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排序方式: 共有412条查询结果,搜索用时 31 毫秒
201.
Joy P. Dunkers Joseph L. Lenhart Sylvia R. Kueh John H. van Zanten Suresh G. Advani Richard S. Parnas 《Optics and Lasers in Engineering》2001,35(2)
The Polymer Composites group at the National Institute of Standards and Technology has efforts in both on-line flow and cure sensing for liquid composite molding. For our flow program, a novel fiber optic real-time sensor system has been developed that can sense resin at various locations on a single fiber using long-period gratings and a polychromatic source. The sensor operation and characterization will be discussed along with sensor performance during mold filling with various types of reinforcement. The cure sensing program focuses on the interface-sensitive fluorescence response of a dye molecule grafted to a high-index glass fiber. The fluorescence emission of the fluorophore undergoes a blue shift as the resin cures. The fluorescence sensor is made by grafting a silane functional fluorophore onto the surface of the glass with close attention to layer thickness. Fluorescence emission of the grafted fluorophore film is shown to be sensitive to epoxy resin cure, co-silane, and layer thickness. The response of the grafted fluorophore to cure on a high-index fiber is demonstrated. 相似文献
202.
As a result of difficulties in scaling up silicon-directed Nazarov cyclization for use in the synthesis of taxane natural products, the effect of additives was examined. Hydroxylic additives were found to convey a consistent beneficial effect on the rate, overall yield, and scalability of these reactions. Optimal conditions (1.5 equiv methanol or water with substoichiometric FeCl3), were successfully applied to a range of β-silyl dienone substrates, including several complex polycyclic examples, as well as simpler dienones previously shown to be low-yielding or completely unreactive under the conventional conditions. 相似文献
203.
Antioxidant activity and total phenolic content of aqueous and methanolic extracts of Jordanian plants: an ICBG project 总被引:2,自引:0,他引:2
Alali FQ Tawaha K El-Elimat T Syouf M El-Fayad M Abulaila K Nielsen SJ Wheaton WD Falkinham JO Oberlies NH 《Natural product research》2007,21(12):1121-1131
As part of an International Cooperative Biodiversity Groups (ICBG) program to study Jordan's biodiversity, the relative levels of antioxidant activity and the total phenolic content of aqueous and methanolic extracts of a total of 95 plant species, all of Jordanian origin and those collected at random, have been measured. The total phenolic content of aqueous and methanolic extracts of the investigated plant species ranged from 4.4 to 78.3 mg and from 2.1 to 52.8 mg gallic acid equivalents g(-1) dry weight, respectively, while the total antioxidant capacity ranged from 20.0 to 916.7 and from 15.1 to 915.6 micromol Trolox equivalents g(-1) dry weight, respectively. Based on this collection, approximately 5% of assayed plants showed high levels of antioxidant activity. There was a significant linear correlation between antioxidant activity and total phenolic content for aqueous and methanolic extracts, suggesting that phenolic compounds were the predominant antioxidant components in the investigated plant species. Interestingly, a few of the collected plants had high-antioxidant activity yet "low" phenolic content includes Ceratonia siliqua and Viscum cruciatum. These plants may serve as sources of antioxidants with new chemotypes. 相似文献
204.
Costello CE Contado-Miller JM Cipollo JF 《Journal of the American Society for Mass Spectrometry》2007,18(10):1799-1812
This communication reports the development of an LC/MS platform for the analysis of permethylated oligosaccharide alditols that, for the first time, demonstrates routine online oligosaccharide isomer separation of these compounds before introduction into the mass spectrometer. The method leverages a high-resolution liquid chromatography system with the superior fragmentation pattern characteristics of permethylated oligosaccharide alditols that are dissociated under low-energy collision conditions using quadrupole orthogonal time-of-flight (QoTOF) instrumentation and up to pseudo MS(3) mass spectrometry. Glycoforms, including isomers, are readily identified and their structures assigned. The isomer-specific spectra include highly informative cross-ring and elimination fragments, branch position specific signatures, and glycosidic bond fragments, thus facilitating linkage, branch, and sequence assignment. The method is sensitive and can be applied using as little as 40 fmol of derivatized oligosaccharide. Because permethylation renders oligosaccharides nearly chemically equivalent in the mass spectrometer, the method is semiquantitative and, in this regard, is comparable to methods reported using high field NMR and capillary electrophoresis. In this postgenomic age, the importance of glycosylation in biological processes has become clear. The nature of many of the important questions in glycomics is such that sample material is often extremely limited, thus necessitating the development of highly sensitive methods for rigorous structural assignment of the oligosaccharides in complex mixtures. The glycomics platform presented here fulfills these criteria and should lead to more facile glycomics analyses. 相似文献
205.
Brajagopal Samanta Joy Chakraborty R.K. Bhubon Singh Manas K. Saha Stuart R. Batten Paul Jensen M. Salah El. Fallah Samiran Mitra 《Polyhedron》2007
Two new cyano bridged Cu–Co and Cu–Fe trinuclear bimetallic assemblies, [(CuL)[Co(CN)6](CuL)]ClO4 · 3.5H2O (1) and [(CuL)[Fe(CN)6](CuL)] · 13H2O (2) where [L = (3E,5E)-N1,N4-bis((pyridin-2-yl)methylene)butane-1,4-diamine] have been prepared using cyanometallates as anion precursors and characterised by elemental analyses, spectroscopic studies, single crystal X-ray diffraction and cryomagnetic susceptibility measurements. Magneto-structural correlations have been drawn from cryomagnetic susceptibility measurements over a wide temperature range (2–300 K) under 0.5 T magnetic fields. Weak antiferromagnetic interactions with J = −0.81 and −0.73 cm−1 are found for 1 and 2, respectively, showing a very weak coupling, as expected from the diamagnetic long chain –NC–Co–CN–CN– and –NC–Fe–CN–CN– bridges revealed from the single crystal X-ray diffraction studies. 相似文献
206.
An analytical method using an optical probe in a photoelectrochemical cell for the sensitive and selective determination of aqueous Hg2+ is presented. A previously synthesized Hg2+ selective chemosensor, proven to be Hg2+ sensitive up to 2 μg L−1, has been immobilized onto indium tin oxide (ITO) electrodes in a composite form with polyaniline. The coated ITO electrode was placed in a photoelectrochemical cell under closed circuit conditions in which the optical recognition of the chemosensor was converted to a measurable signal. A composite of the fluorescent chemosensor, Rhodamine 6G derivative (RS), and polyaniline (PANI) was immobilized on ITO glass plates and subjected to photovoltage measurements in the absence and presence of Hg2+. The optical responses of the coated electrode were used to determine the sensitivity and selectivity of the immobilized sensor to Hg2+ in the presence of background ions. The optical response of the PANI-dye coated electrode increased linearly with increasing Hg2+ concentration in the range 10-150 μg L−1, with a detection limit of 6 μg L−1. 相似文献
207.
Reliable Multicast in Multi-Access Wireless LANs 总被引:8,自引:0,他引:8
Multicast is an efficient paradigm for transmitting data from a sender to a group of receivers. In this paper, we focus on multicast in single channel multi-access wireless local area networks (LANs) comprising several small cells. In such a system, a receiver cannot correctly receive a packet if two or more packets are sent to it at the same time, because the packets collide. Therefore, one has to ensure that only one node sends at a time. We look at two important issues. First, we consider the problem of the sender acquiring the multi-access channel for multicast transmission. Second, for reliable multicast in each cell of the wireless LAN, we examine ARQ-based approaches. The second issue is important because the wireless link error rates can be very high.We present a new approach to overcome the problem of feedback collision in single channel multi-access wireless LANs, both for the purpose of acquiring the channel and for reliability. Our approach involves the election of one of the multicast group members (receivers) as a leader or representative for the purpose of sending feedback to the sender. For reliable multicast, on erroneous reception of a packet, the leader does not send an acknowledgment, prompting a retransmission. On erroneous reception of the packet at receivers other than the leader, our protocol allows negative acknowledgments from these receivers to collide with the acknowledgment from the leader, thus destroying the acknowledgment and prompting the sender to retransmit the packet.Using analytical models, we demonstrate that the leader-based protocol exhibits higher throughput in comparison to two other protocols which use traditional delayed feedback-based probabilistic methods. Last, we present a simple scheme for leader election. 相似文献
208.
209.
In recent years, due to fast development of wireless sensor networks (WSNs), the numbers of nodes are increasing, and their scope of applications is continuously expanding, including environmental monitoring, military and smart home applications. The power supply, memory and computing power of wireless sensor nodes are greatly hampered in WSNs so that the WSNs are classified as a task-oriented framework. This study focused on exploring problems caused by traffic congestion on the WSNs with a large amount of flow, such as packet loss, bandwidth reduction, and waste of energy on the sensor nodes. On the other hand, a cooperative strong node mechanism is presented and named as Cooperative Strong Node Mechanism, in which a threshold is set to determine whether the node traffic is over or not. When the load exceeds, the privilege of corresponding sensor nodes is upgraded so that it can command its child nodes to change the transmission path to distribute the traffic effectively. Furthermore, when the traffic exceeds preset overall network flow, new sensor nodes are added in the network to relieve the traffic. This novel proposed mechanism can not only increase network throughput and effectively prevent the occurrence from congestion problems, but is suitable for a variety of routing protocols. 相似文献
210.
Ronglin Li Gerald Dejean Manos M. Tentzeris Joy Laskar 《Wireless Personal Communications》2007,42(1):1-11
A new type of multi-broadband antennas is developed for wireless applications. The broadband performance of this type of antennas
is achieved by introducing a small gap on a wire loop while the multi-band operation is realized by simultaneously exciting
one or more additional wire elements. Three antenna configurations are investigated for single-, dual-, and triple-broadband
operations. The dual-broadband operation is obtained by employing a combination of a loop and a monopole. The triple-broadband
antenna consists of a loop, a monopole, and an inverted-L structure. It is demonstrated that the multi-broadband antennas
can achieve a bandwidth of 30% in the 1GHz band, 50% in the 2 GHz band, and 40% in the 5 GHz band, which cover the frequency
bands for AMPS/PCS, GSM/DCS, PDC/PHS, and IMT-2000 mobile communications as well as for 2.4-GHz and 5-GHz wireless applications. 相似文献