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121.
G. Baum E. Kisker A. H. Mahan W. Raith B. Reihl 《Applied Physics A: Materials Science & Processing》1977,14(2):149-153
Field emission from a tungsten tip, covered by a layer of crystallized ferromagnetic europium sulfide, leads to an electron
beam with a current of 10−8 A, an energy width of less than 100 meV, and a spin polarization of about 0.85 at a tip temperature of 9 K. Proper annealing
of the EuS layer is crucial. 相似文献
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Simulation of the effects of severe fires on the structural integrity of buildings requires a close coupling between the gas phase energy release and transport phenomena, and the stress analysis in the load-bearing materials. The connection between the two is established primarily through the interaction of the radiative heat transfer between the solid and gas phases with the conduction of heat through the structural elements. This process is made difficult in large, geometrically complex buildings by the wide disparity in length and time scales that must be accounted for in the simulations. A procedure for overcoming these difficulties used in the analysis of the collapse of the World Trade Center towers is presented. The large scale temperature and other thermophysical properties in the gas phase are predicted using the NIST Fire Dynamics Simulator. Heat transfer to subgrid scale structural elements is calculated using a simple radiative transport model that assumes the compartment is locally divided into a hot, soot laden upper layer and a cool relatively clear lower layer. The properties of the two layers are extracted from temporal averages of the results obtained from the Fire Dynamics Simulator. Explicit formulae for the heat flux are obtained as a function of temperature, hot layer depth, soot concentration, and orientation of each structural element. These formulae are used to generate realistic thermal boundary conditions for a coupled transient three-dimensional finite element code. This code is used to generate solutions for the heating of complex structural assemblies. 相似文献
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H. R. Baum O. A. Ezekoye K. B. McGrattan R. G. Rehm 《Theoretical and Computational Fluid Dynamics》1994,6(2-3):125-139
An approach to the study of gas phase combustion and convection processes in fires using a combination of mathematical analysis and computer simulation is presented. It seeks to solve the governing equations directly (if approximately) by decomposing the fire into a large-scale convective and radiative transport problem coupled to a small-scale thermal-element model of combustion and radiative emission. The thermal-element model solves the combustion equations in a local Lagrangian coordinate system convected by the large-scale motion, which in turn is driven by the heat released by the combustion processes. The large-scale flow is studied using finite-difference techniques to solve large-eddy simulations of the Navier-Stokes equations. The basic theory behind the methodology is outlined and sample results of both large- and small-scale phenomena are presented. An analytical model of a large eddy is used to show how the simulation can be assembled to yield radiation feedback from a fire plume to a target surface. 相似文献
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Transition metal-catalyzed carbocyclization reactions represent powerful methods for the construction of complex polycyclic systems. We have developed a regiospecific and diastereoselective intramolecular temporary silicon-tethered rhodium-catalyzed [4+2+2] cycloisomerization reaction of a tethered enyne for the construction of tricyclic eight-membered heterocycles and carbocycles. This methodology also allows (E)- and (Z)-olefins to be utilized in a stereospecific manner. The incorporation of either alkene geometry is particularly significant, since related carbocyclization reactions are often limited in this respect. Finally, the ability to utilize carbon-tethered 1,6-enynes and to regiospecifically incorporate substituted 1,3-butadiene derivatives provides exciting opportunities for future applications toward the total synthesis of cyclooctanoid containing diterpenes. 相似文献
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I. Kovcs G. Baum G. Fritz D. Fenske N. Wiberg H. Schuster K. Karaghiosoff 《无机化学与普通化学杂志》1993,619(3):453-460
Synthesis, Characterization, and Structure of P7(t-Bu3Si)3 (?Tris(supersilyl)heptaphosphane(3)”? Tris(tri-tert-butylsilyl)heptaphosphanortricyclane P7(t-Bu3Si)3 1 is obtained from the reaction of (t-Bu)3Si? Si(t-Bu)3 with white phosphorus and forms colorless to pale yellow thermostable crystals. 1 is identified by the complete analysis of its 31P{1H} NMR spectrum (A[MX]3 spin system) as well as by a single crystal structure determination (space group Pca21, a = 170.76(2)pm, b = 131.14(3)pm, c = 426.61(5)pm, α = β = γ= 90°, Z = 8 formula units in the elementary cell). The steric demand of the (t-Bu)3Si-Groups causes an increase of the exocyclic bond angles at the equatorial phosphorus atoms Pe, while it does not particularly influence the P7-skeleton. Chlorine (r.t.) and bromine (70°C) degrade the P7-cage of 1 with formation of PX3 and (t-Bu)3SiX (X = Cl, Br). 相似文献
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