55,928 research outputs found
Methane Activation by Transition-Metal Oxides, MOx (M=Cr, Mo, W; x=1,2,3)
No full textRecent experiments on the dehydrogenation-aromatization of methane (DHAM) to form benzene using a MoO3/HZSM-5 catalyst stimulated us to examine methane activation by the transition-metal oxide molecules, MOx (M = Cr, Mo, W; x = 1, 2, 3). The present studies use hybrid density functional theory (B3LYP). The reactivity trend is rationalized in terms of changes in the electrophilicity of MO,, the strength of the M-O pi bond, and the bonding properties of MQ(x) to methyl or hydrogen as M and x are varied, It is found that or-bond metathesis to the metal hydride product (H-MOx-1-OCH3) occurs preferentially over the high oxidation state form (MO3) of the heavier metals, as well as all chromium oxides (CrOx). Instead, oxidative addition of MOx leading to methyl metal hydride (H-M(O-x)-CH3) is more favorable over the low oxidation state of MOx (M = Mo, W, x = 2, 1). In particular, it is found that WO2 can undergo oxidative addition with negligible activation barrier and is predicted to be the most reactive compound of this class toward methane activation. Our finding that MO2 (M = W, Mo) is the best oxidation state for this class of metal oxides toward methane activation suggests that the MO3/HZSM-5 catalysts active in the DHAM reaction may be W and Mo oxycarbides (MO2C2). The formation of such intermediates may be the reason that the experiments find an induction period before the catalyst is active for the desired reaction
Osiadanie terenu i jego wpływ na środowisko w chińskich regionach Xu Zhou, Yan Zhou oraz Huainan i Huaibei
No full textThe main coal mining subsidence area of the Xu Zhou, Yan Zhou, Huaihe collapsed state region were investigated including the environment state, water surface area, pollution source, and the utilization state. Some advice wasproposed to protect the environment of the subsidence area.Badaniem objęto główną strefę osiadania w stanie całkowitego zawału w chińskich regionach Xu Zhou, Yan Zhou i Huaihe, w tym stan otoczenia, wody powierzchniowe, źródła zanieczyszczenia oraz utylizację. Zaproponowano kilka rozwiązań dla ochrony środowiska strefy osiadania
Measurement of w-InN/h-BN Heterojunction Band Offsets by X-Ray Photoemission Spectroscopy
No full textX-ray photoelectron spectroscopy has been used to measure the valence band offset (VBO) of the w-InN/h-BN heterojunction. We find that it is a type-II heterojunction with the VBO being -0.30 +/- A 0.09 eV and the corresponding conduction band offset (CBO) being 4.99 +/- A 0.09 eV. The accurate determination of VBO and CBO is important for designing the w-InN/h-BN-based electronic devices
The Effect of Microstructure on the Abrasion Resistance of Low Alloyed Steels
No full textThe thesis attempts to develop advanced high abrasion resistant steels with low hardness in combination with good toughness, processability and low alloying additions. For this purpose, a novel multi-pass dual-indenter (MPDI) scratch test approach has been developed to approach the real continuous abrasion process and unravel abrasion damage formation in construction steels, i.e. carrying out scratch tests using a large indenter with different pre-loads to generate a wide pre-scratch with stable saturated work hardening and then a small indenter sliding over the pre-scratch to evaluate the abrasion resistance and observe the failure mechanism. With this approach, an extensive experimental investigation has been conducted to study the correlation of abrasion resistance and microstructural features (phase volume fraction, morphology, grain size, etc.) so as to understand the response of microstructures on abrasive wear, and eventually provide the knowledge to guide the design of high abrasion resistant steel. A promising type of microstructure with high abrasion resistance but low hardness has been proposed. In addition, a two-stage tensile strain hardening model was applied to interpret the scratch resistance under different pre-load conditions and resulting failure mechanisms. The strength coefficient K in the Hollomon equation (?=K?^n) in this model corresponds well with that of scratch resistance.Aerospace Structures and MaterialsAerospace Engineerin
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
A novel 2D non-interpenetrated copper(I) iodide coordination polymer with trans-1,4-diaminocyclohexane
No full textA novel non-interpenetrated 2D coordination polymer [Cu 4I 4(DACH) 2] n bridged by the flexible trans-1,4-diaminocyclohexane (DACH) ligand has been synthesized under hydrothermal condition and characterized by single crystal X-ray diffraction, TG-DTA and the solid-state luminescence spectrum. In the compound, each Cu 4I 4 cluster is bridged by DACH ligand to form a quadrate-grid 2D network with the inner cavities of approximate size of 6.35 × 18.12 . It is unusual that these 2D networks are stacked and there exist the N-H...I hydrogen bonds between layers. The channels are finally formed along b-axis with small size of cavities. Thermal analysis shows that the complex is stable up to ca. 240 °C. The solid-state luminescence spectrum displays a strong yellow emission band at room temperature, characteristic of the Cu 4I 4 cluster centers. © 2011 Elsevier B.V. All rights reserved
Robust tube-based model predictive control with Koopman operators
Get PDFKoopman operators are of infinite dimension and capture the characteristics of nonlinear dynamics in a lifted global linear manner. The finite data-driven approximation of Koopman operators results in a class of linear predictors, useful for formulating linear model predictive control (MPC) of nonlinear dynamical systems with reduced computational complexity. However, the robustness of the closed-loop Koopman MPC under modeling approximation errors and possible exogenous disturbances is still a crucial issue to be resolved. Aiming at the above problem, this paper presents a robust tube-based MPC solution with Koopman operators, i.e., r-KMPC, for nonlinear discrete-time dynamical systems with additive disturbances. The proposed controller is composed of a nominal MPC using a lifted Koopman model and an off-line nonlinear feedback policy. The proposed approach does not assume the convergence of the approximated Koopman operator, which allows using a Koopman model with a limited order for controller design. Fundamental properties, e.g., stabilizability, observability, of the Koopman model are derived under standard assumptions with which, the closed-loop robustness and nominal point-wise convergence are proven. Simulated examples are illustrated to verify the effectiveness of the proposed approach
Monitoring of oil spill trajectories with COSMO-SkyMed X-band SAR images and model simulation
No full textThe Shell North Sea Gannet Alpha platform oil spill accident occurred on August 10, 2011. This was the largest oil spill accident in United Kingdom waters in the last decade. The spills were observed on four COSMO-SkyMed (CSK) X-band synthetic aperture radar (SAR) images acquired between August 17 and 22, 2011, with revisit time from 11 h to 3 days between the SAR acquisitions. The areas of oil slicks were extracted from SAR images using an existing image classification and segmentation algorithm. It was found that the oil slicks moved toward the southwest with slick size enlarging from 3.69 to 62.01 km2 in the first 24 h between the first and second SAR acquisitions. We tracked the oil spill trajectories using the General NOAA Operational Modeling Environment (GNOME) oil-drifting model. The 6-hourly surface wind fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA) Interim products and the 3-hourly ocean surface current fields from the Navy Coastal Ocean Model (NCOM) global operational model were used to drive the GNOME model. The simulated oil slick movement was in good agreement with that observed by the CSK SAR images. Moreover, the simulation showed that the movement of oil spills was dominated by the surface winds in the North Sea
Effects of Different Nano-Sized Metal Oxide Catalysts on the Properties of Composite Solid Propellants
Get PDFSeveral industrial- and research-type composite solid propellants containing different nano metric metal oxide catalysts (Fe2O3, Co3O4, CuO, and PbO) with similar nominal composition, were prepared and experimentally analyzed. The effects of different nano-sized metal oxide catalysts on the rheological properties and hazardous properties were investigated. The strand burning rate and the associated combustion flame structure of composite propellants were determined. The results show that the nano-sized metal oxide powders can be sufficiently dispersed in hydroxyl terminated polybutadiene binder. The propellant formulations containing nano metal oxide particles are sensitive to impact and friction except for the base propellant without nano-sized powders, which is less sensitive to friction as compared to the other compositions. The nano-sized metal oxide additives can affect the combustion behavior and increase the burning rate of propellants compared with the reference propellant composition
Relations between x-ray timing features and spectral parameters of galactic black hole x-ray binaries
Get PDFWe present a study of correlations between spectral and timing parameters for a sample of black hole X-ray binary candidates. Data are taken from GX
339-4, H 1743-322, and XTE J1650-500, as the Rossi X-ray Timing Explorer
(RXTE) observed complete outbursts of these sources. In our study we investigate outbursts that happened before the end of 2009 to make use of the high-energy coverage of the HEXTE detector and select observations that
show a certain type of quasi-periodic oscillations (type-C QPOs). The spectral parameters are derived using the empirical convolution model simpl to model the Comptonized component of the emission together with a disc blackbody for the emission of the accretion disc. Additional spectral features, namely a reflection component, a high-energy cut-off, and excess emission at 6.4 keV, are taken into account. Our investigations confirm the known positive
correlation between photon index and centroid frequency of the QPOs and reveal an anti-correlation between the fraction of up-scattered photons and the QPO frequency. We show that both correlations behave as expected in the “sombrero”
geometry. Furthermore, we find that during outburst decay the correlation between photon index and QPO frequency follow a general track, independent of individual outbursts
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