全文获取类型
收费全文 | 35037篇 |
免费 | 3616篇 |
国内免费 | 2339篇 |
专业分类
化学 | 19936篇 |
晶体学 | 580篇 |
力学 | 2173篇 |
综合类 | 189篇 |
数学 | 3468篇 |
物理学 | 14646篇 |
出版年
2024年 | 39篇 |
2023年 | 505篇 |
2022年 | 590篇 |
2021年 | 790篇 |
2020年 | 884篇 |
2019年 | 870篇 |
2018年 | 819篇 |
2017年 | 761篇 |
2016年 | 1195篇 |
2015年 | 1103篇 |
2014年 | 1381篇 |
2013年 | 2038篇 |
2012年 | 2634篇 |
2011年 | 2964篇 |
2010年 | 2039篇 |
2009年 | 1877篇 |
2008年 | 2160篇 |
2007年 | 2134篇 |
2006年 | 2020篇 |
2005年 | 1714篇 |
2004年 | 1357篇 |
2003年 | 1124篇 |
2002年 | 1030篇 |
2001年 | 1297篇 |
2000年 | 998篇 |
1999年 | 951篇 |
1998年 | 674篇 |
1997年 | 702篇 |
1996年 | 570篇 |
1995年 | 511篇 |
1994年 | 503篇 |
1993年 | 396篇 |
1992年 | 359篇 |
1991年 | 348篇 |
1990年 | 317篇 |
1989年 | 226篇 |
1988年 | 178篇 |
1987年 | 173篇 |
1986年 | 129篇 |
1985年 | 121篇 |
1984年 | 98篇 |
1983年 | 61篇 |
1982年 | 53篇 |
1981年 | 46篇 |
1980年 | 29篇 |
1977年 | 18篇 |
1976年 | 27篇 |
1975年 | 29篇 |
1974年 | 20篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
91.
Song Jiang 《Mathematische Nachrichten》1998,190(1):169-183
We consider initial boundary value problems for the equations of the one-dimensional motion of a viscous, heat-conducting gas with density-dependent viscosity that decreases (to zero) with decreasing density. We prove that if the viscosity does not decrease to zero too rapidly, then smooth solutions exist globally in time. 相似文献
92.
本文证明了周期单元中带有某些硬相互作用的相对论Boltzmann方程初值问题在初值满足质量、能量和熵有限的条件下具有一个整体温和解. 相似文献
93.
94.
95.
We established that acetylacetone and acetone photolytically sensitize norbornene to undergo an efficient radical addition
of solvent (ranging from hexane, cyclic ethers, haloalkanes, acetone, alcohols and acetonitrile) across the double bond. In
view of its synthetic applicability, sensitized photoreactions of norbornene were reviewed and their mechanisms were compared.
Photolysis of acetylacetone in the presence of norbornene in hexane induced i) acetylacetone to cycloadd to norbornene giving
the expected 1,5-diketone, and ii) sensitization by triplet excited acetylacetone to generate reactive norbornene, which underwent
dimerization as well as the addition of a solvent molecule by radical chain processes. In other solvents, the radical chain
addition of solvent dominated the photoreaction, and superseded the cycloaddition, to give excellent to good yields of adducts
to norbornene. While the excited species of acetylacetone for the sensitization was deduced to be its spectroscopic triplet
excited state, that for the cycloaddition should involve a different one which may be a twisted triplet acetylacetone; sensitization
experiments showed that the cycloaddition did not occur from the spectroscopic triplet state. Triplet excited acetone sensitized
norbornene to undergo the same solvent addition more efficiently and cleanly than acetylacetone did. In view of various conflicts
existing in the proposed energy transfer mechanism, the sensitized norbornene reactions were rationalized with electron transfer
and a cation radical chain mechanism. 相似文献
96.
Zhai X. Garate E. Prohaska R. Benford G. Fisher A. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1993,21(1):142-150
Experimental studies of a plasma-filled X -band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ⩽0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X -band Tm01 mode prevailed when the background plasma density was below 6×1011 cm-3 . At a critical background plasma density of ≃8×1011 cm-3 power enhancement appeared in both X -band and the T-G modes. Power enhancement of the S -band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse 相似文献
97.
98.
The standard model on non-commutative space-time 总被引:1,自引:0,他引:1
X. Calmet B. Jurčo P. Schupp J. Wess M. Wohlgenannt 《The European Physical Journal C - Particles and Fields》2002,23(2):363-376
We consider the standard model on a non-commutative space and expand the action in the non-commutativity parameter . No new particles are introduced; the structure group is . We derive the leading order action. At zeroth order the action coincides with the ordinary standard model. At leading order
in we find new vertices which are absent in the standard model on commutative space-time. The most striking features are couplings
between quarks, gluons and electroweak bosons and many new vertices in the charged and neutral currents. We find that parity
is violated in non-commutative QCD. The Higgs mechanism can be applied. QED is not deformed in the minimal version of the
NCSM to the order considered.
Received: 29 November 2001 / Published online: 25 January 2002 相似文献
99.
The structural stability of heat-treated CoN/CN soft X-ray multilayers fabricated by dual-facing-target sputtering has been investigated by using complementary measurement techniques. The high temperature annealing results imply that the destructive threshold of the Co/C multilayers is improved by 100–200 degrees centigrade through doping with N. The low-angle X-ray diffraction of CoN/CN soft X-ray multilayers indicates that the period expansion of the multilayers is only 4 % at 400°C, and the interface pattern still exists even if they were annealed at 700°C. The Raman spectra analyses give the evidence that the formation of the sp3 bonding in the CN sublayers can be suppressed effectively by doping N with atoms, and thus the period expansion resulting from the changes in the density of CN layers can be decreased considerably. The X-ray photoelectron spectra analyses present the information of the existence of the strong covalent bonding betweenC andN atoms, and the ionic bonding between Co and N atoms, which can slow down the tendency of the structural relaxation. The interstitial N atoms decrease the mobility of Co atoms, and thus the fcc-Co and hcp-Co coexist even though the annealing temperature is much higher than the phase transformation temperature of 420°C, leading to the suppression of the grain growth. 相似文献
100.