Showing papers by "Chalmers University of Technology published in 1994"

Light-emitting diodes with variable colours from polymer blends

Abstract: THE range of materials now available for polymer-based light-emitting diodes (LEDs) is such that electroluminescence can be obtained throughout the visible spectrum(1-12). Here we show that, by ble ...

An experimental investigation of the flow around a circular cylinder : influence of aspect ratio

Abstract: The investigation is concentrated on two important quantities – the Strouhal number and the mean base suction coefficient, both measured at the mid-span position Reynolds numbers from about 50 to 4 × 104 were investigated Different aspect ratios, at low blockage ratios, were achieved by varying the distance between circular end plates (end plate diameter ratios between 10 and 30) It was not possible, by using these end plates in uniform flow and at very large aspect ratios, to produce parallel shedding all over the laminar shedding regime However, parallel shedding at around mid-span was observed throughout this regime in cases when there was a slight but symmetrical increase in the free-stream velocity towards both ends of the cylinder At higher Re, the results at different aspect ratios were compared with those of a ‘quasi-infinite cylinder’ and the required aspect ratio to reach conditions independent of this parameter, within the experimental uncertainties, are given For instance, aspect ratios as large as L/D = 60–70 were needed in the range Re ≈ 4 × 103–104 With the smallest relative end plate diameter and for aspect ratios smaller than 7, a bi-stable flow switching between regular vortex shedding and ‘irregular flow’ was found at intermediate Reynolds number ranges in the subcritical regime (Re ≈ 2 × 103)

DNA-like double helix formed by peptide nucleic acid

Abstract: ALTHOUGH the importance of the nucleobases in the DNA double helix is well understood, the evolutionary significance of the deoxyribose phosphate backbone and the contribution of this chemical entity to the overall helical structure and stability of the double helix is not so clear. Peptide nucleic acid (PNA)1-7 is a DNA analogue with a backbone consisting of N-(2-aminoethyl)glycine units (Fig. 1) which has been shown to mimic DNA in forming Watson-Crick complementary duplexes with normal DNA7. Using circular dichroism spectroscopy we show here that two complementary PNA strands can hybridize to one another to form a helical duplex. There is a seeding of preferred chirality which is induced by the presence of an L- (or D-) lysine residue attached at the carboxy terminus of the PNA strand. These results indicate that a (deoxy)ribose phosphate backbone is not an essential requirement for the formation of double helical DNA-like structures in solution.

Fault injection into VHDL models: the MEFISTO tool

Abstract: This paper focuses on the integration of the fault injection methodology within the design process of fault-tolerant systems. Due to its wide spectrum of application and hierarchical features, VHDL has been selected as the simulation language to support such an integration. Suitable techniques for injecting faults into VHDL models are identified and depicted. Then, the main features of the MEFISTO environment aimed at supporting these techniques are described. Finally, some preliminary results obtained with MEFISTO are presented and analyzed. >

Rack-induced bonding in blue-copper proteins

Abstract: The unique spectroscopic properties of blue-copper centers, i.e. the strong charge-transfer band at approximately 600 nm and the narrow hyperfine coupling in the EPR spectrum, are reviewed. The concept of rack-induced bonding is summarized. The tertiary structure of the protein creates a preformed chelating site with very little flexibility, the geometry of which is in conflict with that preferred by Cu2+. The structure of the metal site in azurin is discussed. It is shown that the three strong ligands, one thiolate S and two imidazole N, are in a configuration intermediate between those preferred by Cu2+ and Cu+. It is emphasized that cysteine is an obligatory component of a blue site, whereas the weak interaction with a methionine S is not necessary. The minimum rack energy is estimated to be 70 kJ · mol-1. It is pointed out that the high reduction potentials of blue- copper centers are a result of the protein-forced ligand-field-destabilized site structure. It is suggested that the potentials are tuned by variations in π back bonding, and this is supported by a linear increase in ΔLF (ligand field) with decreasing electron-transfer enthalpy. Site-directed mutagenesis has shown that large hydrophobic residues in the site increase the potential, whereas negative groups or water decrease it. It is also shown that the fine-tuning of the properties of the metal site by rack- induced bonding can alter the electron-transfer reorganization energy. Kinetic results with azurin mutants support a through-bond tunneling mechanism for intramolecular electron transfer in proteins. Finally, it is pointed out that the concept of rack-induced bonding is a universal principle of macromolecular structure/function relationships, which should be applied also to other systems.

Analysing DNA complexes by circular and linear dichroism

Abstract: The application of linear and circular dichroism (LD and CD) in nucleic acid research id illustrated by recent results aimed at answering specific structural problem in the interaction of DNA with molecules of biological importance. We first consider the circumstances under which ligands, such as DAPI (4′, 6-diamidino-2-phenylindole), change their preferred binding mode in the minor groove to major groove binding or intercalation. As an extension of this problem we refer to the switch between groove binding and intercalation of structurally similar ligands such as ellipticines and trigonal ruthenium complexes. We also explore the use of LD and CD in the determination of the structure of the complex formed between the polynucleotide poly(dA) and the novel ‘peptide nucleic acid’, consisting of nucleic acid bases joined by a polyamide homomorphous with the deoxyribose-phosphate backbone of DNA. Finally, the structure and interaction of the recombination enzyme RecA with DNA is discussed, in particular the influence of the presence of the intercalators, groove binders or covalent DNA adducts.

Infinite objects in type theory

Abstract: We show that infinite objects can be constructively understood without the consideration of partial elements, or greatest fixed-points, through the explicit consideration of proof objects. We present then a proof system based on these explanations. According to this analysis, the proof expressions should have the same structure as the program expressions of a pure functional lazy language: variable, constructor, application, abstraction, case expressions, and local let expressions.

Using heavy-ion radiation to validate fault-handling mechanisms

Abstract: Fault injection is an effective method for studying the effects of faults in computer systems and for validating fault-handling mechanisms. The approach presented involves injecting transient faults into integrated circuits by using heavy-ion radiation from a Californium-252 source. The proliferation of safety-critical and fault-tolerant systems using VLSI technology makes such attempts to inject faults at internal locations in VLSI circuits increasingly important. >

Interaction of Cationic Porphyrins with DNA

Abstract: Utilizing linear dichroism (LD), circular dichroism (CD), and fluorescence energy transfer, the binding geometries of a series of Co(3+)-porphyrins and their free ligands were examined. The compounds studied were Co-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (CoTMPyP) and its free ligand (H2-TMPyP), Co-meso-tetrakis(N-n-butylpyridinium-4-yl)porphyrin (CoTBPyP) and its free ligand (H2TBPyP), and Co-meso-tetrakis(N-n-octylpyridinium-4-yl)porphyrin (CoTOPyP). The two non-metalloporphyrins exhibit negative LD, having angles of roughly 75 degrees relative to the DNA helix axis. They also display negative CD and a significant contact energy transfer from the DNA bases. On the other hand, the three metalloporphyrins display orientation angles of roughly 45 degrees between the porphyrin plane and the helix axis of DNA. Furthermore, they exhibit positive CD and no contact energy transfer from DNA bases. These observations show that the metalloporphyrins are not intercalated whereas non-metalloporphyrins having four freely rotating meso-aryl groups intercalate between the base pairs of DNA. In the presence of KHSO5, the cobalt porphyrins cleave closed circular PM2 DNA in a single strand manner, i.e., a single activation event on the porphyrin leads to a break in one of the DNA strands. A kinetic analysis of the cleavage data revealed that cleavage rates are in the order CoTMPyP > CoTBPyP > CoTOPyP with the difference being due to different DNA affinities rather than differences in cleavage rate-constants. Based on these and earlier observations, the metalloporphyrins appear bound to a partially melted region of DNA.

A Bayesian approach to auto-calibration for parametric array signal processing

Abstract: A number of techniques for parametric (high-resolution) array signal processing have been proposed in the last few decades. With few exceptions, these algorithms require an exact characterization of the array, including knowledge of the sensor positions, sensor gain/phase response, mutual coupling, and receiver equipment effects. Unless all sensors are identical, this information must typically be obtained by experimental measurements (calibration). In practice, of course, all such information is inevitably subject to errors. Several different methods have been proposed for alleviating the inherent sensitivity of parametric methods to such modelling errors. The technique proposed in the present paper is related to the class of so-called auto-calibration procedures, but it is assumed that certain prior knowledge of the array response errors is available. This is a reasonable assumption in most applications, and it allows for more general perturbation models than does pure auto-calibration. The optimal maximum a posteriori (MAP) estimator for the problem at hand is formulated, and a computationally more attractive large-sample approximation is derived. The proposed technique is shown to be statistically efficient, and the achievable performance is illustrated by numerical evaluation and computer simulation. >

Surface analysis of oxidized aluminium. 1. Hydration of Al2O3 and decomposition of Al(OH)3 in a vacuum as studied by ESCA

Abstract: Pure polycrystalline Al (99.998%) was oxidized in dry oxygen and water. The surface products formed were analysed by ESCA. The thicknesses of the amorphous Al2O3 formed during oxidation for 1 h in dry oxygen at 25°C and 250°C are 14 and 20 A, respectively. During exposure of the oxidized sample to water a surface layer of amorphous Al2O3 in hydrated and Al(OH)3 is formed by the reaction: Al2O3 + 3H2O⇄2Al(OH)3. The hydroxide is not stable in a vacuum. During analysis in the ESCA instrument, Al(OH)3 is decomposed to Al2O3 and H2O because the equilibrium pressure of H2O is higher than the partial pressure of water in the vacuum system. It is found that a 7 A thick layer of Al2O3 is formed on the surface of Al(OH)3. The binding energies of the Al3+(2p) and O2−(1s) signals recorded from Al2O3 formed on Al at 22°C and at 250°C are 75.8 and 532.7 eV, respectively. The binding energies of these peaks are 0.6 eV lower for a sample preoxidized at 250°C and then exposed to a humid atmosphere and immediately analysed. It is suggested that the change of the binding energy is due to set up of a space charge during exposure to a humid atmosphere. After exposure for a long time to a humid atmosphere the charging decays and the binding energies approach their normal values.

Enhanced Superconvergent Patch Recovery incorporating equilibrium and boundary conditions

Abstract: Patch recovery based on superconvergent derivatives and equilibrium (SPRE), an enhancement of the Superconvergent Patch Recovery (SPR), is studied for linear elasticity problems. The paper also presents a further improvement for recovery of derivatives near boundaries, SPREB, where either tractions or displacements are prescribed. This is made by inclusion of weighted residual errors at boundary points in the patch recovery. A pronounced improvement in the post processed gradients of the finite element solution is observed by this method.

On latching probability of particle induced transients in combinational networks

Abstract: The question to what extent particle induced transients in combinational parts of a circuit propagate into memory elements is addressed in this paper An experimental method is presented in which the proportion of bit flips originating from heavy-ion hits in combinational logic is determined. It is proposed that a voltage pulse may only propagate through a limited number of transistor stages and still be latched. The proportion of all transients in combinational logic that were latched into registers was experimentally, estimated to be between 0.7/spl middot/10/sup -3/ and 2/spl middot/10/sup -3/ for a custom designed CMOS circuit. Very few multiple bit flips were observed during the experiments which indicates that the single bit flip model used in many high-level simulations is reasonable accurate. >

Material-tissue interfaces: the role of surface properties and processes.

Abstract: The introduction of a foreign material into living tissue--intentionally as in biomedical applications (implants, protheses, drugs) or unintentionally as when minerals or fibers are inhaled--results in the creation of interfaces between the material and the surrounding tissue. This article identifies and discusses the possible role of material surface properties and molecular processes occurring at such interfaces. For kinetic and thermodynamic reasons, surfaces are different from the corresponding bulk of the material, and contain reactive (unsaturated) bonds, which in turn lead to the formation of surface reactive layers (e.g., surface oxides on metals) and adsorbed contamination layers. The encounter with the biological environment leads to further surface reactions modifying the surface, and to the adsorption of water, ions, and biomolecules, which are continuously exchanged. The exact nature of the dynamic, adsorbed water, ions, and biomolecule coating in turn influences the behavior of cells approaching the material surface, and hence the tissue response.

A Study of the DRASTIC Methodology with Emphasis on Swedish Conditions

Abstract: A study of the DRASTIC classification methodology, with special emphasis on Swedish conditions, indicated both favorable and unfavorable properties of the system. The study showed that the fairly large number of parameters (seven) and the correlations between these parameters give DRASTIC some advantageous statistical properties. A comparison of the DRASTIC parameters with parameters found to determine the effects on ground-water quality in surficial aquifers in southwestern Sweden from acid deposition showed a good agreement between these two parameter sets; the study thus indicated that the selection of parameters is relevant for general ground-water vulnerability assessments in Sweden. The concept of hydrogeologic settings and the possibility to trace results backwards in the system is excellent, but the qualitative weight functions in DRASTIC result in a simplistic index of unclear meaning that is less useful and less distinctive than desired. The system tends to overestimate the vulnerability of porous media aquifers compared to aquifers in fractured media; the rating tables need to be revised so that more weight is given to specific surface and effective porosity, and less to hydraulic conductivity. The applicability of the results would be enhanced and the risk of misuse reduced if the results were more clearly directed toward scientifically defined factors, e.g. sorption capacity, travel time, and dilution.

Molecular-dynamics study of supercooled ortho-terphenyl.

Abstract: We present the results of a detailed molecular-dynamics study of relaxation in the van der Waals system ortho-terphenyl in the supercooled regime. The molecule is described by a simple rigid three-site model, with interactions between different molecules of the Lennard-Jones form. We find that the long-time (\ensuremath{\alpha}) relaxation, as determined from the intermediate scattering function, is well described by a Kohlrausch law. The zero-time amplitude of this process, often referred to as the ``nonergodicity parameter,'' which can be interpreted as a Debye-Waller factor, is seen to depart from a linear temperature dependence and seems to exhibit a ``singularity'' at a temperature substantially larger than the conventional glass transition temperature, in accord with neutron-scattering data, and as predicted by mode-coupling theory. We find that the observed anharmonic behavior around ${\mathit{T}}_{\mathit{c}}$, as well as the decay of the scattering functions, is equally evident from both orientational and translational correlations. The latter observation indicates that the structural changes that take place during \ensuremath{\alpha} relaxation are neither specifically orientational nor specifically translational. Investigation of the van Hove self-correlation for both center-of-mass and orientational motion reveals the existence of a strong non-Gaussian spatial dependence at intermediate times and of processes, even at low temperatures, that cannot be described as simple vibrational motion, and which consist of rapid reorientations of the molecules about their essentially frozen centers of mass. This motion may be related to the fast secondary mode seen in experiments.

The rational Krylov algorithm for nonsymmetric eigenvalue problems. III: Complex shifts for real matrices

Abstract: A new algorithm for the computation of eigenvalues of a nonsymmetric matrix pencil is described. It is a generalization of the shifted and inverted Lanczos (or Arnoldi) algorithm, in which several shifts are used in one run. It computes an orthogonal basis and a small Hessenberg pencil. The eigensolution of the Hessenberg pencil, gives Ritz approximations to the solution of the original pencil. It is shown how complex shifts can be used to compute a real block Hessenberg pencil to a real matrix pair. Two applicationx, one coming from an aircraft stability problem and the other from a hydrodynamic bifurcation, have been tested and results are reported.

Possibility of coherent multiple excitation in atom transfer with a scanning tunneling microscope.

Abstract: We examine atom transfer resulting from coherent multiple excitation of the adsorbate-substrate bond caused by inelastic tunneling of a single electron (or hole) via a negative- (or positive-) ion resonance. At low biases and in particular for the transitions resulting in atom transfer, the rates of coherent multiple excitation are nonlinear and also highly asymmetrical with respect to the polarity of the bias. We establish a simple criterion for the regime in which this mechanism dominates over earlier proposed mechanisms for atom transfer resulting from vibrational heating by sequential (incoherent) inelastic resonance tunneling. In the case of the atomic switch, where a Xe atom is transferred between a Ni surface and a tip, the vibrational heating mechanism is found to dominate over the coherent mechanism. For other systems, such as Na adsorption on Cu or ${\mathrm{O}}_{2}$ adsorbed on Pt, the coherent mechanism is argued to play a role in bond breaking.

Non-uniqueness of measures of maximal entropy for subshifts of finite type

Abstract: It is known that in one dimension an irreducible subshift of finite type has a unique measure of maximal entropy, the so-called Parry measure. Here we give a counterexample to this in higher dimensions. For this example, we also describe the geometric structure of the measures of maximal entropy and show that there are exactly two extremal measures.