Showing papers by "Jožef Stefan Institute published in 2007"

Beyond One-Electron Reaction in Li Cathode Materials: Designing Li2MnxFe1-xSiO4

Abstract: Li2MnSiO4 has been identified recently as one of the first cathode battery materials that, at least in principle, could exchange more than 1 lithium per redox-active transition metal ion. In this article, we analyze experimentally and by computer simulations based on density functional theory (DFT) why actual experiments have not confirmed these expectations. We show that Li2MnSiO4 is unstable upon delithiation, with a strong tendency to amorphize. Detailed DFT calculations further indicate that it might be possible to obtain a stable material with a reversible exchange of more than one Li per formula unit (FU) by using an appropriate Mn/Fe mixture (solid solution) with a general formula Li2MnxFe1-xSiO4.

Vogel-Fulcher freezing in relaxor ferroelectrics

Abstract: A physical mechanism for the freezing of polar nanoregions (PNRs) in relaxor ferroelectrics is presented. Assuming that the activation energy for the reorientation of a cluster of PNRs scales with the mean volume of the cluster, the characteristic relaxation time $\ensuremath{\tau}$ is found to diverge as the cluster volume reaches the percolation limit. Applying the mean field theory of continuum percolation, the familiar Vogel-Fulcher equation for the temperature dependence of $\ensuremath{\tau}$ is derived.

Wired Porous Cathode Materials: A Novel Concept for Synthesis of LiFePO4

Abstract: To increase the power density of battery materials, without significantly affecting their main advantage of a high energy density, novel material architectures need to be developed. Using the example of LiFePO4, we demonstrate a simple, sol−gel-based route that leads to large (up to 20 μm) primary LiFePO4 particles, each of which contains hierarchically organized pores in the meso and macro range. As the pores are formed due to vigorous gas evolution (mainly CO and CO2) during degradation of a citrate precursor, they are perfectly interconnected within each particle. Elementary carbon, the other citrate-degradation product, is deposited on the walls of emerging pores. The superposition of a continuous 1−2 nm thick carbon film (electron conductor) on pores (ion conductor when filled with electrolyte) represents a unique architecture in which the electrons and ions are simultaneously supplied to the site of insertion in the particle interior. The material can operate at current rates up to 50 C while preser...

Ensembles of Multi-Objective Decision Trees

Abstract: Ensemble methods are able to improve the predictive performance of many base classifiers. Up till now, they have been applied to classifiers that predict a single target attribute. Given the non-trivial interactions that may occur among the different targets in multi-objective prediction tasks, it is unclear whether ensemble methods also improve the performance in this setting. In this paper, we consider two ensemble learning techniques, bagging and random forests, and apply them to multi-objective decision trees (MODTs), which are decision trees that predict multiple target attributes at once. We empirically investigate the performance of ensembles of MODTs. Our most important conclusions are: (1) ensembles of MODTs yield better predictive performance than MODTs, and (2) ensembles of MODTs are equally good, or better than ensembles of single-objective decision trees, i.e., a set of ensembles for each target. Moreover, ensembles of MODTs have smaller model size and are faster to learn than ensembles of single-objective decision trees.

Entangled nematic colloidal dimers and wires.

Abstract: It has been predicted, but never confirmed, that colloidal particles in a nematic liquid crystal could be self-assembled by delocalized topological defects and entangled disclinations. We show experimentally and theoretically that colloidal dimers and 1D structures bound by entangled topological defect loops can indeed be created by locally thermally quenching a thin layer of the nematic liquid crystal around selected colloidal particles. The topological entanglement provides a strong stringlike binding, which is ten thousand times stronger compared to water-based colloids. This unique binding mechanism could be used to assemble resonator optical waveguides and robust chiral and achiral structures of topologically entangled colloids that we call colloidal wires.

The meaning of action: a review on action recognition and mapping

Abstract: In this paper, we analyze the different approaches taken to date within the computer vision, robotics and artificial intelligence communities for the representation, recognition, synthesis and understanding of action. We deal with action at different levels of complexity and provide the reader with the necessary related literature references. We put the literature references further into context and outline a possible interpretation of action by taking into account the different aspects of action recognition, action synthesis and task-level planning.

Seesaw at LHC

Abstract: We study the implementation of the type III seesaw in the ordinary nonsupersymmetric SU(5) grand unified theory. This allows for an alternative definition of the minimal SU(5) model, with the inclusion of the adjoint fermionic multiplet. The main prediction of the theory is the light fermionic SU(2) triplet with mass at the electroweak scale. Due to their gauge couplings, these triplets can be produced pair-wise via Drell-Yan, and due to the Majorana nature of the neutral component their decays leave a clear signature of same sign di-leptons and four jets. This allows for their possible discovery at LHC and provides an example of directly measurable seesaw parameters.

Probing the seesaw mechanism at CERN LHC

Abstract: We have recently proposed a simple SU(5) theory with an adjoint fermionic multiplet on top of the usual minimal spectrum. This leads to the hybrid scenario of both type I and type III seesaw and it predicts the existence of the fermionic SU(2) triplet between 100 GeV and 1 TeV for a conventional grand unified theory scale of about ${10}^{16}\text{ }\text{ }\mathrm{GeV}$, with main decays into $W$ ($Z$) and leptons, correlated through Dirac Yukawa couplings, and lifetimes shorter than about ${10}^{\ensuremath{-}12}\mathrm{sec} $. These decays are lepton number violating and they offer an exciting signature of $\ensuremath{\Delta}L=2$ dilepton events together with 4 jets at future $pp$ ($p\overline{p}$) colliders. Increasing the triplet mass endangers the proton stability and so the seesaw mechanism could be directly testable at LHC.

A mild chemically cleavable linker system for functional proteomic applications.

Abstract: One of the primary goals in the field of proteomics is finding ways to enrich specific protein targets from complex mixtures. Generally, this is accomplished with activity-based probes (ABPs) that allow specific modification by the formation of a stable covalent bond with reactive groups on a target protein. Such proteomic probes often carry biotin tags and can either be generally reactive towards free nucleophiles, such as thiols (i.e, isotope-coded affinity tagging (ICAT) reagents), or react through a specific enzymatic process with a key catalytic residue. Although enrichment by immobilized streptavidin allows efficient isolation of even highly dilute targets, one of the primary limitations is the need for harsh, denaturing conditions to disrupt the biotin–streptavidin interaction. This elution generally results in contamination of the desired probe-labeled proteins with avidin monomers, proteins that were nonselectively bound to the streptavidin, and endogenously biotinylated proteins. Hence, additional purification techniques such as gel electrophoresis are required prior to identification with MS. The incorporation of a cleavable linker between the biotin tag and the site of attachment to the target protease provides a significant advance as it allows specific elution of probelabeled proteins or peptides. Recently, a number of cleavable linkers have been reported with a focus on applications in MS and ICAT. However, these reagents require a strong acid (trifluoroacetic acid), making cleavage of labeled proteins directly from a strepavidin resin problematic as nonselective release can also occur. Disulfide cleavable linker systems, though useful for a variety of applications, are subject to disulfide exchange and show premature cleavage in cellular systems and reducing buffer solutions. Herein, we report a novel diazobenzene derivative and its application as a chemoselective cleavable linker system (Scheme 1). This linker can be easily incorporated into small-molecule probes that allow the isolation of specific protein targets by streptavidin affinity purification

Hip, thigh and calf muscle atrophy and bone loss after 5-week bedrest inactivity

Abstract: Unloaded inactivity induces atrophy and functional deconditioning of skeletal muscle, especially in the lower extremities. Information is scarce, however, regarding the effect of unloaded inactivity on muscle size and function about the hip. Regional bone loss has been demonstrated in hips and knees of elderly orthopaedic patients, as quantified by computerized tomography (CT). This method remains to be validated in healthy individuals rendered inactive, including real or simulated weightlessness. In this study, ten healthy males were subjected to 5 weeks of experimental bedrest and five matched individuals served as ambulatory controls. Maximum voluntary isometric hip and knee extension force were measured using the strain gauge technique. Cross-sectional area (CSA) of hip, thigh and calf muscles, and radiological density (RD) of the proximal tibial bone were measured using CT. Bedrest decreased (P 0.05), when examined at similar time intervals. The present findings of a substantial loss in hip extensor strength and a smaller, yet significant atrophy of these muscles, demonstrate that hip muscle deconditioning accompanies losses in thigh and calf muscle mass after bedrest. This suggests that comprehensive quantitative studies on impaired locomotor function after inactivity should include all joints of the lower extremity. Our results also demonstrate that a decreased RD, indicating bone mineral loss, can be shown already after 5 weeks of unloaded bedrest, using a standard CT technique.

Lysosomal cysteine cathepsins: signaling pathways in apoptosis

Abstract: Apoptosis is the major mechanism by which eukaryotic organisms eliminate potentially dangerous, superfluous and damaged cells. Initially, nuclei and mitochondria were found to be the key organelles involved in the process. However, recent data suggest that lysosomes and the endoplasmic reticulum also play important roles in the process. A number of different stimuli were found to directly or indirectly target the lysosomal membrane, thereby inducing lysosomal permeabilization and the release of cysteine cathepsins and the aspartic protease cathepsin D into the cytosol. Once in the cytosol, cathepsins can trigger cell death by different mechanisms. Here we discuss the different signaling pathways used by lysosomal proteases to trigger apoptosis and their potential role in physiological processes.

Electric field induced critical points and polarization rotations in relaxor ferroelectrics

Abstract: The giant electromechanical response in ferroelectric relaxors such as $\mathrm{Pb}({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ (PMN-PT) is of great importance for a number of ultrasonic and medical applications as well as in telecommunications. On the basis of the dielectric, heat capacity, and piezoelectric investigations on PMN-PT crystals of various PT compositions and bias fields, we have recently shown the existence of a line of critical points for the paraelectric to ferroelectric transformations in the composition-temperature-electric field $(x\text{\ensuremath{-}}T\text{\ensuremath{-}}E)$ phase diagram. Here, we show the piezobehavior in more detail and present a theoretical evaluation of the Widom line and the critical line. This line effectively terminates a surface of first order transitions. Above this line, supercritical evolution has been observed. On approaching the critical point, both the enthalpy cost to induce the intermediate monoclinic states and thus the barrier for polarization rotations decrease significantly. The maximum of the piezoelectric response is not at $E=0$, but at the critical field values. It is shown that the critical fluctuations in the proximity of the critical points are directly responsible for the observed enhancement of the electromechanical response in the PMN-PT system. In view of the large electric field dependence of the dielectric constant near the critical point, these systems may also be important as electric field tunable elements.

Sodium deficiency in Na0.5Bi0.5TiO3

Abstract: The mechanism of solid-state synthesis for Na0.5Bi0.5TiO3 was investigated using X-ray powder diffraction, scanning electron microscopy and wavelength-dispersive X-ray spectroscopy. The results showed that a Na0.5−xBi0.5TiO3−x/2 homogeneity region, with Na0.5Bi0.5TiO3 and Na0.47Bi0.5TiO3−δ as the end-members (0 ≤ x ≤ 0.03), exists in the Na2O–Bi2O3–TiO2 system. The formation of any member from the homogeneity region starts with the formation of the deficient end-member, i.e., Na0.47Bi0.5TiO3−δ. However, the reaction continues toward a nominal composition when a sufficiently high temperature and/or a long firing time are used. The conversion of the latter reaction may be followed by the observation of the crystal symmetry, which changes from the cubic-like toward the rhombohedral as the reaction proceeds from Na0.47Bi0.5TiO3−δ to Na0.5Bi0.5TiO3. Differences in the dielectric, ferroelectric and piezoelectric properties were observed for the samples from the homogeneity region, and these can be related to the stoichiometry variation along the homogeneity region and thus to the variation in the vacancy concentration. The most striking difference is the change in the piezoelectric constant d33, which increases from 82 pC N−1 to 97 pC N−1 as the composition of the ceramics changes from Na0.5Bi0.5TiO3 to Na0.47Bi0.5TiO3−δ.

OntoGen: semi-automatic ontology editor

Abstract: In this paper we present a semi-automatic ontology editor as implemented in a new version of OntoGen system. The system integrates machine learning and text mining algorithms into an efficient user interface lowering the entry barrier for users who are not professional ontology engineers. The main features of the systems include unsupervised and supervised methods for concept suggestion and concept naming, as well as ontology and concept visualization. The system was tested in extensive user trails and in several real-world scenarios with very positive results.

Interactions between cardiac, respiratory and EEG-delta oscillations in rats during anaesthesia.

Abstract: We hypothesized that, associated with the state of anaesthesia, characteristic changes exist in both cardio-respiratory and cerebral oscillator parameters and couplings, perhaps varying with depth of anaesthesia. Electrocardiograms (ECGs), respiration and electroencephalograms (EEGs) were recorded from two groups of 10 rats during the entire course of anaesthesia following the administration of a single bolus of ketamine–xylazine (KX group) or pentobarbital (PB group). The phase dynamics approach was then used to extract the instantaneous frequencies of heart beat, respiration and slow δ-waves (within 0.5–3.5 Hz). The amplitudes of δ- and θ-waves were analysed by use of a time–frequency representation of the EEG signal within 0.5–7.5 Hz obtained by wavelet transformation, using the Morlet mother wavelet. For the KX group, where slow δ-waves constituted the dominant spectral component, the Hilbert transform was applied to obtain the instantaneous δ-frequency. The θ-activity was spread over too wide a spectral range for its phase to be meaningfully defined. For both agents, we observed two distinct phases of anaesthesia, with a marked increase in θ-wave activity occurring on passage from a deeper phase of anaesthesia to a shallower one. In other respects, the effects of the two anaesthetics were very different. For KX anaesthesia, the two phases were separated by a marked change in all three instantaneous frequencies: stable, deep, anaesthesia with small frequency variability was followed by a sharp transition to shallow anaesthesia with large frequency variability, lasting until the animal awoke. The transition occurred 16–76 min after injection of the anaesthetic, with simultaneous reduction in the δ-wave amplitude. For PB anaesthesia, the two epochs were separated by the return of a positive response to the pinch test at 53–94 min, following which it took a further period of 45–70 min for the animal to awaken. δ-Waves were not apparent at any stage of PB anaesthesia. We applied non-linear dynamics and information theory to seek evidence of causal relationships between the cardiac, respiratory and slow δ-oscillations. We demonstrate that, for both groups, respiration drives the cardiac oscillator during deep anaesthesia. During shallow KX anaesthesia the direction either reverses, or the cardio-respiratory interaction becomes insignificant; in the deep phase, there is a unidirectional deterministic interaction of respiration with slow δ-oscillations. For PB anaesthesia, the cardio-respiratory interaction weakens during the second phase but, otherwise, there is no observable change in the interactions. We conclude that non-linear dynamics and information theory can be used to identify different stages of anaesthesia and the effects of different anaesthetics.