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31.
Short TE in vivo (1)H MR spectroscopic imaging at 1.5 T: acquisition and automated spectral analysis
Soher BJ Vermathen P Schuff N Wiedermann D Meyerhoff DJ Weiner MW Maudsley AA 《Magnetic resonance imaging》2000,18(9):1159-1165
Spectral analysis of short TE in vivo proton magnetic resonance spectroscopic imaging (MRSI) data are complicated by the presence of spectral overlap, low signal to noise and uncharacterized signal contributions. In this study, it is shown that an automated data analysis method can be used to generate metabolite images from MRSI data obtained from human brain at TE = 25 ms and 1.5 T when optimized pulse sequences and a priori metabolite knowledge are used. The analysis approach made use of computer simulation methods to obtain a priori spectral information of the metabolites of interest and utilized a combination of parametric spectral modeling and non-parametric signal characterization for baseline fitting. This approach was applied to data from optimized PRESS-SI and multi-slice spin-echo SI acquisitions, for which sample spectra and metabolite images are shown. 相似文献
32.
We developed a differential group delay (DGD) emulator which is based on a high spectral resolution phase pulse shaper. DGD generation has been demonstrated for up to four wavelength channels independently and simultaneously, with DGD accuracy of approximately 1 ps for up to 400-ps DGD range. For each channel, DGD versus frequency profiles can be arbitrarily programmed to user specifications. Multichannel DGD compensation has also been demonstrated using the same setup. 相似文献
33.
Because of limits on the speed of the photodetector, a nonlinear thresholder is needed at the receiver of a spectral-phase-encoding optical code-division multiple-access system to discriminate between the correctly decoded short pulse and the low-intensity interference. The two most common nonlinear receivers based on second harmonic generation and self-phase modulation effects are analyzed in this paper. Mathematical models are provided, and analytical results are obtained to estimate the receivers' performances. Numerical simulations are carried out for both receivers with different system parameters. Both m-sequences and random binary codes are examined for spreading. The results provide a profile of how these nonlinear receivers perform with various system settings. It is found that, when an m-sequence is used as the spreading code, the encoded signal does not obey Gaussian statistics, and the system performs better than an equivalent system using a random code. 相似文献
34.
Leaird D.E. Shen S. Weiner A.M. Sugita A. Kamei S. Ishii M. Okamoto K. 《Photonics Technology Letters, IEEE》2001,13(3):221-223
We present what we believe to be the first demonstration of femtosecond pulse train generation from an arrayed-waveguide grating. Terahertz-rate bursts of femtosecond pulses are produced with the rate determined by the arrayed-waveguide delay spacing 相似文献
35.
Use of the Longley-Rice and Johnson-Gierhart Tropospheric Radio Propagation Programs:0.02-20 GHz 总被引:2,自引:0,他引:2
The Longley-Rice and Johnson-Gierhart tropospheric propagation programs are particularly useful in predicting propagation losses for paths over irregular terrain commonly encountered in ground-to-ground, air-to-ground, and air-to-air military tactical scenarios. These programs use an empirical database to statistically weigh knife-edge diffraction losses with losses from multipath interference, smooth-spherical Earth diffraction, and troposcatter modes of propagation. The purpose of this paper is to provide perspective concerning the use of the two programs. Emphasis is placed upon the regions of applicability for the programs and on factors that must be considered in selecting and specifying the input parameters. Numerical results are given for various scenarios and ranges of input parameters. 相似文献
36.
37.
For many practical problems an efficient solution of the one-dimensional shallow-water equations (Saint–Venant equations) is important, especially when large networks of rivers, channels or pipes are considered. In order to test and develop numerical methods four test problems are formulated. These tests include the well-known dam break and hydraulic jump problems and two steady state problems with varying channel bottom, channel width and friction.The space discretization of the partial differential equations is based on a finite volume approach with central WENO interpolation and local Lax–Friedrich fluxes (Kurganov and Levy, 2000) [7]. For time-integration new linearly-implicit two-step peer methods of orders three and four are developed. These methods are especially adapted to the usage within the method of lines framework. They show a good performance compared to the well-established methods like ode15s, radau5 or rodasp. 相似文献
38.
The direct space-to-time (DST) pulse shaping apparatus is investigated both theoretically and experimentally. The discussion shows how the operation of the DST pulse shaper may be understood in terms of the femtosecond response of a generalized spectrometer. A complete discussion of the advantages and tradeoffs of utilizing this pulse-shaping configuration is given, including the direct scaling between the masking function and the output temporal intensity profile, system efficiency, spectral resolution, and temporal window, compensation/cancellation of chirp, and generation of multiple output waveforms 相似文献
39.
B-consistency andB-convergence of linearly implicit one step methods with respect to a class of arbitrarily stiff semi-linear problems are considered. Order conditions are derived. An algorithm for constructing methods of order>1 is shown and examples are given. By suitable modifications of the methods the occurring order reduction is decreased. 相似文献
40.
Alexander M. Khenkin Dr. Irena Efremenko Dr. Lev Weiner Dr. Jan M. L. Martin Prof. Ronny Neumann Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(4):1356-1364
A polyoxometalate of the Keggin structure substituted with RuIII, 6Q5[RuIII(H2O)SiW11O39] in which 6Q=(C6H13)4N+, catalyzed the photoreduction of CO2 to CO with tertiary amines, preferentially Et3N, as reducing agents. A study of the coordination of CO2 to 6Q5[RuIII(H2O)SiW11O39] showed that 1) upon addition of CO2 the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (gx=2.146, gy=2.100, and gz=1.935), and 3) the 13C NMR spectrum had a broadened peak of bound CO2 at 105.78 ppm (Δ1/2=122 Hz). It was concluded that CO2 coordinates to the RuIII active site in both the presence and absence of Et3N to yield 6Q5[RuIII(CO2)SiW11O39]. Electrochemical measurements showed the reduction of RuIII to RuII in 6Q5[RuIII(CO2)SiW11O39] at ?0.31 V versus SCE, but no such reduction was observed for 6Q5[RuIII(H2O)SiW11O39]. DFT‐calculated geometries optimized at the M06/PC1//PBE/AUG‐PC1//PBE/PC1‐DF level of theory showed that CO2 is preferably coordinated in a side‐on manner to RuIII in the polyoxometalate through formation of a Ru? O bond, further stabilized by the interaction of the electrophilic carbon atom of CO2 to an oxygen atom of the polyoxometalate. The end‐on CO2 bonding to RuIII is energetically less favorable but CO2 is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp2 hybridization state. Formation of a O2C–NMe3 zwitterion, in turn, causes bending of CO2 and enhances the carbon sp2 hybridization. The synergetic effect of these two interactions stabilizes both Ru–O and C–N interactions and probably determines the promotional effect of an amine on the activation of CO2 by [RuIII(H2O)SiW11O39]5?. Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO2 and Et3N. A mechanistic pathway for photoreduction of CO2 is suggested based on the experimental and computed results. 相似文献