Einfluß des Erweichungsprozesses auf die Geschwindigkeit der Reduktion von Sinter mit Wasserstoff

Im vorliegenden Beitrag wurde der Einfluß des Erweichungsprozesses auf die Reduktionsgeschwindigkeit von Sinter untersucht. Dabei wurde die Reduktionsgeschwindigkeit von Hochofensinter in einem Temperaturbereich um den Erweichungspunkt bestimmt. Es wurde ein rapider, mit dem Erweichungseffekt wie auch mit dem Porositätsschwund verknüpfter Geschwindigkeitsrückgang festgestellt. Diese Erscheinung findet in einem Temperaturbereich statt, der den Temperaturen des Erweichungsbeginns beim untersuchten Sintermaterial im vorreduzierten Zustand entspricht. Somit ist der Erweichungsvorgang als grundlegende Ursache für die festgestellte Abnahme der Reduktionsgeschwindigkeit zu betrachten. Mikroskopische Direktbeobachtungen der Reduktion von Sinter im Erweichungszustand zeigten Erscheinungen, die zu einem Porositätsschwund führen. Messungen des Porenvolumens und der spezifischen Porenoberfläche an Sinter – vor wie nach abgeschlossener Erweichung und Reduktion – bestätigten die Porositätsabnahme. Diese erschwert den Reduktionsgaszufluß zu den Eisenoxiden und verlangsamt so die Reduktionsgeschwindigkeit.     

The phase stability of solid LiAlO2 used for the electrolyte matrix of molten carbonate fuel cells

LiAlO₂ is used as a solid matrix for molten carbonate fuel cell (MCFC) electrolyte tiles. These devices operate within the temperature range of 870–970 K. The -crystallographic form of this compound is commonly used for fabrication of these matrix tiles. The thermodynamic stability of this phase within the above temperature range is not clear and it is reasonable to consider the transformation to take place in solid LiAlO₂ in presence of molten alkali carbonates. In order to establish the crystallographic form of the compound that forms as a product of reaction between the liquid Li₂CO₃ and solid Al₂O₃, the kinetics of LiAlO₂ formation has been investigated. Values of conversion factor as a function of the reaction time have been determined both for a stoichiometric reaction mixture and for mixtures with an excess of the liquid 0.53 Li₂CO₃ + 0.47 Na₂CO₃ eutectic. The simultaneous determination of the reaction conversion factor and the relative intensity of the characteristic x-ray peak for the – LiAlO₂ form in the reaction mixture have shown that, within 843–973 K, in experiments of ca 100–10 hours, respectively, only this phasev of LiAlO₂ is formed. These results may explain the presence of some – LiAlO₂ amount in the MCFC electrolyte tiles after long run tests of these devices.     

Processing high critical current density Bi-2223 wires and tapes

Considerable progress has been made in fabricating (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ in high-T_c superconductor wires or tapes with high critical current densities that are attractive for electric power and high-field magnet applications. The powder-in-tube technique appears to be useful for making silver-clad Bi-2223 composites. This article discusses the processing and the excellent superconducting properties of the resulting wires and tapes.     

Phase equilibria in the cobalt oxide-copper oxide system

Thermodynamic properties were determined for the system cobalt oxide-copper oxide by means of an electromotive force (EMF) measurement techniques using galvanic cells with calciastabilized zirconia (CSZ) as the solid electrolyte and with air as the reference electrode according to the following schemes: CuO, Cu₂O | CSZ | air and CoO-CuO, Cu₂O CSZ | air for composition variables y=X_Cu/(X_Co+X_cu equal to 0.05, 0.15, 0.25, 0.35, 0.45, 0.667, and 0.8; and within the temperature interval 1200–1350 K. Thermodynamic properties calculated directly from EMF values were combined with the available literature data on phase equilibria, and thermodynamic properties of solid phases in the Co-Cu-O system were assessed. Both terminal solid solutions, (Co,Cu)O and (Cu,Co)O, were described by a sublattice model with Redlich-Kister excess term. The interaction parameters for both (Co,Cu)O and (Cu,Co)O solid solutions and the Gibbs energy of formation for the intermediate phase Cu₂CoO₃ were obtained. The Gibbs energies of fictive end-members: monoclinic “CoO” and “CuO” with rock salt structure were derived as well. The phase diagrams were calculated using the assessed thermodynamic parameters. The (T, y) phase diagram was calculated for existence under ambient air. The property diagrams log₁₀P(O₂) versus composition and activity of CuO versus composition were calculated at 1273 K. The results of our calculations were in a good agreement with available experimental data. This paper was presented at CALPHAD XXX International Conference on Phase Diagram Calculations in York, UK, May 27–June 1, 2001, and appeared in the Conference Abstracts on Page 75.     

Equilibrium between ammonia-hydrogen mixtures and the ε phase of iron

Foils made from pure iron with a porous structure were nitrided in mixtures of pure and entirely decomposed ammonia until constant weight was reached. The relationship between nitriding potential, temperature and concentration was measured in the ε region between 350 and 725°C and at concentrations between and . This relationship can be mathematically represented, if the ε phase is considered to be a subregular solution of nitrogen in hexagonal iron. From the Gibbs free energy of the nitrogen in the gas phase, the numerical relation and the parameters L′ = ? 600 000 + 775.6 T J/mole L″ = 1270000 ? 1423 T J/mole are deduced. The results of a calculation of the (γ + ε)/ε phase boundary using this equations are in accordance with an experimental measurement.     

Discontinuous Petrov–Galerkin method with optimal test functions for thin-body problems in solid mechanics

We study the applicability of the discontinuous Petrov–Galerkin (DPG) variational framework for thin-body problems in structural mechanics. Our numerical approach is based on discontinuous piecewise polynomial finite element spaces for the trial functions and approximate, local computation of the corresponding ‘optimal’ test functions. In the Timoshenko beam problem, the proposed method is shown to provide the best approximation in an energy-type norm which is equivalent to the L₂-norm for all the unknowns, uniformly with respect to the thickness parameter. The same formulation remains valid also for the asymptotic Euler–Bernoulli solution. As another one-dimensional model problem we consider the modelling of the so called basic edge effect in shell deformations. In particular, we derive a special norm for the test space which leads to a robust method in terms of the shell thickness. Finally, we demonstrate how a posteriori error estimator arising directly from the discontinuous variational framework can be utilized to generate an optimal hp-mesh for resolving the boundary layer.     

USING DATA MINING ALGORITHMS IN WEB PERFORMANCE PREDICTION

This paper presents the application of data mining algorithms to the prediction of Web performance. Our domain-driven data mining uses historic HTTP transactions data reflecting Web performance as perceived by the end-users located in the Internet domain of Wroclaw University of Technology, Wroclaw, Poland. The predictive modeling features of two general data mining systems, Microsoft SQL Server and IBM Intelligent Miner, are compared. The neural networks, decision tree, time series, and transform regression models are evaluated. It is shown that the data mining algorithms return quite accurate prediction results. The best results are achieved using the IBM's transform regression algorithm.     

THE USE OF DATA MINING TO PREDICT WEB PERFORMANCE

Web mining is the area of data mining that deals with the extraction of interesting knowledge from World Wide Web data. The purpose of this article is to show how data mining may offer a promising strategy for discovering and building knowledge usable in the prediction of Web performance. We introduce a novel Web mining dimension—a Web performance mining that discovers the knowledge about Web performance issues using data mining. The analysis is aimed at the characterization of Web performance as seen by the end users. Our strategy involves discovering knowledge that characterizes Web performance perceived by end users and then making use of this knowledge to guide users in future Web surfing. For that, the predictive model using a two-phase mining procedure is constructed on the basis of the clustering and decision tree techniques. The usefulness of the method for the prediction the future Web performance has been confirmed in a real-world experiment, which showed the average correct prediction ratio of about 80%. The WING (Web pING) measurement infrastructure was used for active measurements and data gathering.     

Fast periodic graph exploration with constant memory

We consider the problem of periodic exploration of all nodes in undirected graphs by using a finite state automaton called later a robot. The robot, using a constant number of states (memory bits), must be able to explore any unknown anonymous graph. The nodes in the graph are neither labelled nor coloured. However, while visiting a node v the robot can distinguish between edges incident to it. The edges are ordered and labelled by consecutive integers 1,…,d(v) called port numbers, where d(v) is the degree of v. Periodic graph exploration requires that the automaton has to visit every node infinitely many times in a periodic manner. In this paper, we are interested in minimisation of the length of the exploration period. In other words, we want to minimise the maximum number of edge traversals performed by the robot between two consecutive visits of a generic node, in the same state and entering the node by the same port. Note that the problem is unsolvable if the local port numbers are set arbitrarily, see [L. Budach, Automata and labyrinths, Math. Nachr. 86 (1978) 195–282]. In this context, we are looking for the minimum function π(n), such that, there exists an efficient deterministic algorithm for setting the local port numbers allowing the robot to explore all graphs of size n along a traversal route with the period π(n). Dobrev et al. proved in [S. Dobrev, J. Jansson, K. Sadakane, W.-K. Sung, Finding short right-hand-on-the-wall walks in graphs, in: Proc. 12th Colloquium on Structural Information and Communication Complexity, SIROCCO 2005, in: Lecture Notes in Comput. Sci., vol. 3499, Springer, Berlin, 2005, pp. 127–139] that for oblivious robots π(n)10n. Recently Ilcinkas proposed another port labelling algorithm for robots equipped with two extra memory bits, see [D. Ilcinkas, Setting port numbers for fast graph exploration, in: Proc. 13th Colloquium on Structural Information and Communication Complexity, SIROCCO 2006, in: Lecture Notes in Comput. Sci., vol. 4056, Springer, Berlin, 2006, pp. 59–69], where the exploration period π(n)4n−2. In the same paper, it is conjectured that the bound 4n−O(1) is tight even if the use of larger memory is allowed. In this paper, we disprove this conjecture presenting an efficient deterministic algorithm arranging the port numbers, such that, the robot equipped with a constant number of bits is able to complete the traversal period in π(n)<3.75n−2 steps hence decreasing the existing upper bound. This reduces the gap with the lower bound of π(n)2n−2 holding for any robot.