全文获取类型
收费全文 | 21172篇 |
免费 | 177篇 |
国内免费 | 42篇 |
专业分类
化学 | 19680篇 |
晶体学 | 21篇 |
力学 | 147篇 |
数学 | 808篇 |
物理学 | 735篇 |
出版年
2014年 | 182篇 |
2013年 | 267篇 |
2012年 | 315篇 |
2011年 | 378篇 |
2010年 | 204篇 |
2009年 | 198篇 |
2008年 | 290篇 |
2007年 | 314篇 |
2006年 | 325篇 |
2005年 | 286篇 |
2004年 | 244篇 |
2003年 | 222篇 |
2002年 | 235篇 |
1987年 | 156篇 |
1985年 | 241篇 |
1984年 | 240篇 |
1982年 | 179篇 |
1980年 | 360篇 |
1979年 | 298篇 |
1978年 | 1084篇 |
1977年 | 832篇 |
1976年 | 264篇 |
1972年 | 147篇 |
1970年 | 141篇 |
1968年 | 225篇 |
1967年 | 187篇 |
1966年 | 236篇 |
1965年 | 261篇 |
1964年 | 237篇 |
1963年 | 184篇 |
1962年 | 350篇 |
1961年 | 173篇 |
1960年 | 221篇 |
1959年 | 210篇 |
1958年 | 210篇 |
1957年 | 155篇 |
1956年 | 243篇 |
1955年 | 227篇 |
1954年 | 166篇 |
1943年 | 161篇 |
1939年 | 166篇 |
1938年 | 178篇 |
1937年 | 232篇 |
1936年 | 214篇 |
1935年 | 231篇 |
1934年 | 187篇 |
1933年 | 251篇 |
1930年 | 220篇 |
1928年 | 156篇 |
1927年 | 153篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The applicability of glass chips with powder-blasted microchannels for electrophoretic separations was examined, and the performance was compared to microchannels etched with hydrogen fluoride (HF), using bicarbonate buffer and rhodamine B and fluorescein as model compounds. The measured electroosmotic mobilities in all chips were comparable, with values of ca. 7 x 10(-4) cm(2) V(-1)s(-1). The effect of electrical field strength and detection length on the separation efficiency was monitored. It was found that the main source of dispersion is of the Taylor-Aris type, which was discussed in relation to channel roughness differences. Although in powder-blasted channels with a separation length of 8.20 cm, 7-9 times lower plate numbers were obtained than in a HF-etched channel with similar dimensions, successful separation of five fluorescein isothiocyanate (FITC)-labeled amino acids was obtained on a powder-blasted chip within 80 s. Efficiencies of up to 360 000 plates/m were demonstrated on this chip, when a higher buffer concentration was used at a field strength of 664 V/cm. It can be concluded that powder-blasted microchannel chips, although they have a lower separation efficiency compared to HF-etched chips, perform well enough for many applications. Powder blasting can therefore be considered a low-cost and efficient alternative to HF etching, in particular because of the possibility to fabricate access holes through the glass with the same process. 相似文献
992.
Busico V Van Axel Castelli V Aprea P Cipullo R Segre A Talarico G Vacatello M 《Journal of the American Chemical Society》2003,125(18):5451-5460
The 150 MHz (13)C NMR microstructural analysis of polypropylene samples produced with two representative "oscillating" metallocene catalysts of largely different steric hindrance, namely [(2-(3,5-di-tert-butyl-4-methoxyphenyl)indenyl)(2)ZrP](+) and [(2-phenylindenyl)(2)ZrP](+) (P = polymeryl), and the implications on the origin of the stereocontrol are presented and discussed in detail. The original mechanistic proposal of an "oscillation" between a rac-like (isotactic-selective) and a meso-like (nonstereoselective) conformation cannot explain the observed polymer configuration. The isotactic-stereoblock nature of the polymers obtained with the former catalyst proves unambiguously that the active cation "oscillates" between the two enantiomorphous rac-like conformations at an average frequency that, even at high propene concentration, is only slightly lower than that of monomer insertion. The less-hindered [(2-phenylindenyl)(2)ZrP](+) gives instead a largely stereoirregular polypropylene, which is the logical consequence of a faster ligand rotation; however, depending on the use conditions (in particular, on the nature of the cocatalyst and the polarity of the solvent), the polymerization products may also contain appreciable amounts of a fairly isotactic fraction. The peculiar microstructure of this fraction, with isotactic blocks of the same relative configuration spanned by very short atactic ones, rules out the possibility that the latter are due to an active species in meso-like conformation and points rather to a conformationally "locked" rac-like species with restricted ring mobility. The hypothesis of a stereorigidity induced by the proximity to a counteranion, which would play the role of the interannular bridge in the rac-bis(indenyl) ansa-metallocenes, was tested by computer modeling on a [rac-(2-phenylindenyl)(2)ZrMe(C(3)H(6))][B(C(6)F(5))(4)] ion couple and found viable. 相似文献
993.
Cossu S Cimenti C Peluso P Paulon A De Lucchi O 《Angewandte Chemie (International ed. in English)》2001,40(21):4086-4089
994.
995.
Let D Hn(k2) be a convex compact subset of the hyperbolicspace Hn(k2) with non-empty interior and smooth boundary.It is shown that the volume of D can be estimated by the totalcurvature of D. More precisely, , where K denotes the GaussKronecker curvature of D andVol(Sn1) denotes the Euclidean volume of the sphere.2000 Mathematics Subject Classification 53C21. 相似文献
996.
Raffaele Cerulli Paola Festa Giancarlo Raiconi 《Computational Optimization and Applications》2003,26(2):191-208
In this paper, the problem of finding a shortest path tree rooted at a given source node on a directed graph (SPT) is considered. A new efficient algorithm based on a primal-dual approach is presented, which improves both the convergence and the complexity of the best known auction-like algorithm. It uses the virtual source (VS) concept based on the following consideration: when a node i is visited for the first time by any algorithm which preserves verified the dual admissibility conditions, then the shortest path (SP) from the source node to i is found. Therefore, the SP from the source to the remaining nodes may be computed by considering i as a virtual source.We propose a very efficient implementation of an auction-like algorithm that uses this concept and enables us to obtain a computational cost of O(n
2), where n is the number of nodes.Numerical experimentsare reported showing that the new method outdoes previously proposed auction-like algorithms and is highly competitive with other state-of-art SP approaches. 相似文献
997.
The space of obstacles (i.e. p-quasi upper semicontinuous functions) is endowed with a distance which is topologically equivalent to the -convergence. We find the metric completion of this space and we give some application for minimization problems of cost functionals depending on obstacles via their level sets. An element of the completion is a decreasing and
p
-continuous on the left mapping Rt
t
, where
t
are positive Borel measures vanishing on sets of zero p-capacity. 相似文献
998.
We compute the -limit of a sequence obstacle functionals in the case of periodic obstacles. 相似文献
999.
Roberto Gianni Paola Mannucci 《NoDEA : Nonlinear Differential Equations and Applications》2001,8(2):219-235
We prove a well posedness result for a free boundary problem describing the filtration of an incompressible viscous fluid
in a porous medium containing water absorbing granules.?The location of the wetting front (the free boundary) is described
by a Stefan like problem for a parabolic equation which is coupled with an hyperbolic equation describing the absorption kinetic
of the granules.
Received December 1999 相似文献
1000.
Albert DG 《The Journal of the Acoustical Society of America》2001,109(1):91-101
The amplitude and waveform shape of atmospheric acoustic pulses propagating horizontally over a seasonal snow cover are profoundly changed by the air forced into the snow pores as the pulses move over the surface. This interaction greatly reduces the pulse amplitude and elongates the waveform compared to propagation above other ground surfaces. To investigate variations in snow-cover effects, acoustic pulses were recorded while propagating horizontally over 11 different naturally occurring snow covers during two winters. Two inversion procedures were developed to automatically match the observed waveforms by varying the snow-cover parameters in theoretical calculations. A simple frequency-domain technique to match the dominant frequency of the measured waveform suffered from multiple solutions and poor waveform matching, while a time-domain minimization method gave unique solutions and excellent waveform agreement. Results show that the effective flow resistivity and depth of the snow are the parameters controlling waveform shape, with the pore shape factor ratio of secondary importance. Inversion estimates gave flow resistivities ranging from 11 to 29 kN s m(-4), except for two late-season cases where values of 60 and 140 were determined (compared to 345 for the vegetation-covered site in the summer). Acoustically determined snow depths agreed with the measured values in all but one case, when the depth to a snow layer interface instead of the total snow depth was determined. Except for newly fallen snow, the pore shape factor ratio values clustered near two values that appear to correspond to wet (1.0) or dry (0.8) snow. 相似文献