Showing papers by "Technical University of Denmark published in 1995"

Why gold is the noblest of all the metals

Abstract: THE unique role that gold plays in society is to a large extent related to the fact that it is the most noble of all metals: it is the least reactive metal towards atoms or molecules at the interface with a gas or a liquid. The inertness of gold does not reflect a general inability to form chemical bonds, however—gold forms very stable alloys with many other metals. To understand the nobleness of gold, we have studied a simple surface reaction, the dissociation of H2 on the surface of gold and of three other metals (copper, nickel and platinum) that lie close to it in the periodic table. We present self-consistent density-functional calculations of the activation barriers and chemisorption energies which clearly illustrate that nobleness is related to two factors: the degree of filling of the antibonding states on adsorption, and the degree of orbital overlap with the adsorbate. These two factors, which determine both the strength of the adsorbate-metal interaction and the energy barrier for dissociation, operate together to the maxima] detriment of adsorbate binding and subsequent reactivity on gold.

Polyisoprene-Polystyrene Diblock Copolymer Phase Diagram near the Order-Disorder Transition

Abstract: The phase behavior of ten polyisoprene-polystyrene (PI-PS) diblock copolymers, spanning the composition range from 0.24 to 0.82 polyisoprene volume fraction (f PI ), has been studied near the order-disorder transition (ODT). Dynamic mechanical spectroscopy, transmission electron microscopy, and neutron and X-ray scattering have been used to characterize phase transition temperatures and ordered state symmetries. Five distinct microstructures were observed for this chemical system : spheres, hexagonally packed cylinders (HEX), lamellae (LAM), hexagonally perforated layers (HPL), and a bicontinuous cubic phase having an Ia3d space group symmetry. The bicontinuous Ia3d phase only occurs in the vicinity of the ODT between the HEX and LAM states at compositions of 0.65 ≤ f pI ≤ 0.68 and 0.36 ≤ f pI ≤ 0.39 (prior report). Farther from the ODT, within these composition ranges, the HPL phase occurs. We did not find the ordered bicontinuous double diamond (OBDD) morphology at any composition or temperature studied, and the overall phase diagram is qualitatively different from those reported previously for PI-PS block copolymers.

Volatile fatty acids as indicators of process imbalance in anaerobic digestors

Abstract: In continuously stirred tank reactor experiments, with manure as substrate at thermophilic temperatures, the use of volatile fatty acids (VFA) as process indicators was investigated. Changes in VFA level were shown to be a good parameter for indicating process instability. The VFA were evaluated according to their relative changes caused by changes in hydraulic loading, organic loading or temperature. Butyrate and isobutyrate together were found to be particularly good indicators. Butyrate and isobutyrate concentrations increased significantly 1 or 2 days after the imposed perturbation, which makes these acids suitable for process monitoring and important for process control of the anaerobic biological system. In addition it was shown in a batch experiment that VFA at concentrations up to 50 mM did not reduce the overall methane production rate. This showed that VFA accumulation in anaerobic reactors was the result of process imbalance, not the cause of inhibition, thus justifying the use of VFA as process indicators.

Checkerboard patterns in layout optimization

Abstract: Effective properties of arrangements of strong and weak materials in a checkerboard fashion are computed. Kinematic constraints are imposed so that the displacements are consistent with typical finite element approximations. It is shown that when four-node quatrilateral elements are involved, these constraints result in a numerically induced, artificially high stiffness. This can account for the formation of checkerboard patterns in continuous layout optimization problems of compliance minimization.

Electromagnetic optimization exploiting aggressive space mapping

Abstract: We propose a significantly improved space mapping (SM) strategy for electromagnetic (EM) optimization. Instead of waiting for upfront EM analyses at several base points, our new approach aggressively exploits every available EM analysis, producing dramatic results right from the first step. We establish a relationship between the novel SM optimization and the quasi-Newton iteration for solving a system of nonlinear equations. Approximations to the matrix of first-order derivatives are updated by the classic Broyden formula. A high-temperature superconducting microstrip filter design solution emerges after only six EM simulations with sparse frequency sweeps. Furthermore, less CPU effort is required to optimize the filter than is required by one single detailed frequency sweep. We also extend the SM concept to the parameter extraction phase, overcoming severely misaligned responses induced by inadequate empirical models. This novel concept should have a significant impact on parameter extraction of devices.

A heuristic algorithm for a dial-a-ride problem with time windows, multiple capacities, and multiple objectives

Abstract: The paper describes a system for the solution of a static dial-a-ride routing and scheduling problem with time windows (DARPTW). The problem statement and initialization of the development project was made by the Copenhagen Fire-Fighting Service (CFFS). The CFFS needed a new system for scheduling elderly and disabled persons, involving about 50.000 requests per year. The problem is characterized by, among other things, multiple capacities and multiple objectives. The capacities refer to the fact that a vehicle may be equipped with e.g. normal seats, children seats or wheel chair places. The objectives relate to a number of concerns such as e.g. short driving time, high vehicle utilization or low costs. A solution algorithm REBUS based on an insertion heuristics was developed. The algorithm permits in a flexible way weighting of the various goals such that the solution reflects the user's preferences. The algorithm is implemented in a dynamic environment intended for on-line scheduling. Thus, a new request for service is treated in less than 1 second, permitting an interactive user interface.

Planktivorous feeding in calm and turbulent environments, with emphasis on copepods

Abstract: ABSTFUCT Turbulence may enhance contact rates between planktonic predators and their prey We formulate slmple and general models of prey encounter rates, taklng into account the behaviours and motility patterns of both prey and predator as well as turbulent fluld motlon Uslng these models we determine the levels of turbulence (as clmipation rate) at which ambient fluid motlon is lmportant in enhancing prey encounter rates for vanous types of predators (e g ambush and cruise predators, suspension feeders) Generally, turbulence has the largest effect on prey encounters for predators with low motility and long reactlon distances Also, turbulence is most important for meso-sized (mm to cm) predators and insignificant for smaller and larger predators The effect of turbulence on copepods is specifically examined For copepods that establish feeding currents, turbulence is of minor importance, for ambush feeding copepods, such as Acartldae and many cyclopoids, turbulence has a dominant influence on prey encounter rates The effect on cruising predators is intermediate Application of the models to situations examined experimentally demonstrates a hlgh predictive performance Finally we explore and model the potentially negatlve effects of turbulence on copepod feedlng currents, prey perception and capture success At typical and even high turbulent lntenslties none of these IS slgnificantly affected

Novel Side-Chain Liquid Crystalline Polyester Architecture for Reversible Optical Storage

Abstract: New side-chain liquid crystalline polyesters have been prepared by melt transesterification of diphenyl tetradecanedioate and a series of mesogenic 2-[ω-[4-[(4-cyanophenyl)azo]phenoxy]alkyl]-1,3-propanediols, where the alkyl spacer is hexa-, octa-, and decamethylene in turn. The polyesters have molecular masses in the range 5000-89 000. Solution 13 C NMR spectroscopy has been employed to identify carbons of polyester repeat units and of both types of end groups. Polyester phases and phase transitions have been investigated in detail by polarizing optical microscopy and differential scanning calorimetry for the hexamethylene spacer architecture with different molecular masses. Using FTIR polarization spectroscopy, the segmental orientation in unoriented polyester films induced by argon ion laser irradiation has been followed and an irradiation-dependent order parameter for the cyanoazobenzene mesogens calculated. FTIR is also utilized to follow the temperature-dependent erasure of the induced orientation. Optical storage properties of thin unoriented polyester films are examined through measurements of polarization anisotropy and holography. A resolution of over 5000 lines/mm and diffraction efficiencies of about 40% have been achieved. Lifetimes greater than 30 months for information stored have been obtained, even though the glass transition temperatures are about 20 °C. Complete erasure of the information can be obtained by heating the films to about 80 °C, and the films can be reused many times without fatigue.

Prediction of O-glycosylation of mammalian proteins: specificity patterns of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase.

Abstract: The specificity of the enzyme(s) catalysing the covalent link between the hydroxyl side chains of serine or threonine and the sugar moiety N-acetylgalactosamine (GalNAc) is unknown. Pattern recognition by artificial neural networks and weight matrix algorithms was performed to determine the exact position of in vivo O-linked GalNAc-glycosylated serine and threonine residues from the primary sequence exclusively. The acceptor sequence context for O-glycosylation of serine was found to differ from that of threonine and the two types were therefore treated separately. The context of the sites showed a high abundance of proline, serine and threonine extending far beyond the previously reported region covering positions -4 through +4 relative to the glycosylated residue. The O-glycosylation sites were found to cluster and to have a high abundance in the N-terminal part of the protein. The sites were also found to have an increased preference for three different classes of beta-turns. No simple consensus-like rule could be deduced for the complex glycosylation sequence acceptor patterns. The neural networks were trained on the hitherto largest data material consisting of 48 carefully examined mammalian glycoproteins comprising 264 O-glycosylation sites. For detection neural network algorithms were much more reliable than weight matrices. The networks correctly found 60-95% of the O-glycosylated serine/threonine residues and 88-97% of the non-glycosylated residues in two independent test sets of known glycoproteins. A computer server using E-mail for prediction of O-glycosylation sites has been implemented and made publicly available. The Internet address is NetOglyc@cbs.dtu.dk.

Reduction of broad-band noise in speech by truncated QSVD

Abstract: We consider an algorithm for reduction of broadband noise in speech based on signal subspaces. The algorithm is formulated by means of the quotient singular value decomposition (QSVD). With this formulation, a prewhitening operation becomes an integral part of the algorithm. We demonstrate that this is essential in connection with updating issues in real-time recursive applications. We also illustrate by examples that we are able to achieve a satisfactory quality of the reconstructed signal.

Distribution of redox-sensitive groundwater quality parameters downgradient of a landfill (Grindsted, Denmark).

Abstract: The leachate plume stretching 300 m downgradient from the Grindsted Landfill (Denmark) has been characterized in terms of redox-sensitive groundwater quality parameters along two longitudinal transects (285 samples). Variations in the levels of methane, sulfide, iron(II), manganese(II), ammonium, dinitrogen oxide, nitrite, nitrate, and oxygen in the groundwater samples indicate that methane production, sulfate reduction, iron reduction, manganese reduction, and nitrate reduction take place in the plume. Adjacent to the landfill, methanogenic and sulfate-reducing zones were identified, while aerobic environments were identified furthest away from the landfill. In between, different redox environments, including apparent transition zones, were identified in a sequence in accordance with the thermodynamic principles. The redox zones are believed to constitute an important chemical framework for the attenuation processes in the plume

Physiological state of Escherichia coli BJ4 growing in the large intestines of streptomycin-treated mice.

Abstract: Growth rates of Escherichia coli BJ4 colonizing the large intestine of streptomycin-treated mice were estimated by quantitative hybridization with rRNA target probes and by epifluorescence microscopy. The ribosomal contents in bacteria isolated from the cecal mucus, cecal contents, and feces were measured and correlated with the ribosomal contents of bacteria growing in vitro at defined rates. The data suggest that E. coli BJ4 grows at an overall high rate in the intestine. However, when taking into account the total intestinal volume and numbers of bacteria present in cecal mucus, cecal contents, and feces, we suggest that E. coli BJ4 in the intestine consists of two populations, one in the mucus which has an apparent generation time of 40 to 80 min and one in the luminal contents which is static.

A dilatometer for measuring autogenous deformation in hardening portland cement paste

Abstract: Different techniques for measuring autogenous deformation in cement paste are discussed, and a newly developed dilatometer designed specially for measuring this deformation is described. A particular characteristic of the dilatometer is the encapsulation of the cement paste in corrugated moulds. This restricts moisture loss and ensures that the hardening cement paste suffers insignificant restraint. In addition, the encapsulation permits measurements to commence shortly after casting.

Metabolic flux distributions in Penicillium chrysogenum during fed‐batch cultivations

Abstract: Based on a review of the Penicillium chrysogenum biochemistry a stoichiometric model has been set up. The model considers 61 internal fluxes and there are 49 intracellular metabolites which are assumed to be in pseudo-steady state. In addition to the intracellular fluxes the model considers the uptake of 21 amino acids. From the stoichiometric model the maximum theoretical yield of penicillin V is calculated to 0.43 mol/mol glucose. If biosynthesis of cysteine is by direct sulfhydrylation rather than by transsulfuration, the maximum theoretical yield is about 20% higher, i.e., 0.50 mol/mol glucose. The theoretical yield decreases substantially if alpha-aminoadipate is converted to 6-oxo-piperidine-2-carboxylic acid (OPC). If only 40% of the alpha-aminoadipate is recycled, the maximum theoretical yield is 0.31 mol/mol glucose. The uptake rates of glucose, lactate, gamma-aminobutyrate, and 21 amino acids were measured during fed-batch cultivations. The rates of formation of penicillin V, delta-(L-alpha)-aminoadipyl-L-cysteinyl-D-valine (ACV), OPC, and the pool of isopenicillin N, 6-APA, and 8-HPA were also measured. Finally the synthesis rates of the biomass constituents RNA/DNA, protein, lipid, carbohydrate, and amino carbohydrate were measured. From these measured rates and the stoichiometric model the metabolic fluxes through the different intracellular pathways are calculated. The calculations show that penicillin formation is accompanied by a large flux through the pentose phosphate (PP) pathway due to a large requirement for nicotinamide-adenine dinucleotide phosphate (NADPH) used in the biosynthesis of cysteine. If cysteine is added to the medium, the flux through the PP pathway decreases. From the stoichiometric model Y(xATP) is calculated to 87 mmol adenosine triphosphate (ATP)/g dry weight (DW), and from the flux calculations m(ATP) is found to 3 mmol ATP/g DW/h. (c) 1995 John Wiley & Sons, Inc.

Order and disorder in symmetric diblock copolymer melts

Abstract: The thermodynamic and dynamic properties of several partially deuterated symmetric (equal block volume fraction) polyethylene-poly(ethylethylene) (PE-PEE), poly(ethylene-propylene)-poly(ethylethylene) (PEP-PEE), and polyethylene-poly(ethylene-propylene) (PE-PEP) diblock copolymers were characterized above and below the order-disorder transition (ODT). A lamellar morphology was established for all temperatures below the ODT using small-angle neutron scattering (SANS) measurements with shear-aligned specimens. As T ODT is approached, the lamellar order weakens as evidenced by the loss of higher order SANS reflections and azimuthal smearing of the principal equitorial scattering peaks. SANS and rheology experiments also provided unambiguous evidence of composition fluctuations above the order-disorder transition temperature in each system. However, the temperature range of the fluctuations in the disordered state depends on the degree of polymerization, in accordance with fluctuation theory. A crossover in the disordered state between slightly and moderately stretched coils was also documented by SANS, coincident with the temperature where fluctuations become apparent rheologically.