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排序方式: 共有368条查询结果,搜索用时 171 毫秒
51.
David Lehotzky Tamas Insperger Firas Khasawneh Gabor Stepan 《The International Journal of Advanced Manufacturing Technology》2017,89(9-12):2503-2514
This paper presents an application of the spectral element method for the stability analysis of regenerative machine tool chatter models in milling operations. An extension of the spectral element method is introduced in order to handle the discontinuities in the cutting force in an efficient way. The efficiency of the method is demonstrated on some well-known machine tool chatter models taken from the literature. Efficiency is characterized by the computational time, the convergence of the stability boundaries, and the convergence of critical characteristic multipliers. Results show that compared to the most widespread methods in machining literature, the spectral element method provides significant improvements in computational time while maintaining high accuracy levels. 相似文献
52.
Morphological control of nanocrystals has become increasingly important, as many of their physical and chemical properties are highly shape dependent. Nanocrystal shape control for both single- and multiple-material systems, however, remains empirical and challenging. New methods need to be explored for the rational synthetic design of heterostructures with controlled morphology. Overgrowth of a different material on well-faceted seeds, for example, allows for the use of the defined seed morphology to control nucleation and growth of the secondary structure. Here, we have used highly faceted cubic Pt seeds to direct the epitaxial overgrowth of a secondary metal. We demonstrate this concept with lattice-matched Pd to produce conformal shape-controlled core-shell particles, and then extend it to lattice-mismatched Au to give anisotropic growth. Seeding with faceted nanocrystals may have significant potential towards the development of shape-controlled heterostructures with defined interfaces. 相似文献
53.
Xingcai Su Kyle Kung Jouko Lahtinen Ron Y. Shen Gabor A. Somorjai 《Catalysis Letters》1998,54(1-2):9-15
The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(111) crystal surfaces were investigated by surface vibrational
spectroscopy via sum frequency generation (SFG) both under vacuum and high pressure conditions with 10 Torr cyclohexene and
various hydrogen pressures from 30 up to ~600 Torr. At high pressures, the gas composition and turnover rate (TOR) were measured
by gas chromatography. In vacuum, cyclohexene on Pt(111) undergoes a change from π/σ‐bonded, σ‐bonded cyclohexene and c‐C6H9 surface species to adsorbed benzene when the surface was heated from 130 to 330 K. A site‐blocking effect was observed at
saturation coverage of cyclohexene that caused dehydrogenation to shift to somewhat higher surface temperature. At high pressures,
however, none of the species observed in vacuum conditions were detectable. 1,4‐cyclohexadiene (1,4‐CHD) was found to be the
major species on the surface at 295 K, even with the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable
reaction at the temperature range between 300 and 400 K with 1,3‐cyclohexadiene (1,3‐CHD) on the surface, as revealed by SFG.
Further increasing the surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation
reaction rate and both 1,3‐CHD and 1,4‐CHD were present on the surface simultaneously. The simultaneous observation of the
reaction kinetic data and the chemical nature of surface species allows us to postulate a reaction mechanism at high pressures:
cyclohexene hydrogenates to cyclohexane via a 1,3‐CHD intermediate and dehydrogenates to benzene through both 1,4‐CHD and
1,3‐CHD intermediates. Isomerisation of the 1,4‐CHD and 1,3‐CHD surface species is negligible.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
54.
Judit E. Puskas Prince Antony Yongmoon Kwon Christophe Paulo Milan Kovar Peter R. Norton Gabor Kaszas Volker Altstdt 《大分子材料与工程》2001,286(10):565-582
This article features macromolecular engineering via carbocationic polymerization, the focus of research of the recently established Macromolecular Engineering Research Centre (MERC) at the University of Western Ontario. The fundamental philosophy of MERC is interdisciplinary research with a strong industrial orientation, while emphasizing the quest for fundamental understanding of polymerization processes and polymer structure‐property relationships. First, a brief overview of living polymerizations in general, and living carbocationic polymerizations in particular will be given. This latter technique is of interest because some monomers (e. g., isobutylene) can be polymerized by cationic techniques only, to yield polymers with unique properties (e. g., polyisobutylene with superior chemical and oxidative stability, low permeability and high damping). This will be followed by an overview of our research strategy and a summary of our latest results. These include the development of a fiber‐optic mid‐FTIR method for the real‐time monitoring of low temperature polymerization processes, the discovery that selected epoxides initiate effectively the living carbocationic polymerization of isobutylene, fundamental studies into the mechanism and kinetics of living carbocationic polymerization, and the design and synthesis of various polymer architectures (e. g., branched homo‐ and block copolymers) with improved properties and nanostructured phase morphologies. 相似文献
55.
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57.
Using a Pt/Si catalytic nanodiode, we externally control the rate of CO oxidation on a Pt nanofilm. The catalytic reaction can be turned on and off by alternating between bias states of the device. Additionally, the reaction rate is sensitive to photocurrent induced by visible light. The effects of both bias and light show that negative charge on the Pt increases catalytic activity, while positive charge on the Pt decreases catalytic activity for CO oxidation. 相似文献
58.
Giovanetti LJ Ramallo-López JM Foxe M Jones LC Koebel MM Somorjai GA Craievich AF Salmeron MB Requejo FG 《Small (Weinheim an der Bergstrasse, Germany)》2012,8(3):468-473
Polyvinylpyrollidone (PVP)-capped platinum nanoparticles (NPs) are found to change shape from spherical to flat when deposited on mesoporous silica substrates (SBA-15). Transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and extended X-ray absorption fine structure (EXAFS) analyses are used in these studies. The SAXS results indicate that, after deposition, the 2 nm NPs have an average gyration radius 22% larger than in solution, while the EXAFS measurements indicate a decrease in first neighbor co-ordination number from 9.3 to 7.4. The deformation of these small capped NPs is attributed to interactions with the surface of the SBA-15 support, as evidenced by X-ray absorption near-edge structure (XANES). 相似文献
59.
Kaitlin M. Bratlie Yimin Li Ragnar Larsson Gabor A. Somorjai 《Catalysis Letters》2008,121(3-4):173-178
Kinetic measurements at low temperatures (310–360 K) using gas chromatography (GC) for benzene hydrogenation on Pt(100) and
Pt(111) single crystal surfaces have been carried out at Torr pressures. These kinetic measurements demonstrated a linear
compensation effect for the production of cyclohexane. A detailed application of the model of selective energy transfer to
the experimentally obtained results yields the vibrational frequency of the adsorbate leading to reaction. This frequency
is attributed to ring distortion modes. The vibrational frequency of the heat bath, or catalyst, is ascribed to a Pt-H mode.
An approximate heat of adsorption of the reacting molecule is also calculated from the model. 相似文献
60.