Showing papers by "Helsinki University of Technology published in 2004"

Irradiation-induced magnetism in graphite: a density functional study.

Abstract: Recent experiments indicate that proton irradiation triggers ferromagnetism in originally nonmagnetic graphite samples while He ion bombardment has a much smaller effect. To understand the origin of irradiation-induced magnetism, we have performed spin-polarized density functional theory calculations of the magnetic properties of the defects which are most likely to appear under irradiation vacancies and vacancy-hydrogen complexes. Both defects are magnetic, but as for the latter we find that H adsorption on one of the vacancy dangling bonds gives rise to a magnetic moment double that of the naked vacancy. We show that for small irradiation doses vacancy-hydrogen complexes result in a macroscopic magnetic signal which agrees well with the experimental values.

Laser scanning of forest resources: the nordic experience

Abstract: This article reviews the research and application of airborne laser scanning for forest inventory in Finland, Norway and Sweden. The first experiments with scanning lasers for forest inventory were conducted in 1991 using the FLASH system, a full-waveform experimental laser developed by the Swedish Defence Research Institute. In Finland at the same time, the HUTSCAT profiling radar provided experiences that inspired the following laser scanning research. Since 1995, data from commercially operated time-of-flight scanning lasers (e.g. TopEye, Optech ALTM and TopoSys) have been used. Especially in Norway, the main objective has been to develop methods that are directly suited for practical forest inventory at the stand level. Mean tree height, stand volume and basal area have been the most important forest mensurational parameters of interest. Laser data have been related to field training plot measurements using regression techniques, and these relationships have been used to predict corresponding properti...

A multi-national study of reading and tracing skills in novice programmers

Abstract: A study by a ITiCSE 2001 working group ("the McCracken Group") established that many students do not know how to program at the conclusion of their introductory courses. A popular explanation for this incapacity is that the students lack the ability to problem-solve. That is, they lack the ability to take a problem description, decompose it into sub-problems and implement them, then assemble the pieces into a complete solution. An alternative explanation is that many students have a fragile grasp of both basic programming principles and the ability to systematically carry out routine programming tasks, such as tracing (or "desk checking") through code. This ITiCSE 2004 working group studied the alternative explanation, by testing students from seven countries, in two ways. First, students were tested on their ability to predict the outcome of executing a short piece of code. Second, students were tested on their ability, when given the desired function of short piece of near-complete code, to select the correct completion of the code from a small set of possibilities. Many students were weak at these tasks, especially the latter task, suggesting that such students have a fragile grasp of skills that are a prerequisite for problem-solving.

Gas sensing using air-guiding photonic bandgap fibers

Abstract: We report on experimental studies of gas sensing using air-guiding photonic bandgap fibers. The photonic bandgap fibers have at one end been spliced to standard single mode fibers for ease of use and improved stability

On private scalar product computation for privacy-preserving data mining

Abstract: In mining and integrating data from multiple sources, there are many privacy and security issues. In several different contexts, the security of the full privacy-preserving data mining protocol depends on the security of the underlying private scalar product protocol. We show that two of the private scalar product protocols, one of which was proposed in a leading data mining conference, are insecure. We then describe a provably private scalar product protocol that is based on homomorphic encryption and improve its efficiency so that it can also be used on massive datasets.

Mechanical properties of carbon nanotubes with vacancies and related defects

Abstract: Although as-grown carbon nanotubes have relatively few defects, defects can appear at the purification stage or be deliberately introduced by irradiation with energetic particles or by chemical treatment when aiming at the desired functionality. The defects, especially vacancies, give also rise to a deleterious effect – deterioration of axial mechanical properties of nanotubes. By employing molecular dynamics simulations and continuum theory we study how the Young’s modulus and tensile strength of nanotubes with vacancy-related defects depend on the concentration of defects and defect characteristics. We derive an analytical expression, with coefficients parametrized from atomistic computer simulations, which relates the Young’s modulus and defect density in carbon nanotubes. We further show that the tensile strength and critical strain of single-walled nanotubes decrease by nearly a factor of two if an unreconstructed vacancy is present. However, this deterioration in the mechanical characteristics is partly alleviated by the ability of nanotubes to heal vacancies in the atomic network by saturating dangling bonds.

Magnetic properties of vacancies in graphene and single-walled carbon nanotubes

Abstract: Spin-polarized density functional theory has been used to study the properties of vacancies in a graphene sheet and in single-walled carbon nanotubes (SWNTs). For graphene, we find that the vacancies are magnetic and the symmetry of the sheet is broken by the distortion of an atom next to the vacancy site. We also studied vacancies in four armchair SWNTs from (3,3) to (6,6) and six zigzag SWNTs from (5,0) to (10,0). Our calculations demonstrate that vacancies can change the electronic structure of SWNTs, converting some metallic nanotubes to semiconductors and vice versa. Metallic nanotubes with vacancies exhibit ferro- or ferrimagnetism, whereas some semiconducting nanotubes with vacancies show an antiferromagnetic order. The magnetic properties depend on chiralities of the tubes, the configuration of the vacancy and the concentration of the vacancies.

Advanced electronic and optoelectronic materials by Atomic Layer Deposition: An overview with special emphasis on recent progress in processing of high-k dielectrics and other oxide materials

Abstract: The Feature Article [1] describes the current state of the Atomic Layer Deposition (ALD) technique for producing high-quality thin layers with the focus on oxide materials such as ZrO2 and other rare earth oxides, SnO2 and ZnO for high-k dielectrics, gas sensors and various optoelectronic applications. The cover picture is a photograph of a thin-film-on-silicon sample. The inset schematically illustrates the ALD growth of an oxide film on a surface where L refers to the precursor ligand. The first author, Lauri Niinisto, is Professor of Inorganic Chemistry at Helsinki University of Technology. His current research is focused on the preparation and characterization of thin films for optoelectronic and electronic devices.

Manufacturing practices, strategic fit and performance: A routine‐based view

Abstract: This article challenges and advances the extant manufacturing practice‐performance research in three ways. First and most fundamentally, the article offers a sound theoretical foundation for the proposition that manufacturing practices have competitive value. Second, typical studies do not pay enough attention to the multidimensional nature of performance and often collapse strategic position (performance) into a one‐dimensional index. The article will show that this does not do justice to the multidimensional nature of operational performance. Third, extant research, aside from a few exceptions, pays little attention to the strategic contingencies involved in adopting and implementing specific practices. The overarching goal of this article is to move us toward better‐informed empirical inquiry of the strategic contingency argument in operations strategy research. The article builds a theoretical argument of manufacturing practices, strategic contingency and performance and tests it in a sample of 164 manufacturing plants using a series of regression analyses. Results show that both the best practice and strategic contingency arguments have merit in explaining operational performance; however, the contingency argument has stronger support.

Clustering and information in correlation based financial networks

Abstract: Networks of companies can be constructed by using return correlations. A crucial issue in this approach is to select the relevant correlations from the correlation matrix. In order to study this problem, we start from an empty graph with no edges where the vertices correspond to stocks. Then, one by one, we insert edges between the vertices according to the rank of their correlation strength, resulting in a network called asset graph. We study its properties, such as topologically different growth types, number and size of clusters and clustering coefficient. These properties, calculated from empirical data, are compared against those of a random graph. The growth of the graph can be classified according to the topological role of the newly inserted edge. We find that the type of growth which is responsible for creating cycles in the graph sets in much earlier for the empirical asset graph than for the random graph, and thus reflects the high degree of networking present in the market. We also find the number of clusters in the random graph to be one order of magnitude higher than for the asset graph. At a critical threshold, the random graph undergoes a radical change in topology related to percolation transition and forms a single giant cluster, a phenomenon which is not observed for the asset graph. Differences in mean clustering coefficient lead us to conclude that most information is contained roughly within 10% of the edges.

Composition and temperature dependence of the crystal structure of Ni–Mn–Ga alloys

Abstract: The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (TM) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, TM and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or TM has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni–Mn–Ga system has been confirmed.

Comparison of different methods for measuring strain induced α-martensite content in austenitic steels

Abstract: Several techniques for measuring the strain induced α-martensite content in EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) austenitic stainless steels were compared in order to determine a correlation curve between Ferritescope measurement results and actual α-martensite contents. The studied methods involved Satmagan measurement, magnetic balance measurement, X-ray diffraction, density measurement and quantitative optical metallography. Satmagan, magnetic balance and density measurements were found to give equal α-martensite contents. X-ray diffraction results were affected by the texture but averaging of several diffraction peaks improved the reliability of the results. It was shown that α-martensite can be detected by means of optical metallography, but quantitative analysis is time consuming and inaccurate. The relationship between the Ferritescope results and actual α-martensite contents measured with the other techniques was found to be linear. According to the results, the Ferritescope rea...

Imbibition in disordered media

Abstract: The physics of liquids in porous media gives rise to many interesting phenomena, including imbibition where a viscous fluid displaces a less viscous one. Here we discuss the theoretical and experimental progress made in recent years in this field. The emphasis is on an interfacial description, akin to the focus of a statistical physics approach. Coarse-grained equations of motion have been recently presented in the literature. These contain terms that take into account the pertinent features of imbibition: non-locality and the quenched noise that arises from the random environment, fluctuations of the fluid flow and capillary forces. The theoretical progress has highlighted the presence of intrinsic length-scales that invalidate scale invariance often assumed to be present in kinetic roughening processes such as that of a two-phase boundary in liquid penetration. Another important fact is that the macroscopic fluid flow, the kinetic roughening properties, and the effective noise in the problem are all coupled. Many possible deviations from simple scaling behaviour exist, and we outline the experimental evidence. Finally, prospects for further work, both theoretical and experimental, are discussed.

Abnormal imitation-related cortical activation sequences in Asperger's syndrome.

Abstract: Subjects with Asperger's syndrome (AS) are impaired in social interaction and imitation, but the underlying brain mechanisms are poorly understood. Because the mirror-neuron system (MNS) that matches observed and executed actions has been suggested to play an important role in imitation and in reading of other people's intentions, we assessed MNS functions in 8 adult AS subjects and in 10 healthy control subjects during imitation of still pictures of lip forms. In the control subjects, cortical activation progressed in 30 to 80-millisecond steps from the occipital cortex to the superior temporal sulcus, to the inferior parietal lobe, and to the inferior frontal lobe, and finally, 75 to 90 milliseconds later, to the primary motor cortex of both hemispheres. Similar activation sites were found in AS subjects but with slightly larger scatter. Activation of the inferior frontal lobe was delayed by 45 to 60 milliseconds and activations in the inferior frontal lobe and in the primary motor cortex were weaker than in control subjects. The observed abnormal premotor and motor processing could account for a part of imitation and social impairments in subjects with AS.

Understanding mobile contexts

Abstract: Mobile urban environments present a challenge for context-aware computers because they differ from fixed indoor contexts such as offices, meeting rooms, and lecture halls in many important ways. Internal factors such as tasks and goals are different—external factors such as social resources are dynamic and unpredictable. An empirical, user-centred approach is needed to understand mobile contexts. In this paper, we present insights from an ethnomethodologically inspired study of 25 adult urbanites in Helsinki. The results describe typical phenomena in mobility: how situational and planned acts intermesh in navigation, how people construct personal and group spaces, and how temporal tensions develop and dissolve. Furthermore, we provide examples of social solutions to navigation problems, examine mobile multitasking, and consider design implications for mobile and context-aware human–computer interaction.

Quantum circuits for general multiqubit gates.

Abstract: We consider a generic elementary gate sequence which is needed to implement a general quantum gate acting on $n$ qubits---a unitary transformation with ${4}^{n}$ degrees of freedom. For synthesizing the gate sequence, a method based on the so-called cosine-sine matrix decomposition is presented. The result is optimal in the number of elementary one-qubit gates, ${4}^{n}$, and scales more favorably than the previously reported decompositions requiring ${4}^{n}\ensuremath{-}{2}^{n+1}$ controlled NOT gates.

Reproducible XPS on biopolymers: cellulose studies

Abstract: The development of 'user-friendly' surface analytical instrumentation has expanded the application of x-ray photoelectron spectroscopy (XPS) towards softer and more complex composite materials. However, the knowledge of fundamental phenomena in practical, interdisciplinary applications is dearly needed. In the case of polymers and polymer-based or -reinforced composites, the analytical problems are not only related to detection limits and resolution; indeed, the effects of contamination and experimental artefacts on interpretation of the carbon signal are often the more crucial challenge. As a biopolymer, natural cellulose fibres provide an interesting model system for polymer studies. Numerous published XPS studies on lignocelluloses highlight both the possibilities and the problems of the surface analytical probes in analysis of air-exposed organic materials. We report XPS experiments of cellulosic natural fibre materials, discussing the effects of sample storage, UHV exposure, radiation doses, charging and data analysis. Based on a large body of experimental data we also propose that clean paper specimens could be used as an in-situ reference in polymer studies.

Self-organization of collaboration networks.

Abstract: We study collaboration networks in terms of evolving, self-organizing bipartite graph models. We propose a model of a growing network, which combines preferential edge attachment with the bipartite structure, generic for collaboration networks. The model depends exclusively on basic properties of the network, such as the total number of collaborators and acts of collaboration, the mean size of collaborations, etc. The simplest model defined within this framework already allows us to describe many of the main topological characteristics (degree distribution, clustering coefficient, etc.) of one-mode projections of several real collaboration networks, without parameter fitting. We explain the observed dependence of the local clustering on degree and the degree‐degree correlations in terms of the “aging” of collaborators and their physical impossibility to participate in an unlimited number of collaborations.

Mechanical properties of structural steel at elevated temperatures and after cooling down

Abstract: An experimental research programme was carried out during the years 1994-2001 in the Laboratory of Steel Structures at Helsinki University of Technology in order to investigate the mechanical properties of several structural steels at elevated temperatures by using mainly the transient state tensile test method The aim was to produce accurate material data for use in different structural analyses The main test results are public and they are available for other researchers In this paper the experimental test results for the mechanical properties of the studied steel grades S350GD+Z, S355 and S460M at fire temperatures are presented with a short description of the testing facilities A test series was also carried out for cold-formed material taken from rectangular hollow sections of structural steel S355J2H and these test results are also given in this report The mechanical properties of structural steel after cooling down have also been examined briefly and these test results are given in this report The test results were used to determine the temperature dependencies of the mechanical properties, ie yield strength, modulus of elasticity and thermal elongation, of the studied steel material at temperatures up to 950degreesC The test results are compared with the material model for steel according to Eurocode 3: Part 12

Summary of human responses to ventilation

Abstract: The effects of ventilation on indoor air quality and health is a complex issue. It is known that ventilation is necessary to remove indoor generated pollutants from indoor air or dilute their concentration to acceptable levels. But, as the limit values of all pollutants are not known, the exact determination of required ventilation rates based on pollutant concentrations and associated risks is seldom possible. The selection of ventilation rates has to be based also on epidemiological research (e.g. Seppanen et al., 1999), laboratory and field experiments (e.g. CEN 1996, Wargocki et al., 2002a) and experience (e.g. ECA 2003). Ventilation may also have harmful effects on indoor air quality and climate if not properly designed, installed, maintained and operated as summarized by Seppdnen (2003). Ventilation may bring indoors harmful substances that deteriorate the indoor environment. Ventilation also affects air and moisture flow through the building envelope and may lead to moisture problems that deteriorate the structures of the building. Ventilation changes the pressure differences over the structures of building and may cause or prevent the infiltration of pollutants from structures or adjacent spaces. Ventilation is also in many cases used to control the thermal environment or humidity in buildings. Ventilation can be implemented with various methods which may also affect health (e.g. Seppdnen and Fisk, 2002, Wargocki et al., 2002a). In non residential buildings and hot climates, ventilation is often integrated with air-conditioning which makes the operation of ventilation system more complex. As ventilation is used for many purposes its health effects are also various and complex. This paper summarizes the current knowledge on positive and negative effects of ventilation on health and other human responses. The focus of the paper is on office-type working environment and residential buildings. In the industrial premises the problems of air quality are usually more complex and case specific. They are subject to occupational safety legislation and not discussed here.

Flooding of Gas Diffusion Backing in PEFCs : Physical and Electrochemical Characterization

Abstract: In polymer electrolyte fuel cells (PEFCs) gas diffusion backings (GDBs) have a significant effect on water management and cell performance. In this study, methods for characterizing GDB performance by fuel cell testing and ex situ measurements are presented. The performance of four different commercial GDB materials was tested and significant differences were found between the materials. While the performance and behavior are almost similar in the single-phase region, the flooding behavior of different GDBs in the two-phase region varies widely. The results show that using high clamping pressures increases cell flooding, but the increase varies from material to material. Increased flooding is caused by the combination of decreased porosity and a temperature difference between GDB and current collector. Furthermore, it was observed that the decrease in porosity due to cell compression and corresponding increase in mass-transfer resistance should be studied in the single-phase region, because flooding of the GDB easily becomes the dominating source of mass-transfer resistance. In addition, a literature review on GDB studies and characterization methods was carried out. The review revealed a lack of an established GDB testing regime and the absence of a relation between physical properties of the GDB and fuel cell performance.