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Ashraf M.A.,University of Massachusetts Amherst | Copeland C.W.,University of Massachusetts Amherst | Kocak A.,University of Massachusetts Amherst | Kocak A.,Gebze Technical University | And 2 more authors.
Physical Chemistry Chemical Physics

Vibrational spectra are measured for Fe2+(CH4)n (n = 1-3) in the C-H stretching region (2650-3100 cm-1) using photofragment spectroscopy, by monitoring the loss of CH4. All of the spectra exhibit an intense peak corresponding to the symmetric C-H stretch around 2800 cm-1. The presence of a single peak suggests a nearly equivalent interaction between the iron dimer and the methane ligands. The peak becomes slightly blue shifted as the number of methane ligands increases. Density functional theory calculations, B3LYP and BPW91, are used to identify possible structures and predict the spectra. Results suggest that the methane(s) bind in a terminal configuration and the complexes are in the octet spin state. This journal is © the Owner Societies. Source

Kayabol K.,Gebze Technical University
2015 23rd European Signal Processing Conference, EUSIPCO 2015

We propose a Bayesian Gaussian mixture model for hyper-spectral image classification. The model provides a robust estimation framework for small size training samples. Defining prior distributions for the mean vector and the covariance matrix, we are able to regularize the parameter estimation problem. Especially, we can obtain invertible positive definite covariance matrices. The mixture model also takes into account the spatial alignments of the pixels by using non-stationary mixture proportions. Based on the classification results obtained on Indian Pine data set, the proposed method yields better classification performance especially for small size training samples compared to state-of-the-art linear and quadratic classifiers. © 2015 EURASIP. Source

Kayabol K.,Gebze Technical University
2015 23rd European Signal Processing Conference, EUSIPCO 2015

We propose a superpixel segmentation method for synthetic aperture radar (SAR) images. The method uses the SAR image amplitudes and pixels coordinates as features. The feature vectors are modeled statistically by taking into account the SAR image statistics. Nakagami and bivariate Gaussian distributions are used for amplitudes and position vectors, respectively. A finite mixture model (FMM) is proposed for pixel clustering. Learning and clustering steps are performed using posterior distributions. Based on the classification results obtained on real TerraSAR-X image, it is shown that the proposed method is capable of obtaining more accurate superpixels compared to state-of-the-art superpixel segmentation methods. © 2015 EURASIP. Source

Demir-Cakan R.,Gebze Technical University | Demir-Cakan R.,French National Center for Scientific Research
Journal of Power Sources

This work aims to study the reasons for the rapid capacity fading in the field of lithium-sulphur batteries. It is found that the formation of non-soluble and insulating lithium sulphide, Li2S, upon sulphur reduction is one of the main reasons to account for the capacity fade due to its relatively low electrochemical activity and blocking of the carbon framework. A variety of analytical techniques are used to assess the formation of Li2S and visualize the resulting morphological changes in a mesoporous carbon/sulphur composite (MCS) during battery operation. It is shown that simply changing the cut-off voltages can alter retention and resistance of the cells during cycling. Since the electrochemical activity of lithium polysulphides decreases with the order, elimination of low order of polysulphides is considered critical. Thus, in order to oxidize the least sulphur reduction specie, Li2S, two different additives are suggested; namely an electrolyte additive, LiI, or an electrode additive, V2O5. Incorporation of the one of these components into the MCS composite enables the delivery of higher capacities (i.e. as compared to a bare C/S composite). The increased capacity is linked not only to the co-cathode materials, which contribute to the total capacity, but also to the oxidizing agent and internal redox mediator role of the additives. © 2015 Elsevier B.V. Source

Shen J.,Iowa State University | Seker O.,Iowa State University | Sutchiewcharn N.,Aon Corporation | Akbas B.,Gebze Technical University
Journal of Constructional Steel Research

A parametric study is presented to quantify essential factors influencing cyclic behavior of a steel buckling-controlled brace (BCB) with a tube carrying axial load surrounded by an outer tube to control buckling of the load-bearing tube. A small-scale experiment helped observe overall cyclic behavior, and develop finite-element models for numerical simulations. The model-based simulations identified the interaction of the friction, gap and thickness ratio between the two tubes as the essential factor. The paper concludes that (1) the gap is a sensitive parameter influencing local and global buckling. The smaller the gap, the less likely the local and global buckling will occur, but the more participation of the outer tube in load bearing due to adverse interaction between the two tubes; (2) Friction between the two tubes is a very delicate factor because its impact on the cyclic behavior of BCB varies depending on thickness ratio and friction; (3) Thickness ratio of the two tubes decides the effectiveness of controlling buckling. The thickness ratio of 1.0 is sufficient to control global buckling, but a larger than 1.0 ratio is needed to control both local and global buckling; (4) Interaction among the gap, friction and thickness ratio is strong, and shall be considered in design; and (5) Optimal performance results from a system with smallest gap possible, low friction, and heavier outer tube. Some less optimal but lower costly design combinations may have moderate gaps and various outer tube sizes to control brace buckling within targeted drift limits in performance-based design. © 2015 Elsevier Ltd. All rights reserved. Source

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