Thermal properties of rare earth dodecaborides

We have measured heat capacity and thermal expansion of rare earth dodecaborides REB₁₂ (RE=Y, Tb-Tm, Lu). YB₁₂ and LuB₁₂ are diamagnetics whereas the other dodecaborides are ordered antiferromagnetically. The amplitude of the heat capacity discontinuity at the Néel temperature and the shape of the heat capacity variation in the critical region for all these antiferromagnetics are characteristics for amplitude-modulated magnetic structures. In the ordered state TbB₁₂ reveals two first-order phase transitions, most likely due to magnetic structure changes. The heat capacity of ErB₁₂ just below the Néel point shows an anomaly of unclear origin. From the Schottky contribution to the heat capacity we have determined crystal field parameters. They are completely different than that is estimated from Point Charge Model.     

On clique relaxation models in network analysis

Increasing interest in studying community structures, or clusters in complex networks arising in various applications has led to a large and diverse body of literature introducing numerous graph-theoretic models relaxing certain characteristics of the classical clique concept. This paper analyzes the elementary clique-defining properties implicitly exploited in the available clique relaxation models and proposes a taxonomic framework that not only allows to classify the existing models in a systematic fashion, but also yields new clique relaxations of potential practical interest. Some basic structural properties of several of the considered models are identified that may facilitate the choice of methods for solving the corresponding optimization problems. In addition, bounds describing the cohesiveness properties of different clique relaxation structures are established, and practical implications of choosing one model over another are discussed.     

The effect of point defects on the ionic conductivity of RbAg4I5 solid electrolytes

Abstract

The effect has been studied of additive coloring on the magnitude of ion conductivity of RbAg₄I₅ crystals. It has been found that slight changes of silver stoichiometry of 10^?3 at.% can lead to considerable variations of the ionic conductivity Δ[sgrave]_i/[sgrave]_i ? 0.1. The dependence has been observed of the magnitude of ion conductivity on the ratio between the integral intensities of the main bands in the photoluminescence spectrum of the γ-phase of RbAg₄I₅ which associated with the luminescence centres containing vacancies and interstitials of silver cations.     

Stochastic optimization in supply chain networks: averaging robust solutions

We propose a novel robust optimization approach to analyze and optimize the expected performance of supply chain networks. We model uncertainty in the dema     

Forces between surfaces across nanoparticle solutions: role of size, shape, and concentration

Using a surface force apparatus, we have measured the normal forces between mica surfaces across various types of nanoparticles consisting of ZnS cores coated with a monolayer of physisorbed surfactant, dispersed in organic solvents. We focused on the effects of nanoparticle size, shape, and concentration on the force-distance profiles. Forces were exponentially repulsive when the surfactant layers were strongly bound to the nanoparticles and were roughly linear when there was adhesion between the nanoparticle cores, i.e., when the surfactant layers detached from the nanoparticles. In both cases, the range and magnitude of the forces were dependent upon the particle size, shape, and solution concentration. Fine details in the otherwise smooth force-distance profiles indicate specific effects due to particle chemistry and geometry and the existence of first-order disorder-order phase transitions upon confinement. Small amounts of water in the (hydrophobic) organic solvents had dramatic effects on the measured forces. Understanding and controlling the effects of particle shape, size, and concentration and the presence of water (or other surface-active solutes) on particle-particle and particle-surface interactions are important for the processing of nanoparticles into ordered superstructured materials.     

Biological ferroelectricity uncovered in aortic walls by piezoresponse force microscopy

Many biological tissues are piezoelectric and pyroelectric with spontaneous polarization. Ferroelectricity, however, has not been reported in soft biological tissues yet. Using piezoresponse force microscopy, we discover that the porcine aortic walls are not only piezoelectric, but also ferroelectric, with the piezoelectric coefficient in the order of 1 pm/V and coercive voltage approximately 10 V. Through detailed switching spectroscopy mapping and relaxation studies, we also find that the polarization of the aortic walls is internally biased outward, and the inward polarization switched by a negative voltage is unstable, reversing spontaneously to the more stable outward orientation shortly after the switching voltage is removed. The discovery of ferroelectricity in soft biological tissues adds an important dimension to their biophysical properties, and could have physiological implications as well.     

Calibration of laser-ablation ICP-MS. Can we use synthetic standards with pneumatic nebulization?

An approach for the determination of trace element concentrations in high purity metals, using an inductively coupled plasma mass spectrometer (ICP-MS) with a laser-ablation system for direct solid sample introduction after calibration with nebulized liquid standards was made. Due to the inherent differences in the rate of sample introduction with laser-ablation and pneumatic nebulization, a matrix element must be used as an internal standard. This is problematical for elements that have no isotope with a relative abundance of less than 0.1 %, since the ion signals would be too high for direct measurement, and reduction of the ablation rate would compromise the sensitivity for trace elements. Due to the high stability of ICP-unit and mass filter of the instrument used, it was found that the tail of a mass-peak of the matrix element could be used as an internal standard. Therefore, a position at –0.5 amu from the matrix-isotope (e.g. ^62.5Cu in copper samples) was used for internal standardization. The standard deviation of this signal in a period of 2.5 h was 3.6% RSD with no notable drift when the laser ablation was used for sample introduction. The calibration of the matrix-element by nebulizing liquid standards showed that the ion signal measured on the peak-tail is directly proportional to the element concentration in the ICP. This indicates that the peak shape is not only stable, but also independent of the peak height. The advantages of this method lie in the easy preparation of calibration standards for quantitative measurements with a laser-ablation system and access to homogeneous standards for materials, that are difficult to homogenize in the solid state. The calibration of the traces is performed relatively to a fixed concentration of the matrix element. Calibrations were carried out for trace concentrations in high purity copper and good recoveries were obtained for high-purity reference standards. Received: 23 February 1998 / Revised: 20 July 1998 / Accepted: 25 July 1998     

Chemometrics‐enhanced laser‐induced thermal emission detection of PETN and other explosives on various substrates

Infrared emissions (IREs) of samples of pentaerythritol tetranitrate (PETN) deposited as contamination residues on various substrates were measured to generate models for the detection and discrimination of the important nitrate ester from the emissions of the substrates. Mid‐infrared emissions were generated by heating the samples remotely using laser‐induced thermal emission (LITE). Chemometrics multivariate analysis techniques such as principal component analysis (PCA), soft independent modeling by class analogy (SIMCA), partial least squares‐discriminant analysis (PLS‐DA), support vector machines (SVMs), and neural network (NN) were employed to generate the models for the classification and discrimination of PETN IREs from substrate thermal emissions. PCA exhibited less variability for the LITE spectra of PETN/substrates. SIMCA was able to predict only 44.7% of all samples, while SVM proved to be the most effective statistical analysis routine, with a discrimination performance of 95%. PLS‐DA and NN achieved prediction accuracies of 94% and 88%, respectively. High sensitivity and specificity values were achieved for five of the seven substrates investigated. Copyright © 2015 John Wiley & Sons, Ltd.