also present results for thyristor and GaAs MESFET simulations.

Structure and vibrational dynamics of the metallic glass Ca 70 Mg 30

Abstract: The atomic structure and the vibrational dynamics are investigated for a realistic model of the metallic glass ${\mathrm{Ca}}_{70}$${\mathrm{Mg}}_{30}$. The calculation is based on a first-principles pseudopotential treatment of the interatomic potentials and a cluster relaxation technique for the determination of the equilibrium atomic structure and density. The recursion method has been used to compute the vibrational spectra. For the first time, a quantitative comparison between theory and experiment on the dynamical properties of a metallic glass is now possible. The theory explains successfully the low-energy vibrational modes characteristic of the amorphous state and the dispersion law for propagating collective modes.

A Spread-Spectrum Concept Combining Chirp Modulation and Pseudonoise Coding

Abstract: This paper investigates the combination of chirp modulation and pseudonoise phase shift keying for spread-spectrum transmission of digital data. Data encoding is accomplished by assignment of positive and negative dispersive slope chirps for binary ones and zeros. Each chirp is then modulated by one period of a PN signature sequence. The performance in additive white Gaussian noise is analyzed, measures of performance being mean acquisition time, mean hold-in time, and bit error probability. Calculated and experimental results are presented for a 105 kbit/s modem. In the test system, SAW dispersive delay lines with a bandwidth of 16.4 MHz and 9.5 μs dispersion time serve for the key operations of chirp generation and pulse compression, and the signature codes are 127 bit m -sequences.

Application of the theory of kinematic waves to the centrifugation of suspensions

Abstract: The settling of solid particles in a liquid due to centrifugal force is described by using the equations of one-dimensional unsteady two-phase flow. By neglecting the acceleration terms the momentum equation can be reduced to a functional relationship between the dependant variables of the problem (volumetric concentration α of the solid particles and volumetric flux j). As an additional relation for the unknown quantities, a first order partial differential equation is obtained from the equation of continuity for the solid particles. Concentration jumps (e.g. discontinuities between suspension and clear liquid or sediment) are described as kinematic shock waves. Analytical solutions are obtained for the kinematic wave fronts and for certain cases of shock waves. The results for the centrifugation process with uniform particle size show that several cases are to be distinguished. Under certain conditions the concentration of the suspension depends on time only and not on the radial coordinate of the rotating system. Other initial conditions give additional discontinuities within the suspension.

Imaging XPS—A new technique, I—principles

Abstract: XPS imaging promises to be a powerful analytic tool because it enables specific information on both elements and bonding to be recorded on a two-dimensional distribution map. As far as the authors are aware, the only scanning XPS method to date which has been found to be practical is essentially a scanned-particle-beam method, like scanning AES, and it is only applicable to thin film specimens. This paper provides the basic ideas of a new imaging XPS technique based on a quite different concept. It will be applicable to any kind of specimen that can be analysed in a conventional XPS system. It makes use of the dispersion properties of a spherical condenser-type spectrometer and applies a two-dimensional electron detection device for decoding the energy and emission position of an analysed photoelectron. Experimental arrangement and theory of operation are presented.

Costas Loop Experiments for a 10.6 µm Communications Receiver

Abstract: We have transferred the principle of the Costas-type nonlinear phase-locked loop to the \lambda = 10.6 \mu m wavelength of CO 2 lasers. The ability of the optical Costas loop to regenerate the carrier of a binary phase-shift-keyed input signal and to perform coherent demodulation was demonstrated at a data rate of 20 Mbits/s.

A singular perturbation approach for the analysis of the fundamental semiconductor equations

Abstract: This paper is concerned with a singular perturbation analysis of the two-dimensional steady-state semiconductor equations and of the usual finite difference scheme consisting of the five point discretization of Poisson's equation and of the Scharfetter--Gummel discretization of the continuity equations. By appropriate scaling we transform the semiconductor equations into a singularly perturbed elliptic system with nonsmooth data. The singular perturbation parameter is defined as the minimal Debeye-length of the device under consideration. Singular perturbation theory allows to distinguish between regions of strong and of weak variation of solutions, so called layers and smooth regions, and to describe solutions qualitatively in these regions. This information is used to analyze the stability and convergence of the discretization scheme. Particular emphasis is put on the construction of efficient grids. It is shown that the Scharfetter-Gummel method is uniformly convergent, i.e., the global error contribution coming from the continuity equations is small when the maximal mesh size is small, independent of the gradient of the solution. Layer jumps are automatically resolved. The five point scheme however is not uniformly convergent. Large gradients of solutions require a graded mesh if solutions inside the layers are to be resolved accurately. This can lead to an intolerably large number of gridpoints. Therefore, we present a modification of the five point scheme which is uniformly convergent.

Nucleotide pools of growing, synchronized and stressed cultures of Saccharomyces cerevisiae

Abstract: High pressure liquid chromatography has been used to study the acid soluble nucleotide pool of Saccharomyces cerevisiae under different conditions of growth. ATP, ADP, AMP, NAD, GTP, UTP, UDP, CTP, CDP, and UDP-sugars plus UMP could be separated and were found in concentrations higher than 0.1 mumol per g yeast cell dry weight (= detection limit). During glucose-limited continuous culture the levels of individual nucleotides depended on the growth rate, which was most pronounced with pyrimidine (uridine, cytidine) nucleotides. The energy charge (E.C.) remained high (0.9) at all growth rates (0.07-0.3 h-1). During synchronized growth at a constant growth rate (0.11 h-1) almost all nucleotide levels and the E.C. remained at constant values with the only exception of UDP-sugars and UMP of which increased levels were found during the phase of budding. Under conditions of metabolic stress (addition of antimycin A, deoxyglucose plus iodoacetate) pronounced changes in the levels of purine (adenine and guanine) nucleotides and the E.C. were observed. All other nucleotides were less influenced by these conditions. Only under these conditions IMP accumulation was observed. The results strongly argue against the significance of purine nucleotide or E.C. measurements under viable conditions. In contrast, changes in the levels of pyrimidine nucleotides seem to be indicative of changes in the flux through the metabolic pathways where they act as coenzymes.

Energy Distribution of Sputtered Metastable Ca Atoms

Abstract: The energy distribution of sputtered metastable Ca atoms in the $^{3}P_{2}$ sublevel of the lowest-lying triplet state has been measured by Doppler-shift laser-fluorescence spectroscopy. The energy distribution was found to peak at values higher than known for sputtered ground-state atoms. No threshold energy has been found. With increasing oxygen partial pressure the flux of sputtered metastable atoms increased by an order of magnitude; however, the shape of the energy distribution remained unchanged.

Stress distribution in a rotating elastic-plastic tube

Abstract: Subject of the investigation is the distribution of displacement and stresses in a tube with fixed ends of elastic-plastic material under the assumption of Tresca's yield condition, its associated flow rule and linear strain-hardening.

Theory of formation of metallic glasses. II

Abstract: Low-temperature limits for the metastability of the supercooled liquid state are derived from a thermodynamic perturbation calculation. Together with a previously published calculation of the energetics of solid and liquid alloy phases, this allows for an ab initio determination of the glass-forming ability of metallic alloys.

Small‐amplitude Finite‐rate Waves in Suspensions of Particles in Fluids

Abstract: The paper deals with the propagation of weakly nonlinear waves in homogeneous suspensions. Under the assumption that a one-dimensional treatment is adequate the properties of these waves are investigated in stable as well as unstable flow and near the stability limit. Special emphasis is placed on the discussion of waves of constant form. Der Artikel beschaftigt sich mit der Ausbreitung schwach nichtlinearer Wellen in homogener Suspension. Mit der Annahme, das eine eindimensionale Behandlung angemessen ist, werden die Eigenschaften dieser Wellen sowohl in stabiler als auch instabiler Stromung und nahe der Stabilitatsgrenze untersucht. Insbesondere werden Wellen konstanter Form behandelt.

Collocation methods for boundary value problems on “long” intervals

Abstract: This paper deals with the numerical solution of boundary value problems of ordinary differential equations posed on infinite intervals. We cut the infinite interval at a finite, large enough point and insert additional, so-called asymptotic boundary conditions at the far (right) end and then solve the resulting two-point boundary value problem by an A-stable symmetric collocation method. Problems arise, because standard theory predicts the use of many grid points as the length of the interval increases. Using the exponential decay of the 'infinite' solution, an 'asymptotic' a priori mesh-size sequence which increases exponentially, and which therefore only employs a reasonable number of meshpoints, is developed and stability, as the length of the interval tends to infinity, is shown. We also show that the condition number of the collocation equations is asymptotically proportional to the number of meshpoints employed when using this exponentially graded mesh. Using k-stage collocation at Gaussian points and requiring an accuracy 0(E-) at the knots implies that the number of meshpoints is Q(g-i/2k) as E O 0.

Infra-red homodyne receiver with acousto-optically controlled local oscillator

Abstract: A homodyne receiver for 10 μm radiation has been built, based on the phase-locked-loop principle. Fast frequency control of the local CO2 laser is achieved by an acoustooptic frequency shifter. Demodulation of a 100 Mbit/s phase-modulated input signal has been demonstrated.

Magnetism and Hyperfine Interactions in Ti – Mn Intermetallic Hydrides

Abstract: We have studied the magnetic properties of the C14 Laves phase Ti0.40Mn0.60 (homogeneity range from Ti0.33Mn0.67 to Ti0.45Mn0.55), of the p phase Ti0.46Mn0.54, and of the ϕ phase Ti0.48Mn0.52, as well as the magnetic properties of their hydrides. The investigation includes susceptibility measurements in the temperature range 5 to 360 K, magnetization measurements between 4.2 K and 250 K with fields up to 7.2 T, and Mossbauer effect measurements on 57Fe doped samples at 4.2 K and 295 K. Additionally, low temperature heat capacity measurements have been performed. While the Ti – Mn intermetallic C14, ρ and ϕ phases themselves are paramagnetic with susceptibilities independent of temperature, their hydrides are ferromagnetic with Curie temperatures of about 213 K, 87 and 155 K, respectively. The ρ and ϕ phase hydrides possess a weak itinerant ferromagnetism, whereas the ferromagnetism of the C14 Laves phase hydride is of a more localized type. The susceptibilities above Tc consist of Curie Weiss terms and large temperature independent contributions. The non-symmetric quadrupole split Mossbauer spectrum at room temperature indicates a distortion of a fraction of the tetrahedrons in the uncharged Ti0.40Mn0.60 Laves phase which is removed by hydrogenation. The onset of ferromagnetism in the Ti – Mn ternary hydrides is due to the volume expansion which causes a narrowing of the d bands and thus increases the density of states at the Fermi level upon hydrogenation.

The application of iterated defect correction to variational methods for elliptic boundary value problems

Abstract: We construct a method which makes it possible to apply the idea of iterated defect correction to finite element methods. The construction is motivated heuristically. We believe that the significance of our method lies in the possibility to write “metaalgorithms” for existing finite element program packages which entail a substantial improvement of the accuracy of these program packages. The efficiency of the method is demonstrated in a number of examples.

Beiträge zur Diskrepanz bezüglich gewichteter Mittel

Abstract: This paper contains results on the discrepancy of sequences with respect to weighted means. Upper and lower bounds for this discrepancy are established. A metric theorem which extends a classical result of Erdos and Koksma is shown. Applications to numerical integration are presented.

Vapor flow through a porous membrane — A throttling process with condensation and evaporation

Abstract: The one-dimensional flow of a vapor through a porous membrane is considered. Upstream the membrane the vapor is assumed to be in a state of saturation, downstream the membrane there may be superheated vapor, saturated vapor or a two-phase mixture. It is shown that heat conduction in flow direction is important if the Joule-Kelvin coefficient of the vapor is positive and the permeability of the membrane is sufficiently small. This results in condensation at the front surface of the membrane, liquid flow in the membrane or part of it, and re-evaporation at the rear surface or in an evaporation front in the interior of the membrane. If the permeability of the membrane is below a critical value that depends on thermodynamic quantities only, the energy balance requires the formation of a condensate film in front of the membrane. Four possible types of throttling processes are analyzed for small pressure variations and an equation for the mass flow rate in terms of the pressure difference across the membrane is given. The predicted mass flow rate of isobutane through a microporous polypropylene membrane compares favourably with measurements.

Time - harmonic and nonstationary stochastic vibrations of arch dam - reservoir - systems

Abstract: Solutions for the interacting vibrations of a linear elastic arch dam with a linear compressible, three-dimensional, irregularly shaped fluidbody are presented. The vibration response is derived for a time harmonic excitation of the arch dam and, with regard to an earthquake analysis, for nonstationary stochastic excitation processes. The expansions of the stochastic responses are based on time-dependent power spectral density functions, demanding the evaluation of the frequency response spectras in advance. These time-harmonic solutions are obtained by means of substructure synthesis method, thereby applying a boundary integral equation formulation for the vibrating fluidbody.