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排序方式: 共有775条查询结果,搜索用时 125 毫秒
41.
Gabriela Ben-Melech Stan Kapil Dhaka Maytal Caspary Toroker 《Israel journal of chemistry》2020,60(8-9):888-896
Two-dimensional transistors are promising candidates for the next generation of nanoscale devices. Like the other alternatives, they also encounter problems such as instability under standard condition (STP), low channel mobility, small band gaps, and difficulty to integrate metal contacts. The latter poses a great challenge since metal/semiconductor interface significantly affects the transistor‘s performance. Some of these obstacles can be solved by using two-dimensional transition metal di-chalcogenides (TMDC) materials. In this study, we performed charge transport calculation based on density functional theory (DFT) followed by wave dynamics to evaluate the performance of six two-dimensional TMDC metal/semiconductor/metal systems. Each semiconductor monolayer was laterally connected, at both ends to metal contacts consisting of VS2 or FeS2 monolayers. We found that charge transport was more efficient in systems containing a CrS2 semiconductor monolayer compared to systems with MoS2 or WS2 as the semiconductor monolayer. The electronic characterization of the monolayer TMDC materials by DFT estimates well the trend in charge transport efficiency calculated using wave packet dynamics. 相似文献
42.
Michael?J.?HaasEmail author Paul?J.?Michalski Stan?Runyon Alberto?Nunez Karen?M.?Scott 《Journal of the American Oil Chemists' Society》2003,80(1):97-102
Simple alkyl FA esters have numerous uses, including serving as biodiesel, a fuel for compression ignition (diesel) engines.
The use of acid-catalyzed esterification for the synthesis of FAME from acid oil, a by-product of edible vegetable oil refining
that is produced from soapstock, was investigated. Soybean acid oil contained 59.3 wt% FFA, 28.0 wt% TAG, 4.4 wt% DAG, and
less than 1% MAG. Maximum esterification occurred at 65°C and 26 h reaction at a molar ratio of total FA/methanol/sulfuric
acid of 1∶15∶1.5. Residual unreacted species under these conditions, as a fraction of their content in unesterified acid oil,
were FFA, 6.6%; TAG, 5.8%; and DAG, 2.6%. This corresponds to estimated concentrations of FFA, 3.2%; TAG, 1.3%; and DAG, 0.2%,
on a mass basis, in the ester product. In an alternative approach, the acylglycerol species in soapstock were saponified prior
to acidulation. High-acid (HA) acid oil made from this saponified soapstock had an FFA content of 96.2 wt% and no detectable
TAG, DAG, or MAG. Optimal esterification conditions for HA acid oil at 65°C were a mole ratio of FFA/methanol/acid of 1∶1.8∶0.17,
and 14 h incubation. FAME recovery under these conditions was 89% of theoretical, and the residual unesterified FFA content
was approximately 20 mg/g. This was reduced to 3.5 mg/g, below the maximum FFA level allowed for biodiesel, by washing with
NaCl, NaHCO3, and Ca(OH)2 solutions. Alternatively, by subjecting the unwashed ester layer to a second esterification, the FFA level was reduced to
less than 2 mg/g. The acid value of this material exceeded the maximum allowed for biodiesel, but was reduced to an acceptable
value by a brief wash with 0.5 N NaOH. 相似文献
43.
44.
Iulian Riposan Stelian Stan Valentin Uta Ion Stefan 《Journal of Materials Engineering and Performance》2017,26(9):4217-4226
Inoculation is one of the most important metallurgical treatments applied to the molten cast iron immediately prior to casting, to promote solidification without excessive eutectic undercooling, which favors carbides formation usually with undesirable graphite morphologies. The paper focused on the separate addition of an inoculant enhancer alloy [S, O, oxy-sulfides forming elements] with a conventional Ca-FeSi alloy, in the production of gray and ductile cast irons. Carbides formation tendency decreased with improved graphite characteristics as an effect of the [Ca-FeSi + Enhancer] inoculation combination, when compared to other Ca/Ca, Ba/Ca, RE-FeSi alloy treatments. Adding an inoculant enhancer greatly enhances inoculation, lowers inoculant consumption up to 50% or more and avoids the need to use more costly inoculants, such as a rare earth bearing alloy. The Inoculation Specific Factor [ISF] was developed as a means to more realistically measure inoculant treatment efficiency. It compares the ratio between the improved characteristic level and total inoculant consumption for this effect. Addition of any of the commercial inoculants plus the inoculant enhancer offered outstanding inoculation power [increased ISF] even at higher solidification cooling rates, even though the total enhancer addition was at a small fraction of the amount of commercial inoculant used. 相似文献
45.
L. Peksa D. Pražák T. Gronych P. Řepa M. Vièar J. Tesař Z. Krajíèek F. Stanìk 《Mapan》2009,24(2):77-88
Accurate calibrations of gauges to ever lower pressures (higher vacuum) are increasingly becoming necessary. The calibrations
over the entire ultra high vacuum (UHV) range i.e. up to 10−10 Pa are already sometimes demanded by the users of vacuum measuring devices. Standards covering part of this range are available
in some national metrological laboratories while primary standards covering the entire UHV range are under development. However,
the most suitable principle of the primary standard for generating such a low pressure is still not clear. In spite of some
attempts during the last century no sufficiently accurate absolute (“calibration-free”) gauge for high vacuum (HV) and UHV
ranges has been developed to date. Apparatuses for generating gas pressure in a dynamic flow control method will continue
to be used as primary standards for the UHV range, but, by virtue of technical difficulties, the equilibrium (Maxwellian velocity
distribution) will be severely disturbed during the process of pressure generation that the pressure in the chamber will not
be defined as a simple scalar quantity. This principal problem must be solved first by the development of a primary UHV standard.
Another serious problem in the design of an apparatusfor generating controlled pressures on the order of 10−10 Pa requires the uncontrolled changes of gas pressure to 10−12 Pa or lower. Hence an extremely high vacuum (XHV) apparatus must be used in which the requirements concerning the pumps and
the outgassing rate from the materials are kept very strict. 相似文献
46.
当我们即将迎来喜气洋洋的春节时,逐渐增多的室外活动和朋友聚会几乎又开始占满我们的休息时间。DC,尤其是随身携带的卡片机型肯定成为了记录2010年春天的必备之物。 相似文献
47.
Lenka Slavíková Vítězslav Malý Michael Rost Lubomír Petružela Ondřej Vojáček 《Water Resources Management》2013,27(2):365-379
The paper investigates whether there is a statistically significant impact of short-term climate variables (specifically air temperature and rainfall) on residential water consumption at two selected case sites in the Czech Republic. The analysis is based on a unique time series of daily data from 2004–2009. The statistical methods used are CART methodology and a decomposition of these time series based on a locally weighted regression method. Apart from the data analysis results, the investigation raises several methodological questions regarding the use of daily data and the scope of analysis based on such data sets. 相似文献
48.
49.
The effects of vitamin E and selenium intake on oxidative stress and plasma lipids in hamsters fed fish oil 总被引:5,自引:0,他引:5
The aim of the present work was to test the effects of large-dose supplementation of vitamin E (Vit E) and selenium (Se),
either singly or in combination, on fish oil (FO)-induced tissue lipid peroxidation and hyperlipidemia. The supplementation
of Se has been shown to lower blood cholesterol and increase tissue concentrations of the antioxidant glutathione (GSH); however,
the effects of Se supplementation, either alone or in combination with supplemental Vit E, on FO-induced oxidative stress
and hyperlipidemia have not been studied. Male Syrian hamsters received FO-based diets that contained 14.3 wt% fat and 0.46
wt% cholesterol supplemented with Vit E (129 IU d-α-tocopheryl acetate/kg diet) and/or Se (3.4 ppm as sodium selenate) or that contained basal requirements of both nutrients.
The cardiac tissue of hamsters fed supplemental Se showed increased concentrations of lipid hydroperoxides (LPO) but decreased
oxidized glutathione (GSSG) concentrations. The higher concentrations of LPO in the hearts of Se-supplemented hamsters were
not lowered with concurrent Vit E supplementation. In the liver, Se supplementation was associated with higher Se-dependent
glutathione peroxidase activity and an increase in the GSH/GSSG ratio, whereas a lower hepatic non-Se-dependent glutathione
peroxidase activity was seen with Vit E supplementation. Supplemental intake of Se was associated with lower plasma concentrations
of total cholesterol and low density lipoprotein cholesterol plus very low density lipoprotein cholesterol. In view of the
pro-oxidative effects of Se supplementation on cardiac tissue, a cautionary approach needs to be taken regarding the plasma
lipid-lowering properties of supplemental Se. 相似文献
50.
Associative memory with dynamic synapses 总被引:5,自引:0,他引:5
We have examined a role of dynamic synapses in the stochastic Hopfield-like network behavior. Our results demonstrate an appearance of a novel phase characterized by quick transitions from one memory state to another. The network is able to retrieve memorized patterns corresponding to classical ferromagnetic states but switches between memorized patterns with an intermittent type of behavior. This phenomenon might reflect the flexibility of real neural systems and their readiness to receive and respond to novel and changing external stimuli. 相似文献