New hybrid catalysts for olefin metathesis were prepared by immobilization of Hoveyda–Grubbs type Ru alkylidenes on a siliceous SBA-15 support via exchange of chloro ligands for substituted carboxylates. Catalysts proved a high activity in ring closing metathesis of 1,7-octadiene and diethyl diallylmalonate, reusability and low Ru leaching. In Ring Opening Metathesis Polymerization of norbornene and cyclooctene, SBA-15 supported catalyst gave higher yields of high-molecular-weight polymers than corresponding catalyst with conventional silica support. 相似文献
New hybrid olefin metathesis catalysts were prepared by immobilization of Hoveyda-Grubbs type catalyst (commercially available as Zhan catalyst-1B) on the surface of mesoporous molecular sieves differing in pore size and architecture (MCM-41, MCM-48, and SBA-15) and conventional silica for a comparison. The activity of these catalysts was tested in RCM of (−)-β-citronellene, metathesis of 1-decene, ADMET of 1,9-decadiene, and in ROMP of cyclooctene and was found to increase significantly with the increasing pore size of the supports used. In all reactions, the activity of hybrid catalysts based on mesoporous molecular sieves was higher than that of catalyst using conventional silica as a support. In ROMP of cyclooctene, high molecular weight polymer (Mw = 300,000) in high yield (70-80%) was obtained with catalysts based on mesoporous supports, however, only 40% polymer yield was obtained using catalyst based on conventional silica. 相似文献
Speaker variability is known to have an adverse impact on speech systems that process linguistic content, such as speech and language recognition. However, speech production changes in individuals due to stress and emotions have similarly detrimental effect also on the task of speaker recognition as they introduce mismatch with the speaker models typically trained on modal speech. The focus of this study is on the analysis of stress-induced variations in speech and design of an automatic stress level assessment scheme that could be used in directing stress-dependent acoustic models or normalization strategies. Current stress detection methods typically employ a binary decision based on whether the speaker is or not under stress. In reality, the amount of stress in individuals varies and can change gradually. Using speech and biometric data collected in a real-world, variable-stress level law enforcement training scenario, this study considers two methods for stress level assessment. The first approach uses a nearest neighbor clustering scheme at the vowel token and sentence levels to classify speech data into three levels of stress. The second approach employs Euclidean distance metrics within the multi-dimensional feature space to provide real-time stress level tracking capability. Evaluations on audio data confirmed by biometric readings show both methods to be effective in assessment of stress level within a speaker (average accuracy of 55.6?% in a 3-way classification task). In addition, an impact of high-level stress on in-set speaker recognition is evaluated and shown to reduce the accuracy from 91.7?% (low/mid stress) to 21.4?% (high level stress). 相似文献
Using the MoS2‐WTe2 heterostructure as a model system combined with electrochemical microreactors and density function theory calculations, it is shown that heterostructured contacts enhance the hydrogen evolution reaction (HER) activity of monolayer MoS2. Two possible mechanisms are suggested to explain this enhancement: efficient charge injection through large‐area heterojunctions between MoS2 and WTe2 and effective screening of mirror charges due to the semimetallic nature of WTe2. The dielectric screening effect is proven minor, probed by measuring the HER activity of monolayer MoS2 on various support substrates with dielectric constants ranging from 4 to 300. Thus, the enhanced HER is attributed to the increased charge injection into MoS2 through large‐area heterojunctions. Based on this understanding, a MoS2/WTe2 hybrid catalyst is fabricated with an HER overpotential of ?140 mV at 10 mA cm?2, a Tafel slope of 40 mV dec?1, and long stability. These results demonstrate the importance of interfacial design in transition metal dichalcogenide HER catalysts. The microreactor platform presents an unambiguous approach to probe interfacial effects in various electrocatalytic reactions. 相似文献
Global digital elevation models of Mars, Venus and the Moon have been used to study the evolution of these planetary bodies and the early history of our solar system. A global digital elevation model of Mars has recently been produced from altimetry data collected by the Mars Orbiter Laser Altimeter on the Mars Global Surveyor orbiter. Over half a billion data points were collected over the five years of operation and the end result was the first centre of mass-based geodetic grid of Mars. This product has vertical resolution of ~1 m and spatial resolution ≤500 m and has helped to define and test many hypotheses related to the planet's interior and surface evolution. Higher resolution digital elevations models (DEMs) have also been created from stereo images taken by the High Resolution Stereo Camera on the Mars Express spacecraft. These data have spatial resolution of up to 10 m, but only cover ~25% of the planet. Both sets of DEMs have been applied to a broad range of geological and geophysical problems. Here, I focus on the application of Martian DEMs to assess the history of water on the Red Planet, which allows improved understanding of Mars' astrobiological potential. 相似文献
This work aims to describe the effect of magnesium on the microstructure, phase composition, amount of undesirable Ti2Ni phase, martensitic transformation, mechanical properties, and corrosion resistance of NiTi alloy. To minimize the quantity of Ti2Ni phase, we use the magnesium as an element with high affinity to oxygen, because this phase is stabilized by oxygen. Various quantities of magnesium (1, 3, and 5 wt pct) were tested. Self-propagating high-temperature synthesis (SHS) was used as a production method of the alloys. The samples prepared by SHS were pulverized by a vibrating mill, and the obtained powders were used for consolidation by means of spark plasma sintering. Results showed a significant reduction of the content of undesirable Ti2Ni phase by the addition of magnesium. Further, magnesium increased corrosion resistance and yield strength.
The RF magnetron sputtering of polytetrafluoroethylene target is studied with the aim to find out conditions leading to the deposition of super-hydrophobic thin films. It is shown that such coatings can be prepared at elevated pressures and a longer distance between the sputtered target and the substrate. This is explained by an increase in the density of longer CxFy molecules that reach the substrate and a lower flux of ions and CF2 radicals on the surface of growing film under such deposition conditions, as observed by optical emission spectroscopy and mass spectrometry. Such changes in plasma composition result in a deposition of rough films having F/C ratio close to 2 as observed by scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. These findings clearly distinguish our results from the previous investigations of polytetrafluoroethylene sputtering performed at shorter distances from the target, where either low F/C ratio or low roughness of the deposited films did not allow reaching super-hydrophobic character of the coatings. 相似文献