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151.
In the interaction of cyclohexa-1,4-diene (1,4-CHD) with a mixture of biphenyl and metallic lithium or sodium in THF at 20 °C, three processes occur, viz., disproportionation of 1,4-CHD to form benzene and cyclohexene, dehydrogenation of 1,4-CHD to form benzene and molecular hydrogen, and dehydrogenation of 1,4-CHD to form benzene and lithium or sodium hydride. In the case of lithium on the use of an equimolar amount of biphenyl, the isomerization of 1,4-CHD to cyclohexa-1,3-diene is also observed. When the molar ratio Li(Na): Ph2 increases from 1 : 1 to 2 : 1, i.e., when the reaction is carried out in the presence of an alkali metal solid phase, the overall conversion of 1,4-CHD into benzene and cyclohexene increases. The use of mixtures of lithium and sodium leads to acceleration of the processes of the formation of benzene and cyclohexene. The possible mechanism of the synergistic effect found is discussed. 相似文献
152.
Results of carbon nanoribbons and nanotubes simulation by means of hybrid density functional method and using empirical potentials have been compared. Energy of the nanoribbons formation and their citting from graphene sheet as well as energy of the nanotubes folding from graphene and nanoribbons have been determined. The REBO force field satisfactorily reflects the result of quantum chemical simulations; however, it cannot reproduce the formation of triple bonds between the edge atoms of the nanoribbons in the armchair conformation and thus leads to underestimated stability of the latter. Energy of the nanotubes folding from the nanoribbons is linear with the nanotube diameter. 相似文献
153.
Philippe Spalart Linda Hedges Michael Shur Andrey Travin 《Flow, Turbulence and Combustion》2003,71(1-4):361-373
An Active-Flow-Control (AFC) device, designed to unload the wing of the V-22 tilt-rotor aircraft in hover when the rotors blow down on it, is studied by Detached-Eddy Simulation (DES). The device is a zero-net-flux jet through a slot on the flap shoulder, designed to help the flow turn and follow the highly-deflected flap more closely. The details of the DES are presented, including boundary conditions, grid, slot rendition, and algorithm. Meaningful grid-refinement studies were not possible. As AFC is applied, the flow pattern is altered, the response of the pressure distribution is qualitatively correct, and a drag reduction results. However the agreement on drag is poor if the boundary layers are treated as turbulent in the DES; discrepancies are in the 15–20% range. The boundary layers were tripped in the experiment and therefore expected to be turbulent. However the Reynolds numbers, pressure gradients, and shape of the pressure distributions (experimental and computed) at the leading edge all argue that the tripping failed. If the boundary layers are treated as laminar, the agreement is considerably improved with AFC off, but the response to AFC still differs from that in the wind tunnel, and is not monotonic with respect to the blowing level. Other issues related to the numerical and experimental approaches are discussed, particularly the nature of the simulation in the neighborhood of the slot, transition, and two-dimensionality. 相似文献
154.
V. A. Vazhenin V. B. Guseva V. Ya. Shur E. V. Nikolaeva M. Yu. Artemov 《Physics of the Solid State》2001,43(10):1952-1957
The temperature dependence of the spontaneous polarization in the Pb5Ge3O11 lead germanate (PGO) is experimentally investigated using optical, magnetic resonance, and conventional electrical measurements. The deviations from the temperature dependence typical of second-order phase transitions at temperatures below 420 K are explained in terms of incomplete polarization switching and polarization induced by a residual depolarization field. The low-temperature anomalies are interpreted without consideration of additional structural transformations. The internal bias field is determined from the experimental temperature dependence of the perfect polarization of PGO single crystals in an electric field. 相似文献
155.
156.
Detached-Eddy Simulations Past a Circular Cylinder 总被引:12,自引:0,他引:12
Andrei Travin Michael Shur Michael Strelets Philippe Spalart 《Flow, Turbulence and Combustion》2000,63(1-4):293-313
The flow is calculated with laminar separation (LS) at Reynolds numbers 50,000 and 140,000, and with turbulent separation
(TS) at140,000 and 3 × 106. The TS cases are effectively tripped, but compared with untripped experiments at very high Reynolds numbers. The finest
grid has about 18,000 points in each of 56 grid planes spanwise; the resolution is far removed from Direct Numerical Simulations,
and the turbulence model controls the separation if turbulent. The agreement is quite good for drag, shedding frequency, pressure,
and skin friction. However the comparison is obscured by large modulations of the vortex shedding and drag which are very
similar to those seen in experiments but also, curiously, durably different between cases especially of the LS type. The longest
simulations reach only about 50 shedding cycles. Disagreement with experimental Reynolds stresses reaches about 30%, and the
length of the recirculation bubble is about double that measured. The discrepancies are discussed, as are the effects of grid
refinement, Reynolds number, and a turbulence-model curvature correction. The finest grid does not give the very best agreement
with experiment. The results add to the validation base of the Detached-Eddy Simulation (DES) technique for smooth-surface
separation. Unsteady Reynolds-averaged simulations are much less accurate than DES for LS cases, but very close for TS cases.
Cases with a more intricate relationship between transition and separation are left for future study.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
157.
J. Lusakowski W. Knap N. Dyakonova E. Kaminska A. Piotrowska K. Golaszewska M. S. Shur D. Smirnov V. Gavrilenko A. Antonov S. Morozov 《Physics of the Solid State》2004,46(1):138-145
Magnetotransport characterization of field-effect transistors in view of their application as resonant detectors of THz radiation
is presented. Three groups of different transistors based on GaAs/GaAlAs or GaInAs/AlGaAs heterostructures are investigated
at liquid-helium temperatures and for magnetic fields of up to 14 T. The magnetic-field dependence of the transistor resistance
is used for evaluation of the electron density and mobility in the transistor channel. The electron mobility and concentration
determined from magnetotransport measurements are used for the interpretation of recently observed resonant detection of terahertz
radiation in 0.15 μm gate length GaAs transistors and for the determination of the parameters of other field-effect transistors
processed for resonant and voltage tunable detection of THz radiation.
From Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 138–145.
Original English Text Copyright ? 2004 by Lusakowski, Knap, Dyakonova, Kaminska, Piotrowska, Golaszewska, Shur, Smirnov, Gavrilenko,
Antonov, Morozov.
This article was submitted by the authors in English. 相似文献
158.
159.
160.
Batchko RG Fejer MM Byer RL Woll D Wallenstein R Shur VY Erman L 《Optics letters》1999,24(18):1293-1295
We report continuous-wave single-pass second-harmonic generation (SHG) in 4-mum -period 0.5-mm-thick backswitch-poled lithium niobate. Pump sources at 920-930 nm include both Ti:sapphire and diode-oscillator-amplifier lasers. SHG of a Ti:sapphire laser at 6.1%/W efficiency, producing 61 mW of power at 460 nm, is demonstrated in 50-mm-long periodically poled lithium niobate samples with a nonlinear coefficient d(eff) approximately 9pm/V, and 60 mW at 465 nm and 2.8%/W efficiency is obtained by SHG of a laser-diode source. 相似文献